Hot stamped compact

Abstract

Provided is a hot-stamp-formed article which has a chemical composition formed from, by mass%, 0.40 to 0.70% C, 0.100% or less P, 0.0100% or less S, 0.0200% or less N, 0.0200% or less O, 0.0010 to 0.500% Al, 0.0010 to 0.100% Nb, 0.010 to 0.200% Ti, 0.010 to 2.00% Mo, 0.0005 to 0.0200% B, and Fe and impurities as the balance. The hot-stamp-formed article has a microstructure in which the total segregation of least one of Mo, W, Ta, Re, Os, Ir, and Tc at the prior austenite grain boundary is 0.10 atom% or greater.

Description

[Technical Field]

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

[0002] In recent years, the automotive industry has been seeking to reduce the weight of vehicle bodies in order to improve fuel efficiency. Increasing the strength of the steel sheets used is one effective way to achieve both a lighter vehicle body and collision safety, and against this background, the development of high-strength steel sheets is underway.

[0003] Increasing the strength of steel sheets reduces their formability, making it generally difficult to achieve both strength and formability in steel sheets. Hot stamping is known as a technique for press-forming difficult-to-form materials such as high-strength steel sheets. Hot stamping is a hot forming technique in which the material to be formed is heated before being formed. With this technique, the material is heated before being formed, so the steel is soft and has good formability at the time of forming. Therefore, even high-strength steel can be formed with high precision into complex shapes. In addition, because the steel is quenched simultaneously with forming using a press die, the steel after forming is known to have sufficient strength.

[0004] In this regard, Patent Document 1 describes a hot-stamped steel sheet having a predetermined chemical composition, an average grain size of prior austenite grains in the microstructure of 5.0 μm or less, and an average Mn concentration of the prior austenite grain boundaries of 1.0 mass % or less. Patent Document 1 also describes that the above configuration can provide a hot-stamped steel sheet having a tensile strength of 2000 MPa or more and excellent toughness. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2020 / 189767 Summary of the Invention [Problem to be solved by the invention]

[0006] Hot-stamped steel sheets having high strength, such as those described in Patent Document 1, may suffer from hydrogen embrittlement cracking (also known as delayed fracture). Hydrogen embrittlement cracking is a phenomenon in which a steel member subjected to high stress during use suddenly fractures due to hydrogen that penetrates 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. Meanwhile, the automotive industry and other industries are also demanding further weight reduction in steel materials. To achieve such weight reduction, steel materials must be strengthened even more than ever before. Therefore, there is a high demand for steel materials, more specifically, hot-stamped steel sheets, that can solve the problem of hydrogen embrittlement even when strength is increased to the same or greater levels than conventional steel materials.

[0007] 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]

[0008] In order to achieve the above object, the present inventors discovered that grain boundaries can be strengthened by reducing the Mn content and segregating specific elements to the grain boundaries, and as a result, it was discovered that hydrogen embrittlement resistance can be significantly improved despite the hot stamped steel having high tensile strength, and thus completed the present invention.

[0009] The present invention, which has achieved the above object, is as follows. (1) In mass%, C: 0.40~0.70%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0200% or less, Al: 0.0010~0.500%, Nb: 0.0010~0.100%, Ti: 0.010 to 0.200%, Mo: 0.010 to 2.000%, B: 0.0005~0.0200%, Si: 0 to 3.00% Mn: 0 to less than 0.50% Cr: 0~1.00%, Co: 0-4.00%, Ni: 0-3.00% Cu: 0-3.00% V: 0~3.00%, Ca: 0 to 1.000%, Mg: 0 to 1.000%, REM: 0 to 1.000%, Sb: 0 to 1.00% Zr: 0 to 1.00%, Sn: 0 to 1.00% As: 0~0.100%, W: 0 to 3.000%, At least one of Ta, Re, Os, Ir, and Tc: 0 to 1.00% in total; Se: 0-1.00%, Bi: 0 to 1.00%, and The balance has a chemical composition consisting of Fe and impurities, A hot stamped body having a microstructure in which the total segregation amount of at least one of Mo, W, Ta, Re, Os, Ir and Tc in prior austenite grain boundaries is 0.10 atomic % or more. (2) The hot stamped body according to (1) above, which contains, by area ratio, at least one of martensite, bainite, and tempered martensite: 70% or more in total. (3) The hot stamped steel according to (1) or (2) above, wherein the amount of Mo segregated in the prior austenite grain boundaries is 0.10 atomic % or more. (4) The hot stamped steel according to (1) or (2) above, wherein the amount of W segregated at the prior austenite grain boundaries is 0.10 atomic % or more. (5) The hot-stamped product according to any one of (1) to (4) above, wherein the total segregation amount is 0.15 atomic % or more. (6) The hot-stamped product according to any one of (1) to (5) above, which has a coating on its surface. (7) The hot stamped product according to (6) above, wherein the coating is mainly composed of an Fe—Al alloy. (8) The hot-stamped product according to (6) above, wherein the coating is mainly composed of an Fe—Zn alloy. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a hot stamped steel sheet that has high strength and is capable of suppressing hydrogen embrittlement. DETAILED DESCRIPTION OF THE INVENTION

[0011] <Hot stamped compact> The hot-stamped steel according to the embodiment of the present invention comprises, in mass %, C: 0.40~0.70%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0200% or less, Al: 0.0010~0.500%, Nb: 0.0010~0.100%, Ti: 0.010 to 0.200%, Mo: 0.010 to 2.000%, B: 0.0005~0.0200%, Si: 0 to 3.00% Mn: 0 to less than 0.50% Cr: 0~1.00%, Co: 0-4.00%, Ni: 0-3.00% Cu: 0-3.00% V: 0~3.00%, Ca: 0 to 1.000%, Mg: 0 to 1.000%, REM: 0 to 1.000%, Sb: 0 to 1.00% Zr: 0 to 1.00%, Sn: 0 to 1.00% As: 0~0.100%, W: 0 to 3.000%, At least one of Ta, Re, Os, Ir, and Tc: 0 to 1.00% in total; Se: 0-1.00%, Bi: 0 to 1.00%, and The balance has a chemical composition consisting of Fe and impurities, The alloy is characterized by having a microstructure in which the total segregation amount of at least one of Mo, W, Ta, Re, Os, Ir, and Tc at prior austenite grain boundaries is 0.10 atomic % or more.

[0012] As mentioned above, it is known that hydrogen embrittlement cracking is more likely to occur as the strength of a steel increases. In particular, in steels with extremely high strength, such as those with a tensile strength of 2000 MPa or more, the microstructure of the steel generally contains martensite to ensure high strength. However, in such high-strength steels, hydrogen embrittlement cracking is thought to occur primarily due to hydrogen segregation at prior austenite grain boundaries in the martensite structure. Therefore, in order to address the deterioration of hydrogen embrittlement resistance associated with intergranular cracking in steels with extremely high strength, such as those with a tensile strength of 2000 MPa or more, more specifically, in hot-stamped steels, the inventors conducted research focusing on specific elements contained in the hot-stamped steels, with the aim of strengthening the prior austenite grain boundaries, which are the initiation sites of hydrogen embrittlement cracking in the microstructure. First, the inventors conducted research with the aim of suppressing embrittlement of the prior austenite grain boundaries and thereby strengthening the prior austenite grain boundaries. More specifically, as the strength of steel materials increases, a relatively large amount of Mn is generally added to improve the hardenability of the steel material. However, research by the present inventors has revealed that, although a relatively high Mn content improves hardenability, Mn embrittles prior austenite grain boundaries, accelerating hydrogen embrittlement cracking at the prior austenite grain boundaries, which may result in deterioration of the hydrogen embrittlement resistance of the hot-stamped steel. In response to this, the present inventors have found that by limiting the Mn content in the hot-stamped steel to less than 0.50 mass%, it is possible to sufficiently suppress or reduce the embrittlement of prior austenite grain boundaries caused by Mn, and as a result, it is possible to strengthen the prior austenite grain boundaries and improve the hydrogen embrittlement resistance of the hot-stamped steel compared to steels containing a relatively high amount of Mn.

[0013] Next, the inventors conducted further studies from the perspective of actively strengthening the prior austenite grain boundaries and found that the prior austenite grain boundaries in the microstructure of a hot-stamped steel can be strengthened by segregating specific elements, more specifically at least one of Mo, W, Ta, Re, Os, Ir, and Tc, and particularly Mo and W, to the prior austenite grain boundaries in an amount such that the total segregation amount is 0.10 atomic % or more. In addition, the inventors found that the grain boundary segregation of these grain boundary strengthening elements not only suppresses a decrease in hardenability, but can actually improve hardenability to a level equivalent to or even higher than that of a high Mn content, despite the Mn content being limited to less than 0.50 mass %, and as a result, high tensile strength, for example, of 2200 MPa or more can be reliably achieved despite a relatively low Mn content of less than 0.50 mass %.

[0014] While not intending to be bound by any particular theory, it is believed that segregating the above-mentioned grain boundary strengthening elements to prior austenite grain boundaries can significantly reduce grain boundary energy. Reducing grain boundary energy generally suppresses ferrite nucleation. Therefore, it is believed that segregating the above-mentioned grain boundary strengthening elements to prior austenite grain boundaries can suppress the decrease in hardenability due to a relatively low Mn content and achieve hardenability equivalent to or better than that of a high Mn content. It has been known to add some of the above-mentioned grain boundary strengthening elements to hot-stamped steels, for example, to improve hardenability. However, in high-strength hot-stamped steels with a tensile strength exceeding 2000 MPa, the C content of the hot-stamped steel is high. Therefore, in conventional manufacturing methods, these grain boundary strengthening elements form carbides and / or intermetallic compounds, making it impossible to sufficiently segregate these grain boundary strengthening elements in a solid solution state to prior austenite grain boundaries. The present inventors have now found that, as will be described in detail later in connection with the manufacturing method of a hot-stamped steel, it is possible to segregate at least one of Mo, W, Ta, Re, Os, Ir, and Tc to the prior austenite grain boundaries in a predetermined total segregation amount by appropriately controlling the heat treatment conditions, particularly in the preheating step before the hot stamping step and in the hot stamping step. Thus, the present inventors have now discovered for the first time that in a high-strength hot-stamped steel containing a relatively high carbon content of 0.40 mass% or more, at least one of Mo, W, Ta, Re, Os, Ir, and Tc can be segregated to the prior austenite grain boundaries in a predetermined total segregation amount to strengthen the grain boundaries, thereby improving hydrogen embrittlement resistance while maintaining high strength despite a low Mn content.Therefore, with the hot stamped steel according to the embodiment of the present invention, the combination of suppression of embrittlement of the prior austenite grain boundaries due to the reduced Mn content and active strengthening of the prior austenite grain boundaries and improved hardenability due to grain boundary segregation of a grain boundary strengthening element selected from at least one of Mo, W, Ta, Re, Os, Ir, and Tc makes it possible to significantly improve hydrogen embrittlement resistance in the hot stamped steel, despite the hot stamped steel having a high tensile strength, for example, a high tensile strength of 2200 MPa or more.

[0015] Hereinafter, a hot-stamped steel according to an 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. Furthermore, in this specification, unless otherwise specified, the symbol "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits.

[0016] [C: 0.40~0.70%] C is an element that improves the strength of a hot-stamped steel sheet. If the C content is less than 0.40%, the desired strength cannot be obtained in the hot-stamped steel sheet. 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.

[0017] [P:0.100% or less] P is an impurity element that segregates at grain boundaries and deteriorates hydrogen embrittlement resistance. Therefore, the P content is set to 0.100% or less. The P content is 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% significantly increases the cost of dephosphorization, which is economically undesirable, so the P content may be set to 0.0001% or more.

[0018] [S:0.0100% or less] S is an impurity element that forms inclusions in steel. These inclusions deteriorate hydrogen embrittlement resistance, so 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% significantly increases the cost of desulfurization, which is economically undesirable, so the S content may be set to 0.0001% or more.

[0019] [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% significantly increases the cost of denitrification, which is economically undesirable. Therefore, the N content may be set to 0.0001% or more.

[0020] [O:0.0200% or less] If O is contained in a steel in a large amount, 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 set to 0.0001% or more. In order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be set to 0.0005% or more.

[0021] [Al: 0.0010~0.500%] 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 0.0010%, deoxidation is insufficient, and coarse oxides are generated, deteriorating the hydrogen embrittlement resistance. Therefore, the Al content is set to 0.0010% 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 0.500%, coarse oxides are formed in the steel, which reduces the hydrogen embrittlement resistance of the hot-stamped steel. Therefore, the Al content is set to 0.500% or less. The Al content is preferably 0.400% or less, 0.300% or less, 0.200% or less, 0.150% or less, or 0.100% or less.

[0022] [Nb: 0.0010~0.100%] Nb is an element that forms carbonitrides in steel and improves the strength of hot-stamped steel through precipitation strengthening, and also contributes to refining the structure through its pinning effect. If the Nb content is less than 0.0010%, these effects cannot be fully achieved. 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%, coarse carbonitrides are formed in the steel, which reduces the hydrogen embrittlement 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, 0.060% or less, or 0.050% or less.

[0023] [Ti: 0.010~0.200%] Ti is an element that forms carbonitrides in steel and improves the strength of hot-stamped steel through precipitation strengthening, and also contributes to refining the structure through its pinning effect. If the Ti content is less than 0.010%, these effects cannot be fully achieved. Therefore, the Ti content is set to 0.010% or more. The Ti content is preferably 0.015% or more, 0.020% or more, or 0.025% or more. On the other hand, if the Ti content exceeds 0.200%, coarse carbonitrides are formed in the steel, which reduces the hydrogen embrittlement resistance of the hot-stamped steel. Therefore, the Ti content is set to 0.200% or less. The Ti content is preferably 0.180% or less, 0.150% or less, 0.100% or less, 0.060% or less, or 0.050% or less.

[0024] [Mo: 0.010~2.000%] Mo is an element that segregates to austenite grain boundaries during heating in the hot stamping process, thereby improving hardenability and increasing the strength of prior austenite grain boundaries, thereby improving the hydrogen embrittlement resistance of the hot stamped steel. If the Mo content is less than 0.010%, these effects cannot be fully achieved, and the desired hydrogen embrittlement resistance may not be achieved. Therefore, the Mo content is set to 0.010% or more. The Mo content is preferably 0.050% or more, 0.100% or more, 0.150% or more, 0.200% or more, 0.300% or more, or 0.500% or more. On the other hand, if the Mo content exceeds 2.000%, coarse intermetallic compounds and carbides are formed in the hot-stamped steel sheet, deteriorating the hydrogen embrittlement resistance of the hot-stamped steel sheet. Therefore, the Mo content is set to 2.000% or less. The Mo content is preferably 1.800% or less, 1.500% or less, 1.300% or less, 1.000% or less, or 0.800% or less.

[0025] [B:0.0005~0.0200%] 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 0.0010% or more, 0.0015% or more, or 0.0020% or more. On the other hand, if the B content exceeds 0.0200%, coarse borides are formed in the hot-stamped steel, and the hydrogen embrittlement resistance of the hot-stamped steel deteriorates. Therefore, the B content is set to 0.0200% or less. The B content is preferably 0.0150% or less, 0.0100% or less, 0.0050% or less, 0.0040% or less, or 0.0030% or less.

[0026] 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, as needed, in place of a portion of the remaining Fe. For example, the hot-stamped steel may contain at least one element selected from the group consisting of 0-3.00% Si, 0-less than 0.50% Mn, 0-1.00% Cr, 0-4.00% Co, 0-3.00% Ni, 0-3.00% Cu, and 0-3.00% V. The hot-stamped steel may also contain at least one element selected from the group consisting of 0-1.000% Ca, 0-1.000% Mg, and 0-1.000% REM. The hot-stamped steel may also contain at least one element selected from the group consisting of 0-1.00% Sb, 0-1.00% Zr, and 0-1.00% Sn. The hot-stamped steel may also contain 0 to 0.100% As. The hot-stamped steel may also contain 0 to 3.000% W. The hot-stamped steel may also contain at least one of Ta, Re, Os, Ir, and Tc in a total amount of 0 to 1.00%. The hot-stamped steel may also contain at least one element selected from the group consisting of 0 to 1.00% Se and 0 to 1.00% Bi. These optional elements will be described in detail below.

[0027] [Si: 0-3.00%] Silicon is an element that improves the strength of a hot-stamped steel sheet through solid solution strengthening. The silicon content may be 0.001% or more, but to ensure this effect, the silicon content is preferably 0.01% or more. The silicon content may be 0.05% or more, 0.10% or more, 0.20% or more, 0.30% or more, or 0.40% or more. On the other hand, if Si is contained in an excessive amount, the amount of ferrite in the hot stamped steel increases, and the desired strength may not be obtained. Therefore, the Si content is set to 3.00% or less. The Si content may be 2.50% or less, 2.00% or less, 1.00% or less, or 0.70% or less.

[0028] [Mn: 0 to less than 0.50%] Mn is an element that improves the hardenability of steel and contributes to improving its strength. The Mn content may be 0.001% or more, but to ensure this effect, the Mn content is preferably 0.01% or more. The Mn content may be 0.05% or more, 0.10% or more, 0.15% or more, or 0.20% or more. On the other hand, excessive Mn content may embrittle prior austenite grain boundaries and promote hydrogen embrittlement cracking at the prior austenite grain boundaries. Therefore, the Mn content is set to less than 0.50%. The Mn content may be 0.49% or less, 0.48% or less, 0.47% or less, 0.46% or less, 0.45% or less, 0.43% or less, 0.40% or less, 0.35% or less, or 0.30% or less.

[0029] [Cr: 0~1.00%] Cr is an element that dissolves in prior austenite grains during heating before hot stamping, thereby increasing the strength of the hot stamped steel. The Cr content may be 0.001% or more, but to ensure this effect, the Cr content is preferably 0.01% or more or 0.05% or more. On the other hand, excessive Cr content may cause the formation of coarse carbides in the hot-stamped steel sheet, which may reduce the hydrogen embrittlement resistance of the hot-stamped steel sheet. Therefore, the Cr content is set to 1.00% or less. The Cr content may be 0.80% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.08% or less.

[0030] [Co: 0-4.00%] Co is an element that improves the strength of the hot stamped steel by solid solution strengthening. The Co content may be 0.001% or more, but to ensure 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 when a large amount of Co is added, 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, 0.50% or less, or 0.10% or less.

[0031] [Ni: 0-3.00%] Ni has the effect of increasing the strength of the hot stamped steel by dissolving in austenite grains during heating in the hot stamping process. The Ni content may be 0.001% or more, but to ensure this effect, the Ni content is preferably 0.01% or more or 0.05% or more. On the other hand, since the above effects are saturated even when the Ni content is large, the Ni content is preferably 3.00% or less. The Ni content may be 2.00% or less, 1.00% or less, 0.60% or less, 0.30% or less, or 0.10% or less.

[0032] [Cu: 0-3.00%] Cu dissolves in austenite grains during heating in the hot stamping process, thereby enhancing the strength of the hot stamped steel. The Cu content may be 0.001% or more, but to ensure this effect, the Cu content is preferably 0.01% or more or 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 3.00% or less. The Cu content may be 2.00% or less, 1.00% or less, 0.60% or less, 0.30% or less, or 0.10% or less.

[0033] [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. The V content may be 0.001% or more, but to ensure 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 when a large amount of V is added, the V content is preferably 3.00% or less. The V content may be 2.00% or less, 1.00% or less, 0.60% or less, 0.30% or less, or 0.10% or less.

[0034] [Ca: 0-1.000%] Ca is an element that suppresses the formation of oxides. The Ca content may be 0.0001% or more, but to ensure this effect, the Ca content is preferably 0.0005% or more or 0.001% or more. On the other hand, since the above effects are saturated even when Ca is added in a large amount, 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.

[0035] [Mg: 0-1.000%] Mg forms oxides and sulfides in molten steel, suppresses the formation of coarse MnS, disperses many fine oxides, and contributes to the refinement of the metal structure. The Mg content may be 0.0001% or more, but to ensure these effects, the Mg content is preferably 0.0005% or more or 0.001% or more. On the other hand, since the above effects are saturated even when Mg is added 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.

[0036] [REM:0~1.000%] REM is an element that suppresses the formation of oxides. The REM content may be 0.0001% or more, but to ensure 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 contained in a large amount, 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.

[0037] [Sb: 0~1.00%] Sb is an element that suppresses the formation of oxides. To reliably obtain this effect, the Sb content is preferably 0.001% or more, or 0.005% or more. On the other hand, since the above effects are saturated even when Sb is contained in a large amount, the Sb content is preferably 1.00% or less. The Sb content may be 0.80% or less, 0.50% or less, 0.20% or less, or 0.10% or less.

[0038] [Zr: 0~1.00%] Zr is an element that suppresses the formation of oxides. To reliably obtain this effect, the Zr content is preferably 0.001% or more, or 0.005% or more. On the other hand, since the above effects are saturated even when Zr is contained in a large amount, the Zr content is preferably 1.00% or less. The Zr content may be 0.80% or less, 0.50% or less, 0.20% or less, or 0.10% or less.

[0039] [Sn: 0~1.00%] Sn is an element that suppresses the formation of oxides. To reliably obtain this effect, the Sn content is preferably 0.001% or more, or 0.005% or more. On the other hand, since the above effects are saturated even when Sn is contained in a large amount, the Sn content is preferably 1.00% or less. The Sn content may be 0.80% or less, 0.50% or less, 0.20% or less, or 0.10% or less.

[0040] [As:0~0.100%] As contributes to the refinement of prior austenite grains by lowering the austenite single-phase transformation temperature. To reliably obtain this effect, the As content is preferably 0.001% or more, or 0.005% or more. On the other hand, since the above effects are saturated even when As is contained in a large amount, 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, or 0.010% or less.

[0041] [W:0~3.000%] W is an element that segregates at austenite grain boundaries during heating in the hot stamping process, thereby improving hardenability and increasing the strength of prior austenite grain boundaries, thereby improving the hydrogen embrittlement resistance of the hot stamped steel. The W content may be 0.001% or more, but to ensure this effect, the W content is preferably 0.005%. The W content may be 0.010% or more, 0.050% or more, 0.100% or more, 0.200% or more, 0.400% or more, 0.500% or more, or 0.800% or more. On the other hand, even if W is added in a large amount, the above effects may saturate and / or the W that cannot exist due to segregation in a solid solution state may form intermetallic compounds and carbides. Such intermetallic compounds and carbides may become crack initiation sites and may reduce the hydrogen embrittlement resistance of the hot-stamped steel. For this reason, the W content is preferably 3.000% or less. The W content may also be 2.500% or less, 2.000% or less, 1.800% or less, 1.500% or less, or 1.000% or less.

[0042] [At least one of Ta, Re, Os, Ir, and Tc: 0 to 1.00% in total] Like Mo and W, Ta, Re, Os, Ir, and Tc segregate to prior austenite grain boundaries during heating in the hot stamping process, thereby improving hardenability and increasing the strength of prior austenite grain boundaries, thereby enhancing hydrogen embrittlement resistance in hot stamped steel. The total content of at least one of Ta, Re, Os, Ir, and Tc may be 0%, but to achieve this effect, it is preferably 0.001% or more. The total content of at least one of Ta, Re, Os, Ir, and Tc is preferably 0.01% or more, more preferably 0.10% or more, and even more preferably 0.15% or more. However, even if these elements are contained in excess, the effects saturate. Therefore, the inclusion of more than necessary of these elements in a steel material may increase production costs. Therefore, the total content of at least one of Ta, Re, Os, Ir, and Tc is preferably 1.00% or less. It may also be 0.80% or less, 0.60% or less, or 0.40% or less.

[0043] [Se: 0-1.00%] Se is an element that improves hydrogen embrittlement resistance. Therefore, Se may be contained. To obtain the above effects, the Se content is preferably 0.001% or more, or 0.01% or more. On the other hand, if the Se content exceeds 1.00%, the effect saturates and the cost increases. Therefore, when Se is contained, the Se content is preferably 1.00% or less. The Se content may be 0.80% or less, 0.50% or less, 0.20% or less, or 0.10% or less.

[0044] [Bi: 0-1.00%] Bi is an element that improves hydrogen embrittlement resistance. Therefore, Bi may be contained. To obtain the above effects, the Bi content is preferably 0.001% or more, or 0.01% or more. On the other hand, if the Bi content exceeds 1.00%, the effect saturates and the cost increases. Therefore, when Bi is contained, the Bi content is preferably 1.00% or less. The Bi content may be 0.80% or less, 0.50% or less, 0.20% or less, or 0.10% or less.

[0045] 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 ore and scrap, when the hot-stamped steel is industrially manufactured. The industrial manufacturing methods include the blast furnace steelmaking method and the electric furnace steelmaking method, and include the levels (impurity levels) of components mixed in when manufactured by either method.

[0046] The chemical composition of the hot-stamped body may be measured by a common analytical method, such as inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method. When the hot stamped body has a plated layer on its surface, the plated layer may be removed by mechanical grinding before analyzing the chemical composition.

[0047] [At least one of martensite, bainite and tempered martensite: 70% or more in total] The microstructure of the hot-stamped steel preferably contains at least one of martensite, bainite, and tempered martensite in a total area fraction of 70% or more. The remaining structure is not particularly limited, but may consist of at least one of ferrite, retained austenite, and pearlite in an amount of 30% or less. Martensite, bainite, and tempered martensite are very hard structures. Therefore, by containing at least one of martensite, bainite, and tempered martensite in a total area fraction of 70% or more in the hot-stamped steel, high tensile strength, specifically tensile strength of 2200 MPa or more, can be achieved. The total area fraction of at least one of martensite, bainite, and tempered martensite is preferably 75% or more, 80% or more, 85% or more, 90% or more, 92% or more, or 94% or more, and more preferably 95% or more or 97% or more. The upper limit of the total area fraction of at least one of martensite, bainite, and tempered martensite is not particularly limited and may be 100%.

[0048] [Identification of microstructure and calculation of area ratio] Identification of the microstructure and calculation of the area ratio in a hot-stamped steel product are carried out as follows. First, a sample is cut from an arbitrary position at least 50 mm away from the end face of the steel material (if a sample cannot be taken from this position, a position avoiding the end) so that a thickness cross section perpendicular to the surface 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 direction perpendicular to the thickness direction.

[0049] 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, 50 μm thick area is measured 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.

[0050] The obtained crystal orientation information is used with the "Phase Map" function of the "OIM Analysis®" software attached to 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 bcc crystal structures are determined to be bainite, tempered martensite, martensite, and ferrite. For these regions, the "Grain Average Misorientation" function of the "OIM Analysis®" software attached to 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 calculated to determine the area fraction of ferrite.

[0051] Next, the remaining region (the region where "Grain Average Misorientation" exceeds 0.5°) is taken as the total area fraction of martensite, tempered martensite, and bainite. The area fraction of pearlite is calculated by subtracting the area fraction of retained austenite, the area fraction of bainite, tempered martensite, martensite, and ferrite from 100%.

[0052] [Total segregation amount of at least one of Mo, W, Ta, Re, Os, Ir, and Tc at prior austenite grain boundaries: 0.10 atomic % or more] In an embodiment of the present invention, the total segregation amount of at least one of Mo, W, Ta, Re, Os, Ir, and Tc at the prior austenite grain boundaries is 0.10 atomic % or more. By segregating at least one of Mo, W, Ta, Re, Os, Ir, and Tc at the prior austenite grain boundaries in an amount such that the total segregation amount is 0.10 atomic % or more, hardenability can be improved and the prior austenite grain boundaries in the microstructure of the hot-stamped steel can be strengthened. According to an embodiment of the present invention, the combination of suppressing embrittlement of the prior austenite grain boundaries by limiting the Mn content of the hot-stamped steel to less than 0.50% and actively strengthening the prior austenite grain boundaries by segregating these specific grain boundary strengthening elements can significantly increase the strength of the prior austenite grain boundaries compared to the application of either one alone. Therefore, even if the hot-stamped steel has an extremely high tensile strength, for example, 2200 MPa or more, the extremely high resistance to intergranular cracking can significantly improve hydrogen embrittlement resistance. From the viewpoint of grain boundary strengthening, the higher the total segregation amount of at least one of Mo, W, Ta, Re, Os, Ir, and Tc at the prior austenite grain boundaries, the better, and it may be, for example, 0.13 atomic % or more, 0.15 atomic % or more, 0.18 atomic % or more, or 0.20 atomic % or more. The upper limit of the total content is not particularly limited, and may be, for example, 3.00 atomic % or less, 2.00 atomic % or less, 1.50 atomic % or less, 1.00 atomic % or less, 0.80 atomic % or less, 0.60 atomic % or less, or 0.40 atomic % or less.

[0053] In one embodiment, the amount of Mo segregated at the prior austenite grain boundaries may be 0.10 atomic % or more, 0.13 atomic % or more, 0.15 atomic % or more, 0.18 atomic % or more, or 0.20 atomic % or more. Similarly, the amount of Mo segregated at the prior austenite grain boundaries may be 3.00 atomic % or less, 2.00 atomic % or less, 1.50 atomic % or less, 1.00 atomic % or less, 0.80 atomic % or less, 0.60 atomic % or less, or 0.40 atomic % or less. In another embodiment, the amount of W segregated at the prior austenite grain boundaries may be 0.10 atomic % or more, 0.13 atomic % or more, 0.15 atomic % or more, 0.18 atomic % or more, or 0.20 atomic % or more. Similarly, the amount of W segregation at prior austenite grain boundaries may be 3.00 atomic % or less, 2.00 atomic % or less, 1.50 atomic % or less, 1.00 atomic % or less, 0.80 atomic % or less, 0.60 atomic % or less, or 0.40 atomic % or less. In yet another embodiment, the total amount of Mo segregation and W segregation at prior austenite grain boundaries may be 0.10 atomic % or more, 0.13 atomic % or more, 0.15 atomic % or more, 0.18 atomic % or more, or 0.20 atomic % or more, and / or 3.00 atomic % or less, 2.00 atomic % or less, 1.50 atomic % or less, 1.00 atomic % or less, 0.80 atomic % or less, 0.60 atomic % or less, or 0.40 atomic % or less. In still another embodiment, the total amount of segregation of Mo, W, and at least one of Ta, Re, Os, Ir, and Tc at the prior austenite grain boundaries may be 0.10 atomic % or more, 0.13 atomic % or more, 0.15 atomic % or more, 0.18 atomic % or more, or 0.20 atomic % or more, and / or 3.00 atomic % or less, 2.00 atomic % or less, 1.50 atomic % or less, 1.00 atomic % or less, 0.80 atomic % or less, 0.60 atomic % or less, or 0.40 atomic % or less.

[0054] [Method for determining the total segregation amount of at least one of Mo, W, Ta, Re, Os, Ir, and Tc at prior austenite grain boundaries] The total segregation amount of at least one of Mo, W, Ta, Re, Os, Ir, and Tc at prior austenite grain boundaries is determined as follows. First, a test specimen is taken from a position at least 50 mm away from the end face of the hot-stamped steel. The front and back surfaces of the test specimen are then mechanically ground. If a coating layer is present on the steel sheet surface, the coating layer is removed, and the front and back surfaces of the steel sheet test specimen are then mechanically ground. The thickness of the steel sheet is not particularly specified as long as it is possible to measure the depth of 1 / 4 of the sheet thickness. However, it is also possible to remove equal amounts of the front and back surfaces of the test specimen by mechanical grinding to achieve a thickness of 1.2 mm. A test specimen measuring 20 mm in length and 3.2 mm in width is machined, and a 45° V-notch is inserted at a position 11.5 mm in length. The test specimen is then immersed in a 20% ammonium thiocyanate solution. The immersion time is not particularly limited, as long as the prior austenite grain boundaries are exposed when the specimen is placed in an Auger electron emission spectrometer and fractured, and may be, for example, 48 hours. Within 10 minutes of the completion of immersion, the front and back surfaces of the specimen are zinc-plated. After plating, the specimen is immediately subjected to Auger electron emission spectroscopic analysis and fractured. The time from plating to fracture is preferably within 1.5 hours, more preferably within 0.5 hours. The specimen is placed in the Auger electron emission spectrometer and fractured from the notch to expose the prior austenite grain boundaries. The equipment used for this purpose is not particularly limited as long as it is a field-emission Auger electron spectrometer, and the model may be, for example, a PHI680 manufactured by ULVAC-PHI. Measurement conditions may be an accelerating voltage of 10 keV and a probe current of 10 nA. The exposed prior austenite grain boundaries are irradiated with an electron beam at an accelerating voltage of 1 to 30 kV, and the atomic percentage of a specific element (specifically, at least one of Mo, W, Ta, Re, Os, Ir, and Tc) at the grain boundaries is measured. Measurements are performed at 10 prior austenite grain boundaries located at a depth of 1 / 4 of the plate thickness from the surface. To prevent contamination of the grain boundaries, it is preferable to complete the measurement quickly after fracture, and it may be completed within 30 minutes. The average atomic percentage of the specific element obtained is calculated and determined as the total segregation amount of at least one of Mo, W, Ta, Re, Os, Ir, and Tc.

[0055] [Average grain size of prior austenite grains: 15 μm or less] In the present embodiment, the average grain size of the prior austenite grains is not particularly limited, but may be, for example, 15 μm or less. The hot-stamped steel according to the present embodiment contains Nb and Ti. These elements form carbides, nitrides, and / or carbonitrides, and their pinning effect contributes to microstructural refinement. Furthermore, in the hot-stamped steel according to the present embodiment, a grain boundary strengthening element selected from at least one of Mo, W, Ta, Re, Os, Ir, and Tc segregates at the grain boundaries, slowing the grain growth rate through the solute drag effect. Therefore, in the hot-stamped steel according to the present embodiment, the pinning effect due to Nb and Ti and the solute drag effect due to the grain boundary segregation of the specific grain boundary strengthening element enable refinement of the prior austenite grains. For example, the average grain size of the prior austenite grains may be 12 μm or less, 10 μm or less, or 8 μm or less. Although there is no particular lower limit, the average grain size of the prior austenite grains may be, for example, 1 μm or more, 2 μm or more, or 3 μm or more.

[0056] [Method for determining the average grain size of prior austenite grains] The average grain size of prior austenite grains is determined as follows. First, a sample is cut from a location at least 50 mm away from the end face of the hot-stamped product (or from a location that avoids the end if a sample cannot be taken from this location) so that a cross-section perpendicular to the surface can be observed. The size of the sample depends on the measurement device, but it should be large enough to allow observation of approximately 10 mm in the direction perpendicular to the thickness direction. The cross-section of the sample is 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 is dispersed in a diluted solution such as alcohol or pure water. Next, the observation surface is polished by electrolytic polishing. At a depth of 1 / 4 of the thickness at any position along the longitudinal direction of the sample cross-section, a 50 μm long area with a 50 μm thickness direction is measured at 0.1 μm measurement intervals using electron backscatter diffraction to obtain crystal orientation information. For the measurement, an EBSD analyzer consisting of a thermal field emission scanning electron microscope and an EBSD detector may be used, for example, an EBSD analyzer consisting of a JEOL JSM-7001F and a TSL DVC5 detector. In this case, the degree of vacuum inside the EBSD analyzer is 9.6 × 10 -5 The crystal orientation of the prior austenite grains may be calculated using the crystal orientation information obtained from the crystal orientation relationship between typical prior austenite grains and the transformed body-centered grains. 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 using 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 and longest diameters is calculated, and this average value is used as the grain size of the prior austenite grain. The above operation is performed for all prior austenite grains, excluding prior austenite grains whose entirety is not included in the observation field, such as those at the edge of the observation field, to determine the grain sizes of all prior austenite grains in the observation field. The average grain size of the prior austenite grains is determined by calculating the average grain size from the obtained grain sizes of all prior austenite grains.

[0057] [Covering] The hot stamped steel according to this embodiment may have a coating on a part or the whole of its surface. The coating may be a coating mainly made of an Fe-Al alloy or a coating mainly made of an Fe-Zn alloy. The coating is also called a film, an alloyed plating layer, or an intermetallic compound layer. A coating primarily made of an Fe-Al alloy is a coating containing 70% by mass or more of Fe and Al, and a coating primarily made of an Fe-Zn alloy is a coating containing 70% by mass or more of Fe and Zn. A coating primarily made of an Fe-Al alloy may contain, in addition to Fe and Al, Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, and REM, with the balance being impurities. A coating primarily made of an Fe-Zn alloy may contain, in addition to Fe and Zn, Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Al, Co, In, Bi, Zr, Se, As, and REM, with the balance being impurities. The coating provides corrosion resistance, which has the effect of improving hydrogen embrittlement resistance when used in automobiles. The thickness of the coating is preferably 10 to 100 μm.

[0058] [Shape of hot stamped compact] The shape of the hot-stamped steel according to this embodiment is not particularly limited. That is, the hot-stamped steel may be a flat plate or a steel plate formed into a predetermined shape. Steel members formed by hot stamping (hot forming) are often formed bodies, but in this embodiment, the term "hot-stamped steel" refers to both formed bodies and flat plates. The hot-stamped steel may also be a tailored property steel having different strengths depending on the region. In this case, at least a portion of the hot-stamped steel must have a tensile strength of 2200 MPa or more. The tailored property steel may be formed by joining steel plates with different chemical compositions, strengths, or thicknesses, or may be a steel plate partially subjected to heat treatment. The hot-stamped steel may also have a decarburized layer or a soft layer in a portion of its surface.

[0059] [Mechanical properties] The hot-stamped steel according to the embodiment of the present invention can achieve excellent mechanical properties, such as a tensile strength of 2200 MPa or more. The tensile strength is preferably 2300 MPa or more, more preferably 2400 MPa or more, and most preferably 2500 MPa or more. There is no particular upper limit, but the tensile strength may be, for example, 3500 MPa or less, 3300 MPa or less, or 3000 MPa or less. The tensile strength of the hot-stamped steel is measured by preparing a No. 5 test piece and conducting a tensile test in accordance with JIS Z 2241:2011. In this case, the surface layers on both the front and back surfaces may be removed by machining or chemical polishing to remove any irregularities on the test piece surface.

[0060] As described above, the hot-stamped steel according to the embodiment of the present invention has excellent hydrogen embrittlement resistance despite having a high tensile strength of, for example, 2200 MPa or more. Therefore, it is very useful for use as, for example, automotive frame members, bumpers, and other structural and reinforcing members that require strength.

[0061] <Method of manufacturing hot-stamped body> Next, a preferred method for producing a hot-stamped steel according to an embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for producing a hot-stamped steel according to an embodiment of the present invention, but is not intended to limit the hot-stamped steel to steels produced by the production method described below.

[0062] A method for producing a hot-stamped steel according to an embodiment of the present invention is characterized by appropriately controlling, in particular, the coiling conditions in the hot rolling step, and the heat treatment conditions in the preheating step before the hot stamping step and the hot stamping step, in order to segregate specific grain boundary strengthening elements at the prior austenite grain boundaries. More specifically, the method for producing a hot-stamped steel according to an embodiment of the present invention includes: hot rolling a slab having the chemical composition described above in relation to the hot stamped body, followed by coiling at a temperature of 450°C or less (hot rolling step); A step of preheating the obtained steel plate to a temperature of more than 1200 ° C. and then cooling it to less than 350 ° C. at an average cooling rate of 10 ° C. / second or more (preheating step); a step of hot stamping the steel sheet, the step including heating the steel sheet to a temperature range of 800 to 1000°C and then holding the temperature for 60 to 600 seconds (hot stamp forming step); Each step will be described in detail below.

[0063] [Hot rolling process] In the hot rolling process, a slab having the chemical composition described above in relation to the hot-stamped steel is first heated. The casting method for the molten steel is not particularly limited, and it may be produced by continuous casting, ingot casting, or thin slab casting. The heating method before hot rolling is not particularly limited, but the slab used contains a relatively large amount of alloying elements to obtain a high-strength steel plate. Therefore, the slab may be heated to a heating temperature of 1100°C or higher before hot rolling in order to dissolve the alloying elements in the slab. Furthermore, the heated slab may optionally be subjected to rough rolling before finish rolling for thickness adjustment or the like. The conditions for rough rolling are not particularly limited as long as the desired sheet bar dimensions are secured. The heated slab, or a slab that has been optionally rough-rolled, is then subjected to finish rolling. While not particularly limited, finish rolling is generally performed under conditions such that the finish rolling completion temperature is 650°C or higher. If the temperature at the end of finish rolling 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 temperature at the end of finish rolling is generally 950°C or lower.

[0064] [Rewind] Next, the finish-rolled hot-rolled steel sheet is coiled at a temperature of 450°C or lower. The grain boundary strengthening element selected from at least one of Mo, W, Ta, Re, Os, Ir, and Tc is present in the steel sheet in the form of a carbide or intermetallic compound before the preheating and hot stamping processes. Examples of such carbides include carbides formed by the grain boundary strengthening element alone bonding with carbon (e.g., WC), and carbides in which the grain boundary strengthening element is partially dissolved in cementite (FeC) in the microstructure. As will be described in detail later, this method involves sufficiently dissolving the carbides and intermetallic compounds of the grain boundary strengthening element in the steel sheet during the preheating process to dissolve the grain boundary strengthening element in the steel sheet. The grain boundary strengthening element then diffuses and segregates to the austenite grain boundaries during the subsequent hot stamping process, thereby enabling the realization of a microstructure in which the grain boundary strengthening element is segregated to the prior austenite grain boundaries in the hot-stamped steel finally obtained. However, because carbides and intermetallic compounds of grain boundary strengthening elements are thermally stable, they may not be sufficiently dissolved by heat treatment alone in the preheating step. In such cases, the grain boundary strengthening elements cannot be fully solid-dissolved in the steel sheet. Therefore, in order to facilitate the dissolution operation in the preheating step, it is extremely important to refine the carbides and / or intermetallic compounds of the grain boundary strengthening elements before the preheating step, thereby making them more easily soluble. In this regard, by setting the coiling temperature after finish rolling to 450°C or less, the carbides and / or intermetallic compounds of the grain boundary strengthening elements can be refined in the hot-rolled steel sheet after coiling. For example, in the case of carbides in which the grain boundary strengthening elements are partially solid-dissolved in cementite, the carbides are formed by the grain boundary strengthening elements concentrating in the cementite during coiling. Therefore, by controlling the coiling temperature to a relatively low temperature of 450°C or less, not only can the carbides be refined, but the amount of the grain boundary strengthening elements solid-dissolved in cementite can be reduced, thereby further facilitating the dissolution operation in the subsequent preheating step. The coiling temperature is preferably 420° C. or lower. There is no particular lower limit to the coiling temperature, but the coiling temperature may be, for example, 250° C. or higher or 300° C. or higher.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.

[0065] When a hot-rolled steel sheet is coiled at a relatively low temperature of 450°C or less, preferably 420°C or less, the proportion of hard structures such as bainite and martensite generally increases in the hot-rolled steel sheet, significantly increasing the rolling load on the rolling mill during subsequent cold rolling processes. Furthermore, preheating at temperatures above 1200°C before the hot stamping process, which will be described in detail later, and the resulting effects, i.e., dissolution and solid solution of carbides and / or intermetallic compounds of grain boundary strengthening elements, have not been known until now. Therefore, the technical idea of ​​segregating specific grain boundary strengthening elements to the prior austenite grain boundaries of a hot-stamped steel sheet, thereby improving the hydrogen embrittlement resistance of the hot-stamped steel sheet by combining low-temperature coiling at 450°C or less, preferably 420°C or less, in the hot rolling process with a preheating step at a temperature above 1200°C and a heat treatment in the hot stamping process, is a novel concept that has been discovered for the first time by the present inventors. In particular, it is generally recognized that preheating at high temperatures before the hot stamping process simply results in coarsening of austenite grains, and for this reason, it is believed that preheating at temperatures above 1200°C has not been performed in the prior art. Furthermore, in the present manufacturing method, as described above, a combination of low-temperature coiling at 450°C or less in the hot rolling process, a preheating step at a temperature above 1200°C, and a heat treatment in the hot stamping process segregates specific grain boundary strengthening elements to the prior austenite grain boundaries of the hot stamped body, thereby improving the hydrogen embrittlement resistance of the hot stamped body. However, it goes without saying that other manufacturing conditions can be applied instead of the above combination, as long as they can segregate specific grain boundary strengthening elements to the prior austenite grain boundaries of the hot stamped body, thereby improving the hydrogen embrittlement resistance of the hot stamped body.

[0066] [Pickling process] After the coiling step and before the optional cold rolling step, pickling may be performed to remove oxide scale formed on the surface of the hot-rolled steel sheet. Pickling may be performed under conditions suitable for removing oxide scale, and may be performed once or multiple times to ensure complete removal of oxide scale.

[0067] [Cold rolling process] After the coiling step, cold rolling may be optionally performed. There are no particular limitations on the cold rolling, and it may be performed under any appropriate conditions. For example, the cold rolling reduction may be 30 to 80%. There are no particular limitations on the number of rolling passes or the reduction per pass, and the reduction of the entire cold rolling may be appropriately set so as to be within the above range.

[0068] [Annealing process] For example, after the cold rolling step, annealing may be optionally performed to adjust the microstructure and / or properties. The heating temperature in the annealing step is not particularly limited, but may be, for example, 800°C or less.

[0069] [Coating process] For the purpose of improving corrosion resistance, etc., a coating treatment may be applied to the surface of a hot-rolled steel sheet or a cold-rolled steel sheet. The coating treatment may be a treatment such as hot-dip galvanizing, alloying hot-dip galvanizing, or electroplating. For example, the coating treatment may involve hot-dip galvanizing of the steel sheet, or hot-dip galvanizing may be followed by an alloying treatment. Examples of coatings include coatings mainly made of Fe-Al alloys and coatings mainly made of Fe-Zn alloys. Specific conditions for the coating treatment and alloying treatment are not particularly limited and may be any appropriate conditions known to those skilled in the art.

[0070] [Temper rolling process] For the purpose of correcting the shape of the steel sheet or adjusting the surface roughness, the steel sheet may be subjected to temper rolling after, for example, the annealing step or the plating step.

[0071] [Preheating process] In this method, the obtained hot-rolled or cold-rolled steel sheet is preheated to a temperature above 1200°C before the hot stamping process and then cooled to below 350°C at an average cooling rate of 10°C / s or more. In the hot-stamped steel sheet according to the embodiment of the present invention, it is extremely important that a specific grain boundary strengthening element, more specifically, at least one of Mo, W, Ta, Re, Os, Ir, and Tc, be segregated in a predetermined amount at the prior austenite grain boundaries. However, the hot-stamped steel sheet according to the embodiment of the present invention has a relatively high C content of 0.40% or more. Therefore, in the hot-rolled steel sheet after the hot rolling process or the cold-rolled steel sheet after the optional cold rolling or annealing process, these grain boundary strengthening elements exist as carbides and / or intermetallic compounds. Therefore, even if such a steel sheet is subjected to the hot stamping process without a preheating process and subjected to normal heating and forming operations, these grain boundary strengthening elements cannot be sufficiently segregated at the prior austenite grain boundaries. In this case, the grain boundary strengthening effect of these elements based on the grain boundary segregation cannot be fully exerted. Therefore, in this method, it is extremely important to preheat the steel sheet at a relatively high temperature above 1200°C before the hot stamping process, thereby sufficiently dissolving the carbides and / or intermetallic compounds of the grain boundary strengthening elements and solid-solving the grain boundary strengthening elements in the steel sheet. The upper limit of the heating temperature for preheating is not particularly limited, but the heating temperature may be, for example, 1400°C or lower. After heating, the steel sheet is cooled to less than 350°C at an average cooling rate of 10°C / s or higher. By cooling to less than 350°C at an average cooling rate of 10°C / s or higher, precipitation of the grain boundary strengthening elements solid-solubilized in the steel sheet as compounds can be suppressed. The upper limit of the average cooling rate is not particularly limited, but may be, for example, 3000°C / s or lower, 1500°C / s or lower, or 1200°C / s or lower. The upper limit of the cooling rate is not limited by the characteristics. The cooling method is also not particularly limited, and may be die cooling, water cooling, oil cooling, or gas cooling. In particular, even very high average cooling rates can be achieved relatively easily using mold cooling or water-cooled mold cooling.

[0072] [Hot stamping process] Finally, the steel sheet after the preheating step is hot stamped in a hot stamp forming step to produce a hot stamped body having the desired chemical composition and microstructure. In particular, the grain boundary strengthening elements dissolved in the steel sheet in the previous preheating step diffuse and segregate to the austenite grain boundaries during heating in the hot stamp forming step. Therefore, the desired total segregation amount of the grain boundary strengthening elements can be achieved at the prior austenite grain boundaries after martensitic transformation through the subsequent forming and cooling operations. To achieve this diffusion and segregation of the grain boundary strengthening elements and obtain a high area ratio of hard structure, the steel sheet for hot stamping must be heated to a temperature range of 800°C to 1000°C and held at this temperature range for 60 to 600 seconds. If the heating temperature is less than 800°C, the grain boundary strengthening elements will not diffuse sufficiently into the austenite grain boundaries, and therefore the desired total segregation amount of the grain boundary strengthening elements will not be achieved, which may result in a deterioration of hydrogen embrittlement resistance and / or insufficient austenitization, resulting in a low area ratio of hard structures (at least one of martensite, bainite, and tempered martensite) and a deterioration of tensile strength.On the other hand, if the heating temperature exceeds 1000°C, grain boundary segregation will proceed excessively, and the grain boundary strengthening elements that have segregated at the grain boundaries will precipitate as carbides or intermetallic compounds, reducing the amount of grain boundary segregation.As a result, the desired total segregation amount of the grain boundary strengthening elements will not be achieved, and hydrogen embrittlement resistance may be deteriorated. If the holding time is less than 60 seconds, as in the case where the heating temperature is less than 800°C, the grain boundary strengthening elements do not diffuse sufficiently into the austenite grain boundaries, and therefore the desired total segregation amount of the grain boundary strengthening elements cannot be achieved, which may result in a deterioration of hydrogen embrittlement resistance and / or insufficient austenitization, resulting in a low area ratio of hard structures (at least one of martensite, bainite, and tempered martensite) and a deterioration of tensile strength.If the holding time exceeds 600 seconds, long-term heating may cause excessive grain boundary segregation, resulting in the precipitation of grain boundary strengthening elements, and these precipitates may become the starting points of fracture, resulting in a deterioration of hydrogen embrittlement resistance.

[0073] The heating atmosphere is not particularly limited and may be under ordinary conditions, such as air, a gas combustion atmosphere with a controlled air-to-fuel ratio, or a nitrogen atmosphere, and the dew point of these gases may be controlled. The material is held in a temperature range of 800°C to 1000°C, and then hot stamped. After hot stamping, the material may be cooled to a temperature range of 250°C or lower at an average cooling rate of 20°C / second or higher.

[0074] Examples of the heating method before hot stamping include furnace heating using an electric furnace or gas furnace, flame heating, electrical heating, high frequency heating, and induction heating.

[0075] The hot-stamped product according to this embodiment can be obtained by the above method. After hot stamping, tempering treatment at 130 to 600°C or bake hardening treatment (BH treatment) after painting can be performed. Furthermore, a portion of the hot-stamped product can be tempered by laser irradiation or the like to provide a partially softened region.

[0076] 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. [Example]

[0077] In the following examples, hot stamped products according to the embodiments of the present invention were produced under various conditions, and the tensile strength and hydrogen embrittlement resistance of the obtained hot stamped products were examined.

[0078] First, molten steel having the chemical composition shown in Table 1 was cast by continuous casting to produce slabs. The balance other than the components shown in Table 1 consisted of Fe and impurities. These slabs were heated to a temperature of 1100°C or higher and rough-rolled under specified conditions. Then, finish-rolling was performed under conditions such that the temperature at the end of finish-rolling was 650°C or higher, and the slabs were coiled at the coiling temperature shown in Table 2. After coiling, some of the hot-rolled steel sheets were subjected to a specified softening heat treatment. The obtained hot-rolled steel sheets were then cold-rolled at a specified reduction of 30 to 80%. Some of the steel sheets were then annealed, coated, or temper-rolled under specified conditions. The obtained steel sheets were then hot-stamped under the conditions shown in Table 2. The heating atmosphere and heating method in the hot-stamping process were a gas combustion atmosphere (air-fuel ratio 0.85) and furnace heating, unless otherwise specified. After hot-stamping, some of the hot-stamped steel sheets were tempered or partially softened.

[0079] [Table 1-1]

[0080] [Table 1-2]

[0081] [Table 1-3]

[0082] [Table 1-4]

[0083] [Table 1-5]

[0084] [Table 1-6]

[0085]

Table 1-7

[0086]

Table 1-8

[0087]

Table 1-9

[0088]

Table 1-10

[0089]

Table 2-1

[0090]

Table 2-2

[0091]

Table 2-3

[0092]

Table 2-4

[0093]

Table 2-5

[0094]

Table 2-6

[0095] [Table 2-7]

[0096] [Table 2-8]

[0097] The properties of the hot stamped steel sheets thus obtained were measured and evaluated by the following methods.

[0098] [Tensile strength] The tensile strength 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.

[0099] [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 measuring 1.0 mm thick x 9.0 mm wide x 120 mm long (mm) was prepared. The parallel portion of the test piece had a length of 20 mm and a diameter of 2.0 mm. U-notches with a notch depth of 0.35 mm and a notch bottom radius of 0.1 mm were provided on both sides of the center of the parallel portion. This test piece was immersed in a 3% NaCl solution, and a galvanostat was used as the power source to apply a current density of 0.1 mA / cm2 to the immersed portion of the test piece surface. 2 The hydrogen charging was controlled so that the strain rate was 0.0060 mm / min. Next, the hydrogen-charged test specimens were subjected to a slow strain rate tensile test at a tensile speed of 0.0060 mm / min, and the load at break was investigated. The same test was performed three times for a sample with the same test number, and a sample with an average breaking load of 500 MPa or more in such a hydrogen environment was evaluated as passing, and a sample with a breaking load of less than 500 MPa was evaluated as failing.

[0100] Hot-stamped steel sheets with a tensile strength of 2200 MPa or more and passing the evaluation of hydrogen embrittlement resistance were evaluated as having high strength and capable of suppressing hydrogen embrittlement. The area ratio of the hard structure in Table 3 refers to the total area ratio of martensite, bainite, and tempered martensite. The remaining structure other than the hard structure was ferrite, retained austenite, and / or pearlite. Although not shown in Table 3, the average grain size of the prior austenite grains in the hot-stamped steel sheets in the invention examples in Table 3 was measured, and all of them had an average grain size of 8 μm or less.

[0101] [Table 3-1]

[0102] [Table 3-2]

[0103] [Table 3-3]

[0104] [Table 3-4]

[0105] [Table 3-5]

[0106] [Table 3-6]

[0107] [Table 3-7]

[0108] [Table 3-8]

[0109] Referring to Tables 1 to 3, in Comparative Example 1, the tensile strength was reduced due to the low C content. In Comparative Example 14, the strength was too high due to the high C content, resulting in reduced hydrogen embrittlement resistance. In Comparative Example 29, the ferrite amount was increased due to the high Si content, resulting in reduced tensile strength. In Comparative Example 43, the prior austenite grain boundaries were presumably embrittled due to the high Mn content. As a result, the hydrogen embrittlement resistance was reduced. In Comparative Examples 52, 61, 70, 78, 79, and 92, the P, S, N, O, or Al contents were inappropriate, resulting in reduced hydrogen embrittlement resistance. In Comparative Examples 93, 107, and 146, the Nb, Ti, and B contents were low, respectively, so the strength could not be sufficiently improved, resulting in reduced tensile strength. In Comparative Examples 106, 118, 132, 145, and 156, the high contents of Nb, Ti, Cr, Mo, and B are thought to have caused the formation of coarse carbonitrides, coarse intermetallic compounds, or coarse borides in the steel, resulting in a deterioration in hydrogen embrittlement resistance.In Comparative Example 133, the low Mo content reduced the total segregation amount of grain boundary strengthening elements at the prior austenite grain boundaries, resulting in a deterioration in hydrogen embrittlement resistance.

[0110] In Comparative Example 336, the high coiling temperature presumably prevented the carbides and / or intermetallic compounds of the grain boundary strengthening elements from being sufficiently refined, and the grain boundary strengthening elements were unable to be sufficiently solid-dissolved in the steel sheet during the subsequent preheating process. As a result, the total segregation amount of the grain boundary strengthening elements at the prior austenite grain boundaries was reduced, resulting in poor hydrogen embrittlement resistance. In Comparative Example 351, the low heating temperature in the preheating process presumably prevented the grain boundary strengthening elements from being sufficiently solid-dissolved in the steel sheet. As a result, the total segregation amount of the grain boundary strengthening elements at the prior austenite grain boundaries was reduced, resulting in poor hydrogen embrittlement resistance. In Comparative Example 357, the slow average cooling rate in the preheating process presumably caused the grain boundary strengthening elements solid-dissolved in the steel sheet during preheating to precipitate as compounds. As a result, the total segregation amount of the grain boundary strengthening elements at the prior austenite grain boundaries was reduced, resulting in poor hydrogen embrittlement resistance. In Comparative Example 364, the low heating temperature in the hot stamping process presumably prevented the grain boundary strengthening elements from being sufficiently diffused into the austenite grain boundaries. As a result, the total segregation amount of grain boundary strengthening elements at the prior austenite grain boundaries decreased, resulting in a deterioration in hydrogen embrittlement resistance. In Comparative Example 378, the high heating temperature in the hot stamping process caused excessive grain boundary segregation, and the grain boundary segregated grain boundary strengthening elements precipitated as carbides and intermetallic compounds, reducing the amount of grain boundary segregation. As a result, the desired total segregation amount of the grain boundary strengthening elements could not be achieved, resulting in a deterioration in hydrogen embrittlement resistance. In Comparative Example 379, it is believed that the short holding time in the hot stamping process prevented the grain boundary strengthening elements from sufficiently diffusing to the austenite grain boundaries. As a result, the total segregation amount of grain boundary strengthening elements at the prior austenite grain boundaries decreased, resulting in a deterioration in hydrogen embrittlement resistance. In Comparative Example 395, the long holding time in the hot stamping process caused excessive grain boundary segregation, and the grain boundary segregated grain boundary strengthening elements precipitated as carbides and intermetallic compounds, reducing the amount of grain boundary segregation. As a result, the desired total segregation amount of the grain boundary strengthening elements could not be achieved, and the hydrogen embrittlement resistance was reduced.

[0111] In contrast, all of the hot stamped steel sheets according to the examples of the present invention had a predetermined chemical composition, and by controlling the total segregation amount of at least one of the grain boundary strengthening elements Mo, W, Ta, Re, Os, Ir, and Tc at the prior austenite grain boundaries to be 0.10 atomic % or more, hydrogen embrittlement could be reliably suppressed despite having a high tensile strength of 2200 MPa or more.

Claims

1. In mass%, C: 0.40-0.70%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0200% or less, Al: 0.0010-0.500%, Nb: 0.0010-0.100%, Ti: 0.010-0.200%, Mo: 0.010-2.000%, B: 0.0005-0.0200%, Si: 0-3.00%, Mn: 0 to less than 0.50% Cr: 0-1.00%, Co: 0-4.00%, Ni: 0-3.00%, Cu: 0 to 3.00%, V: 0-3.00%, Ca: 0-1.000%, Mg: 0-1.000%, REM: 0-1.000%, Sb: 0 to 1.00%, Zr: 0 to 1.00%, Sn: 0 to 1.00%, As: 0 to 0.100%, W: 0-3.000%, at least one of Ta, Re, Os, Ir, and Tc: 0 to 1.00% in total; Se: 0-1.00%, Bi: 0 to 1.00%, and The balance has a chemical composition consisting of Fe and impurities, A hot stamped body having a microstructure in which the total segregation amount of at least one of Mo, W, Ta, Re, Os, Ir, and Tc at prior austenite grain boundaries is 0.10 atomic % or more.

2. The hot-stamped steel according to claim 1, comprising, by area ratio, at least one of martensite, bainite, and tempered martensite in a total amount of 70% or more.

3. The hot-stamped steel according to claim 1 or 2, wherein an amount of Mo segregated at the prior austenite grain boundaries is 0.10 atomic % or more.

4. The hot-stamped steel according to claim 1 or 2, wherein an amount of W segregated at the prior austenite grain boundaries is 0.10 atomic % or more.

5. The hot-stamped steel according to claim 1 or 2, wherein the total segregation amount is 0.15 atomic % or more.

6. The hot-stamped product according to claim 1 or 2, which has a coating on its surface.

7. The hot-stamped product according to claim 6, wherein the coating is mainly composed of an Fe—Al alloy.

8. The hot-stamped product according to claim 6, wherein the coating is mainly composed of an Fe-Zn alloy.

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