Steel sheet, member, and production methods for these

Abstract

The present invention provide: a steel sheet which has a tensile strength TS of 1,320 MPa or more, a yield ratio YR of 75% or more, and excellent stretch flangeability, excellent fatigue resistance of a shear end surface part, and excellent delayed fracture resistance in a corrosive environment; a member; and production methods for these.　A steel sheet according to the present invention has a specific component composition, and has a structure in which, at the position of 1 / 4 the sheet thickness, the amount of tempered martensite is 95% or more in terms of area fraction, the amount of retained austenite is less than 3% in terms of volume fraction, and the total amount of ferrite and bainitic ferrite is less than 5% in terms of area fraction. When P(C) is the average value of the occupancies of packets that each have the maximum occupancy in prior austenite grains at the central position in the sheet thickness direction of the steel sheet and P(S) is the average value of the occupancies of packets that each have the maximum occupancy in prior austenite grains at a depth of 100 µm from the surface of the steel sheet, the structure satisfies P(C) ≤ 70% and {P(S) - P(C)} ≤ 20%.

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, fatigue resistance at sheared edges, and delayed fracture resistance in a corrosive environment, and to 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 an excellent yield ratio, excellent stretch-flange formability, excellent fatigue resistance at sheared edges, and excellent resistance to delayed fracture in corrosive environments. For example, for structural components such as floor cross members of automobiles, it is preferable to use steel sheets with excellent stretch-flange formability from the viewpoint of formability. Furthermore, from the viewpoint of component performance, excellent yield ratios, excellent fatigue resistance at sheared edges, and excellent resistance to delayed fracture in corrosive environments are required.

[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, fatigue resistance at sheared edges, and delayed fracture resistance in a corrosive environment.

[0005] Patent Document 2 discloses a high-strength steel sheet of 1180 MPa or more having excellent yield ratio and stretch flangeability, and a method for manufacturing the same. However, the technology described in Patent Document 2 does not take into consideration high-strength steel sheets having excellent fatigue resistance at sheared edges and delayed fracture resistance in a corrosive environment.

[0006] Patent Document 3 discloses a high-strength steel sheet of 980 MPa or more that is excellent in surface properties, steel sheet shape, and fatigue strength, and a manufacturing method thereof. However, the technology described in Patent Document 3 does not take into consideration high-strength steel sheets that are excellent in stretch flangeability, fatigue resistance of sheared edge surfaces, and delayed fracture resistance in a corrosive environment.

[0007] Patent Document 4 discloses a high-strength steel sheet of 1180 MPa or more that is excellent in delayed fracture resistance in a corrosive environment, corrosion resistance, and weldability, and a method for manufacturing the same. However, the technology described in Patent Document 4 does not take into consideration high-strength steel sheets that are excellent in yield ratio, stretch flangeability, and fatigue resistance of sheared edge portions.

[0008] Patent No. 7323093 Patent No. 6879441 Patent No. 6928112 Patent No. 6638694

[0009] Journal of the Society for Smart Processing 2013, Vol. 2, No. 3, pp. 110-118

[0010] The present invention has been 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, fatigue resistance at sheared edge portions, and delayed fracture resistance in a corrosive environment.

[0011] 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 means that the limiting hole expansion ratio: λ (%) obtained by a hole expansion test in accordance with JIS Z 2256 (2020) is 30% or more. Excellent fatigue resistance of the sheared end face means that the fatigue strength ratio of the sheared end face (= fatigue limit of the sheared end face / TS) obtained by a full reverse bending test in accordance with JIS Z 2275 (1978) is 0.25 or more. Excellent delayed fracture resistance in a corrosive environment means that the delayed fracture resistance in the corrosive environment specified in the present invention is excellent, that is, the number of days to crack in the evaluation of the metallic material is 63 days or more.

[0012] 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 P(C) to 70% or less and {P(S) - P(C)} to 20% or less, excellent fatigue resistance at the sheared edge and delayed fracture resistance in a corrosive environment can be achieved.

[0013] 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% to 0.500%, Si: 0.010% to 2.500%, Mn: 0.10% to 5.00%, 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 having, at a 1 / 4 position in the sheet thickness direction, 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%, and having a structure satisfying the following formulas (1) and (2): P(C)≦70% (1) {P(S)−P(C)}≦20% (2) In the formula, P(S): average value of the occupancy rate of packets having the maximum occupancy rate in prior austenite grains at a depth of 100 μm from the surface of the steel plate, and P(C): average value of the occupancy rate of packets having the maximum occupancy rate in prior austenite grains at the center position of the thickness of the steel plate. [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, containing 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, having a plating layer on the surface of the steel sheet. [4] The steel sheet according to any one of [1] to [3] above, having a number of days to cracking of 63 days or more in an evaluation of metallic material. [5] A member made using the steel sheet according to any one of [1] to [4] above.[6] 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 is subjected to an annealing process in which the steel 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 longer, and cooling from 700 to 600°C at an average cooling rate CR1 of 5°C / s or higher, cooling from (Ms+100°C) to a quenching start temperature T2 of (Ms-50°C) or higher but lower than (Ms+50°C) at an average cooling rate CR2 of 5°C / s or higher and 30°C / s or lower, and cooling by water quenching from the quenching start temperature T2 to 80°C at an average cooling rate CR3 of 300°C / s or higher, and tempering temperature T3 of 100°C or higher and 400°C or lower, and a holding time t3 at the tempering temperature T3 of 10 seconds or higher and 10,000 seconds or shorter.

[0013]

[0014]

[0015]

[0016]

[0017]

[0018]

[0019]

[0020]

[0021]

[0022]

[0023]

[0024]

[0025]

[0026]

[0027]

[0028]

[0029]

[0030]

[0031]

[0032]

[0033]

[0034]

[0035]

[0036]

[0037]

[0038]

[0039]

[0040]

[0041]

[0042]

[0043]

[0044]

[0050]

[0051]

[0052]

[0053]

[0054]

[0055]

[0056]

[0057]

[0058]

[0060]

[0061]

[0062]

[0063]

[0064]

[0065]

[0066]

[0070]

[0071]

[0072]

[0073]

[0074]

[0075]

[0076]

[0077]

[0078]

[0080]

[0081]

[0082]

[0083]

[0084]

[0085]

[0086]

[0087]

[0088]

[0089]

[0090]

[0091]

[0092]

[0093]

[0094]

[0095]

[0096]

[0097]

[0098]

[0099]

[0100]

[0099]

[0099]

[0101]

[0099]

[0099]

[0099]

[0099] [

[0014] 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, fatigue resistance at sheared edges, and delayed fracture resistance in a corrosive environment. 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 industrial value of the steel sheet is extremely great.

[0015] FIG. 1 is a schematic diagram illustrating a packet having the maximum occupancy rate within a prior austenite grain. FIG. 2 is a schematic diagram illustrating a method for evaluating the delayed fracture properties of a metallic material (HeTsAce). FIG. 3 is a schematic diagram illustrating an example of an image photographed showing the distribution of droplets on an evaluation surface of a metallic material. FIG. 4 is a schematic diagram illustrating a case in which a shielding material is placed between a spray nozzle and the metallic material in the method for evaluating the delayed fracture properties of a metallic material. FIG. 5 is a schematic diagram illustrating how the sprayed liquid in the atmosphere re-adheres to the evaluation surface when the distance between the shielding material and the evaluation surface of the metallic material is changed. FIG. 6 is a diagram illustrating one embodiment of a corrosion test cycle related to the method for evaluating the delayed fracture properties of a metallic material. FIG. 7 is a diagram illustrating another embodiment of a corrosion test cycle related to the method for evaluating the delayed fracture properties of a metallic material. FIG. 8 is a diagram illustrating a test piece for evaluating delayed fracture properties used in the examples. FIG. 9 is a schematic diagram illustrating a pressurization method during water cooling in the steel sheet manufacturing method of the present invention.

[0016] Hereinafter, an embodiment of the present invention will be described.

[0017] [Steel Plate] The steel plate of the present invention has a chemical composition containing, by mass%, C: 0.030% or more and 0.500% 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; at a 1 / 4 position in the plate thickness direction, 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%; and a structure satisfying the following formulas (1) and (2): P(C)≦70% (1) {P(S)−P(C)}≦20% (2) In the formula, P(S): average value of the occupancy rate of packets having the maximum occupancy rate in prior austenite grains at a depth of 100 μm from the surface of the steel plate, and P(C): average value of the occupancy rate of packets having the maximum occupancy rate in prior austenite grains at the center position of the thickness of the steel plate.

[0018] 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.

[0019] C: 0.030% or more and 0.500% or less. C is one of the important basic components of steel. In particular, in the present invention, C is an important element that affects the area fraction (hereinafter also referred to as fraction) of tempered martensite and the fatigue resistance properties of the sheared edge. 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 0.500%, the tempered martensite becomes embrittled, making it difficult to obtain excellent fatigue resistance properties of the sheared edge. Therefore, the C content is set to 0.030% or more and 0.500% 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. Furthermore, the C content is preferably set to 0.400% or less. The C content is more preferably set to 0.350% or less.

[0020] 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 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. Furthermore, 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 set to 0.500% or less, and even more preferably set to 0.300% or less.

[0021] 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 delayed fracture resistance in a corrosive environment. 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 steel sheet is accelerated, and hydrogen generation associated with corrosion is promoted, making it difficult to achieve excellent delayed fracture resistance in a corrosive environment. 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. Furthermore, the Mn content is preferably set to 4.50% or less. The Mn content is more preferably set to 4.00% or less.

[0022] 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%, the ultimate deformability of the steel sheet is reduced, making it difficult to achieve excellent fatigue resistance at the sheared edge. 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 0.002% or more.

[0023] S: 0.0200% or less S exists as sulfides, and if the content exceeds 0.0200%, it reduces the ultimate deformability of the steel sheet, making it difficult to achieve excellent fatigue resistance at the sheared edge. 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, the S content is preferably 0.0001% or more. The S content is more preferably 0.0002% or more.

[0024] Al: 1.000% or less. Al exists as an oxide, and if it is contained in an amount exceeding 1.000%, it reduces the ultimate deformability of the steel sheet, making it difficult to achieve excellent fatigue resistance at the sheared edge. Therefore, the Al content must be 1.000% or less. Therefore, the Al content is set to 1.000% or less. The Al content is preferably set to 0.500% or less. The Al content is more preferably set to 0.200% or less, and even more preferably set to 0.100% or less. Although there is no particular lower limit for the Al content, due to production technology constraints, the Al content is preferably set to 0.001% or more. The Al content is more preferably set to 0.002% or more. The Al content is more preferably set to 0.005% or more, and even more preferably set to 0.010% or more.

[0025] N: 0.0100% or less N exists as a nitride, and if it is contained in an amount exceeding 0.0100%, it reduces the ultimate deformability of the steel sheet, making it difficult to achieve excellent fatigue resistance at the sheared edge. 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 0.0010% or more, and even more preferably 0.0020% or more.

[0026] O: 0.0100% or less O exists as an oxide, and if it is contained in an amount exceeding 0.0100%, it reduces the ultimate deformability of the steel sheet, making it difficult to achieve excellent fatigue resistance at the sheared edge. Therefore, the O content must be 0.0100% or less. Therefore, the O content is set to 0.0100% or less. The O content is preferably set to 0.0050% or less. Although there is no particular lower limit for the O content, due to constraints on production technology, the O content is preferably set to 0.0001% or more. The O content is more preferably 0.0010% or more, and even more preferably 0.0015% or more.

[0027] 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. 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. It is preferable that 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.

[0028] 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.

[0029] 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 the ultimate deformability of the steel sheet is not reduced, resulting in excellent fatigue resistance at the sheared edge. 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. The content of each of these elements is more preferably 0.002% or more, and further preferably 0.003% or more.

[0030] Ta: 0.10% or less, W: 0.10% or less. Ta and W, if present in an amount of 0.10% or less, do not form large amounts of coarse precipitates or inclusions, thereby preventing a decrease in the ultimate deformability of the steel sheet and thus preventing a decrease in the fatigue resistance of the sheared edge. 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, these elements 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 elements preferably has a content of 0.02% or more, and even more preferably 0.03% or more.

[0031] 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 the ultimate deformability of the steel sheet will not be reduced, so the fatigue resistance of the sheared end surface 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.

[0032] 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 the ultimate deformability of the steel sheet is not reduced, resulting in a decrease in the fatigue resistance of the sheared edge. 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 0.80% or less. More preferably, each of these contents is 0.75% or less, and even more preferably, 0.70% or less. While there are no particular lower limits for the Cr, Mo, and Ni contents, since these elements improve hardenability, it is preferable that each of the Cr, Mo, and Ni contents be 0.01% or more. More preferably, each of these contents is 0.02% or more.

[0033] Co: 0.010% or less If Co is 0.010% or less, coarse precipitates and inclusions do not increase, and the ultimate deformability of the steel sheet is not reduced, so the fatigue resistance of the sheared edge portion is not reduced. Therefore, when Co is contained, the Co content is set to 0.010% or less. The Co content is preferably set to 0.008% or less. The Co content is more preferably set to 0.007% 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 preferably set to 0.001% or more.

[0034] Cu: 1.00% or less If Cu is 1.00% or less, coarse precipitates and inclusions do not increase, and the ultimate deformability of the steel sheet is not reduced, so the fatigue resistance of the sheared edge portion is not reduced. 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.

[0035] 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 ultimate deformability of the steel sheet will not be reduced, so the fatigue resistance of the sheared edge portion will not be reduced. Therefore, if Sn is contained, the Sn content is set to 0.200% or less. The Sn content is preferably set to 0.150% or less. Note that there is no particular lower limit for the Sn content, but since Sn is an element that improves hardenability (generally an element that improves corrosion resistance), the Sn content is preferably set to 0.001% or more. The Sn content is more preferably set to 0.002% or more.

[0036] Sb: 0.200% or less If Sb is 0.200% or less, coarse precipitates and inclusions do not increase, and the ultimate deformability of the steel sheet is not reduced, so the fatigue resistance of the sheared edge portion is not reduced. Therefore, when Sb is contained, the Sb content is set to 0.200% or less. The Sb content is preferably set to 0.150% or less. The Sb content is more preferably set to 0.100% or less. Note that there is no particular lower limit for the Sb content, but since Sb is an element that controls the softened surface thickness and enables strength adjustment, the Sb content is preferably set to 0.001% or more. The Sb content is more preferably set to 0.002% or more.

[0037] Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less. Ca, Mg, and REM, each present at 0.0100% or less, do not increase coarse precipitates or inclusions, and do not reduce the ultimate deformability of the steel sheet, thereby preventing a decrease in the fatigue resistance of the sheared edge. Therefore, when at least one of Ca, Mg, and REM is contained, the Ca, Mg, and REM contents are preferably each 0.0100% or less. Preferably, each of these contents is 0.0050% or less. While there are no particular lower limits for the Ca, Mg, and REM contents, these elements spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet, so the Ca, Mg, and REM contents are preferably 0.0001% or more. The content of each of these elements is more preferably 0.0002% or more, and further preferably 0.0005% or more.

[0038] 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 reduce the ultimate deformability of the steel sheet, thereby preventing a decrease in the fatigue resistance of the sheared edge. Therefore, when at least one of Zr and Te is contained, the Zr and Te contents are set to 0.100% or less. Preferably, each of these contents is set to 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 improve the ultimate deformability of the steel sheet, it is 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.

[0039] Hf: 0.10% or less If Hf is 0.10% or less, coarse precipitates and inclusions do not increase, and the ultimate deformability of the steel sheet is not reduced, so the fatigue resistance of the sheared edge portion is not reduced. Therefore, when Hf is contained, the Hf content is set to 0.10% or less. The Hf content is preferably set to 0.09% or less. The Hf content is more preferably set to 0.08% or less, and even more preferably set to 0.07% or less. Note that there is no particular lower limit for the Hf content, but since Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of the steel sheet, the Hf content is preferably set to 0.001% or more, and more preferably set to 0.010% or more.

[0040] Bi: 0.200% or less If Bi is 0.200% or less, coarse precipitates and inclusions do not increase, and the ultimate deformability of the steel sheet is not reduced, so the fatigue resistance of the sheared edge portion is not reduced. Therefore, when Bi is contained, the Bi content is set to 0.200% or less. The Bi content is preferably set to 0.190% 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 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.

[0041] 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.

[0042] Next, the steel structure of the steel sheet of the present invention will be described.

[0043] 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 must be 95% or more. Therefore, the area fraction of tempered martensite is 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. There is no particular upper limit to the area fraction of tempered martensite, but it may be 100%.

[0044] 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.

[0045] 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 volume fraction of retained austenite is set to less than 3%. Preferably, the volume fraction of retained austenite is set to 1% or less. Note that there is no particular limitation on the lower limit of the volume fraction of retained austenite, and it may be 0%.

[0046] 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.

[0047] 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%. The total area fraction of these is preferably 3% or less. The total area fraction of these is more preferably 2% or less. The lower limit of the total area fraction of ferrite and bainitic ferrite is not particularly limited, and may be 0%.

[0048] 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.

[0049] The remaining structure other than the entire structure may be pearlite, fresh martensite, acicular ferrite, etc. These remaining structures may be contained in an area fraction of 5% or less, as they do not affect the properties.

[0050] P(C)≦70% (1) P(C): Average value of the occupancy rate of the packet with the largest occupancy rate within the prior austenite grain at the center of the thickness of the steel plate. This is one of the important constituent elements of the present invention. The occupancy rate of the packet with the largest occupancy rate within the prior austenite grain at the center of the thickness of the steel plate affects the fatigue resistance of the sheared edge surface and the delayed fracture resistance in a corrosive environment. As shown in Figure 1, there are up to four regions called packets within a prior austenite grain that have the same habit plane at the time of transformation, and the packet with the largest occupancy rate within these regions refers to the packet with the largest occupancy rate. The occupancy rate of one packet within a prior austenite grain is calculated by dividing the area of ​​the specified packet by the total area within the prior austenite grain. As a result of extensive research, the inventors have found that by reducing the occupancy rate of packets with the highest occupancy rate within prior austenite grains at the thickness center of a steel plate, specifically by setting the average occupancy rate of packets with the highest occupancy rate within prior austenite grains at the thickness center of a steel plate to 70% or less, the structure is refined and crack propagation is suppressed, thereby improving the fatigue resistance of the sheared edge and the delayed fracture resistance in a corrosive environment. Therefore, the average occupancy rate of packets with the highest occupancy rate within prior austenite grains at the thickness center of a steel plate: P(C) is set to 70% or less. Preferably, this average occupancy rate: P(C) is set to 60% or less. The lower limit of the average occupancy rate of packets with the highest occupancy rate within prior austenite grains is not particularly limited. The number of packet types is up to four, and when the four packets are evenly distributed, the occupancy rate of packets with the highest occupancy rate within prior austenite grains is 25%. Therefore, the average value of the occupancy rate of packets having the maximum occupancy rate in the prior austenite grains at the center position of the thickness of the steel plate: P(C) is preferably 25% or more, but is not necessarily limited to this.

[0051] {P(S) - P(C)} ≦ 20% (2) P(S): The average value of the packet occupancy rate with the highest occupancy rate within prior austenite grains at a depth of 100 μm from the steel sheet surface. P(C): The average value of the packet occupancy rate with the highest occupancy rate within prior austenite grains at the center of the steel sheet thickness. This is one of the important constituent elements of the present invention. The difference between P(S) and P(C) affects the fatigue resistance of the sheared edge and the delayed fracture resistance in a corrosive environment. Failure to satisfy {P(S) - P(C)} ≦ 20% means that the packet occupancy rate in the surface layer of the steel sheet is excessively large compared to the packet occupancy rate in the center of the steel sheet, accelerating the initiation of fatigue-induced cracks and delayed fracture-induced cracks in the surface layer of the steel sheet. Therefore, {P(S) - P(C)} is set to 20% or less. {P(S) - P(C)} is preferably set to 15% or less.

[0052] Here, the method for measuring the average value of the occupancy rate of packets having the maximum occupancy rate within prior austenite grains is as follows. First, a test specimen for microstructure observation is taken from the steel sheet. Next, the taken test specimen is polished by colloidal silica vibration polishing so that the rolling direction cross section (L cross section) becomes the observation surface. The observation surface is made a mirror finish. Next, electron backscatter diffraction (EBSD) measurement is performed at a position 100 μm deep from the steel sheet surface and at the center position of the steel sheet thickness to obtain local crystal orientation data. In this case, the SEM magnification is 1000x, the step size is 0.2 μm, the measurement area is 80 μm square, and the WD is 15 mm. The obtained local orientation data is analyzed using OIM Analysis 7 (OIM), and a diagram (CP map) is created in which each close-packed plane group (CP group) is color-coded using the method described in Non-Patent Document 1 (Smart Process Society Journal, 2013, Vol. 2, No. 3, pp. 110-118). In the present invention, a packet is defined as an area to which the same CP group belongs. The area of ​​the packet with the largest occupancy rate is determined from the obtained CP map, and this is divided by the total area within the prior austenite grain to determine the occupancy rate of the packet with the largest occupancy rate within the prior austenite grain. This analysis is performed on 10 or more adjacent prior austenite grains, and the average value is used as the average occupancy rate of the packet with the largest occupancy rate within the prior austenite grain.

[0053] 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.

[0054] The thickness of the steel plate of the present invention is preferably 0.5 mm or more, and more preferably 3.0 mm or less.

[0055] The steel sheet of the present invention can be determined to have excellent delayed fracture resistance in a corrosive environment in accordance with the method for evaluating delayed fracture properties of metallic materials in a corrosive environment (HeTsAce). Specifically, the steel sheet of the present invention has a number of days to crack of 63 days or more in accordance with the method for evaluating delayed fracture properties of metallic materials in a corrosive environment (HeTsAce) shown below.

[0056] (HeTsAce) The amount of chloride attached to the evaluation surface of the metal material to be evaluated is 1000 to 20000 mg / m ２ and a corrosion step (B) in which one cycle is performed one or more times in an atmosphere at a temperature Tb1 of 60°C or less and within a certain range, the cycle comprising the following steps: a drying step (b1), a wetting step (b2), a transition step (b3), and a transition step (b4), and the cycle is performed one or more times, and the distribution of droplets of the chloride-containing aqueous solution on the evaluation surface of the metal material in at least the first chloride adhesion step (A) is such that: an average contact area of ​​the droplets on the evaluation surface of the metal material is 0.1 mm ２ More than 3.0 mm ２ less than 1.0 mm, an area ratio of the total contact area of ​​the droplets to the area of ​​the evaluation surface of the metal material: 40% or more and 80% or less, and a standard deviation of the contact area of ​​the droplets on the evaluation surface of the metal material: 3.0 mm ２ A method for evaluating the delayed fracture properties of a metallic material, comprising: a drying step (b1): a step of drying a metallic material by maintaining an atmosphere of a relative humidity Hb1 of 45% or less for 1.0 hour or more and 5.0 hours or less; a wetting step (b2): a step of wetting a metallic material by maintaining an atmosphere of a relative humidity Hb2 of 80% or more for 1.0 hour or more and 5.0 hours or less; a transition step (b3): ​​a step of transitioning from the atmosphere of the relative humidity Hb1 to the atmosphere of the relative humidity Hb2 at a rate of change of the relative humidity of 30% / h or less; and a transition step (b4): a step of transitioning from the atmosphere of the relative humidity Hb2 to the atmosphere of the relative humidity Hb1 at a rate of change of the relative humidity of 30% / h or less.

[0057] In the present invention, a metallic material extracted from a steel plate is evaluated. The method for extracting the metallic material from the steel plate is not particularly limited. For example, the metallic material can be extracted by shearing the steel plate to a predetermined size. In HeTsAce, it is preferable to apply stress to the metallic material to be evaluated. Examples of methods for applying stress to the metallic material include a method of processing the metallic material (a processing method). Examples of processing methods include bending, bulging, tensioning, and twisting. Other examples include a method of fixing the metallic material in a stressed shape using bolts or the like, and a method of using residual stress remaining after processing. In the evaluation method of the present invention, a process including a chloride adhesion process (A) and a corrosion process (B) can be performed at least once (once or twice or more) on the metallic material to which stress has been applied as described above. Furthermore, in the evaluation method of the present invention, the state of the metallic material is confirmed after the process including the chloride adhesion process (A) and the corrosion process (B) is performed once or more times, and the delayed fracture properties of the metallic material can be evaluated based on the confirmed state of the metallic material. The confirmation can be carried out, for example, by visually observing the presence or absence of cracks in the metal material and the extent of the cracks.

[0058] In the present invention, in order to evaluate the delayed fracture properties of a metallic material, a chloride adhesion step (A) and a corrosion step (B) are carried out while applying stress to the metallic material. After carrying out each of these steps one or more times, the presence or absence and degree of cracking in the metallic material are confirmed, thereby evaluating the delayed fracture properties.

[0059] (Chloride Adhesion Step (A)) In the chloride adhesion step (A), a chloride adhesion amount of 1000 to 20000 mg / m is applied to the evaluation surface of the metal material. ２ In the evaluation method of the present invention, the distribution of the droplets of the chloride-containing aqueous solution on the evaluation surface of the metal material in at least the first (first) chloride deposition step (A) (deposition distribution of the droplets) is such that the average contact area of ​​the droplets on the evaluation surface of the metal material is 0.1 mm ２ More than 3.0 mm ２less than 1.0 mm, the area ratio of the total contact area of ​​the droplets to the area of ​​the evaluation surface of the metal material: 40% or more and 80% or less, and the standard deviation of the contact area of ​​the droplets on the evaluation surface of the metal material: 3.0 mm ２ The following applies.

[0060] <Chloride deposition amount: 1,000 to 20,000 mg / m ２ The amount of chloride to be attached to the metal material (the amount of chloride solids not including solvents such as water) is 1,000 to 20,000 mg / m ２ The adhesion amount corresponds to the amount of chlorides that is assumed to be adhered in the atmospheric corrosive environment in which an actual automobile runs. ２ In a corrosive environment where the coating amount is less than 20,000 mg / m, corrosion hardly progresses, and therefore hydrogen generation and penetration into the metal material are minimal, making delayed fracture unlikely to occur. ２ If the coating amount exceeds 20,000 mg / m, the corrosion rate will be significantly different from that in the actual environment, resulting in an excessive durability test, which will not serve the purpose. ２ From the viewpoint of simulating the corrosion pattern in the atmospheric corrosive environment in which an actual automobile runs and promoting corrosion, the coating amount is set to 5000 mg / m ２ From the above viewpoint, the amount of adhesion is preferably more than 12,000 mg / m ２ The following is preferred:

[0061] The amount of chloride adhesion can be calculated by multiplying the mass difference between the test piece (metal material) before and after application of the chloride-containing aqueous solution in the chloride adhesion step (A) by the chloride concentration of the chloride-containing aqueous solution and dividing the result by the area of ​​the test piece's evaluation surface. When measuring the mass difference, if the chloride-containing aqueous solution adheres to areas other than the evaluation surface of the test piece, appropriate measures can be taken, such as masking the areas other than the evaluation surface or wiping off the chloride-containing aqueous solution that has adhered to the areas other than the evaluation surface. The amount of chloride adhesion can be controlled, for example, by changing the chloride concentration of the chloride-containing aqueous solution or by changing the time for applying the chloride-containing aqueous solution (the process time of the chloride adhesion step (A)) to change the amount of chloride-containing aqueous solution applied to the metal material.

[0062] In the chloride deposition step (A), chloride is deposited on the metal material to obtain a desired amount of chloride. Examples of chloride include sodium salt (NaCl), potassium salt (KCl), and calcium salt (CaCl), which are present in the atmospheric environment where general metal materials are used. ２ ), magnesium salts (MgCl ２ ) is preferably included. In the chloride adhering step (A), when adhering chloride to the metal material, a component mainly composed of chloride, which contains chloride and components other than chloride, may be adhered. Here, the component mainly composed of chloride refers to a component in which chloride is more than 50 mass% of all components in terms of solid content. Examples of components other than chloride include, but are not limited to, sulfides and nitrate compounds. In consideration of an actual atmospheric corrosion environment, it is preferable to adhere a component mainly composed of NaCl (a component in which NaCl is more than 50 mass% of all components) to the metal material.

[0063] In addition, when simulating delayed fracture characteristics in an area where snow-melting agents are frequently sprayed in winter, it is preferable that the chlorides to be attached to the metal material have a composition similar to that of the snow-melting agents sprayed in that area. ２ Components mainly composed of (CaCl ２ is more than 50% by mass of all components), MgCl ２ A component mainly composed of MgCl ２ Examples of the component include a component containing NaCl as the main component (a component in which NaCl accounts for more than 50% by mass of all components), a component containing NaCl as the main component (a component in which NaCl accounts for more than 50% by mass of all components), and the like.

[0064] Furthermore, a component containing a combination of multiple metal salts may be used as the chloride to be attached to the metal material. An example of a component containing a combination of multiple metal salts is the SAE J2334 (0.5% by mass NaCl - 0.1% by mass CaCl ２ -0.075% by mass NaHCO ３ ), artificial seawater (2.5% by mass NaCl-0.5% by mass MgCl ２ -0.12% by mass CaCl ２Examples include 0.07% by mass KCl and others (for example, an aqueous solution of Aquamarine (registered trademark) manufactured by Yashima Pharmaceutical Co., Ltd.).

[0065] The method for depositing chloride on a metal material (chloride deposition method) is not particularly limited as long as it is a method that can achieve a desired distribution of chloride-containing aqueous solution droplets for evaluation of the metal material. Examples of the chloride-containing aqueous solution include a chloride-containing aqueous solution containing a component mainly composed of chloride (usually an aqueous solution such as salt water, hereinafter also referred to as salt water). The following description will be given taking the case where salt water is used as the chloride-containing aqueous solution as an example.

[0066] The spray method is an example of a chloride deposition method. One example of a spray method is the deposition of salt water using a spray nozzle. Types of spray nozzles include single-fluid spray nozzles (nozzles in which a liquid fed under pressure is atomized and sprayed) and two-fluid spray nozzles (nozzles that atomize the liquid using a high-speed fluid such as compressed air). Two-fluid spray nozzles also differ in the liquid supply method, and are classified into liquid pressure types (liquid is pressurized and supplied to the two-fluid nozzle) and suction types (liquid is sucked up and sprayed using the force of compressed air). It is preferable to select a spray nozzle that ensures uniform deposition distribution of droplets. Furthermore, since salt water is used, it is preferable to use a corrosion-resistant metal such as stainless steel for the spray nozzle material.

[0067] The chloride concentration in the saltwater is not particularly limited. However, when controlling the saltwater droplet distribution using a spray nozzle, if saltwater with a chloride concentration of less than 2.0 mass% is used to deposit saltwater on a metal material, the spray time is long to obtain a suitable chloride deposition amount, making it difficult to obtain the desired droplet distribution on the evaluation surface of the metal material. Therefore, the chloride concentration in the saltwater is preferably 2.0 mass% or more, and more preferably 5.0 mass% or more. On the other hand, if saltwater with a chloride concentration of more than 20 mass% is used to deposit saltwater on a metal material, chlorides are likely to precipitate in the spray nozzle, causing clogging of the spray nozzle and making it difficult to spray saltwater droplets of consistent size, making it difficult to obtain the desired droplet distribution on the evaluation surface of the metal material. As a result, the amount of chloride deposition on the evaluation surface of the metal material varies depending on the location, and in areas with a high chloride deposition amount, localized corrosion occurs and the amount of hydrogen penetration increases. This results in changes in the delayed fracture properties within the evaluation surface of the metal material, reducing the accuracy of the evaluation of delayed fracture properties. This tendency is particularly pronounced when the chloride deposition amount is high. Therefore, the chloride concentration in the salt water is preferably 20% by mass or less, and more preferably 15% by mass or less.

[0068] It is recommended to adjust the chloride concentration in the saltwater according to the target chloride deposition amount. It is preferable to use low-concentration saltwater when the chloride deposition amount is relatively low, and high-concentration saltwater when the chloride deposition amount is relatively high. To reduce the chloride deposition amount on the test surface of the metal material using high-concentration saltwater, the total amount of saltwater sprayed must be reduced, which can lead to an undersized average contact area of ​​saltwater droplets on the test surface of the metal material and a ratio of the total contact area of ​​saltwater droplets to the total area of ​​the test surface. On the other hand, to increase the chloride deposition amount on the test surface of the metal material using low-concentration saltwater, the total amount of saltwater sprayed must be increased, which can lead to an oversized average contact area of ​​saltwater droplets on the test surface of the metal material and a ratio of the total contact area of ​​saltwater droplets to the total area of ​​the test surface.

[0069] [Distribution of Droplets of Chloride-Containing Aqueous Solution Adhered to the Test Surface of the Metallic Material in the Initial Chloride Adhesion Step (A)] In the evaluation method of the present invention, it is necessary to control the distribution of saltwater droplets on the test surface of the metallic material in at least the initial (i.e., at least the first) chloride adhe- sion step (A) within the range described below. It is believed that the locations where saltwater was present on the test surface of the metallic material in the first chloride adhe- sion step primarily become the starting points of corrosion on the test surface. Furthermore, in the second chloride adhe- sion step, the test surface becomes corroded after the first chloride adhe- sion step, forming corrosion products, resulting in saltwater spray. Even if saltwater droplets are uniformly applied to the test surface of the metallic material, wetting and spreading occurs due to the influence of the corrosion products. Therefore, it is important to uniformly control the distribution of saltwater droplets on the test surface of the metallic material in the first chloride adhe- sion step.

[0070] Fig. 2 is a schematic diagram illustrating an example of the evaluation method of the present invention. As shown in Fig. 2, in the evaluation method of the present invention, salt water is sprayed from a spray nozzle to deposit droplets of salt water onto the evaluation surface of the metal material. In this process, in the initial chloride deposition step (A), the distribution of salt water droplets deposited on the evaluation surface of the metal material (droplet deposition distribution) is controlled within a predetermined range.

[0071] <Average contact area of ​​droplets on the evaluation surface of the metal material: 0.1 mm ２ More than 3.0 mm ２ In the evaluation method of the present invention, in at least the first chloride adhesion step (A), the average contact area of ​​the droplets on the evaluation surface of the metal material (average contact area per droplet) is set to 0.1 mm ２ More than 3.0 mm ２ The average contact area is less than 0.1 mm ２ If the average contact area is less than 0.1 mm, the volume of the droplets is too small to achieve the target chloride deposition amount. ２ The average contact area is 0.5 mm ２ It is preferable that the thickness is 1.0 mm or more. ２ On the other hand, it is more preferable that the average contact area is 3.0 mm or more. ２If the average contact area is more than 3.0 mm, the droplet adhesion distribution becomes non-uniform, resulting in large variations in the delayed fracture evaluation. ２ The average contact area is less than 2.8 mm ２ It is preferable that the thickness is 2.5 mm or less. ２ It is more preferable that the average contact area of ​​the droplets on the evaluation surface of the metal material be as follows: The average contact area of ​​the droplets on the evaluation surface of the metal material can be measured by the measurement method described below.

[0072] <Area Ratio of Total Contact Area of ​​Droplets to Area of ​​Evaluation Surface of Metal Material: 40% to 80%> In the evaluation method of the present invention, in at least the first chloride adhesion step (A), the area ratio of the total contact area of ​​droplets to the area of ​​the evaluation surface of the metal material (total contact area ratio of droplets) is set to 40% to 80%. If the total contact area ratio of droplets is less than 40%, the droplet adhesion distribution will be non-uniform, resulting in large variations in delayed fracture evaluation. Therefore, the total contact area ratio of droplets is set to 40% or more. The total contact area ratio of droplets is preferably set to 50% or more, more preferably 55% or more. On the other hand, if the total contact area ratio of droplets is greater than 80%, adjacent droplets on the evaluation surface will be more likely to bond with each other, resulting in coarsening of the average contact area of ​​droplets (average contact area per droplet). Therefore, the total contact area ratio of droplets is set to 80% or less. The total contact area ratio of droplets is preferably set to 75% or less, more preferably 70% or less. The total contact area ratio of the droplets can be measured by the measurement method described below.

[0073] <Standard deviation of the contact area of ​​the droplet on the evaluation surface of the metal material: 3.0 mm ２ In the evaluation method of the present invention, in at least the first chloride adhesion step (A), the standard deviation in the distribution of the contact area of ​​the droplet on the evaluation surface of the metal material is 3.0 mm ２ The standard deviation of the contact area of ​​the droplet on the evaluation surface of the metal material is 3.0 mm or less. ２ If the contact area is larger than this, the droplet adhesion distribution becomes non-uniform, resulting in a large variation in the delayed fracture evaluation. ２The standard deviation of the contact area of ​​the droplets is 2.8 mm ２ It is preferable that the thickness is 2.5 mm or less. ２ It is more preferable that the standard deviation of the contact area of ​​the droplets is as follows: The standard deviation of the contact area of ​​the droplets can be measured by the measurement method described below.

[0074] The distribution of saltwater droplets attached to the evaluation surface of the metal material (average contact area of ​​the droplets, total contact area ratio of the droplets, standard deviation of the droplet contact area) can be determined by attaching saltwater droplets to the evaluation surface of the metal material in the chloride attachment step (A), acquiring an image of the droplet distribution over the entire evaluation surface of the metal material, and performing image analysis. The image can be acquired using a digital camera, a microscope, an optical microscope, or the like. The image can also be acquired by photographing the evaluation surface from above (from the direction of the spray nozzle shown in Figure 2) the evaluation surface of the metal material. The image of the evaluation surface can be acquired within a test tank equipped with a spray nozzle, or by removing the metal material from the test tank. Preferably, in the latter case, the metal material is removed from the test tank equipped with a spray nozzle and the evaluation surface of the metal material is photographed. The image is also acquired immediately (within 30 seconds) after the saltwater droplets are attached to the evaluation surface of the metal material.

[0075] Figure 3 is a schematic diagram showing an image of the droplet distribution on the evaluation surface of the metal material obtained as described above. In Figure 3, the areas indicated by circles are the contact areas of the droplets. From such an image, the average contact area of ​​the droplets (average contact area per droplet), the total contact area ratio of the droplets, and the standard deviation of the contact area of ​​the droplets are determined by image analysis.

[0076] Here, the evaluation surface of a metallic material refers to the surface of the metallic material for evaluating the delayed fracture properties. The evaluation surface can be determined appropriately depending on the metallic material to be evaluated. For example, if the metallic material is a plate, the evaluation surface can be the surface of the plate facing the spray nozzle (see FIG. 2). Furthermore, if stress is applied to the metallic material, the evaluation surface can be the surface of the stressed portion facing the spray nozzle (the surface corresponding to the plan view (top view) of the metallic material when the direction in which the spray nozzle is installed relative to the metallic material is upward). More specifically, for example, as described below, if the evaluation target is a bent metallic material, the evaluation surface can be the surface of the bent portion facing the spray nozzle (see FIG. 8).

[0077] As a method for achieving the above-mentioned distribution of saltwater droplets, there is a method in which saltwater is applied to the evaluation surface of the metal material using a spray nozzle, as described above. As the spray nozzle, a two-fluid nozzle is preferable. Examples of the two-fluid nozzle include KSMMS (product name) manufactured by Kyoritsu Alloy Manufacturing Co., Ltd., a two-fluid air atomizing nozzle (product name) manufactured by Spraying Systems Japan LLC, and a fine mist generating nozzle (product name) manufactured by Ikeuchi Co., Ltd.

[0078] As an example of specific conditions when using a spray nozzle, the distance from the tip of the spray nozzle to the evaluation surface of the metal material (X in Figure 2) is preferably 10 to 30 cm. The spray pressure of the spray nozzle is preferably 0.05 to 0.7 MPa. The spray angle (θ in Figure 2) is preferably 30 to 120°. The salt water spray time is preferably 10 seconds or less. As mentioned above, it is also preferable to adjust the chloride concentration in the salt water according to the target chloride adhesion amount. Note that, as shown in Figure 8, when a metal material that has been bent is to be evaluated, the distance from the tip of the spray nozzle to the evaluation surface of the metal material is the shortest distance from the tip of the spray nozzle to the evaluation surface of the metal material.

[0079] A particularly preferred method for achieving the above-described saltwater droplet distribution is to place a shielding material having an opening between the spray nozzle and the metal material, and allow droplets of the chloride-containing aqueous solution sprayed from the spray nozzle to adhere to the evaluation surface of the metal material through the opening of the shielding material. The opening of the shielding material preferably has a shape and size substantially identical to the shape and size of the evaluation surface of the metal material. "Substantially identical" means that the opening of the shielding material is equivalent to the peripheral shape and size of the evaluation surface of the metal material when viewed from above, or that the area of ​​the opening of the shielding material is within ±10% of the area of ​​the evaluation surface of the metal material. Furthermore, it is preferable that the shielding material be capable of shielding areas other than the evaluation surface of the metal material. That is, when viewed from above, the evaluation surface of the metal material can be seen through the opening of the shielding material, while other areas are not visible (are shielded).

[0080] Fig. 4 is a schematic diagram illustrating the case where a shielding material is placed between the spray nozzle and the metal material in the evaluation method of the present invention. As shown in Fig. 4, by placing the above-mentioned shielding material between the spray nozzle and the metal material, it is possible to prevent the sprayed liquid (atomized salt water) that is sprayed from the spray nozzle but floats in the atmosphere without adhering to the evaluation surface of the metal material from adhering (re-adhering) to the evaluation surface of the metal material after the salt water spray has ended, i.e., to which droplets have already adhered. As a result, it is possible to achieve a desired droplet distribution on the evaluation surface of the metal plate with high precision.

[0081] When a shielding material is used, the distance X from the tip of the spray nozzle to the evaluation surface of the metal material is preferably 10 to 30 cm. The distance between the shielding material and the evaluation surface of the metal material (Y in Figure 4) is preferably 1 cm or more and 0.3X cm or less. If the distance Y is less than 1 cm, the sprayed liquid floating in the atmosphere without adhering to the evaluation surface of the metal material remains near the evaluation surface of the metal material, thereby reducing the effect of suppressing redeposition (Figure 5(a)). On the other hand, if the distance Y is greater than 0.3X cm, the sprayed liquid that passes through the opening of the shielding material will scatter below the shielding material, reducing the effect of suppressing redeposition (Figure 5(b)). Note that the distance Y is the shortest distance from the shielding material to the evaluation surface of the metal material. The material of the shielding material is not limited as long as it can prevent the transmission of the sprayed liquid. Examples of materials include resin, ceramic, metal, and wood. These materials can be processed and used as the shielding material.

[0082] As described above, in the evaluation method of the present invention, it is important to uniformly control the distribution of saltwater droplets on the evaluation surface of the metal material in the initial (first) chloride deposition step (A). The droplet distribution on the evaluation surface as described above is controlled at least in the initial chloride deposition step (A). The average droplet contact area, total droplet contact area ratio, and standard deviation of the droplet contact area can be measured at the end of the initial chloride deposition step (A) (within 30 seconds after the end of saltwater spraying). Alternatively, a test specimen other than the one to be tested may be used, and the conditions for the average droplet contact area, total droplet contact area ratio, and standard deviation of the droplet contact area may be set in advance to achieve a predetermined average droplet contact area, total droplet contact area ratio, and standard deviation of the droplet contact area. The evaluation test of the test specimen may then be performed under the same conditions. In the second or subsequent chloride deposition steps (A), chloride deposition may be performed under the same conditions as in the initial chloride deposition step (A), or may be performed under different conditions from the initial chloride deposition step (A) as long as the desired chloride deposition amount is achieved. Preferably, conditions for achieving a predetermined average contact area of ​​droplets, a total contact area ratio of droplets, and a standard deviation of the contact areas of droplets are set in advance, and the first chloride deposition step (A) is carried out under the set conditions. When the chloride deposition step (A) is carried out two or more times, it is preferable to carry out the second and subsequent chloride deposition steps (A) under the set conditions.

[0083] The chloride adhesion step (A) is preferably carried out in an atmosphere with a relative humidity Ha1 of 30% or more. If the relative humidity Ha1 in the chloride adhesion step (A) is less than 30%, particularly when droplets of a chloride-containing aqueous solution are sprayed using a spray nozzle, the droplets sprayed from the spray nozzle tend to dry before reaching the evaluation surface of the metal material. As a result, it may be difficult to control the droplet distribution to obtain a desired distribution on the evaluation surface of the metal material. Furthermore, the chloride adhesion step (A) is preferably carried out in an atmosphere with a relative humidity of 80% or less. If the relative humidity in the chloride adhesion step (A) is greater than 80%, the droplets adhering to the metal material tend to become coarse.

[0084] Furthermore, the chloride deposition step (A) is preferably performed in an atmosphere having a temperature Ta1 of 50°C or less. If the temperature Ta1 in the chloride deposition step (A) exceeds 50°C, particularly when droplets of a chloride-containing aqueous solution are sprayed using a spray nozzle, the droplets sprayed from the spray nozzle tend to dry before reaching the evaluation surface of the metal material. As a result, it may be difficult to control the droplet distribution to obtain the desired droplet distribution on the evaluation surface of the metal material. On the other hand, the lower limit of the temperature Ta1 is not limited as long as it is a temperature at which the saltwater state can be maintained. As an example, the lower limit of the temperature Ta1 is 25°C.

[0085] A characteristic of atmospheric corrosion environments is the repeated alternation of wet (humid) and dry (dry) conditions, and simulating this environmental change is important for approximating the corrosion patterns in the actual environment in which a vehicle runs. For example, in the case of steel materials, it is known that the corrosion products formed on the steel material change depending on the wet and dry conditions, and hydrogen is generated during the process of changing from a wet state to a dry state, or from a dry state to a wet state. Therefore, the conditions in the cycle of relative humidity change (corrosion process (B)) are also important for evaluating delayed fracture properties.

[0086] (Corrosion Step (B)) The corrosion step (B) is a step of performing a cycle at least once (once or twice or more) in an atmosphere at a temperature Tb1 that is 60°C or less and within a certain range, the cycle including the following drying step (b1), the following wetting step (b2), the following transition step (b3), and the following transition step (b4).

[0087] <Temperature Tb1 of Corrosion Step (B): 60°C or Less and Within a Certain Range> The corrosion step (B) is performed in an atmosphere with a temperature Tb1 of 60°C or less and within a certain range. If the temperature Tb1 of the corrosion step (B) exceeds 60°C, not only will the evaluation be performed in an environment far removed from the corrosive environment in which the metal material is actually used, but the corrosion mechanism may also change. Therefore, the temperature Tb1 of the corrosion step (B) is set to 60°C or less, preferably 50°C or less. On the other hand, the lower limit of the temperature Tb1 of the corrosion step (B) is not particularly limited. If the temperature Tb1 of the corrosion step (B) is less than 5°C, it may be difficult to control the relative humidity in the corrosion test chamber (constant temperature and humidity chamber) used when conducting the corrosion test. In addition, the corrosion rate of the metal material will be significantly reduced, resulting in a longer evaluation time. Therefore, the temperature Tb1 of the corrosion step (B) is preferably set to 5°C or more, and more preferably 10°C or more.

[0088] In addition, delayed fracture properties are strongly affected by the temperature of the environment (atmosphere). Therefore, in order to properly evaluate the delayed fracture properties of metallic materials, taking into account the application location and the environment in which the metallic material is used, it is necessary to keep the temperature Tb1 of the corrosion step (B) within a certain range. When the temperature Tb1 of the corrosion step (B) fluctuates within ±5°C, the amount of hydrogen penetrating from the environment into the metallic material (hydrogen penetration) can be evaluated within a fluctuation range of 30% of the hydrogen penetration amount at the target environmental temperature, allowing for accurate evaluation of delayed fracture properties. When the temperature Tb1 of the corrosion step (B) fluctuates within ±2°C, the fluctuation range of the hydrogen penetration amount is within 15%. Therefore, the fluctuation range of the temperature Tb1 of the step (B) is preferably within ±5°C, and more preferably within ±2°C.

[0089] [Drying Step (b1)] The drying step (b1) is a step of drying the metal material in an atmosphere with a relative humidity Hb1 of 45% or less for 1.0 to 5.0 hours. The relative humidity Hb1 in the drying step (b1) is set to 45% or less. This is to simulate the dry state, which is one of the characteristics of an atmospheric corrosive environment. Furthermore, if the relative humidity Hb1 in the drying step (b1) exceeds 45%, a long period of time is required to sufficiently dry the metal material surface, resulting in a longer evaluation time. The relative humidity Hb1 in the drying step (b1) is preferably 40% or less. On the other hand, the lower limit of the relative humidity Hb1 in the drying step (b1) is not particularly limited. From the viewpoint of relative humidity controllability, the relative humidity Hb1 in the drying step (b1) is preferably 20% or more. Furthermore, if the components to be attached to the metal material surface contain substances that exhibit deliquescent properties at lower relative humidities, such as magnesium chloride or calcium chloride, it is preferable to set the relative humidity Hb1 in the drying step (b1) low.

[0090] The process time of the drying step (b1) (the time for maintaining the sample in an atmosphere of relative humidity Hb1) is 1.0 hour or more and 5.0 hours or less. If the process time of the drying step (b1) is less than 1.0 hour, an actual corrosive environment cannot be simulated. On the other hand, if the process time of the drying step (b1) is more than 5.0 hours, an actual corrosive environment can be simulated, but it takes a long time to evaluate the delayed fracture properties.

[0091] [Wetting Step (b2)] The wetting step (b2) is a step of wetting a metal material by maintaining an atmosphere with a relative humidity Hb2 of 80% or higher for 1.0 to 5.0 hours. The relative humidity Hb2 in the wetting step (b2) is set to 80% or higher. This is to simulate the wet state, which is one of the characteristics of an atmospheric corrosive environment. If the relative humidity Hb2 in the wetting step (b2) is less than 80%, the effect of wetting will be insufficient, making it impossible to simulate an actual corrosive environment. Among chlorides, sodium chloride has the highest saturated critical vapor pressure, which is approximately 75 to 78% in relative humidity terms. Therefore, for any chloride, if the relative humidity is set to 80% or higher, a water film will form on the metal material surface due to moisture absorption by the chloride, allowing the metal material to maintain a wet state. Therefore, the relative humidity Hb2 in the wetting step (b2) is set to 80% or higher. On the other hand, although there is no particular upper limit for the relative humidity Hb2 in the wetting step (b2), it is preferable that the relative humidity Hb2 in the wetting step (b2) be less than 98%. This is because if the relative humidity Hb2 is 98% or higher, the water film formed by condensation becomes too thick, making it easier for the attached chlorides to be washed away. This phenomenon is particularly likely to occur when evaluating processed test specimens. Therefore, when evaluating processed test specimens, it is preferable that the relative humidity Hb2 in the wetting step (b2) be less than 98%.

[0092] The process time of the wetting step (b2) (the time for maintaining the specimen in an atmosphere with a relative humidity Hb2 of 80% or more) is set to 1.0 hour or more and 5.0 hours or less. If the process time of the wetting step (b2) is less than 1.0 hour, it is not possible to simulate an actual corrosive environment. On the other hand, if the process time of the wetting step (b2) is more than 5.0 hours, it is possible to simulate an actual corrosive environment, but it takes a long time to evaluate the delayed fracture properties.

[0093] [Transition Step (b3) and Transition Step (b4)] The transition step (b3) is a step of transitioning from an atmosphere with the relative humidity Hb1 to an atmosphere with the relative humidity Hb2, and the transition step (b4) is a step of transitioning from an atmosphere with the relative humidity Hb2 to an atmosphere with the relative humidity Hb1. It is known that the amount of hydrogen penetration into a metal material, particularly a steel material, increases when the relative humidity changes. That is, a large amount of hydrogen penetrates into the steel material during the transition steps (b3) and (b4). The reason why the amount of hydrogen penetration into the metal material increases when the relative humidity changes is not entirely clear, but it can be considered as follows. In the transition step (b3), which is a step of transitioning from an atmosphere with the relative humidity Hb1 to an atmosphere with the relative humidity Hb2, moisture absorption begins due to deliquescence of chlorides present on the surface of the metal material, and corrosion of the metal material begins. It is known that corrosion products present on the surface of the metal material change at this time, and hydrogen is thought to be generated along with this change in corrosion products. Furthermore, in the transition step (b4), which is a step of transitioning from an atmosphere with the relative humidity Hb2 to an atmosphere with the relative humidity Hb1, the moisture becomes a concentrated solution containing a large amount of chlorides and metal ions eluted by corrosion, and in the case of steel materials, iron ions, and the pH of the solution is thought to decrease. In other words, a large amount of hydrogen ions is contained in the solution during the drying process, which is thought to facilitate hydrogen penetration into the metal material. For this reason, in the transition steps (b3) and (b4), the rate of change of the relative humidity when changing the relative humidity is set to 30% / h or less. If the rate of change of the relative humidity in the transition steps (b3) and (b4) is 30% / h or less, hydrogen generated by corrosion can be sufficiently penetrated into the metal material, enabling appropriate evaluation of delayed fracture properties. On the other hand, the lower limit of the rate of change of the relative humidity is not particularly specified. However, if the transition steps (b3) and (b4) are too long, it will take a long time to evaluate the delayed fracture properties. Therefore, the rate of change of the relative humidity is preferably 1.5% / h or more, and more preferably 10% / h or more.

[0094] The purpose of the method for evaluating the delayed fracture properties of metallic materials of the present invention is to simulate the daytime and nighttime changes in relative humidity in an actual environment. Therefore, if the process time (time for one cycle) of the corrosion step (B), which simulates the daytime and nighttime changes in relative humidity in an actual environment, exceeds 24 hours, corrosion will be slower than in an actual environment, and the evaluation of delayed fracture properties will require a long time. In other words, the process time of the corrosion step (B) is preferably set to 24 hours or less. To expedite the evaluation, the process time of the corrosion step (B) is more preferably set to 12 hours or less. On the other hand, if the process time of the corrosion step (B) is shortened, the relative humidity will change rapidly, reducing the correlation with corrosion in an actual environment and resulting in inconsistencies in the delayed fracture properties in an actual environment. Therefore, the process time of the corrosion step (B) is preferably set to 5 hours or more.

[0095] In the evaluation method of the present invention, the chloride adhesion step (A) and the corrosion step (B) are each performed at least once. The chloride adhesion step (A) may be performed every random number of cycles of the corrosion step (B) or every predetermined number of cycles of the corrosion step (B). The upper limit of the number of times the process including the chloride adhesion step (A) and the corrosion step (B) is performed is not particularly limited. For example, the process including the chloride adhesion step (A) and the corrosion step (B) may be performed until cracks occur in the metal material. Alternatively, the number of test days may be determined in advance, and the process may be performed for a number of days corresponding to the number of test days. The number of times the process is performed can be appropriately set, taking into consideration, for example, simulating corrosion patterns in an actual environment. As an example, the process may be performed 200 times or less, or may be performed 100 times or less.

[0096] Next, a process including a chloride adhesion step (A) and a corrosion step (B) will be described. FIG. 6 is a diagram illustrating one embodiment of a corrosion test cycle according to the evaluation method of the present invention. The corrosion test cycle shown in FIG. 6 shows an example of a corrosion test cycle in which the chloride adhesion step (A) and the corrosion step (B) are each performed once. In this example, the corrosion step (B) includes a drying step (b1), a transition step (b3), a wetting step (b2), and a transition step (b4) as one cycle.

[0097] The corrosion process (B) cycle following the chloride deposition process (A) preferably begins with the drying process (b1). By drying the saltwater applied in the chloride deposition process (A) in the drying process (b1), condensation initiation points are uniformly dispersed when humidity increases, reducing the variability in the evaluation of delayed fracture properties. If the corrosion process (B) cycle begins with the transition process (b3), the wetting process (b2), or the transition process (b4), the saltwater applied in the chloride deposition process (A) may not be sufficiently dried, or the saltwater applied in a high-humidity environment may absorb moisture and become coarse, resulting in uneven dispersion of condensation initiation points. Therefore, it is best to avoid starting the corrosion process (B) cycle from any process other than the drying process (b1).

[0098] When a cycle of the chloride deposition step (A) followed by the corrosion step (B) is performed and then the chloride deposition step (A) is performed again, it is preferable to include a water rinsing step (C) before the chloride deposition step (A). Figure 7 shows an example of a corrosion test cycle in which a cycle of the chloride deposition step (A) followed by the corrosion step (B) is performed, and then a water rinsing step (C) is performed before the chloride deposition step (A) is performed again. If the chloride deposition step (A) is performed again without the water rinsing step (C), the amount of chloride deposited on the surface of the metal material tends to increase as the amount of chloride deposition increases, which may make it impossible to continue the same corrosive environment. This may result in the possibility of evaluating delayed fracture properties in an environment different from the intended corrosive environment. Therefore, it is preferable to include a water rinsing step (C) before performing the chloride deposition step (A) again. The water rinsing step (C) is a step of rinsing the evaluation surface of the metal material with water. The water-washing method in the water-washing step (C) is not particularly limited, but examples thereof include a method in which water is sprayed onto the evaluation surface of the metal material from a spray nozzle to wash the evaluation surface, and a method in which the evaluation surface is immersed in water to wash the evaluation surface.

[0099] In the evaluation method of the present invention, a cycle of the chloride adhesion step (A) and the corrosion step (B) as described above is carried out, and then a corrosion test cycle in which the water rinsing step (C) is carried out before the chloride adhesion step (A) is carried out again is carried out at least once. After that, the condition of the metallic material (the presence or absence of cracks in the metallic material, the degree of cracks, etc.) is confirmed, and the delayed fracture properties of the metallic material are evaluated based on the confirmed condition of the metallic material.

[0100] Specifically, the steel sheet of the present invention has a cracking period of 63 days or more under the following conditions in the evaluation method for delayed fracture properties of the above-mentioned metallic material (HeTsAce).

[0101] Step (A) Chloride-containing aqueous solution: 15 mass% NaCl aqueous solution sprayed by a spray method. Chloride adhesion amount (solid content equivalent): 10,000 mg / m ２ Distance between the nozzle and the evaluation surface (X in Figure 2): 30 cm Distance between the shielding material and the evaluation surface (Y in Figure 5): 5 cm Temperature: 22°C, relative humidity: 50% Distribution of droplets on the evaluation surface Average contact area of ​​droplets: 1.3 mm ２ , Standard deviation of droplet contact area: 1.3 mm ２ , total contact area ratio of droplets: 71% Step (B) Drying step (b1) Temperature: 30°C, relative humidity: 40%, retention time: 2.0 hours Wetting step (b2) Temperature: 30°C, relative humidity: 90%, retention time: 2.0 hours Transition step (b3) Relative humidity change rate: 25% / h Transition step (b4) Relative humidity change rate: 25% / h Temperature fluctuation range: within ±5°C

[0102] The water-washing step (C) is a step of rinsing the evaluation surface of the metal material with water. The water-washing method in the water-washing step (C) is not particularly limited, but the evaluation surface of the metal material is rinsed with water by spraying water onto the evaluation surface from a spray nozzle.

[0103] Corrosion test cycle: After the first step (A) was performed, the droplet distribution on the evaluation surface was confirmed. Subsequent steps (A) were performed under the same conditions as the first step (A). One cycle of step (B) consisted of the drying step (b1) → transition step (b3) → wetting step (b2) → transition step (b4) in this order. This cycle was repeated four times, and then a water-washing step (C) was inserted before step (A) was performed, followed by step (A). Metallic materials used for evaluation: The obtained steel sheets were sheared to a size of 16 mm x 75 mm, with the longitudinal direction perpendicular to the rolling direction, to prepare test specimens. The shear clearance was 15% in both cases. Next, a four-point bending test was performed according to ASTM (G39-99), and stresses equivalent to YS and TS were applied to the bend vertices of the test specimens. Next, the above-mentioned corrosion environment cycle (HeTsAce) was performed for 63 days. After the test, each test piece is visually inspected for cracks, and if no cracks are found in the sample to which a stress equivalent to YS is applied, it is judged to have excellent delayed fracture resistance in a corrosive environment.

[0104] In the above-mentioned evaluation method for delayed fracture properties of metallic materials (HeTsAce), those materials that take a long time until cracks occur tend to have a small amount of diffusible hydrogen in the steel.

[0105] [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 comprises the steps of: heating a cold-rolled sheet obtained by hot-rolling, pickling, and cold-rolling a steel having the above-described chemical composition 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 longer; cooling at an average cooling rate CR1 of 5°C / s or higher from 700 to 600°C; cooling from (Ms + 100°C) to a quenching start temperature T2 of (Ms - 50°C) or higher but lower than (Ms + 50°C) at an average cooling rate CR2 of 5°C / s or higher but 30°C / s or lower; and cooling from the quenching start temperature T2 to a quenching start temperature T2 of (Ms - 50°C) or higher but lower than (Ms + 50°C) at an average cooling rate CR2 of 5°C / s or higher but 30°C / s or lower. The method includes an annealing step in which the steel sheet is cooled by water quenching from 0°C to 80°C at an average cooling rate CR3 of 300°C / s or more, and heated under the conditions of a tempering temperature T3 of 100°C to 400°C and a holding time t3 at the tempering temperature T3 of 10 seconds to 10,000 seconds, and during the water quenching cooling in the annealing step, the steel sheet is pressed from the front and back sides by two rolls placed on either side of the steel sheet, and the pressing is performed under the conditions of a distance between the two rolls in the steel sheet conveying direction of 20 mm to 250 mm and a pressing force of 196 N or more.

[0106] 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.

[0107] 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.

[0108] 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.

[0109] 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.

[0110] The cold-rolled sheet obtained as described above is subjected to annealing under the following annealing conditions.

[0111] 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.

[0112] 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.

[0113] Average cooling rate CR1 from 700 to 600 ° C: 5 ° C / s or more If the average cooling rate CR1 from 700 to 600 ° 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 600 ° C is set to 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 (600 ° 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.

[0114] Average cooling rate CR2 from (Ms + 100°C) to the quenching start temperature T2: 5°C / s or more and 30°C / s or less. This is one of the important constituent elements of the present invention. The average cooling rate CR2 from (Ms + 100°C) to the quenching start temperature T2 affects the total area fraction of ferrite and bainitic ferrite and the average occupancy rate of the packet with the largest occupancy rate within the prior austenite grains at the center of the steel plate thickness. If the average cooling rate CR2 from (Ms + 100°C) to the quenching start temperature T2 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 excellent stretch flangeability. On the other hand, if the average cooling rate CR2 from (Ms + 100°C) to the quenching start temperature T2 exceeds 30°C / s, the average occupancy rate of packets with the largest occupancy rate within the prior austenite grains at the center of the steel plate thickness exceeds 70%, resulting in a decrease in the fatigue resistance of the sheared edge and delayed fracture resistance in a corrosive environment. Therefore, the average cooling rate CR2 from (Ms + 100°C) to the quenching start temperature T2 is set to 5°C / s or more and 30°C / s or less. The average cooling rate CR2 is preferably 10°C / s or more. The average cooling rate CR2 is preferably 20°C / s or less. Here, the average cooling rate CR2 is calculated by (cooling start temperature (Ms + 100°C) - cooling stop temperature (quenching start temperature T2)) / cooling time (s) from the cooling start temperature (Ms + 100°C) to the cooling stop temperature (quenching start temperature T2). Specific examples of cooling at the average cooling rate CR2 include mist cooling and gas cooling.

[0115] Rapid cooling start temperature T2: (Ms - 50°C) or more but less than (Ms + 50°C) This is one of the important constituent features of the present invention. By setting the rapid cooling start temperature T2 to (Ms - 50°C) or more but less than (Ms + 50°C), it is possible to obtain a structure in which the average occupancy rate of packets having the largest occupancy rate within prior austenite grains at the center of the thickness of the steel plate is 70% or less and the volume fraction of retained austenite is less than 3%. When the rapid cooling start temperature T2 is less than (Ms - 50°C), the volume fraction of retained austenite becomes 3% or more, making it difficult to achieve a YR of 75% or more. On the other hand, when the rapid cooling start temperature T2 is (Ms + 50°C) or more, the average occupancy rate of packets having the largest occupancy rate within prior austenite grains at the center of the thickness of the steel plate exceeds 70%, resulting in reduced fatigue resistance at the sheared edge and delayed fracture resistance in a corrosive environment. Therefore, the quenching start temperature T2 is set to be equal to or higher than (Ms - 50°C) and lower than (Ms + 50°C). 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 + 40°C). Ms is the martensitic transformation start temperature (°C), and the martensitic transformation start temperature Ms (°C) is defined by the following formula (3): Ms (°C) = 519 - 474 x [%C] - 30.4 x [%Mn] - 12.1 x [%Cr] - 7.5 x [%Mo] - 17.7 x [%Ni] (3) 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 of these elements is contained.

[0116] Average cooling rate CR3 from quenching start temperature T2 to 80°C: 300°C / s or more If the average cooling rate CR3 from quenching start temperature T2 to 80°C is less than 300°C / s during water quenching in the annealing process, 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 CR3 from quenching start temperature T2 to 80°C is set to 300°C / s or more. The average cooling rate CR3 is preferably 800°C / s or more. There is no particular upper limit to the average cooling rate CR3, but it is preferable that the average cooling rate CR3 be 3000°C / s or less. Here, the average cooling rate CR3 is calculated by (cooling start temperature (quenching start temperature T2) - cooling stop temperature (80°C) / cooling time (s) from the cooling start temperature (quenching start temperature T2) to the cooling stop temperature (80°C).

[0117] Tempering temperature T3: 100°C or higher and 400°C or lower 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 and a holding time of 10 seconds or longer. In the tempering step during the annealing process, 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, which deteriorates the fatigue resistance of the sheared edge surface. On the other hand, if the tempering temperature T3 exceeds 400°C, the martensite is excessively tempered, making it difficult to achieve a TS of 1320 MPa or higher. Therefore, the tempering temperature T3 is set to 100°C or higher and 400°C or lower. The tempering temperature T3 is preferably set to 150°C or higher. The tempering temperature T3 is preferably set to 350°C or lower.

[0118] 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 heat treatment at a tempering temperature of 100°C or more and a holding time of 10 seconds or more. In the tempering process during the annealing process, if the holding time t3 at the tempering temperature T3 is less than 10 seconds, the martensite is not sufficiently tempered, resulting in a structure mainly composed of as-quenched martensite, which deteriorates the fatigue resistance properties of the sheared end surface. On the other hand, if the holding time t3 exceeds 10,000 seconds, the martensite tempering proceeds excessively, making it difficult to achieve a TS of 1320 MPa or more. Therefore, the holding time t3 at the tempering temperature T3 is set to 10 seconds or more and 10,000 seconds or less. The holding time t3 is preferably set to 50 seconds or more. The holding time t3 is preferably set to 5,000 seconds or less.

[0119] 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.

[0120] 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.

[0121] When steel sheets (high-strength steel sheets) are traded, they are usually cooled to room temperature before being traded.

[0122] 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 9. If pressure is not applied during the water-quenching process, P(S) becomes excessively large relative to P(C), making it difficult to achieve {P(S) - P(C)} ≦ 20%, and the fatigue resistance of the sheared edge and the delayed fracture resistance in a corrosive environment are reduced. After extensive research, the inventors discovered that applying pressure during the water-quenching process affects the difference between P(S) and P(C). 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 the nucleation of martensitic transformation near the surface of the steel sheet relative to the center, contributing to a reduction in {P(S) - P(C)}. Therefore, {P(S) - P(C)} ≦ 20% is achieved, improving the fatigue resistance of the sheared edge and the delayed fracture resistance in a corrosive environment. To achieve this effect, the roll-to-roll distance (see Figure 9) must be at least 20 mm. 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.

[0123] 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.

[0124]

[0033] 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 can be obtained which has a TS of 1320 MPa or more, a YR of 75% or more, and excellent stretch flangeability, fatigue resistance at sheared edges, and delayed fracture resistance in a corrosive environment. The obtained steel sheet can be suitably used, for example, as a material for automobile parts.

[0125] 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 400°C or less.

[0126] [Member and Manufacturing Method of Member] Next, the member of the present invention and its manufacturing method will be described.

[0127] 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.

[0128] 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, fatigue resistance at the sheared edge, and delayed fracture resistance in a corrosive environment. Therefore, a member obtained using the steel sheet of the present invention also has a tensile strength of 1320 MPa or more, a YR of 75% or more, and is excellent in stretch flangeability, fatigue resistance at the sheared edge, and delayed fracture resistance in a corrosive environment. The 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.

[0129] 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.

[0130] 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) as a final product. Some of the steel sheets (those with the type column in Table 2 as GI, GA, and EG) were also 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.

[0131]

[0132]

[0133]

[0134]

[0135] The steel sheets (high-strength cold-rolled steel sheets) obtained as described above were used as test steels, and the tensile properties, stretch flangeability, fatigue resistance of the sheared end surface and fatigue resistance of the sheared end surface were evaluated according to the following test methods.

[0136] (Structural Observation) According to the method described above, the amount of tempered martensite (area fraction), the amount of retained austenite (volume fraction), and the total amount of ferrite and bainitic ferrite (area fraction) were determined.

[0137] (P(S): average value of the occupancy rate of packets having the maximum occupancy rate within prior austenite grains at a depth of 100 μm from the surface of the steel plate, P(C): average value of the occupancy rate of packets having the maximum occupancy rate within prior austenite grains at the center position of the thickness of the steel plate) According to the method described above, the average values ​​of the occupancy rate of packets having the maximum occupancy rate within prior austenite grains at a depth of 100 μm from the surface of the steel plate and at the center position of the thickness of the steel plate were determined.

[0138] (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 (4): YR = 100 × YS / TS (4)

[0139] (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.

[0140] (Fatigue resistance of sheared end surface) The fatigue resistance of the sheared end surface was evaluated by a complete reverse bending test in accordance with JIS Z 2275 (1978). The test shape was a punched hole in the center of the fatigue test piece described in JIS Z 2275 (1978) with a punch diameter of 10 mm and a clearance of 12%. The obtained test piece was subjected to a complete reverse bending fatigue test (stress ratio R = -1) with a constant stress amplitude, and the number of repetitions was 1 x 10. ７ The upper limit of fatigue strength after 100 cycles was measured as the fatigue limit of the sheared end face, and the fatigue strength ratio of the sheared end face was calculated using the following formula (5): Fatigue strength ratio of sheared end face = Fatigue limit of sheared end face / TS (5) The fatigue resistance of the sheared end face was evaluated according to the following criteria: ⊚ (pass, particularly excellent): The fatigue strength ratio of the sheared end face was 0.30 or more ◯ (pass, excellent): The fatigue strength ratio of the sheared end face was 0.25 or more and less than 0.30 × (fail): The fatigue strength ratio of the sheared end face was less than 0.25

[0141] (Delayed fracture resistance in a corrosive environment) The obtained steel sheets were sheared into a size of 16 mm x 75 mm with the longitudinal direction perpendicular to the rolling direction to prepare test specimens. The clearance during shearing was 15% in all cases. Next, a four-point bending test was performed in accordance with ASTM (G39-99), and stresses equivalent to YS and TS were applied to the bending vertices of the test specimens. Next, a delayed fracture test was performed using the corrosive environment cycle described below.

[0142] (Corrosion Test Cycle) The above-mentioned test pieces were subjected to a corrosion test cycle (corrosion test) consisting of the following chloride adhesion step (A) and corrosion step (B), and a water washing step (C). In the chloride adhesion step (A), the amount of chloride adhesion (in terms of solid content) was 10,000 mg / m ２Droplets of the chloride-containing aqueous solution were adhered so that the measured surface of the test piece was measured. The distance between the spray nozzle and the test piece's evaluation surface (X in Figure 2) was 30 cm. The spray pressure of the spray nozzle was 0.2 MPa. The saltwater spraying time was 5 seconds. The amount of chloride adhesion was calculated by calculating the difference in mass of the test piece before and after saltwater spraying, dividing this by the area of ​​the test piece's evaluation surface to calculate the amount of aqueous solution adhesion, and then calculating it from the chloride concentration of the aqueous solution. In addition, the following conditions were used. Chloride-containing aqueous solution: 15 mass % NaCl aqueous solution sprayed by a spray method. Distance between the shielding material and the test surface (Y in Figure 5): 5 cm. Temperature: 22°C, relative humidity: 50%. Distribution of droplets on the test surface. Average contact area of ​​droplets: 1.3 mm ２ , Standard deviation of droplet contact area: 1.3 mm ２ , Total contact area ratio of droplets: 71%. Note that the chloride deposition step (A) from the second time onwards was carried out under the same conditions as the first chloride deposition step (A).

[0143] The corrosion step (B) is a process in which the drying step (b1) → transition step (b3) → wetting step (b2) → transition step (b4) are performed in this order, with one cycle consisting of the four steps described above. In this example, the temperature fluctuation range in the corrosion step (B) was set to within 30±5°C. Drying step (b1): A process of drying the metal material by holding it in an atmosphere of 40% relative humidity Hb1 for 2.0 hours. Wetting step (b2): A process of wetting the metal material by holding it in an atmosphere of 90% relative humidity Hb2 for 2.0 hours. Transition step (b3): ​​A process of transitioning from the atmosphere of relative humidity Hb1 to the atmosphere of relative humidity Hb2 at a relative humidity change rate of 25% / h. Transition step (b4): A process of transitioning from the atmosphere of relative humidity Hb2 to the atmosphere of relative humidity Hb1 at a relative humidity change rate of 25% / h.

[0144] The water-washing step (C) is a step of rinsing the evaluation surface of the metal material with water. The water-washing method in the water-washing step (C) is not particularly limited, but the evaluation surface of the metal material was rinsed with water by spraying water onto the evaluation surface from a spray nozzle.

[0145] In this example, the corrosion test cycle was such that the chloride adhesion step (A) was followed by the corrosion step (B), and then the water-rinsing step (C) was performed before the chloride adhesion step (A) was performed again. That is, in this example, the delayed fracture properties were evaluated using the following corrosion test cycle: a cycle in which the chloride adhesion step (A) → corrosion step (B) → water-rinsing step (C) were repeatedly performed. In this example, the period for evaluating delayed fracture resistance using the corrosive environment cycle was 63 days.

[0146] After the test, each test piece was visually inspected for the presence or absence of cracks. Then, the delayed fracture resistance in a corrosive environment was evaluated according to the following criteria. In the present invention, a sample loaded with a stress equivalent to YS and showing no cracks was judged to have excellent delayed fracture resistance in a corrosive environment. ⊚ (Pass, particularly excellent): No cracks in the sample loaded with stress equivalent to YS and TS. ◯ (Pass, excellent): No cracks in the sample loaded with stress equivalent to YS. × (Fail): Cracks in both the sample loaded with stress equivalent to YS and TS.

[0147] The examples of the present invention shown in Table 3 (Table 3-1, Table 3-2) had a tensile strength TS of 1,320 MPa or more, a yield ratio YR of 75% or more, and were excellent in stretch flangeability, fatigue resistance at the sheared end face, and delayed fracture resistance in a corrosive environment, whereas the comparative examples were inferior in at least one of these properties.

[0148] Furthermore, the members 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 are excellent in stretch flangeability, fatigue resistance of the sheared edge surface, and delayed fracture resistance in a corrosive environment. Therefore, it was found that, like the steel plates of the present invention, the members obtained by forming and joining the steel plates of the present invention have a tensile strength of 1320 MPa or more, a yield ratio YR of 75% or more, and are excellent in stretch flangeability, fatigue resistance of the sheared edge surface, and delayed fracture resistance in a corrosive environment.

[0149]

[0150]

Claims

1. In mass%, C: 0.030% or more and 0.500% 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%; A steel sheet having a structure that satisfies the following formulas (1) and (2): P(C)≦70%...(1) {P(S)-P(C)}≦20%...(2) During the ceremony, P(S): Depth from the steel sheet surface: Average value of the occupancy rate of packets having the maximum occupancy rate in prior austenite grains at a position of 100 μm, P(C): The average value of the occupation ratio of the packet having the maximum occupation ratio in the prior austenite grain at the center position of the thickness of the steel plate.

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 of 700 to 600 ° C. CR1: 5 ° C. / s or more; Cooling from (Ms + 100 ° C.) to a quenching start temperature T2 that is (Ms - 50 ° C.) or higher but lower than (Ms + 50 ° C.) at an average cooling rate CR2 of 5 ° C. / s or higher and 30 ° C. / s or lower; and Cooling is performed by water quenching at an average cooling rate CR3 of 300 ° C. / s or more from the quenching start temperature T2 to 80 ° C. Tempering temperature T3: 100°C or more and 400°C or less, 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. 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.