Steel plates and components

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

[Technical Field]

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

[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). Hydrogen embrittlement cracking is a phenomenon in which a steel component 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, 45% to 83% tempered martensite, 15% to 53% bainite, and 2% or more retained austenite, in which 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 a tensile strength of 1470 MPa or more. Furthermore, Patent Document 1 teaches that the delayed fracture resistance of a high-strength steel plate having a TS≧1470 MPa can be improved by suppressing the coarsening of carbides in tempered martensite. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2022 / 185804 Summary of the Invention [Problem to be solved by the invention]

[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. [Means for solving the problem]

[0008] In order to achieve the above object, the present inventors have conducted research, focusing particularly on the metallographic structure of steel sheets. Specifically, the present inventors have discovered that by configuring the metallographic structure of a 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, 1660 MPa or more, and have completed the present invention.

[0009] The present invention, which has achieved the above object, is as follows. (1) The tensile strength is 1660 MPa or more, The metal structure is, in area%, Martensite: 85.0% or more, Retained austenite: 1.0 to 7.0%, and Remaining structure: 10.0% or less, The chemical composition is, in mass%, C: 0.25~0.45%, Si: 0.01 to 1.30%, Mn: 1.00~3.50%, 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 to 0.500% B: 0~0.0100%, Nb: 0 to 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: 0~0.100%, 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%, and Ce: 0 to 0.050% A steel plate, the balance of which consists of Fe and impurities. (2) The steel sheet according to (1), 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 1: A s >3.5800Å A q >3.5800Å A c >3.5800Å...Formula 1 where A s , A q and A care lattice constants calculated from the diffraction peaks derived from retained austenite at the surface layer, the 1 / 4 sheet thickness position, and the 1 / 2 sheet thickness position, respectively. (3) Area fraction of retained austenite in the surface layer, RA s and the area fraction RA of retained austenite at the 1 / 2 thickness position c However, RA s / RA c The steel sheet according to (1) or (2) above, wherein the steel sheet satisfies <0.75. (4) The steel sheet according to any one of the above (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, 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: 0.0001~0.0100%, 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%, 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% The steel sheet according to any one of the above items (1) to (4), characterized in that it contains at least one of the following. (6) A member comprising the steel plate according to any one of (1) to (5) above. [Effects of the Invention]

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

[0011] <Steel plate> The steel plate according to the embodiment of the present invention has a tensile strength of 1660 MPa or more, The metal structure is, in area%, Martensite: 85.0% or more, Retained austenite: 1.0 to 7.0%, and Remaining structure: 10.0% or less, The chemical composition is, in mass%, C: 0.25~0.45%, Si: 0.01 to 1.30%, Mn: 1.00~3.50%, 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 to 0.500% B: 0~0.0100%, Nb: 0 to 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: 0~0.100%, 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%, and Ce: 0 to 0.050% The remainder is Fe and impurities.

[0012] As mentioned above, hydrogen embrittlement cracking is known to occur more easily as the strength of steel increases. In particular, for steel sheets with extremely high strength, such as those with tensile strengths of 1660 MPa or higher, 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 penetrates the steel and 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 address the issue of hydrogen embrittlement in high-strength steel sheets with tensile strengths of 1660 MPa or higher, 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 when the tensile strength is high, such as 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 previously unknown 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, but 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 with the longitudinal direction of the test piece 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 achieved as long as the steel sheet has the above-mentioned tensile strength in any direction within the steel sheet plane. If the surface of the steel sheet is provided with a coating layer such as plating, the coating layer shall be removed before conducting the tensile test.

[0015] [Metal structure] Next, the metallographic structure of the steel sheet according to an embodiment of the present invention will be described. In the following description, the unit of structure fraction, "%", means "area %" unless otherwise specified. The metallographic structure is controlled in the surface layer portion, the 1 / 4 thickness position, and the 1 / 2 thickness position of the steel sheet. Here, the surface layer portion refers to a depth position of 50 μm in the thickness direction from the surface of the steel sheet. Hereinafter, unless otherwise specified, the structure fraction refers to the average value of the structure 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 will be 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, but, for example, the area fraction of martensite may be 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 %, 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. This makes it possible to significantly suppress 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, it may be difficult to ensure a sufficient area fraction of martensite, making it impossible to achieve the desired strength. 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 an excessive amount, the amount of hydrogen released may increase, thereby promoting hydrogen embrittlement. Therefore, the area ratio of 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 tissue: 10.0% or less] The area fraction of the remaining structure other than martensite and retained austenite may be 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, resulting in failure to obtain the desired strength and / or hydrogen embrittlement resistance. 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 total area fraction 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 fraction of the remaining structure requires advanced control during 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 total area ratio 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 at least one of ferrite, bainite, and pearlite, for example, or may be at least one of these.

[0019] [Identification and calculation of metal structure] [Martensite] The metallographic structure is identified and calculated as follows. First, a sample is taken with a cross-section perpendicular to the plate surface, and this cross-section is used as the observation surface. The cross-section is polished carefully to remove any scratches caused by polishing. The strain introduced by surface polishing is then removed by chemical polishing to obtain a cross-sectional observation sample for EBSD (Electron-Backscatter-Diffraction) analysis, in which the crystal orientation has not been altered by polishing. Here, crystal orientation scratches caused by polishing refer to areas that appear linearly through the metallographic structure in the inverse-pole-figure (IPF) map in EBSD analysis. Since such scratches are not inherent to the structure, it is natural that they should not be included in the observation field of view. It is recommended to search for a field of view that is free of scratches, or if scratches are present within the field of view, to polish the sample again.

[0020] The above sample for EBSD analysis is subjected to electron backscattering analysis. While EBSD analysis conditions are within the scope of common sense for 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, tilt the polished surface of the sample 60–70° 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 to maximize 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. Adjusting the detection sensitivity depends on the performance of the FE-SEM electron gun and EBSD detector used, so it is recommended to perform the adjustment within the scope of common sense for those skilled in the art. The adjustment should ultimately result in conditions that allow for clear observation of the EBSD pattern. The electron beam is then irradiated onto the observation field at 0.3 μm intervals, and an EBSD pattern is collected at each measurement point. Indexing and crystal orientation calculations are performed based on the EBSD patterns at each measurement point. AMETEK's APEX software is recommended for indexing and crystal orientation calculations. The EBSD data obtained in this manner is analyzed using AMETEK's OIMAnalysis (Orientation Imaging Microscopy) software, version 7 or later, which is EBSD data analysis software. The obtained EBSD data is opened in OIMAnalysis, and only regions with a CI (Confidence Index) value of 0.1 or higher are extracted. The CI value is an indicator of the reliability of the indexing and crystal orientation analysis results. Areas with a CI value below 0.1 are likely to be areas where contamination on the sample surface or overlapping orientations such as grain boundaries occur during electron beam irradiation, and can be considered areas that do not have the original crystal orientation of the metallographic structure. Then, regions with a GAM (Grain Average Misorientation) value of 0.5° or more are considered to be martensite, and the area fraction is calculated. Note that a grain boundary refers to a boundary between measurement points where the misorientation between the measurement points is 15° or more.Here, the GAM value is the average misorientation between measurement points of crystal orientation in the area surrounded by grain boundaries, and since structures formed at low temperatures such as martensite are characterized by misorientation within the grains due to transformation strain, etc., it is possible to distinguish them using the GAM value. This type of investigation is carried out at five or more locations within a 100 x 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] Retained austenite is formed between the laths of martensite at sub-nano size. Therefore, it is difficult to separate the two with the resolution of EBSD analysis, and the area fraction determined by the above method is the combined area fraction of martensite and retained austenite. 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 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 ratio of martensite is measured in the same manner at the 1 / 4 thickness position and the surface layer portion. However, when measuring the area ratio 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 ratios measured at the surface layer portion, the 1 / 4 thickness position, and the 1 / 2 thickness position is calculated and determined as the area ratio of martensite.

[0023] [Residual austenite] The area fraction of retained austenite is calculated by measurement using X-rays. First, a sample is taken from the same component as the sample taken to identify martensite. Next, the sample is removed from the plate surface to the half-thickness position in the plate thickness direction by mechanical polishing and chemical polishing. Next, MoKα rays are used as characteristic X-rays on the polished sample, 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 taken as the area fraction of retained austenite at the half-thickness position. The area fraction of retained austenite is obtained in the same manner for the surface layer and the quarter-thickness position, and finally, the average value is calculated and determined as the area fraction of retained austenite. The area fraction of retained austenite at the half-thickness position is calculated using the "RA" method described below. c ", and the area ratio of retained austenite in the surface layer is equivalent to "RA s " is equivalent to

[0024] [Remaining tissue] 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 would not be 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 >3.5800Å, and A c >3.5800Å] 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 thickness position, and the 1 / 2 thickness position satisfies the following formula 1. A s >3.5800Å A q>3.5800Å A c >3.5800Å...Formula 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, 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 the 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 s , A q and A csatisfying the above formula 1, the present inventors have found that the stability of retained austenite contained in the metallographic structure of a steel sheet can be improved throughout the steel sheet. In relation to this, the inventors have found 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 retained austenite can be increased, thereby improving the stability of retained austenite. More specifically, as will be explained 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. In the subsequent annealing step, austenite-stabilizing elements such as Mn can be concentrated in the cementite, 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 present inventors have conducted extensive research and have found that 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 when measured by X-ray diffraction, 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 to satisfy 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, etc., 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 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 less than 3.7000Å (i.e., A s ≦3.7000Å, A q ≦3.7000Å, 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 specimen is mechanically polished and chemically polished from the surface to half the thickness in the thickness direction. 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 radiation as characteristic X-rays for the polished specimen, 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 equation 5 to obtain the lattice constant A c It is determined as follows. d c(hkl) =λ / 2sin(2θ c(hkl) / 2) ...expression 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...expression 5 where d c(hkl) is the lattice spacing (Å) of the (hkl) plane at the half-thickness position, λ is the wavelength of the light source (MoKα ray wavelength) of the X-ray diffraction device (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 using the following formula 6 to obtain a three-point weighted average. Then, the 2θ with the highest diffraction intensity on each diffraction plane is calculated as the 2θ of that diffraction plane. c(hkl) Let's say. 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 P n is the diffraction angle 2θ in the "diffraction angle vs. diffraction intensity" data set 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 this smoothing process is performed on all measurement points. The same process is performed on the surface layer 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 is s and the area fraction RA of retained austenite at the 1 / 2 thickness position c RA s / RA c Meets <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 of the steel sheet should be controlled to be smaller by a predetermined percentage 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 of the steel sheet should be controlled to be smaller by a predetermined percentage 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 to satisfy <0.75 significantly improves 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 naturally reduces the amount of unstable retained austenite in the surface layer. Reducing the amount of unstable retained austenite in the surface layer in this way suppresses the reduction or disappearance of the retained austenite in the surface layer 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 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 to a range of 1.0 to 7.0%.

[0032] From the viewpoint of further improving hydrogen embrittlement resistance, the area fraction 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 of RA to RA may be greater than 0, or greater than 0.10 (i.e., RA s / RAc ≧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 c >0.9970] In 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 1 / 4 sheet thickness position, and the 1 / 2 sheet 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, 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] The inventors have 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 metallographic structure 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 increasing the stability of the retained austenite in the surface layer and the quarter-thickness position, it is possible to prevent the retained austenite in the surface layer and the quarter-thickness position from being reduced or lost due to strain-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 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. Therefore, it is preferable to reduce 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 better. For example, each of these ratios should be 0.9975 or more (i.e., A q / A c ≧0.9975, and A q / A c The upper limit may be, but is not particularly limited to, for example, A s / A c and A q / A c The ratio is less than 1.1000 (i.e., A q / Ac ≦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 , see [Lattice constant A s , A q and A c This is as explained above in the section on calculation of

[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 fraction 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 in particular can be reduced, while the stability of the retained austenite present in the surface layer portion and the quarter thickness position can be further increased. 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 plate] In the following description, the unit of content of each element, "%," means "% by mass" unless otherwise specified. Furthermore, in this specification, "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits, unless otherwise specified.

[0039] In a particular embodiment of the present invention, the chemical composition of the steel sheet is, in mass %, The chemical composition is, in mass%, C: 0.25~0.45%, Si: 0.01 to 1.30%, Mn: 1.00~3.50%, 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 to 0.500% B: 0~0.0100%, Nb: 0 to 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: 0~0.100%, 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 Remainder: Fe and impurities Each element will be explained in more detail below.

[0040] [C: 0.25~0.45%] C is an element that is effective for increasing tensile strength inexpensively. C is also an element that is 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, if the C content is excessive, the weldability may be reduced. 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%] Silicon acts as a deoxidizer and is an element that affects the morphology of carbides and retained austenite after heat treatment. Without silicon, it may be difficult to suppress the formation of coarse oxides. Therefore, the silicon content is set to 0.01% or more. The silicon 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 reduce 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: 1.00~3.50%] Mn is an element effective in improving the hardenability of steel and increasing the strength of steel sheet. 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. Therefore, the Mn content is set to 3.50% or less. The Mn content may also be 3.20% or less, 3.00% or less, 2.80% or less, or 2.60% or less.

[0043] [P:0.0001~0.0200%] P is an element that embrittles welds and deteriorates platability. Therefore, the P content is set to 0.0200% or less. The P content may be 0.0180% or less, 0.0150% or less, 0.0120% or less, or 0.0100% or less. Although a lower P content is preferable, 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:0.0001~0.0200%] 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 can become crack initiation sites during cold working. For this reason, the S content is set to 0.0200% or less. The S content may also be 0.0180% or less, 0.0150% or less, 0.0120% or less, or 0.0100% or less. Although the lower the S content, the better. However, 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 can generate coarse Al oxides that can become crack initiation points. Therefore, 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:0.0001~0.0200%] 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 the N content to less than 0.0001% would result in a significant increase in manufacturing costs. For this reason, 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:0.0001~0.0200%] 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 better. However, reducing the O content to less than 0.0001% would result in a significant increase in manufacturing costs. For this reason, 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: 0-0.0500%, Ca: 0-0.050%, Y: 0-0.050%, Zr: 0-0.050%, La: 0-0.050%, and Ce: 0-0.050%. These optional elements will be described in detail below.

[0049] [Co: 0-0.50%] Co is an element effective for controlling the morphology of carbides and increasing the strength of steel sheet. The Co content may be 0%, but to obtain these effects, the Co content is preferably 0.001% or more. The Co content may be 0.01% or more, 0.02% or more, 0.05% or more, or 0.10% or more. On the other hand, excessive Co content may cause precipitation of coarse Co carbides. Therefore, the Co content is preferably 0.50% or less. The Co content may be 0.40% or less, 0.30% or less, or 0.20% or less.

[0050] [Ni: 0-1.00%] Ni is an element effective in increasing the strength of steel sheets. Ni is also effective in improving wettability and accelerating 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 deteriorate weldability. Therefore, 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: 0-1.00%] Mo is an element effective in increasing the strength of steel sheets. Mo also 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 form. 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:0~2.000%] 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: 0~0.500%] Ti is an element that contributes to increasing the strength of steel sheets by strengthening precipitates, strengthening fine grains by inhibiting ferrite grain growth, and strengthening dislocations by inhibiting 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: 0~0.0100%] B is an element that suppresses the formation of ferrite and pearlite during the cooling process from the austenite temperature range and promotes the formation of low-temperature transformation structures such as martensite. B is also an element that is beneficial for increasing the strength of steel. The B content may be 0%, but 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: 0~0.500%] 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. Therefore, 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:0~0.500%] V is an element that contributes to increasing the strength of steel sheets by strengthening precipitates, strengthening fine grains by inhibiting ferrite grain growth, and strengthening dislocations by inhibiting recrystallization. The V content may be 0%, but to obtain these effects, the V content is preferably 0.001% or more. The V content may be 0.002% or more, 0.010% or more, or 0.020% or more. On the other hand, excessive V content may increase the precipitation of carbonitrides. Therefore, the V content is preferably 0.500% or less. The V content may be 0.450% or less, 0.400% or less, 0.300% or less, or 0.100% or less.

[0057] [Cu: 0-0.500%] 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 can embrittle the steel material during hot rolling, making hot rolling difficult. Therefore, 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:0~0.100%] W is an element effective in increasing the strength of steel sheets. 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 particles. 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 for controlling the morphology of carbides and increasing the strength of 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, resulting in a decrease in ductility as the strength of the steel sheet increases. Therefore, 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: 0~0.100%] 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 better, and even 0% is 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: 0~0.100%] Like Sn, Sb is an element contained when scrap is used as a steel raw material. Sb is also an element that 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 even 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:0~0.100%] Like Sn and Sb, As is an element contained when scrap is used as a steel raw material. 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 better, and 0% is 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: 0~0.0500%] Mg is an element that can control the morphology of sulfides even 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. Therefore, 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: 0~0.050%] 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. Therefore, 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:0~0.050%] [Zr: 0~0.050%] [La: 0~0.050%] [Ce: 0~0.050%] Y, Zr, La, and Ce are elements that can control the morphology of sulfides when contained in small amounts, similar to Mg, etc. 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 content of these elements may result in the formation of coarse oxides, and therefore the content of Y, Zr, La and Ce is 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 during the industrial production of steel sheet due to various factors in the production process, including raw materials such as ore and scrap.

[0067] The chemical composition of the steel sheet according to the present invention may be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) of chips in accordance with JIS G 1201:2014. Specifically, for example, a 35 mm square test piece is obtained from the steel sheet at approximately half the thickness position, and the test piece is measured using a Shimadzu ICPS-8100 (measuring device) or the like under conditions based on a pre-established 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 sheet 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] Plate 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 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 described 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] <Steel sheet manufacturing method> 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 manufacturing a steel sheet according to an embodiment of the present invention includes: a hot rolling process including heating a slab having the chemical composition described above in relation to the steel plate to a temperature of 1100 to 1300°C, then finish rolling the slab under the condition of a final temperature of 850 to 1050°C, and cooling the finish-rolled steel plate to 500°C or less at an average cooling rate of 20°C / sec or more, and coiling the steel plate; a pickling step of pickling the obtained hot-rolled steel sheet; A cold rolling process in which the pickled hot-rolled steel sheet is cold-rolled at a reduction ratio 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; a cooling step of cooling the cold-rolled steel sheet to the Ms point or lower at an average cooling rate of 1.0°C / second or higher 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 plate is heated. From the viewpoint of productivity, the slab used is preferably cast by a continuous casting method, but may also be produced by an ingot casting method or a thin slab casting method. The slab used contains a relatively large amount of alloying elements in order to obtain a high-strength steel plate. Therefore, the slab must be heated before being subjected to hot rolling to dissolve the alloying elements. 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] [Finishing 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 a large rolling load is required during hot rolling. For this reason, hot rolling is preferably performed at a high temperature. In particular, the finish temperature of finish rolling is important in terms of controlling the metal structure of the steel sheet. If the finish rolling temperature is low, the metal structure may become non-uniform and formability may decrease. For this reason, the finish rolling temperature is preferably 850°C or higher. On the other hand, in order to suppress coarsening of austenite, the finish rolling temperature is preferably 1050°C or lower.

[0076] [Cooling and winding] Next, 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. If the average cooling rate is less than 20°C / second 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 / second or more and the coiling temperature is preferably 480°C or less. For example, the average cooling rate is preferably 100°C / second or less and the coiling temperature is preferably 300°C or more.

[0077] [Hot rolling post-treatment process] In a preferred embodiment of the present manufacturing method, the 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 in the subsequent annealing step, elements that stabilize austenite, such as Mn, can be concentrated in the cementite, further increasing the stability of the 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 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 >3.5800Å...Formula 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, 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 one 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 longer than 24 hours is not preferable from an economic standpoint. When the temperature of the hot-rolled steel sheet after coiling is lower than 400°C, the hot-rolled steel sheet may be reheated as necessary.

[0079] [Pickling process] 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 set appropriately so that the reduction of the entire cold rolling is within the above range.

[0081] [Annealing process] [Hold at maximum heating temperature of 830-900℃ for 20-150 seconds] The resulting cold-rolled steel sheet is heated, for example, in a heating furnace and 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, resulting in the desired hard structure, mainly composed of martensite, in the subsequent cooling process, thereby reliably achieving a tensile strength of 1660 MPa or higher. If the maximum heating temperature is lower than 830°C or the holding time at the maximum heating temperature is shorter 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 higher. 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 process] 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 process] [Cool down to below the Ms point at an average cooling rate of 1.0°C / sec or more] Finally, the cold-rolled steel sheet after the annealing or coating process is cooled to the Ms point or below at an average cooling rate of 1.0°C / sec or more in the next cooling process. This allows the final metal structure to be composed of a structure containing 85.0% or more martensite by area, while the metal structure can contain 1.0 to 7.0% by area of ​​retained austenite, thereby making it possible to achieve both a tensile strength of 1660 MPa or more and excellent hydrogen embrittlement resistance. Here, the Ms point (°C) is calculated using the following formula 7. Ms=550-350×[C]-40×[Mn]-20×[Cr]-10×[Mo]-17×[Ni]-10×[Cu]-35×[V]-10×[W]-15×[Co]...Formula 7 In the formula, [C], [Mn], [Cr], [Mo], [Ni], [Cu], [V], [W] and [Co] are the contents (mass%) of each element in the steel sheet.

[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 in the surface layer, the 1 / 4 thickness position, and the 1 / 2 thickness position can be improved. s , A q and A c It can be ensured that satisfies the above formula 1.

[0085] [Preferred embodiment of the 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 / sec 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 / sec. By performing such a cooling treatment, it becomes possible to further improve 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 thickness position, and the 1 / 2 thickness position satisfies the following formula 1: A s >3.5800Å A q >3.5800Å A c >3.5800Å...Formula 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, 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 will be 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 will be 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 will be difficult to promote the enrichment of elements such as carbon in austenite, and similarly, it may be difficult to sufficiently stabilize the retained austenite in the final metallographic structure. In this case, it will also be 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 process] Preferably, the cold-rolled steel sheet after the cooling step is subjected to a stabilization treatment. Specifically, the stabilization treatment involves 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 to less than 280°C for 1 hour to less than 200 hours. By treating the cold-rolled steel sheet containing 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 >3.5800Å...Formula 1 where A s , A q and A care lattice constants calculated from the diffraction peaks derived from retained austenite at the surface layer, 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 a 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 step or the cold-rolled steel sheet after the stabilization step is subjected to skin-pass rolling. Specifically, skin-pass rolling involves rolling the cold-rolled steel sheet after the cooling step or the cold-rolled steel sheet after the stabilization step 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 / s) and the delivery speed V2 (m / s) of the steel sheet. Elongation rate = (V2 - V1) / V1 x 100 Equation 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, 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 fraction 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 metallographic structure in which the lattice constant calculated from the diffraction peaks derived from 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%. 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, 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 is insufficient, resulting in a high 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. [Example]

[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. These slabs were then heated to the heating temperatures shown in Table 2 and hot-rolled. Hot rolling was performed by rough rolling and finish rolling, with the end temperatures of finish rolling being 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, followed by pickling and cold rolling at the rolling reductions 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 the heating furnace and 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 galvannealed hot-dip galvannealing (GA) as a coating treatment. Finally, the cold-rolled steel sheets or plated steel sheets were cooled in the cooling process under the conditions shown in Table 2, and then appropriately subjected to the stabilization treatment process and / or skin-pass rolling process shown in Table 2. In this example, in all examples, the "average cooling rate to the Ms point or below" is 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 in a direction such that 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 using 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° to obtain 15 mm-wide steel plates. The test specimens were cut so that their longitudinal direction was parallel to the direction perpendicular to the rolling direction of the steel plate, i.e., so that 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 can be cut 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 was set to 2r + 2t (specimen thickness) ± 1 mm. The test specimen was pushed through the supports to obtain a 180° bent sample. Next, a strain gauge was attached to the center of the obtained test specimen, and stress was applied by tightening both ends of the test specimen with bolts. 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 top of the bend, i.e., perpendicular to the bend ridge. Load stress = strain at the top of the bend measured by the strain gauge × Young's modulus (20,500 N / mm 2 The applied stress was set at a constant value (at 2000 psi), and a stress corresponding to 80% of the specimen's tensile strength was applied. This was because the residual stress introduced during forming is thought to correspond to the tensile strength of the steel sheet. The obtained U-bend specimens were immersed in an HCl solution with a pH of 2 at a liquid temperature of 35°C and then held for 72 hours to check for cracks. The lower the pH of the HCl solution and the longer the immersion time, the greater the amount of hydrogen that penetrated into the steel sheet, resulting in a severe hydrogen embrittlement environment. After immersion, the maximum length of cracks that occurred in the U-bend specimens was measured, and cracks longer than 8 mm were evaluated as NG (fail), cracks between 3 mm and 8 mm were evaluated as A (passable), cracks between 1 mm and 3 mm were evaluated as AA (good), and cracks less than 1 mm were evaluated as AAA (excellent).

[0098] High strength steel sheets with a tensile strength of 1660 MPa or more and a hydrogen embrittlement resistance rating 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] [Table 1-1]

[0100] [Table 1-2]

[0101] [Table 2]

[0102] [Table 3]

[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. As a result, the 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. As a result, the 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, preventing the desired martensite area ratio from being obtained, and the 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 was not obtained, and the tensile strength decreased. In Example F-2, the high treatment temperature in the stabilization treatment step is thought to have caused much of the retained austenite to decompose. As a result, the desired area fraction 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 due to the long holding time in the stabilization treatment process. As a result, the desired area fraction 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 process. As a result, the desired area fraction of retained austenite could not be obtained in the final structure, and hydrogen embrittlement resistance deteriorated.

[0104] In contrast, in all of the examples of the present invention, the metal structure of the steel plate is composed mainly of martensite, but the metal structure contains a predetermined amount of retained austenite, which has the property of easily absorbing hydrogen.More specifically, by controlling the metal structure of the steel plate to contain, by area percentage, 85.0% or more of martensite and 1.0 to 7.0% of retained austenite, the hydrogen embrittlement resistance, particularly of the bent portion, was significantly improved despite the steel plate 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 process was carried out after the hot rolling process and before the pickling process, and in the cooling process, the steel was cooled at an average cooling rate of 1.0°C / sec 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 / sec, and further an appropriate stabilization treatment process and skin pass rolling process were carried out, 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 It was possible to obtain steel sheets having a metal structure satisfying the formulas 1 and 2, where β<0.75. As a result, in these examples, the hydrogen embrittlement resistance was significantly improved.

Claims

1. The tensile strength is 1660 MPa or more, The metal structure is, in area%, Martensite: 85.0% or more, Retained austenite: 1.0 to 7.0%, and Remaining structure: 10.0% or less, The chemical composition, in mass%, is C: 0.25-0.45%, Si: 0.01 to 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 to 0.0200%, O: 0.0001-0.0200%, Co: 0 to 0.50%, Ni: 0 to 1.00%, Mo: 0-1.00%, Cr: 0-2.000%, Ti: 0 to 0.500%, B: 0 to 0.0100%, Nb: 0 to 0.500%, V: 0 to 0.500%, Cu: 0-0.500%, W: 0-0.100%, Ta: 0-0.100%, Sn: 0-0.100%, Sb: 0 to 0.100%, As: 0 to 0.100%, Mg: 0 to 0.0500%, Ca: 0-0.050%, Y: 0 to 0.050%, Zr: 0 to 0.050%, La: 0 to 0.050%, and Ce: 0 to 0.050%; the balance being Fe and impurities; A steel sheet characterized in that, when measured by X-ray diffraction, the lattice constant calculated from diffraction peaks derived from retained austenite in a surface layer portion, a quarter-thickness position, and a half-thickness position satisfies the following formula 1: A s >3.5800Å A q >3.5800Å A c >3.5800Å ・・・Equation 1 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.

2. Area ratio RA of 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 2. The steel sheet according to claim 1, wherein the σ is less than 0.

75.

3. 3. The steel sheet according to claim 1, wherein, when measured by X-ray diffraction, a lattice constant calculated from diffraction peaks derived from retained austenite at a surface layer portion, a quarter-thickness position, and a half-thickness position satisfies the following formula 2: A s / A c >0.9970 A q / A c >0.9970...Formula 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.

4. The chemical composition is, in mass %, Co: 0.01 to 0.50%, Ni: 0.01-1.00%, Mo: 0.01-1.00%, Cr: 0.001-2.000%, Ti: 0.001 to 0.500%, B: 0.0001 to 0.0100%, Nb: 0.001-0.500%, V: 0.001-0.500%, Cu: 0.001 to 0.500%, W: 0.001-0.100%, Ta: 0.001 to 0.100%, Sn: 0.001 to 0.100%, Sb: 0.001 to 0.100%, As: 0.001 to 0.100%, Mg: 0.0001-0.0500%, Ca: 0.001-0.050%, Y: 0.001-0.050%, Zr: 0.001 to 0.050%, La: 0.001 to 0.050%, and Ce: 0.001-0.050% The steel sheet according to claim 1 or 2, characterized in that it contains at least one of the following:

5. A member comprising the steel plate according to claim 1 or 2.

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