Steel sheet, member, and methods for producing same

Abstract

Provided are: a steel sheet having a tensile strength TS of at least 1320 MPa, a yield ratio YR of at least 75%, and excellent stretch flangeability, stress corrosion cracking resistance characteristics, and material stability; a member; and methods for producing the steel sheet and the member.　This steel sheet has a specific component composition, and has a structure in which: at a position 1 / 4 of the thickness of the steel sheet, the area fraction of tempered martensite is at least 95%, the volume fraction of residual austenite is less than 3%, and the total area fraction of ferrite and bainitic ferrite is less than 5%; and, in the tempered martensite, the area fraction of tempered martensite including at least five carbide particles having a particle size of 0.1-1.0 μm in particles surrounded by a grain boundary of at least 15° is 5-50%.

Description

Steel plates, components, and their manufacturing methods

[0001] The present invention relates to a steel sheet and a member having excellent tensile strength, yield ratio, stretch flangeability, stress corrosion cracking resistance, and material stability, and a method for manufacturing the same. The steel sheet of the present invention can be suitably used as a structural member for automobile parts and the like.

[0002] CO2 emissions from vehicle weight reduction ２ With the aim of reducing emissions and improving crashworthiness by reducing the weight of the vehicle body, the strength of steel sheets for automobiles is being increased, and new regulations are being introduced one after another. Therefore, in order to increase the strength of the vehicle body, there are increasing cases of using high-strength steel sheets with a tensile strength of 1320 MPa or more in the main structural parts that make up an automobile.

[0003] High-strength steel sheets used in automobiles are required to have excellent yield ratios, excellent stretch-flangeability, and excellent stress corrosion cracking resistance. For example, for structural components such as automobile floor cross members, steel sheets with excellent stretch-flangeability are preferred from the standpoint of formability. Furthermore, from the standpoint of component performance, excellent yield ratios and excellent stress corrosion cracking resistance are required. Furthermore, a challenge in using high-strength steel sheets for components is the significant decrease in shape fixability during press forming due to the increased springback of the steel sheets caused by the increased strength of the steel sheets. Therefore, in the field of press technology, in order to ensure shape fixability, it is common to predict the amount of shape change after demolding during press forming and design the press die shape taking this into account. However, as the TS variation of the steel sheet used as the press material increases, the springback variation also increases, making it difficult to obtain parts with the same shape even when press-formed using the same die. This necessitates individual adjustments, such as sheet metal processing, after press forming, significantly reducing mass production efficiency. Therefore, there is a demand for minimizing TS variation, i.e., improving material stability.

[0004] Patent Document 1 discloses a high-strength steel sheet of 1180 MPa or more that is excellent in yield ratio, flatness in the sheet width direction, and work embrittlement resistance, and a method for manufacturing the same. However, the technology described in Patent Document 1 does not take into consideration high-strength steel sheets that are excellent in stretch flangeability, stress corrosion cracking resistance, and material stability.

[0005] Patent Document 2 discloses a high-strength steel sheet of 1180 MPa or more having excellent yield ratio and stretch flangeability, and a manufacturing method thereof. However, the technology described in Patent Document 2 does not take into consideration high-strength steel sheets having excellent stress corrosion cracking resistance and material stability.

[0006] Patent Document 3 discloses a method for manufacturing a high-strength steel sheet of 980 MPa or more that is excellent in ductility, stretch flangeability, and stability of mechanical properties. However, the technology described in Patent Document 3 does not take into consideration high-strength steel sheets that are excellent in yield ratio and stress corrosion cracking resistance.

[0007] Patent No. 7323093 Patent No. 6879441 Patent No. 5333298

[0008] The present invention was developed in view of the above circumstances, and aims to provide a steel plate, a member, and a method for manufacturing the same, which have a tensile strength TS of 1320 MPa or more, a yield ratio YR of 75% or more, and excellent stretch flangeability, stress corrosion cracking resistance, and material stability.

[0009] Here, the tensile strength TS (hereinafter also referred to as TS) and the yield ratio YR (hereinafter also referred to as YR) can be measured according to JIS Z 2241 (2022). Excellent stretch flangeability refers to a limiting hole expansion ratio (λ) (%) of 30% or more obtained by a hole expansion test in accordance with JIS Z 2256 (2020). Excellent stress corrosion cracking resistance refers to a test piece obtained from a steel plate being subjected to four-point bending according to ASTM (G39-99), stresses equivalent to YS and TS being applied to the bend apex of the test piece, and the test piece in the stressed state being immersed in 1% by mass sulfuric acid at 25 ° C. for 100 hours, and the test piece to which a stress equivalent to YS has been applied shows no cracks. Excellent material stability means that TS is measured at 10 or more locations at least 50 mm apart in the width direction of the plate, and the difference between the maximum TS and the minimum TS among the measured TS is calculated as TS variation (ΔTS = maximum TS - minimum TS), and ΔTS is less than 60 MPa.

[0010] The present inventors conducted extensive research to achieve the above-mentioned objectives and found the following: (1) By setting the amount of tempered martensite to 95% or more, a TS of 1320 MPa or more can be achieved. (2) By setting the total amount of ferrite and bainitic ferrite to less than 5%, excellent stretch flangeability can be achieved. (3) By setting the amount of retained austenite to less than 3%, a YR of 75% or more can be achieved. (4) By setting the amount of tempered martensite containing five or more carbides with a particle size of 0.1 μm to 1.0 μm within grains surrounded by grain boundaries at an angle of 15° or more to 5% to 50%, excellent stress corrosion cracking resistance and material stability can be achieved.

[0011] The present invention has been made based on the above findings. That is, the gist of the present invention is as follows: [1] A steel sheet having a chemical composition containing, by mass%, C: 0.030% or more and 0.450% or less, Si: 0.010% or more and 2.500% or less, Mn: 0.10% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, with the balance being Fe and unavoidable impurities, and at a 1 / 4 position in the sheet thickness, an area fraction of tempered martensite: 95% or more, a volume fraction of retained austenite: less than 3%, and a total area fraction of ferrite and bainitic ferrite: less than 5%, The steel plate has a structure satisfying that the area fraction of the tempered martensite containing five or more carbides with a grain size of 0.1 μm or more and 1.0 μm or less within grains surrounded by grain boundaries at an angle of 15° or more is 5% or more and 50% or less. [2] The composition further includes, in mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, [3] The steel sheet according to [1] above, which contains at least one element selected from the group consisting of: Bi: 0.200% or less. [3] The steel sheet according to [1] above or [2] above, which has a plating layer on the surface of the steel sheet. [4] A member made using the steel sheet according to any one of [1] to [3] above.[5] A cold-rolled sheet is produced by hot-rolling, pickling, and cold-rolling a steel having the chemical composition described in [1] or [2] above, and the steel sheet is heated under the conditions of an annealing temperature T1 of 800°C or higher, a holding time t1 at the annealing temperature T1 of 10 seconds or higher, and a cooling treatment including cooling at an average cooling rate CR1 of 700 to 500°C of 5°C / s or higher, and cooling by water quenching from a quenching start temperature T2 of (Ms-80°C) or higher to Ms or lower to 80°C of 300°C / s or higher, and a tempering temperature T3 of 100°C or higher but lower than 250°C, and a holding time t3 at the tempering temperature T3 of 10 seconds or higher and 10,000 seconds or lower, and in the cooling treatment in the annealing step, the time t2 during which the steel sheet is held in a temperature range of 250°C or higher and Ms or lower is 1.0 seconds or higher and 10.0 seconds or lower,

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[0012] According to the present invention, it is possible to obtain a steel sheet having a TS of 1320 MPa or more, a YR of 75% or more, and excellent stretch flangeability, stress corrosion cracking resistance, and material stability. Furthermore, by applying the steel sheet of the present invention to, for example, automotive structural members, it is possible to reduce the weight of the vehicle body and thereby improve fuel economy. Therefore, the steel sheet has extremely great industrial utility value.

[0013] FIG. 1 is a schematic diagram illustrating a method of applying pressure during water cooling in the method of manufacturing a steel sheet according to the present invention.

[0014] Hereinafter, an embodiment of the present invention will be described.

[0015] [Steel Sheet] The steel sheet of the present invention has a component composition containing C: 0.030% or more and 0.450% or less, Si: 0.010% or more and 2.500% or less, Mn: 0.10% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, with the balance being Fe and unavoidable impurities; The steel sheet has a structure that satisfies the following conditions at a 1 / 4 position in the sheet thickness: an area fraction of tempered martensite: 95% or more, a volume fraction of retained austenite: less than 3%, a total area fraction of ferrite and bainitic ferrite: less than 5%, and an area fraction of tempered martensite containing five or more carbides with a particle size of 0.1 μm or more and 1.0 μm or less, within grains of tempered martensite surrounded by grain boundaries at an angle of 15° or more, of 5% or more and 50% or less.

[0016] First, the appropriate range of the chemical composition of the steel sheet and the reasons for limiting it will be explained. In the following explanation, "%" representing the content of the component elements of the steel means "mass %" unless otherwise specified.

[0017] C: 0.030% or more and 0.450% or less C is one of the important basic components of steel, and in the present invention, it is an important element that affects the area fraction of tempered martensite (hereinafter also referred to as fraction) and stress corrosion cracking resistance. If the C content is less than 0.030%, the fraction of tempered martensite decreases, making it difficult to achieve a TS of 1320 MPa or more. On the other hand, if the C content exceeds 0.450%, the tempered martensite becomes embrittled, making it difficult to achieve excellent stress corrosion cracking resistance. Therefore, the C content is set to 0.030% or more and 0.450% or less. The C content is preferably set to 0.050% or more. The C content is more preferably set to 0.100% or more. The C content is preferably set to 0.400% or less. The C content is more preferably set to 0.350% or less.

[0018] Si: 0.010% or more and 2.500% or less Si is one of the important basic components of steel. In particular, in the present invention, Si suppresses carbide formation during continuous annealing and promotes the formation of retained austenite, making it an important element that affects TS and the amount of retained austenite. If the Si content is less than 0.010%, it becomes difficult to achieve a TS of 1320 MPa or more. On the other hand, if the Si content exceeds 2.500%, the retained austenite increases excessively, making it difficult to achieve a YR of 75% or more. Therefore, the Si content is set to 0.010% or more and 2.500% or less. The Si content is preferably set to 0.050% or more. The Si content is more preferably set to 0.100% or more. The Si content is preferably set to 2.000% or less. The Si content is more preferably set to 1.200% or less. The Si content is more preferably 0.500% or less, and further preferably 0.300% or less.

[0019] Mn: 0.10% or more and 5.00% or less Mn is one of the important basic components of steel, and in the present invention, it is an important element that affects the fraction of tempered martensite and stress corrosion cracking resistance. If the Mn content is less than 0.10%, the fraction of tempered martensite decreases, making it difficult to achieve a TS of 1320 MPa or more. On the other hand, if the Mn content exceeds 5.00%, corrosion of the grain boundaries of the steel sheet is promoted, making it difficult to achieve excellent stress corrosion cracking resistance. Therefore, the Mn content is set to 0.10% or more and 5.00% or less. The Mn content is preferably set to 0.50% or more. The Mn content is more preferably set to 0.80% or more. The Mn content is preferably set to 4.50% or less. The Mn content is more preferably set to 4.00% or less.

[0020] P: 0.100% or less P segregates at prior austenite grain boundaries and embrittles the grain boundaries. Therefore, if the P content exceeds 0.100%, it becomes difficult to achieve excellent stress corrosion cracking resistance. Therefore, the P content must be 0.100% or less. The P content is preferably 0.070% or less. The P content is more preferably 0.050% or less, and even more preferably 0.020% or less. Although there is no particular lower limit for the P content, since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferably 0.001% or more. The P content is more preferably 0.003% or more, and even more preferably 0.005% or more.

[0021] S: 0.0200% or less S exists as sulfides and becomes the initiation site of stress corrosion cracking. Therefore, if the S content exceeds 0.0200%, it becomes difficult to achieve excellent stress corrosion cracking resistance. Therefore, the S content must be 0.0200% or less. The S content is preferably 0.0050% or less. The S content is more preferably 0.0030% or less, and even more preferably 0.0020% or less. There is no particular lower limit for the S content, but due to production technology constraints, it is preferably 0.0001% or more. The S content is more preferably 0.0002% or more.

[0022] Al: 1.000% or less Since Al exists as an oxide and serves as the initiation site for stress corrosion cracking, if the Al content exceeds 1.000%, it becomes difficult to achieve excellent stress corrosion cracking resistance. Therefore, the Al content must be 1.000% or less. The Al content is preferably 0.500% or less. The Al content is more preferably 0.200% or less, and even more preferably 0.100% or less. There is no particular lower limit for the Al content, but due to production technology constraints, it is preferably 0.001% or more. The Al content is more preferably 0.002% or more. The Al content is more preferably 0.005% or more, and even more preferably 0.010% or more.

[0023] N: 0.0100% or less N exists as a nitride and becomes the initiation site of stress corrosion cracking. Therefore, if the N content exceeds 0.0100%, it becomes difficult to achieve excellent stress corrosion cracking resistance. Therefore, the N content must be 0.0100% or less. The N content is preferably 0.0050% or less. Although there is no particular lower limit for the N content, due to constraints on production technology, the N content is preferably 0.0001% or more. The N content is more preferably 0.0002% or more. The N content is more preferably 0.0010% or more, and even more preferably 0.0020% or more.

[0024] O: 0.0100% or less O exists as an oxide and can become the starting point of stress corrosion cracking. Therefore, if the content of O exceeds 0.0100%, it becomes difficult to achieve excellent stress corrosion cracking resistance. Therefore, the O content must be 0.0100% or less. The O content is preferably 0.0050% or less. There is no particular lower limit for the O content, but due to constraints on production technology, the O content is preferably 0.0001% or more.

[0025] A steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components, with the balance including Fe and unavoidable impurities. In the steel sheet of the present invention, the balance preferably consists of Fe and unavoidable impurities. Examples of unavoidable impurities include Zn, Pb, As, Ge, Sr, and Cs. A total content of 0.100% or less of these impurities is permitted.

[0026] In addition to the above-mentioned chemical composition, the steel sheet of the present invention further contains, in mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, At least one element selected from Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less may be contained alone or in combination.

[0027] Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less. When Ti, Nb, and V are each 0.200% or less, large amounts of coarse precipitates and inclusions are not formed, and they do not become the initiation sites of stress corrosion cracking, so stress corrosion cracking resistance is not degraded. Therefore, when at least one of Ti, Nb, and V is contained, the Ti, Nb, and V contents are each 0.200% or less. Preferably, each of these contents is 0.100% or less. While there are no particular lower limits for the Ti, Nb, and V contents, the Ti, Nb, and V contents are preferably 0.001% or more because they increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Each of these contents is more preferably 0.002% or more, and even more preferably 0.003% or more.

[0028] Ta: 0.10% or less, W: 0.10% or less. Ta and W, when each is 0.10% or less, do not form large amounts of coarse precipitates or inclusions, which do not serve as initiation sites for stress corrosion cracking, and therefore stress corrosion cracking resistance is not impaired. Therefore, when at least one of Ta and W is contained, the Ta and W contents are each set to 0.10% or less. Preferably, each of these contents is set to 0.08% or less. While there are no particular lower limits for the Ta and W contents, Ta and W increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing, so the Ta and W contents are preferably set to 0.01% or more. Each of these contents is more preferably set to 0.02% or more, and even more preferably set to 0.03% or more.

[0029] B: 0.0100% or less If B is 0.0100% or less, cracks will not form inside the steel sheet during casting or hot rolling, and stress corrosion cracking resistance will not be reduced. Therefore, when B is contained, the B content is set to 0.0100% or less. The B content is preferably set to 0.0080% or less. Note that there is no particular lower limit for the B content, but since B is an element that segregates to austenite grain boundaries during annealing and improves hardenability, the B content is preferably set to 0.0003% or more.

[0030] Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less. When Cr, Mo, and Ni are each 1.00% or less, coarse precipitates and inclusions do not increase and do not become the initiation sites of stress corrosion cracking, thereby preventing a decrease in stress corrosion cracking resistance. Therefore, when at least one of Cr, Mo, and Ni is contained, the Cr, Mo, and Ni contents are each 1.00% or less. Preferably, each of these contents is 0.90% or less. Each of these contents is preferably 0.80% or less. While there are no particular lower limits for the Cr, Mo, and Ni contents, since these elements improve hardenability, it is more preferable that each of the Cr, Mo, and Ni contents be 0.01% or more. Each of these contents is more preferably 0.02% or more, and even more preferably 0.03% or more.

[0031] Co: 0.010% or less If Co is 0.010% or less, coarse precipitates and inclusions do not increase and do not become the initiation point of stress corrosion cracking, so stress corrosion cracking resistance does not deteriorate. Therefore, when Co is contained, the Co content is preferably 0.010% or less. The Co content is preferably 0.008% or less. Note that there is no particular lower limit for the Co content, but since Co is an element that improves hardenability, the Co content is more preferably 0.001% or more. The Co content is more preferably 0.002% or more.

[0032] Cu: 1.00% or less If Cu is 1.00% or less, coarse precipitates and inclusions do not increase and do not become the starting point of stress corrosion cracking, so stress corrosion cracking resistance does not deteriorate. Therefore, when Cu is contained, the Cu content is set to 1.00% or less. The Cu content is preferably set to 0.80% or less. Note that there is no particular lower limit for the Cu content, but since Cu is an element that improves hardenability, the Cu content is preferably set to 0.01% or more.

[0033] Sn: 0.200% or less If the Sn content is 0.200% or less, cracks will not form inside the steel sheet during casting or hot rolling, and the Sn content will not become the initiation point for stress corrosion cracking, so stress corrosion cracking resistance will not be reduced. Therefore, when Sn is contained, the Sn content is set to 0.200% or less. The Sn content is preferably set to 0.190% or less. Although there is no particular lower limit for the Sn content, since Sn is an element that improves hardenability (generally an element that improves corrosion resistance), the Sn content is more preferably set to 0.001% or more. The Sn content is more preferably set to 0.002% or more, and even more preferably set to 0.003% or more.

[0034] Sb: 0.200% or less If Sb is 0.200% or less, coarse precipitates and inclusions do not increase and do not become the starting point of stress corrosion cracking, so stress corrosion cracking resistance does not deteriorate. Therefore, when Sb is contained, the Sb content is set to 0.200% or less. The Sb content is preferably set to 0.190% or less. Although there is no particular lower limit for the Sb content, since Sb is an element that controls the surface softening thickness and enables strength adjustment, the Sb content is more preferably set to 0.001% or more. Each of these contents is more preferably set to 0.002% or more, and even more preferably set to 0.003% or more.

[0035] Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less. When the Ca, Mg, and REM contents are each 0.0100% or less, coarse precipitates and inclusions do not increase and do not become initiation sites for stress corrosion cracking, thereby preventing a decrease in stress corrosion cracking resistance. Therefore, when at least one of Ca, Mg, and REM is contained, the Ca, Mg, and REM contents are set to 0.0100% or less. Preferably, each of these contents is set to 0.0070% or less. More preferably, each of these contents is set to 0.0050% or less. Note that there are no particular lower limits for the Ca, Mg, and REM contents. However, because these elements spheroidize the shape of nitrides and sulfides and reduce the number of initiation sites for stress corrosion cracking, it is preferable that the Ca, Mg, and REM contents are set to 0.0005% or more. The content of each of these elements is more preferably 0.0006% or more, and further preferably 0.0007% or more.

[0036] Zr: 0.100% or less, Te: 0.100% or less. Zr and Te, if present in an amount of 0.100% or less, do not increase coarse precipitates or inclusions, and do not become initiation sites for stress corrosion cracking. Therefore, stress corrosion cracking resistance is not impaired. Therefore, when at least one of Zr and Te is contained, the Zr and Te contents are preferably 0.100% or less. Preferably, each of these contents is 0.080% or less. While there are no particular lower limits for the Zr and Te contents, since these elements spheroidize the shape of nitrides and sulfides and reduce the number of initiation sites for stress corrosion cracking, it is more preferable that each of the Zr and Te contents be 0.001% or more. Each of these contents is more preferably 0.002% or more, and even more preferably 0.003% or more.

[0037] Hf: 0.10% or less If the Hf content is 0.10% or less, coarse precipitates and inclusions do not increase and do not become initiation points for stress corrosion cracking, so stress corrosion cracking resistance does not deteriorate. Therefore, when Hf is contained, the Hf content is set to 0.10% or less. The Hf content is preferably set to 0.08% or less. Although there is no particular lower limit for the Hf content, since Hf is an element that spheroidizes the shape of nitrides and sulfides and reduces the number of sites that become initiation points for stress corrosion cracking, the Hf content is preferably set to 0.001% or more, and more preferably set to 0.010% or more.

[0038] Bi: 0.200% or less If Bi is 0.200% or less, coarse precipitates and inclusions do not increase and do not become the initiation point of stress corrosion cracking, so stress corrosion cracking resistance does not deteriorate. Therefore, when Bi is contained, the Bi content is set to 0.200% or less. The Bi content is preferably set to 0.100% or less. Although there is no particular lower limit for the Bi content, since Bi is an element that reduces segregation, the Bi content is more preferably set to 0.001% or more. The Bi content is more preferably set to 0.002% or more, and even more preferably set to 0.003% or more.

[0039] In addition, when the content of each of the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi is less than the preferred lower limit, the effect of the present invention is not impaired, and therefore, these elements are included as unavoidable impurities.

[0040] Next, the steel structure of the steel sheet of the present invention will be described. Tempered martensite: area fraction of 95% or more This is one of the important constituent elements of the present invention. By using tempered martensite as the main phase, it is possible to achieve a TS of 1320 MPa or more. In order to achieve this effect, the area fraction of tempered martensite needs to be 95% or more. Therefore, the area fraction of tempered martensite is set to 95% or more. The area fraction of tempered martensite is preferably 96% or more. The area fraction of tempered martensite is more preferably 97% or more. The upper limit of the area fraction of tempered martensite is not particularly limited, but may be 100%.

[0041] The area fraction of tempered martensite is measured as follows. The L-section of a steel sheet is polished and then corroded with 1 vol. % nital. A quarter-thickness portion (a position corresponding to one-quarter of the sheet thickness in the depth direction from the surface of the steel sheet) is observed using an SEM at a magnification of 2000x, with a field of view of 30 μm × 30 μm, for a total of 10 observations. In the above structural image, the tempered martensite has fine irregularities within the structure and contains carbides. The area fraction of tempered martensite can be calculated from the average of these values.

[0042] Retained austenite: volume fraction less than 3% This is one of the important constituent elements of the present invention. If the volume fraction of retained austenite is 3% or more, it becomes difficult to achieve YR ≥ 75%. The reason why it becomes difficult to achieve YR ≥ 75% is that the retained austenite transforms into martensite during the tensile test, resulting in a decrease in YS. Therefore, the retained austenite is set to less than 3%. Preferably, the volume fraction of retained austenite is set to 1% or less. There is no particular lower limit for the retained austenite. The volume fraction of retained austenite may be 0%.

[0043] The volume fraction of retained austenite is determined by polishing the surface of a steel sheet to a depth of 1 / 4 of the sheet thickness, and then chemically polishing the surface by a further 0.1 mm, measuring the integrated intensity ratios of the diffraction peaks of the {200}, {220}, and {311} planes of fcc iron and the {200}, {211}, and {220} planes of bcc iron using CoKα radiation in an X-ray diffractometer, and averaging the nine integrated intensity ratios obtained.

[0044] Total area fraction of ferrite and bainitic ferrite: less than 5% This is one of the important constituent elements of the present invention. If the total area fraction of ferrite and bainitic ferrite is 5% or more, it becomes difficult to achieve excellent stretch flangeability. Therefore, the total area fraction of ferrite and bainitic ferrite is less than 5%. This total area fraction is preferably 3% or less. This total area fraction is more preferably 2% or less. There is no particular lower limit for the total area fraction of ferrite and bainitic ferrite. The total area fraction of ferrite and bainitic ferrite may even be 0%.

[0045] The method for measuring the total area fraction of ferrite and bainitic ferrite is as follows. After polishing the L-section of a steel sheet, it is corroded with 1 vol. % nital, and a 1 / 4 portion of the sheet thickness (a position corresponding to 1 / 4 of the sheet thickness in the depth direction from the steel sheet surface) is observed using an SEM at 2000x magnification with a field of view of 30 μm × 30 μm, with 10 fields of view. Note that in the above structural image, the ferrite and bainitic ferrite are recessed and the interior of the structure is flat, and the structure does not contain carbides. The total area fraction of ferrite and bainitic ferrite can be calculated from the average of these values.

[0046] The remaining structure other than the entire structure may be pearlite, fresh martensite, etc. These remaining structures may be contained in an area fraction of 5% or less, as they do not affect the properties.

[0047] The area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 μm to 1.0 μm within grains surrounded by grain boundaries at an angle of 15° or more: 5% to 50%. This is one of the important constituent elements of the present invention. The area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 μm to 1.0 μm within grains surrounded by grain boundaries at an angle of 15° or more affects stress corrosion cracking resistance and material stability. If the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 μm to 1.0 μm within grains surrounded by grain boundaries at an angle of 15° or more exceeds 50%, stress corrosion cracking will begin. This makes it difficult to achieve excellent stress corrosion cracking resistance. As a result of extensive research, the inventors have discovered that by increasing the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 μm to 1.0 μm, an excessive increase in TS due to an increase in the area fraction of tempered martensite can be suppressed, thereby suppressing changes in TS when the tempered martensite fraction changes in the sheet width direction, thereby improving material stability. Therefore, within the tempered martensite grains surrounded by grain boundaries at an angle of 15° or more, the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 μm to 1.0 μm is set to 5% to 50%. This area fraction is preferably 7% or more. Furthermore, this area fraction is preferably 30% or less.

[0048] Here, the method for measuring the fraction of tempered martensite containing five or more carbides with a grain size of 0.1 μm to 1.0 μm is as follows. Using a TEM (transmission electron microscope), the steel sheet structure is observed at a position 1 / 4 of the sheet thickness from the steel sheet surface at 20,000x magnification with a field of view of 4 μm x 4 μm, and the grain size and number of carbides present within all tempered martensite grains in the field of view are calculated. The grain size of the carbides is determined by importing data in which carbides have been identified in advance into Image-Pro from MediaCybernetics and calculating the circle equivalent diameter. Within grains surrounded by grain boundaries at an angle of 15° or more, the total area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 μm to 1.0 μm is calculated. The total area fraction of all tempered martensite is also calculated. The total area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 μm to 1.0 μm is divided by the total area fraction of all tempered martensite to calculate the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 μm to 1.0 μm. Note that the number of tempered martensite grains surrounded by grain boundaries at an angle of 15° or more is determined, for example, as follows. Transmission electron backscatter diffraction (transmission EBSD) measurements are performed using the test specimens that have undergone TEM observation to obtain local crystal orientation data. The obtained local crystal orientation data is then analyzed using analysis software: OIM Analysis 7. Prior to the analysis of the local crystal orientation data, a cleanup process was performed once using the grain dilation function of the analysis software (grain tolerance angle: 5, minimum grain size: 2, single iteration: ON). Next, the grain boundaries in the tempered martensite with an angle of 15° or more were displayed to identify the grains in the tempered martensite that were surrounded by grain boundaries with an angle of 15° or more. For each carbide, a region in the microscope image that was surrounded by material other than carbide and formed seamlessly and integrally was counted as one carbide.

[0049] The steel sheet of the present invention may have a plating layer on its surface. Examples of the plating layer include a zinc plating layer such as a hot-dip galvanized layer, a galvannealed hot-dip galvanized layer, and an electrogalvanized layer. Examples of plating layers other than a zinc plating layer include an aluminum plating layer and an alloy plating layer. Examples of alloy plating layers include a hot-dip zinc-aluminum-magnesium alloy plating layer and a Zn—Ni electroalloy plating layer.

[0050] The steel plate of the present invention preferably has a thickness of 0.5 mm or more, and a width of 600 mm or more, and a width of 1500 mm or less.

[0051] [Method of Manufacturing Steel Sheet] Next, a method of manufacturing a steel sheet of the present invention will be described. The method of manufacturing a steel sheet of the present invention includes an annealing step in which a cold-rolled sheet produced by hot-rolling, pickling, and cold-rolling a steel having a chemical composition is heated under conditions of an annealing temperature T1 of 800°C or higher and a holding time t1 at the annealing temperature T1 of 10 seconds or higher, and cooling at 700 to 500°C at an average cooling rate CR1 of 5°C / s or higher, and cooling by water quenching from a quenching start temperature T2 of (Ms-80°C) or higher to Ms or lower to 80°C at an average cooling rate CR2 of 300°C / s or higher, and a tempering temperature T3 of 100°C or higher but lower than 250°C and a holding time t3 at the tempering temperature T3 of 10 seconds or higher and 10,000 seconds or lower, and in the cooling treatment in the annealing step, the time t2 during which the steel sheet is held in a temperature range of 250°C or higher but lower than Ms is 1.0 second or higher and 10.0 seconds or lower, and During cooling of water quenching in the annealing step, pressure is applied to the front and back surfaces of the steel sheet using two rolls placed on either side of the steel sheet, and the pressure is applied under the conditions of a distance between the two rolls in the steel sheet conveying direction of 20 mm or more and 250 mm or less, and a pressure of 196 N or more.

[0052] In the present invention, the method for producing the steel material (steel slab) is not particularly limited, and any known method such as a converter or an electric furnace is suitable. The steel slab (slab) is preferably produced by a continuous casting method in order to prevent macrosegregation.

[0053] In the present invention, the slab heating temperature, slab soaking time, and coiling temperature in hot rolling are not particularly limited. Methods of hot rolling a steel slab include a method of heating the slab and then rolling it, a method of directly rolling the slab after continuous casting without heating it, and a method of rolling the slab after continuous casting after subjecting it to a short-term heat treatment. The slab heating temperature, slab soaking time, finish rolling temperature, and coiling temperature in hot rolling are not particularly limited, but the slab heating temperature is preferably 1100°C or higher. The slab heating temperature is preferably 1300°C or lower. The slab soaking time is preferably 30 minutes or longer. The slab soaking time is preferably 250 minutes or shorter. The finish rolling temperature is Ar ３ The coiling temperature is preferably 350°C or higher and 650°C or lower.

[0054] The hot-rolled steel sheet produced in this manner is subjected to pickling. Pickling can remove oxides from the steel sheet surface, and is therefore important for ensuring good chemical conversion treatability and plating quality in the final high-strength steel sheet product. Pickling may be performed once or multiple times. After hot rolling, the pickled steel sheet may be cold-rolled as is, or may be heat-treated and then cold-rolled.

[0055] The reduction rate (cumulative reduction rate) in cold rolling and the thickness after rolling are not particularly limited, but the reduction rate is preferably 30% or more. The reduction rate is preferably 80% or less. The number of rolling passes and the reduction rate of each pass are not particularly limited, and the effects of the present invention can be obtained.

[0056] The cold-rolled sheet obtained as described above is subjected to annealing under the following annealing conditions.

[0057] Annealing temperature T1: 800°C or higher If the annealing temperature T1 is lower than 800°C, the total area fraction of ferrite and bainitic ferrite will be 5% or higher, making it difficult to achieve a TS of 1320 MPa or higher and also difficult to achieve excellent stretch flangeability. Therefore, the annealing temperature T1 is set to 800°C or higher. The annealing temperature T1 is preferably 820°C or higher. There is no particular need to limit the upper limit, but the annealing temperature T1 is preferably 1000°C or lower. The annealing temperature here refers to the holding temperature in the annealing process. Note that the annealing temperature may be constant during holding. Furthermore, the annealing temperature does not have to be constant during holding as long as it is in a temperature range of 800°C or higher and the temperature fluctuation is within ±10°C of the set temperature.

[0058] Holding time t1 at annealing temperature T1: 10 seconds or more If the holding time t1 at annealing temperature T1 is less than 10 seconds, the total area fraction of ferrite and bainitic ferrite will be 5% or more, making it difficult to achieve a TS of 1320 MPa or more and difficult to achieve excellent stretch flangeability. Therefore, the holding time t1 at annealing temperature T1 is set to 10 seconds or more. The holding time t1 at annealing temperature T1 is preferably 30 seconds or more. There is no particular need to limit the upper limit, but the holding time t1 at annealing temperature T1 is preferably 1000 seconds or less.

[0059] Average cooling rate CR1 from 700 to 500 ° C: 5 ° C / s or more If the average cooling rate CR1 from 700 to 500 ° C is less than 5 ° C / s, the total area fraction of ferrite and bainitic ferrite will be 5% or more, making it difficult to achieve a TS of 1320 MPa or more and difficult to achieve excellent stretch flangeability. Therefore, the average cooling rate CR1 from 700 to 500 ° C is 5 ° C / s or more. The average cooling rate CR1 is preferably 10 ° C / s or more. There is no particular need to limit the upper limit, but the average cooling rate CR1 is preferably 50 ° C / s or less. Here, the average cooling rate CR1 is calculated by (cooling start temperature (700 ° C) - cooling stop temperature (500 ° C)) / cooling time (s) from the cooling start temperature (700 ° C) to the cooling stop temperature. Specific examples of cooling at the average cooling rate CR1 include water cooling and mist cooling.

[0060] Quenching start temperature T2: (Ms - 80°C) or higher and Ms or lower. This is one of the important constituent elements of the present invention. By setting the quenching start temperature T2 to (Ms - 80°C) or higher and Ms or lower, it is possible to obtain a structure in which the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 μm to 1.0 μm is 5% to 50% within grains surrounded by grain boundaries at an angle of 15° or more. If the quenching start temperature T2 is lower than (Ms - 80°C), the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 μm to 1.0 μm will exceed 50%, making it difficult to achieve excellent stress corrosion cracking resistance. On the other hand, if the quenching start temperature T2 exceeds Ms, the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 μm to 1.0 μm will be less than 5%, making it difficult to achieve excellent material stability. Therefore, the quenching start temperature T2 is set to be equal to or higher than (Ms - 80°C) and equal to or lower than Ms. The quenching start temperature T2 is preferably equal to or higher than (Ms - 40°C). The quenching start temperature T2 is preferably equal to or lower than (Ms - 10°C). Ms is the martensitic transformation start temperature (°C), and the martensitic transformation start temperature Ms (°C) is defined by the following formula (1): Ms (°C) = 519 - 474 x [%C] - 30.4 x [%Mn] - 12.1 x [%Cr] - 7.5 x [%Mo] - 17.7 x [%Ni] (1) Here, [%C], [%Mn], [%Cr], [%Mo], and [%Ni] represent the respective contents (mass%) of C, Mn, Cr, Mo, and Ni in the steel (steel plate), and are set to 0 if none are contained.

[0061] Average cooling rate CR2 from quenching start temperature T2 to 80°C: 300°C / s or more If the average cooling rate CR2 from quenching start temperature T2 to 80°C is less than 300°C / s, the volume fraction of retained austenite will be 3% or more, making it difficult to achieve a YR of 75% or more. Therefore, the average cooling rate CR2 from quenching start temperature T2 to 80°C is set to 300°C / s or more. The average cooling rate CR2 is preferably 800°C / s or more. There is no particular need to limit the upper limit, but the average cooling rate CR2 is preferably 3000°C / s or less.

[0062] In the cooling treatment, the time t2 during which the steel sheet is held in the temperature range of 250°C to Ms is 1.0 second to 10.0 seconds. This is one of the important constituent features of the present invention. By setting the time during which the steel sheet is held in the temperature range of 250°C to Ms in the cooling treatment to 1.0 second to 10.0 seconds, it is possible to obtain a structure in which the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 μm to 1.0 μm inclusive in grains surrounded by grain boundaries at an angle of 15° or more is 5% to 50%. If the time during which the steel sheet is held in the temperature range of 250°C to Ms in the cooling treatment exceeds 10.0 seconds, the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 μm to 1.0 μm inclusive in grains surrounded by grain boundaries at an angle of 15° or more exceeds 50%, making it difficult to achieve excellent stress corrosion cracking resistance. On the other hand, if the cooling treatment involves holding the steel sheet in the temperature range of 250°C to Ms for less than 1.0 second, the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 μm to 1.0 μm within grains surrounded by grain boundaries at an angle of 15° or more will be less than 5%, making it difficult to achieve excellent material stability. Therefore, the cooling treatment involves holding the steel sheet in the temperature range of 250°C to Ms for 1.0 second to 10.0 seconds. The cooling treatment involves holding the steel sheet in the temperature range of 250°C to Ms for preferably 1.5 seconds or more. The cooling treatment involves holding the steel sheet in the temperature range of 250°C to Ms for preferably 5.0 seconds or less.

[0063] Tempering temperature T3: 100°C or higher but lower than 250°C. This is one of the important constituent elements of the present invention. In the present invention, tempered martensite refers to a structure in which martensite at 80°C or lower is subjected to a heat treatment at a tempering temperature of 100°C or higher for a holding time of 10 seconds or longer. Therefore, if the tempering temperature T3 is lower than 100°C, the martensite is not sufficiently tempered, resulting in a structure mainly composed of as-quenched martensite, and the as-quenched martensite has poor stress corrosion cracking resistance. On the other hand, if the tempering temperature T3 is higher than 250°C, the tempering of martensite proceeds excessively, and the area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 μm to 1.0 μm in the grains surrounded by grain boundaries at an angle of 15° or higher exceeds 50%, making it difficult to achieve excellent stress corrosion cracking resistance. Therefore, the tempering temperature T3 is set to 100°C or higher but lower than 250°C. The tempering temperature T3 is preferably 150° C. or higher, and 220° C. or lower.

[0064] Holding time t3 at tempering temperature T3: 10 seconds or more and 10,000 seconds or less. In the present invention, tempered martensite refers to a structure in which martensite at 80°C or less is subjected to a heat treatment at a tempering temperature of 100°C or more and a holding time of 10 seconds or more. Therefore, if the holding time t3 at tempering temperature T3 is less than 10 seconds, the martensite is not sufficiently tempered, resulting in a structure mainly composed of as-quenched martensite, and the as-quenched martensite has poor stress corrosion cracking resistance. On the other hand, if the holding time t3 exceeds 10,000 seconds, the tempering of martensite proceeds excessively, and the proportion of tempered martensite containing five or more carbides with a grain size of 0.1 μm to 1.0 μm in the grains surrounded by grain boundaries of 15° or more exceeds 50%. This makes it difficult to achieve excellent stress corrosion cracking resistance. Therefore, the holding time t3 at tempering temperature T3 is set to 10 seconds or more and 10,000 seconds or less. The holding time t3 at the tempering temperature T3 is preferably 50 seconds or more, and 5000 seconds or less.

[0065] The cooling after tempering does not need to be particularly specified, and the material may be cooled to a desired temperature by any method, which is preferably about room temperature.

[0066] The steel sheet (high-strength steel sheet) may be processed under conditions that result in an equivalent plastic strain of 0.05% to 5.00%. After processing, the steel sheet may be reheated again under conditions that result in a temperature of 100°C to 400°C.

[0067] When high-strength steel sheets are traded, they are usually cooled to room temperature before being traded.

[0068] During cooling after water quenching, pressure is applied from the front and back sides of the steel sheet by two rolls placed on either side of the steel sheet, with the distance between the two rolls in the steel sheet transport direction during the pressure application being 20 mm or more and 250 mm or less, and the pressure being 196 N or more. During cooling (water cooling) after water quenching in the annealing process, pressure is applied from the front and back sides of the steel sheet by two rolls placed on either side of the steel sheet. At that time, the distance between the two rolls in the steel sheet transport direction (hereinafter simply referred to as the roll distance) is 20 mm or more and 250 mm or less, and the pressure is 196 N or more. In the present invention, the "roll distance between two rolls" refers to the distance between the contact points of one roll and the steel sheet and the contact point of the other roll and the steel sheet, as shown in Figure 1. If pressure is not applied during the water-quenching process, the proportion of tempered martensite containing five or more carbides with a grain size of 0.1 μm to 1.0 μm within grains surrounded by grain boundaries at an angle of 15° or greater will be less than 5%, making it difficult to achieve excellent material stability. After extensive research, the inventors discovered that applying pressure during the cooling process (water-quenching) of water-quenching affects the progression of martensitic transformation. Applying pressure under conditions of a roll-to-roll distance of 20 mm to 250 mm and a pressure of 196 N or greater promotes nucleation of martensitic transformation, increasing the area fraction of martensite that transforms at 250°C or higher. This results in an area fraction of tempered martensite containing five or more carbides with a grain size of 0.1 μm to 1.0 μm of greater than 5%, thereby achieving excellent material stability. To achieve this effect, the roll-to-roll distance (see FIG. 1) must be 20 mm or greater. On the other hand, if the distance between the rolls exceeds 250 mm, the pressure weakens, so the distance between the rolls must be 250 mm or less. Pressure is applied by sandwiching the material between rolls spaced apart from each other, and the pressure required to achieve the above effect is 196 N or more. This pressure corresponds to the load of one roll. In other words, a pressure of 196 N or more means that each of the two rolls applies pressure with a force of 196 N or more. As pressure conditions, the preferred distance between the rolls is 30 mm or more. The preferred distance between the rolls is 220 mm or less. Furthermore, the preferred pressure is 294 N or more. The preferred pressure is 4900 N or less.Furthermore, the pressure varies depending on the strength and tension of the steel sheet, but the pressure can be adjusted by the tension, the amount of pressure indentation, etc., and it can be confirmed by a load meter or the like attached to the roll that the pressure is within the above range, and the amount of pressure indentation can be calculated from the roll diameter and roll position. Based on the above, in the present invention, during cooling (water cooling) of water quenching, pressure is applied from the front and back sides of the steel sheet by two rolls placed on either side of the steel sheet, and the pressure is applied under the conditions of a roll distance of 20 mm to 250 mm between the two rolls and a pressure of 196 N or more.

[0069] Optionally, the steel sheet may be subjected to a plating process. The plating process is not particularly limited. Examples of plating processes include galvanizing processes such as hot-dip galvanizing, galvannealed hot-dip galvanizing, and electrogalvanizing. Examples of plating processes other than galvanizing include aluminum plating and alloy plating. Examples of alloy plating processes include hot-dip zinc-aluminum-magnesium alloy plating and Zn-Ni electroalloy plating. Conventional treatment conditions may be used for all of these processes. As described above, the plating process is preferably performed during cooling from the annealing temperature T1 to the quenching start temperature T2 or after the tempering process. For example, the hot-dip galvanizing process and the galvannealed hot-dip galvanizing process are preferably performed during cooling from the annealing temperature T1 to the quenching start temperature T2. The electrogalvanizing process and the Zn-Ni electroalloy plating process are preferably performed after the tempering process. In the case of hot-dip galvanizing and galvannealed hot-dip galvanizing, from the viewpoint of productivity, it is preferable to carry out a series of processes such as the heating step, annealing step, and plating step in a continuous galvanizing line (CGL). After hot-dip galvanizing, wiping can be performed to adjust the coating weight of the coating.

[0070] The conditions other than those described above are not particularly limited and may be performed in accordance with conventional methods. According to the method for producing a steel sheet according to one embodiment of the present invention described above, a steel sheet having a TS of 1320 MPa or more, a YR of 75% or more, and excellent stretch flangeability, stress corrosion cracking resistance, and material stability can be obtained. The obtained steel sheet can be suitably used, for example, as a material for automobile parts.

[0071] After the plating treatment following annealing, the steel sheet may be again subjected to working under conditions that result in an equivalent plastic strain of 0.05% or more and 5.00 or less.Furthermore, after working, the steel sheet may be reheated under conditions that result in a temperature of 100°C or more and less than 250°C.

[0072] [Member and Manufacturing Method of Member] Next, the member of the present invention and its manufacturing method will be described.

[0073] The member of the present invention is obtained by subjecting the steel plate of the present invention to at least one of forming and joining. Also, the method for manufacturing the member of the present invention includes a step of subjecting the steel plate of the present invention to at least one of forming and joining to form the member.

[0074] The steel sheet of the present invention has a tensile strength of 1320 MPa or more, a YR of 75% or more, and is excellent in stretch flangeability, stress corrosion cracking resistance, and material stability. Therefore, members obtained using the steel sheet of the present invention also have a tensile strength of 1320 MPa or more, a YR of 75% or more, and are excellent in stretch flangeability, stress corrosion cracking resistance, and material stability. Use of the member of the present invention enables weight reduction. Therefore, the member of the present invention can be suitably used, for example, in automotive structural members.

[0075] The forming process can be performed using a general processing method such as press working without any restrictions, and the joining process can be performed using general welding methods such as spot welding and arc welding, riveting, crimping, etc. without any restrictions.

[0076] Steel having the chemical composition shown in Table 1 (Table 1-1, Table 1-2) (the balance being Fe and unavoidable impurities) was melted in a converter and formed into a steel slab by continuous casting. The steel slab was then heated. The steel slab was then hot-rolled to form a hot-rolled steel sheet. The hot-rolled steel sheet was then pickled. The hot-rolled steel sheet was then cold-rolled to form a cold-rolled steel sheet. A base steel sheet was thus prepared. The prepared base steel sheet was then annealed under the conditions shown in Table 2 (Table 2-1, Table 2-2) to obtain a steel sheet (thickness: 1.4 mm, width: 1000 mm) as a final product. In addition, some of the steel sheets (those with the type column in Table 2 as GI, GA, and EG) were plated. Of these, those with the type column in Table 2 as GI and GA were plated during cooling from the annealing temperature T1 to the quenching start temperature T2. In addition, for those in the type column of Table 2 that are marked EG, plating was carried out after the tempering process. Conditions not specified were those according to conventional methods.

[0077]

[0078]

[0079]

[0080]

[0081] The steel sheets (high strength cold rolled steel sheets) obtained as described above were used as test steels to evaluate tensile properties, stretch flangeability, stress corrosion cracking resistance and material stability according to the following test methods.

[0082] (Structural Observation) According to the above-described method, the amount of tempered martensite (area fraction), the amount of retained austenite (volume fraction), the total amount of ferrite and bainitic ferrite (area fraction), and the fraction (area fraction) of the tempered martensite containing five or more carbides with a particle size of 0.1 μm or more and 1.0 μm or less were determined.

[0083] (Tensile Test) A tensile test was performed in accordance with JIS Z 2241 (2022) using a JIS No. 5 test piece (gauge length 50 mm, parallel portion width 25 mm) with the longitudinal direction of the test piece perpendicular to the rolling direction.－１ A tensile test was carried out under the condition of tensile strength TS measured at 1000 mm / sec. In the present invention, a TS of 1320 MPa or more was judged as passing. A yield ratio YR of 75% or more was judged as passing. YR can be calculated using the following formula (2): YR = 100 × YS / TS (2)

[0084] (Stretch flangeability) The hole expansion test was carried out in accordance with JIS Z 2256 (2020). After shearing the obtained steel sheet to 100 mm x 100 mm, a hole of 10 mm diameter was punched with a clearance of 12.5%, and then a die with an inner diameter of 75 mm was used to hold down the blank holder force of 9 ton (88.26 kN) and a conical punch with an apex angle of 60 ° was pressed into the hole to measure the hole diameter at the crack initiation limit, and the limit hole expansion ratio: λ (%) was calculated from the following formula, and the hole expandability was evaluated from the value of this limit hole expansion ratio. Limit hole expansion ratio: λ (%) = {(Df - D0) / D0} × 100 where Df is the hole diameter (mm) at the time of crack initiation, and D0 is the initial hole diameter (mm). In the present invention, when the value of the hole expansion ratio (λ), which is an indicator of stretch flangeability, is 30% or more regardless of the strength of the steel sheet, the stretch flangeability was determined to be good.

[0085] (Stress corrosion cracking resistance) The obtained steel plate was sheared to a size of 20 mm x 75 mm with the direction perpendicular to the rolling direction as the longitudinal direction, and 2 mm was removed from both sides of the longitudinal end face by mechanical grinding to process into 16 mm x 75 mm test specimens. The obtained test specimens were subjected to four-point bending in accordance with ASTM (G39-99), and stresses equivalent to YS and TS were applied to the bend vertices of the test specimens. The test specimens in the stressed state were immersed in 1 mass % sulfuric acid at 25°C for 100 hours. After the test, the presence or absence of cracks was confirmed visually for each test specimen. Then, the stress corrosion cracking resistance was evaluated according to the following criteria. ⊚ (Pass, particularly excellent): No cracks in samples loaded with stresses equivalent to YS and TS. ○ (Pass, excellent): No cracks in samples loaded with stresses equivalent to YS. × (Fail): Cracks present in both samples loaded with stresses equivalent to YS and TS.

[0086] (Material stability) Material stability can be evaluated using coils and cut sheets. TS was measured at 10 or more locations in the coil at positions 50 mm or more apart in the sheet width direction, and the difference between the maximum TS and the minimum TS among the measured TS was calculated as TS variation (ΔTS = maximum TS - minimum TS). Material stability was then evaluated according to the following criteria: ◎ (Pass, particularly excellent): ΔTS is less than 40 MPa ○ (Pass, excellent): ΔTS is less than 60 MPa × (Fail): ΔTS is 60 MPa or more

[0087]

[0088]

[0089] The examples of the present invention shown in Table 3 (Table 3-1, Table 3-2) had a tensile strength TS of 1320 MPa or more, a yield ratio YR of 75% or more, and excellent stretch flangeability, stress corrosion cracking resistance, and material stability, whereas the comparative examples were inferior in at least one of these.

[0090] Furthermore, the components obtained by forming and joining the steel plates of the present invention have a tensile strength TS of 1320 MPa or more, a yield ratio YR of 75% or more, and excellent stretch flangeability, stress corrosion cracking resistance, and material stability, similar to the steel plates of the present invention, and have a tensile strength of 1320 MPa or more, a yield ratio YR of 75% or more, and excellent stretch flangeability, stress corrosion cracking resistance, and material stability.

Claims

1. In mass%, C: 0.030% or more and 0.450% or less, Si: 0.010% or more and 2.500% or less, Mn: 0.10% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less and the balance being Fe and unavoidable impurities, At the 1 / 4 position of the plate thickness, Area fraction of tempered martensite: 95% or more, Volume fraction of retained austenite: less than 3%; The total area fraction of ferrite and bainitic ferrite is less than 5%; The steel plate has a structure satisfying that the area fraction of the tempered martensite containing five or more carbides with a grain size of 0.1 μm or more and 1.0 μm or less within grains surrounded by grain boundaries at an angle of 15° or more is 5% or more and 50% or less.

2. The component composition further comprises, in mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, Bi: 0.200% or less, The steel sheet according to claim 1, further comprising at least one element selected from the group consisting of:

3. The steel sheet according to claim 1 , having a plating layer on the surface of the steel sheet.

4. A steel plate as described in claim 2, having a plating layer on the surface of the steel plate.

5. A member made using the steel plate according to any one of claims 1 to 4.

6. A cold-rolled sheet produced by subjecting a steel having the component composition according to claim 1 or 2 to hot rolling, pickling and cold rolling, Annealing temperature T1: 800 ° C. or higher, Heating is performed under the condition that the holding time t1 at the annealing temperature T1 is 10 seconds or more, Cooling at an average cooling rate CR1 of 700 to 500 ° C.: 5 ° C. / s or more; and cooling by water quenching at an average cooling rate CR2 of 300 ° C. / s or more from a quenching start temperature T2 of (Ms - 80 ° C.) or more to 80 ° C., which is equal to or higher than Ms. Tempering temperature T3: 100 ° C or more and less than 250 ° C, An annealing process is performed at a tempering temperature T3 for a holding time t3 of 10 seconds or more and 10,000 seconds or less. In the cooling treatment in the annealing step, the time t2 during which the steel sheet is held in a temperature range of 250°C or higher and Ms or lower is set to 1.0 seconds or higher and 10.0 seconds or lower, a method for producing a steel sheet, wherein, during cooling of the water quenching in the annealing step, pressure is applied from the front and back surfaces of the steel sheet with two rolls placed on either side of the steel sheet, and the pressure is applied under the conditions of a roll-to-roll distance of 20 mm or more and 250 mm or less in the steel sheet transport direction of the two rolls and a pressure of 196 N or more.

7. The method for producing a steel sheet according to claim 6, further comprising the step of subjecting the steel sheet to a plating treatment.

8. A method for manufacturing a component, comprising the step of subjecting the steel plate according to any one of claims 1 to 4 to at least one of forming and joining to form a component.