Steel sheet

Abstract

The present invention provides a high-strength steel sheet that has excellent hydrogen embrittlement resistance after spot welding because of a novel constitution.　This steel sheet is characterized by having a tensile strength of at least 1470 MPa and in that the roughness of at least one surface has an aspect ratio Str that satisfies 0<Str<0.60.

Description

steel plate

[0001] The present invention relates to a steel sheet.

[0002] In recent years, the automotive industry has been seeking to reduce the weight of vehicle bodies in order to improve fuel efficiency. Increasing the strength of the steel sheets used is one effective way to achieve both a lighter vehicle body and collision safety, and against this background, the development of high-strength steel sheets is underway.

[0003] On the other hand, spot welding is mainly used for welding in the manufacture and assembly of welded structural components for automobiles, etc., but when high-strength steel sheets are used for spot welding, hydrogen embrittlement cracking (sometimes called "delayed fracture") can occur. Hydrogen embrittlement cracking is a phenomenon in which a steel component subjected to high stress during use suddenly fractures due to hydrogen that has penetrated into the steel from the environment.

[0004] When high-strength steel sheets are spot-welded, water vapor in the atmosphere and oils, such as processing oils, adhering to the surface of the steel sheet are decomposed during welding to generate hydrogen gas, and it is believed that some of this hydrogen gas is adsorbed on the surface of the steel sheet and further decomposed, penetrating the steel sheet as hydrogen atoms. In particular, spot welds using high-strength steel sheets are highly susceptible to hydrogen embrittlement due to their high hardness and large tensile residual stress, making them areas prone to hydrogen embrittlement cracking.

[0005] Regarding hydrogen embrittlement cracking in spot welding using such high-strength steel plates, for example, Patent Document 1 discloses a steel plate for spot welding, in which the tensile strength of the steel plate is 980 MPa or more, the steel plate has a textured surface, and when any location on the steel plate surface is divided into concentric circles with diameters of 5 mm and 2 mm, recesses in the textured pattern form a plurality of continuous passages passing through the 2 mm diameter circle and penetrating the 5 mm diameter circle, the width of the recesses is 500 μm or less, and the passages serve as passages through which oil present on the mating surfaces of the steel plates during spot welding escapes to the outside of the contact portion of the mating surfaces of the steel plates. According to the steel plate for spot welding disclosed in Patent Document 1, when a welded structural member is manufactured by spot welding, "oil" is discharged to the outside of the weld through the continuous recesses formed in advance on the mating surfaces of the weld, making it difficult for hydrogen to penetrate the spot weld, and it is said that a welded structural member can be obtained in which hydrogen embrittlement cracking due to hydrogen that penetrates during welding is suppressed.

[0006] Patent No. 7163018

[0007] It is generally known that hydrogen embrittlement cracking is more likely to occur as the strength of steel increases. Meanwhile, the automotive industry is demanding further weight reduction of steel, and to achieve such weight reduction, steel needs to be made stronger than ever before. Therefore, there is a strong demand for steel that is less susceptible to hydrogen embrittlement cracking after spot welding, even when strength is increased to the same level as or greater than conventional steels, i.e., for high-strength steel sheets that have excellent hydrogen embrittlement resistance after spot welding.

[0008] Therefore, an object of the present invention is to provide a high-strength steel sheet having excellent hydrogen embrittlement resistance after spot welding, by using a novel structure.

[0009] The present invention includes at least the following aspects.

[0010] (Aspect 1) A steel sheet having a tensile strength of 1470 MPa or more, and characterized in that the aspect ratio Str of the roughness of at least one surface satisfies 0<Str<0.60.

[0011] (Aspect 2) The steel sheet according to Aspect 1, wherein the at least one surface includes a plurality of recesses, the plurality of recesses having an average depth of 1.00 to 10.00 μm and an average width of 1.00 to 20.00 μm, and an average spacing between adjacent recesses of the plurality of recesses is 1.00 to 30.00 μm.

[0012] (Embodiment 3) The steel sheet according to embodiment 1 or 2, wherein the arithmetic mean height Sa of the at least one surface satisfies 0.40 µm < Sa < 4.00 µm.

[0013] (Embodiment 4) The steel sheet according to any one of the above-mentioned embodiments 1 to 3, characterized in that the ratio HVs / HVb of the average Vickers hardness HVs from the at least one surface to 100 μm in the sheet thickness to the average Vickers hardness HVb at 1 / 2 the sheet thickness satisfies HVs / HVb<0.90.

[0014] (Embodiment 5) The chemical composition of the steel sheet is, in mass%, C: 0.15 to 0.60%, Si: 0.01 to 1.30%, Mn: 2.000 to 3.500%, P: 0.0001 to 0.0200%, S: 0.0001 to 0.0200%, Al: 0.001 to 1.000%, N: 0.0001 to 0.0200%, O: 0.0001 to 0.0200%, Co: 0 to 0.50%, Ni: 0 to 1.00%, Mo: 0 to 1.00%, Cr: 0 to 2.000%, Ti: 0 to 0.500%, B: 0 to 0.0100%, Nb: 0 to 0.500%, V 5. The steel sheet according to any one of Aspects 1 to 4, characterized in that the steel sheet contains: 0 to 0.500%, Cu: 0 to 0.500%, W: 0 to 0.100%, and the balance: Fe and impurities.

[0015] (Aspect 6) The steel sheet according to Aspect 5, characterized in that the chemical composition includes, in mass%, at least one of Co: 0.01 to 0.50%, Ni: 0.01 to 1.00%, Mo: 0.01 to 1.00%, Cr: 0.001 to 2.000%, O: 0.0001 to 0.0200%, Ti: 0.001 to 0.500%, B: 0.0001 to 0.0100%, Nb: 0.001 to 0.500%, V: 0.001 to 0.500%, Cu: 0.001 to 0.500%, and W: 0.001 to 0.100%.

[0016] (Aspect 7) The steel sheet according to any one of Aspects 1 to 6, characterized in that the metal structure of the steel sheet is, in area %, martensite: 85.0% or more, retained austenite: 1.0 to 7.0%, and balance: 10.0% or less.

[0017] (Embodiment 8) The steel sheet according to any one of the above-mentioned embodiments 1 to 7, characterized in that the steel sheet has a coating layer on at least one surface thereof, and the coating layer contains at least one metal selected from the group consisting of zinc, aluminum, magnesium, and alloys thereof.

[0018] According to the present invention, a high-strength steel sheet having excellent hydrogen embrittlement resistance after spot welding can be provided.

[0019] Fig. 1 is a schematic diagram for explaining a method for measuring the average depth, average width, and average spacing of recesses on the surface of a steel sheet. Fig. 2 is a graph showing an example of data in the Z direction obtained by the measurement method shown in Fig. 1. Fig. 3 is a perspective view schematically showing a spot-welded test piece 3 used for evaluating hydrogen embrittlement resistance.

[0020] Hereinafter, preferred embodiments of the steel sheet of the present invention will be described in detail. In this specification, various numerical ranges mean ranges including the upper and lower limit values ​​unless otherwise specified.

[0021] To achieve the above object, the present inventors conducted extensive research, particularly focusing on the surface morphology of steel sheets. Specifically, the present inventors conducted extensive research into the relationship between the amount of hydrogen penetration during spot welding and the surface roughness of steel sheets. As a result, the present inventors found that the amount of hydrogen penetration decreases as the aspect ratio Str of the surface roughness of the steel sheet approaches 0 (i.e., as the surface depressions approach a streak-like shape). The present inventors also found that the amount of hydrogen penetration decreases as the arithmetic mean height Sa of the steel sheet surface decreases. Although the mechanism behind these findings is unclear, it is thought that, for example, when the aspect ratio of the surface roughness of a steel sheet is close to 0, the streak-like depressions present on the surface of the steel sheet function as exhaust channels for "hydrogen gas" generated due to water vapor in the atmosphere and oils such as processing oil. The generated hydrogen gas is then released from the surface of the steel sheet through the streak-like depressions to the outside, thereby reducing the amount of hydrogen penetration. Furthermore, when the arithmetic mean height Sa of the surface of the steel sheet is small, oil such as processing oil, which is one of the causes of hydrogen gas generation, is less likely to accumulate on the surface of the steel sheet, and this is thought to result in a smaller amount of hydrogen gas generated and a reduced amount of hydrogen penetration.

[0022] The present invention has been completed based on these findings and includes the following embodiments.

[0023] <Steel Plate> A steel plate according to one embodiment of the present invention has a unique characteristic configuration in which the tensile strength is 1470 MPa or more and the aspect ratio Str of the roughness of at least one surface satisfies 0<Str<0.60.

[0024] As described above, it is known that hydrogen embrittlement cracking is more likely to occur as the strength of the steel increases. In particular, hydrogen embrittlement cracking is more likely to occur in steel sheets with extremely high strength, such as a tensile strength of 1470 MPa or more. However, even if the steel sheet of this embodiment has an extremely high strength, such as a tensile strength of 1470 MPa or more, the surface roughness aspect ratio Str is within the above-mentioned specific range, i.e., the steel sheet has specific streak-like recesses on its surface. These specific streak-like recesses function as exhaust channels for "hydrogen gas" generated during spot welding due to water vapor in the atmosphere and oils such as processing oil, allowing the hydrogen gas generated during spot welding to be discharged from the surface of the steel sheet to the outside. As a result, the steel sheet of this embodiment can reduce the amount of hydrogen penetration into the steel sheet during spot welding while maintaining high strength, making it less likely to cause hydrogen embrittlement cracking after welding. In other words, the steel sheet of this embodiment can exhibit excellent hydrogen embrittlement resistance after spot welding while maintaining high strength.

[0025] Hereinafter, each component of the steel sheet of this embodiment will be described in detail.

[0026] [Tensile strength: 1470 MPa or more] The steel sheet of this embodiment has a tensile strength of 1470 MPa or more. The tensile strength is preferably 1660 MPa or more, 1700 MPa or more, 1760 MPa or more, 1800 MPa or more, 1900 MPa or more, or 2000 MPa or more. Despite having such an extremely high tensile strength, the steel sheet of this embodiment can exhibit excellent hydrogen embrittlement resistance after spot welding because the aspect ratio Str of the surface roughness of the steel sheet is within the specific range as described above, i.e., the steel sheet has specific streak-like recesses on its surface.

[0027] The upper limit of the tensile strength of the steel sheet may be, for example, 2500 MPa or less, 2300 MPa or less, 2200 MPa or less, or 2100 MPa or less, from the viewpoint of workability and the like.

[0028] The tensile strength of a steel sheet is measured by conducting a tensile test in accordance with JIS Z 2241:2022 using a JIS No. 5 test piece taken from a steel sheet with its longitudinal direction preferably parallel to the directions perpendicular to the rolling direction and the thickness direction of the steel sheet. Even if the rolling direction of the steel sheet cannot be specified, the effects of the present invention can be enjoyed as long as the steel sheet has the above-mentioned tensile strength in any direction within the steel sheet surface. If the steel sheet has a coating layer such as a plating layer on its surface, the coating layer is removed by mechanical grinding before conducting the tensile test.

[0029] If a JIS No. 5 test piece cannot be obtained from the welded steel plate, the Vickers hardness of the steel plate can be measured first, and the tensile strength (MPa) of the steel plate can be calculated from the Vickers hardness using the formula: tensile strength (MPa) = 3.3 × Vickers hardness (HV). The Vickers hardness of the steel plate is measured using a method conforming to JIS Z 2244-1:2020 "Vickers Hardness Test - Part 1: Test Method." To measure the Vickers hardness of a steel plate, a sample for the Vickers hardness test is first obtained from a location on the steel plate to be measured that is considered to be free from the effects of press processing or heat from the welded portion. The direction of the sample cross section within the steel plate plane is not important. The sample cross section is then polished to a mirror finish. Then, Vickers hardness measurements are performed on the sample cross section at five or more locations at 1 / 2 the plate thickness of the steel plate, for example, under a test load of 1 kgf. The average value of the Vickers hardness measured at five or more positions is defined as the Vickers hardness (HV) of the steel plate. The Vickers hardness measurements at the five or more positions are performed at positions spaced apart by at least three times the indentation size. Here, the distance three times the indentation size means a distance three times the diagonal length of the rectangular opening of the indentation made by the diamond indenter during the Vickers hardness measurement.

[0030] [Surface Roughness Aspect Ratio Str: 0<Str<0.60] In the steel sheet of this embodiment, the roughness aspect ratio Str of at least one surface satisfies 0<Str<0.60. The surface roughness aspect ratio Str may be 0<Str<0.60 for only one surface (i.e., one side) of the steel sheet, or may be 0<Str<0.60 for both surfaces (i.e., both sides) of the steel sheet. In any case, the effects of the present invention can be achieved by arranging the steel sheet so that the surface with controlled Str becomes the overlapping surface of the steel sheets in the weld during spot welding.

[0031] In this embodiment, the region on the surface of the steel sheet where the aspect ratio Str of the surface roughness satisfies 0<Str<0.60 may be the entire surface of the steel sheet, or may be at least a portion including the region to be welded by spot welding, i.e., the region to become the weld. In either case, the effects of the present invention can be enjoyed by positioning the steel sheet during spot welding so that the region where Str is controlled overlaps the region to become the weld and is located on the overlapping surface of the steel sheets in the weld.

[0032] The aspect ratio Str of the surface roughness of a steel plate is an index that takes a value in the range of 0 to 1 and represents the anisotropy of the surface unevenness of the steel plate. The aspect ratio Str of the surface roughness of a steel plate is one of the spatial parameters of the surface shape defined in JIS B0681-2:2018, 4.2.2, "Texture aspect ratio," and indicates the strength of the surface anisotropy, taking a value in the range of 0 to 1. When this aspect ratio Str is close to 0, the surface shape is considered to be a highly anisotropic surface shape such as a streak pattern, and when the aspect ratio Str is close to 1, the surface shape is considered to be an isotropic surface shape that is not direction-dependent.

[0033] As described above, the steel sheet of this embodiment has a surface roughness aspect ratio Str within the specific range of 0 < Str < 0.60, i.e., has specific streak-like recesses on the surface of the steel sheet, and these specific streak-like recesses function as exhaust channels for "hydrogen gas" generated during spot welding due to water vapor in the atmosphere and oils such as processing oil, allowing the hydrogen gas generated during spot welding to be discharged from the surface of the steel sheet to the outside. This makes it possible for the steel sheet of this embodiment to reduce the amount of hydrogen that penetrates into the steel sheet during spot welding, and as a result, makes it less likely for hydrogen embrittlement cracking to occur after welding.

[0034] Furthermore, when the steel sheet has a coating layer such as a plating layer, the aspect ratio Str of the surface roughness of the base steel sheet after the coating layer such as the plating layer is removed by acid stripping satisfies 0 < Str < 0.60. Since the coating layer such as the plating layer dissolves and evaporates during welding, if the aspect ratio Str of the surface roughness of the base steel sheet is within the above-mentioned specific range, hydrogen gas generated during spot welding can be discharged from the surface of the base steel sheet to the outside, and the amount of hydrogen penetrating into the base steel sheet during spot welding can be reduced.

[0035] The aspect ratio Str of the surface roughness of the steel sheet is preferably 0.15 or more, 0.20 or more, or 0.25 or more. Furthermore, the aspect ratio Str of the surface roughness of the steel sheet is preferably 0.55 or less, 0.50 or less, or 0.45 or less. In particular, when the steel sheet has a coating layer such as a plating layer, i.e., when hydrogen embrittlement cracking is more likely to occur, it is preferable that the aspect ratio Str of the surface roughness of the base steel sheet after removing the coating layer such as a plating layer is 0.15 or more and / or 0.55 or less.

[0036] The aspect ratio Str of the surface of a steel sheet is measured as follows. First, a test piece of any size sufficient to ensure an observation area with a laser microscope, as described below, is taken from the steel sheet to be measured. The test piece is taken from a position at least 100 mm away from the end face of the steel sheet. If a coating layer, such as a plating layer, is present on the surface of the steel sheet, the coating layer is removed from the surface of the steel sheet by acid stripping. When removing the coating layer by acid stripping, an inhibitor is added to minimize dissolution of Fe in the base steel sheet by the acid. Next, using a laser microscope (e.g., Keyence Corporation's "VK-X3000"), the surface irregularities of the test piece are measured in an 8 mm x 8 mm area. The measurement conditions are a measurement magnification of 20x, resolution of 5 μm in the X and Y directions, and resolution of 0.1 nm in the Z direction, and the measurement is performed by connecting the measurement range. The X and Y directions are two perpendicular directions in the plane of the steel plate, and the Z direction is the thickness direction of the steel plate. The entire measurement area is then subjected to a filtering process (i.e., a low-pass filter λs of 0.25 mm) to remove irregularities with a period of 0.25 mm or less, and the aspect ratio Str is calculated. The aspect ratio Str obtained in this way is defined as the aspect ratio Str of the surface of the steel plate.

[0037] [Multiple Recesses: Average Depth 1.00 to 10.00 μm, Average Width 1.00 to 20.00 μm, Average Spacing 1.00 to 30.00 μm] The aspect ratio Str of the surface roughness of the steel sheet is achieved by controlling the surface shape of the steel sheet, for example, by including multiple recesses of a specific shape. A specific example of such a specific shape is a structure in which the multiple recesses have an average depth of 1.00 to 10.00 μm and an average width of 1.00 to 20.00 μm, with the average spacing between adjacent recesses being 1.00 to 30.00 μm. When the surface of the steel sheet includes multiple recesses of such a specific shape, hydrogen gas generated during spot welding can be more easily discharged to the outside, further reducing the amount of hydrogen penetration into the steel sheet during spot welding. This makes it more difficult for hydrogen embrittlement cracking to occur after welding.

[0038] Identification of the recesses and measurement of the average depth, average width, and average spacing of the recesses are carried out as follows. Here, FIG. 1 is a schematic diagram for explaining a method for measuring the average depth, average width, and average spacing of recesses on the surface of a steel sheet. Furthermore, FIG. 2 is a graph schematically showing an example of Z-direction data obtained by the measurement method shown in FIG. 1. In the graph of FIG. 2, the vertical axis represents the Z-direction value Zd, and the horizontal axis represents the circumferential distance Tm (μm) from the measurement start point P1 in FIG. 1. FIG. 2 also shows the Z-direction value Zd in a partial section between the measurement start point P1 and the measurement end point P2. First, as shown in FIG. 1, an 8 mm × 8 mm field of view on the surface of the test piece 1 used in the above-mentioned aspect ratio Str measurement is observed at 20x magnification using a laser microscope (e.g., "VK-X3000" manufactured by Keyence Corporation), and Z-direction data is collected on a circumference with a diameter of 6 mm. The Z direction is the thickness direction of the steel sheet. Furthermore, at this time, data is collected on a circumference of 6 mm diameter from the measurement start point P1 to the measurement end point P2, as shown in FIG. 1, with the center of the 6 mm diameter circle aligned with the center of the 8 mm x 8 mm field of view. From this circumference data, Z-direction data Zd in the circumferential direction as shown in FIG. 2 is extracted. The Z-direction data Zd is the value of the depth position at the circumferential distance Tm from the measurement start point P1, and hereinafter will be simply referred to as the "Z-direction value." Furthermore, an average value Lave is calculated for the Z-direction values ​​from the measurement start point P1 to the measurement end point P2, as shown in FIG. 2. From the Z-direction value and its average value Lave thus obtained, recesses 2 are identified as follows, and the average depth, average width, and average spacing of recesses 2 are further calculated.

[0039] (Identification of recesses) As shown in Fig. 2, a region where the Z-direction value is lower than the average value Lave is defined as one recess 2. The number of recesses 2 is determined by counting one continuous region where the Z-direction value is lower than the average value Lave from the measurement start point P1 to the measurement end point P2 as one recess 2. For example, in the graph shown in Fig. 2 (part of the measurement range), the number of recesses 2 is 10.

[0040] 2 , the Z-direction value of a portion of one recess 2 where the Z-direction value is smallest is defined as the depth d of that recess 2. The depth d of each recess 2 from the measurement start point P1 to the measurement end point P2 is then measured, and the average value of the depths of each recess 2 on a circumference with a diameter of 6 mm is defined as the average depth of the plurality of recesses 2 in the steel plate 1.

[0041] (Average Width of Recesses) The width w of one recess 2 is defined as the distance between the ends of regions that are lower than the average value of the Z-direction value. The width w of each recess 2 is then measured from the measurement start point P1 to the measurement end point P2, and the average value of the widths of each recess 2 on a circumference with a diameter of 6 mm is defined as the average width of the multiple recesses 2 in the steel sheet 1.

[0042] (Average Spacing Between Recesses) The average spacing between adjacent recesses 2 among the plurality of recesses 2 is the value obtained by dividing the length of the circumference of a circle with a diameter of 6 mm by the number of recesses 2 from the measurement start point P1 to the measurement end point P2.

[0043] The means for controlling the aspect ratio Str of the roughness of the surface of the steel sheet and the shape of the recesses will be described in detail in the steel sheet manufacturing method described later.

[0044] [Arithmetic mean height Sa of at least one surface: 0.40 μm<Sa<4.00 μm] In the steel sheet of this embodiment, the arithmetic mean height Sa of at least one surface preferably satisfies 0.40 μm<Sa<4.00 μm. When the arithmetic mean height Sa of the steel sheet surface is within this specific range, oil such as processing oil, which is one of the causes of hydrogen gas generation, is less likely to accumulate on the surface of the steel sheet, thereby reducing the amount of hydrogen gas generated during spot welding and the amount of hydrogen penetration into the steel sheet. This makes it even less likely that hydrogen embrittlement cracking will occur after welding.

[0045] Furthermore, when the steel sheet has a coating layer such as a plating layer, the arithmetic mean height Sa of the surface of the base steel sheet after the coating layer such as the plating layer is removed by acid stripping satisfies 0.40 μm < Sa < 4.00 μm. Since the coating layer such as the plating layer dissolves and evaporates during welding, if the arithmetic mean height Sa of the surface of the base steel sheet is within the above-mentioned specific range, oil such as processing oil, which is one of the causes of hydrogen gas generation, is less likely to accumulate on the surface of the base steel sheet. This reduces the amount of hydrogen gas generated during spot welding, and the amount of hydrogen penetrating into the base steel sheet can be reduced.

[0046] The arithmetic mean height Sa of the surface of the steel sheet is preferably 0.60 or more, 0.70 or more, or 0.80 or more. Furthermore, the arithmetic mean height Sa of the surface of the steel sheet is preferably 3.80 or less, 3.60 or less, or 3.40 or less. In particular, when the steel sheet has a coating layer such as a plating layer, that is, when hydrogen embrittlement cracking is more likely to occur, it is preferable that the arithmetic mean height Sa of the surface of the base steel sheet after removing the coating layer such as a plating layer is 0.60 or more and / or 3.80 or less.

[0047] The arithmetic mean height Sa (μm) of the surface of a steel sheet is measured as follows. First, a test piece is taken from the steel sheet to be measured. The test piece is taken from a position at least 100 mm away from the end face of the steel sheet. If a coating layer such as a plating layer is present on the surface of the steel sheet, the coating layer is removed from the surface of the steel sheet by acid stripping. When removing the coating layer by acid stripping, an inhibitor is added to minimize dissolution of Fe in the base steel sheet by the acid. Next, using a laser microscope, the unevenness of the surface of the test piece is measured in an 8 mm × 8 mm area. The measurement conditions are a measurement magnification of 20x, resolution of 5 μm in the X and Y directions, and resolution of 0.1 nm in the Z direction, and the measurement is performed in a linked manner. Thereafter, the entire measurement area is subjected to a filtering process to remove irregularities with a period of 0.25 mm or less (i.e., a low-pass filter λs of 0.25 mm), and the arithmetic mean height Sa is determined in accordance with JIS B0681-2:2018, 4.1.7, "Arithmetic mean height of the scale limited surface." The arithmetic mean height Sa thus obtained is defined as the arithmetic mean height Sa of the surface of the steel sheet.

[0048] The means for controlling the arithmetic mean height Sa of the surface of the steel sheet will be described in detail in the steel sheet manufacturing method described later.

[0049] [Average Vickers Hardness Ratio HVs / HVb: HVs / HVb<0.90] In the steel sheet of this embodiment, it is preferable that the ratio HVs / HVb of the average Vickers hardness HVs from at least one surface (if a coating layer such as a plating layer is present on the surface of the steel sheet, the interface between the coating layer and the steel sheet) to 100 μm in the sheet thickness to the average Vickers hardness HVb at a position halfway through the sheet thickness satisfies HVs / HVb<0.90.

[0050] Generally, high-strength steel sheets having a strength of 1470 MPa or more have high hardenability and therefore tend to have high strength in welds after spot welding. Furthermore, areas with such high strength are highly susceptible to hydrogen embrittlement and are prone to hydrogen embrittlement cracking. Therefore, it is desirable to suppress the strength of welds with such high strength within an appropriate range. Therefore, in the steel sheet of this embodiment, the surface layer is decarburized to control the hardness within an appropriate range. More specifically, the average Vickers hardness HVs from at least one surface (preferably both surfaces) of the steel sheet to 100 μm in thickness is controlled to less than 0.90 times the average Vickers hardness HVb at the half-thickness position. This softens the surface layer of the steel sheet and reduces the susceptibility to hydrogen embrittlement in the welds, thereby enabling the steel sheet to exhibit even better hydrogen embrittlement resistance.

[0051] As described above, the relationship between the average Vickers hardness HVs from the surface of the steel sheet to 100 μm in thickness and the average Vickers hardness HVb at half the thickness position preferably satisfies HVs / HVb<0.90 on both sides of the steel sheet, but it may also satisfy HVs / HVb<0.90 on only one side of the steel sheet. Even if only one side of the steel sheet satisfies HVs / HVb<0.90, the same effect can be achieved by positioning the steel sheet so that the surface with the controlled hardness of the surface layer becomes the overlapping surface of the steel sheets in the weld during spot welding.

[0052] From the viewpoint of further improving the above-mentioned effects, it is preferable that the average Vickers hardness HVs from the surface of the steel sheet to 100 μm in thickness be smaller than the average Vickers hardness HVb at 1 / 2 the sheet thickness. For example, HVs may be 0.85 times or less, 0.80 times or less, 0.75 times or less, or 0.70 times or less of HVb. Meanwhile, any appropriate value can be selected as the lower limit. For example, HVs may be 0.20 times or more, 0.25 times or more, 0.30 times or more, 0.40 times or more, 0.45 times or more, or 0.50 times or more of HVb.

[0053] (Measurement of HVs and HVb) The average Vickers hardness HVs from the surface of the steel sheet to a thickness of 100 μm and the average Vickers hardness HVb at a position halfway through the thickness are measured as follows: The Vickers hardness test is performed in accordance with JIS Z 2244-1:2020.

[0054] First, a test specimen is cut from an arbitrary position at least 50 mm away from the end of the steel sheet so that a cross section perpendicular to the surface (i.e., a cross section through the sheet thickness) can be observed. The Vickers hardness is measured at 10 depth positions, which are divided into 9 equal parts from a depth position of 2 μm to a depth position of 100 μm from the surface of the test specimen (if a coating layer such as a plating layer is present on the surface of the test specimen, the interface between the coating layer and the steel sheet). This Vickers hardness measurement is performed with an indentation load of 10 gf. The average value of the Vickers hardness measured at the 10 depth positions in this way is taken as the average Vickers hardness HVs in the region from the surface to 100 μm in sheet thickness.

[0055] Next, the Vickers hardness is measured at five or more (e.g., ten) positions at half the thickness of the test piece under a test load of 10 gf. The average value of the Vickers hardness values ​​measured at the five or more measurement positions is defined as the average Vickers hardness HVb at half the thickness.

[0056] The distance between each measurement point at the half-thickness position is set to be at least three times the size of the indentation.

[0057] Using the average Vickers hardness HVs from the surface of the steel plate obtained as described above to a thickness of 100 μm and the average Vickers hardness HVb at a position halfway through the thickness, HVs / HVb can be calculated.

[0058] As described above, the steel sheet of this embodiment has a tensile strength of 1470 MPa or more, and the aspect ratio Str of the roughness of at least one surface satisfies 0 < Str < 0.60. As long as the steel sheet has the unique characteristic configuration, the chemical composition and metal structure of the steel sheet are not particularly limited.

[0059] A preferred chemical composition of the steel sheet of this embodiment will be described in detail below. However, the following description is intended merely as an example of a preferred chemical composition and is not intended to limit the steel sheet of the present invention to a steel sheet having such a specific chemical composition. Furthermore, in the following description, the unit of content of each element, "%," means "mass %" unless otherwise specified. Furthermore, in this specification, the term "to" indicating a numerical range means that the numerical values ​​before and after the range are included as the lower and upper limits, unless otherwise specified.

[0060] [Chemical Composition] In this embodiment, the chemical composition of the steel sheet is, in mass%, C: 0.15 to 0.60%, Si: 0.01 to 1.30%, Mn: 2.000 to 3.500%, P: 0.0001 to 0.0200%, S: 0.0001 to 0.0200%, Al: 0.001 to 1.000%, N: 0.0001 to 0.0200%, O: 0.0001 to 0.0200%, Co: 0 to 0.50%, Ni: 0 to 1.00%, Mo: 0 to 1.00%, Cr: 0 to 2.000%, Ti: 0 to 0.500%, B: 0 to 0.0100%, Nb: 0 to 0.500%, V : 0 to 0.500%, Cu: 0 to 0.500%, W: 0 to 0.100%, and the balance: Fe and impurities.

[0061] These elements are described in more detail below.

[0062] [C: 0.15 to 0.60%] C is an element that is effective for increasing tensile strength inexpensively. To fully obtain these effects, the C content is set to 0.15% or more. The C content may be 0.20% or more, 0.25% or more, 0.30% or more, or 0.35% or more. On the other hand, excessive C content may degrade weldability. For this reason, the C content is set to 0.60% or less. The C content may be 0.55% or less, 0.50% or less, 0.45% or less, or 0.40% or less.

[0063] [Si: 0.01 to 1.30%] Si acts as a deoxidizer and is an element that affects the morphology of carbides and retained austenite after heat treatment. Without Si, it may be difficult to suppress the generation of coarse oxides. Therefore, the Si content is set to 0.01% or more. The Si content may be 0.05% or more, 0.10% or more, 0.30% or more, or 0.50% or more. On the other hand, excessive Si content may decrease local ductility. Therefore, the Si content is set to 1.30% or less. The Si content may be 1.20% or less, 1.00% or less, 0.80% or less, or 0.60% or less.

[0064] [Mn: 2.000 to 3.500%] Mn is an element effective in improving the hardenability of steel and increasing the strength of steel plate. Mn is also an element effective in stabilizing austenite. To fully obtain these effects, the Mn content is set to 2.000% or more. The Mn content may be 2.100% or more, 2.200% or more, 2.300% or more, 2.400% or more, or 2.500% or more. On the other hand, excessive Mn content not only promotes co-segregation with P and S, but may also deteriorate corrosion resistance. For this reason, the Mn content is set to 3.500% or less. The Mn content may be 3.400% or less, 3.300% or less, 3.200% or less, 3.100% or less, or 3.000% or less.

[0065] [P: 0.0001 to 0.0200%] P is an element that embrittles welds and deteriorates platability. Therefore, the P content is set to 0.0200% or less. The P content may be 0.0180% or less, 0.0150% or less, 0.0120% or less, or 0.0100% or less. A lower P content is preferable, but reducing the P content to less than 0.0001% requires a long refining time, resulting in a significant increase in costs. Therefore, the P content is set to 0.0001% or more. The P content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more.

[0066] [S: 0.0001 to 0.0200%] S is an element that forms non-metallic inclusions such as MnS in steel. Excessive S content significantly increases the formation of non-metallic inclusions that serve as crack initiation sites during cold working. For this reason, the S content is set to 0.0200% or less. The S content may be 0.0180% or less, 0.0150% or less, 0.0120% or less, or 0.0100% or less. A lower S content is preferable, but reducing the S content to less than 0.0001% requires a long refining time, resulting in a significant increase in costs. For this reason, the S content is set to 0.0001% or more. The S content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more.

[0067] [Al: 0.001 to 1.000%] Al is an element that acts as a deoxidizer for steel. To fully obtain this effect, the Al content is set to 0.001% or more. The Al content may be 0.005% or more, 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, excessive Al content may generate coarse Al oxides that may become the starting point for cracks. For this reason, the Al content is set to 1.000% or less. The Al content may be 0.950% or less, 0.900% or less, 0.800% or less, or 0.600% or less.

[0068] [N: 0.0001 to 0.0200%] N is an element that causes blowholes during welding. Therefore, the N content is set to 0.0200% or less. The N content may be 0.0180% or less, 0.0160% or less, 0.0120% or less, or 0.0100% or less. The lower the N content, the more preferable it is, but reducing the N content to less than 0.0001% results in a significant increase in manufacturing costs. Therefore, the N content is set to 0.0001% or more. The N content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more.

[0069] [O: 0.0001 to 0.0200%] O is an element that causes blowholes during welding. Therefore, the O content is set to 0.0200% or less. The O content may be 0.0180% or less, 0.0150% or less, 0.0120% or less, or 0.0100% or less. The lower the O content, the more preferable it is, but reducing the O content to less than 0.0001% results in a significant increase in manufacturing costs. Therefore, the O content is set to 0.0001% or more. The O content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more.

[0070] In this embodiment, the preferred basic chemical composition of the steel sheet is as described above. Furthermore, in this embodiment, the steel sheet may contain at least one element selected from the following optional elements in place of a portion of the remaining Fe, as necessary. These optional elements will be described in detail below.

[0071] [Co: 0 to 0.50%] Co is an element effective for controlling the morphology of carbides and increasing the strength of steel sheet. The Co content may be 0%, but to obtain these effects, the Co content is preferably 0.001% or more. The Co content may be 0.01% or more, 0.02% or more, 0.05% or more, or 0.10% or more. On the other hand, excessive Co content may cause precipitation of coarse Co carbides. Therefore, the Co content is preferably 0.50% or less. The Co content may be 0.40% or less, 0.30% or less, or 0.20% or less.

[0072] [Ni: 0 to 1.00%] Ni is an element effective in increasing the strength of steel sheet. Ni is also effective in improving wettability and promoting alloying reactions. The Ni content may be 0%, but to obtain these effects, the Ni content is preferably 0.001% or more. The Ni content may be 0.01% or more, 0.02% or more, 0.05% or more, or 0.10% or more. On the other hand, excessive Ni content may decrease weldability. For this reason, the Ni content is preferably 1.00% or less. The Ni content may be 0.90% or less, 0.80% or less, 0.60% or less, or 0.30% or less.

[0073] [Mo: 0 to 1.00%] Mo is an element effective in increasing the strength of steel sheets. Furthermore, Mo has the effect of suppressing ferrite transformation that occurs during heat treatment in continuous annealing equipment or continuous hot-dip galvanizing equipment. The Mo content may be 0%, but to obtain these effects, the Mo content is preferably 0.001% or more. The Mo content may be 0.01% or more, 0.02% or more, 0.05% or more, or 0.08% or more. On the other hand, excessive Mo content may saturate the effect of suppressing ferrite transformation or may result in the formation of coarse intermetallic compounds and carbides. Therefore, the Mo content is preferably 1.00% or less. The Mo content may be 0.90% or less, 0.80% or less, 0.60% or less, or 0.30% or less.

[0074] [Cr: 0 to 2.000%] Cr is an element that suppresses pearlite transformation and is effective in increasing the strength of steel. The Cr content may be 0%, but to achieve this effect, the Cr content is preferably 0.001% or more. The Cr content may be 0.005% or more, 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, excessive Cr content may cause the formation of coarse Cr carbides in the central segregation region. Therefore, the Cr content is preferably 2.000% or less. The Cr content may be 1.800% or less, 1.500% or less, 1.000% or less, or 0.500% or less.

[0075] [Ti: 0 to 0.500%] Ti is an element that contributes to increasing the strength of steel sheets through precipitation strengthening, grain refinement due to suppression of ferrite grain growth, and dislocation strengthening through suppression of recrystallization. The Ti content may be 0%, but to obtain these effects, the Ti content is preferably 0.001% or more. The Ti content may be 0.005% or more, 0.010% or more, or 0.050% or more. On the other hand, excessive Ti content may increase the precipitation of carbonitrides. Therefore, the Ti content is preferably 0.500% or less. The Ti content may be 0.450% or less, 0.400% or less, 0.300% or less, or 0.100% or less.

[0076] [B: 0 to 0.0100%] B is an element that suppresses the formation of ferrite and pearlite during cooling from the austenite temperature range and promotes the formation of low-temperature transformation structures such as martensite. B is also a beneficial element for increasing the strength of steel. While the B content may be 0%, to obtain these effects, the B content is preferably 0.0001% or more. The B content may be 0.0003% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive B content may cause the formation of coarse B oxides in the steel. Therefore, the B content is preferably 0.0100% or less. The B content may be 0.0080% or less, 0.0060% or less, 0.0050% or less, or 0.0020% or less.

[0077] [Nb: 0 to 0.500%] Nb is an element effective in controlling the morphology of carbides and is also effective in improving toughness by refining the structure. The Nb content may be 0%, but to obtain these effects, the Nb content is preferably 0.001% or more. The Nb content may be 0.002% or more, 0.010% or more, or 0.020% or more. On the other hand, excessive Nb content may cause the formation of coarse Nb carbides. For this reason, the Nb content is preferably 0.500% or less. The Nb content may be 0.450% or less, 0.400% or less, 0.300% or less, or 0.100% or less.

[0078] [V: 0 to 0.500%] V is an element that contributes to increasing the strength of steel sheet through precipitation strengthening, grain refinement due to suppression of ferrite grain growth, and dislocation strengthening through suppression of recrystallization. The V content may be 0%, but to obtain these effects, the V content is preferably 0.001% or more. The V content may be 0.002% or more, 0.010% or more, or 0.020% or more. On the other hand, excessive V content may increase the precipitation of carbonitrides. Therefore, the V content is preferably 0.500% or less. The V content may be 0.450% or less, 0.400% or less, 0.300% or less, or 0.100% or less.

[0079] [Cu: 0 to 0.500%] Cu is an element effective in improving the strength of steel sheet. The Cu content may be 0%, but to obtain this effect, the Cu content is preferably 0.001% or more. The Cu content may be 0.002% or more, 0.010% or more, or 0.030% or more. On the other hand, excessive Cu content may embrittle the steel material during hot rolling, making hot rolling difficult. For this reason, the Cu content is preferably 0.500% or less. The Cu content may be 0.450% or less, 0.400% or less, 0.300% or less, or 0.100% or less.

[0080] [W: 0 to 0.100%] W is an element effective in increasing the strength of steel sheet. W also forms precipitates and crystallized products. W-containing precipitates and crystallized products act as hydrogen trapping sites, so W is an element effective in improving hydrogen embrittlement resistance. The W content may be 0%, but to obtain these effects, the W content is preferably 0.001% or more. The W content may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, excessive W content may cause the formation of coarse W precipitates or crystallized products. Therefore, the W content is preferably 0.100% or less. The W content may be 0.080% or less, 0.060% or less, 0.050% or less, or 0.030% or less.

[0081] Regarding the above optional elements, in this embodiment, the chemical composition of the steel sheet may include, in mass%, at least one of Co: 0.01 to 0.50%, Ni: 0.01 to 1.00%, Mo: 0.01 to 1.00%, Cr: 0.001 to 2.000%, O: 0.0001 to 0.0200%, Ti: 0.001 to 0.500%, B: 0.0001 to 0.0100%, Nb: 0.001 to 0.500%, V: 0.001 to 0.500%, Cu: 0.001 to 0.500%, and W: 0.001 to 0.100%.

[0082] [Balance: Fe and Impurities] In the steel sheet of this embodiment, the balance other than the above elements consists of Fe and impurities. Here, the impurities refer to components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when industrially manufacturing the steel sheet.

[0083] The chemical composition of steel plate may be measured by a common analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) on chips in accordance with JIS G 1201:2014. Specifically, for example, a 35 mm square test piece is obtained from the steel plate at approximately half the plate thickness position, and the composition is determined by measuring it using a measuring device such as Shimadzu Corporation's ICPS-8100 under conditions based on a pre-created calibration curve. C and S, which cannot be measured by ICP-AES, may be measured using the combustion-infrared absorption method, N may be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-non-dispersive infrared absorption method. If the steel plate has a coating layer on its surface, the coating layer may be removed by mechanical grinding or the like before analyzing the chemical composition.

[0084] Next, a preferred metallographic structure of the steel sheet of this embodiment will be described in detail. However, the following description is intended merely as an example of a preferred metallographic structure and is not intended to limit the steel sheet of the present invention to a steel sheet having such a specific metallographic structure. In the following description, the unit of microstructure fraction, "%," means "area %" unless otherwise specified. Furthermore, the metallographic structure is controlled in the surface layer portion, the 1 / 4 thickness position, and the 1 / 2 thickness position of the steel sheet. Here, the surface layer portion refers to a position 50 μm deep in the thickness direction from the surface of the steel sheet. Hereinafter, unless otherwise specified, the microstructure fraction refers to the average value of the microstructure fractions measured in the surface layer portion, the 1 / 4 thickness position, and the 1 / 2 thickness position. In addition, when a coating layer such as a plating layer is provided on the surface of the steel sheet, the position in the thickness direction of the region excluding the coating layer is specified.

[0085] [Metal Structure] The steel sheet of this embodiment preferably has a metal structure consisting of, by area percentage, martensite: 85.0% or more, retained austenite: 1.0 to 7.0%, and balance: 10.0% or less.

[0086] These metal structures will be described in detail below.

[0087] (Martensite: 85.0% or more) In this embodiment, the metal structure of the steel plate preferably contains, in area %, 85.0% or more of martensite. By containing 85.0% or more of martensite, it becomes easier to achieve a tensile strength of 1470 MPa or more. From the viewpoint of increasing strength, the higher the area fraction of martensite, the more preferable it is, and it may be, for example, 87.0% or more, 90.0% or more, 92.0% or more, or 95.0% or more. The area fraction of martensite may be 99.0% or less, or 97.0% or less.

[0088] In this specification, "martensite" includes not only as-quenched martensite (so-called fresh martensite) but also tempered martensite.

[0089] (Residual austenite: 1.0 to 7.0%) In this embodiment, the metal structure of the steel sheet preferably contains, in area %, 1.0 to 7.0% retained austenite. By containing 1.0 to 7.0% retained austenite, even if hydrogen penetrates into the steel, the penetrated hydrogen can be appropriately trapped by the retained austenite, making it possible to more significantly suppress the occurrence of hydrogen embrittlement cracking even when the tensile strength is high, such as 1470 MPa or more. From the viewpoint of improving hydrogen embrittlement resistance, the higher the area fraction of retained austenite, the more preferable it is, and it may be, for example, 2.0% or more, 3.0% or more, or 4.0% or more. Furthermore, the area fraction of retained austenite may be 6.0% or less or 5.0% or less.

[0090] (Remaining structure: 10.0% or less) The remaining structure other than martensite and retained austenite may have an area ratio of 0%, but if a remaining structure is present, it is preferable that the area ratio of the remaining structure be 10.0% or less. From the viewpoint of improving strength and hydrogen embrittlement resistance, the area ratio of the remaining structure may be 8.0% or less, 6.0% or less, 5.0% or less, 4.0% or less, 3.0% or less, or 2.0% or less. On the other hand, achieving an area ratio of the remaining structure of 0% requires advanced control in the steel plate manufacturing process, which may result in a decrease in yield. Therefore, the area ratio of the remaining structure may be 0.5% or more, or 1.0% or more.

[0091] The remaining structure is at least one of ferrite, bainite, and pearlite, or a structure containing at least one of these.

[0092] <Identification and Calculation of Metallographic Structure> (Martensite) Identification and calculation of the metallographic structure are performed as follows. First, a sample having a thickness cross section perpendicular to the surface of the steel plate is collected, and the thickness cross section of the sample is used as the observation surface. The thickness cross section is polished carefully so as not to leave scratches due to polishing. Then, strain introduced by surface polishing is removed by chemical polishing to obtain a cross-sectional observation sample for EBSD (Electron-Back-Scatter-Diffraction) analysis, in which unintended changes in the crystal structure due to polishing have not occurred. Here, scratches due to polishing refer to a linear region observed in an IPF (Inverse-Pole-Figure) map in EBSD analysis, penetrating the boundary between regions with different crystal orientations. Since such scratches are not inherent to the structure, it is necessary to ensure that they are not included in the observation field. It is preferable to search for a field of view that is free of scratches, or if scratches inevitably appear in the field of view, to polish again.

[0093] The above sample for EBSD analysis is subjected to electron backscattering analysis. The EBSD analysis conditions are not particularly limited as long as they are within the common knowledge of those skilled in the art, but an example of detailed conditions will be described below.

[0094] First, a sample is inserted into an FE-SEM capable of EBSD measurement, and then the polished surface of the sample is tilted 60 to 70 degrees relative to the direction of incidence of the electron beam. The sample must be tilted so that the cross section of the object to be measured faces the EBSD detector that will be inserted later.

[0095] After tilting, the EBSD detector is inserted into the FE-SEM chamber and brought close to the sample. The EBSD detector can be positioned where it has high sensitivity to detect electron beams, but it is desirable to bring it as close as possible without hitting the inside of the FE-SEM chamber, the sample, or the jig or table that holds the sample.

[0096] Thereafter, the electron beam detection sensitivity of the EBSD detector is adjusted. The adjustment of the detection sensitivity varies depending on the performance of the electron gun of the FE-SEM and the EBSD detector used, and therefore it is desirable to perform the adjustment within the scope of common sense of a person skilled in the art. It is sufficient if the adjustment results in conditions that allow the EBSD pattern to be clearly observed.

[0097] Then, the electron beam is irradiated onto the observation field at step size intervals of 0.3 μm, and an EBSD pattern is collected at each measurement point. Based on the EBSD pattern at each measurement point, indexing and calculation of crystal orientation are performed. For indexing and calculation of crystal orientation, it is preferable to use APEX software manufactured by AMETEK Corporation.

[0098] The EBSD data thus obtained is analyzed using version 7 or later of OIM Analysis software (Orientation Imaging Microscopy), an EBSD data analysis software manufactured by AMETEK. The obtained EBSD data is opened in OIM Analysis, and only regions with a CI value (Confidence Index) of 0.1 or more are extracted.

[0099] The CI value is an index of the reliability of the indexing and crystal orientation analysis results. Areas with a CI value lower than 0.1 are likely to be areas where the orientations of impurities on the sample surface or grain boundaries overlap during electron beam irradiation, and can be considered to be areas that do not have the original crystal orientation of the metal structure.

[0100] Then, regions with a GAM (Grain Average Misorientation) value of 0.5° or more are considered to be martensite, and the area ratio is calculated. Note that the grain boundary refers to a boundary line where the misorientation between measurement points is 15° or more. Here, the GAM value is the average misorientation between measurement points of the crystal orientation of the region surrounded by the grain boundary. Since structures formed at low temperatures such as martensite are characterized by the occurrence of misorientation within the grains due to transformation strain, etc., it is possible to distinguish them by the GAM value.

[0101] This operation is carried out at five or more locations within a 100×100 μm field of view centered at a position halfway through the sheet thickness from the surface of the steel sheet, and the arithmetic average area ratio is calculated.

[0102] The retained austenite is formed between the laths of martensite in sub-nano size. Therefore, it is difficult to separate the two with the resolution of EBSD analysis, and the area fraction determined by the above method is the combined area fraction of martensite and retained austenite. The area fraction of martensite at the half-thickness position is determined by subtracting the area fraction of retained austenite measured by the procedure described below from the area fraction determined by the above method.

[0103] As described above, in this specification, martensite refers to a structure including tempered martensite, and no particular distinction is made regarding the tempered state of martensite.

[0104] Next, the area ratio of martensite at the 1 / 4 thickness position and the surface layer portion is measured in the same manner. However, when measuring the area ratio of martensite at the surface layer portion, the observation region is a 50 μm (thickness direction) × 200 μm (direction perpendicular to the thickness direction) region centered at a depth of 50 μm from the surface of the steel plate in the thickness direction. Finally, the average value of the area ratios measured at the surface layer portion, the 1 / 4 thickness position, and the 1 / 2 thickness position is calculated, and this is defined as the area ratio of martensite.

[0105] (Retained Austenite) The area fraction of retained austenite is calculated by measurement using X-rays. First, the sample is removed from the surface to the half-thickness position in the thickness direction by mechanical polishing and chemical polishing. Next, the polished sample is subjected to MoKα radiation as characteristic X-rays, and the structural fraction of retained austenite is calculated from the integrated intensity ratio of the diffraction peaks of (200) and (211) of the bcc phase and (200), (220), and (311) of the fcc phase. This is the area fraction of retained austenite at the half-thickness position. The area fractions of retained austenite are obtained in the same manner for the surface layer portion and the quarter-thickness position, and finally, the arithmetic average value of these is calculated, and this is the area fraction of retained austenite. Note that when measuring the area fractions of retained austenite at the surface layer portion and the quarter-thickness position, the sample is removed from the surface to a predetermined depth position in the thickness direction by mechanical polishing and chemical polishing.

[0106] (Remaining structure) The area fraction of the remaining structure is obtained by subtracting the area fraction of martensite and the area fraction of retained austenite obtained as described above from 100%. As described above, the remaining structure is at least one of ferrite, bainite, and pearlite, or a structure containing at least one of these, but it is not necessary to specify these structure types or to determine the area fraction of each structure.

[0107] [Thickness] In this embodiment, the thickness of the steel plate can be appropriately determined depending on the type of final product, etc. The steel plate may have a thickness of 0.6 to 6.0 mm. The thickness of the steel plate may be 1.0 mm or more, 1.2 mm or more, or 1.4 mm or more. The thickness of the steel plate may also be 5.0 mm or less, 4.0 mm or less, 3.0 mm or less, or 2.5 mm or less.

[0108] [Coating Layer] In this embodiment, the steel sheet may have a coating layer on at least one surface, preferably both surfaces, for the purpose of improving corrosion resistance, etc. Typical examples of the coating layer include a plating layer. The coating layer may contain, for example, at least one metal selected from zinc, aluminum, magnesium, and alloys thereof. More specifically, the coating layer may be a hot-dip plating layer or an electroplating layer containing at least one of these metals.

[0109] Examples of the hot-dip plated layer include a hot-dip galvanized layer, a galvannealed layer, a hot-dip aluminum plated layer, a hot-dip Zn—Al alloy plated layer, a hot-dip Zn—Al—Mg alloy plated layer, a hot-dip Zn—Al—Mg—Si alloy plated layer, etc. Examples of the electroplated layer include an electrogalvanized layer, an electrogalvanized Zn—Ni alloy plated layer, etc. Preferably, the coating layer is a hot-dip galvanized layer, a galvannealed layer, or an electrogalvanized layer.

[0110] The coating weight of the coating layer is not particularly limited and may be a general coating weight. For example, the coating weight may be 20 g / m per side of the steel sheet. ２ Above, 25g / m ２ or more than 30 g / m ２ The coating amount of the coating layer is 120 g / m per one side of the steel sheet. ２ Below, 110g / m ２ or less than 100 g / m ２ It may be the following:

[0111] The steel sheet of this embodiment is a high-strength steel sheet having an extremely high tensile strength of 1470 MPa or more as described above, yet has excellent hydrogen embrittlement resistance after spot welding, and is therefore very useful for, for example, automotive frame members and bumpers, and other structural and reinforcing members that require strength.

[0112] <Method for manufacturing steel sheet> Next, a preferred method for manufacturing a steel sheet according to one embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing a steel sheet according to one embodiment of the present invention, and is not intended to limit the steel sheet to one manufactured by the manufacturing method described below.

[0113] The method for producing a steel sheet of this embodiment includes a hot rolling step in which a slab having the chemical composition described above in relation to the steel sheet is heated to a temperature of 1100 to 1300°C, then finish-rolled under conditions of a final temperature of 850 to 1050°C, and the finish-rolled steel sheet is cooled to 500°C or less at an average cooling rate of 20°C / sec or more and coiled; a pickling step in which the obtained hot-rolled steel sheet is pickled; a cold rolling step in which the pickled hot-rolled steel sheet is cold-rolled at a reduction of 35 to 80%; The method includes an annealing step of heating the cold-rolled steel sheet and then holding it at a maximum heating temperature of 830 to 900°C for 20 to 150 seconds; a cooling step of cooling the obtained cold-rolled steel sheet from the maximum heating temperature to room temperature at an average cooling rate of 0.10°C / sec or more and 30.00°C / sec or less; and a surface shape adjustment step of imparting a predetermined uneven structure to at least one surface of the obtained steel sheet so that the aspect ratio Str of the roughness of at least one surface of the steel sheet satisfies 0 < Str < 0.60.

[0114] Furthermore, in the steel sheet manufacturing method of this embodiment, a decarburization treatment is performed in the annealing step. Specifically, in the annealing step, when the cold-rolled steel sheet is heated and held at the maximum heating temperature, the atmosphere is controlled to a hydrogen concentration of 2 vol% to 15 vol% and a dew point of -30°C to 15°C to decarburize the steel sheet. The atmosphere in the annealing step is a mixed gas composed of 2 vol% to 15 vol% hydrogen, 0.05 vol% to 4.0 vol% water vapor, and the remainder: nitrogen. Such a mixed gas can be obtained as follows. First, dry nitrogen gas is bubbled in water adjusted to a predetermined temperature. The number of bubbling times and the water temperature are not particularly limited as long as the water vapor is saturated in the nitrogen gas. In this way, nitrogen gas saturated with water vapor is obtained. Then, by mixing this water vapor-saturated nitrogen gas with dry nitrogen gas and hydrogen gas while adjusting the volume concentration, an atmosphere with a desired dew point can be obtained.

[0115] Furthermore, the method for manufacturing a steel sheet according to this embodiment may include, in addition to the above-described steps, a coating step of forming a coating layer on at least one surface of the steel sheet.

[0116] Preferred conditions for these steps will be described below.

[0117] [Hot Rolling Step] In the method for producing a steel sheet according to this embodiment, the hot rolling step is a step of hot rolling a slab. In this step, a slab having the chemical composition described above in relation to the steel sheet is first heated to a temperature of 1100 to 1300°C. Next, the slab is finish-rolled under conditions of a final temperature of 850 to 1050°C. The finish-rolled steel sheet is then cooled to 500°C or less at an average cooling rate of 20°C / s or more and coiled.

[0118] The slab used in the hot rolling process is preferably cast by a continuous casting method from the viewpoint of productivity, but may also be produced by an ingot casting method or a thin slab casting method. The slab used contains a relatively large amount of alloying elements in order to obtain a high-strength steel plate. Therefore, the slab needs to be heated before being subjected to hot rolling to dissolve the alloying elements in the slab. If the heating temperature of the slab is 1100°C or higher, the alloying elements are sufficiently dissolved in the slab, making it difficult for coarse alloy carbides to remain, thereby making it difficult for embrittlement cracking to occur during hot rolling. The upper limit of the heating temperature of the slab is preferably 1300°C or lower from the viewpoint of the capacity of the heating equipment and productivity.

[0119] (Rough Rolling) In the hot rolling process, the heated slab is subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc. The rough rolling is performed under conditions that ensure the desired sheet bar dimensions.

[0120] (Finish Rolling) The heated slab, or the slab that has been rough-rolled as needed, is then subjected to finish rolling. The slab used contains a relatively large amount of alloying elements. Therefore, it is necessary to increase the rolling load during hot rolling. For this reason, hot rolling is performed at a high temperature. The end temperature of finish rolling is particularly important in terms of controlling the metal structure of the steel sheet. If the end temperature of finish rolling is low, the metal structure may become non-uniform and formability may decrease. For this reason, the end temperature of finish rolling is set to 850°C or higher. On the other hand, in order to suppress coarsening of austenite, the end temperature of finish rolling is set to 1050°C or lower.

[0121] (Cooling and Coiling) Next, the finish-rolled steel sheet is cooled to 500°C or less at an average cooling rate of 20°C / s or more and then coiled. If the average cooling rate is less than 20°C / s or the coiling temperature exceeds 500°C, P segregation occurs in the hot rolling step, making the hot-rolled steel sheet embrittled and making subsequent cold rolling difficult. The average cooling rate is preferably 25°C / s or more, and the coiling temperature is preferably 480°C or less. The average cooling rate is preferably 100°C / s or less, and the coiling temperature is preferably 300°C or more.

[0122] [Pickling Process] In the steel sheet manufacturing method of this embodiment, the pickling process is a process for removing oxide scale formed on the surface of the hot-rolled steel sheet during hot rolling. In the pickling process, the hot-rolled steel sheet is continuously transported and immersed in a pickling tank containing an acidic cleaning solution, thereby removing the oxide scale formed on the surface of the hot-rolled steel sheet. Examples of the acidic cleaning solution that can be used include hydrochloric acid and sulfuric acid. Pickling may be performed under conditions appropriate for removing the oxide scale. Furthermore, pickling may be performed once, or may be performed multiple times in order to remove as much oxide scale as possible.

[0123] [Cold Rolling Step] In the steel sheet manufacturing method of this embodiment, the cold rolling step is a step of cold rolling the hot-rolled steel sheet after pickling. In the cold rolling step, the pickled hot-rolled steel sheet is cold-rolled at a reduction ratio of 35 to 80%. By setting the cold rolling reduction ratio to 35% or more, the shape of the cold-rolled steel sheet can be kept flat and a decrease in ductility in the final product can be suppressed. The cold rolling reduction ratio is preferably 50% or more. On the other hand, by setting the cold rolling reduction ratio to 80% or less, it is possible to prevent the rolling load from becoming excessively large, making rolling difficult. The cold rolling reduction ratio is preferably 70% or less. The number of rolling passes and the reduction ratio per pass may be appropriately set so that the reduction ratio of the entire cold rolling is within the above range.

[0124] [Annealing step] In the method for producing a steel sheet according to the present embodiment, the annealing step is a step of holding the obtained cold-rolled steel sheet in a predetermined atmosphere at a predetermined temperature range. In the annealing step, the obtained cold-rolled steel sheet is heated in a heating furnace and a soaking furnace of a continuous annealing line, and then held at a maximum heating temperature of 830 to 900°C for 20 to 150 seconds, for example.

[0125] If the maximum heating temperature is lower than 830°C, the austenite fraction at the maximum heating temperature decreases, and the martensite fraction may not be 85.0% or more. On the other hand, if the maximum heating temperature is higher than 900°C, special furnace body protection is required, which is not preferable from the viewpoint of productivity.

[0126] Furthermore, if the holding time at the maximum heating temperature is shorter than 20 seconds, austenitization does not proceed sufficiently, and the martensite fraction may not be increased to 85.0% or more. On the other hand, even if the holding time at the maximum heating temperature is longer than 150 seconds, the austenite fraction saturates and does not change, which may lead to a decrease in productivity.

[0127] (Decarburization Treatment) In the method for producing a steel sheet according to this embodiment, it is preferable to perform a decarburization treatment in the annealing step. Specifically, in the annealing step, when the cold-rolled steel sheet is heated and held at the maximum heating temperature, it is preferable to control the atmosphere to have a hydrogen concentration of 2% by volume to 15% by volume and a dew point of −30.0°C to 15.0°C to decarburize the steel sheet. The dew point at this time is preferably −12.0°C or higher, more preferably −11.0°C or higher, −10.0°C or higher, −6.0°C or higher, or 0.0°C or higher. By performing such a decarburization treatment, it is possible to adjust the average Vickers hardness ratio HVs / HVb to less than 0.90.

[0128] In this decarburization treatment, if the hydrogen concentration is lower than 2% by volume, the decarburization may not proceed sufficiently or oxidation of the furnace body may occur. Furthermore, from an economical viewpoint, the upper limit of the hydrogen concentration is set to 15% by volume or less.

[0129] Furthermore, in this decarburization treatment, if the dew point is lower than −30° C., decarburization may not proceed sufficiently. On the other hand, if the dew point is higher than 15° C., decarburization may proceed excessively, resulting in a decrease in strength, or moisture may adhere to the steel sheet surface due to condensation, damaging the appearance.

[0130] [Cooling Step] In the steel sheet manufacturing method of this embodiment, the cooling step is a step of cooling the cold-rolled steel sheet after the annealing step or the coating step described below. The average cooling rate in the cooling step is an important factor for maintaining the shape of the steel sheet. If the average cooling rate is too small, ferrite or bainite transformation occurs during the cooling process, and the martensite fraction described above does not reach 85.0% or more, which may result in a decrease in the strength of the steel sheet. On the other hand, if the average cooling rate is too large, the martensitic transformation of the steel sheet may occur unevenly, or the cooling may occur unevenly, which may result in uneven internal stress and thermal stress due to the transformation, resulting in deformation of the steel sheet. With such a deformed steel sheet, it may be difficult to adjust the surface shape, such as the aspect ratio Str described below.

[0131] From the above viewpoints, in the cooling step, the average cooling rate from the maximum heating temperature to room temperature is preferably 0.10° C. / sec or more and 30.00° C. / sec or less.

[0132] When a plating treatment described below is performed in the cooling step, the average cooling rate up to the plating treatment (hereinafter sometimes referred to as the "first cooling rate") and the average cooling rate after the plating treatment (hereinafter sometimes referred to as the "second cooling rate") may be within the above-mentioned range. The first cooling rate and the second cooling rate do not need to be the same.

[0133] [Surface Shape Adjustment Step] In the steel sheet manufacturing method of this embodiment, the surface shape adjustment step is a step of imparting a predetermined uneven structure to at least one surface of the obtained steel sheet so that the aspect ratio Str of the roughness of at least one surface of the steel sheet satisfies 0 < Str < 0.60. In the surface shape adjustment step, means for imparting such an uneven structure to at least one surface of the steel sheet include a method of rolling and processing with a skin-pass rolling roll and a method of grinding with a brush.

[0134] In the steel sheet manufacturing method of this embodiment, skin-pass rolling is utilized as the surface shape adjustment step, and in this case, it is possible to adjust the surface shape taking into account the following skin-pass rolling conditions.

[0135] (Control of Str in Skin-Pass Rolling) In order to impart the above-mentioned specific Str to the steel sheet surface, it is necessary to use a skin-pass rolling roll having a specific surface shape. Specifically, the skin-pass rolling roll used has a surface roughness aspect ratio Str that satisfies 0 < Str < 0.60. More specifically, the skin-pass rolling roll used has a structure in which multiple recesses have an average depth of 1.00 to 10.00 μm and an average width of 1.00 to 20.00 μm, and the average spacing between adjacent recesses is 1.00 to 30.00 μm. That is, the average depth, average width, and average spacing dimensions of the skin-pass rolling roll are design dimensions on the surface of the steel sheet, which is the transferred surface. In other words, the average depth, average width, and average spacing dimensions of the skin-pass rolling roll correspond to the dimensions of each recess formed on the surface of the steel sheet, which is the transferred surface.

[0136] Str varies depending on the elongation rate during rolling in roll rolling. This is because, as the elongation rate increases, the surface shape of the roll is more easily transferred to the surface of the steel sheet. In the method for producing a steel sheet of this embodiment, the elongation rate is set to 0.05% or more. When the elongation rate is 0.05% or more, the surface shape of the roll is transferred to the surface of the steel sheet, making it easier to control the Str of the steel sheet surface within a predetermined range. On the other hand, the elongation rate is preferably 2.00% or less. When the elongation rate is 2.00% or less, it is easier to maintain the flatness of the steel sheet and ensure the desired appearance.

[0137] (Control of the average depth of recesses in skin-pass rolling) The average depth of recesses is easily affected by the line speed during rolling. This is thought to be because the slower the line speed, the greater the depth transferred to the roll. In the method for producing a steel sheet of this embodiment, the line speed during rolling is set to 80 mpm or less. If the line speed is 80 mpm or less, the average depth of recesses, of the predetermined uneven structure possessed by the roll, is more likely to be transferred. The lower limit of the line speed is set to 20 mpm from the viewpoint of ensuring the transferability of the average depth of recesses and productivity.

[0138] On the other hand, when brush grinding is also utilized as a surface shape adjustment step, a roll brush is used as the brush, and the surface shape, particularly the width and spacing of the recesses, and Sa, can be finely adjusted by adjusting the brush pressure, rotation speed (contact time between the brush and the steel sheet surface), and brush length (bristle length). In other words, by devising the brush grinding conditions, the shape of the recesses on the steel sheet surface can be finely adjusted, and the hydrogen penetration suppression effect can be further enhanced. In particular, by performing brush grinding after skin-pass rolling, an even greater hydrogen penetration suppression effect can be expected.

[0139] (Roll Brush) The roll brush used in the surface shape adjustment process is a brush whose wire material is a chemical fiber. Examples of chemical fiber wire materials include nylon, PP, and aramid fiber. These wire materials containing abrasive grains may also be used. The brush wire diameter is preferably 0.1 to 3.0 mm. If the wire diameter is less than 0.1 mm, the effect of adjusting the surface shape may be reduced. On the other hand, if the wire diameter is 3.0 mm or more, brush marks may be left, which may impair the appearance quality of the steel sheet surface. Furthermore, a close wire density is preferable for the brush, and a close-wound roll brush is preferably used. Furthermore, brush winding methods such as pitch winding and single-row winding may cause brush marks, which may impair the appearance quality of the steel sheet surface.

[0140] (Control of the Average Spacing and Width of Recesses by Adjusting the Brush Rotation Speed) In this specification, the brush rotation speed refers to the number of rotations (rpm) per unit time of a roll equipped with a brush. Because the rotation of the brush widens the recesses in the width direction, the average spacing and average width between the recesses created in the skin-pass rolling process increase. This is because the corners of the recesses and flat portions are removed by brush grinding. The lower the rotation speed, the less corners are removed. On the other hand, if the rotation speed is too high, the corners are removed and the recesses on the steel sheet surface are smoothed, making it difficult to achieve a good hydrogen penetration suppression effect. Therefore, in the steel sheet manufacturing method of this embodiment, the brush rotation speed is set to 200 rpm or more. If the brush rotation speed is less than 200 rpm, the effect on the surface shape is reduced, and as a result, a good hydrogen penetration suppression effect may not be achieved. Furthermore, in the steel sheet manufacturing method of this embodiment, the brush rotation speed is set to 1100 rpm or less. If the brush rotation speed is greater than 1100 rpm, the recesses on the surface may become too wide, making it difficult to achieve a good hydrogen penetration suppression effect.

[0141] (Control of the average spacing and average width between recesses by brush pressure) In this specification, the brush pressure refers to the length (bristle length) (mm / mm) of the contact area when the brush comes into contact with the steel sheet surface relative to the total length of the brush bristles. As with the brush rotation speed, the greater the brush pressure, the more the corners of the recesses are ground away, and the larger the average spacing and average width of the recesses tend to be. In the steel sheet manufacturing method of this embodiment, the brush pressure is set to 0.50 mm or more. If the brush pressure is less than 0.50 mm, the amount of grinding decreases, and the average spacing and average width hardly change, which may make it difficult to control the average spacing and average width within the specified range. Furthermore, in the steel sheet manufacturing method of this embodiment, the brush pressure is set to 3.00 mm or less. If the brush pressure is greater than 3.00 mm, the average spacing becomes small and the average width becomes too large, which may result in a poor hydrogen penetration suppression effect.

[0142] (Control of arithmetic mean height Sa by brush length) The brush length (bristle length) is a factor that affects the amount of grinding in the depth direction of the steel sheet. The shorter the brush length, the higher the brush rigidity, and the greater the amount of grinding in the depth direction. On the other hand, the longer the brush length, the lower the brush rigidity, and the less the amount of grinding in the depth direction. In the steel sheet manufacturing method of this embodiment, the brush length is 100 mm or more. If the brush length is less than 100 mm, the amount of grinding becomes too large, making it difficult to control Sa within the specified range, which may make it difficult to achieve a good hydrogen penetration suppression effect or impair the surface appearance. Furthermore, in the steel sheet manufacturing method of this embodiment, the brush length is 500 mm or less. If the brush length exceeds 500 mm, the amount of grinding becomes small, making it difficult to control Sa within the specified range, and may make it difficult to achieve a good hydrogen penetration suppression effect.

[0143] [Coating Step] In the steel sheet manufacturing method of this embodiment, the coating step is a step of forming a coating layer on the surface of the steel sheet after the annealing step, the cooling step, or the surface shape adjustment step, for the purpose of improving corrosion resistance. Examples of coating treatments for forming the coating layer include plating treatments such as hot-dip galvanizing, alloying hot-dip galvanizing, and electroplating. For example, the coating treatment may involve subjecting the surface of the steel sheet to a hot-dip galvanizing treatment, or may involve subjecting the surface of the steel sheet to an alloying treatment after the hot-dip galvanizing treatment. Specific conditions for the coating treatment and the alloying treatment may be any appropriate conditions known to those skilled in the art.

[0144] In the method for manufacturing a steel sheet according to this embodiment, the coating step is not an essential step, and therefore, if the steel sheet does not include a coating layer, there is no need to perform such a coating step.

[0145] By the manufacturing method described above, it is possible to manufacture the high-strength steel plate of the above-described embodiment that has excellent hydrogen embrittlement resistance after spot welding.

[0146] The present invention is not limited to the above-described embodiments or the following examples, and appropriate combinations, substitutions, modifications, etc. are possible within the scope that does not deviate from the object and intent of the present invention.

[0147] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0148] In the following examples, steel plates according to the embodiments of the present invention were produced under various conditions, and the tensile strength and hydrogen embrittlement resistance of the obtained steel plates were examined.

[0149] (Production of Steel Plate) First, molten steel was cast by a continuous casting method to form slabs having various chemical compositions shown in Table 1. These slabs were heated to the heating temperatures shown in Table 2-1 and hot-rolled. The hot-rolling was carried out by performing rough rolling and finish rolling, and the finishing temperatures of the finish rolling were as shown in Table 2-1.

[0150] Next, the finish-rolled steel sheet was cooled under the conditions shown in Table 2-1 and coiled. Furthermore, the obtained hot-rolled steel sheet having a thickness of 3.2 mm was appropriately subjected to the post-hot-rolling treatment shown in Table 2-1. Then, the hot-rolled steel sheet was subjected to pickling and cold-rolled at the cold-rolling reduction shown in Table 2-1 to obtain a cold-rolled steel sheet.

[0151] Next, the obtained cold-rolled steel sheets were heated in a heating furnace and a soaking furnace of a continuous annealing line, and were heated and held under the conditions shown in Table 2-1. Then, the annealed cold-rolled steel sheets were cooled at a predetermined average cooling rate shown in Table 2-1, and some of the steel sheets were subjected to hot-dip galvanizing (GI) or galvannealed hot-dip galvannealing (GA) as a coating treatment.

[0152] Then, as a surface shape adjustment step, the cooled steel sheets were subjected to skin-pass rolling using a rolling roll having a surface shape and rolling conditions shown in Table 2-2. Note that the various dimensions of the surface shape of the rolling roll shown in Table 2-2 are design dimensions on the surface side of the steel sheet, which is the transferred surface. Furthermore, brush grinding was performed on some of the steel sheets under the conditions shown in Table 2-2.

[0153] In this manner, steel plates Nos. 1 to 22 were produced.

[0154]

[0155]

[0156]

[0157] The chemical composition of samples taken from the obtained steel plates was analyzed, and it was confirmed that there was no change from the chemical composition of the slab. Furthermore, various properties of the obtained steel plates were measured and evaluated according to the above-mentioned methods or the following methods. The measurement results and evaluation results of various properties of the obtained steel plates are shown in Table 3 below.

[0158] In Tables 2-1 and 2-2, underlines next to various values ​​indicate that the steel sheet of the present invention cannot be obtained under the manufacturing conditions or that the manufacturing conditions are unfavorable. In Table 3, underlines next to the aspect ratio Str of surface roughness indicate that the condition is outside the range of the present invention, and underlines next to other values ​​indicate that the various properties of the steel sheet are unfavorable.

[0159] (Tensile Strength) The tensile strength of the obtained steel plate was measured by conducting a tensile test in accordance with JIS Z 2241:2022 using a JIS No. 5 test piece taken from a direction in which the longitudinal direction of the test piece was parallel to the directions perpendicular to each of the rolling direction and the plate thickness direction of the steel plate.

[0160] (Evaluation of Hydrogen Embrittlement Resistance) The hydrogen embrittlement resistance of the spot welded portion of the obtained steel sheet was evaluated by the following method. Here, FIG. 3 is a perspective view schematically showing a test piece 3 after spot welding used for evaluating hydrogen embrittlement resistance. First, a 30 × 50 mm test piece was cut out from the steel sheet, and two of the test pieces were stacked. Spot welding was performed at the center of the stacked test pieces (steel sheets) under the condition that the nugget diameter was 4 mm. The test pieces were positioned so that the surface where Str was controlled was the mating surface of the steel sheet (the surface facing inward of the two stacked steel sheets). The electrode impact angle during spot welding was perpendicular to the surface of the test piece.

[0161] In addition, in order to reproduce a condition in which hydrogen can be easily added, anti-rust oil (NOX-RUST530F(60) manufactured by Parker Industries, Ltd.) was applied to the mating surfaces of the steel sheets. Furthermore, the welding conditions were set so that the initial contact area during spot welding was a circle with a diameter of 3 mm and the nugget diameter was 4 mm.

[0162] The spot-welded test piece 3 shown in Fig. 3 was left at room temperature for one day. The test piece 3 was then heated at 400°C for one hour. Furthermore, the test piece 3 was cut along the dashed-dotted line CS shown in Fig. 3 , passing through the center of the 30 mm width and parallel to the 50 mm length (longitudinal direction), so as to cut the spot welded portion 4. The cross section of the cut test piece 3 was then mirror-polished and observed to confirm the length of the cracks generated by hydrogen embrittlement.

[0163] Those with no cracks were rated as "AAA," those with cracks of 0.05 mm or less were rated as "AA," those with cracks greater than 0.05 mm but less than 0.3 mm were rated as "A," and those with cracks greater than 0.3 mm were rated as "NG." The reason for heating at 400°C for one hour is to prevent hydrogen embrittlement that occurs during cutting of the cross section and mirror finishing.

[0164]

[0165] As shown in Table 3, the steel sheets of Nos. 1 to 3, 5 to 9, and 11 to 22, which are invention examples having a tensile strength of 1470 MPa or more and a surface roughness aspect ratio Str satisfying 0 < Str < 0.60, were all found to have excellent hydrogen embrittlement resistance. In particular, the steel sheets of Nos. 9, 11, 13 to 15, 17, and 20, which have a specific metal structure, a specific surface shape in which multiple recesses on the steel sheet surface have an average depth of 1.00 to 10.00 μm, an average width of 1.00 to 20.00 μm, and an average spacing between adjacent recesses of 1.00 to 30.00 μm, an arithmetic mean surface height Sa of 0.40 μm < Sa < 4.00 μm, and an average Vickers hardness ratio HVs / HVb of HVs / HVb < 0.90, were all found to have extremely excellent hydrogen embrittlement resistance.

[0166] On the other hand, it was found that the steel sheets of Comparative Examples Nos. 4 and 10, in which the aspect ratio Str of the surface roughness did not satisfy 0<Str<0.60, were both poor in hydrogen embrittlement resistance.

[0167] In particular, for steel sheet No. 4, although the aspect ratio of the surface roughness of the skin-pass rolling roll was within the appropriate range, the elongation during skin-pass rolling was low and the pressing force during brush grinding was large, resulting in an excessively high aspect ratio Str of the steel sheet surface roughness, and consequently poor hydrogen embrittlement resistance. Furthermore, for steel sheet No. 4, the maximum heating temperature during the annealing process was low, resulting in a small martensite fraction.

[0168] In addition, although the aspect ratio of the surface roughness of the skin-pass rolling roll of steel sheet No. 10 was within the appropriate range, the elongation during skin-pass rolling was low, so the aspect ratio Str of the surface roughness of the steel sheet was too high, resulting in poor hydrogen embrittlement resistance. Furthermore, the holding time at the maximum heating temperature during the annealing process was short for steel sheet No. 10, resulting in a small martensite fraction.

[0169] As described above, by controlling the surface shape of the steel sheet, it is possible to exert an effect of suppressing hydrogen penetration and significantly suppress hydrogen embrittlement of the welded portion.

[0170] 1 Steel plate 2 Recess 3 Test piece after spot welding 4 Spot welded part

Claims

1. A steel sheet having a tensile strength of 1470 MPa or more, and characterized in that the aspect ratio Str of the roughness of at least one surface satisfies 0<Str<0.

60.

2. The steel sheet according to claim 1, wherein the at least one surface includes a plurality of recesses, the plurality of recesses having an average depth of 1.00 to 10.00 μm and an average width of 1.00 to 20.00 μm, and the average spacing between adjacent recesses of the plurality of recesses is 1.00 to 30.00 μm.

3. The steel sheet according to claim 1 or 2, wherein the arithmetic mean height Sa of said at least one surface satisfies 0.40 μm<Sa<4.00 μm.

4. A steel sheet according to any one of claims 1 to 3, characterized in that the ratio HVs / HVb of the average Vickers hardness HVs from said at least one surface up to 100 μm in the sheet thickness to the average Vickers hardness HVb at 1 / 2 the sheet thickness satisfies HVs / HVb<0.

90.

5. The chemical composition of the steel plate is, in mass%, C: 0.15 to 0.60%, Si: 0.01 to 1.30%, Mn: 2.000 to 3.500%, P: 0.0001 to 0.0200%, S: 0.0001 to 0.0200%, Al: 0.001 to 1.000%, N: 0.0001 to 0.0200%, O: 0.0001 to 0.0200%, Co: 0 to 0.50%, Ni: 0 to 1.00%, Mo: 0 to 1.00%, Cr: 0 to 2.000%, Ti: 0 to 0.500%, B: 0 to 0.0100%, Nb: 0 to 0.500%, V 5. The steel sheet according to claim 1, characterized in that the steel sheet contains: Cu: 0 to 0.500%, W: 0 to 0.100%, and the balance: Fe and impurities.

6. The steel sheet according to claim 5, characterized in that the chemical composition includes, in mass%, at least one of Co: 0.01 to 0.50%, Ni: 0.01 to 1.00%, Mo: 0.01 to 1.00%, Cr: 0.001 to 2.000%, O: 0.0001 to 0.0200%, Ti: 0.001 to 0.500%, B: 0.0001 to 0.0100%, Nb: 0.001 to 0.500%, V: 0.001 to 0.500%, Cu: 0.001 to 0.500%, and W: 0.001 to 0.100%.

7. The steel plate according to any one of claims 1 to 6, characterized in that the metal structure of the steel plate is, in area percentages, martensite: 85.0% or more, retained austenite: 1.0 to 7.0%, and balance: 10.0% or less.

8. The steel sheet according to any one of claims 1 to 7, characterized in that it has a coating layer on at least one of the surfaces, and the coating layer contains at least one metal selected from the group consisting of zinc, aluminum, magnesium, and alloys thereof.

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