Steel plates, components, and methods for manufacturing them.

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

[Technical Field]

[0001] This invention relates to steel sheets, components, and methods for manufacturing them, which are excellent in tensile strength, yield ratio, elongation flangeability, fatigue resistance of the shear end face, and delayed fracture resistance in corrosive environments. The steel sheets of this invention can be suitably used as structural components for automobile parts and the like. [Background technology]

[0002] With the aim of simultaneously reducing CO2 emissions through vehicle weight reduction and improving collision resistance through vehicle body weight reduction, the strengthening of thin steel sheets for automobiles is progressing, and new legal regulations are being introduced one after another. As a result, in order to increase the strength of the vehicle body, the application of high-strength steel sheets with a tensile strength of 1320 MPa or higher (TS) is increasing in major structural components that make up automobiles.

[0003] High-strength steel sheets used in automobiles are required to have excellent yield ratio, excellent tensile flange properties, excellent fatigue resistance at the shear end face, and excellent delayed fracture resistance in corrosive environments. For example, in structural components such as floor cross members of automobiles, it is preferable to use steel sheets with excellent tensile flange properties from the viewpoint of formability. Furthermore, from the viewpoint of component performance, excellent yield ratio, excellent fatigue resistance at the shear end face, and excellent delayed fracture resistance in corrosive environments are required.

[0004] Patent Document 1 discloses a high-strength steel sheet with a yield ratio of 1180 MPa or higher, excellent flatness in the width direction, and resistance to work-induced embrittlement, and a method for manufacturing the same. However, the technology described in Patent Document 1 does not consider high-strength steel sheets with excellent tensile flange properties, fatigue resistance of the shear end face, and delayed fracture resistance in corrosive environments.

[0005] Patent Document 2 discloses a high-strength steel sheet with a yield ratio and elongation flangibility of 1180 MPa or more and a method for producing the same. However, the technology described in Patent Document 2 does not consider a high-strength steel sheet having excellent fatigue resistance characteristics at the sheared end face and excellent delayed fracture resistance characteristics in a corrosive environment.

[0006] Patent Document 3 discloses a high-strength steel sheet with a surface property, steel sheet shape, and fatigue strength of 980 MPa or more and a method for producing the same. However, the technology described in Patent Document 3 does not consider a high-strength steel sheet having excellent elongation flangibility, excellent fatigue resistance characteristics at the sheared end face, and excellent delayed fracture resistance characteristics in a corrosive environment.

[0007] Patent Document 4 discloses a high-strength steel sheet with excellent delayed fracture resistance characteristics, corrosion resistance, and weldability in a corrosive environment and a method for producing the same. However, the technology described in Patent Document 4 does not consider a high-strength steel sheet having an excellent yield ratio, elongation flangibility, and fatigue resistance characteristics at the sheared end face.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

[0010] The present invention has been developed in view of such circumstances, and aims to provide a steel plate, a member, and a method for manufacturing them, which have a tensile strength TS of 1320 MPa or more, a yield ratio YR of 75% or more, and excellent elongation flanging property, fatigue resistance characteristics of the sheared end face portion, and stress corrosion cracking resistance characteristics in a corrosive environment.

[0011] Here, the tensile strength TS (hereinafter, also simply referred to as TS) and the yield ratio YR (hereinafter, also simply referred to as YR) can be measured in accordance with JIS Z 2241 (2022). Excellent elongation flanging property means that the limiting hole expansion ratio: λ (%) obtained by the hole expansion test based on JIS Z 2256 (2020) is 30% or more. Excellent fatigue resistance characteristics of the sheared end face portion means that the fatigue strength ratio of the sheared end face portion (= fatigue limit of the sheared end face portion / TS) obtained by the complete double bending test according to JIS Z 2275 (1978) is 0.25 or more. Excellent stress corrosion cracking resistance characteristics in a corrosive environment means excellent stress corrosion cracking resistance characteristics in the corrosive environment specified in the present invention, that is, in the evaluation of metal materials, the number of days to cracking is 63 days or more.

Means for Solving the Problems

[0012] As a result of intensive studies to achieve the above problems, the present inventors have found the following. (1) By setting the amount of tempered martensite to 95% or more, TS of 1320 MPa or more can be realized. (2) By setting the total amount of ferrite and bainitic ferrite to less than 5%, excellent elongation flanging property can be realized. (3) By setting the amount of retained austenite to less than 3%, YR of 75 % or more can be realized. (4) By setting P(C) to 70% or less and {P(S) - P(C)} to 20% or less, excellent fatigue resistance characteristics of the sheared end face portion and stress corrosion cracking resistance characteristics in a corrosive environment can be realized.

[0013] This invention is based on the above findings. Specifically, the gist of this invention is as follows: [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 It contains, with the remainder being Fe and unavoidable impurities, and has a component composition. At the point where the plate thickness is 1 / 4, 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 plate having a structure that satisfies the following equations (1) and (2). P(C) ≤ 70% ···(1) {P(S)-P(C)}≦20% ···(2) During the ceremony, P(S): The average occupancy rate of packets with the maximum occupancy rate within prior austenite grains at a depth of 100 μm from the surface of the steel plate. P(C): This is the average occupancy rate of the packet with the maximum occupancy rate within the prior austenite grains at the center of the steel plate thickness. [2] The above component composition is further, 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 [1], which contains at least one element selected from among the following. [3] The steel sheet according to [1] or [2], having a plating layer on the surface of the steel sheet. [4] In the evaluation of metallic materials, a steel plate according to any of [1] to [3] above, wherein the number of days to crack is 63 days or more. [5] A member made of a steel plate as described in any of [1] to [4] above. [6] A cold-rolled sheet made by hot-rolling, pickling and cold-rolling a steel having the component composition described in [1] or [2] above, Annealing temperature T1: 800℃ or higher, Heat under conditions of holding time t1: 10 seconds or more at annealing temperature T1. Cooling with an average cooling rate of CR1: 5°C / s or higher at 700-600°C. Cooling from (Ms+100℃) to a rapid cooling start temperature T2 which is between (Ms-50℃) and (Ms+50℃), with an average cooling rate CR2 of 5℃ / s or more and 30℃ or less, and Cooling is performed by water quenching, with an average cooling rate CR3 of 300°C / s or more from the rapid cooling start temperature T2 to 80°C. Tempering temperature T3: 100℃ or more and 400℃ or less, The process includes an annealing step in which heating is performed under the condition that the holding time t3 at tempering temperature T3 is between 10 seconds and 10,000 seconds. A method for manufacturing a steel sheet, wherein, during the cooling of the water quenching in the annealing process, pressure is applied to the front and back surfaces of the steel sheet using two rolls placed on either side of the steel sheet, and this pressure is applied under the conditions that the distance between the two rolls in the steel sheet conveying direction is 20 mm or more and 250 mm or less, and the applied pressure is 196 N or more. [7] A method for manufacturing a steel sheet according to [6], wherein a plating treatment is applied. [8] A method for manufacturing a component, comprising the step of forming and joining a steel plate according to any one of [1] to [4] above to obtain a component. [Effects of the Invention]

[0014] According to the present invention, a steel sheet can be obtained that has a TS of 1320 MPa or higher, a YR of 75% or higher, and excellent tensile flange properties, fatigue resistance of the shear end face, and delayed fracture resistance in corrosive environments. Furthermore, by applying the steel sheet of the present invention to, for example, automotive structural members, it is possible to improve fuel efficiency by reducing the weight of the vehicle body. Therefore, its industrial utility is extremely large. [Brief explanation of the drawing]

[0015] [Figure 1] Figure 1 is a schematic diagram illustrating the packet with the maximum occupancy rate within the prior austenite grain. [Figure 2] Figure 2 is a schematic diagram illustrating the evaluation method (HeTsAce) for the delayed fracture characteristics of metallic materials. [Figure 3] Figure 3 is a schematic diagram showing an example of an image capturing the distribution of liquid droplets on an evaluation surface of a metallic material. [Figure 4] Figure 4 is a schematic diagram illustrating the method for evaluating the delayed fracture characteristics of a metallic material, specifically when a shielding material is placed between the spray nozzle and the metallic material. [Figure 5] Figure 5 schematically shows how the spray liquid in the atmosphere re-adheses to the evaluation surface when the distance between the shielding material and the evaluation surface of the metal material is changed. [Figure 6]Figure 6 illustrates one embodiment of a corrosion test cycle related to a method for evaluating the delayed fracture characteristics of metallic materials. [Figure 7] Figure 7 illustrates another embodiment of a corrosion test cycle related to a method for evaluating the delayed fracture characteristics of metallic materials. [Figure 8] Figure 8 schematically shows the test specimen used to evaluate the delayed fracture characteristics in the example. [Figure 9] Figure 9 is a schematic diagram illustrating the pressurization method during water cooling in the steel plate manufacturing method of the present invention. [Modes for carrying out the invention]

[0016] Embodiments of the present invention will be described below.

[0017] [Steel plate] The steel sheet of the present invention has a composition in mass% of 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 remainder being Fe and unavoidable impurities. At the 1 / 4 thickness position, the area fraction of tempered martensite is 95% or more, the volume fraction of retained austenite is less than 3%, and the total area fraction of ferrite and bainitic ferrite is less than 5%. The steel plate has a structure that satisfies both equation (1) and equation (2) below. P(C) ≤ 70% ···(1) {P(S)-P(C)}≦20% ···(2) During the ceremony, P(S): The average occupancy rate of packets with the maximum occupancy rate within prior austenite grains at a depth of 100 μm from the surface of the steel plate. P(C): This is the average occupancy rate of the packet with the maximum occupancy rate within the prior austenite grains at the center of the steel plate thickness.

[0018] First, we will explain the appropriate range of the steel plate's composition and the reasons for its limitations. In the following explanation, the "%" used to represent the content of the steel's constituent elements means "mass percent" unless otherwise specified.

[0019] C: 0.030% or more and 0.500% or less Carbon (C) is one of the important basic components of steel, and in this invention in particular, tempered martensite is an important element that affects the area fraction (hereinafter also referred to as fraction) and the fatigue resistance of the shear end face. If the carbon content is less than 0.030%, the fraction of tempered martensite decreases, making it difficult to achieve a total stress of 1320 MPa or higher. On the other hand, if the carbon content exceeds 0.500%, the tempered martensite becomes brittle, making it difficult to obtain excellent fatigue resistance properties at the shear end face. Therefore, the C content should be between 0.030% and 0.500%. Preferably, the C content should be 0.050% or more. More preferably, the C content should be 0.100% or more. Also, preferably, the C content should be 0.400% or less. More preferably, the C content should be 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, and in this invention in particular, it is an important element that affects the amount of retained austenite because it suppresses carbide formation during continuous annealing and promotes the formation of retained austenite. If the Si content is less than 0.010%, it becomes difficult to achieve a total annealing pressure (TS) of 1320 MPa or higher. On the other hand, if the Si content exceeds 2,500%, the amount of retained austenite increases excessively, making it difficult to achieve a YR ≥ 75%. Therefore, the Si content should be between 0.010% and 2.500%. Preferably, the Si content should be 0.050% or more. More preferably, the Si content should be 0.100% or more. Also, preferably, the Si content should be 2.000% or less. More preferably, the Si content should be 1.200% or less. More preferably, the Si content should be 0.500% or less, and even more preferably 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 this invention in particular, it is an important element that affects the fraction of tempered martensite and the delayed fracture resistance in corrosive environments. If the Mn content is less than 0.10%, the fraction of tempered martensite decreases, making it difficult to achieve a total stress of 1320 MPa or higher. On the other hand, if the Mn content exceeds 5.00%, corrosion of the steel plate is accelerated, and hydrogen generation associated with corrosion is also promoted, making it difficult to achieve excellent delayed fracture resistance in corrosive environments. Therefore, the Mn content should be between 0.10% and 5.00%. Preferably, the Mn content should be 0.50% or more. More preferably, the Mn content should be 0.80% or more. Also, preferably, the Mn content should be 4.50% or less. More preferably, the Mn content should be 4.00% or less.

[0022] P:0.100% or less Since phosphorus (P) segregates at prior austenite grain boundaries and embrittles them, if its content exceeds 0.100%, it reduces the ultimate deformability of the steel sheet, making it difficult to achieve excellent fatigue resistance at the shear end face. Therefore, the P content must be 0.100% or less. Preferably, the P content is 0.070% or less. More preferably, the P content is 0.050% or less, and even more preferably, 0.020% or less. Although there is no specific 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 preferable that the P content be 0.001% or more. More preferably, the P content should be 0.002% or more.

[0023] S: 0.0200% or less S exists as a sulfide, and if its content exceeds 0.0200%, it reduces the ultimate deformability of the steel plate, making it difficult to achieve excellent fatigue resistance of the shear end face. Therefore, the S content must be 0.0200% or less. Preferably, the S content is 0.0050% or less. More preferably, the S content is 0.0030% or less, and even more preferably, 0.0020% or less. Although there is no specific lower limit for the sulfur content, due to production technology constraints, it is preferable that the sulfur content be 0.0001% or more. More preferably, the sulfur content is 0.0002% or more.

[0024] Al: 1.000% or less Al exists as an oxide, and when present in a concentration exceeding 1,000%, it impairs the ultimate deformability of the steel sheet. Reducing the Al content makes it difficult to achieve excellent fatigue resistance of the shear end face. Therefore, the Al content needs to be 1.000% or less. Accordingly, the Al content is 1.000% or less. Preferably, the Al content is 0.500% or less. More preferably, the Al content is 0.200% or less, and even more preferably, 0.100% or less. Although there is no specific lower limit for the Al content, due to production technology constraints, it is preferable that the Al content be 0.001% or more. More preferably, the Al content is 0.002% or more. More preferably, the Al content is 0.005% or more, and even more preferably, 0.010% or more.

[0025] N: 0.0100% or less N exists as a nitride, and if its content exceeds 0.0100%, it reduces the ultimate deformability of the steel sheet, making it difficult to achieve excellent fatigue resistance of the shear end face. Therefore, the N content must be 0.0100% or less. Preferably, the N content is 0.0050% or less. Although there is no specific lower limit for the N content, due to production technology constraints, it is preferable that the N content be 0.0001% or more. More preferably, the N content is 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 its content exceeds 0.0100%, it reduces the ultimate deformability of the steel sheet, making it difficult to achieve excellent fatigue resistance of the shear end face. Therefore, the O content must be 0.0100% or less. Accordingly, the O content should be 0.0100% or less. Preferably, the O content should be 0.0050% or less. Although there is no specific lower limit for the O content, due to production technology constraints, it is preferable that the O content be 0.0001% or more. More preferably, the O content is 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 component composition containing the above-mentioned components, with the remainder being Fe and unavoidable impurities. Examples of unavoidable impurities include Zn, Pb, As, Ge, Sr, and Cs. The total amount of these impurities is acceptable to be 0.100% or less. Preferably, a steel sheet according to one embodiment of the present invention has a component composition containing the above-mentioned components, with the remainder being Fe and unavoidable impurities.

[0028] In addition to the above component composition, the steel sheet of the present invention may further contain, by mass%, at least one element selected from 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, and Bi: 0.200% or less, either alone or in combination.

[0029] Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less If Ti, Nb, and V are present in amounts of 0.200% or less, large amounts of coarse precipitates and inclusions will not be generated, and the ultimate deformability of the steel sheet will not be reduced, thus preserving the excellent fatigue resistance of the shear end face. Therefore, when at least one of Ti, Nb, and V is included, the content of Ti, Nb, and V should be 0.200% or less for each. Preferably, the content of each should be 0.100% or less. While there is no specific lower limit for the Ti, Nb, and V content, it is preferable that the Ti, Nb, and V content be 0.001% or more, since the formation of fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing increases the strength of the steel sheet. More preferably, the content of each is 0.002% or more, and even more preferably 0.003% or more.

[0030] Ta: 0.10% or less, W: 0.10% or less If the content of Ta and W is 0.10% or less, large amounts of coarse precipitates and inclusions will not be generated, and the ultimate deformability of the steel sheet will not decrease, thus preventing a decrease in the fatigue resistance of the shear end face. For this reason, when at least one of Ta and W is included, the content of Ta and W should be 0.10% or less each. Preferably, the content of each should be 0.08% or less. Although there is no specific lower limit for the content of Ta and W, it is preferable that the content of Ta and W be 0.01% or more, since the strength of the steel sheet is increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. More preferably, the content of each is 0.02% or more, and even more preferably 0.03% or more.

[0031] B: 0.0100% or less If B is present in a quantity of 0.0100% or less, it does not cause cracks to form inside the steel sheet during casting or hot rolling, and does not reduce the ultimate deformability of the steel sheet, thus preventing a decrease in the fatigue resistance of the shear end face. Therefore, when B is included, its content should be 0.0100% or less. Preferably, the B content is 0.0080% or less. Although there is no specific lower limit for the B content, it is preferable that the B content be 0.0003% or higher, as it is an element that segregates at austenite grain boundaries during annealing and improves hardenability.

[0032] Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less If the content of Cr, Mo, and Ni is 1.00% or less, the amount of coarse precipitates and inclusions does not increase, and the ultimate deformability of the steel sheet does not decrease, thus the fatigue resistance of the shear end face does not decrease. For this reason, when at least one of Cr, Mo, and Ni is included, the content of each of Cr, Mo, and Ni should be 1.00% or less. Preferably, the content of each is 0.80% or less. More preferably, the content of each is 0.75% or less, and even more preferably 0.70% or less. While there are no specific lower limits for the content of Cr, Mo, and Ni, it is preferable that the content of each of these elements be 0.01% or more, as they are elements that improve hardenability. More preferably, the content of each of these elements is 0.02% or more.

[0033] Co:0.010% or less If the Co content is 0.010% or less, the amount of coarse precipitates and inclusions does not increase, and the ultimate deformability of the steel plate does not decrease, so the fatigue resistance of the shear end face does not decrease. For this reason, when Co is included, the Co content should be 0.010% or less. Preferably, the Co content should be 0.008% or less. More preferably, the Co content should be 0.007% or less. Although there is no specific lower limit for the Co content, it is preferable that the Co content be 0.001% or higher, as it is an element that improves hardenability.

[0034] Cu: 1.00% or less If the Cu content is 1.00% or less, it does not increase coarse precipitates or inclusions and does not reduce the ultimate deformability of the steel plate, thus not reducing the fatigue resistance of the shear end face. For this reason, when Cu is included, the Cu content should be 1.00% or less. Preferably, the Cu content should be 0.80% or less. Although there is no specific lower limit for the Cu content, it is preferable that the Cu content be 0.01% or more, as it is an element that improves hardenability.

[0035] Sn: 0.200% or less If the Sn content is 0.200% or less, it does not cause cracks to form inside the steel sheet during casting or hot rolling, and does not reduce the ultimate deformability of the steel sheet, thus preventing a decrease in the fatigue resistance of the shear end face. For this reason, when Sn is included, the Sn content should be 0.200% or less. Preferably, the Sn content should be 0.150% or less. Although there is no specific lower limit for the Sn content, it is preferable that the Sn content be 0.001% or more, since Sn is an element that improves hardenability (generally an element that improves corrosion resistance). More preferably, the Sn content should be 0.002% or more.

[0036] Sb: 0.200% or less If the Sb content is 0.200% or less, the amount of coarse precipitates and inclusions does not increase, and the ultimate deformability of the steel plate does not decrease, so the fatigue resistance of the shear end face does not decrease. For this reason, when Sb is included, the Sb content should be 0.200% or less. Preferably, the Sb content should be 0.150% or less. More preferably, the Sb content should be 0.100% or less. Although there is no specific lower limit for the Sb content, it is preferable that the Sb content be 0.001% or more, as it is an element that allows control of the surface softening thickness and strength adjustment. More preferably, the Sb content is 0.002% or more.

[0037] Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less If Ca, Mg, and REM are present in amounts of 0.0100% or less, the amount of coarse precipitates and inclusions will not increase, and the ultimate deformability of the steel sheet will not decrease, thus preventing a reduction in the fatigue resistance of the shear end face. Therefore, when at least one of Ca, Mg, and REM is present, it is preferable that the content of Ca, Mg, and REM be 0.0100% or less, respectively. Preferably, the content of each is 0.0050% or less. Although there is no specific lower limit for the content of Ca, Mg, and REM, it is preferable that the content of Ca, Mg, and REM be 0.0001% or more, as these elements spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of steel sheets. The content of each of these is more preferably 0.0002% or more, and even more preferably 0.0005% or more.

[0038] Zr: 0.100% or less, Te: 0.100% or less If the content of Zr and Te is 0.100% or less, the amount of coarse precipitates and inclusions will not increase, and the ultimate deformability of the steel sheet will not decrease, thus preventing a decrease in the fatigue resistance of the shear end face. Therefore, when at least one of Zr and Te is included, the content of Zr and Te should be 0.100% or less. Preferably, the content of each should be 0.080% or less. While there is no specific lower limit for the content of Zr and Te, it is preferable that the content of Zr and Te be 0.001% or more, since these elements spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of steel sheets. More preferably, the content of each is 0.002% or more, and even more preferably 0.003% or more.

[0039] Hf: 0.10% or less If Hf is present at 0.10% or less, the amount of coarse precipitates and inclusions does not increase, and the ultimate deformability of the steel plate does not decrease, thus preventing a decrease in the fatigue resistance of the shear end face. For this reason, when Hf is included, the Hf content should be 0.10% or less. Preferably, the Hf content should be 0.09% or less. More preferably, the Hf content should be 0.08% or less, and even more preferably, 0.07% or less. Although there is no specific lower limit for the Hf content, it is preferable that the Hf content be 0.001% or more, and more preferably 0.010% or more, since Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of steel sheets.

[0040] Bi:0.200% or less If the Bi content is 0.200% or less, the amount of coarse precipitates and inclusions does not increase, and the ultimate deformability of the steel plate does not decrease, thus the fatigue resistance of the shear end face does not deteriorate. For this reason, when Bi is included, the Bi content should be 0.200% or less. Preferably, the Bi content should be 0.190% or less. Although there is no specific lower limit for the Bi content, it is preferable that the Bi content be 0.001% or more, as it is an element that reduces segregation. More preferably, the Bi content is 0.002% or more, and even more preferably 0.003% or more.

[0041] Furthermore, regarding the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi, if their respective contents are below the preferred lower limit, they will not impair the effects of the present invention and will therefore be included as unavoidable impurities.

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

[0043] Tempered martensite: over 95% by area fraction 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 total stress point (TS) of 1320 MPa or higher. In order to obtain this effect, the area fraction of tempered martensite must be 95% or more. Therefore, the area fraction of tempered martensite is set to 95% or more. Preferably, the area fraction of tempered martensite is 96% or more. More preferably, the area fraction of tempered martensite is 97% or more. There is no particular upper limit to the area fraction of tempered martensite, but it may be 100%.

[0044] The method for measuring the area fraction of tempered martensite is as follows: After polishing the L-shaped cross section of the steel plate, it is etched with 1 vol.% nital, and 10 fields of view are observed using a SEM at a magnification of 2000x with a field of view of 30 μm × 30 μm at a position corresponding to 1 / 4 of the plate thickness (from the surface of the steel plate in the depth direction). In the above microstructure image, tempered martensite is a structure with fine irregularities within its interior and contains carbides. The area fraction of tempered martensite can be determined from the average of these values.

[0045] Retained austenite: less than 3% by volume fraction 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, causing a decrease in YS. Therefore, the volume fraction of retained austenite should be less than 3%. Preferably, the volume fraction of retained austenite should be 1% or less. The lower limit of the volume fraction of retained austenite is not particularly limited and may be 0%.

[0046] The method for measuring the volume fraction of retained austenite is as follows: After polishing the steel plate to a surface that is 1 / 4 of its thickness, the surface is further polished by 0.1 mm using chemical polishing. Using an X-ray diffractometer with CoKα rays, the integral 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 are measured, and the retained austenite is determined by averaging the nine obtained integral intensity ratios.

[0047] Total of ferrite and bainitic ferrite: less than 5% by area fraction In this invention, this is one of the important constituent elements of the invention. When the total area fraction of ferrite and bainitic ferrite is 5% or more, it becomes difficult to achieve excellent stretch flange properties. Therefore, the total area fraction of ferrite and bainitic ferrite should be less than 5%. Preferably, the total area fraction of these should be 3% or less. More preferably, the total area fraction of these should be 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 the steel plate, it is etched with 1 vol.% nital, and 1 / 4 of the plate thickness (a position corresponding to 1 / 4 of the plate thickness in the depth direction from the steel plate surface) is observed using a SEM at 2000x magnification with a field of view of 30 μm × 30 μm for 10 fields. In the above microstructure image, ferrite and bainitic ferrite are structures with a flat interior in the recesses and do not contain carbides. The total of ferrite and bainitic ferrite can be determined from the average value of these values.

[0049] Other tissues besides those listed above may include pearlite, fresh martensite, and acicular ferrite. These remaining tissues do not affect the properties as long as their area fraction is 5% or less, so their inclusion is acceptable.

[0050] P(C) ≤ 70% ···(1) P(C): The average occupancy rate of the packet with the maximum occupancy rate within the prior austenite grains at the center of the steel plate thickness. In this invention, this is one of the important constituent elements of the invention. The occupancy rate of the packet having the maximum occupancy rate within the prior austenite grain at the center of the thickness of the steel plate affects the fatigue resistance of the shear end face and the delayed fracture resistance in a corrosive environment. The packet having the maximum occupancy rate within the prior austenite grain refers to the packet with the largest occupancy rate among up to four regions with the same crystal habit plane during transformation, called packets, within the prior austenite grain, as shown in Figure 1. The occupancy rate of one packet within the prior austenite grain can be obtained by dividing the area of ​​the specified packet by the total area within the prior austenite grain. As a result of diligent research, the inventors have found that by reducing the occupancy rate of packets with the highest occupancy rate within the prior austenite grains at the center of the steel plate thickness, specifically by setting the average occupancy rate of packets with the highest occupancy rate within the prior austenite grains at the center of the steel plate thickness to 70% or less, the microstructure is refined and crack propagation is suppressed, thereby improving the fatigue resistance of the shear end face and the delayed fracture resistance in corrosive environments. Therefore, the average occupancy rate P(C) of the packet having the maximum occupancy rate within the prior austenite grains at the center of the steel plate thickness should be 70% or less. Preferably, this average occupancy rate P(C) should be 60% or less. Furthermore, there is no particular lower limit to the average value of the occupancy rate of the packet having the maximum occupancy rate within the prior austenite grains. There are a maximum of four types of packets, and when the four packets are evenly distributed, the occupancy rate of the packet having the maximum occupancy rate within the prior austenite grains is 25%. Therefore, the average value of the occupancy rate of the packet having the maximum occupancy rate within the prior austenite grains at the center of the thickness of the steel plate, P(C), is preferably 25% or more, but it is not necessary to limit it to this.

[0051] {P(S)-P(C)}≦20% ···(2) P(S): The average occupancy rate of packets with the maximum occupancy rate within prior austenite grains at a depth of 100 μm from the steel plate surface. P(C): The average occupancy rate of the packet with the maximum occupancy rate within the prior austenite grains at the center of the steel plate thickness. In this invention, the difference between P(S) and P(C) affects the fatigue resistance of the shear end face and the delayed fracture resistance in a corrosive environment. If {P(S)-P(C)} ≤ 20%, it means that the packet occupancy rate in the surface layer of the steel plate is excessively large compared to the packet occupancy rate in the center of the steel plate, which promotes the occurrence of cracks due to steel plate fatigue and cracks due to delayed fracture in the surface layer of the steel plate. Therefore, {P(S)-P(C)} should be 20% or less. Preferably, {P(S)-P(C)} should be 15% or less.

[0052] The method for measuring the average occupancy rate of packets with the highest occupancy rate within the prior austenite grains is as follows. First, a specimen for microstructure observation is taken from the steel plate. Next, the taken specimen is polished by colloidal silica vibration polishing so that the rolling direction cross section (L section) becomes the observation surface. The observation surface is made mirror-finish. Then, electron beam backscatter diffraction (EBSD) measurements are performed at a depth of 100 μm from the surface of the steel plate and at the center of the thickness of the steel plate to obtain local crystal orientation data. At this time, 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 OIMAnalysis7 (OIM), and a color-coded figure (CP map) is created for each Close-packed Plane group (CP group) using the method described in Non-Patent Literature 1 (Journal of the Japan Society for Smart Processes, 2013, Vol. 2, No. 3, pp. 110-118). In this 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 the occupancy rate of the packet with the maximum occupancy rate within the austenite grain is obtained by dividing this area by the total area within the austenite grain. This analysis is performed for 10 or more adjacent austenite grains, and the average value is taken as the average occupancy rate of the packet with the maximum occupancy rate within the austenite grain.

[0053] The steel sheet of the present invention may have a plating layer on its surface. Examples of plating layers include zinc plating layers such as hot-dip galvanized layers, alloyed hot-dip galvanized layers, and electroplated zinc plating layers. Examples of plating layers other than zinc plating layers include aluminum plating layers and alloy plating layers. Examples of alloy plating layers include hot-dip zinc-aluminum-magnesium alloy plating layers and Zn-Ni electroalloy plating layers.

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

[0055] In the evaluation method for the delayed fracture characteristics of metallic materials (HeTsAce), the number of days to crack is 63 days or longer. The steel sheet of the present invention can be determined to have excellent resistance to delayed fracture in a corrosive environment (HeTsAce) in a method for evaluating the delayed fracture characteristics of metal materials in a corrosive environment (HeTsAce). Specifically, the steel sheet of the present invention exhibits a cracking time of 63 days or more in the delayed fracture characteristics evaluation method (HeTsAce) for metallic materials shown below.

[0056] (HeTsAce) The amount of chloride adhering to the evaluation surface of the metal material being evaluated is 1,000 to 20,000 mg / m². 2 Chloride adhesion step (A) involves attaching droplets of a chloride-containing aqueous solution in such a manner, A corrosion process (B) is performed at least once, comprising a cycle consisting of the following: drying step (b1), wetting step (b2), transition step (b3), and transition step (b4), in an atmosphere at a temperature Tb1 of 60°C or lower and within a certain range; and repeating this cycle at least once. At least in the initial chloride deposition step (A), the distribution of droplets of the chloride-containing aqueous solution on the evaluation surface of the metal material is, Average contact area of ​​the droplet on the evaluation surface of the metal material: 0.1 mm 2 3.0mm or more 2 The area ratio of the total contact area of ​​the droplets to the area of ​​the evaluation surface of the metal material is 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 is 3.0 mm. 2 The following is a method for evaluating the delayed fracture characteristics of metallic materials. Drying process (b1): A process of drying metal materials by holding them in an atmosphere with a relative humidity of 45% or less (Hb1) for 1.0 hour to 5.0 hours. Wetting process (b2): A process of wetting a metal material by holding it in an atmosphere with a relative humidity of 80% or more (Hb2) for 1.0 hour to 5.0 hours. Transition process (b3): ​​A process in which the relative humidity changes from an atmosphere with relative humidity Hb1 to an atmosphere with relative humidity Hb2 at a rate of change of relative humidity of 30% / h or less. Transition process (b4): A process of transitioning from an atmosphere with relative humidity Hb2 to an atmosphere with relative humidity Hb1 at a rate of change of relative humidity of 30% / h or less.

[0057] In this invention, a metallic material sampled from a steel plate is used for evaluation. The method for sampling the metallic material from the steel plate is not particularly limited. For example, the metallic material can be sampled by shearing the steel plate to a predetermined size. In HeTsAce, it is preferable to apply stress to the metallic material to be evaluated. Methods for applying stress to the metallic material include processing the metallic material (processing method). Examples of such processing methods include bending, bulging, tensioning, and torsion. Other methods include fixing the material in a stressed shape using bolts, etc., and using residual stress remaining after processing. In the evaluation method of this invention, the metallic material to which stress has been applied as described above can be subjected to at least one (one or two or more) steps comprising a chloride deposition step (A) and a corrosion step (B). Furthermore, in the evaluation method of this invention, after performing the steps comprising the chloride deposition step (A) and the corrosion step (B) once or more, the state of the metallic material can be checked, and the delayed fracture characteristics of the metallic material can be evaluated based on the checked state of the metallic material. The aforementioned verification can be performed, for example, by visually observing the presence and extent of cracks in the metal material.

[0058] In this invention, in order to evaluate the delayed fracture characteristics of a metallic material, a chloride deposition process (A) and a corrosion process (B) are performed on the metallic material while it is under stress. After each of these processes is performed at least once, the presence and extent of cracks in the metallic material are checked to evaluate the delayed fracture characteristics.

[0059] (Chloride adhesion process (A)) Chloride deposition process (A) involves depositing a chloride amount of 1,000 to 20,000 mg / m² onto the evaluation surface of the metal material. 2This is a step of attaching droplets of an aqueous chloride-containing solution so as to achieve this. In the evaluation method of the present invention, at least in the first (first time) chloride attachment step (A), the distribution of droplets of the aqueous chloride-containing solution (droplet attachment distribution) on the evaluation surface of the metal material is such that the average contact area of the droplets on the evaluation surface of the metal material is 0.1 mm 2 or more and 3.0 mm 2 or less, the area ratio of the total contact area of the droplets to the area of the evaluation surface of the metal material is 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 is 3.0 mm 2 or less.

[0060] <Amount of chloride attachment: 1000 - 20000 mg / m 2 > The amount of chloride to be attached to the metal material (the amount of the solid content of chloride not including solvents such as water) is 1000 - 20000 mg / m 2 or less. The above attachment amount corresponds to the amount of chloride attachment assumed in the atmospheric corrosion environment in which an actual automobile travels. In a corrosion environment where the attachment amount is less than 1000 mg / m 2 , since corrosion hardly progresses, hydrogen generation and hydrogen intrusion into the metal material are also slight, and delayed fracture is unlikely to occur. On the other hand, when the attachment amount exceeds 20000 mg / m 2 , it deviates significantly from the corrosion in the actual environment and becomes an excessive durability test, which does not meet the purpose. Therefore, the attachment amount is 20000 mg / m 2 or less. From the perspective of simulating the corrosion form in the atmospheric corrosion environment in which an actual automobile travels and promoting corrosion, the attachment amount is preferably more than 5000 mg / m 2 . Also, from the above perspective, the attachment amount is preferably 12000 mg / m 2 or less.

[0061] The amount of chloride deposited can be calculated by multiplying the difference in mass of the test specimen (metal material) before and after the application of the chloride-containing aqueous solution in the chloride deposition process (A) by the chloride concentration of the chloride-containing aqueous solution and dividing by the evaluation surface area of ​​the test specimen. When measuring the mass difference, if the chloride-containing aqueous solution adheres to areas other than the evaluation surface of the test specimen, 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. Furthermore, to change the amount of chloride deposited, it can be controlled by changing the chloride concentration of the chloride-containing aqueous solution or by changing the application time of the chloride-containing aqueous solution (process time of the chloride deposition process (A)) to change the amount of chloride-containing aqueous solution applied to the metal material.

[0062] In the chloride deposition step (A), chlorides are deposited onto the metal material to obtain the desired amount of chloride. Preferably, the chlorides include one or more selected from sodium salts (NaCl), potassium salts (KCl), calcium salts (CaCl2), and magnesium salts (MgCl2) that are present in the atmospheric environment in which the metal material is commonly used. In addition, in the chloride deposition step (A), when depositing chlorides onto the metal material, a chloride-based component containing both chlorides and other components may be deposited. Here, a chloride-based component refers to a component in which chlorides account for more than 50% by mass of the total components on a solid content basis. Other components include, but are not limited to, sulfides and nitrate compounds. Considering the actual atmospheric corrosion environment, it is preferable to deposit a NaCl-based component (a component in which NaCl accounts for more than 50% by mass of the total components) onto the metal material.

[0063] Furthermore, when simulating delayed fracture characteristics in areas where de-icing agents are frequently spread during winter, it is preferable to use a chloride adhering to the metal material that has a composition similar to that of the de-icing agent spread in that area. Examples of components with a composition similar to that of a de-icing agent include components mainly composed of CaCl2 (components in which CaCl2 accounts for more than 50% by mass of the total components), components mainly composed of MgCl2 (components in which MgCl2 accounts for more than 50% by mass of the total components), and components mainly composed of NaCl (components in which NaCl accounts for more than 50% by mass of the total components).

[0064] Furthermore, a component containing a combination of multiple metal salts may be used as the chloride to be attached to the metal material. Examples of components containing a combination of multiple metal salts include, for example, the Society of Automotive Engineers standard (SAE J2334) (0.5 mass%NaCl-0.1 mass%CaCl2-0.075 mass%NaHCO3), artificial seawater (2.5 mass%NaCl-0.5 mass%MgCl2-0.12 mass%CaCl2-0.07 mass%KCl etc. (for example, an aqueous solution of Aquamarine® manufactured by Yashima Pharmaceutical Co., Ltd.))).

[0065] The method for depositing chlorides onto a metal material (chloride deposition method) is not particularly limited as long as it is a method that can achieve the desired distribution of chloride-containing aqueous solution droplets on the evaluation surface of the metal material. Examples of the chloride-containing aqueous solution include a chloride-containing aqueous solution containing a chloride-based component (usually an aqueous solution such as saltwater, hereinafter also referred to as saltwater). The following explanation will use the case where saltwater is used as the chloride-containing aqueous solution as an example.

[0066] One method for attaching chlorides is the spray method. The spray method involves applying saltwater using a spray nozzle. Types of spray nozzles include single-fluid spray nozzles (nozzles that atomize and spray liquid delivered under pressure) and two-fluid spray nozzles (nozzles that atomize liquid using a high-speed fluid such as compressed air). Two-fluid spray nozzles also have different liquid supply methods, such as pressurized liquid type (liquid is pressurized and supplied to the two-fluid nozzle) and suction type (liquid is drawn up and sprayed using the force of compressed air). It is preferable to select a spray nozzle that ensures a uniform distribution of droplet attachment. Furthermore, since saltwater is used, it is preferable to use corrosion-resistant metals such as stainless steel for the spray nozzle material.

[0067] The chloride concentration in the saltwater is not particularly limited. However, when controlling the droplet distribution of the saltwater using a spray nozzle, if saltwater with a chloride concentration of less than 2.0 mass% is used to adhere the saltwater to the metal material, the spraying time becomes longer to obtain a suitable amount of chloride adhesion, making it difficult to obtain the desired droplet distribution on the evaluation surface of the metal material. Therefore, it is preferable that the chloride concentration in the saltwater be 2.0 mass% or higher, and more preferably 5.0 mass% or higher. On the other hand, if saltwater with a chloride concentration exceeding 20 mass% is used to adhere the saltwater to the metal material, chloride is more likely to precipitate in the spray nozzle, causing clogging of the spray nozzle, making it difficult to spray saltwater droplets of a stable size, and making it difficult to obtain the desired droplet distribution on the evaluation surface of the metal material. As a result, the amount of chloride adhesion on the evaluation surface of the metal material varies from place to place, and in areas with a large amount of chloride adhesion, localized corrosion occurs and the amount of hydrogen penetration increases. Therefore, the delayed fracture characteristics change within the evaluation surface of the metal material, and the accuracy of evaluating the delayed fracture characteristics decreases. This tendency is particularly pronounced when the amount of chloride adhesion is high. Therefore, the chloride concentration in the saltwater is preferably 20% by mass or less, and more preferably 15% by mass or less.

[0068] Furthermore, it is preferable to adjust the chloride concentration in the brine according to the desired amount of chloride deposition. A low-concentration brine is preferable under conditions where the amount of chloride deposition is relatively small, while a high-concentration brine is preferable under conditions where the amount of chloride deposition is relatively large. In order to reduce the amount of chloride deposition on the evaluation surface of the metal material using a high-concentration brine, the total amount of brine sprayed must be reduced, which tends to result in an underestimation of the average contact area of ​​the brine droplets adhering to the evaluation surface of the metal material and the ratio of the total contact area of ​​the brine droplets to the total area of ​​the evaluation surface. On the other hand, in order to increase the amount of chloride deposition on the evaluation surface of the metal material using a low-concentration brine, the total amount of brine sprayed must be increased, which tends to result in an underestimation of the average contact area of ​​the brine droplets adhering to the evaluation surface of the metal material and the ratio of the total contact area of ​​the brine droplets to the area of ​​the evaluation surface.

[0069] [Distribution of chloride-containing aqueous solution droplets applied to the evaluation surface of the metal material in the initial chloride application step (A)] In the evaluation method of the present invention, it is necessary to control the distribution of saltwater droplets on the evaluation surface of the metal material in at least the first (i.e., at least one) chloride deposition step (A) to the range described below. In the first chloride deposition step, the areas where saltwater was present on the evaluation surface of the metal material are considered to be the main corrosion initiation points on that evaluation surface. Then, in the second chloride deposition step, the saltwater spray occurs in a state where corrosion has occurred after the first chloride deposition step and corrosion products have formed on the evaluation surface. Even if the saltwater droplets are uniformly deposited on the evaluation surface of the metal material, wetting will spread due to the influence of the corrosion products. Therefore, it is important to uniformly control the distribution of saltwater droplets on the evaluation surface of the metal material in the first chloride deposition step.

[0070] Figure 2 is a schematic diagram illustrating an example of the evaluation method of the present invention. As shown in Figure 2, in the evaluation method of the present invention, saltwater is sprayed from a spray nozzle to adhere saltwater droplets to the evaluation surface of a metal material. At that time, in the first chloride adhesion step (A), the distribution of saltwater droplets (droplet adhesion distribution) to be adhered to the evaluation surface of the metal material is controlled to a predetermined range.

[0071] <Average contact area of ​​droplets on the evaluation surface of the metal material: 0.1 mm 2 3.0mm or more 2 Less than > In the evaluation method of the present invention, in at least the first chloride deposition 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 mm². 2 3.0mm or more 2 The average contact area shall be less than 0.1 mm². 2 If the amount is less than 0.1 mm², the droplet volume is too small to reach the target chloride deposition amount. Therefore, the average contact area is 0.1 mm². 2 The above is the average contact area, which is 0.5 mm². 2 Preferably, it should be 1.0 mm or more. 2 It is more preferable to have the above. On the other hand, the average contact area is 3.0 mm 2 If the average contact area exceeds 3.0 mm², the droplet adhesion distribution becomes uneven, leading to greater variability in the delayed fracture evaluation. 2 It shall be less than 2.8 mm². The average contact area is 2.8 mm². 2 The following is preferable: 2.5 mm 2 The following is more preferable. The average contact area of ​​the droplet on the evaluation surface of the metal material can be measured by the measurement method described later.

[0072] <Ratio of total droplet contact area to the surface area of ​​the metal material being evaluated: 40% to 80%> In the evaluation method of the present invention, in at least the first chloride deposition 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% or more and 80% or less. If the total contact area ratio of droplets is less than 40%, the droplet deposition distribution becomes uneven, and the variability of the delayed fracture evaluation increases. For this reason, the total contact area ratio of droplets is set to 40% or more. Preferably, the total contact area ratio of droplets is 50% or more, and more preferably 55% or more. On the other hand, if the total contact area ratio of droplets is greater than 80%, bonding between adjacent droplets is more likely to occur on the evaluation surface, and the average contact area of ​​droplets (average contact area per droplet) tends to become coarser. For this reason, the total contact area ratio of droplets is set to 80% or less. Preferably, the total contact area ratio of droplets is 75% or less, and more preferably 70% or less. The total contact area ratio of droplets can be measured by the measurement method described later.

[0073] Standard deviation of the contact area of ​​liquid droplets on the evaluation surface of a metal material: 3.0 mm 2 Below> In the evaluation method of the present invention, in at least the first chloride deposition step (A), the standard deviation of the distribution of the contact area of ​​the droplets on the evaluation surface of the metal material is 3.0 mm. 2 The following conditions apply: The standard deviation of the contact area of ​​the droplet on the evaluation surface of the metal material is 3.0 mm. 2 If the standard deviation is larger, the droplet adhesion distribution becomes uneven, leading to greater variability in the delayed fracture evaluation. Therefore, the standard deviation of the droplet contact area is 3.0 mm. 2 The following applies: The standard deviation of the contact area of ​​the droplet is 2.8 mm. 2 The following is preferable: 2.5 mm 2 The following is more preferable. The standard deviation of the contact area of ​​the above droplets can be measured by the measurement method described later.

[0074] The distribution of saltwater droplets adhering to the evaluation surface of a metal material (average contact area of ​​droplets, total contact area ratio of droplets, and standard deviation of the contact area of ​​droplets) can be determined in the chloride adhesion process (A) by acquiring an image of the droplet distribution across the entire evaluation surface of the metal material after the saltwater droplets have adhered to the evaluation surface of the metal material, and then performing image analysis. The image can be acquired using a digital camera, microscope, optical microscope, etc. The image can also be acquired by photographing the evaluation surface of the metal material from above (from the direction of the spray nozzle shown in Figure 2). The photograph of the evaluation surface may be taken inside the test tank where the spray nozzle is installed, or it may be taken after removing the metal material from the test tank. Preferably, the latter is used, where the metal material is removed from the test tank where the spray nozzle is installed, and the evaluation surface of the metal material is photographed. The image should be acquired immediately after the saltwater droplets have adhered to the evaluation surface of the metal material (within 30 seconds).

[0075] Figure 3 schematically shows an image of the droplet distribution on the evaluation surface of the metal material acquired as described above. In Figure 3, the area indicated by a circle represents the droplet contact area. 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 droplet contact area are determined by image analysis.

[0076] Here, the evaluation surface of the metallic material is the surface on which the delayed fracture characteristics of the metallic material are evaluated. The evaluation surface can be appropriately determined depending on the metallic material being evaluated. For example, if the metallic material is a plate, the surface of the plate facing the spray nozzle can be used as the evaluation surface (see Figure 2). Also, if stress is applied to the metallic material, the surface of the stressed area 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 considered upward) can be used as the evaluation surface. More specifically, for example, when evaluating a bent metallic material as described later, the surface of the bent area facing the spray nozzle can be used as the evaluation surface (see Figure 8).

[0077] As mentioned above, one method for achieving the distribution of saltwater droplets is to apply saltwater to the evaluation surface of the metal material using a spray nozzle. A two-fluid nozzle is preferred as the spray nozzle. Examples of two-fluid nozzles include KSMMS (product name) manufactured by Kyoritsu Alloy Manufacturing Co., Ltd., two-fluid air atomizing nozzle (product name) manufactured by Spraying Systems Japan LLC, and 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 spray time of the saltwater is preferably within 10 seconds. Furthermore, as mentioned above, it is preferable to adjust the chloride concentration in the saltwater according to the target amount of chloride adhesion. Note that, as shown in Figure 8, when a bent metal material 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] As a method for achieving the distribution of the above-mentioned saltwater droplets, it is particularly preferable to place a shielding material having an opening between the spray nozzle and the metal material, and to 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. It is preferable that the opening of the shielding material be approximately the same shape and size as the evaluation surface of the metal material. By "approximately the same," it means that the opening of the shielding material is equivalent to the outer circumference shape and size of the evaluation surface of the metal material when viewed from above from the spray side, 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, it is preferable that when the shielding material is viewed from above, the evaluation surface of the metal material can be seen through the opening of the shielding material, while other parts are not visible (shielded).

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

[0081] When using a shielding material, the distance X from the tip of the spray nozzle to the evaluation surface of the metal material is preferably 10 to 30 cm. Furthermore, 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 spray liquid that does not adhere to the evaluation surface of the metal material remains in the vicinity of the evaluation surface of the metal material, so the effect of suppressing re-adhesion is reduced (Figure 5(a)). On the other hand, if the distance Y is greater than 0.3X cm, the spray liquid that passes through the opening of the shielding material scatters below the shielding material, so the effect of suppressing re-adhesion is reduced (Figure 5(b)). Note that the distance Y is the shortest distance from the shielding material to the evaluation surface of the metal material. Note that the material of the shielding material is not limited as long as it can prevent the permeation of the spray liquid. Examples of materials include resin, ceramic, metal, and wood. The shielding material can be made by processing these materials.

[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 during the first (1st) chloride deposition step (A), and this control of droplet distribution on the evaluation surface is performed at least during the first chloride deposition step (A). Furthermore, the average contact area of ​​the droplets, the total contact area ratio of the droplets, and the standard deviation of the droplet contact area can be measured at the time the first chloride deposition step (A) is completed (within 30 seconds after the end of saltwater spraying). Alternatively, conditions may be set in advance using a test specimen separate from the test specimen to be used for the test, so that the average contact area of ​​the droplets, the total contact area ratio of the droplets, and the standard deviation of the droplet contact area are predetermined, and the evaluation test of the test specimen may be performed under the same conditions. In the second and subsequent chloride deposition steps (A), chloride deposition may be performed under the same conditions as the first chloride deposition step (A), or chloride deposition may be performed under different conditions from the first chloride deposition step (A), as long as the predetermined amount of chloride deposition can be obtained. Preferably, conditions are set in advance to determine the average contact area of ​​the droplets, the total contact area ratio of the droplets, and the standard deviation of the contact area of ​​the droplets, and the first chloride adhesion step (A) is performed under these set conditions. If the chloride adhesion step (A) is performed two or more times, it is preferable to perform the second and subsequent chloride adhesion steps (A) under the same set conditions.

[0083] The chloride deposition process (A) is preferably carried out in an atmosphere with a relative humidity Ha1 of 30% or higher. If the relative humidity Ha1 in the chloride deposition process (A) is less than 30%, the droplets of the chloride-containing aqueous solution will tend to dry out before reaching the evaluation surface of the metal material, especially when droplets are sprayed using a spray nozzle. As a result, it may become difficult to control the distribution of droplets to obtain the desired distribution on the evaluation surface of the metal material. Furthermore, the chloride deposition process (A) is preferably carried out in an atmosphere with a relative humidity of 80% or lower. If the relative humidity in the chloride deposition process (A) is greater than 80%, the droplets that adhere to the metal material tend to become coarser.

[0084] Furthermore, the chloride deposition process (A) is preferably carried out in an atmosphere with a temperature Ta1 of 50°C or lower. If the temperature Ta1 in the chloride deposition process (A) exceeds 50°C, the droplets of the chloride-containing aqueous solution tend to dry out before reaching the evaluation surface of the metal material, especially when droplets are sprayed using a spray nozzle. As a result, it may become difficult to control the distribution of droplets to obtain the desired 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 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 cycles of wetting (humid) and drying (dry) states. Simulating these environmental changes is crucial for simulating corrosion patterns in real-world environments where automobiles operate. For example, in the case of steel materials, the corrosion products formed on the steel material change depending on whether it is wet or dry, and it is known that hydrogen is generated during the transition from wet to dry or vice versa. Therefore, the conditions in the cycle of relative humidity change (corrosion process (B)) are also important in evaluating delayed fracture characteristics.

[0086] (Corrosion process (B)) The corrosion process (B) is a process in which a cycle consisting of the following drying process (b1), the following wetting process (b2), the following transition process (b3), and the following transition process (b4) is performed in an atmosphere at a temperature Tb1 of 60°C or lower and within a certain range, and this cycle is performed at least once (once or twice or more).

[0087] <Corrosion process (B) temperature Tb1: 60°C or less, and within a certain range> The corrosion process (B) is carried out in an atmosphere with a temperature Tb1 of 60°C or lower and within a certain range. If the temperature Tb1 of the corrosion process (B) exceeds 60°C, not only will the evaluation be conducted in an environment far removed from the actual corrosive environment in which the metal material is used, but the corrosion mechanism may also change. For this reason, the temperature Tb1 of the corrosion process (B) should be 60°C or lower. Preferably, it should be 50°C or lower. On the other hand, there is no particular lower limit to the temperature Tb1 of the corrosion process (B). If the temperature Tb1 of the corrosion process (B) is below 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. Also, the evaluation time will be longer because the corrosion rate of the metal material will be significantly reduced. For this reason, the temperature Tb1 of the corrosion process (B) is preferably 5°C or higher, and more preferably 10°C or higher.

[0088] Furthermore, delayed fracture characteristics are strongly influenced by the temperature of the environment (atmosphere). Therefore, in order to properly evaluate the delayed fracture characteristics of a metallic material, taking into account the application site of the metallic material component and the environment in which it is used, it is necessary to keep the temperature Tb1 of the corrosion process (B) within a certain range. When the fluctuation range of the temperature Tb1 of the corrosion process (B) is within ±5°C, the amount of hydrogen that penetrates into the interior of the metallic material from the environment (hydrogen intrusion amount) can be evaluated with a fluctuation range of within 30% of the hydrogen intrusion amount at the target environmental temperature, and the delayed fracture characteristics can be accurately evaluated. When the fluctuation range of the temperature Tb1 of the corrosion process (B) is within ±2°C, the fluctuation range of the hydrogen intrusion amount is within 15%. For this reason, the fluctuation range of the temperature Tb1 of process (B) is preferably within ±5°C, and more preferably within ±2°C.

[0089] [Drying process (b1)] The drying process (b1) is a process of drying the metal material by holding it in an atmosphere with a relative humidity Hb1 of 45% or less for a period of 1.0 hour to 5.0 hours. The relative humidity Hb1 in the drying process (b1) is set to 45% or less. This is to simulate the dry state, which is one of the characteristics of an atmospheric corrosion environment. Furthermore, if the relative humidity Hb1 in the drying process (b1) exceeds 45%, it will be necessary to hold it for a longer period of time in order to sufficiently dry the surface of the metal material, and the evaluation time will be longer. The relative humidity Hb1 in the drying process (b1) is preferably 40% or less. On the other hand, there is no particular lower limit to the relative humidity Hb1 in the drying process (b1). From the viewpoint of controllability of relative humidity, the relative humidity Hb1 in the drying process (b1) is preferably 20% or more. In addition, if the components to be attached to the surface of the metal material include substances that exhibit deliquescence at lower relative humidity, such as magnesium chloride or calcium chloride, it is preferable to set the relative humidity Hb1 in the drying process (b1) lower.

[0090] The drying process (b1) duration (the time spent in an atmosphere with relative humidity Hb1) should be between 1.0 hour and 5.0 hours. If the drying process (b1) duration is less than 1.0 hour, it is not possible to simulate an actual corrosive environment. On the other hand, if the drying process (b1) duration exceeds 5.0 hours, it is possible to simulate an actual corrosive environment, but it takes too long to evaluate the delayed fracture characteristics.

[0091] [Wetting process (b2)] The wetting process (b2) is a process of wetting a metal material by maintaining it in an atmosphere with a relative humidity of 80% or more (Hb2) for 1.0 hour to 5.0 hours. The relative humidity (Hb2) in the wetting process (b2) is set to 80% or more. This is to simulate the wet state, which is one of the characteristics of an atmospheric corrosion environment. If the relative humidity (Hb2) in the wetting process (b2) is less than 80%, the effect of wetting will be insufficient, and it will not be possible to simulate an actual corrosion environment. Among chlorides, sodium chloride has the highest saturated critical vapor pressure, which is approximately 75-78% in terms of relative humidity. Therefore, if the relative humidity is set to 80% or more for any chloride, a water film will form on the surface of the metal material due to moisture absorption by the chloride, and the wet state can be maintained. For this reason, the relative humidity (Hb2) in the wetting process (b2) is set to 80% or more. On the other hand, there is no particular upper limit to the relative humidity (Hb2) in the wetting process (b2), but it is preferable that the relative humidity (Hb2) in the wetting process (b2) is less than 98%. This is because when the relative humidity Hb2 exceeds 98%, the water film thickness generated 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 to keep the relative humidity Hb2 in the wetting process (b2) below 98%.

[0092] The process time for the wetting process (b2) (the time spent in an atmosphere with a relative humidity of 80% or higher Hb2) shall be between 1.0 hour and 5.0 hours. If the process time for the wetting process (b2) is less than 1.0 hour, it is not possible to simulate an actual corrosion environment. On the other hand, if the process time for the wetting process (b2) exceeds 5.0 hours, an actual corrosion environment can be simulated, but it takes time to evaluate the delayed fracture characteristics.

[0093] [Transition process (b3), transition process (b4)] Transition process (b3) is a process of transitioning from an atmosphere with relative humidity Hb1 to an atmosphere with relative humidity Hb2, and transition process (b4) is a process of transitioning from an atmosphere with relative humidity Hb2 to an atmosphere with relative humidity Hb1. It is known that the amount of hydrogen entering the interior of metallic materials, especially steel materials, increases when the relative humidity changes. In other words, a large amount of hydrogen enters the interior of the steel material in transition processes (b3) and (b4). The reason why the amount of hydrogen entering the metallic material increases when the relative humidity changes is not entirely clear, but it can be considered as follows. In the case of transition process (b3), which is a process of transitioning from an atmosphere with relative humidity Hb1 to an atmosphere with relative humidity Hb2, moisture is absorbed due to the deliquescence of chlorides present on the surface of the metallic material, and corrosion of the metallic material begins. It is known that the corrosion products present on the surface of the metallic material change at this time, and it is thought that hydrogen is generated along with this change in corrosion products. Furthermore, in the transition step (b4), which is the process of moving from an atmosphere with relative humidity Hb2 to an atmosphere with relative humidity Hb1, the moisture becomes a concentrated solution containing a large amount of chloride and metal ions dissolved by corrosion, and in the case of steel materials, a large amount of iron ions, so the pH of the solution is thought to decrease. In other words, a large amount of hydrogen ions will be contained in the solution during the drying process, and it is thought that hydrogen will easily penetrate into the metal material. For this reason, in transition steps (b3) and (b4), the rate of change of relative humidity when changing the relative humidity should be 30% / h or less. If the rate of change of relative humidity in transition steps (b3) and (b4) is 30% / h or less, it will be possible to sufficiently penetrate the metal material with hydrogen generated by corrosion, and it will be possible to properly evaluate the delayed fracture characteristics. On the other hand, there is no specific lower limit for the rate of change of relative humidity, but if the transition process (b3) and transition process (b4) are too long, it will take a long time to evaluate the delayed failure characteristics. Therefore, the rate of change of relative humidity is preferably 1.5% / h or more, and more preferably 10% / h or more.

[0094] The present invention's method for evaluating the delayed fracture characteristics of a metal material aims to simulate the diurnal relative humidity changes in a real environment. Therefore, if the process time (time of one cycle) of the corrosion process (B), which simulates the diurnal relative humidity changes in a real environment, exceeds 24 hours, it means that the corrosion will be slower than in a real environment, and thus the evaluation of the delayed fracture characteristics will take a long time. In other words, it is preferable to set the process time of the corrosion process (B) to 24 hours or less. To expedite the evaluation, it is even more preferable to set the process time of the corrosion process (B) to 12 hours or less. On the other hand, if the process time of the corrosion process (B) is shortened, the relative humidity will change rapidly, resulting in a low correlation with corrosion in a real environment and a discrepancy with the results of delayed fracture characteristics in a real environment. For this reason, it is preferable to set the process time of the corrosion process (B) to 5 hours or more.

[0095] In the evaluation method of the present invention, the chloride deposition step (A) and the corrosion step (B) are each performed at least once. The chloride deposition step (A) may be placed at random intervals of the corrosion step (B), or at predetermined intervals of the corrosion step (B). There is no particular upper limit to the number of times the process comprising the chloride deposition step (A) and the corrosion step (B) is performed. For example, the process comprising the chloride deposition step (A) and the corrosion step (B) can be performed until cracks occur in the metal material. Alternatively, the number of test days may be predetermined, and the process may be performed for a number of days corresponding to that test period. The number of times the process is performed can be set appropriately, for example, by considering the simulation of corrosion patterns in a real environment. As an example, the process may be 200 times or less, or 100 times or less.

[0096] Next, a process comprising a chloride adhesion process (A) and a corrosion process (B) will be described. Figure 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 Figure 6 is an example of a corrosion test cycle in which the chloride adhesion process (A) and the corrosion process (B) are performed once each. In this example, the corrosion process (B) has a process consisting of a drying process (b1), a migration process (b3), a wetting process (b2), and a migration process (b4) as one cycle.

[0097] It is preferable that the corrosion cycle (B) following the chloride deposition process (A) begin with the drying process (b1). By drying the saltwater deposited in the chloride deposition process (A) during the drying process (b1), the points at which condensation occurs when humidity rises are uniformly dispersed, reducing variability in the evaluation of delayed fracture characteristics. If the corrosion cycle (B) is started with the transition process (b3), the wetting process (b2), or the transition process (b4), the saltwater deposited in the chloride deposition process (A) may not be sufficiently dried, or the saltwater deposited in a high-humidity environment may absorb moisture and become coarser, leading to an uneven dispersion of condensation points. Therefore, it is best to avoid starting the corrosion cycle (B) with any process other than the drying process (b1).

[0098] When performing the chloride deposition process (A) followed by the corrosion process (B) cycle, and then repeating the chloride deposition process (A), it is preferable to include a water washing process (C) before the chloride deposition process (A). Figure 7 shows an example of a corrosion test cycle when performing the chloride deposition process (A), followed by the corrosion process (B), and then repeating the chloride deposition process (A) with a water washing process (C). If the chloride deposition process (A) is repeated without including the water washing process (C), the amount of chloride deposited on the surface of the metal material tends to increase as the amount of chloride deposition increases, and it may become impossible to maintain the same corrosion environment. Therefore, it may become possible to evaluate the delayed fracture characteristics in an environment different from the assumed corrosion environment. For this reason, it is preferable to include a water washing process (C) before repeating the chloride deposition process (A). Note that the water washing process (C) is a process of washing the evaluation surface of the metal material with water. The washing method in the washing step (C) is not particularly limited, but examples include a method of washing the evaluation surface of a metal material by spraying water from a spray nozzle, or a method of washing the evaluation surface by immersing the metal material in water.

[0099] In the evaluation method of the present invention, the above-described chloride deposition step (A) and corrosion step (B) cycle is performed, and before performing the chloride deposition step (A) again, a corrosion test cycle comprising a water washing step (C) is performed at least once. After that, the condition of the metal material (presence and extent of cracks in the metal material, etc.) is checked, and the delayed fracture characteristics of the metal material are evaluated based on the checked condition of the metal material.

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

[0101] ·Process (A) Chloride-containing aqueous solution: 15% by mass NaCl aqueous solution is sprayed. Chloride deposition amount (based on solid content): 10,000 mg / m² 2 Distance between nozzle and evaluation surface (X in Figure 2): 30 cm Distance between shielding material and evaluation surface (Y in Figure 5): 5 cm Temperature: 22℃, Relative humidity: 50% Droplet distribution on the evaluation surface; Average contact area of ​​droplets: 1.3 mm² 2 Standard deviation of droplet contact area: 1.3 mm 2 Total contact area ratio of droplets: 71% ·Process (B) Drying process (b1): Temperature: 30°C, relative humidity: 40%, holding time: 2.0 hours Wetting process (b2): Temperature: 30°C, Relative humidity: 90%, Holding time: 2.0 hours Transition process (b3) Rate of change in relative humidity: 25% / h Transition process (b4) Rate of change in relative humidity: 25% / h Temperature fluctuation range: within ±5℃

[0102] The water washing step (C) is a step of washing the evaluation surface of the metal material with water. The water washing method in the water washing step (C) is not particularly limited, but water is sprayed onto the evaluation surface of the metal material from a spray nozzle to wash the evaluation surface.

[0103] • Corrosion test cycle: After performing the first step (A), check the droplet distribution on the evaluation surface. Subsequent steps (A) are performed under the same conditions as the first step (A). Step (B) consists of a drying step (b1) → transition step (b3) → wetting step (b2) → transition step (b4), with one cycle being performed. After repeating this cycle four times, a water rinsing step (C) is added before performing step (A), and then step (A) is performed. • Metal materials used for evaluation: The obtained steel plate was sheared to a size of 16 mm x 75 mm with the longitudinal side perpendicular to the rolling direction, and test specimens were prepared. The clearance during shearing was 15% in all cases. Then, a four-point bending test was performed according to ASTM (G39-99), and stresses equivalent to YS and TS were applied to the bending apex of the test specimens. Then, the above-mentioned corrosion environment cycle (HeTsAce) was carried out for 63 days. After the test, the presence or absence of cracks was checked visually for each test specimen, and if no cracks were found in the sample subjected to stress equivalent to YS, it was judged to have excellent delayed fracture resistance in a corrosive environment.

[0104] In the aforementioned evaluation method for the delayed fracture characteristics of metallic materials (HeTsAce), materials with a longer time until cracking occurs tend to have a lower amount of diffusible hydrogen in the steel.

[0105] [Method of manufacturing steel plates] Next, the method for manufacturing steel sheets according to the present invention will be described. The present invention provides a method for manufacturing steel sheets, in which a cold-rolled sheet is produced by hot-rolling, pickling, and cold-rolling a steel having the aforementioned component composition, and the sheet is heated under the conditions of an annealing temperature T1 of 800°C or higher and a holding time t1 at the annealing temperature T1 of 10 seconds or higher, followed by cooling at an average cooling rate CR1 of 5°C / s or higher from 700 to 600°C, and cooling at an average cooling rate CR2 of 5°C / s or higher and 30°C / s or lower from (Ms+100°C) to a rapid cooling start temperature T2 which is between (Ms-50°C) and (Ms+50°C), and from the rapid cooling start temperature T2... The process includes water quenching to an average cooling rate CR3 of 300°C / s or more down to 80°C, followed by an annealing process with a tempering temperature T3 of 100°C to 400°C and a holding time t3 at tempering temperature T3 of 10 seconds to 10,000 seconds. During the water quenching cooling in the annealing process, pressure is applied to the front and back surfaces of the steel plate using two rolls placed on either side of the steel plate, with the distance between the two rolls in the steel plate conveying direction being 20 mm to 250 mm and the applied pressure being 196 N or more.

[0106] In this invention, the method for melting the steel material (steel slab) is not particularly limited, and any known melting method, such as a converter or electric furnace, is suitable. The steel slab (slab) is preferably manufactured by a continuous casting method 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 for hot rolling a steel slab include a method of heating the slab before rolling, a method of directly rolling a slab after continuous casting without heating it, and a method of subjecting a slab after continuous casting to a short-time heat treatment before rolling. 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. A slab heating time of 30 minutes or more is preferable. A slab heating time of 250 minutes or less is preferable. The finishing rolling temperature is preferably above the Ar3 transformation point. Furthermore, a winding temperature of 350°C or higher is preferable. Also, a winding temperature of 650°C or lower is preferable.

[0108] The hot-rolled steel sheets produced in this manner are then pickled. Pickling is important for ensuring good chemical conversion treatment properties and plating quality in the final high-strength steel sheet product because it removes oxides from the surface of the steel sheet. Pickling may be performed once or in multiple stages. Furthermore, the hot-rolled and pickled sheets may be cold-rolled immediately, or they may be cold-rolled after heat treatment.

[0109] The reduction ratio (cumulative reduction ratio) in cold rolling and the thickness of the sheet after rolling are not particularly limited, but a reduction ratio of 30% or more is preferred. Furthermore, a reduction ratio of 80% or less is preferred. The effects of the present invention can be obtained without any particular limitations on the number of rolling passes or the reduction ratio of each pass.

[0110] The cold-rolled sheet obtained as described above is then annealed. The annealing conditions are as follows:

[0111] Annealing temperature T1: 800℃ or higher If the annealing temperature T1 is less than 800°C, 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 higher, and also difficult to achieve excellent elongation flange properties. Therefore, the annealing temperature T1 should be 800°C or higher. The annealing temperature T1 is preferably 820°C or higher. There is no particular upper limit, but the annealing temperature T1 is preferably 1000°C or lower. The annealing temperature referred to here is the holding temperature during the annealing process. The annealing temperature may remain constant during the holding period. Furthermore, the annealing temperature does not need to be constant throughout the holding period, as long as it is in the temperature range of 800°C or higher and the temperature fluctuation is within ±10°C of the set temperature.

[0112] Holding time 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 higher, and also making it difficult to achieve excellent elongation flange properties. Therefore, the holding time t1 at annealing temperature T1 should be 10 seconds or more. Preferably, the holding time t1 at annealing temperature T1 is 30 seconds or more. There is no particular upper limit, but preferably the holding time t1 at annealing temperature T1 is 1000 seconds or less.

[0113] Average cooling rate CR1 at 700-600℃: 5℃ / s or higher If the average cooling rate CR1 at 700-600°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 higher, and also difficult to achieve excellent scalability. Therefore, the average cooling rate CR1 at 700-600°C should be 5°C / s or higher. The average cooling rate CR1 is preferably 10°C / s or higher. There is no particular upper limit, but the average cooling rate CR1 is preferably 50°C / s or lower. Here, the average cooling rate CR1 is calculated as (cooling start temperature (700°C) - cooling stop temperature (600°C)) / cooling time (s) from cooling start temperature (700°C) to cooling stop temperature. Cooling at an average cooling rate of CR1 specifically includes methods such as water cooling and mist cooling.

[0114] (Ms+100℃) ~ Average cooling rate CR2 of rapid cooling start temperature T2: 5℃ / s or more and 30℃ / s or less In this invention, the average cooling rate CR2 at the rapid cooling start temperature T2 (Ms+100℃) affects the total area fraction of ferrite and bainitic ferrite and the average value of the occupancy rate of packets having the maximum occupancy rate within the prior austenite grains at the center of the steel sheet thickness. If the average cooling rate CR2 at the rapid cooling start temperature T2 (Ms+100℃) is less than 5℃ / s, the total area fraction of ferrite and bainitic ferrite will be 5% or more, making it difficult to achieve excellent ductility flange properties. On the other hand, if the average cooling rate CR2 from (Ms+100℃) to the rapid cooling start temperature T2 exceeds 30℃ / s, the average occupancy rate of packets with the maximum occupancy rate within the prior austenite grains at the center of the steel plate thickness exceeds 70%, and the fatigue resistance of the shear end face and the delayed fracture resistance in a corrosive environment decrease. Therefore, the average cooling rate CR2 from (Ms+100℃) to the rapid cooling start temperature T2 should be 5℃ / s or more and 30℃ / s or less. The average cooling rate CR2 is preferably 10℃ / s or more. The average cooling rate CR2 is preferably 20℃ / s or less. Here, the average cooling rate CR2 is calculated as (cooling start temperature (Ms+100℃) - cooling stop temperature (rapid cooling start temperature T2)) / cooling time (s) from cooling start temperature (Ms+100℃) to cooling stop temperature (rapid cooling start temperature T2). Cooling at an average cooling rate of CR2 can be achieved using methods such as mist cooling or gas cooling.

[0115] Rapid cooling start temperature T2: (Ms -50℃) or higher and (Ms +50℃) or lower. This is one of the important constituent elements of the present invention. By setting the rapid cooling start temperature T2 to (Ms-50°C) or higher and less than (Ms+50°C), it is possible to obtain a structure in which the average occupancy rate of packets having the maximum occupancy rate within the prior austenite grains at the center of the thickness of the steel sheet is 70% or less, and retained austenite is less than 3% by volume fraction. If the rapid cooling start temperature T2 is less than (Ms-50°C), retained austenite will be 3% or more by volume fraction, making it difficult to achieve a YR of 75% or more. On the other hand, if the rapid cooling start temperature T2 is (Ms + 50°C) or higher, the average occupancy rate of packets with the maximum occupancy rate within the prior austenite grains at the center of the steel plate thickness exceeds 70%, and the fatigue resistance of the shear end face and the delayed fracture resistance in a corrosive environment decrease. Therefore, the rapid cooling start temperature T2 should be (Ms - 50°C) or higher and less than (Ms + 50°C). The rapid cooling start temperature T2 is preferably (Ms - 40°C) or higher. The rapid cooling start temperature T2 is preferably (Ms + 40°C) or lower. Note that Ms is the martensitic transformation start temperature (°C), and the martensitic transformation start temperature Ms (°C) is defined by the following equation (3). Ms(℃)=519-474×[%C]-30.4×[%Mn]-12.1×[%Cr]-7.5×[%Mo]-17.7×[%Ni] ···(3) Here, [%C], [%Mn], [%Cr], [%Mo], and [%Ni] represent the mass percentage of C, Mn, Cr, Mo, and Ni contained in the steel (steel sheet), with 0 indicating that they are not present.

[0116] Average cooling rate CR3 from the rapid cooling start temperature T2 to 80°C: 300°C / s or higher. In the water quenching cooling process during annealing, if the average cooling rate CR3 from the rapid cooling start temperature T2 to 80°C is less than 300°C / s, the retained austenite will be 3% or more by volume fraction, making it difficult to achieve a YR of 75% or more. Therefore, the average cooling rate CR3 from the rapid cooling start temperature T2 to 80°C should be 300°C / s or higher. Preferably, the average cooling rate CR3 is 800°C / s or higher. There is no particular upper limit, but it is preferable that the average cooling rate CR3 is 3000°C / s or less. Here, the average cooling rate CR3 is calculated as (cooling start temperature (rapid cooling start temperature T2) - cooling stop temperature (80°C) / cooling time (s) from cooling start temperature (rapid cooling start temperature T2) to cooling stop temperature (80°C).

[0117] Tempering temperature T3: 100℃ or more and 400℃ or less In this invention, tempered martensite refers to a structure in which martensite below 80°C has been subjected to a heat treatment at a tempering temperature of 100°C or higher and a holding time of 10 seconds or more. In the tempering process during the annealing process, if the tempering temperature T3 is less than 100°C, the martensite is not sufficiently tempered, resulting in a structure mainly composed of as-quenched martensite, and as-quenched martensite deteriorates the fatigue resistance of the shear end face. On the other hand, if the tempering temperature T3 exceeds 400°C, the tempering of the martensite proceeds excessively, making it difficult to achieve a TS of 1320 MPa or higher. Therefore, the tempering temperature T3 should be between 100°C and 400°C. Preferably, the tempering temperature T3 should be 150°C or higher. Preferably, the tempering temperature T3 should be 350°C or lower.

[0118] Holding time 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 heated at a temperature of 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 more. During the tempering process in the annealing process, if the holding time t3 at tempering temperature T3 is less than 10 seconds, the martensite is not sufficiently tempered, resulting in a structure mainly composed of as-quenched martensite. This as-quenched martensite degrades the fatigue resistance of the shear end face. On the other hand, if the holding time t3 exceeds 10,000 seconds, the tempering of the martensite proceeds excessively, making it difficult to achieve a TS of 1,320 MPa or higher. Therefore, the holding time t3 at tempering temperature T3 should be between 10 seconds and 10,000 seconds. The holding time t3 should preferably be 50 seconds or more. The holding time t3 should preferably be 5,000 seconds or less.

[0119] Cooling after tempering does not need to be specifically defined and may be cooled to the desired temperature by any method. The desired temperature is preferably around room temperature.

[0120] Furthermore, the above-mentioned steel plate (high-strength steel plate) may be processed under conditions that result in an equivalent plastic strain of 0.05% to 5.00%. In addition, after processing, it may be reheated again under conditions of 100°C to 400°C.

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

[0122] During the cooling process of water quenching, pressure is applied to the front and back surfaces of the steel plate by two rolls positioned on either side of the plate. The distance between the two rolls in the steel plate transport direction during this pressurization is 20 mm to 250 mm, and the applied pressure is 196 N or more. During the cooling (water cooling) phase of the water quenching process in the annealing process, the front and back surfaces of the steel plate are pressed by two rolls placed on either side of the steel plate. At this time, the distance between the two rolls in the steel plate conveying direction (hereinafter also simply referred to as the distance between the rolls) is set to 20 mm or more and 250 mm or less, and the pressing force is set to 196 N or more. In this invention, "distance between the two rolls" refers to the distance between the contact point between one roll and the steel plate and the contact point between the other roll and the steel plate, as shown in Figure 9. In the water quenching process described above, if no pressure is applied, P(S) becomes excessively large compared to P(C), making it difficult to satisfy {P(S)-P(C)}≦20%, thus reducing the fatigue resistance of the shear end face and the delayed fracture resistance in a corrosive environment. After extensive research by the inventors, it was found that applying pressure during the water quenching process described above affects the difference between P(S) and P(C). By applying this pressure under the conditions of a roll-to-roll distance of 20 mm to 250 mm and a pressure of 196 N or more, nucleation of martensitic transformation near the surface of the steel sheet is promoted relative to the center of the steel sheet, contributing to a decrease in {P(S)-P(C)}. Therefore, {P(S)-P(C)}≦20% is achieved, improving the fatigue resistance of the shear end face and the delayed fracture resistance in a corrosive environment. To obtain this effect, the roll-to-roll distance (see Figure 9) must be 20 mm or more. On the other hand, if the distance between the rolls exceeds 250 mm, the pressure will weaken, so the distance between the rolls must be 250 mm or less. Pressure is applied by sandwiching the material between rolls that are far apart from each other, and the pressure required to achieve the above effect is 196 N or more. This pressure is equivalent 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 of 196 N or more. For pressurization conditions, the preferred distance between rolls is 30 mm or more. Alternatively, the preferred distance between rolls is 220 mm or less. Furthermore, a preferred pressing force is 294N or higher. A preferred pressing force is 4900N or lower. The pressing force varies depending on the steel plate strength and tension, but it can be adjusted by tension, indentation amount, etc. The pressing force being within the above range can be confirmed using a load cell attached to the roll, and the indentation amount can be calculated from the roll diameter and roll position. Based on the above, in the present invention, during the cooling (water cooling) of water quenching, pressure is applied to the front and back surfaces of the steel plate by two rolls placed on either side of the steel plate, and this pressure is applied under the conditions that the distance between the two rolls is 20 mm or more and 250 mm or less, and the applied pressure is 196 N or more.

[0123] Furthermore, the steel plate may optionally be plated. The plating treatment is not particularly limited. Examples of plating treatments include zinc plating treatments such as hot-dip galvanizing, alloyed hot-dip galvanizing, and electroplating. Examples of plating treatments other than zinc plating include aluminum plating and alloy plating. Examples of alloy plating treatments include hot-dip zinc-aluminum-magnesium alloy plating and Zn-Ni electroalloy plating. The treatment conditions in all cases should follow conventional methods. As mentioned above, it is preferable to perform the plating treatment during the cooling from the annealing temperature T1 to the rapid cooling start temperature T2, or after the tempering process. For example, hot-dip galvanizing and alloyed hot-dip galvanizing are preferable to perform during the cooling from the annealing temperature T1 to the rapid cooling start temperature T2. Also, it is preferable to perform electroplating and Zn-Ni electroalloy plating after the tempering process. Furthermore, in the case of hot-dip galvanizing and alloyed hot-dip galvanizing, from the viewpoint of productivity, it is preferable to perform the series of processes, including the heating, annealing, and plating processes, on a CGL (Continuous Galvanizing Line), which is a hot-dip galvanizing line. After hot-dip galvanizing, wiping is possible to adjust the plating thickness.

[0124] Other than the conditions mentioned above, there are no particular limitations, and conventional methods may be followed. According to the steel sheet manufacturing method of the embodiment described above, a steel sheet can be obtained in which the TS is 1320 MPa or higher, the YR is 75% or higher, and which has excellent tensile flange properties, fatigue resistance of the shear end face, 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 annealing and plating, the material may be processed again under conditions that result in an equivalent plastic strain of 0.05% to 5.00%. Alternatively, after processing, it may be reheated again under conditions of 100°C to 400°C.

[0126] [Components and methods for manufacturing components] Next, the component of the present invention and its manufacturing method will be described.

[0127] The component of the present invention is obtained by subjecting a steel sheet of the present invention to at least one of forming and joining processes. Furthermore, the method for manufacturing the component of the present invention includes the step of subjecting a steel sheet of the present invention to at least one of forming and joining processes to form the component.

[0128] The steel sheet of the present invention has a tensile strength of 1320 MPa or more, a YR of 75% or more, and excellent stretch flange properties, fatigue resistance of the shear end face, and delayed fracture resistance in corrosive environments. Therefore, members obtained using the steel sheet of the present invention also have a tensile strength of 1320 MPa or more, a YR of 75% or more, and excellent stretch flange properties, fatigue resistance of the shear end face, and delayed fracture resistance in corrosive environments. Weight reduction is possible by using the members of the present invention. Therefore, the members of the present invention can be suitably used, for example, in automotive structural members.

[0129] Forming processes can utilize general processing methods such as press working without restriction. Joining processes can utilize general welding methods such as spot welding and arc welding, as well as riveting and crimping without restriction. [Examples]

[0130] Steel having the component composition shown in Table 1 (Table 1-1, Table 1-2) (the remainder being Fe and unavoidable impurities) was melted in a converter and formed into steel slabs by continuous casting. The steel slabs were then heated. The steel slabs were then hot-rolled to form hot-rolled steel sheets. The hot-rolled steel sheets were then pickled. The hot-rolled steel sheets were then cold-rolled to form cold-rolled steel sheets. In this way, the raw steel sheets were prepared. The prepared raw steel sheets were then annealed under the conditions shown in Table 2 (Table 2-1, Table 2-2) to obtain the final product steel sheets (thickness: 1.4 mm). In addition, some of the steel sheets (those with type columns GI, GA, and EG in Table 2) were plated. Of these, those with type columns GI and GA in Table 2 were plated during the cooling from the annealing temperature T1 to the rapid cooling start temperature T2. Furthermore, for items with "EG" in the Type column of Table 2, plating was performed after the tempering process. Conditions not specified were governed by conventional methods.

[0131] [Table 1-1]

[0132] [Table 1-2]

[0133] [Table 2-1]

[0134] [Table 2-2]

[0135] The steel sheets (high-strength cold-rolled steel sheets) obtained as described above were used as test materials, and their tensile properties, elongation flange properties, fatigue resistance of the shear end face, and fatigue resistance of the shear end face were evaluated according to the following test methods.

[0136] (Tissue observation) Following the method described above, the amount of tempered martensite (area fraction), the amount of retained austenite (volume fraction), and the sum of the ferrite and bainitic ferrite amounts (area fraction) were determined.

[0137] (P(S): Average value of the occupancy rate of packets with the maximum occupancy rate within the 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 with the maximum occupancy rate within the prior austenite grains at the center of the thickness of the steel plate) Following the method described above, the average occupancy rate of packets with the maximum occupancy rate within the prior austenite grains was determined at a depth of 100 μm from the surface of the steel plate and at the center of the thickness of the steel plate.

[0138] (Tensile test) For the tensile test, a JIS No. 5 test specimen (gauge length 50 mm, parallel section width 25 mm) was taken so that the longitudinal direction of the specimen was perpendicular to the rolling direction, and the test was performed according to JIS Z 2241 (2022). The crosshead speed was 1.67 × 10⁻⁶. -1 A tensile test was performed under conditions of mm / second, and the tensile strength TS was measured. In this invention, a TS of 1320 MPa or higher was considered acceptable. A yield ratio YR of 75% or higher was also considered acceptable. YR is calculated using the following formula (4). YR = 100 × YS / TS ... (4)

[0139] (Stretchable flange properties) The hole expansion test was conducted in accordance with JIS Z 2256 (2020). After shearing the obtained steel plate to 100 mm x 100 mm, a 10 mm diameter hole was punched with a clearance of 12.5%. Then, using a die with an inner diameter of 75 mm, a wrinkle-holding force of 9 tons (88.26 kN) was applied, and a conical punch with a vertex angle of 60° was pressed into the hole to measure the hole diameter at the crack initiation limit. The limit hole expansion ratio: λ (%) was calculated from the following formula, and the hole expansion performance was evaluated from this limit hole expansion ratio value. Limit hole expansion rate: λ(%)={(Df-D0) / D0}×100 However, Df is the hole diameter at the time of crack initiation (mm), and D0 is the initial hole diameter (mm). In this invention, a good elongation flange property was determined when the hole expansion ratio (λ), which is an indicator of elongation flange property, was 30% or more, regardless of the strength of the steel plate.

[0140] (Fatigue resistance characteristics of the sheared end face) The fatigue resistance characteristics of the shear end face were evaluated by a full double-bend test in accordance with JIS Z 2275 (1978). The test shape was a fatigue test specimen as described in JIS Z 2275 (1978), with a punch hole punched in the center with a punch diameter of Φ10 mm and a clearance of 12%. A full double-bend fatigue test with constant stress amplitude (stress ratio R = -1) was performed on the obtained specimen, with 1 × 10 cycles. 7 The upper limit of fatigue strength in one cycle was measured as the fatigue limit of the shear end face, and the fatigue strength ratio of the shear end face was determined using the following equation (5). Fatigue strength ratio of the shear end face = Fatigue limit of the shear end face / TS ... (5) The fatigue resistance characteristics of the shear end face were then evaluated according to the following criteria. ◎ (Pass, particularly excellent): Fatigue strength ratio of the shear end face is 0.30 or higher. ○ (Pass, Excellent): Fatigue strength ratio of the shear end face is 0.25 or higher and less than 0.30. × (Failure): Fatigue strength ratio of the shear end face is less than 0.25

[0141] (Delayed fracture resistance in corrosive environments) The obtained steel plates were sheared to a size of 16 mm × 75 mm with the longitudinal side perpendicular to the rolling direction, and test specimens were prepared. The clearance during shearing was 15% in all 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 bending apex of the test specimens. Then, a delayed fracture test was performed using the corrosion environment cycle described later.

[0142] (Corrosion test cycle) The above-mentioned test specimens were subjected to the following corrosion test cycle (corrosion test) consisting of a chloride deposition step (A), a corrosion step (B), and a water washing step (C). In the chloride deposition process (A), the amount of chloride deposited (in terms of solid content) is 10,000 mg / m². 2 A droplet of chloride-containing aqueous solution was applied to the specimen in the manner described. The distance between the spray nozzle and the evaluation surface of the test specimen (X in Figure 2) was 30 cm. The spray pressure of the spray nozzle was 0.2 MPa. The spraying time of the saltwater was 5 seconds. The amount of chloride deposited was calculated by determining the mass difference of the test specimen before and after the saltwater spraying, dividing it by the evaluation surface area of ​​the test specimen to calculate the amount of aqueous solution deposited, and then calculating the chloride concentration of the aqueous solution. In addition, the following conditions were adopted. Chloride-containing aqueous solution: 15% by mass NaCl aqueous solution is sprayed. Distance between shielding material and evaluation surface (Y in Figure 5): 5 cm Temperature: 22℃, Relative humidity: 50% Droplet distribution on the evaluation surface; Average contact area of ​​droplets: 1.3 mm² 2 Standard deviation of droplet contact area: 1.3 mm 2 Total contact area ratio of droplets: 71% Furthermore, the chloride deposition process (A) from the second time onward was carried out under the same conditions as the first chloride deposition process (A).

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

[0144] The water washing step (C) is a step of washing the evaluation surface of the metal material with water. The water washing method in the water washing step (C) is not particularly limited, but in this case, water was sprayed onto the evaluation surface of the metal material from a spray nozzle to wash the evaluation surface.

[0145] Furthermore, in this embodiment, the corrosion test cycle consisted of a chloride deposition process (A), followed by a corrosion process (B), and then a water washing process (C) before repeating the chloride deposition process (A). In other words, in this embodiment, the evaluation of delayed fracture characteristics was performed using the following corrosion test cycle. A cycle is repeated, consisting of a chloride deposition process (A) → corrosion process (B) → water washing process (C). In this embodiment, the period for evaluating delayed fracture resistance using a corrosion environment cycle was set to 63 days.

[0146] After the test, each test specimen was visually inspected for cracks. Then, the delayed fracture resistance in a corrosive environment was evaluated according to the following criteria. In this invention, a sample that showed no cracks when subjected to a stress equivalent to YS was judged to have excellent delayed fracture resistance in a corrosive environment. ◎ (Pass, particularly excellent): No cracking in samples subjected to stress equivalent to YS and TS. ○ (Pass, Excellent): No cracks were found in the sample subjected to stress equivalent to YS. × (Failure): Cracking was observed in both samples subjected to stress equivalent to YS and TS.

[0147] The examples of the present invention shown in Table 3 (Tables 3-1 and 3-2) have a tensile strength TS of 1320 MPa or higher, a yield ratio YR of 75% or higher, and excellent tensile flange properties, fatigue resistance of the shear end face, and delayed fracture resistance in corrosive environments, whereas the comparative examples were inferior in at least one of these aspects.

[0148] Furthermore, it was found that members obtained by forming and joining using the steel plate of the present invention example exhibited the same characteristics as the steel plate of the present invention: a tensile strength TS of 1320 MPa or higher, a yield ratio YR of 75% or higher, and excellent stretch flange properties, fatigue resistance of the shear end face, and delayed fracture resistance in corrosive environments.

[0149] [Table 3-1]

[0150] [Table 3-2]

Claims

1. In mass percent, 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 It contains, with the remainder being Fe and unavoidable impurities, and has a component composition. At the point where the plate thickness is 1 / 4, 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%. Having an organization that satisfies the following equations (1) and (2), Steel plates with a thickness of 0.5 to 3.0 mm. 25%≦P(C)≦70%...(1) {P(S)-P(C)}≦20%...(2) During the ceremony, P(S): The average occupancy rate of packets having the maximum occupancy rate within the prior austenite grains at a depth of 100 μm from the surface of the steel plate. P(C): This is the average occupancy rate of packets having the maximum occupancy rate within the prior austenite grains at the center of the steel plate thickness.

2. The aforementioned component composition is further expressed 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, comprising at least one element selected from among the following.

3. The steel sheet according to claim 1, wherein the steel sheet surface has a plating layer.

4. The steel sheet according to claim 2, wherein the steel sheet surface has a plating layer.

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

6. A cold-rolled sheet is made by hot-rolling, pickling, and cold-rolling a steel having the component composition described in claim 1 or 2. Annealing temperature T1: 800°C or higher, Heating is performed under conditions where the holding time t1 at the annealing temperature T1 is 10 seconds or more. Cooling with an average cooling rate CR1 of 5°C / s or more between 700°C and 600°C. Cooling from (Ms + 100°C) to a rapid cooling start temperature T2 which is between (Ms - 50°C) and (Ms + 50°C), with an average cooling rate CR2 of 5°C / s or more and 30°C / s or less, and Cooling is performed by water quenching with an average cooling rate CR3 of 300°C / s or more from the rapid cooling start temperature T2 to 80°C. Tempering temperature T3: 100°C or more and 400°C or less, The process includes an annealing step in which the heating is performed under the condition that the holding time t3 at the tempering temperature T3 is between 10 seconds and 10,000 seconds. During the cooling of the water quenching in the annealing process, pressure is applied to the front and back surfaces of the steel plate by two rolls placed on either side of the steel plate, and this pressure is applied under the following conditions: the distance between the two rolls in the steel plate conveying direction is 20 mm to 250 mm, and the applied pressure is 196 N or more. At the point where the plate thickness is 1 / 4, 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%. Having an organization that satisfies the following equations (1) and (2), A method for manufacturing steel plates with a thickness of 0.5 to 3.0 mm. 25%≦P(C)≦70%...(1) {P(S)-P(C)}≦20%...(2) During the ceremony, P(S): The average occupancy rate of packets having the maximum occupancy rate within the prior austenite grains at a depth of 100 μm from the surface of the steel plate. P(C): This is the average occupancy rate of packets having the maximum occupancy rate within the prior austenite grains at the center of the steel plate thickness.

7. A method for manufacturing a steel sheet according to claim 6, wherein a plating treatment is applied.

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

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