US20180010204A1 - High yield strength steel - Google Patents

High yield strength steel Download PDF

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Publication number
US20180010204A1
US20180010204A1 US15/643,679 US201715643679A US2018010204A1 US 20180010204 A1 US20180010204 A1 US 20180010204A1 US 201715643679 A US201715643679 A US 201715643679A US 2018010204 A1 US2018010204 A1 US 2018010204A1
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alloy
rolling
mpa
yield strength
tensile
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US15/643,679
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Daniel James Branagan
Andrew E. Frerichs
Brian E. MEACHAM
Jason K. Walleser
Andrew T. BALL
Grant G. JUSTICE
Kurtis R. Clark
Sheng Cheng
Scott T. ANDERSON
Scott T. Larish
Taylor L. GIDDENS
Logan J. TEW
Alla V. Sergueeva
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Nanosteel Co Inc
United States Steel Corp
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Nanosteel Co Inc
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Priority to US15/643,679 priority Critical patent/US20180010204A1/en
Assigned to THE NANOSTEEL COMPANY, INC. reassignment THE NANOSTEEL COMPANY, INC. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: BALL, ANDREW T., BRANAGAN, DANIEL JAMES, CHENG, SHENG, TEW, Logan J., WALLESER, JASON K., CLARK, Kurtis R., SERGUEEVA, ALLA V., ANDERSON, Scott T., FRERICHS, ANDREW E., GIDDENS, TAYLOR L., JUSTICE, GRANT G., LARISH, Scott T., MEACHAM, BRIAN E.
Publication of US20180010204A1 publication Critical patent/US20180010204A1/en
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Assigned to UNITED STATES STEEL CORPORATION reassignment UNITED STATES STEEL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: HORIZON TECHNOLOGY FINANCE CORPORATION
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D7/00—Modifying the physical properties of iron or steel by deformation
    • C21D7/02—Modifying the physical properties of iron or steel by deformation by cold working
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
    • B21B3/02—Rolling special iron alloys, e.g. stainless steel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

  • This disclosure is related to high yield strength steel. Due to the unique structures and mechanisms, yield strength can be increased without significantly affecting ultimate tensile strength (UTS) and in some cases, higher yield strength can be obtained without significant decrease in ultimate tensile strength and total elongation. These new steels can offer advantages for a myriad of applications where relatively high yield strength is desirable along with relatively high UTS and total elongation such as the passenger cage in automobiles.
  • UTS ultimate tensile strength
  • AHSS Third Generation Advanced High Strength Steels
  • MS martensitic steels
  • DP dual phase steels
  • TRIP transformation induced plasticity
  • CP complex phase
  • Example targets for 3 rd Generation AHSS are provided in the banana chart for autobody steels which is published by World Auto Steel ( FIG. 1 ).
  • AHSS ultimate tensile strength
  • AHSS materials are not generally classified by the yield strength (YS). Yield strength of a material is also of large importance to automobile designers since once a part is in service and if the part is stressed beyond yield, the part will permanently (plastically) deform. Materials that have high yield strength resist permanent deformation to higher stress levels than those with lower yield strength. This resistance to deformation is useful by allowing structures made from the material to withstand greater loads before the structure permanently deflects and deforms. Materials with higher yield strength can thereby enable automobile designers to reduce associated part weight through gauge reduction while maintaining the same resistance to deformation in the part. Many types of emerging grades of third generation AHSS suffer from low initial yield strengths, despite having various combinations of tensile strength and ductility.
  • the yield strength of a material can be increased in a number of ways on the industrial scale.
  • the material can be cold rolled a small amount (with a reduction ⁇ 2%) in a process called temper rolling. This process introduces a small amount of plastic strain in the material, and the yield strength of the material is increased slightly corresponding to the amount of strain that the material was subjected to during the temper pass.
  • Another method of increasing the yield strength in the material is through a reduction in the material's crystal grain size, known as Hall-Petch strengthening. Smaller crystal grains increase the required shear stress for the initial dislocation movement in the material, and the initial deformation is delayed until higher applied loads.
  • the grain size can be reduced through process modifications such as altered annealing schedules to limit grain growth during the recrystallization and growth process that occurs during annealing after plastic deformation.
  • Chemistry modifications to an alloy such as the addition of alloying elements that exist in solid solution can also increase the yield strength of a material, however the addition of these alloying elements must take place while the material is molten and may result in increased costs.
  • Cold working steel from a fully annealed state is a known route to increase yield strength and tensile strength. It can be applied uniformly across a sheet during processing through cold rolling increasing the yield strength and tensile strength. However, this approach results in a decrease in total elongation and often to levels much below 20%. As elongation decreases, the cold forming ability also decreases, reducing the ability to produce parts with complex geometries resulting in a decrease in the usefulness of the AHSS. Higher ductility with a minimum of 30% total elongation is generally needed to form complex geometries through cold stamping processes. While processes such as roll forming can be used to create parts from lower elongation material, the geometric complexity of parts from these processes is limited. Cold rolling also can introduce anisotropy into the material which will farther reduce its ability to be cold formed into parts.
  • a method to increase yield strength in a metallic alloy comprising:
  • the second sheet formed in step (c) indicating either tensile property combination A or B above may then be exposed to a permanent deformation at a temperature of ⁇ 150° C.
  • the invention herein also relates to a method to increase yield strength in a metallic alloy comprising:
  • the metallic alloys produced herein provide particular utility in vehicles, railway cars, railway tank cars/wagons, drill collars, drill pipe, pipe casing, tool joints, wellheads, compressed gas storage tanks or liquefied natural gas canisters. More specifically, the alloys find utility in vehicular bodies in white, vehicular frames, chassis, or panels.
  • FIG.s are provided for illustrative purposes and are not to be considered as limiting any aspect of this invention.
  • FIG. 1 World Auto Steel “Banana Plot” with targeted properties for 3 rd Generation AHSS.
  • FIG. 2 Summary of Method 1 to produce high yield strength in alloys herein.
  • FIG. 3 Summary of Method 2 to produce high yield strength and targeted combinations of properties in the alloys herein.
  • FIG. 4 Ultimate tensile strength in alloys herein before and after cold rolling.
  • FIG. 5 Tensile elongation in alloys herein before and after cold rolling.
  • FIG. 6 Yield strength in alloys herein before and after cold rolling.
  • FIG. 7 Magnetic phase volume percent in alloys herein before and after cold rolling.
  • FIG. 8 Tensile stress-strain curves for Alloy 2 after cold rolling with various reductions.
  • FIG. 9 Back-scattered SEM micrograph of the microstructure in the hot band from Alloy 2: a) low magnification image; b) high magnification image.
  • FIG. 10 Bright-field TEM micrograph of the microstructure in the hot band from Alloy 2: a) low magnification image; b) high magnification image.
  • FIG. 11 TEM micrograph showing nanoscale precipitates in the hot band from Alloy 2.
  • FIG. 12 Back-scattered SEM micrograph of the microstructure in the cold rolled sheet from Alloy 2: a) low magnification image; b) high magnification image.
  • FIG. 13 TEM micrograph of the microstructure in the cold rolled sheet from Alloy 2: a) low magnification image; b) high magnification image.
  • FIG. 14 TEM micrograph showing nanoscale precipitates found in Alloy 2 sheet after cold deformation.
  • FIG. 15 Engineering tensile stress—strain curves for Alloy 2 after rolling with 20% reduction at different temperatures.
  • FIG. 16 Change in magnetic phases volume percent (Fe %) during tensile testing in Alloy 2.
  • FIG. 17 Engineering stress—strain curves for Alloy 7 after rolling with 20% reduction at different temperatures.
  • FIG. 18 Engineering stress—strain curves for Alloy 18 after rolling with 20% reduction at different temperatures.
  • FIG. 19 Engineering stress—strain curves for Alloy 34 after rolling with 20% reduction at different temperatures.
  • FIG. 20 Engineering stress—strain curves for Alloy 37 after rolling with 20% reduction at different temperatures.
  • FIG. 21 Representative engineering stress—strain curves for Alloy 2 that was rolled at 200° C. to various rolling reductions.
  • FIG. 22 The yield and ultimate tensile strength of Alloy 2 as a function of rolling reduction at 200° C.
  • FIG. 23 The yield strength and total elongation of Alloy 2 as a function of rolling reduction at 200° C.
  • FIG. 24 The effect of rolling at 200° C. on the deformation induced phase transformation in Alloy 2 as a function of rolling reduction.
  • FIG. 25 Backscattered SEM micrograph of microstructure in hot band from Alloy 2: a) low magnification image; b) high magnification image.
  • FIG. 26 Backscattered SEM micrographs of microstructure in Alloy 2 after rolling at 200° C. to 30% reduction: a) low magnification image; b) high magnification image.
  • FIG. 27 Backscattered SEM micrographs of microstructure in Alloy 2 after rolling at 200° C. to 70% reduction: a) low magnification image; b) high magnification image.
  • FIG. 28 Bright-field TEM micrographs of the microstructure in Alloy 2 after rolling at 200° C. with 10% reduction: a) low magnification image and b) high magnification image.
  • FIG. 29 Bright-field TEM micrographs of the microstructure in Alloy 2 after rolling at 200° C. with 30% reduction: a) low magnification image and b) high magnification image.
  • FIG. 30 Bright-field TEM micrographs of the microstructure in Alloy 2 after rolling at 200° C. with 70% reduction: a) low magnification image and b) high magnification image.
  • FIG. 31 Engineering stress-strain curves for Alloy 2 processed by combination of rolling methods.
  • FIG. 32 Engineering stress-strain curves for Alloy 7 processed by combination of rolling methods.
  • FIG. 33 Engineering stress-strain curves for Alloy 18 processed by combination of rolling methods.
  • FIG. 34 Engineering stress-strain curves for Alloy 34 processed by combination of rolling methods.
  • FIG. 35 Comparison of engineering stress-strain curves for Alloy 2 sheet processed by different methods and their combination.
  • FIG. 36 Tensile elongation and magnetic phases volume percent in a tensile sample gauge after testing of Alloy 2 at different temperatures.
  • FIG. 37 Magnetic phases volume percent as a function of rolling reduction at ambient temperature and at 200° C.
  • FIG. 38 Examples of engineering stress-strain curves for the annealed sheet produced by both cold rolling and rolling at 200° C.
  • FIG. 39 Rolling reduction limit vs rolling temperature for Alloy 2.
  • FIG. 2 represents a summary of preferred Method 1 to develop high yield strengths from a low yield strength material by a route which results in either of two conditions as provided in conditions 3a or 3b.
  • the starting condition is to supply a metal alloy.
  • This metal alloy will comprise at least 70 atomic % iron and at least four or more elements selected from Si, Mn, Cr, Ni, Cu or C.
  • the alloy chemistry is melted and preferably cooled at a rate of 10 ⁇ 4 K/s to 10 3 K/s and solidified to a thickness of >5.0 mm to 500 mm.
  • the casting process can be done in a wide variety of processes including ingot casting, bloom casting, continuous casting, thin slab casting, thick slab casting, thin strip casting, belt casting etc.
  • Preferred methods would be continuous casting in sheet form by thin slab casting, thick slab casting, and thin strip casting.
  • Preferred alloys would exhibit a fraction of austenite (y-Fe) at least 10 volume percent up to 100 volume percent and all increments in between in the temperature range from 150 to 400° C.
  • Step 2 of Method 1 the alloy is preferably processed into sheet form with thickness from 0.5 to 5.0 mm.
  • This step 2 can involve hot rolling or hot rolling and cold rolling. If hot rolling the preferred temperature range would be at a temperature of 700° C. and below the Tm of said alloy. If cold rolling is employed, such is understood to be at ambient temperature. Note that after hot rolling or hot rolling and cold rolling, the sheet can be additionally heat treated, preferably in the range from at a temperature of 650° C. to a temperature below the melting point (Tm) of said alloy.
  • Tm melting point
  • the steps to produce sheet from the cast product can therefore vary depending on specific manufacturing routes and specific targeted goals.
  • the alloy would preferably be cast going through a water cooled mold typically in a thickness range of 150 to 300 mm in thickness.
  • the cast ingot after cooling would then be preferably prepared for hot rolling which may involve some surface treatment to remove surface defects including oxides.
  • the ingot would then go through a roughing mill hot roller which may involve several passes resulting in a transfer bar slab typically from 15 to 100 mm in thickness. This transfer bar would then go through successive/tandem hot rolling finishing stands to produce hot band coils which are typically from 1.5 to 5.0 mm in thickness.
  • cold rolling can be done at various reductions per pass, variable number of passes and in different mills including tandem mills, Z-mills, and reversing mills.
  • cold rolled thickness would be 0.5 to 2.5 mm thick.
  • the cold rolled material is annealed to restore the ductility lost from the cold rolling process either partially or completely at a temperature range from 650° C. to a temperature below the melting point (Tm) of said alloy.
  • Another example would be to preferably process the cast material through a thin slab casting process.
  • the newly formed slab goes directly to hot rolling without cooling down with auxiliary tunnel furnace or induction heating applied to bring the slab directly up to targeted temperature.
  • the slab is then hot rolled directly in multi-stand finishing mills which are preferably from 1 to 10 in number.
  • the strip is rolled into hot band coils with typical thickness from 1 to 5 mm in thickness. If further processing is needed, cold rolling can be applied in a similar manner as above.
  • bloom casting would be similar to the examples above but higher thickness might be cast typically from 200 to 500 mm thick and initial breaker steps would be needed to reduce initial cast thickness to allow it to go through a hot rolling roughing mill.
  • the sheet will then exhibit a total elongation of X 1 (%), an ultimate tensile strength of Y 1 (MPa), and a yield strength of Z 1 (MPa).
  • Preferred properties for this alloy would be ultimate tensile strength values from 900 to 2050 MPa, tensile elongation from 10 to 70%, and yield strength is in a range from 200 to 750 MPa.
  • the alloy is permanently (i.e. plastically) deformed in the temperature range from 150° C. to 400° C.
  • Such permanent deformation may be provided by rolling and causing a reduction in thickness. This can be done for example during the final stages of the development of a steel coil.
  • elevated temperature rolling is now preferably done in the targeted temperature range of 150 to 400° C.
  • One method would be to heat the sheet to the targeted temperature range prior to going through the cold rolling mill.
  • the sheet could be heated by a variety of methods including going through a tunnel mill, a radiative heater, a resistance heater, or an induction heater. Another method would be to heat directly the reduction rollers.
  • a third example for illustration would be to low temperature batch anneal the sheet and then send this through the cold rolling mill(s) at the targeted temperature range.
  • the sheet may be deformed at the elevated temperature range into parts using a variety of processes providing permanent deformation during the making of parts by various methods including roll forming, metal stamping, metal drawing, hydroforming etc.
  • Condition 3a comparing said alloy in Step 2 and after Step 3, the total elongation and ultimate tensile strength are relatively unaffected but the yield strength is increased. Specifically, the total elongation X 2 is equal to X 1 ⁇ 7.5%, the tensile strength Y 2 is equal to Y 1 ⁇ 100 MPa, and the yield strength Z 2 is ⁇ Z 1 +100 MPa.
  • Preferred properties for this alloy in Condition 3a would be ultimate tensile strength values (Y 2 ) from 800 to 2150 MPa, tensile elongation (X 2 ) from 2.5% to 77.5%, and yield strength (Z 2 ) ⁇ 300 MPa. More preferably, yield strength may fall in the range of 300 to 1000 MPa.
  • Condition 3b comparing said alloy in Step 2 and after Step 3, the ultimate tensile strength is relatively unaffected but the yield strength is increased. Specifically, the ultimate tensile strength Y 3 is equal to Y 1 ⁇ 300 MPa and yield strength Z 3 is ⁇ Z 1 ⁇ F_200 MPa. Preferred properties for this alloy in Condition 3b would be ultimate tensile strength values (Y 3 ) from 800 to 2150 MPa and yield strength (Z 3 ) ⁇ 400 MPa. More preferably, yield strength may fall in the range of 400 to 1200 MPa. In addition, unlike Condition 3a, the total elongation drop is greater than 7.5%, that is, in Step B, the total elongation (X 3 ) is defined as follows: X 3 ⁇ X 1 ⁇ 7.5%.
  • FIG. 3 identifies a summary of Method 2 of the present disclosure.
  • the first 3 steps in Method 2 are identical to Method 1 with Step 4 being an additional step for Method 2.
  • Step 4 can be applied to the alloys herein in either Condition 3a or Condition 3b.
  • Condition 3a or 3b various combinations of properties (i.e. total elongation, ultimate tensile strength, and yield strength) are provided for each Condition 3a or 3b.
  • alloys in Condition 3a or 3b may be further characterized by their particular structure. This then allows further tailoring of the final properties by the use of a further optional step of permanently deforming the alloys at temperatures from ambient to ⁇ 150° C., or more preferably at a range of temperatures of 0° C. to 150° C. This can be done for example by adding another step during the production of steel coils as illustrated in FIG. 3 .
  • Step 4 can be a skin pass (i.e.
  • Step 4 of Method 2 the sheet could be subsequently made into parts using a variety of deformation processes including roll forming, metal stamping, metal drawing, hydroforming etc. Notwithstanding the exact process to activate Step 4 in Method 2, final properties can be developed with the said alloy which are contemplated to exhibit properties with tensile elongation from 10 to 40%, ultimate tensile strength from 1150 to 2000 MPa, and yield strength from 550 to 1600 MPa).
  • the alloys herein are iron based metal alloys, having greater than 70 at. % Fe.
  • the alloys herein are such that they comprise Fe and at least four or more, or five or more, or six elements selected from Si, Mn, Cr, Ni, Cu or C. Accordingly, with respect to the presence of four or more, or five or more elements selected from Si, Mn, Cr, Ni, Cu or C, such elements are present at the following indicated atomic percents: Si (0 to 6.13 at. %); Mn (0 to 15.17 at. %); Cr (0 to 8.64 at. %); Ni (0 to 9.94 at. %); Cu (0 to 1.86 at. %); and C (0 to 3.68 at. %).
  • the alloys herein are such that they comprise, consist essentially of, or consist of Fe at a level of 70 at. % or greater along with Si, Mn, Cr, Ni, Cu and C, wherein the level of impurities of all other elements is in the range from 0 to 5000 ppm.
  • Impurities can be present at various levels depending on the feedstock used. Impurity elements would commonly include the following elements; Al, Co, Mo, N, Nb, P, Ti, V, W, and S which if present would be in the range from 0 to 5000 ppm (parts per million) with preferred ranges of 0 to 500 ppm.
  • the casting machine then evacuated the chamber and tilted the crucible and poured the melt into a 50 mm thick, 75 to 80 mm wide, and 125 mm deep channel in a water cooled copper die and would represent Step 1 in FIGS. 2 and 3 .
  • the process can be adapted to a preferred as-cast thickness at a range from >5.0 to 500 mm.
  • the melt was allowed to cool under vacuum for 200 seconds before the chamber was filled with argon to atmospheric pressure.
  • the alloys herein were preferably processed into a laboratory sheet.
  • Laboratory alloy processing is developed to simulate the hot band production from slabs produced by continuous casting and would represent Step 2 in FIGS. 2 and 3 .
  • Industrial hot rolling is performed by heating a slab in a tunnel furnace to a target temperature, then passing it through a either a reversing mill or a multi-stand mill or a combination of both to reach the target gauge in a preferred temperature range from 700° C. up to the melting point (Tm) of the alloy.
  • Tm melting point
  • This is simulated in the laboratory by heating in a tunnel furnace to between 1100° C.
  • the laboratory mill is slower than industrial mills causing greater loss of heat during each hot rolling pass so the slab is reheated for 4 minutes between passes to reduce the drop in temperature, the final temperature at target gauge when exiting the laboratory mill commonly is in the range from 1000° C. to 800° C., depending on furnace temperature and final thickness.
  • the slab thickness has been reduced to a final thickness of the hot band from 1.8 to 2.3 mm.
  • Processing conditions can be adjusted by changing the amount of hot rolling and/or adding cold rolling steps to produce the preferred thickness range from 0.5 to 5.0 mm.
  • Tensile specimens were cut from laboratory hot band using wire EDM.
  • Tensile properties were measured on an Instron mechanical testing frame (Model 3369), utilizing Instron's Bluehill control and analysis software.
  • Tensile properties of the alloys in the hot rolled condition are listed in Table 2 which have been processed to a thickness from 1.8 to 2.3 mm.
  • the ultimate tensile strength values may vary from 913 to 2000 MPa with tensile elongation from 13.8 to 68.5%.
  • the yield strength is in a range from 250 to 711 MPa.
  • Mechanical properties of the hot band from steel alloys herein depend on alloy chemistry, processing conditions, and material mechanistic response to the processing conditions.
  • the hot band from alloys herein listed in Table 1 was, for comparison purposes, cold rolled to final target gauge thickness of 1.2 mm through multiple cold rolling passes.
  • Tensile specimens were cut from each cold rolled sheet using wire EDM.
  • Tensile properties were measured on an Instron mechanical testing frame (Model 3369), utilizing Instron's Bluehill control and analysis software. All tests were run at ambient temperature in displacement control.
  • Tensile properties of alloys herein after cold rolling are listed in Table 3. As it can be seen, the yield strength is significantly increased over the range in a hot band with maximum at 711 MPa (Table 2). After cold rolling yield strength varies from 1037 to 2000 MPa. The ultimate tensile strength values after cold rolling are in a range from 1431 to 2222 MPa. However, a drop in tensile elongation is recorded for each alloy herein after cold rolling with variation from 4.2 to 31.1%. The general trends in effect of cold rolling on tensile properties of alloys herein are illustrated in FIG. 4 to FIG. 6 .
  • the relative magnetic phases content was measured by Feritscope in both a hot band and after cold rolling for each alloy herein that is listed in Table 4 and illustrated in FIG. 7 for selected alloys.
  • the magnetic phases volume percent of 0.1 to 56.4 Fe % in a hot band increases to the range from 1.6 to 84.9 Fe % after cold rolling confirming a phase transformation during deformation.
  • This comparative Case Example demonstrates that yield strength can be increased in alloys herein by cold rolling (i.e. at ambient temperature). Ultimate tensile strength is also increasing but cold rolling leads to a significant decrease in alloy ductility indicated by a drop in tensile elongation that can be a limiting factor in certain applications. Strengthening, as shown by the increase in ultimate tensile strength, is related to a phase transformation of austenite to ferrite as depicted by measurements of magnetic phases volume percent before and after cold rolling.
  • Alloy 2 was processed into a hot band with a thickness of 4.4 mm.
  • the hot band was then cold rolled with different reduction through multiple cold rolling (i.e. at ambient temperature) passes. After cold rolling the samples were heat treated with intermediate annealing at 850° C. for 10 min. This represented a start condition for each sample which represented a fully annealed condition to remove the prior cold work. From this start condition, subsequent cold rolling at different percentages (i.e. 0%, 4.4%, 9.0%, 15.1%, 20.1%, 25.1% and 29.7%) as provided in Table 5 was applied so that the final gauge for tensile testing would be at a targeted constant thickness of 1.2 mm.
  • This Comparative Case Example #2 demonstrates that yield strength in alloys herein can be altered by cold rolling reduction to achieve relatively higher yield strength values with increase in tensile strength but with decrease in ductility. The higher cold rolling reduction that is applied, the higher yield strength achieved and the lower tensile elongation recorded.
  • Hot band from Alloy 2 with thickness of 4 mm was cold rolled to a final thickness of 1.2 mm through multiple cold rolling passes with intermediate annealing at 850° C. for 10 min. Microstructures of the hot band and the cold rolled sheet were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
  • the TEM specimens may be ion-milled using a Gatan Precision Ion Polishing System (PIPS).
  • PIPS Gatan Precision Ion Polishing System
  • the ion-milling usually is done at 4.5 keV, and the inclination angle is reduced from 4° to 2° to open up the thin area.
  • the TEM studies were done using a JEOL 2100 high-resolution microscope operated at 200 kV.
  • This Case Example demonstrates a microstructure evolution from the initial hot band austenitic structure during cold rolling leading to alloy strengthening (increase in ultimate tensile strength) by grain refinement due to phase transformation into ferrite with nanoprecipitation as well as dislocation density increase and deformation twinning.
  • the starting material was a hot band from Alloy 2 with approximately 2.5 mm thickness prepared by hot rolling of 50 mm thick laboratory cast slab mimicking processing at commercial hot band production.
  • the starting material had an average ultimate tensile strength of 1166 MPa, an average tensile elongation of 53.0% and an average yield strength of 304 MPa.
  • the starting material also had a magnetic phases volume percent of 0.9 Fe %.
  • the hot band was media blasted to remove oxide and loaded into a Yamato DKN810 mechanical convection oven for at least 30 minutes prior to rolling to allow the plate to reach temperature.
  • the hot band was rolled on a Fenn Model 061 rolling mill with steadily decreasing roll gaps, and was loaded into the furnace for at least 10 minutes between passes to ensure a constant starting temperature (i.e. 50, 100, 150, 200, 250° C., 300° C., 350° C., and 400° C.) for each subsequent rolling pass for a total targeted 20% reduction.
  • Samples were EDM cut in the ASTM E8 Standard geometry.
  • Tensile properties were measured on an Instron mechanical testing frame (Model 5984), utilizing Instron's Bluehill control and analysis software. All tensile tests were run at ambient temperature in displacement control with the bottom fixture held rigid and the top fixture moving; the load cell is attached to the top fixture.
  • Tensile properties of the Alloy 2 after rolling at identified temperatures are listed in Table 6. Depending on rolling temperature, the yield strength is increased to a range from 589 to 945 MPa as compared to the values of 250 to 711 MPa in a hot band (Table 2). The ultimate tensile strength of the Alloy 2 varies from 1132 to 1485 MPa with tensile elongation from 21.2 to 60.5%. An example stress-strain curves are shown in FIG. 15 . As can be seen, rolling at temperature of 200° C. of the hot band from Alloy 2 demonstrates the possibility to increase yield strength with minimal changes in ductility and ultimate strength consistent with Step 3a in FIG. 3 .
  • This Case Example demonstrates that yield strength in alloys herein can be increased by rolling at elevated temperatures whereby phase transformation of austenite into ferrite is reduced. Significant drops in Fe % occur when rolling temperature is greater than 100° C.
  • rolling of the hot band from alloys herein at temperatures of 150° C. to 400° C. demonstrates the ability to increase yield strength (e.g. increasing yield strength to a value of at least 100 MPa or more over the original value) without significant change in ductility (i.e. change limited to plus or minus seven and one half percent ( ⁇ 7.5% tensile elongation) and maintain the ultimate tensile strength at about the same level (i.e. ⁇ 100 MPa as compared to the original value).
  • the starting material was a hot band from each of Alloy 7, Alloy 18, Alloy 34, and Alloy 37 with approximately 2.5 mm initial thickness prepared by hot rolling of 50 mm thick laboratory cast slab mimicking commercial processing. Alloys 7, 18, 34, and 37 were processed into hot bands with a thickness of approximately 2.5 mm by hot rolling at temperatures between 1100° C. and 1250° C. and subsequently media blasted to remove the oxide. The tensile properties of hot band material were previously listed in Table 2. The hot band was media blasted to remove oxide and loaded into a Yamato DKN810 mechanical convection oven for at least 30 minutes prior to rolling to allow the plate to reach the desired temperature.
  • the resulting cleaned hot band was rolled on a Fenn Model 061 rolling mill with steadily decreasing roll gaps, and was loaded into the furnace for at least 10 minutes between passes to ensure constant temperature.
  • the hot band was rolled to a targeted 20% reduction and samples were EDM cut in the ASTM E8 Standard geometry.
  • Tensile properties were measured on an Instron mechanical testing frame (Model 5984), utilizing Instron's Bluehill control and analysis software. All tensile tests were run at ambient temperature in displacement control with the bottom fixture held rigid and the top fixture moving; the load cell is attached to the top fixture.
  • tensile elongation ranged from 43.0% to 51.9%, ultimate tensile strength ranged from 1083 MPa to 1263 MPa, and yield strength ranged from 772 MPa to 924 MPa from 150 to 400° C. (Table 10), with Fe % numbers ranging from 6.8 to 12.3 before tensile testing and from 31.5 to 39.6 after testing in the 150 to 400° C. range (Table 11).
  • tensile elongation ranged from 21.1% to 31.1%
  • ultimate tensile strength ranged from 1080 MPa to 1140 MPa
  • yield strength ranged from 869 MPa to 966 MPa in the 150 to 400° C.
  • This Case Example demonstrates that yield strength in alloys herein can be increased although phase transformation of austenite into ferrite is reduced when rolling at temperatures of 100° C. or greater up to 400° C. Examples of changes in yield strength, ultimate tensile strength, and tensile elongation were provided for both Steps 3a and 3b in FIG. 2 .
  • Alloy 2 was processed into a hot band with thickness of approximately 2.5 mm from the laboratory cast. Following hot rolling, Alloy 2 was rolled at 200° C. to varying rolling reductions ranging from approximately 10% to 40%. Between rolling passes, the Alloy 2 sheet material was placed in a convection furnace at 200° C. for 10 minutes to maintain the temperature. When the desired rolling reduction was achieved, ASTM E8 tensile samples were cut via wire-EDM and tested.
  • FIG. 21 shows the representative tensile curves for Alloy 2 as a function of rolling reduction at 200° C. It is observed that the yield strength of the material increases rapidly with increasing reduction, without changing the ultimate tensile strength (i.e. a change of plus or minus 100 MPa) up to 30% reduction.
  • FIG. 22 provides a comparison of the trends for yield strength and ultimate tensile strength as a function of rolling reduction at 200° C., showing that while the yield strength increase is relatively rapid, the ultimate tensile strength change is consistent with step 3a property changes in FIG. 2 up to 30.4% rolling reduction and is consistent with step 3b property changes at 39.0% rolling reduction.
  • the total elongation of Alloy 2 is plotted as a function of rolling reduction at 200° C. in FIG. 23 . It demonstrates that while the yield strength of Alloy 2 is increasing with additional reduction during rolling at 200° C., the available ductility does not decrease rapidly until >30% reduction. Note that this is simulated using laboratory rolling and commercial rolling methods including tandem mill rolling, Z-mill rolling, and reversing mill rolling will additionally apply a strip tension during rolling so the exact amount of reduction whereby ductility decreases may change.
  • the magnetic phases volume percent (Fe %) was measured using a Fischer Feritscope FMP30 for the samples after rolling at 200° C. and again after tensile testing in the tensile gauge (i.e. the reduced gauge section present in the tensile specimen). These measurements, shown in Table 17, are indicative of the amount of deformation-induced phase transformation that is occurring in the alloy during the rolling process and during subsequent tensile testing.
  • the amount of deformation-induced phase transformation in Alloy 2 after rolling and tensile testing is shown in FIG. 24 . It can be seen that the deformation-induced phase transformation is largely suppressed at 200° C., as the magnetic phases volume percent only increases slightly with increasing rolling reduction. Rolling at 200° C. is demonstrated to have an effect on the deformation-induced phase transformation during tensile testing also, with increasing rolling reductions suppressing the amount of transformation in the material.
  • This Case Example demonstrates that the yield strength of the alloys described herein may be tailored by varying the rolling reduction at temperatures greater than ambient as shown here for Alloy 2 by rolling at 200° C.
  • the temperature range is contemplated to be between 150° C. to 400° C. as provided in the previous case example for Table 7.
  • the deformation pathway is modified such that relatively limited deformation-induced phase transformation is occurring, which results in the ability to retain significant ductility and maintain ultimate tensile strength while increasing yield strength in the cold rolled state.
  • the parameters of the rolling can be optimized to improve the yield strength of the material without sacrificing the ductility or ultimate tensile strength.
  • Alloy 2 was processed into a hot band with thickness of 9 mm from the laboratory cast mimicking processing at commercial hot band production.
  • the hot band was cold rolled with 50% reduction and annealed at 850° C. for 10 minutes with air cooling mimicking cold rolling processing at commercial sheet production. Media blasting was used to remove the oxides which formed during annealing. Then the alloys were cold rolled again until failure or the mill limited reduction. Samples were heated to 200° C. in a convection oven for at least 30 minutes prior to cold rolling to ensure they were at uniform temperature, and reheated for 10 minutes between passes to ensure constant temperature. Alloy 2 sheet was cold rolled first with reduction of 30% and then to a maximum reduction of 70%.
  • SEM scanning electron microscopy
  • FIG. 25 shows the backscattered SEM images of the microstructure before cold rolling that is mostly austenitic with annealing twins inside micron-sized grains.
  • FIG. 26 shows the bands with similar orientation.
  • the bands with similar orientation are deformation twins in one austenitic grain while bands in different direction are twins in another crystal orientation grain. Some grain refinement can be observed in selected areas.
  • FIG. 27 After the rolling reduction is increased to 70%, the bands are no longer visible, and refined structure through the volume can be seen ( FIG. 27 ). As shown in the high magnification image in FIG. 27 b , fine islands with size much smaller than 10 ⁇ m can be discerned. Considering the high deformation exerted in the stable austenite during the rolling process, the austenite could be dramatically refined typically in the range of 100 to 500 nm. Feritscope measurements suggest that the austenite is stable at 200° C. with nearly 100% austenite maintained after rolling.
  • This Case Example demonstrates austenite stabilization (i.e. the resistance to transformation to ferrite) in alloys herein during the rolling at 200° C. even at high rolling reduction of 70% and microstructural refinement of the austenite in contrast to cold rolling when refinement occurs through austenite transformation to ferrite.
  • FIG. 28 shows the bright-field TEM images of the microstructure in the Alloy 2 rolled at 200° C. with 10% reduction. It can be seen that the austenite grains are filled with tangled dislocations, and dislocation cell structure is exhibited. However, due to the relatively low rolling strain, the original austenite grain boundaries are still visible. It is noted that the austenite is stable during the rolling at 200° C. Electron diffraction suggests that austenite is the predominant phase that was also consistent with Feritscope measurement. Rolling at 200° C. with 10% reduction increases the average yield strength from 303 MPa in the hot band to 529 MPA (see Table 16). When the sheet is rolled to 30%, TEM qualitatively shows higher dislocations density in the grains, as shown in FIG.
  • Alloys 2, Alloy 7, Alloy 18, and Alloy 34 were processed into hot band with a thickness of ⁇ 2.7 mm, this was media blasted to remove the oxide and rolled at 200° C. to 20% reduction. The material was sectioned and then rolled at a range of reductions at ambient temperature. ASTM E8 tensile samples were cut by wire EDM and tested in an Instron 5984 frame using Instron's Bluehill software.
  • Tensile properties of the selected alloys after combined rolling are listed in Table 18 through Table 21. Significant increase in yield strength after combination of rolling methods was observed in all three alloys as compared to the hot band state or just after rolling with ⁇ 20% reduction in rolling thickness at 200° C. and subsequent rolling reduction at ambient temperature.
  • Yield strength up to 1216 MPa recorded for Alloy 2 Yield strength in hot band is 309 MPa and 803 MPa after rolling at 200° C.
  • Yield strength in hot band is 309 MPa and 803 MPa after rolling at 200° C.
  • 1571 MPa in Alloy 7 Yield strength in hot band is 333 MPa and 575 MPa after rolling at 200° C.
  • 1080 MPa in Alloy 18 Yield strength in hot band is 390 MPa and 834 MPa after rolling at 200° C.
  • up to 1248 MPa in Alloy 34 Yield strength in hot band is 970 MPa and 1120 MPa after rolling at 200° C.
  • FIG. 31 through FIG. 34 shows the corresponding tensile curves for alloys 2, 7, 18, and 34, respectively.
  • This Case Example demonstrates a pathway to creating a third distinct set of property combinations, which may be achieved by processing the alloy into a sheet at a thickness of 0.5 mm to 5.0 mm, followed by deforming (rolling) and reducing thickness in one pass at a temperature in the range of 150° C. to 400° C., and then subsequent reductions in thickness at temperatures ⁇ 150° C. temperature. This is observed to provide relatively higher yield strength compared to only cold rolling, and higher tensile strengths compared to only rolling at temperature.
  • a hot band from Alloy 2 was processed into a sheet by different methods herein towards higher yield strength and property combination according to the steps provided in FIG. 2 and FIG. 3 .
  • Alloy 2 was first cast and then processed into a sheet via hot rolling which was from 2.5 to 2.7 mm thick.
  • the reference hot band material was hot rolled to ⁇ 1.8 mm to reduce gauge prior to testing.
  • the hot band was rolled with a 20% reduction at 200° C. Prior to rolling, it was heated up to 200° C. for 30 minutes before being rolled 20% at 200° C. with a 10 minute reheat between rolling passes to maintain temperature.
  • FIG. 3 example i.e.
  • FIG. 35 Representative stress-strain curves with property combination achieved at each processing method close to optimal are shown in FIG. 35 .
  • the yield strength can be significantly increased (i.e. 469 MPa increase) by rolling at 200° C. with minimal change in alloy ultimate tensile strength (i.e. 34 MPa increase) and elongation (i.e. 1.8% decrease).
  • This is provided by the example condition 3a in FIG. 2 .
  • this is a route to higher yield strength (i.e. 688 MPa increase) and tensile strength (i.e.
  • Step 4 in FIG. 3 could also be done by for example by cold stamping the part by various processes whereby the areas in the stamped part would experience higher yield strength and tensile strength with commensurate lower ductility which was used up partially in forming the part.
  • This Case Example demonstrates an achievement of high yield strength in alloys herein by various methods or their combination which provides a variety of the strength/elongation combinations in the resultant sheet from alloys herein.
  • Alloy 2 was produced in a sheet form with 1.4 mm thickness from the slab by hot rolling and cold rolling to a targeted thickness with subsequent annealing. Tensile specimens were cut from the Alloy 2 sheet using wire EDM. Tensile properties were measured at different temperatures in a range from ⁇ 40° C. to 200° C.
  • Tensile properties of the Alloy 2 sheet at different temperatures are listed in Table 26.
  • the magnetic phases volume percent was measured in the tensile sample gauge after testing at each temperature using Feritscope that is also listed in Table 26. As it can be seen, yield and ultimate tensile strength are decreasing with increasing test temperature while tensile elongation is increasing.
  • Tensile elongation and magnetic phases volume percent (Fe %) as a function of test temperature are plotted in FIG. 36 showing that despite higher elongation at elevated temperatures, the magnetic phases volume percent in a tensile sample gauge after testing drops significantly and close to zero after testing at 200° C.
  • a decrease in the magnetic phases volume percent in a tensile sample gauge after testing indicates higher austenite stability at elevated temperatures suppressing its transformation to ferrite under the stress.
  • Alloy 2 was processed into a hot band with thickness of 4.4 mm. Two sections of the hot band were then rolled, one at ambient temperature and one at 200° C. The plate at 200° C. was heated in a mechanical convection oven for 30 minutes prior to rolling and reheated for 10 minutes between passes to ensure constant temperature.
  • the magnetic phases volume percent (Fe %) was measured by Feritscope at different levels of reductions during cold rolling and rolling at 200° C. The data are shown in FIG. 37 . As it can be seen, the magnetic phases volume percent (Fe %) increases rapidly with reduction at ambient temperature leading to the material limit for rolling at ⁇ 42%. In a case of the rolling at 200° C., the magnetic phases volume percent (Fe %) remains under 3 Fe % even at maximum rolling reduction of >70%.
  • a sheet from Alloy 2 with final thickness of 1.2 mm was produced by utilizing both cold rolling and rolling at 200° C. In a case of cold rolling, the rolling was cycled with intermediate annealing to restore the alloy ductility and achieve the targeted thickness with reduction of 29% at final rolling step.
  • Tensile samples were EDM cut from the sheet with 1.2 mm thickness produced by both rolling methods and annealed at 1000° C. for 135 sec.
  • Tensile properties were measured on an Instron mechanical testing frame (Model 3369), utilizing Instron's Bluehill control and analysis software. All tests were run at ambient temperature in displacement control with the bottom fixture held rigid and the top fixture moving; the load cell is attached to the top fixture.
  • FIG. 38 Examples of the engineering stress-strain curves for the annealed sheet produced by both cold rolling and rolling at 200° C. are shown in FIG. 38 . As it can be seen, despite different rolling methods towards targeted thickness, the final properties of the sheet after annealing are similar.
  • This Case Example demonstrates that rolling where the austenite is stable and does not transfer to ferrite as demonstrated here for Alloy 2 at 200° C. significantly improves rolling ability of the alloys herein that will allow reduction in processing steps towards targeted sheet gauges.
  • this elevated temperature rolling can be used to hit a near final targeted gauge with high cold rolling reduction as provided in this example of >70%.
  • This near final gauge material can then be annealed to restore the starting properties (i.e. the initial condition).
  • the final targeted gauge can be obtained by rolling in the temperature range provided in this application from 150 to 400° C. following the steps and procedures in FIG. 2 or FIG. 3 .
  • Hot band was prepared from Alloy 2 with approximately 9 mm thickness. It was heated to 200 to 250° C. for 60 minutes and rolled to approximately 4.5 mm with 10 minute reheats between rolling passes to ensure consistent temperature. Once at 4.5 mm it was sectioned and annealed at 850° C. for 10 minutes and allowed to air cool. The material was media blasted to remove the oxide and heated to the desired temperature for at least 30 minutes prior to rolling, and reheated for 10 minutes between passes to ensure consistent temperature. The material was rolled until failure (visible cracking) characterized by such visible cracks propagating in from the ends of the sheet at least 2 inches. At around 70% reduction the mill had difficulty achieving the loads necessary to reduce the material and rolling was stopped, this is an equipment limitation and not a material limitation. The control material for room temperature rolling was hot band at 4.4 mm thick which was rolled at room temperature until failure. The results of the maximum rolling reduction as a function of rolling temperature are provided in Table 27 and FIG. 39 .
  • This Case Example demonstrates for the alloys herein that the limiting rolling reduction increases as temperature increases. It therefore can be seen that the alloys herein are contemplated to allow for permanent deformation with a reduction in thickness of greater than 20% before failure when heated to a temperature falling in the range of 150° C. to 400° C. More preferably, the alloys herein are such that they are contemplated to be capable of permanent deformation with a reduction in thickness of greater than 40% before failure when heated in such temperature range. This provides much greater potential deformation for rolling operations, including processing of industrial material to reach a target gauge. Greater reductions before cracking means that less steps (i.e. cold rolling and recrystallization annealing) may be required to hit a specific targeted gauge during steel production. Additionally, the greater formability demonstrated at elevated temperatures would be beneficial in making parts from a variety of forming operations including, stamping, roll forming, drawing, hydroforming etc.

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Abstract

This disclosure is related to high yield strength steel where yield strength can be increased without significantly affecting ultimate tensile strength (UTS) and in some cases, higher yield strength can be obtained without significant decrease in ultimate tensile strength and total elongation.

Description

    CROSS-REFERENCE TO RELATED APPLICATIONS
  • This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/359,844, filed on Jul. 8, 2016 and U.S. Provisional Patent Application Ser. No. 62/482,954, filed on Apr. 7, 2017, which are fully incorporated herein by reference.
    • FIELD OF INVENTION
  • This disclosure is related to high yield strength steel. Due to the unique structures and mechanisms, yield strength can be increased without significantly affecting ultimate tensile strength (UTS) and in some cases, higher yield strength can be obtained without significant decrease in ultimate tensile strength and total elongation. These new steels can offer advantages for a myriad of applications where relatively high yield strength is desirable along with relatively high UTS and total elongation such as the passenger cage in automobiles.
    • BACKGROUND
  • Third Generation Advanced High Strength Steels (AHSS) are currently being developed for automobile uses, and in particular automobile body applications. Advanced High-Strength Steels (AHSS) steels are classified by tensile strengths greater than 700 MPa with elongations from 4% to 30% and include such types as martensitic steels (MS), dual phase (DP) steels, transformation induced plasticity (TRIP) steels, and complex phase (CP) steels. Example targets for 3rd Generation AHSS are provided in the banana chart for autobody steels which is published by World Auto Steel (FIG. 1).
  • Tensile properties such as ultimate tensile strength (UTS) and total elongation are important benchmarks for establishing combinations of properties. However, AHSS materials are not generally classified by the yield strength (YS). Yield strength of a material is also of large importance to automobile designers since once a part is in service and if the part is stressed beyond yield, the part will permanently (plastically) deform. Materials that have high yield strength resist permanent deformation to higher stress levels than those with lower yield strength. This resistance to deformation is useful by allowing structures made from the material to withstand greater loads before the structure permanently deflects and deforms. Materials with higher yield strength can thereby enable automobile designers to reduce associated part weight through gauge reduction while maintaining the same resistance to deformation in the part. Many types of emerging grades of third generation AHSS suffer from low initial yield strengths, despite having various combinations of tensile strength and ductility.
  • A component in an automobile that experiences early yielding during normal service and undergoes permanent plastic deformation would be unacceptable based on most design criteria. In a crash event however, lower yield strengths, especially when coupled with a high strain hardening coefficient can be advantageous. This is especially true in the front and back ends of a passenger compartment which are often called the crumple zones. In these areas, a lower yield strength material with higher ductility can deform and strain harden increasing strength during the crash event leading to high levels of energy absorption due to the high starting ductility.
  • For other areas of the automobile, low yield strength would be unacceptable. Specifically, this would include what is called the passenger cage of an automobile. In the passenger cage, the materials utilized must have high yield strength since only very limited deformation/intrusion into the passenger cage is allowed. Once the passenger cage is penetrated this can lead to injury or death to the occupant(s). Thus, a material with high yield strength is required for these areas.
  • The yield strength of a material can be increased in a number of ways on the industrial scale. The material can be cold rolled a small amount (with a reduction <2%) in a process called temper rolling. This process introduces a small amount of plastic strain in the material, and the yield strength of the material is increased slightly corresponding to the amount of strain that the material was subjected to during the temper pass. Another method of increasing the yield strength in the material is through a reduction in the material's crystal grain size, known as Hall-Petch strengthening. Smaller crystal grains increase the required shear stress for the initial dislocation movement in the material, and the initial deformation is delayed until higher applied loads. The grain size can be reduced through process modifications such as altered annealing schedules to limit grain growth during the recrystallization and growth process that occurs during annealing after plastic deformation. Chemistry modifications to an alloy such as the addition of alloying elements that exist in solid solution can also increase the yield strength of a material, however the addition of these alloying elements must take place while the material is molten and may result in increased costs.
  • Developing high yield strength in the passenger cage from a low yield strength version of AHSS is a possible route. However, it is difficult in many metalworking operations to strain harden the finished part uniformly. This means that while the heavily cold worked areas of a part are much higher yield, there would still be lower yield strength areas which might then deform and cause an unacceptable intrusion into the passenger space.
  • Cold working steel from a fully annealed state is a known route to increase yield strength and tensile strength. It can be applied uniformly across a sheet during processing through cold rolling increasing the yield strength and tensile strength. However, this approach results in a decrease in total elongation and often to levels much below 20%. As elongation decreases, the cold forming ability also decreases, reducing the ability to produce parts with complex geometries resulting in a decrease in the usefulness of the AHSS. Higher ductility with a minimum of 30% total elongation is generally needed to form complex geometries through cold stamping processes. While processes such as roll forming can be used to create parts from lower elongation material, the geometric complexity of parts from these processes is limited. Cold rolling also can introduce anisotropy into the material which will farther reduce its ability to be cold formed into parts.
    • SUMMARY
  • A method to increase yield strength in a metallic alloy comprising:
      • a. supplying a metal alloy comprising at least 70 atomic % iron and at least four or more elements selected from Si, Mn, Cr, Ni, Cu or C, melting said alloy, cooling at a rate of 10−4 K/s to 103 K/s and solidifying to a thickness of >5.0 mm to 500 mm;
      • b. processing said alloy into a first sheet form with thickness from 0.5 to 5.0 mm with the first sheet having a total elongation of X1 (%), an ultimate tensile strength of Y1 (MPa), and a yield strength of Z1 (MPa);
      • c. permanently deforming said alloy in the temperature range of 150° C. to 400° C. into a second sheet form exhibiting one of the following tensile property combinations A or B:
      • A. (1) total elongation X2=X1±7.5%;
        • (2) ultimate tensile strength Y2=Y1±100 MPa; and
        • (3) yield strength Z2≧Z1+100 MPa.
      • B. (1) ultimate tensile strength Y3=Y1±100 MPa; and
        • (2) yield strength Z3≧Z1+200 MPa.
  • Optionally, the second sheet formed in step (c) indicating either tensile property combination A or B above may then be exposed to a permanent deformation at a temperature of ≦150° C.
  • Accordingly, the invention herein also relates to a method to increase yield strength in a metallic alloy comprising:
      • a. supplying a metal alloy comprising at least 70 atomic % iron and at least four or more elements selected from Si, Mn, Cr, Ni, Cu or C, melting said alloy, cooling at a rate of 10−4K/sec to 103 K/sec and solidifying to a thickness of >5.0 mm to 500 mm;
      • b. processing said alloy into a first sheet form with thickness from 5.0 to 0.5 mm;
      • c. permanently deforming said alloy in the temperature range of 150° C. to 400° C. into a second sheet form;
      • d. permanently deforming said alloy in a temperature of <150° C. into a second sheet form exhibiting the following tensile property combinations:
        • (1) total elongation=10.0 to 40.0%;
        • (2) ultimate tensile strength=1150 to 2000 MPa;
        • (3) yield strength=550 to 1600 MPa.
  • The metallic alloys produced herein provide particular utility in vehicles, railway cars, railway tank cars/wagons, drill collars, drill pipe, pipe casing, tool joints, wellheads, compressed gas storage tanks or liquefied natural gas canisters. More specifically, the alloys find utility in vehicular bodies in white, vehicular frames, chassis, or panels.
    • BRIEF DESCRIPTION OF THE DRAWINGS
  • The detailed description below may be better understood with reference to the accompanying FIG.s which are provided for illustrative purposes and are not to be considered as limiting any aspect of this invention.
  • FIG. 1 World Auto Steel “Banana Plot” with targeted properties for 3rd Generation AHSS.
  • FIG. 2 Summary of Method 1 to produce high yield strength in alloys herein.
  • FIG. 3 Summary of Method 2 to produce high yield strength and targeted combinations of properties in the alloys herein.
  • FIG. 4 Ultimate tensile strength in alloys herein before and after cold rolling.
  • FIG. 5 Tensile elongation in alloys herein before and after cold rolling.
  • FIG. 6 Yield strength in alloys herein before and after cold rolling.
  • FIG. 7 Magnetic phase volume percent in alloys herein before and after cold rolling.
  • FIG. 8 Tensile stress-strain curves for Alloy 2 after cold rolling with various reductions.
  • FIG. 9 Back-scattered SEM micrograph of the microstructure in the hot band from Alloy 2: a) low magnification image; b) high magnification image.
  • FIG. 10 Bright-field TEM micrograph of the microstructure in the hot band from Alloy 2: a) low magnification image; b) high magnification image.
  • FIG. 11 TEM micrograph showing nanoscale precipitates in the hot band from Alloy 2.
  • FIG. 12 Back-scattered SEM micrograph of the microstructure in the cold rolled sheet from Alloy 2: a) low magnification image; b) high magnification image.
  • FIG. 13 TEM micrograph of the microstructure in the cold rolled sheet from Alloy 2: a) low magnification image; b) high magnification image.
  • FIG. 14 TEM micrograph showing nanoscale precipitates found in Alloy 2 sheet after cold deformation.
  • FIG. 15 Engineering tensile stress—strain curves for Alloy 2 after rolling with 20% reduction at different temperatures.
  • FIG. 16 Change in magnetic phases volume percent (Fe %) during tensile testing in Alloy 2.
  • FIG. 17 Engineering stress—strain curves for Alloy 7 after rolling with 20% reduction at different temperatures.
  • FIG. 18 Engineering stress—strain curves for Alloy 18 after rolling with 20% reduction at different temperatures.
  • FIG. 19 Engineering stress—strain curves for Alloy 34 after rolling with 20% reduction at different temperatures.
  • FIG. 20 Engineering stress—strain curves for Alloy 37 after rolling with 20% reduction at different temperatures.
  • FIG. 21 Representative engineering stress—strain curves for Alloy 2 that was rolled at 200° C. to various rolling reductions.
  • FIG. 22 The yield and ultimate tensile strength of Alloy 2 as a function of rolling reduction at 200° C.
  • FIG. 23 The yield strength and total elongation of Alloy 2 as a function of rolling reduction at 200° C.
  • FIG. 24 The effect of rolling at 200° C. on the deformation induced phase transformation in Alloy 2 as a function of rolling reduction.
  • FIG. 25 Backscattered SEM micrograph of microstructure in hot band from Alloy 2: a) low magnification image; b) high magnification image.
  • FIG. 26 Backscattered SEM micrographs of microstructure in Alloy 2 after rolling at 200° C. to 30% reduction: a) low magnification image; b) high magnification image.
  • FIG. 27 Backscattered SEM micrographs of microstructure in Alloy 2 after rolling at 200° C. to 70% reduction: a) low magnification image; b) high magnification image.
  • FIG. 28 Bright-field TEM micrographs of the microstructure in Alloy 2 after rolling at 200° C. with 10% reduction: a) low magnification image and b) high magnification image.
  • FIG. 29 Bright-field TEM micrographs of the microstructure in Alloy 2 after rolling at 200° C. with 30% reduction: a) low magnification image and b) high magnification image.
  • FIG. 30 Bright-field TEM micrographs of the microstructure in Alloy 2 after rolling at 200° C. with 70% reduction: a) low magnification image and b) high magnification image.
  • FIG. 31 Engineering stress-strain curves for Alloy 2 processed by combination of rolling methods.
  • FIG. 32 Engineering stress-strain curves for Alloy 7 processed by combination of rolling methods.
  • FIG. 33 Engineering stress-strain curves for Alloy 18 processed by combination of rolling methods.
  • FIG. 34 Engineering stress-strain curves for Alloy 34 processed by combination of rolling methods.
  • FIG. 35 Comparison of engineering stress-strain curves for Alloy 2 sheet processed by different methods and their combination.
  • FIG. 36 Tensile elongation and magnetic phases volume percent in a tensile sample gauge after testing of Alloy 2 at different temperatures.
  • FIG. 37 Magnetic phases volume percent as a function of rolling reduction at ambient temperature and at 200° C.
  • FIG. 38 Examples of engineering stress-strain curves for the annealed sheet produced by both cold rolling and rolling at 200° C.
  • FIG. 39 Rolling reduction limit vs rolling temperature for Alloy 2.
    •  DETAILED DESCRIPTION
  • FIG. 2 represents a summary of preferred Method 1 to develop high yield strengths from a low yield strength material by a route which results in either of two conditions as provided in conditions 3a or 3b. In Step 1 of Method 1, the starting condition is to supply a metal alloy. This metal alloy will comprise at least 70 atomic % iron and at least four or more elements selected from Si, Mn, Cr, Ni, Cu or C. The alloy chemistry is melted and preferably cooled at a rate of 10−4 K/s to 103 K/s and solidified to a thickness of >5.0 mm to 500 mm. The casting process can be done in a wide variety of processes including ingot casting, bloom casting, continuous casting, thin slab casting, thick slab casting, thin strip casting, belt casting etc. Preferred methods would be continuous casting in sheet form by thin slab casting, thick slab casting, and thin strip casting. Preferred alloys would exhibit a fraction of austenite (y-Fe) at least 10 volume percent up to 100 volume percent and all increments in between in the temperature range from 150 to 400° C.
  • In Step 2 of Method 1, the alloy is preferably processed into sheet form with thickness from 0.5 to 5.0 mm. This step 2 can involve hot rolling or hot rolling and cold rolling. If hot rolling the preferred temperature range would be at a temperature of 700° C. and below the Tm of said alloy. If cold rolling is employed, such is understood to be at ambient temperature. Note that after hot rolling or hot rolling and cold rolling, the sheet can be additionally heat treated, preferably in the range from at a temperature of 650° C. to a temperature below the melting point (Tm) of said alloy.
  • The steps to produce sheet from the cast product can therefore vary depending on specific manufacturing routes and specific targeted goals. As an example, consider thick slab casting as one process route to get to sheet of this targeted thickness. The alloy would preferably be cast going through a water cooled mold typically in a thickness range of 150 to 300 mm in thickness. The cast ingot after cooling would then be preferably prepared for hot rolling which may involve some surface treatment to remove surface defects including oxides. The ingot would then go through a roughing mill hot roller which may involve several passes resulting in a transfer bar slab typically from 15 to 100 mm in thickness. This transfer bar would then go through successive/tandem hot rolling finishing stands to produce hot band coils which are typically from 1.5 to 5.0 mm in thickness. If additional gauge reduction is needed, cold rolling can be done at various reductions per pass, variable number of passes and in different mills including tandem mills, Z-mills, and reversing mills. Typically cold rolled thickness would be 0.5 to 2.5 mm thick. Preferably, the cold rolled material is annealed to restore the ductility lost from the cold rolling process either partially or completely at a temperature range from 650° C. to a temperature below the melting point (Tm) of said alloy.
  • Another example would be to preferably process the cast material through a thin slab casting process. In this case, after casting typically forms 35 to 150 mm in thickness by going through a water cooled mold, the newly formed slab goes directly to hot rolling without cooling down with auxiliary tunnel furnace or induction heating applied to bring the slab directly up to targeted temperature. The slab is then hot rolled directly in multi-stand finishing mills which are preferably from 1 to 10 in number. After hot rolling, the strip is rolled into hot band coils with typical thickness from 1 to 5 mm in thickness. If further processing is needed, cold rolling can be applied in a similar manner as above. Note that bloom casting would be similar to the examples above but higher thickness might be cast typically from 200 to 500 mm thick and initial breaker steps would be needed to reduce initial cast thickness to allow it to go through a hot rolling roughing mill.
  • Notwithstanding the specific process in going from the cast material in Step 1 to Step 2, once the sheet is formed in the preferred range from 0.5 mm to 5.0 mm, the sheet will then exhibit a total elongation of X1 (%), an ultimate tensile strength of Y1 (MPa), and a yield strength of Z1 (MPa). Preferred properties for this alloy would be ultimate tensile strength values from 900 to 2050 MPa, tensile elongation from 10 to 70%, and yield strength is in a range from 200 to 750 MPa.
  • In Step 3 of Method 1, the alloy is permanently (i.e. plastically) deformed in the temperature range from 150° C. to 400° C. Such permanent deformation may be provided by rolling and causing a reduction in thickness. This can be done for example during the final stages of the development of a steel coil. Rather than doing the traditional cold rolling for final gauge reduction with the sheet starting at ambient temperature, elevated temperature rolling is now preferably done in the targeted temperature range of 150 to 400° C. One method would be to heat the sheet to the targeted temperature range prior to going through the cold rolling mill. The sheet could be heated by a variety of methods including going through a tunnel mill, a radiative heater, a resistance heater, or an induction heater. Another method would be to heat directly the reduction rollers. A third example for illustration would be to low temperature batch anneal the sheet and then send this through the cold rolling mill(s) at the targeted temperature range. Alternatively, the sheet may be deformed at the elevated temperature range into parts using a variety of processes providing permanent deformation during the making of parts by various methods including roll forming, metal stamping, metal drawing, hydroforming etc.
  • Notwithstanding the specific process to permanently deform the alloy in the temperature range of 150 to 400° C., two distinct conditions can be formed which are shown in Condition 3a and Condition 3b in FIG. 2. In Condition 3a, comparing said alloy in Step 2 and after Step 3, the total elongation and ultimate tensile strength are relatively unaffected but the yield strength is increased. Specifically, the total elongation X2 is equal to X1±7.5%, the tensile strength Y2 is equal to Y1±100 MPa, and the yield strength Z2 is ≧Z1+100 MPa. Preferred properties for this alloy in Condition 3a would be ultimate tensile strength values (Y2) from 800 to 2150 MPa, tensile elongation (X2) from 2.5% to 77.5%, and yield strength (Z2)≧300 MPa. More preferably, yield strength may fall in the range of 300 to 1000 MPa.
  • In Condition 3b, comparing said alloy in Step 2 and after Step 3, the ultimate tensile strength is relatively unaffected but the yield strength is increased. Specifically, the ultimate tensile strength Y3 is equal to Y1±300 MPa and yield strength Z3 is ≧Z1±F_200 MPa. Preferred properties for this alloy in Condition 3b would be ultimate tensile strength values (Y3) from 800 to 2150 MPa and yield strength (Z3)≧400 MPa. More preferably, yield strength may fall in the range of 400 to 1200 MPa. In addition, unlike Condition 3a, the total elongation drop is greater than 7.5%, that is, in Step B, the total elongation (X3) is defined as follows: X3<X1−7.5%.
  • As will be shown by various case examples, with normal deformation, a metallic material will strain harden/work harden. This is shown for example by the strain hardening exponent (n) in the relationship σ=Kεn between stress (σ) and strain (ε). The ramifications of this is that as a material is permanently deformed the basic material properties change. Comparing the initial condition to the final condition will show the typical and expected behavior where yield strength and tensile strength is increased with commensurate reductions in total ductility. Specific case examples are provided to illustrate this effect and then contrast this with the new material behavior noted in this disclosure.
  • FIG. 3 identifies a summary of Method 2 of the present disclosure. The first 3 steps in Method 2 are identical to Method 1 with Step 4 being an additional step for Method 2. As shown Step 4 can be applied to the alloys herein in either Condition 3a or Condition 3b.
  • As presented previously, in the description of FIG. 2, various combinations of properties (i.e. total elongation, ultimate tensile strength, and yield strength) are provided for each Condition 3a or 3b. As will be further illustrated in the detailed description and subsequent case examples, that alloys in Condition 3a or 3b may be further characterized by their particular structure. This then allows further tailoring of the final properties by the use of a further optional step of permanently deforming the alloys at temperatures from ambient to ≦150° C., or more preferably at a range of temperatures of 0° C. to 150° C. This can be done for example by adding another step during the production of steel coils as illustrated in FIG. 3. In this case Step 4 can be a skin pass (i.e. a small reduction rolling pass sometimes used also for improvements in surface quality or leveling) from 0.5 to 2.0% reduction or at greater reductions from >2% to 50% to develop specific combinations of properties. Alternate approaches can be done for example in making parts out of sheet which has been processed by Method 1. In optional Step 4 of Method 2, the sheet could be subsequently made into parts using a variety of deformation processes including roll forming, metal stamping, metal drawing, hydroforming etc. Notwithstanding the exact process to activate Step 4 in Method 2, final properties can be developed with the said alloy which are contemplated to exhibit properties with tensile elongation from 10 to 40%, ultimate tensile strength from 1150 to 2000 MPa, and yield strength from 550 to 1600 MPa).
    • Alloys
  • The structures and mechanisms in this application leading to the new process route for developing high yield strength are tied to the following chemistries of alloys provided in Table 1.
  • TABLE 1
    Chemical Composition of Alloys (Atomic %)
    Alloy Fe Cr Ni Mn Si Cu C
    Alloy 1 75.75 2.63 1.19 13.86 5.13 0.65 0.79
    Alloy 2 74.75 2.63 1.19 14.86 5.13 0.65 0.79
    Alloy 3 77.31 2.63 8.49 5.00 5.13 0.65 0.79
    Alloy 4 77.14 2.63 6.49 7.17 5.13 0.65 0.79
    Alloy 5 76.24 2.63 4.49 10.07 5.13 0.65 0.79
    Alloy 6 75.34 2.63 2.49 12.97 5.13 0.65 0.79
    Alloy 7 78.92 2.63 6.49 5.39 5.13 0.65 0.79
    Alloy 8 77.34 2.63 4.49 8.97 5.13 0.65 0.79
    Alloy 9 75.77 2.63 2.49 12.54 5.13 0.65 0.79
    Alloy 10 75.90 2.63 3.74 11.16 5.13 0.65 0.79
    Alloy 11 77.73 2.63 3.74 9.33 5.13 0.65 0.79
    Alloy 12 79.57 2.63 3.74 7.49 5.13 0.65 0.79
    Alloy 13 75.97 2.63 3.74 10.09 5.13 1.65 0.79
    Alloy 14 77.80 2.63 3.74 8.26 5.13 1.65 0.79
    Alloy 15 79.64 2.63 3.74 6.42 5.13 1.65 0.79
    Alloy 16 76.88 2.63 3.74 9.18 5.13 1.65 0.79
    Alloy 17 76.83 2.63 3.74 9.85 5.13 1.03 0.79
    Alloy 18 76.57 2.63 3.06 10.17 5.13 1.65 0.79
    Alloy 19 76.52 2.63 3.06 10.84 5.13 1.03 0.79
    Alloy 20 78.02 1.13 3.06 10.84 5.13 1.03 0.79
    Alloy 21 80.02 1.13 3.06 10.84 3.13 1.03 0.79
    Alloy 22 76.70 2.63 3.40 10.01 5.13 1.34 0.79
    Alloy 23 76.20 3.13 3.40 10.01 5.13 1.34 0.79
    Alloy 24 75.70 3.63 3.40 10.01 5.13 1.34 0.79
    Alloy 25 77.70 2.63 3.40 10.01 4.13 1.34 0.79
    Alloy 26 75.70 2.63 3.40 10.01 6.13 1.34 0.79
    Alloy 27 77.20 2.63 3.40 10.01 4.13 1.34 1.29
    Alloy 28 75.20 2.63 3.40 10.01 6.13 1.34 1.29
    Alloy 29 76.98 2.88 3.40 10.01 4.63 1.34 0.76
    Alloy 30 77.23 2.88 3.15 10.01 4.63 1.34 0.76
    Alloy 31 77.48 2.88 2.90 10.01 4.63 1.34 0.76
    Alloy 32 77.73 2.88 2.65 10.01 4.63 1.34 0.76
    Alloy 33 77.98 2.88 2.40 10.01 4.63 1.34 0.76
    Alloy 34 74.59 2.61 0.00 15.17 3.59 1.86 2.18
    Alloy 35 82.22 3.69 9.94 0.00 2.26 0.37 1.52
    Alloy 36 76.17 8.64 0.90 11.77 0.00 1.68 0.84
    Alloy 37 82.77 4.41 6.66 3.19 1.14 1.16 0.67
    Alloy 38 76.55 0.78 0.72 14.43 3.42 0.42 3.68
    Alloy 39 81.44 0.00 4.42 10.33 2.87 0.00 0.94
    Alloy 40 81.00 1.22 0.89 13.45 2.66 0.78 0.00
    Alloy 41 81.68 2.24 3.25 9.87 0.00 1.55 1.41
  • As can be seen from Table 1, the alloys herein are iron based metal alloys, having greater than 70 at. % Fe. In addition, it can be appreciated that the alloys herein are such that they comprise Fe and at least four or more, or five or more, or six elements selected from Si, Mn, Cr, Ni, Cu or C. Accordingly, with respect to the presence of four or more, or five or more elements selected from Si, Mn, Cr, Ni, Cu or C, such elements are present at the following indicated atomic percents: Si (0 to 6.13 at. %); Mn (0 to 15.17 at. %); Cr (0 to 8.64 at. %); Ni (0 to 9.94 at. %); Cu (0 to 1.86 at. %); and C (0 to 3.68 at. %). Most preferably, the alloys herein are such that they comprise, consist essentially of, or consist of Fe at a level of 70 at. % or greater along with Si, Mn, Cr, Ni, Cu and C, wherein the level of impurities of all other elements is in the range from 0 to 5000 ppm.
    • Laboratory Slab Casting
  • Alloys were weighed out into 3,400 gram charges using commercially available ferroadditive powders and a base steel feedstock with known chemistry according to the atomic ratios in Table 1. As alluded to above, impurities can be present at various levels depending on the feedstock used. Impurity elements would commonly include the following elements; Al, Co, Mo, N, Nb, P, Ti, V, W, and S which if present would be in the range from 0 to 5000 ppm (parts per million) with preferred ranges of 0 to 500 ppm.
  • Charges were loaded into a zirconia coated silica crucible which was placed into an Indutherm VTC800V vacuum tilt casting machine. The machine then evacuated the casting and melting chambers and flushed with argon to atmospheric pressure twice prior to casting to prevent oxidation of the melt. The melt was heated with a 14 kHz RF induction coil until fully molten, approximately from 5 to 7 minutes depending on the alloy composition and charge mass. After the last solids were observed to melt it was allowed to heat for an additional 30 to 45 seconds to provide superheat and ensure melt homogeneity. The casting machine then evacuated the chamber and tilted the crucible and poured the melt into a 50 mm thick, 75 to 80 mm wide, and 125 mm deep channel in a water cooled copper die and would represent Step 1 in FIGS. 2 and 3. The process can be adapted to a preferred as-cast thickness at a range from >5.0 to 500 mm. The melt was allowed to cool under vacuum for 200 seconds before the chamber was filled with argon to atmospheric pressure.
    • Laboratory Hot Rolling
  • The alloys herein were preferably processed into a laboratory sheet. Laboratory alloy processing is developed to simulate the hot band production from slabs produced by continuous casting and would represent Step 2 in FIGS. 2 and 3. Industrial hot rolling is performed by heating a slab in a tunnel furnace to a target temperature, then passing it through a either a reversing mill or a multi-stand mill or a combination of both to reach the target gauge in a preferred temperature range from 700° C. up to the melting point (Tm) of the alloy. During rolling on either mill type the temperature of the slab is steadily decreasing due to heat loss to the air and to the work rolls so the final hot band is at a much reduced temperature. This is simulated in the laboratory by heating in a tunnel furnace to between 1100° C. and 1250° C., then hot rolling. The laboratory mill is slower than industrial mills causing greater loss of heat during each hot rolling pass so the slab is reheated for 4 minutes between passes to reduce the drop in temperature, the final temperature at target gauge when exiting the laboratory mill commonly is in the range from 1000° C. to 800° C., depending on furnace temperature and final thickness.
  • Prior to hot rolling, laboratory slabs were preheated in a Lucifer EHS3GT-B18 furnace to heat. The furnace set point varies between 1100° C. to 1250° C., depending on alloy melting point and point in the hot rolling process, with the initial temperatures set higher to facilitate higher reductions, and later temperatures set lower to minimize surface oxidation on the hot band. The slabs were allowed to soak for 40 minutes prior to hot rolling to ensure they reach the target temperature and then pushed out of the tunnel furnace into a Fenn Model 061 2 high rolling mill. The 50 mm casts are hot rolled for 5 to 10 passes though the mill before being allowed to air cool. Final thickness ranges after hot rolling are preferably from 1.8 mm to 4.0 mm with variable reduction per pass ranging from 20% to 50%.
  • After the hot rolling, the slab thickness has been reduced to a final thickness of the hot band from 1.8 to 2.3 mm. Processing conditions can be adjusted by changing the amount of hot rolling and/or adding cold rolling steps to produce the preferred thickness range from 0.5 to 5.0 mm. Tensile specimens were cut from laboratory hot band using wire EDM. Tensile properties were measured on an Instron mechanical testing frame (Model 3369), utilizing Instron's Bluehill control and analysis software. Tensile properties of the alloys in the hot rolled condition, are listed in Table 2 which have been processed to a thickness from 1.8 to 2.3 mm.
  • The ultimate tensile strength values may vary from 913 to 2000 MPa with tensile elongation from 13.8 to 68.5%. The yield strength is in a range from 250 to 711 MPa. Mechanical properties of the hot band from steel alloys herein depend on alloy chemistry, processing conditions, and material mechanistic response to the processing conditions.
  • TABLE 2
    Tensile Properties of Alloys in Hot Rolled Condition
    Strain Hardening
    Exponent (n)
    Ultimate in a Strain Range
    Tensile Tensile Yield Yield
    Elongation Strength Strength Point 5% to 25% to
    Alloy (%) (MPa) (MPa) to 5% 15% Max Stress
    Alloy 1 51.4 1248 294 0.29 0.38 0.67
    49.2 1253 310 0.31 0.42 0.64
    31.2 1093 396 0.28 0.39 0.71
    Alloy 2 57.6 1175 311 0.29 0.38 0.83
    58.6 1209 294 0.31 0.40 0.64
    56.6 1167 302 0.29 0.38 0.45
    Alloy 3 55.4 1163 330 0.08 0.52 0.82
    59.5 1154 373 0.06 0.47 0.73
    58.1 1165 347 0.07 0.44 0.84
    Alloy 4 59.8 1220 342 0.12 0.40 0.78
    51.6 1241 338 0.12 0.41 0.78
    55.5 1245 375 0.10 0.38 0.80
    54.6 1324 377 0.11 0.41 0.77
    Alloy 5 54.3 1248 325 0.18 0.40 0.80
    53.1 1218 313 0.18 0.42 0.74
    50.6 1258 304 0.21 0.42 0.79
    54.1 1242 331 0.18 0.39 0.75
    Alloy 6 58.3 1212 330 0.21 0.38 0.71
    53.7 1212 283 0.26 0.42 0.72
    58.7 1193 315 0.23 0.40 0.72
    Alloy 7 28.1 1508 333 0.28 0.89 —
    28.5 1516 331 0.26 0.93 —
    26.0 1520 317 0.26 0.90 —
    Alloy 8 41.2 1343 330 0.17 0.44 0.78
    32.8 1281 328 0.17 0.44 0.94
    45.7 1387 336 0.16 0.42 0.71
    41.4 1375 328 0.17 0.42 0.84
    Alloy 9 48.1 1248 300 0.25 0.40 0.75
    50.5 1293 304 0.27 0.41 0.70
    52.0 1280 303 0.25 0.40 0.72
    Alloy 10 58.5 1229 379 0.18 0.31 0.73
    57.8 1223 384 0.18 0.32 0.72
    59.0 1220 389 0.19 0.31 0.71
    Alloy 11 45.3 1411 360 0.15 0.44 0.74
    40.2 1460 359 0.17 0.45 0.74
    41.3 1429 325 0.20 0.53 0.74
    47.1 1448 347 0.17 0.48 0.70
    Alloy 12 31.3 1624 250 — 1.34 —
    31.7 1581 304 0.19 1.24 —
    28.7 1610 319 0.16 1.23 —
    Alloy 13 57.1 1101 358 0.16 0.34 0.79
    66.1 1120 362 0.14 0.34 0.82
    68.5 1114 362 0.15 0.33 0.80
    60.1 1120 350 0.14 0.34 0.83
    Alloy 14 45.1 1371 354 0.11 0.59 0.69
    40.6 1403 363 0.11 0.62 0.66
    42.3 1403 364 0.11 0.55 0.69
    46.9 1379 341 0.12 0.63 0.65
    Alloy 15 26.2 1579 295 0.47 0.89 —
    25.2 1593 264 — 0.98 —
    24.6 1588 302 0.45 0.84 —
    Alloy 16 54.8 1239 379 0.13 0.34 0.76
    58.5 1207 341 0.15 0.42 0.80
    55.8 1207 359 0.13 0.39 0.82
    Alloy 17 51.3 1270 354 0.16 0.36 0.80
    50.1 1328 384 0.15 0.35 0.81
    Alloy 18 58.8 1224 384 0.14 0.33 0.78
    56.1 1245 390 0.14 0.32 0.79
    50.7 1190 365 0.14 0.33 0.82
    Alloy 19 47.4 1263 348 0.17 0.34 0.79
    50.7 1260 362 0.17 0.34 0.79
    51.8 1277 363 0.17 0.34 0.80
    Alloy 20 40.1 1337 376 0.15 0.36 0.85
    43.9 1343 375 0.14 0.35 0.83
    44.7 1328 394 0.15 0.36 0.88
    Alloy 21 45.2 1277 327 0.18 0.45 0.76
    46.1 1318 340 0.17 0.44 0.76
    54.2 1310 325 0.18 0.46 0.71
    Alloy 22 49.6 1272 369 0.15 0.36 0.83
    54.9 1275 354 0.14 0.36 0.77
    54.8 1271 319 0.17 0.42 0.73
    52.4 1297 340 0.16 0.38 0.79
    Alloy 23 53.5 1246 344 0.16 0.4 0.78
    55.9 1226 359 0.15 0.34 0.76
    51.2 1232 346 0.16 0.36 0.77
    52.7 1228 375 0.14 0.34 0.78
    Alloy 24 57.0 1209 356 0.15 0.35 0.77
    54.6 1202 348 0.15 0.36 0.83
    55.1 1207 363 0.15 0.34 0.80
    56.9 1225 338 0.16 0.38 0.78
    Alloy 25 53.4 1227 357 0.15 0.37 0.78
    56.5 1249 325 0.16 0.39 0.77
    54.5 1214 345 0.14 0.37 0.79
    49.5 1220 343 0.15 0.38 0.83
    Alloy 26 49.0 1319 340 0.16 0.37 0.79
    48.4 1320 344 0.17 0.35 0.79
    50.5 1304 331 0.19 0.38 0.79
    51.1 1296 346 0.16 0.36 0.77
    Alloy 27 56.5 967 404 0.11 0.31 0.66
    54.5 956 421 0.11 0.31 0.66
    67.6 979 417 0.11 0.31 0.66
    52.0 942 390 0.12 0.33 0.66
    Alloy 28 50.4 1121 442 0.11 0.30 0.77
    49.8 1088 407 0.13 0.33 0.78
    51.8 1116 423 0.13 0.32 0.77
    Alloy 29 56.0 1229 422 0.14 0.30 0.70
    56.3 1247 409 0.15 0.30 0.74
    54.6 1226 405 0.15 0.31 0.71
    50.0 1196 421 0.18 0.32 0.73
    56.3 1199 412 0.15 0.31 0.69
    53.3 1205 402 0.16 0.33 0.67
    Alloy 30 52.1 1271 421 0.16 0.30 0.74
    51.4 1284 416 0.14 0.32 0.74
    50.6 1269 407 0.15 0.33 0.72
    53.9 1248 418 0.14 0.32 0.68
    49.9 1237 399 0.16 0.34 0.69
    54.8 1241 407 0.17 0.31 0.71
    Alloy 31 48.6 1326 379 0.17 0.34 0.74
    51.3 1323 390 0.16 0.33 0.71
    51.6 1293 372 0.17 0.35 0.72
    51.4 1314 374 0.17 0.34 0.72
    Alloy 32 49.5 1347 383 0.17 0.37 0.65
    47.0 1367 388 0.17 0.36 0.68
    47.9 1341 381 0.17 0.36 0.75
    47.8 1391 431 0.15 0.33 0.67
    Alloy 33 44.8 1373 372 0.18 0.38 0.68
    42.3 1392 381 0.17 0.40 0.72
    40.7 1388 381 0.17 0.40 0.69
    Alloy 34 65.9 963 515 0.09 0.27 0.47
    58.7 954 485 0.10 0.28 0.47
    62.1 970 545 0.08 0.26 0.46
    Alloy 35 19.6 2000 533 0.29 0.31 —
    22.3 1976 511 0.20 0.30 —
    19.8 1995 526 0.31 0.29 —
    Alloy 36 60.1 1091 439 0.11 0.31 0.60
    61.0 1114 469 0.10 0.28 0.61
    59.4 1137 481 0.10 0.29 0.62
    Alloy 37 13.8 1572 649 0.13 — —
    14.1 1619 711 0.18 — —
    14.6 1610 692 0.19 — —
    Alloy 38 58.9 1105 531 0.11 0.30 0.52
    61.4 1108 524 0.10 0.30 0.52
    58.6 1106 511 0.10 0.30 0.52
    Alloy 39 51.0 1317 354 0.16 0.39 0.71
    50.5 1334 370 0.15 0.38 0.71
    50.5 1325 368 0.14 0.38 0.69
    Alloy 40 47.9 1374 330 0.22 0.38 0.74
    48.8 1336 317 0.24 0.39 0.64
    41.5 1362 321 0.23 0.39 0.77
    Alloy 41 51.1 963 472 0.08 0.29 0.58
    48.4 913 463 0.08 0.29 0.55
    • CASE EXAMPLES Comparative Case Example #1 Conventional Response to Rolling at Ambient Temperature
  • The hot band from alloys herein listed in Table 1 was, for comparison purposes, cold rolled to final target gauge thickness of 1.2 mm through multiple cold rolling passes. Tensile specimens were cut from each cold rolled sheet using wire EDM. Tensile properties were measured on an Instron mechanical testing frame (Model 3369), utilizing Instron's Bluehill control and analysis software. All tests were run at ambient temperature in displacement control.
  • Tensile properties of alloys herein after cold rolling are listed in Table 3. As it can be seen, the yield strength is significantly increased over the range in a hot band with maximum at 711 MPa (Table 2). After cold rolling yield strength varies from 1037 to 2000 MPa. The ultimate tensile strength values after cold rolling are in a range from 1431 to 2222 MPa. However, a drop in tensile elongation is recorded for each alloy herein after cold rolling with variation from 4.2 to 31.1%. The general trends in effect of cold rolling on tensile properties of alloys herein are illustrated in FIG. 4 to FIG. 6.
  • TABLE 3
    Tensile Properties of Alloys at Final Gauge after Cold Rolling
    Cold Ultimate
    Rolling Tensile Tensile
    Reduction Elongation Strength Yield Strength
    Alloy (%) (%) (MPa) (MPa)
    Alloy 1 38.0 20.5 1712 1114
    20.4 1712 1131
    Alloy 2 29.4 21.8 1603 1135
    23.2 1612 1111
    25.7 1589 1120
    Alloy 10 35.1 20.1 1715 1038
    20.5 1716 1280
    20.5 1729 1173
    Alloy 11 32.7 13.9 1893 1320
    15 1906 1467
    15.6 1875 1536
    Alloy 12 33.8 5.5 2125 1913
    5.9 2116 1720
    4.2 2114 1675
    Alloy 13 36.5 22.8 1500 1182
    24.0 1523 1204
    23.9 1518 1098
    Alloy 14 34.5 18.6 1790 1561
    20.2 1793 1436
    17.9 1726 1491
    Alloy 15 37.3 5.0 2051 1784
    6.2 2073 2000
    6.3 2057 1957
    Alloy 16 36.9 19.9 1700 1413
    19.7 1689 1436
    21.1 1704 1302
    Alloy 17 36.0 20.1 1765 1379
    20.2 1759 1306
    17.2 1764 1374
    Alloy 18 37.3 20.6 1708 1388
    20.0 1721 1326
    18.9 1709 1369
    Alloy 19 38.0 18.9 1810 1213
    19.3 1807 1324
    19.2 1806 1260
    Alloy 20 38.3 15.1 1864 1404
    16.2 1884 1461
    17.1 1879 1512
    Alloy 21 34.1 18.6 1780 1374
    18.0 1785 1414
    18.6 1786 1423
    Alloy 22 38.0 17.3 1759 1356
    21.3 1736 1196
    18.8 1757 1304
    Alloy 23 37.4 19.3 1718 1240
    20.4 1728 1283
    19.0 1727 1271
    Alloy 24 37.5 22.0 1709 1136
    12.6 1695 1256
    14.8 1706 1258
    Alloy 25 33.5 19.8 1715 1326
    20.2 1704 1320
    21.0 1700 1316
    Alloy 29 38.8 18.1 1718 1483
    18.6 1712 1454
    19.4 1720 1407
    Alloy 30 39.9 17.7 1770 1335
    17.7 1764 1430
    17.9 1765 1515
    Alloy 31 40.5 17.5 1834 1524
    16.9 1831 1707
    16.0 1837 1578
    Alloy 32 41.1 15.7 1890 1442
    14.8 1897 1563
    15.4 1886 1676
    Alloy 33 38.2 15.4 1891 1533
    16.3 1889 1604
    15.8 1895 1419
    Alloy 34 39.0 10.9 1519 1249
    9.4 1515 1037
    10.8 1519 1345
    Alloy 35 19.6 16.2 2222 1693
    16.4 2216 1735
    16.2 2217 1657
    Alloy 36 36.7 16.4 1641 1116
    20.6 1604 1187
    19.1 1623 1295
    Alloy 37 36.3 7.1 1949 1617
    6.6 1977 1824
    6.5 1975 1834
    Alloy 38 43.0 7.0 1727 1539
    9.7 1721 1373
    10.0 1717 1490
    Alloy 39 36.5 16.0 1869 1289
    19.0 1840 1471
    19.0 1837 1245
    Alloy 40 37.4 15.6 1917 1238
    17.2 1913 1361
    17.7 1917 1192
    Alloy 41 39.2 28.6 1452 1121
    31.1 1445 1101
    31.1 1431 1231
  • The relative magnetic phases content was measured by Feritscope in both a hot band and after cold rolling for each alloy herein that is listed in Table 4 and illustrated in FIG. 7 for selected alloys. The magnetic phases volume percent of 0.1 to 56.4 Fe % in a hot band increases to the range from 1.6 to 84.9 Fe % after cold rolling confirming a phase transformation during deformation.
  • TABLE 4
    Magnetic Phases Volume Percent (Fe %) in Alloys after Cold Rolling
    Hot Band Cold Rolled Sheet
    Alloy (Fe %) (Fe %)
    Alloy 1 1.7 14.7
    Alloy 2 1.3 18.0
    Alloy 3 3.2 43.5
    Alloy 4 0.3 55.8
    Alloy 5 0.5 53.0
    Alloy 6 0.4 45.0
    Alloy 7 10.4 67.7
    Alloy 8 0.9 57.8
    Alloy 9 1.4 44.8
    Alloy 10 2.7 40.3
    Alloy 11 0.8 57.1
    Alloy 12 1.5 70.6
    Alloy 13 0.1 25.6
    Alloy 14 0.4 52.2
    Alloy 15 1.6 65.6
    Alloy 16 0.2 43.2
    Alloy 17 0.6 56.9
    Alloy 18 0.3 45.3
    Alloy 19 0.4 55.9
    Alloy 20 0.3 60.9
    Alloy 21 0.5 56.3
    Alloy 22 0.3 43.9
    Alloy 23 0.3 53.5
    Alloy 24 0.2 36.8
    Alloy 25 0.4 42.6
    Alloy 26 0.5 48.5
    Alloy 27 0.1 12.6
    Alloy 28 0.4 20.6
    Alloy 29 4.2 42.8
    Alloy 30 5.5 44.6
    Alloy 31 6.5 49.3
    Alloy 32 5.7 51.5
    Alloy 33 7.3 56.3
    Alloy 34 0.3 1.6
    Alloy 35 43.3 67.7
    Alloy 36 2.0 29.6
    Alloy 37 56.4 84.9
    Alloy 38 0.7 3.8
    Alloy 39 8.2 50.0
    Alloy 40 5.8 45.8
    Alloy 41 5.2 26.4
  • This comparative Case Example demonstrates that yield strength can be increased in alloys herein by cold rolling (i.e. at ambient temperature). Ultimate tensile strength is also increasing but cold rolling leads to a significant decrease in alloy ductility indicated by a drop in tensile elongation that can be a limiting factor in certain applications. Strengthening, as shown by the increase in ultimate tensile strength, is related to a phase transformation of austenite to ferrite as depicted by measurements of magnetic phases volume percent before and after cold rolling.
    • Comparative Case Example #2 Cold Rolling Reduction Effect on Yield Strength in Allow 2
  • Alloy 2 was processed into a hot band with a thickness of 4.4 mm. The hot band was then cold rolled with different reduction through multiple cold rolling (i.e. at ambient temperature) passes. After cold rolling the samples were heat treated with intermediate annealing at 850° C. for 10 min. This represented a start condition for each sample which represented a fully annealed condition to remove the prior cold work. From this start condition, subsequent cold rolling at different percentages (i.e. 0%, 4.4%, 9.0%, 15.1%, 20.1%, 25.1% and 29.7%) as provided in Table 5 was applied so that the final gauge for tensile testing would be at a targeted constant thickness of 1.2 mm. With increasing cold reduction as a final step after annealing, a corresponding increase of the material yield strength is demonstrated by tensile stress-strain curves in FIG. 8. Tensile properties from the tests are listed in Table 5. The yield strength of the Alloy 2 increases to a range from 666 to 1140 MPa depending on the level of reduction as compared to initial values in annealed state (Table 5). Also, the magnetic phases volume percent measured by Feritscope increases up to 12.9 Fe % as shown in Table 5 in comparison with initial value of 1.0 Fe % in the annealed state. It should be noted that yield strength increase is achieved at expense of alloy ductility with decreased tensile elongation after cold rolling.
  • TABLE 5
    Tensile Properties and Magnetic Phases Volume Percent in
    Alloy 2 After Cold Rolling
    Magnetic
    Ultimate Phases
    Tensile Tensile Yield Volume
    Cold Rolling Elongation Strength Strength Percent
    Reduction (%) (%) (MPa) (MPa) (Fe %)
    0.0 60.1 1200 445 1.0
    (fully annealed, i.e. 58.1 1192 433
    starting condition) 61.6 1222 444
    55.2 1197 444
    64.1 1212 446
    4.4 49.5 1262 667 1.7
    35.2 1230 666
    43.4 1268 673
    49.3 1298 679
    9.0 43.6 1325 736 2.3
    33.0 1340 738
    40.3 1342 732
    40.3 1346 737
    15.1 28.2 1422 865 6.2
    27.8 1441 865
    30.0 1454 867
    33.5 1445 869
    20.1 27.2 1510 980 9.1
    21.0 1512 960
    20.4 1524 970
    20.2 1515 990
    25.1 21.2 1555 1036 11.5
    22.7 1565 1037
    24.5 1563 1051
    25.1 1566 1058
    29.7 17.8 1628 1121 12.9
    21.0 1629 1105
    19.0 1627 1137
    20.0 1631 1140
  • This Comparative Case Example #2 demonstrates that yield strength in alloys herein can be altered by cold rolling reduction to achieve relatively higher yield strength values with increase in tensile strength but with decrease in ductility. The higher cold rolling reduction that is applied, the higher yield strength achieved and the lower tensile elongation recorded.
    • Comparative Case Example #3 Structural Transformation During Cold Rolling in a Hot Band from Alloy 2
  • Hot band from Alloy 2 with thickness of 4 mm was cold rolled to a final thickness of 1.2 mm through multiple cold rolling passes with intermediate annealing at 850° C. for 10 min. Microstructures of the hot band and the cold rolled sheet were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
  • To prepare SEM samples, pieces were cut by EDM and mounted in epoxy, and polished progressively with 9 μm, 6 μm and 1 μm diamond suspension solution, and finally with 0.02 μm silica. To prepare TEM specimens, the samples were cut from the sheet with EDM, and then thinned by grinding with pads of reduced grit size every time. Further thinning to 60 to 70 μm thick is done by polishing with 9 μm, 3 μm and 1 μm diamond suspension solution respectively. Discs of 3 mm in diameter were punched from the foils and the final polishing was fulfilled with electropolishing using a twin-jet polisher. The chemical solution used was a 30% nitric acid mixed in methanol base. In case of insufficient thin area for TEM observation, the TEM specimens may be ion-milled using a Gatan Precision Ion Polishing System (PIPS). The ion-milling usually is done at 4.5 keV, and the inclination angle is reduced from 4° to 2° to open up the thin area. The TEM studies were done using a JEOL 2100 high-resolution microscope operated at 200 kV.
  • SEM analysis of the hot band structure revealed relatively large austenite grains with straight boundaries (FIG. 9). Bright-field TEM image shows that the hot band structure contains very few dislocations and the grains boundaries are straight and sharp (FIG. 10) that is typical for recrystallized structures. TEM studies also showed that nanoprecipitates are present in the microstructure (FIG. 11).
  • When the hot band was subjected to cold rolling, the austenite phase in selected areas of the hot band structure transforms to refined ferrite phase under stress. Backscattered SEM images of the cold rolled sheet show the transformed and refined structure, and the presence of deformation twins (FIG. 12). As shown by TEM images in FIG. 13, high dislocation density is generated in retained austenite grains and refined grains of ferrite with a size of 200 to 300 nm are formed. Deformation twinning was also observed in the retained austenite grains. Additional nanoprecipitation as a part of phase transformation process during cold rolling was also observed (FIG. 14).
  • This Case Example demonstrates a microstructure evolution from the initial hot band austenitic structure during cold rolling leading to alloy strengthening (increase in ultimate tensile strength) by grain refinement due to phase transformation into ferrite with nanoprecipitation as well as dislocation density increase and deformation twinning.
    • Case Example #4 Rolling Temperature Effect on Yield Strength of Alloy 2
  • The starting material was a hot band from Alloy 2 with approximately 2.5 mm thickness prepared by hot rolling of 50 mm thick laboratory cast slab mimicking processing at commercial hot band production. The starting material had an average ultimate tensile strength of 1166 MPa, an average tensile elongation of 53.0% and an average yield strength of 304 MPa. The starting material also had a magnetic phases volume percent of 0.9 Fe %.
  • The hot band was media blasted to remove oxide and loaded into a Yamato DKN810 mechanical convection oven for at least 30 minutes prior to rolling to allow the plate to reach temperature. The hot band was rolled on a Fenn Model 061 rolling mill with steadily decreasing roll gaps, and was loaded into the furnace for at least 10 minutes between passes to ensure a constant starting temperature (i.e. 50, 100, 150, 200, 250° C., 300° C., 350° C., and 400° C.) for each subsequent rolling pass for a total targeted 20% reduction. Samples were EDM cut in the ASTM E8 Standard geometry. Tensile properties were measured on an Instron mechanical testing frame (Model 5984), utilizing Instron's Bluehill control and analysis software. All tensile tests were run at ambient temperature in displacement control with the bottom fixture held rigid and the top fixture moving; the load cell is attached to the top fixture.
  • Tensile properties of the Alloy 2 after rolling at identified temperatures are listed in Table 6. Depending on rolling temperature, the yield strength is increased to a range from 589 to 945 MPa as compared to the values of 250 to 711 MPa in a hot band (Table 2). The ultimate tensile strength of the Alloy 2 varies from 1132 to 1485 MPa with tensile elongation from 21.2 to 60.5%. An example stress-strain curves are shown in FIG. 15. As can be seen, rolling at temperature of 200° C. of the hot band from Alloy 2 demonstrates the possibility to increase yield strength with minimal changes in ductility and ultimate strength consistent with Step 3a in FIG. 3.
  • The magnetic phases volume percent (Fe %) was measured after rolling, in the tensile gauge at least 10 mm from fracture are reported in Table 7. As it can be seen, the magnetic phases volume percent after rolling at temperature of 100° C. and above is significantly lower in a range from 0.3 to 9.7 Fe % as compared to that after cold rolling Alloy 2 at ambient temperature (18.0 Fe %, Table 4). A significant increase in a magnetic phases volume percent was measured in the Alloy 2 after rolling at temperature and tensile tested (Table 7, FIG. 16). After tensile testing, magnetic phases volume percent in tensile gauge of the samples varies from 25.2 to 52.1 Fe % depending on rolling temperature.
  • TABLE 6
    Tensile Properties of Alloy 2 After ~20% Rolling Reduction at
    Different Temperatures
    Ultimate
    Tensile Tensile Yield Strength Rolling Rolling
    Elongation Strength (Offset 0.2%) Reduction Temperature
    (%) (MPa) (MPa) (%) (° C.)
    47.4 1165 296 0
    51.5 1171 309
    60.2 1162 306
    27.5 1485 945 21.1 50
    32.1 1481 942
    21.2 1468 934
    40.9 1326 819 19.4 100
    36.6 1321 825
    39.5 1334 823
    51.8 1224 804 19.6 150
    48.3 1219 803
    48.1 1225 809
    52.3 1205 803 20.1 200
    58.0 1196 775
    53.3 1218 773
    50.6 1158 745 22.0 250
    53.0 1166 733
    53.4 1152 723
    53.2 1157 738 20.6 300
    55.4 1145 752
    52.0 1157 724
    52.9 1186 691 19.8 350
    56.2 1168 686
    57.7 1168 695
    60.5 1150 651 18.6 400
    53.0 1144 621
    60.2 1158 655
  • TABLE 7
    Magnetic Phases Volume Percent (Fe %) as a Function of Rolling
    Temperature Before and After Tensile Testing of Alloy 2
    Rolling Temperature Fe % After Fe % in Tested
    (° C.) Rolling Tensile Gauge
    Hot Band 18.0 54.3
    50 18.1 52.1
    100 9.7 44.8
    150 7.1 37.7
    200 4.1 25.2
    250 4.1 30.5
    300 2.3 30.5
    350 1.8 32.8
    400 1.0 31.1
  • This Case Example demonstrates that yield strength in alloys herein can be increased by rolling at elevated temperatures whereby phase transformation of austenite into ferrite is reduced. Significant drops in Fe % occur when rolling temperature is greater than 100° C. Moreover, rolling of the hot band from alloys herein at temperatures of 150° C. to 400° C. demonstrates the ability to increase yield strength (e.g. increasing yield strength to a value of at least 100 MPa or more over the original value) without significant change in ductility (i.e. change limited to plus or minus seven and one half percent (±7.5% tensile elongation) and maintain the ultimate tensile strength at about the same level (i.e. ±100 MPa as compared to the original value).
    • Case Example #5 Rolling Temperature Effect on Yield Strength of Alloy 7, Alloy 18, Alloy 34 and Alloy 37
  • The starting material was a hot band from each of Alloy 7, Alloy 18, Alloy 34, and Alloy 37 with approximately 2.5 mm initial thickness prepared by hot rolling of 50 mm thick laboratory cast slab mimicking commercial processing. Alloys 7, 18, 34, and 37 were processed into hot bands with a thickness of approximately 2.5 mm by hot rolling at temperatures between 1100° C. and 1250° C. and subsequently media blasted to remove the oxide. The tensile properties of hot band material were previously listed in Table 2. The hot band was media blasted to remove oxide and loaded into a Yamato DKN810 mechanical convection oven for at least 30 minutes prior to rolling to allow the plate to reach the desired temperature. The resulting cleaned hot band was rolled on a Fenn Model 061 rolling mill with steadily decreasing roll gaps, and was loaded into the furnace for at least 10 minutes between passes to ensure constant temperature. The hot band was rolled to a targeted 20% reduction and samples were EDM cut in the ASTM E8 Standard geometry. Tensile properties were measured on an Instron mechanical testing frame (Model 5984), utilizing Instron's Bluehill control and analysis software. All tensile tests were run at ambient temperature in displacement control with the bottom fixture held rigid and the top fixture moving; the load cell is attached to the top fixture.
  • The responses of each alloy, in particular of their elongation, yield strength, and ultimate tensile strength were monitored across the entire range of temperatures investigated. Each alloy was tested after rolling at temperatures ranging from 100° C. at the lowest to 400° C. at the highest. For Alloy 7, tensile elongation ranged from 14.7% to 35.5%, ultimate tensile strength ranged from 1218 MPa to 1601 MPa, and yield strength ranged from 557 MPa to 678 MPa across the investigated temperature range (Table 8), with Fe % numbers ranging from 29.9 to 41.7 before tensile testing, and 57.7 to 65.4 after testing (Table 9). For Alloy 18, tensile elongation ranged from 43.0% to 51.9%, ultimate tensile strength ranged from 1083 MPa to 1263 MPa, and yield strength ranged from 772 MPa to 924 MPa from 150 to 400° C. (Table 10), with Fe % numbers ranging from 6.8 to 12.3 before tensile testing and from 31.5 to 39.6 after testing in the 150 to 400° C. range (Table 11). For Alloy 34, tensile elongation ranged from 21.1% to 31.1%, ultimate tensile strength ranged from 1080 MPa to 1140 MPa, and yield strength ranged from 869 MPa to 966 MPa in the 150 to 400° C. range (Table 12), with Fe % numbers ranging from 0.4 to 1.0 before tensile testing and 0.8 to 2.1 after testing (Table 13). For Alloy 37, tensile elongation ranged from 1.5% to 9.0%, ultimate tensile strength ranged from 1537 MPa to 1750 MPa, and yield strength ranged from 1384 MPa to 1708 MPa in the 150 to 400° C. range (Table 14), with Fe % numbers ranging from 74.5 to 84.3 before tensile testing and 71.1 to 85.6 after testing (Table 15).
  • TABLE 8
    Tensile Properties of Alloy 7 After ~20% Rolling Reduction at
    Different Temperatures
    Yield
    Ultimate Strength
    Tensile Tensile (Offset Rolling Rolling
    Elongation Strength 0.2%) Reduction Temperature
    (%) (MPa) (MPa) (%) (° C.)
    32.9 1396 389 0
    34.7 1425 373
    33.3 1392 382
    25.4 1575 676 20.2 100
    27.9 1601 678
    26.5 1597 665
    28.1 1519 593 21.2 150
    30.7 1529 586
    28.8 1503 609
    33.8 1478 557 19.3 200
    31.9 1458 575
    35.1 1501 567
    31.8 1464 631 19.8 250
    33.5 1491 607
    31.7 1491 583
    35.5 1449 647 19.5 300
    33.5 1462 645
    34.0 1468 647
    33.9 1468 663 19.2 350
    34.5 1428 673
    30.2 1469 673
    14.7 1218 651 20.2 400
    17.4 1287 648
    17.7 1270 665
  • TABLE 9
    Fe % Before and After Testing of Alloy 7 at Different Temperatures
    Rolling Temperature Fe % After Fe % in Tested
    (° C.) Rolling Gauge
    100 41.7 65.4
    150 33.5 65.2
    200 29.9 64.5
    250 30.4 62.7
    300 32.0 61.9
    350 30.5 60.6
    400 30.5 57.7
  • TABLE 10
    Tensile Properties of Alloy 18 After ~20% Rolling Reduction
    at Different Temperatures
    Yield
    Ultimate Strength
    Tensile Tensile (Offset Rolling Rolling
    Elongation Strength 0.2%) Reduction Temperature
    (%) (MPa) (MPa) (%) (C.)
    54.3 1145 415 0
    53.8 1168 401
    53.3 1167 401
    39.6 1243 911 20.4 100
    37.3 1242 922
    37.6 1263 924
    46.5 1184 856 20.3 150
    43.4 1155 869
    47.4 1195 859
    43.0 1142 828 20.5 200
    50.5 1153 830
    47.2 1155 834
    48.6 1125 797 19.9 250
    49.4 1138 808
    47.9 1118 801
    51.7 1144 812 20.3 300
    49.6 1100 798
    51.9 1123 825
    50.3 1139 784 19.3 350
    49.1 1127 811
    46.8 1145 812
    43.0 1083 782 20.5 400
    46.6 1130 778
    46.5 1097 772
  • TABLE 11
    Fe % Before and After Testing of Alloy 18 at Different Temperatures
    Rolling Fe % After Fe % in Tested
    Temperature (° C.) Rolling Gauge
    100 14.9 42.7
    150 12.3 39.6
    200 10.2 37.3
    250 9.5 36.6
    300 8.7 34.7
    350 7.7 33.2
    400 6.8 31.5
  • TABLE 12
    Tensile Properties of Alloy 34 After ~20% Rolling Reduction at
    Different Temperatures
    Ultimate
    Tensile Tensile Yield Strength Rolling Rolling
    Elongation Strength (Offset 0.2%) Reduction Temperature
    (%) (MPa) (MPa) (%) (C.)
    50.3 944 509 0
    52.7 946 524
    52.1 942 520
    20.3 1194 1031 20.5 100
    20.8 1189 1039
    20.6 1199 1040
    25.7 1136 962 19.9 150
    24.2 1140 966
    24.9 1136 961
    25.6 1120 948 20.3 200
    25.4 1115 942
    24.4 1112 947
    29.8 1092 904 19.3 250
    29.7 1097 911
    29.0 1099 899
    24.0 1115 945 19 300
    23.8 1111 957
    24.0 1105 955
    30.7 1088 869 20.3 350
    21.1 1088 913
    28.6 1081 881
    31.1 1080 877 19.8 400
    29.3 1084 883
    30.7 1081 898
  • TABLE 13
    Fe % Before and After Testing of Alloy
    34 at Different Temperatures
    Rolling Temperature Fe % After Fe % in Tested
    (° C.) Rolling Gauge
    100 1.5 3.5
    150 1.0 2.1
    200 0.9 1.6
    250 0.4 0.8
    300 0.4 1.0
    350 0.6 1.0
    400 0.5 0.8
  • TABLE 14
    Tensile Properties of Alloy 37 After ~20%
    Rolling Reduction at Different Temperatures
    Yield
    Tensile Ultimate Tensile Strength Rolling Rolling
    Elongation Strength (Offset 0.2%) Reduction Temperature
    (%) (MPa) (MPa) (%) (C.)
    8.2 1612 998 0
    7.7 1617 1004
    7.8 1607 995
    7.4 1780 1483 20.5 100
    4.8 1763 1469
    7.3 1771 1484
    8.5 1645 1420 20.1 150
    8.4 1634 1384
    9.0 1642 1413
    7.5 1631 1494 20.7 200
    7.4 1635 1499
    7.3 1629 1474
    6.5 1537 1481 19.4 250
    6.9 1542 1484
    7.5 1546 1482
    4.8 1591 1561 20.2 300
    5.0 1588 1558
    5.2 1596 1559
    4.1 1649 1618 20.6 350
    1.5 1644 1616
    4.1 1647 1615
    3.7 1750 1706 20 400
    4.1 1742 1698
    4.1 1747 1708
  • TABLE 15
    Fe % Before and After Testing of Alloy
    37 at Different Temperatures
    Rolling Temperature Fe % After Fe % in Tensile
    (° C.) Rolling Gauge
    100 84.3 85.6
    150 77.2 84.4
    200 79.9 76.8
    250 75.1 80.9
    300 76.7 71.1
    350 77.5 75.7
    400 74.5 72.8
  • Representative curves for each alloy herein are shown in FIG. 17 through FIG. 20 with reference curves from tested hot band and after cold rolling to the same approximate 20% reduction for parallel comparison.
  • This Case Example demonstrates that yield strength in alloys herein can be increased although phase transformation of austenite into ferrite is reduced when rolling at temperatures of 100° C. or greater up to 400° C. Examples of changes in yield strength, ultimate tensile strength, and tensile elongation were provided for both Steps 3a and 3b in FIG. 2.
    • Case Example #6 Effect of Reduction of Rolling at 200° C. on Yield Strength of Alloy 2
  • Alloy 2 was processed into a hot band with thickness of approximately 2.5 mm from the laboratory cast. Following hot rolling, Alloy 2 was rolled at 200° C. to varying rolling reductions ranging from approximately 10% to 40%. Between rolling passes, the Alloy 2 sheet material was placed in a convection furnace at 200° C. for 10 minutes to maintain the temperature. When the desired rolling reduction was achieved, ASTM E8 tensile samples were cut via wire-EDM and tested.
  • Tensile properties of Alloy 2 after rolling at 200° C. with different rolling reduction (0.0 to 70.0%) are listed in Table 16, which also includes data prior to any rolling experiments. FIG. 21 shows the representative tensile curves for Alloy 2 as a function of rolling reduction at 200° C. It is observed that the yield strength of the material increases rapidly with increasing reduction, without changing the ultimate tensile strength (i.e. a change of plus or minus 100 MPa) up to 30% reduction. FIG. 22 provides a comparison of the trends for yield strength and ultimate tensile strength as a function of rolling reduction at 200° C., showing that while the yield strength increase is relatively rapid, the ultimate tensile strength change is consistent with step 3a property changes in FIG. 2 up to 30.4% rolling reduction and is consistent with step 3b property changes at 39.0% rolling reduction.
  • The total elongation of Alloy 2 is plotted as a function of rolling reduction at 200° C. in FIG. 23. It demonstrates that while the yield strength of Alloy 2 is increasing with additional reduction during rolling at 200° C., the available ductility does not decrease rapidly until >30% reduction. Note that this is simulated using laboratory rolling and commercial rolling methods including tandem mill rolling, Z-mill rolling, and reversing mill rolling will additionally apply a strip tension during rolling so the exact amount of reduction whereby ductility decreases may change.
  • The magnetic phases volume percent (Fe %) was measured using a Fischer Feritscope FMP30 for the samples after rolling at 200° C. and again after tensile testing in the tensile gauge (i.e. the reduced gauge section present in the tensile specimen). These measurements, shown in Table 17, are indicative of the amount of deformation-induced phase transformation that is occurring in the alloy during the rolling process and during subsequent tensile testing. The amount of deformation-induced phase transformation in Alloy 2 after rolling and tensile testing is shown in FIG. 24. It can be seen that the deformation-induced phase transformation is largely suppressed at 200° C., as the magnetic phases volume percent only increases slightly with increasing rolling reduction. Rolling at 200° C. is demonstrated to have an effect on the deformation-induced phase transformation during tensile testing also, with increasing rolling reductions suppressing the amount of transformation in the material.
  • TABLE 16
    Average Tensile Properties of Alloy 2 after
    Rolling at 200° C. to Various Reductions
    Rolling Yield Ultimate Tensile Tensile
    Reduction Strength Strength Elongation
    (%) (MPa) (MPa) (%)
    0.0 296 1165 47.4
    309 1171 51.5
    306 1162 60.2
    10.7 496 1175 60.8
    556 1223 63.5
    536 1187 61.0
    20.1 803 1205 52.3
    775 1196 58.0
    773 1218 53.3
    30.4 986 1226 42.3
    938 1209 42.7
    979 1233 42.6
    39.0 1123 1274 5.5
    1148 1290 7.2
    1147 1285 9.4
    50.4* 805 1425 5.11
    1107 1445 5.17
    786 1427 3.1
    60.1* 1258 1520 6.92
    1200 1520 6.93
    1216 1524 4.29
    70.0* 1299 1623 6.06
    1361 1625 6.58
    1348 1626 6.14
    *Different processing was applied: Alloy 2 was processed into hot band at 1250° C. with a thickness of approximately 9.3 mm, subsequently media blasted to remove the oxide and then rolled at 200° C. to 4.6 mm (~50% reduction). The material was then annealed at 850° C. for 10 minutes and rolled at 200° C. to approximately 50.4, 60.1, and 70% reduction.
  • TABLE 17
    Magnetic Phases Volume Percent (Fe %)
    as a Function of Rolling Reduction
    Rolling Reduction Fe % After Fe % in Tensile
    (%) Rolling Gauge
    0.0 0.9 42.6
    10.7 3.0 46.7
    20.1 4.2 37.9
    30.4 5.8 26.7
    39.0 5.1 16.2
    50.4 2.5 15.3
    60.1 2.4 13.5
    70.0 2.3 16.1
  • This Case Example demonstrates that the yield strength of the alloys described herein may be tailored by varying the rolling reduction at temperatures greater than ambient as shown here for Alloy 2 by rolling at 200° C. In the broad context of the present disclosure, the temperature range is contemplated to be between 150° C. to 400° C. as provided in the previous case example for Table 7. During this rolling, the deformation pathway is modified such that relatively limited deformation-induced phase transformation is occurring, which results in the ability to retain significant ductility and maintain ultimate tensile strength while increasing yield strength in the cold rolled state. Thereby, the parameters of the rolling can be optimized to improve the yield strength of the material without sacrificing the ductility or ultimate tensile strength.
    • Case Example #7 Microstructure in Alloy 2 after Rolling at 200° C.
  • Alloy 2 was processed into a hot band with thickness of 9 mm from the laboratory cast mimicking processing at commercial hot band production. The hot band was cold rolled with 50% reduction and annealed at 850° C. for 10 minutes with air cooling mimicking cold rolling processing at commercial sheet production. Media blasting was used to remove the oxides which formed during annealing. Then the alloys were cold rolled again until failure or the mill limited reduction. Samples were heated to 200° C. in a convection oven for at least 30 minutes prior to cold rolling to ensure they were at uniform temperature, and reheated for 10 minutes between passes to ensure constant temperature. Alloy 2 sheet was cold rolled first with reduction of 30% and then to a maximum reduction of 70%. Microstructure of the initial structure and after rolling was studied by scanning electron microscopy (SEM). To prepare SEM samples, pieces were cut by EDM and mounted in epoxy, and polished progressively with 9 μm, 6 μm and 1 μm diamond suspension solution, and finally with 0.02 μm silica.
  • FIG. 25 shows the backscattered SEM images of the microstructure before cold rolling that is mostly austenitic with annealing twins inside micron-sized grains. After cold rolling with 30% reduction, as shown in FIG. 26, a band structure can be seen in different areas with different orientations. Presumably, the bands with similar orientation are deformation twins in one austenitic grain while bands in different direction are twins in another crystal orientation grain. Some grain refinement can be observed in selected areas.
  • After the rolling reduction is increased to 70%, the bands are no longer visible, and refined structure through the volume can be seen (FIG. 27). As shown in the high magnification image in FIG. 27b , fine islands with size much smaller than 10 μm can be discerned. Considering the high deformation exerted in the stable austenite during the rolling process, the austenite could be dramatically refined typically in the range of 100 to 500 nm. Feritscope measurements suggest that the austenite is stable at 200° C. with nearly 100% austenite maintained after rolling.
  • This Case Example demonstrates austenite stabilization (i.e. the resistance to transformation to ferrite) in alloys herein during the rolling at 200° C. even at high rolling reduction of 70% and microstructural refinement of the austenite in contrast to cold rolling when refinement occurs through austenite transformation to ferrite.
    • Case Example #8 Effect of Rolling Reduction at 200° C. on Microstructure in Alloy 2
  • Rolling at temperature resulted in significant increase in yield strength of the Alloy 2 while high tensile elongation was maintained. TEM study was conducted on the Alloy 2 rolled at 200° C. to analyze the structural changes during the rolling at 200° C. as a function of rolling strain. In this case example, 50 mm thick laboratory cast slab was hot rolled first, and the resultant hot band was then rolled at 200° C. to different strains. To show structural evolution, microstructures of the rolled sheets were studied by transmission electron microscopy (TEM). To prepare TEM specimens, the samples were cut from the sheet using wire-EDM, and then thinned by grinding with pads of reduced grit size every time. Further thinning to 60 to 70 μm thick samples was done by polishing with 9 μm, 3 μm and 1 μm diamond suspension solutions, respectively. Discs of 3 mm in diameter were punched from the foils and the final polishing was fulfilled by electropolishing using a twin-jet polisher. The chemical solution used was a 30% nitric acid mixed in methanol base. In case of insufficient thin area for TEM observation, the TEM specimens were ion-milled using a Gatan Precision Ion Polishing System (PIPS). The ion-milling usually is done at 4.5 keV, and the inclination angle is reduced from 4° to 2° to open up the thin area. The TEM studies were done using a JEOL 2100 high-resolution microscope operated at 200 kV.
  • FIG. 28 shows the bright-field TEM images of the microstructure in the Alloy 2 rolled at 200° C. with 10% reduction. It can be seen that the austenite grains are filled with tangled dislocations, and dislocation cell structure is exhibited. However, due to the relatively low rolling strain, the original austenite grain boundaries are still visible. It is noted that the austenite is stable during the rolling at 200° C. Electron diffraction suggests that austenite is the predominant phase that was also consistent with Feritscope measurement. Rolling at 200° C. with 10% reduction increases the average yield strength from 303 MPa in the hot band to 529 MPA (see Table 16). When the sheet is rolled to 30%, TEM qualitatively shows higher dislocations density in the grains, as shown in FIG. 29, and clear dislocation cell structure is exhibited. In addition, some deformation twins are seen within the austenite grains. Similar to the 10% rolled sample, the austenite phase is maintained, as confirmed by the electron diffraction. However, the original grain boundaries of austenite are no longer visible. Rolling at 200° C. with 30% reduction results in average yield strength of 968 MPa (Table 16). After rolling with 70% reduction (FIG. 30), a qualitatively higher dislocation density continues can be seen from TEM, and dislocation cells are similar to that in the 30% rolled sample (FIG. 29). In addition, deformation twins are also present in the sample. Similar to the 30% rolled sample, the austenite still remains stable during rolling that is verified by electron diffraction.
  • This Case Example demonstrates that the alloys herein maintain austenite structure during rolling at 200° C. with up to 70% reduction. Structural changes including dislocation cell formation and twinning leads to increase in yield strength after rolling at 200° C.
    • Case Example #9 Process Route by Combination of Rolling Methods
  • Alloys 2, Alloy 7, Alloy 18, and Alloy 34 were processed into hot band with a thickness of ˜2.7 mm, this was media blasted to remove the oxide and rolled at 200° C. to 20% reduction. The material was sectioned and then rolled at a range of reductions at ambient temperature. ASTM E8 tensile samples were cut by wire EDM and tested in an Instron 5984 frame using Instron's Bluehill software.
  • Tensile properties of the selected alloys after combined rolling are listed in Table 18 through Table 21. Significant increase in yield strength after combination of rolling methods was observed in all three alloys as compared to the hot band state or just after rolling with ˜20% reduction in rolling thickness at 200° C. and subsequent rolling reduction at ambient temperature. Yield strength up to 1216 MPa recorded for Alloy 2 (yield strength in hot band is 309 MPa and 803 MPa after rolling at 200° C.), up to 1571 MPa in Alloy 7 (yield strength in hot band is 333 MPa and 575 MPa after rolling at 200° C.), up to 1080 MPa in Alloy 18 (yield strength in hot band is 390 MPa and 834 MPa after rolling at 200° C.), and up to 1248 MPa in Alloy 34 (yield strength in hot band is 970 MPa and 1120 MPa after rolling at 200° C.). FIG. 31 through FIG. 34 shows the corresponding tensile curves for alloys 2, 7, 18, and 34, respectively. An increase in ultimate tensile strength after cold rolling was also observed in all alloys herein with decrease in tensile elongation (see Tables 18 through 21). Analysis of the magnetic phases volume percent of the selected alloys herein in each examined condition, both prior to and after tensile testing is listed in Table 22 through Table 25. Cold rolling leads to higher Fe % in the processed sheet from the alloys herein followed by further increase in Fe % due to the transformation occurring during tensile testing.
  • TABLE 18
    Tensile Properties of Alloy 2 after
    Combination of Rolling Methods
    Ultimate
    First Reduction Second Reduction Tensile Tensile Yield
    by Rolling by Cold Elongation Strength Strength
    at 200° C. Rolling (%) (MPa) (MPa)
    Hot Band 47.4 1165 296
    51.5 1171 309
    60.2 1162 306
    20.1 — 52.3 1205 803
    20.1 — 58.0 1196 775
    20.1 — 53.3 1218 773
    19.2 4.4 36.1 1298 951
    19.2 4.4 39.0 1303 974
    19.2 4.4 37.4 1275 944
    20.0 10.2 35.1 1386 994
    20.0 10.2 31.8 1393 1018
    20.0 10.2 34.0 1409 999
    20.0 19.8 19.2 1544 1064
    20.0 19.8 23.1 1542 1079
    20.0 19.8 18.5 1541 1068
    20.0 30.7 21.3 1662 1199
    20.0 30.7 15.2 1665 1216
    20.0 30.7 20.3 1672 1212
  • TABLE 19
    Tensile Properties of Alloy 7 after
    Combination of Rolling Methods
    Ultimate
    First Reduction Second Reduction Tensile Tensile Yield
    by Rolling by Cold Elongation Strength Strength
    at 200° C. Rolling (%) (MPa) (MPa)
    Hot Band 28.1 1508 333
    28.5 1516 331
    26.0 1520 317
    19.3 — 33.8 1478 557
    19.3 — 31.9 1458 575
    19.3 — 35.1 1501 567
    19.3 5 27.4 1598 625
    19.3 5 26.1 1619 608
    19.3 5 27.4 1629 597
    19.3 10.7 23.4 1689 795
    19.3 10.7 20.4 1710 774
    19.3 10.7 21.7 1737 778
    19.3 19.9 15.1 1817 1199
    19.3 19.9 16.3 1802 1217
    19.3 19.9 16.5 1838 1265
    19.3 29.7 12.0 1872 1510
    19.3 29.7 14.4 1907 1492
    19.3 29.7 13.1 1920 1571
  • TABLE 20
    Tensile Properties of Alloy 18 after
    Combination of Rolling Methods
    Ultimate
    First Reduction Second Reduction Tensile Tensile Yield
    by Rolling by Cold Elongation Strength Strength
    at 200° C. Rolling (%) (MPa) (MPa)
    Hot Band 58.8 1224 384
    56.1 1245 390
    50.7 1190 365
    20.5 — 43.0 1142 828
    20.5 — 50.5 1153 830
    20.5 — 47.2 1155 834
    20.5 4.9 35.7 1244 846
    20.5 4.9 37.5 1243 856
    20.5 4.9 34.8 1251 769
    20.5 10.3 30.7 1339 830
    20.5 10.3 31.6 1340 905
    20.5 10.3 26.6 1337 819
    20.5 19.3 22.4 1529 1025
    20.5 19.3 22.3 1523 898
    20.5 19.3 22.0 1521 885
    20.6 29.4 17.0 1625 1008
    20.6 29.4 17.3 1641 1080
    20.6 29.4 18.8 1622 1074
  • TABLE 21
    Tensile Properties of Alloy 34 after
    Combination of Rolling Methods
    Ultimate
    First Reduction Second Reduction Tensile Tensile Yield
    by Rolling by Cold Elongation Strength Strength
    at 200° C. Rolling (%) (MPa) (MPa)
    Hot Band 65.9 963 515
    58.7 954 485
    62.1 970 545
    20.3 — 25.6 1120 948
    20.3 — 25.4 1115 942
    20.3 — 24.4 1112 947
    19.7 5.9 18.2 1173 1037
    19.7 5.9 18.8 1163 1020
    19.7 5.9 19.3 1162 1005
    19.7 11 12.4 1247 866
    19.7 11 11.9 1243 1028
    19.7 11 12.2 1248 1055
  • TABLE 22
    Magnetic Phases Volume Percent (Fe %) in Alloy
    2 after Combination of Rolling Methods
    First Reduction Second Reduction Fe % in
    by Rolling by Cold Fe % After Tensile
    at 200° C. Rolling Rolling Gauge
    (%) (%) (Fe %) (Fe %)
    0.0 0.0 0.9 42.6
    20.1 0.0 4.2 37.6
    19.2 4.4 3.6 34.0
    20.0 10.2 6.1 40.0
    20.0 19.8 11.6 44.9
    20.0 30.7 16.8 49.3
  • TABLE 23
    Magnetic Phases Volume Percent (Fe %) in Alloy
    7 after Combination of Rolling Methods
    Rolling Reduction Fe % in
    Reduction at Cold Fe % After Tensile
    at 200° C. Rolling Rolling Gauge
    (%) (%) (Fe %) (Fe %)
    0 0 10.4 63.6
    19.3 0 29.9 64.5
    19.3 5.0 33.8 64.9
    19.3 10.7 44.0 66.2
    19.3 19.9 56.4 67.9
    19.3 29.7 59.8 67.3
  • TABLE 24
    Magnetic Phases Volume Percent (Fe %) in Alloy
    18 after Combination of Rolling Methods
    First Reduction Second Reduction Fe % in
    by Rolling by Cold Fe % After Tensile
    at 200° C. Rolling Rolling Gauge
    (%) (%) (Fe %) (Fe %)
    0.0 0.0 0.3 48.6
    20.5 0.0 10.2 37.3
    20.5 4.9 9.9 38.5
    20.5 10.3 14.4 42.0
    20.5 19.3 23.0 48.2
    20.6 29.4 32.5 49.2
  • TABLE 25
    Magnetic Phases Volume Percent (Fe %) in Alloy
    34 after Combination of Rolling Methods
    First Reduction Second Reduction Fe % in
    by Rolling by Cold Fe % After Tensile
    at 200° C. Rolling Rolling Gauge
    (%) (%) (Fe %) (Fe %)
    0.0 0.0 0.3 2.2
    20.3 0.0 0.9 1.6
    19.7 5.9 1.1 1.6
    19.7 11.0 1.4 2.9
    19.7 19.7 1.8 2.7
    19.7 29.7 2.0 2.7
  • This Case Example demonstrates a pathway to creating a third distinct set of property combinations, which may be achieved by processing the alloy into a sheet at a thickness of 0.5 mm to 5.0 mm, followed by deforming (rolling) and reducing thickness in one pass at a temperature in the range of 150° C. to 400° C., and then subsequent reductions in thickness at temperatures≦150° C. temperature. This is observed to provide relatively higher yield strength compared to only cold rolling, and higher tensile strengths compared to only rolling at temperature.
    • Case Example #10 Example Methods to Tailor Property Combinations
  • A hot band from Alloy 2 was processed into a sheet by different methods herein towards higher yield strength and property combination according to the steps provided in FIG. 2 and FIG. 3. Alloy 2 was first cast and then processed into a sheet via hot rolling which was from 2.5 to 2.7 mm thick. For tensile comparison, the reference hot band material was hot rolled to ˜1.8 mm to reduce gauge prior to testing. For the FIG. 2 example (i.e. Rolled 20% at 200° C.), the hot band was rolled with a 20% reduction at 200° C. Prior to rolling, it was heated up to 200° C. for 30 minutes before being rolled 20% at 200° C. with a 10 minute reheat between rolling passes to maintain temperature. For the FIG. 3 example (i.e. rolled 20% at 200° C. and then 10% cold roll at ambient temperature), the process steps were repeated which included a 20% reduction at 200° C. and with the additional step of a 10% ambient temperature rolling reduction applied. Tensile specimens were cut from the sheet processed by each method using wire EDM. Tensile properties were measured on an Instron mechanical testing frame (Model 5984), utilizing Instron's Bluehill control and analysis software. All tests were run at ambient temperature in displacement control.
  • Representative stress-strain curves with property combination achieved at each processing method close to optimal are shown in FIG. 35. As it can be seen, the yield strength can be significantly increased (i.e. 469 MPa increase) by rolling at 200° C. with minimal change in alloy ultimate tensile strength (i.e. 34 MPa increase) and elongation (i.e. 1.8% decrease). This is provided by the example condition 3a in FIG. 2. For the sample additionally rolled at 10% at ambient temperature from the starting condition of Step 3, then this would satisfy Step 4 in FIG. 3. As can be seen, in this case, this is a route to higher yield strength (i.e. 688 MPa increase) and tensile strength (i.e. 224 MPa increase) but comes with a reduction in total elongation (i.e. 25.1% decrease). Note that satisfying Step 4 in FIG. 3 could also be done by for example by cold stamping the part by various processes whereby the areas in the stamped part would experience higher yield strength and tensile strength with commensurate lower ductility which was used up partially in forming the part.
  • This Case Example demonstrates an achievement of high yield strength in alloys herein by various methods or their combination which provides a variety of the strength/elongation combinations in the resultant sheet from alloys herein.
    • Case Example #11 Effect of Test Temperature on Tensile Properties of Alloy 2
  • Alloy 2 was produced in a sheet form with 1.4 mm thickness from the slab by hot rolling and cold rolling to a targeted thickness with subsequent annealing. Tensile specimens were cut from the Alloy 2 sheet using wire EDM. Tensile properties were measured at different temperatures in a range from −40° C. to 200° C.
  • Tensile properties of the Alloy 2 sheet at different temperatures are listed in Table 26. The magnetic phases volume percent was measured in the tensile sample gauge after testing at each temperature using Feritscope that is also listed in Table 26. As it can be seen, yield and ultimate tensile strength are decreasing with increasing test temperature while tensile elongation is increasing. Tensile elongation and magnetic phases volume percent (Fe %) as a function of test temperature are plotted in FIG. 36 showing that despite higher elongation at elevated temperatures, the magnetic phases volume percent in a tensile sample gauge after testing drops significantly and close to zero after testing at 200° C. A decrease in the magnetic phases volume percent in a tensile sample gauge after testing indicates higher austenite stability at elevated temperatures suppressing its transformation to ferrite under the stress.
  • TABLE 26
    Tensile Properties of Alloy 2 Tested at Different Temperatures
    Test Ultimate Tensile Yield Tensile Magnetic Phases
    Temperature Strength Strength Elongation Volume Percent
    (° C.) (MPa) (MPa) (%) (Fe %)
    −40 1240 358 59 56.3
    1180 345 46 52.7
    1180 340 57 58.0
    1190 338 46 53.8
    1120 364 31 45.1
    23 1210 370 62 48.7
    1220 355 62 49.3
    1220 371 57 47.0
    1230 362 56 48.4
    1210 353 56 50.6
    1230 376 56 46.6
    1230 369 54 48.6
    1200 361 52 49.2
    1200 359 56 47.9
    1200 364 62 49.4
    100 890 329 66 10.0
    905 333 71 10.8
    900 332 67 11.0
    905 342 66 9.7
    905 334 60 11.1
    200 685 226 67 0.5
    690 230 66 0.6
    695 224 71 0.6
    695 217 64 0.7
    710 228 66 0.6
  • This Case Example demonstrates that multicomponent alloying of the alloys herein resulted in significant increase of austenite stability and transformation to ferrite during rolling is shown to be suppressed at elevated temperatures as compared to cold rolling as clearly provided in the last column in Table 26. It provides higher ductility during rolling itself and higher formability at subsequent sheet forming operations such as stamping, drawing, etc.
    • Case Example #12 Reduction in Processing Steps Towards Targeted Gauge
  • Alloy 2 was processed into a hot band with thickness of 4.4 mm. Two sections of the hot band were then rolled, one at ambient temperature and one at 200° C. The plate at 200° C. was heated in a mechanical convection oven for 30 minutes prior to rolling and reheated for 10 minutes between passes to ensure constant temperature.
  • In a case of rolling at ambient temperature, the failure occurred at approximately 42% reduction while a reduction of more than 70% was applied during rolling at 200° C. without the failure when the limit of the mill achieved. Mill limitations occurred when the Fenn Model 061 rolling mill could no longer make significant reductions per pass during cold rolling while the material still has ability for further rolling reduction.
  • The magnetic phases volume percent (Fe %) was measured by Feritscope at different levels of reductions during cold rolling and rolling at 200° C. The data are shown in FIG. 37. As it can be seen, the magnetic phases volume percent (Fe %) increases rapidly with reduction at ambient temperature leading to the material limit for rolling at ˜42%. In a case of the rolling at 200° C., the magnetic phases volume percent (Fe %) remains under 3 Fe % even at maximum rolling reduction of >70%.
  • A sheet from Alloy 2 with final thickness of 1.2 mm was produced by utilizing both cold rolling and rolling at 200° C. In a case of cold rolling, the rolling was cycled with intermediate annealing to restore the alloy ductility and achieve the targeted thickness with reduction of 29% at final rolling step. Tensile samples were EDM cut from the sheet with 1.2 mm thickness produced by both rolling methods and annealed at 1000° C. for 135 sec. Tensile properties were measured on an Instron mechanical testing frame (Model 3369), utilizing Instron's Bluehill control and analysis software. All tests were run at ambient temperature in displacement control with the bottom fixture held rigid and the top fixture moving; the load cell is attached to the top fixture.
  • Examples of the engineering stress-strain curves for the annealed sheet produced by both cold rolling and rolling at 200° C. are shown in FIG. 38. As it can be seen, despite different rolling methods towards targeted thickness, the final properties of the sheet after annealing are similar.
  • This Case Example demonstrates that rolling where the austenite is stable and does not transfer to ferrite as demonstrated here for Alloy 2 at 200° C. significantly improves rolling ability of the alloys herein that will allow reduction in processing steps towards targeted sheet gauges. Thus, this elevated temperature rolling can be used to hit a near final targeted gauge with high cold rolling reduction as provided in this example of >70%. This near final gauge material can then be annealed to restore the starting properties (i.e. the initial condition). Subsequently, the final targeted gauge can be obtained by rolling in the temperature range provided in this application from 150 to 400° C. following the steps and procedures in FIG. 2 or FIG. 3.
    • Case Example #13 Change in Limiting Rolling Reduction
  • Hot band was prepared from Alloy 2 with approximately 9 mm thickness. It was heated to 200 to 250° C. for 60 minutes and rolled to approximately 4.5 mm with 10 minute reheats between rolling passes to ensure consistent temperature. Once at 4.5 mm it was sectioned and annealed at 850° C. for 10 minutes and allowed to air cool. The material was media blasted to remove the oxide and heated to the desired temperature for at least 30 minutes prior to rolling, and reheated for 10 minutes between passes to ensure consistent temperature. The material was rolled until failure (visible cracking) characterized by such visible cracks propagating in from the ends of the sheet at least 2 inches. At around 70% reduction the mill had difficulty achieving the loads necessary to reduce the material and rolling was stopped, this is an equipment limitation and not a material limitation. The control material for room temperature rolling was hot band at 4.4 mm thick which was rolled at room temperature until failure. The results of the maximum rolling reduction as a function of rolling temperature are provided in Table 27 and FIG. 39.
  • This Case Example demonstrates for the alloys herein that the limiting rolling reduction increases as temperature increases. It therefore can be seen that the alloys herein are contemplated to allow for permanent deformation with a reduction in thickness of greater than 20% before failure when heated to a temperature falling in the range of 150° C. to 400° C. More preferably, the alloys herein are such that they are contemplated to be capable of permanent deformation with a reduction in thickness of greater than 40% before failure when heated in such temperature range. This provides much greater potential deformation for rolling operations, including processing of industrial material to reach a target gauge. Greater reductions before cracking means that less steps (i.e. cold rolling and recrystallization annealing) may be required to hit a specific targeted gauge during steel production. Additionally, the greater formability demonstrated at elevated temperatures would be beneficial in making parts from a variety of forming operations including, stamping, roll forming, drawing, hydroforming etc.
  • TABLE 27
    Rolling Reduction Limit vs Rolling Temperature for Alloy 2
    Temperature Rolling
    (° C.) Reduction Limit
    23 41.4%
    100 53.8%
    150 68.6%
    200  >70%
    250  >70%

Claims (19)

1. A method to increase yield strength in a metallic alloy comprising:
a. supplying a metal alloy comprising at least 70 atomic % iron and at least four or more elements selected from Si, Mn, Cr, Ni, Cu or C, melting said alloy, cooling at a rate of 10−4K/sec to 103 K/sec and solidifying to a thickness of >5.0 mm to 500 mm;
b. processing said alloy into a first sheet form with thickness from 0.5 to 5.0 mm with the first sheet having a total elongation of X1 (%), an ultimate tensile strength of Y1 (MPa), and a yield strength of Z1 (MPa);
c. permanently deforming said alloy in the temperature range of 150° C. to 400° C. into a second sheet form exhibiting one of the following tensile property combinations A or B:
A. (1) total elongation X2=X1±7.5%;
(2) ultimate tensile strength Y2=Y1±100 MPa; and
(3) yield strength Z2≧Z1+100 MPa.
B. (1) ultimate tensile strength Y3=Y1±100 MPa; and
(2) yield strength Z3≧Z1+200 MPa
2. The method of claim 1, wherein said alloy contains at least 70 atomic % iron and five or more elements selected from Si, Mn, Cr, Ni, Cu or C.
3. The method of 1 wherein said alloy contains at least 70 atomic % iron and Si, Mn, Cr, Ni, Cu and C.
4. The method of claim 1 wherein said alloy formed in step (b) exhibits X1 values of 10.0 to 70.0%, Y1 values of 900 MPa to 2050 MPa and Z1 values of 200 MPa to 750 MPa.
5. The method of claim 1 wherein said tensile property combination A is as follows: X2=2.5% to 77.5%, Y2=800 MPa to 2150 MPa and Z2≧300 MPa.
6. The method of claim 1 wherein said tensile property combination B is as follows: Y3=800 MPa to 2150 MPa and Z3≧300 MPa.
7. The method of claim 1 wherein said first sheet formed in step (c) is permanently deformed into said second alloy sheet by reducing thickness of said first alloy sheet.
8. The method of claim 1 wherein step (b) is carried out at a temperature of 700° C. to a temperature below the melting point (Tm) of said alloy.
9. The method of claim 1, wherein after step (b) the alloy is heat treated at a temperature of 650° C. to a temperature below the melting point (Tm) of the alloy.
10. The method of claim 1 wherein in step (c) the alloy is permanently deformed with a reduction in thickness of greater than 20% before failure.
11. The method of claim 1 wherein said first sheet formed in step (c) is permanently deformed into said second alloy sheet by the process of roll forming, metal stamping, metal drawing, or hydroforming.
12. The method of claim 1 wherein said alloy formed at a thickness of >5.0 mm to 500 mm contains greater than 10 volume percent of austenite.
13. The method of claim 1 wherein after permanently deforming said alloy in the temperature range of 150° C. to 400° C. in step (c) into a second sheet, said second sheet is permanently deformed at a temperature range of ≦150° C.
14. A method to increase yield strength in a metallic alloy comprising:
a. supplying a metal alloy comprising at least 70 atomic % iron and at least four or more elements selected from Si, Mn, Cr, Ni, Cu or C, melting said alloy, cooling at a rate of 10−4K/sec to 103 K/sec and solidifying to a thickness of >5.0 mm to 500 mm;
b. processing said alloy into a first sheet form with thickness from 5.0 to 0.5 mm;
c. permanently deforming said alloy in the temperature range of 150° C. to 400° C. into a second sheet form;
d. permanently deforming said alloy in a temperature of <150° C. into a second sheet form exhibiting the following tensile property combinations:
(1) total elongation=10.0 to 40.0%;
(2) ultimate tensile strength=1150 to 2000 MPa;
(3) yield strength=550 to 1600 MPa.
15. The method of claim 14 wherein step (b) is carried out at a temperature of 700° C. to a temperature below the melting point (Tm) of said alloy.
16. The method of claim 14, wherein after step (b) the alloy is heat treated at a temperature of 650° C. to a temperature below the melting point (Tm) of the alloy.
17. The method of claim 14 wherein said step of permanently deforming comprises the process of roll forming, metal stamping, metal drawing, or hydroforming.
18. The method of claim 14 wherein said permanently deformed part formed in step (d) is positioned in a vehicular frame, vehicular chassis, or vehicular panel.
19. The method of claim 14 wherein said permanently deformed part in step (d) is positioned in one of a drill collar, drill pipe, pipe casing, tool joint, wellhead, compressed gas storage tank, railway tank car/tank wagon or liquefied natural gas canister.
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