Steel sheet and member

Abstract

Provided is a steel sheet characterized in that the tensile strength thereof is 1660 MPa or more, the metal structure thereof has, in terms of area%, 85.0% or more of martensite and 1.0%-7.0% of residual austenite, the balance structure making up 10.0% or less.

Description

Steel plates and components

[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] High-strength steel sheets can suffer from hydrogen embrittlement cracking (also known as delayed fracture), a phenomenon in which a steel member subjected to high stress during use suddenly fractures due to hydrogen that penetrates into the steel from the environment.

[0004] In this regard, Patent Document 1 describes a steel sheet having a composition containing, by mass%, C: 0.20% to 0.40%, Si: more than 1.0% to 3.0%, Mn: 1.5% to 3.5%, P: 0.002% to 0.010%, S: 0.0002% to 0.0020%, sol.Al: 0.40% or less (excluding 0%), N: 0.0100% or less, with the balance being Fe and unavoidable impurities, and having a steel structure containing, by area ratio, tempered martensite: 45% to 83%, bainite: 15% to 53%, and retained austenite: 2% or more, wherein the average grain size of carbides in the tempered martensite is 0.40 μm or less, the average C content in the retained austenite is 0.5% by mass or more, and wherein the tensile strength is 1470 MPa or more. Furthermore, Patent Document 1 teaches that the delayed fracture resistance of a high-strength steel sheet having TS≧1470 MPa can be improved by suppressing the coarsening of carbides in tempered martensite.

[0005] International Publication No. 2022 / 185804

[0006] It is generally known that hydrogen embrittlement cracking is more likely to occur as the strength of steel increases. On the other hand, the automotive industry and other industries are also demanding further weight reduction of steel, and to achieve such weight reduction, steel must be made stronger than ever before. Therefore, there is a strong demand for steel that can solve the problem of hydrogen embrittlement even when strength is increased to the same level or even higher than conventional steels, i.e., high-strength steel sheets with excellent hydrogen embrittlement resistance.

[0007] Therefore, an object of the present invention is to provide a high-strength steel sheet having excellent hydrogen embrittlement resistance through a novel structure.

[0008] In order to achieve the above object, the present inventors have conducted research, focusing particularly on the metallographic structure of the steel sheet. Specifically, the present inventors have discovered that by configuring the metallographic structure of the steel sheet to be mainly composed of martensite in order to ensure a tensile strength of 1660 MPa or more, and by including a predetermined amount of retained austenite, which has the property of easily absorbing hydrogen, in the metallographic structure, it is possible to significantly suppress the occurrence of hydrogen embrittlement cracking even when the tensile strength is high, such as 1660 MPa or more, and have completed the present invention.

[0009] The present invention, which has achieved the above object, is as follows. (1) Tensile strength is 1660 MPa or more, and the metal structure is, in area %, martensite: 85.0% or more, retained austenite: 1.0 to 7.0%, and balance: 10.0% or less, and the chemical composition is, in mass %, C: 0.25 to 0.45%, Si: 0.01 to 1.30%, Mn: 1.00 to 3.50%, 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%, 1. A steel sheet comprising: Cr: 0-2.000%, Ti: 0-0.500%, B: 0-0.0100%, Nb: 0-0.500%, V: 0-0.500%, Cu: 0-0.500%, W: 0-0.100%, Ta: 0-0.100%, Sn: 0-0.100%, Sb: 0-0.100%, As: 0-0.100%, Mg: 0-0.0500%, Ca: 0-0.050%, Y: 0-0.050%, Zr: 0-0.050%, La: 0-0.050%, and Ce: 0-0.050%, with the balance being Fe and impurities. (2) The steel sheet according to (1) above, characterized in that, when measured by X-ray diffraction, the lattice constant calculated from diffraction peaks derived from retained austenite at the surface layer portion, the 1 / 4 position in the sheet thickness direction, and the 1 / 2 position in the sheet thickness direction satisfies the following formula 1. s >3.5800Å A q >3.5800Å A c >3.5800 Å ...Equation 1 where A s , A q and A c are lattice constants calculated from the diffraction peaks at the surface layer, the 1 / 4 thickness position, and the 1 / 2 thickness position, respectively, resulting from the retained austenite. (3) Area fraction RA of the retained austenite in the surface layer s and the area fraction RA of retained austenite at the 1 / 2 thickness position c However, R.A. s / RA c(4) The steel sheet according to any one of (1) to (3), characterized in that, when measured by X-ray diffraction, the lattice constant calculated from diffraction peaks derived from retained austenite at the surface layer portion, at the 1 / 4 position in the sheet thickness direction, and at the 1 / 2 position in the sheet thickness direction satisfies the following formula 2: A s / A c >0.9970 A q / A c >0.9970 ... Equation 2 where A s , A q and A c are lattice constants calculated from the diffraction peaks derived from retained austenite at the surface layer portion, the 1 / 4 sheet thickness position, and the 1 / 2 sheet thickness position, respectively. (5) The chemical composition is, in mass%, Co: 0.01 to 0.50%, Ni: 0.01 to 1.00%, Mo: 0.01 to 1.00%, Cr: 0.001 to 2.000%, 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%, W: 0.001 to 0.100%, Ta: 0.001 to 0.100%, Sn: 0.001 to 0.100%, Sb: 0.001 to 0.100%, As: 0.001 to 0.100%, The steel sheet according to any one of (1) to (4) above, characterized in that it contains at least one of Mg: 0.0001 to 0.0500%, Ca: 0.001 to 0.050%, Y: 0.001 to 0.050%, Zr: 0.001 to 0.050%, La: 0.001 to 0.050%, and Ce: 0.001 to 0.050%. (6) A member comprising the steel sheet according to any one of (1) to (5) above.

[0010] According to the present invention, a high-strength steel sheet having excellent hydrogen embrittlement resistance can be provided.

[0011] <Steel Sheet> A steel sheet according to an embodiment of the present invention has a tensile strength of 1660 MPa or more, and a metallographic structure consisting of, in area %, martensite: 85.0% or more, retained austenite: 1.0 to 7.0%, and a balance: 10.0% or less, and a chemical composition consisting, in mass %, of C: 0.25 to 0.45%, Si: 0.01 to 1.30%, Mn: 1.00 to 3.50%, 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: 0-1.00%, Cr: 0-2.000%, Ti: 0-0.500%, B: 0-0.0100%, Nb: 0-0.500%, V: 0-0.500%, Cu: 0-0.500%, W: 0-0.100%, Ta: 0-0.100%, Sn: 0-0.100%, Sb: 0-0.100%, As: 0-0.100%, Mg: 0-0.0500%, Ca: 0-0.050%, Y: 0-0.050%, Zr: 0-0.050%, La: 0-0.050%, and Contains Ce: 0 to 0.050%, The balance is Fe and impurities.

[0012] As mentioned above, it is known that hydrogen embrittlement cracking is more likely to occur as the strength of steel increases. In particular, in steel sheets with extremely high strength, such as those with a tensile strength of 1660 MPa or more, the metallographic structure of the steel sheet generally contains martensite as a major component to ensure high strength. However, in such high-strength steel sheets primarily composed of martensite, hydrogen embrittlement cracking is thought to occur when hydrogen that penetrates the steel accumulates primarily at prior austenite grain boundaries in the martensite structure, reducing the bonding strength of the grain boundaries. These weakened grain boundaries are thought to be the origin of embrittlement. Generally, as tensile strength increases, hydrogen embrittlement occurs even with small amounts of hydrogen, and hydrogen penetration into steel can occur even at room temperature. However, there is no method for completely suppressing hydrogen penetration into steel, or it is extremely difficult to completely suppress it. Therefore, to fundamentally solve the issue of hydrogen embrittlement in high-strength steel sheets with a tensile strength of 1660 MPa or more, it is important to modify the metallographic structure of the steel sheet.

[0013] Therefore, the present inventors conducted research, focusing particularly on the metallographic structure of steel sheets, in order to improve the hydrogen embrittlement resistance of steel sheets having extremely high strengths, such as tensile strengths of 1660 MPa or more. First, the present inventors noted that retained austenite, which is inevitably formed when martensite is transformed from austenite, has a tendency to easily absorb hydrogen. In relation to this, the present inventors discovered that, in order to ensure a tensile strength of 1660 MPa or more, the metallographic structure of a steel sheet is mainly composed of martensite, more specifically, a structure containing 85.0% or more martensite by area percentage, and that the metallographic structure contains a predetermined amount of retained austenite, more specifically, a retained austenite content of 1.0 to 7.0% by area percentage, thereby significantly suppressing the occurrence of hydrogen embrittlement cracking even in steel sheets having a high tensile strength of 1660 MPa or more. While not intending to be bound by any particular theory, it is believed that, because retained austenite has the property of easily absorbing hydrogen as described above, the presence of an appropriate amount of retained austenite in the metallographic structure allows the retained austenite to appropriately trap hydrogen that has penetrated into the steel. In this case, it is possible to prevent hydrogen that has penetrated into the steel from accumulating at prior austenite grain boundaries in a metallographic structure mainly composed of martensite, thereby reducing the bonding strength of the grain boundaries. As a result, it is believed that the occurrence of hydrogen embrittlement cracking can be significantly suppressed even at high tensile strengths of 1660 MPa or more. Since the amount of retained austenite generally tends to increase with increasing strength, the fact that controlling the amount of retained austenite to within a range of 1.0 to 7.0% in a hard metallographic structure containing 85.0% or more of martensite can achieve both a tensile strength of 1660 MPa or more and excellent hydrogen embrittlement resistance was not previously known and has now been discovered for the first time by the present inventors. Therefore, steel sheets according to embodiments of the present invention are particularly useful in the automotive field, where high strength is required. Hereinafter, each component of the steel plate according to the embodiment of the present invention will be described in more detail.

[0014] [Tensile Strength: 1660 MPa or More] The steel sheet according to the embodiment of the present invention has a tensile strength of 1660 MPa or more. The tensile strength is preferably 1700 MPa or more, 1760 MPa or more, 1800 MPa or more, 1900 MPa or more, or 2000 MPa or more. Despite having such a very high tensile strength, the steel sheet according to the embodiment of the present invention can significantly improve its hydrogen embrittlement resistance by including a predetermined amount of retained austenite in a metal structure mainly composed of martensite, as described above. The upper limit of the tensile strength is not particularly limited, and may be, for example, 2500 MPa or less, 2300 MPa or less, 2200 MPa or less, or 2100 MPa or less. The tensile strength is 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 is preferably parallel to the rolling 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 sheet surface. When the steel sheet has a coating layer such as a plating on its surface, the coating layer is removed before the tensile test.

[0015] [Metallic Structure] Next, the metallic structure of the steel sheet according to an embodiment of the present invention will be described. In the following description, the unit of microstructure fraction, "%", means "area %" unless otherwise specified. The metallic 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 depth position of 50 μm 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 fraction measured in the surface layer portion, the 1 / 4 thickness position, and the 1 / 2 thickness position. When a coating layer such as a plating is provided on the surface of the steel sheet, the position in the thickness direction of the region excluding the coating layer is specified.

[0016] [Martensite: 85.0% or more] In the steel sheet according to the embodiment of the present invention, the metal structure contains, in area %, 85.0% or more of martensite. By containing 85.0% or more of martensite, it is possible to achieve a tensile strength of 1660 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. There is no particular upper limit, and the area fraction of martensite may be, for example, 99.0% or less or 97.0% or less. In the present invention, "martensite" includes not only as-quenched martensite (so-called fresh martensite) but also tempered martensite.

[0017] [Residual Austenite: 1.0 to 7.0%] In the steel sheet according to the embodiment of the present invention, the metallographic structure contains, in area percentage, 1.0 to 7.0% retained austenite. By containing 1.0 to 7.0% retained austenite, hydrogen that has penetrated into the steel can be appropriately trapped by the retained austenite, thereby significantly suppressing the occurrence of hydrogen embrittlement cracking even when the tensile strength is high, such as 1660 MPa or higher. From the viewpoint of improving hydrogen embrittlement resistance, a higher area fraction of retained austenite is preferable, and may be, for example, 2.0% or more, 3.0% or more, or 4.0% or more. On the other hand, if the retained austenite is contained in an excessive amount, a sufficient area fraction of martensite cannot be ensured, and the desired strength may not be achieved. Alternatively, some of the retained austenite that has trapped hydrogen may transform into martensite by cold working or the like, or may decompose due to aging or the like, thereby releasing some of the trapped hydrogen. If the retained austenite is contained in excess, the amount of hydrogen released may increase, which may accelerate hydrogen embrittlement. Therefore, the area ratio of the retained austenite is set to 7.0% or less, and may be, for example, 6.0% or less or 5.0% or less.

[0018] [Remaining structure: 10.0% or less] The remaining structure other than martensite and retained austenite may have an area fraction of 0%, but if a remaining structure is present, the area fraction of the remaining structure should be 10.0% or less. If the remaining structure is excessively contained, it may be impossible to control the area fraction of martensite and / or retained austenite within the desired range, and as a result, the desired strength and / or hydrogen embrittlement resistance may not be obtained. Therefore, the area fraction of the remaining structure should be 10.0% or less, and 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. In other words, the sum of the area fractions of martensite and retained austenite should be 90.0% or more, and may be 92.0% or more, 94.0% or more, 95.0% or more, 96.0% or more, 97.0% or more, or 98.0% or more. On the other hand, achieving a 0% area ratio of the remaining structure requires advanced control in the steel sheet 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. In other words, the sum of the area ratios of martensite and retained austenite may be 99.5% or less, or 99.0% or less. The remaining structure is not particularly limited, but may include, for example, at least one of ferrite, bainite, and pearlite, or at least one of these.

[0019] [Identification and Calculation of Metallographic Structure] [Martensite] Identification and calculation of the metallographic structure are performed as follows. First, a sample having a cross-section through the plate thickness perpendicular to the plate surface is collected, and this cross-section is used as the observation surface. The cross-section is polished carefully so that scratches due to polishing do not remain. 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 the crystal orientation has not changed due to polishing. Here, scratches in the crystal orientation due to polishing refer to a region observed as a straight line penetrating the metallographic structure in the IPF (Inverse-Pole-Figure) map in EBSD analysis. Since such scratches are not inherent to the structure, it is natural that they must not be included in the observation field of view. It is preferable to search for a field of view free of scratches, or if scratches inevitably appear in the field of view, to polish again.

[0020] The above-described sample for EBSD analysis is subjected to electron backscattering analysis. While EBSD analysis conditions are within the scope of common knowledge of those skilled in the art, detailed conditions are described below as an example. After inserting the sample into an FE-SEM capable of EBSD measurement, 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, which will be inserted later. After tilting, the EBSD detector is inserted into the FE-SEM chamber and brought close to the sample. The EBSD detector can be positioned in any position that maximizes its electron beam detection sensitivity; however, it is preferable to position it as close as possible without colliding with the FE-SEM chamber, the sample, or the jig or table holding the sample. The electron beam detection sensitivity of the EBSD detector is then adjusted. Adjustment of the detection sensitivity depends on the performance of the FE-SEM electron gun and EBSD detector used, and therefore should be performed within the scope of common knowledge of those skilled in the art. The adjustment should ultimately result in conditions that allow for clear observation of the EBSD pattern. Thereafter, the observation field is irradiated with an electron beam 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 crystal orientation calculation are performed. For indexing and crystal orientation calculation, it is desirable to use APEX software manufactured by AMETEK. The EBSD data obtained in this manner is analyzed using OIMANALYSIS software (Orientation Imaging Microscopy) version 7 or later, which is EBSD data analysis software manufactured by AMETEK. The obtained EBSD data is opened in OIMANALYSIS, and only regions with a CI (Confidence Index) value of 0.1 or more are extracted. 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 contaminants 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. Areas with a GAM (Grain Average Misorientation) value of 0.5° or more are then considered to be martensite, and the area fraction is calculated. Note that a grain boundary refers to a boundary between measurement points where the orientation difference between the measurement points is 15° or more.Here, the GAM value is the average value of the misorientation between measurement points of crystal orientation in the region surrounded by grain boundaries, and since a structure formed at low temperatures such as martensite is characterized by the occurrence of misorientation within the grains due to transformation strain, etc., it is possible to distinguish by the GAM value. Such an investigation is carried out at five or more locations within a 100 × 100 μm field centered at a position halfway through the plate thickness from the surface of the steel plate, and the average area ratio is derived.

[0021] 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-mentioned method is the combined area fraction of martensite and retained austenite. Therefore, the area fraction of martensite at the half-thickness position is determined by subtracting the area fraction of retained austenite measured using the procedure described below from the area fraction determined by the above-mentioned method, i.e., by calculating [area fraction determined by EBSD (area fraction of regions with a GAM value of 0.5° or more)] - [area fraction of retained austenite determined by X-ray diffraction]. As mentioned above, martensite in this application refers to a structure including tempered martensite, and no particular distinction is made regarding the tempered state of martensite.

[0022] Next, the area fraction of martensite is measured in the same manner at the 1 / 4 thickness position and the surface layer portion. However, when measuring the area fraction of martensite in 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 steel sheet surface in the thickness direction. Finally, the average value of the area fractions measured at the surface layer portion, the 1 / 4 thickness position, and the 1 / 2 thickness position is calculated and determined as the area fraction of martensite.

[0023] [Retained Austenite] The area fraction of retained austenite is calculated by measurement using X-rays. First, a sample is taken from the same member as the sample taken for identifying martensite, and then the sample is removed from the plate surface to the 1 / 2 position in the plate 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 the (200) and (211) planes of the bcc phase and the (200), (220), and (311) planes of the fcc phase. This is defined as the area fraction of retained austenite at the 1 / 2 position in the plate thickness. The area fractions of retained austenite are obtained in the same manner for the surface layer portion and the 1 / 4 position in the plate thickness, and finally, the average value is calculated and determined as the area fraction of retained austenite. The area fraction of retained austenite at the 1 / 2 position in the plate thickness is determined in accordance with the "RA" method described below. c ", and similarly, the area ratio of retained austenite in the surface layer portion corresponds to "RA s " is equivalent to

[0024] [Remaining structure] The area fraction of the remaining structure is determined by subtracting the area fraction of martensite and the area fraction of retained austenite obtained above from 100%. The remaining structure may contain at least one of ferrite, bainite, and pearlite, for example, or may be at least one of these. However, in the present invention, the identification of these structure types and the determination of the area fraction of each structure are not particularly necessary for achieving the object of the present invention. If there is any need, it is not difficult for a person skilled in the art to identify them using a method normally applied.

[0025] [Formula 1: A s >3.5800Å, A q >3.5800 Å, and A c In a preferred embodiment of the present invention, when measured by X-ray diffraction, the lattice constant calculated from the diffraction peaks derived from retained austenite at the surface layer portion, the 1 / 4 sheet thickness position, and the 1 / 2 sheet thickness position satisfies the following formula 1: A s >3.5800Å A q >3.5800Å A c >3.5800 Å ...Equation 1 where As , A q and A c are lattice constants calculated from the diffraction peaks derived from retained austenite at the surface layer portion, the 1 / 4 sheet thickness position, and the 1 / 2 sheet thickness position, respectively.

[0026] The retained austenite in a metal structure is composed of multiple retained austenite phases with various degrees of stability. Therefore, for example, a metal structure may contain both relatively low-stability retained austenite and relatively high-stability retained austenite. Both retained austenite phases have hydrogen storage capacity and can therefore trap hydrogen that penetrates into the steel. However, if there is a large amount of low-stability retained austenite, some of the retained austenite may undergo strain-induced martensite transformation during cold working, such as cold pressing, or may decompose due to aging. In such cases, the partial loss of the retained austenite reduces the hydrogen storage capacity and releases some of the hydrogen trapped in the retained austenite. Furthermore, the released hydrogen may accumulate at prior austenite grain boundaries in a metal structure primarily composed of martensite, reducing the grain boundary bonding strength and promoting hydrogen embrittlement. Therefore, from the viewpoint of further improving hydrogen embrittlement resistance, it is preferable to control the area fraction of retained austenite to within the range of 1.0 to 7.0%, and also to stabilize the retained austenite, thereby suppressing or reducing the loss of the retained austenite and the associated release of hydrogen.

[0027] Therefore, the present inventors have investigated the relationship between hydrogen embrittlement resistance and stabilization of retained austenite in order to further improve the hydrogen embrittlement resistance of steel sheets. More specifically, the present inventors have conducted investigations focusing on the fact that the lattice constant calculated from the X-ray diffraction peaks derived from retained austenite is an index of the stability of the retained austenite. As a result, the present inventors have found that, when measured by X-ray diffraction, the lattice constant, i.e., A, calculated from the diffraction peaks derived from retained austenite at the surface layer, the 1 / 4 position, and the 1 / 2 position s , A qand A c satisfying the above formula 1, the stability of retained austenite contained in the metallographic structure of the steel sheet can be improved throughout the steel sheet. In relation to this, the inventors have discovered that it is possible to significantly suppress the loss of retained austenite due to strain-induced martensitic transformation caused by cold working, etc., and the decomposition of retained austenite due to aging, etc. Without intending to be bound by any particular theory, it is believed that by concentrating austenite-stabilizing elements such as C and / or Mn in retained austenite, the value of the lattice constant calculated from the diffraction peaks derived from the retained austenite can be increased, thereby improving the stability of the retained austenite. More specifically, as will be described in detail later in relation to the manufacturing method, cementite can be formed in the hot-rolled steel sheet by performing an appropriate heat treatment in the post-hot-rolling treatment step. Therefore, an austenite-stabilizing element such as Mn can be concentrated in the cementite in the subsequent annealing step, thereby further improving the stability of the austenite. Similarly, as will be described in detail later in connection with the manufacturing method, by performing an appropriate heat treatment in the stabilization treatment step, austenite-stabilizing elements such as C and Mn in the steel sheet can be concentrated in the retained austenite, thereby sufficiently stabilizing the retained austenite. In this case, the concentrated austenite-stabilizing elements such as C and / or Mn are thought to act as interstitial solid solution elements, entering voids where atoms would not normally exist and widening the lattice, while Mn acts as substitutional solid solution elements, entering lattice positions where Fe atoms would normally exist and widening the lattice, resulting in an increase in the lattice constant. Therefore, a certain correlation is observed between the stabilization of retained austenite due to austenite-stabilizing elements and the value of the lattice constant. Based on these findings, the inventors have conducted extensive research and have found that, when measured by X-ray diffraction, the lattice constant, i.e., A, calculated from diffraction peaks derived from retained austenite at the surface layer, the 1 / 4 thickness position, and the 1 / 2 thickness position, is s , A q and A cIt has been found that by controlling so that A satisfies the above formula 1, the stability of the retained austenite contained in the metal structure of the steel sheet can be increased throughout the steel sheet. s , A q and A c By controlling the amount of A so that it satisfies the above formula 1, it is possible to significantly suppress the disappearance of retained austenite due to deformation-induced martensitic transformation and the decomposition of retained austenite due to aging and the like, and therefore it is possible to further improve the hydrogen embrittlement resistance of the steel sheet compared to when the amount of retained austenite is simply controlled to be within the range of 1.0 to 7.0%. s , A q and A c The higher the value of A, the better. s , A q and A c are 3.5830 Å or more (i.e., A s ≧3.5830Å, A q ≧3.5830 Å, and A c ≧3.5830 Å, the same applies below), 3.5850 Å or more, 3.5880 Å or more, or 3.5900 Å or more. s , A q and A c are 3.7000 Å or less (i.e., A s ≦3.7000Å, A q ≦3.7000 Å, and A c ≦3.7000 Å, the same applies below), 3.6500 Å or less, or 3.6000 Å or less.

[0028] [Lattice constant A s , A q and A c Calculation of Lattice Constant A s , A q and A cis calculated as follows. First, the sample is removed from the plate surface to a position halfway through the plate thickness in the plate thickness direction by mechanical polishing and chemical polishing. Next, the lattice constant A of each plane of the retained austenite is calculated from the diffraction peaks of the (200), (220) and (311) planes of the fcc phase obtained by using MoKα rays as characteristic X-rays for the polished sample, based on the following formulas 3 and 4. c(200) , A c(220) and A c(311) are calculated, and then averaged based on the following formula 5 to obtain the lattice constant A c Determined as: d c(hkl) = λ / 2 sin (2θ c(hkl) / 2) ...Formula 3 A c(hkl) = d c(hkl) × (h 2 +k 2 +l 2 ) 0.5 ...Formula 4 A c = (A c(200) +A c(220) +A c(311) ) / 3 ... Equation 5 where d c(hkl) is the lattice spacing (Å) of the (hkl) plane at the half-thickness position, λ is the light source wavelength of the X-ray diffraction device (wavelength of MoKα ray) (Å), and A c(hkl) is the lattice constant (Å) of the (hkl) plane at the half-thickness position. c(hkl) is the diffraction angle (°) of the diffraction peak of the (hkl) plane at the half-thickness position. c(hkl) is calculated from the diffraction intensity and diffraction angle 2θ obtained by X-ray diffraction. First, the obtained 2θ values ​​are smoothed by a three-point weighted average using the following formula 6. Then, the 2θ with the highest diffraction intensity on each diffraction plane is calculated as the 2θ of that diffraction plane. c(hkl) Let 2θ n = (2θ n-1 ×P n-1 +2θ n ×P n +2θ n+1 ×P n+1 ) / (P n-1 +P n +P n+1 ) ...Equation 6 where Pn is the diffraction angle 2θ in the data group of "diffraction angle vs. diffraction intensity" n refers to the diffraction intensity of n In addition, the diffraction angle 2θ n The diffraction intensity before and after (i.e., P n-1 and P n+1 ) A three-point weighted average smoothing process is performed using a total of three data points, and the smoothing process is performed for all measurement points. The same process is performed for the surface layer portion and the 1 / 4 position of the plate thickness to obtain the lattice constants of the (200), (220), and (311) planes of the retained austenite at each position, and these are averaged to obtain the lattice constant A s and A q Determine.

[0029] [RA s / RA c <0.75] In another preferred embodiment of the present invention, the area ratio RA of the retained austenite in the surface layer portion s and the area fraction RA of retained austenite at the 1 / 2 thickness position c RA s / RA c <0.75.

[0030] As mentioned above, it is known that hydrogen embrittlement cracking is more likely to occur as the strength of the steel increases, and is particularly likely to occur in bent portions where large plastic strain is applied. In particular, for high-strength steel sheets mainly composed of martensitic structures, bending is the main process used in cold working and other processes. Therefore, in order to further improve hydrogen embrittlement resistance, it is particularly effective to improve the metal structure of the surface layer of the steel sheet, which is prone to hydrogen embrittlement during bending.

[0031] Therefore, the inventors have determined that the amount of retained austenite in the surface layer portion of the steel sheet is controlled to be smaller by a predetermined ratio than the amount of retained austenite in the metal structure inside the steel sheet, while maintaining the amount of retained austenite in the entire steel sheet within the range of 1.0 to 7.0%. More specifically, the area ratio RA of the retained austenite in the surface layer portion of the steel sheet is controlled to be smaller by a predetermined ratio than the amount of retained austenite in the metal structure inside the steel sheet. s and the area fraction RA of retained austenite at the 1 / 2 thickness position c RA s / RA c It has been found that controlling the amount of retained austenite in the surface layer portion to satisfy the condition <0.75 can significantly improve hydrogen embrittlement resistance during bending. While not intending to be bound by any particular theory, it is believed that reducing the amount of retained austenite in the surface layer portion naturally reduces the amount of unstable retained austenite in the surface layer portion. Reducing the amount of unstable retained austenite in the surface layer portion in this manner suppresses the reduction or disappearance of the retained austenite in the surface layer portion due to deformation-induced martensitic transformation caused by bending, such as cold pressing. As a result, the amount of hydrogen released from the retained austenite in the surface layer portion caused by bending, can be reduced, thereby enabling the hydrogen embrittlement resistance of the steel sheet to be further improved compared to when the amount of retained austenite is simply controlled to be within the range of 1.0 to 7.0%.

[0032] From the viewpoint of further improving hydrogen embrittlement resistance, the area ratio RA of the retained austenite in the surface layer s It is preferable to reduce the amount of hydrogen released from the retained austenite in the surface layer by reducing RA. s / RA c The smaller the ratio, the more preferable. For example, the ratio is 0.72 or less (i.e., RA s / RA c ≦0.72, the same applies below), 0.70 or less, 0.68 or less, 0.65 or less, 0.62 or less, or 0.60 or less. s / RA c The ratio may be greater than 0, or may be 0.10 or greater (i.e., RA s / RA c ≧0.10, the same applies below), 0.20 or more, or 0.30 or more. s and R.A. c The calculation method is as explained above in the section [Identification and calculation of metal structure].

[0033] [Formula 2: A s / A c >0.9970, and A q / A cIn yet another preferred embodiment of the present invention, when measured by X-ray diffraction, the lattice constant calculated from diffraction peaks derived from retained austenite at the surface layer portion, the quarter thickness position, and the half thickness position satisfies the following formula 2: A s / A c >0.9970 A q / A c >0.9970 ... Equation 2 where A s , A q and A c are lattice constants calculated from the diffraction peaks derived from retained austenite at the surface layer portion, the 1 / 4 sheet thickness position, and the 1 / 2 sheet thickness position, respectively.

[0034] As described above, in high-strength steel sheets mainly composed of martensitic structures, bending is the main process used for cold working, etc. Therefore, in order to further improve hydrogen embrittlement resistance, it is particularly effective to improve the metal structure of the surface layer of the steel sheet, which is prone to hydrogen embrittlement during bending.

[0035] Therefore, the inventors discovered that hydrogen embrittlement resistance during bending can be significantly improved by maintaining the amount of retained austenite throughout the steel sheet within a range of 1.0 to 7.0% while controlling the stability of the retained austenite in the metal structure in regions closer to the surface of the steel sheet to be relatively high. More specifically, by controlling the lattice constants calculated from the diffraction peaks derived from the retained austenite in the surface layer, the quarter-thickness position, and the half-thickness position so as to satisfy the above-mentioned formula 2. By further increasing the stability of the retained austenite in the surface layer and the quarter-thickness position, it is possible to suppress the reduction or disappearance of the retained austenite in the surface layer and the quarter-thickness position due to deformation-induced martensitic transformation caused by bending, such as cold pressing. As a result, the amount of hydrogen released from the retained austenite in the surface layer and the quarter-thickness position due to bending, etc., can be reduced, thereby enabling the hydrogen embrittlement resistance of the steel sheet to be further improved compared to when the amount of retained austenite is simply controlled within a range of 1.0 to 7.0%.

[0036] From the viewpoint of further improving hydrogen embrittlement resistance, it is preferable to further increase the stability of the retained austenite in the surface layer portion and at the 1 / 4 position of the plate thickness, thereby increasing the value of the lattice constant calculated from the diffraction peaks derived from the retained austenite in the surface layer portion and at the 1 / 4 position of the plate thickness, thereby reducing the amount of hydrogen released from the retained austenite in the surface layer portion and at the 1 / 4 position of the plate thickness. s / A c and A q / A c The larger the ratio, the more preferable. For example, each of these ratios is 0.9975 or more (i.e., A q / A c ≧0.9975, and A q / A c The upper limit is not particularly limited, but may be, for example, A s / A c and A q / A c The ratio is 1.1000 or less (i.e., A q / A c ≦1.1000, and A q / A c ≦1.1000, the same applies below), 1.0500 or less, or 1.0200 or less. s , A q and A c For the calculation method of lattice constant A s , A q and A c This is as explained above in the section on calculation of [value].

[0037] In the most preferred embodiment of the present invention, when measured by X-ray diffraction, the lattice constants calculated from the diffraction peaks derived from the retained austenite at the surface layer portion, the quarter thickness position, and the half thickness position satisfy the above formula 1, and the area ratio RA of the retained austenite at the surface layer portion is s and the area fraction RA of retained austenite at the 1 / 2 thickness position c RA s / RA c<0.75, and when measured by X-ray diffraction, the lattice constant calculated from the diffraction peaks derived from the retained austenite in the surface layer portion, the quarter thickness position, and the half thickness position satisfies the above formula 2. According to such an embodiment, the stability of the retained austenite contained in the metallographic structure of the steel sheet is increased throughout the steel sheet, and the amount of unstable retained austenite in the surface layer portion and the quarter thickness position can be further increased while reducing the amount of unstable retained austenite in the surface layer portion in particular. Therefore, the amount of hydrogen released from the retained austenite due to bending or the like can be significantly reduced, particularly in the surface layer portion and the quarter thickness position of the steel sheet, which are prone to hydrogen embrittlement during bending, and the hydrogen embrittlement resistance of the steel sheet can be more significantly improved.

[0038] [Chemical Composition of Steel Sheet] In the following description, the unit of content of each element, "%", means "mass%" unless otherwise specified. Furthermore, in this specification, unless otherwise specified, the term "to" indicating a numerical range means that the numerical values ​​before and after it are included as the lower and upper limits.

[0039] In a specific embodiment of the present invention, the chemical composition of the steel sheet is, in mass%, C: 0.25-0.45%, Si: 0.01-1.30%, Mn: 1.00-3.50%, P: 0.0001-0.0200%, S: 0.0001-0.0200%, Al: 0.001-1.000%, N: 0.0001-0.0200%, O: 0.0001-0.0200%, Co: 0-0.50%, Ni: 0-1.00%, Mo: 0-1.00%, Cr: 0-2.000%, Ti: 0-0.500%, B: 0-0.0100%, Nb: 0-0.500%, V: 0 to 0.500%, Cu: 0 to 0.500%, W: 0 to 0.100%, Ta: 0 to 0.100%, Sn: 0 to 0.100%, Sb: 0 to 0.100%, As: 0 to 0.100%, Mg: 0 to 0.0500%, Ca: 0 to 0.050%, Y: 0 to 0.050%, Zr: 0 to 0.050%, La: 0 to 0.050%, Ce: 0 to 0.050%, and the balance: Fe and impurities. Each element will be described in more detail below.

[0040] [C: 0.25 to 0.45%] C is an element effective for increasing tensile strength inexpensively. C is also an element effective for stabilizing austenite. To fully obtain these effects, the C content is set to 0.25% or more. The C content may be 0.26% or more, 0.28% or more, 0.29% or more, or 0.30% or more. On the other hand, excessive C content may degrade weldability. For this reason, the C content is set to 0.45% or less. The C content may be 0.42% or less, 0.40% or less, or 0.38% or less.

[0041] [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.

[0042] [Mn: 1.00 to 3.50%] 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 1.00% or more. The Mn content may be 1.20% or more, 1.50% or more, 1.80% or more, 2.00% or more, 2.20% or more, 2.40% or more, or 2.50% 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 3.50% or less. The Mn content may be 3.20% or less, 3.00% or less, 2.80% or less, or 2.60% or less.

[0043] [P: 0.0001 to 0.0200%] P is an element that embrittles welds and deteriorates platability. For this reason, 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. The lower the P content, the more preferable it is; however, reducing the P content to less than 0.0001% requires a long refining time, resulting in a significant increase in costs. For this reason, 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.

[0044] [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. While a lower S content is preferable, 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.

[0045] [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.

[0046] [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. On the other hand, reducing N to less than 0.0001% would result 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.

[0047] [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. However, 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.

[0048] The basic chemical composition of the steel sheet according to the embodiment of the present invention is as described above. Furthermore, the steel sheet may contain at least one of the following optional elements, as needed, in place of a portion of the remaining Fe. For example, the steel sheet may contain at least one element selected from the group consisting of Co: 0-0.50%, Ni: 0-1.00%, Mo: 0-1.00%, Cr: 0-2.000%, Ti: 0-0.500%, B: 0-0.0100%, Nb: 0-0.500%, V: 0-0.500%, Cu: 0-0.500%, W: 0-0.100%, and Ta: 0-0.100%. The steel sheet may also contain at least one element selected from the group consisting of Sn: 0-0.100%, Sb: 0-0.100%, and As: 0-0.100%. The steel sheet may also contain at least one element selected from the group consisting of Mg: 0 to 0.0500%, Ca: 0 to 0.050%, Y: 0 to 0.050%, Zr: 0 to 0.050%, La: 0 to 0.050%, and Ce: 0 to 0.050%. These optional elements will be described in detail below.

[0049] [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. For this reason, 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.

[0050] [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.

[0051] [Mo: 0 to 1.00%] Mo is an element effective in increasing the strength of steel sheets. Furthermore, Mo is an element that 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, even if Mo is contained in an excessive amount, the effect of suppressing ferrite transformation may saturate, or coarse intermetallic compounds and carbides may be formed. 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.

[0052] [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 obtain 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.

[0053] [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.

[0054] [B: 0 to 0.0100%] B is an element that suppresses the formation of ferrite and pearlite and promotes the formation of low-temperature transformation structures such as martensite during cooling from the austenite temperature range. 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.

[0055] [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.

[0056] [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. For this reason, 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.

[0057] [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.

[0058] [W: 0 to 0.100%] W is an element effective in increasing the strength of steel sheet. W also forms precipitates and crystallized products. Precipitates and crystallized products containing W 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.

[0059] [Ta: 0 to 0.100%] Ta is an element effective in controlling the morphology of carbides and increasing the strength of the steel sheet. The Ta content may be 0%, but to obtain these effects, the Ta content is preferably 0.001% or more. The Ta content may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, excessive Ta content may cause the precipitation of many fine Ta carbides, which may decrease the ductility as the strength of the steel sheet increases. For this reason, the Ta content is preferably 0.100% or less. The Ta content may be 0.080% or less, 0.060% or less, 0.050% or less, or 0.020% or less.

[0060] [Sn: 0 to 0.100%] Sn is an element contained in steel when scrap is used as the steel raw material. A high Sn content may cause ferrite embrittlement. Therefore, the Sn content is preferably 0.100% or less. The Sn content may be 0.060% or less, 0.030% or less, or 0.020% or less. The lower the Sn content, the more preferable it is, and 0% may be acceptable. However, reducing the Sn content to less than 0.001% requires a long refining time, resulting in a significant increase in costs. Therefore, the Sn content may be 0.001% or more. The Sn content may be 0.002% or more, 0.005% or more, or 0.010% or more.

[0061] [Sb: 0 to 0.100%] Like Sn, Sb is an element contained when scrap is used as a steel raw material. Sb also reduces ductility. Therefore, the Sb content is preferably 0.100% or less. The Sb content may be 0.040% or less, 0.030% or less, or 0.020% or less. The lower the Sb content, the better, and 0% is acceptable. However, reducing the Sb content to less than 0.001% requires a long refining time, resulting in a significant increase in costs. Therefore, the Sb content may be 0.001% or more. The Sb content may be 0.002% or more, 0.005% or more, or 0.008% or more.

[0062] [As: 0 to 0.100%] Like Sn and Sb, As is an element contained when scrap is used as a steel raw material. Furthermore, As is also an element that reduces ductility. Therefore, the As content is preferably 0.100% or less. The As content may be 0.040% or less, 0.030% or less, or 0.020% or less. The lower the As content, the more preferable it is, and 0% is also acceptable. However, reducing the As content to less than 0.001% requires a long refining time, resulting in a significant increase in costs. Therefore, the As content may be 0.001% or more. The As content may be 0.002% or more, 0.003% or more, or 0.005% or more.

[0063] [Mg: 0 to 0.0500%] Mg is an element that can control the morphology of sulfides when contained in small amounts. The Mg content may be 0%, but to obtain this effect, the Mg content is preferably 0.0001% or more. The Mg content may be 0.0005% or more, 0.0010% or more, 0.0015% or more, or 0.0020% or more. On the other hand, excessive Mg content may cause the formation of coarse inclusions. For this reason, the Mg content is preferably 0.0500% or less. The Mg content may be 0.0300% or less, 0.0100% or less, 0.0050% or less, or 0.0030% or less.

[0064] [Ca: 0 to 0.050%] Ca is useful as a deoxidizing element and is also effective in controlling the morphology of sulfides. The Ca content may be 0%, but to obtain these effects, the Ca content is preferably 0.0001% or more. The Ca content may be 0.0005% or more, 0.001% or more, or 0.002% or more. On the other hand, excessive Ca content may cause the formation of coarse inclusions. For this reason, the Ca content is preferably 0.050% or less. The Ca content may be 0.030% or less, 0.010% or less, 0.005% or less, or 0.003% or less.

[0065] [Y: 0 to 0.050%] [Zr: 0 to 0.050%] [La: 0 to 0.050%] [Ce: 0 to 0.050%] Y, Zr, La, and Ce are elements that can control the form of sulfides when contained in small amounts, similar to Mg and the like. The Y, Zr, La, and Ce contents may be 0%, but in order to obtain such effects, the Y, Zr, La, and Ce contents are preferably 0.0001% or more, and may be 0.0005% or more, 0.001% or more, or 0.002% or more. On the other hand, excessive inclusion of these elements may cause the formation of coarse oxides. For this reason, the Y, Zr, La, and Ce contents are preferably 0.050% or less, and may be 0.030% or less, 0.010% or less, 0.005% or less, or 0.003% or less.

[0066] In the steel sheet according to the embodiment of the present invention, the balance other than the above elements consists of Fe and impurities. The impurities are 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.

[0067] The chemical composition of the steel plate according to the embodiment of the present invention may be measured by a general analytical method. For example, measurement may be performed 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 measurement is performed using a Shimadzu ICPS-8100 (measuring device) or the like under conditions based on a pre-created calibration curve. C and S, which cannot be measured by ICP-AES, may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an 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.

[0068] [Thickness] The steel sheet according to the embodiment of the present invention generally has a thickness of 0.6 to 6.0 mm. Although not particularly limited, the thickness may be 1.0 mm or more, 1.2 mm or more, or 1.4 mm or more, and / or 5.0 mm or less, 4.0 mm or less, 3.0 mm or less, or 2.5 mm or less.

[0069] [Coating Layer] The steel sheet according to the embodiment of the present invention may further have a coating layer, such as a plating layer, on at least one surface, preferably both surfaces, for the purpose of improving corrosion resistance, etc. While not particularly limited, the coating layer may include, for example, at least one selected from the group consisting of zinc, aluminum, magnesium, and alloys thereof. More specifically, the coating layer may be a hot-dip plating layer or an electroplated layer. Hot-dip plating layers include, for example, hot-dip galvanized layers, alloyed hot-dip galvanized layers, hot-dip aluminum plating layers, hot-dip Zn—Al alloy plating layers, hot-dip Zn—Al—Mg alloy plating layers, and hot-dip Zn—Al—Mg—Si alloy plating layers. Electroplated layers include, for example, electrogalvanized layers and electrogalvanized Zn—Ni alloy plating layers. Preferably, the coating layer is a hot-dip galvanized layer, alloyed hot-dip galvanized layer, or electrogalvanized layer. The coating weight of the coating layer is not particularly limited and may be a general coating weight. As mentioned above, such coating layers are excluded from the determination of the tensile strength, thickness direction position, and chemical composition defined in the present invention.

[0070] As described above, the steel sheet according to the embodiment of the present invention has an extremely high tensile strength of 1660 MPa or more, yet has excellent hydrogen embrittlement resistance, and is therefore very useful for use as, for example, automotive frame members, bumpers, and other structural and reinforcing members that require strength.

[0071] <Method for manufacturing steel sheet> Next, a preferred method for manufacturing a steel sheet according to an 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 an embodiment of the present invention, but is not intended to limit the steel sheet to one manufactured by the manufacturing method described below.

[0072] A method for producing a steel sheet according to an embodiment of the present invention includes the following steps: 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 / second or more, and then 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 rolling reduction of 35 to 80%; an annealing step in which the obtained cold-rolled steel sheet is heated and then held at a maximum heating temperature of 830 to 900°C for 20 to 150 seconds; and a cooling step in which the cold-rolled steel sheet is cooled to the Ms point or less at an average cooling rate of 1.0°C / second or more. When a stabilization treatment step and / or a skin-pass rolling step are further included, the treatment temperature and holding time in the stabilization treatment step are lower than 280°C and shorter than 200 hours, respectively, and the elongation in the skin-pass rolling step is lower than 2.00%. Each step will be described in detail below.

[0073] [Hot Rolling Process] [Slab Heating] First, a slab having the chemical composition described above in relation to the steel sheet is heated. The slab used 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 sheet. 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 is less than 1100°C, the alloying elements may not be sufficiently dissolved in the slab, leaving coarse alloy carbides, which may cause embrittlement cracking during hot rolling. Therefore, the heating temperature is preferably 1100°C or higher. The upper limit of the heating temperature is not particularly limited, but is preferably 1300°C or lower from the viewpoint of the capacity of the heating equipment and productivity.

[0074] [Rough rolling] In this method, for example, the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc. The conditions for rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.

[0075] [Finish Rolling] The heated slab, or the slab that has been rough-rolled as needed, is then subjected to finish rolling. As described above, the slab used contains a relatively large amount of alloying elements, so it is necessary to increase the rolling load during hot rolling. For this reason, it is preferable to perform hot rolling at a high temperature. In particular, the end temperature of finish rolling is 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 preferably 850°C or higher. On the other hand, in order to suppress coarsening of austenite, it is preferable that the end temperature of finish rolling be 1050°C or lower.

[0076] [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. For example, the average cooling rate is preferably 25°C / s or more and the coiling temperature is preferably 480°C or less. For example, the average cooling rate is preferably 100°C / s or less and the coiling temperature is preferably 300°C or more.

[0077] [Post-hot rolling treatment step] A preferred embodiment of the present manufacturing method optionally further includes a post-hot rolling treatment step, after the hot rolling step and before the pickling step, in which the obtained hot-rolled steel sheet is retained in a temperature range of 400 to 680°C for at least 1 hour but less than 24 hours. By performing such heat treatment, cementite can be formed in the hot-rolled steel sheet, and elements that stabilize austenite, such as Mn, can be concentrated in the cementite in the subsequent annealing step, making it possible to further increase the stability of the austenite. As a result, it is possible to obtain a steel sheet having a metal structure in which, when measured by X-ray diffraction, the lattice constant calculated from the diffraction peaks derived from retained austenite at the surface layer portion, the 1 / 4 thickness position, and the 1 / 2 thickness position satisfies the following formula 1: A s >3.5800Å A q >3.5800Å Ac >3.5800 Å ...Equation 1 where A s , A q and A c are lattice constants calculated from the diffraction peaks derived from retained austenite at the surface layer portion, the 1 / 4 sheet thickness position, and the 1 / 2 sheet thickness position, respectively.

[0078] If the residence temperature is lower than 400°C, the diffusion of elements such as Mn into cementite becomes insufficient, and austenite cannot be sufficiently stabilized in the annealing process. On the other hand, if the residence temperature is higher than 680°C, cementite dissolves, and the effect of element enrichment in cementite cannot be fully achieved. Furthermore, if the residence time is less than 1 hour, cementite is not sufficiently formed, and similarly, the effect of element enrichment in cementite cannot be fully achieved. In either case, it becomes difficult to obtain a steel sheet having a metal structure that satisfies the above formula 1. On the other hand, even if the residence time is longer than 24 hours, the effect of element enrichment in cementite saturates, and therefore, heat treatment for a long period of time of 24 hours or more is not preferable from the viewpoint of economy. When the temperature of the hot-rolled steel sheet after coiling is lower than 400°C, for example, the hot-rolled steel sheet may be reheated as necessary.

[0079] [Pickling Step] Next, the obtained hot-rolled steel sheet is pickled to remove oxide scale formed on the surface of the hot-rolled steel sheet. Pickling may be carried out under conditions suitable for removing oxide scale, and may be carried out once or in multiple steps to ensure complete removal of oxide scale.

[0080] [Cold Rolling Process] The pickled hot-rolled steel sheet is cold-rolled at a reduction of 35 to 80% in the cold rolling process. By setting the cold rolling reduction 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 is preferably 50% or more. On the other hand, by setting the cold rolling reduction to 80% or less, it is possible to prevent the rolling load from becoming excessively large, making rolling difficult. The cold rolling reduction is preferably 70% or less. The number of rolling passes and the reduction per pass are not particularly limited, and may be appropriately set so that the reduction of the entire cold rolling is within the above range.

[0081] [Annealing Process] [Holding at a Maximum Heating Temperature of 830 to 900°C for 20 to 150 Seconds] The obtained cold-rolled steel sheet is heated, for example, 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. This promotes austenitization, and in the subsequent cooling process, a desired hard structure consisting mainly of martensite or the like is obtained, thereby reliably achieving a tensile strength of 1660 MPa or more. If the maximum heating temperature is less than 830°C or the holding time at the maximum heating temperature is less than 20 seconds, austenitization is likely to be insufficient, making it difficult to obtain the desired area ratio of martensite in the final steel sheet and thus difficult to achieve a tensile strength of 1660 MPa or more. On the other hand, if the maximum heating temperature exceeds 900°C or the holding time at the maximum heating temperature exceeds 150 seconds, austenite coarsens, reducing hardenability and making ferrite transformation and bainite transformation more likely to occur. As a result, it becomes difficult to obtain the desired structure fraction in the final structure.

[0082] [Coating Step] After the annealing step, the surface of the cold-rolled steel sheet may be subjected to a coating treatment for the purpose of improving corrosion resistance, etc. The coating treatment may be a treatment such as hot-dip galvanizing, alloyed hot-dip galvanizing, or electroplating. For example, the steel sheet may be subjected to a hot-dip galvanizing treatment as the coating treatment, or an alloying treatment may be performed after the hot-dip galvanizing treatment. The coating layer may contain, for example, at least one element selected from the group consisting of zinc, aluminum, magnesium, and alloys thereof. More specifically, the coating layer may be a hot-dip galvanized layer or an electroplated layer. Examples of hot-dip galvanized layers include a hot-dip galvanized (GI) layer, a hot-dip galvannealed (GA) layer, a hot-dip aluminum plating layer, a hot-dip Zn—Al alloy plating layer, a hot-dip Zn—Al—Mg alloy plating layer, and a hot-dip Zn—Al—Mg—Si alloy plating layer. Examples of electroplated layers include an electrogalvanized layer and an electrogalvanized Zn—Ni alloy plating layer. Preferably, the coating layer is a hot-dip galvanized layer, a hot-dip galvannealed layer, or an electrogalvanized layer. The specific conditions for the coating treatment and alloying treatment are not particularly limited, and may be any appropriate conditions known to those skilled in the art.

[0083] [Cooling Step] [Cooling to the Ms Point or Below at an Average Cooling Rate of 1.0°C / Second or More] Finally, the cold-rolled steel sheet after the annealing step or the coating step is cooled to the Ms point or below at an average cooling rate of 1.0°C / second or more in the next cooling step. This allows the final metal structure to be composed of a structure containing 85.0% or more of martensite by area, while the metal structure can contain 1.0 to 7.0% by area of ​​retained austenite, thereby achieving both a tensile strength of 1660 MPa or more and excellent hydrogen embrittlement resistance. Here, the Ms point (°C) is calculated based on the following formula 7. Ms = 550 - 350 x [C] - 40 x [Mn] - 20 x [Cr] - 10 x [Mo] - 17 x [Ni] - 10 x [Cu] - 35 x [V] - 10 x [W] - 15 x [Co] Equation 7 In the formula, [C], [Mn], [Cr], [Mo], [Ni], [Cu], [V], [W] and [Co] are the contents (mass%) of each element in the steel plate.

[0084] On the other hand, if the average cooling rate is less than 1.0°C / sec, a relatively large amount of ferrite and bainite will be formed during the cooling process, making it difficult to obtain the desired structure fraction in the final structure and making it impossible to achieve sufficient tensile strength. Preferably, the average cooling rate is 10.0°C / sec or more and 20.0°C / sec or less. In particular, by controlling the average cooling rate to 20.0°C / sec or less, the stability of the retained austenite contained in the metal structure of the steel sheet can be increased throughout the steel sheet. As a result, the lattice constant, i.e., A, calculated from the diffraction peaks derived from the retained austenite at the surface layer portion, the 1 / 4 thickness position, and the 1 / 2 thickness position can be reduced. s , A q and A c It can be ensured that satisfies the above formula 1.

[0085] [Preferred embodiment of cooling step] In a preferred embodiment of the cooling step, the cold-rolled steel sheet after the annealing step or the coating step is cooled to a controlled temperature T of 100°C or higher and Ms point -100°C or lower at an average cooling rate of 1.0°C / second or higher, and then cooled from the controlled temperature T to room temperature (25°C) at an average cooling rate of less than 1.0°C / second. By performing such a cooling treatment, it becomes possible to further increase the stability of retained austenite due to the concentration of elements such as carbon. As a result, it becomes possible to obtain a steel sheet having a metal structure in which, when measured by X-ray diffraction, the lattice constant calculated from the diffraction peaks derived from retained austenite at the surface layer portion, the 1 / 4 sheet thickness position, and the 1 / 2 sheet thickness position satisfies the following formula 1. A s >3.5800Å A q >3.5800Å A c >3.5800 Å ...Equation 1 where A s , A q and A c are lattice constants calculated from the diffraction peaks derived from retained austenite at the surface layer portion, the 1 / 4 sheet thickness position, and the 1 / 2 sheet thickness position, respectively.

[0086] If the control temperature T is higher than the Ms point −100°C, the amount of untransformed austenite is relatively large, and even if carbon enrichment occurs in the untransformed austenite, the retained austenite in the final metallographic structure may not be sufficiently stabilized. In this case, it becomes difficult to obtain a steel sheet having a metallographic structure that satisfies the above formula 1. On the other hand, if the control temperature T is less than 100°C or the average cooling rate is less than 1.0°C / sec, it becomes difficult to promote the enrichment of elements such as carbon in austenite, and similarly, it becomes difficult to sufficiently stabilize the retained austenite in the final metallographic structure. In this case, it also becomes difficult to obtain a steel sheet having a metallographic structure that satisfies the above formula 1. The lower limit of the average cooling rate from the control temperature T to room temperature is preferably, for example, 0.1°C / sec or more.

[0087] [Stabilization Treatment Step] Preferably, the cold-rolled steel sheet after the cooling step is subjected to a stabilization treatment. Specifically, the stabilization treatment includes heating the cold-rolled steel sheet cooled to the Ms point or below or room temperature in the cooling step as needed, and then holding it in a temperature range of 70°C or higher and lower than 280°C for 1 hour or higher and lower than 200 hours. By treating the cold-rolled steel sheet containing the retained austenite generated in the previous cooling step in such a temperature range for a relatively long time, austenite-stabilizing elements in the steel sheet, such as C and Mn, can be concentrated in the retained austenite, thereby sufficiently stabilizing the retained austenite. As a result, it is possible to obtain a steel sheet having a metallographic structure in which, when measured by X-ray diffraction, the lattice constant calculated from the diffraction peaks derived from the retained austenite at the surface layer, the 1 / 4 thickness position, and the 1 / 2 thickness position satisfies the following formula 1: A s >3.5800Å A q >3.5800Å A c >3.5800 Å ...Equation 1 where A s , A q and A c are lattice constants calculated from the diffraction peaks derived from retained austenite at the surface layer portion, the 1 / 4 sheet thickness position, and the 1 / 2 sheet thickness position, respectively.

[0088] If the treatment temperature is below 70°C or the holding time is less than 1 hour, the stabilization of the retained austenite will be insufficient, making it difficult to obtain a steel sheet having a metal structure that satisfies the above formula 1. On the other hand, if the treatment temperature is 280°C or higher or the holding time is 200 hours or longer when the stabilization treatment step is carried out, much of the retained austenite will decompose due to excessive heat treatment, and the area fraction of the retained austenite in the final structure may be less than 1.0% and / or the desired strength may not be achieved due to softening. Preferably, the treatment temperature is 100 to 200°C, and the holding time is 10 to 100 hours.

[0089] [Skin-pass rolling process] Preferably, the cold-rolled steel sheet after the cooling process or the cold-rolled steel sheet after the stabilization treatment process is subjected to skin-pass rolling. Specifically, skin-pass rolling involves rolling the cold-rolled steel sheet after the cooling process or the cold-rolled steel sheet after the stabilization treatment process to an elongation of more than 0.05% and less than 2.00%. Here, the elongation is defined by the following formula 8 using the entry speed V1 (m / sec) and the delivery speed V2 (m / sec) of the steel sheet: Elongation = (V2 - V1) / V1 × 100 ... formula 8

[0090] By performing skin-pass rolling under these conditions to lightly process the surface of the cold-rolled steel sheet, it is possible to decompose the unstable retained austenite present in the surface layer portion and at the 1 / 4 position of the sheet thickness of the steel sheet, thereby reducing the amount of unstable retained austenite in the surface layer portion and at the 1 / 4 position of the sheet thickness, and ultimately increasing the stability of the retained austenite present in the surface layer portion and at the 1 / 4 position of the sheet thickness. As a result, the area ratio RA of the retained austenite in the surface layer portion s and the area fraction RA of retained austenite at the 1 / 2 thickness position c RA s / RA cIt is possible to obtain a steel sheet having a metal structure in which the lattice constant calculated from the diffraction peaks derived from the retained austenite in the surface layer portion, the quarter thickness position, and the half thickness position satisfies the following formula 2. In this case, the amount of hydrogen released from the retained austenite in the surface layer portion and the quarter thickness position due to bending or the like can be significantly reduced, thereby making it possible to further improve the hydrogen embrittlement resistance of the steel sheet compared to when the amount of retained austenite is simply controlled to be within the range of 1.0 to 7.0%. s / A c >0.9970 A q / A c >0.9970 ... Equation 2 where A s , A q and A c are lattice constants calculated from the diffraction peaks derived from retained austenite at the surface layer portion, the 1 / 4 sheet thickness position, and the 1 / 2 sheet thickness position, respectively.

[0091] If the elongation rate is 0.05% or less, the decomposition of unstable retained austenite present in the surface layer of the steel sheet becomes insufficient, and the RA s / RA c <0.75 and / or it becomes difficult to obtain a steel sheet having a metal structure that satisfies the above formula 2. On the other hand, when a skin-pass rolling step is performed, if the elongation is 2.00% or more, much of the retained austenite may be decomposed due to excessive skin-pass rolling, and the area ratio of the retained austenite in the final structure may become less than 1.0%. Preferably, the elongation is 0.10 to 1.00%.

[0092] 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.

[0093] 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.

[0094] First, molten steel was cast by a continuous casting method to form slabs having various chemical compositions shown in Table 1, and these slabs were heated to the heating temperatures shown in Table 2 and hot-rolled. Hot rolling was performed by rough rolling and finish rolling, and the end temperatures of finish rolling were as shown in Table 2. The finish-rolled steel sheets were then cooled under the conditions shown in Table 2 and coiled. Next, the obtained hot-rolled steel sheets having a thickness of 2.6 mm were appropriately subjected to the post-hot rolling treatments shown in Table 2, then pickled, and cold-rolled at the rolling reduction shown in Table 2 to obtain cold-rolled steel sheets having a thickness of 1.4 mm. Next, the obtained cold-rolled steel sheets were heated in a heating furnace and a soaking furnace of a continuous annealing line, heated and held under the conditions shown in Table 2, and then appropriately subjected to hot-dip galvanizing (GI) or galvannealing (GA) as a coating treatment. Finally, the cold-rolled steel sheet or plated steel sheet was cooled in the cooling step under the conditions shown in Table 2, and then appropriately subjected to a stabilization treatment step and / or a skin-pass rolling step shown in Table 2. In all examples, the "average cooling rate to the Ms point or lower" was evaluated as the "average cooling rate to the control temperature T."

[0095] The properties of the obtained steel sheets were measured and evaluated by the following methods.

[0096] [Tensile Strength] The tensile strength 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 direction perpendicular to the rolling direction of the steel plate.

[0097] [Evaluation of Hydrogen Embrittlement Resistance] The hydrogen embrittlement resistance of the bent portion of the obtained steel plate was evaluated by the following method. Specifically, the steel plate was first cut by applying a shear force with a clearance of 12.5% ​​and a shear angle of 0 degrees to obtain a 15 mm wide steel plate. The test specimens were taken so that the longitudinal direction was parallel to the direction perpendicular to the rolling direction of the steel plate, i.e., the bending ridge was parallel to the rolling direction of the steel plate. If the rolling direction of the steel plate cannot be identified, the test specimens may be taken in any direction within the steel plate surface. Next, a U-bend test was performed at 8R. Specifically, the U-bend test was performed using the push-bending method described in JIS Z 2248:2022. The inner radius r was set to 8 mm, and the distance between the supports L = 2r + 2t (thickness of the test plate) ± 1 mm. The test specimen was pushed until it passed through the supports, and a 180° bent sample was obtained. Next, a strain gauge was attached to the center of the obtained test piece, and both ends of the test piece were fastened with bolts to apply stress. The applied stress was calculated from the strain of the monitored strain gauge. The strain gauge used had a gauge length of 1.0 mm, and was attached so that it was parallel to the longitudinal direction of the steel plate at the bend apex, i.e., perpendicular to the bend ridge line. Load stress = strain amount at the bend apex measured by the strain gauge × Young's modulus (20,500 N / mm 2 The applied stress was a constant value (constant at 100°C), and a stress corresponding to 80% of the tensile strength of the test specimen was applied. This is because the residual stress introduced during forming is thought to correspond to the tensile strength of the steel sheet. The obtained U-bend test specimens were immersed in an HCl aqueous solution with a pH of 2 at a liquid temperature of 35°C and then held for 72 hours to check for the presence or absence of cracks. The lower the pH of the HCl aqueous solution and the longer the immersion time, the greater the amount of hydrogen that penetrates into the steel sheet, resulting in a severe hydrogen embrittlement environment. After immersion, the maximum length of cracks that occurred in the U-bend test specimens was measured, and they were evaluated as follows: NG (fail) if a crack longer than 8 mm was observed; A (passable) if a crack longer than 3 mm but shorter than 8 mm was observed; AA (good) if a crack longer than 1 mm but shorter than 3 mm was observed; and AAA (excellent) if a crack shorter than 1 mm was observed.

[0098] High strength steel sheets having a tensile strength of 1660 MPa or more and an evaluation of hydrogen embrittlement resistance of A, AA or AAA were evaluated as having excellent hydrogen embrittlement resistance. The results are shown in Table 3. In Table 3, RAs , R.A. q , A s and A q The values ​​of σ represent calculated values ​​based on measurements from only one surface of the steel plate. However, since all steel plates are manufactured using the same treatment on both sides, these values ​​are substantially the same on both sides of the steel plate, and in fact, it has been confirmed that for some steel plates these values ​​are the same on both sides of the steel plate.

[0099]

[0100]

[0101]

[0102]

[0103] Referring to Tables 2 and 3, in Example A-2, the maximum heating temperature in the annealing step was low, resulting in insufficient austenitization and the desired martensite area ratio could not be obtained. As a result, tensile strength decreased. In Example B-2, the holding time in the annealing step was short, resulting in insufficient austenitization and the desired martensite area ratio could not be obtained. As a result, tensile strength decreased. In Example C-2, the holding time in the annealing step was long, which is thought to have caused austenite to coarsen and reduced hardenability. As a result, ferrite transformation and bainite transformation progressed, making it impossible to obtain the desired martensite area ratio, and tensile strength decreased. In Example D-2, the average cooling rate to the Ms point or below in the cooling step was slow, resulting in the formation of relatively large amounts of ferrite and bainite during the cooling process. As a result, the desired martensite area ratio could not be obtained, and tensile strength decreased. In Example F-2, it is thought that the high treatment temperature in the stabilization treatment step caused much of the retained austenite to decompose. As a result, the desired area ratio of retained austenite could not be obtained in the final structure, and hydrogen embrittlement resistance deteriorated. In Example G-2, it is believed that much of the retained austenite was decomposed similarly due to the long holding time in the stabilization treatment step. As a result, the desired area ratio of retained austenite could not be obtained in the final structure, and hydrogen embrittlement resistance deteriorated. In Example H-2, it is believed that much of the retained austenite was decomposed due to the high elongation in the skin-pass rolling step. As a result, the desired area ratio of retained austenite could not be obtained in the final structure, and hydrogen embrittlement resistance deteriorated.

[0104] In contrast to this, in all of the inventive examples according to the present invention, the metal structure of the steel plate was composed mainly of martensite, and a predetermined amount of retained austenite, which has the property of easily absorbing hydrogen, was included in the metal structure; more specifically, the metal structure of the steel plate was controlled to contain, in area percentages, 85.0% or more of martensite and 1.0 to 7.0% of retained austenite. This made it possible to significantly improve the hydrogen embrittlement resistance, particularly the hydrogen embrittlement resistance of the bent portion, despite having an extremely high tensile strength of 1660 MPa or more.

[0105] In particular, in Examples F-1, H-1, I-1, and K-1 to P-1, an appropriate post-hot rolling treatment step was performed after the hot rolling step and before the pickling step, and in the cooling step, the steel was cooled at an average cooling rate of 1.0 ° C. / second or more to a controlled temperature T of 100 ° C. or higher and Ms point -100 ° C. or lower, and then cooled from the controlled temperature T to room temperature at an average cooling rate of less than 1.0 ° C. / second, and further an appropriate stabilization treatment step and skin pass rolling step were performed, thereby achieving the characteristics of martensite: 85.0% or higher and retained austenite: 1.0 to 7.0%, as well as RA s / RA c <0.75, it was possible to obtain steel sheets having metal structures that satisfied Formula 1 and Formula 2. As a result, in these examples, the hydrogen embrittlement resistance was particularly significantly improved.

Claims

1. The tensile strength is 1660 MPa or more, the metal structure is, in area percentage, martensite: 85.0% or more, retained austenite: 1.0 to 7.0%, and the remaining structure: 10.0% or less, and the chemical composition is, in mass percentage, C: 0.25 to 0.45%, Si: 0.01 to 1.30%, Mn: 1.00 to 3.50%, 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%, 1. A steel plate comprising: Cr: 0-2.000%, Ti: 0-0.500%, B: 0-0.0100%, Nb: 0-0.500%, V: 0-0.500%, Cu: 0-0.500%, W: 0-0.100%, Ta: 0-0.100%, Sn: 0-0.100%, Sb: 0-0.100%, As: 0-0.100%, Mg: 0-0.0500%, Ca: 0-0.050%, Y: 0-0.050%, Zr: 0-0.050%, La: 0-0.050%, and Ce: 0-0.050%, with the balance being Fe and impurities.

2. The steel sheet according to claim 1, characterized in that, when measured by X-ray diffraction, the lattice constant calculated from the diffraction peaks derived from the retained austenite at the surface layer portion, the 1 / 4 position at the sheet thickness, and the 1 / 2 position at the sheet thickness satisfies the following formula 1. s >3.5800Å A q >3.5800Å A c >3.5800 Å...Equation 1 where A s , A q and A c are lattice constants calculated from the diffraction peaks derived from retained austenite at the surface layer portion, the 1 / 4 sheet thickness position, and the 1 / 2 sheet thickness position, respectively.

3. Area ratio RA of retained austenite in the surface layer s and the area fraction RA of retained austenite at the 1 / 2 position of the plate thickness c But R.A. s / R.A. c The steel sheet according to claim 1 or 2, characterized in that it satisfies < 0.

75.

4. The steel sheet according to any one of claims 1 to 3, characterized in that, when measured by X-ray diffraction, the lattice constant calculated from the diffraction peaks derived from retained austenite in the surface layer portion, the 1 / 4 position in the sheet thickness, and the 1 / 2 position in the sheet thickness satisfies the following formula 2. s / A c >0.9970 A q / A c >0.9970 ... Equation 2 Here, A s , A q and A c are lattice constants calculated from the diffraction peaks derived from retained austenite at the surface layer portion, the 1 / 4 sheet thickness position, and the 1 / 2 sheet thickness position, respectively.

5. The chemical composition is, in mass%, Co: 0.01 to 0.50%, Ni: 0.01 to 1.00%, Mo: 0.01 to 1.00%, Cr: 0.001 to 2.000%, 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%, W: 0.001 to 0.100%, Ta: 0.001 to 0.100%, Sn: 0.001 to 0.100%, Sb: 0.001 to 0.100%, As: 0.001 to 0.100%, The steel sheet according to any one of claims 1 to 4, characterized in that it contains at least one of Mg: 0.0001 to 0.0500%, Ca: 0.001 to 0.050%, Y: 0.001 to 0.050%, Zr: 0.001 to 0.050%, La: 0.001 to 0.050%, and Ce: 0.001 to 0.050%.

6. A member comprising the steel plate according to any one of claims 1 to 5.

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