Hot stamped compact

Abstract

This hot-stamp-formed article has a specified chemical composition, in which the standard deviation of crystal grain diameters of prior austenite grains in an internal region is 5.0 μm or less, the area ratio of bainite in a surface layer region is more than 10%, the maximum value of a pole density in a texture is 4.0 or less, and the de-B index is 0.05 or more.

Description

[Technical Field]

[0001] The present invention relates to a hot stamped product. This application claims priority based on Japanese Patent Application No. 2022-067020, 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 lighter automobile bodies from the perspectives of environmental protection and resource conservation, and high-strength steel sheets are being used in automobile components. Automotive components are manufactured by press forming, but as the strength of steel sheets increases, not only do the forming loads increase but also formability decreases. Therefore, the formability of high-strength steel sheets into components with complex shapes becomes an issue.

[0003] To solve the above-mentioned problems, the application of hot stamping technology, in which steel sheets are heated to a high temperature in the austenite region where they soften and then press-formed, is being promoted. Hot stamping is attracting attention as a technology that achieves both formability into automotive parts and strength by performing quenching treatment in a die simultaneously with press working.

[0004] For example, Patent Document 1 discloses a high-strength electrogalvanized steel sheet with a high yield ratio and excellent bendability, in which the amount of diffusible hydrogen in the steel is 0.20 mass ppm or less. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2020 / 079925 [Non-patent literature]

[0006] [Non-Patent Document 1] Acta Materialia, 58(2010), 6393-6403 Summary of the Invention [Problem to be solved by the invention]

[0007] Increasing the strength of steel sheets is an effective way to further reduce the weight of automobile bodies. One way to increase the strength of steel sheets is to increase the Mn content to improve the hardenability of the steel sheets, but increasing the Mn content can lead to issues such as hydrogen embrittlement cracking and premature fracture.

[0008] Hydrogen embrittlement cracking is a phenomenon in which a steel component subjected to high stress during use is destroyed due to hydrogen that penetrates into the steel from the external environment. This phenomenon is also called delayed fracture due to the type of fracture that occurs. It is generally known that hydrogen embrittlement cracking in steel plate is more likely to occur as the tensile strength of the steel plate increases. This is thought to be because the higher the tensile strength of the steel plate, the greater the residual stress in the steel plate after part formation. The susceptibility to this hydrogen embrittlement cracking (delayed fracture) is called hydrogen embrittlement resistance.

[0009] Premature fracture is a phenomenon in which a steel member breaks at a stress lower than the tensile strength estimated from its hardness. The sensitivity to this premature fracture is called premature fracture resistance.

[0010] In Patent Document 1, bendability is taken into consideration, but hydrogen embrittlement resistance and early fracture resistance are not taken into consideration.

[0011] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a hot-stamped steel sheet having high strength and excellent resistance to hydrogen embrittlement and premature fracture. [Means for solving the problem]

[0012] The gist of the present invention is as follows. (1) A hot-stamped steel according to one aspect of the present invention has a chemical composition, in mass%, of C: more than 0.40%, less than 0.70%, Si: 0.010 to 3.00%, Mn: 0.60-3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0200% or less, Al: 0.0010~0.5000%, Nb: 0.0010~0.100%, Ti: 0.010 to 0.200%, Cr: 0.01 to 0.80%, Mo: 0.0010 to 1.000%, B: 0.0005~0.0200%, Co: 0-4.00%, Ni: 0-3.00% Cu: 0-3.00% V: 0~3.00%, W: 0~3.00%, Ca: 0 to 1.000%, Mg: 0 to 1.000%, REM: 0 to 1.000%, Sb: 0 to 1.000%, Sn: 0 to 1.000%, Zr: 0 to 1.000%, As: 0 to 0.100%, and The balance is Fe and impurities. In the internal region, which is a region from a depth of 4 / 16 of the plate thickness from the surface to a depth of 5 / 16 of the plate thickness from the surface, The standard deviation of the grain size of the prior austenite grains is 5.0 μm or less, In a surface layer region which is a region from the surface to a depth of 1 / 25 of the plate thickness, The area fraction of bainite is more than 10%, The maximum value of the pole density of the texture is 4.0 or less, It is an index that quantitatively shows the decrease in the concentration of B. The de-B index is 0.05 or higher. (2) The hot stamped steel according to (1) above, wherein the chemical composition is, in mass%, Co: 0.01 to 4.00%, Ni: 0.01 to 3.00% Cu: 0.01 to 3.00%, V: 0.01~3.00%, W: 0.01 to 3.00%, Ca: 0.001 to 1.000%, Mg: 0.001 to 1.000%, REM: 0.001 to 1.000%, Sb: 0.001 to 1.000%, Sn: 0.001 to 1.000%, Zr: 0.001 to 1.000%, and As: 0.001 to 0.100% The compound may contain one or more selected from the group consisting of: [Effects of the Invention]

[0013] According to the above aspects of the present invention, it is possible to provide a hot stamped steel sheet having high strength, and excellent resistance to hydrogen embrittlement and premature fracture. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 10 is a diagram for explaining how to determine the de-B index. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present inventors have found that the hydrogen embrittlement resistance and premature fracture resistance of a hot-stamped steel can be improved by reducing the standard deviation of the grain size of prior austenite grains in the inner region. The present inventors have also found that the hydrogen embrittlement resistance can be further improved by generating a desired amount of bainite in the surface layer region, forming a texture with a desired crystal orientation, and achieving a desired deboronation index.

[0016] The present inventors have found that, in order to obtain a hot stamped steel sheet having the above-described characteristics, it is particularly effective to perform finish rolling and annealing under desired conditions when producing a steel sheet to be subjected to hot stamping.

[0017] 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. In the following, the numerical ranges indicated by "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 %.

[0018] The hot stamped steel according to this embodiment has a chemical composition, in mass%, of C: more than 0.40% but not more than 0.70%, Si: 0.010 to 3.00%, Mn: 0.60 to 3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0200% or less, Al: 0.0010 to 0.5000%, Nb: 0.0010 to 0.100%, Ti: 0.010 to 0.200%, Cr: 0.01 to 0.80%, Mo: 0.0010 to 1.000%, B: 0.0005 to 0.0200%, and the balance: Fe and impurities. Each element will be explained below.

[0019] C: More than 0.40%, less than 0.70% C is an element that improves the strength of a hot-stamped steel sheet. If the C content is 0.40% or less, the desired strength cannot be obtained in the hot-stamped steel sheet. Therefore, the C content is set to more than 0.40%. The C content is preferably 0.42% or more or 0.44% or more. On the other hand, if the C content exceeds 0.70%, the toughness of martensite is too low to obtain excellent premature fracture resistance. Therefore, the C content is set to 0.70% or less. Preferably, the C content is 0.65% or less or 0.60% or less.

[0020] Si: 0.010 to 3.00% Silicon is an element that improves the strength of a hot stamped steel sheet through solid solution strengthening. If the Si content is less than 0.010%, the desired strength cannot be obtained. Therefore, the Si content is set to 0.010% or more. The Si content is preferably 0.05% or more, 0.10% or more, or 0.15% or more. On the other hand, if the Si content exceeds 3.00%, the amount of ferrite increases and the desired metal structure cannot be obtained. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.00% or less, 1.00% or less, or 0.70% or less.

[0021] Mn: 0.60 to 3.00% Mn is an element that promotes the transformation from prior austenite to pearlite in a hot-rolled steel sheet having the chemical composition according to this embodiment and contributes to controlling the prior austenite grain size distribution of the hot-stamped steel. To ensure that the standard deviation of the grain size of the prior austenite grains falls within a desired range, the Mn content is set to 0.60% or more. The Mn content is preferably 0.70% or more or 1.00% or more. On the other hand, if the Mn content exceeds 3.00%, the transformation from prior austenite to pearlite is excessively accelerated in the hot-rolled steel sheet having the chemical composition according to this embodiment, and the standard deviation of the grain size of the prior austenite grains in the hot-stamped steel sheet cannot be within the desired range. Therefore, the Mn content is set to 3.00% or less. Preferably, the Mn content is 2.50% or less or 2.30% or less.

[0022] P:0.100% or less P is an impurity element that segregates at grain boundaries, becoming the origin of fracture and degrading premature fracture resistance, so the P content is set to 0.100% or less, preferably 0.050% or less or 0.010% or less. The lower limit of the P content is not particularly limited, but may be 0%. However, if the P content is reduced to less than 0.0001%, the dephosphorization cost increases significantly, which is economically undesirable. Therefore, the P content may be set to 0.0001% or more, 0.001% or more, or 0.005% or more.

[0023] S: 0.0100% or less S is an impurity element that forms inclusions in steel. These inclusions act as fracture initiation sites, deteriorating premature fracture resistance. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less, 0.0050% or less, or 0.0030% or less. The lower limit of the S content is not particularly limited, but may be 0%. However, reducing the S content to less than 0.0001% significantly increases the desulfurization cost, which is economically undesirable. Therefore, the S content may be 0.0001% or more, 0.0002% or more, 0.0003% or more, or 0.0010% or more.

[0024] N: 0.0200% or less Nitrogen (N) is an impurity element that forms nitrides in steel. These nitrides act as fracture initiation sites, deteriorating premature fracture resistance. Therefore, the N content is set to 0.0200% or less. The N content is preferably 0.0150% or less, 0.0100% or less, 0.0060% or less, or 0.0040% or less. The lower limit of the N content is not particularly limited, but may be 0%. However, if the N content is reduced to less than 0.0001%, the cost of denitrification increases significantly, which is economically undesirable. Therefore, the N content may be set to 0.0001% or more or 0.0010% or more.

[0025] O: 0.0200% or less If the O content in steel is too high, it forms coarse oxides that act as fracture initiation sites, degrading the premature fracture resistance of the hot-stamped steel. Therefore, the O content is set to 0.0200% or less. The O content is preferably set to 0.00100% or less, 0.0070% or less, or 0.0040% or less. The O content may be 0%, but in order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be 0.0005% or more or 0.0010% or more.

[0026] Al: 0.0010 to 0.5000% Al is an element that has the effect of deoxidizing molten steel and improving the quality of the steel. If the Al content is less than 0.0010%, deoxidation is insufficient, resulting in the formation of coarse oxides and a deterioration in premature fracture resistance. Therefore, the Al content is set to 0.0010% or more. The Al content is preferably 0.0050% or more, 0.0100% or more, or 0.0300% or more. On the other hand, if the Al content exceeds 0.5000%, coarse oxides are formed in the steel, which reduces the premature fracture resistance of the hot-stamped steel. Therefore, the Al content is set to 0.5000% or less. The Al content is preferably 0.4000% or less, 0.3000% or less, 0.2000% or less, or 0.1000% or less.

[0027] Nb: 0.0010 to 0.100% Nb is an element that forms carbonitrides in steel and improves the strength of hot-stamped steel through precipitation strengthening. If the Nb content is less than 0.0010%, the desired strength cannot be obtained. Therefore, the Nb content is set to 0.0010% or more. The Nb content is preferably 0.005% or more, 0.009% or more, or 0.015% or more. On the other hand, if the Nb content exceeds 0.100%, a large amount of carbonitrides is formed in the steel, deteriorating the premature fracture resistance of the hot-stamped steel. Therefore, the Nb content is set to 0.100% or less. The Nb content is preferably 0.080% or less or 0.060% or less.

[0028] Ti: 0.010 to 0.200% Ti is an element that forms carbonitrides in steel and improves the strength of hot-stamped steel through precipitation strengthening. If the Ti content is less than 0.010%, the desired strength cannot be obtained. Therefore, the Ti content is set to 0.010% or more. The Ti content is preferably 0.020% or more or 0.025% or more. On the other hand, if the Ti content exceeds 0.200%, a large amount of carbonitrides is formed in the steel, deteriorating the premature fracture resistance of the hot-stamped steel. Therefore, the Ti content is set to 0.200% or less. The Ti content is preferably 0.150% or less, 0.100% or less, 0.080% or less, 0.060% or less, or 0.050% or less.

[0029] Cr: 0.01 to 0.80% Cr is an element that dissolves in prior austenite grains during heating before hot stamping, thereby increasing the strength of the hot stamped steel. If the Cr content is less than 0.01%, the desired strength cannot be obtained. Therefore, the Cr content is set to 0.01% or more. The Cr content is preferably set to 0.10% or more, 0.15% or more, or 0.20% or more. On the other hand, if the Cr content exceeds 0.80%, coarse intermetallic compounds are formed in the hot stamped steel, which deteriorates the premature fracture resistance. Therefore, the Cr content is set to 0.80% or less. The Cr content is preferably 0.70% or less, 0.50% or less, or 0.40% or less.

[0030] Mo: 0.0010 to 1.000% Mo is an element that dissolves in prior austenite grains during heating before hot stamping, thereby increasing the strength of the hot stamped steel. If the Mo content is less than 0.0010%, the desired strength cannot be obtained. Therefore, the Mo content is set to 0.0010% or more. The Mo content is preferably set to 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, if the Mo content exceeds 1.000%, coarse intermetallic compounds are formed in the hot stamped steel, which deteriorates the premature fracture resistance. Therefore, the Mo content is set to 1.000% or less. The Mo content is preferably 0.800% or less, 0.600% or less, or 0.400% or less.

[0031] B: 0.0005 to 0.0200% B is an element that improves the hardenability of steel. If the B content is less than 0.0005%, the desired strength cannot be obtained. Therefore, the B content is set to 0.0005% or more. The B content is preferably set to 0.0010% or more or 0.0015% or more. On the other hand, if the B content exceeds 0.0200%, coarse intermetallic compounds are formed in the hot stamped steel, deteriorating the premature fracture resistance. Therefore, the B content is set to 0.0200% or less. The B content is preferably 0.0150% or less, 0.0100% or less, 0.0080% or less, 0.0040% or less, or 0.0030% or less.

[0032] The remainder of the chemical composition of the hot stamped steel 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.

[0033] The hot stamped steel may contain the following elements as optional elements. When the following optional elements are not contained, the content is 0%.

[0034] Co: 0-4.00% Co is an element that improves the strength of the hot stamped steel by solid solution strengthening. To ensure this effect, the Co content is preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, since the above effects are saturated even when Co is contained in a large amount, the Co content is set to 4.00% or less. If necessary, the upper limit of the Co content may be set to 1.00%, 0.50%, 0.10%, 0.05%, or 0.02%.

[0035] Ni: 0 to 3.00% Ni has the effect of increasing the strength of the hot stamped body by dissolving in prior austenite grains during heating before hot stamping. To ensure this effect, the Ni content is preferably 0.01% or more. On the other hand, since the above effects are saturated even if the Ni content is large, the Ni content is preferably 3.00% or less. If necessary, the upper limit of the Ni content may be set to 1.50%, 1.00%, 0.50%, 0.10%, 0.05%, or 0.02%.

[0036] Cu: 0 to 3.00% Cu dissolves in prior austenite grains during heating before hot stamping, thereby enhancing the strength of the hot stamped steel. To ensure this effect, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.05% or more. On the other hand, since the above effects are saturated even when Cu is contained in a large amount, the Cu content is preferably set to 3.00% or less. If necessary, the upper limit of the Cu content may be set to 1.50%, 1.00%, 0.50%, 0.10%, 0.05%, or 0.02%.

[0037] V: 0~3.00% V forms carbonitrides in steel and has the effect of improving the strength of the hot-stamped steel by precipitation strengthening. To ensure this effect, the V content is preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, if the V content exceeds 3.00%, a large amount of carbonitrides is formed in the steel, deteriorating the premature fracture resistance of the hot-stamped steel. Therefore, the V content is set to 3.00% or less. If necessary, the upper limit of the V content may be set to 1.50%, 1.00%, 0.50%, 0.10%, 0.05%, or 0.02%.

[0038] W: 0~3.00% W has the effect of improving the strength of the hot-stamped steel. To reliably obtain this effect, the W content is preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, since the above effect saturates even if W is contained in a large amount, the W content is set to 3.00% or less. If necessary, the upper limit of the W content may be set to 1.50%, 1.00%, 0.50%, 0.10%, 0.05%, or 0.02%.

[0039] Ca: 0 to 1.000% Ca is an element that suppresses the generation of oxides that serve as fracture initiation sites, and contributes to improving premature fracture resistance. To reliably obtain this effect, the Ca content is preferably 0.001% or more. On the other hand, since the above effect saturates even if a large amount of Ca is added, the Ca content is set to 1.000% or less. If necessary, the upper limit of the Ca content may be set to 0.100%, 0.010%, 0.005%, 0.001%, 0.0005%, or 0.0002%.

[0040] Mg: 0 to 1.000% Mg forms oxides and sulfides in molten steel, suppresses the formation of coarse MnS, disperses many fine oxides, refines the metal structure, and contributes to improving premature fracture resistance. To reliably obtain these effects, the Mg content is preferably 0.001% or more. On the other hand, since the above effect saturates even if Mg is contained in a large amount, the Mg content is set to 1.000% or less. If necessary, the upper limit of the Mg content may be set to 0.100%, 0.010%, 0.005%, 0.001%, 0.0005%, or 0.0002%.

[0041] REM: 0 to 1.000% REM suppresses the generation of oxides that serve as fracture initiation sites and contributes to improving premature fracture resistance. To reliably obtain this effect, the REM content is preferably 0.001% or more. On the other hand, since the above effect saturates even if the REM content is contained in a large amount, the REM content is set to 1.000% or less. If necessary, the upper limit of the REM content may be set to 0.100%, 0.010%, 0.005%, 0.001%, 0.0005%, or 0.0002%. 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.

[0042] Sb: 0 to 1.000% Sb suppresses the generation of oxides that serve as the starting point for fracture and contributes to improving premature fracture resistance. To reliably obtain this effect, the Sb content is preferably 0.001% or more. On the other hand, since the above effect saturates even if Sb is added in a large amount, the Sb content is set to 1.000% or less. If necessary, the upper limit of the Sb content may be set to 0.100%, 0.050%, 0.020%, 0.010%, 0.005%, or 0.002%.

[0043] Sn: 0 to 1.000% Sn suppresses the generation of oxides that serve as the starting point for fracture and contributes to improving premature fracture resistance. To reliably obtain this effect, the Sn content is preferably 0.001% or more. On the other hand, since the above effect saturates even if Sn is contained in a large amount, the Sn content is set to 1.000% or less. If necessary, the upper limit of the Sn content may be set to 0.100%, 0.050%, 0.020%, 0.010%, 0.005%, or 0.002%.

[0044] Zr: 0 to 1.000% Zr suppresses the generation of oxides that serve as fracture initiation sites and contributes to improving premature fracture resistance. To reliably obtain this effect, the Zr content is preferably 0.001% or more. On the other hand, since the above effect saturates even if the Zr content is large, the Zr content is set to 1.000% or less. If necessary, the upper limit of the Zr content may be set to 0.100%, 0.050%, 0.020%, 0.010%, 0.005%, or 0.002%.

[0045] As: 0 to 0.100% As reduces the austenite single-phase temperature, thereby refining prior austenite grains and contributing to improving premature fracture resistance. To reliably obtain this effect, the As content is preferably 0.001% or more. On the other hand, since the above effect saturates even if the As content is large, the As content is set to 0.100% or less. If necessary, the upper limit of the As content may be set to 0.100%, 0.050%, 0.020%, 0.010%, 0.005%, or 0.002%.

[0046] The chemical composition of the hot-stamped body can be measured by a common analytical method, such as 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. When the surface of the hot stamped body is provided with a plating layer, a paint film, etc., the plating layer, paint film, etc. is removed by mechanical grinding before analyzing the chemical composition.

[0047] Next, the metal structure of the hot stamped steel according to this embodiment will be described. In the hot-stamped steel according to this embodiment, in an internal region extending from a depth of 4 / 16 of the plate thickness (thickness of the hot-stamped steel) from the surface of the hot-stamped steel to a depth of 5 / 16 of the plate thickness from the surface, the standard deviation of the grain size of prior austenite grains is 5.0 μm or less; in a surface layer region extending from the surface to a depth of 1 / 25 of the plate thickness from the surface, the area fraction of bainite exceeds 10%, the maximum value of the pole density of the texture is 4.0 or less, and the deB index is 0.05 or more.

[0048] In this embodiment, the internal region refers to a region from a depth of 4 / 16 of the plate thickness from the surface of the hot stamped steel to a depth of 5 / 16 of the plate thickness from the surface. The surface layer region refers to a region extending from the surface of the hot stamped body to a depth of 1 / 25 of the plate thickness. In addition, when the hot-stamped product has a plating layer, a paint film, etc. on its surface, the term "surface" here refers to the interface between the plating layer and the base steel sheet, and for convenience, the plating layer, paint film, etc. are excluded from the hot-stamped product. Specifically, when the hot-stamped product has a plating layer, paint film, etc. on its surface, as described below, for convenience, the region where the iron concentration is less than 90 mass% in GD-OES measurement, that is, the plating layer, paint film, etc., is excluded from the hot-stamped product, and the measurement point where the iron concentration is 90 mass% (that is, the interface between the base steel material and the plating layer, etc.) is considered to be the surface of the hot-stamped product. As mentioned above, the plated layer, paint film, etc. are excluded from the hot-stamped body. However, if the thickness of the plated layer, paint film, etc. is so small as to be negligible compared to the thickness of the hot-stamped body (however, if only the plated layer is present, the thickness of the plated layer is often so small that it can be ignored in most cases), then when measuring the thickness of the hot-stamped body, the thickness of the hot-stamped body may be taken as the thickness including the plated layer, paint film, etc.

[0049] (internal area) Standard deviation of prior austenite grain size: 5.0 μm or less By reducing the variation in the grain size of the prior austenite grains in the internal region, i.e., by reducing the standard deviation, it is possible to suppress an increase in local residual stress. As a result, it is possible to improve the hydrogen embrittlement resistance and premature fracture resistance of the hot-stamped steel. If the standard deviation of the grain size of the prior austenite grains exceeds 5.0 μm, the hydrogen embrittlement resistance and premature fracture resistance deteriorate. Therefore, the standard deviation of the grain size of the prior austenite grains is set to 5.0 μm or less. Preferably, it is 4.0 μm or less, 3.0 μm or less, or 2.5 μm or less. The lower limit of the standard deviation of the grain size of the prior austenite grains does not need to be particularly limited, but may be 0.1 μm, 0.5 μm, 1.0 μm, or 1.5 μm.

[0050] The standard deviation of the grain size of the prior austenite grains is obtained by the following method. A sample is cut out from any position at least 50 mm away from the end face of the hot-stamped product (if a sample cannot be taken from this position, a position avoiding the end) so that a thickness cross section parallel to the rolling direction can be observed. The size of the sample depends on the measuring device, but it should be large enough to allow observation of about 10 mm in the rolling direction.

[0051] The cross section of the sample is polished using #600 to #1500 silicon carbide paper, then polished to a mirror finish using a liquid in which diamond powder with a grain size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water. The observation surface is then polished by electrolytic polishing. At any position along the longitudinal direction of the sample cross section, a 50 μm long region is measured using electron backscatter diffraction at 0.1 μm measurement intervals, from a depth of 4 / 16 of the plate thickness from the surface to a depth of 5 / 16 of the plate thickness from the surface, to obtain crystal orientation information. Measurements can be performed using an EBSD analyzer consisting of a thermal field emission scanning electron microscope and an EBSD detector, such as a JEOL JSM-7001F and a TSL DVC5 detector. The vacuum level inside the EBSD analyzer is 9.6 × 10 -5 The acceleration voltage may be 15 kV and the probe current level may be 13.

[0052] Using the obtained crystal orientation information, the crystal orientation of the prior austenite grains is calculated from the crystal orientation relationship between general prior austenite grains and crystal grains having a body-centered structure after transformation, and this is used to calculate the average crystal grain size of the prior austenite grains, and then its standard deviation is calculated.

[0053] The crystal orientation of the prior austenite grains is calculated as follows. First, a crystal orientation map of the prior austenite grains is created using the method described in Non-Patent Document 1. For one of the prior austenite grains included in the observation field, the average value of the shortest and longest diameters is calculated, and this average value is used as the grain size of that prior austenite grain. The above operation is performed for all prior austenite grains, excluding prior austenite grains that are not entirely included in the observation field, such as those at the edges of the observation field, to determine the grain sizes of all prior austenite grains in the observation field. The standard deviation is calculated from the obtained grain sizes of all prior austenite grains to obtain the standard deviation of the grain sizes of the prior austenite grains. In this embodiment, the rolling direction of the hot-stamped steel is determined by the following method. First, a test piece is taken from any position at least 50 mm away from the end of the hot-stamped product so that the thickness cross section can be observed. The thickness cross section of the taken test piece is mirror-polished and then observed using an optical microscope at magnifications of 100x, 200x, 500x, and 1000x. An appropriate magnification at which the dimensions of the inclusions can be measured is selected depending on the size of the inclusions. The observation range is 500 μm or more in width and across the entire thickness of the product, and dark areas are determined to be inclusions. Observation may be performed using multiple fields of view. Next, using the thickness cross section initially observed using the above method as a reference, the test piece is rotated in 5° increments in the range of 0° to 180° around the thickness direction, and the cross sections parallel to the plane are observed using the same method. The average length of the major axes of the multiple inclusions in each cross section is calculated for each cross section. The cross section with the largest average value of the major axis length of the inclusions obtained is identified. The direction parallel to the longitudinal axis of the inclusions in the cross section is determined to be the rolling direction.

[0054] The metal structure of the inner region is not particularly limited as long as it can provide the desired strength, hydrogen embrittlement resistance, and early fracture resistance, but may be, for example, composed of a total of 90 to 100% (90% or more and 100% or less) of martensite and bainite by area, and 0 to 10% (0% or more and 10% or less) of ferrite and retained austenite. Note that martensite in this embodiment includes untempered martensite (fresh martensite) and tempered martensite. The metal structure of the hot stamped body is measured by the following method.

[0055] A sample is cut out from any position at least 50 mm away from the end face of the hot-stamped product (if a sample cannot be taken from this position, a position avoiding the end) so that a thickness cross section parallel to the rolling direction can be observed. The size of the sample depends on the measuring device, but it should be large enough to allow observation of about 10 mm in the rolling direction.

[0056] The cross section of the sample is polished using #600 to #1500 silicon carbide paper, then mirror-finished using a diluent such as alcohol or a liquid in which diamond powder with a grain size of 1 to 6 μm is dispersed in pure water. The observation surface is then polished by electrolytic polishing. At any longitudinal position on the sample cross section, a 50 μm long region extending from a depth of 4 / 16 of the plate thickness from the surface of the hot-stamped compact to a depth of 5 / 16 of the plate thickness from the surface is measured using electron backscatter diffraction at 0.1 μm measurement intervals to obtain crystal orientation information. Measurements can be performed using an EBSD analyzer consisting of a thermal field emission scanning electron microscope and an EBSD detector, such as a JEOL JSM-7001F and a TSL DVC5 detector. The vacuum level inside the EBSD analyzer is 9.6 × 10 -5 The acceleration voltage may be 15 kV and the probe current level may be 13.

[0057] The obtained crystal orientation information is used with the "Phase Map" function of the "OIM Analysis®" software provided with the EBSD analyzer to determine whether the crystal structure is fcc or retained austenite. The area fraction of this retained austenite is calculated to determine the area fraction of retained austenite. Next, regions with a bcc crystal structure are determined to be bainite, martensite, and ferrite. For these regions, the "Grain Average Misorientation" function of the "OIM Analysis®" software provided with the EBSD analyzer is used to extract regions with a "Grain Average Misorientation" of 0.5° or less as ferrite, under the condition that 5° grain boundaries are considered to be grain boundaries. The area fraction of the extracted ferrite is then calculated to determine the area fraction of ferrite.

[0058] Next, the area ratio of the remaining region (the region where "Grain Average Misorientation" exceeds 0.5°) is calculated, and this area ratio is taken as the total area ratio of martensite and bainite.

[0059] (Surface region) Area ratio of bainite: over 10% By forming bainite in the surface layer region, the dislocation density in the surface layer region can be reduced. As a result, the penetration of hydrogen from the external environment can be suppressed, and the hydrogen embrittlement resistance of the hot-stamped body can be improved. Furthermore, by forming bainite in the surface layer region, excessive softening of the surface layer can be suppressed, thereby improving the hydrogen embrittlement resistance while maintaining the load-bearing capacity of the member. If the area fraction of bainite in the surface layer region is 10% or less, the hydrogen embrittlement resistance deteriorates. Therefore, the area fraction of bainite is set to more than 10%. Preferably, it is 20% or more, 40% or more, or 60% or more. The upper limit of the area ratio of bainite is not particularly limited, but may be 100%, 90%, or 80%.

[0060] The metal structure of the surface layer region may contain, in addition to bainite, 0 to 90% (0% or more and 90% or less) martensite, and a total of 0 to 65% (0% or more and 65% or less) ferrite and retained austenite. The area ratio of the metal structure is calculated for the surface region (the region from the surface to a depth of 1 / 25 of the plate thickness) by the following method.

[0061] A sample is cut out from any position at least 50 mm away from the end face of the hot-stamped product (if a sample cannot be taken from this position, a position avoiding the end) so that a thickness cross section parallel to the rolling direction can be observed. The size of the sample depends on the measuring device, but it should be large enough to allow observation of about 10 mm in the rolling direction.

[0062] The cross section of the sample is polished using #600 to #1500 silicon carbide paper, and then mirror-finished using a liquid in which diamond powder with a grain size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water. The observation surface is then polished by electrolytic polishing. At any position along the longitudinal direction of the sample cross section, a 50 μm long region extending from the surface of the hot-stamped compact to a depth of 1 / 25 of the plate thickness is measured using electron backscatter diffraction at 0.1 μm measurement intervals to obtain crystal orientation information. An EBSD analyzer consisting of a thermal field emission scanning electron microscope and an EBSD detector can be used for the measurement. For example, an EBSD analyzer consisting of a JEOL JSM-7001F and a TSL DVC5 detector can be used. The vacuum level inside the EBSD analyzer is 9.6 × 10 -5 The acceleration voltage may be 15 kV and the probe current level may be 13.

[0063] The obtained crystal orientation information is used with the "Phase Map" function of the "OIM Analysis®" software attached to the EBSD analyzer to determine that the crystal structure is fcc and to determine the area fraction of retained austenite. The area fraction of this retained austenite is then calculated to obtain the area fraction of retained austenite. Next, in the region with a bcc crystal structure, the "Grain Average Misorientation" function of the "OIM Analysis®" software attached to the EBSD analyzer is used to extract, as bainite, regions where the "Grain Average Misorientation" is greater than 0.50° and less than 0.75° under the condition that 5° grain boundaries are considered to be grain boundaries. The area fraction of bainite is then calculated to obtain the area fraction of bainite. Next, the area where "Grain Average Misorientation" is 0.5° or less is extracted as ferrite. The area ratio of the extracted ferrite is calculated to obtain the ferrite area ratio. The remaining area (the area where "Grain Average Misorientation" is over 0.75°) is extracted as martensite, and the area ratio of martensite is calculated to obtain the martensite area ratio.

[0064] (Surface region) Crystal orientation in the surface region: Maximum value of the pole density of the texture is 4.0 or less By controlling the texture in the surface layer region, it is possible to suppress the penetration of hydrogen into the surface layer region from the external environment, thereby improving the hydrogen embrittlement resistance of the hot-stamped body. If the maximum value of the pole density of the texture in the surface layer region exceeds 4.0, the hydrogen embrittlement resistance of the hot-stamped body deteriorates. Therefore, the maximum value of the pole density of the texture in the surface layer region is set to 4.0 or less. Preferably, it is 3.5 or less, 3.0 or less, or 2.5 or less. The lower limit of the pole density of the texture in the surface layer region is not particularly limited, but may be 1.0 or 1.2.

[0065] The texture in the surface region (the region from the surface to a depth of 1 / 25 of the plate thickness) is obtained by the following method. A sample is cut out from any position at least 50 mm away from the end face of the hot-stamped product (if a sample cannot be taken from this position, a position avoiding the end) so that a thickness cross section parallel to the rolling direction can be observed. The size of the sample depends on the measuring device, but it should be large enough to allow observation of about 10 mm in the rolling direction.

[0066] The cross section of the sample is polished using #600 to #1500 silicon carbide paper, then polished to a mirror finish using a liquid in which diamond powder with a grain size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water. It is then finished by electrolytic polishing. At any position along the longitudinal direction of the sample cross section, a region 1000 μm long, extending from the surface to a depth of 1 / 25 of the plate thickness, is measured using electron backscatter diffraction at 5.0 μm measurement intervals to obtain crystal orientation information. Measurements can be performed using an EBSD analyzer consisting of a thermal field emission scanning electron microscope and an EBSD detector, such as a JEOL JSM-7001F and a TSL DVC5 detector. The vacuum level inside the EBSD analyzer is 9.6 × 10 -5 The acceleration voltage may be 15 kV and the probe current level may be 13.

[0067] The obtained crystal orientation information is used with the "Texture" function of the "OIM Analysis (registered trademark)" software attached to the EBSD analyzer to perform intensity calculations using harmonic series expansion for crystal grains with a bcc crystal structure. In this case, the expansion order is set to 16, and the half-width when applying a Gaussian distribution is set to 5°. Next, the "Texture Plot" function is used for the output file after intensity calculation to output the φ2 = 45° cross section in the crystal orientation distribution function (ODF). The maximum value of the pole density in the φ2 = 45° cross section is taken as the pole density of the texture in the surface region.

[0068] (Surface area) B removal index: 0.05 or more The deB index is an index that quantitatively represents the amount of decrease in the B concentration in the surface layer region. By reducing the B concentration in the surface layer region, the strength of the prior austenite before transformation decreases, improving the deformability of the prior austenite grains and facilitating the generation of randomly oriented crystal grains in the surface layer region. If the deB index in the surface layer region is less than 0.05, it is not possible to obtain crystal grains with a desired texture in the surface layer region. Therefore, the deB index is set to 0.05 or more. Preferably, it is 0.20 or more, 0.30 or more, or 0.35 or more. The upper limit of the de-B index is not particularly limited, but may be set to 1.00, 0.80, or 0.60.

[0069] The de-B index in the surface layer region can be obtained by the following method. The element concentration distribution in the thickness direction of the hot-stamped compact is measured using a glow discharge optical emission spectrometer (GD-OES: a Marcus-type high-frequency glow discharge optical emission spectrometer, GD-PROFILER-HR, manufactured by HORIBA, Ltd.). The measurement conditions are an analysis diameter of 4 mm, a sputtering rate of 4 μm / min, an argon pressure of 600 Pa, an RF output of 35 W, and a measurement interval of 0.02 μm or less. Measurements are performed for all elements contained in the hot-stamped compact.

[0070] In addition, when the hot-stamped product has a plating layer or the like on its surface, the "surface" here refers to the interface between the plating layer or the like and the base steel sheet. In cases where the surface has a plating layer, paint film, or the like, the plating layer or paint film is partially or completely removed by mechanical polishing or chemical polishing so that measurements can be made up to a depth of 200 μm from the surface of the base steel sheet (the interface between the base steel sheet and the plating layer or paint film, etc.) before GD-OES measurement. In GD-OES measurement, the measurement point where the iron concentration is 90 mass% is considered to be the surface of the hot-stamped product. In the following explanation, for convenience of explanation, the hot-stamped product may also be referred to as the base steel sheet.

[0071] Next, the B concentration is measured from the surface of the hot-stamped body to a depth of at least 100 μm from the surface. After measuring the B concentration at a depth of 100 μm from the surface, if the absolute value of the difference between the average B concentration in a region 80 to 100 μm and the maximum measured B concentration in the region 80 to 100 μm is 0.0006 mass% or less, and the absolute value of the difference between the average B concentration in the region 80 to 100 μm and the minimum measured B concentration in the region 80 to 100 μm is 0.0006 mass% or less, the measurement of the B concentration in the depth direction is terminated at a depth of 100 μm from the surface. If this requirement for ending measurement is not met, measurement of the B concentration in the depth direction is continued. Then, each time a new measurement value of the B concentration in the depth direction is obtained, the average B concentration in the region from the deepest point to 20 μm from the deepest point toward the surface is calculated, and if the absolute value of the difference between the average B concentration in the region from the deepest point to 20 μm from the deepest point toward the surface and the maximum measurement value of the B concentration in the region from the deepest point to 20 μm from the deepest point toward the surface is 0.0006 mass% or less, and the absolute value of the difference between the average B concentration in the region from the deepest point to 20 μm from the deepest point toward the surface and the minimum measurement value of the B concentration in the region from the deepest point to 20 μm from the deepest point toward the surface is 0.0006 mass% or less, measurement of the B concentration in the depth direction is ended at that position. For example, when a measurement value of the B concentration is obtained at a depth of 150 μm from the surface, if the absolute value of the difference between the average B concentration in a region 130 to 150 μm deep from the surface and the maximum measured B concentration value in the region 130 to 150 μm deep from the surface is 0.0006 mass% or less, and if the absolute value of the difference between the average B concentration in the region 130 to 150 μm deep from the surface and the minimum measured B concentration value in the region 130 to 150 μm deep from the surface is 0.0006 mass% or less, the measurement of the B concentration in the depth direction is terminated at a depth of 150 μm from the surface. Even if the requirements for ending measurement are not met and measurement of the B concentration in the depth direction cannot be completed, measurement of the B concentration in the depth direction is completed when measurement of the B concentration at a depth of 200 μm from the surface is completed. Then, when measurement of the B concentration in the depth direction is completed, the average value of the B concentration in the region from the deepest part (the deepest position where the B concentration used to calculate the B removal index was obtained) to a position 20 μm toward the surface from the deepest part (hereinafter, the average value of the B concentration in this region is referred to as the average B concentration in the deepest 20 μm) is used to calculate the B removal index below. For convenience of measurement, for example, the B concentration may be measured from the surface to a depth of 200 μm, and then the shallowest depth position that satisfies the above-mentioned condition for ending the B concentration measurement in the depth direction may be found in the region from the surface to a depth of 100 to 200 μm, and if such a position is found, the B desorption index may be calculated without using the B concentration measurement results at positions deeper than that depth position. For example, the B concentration may be measured from the surface to a depth of 200 μm, and in this case, if the shallowest depth position that satisfies the above-mentioned condition for ending the B concentration measurement in the depth direction is found in the region 100 μm or more from the surface, the measurement may be considered to have ended at that depth position, and the B desorption index may be calculated.

[0072] Hot stamped compact Surface to deepest part In this region, the decrease in B concentration per unit depth (the average B concentration in the deepest 20 μm minus the B concentration at each measurement point) is calculated, and the integral of the product of the decrease in B concentration and unit depth is calculated to determine the area of ​​the B-deficient region (area of ​​region A in Figure 1). However, if the value obtained by subtracting the B concentration at each measurement point from the average B concentration in the deepest 20 μm is negative, the integral is set to 0 (due to the de-B phenomenon near the surface, the B concentration at each measurement point is almost always lower than the average B concentration in the deepest 20 μm, and this integral value is positive). Next, the product of the average B concentration in the deepest 20 μm and the length of 200 μm is calculated as the reference area (area of ​​the rectangular region B in Figure 1). The B-deficient area (area of ​​region A) divided by the reference area (area of ​​region B) is determined to be the de-B index (area of ​​region A / area of ​​region B). Even when the requirement for measurement to be completed within 200 μm from the surface is met as described above, the length by which the average B concentration in the deepest 20 μm is multiplied is set to 200 μm to calculate the reference area (area of ​​region B).

[0073] The hot-stamped product according to this embodiment may have a plating layer on its surface. By having a plating layer on its surface, corrosion resistance can be improved after hot stamping. Examples of the plating layer include an aluminum plating layer, an aluminum-zinc plating layer, an aluminum-silicon plating layer, a hot-dip galvanized layer, an electrogalvanized layer, a galvannealed hot-dip galvanized layer, a zinc-nickel plating layer, and an aluminum-magnesium-zinc based plating layer.

[0074] Next, a steel sheet for hot stamping for obtaining the hot stamped steel according to this embodiment will be described. The steel sheet for hot stamping has the above-mentioned chemical composition. The metal structure of the steel sheet for hot stamping is not particularly limited as long as it can provide the desired strength, hydrogen embrittlement resistance, and premature fracture resistance after hot stamping, but may, for example, be composed of, in area ratios, 5 to 90% ferrite, 0 to 100% bainite and martensite, 10 to 95% pearlite, and 0 to 5% retained austenite. In addition, the steel sheet may contain iron carbides, alloy carbides, intermetallic compounds, and inclusions.

[0075] Furthermore, the steel sheet for hot stamping may have a plating layer on its surface. By having a plating layer on its surface, corrosion resistance can be improved after hot stamping. Examples of the plating layer include an aluminum plating layer, an aluminum-zinc plating layer, an aluminum-silicon plating layer, a hot-dip galvanized layer, an electrogalvanized layer, a galvannealed hot-dip galvanized layer, a zinc-nickel plating layer, and an aluminum-magnesium-zinc based plating layer.

[0076] Manufacturing method of steel sheet for hot stamping Hereinafter, a method for producing a steel sheet for hot stamping for obtaining the hot-stamped steel according to this embodiment will be described. In order to obtain the hot-stamped steel, it is effective to control the finish rolling conditions and annealing conditions in particular in the method for producing a steel sheet for hot stamping.

[0077] Finishing rolling In finish rolling, the reduction ratio of the final pass (final reduction ratio) is preferably 20% or more. The final reduction ratio can be expressed as {(t0-t1) / t0} × 100 (%), where t0 is the thickness before the final rolling pass and t1 is the thickness after the final rolling pass. By increasing the final reduction ratio, pearlite is uniformly dispersed in the hot-rolled steel sheet after rolling. This pearlite becomes a reverse transformation site for prior austenite during heating in hot stamping. Therefore, uniform dispersion of pearlite reduces the standard deviation of the grain size of prior austenite grains in the hot-stamped steel sheet. As a result, the premature fracture resistance of the hot-stamped steel sheet can be improved. A final reduction ratio of 30% or more, 40% or more, or 45% or more is more preferable. When the Mn content is 0.60% or more as in the chemical composition of the hot-stamped steel according to this embodiment, it is important to increase the final reduction rate of the finish rolling as described above in order to favorably control the texture in the surface layer region of the hot-stamped steel.

[0078] The conditions for the casting method of molten steel, heating before hot rolling, rough rolling, coiling, and cold rolling are not particularly limited, and general conditions may be used. The coiling temperature may be 750°C or lower. By setting the coiling temperature to 750°C or lower, it is possible to prevent ferrite from being linked and arranged in the hot-rolled steel sheet after rolling, and pearlite is uniformly dispersed. This pearlite becomes a reverse transformation site for prior austenite during heating for hot stamping. Therefore, uniform dispersion of pearlite reduces the standard deviation of the crystal grain size of prior austenite grains in the hot-stamped steel. As a result, the premature fracture resistance of the hot-stamped steel can be improved. Furthermore, in order to soften the hot-rolled steel sheet, the coil after coiling may be subjected to a softening heat treatment. The method of the softening heat treatment is not particularly limited, and general conditions may be used.

[0079] annealing After cold rolling, it is preferable to perform annealing by heating in an oxidizing atmosphere for 15 seconds or more. Normally, annealing is preferably performed in a reducing atmosphere to suppress scale formation, but in this embodiment, annealing is performed in an oxidizing atmosphere to promote scale formation on the steel sheet surface. During heating for hot stamping, the scale formed on the steel sheet surface acts as an oxidation source, oxidizing C and B in the surface layer region. The oxidized C and B are removed from the steel sheet surface, reducing the amounts of C and B in the surface layer region. As a result, the strength of prior austenite grains decreases, making them more susceptible to deformation, and randomly oriented crystal grains are more likely to be generated. This allows crystal grains with a desired texture to be generated in the surface layer region. The heating temperature during annealing may be in the range of 730 to 900°C, and by maintaining the steel in this heating temperature range for 15 seconds or longer, it is possible to promote scale formation while suppressing scale spalling. The annealing time is preferably 100 seconds or longer, more preferably 200 seconds or longer, and even more preferably 300 seconds or longer. On the other hand, annealing for more than 3600 seconds is not preferred because the prior austenite grain size becomes coarse, the grain boundary diffusion rate of B decreases, deboronation does not progress, and the deboronation index does not reach 0.05 or more. Therefore, the annealing time is preferably 3600 seconds or shorter. After annealing in an oxidizing atmosphere, the steel sheet may be subjected to another annealing step in an oxidizing or non-oxidizing atmosphere, as long as no treatment (such as pickling) for removing oxide scale is performed.

[0080] In this embodiment, the oxidizing atmosphere may be any heating atmosphere in which oxide scale is formed on the surface of the steel sheet, and may be any general condition. For example, in a gas combustion atmosphere, the mixture ratio of air to fuel (air-fuel ratio) is preferably controlled to 0.80 or more, and more preferably to more than 1.00. Annealing in an oxidizing atmosphere preferably forms oxide scale of 15 μm or more on the surface of the steel sheet. It is preferable that the oxide scale on the steel sheet surface is left in subsequent processes. That is, it is preferable to perform the hot stamping described below with the oxide scale remaining. The oxide scale is removed by shot blasting after hot stamping. Furthermore, even when a plating layer is formed on the surface of a steel sheet for hot stamping, an oxide scale remains at the interface between the base steel sheet and the plating layer. When a plating layer is formed, the oxide scale disappears after hot stamping due to an alloying reaction during heating before hot stamping.

[0081] The hot stamped steel sheet for hot stamping manufactured by the above-described method is hot stamped to obtain a hot-stamped steel according to this embodiment. The hot stamping conditions are not particularly limited, but for example, it is preferable to heat the steel sheet for hot stamping to a temperature range of 800°C to 1000°C and hold it at this temperature range for 60 to 600 seconds. If the heating temperature is less than 800°C, austenitization may be insufficient, the desired prior austenite grain size distribution may not be obtained, and premature fracture resistance may deteriorate. On the other hand, if the heating temperature exceeds 1000°C, prior austenite may undergo excessive grain growth, the desired prior austenite grain size distribution may not be obtained, and premature fracture resistance may deteriorate. If the holding time is less than 60 seconds, austenitization may be insufficient, the desired prior austenite grain size distribution may not be obtained, and premature fracture resistance may deteriorate. If the holding time exceeds 600 seconds, prior austenite may undergo excessive grain growth, the desired prior austenite grain size distribution may not be obtained, and premature fracture resistance may deteriorate.

[0082] The heating atmosphere is not particularly limited and may be under normal conditions, such as in the atmosphere, a gas combustion atmosphere in which the ratio of air to fuel is controlled, or a nitrogen atmosphere, and the dew point of these gases may be controlled. After holding the material in this temperature range, it is hot stamped. After hot stamping, it is sufficient to cool the material to a temperature range of 250°C or lower at an average cooling rate of 20°C / s or more.

[0083] Examples of the heating method before hot stamping include heating with an electric furnace or gas furnace, flame heating, electrical heating, high frequency heating, and induction heating.

[0084] The hot-stamped product according to this embodiment is obtained by the above method. After hot stamping, tempering treatment at 130 to 600°C or bake hardening treatment after painting may be performed. Furthermore, a part of the hot-stamped product may be tempered by laser irradiation or the like to provide a partially softened region. [Example]

[0085] 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.

[0086] Slabs produced by casting molten steel having the chemical compositions shown in Tables 1A to 1T were held at a temperature of 1200°C or higher for 20 minutes or longer, and then finish-rolled, coiled, and annealed under the conditions shown in Tables 2A to 2H. Annealing was carried out in an oxidizing atmosphere, with the exception of some examples. For examples not specifically noted in the remarks column in the tables, the air-to-fuel mixture ratio (air-fuel ratio) in the oxidizing atmosphere was controlled to 1.05 in the gas combustion atmosphere during annealing. For some examples, annealing was carried out in a reducing atmosphere, as shown in the tables, and the coils were subjected to softening heat treatment after coiling.

[0087] The obtained steel sheet for hot stamping was heated to a temperature range above 800°C (hot stamp heating) in a furnace to which nitrogen gas was continuously supplied, and after holding at that temperature range, hot stamped, and then hot stamped under conditions of cooling to a temperature range of 250°C or less at an average cooling rate of 20°C / s or more. In this way, hot stamped products shown in Tables 3A to 3H were obtained. Note that, for examples not specifically noted in the remarks column in the tables, a gas combustion atmosphere was used in which the mixture ratio of air to fuel (air-fuel ratio) was controlled to 0.85. However, in some examples, heating in a furnace adjusted to a different atmosphere, re-annealing, plating, tempering or hot stamp heating was performed as shown in the table.

[0088] The underlines in the table indicate that the product is outside the range of the present invention, that the production conditions are not within the preferred range, or that the characteristic values ​​are not preferred. The metal structure of the hot-stamped body (including the standard deviation of the austenite grain size), the deboronation index, and the pole density of the texture were measured by the methods described above. The mechanical properties of the hot-stamped body were evaluated by the following methods.

[0089] Tensile strength The tensile (maximum) strength TS of the hot-stamped compact was measured by preparing a No. 5 test piece from any position on the hot-stamped compact and conducting a tensile test in accordance with JIS Z 2241:2011. The crosshead speed was 1 mm / min. A tensile strength TS of 2200 MPa or more was judged to be high strength and pass, while a tensile strength of less than 2200 MPa was judged to be low strength and fail. For examples that failed the early fracture resistance test described below, the tensile strength was determined by multiplying the Vickers hardness measured by the method for evaluating early fracture resistance described below by 3.3 (=Vickers hardness × 3.3).

[0090] Hydrogen embrittlement resistance The hydrogen embrittlement resistance of the hot-stamped compact was evaluated using the following method. A 68 mm long, 6 mm wide test piece was taken from a random position on the hot-stamped compact. The edges of the test piece were polished using #200 to #1500 silicon carbide paper, and then polished to a mirror finish using a liquid in which diamond powder with a particle size of 1 to 6 μm was dispersed in a diluted solution such as alcohol or pure water. The corners of the test piece were then chamfered using #200 to #1500 silicon carbide paper. A stress of 800 MPa or more was applied to the test piece, and it was immersed in 1 liter of hydrochloric acid adjusted to pH 4 at room temperature for 48 hours. The presence or absence of cracks was evaluated. A test piece that did not develop cracks even under a load stress of 800 MPa or more was deemed to have passed the test. In the table, a specimen with no cracks at 800 MPa was marked "Fair," a specimen with no cracks at 900 MPa was marked "Good," a specimen with no cracks at 1000 MPa was marked "Very Good," and a specimen with no cracks at 1100 MPa or more was marked "Excellent." On the other hand, a specimen with cracks at a load stress of 800 MPa was judged to have failed and marked "Bad" in the table.

[0091] Early break resistance The premature fracture resistance was evaluated by dividing the tensile strength of the hot stamped body obtained by the above method by the value obtained by multiplying the Vickers hardness obtained by the following method by 3.3 (tensile strength / (Vickers hardness × 3.3)). A value of 0.60 or more was judged to have excellent premature fracture resistance and to have passed, while a value of less than 0.60 was judged to have failed. The value obtained by multiplying the Vickers hardness by 3.3 is the tensile strength estimated from the hardness, and if the measured tensile strength is 0.60 times or more the estimated tensile strength, it can be judged to have excellent premature fracture resistance.

[0092] The Vickers hardness used to evaluate premature fracture resistance was obtained using the following method. First, a sample was cut from a location at least 50 mm away from the end face of the hot-stamped compact so that a cross section perpendicular to the surface (thickness cross section) could be observed. The sample size, depending on the measurement device, was adjusted to allow observation of a 10 mm section in the rolling direction. The cross section of the sample was polished using #600 to #1500 silicon carbide paper and then polished to a mirror finish using a liquid in which diamond powder with a grain size of 1 to 6 μm was dispersed in a diluent such as alcohol or pure water. The hardness of the mirror-finished cross section was measured at a depth ranging from 4 / 16 of the thickness from the surface to 5 / 16 of the thickness from the surface using a micro-Vickers hardness tester in a direction parallel to the surface (rolling direction) with a load of 1 kgf, at intervals at least three times the indentation width. A total of 20 measurements were made, and the Vickers hardness was calculated by averaging the measurements.

[0093] [Table 1A]

[0094] [Table 1B]

[0095] [Table 1C]

[0096] [Table 1D]

[0097] [Table 1E]

[0098] [Table 1F]

[0099]

Table 1G

[0100] Table 1H

[0101]

Table 1I

[0102]

Table 1J

[0103] Table 1K

[0104]

Table 1L

[0105]

Table 1M

[0106]

Table 1N

[0107]

Table 10

[0108] [Table 1P]

[0109]

Table 1Q

[0110]

Table 1R

[0111]

Table 1S

[0112]

Table 1T

[0113] Table 2A

[0114] Table 2B

[0115] Table 2C

[0116]

Table 2D

[0117] Table 2E

[0118] Table 2F

[0119]

Table 2G

[0120] [Table 2H]

[0121] [Table 3A]

[0122] [Table 3B]

[0123] [Table 3C]

[0124] [Table 3D]

[0125] [Table 3E]

[0126] [Table 3F]

[0127] [Table 3G]

[0128] [Table 3H]

[0129] Tables 3A to 3H show that the hot-stamped steels of the examples of the present invention have high strength and excellent resistance to hydrogen embrittlement and premature fracture, while the hot-stamped steels of the comparative examples are inferior in one or more properties. [Industrial Applicability]

[0130] According to the above aspects of the present invention, it is possible to provide a hot stamped steel sheet having high strength, and excellent resistance to hydrogen embrittlement and premature fracture.

Claims

1. The chemical composition, in mass%, is C: more than 0.40%, less than 0.70%, Si: 0.010-3.00%, Mn: 0.60-3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0200% or less, Al: 0.0010-0.5000%, Nb: 0.0010-0.100%, Ti: 0.010-0.200%, Cr: 0.01-0.80%, Mo: 0.0010-1.000%, B: 0.0005-0.0200%, Co: 0-4.00%, Ni: 0-3.00%, Cu: 0-3.00%, V: 0 to 3.00%, W: 0-3.00%, Ca: 0-1.000%, Mg: 0-1.000%, REM: 0-1.000%, Sb: 0 to 1.000%, Sn: 0-1.000%, Zr: 0 to 1.000%, As: 0 to 0.100%, and The balance is Fe and impurities. In an internal region which is a region from a depth of 4 / 16 of the plate thickness from the surface of the hot stamped body to a depth of 5 / 16 of the plate thickness from the surface, The standard deviation of the grain size of the prior austenite grains is 5.0 μm or less, In a surface layer region which is a region from the surface to a depth of 1 / 25 of the plate thickness, The area ratio of bainite is more than 10%, The maximum value of the pole density of the texture is 4.0 or less, The B removal index, which is an index that quantitatively indicates the amount of decrease in B concentration, is 0.05 or more. A hot stamped product characterized by:

2. The chemical composition is, in mass %, Co: 0.01-4.00%, Ni: 0.01 to 3.00%, Cu: 0.01-3.00%, V: 0.01 to 3.00%, W: 0.01-3.00%, Ca: 0.001-1.000%, Mg: 0.001-1.000%, REM: 0.001-1.000%, Sb: 0.001 to 1.000%, Sn: 0.001 to 1.000%, Zr: 0.001 to 1.000%, and As: 0.001-0.100% The hot-stamped product according to claim 1, characterized in that it contains one or more selected from the group consisting of

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