Hot-stamp molded body

Abstract

This hot-stamp molded body is characterized by having a predetermined chemical composition, and characterized in that: the average Vickers hardness (Hvi) at a position one-quarter of the plate thickness from the surface is 615 HV or more; the ratio of the average Vickers hardness (Hvs) at a depth of 20 μm from the surface to Hvi (Hvs / Hvi) is 0.50 to 0.75; and the gradient of Vickers hardness in a region from a depth of 20 μm to a depth of 60 μm from the surface is 3.5 HV / μm or less.

Description

Hot stamped compact

[0001] This application claims priority to Japanese Patent Application No. 2024-100138, filed on June 21, 2024, the contents of which are incorporated herein by reference.

[0002] In the field of automotive steel sheets, in response to recent tightening of environmental regulations and collision safety standards, the application of steel sheets having high tensile strength (high-strength steel sheets) is expanding in order to improve both fuel economy and collision safety. However, as the strength of steel sheets increases, the press formability of the steel sheets decreases, making it difficult to manufacture products with complex shapes.

[0003] Specifically, as the strength of steel sheets increases, their ductility decreases, resulting in the problem of fracture at highly processed locations when they are processed into complex shapes. Furthermore, as the strength of steel sheets increases, residual stress after processing causes springback and wall warping, resulting in poor dimensional accuracy. Therefore, it is not easy to press-form high-strength steel sheets, particularly those with a tensile strength of 780 MPa or more, into products with complex shapes. While roll forming, rather than press forming, makes it easier to process high-strength steel sheets, its application is limited to parts with a uniform cross section in the longitudinal direction.

[0004] Therefore, in recent years, hot stamping technology has been adopted as a technique for press-forming difficult-to-form materials such as high-strength steel sheets.Hot stamping technology is a hot forming technology in which the material to be formed is heated and then formed.

[0005] In this technology, the material is heated before being formed. Therefore, the steel is soft during forming and has good formability. This allows even high-strength steel sheets to be formed into complex shapes with high precision. In addition, with hot stamping technology, the steel is quenched at the same time as it is formed using a press die, so the steel part after forming has sufficient strength.

[0006] For example, Patent Document 1 discloses that hot stamping technology can impart a tensile strength of 1400 MPa or more to a formed steel member.

[0007] On the other hand, automobiles are also required to have crashworthiness. The crashworthiness of an automobile is evaluated based on the crushing strength and absorbed energy in crash tests of the entire vehicle body and some of its components. Since crushing strength in particular is highly dependent on the strength of the material, the demand for ultra-high strength steel components for automobiles is increasing dramatically.

[0008] However, in general, the deformability of steel members decreases as their strength increases. Therefore, they may fracture early during a collision or at a location where deformation is concentrated, resulting in an insufficient crushing strength commensurate with the material strength, and insufficient energy absorption. Therefore, in order to improve the collision safety of automobiles, it is necessary to improve not only the material strength but also the deformability of the steel members used. Therefore, in order to apply ultra-high strength steel members such as hot-stamped steel bodies to vehicle bodies, a technology is needed to provide steel members that have higher deformability than conventional steel members and that exhibit sufficient energy absorption during a collision.

[0009] Patent Document 2 discloses a hot press-formed product having excellent toughness and a tensile strength of 1.8 GPa or more. Patent Document 3 discloses a steel material having an extremely high tensile strength of 2.0 GPa or more and also having good toughness and ductility. Patent Document 4 discloses a steel material having a high tensile strength of 1.8 GPa or more and also having good toughness. Patent Document 5 discloses a steel material having an extremely high tensile strength of 2.0 GPa or more and also having good toughness.

[0010] Japanese Patent Publication No. 2002-102980 Japanese Patent Publication No. 2012-180594 Japanese Patent Publication No. 2012-1802 International Publication No. 2015 / 182596 International Publication No. 2015 / 182591

[0011] However, Patent Documents 1 to 5 have room for improvement in terms of obtaining better deformability when using ultra-high strength steel materials as automobile parts.

[0012] The present disclosure has been made in view of the above circumstances, and has an object to provide a hot-stamped steel sheet having high strength and excellent deformability.

[0013] The gist of the present invention is as follows. [1] Chemical composition, in mass%, C: 0.380 to 1.000%, Si: 0.01 to 2.00%, Mn: 0.01% or more, less than 1.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, N: 0.0150% or less, O: 0.1000% or less, B: 0.0003 to 0.0100%, Cr: 0 to 0.50%, Mo: 0 to 0.50%, W: 0 to 3.00%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, Co: 0 to 1.000%, 1. A hot stamped product comprising: V: 0 to 0.500%, Zr: 0 to 0.100%, Ca: 0 to 1.0000%, Mg: 0 to 1.0000%, REM: 0 to 0.0050%, Sn: 0 to 0.100%, Sb: 0 to 0.020%, and As: 0 to 0.100%, with the balance being Fe and impurities; Hvi, which is an average value of Vickers hardness at a position from the surface to a quarter of the sheet thickness, is 615 HV or more; Hvs / Hvi, which is a ratio of Hvs, which is an average value of Vickers hardness at a depth of 20 μm from the surface, to Hvi, is 0.50 to 0.75; and a gradient of Vickers hardness in a region from a depth of 20 μm from the surface to a depth of 60 μm from the surface, is 3.5 HV / μm or less. [2] The hot-stamped steel according to the above item [1], wherein Bs_max / Bi, which is a ratio of Bs_max, which is the maximum value of the B concentration in a region from the surface to a depth of 10 μm from the surface, to Bi, which is the B concentration at a depth of 100 μm from the surface, is 0.75 or more.[3] The chemical composition, in mass%, is: Cr: 0.01 to 0.50%, Mo: 0.01 to 0.50%, W: 0.01 to 3.00%, Ti: 0.001 to 0.100%, Nb: 0.001 to 0.100%, Ni: 0.01 to 1.00%, Cu: 0.01 to 1.00%, Co: 0.001 to 1.000%, V: 0.001 to 0.500%, Zr: 0.001 to 0.100%, Ca: 0.0001 to 1.0000%, Mg: 0.0001 to 1.0000%, REM: 0.0001 to 0.0050%, Sn: 0.001 to 0.100%, The hot-stamped steel according to the above [1] or [2], characterized by containing one or more of Sb: 0.001 to 0.020%, and As: 0.001 to 0.100%.

[0014] According to the above aspects of the present disclosure, it is possible to provide a hot-stamped steel sheet having high strength and excellent deformability.

[0015] The hot stamped steel according to this embodiment will be described in detail below. First, the reasons for limiting the chemical composition of the hot stamped steel according to this embodiment will be described.

[0016] The hot-stamped steel according to this embodiment has the following chemical composition. When the hot-stamped steel consists only of a steel plate, it can be said that the hot-stamped steel has the chemical composition shown below. Note that the numerical ranges described below, separated by "to" include the lower and upper limits. Numerical values ​​indicated as "less than" and "greater than" do not include the numerical range. All percentages regarding the chemical composition represent mass %.

[0017] The hot-stamped steel according to this embodiment has a chemical composition, in mass%, of 0.380 to 1.000% C, 0.01 to 2.00% Si, 0.01% or more but less than 1.00% Mn, 0.100% or less P, 0.0100% or less S, 0.001 to 1.000% Al, 0.0150% or less N, 0.1000% or less O, 0.0003 to 0.0100% B, and the balance being Fe and impurities. Each element will be described in detail below.

[0018] C: 0.380 to 1.000% C is an element that significantly affects the strength of the hot-stamped steel. If the C content is less than 0.380%, the desired strength cannot be obtained in the hot-stamped steel. Therefore, the C content is set to 0.380% or more. The C content is preferably more than 0.400%, 0.450% or more, or 0.500% or more. On the other hand, if the C content exceeds 1.000%, the strength of the hot-stamped steel becomes too high, and the deformability of the hot-stamped steel decreases. Therefore, the C content is set to 1.000% or less. The C content is preferably 0.800% or less, 0.750% or less, 0.700% or less, or 0.650% or less.

[0019] Si: 0.01 to 2.00% Si has temper softening resistance and is effective in suppressing strength reduction due to autotempering during hot stamp quenching. Therefore, the Si content is set to 0.01% or more. The Si content is preferably 0.02% or more, 0.03% or more, 0.15% or more, or 0.20% or more. On the other hand, if the Si content exceeds 2.00%, problems with surface scale occur. That is, after pickling to remove scale formed during hot rolling, patterns due to surface irregularities occur, resulting in poor surface appearance. Furthermore, when plating is performed on the surface of a steel sheet for hot stamping, platability deteriorates. Therefore, the Si content is set to 2.00% or less. The Si content is preferably 1.50% or less, 1.00% or less, 0.70% or less, 0.50% or less, 0.45% or less, or 0.40% or less.

[0020] Mn: 0.01% or more, less than 1.00% Mn is an element that improves the strength of the hot-stamped body and the hardenability of the steel. If the Mn content is less than 0.01%, the strength of the hot-stamped body decreases. Therefore, the Mn content is set to 0.01% or more. The Mn content is preferably 0.10% or more, 0.20% or more, 0.30% or more, or 0.40% or more. On the other hand, if the Mn content is 1.00% or more, the deformability of the hot-stamped body deteriorates. Therefore, the Mn content is set to less than 1.00%. The Mn content is preferably 0.80% or less or 0.60% or less.

[0021] P: 0.100% or less P is an element that segregates at grain boundaries and reduces the strength of the grain boundaries. If the P content exceeds 0.100%, the strength of the grain boundaries is significantly reduced, resulting in a decrease in the deformability of the hot-stamped steel. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.050% or less, 0.040% or less, or 0.030% or less. The P content may be 0%. However, since excessive reduction of the P content increases refining costs, the P content may be set to 0.001% or more.

[0022] S: 0.0100% or less S is an element that affects non-metallic inclusions in steel and deteriorates the deformability of the hot-stamped steel. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less or 0.0050% or less. The S content may be 0%. However, since excessive reduction of the S content increases the manufacturing cost of the desulfurization process, the S content may be set to 0.0001% or more.

[0023] Al: 0.001 to 1.000% Al is an element used as a deoxidizer for molten steel. If deoxidation is insufficient, the deformability of the hot-stamped steel will deteriorate due to the excess oxides produced. In order to sufficiently deoxidize the molten steel, the Al content is set to 0.001% or more. The Al content is preferably 0.010% or more or 0.030% or more. On the other hand, if the Al content exceeds 1.000%, many nonmetallic inclusions will be formed, making the hot-stamped steel more susceptible to surface defects. Therefore, the Al content is set to 1.000% or less. The Al content is preferably 0.700% or less, 0.500% or less, 0.300% or less, 0.200% or less, or 0.100% or less.

[0024] N: 0.0150% or less If the N content exceeds 0.0150%, coarse nitrides are formed in the steel, significantly deteriorating the deformability of the hot-stamped steel. Therefore, the N content is set to 0.0150% or less. The N content is preferably 0.0100% or less, 0.0080% or less, or 0.0060% or less. The N content may be 0%. However, reducing the N content to less than 0.0005% significantly increases the denitrification cost, which is economically undesirable. Therefore, the N content is preferably set to 0.0005% or more. The N content is more preferably set to 0.0010% or more.

[0025] O: 0.1000% or less If O is contained in a large amount in steel, it forms coarse oxides that become the starting points of fracture. As a result, the deformability of the hot-stamped steel deteriorates. Therefore, the O content is set to 0.1000% or less. The O content is preferably 0.0080% or less, 0.0050% or less, or 0.0020% or less. The O content may be 0%. However, reducing the O content to less than 0.0005% significantly increases the cost of deoxidation, which is economically undesirable. Therefore, the O content may be set to 0.0005% or more, or 0.0010% or more.

[0026] B: 0.0003 to 0.0100% B is an element that improves the hardenability of steel and thereby improves the strength of the hot-stamped body. If the B content is less than 0.0003%, the above effect cannot be obtained, and the strength of the hot-stamped body decreases. Therefore, the B content is set to 0.0003% or more. The B content is preferably 0.0005% or more, 0.0007% or more, or 0.0010% or more. On the other hand, if the B content exceeds 0.0100%, the above effect saturates, and cracks may occur during hot rolling and borides may deteriorate the deformability of the hot-stamped body. Therefore, the B content is set to 0.0100% or less. The B content is preferably 0.0080% or less or 0.0050% or less.

[0027] The balance of the chemical composition of the hot stamped steel according to this embodiment may be Fe and impurities. Examples of impurities include elements that are inevitably mixed in from steel raw materials or scrap and / or during the steelmaking process and are permissible to the extent that they do not impair the properties of the hot stamped steel according to this embodiment.

[0028] The hot-stamped steel according to this embodiment may contain the following elements as optional elements in place of a portion of Fe. When the following optional elements are not contained, the content is 0%.

[0029] Cr: 0 to 0.50% Cr is an element that improves the hardenability of steel and has the effect of improving the strength of hot-stamped steel. Cr also has the effect of improving the corrosion resistance of hot-stamped steel. Therefore, Cr may be added as necessary. To ensure the above effects, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.02% or more, 0.03% or more, 0.04% or more, or 0.05% or more. On the other hand, if the Cr content exceeds 0.50%, carbides present after hot rolling, cold rolling, or annealing (including after plating) may be stabilized, delaying the dissolution of carbides during heating during hot stamping and reducing hardenability. As a result, the strength and deformability of the hot-stamped steel may be reduced. Therefore, the Cr content is set to 0.50% or less. The Cr content is preferably set to 0.45% or less, 0.40% or less, or 0.30% or less.

[0030] Mo: 0 to 0.50% Mo is an element that improves the hardenability of steel and has the effect of improving the strength of hot-stamped steel. Mo also has the effect of improving the corrosion resistance of hot-stamped steel. Therefore, Mo may be added as needed. To ensure the above effects, the Mo content is preferably 0.01% or more. The Mo content is more preferably 0.05% or more, 0.07% or more, 0.10% or more, or 0.15% or more. On the other hand, if the Mo content exceeds 0.50%, carbides present after hot rolling, cold rolling, or annealing (including after plating) may be stabilized, delaying the dissolution of carbides during heating during hot stamping and reducing hardenability. As a result, the strength and deformability of the hot-stamped steel may be reduced. Therefore, the Mo content is set to 0.50% or less. The Mo content is preferably 0.45% or less, 0.40% or less, or 0.35% or less.

[0031] W: 0 to 3.00% W is an element that improves the hardenability of steel and has the effect of improving the strength of hot-stamped steel. W also has the effect of improving the corrosion resistance of hot-stamped steel. Therefore, W may be added as necessary. To ensure the above effects, the W content is preferably 0.01% or more. The W content is more preferably 0.10% or more, 0.15% or more, or 0.20% or more. On the other hand, a W content of more than 3.00% may deteriorate hot workability and reduce productivity, or may delay the dissolution of carbides during heating during hot stamping, reducing hardenability and reducing the strength and deformability of the hot-stamped steel. Therefore, the W content is set to 3.00% or less. The W content is preferably 2.00% or less, 1.00% or less, or 0.50% or less.

[0032] Ti: 0 to 0.100% Ti forms carbonitrides in steel and improves the strength of hot-stamped steel through precipitation strengthening. Furthermore, Ti fixes N as nitrides, suppressing BN formation and enhancing the hardenability-improving effect of B. Therefore, Ti may be added as needed. To ensure the above effects, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.010% or more, 0.015% or more, or 0.020% or more. On the other hand, if the Ti content exceeds 0.100%, excessive carbonitrides are formed, degrading the deformability of the hot-stamped steel. Therefore, the Ti content is set to 0.100% or less. The Ti content is preferably 0.080% or less, 0.060% or less, or 0.040% or less.

[0033] Nb: 0 to 0.100% Nb forms fine carbides in steel and has the effect of increasing the strength of the hot-stamped body through precipitation strengthening. Nb also has the effect of improving the strength and deformability of the hot-stamped body by refining the metal structure. Therefore, Nb may be added as necessary. To ensure the above effect, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.010% or more, 0.020% or more, 0.025% or more, or 0.035% or more. On the other hand, if the Nb content exceeds 0.100%, a large amount of carbonitrides is formed, degrading the deformability of the hot-stamped body. Therefore, the Nb content is set to 0.100% or less. The Nb content is preferably 0.090% or less, 0.070% or less, or 0.050% or less.

[0034] Ni: 0 to 1.00% Ni is an element that improves the hardenability of steel and has the effect of improving the strength of hot-stamped steel. Ni also has the effect of improving the corrosion resistance of hot-stamped steel. Therefore, Ni may be added as necessary. To ensure the above effects, the Ni content is preferably 0.01% or more. The Ni content is more preferably 0.04% or more, 0.08% or more, or 0.10% or more. On the other hand, even if a large amount of Ni is added, the above effects saturate and alloy costs increase. Furthermore, the deformability of the hot-stamped steel may deteriorate. Therefore, the Ni content is set to 1.00% or less. The Ni content is preferably 0.90% or less, 0.80% or less, 0.50% or less, or 0.20% or less.

[0035] Cu: 0 to 1.00% Cu is an element that improves the hardenability of steel and has the effect of improving the strength of hot-stamped steel. Cu also has the effect of improving the corrosion resistance of hot-stamped steel. Therefore, Cu may be added as needed. To ensure the above effects, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.05% or more, 0.10% or more, 0.15% or more, or 0.20% or more. On the other hand, even if a large amount of Cu is added, the above effects saturate and alloy costs increase. Furthermore, the deformability of the hot-stamped steel may deteriorate. Therefore, the Cu content is set to 1.00% or less. The Cu content is preferably 0.80% or less, 0.60% or less, or 0.40% or less.

[0036] Co: 0 to 1.000% Co is an element that has the effect of raising the Ms point and improving the deformability of the hot-stamped steel. Therefore, Co may be added as needed. To ensure the above effect, the Co content is preferably 0.001% or more. The Co content is more preferably 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, Co is an expensive element, and if the Co content exceeds 1.000%, the alloy cost increases. Therefore, the Co content is set to 1.000% or less. The Co content is preferably 0.800% or less, 0.600% or less, 0.400% or less, or 0.200% or less.

[0037] V: 0 to 0.500% V forms carbonitrides in steel and has the effect of improving the strength of the hot-stamped body through precipitation strengthening. Furthermore, V has the effect of improving the strength and deformability of the hot-stamped body by refining the metal structure. Therefore, V may be added as necessary. To ensure the above effect, the V content is preferably 0.001% or more. The V content is more preferably 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, if the V content exceeds 0.500%, excessive carbonitrides are formed, degrading the deformability of the hot-stamped body. Therefore, the V content is set to 0.500% or less. The V content is preferably 0.400% or less, 0.300% or less, or 0.200% or less.

[0038] Zr: 0 to 0.100% Zr forms carbonitrides in steel and improves the strength of hot-stamped steel through precipitation strengthening. Furthermore, Zr fixes N as nitrides, suppressing BN formation and enhancing the hardenability-improving effect of B. Therefore, Zr may be added as needed. To ensure the above effects, the Zr content is preferably 0.001% or more. The Zr content is more preferably 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, if the Zr content exceeds 0.100%, excessive carbonitrides are formed, resulting in a decrease in the bendability of the hot-stamped steel. Therefore, the Zr content is set to 0.100% or less. The Zr content is preferably 0.080% or less or 0.050% or less.

[0039] Ca: 0 to 1.0000% Ca has the effect of refining inclusions in steel and preventing cracks caused by the inclusions during hot stamping and collision. Therefore, Ca may be added as necessary. To ensure the above effect, the Ca content is preferably 0.0001% or more. The Ca content is more preferably 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if the Ca content exceeds 1.0000%, the effect of refining inclusions in steel saturates, and alloy costs increase. Therefore, the Ca content is set to 1.0000% or less. The Ca content is preferably 0.0400% or less, 0.0200% or less, 0.0100% or less, or 0.0050% or less.

[0040] Mg: 0 to 1.0000% Mg has the effect of refining inclusions in steel and preventing cracks caused by the inclusions during hot stamping and collision. Therefore, Mg may be added as necessary. To ensure the above effect, the Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if the Mg content exceeds 1.0000%, the effect of refining inclusions in steel saturates, and alloy costs increase. Therefore, the Mg content is set to 1.0000% or less. The Mg content is preferably 0.0400% or less, 0.0200% or less, 0.0100% or less, or 0.0050% or less.

[0041] REM: 0 to 0.0050% REM has the effect of refining inclusions in steel and preventing cracks caused by the inclusions during hot stamping and collision. Therefore, REM may be added as needed. To ensure the above effect, the REM content is preferably 0.0001% or more. The REM content is more preferably 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if the REM content exceeds 0.0050%, the effect of refining inclusions in steel saturates and alloy costs increase. Therefore, the REM content is set to 0.0050% or less. The REM content is preferably 0.0100% or less or 0.0050% or less. In this embodiment, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the REM content refers to the total content of these elements.

[0042] Sn: 0 to 0.100% Sn has the effect of improving the corrosion resistance of the hot-stamped body. Therefore, Sn may be contained as necessary. To ensure this effect, the Sn content is preferably 0.001% or more. The Sn content is more preferably 0.010% or 0.030% or more. On the other hand, even if more than 0.100% of Sn is contained, the above effect saturates, so the Sn content is set to 0.100% or less. The Sn content is preferably 0.080% or less, 0.060% or less, or 0.040% or less.

[0043] Sb: 0 to 0.020% Sb has the effect of improving the corrosion resistance of a hot-stamped steel. Therefore, Sb may be contained as necessary. To ensure that the above effect is exerted, the Sb content is preferably 0.001% or more. The Sb content is more preferably 0.005% or 0.010% or more. On the other hand, even if the Sb content exceeds 0.020%, the above effect saturates, so the Sb content is set to 0.020% or less. The Sb content is preferably 0.015% or less.

[0044] As: 0 to 0.100% As has the effect of inhibiting grain boundary migration and suppressing grain growth. This reduces the ferrite grain size after annealing and the prior austenite grain size after hot stamping, thereby improving the deformability of the hot-stamped steel. Therefore, As may be added as necessary. To ensure the above effect, the As content is preferably 0.001% or more. The As content is more preferably 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, if the As content exceeds 0.100%, the hot ductility decreases, which may result in cracking during casting and hot rolling. Therefore, the As content is set to 0.100% or less. The As content is preferably 0.080% or less, 0.060% or less, or 0.040% or less.

[0045] The chemical composition of the hot-stamped steel can be measured by taking a test piece from the surface of the hot-stamped steel in the thickness direction, extending from ¼ to ¾ of the thickness, and then measuring the test piece using a common analytical method. For example, the measurement can be performed using inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S can be measured using a combustion-infrared absorption method, N can be measured using an inert gas fusion-thermal conductivity method, and O can be measured using an inert gas fusion-non-dispersive infrared absorption method.

[0046] Next, the Vickers hardness of the hot-stamped steel according to this embodiment will be described. The hot-stamped steel according to this embodiment has an average Vickers hardness Hvi of 615 HV or more at a position ¼ of the plate thickness from the surface, an average Vickers hardness Hvs at a depth of 20 μm from the surface, and a ratio Hvs / Hv i of 0.50 to 0.75 between the average Vickers hardness Hv i and the Hv i , and a Vickers hardness gradient of 3.5 HV / μm or less in a region from a depth of 20 μm to a depth of 60 μm from the surface.

[0047] In this embodiment, the surface of the hot-stamped product refers to the surface of the steel sheet when the hot-stamped product consists only of a steel sheet, and refers to the interface between the coating layer and the steel sheet when the hot-stamped product has a coating layer on the surface. The interface between the coating layer and the steel sheet is defined by GDS measurement, which will be described later.

[0048] Hvi: 615 HV or more If Hvi, which is the average value of the Vickers hardness at a position 1 / 4 of the plate thickness from the surface, is less than 615 HV, the desired strength of the hot-stamped steel sheet cannot be obtained. Therefore, Hvi is set to 615 HV or more. Hvi is preferably set to 630 HV or more, 650 HV or more, 680 HV or more, or 715 HV or more. If Hvi is set too high, the deformability of the hot-stamped steel sheet may deteriorate. Therefore, Hvi is preferably set to less than 1000 HV or less than 900 HV.

[0049] Hvs / Hvi: 0.50 to 0.75 If Hvs / Hvi, which is the ratio of Hvs, the average Vickers hardness at a depth of 20 μm from the surface, to Hvi, is less than 0.50, only the very surface layer will be excessively elongated when subjected to bending deformation, resulting in a deterioration in the deformability of the hot-stamped steel sheet. Therefore, Hvs / Hvi is set to 0.50 or more. Hvs / Hvi is preferably 0.52 or more or 0.55 or more. On the other hand, if Hvs / Hvi exceeds 0.75, fine cracks are likely to occur near the surface layer, resulting in a deterioration in the deformability of the hot-stamped steel sheet. Therefore, Hvs / Hvi is set to 0.75 or less. Hvs / Hvi is preferably 0.72 or less or 0.70 or less.

[0050] The Vickers hardness of a hot-stamped body is measured by the following method. A test specimen is cut from an arbitrary position at least 50 mm away from the end face of the hot-stamped body so that a cross section perpendicular to the surface (sheet thickness cross section) can be observed. The sheet thickness cross section of the steel sheet is buffed and then subjected to a Vickers hardness test in accordance with JIS Z 2244-1:2020. The Vickers hardness test is performed at a depth of 20 μm from the surface using a micro Vickers hardness tester, with a load of 0.098 N and a load holding time of 10 seconds. Because measured values ​​near the surface tend to vary depending on the load, in this embodiment, the load is set to 0.098 N. At least five measurement points are measured at a depth of 20 μm from the surface. The spacing between measurement points is at least three times the size of the indentation. If it is not possible to measure five or more points on one sample, measurements are performed on multiple samples. Vickers hardness is measured at five or more measurement points, and the average value is calculated to obtain Hvs, which is the average Vickers hardness at a depth of 20 μm from the surface.

[0051] The average Vickers hardness Hvi at a position 1 / 4 of the plate thickness from the surface is measured at a depth of 1 / 4 of the plate thickness from the surface using a method similar to that for measuring Vickers hardness at a depth of 20 μm from the surface. Vickers hardness is measured at at least five or more measurement points at a depth of 1 / 4 of the plate thickness from the surface, and Hvi is obtained by calculating the average value.

[0052] Vickers hardness gradient in the region from 20 μm deep to 60 μm deep from the surface: 3.5 HV / μm or less If the Vickers hardness gradient in the region from 20 μm deep to 60 μm deep from the surface exceeds 3.5 HV / μm, fine cracks will occur at the boundary between the soft layer near the surface and the inner layer, degrading the deformability of the hot-stamped body. Therefore, the Vickers hardness gradient in the region from 20 μm deep to 60 μm deep from the surface is set to 3.5 HV / μm or less. The Vickers hardness gradient in the region from 20 μm deep to 60 μm deep from the surface is preferably 3.2 HV / μm or less, 3.0 HV / μm or less, less than 2.5 HV / μm, or less than 2.0 HV / μm. If the Vickers hardness gradient is too low, fine cracks may occur in the vicinity of the surface layer during bending deformation, and the deformability of the hot-stamped body may deteriorate. Therefore, the Vickers hardness gradient in the region from a depth of 20 μm to a depth of 60 μm from the surface is preferably 1.0 HV / μm or more, 1.2 HV / μm or more, or 1.5 HV / μm or more.

[0053] The Vickers hardness gradient of the hot-stamped body is measured by the following method. Similar to the method for measuring Vickers hardness at a depth of 20 μm from the surface, a test piece is cut out, polished, and then subjected to a Vickers hardness test. Measurement points are located on a line (straight line) extending from the surface in the thickness direction (perpendicular to the surface), at a depth of 20 μm from the surface and a depth of 60 μm from the surface. The difference between the Vickers hardness at a depth of 60 μm from the surface and the Vickers hardness at a depth of 20 μm from the surface is divided by the measurement distance (40 μm) to obtain the rate of change of Vickers hardness with respect to depth. The Vickers hardness is measured on at least five or more lines to determine the respective rates of change, and the average value is calculated to obtain the Vickers hardness gradient in the region from a depth of 20 μm to a depth of 60 μm from the surface.

[0054] Bs_max / Bi: 0.75 or more In the hot-stamped steel according to this embodiment, Bs_max / Bi, which is the ratio of Bs_max, the maximum B concentration in the region from the surface to a depth of 10 μm from the surface, to Bi, the B concentration at a depth of 100 μm from the surface, is preferably 0.75 or more. By setting Bs_max / Bi to 0.75 or more, a sudden change in the strength of the grain boundaries from the surface to the interior is suppressed, thereby further improving the deformability of the hot-stamped steel. Bs_max / Bi is preferably 0.80 or more. If Bs_max / Bi is too high, the productivity of the hot-stamped steel is significantly impaired. Furthermore, the hardness distribution of the hot-stamped steel may not be favorably controlled. Therefore, Bs_max / Bi is preferably less than 5.00 or less than 4.50.

[0055] Bs_max, which is the maximum B concentration in the region from the surface of the hot-stamped body to a depth of 10 μm from the surface, and Bi, which is the B concentration at a depth of 100 μm from the surface, are measured by the following method. A test specimen is taken from an arbitrary position at least 50 mm away from the end face of the hot-stamped body. If the steel sheet is painted, the paint is removed, and then the concentration (mass%) of each element is measured from the measurement start surface to a depth of 100 μm or more in the depth direction (sheet thickness direction) by glow discharge optical emission spectrometry (GDS analysis). Note that the "measurement start surface" referred to here is different from the "surface of the steel sheet constituting the formed body."

[0056] In GDS analysis, the measurement pitch is adjusted so that there are 1,200 to 1,800 measurement points from the surface of the steel sheet to a depth of 100 μm. To eliminate the influence of foreign matter such as oil adhering to the measurement start surface, the surface of the steel sheet is defined as the depth at which the Fe concentration first reaches 95% or more of the "Fe concentration at a depth of 100 μm from the measurement start surface." Similarly, when a hot-stamped body has a coating layer on its surface, the depth at which the Fe concentration first reaches 95% or more of the "Fe concentration at a depth of 100 μm from the measurement start surface" is defined as the interface between the coating layer and the steel sheet, i.e., the surface of the steel sheet. If there is no measurement point at a depth of 100 μm from the measurement start surface, the surface of the steel sheet is similarly defined based on the Fe concentration at the measurement point closest to a depth of 100 μm from the measurement start surface. From the obtained measurement results, the maximum value of the B concentration in the region from the surface to a depth of 10 μm from the surface, and the B concentration at a depth of 100 μm from the surface are obtained, and the ratio of Bs_max to Bi is calculated. If there is no measurement point at a depth of 100 μm from the surface, the B concentration at the measurement point closest to a depth of 100 μm from the surface is taken as Bi. This measurement is carried out five times, changing the measurement location. The average value of the ratios of Bs_max to Bi obtained from the five measurements is calculated to obtain Bs_max / Bi.

[0057] The metallographic structure of the steel sheet constituting the hot-stamped steel is not particularly limited as long as the desired deformability can be obtained, but it is preferable that the steel sheet constituting the hot-stamped steel according to this embodiment has a metallographic structure containing the amount of martensite shown below. In the following description of the metallographic structure, "%" means "volume %".

[0058] The metal structure at a position 1 / 4 of the plate thickness from the surface (also referred to as the inner layer portion) preferably contains martensite at a volume fraction of more than 90.0%. Since martensite is an effective structure for increasing the strength of a hot-stamped body, the volume fraction of martensite is preferably more than 90.0%. If the volume fraction of martensite is 90.0% or less, the Vickers hardness (Hvi) of the inner layer portion of the hot-stamped body may be less than 615 HV, resulting in insufficient strength. The volume fraction of martensite is more preferably more than 91.0%, more than 93.0%, or more than 95.0%. There is no need to particularly set an upper limit for the volume fraction of martensite. However, in order to significantly increase the volume fraction of martensite, it is necessary to excessively increase the heating temperature or cooling rate in the hot stamping process, which significantly impairs the productivity of the hot-stamped body. Therefore, the volume fraction of martensite is preferably 99.0% or less or 98.0% or less.

[0059] In this embodiment, martensite includes not only fresh martensite that has not been tempered, but also tempered martensite that has been tempered and contains iron carbides. The remainder of the metal structure of the inner layer portion may contain ferrite, pearlite, bainite, or retained austenite, and may further contain precipitates such as cementite or oxides that exist alone. Since it is not necessary to contain ferrite, pearlite, bainite, retained austenite, and precipitates, the lower limits of the volume fractions of ferrite, pearlite, bainite, retained austenite, and precipitates are all 0%.

[0060] Retained austenite has the effect of improving the ductility of a steel sheet after hot stamping. To achieve this effect, the volume fraction of retained austenite is preferably 0.5% or more, 1.0% or more, or 2.0% or more. On the other hand, excessively increasing the volume fraction of retained austenite requires austempering at high temperatures after hot stamping, which significantly reduces the productivity of the hot-stamped steel sheet. Furthermore, excessive retained austenite may deteriorate the deformability of the hot-stamped steel sheet. Therefore, the volume fraction of retained austenite is preferably less than 9.0%, less than 7.0%, less than 5.0%, or less than 4.0%.

[0061] In this embodiment, the volume fraction of each structure is measured by the following method: A test piece is taken from an arbitrary position 50 mm or more away from the end face of the hot-stamped steel sheet, and the cross section of the steel sheet through the plate thickness is buffed, and then the structure is observed at a position 1 / 4 of the plate thickness from the surface (it is acceptable if the observation region includes the position 1 / 4 of the plate thickness from the surface and is in the range of 1 / 8 to 3 / 8 of the plate thickness from the surface in the thickness direction).

[0062] Specifically, after the polished surface is subjected to nital etching or electrolytic polishing, a structural photograph is taken using an optical microscope and a scanning electron microscope (SEM). Image analysis is performed on the obtained structural photograph based on brightness differences or differences in the morphology of iron carbides present within the phase to obtain the area ratios of ferrite, pearlite, bainite, tempered martensite, and precipitates. Then, after Répéra corrosion at the same observation position, a structural photograph is taken using an optical microscope or a scanning electron microscope (SEM). Image analysis is performed on the obtained structural photograph to calculate the total area ratio of "retained austenite and fresh martensite." Furthermore, after electrolytic polishing at the same observation position, the thickness cross section is measured using an SEM equipped with an electron backscatter pattern analyzer (EBSP). The area ratio of retained austenite is obtained by calculating the area ratio of regions with an fcc crystal structure from the crystal orientation information obtained by EBSP analysis.

[0063] The area fraction of fresh martensite is obtained by subtracting the area fraction of retained austenite from the sum of the area fractions of the above-mentioned "retained austenite and fresh martensite." Based on these results, the area fractions of ferrite, pearlite, bainite, martensite (tempered martensite and fresh martensite), retained austenite, and precipitates are obtained. The area fractions are then considered to be equal to the volume fractions, and the obtained area fractions are regarded as the volume fractions of each structure.

[0064] In structural observation, tempered martensite can be distinguished from fresh martensite by the presence of iron carbides inside. Tempered martensite can also be distinguished from bainite by the fact that the iron carbides inside are elongated in multiple directions rather than in a single direction. Elongation in a single direction means that the difference in the elongation direction is within 5°.

[0065] The thickness of the hot-stamped product according to this embodiment (the thickness of the steel plate when the hot-stamped product is made of only a steel plate) is not particularly limited, but from the viewpoint of reducing the vehicle body weight, it is preferably 2.5 mm or less, 2.0 mm or less, 1.8 mm or less, or 1.6 mm or less. On the other hand, from the viewpoint of ensuring the amount of impact absorption, the thickness is preferably 0.4 mm or more, 0.6 mm or more, 0.8 mm or more, or 1.0 mm or more.

[0066] The hot-stamped product according to this embodiment may have a plating layer on its surface. Having a plating layer on its surface can improve corrosion resistance after hot stamping. The plating layer may be formed on both sides or one side of the hot-stamped product. Examples of the plating layer include an aluminum-based plating layer formed by hot stamping a steel sheet for hot stamping having a hot-dip aluminum-plated layer, a hot-dip aluminum-zinc alloy-based plating layer, a hot-dip aluminum-silicon alloy-plated layer, or the like; and a zinc-based plating layer formed by hot stamping a steel sheet for hot stamping having a hot-dip galvanized layer, an electrogalvanized layer, a galvannealed layer, a hot-dip zinc-aluminum alloy-based plating layer, an electrogalvanized-nickel alloy-based plating layer, or the like.

[0067] Next, a preferred method for manufacturing the hot-stamped steel according to this embodiment will be described. The hot-stamped steel according to this embodiment can be manufactured by hot stamping a steel sheet for hot stamping that has been manufactured by a manufacturing method in which the pickling conditions after hot rolling and the annealing conditions after cold rolling are preferably controlled. First, a method for manufacturing a steel sheet for hot stamping that can manufacture the hot-stamped steel according to this embodiment will be described.

[0068] The steel sheet for hot stamping is produced by a production method including a hot rolling process in which a slab having the above-mentioned chemical composition is hot rolled to form a hot rolled steel sheet, a pickling process in which the hot rolled steel sheet is pickled, a cold rolling process in which the hot rolled steel sheet after pickling is cold rolled to form a cold rolled steel sheet, and an annealing process in which the cold rolled steel sheet is annealed to form an annealed steel sheet.

[0069] The method for producing a slab used in the method for producing a steel sheet for hot stamping is not particularly limited. Steel having the above-described chemical composition is melted by a known means and then formed into a steel ingot by a continuous casting method, or formed into a steel billet by a method such as blooming after being formed into a steel ingot by any casting method. In the continuous casting process, it is preferable to generate an external additional flow, such as electromagnetic stirring, in the molten steel in the mold to suppress the occurrence of surface defects due to inclusions. The steel ingot or billet may be cooled and then reheated for hot rolling. Alternatively, the steel ingot in a high temperature state after continuous casting or the billet in a high temperature state after blooming may be subjected to hot rolling directly, with warming, or with supplementary heating. Such steel ingots and billets are collectively referred to as "slabs" as raw materials for hot rolling.

[0070] The heating temperature of the slab to be subjected to hot rolling is not particularly limited, but in order to prevent coarsening of austenite, it is preferably less than 1250° C., more preferably less than 1200° C. If the slab heating temperature is low, rolling becomes difficult, so the slab heating temperature may be 1050° C. or higher.

[0071] The heated slab is hot-rolled to obtain a hot-rolled steel sheet. Hot-rolling is carried out by using Ar to transform austenite after the rolling is completed, thereby refining the metal structure of the hot-rolled steel sheet. ３ It is preferable to complete the reaction in a temperature range above this point.

[0072] When the hot-rolled steel sheet is coiled after hot rolling, the coiling temperature is preferably less than 600°C. If the coiling temperature is 600°C or higher, thermally stable iron carbides are formed, which may deteriorate the deformability of the hot-stamped steel sheet. Furthermore, an excessive internal oxide layer is formed, and this internal oxide layer remains even after pickling, which will be described later, making it impossible to preferably control the hardness distribution and B concentration distribution of the hot-stamped steel sheet. On the other hand, if the coiling temperature is too low, the hot-rolled steel sheet becomes excessively hard, making it difficult to perform cold rolling, and therefore the coiling temperature is preferably more than 400°C.

[0073] The hot-rolled and coiled hot-rolled steel sheet is subjected to pickling. The pickling conditions include using an aqueous solution of hydrogen chloride as the pickling bath, preferably with a hydrogen chloride concentration of 5 mass% or more, a pickling temperature of 60°C or more, and a pickling time of 30 seconds or more. By performing pickling under preferred conditions, the internal oxide layer is adequately removed, and the hardness distribution and B concentration distribution of the hot-stamped steel sheet can be favorably controlled. If the concentration of the pickling bath or the pickling temperature is too high or the pickling time is too long, excessive pickling occurs, suppressing decarburization in the annealing step described below, and the hardness distribution of the hot-stamped steel sheet may not be favorably controlled. Therefore, the hydrogen chloride concentration is preferably 15 mass% or less, the pickling temperature is preferably 95°C or less, and the pickling time is preferably 120 seconds or less.

[0074] The hot-rolled steel sheet after pickling is cold-rolled according to a conventional method to obtain a cold-rolled steel sheet. In the cold-rolling step, the cumulative reduction in cold rolling is preferably 30% or more. If the cumulative reduction is less than 30%, the metal structure of the steel sheet for hot stamping may become coarse. If the metal structure of the steel sheet for hot stamping is coarse, the metal structure of the hot-stamped body will become coarse after hot stamping, causing a decrease in the deformability of the body. On the other hand, an excessive increase in the cumulative reduction increases the load on the rolling equipment and causes a decrease in productivity, so the cumulative reduction is preferably less than 70%. After cold rolling, treatment such as degreasing may be performed according to a conventional method.

[0075] Annealing is performed on the cold-rolled steel sheet to produce an annealed steel sheet. In the annealing process, the surface layer of the steel sheet is appropriately decarburized by controlling the soaking temperature, soaking time, annealing atmosphere, and primary cooling conditions. As a result, the hardness distribution of the hot-stamped steel can be favorably controlled. As annealing conditions, the soaking temperature is preferably 730°C or higher and lower than 820°C, the soaking time is preferably 200 seconds or higher and lower than 400 seconds, and the dew point of the atmosphere during soaking is preferably 0 to 20°C. Furthermore, the atmosphere during soaking is preferably a nitrogen-hydrogen atmosphere containing 2% by volume or higher and less than 15% by volume of hydrogen.

[0076] If the soaking temperature is less than 730°C or equal to or greater than 820°C, if the soaking time is less than 200 seconds, or if the dew point of the atmosphere during soaking is less than 0°C or greater than 20°C, the amount of decarburization may be insufficient, and the hardness distribution of the hot-stamped steel sheet may not be favorably controlled. If the soaking temperature is 820°C or higher, the metal structure of the annealed steel sheet may coarsen due to grain growth, and the deformability of the hot-stamped steel sheet may decrease. If the soaking time is 400 seconds or longer, decarburization may be excessive, and the hardness distribution of the hot-stamped steel sheet may not be favorably controlled. Furthermore, if the dew point of the atmosphere during soaking is greater than 20°C, Fe in the surface layer of the steel sheet may oxidize, and the amount of decarburization may be insufficient.

[0077] After holding at the soaking temperature, it is preferable to perform primary cooling to a temperature range of 650°C or lower at an average cooling rate of 5°C / s or lower. The dew point of the atmosphere during primary cooling is preferably -20°C or lower. By performing primary cooling under these conditions, the C concentration distribution in the surface layer of the steel sheet is improved, and the hardness distribution of the hot-stamped steel can be preferably controlled.

[0078] In this embodiment, the average cooling rate is the temperature difference between the start point and the end point of the set range divided by the elapsed time from the start point to the end point.

[0079] The annealed steel sheet produced by the above-mentioned method may be plated in a conventional manner to obtain a plated steel sheet. The annealed steel sheet or plated steel sheet thus obtained may be temper rolled in a conventional manner.

[0080] The hot-stamped steel according to this embodiment can be obtained by a production method including a heating step of heating a steel sheet for hot stamping (annealed steel sheet or plated steel sheet) produced by the above-described method, and a hot stamping step of hot stamping the heated steel sheet for hot stamping. In order to stably obtain the hot-stamped steel according to this embodiment, it is preferable to perform hot stamping by the following method.

[0081] In the heating step, prior to the hot stamping step, the hot stamping steel sheet having the above-mentioned chemical composition is heated. In the heating step, the maximum heating temperature is 800°C or more and Ac ３ The holding time at the maximum heating temperature (the residence time in the temperature range from (maximum heating temperature - 10°C) to the maximum heating temperature) is preferably more than 60 seconds. １ Point ~ Ac ３ It is more preferable that the time spent in the temperature range of the point is 30 seconds or more. １ Points and Ac ３ The points are calculated using the following formulas (1) and (2).

[0082] A.C. １(℃)=723+29.1×Si-10.7×Mn+16.9×Cr+16.9×Ni+6.38×W+290×As…(1) Ac ３ (°C) = 910 - 203 x √C + 44.7 x Si - 30 x Mn + 700 x P + 400 x Al - 11 x Cr + 31.5 x Mo - 15.2 x Ni - 20 x Cu + 400 x Ti + 104 x V + 13.1 x W + 120 x As ... (2) where the element symbols in formulas (1) and (2) indicate the content (mass%) of each element in the chemical composition of the steel sheet for hot stamping, and 0 is substituted if the element is not contained.

[0083] A.C. １ Point ~ Ac ３ By setting the residence time in the temperature range of the point to 30 seconds or more, the B concentration distribution in the surface layer of the hot stamped body is improved, and the deformability of the hot stamped body is further improved. ３ If the maximum heating temperature is below 1050°C or less or is less than 800°C, or if the holding time at the maximum heating temperature is 60 seconds or less, the volume fraction of martensite in the metallographic structure of the inner layer of the hot-stamped body may be insufficient, resulting in a decrease in the strength of the hot-stamped body. On the other hand, if the maximum heating temperature is too high or the holding time at the maximum heating temperature is too long, the metallographic structure of the hot-stamped body may coarsen, resulting in a decrease in the deformability of the hot-stamped body and a decrease in strength. Therefore, the maximum heating temperature is preferably less than 1050°C, and the holding time at the maximum heating temperature is preferably less than 600 seconds.

[0084] In the hot stamping process, it is preferable that the heated steel sheet for hot stamping is removed from the heating furnace and allowed to cool in the atmosphere, and then hot stamping is started in a temperature range above 750°C. If the hot stamping starting temperature is 750°C or lower, ferrite is excessively formed in the metal structure of the inner layer of the hot stamped body, which may result in a decrease in the strength of the hot stamped body. After forming by hot stamping, the hot stamped body is cooled while held in the die, and / or the hot stamped body is removed from the die and cooled by any method.

[0085] If the cooling rate is too slow, the volume fraction of martensite in the metallographic structure of the inner layer portion of the hot-stamped body may be insufficient, resulting in a decrease in the strength of the hot-stamped body. Therefore, the average cooling rate from the hot stamping start temperature to 400°C is preferably 30°C / sec or more, 60°C / sec or more, or 90°C / sec or more. Furthermore, if the cooling stop temperature is too high, the volume fraction of martensite in the metallographic structure of the inner layer portion of the hot-stamped body may be insufficient, resulting in a decrease in the strength of the hot-stamped body. Therefore, the cooling stop temperature for the above cooling is preferably less than 90°C.

[0086] The hot-stamped steel according to this embodiment is obtained by the above method. The hot-stamped steel may be subjected to a reheat treatment. The reheat treatment further improves the deformability of the hot-stamped steel. The reheat treatment is preferably performed at a heating temperature of 120 to 300°C for a holding time at the heating temperature of 10 to 30 minutes.

[0087] Next, examples of the present invention will be described, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0088] Molten steel was cast using a vacuum melting furnace to obtain steel materials having the chemical compositions shown in Tables 1A and 1B. Note that blank cells in Tables 1A and 1B indicate that the content of the corresponding element was below the lower limit of measurement.

[0089] The obtained steel material was heated to 1200°C and held there for 60 minutes, and then subjected to 10 passes of hot rolling in a temperature range of 900°C or higher to obtain a hot-rolled steel sheet having a thickness of 3.5 mm. After hot rolling, the hot-rolled steel sheet was cooled to 550°C with a water spray. In test number 47, the hot-rolled steel sheet was cooled to 620°C. The cooling end temperature was set as the coiling temperature, and the hot-rolled steel sheet was charged into an electric heating furnace held at this coiling temperature and held there for 60 minutes. Thereafter, the hot-rolled steel sheet was furnace-cooled to room temperature at an average cooling rate of 20°C / hour to simulate slow cooling after coiling.

[0090] The furnace-cooled hot-rolled steel sheets were pickled under the conditions shown in Tables 2A and 2B to obtain pickled steel sheets. The obtained pickled steel sheets were cold-rolled to obtain cold-rolled steel sheets with a thickness of 1.4 mm. The cumulative reduction during cold rolling was 60%. The obtained cold-rolled steel sheets were annealed using a continuous annealing simulator or a hot-dip galvanizing simulator under the conditions shown in Tables 2A and 2B. After primary cooling, the steel sheets were cooled to room temperature to obtain annealed steel sheets. The atmosphere during soaking was a nitrogen-hydrogen atmosphere containing 2 vol% or more and less than 15 vol% hydrogen. Some cold-rolled steel sheets were cooled after primary cooling and immersed in a hot-dip galvanizing bath or a hot-dip aluminum galvanizing bath to undergo plating treatment, resulting in hot-dip galvanized steel sheets (GI), galvannealed hot-dip galvannealed steel sheets (GA), or hot-dip aluminum galvanized steel sheets (Al).

[0091] A hot stamping blank was taken from the obtained annealed steel sheet or plated steel sheet (hot stamping steel sheet), and subjected to hot stamping and reheating to obtain a plate-shaped hot stamped product. In the hot stamping heating step, the hot stamping blank was heated using an electric heating furnace under the conditions shown in Tables 3A and 3B. The atmosphere in the heating furnace was 100% nitrogen. After heating, the hot stamping blank was removed from the heating furnace and allowed to cool. It was then clamped between dies equipped with a cooling device and hot stamped at a forming start temperature of 770°C or higher (except for test number 42). Next, the blank was cooled in the die from the forming start temperature to 400°C at an average cooling rate of 50°C / sec or higher to a cooling stop temperature of 80°C or lower. A reheating treatment was then performed by holding the blank at 170°C for 20 minutes. Note that the "residence time" in Tables 3A and 3B refers to the Ac in the heating step. １ Point ~ Ac ３ The time spent in the temperature range of the point is shown.

[0092] Test specimens were taken from the obtained hot stamped bodies, and the chemical composition, Vickers hardness, and distribution of B concentration were measured by the methods described above.

[0093] Furthermore, a 60 mm square test piece for a bending test was taken from the hot-stamped product, and a bending test was carried out in accordance with the German Association of the Automotive Industry standard VDA 238-100. The test piece was bent so that the ridge direction of the bend was perpendicular to the rolling direction of the steel sheet for hot stamping, and the bend angle (VDA bend angle) was determined when the bending load had decreased by 60 N from the maximum point. If a crack occurred before the bending load reached the maximum point, the bend angle at the time when the crack occurred was determined and used as the VDA bend angle.

[0094] When the average Vickers hardness (Hvi) of the steel sheet constituting the hot stamped body at a position from the surface to ¼ of the sheet thickness was 615 HV or more, the steel sheet was judged to have high strength and pass, whereas when the Hvi was less than 615 HV, the steel sheet was judged to have low strength and fail.

[0095] When the product of the average Vickers hardness (Hvi) at a position 1 / 4 of the plate thickness from the surface of the steel plate constituting the hot stamped body was 30,000 HV·° or more, the steel plate was judged to have excellent deformability and to pass. On the other hand, when the product of Hvi and VDA bending angle was less than 30,000 HV·°, the steel plate was judged to have no excellent deformability and to fail.

[0096] Tables 3A and 3B show the results of measuring the Vickers hardness and B concentration distribution of the hot-stamped bodies, as well as the results of evaluating the mechanical properties of the hot-stamped bodies. The contents of each element in the chemical composition of the hot-stamped bodies are omitted because they were the same as the contents of the elements shown in Tables 1A and 1B. Furthermore, the metal structure of the hot-stamped bodies according to the present invention had a martensite volume fraction of 92.0% or more at a position one-quarter of the plate thickness from the surface, and a total volume fraction of components other than martensite of 8.0% or less. Furthermore, in Test No. 42, the maximum heating temperature was as low as 780°C, so the forming start temperature was 750°C or less, resulting in a martensite volume fraction of 70.0% or less.

[0097]

[0098]

[0099]

[0100]

[0101]

[0102]

[0103] As shown in Tables 3A and 3B, the hot-stamped steel according to the examples of the present invention has high strength, with an Hvi of 615 HV or more, and excellent deformability, with a product of Hvi and VDA bend angle of 30,000 HV·° or more. Furthermore, hot-stamped steels having a Bs_max / Bi ratio (Bs_max / Bi, the ratio of Bs_max, the maximum B concentration in the region from the surface to a depth of 10 μm from the surface, to Bi, the B concentration at a depth of 100 μm from the surface) of 0.75 or more have a product of Hvi and VDA bend angle of 35,000 HV·° or more, and are therefore particularly excellent in deformability. On the other hand, the hot-stamped steel according to the comparative examples has an Hvi of less than 615 HV, which means that the strength is insufficient, or a product of Hvi and VDA bend angle of less than 30,000 HV·°, which means that the deformability is poor.

[0104] According to the above aspects of the present disclosure, it is possible to provide a hot-stamped steel sheet having high strength and excellent deformability.

Claims

1. Chemical composition, in mass%, is: C: 0.380 to 1.000%, Si: 0.01 to 2.00%, Mn: 0.01% or more, less than 1.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001 to 1.000%, N: 0.0150% or less, O: 0.1000% or less, B: 0.0003 to 0.0100%, Cr: 0 to 0.50%, Mo: 0 to 0.50%, W: 0 to 3.00%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, Co: 0 to 1.000%, 1. A hot stamped product comprising: V: 0 to 0.500%, Zr: 0 to 0.100%, Ca: 0 to 1.0000%, Mg: 0 to 1.0000%, REM: 0 to 0.0050%, Sn: 0 to 0.100%, Sb: 0 to 0.020%, and As: 0 to 0.100%, with the balance being Fe and impurities; Hvi, which is an average value of Vickers hardness at a position from the surface to a quarter of the sheet thickness, is 615 HV or more; Hvs / Hvi, which is a ratio of Hvs, which is an average value of Vickers hardness at a depth of 20 μm from the surface, to Hvi, is 0.50 to 0.75; and a gradient of Vickers hardness in a region from a depth of 20 μm from the surface to a depth of 60 μm from the surface, is 3.5 HV / μm or less.

2. The hot-stamped steel according to claim 1, characterized in that Bs_max / Bi, which is the ratio of Bs_max, the maximum value of the B concentration in the region from the surface to a depth of 10 μm from the surface, to Bi, the B concentration at a depth of 100 μm from the surface, is 0.75 or more.

3. The chemical composition is, in mass%, Cr: 0.01 to 0.50%, Mo: 0.01 to 0.50%, W: 0.01 to 3.00%, Ti: 0.001 to 0.100%, Nb: 0.001 to 0.100%, Ni: 0.01 to 1.00%, Cu: 0.01 to 1.00%, Co: 0.001 to 1.000%, V: 0.001 to 0.500%, Zr: 0.001 to 0.100%, Ca: 0.0001 to 1.0000%, Mg: 0.0001 to 1.0000%, REM: 0.0001 to 0.0050%, Sn: 0.001 to 0.100%, The hot-stamped steel according to claim 1 or 2, characterized in that it contains one or more of Sb: 0.001 to 0.020%, and As: 0.001 to 0.100%.