Steel sheet and hot press molded body

Abstract

Provided is a steel sheet characterized by having a prescribed chemical composition, and by having a depth of 3-100 [mu] m at which the areal proportion of pearlite in the sheet thickness direction from the surface is 0-20%, and by having an areal proportion of pearlite having an equivalent circle diameter of 5 [mu] m or more at a depth of 0-20% of the areal proportion of pearlite in the sheet thickness direction, the areal proportion of pearlite is 0-30%. Further provided is a hot press molded article characterized by having a prescribed chemical composition, having an average C concentration of 0.20 mass% or less from the surface to 20 [mu] m in the thickness direction, and containing 90% or more of martensite in terms of areal proportion.

Description

Technical Field

[0001] The present invention relates to a steel sheet and a hot-stamped formed body, and more particularly, to a steel sheet and a hot-stamped formed body that can be obtained by hot-stamping the steel sheet. Background Art

[0002] As a technique for press-forming a material that is difficult to form, such as a high-strength steel sheet, hot stamping (hot pressing) is known. Hot stamping is a hot forming technique in which a material to be formed is heated and then formed. In this technique, since the material is heated and then formed, the steel material is soft and has good formability during forming. Therefore, even a high-strength steel material can be accurately formed into a complex shape. In addition, it is known that since quenching is performed simultaneously with forming by a pressing die, the formed steel material has sufficient strength.

[0003] In connection with this, various studies have been conducted on steel sheets for hot stamping and plated steel sheets in the past.

[0004] For example, in Patent Document 1, a galvanized steel sheet for hot stamping is described. The galvanized steel sheet is characterized in that it includes a base steel sheet and a plating layer provided on the surface of the base steel sheet. The base steel sheet contains C: 0.10 to 0.5%, Si: 0.7 to 2.5%, Mn: 1.0 to 3%, and Al: 0.01 to 0.5% by mass, and the balance is iron and inevitable impurities. The base steel sheet has an internal oxide layer containing at least one of Si and Mn with a thickness of 1 μm or more inside and a decarburized layer with a thickness of 20 μm or less from the interface with the plating layer toward the inside of the base steel sheet. In addition, in Patent Document 1, it is taught that by setting the thickness of the internal oxide layer of the base steel sheet to 1 μm or more, the generation of unplated portions in the galvanized steel sheet can be sufficiently suppressed, and the adhesion between the formed plating layer and the base steel sheet can be made sufficiently high. In addition, in Patent Document 1, it is taught that the internal oxide layer is formed near the surface of the base steel sheet by high dew point annealing. On the other hand, a decarburized layer is formed on the surface and near the surface of the base steel sheet by this high dew point annealing. Regarding this decarburized layer, since the carbon content is low, although the tensile strength is lower than that of the non-decarburized portion, as long as the thickness of the decarburized layer is 20 μm or less, the influence of the decarburized layer on the strength of the galvanized steel sheet and the hot-stamped formed product manufactured using the same can be suppressed.

[0005] Prior Art Documents

[0006] Patent Documents

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 2019-151883 Summary of the Invention

[0008] Problems to be Solved by the Invention

[0009] In a conventional hot-stamped formed body manufactured by press quenching a steel sheet, since the entire region in the plate thickness direction is formed of a hard structure (mainly martensite), there is generally a problem of insufficient formability. When the hot-stamped formed body is applied to automotive components or the like, in order to obtain excellent collision resistance characteristics, it is necessary to improve the impact energy absorption ability. Considering the deformation mode during a collision, it is important to improve the bendability, in particular, among the formabilities. On the other hand, in the automotive industry and the like, further weight reduction of steel materials is also required. In order to achieve such weight reduction, it becomes necessary to increase the strength of the steel material more than ever before. Therefore, even when the strength is increased to the same level as or higher than that of the past, there is a high demand for steel materials having excellent bendability, more specifically, hot-stamped formed bodies and steel sheets for obtaining them.

[0010] Accordingly, an object of the present invention is to provide a steel sheet that is high-strength even when applied to hot stamping and can achieve excellent bendability, and a hot-stamped formed body that is such high-strength and has excellent bendability.

[0011] Means for Solving the Problems

[0012] The inventors of the present invention conducted research to achieve the above object, and as a result, found that excellent bendability can be achieved even when applied to hot stamping by appropriately modifying the structure of the surface layer portion of the steel sheet, thereby completing the present invention.

[0013] The present invention that can achieve the above object is as follows.

[0014] (1) A steel sheet, characterized by having the following chemical composition: containing, by mass%:

[0015] C: 0.27 to 0.60%,

[0016] Si: 0.001 to 3.00%,

[0017] Mn: 0.30 to 3.00%,

[0018] Al: 0.0002 to 2.000%,

[0019] P: 0.1000% or less,

[0020] S: 0.1000% or less,

[0021] N: 0.0100% or less,

[0022] B: 0 to 0.0100%,

[0023] Ti: 0 to 0.1500%,

[0024] Nb: 0 to 0.150%,

[0025] V: 0 to 0.150%,

[0026] Mo: 0 to 1.00%,

[0027] Cr: 0 to 1.0%,

[0028] Cu: 0 to 1.000%,

[0029] Ni: 0 to 1.00%,

[0030] W: 0 to 1.000%,

[0031] Hf: 0 to 0.050%,

[0032] Mg: 0 to 0.050%,

[0033] Zr: 0 to 0.050%,

[0034] Ca: 0 to 0.0100%,

[0035] REM: 0 to 0.3000%,

[0036] Ir: 0 to 1.000%, and

[0037] the balance: consisting of Fe and impurities,

[0038] the depth at which the area ratio of pearlite in the thickness direction from the surface is 0 to 20% is 3 to 100 μm,

[0039] the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth where the area ratio of the above pearlite is 0 to 20% is 0 to 30%.

[0040] (2) The steel plate according to the above (1), characterized in that the depth at which the area ratio of the above pearlite is 0 to 20% is 10 to 100 μm.

[0041] (3) The steel plate according to the above (2), characterized in that the depth at which the area ratio of the above pearlite is 0 to 20% is 30 to 100 μm.

[0042] (4) The steel plate according to any one of the above (1) to (3), characterized in that the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth where the area ratio of the above pearlite is 0 to 20% is 0 to 15%.

[0043] (5) A hot stamping formed body, characterized in that it has the following chemical composition: by mass% including:

[0044] C: 0.27 to 0.60%,

[0045] Si: 0.001 to 3.00%,

[0046] Mn: 0.30 to 3.00%,

[0047] Al: 0.0002 to 2.000%,

[0048] P: 0.1000% or less,

[0049] S: 0.1000% or less,

[0050] N: 0.0100% or less,

[0051] B: 0 to 0.0100%,

[0052] Ti: 0 to 0.1500%,

[0053] Nb: 0 to 0.150%,

[0054] V: 0 to 0.150%,

[0055] Mo: 0 to 1.00%,

[0056] Cr: 0 to 1.0%,

[0057] Cu: 0 to 1.000%,

[0058] Ni: 0 to 1.00%,

[0059] W: 0 to 1.000%,

[0060] Hf: 0 to 0.050%,

[0061] Mg: 0 to 0.050%,

[0062] Zr: 0 to 0.050%,

[0063] Ca: 0 to 0.0100%,

[0064] REM: 0 to 0.3000%,

[0065] Ir: 0 to 1.000%, and

[0066] The balance: consists of Fe and impurities,

[0067] The average C concentration from the surface to 20 μm in the thickness direction is 0.20 mass% or less, and it contains 90% or more martensite by area ratio.

[0068] (6) The hot stamping formed body according to (5) above, characterized in that the average C concentration is 0.10% by mass or less.

[0069] (7) The hot stamping formed body according to (5) or (6) above, characterized in that the average C concentration is 0.05% by mass or less.

[0070] Advantages of the Invention

[0071] According to the present invention, it is possible to provide a steel sheet that is high-strength even when applied to hot stamping and can achieve excellent bendability, and a hot stamping formed body that is such high-strength and has excellent bendability. Detailed Embodiments

[0072] <Steel Sheet>

[0073] The steel sheet according to the embodiment of the present invention is characterized by having the following chemical composition: by mass% including:

[0074] C: 0.27 - 0.60%,

[0075] Si: 0.001 - 3.00%,

[0076] Mn: 0.30 - 3.00%,

[0077] Al: 0.0002 - 2.000%,

[0078] P: 0.1000% or less,

[0079] S: 0.1000% or less,

[0080] N: 0.0100% or less,

[0081] B: 0 - 0.0100%,

[0082] Ti: 0 - 0.1500%,

[0083] Nb: 0 - 0.150%,

[0084] V: 0 - 0.150%,

[0085] Mo: 0 - 1.00%,

[0086] Cr: 0 - 1.0%,

[0087] Cu: 0 - 1.000%,

[0088] Ni: 0 - 1.00%,

[0089] W: 0 - 1.000%,

[0090] Hf: 0 to 0.050%,

[0091] Mg: 0 to 0.050%,

[0092] Zr: 0 to 0.050%,

[0093] Ca: 0 to 0.0100%,

[0094] REM: 0 to 0.3000%,

[0095] Ir: 0 to 1.000%, and

[0096] The balance: consisting of Fe and impurities,

[0097] The depth at which the area ratio of pearlite is 0 to 20% in the plate thickness direction from the surface is 3 to 100 μm,

[0098] The area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth where the area ratio of the above-mentioned pearlite is 0 to 20% is 0 to 30%.

[0099] As described above, in a conventional hot stamping formed body manufactured by press quenching a steel sheet, since the entire region in the plate thickness direction is formed of a hard structure (mainly martensite), there is generally a problem of insufficient deformability. On the other hand, for example, it is considered to soften the surface layer portion of the steel sheet before hot stamping by reducing the carbon concentration in the surface layer portion of the steel sheet by using decarburization or the like, thereby improving the bendability of the formed body after hot stamping. However, the inventors have conducted various studies and found that: even from the viewpoint of improving bendability, even if the carbon concentration in the surface layer portion of the steel sheet before hot stamping is reduced by decarburization or the like, the carbon contained in the body of the steel sheet during high-temperature heating in hot stamping diffuses into the surface layer portion, and the initial bendability improvement effect brought about by the low carbon concentration in the surface layer portion disappears or is reduced due to such re-carbonization of the surface layer portion. Then, the inventors further conducted research and found that: by forming a structure in the surface layer portion of the steel sheet before hot stamping that can suppress such re-carbonization, the initial bendability improvement effect brought about by the low carbon concentration in the surface layer portion of the steel sheet can be sufficiently maintained, and the bendability of the formed body obtained after hot stamping can be significantly improved. More specifically, the inventors found that: by forming a structure in the surface layer portion of the steel sheet that sets the depth at which the area ratio of pearlite is 0 to 20% in the plate thickness direction from the surface to 3 to 100 μm and controls the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth where the area ratio of the pearlite is 0 to 20% to 0 to 30%, the bendability of the formed body obtained after hot stamping can be significantly improved.

[0100] Although not intending to be bound by any particular theory, it is considered that in the steel sheet according to the embodiment of the present invention, the structure in the surface layer portion of the steel sheet functions as follows, suppressing or reducing the re-carbonization due to the diffusion of carbon contained in the steel sheet to the surface layer portion of the steel during high-temperature heating in hot stamping forming. More specifically, if the carbon concentration in the surface layer portion of the steel sheet is reduced by decarburization or the like, the amount of pearlite generated in the microstructure in the surface layer portion of the steel sheet becomes relatively small in association with such a low-carbon concentration. Here, in the steel sheet according to the embodiment of the present invention, first, it is important to decarbonize the surface layer portion of the steel sheet to a low-carbon concentration such that the depth at which the area ratio of pearlite becomes 0 to 20% in the thickness direction from the surface of the steel sheet, that is, the depth of the region where the area ratio of pearlite is relatively low, becomes 3 to 100 μm. Thereby, the effect of improving bendability based on low-carbon concentration can be fully exerted. However, it is considered that when only the area ratio of pearlite is simply reduced to a specified range, in the case where such pearlite precipitates along the grain boundaries, during high-temperature heating in hot stamping forming, the pearlite transforms into austenite to form a diffusion path of carbon based on austenite along the grain boundaries (i.e., a re-carbonization path of carbon). During high-temperature heating in hot stamping forming, it is desired that carbon in the body of the steel sheet diffuses to the surface side based on the concentration gradient between the high-carbon concentration in the body and the low-carbon concentration on the surface side. At this time, if there is a re-carbonization path of carbon based on austenite along the grain boundaries as described above, the carbon in the body diffuses to the surface side via this re-carbonization path, thereby promoting re-carbonization to the surface layer portion of the steel. As a result, it becomes impossible to sufficiently maintain the initial effect of improving bendability brought about by the low-carbon concentration of the steel sheet surface layer portion. More specifically, although decarburization may also occur on the surface side of the steel sheet during high-temperature heating in hot stamping forming, since the above-mentioned re-carbonization is dominant, it becomes impossible to sufficiently maintain the initial effect of improving bendability brought about by the low-carbon concentration of the steel sheet surface layer portion. In contrast, if the steel sheet according to the embodiment of the present invention is used, in the above-mentioned depth region where the area ratio of pearlite is relatively low, by controlling the area ratio of pearlite with an equivalent circle diameter of 5 μm or more to be within the range of 0 to 30%, the amount of relatively large pearlite is reduced, so that even during high-temperature heating in hot stamping forming, the austenite transformed from pearlite can be dispersed and present at the grain boundaries, thereby reliably truncating the re-carbonization path of carbon based on austenite.

[0101] More specifically, when pearlite transforms into austenite during high-temperature heating in hot stamping forming, a two-phase structure of ferrite and austenite is formed. In such a case, a carbon re-carbonization path is formed by austenite existing at the heterogeneous interface between ferrite and austenite connecting to the surface side of the steel sheet. As a result, the diffusion of carbon from the bulk of the steel sheet to the surface side is promoted. Associated therewith, in the steel sheet according to the embodiment of the present invention, it is important to reduce the pearlite in the surface layer portion of the steel sheet, that is, in the depth region from the surface of the steel sheet in the plate thickness direction of 3 to 100 μm, to 0 to 20% in terms of area ratio, and to limit the relatively coarse pearlite in this depth region, that is, pearlite with an equivalent circle diameter of 5 μm or more, to 0 to 30% in terms of area ratio. With such a surface layer portion structure, even during high-temperature heating in hot stamping forming, the amount of austenite transformed from pearlite can be reduced, and further, the austenite can be dispersed and present at the grain boundaries. Therefore, the carbon re-carbonization path based on austenite can be reliably cut off. Therefore, if the steel sheet according to the embodiment of the present invention is used, by significantly suppressing or reducing re-carbonization during high-temperature heating in hot stamping forming, the bending property improvement effect initially brought about by the low carbon concentration in the surface layer portion of the steel sheet can be sufficiently maintained, and the bending property of the formed body obtained after hot stamping forming can be significantly improved. The fact that the re-carbonization during high-temperature heating in hot stamping forming can be suppressed or reduced by appropriately modifying the surface layer portion structure of the steel sheet was first clarified by the inventors this time. In addition, if the steel sheet according to the embodiment of the present invention is used, by improving the bending property based on the suppression or reduction of such re-carbonization, a hot stamping formed body with excellent collision resistance characteristics can be obtained despite its high strength. Therefore, the steel sheet according to the embodiment of the present invention is particularly useful in automotive applications.

[0102] 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 the content of each element means “mass %” unless otherwise specified. In addition, in this specification, “~” indicating a numerical range is used to mean including the values described before and after it as the lower limit value and the upper limit value unless otherwise specified.

[0103] [Chemical composition of the steel sheet (common to the chemical composition of the hot stamping formed body described below)]

[0104] [C: 0.27 to 0.60%]

[0105] C is an element that cheaply increases the tensile strength and is an important element for controlling the strength of steel. To fully obtain such an effect, the C content is set to 0.27% or more. The C content can also be 0.28% or more, 0.30% or more, 0.32% or more, 0.35% or more, 0.38% or more, or 0.40% or more. On the other hand, if C is excessively contained, sometimes a decrease in elongation may occur. Therefore, the C content is set to 0.60% or less. The C content can also be 0.55% or less, 0.50% or less, 0.48% or less, or 0.45% or less.

[0106] [Si: 0.001 - 3.00%]

[0107] Si is an element that acts as a deoxidizer and inhibits the precipitation of carbides during the cooling process in the annealing of cold-rolled sheets. To fully obtain such an effect, the Si content is set to 0.001% or more. The Si content can also be 0.01% or more, 0.10% or more, 0.20% or more, 0.50% or more, 1.00% or more, or 1.20% or more. On the other hand, if Si is excessively contained, sometimes a decrease in elongation may occur as the strength of the steel increases. Therefore, the Si content is set to 3.00% or less. The Si content can also be 2.50% or less, 2.00% or less, 1.70% or less, or 1.50% or less.

[0108] [Mn: 0.30 - 3.00%]

[0109] Mn is an element that improves the hardenability of steel and is effective for increasing the strength. To fully obtain such an effect, the Mn content is set to 0.30% or more. The Mn content can also be 0.50% or more, 1.00% or more, 1.20% or more, or 1.50% or more. On the other hand, if Mn is excessively contained, sometimes a decrease in elongation may occur as the strength of the steel increases. Therefore, the Mn content is preferably set to 3.00% or less. The Mn content can also be 2.80% or less, 2.50% or less, 2.00% or less, or 1.70% or less.

[0110] [Al: 0.0002 - 2.000%]

[0111] Al is an element that acts as a deoxidizer for steel and has the effect of soundifying the steel. To fully obtain such an effect, the Al content is set to 0.0002% or more. The Al content can also be 0.001% or more, 0.010% or more, 0.040% or more, or 0.100% or more. On the other hand, if Al is excessively contained, sometimes coarse Al oxides are generated and the toughness of the steel plate decreases. Therefore, the Al content is set to 2.000% or less. The Al content can also be 1.500% or less, 1.000% or less, 0.800% or less, or 0.500% or less.

[0112] [P: 0.1000% or less]

[0113] P is an element that segregates at grain boundaries and promotes embrittlement of steel. The lower the P content, the more preferable it is, and ideally it is 0%. However, an excessive reduction in the P content sometimes leads to a significant increase in cost. Therefore, the P content can also be set to 0.0001% or more, and can also be 0.0005% or more, 0.0010% or more, or 0.0050% or more. On the other hand, if P is excessively contained, as described above, embrittlement of steel sometimes occurs due to grain boundary segregation. Therefore, the P content is set to 0.1000% or less. The P content can also be 0.0500% or less, 0.0300% or less, or 0.0100% or less.

[0114] [S: 0.1000% or less]

[0115] S is an element that forms non-metallic inclusions such as MnS in steel and causes a reduction in the ductility of steel components. The lower the S content, the more preferable it is, and ideally it is 0%. However, an excessive reduction in the S content sometimes leads to a significant increase in cost. Therefore, the S content can also be set to 0.0001% or more, and can also be 0.0002% or more, 0.0010% or more, or 0.0050% or more. On the other hand, if S is excessively contained, cracking sometimes occurs starting from non-metallic inclusions during cold forming. Therefore, the S content is set to 0.1000% or less. The S content can also be 0.0500% or less, 0.0200% or less, or 0.0100% or less.

[0116] [N: 0.0100% or less]

[0117] N is an element that forms coarse nitrides in the steel plate and reduces the workability of the steel plate. The lower the N content, the more preferable it is, and ideally it is 0%. However, an excessive reduction in the N content sometimes leads to a significant increase in manufacturing cost. Therefore, the N content can also be set to 0.0001% or more, and can also be 0.0005% or more, or 0.0010% or more. On the other hand, if N is excessively contained, as described above, coarse nitrides sometimes form and reduce the workability of the steel plate. Therefore, the N content is set to 0.0100% or less. The N content can also be 0.0080% or less, or 0.0050% or less.

[0118] The basic chemical composition of the steel sheet according to the embodiment of the present invention is as described above. Further, the steel sheet may also contain, as required, one or more selected from the group consisting of B: 0 to 0.0100%, Ti: 0 to 0.1500%, Nb: 0 to 0.150%, V: 0 to 0.150%, Mo: 0 to 1.00%, Cr: 0 to 1.0%, Cu: 0 to 1.000%, Ni: 0 to 1.00%, W: 0 to 1.000%, Hf: 0 to 0.050%, Mg: 0 to 0.050%, Zr: 0 to 0.050%, Ca: 0 to 0.0100%, REM: 0 to 0.3000%, and Ir: 0 to 1.000% to replace a part of the remaining Fe. Hereinafter, these optional elements will be described in detail.

[0119] [B: 0 to 0.0100%]

[0120] B is an element that improves the hardenability of steel and contributes to the improvement of strength. The B content may also be 0%, but in order to obtain such an effect, the B content is preferably 0.0001% or more. The B content may also be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, if B is contained excessively, the toughness and / or weldability may sometimes decrease. Therefore, the B content is preferably 0.0100% or less. The B content may also be 0.0050% or less, 0.0030% or less, 0.0015% or less, or 0.0010% or less.

[0121] [Ti: 0 to 0.1500%]

[0122] Ti is an element that forms carbonitrides in steel and contributes to the improvement of strength through precipitation strengthening. The Ti content may also be 0%, but in order to obtain such an effect, the Ti content is preferably 0.0001% or more. The Ti content may also be 0.0010% or more, 0.0100% or more, 0.0300% or more, or 0.0500% or more. On the other hand, even if Ti is contained excessively, the effect saturates and the manufacturing cost may increase. Therefore, the Ti content is preferably 0.1500% or less. The Ti content may also be 0.1200% or less, 0.1000% or less, or 0.0800% or less.

[0123] [Nb: 0 to 0.150%]

[0124] Nb is an element that forms carbides, nitrides, and / or carbonitrides in steel and contributes to the micro-refinement of the structure through the pinning effect. The Nb content can also be 0%, but in order to achieve such an effect, the Nb content is preferably 0.001% or more. The Nb content can also be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, if Nb is excessively contained, sometimes coarse carbides, etc. are generated in the steel, resulting in a decrease in the toughness of the steel plate. Therefore, the Nb content is preferably 0.150% or less. The Nb content can also be 0.120% or less, 0.100% or less, or 0.080% or less.

[0125] [V: 0 to 0.150%]

[0126] V is an element that contributes to the increase in strength through precipitation strengthening, etc. The V content can also be 0%, but in order to achieve such an effect, the V content is preferably 0.001% or more. The V content can also be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, if V is excessively contained, sometimes a large amount of precipitates are generated, resulting in a decrease in toughness. Therefore, the V content is preferably 0.150% or less. The V content can also be 0.120% or less, 0.100% or less, or 0.080% or less.

[0127] [Mo: 0 to 1.00%]

[0128] Mo is an element that improves the hardenability of steel, contributes to the increase in strength, and also contributes to the improvement of corrosion resistance. The Mo content can also be 0%, but in order to achieve these effects, the Mo content is preferably 0.001% or more. The Mo content can also be 0.01% or more, 0.02% or more, or 0.05% or more. On the other hand, if Mo is excessively contained, sometimes the deformation resistance during hot working increases, and the equipment load becomes larger. Therefore, the Mo content is preferably 1.00% or less. The Mo content can also be 0.80% or less, 0.50% or less, 0.20% or less, 0.10% or less, or 0.08% or less.

[0129] [Cr: 0 to 1.0%]

[0130] Cr is an element that improves the hardenability of steel and contributes to the increase in strength and / or corrosion resistance. The Cr content can also be 0%, but in order to achieve these effects, the Cr content is preferably 0.001% or more. The Cr content can also be 0.01% or more, 0.05% or more, or 0.1% or more. On the other hand, even if Cr is excessively contained, the effect saturates, and it may lead to an increase in manufacturing cost. Therefore, the Cr content is preferably 1.0% or less. The Cr content can also be 0.8% or less, 0.5% or less, 0.3% or less, or 0.2% or less.

[0131] [Cu: 0 to 1.000%]

[0132] Cu is an element that helps improve strength and / or corrosion resistance. The Cu content can also be 0%, but in order to obtain these effects, the Cu content is preferably 0.001% or more. The Cu content can also be 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, if Cu is excessively contained, deterioration of toughness and weldability may sometimes occur. Therefore, the Cu content is preferably 1.000% or less. The Cu content can also be 0.800% or less, 0.700% or less, 0.500% or less, 0.300% or less, or 0.150% or less.

[0133] [Ni: 0 to 1.00%]

[0134] Ni is an element that improves the hardenability of steel and helps improve strength and / or corrosion resistance. The Ni content can also be 0%, but in order to obtain these effects, the Ni content is preferably 0.001% or more. The Ni content can also be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, even if Ni is excessively contained, the effect saturates and the manufacturing cost may increase. Therefore, the Ni content is preferably 1.00% or less. The Ni content can also be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[0135] [W: 0 to 1.000%]

[0136] W is an element that improves the hardenability of steel and helps improve strength. The W content can also be 0%, but in order to obtain such an effect, the W content is preferably 0.001% or more. The W content can also be 0.005% or more, 0.010% or more, or 0.050% or more. On the other hand, if W is excessively contained, weldability sometimes decreases. Therefore, the W content is preferably 1.000% or less. The W content can also be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, or 0.080% or less.

[0137] [Hf: 0 to 0.050%]

[0138] [Mg: 0 to 0.050%]

[0139] [Zr: 0 to 0.050%]

[0140] Hf, Mg and Zr are elements capable of controlling the morphology of sulfides. The contents of Hf, Mg and Zr can also be 0%, but in order to achieve such an effect, the contents of these elements are preferably 0.0001% or more, and can also be 0.0005% or more, 0.001% or more, or 0.002% or more. On the other hand, even if these elements are excessively contained, the effect is saturated, and containing them in the steel sheet more than necessary will lead to an increase in manufacturing cost. Therefore, the contents of Hf, Mg and Zr are preferably 0.050% or less, and can also be 0.010% or less, 0.005% or less, or 0.003% or less.

[0141] [Ca: 0 to 0.0100%]

[0142] Ca is an element capable of controlling the morphology of sulfides. The content of Ca can also be 0%, but in order to achieve such an effect, the content of Ca is preferably 0.0001% or more. The content of Ca can also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if Ca is excessively contained, the effect is saturated, and containing it in the steel sheet more than necessary will lead to an increase in manufacturing cost. Therefore, the content of Ca is preferably 0.0100% or less, and can also be 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[0143] [REM: 0 to 0.3000%]

[0144] REM is an element capable of controlling the morphology of sulfides. The content of REM can also be 0%, but in order to achieve such an effect, the content of REM is preferably 0.0001% or more. The content of REM can also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if REM is excessively contained, the effect is saturated, and containing it in the steel sheet more than necessary will lead to an increase in manufacturing cost. Therefore, the content of REM is preferably 0.3000% or less. The content of REM can also be 0.1000% or less, 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less. REM in this specification is a general term for 17 elements including scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 among the lanthanide elements, and the content of REM is the total content of these elements.

[0145] [Ir: 0 to 1.000%]

[0146] Ir is an element that forms fine oxides and contributes to the improvement of strength. The Ir content may be 0%, but in order to obtain such an effect, the Ir content is preferably 0.001% or more. The Ir content may also be 0.005% or more, 0.010% or more, or 0.050% or more. On the other hand, even if Ir is excessively contained, the effect saturates, and containing it in the steel sheet more than necessary will lead to an increase in manufacturing cost. Therefore, the Ir content is preferably 1.000% or less. The Ir content may also be 0.500% or less, 0.100% or less, 0.080% or less, or 0.060% or less.

[0147] In the steel sheet, the remaining part other than the above elements is composed of Fe and impurities. The impurities in the steel sheet are components mixed in through various factors in the manufacturing process represented by raw materials such as ores and scraps when manufacturing the steel sheet industrially.

[0148] The chemical composition of the steel sheet can be determined by general analysis methods. For example, for the chemical composition of the steel sheet, it is only necessary to perform measurement using ICP - AES (Inductively Coupled Plasma - Atomic Emission Spectrometry) on the cut powder according to JIS G 1201:2014. Specifically, for example, a test piece of 35 mm square is taken from the vicinity of the 1 / 2 thickness position of the steel sheet, and using an ICPS - 8100 etc. (measurement device) manufactured by Shimadzu Corporation, measurement is performed under the conditions based on the standard line prepared in advance, whereby it can be determined. For C and S that cannot be measured by ICP - AES, combustion - infrared absorption method can be used, for N, inert gas fusion - thermal conductivity method can be used, and for O, inert gas fusion - non - dispersive infrared absorption method can be used for measurement. When a plating layer is provided on the surface of the steel sheet, it is only necessary to remove the plating layer by mechanical grinding and then perform chemical composition analysis.

[0149] [Depth where the area ratio of pearlite in the steel sheet in the thickness direction from the surface is 0 - 20%: 3 - 100 μm]

[0150] In an embodiment of the present invention, the depth at which the area ratio of pearlite is 0 to 20% in the plate thickness direction from the surface of the steel plate is 3 to 100 μm. This feature is associated with the reduction of carbon concentration in the surface layer of the steel plate. Therefore, by having this feature, the bendability of the formed body obtained after hot stamping can be improved based on the effect of improved bendability brought about by the reduction of carbon concentration in the surface layer of the steel plate. In addition, by reducing the amount of pearlite in the surface layer of the steel plate to the above range, the amount of austenite transformed from pearlite during high-temperature heating in hot stamping can be reduced. Therefore, this feature can also be said to be a very important feature in preventing the formation of a carbide re-carbonization path based on austenite along the grain boundaries during hot stamping. From the viewpoint of further enhancing these effects, it is preferable to increase the area of the surface layer with less pearlite. More specifically, the depth at which the area ratio of pearlite is 0 to 20% in the plate thickness direction from the surface of the steel plate is preferably 5 μm or more, or 10 μm or more, more preferably 20 μm or more, or 30 μm or more, and most preferably 40 μm or more, or 50 μm or more. The upper limit of the depth can be, for example, 90 μm or 80 μm.

[0151] [The area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth where the area ratio of pearlite is 0 to 20% in the plate thickness direction from the surface of the steel plate: 0 to 30%]

[0152] In an embodiment of the present invention, the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth where the area ratio of pearlite is 0 to 20% in the plate thickness direction from the surface of the steel plate is 0 to 30%. By controlling the area ratio of pearlite with an equivalent circle diameter of 5 μm or more to be in the range of 0 to 30% in the depth region where the area ratio of pearlite is relatively low as described above, the amount of relatively large pearlite is reduced. Thus, even during high-temperature heating in hot stamping, austenite transformed from pearlite can be dispersed at the grain boundaries, thereby reliably truncating the carbide re-carbonization path based on austenite. Therefore, by significantly suppressing or reducing re-carbonization during high-temperature heating in hot stamping, the effect of improved bendability brought about by the initial reduction of carbon concentration in the surface layer of the steel plate can be fully maintained, and the bendability of the formed body obtained after hot stamping can be significantly improved. From the viewpoint of further enhancing such effects, the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth where the area ratio of pearlite is 0 to 20% is preferably 25% or less, or 20% or less, more preferably 15% or less, or 12% or less, and most preferably 10% or less, or 8% or less. The lower limit of the area ratio of pearlite with an equivalent circle diameter of 5 μm or more can be, for example, 1% or 3%.

[0153] [Measurement of the depth where the area ratio of pearlite is 0 to 20% and the area ratio of pearlite with an equivalent circle diameter of 5 μm or more]

[0154] The depth at which the area ratio of pearlite in the microstructure in the surface layer of the steel sheet is 0 to 20% and the area ratio of pearlite with an equivalent circle diameter of 5 μm or more are determined as follows. First, five samples are collected from the surface of the steel sheet in such a way that a cross-section parallel to the rolling direction and the sheet thickness direction can be observed. Next, these observation surfaces are mirror-polished, etched with picral etching solution, and then the microstructure is observed using a scanning electron microscope (SEM). Regarding the measurement range, for each sample, a rectangular range of 100 μm in the sheet thickness direction and 500 μm in the direction perpendicular to the sheet thickness direction starting from the surface of the steel sheet (in the case where the steel sheet includes a coating, the interface between the steel sheet and the coating) is set as one field of view, and a total of five fields of view of the five samples are measured. In the case where the steel sheet includes a coating, the interface between the steel sheet and the coating can be discriminated by the difference in the hues of the steel sheet and the coating in the backscattered electron image (BSE image) of the SEM. The area ratio of pearlite is calculated, for example, using the point algorithm from a microstructure photograph with a magnification of about 5000 times. Here, a region surrounded by grain boundaries with a crystal orientation difference of 15° or more in ferrite, and in which a ferrite phase and a cementite phase coexist, and the morphology of the cementite is lamellar and / or spherical is defined as pearlite, and its area ratio is calculated. For each sample, the depth position at which the area ratio of pearlite gradually increases to 20% from the surface of the steel sheet is determined. Then, the distance from the determined depth position to the surface is calculated, and their arithmetic mean is determined as "the depth at which the area ratio of pearlite in the steel sheet in the sheet thickness direction from the surface is 0 to 20%". Similarly, for each sample, in the depth region starting from the interface where the area ratio of pearlite becomes 20%, the area ratio of pearlite with an equivalent circle diameter of 5 μm or more is calculated by image processing, and their arithmetic mean is determined as "the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth at which the area ratio of pearlite in the steel sheet in the sheet thickness direction from the surface is 0 to 20%".

[0155] [Area ratio of martensite: less than 1%]

[0156] Regarding the microstructure of the steel plate, as described above, in the surface layer portion of the steel plate, a structure is formed in which the depth where the area ratio of pearlite is 0 to 20% in the plate thickness direction from the surface of the steel plate is set to 3 to 100 μm, and the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth where the area ratio of the pearlite is 0 to 20% is controlled to 0 to 30%. Therefore, there is no particular limitation on other structures. For example, in a preferred embodiment of the present invention, the area ratio of martensite contained in the steel plate is less than 1%. As will be described in detail below regarding the manufacturing method of the steel plate, in order to obtain the above-mentioned pearlite structure, in the cooling process after the annealing process, it is particularly preferable to cool from the controlled temperature of 620 to 670 °C to room temperature at a relatively slow average cooling rate of 10 °C / second or less. In such a case of a relatively slow average cooling rate, almost no martensite precipitates, and even if it precipitates, its area ratio becomes less than 1%. The area ratio of martensite can also be 0.5% or less or 0%.

[0157] [Identification and calculation of the area ratio of martensite]

[0158] The identification and calculation of the area ratio of martensite are carried out as follows. First, a specimen is collected in such a way that the cross-section parallel to the rolling direction and the plate thickness direction of the steel plate becomes the observation surface. Next, the observation surface is mirror-polished, etched with a nitric acid ethanol etching solution, and then the microstructure is observed using a scanning electron microscope (SEM). At the position of 1 / 2 of the plate thickness depth of the above observation surface, a range of 300 μm × 300 μm is photographed at 1000 times magnification. After performing black-and-white binarization processing on the obtained microstructure photograph, image analysis is carried out to determine pearlite, bainite, and ferrite, and the total area ratio of them is obtained using the method based on the "Microscopic test method for crystal grain size of steel" specified in JIS G0551:2020. Since retained austenite is difficult to distinguish from martensite by SEM, the area ratio of retained austenite is measured by X-ray diffraction method. Finally, the area ratio of martensite is determined by subtracting the total area ratio of pearlite, bainite, ferrite, and retained austenite obtained by the above method from 100%.

[0159] [Thickness of the steel plate]

[0160] The thickness of the steel plate is not particularly limited. For example, it is 0.2 mm or more, and it can also be 0.3 mm or more, 0.6 mm or more, 1.0 mm or more, or 2.0 mm or more. Similarly, the thickness of the steel plate is, for example, 6.0 mm or less, and it can also be 5.0 mm or less or 4.0 mm or less.

[0161] [Plating]

[0162] The steel sheet according to an embodiment of the present invention is for the purpose of improving corrosion resistance etc., and may further include a plating layer on the surface. The plating layer may be either a hot-dip plating layer or an electroplating layer. The hot-dip plating layer includes, for example, a hot-dip galvanized layer (GI), an alloyed hot-dip galvanized layer (GA), a hot-dip aluminized layer, a hot-dip Zn-Al alloy layer, a hot-dip Zn-Al-Mg alloy layer, a hot-dip Zn-Al-Mg-Si alloy layer, etc. The electroplating layer includes, for example, an electrogalvanized layer (EG), an electroplated Zn-Ni alloy layer, etc. Preferably, the plating layer is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electrogalvanized layer. The coating amount of the plating layer is not particularly limited and may be a general coating amount. In addition, the steel sheet according to an embodiment of the present invention may not include a plating layer on the surface, specifically, may not include a hot-dip plating layer on the surface, or may not include an electroplating layer on the surface.

[0163] [Mechanical properties]

[0164] The steel sheet according to an embodiment of the present invention, more specifically, the steel sheet before hot stamping is not particularly limited, but for example, has a tensile strength of less than 980 MPa. The tensile strength may also be 950 MPa or less, 900 MPa or less, 850 MPa or less, or 800 MPa or less. The lower limit is not particularly limited, but for example, the tensile strength may also be 500 MPa or more, 550 MPa or more, or 590 MPa or more. According to an embodiment of the present invention, even if the tensile strength of the steel sheet before hot stamping is less than 980 MPa, in the formed body after hot stamping, by having a microstructure mainly composed of martensite, a Vickers hardness of 500 HV or more can be sufficiently achieved. The tensile strength is measured by a tensile test in accordance with JIS Z 2241:2011 based on a JIS No. 5 test piece collected in a direction where the length direction of the test piece becomes parallel to the direction perpendicular to the rolling direction of the steel sheet.

[0165] <Hot stamping formed body>

[0166] In an embodiment of the present invention, in addition to the above-mentioned steel sheet, a hot stamping formed body that can be obtained by hot stamping the steel sheet is further provided. The hot stamping formed body is characterized in that it has the following chemical composition: by mass% including:

[0167] C: 0.27 - 0.60%,

[0168] Si: 0.001 - 3.00%,

[0169] Mn: 0.30 - 3.00%,

[0170] Al: 0.0002 - 2.000%,

[0171] P: 0.1000% or less,

[0172] S: 0.1000% or less,

[0173] N: 0.0100% or less,

[0174] B: 0 to 0.0100%,

[0175] Ti: 0 to 0.1500%,

[0176] Nb: 0 to 0.150%,

[0177] V: 0 to 0.150%,

[0178] Mo: 0 to 1.00%,

[0179] Cr: 0 to 1.0%,

[0180] Cu: 0 to 1.000%,

[0181] Ni: 0 to 1.00%,

[0182] W: 0 to 1.000%,

[0183] Hf: 0 to 0.050%,

[0184] Mg: 0 to 0.050%,

[0185] Zr: 0 to 0.050%,

[0186] Ca: 0 to 0.0100%,

[0187] REM: 0 to 0.3000%,

[0188] Ir: 0 to 1.000%, and

[0189] The balance: composed of Fe and impurities,

[0190] The average C concentration from the surface to a depth of 20 μm in the thickness direction is 0.20 mass% or less,

[0191] It contains 90% or more martensite by area ratio.

[0192] Since the steel plate described above is suitable for manufacturing the hot stamping formed body according to the embodiment of the present invention, hereinafter, the hot stamping formed body according to the embodiment of the present invention will be described in more detail in association with the steel plate described above. However, the following description is merely an illustrative description of the characteristic embodiments of the hot stamping formed body according to the embodiment of the present invention, and is not intended to limit the hot stamping formed body to the one obtained by hot stamping the steel plate described above.

[0193] When the steel sheet according to the embodiment of the present invention described above is applied to hot stamping forming, due to the appropriately modified surface layer structure, even during the high-temperature heating of hot stamping forming, the amount of austenite resulting from pearlite transformation can be reduced, and further, the austenite can be dispersed and present at grain boundaries. Therefore, the carbon re-carbonization path based on austenite can be reliably interrupted. As a result, in the finally obtained hot stamping formed body, the average C concentration from the surface to 20 μm in the thickness direction can be suppressed to 0.20 mass% or less. More specifically, in the case of a steel sheet in which the surface layer of the steel sheet has been made to have a low carbon concentration by ordinary decarburization treatment, although decarburization may also occur on the surface side of the steel sheet during the high-temperature heating of hot stamping forming, the re-carbonization of carbon contained in the body of the steel sheet diffusing to the surface layer becomes dominant, and the state of the initially low-carbon-concentrated surface layer of the steel sheet cannot be maintained. In such a case, in the obtained hot stamping formed body, the carbon concentration of the surface layer becomes relatively high, and the effect of improving the bendability initially brought about by the low-carbon concentration of the steel sheet surface layer cannot be sufficiently exerted. However, according to the embodiment of the present invention, for example, by applying a steel sheet having a structure in which the depth of the area ratio of pearlite from the surface of the steel sheet in the plate thickness direction is set to 3 to 100 μm and the equivalent circle diameter at the depth where the area ratio of pearlite is 0 to 20% is 5 μm or more, and the area ratio of pearlite is controlled to 0 to 30%, the re-carbonization during the high-temperature heating of hot stamping forming can be significantly suppressed or reduced, and the carbon concentration in the surface layer of the obtained hot stamping formed body can be maintained at a relatively low level. Specifically, as described above, the average C concentration from the surface to 20 μm in the thickness direction of the hot stamping formed body can be suppressed to 0.20 mass% or less. As a result, the effect of improving the bendability initially brought about by the low-carbon concentration of the steel sheet surface layer can be sufficiently maintained, and the bendability of the hot stamping formed body can be significantly improved. In addition, if the hot stamping formed body according to the embodiment of the present invention is used, excellent collision resistance characteristics can be achieved by such an improvement in bendability despite the high strength. Therefore, the hot stamping formed body according to the embodiment of the present invention is particularly useful in the automotive field. Hereinafter, the hot stamping formed body according to the embodiment of the present invention will be described in more detail.

[0194] [Chemical composition of hot stamping formed body]

[0195] The chemical composition of the hot stamping formed body is related to the steel sheet according to the embodiments of the present invention not only for the basic components of C, Si, Mn, Al, P, S and N, but also for the optional elements of B, Ti, Nb, V, Mo, Cr, Cu, Ni, W, Hf, Mg, Zr, Ca, REM and Ir as described above. In addition, in the hot stamping formed body, the remaining portion other than these elements is composed of Fe and impurities. The impurities in the hot stamping formed body refer to components such as those mixed in through various factors in the manufacturing process represented by raw materials such as ore and scrap during the industrial production of the hot stamping formed body.

[0196] The chemical composition of the hot stamping formed body can be determined by general analysis methods. For example, the chemical composition of the hot stamping formed body can be determined by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) for the cut powder according to JIS G 1201:2014. Specifically, for example, a test piece of 35 mm square is taken from the vicinity of the 1 / 2 thickness position of the steel sheet, and it can be determined by measuring under the conditions based on the pre-prepared standard line using an ICPS-8100 (measurement device) manufactured by Shimadzu Corporation. C and S that cannot be measured by ICP-AES can be measured by combustion-infrared absorption method, N can be measured by inert gas fusion-thermal conductivity method, and O can be measured by inert gas fusion-non-dispersive infrared absorption method. In the case where a coating is provided on the surface of the hot stamping formed body, the chemical composition can be analyzed after removing the coating by mechanical grinding.

[0197] [Average C concentration of the hot stamping formed body from the surface to 20 μm in the thickness direction: 0.20 mass% or less]

[0198] In the hot stamping formed body according to the embodiment of the present invention, the average C concentration from the surface to 20 μm in the thickness direction is 0.20% by mass or less. By controlling the average C concentration of the hot stamping formed body from the surface to 20 μm in the thickness direction to 0.20% by mass or less, the surface layer portion of the hot stamping formed body is softened, and the bendability of the hot stamping formed body can be significantly improved. From the viewpoint of improving the bendability of the hot stamping formed body, the lower the average C concentration of the hot stamping formed body from the surface to 20 μm in the thickness direction, the more preferable. For example, it may also be 0.18% by mass or less, 0.15% by mass or less, 0.12% by mass or less, 0.10% by mass or less, 0.08% by mass or less, 0.06% by mass or less, or 0.05% by mass or less. The lower limit is not particularly limited. For example, the average C concentration of the hot stamping formed body from the surface to 20 μm in the thickness direction may also be 0.001% by mass or more, 0.005% by mass or more, or 0.01% by mass or more.

[0199] [Measurement of the average C concentration of the hot stamping formed body from the surface to 20 μm in the thickness direction]

[0200] The average C concentration of the hot stamping formed body from the surface to 20 μm in the thickness direction is determined by operating a high-frequency glow discharge optical emission analysis device (GDS) as follows. Specifically, the following method is adopted: The surface of the hot stamping formed body is set in an Ar atmosphere, and in a state where a glow plasma is generated by applying a voltage, the surface of the hot stamping formed body is analyzed in the depth direction while sputtering. Then, the elements contained in the material are identified from the characteristic emission spectral wavelengths of the elements emitted when the atoms are excited in the glow plasma, and the emission intensity of the identified elements is estimated. The data in the depth direction can be estimated from the sputtering time. Specifically, by previously obtaining the relationship between the sputtering time and the sputtering depth using a standard sample, the sputtering time can be converted into the sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the surface of the material. The obtained emission intensity is converted into mass% by making a standard curve. The average C concentration in the region from the surface to 20 μm in the thickness direction measured by operating like this is determined as "the average C concentration of the hot stamping formed body from the surface to 20 μm in the thickness direction". In the GDS measurement, the peaks presumably associated with the contamination of the surface of the hot stamping formed body, etc. are ignored when calculating the average C concentration.

[0201] As described above, in the case of a steel sheet whose surface layer portion has been made to have a low carbon concentration by ordinary decarburization treatment, although decarburization may also occur on the surface side of the steel sheet during high-temperature heating in hot stamping forming, the re-carbonization in which carbon contained in the main body of the steel sheet diffuses to the surface layer portion becomes dominant, and it becomes impossible to maintain the state of the surface layer portion of the steel sheet that has been initially made to have a low carbon concentration. Therefore, even if the surface layer portion of the steel sheet is sufficiently made to have a low carbon concentration by decarburization or the like before hot stamping forming, the C concentration in the surface layer portion of the steel sheet after hot stamping forming sometimes becomes a value closer to the C concentration in the main body. However, according to the embodiment of the present invention, since the diffusion of C from the main body to the surface layer portion is suppressed due to the suppression of re-carbonization, the C concentration in the surface layer portion of the hot stamping formed body can be sufficiently reduced with respect to the C concentration in the main body as compared with the case of a conventional hot stamping formed body. From the viewpoint of improving bendability, the lower the average C concentration of the hot stamping formed body from the surface to 20 μm in the thickness direction is compared with the C content of the hot stamping formed body (main body), the more preferable it is. More specifically, the average C concentration of the hot stamping formed body from the surface to 20 μm in the thickness direction is preferably 0.90 times or less of the C content of the hot stamping formed body, and for example, it may also be 0.85 times or less, 0.80 times or less, 0.78 times or less, 0.75 times or less, 0.70 times or less, 0.60 times or less, 0.50 times or less, or 0.40 times or less. The lower limit is not particularly limited, but for example, the average C concentration of the hot stamping formed body from the surface to 20 μm in the thickness direction may also be 0.01 times or more, 0.03 times or more, 0.05 times or more, or 0.10 times or more of the C content of the hot stamping formed body. In the present invention, the "C content of the hot stamping formed body" means a value measured by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) for the cut powder in accordance with JIS G 1201:2014 using a test piece obtained from the vicinity of the 1 / 2 position of the thickness of the hot stamping formed body.

[0202] [Martensite area ratio of hot stamping formed body: 90% or more]

[0203] The hot stamping formed body according to the embodiment of the present invention contains 90% or more martensite by area ratio. The remaining structure is not particularly limited, but may also be composed of at least one of bainite, ferrite, retained austenite, and pearlite of 10% or less. Since martensite is a very hard structure, in the hot stamping formed body, by containing 90% or more martensite by area ratio, high strength, specifically, a Vickers hardness of 400 HV can be achieved. On the other hand, if the area ratio of martensite becomes low and the proportion of soft structures such as ferrite becomes high, sometimes the Vickers hardness of 400 HV cannot be achieved. Therefore, the larger the area ratio of martensite, the more preferable it is. For example, it may also be 92% or more, 94% or more, 96% or more, or 98% or more. The upper limit of the area ratio of martensite is not particularly limited and may also be 100%.

[0204] [Identification and calculation of area ratio of martensite]

[0205] The identification and calculation of the area ratio of martensite are carried out as follows. First, a specimen is collected in such a way that the cross-section parallel to the rolling direction and the plate thickness direction of the hot stamping formed body becomes the observation surface. Then, the observation surface is mirror-polished, etched with a nitric acid ethanol etching solution, and the microstructure is observed using a scanning electron microscope (SEM). At the position of 1 / 4 depth of the plate thickness on the above observation surface, a range of 300 μm × 300 μm is photographed at a magnification of 1000 times. After performing black-and-white binary processing on the obtained microstructure photograph, image analysis is carried out to identify pearlite, bainite, and ferrite, and the total area ratio of them is obtained using the method based on the "Microscopic test method for crystal grain size of steel" specified in JIS G 0551:2020. Since it is difficult to distinguish retained austenite from martensite by SEM, the area ratio of retained austenite is measured by X-ray diffraction method. Finally, the area ratio of martensite is determined by subtracting the total area ratio of pearlite, bainite, ferrite, and retained austenite obtained by the above method from 100%.

[0206] [Plating]

[0207] The hot stamping formed body according to the embodiment of the present invention aims at improving corrosion resistance, etc., and may further include a plating layer on the surface. The plating layer can be either a hot-dip plating layer or an electroplated layer. The hot-dip plating layer includes, for example, a hot-dip galvanized layer (GI), an alloyed hot-dip galvanized layer (GA), a hot-dip aluminized layer, a hot-dip Zn-Al alloy layer, a hot-dip Zn-Al-Mg alloy layer, a hot-dip Zn-Al-Mg-Si alloy layer, etc. The electroplated layer includes, for example, an electrogalvanized layer (EG), an electroplated Zn-Ni alloy layer, etc. The plating layer is preferably a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electrogalvanized layer. The coating amount of the plating layer is not particularly limited and can be a general coating amount. In addition, the hot stamping formed body according to the embodiment of the present invention may not include a plating layer on the surface, specifically, may not include a hot-dip plating layer on the surface, or may not include an electroplated layer on the surface.

[0208] [Mechanical properties]

[0209] If the hot stamping formed body according to the embodiment of the present invention is used, excellent mechanical properties can be achieved, such as a Vickers hardness of 500 HV or more. More specifically, a Vickers hardness of 500 HV or more can be achieved at the 1 / 2 position of the thickness of the hot stamping formed body. The Vickers hardness is preferably 530 HV or more, more preferably 550 HV or more or 600 HV. The upper limit is not particularly limited, but for example, the Vickers hardness can also be 700 HV or less or 650 HV or less.

[0210] [Measurement of hardness]

[0211] The Vickers hardness is determined as follows. First, a test piece is cut out from an arbitrary position other than the end of the hot stamping formed body in such a way that a cross-section perpendicular to the surface (thickness cross-section) can be observed. The plate thickness cross-section of the test piece is polished using silicon carbide paper with #600 to #1500, and then polished into a mirror surface using a liquid obtained by dispersing diamond powder with a particle size of 1 to 6 μm in a diluent such as alcohol or pure water. This thickness cross-section is used as the measurement surface. Then, a micro-Vickers hardness tester is used to measure the Vickers hardness at a load of 1 kgf and at intervals of more than 3 times the indentation. A total of 20 points are randomly measured near the 1 / 2 position of the thickness of the hot stamping formed body in such a way that the surface layer portion with low carbon concentration is not included, and their arithmetic mean is determined as the hardness of the hot stamping formed body.

[0212] <Manufacturing method of steel plate>

[0213] Next, a preferred manufacturing method of the steel plate according to the embodiment of the present invention will be described. The following description is an illustrative description of a characteristic method intended to manufacture the steel plate according to the embodiment of the present invention, and does not intend to limit the steel plate to the steel plate manufactured by the manufacturing method described below.

[0214] The steel sheet related to the embodiments of the present invention can be manufactured, for example, by performing the following processes: a casting process of casting molten steel with adjusted chemical composition to form a steel slab; a hot rolling process of hot rolling the steel slab to obtain a hot rolled steel sheet; a coiling process of coiling the hot rolled steel sheet; a cold rolling process of cold rolling the coiled hot rolled steel sheet to obtain a cold rolled steel sheet; an annealing process of annealing the cold rolled steel sheet; a cooling process of cooling the annealed cold rolled steel sheet; and a plating process of forming a plating layer on the obtained steel sheet as needed. Instead, it is also possible not to coil after the hot rolling process, perform pickling, and directly perform the cold rolling process. Hereinafter, each process will be described in detail.

[0215] [Casting Process]

[0216] The conditions of the casting process are not particularly limited. For example, immediately following smelting using a blast furnace, an electric furnace, etc., various secondary refining processes are carried out, and then casting can be performed by methods such as ordinary continuous casting and casting using the ingot method.

[0217] [Hot Rolling Process]

[0218] The cast steel slab can be hot rolled to obtain a hot rolled steel sheet. The hot rolling process is carried out by directly or temporarily cooling the cast steel slab and then reheating it for hot rolling. In the case of reheating, the heating temperature of the steel slab can be, for example, 1100 - 1250 °C. In the hot rolling process, rough rolling and finish rolling are usually carried out. The temperature and reduction ratio of each rolling can be appropriately determined according to the desired metal structure and plate thickness. For example, the finish rolling end temperature can be 900 - 1050 °C, and the finish rolling reduction ratio can be 10 - 50%.

[0219] [Coiling Process]

[0220] The hot rolled steel sheet can be coiled at a specified temperature. The coiling temperature can be appropriately determined according to the desired metal structure, etc., and can be, for example, 500 - 800 °C. It is also possible to uncoil before or after coiling and give the hot rolled steel sheet a specified heat treatment. Instead, it is also possible not to perform the coiling process, perform pickling after the hot rolling process, and perform the subsequent cold rolling process.

[0221] [Cold Rolling Process]

[0222] After pickling the hot rolled steel sheet, etc., the hot rolled steel sheet can be cold rolled to obtain a cold rolled steel sheet. The reduction ratio of cold rolling can be appropriately determined according to the desired metal structure and plate thickness, and can be, for example, 20 - 80%. After the cold rolling process, it is also possible to perform air cooling and cool to room temperature, for example.

[0223] [Annealing Process]

[0224] Next, the obtained cold-rolled steel sheet is annealed. The annealing process includes heating the cold-rolled steel sheet to a temperature of 730 to 900°C and holding it for 10 to 300 seconds in an atmosphere with a dew point of -20 to 10°C. By performing the annealing process under such relatively high dew point conditions, the surface layer of the cold-rolled steel sheet can be appropriately decarburized. Therefore, in the finally obtained steel sheet, the depth at which the area ratio of pearlite in the thickness direction from the surface is 0 to 20% can be controlled within the range of 3 to 100 μm. By forming such a structure with a relatively small amount of pearlite in the surface layer of the steel sheet, the average C concentration from the surface to 20 μm in the thickness direction in the hot stamping formed body obtained by subsequent hot stamping can be reliably suppressed to 0.20 mass% or less. If the dew point is lower than -20°C, or the heating temperature is lower than 730°C and / or the holding time is shorter than 10 seconds, the decarburization in the surface layer of the cold-rolled steel sheet becomes insufficient. As a result, in the finally obtained steel sheet, it becomes impossible to set the depth at which the area ratio of pearlite in the thickness direction from the surface is 0 to 20% to 3 μm or more. On the other hand, if the dew point exceeds 10°C, or the heating temperature exceeds 900°C and / or the holding time exceeds 300 seconds, an external oxide layer may be generated on the steel sheet surface, the plating property may be reduced, or the strength of the finally obtained steel sheet and further the hot stamping formed body may be reduced due to excessive decarburization. The dew point is preferably -10 to 5°C, more preferably -5 to 5°C. In addition, the atmosphere in the annealing process can be a reducing atmosphere, more specifically a reducing atmosphere containing nitrogen and hydrogen, such as a reducing atmosphere with 1 to 10% hydrogen (for example, 4% hydrogen and nitrogen balance).

[0225] [Cooling process]

[0226] In order to obtain the desired surface layer structure, the cold-rolled steel sheet after decarburizing the surface layer in the annealing process needs to be appropriately cooled in the subsequent cooling process. Specifically, the cooling process includes cooling from the heating temperature of the annealing process to the control temperature of 620 to 670°C at an average cooling rate of 20°C / second or more (first cooling), and cooling from this control temperature to room temperature at an average cooling rate of 10°C / second or less (second cooling). Hereinafter, the first cooling and the second cooling will be described in more detail.

[0227] [First cooling]

[0228] In the first cooling, it is important to suppress the precipitation of pearlite at high temperatures. More specifically, pearlite that precipitates at high temperatures from the heating temperature of 730 - 900°C to the control temperature of 620 - 670°C in the annealing process diffuses quickly, and thus is likely to diffuse to the grain boundaries after precipitation and form pearlite along the grain boundaries. The pearlite formed along the grain boundaries undergoes austenite phase transformation during the high-temperature heating of hot stamping, thereby forming a carbide re-carbonization path of austenite-based carbon along the grain boundaries, which promotes the re-carbonization of carbon in the body to the steel surface layer. Therefore, in the temperature range from the heating temperature of the annealing process to the above control temperature, it is extremely important to cool the cold-rolled steel sheet at a relatively fast average cooling rate of 20°C / second or more to suppress the precipitation of pearlite at high temperatures in the steel sheet surface layer. If the above average cooling rate is lower than 20°C / second and / or the control temperature exceeds 670°C, pearlite precipitates at high temperatures where diffusion is fast, thus promoting the formation of pearlite along the grain boundaries. As a result, in the finally obtained steel sheet, the area ratio of pearlite with an equivalent circle diameter of 5μm or more at a depth where the area ratio of pearlite in the plate thickness direction from the surface is 0 - 20% exceeds 30%, making it impossible to sufficiently suppress or reduce re-carbonization during the high-temperature heating of hot stamping.

[0229] [Second cooling]

[0230] On the other hand, in the second cooling after the first cooling, it is important to precipitate pearlite at low temperatures where diffusion is relatively slow. More specifically, by cooling from the control temperature of 620 - 670°C to room temperature at an average cooling rate of 10°C / second or less, pearlite can be precipitated. Pearlite that precipitates in the low-temperature range below such a control temperature does not form in a shape that connects along the grain boundaries due to relatively slow diffusion, and pearlite can be dispersed at the grain boundaries. In such a microstructure, even during the high-temperature heating of hot stamping, austenite transformed from pearlite above the A c1 point can also be dispersed at the grain boundaries, so that the carbide re-carbonization path of austenite-based carbon can be reliably truncated. On the other hand, if the above average cooling rate exceeds 10°C / second and / or the control temperature is lower than 620°C, martensite and bainite precipitate mainly instead of pearlite, and in the finally obtained steel sheet, it becomes impossible to set the depth where the area ratio of pearlite in the plate thickness direction from the surface is 0 - 20% to 100μm or less. The phase transformation rate of martensite and bainite to austenite is faster than that of pearlite, and they immediately transform to austenite just above the A c1 point. Therefore, compared with the case of pearlite, the time during which the ferrite-austenite two-phase structure is exposed to high temperatures during hot stamping becomes longer. In such a situation, it also becomes impossible to sufficiently suppress or reduce re-carbonization because re-carbonization paths are likely to form at the grain boundaries.

[0231] [Coating process]

[0232] For the purpose of improving corrosion resistance, etc., the surface of the obtained cold-rolled steel sheet can also be subjected to a coating treatment. The coating treatment can be hot-dip coating, alloyed hot-dip coating, electroplating, etc. For example, as the coating treatment, the steel sheet can be subjected to hot-dip galvanizing treatment, or alloying treatment can be carried out after hot-dip galvanizing treatment. The specific conditions of the coating treatment and the alloying treatment are not particularly limited and can be any suitable conditions known to those skilled in the art. For example, in addition, the coating treatment can also be a hot-dip coating method, an electroplating method, a vapor deposition coating method, spraying, or a cold spraying method, etc. Other conditions of the coating process can be appropriately set as long as the thickness and adhesion amount of the coating layer are considered. For example, by immersing the cold-rolled steel sheet in a plating bath and then lifting it out, immediately blowing N2 gas or air by the gas wiping method, and then cooling it, the adhesion amount of the coating layer can be adjusted to within a specified range, such as 20 to 200 g / m per single side 2 in the range of.

[0233] The steel sheet manufactured by this manufacturing method can form the following structure in the surface layer part of the steel sheet: The depth where the area ratio of pearlite is 0 to 20% in the plate thickness direction from the surface is set to 3 to 100 μm, and the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth where the area ratio of pearlite is 0 to 20% is controlled to 0 to 30%. Therefore, even when exposed to a high temperature such as during hot stamping forming, the re-carbonization of carbon in the body to the steel surface layer part can be significantly suppressed or reduced. Therefore, the bending property improvement effect brought about by the initial low carbon concentration of the steel surface layer part can be fully maintained, and the bending property of the formed body obtained after hot stamping forming can be significantly improved.

[0234] [Manufacturing method of hot stamping formed body]

[0235] The hot stamping formed body according to an embodiment of the present invention can be manufactured by performing a hot stamping process of hot stamping the steel sheet obtained by the method described above. In particular, from the viewpoint of obtaining a desired hard structure, it is preferable to load the steel sheet into a furnace at 800 to 1000 °C, and after the temperature of the steel sheet reaches a specified temperature, for example, the furnace temperature - 10 °C, hold it in the furnace for 60 to 600 seconds. If the heating temperature is lower than 800 °C and / or the holding time is lower than 60 seconds, austenitization may sometimes be insufficient, and the area ratio of the desired hard structure (that is, the area ratio of martensite is 90% or more) cannot be obtained, and a Vickers hardness of 500 HV or more cannot be achieved in the finally obtained hot stamping formed body. The heating atmosphere is not particularly limited and can be normal conditions. For example, as long as it is in the atmosphere, a gas combustion atmosphere in which the ratio of air to fuel is controlled, or a nitrogen atmosphere, the dew point can also be controlled in these gases. After heating and holding in the furnace, the steel sheet is taken out of the furnace, and then after the steel sheet reaches a specified temperature, for example, a specified temperature of 850 °C or lower, hot stamping can be performed under normal conditions. After hot stamping, although not particularly limited, for example, it is sufficient to cool to a temperature range of 250 °C or lower at an average cooling rate of 20 °C / second or more.

[0236] The hot stamping formed body manufactured by this manufacturing method can control the average C concentration from the surface to 20 μm in the thickness direction to 0.20 mass% or less, and contains 90% or more of martensite in terms of area ratio. Therefore, compared with the conventional hot stamping formed body, although it has high strength, due to the improvement of bendability based on the softening of the surface layer part, more excellent collision resistance characteristics can be achieved. Therefore, it is particularly useful in the field of automobiles and can contribute to the development of the industry.

[0237] Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited by any of these examples.

[0238] Examples

[0239] In the following examples, the steel sheets and hot stamping formed bodies according to the embodiments of the present invention were manufactured under various conditions, and the characteristics of the manufactured steel sheets and hot stamping formed bodies were investigated.

[0240] [Manufacture of steel sheet]

[0241] First, the molten steel is cast by continuous casting to form a steel billet with the chemical composition shown in Table 1. After the steel billet is temporarily cooled, it is reheated to 1200 °C and hot-rolled, and then coiled at a temperature below 600 °C. The hot rolling is carried out by rough rolling and finish rolling. The finishing temperature of the finish rolling is 900 - 1050 °C, and the reduction ratio of the finish rolling is 30%. Then, pickling is performed on the obtained hot-rolled steel sheet, and then cold-rolled with a reduction ratio of 50% to obtain a cold-rolled steel sheet with a thickness of 1.6 mm. Then, for the obtained cold-rolled steel sheet, an annealing process is carried out in a furnace with an oxygen concentration of 20 ppm or less in a mixed gas atmosphere of 4% hydrogen and nitrogen balance under the conditions shown in Table 1, and then a cooling process is carried out under the conditions shown in Table 1 in the same way to manufacture the steel sheet.

[0242] [Manufacture of hot stamping formed body]

[0243] First, the manufactured steel sheet is loaded into an atmospheric heating furnace at 900 °C. After the temperature of the steel sheet reaches the furnace temperature - 10 °C, it is held for 100 seconds. Then, the steel sheet is taken out of the furnace and clamped and quenched with a flat die at a temperature around room temperature to obtain a hot stamping formed body.

[0244] [Evaluation of hardness after hot stamping (HS)]

[0245] A test piece is cut out from an arbitrary position except the end of the obtained hot stamping formed body in a manner that allows observation of a cross-section perpendicular to the surface (plate thickness cross-section). The plate thickness cross-section of the test piece is ground using #600 - #1500 silicon carbide paper, and then liquid-finished into a mirror surface using a liquid obtained by dispersing diamond powder with a particle size of 1 - 6 μm in a diluent such as alcohol or pure water. This plate thickness cross-section is used as the measurement surface. Then, a micro-Vickers hardness tester is used to measure the Vickers hardness at a load of 1 kgf at intervals of more than 3 times the indentation. A total of 20 points are randomly measured near the 1 / 2 position of the thickness of the hot stamping formed body in a manner that does not include the surface layer portion carbon-concentrated to a low carbon concentration, and their arithmetic mean is determined as the hardness after hot stamping (HS), and the evaluation is carried out as follows.

[0246] AAA: Hardness after HS exceeds 600 HV

[0247] AA: Hardness after HS exceeds 530 HV and is 600 HV or less

[0248] A: Hardness after HS is 500 - 530 HV

[0249] B: Hardness after HS is less than 500 HV

[0250] [Evaluation of bendability]

[0251] The bendability is evaluated by a bending test in accordance with VDA (Verband der Automobilindustrie) 238-100:2017-04. More specifically, the maximum bending angle α (°) is obtained by converting the displacement at the maximum load obtained in the bending test into an angle based on the VDA standard, and the evaluation is carried out as follows.

[0252] AAA: The maximum bending angle α exceeds 80°

[0253] AA: The maximum bending angle α exceeds 70° and is 80° or less

[0254] A: The maximum bending angle α is 60 - 70°

[0255] B: The maximum bending angle α is less than 60°

[0256] The hot stamping formed body having a hardness after HS evaluated as AAA, AA, and A and a bendability evaluated as AAA, AA, and A is evaluated as a high-strength and excellent bendability hot stamping formed body. The results are shown in Table 1. In the hot stamping formed body shown in Table 1, the remaining structure other than martensite is bainite, ferrite, retained austenite, and / or pearlite.

[0257] [Table 1-1]

[0258]

[0259] [Table 1-2]

[0260]

[0261] Referring to Table 1, in the case of Comparative Example 35, since the C content is low, the hardness after HS decreases. In the case of Comparative Example 36, it is considered that since the heating temperature in the annealing process is low, decarburization in the surface layer portion of the cold-rolled steel sheet is insufficient. As a result, it is impossible to set the depth at which the area ratio of pearlite in the plate thickness direction from the surface of the steel sheet is 0 to 20% to 3 μm or more, and the average C concentration of the hot stamping formed body obtained from this steel sheet from the surface to 20 μm in the thickness direction also becomes high, and the bendability decreases. In the case of Comparative Example 37, it is considered that since the holding time in the annealing process is short, decarburization in the surface layer portion of the cold-rolled steel sheet is insufficient in the same way. As a result, it is impossible to set the depth at which the area ratio of pearlite in the plate thickness direction from the surface of the steel sheet is 0 to 20% to 3 μm or more, and the average C concentration of the hot stamping formed body obtained from this steel sheet from the surface to 20 μm in the thickness direction also becomes high, and the bendability decreases. In the case of Comparative Example 38, it is considered that since the dew point in the annealing process is low, decarburization in the surface layer portion of the cold-rolled steel sheet is insufficient in the same way. As a result, it is impossible to set the depth at which the area ratio of pearlite in the plate thickness direction from the surface of the steel sheet is 0 to 20% to 3 μm or more, and the average C concentration of the hot stamping formed body obtained from this steel sheet from the surface to 20 μm in the thickness direction also becomes high, and the bendability decreases. In the case of Comparative Example 39, it is considered that since the average cooling rate of the first cooling in the cooling process is low, pearlite precipitates at a high temperature and forms along the grain boundaries. As a result, the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth where the area ratio of pearlite in the plate thickness direction from the surface of the steel sheet is 0 to 20% exceeds 30%. In addition, in connection with this, it is considered that the re-carbonization during the high-temperature heating of the hot stamping is promoted, and the average C concentration of the hot stamping formed body from the surface to 20 μm in the thickness direction also becomes high, and the bendability decreases. In the case of Comparative Example 40, it is considered that since the control temperature of the first cooling in the cooling process is high, pearlite also precipitates at a high temperature and forms along the grain boundaries in the same way. As a result, the area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth where the area ratio of pearlite in the plate thickness direction from the surface of the steel sheet is 0 to 20% exceeds 30%. In addition, in connection with this, it is considered that the re-carbonization during the high-temperature heating of the hot stamping is promoted, and the average C concentration of the hot stamping formed body from the surface to 20 μm in the thickness direction also becomes high, and the bendability decreases. In the case of Comparative Example 41, since the control temperature of the second cooling in the cooling process is low, bainite mainly precipitates instead of pearlite. As a result, it is impossible to set the depth at which the area ratio of pearlite in the plate thickness direction from the surface of the steel sheet is 0 to 20% to the desired depth. In addition, in connection with this, it is considered that the re-carbonization during the high-temperature heating of the hot stamping is promoted, and the average C concentration of the hot stamping formed body from the surface to 20 μm in the thickness direction also becomes high, and the bendability decreases.In Comparative Example 42, since the average cooling rate during the two-stage cooling in the cooling process was high, bainite, rather than pearlite, mainly precipitated in the same manner. As a result, it was not possible to set the depth at which the area ratio of pearlite in the plate thickness direction from the surface of the steel sheet was 0 to 20% to the desired depth. In addition, associated therewith, it was considered that the carbide re-precipitation during the high-temperature heating for hot stamping forming was promoted, and the average C concentration from the surface to 20 μm in the thickness direction of the hot stamping formed body also increased, resulting in a decrease in bendability.

[0262] In contrast, in the steel sheets according to all the examples, by having a specified chemical composition, the depth at which the area ratio of pearlite in the plate thickness direction from the surface of the steel sheet was 0 to 20% was set to 3 to 100 μm, and the area ratio of pearlite having an equivalent circle diameter of 5 μm or more at the depth where the area ratio of pearlite was 0 to 20% was controlled to 0 to 30%. Thus, even when exposed to a high temperature of 900 °C by hot stamping forming, in the obtained hot stamping formed body, the average C concentration from the surface to 20 μm in the thickness direction could be controlled to 0.20 mass% or less. As a result, high strength and high bendability could be achieved. In particular, for Examples 8 to 12 in which the depth at which the area ratio of pearlite was 0 to 20% was set to 10 to 100 μm and the area ratio of pearlite having an equivalent circle diameter of 5 μm or more at that depth was controlled to 0 to 15%, the average C concentration from the surface to 20 μm in the thickness direction of the hot stamping formed body was reduced to 0.10 mass% or less. As a result, the evaluation of bendability became AA, and the bendability was further improved. Furthermore, for Examples 13 to 34 in which the depth at which the area ratio of pearlite was 0 to 20% was set to 30 to 100 μm and the area ratio of pearlite having an equivalent circle diameter of 5 μm or more at that depth was controlled to 0 to 15%, the average C concentration from the surface to 20 μm in the thickness direction of the hot stamping formed body was reduced to 0.05 mass% or less. As a result, the evaluation of bendability became AAA, and the bendability was further improved.

Claims

1. A steel plate, characterized in that, It has the following chemical composition: by mass% it contains: C：0.27～0.60％、 Si: 0.001 to 3.00%, Mn: 0.30 to 3.00%, Al：0.0002～2.000％、 P: 0.1000% or less, S: 0.1000% or less, N: 0.0100% or less, B：0～0.0100％、 Ti: 0 to 0.1500%, Nb: 0 to 0.150%, V：0～0.150％、 Mo: 0 to 1.00%, Cr：0～1.0％、 Cu: 0 to 1.000%, Ni: 0 to 1.00%, W：0～1.000％、 Hf: 0 to 0.050%, Mg: 0 to 0.050%, Zr：0～0.050％、 Ca: 0 to 0.0100%, REM: 0 to 0.3000%, Ir: 0 to 1.000%, and The balance: consists of Fe and impurities, The depth at which the area ratio of pearlite is 0 to 20% in the plate thickness direction from the surface is 3 to 100 μm, The area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth where the area ratio of pearlite is 0 to 20% is 0 to 30%.

2. The steel plate according to claim 1, characterized in that, The depth at which the area ratio of pearlite is 0 to 20% is 10 to 100 μm.

3. The steel plate according to claim 2, characterized in that, The depth at which the area ratio of pearlite is 0 to 20% is 30 to 100 μm.

4. The steel plate according to any one of claims 1 to 3, characterized in that, The area ratio of pearlite with an equivalent circle diameter of 5 μm or more at the depth where the area ratio of pearlite is 0 to 20% is 0 to 15%.

5. A hot stamping formed body, characterized in that, It has the following chemical composition: by mass% it contains: C：0.27～0.60％、 Si: 0.001 to 3.00%, Mn: 0.30 to 3.00%, Al：0.0002～2.000％、 P: 0.1000% or less, S: 0.1000% or less, N: 0.0100% or less, B：0～0.0100％、 Ti: 0 to 0.1500%, Nb: 0 to 0.150%, V：0～0.150％、 Mo: 0 to 1.00%, Cr：0～1.0％、 Cu: 0 to 1.000%, Ni: 0 to 1.00%, W：0～1.000％、 Hf: 0 to 0.050%, Mg: 0 to 0.050%, Zr：0～0.050％、 Ca: 0 to 0.0100%, REM: 0 to 0.3000%, Ir: 0 to 1.000%, and The balance: consists of Fe and impurities, The average C concentration from the surface to 20 μm in the thickness direction is 0.20 mass% or less, It contains 90% or more martensite by area ratio.

6. The hot stamping formed body according to claim 5, wherein The average C concentration is 0.10 mass% or less.

7. The hot stamping formed body according to claim 5 or 6, characterized in that, The average C concentration is 0.05 mass% or less.

Images