Steel sheet and member

Abstract

Provided is a steel sheet characterized by a tensile strength of 1660 MPa or more, a metal structure of 85.0% or more by area of martensite, 1.0 to 7.0% by area of retained austenite, and 10.0% or less by area of the balance.

Description

Technical Field

[0001] This invention relates to steel plates. Background Technology

[0002] In recent years, the automotive industry has seen a demand for lighter vehicle bodies from the perspective of improving fuel efficiency. To balance lightweighting and crash safety, using high-strength steel sheets is one effective method, driving the development of high-strength steel sheets.

[0003] In high-strength steel plates, hydrogen embrittlement (also known as delayed fracture, etc.) sometimes becomes a problem. Hydrogen embrittlement is a phenomenon in which steel components subjected to high stress under service conditions suddenly fail due to hydrogen intruding into the steel from the environment.

[0004] Relatedly, Patent Document 1 describes a steel plate having the following composition by mass: C: 0.20% or more and 0.40% or less, Si: more than 1.0% and 3.0% or less, Mn: 1.5% or more and 3.5% or less, P: 0.002% or more and 0.010% or less, S: 0.0002% or more and 0.0020% or less, sol.Al: 0.40% or less (excluding 0%), N: The steel plate has a steel structure containing, by area, 45% to 83% tempered martensite, 15% to 53% bainite, and 2% or more retained austenite, with an average carbide particle size of 0.40 μm or less in the tempered martensite and an average carbon content of 0.5% by mass or more in the retained austenite, and a tensile strength of 1470 MPa or more. Furthermore, Patent Document 1 teaches that by suppressing the coarsening of carbides in the tempered martensite, it is possible to improve the delayed fracture resistance of high-strength steel plates with a tensile strength of 1470 MPa.

[0005] Existing technical documents Patent documents Patent Document 1: International Publication No. 2022 / 185804 Summary of the Invention

[0006] The problem that the invention aims to solve

[0007] It is generally known that the higher the strength of steel, the more prone it is to hydrogen embrittlement. On the other hand, in industries such as automotive, there is a demand for further lightweighting of steel. To achieve this, the strength of steel needs to be increased to levels higher than currently available. Therefore, there is a high demand for high-strength steel sheets with excellent resistance to hydrogen embrittlement, which can solve the problem of hydrogen embrittlement even when the strength is increased to the same level or higher than before.

[0008] Therefore, the object of the present invention is to provide a high-strength steel plate with excellent resistance to hydrogen embrittlement through a new composition.

[0009] means for solving problems

[0010] To achieve the aforementioned objectives, the inventors specifically focused on the microstructure of steel plates. Specifically, they discovered that by constructing a steel plate with a martensitic microstructure to ensure a tensile strength of 1660 MPa or higher, and by including a predetermined amount of retained austenite with a tendency to store hydrogen, hydrogen embrittlement cracking can be significantly suppressed even at high strengths of 1660 MPa or higher, thus completing this invention.

[0011] The present invention, which achieves the above objectives, is described below.

[0012] (1) A steel plate, characterized in that, Tensile strength above 1660MPa Metal structure as a percentage of area Martensite: 85.0% or more Residual austenite: 1.0~7.0%, and Balance of microstructure: below 10.0% Chemical composition by mass% contains C: 0.25~0.45% Si: 0.01~1.30% Mn: 1.00~3.50%, P: 0.0001~0.0200% S: 0.0001~0.0200% Al: 0.001~1.000% N: 0.0001~0.0200% O: 0.0001~0.0200% Co: 0~0.50%, Ni: 0~1.00% Mo: 0~1.00% Cr: 0~2.000% Ti: 0~0.500%, B: 0~0.0100% Nb: 0~0.500%, V: 0~0.500% Cu: 0~0.500%, W: 0~0.100% Ta: 0~0.100% Sn: 0~0.100% Sb: 0~0.100% As: 0~0.100% Mg: 0~0.0500%, Ca: 0~0.050%, Y: 0~0.050% Zr: 0~0.050%, La: 0~0.050% sum Ce: 0~0.050%, The balance consists of Fe and impurities.

[0013] (2) The steel plate according to (1) above, characterized in that, When measured by X-ray diffraction, the lattice constants calculated from the diffraction peaks from the retained austenite at the surface layer, the 1 / 4 thickness position, and the 1 / 2 thickness position satisfy the following equation 1. A s >3.5800Å A q >3.5800Å A c >3.5800Å…Equation 1 Here, A s A q and A c The lattice constants are calculated based on the diffraction peaks from the retained austenite at the surface, 1 / 4 of the plate thickness, and 1 / 2 of the plate thickness, respectively.

[0014] (3) The steel plate according to (1) or (2) above, characterized in that the area ratio RA of the retained austenite in the surface layer is s The area ratio RA of the retained austenite at the 1 / 2 position of the plate thickness c Satisfy RA s / RA c <0.75.

[0015] (4) The steel plate according to any one of (1) to (3) above, characterized in that, When measured by X-ray diffraction, the lattice constants calculated from the diffraction peaks from the retained austenite at the surface, 1 / 4 of the plate thickness, and 1 / 2 of the plate thickness satisfy the following Equation 2. A s / A c >0.9970 A q / A c >0.9970…Equation 2 Here, As A q and A c The lattice constants are calculated based on the diffraction peaks from the retained austenite at the surface, 1 / 4 of the plate thickness, and 1 / 2 of the plate thickness, respectively.

[0016] (5) The steel plate according to any one of (1) to (4) above, characterized in that, The chemical composition, expressed in % by mass, includes Co: 0.01~0.50%, Ni: 0.01~1.00% Mo: 0.01~1.00% Cr: 0.001~2.000% Ti: 0.001~0.500%, B: 0.0001~0.0100% Nb: 0.001~0.500%, V: 0.001~0.500% Cu: 0.001~0.500%, W: 0.001~0.100% Ta: 0.001~0.100% Sn: 0.001~0.100% Sb: 0.001~0.100% As: 0.001~0.100% Mg: 0.0001~0.0500%, Ca: 0.001~0.050% Y: 0.001~0.050% Zr: 0.001~0.050% La: 0.001~0.050%, and Ce: 0.001~0.050% At least one of them.

[0017] (6) A component comprising the steel plate of any one of (1) to (5) above.

[0018] Invention Effects

[0019] According to the present invention, a high-strength steel plate with excellent resistance to hydrogen embrittlement can be provided. Detailed Implementation

[0020] <steel plate>

[0021] The steel plate of the embodiments of the present invention is characterized in that, Tensile strength above 1660MPa Metal structure as a percentage of area Martensite: 85.0% or more Residual austenite: 1.0~7.0%, and Balance of microstructure: below 10.0% Chemical composition by mass% contains C: 0.25~0.45% Si: 0.01~1.30% Mn: 1.00~3.50%, P: 0.0001~0.0200% S: 0.0001~0.0200% Al: 0.001~1.000% N: 0.0001~0.0200% O: 0.0001~0.0200% Co: 0~0.50%, Ni: 0~1.00% Mo: 0~1.00% Cr: 0~2.000% Ti: 0~0.500%, B: 0~0.0100% Nb: 0~0.500%, V: 0~0.500% Cu: 0~0.500%, W: 0~0.100% Ta: 0~0.100% Sn: 0~0.100% Sb: 0~0.100% As: 0~0.100% Mg: 0~0.0500%, Ca: 0~0.050%, Y: 0~0.050% Zr: 0~0.050%, La: 0~0.050% sum Ce: 0~0.050%, The balance consists of Fe and impurities.

[0022] As mentioned above, it is known that the higher the strength of steel, the more prone it is to hydrogen embrittlement. In particular, in steel sheets with very high strength, such as those with tensile strengths of 1660 MPa and above, the microstructure is generally predominantly martensite to ensure high strength. On the other hand, in such high-strength steel sheets with a predominantly martensite structure, hydrogen embrittlement is considered to occur because hydrogen penetrating the steel mainly accumulates at the old austenite grain boundaries within the martensite microstructure, resulting in weak grain boundary bonding. These weakly bonded grain boundaries become the starting point for embrittlement. Generally, as tensile strength increases, hydrogen embrittlement occurs even with small amounts of hydrogen, and hydrogen intrusion into the steel also occurs at room temperature. There is no perfect method to suppress hydrogen intrusion into the steel, and it is extremely difficult to completely suppress hydrogen intrusion itself. Therefore, in order to completely solve the problem of hydrogen embrittlement in high-strength steel sheets with tensile strengths of 1660 MPa and above, it is important to modify the microstructure of the steel sheet.

[0023] Therefore, in order to improve the resistance to hydrogen embrittlement in steel sheets with very high strength, such as tensile strength of 1660 MPa or higher, the inventors have focused on the microstructure of the steel sheet. First, the inventors have focused on the characteristic that retained austenite, which is inevitably formed during the formation of martensite from austenite, readily stores hydrogen. Relatedly, the inventors have discovered that, in order to ensure a tensile strength of 1660 MPa or higher, the microstructure of the steel sheet must consist of a martensite-dominated microstructure, more specifically, a microstructure containing 85.0% or more martensite by area, and containing a specified amount of retained austenite, more specifically, by containing 1.0 to 7.0% retained austenite by area, even at high strengths of 1660 MPa or higher, hydrogen embrittlement cracking can be significantly suppressed. While not intended to be bound by a specific theory, retained austenite, as described above, readily stores hydrogen. Therefore, it is believed that by ensuring retained austenite exists in an appropriate amount within the metal structure, hydrogen intruding into the steel can be appropriately trapped using this retained austenite. In this case, the accumulation of hydrogen intruding into the steel at the old austenite grain boundaries in the martensite-dominated metal structure can be suppressed, thus preventing a decrease in grain boundary bonding strength. As a result, it is believed that even at high strengths of 1660 MPa or higher, hydrogen embrittlement cracking can be significantly suppressed. With increasing strength, the amount of retained austenite generally tends to increase. Therefore, the fact that it is possible to achieve both tensile strength of 1660 MPa or higher and excellent resistance to hydrogen embrittlement by controlling the retained austenite content in a hard metal structure containing 85.0% or more martensite within the range of 1.0 to 7.0% is currently unknown, but is clarified for the first time by the inventors. Therefore, the steel sheet according to the embodiments of the present invention is particularly useful in the automotive field where high strength requirements are high. The constituent elements of the steel plate according to the embodiments of the present invention will be described in more detail below.

[0024] [Tensile strength: ≥1660MPa]

[0025] The steel plate according to embodiments of the present invention has a tensile strength of 1660 MPa or higher. Preferably, the tensile strength is 1700 MPa or higher, 1760 MPa or higher, 1800 MPa or higher, 1900 MPa or higher, or 2000 MPa or higher. Despite having such a very high tensile strength, the steel plate according to embodiments of the present invention, as explained above, significantly improves its resistance to hydrogen embrittlement by including a specified amount of retained austenite in the martensitic metal structure. There is no particular upper limit to the tensile strength; for example, the tensile strength can be 2500 MPa or lower, 2300 MPa or lower, 2200 MPa or lower, or 2100 MPa or lower. The tensile strength is determined by a tensile test according to JIS Z 2241:2022 on a JIS No. 5 test piece taken from a direction preferably parallel to the rolling right angle direction of the steel plate along the length of the test piece. Even when the rolling direction of the steel plate cannot be specified, the effects of the present invention can be enjoyed as long as the steel plate has the above-mentioned tensile strength in any direction within the surface of the steel plate. When the surface of the steel plate has a coating such as a plating layer, a tensile test is performed after removing the coating layer.

[0026] [Metal Structure]

[0027] Next, the metal structure of the steel plate according to an embodiment of the present invention will be described. In the following description, unless otherwise specified, the unit of the microstructure fraction, "%", refers to "area %". Furthermore, the metal structure is controlled at the surface portion, the 1 / 4 thickness position, and the 1 / 2 thickness position of the steel plate. Here, the surface portion refers to a depth of 50 μm along the thickness direction from the surface of the steel plate. Hereafter, unless otherwise specified, the microstructure fraction refers to the average value of the microstructure fraction measured at the surface portion, the 1 / 4 thickness position, and the 1 / 2 thickness position. When the surface of the steel plate has a coating such as a plating layer, the position in the thickness direction is specified for the area where the coating layer has been removed.

[0028] [Martensite: 85.0% or higher]

[0029] In the steel plate of the embodiments of the present invention, the metal microstructure contains 85.0% or more martensite by area. By containing 85.0% or more martensite, a tensile strength of 1660 MPa or more can be achieved. From the viewpoint of maximizing strength, a higher martensite area percentage is preferred, 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; for example, the martensite area percentage can be 99.0% or less or 97.0% or less. In the present invention, "martensite" includes not only quenched martensite (so-called primary martensite) but also tempered martensite.

[0030] [Retained austenite: 1.0~7.0%]

[0031] In the steel sheet of the embodiment of the present invention, the metal microstructure contains 1.0 to 7.0% retained austenite by area. By containing 1.0 to 7.0% retained austenite, hydrogen intrusion into the steel can be appropriately trapped by the retained austenite, thus significantly suppressing hydrogen embrittlement even at high strengths of 1660 MPa or higher. From the viewpoint of improving resistance to hydrogen embrittlement, a higher area ratio of retained austenite is preferred, for example, 2.0% or more, 3.0% or more, or 4.0% or more. On the other hand, when retained austenite is excessively contained, a sufficient area ratio of martensite cannot be ensured, and sometimes the desired strength cannot be achieved. Alternatively, sometimes a portion of the retained austenite that has trapped hydrogen may transform into martensite through cold working or the like, or decompose due to changes over time, releasing some of the trapped hydrogen. When retained austenite is excessively contained, there is a situation where the amount of hydrogen released increases, promoting hydrogen embrittlement. Therefore, the area fraction of retained austenite is less than 7.0%, for example, it can be less than 6.0% or less than 5.0%.

[0032] [Balance in texture: below 10.0%]

[0033] The remaining microstructure other than martensite and retained austenite can be considered 0% in terms of area ratio, but in the presence of remaining microstructure, this remaining microstructure must be 10.0% or less in terms of area ratio. When the remaining microstructure is excessively included, it becomes impossible to control the area ratio of martensite and / or retained austenite within the desired range, sometimes resulting in unsatisfactory strength and / or resistance to hydrogen embrittlement. Therefore, the area ratio of the remaining microstructure must be 10.0% or less, which can be 8.0%, 6.0%, 5.0%, 4.0%, 3.0%, or 2.0% or less. In other words, the combined area ratio of martensite and retained austenite must be 90.0% or more, which can 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, to achieve a 0% area ratio of the remaining microstructure, a high degree of control is required during the steel sheet manufacturing process, which sometimes leads to low yield. Therefore, the area ratio of the retained microstructure can be 0.5% or more, or 1.0% or more. In other words, the combined area ratio of martensite and retained austenite can be 99.5% or less, or 99.0% or less. The retained microstructure is not particularly limited, and may include at least one of ferrite, bainite, and pearlite, or at least one of these.

[0034] [Identification and calculation of metal structure]

[0035] [Martensite]

[0036] The identification and calculation of the metal structure are performed as follows. First, a specimen with a thickness section perpendicular to the plate surface is taken and used as the observation surface. Care is taken to grind this section without leaving any damage caused by grinding. Then, the strain introduced by the grinding through the surface is removed by chemical grinding, thereby obtaining a cross-sectional observation sample for EBSD (Electron-Back-Scatter-Diffraction) analysis without any change in crystal orientation caused by grinding. Here, damage based on crystal orientation caused by grinding refers to a region observed as a straight line through the metal structure in the IPF (Inverse-Pole-Figure) of the EBSD analysis. Such damage is not inherent to the structure and therefore must be absent from the field of view. If a field of view without damage is found, or if damage is present in the field of view regardless, grinding is preferably performed again.

[0037] The EBSD analysis sample described above is subjected to electron backscatter diffraction analysis. The analysis conditions for EBSD can be as simple as those familiar with the art; detailed conditions are described as an example. After inserting the sample into the FE-SEM capable of EBSD measurement, the polished surface of the sample is tilted at 60-70° relative to the incident direction of the electron beam. The tilting direction of the sample should be set to the direction opposite to the cross-section of the object being measured and the subsequently inserted EBSD detector. After tilting, the EBSD detector is inserted into the FE-SEM chamber and brought close to the sample. The position of the EBSD detector can be any position where the detection sensitivity of the electron beam is high, but it is preferably closest within the FE-SEM chamber, within a range that does not collide with the sample, the sample holder, or the stage. Then, the electron beam detection sensitivity of the EBSD detector is adjusted. The adjustment of the detection sensitivity varies depending on the performance of the electron gun of the FE-SEM used and the EBSD detector; therefore, it is preferable to perform the adjustment within the scope of those familiar with the art. The adjustment is performed to achieve conditions under which the EBSD pattern can be clearly observed. Then, electron beams were applied to the observation field at intervals of 0.3 μm, and EBSD patterns were captured at each measurement point. Based on the EBSD patterns at each measurement point, index calibration and crystal orientation calculation were performed. For index calibration and crystal orientation calculation, APEX software manufactured by AMETEK was preferred. The EBSD data obtained in this way was analyzed using OIMAnalysis software (Orientation Imaging Microscopy), version 7 or later, which is AMETEK's EBSD data analysis software. The obtained EBSD data was opened in OIMAnalysis, and only regions with a CI value (Confidence Index) of 0.1 or higher were extracted. The CI value is an indicator of the reliability of the index calibration and crystal orientation analysis results. Regions with a CI value below 0.1 are likely areas of overlapping orientations such as contaminants and grain boundaries on the sample surface during electron beam irradiation and are considered regions that are not part of the original crystal orientation of the metal structure. Then, regions with a GAM value (Grain Average Misorientation) of 0.5° or higher were considered martensite, and the area fraction was calculated. It should be noted that grain boundaries refer to the boundaries between measurement points where the orientation difference between the measurement points is 15° or more. Here, the GAM value is the value obtained by averaging the orientation difference between measurement points of crystal orientation within a region surrounded by grain boundaries. Microstructures formed at low temperatures, such as martensite, exhibit intragranular orientation differences due to phase transformation strain, thus enabling discrimination based on GAM values. At least five such investigations were conducted within a 100×100μm field of view centered at half the plate thickness from the steel plate surface, and the average area ratio was derived.

[0038] Retained austenite forms at sub-nanometer scale between the martensite laths. Therefore, at the resolution of EBSD analysis, it is difficult to separate the two. The area ratio obtained by the above method becomes the area ratio obtained by combining martensite and retained austenite. Therefore, by subtracting the area ratio of retained austenite determined according to the steps described later from the area ratio obtained by the above method, i.e., calculating [the area ratio obtained by EBSD (the area ratio of the region with a GAM value of 0.5° or higher)] - [the area ratio of retained austenite obtained by X-ray diffraction], the area ratio of martensite at the 1 / 2 thickness position is obtained. It should be noted that, as mentioned above, the martensite in this application is a microstructure containing tempered martensite, and there is no particular difference in the state of the martensite after tempering.

[0039] Next, the martensite area ratio at the 1 / 4 thickness position and the surface layer was measured similarly. However, when measuring the martensite area ratio at the surface layer, a 50μm (thickness direction) × 200μm (perpendicular to the thickness direction) region centered at a depth of 50μm along the thickness direction from the steel plate surface was used as the observation area. Finally, the average of the area ratios measured at the surface layer, the 1 / 4 thickness position, and the 1 / 2 thickness position was calculated and determined as the martensite area ratio.

[0040] [Retained austenite]

[0041] The area ratio of retained austenite was calculated using X-ray determination. First, a sample was taken from the same component used to identify martensite. Then, the sample was removed from its surface along the thickness direction to half the plate thickness by mechanical and chemical grinding. Next, the fraction of retained austenite was calculated based on the integral 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 obtained by using MoKα rays as characteristic X-rays. This fraction was used as the area ratio of retained austenite at the half-thickness position. The area ratios of retained austenite were similarly obtained for the surface portion and the quarter-thickness position, and their average value was calculated and determined as the area ratio of retained austenite. The area ratio of retained austenite at the half-thickness position corresponds to "RA" as described later. c Similarly, the area ratio of retained austenite in the surface layer is equivalent to "RA" as described later. s ".

[0042] [Balance Structure]

[0043] The area ratio of the remaining microstructure is determined by subtracting the area ratio of martensite and the area ratio of retained austenite obtained above from 100%. The remaining microstructure may, for example, comprise at least one or more of ferrite, bainite, and pearlite, but in this invention, there is no particular need to specify these microstructure types or determine the area ratio of each microstructure in order to achieve the objectives of this invention. If there is a need, it is not difficult to specify them using methods commonly employed by those skilled in the art.

[0044] [Formula 1: A] s >3.5800Å, A q >3.5800Å and A c >3.5800Å]

[0045] In a preferred embodiment of the present invention, when measured by X-ray diffraction, the lattice constant calculated from the diffraction peaks from the retained austenite at the surface portion, the 1 / 4 plate thickness position, and the 1 / 2 plate 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 The lattice constants are calculated based on the diffraction peaks from the retained austenite at the surface, 1 / 4 of the plate thickness, and 1 / 2 of the plate thickness, respectively.

[0046] Retained austenite in a metallic microstructure consists of multiple retained austenites with varying degrees of stability. Therefore, a metallic microstructure may contain both low-stability and high-stability retained austenite. All retained austenite possesses hydrogen storage capacity, thus trapping hydrogen that has infiltrated the steel. However, when there is a predominance of low-stability retained austenite, some of it may undergo work-induced martensitic transformation due to cold working such as cold pressing, or decompose over time. In such cases, the partial disappearance of retained austenite reduces hydrogen storage capacity, and some of the hydrogen trapped within is released. This released hydrogen then accumulates at the old austenite grain boundaries in a martensitic-dominated metallic microstructure, weakening the grain boundary bonding strength and sometimes promoting hydrogen embrittlement. Therefore, from the viewpoint of further improving the resistance to hydrogen embrittlement, it is preferable not only to control the area ratio of retained austenite in the range of 1.0 to 7.0%, but also to stabilize the retained austenite, thereby inhibiting or reducing the disappearance of retained austenite and the release of hydrogen associated with it.

[0047] Therefore, in order to further improve the hydrogen embrittlement resistance of steel plates, the inventors investigated the relationship between hydrogen embrittlement resistance and the stabilization of retained austenite. More specifically, the inventors focused on the fact that the lattice constant calculated from the X-ray diffraction peaks of retained austenite serves as an indicator of the stability of the retained austenite. As a result, the inventors found that, when measured by X-ray diffraction, the lattice constant A, calculated from the diffraction peaks of retained austenite at the surface portion, the 1 / 4 thickness position, and the 1 / 2 thickness position, is a reliable indicator of the stability of the retained austenite. s A q and A c By controlling the process in accordance with Equation 1 above, the stability of the retained austenite contained in the metal structure of the steel sheet can be improved as a whole. Relatedly, the inventors have discovered that the disappearance of retained austenite caused by processing-induced martensitic transformation due to cold working, and the decomposition of retained austenite caused by aging, can be significantly suppressed. While not intended to be bound by a specific theory, it is believed that by enriching austenite-stabilizing elements such as C and / or Mn in the retained austenite, the value of the lattice constant calculated from the diffraction peaks from the retained austenite can be increased, thereby improving the stability of the retained austenite. More specifically, as will be described in detail later in connection with the manufacturing method, by performing appropriate heat treatment in the hot-rolled post-processing step, cementite can be formed in the hot-rolled steel sheet. Therefore, in the subsequent annealing process, austenite-stabilizing elements such as Mn can be enriched in the cementite, further improving the stability of the austenite. Similarly, as will be explained in detail later in connection with the manufacturing method, by performing appropriate heat treatment in the stabilization process, it is possible to enrich austenite stabilizing elements such as C and Mn in the steel sheet within the retained austenite, thereby fully stabilizing the retained austenite. In this case, the enriched austenite stabilizing elements such as C and / or Mn, for example, in the case of C, act as intrusive solid solution elements, entering the voids where atoms were not originally present, causing lattice expansion; in the case of Mn, they act as substitutional solid solution elements, entering the lattice positions where Fe atoms were originally present, causing lattice expansion. As a result, the value of the lattice constant is considered to increase. Therefore, a certain correlation has been established between the stabilization of retained austenite caused by austenite stabilizing elements and the value of the lattice constant. Based on these insights, the inventors have repeatedly studied the results and discovered that, when measured by X-ray diffraction, the lattice constant, A, calculated from the diffraction peaks from the retained austenite at the surface portion, the 1 / 4 thickness position, and the 1 / 2 thickness position, is... s A q and A c Controlling the process in accordance with Equation 1 above can improve the overall stability of the retained austenite within the microstructure of the steel plate. Therefore, by using A... s Aq and A c Controlling the process according to Equation 1 above can significantly suppress the disappearance of retained austenite caused by processing-induced martensitic transformation and the decomposition of retained austenite caused by years of change. Therefore, compared with simply controlling the amount of retained austenite within the range of 1.0~7.0%, it can further improve the hydrogen embrittlement resistance of the steel plate. From the perspective of further improving hydrogen embrittlement resistance, A s A q and A c The higher the value, the better. For example, A s A q and A c They can be above 3.5830 Å (i.e., A). s ≥3.5830Å, A q ≥3.5830Å and A c ≥3.5830 Å (the same applies below), ≥3.5850 Å, ≥3.5880 Å, or ≥3.5900 Å. There is no specific upper limit; for example, A... s A q and A c They can be below 3.7000 Å (i.e., A) s ≤3.7000Å, A q ≤3.7000Å and A c ≤3.7000Å (the same applies below), below 3.6500Å or below 3.6000Å.

[0048] [Lattice constant A] s A q and A c [Calculated]

[0049] The lattice constant A is calculated as follows. s A q and A c First, the sample was mechanically and chemically ground to remove material along the thickness direction up to half the thickness of the plate. Then, based on the diffraction peaks of the (200), (220), and (311) planes of the fcc phase obtained using MoKα rays as characteristic X-rays on the ground sample, the lattice constants A of the retained austenite on each plane were calculated using Equations 3 and 4 below. c（200） A c（220） and A c（311） Then, the value obtained by averaging them based on Equation 5 below is determined as the lattice constant A. c . d c（hkl） =λ / 2sin(2θ c（hkl） / 2)...Form 3 A c（hkl） =d c（hkl）×(h 2 +k 2 +l 2 ) 0.5 …Form 4 A c = (A c（200） +A c（220） +A c（311） ) / 3…Formula 5 Here, d c（hkl） λ is the lattice spacing (Å) of the (hkl) plane at the 1 / 2 position of the plate thickness, λ is the wavelength of the X-ray diffraction source (the wavelength of MoKα rays) (Å), and A c（hkl） Let θ be the lattice constant (Å) of the (hkl) plane at the position of 1 / 2 thickness. c（hkl） 2θ is the diffraction angle (°) of the diffraction peak on the (hkl) plane at half the plate thickness. c（hkl） The value of 2θ is calculated based on the diffraction intensity and diffraction angle obtained through X-ray diffraction. First, the obtained 2θ values ​​are smoothed using a three-point weighted average according to Equation 6 below. Then, the 2θ with the highest diffraction intensity for each diffraction plane is taken as the 2θ of that plane. c（hkl） . 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 Formula 6 Here, P n This refers to the diffraction angle 2θ in the data set of "diffraction angle vs. diffraction intensity". n The diffraction intensity, except for P n In addition, the diffraction angle 2θ is also used. n The diffraction intensity before and after (i.e., P) n-1 and P n+1 The data from the three points were smoothed by a three-point weighted average, and this smoothing process was applied to all measurement points. Similarly, the lattice constants of the (200), (220), and (311) planes of the retained austenite at the surface layer and the 1 / 4 thickness location were obtained, and the lattice constant A was determined by averaging these values. s and A q .

[0050] [RA s / RA c <0.75]

[0051] In another preferred embodiment of the present invention, the area ratio RA of the retained austenite in the surface layer is... s The area ratio RA of retained austenite at the 1 / 2 position of the plate thickness c Satisfy RA s / RA c <0.75.

[0052] As is known, hydrogen embrittlement cracking is more likely to occur with higher steel strength, and it is particularly prone to occur in bending processes where large plastic strains are applied. Especially in high-strength steel sheets primarily composed of martensitic structures, bending is the main processing method, including cold working. Therefore, improving the microstructure of the surface layer of the steel sheet, which is prone to hydrogen embrittlement during bending, is particularly effective in further enhancing its resistance to hydrogen embrittlement.

[0053] Therefore, the inventors have discovered that by maintaining the total amount of retained austenite in the steel sheet within the range of 1.0% to 7.0%, and by controlling the amount of retained austenite in the surface layer of the steel sheet relative to the amount of retained austenite in the internal structure of the steel sheet by a predetermined ratio, more specifically, by controlling the amount of retained austenite in the surface layer relative to the area ratio RA of the retained austenite in the surface layer... s The area ratio RA of retained austenite at the 1 / 2 position of the plate thickness c Satisfy RA s / RA c Controlling the content of austenite in the surface layer to <0.75% significantly improves the resistance to hydrogen embrittlement during bending. While not intending to be bound by a specific theory, it is believed that by reducing the amount of retained austenite in the surface layer, the amount of unstable retained austenite in the surface layer will naturally also decrease. Thus, when the amount of unstable retained austenite in the surface layer is reduced, the reduction or disappearance of the retained austenite in the surface layer caused by bending processes such as cold pressing can be suppressed. As a result, the amount of hydrogen released from the retained austenite in the surface layer caused by bending processes can be reduced, thereby further improving the hydrogen embrittlement resistance of the steel sheet compared to simply controlling the retained austenite content within the range of 1.0% to 7.0%.

[0054] From the perspective of further improving resistance to hydrogen embrittlement, it is preferable to reduce the area ratio RA of retained austenite in the surface layer. s This reduces the amount of hydrogen released from the residual austenite in the surface layer. Therefore, RA s / RA c The smaller the ratio, the better; for example, it can be below 0.72 (i.e., RA). s / RA c ≤0.72 (and so on), below 0.70, below 0.68, below 0.65, below 0.62, or below 0.60. The lower limit is not specifically defined; for example, RA... s / RAc The ratio only needs to be greater than 0, or it can be greater than 0.10 (i.e., RA). s / RA c ≥0.10 (same below), ≥0.20 or ≥0.30. Regarding RA s and RA c The calculation method is as previously described in the section on [Identification and Calculation of Metal Structures].

[0055] [Equation 2: A] s / A c >0.9970 and A q / A c >0.9970]

[0056] In another preferred embodiment of the present invention, when measured by X-ray diffraction, the lattice constant calculated from the diffraction peaks from the retained austenite at the surface portion, the 1 / 4 position of the plate thickness, and the 1 / 2 position of the plate thickness satisfies the following formula 2. A s / A c >0.9970 A q / A c >0.9970…Equation 2 Here, A s A q and A c The lattice constants are calculated based on the diffraction peaks from the retained austenite at the surface, 1 / 4 of the plate thickness, and 1 / 2 of the plate thickness, respectively.

[0057] As mentioned above, in high-strength steel sheets mainly composed of martensitic microstructure, cold working and other processing are primarily bending processes. Therefore, to further improve resistance to hydrogen embrittlement, it is particularly effective to improve the microstructure of the surface layer of the steel sheet, which is prone to hydrogen embrittlement during bending.

[0058] Therefore, the inventors have discovered that by maintaining the total amount of retained austenite in the steel sheet within the range of 1.0% to 7.0%, and by controlling this in a way that the stability of retained austenite in the microstructure of the region closer to the surface of the steel sheet becomes relatively high, and more specifically, by controlling this in a way that the lattice constants calculated from the diffraction peaks of retained austenite at the surface portion, the 1 / 4 thickness position, and the 1 / 2 thickness position satisfy Equation 2 above, the resistance to hydrogen embrittlement during bending processing can be significantly improved. By further improving the stability of retained austenite at the surface portion and the 1 / 4 thickness position, the reduction or disappearance of retained austenite at the surface portion and the 1 / 4 thickness position due to processing-induced martensitic phase transformation caused by bending processing such as cold pressing can be suppressed. As a result, the amount of hydrogen released from the surface layer and the 1 / 4 position of the plate thickness due to bending processes can be reduced. Thus, compared with the case where the amount of retained austenite is only controlled within the range of 1.0 to 7.0%, the hydrogen embrittlement resistance of the steel plate can be further improved.

[0059] From the viewpoint of further improving resistance to hydrogen embrittlement, it is preferable to further improve the stability of the retained austenite in the surface layer and at the 1 / 4 thickness position, and increase the value of the lattice constant calculated from the diffraction peaks of the retained austenite in the surface layer and at the 1 / 4 thickness position, thereby reducing the amount of hydrogen released from the retained austenite in the surface layer and at the 1 / 4 thickness position. Therefore, A s / A c and A q / A c The higher the ratio, the better; for example, it can be 0.9975 or higher (i.e., A). q / A c ≥0.9975 and A q / A c ≥0.9975 (and so on below), ≥0.9980, ≥0.9985, or ≥0.9990. There is no specific upper limit; for example, A... s / A c and A q / A c The ratio can be below 1.1000 (i.e., A). q / A c ≤1.1000 and A q / A c ≤1.1000 (same below), below 1.0500 or below 1.0200. Regarding A s A q and A c The calculation method, such as in [lattice constant A], is as follows: s A q and A c As previously stated in the section on [calculation].

[0060] In the most preferred embodiment of the present invention, when measured by X-ray diffraction, the lattice constant calculated from the diffraction peaks of retained austenite at the surface portion, the 1 / 4 thickness position, and the 1 / 2 thickness position satisfies Equation 1 above, and the area ratio RA of retained austenite at the surface portion... s The area ratio RA of retained austenite at the 1 / 2 position of the plate thickness c Satisfy RA s / RA c The lattice constant, calculated from the diffraction peaks of retained austenite at the surface layer, the 1 / 4 thickness position, and the 1 / 2 thickness position, is less than 0.75 and satisfies Equation 2 above, as measured by X-ray diffraction. According to this embodiment, the stability of retained austenite contained in the metal structure of the steel sheet can be improved overall, and the amount of unstable retained austenite in the surface layer can be reduced in particular, and the stability of retained austenite present in the surface layer and the 1 / 4 thickness position can be further improved. Therefore, especially in the surface layer and the 1 / 4 thickness position of the steel sheet, which are prone to hydrogen embrittlement during bending, the amount of hydrogen released from retained austenite due to bending and other processes can be significantly reduced, thus significantly improving the hydrogen embrittlement resistance of the steel sheet.

[0061] [Chemical composition of steel plate]

[0062] In the following description, unless otherwise specified, the unit of content of each element, i.e., "%", refers to "mass%". Furthermore, in this specification, the "~" indicating a numerical range is used to mean the lower limit and upper limit of the values ​​listed before and after it, unless otherwise specified.

[0063] In a specific embodiment of the present invention, the chemical composition of the steel plate is expressed as a percentage by mass. Chemical composition by mass% is C: 0.25~0.45% Si: 0.01~1.30% Mn: 1.00~3.50%, P: 0.0001~0.0200% S: 0.0001~0.0200% Al: 0.001~1.000% N: 0.0001~0.0200% O: 0.0001~0.0200% Co: 0~0.50%, Ni: 0~1.00% Mo: 0~1.00% Cr: 0~2.000% Ti: 0~0.500%, B: 0~0.0100% Nb: 0~0.500%, V: 0~0.500% Cu: 0~0.500%, W: 0~0.100% Ta: 0~0.100% Sn: 0~0.100% Sb: 0~0.100% As: 0~0.100% Mg: 0~0.0500%, Ca: 0~0.050%, Y: 0~0.050% Zr: 0~0.050%, La: 0~0.050% Ce: 0~0.050%, and Balance: Fe and impurities. The following provides a more detailed explanation of each element.

[0064] [C: 0.25~0.45%] Carbon (C) is an effective element for increasing tensile strength at low cost. Additionally, C is also effective for stabilizing austenite. To fully achieve these effects, the C content is set to 0.25% or more. The C content can be 0.26% or more, 0.28% or more, 0.29% or more, or 0.30% or more. On the other hand, excessive carbon content can sometimes lead to poor weldability. Therefore, the carbon content is set to 0.45% or less. The carbon content can be 0.42% or less, 0.40% or less, or 0.38% or less.

[0065] [Si: 0.01~1.30%] Si acts as a deoxidizer, influencing the morphology of carbides and retained austenite after heat treatment. Without Si, it is sometimes difficult to suppress the formation of coarse oxides. Therefore, the Si content is set to 0.01% or higher. The Si content can be 0.05% or higher, 0.10% or higher, 0.30% or higher, or 0.50% or higher. On the other hand, excessive Si content can sometimes lead to localized low ductility. Therefore, the Si content is set to 1.30% or less. The Si content can be 1.20% or less, 1.00% or less, 0.80% or less, or 0.60% or less.

[0066] [Mn: 1.00~3.50%] Mn is an effective element for improving the hardenability of steel and thus increasing the strength of steel plates. Additionally, Mn is also effective for stabilizing austenite. To fully achieve these effects, the Mn content is set to 1.00% or more. The Mn content can 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 the co-segregation of M, P, and S, but also deteriorates corrosion resistance. Therefore, the Mn content is set to 3.50% or less. The Mn content can be 3.20% or less, 3.00% or less, 2.80% or less, or 2.60% or less.

[0067] [P: 0.0001~0.0200%] P is an element that causes embrittlement of the weld and deterioration of the plating. Therefore, the P content is set to be below 0.0200%. The P content can be below 0.0180%, 0.0150%, 0.0120%, or 0.0100%. A lower phosphorus (P) content is preferred, but refining to less than 0.0001% requires a significant amount of time, leading to a substantial increase in costs. Therefore, the P content is set to 0.0001% or higher. The P content can be 0.0005% or higher, 0.0010% or higher, or 0.0020% or higher.

[0068] [S: 0.0001~0.0200%] Sulfur (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 become the initiation point for cracking during cold working. Therefore, the S content is set to be below 0.0200%. The S content can be below 0.0180%, 0.0150%, 0.0120%, or 0.0100%. The lower the sulfur content, the better. However, refining to reduce the sulfur content to less than 0.0001% requires a significant amount of time, leading to a substantial increase in costs. Therefore, the sulfur content is set to be 0.0001% or higher. The sulfur content can be 0.0005% or higher, 0.0010% or higher, or 0.0020% or higher.

[0069] [Al: 0.001~1.000%] Al is an element that acts as a deoxidizer in steel. To achieve this effect, the Al content is set to 0.001% or more. The Al content can be 0.005% or more, 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, when there is an excessive amount of Al, large Al oxides can sometimes form, becoming the starting point for cracking. Therefore, the Al content is set to 1.000% or less. The Al content can be 0.950% or less, 0.900% or less, 0.800% or less, or 0.600% or less.

[0070] [N: 0.0001~0.0200%] Nitrogen (N) is an element that causes porosity during welding. Therefore, the N content is set to be 0.0200% or less. The N content can be 0.0180%, 0.0160%, 0.0120%, or 0.0100% or less. On the other hand, reducing nitrogen (N) to less than 0.0001% leads to a significant increase in manufacturing costs. Therefore, the nitrogen content is set to 0.0001% or higher. The nitrogen content can be 0.0005% or higher, 0.0010% or higher, or 0.0020% or higher.

[0071] [O: 0.0001~0.0200%] O is an element that causes porosity during welding. Therefore, the O content is set to 0.0200% or less. The O content can be 0.0180%, 0.0150%, 0.0120%, or 0.0100% or less. The lower the oxygen content, the better. However, reducing the oxygen content to less than 0.0001% results in a significant increase in manufacturing costs. Therefore, the oxygen content is set to 0.0001% or higher. The oxygen content can be 0.0005% or higher, 0.0010% or higher, or 0.0015% or higher.

[0072] The basic chemical composition of the steel plate according to embodiments of the present invention is as described above. Furthermore, the steel plate may, as needed, contain at least one of the following elements to replace a portion of the balance Fe. For example, the steel plate may contain at least one element selected from 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%. Additionally, the steel plate may contain at least one element selected from Sn: 0~0.100%, Sb: 0~0.100%, and As: 0~0.100%. In addition, the steel plate may also contain at least one element selected from 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 are described in detail below.

[0073] [Co: 0~0.50%] Co is an effective element for controlling the morphology of carbides and increasing the strength of steel plates. The Co content can also be 0%, but to achieve these effects, the Co content is preferably 0.001% or more. The Co content can be 0.01% or more, 0.02% or more, 0.05% or more, or 0.10% or more. On the other hand, when there is an excessive amount of Co, large Co carbides may sometimes precipitate. Therefore, the Co content is preferably 0.50% or less. The Co content can be 0.40% or less, 0.30% or less, or 0.20% or less.

[0074] [Ni: 0~1.00%] Ni is an effective element for increasing the strength of steel sheets. In addition, Ni also improves wettability and promotes alloying reactions. The Ni content can be 0%, but to achieve these effects, the Ni content is preferably 0.001% or more. The Ni content can 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 can sometimes result in poor weldability. Therefore, the Ni content is preferably 1.00% or less. The Ni content can be 0.90% or less, 0.80% or less, 0.60% or less, or 0.30% or less.

[0075] [Mo: 0~1.00%] Mo is an effective element for increasing the strength of steel sheets. Furthermore, Mo is an element that suppresses ferrite phase transformation that occurs during heat treatment in continuous annealing or continuous molten zinc plating equipment. The Mo content can be 0%, but to achieve these effects, the Mo content is preferably 0.001% or more. The Mo content can be 0.01% or more, 0.02% or more, 0.05% or more, or 0.08% or more. On the other hand, even with excessive Mo content, the effect of suppressing the ferrite phase transformation becomes saturated, or sometimes coarse intermetallic compounds and carbides are formed. Therefore, the Mo content is preferably 1.00% or less. The Mo content can be 0.90% or less, 0.80% or less, 0.60% or less, or 0.30% or less.

[0076] [Cr: 0~2.000%] Cr is an effective element for suppressing pearlite phase transformation and increasing the strength of steel. The Cr content can be 0%, but to achieve this effect, the Cr content is preferably 0.001% or higher. The Cr content can be 0.005% or higher, 0.010% or higher, 0.020% or higher, or 0.050% or higher. On the other hand, when there is an excessive amount of Cr, coarse Cr carbides sometimes form in the central segregation region. Therefore, the Cr content is preferably 2.000% or less. The Cr content can be 1.800% or less, 1.500% or less, 1.000% or less, or 0.500% or less.

[0077] [Ti: 0~0.500%] Ti is an element that contributes to increased strength in steel sheets through precipitation strengthening, fine-grain strengthening based on ferrite grain growth inhibition, and dislocation strengthening through recrystallization inhibition. The Ti content can be 0%, but to achieve these effects, a Ti content of 0.001% or higher is preferred. The Ti content can be 0.005% or higher, 0.010% or higher, or 0.050% or higher. On the other hand, excessive Ti content can sometimes lead to increased precipitation of carbonitrides. Therefore, the Ti content is preferably 0.500% or less. The Ti content can be 0.450% or less, 0.400% or less, 0.300% or less, or 0.100% or less.

[0078] [B: 0~0.0100%] Boron (B) is an element that inhibits the formation of ferrite and pearlite and promotes the formation of low-temperature phase transformation structures such as martensite during cooling from the austenitic temperature range. Additionally, B is beneficial for increasing the strength of steel. The B content can be 0%, but to achieve these effects, a B content of 0.0001% or more is preferred. The B content can be 0.0003% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive B content can sometimes lead to the formation of coarse B oxides in steel. Therefore, the B content is preferably 0.0100% or less. The B content can be 0.0080% or less, 0.0060% or less, 0.0050% or less, or 0.0020% or less.

[0079] [Nb: 0~0.500%] Nitrogen (Nb) is an effective element for controlling the morphology of carbides and for improving the toughness of microstructures. The Nb content can be 0%, but to achieve these effects, the Nb content is preferably 0.001% or higher. The Nb content can be 0.002% or higher, 0.010% or higher, or 0.020% or higher. On the other hand, excessive Nb content can sometimes lead to the formation of coarse Nb carbides. Therefore, the Nb content is preferably 0.500% or less. The Nb content can be 0.450% or less, 0.400% or less, 0.300% or less, or 0.100% or less.

[0080] [V: 0~0.500%] Valium (V) is an element that contributes to increased strength in steel sheets through precipitation strengthening, fine-grain strengthening based on ferrite grain growth inhibition, and dislocation strengthening through recrystallization inhibition. The V content can be 0%, but to achieve these effects, the V content is preferably 0.001% or more. The V content can be 0.002% or more, 0.010% or more, or 0.020% or more. On the other hand, excessive V content can sometimes lead to increased precipitation of carbonitrides. Therefore, the V content is preferably 0.500% or less. The V content can be 0.450% or less, 0.400% or less, 0.300% or less, or 0.100% or less.

[0081] [Cu: 0~0.500%] Cu is an effective element for improving the strength of steel plates. The Cu content can also be 0%, but to achieve this effect, the Cu content is preferably 0.001% or more. The Cu content can be 0.002% or more, 0.010% or more, or 0.030% or more. On the other hand, excessive Cu content can cause steel to become brittle during hot rolling, sometimes making hot rolling difficult. Therefore, the Cu content is preferably 0.500% or less. The Cu content can be 0.450% or less, 0.400% or less, 0.300% or less, or 0.100% or less.

[0082] [W: 0~0.100%] W is an effective element for increasing the strength of steel plates. Furthermore, W forms precipitates and crystals. Since these W-containing precipitates and crystals become hydrogen trapping sites, W is an effective element for improving resistance to hydrogen embrittlement. The W content can be 0%, but to achieve these effects, the W content is preferably 0.001% or more. The W content can be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, excessive W content can sometimes lead to the formation of coarse W precipitates or crystals. Therefore, the W content is preferably 0.100% or less. The W content can be 0.080% or less, 0.060% or less, 0.050% or less, or 0.030% or less.

[0083] [Ta: 0~0.100%] Ta is an effective element for controlling the morphology of carbides and increasing the strength of steel plates. The Ta content can be 0%, but to achieve these effects, the Ta content is preferably 0.001% or more. The Ta content can be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, excessive Ta content leads to the precipitation of a large amount of fine Ta carbides, which increases the strength of the steel plate but sometimes reduces its ductility. Therefore, the Ta content is preferably 0.100% or less. The Ta content can be 0.080% or less, 0.060% or less, 0.050% or less, or 0.020% or less.

[0084] [Sn: 0~0.100%] Sn is an element present in steel when scrap is used as a raw material. High Sn content can potentially cause embrittlement of ferrite. Therefore, the Sn content is preferably below 0.100%. The Sn content can be below 0.060%, 0.030%, or 0.020%. The lower the Sn content, the better; 0% is also preferable. However, refining to less than 0.001% requires a significant amount of time, leading to a substantial increase in costs. Therefore, Sn content can also be above 0.001%. Sn content can be above 0.002%, 0.005%, or 0.010%.

[0085] [Sb: 0~0.100%] Like Sn, Sb is an element present when scrap is used as a raw material for steelmaking. Furthermore, Sb contributes to low ductility. Therefore, the Sb content is preferably 0.100% or less. The Sb content can be 0.040% or less, 0.030% or less, or 0.020% or less. The lower the Sb content, the better; 0% is also preferable. However, reducing the Sb content to less than 0.001% requires a significant amount of refining time, leading to a substantial increase in costs. Therefore, the Sb content can also be above 0.001%. The Sb content can be above 0.002%, 0.005%, or 0.008%.

[0086] [As: 0~0.100%] As, like Sn and Sb, is an element present when scrap is used as a raw material for steelmaking. Furthermore, As also contributes to low ductility. Therefore, the As content is preferably 0.100% or less. The As content can be 0.040% or less, 0.030% or less, or 0.020% or less. The lower the asperformance (As) content, the better; 0% is also preferable. However, reducing the As content to less than 0.001% requires a significant amount of refining time, leading to a substantial increase in costs. Therefore, the As content can also be above 0.001%. The As content can be above 0.002%, 0.003%, or 0.005%.

[0087] [Mg: 0~0.0500%] Mg is an element whose presence, even in trace amounts, allows control over the form of sulfides. While the Mg content can be 0%, it is preferable to have a content of 0.0001% or higher to achieve this effect. The Mg content can be 0.0005% or higher, 0.0010% or higher, 0.0015% or higher, or 0.0020% or higher. On the other hand, excessive Mg content can sometimes lead to the formation of large inclusions. Therefore, the Mg content is preferably 0.0500% or less. The Mg content can be 0.0300% or less, 0.0100% or less, 0.0050% or less, or 0.0030% or less.

[0088] [Ca: 0~0.050%] Ca is useful as a deoxidizing element and also plays an effective role in controlling the speciation of sulfides. The Ca content can be 0%, but to achieve these effects, the Ca content is preferably set to 0.0001% or more. The Ca content can be 0.0005% or more, 0.001% or more, or 0.002% or more. On the other hand, excessive Ca content can sometimes lead to the formation of large inclusions. Therefore, the Ca content is preferably 0.050% or less. The Ca content can be 0.030% or less, 0.010% or less, 0.005% or less, or 0.003% or less.

[0089] [Y: 0~0.050%] [Zr: 0~0.050%] [La: 0~0.050%] [Ce: 0~0.050%] Like Mg, Y, Zr, La, and Ce are elements whose sulfide forms can be controlled by their trace amounts. The contents of Y, Zr, La, and Ce can be 0%, but to achieve this effect, the contents of Y, Zr, La, and Ce are preferably 0.0001% or more, or 0.0005% or more, 0.001% or more, or 0.002% or more, respectively. On the other hand, excessive content of these elements can sometimes result in the formation of coarse oxides. Therefore, the contents of Y, Zr, La and Ce are preferably 0.050% or less, but can also be 0.030% or less, 0.010% or less, 0.005% or less, or 0.003% or less.

[0090] In the steel sheet of the embodiments of the present invention, the balance other than the aforementioned elements consists of Fe and impurities. Impurities refer to components that are mixed in during the industrial manufacturing of steel sheets, such as raw materials like ores and waste, due to various reasons in the manufacturing process.

[0091] The chemical composition of the steel plate according to the embodiments of the present invention can be determined by general analytical methods. For example, according to JIS G 1201:2014, the powder can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Specifically, for example, a 35 mm square test piece can be obtained from near half the thickness of the steel plate, and the measurement can be performed using a Shimadzu ICPS-8100 or similar measuring device under conditions based on a pre-prepared standard curve. C and S, which cannot be measured by ICP-AES, can be measured using the combustion-infrared absorption method, N using the inactive gas melting-thermal conductivity method, and O using the inactive gas melting-non-dispersive infrared absorption method. If the surface of the steel plate has a coating, the chemical composition can be analyzed after removing the coating by mechanical grinding or the like.

[0092] [Plate thickness] The steel plate in embodiments of the present invention typically has a thickness of 0.6 to 6.0 mm. Although not particularly limited, the plate thickness can 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.

[0093] [Covering layer] The steel sheet of the embodiments of the present invention, for the purpose of improving corrosion resistance, may further have a coating, such as a plating layer, on at least one surface, preferably on two surfaces. Without particular limitation, the coating layer may, for example, contain at least one selected from zinc, aluminum, magnesium, and their alloys. More specifically, the coating layer may be a molten plating layer or an electroplating layer. Molten plating layers include, for example, molten zinc plating, alloyed molten zinc plating, molten aluminum plating, molten Zn-Al alloy plating, molten Zn-Al-Mg alloy plating, molten Zn-Al-Mg-Si alloy plating, etc. Electroplating layers include, for example, electroplated zinc plating, electroplated Zn-Ni alloy plating, etc. Preferably, the coating layer is a molten zinc plating, alloyed molten zinc plating, or electroplated zinc plating. The amount of coating layer adhered is not particularly limited and may be a general amount. It should be noted that, as described above, such a coating layer is excluded from the determination of tensile strength, thickness direction position, and chemical composition specified in the present invention.

[0094] As described above, the steel plate of the embodiments of the present invention has a very high tensile strength of over 1660 MPa, but excellent resistance to hydrogen embrittlement, and is therefore very useful as a skeleton component, bumper, or other structural component and reinforcing component of an automobile.

[0095] <Methods for manufacturing steel plates>

[0096] Next, a preferred method for manufacturing the steel plate according to an embodiment of the present invention will be described. The following description is an example of a characteristic method for manufacturing the steel plate according to an embodiment of the present invention, and is not intended to limit the steel plate to the steel plate manufactured by the manufacturing method described below.

[0097] The method for manufacturing a steel plate according to an embodiment of the present invention is characterized by comprising: In the hot rolling process, a slab having the chemical composition described above in relation to the steel sheet is heated to a temperature of 1100~1300°C, and then finished rolled at a finishing temperature of 850~1050°C. The finished rolled steel sheet is then cooled to below 500°C at an average cooling rate of 20°C / second or higher and then coiled. The pickling process involves pickling the obtained hot-rolled steel sheet; The cold rolling process involves cold rolling the pickled hot-rolled steel sheet with a reduction rate of 35-80%. The annealing process involves heating the resulting cold-rolled steel sheet and then holding it at a maximum heating temperature of 830~900℃ for 20~150 seconds; and The cooling process involves cooling the cold-rolled steel sheet to below the Ms point at an average cooling rate of 1.0°C / second or higher. When the process also includes a stabilization treatment step and / or a surface rolling step, the stabilization treatment step has a treatment temperature of less than 280°C and a holding time of less than 200 hours, respectively, and the surface rolling step has an elongation of less than 2.00%. The following provides a detailed description of each step.

[0098] [Hot rolling process]

[0099] [Heating of slab] First, a slab having the chemical composition described above in relation to the steel sheet is heated. From a productivity point of view, it is preferable to cast the slab using a continuous casting method, but it can also be manufactured by ingot casting or thin-slab casting. The slab used contains a large number of alloying elements in order to obtain a high-strength steel sheet. Therefore, it is necessary to heat the slab before supplying it for hot rolling to dissolve the alloying elements in the slab. When the heating temperature is below 1100°C, the alloying elements are not sufficiently dissolved in the slab, leaving coarse alloy carbides, which sometimes cause embrittlement and cracking during hot rolling. Therefore, the heating temperature is preferably 1100°C or higher. There is no particular upper limit to the heating temperature, but from the viewpoint of heating equipment capacity and productivity, it is preferably 1300°C or lower.

[0100] [Rough rolling] In this method, for example, rough rolling can be performed before finish rolling to adjust the thickness of the heated slab. Rough rolling is not particularly limited by conditions, as long as the desired slab size is ensured.

[0101] [Precision rolling] The heated slab, or slabs that have been rough-rolled as needed, are then subjected to finish rolling. As described above, the slabs contain a significant amount of alloying elements, thus requiring increased rolling loads during hot rolling. Therefore, hot rolling is preferably performed at high temperatures. In particular, the finish rolling temperature is crucial for controlling the microstructure of the steel sheet. At low finish rolling temperatures, the microstructure may become inhomogeneous, resulting in poor formability. Therefore, the finish rolling temperature is preferably 850°C or higher. On the other hand, to suppress austenite coarsening, the finish rolling temperature is preferably 1050°C or lower.

[0102] [Cooling & Rolling] Next, the finished steel sheet is cooled to below 500°C at an average cooling rate of 20°C / second or higher and then coiled. If the average cooling rate is below 20°C / second or the coiling temperature exceeds 500°C, phosphorus segregation occurs during the hot rolling process, causing the hot-rolled steel sheet to become brittle, and sometimes subsequent cold rolling becomes difficult. For example, the average cooling rate is preferably 25°C / second or higher, and the coiling temperature is preferably below 480°C. Alternatively, the average cooling rate is preferably 100°C / second or lower, and the coiling temperature is preferably 300°C or higher.

[0103] [Hot Rolling Post-Processing] In a preferred embodiment of this manufacturing method, optionally, after the hot rolling process and before the pickling process, a post-hot rolling treatment process is included, in which the obtained hot-rolled steel sheet is held in a temperature range of 400~680°C for 1 hour or more but less than 24 hours. By performing such heat treatment, cementite can be formed in the hot-rolled steel sheet, thus allowing elements that stabilize austenite, such as Mn, to accumulate in the cementite during subsequent annealing processes, further improving the stability of the austenite. As a result, when measured by X-ray diffraction, a steel sheet with a lattice constant calculated from the diffraction peaks of the retained austenite at the surface portion, 1 / 4 of the plate thickness, and 1 / 2 of the plate thickness, satisfying the following formula 1, can be obtained. A s >3.5800Å A q >3.5800Å A c >3.5800Å…Equation 1 Here, A s A q and A cThe lattice constants are calculated based on the diffraction peaks from the retained austenite at the surface, 1 / 4 of the plate thickness, and 1 / 2 of the plate thickness, respectively. When the holding temperature is below 400°C, the diffusion of elements such as Mn into cementite is insufficient, and austenite cannot be sufficiently stabilized during the annealing process. On the other hand, when the holding temperature is above 680°C, the cementite melts, and the effect of element enrichment into the cementite cannot be fully achieved. Furthermore, when the holding time is less than 1 hour, the formation of cementite becomes insufficient, and the effect of element enrichment into the cementite is also insufficient. In either case, it is difficult to obtain a steel sheet with a metal structure that satisfies Formula 1 above. On the other hand, even if the holding time exceeds 24 hours, the effect of element enrichment into the cementite becomes saturated; therefore, from an economic point of view, long heat treatment of more than 24 hours is not preferred. If the temperature of the hot-rolled steel sheet after coiling is below 400°C, the hot-rolled steel sheet can be reheated as needed.

[0104] [Pickling process] Next, in order to remove the oxide scale formed on the surface of the hot-rolled steel sheet, the obtained hot-rolled steel sheet is pickled. Pickling can be carried out under suitable conditions for removing oxide scale, and can be carried out in one go or in multiple stages to reliably remove oxide scale.

[0105] [Cold rolling process] After pickling, the hot-rolled steel sheet is cold-rolled at a reduction rate of 35% to 80% in the cold rolling process. By maintaining a cold rolling reduction rate of 35% or more, the shape of the cold-rolled steel sheet can be kept flat, suppressing the decrease in ductility in the final product. The cold rolling reduction rate is preferably 50% or more. On the other hand, by maintaining a cold rolling reduction rate of 80% or less, it is possible to prevent the rolling load from becoming too large and making rolling difficult. The cold rolling reduction rate is preferably 70% or less. There are no particular limitations on the number of rolling passes and the reduction rate per pass; they can be appropriately set so that the overall cold rolling reduction rate falls within the above range.

[0106] [Annealing process]

[0107] [Hold at the 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 production line, and then held at a maximum heating temperature of 830–900°C for 20–150 seconds. This promotes austenitization, resulting in a desired hard microstructure dominated by martensite and other materials during the subsequent cooling process, reliably achieving a tensile strength of 1660 MPa or higher. When the maximum heating temperature is below 830°C or the holding time at the maximum heating temperature is less than 20 seconds, austenitization tends to be insufficient, making it difficult to obtain the desired martensite area ratio in the final steel sheet and thus difficult to achieve a tensile strength of 1660 MPa or higher. On the other hand, when the maximum heating temperature exceeds 900°C or the holding time at the maximum heating temperature exceeds 150 seconds, the austenite coarsens, resulting in low hardenability and a tendency to induce ferrite and bainite phase transformations. Consequently, it is difficult to obtain the desired microstructure fraction in the final microstructure.

[0108] [Coating process] After annealing, the surface of cold-rolled steel sheets can be coated to improve corrosion resistance, among other things. Coating treatments can include melt plating, alloyed melt plating, electroplating, etc. For example, as a coating treatment, the steel sheet can be subjected to melt zinc plating, or alloying can be performed after melt zinc plating. The coating layer may contain at least one element selected from zinc, aluminum, magnesium, and their alloys. More specifically, the coating layer can be a melt coating or an electroplated layer. Melt coatings may include, for example, melt zinc (GI) coatings, alloyed melt zinc (GA) coatings, melt aluminum coatings, melt Zn-Al alloy coatings, melt Zn-Al-Mg alloy coatings, melt Zn-Al-Mg-Si alloy coatings, etc. Electroplated layers may include, for example, electroplated zinc coatings, electroplated Zn-Ni alloy coatings, etc. Preferably, the coating layer is a melt zinc coating, an alloyed melt zinc coating, or an electroplated zinc coating. The specific conditions for coating and alloying are not particularly limited and can be any suitable conditions known to those skilled in the art.

[0109] [Cooling Process]

[0110] [Cooled to below Ms point at an average cooling rate of 1.0℃ / second or higher] Finally, the cold-rolled steel sheet after the annealing or coating process is cooled to below the Ms point in the next cooling process at an average cooling rate of 1.0°C / second or higher. The resulting microstructure consists of at least 85.0% martensite by area, and contains 1.0 to 7.0% retained austenite by area, thus achieving a tensile strength of over 1660 MPa and excellent resistance to hydrogen embrittlement. Here, the Ms point (°C) is calculated based on Equation 7 below. Ms=550-350×[C]-40×[Mn]-20×[Cr]-10×[Mo]-17×[Ni]-10×[Cu]-35×[V]-10×[W]-15×[Co]…Equation 7 In the formula, [C], [Mn], [Cr], [Mo], [Ni], [Cu], [V], [W] and [Co] are the contents (mass%) of each element in the steel plate. On the other hand, when the average cooling rate is less than 1.0 °C / s, more ferrite and bainite are formed during the cooling process, making it difficult to obtain the desired microstructure fraction in the final microstructure and failing to achieve sufficient tensile strength. An average cooling rate of 10.0 °C / s or higher and 20.0 °C / s or lower is preferred. In particular, by controlling the average cooling rate to below 20.0 °C / s, the stability of the retained austenite contained in the metal microstructure of the steel sheet can be improved overall. As a result, the lattice constant A, calculated from the diffraction peaks of the retained austenite at the surface, 1 / 4 of the sheet thickness, and 1 / 2 of the sheet thickness, can be reliably obtained. s A q and A c It satisfies Equation 1 above.

[0111] [Preferred Implementation of the Cooling Process] In a preferred embodiment of the cooling process, after the annealing or coating process, the cold-rolled steel sheet is cooled at an average cooling rate of 1.0°C / second or higher to a control temperature T of 100°C or higher and Ms point below -100°C, and then cooled from this control temperature T to room temperature (25°C) at an average cooling rate of less than 1.0°C / second. By performing such a cooling treatment, the stability of the retained austenite can be further improved due to the enrichment of elements such as carbon. As a result, when measured by X-ray diffraction, a steel sheet with a lattice constant calculated from the diffraction peaks from the retained austenite at the surface portion, the 1 / 4 position of the plate thickness, and the 1 / 2 position of the plate thickness that satisfies the following formula 1 can be obtained. A s >3.5800Å A q >3.5800Å A c >3.5800Å…Equation 1 Here, A s A q and A c The lattice constants are calculated based on the diffraction peaks from the retained austenite at the surface, 1 / 4 of the plate thickness, and 1 / 2 of the plate thickness, respectively. When the controlled temperature T is above Ms point -100°C, there is a relatively large amount of untransformed austenite. Therefore, even if carbon enrichment occurs in this untransformed austenite, it is sometimes impossible to sufficiently stabilize the retained austenite in the final metal structure. In this case, it is difficult to obtain a steel sheet with a metal structure that satisfies Formula 1 above. On the other hand, when the controlled temperature T is below 100°C or the average cooling rate is below 1.0°C / second, it is not possible to promote the enrichment of elements such as carbon into austenite. Similarly, it is sometimes impossible to sufficiently stabilize the retained austenite in the final metal structure. In this case, it is also difficult to obtain a steel sheet with a metal structure that satisfies Formula 1 above. The lower limit of the average cooling rate from the controlled temperature T to room temperature is preferably, for example, 0.1°C / second or more.

[0112] [Stabilization Process] It is preferable to perform a stabilization treatment on the cold-rolled steel sheet after the cooling process. Specifically, the stabilization treatment includes, as needed, heating the cold-rolled steel sheet that has been cooled to below the Ms point or room temperature during the cooling process, and then holding it in a temperature range of 70°C or higher and below 280°C for at least 1 hour and less than 200 hours. By treating the cold-rolled steel sheet containing retained austenite generated in the previous cooling process in such a temperature range for a relatively long time, elements such as C and Mn, which are austenite stabilizing elements in the steel sheet, can be enriched in the retained austenite, thereby sufficiently stabilizing the retained austenite. As a result, when measured by X-ray diffraction, a steel sheet with a metal structure having a lattice constant calculated from the diffraction peaks from the retained austenite at the surface portion, 1 / 4 of the plate thickness position, and 1 / 2 of the plate thickness position that satisfies the following formula 1 can be obtained. A s >3.5800Å A q >3.5800Å A c >3.5800Å…Equation 1 Here, A s A q and A c The lattice constants are calculated based on the diffraction peaks from the retained austenite at the surface, 1 / 4 of the plate thickness, and 1 / 2 of the plate thickness, respectively. When the processing temperature is below 70°C or the holding time is less than 1 hour, the stabilization of retained austenite becomes insufficient, making it difficult to obtain a steel sheet with a microstructure satisfying Formula 1 above. On the other hand, when the stabilization process is performed, if the processing temperature is above 280°C or the holding time is above 200 hours, excessive heat treatment may sometimes lead to the decomposition of most of the retained austenite, resulting in a final microstructure with a retained austenite area ratio of less than 1.0%, and / or the desired strength cannot be achieved due to softening. A processing temperature of 100~200°C and a holding time of 10~100 hours are preferred.

[0113] [Surface smoothing process] Preferably, surface finishing rolling is performed on cold-rolled steel sheets after the cooling process or the stabilization process. Specifically, surface finishing rolling includes rolling the cold-rolled steel sheets after the cooling process or the stabilization process with an elongation of more than 0.05% and less than 2.00%. Here, the elongation is defined using the steel sheet's entry speed V1 (m / s) and exit speed V2 (m / s) by Equation 8 below. Elongation = (V2 - V1) / V1 × 100 … Equation 8 By performing surface finishing on cold-rolled steel sheets under these conditions, unstable retained austenite present in the surface layer and the quarter-thickness region of the steel sheet can be decomposed. This reduces the amount of unstable retained austenite in these regions and ultimately improves its stability. As a result, a steel sheet with the following microstructure can be obtained: RA of retained austenite in the surface layer. s The area ratio RA of retained austenite at the 1 / 2 position of the plate thickness c Satisfy RA s / RA c <0.75, and / or the lattice constant calculated from the diffraction peaks from retained austenite at the surface portion, the 1 / 4 thickness position, and the 1 / 2 thickness position satisfies Equation 2 below. In this case, the amount of hydrogen released from retained austenite at the surface portion and the 1 / 4 thickness position due to bending processing, etc., can be significantly reduced. Therefore, compared with the case where the amount of retained austenite is only controlled within the range of 1.0 to 7.0%, the hydrogen embrittlement resistance of the steel plate can be further improved. A s / A c >0.9970 A q / A c >0.9970…Equation 2 Here, A s A q and A c The lattice constants are calculated based on the diffraction peaks from the retained austenite at the surface, 1 / 4 of the plate thickness, and 1 / 2 of the plate thickness, respectively. When the elongation is below 0.05%, the decomposition of unstable retained austenite in the surface layer of the steel plate becomes incomplete, making it difficult to obtain steel with elongation that meets the RA (Range Requirement). s / RA cSteel plates with a metal structure of <0.75 and / or the metal structure of Formula 2 above. On the other hand, when the surface rolling process is performed, if the elongation is 2.00% or more, excessive surface rolling may sometimes lead to the decomposition of most of the retained austenite, resulting in a final microstructure with a retained austenite area ratio of less than 1.0%. An elongation of 0.10 to 1.00% is preferred.

[0114] The present invention will be described in more detail below through embodiments, but the present invention is not limited to these embodiments in any way.

[0115] Example

[0116] In the following embodiments, steel plates according to embodiments of the present invention were manufactured under various conditions, and the tensile strength and hydrogen embrittlement resistance of the obtained steel plates were investigated.

[0117] First, molten steel is cast using a continuous casting method to form slabs with various chemical compositions as shown in Table 1. These slabs are then heated to the heating temperatures shown in Table 2 and hot-rolled. Hot rolling is performed by roughing and finishing rolling, with the finishing rolling end temperatures shown in Table 2. Next, the finished steel sheet is cooled and coiled under the conditions shown in Table 2. Then, for the resulting hot-rolled steel sheet with a thickness of 2.6 mm, the appropriate hot-rolling post-treatment shown in Table 2 is applied, followed by pickling, and then cold rolling at the reduction rate shown in Table 2 to obtain a cold-rolled steel sheet with a thickness of 1.4 mm. Next, the resulting cold-rolled steel sheet is heated in the heating furnace and soaking furnace of a continuous annealing production line under the conditions shown in Table 2 and held, followed by appropriate molten zinc plating (GI) or alloyed molten zinc plating (GA) as a coating treatment. Finally, in the cooling process, the cold-rolled or coated steel sheet is cooled under the conditions shown in Table 2, followed by appropriate stabilization treatment and / or surface finishing rolling as shown in Table 2. It should be noted that, in this embodiment, in all examples, the "average cooling rate up to the Ms point" is evaluated as the "average cooling rate up to the control temperature T".

[0118] The properties of the obtained steel plate were measured and evaluated using the following methods.

[0119] [tensile strength] Tensile strength was determined by tensile testing according to JIS Z 2241:2022 on a JIS 5 test piece taken from a direction parallel to the rolling right angle of the steel plate along the length of the test piece.

[0120] [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, firstly, the steel plate was cut with a shearing force applied at a punching gap of 12.5% ​​and a shearing angle of 0 degrees to obtain a 15 mm wide steel plate. The test piece was taken in a direction parallel to the rolling right angle of the steel plate, i.e., the bending edge was parallel to the rolling direction of the steel plate. If the rolling direction of the steel plate could not be specified, the test piece could be taken in any direction within the surface of the steel plate. Next, a U-shaped bending test was performed with an 8R radius. Specifically, the U-shaped bending test was performed using the pressure bending method described in JIS Z 2248:2022. The inner radius r was set to 8 mm, and the distance between supports was set to L = 2r + 2t (thickness of the test piece) ± 1 mm. The test piece was pressed in until it passed through the supports, resulting in a 180° bent sample. Next, a strain gauge was attached to the center of the obtained test piece, and the two ends of the test piece were bolted together, thereby applying stress. The applied stress was calculated from the strain monitored by the strain gauge. It should be noted that the strain gauge used has a 1.0 mm spacing between its gauge points and is attached to the top of the bend in a manner parallel to the length of the steel plate, i.e., perpendicular to the bend's edge. Let the load stress be defined as: load stress = strain measured at the top of the bend by the strain gauge × Young's modulus (constantly 20500 N / mm²). 2 The load stress is the stress corresponding to 80% of the tensile strength of the test piece. This is because the residual stress introduced during forming is considered to correspond to the tensile strength of the steel plate. The obtained U-shaped bending test pieces were immersed in an HCl aqueous solution with pH 2 at a liquid temperature of 35°C, and then kept for 72 hours to check for cracking. The pH of the HCl aqueous solution is low, and the longer the immersion time, the more hydrogen penetrates into the steel plate, thus making the hydrogen embrittlement environment a severe condition. After immersion, the maximum length of cracks generated in the U-shaped bending test pieces was measured. Cracks exceeding 8 mm in length were rated NG (Not Acceptable), cracks between 3 mm and 8 mm in length were rated A (Acceptable), cracks between 1 mm and 3 mm in length were rated AA (Good), and cracks less than 1 mm in length were rated AAA (Excellent).

[0121] High-strength steel plates with tensile strength above 1660 MPa and hydrogen embrittlement resistance rated A, AA, or AAA are considered to have excellent hydrogen embrittlement resistance. The results are shown in Table 3. In Table 3, RA... s RA q A s and A q The values ​​shown are only those calculated based on measurements from one surface of the steel plate. However, all the steel plates are manufactured with the same treatment on both sides, thus confirming that the steel plates are substantially the same on both sides; in fact, in several steel plates, these values ​​are the same on both sides.

[0122]

[0123]

[0124]

[0125]

[0126] Referring to Tables 2 and 3, in Example A-2, due to the low maximum heating temperature of the annealing process, austenitization was insufficient, and the desired martensite area ratio could not be obtained. As a result, tensile strength was low. Similarly, in Example B-2, due to the short holding time of the annealing process, austenitization was also insufficient, and the desired martensite area ratio could not be obtained. As a result, tensile strength was low. In Example C-2, due to the long holding time of the annealing process, the austenite was considered to have coarsened, resulting in low hardenability. As a result, ferrite and bainite phase transformations occurred, and the desired martensite area ratio could not be obtained, leading to low tensile strength. In Example D-2, the average cooling rate below the Ms point in the cooling process was slow, resulting in the formation of more ferrite and bainite during cooling. As a result, the desired martensite area ratio could not be obtained, leading to low tensile strength. In Example F-2, due to the high processing temperature of the stabilization treatment process, it was believed that most of the retained austenite decomposed. As a result, the desired retained austenite area ratio cannot be obtained in the final microstructure, leading to poor resistance to hydrogen embrittlement. Example G-2 suggests that the long holding time in the stabilization process caused the decomposition of most of the retained austenite. Consequently, the desired retained austenite area ratio cannot be obtained in the final microstructure, resulting in poor resistance to hydrogen embrittlement. Example H-2, due to the high elongation in the surface rolling process, suggests that much of the retained austenite decomposed. Consequently, the desired retained austenite area ratio cannot be obtained in the final microstructure, resulting in poor resistance to hydrogen embrittlement.

[0127] In contrast, in all the inventive examples of the present invention, by having the metal structure of the steel plate consist of a predominantly martensite structure and including a specified amount of retained austenite with the characteristic of easily storing hydrogen in the metal structure, more specifically, by controlling the metal structure of the steel plate to contain at least 85.0% martensite and 1.0 to 7.0% retained austenite by area percent, it is possible to significantly improve the hydrogen embrittlement resistance, especially the hydrogen embrittlement resistance of the bending part, even though it has a very high tensile strength of 1660 MPa or more.

[0128] In particular, in Examples F-1, H-1, I-1, and K-1 to P-1, by performing an appropriate hot-rolling post-treatment process after the hot rolling process and before the pickling process, cooling to a control temperature T of 100°C or higher and Ms point -100°C or lower at an average cooling rate of 1.0°C / s or higher in the cooling process, and then cooling to room temperature from the control temperature T at an average cooling rate of less than 1.0°C / s, followed by an appropriate stabilization treatment process and a surface finishing rolling process, it is possible to obtain a product that, in addition to having a martensite content of 85.0% or higher and a retained austenite content of 1.0 to 7.0%, also satisfies the RA (Rapid Aspect Rating). s / RA c Steel plates with a metal structure of <0.75, Formula 1, and Formula 2. As a result, in these examples, the resistance to hydrogen embrittlement is particularly significantly improved.

Claims

1. A steel plate, characterized in that, Tensile strength above 1660MPa Metal structure as a percentage of area Martensite: 85.0% or more Residual austenite: 1.0~7.0%, and Balance of microstructure: below 10.0% Chemical composition by mass% contains C：0.25~0.45%、 Si: 0.01~1.30% Mn: 1.00~3.50%, P：0.0001~0.0200%、 S：0.0001~0.0200%、 Al：0.001~1.000%、 N：0.0001~0.0200%、 O：0.0001~0.0200%、 Co: 0~0.50%, Ni: 0~1.00% Mo: 0~1.00% Cr：0~2.000%、 Ti: 0~0.500%, B：0~0.0100%、 Nb: 0~0.500%, V：0~0.500%、 Cu: 0~0.500%, W：0~0.100%、 Ta: 0~0.100% Sn: 0~0.100% Sb: 0~0.100% As: 0~0.100% Mg: 0~0.0500%, Ca: 0~0.050%, Y：0~0.050%、 Zr：0~0.050%、 La: 0~0.050% sum Ce: 0~0.050%, The balance consists of Fe and impurities.

2. The steel plate according to claim 1, characterized in that, When measured by X-ray diffraction, the lattice constant calculated from the diffraction peaks of the retained austenite at the surface layer, the 1 / 4 thickness position, and the 1 / 2 thickness position satisfies the following equation: A s >3.5800Å A q >3.5800Å A c >3.5800Å…Equation 1 Here, A s A q and A c The lattice constants are calculated based on the diffraction peaks from the retained austenite at the surface, 1 / 4 of the plate thickness, and 1 / 2 of the plate thickness, respectively.

3. The steel plate according to claim 1 or 2, characterized in that, The area ratio of retained austenite in the surface layer, RA s The area ratio RA of the retained austenite at the 1 / 2 position of the plate thickness c Satisfy RA s / RA c <0.

75.

4. The steel plate according to any one of claims 1 to 3, characterized in that, When measured by X-ray diffraction, the lattice constant calculated from the diffraction peaks of the retained austenite at the surface layer, the 1 / 4 thickness position, and the 1 / 2 thickness position satisfies the following equation 2: A s / A c >0.9970 A q / A c >0.9970…Formula 2 Here, A s A q and A c The lattice constants are calculated based on the diffraction peaks from the retained austenite at the surface, 1 / 4 of the plate thickness, and 1 / 2 of the plate thickness, respectively.

5. The steel plate according to any one of claims 1 to 4, characterized in that, The chemical composition, expressed in % by mass, includes Co: 0.01~0.50%, Ni: 0.01~1.00% Mo: 0.01~1.00% Cr：0.001~2.000%、 Ti: 0.001~0.500%, B：0.0001~0.0100%、 Nb: 0.001~0.500%, V：0.001~0.500%、 Cu: 0.001~0.500%, W：0.001~0.100%、 Ta: 0.001~0.100% Sn: 0.001~0.100% Sb: 0.001~0.100% As: 0.001~0.100% Mg: 0.0001~0.0500%, Ca: 0.001~0.050% Y：0.001~0.050%、 Zr：0.001~0.050%、 La: 0.001~0.050%, and Ce: 0.001~0.050% At least one of them.

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

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