Hot stamped compact

Abstract

This hot-stamp formed article comprises a steel sheet. The steel sheet has a prescribed chemical composition. In the steel sheet, the average B-concentration in a region between a depth of 5.0 μm from the surface and a depth of 25.0 μm from the surface is at most 0.700 times the B-concentration in a position at a depth of 100 μm from the surface. The average B-concentration in a region between a depth of 0.5 μm from the surface and a depth of 4.0 μm from the surface is at least 1.600 times the B-concentration in a position at a depth of 100 μm from the surface. The average O-concentration in said region between a depth of 0.5 μm from the surface and a depth of 4.0 μm from the surface is more than 0.0150 mass%.

Description

[Technical Field]

[0001] The present invention relates to a hot stamped product. This application claims priority based on Japanese Patent Application No. 2022-067026, filed on April 14, 2022, the contents of which are incorporated herein by reference. [Background technology]

[0002] In recent years, there has been a demand for reducing the weight of automobile bodies from the perspectives of environmental protection and resource conservation, and high-strength steel sheets have been used in automobile components. The use of high-strength steel sheets allows for the reduction of the steel sheet thickness, thereby reducing the weight of the automobile body while providing the desired strength. Automotive components are manufactured by press-forming steel sheets. However, as the strength of steel sheets increases, not only does the forming load increase, but formability also decreases, making the components more susceptible to cracking and wrinkling. Furthermore, when high-strength steel sheets are press-formed, the shape of the components changes significantly due to springback when the components are removed from the mold, making it difficult to ensure the dimensional accuracy of the components. Thus, it is not easy to manufacture high-strength automobile body components by press forming.

[0003] To solve the above problems, a technique has been proposed in the past, as disclosed in Patent Document 1, for example, in which heated steel sheet is press-formed using a low-temperature press die. This technique is called hot stamping or hot pressing, and since steel sheet heated to a high temperature and in a soft state is press-formed, it is possible to manufacture components with complex shapes with high dimensional accuracy. In addition, since the steel sheet is rapidly cooled by contact with the die, it is possible to significantly increase the strength by quenching at the same time as press forming. Patent Document 1 discloses that by hot stamping a steel sheet with a tensile strength of 500 to 600 MPa, a component with a tensile strength of 1400 MPa or more can be obtained.

[0004] The strength of a hot-stamped component can be further increased by increasing the C content of the steel sheet. However, increasing the C content of the steel sheet reduces the deformability of the component as the component strength increases, and cracks are more likely to occur in the early stages of deformation when the component deforms during a collision. As such, it is not easy to manufacture a high-strength hot-stamped component with excellent crash resistance. In particular, when the tensile strength of the component exceeds 1900 MPa, it is difficult to achieve both strength and crash resistance.

[0005] As a technique for manufacturing hot-stamped components with excellent crashworthiness, Patent Document 2 discloses a high-strength pressed part having a tensile strength of 1300 MPa or more and high impact absorption, and a method for manufacturing the same. Patent Document 3 also discloses a hot-stamped automotive component having a tensile strength of 1100 MPa or more and improved bendability from the perspective of impact energy absorption, and a method for manufacturing the same. The methods disclosed in Patent Documents 2 and 3 involve hot stamping a steel sheet for hot stamping having a decarburized surface layer under specified conditions, forming a soft layer on the surface of the hot-stamped component, thereby improving the crashworthiness of the component. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2002-102980 [Patent Document 2] Japanese Patent Application Publication No. 2015-30890 [Patent Document 3] International Publication No. 2018 / 179839 Summary of the Invention [Problem to be solved by the invention]

[0007] However, according to investigations by the present inventors, it has been found that when hot stamping is performed using a steel sheet with a decarburized surface layer, C flows from the interior of the steel sheet into the surface layer during the heating process of hot stamping, increasing the C concentration in the decarburized layer, preventing the surface layer of the part from softening and resulting in insufficient improvement in crash resistance.

[0008] The present invention has been made in view of the above circumstances, and has an object to provide a hot-stamped steel sheet having high strength, with a tensile strength of 1900 MPa or more, and excellent crash resistance. [Means for solving the problem]

[0009] The gist of the present invention is as follows.

[0010] (1) A hot-stamped steel according to one aspect of the present invention is a hot-stamped steel comprising a steel plate, wherein all or a part of the steel plate has a chemical composition, in mass%, of: C: more than 0.32%, less than 0.70%, Si: less than 2.00% Mn: 0.01 to 3.00%, P: 0.200% or less, S: 0.0200% or less, sol.Al: 0.001~1.000%, N: 0.0200% or less, O: 0.0005 to 0.0200%, B: 0.0005~0.0200%, Cr: 0~2.00%, Mo: 0-2.00%, W: 0~2.00%, Cu: 0-2.00% Ni: 0-2.00% Ti: 0 to 0.200%, Nb: 0 to 0.200%, V: 0 to 0.200%, Zr: 0 to 0.200%, Ca: 0 to 0.1000%, Mg: 0 to 0.1000%, REM: 0 to 0.1000%, Sn: 0 to 0.200% As: 0 to 0.100%, and Bi: 0 to 0.0500% the balance being Fe and impurities, The tensile strength is 1900 MPa or more, an average B concentration in a region from a depth of 5.0 μm from the surface of the steel plate to a depth of 25.0 μm from the surface is 0.700 times or less of the B concentration at a depth of 100 μm from the surface; an average B concentration in a region from a depth of 0.5 μm to a depth of 4.0 μm from the surface is 1.600 times or more of the B concentration at the position at a depth of 100 μm from the surface; The average O concentration in the region from a depth of 0.5 μm to a depth of 4.0 μm from the surface exceeds 0.0150 mass %. (2) The hot stamped steel according to (1) above, wherein the chemical composition is, in mass%, Cr: 0.01 to 2.00%, Mo: 0.01 to 2.00%, W: 0.01 to 2.00%, Cu: 0.01 to 2.00%, Ni: 0.01 to 2.00% Ti: 0.001 to 0.200%, Nb: 0.001 to 0.200%, V: 0.001~0.200%, Zr: 0.001 to 0.200%, Ca: 0.0001 to 0.1000%, Mg: 0.0001 to 0.1000%, REM: 0.0001 to 0.1000%, Sn: 0.001 to 0.200%, As: 0.001 to 0.100%, and Bi: 0.0001 to 0.0500% The compound may contain one or more of the group consisting of: [Effects of the Invention]

[0011] According to the above aspect of the present invention, it is possible to provide a hot-stamped steel having high strength, such as a tensile strength of 1900 MPa or more, and excellent crash resistance. The hot-stamped steel according to the above aspect has excellent crash resistance and is free from cracks in the early stages of deformation during a collision, making it suitable for use in automotive components such as pillars and bumpers. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 10 is a view showing a hat member manufactured in an example. [Figure 2] FIG. 2 is a diagram showing a test specimen for a three-point bending test manufactured in an example. [Figure 3] FIG. 2 is a diagram illustrating a three-point bending test performed in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present inventors have investigated methods for suppressing the occurrence of cracks in the early stages of deformation during a collision in a hot-stamped steel sheet having a tensile strength of 1900 MPa or more, and have obtained the following findings.

[0014] (A) By softening the surface layer of the hot stamped body, the impact resistance of the hot stamped body is improved.

[0015] (B) In order to soften the surface layer of a hot-stamped steel sheet, it is effective to perform hot stamping using a steel sheet for hot stamping that has a decarburized layer in the surface layer. However, during the heating process of the steel sheet for hot stamping, carbon diffuses from the interior of the steel sheet to the surface layer, causing an increase in the carbon concentration in the surface layer (a phenomenon known as recarburization). As a result, the surface layer of the hot-stamped steel sheet may not be softened sufficiently, and the impact resistance of the hot-stamped steel sheet may not be sufficiently improved.

[0016] (C) In a hot-stamped steel, (a) the average B concentration in a region from 5.0 μm deep to 25.0 μm deep from the surface of the steel plate constituting the hot-stamped steel is reduced, (b) the average B concentration in a region from 0.5 μm deep to 4.0 μm deep from the surface is increased, and (c) the average O concentration in the region from 0.5 μm deep to 4.0 μm deep from the surface is increased, thereby making it possible to improve the impact resistance of the hot-stamped steel even if recarburization occurs during the heating process of hot stamping.

[0017] (D) The reason for this is not clear, but it is presumed that this is because (a) in the region from 5.0 μm deep from the surface of the steel sheet constituting the hot stamped body to a depth of 25.0 μm from the surface, a decrease in the average B concentration reduces the hardenability of the steel sheet, and (b) in the region from 0.5 μm deep from the surface to a depth of 4.0 μm from the surface, B and O combine to reduce the hardenability of the steel sheet, resulting in softening of the surface layer of the hot stamped body even though the C concentration in the surface layer is increased due to the recarburization phenomenon.

[0018] Based on the findings of (A) to (D) above, the present inventors have found that by adjusting the B concentration and O concentration within specific ranges in the surface layer portion (including the surface layer region and the outermost surface layer region described below) of a hot-stamped body, a hot-stamped body having a tensile strength of 1900 MPa or more and excellent crash resistance can be obtained. The hot stamped steel according to this embodiment will be described in detail below. First, the reasons for limiting the chemical composition of the steel sheet that constitutes the hot stamped steel according to this embodiment will be described.

[0019] All or part of the steel sheet included in the hot-stamped steel according to this embodiment has the following chemical composition. When the hot-stamped steel is made of only a steel sheet, it can be said that all or part of the hot-stamped steel has the chemical composition shown below. In the following, the numerical ranges indicated with "to" include the lower and upper limits. Numerical values ​​indicated as "less than" and "greater than" are not included in the numerical range. All % in chemical compositions indicate mass %.

[0020] When the hot-stamped steel has a portion having a tensile strength of 1900 MPa or more and a portion having a tensile strength of less than 1900 MPa, it is sufficient that at least the portion having a tensile strength of 1900 MPa or more has the following chemical composition.

[0021] The chemical composition of all or a portion of the steel plate provided with the hot stamped steel according to this embodiment is, in mass%, C: more than 0.32% and 0.70% or less, Si: less than 2.00%, Mn: 0.01 to 3.00%, P: 0.200% or less, S: 0.0200% or less, sol. Al: 0.001 to 1.000%, N: 0.0200% or less, O: 0.0005 to 0.0200%, B: 0.0005 to 0.0200%, and the balance: Fe and impurities. Each element will be explained below.

[0022] C: More than 0.32%, less than 0.70% C is an element that improves the tensile strength of the steel sheet after hot stamping (the steel sheet that constitutes the hot-stamped body). If the C content is 0.32% or less, the tensile strength of the steel sheet after hot stamping will be less than 1900 MPa, resulting in insufficient strength of the hot-stamped body. In addition, the impact resistance of the hot-stamped body will deteriorate. Therefore, the C content is set to more than 0.32%. The C content is preferably more than 0.34%, more than 0.38%, more than 0.42%, or more than 0.45%. On the other hand, if the C content exceeds 0.70%, the strength of the hot-stamped steel becomes too high and excellent crash resistance cannot be obtained. Therefore, the C content is set to 0.70% or less. Preferably, the C content is 0.65% or less, 0.60% or less, 0.55% or less, or 0.50% or less.

[0023] Si: Less than 2.00% Silicon (Si) is an element that may be contained in steel as an impurity and embrittles the steel. The adverse effects become particularly significant when the Si content exceeds 2.00%. Therefore, the Si content is set to less than 2.00%. The Si content is preferably less than 1.00%, less than 0.75%, less than 0.50%, or less than 0.20%. The lower limit of the Si content is not particularly limited, but may be 0%. Since excessively reducing the Si content increases steelmaking costs, the Si content is preferably 0.001% or more. Furthermore, since Si has the effect of improving the hardenability of steel, it may be intentionally added. From the viewpoint of improving hardenability, the Si content is preferably 0.05% or more, 0.10% or more, or 0.15% or more.

[0024] Mn: 0.01 to 3.00% Mn is an element that combines with the impurity S to form MnS, thereby suppressing the adverse effects of S. If the Mn content is less than 0.01%, the above effect cannot be obtained. Therefore, the Mn content is set to 0.01% or more. Furthermore, Mn is an element that improves the hardenability of steel, and is an element that forms a metal structure mainly composed of martensite inside the steel sheet after hot stamping, and is effective in ensuring the strength of the hot stamped body. From the viewpoint of ensuring strength, the Mn content is preferably 0.50% or more, 0.75% or more, 1.00% or more, or 1.25% or more. On the other hand, if the Mn content exceeds 3.00%, excellent impact resistance cannot be obtained in the hot-stamped steel. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.50% or less, 2.00% or less, or 1.50% or less.

[0025] P:0.200% or less P is sometimes contained in steel as an impurity and is an element that embrittles steel. If the P content exceeds 0.200%, the adverse effects become particularly significant, and weldability also deteriorates significantly. Therefore, the P content is set to 0.200% or less. The P content is preferably less than 0.100%, less than 0.050%, or less than 0.020%. The P content may be 0%, but reducing the P content to less than 0.001% significantly increases the dephosphorization cost, which is not economically preferable, so it may be 0.001% or more or 0.005% or more.

[0026] S: 0.0200% or less S may be contained in steel as an impurity and is an element that embrittles steel. When the S content exceeds 0.0200%, the adverse effects become particularly significant. Therefore, the S content is set to 0.0200% or less. The S content is preferably less than 0.0050%, less than 0.0020%, or less than 0.0010%. The S content may be 0%, but reducing the S content to less than 0.0001% significantly increases the desulfurization cost, which is not economically preferable, so it may be 0.0001% or more or 0.0002% or more.

[0027] sol.Al:0.001~1.000% Al is an element that has the effect of deoxidizing molten steel. If the sol. Al content (acid-soluble Al content) is less than 0.001%, deoxidation will be insufficient. Therefore, the sol. Al content is set to 0.001% or more. The sol. Al content is preferably 0.005% or more, 0.010% or more, or 0.020% or more. On the other hand, if the sol.Al content is too high, the transformation point rises, making it difficult to heat the steel sheet to a temperature above the Ac3 point during the hot stamping heating process. Furthermore, the strength and crash resistance of the hot-stamped steel deteriorate. Therefore, the sol.Al content is set to 1.000% or less. The sol.Al content is preferably less than 0.500%, less than 0.100%, less than 0.060%, or less than 0.040%.

[0028] N: 0.0200% or less N may be contained in steel as an impurity and is an element that forms nitrides during continuous casting of steel. These nitrides deteriorate the ductility of steel sheets after hot stamping, so a low N content is preferable. If the N content exceeds 0.0200%, the adverse effects become particularly significant. Therefore, the N content is set to 0.0200% or less. The N content is preferably less than 0.0100%, less than 0.0080%, or less than 0.0050%. The N content may be 0%, but if the N content is reduced too much, the denitrification cost will increase significantly and this is not economically preferable, so the N content may be set to 0.0005% or more, 0.0010% or more, or 0.0020% or more.

[0029] O: 0.0005 to 0.0200% O is an element that combines with B to form B oxides, which reduces the hardenability of steel, and is an element that is effective in softening the surface layer of the steel sheet after hot stamping. If the O content is less than 0.0005%, the surface layer of the steel sheet after hot stamping does not soften, and the impact resistance of the hot-stamped steel sheet deteriorates. Therefore, the O content is set to 0.0005% or more. The O content is preferably 0.0010% or more, 0.0015% or more, or 0.0020% or more. On the other hand, if the O content exceeds 0.0200%, a large amount of coarse oxide-based inclusions is formed in the steel, which deteriorates the impact resistance of the hot-stamped steel. Therefore, the O content is set to 0.0200% or less. The O content is preferably 0.0150% or less, 0.0100% or less, 0.0060% or less, or 0.0040% or less.

[0030] B: 0.0005 to 0.0200% B is an element that improves the hardenability of steel, forms a metallographic structure mainly composed of martensite inside the steel sheet after hot stamping, and is an effective element for ensuring the strength of the hot-stamped body. If the B content is less than 0.0005%, the desired strength cannot be obtained in the hot-stamped body. Furthermore, the impact resistance of the hot-stamped body deteriorates. Therefore, the B content is set to 0.0005% or more. The B content is preferably 0.0010% or more, 0.0015% or more, or 0.0020% or more. On the other hand, if the B content exceeds 0.0200%, boron dioxide is formed in the hot-stamped steel, impairing the hardenability-improving effect of B. Therefore, the B content is set to 0.0200% or less. The B content is preferably less than 0.0050%, less than 0.0040%, or less than 0.0030%.

[0031] The balance of the chemical composition of the steel sheet constituting 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 within a range that does not impair the properties of the hot-stamped steel according to this embodiment.

[0032] The steel sheet constituting 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%.

[0033] Cr: 0.01 to 2.00% Cr is an element that improves the hardenability of steel, thereby increasing the strength of the hot-stamped steel. To ensure this effect, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.05% or more or 0.10% or more. On the other hand, if the Cr content exceeds 2.00%, the impact resistance of the hot-stamped steel deteriorates. Therefore, the Cr content is set to 2.00% or less. The Cr content is preferably less than 0.50%, less than 0.40%, or less than 0.30%.

[0034] Mo: 0.01 to 2.00% Mo is an element that improves the hardenability of steel, thereby increasing the strength of hot-stamped steel. To ensure this effect, the Mo content is preferably 0.01% or more. The Mo content is more preferably 0.05% or more, 0.10% or more, or 0.15% or more. On the other hand, if the Mo content exceeds 2.00%, the impact resistance of the hot stamped steel deteriorates. Therefore, the Mo content is set to 2.00% or less. The Mo content is preferably less than 0.50%, less than 0.40%, or less than 0.30%.

[0035] W: 0.01 to 2.00% W is an element that improves the hardenability of steel, thereby increasing the strength of the hot-stamped steel. To ensure this effect, the W content is preferably 0.01% or more. The W content is more preferably 0.05% or more or 0.10% or more. On the other hand, if the W content exceeds 2.00%, the impact resistance of the hot-stamped steel deteriorates. Therefore, the W content is set to 2.00% or less. The W content is preferably less than 0.50%, less than 0.40%, or less than 0.30%.

[0036] Cu: 0.01 to 2.00% Cu is an element that improves the hardenability of steel, thereby increasing the strength of the hot-stamped body. To ensure these effects, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.10% or more. On the other hand, if the Cu content exceeds 2.00%, the impact resistance of the hot stamped steel deteriorates. Therefore, the Cu content is set to 2.00% or less. The Cu content is preferably less than 1.00% or less than 0.50%.

[0037] Ni: 0.01 to 2.00% Ni is an element that improves the hardenability of steel, thereby increasing the strength of the hot-stamped body. To ensure this effect, the Ni content is preferably 0.01% or more, and more preferably 0.10% or more. On the other hand, if the Ni content exceeds 2.00%, the impact resistance of the hot stamped steel deteriorates. Therefore, the Ni content is preferably 2.00% or less. The Ni content is preferably less than 1.00% or less than 0.50%.

[0038] Ti: 0.001 to 0.200% Ti is an element that forms carbonitrides in steel and increases the strength of hot-stamped steel through precipitation strengthening. Ti also improves the impact resistance of hot-stamped steel through refinement of the metal structure. To ensure these effects, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more or 0.010% or more. On the other hand, if the Ti content exceeds 0.200%, a large amount of coarse carbonitrides is formed in the steel, deteriorating the impact resistance of the hot-stamped steel. Therefore, the Ti content is set to 0.200% or less. The Ti content is preferably less than 0.050% or less than 0.030%.

[0039] Nb: 0.001 to 0.200% Nb is an element that forms carbonitrides in steel and increases the strength of hot-stamped steel through precipitation strengthening. Nb also improves the impact resistance of hot-stamped steel through refinement of the metal structure. To ensure these effects, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more or 0.010% or more. On the other hand, if the Nb content exceeds 0.200%, a large amount of coarse carbonitrides is formed in the steel, deteriorating the impact resistance of the hot-stamped steel. Therefore, the Nb content is set to 0.200% or less. The Nb content is preferably less than 0.050%, less than 0.030%, or less than 0.020%.

[0040] V: 0.001 to 0.200% V is an element that forms carbonitrides in steel and enhances the strength of hot-stamped steel through precipitation strengthening. V also refines the metal structure, improving the impact resistance of the hot-stamped steel. To ensure these effects, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more or 0.010% or more. On the other hand, if the V content exceeds 0.200%, a large amount of coarse carbonitrides is formed in the steel, deteriorating the impact resistance of the hot-stamped steel. Therefore, the V content is set to 0.200% or less. The V content is preferably less than 0.100% or less than 0.050%.

[0041] Zr: 0.001 to 0.200% Zr is an element that forms carbonitrides in steel and increases the strength of hot-stamped steel through precipitation strengthening. Zr also improves the impact resistance of hot-stamped steel through refinement of the metal structure. To ensure these effects, the Zr content is preferably 0.001% or more. The Zr content is more preferably 0.005% or more or 0.010% or more. On the other hand, if the Zr content exceeds 0.200%, a large amount of coarse carbonitrides is formed in the steel, deteriorating the impact resistance of the hot-stamped steel. Therefore, the Zr content is set to 0.200% or less. The Zr content is preferably less than 0.100% or less than 0.050%.

[0042] Ca: 0.0001 to 0.1000% Ca is an element that improves the ductility of the steel sheet after hot stamping by adjusting the shape of inclusions. To reliably obtain this effect, the Ca content is preferably 0.0001% or more. On the other hand, even if a large amount of Ca is added, the above effects are saturated and, furthermore, excessive costs are incurred, so the Ca content is set to 0.1000% or less, and preferably less than 0.0100%.

[0043] Mg: 0.0001 to 0.1000% Mg is an element that improves the ductility of the steel sheet after hot stamping by adjusting the shape of inclusions. To reliably obtain these effects, the Mg content is preferably 0.0001% or more. On the other hand, even if Mg is added in a large amount, the above effects are saturated and, furthermore, excessive costs are incurred, so the Mg content is set to 0.1000% or less, and preferably less than 0.0100%.

[0044] REM: 0.0001 to 0.1000% REM is an element that improves the ductility of the steel sheet after hot stamping by adjusting the shape of inclusions. To reliably obtain this effect, the REM content is preferably 0.0001% or more. On the other hand, even if REM is contained in a large amount, the above effects are saturated and, furthermore, excessive costs are incurred, so the REM content is set to 0.1000% or less, and preferably less than 0.0100%. In this embodiment, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids, and the content of REM refers to the total content of these elements.

[0045] Sn: 0.001 to 0.200% Sn is an element that has the effect of improving the corrosion resistance of the hot-stamped steel. To reliably obtain this effect, the Sn content is preferably 0.001% or more. The Sn content is more preferably 0.005% or more, 0.015% or more, or 0.030% or more. On the other hand, even if a large amount of Sn is added, the above effects are saturated and, furthermore, excessive costs are incurred, so the Sn content is set to 0.200% or less, and preferably 0.150% or less or 0.100% or less.

[0046] As: 0.001 to 0.100% As is an element that has the effect of increasing the strength of the hot stamped steel sheet. To reliably obtain this effect, the As content is preferably 0.001% or more. On the other hand, even if As is contained in a large amount, the above effects become saturated and, furthermore, excessive costs are incurred, so the As content is set to 0.100% or less.

[0047] Bi: 0.0001 to 0.0500% Bi is an element that refines the solidification structure and thereby improves the impact resistance of the hot stamped steel. To reliably obtain this effect, the Bi content is preferably 0.0001% or more. On the other hand, even if a large amount of Bi is added, the above effects saturate and, furthermore, excessive costs are incurred, so the Bi content is set to 0.0500% or less, and preferably 0.0100% or less or 0.0050% or less.

[0048] The chemical composition of the steel sheet constituting the hot-stamped product described above can be determined by taking a test specimen from the steel sheet constituting the hot-stamped product, removing the paint if the steel sheet is coated, and then measuring the average elemental content across the entire thickness using a common analytical method. For example, this can be measured using inductively coupled plasma optical emission spectrometry or inductively coupled plasma mass spectrometry. C and S can be measured using the combustion-infrared absorption method, and O and N can be measured using the inert gas fusion-infrared absorption method or the inert gas fusion-thermal conductivity method. If the steel sheet constituting the hot-stamped product has a coating layer on its surface, this coating layer can be removed before measuring the chemical composition.

[0049] As described above, when a hot-stamped steel has a portion having a tensile strength of 1900 MPa or more and a portion having a tensile strength of less than 1900 MPa, it is sufficient that at least the portion having a tensile strength of 1900 MPa or more has the above-described chemical composition. In order to analyze the chemical composition of the portion having a tensile strength of 1900 MPa or more, a tensile test described below is performed, and a test specimen for chemical composition analysis is taken from a tensile test specimen that has a tensile strength of 1900 MPa or more, or from a portion adjacent to the portion from which the tensile test specimen was taken.

[0050] Next, the B concentration distribution and the O concentration distribution in the sheet thickness direction of the steel sheet constituting the hot stamped steel according to this embodiment will be described. In the present embodiment, in the step of heating the steel sheet for hot stamping, the steel sheet for hot stamping is heated under specific conditions, whereby B present in the surface layer region can be diffused to the outermost surface layer region, and B and O can be bonded to each other in the outermost surface layer region, thereby improving the crash resistance of the hot-stamped steel.

[0051] In the hot-stamped steel according to this embodiment, the average B concentration in a region from 5.0 μm deep from the surface to 25.0 μm deep from the surface of the steel sheet constituting the hot-stamped steel is 0.700 times or less the B concentration at a position 100 μm deep from the surface, the average B concentration in a region from 0.5 μm deep from the surface to 4.0 μm deep from the surface is 1.600 times or more the B concentration at the position 100 μm deep from the surface, and the average O concentration in the region from 0.5 μm deep from the surface to 4.0 μm deep from the surface exceeds 0.0150 mass%. The region from a depth of 5.0 μm from the surface of the steel sheet to a depth of 25.0 μm from the surface can be rephrased as a region starting from a position 5.0 μm deep from the surface of the steel sheet and ending at a position 25.0 μm deep from the surface. Furthermore, the region from a depth of 0.5 μm from the surface of the steel sheet to a depth of 4.0 μm from the surface can be rephrased as a region starting from a position 0.5 μm deep from the surface of the steel sheet and ending at a position 4.0 μm deep from the surface.

[0052] When the hot-stamped steel has a portion having a tensile strength of 1900 MPa or more and a portion having a tensile strength of less than 1900 MPa, it is sufficient that at least the portion having a tensile strength of 1900 MPa or more has the following B concentration distribution and O concentration distribution. Each provision will be explained below.

[0053] Average B concentration in the region from 5.0 μm deep to 25.0 μm deep from the surface: Less than 0.700 times the B concentration at a position 100 μm deep from the surface If the average B concentration in the region from 5.0 μm deep to 25.0 μm deep from the surface (hereinafter sometimes referred to as the surface region) exceeds 0.700 times the B concentration at a depth of 100 μm from the surface, the surface region will not be softened, and the hot-stamped steel will not be able to achieve the desired impact resistance characteristics. Therefore, the average B concentration in the surface region is set to 0.700 times or less the B concentration at a depth of 100 μm from the surface. The average B concentration in the surface region is preferably 0.700 times or less and 0.0015% by mass or less the B concentration at a depth of 100 μm from the surface. More preferably, the average B concentration in the surface region is 0.500 times or less or 0.300 times or less the B concentration at a depth of 100 μm from the surface. Furthermore, the average B concentration in the surface region is more preferably 0.0010% by mass or less or 0.0006% by mass or less. Although there is no particular lower limit, if the concentration is reduced too much, not only will the above effect saturate, but the strength of the hot stamped steel will decrease, so the average B concentration in the surface layer region may be set to 0.0002 mass % or more.

[0054] The surface refers to the surface of the steel sheet that constitutes the hot-stamped body. When the hot-stamped body has a plating layer on the surface, the surface refers to the interface between the plating layer and the steel sheet.

[0055] Average B concentration in the region from 0.5 μm deep to 4.0 μm deep from the surface: More than 1,600 times the B concentration at a depth of 100 μm from the surface If the average B concentration in the region from 0.5 μm deep to 4.0 μm deep from the surface (hereinafter sometimes referred to as the outermost layer region) is less than 1.600 times the B concentration at a depth of 100 μm from the surface, as described below, B and O do not bond sufficiently in the outermost layer region, and the outermost layer region does not soften. As a result, the desired impact resistance characteristics cannot be obtained in the hot-stamped steel. Therefore, the average B concentration in the outermost layer region is set to be 1.600 times or more the B concentration at a depth of 100 μm from the surface. The average B concentration in the outermost layer region is preferably 1.600 times or more and 0.0040 mass% or more the B concentration at a depth of 100 μm from the surface. More preferably, the average B concentration in the outermost layer region is 2.000 times or more, 3.000 times or more, or 4.000 times or more the B concentration at a depth of 100 μm from the surface. Furthermore, the average B concentration in the outermost surface layer region is more preferably 0.0050% by mass or more, 0.0060% by mass or more, or 0.0070% by mass or more. Although there is no particular upper limit, if the average B concentration in the outermost layer region is excessively increased, not only will the above effect saturate, but the productivity of the hot stamped body will be significantly impaired, so the upper limit may be set to 0.2000 mass% or less.

[0056] Average O concentration in the region from 0.5 μm deep to 4.0 μm deep from the surface: More than 0.0150 mass% If the average O concentration in the region from 0.5 μm deep to 4.0 μm deep from the surface (the outermost layer region) is 0.0150 mass% or less, the amount of O bonding with B will be insufficient, the amount of B in solid solution will increase, and the outermost layer region will not soften. As a result, the desired impact resistance characteristics cannot be obtained in the hot-stamped steel. Therefore, the average O concentration in the outermost layer region is set to be greater than 0.0150 mass%. The average O concentration in the outermost layer region is preferably greater than 0.0200 mass%, greater than 0.0300 mass%, or greater than 0.0400 mass%. Although there is no particular upper limit, if the average O concentration in the outermost layer region is excessively increased, not only will the above effect saturate, but the productivity of the hot-stamped steel will be significantly impaired, so the upper limit may be set to 1.0000 mass% or less.

[0057] The average B concentration in the surface layer region, the average B concentration and average O concentration in the outermost surface layer region, and the B concentration at a depth of 100 μm from the surface are measured by the following method. A test piece is taken from the hot-stamped product, and if the steel sheet is painted, the paint is removed. Then, using glow discharge optical emission spectrometry (GDS analysis), 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). Note that the "measurement start surface" here is different from the "surface of the steel sheet."

[0058] 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 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 surface of the steel sheet. 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.

[0059] From the obtained measurement results, the average B concentration in the surface region is calculated by calculating the average value of the B concentration in the region from 5.0 μm deep from the surface of the steel sheet to 25.0 μm deep from the surface of the steel sheet. The average B concentration and average O concentration in the outermost surface region are calculated by calculating the average values ​​of the B concentration and O concentration in the region from 0.5 μm deep from the surface of the steel sheet to 4.0 μm deep from the surface of the steel sheet. The B concentration at a depth of 100 μm from the surface is also calculated by determining the B concentration at a depth of 100 μm from the surface of the steel sheet. If no GDS analysis measurement value is available at a depth of 100 μm from the surface of the steel sheet, the first measurement value beyond a depth of 100 μm from the surface can be used as the B concentration at a depth of 100 μm from the surface.

[0060] The GDS analysis is preferably performed on test specimens collected from three or more locations on the hot-stamped body, and the average values ​​of the obtained results are used as the concentrations of B and O. The test specimens may be collected from a location adjacent to a location from which a tensile test specimen that exhibited a tensile strength of 1900 MPa or more was collected, as will be described later.

[0061] The metal structure of the steel sheet constituting the hot stamped steel is not particularly limited as long as the desired strength and crash resistance can be obtained, but it is preferable that the steel sheet has the metal structure shown below.

[0062] All or part of the steel sheet constituting the hot-stamped steel according to this embodiment preferably has a metallographic structure containing the following amount of martensite. In the following description of the metallographic structure, "%" means "volume %." When the hot-stamped steel has a portion having a tensile strength of 1900 MPa or more and a portion having a tensile strength of less than 1900 MPa, it is sufficient that at least the portion having a tensile strength of 1900 MPa or more has the following metallographic structure.

[0063] Metal structure of the inner layer region The metal structure of the inner layer region (region from the surface of the steel sheet constituting the hot stamped body to a depth of 100 μm to the center of the sheet thickness (position 1 / 2 of the sheet thickness)) preferably contains more than 90.0% martensite.

[0064] Since martensite is an effective structure for increasing the tensile strength of a steel sheet after hot stamping, it is preferable that the volume fraction of martensite is greater than 90.0% in the region from the surface of the steel sheet constituting the hot-stamped steel sheet to a depth of 100 μm to the center of the sheet thickness (hereinafter sometimes referred to as the inner layer region). If the volume fraction of martensite in the inner layer region is 90.0% or less, the tensile strength of the hot-stamped steel sheet may be less than 1900 MPa, resulting in insufficient strength. Therefore, it is preferable that the volume fraction of martensite in the inner layer region is greater than 90.0%. The volume fraction of martensite in the inner layer region is more preferably greater than 91.0%, greater than 93.0%, or greater than 95.0%.

[0065] Although there is no particular need to set an upper limit for the volume fraction of martensite in the inner layer region, in order to significantly increase the volume fraction of martensite, it would be necessary to excessively increase the heating temperature or cooling rate of the hot stamping steel sheet in the hot stamping process, which would significantly impair the productivity of the hot stamped steel. Therefore, it is preferable that the volume fraction of martensite in the inner layer region is 99.0% or less or 98.0% or less.

[0066] In this embodiment, the martensite includes not only fresh martensite that has not been tempered, but also tempered martensite that has been tempered and contains iron carbides therein.

[0067] The remainder of the metal structure of the inner layer region may contain ferrite, pearlite, bainite, or retained austenite, and may further contain precipitates such as cementite or oxides present 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%.

[0068] Retained austenite has the effect of improving the ductility of the steel sheet after hot stamping. To obtain this effect, the volume fraction of retained austenite in the inner layer region 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. Furthermore, excessive retained austenite content can degrade the impact resistance of the hot-stamped steel. Therefore, it is preferable that the volume fraction of retained austenite in the inner layer region be less than 9.0%, less than 7.0%, less than 5.0%, or less than 4.0%.

[0069] Metal structure of the outermost layer The metal structure of the outermost layer region preferably contains more than 5.0% ferrite.

[0070] Ferrite is soft and has the effect of improving the impact resistance of the hot-stamped body. To achieve this effect, the volume fraction of ferrite in the outermost layer region is preferably greater than 5.0%. The volume fraction of ferrite in the outermost layer region is more preferably greater than 10.0%, 20.0%, 40.0%, or 60.0%. In this embodiment, the ferrite includes, in addition to polygonal ferrite, acicular ferrite and bainitic ferrite, which have high dislocation densities. In order to efficiently improve the impact resistance of the hot-stamped steel, it is more preferable that the outermost surface region contains the softest polygonal ferrite at the volume fraction described above.

[0071] The remainder of the metal structure of the outermost layer region may contain pearlite, bainite, retained austenite, or martensite, and may further contain precipitates such as cementite or oxides present alone. Since pearlite, bainite, retained austenite, martensite, and precipitates are not required to be present, the lower limits of the volume fractions of pearlite, bainite, retained austenite, martensite, and precipitates are all 0%. Excessive martensite content may harden the outermost layer region, potentially degrading the impact resistance of the hot-stamped steel. Therefore, it is preferable that the volume fraction of martensite in the outermost layer region be less than 90.0%, less than 40.0%, less than 10.0%, or less than 5.0%. Bainite is softer than martensite and has the effect of improving the impact resistance of the hot-stamped article. To achieve this effect, the volume fraction of bainite in the outermost layer region is preferably greater than 10.0%. The volume fraction of bainite in the outermost layer region is more preferably greater than 20.0% or greater than 40.0%. On the other hand, if bainite is contained in excess, the outermost layer region becomes hard, which may deteriorate the impact resistance of the hot-stamped article. Therefore, the volume fraction of bainite in the outermost layer region is preferably less than 80.0% or less than 60.0%.

[0072] In this embodiment, the volume fraction of each structure is measured by the following method. First, a test piece is taken from the hot-stamped body, and the longitudinal section (thickness section) of the steel sheet is buffed, after which the structure of the inner layer region and the outermost surface region is observed. For the inner layer region, the structure is observed from a region 100 μm deep from the surface of the steel sheet constituting the hot-stamped body to the center of the sheet thickness (1 / 2 position of the sheet thickness). For the outermost surface region, the structure is observed from a region 0.5 μm deep from the surface of the steel sheet to a depth of 4.0 μm from the surface.

[0073] Specifically, the polished surface is subjected to nital etching or electrolytic polishing, and then microstructure photographs are taken using an optical microscope and a scanning electron microscope (SEM). The obtained microstructure photographs are then subjected to image analysis based on brightness differences or differences in the morphology of iron carbides present within the phases to determine the area ratios of ferrite, pearlite, bainite, tempered martensite, and precipitates. Subsequently, similar observation positions are subjected to Lepera etching, and microstructure photographs are taken using an optical microscope and a scanning electron microscope (SEM). The obtained microstructure photographs are then subjected to image analysis to calculate the total area ratio of "retained austenite and fresh martensite."

[0074] In addition, after electrolytic polishing of the longitudinal section at the same observation position, the area fraction of retained austenite was measured using an SEM equipped with an electron backscatter pattern analyzer (EBSP). The area fraction of retained austenite was obtained by calculating the area fraction of the region with an fcc crystal structure from the crystal orientation information obtained by EBSP analysis. 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."

[0075] 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 equivalent to the volume fractions of each structure.

[0076] In structural observation, tempered martensite can be distinguished from fresh martensite by the presence of iron carbides within it. Tempered martensite can also be distinguished from bainite by the fact that the iron carbides within it 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°.

[0077] Plate thickness The thickness of the hot stamped body according to this embodiment (the thickness of the steel plate when the hot stamped body consists only of 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 sufficient shock absorption, the plate thickness is preferably 0.4 mm or more, 0.6 mm or more, 0.8 mm or more, or 1.0 mm or more.

[0078] Tensile strength All or part of the hot-stamped steel according to this embodiment has a tensile strength of 1900 MPa or more. To achieve this, it is necessary that all or part of the steel sheet constituting the hot-stamped steel according to this embodiment has a tensile strength of 1900 MPa or more. If the tensile strength of at least part of the hot-stamped steel is not 1900 MPa or more, it will be impossible to ensure the deformation load of the hot-stamped steel. As a result, the impact resistance of the hot-stamped steel will deteriorate. Therefore, the tensile strength of all or part of the hot-stamped steel is set to 1900 MPa or more. Preferably, the tensile strength of all or part of the hot-stamped steel is 2000 MPa or more, 2100 MPa or more, 2300 MPa or more, or 2500 MPa or more. On the other hand, excessively increasing the strength of the hot stamped steel sheet will result in a decrease in impact resistance, so the tensile strength of the hot stamped steel sheet is preferably less than 3000 MPa or less than 2800 MPa.

[0079] The hot-stamped steel according to this embodiment may have a tensile strength of 1900 MPa or more throughout (the entire hot-stamped steel), or it may have a mixture of portions with a tensile strength of 1900 MPa or more and portions with a tensile strength of less than 1900 MPa. By providing portions with different strengths, it becomes possible to control the deformation state of the hot-stamped steel during collision. A hot-stamped steel having portions with different strengths can be produced by a method of joining two or more types of steel sheets with different chemical compositions and then hot stamping them, a method of partially changing the heating temperature of the steel sheets or the cooling rate after hot stamping in the hot stamping step, and a method of partially reheating the hot-stamped steel.

[0080] The tensile strength of the hot-stamped steel is measured by taking a small rectangular piece from the hot-stamped steel, processing it into a tensile test piece without surface grinding, and conducting a tensile test. Specifically, it is preferable to take a No. 13B plate test piece from the hot-stamped steel in accordance with JIS Z 2241:2011 and conduct a tensile test at a tension rate of 10 mm / min. If a No. 13B plate test piece cannot be taken from the hot-stamped steel due to its small size or complex shape, a small rectangular piece having a parallel portion of any width may be taken, and a tensile test may be conducted at a tension rate of 10 mm / min, and the tensile strength may be determined from the maximum test force and the original cross-sectional area of ​​the parallel portion. When high strength portions and low strength portions are mixed in the hot stamped product, the tensile test specimen is taken from the high strength portion.

[0081] The hot-stamped steel 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. Examples of the plating layer include a zinc-based plating layer and an aluminum-based plating layer. A hot-stamped steel having such a plating layer can be obtained by hot stamping a zinc-based plated steel sheet or an aluminum-based plated steel sheet. The plating layer may be formed on both sides or one side of the hot-stamped steel sheet. The plating layer of the hot-stamped steel sheet can be formed by hot stamping a plated steel sheet having a plating layer. However, the plating layer of the plated steel sheet prevents the formation of desirable B and O concentration distributions in the surface and outermost surface regions of the hot-stamped steel sheet during the manufacturing process of the hot-stamped steel sheet. This requires stricter control of the manufacturing method, which may significantly reduce the productivity of the hot-stamped steel sheet. Therefore, from the viewpoint of productivity, it is preferable that the hot-stamped steel sheet does not have a plating layer on its surface.

[0082] Next, a steel sheet for hot stamping suitable for obtaining the hot stamped steel according to this embodiment will be described. Since the change in chemical composition due to hot stamping is negligibly small, the chemical composition of the steel sheet for hot stamping may be the same as that of the hot-stamped body described above. The chemical composition of the steel sheet for hot stamping may be measured by taking a test piece from the steel sheet for hot stamping and measuring it in the same manner as for the hot-stamped body.

[0083] In the steel sheet for hot stamping, the average B concentration in the region from 5.0 μm deep to 25.0 μm deep from the surface (surface region) of the steel sheet is preferably 0.850 times or less the B concentration at a position 100 μm deep from the surface of the steel sheet. If the average B concentration in the surface region exceeds 0.850 times the B concentration at a position 100 μm deep from the surface of the steel sheet, it is not possible to preferably control the B concentration distribution and O concentration distribution in the surface region and outermost surface region of the hot-stamped product even when the hot stamping conditions described below are applied. As a result, the hot-stamped product cannot achieve the desired crash resistance characteristics. Note that, when the steel sheet for hot stamping has a coating layer, the surface refers to the interface between the coating layer and the steel sheet.

[0084] The distribution of B concentration in the thickness direction of a steel sheet for hot stamping can be determined by taking a test piece from the steel sheet for hot stamping and performing GDS analysis in the same manner as in the case of a hot-stamped steel.

[0085] Hereinafter, a method for producing a steel sheet for hot stamping for obtaining a hot-stamped steel according to this embodiment will be described.

[0086] 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 cold rolling process in which the hot rolled steel sheet 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.

[0087] 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 continuous casting, or formed into a steel billet by any casting method and then blooming. 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.

[0088] The heating temperature of the slab to be subjected to hot rolling is preferably less than 1250°C, more preferably less than 1200°C, in order to prevent coarsening of austenite. If the slab heating temperature is too low, rolling becomes difficult, so the slab heating temperature may be 1050°C or higher.

[0089] The heated slab is hot-rolled to obtain a hot-rolled steel sheet. The hot-rolling is preferably completed in a temperature range of Ar3 point or higher in order to refine the metal structure of the hot-rolled steel sheet by transforming austenite after the rolling is completed.

[0090] When the hot-rolled steel sheet is coiled after hot rolling, the coiling temperature is preferably set to less than 550° C. If the coiling temperature is 550° C. or higher, thermally stable iron carbides are formed, which may deteriorate the impact resistance of the hot-stamped steel. 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. Therefore, the coiling temperature is preferably set to more than 500°C.

[0091] The hot-rolled and coiled hot-rolled steel sheet is pickled in a conventional manner and then cold-rolled in a conventional manner to form a cold-rolled steel sheet. In the cold-rolling process, the cumulative reduction in cold rolling is preferably 40% or more. If the cumulative reduction is less than 40%, 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 also become coarse after hot stamping, causing a decrease in the impact resistance of the body. On the other hand, an excessive increase in the cumulative reduction rate increases the load on the rolling equipment and causes a decrease in productivity, so the cumulative reduction rate is preferably less than 70%. After cold rolling, treatment such as degreasing may be carried out in accordance with a conventional method.

[0092] The cold-rolled steel sheet is annealed to become an annealed steel sheet. In the annealing process, the soaking temperature is preferably set to more than 700°C in order to refine the metal structure of the annealed steel sheet (steel sheet for hot stamping) by recrystallization. If the soaking temperature is 700°C or lower, it may not be possible to favorably control the B concentration distribution in the surface layer region of the steel sheet for hot stamping. As a result, it may not be possible to obtain the desired crash resistance properties in the hot-stamped steel. On the other hand, if the heating rate is too slow, the soaking temperature is too high, or the soaking time is too long, grain growth may cause the metal structure of the annealed steel sheet to coarsen, resulting in a decrease in the impact resistance of the hot-stamped steel sheet. Therefore, the average heating rate to the soaking temperature is preferably 1°C / s or higher, the soaking temperature is preferably 800°C or lower, and the soaking time (holding time at the soaking temperature) is preferably less than 600 seconds. Furthermore, it is preferable that the dew point of the atmosphere in the annealing furnace be -20°C or higher but less than 0°C, and that the residence time in the temperature range of 700°C or higher but less than (Ac3 point -30°C) be more than 360 seconds but less than 600 seconds. Furthermore, it is preferable that the atmosphere in the annealing furnace be a nitrogen-hydrogen atmosphere containing 1% or more but less than 4% by volume of hydrogen.

[0093] If the dew point is lower than -20°C or higher than 0°C, or if the residence time in the temperature range of 700°C or higher and lower than (Ac3 point -30°C) is 360 seconds or shorter, the B concentration distribution in the surface layer region of the steel sheet for hot stamping may not be preferably controlled, and as a result, the desired crash resistance may not be obtained in the hot-stamped steel sheet. On the other hand, if the residence time in the above temperature range is 600 seconds or more, excessive decarburization occurs in the steel sheet for hot stamping, and the strength of the hot-stamped product after hot stamping may be insufficient. The annealed steel sheet produced by the above method may be plated in a conventional manner to obtain a plated steel sheet. The annealed steel sheet or plated steel sheet obtained in this manner may be subjected to temper rolling in a conventional manner.

[0094] The Ac3 point is the temperature at which ferrite disappears from the metal structure when the base steel sheet is heated, and can be determined from the change in thermal expansion when the cold-rolled steel sheet is heated at a heating rate of 8°C / second.

[0095] 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, the hot stamping is preferably performed by the following method.

[0096] In the heating step, prior to the hot stamping step, a steel sheet for hot stamping having the above-mentioned chemical composition and B concentration distribution in the sheet thickness direction is heated. In the heating step, a gas combustion furnace using a combustible gas containing propane gas is preferably used to heat the steel sheet for hot stamping at an air ratio of 0.84 or less. The heating temperature is preferably above 950°C and above the Ac3 point, and the holding time at the heating temperature is preferably above 360 ​​seconds. The air ratio is the ratio (A / A0) of the amount of air actually charged (A) to the theoretical amount of air (A0). The Ac3 point in the heating step refers to the Ac3 point of the inner layer region of the steel sheet for hot stamping, and may be the same value as the Ac3 point of the cold-rolled steel sheet determined by the above-mentioned method.

[0097] If the air ratio exceeds 0.84, the heating temperature is 950°C or lower, or the holding time is 360 seconds or shorter, it may not be possible to favorably control the B concentration distribution and the O concentration distribution in the surface layer region and the outermost surface layer region of the hot-stamped body. Furthermore, if the heating temperature is Ac3 point or lower, the volume fraction of martensite in the metal structure of the inner layer region of the hot-stamped body may be insufficient, which may result in a decrease in the strength of the hot-stamped body. On the other hand, if the heating temperature is too high or the holding time at the heating temperature is too long, the metal structure of the hot-stamped steel may become coarse, resulting in a decrease in the impact resistance and strength of the hot-stamped steel. Therefore, the heating temperature is preferably set to less than 1050°C, and the holding time is preferably set to less than 600 seconds.

[0098] In the hot stamping process, it is preferable to start hot stamping in a temperature range above 750°C after removing the heated steel sheet for hot stamping from the heating furnace and allowing it to cool in the atmosphere. If the hot stamping starting temperature is 750°C or lower, excessive ferrite may be formed in the metal structure of the inner layer region 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.

[0099] If the cooling rate is low, the volume fraction of martensite in the metal structure of the inner layer region 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 high, the volume fraction of martensite in the metal structure of the inner layer region 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.

[0100] The hot-stamped product according to this embodiment is obtained by the above method. After hot stamping, a reheating treatment may be performed as long as the strength of the hot-stamped product is ensured. When the reheating treatment is performed, the heating temperature is preferably less than (Ac3 point - 100°C). If the heating temperature of the reheating treatment is equal to or higher than (Ac3 point - 100°C), the surface and outermost surface regions of the hot-stamped product may not be sufficiently softened, which may result in a decrease in the impact resistance of the hot-stamped product. A portion of the hot-stamped product may be reheated by laser irradiation or the like to provide a partially softened region. The hot-stamped product may also be subjected to a blasting treatment, or to painting and baking treatment. [Example]

[0101] 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.

[0102] Molten steel was cast using a vacuum melting furnace to obtain steel with the chemical composition shown in Table 1. The resulting steel was heated to 1200°C and held there for 60 minutes. It was then subjected to 10 passes of hot rolling at temperatures above 900°C to obtain a 3.5 mm thick hot-rolled steel sheet. After hot rolling, the hot-rolled steel sheet was cooled to 540°C using a water spray. The cooling end temperature was the coiling temperature, and the hot-rolled steel sheet was placed in an electric heating furnace held at this coiling temperature and held there for 60 minutes. The hot-rolled steel sheet was then furnace-cooled to room temperature at an average cooling rate of 20°C / hour to simulate slow cooling after coiling. After furnace cooling, the hot-rolled steel sheet was pickled and then cold-rolled to obtain a 1.4 mm thick cold-rolled steel sheet. The cumulative reduction during cold rolling was 60%.

[0103] In Table 1, "-" indicates that the content of the element in question was below the detection limit. The Ac3 points in Table 1 were determined from the change in thermal expansion when the cold-rolled steel sheets of Steels A to N were heated at 8°C / second.

[0104] The obtained cold-rolled steel sheets were annealed using a continuous annealing simulator under the annealing conditions shown in Tables 2A and 2B. The steel sheets were heated to the soaking temperatures shown in Tables 2A and 2B at an average heating rate of 8°C / s. The atmosphere in the annealing furnace was a nitrogen-hydrogen atmosphere containing 3% by volume of hydrogen, with a dew point as shown in Tables 2A and 2B. After soaking, the steel sheets were cooled to room temperature to obtain annealed steel sheets (steel sheets for hot stamping).

[0105] Test pieces for GDS analysis were taken from three locations on the obtained hot stamping steel sheet. The surface of the test piece was used as the measurement starting surface, and GDS analysis was performed using the method described above from the measurement starting surface to a depth of 120 μm in the sheet thickness direction. This resulted in the average B concentration in the region from a depth of 5.0 μm to a depth of 25.0 μm from the surface (surface region) of the hot stamping steel sheet, as well as the B concentration at a depth of 100 μm from the surface. A total of 1,500 measurement points were measured from the surface of the steel sheet to a depth of 100 μm. The results are shown in Table 2.

[0106] Next, a hot stamping blank measuring 240 mm in width and 800 mm in length was cut from the obtained hot stamping steel sheet, and hot stamping was performed to obtain a hat member (hot-stamped product) with the shape shown in Figure 1. In the hot stamping process, the hot stamping blank was heated using a gas combustion furnace under the conditions shown in Table 3. Specifically, propane gas was used as the combustion gas, and the heating temperature, holding time, and air ratio were set as shown in Table 3. The hot stamping blank was then removed from the heating furnace and allowed to cool. It was then sandwiched between a die equipped with a cooling device and subjected to hat forming with a forming start temperature of 770°C or higher. Next, the blank was cooled in the die from the forming start temperature to 400°C at an average cooling rate of 50°C / s or higher to a cooling stop temperature of 80°C or lower. In addition, oxide scale (iron oxide) formed on the surface of the hat member was removed by shot blasting.

[0107] Test pieces were taken from the vertical wall portion of the obtained hat member, and the chemical compositions were measured by the method described above.

[0108] In addition, No. 13B plate-shaped test pieces were taken from the vertical wall portion of the hat member along the longitudinal direction of the hat member in accordance with JIS Z 2241:2011, and tensile strength was determined by performing a tensile test at a tensile speed of 10 mm / min. When the obtained tensile strength was 1900 MPa or more, it was judged to have high strength and pass, while when the obtained tensile strength was less than 1900 MPa, it was judged to not have high strength and fail.

[0109] In addition, test pieces for GDS analysis were taken from three locations on the vertical wall of the hat member, and GDS analysis was performed using the method described above, starting from the surface of the test piece and extending from the measurement starting surface to a depth of 120 μm in the sheet thickness direction. This resulted in the determination of the average B concentration in the region from 5.0 μm deep to 25.0 μm deep from the surface (surface region), the average B concentration and average O concentration in the region from 0.5 μm deep to 4.0 μm deep from the surface (outermost layer region), and the B concentration at a depth of 100 μm from the surface. A total of 1,500 measurement points were measured from the surface of the steel sheet to a depth of 100 μm.

[0110] In addition, test pieces for microstructure observation were taken from the vertical wall portion of the hat member, and the vertical cross section of this test piece was polished. After that, the metal structure was observed in the region from 0.5 μm deep from the surface to 4.0 μm deep from the surface (the outermost layer region) and from 100 μm deep from the surface to a position 1 / 2 the plate thickness (the inner layer region) using the method described above.

[0111] Additionally, a specimen for the three-point bending test was obtained by welding a closing plate with a thickness of 1.4 mm, width of 130 mm, and length of 800 mm to the hat member, as shown in Figure 2. A steel plate with a tensile strength of 1553 MPa was used for the closing plate.

[0112] The obtained test specimen was placed on two support rolls arranged with a roll distance of 700 mm, with the closing plate facing downwards, and a three-point bending test was carried out at a test speed of 2 m / s, as shown in Figure 3. From this, the maximum load and the displacement from when the test specimen came into contact with the impactor until cracks began to appear in the test specimen (crack initiation displacement) were determined. When the tensile strength of the steel sheet constituting the hot-stamped body was less than 2300 MPa, the maximum load was 18.0 kN or more and the crack initiation displacement was 50 mm or more, and the steel was judged to have excellent crash resistance and pass. When the tensile strength was 2300 MPa or more, the maximum load was 23.0 kN or more and the crack initiation displacement was 35 mm or more, and the steel was judged to have excellent crash resistance and pass. When these conditions were not met, the steel was judged to have poor crash resistance and fail.

[0113] Tables 4A and 4B show the results of measuring the chemical composition of the hot-stamped compacts, the results of measuring the mechanical properties of the hot-stamped compacts, the results of measuring the B and O concentration distributions in the hot-stamped compacts, and the results of evaluating the impact resistance of the hot-stamped compacts. The contents of elements other than C in the hot stamped steel were the same as those shown in Table 1, and therefore are omitted.

[0114] [Table 1]

[0115] [Table 2A]

[0116] [Table 2B]

[0117] [Table 3]

[0118] [Table 4A]

[0119] [Table 4B]

[0120] The hot-stamped steel according to the present invention had a high tensile strength of 1900 MPa or more. In addition, the average B concentration in the surface layer region was low, and the average B concentration and average O concentration in the outermost surface layer region were high, indicating excellent impact resistance. In the metal structure of the hot stamped steel according to the present invention, the volume fraction of martensite in the inner layer region was 91.0% or more, and the total volume fraction of structures other than martensite was 9.0% or less. In the outermost surface region, the volume fraction of ferrite was 6.0% or more, and the total volume fraction of structures other than ferrite was 94.0% or less.

[0121] In contrast, in the comparative examples (Test Nos. 1, 5, and 17) in which the chemical composition of the hot stamped body was outside the range of the invention, the C content was too low or the sol.Al content was too high, resulting in a tensile strength of less than 1900 MPa, poor strength, and a low maximum load, resulting in poor crash resistance.

[0122] In test number 15, the C content was too high, so the crack initiation displacement of the hot stamped body was low and the impact resistance properties were poor. In addition, premature fracture occurred in the tensile test, making it impossible to determine the tensile strength, and the fracture strength was less than 1900 MPa.

[0123] In Test No. 16, the B content was too low, so the tensile strength of the hot-stamped product was less than 1900 MPa, resulting in poor strength. In addition, the average B concentration and average O concentration in the outermost layer region were low, resulting in low maximum load and crack initiation displacement, and poor crash resistance.

[0124] Although the chemical compositions of the hot-stamped steel sheets were within the preferred ranges, the comparative examples, test numbers 3, 4, 7, 8, 10, 11, 13, 14, 24, 25, and 27, in which the manufacturing conditions were outside the preferred conditions, had one or more of the average B concentration in the surface layer region, the average B concentration in the outermost surface layer region, and the average O concentration in the outermost surface layer region outside the range of the invention. As a result, the hot-stamped steel sheets had low crack initiation displacements, or low maximum loads and crack initiation displacements, and poor crash resistance. [Industrial Applicability]

[0125] According to the above aspects of the present invention, it is possible to provide a hot-stamped steel sheet having high strength and excellent crash resistance.

Claims

1. A hot stamped body comprising a steel plate, All or part of the steel plate has a chemical composition, in mass%, C: more than 0.32%, less than 0.70%, Si: less than 2.00% Mn: 0.01 to 3.00%, P: 0.200% or less, S: 0.0200% or less, sol. Al: 0.001 to 1.000%, N: 0.0200% or less, O: 0.0005-0.0200%, B: 0.0005-0.0200%, Cr: 0-2.00%, Mo: 0-2.00%, W: 0-2.00%, Cu: 0-2.00%, Ni: 0-2.00%, Ti: 0-0.200%, Nb: 0 to 0.200%, V: 0 to 0.200%, Zr: 0-0.200%, Ca: 0-0.1000%, Mg: 0 to 0.1000%, REM: 0-0.1000%, Sn: 0-0.200%, As: 0 to 0.100%, and Bi: 0 to 0.0500% the balance being Fe and impurities; The tensile strength is 1900 MPa or more, an average B concentration in a region from a depth of 5.0 μm from the surface of the steel plate to a depth of 25.0 μm from the surface is 0.700 times or less of the B concentration at a depth of 100 μm from the surface; an average B concentration in a region from a depth of 0.5 μm to a depth of 4.0 μm from the surface is 1.600 times or more of the B concentration at the position at a depth of 100 μm from the surface; The hot-stamped product has an average O concentration of more than 0.0150 mass% in the region from a depth of 0.5 μm to a depth of 4.0 μm from the surface.

2. The chemical composition is, in mass %, Cr: 0.01-2.00%, Mo: 0.01-2.00%, W: 0.01-2.00%, Cu: 0.01-2.00%, Ni: 0.01-2.00%, Ti: 0.001 to 0.200%, Nb: 0.001-0.200%, V: 0.001-0.200%, Zr: 0.001 to 0.200%, Ca: 0.0001-0.1000%, Mg: 0.0001-0.1000%, REM: 0.0001-0.1000%, Sn: 0.001-0.200%, As: 0.001 to 0.100%, and Bi:0.0001~0.0500% The hot-stamped product according to claim 1, characterized in that it contains one or more members selected from the group consisting of

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