Steel plate and its manufacturing method

Abstract

A material having a specified chemical composition, in which the number density of recesses on the surface with a depth of more than 2 μm and a vertex angle of 45 degrees or less is 10.0 / mm or less, and which is immersed in an aqueous solution of ammonium thiocyanate with a concentration of 100 g / L at 25°C for 48 hours, and then heated from room temperature to 300°C at a heating rate of 100°C / h by thermal desorption analysis, shows no hydrogen release. 100-200 / H 20-300 <0.30 (in the formula, H 100-200 is the amount of hydrogen released at 100-200°C, H 20-300 The present invention provides a steel sheet that satisfies the above requirements (amount of hydrogen released at 20 to 300°C) and a method for manufacturing the same.

Description

[Technical Field]

[0001] The present invention relates to a steel sheet and a method for manufacturing the same. [Background technology]

[0002] In recent years, there has been a demand for improved fuel efficiency in automobiles in light of greenhouse gas emission regulations as part of measures to combat global warming, and the use of high-strength steel sheets has been expanding to reduce the weight of vehicle bodies and ensure collision safety.

[0003] Hydrogen embrittlement cracking (also known as delayed fracture) can be a problem for high-strength steel plates. Hydrogen embrittlement cracking is a phenomenon in which a steel component subjected to high stress during use suddenly breaks due to hydrogen that penetrates into the steel from the environment. It is generally known that hydrogen embrittlement cracking in steel plates is more likely to occur as the strength of the steel plate increases. This is thought to be because the higher the tensile strength of the steel plate, the greater the residual stress in the steel plate after part formation. The susceptibility to this hydrogen embrittlement cracking is called hydrogen embrittlement resistance.

[0004] Various attempts have been made to improve the hydrogen embrittlement resistance of steel sheets.

[0005] For example, Patent Document 1 describes a zinc-based plated steel sheet characterized in that the steel structure in a range from 1 / 8 to 3 / 8 of the thickness from the surface of the steel sheet, centered on the 1 / 4 thickness, contains, by volume, ferrite: 0 to 10%, bainite: 0 to 20%, tempered martensite: 70% or more, fresh martensite: 0 to 10%, retained austenite: 0 to 10%, and pearlite: 0 to 5%, and that when the zinc-based plated layer is removed and the steel sheet is heated from room temperature to 200°C, the amount of hydrogen released is 0.40 ppm or less per steel sheet mass, the tensile strength is 1470 MPa or more, and no cracks occur in a U-bend test in which a stress equivalent to 1000 MPa is applied for 24 hours. Furthermore, Patent Document 1 teaches that the hydrogen that affects hydrogen embrittlement is hydrogen that is released when a steel sheet is heated at a relatively low temperature, and in this regard, it teaches that in order to prevent hydrogen embrittlement cracking, the amount of hydrogen that is released when a steel sheet is heated from room temperature to 200°C should be limited to 0.40 ppm or less.

[0006] Patent Document 2 describes a high-strength cold-rolled steel sheet having a predetermined chemical composition, wherein the structure at a position from the surface to one-quarter of the plate thickness contains, by volume, 70.0% or more tempered martensite, more than 3.0% but less than 10.0% retained austenite, a total of 25.0% or less ferrite and bainite, and 5.0% or less martensite; the structure at a position 25 μm from the surface contains, by volume, a total of 70% or more ferrite and bainite, and a total of 30% or less martensite and tempered martensite; the average grain size of the martensite and the tempered martensite at a position 25 μm from the surface is 5.0 μm or less; the tensile strength is 1310 MPa or more; the uniform elongation is 5.0% or more; and R / t, which is the ratio of the limiting bending radius R to the plate thickness t in a 90° V-bend, is 5.0 or less. Furthermore, Patent Document 2 teaches that if the volume fraction of ferrite and bainite is 70% or more in total at a position 25 μm from the surface of the steel plate in the plate thickness direction, the volume fraction of martensite and tempered martensite is 30% or less in total, and the average grain size of martensite and tempered martensite is 5.0 μm or less, the steel plate surface will be soft, and the hard phase in the surface layer, which is the starting point for cracks, will be reduced and made fine and uniform, thereby reducing the number of starting points for cracks and suppressing hydrogen embrittlement. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2019 / 212047 [Patent Document 2] International Publication No. 2019 / 181950 Summary of the Invention [Problem to be solved by the invention]

[0008] As mentioned above, it is known that hydrogen embrittlement cracking is more likely to occur as the strength of the steel plate increases, and is particularly likely to occur in bent sections (hereinafter simply referred to as "bent sections") that are subjected to large plastic strain.

[0009] Therefore, an object of the present invention is to provide a steel sheet having high strength and excellent resistance to hydrogen embrittlement in bent portions, and a method for manufacturing the same, by using a novel structure. [Means for solving the problem]

[0010] To achieve the above object, the present inventors conducted research, focusing particularly on the microstructure of steel sheets. Specifically, the present inventors first discovered that by optimizing the chemical composition of steel sheets and configuring the steel sheet's microstructure to be primarily martensite, it is possible to achieve high strength of the steel sheet, for example, a tensile strength of 1470 MPa or more, while improving the hydrogen embrittlement resistance of the steel sheet. In addition, the present inventors discovered that by reducing relatively large and sharp depressions on the steel sheet surface, stress concentration areas that can serve as initiation points for hydrogen embrittlement cracking can be reduced, and by reducing hydrogen trapping sites in the steel, it is possible to significantly suppress the occurrence of hydrogen embrittlement cracking in bent parts, even in steel sheets with a high tensile strength of 1470 MPa or more, and thus completed the present invention.

[0011] The present invention, which has achieved the above object, is as follows. (1) In mass%, C: 0.16~0.35%, Si: 0.001 to 0.80%, Mn: 1.00~3.50%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, Ti: 0.001 to 0.100%, B: 0.0005~0.0050%, N: 0.0100% or less, O: 0.0100% or less, Cr: 0~1.00%, Mo: 0-1.00%, Cu: 0-1.00% Ni: 0 to 1.00% Co: 0 to 1.00%, W: 0~1.00%, Ta: 0 to 1.000%, Sn: 0 to 1.00% Sb: 0 to 0.50% Nb: 0 to 0.200%, V: 0~1.00%, As: 0~0.100%, Zn: 0 to 1.000%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Hf: 0 to 0.0100%, Bi: 0 to 0.0100%, REM: 0 to 0.0100%, and The balance has a chemical composition consisting of Fe and impurities, The steel structure at the 1 / 4 thickness position from the surface is, in area%, Martensite: 70% or more, Sum of ferrite and bainite: 0 to 20%, and Retained austenite: 0-10% The number density of recesses on the surface that are more than 2 μm deep and have a vertex angle of 45 degrees or less is 10.0 / mm or less, and A steel sheet characterized in that, when immersed in an aqueous solution of ammonium thiocyanate having a concentration of 100 g / L at 25°C for 48 hours and then heated from room temperature to 300°C at a heating rate of 100°C / h by thermal desorption analysis, the amount of released hydrogen is measured, and the steel sheet satisfies the following formula (1): H 100-200 / H 20-300 <0.30 (1) H 100-200 : Hydrogen release amount (mass ppm) at 100-200°C H 20-300 : Hydrogen release amount (mass ppm) at 20 to 300°C (2) The steel sheet according to (1) above, characterized in that when the C concentration of the steel sheet is measured in the depth direction from the surface thereof using a high-frequency glow discharge optical emission spectrometer (GDS), the following formula (2) is satisfied:

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[0012] According to the present invention, it is possible to provide a steel sheet having high strength and excellent resistance to hydrogen embrittlement at bent portions, and a method for manufacturing the same. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a diagram illustrating a method for measuring the number density of recesses having a depth of more than 2 μm and an apex angle of 45 degrees or less on the surface of a steel plate according to an embodiment of the present invention. [Figure 2] FIG. 10 is a diagram schematically showing a test method for evaluating the hydrogen embrittlement resistance of a bent portion. DETAILED DESCRIPTION OF THE INVENTION

[0014] <Steel plate> The steel plate according to the embodiment of the present invention has, in mass%, C: 0.16~0.35%, Si: 0.001 to 0.80%, Mn: 1.00~3.50%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, Ti: 0.001 to 0.100%, B: 0.0005~0.0050%, N: 0.0100% or less, O: 0.0100% or less, Cr: 0~1.00%, Mo: 0-1.00%, Cu: 0-1.00% Ni: 0 to 1.00% Co: 0 to 1.00%, W: 0~1.00%, Ta: 0 to 1.000%, Sn: 0 to 1.00% Sb: 0 to 0.50% Nb: 0 to 0.200%, V: 0~1.00%, As: 0~0.100%, Zn: 0 to 1.000%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Hf: 0 to 0.0100%, Bi: 0 to 0.0100%, REM: 0 to 0.0100%, and The balance has a chemical composition consisting of Fe and impurities, The steel structure at the 1 / 4 thickness position from the surface is, in area%, Martensite: 70% or more, Sum of ferrite and bainite: 0 to 20%, and Retained austenite: 0-10% The number density of recesses on the surface that are more than 2 μm deep and have a vertex angle of 45 degrees or less is 10.0 / mm or less, and The material is characterized by satisfying the following formula (1) when it is immersed in a 100 g / L aqueous solution of ammonium thiocyanate at 25°C for 48 hours, and then heated from room temperature to 300°C at a temperature increase rate of 100°C / h by thermal desorption analysis to measure the amount of released hydrogen. H 100-200 / H 20-300 <0.30 (1) H 100-200 : Hydrogen release amount (mass ppm) at 100-200°C H 20-300 : Hydrogen release amount (mass ppm) at 20 to 300°C

[0015] As mentioned above, it is known that hydrogen embrittlement cracking is more likely to occur as the strength of steel plate increases. In particular, in steel plates with extremely high strength, such as those with a tensile strength of 1470 MPa or more, the steel plate generally has a steel structure that mainly contains martensite in order to ensure high strength. However, in such high-strength steel plates mainly composed of martensite, hydrogen embrittlement cracking can occur when hydrogen that has penetrated the steel accumulates at the prior austenite grain boundaries in the martensite structure, reducing the bonding strength of the grain boundaries. This weakened grain boundaries can then become the starting point for embrittlement.

[0016] Therefore, the present inventors first found that optimizing the chemical composition of a steel sheet, particularly by adding predetermined amounts of Ti and B, more specifically, 0.001 to 0.100 mass% and 0.0005 to 0.0050 mass%, respectively, ensures sufficient solute B, thereby improving hardenability and strengthening prior austenite grain boundaries. More specifically, adding Ti fixes N, which is present as an impurity in the steel, as titanium nitride (TiN), thereby preventing the solute B from being consumed in the formation of boron nitride (BN). As a result, it becomes possible to ensure sufficient solute B in the steel. Here, solute B improves the hardenability of the steel sheet and contributes to the formation of a steel structure mainly composed of martensite, more specifically, a steel structure containing 70% or more martensite by area. It also strengthens the prior austenite grain boundaries, thereby improving the hydrogen embrittlement resistance of the steel sheet. Therefore, according to the steel plate according to the embodiment of the present invention, by using a specific combination of a predetermined chemical composition of the steel plate containing, in particular, Ti and B, and a steel structure containing martensite in an area percentage of 70% or more, it is possible to achieve high strength in the steel plate, for example, a tensile strength of 1470 MPa or more, while improving the hydrogen embrittlement resistance of the steel plate.

[0017] In addition, because hydrogen embrittlement cracking is particularly likely to occur in bent parts that are subjected to large plastic strain, as mentioned above, the present inventors conducted further studies to improve the hydrogen embrittlement resistance of such bent parts. As a result, the present inventors found that by reducing the number of relatively large and sharp depressions on the steel sheet surface, more specifically, by controlling the number density of depressions on the steel sheet surface that are more than 2 μm deep and have an apex angle of 45 degrees or less to 10.0 / mm or less, it is possible to reduce stress concentration points in bent parts that could become the starting points for hydrogen embrittlement cracking, thereby improving the hydrogen embrittlement resistance of bent parts.

[0018] FIG. 1 is a diagram illustrating a method for measuring the number density of recesses having a depth of more than 2 μm and an apex angle of 45° or less on the surface of a steel sheet according to an embodiment of the present invention. Referring particularly to FIG. 1(c), steel sheet 1 has recesses 2 on its surface. In recesses 2, the distance (depth) D from center line CL to deepest position A, determined by the least squares method from the profile of surface height SH, exceeds 2 μm. Additionally, in recesses 2, an apex angle θ is formed by line segments connecting position A with intersections B and C of center line CL and recess 2. When the apex angle θ is 45° or less, such relatively large and sharp recesses 2 can become stress concentration areas, which are likely to be the initiation points for hydrogen embrittlement cracking in bent portions. Therefore, the present inventors have discovered that, as will be described in detail later in connection with the manufacturing method, the surface irregularities of a steel sheet can be sufficiently reduced by appropriately controlling the hot rolling process and the pickling process in particular. More specifically, the number density of recesses having a depth of more than 2 μm and an apex angle of 45° or less on the surface can be controlled to 10.0 / mm or less. As a result, in the steel plate according to the embodiment of the present invention, the number of stress concentration points that can become the starting points for hydrogen embrittlement cracking is sufficiently reduced, and therefore it is possible to improve the hydrogen embrittlement resistance of the bent portion. The method for measuring the number density of recesses will be described in more detail later.

[0019]

[0005] On the other hand, studies by the present inventors have revealed that simply reducing the number of relatively large and sharp recesses on the surface of a steel sheet may not necessarily sufficiently improve the hydrogen embrittlement resistance of a bent portion. Therefore, the present inventors conducted further studies focusing on the steel structure of the steel sheet. As a result, the present inventors have found that, as will be described in detail later in connection with the manufacturing method, it is possible to reduce hydrogen trapping sites in the steel by appropriately heat treating the steel sheet after cold rolling, and that a specific combination of such reduction in hydrogen trapping sites and the control of the number density of recesses on the surface of the steel sheet as described above can significantly improve the hydrogen embrittlement resistance of a bent portion.

[0020] More specifically, the steel sheet according to the embodiment of the present invention is primarily composed of martensite, which is generally known to have a high dislocation density and a hard microstructure. Because dislocations have the property of trapping hydrogen that penetrates into steel, hydrogen easily accumulates in steel sheets having a steel microstructure primarily composed of martensite, making it extremely difficult to improve hydrogen embrittlement resistance. The present inventors have discovered that dislocation-related hydrogen trapping sites can be significantly reduced by heat-treating the cold-rolled steel sheet under appropriate conditions. While not intending to be bound by any particular theory, it is believed that heat-treating the cold-rolled steel sheet under appropriate conditions ensures sufficient solute carbon (C) in the steel, allowing the solute C to attach to dislocations before hydrogen. As a result, it is believed that the function of dislocations in the steel as hydrogen trapping sites can be significantly reduced or suppressed. In relation to the reduction of such hydrogen trapping sites in steel, the present inventors have found that when a steel sheet is immersed in an aqueous solution of ammonium thiocyanate with a concentration of 100 g / L at 25°C for 48 hours and then heated from room temperature to 300°C at a heating rate of 100°C / h by thermal desorption analysis to measure the amount of released hydrogen, a steel structure that satisfies the following formula (1) can be formed. H 100-200 / H 20-300 <0.30 (1) H 100-200: Hydrogen release amount (mass ppm) at 100-200°C H 20-300 : Hydrogen release amount (mass ppm) at 20 to 300°C

[0021] To explain the above formula (1) in more detail, experiments by the present inventors have shown that hydrogen trapped in dislocations is released in the temperature range of 100 to 200°C when thermal desorption analysis is performed under the above conditions. 100-200 / H 20-300 It can be understood that the smaller the value of H, more specifically, the smaller the ratio of the amount of hydrogen released (mass ppm) at 100 to 200°C to the total amount of hydrogen released (mass ppm) at 20 to 300°C, the smaller the amount of hydrogen trapped in dislocations in the steel. Since the steel sheet according to the embodiment of the present invention is mainly composed of martensite, it is clear that the number of dislocations is relatively large. Nevertheless, H 100-200 / H 20-300 The fact that the value of is small supports the idea that dislocations in the steel are pinned by solute C, thereby reducing the number of hydrogen trapping sites associated with dislocations in the steel.

[0022] As a result of further investigation from this perspective, the present inventors have found that H in the above formula (1) 100-200 / H 20-300 It has been found that by reducing the number of hydrogen trapping sites in the steel to a level where the value of σ is less than 0.30, in specific combination with the control of the number density of recesses on the steel sheet surface as described above, it is possible to significantly improve the hydrogen embrittlement resistance of bent portions. Therefore, with the steel sheet according to the embodiment of the present invention, a steel structure mainly composed of martensite can be used to achieve high strength, more specifically, a high strength of 1470 MPa or more, while also significantly improving the hydrogen embrittlement resistance of bent portions. Therefore, the steel sheet according to the embodiment of the present invention is particularly useful in the automotive field, where a high level of both high strength and hydrogen embrittlement resistance is required.

[0023] Hereinafter, the steel sheet according to the embodiment of the present invention will be described in more detail. In the following description, the unit of content of each element, "%", means "mass%" unless otherwise specified. Furthermore, in this specification, "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower limit and upper limit, unless otherwise specified.

[0024] [C:0.16~0.35%] Carbon (C) is an essential element for ensuring the strength of steel sheets. To fully obtain this effect, the C content is set to 0.16% or more. The C content may be 0.18% or more, 0.20% or more, 0.22% or more, or 0.24% or more. On the other hand, excessive C content may result in an excessive increase in strength, which may result in a decrease in hydrogen embrittlement resistance and uniform elongation at bent portions. For this reason, the C content is set to 0.35% or less. The C content may be 0.32% or less, 0.30% or less, 0.28% or less, or 0.26% or less.

[0025] [Si: 0.001 to 0.80%] Silicon (Si) is an element that suppresses the formation of iron carbides and contributes to improving strength and formability. To fully obtain these effects, the Si content is set to 0.001% or more. The Si content may be 0.01% or more, 0.05% or more, 0.10% or more, 0.20% or more, 0.30% or more, or 0.40% or more. On the other hand, excessive Si content may reduce local ductility and reduce hydrogen embrittlement resistance at bent portions. Therefore, the Si content is set to 0.80% or less. The Si content may also be 0.70% or less, 0.60% or less, or 0.50% or less.

[0026] [Mn: 1.00~3.50%] Manganese (Mn) is a powerful austenite-stabilizing element and is effective in increasing the strength of steel sheets. To fully achieve this effect, the Mn content is set to 1.00% or more. The Mn content may be 1.20% or more, 1.50% or more, 1.80% or more, 2.00% or more, 2.20% or more, or 2.40% or more. On the other hand, excessive Mn content may lead to excessive formation of martensite, which may deteriorate hydrogen embrittlement resistance and / or uniform elongation. Therefore, the Mn content is set to 3.50% or less. The Mn content may be 3.40% or less, 3.20% or less, 3.00% or less, 2.80% or less, 2.60% or less, or 2.50% or less.

[0027] [P:0.050% or less] P (phosphorus) is a solid solution strengthening element and is effective in increasing the strength of steel sheets, but excessive addition may deteriorate weldability and toughness. Therefore, the P content is set to 0.050% or less. The P content is preferably 0.045% or less, 0.035% or less, or 0.020% or less. The P content may be 0%, but excessive reduction of the P content increases the cost of dephosphorization. Therefore, from an economical viewpoint, the P content may be 0.0001% or more, 0.0005% or more, or 0.001% or more.

[0028] [S:0.0100% or less] S (sulfur) is an element contained as an impurity and may form MnS in steel, which may deteriorate toughness and hole expandability. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0050% or less, 0.0040% or less, or 0.0030% or less. The S content may be 0%, but reducing the S content too much increases the cost of desulfurization. Therefore, from an economical standpoint, the S content may be 0.0001% or more, 0.0003% or more, or 0.0005% or more.

[0029] [Al: 0.001 to 1.000%] Al (aluminum) is an element that functions as a deoxidizer. To fully obtain this effect, the Al content is set to 0.001% or more. The Al content may be 0.010% or more, 0.020% or more, 0.030% or more, 0.040% or more, or 0.050% or more. On the other hand, even if an excessive amount of Al is added, the effect saturates, and adding more than necessary to the steel sheet increases manufacturing costs. Furthermore, excessive Al content increases the transformation temperature of the steel, increasing the load during hot rolling and, as a result, may degrade the mechanical properties of the steel sheet. Therefore, the Al content is set to 1.000% or less. The Al content may be 0.800% or less, 0.600% or less, 0.300% or less, or 0.100% or less.

[0030] [Ti: 0.001~0.100%] Ti (titanium) is an element effective in fixing N (nitrogen) present as an impurity in steel as TiN and suppressing the precipitation of B (boron) as nitride (BN). To fully obtain these effects, the Ti content is set to 0.001% or more. The Ti content may be 0.005% or more, 0.010% or more, 0.015% or more, or 0.020% or more. However, even if an excessive amount of Ti is added, the effect saturates, and adding more than necessary to the steel sheet increases manufacturing costs. Therefore, the Ti content is set to 0.100% or less. The Ti content may be 0.090% or less, 0.080% or less, 0.060% or less, or 0.040% or less.

[0031] [B:0.0005~0.0050%] Boron (B) is an element that improves hardenability and contributes to improving strength. Furthermore, B segregates at prior austenite grain boundaries to strengthen the prior austenite grain boundaries, thereby effectively improving hydrogen embrittlement resistance. To fully achieve these effects, the B content is set to 0.0005% or more. The B content may be 0.0008% or more, 0.0010% or more, 0.0012% or more, 0.0015% or more, 0.0018% or more, 0.0020% or more, or 0.0022% or more. On the other hand, excessive B content may result in excessive formation of borides in the steel, which may reduce the hardenability of the steel sheet. Therefore, the B content is set to 0.0050% or less. The B content may be 0.0045% or less, 0.0040% or less, 0.0035% or less, 0.0030% or less, or 0.0028% or less.

[0032] [N:0.0100% or less] N (nitrogen) is an element contained as an impurity, and if the N content is high, coarse nitrides may form in the steel, which may reduce bendability and hole expandability. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0080% or less, 0.0060% or less, or 0.0050% or less. The N content may be 0%, but reducing the N content too much increases the cost of denitrification. Therefore, from an economical standpoint, the N content may be 0.0001% or more, 0.0003% or more, or 0.0005% or more.

[0033] [O:0.0100% or less] O (oxygen) is an element contained as an impurity, and if the O content is high, coarse oxides may form in the steel, resulting in reduced bendability and hole expandability. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less, 0.0060% or less, or 0.0050% or less. The O content may be 0%, but reducing the O content too much increases production costs. Therefore, from the viewpoint of production costs, the O content may be 0.0001% or more, 0.0003% or more, or 0.0005% or more.

[0034] The basic chemical composition of the steel sheet according to the embodiment of the present invention is as described above. Furthermore, the steel sheet may contain at least one of the following elements in place of a portion of the remaining Fe, as necessary.

[0035] [Cr:0~1.00%, Mo:0~1.00%, Cu:0~1.00%, Ni:0~1.00%, Co:0~1.00%, W:0~1.00%, Ta:0~1.000%, Sn:0~1.00%, Sb:0~0.50%, Nb:0~0.200%, and V:0~1.00%] Cr (chromium), Mo (molybdenum), Cu (copper), Ni (nickel), Co (cobalt), W (tungsten), Ta (tantalum), Sn (tin), Sb (antimony), Nb (niobium), and V (vanadium) are all elements effective in increasing the strength of steel sheets. The content of these elements may be 0%, but to achieve this effect, at least one of these elements may be contained in the steel sheet as needed. However, excessive inclusion of these elements may saturate the effect and increase manufacturing costs. Therefore, the contents of Cr, Mo, Cu, Ni, Co, W, Sn, and V may be 1.00% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.20% or less, respectively. Similarly, the Ta content may be 1.000% or less, 0.600% or less, 0.500% or less, 0.300% or less, or 0.200% or less. Similarly, the Sb content is set to 0.50% or less, and may be set to 0.30% or less, or 0.10% or less. Similarly, the Nb content is set to 0.200% or less, and may be set to 0.100% or less, or 0.060% or less. Regarding the lower limits of these elements, for example, the contents of Cr, Mo, Cu, Ni, Co, W, Sn, Sb, and V may be 0.001% or more, or 0.01% or more. Similarly, the contents of Ta and Nb may be 0.001% or more, or 0.005% or more.

[0036] [As:0~0.100%] As (arsenic) is an element effective in improving corrosion resistance. The As content may be 0%, but to obtain this effect, the As content is preferably 0.001% or more, and may be 0.005% or more or 0.010% or more. On the other hand, even if an excessive amount of As is contained, the effect saturates, and adding more As than necessary to the steel sheet increases the manufacturing cost. Therefore, the As content is set to 0.100% or less, and may be 0.050% or less, 0.030% or less, or 0.020% or less.

[0037] [Zn: 0-1.000%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, Hf: 0-0.0100%, Bi: 0-0.0100%, and REM: 0-0.0100%] Zn (zinc) is an element effective in controlling the shape of inclusions in steel, Ca (calcium), Mg (magnesium), Zr (zirconium), Hf (hafnium), and REM (rare earth metals) are elements that contribute to the fine dispersion of inclusions in steel, and Bi (bismuth) is an element that reduces the microsegregation of substitutional alloying elements such as Mn and Si in steel. While the content of these elements may be 0%, since each contributes to improving the workability of steel sheets, at least one of these elements may be contained in the steel sheet as necessary. However, excessive inclusion of these elements may saturate the effect and increase the manufacturing cost. Therefore, the Zn content is set to 1.000% or less, and may also be 0.500% or less, 0.200% or less, 0.100% or less, or 0.050% or less. Similarly, the Ca, Mg, Zr, Hf, Bi, and REM contents are each 0.0100% or less, and may be 0.0080% or less, 0.0060% or less, or 0.0030% or less. Regarding the lower limits of these elements, for example, the Zn content may be 0.001% or more or 0.005% or more. Similarly, the Ca, Mg, Zr, Hf, Bi, and REM contents may be 0.0001% or more or 0.0005% or more. In this specification, REM is a collective term for 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content is the total content of these elements.

[0038] In the steel sheet according to the embodiment of the present invention, the balance other than the above elements consists of Fe and impurities, which are components that are mixed in during the industrial production of steel sheet due to various factors in the production process, including raw materials such as ore and scrap.

[0039] The chemical composition of the steel sheet according to the embodiment of the present invention may be measured by a general analytical method. For example, the chemical composition of the steel sheet may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method.

[0040] [Steel structure] [Martensite: 70% or more, total of ferrite and bainite: 0-20%, and retained austenite: 0-10%] In the steel plate according to the embodiment of the present invention, the steel structure in a cross section at a quarter thickness position from the surface of the steel plate contains, in area percentages, martensite: 70% or more, ferrite and bainite: 0 to 20% in total, and retained austenite: 0 to 10%.

[0041] In an embodiment of the present invention, martensite encompasses as-quenched martensite (fresh martensite) and tempered martensite. Martensite is a hard structure with a high dislocation density, which contributes to improving strength. To obtain a desired high strength, the area fraction of martensite is set to 70% or more. The area fraction of martensite may be 75% or more, 80% or more, 85% or more, or 90% or more. There is no particular upper limit, and therefore the area fraction of martensite may be 100% or 98% or less. However, from the viewpoint of improving uniform elongation, a lower area fraction of martensite is preferable, and may be, for example, 97% or less, 95% or less, or 92% or less.

[0042] Of ferrite and bainite, ferrite is particularly excellent in ductility and contributes to improving elongation. However, if the total area ratio of ferrite and bainite becomes too high, the area ratio of martensite decreases, and therefore the desired strength cannot be achieved. Therefore, the total area ratio of ferrite and bainite is set to 20% or less. The total area ratio of ferrite and bainite may be 18% or less, 15% or less, 12% or less, 10% or less, or 8% or less. The total area ratio of ferrite and bainite may be 0%, but from the viewpoint of improving uniform elongation, it is preferably 1% or more or 2% or more, and may be 3% or more or 5% or more.

[0043] Retained austenite improves the ductility of steel sheets through the TRIP effect, which transforms into martensite through stress-induced transformation during deformation of the steel sheet. However, if the area fraction of retained austenite becomes too high, the area fraction of martensite decreases, making it impossible to achieve the desired strength. Therefore, the area fraction of retained austenite is set to 10% or less. The area fraction of retained austenite may be 9% or less, 8% or less, 7% or less, 6% or less, or 5% or less. The area fraction of retained austenite may be 0%, but from the viewpoint of improving uniform elongation, it is preferably 1% or more or 2% or more, and may be 3% or more or 4% or more.

[0044] [Remaining structure: 0-10% in total] The area ratio of the remaining structure other than martensite, ferrite, bainite, and retained austenite may be 0%. If a remaining structure exists, the remaining structure is pearlite. From the viewpoint of ensuring the above effects based on martensite, ferrite, bainite, and retained austenite, the area ratio of the remaining structure is preferably 10% or less in total, and may be, for example, 8% or less, 6% or less, 4% or less, 3% or less, or 2% or less. On the other hand, the area ratio of the remaining structure may be 0.5% or more, or 1% or more.

[0045] [Identification of steel structure and calculation of area ratio] The steel structure is identified and the area ratio is calculated using a secondary electron image taken using an FE-SEM (field emission scanning electron microscope, for example, JEOL JSM-7200F, measured at an acceleration voltage of 15 kV) and X-ray diffraction. First, a sample is taken from the cross section of the steel plate in the thickness direction perpendicular to the plate surface, and the observation surface is mechanically polished to a mirror finish, and then etched using a nital solution. Next, a total of 2.0 × 10 -9 m 2 Secondary electron images are taken of the above areas. From the obtained secondary electron images, the area ratios of the total of martensite and retained austenite, pearlite (if present), and the total of ferrite and bainite are measured. First, regions with high brightness and where the substructure is not revealed by etching are judged to be fresh martensite and retained austenite. Next, regions with substructure and where multiple cementites with different elongation directions are precipitated are judged to be tempered martensite. Next, regions where cementite is precipitated in a lamellar form are judged to be pearlite (the total of pearlite and cementite). The remainder other than the above structures are judged to be ferrite and bainite. For reference, regions with low brightness and where no substructure is observed can be judged to be ferrite, and regions that do not fall into any of the above categories can be judged to be bainite. The area ratios of each structure identified in this way are calculated using the point counting method. The area ratio of martensite can be determined by subtracting the area ratio of retained austenite determined by the X-ray diffraction method described below from the total area ratio of tempered martensite, fresh martensite, and retained austenite.

[0046] The area fraction of retained austenite is measured by X-ray diffraction. Specifically, the steel plate is mechanically polished and chemically polished from the plate surface to a depth of 1 / 4 of the way through the plate thickness. The polished sample is then subjected to MoKα1 radiation as characteristic X-rays. The structural fraction of retained austenite is calculated from the integrated intensity ratio of the diffraction peaks of (200), (211) of the bcc phase and (200), (220), and (311) of the fcc phase. This is the area fraction of retained austenite.

[0047] [Number density of recesses on the surface with a depth of more than 2 μm and a vertex angle of 45 degrees or less: 10.0 / mm or less] In the steel sheet according to the embodiment of the present invention, the number density of recesses at the surface having a depth of more than 2 μm and a apex angle of 45 degrees or less is controlled to 10.0 / mm or less. By controlling the number density of recesses at the surface having a depth of more than 2 μm and a apex angle of 45 degrees or less within this range, as described above, stress concentration points that may serve as starting points for hydrogen embrittlement cracking in the bent portion can be reduced, thereby improving the hydrogen embrittlement resistance of the bent portion. From the viewpoint of further improving the hydrogen embrittlement resistance of the bent portion, a lower number density of the recesses is preferable, and may be, for example, 8.0 / mm or less, 6.0 / mm or less, 5.0 / mm or less, 4.0 / mm or less, 3.0 / mm or less, 2.0 / mm or less, or 1.5 / mm or less. The lower limit is not particularly limited and may be 0 / mm. For example, the number density of recesses at the surface having a depth of more than 2 μm and a apex angle of 45 degrees or less may be 0.1 / mm or more, 0.3 / mm or more, or 0.5 / mm or more.

[0048] [Method for measuring the number density of recesses on a surface with a depth of more than 2 μm and a vertex angle of 45 degrees or less] The number density of recesses with a depth of more than 2 μm and a apex angle of 45° or less on the surface of a steel sheet is measured as follows. First, a specimen is taken from the cross-section of the steel sheet perpendicular to its surface. The specimen is then mechanically polished to a mirror finish. A backscattered electron image of the steel sheet surface (or the coating / steel sheet interface if the steel sheet has a coating layer) is taken at a magnification of 500x using a field emission scanning electron microscope (FE-SEM, e.g., a JEOL JSM-7200F, measured at an accelerating voltage of 15 kV) (Figure 1(a)). The backscattered electron image is then binarized to clarify the steel sheet surface (Figure 1(b)). In the backscattered electron image, the coating appears bright and the base steel appears dark. During binarization, the threshold value is adjusted to distinguish between the bright and dark areas. The converted binary image is then converted to numerical data, and a surface height profile (SH in Figure 1(c)) is obtained. Image analysis software capable of this operation includes Image J, for example. The center line (CL in Figure 1(c)) is determined from the surface height profile using the least squares method. Regions where the distance (D in Figure 1(c)) from the center line to the deepest point (point A in Figure 1(c)) exceeds 2 μm are defined as "depressions with a depth of more than 2 μm." Regions where the apex angle (θ in Figure 1(c)) formed by the line segment connecting the deepest point (A in Figure 1(c)) and the intersection point (B and C in Figure 1(c)) of the depression is 45 degrees or less are defined as "depressions with a depth of more than 2 μm on the surface and an apex angle of 45 degrees or less." Similar analyses are performed over a measurement range (surface length) exceeding 1 mm in total. For example, if the surface length in one field of view is 200 μm, the above analysis is performed at least five times, changing the field of view. The number of "depressions with a depth of more than 2 μm and a apex angle of 45° or less" obtained in each field of view is summed, and this is converted into a number density per mm of surface length to determine the "number density of depressions with a depth of more than 2 μm and a apex angle of 45° or less on the surface." Here, the surface length refers to the length along the surface height profile described above, and can be measured using image analysis software. Contact or laser roughness meters are commonly used to measure the surface height profile of steel sheets, but if the steel sheet has a coating layer, the coating layer must first be dissolved and stripped using acid.However, these methods are not recommended because they may corrode not only the coating layer but also the interface with the base steel during acid dissolution, resulting in changes to the original unevenness.

[0049] [H 100-200 / H 20-300 <0.30] The steel sheet according to the embodiment of the present invention is immersed in an aqueous solution of ammonium thiocyanate with a concentration of 100 g / L at 25°C for 48 hours, and then heated from room temperature to 300°C at a heating rate of 100°C / h by thermal desorption analysis to measure the amount of released hydrogen. The steel sheet is controlled so as to satisfy the following formula (1): H 100-200 / H 20-300 <0.30 (1) H 100-200 : Hydrogen release amount (mass ppm) at 100-200°C H 20-300 : Hydrogen release amount (mass ppm) at 20 to 300°C

[0050] The steel sheet according to the embodiment of the present invention is mainly composed of martensite. Here, martensite has a high dislocation density, and the dislocations have the property of trapping hydrogen that has penetrated into the steel. As mentioned above, when thermal desorption analysis is performed under the above conditions, the hydrogen trapped in the dislocations is released in the temperature range of 100 to 200°C. Therefore, H in the above formula (1) 100-200 / H 20-300 It can be understood that the smaller the value of H, more specifically, the smaller the ratio of the amount of hydrogen released (mass ppm) at 100 to 200°C to the total amount of hydrogen released (mass ppm) at 20 to 300°C, the smaller the amount of hydrogen trapped in dislocations in the steel. It is clear that the steel sheet according to the embodiment of the present invention is mainly composed of martensite, and therefore the number of dislocations is relatively large. Nevertheless, H 100-200 / H 20-300 When the value of H in the above formula (1) is small, it can be understood that dislocations in the steel are fixed by solute C, and the number of hydrogen trapping sites in the steel is reduced.100-200 / H 20-300 By reducing the number of hydrogen trapping sites in the steel to a level where the value of is less than 0.30, it is possible to significantly improve the hydrogen embrittlement resistance of the bent portion in a specific combination with the control of the number density of recesses on the steel sheet surface as described above. 100-200 / H 20-300 The smaller the value, the more preferable, and may be, for example, 0.28 or less, 0.25 or less, 0.22 or less, 0.20 or less, or 0.18 or less. 100-200 / H 20-300 The value of may be 0.01 or more, 0.05 or more, or 0.10 or more.

[0051] [H 100-200 / H 20-300 Measurement of H 100-200 / H 20-300 The measurement of is performed as follows. First, for unprocessed steel sheets, test pieces of approximately 25 mm × 10 mm are taken from a position other than the end in the width direction. For plated steel sheets, the plating layer is dissolved in a 5% hydrochloric acid solution containing 0.04 vol% inhibitor (Ivit 710K, manufactured by Asahi Chemical Industry Co., Ltd.) to remove the plating. The test piece is then immersed in a 100 g / L ammonium thiocyanate solution at 25°C for 48 hours, and hydrogen is introduced into the test piece. When test pieces are taken from processed parts, test pieces should be taken from flat areas that have not been directly processed or that have been processed to a relatively small extent. Even if it is unclear whether the part has been processed, or even if the part has been processed, the effects of the present invention can be achieved as long as the above formula (1) is satisfied. Next, the test specimens into which hydrogen had been introduced were heated from room temperature to 300°C at a heating rate of 100°C / h using a hydrogen measurement system for steel (JTF-20A manufactured by J Science Lab Co., Ltd.) using thermal desorption analysis. The total amount of hydrogen released during the temperature rise time up to 300°C, H 20-300 (mass ppm) and the amount of hydrogen released between 100 and 200°C, H 100-200(mass ppm) and finally calculate the ratio of H 100-200 / H 20-300 The room temperature should be in the range of 20 to 30°C.

[0052] [C concentration from the steel plate surface to the depth direction by GDS] In a preferred embodiment of the present invention, the steel sheet is controlled so that the following formula (2) is satisfied when the C concentration of the steel sheet is measured in the depth direction from the surface thereof using a high-frequency glow discharge optical emission spectrometer (GDS).

number

[0053] The left side of the above formula (2) can be understood as an index representing the degree of decarburization in the surface layer of the steel sheet. b means the C content (mass%) of the steel plate, and C n means the carbon concentration (mass%) at each measurement point when measured by GDS from a depth of 5 μm to a depth of 30 μm from the steel sheet surface. b -C n The value of becomes larger. The left side of the above equation (2) becomes C b -C nSince this is the sum of the values ​​(mass%) of the test points multiplied by the distance (μm) between each measurement point, the greater the degree of decarburization, the greater the value naturally becomes. On the other hand, hydrogen embrittlement cracking in bent portions is more likely to occur the harder the steel sheet surface, and this tendency becomes particularly pronounced as steel sheets become stronger. In relation to this, the present inventors have also conducted detailed studies on controlling the hardness of the steel sheet surface layer in order to further improve the hydrogen embrittlement resistance of bent portions. As a result, the present inventors have found that, in addition to the specific combination of reducing hydrogen trapping sites and controlling the number density of recesses on the steel sheet surface, decarburizing the steel sheet surface layer within a predetermined range, more specifically, controlling the C concentration in the depth direction of the steel sheet surface layer so as to satisfy the above formula (2), can further significantly improve the hydrogen embrittlement resistance of bent portions.

[0054] From the viewpoint of further improving the hydrogen embrittlement resistance of the bent portion, the larger the value of the left side of the above formula (2), the better, and it may be, for example, 2.2 or more, 2.4 or more, 2.6 or more, 2.8 or more, 3.0 or more, or 3.2 or more. There is no particular upper limit, but, for example, the value of the left side of the above formula (2) may be 6.0 or less, 5.5 or less, 5.0 or less, 4.5 or less, or 4.0 or less.

[0055] [How to determine the value of the left side of equation (2)] The value of the left side of the above equation (2) is determined as follows using a radio-frequency glow discharge optical emission spectrometer (GDS). In this embodiment, a radio-frequency glow discharge optical emission spectrometer, GD-Profiler2, manufactured by HORIBA, Ltd., is used. First, the surface of the steel sheet is placed in an Ar atmosphere, and a voltage is applied to generate glow plasma. The steel sheet surface is then sputtered and analyzed in the depth direction. The elements contained in the steel sheet are then identified from the element-specific emission spectrum wavelengths emitted by excited atoms in the glow plasma, and the emission intensity of the identified elements is estimated. This results in an emission intensity profile of the elements at each measurement time. The measurement interval is 0.1 seconds. Next, the sputtering rate is calculated by measuring the depth of the sputtering mark on the sample after GDS measurement, and the time is converted into depth. The sputtering mark depth can be measured on the surface using a commercially available microscope or laser microscope with a height measurement function, or by cross-sectional observation. The sputtering depth converted from the sputtering time can be defined as the depth from the surface of the steel sheet. The obtained emission intensity is converted to mass% by creating a calibration curve. To create the calibration curve, a steel plate to be measured that has been mechanically polished to a depth of 1 / 4 t and commercially available high-purity electrolytic iron are used. The C emission intensity of both is measured for 300 seconds at a measurement interval of 0.1 seconds, and the average C emission intensity is calculated from 20 seconds to 300 seconds, when the measurement results stabilize. Since the C concentration of the steel plate to be measured is known and the C concentration of the high-purity electrolytic iron can be considered to be 0, a calibration curve is obtained that can convert the C emission intensity to the C concentration (mass%). In this way, the C emission intensity at each measurement point (i.e., X ) when GDS measurement is performed from a depth of 5 μm to a depth of 30 μm from the steel plate surface is calculated. n ) C concentration (i.e., C n ) and calculate the C concentration and C b (i.e., the C content of the steel sheet measured using the combustion-infrared absorption method) and the value of the left side of the above formula (2). When the steel sheet to be measured is a plated steel sheet, the depth position at which the emission intensity of Fe exceeds the emission intensity of the plating metal (Zn in the case of a zinc-plated steel sheet) is defined as 0 μm.

[0056] Plate Thickness The steel sheet according to the embodiment of the present invention generally has a thickness of 0.6 to 6.0 mm, although not particularly limited thereto. For example, the thickness may be 1.0 mm or more, 1.2 mm or more, or 1.4 mm or more, and / or 5.0 mm or less, 4.0 mm or less, 3.0 mm or less, or 2.5 mm or less.

[0057] [Plating layer] The steel sheet according to the embodiment of the present invention may be a plated steel sheet having a plating layer on at least one surface, preferably both surfaces. The plating layer is not particularly limited, but may be, for example, a hot-dip galvanized layer (GI), a galvannealed layer (GA), or an electrogalvanized layer (EG). These galvanized layers may have any composition known to those skilled in the art, and may contain additive elements other than Zn, such as Al and Mg. The coating weight of the plating layer is not particularly limited, and may be a general coating weight.

[0058] As described above, the steel sheet according to the embodiment of the present invention achieves high strength, more specifically, a high strength of 1470 MPa or more, due to a steel structure mainly composed of martensite, while also achieving significantly improved hydrogen embrittlement resistance even in bent portions. Therefore, the steel sheet according to the embodiment of the present invention is particularly useful for use in parts in technical fields that require a high level of both high strength and hydrogen embrittlement resistance, and is particularly useful for use in parts in the automotive field. In a preferred embodiment, an automobile part including the steel sheet according to the embodiment of the present invention is provided. Examples of automobile parts include frame parts such as front pillars, center pillars, side sills, and cross members, as well as bumpers and other structural and reinforcing parts that require strength. It is sufficient for at least a portion of these parts to include the steel sheet according to the embodiment of the present invention, and therefore at least a portion of these parts will satisfy the characteristics of the steel sheet described above. In parts of the steel sheet that do not come into direct contact with a mold during forming, such as press forming, or that come into direct contact with the mold but are relatively little processed, the characteristics of the steel sheet do not change significantly before and after forming.

[0059] [Mechanical properties] [Tensile strength (TS) and uniform elongation (uEL)] A steel sheet having the above-described chemical composition and microstructure can achieve high tensile strength, specifically, a tensile strength (TS) of 1470 MPa or more. The tensile strength is preferably 1500 MPa or more, 1550 MPa or more, or 1600 MPa or more. Despite such extremely high tensile strength, the steel sheet according to the present invention can significantly suppress the occurrence of hydrogen embrittlement cracking in bent portions due to the specific combination of the above-described chemical composition and microstructure. The upper limit of the tensile strength is not particularly limited, and for example, the tensile strength of the steel sheet may be 1800 MPa or less, 1750 MPa or less, or 1700 MPa or less. Furthermore, the steel sheet according to the present invention can achieve improved ductility despite such extremely high tensile strength. In particular, when the total area fraction of ferrite and bainite is 2% or more and the area fraction of retained austenite is 1% or more, a uniform elongation (uEL) of 5.0% or more can be achieved. For example, the uniform elongation may be 5.2% or more, 5.5% or more, 5.8% or more, or 6.0% or more. The upper limit of the uniform elongation is not particularly limited, but for example, the uniform elongation of the steel sheet may be 10.0% or less or 8.0% or less. The tensile strength and uniform elongation are determined by taking a JIS No. 5 test piece from a direction in which the longitudinal direction of the test piece is preferably parallel to the rolling direction perpendicular to the rolling direction of the steel sheet (C direction) and conducting a tensile test in accordance with JIS Z 2241:2022. If the rolling direction of the steel sheet cannot be specified, the JIS No. 5 test piece may be taken from any direction within the surface of the steel sheet. If it is difficult to take a JIS No. 5 test piece, a JIS No. 13B test piece may be used, or a small test piece having a shape similar to that of a JIS No. 13B test piece may be used.

[0060] <Steel sheet manufacturing method> Next, a preferred method for manufacturing a steel sheet according to an embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing a steel sheet according to an embodiment of the present invention, but is not intended to limit the steel sheet to one manufactured by the manufacturing method described below.

[0061] A method for manufacturing a steel sheet according to an embodiment of the present invention includes: (A) a hot rolling process comprising hot rolling a slab having the chemical composition described above in relation to the steel sheet, and then coiling and cooling the resulting hot-rolled steel sheet, wherein the hot-rolling process satisfies the following conditions (A1) and (A2): (A1) The cumulative reduction rate between descaling steps at a steel plate temperature of 1000°C or higher is 50% or less, and (A2) The coiling temperature is higher than 400°C and is 650°C, and the cooling satisfies the following formulas (3) to (5):

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[0062] [(A) Hot rolling process] First, a slab having the chemical composition described above in relation to the steel sheet is hot-rolled in a hot-rolling process, and then the obtained hot-rolled steel sheet is coiled and cooled. The hot-rolling process must satisfy the following conditions (A1) and (A2). (A1) The cumulative reduction rate between descaling steps at a steel plate temperature of 1000°C or higher is 50% or less, and (A2) The coiling temperature is higher than 400°C and is 650°C, and the cooling satisfies the following formulas (3) to (5):

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number

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[0063] The slabs used contain relatively large amounts of alloying elements, and in particular, the slabs after continuous casting contain coarse Ti carbides. Therefore, it is necessary to dissolve the alloying elements in the slab, and Ti in particular needs to be sufficiently solutionized. Therefore, the slabs are heated before hot rolling, and the heating temperature is preferably 1200°C or higher. Although there is no particular upper limit, if the slab heating temperature is too high, the yield decreases due to scaling. Therefore, the slab heating temperature is preferably 1300°C or lower. From the viewpoint of manufacturability, the slabs used are preferably cast by a continuous casting method, but they may also be produced by an ingot casting method or a thin slab casting method. Furthermore, the slabs discharged from the heating furnace are descaled before rolling begins.

[0064] [(A1) Cumulative reduction rate during descaling when steel plate temperature is 1000°C or higher: 50% or less] In a hot rolling process, scale formed on the surface of a steel sheet during the hot rolling process is generally removed by descaling using high-pressure water or the like. In the present manufacturing method, descaling is performed multiple times during the hot rolling process. However, it is necessary to control the cumulative reduction ratio of one or more rolling passes performed between adjacent descaling passes (e.g., between the first and second descaling passes, or between the second and third descaling passes) to 50% or less. When the steel sheet temperature is relatively high, for example, when the steel sheet temperature is 1000°C or higher, scale grows on the steel sheet surface even between descaling passes. Therefore, if the steel sheet is strongly reduced under such conditions, i.e., the cumulative reduction ratio exceeds 50%, the growing scale is pushed into the steel sheet, resulting in noticeable irregularities on the steel sheet surface. As a result, the number density of recesses with a depth of more than 2 μm and a vertex angle of 45° or less on the surface of the finally obtained steel sheet exceeds 10.0 / mm, which reduces the hydrogen embrittlement resistance of the bent portion. In this manufacturing method, by controlling the cumulative reduction ratio between descaling passes when the steel sheet temperature is 1000°C or higher to 50% or less, it is possible to suppress the intrusion of scale into the steel sheet and thereby suppress the formation of irregularities on the steel sheet surface. Here, the steel sheet temperature refers to the temperature at the inlet side of the rolling rolls. The temperature at the inlet side of the rolling rolls can be measured with a thermometer or determined by numerical calculation. As a result, it is possible to reduce the number density of recesses having a depth of more than 2 μm and an apex angle of 45° or less on the surface of the finally obtained steel sheet to 10.0 / mm or less. Preferably, the cumulative reduction ratio between descaling passes is 48% or less. Here, the cumulative reduction ratio between descaling passes is calculated using the following formula, for example, when n rolling passes are included between descaling passes. Note that if the steel sheet temperature drops to less than 1000°C during descaling, it is sufficient that the cumulative reduction ratio of the rolling passes from the immediately preceding descaling pass in which the steel sheet temperature at the inlet side of the rolling rolls is 1000°C or higher is 50% or less. Furthermore, when rolling is performed after final descaling, the cumulative reduction ratio must be 50% or less for rolling passes after final descaling in which the steel sheet temperature is 1000°C or higher. Cumulative reduction rate during descaling (%) = (Strip thickness before n rolling passes - Strip thickness after n rolling passes) / Strip thickness before n rolling passes x 100

[0065] [(A2) Coiling temperature: over 400°C to 650°C and cooling satisfying formulas (3) to (5)] After hot rolling, the obtained hot-rolled steel sheet is coiled at a coiling temperature of more than 400°C to 650°C, and then cooled so as to satisfy the following formulas (3) to (5).

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[0066] If the coiling temperature is higher than 650°C, oxidation of the steel sheet surface will proceed excessively, and the number density of recesses with a depth of more than 2 μm and an apex angle of 45 degrees or less on the surface of the finally obtained steel sheet will exceed 10.0 / mm, resulting in a decrease in hydrogen embrittlement resistance at the bent portion. On the other hand, if the coiling temperature is 400°C or lower, the hot-rolled steel sheet will become too hard, which will increase the rolling load on the rolling mill in the subsequent cold rolling process, which is undesirable. Therefore, in the present production method, the coiling temperature is set to be above 400°C to 650°C, and preferably above 400°C to 550°C.

[0067] On the other hand, the above formula (3) indicates that the larger the value of the middle side, the more the internal oxidation reaction of elements such as Si progresses on the surface of the hot-rolled steel sheet. Σ in the above formula (3) is calculated by the quadrature method of pieces. Δt is a finite value that corresponds to the measurement interval of temperature T(t). For example, it is 100 seconds. Do is the diffusion coefficient [m 2 / sec], No is the amount of oxygen atoms dissolved in the steel at temperature T(t) (atomic fraction), and Nx is the total amount of the main elements to be internally oxidized in the steel. Nx can be calculated by converting the mass fraction of each element (Si, Mn, and Al) into atomic fractions and summing them up. This can be expressed mathematically as in Equation (9) below.

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[0068] The above formula (3) indicates that the internal oxidation reaction proceeds more easily as the diffusion coefficient of oxygen atoms and the amount of oxygen in solid solution increase, and less easily as the amount of the element to be internally oxidized increases. If the internal oxidation reaction proceeds excessively, i.e., if the value of the middle part of the above formula (3) exceeds 1.50, the unevenness of the steel sheet surface becomes large after pickling. Although these unevennesses are smoothed to a certain extent by cold rolling, their influence persists in the final product. As a result, the number density of recesses with a depth of more than 2 μm and a vertex angle of 45° or less on the surface of the final steel sheet exceeds 10.0 / mm, which reduces the hydrogen embrittlement resistance of the bent portion.

[0069] On the other hand, if the value of the middle part of the above equation (3) is 0.05 or less, the surface irregularities of the steel sheet are improved, but the hot-rolled steel sheet becomes too hard, which increases the rolling load on the rolling mill in the subsequent cold rolling process, which is undesirable. For example, the slower the cooling, the longer the time tf to reach 673 K and the longer the residence time in the high-temperature region where Do and No are large. This increases the value of Σ in equation (3), i.e., the value of the middle part of equation (3), and the internal oxidation reaction progresses. On the other hand, the faster the cooling, the smaller the value of the middle part of equation (3), and the more difficult the internal oxidation reaction progresses. Furthermore, as mentioned above, No is the amount of oxygen atoms dissolved in steel at temperature T(t), and Do is the diffusion coefficient of oxygen atoms. As is clear from equations (4) and (5), the higher the steel sheet temperature, the larger the values ​​of No and Do, and the larger the value of Σ in equation (3). Therefore, for example, when the coiling temperature is relatively high and the steel sheet temperature increases accordingly, the value of the middle leg of formula (3) increases, and the internal oxidation reaction progresses. Conversely, when the steel sheet temperature decreases, the internal oxidation reaction progresses less easily. Therefore, by appropriately controlling parameters such as the steel sheet temperature and cooling rate during cooling after coiling, the value of the middle leg of formula (3) can be appropriately controlled within a range of more than 0.05 and less than 1.50. Therefore, in the present manufacturing method, by setting the coiling temperature to more than 400°C and 650°C in the hot rolling step and controlling the subsequent cooling to satisfy the above formulas (3) to (5), and particularly by controlling the value of the middle leg of formula (3) to be more than 0.05 and less than 1.50, it becomes possible to reduce the number density of recesses having a depth of more than 2 μm and an apex angle of 45 degrees or less on the surface of the finally obtained steel sheet to 10.0 / mm or less, while keeping the rolling load in the subsequent cold rolling step appropriate. The value of the middle side of the above formula (3) is preferably 0.10 or more, more preferably 0.20 or more or 0.30 or more. Similarly, the value of the middle side of the above formula (3) is preferably 1.00 or less, more preferably 0.70 or less or 0.60 or less.

[0070] [(B) Pickling process] Next, the obtained hot-rolled steel sheet is subjected to a pickling process, which is carried out using 1.0 to 5.0 mol / L of HCl and less than 0.10 mol / L of Fe. 3+The method includes carrying out a pickling treatment for 30 to 200 seconds by passing the hot-rolled steel sheet through an aqueous solution containing HCl at a temperature of 70 to 90°C at an average speed of 10 m / min or more. If the HCl concentration in the pickling solution is less than 1.0 mol / L, the temperature of the aqueous solution is less than 70°C, the average speed of the hot-rolled steel sheet is less than 10 m / min, or the pickling time is less than 30 seconds, the pickling does not proceed sufficiently, resulting in uneven removal of scale and the internal oxide layer containing silicon oxides and the like. As a result, the number density of recesses with a depth of more than 2 μm and an apex angle of 45° or less on the surface of the finally obtained steel sheet exceeds 10.0 recesses / mm, and the hydrogen embrittlement resistance of the bent portion is reduced. On the other hand, if the HCl concentration exceeds 5.0 mol / L, the temperature of the aqueous solution exceeds 90°C, or the pickling time exceeds 200 seconds, the pickling proceeds excessively, dissolving not only the scale and internal oxide layer but also the base steel, resulting in noticeable unevenness on the steel sheet surface. As a result, the number density of recesses with a depth of more than 2 μm and a apex angle of 45 degrees or less on the surface of the finally obtained steel sheet exceeds 10.0 / mm, and the hydrogen embrittlement resistance of the bent portion is also reduced. 3+ In this regard, Fe in aqueous solution 3+ If the content is 0.10 mol / L or more, dissolution of the base steel becomes significant, and as a result, the number density of recesses having a depth of more than 2 μm and an apex angle of 45 degrees or less on the surface of the finally obtained steel sheet will exceed 10.0 / mm. Therefore, it is important to carry out the pickling treatment appropriately, without excess or deficiency.

[0071] In contrast, in the present production method, 1.0 to 5.0 mol / L of HCl and less than 0.10 mol / L of Fe 3+ By carrying out pickling treatment for 30 to 200 seconds in which the steel sheet is passed through an aqueous solution containing Fe at a temperature of 70 to 90°C at an average speed of 10 m / min or more, it is possible to adequately remove scale and internal oxide layers while suppressing over-pickling, thereby suppressing the formation of irregularities on the surface of the steel sheet. As a result, it is possible to reduce the number density of recesses having a depth of more than 2 μm and an apex angle of 45 degrees or less on the surface of the finally obtained steel sheet to 10.0 / mm or less. Preferably, the Fe in the aqueous solution 3+The content is 0.06 mol / L or less, the pickling temperature is 75 to 85°C, and the pickling time is 50 to 100 seconds. 3+ The lower limit of the content is not particularly limited. For example, Fe in an aqueous solution 3+ The content may be 0 mol / L or more or 0.001 mol / L or less. Similarly, the upper limit of the average speed of the hot-rolled steel sheet is not particularly limited, but for example, the average speed of the hot-rolled steel sheet may be 100 m / min or less.

[0072] [(C) Cold rolling process] The hot-rolled steel sheet after pickling is then subjected to cold rolling. The reduction ratio in cold rolling is 30% or more to promote recrystallization and smooth out irregularities on the steel sheet after pickling. If the reduction ratio is less than 30%, the irregularities on the steel sheet surface cannot be sufficiently smoothed, making it difficult to reduce the number density of recesses having a depth of more than 2 μm and an apex angle of 45 degrees or less on the surface of the finally obtained steel sheet to 10.0 / mm or less. The reduction ratio is preferably 40% or more or 50% or more. On the other hand, excessive reduction increases the rolling load, leading to an increase in the load on the cold rolling mill. For this reason, the reduction ratio is set to 75% or less, preferably 70% or less or 60% or less.

[0073] [(D) First heat treatment step] Next, the obtained cold-rolled steel sheet is heated to a maximum heating temperature of Ac3 to 950° C. in a first heat treatment step, and then cooled to 40° C. or less. In addition, the first heat treatment step must satisfy the following conditions (D1) and (D2). (D1) After reaching the maximum heating temperature, the residence time at 600 to 700°C is 50 seconds or less, the residence time at 450 to 600°C is 500 seconds or less, and the residence time at Ms to 450°C is 100 seconds or less; and (D2) The average cooling rate between 200°C and Ms is 20°C / sec or more.

[0074] [(D1) After reaching the maximum heating temperature of Ac3 to 950°C, the residence time at 600 to 700°C is 50 seconds or less, the residence time at 450 to 600°C is 500 seconds or less, and the residence time at Ms to 450°C is 100 seconds or less] In order to sufficiently advance austenitization and obtain the desired steel structure in the subsequent cooling treatment, the cold-rolled steel sheet needs to be heated to a maximum heating temperature of Ac3 to 950°C. If austenitization is insufficient, a large amount of ferrite will be formed in the final steel structure, and the desired martensite area ratio may not be achieved. Furthermore, if the residence time at 600 to 700°C after reaching the maximum heating temperature of Ac3 to 950°C exceeds 50 seconds, the residence time at 450 to 600°C exceeds 500 seconds, or the residence time at Ms to 450°C exceeds 100 seconds, a relatively large amount of ferrite and / or bainite will be formed, increasing the total area ratio of ferrite and bainite, and in some cases, the area ratio of martensite will decrease, making it impossible to achieve the desired strength. In this manufacturing method, after reaching the maximum heating temperature of Ac3 to 950°C, the residence time at 600 to 700°C is controlled to 50 seconds or less, the residence time at 450 to 600°C to 500 seconds or less, and the residence time at Ms to 450°C to 100 seconds or less. This allows the total area ratio of ferrite and bainite to be controlled to 0 to 20% and the area ratio of martensite to be controlled to 70% or more. Preferably, the residence time at 600 to 700°C is 30 seconds or less, the residence time at 450 to 600°C to 200 seconds or less, and the residence time at Ms to 450°C to 60 seconds or less. Ac3 (°C) and Ms (°C) can be approximately calculated based on the following formulas (10) and (11). The element symbols in the formulas are substituted with the mass% of the element. Elements that are not contained are substituted with 0 mass%. Ac3(℃)=912-230.5×C+31.6×Si-20.4×Mn-39.8×Cu-18.1×Ni-14.8×Cr+16.8×Mo+100.0×Al...(10) Ms(℃)=561-474×C-33×Mn-17×Cr-17×Ni-21×Mo-7.5×Si+10×Co...(11)

[0075] [(D2) Average cooling rate between 200°C and Ms: 20°C / s or more] Controlling the average cooling rate between 200°C and Ms in the first heat treatment step to 20°C / s or more, i.e., by cooling the cold-rolled steel sheet relatively quickly between 200°C and Ms, ensures sufficient solute carbon in the steel. Therefore, in the subsequent second heat treatment step, the solute carbon can be fixed to dislocations, sufficiently reducing the number of hydrogen trapping sites in the steel, more specifically, the number of dislocations that function as hydrogen trapping sites in the steel. Furthermore, setting the average cooling rate to 20°C / s or more suppresses the diffusion of carbon atoms from martensite already formed during martensitic transformation to untransformed austenite, thereby preventing the formation of locally high-carbon austenite regions. The higher the carbon concentration of austenite, the lower the martensite transformation temperature. However, martensite transformed at a lower temperature contains a large number of dislocations. Setting the average cooling rate to 20°C / s or more suppresses the formation of low-temperature transformed martensite. In this regard, when a steel sheet is immersed in an aqueous solution of ammonium thiocyanate with a concentration of 100 g / L at 25°C for 48 hours and then heated from room temperature to 300°C at a heating rate of 100°C / h by thermal desorption analysis to measure the amount of released hydrogen, it becomes possible to form a steel structure that satisfies the following formula (1). H 100-200 / H 20-300 <0.30 (1)

[0076] On the other hand, if the average cooling rate between 200°C and Ms is less than 20°C / s, it becomes impossible to secure sufficient solute C in the steel. Therefore, even the subsequent second heat treatment step cannot sufficiently reduce the number of hydrogen trapping sites, and it may be impossible to form a steel structure that satisfies the above formula (1). Preferably, the average cooling rate between 200°C and Ms is 24°C / s or more. In addition to controlling the average cooling rate between 200°C and Ms, the cooling end temperature is also important for suppressing low-temperature transformed martensite. Specifically, the cooling end temperature must be 40°C or less, and preferably 30°C or less. If the cooling end temperature is high, untransformed austenite remains at the end of cooling. This transforms into martensite during cooling after the second heat treatment step, resulting in the formation of low-temperature transformed martensite containing a large amount of dislocations to which C is not fixed. As a result, it becomes difficult to satisfy formula (1).

[0077] [(D) Preferred embodiment of the first heat treatment step] In a preferred embodiment of the first heat treatment step, the atmosphere between Ac1 and Ac3 when the cold-rolled steel sheet is heated to the maximum heating temperature is controlled so as to satisfy the following formula (8).

number

[0078] The log(pH2O / pH2) in the above formula (8) is also called the oxygen potential, and the larger this value, the more rapidly decarburization can occur in the surface layer of the steel sheet. In this manufacturing method, by controlling the oxygen potential to satisfy the above formula (8), it is possible to appropriately decarburize the C concentration in the surface layer of the steel sheet so that the C concentration satisfies the above formula (2). As a result, in addition to the effect based on the specific combination of reducing hydrogen trapping sites and controlling the number density of recesses on the steel sheet surface, it is possible to further significantly improve the hydrogen embrittlement resistance of the bent portion. To enhance this effect, the log(pH2O / pH2) in the above formula (8) is preferably greater than −1.00. More preferably, the log(pH2O / pH2) is −0.90 or greater. On the other hand, if the log(pH2O / pH2) is −0.10 or greater, not only decarburization but also oxidation of Fe in the steel substrate may occur. Excessive oxidation of the steel substrate can have a detrimental effect on plating. For this reason, the log(pH2O / pH2) is preferably less than −0.10. More preferably, log(pH2O / pH2) is -0.20 or less or -0.30 or less. Ac1 (°C) can be approximately calculated based on the following formula (12). In the formula, the mass% of the element is substituted for the element symbol. For elements that are not contained, 0 mass% is substituted. Ac1(℃)=723-10.7×Mn-16.9×Ni+29.1×Si+16.9×Cr...(12)

[0079] [Plating] When producing a plated steel sheet, for example, cooling of a cold-rolled steel sheet during (D1) treatment can be stopped near the plating bath temperature (approximately 460°C in the case of a Zn bath) and the steel sheet can be immersed in the plating bath. When producing a galvannealed hot-dip galvanized steel sheet (GA), the steel sheet can be reheated after immersion in the plating bath and then alloyed. The alloying temperature can be in the range of 460 to 600°C. Alternatively, hot-dip galvanized steel (GI) can be used without alloying treatment. However, the conditions described in (D1), including the plating treatment time and alloying treatment time, must be satisfied. When producing an electrogalvanized steel sheet (EG), the steel sheet can be cooled to room temperature after (E). These galvanizing processes can be carried out according to any appropriate method known to those skilled in the art. Similarly, these galvanizing processes can have any composition known to those skilled in the art and can contain additive elements other than Zn, such as Al and Mg. Furthermore, the coating weight of these galvanized steel sheets is not particularly limited and can be a general coating weight.

[0080] [(E) Second heat treatment step] The cold-rolled steel sheet after the first heat treatment is heated to a maximum heating temperature of 80 to 300°C in the second heat treatment step, and the second heat treatment step must satisfy the following formulas (6) and (7).

number

number

[0081] The above (6) and (7) can be understood as indicators of the degree of tempering, and therefore, the larger the value of the middle part of the above formula (6), the more the tempering progresses. The present inventors have discovered that by appropriately controlling the second heat treatment step using the above formulas (6) and (7), i.e., by appropriately tempering the cold-rolled steel sheet, it is possible to reduce hydrogen trapping sites in the steel. More specifically, in the first heat treatment step, the steel is cooled relatively quickly between 200°C and Ms to ensure sufficient solute C in the steel. Then, in the second heat treatment step, the steel is heated to a maximum heating temperature of 80°C to 300°C and heat-treated (i.e., tempered) so as to satisfy the above (6) and (7). This allows the previously obtained solute C to be fixed to dislocations in the steel, thereby reducing hydrogen trapping sites in the steel. In other words, by first fixing C to dislocations, it is possible to prevent dislocations from functioning as hydrogen trapping sites. This allows the steel sheet to be constructed with a steel structure that satisfies the above formula (1). By specifically combining this with the previously described control of the number density of recesses on the steel sheet surface, the hydrogen embrittlement resistance of the bent portion of the finally obtained steel sheet can be significantly improved. In the second heat treatment step, it is important to properly temper the cold-rolled steel sheet. Therefore, if the maximum heating temperature in the second heat treatment step is too high or too low, the effect of reducing hydrogen trapping sites cannot be achieved. Similarly, if the value of the middle arm of the above formula (6) is too high or too low, the effect of reducing hydrogen trapping sites cannot be achieved. Preferably, the maximum heating temperature is 150 to 250°C, and the value of the middle arm of the above formula (6) is 9000 to 11100. The interval of t in formula (7) is 1 second. If the temperature measurement interval is not 1 second, linear interpolation can be used to convert the data to 1-second intervals.

[0082] According to the steel sheet manufactured by the above-mentioned manufacturing method, the inclusion of 0.001 to 0.100 mass% Ti and 0.0005 to 0.0050 mass% B in the steel ensures sufficient solute B, thereby improving hardenability and strengthening prior austenite grain boundaries. Therefore, by combining this with a steel structure containing 70% or more martensite by area, it is possible to achieve high strength in the steel sheet, for example, a tensile strength of 1470 MPa or more, while improving the hydrogen embrittlement resistance of the steel sheet. Furthermore, by appropriately controlling the hot rolling and pickling processes in particular, and controlling the number density of recesses on the steel sheet surface with a depth of more than 2 μm and an apex angle of 45° or less to 10.0 / mm or less, stress concentration areas that could serve as initiation points for hydrogen embrittlement cracking can be reduced in bent portions. In addition, by appropriately heat treating the steel sheet after cold rolling in the first and second heat treatment steps, hydrogen trapping sites in the steel can be formed by H 100-200 / H 20-300 The specific combination of reducing hydrogen trapping sites and controlling the number density of recesses on the steel sheet surface can significantly improve the hydrogen embrittlement resistance of bent portions. Therefore, steel sheets manufactured by the above manufacturing method are particularly useful in the automotive field, where high levels of both high strength and hydrogen embrittlement resistance are required.

[0083] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way. [Example]

[0084] In the following examples, steel sheets according to the embodiments of the present invention were manufactured under various conditions, and the tensile strength (TS), uniform elongation (uEL), and hydrogen embrittlement resistance of the bent portion of the obtained steel sheets were investigated.

[0085] First, molten steel was cast by continuous casting under the conditions shown in Table 2 to form slabs having various chemical compositions shown in Table 1. Next, in the hot rolling process, these slabs were heated under the conditions shown in Table 2 and then hot rolled. The obtained hot-rolled steel sheets were coiled and cooled at the temperatures shown in Table 2. The "maximum cumulative reduction during descaling" shown in Table 2 indicates the maximum cumulative reduction during descaling at a steel sheet temperature of 1000°C or higher among the multiple descalings performed in the hot rolling process (including the cumulative reduction after final descaling if rolling was performed after final descaling). Furthermore, "Equation (3)" in Table 2 indicates the middle value of equation (3) in the cooling after coiling performed based on the above equations (3) to (5).

[0086] Next, in the pickling process, the hot-rolled steel sheet was pickled in 3.0 mol / L HCl and the Fe 3+ The cold-rolled steel sheets were passed through an aqueous solution having the concentration and temperature at an average speed of 80 m / min for the time shown in Table 2, followed by cold rolling at the reduction shown in Table 2 in the cold rolling step. The thickness of each cold-rolled steel sheet was 1.4 mm. In the first heat treatment step, the obtained cold-rolled steel sheets were heated to the maximum heating temperature shown in Table 2 and then cooled under the conditions of the residence time in each temperature range shown in Table 2. Next, except for Examples 19 and 20, the cold-rolled steel sheets or plated steel sheets were subjected to plating and alloying treatment while staying at 450 to 600°C, and then cooled between 200°C and Ms at the average cooling rate shown in Table 2, and then cooled to the cooling end temperature shown in Table 2. In the first heat treatment step, the atmosphere between Ac1 and Ac3 when heating the cold-rolled steel sheets to the maximum heating temperature was controlled to have the log(pH2O / pH2) value shown in Table 2.

[0087] Next, the cold-rolled steel sheet after the first heat treatment was heated to the maximum heating temperature shown in Table 2 in the second heat treatment step, and finally, the second heat treatment was performed based on the above formulas (6) and (7) to obtain a cold-rolled steel sheet or a plated steel sheet. Then, electrogalvanization (EG) was performed in Example 20. In Table 2, CR indicates an unplated cold-rolled steel sheet, GI indicates a hot-dip galvanized steel sheet, GA indicates a galvannealed steel sheet, and EG indicates an electrogalvanized steel sheet.

[0088] [Table 1]

[0089] [Table 2-1] [Table 2-2]

[0090] The properties of the obtained steel sheets were measured and evaluated by the following methods.

[0091] [Tensile strength (TS) and uniform elongation (uEL)] The tensile strength (TS) and uniform elongation (uEL) were determined by taking JIS No. 5 test specimens in a direction parallel to the rolling direction (C direction) of the steel plate and conducting tensile tests in accordance with JIS Z 2241:2022.

[0092] [Hydrogen embrittlement resistance of bent parts] The hydrogen embrittlement resistance of the bent portion was evaluated by a U-bend test. First, a 30 mm × 120 mm rectangular test piece 11 was cut from the steel plate, and holes for bolt fastening were drilled at both ends of the test piece 11. Next, as shown in Figure 2, the test piece 11 was bent 180° using a 5 mm radius punch 12. The clearance between the punch 12 and the die (support roll) 13 was the thickness of the test piece 11 + 1.0 mm. Next, stress was applied to the springback U-bend test piece 14 by fastening it with a bolt 15 and nut 16. At this time, a 3 mm GL strain gauge 17 was attached to the top of the U-bend test piece 14, and stress was applied by controlling the strain amount. The applied stress was equivalent to 1050 and 1350 MPa. The strain was converted to stress using the stress-strain curve obtained in advance from a tensile test. Next, each piece was immersed in 1000 mL of a pH 1.0 hydrochloric acid solution for 48 hours. The end faces of the U-bend test pieces 14 were milled. After the test was completed, cracks exceeding 3 mm were observed at the top of the bend and were judged to be cracks, with those that cracked at 1050 MPa being rated "B," those that did not crack at 1050 MPa but cracked at 1300 MPa being rated "A," and those that did not crack at 1300 MPa being rated "AA."

[0093] Steel sheets with a tensile strength of 1470 MPa or more and a hydrogen embrittlement resistance rating of AA or A were evaluated as having high strength and excellent hydrogen embrittlement resistance in bent portions. The results are shown in Table 3. In Table 3, "M," "α+B," and "residual γ" indicate the area ratios of "martensite," "ferrite and bainite," and "residual austenite," respectively. In addition, the "number density of recesses" in Table 3 refers to the number density of recesses on the steel sheet surface that are more than 2 μm deep and have an apex angle of 45 degrees or less.

[0094] [Table 3]

[0095] Referring to Tables 1 to 3, in Comparative Example 11, TS decreased due to the low C content. In Comparative Example 12, TS increased excessively due to the high C content, resulting in decreased hydrogen embrittlement resistance at the bent portion. In Comparative Example 13, hydrogen embrittlement resistance at the bent portion also decreased due to the high Si content. In Comparative Example 14, martensite was not sufficiently formed due to the low Mn content, resulting in decreased TS. In Comparative Example 15, martensite was formed excessively due to the high Mn content, resulting in decreased hydrogen embrittlement resistance at the bent portion. In Comparative Example 16, hardenability decreased due to the low B content, resulting in decreased TS. In Comparative Example 17, the high B content is thought to have caused excessive formation of borides in the steel, resulting in decreased hardenability of the steel sheet. As a result, the desired steel structure fraction was not obtained, resulting in decreased TS. In Comparative Example 18, N, which is present as an impurity in the steel, could not be fixed as TiN due to the absence of Ti, and most of the B precipitated as BN. In relation to this, the amount of solute B could not be sufficiently secured, which resulted in a decrease in hardenability and a decrease in TS.

[0096] In Comparative Example 23, the cumulative reduction rate during descaling at a steel sheet temperature of 1000°C or higher in the hot rolling process was high, which is thought to have caused scale that grew during descaling to be pressed into the steel sheet. As a result, the number density of recesses with a depth of more than 2 μm and a apex angle of 45° or less on the surface of the finally obtained steel sheet exceeded 10.0 / mm, and the hydrogen embrittlement resistance of the bent portion deteriorated. In Comparative Example 24, the coiling temperature was high, which is thought to have caused excessive oxidation of the steel sheet surface. As a result, the number density of recesses with a depth of more than 2 μm and a apex angle of 45° or less on the surface of the finally obtained steel sheet also exceeded 10.0 / mm, and the hydrogen embrittlement resistance of the bent portion deteriorated. In Comparative Example 25, the cooling after coiling did not satisfy the above formula (3), i.e., the value of the middle part of the above formula (3) was 1.50 or more, and the internal oxidation reaction proceeded excessively, resulting in large unevenness on the steel sheet surface after pickling. As a result, the number density of recesses having a depth of more than 2 μm and an apex angle of 45 degrees or less on the surface of the finally obtained steel sheet exceeded 10.0 / mm, and the hydrogen embrittlement resistance of the bent portion deteriorated. 3+It is believed that the high content of Cr caused significant dissolution of the base steel. As a result, the number density of recesses having a depth of more than 2 μm and a apex angle of 45 degrees or less on the surface of the finally obtained steel sheet exceeded 10.0 / mm, and the hydrogen embrittlement resistance of the bent portion was reduced. In Comparative Example 27, the pickling time was too short, and the pickling did not proceed sufficiently, resulting in uneven removal of the internal oxide layer containing scale and silicon oxides. As a result, the number density of recesses having a depth of more than 2 μm and a apex angle of 45 degrees or less on the surface of the finally obtained steel sheet exceeded 10.0 / mm, and the hydrogen embrittlement resistance of the bent portion was reduced. In Comparative Example 28, the temperature of the aqueous solution in the pickling process was too low, and the pickling did not proceed sufficiently, resulting in uneven removal of the internal oxide layer containing scale and silicon oxides. As a result, the number density of recesses having a depth of more than 2 μm and a apex angle of 45 degrees or less on the surface of the finally obtained steel sheet exceeded 10.0 / mm, and the hydrogen embrittlement resistance of the bent portion was reduced. In Comparative Example 29, the temperature of the aqueous solution in the pickling process was high, which presumably caused the pickling to proceed excessively, dissolving not only the scale and internal oxide layer but also the base steel. As a result, the number density of recesses with a depth of more than 2 μm and a apex angle of 45 degrees or less on the surface of the finally obtained steel sheet was more than 10.0 / mm, and the hydrogen embrittlement resistance of the bent portion was reduced. In Comparative Example 30, the reduction rate in the cold rolling process was low, which prevented the unevenness on the steel sheet surface from being sufficiently smoothed. Similarly, the number density of recesses with a depth of more than 2 μm and a apex angle of 45 degrees or less on the surface of the finally obtained steel sheet was more than 10.0 / mm, and the hydrogen embrittlement resistance of the bent portion was reduced.

[0097] In Comparative Example 31, the maximum heating temperature in the first heat treatment step was low, resulting in insufficient austenitization, and a large amount of ferrite was formed in the final steel structure, making it impossible to achieve the desired martensite area ratio. As a result, TS decreased. In Comparative Example 33, the residence time at 600 to 700°C in the first heat treatment step was long, resulting in a high total area ratio of ferrite and bainite, and TS decreased. In Comparative Example 34, the residence time at 450 to 600°C in the first heat treatment step was long, resulting in a high total area ratio of ferrite and bainite, and therefore, the desired martensite area ratio could not be achieved. As a result, TS decreased. In Comparative Example 35, the residence time at Ms to 450°C in the first heat treatment step was long, resulting in a high total area ratio of ferrite and bainite, and therefore, the desired martensite area ratio could not be achieved. As a result, TS decreased. In Comparative Example 36, the average cooling rate between 200°C and Ms in the first heat treatment step was slow, which presumably prevented sufficient solute C from being secured in the steel, and the number of hydrogen trapping sites could not be sufficiently reduced even by the subsequent second heat treatment step. As a result, a steel structure satisfying the above formula (1) could not be formed, and the hydrogen embrittlement resistance of the bent portion was reduced. In Comparative Example 37, the cooling end temperature in the first heat treatment step was high, which presumably resulted in the formation of low-temperature transformation martensite containing a large amount of dislocations to which C was not attached in the final steel structure. As a result, a steel structure satisfying the above formula (1) could not be formed, and the hydrogen embrittlement resistance of the bent portion was reduced. In Comparative Example 38, the maximum heating temperature in the second heat treatment step was low, which presumably prevented sufficient solute C from being attached to dislocations in the steel, and the number of hydrogen trapping sites could not be sufficiently reduced. As a result, a steel structure satisfying the above formula (1) could not be formed, and the hydrogen embrittlement resistance of the bent portion was reduced. In Comparative Example 39, the maximum heating temperature in the second heat treatment step was high and the above formula (6) was not satisfied, so it is thought that solute C could not be properly fixed to dislocations in the steel, and the number of hydrogen trapping sites could not be sufficiently reduced.As a result, it was not possible to form a steel structure satisfying the above formula (1), and the hydrogen embrittlement resistance of the bent portion was reduced. In Comparative Example 40, it is thought that because the above formula (6) was not satisfied in the second heat treatment step, solute C could not be properly fixed to dislocations in the steel, and the number of hydrogen trapping sites could not be sufficiently reduced. As a result, it was not possible to form a steel structure satisfying the above formula (1), and the hydrogen embrittlement resistance of the bent portion was reduced.

[0098] In contrast to this, in the steel sheets according to all of the examples, by having a predetermined chemical composition and further appropriately controlling the conditions in the manufacturing method, it was possible to configure the steel structure to contain, by area %, 70% or more martensite, 0 to 20% total of ferrite and bainite, and 0 to 10% retained austenite, thereby achieving a high tensile strength of 1470 MPa or more. Furthermore, by controlling the number density of recesses on the steel sheet surface with a depth of more than 2 μm and an apex angle of 45 degrees or less to 10.0 / mm or less, it was possible to reduce stress concentration areas in bent parts that could become the starting point of hydrogen embrittlement cracking, and 100-200 / H 20-300 The specific combination of this with the reduction of hydrogen trapping sites satisfying <0.30 significantly improved the hydrogen embrittlement resistance of the bent section.

[0099] In particular, in Examples 1 to 10, 19 to 21, and 41, in which the surface layer of the steel sheet was appropriately decarburized, thereby satisfying the above formula (2), and further in which the number density of recesses on the steel sheet surface with a depth of more than 2 μm and an apex angle of 45 degrees or less was controlled to 5.0 / mm or less, the hydrogen embrittlement resistance of the bent portion was evaluated as AA, and further improved compared to Example 32, which did not satisfy the formula (2) (the hydrogen embrittlement resistance of the bent portion was evaluated as A). In addition, Examples 1 to 7, 9, 10, 21, 22, 32, and 41, in which the steel structure was configured to contain, by area percentage, 2% or more of ferrite and bainite in total and 1% or more of retained austenite, were able to achieve a uniform elongation of 5.0% or more. [Explanation of symbols]

[0100] 1 steel plate 2 recesses SH Surface height CL center line D Depth θ vertical angle 11 Strip-shaped test piece 12 Punch 13 Die 14 U-bend test piece 15 volts 16 Nut 17 Strain gauge

Claims

1. In mass%, C: 0.16-0.35%, Si: 0.001 to 0.80%, Mn: 1.00-3.50%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001-1.000%, Ti: 0.001 to 0.100%, B: 0.0005-0.0050%, N: 0.0100% or less, O: 0.0100% or less, Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Co: 0-1.00%, W: 0-1.00%, Ta: 0-1.000%, Sn: 0 to 1.00%, Sb: 0 to 0.50%, Nb: 0 to 0.200%, V: 0-1.00%, As: 0 to 0.100%, Zn: 0 to 1.000%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Hf: 0-0.0100%, Bi: 0 to 0.0100%, REM: 0 to 0.0100%, and The balance has a chemical composition consisting of Fe and impurities, The steel structure at the 1 / 4 thickness position from the surface is, in area%, Martensite: 70% or more, The sum of ferrite and bainite: 0 to 20%; and Retained austenite: 0 to 10% The number density of recesses on the surface having a depth of more than 2 μm and a vertex angle of 45 degrees or less is 10.0 / mm or less, and A steel sheet characterized in that, when the steel sheet is immersed in an aqueous solution of ammonium thiocyanate having a concentration of 100 g / L at 25°C for 48 hours and then heated from room temperature to 300°C at a temperature increase rate of 100°C / h by thermal desorption analysis, the amount of released hydrogen is measured, and the steel sheet satisfies the following formula (1): H 100-200 / H 20-300 <0.30 ・・・(1) H 100-200 : Hydrogen release amount at 100 to 200 ° C (mass ppm) H 20-300 : Hydrogen release amount at 20 to 300 ° C (mass ppm)

2. 2. The steel sheet according to claim 1, wherein when a C concentration is measured in a depth direction from a surface of the steel sheet using a high-frequency glow discharge optical emission spectrometer (GDS), the following formula (2) is satisfied: [Equation 1] C b C content (by mass%) in steel plate C n : C concentration (mass%) at the nth measurement point x n : Measurement point depth at the nth measurement point (μm) x n-1 : Measurement point depth at the n-1th measurement point (μm) l: n when the measurement point depth reaches 5 μm or more m: n when the measurement point depth reaches 30 μm or more

3. The steel structure at a 1 / 4 thickness position from the surface is, in area%, The sum of ferrite and bainite: 2 to 20%, and The steel plate according to claim 1 or 2, characterized in that it contains 1 to 10% retained austenite.

4. 3. The steel sheet according to claim 1, wherein the steel sheet has a tensile strength of 1470 MPa or more.

5. 3. The steel sheet according to claim 1, wherein at least one surface of the steel sheet is provided with a hot-dip galvanized layer or a hot-dip galvannealed layer.

6. A component, characterized in that it comprises a steel sheet according to claim 1 or 2.

7. (A) a hot rolling step comprising hot rolling a slab having the chemical composition according to claim 1, and then coiling and cooling the resulting hot-rolled steel sheet, wherein the hot-rolling step satisfies the following conditions (A1) and (A2): (A1) The cumulative reduction rate between descaling steps at a steel plate temperature of 1000°C or higher is 50% or less; and (A2) The coiling temperature is higher than 400°C and is in the range of 650°C, and the cooling satisfies the following formulas (3) to (5): [Equation 2] where: [Equation 3] [Equation 4] T(t): Steel plate temperature when t seconds have passed after winding [K] tf: time [seconds] for the steel plate temperature to reach 673K Nx: the sum of the atomic fractions [-] of Si, Mn and Al in the steel Δt: Measurement interval of T(t) [seconds] (B) The hot-rolled steel sheet is treated with 1.0 to 5.0 mol / L of HCl and less than 0.10 mol / L of Fe. 3+ a pickling step comprising: passing the material through an aqueous solution containing the compound at a temperature of 70 to 90°C at an average speed of 10 m / min or more for 30 to 200 seconds; (C) A cold rolling process in which the hot-rolled steel sheet after pickling treatment is cold-rolled at a reduction ratio of 30 to 75%; (D) a first heat treatment step, which includes heating the obtained cold-rolled steel sheet to a maximum heating temperature of Ac3 to 950°C and then cooling it to 40°C or less, and which satisfies the following conditions (D1) and (D2); (D1) After reaching the maximum heating temperature, the residence time at 600 to 700°C is 50 seconds or less, the residence time at 450 to 600°C is 500 seconds or less, and the residence time at Ms to 450°C is 100 seconds or less; and (D2) The average cooling rate between 200°C and Ms is 20°C / sec or more. (E) A second heat treatment step which includes heating the cold-rolled steel sheet after the first heat treatment to a maximum heating temperature of 80 to 300°C, and satisfies the following formulas (6) and (7): The method for producing a steel sheet according to claim 1 or 2, comprising: [Equation 5] where: [Equation 6] t: time elapsed after reaching 80°C [seconds] tf: time at which retention at 80 to 300°C ends [seconds] T: Temperature at time t [K] T max Maximum heating temperature [K]

8. The method according to claim 7, characterized in that, in the first heat treatment step, the atmosphere between Ac1 and Ac3 when heating the cold-rolled steel sheet to the maximum heating temperature satisfies the following formula (8): [Equation 7] pH 2 O: Water vapor partial pressure pH 2 : Hydrogen partial pressure

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