Hot stamped compact

Abstract

Provided is a hot stamped compact that has a prescribed chemical composition, and has a metal structure containing at least 90 vol% martensite, wherein the average grain diameter of the prior austenite grains is 3.0 µm or less, the standard deviation in the grain diameter distribution of the prior austenite grains is 1.5 µm or less, and the difference between the minimum and maximum values of the Vickers hardness distribution in the plate thickness direction is 35% or less of the average value of the Vickers hardness distribution.

Description

[Technical field]

[0001] The present invention relates to a hot stamped product. [Background technology]

[0002] In recent years, the automobile industry has been required to reduce the weight of vehicle bodies in order to improve fuel efficiency. In order to achieve both weight reduction and crashworthiness of vehicle bodies, increasing the strength of the steel plates used is one effective method, and against this background, the development of high-strength steel plates has been promoted. However, increasing the strength of steel plates reduces their formability, so it is generally difficult to achieve both strength and formability in steel plates.

[0003] In this regard, Patent Document 1 describes a steel material having a predetermined chemical composition, a Ceq defined as C+1 / 24Si+1 / 6Mn+1 / 40Ni+1 / 5Cr+1 / 4Mo+1 / 14V of 0.10 to 1.00, a metal structure containing 95.0% or more of martensite by volume, a prior austenite grain size of 5.0 μm or less, and the number of packets in the prior austenite grains being 3.0 or less. Patent Document 1 also teaches that it is possible to obtain a steel material having an ultrafine structure with an average crystal grain size of 5.0 μm or less, and excellent strength, ductility, and toughness.

[0004] Hot stamping is known as a technique for press-forming steel materials that are high in strength and therefore difficult to form, as described in Patent Document 1. Hot stamping is a hot forming technique in which the material to be formed is heated and then formed. With this technique, the material is heated and then formed, so the steel material is soft and has good formability at the time of forming. Therefore, even high-strength steel materials can be precisely formed into complex shapes, and since quenching is performed simultaneously with forming using a press die, the steel material after forming is known to have sufficient strength.

[0005] In this regard, Patent Document 2 describes a hot stamped steel having a predetermined component composition, a microstructure containing prior austenite with an average crystal grain size of 3 μm or less, and containing at least one of lower bainite, martensite, and tempered martensite in an area ratio of 90% or more, and having a grain boundary solid solution ratio Z defined as Z=(mass% of one or both of Nb and Mo at grain boundaries) / (mass% of one or both of Nb and Mo at melting) of 0.3 or more. Patent Document 2 also teaches that by setting the grain size of prior austenite to 3 μm or less and further dissolving one or both of Nb and Mo in the prior austenite grain boundaries to increase the embrittlement strength of the grain boundaries, it is possible to obtain impact absorption ability superior to that of the prior art, and more specifically, it is disclosed that the hot stamped steel having the above configuration has a tensile strength of 2000 MPa or more and is suppressed from early fracture.

[0006] Patent Document 3 discloses a steel having a predetermined component composition, a microstructure containing at least one of lower bainite, martensite, and tempered martensite in an area ratio of 90% or more, a grain boundary solid solution ratio Z defined as Z=(mass% of one or both of Nb and Mo at grain boundaries) / (mass% of one or both of Nb and Mo at the time of melting) of 0.4 or more, and {112} <111> The X-ray random intensity ratio is 2.8 or more, and the number density of cementite and epsilon carbide with a grain size of 50 nm or less is 1×10 16 pieces / cm 3 In addition, in Patent Document 3, the crystal orientation of the grains of lower bainite, martensite, or tempered martensite in the steel sheet for hot stamping is {112} <111> It is taught that by controlling the X-ray random intensity ratio, a crystal orientation having a high crack propagation suppression effect is generated in the hot stamped steel by the texture memory effect of austenite and martensite, and excellent bending deformability is obtained in the hot stamped steel. More specifically, it is disclosed that the hot stamped steel having the above configuration has a tensile strength of 2000 MPa or more and a maximum bending angle of 50° or more. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2020-012173 A [Patent Document 2] International Publication No. 2019 / 186928 [Patent Document 3] International Publication No. 2019 / 186927 Summary of the Invention [Problem to be solved by the invention]

[0008] Since plastic deformation occurs when an impact exceeding the yield strength is received, from the viewpoint of ensuring the collision safety of automobiles, it is required to improve not only the tensile strength but also the yield strength of hot stamped steel. However, in hot stamped steel having such high strength, hydrogen embrittlement cracking (also called delayed fracture) may be a problem. Hydrogen embrittlement cracking is a phenomenon in which a steel member under high stress in use suddenly breaks due to hydrogen that has penetrated into the steel from the environment. It is generally known that hydrogen embrittlement cracking is more likely to occur as the strength of the steel increases. On the other hand, the automobile industry and the like are also required to further reduce the weight of steel, and in order to achieve such weight reduction, it is necessary to make the steel stronger than ever before. Therefore, there is a high demand for steel, more specifically, hot stamped steel, that can solve the problem of hydrogen embrittlement even when the steel is strengthened to the same level or higher than before.

[0009] Therefore, an object of the present invention is to provide a hot stamped steel sheet having high strength and capable of suppressing hydrogen embrittlement through a novel configuration. [Means for solving the problem]

[0010] In order to achieve the above object, the present inventors have conducted research, focusing particularly on the metal structure of a hot stamped steel. As a result, the present inventors have found that microscopic stress concentration can be suppressed or reduced by refining and regulating the grain size of prior austenite grains in a hot stamped steel, and in connection therewith, reducing the variation in hardness in the metal structure of the hot stamped steel, and further that the amount of hydrogen trapped per unit grain boundary area can be reduced by increasing the grain boundary area associated with the refinement and regulating of prior austenite grains, and have found that a combination of such suppression or reduction of microscopic stress concentration and a reduction in the amount of trapped hydrogen per unit grain boundary area can significantly improve hydrogen embrittlement resistance in a hot stamped steel, despite the hot stamped steel having high tensile strength and yield strength, and have completed the present invention.

[0011] The present invention, which has achieved the above object, is as follows. (1) In mass percent, C: 0.40-0.70%, Si: 0.01 to 1.30%, Mn: 0.05 to 3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0200% or less, Al: 0.001 to 1.000%, Cr: 0.01 to 1.00%, Nb: 0 to 0.200%, Ti: 0 to 0.200%, Mo: 0-1.00%, B: 0~0.1000%, Co: 0-4.00%, Ni: 0-3.00%, Cu: 0-3.00%, V: 0 to 3.00%, W: 0 to 1.00%, Ca: 0-1.000%, Mg: 0-1.000%, REM: 0~1.000%, Sb: 0 to 1.000%, Zr: 0 to 1.000%, Sn: 0~1.000%, As: 0 to 0.100%, and The balance has a chemical composition consisting of Fe and impurities, The volume fraction of martensite is 90% or more. The average grain size of prior austenite grains is 3.0 μm or less, The standard deviation in the grain size distribution of prior austenite grains is 1.5 μm or less, A hot stamped product having a metal structure in which the difference between the maximum and minimum values ​​of a Vickers hardness distribution in the sheet thickness direction is 35% or less of the average value of the Vickers hardness distribution. (2) The chemical composition is, in mass%, Nb: 0.001 to 0.200%, Ti: 0.001 to 0.200%, Mo: 0.001 to 1.00%, B: 0.0001 to 0.1000%, Co: 0.001-4.00%, Ni: 0.001 to 3.00%, Cu: 0.001-3.00%, V: 0.001 to 3.00%, W: 0.001 to 1.00%, Ca: 0.0001 to 1.000%, Mg: 0.0001 to 1.000%, REM: 0.0001~1.000%, Sb: 0.001 to 1.000%, Zr: 0.001 to 1.000%, Sn: 0.001 to 1.000%, and As: 0.001 to 0.100% The hot stamped product according to the above (1), comprising one or more selected from the group consisting of: Effect of the Invention

[0012] According to the present invention, it is possible to provide a hot stamped steel sheet which has high strength and is capable of suppressing hydrogen embrittlement. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] <Hot stamped compact> The hot stamped steel according to the embodiment of the present invention has, in mass%, C: 0.40-0.70%, Si: 0.01 to 1.30%, Mn: 0.05 to 3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0200% or less, Al: 0.001 to 1.000%, Cr: 0.01 to 1.00%, Nb: 0 to 0.200%, Ti: 0 to 0.200%, Mo: 0-1.00%, B: 0~0.1000%, Co: 0-4.00%, Ni: 0-3.00%, Cu: 0-3.00%, V: 0 to 3.00%, W: 0 to 1.00%, Ca: 0-1.000%, Mg: 0-1.000%, REM: 0~1.000%, Sb: 0 to 1.000%, Zr: 0 to 1.000%, Sn: 0~1.000%, As: 0 to 0.100%, and The balance has a chemical composition consisting of Fe and impurities, The volume fraction of martensite is 90% or more. The average grain size of prior austenite grains is 3.0 μm or less, The standard deviation in the grain size distribution of prior austenite grains is 1.5 μm or less, The steel sheet is characterized in having a metal structure in which the difference between the maximum and minimum values ​​of the Vickers hardness distribution in the sheet thickness direction is 35% or less of the average value of the Vickers hardness distribution.

[0014] As mentioned above, it is known that hydrogen embrittlement cracking is more likely to occur as the strength of the steel material increases. For example, in high-strength steel materials, the metal structure generally contains martensite to ensure high strength, and in particular, in hot stamped steels having a tensile strength of 2000 MPa or more, the metal structure is often controlled to have martensite as the main phase. Such martensite-based structures have a high dislocation density, while dislocations can become hydrogen trapping sites. Therefore, there is a continuing need to solve the problem of hydrogen embrittlement in high-strength hot stamped steels having martensite as the main phase. Therefore, the present inventors conducted a study, focusing particularly on the metal structure of the hot stamped steel, from the viewpoint of reducing or suppressing regions that can become the starting points of hydrogen embrittlement cracking in such high-strength hot stamped steels having martensite as the main phase. More specifically, the inventors first found that when the prior austenite grain size varies widely in the metal structure of a hot stamped steel, the hardness increases in regions where the prior austenite grain size is smaller, and such localized high-hardness regions can become the starting points of hydrogen embrittlement cracking. In response to this, the inventors found that such a localized increase in hardness can be suppressed and microscopic stress concentration can be reliably suppressed or reduced by refining the prior austenite grains to reduce the average grain size to 3.0 μm or less and controlling the standard deviation in the grain size distribution to 1.5 μm or less for grain regulation, and in connection therewith, controlling the difference between the maximum and minimum values ​​of the Vickers hardness distribution in the sheet thickness direction to 35% or less of the average value of the Vickers hardness distribution.

[0015] Although it is not intended to be bound by any particular theory, it is believed that the martensite transformation start temperature changes depending on the grain size of the austenite grains during hot stamp forming. To explain in more detail, it is believed that austenite grains having a larger grain size have a higher martensite transformation start temperature than austenite grains having a smaller grain size, and that autotempering progresses between the completion of transformation and cooling to room temperature, resulting in a lower hardness. Austenite grains having a smaller grain size undergo martensite transformation at a lower temperature than large grains, resulting in an increase in hardness. Therefore, in order to suppress or reduce such a local increase in hardness, it is important to reduce the variation in the austenite grain size before martensite transformation. In other words, it is believed that by reducing the variation in the austenite grain size before martensite transformation, the variation in the prior austenite grain size after martensite transformation can be reduced, and as a result, the hardness variation in the metal structure of the hot stamped body can be reduced. For these reasons, it is believed that by controlling the difference between the maximum and minimum values ​​of the Vickers hardness distribution in the metal structure of the hot stamped body to 35% or less of the average value of the Vickers hardness distribution and reducing the hardness variation, it is possible to significantly suppress local increases in hardness due to differences in the timing of martensitic transformation. Since it is believed that if there are locally high hardness regions, there is a high possibility that stress will concentrate at the interfaces of prior austenite grains, which have hardness differences, and cause hydrogen embrittlement cracking, reducing the hardness variation in the metal structure of the hot stamped body is very effective in suppressing or reducing microscopic stress concentration and improving hydrogen embrittlement resistance.

[0016] Furthermore, in the hot stamped steel according to the embodiment of the present invention, the prior austenite grains have an average grain size of 3.0 μm or less, and are therefore very fine, and the standard deviation in the grain size distribution of the prior austenite grains is controlled to 1.5 μm or less, that is, the prior austenite grains are highly granular. The fine prior austenite grains are considered to undergo martensitic transformation at a lower temperature as described above, and since they are highly granular, it is considered that the metal structure including such refined and granular prior austenite grains undergoes martensitic transformation at a lower temperature as a whole metal structure. For this reason, the hot stamped steel including such a metal structure can significantly improve the hardness as a whole, and therefore it is possible to achieve extremely high strength, specifically, extremely high tensile strength and yield strength. In addition, in the metal structure including such refined and granular prior austenite grains, the grain boundary area is greatly increased, and as a result, the amount of hydrogen captured per unit grain boundary area can be significantly reduced. Hydrogen embrittlement cracking is caused, for example, by hydrogen that has entered steel from the environment and is trapped at grain boundaries under high stress. Therefore, by reducing the amount of hydrogen trapped per unit grain boundary area, it is possible to significantly reduce the risk of hydrogen embrittlement cracking. Therefore, according to the hot stamped steel according to the embodiment of the present invention, in addition to the above-mentioned effect of improving hydrogen embrittlement resistance due to the reduction in hardness variation, the hydrogen embrittlement resistance of the hot stamped steel can be improved from the viewpoint of the increase in grain boundary area due to the refinement and grain regulation of prior austenite grains.

[0017] Hereinafter, the hot stamped steel according to the embodiment of the present invention will be described in more detail. In the following description, the unit of content of each element, "%", means "mass%" unless otherwise specified. In addition, in this specification, "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower limit and upper limit unless otherwise specified.

[0018] [C:0.40~0.70%] C is an element that improves the strength of a hot stamped steel. If the C content is less than 0.40%, the hot stamped steel cannot obtain the desired strength. Therefore, the C content is set to 0.40% or more. The C content is preferably more than 0.40%, 0.42% or more, 0.44% or more, or 0.45% or more. On the other hand, if the C content exceeds 0.70%, the strength becomes too high and it may not be possible to obtain excellent hydrogen embrittlement resistance. Therefore, the C content is set to 0.70% or less. Preferably, the C content is 0.68% or less, 0.67% or less, 0.65% or less, or 0.60% or less.

[0019] [Si: 0.01-1.30%] Silicon is an element that improves the strength of a hot stamped body by solid solution strengthening. If the silicon content is less than 0.01%, the desired strength cannot be obtained. Therefore, the silicon content is set to 0.01% or more. The silicon content is preferably 0.05% or more, 0.10% or more, 0.20% or more, more than 0.25%, 0.26% or more, 0.27% or more, 0.30% or more, or 0.40% or more. On the other hand, if the Si content exceeds 1.30%, the amount of ferrite increases, and the desired metal structure may not be obtained. Therefore, the Si content is set to 1.30% or less. The Si content is preferably 1.20% or less, 1.00% or less, 0.80% or less, 0.60% or less, or 0.50% or less.

[0020] [Mn: 0.05~3.00%] Mn is an element that improves the hardenability of steel and contributes to improving strength. If the Mn content is less than 0.05%, such effects cannot be sufficiently obtained. Therefore, the Mn content is set to 0.05% or more. The Mn content is preferably 0.10% or more, 0.50% or more, 1.00% or more, 1.30% or more, or 1.50% or more. On the other hand, if the Mn content exceeds 3.00%, Mn segregation becomes significant, which may cause the variation in the prior austenite grain size to be insufficiently suppressed. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.80% or less, 2.50% or less, 2.30% or less, or 2.00% or less.

[0021] [P:0.100% or less] P is an impurity element that segregates at grain boundaries and deteriorates hydrogen embrittlement resistance, so the P content is set to 0.100% or less, and preferably 0.070% or less, 0.050% or less, or 0.010% or less. Although there is no particular lower limit for the P content, reducing it to less than 0.0001% is economically undesirable because the dephosphorization cost increases significantly, so the P content may be set to 0.0001% or more.

[0022] [S:0.0100% or less] S is an impurity element that forms inclusions in steel. Since these inclusions deteriorate hydrogen embrittlement resistance, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less. Although there is no particular lower limit for the S content, reducing it to less than 0.0001% is economically undesirable since the cost of removing S increases significantly, so the S content may be set to 0.0001% or more.

[0023] [N:0.0200% or less] N is an impurity element that forms nitrides in steel. Since these nitrides deteriorate hydrogen embrittlement resistance, the N content is set to 0.0200% or less. The N content is preferably 0.0180% or less, 0.0150% or less, 0.0100% or less, 0.0060% or less, or 0.0040% or less. Although there is no particular lower limit for the N content, reducing it to less than 0.0001% is economically undesirable because the cost of denitrification increases significantly, so the N content may be 0.0001% or more.

[0024] [O:0.0200% or less] If O is contained in a large amount in steel, it forms coarse oxides and deteriorates hydrogen embrittlement resistance. Therefore, the O content is set to 0.0200% or less. The O content is preferably set to 0.0150% or less, 0.0100% or less, 0.0070% or less, or 0.0040% or less. From the viewpoint of reducing refining costs, the O content may be 0.0001% or more. In order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be 0.0005% or more.

[0025] [Al: 0.001-1.000%] Al is an element that has the effect of deoxidizing molten steel to improve the soundness of the steel. If the Al content is less than 0.001%, deoxidation is not performed sufficiently, and coarse oxides are generated, which deteriorates the hydrogen embrittlement resistance. Therefore, the Al content is set to 0.001% or more. The Al content is preferably 0.003% or more, 0.005% or more, 0.010% or more, or 0.030% or more. On the other hand, if the Al content exceeds 1.000%, coarse oxides are generated in the steel, and the hydrogen embrittlement resistance of the hot stamped body is deteriorated. Therefore, the Al content is set to 1.000% or less. The Al content is preferably 0.800% or less, 0.600% or less, 0.400% or less, 0.200% or less, or 0.100% or less.

[0026] [Cr:0.01~1.00%] Cr is an element that dissolves in prior austenite grains during heating before hot stamping, thereby increasing the strength of the hot stamped body. 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 0.05% or more, 0.10% or more, 0.15% or more, or 0.20% or more. On the other hand, if the Cr content exceeds 1.00%, coarse intermetallic compounds are formed in the hot stamped steel, and the hydrogen embrittlement resistance of the hot stamped steel deteriorates. Therefore, the Cr content is set to 1.00% or less. The Cr content is preferably 0.80% or less, 0.60% or less, 0.50% or less, or 0.40% or less.

[0027] The basic chemical composition of the hot stamped steel according to the embodiment of the present invention is as described above. Furthermore, the hot stamped steel may contain at least one of the following optional elements in place of a part of the remaining Fe, as necessary. For example, the hot stamped steel may contain at least one selected from the group consisting of Nb: 0-0.200%, Ti: 0-0.200%, Mo: 0-1.00%, B: 0-0.1000%, Co: 0-4.00%, Ni: 0-3.00%, Cu: 0-3.00%, V: 0-3.00%, and W: 0-1.00%. In addition, the hot stamped steel may contain at least one selected from the group consisting of Ca: 0-1.000%, Mg: 0-1.000%, and REM: 0-1.000%. The hot stamped steel may contain at least one element selected from the group consisting of Sb: 0-1.000%, Zr: 0-1.000%, and Sn: 0-1.000%. The hot stamped steel may contain As: 0-0.100%. These optional elements will be described in detail below.

[0028] [Nb: 0~0.200%] Nb is an element that forms carbonitrides in steel and improves the strength of hot stamped steel by precipitation strengthening. The Nb content may be 0.001% or more, but in order to reliably obtain this effect, the Nb content is preferably 0.010% or more or 0.020% or more. On the other hand, since the above effects are saturated even if the Nb content is large, the Nb content is preferably 0.200% or less. The Nb content may be 0.180% or less, 0.150% or less, 0.100% or less, 0.080% or less, or 0.060% or less.

[0029] [Ti: 0~0.200%] Ti is an element that forms carbonitrides in steel and improves the strength of the hot stamped body by precipitation strengthening. The Ti content may be 0.001% or more, but in order to reliably obtain this effect, the Ti content is preferably 0.010% or more or 0.020% or more. On the other hand, since the above effect is saturated even if Ti is contained in a large amount, the Ti content is preferably 0.200% or less. The Ti content may be 0.180% or less, 0.150% or less, 0.100% or less, 0.080% or less, or 0.060% or less.

[0030] [Mo:0~1.00%] Mo is an element that improves the hardenability of steel. The Mo content may be 0.001% or more, but in order to reliably obtain this effect, the Mo content is preferably 0.005% or more or 0.01% or more. On the other hand, since the above effects are saturated even if the Mo content is large, the Mo content is preferably 1.00% or less. The Mo content may be 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.10% or less.

[0031] [B:0~0.1000%] B is an element that improves the hardenability of steel. The B content may be 0.0001% or more, but in order to reliably obtain this effect, the B content is preferably 0.0005% or more or 0.0010% or more. On the other hand, since the above effects are saturated even if the B content is large, the B content is preferably 0.1000% or less. The B content may be 0.0500% or less, 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0015% or less.

[0032] [Co: 0-4.00%] Co is an element that improves the strength of a hot stamped steel by solid solution strengthening. The Co content may be 0.001% or more, but in order to reliably obtain this effect, the Co content is preferably 0.01% or more or 0.05% or more. On the other hand, since the above effects are saturated even if the Co content is large, the Co content is preferably 4.00% or less. The Co content may be 3.00% or less, 2.00% or less, 1.00% or less, 0.80% or less, or 0.60% or less.

[0033] [Ni:0~3.00%] Ni has the effect of increasing the strength of the hot stamped body by dissolving in austenite grains during heating in the hot stamp forming process. The Ni content may be 0.001% or more, but in order to reliably obtain this effect, the Ni content is preferably 0.01% or more. On the other hand, since the above effect is saturated even if the Ni content is large, the Ni content is preferably 3.00% or less. The Ni content may be 2.80% or less, 2.50% or less, 2.00% or less, 1.50% or less, 1.00% or less, or 0.80% or less.

[0034] [Cu: 0-3.00%] Cu has the effect of increasing the strength of the hot stamped body by dissolving in austenite grains during heating in the hot stamp forming process. The Cu content may be 0.001% or more, but in order to reliably obtain this effect, the Cu content is preferably 0.01% or more or 0.05% or more. On the other hand, since the above effect is saturated even if a large amount of Cu is contained, the Cu content is preferably 3.00% or less. The Cu content may be 2.00% or less, 1.00% or less, 0.50% or less, 0.30% or less, or 0.10% or less.

[0035] [V:0~3.00%] V forms carbonitrides in steel and has the effect of improving the strength of the hot stamped body by precipitation strengthening. The V content may be 0.001% or more, but in order to reliably obtain this effect, the V content is preferably 0.01% or more or 0.05% or more. On the other hand, since the above effects are saturated even if the V content is large, the V content is preferably 3.00% or less. The V content may be 2.00% or less, 1.00% or less, 0.50% or less, 0.30% or less, or 0.10% or less.

[0036] [W:0~1.00%] W is an element that improves the hardenability of steel. The W content may be 0.001% or more, but in order to reliably obtain this effect, the W content is preferably 0.005% or more or 0.01% or more. On the other hand, since the above effect is saturated even if the W content is large, the W content is preferably 1.00% or less. The W content may be 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.10% or less.

[0037] [Ca: 0-1.000%] Ca is an element capable of controlling the morphology of sulfides. The Ca content may be 0.0001% or more, but in order to reliably obtain this effect, the Ca content is preferably 0.0005% or more or 0.001% or more. On the other hand, since the above effect is saturated even if a large amount is contained, the Ca content is preferably 1.000% or less. The Ca content may be 0.500% or less, 0.100% or less, 0.050% or less, 0.010% or less, 0.005% or less, or 0.002% or less.

[0038] [Mg:0~1.000%] Mg is an element capable of controlling the morphology of sulfides. The Mg content may be 0.0001% or more, but in order to reliably obtain this effect, the Mg content is preferably 0.0005% or more or 0.001% or more. On the other hand, since the above effect is saturated even if Mg is contained in a large amount, the Mg content is preferably 1.000% or less. The Mg content may be 0.500% or less, 0.100% or less, 0.050% or less, 0.010% or less, 0.005% or less, or 0.002% or less.

[0039] [REM:0~1.000%] REM is an element that can control the morphology of sulfides. The REM content may be 0.0001% or more, but in order to reliably obtain this effect, the REM content is preferably 0.0005% or more or 0.001% or more. On the other hand, since the above effect saturates even if the REM content is large, the REM content is preferably 1.000% or less. The REM content may be 0.500% or less, 0.100% or less, 0.050% or less, 0.010% or less, 0.005% or less, or 0.002% or less. In this embodiment, REM is a collective term for 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content is the total content of these elements.

[0040] [Sb:0~1.000%] Sb is an element that suppresses the generation of oxides. To reliably obtain this effect, the Sb content is preferably 0.001% or more. On the other hand, since the above effect is saturated even if the Sb content is large, the Sb content is preferably 1.000% or less. The Sb content may be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, or 0.050% or less.

[0041] [Zr:0~1.000%] Zr is an element that suppresses the formation of oxides. To reliably obtain this effect, the Zr content is preferably 0.001% or more. On the other hand, since the above effect is saturated even if the Zr content is large, the Zr content is preferably 1.000% or less. The Zr content may be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, or 0.050% or less.

[0042] [Sn:0~1.000%] Sn is an element that suppresses the generation of oxides. To reliably obtain this effect, the Sn content is preferably 0.001% or more. On the other hand, since the above effect is saturated even if the Sn content is large, the Sn content is preferably 1.000% or less. The Sn content may be 0.800% or less, 0.500% or less, 0.200% or less, 0.100% or less, or 0.050% or less.

[0043] [As:0~0.100%] As contributes to the refinement of prior austenite grains by lowering the austenite single-phase temperature. In order to reliably obtain this effect, the As content is preferably 0.001% or more. On the other hand, since the above effect is saturated even if the As content is large, the As content is preferably 0.100% or less. The As content may be 0.080% or less, 0.050% or less, 0.020% or less, 0.010% or less, or 0.005% or less.

[0044] In the hot stamped steel according to the embodiment of the present invention, the balance other than the above elements consists of Fe and impurities. The impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ores and scraps, during industrial production of the hot stamped steel.

[0045] The chemical composition of the hot stamped body may be measured by a general analytical method. For example, it may be measured by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S may be measured by a combustion-infrared absorption method, N by an inert gas fusion-thermal conductivity method, and O by an inert gas fusion-non-dispersive infrared absorption method. When the hot stamped body has a plated layer on its surface, the plated layer can be removed by mechanical grinding before analyzing the chemical composition.

[0046] [Martensite: 90% or more] The metal structure of the hot stamped steel according to the embodiment of the present invention contains 90% or more martensite by volume. The remaining structure is not particularly limited, but may be at least one of bainite, ferrite, retained austenite, and pearlite at 10% or less. Martensite is a very hard structure, and therefore, by containing 90% or more martensite by volume in the hot stamped steel, it is possible to achieve high tensile strength and yield strength, specifically, tensile strength of 2200 MPa or more and yield strength of 1800 MPa or more. On the other hand, when the volume fraction of martensite is low and the ratio of soft structures such as ferrite is high, the variation in hardness in the sheet thickness direction becomes significant, and the ratio of the difference between the maximum value and the minimum value of the Vickers hardness distribution in the sheet thickness direction to the average value becomes large. Therefore, the larger the volume fraction of martensite, the more preferable it is, and may be, for example, 92% or more, 94% or more, 96% or more, or 98% or more. The upper limit of the volume fraction of martensite is not particularly limited and may be 100%.

[0047] [Identification of metal structure and calculation of volume fraction] The identification of the metal structure and the calculation of the volume fraction of the hot stamped body are performed as follows. First, a sample is taken so that the cross section parallel to the plate thickness direction of the hot stamped body is the observation surface. Next, the observation surface is mirror-polished and etched with a nital etching solution, and then the structure is observed using a scanning electron microscope (SEM). A range of 300 μm × 300 μm is photographed at 1000 times magnification, centered on the 1 / 4 plate thickness depth position of the observation surface. The obtained microstructure photo is subjected to black and white binarization processing, and then image analysis is performed to identify pearlite, bainite, and ferrite, and the total amount of their area fractions is obtained using a method based on "Steel-Microscopic Test Method for Grain Size" defined in JIS G 0551:2020. Furthermore, the area fraction of the structure is converted to a volume fraction by a line segment method. The line segment method is based on the method described in, for example, Robert T. DeHoff, Frederik N. Rhines (ed.) (Quantitative Microscopy, 1968). Since it is difficult to distinguish retained austenite from martensite using an SEM, the volume fraction of retained austenite is measured using X-ray diffraction. Finally, the volume fraction of martensite is determined by subtracting the total volume fraction of pearlite, bainite, ferrite, and retained austenite obtained by the above method from 100%.

[0048] [Average grain size of prior austenite grains: 3.0 μm or less] In an embodiment of the present invention, the average grain size of the prior austenite grains is 3.0 μm or less. Such refinement of the prior austenite grains, in combination with the grain regulation of the prior austenite grains described later, contributes to the high strength of the hot stamped body, and can improve the hydrogen embrittlement resistance of the hot stamped body by increasing the grain boundary area and reducing the amount of hydrogen captured per unit grain boundary area. From the viewpoint of improving the strength and hydrogen embrittlement resistance of the hot stamped body, the smaller the average grain size of the prior austenite grains, the more preferable it is, and may be, for example, 2.8 μm or less, 2.5 μm or less, 2.3 μm or less, or 2.0 μm or less. Although there is no particular lower limit, the average grain size of the prior austenite grains may be, for example, 1.0 μm or more, 1.2 μm or more, or 1.5 μm or more.

[0049] [Standard deviation in grain size distribution of prior austenite grains: 1.5 μm or less] In the embodiment of the present invention, the standard deviation in the grain size distribution of the prior austenite grains is 1.5 μm or less. By suppressing the variation in the prior austenite grain size by such granulation of the prior austenite grains, as described in relation to the average grain size of the prior austenite grains, the strength of the hot stamped steel can be increased, and further, the hydrogen embrittlement resistance can be improved due to the increase in the grain boundary area. In addition, according to the embodiment of the present invention, by controlling the standard deviation in the grain size distribution of the prior austenite grains to 1.5 μm or less to reduce the variation in the prior austenite grain size, the variation in hardness can be reduced, and thus the local increase in hardness in the hot stamped steel can be significantly suppressed. By suppressing the local increase in hardness in the hot stamped steel, it is possible to reliably suppress or reduce microscopic stress concentration, and therefore it is possible to further improve the hydrogen embrittlement resistance of the hot stamped steel. From the viewpoint of improving these effects, the smaller the standard deviation in the grain size distribution of the prior austenite grains, the more preferable it is, and it may be, for example, 1.4 μm or less, 1.2 μm or less, 1.0 μm or less, or 0.8 μm or less. Although there is no particular lower limit, the standard deviation may be, for example, 0.1 μm or more, 0.2 μm or more, or 0.4 μm or more.

[0050] [Method for determining the average grain size and standard deviation in grain size distribution of prior austenite grains] The average grain size and standard deviation in grain size distribution of prior austenite grains are determined as follows. First, a sample is cut out from an arbitrary position 50 mm or more away from the end face of the hot stamped body (if a sample cannot be taken from this position, a position avoiding the end) so that the plate thickness cross section perpendicular to the surface can be observed. The size of the sample depends on the measuring device, but it should be a size that allows observation of about 10 mm in the direction perpendicular to the plate thickness direction. The cross section of the above sample is polished using silicon carbide paper of #600 to #1500, and then finished to a mirror surface using a dilution liquid such as alcohol or a liquid in which diamond powder with a grain size of 1 to 6 μm is dispersed in pure water. Next, the observation surface is finished by electrolytic polishing. At an arbitrary position in the longitudinal direction of the sample cross section, a region of 50 μm in length and 50 μm in the plate thickness direction centered at a depth position of 1 / 4 of the plate thickness is measured by electron backscatter diffraction at a measurement interval of 0.1 μm to obtain crystal orientation information. For the measurement, an EBSD analysis device consisting of a thermal field emission scanning electron microscope and an EBSD detector may be used. For example, an EBSD analysis device consisting of a JEOL JSM-7001F and a TSL DVC5 type detector may be used. In this case, the degree of vacuum in the EBSD analysis device is 9.6×10 -5The axial direction of the prior austenite grains may be 10 Pa or less, the acceleration voltage may be 15 kV, and the irradiation current level may be 13. 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. The following method is used to calculate the crystal orientation of the prior austenite grains. First, a crystal orientation map of the prior austenite grains is created by the method described in Acta Materialia, 58 (2010), 6393-6403. For one of the prior austenite grains included in the observation field, the average value of the shortest diameter and the longest diameter is calculated, and the average value is set as the grain size of the prior austenite grain. The above operation is performed for all prior austenite grains, except for prior austenite grains whose entire grains are not included in the observation field, such as the end of the observation field, to determine the grain size of all prior austenite grains in the observation field. The average grain size and standard deviation are calculated from the grain sizes of all the prior austenite grains obtained, and the average grain size and standard deviation in the grain size distribution of the prior austenite grains are determined.

[0051] [Difference between maximum and minimum values ​​of Vickers hardness distribution in the plate thickness direction: 35% or less of the average value of the Vickers hardness distribution] In an embodiment of the present invention, the difference between the maximum and minimum values ​​of the Vickers hardness distribution in the sheet thickness direction is 35% or less of the average value of the Vickers hardness distribution. By suppressing the variation in the Vickers hardness distribution within such a range, it is possible to significantly suppress the increase in local hardness in the hot stamped steel. As a result, it is possible to reliably suppress or reduce microscopic stress concentration, and it is possible to improve the hydrogen embrittlement resistance of the hot stamped steel. From the viewpoint of improving the hydrogen embrittlement resistance of the hot stamped steel, it is preferable that the difference between the maximum and minimum values ​​of the Vickers hardness distribution is as small as possible, and may be, for example, 30% or less, 25% or less, 20% or less, or 15% or less of the average value of the Vickers hardness distribution. Although the lower limit is not particularly limited, the difference between the maximum and minimum values ​​of the Vickers hardness distribution may be, for example, 1% or more, 3% or more, or 5% or more of the average value of the Vickers hardness distribution.

[0052] As described above, it is possible to reduce the variation in hardness by reducing the variation in the prior austenite grain size. However, in the embodiment of the present invention, the standard deviation in the grain size distribution of the prior austenite grains is measured and determined in the same region centered on the 1 / 4 depth position of the plate thickness, whereas the difference between the maximum and minimum values ​​of the Vickers hardness distribution is determined based on the values ​​of Vickers hardness measured at various depth positions in the plate thickness direction. Therefore, it cannot be said that the difference between the maximum and minimum values ​​of the Vickers hardness distribution in the plate thickness direction is 35% or less of the average value of the Vickers hardness distribution by satisfying the standard deviation in the grain size distribution of the prior austenite grains being 3.0 μm or less. In addition, the effect obtained by reducing the variation in the prior austenite grain size and the effect obtained by reducing the variation in hardness are not completely the same. For example, as explained above, the reduction of the variation in the prior austenite grain size (granulation regulation), in combination with the prior austenite grain size, contributes to improving hydrogen embrittlement resistance due to the increase in the strength and grain boundary area of ​​the hot stamped body, whereas the reduction of the variation in hardness suppresses a local increase in hardness in the hot stamped body and suppresses or reduces microscopic stress concentration, thereby improving the hydrogen embrittlement resistance of the hot stamped body. The object of the present invention to "provide a hot stamped body which has high strength and is capable of suppressing hydrogen embrittlement" is achieved by satisfying all of the requirements, namely, the average grain size and standard deviation in grain size distribution of the prior austenite grains, as well as the chemical composition of the hot stamped body, and the difference between the maximum and minimum values ​​of the Vickers hardness distribution, and cannot be achieved by, for example, only specifying the chemical composition of the hot stamped body and the average grain size and standard deviation in grain size distribution of the prior austenite grains.

[0053] [Method of determining the difference between the maximum and minimum values ​​of the Vickers hardness distribution in the plate thickness direction] The difference between the maximum and minimum values ​​of the Vickers hardness distribution in the thickness direction is determined as follows. First, a sample is cut out so that a cross section perpendicular to the surface (thickness cross section) can be observed from any position 50 mm or more away from the end face of the hot stamped body. The size of the sample is such that 10 mm can be observed in the direction perpendicular to the thickness direction, depending on the measuring device. The cross section of the sample is polished using silicon carbide paper of #600 to #1500, and then finished to a mirror surface using a liquid in which diamond powder with a grain size of 1 to 6 μm is dispersed in a dilution liquid such as alcohol and pure water. For the cross section finished to a mirror surface, the Vickers hardness is measured at a load of 25 gf using a micro Vickers hardness tester at a depth position of 100 μm from the surface of the hot stamped body (the surface of the hot stamped body excluding the plating layer if a plating layer exists on the surface) at intervals of 25 to 30 μm in the thickness direction, and the measured value of the Vickers hardness is obtained at a distance of more than half the thickness excluding each 100 μm from the front and back surfaces of the hot stamped body. The interval between each measurement point arranged in the thickness direction (the interval between the centers of the indentations) must be at least three times the distance of the indentation. The distance of at least three times the indentation means a distance of at least three times the length of the diagonal line of the rectangular opening of the indentation made by the diamond indenter when measuring the Vickers hardness. If the hot stamped body to be measured is thin and it is difficult to stamp each measurement point linearly in the thickness direction while setting the distance between each measurement point at at least three times the distance of the indentation, the measurement points may be stamped in a zigzag manner in the thickness direction while setting the distance between each measurement point at at least three times the distance of the indentation. Finally, the maximum and minimum values ​​are selected from all the measured values ​​of Vickers hardness, and the difference between them is calculated. The ratio of the difference between the calculated maximum and minimum values ​​and the average value obtained by arithmetically averaging all the measured values ​​of Vickers hardness to the average value is determined.

[0054] [Plating] The hot stamped product according to the present embodiment may have a plating layer on the surface. By having a plating layer on the surface, corrosion resistance can be improved. Examples of the plating layer include an aluminum plating layer, an aluminum-zinc plating layer, an aluminum-silicon plating layer, a hot-dip galvanizing layer, an electrolytic zinc plating layer, an alloyed hot-dip galvanizing layer, a zinc-nickel plating layer, and an aluminum-magnesium-zinc based plating layer.

[0055] [Mechanical properties] According to the hot stamped steel according to the embodiment of the present invention, excellent mechanical properties, for example, a tensile strength of 2200 MPa or more can be achieved. The tensile strength is preferably 2300 MPa or more, more preferably 2400 MPa or more, and most preferably 2500 MPa or more. The upper limit is not particularly limited, but for example, the tensile strength may be 3500 MPa or less, 3300 MPa or less, or 3000 MPa or less. Similarly, according to the hot stamped steel according to the embodiment of the present invention, a yield strength of 1800 MPa or more can be achieved. The yield strength is preferably 1900 MPa or more, more preferably 2000 MPa or more, and most preferably 2100 MPa or more. The upper limit is not particularly limited, but for example, the yield strength may be 3000 MPa or less, 2800 MPa or less, or 2500 MPa or less. The tensile strength and yield strength of the hot stamped steel are measured by preparing a No. 5 test piece and performing a tensile test in accordance with JIS Z 2241:2011.

[0056] As described above, the hot stamped steel according to the embodiment of the present invention has a high tensile strength of, for example, 2200 MPa or more and a high yield strength of, for example, 1800 MPa or more, yet has excellent hydrogen embrittlement resistance. Therefore, it is very useful for use as, for example, automobile frame members, bumpers, and other structural members and reinforcing members that require strength.

[0057] <Method of Manufacturing Hot Stamped Body> Next, a preferred method for producing the hot stamped steel according to the embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for producing the hot stamped steel according to the embodiment of the present invention, but is not intended to limit the hot stamped steel to one produced by the production method described below.

[0058] The method for producing a hot stamped steel according to an embodiment of the present invention is characterized in that a hot rolled steel sheet having a metal structure mainly composed of tempered martensite is cold worked and then rapidly heated during hot stamping in order to refine and regulate prior austenite grains and to reduce the associated variation in hardness. More specifically, the method for producing a steel sheet for hot stamping according to an embodiment of the present invention includes the steps of: hot rolling a slab having the chemical composition described above in relation to the hot stamped body, followed by cooling at an average cooling rate of 50° C. / s or more and coiling at a temperature below the Ms point (hot rolling step); A process of tempering the obtained hot-rolled steel sheet at a temperature range of 400 to 600 ° C. (tempering process); A process of cold rolling the tempered hot-rolled steel sheet at a rolling reduction of 30% or more (cold rolling process); and The obtained cold-rolled steel sheet is heated from 600°C to a heat treatment temperature of A3+50 to A3+150°C at an average heating rate of more than 100°C / s and held for 30 seconds or less, and then hot stamp forming is started before the temperature reaches 550°C, and the sheet is cooled at an average cooling rate of 10°C / s or more (hot stamp forming process). The present invention is characterized in that it includes:

[0059] In order to obtain fine prior austenite grains in the metal structure of the finally obtained hot stamped body, it is important to form a large number of austenite nucleation sites during heating in the hot stamp forming process. In relation to this, in the martensite structure, prior austenite grain boundaries can be austenite nucleation sites, but the martensite structure has substructures such as packets, blocks, and laths in the prior austenite grains, and it is known that the boundaries (interfaces) between them can also be austenite nucleation sites. For this reason, the martensite structure has a much larger number of austenite nucleation sites than structures such as ferrite and pearlite. Therefore, in the present manufacturing method, first, in the hot rolling process, the hot rolled slab is cooled at a cooling rate equal to or higher than the critical cooling rate, more specifically, at an average cooling rate of 50° C. / sec or higher, and coiled at a temperature equal to or lower than the Ms point to form a hot rolled steel sheet having a structure mainly composed of martensite. Next, the obtained hot-rolled steel sheet is heat-treated at an appropriate temperature range in the tempering process, which generates (precipitates) a large number of carbides at the prior austenite grain boundaries, packet-block boundaries, and lath spaces in the martensite structure. The interfaces between the numerous precipitated carbides and these substructures also become major austenite nucleation sites during heating in the hot stamp forming process. Therefore, by performing the hot stamp forming process after the tempering process, it becomes possible to generate austenite from a larger number of austenite nucleation sites compared to performing the hot stamp forming process without the tempering process.

[0060] Next, by cold working (cold rolling) the tempered hot-rolled steel sheet at a predetermined rolling reduction, the metal structure can be refined and the interfaces of the above-mentioned substructure can be increased, and as a result, the austenite nucleation sites can be further increased. For example, in the tempering process, carbides precipitated at the interfaces of substructures such as packets, blocks, and laths in the same grain have a close orientation relationship, so if they are heated in the hot stamp forming process as they are, the orientation relationship of the austenite generated at the interfaces between these carbides and the substructure as nucleation sites may also be close. In such a case, the generated austenite grains tend to connect, making it difficult to keep the final metal structure fine. In contrast, by cold rolling at a predetermined rolling reduction, the orientation relationship between the above-mentioned carbides can be changed, and the orientation relationship of the austenite generated at the interfaces between these carbides and the substructure as nucleation sites can be randomized. For this reason, the generated austenite is less likely to connect, and therefore the metal structure finally obtained in the subsequent hot stamp forming process can be kept not only fine but also in a regular grain state. Finally, the cold-rolled steel sheet having a metal structure mainly composed of cold-worked tempered martensite is rapidly heated at a predetermined average heating rate through a temperature zone where recrystallization progresses during heating in hot stamp forming. If recrystallization progresses during heating in hot stamp forming, the austenite nucleation sites formed in the previous steps are reduced or lost, making it impossible to achieve the desired refinement and grain regulation in the final metal structure. In this manufacturing method, recrystallization is suppressed by passing through such a recrystallization progress temperature zone in a short time by a predetermined rapid heating, and thus the reduction or loss of austenite nucleation sites caused by the progress of recrystallization is suppressed, thereby enabling the refinement and grain regulation of the final metal structure. According to this manufacturing method, since the austenite grains refined during the manufacturing process do not or are difficult to be connected, it is possible not only to simply refine the prior austenite grains obtained after heating and cooling in the hot stamp forming process, but also to regulate the grain size, which results in a reduction in the variation in prior austenite grain size and a reliably reduced variation in hardness.Each step will be described in more detail below.

[0061] [Hot rolling process] In the hot rolling process, first, a slab having the chemical composition described above in relation to the hot stamped body is heated. The casting method of the molten steel is not particularly limited, and the slab may be produced by a continuous casting method, an ingot casting method, or a thin slab casting method. The heating before hot rolling is not particularly limited, but the slab used contains a relatively large amount of alloying elements in order to obtain a high-strength steel plate. For this reason, the heating temperature may be 1100 ° C or higher in order to heat the slab before subjecting it to hot rolling and dissolve the alloying elements in the slab. In addition, the heated slab may be optionally subjected to rough rolling before finish rolling for plate thickness adjustment or the like. The conditions of rough rolling are not particularly limited as long as the desired sheet bar dimensions are secured. The heated slab or the slab that has been rough rolled as necessary is then subjected to finish rolling. The finish rolling is not particularly limited, but is generally performed under conditions such that the finish rolling completion temperature is 650 ° C or higher. If the finishing temperature is too low, the rolling reaction force increases, making it difficult to stably obtain the desired plate thickness. Although there is no particular upper limit, the finishing temperature is generally 950° C. or lower.

[0062] [Winding] Next, the finish-rolled hot-rolled steel sheet is cooled at an average cooling rate of 50°C / sec or more and coiled at a temperature of the Ms point or less. By setting the average cooling rate to 50°C / sec or more, preferably 100°C / sec or more, and the coiling temperature to the Ms point or less, a metal structure mainly composed of martensite can be formed in the hot-rolled steel sheet after coiling, and it becomes possible to reliably form a cold-rolled steel sheet having a metal structure mainly composed of tempered martensite in the subsequent tempering process. If the average cooling rate is less than 50°C / sec and / or the coiling temperature is higher than the Ms point, a metal structure mainly composed of martensite cannot be formed in the hot-rolled steel sheet after coiling, and even if the subsequent processes are appropriately performed, the desired hard structure cannot be obtained in the finally obtained hot stamped steel sheet and / or the refinement and granulation of prior austenite grains, and further reduction of hardness variation cannot be achieved. From the viewpoint of refining the metal structure, it is preferable that the average cooling rate is 100°C / sec or more and the coiling temperature is the Mf point or less. By setting the average cooling rate to 100°C / sec or more and the coiling temperature to the Mf point or less, the fraction of the obtained martensite structure can be further increased. As described above, the martensite structure has a plurality of substructures, and many austenite nucleation sites can be created by these substructures and carbides precipitated in the tempering process. Therefore, the martensite structure can be said to be a very effective structure in promoting the refinement of the metal structure. However, on the other hand, if the fraction of the martensite structure, which is a hard structure, is increased in the hot-rolled steel sheet, the rolling load of the rolling machine in the subsequent cold rolling process will increase significantly. Therefore, the technical idea of ​​achieving both refinement and grain regulation of prior austenite grains by combining low-temperature coiling at the Ms point or less with subsequent tempering and cold rolling, and further rapid heating in the hot stamp forming process, has not existed until now, and was discovered for the first time by the present inventors. Here, the Ms point (°C) and the Mf point (°C) can be approximately calculated based on the following formulas 1 and 2. Ms=550-361×[C]-39×[Mn]-35×[V]-20×[Cr]-17×[Ni]-10×[Cu]-5×([Mo]+[W])+15×[Co]+30×[Al]...Formula 1 Mf=410.5-407.3×[C]-7.3×[Si]-37.8×[Mn]-20.5×[Cu]-19.5×[Ni]-19.8×[Cr]-4.5×[Mo] ...Formula 2 In the formula, [C], [Mn], [V], [Cr], [Ni], [Cu], [Mo], [W], [Co], [Al] and [Si] are the contents (mass%) of each element in the hot stamped steel.

[0063] [Tempering process] Next, the obtained hot-rolled steel sheet is tempered in a temperature range of 400 to 600 ° C. in a tempering process. By tempering the hot-rolled steel sheet in the temperature range, a large number of carbides can be precipitated at prior austenite grain boundaries, packet-block boundaries, and laths in the martensite structure, and as a result, a large number of austenite nucleation sites can be generated at the interface between the precipitated carbides and these substructures. If the tempering process is not performed or the tempering temperature is lower than 400 ° C., the carbides cannot be sufficiently precipitated, and therefore, even if the subsequent processes are appropriately performed, the desired refinement and / or grain regulation cannot be achieved in the metal structure. Preferably, the tempering temperature is 450 ° C. or higher. On the other hand, if the tempering temperature is higher than 600 ° C., the precipitated carbides become coarse, and similarly, the desired refinement and grain regulation cannot be achieved in the metal structure. Preferably, the tempering temperature is 550 ° C. or lower. From the viewpoint of sufficiently precipitating the carbides, the tempering time is preferably 1200 seconds or more. The upper limit is not particularly limited, but the tempering time may be, for example, 7200 seconds or less.

[0064] [Pickling process] Before the cold rolling step, pickling may be optionally performed to remove oxide scale formed on the surface of the hot-rolled steel sheet. Pickling may be performed under conditions suitable for removing the oxide scale, and may be performed once or may be performed multiple times to ensure removal of the oxide scale. The pickling step is not necessarily performed before the cold rolling step, and may be performed after the cold rolling step, for example.

[0065] [Cold rolling process] The tempered hot-rolled steel sheet is cold-rolled at a reduction rate of 30% or more. By performing cold rolling at a reduction rate of 30% or more, as described above, the metal structure is refined to increase the interface of the martensite substructure, and the orientation relationship of the carbides precipitated in the tempering process is changed, so that the orientation relationship of the austenite generated at the interface between these carbides and the substructure as a nucleation site can be reliably randomized. When the reduction rate of the cold rolling is low, such an effect cannot be sufficiently obtained. Preferably, the reduction rate of the cold rolling is 40% or more. Although there is no particular upper limit, the reduction rate of the cold rolling may be, for example, 80% or less or 70% or less from the viewpoint of reducing the rolling load of the rolling machine.

[0066] [Annealing process] For example, after the cold rolling process, annealing may be optionally performed to adjust the metal structure and / or properties. Although not particularly limited, from the viewpoint of avoiding the loss of the metal structure created in the previous processes, for example, the loss of austenite nucleation sites due to the progress of recrystallization, the heating temperature in the annealing process is preferably, for example, 600° C. or less.

[0067] [Plating process] For the purpose of improving corrosion resistance, etc., a plating treatment may be applied to the surface of the cold-rolled steel sheet. The plating treatment may be a treatment such as hot-dip plating, alloying hot-dip plating, or electroplating. For example, the steel sheet may be subjected to hot-dip galvanizing treatment as a plating treatment, or an alloying treatment may be performed after the hot-dip galvanizing treatment. Examples of the plating layer include an aluminum plating layer, an aluminum-zinc plating layer, an aluminum-silicon plating layer, a hot-dip galvanizing layer, an electrogalvanizing layer, an alloying hot-dip galvanizing layer, a zinc-nickel plating layer, and an aluminum-magnesium-zinc plating layer. The specific conditions of the plating treatment and the alloying treatment are not particularly limited, and may be any appropriate conditions known to those skilled in the art. However, as in the case of the annealing process, from the viewpoint of avoiding the disappearance of austenite nucleation sites due to the progress of recrystallization, it is preferable to perform the plating treatment and the alloying treatment at a low temperature and for a short time as possible.

[0068] [Temper rolling process] For the purpose of correcting the shape of the steel sheet, adjusting the surface roughness, etc., the steel sheet may be subjected to temper rolling after, for example, the cold rolling process, the annealing process, or the plating process. The rolling reduction in temper rolling is preferably, for example, 1.0% or less.

[0069] [Hot stamp forming process] Finally, in the hot stamp forming process, the obtained cold rolled steel sheet is heated from 600 ° C to a heat treatment temperature: A3 + 50 to A3 + 150 ° C at an average heating rate of more than 100 ° C / s and held at the heat treatment temperature for 30 seconds or less, and then hot stamp forming is started before the temperature reaches 550 ° C, and cooled at an average cooling rate of 10 ° C / s or more. By rapidly heating from 600 ° C to the above heat treatment temperature at an average heating rate of more than 100 ° C / s, it is possible to avoid the temperature range in which recrystallization progresses, and it is possible to suppress the reduction or disappearance of austenite nucleation sites formed in the previous process due to the progress of recrystallization. Therefore, due to the martensitic transformation in the subsequent cooling process, it is possible to obtain a metal structure in the final hot stamp formed body that contains refined and grain-regulated prior austenite grains and has a significantly reduced hardness variation in the sheet thickness direction. As the strength of steel materials increases, a relatively large amount of Mn may be added to improve the hardenability of the steel materials. In such cases, Mn segregation becomes significant and the variation in the prior austenite grain size may not be adequately suppressed. However, according to the present manufacturing method, even if the Mn content is relatively high, the variation in the grain size and hardness of the prior austenite grains can be reduced and a desired metal structure can be obtained by rapidly heating at a high average heating rate of more than 100°C / s. On the other hand, if the average heating rate is 100°C / s or less or the heating start temperature at an average heating rate of more than 100°C / s is higher than 600°C, recrystallization progresses and austenite nucleation sites are reduced or disappeared, making it impossible to achieve the desired refinement and grain regulation in the metal structure. From the viewpoint of achieving the desired refinement and grain regulation, the average heating rate is preferably 120°C / s or more. In addition, when rapid heating is performed at a high average heating rate such as more than 100°C / s, the A3 point generally tends to rise. Therefore, in order to sufficiently austenitize the metal structure to obtain a desired hard structure and to achieve the desired refinement and grain regulation, it is necessary to rapidly heat the metal structure to a temperature that is 50 to 150°C higher than the A3 point (°C) of the following formula 3, which is approximately determined from the chemical composition of the hot stamped body. A3=850+10×([C]+[N])×[Mn]+350×[Nb]+250×[Ti]+40×[B]+10×[Cr]+100×[Mo]...Formula 3 In the formula, [C], [N], [Mn], [Nb], [Ti], [B], [Cr] and [Mo] are the contents (mass%) of each element in the hot stamped steel.

[0070] If the heat treatment temperature is lower than A3+50°C, austenitization is insufficient, and the desired hard structure is not obtained and / or the desired refinement and grain regulation cannot be achieved. On the other hand, if the heat treatment temperature exceeds A3+150°C, the austenite grains grow excessively, and similarly the desired refinement and grain regulation cannot be achieved, and as a result, the desired Vickers hardness distribution in the plate thickness direction cannot be obtained. Also, if the holding time at the heat treatment temperature exceeds 30 seconds, the austenite grains grow excessively, and the desired refinement and grain regulation cannot be achieved. Therefore, the holding time at the heat treatment temperature is preferably 10 seconds or less, more preferably 3 seconds or less. Although the lower limit is not particularly limited, the holding time may be 1 second or more.

[0071] The heating atmosphere is not particularly limited, and may be under normal conditions, for example, in the air, in a gas combustion atmosphere with a controlled air-to-fuel ratio, or in a nitrogen atmosphere, and the dew point may be controlled in these gases. In addition, examples of the heating method include furnace heating using an electric furnace or gas furnace, flame heating, electrical heating, high-frequency heating, and induction heating. After holding at the above heat treatment temperature, hot stamp forming is started before the temperature reaches 550°C. If hot stamp forming is started at a temperature lower than 550°C, bainite transformation and the like progresses, and in such a case, it becomes impossible to obtain a desired hard structure with martensite of 90% by volume or more. After hot stamp forming, it is sufficient to cool at an average cooling rate of 10°C / second or more to a temperature range of 250°C or less in order to obtain a desired hard structure with martensite of 90% by volume or more. In addition, after hot stamp forming, a bake hardening treatment (BH treatment) after painting, for example, a BH treatment at 170 to 200°C for 20 to 30 minutes, may be performed.

[0072] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way. EXAMPLES

[0073] In the following examples, hot stamped steel sheets according to the embodiments of the present invention were produced under various conditions, and the tensile strength, yield strength, and hydrogen embrittlement resistance of the obtained hot stamped steel sheets were examined.

[0074] First, molten steel having the chemical composition shown in Table 1 was cast by a continuous casting method to prepare a slab. The balance other than the components shown in Table 1 is Fe and impurities. These slabs were heated to a temperature of 1100 ° C or more and roughly rolled under predetermined conditions, and then finish rolling, cooling, coiling, tempering and cold rolling were performed under the conditions shown in Table 2. Next, the obtained cold-rolled steel sheet was heated and held from 600 ° C to the heat treatment temperature shown in Table 3 at the average heating rate shown in Table 3, and then hot stamp forming was started before the temperature reached 550 ° C, and cooled to a temperature range of 250 ° C or less at the average cooling rate shown in Table 3. For Comparative Examples 42, 44 and 46 and Invention Examples 43 and 45, the cold-rolled steel sheet obtained by cold rolling was heated and held from the temperature T1 shown in Table 4 to the heat treatment temperature at the average heating rate shown in Table 4, and then hot stamp forming was started before the temperature reached 550 ° C, and cooled to a temperature range of 250 ° C or less at the average cooling rate shown in Table 4. The heating atmosphere and heating method in the hot stamp forming process were a gas combustion atmosphere (air-fuel ratio 0.85) and electrical heating. In order to evaluate the performance under conditions closer to those of the actual product, the obtained hot stamped body was subjected to a heat treatment (without painting) at 170°C for 20 minutes as a post-paint bake hardening treatment (BH treatment).

[0075] [Table 1]

[0076] [Table 2]

[0077] [Table 3]

[0078] [Table 4]

[0079] The properties of the obtained hot stamped steel sheets were measured and evaluated by the following methods.

[0080] [Tensile strength (TS) and yield strength (YS)] The tensile strength (TS) and (YS) of the hot stamped compact were obtained by preparing No. 5 test pieces from any position of the hot stamped compact and conducting tensile tests in accordance with JIS Z 2241: 2011. The crosshead speed was 1 mm / min.

[0081] [Hydrogen embrittlement resistance] The hydrogen embrittlement resistance of the hot stamped compact was evaluated by slow strain rate tensile testing (SSRT) as follows. First, a test piece of 1.0t x 9.0W x 140L (mm) was prepared, with the parallel part having a length of 25 mm and a diameter of 2.0 mm, and a U-notch with a notch depth of 0.35 mm and a notch bottom radius of 0.1 mm was provided on both sides of the center of the parallel part. This test piece was immersed in a 3% NaCl solution, and a galvanostat was used as the power source so that the current density at the immersed part of the test piece surface was 0.1 mA / cm. 2 The hydrogen charging was controlled so that the strain rate was 0.0060 mm / min. The hydrogen-charged test piece was then subjected to a slow strain rate tensile test at a tensile speed of 0.0060 mm / min to investigate the load at break. A specimen with a break load of 600 MPa or more in such a hydrogen environment was evaluated as passing, and a specimen with a break load of less than 600 MPa was evaluated as failing.

[0082] Hot stamped steels with a tensile strength (TS) of 2200 MPa or more, a yield strength (YS) of 1800 MPa or more, and passing the evaluation of hydrogen embrittlement resistance were evaluated as hot stamped steels with high strength and capable of suppressing hydrogen embrittlement. The results are shown in Table 3. Table 3 shows the volume fraction of martensite in a region centered on the 1 / 4 depth position of the plate thickness of the hot stamped steel, as well as the average grain size and standard deviation of the grain size distribution of prior austenite grains (prior γ grains). Table 3 also shows the ratio of the difference between the maximum and minimum values ​​of the Vickers hardness distribution in the plate thickness direction to the average value (hardness regulation). In the hot stamped steels shown in Table 3, the remaining structure other than martensite was bainite, ferrite, retained austenite, and / or pearlite.

[0083] With reference to Tables 1 to 4, in Comparative Example 2, since the average heating rate in the hot stamp forming process was slow, it is considered that recrystallization proceeded and the austenite nucleation sites were reduced. As a result, the desired refinement and grain regulation could not be achieved in the metal structure, and in connection therewith, the desired average grain size of γ grains, the standard deviation of grain size distribution, and the hardness specification could not be obtained, and the hydrogen embrittlement resistance property was deteriorated. In Comparative Example 3, since the tempering process was not performed, it is considered that the austenite nucleation sites due to the precipitation of carbides could not be sufficiently generated. As a result, the desired average grain size and hardness specification of γ grains could not be obtained in the metal structure, and the TS and hydrogen embrittlement resistance property were deteriorated. In Comparative Example 4, since the cold rolling process was not performed, it is considered that the orientation relationship of austenite that generates the interface between the carbide and the martensite substructure as a nucleation site could not be randomized, and the austenite became easily connected. As a result, the desired refinement and grain regulation could not be achieved in the metal structure, and the YS was deteriorated. In Comparative Example 6, the heat treatment temperature in the hot stamp forming process was low, so that austenitization was insufficient, and the desired hard structure and hardness specifications could not be achieved, and the YS and hydrogen embrittlement resistance were reduced. In Comparative Example 7, it is considered that the coiling temperature was high, so that a metal structure mainly composed of martensite could not be formed in the hot rolled steel sheet after coiling. As a result, the desired refinement and grain regulation could not be achieved in the metal structure of the finally obtained hot stamped product, and the YS was reduced.

[0084] In Comparative Example 9, it is considered that the tempering temperature was high, so that the precipitated carbides were coarsened. As a result, it was not possible to achieve the desired refinement and grain regulation in the metal structure, and the YS was reduced. On the other hand, in Comparative Example 10, it is considered that the tempering temperature was low, so that the carbides could not be sufficiently precipitated. As a result, it was not possible to achieve the desired refinement and grain regulation in the metal structure, and the hydrogen embrittlement resistance was reduced. In Comparative Example 11, it is considered that the holding time in the hot stamp forming process was long, so that the austenite grains grew excessively. As a result, it was not possible to achieve the desired refinement and grain regulation in the metal structure, and the hydrogen embrittlement resistance was reduced. In Comparative Example 12, it is considered that the average heating rate in the hot stamp forming process was slow and the holding time was long, so that recrystallization progressed, the austenite nucleation site was reduced, and the generated austenite also grew excessively. As a result, it was not possible to achieve the desired refinement and grain regulation in the metal structure, and the hydrogen embrittlement resistance was reduced. In Comparative Example 14, it is considered that the heat treatment temperature in the hot stamp forming process was high, so that the austenite grains grew excessively. As a result, the desired refinement and grain regulation could not be achieved in the metal structure, and the hydrogen embrittlement resistance property was deteriorated. In Comparative Example 15, the average cooling rate in the hot stamp forming process was slow, so that the volume fraction of martensite in the metal structure was greatly reduced. Therefore, the method specified in this specification could not appropriately measure the average grain size of γ grains, the standard deviation of the grain size distribution, and the hardness specification, and the YS and TS were also greatly reduced. In Comparative Examples 18 to 20, the C content was low, so that the YS and TS were reduced. In Comparative Example 22, it is considered that the coiling temperature was high, so that the metal structure mainly composed of martensite could not be formed in the hot rolled steel sheet after coiling. As a result, the desired hard structure and hardness specification could not be achieved in the metal structure of the finally obtained hot stamped body, and the YS and TS were reduced.In Comparative Example 24, the heat treatment temperature in the hot stamp forming step was low, so that austenitization was insufficient, the desired hard structure and hardness specifications could not be achieved, and the YS and TS were reduced.

[0085] In Comparative Example 38, it is considered that the average cooling rate after the finish rolling in the hot rolling process was slow, so that the metal structure mainly composed of martensite could not be formed in the hot-rolled steel sheet after coiling. As a result, the desired refinement and grain regulation could not be achieved in the metal structure, and the hydrogen embrittlement resistance was deteriorated. In Comparative Example 39, it is considered that the average heating rate in the hot stamp forming process was low, so that recrystallization proceeded and the austenite nucleation site was reduced or disappeared. As a result, the desired refinement and grain regulation could not be achieved in the metal structure, and the hydrogen embrittlement resistance was deteriorated. In Comparative Example 40, it is considered that the reduction rate of the cold rolling was low, so that the orientation relationship of austenite generated at the interface between the carbide and the martensite substructure as the nucleation site could not be randomized, and the austenite was easily connected. As a result, the desired refinement and grain regulation could not be achieved in the metal structure, and the hydrogen embrittlement resistance was deteriorated. In Comparative Examples 42, 44 and 46, the heating start temperature (rate change temperature T1 in Table 4) at an average heating rate of more than 100°C / s in the hot stamp forming process was higher than 600°C, which is believed to have led to the progression of recrystallization and the reduction or disappearance of austenite nucleation sites. As a result, the desired refinement and grain regulation could not be achieved in the metal structure, and hydrogen embrittlement resistance was reduced.

[0086] In contrast, all of the hot stamped steel sheets according to the examples of the present invention have a specified chemical composition and a metal structure in which martensite accounts for 90% by volume or more, and the prior austenite grains are refined to reduce the average grain size to 3.0 μm or less and the standard deviation in the grain size distribution is controlled to 1.5 μm or less for grain regulation. In addition, in connection with this, the difference between the maximum and minimum values ​​of the Vickers hardness distribution in the plate thickness direction is controlled to 35% or less of the average value of the Vickers hardness distribution. As a result, despite the high tensile strength of 2200 MPa or more and the high yield strength of 1800 MPa or more due to the high C content and martensite volume fraction, etc., it is possible to achieve a high breaking load of 600 MPa or more, and therefore excellent resistance to hydrogen embrittlement.

Claims

1. In mass percent, C: 0.40-0.70%, Si: 0.01 to 1.30%, Mn: 0.05-3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0200% or less, Al: 0.001-1.000%, Cr: 0.01-1.00%, Nb: 0 to 0.200%, Ti: 0-0.200%, Mo: 0-1.00%, B: 0 to 0.1000%, Co: 0-4.00%, Ni: 0-3.00%, Cu: 0-3.00%, V: 0 to 3.00%, W: 0-1.00%, Ca: 0-1.000%, Mg: 0-1.000%, REM: 0-1.000%, Sb: 0 to 1.000%, Zr: 0 to 1.000%, Sn: 0-1.000%, As: 0 to 0.100%, and The balance has a chemical composition consisting of Fe and impurities, The volume fraction of martensite is 90% or more, The average grain size of prior austenite grains is 3.0 μm or less, The standard deviation in the grain size distribution of prior austenite grains is 1.5 μm or less; A hot stamped product having a metal structure in which the difference between the maximum and minimum values ​​of a Vickers hardness distribution in the sheet thickness direction is 35% or less of the average value of the Vickers hardness distribution.

2. The chemical composition, in mass%, Nb: 0.001-0.200%, Ti: 0.001 to 0.200%, Mo: 0.001-1.00%, B: 0.0001 to 0.1000%, Co: 0.001 to 4.00%, Ni: 0.001 to 3.00%, Cu: 0.001 to 3.00%, V: 0.001 to 3.00%, W: 0.001-1.00%, Ca: 0.0001-1.000%, Mg: 0.0001-1.000%, REM: 0.0001-1.000%, Sb: 0.001 to 1.000%, Zr: 0.001 to 1.000%, Sn: 0.001 to 1.000%, and As: 0.001-0.100% The hot stamped product according to claim 1 , comprising one or more selected from the group consisting of:

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