WO2023162571A1 - Steel plate and method for manufacturing same - Google Patents

Steel plate and method for manufacturing same Download PDF

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Publication number
WO2023162571A1
WO2023162571A1 PCT/JP2023/002491 JP2023002491W WO2023162571A1 WO 2023162571 A1 WO2023162571 A1 WO 2023162571A1 JP 2023002491 W JP2023002491 W JP 2023002491W WO 2023162571 A1 WO2023162571 A1 WO 2023162571A1
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steel sheet
hardness
steel
content
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PCT/JP2023/002491
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French (fr)
Japanese (ja)
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恭野 安田
和彦 塩谷
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Jfeスチール株式会社
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Priority to JP2023523653A priority Critical patent/JPWO2023162571A1/ja
Priority to KR1020247023941A priority patent/KR20240124991A/en
Priority to AU2023223720A priority patent/AU2023223720A1/en
Priority to EP23759577.2A priority patent/EP4458993A1/en
Priority to CN202380020301.8A priority patent/CN118647744A/en
Publication of WO2023162571A1 publication Critical patent/WO2023162571A1/en

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    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226Hot rolling
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    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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    • C21D9/00Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • C21D2211/00Microstructure comprising significant phases
    • C21D2211/008Martensite

Definitions

  • the present invention relates to a high-strength steel sheet with excellent toughness and corrosion resistance, particularly suitable for structural members such as tanks used in a low-temperature, liquid-ammonia environment.
  • the present invention relates to a steel sheet for low temperature use and a method for manufacturing the same.
  • tanks may carry liquid ammonia as well as LPG.
  • hydrogen carriers for liquid ammonia and liquid ammonia fuel has been promoted, and thus the size of tanks for transportation and storage of liquefied ammonia has been increased.
  • ammonia SCC Stress Corrosion Cracking
  • Patent Documents 1 and 2 disclose techniques for satisfying the low-temperature toughness and the predetermined strength range required for liquefied gas storage tanks as described above.
  • high low-temperature toughness and predetermined strength properties are achieved by heat-treating a steel plate that has been cooled after hot-rolling, or by heat-treating a steel plate that has been water-cooled after hot-rolling several times. is realized.
  • Patent Literatures 1 and 2 above had the economic problem of requiring multiple heat treatments, which required high equipment and energy costs.
  • the present invention solves the above problems and provides a high-strength steel sheet with excellent ammonia SCC resistance and low-temperature toughness, which is used for storage tanks used for storing liquefied gas in energy transport ships, and a method for producing the same. for the purpose.
  • the present inventors used the TMCP process and an online induction heating device to extensively study various factors affecting the low-temperature toughness and strength characteristics of steel sheets.
  • the steel sheet contains a predetermined amount of elements such as C, Si, Mn, and Al, and the metal structure is controlled so that the volume fraction of the bainite structure at a position 0.5 mm from the surface of the steel sheet is 90% or more.
  • the average hardness is 230 HV0.1 or less
  • the variation in hardness is 30 HV0.1 or less
  • the maximum hardness in the thickness direction is the surface of the steel sheet
  • the present invention was made based on the above findings, and the gist of the present invention is as follows. 1. in % by mass, C: 0.010 to 0.200%, Si: 0.01 to 0.50%, Mn: 0.50-2.50%, Al: 0.010 to 0.060%, N: 0.0010% or more and 0.0100% or less, P: 0.020% or less, A steel sheet having a chemical composition containing S: 0.0100% or less and O: 0.0100% or less, with the balance being Fe and inevitable impurities, At a depth of 0.5 mm from the surface of the steel sheet, the average hardness is 230 HV0.1 or less, the hardness variation is 30 HV0.1 or less, and the maximum hardness in the thickness direction is the steel sheet.
  • the component composition further, in mass %, Cu: 0.01-0.50%, Ni: 0.01 to 2.00%, Cr: 0.01 to 1.00%, Sn: 0.01 to 0.50%, Sb: 0.01 to 0.50%, Mo: 0.01-0.50% and W: 0.01-1.00% 2.
  • the component composition further, in mass %, V: 0.01 to 1.00%, Ti: 0.005 to 0.100%, Co: 0.01 to 1.00%, Nb: 0.005 to 0.100%, B: 0.0001 to 0.0100%, Ca: 0.0005 to 0.0200%, Mg: 0.0005-0.0200% and REM: 0.0005-0.0200% 3.
  • V 0.01 to 1.00%
  • Ti 0.005 to 0.100%
  • Co 0.01 to 1.00%
  • Nb 0.005 to 0.100%
  • B 0.0001 to 0.0100%
  • Ca 0.0005 to 0.0200%
  • Mg 0.0005-0.0200%
  • REM 0.0005-0.0200% 3.
  • a method for producing a steel sheet comprising : In the accelerated cooling, the cooling stop temperature is in the range of 200 to 600 ° C., and the cooling rate at the 1/4 position of the plate thickness of the steel plate is 20 to 120 ° C./s, The reheating is performed until the temperature reached at a position 1/4 of the plate thickness of the steel plate is 500 ° C. or less, and the temperature reached at a depth of 0.5 mm from the surface of the steel plate is in the range of 400 to 680 ° C.
  • the steel plate manufacturing method.
  • the chemical composition of the steel material is further, in mass%, Cu: 0.01-0.50%, Ni: 0.01 to 2.00%, Cr: 0.01 to 1.00%, Sn: 0.01 to 0.50%, Sb: 0.01 to 0.50%, Mo: 0.01-0.50% and W: 0.01-1.00% 4.
  • the chemical composition of the steel material is further, in mass%, V: 0.01 to 1.00%, Ti: 0.005 to 0.100%, Co: 0.01 to 1.00%, Nb: 0.005 to 0.100%, B: 0.0001 to 0.0100%, Ca: 0.0005 to 0.0200%, 6.
  • a steel sheet having excellent low-temperature toughness that is, low-temperature impact resistance and ammonia SCC resistance, and having high strength suitable for structural members such as tanks used in a low-temperature and liquid ammonia environment.
  • a steel sheet having excellent low-temperature toughness that is, low-temperature impact resistance and ammonia SCC resistance, and having high strength suitable for structural members such as tanks used in a low-temperature and liquid ammonia environment.
  • % representing the content of the following components (elements) means “% by mass” unless otherwise specified.
  • C 0.010-0.200% C is the most effective element for increasing the strength of steel sheets produced by cooling according to the present invention.
  • the C content is specified to be 0.010% or more.
  • the C content is preferably 0.013% or more from the viewpoint of reducing the content of other alloying elements and manufacturing at a lower cost.
  • the C content is specified at 0.200% or less.
  • the C content is preferably 0.170% or less from the viewpoint of toughness and weldability.
  • Si 0.01-0.50% Si is added for deoxidation.
  • the Si content is specified to be 0.01% or more. Furthermore, it is preferable to make it 0.03% or more.
  • the Si content is specified to be 0.50% or less. Furthermore, the Si content is preferably 0.40% or less from the viewpoint of toughness and weldability.
  • Mn 0.50-2.50%
  • Mn is an element that has the effect of increasing the hardenability of steel, and is one of the important elements that need to be added in order to achieve high strength as in the present invention.
  • the Mn content is specified to be 0.50% or more.
  • the content of Mn is preferably 0.70% or more from the viewpoint of reducing the content of other alloying elements and manufacturing at a lower cost.
  • the Mn content is specified at 2.50% or less.
  • the Mn content is preferably 2.30% or less from the viewpoint of suppressing deterioration of toughness and weldability.
  • Al acts as a deoxidizing agent.
  • the Al content is specified to be 0.010% or more.
  • the Al content exceeds 0.060%, the oxide inclusions increase to lower the cleanliness and toughness. Therefore, the Al content is specified at 0.060% or less.
  • the Al content is preferably 0.050% or less from the viewpoint of further preventing toughness deterioration.
  • N 0.0010 to 0.0100% N contributes to the refinement of the structure and improves the toughness of the steel sheet.
  • the N content is specified to be 0.0010% or more. Preferably, it is 0.0020% or more.
  • the N content is specified at 0.0100% or less.
  • the N content is preferably 0.0080% or less from the viewpoint of further suppressing deterioration of toughness and weldability.
  • Ti when Ti is present, N can bond with Ti and precipitate as TiN.
  • P 0.020% or less
  • P has an adverse effect, such as lowering toughness and weldability, by segregating at grain boundaries. Therefore, it is desirable to make the P content as low as possible, but a P content of 0.020% or less is acceptable.
  • the lower limit of the P content is not particularly limited, and may be 0%. However, since P is an element that can industrially remain in steel, it may exceed 0%. Moreover, since excessive reduction causes a rise in refining cost, it is preferable to set the P content to 0.0005% or more from the viewpoint of cost.
  • S 0.0100% or less S is present in steel as sulfide-based inclusions such as MnS, and is an element that exerts adverse effects, such as deteriorating the toughness of the steel sheet by becoming the origin of fracture. Therefore, it is desirable that the S content be as low as possible, but a content of 0.0100% or less is permissible.
  • the lower limit of the S content is not particularly limited, and may be 0%. However, since S is an element that can industrially remain in steel, it may exceed 0%. Moreover, since an excessive reduction causes a rise in refining cost, it is preferable to set the S content to 0.0005% or more from the viewpoint of cost.
  • O 0.0100% or less
  • O is an element that forms an oxide, becomes a starting point of fracture, and has an adverse effect such as lowering the toughness of the steel sheet.
  • the O content is preferably 0.0050% or less, more preferably 0.0030% or less.
  • the lower limit of the O content is not particularly limited, and may be 0%. However, since O is an element that can industrially remain in steel, it may exceed 0%. Moreover, since an excessive reduction causes a rise in refining cost, it is preferable to set the O content to 0.0010% or more from the viewpoint of cost.
  • the balance other than the above components is Fe and unavoidable impurities.
  • the above component composition can contain the elements described below, if necessary.
  • Cu 0.01-0.50%, Ni: 0.01-2.00%, Cr: 0.01-1.00%, Sn: 0.01-0.50%, Sb: 0.01- 0.50%, Mo: 0.01 to 0.50%, and W: one or more selected from 0.01 to 1.00% Cu, Ni, Cr, Sn, Sb, Mo and W are It is an element that improves strength and ammonia SCC resistance, and one or more of these elements can be contained.
  • the Cu content is 0.01% or more, when Ni is contained, the Ni content is 0.01% or more, and when Cr is contained, When the Cr content is 0.01% or more, the Sn content is 0.01% or more when Sn is contained, and the Sb content is 0.01% or more when Sb is contained.
  • Mo is contained, the Mo content is preferably adjusted to 0.01% or more, and when W is contained, the W content is preferably adjusted to 0.01% or more.
  • an excessive Ni content causes deterioration of weldability and an increase in alloy cost.
  • the Cu content is 0.50% or less
  • the Ni content is 2.00% or less
  • the Cr content is 1.00% or less
  • the Sn content is 0.50% or less
  • the Sb content is It is preferable to adjust the Mo content to 0.50% or less
  • the W content is 0.50% or less
  • the W content is 1.00% or less.
  • the Cu content is 0.40% or less
  • the Ni content is 1.50% or less
  • the Cr content is 0.80% or less
  • the Sn content is 0.40% or less
  • the Sb content is The amount is adjusted to 0.40% or less
  • the Mo content to 0.40% or less
  • the W content to 0.80% or less.
  • V 0.01-1.00%
  • V is an element that has the effect of improving the strength of the steel sheet, and can be optionally added.
  • the V content is preferably 0.01% or more.
  • the V content is preferably 1.00% or less. More preferably, the lower limit of V content is 0.05% and the upper limit is 0.50%.
  • Ti 0.005-0.100%
  • Ti is an element that has a strong tendency to form nitrides and has the action of fixing N and reducing solid solution N, and can be added arbitrarily.
  • Ti can improve the toughness of the base material and the weld zone.
  • the Ti content is preferably 0.005% or more. Furthermore, it is more preferable to make it 0.007% or more.
  • the Ti content exceeds 0.100%, the toughness rather decreases. Therefore, when adding Ti, the Ti content is preferably 0.100% or less. Furthermore, the Ti content is more preferably 0.090% or less.
  • Co 0.01-1.00%
  • Co is an element that has the effect of improving the strength of the steel sheet, and can be optionally added.
  • the Co content is preferably 0.01% or more.
  • the Co content is preferably 1.00% or less. More preferably, the Co content has a lower limit of 0.05% and an upper limit of 0.50%.
  • Nb 0.005-0.100%
  • Nb is an element that has the effect of reducing the grain size of prior austenite and improving the toughness by precipitating as a carbonitride.
  • the Nb content is preferably 0.005% or more. Furthermore, it is more preferable to make it 0.007% or more.
  • the Nb content exceeds 0.100%, a large amount of NbC precipitates, resulting in a decrease in toughness. Therefore, when Nb is added, the Nb content is preferably 0.100% or less. Furthermore, it is more preferable to make it 0.060% or less.
  • B 0.0001 to 0.0100%
  • B is an element that has the effect of significantly improving hardenability even when added in a very small amount. That is, the strength of the steel sheet can be improved.
  • the B content is preferably 0.0001% or more.
  • the B content exceeds 0.0100%, the weldability deteriorates. Therefore, when B is added, the B content is preferably 0.0100% or less. More preferably, the B content has a lower limit of 0.0010% and an upper limit of 0.0030%.
  • Ca 0.0005-0.0200%
  • Ca is an element that binds to S and has the effect of suppressing the formation of MnS or the like elongated in the rolling direction. That is, by adding Ca, it is possible to control the morphology of the sulfide-based inclusions so that they exhibit a spherical shape, and improve the toughness of the weld zone and the like.
  • the Ca content is preferably 0.0005% or more.
  • the Ca content exceeds 0.0200%, the cleanliness of the steel is lowered. A decrease in cleanliness leads to a decrease in toughness. Therefore, when Ca is added, the Ca content is preferably 0.0200% or less. More preferably, the Ca content has a lower limit of 0.0020% and an upper limit of 0.0100%.
  • Mg: 0.0005-0.0200% Mg, like Ca is an element that binds to S and has the effect of suppressing the formation of MnS or the like elongated in the rolling direction. That is, by adding Mg, it is possible to control the morphology of the sulfide-based inclusions so that they exhibit a spherical shape, and improve the toughness of the weld zone and the like. In order to obtain such an effect, when Mg is added, the Mg content is preferably 0.0005% or more. On the other hand, when the Mg content exceeds 0.0200%, the cleanliness of the steel is lowered. A decrease in cleanliness leads to a decrease in toughness. Therefore, when Mg is added, the Mg content is preferably 0.0200% or less. More preferably, the Mg content has a lower limit of 0.0020% and an upper limit of 0.0100%.
  • REM 0.0005-0.0200%
  • REM rare earth metal
  • the REM content is preferably 0.0005% or more.
  • the REM content exceeds 0.0200%, the cleanliness of the steel deteriorates. A decrease in cleanliness leads to a decrease in toughness. Therefore, when REM is added, the REM content is preferably 0.0200% or less. More preferably, the REM content has a lower limit of 0.0020% and an upper limit of 0.0100%.
  • the steel sheet of the present invention has the above chemical composition, and in addition, the average The hardness is 230 HV0.1 or less, the variation in hardness at the 0.5 mm position is 30 HV0.1 or less, and the maximum hardness in the thickness direction is 1.0 mm or more from the surface of the steel sheet. 4 or less, and has a hardness characteristic in which the variation in hardness in the plate thickness direction is 70HV1 or less. Further, the steel sheet of the present invention has a metal structure in which the volume fraction of bainite structure (hereinafter also simply referred to as bainite) at the 0.5 mm position is 90% or more.
  • bainite volume fraction of bainite structure
  • the average hardness is 230HV0.1 or less, and the variation in hardness is 30HV0.1 or less
  • the average hardness is set to 230 HV0.1 or less
  • the variation in hardness is set to 30 HV0.1 or less.
  • the average hardness at the 0.5 mm position is 230 HV0.1 or less, and the hardness variation is 30 HV0.1 or less by adjusting the hardness characteristics.
  • the lower limit of the average hardness at the 0.5 mm position is not particularly limited, it is preferably about 130HV0.1.
  • the lower limit of the variation in hardness at the 0.5 mm position may be 0HV0.1, but industrially it is about 10HV0.1.
  • the average hardness can be calculated by measuring Vickers hardness at a plurality of points (for example, 100 points) at a position of 0.5 mm. Further, the variation in hardness means the standard deviation of the Vickers hardness measured to obtain the average hardness.
  • the maximum hardness in the plate thickness direction is located at a position of 1.0 mm or more and 1/4 or less of the plate thickness from the surface of the steel plate]
  • the maximum hardness of the steel sheet exists at a position distant from the surface to some extent, the hardness of only the surface layer can be reduced while maintaining the hardness of most of the steel sheet. That is, it is possible to ensure excellent ammonia SCC resistance while maintaining the strength of the steel sheet.
  • the maximum value is located at a position less than 1.0 mm from the surface of the steel sheet, the hardness at the 0.5 mm position cannot be sufficiently reduced.
  • the maximum value is at a position exceeding 1/4 of the plate thickness from the surface of the steel plate, the steel plate itself cannot ensure sufficient strength.
  • the maximum value of the hardness in the plate thickness direction is defined as being at a position of 1.0 mm or more and 1/4 or less of the plate thickness from the surface of the steel plate. .
  • the variation in hardness in the plate thickness direction is specified to be 70HV1 or less.
  • the variation is calculated by measuring the Vickers hardness (HV1) at a pitch of 0.5 mm in the plate thickness direction and obtaining the difference between the maximum value and the minimum value.
  • bainite includes a structure called bainitic ferrite or granular ferrite that transforms during or after accelerated cooling that contributes to transformation strengthening, and a structure obtained by tempering them.
  • the remaining structure occupying 10% or less in volume fraction may include a martensite structure in addition to the ferrite, pearlite, and austenite structures.
  • the fraction of each structure in the remaining structure is not particularly limited, but the remaining structure is preferably a pearlite structure.
  • the volume fraction of various metal structures can be measured by the method described in Examples below.
  • the manufacturing conditions of the steel material need not be particularly limited. It is preferable to use a steel material such as a slab of predetermined dimensions in the method. It should be noted that there is no problem in making a steel material such as a slab having a predetermined size by the ingot casting-decomposition rolling method.
  • the steel material thus obtained is directly hot-rolled without cooling or hot-rolled after reheating.
  • Hot rolling is performed with a rolling end temperature equal to or higher than the Ar 3 transformation point, then accelerated cooling from a cooling start temperature equal to or higher than the Ar 3 transformation point is performed under predetermined conditions, and then reheating is performed under predetermined conditions.
  • the heating temperature of the steel material is not particularly limited, but if the heating temperature is too low, the deformation resistance increases, the load on the hot rolling mill increases, and hot rolling may become difficult. On the other hand, if the temperature exceeds 1300° C., the oxidation becomes significant, the oxidation loss increases, and the yield increases. For these reasons, the heating temperature is preferably 950° C. or higher and 1300° C. or lower.
  • hot rolling [Rolling end temperature: Ar 3 transformation point or higher]
  • the rolling end temperature is set to the Ar 3 transformation point or higher.
  • the rolling end temperature is more preferably Ar 3 transformation point +10°C or higher.
  • the rolling end temperature exceeds 950°C, the structure may coarsen and the toughness may deteriorate, so the rolling end temperature is preferably 950°C or less.
  • each element indicates the content of the element in steel (% by mass).
  • the hot-rolled steel sheet is subjected to accelerated cooling from a cooling start temperature equal to or higher than the Ar 3 transformation point. If the cooling start temperature is less than the Ar 3 transformation point, ferrite is excessively formed and the cooling rate increases, so it coexists with the martensitic structure or bainite, which has a large difference in strength, resulting in insufficient strength and deterioration of toughness. is generated, and further the ammonia SCC resistance deteriorates. Therefore, the cooling start temperature should be the Ar 3 transformation point or higher.
  • the cooling rate at the 1/4 position of the plate thickness of the steel plate is specified to be 120° C./s or less.
  • the cooling rate can be increased by active cooling operation such as water cooling, and can be controlled by intermittently performing the cooling operation as appropriate (providing a period in which the cooling operation is stopped). .
  • the temperature in the thickness cross section The distribution, particularly the temperature at the 1/4 position of the plate thickness, can be obtained in real time.
  • cooling stop temperature 200 to 600°C
  • a predetermined accelerated cooling is performed to a cooling stop temperature arbitrarily set in the range of 200 to 600 ° C., so that ferrite and bainite are reduced to a predetermined volume ratio at the center of the plate thickness. It is possible to improve strength and toughness satisfactorily.
  • the cooling stop temperature is less than 200° C., the volume fraction of the island-shaped martensite structure becomes too large, resulting in a decrease in toughness.
  • the cooling stop temperature exceeds 600° C., ferrite and pearlite structures are excessively formed, resulting in insufficient strength and deterioration of toughness. Therefore, the cooling stop temperature is specified in the range of 200 to 600°C. Further, the cooling stop temperature in the present invention is the temperature at the 1/4 position of the plate thickness of the steel plate.
  • reheat [Attainment temperature at 0.5 mm position from the surface is 400 to 680 ° C.]
  • Accelerated cooling of a thick steel plate increases the cooling rate of the surface layer of the steel plate and cools the surface layer of the steel plate to a lower temperature than the inside of the steel plate. Therefore, a hard structure such as martensite is likely to form in the surface layer of the steel sheet, and the ammonia SCC resistance may be deteriorated. Therefore, in the present invention, the surface layer portion of the steel sheet is reheated after accelerated cooling. This is because the hardness of the surface layer portion can be reduced. Preferably, reheating is performed immediately after accelerated cooling.
  • the temperature reached at a position 0.5 mm from the surface during reheating after accelerated cooling is specified in the range of 400 to 680.degree.
  • induction heating it is preferable to use induction heating as the reheating means after accelerated cooling.
  • high-frequency induction heating so that the heating concentrates on the surface layer of the steel sheet.
  • cooling can be performed as appropriate. Cooling after reheating is not particularly limited, but in a thick steel plate having a thickness exceeding about 40 mm, the cooling rate becomes slow, and there is a concern that toughness is deteriorated due to agglomeration and coarsening of carbides. In such a case, water cooling or mist cooling may be performed after the reheating treatment.
  • the steel sheet thus obtained has excellent strength characteristics and toughness, and is excellent in ammonia SCC resistance.
  • the excellent strength characteristics are yield strength YS (yield point YP when there is a yield point, 0.2% yield strength ⁇ 0.2 when there is no yield point): 450 MPa or more, tensile strength (TS): 570 MPa or more and uniform elongation (uEl): 10% or more.
  • excellent toughness means that vTrs conforming to JIS Z 2241 is -30°C or less.
  • a steel sheet having these properties is the steel sheet of the present invention having excellent ammonia SCC resistance.
  • any item not described in this specification can be used by a conventional method.
  • a slab is formed by continuous casting of steel having the chemical composition shown in Table 1 (steel grades A to AI, the balance being Fe and unavoidable impurities), and hot rolling, accelerated cooling, and reheating are sequentially performed under the conditions shown in Table 2, Thick steel plates (No. 1 to 50) with a thickness of 30 mm were obtained.
  • the obtained steel sheet was subjected to measurement of the metal structure fraction at a position of 0.5 mm from the surface of the steel sheet, evaluation of hardness characteristics, evaluation of strength characteristics and toughness, and evaluation of ammonia SCC resistance. Each test method is as follows. These results are also shown in Table 2.
  • the determination when obtaining the fraction of the metal structure of the sample was performed as follows. That is, in the photographed images described above, the polygonal ferrite is discriminated as ferrite, and the bainite (Table 2 It was determined as B) in
  • HV0.1 Vickers hardness
  • the Vickers hardness (HV1) was measured in the plate thickness direction, and the position in the plate thickness direction (distance from the surface) where the maximum value exists was measured. Further, the difference between the maximum value and the minimum value of Vickers hardness (HV1) in such measurement was calculated and used as variation in hardness in the plate thickness direction.
  • Ammonia SCC resistance was evaluated by an accelerated test in which a four-point bending test was performed in a test solution and constant potential anodic electrolysis was performed to promote corrosion. Specifically, we performed the following steps: A test piece having a thickness of 5 mm ⁇ 15 mm ⁇ 115 mm was taken from the surface of the steel plate, subjected to ultrasonic degreasing in acetone for 5 minutes, and stress equal to the yield strength of each steel plate was applied by four-point bending.
  • the invention examples all have a yield strength YS of 450 MPa or more, a tensile strength TS of 570 MPa or more, and a uniform elongation uEl of 10% or more, vTrs is -30 ° C. or less, A steel sheet excellent in low-temperature toughness and ammonia SCC resistance is obtained.
  • the chemical composition of the steel is outside the range of the present invention, so the yield strength YS, tensile strength TS, toughness at low temperatures, or ammonia SCC resistance are inferior.
  • the chemical composition of the steel may be considered as the chemical composition of the steel sheet.

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Abstract

Provided is a high-strength steel plate which has excellent ammonia SCC resistance and low-temperature toughness, and is used for storage tanks for storing liquefied gas in energy transport ships. The steel plate: has a predetermined composition; has hardness properties in which the average hardness is 230 HV0.1 or less and the hardness variation is 30 HV0.1 or less at a depth of 0.5 mm from the surface of the steel plate, the maximum hardness in the plate thickness direction is at a position of 1.0 mm or more and 1/4 or less of the plate thickness from the surface of the steel plate, and the hardness variation in the plate thickness direction is 70 HV1 or less; and has a metal structure in which the volume fraction of a bainite structure at a depth of 0.5 mm from the surface of the steel plate is 90% or more.

Description

鋼板およびその製造方法Steel plate and its manufacturing method
 本発明は、靭性および耐食性に優れた高強度鋼板、特に低温かつ液体アンモニア環境下で使用する、タンクなどの構造用部材に好適な、低温靱性および耐液体アンモニア応力腐食割れ性に優れた高強度低温用鋼板およびその製造方法に関するものである。 The present invention relates to a high-strength steel sheet with excellent toughness and corrosion resistance, particularly suitable for structural members such as tanks used in a low-temperature, liquid-ammonia environment. The present invention relates to a steel sheet for low temperature use and a method for manufacturing the same.
 近年のエネルギー需要の増加に伴い、エネルギー輸送船による液化ガスの輸送が盛んに行われている。エネルギー輸送船の効率的な運用のため、タンクにはLPGだけでなく液体アンモニアが共に運搬される場合がある。
 また、最近では、かかる液体アンモニアの水素キャリアや液体アンモニア燃料の利用が進められているため、液化アンモニアの輸送や貯蔵用タンクの大型化が図られている。
With the recent increase in energy demand, liquefied gas is being actively transported by energy transport ships. For efficient operation of energy carriers, tanks may carry liquid ammonia as well as LPG.
Recently, the use of hydrogen carriers for liquid ammonia and liquid ammonia fuel has been promoted, and thus the size of tanks for transportation and storage of liquefied ammonia has been increased.
 ここで、液化アンモニアを取り扱う炭素鋼製の配管、貯槽、タンク車、ラインパイプなどにおいては、液体アンモニアによる応力腐食割れ(以下、アンモニアSCC(Stress Corrosion Cracking)を引き起こすことが知られている。このため、液体アンモニア環境下で使用される鋼材に対しては、アンモニアSCC感受性の低い鋼材の適用や、アンモニアSCCを抑制するエンジニアリング措置が講じられてきた。 Here, in carbon steel pipes, storage tanks, tank cars, line pipes, etc. that handle liquefied ammonia, stress corrosion cracking (hereinafter referred to as ammonia SCC (Stress Corrosion Cracking)) due to liquid ammonia is known to occur. Therefore, for steel materials used in a liquid ammonia environment, application of steel materials with low ammonia SCC susceptibility and engineering measures to suppress ammonia SCC have been taken.
 例えば、アンモニアSCCの発生については、材料の強度と相関があることが知られており、炭素鋼の使用にあたっては、440MPa以下の降伏強度(YS)に制御することで、アンモニアによる応力腐食割れの回避が図られている。その一方で、近年のタンク大型化、鋼材使用量の削減の観点から、鋼板の高強度化の要求が高まっている。 For example, it is known that the occurrence of ammonia SCC is correlated with the strength of the material. Avoidance is being attempted. On the other hand, from the standpoint of increasing the size of tanks and reducing the amount of steel used in recent years, there is an increasing demand for higher strength steel sheets.
 また、LPGや液体アンモニアといった液化ガスは低温で輸送および貯蔵されるため、これらの液化ガスの貯蔵用タンクに使用される鋼板は、優れた低温靱性が要求される。 In addition, since liquefied gases such as LPG and liquid ammonia are transported and stored at low temperatures, the steel plates used in storage tanks for these liquefied gases are required to have excellent low temperature toughness.
 前述したような、液化ガス貯蔵用タンクに必要な、低温靱性と所定の強度範囲とを満たすための技術が、特許文献1および2に開示されている。これらの文献に記載の技術では、熱間圧延後冷却した厚鋼板を数回熱処理する、あるいは熱間圧延後水冷した厚鋼板を数回熱処理するという方法にて、高い低温靱性および所定の強度特性を実現している。 Patent Documents 1 and 2 disclose techniques for satisfying the low-temperature toughness and the predetermined strength range required for liquefied gas storage tanks as described above. In the techniques described in these documents, high low-temperature toughness and predetermined strength properties are achieved by heat-treating a steel plate that has been cooled after hot-rolling, or by heat-treating a steel plate that has been water-cooled after hot-rolling several times. is realized.
特開平10-140235号公報JP-A-10-140235 特開平10-168516号公報JP-A-10-168516
 しかしながら、上記の特許文献1および2に記載された方法では、複数回の熱処理を行う必要があり、そのための設備やエネルギーにかかるコストが大きいという経済的な問題があった。 However, the methods described in Patent Literatures 1 and 2 above had the economic problem of requiring multiple heat treatments, which required high equipment and energy costs.
 本発明は、上記の問題を解決し、エネルギー輸送船において液化ガスの収容に使用される貯蔵用タンク等に供する、耐アンモニアSCC性および低温靭性に優れる高強度の鋼板並びにその製造方法を提供することを目的とする。 The present invention solves the above problems and provides a high-strength steel sheet with excellent ammonia SCC resistance and low-temperature toughness, which is used for storage tanks used for storing liquefied gas in energy transport ships, and a method for producing the same. for the purpose.
 本発明者らは、上記目的を達成するために、TMCPプロセスとオンライン誘導加熱装置とを用いて、鋼板の低温靱性、強度特性に対する各種要因について、鋭意検討を重ねた。その結果、鋼板に対し、C、Si、Mn、Al等の元素を所定量含有させ、前記鋼板の表面から0.5mm位置におけるベイナイト組織の体積率が90%以上となるように金属組織を制御し、前記鋼板の表面から0.5mm深さ位置において、平均硬さを230HV0.1以下、硬さのばらつきを30HV0.1以下とし、さらに、板厚方向の硬さの最大値が鋼板の表面から1.0mm以上かつ板厚1/4以下の位置に存在するようにし、当該板厚方向の硬さのばらつきを70HV1以下とすることで、液体アンモニア環境下での耐SCC性が効果的に得られ、コストがかかる複数回の熱処理が省略できることを知見した。 In order to achieve the above objectives, the present inventors used the TMCP process and an online induction heating device to extensively study various factors affecting the low-temperature toughness and strength characteristics of steel sheets. As a result, the steel sheet contains a predetermined amount of elements such as C, Si, Mn, and Al, and the metal structure is controlled so that the volume fraction of the bainite structure at a position 0.5 mm from the surface of the steel sheet is 90% or more. At a depth of 0.5 mm from the surface of the steel sheet, the average hardness is 230 HV0.1 or less, the variation in hardness is 30 HV0.1 or less, and the maximum hardness in the thickness direction is the surface of the steel sheet By making it exist at a position of 1.0 mm or more and 1/4 or less of the plate thickness, and the hardness variation in the plate thickness direction is 70 HV1 or less, the SCC resistance in the liquid ammonia environment is effectively It has been found that multiple heat treatments, which are obtained and costly, can be omitted.
 すなわち、本発明は、上記の知見に基づきなされたものであって、本発明の要旨は次のとおりである。
 1.質量%で、
 C:0.010~0.200%、
 Si:0.01~0.50%、
 Mn:0.50~2.50%、
 Al:0.010~0.060%、
 N:0.0010%以上0.0100%以下、
 P:0.020%以下、
 S:0.0100%以下および
 O:0.0100%以下
を含有し、残部がFeおよび不可避的不純物である成分組成を有する鋼板であって、
 前記鋼板の表面から0.5mm深さの位置において、平均硬さが230HV0.1以下で、硬さのばらつきが30HV0.1以下であり、かつ、板厚方向の硬さの最大値が鋼板の表面から1.0mm以上板厚1/4以下の位置に在り、当該板厚方向の硬さのばらつきが70HV1以下である硬さ特性と、
 前記鋼板の表面から0.5mm深さの位置におけるベイナイト組織の体積率が90%以上である金属組織と、を有する鋼板。
That is, the present invention was made based on the above findings, and the gist of the present invention is as follows.
1. in % by mass,
C: 0.010 to 0.200%,
Si: 0.01 to 0.50%,
Mn: 0.50-2.50%,
Al: 0.010 to 0.060%,
N: 0.0010% or more and 0.0100% or less,
P: 0.020% or less,
A steel sheet having a chemical composition containing S: 0.0100% or less and O: 0.0100% or less, with the balance being Fe and inevitable impurities,
At a depth of 0.5 mm from the surface of the steel sheet, the average hardness is 230 HV0.1 or less, the hardness variation is 30 HV0.1 or less, and the maximum hardness in the thickness direction is the steel sheet. A hardness characteristic that is located at a position of 1.0 mm or more and 1/4 or less of the plate thickness from the surface and has a hardness variation of 70 HV1 or less in the plate thickness direction;
A steel sheet having a metal structure in which the volume fraction of the bainite structure at a position 0.5 mm deep from the surface of the steel sheet is 90% or more.
 2.前記成分組成が、さらに、質量%で、
 Cu:0.01~0.50%、
 Ni:0.01~2.00%、
 Cr:0.01~1.00%、
 Sn:0.01~0.50%、
 Sb:0.01~0.50%、
 Mo:0.01~0.50%および
 W:0.01~1.00%
のうちから選ばれる1種以上を含有する、前記1に記載の鋼板。
2. The component composition further, in mass %,
Cu: 0.01-0.50%,
Ni: 0.01 to 2.00%,
Cr: 0.01 to 1.00%,
Sn: 0.01 to 0.50%,
Sb: 0.01 to 0.50%,
Mo: 0.01-0.50% and W: 0.01-1.00%
2. The steel sheet according to 1 above, containing one or more selected from.
 3.前記成分組成が、さらに、質量%で、
 V:0.01~1.00%、
 Ti:0.005~0.100%、
 Co:0.01~1.00%、
 Nb:0.005~0.100%、
 B:0.0001~0.0100%、
 Ca:0.0005~0.0200%、
 Mg:0.0005~0.0200%および
 REM:0.0005~0.0200%
のうちから選ばれる1種以上を含有する、前記1または2に記載の鋼板。
3. The component composition further, in mass %,
V: 0.01 to 1.00%,
Ti: 0.005 to 0.100%,
Co: 0.01 to 1.00%,
Nb: 0.005 to 0.100%,
B: 0.0001 to 0.0100%,
Ca: 0.0005 to 0.0200%,
Mg: 0.0005-0.0200% and REM: 0.0005-0.0200%
3. The steel sheet according to 1 or 2 above, containing one or more selected from.
 4.質量%で、
 C:0.010~0.200%、
 Si:0.01~0.50%、
 Mn:0.50~2.50%、
 Al:0.010~0.060%、
 N:0.0010%以上0.0100%以下、
 P:0.020%以下、
 S:0.0100%以下および
 O:0.0100%以下
を含有し、残部がFeおよび不可避的不純物である成分組成を有する鋼素材について、圧延終了温度をAr変態点以上として熱間圧延を行い、次いでAr変態点以上の冷却開始温度からの加速冷却を行い、次いで再加熱を行う、鋼板の製造方法であって、
 前記加速冷却では、冷却停止温度を200~600℃の範囲とし、かつ、鋼板の板厚の1/4位置における冷却速度を20~120℃/sとし、
 前記再加熱は、鋼板の板厚の1/4位置における到達温度を500℃以下として、鋼板の表面から0.5mm深さの位置における到達温度が400~680℃の範囲となるまで行う、鋼板の製造方法。
4. in % by mass,
C: 0.010 to 0.200%,
Si: 0.01 to 0.50%,
Mn: 0.50-2.50%,
Al: 0.010 to 0.060%,
N: 0.0010% or more and 0.0100% or less,
P: 0.020% or less,
A steel material having a chemical composition containing S: 0.0100% or less and O: 0.0100% or less, with the balance being Fe and inevitable impurities, is subjected to hot rolling at a rolling end temperature of Ar 3 transformation point or higher. A method for producing a steel sheet, comprising :
In the accelerated cooling, the cooling stop temperature is in the range of 200 to 600 ° C., and the cooling rate at the 1/4 position of the plate thickness of the steel plate is 20 to 120 ° C./s,
The reheating is performed until the temperature reached at a position 1/4 of the plate thickness of the steel plate is 500 ° C. or less, and the temperature reached at a depth of 0.5 mm from the surface of the steel plate is in the range of 400 to 680 ° C. The steel plate. manufacturing method.
 5.前記鋼素材の成分組成が、さらに、質量%で、
 Cu:0.01~0.50%、
 Ni:0.01~2.00%、
 Cr:0.01~1.00%、
 Sn:0.01~0.50%、
 Sb:0.01~0.50%、
 Mo:0.01~0.50%および
 W:0.01~1.00%
のうちから選ばれる1種以上を含有する、前記4に記載の鋼板の製造方法。
5. The chemical composition of the steel material is further, in mass%,
Cu: 0.01-0.50%,
Ni: 0.01 to 2.00%,
Cr: 0.01 to 1.00%,
Sn: 0.01 to 0.50%,
Sb: 0.01 to 0.50%,
Mo: 0.01-0.50% and W: 0.01-1.00%
4. The method for producing a steel sheet according to 4 above, containing one or more selected from.
 6.前記鋼素材の成分組成が、さらに、質量%で、
 V:0.01~1.00%、
 Ti:0.005~0.100%、
 Co:0.01~1.00%、
 Nb:0.005~0.100%、
 B:0.0001~0.0100%、
 Ca:0.0005~0.0200%、
 Mg:0.0005~0.0200%および
 REM:0.0005~0.0200%のうちから選ばれる1種以上を含有する、前記4または5に記載の鋼板の製造方法。
6. The chemical composition of the steel material is further, in mass%,
V: 0.01 to 1.00%,
Ti: 0.005 to 0.100%,
Co: 0.01 to 1.00%,
Nb: 0.005 to 0.100%,
B: 0.0001 to 0.0100%,
Ca: 0.0005 to 0.0200%,
6. The method for producing a steel sheet according to 4 or 5 above, containing one or more selected from Mg: 0.0005 to 0.0200% and REM: 0.0005 to 0.0200%.
 本発明によれば、低温での靭性すなわち低温での耐衝撃特性および耐アンモニアSCC性に優れ、低温かつ液体アンモニア環境下で使用されるタンクなどの構造用部材に好適な高い強度を有する鋼板を、安価な工程で提供することができる。 According to the present invention, a steel sheet having excellent low-temperature toughness, that is, low-temperature impact resistance and ammonia SCC resistance, and having high strength suitable for structural members such as tanks used in a low-temperature and liquid ammonia environment. , can be provided in an inexpensive process.
 以下に、本発明の実施形態を説明する。なお、以下の成分(元素)の含有量を表す「%」は、特に断らない限り「質量%」を意味する。 An embodiment of the present invention will be described below. In addition, "%" representing the content of the following components (elements) means "% by mass" unless otherwise specified.
(1)成分組成について
 以下、鋼板の成分組成(化学成分)について説明する。
(1) Regarding chemical composition The chemical composition (chemical composition) of the steel sheet will be described below.
C:0.010~0.200%
 Cは、本発明に従う冷却によって製造される鋼板の強度を高めるために最も有効な元素である。かかる効果を得るため、C含有量を0.010%以上に規定する。さらに、他の合金元素の含有量を少なくし、より低コストで製造するという観点からは、C含有量は0.013%以上とすることが好ましい。一方、C含有量が0.200%を超えると鋼板の靭性および溶接性の劣化を招く。従って、C含有量を0.200%以下に規定する。さらに、C含有量は、靭性および溶接性の観点から、0.170%以下とすることが好ましい。
C: 0.010-0.200%
C is the most effective element for increasing the strength of steel sheets produced by cooling according to the present invention. In order to obtain such effects, the C content is specified to be 0.010% or more. Furthermore, the C content is preferably 0.013% or more from the viewpoint of reducing the content of other alloying elements and manufacturing at a lower cost. On the other hand, if the C content exceeds 0.200%, the toughness and weldability of the steel sheet deteriorate. Therefore, the C content is specified at 0.200% or less. Furthermore, the C content is preferably 0.170% or less from the viewpoint of toughness and weldability.
Si:0.01~0.50%
 Siは、脱酸のため添加する。かかる効果を得るため、Si含有量を0.01%以上に規定する。さらに、0.03%以上とすることが好ましい。一方、Si含有量が0.50%を超えると鋼板の靭性や溶接性の劣化を招く。従って、Si含有量を0.50%以下に規定する。さらに、Si含有量は、靭性および溶接性の観点から、0.40%以下とすることが好ましい。
Si: 0.01-0.50%
Si is added for deoxidation. In order to obtain such effects, the Si content is specified to be 0.01% or more. Furthermore, it is preferable to make it 0.03% or more. On the other hand, if the Si content exceeds 0.50%, the toughness and weldability of the steel sheet are deteriorated. Therefore, the Si content is specified to be 0.50% or less. Furthermore, the Si content is preferably 0.40% or less from the viewpoint of toughness and weldability.
Mn:0.50~2.50%
 Mnは、鋼の焼入れ性を増加させる作用を有する元素であり、本発明のように高強度を満足するためには添加が必要になる重要な元素の1つである。かかる効果を得るため、Mn含有量を0.50%以上に規定する。さらに、他の合金元素の含有量を少なくし、より低コストで製造するという観点からは、Mn含有量は0.70%以上とすることが好ましい。一方、Mn含有量が2.50%を超えると、鋼板の靭性や溶接性が低下することに加えて、合金コストが過度に高くなってしまう。従って、Mn含有量を2.50%以下に規定する。さらに、Mn含有量は、靭性および溶接性の低下を抑制する観点から、2.30%以下とすることが好ましい。
Mn: 0.50-2.50%
Mn is an element that has the effect of increasing the hardenability of steel, and is one of the important elements that need to be added in order to achieve high strength as in the present invention. In order to obtain such effects, the Mn content is specified to be 0.50% or more. Furthermore, the content of Mn is preferably 0.70% or more from the viewpoint of reducing the content of other alloying elements and manufacturing at a lower cost. On the other hand, if the Mn content exceeds 2.50%, the toughness and weldability of the steel sheet deteriorate, and the alloy cost becomes excessively high. Therefore, the Mn content is specified at 2.50% or less. Furthermore, the Mn content is preferably 2.30% or less from the viewpoint of suppressing deterioration of toughness and weldability.
Al:0.010~0.060%
 Alは、脱酸剤として作用する。かかる効果を得るため、Al含有量を0.010%以上に規定する。一方、Al含有量が0.060%を超えると、酸化物系介在物が増加して清浄度が低下すると共に、靭性が低下する。従って、Al含有量を0.060%以下に規定する。さらに、Al含有量は、靭性劣化をより一層防止する観点から、0.050%以下とすることが好ましい。
Al: 0.010-0.060%
Al acts as a deoxidizing agent. In order to obtain such effects, the Al content is specified to be 0.010% or more. On the other hand, when the Al content exceeds 0.060%, the oxide inclusions increase to lower the cleanliness and toughness. Therefore, the Al content is specified at 0.060% or less. Furthermore, the Al content is preferably 0.050% or less from the viewpoint of further preventing toughness deterioration.
N:0.0010~0.0100%
 Nは、組織の微細化に寄与し、鋼板の靭性を向上させる。かかる効果を得るため、N含有量を0.0010%以上に規定する。好ましくは、0.0020%以上である。一方、N含有量が0.0100%を超えると、かえって靭性の低下を招く。従って、N含有量を0.0100%以下に規定する。さらに、N含有量は、靭性や溶接性の低下をより一層抑制する観点から、0.0080%以下とすることが好ましい。なお、Nは、Tiが存在する場合には、そのTiと結合して、TiNとして析出し得る。
N: 0.0010 to 0.0100%
N contributes to the refinement of the structure and improves the toughness of the steel sheet. In order to obtain such effects, the N content is specified to be 0.0010% or more. Preferably, it is 0.0020% or more. On the other hand, if the N content exceeds 0.0100%, the toughness is rather lowered. Therefore, the N content is specified at 0.0100% or less. Furthermore, the N content is preferably 0.0080% or less from the viewpoint of further suppressing deterioration of toughness and weldability. Incidentally, when Ti is present, N can bond with Ti and precipitate as TiN.
P:0.020%以下
 Pは、粒界に偏析することによって靱性や溶接性を低下させるなど、悪影響を及ぼす。そのため、P含有量は、できる限り低くすることが望ましいが、0.020%以下であれば許容できる。なお、P含有量の下限は特に限定されず、0%であってよいが、通常、Pは工業的には鋼中に残存し得る元素であるため、0%超であってよい。また、過剰の低減は精錬コストの高騰を招くため、コストの観点からはP含有量を0.0005%以上とすることが好ましい。
P: 0.020% or less P has an adverse effect, such as lowering toughness and weldability, by segregating at grain boundaries. Therefore, it is desirable to make the P content as low as possible, but a P content of 0.020% or less is acceptable. The lower limit of the P content is not particularly limited, and may be 0%. However, since P is an element that can industrially remain in steel, it may exceed 0%. Moreover, since excessive reduction causes a rise in refining cost, it is preferable to set the P content to 0.0005% or more from the viewpoint of cost.
S:0.0100%以下
 Sは、MnS等の硫化物系介在物として鋼中に存在し、破壊の発生起点となって鋼板の靭性を低下させるなど、悪影響を及ぼす元素である。そのため、S含有量は、できる限り低くすることが望ましいが、0.0100%以下であれば許容できる。なお、S含有量の下限は特に限定されず、0%であってよいが、通常、Sは工業的には鋼中に残存し得る元素であるため、0%超であってもよい。また、過剰の低減は精錬コストの高騰を招くため、コストの観点からはS含有量を0.0005%以上とすることが好ましい。
S: 0.0100% or less S is present in steel as sulfide-based inclusions such as MnS, and is an element that exerts adverse effects, such as deteriorating the toughness of the steel sheet by becoming the origin of fracture. Therefore, it is desirable that the S content be as low as possible, but a content of 0.0100% or less is permissible. The lower limit of the S content is not particularly limited, and may be 0%. However, since S is an element that can industrially remain in steel, it may exceed 0%. Moreover, since an excessive reduction causes a rise in refining cost, it is preferable to set the S content to 0.0005% or more from the viewpoint of cost.
O:0.0100%以下
 Oは、酸化物を形成し、破壊の発生起点となり、鋼板の靭性を低下させるなど、悪影響を及ぼす元素であることから、0.0100%以下に制限する。O含有量は、0.0050%以下とすることが好ましく、0.0030%以下とすることがより好ましい。一方、O含有量の下限は特に限定されず、0%であってよいが、通常、Oは工業的には鋼中に残存し得る元素であるため、0%超であってよい。また、過剰の低減は精錬コストの高騰を招くため、コストの観点からはO含有量を0.0010%以上とすることが好ましい。
O: 0.0100% or less O is an element that forms an oxide, becomes a starting point of fracture, and has an adverse effect such as lowering the toughness of the steel sheet. The O content is preferably 0.0050% or less, more preferably 0.0030% or less. On the other hand, the lower limit of the O content is not particularly limited, and may be 0%. However, since O is an element that can industrially remain in steel, it may exceed 0%. Moreover, since an excessive reduction causes a rise in refining cost, it is preferable to set the O content to 0.0010% or more from the viewpoint of cost.
 本発明の鋼板の成分組成において、上記成分以外の残部は、Feおよび不可避的不純物である。ただし、上記成分組成は、必要に応じて、以下に記載する元素を含有することができる。 In the chemical composition of the steel sheet of the present invention, the balance other than the above components is Fe and unavoidable impurities. However, the above component composition can contain the elements described below, if necessary.
 Cu:0.01~0.50%、Ni:0.01~2.00%、Cr:0.01~1.00%、Sn:0.01~0.50%、Sb:0.01~0.50%、Mo:0.01~0.50%、およびW:0.01~1.00%のうちから選ばれる1種以上
 Cu、Ni、Cr、Sn、Sb、MoおよびWは、強度や耐アンモニアSCC性を向上させる元素であり、これらのうちの1種以上を含有させることができる。かかる効果を得るため、Cuを含有させる場合には、Cu含有量を0.01%以上に、Niを含有させる場合には、Ni含有量を0.01%以上に、Crを含有させる場合には、Cr含有量を0.01%以上に、Snを含有させる場合には、Sn含有量を0.01%以上に、Sbを含有させる場合には、Sb含有量を0.01%以上に、Moを含有させる場合には、Mo含有量を0.01%以上に、また、Wを含有させる場合には、W含有量を0.01%以上に、それぞれ調整するのが好ましい。一方、Niを過剰に含有させると、溶接性の劣化や合金コストの上昇を招く。また、Cu、Cr、Sn、Sb、MoおよびWを過剰に含有させると、溶接性や靱性が劣化し、合金コストの観点からも不利になる。従って、Cu含有量を0.50%以下に、Ni含有量を2.00%以下に、Cr含有量を1.00%以下に、Sn含有量を0.50%以下に、Sb含有量を0.50%以下に、Mo含有量を0.50%以下に、また、W含有量を1.00%以下に、それぞれ調整するのが好ましい。より好ましくは、Cu含有量を0.40%以下に、Ni含有量を1.50%以下に、Cr含有量を0.80%以下に、Sn含有量を0.40%以下に、Sb含有量を0.40%以下に、Mo含有量を0.40%以下に、また、W含有量を0.80%以下に、それぞれ調整する。
Cu: 0.01-0.50%, Ni: 0.01-2.00%, Cr: 0.01-1.00%, Sn: 0.01-0.50%, Sb: 0.01- 0.50%, Mo: 0.01 to 0.50%, and W: one or more selected from 0.01 to 1.00% Cu, Ni, Cr, Sn, Sb, Mo and W are It is an element that improves strength and ammonia SCC resistance, and one or more of these elements can be contained. In order to obtain such an effect, when Cu is contained, the Cu content is 0.01% or more, when Ni is contained, the Ni content is 0.01% or more, and when Cr is contained, When the Cr content is 0.01% or more, the Sn content is 0.01% or more when Sn is contained, and the Sb content is 0.01% or more when Sb is contained. When Mo is contained, the Mo content is preferably adjusted to 0.01% or more, and when W is contained, the W content is preferably adjusted to 0.01% or more. On the other hand, an excessive Ni content causes deterioration of weldability and an increase in alloy cost. Also, if Cu, Cr, Sn, Sb, Mo and W are contained excessively, weldability and toughness deteriorate, which is disadvantageous from the viewpoint of alloy cost. Therefore, the Cu content is 0.50% or less, the Ni content is 2.00% or less, the Cr content is 1.00% or less, the Sn content is 0.50% or less, and the Sb content is It is preferable to adjust the Mo content to 0.50% or less, the W content to 0.50% or less, and the W content to 1.00% or less. More preferably, the Cu content is 0.40% or less, the Ni content is 1.50% or less, the Cr content is 0.80% or less, the Sn content is 0.40% or less, and the Sb content is The amount is adjusted to 0.40% or less, the Mo content to 0.40% or less, and the W content to 0.80% or less.
V:0.01~1.00%
 Vは、鋼板の強度を向上させる作用を有する元素であり、任意に添加することができる。かかる効果を得るため、Vを添加する場合には、V含有量を0.01%以上とするのが好ましい。一方、V含有量が1.00%を超えると、溶接性の劣化や合金コストの上昇を招く。従って、Vを添加する場合には、V含有量を1.00%以下とするのが好ましい。より好ましくは、V含有量の下限が0.05%であり、上限が0.50%である。
V: 0.01-1.00%
V is an element that has the effect of improving the strength of the steel sheet, and can be optionally added. In order to obtain such an effect, when V is added, the V content is preferably 0.01% or more. On the other hand, if the V content exceeds 1.00%, the weldability deteriorates and the alloy cost increases. Therefore, when V is added, the V content is preferably 1.00% or less. More preferably, the lower limit of V content is 0.05% and the upper limit is 0.50%.
Ti:0.005~0.100%
 Tiは、窒化物の形成傾向が強く、Nを固定して固溶Nを低減する作用を有する元素であり、任意に添加することができる。また、Tiは、母材および溶接部の靭性を向上させることができる。かかる効果を得るため、Tiを添加する場合には、Ti含有量を0.005%以上とするのが好ましい。さらに、0.007%以上とすることがより好ましい。一方、Ti含有量が0.100%を超えると、かえって靭性が低下する。従って、Tiを添加する場合には、Ti含有量を0.100%以下とするのが好ましい。さらに、Ti含有量は、0.090%以下とすることがより好ましい。
Ti: 0.005-0.100%
Ti is an element that has a strong tendency to form nitrides and has the action of fixing N and reducing solid solution N, and can be added arbitrarily. In addition, Ti can improve the toughness of the base material and the weld zone. In order to obtain such an effect, when adding Ti, the Ti content is preferably 0.005% or more. Furthermore, it is more preferable to make it 0.007% or more. On the other hand, when the Ti content exceeds 0.100%, the toughness rather decreases. Therefore, when adding Ti, the Ti content is preferably 0.100% or less. Furthermore, the Ti content is more preferably 0.090% or less.
Co:0.01~1.00%
 Coは、鋼板の強度を向上させる作用を有する元素であり、任意に添加することができる。かかる効果を得るため、Coを添加する場合には、Co含有量を0.01%以上とするのが好ましい。一方、Co含有量が1.00%を超えると、溶接性の劣化や合金コストの上昇を招く。従って、Coを添加する場合には、Co含有量を1.00%以下とするのが好ましい。より好ましくは、Co含有量の下限が0.05%であり、上限が0.50%である。
Co: 0.01-1.00%
Co is an element that has the effect of improving the strength of the steel sheet, and can be optionally added. In order to obtain such an effect, when Co is added, the Co content is preferably 0.01% or more. On the other hand, if the Co content exceeds 1.00%, the weldability deteriorates and the alloy cost increases. Therefore, when Co is added, the Co content is preferably 1.00% or less. More preferably, the Co content has a lower limit of 0.05% and an upper limit of 0.50%.
Nb:0.005~0.100%
 Nbは、炭窒化物として析出することで旧オーステナイト粒径を小さくし、靭性を向上させる効果を有する元素である。かかる効果を得るため、Nbを添加する場合には、Nb含有量を0.005%以上とするのが好ましい。さらに、0.007%以上とすることがより好ましい。一方、Nb含有量が0.100%を超えるとNbCが多量に析出し、靭性が低下する。従って、Nbを添加する場合には、Nb含有量を0.100%以下とするのが好ましい。さらに、0.060%以下とすることがより好ましい。
Nb: 0.005-0.100%
Nb is an element that has the effect of reducing the grain size of prior austenite and improving the toughness by precipitating as a carbonitride. In order to obtain such an effect, when Nb is added, the Nb content is preferably 0.005% or more. Furthermore, it is more preferable to make it 0.007% or more. On the other hand, when the Nb content exceeds 0.100%, a large amount of NbC precipitates, resulting in a decrease in toughness. Therefore, when Nb is added, the Nb content is preferably 0.100% or less. Furthermore, it is more preferable to make it 0.060% or less.
B:0.0001~0.0100%
 Bは、微量の添加でも焼入れ性を著しく向上させる作用を有する元素である。すなわち、鋼板の強度を向上させることができる。かかる効果を得るため、Bを添加する場合には、B含有量を0.0001%以上とするのが好ましい。一方、B含有量が0.0100%を超えると溶接性が低下する。従って、Bを添加する場合には、B含有量を0.0100%以下とするのが好ましい。より好ましくは、B含有量の下限が0.0010%であり、上限が0.0030%である。
B: 0.0001 to 0.0100%
B is an element that has the effect of significantly improving hardenability even when added in a very small amount. That is, the strength of the steel sheet can be improved. In order to obtain such an effect, when B is added, the B content is preferably 0.0001% or more. On the other hand, when the B content exceeds 0.0100%, the weldability deteriorates. Therefore, when B is added, the B content is preferably 0.0100% or less. More preferably, the B content has a lower limit of 0.0010% and an upper limit of 0.0030%.
Ca:0.0005~0.0200%
 Caは、Sと結合し、圧延方向に長く伸びるMnS等の形成を抑制する作用を有する元素である。すなわち、Caを添加することにより、硫化物系介在物が球状を呈するように形態制御し、溶接部等の靭性を向上させることができる。かかる効果を得るために、Caを添加する場合には、Ca含有量を0.0005%以上とするのが好ましい。一方、Ca含有量が0.0200%を超えると、鋼の清浄度が低下する。清浄度の低下は、靭性の低下を招く。従って、Caを添加する場合、Ca含有量を0.0200%以下とするのが好ましい。より好ましくは、Ca含有量の下限が0.0020%であり、上限が0.0100%である。
Ca: 0.0005-0.0200%
Ca is an element that binds to S and has the effect of suppressing the formation of MnS or the like elongated in the rolling direction. That is, by adding Ca, it is possible to control the morphology of the sulfide-based inclusions so that they exhibit a spherical shape, and improve the toughness of the weld zone and the like. In order to obtain such effects, when Ca is added, the Ca content is preferably 0.0005% or more. On the other hand, when the Ca content exceeds 0.0200%, the cleanliness of the steel is lowered. A decrease in cleanliness leads to a decrease in toughness. Therefore, when Ca is added, the Ca content is preferably 0.0200% or less. More preferably, the Ca content has a lower limit of 0.0020% and an upper limit of 0.0100%.
Mg:0.0005~0.0200%
 Mgは、Caと同様、Sと結合し、圧延方向に長く伸びるMnS等の形成を抑制する作用を有する元素である。すなわち、Mgを添加することにより、硫化物系介在物が球状を呈するように形態制御し、溶接部等の靭性を向上させることができる。かかる効果を得るために、Mgを添加する場合には、Mg含有量を0.0005%以上とするのが好ましい。一方、Mg含有量が0.0200%を超えると、鋼の清浄度が低下する。清浄度の低下は、靭性の低下を招く。従って、Mgを添加する場合には、Mg含有量を0.0200%以下とするのが好ましい。より好ましくは、Mg含有量の下限が0.0020%であり、上限が0.0100%である。
Mg: 0.0005-0.0200%
Mg, like Ca, is an element that binds to S and has the effect of suppressing the formation of MnS or the like elongated in the rolling direction. That is, by adding Mg, it is possible to control the morphology of the sulfide-based inclusions so that they exhibit a spherical shape, and improve the toughness of the weld zone and the like. In order to obtain such an effect, when Mg is added, the Mg content is preferably 0.0005% or more. On the other hand, when the Mg content exceeds 0.0200%, the cleanliness of the steel is lowered. A decrease in cleanliness leads to a decrease in toughness. Therefore, when Mg is added, the Mg content is preferably 0.0200% or less. More preferably, the Mg content has a lower limit of 0.0020% and an upper limit of 0.0100%.
REM:0.0005~0.0200%
 REM(希土類金属)は、CaやMgと同様、Sと結合し、圧延方向に長く伸びるMnS等の形成を抑制する作用を有する元素である。すなわち、REMを添加することにより、硫化物系介在物が球状を呈するように形態制御し、溶接部等の靭性を向上させることができる。かかる効果を得るために、REMを添加する場合には、REM含有量は0.0005%以上が好ましい。一方、REM含有量が0.0200%を超えると、鋼の清浄度が低下する。清浄度の低下は、靭性の低下を招く。従って、REMを添加する場合、REM含有量は0.0200%以下が好ましい。より好ましくは、REM含有量の下限が0.0020%であり、上限が0.0100%である。
REM: 0.0005-0.0200%
Like Ca and Mg, REM (rare earth metal) is an element that binds to S and suppresses the formation of MnS or the like elongated in the rolling direction. That is, by adding REM, it is possible to control the morphology of the sulfide-based inclusions so that they exhibit a spherical shape, and improve the toughness of the weld zone and the like. In order to obtain such an effect, when REM is added, the REM content is preferably 0.0005% or more. On the other hand, when the REM content exceeds 0.0200%, the cleanliness of the steel deteriorates. A decrease in cleanliness leads to a decrease in toughness. Therefore, when REM is added, the REM content is preferably 0.0200% or less. More preferably, the REM content has a lower limit of 0.0020% and an upper limit of 0.0100%.
(2)硬さ特性および金属組織について
 本発明の鋼板は、上記成分組成を有することに加えて、鋼板の表面から0.5mm深さの位置(本発明において0.5mm位置ともいう)における平均硬さが230HV0.1以下で、0.5mm位置における硬さのばらつきが30HV0.1以下であり、かつ、板厚方向の硬さの最大値が鋼板の表面から1.0mm以上板厚1/4以下の位置に在り、当該板厚方向の硬さのばらつきが70HV1以下である硬さ特性を有する。
 さらに、本発明の鋼板は、0.5mm位置におけるベイナイト組織(以下、単にベイナイトともいう)の体積率が90%以上である金属組織を有する。
 鋼板の硬さ特性および金属組織を上記のように限定する理由を、以下に説明する。
(2) Hardness characteristics and metal structure The steel sheet of the present invention has the above chemical composition, and in addition, the average The hardness is 230 HV0.1 or less, the variation in hardness at the 0.5 mm position is 30 HV0.1 or less, and the maximum hardness in the thickness direction is 1.0 mm or more from the surface of the steel sheet. 4 or less, and has a hardness characteristic in which the variation in hardness in the plate thickness direction is 70HV1 or less.
Further, the steel sheet of the present invention has a metal structure in which the volume fraction of bainite structure (hereinafter also simply referred to as bainite) at the 0.5 mm position is 90% or more.
The reasons for limiting the hardness properties and metallographic structure of the steel sheet as described above will be explained below.
[0.5mm位置において、平均硬さが230HV0.1以下で、硬さのばらつきが30HV0.1以下]
 0.5mm位置において、平均硬さを230HV0.1以下とし、かつ、硬さのばらつきを30HV0.1以下とする。鋼板の極表層、具体的には鋼板の表面から0.5mm位置に高硬度領域が存在すると、液体アンモニア環境中での応力腐食割れが助長されてしまう。また、局所的な高硬度領域が存在した場合、鋼板に応力が付与された際に、応力集中が生じ、応力腐食割れが助長されてしまう。そこで、本発明の鋼板では、0.5mm位置において、平均硬さを230HV0.1以下とし、かつ、硬さのばらつきを30HV0.1以下として硬さ特性を調整することによって、優れた耐アンモニアSCC性を確保することができる。なお、0.5mm位置における平均硬さの下限は、特に限定されないが、130HV0.1程度が好ましい。また0.5mm位置における硬さのばらつきの下限は、0HV0.1であって良いが、工業的には10HV0.1程度である。
 ここで、上記平均硬さは、0.5mm位置におけるビッカース硬さを複数箇所(例えば、100点)測定して算出することができる。また、硬さのばらつきは、平均硬さを求めるために測定したビッカース硬さの標準偏差を意味する。
[At the 0.5 mm position, the average hardness is 230HV0.1 or less, and the variation in hardness is 30HV0.1 or less]
At the 0.5 mm position, the average hardness is set to 230 HV0.1 or less, and the variation in hardness is set to 30 HV0.1 or less. If a high-hardness region exists in the extreme surface layer of the steel sheet, specifically, at a position of 0.5 mm from the surface of the steel sheet, stress corrosion cracking in a liquid ammonia environment is promoted. Moreover, when a local high-hardness region exists, stress concentration occurs when stress is applied to the steel sheet, and stress corrosion cracking is promoted. Therefore, in the steel sheet of the present invention, the average hardness at the 0.5 mm position is 230 HV0.1 or less, and the hardness variation is 30 HV0.1 or less by adjusting the hardness characteristics. can ensure the integrity of the Although the lower limit of the average hardness at the 0.5 mm position is not particularly limited, it is preferably about 130HV0.1. The lower limit of the variation in hardness at the 0.5 mm position may be 0HV0.1, but industrially it is about 10HV0.1.
Here, the average hardness can be calculated by measuring Vickers hardness at a plurality of points (for example, 100 points) at a position of 0.5 mm. Further, the variation in hardness means the standard deviation of the Vickers hardness measured to obtain the average hardness.
[板厚方向の硬さの最大値が鋼板の表面から1.0mm以上板厚1/4以下の位置に在る]
 鋼板の硬さの最大値が表面からある程度離れた位置に存在すると、鋼板の大部分の硬さを維持しつつ、表層のみの硬さを低減させることができる。すなわち、鋼板の強度を維持しつつ、優れた耐アンモニアSCC特性を確保することができることになる。
 具体的に、かかる最大値が鋼板の表面から1.0mm未満の位置に在ると、0.5mm位置での硬さを十分に低減することができない。一方、かかる最大値が鋼板の表面から板厚1/4を超える位置に在ると、鋼板自体の十分な強度を確保することができない。よって、本発明の鋼板において、板厚方向の硬さ(ビッカース硬さ(HV1))の最大値は、鋼板の表面から1.0mm以上板厚1/4以下の位置に在るものと規定する。
[The maximum hardness in the plate thickness direction is located at a position of 1.0 mm or more and 1/4 or less of the plate thickness from the surface of the steel plate]
When the maximum hardness of the steel sheet exists at a position distant from the surface to some extent, the hardness of only the surface layer can be reduced while maintaining the hardness of most of the steel sheet. That is, it is possible to ensure excellent ammonia SCC resistance while maintaining the strength of the steel sheet.
Specifically, if the maximum value is located at a position less than 1.0 mm from the surface of the steel sheet, the hardness at the 0.5 mm position cannot be sufficiently reduced. On the other hand, if the maximum value is at a position exceeding 1/4 of the plate thickness from the surface of the steel plate, the steel plate itself cannot ensure sufficient strength. Therefore, in the steel plate of the present invention, the maximum value of the hardness in the plate thickness direction (Vickers hardness (HV1)) is defined as being at a position of 1.0 mm or more and 1/4 or less of the plate thickness from the surface of the steel plate. .
[板厚方向の硬さのばらつきが70HV1以下]
 板厚方向の硬さのばらつきが大きい場合、鋼板の均一伸びが低下するばかりか、加速冷却で導入される内部応力に起因する残留応力が大きくなるため、耐アンモニアSCC特性の劣化が懸念される。よって、本発明では、板厚方向の硬さのばらつきは70HV1以下に規定する。
 ここで、上記ばらつきは、板厚方向に、0.5mmピッチでビッカース硬さ(HV1)を測定し、その最大値と最小値の差を求めることにより算出する。
[Hardness variation in plate thickness direction is 70HV1 or less]
If the hardness variation in the sheet thickness direction is large, not only will the uniform elongation of the steel sheet decrease, but also the residual stress due to the internal stress introduced by accelerated cooling will increase, so there is concern that the ammonia SCC resistance will deteriorate. . Therefore, in the present invention, the variation in hardness in the plate thickness direction is specified to be 70HV1 or less.
Here, the variation is calculated by measuring the Vickers hardness (HV1) at a pitch of 0.5 mm in the plate thickness direction and obtaining the difference between the maximum value and the minimum value.
[0.5mm位置におけるベイナイトの体積率が90%以上]
 強度特性や耐アンモニアSCC性を満足させるためには、0.5mm位置における組織を、ベイナイトの体積率が90%以上とする必要がある。表層部は、マルテンサイト組織や島状マルテンサイト(MA)組織等の硬質相が生成すると、表層硬さが上昇し、鋼板内の硬さのばらつきが増大して材質均一性が阻害される。すなわち、ベイナイトの体積率が90%未満であると、これ以外の組織、すなわちフェライト、島状マルテンサイト組織、マルテンサイト組織、パーライト組織、オーステナイト組織の体積分率が増加することになり、十分な強度/または耐アンモニアSCC性が得られない。
[Bainite volume ratio at 0.5 mm position is 90% or more]
In order to satisfy strength characteristics and ammonia SCC resistance, it is necessary to make the volume fraction of bainite 90% or more in the structure at the 0.5 mm position. When a hard phase such as a martensite structure or an island-shaped martensite (MA) structure is generated in the surface layer, the surface layer hardness increases, and the variation in hardness within the steel sheet increases, impairing the uniformity of the material quality. That is, when the volume fraction of bainite is less than 90%, the volume fractions of other structures, that is, ferrite, island-shaped martensite, martensite, pearlite, and austenite, increase. Strength/or ammonia SCC resistance is not obtained.
 ここで、ベイナイトは、変態強化に寄与する加速冷却時あるいは加速冷却後に変態するベイニティックフェライトまたはグラニュラーフェライトと称される組織、またそれらが焼き戻された組織を含むものとする。 Here, bainite includes a structure called bainitic ferrite or granular ferrite that transforms during or after accelerated cooling that contributes to transformation strengthening, and a structure obtained by tempering them.
 体積率で10%以下を占める残部組織には、フェライト、パーライト組織およびオーステナイト組織の他、マルテンサイト組織が含まれていてもよい。残部組織における各組織の分率は特に限定する必要はないが、残部組織はパーライト組織であることが好ましい。
 なお、各種金属組織の体積率は、後述の実施例に記載した方法で測定することができる。
The remaining structure occupying 10% or less in volume fraction may include a martensite structure in addition to the ferrite, pearlite, and austenite structures. The fraction of each structure in the remaining structure is not particularly limited, but the remaining structure is preferably a pearlite structure.
In addition, the volume fraction of various metal structures can be measured by the method described in Examples below.
(3)製造条件について
 本発明における製造方法は、鋼板について前述したものと同様の成分組成を有する鋼素材について、加熱し熱間圧延を行った後、加速冷却を行い、次いで再加熱を行うものである。以下に、鋼板の製造条件の限定理由について説明する。
 まず、鋼素材の製造条件は、特に限定する必要はないが、例えば、前述した成分組成を有する溶鋼を、転炉等の公知の溶製方法で溶製し、連続鋳造法等の公知の鋳造方法にて、所定寸法のスラブ等の鋼素材とすることが好ましい。なお、造塊-分解圧延法により、所定寸法のスラブ等の鋼素材としても何ら問題はない。
(3) Manufacturing conditions In the manufacturing method of the present invention, a steel material having the same chemical composition as that described above for the steel sheet is heated and hot-rolled, then accelerated cooling is performed, and then reheating is performed. is. Reasons for limiting the manufacturing conditions of the steel sheet will be described below.
First, the manufacturing conditions of the steel material need not be particularly limited. It is preferable to use a steel material such as a slab of predetermined dimensions in the method. It should be noted that there is no problem in making a steel material such as a slab having a predetermined size by the ingot casting-decomposition rolling method.
 かようにして得られた鋼素材は、冷却することなく直接熱間圧延するか、あるいは再度加熱してから熱間圧延する。熱間圧延は、圧延終了温度をAr変態点以上として行い、次いで、Ar変態点以上の冷却開始温度からの加速冷却を所定条件で行い、次いで、再加熱を所定条件で行う。 The steel material thus obtained is directly hot-rolled without cooling or hot-rolled after reheating. Hot rolling is performed with a rolling end temperature equal to or higher than the Ar 3 transformation point, then accelerated cooling from a cooling start temperature equal to or higher than the Ar 3 transformation point is performed under predetermined conditions, and then reheating is performed under predetermined conditions.
 鋼素材の加熱温度は特に限定されないが、加熱温度が低すぎると変形抵抗が高くなって、熱間圧延機への負荷が増大し、熱間圧延が困難になるおそれがある。一方、1300℃を超える高温になると、酸化が著しくなって酸化ロスが増大し、歩留りが低下するおそれが増える。このような理由から、加熱温度は、950℃以上1300℃以下にすることが好ましい。 The heating temperature of the steel material is not particularly limited, but if the heating temperature is too low, the deformation resistance increases, the load on the hot rolling mill increases, and hot rolling may become difficult. On the other hand, if the temperature exceeds 1300° C., the oxidation becomes significant, the oxidation loss increases, and the yield increases. For these reasons, the heating temperature is preferably 950° C. or higher and 1300° C. or lower.
(熱間圧延)
[圧延終了温度:Ar変態点以上]
 本発明では、鋼素材を上記温度に加熱後、熱間圧延を開始して、Ar3変態点以上で当該熱間圧延を終了する。
 圧延終了温度がAr3変態点未満となると、フェライトが生成し、鋼板表層部での材質均一性が阻害され、硬さのばらつきが増大するため、耐アンモニアSCC性が劣化する。また、生成したフェライトが加工の影響を受けるため、靭性が悪化することになる。さらには、熱間圧延機への負荷が大きくなる。
 従って、本発明における熱間圧延における圧延終了温度は、Ar3変態点以上とする。上記圧延終了温度は、より好ましくは、Ar3変態点+10℃以上である。一方、圧延終了温度が950℃を超えると、組織が粗大化し靭性が劣化するおそれがあるため、圧延終了温度は、950℃以下とすることが好ましい。
 ここで、Ar3変態点(℃)は、次式で求めることが可能である。
 Ar3(℃)=910-310×C-80×Mn-20×Cu-15×Cr-55×Ni-80×Mo
  ただし、各元素は当該元素の鋼中含有量(質量%)を示す。
(hot rolling)
[Rolling end temperature: Ar 3 transformation point or higher]
In the present invention, after heating the steel material to the above temperature, hot rolling is started, and the hot rolling is finished at the Ar 3 transformation point or higher.
When the rolling end temperature is lower than the Ar 3 transformation point, ferrite is generated, the uniformity of the material in the surface layer of the steel sheet is hindered, and the variation in hardness increases, thereby deteriorating the ammonia SCC resistance. In addition, since the generated ferrite is affected by working, the toughness deteriorates. Furthermore, the load on the hot rolling mill increases.
Therefore, the rolling end temperature in hot rolling in the present invention is set to the Ar 3 transformation point or higher. The rolling end temperature is more preferably Ar 3 transformation point +10°C or higher. On the other hand, if the rolling end temperature exceeds 950°C, the structure may coarsen and the toughness may deteriorate, so the rolling end temperature is preferably 950°C or less.
Here, the Ar 3 transformation point (°C) can be obtained by the following formula.
Ar 3 (° C.)=910-310×C-80×Mn-20×Cu-15×Cr-55×Ni-80×Mo
However, each element indicates the content of the element in steel (% by mass).
(加速冷却)
[冷却開始温度:Ar変態点以上]
 次に、熱間圧延後の鋼板について、Ar3変態点以上の冷却開始温度からの加速冷却を行う。冷却開始温度がAr3変態点未満では、フェライトが過剰に生成し、また、冷却速度が大きくなるため、強度差が大きいマルテンサイト組織あるいはベイナイトと共存することになる結果、強度不足や靭性の劣化が生じ、さらには耐アンモニアSCC性が劣化する。そのため、冷却開始温度はAr3変態点以上とする。
(accelerated cooling)
[Cooling start temperature: Ar 3 transformation point or higher]
Next, the hot-rolled steel sheet is subjected to accelerated cooling from a cooling start temperature equal to or higher than the Ar 3 transformation point. If the cooling start temperature is less than the Ar 3 transformation point, ferrite is excessively formed and the cooling rate increases, so it coexists with the martensitic structure or bainite, which has a large difference in strength, resulting in insufficient strength and deterioration of toughness. is generated, and further the ammonia SCC resistance deteriorates. Therefore, the cooling start temperature should be the Ar 3 transformation point or higher.
[鋼板の板厚の1/4位置における冷却速度:20~120℃/s]
 鋼板の板厚の1/4位置における冷却速度を20℃/s以上で行う加速冷却は、高強度で高靱性の鋼板を得るために不可欠なプロセスであり、高い冷却速度で冷却することで変態強化による強度上昇効果が得られる。よって、かかる効果を得るため、本発明に従う加速冷却時の上記鋼板の板厚の1/4位置における冷却速度を20℃/s以上に規定する。一方、上記冷却速度が120℃/sを超えると、マルテンサイトの体積率が多くなりすぎてしまい、靭性が低下する。従って、上記鋼板の板厚の1/4位置における冷却速度は、120℃/s以下に規定する。
 なお、上記の冷却速度は、水冷等の積極的な冷却操作により高めることができ、また、適宜上記冷却操作を間欠的に行う(冷却操作を停止する期間を設ける)ことで、制御可能である。また、上記鋼板の板厚の1/4位置における温度は、物理的に直接測定することは困難である。しかし、放射温度計にて測定された冷却開始時の表面温度と目標の冷却停止時の表面温度とをもとに、例えばプロセスコンピューターを用いて差分計算を行うことにより、板厚断面内の温度分布、特には板厚の1/4位置における温度を、リアルタイムに求めることができる。
[Cooling rate at 1/4 position of steel plate thickness: 20 to 120 ° C./s]
Accelerated cooling in which the cooling rate at the position of 1/4 of the plate thickness of the steel plate is 20 ° C./s or more is an indispensable process for obtaining a high-strength and high-toughness steel plate. A strength increase effect can be obtained by strengthening. Therefore, in order to obtain such an effect, the cooling rate at the position of 1/4 of the plate thickness of the steel plate during accelerated cooling according to the present invention is specified to be 20° C./s or more. On the other hand, when the cooling rate exceeds 120° C./s, the volume fraction of martensite becomes too large, resulting in a decrease in toughness. Therefore, the cooling rate at the 1/4 position of the plate thickness of the steel plate is specified to be 120° C./s or less.
The cooling rate can be increased by active cooling operation such as water cooling, and can be controlled by intermittently performing the cooling operation as appropriate (providing a period in which the cooling operation is stopped). . Moreover, it is difficult to physically and directly measure the temperature at the 1/4 position of the plate thickness of the steel plate. However, based on the surface temperature at the start of cooling measured by a radiation thermometer and the target surface temperature at the end of cooling, for example, by using a process computer to calculate the difference, the temperature in the thickness cross section The distribution, particularly the temperature at the 1/4 position of the plate thickness, can be obtained in real time.
[冷却停止温度:200~600℃]
 本発明では、熱間圧延の終了後に、200~600℃の範囲で任意に設定した冷却停止温度まで所定の加速冷却を行うことにより、板厚中心部にてフェライトおよびベイナイトを所定の体積率にすることができ、強度や靭性を良好に向上させることができる。
 ここで、上記冷却停止温度が200℃未満では、島状マルテンサイトの組織の体積率が多くなりすぎてしまい、靭性が低下する。一方、上記冷却停止温度が600℃を超えると、フェライトやパーライトの組織が過剰に生成して、強度不足や靭性の劣化を招く。従って、冷却停止温度は200~600℃の範囲に規定する。また、本発明における冷却停止温度は、鋼板の板厚の1/4位置における温度である。
[Cooling stop temperature: 200 to 600°C]
In the present invention, after hot rolling is completed, a predetermined accelerated cooling is performed to a cooling stop temperature arbitrarily set in the range of 200 to 600 ° C., so that ferrite and bainite are reduced to a predetermined volume ratio at the center of the plate thickness. It is possible to improve strength and toughness satisfactorily.
Here, if the cooling stop temperature is less than 200° C., the volume fraction of the island-shaped martensite structure becomes too large, resulting in a decrease in toughness. On the other hand, if the cooling stop temperature exceeds 600° C., ferrite and pearlite structures are excessively formed, resulting in insufficient strength and deterioration of toughness. Therefore, the cooling stop temperature is specified in the range of 200 to 600°C. Further, the cooling stop temperature in the present invention is the temperature at the 1/4 position of the plate thickness of the steel plate.
(再加熱)
[表面から0.5mm位置における到達温度が400~680℃]
 本発明では、前記加速冷却の後、再加熱する必要がある。厚鋼板を加速冷却すると、鋼板表層部の冷却速度が速くなり、また鋼板内部に比べかかる鋼板表層部が低い温度まで冷却される。そのため、鋼板表層部は、マルテンサイトなどの硬い組織が生成しやすく、耐アンモニアSCC性が劣化するおそれがある。よって、本発明では、加速冷却後に鋼板表層部を再加熱する。表層部の硬さを低下することが可能となるからである。好ましくは、加速冷却の後、直ちに再加熱を行う。
 ここで、かかる表面から0.5mm位置における再加熱の温度が、400℃未満であると硬さの低下が十分ではない一方、680℃を超えると、鋼板全体の強度の低下が生じるため、所定の強度を得ることが困難となる。
 従って、加速冷却後の再加熱時の表面から0.5mm位置における到達温度は、400~680℃の範囲に規定する。
(reheat)
[Attainment temperature at 0.5 mm position from the surface is 400 to 680 ° C.]
In the present invention, it is necessary to reheat after the accelerated cooling. Accelerated cooling of a thick steel plate increases the cooling rate of the surface layer of the steel plate and cools the surface layer of the steel plate to a lower temperature than the inside of the steel plate. Therefore, a hard structure such as martensite is likely to form in the surface layer of the steel sheet, and the ammonia SCC resistance may be deteriorated. Therefore, in the present invention, the surface layer portion of the steel sheet is reheated after accelerated cooling. This is because the hardness of the surface layer portion can be reduced. Preferably, reheating is performed immediately after accelerated cooling.
Here, if the reheating temperature at the position 0.5 mm from the surface is less than 400°C, the hardness is not sufficiently reduced, while if it exceeds 680°C, the strength of the entire steel sheet is reduced. It becomes difficult to obtain the strength of
Therefore, the temperature reached at a position 0.5 mm from the surface during reheating after accelerated cooling is specified in the range of 400 to 680.degree.
[鋼板の板厚の1/4位置における到達温度が500℃以下]
 なお、再加熱時に鋼板の板厚の1/4位置における到達温度が500℃を超えた場合、強度の低下や靭性の劣化が生じる。従って、再加熱時の鋼板の板厚の1/4位置における到達温度は、500℃以下に規定する。
[Attainment temperature at 1/4 position of steel plate thickness is 500 ° C. or less]
If the temperature reached at the position of 1/4 of the plate thickness of the steel plate during reheating exceeds 500° C., the strength and toughness are deteriorated. Therefore, the temperature reached at the position of 1/4 of the plate thickness of the steel plate during reheating is specified to be 500° C. or less.
 加速冷却後における、前記再加熱の手段としては、誘導加熱を用いることが好ましい。特に、加熱が鋼板表層部に集中するよう、高周波誘導加熱を用いることが好ましい。また、再加熱後には、適宜、冷却を行うことができる。再加熱後の冷却については特に限定しないが、板厚40mm程度を超えるような厚鋼板において、冷却速度が遅くなり、炭化物の凝集粗大化による靭性劣化が懸念される場合がある。かかる場合には、再加熱処理後に水冷やミストによる冷却を行ってもよい。 It is preferable to use induction heating as the reheating means after accelerated cooling. In particular, it is preferable to use high-frequency induction heating so that the heating concentrates on the surface layer of the steel sheet. Moreover, after reheating, cooling can be performed as appropriate. Cooling after reheating is not particularly limited, but in a thick steel plate having a thickness exceeding about 40 mm, the cooling rate becomes slow, and there is a concern that toughness is deteriorated due to agglomeration and coarsening of carbides. In such a case, water cooling or mist cooling may be performed after the reheating treatment.
 上記した成分組成を有する鋼素材を、上記した製造条件に従って製造することによって、本発明に従う成分組成並びに硬さ特性および金属組織を有する鋼板を得ることができる。かくして得られた鋼板は、優れた強度特性と靭性とを備え、耐アンモニアSCC性に優れた鋼板になる。ここで、優れた強度特性とは、降伏強さYS(降伏点があるときは降伏点YP、ないときは0.2%耐力σ0.2):450MPa以上、引張強さ(TS):570MPa以上および均一伸び(uEl):10%以上である。また、優れた靭性とは、JIS Z 2241に準拠するvTrsが-30℃以下である。そしてこれらの特性を有する鋼板が、本発明の耐アンモニアSCC性に優れた鋼板である。 By manufacturing a steel material having the above chemical composition according to the above manufacturing conditions, it is possible to obtain a steel plate having the chemical composition, hardness characteristics and metallographic structure according to the present invention. The steel sheet thus obtained has excellent strength characteristics and toughness, and is excellent in ammonia SCC resistance. Here, the excellent strength characteristics are yield strength YS (yield point YP when there is a yield point, 0.2% yield strength σ0.2 when there is no yield point): 450 MPa or more, tensile strength (TS): 570 MPa or more and uniform elongation (uEl): 10% or more. In addition, excellent toughness means that vTrs conforming to JIS Z 2241 is -30°C or less. A steel sheet having these properties is the steel sheet of the present invention having excellent ammonia SCC resistance.
 なお、本発明に従う製造方法では、本明細書に記載のない項目は、いずれも常法を用いることができる。 In addition, in the manufacturing method according to the present invention, any item not described in this specification can be used by a conventional method.
 表1に示す成分組成の鋼(鋼種A~AI、残部はFeおよび不可避的不純物)を連続鋳造法によりスラブとし、表2に示す条件で、熱間圧延、加速冷却、再加熱を順次行い、板厚30mmの厚鋼板(No.1~50)を得た。得られた鋼板について、板厚の鋼板表面から0.5mm位置における金属組織の組織分率の測定、硬さ特性の評価、強度特性および靭性の評価、耐アンモニアSCC性の評価を実施した。各試験方法は次のとおりである。また、これらの結果を、表2に併記する。 A slab is formed by continuous casting of steel having the chemical composition shown in Table 1 (steel grades A to AI, the balance being Fe and unavoidable impurities), and hot rolling, accelerated cooling, and reheating are sequentially performed under the conditions shown in Table 2, Thick steel plates (No. 1 to 50) with a thickness of 30 mm were obtained. The obtained steel sheet was subjected to measurement of the metal structure fraction at a position of 0.5 mm from the surface of the steel sheet, evaluation of hardness characteristics, evaluation of strength characteristics and toughness, and evaluation of ammonia SCC resistance. Each test method is as follows. These results are also shown in Table 2.
[鋼板表面から0.5mm位置における金属組織の組織分率]
 各鋼板より、その0.5mm位置が観察面となるように、サンプルを採取した。次いで、かかるサンプルを鏡面研磨し、さらにナイタール腐食をした後、走査型電子顕微鏡(SEM)を用いて10mm×10mmの範囲を倍率:500~3000倍で撮影した。そして、撮影された像について、画像解析装置を用いて解析することによって、ミクロ組織の面分率(金属組織の組織分率)を求めた。ミクロ組織の異方性が小さい場合、面分率は体積率に相当するため、本発明では面分率を体積率と見なした。
[Structural fraction of metal structure at 0.5 mm position from steel plate surface]
A sample was taken from each steel plate so that the 0.5 mm position was the observation surface. Then, the sample was mirror-polished, and after being subjected to nital corrosion, a scanning electron microscope (SEM) was used to photograph an area of 10 mm×10 mm at a magnification of 500 to 3000. Then, the photographed image was analyzed using an image analyzer to determine the area fraction of the microstructure (structure fraction of the metal structure). When the anisotropy of the microstructure is small, the area fraction corresponds to the volume fraction, so in the present invention the area fraction is regarded as the volume fraction.
 なお、本実施例において、サンプルの金属組織の分率を求める際の判別は、次のとおりに行った。
 すなわち、上述の撮影された像において、ポリゴナル状のフェライトをフェライトと判別し、また細長く成長したラス状のフェライトを有し、円相当径で0.05μm以上の炭化物を含む組織をベイナイト(表2におけるB)と判別した。
In addition, in the present example, the determination when obtaining the fraction of the metal structure of the sample was performed as follows.
That is, in the photographed images described above, the polygonal ferrite is discriminated as ferrite, and the bainite (Table 2 It was determined as B) in
[硬さ特性]
 各鋼板の圧延方向に垂直な断面について、JIS Z 2244に準拠して、0.5mm位置において100点のビッカース硬さ(HV0.1)を測定し、その平均値を求めた。また、かかる100点のビッカース硬さの標準偏差を求め、0.5mm位置における硬さのばらつきとした。ここで、通常、鋼板の硬度測定に用いられるHV10に代えてHV0.1を用いたのは、HV0.1で測定することにより圧痕が小さくなるので、より表面に近い位置での硬さ情報や、よりミクロ組織に敏感な硬さ情報を得ることが可能となるからである。
 また、板厚方向でビッカース硬さ(HV1)を測定し、その最大値が在る板厚方向の位置(表面からの距離)を測定した。さらに、かかる測定でのビッカース硬さ(HV1)の最大値と最小値の差を算出し、板厚方向の硬さのばらつきとした。
[Hardness characteristics]
Regarding the cross section perpendicular to the rolling direction of each steel plate, the Vickers hardness (HV0.1) was measured at 100 points at a position of 0.5 mm according to JIS Z 2244, and the average value was obtained. Also, the standard deviation of the Vickers hardness of 100 points was obtained, and the variation in hardness at the 0.5 mm position was obtained. Here, the reason why HV0.1 is used instead of HV10, which is usually used for hardness measurement of a steel plate, is that the indentation becomes smaller by measuring with HV0.1, so that hardness information at a position closer to the surface can be obtained. , it is possible to obtain hardness information more sensitive to the microstructure.
Also, the Vickers hardness (HV1) was measured in the plate thickness direction, and the position in the plate thickness direction (distance from the surface) where the maximum value exists was measured. Further, the difference between the maximum value and the minimum value of Vickers hardness (HV1) in such measurement was calculated and used as variation in hardness in the plate thickness direction.
[強度特性]
 各鋼板の全厚から、圧延方向に直角の方向が試験片長手方向となるようにJIS Z 2201の1B号試験片を採取して、JIS Z 2241に記載の要領で引張試験を行い、降伏強さYS(降伏点があるときは降伏点YP、ないときは0.2%耐力σ0.2)、引張強さ(TS)および均一伸び(uEl)を測定した。そして降伏強さが450MPa以上、引張強さが570MPa以上および均一伸びが10%以上のものを強度特性に優れた鋼板と評価した。
[Strength characteristics]
From the total thickness of each steel plate, a JIS Z 2201 No. 1B test piece is taken so that the direction perpendicular to the rolling direction is the longitudinal direction of the test piece, and a tensile test is performed according to the procedure described in JIS Z 2241 to obtain the yield strength. YS (yield point YP when there is a yield point, 0.2% yield strength σ0.2 when there is no yield point), tensile strength (TS) and uniform elongation (uEl) were measured. A steel sheet having a yield strength of 450 MPa or more, a tensile strength of 570 MPa or more, and a uniform elongation of 10% or more was evaluated as having excellent strength characteristics.
[靭性]
 各鋼板の表面側から1mm削った部位から、圧延方向が試験片長手方向となるようにJIS Z 2202のVノッチ試験片を採取して、JIS Z 2242の要領でシャルピー衝撃試験を行い、vTrs(破面遷移温度)を測定した。そして、かかるvTrsが-30℃以下のものを、靭性に優れた鋼板と評価した。
[Toughness]
A V-notch test piece of JIS Z 2202 was collected from a portion cut by 1 mm from the surface side of each steel plate so that the rolling direction was the longitudinal direction of the test piece, and a Charpy impact test was performed according to JIS Z 2242. fracture surface transition temperature) was measured. A steel sheet with such vTrs of -30°C or less was evaluated as a steel sheet having excellent toughness.
[耐アンモニアSCC性]
 耐アンモニアSCC性は、試験溶液内で4点曲げ試験を実施し、腐食を促進させるため定電位アノード電解した促進試験により評価した。
 具体的には、以下の手順で実施した:
 鋼板表面から、5mm厚×15mm×115mmの試験片を採取して、アセトン中で超音波脱脂を5分間行い、4点曲げにより各鋼板の降伏強さに等しい応力を負荷した。かかる4点曲げの試験片を設置した試験セルに、カルバミン酸アンモニウム12.5gと液体アンモニア1Lとを混合した溶液を充填した後、ポテンショスタットにより、試験片に+2.0V vs Ptの電圧を印加し、室温(25℃)で浸漬した。168時間の浸漬後に、割れが認められない場合を、耐アンモニアSCC性が「良」と判定し、また割れが発生した場合を、耐アンモニアSCC性が「不良」と判定した。
[Ammonia SCC resistance]
Ammonia SCC resistance was evaluated by an accelerated test in which a four-point bending test was performed in a test solution and constant potential anodic electrolysis was performed to promote corrosion.
Specifically, we performed the following steps:
A test piece having a thickness of 5 mm×15 mm×115 mm was taken from the surface of the steel plate, subjected to ultrasonic degreasing in acetone for 5 minutes, and stress equal to the yield strength of each steel plate was applied by four-point bending. After filling a test cell in which such a 4-point bending test piece was installed with a mixed solution of 12.5 g of ammonium carbamate and 1 L of liquid ammonia, a potentiostat applied a voltage of +2.0 V vs Pt to the test piece. and immersed at room temperature (25°C). After immersion for 168 hours, the ammonia SCC resistance was determined to be "good" when cracks were not observed, and the ammonia SCC resistance was determined to be "poor" when cracks occurred.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-I000003
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-I000003
 表1および表2から分かるように、発明例は、いずれも、450MPa以上の降伏強度YSと570MPa以上の引張強度TSと10%以上の均一伸びuElをもち、vTrsが-30℃以下であり、低温での靭性および耐アンモニアSCC性に優れた鋼板が得られている。 As can be seen from Tables 1 and 2, the invention examples all have a yield strength YS of 450 MPa or more, a tensile strength TS of 570 MPa or more, and a uniform elongation uEl of 10% or more, vTrs is -30 ° C. or less, A steel sheet excellent in low-temperature toughness and ammonia SCC resistance is obtained.
 一方、No.31~39は、成分組成が本発明の範囲内であるものの、製造方法が本発明の範囲外であるため、所望の金属組織および/または硬さ特性が得られていない。その結果、降伏強度YS、引張強度TS、低温での靱性、あるいは耐アンモニアSCC性のいずれかが劣っている。 On the other hand, No. In Nos. 31 to 39, although the component composition is within the scope of the present invention, the manufacturing method is outside the scope of the present invention, so the desired metallographic structure and/or hardness properties are not obtained. As a result, the yield strength YS, tensile strength TS, toughness at low temperature, or resistance to ammonia SCC is inferior.
 また、No.40~50は、鋼の成分組成が本発明の範囲外であるため、降伏強度YS、引張強度TS、低温での靱性、あるいは耐アンモニアSCC性のいずれかが劣っている。なお、本発明では、鋼の成分組成は、そのまま鋼板の成分組成と考えてよい。 Also, No. In Nos. 40 to 50, the chemical composition of the steel is outside the range of the present invention, so the yield strength YS, tensile strength TS, toughness at low temperatures, or ammonia SCC resistance are inferior. In addition, in the present invention, the chemical composition of the steel may be considered as the chemical composition of the steel sheet.

Claims (6)

  1.  質量%で、
     C:0.010~0.200%、
     Si:0.01~0.50%、
     Mn:0.50~2.50%、
     Al:0.010~0.060%
     N:0.0010%以上0.0100%以下、
     P:0.020%以下、
     S:0.0100%以下および
     O:0.0100%以下
    を含有し、残部がFeおよび不可避的不純物である成分組成を有する鋼板であって、
     前記鋼板の表面から0.5mm深さの位置において、平均硬さが230HV0.1以下で、硬さのばらつきが30HV0.1以下であり、かつ、板厚方向の硬さの最大値が鋼板の表面から1.0mm以上板厚1/4以下の位置に在り、当該板厚方向の硬さのばらつきが70HV1以下である硬さ特性と、
     前記鋼板の表面から0.5mm深さの位置におけるベイナイト組織の体積率が90%以上である金属組織と、を有する、鋼板。
    in % by mass,
    C: 0.010 to 0.200%,
    Si: 0.01 to 0.50%,
    Mn: 0.50-2.50%,
    Al: 0.010-0.060%
    N: 0.0010% or more and 0.0100% or less,
    P: 0.020% or less,
    A steel sheet having a chemical composition containing S: 0.0100% or less and O: 0.0100% or less, with the balance being Fe and inevitable impurities,
    At a depth of 0.5 mm from the surface of the steel sheet, the average hardness is 230 HV0.1 or less, the hardness variation is 30 HV0.1 or less, and the maximum hardness in the thickness direction is the steel sheet. A hardness characteristic that is located at a position of 1.0 mm or more and 1/4 or less of the plate thickness from the surface and has a hardness variation of 70 HV1 or less in the plate thickness direction;
    A steel sheet having a metal structure in which the volume fraction of the bainite structure at a position 0.5 mm deep from the surface of the steel sheet is 90% or more.
  2.  前記成分組成が、さらに、質量%で、
     Cu:0.01~0.50%、
     Ni:0.01~2.00%、
     Cr:0.01~1.00%、
     Sn:0.01~0.50%、
     Sb:0.01~0.50%、
     Mo:0.01~0.50%および
     W:0.01~1.00%
    のうちから選ばれる1種以上を含有する、請求項1に記載の鋼板。
    The component composition further, in mass %,
    Cu: 0.01-0.50%,
    Ni: 0.01 to 2.00%,
    Cr: 0.01 to 1.00%,
    Sn: 0.01 to 0.50%,
    Sb: 0.01 to 0.50%,
    Mo: 0.01-0.50% and W: 0.01-1.00%
    The steel sheet according to claim 1, containing one or more selected from.
  3.  前記成分組成が、さらに、質量%で、
     V:0.01~1.00%、
     Ti:0.005~0.100%、
     Co:0.01~1.00%、
     Nb:0.005~0.100%、
     B:0.0001~0.0100%、
     Ca:0.0005~0.0200%、
     Mg:0.0005~0.0200%および
     REM:0.0005~0.0200%
    のうちから選ばれる1種以上を含有する、請求項1または請求項2に記載の鋼板。
    The component composition further, in mass %,
    V: 0.01 to 1.00%,
    Ti: 0.005 to 0.100%,
    Co: 0.01 to 1.00%,
    Nb: 0.005 to 0.100%,
    B: 0.0001 to 0.0100%,
    Ca: 0.0005 to 0.0200%,
    Mg: 0.0005-0.0200% and REM: 0.0005-0.0200%
    The steel sheet according to claim 1 or 2, containing one or more selected from.
  4.  質量%で、
     C:0.010~0.200%、
     Si:0.01~0.50%、
     Mn:0.50~2.50%、
     Al:0.010~0.060%、
     N:0.0010%以上0.0100%以下、
     P:0.020%以下、
     S:0.0100%以下および
     O:0.0100%以下
    を含有し、残部がFeおよび不可避的不純物である成分組成を有する鋼素材について、圧延終了温度をAr変態点以上として熱間圧延を行い、次いでAr変態点以上の冷却開始温度からの加速冷却を行い、次いで再加熱を行う、鋼板の製造方法であって、
     前記加速冷却では、冷却停止温度を200~600℃の範囲とし、かつ、鋼板の板厚の1/4位置における冷却速度を20~120℃/sとし、
     前記再加熱は、鋼板の板厚の1/4位置における到達温度を500℃以下として、鋼板の表面から0.5mm深さの位置における到達温度が400~680℃の範囲となるまで行う、鋼板の製造方法。
    in % by mass,
    C: 0.010 to 0.200%,
    Si: 0.01 to 0.50%,
    Mn: 0.50-2.50%,
    Al: 0.010 to 0.060%,
    N: 0.0010% or more and 0.0100% or less,
    P: 0.020% or less,
    A steel material having a chemical composition containing S: 0.0100% or less and O: 0.0100% or less, with the balance being Fe and inevitable impurities, is subjected to hot rolling at a rolling end temperature of Ar 3 transformation point or higher. A method for producing a steel sheet, comprising :
    In the accelerated cooling, the cooling stop temperature is in the range of 200 to 600 ° C., and the cooling rate at the 1/4 position of the plate thickness of the steel plate is 20 to 120 ° C./s,
    The reheating is performed until the temperature reached at a position 1/4 of the plate thickness of the steel plate is 500 ° C. or less, and the temperature reached at a depth of 0.5 mm from the surface of the steel plate is in the range of 400 to 680 ° C. The steel plate. manufacturing method.
  5.  前記鋼素材の成分組成が、さらに、質量%で、
     Cu:0.01~0.50%、
     Ni:0.01~2.00%、
     Cr:0.01~1.00%、
     Sn:0.01~0.50%、
     Sb:0.01~0.50%、
     Mo:0.01~0.50%および
     W:0.01~1.00%
    のうちから選ばれる1種以上を含有する、請求項4に記載の鋼板の製造方法。
    The chemical composition of the steel material is further, in mass%,
    Cu: 0.01-0.50%,
    Ni: 0.01 to 2.00%,
    Cr: 0.01 to 1.00%,
    Sn: 0.01 to 0.50%,
    Sb: 0.01 to 0.50%,
    Mo: 0.01-0.50% and W: 0.01-1.00%
    The method for producing a steel sheet according to claim 4, containing one or more selected from.
  6.  前記鋼素材の成分組成が、さらに、質量%で、
     V:0.01~1.00%、
     Ti:0.005~0.100%、
     Co:0.01~1.00%、
     Nb:0.005~0.100%、
     B:0.0001~0.0100%、
     Ca:0.0005~0.0200%、
     Mg:0.0005~0.0200%および
     REM:0.0005~0.0200%
    のうちから選ばれる1種以上を含有する、請求項4または請求項5に記載の鋼板の製造方法。
    The chemical composition of the steel material is further, in mass%,
    V: 0.01 to 1.00%,
    Ti: 0.005 to 0.100%,
    Co: 0.01 to 1.00%,
    Nb: 0.005 to 0.100%,
    B: 0.0001 to 0.0100%,
    Ca: 0.0005 to 0.0200%,
    Mg: 0.0005-0.0200% and REM: 0.0005-0.0200%
    The method for producing a steel sheet according to claim 4 or 5, containing one or more selected from.
PCT/JP2023/002491 2022-02-24 2023-01-26 Steel plate and method for manufacturing same WO2023162571A1 (en)

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