CN111441000A - 690 MPa-yield-strength low-yield-ratio high-strength steel plate and manufacturing method thereof - Google Patents
690 MPa-yield-strength low-yield-ratio high-strength steel plate and manufacturing method thereof Download PDFInfo
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Abstract
The invention relates to a 690 MPa-yield-strength low-yield-ratio high-strength steel plate, which comprises the chemical components of 0.10-0.20% of C, 0.10-0.40% of Si, 0.90-1.40% of Mn, no more than 0.030% of Nb, no more than 0.010% of V, no more than 0.025% of Ti, 0.02-0.06% of Al, no more than 0.10% of Ni, no more than 0.10% of Cu, 0.10-0.80% of Cr, 0.10-0.70% of Mo, 0.001-0.005% of B, 0.001-0.005% of Ca, no more than 0.015% of P, no more than 0.003% of S, no more than 0.002% of O, no more than 0.004% of N, no more than 0.0002% of H, the balance Fe. tissue of tempered cable body and bainite tissue, 0.2 MPa-690 MPa-yield strength of RmMPa, no less than 0.770% of RmK, 0.84% of RmJ, 0.84-100 ℃ of RH, 0.84-16 ℃ of dry quenching and continuous annealing and smelting in an electric furnace or continuous furnace with a low-temperature annealing process of smelting and a secondary quenching and a quenching process of annealing and a quenching process of smelting.
Description
Technical Field
The invention relates to a steel-making method of iron-based steel, in particular to a manufacturing method of a low-yield-ratio and high-strength steel plate.
Background
The high-speed development of industries such as steel structure industry, building bridges, engineering machinery, mining machinery and the like can initially increase the demand of high-strength steel plates with the yield strength of 690 MPa. The traditional 690 MPa-grade high-strength steel plate with yield strength is generally produced by adopting a quenching and high-temperature tempering mode, the strength is improved, and meanwhile, the toughness can meet the requirement and the use requirement is met. However, the yield ratio of the high-strength steel produced in the mode is generally over 0.90, and the high yield ratio has certain influence on the safety of the industries such as bridges, constructional engineering and the like. In recent years, the reduction of the yield ratio of quenched and tempered high-strength steel is becoming one of the important technological improvements of large steel enterprises.
Chinese patent document CN103952643A discloses a method for manufacturing a steel sheet with a yield strength of 690MPa grade and a low yield ratio. The invention adopts a low-carbon high-Mn design (C: 0.04-0.10%, Mn: 1.4-1.9%), and TMCP on-line cooling mode for production. The microstructure of the steel plate mainly comprises acicular ferrite, quasi-polygonal ferrite and lath bainite, and the yield ratio is less than or equal to 0.83. However, the steel plates delivered by TMCP have the problems of large head-tail performance difference, uneven cooling, poor whole plate performance uniformity, large residual stress and easy deformation during cutting. Meanwhile, due to the existence of a multi-phase structure, each phase proportion is difficult to control accurately, and the performance difference of different batches of steel plates can be caused.
Chinese patent document CN103422025A adopts an ultra-low carbon design (C: 0.01-0.05%) to produce low yield ratio high-strength steel with yield strength of 690MPa grade, and the yield ratio is less than or equal to 0.83. In order to improve the strength of steel, the types and the content of alloy are increased, so that the cost of raw materials is increased, wherein the content of Mn reaches 2.1-3.5%, and the content of Cr reaches 0.51-0.80%. The high Mn content brings great difficulty to steel-making continuous casting, and the high Mn content has the risk of a large amount of MnS strip-shaped precipitation, and the problems of performance uniformity and cutting deformation also exist when the TMCP process is adopted for production.
In summary, the current production technology related to a high-strength steel plate with a yield strength of 690MPa mainly comprises quenching and tempering or online TMCP; the yield ratio of the traditional quenching and tempering process is higher than 0.90; the TMCP process adopts a low-carbon (C is less than 0.10%) high-alloy design, the yield ratio is low, but the alloy cost is high, the uniformity of the integral performance is poor, and the residual stress is large.
Disclosure of Invention
The invention aims to provide a quenched and tempered high-strength steel plate with a yield ratio of less than or equal to 0.88 and a yield strength of more than or equal to 690MPa and a production method thereof, wherein the microstructure of the steel plate is a tempered sorbite and bainite complex phase structure.
The technical scheme of the invention is as follows: the chemical components of the 690 MPa-yield-strength low-yield-ratio high-strength steel plate are as follows by mass percent: 0.10 to 0.20%, Si: 0.10 to 0.40%, Mn: 0.90-1.40%, Nb: less than or equal to 0.030 percent, V: less than or equal to 0.010 percent, less than or equal to 0.025 percent of Ti, Al: 0.02-0.06%, Ni: less than or equal to 0.10 percent, Cu: less than or equal to 0.10 percent, Cr: 0.10-0.80%, Mo: 0.10-0.70%, B: 0.001 to 0.005%, Ca: 0.001-0.005%, P: less than or equal to 0.015 percent, S: less than or equal to 0.003 percent, O: less than or equal to 0.002%, N: less than or equal to 0.004%, H: less than or equal to 0.0002 percent, and the balance of Fe and inevitable impurity elements; carbon equivalent CEV: less than or equal to 0.56 percent.
The production thickness of the low-yield-ratio high-strength steel plate is not more than 50 mm.
The reasons for the limitation of the elemental composition in the steel of the invention are explained below:
c: carbon as a solid solution element can significantly improve the strength of the steel sheet, but the toughness, plasticity, cold formability, and weldability of the steel sheet are adversely affected. Based on the requirements of steel plate strength and toughness matching and weldability, the carbon content is controlled to be 0.10-0.20%.
Si: plays a role of solid solution strengthening in the steel. But the toughness of the martensite high-strength steel is deteriorated due to the excessively high Si content, and the cold crack sensitivity of the steel plate is increased, wherein the content is controlled to be 0.10-0.40%.
Mn: the steel has improved hardenability, promotes martensitic transformation, and has improved strength. However, Mn is a main segregation element, and excessively high content of Mn causes central segregation of a continuous casting slab to form MnS, which adversely affects toughness, lamellar tearing resistance and weldability of a steel plate, and particularly reduces uniformity of the performance of the whole plate. The invention limits the manganese content to 0.90-1.40%.
Nb/V/Ti: the microalloying element, C \ N compound thereof, on one hand, hinders the growth of austenite grains in the heating process and plays a role in refining the grains; on the other hand, the strength of the steel plate can be obviously improved. However, the Nb/V/Ti C \ N compound is precipitated, especially V (C/N) is precipitated by tempering at about 600 ℃, so that the yield strength of the steel plate can be obviously improved, and the yield ratio is improved. The strength of the steel plate is achieved through the design of structure strengthening and solid solution strengthening, so that the adding amount of Nb/Ti is lower, and the content of Nb is less than or equal to 0.030 percent and the content of Ti is less than or equal to 0.025 percent; the addition of V is cancelled, and V is regulated to be less than or equal to 0.010 percent.
Al: deoxidizing and refining the grain elements. However, too high Al content results in the formation of too many Al2O3 inclusions, which affect the toughness of the steel sheet. The Al content is 0.02-0.06%.
Cr: is an element that improves the hardenability of steel, suppresses the formation of polygonal ferrite and pearlite, promotes the transformation of low-temperature structure bainite or martensite, and improves the strength of steel. However, too high Cr content affects the toughness of steel and reduces the weldability of steel sheets. The chromium content is controlled to be 0.10-0.80%.
Mo: is an element for improving the hardenability of steel and is beneficial to the formation of martensite during quenching. The addition of a certain amount of Mo increases the strength of the steel sheet without affecting the low-temperature impact properties of the steel sheet. Mo increases the tempering resistance of the steel sheet, and can ensure that the steel sheet does not reduce the strength at higher temperature. In the invention, the content of Mo is controlled to be 0.10-0.70%.
B: the invention adds 0.001-0.005% of trace B, and mainly aims to improve the hardenability of the steel plate, thereby reducing the addition of other precious metals and lowering the cost. More than 0.005% of B easily causes segregation to form boride, seriously deteriorates the toughness of the steel sheet and lowers hardenability.
Ca: the inclusion modified elements can react with long-strip MnS to generate spherical CaS, change the anisotropy of the steel plate and deoxidize Al to generate Al2O3The inclusion denaturation is spherical low-melting-point inclusion, so that the inclusion is promoted to float upwards and be removed, and the impact toughness of the steel plate is improved.
P, S: sulfur and phosphorus are harmful elements of steel grades, are easy to segregate, have adverse effects on the plasticity and toughness of materials, have high P, S content, and are easy to cause layered cracking of the steel plates in the thickness direction. The invention specifies that P: less than or equal to 0.015 percent, S: less than or equal to 0.003 percent.
O, N, H: harmful gas elements, high content and more inclusions, reduce the plasticity and toughness of the steel plate and have high cracking risk. The invention strictly controls the O content not to be higher than 0.002%; the content of N is not higher than 0.004%; the H content is not higher than 0.0002%.
CEV: the invention adopts a carbon equivalent formula CEV which is C + Mn/6+ (Cr + Mo + V)/5+ (Ni + Cu)/15; the carbon equivalent has a great influence on the strength and weldability of steel. CEV has high strength but reduced weldability; the invention controls the CEV to be less than or equal to 0.56.
Further, the preparation method of the low-yield-ratio high-strength steel plate comprises the following steps:
the smelting and continuous casting process comprises the steps of carrying out molten iron pretreatment, smelting in an electric furnace or a converter mode, then sending into an L F refining furnace for refining, carrying out VD or RH vacuum treatment, carrying out trace Ca treatment after molten steel degassing, carrying out continuous casting by adopting low superheat degree pouring, carrying out whole-course argon protection pouring, and controlling under dynamic soft pressure, wherein the superheat degree of the molten steel is controlled to be 5-20 ℃, the central segregation is not higher than B1.0 level, and the central porosity is not higher than 1.0 level.
The heating rolling process comprises the following steps: the casting blank is put into a step-by-step heating furnace and heated and homogenized within the range of higher temperature of 1200-1300 ℃, the heating time is 8-15min/cm, and the alloy elements in the steel are fully dissolved to ensure the uniformity of the components and the performance of the final product. And after the steel billet is taken out of the furnace, carrying out high-pressure water descaling treatment and then carrying out two-stage controlled rolling of rough rolling and finish rolling. The rolling temperature of the rough rolling recrystallization zone is controlled at 1000-1100 ℃. The thickness of the intermediate blank to be heated is more than or equal to 2.5 times of the thickness of the finished steel plate. The initial rolling temperature of finish rolling is controlled to be 850-940 ℃, the final rolling temperature is controlled to be 760-840 ℃, fine original austenite grains are ensured to be finally obtained by the steel plate, and the strength and the impact toughness of the steel plate are improved.
Quenching process: the steel plate is subjected to quenching heat treatment twice.
The first quenching is carried out at 900-940 ℃; heating the steel plate to a temperature, and then preserving heat for 30-60 min; followed by quenching (water quenching) to room temperature. The first quenching temperature is higher than Ac3 temperature, so that the steel plate is ensured to obtain a full martensite structure after quenching, and the high strength of the steel plate is ensured.
In order to obtain a low yield ratio, the steel sheet needs to be quenched for a second time. The quenching heating temperature is 760-840 ℃, and the temperature is kept for 30-60 min. The temperature interval is in a two-phase region between Ac1 and Ac3, and the complex phase structure of martensite and bainite is obtained after quenching (water quenching). The bainite in the second quenching ensures the toughness and reduces the yield ratio of the original martensite steel plate obtained by the first quenching.
And (3) tempering process: the tempering temperature of the steel plate is 560-630 ℃, and the heat preservation time is 30-90 min after the furnace temperature reaches the temperature. The temperature control precision is +/-10 ℃ to ensure the uniformity of the steel plate.
Compared with the prior art, the invention has the advantages that:
the method optimizes the chemical components, adopts the medium carbon (C: 0.10-0.20%) quenching and tempering component design, takes Cr and Mo solid solution strengthening elements as main components, reduces the content of Nb/Ti precipitation strengthening elements, cancels the addition of V elements, avoids the yield ratio increase caused by precipitation strengthening in the high-temperature tempering process, and simultaneously reduces the alloy cost.
The invention adopts the off-line two-time quenching and tempering process to carry out heat treatment, and the steel plate structure is tempered into a tempered sorbite and bainite complex phase structure. The off-line heat treatment process ensures the whole plate uniformity of the steel plate. The bainite ensures the toughness of the steel plate and reduces the yield ratio.
The yield strength Rp0.2 of the steel plate is more than or equal to 690MPa, the tensile strength Rm is more than or equal to 770MPa, and the yield ratio Rp0.2/Rm is less than or equal to 0.88; the elongation A is more than or equal to 16 percent; the low-temperature impact Akv at minus 40 ℃ is more than or equal to 100J.
The method can be popularized and applied to other high-strength steel plates, such as high-strength marine ship plate steel, high-rise building steel, bridge steel, engineering machinery steel, pressure vessel steel and the like.
Drawings
FIG. 1 is a typical microstructure diagram of an example 1 of the present invention.
Detailed Description
The present invention is described in further detail below with reference to examples, which are intended to be illustrative and not to be construed as limiting the invention.
The production process flow of the 690 MPa-grade steel plate with the low yield ratio comprises the steps of converter or electric furnace steelmaking- > L F refining- > VD or RH vacuum degassing- > Ca treatment- > continuous casting- > heating- > rolling- > primary quenching- > secondary quenching- > tempering.
The manufacturing method of the 690 MPa-yield-strength low-yield-ratio high-strength steel plate in the embodiment 1-2 of the invention adopts the following steps:
(1) continuous casting of steel making, smelting in a 150 ton converter, then refining in an L F furnace, RH vacuum degassing treatment, Ca treatment after vacuum breaking, casting the molten steel into a continuous casting slab with the thickness of 370mm, controlling the casting temperature to be 5-20 ℃ above the liquidus, and implementing dynamic soft reduction in the casting process to ensure that the central segregation is not higher than B1.0 level and the central porosity is not higher than 1.0 level, wherein the final components of the molten steel smelted in the two examples are controlled as shown in Table 1.
(2) Rolling: and (3) putting the continuous casting slab obtained in the step (1) into a stepping heating furnace, heating the whole body to 1200-1300 ℃, and keeping the temperature for 8-14min/cm, so that alloy elements in the steel are fully dissolved in solid to ensure the uniformity of the components and the performance of a final product. And after the steel billet is taken out of the furnace, carrying out high-pressure water descaling treatment and then carrying out two-stage controlled rolling of rough rolling and finish rolling. The rolling temperature of the rough rolling recrystallization zone is controlled at 1000-1100 ℃; the temperature-waiting thickness of the intermediate blank is more than or equal to 2.5 times of the thickness of the finished steel plate; the initial rolling temperature of finish rolling is controlled to be 850-940 ℃, the final rolling temperature is controlled to be 760-840 ℃, and fine original austenite grains are finally obtained from the steel plate through controlled rolling, so that the strength and the impact toughness of the steel plate are improved. The specific hot rolling process parameters are shown in Table 2.
(3) Quenching for the first time: and (3) heating the rolled steel plate obtained in the step (2) to 900-940 ℃, preserving heat for 30-60 min, and quenching to room temperature by using water as a quenching medium. The steel plate after quenching obtains a full martensite structure, thereby ensuring the high strength of the steel plate.
(4) And (3) quenching for the second time: and (4) heating the first quenched steel plate obtained in the step (3) to 760-840 ℃, preserving heat for 30-60 min, and performing second quenching, wherein a quenching medium is water. The steel plate after quenching is a martensite + bainite complex phase structure. The bainite ensures the toughness and reduces the yield ratio of the original martensite steel plate.
(5) Tempering: the tempering temperature of the steel plate is 560-630 ℃, and the heat preservation time is 30-90 min after the furnace temperature reaches the temperature.
The specific components and process parameters are shown in tables 1-3. The detection properties of the samples obtained in the respective examples are shown in Table 4.
FIG. 1 shows a schematic microstructure of the steel sample of example 1. The microstructure of the finished steel plate is a tempered sorbite and bainite complex phase structure, and the crystal grains are fine.
The invention adopts a high-cleanliness steelmaking continuous casting process, controls rolling, secondary off-line quenching and tempering processes, controls the chemical composition design, the base metal structure, the rolling deformation, the quenching and tempering temperature and time and other angles, ensures that the steel plate has high strength and toughness, has lower yield ratio, and can be widely applied to the manufacture of large-scale engineering mechanical equipment or steel structures with strict requirements on the shock resistance, such as high-rise buildings, bridges, coal mine hydraulic supports, ocean platforms and the like.
Table 1 chemical composition (wt%) of super strength steel plate of example
TABLE 2 Rolling Process control
TABLE 3 Heat treatment Process control
TABLE 4 tensile and impact properties of the examples of the invention
Claims (5)
1. A low yield ratio high strength steel plate with 690MPa grade yield strength is characterized in that: the steel plate comprises the following chemical components in percentage by mass: 0.10 to 0.20%, Si: 0.10 to 0.40%, Mn: 0.90-1.40%, Nb: less than or equal to 0.030 percent, V: less than or equal to 0.010 percent, less than or equal to 0.025 percent of Ti, Al: 0.02-0.06%, Ni: less than or equal to 0.10 percent, Cu: less than or equal to 0.10 percent, Cr: 0.10-0.80%, Mo: 0.10-0.70%, B: 0.001 to 0.005%, Ca: 0.001-0.005%, P: less than or equal to 0.015 percent, S: less than or equal to 0.003 percent, O: less than or equal to 0.002%, N: less than or equal to 0.004%, H: not more than 0.0002 percent, and the balance of Fe and inevitable impurity elements, and the microstructure is a tempered sorbite and bainite complex phase structure.
2. The 690MPa grade low yield ratio high strength steel plate of yield strength of claim 1, wherein: carbon equivalent CEV of steel sheet: less than or equal to 0.56 percent, CEV is C + Mn/6+ (Cr + Mo + V)/5+ (Ni + Cu)/15.
3. The 690MPa grade low yield ratio high strength steel plate of yield strength of claim 1, wherein: the production thickness of the steel plate is less than or equal to 50 mm.
4. The 690MPa grade low yield ratio high strength steel plate of yield strength of claim 1, wherein: the yield strength Rp0.2 is more than or equal to 690MPa, the tensile strength Rm is more than or equal to 770MPa, and the yield ratio Rp0.2/Rm is less than or equal to 0.88; the elongation A is more than or equal to 16 percent; the low-temperature impact Akv at minus 40 ℃ is more than or equal to 100J.
5. A method for manufacturing a low yield ratio high strength steel plate with a yield strength of 690MPa grade according to any one of claims 1 to 4, wherein: comprises the following steps
(1) Smelting and continuous casting, wherein molten iron is pretreated, smelted by an electric furnace or a converter, then sent into an L F refining furnace for refining, and then subjected to vacuum degassing treatment by a VD furnace or an RH furnace, a calcium-silicon wire is fed for Ca treatment after the molten steel is degassed, the superheat degree of the molten steel is controlled to be 5-20 ℃ during continuous casting, the molten steel is poured under argon protection in the whole process, the molten steel is controlled under dynamic soft pressure, and a blank with the center segregation not higher than B1.0 level and the center porosity not higher than 1.0 level is taken as a qualified blank;
(2) heating and rolling: reheating a casting blank, starting heat preservation when the temperature of the whole casting blank reaches 1200-1300 ℃, wherein the heat preservation time is 8-15min/cm, performing high-pressure water descaling treatment after the steel blank is discharged from a furnace, and performing two-stage controlled rolling of rough rolling and finish rolling: the rolling temperature range of the rough rolling recrystallization zone is 1000-1100 ℃, the thickness of the intermediate blank to be rolled is more than 2.5 times of the thickness of the steel plate finished product, the finish rolling start temperature is controlled to be 850-940 ℃, the finish rolling temperature is controlled to be 760-840 ℃, and fine original austenite grains are obtained on the steel plate after finish rolling;
(3) quenching process: quenching the steel plate twice, wherein the first quenching temperature is 900-940 ℃ higher than Ac3, heating the steel plate to the temperature, then keeping the temperature for 30-60 min, quenching to the room temperature, and obtaining a full martensite structure after the first quenching; the secondary quenching temperature is 760-840 ℃, the secondary quenching temperature is in a two-phase region between Ac1 and Ac3, the temperature is kept for 30-60 min, and a complex phase structure of martensite and bainite is obtained after quenching;
(4) and (3) tempering process: the tempering temperature of the steel plate is 560-630 ℃, and the heat preservation time is 30-90 min after the furnace temperature reaches the temperature.
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