Steel sheet and method for producing same

Abstract

Provided is a steel sheet which has a specific chemical composition, in which the number density of recesses that have a depth of more than 2 μm and an apex angle of 45° or less in the surface is 10.0 / mm or less, and which satisfies H100-200 / H20-300 < 0.30 (in the formula, H100-200 is the hydrogen release amount from 100°C to 200°C, and H20-300 is the hydrogen release amount from 20°C to 300°C) when the steel sheet is immersed in an aqueous ammonium thiocyanate solution having a concentration of 100 g / L at 25°C for 48 hours, and subsequently the hydrogen release amount is measured by temperature desorption spectroscopy by heating the steel sheet from room temperature to 300°C at a temperature increase rate of 100°C / h. Also provided is a method for producing the same.

Description

Steel plate and its manufacturing method

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

[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 member 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 of 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 having a predetermined chemical composition, wherein the steel structure in a range of ⅛ to ⅜ thickness from the surface of the steel sheet, centered at ¼ 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%, wherein, after the zinc-based plating layer is removed, the amount of hydrogen released when the steel sheet is heated from room temperature to 200°C 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 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 is soft, and the hard phase in the surface layer, which is the starting point for cracks, is reduced and made fine and uniform, thereby reducing the number of starting points for cracks and suppressing hydrogen embrittlement.

[0007] International Publication No. 2019 / 212047 International Publication No. 2019 / 181950

[0008] As described 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, for example, bent portions (hereinafter simply referred to as "bent portions") 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.

[0010]

[0006] In order to achieve the above object, the present inventors have conducted research, focusing particularly on the steel structure of steel sheets. Specifically, the present inventors first discovered that by optimizing the chemical composition of a steel sheet and configuring the steel structure of the steel sheet 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 to 0.35%, Si: 0.001 to 0.80%, Mn: 1.00 to 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 to 0.0050%, N: 0.0100% or less, O: 0.0100% or less, Cr: 0 to 1.00%, Mo: 0 to 1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Co: 0 to 1.00%, W: 0 to 1.00%, Ta: 0 to 1.000%, Sn: 0 to 1.00%, a chemical composition consisting of Sb: 0 to 0.50%, Nb: 0 to 0.200%, V: 0 to 1.00%, As: 0 to 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: Fe and impurities; a steel structure at a quarter thickness position from the surface contains, in area %, martensite: 70% or more, a total 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 pieces / mm or less; 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 to measure the amount of released hydrogen, the steel sheet satisfies the following formula (1): 100-200 / H 20-300 <0.30...(1)H 100-200 : Amount of hydrogen released at 100 to 200 ° C (mass ppm) H 20-300 (2) The steel sheet according to (1), characterized in that when a C concentration is measured in a depth direction from the surface of the steel sheet using a high-frequency glow discharge optical emission spectrometer (GDS), the following formula (2) is satisfied: C b : C content (mass%) of steel plate Cn : C concentration (mass%) at the nth measurement point x n : Measurement point depth (μm) at the nth measurement point x n-1 : measurement point depth (μm) at the n-1th measurement point 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 sheet according to (1) or (2) above, characterized in that the steel structure at a ¼ thickness position from the surface contains, in area %, a total of ferrite and bainite: 2 to 20%, and retained austenite: 1 to 10%. (4) The steel sheet according to any one of (1) to (3) above, characterized in that it has a tensile strength of 1470 MPa or more. (5) The steel sheet according to any one of (1) to (4) above, characterized in that it has a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on at least one surface. (6) A part characterized in that it includes the steel sheet according to any one of (1) to (5) above. (7) (A) A hot rolling process comprising hot rolling a slab having the chemical composition described in (1) above, and then coiling and cooling the resulting hot-rolled steel sheet, the hot-rolling process satisfying the following conditions (A1) and (A2): (A1) the cumulative reduction during descaling at a steel sheet temperature of 1000°C or higher is 50% or less, and (A2) the coiling temperature is higher than 400°C to 650°C, and the cooling satisfies the following formulas (3) to (5): where: T(t): steel sheet temperature [K] when t seconds have elapsed after coiling; tf: time [seconds] for the steel sheet temperature to reach 673K; Nx: total atomic fraction [-] of Si, Mn and Al in the steel; Δt: measurement interval [seconds] of T(t). (B) The hot-rolled steel sheet was treated with HCl of 1.0 to 5.0 mol / L and Fe of less than 0.10 mol / L. 3+(C) a cold rolling step of cold rolling the hot-rolled steel sheet after the pickling treatment at a rolling reduction of 30 to 75%; (D) a first heat treatment step of 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, the first heat treatment step satisfying 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 / second or more. (E) A method for producing a steel sheet according to any one of the above (1) to (6), characterized in that it comprises 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): where: t: time elapsed after reaching 80°C [seconds] tf: time at which retention between 80 and 300°C ends [seconds] T: temperature at time t [K] T max : Maximum heating temperature [K] (8) The method according to (7) above, 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): pH2O: Water vapor partial pressure pH2: Hydrogen partial pressure

[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.

[0013] 1 is a diagram for explaining 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. FIG. 2 is a diagram schematically showing a test method for evaluating hydrogen embrittlement resistance of a bent portion.

[0014] <Steel Sheet> A steel sheet according to an embodiment of the present invention contains, in mass%, C: 0.16 to 0.35%, Si: 0.001 to 0.80%, Mn: 1.00 to 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 to 0.0050%, N: 0.0100% or less, O: 0.0100% or less, Cr: 0 to 1.00%, Mo: 0 to 1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Co: 0 to 1.00%, W: 0 to 1.00%, Ta: 0 to 1.000%, a chemical composition consisting of Sn: 0 to 1.00%, Sb: 0 to 0.50%, Nb: 0 to 0.200%, V: 0 to 1.00%, As: 0 to 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: Fe and impurities, wherein the steel structure at a position of one-quarter thickness from the surface contains, in area %, martensite: 70% or more, ferrite and bainite: 0 to 20% in total, 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 when the sample 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 following formula (1) is satisfied: 100-200 / H 20-300 <0.30...(1)H 100-200 : Amount of hydrogen released at 100 to 200 ° C (mass ppm) H 20-300 : Hydrogen release amount at 20 to 300 ° C (mass ppm)

[0015] As mentioned above, it is known that hydrogen embrittlement cracking is more likely to occur as the strength of a steel plate increases. In particular, in steel plates having extremely high strength, such as a tensile strength of 1470 MPa or more, the steel plate generally has a steel structure containing martensite as a major component in order to ensure high strength. However, in the case of such high-strength steel plates containing martensite as a major component, hydrogen embrittlement cracking may occur when hydrogen that has penetrated into the steel accumulates at prior austenite grain boundaries in the martensite structure, reducing the bonding strength of the grain boundaries, and the grain boundaries with such reduced bonding strength may become the starting point of 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 primarily composed of martensite, more specifically, a steel structure containing 70% or more martensite by area. It also strengthens prior austenite grain boundaries, thereby contributing to 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, particularly containing 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 portions 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 portions. 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 depressions / mm or less, it is possible to reduce stress concentration points in bent portions that could become the starting points for hydrogen embrittlement cracking, thereby improving the hydrogen embrittlement resistance of bent portions.

[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 degrees or less on the surface of a steel sheet according to an embodiment of the present invention. Referring particularly to FIG. 1( c), a steel sheet 1 has a recess 2 on its surface. In the recess 2, the distance (depth) D from the center line CL to the deepest position A, calculated by the least squares method from the profile of the surface height SH, exceeds 2 μm. Additionally, in the recess 2, an apex angle θ is formed by a line segment connecting the position A and the intersections B and C of the center line CL and the recess 2. When the apex angle θ is 45 degrees or less, such relatively large and sharp recess 2 can become a stress concentration area, and is therefore likely to become a starting point for hydrogen embrittlement cracking in the bent portion. Therefore, the present inventors have discovered that, as will be described in detail later in connection with the manufacturing method, the surface irregularities of the 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 degrees 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 the steel, hydrogen easily accumulates in steel sheets having a steel microstructure primarily composed of martensite, making it extremely difficult to improve hydrogen embrittlement resistance. In response to this, the present inventors have discovered that heat-treating a cold-rolled steel sheet under appropriate conditions can significantly reduce dislocation-related hydrogen trapping sites. While not intending to be bound by any particular theory, it is believed that heat-treating a cold-rolled steel sheet under appropriate conditions can ensure sufficient solute carbon (C) in the steel, allowing the solute C to attach to dislocations before hydrogen. As a result, it is believed that dislocations in the steel can be significantly reduced or inhibited from functioning as hydrogen trapping sites. 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 temperature increase rate of 100°C / h by thermal desorption analysis to measure the amount of released hydrogen, a steel structure can be formed that satisfies the following formula (1): H 100-200 / H 20-300 <0.30...(1)H 100-200 : Amount of hydrogen released at 100 to 200 ° C (mass ppm) H20-300 : Hydrogen release amount at 20 to 300 ° C (mass ppm)

[0021] To explain the above formula (1) in more detail, experiments by the present inventors have revealed 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 made 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 trap sites associated with dislocations in the steel.

[0022] As a result of further investigation from this perspective, the present inventors have found that H 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, it is possible to achieve high strength, more specifically a high strength of 1470 MPa or more, by using a steel structure mainly composed of martensite, 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 and upper limits, unless otherwise specified.

[0024] [C: 0.16 to 0.35%] C (carbon) is an essential element for ensuring the strength of the steel sheet. 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, if excessive C is contained, the hydrogen embrittlement resistance and uniform elongation of the bent portion may be reduced due to an excessive increase in strength. 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%] Si (silicon) 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 be 0.70% or less, 0.60% or less, or 0.50% or less.

[0026] [Mn: 1.00 to 3.50%] Mn (manganese) is a powerful austenite-stabilizing element and is effective in increasing the strength of steel sheet. To fully obtain 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 reduce 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 the viewpoint of economic efficiency, 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, deteriorating 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 extremely reducing the S content increases the desulfurization cost. Therefore, from an economical viewpoint, 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 acts 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 contained, 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 to 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 this effect, 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. On the other hand, even if an excessive amount of Ti is contained, 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 to 0.0050%] B (boron) 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, resulting in reduced 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 excessive reduction of the N content 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 the manufacturing cost. Therefore, from the viewpoint of manufacturing cost, 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 in order to obtain such effects, at least one of these elements may be contained in the steel sheet as necessary. However, excessive inclusion of these elements may saturate the effects and increase manufacturing costs. Therefore, the contents of Cr, Mo, Cu, Ni, Co, W, Sn, and V are each set to 1.00% or less, and may be set to 0.60%, 0.50%, 0.30%, or 0.20% or less. Similarly, the Ta content is set to 1.000% or less, and may be set to 0.600%, 0.500%, 0.300%, or 0.200% or less. Similarly, the Sb content is set to 0.50% or less, and may be set to 0.30%, or 0.10% or less. Similarly, the Nb content is set to 0.200% or less, and may be set to 0.100%, or 0.060% or less. The lower limits of these elements may be, for example, 0.001% or more or 0.01% or more for Cr, Mo, Cu, Ni, Co, W, Sn, Sb, and V. Similarly, the lower limits of Ta and Nb may be 0.001% or more or 0.005% or more for Ta and Nb.

[0036] [As: 0 to 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 excessive As is contained, the effect saturates, and containing more As than necessary in the steel sheet increases manufacturing costs. 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 alloy elements such as Mn and Si in steel. The content of these elements may be 0%, but since each contributes to improving the workability of the steel sheet, at least one of these elements may be contained in the steel sheet if necessary. However, excessive inclusion of these elements may saturate the effects and increase manufacturing costs. Therefore, the Zn content is set to 1.000% or less, and may be set to 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 may be set to 0.0100% or less, and may be set to 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 set to 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, and 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. The impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when industrially manufacturing the steel sheet.

[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 to 20%, and retained austenite: 0 to 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 %, martensite: 70% or more, total of ferrite and bainite: 0 to 20%, and retained austenite: 0 to 10%.

[0041] In an embodiment of the present invention, martensite includes 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 is a structure that 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 due to 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, and therefore the desired strength cannot be achieved. 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 to 10% in total] The remaining structure other than martensite, ferrite, bainite, and retained austenite may have an area ratio of 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 Fraction] Identification of the steel structure and calculation of the area fraction are performed using a secondary electron image taken using an FE-SEM (field emission scanning electron microscope, for example, JSM-7200F manufactured by JEOL, measured at an acceleration voltage of 15 kV) and an X-ray diffraction method. First, a sample is taken from a cross section of the steel plate thickness in a 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 sum of martensite and retained austenite, pearlite (if present), and the sum of ferrite and bainite are measured. First, regions with high brightness and where the substructure is not revealed by etching are determined to be fresh martensite and retained austenite. Next, regions with substructure and where multiple cementites with different elongation directions are precipitated are determined to be tempered martensite. Next, regions where cementite is precipitated in a lamellar form are determined to be pearlite (the sum of pearlite and cementite). The remainder other than the above structures are determined to be ferrite and bainite. For reference, regions with low brightness and where no substructure is observed can be determined to be ferrite, and regions that do not fall into any of the above categories can be determined 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 to remove a portion of the steel plate from the plate surface to a depth of 1 / 4 in the plate thickness direction. Then, the polished sample is subjected to a characteristic X-ray scan using MoKα1 radiation. 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 at the Surface Having a Depth of More than 2 μm and an Apex Angle of 45° 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 an Apex Angle of 45° 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 an Apex Angle of 45° 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, the lower the number density of the recesses, the more preferable it is. For example, it may be 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 on the surface having a depth of more than 2 μm and an 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 the Surface with a Depth of More than 2 μm and a Vertical Angle of 45° or Less] The number density of recesses on the surface of a steel sheet with a depth of more than 2 μm and a vertical angle of 45° or less is measured as follows. First, a sample is taken from a cross section of the steel sheet through its thickness in a direction perpendicular to its surface, and the observation surface is 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 an FE-SEM (field emission scanning electron microscope, e.g., a JSM-7200F manufactured by JEOL, measured at an acceleration voltage of 15 kV) ( FIG. 1( a) ). The obtained backscattered electron image is binarized to clarify the surface of the steel sheet ( FIG. 1( b) ). In the backscattered electron image, the coating is observed as a light color and the base steel as a dark color; however, during binarization, the threshold value is adjusted so that the light and dark areas can be distinguished. The converted binary image is converted into numerical data to obtain a profile of the surface height (SH in FIG. 1(c)). Image analysis software capable of such operations includes, for example, Image J. The center line (CL in FIG. 1(c)) is determined from the surface height profile using the least squares method, and the region where the distance (D in FIG. 1(c)) from the center line to the deepest position (point A in FIG. 1(c)) exceeds 2 μm is defined as a "recess with a depth of more than 2 μm." The region where the apex angle (θ in FIG. 1(c)) formed by the line segment connecting the deepest position of this recess (A in FIG. 1(c)) and the intersection point of the center line and the recess (B and C in FIG. 1(c)) is 45 degrees or less is determined as a "recess with a depth of more than 2 μm on the surface and an apex angle of 45 degrees or less." A similar analysis is performed so that the measurement range (surface length) exceeds 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 "recesses with a depth of more than 2 μm and a vertex angle of 45 degrees or less" obtained in each field of view is summed, and this is converted into a number density per mm of surface length, which is determined as the "number density of recesses with a depth of more than 2 μm and a vertex angle of 45 degrees 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. However, if the steel sheet has a coating layer, the coating layer must first be dissolved and stripped using acid. However, this method raises concerns that the acid dissolution will corrode not only the coating layer but also the interface with the base steel, potentially altering the original unevenness. For this reason, these methods are not recommended.

[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 temperature increase 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 : Amount of hydrogen released at 100 to 200 ° C (mass ppm) H 20-300 : Hydrogen release amount at 20 to 300 ° C (mass ppm)

[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 has a relatively large number of dislocations. Nevertheless, H 100-200 / H 20-300When 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 becomes 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, in the case of unprocessed steel sheet, a test piece approximately 25 mm x 10 mm in size is taken from a position other than the end in the width direction. In the case of plated steel sheet, 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 a test piece is taken from a processed part, a test piece taken from a flat area that has not been directly processed or that has been processed to a relatively small extent should be used. 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 piece into which hydrogen had been introduced was heated from room temperature to 300°C at a temperature increase rate of 100°C / h by thermal desorption analysis using a hydrogen measurement system for steel (JTF-20A manufactured by J Science Lab Co., Ltd.), and the total amount of released hydrogen H released during the temperature rise to 300°C was measured. 20-300(mass ppm) and the amount of hydrogen released between 100 ° C 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 in Depth Direction from Surface of Steel Sheet by GDS] In a preferred embodiment of the present invention, the C concentration of the steel sheet is controlled so as to satisfy the following formula (2) when the C concentration is measured in the depth direction from the surface of the steel sheet by a high-frequency glow discharge optical emission spectrometer (GDS). C b : C content (mass%) of steel plate C n : C concentration (mass%) at the nth measurement point x n : Measurement point depth (μm) at the nth measurement point x n-1 : Measurement point depth (μm) at the n-1th measurement point; l: n when the measurement point depth reaches 5 μm or more; m: n when the measurement point depth reaches 30 μm or more

[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 sheet, and C n means the C 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 C becomes larger. 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 value naturally increases as the degree of decarburization increases. On the other hand, hydrogen embrittlement cracking in bent portions is more likely to occur as the steel sheet surface becomes harder, and this tendency becomes particularly pronounced as the strength of steel sheets increases. 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 more preferable, 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 the value of the left side of the above formula (2) may be, for example, 6.0 or less, 5.5 or less, 5.0 or less, 4.5 or less, or 4.0 or less.

[0055] [Method for Determining the Value of the Left Side of Equation (2)] The value of the left side of Equation (2) is determined as follows using a high-frequency glow discharge optical emission spectrometer (GDS). In this embodiment, a high-frequency glow discharge optical emission spectrometer GD-Profiler 2 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 analyzed in the depth direction while sputtering. Then, the elements contained in the steel sheet are 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 determined by measuring the depth of the sputtering marks on the sample after GDS measurement, and the time is converted to depth. The depth of the sputtering marks can be measured on the surface using a commercially available microscope or laser microscope equipped 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 value of the C emission intensity from 20 seconds to 300 seconds, when the measurement results stabilize, is calculated. Since the C concentration of the steel plate to be measured is known and the C concentration of high-purity electrolytic iron can be considered to be 0, a calibration curve that can convert the C emission intensity to the C concentration (mass %) can be obtained. In this way, the C emission intensity at each measurement point (i.e., X n ) C concentration (i.e., C n ) and these C concentrations and C b (i.e., the C content of the steel sheet measured using a 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 type (Zn in the case of a zinc-plated steel sheet) is defined as 0 μm.

[0056] [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] [Plated Layer] The steel sheet according to the embodiment of the present invention may be a plated steel sheet having a plated layer on at least one surface, preferably both surfaces. The plated layer is not particularly limited, and 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 additional elements other than Zn, such as Al or Mg. The coating weight of the plated 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, 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 processed relatively little, the characteristics of the steel sheet do not change particularly before and after forming.

[0059] [Mechanical Properties] [Tensile Strength (TS) and Uniform Elongation (uEL)] A steel sheet having the above-described chemical composition and steel structure 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 having such extremely high tensile strength, the steel sheet according to the embodiment of the present invention can significantly suppress the occurrence of hydrogen embrittlement cracking in bent portions due to the specific combination of the chemical composition and steel structure described above. The upper limit of the tensile strength is not particularly limited, and the tensile strength of the steel sheet may be, for example, 1800 MPa or less, 1750 MPa or less, or 1700 MPa or less. Furthermore, despite having such extremely high tensile strength, the steel sheet according to the embodiment of the present invention can achieve improved ductility. 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 or a small test piece having a shape similar to that of the JIS No. 13B test piece may be used.

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

[0061] A method for producing a steel sheet according to an embodiment of the present invention comprises: (A) a hot rolling step comprising hot rolling a slab having the chemical composition described above in relation to the steel sheet, and then coiling and cooling the obtained hot-rolled steel sheet, the hot-rolled steel sheet satisfying the following conditions (A1) and (A2): (A1) the cumulative reduction during descaling at a steel sheet temperature of 1000°C or higher is 50% or less; and (A2) the coiling temperature is higher than 400°C to 650°C, and the cooling satisfies the following formulas (3) to (5): where: T(t): steel sheet temperature [K] when t seconds have elapsed after coiling; tf: time [seconds] for the steel sheet temperature to reach 673K; Nx: total atomic fraction [-] of Si, Mn and Al in the steel; Δt: measurement interval [seconds] of T(t). (B) The hot-rolled steel sheet was treated with HCl of 1.0 to 5.0 mol / L and Fe of less than 0.10 mol / L. 3+ (C) a cold rolling step of cold rolling the hot-rolled steel sheet after the pickling treatment at a rolling reduction of 30 to 75%; (D) a first heat treatment step of 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, the first heat treatment step satisfying 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 / second or more. (E) A second heat treatment step 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). where: t: time elapsed after reaching 80°C [seconds] tf: time at which retention between 80 and 300°C ends [seconds] T: temperature at time t [K] T max : Maximum heating temperature [K]

[0062] [(A) Hot Rolling Step] First, a slab having the chemical composition described above in relation to the steel sheet is hot rolled in a hot rolling step, and then the obtained hot-rolled steel sheet is coiled and cooled, and the hot rolling step must satisfy the following conditions (A1) and (A2): (A1) the cumulative reduction during descaling at a steel sheet temperature of 1000°C or higher is 50% or less, and (A2) the coiling temperature is higher than 400°C to 650°C, and the cooling satisfies the following formulas (3) to (5). where: T(t): steel sheet temperature [K] when t seconds have elapsed after coiling; tf: time [seconds] for the steel sheet temperature to reach 673K; Nx: total atomic fraction [-] of Si, Mn, and Al in the steel; Δt: measurement interval [seconds] of T(t).

[0063] The slab used contains a relatively large amount of alloying elements, and in particular, the slab after continuous casting contains coarse Ti carbides. Therefore, it is necessary to dissolve the alloying elements in the slab, and in particular, it is necessary to sufficiently dissolve Ti. Therefore, the slab is 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 heating temperature of the slab is preferably 1300°C or lower. From the viewpoint of manufacturability, the slab used is preferably cast by a continuous casting method, but it may also be produced by an ingot casting method or a thin slab casting method. Furthermore, the slab discharged from the heating furnace is descaled before rolling begins.

[0064] [(A1) Cumulative Reduction Between Descaling Passes When the Steel Sheet 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, and it is necessary to control the cumulative reduction of one or more rolling passes performed between adjacent descaling passes (e.g., between the first descaling pass and the second descaling pass, or between the second descaling pass and the third descaling pass) 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., when the cumulative reduction rate exceeds 50%, the grown scale is pushed into the steel sheet, resulting in noticeable unevenness on the steel sheet surface. As a result, 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 exceeds 10.0 / mm, and the hydrogen embrittlement resistance of the bent portion is reduced. In this manufacturing method, by controlling the cumulative reduction ratio between descaling steps to 50% or less when the steel sheet temperature is 1000°C or higher, it is possible to suppress indentation 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 entry temperature into the rolling rolls. The entry temperature into 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 steps is 48% or less. Here, the cumulative reduction ratio between descaling steps is calculated, for example, by the following formula when n rolling passes are included between descaling steps. In addition, when the steel sheet temperature drops to less than 1000°C during descaling, it is sufficient that the cumulative reduction ratio of the rolling passes in which the steel sheet temperature on the inlet side to the rolling rolls is 1000°C or higher since the immediately preceding descaling is 50% or less.Furthermore, when rolling is performed after final descaling, the cumulative reduction must be 50% or less for rolling passes after final descaling in which the steel sheet temperature is 1000°C or higher. Cumulative reduction between descaling passes (%) = (sheet thickness before n rolling passes - sheet thickness after n rolling passes) / sheet thickness before n rolling passes x 100.

[0065] [(A2) Coiling Temperature: More than 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). where: T(t): steel sheet temperature [K] when t seconds have elapsed after coiling; tf: time [seconds] for the steel sheet temperature to reach 673K; Nx: total atomic fraction [-] of Si, Mn, and Al in the steel; Δt: measurement interval [seconds] of T(t).

[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 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, 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 is undesirable as it increases the rolling load on the rolling mill in the subsequent cold rolling process. Therefore, in this production method, the coiling temperature is set to be higher than 400°C to 650°C, and preferably higher than 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 the temperature T(t), for example, 100 seconds. Do is the diffusion coefficient [m ２ / sec], No is the amount of oxygen atoms dissolved in the steel (atomic fraction) at temperature T(t), 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 adding them up. This can be expressed mathematically as in the following formula (9). Here, [X] is the mass fraction of element X, and Mx is the atomic weight of element X. The denominator of the above formula (9) is the sum of all elements added to the steel of interest.

[0068] The above formula (3) means 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 the reaction proceeds more slowly as the amount of the internally oxidized element increases. If the internal oxidation reaction proceeds excessively, i.e., if the value of the middle part of the above formula (3) becomes 1.50 or more, the unevenness of the steel sheet surface after pickling becomes large. Although these unevennesses are smoothed to a certain extent by cold rolling, their influence remains in the final product. 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 exceeds 10.0 / mm, and the hydrogen embrittlement resistance of the bent portion is reduced.

[0069] On the other hand, if the value of the middle part of the above formula (3) is 0.05 or less, although the unevenness of the steel sheet surface is improved, the hot-rolled steel sheet becomes too hard, which is undesirable as it increases the rolling load of the rolling mill in the subsequent cold rolling process. 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. Therefore, the value of Σ in formula (3) increases, i.e., the value of the middle part of formula (3) increases, and the internal oxidation reaction progresses. On the other hand, the faster the cooling, the smaller the value of the middle part of formula (3) becomes, 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 formulas (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 formula (3). Therefore, for example, when the coiling temperature is relatively high and the steel sheet temperature increases accordingly, the value of the middle part of formula (3) increases, and the internal oxidation reaction progresses, whereas 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 part of formula (3) can be appropriately controlled within a range of more than 0.05 and less than 1.50. Therefore, in the present production method, the coiling temperature in the hot rolling step is set to more than 400°C and 650°C, and the subsequent cooling is controlled to satisfy the above formulas (3) to (5), and in particular, the value of the middle part of formula (3) is controlled to more than 0.05 and less than 1.50. This makes it 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 recesses / 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 Step] Next, the obtained hot-rolled steel sheet is subjected to a pickling step, in which the pickling step is carried out using a solution of 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 the above-mentioned 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 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 exceeds 10.0 / 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 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 exceeds 10.0 / mm, and the hydrogen embrittlement resistance of the bent portion is also reduced. 3+ In this regard, the Fe in the 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 similarly exceeds 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 appropriately remove scale and internal oxide layers and suppress 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, but for example, Fe in the 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 Step] 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 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 the 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 / second or more.

[0074] [(D1) After reaching a maximum heating temperature of Ac3 to 950°C, residence time at 600 to 700°C: 50 seconds or less, residence time at 450 to 600°C: 500 seconds or less, and residence time at Ms to 450°C: 100 seconds or less] In order to sufficiently advance austenitization and obtain a desired steel structure in the subsequent cooling treatment, it is necessary to heat the cold-rolled steel sheet to a maximum heating temperature of Ac3 to 950°C. If austenitization is insufficient, a large amount of ferrite will be produced in the final steel structure, and the desired martensite area ratio may not be achieved. Furthermore, if, after reaching the maximum heating temperature of Ac3 to 950°C, the residence time at 600 to 700°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 is produced, resulting in a high total area ratio of ferrite and bainite, and in some cases a decrease in the area ratio of martensite, making it impossible to achieve the desired strength. In this production method, by controlling the residence time at 600 to 700°C 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, it becomes possible to control the total area ratio of ferrite and bainite to 0 to 20% and the area ratio of martensite 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 is 200 seconds or less, and the residence time at Ms to 450°C is 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, sufficient solute C can be secured in the steel. Therefore, in the subsequent second heat treatment step, the solute C 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 C atoms from martensite already formed during martensitic transformation to untransformed austenite, thereby preventing the formation of austenite regions with locally high C concentrations. The higher the C 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 such 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 temperature increase 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 with the subsequent second heat treatment step, the number of hydrogen trapping sites cannot be sufficiently reduced, and a steel structure satisfying the above formula (1) may not be formed. 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, preferably 30°C or less. If the cooling end temperature is high, untransformed austenite remains at the end of cooling, and 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, which makes it 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). pH2O: Water vapor partial pressure pH2: Hydrogen partial pressure

[0078] 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 it 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 bent portions. To enhance this effect, log(pH2O / pH2) in the above formula (8) is preferably greater than −1.00. More preferably, log(pH2O / pH2) is −0.90 or greater. On the other hand, when 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, it is preferable that log(pH2O / pH2) is 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 (°C) = 723 - 10.7 x Mn - 16.9 x Ni + 29.1 x Si + 16.9 x 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 alloyed hot-dip galvanized (GA) steel, 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 (GI) steel without alloying treatment can be used. However, the conditions described in (D1), including the plating treatment time and alloying treatment time, must be satisfied. When producing electrogalvanized (EG) steel, the steel sheet can be cooled to room temperature after (E). These zinc platings can be performed according to any appropriate method known to those skilled in the art. Similarly, these zinc platings can have any composition known to those skilled in the art and may contain additional elements other than Zn, such as Al and Mg. Furthermore, the coating weight of these zinc platings is not particularly limited and may 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 needs to satisfy the following formulas (6) and (7). where: t: time elapsed after reaching 80°C [seconds] tf: time at which retention between 80 and 300°C ends [seconds] T: temperature at time t [K] T max : Maximum heating temperature [K]

[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 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 the 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 final 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, data can be reshaped to 1-second intervals by linear interpolation.

[0082] According to the steel sheet manufactured by the above-mentioned manufacturing method, by adding Ti and B in amounts of 0.001 to 0.100 mass% and 0.0005 to 0.0050 mass%, respectively, the amount of solute B in the steel is sufficiently secured, thereby improving hardenability and strengthening prior austenite grain boundaries. Therefore, by combining this with a steel structure containing martensite in an area percentage of 70% or more, 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 process and the pickling process 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 degrees or less to 10.0 / mm or less, stress concentration areas that could serve as initiation points for hydrogen embrittlement cracking in bent portions can be reduced. In addition, by appropriately heat treating the steel sheet after cold rolling in the first and second heat treatment processes, 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.

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

[0085] First, molten steel was cast by a continuous casting method 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, and the obtained hot-rolled steel sheets were coiled and cooled at the temperatures shown in Table 2. The "maximum cumulative reduction between descaling" shown in Table 2 indicates the maximum value of the cumulative reduction between descalings 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 of 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. The thickness of each cold-rolled steel sheet was 1.4 mm. In a 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 residence time conditions for 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 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 during heating of the cold-rolled steel sheets to the maximum heating temperature was controlled to achieve 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 a second heat treatment step, and finally, a 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]

[0089]

[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 a JIS No. 5 test piece in a direction in which the longitudinal direction of the test piece was parallel to the direction perpendicular to the rolling direction of the steel sheet (C direction) and conducting a tensile test in accordance with JIS Z 2241:2022.

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

[0093] Steel sheets with a tensile strength of 1,470 MPa or more and an evaluation of hydrogen embrittlement resistance of AA or A were evaluated as having high strength and excellent hydrogen embrittlement resistance at bent portions. The results are shown in Table 3. "M," "α+B," and "residual γ" in Table 3 indicate the area ratios of "martensite," "ferrite and bainite," and "residual austenite," respectively. Furthermore, 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]

[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, TS 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, because Ti was not included, N present as an impurity in the steel could not be fixed as TiN, and most of the B precipitated as BN. In relation to this, the amount of solute B could not be sufficiently secured, resulting 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 grown during descaling to be pressed into the steel sheet. 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. 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 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 similarly 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 it is thought that 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 was more than 10.0 / mm, and the hydrogen embrittlement resistance of the bent portion was 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 an apex angle of 45 degrees or less on the surface of the finally obtained steel sheet similarly exceeded 10.0 / mm, and the hydrogen embrittlement resistance of the bent portion was reduced. In Comparative Example 27, the pickling time was short, and it is believed that the pickling did not proceed sufficiently, resulting in uneven removal of the internal oxide layer containing scale and silicon oxides, etc. 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 similarly 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 step was low, and it is believed that the pickling did not proceed sufficiently, resulting in uneven removal of the internal oxide layer containing scale and silicon oxides, etc. 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 similarly 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 is thought to have caused the pickling to proceed excessively, resulting in the dissolution of not only the scale and internal oxide layer but also the base steel. 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 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 failed to sufficiently smooth the unevenness of the steel sheet surface. Similarly, 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 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 plates 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 of martensite, 0 to 20% of the total of ferrite and bainite, and 0 to 10% of retained austenite, thereby achieving a high tensile strength of 1470 MPa or more. Furthermore, by controlling the number density of recesses on the steel plate 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 points in bent parts that could become the starting point of hydrogen embrittlement cracking, and 100-200 / H 20-300 By combining this with a specific reduction in hydrogen trapping sites satisfying <0.30, the hydrogen embrittlement resistance of the bent portion could be significantly improved.

[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 and therefore satisfied the above formula (2), and further in which the number density of recesses on the steel sheet surface having 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 (in which the hydrogen embrittlement resistance of the bent portion was evaluated as A), which did not satisfy the formula (2). 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, a total of ferrite and bainite of 2% or more and retained austenite of 1% or more, were able to achieve a uniform elongation of 5.0% or more.

[0100] REFERENCE SIGNS LIST 1 steel plate 2 recess SH surface height CL center line D depth θ apex angle 11 rectangular test piece 12 punch 13 die 14 U-bend test piece 15 bolt 16 nut 17 strain gauge

Claims

1. In mass%, C: 0.16 to 0.35%, Si: 0.001 to 0.80%, Mn: 1.00 to 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 to 0.0050%, N: 0.0100% or less, O: 0.0100% or less, Cr: 0 to 1.00%, Mo: 0 to 1.00%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Co: 0 to 1.00%, W: 0 to 1.00%, Ta: 0 to 1.000%, Sn: 0 to 1.00%, a chemical composition consisting of Sb: 0 to 0.50%, Nb: 0 to 0.200%, V: 0 to 1.00%, As: 0 to 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: Fe and impurities; a steel structure at a quarter thickness position from the surface contains, in area %, martensite: 70% or more, a total 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 pieces / mm or less; 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 to measure the amount of released hydrogen, the steel sheet satisfies the following formula (1): 100-200 / H 20-300 <0.30...(1)H 100-200 : Amount of hydrogen released at 100 to 200 ° C (mass ppm) H 20-300 : Hydrogen release amount at 20 to 300 ° C (mass ppm) 2. The steel sheet according to claim 1, wherein the carbon concentration measured in the depth direction from the surface of the steel sheet using a high-frequency glow discharge optical emission spectrometer (GDS) satisfies the following formula (2): C b : C content (mass%) of steel plate C n : C concentration (mass%) at the nth measurement point x n : Measurement point depth (μm) at the nth measurement point x n-1 : Measurement point depth (μm) at the n-1th measurement point; 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 plate according to claim 1 or 2, characterized in that the steel structure at a quarter-thickness position from the surface contains, by area percentage, 2 to 20% of ferrite and bainite in total, and 1 to 10% of retained austenite.

4. The steel sheet according to claim 1 or 2, characterized in that the tensile strength is 1470 MPa or more.

5. A steel sheet according to claim 1 or 2, characterized in that it has a hot-dip galvanized layer or a hot-dip galvannealed layer on at least one surface.

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

7. (A) A hot rolling process comprising hot rolling a slab having the chemical composition described in claim 1, then coiling and cooling the resulting hot-rolled steel sheet, which satisfies the following conditions (A1) and (A2): (A1) the cumulative reduction during descaling at a steel sheet temperature of 1000°C or higher is 50% or less, and (A2) the coiling temperature is between 400°C and 650°C, and the cooling satisfies the following formulas (3) to (5): where: T(t): steel sheet temperature [K] when t seconds have elapsed after coiling; tf: time [seconds] for the steel sheet temperature to reach 673K; Nx: total atomic fraction [-] of Si, Mn and Al in the steel; Δt: measurement interval [seconds] of T(t). (B) The hot-rolled steel sheet was treated with HCl of 1.0 to 5.0 mol / L and Fe of less than 0.10 mol / L. 3+ (C) a cold rolling step of cold rolling the hot-rolled steel sheet after the pickling treatment at a rolling reduction of 30 to 75%; (D) a first heat treatment step of 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, the first heat treatment step satisfying 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 / second 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 which satisfies the following formulas (6) and (7): where: t: time elapsed after reaching 80°C [seconds] tf: time at which retention between 80 and 300°C ends [seconds] T: temperature at time t [K] T max : Maximum heating temperature [K] 8. The method according to claim 7, wherein in the first heat treatment step, the atmosphere between Ac1 and Ac3 when the cold-rolled steel sheet is heated to the maximum heating temperature satisfies the following formula (8): pH2O: Water vapor partial pressure pH2: Hydrogen partial pressure

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