Hot stamping steel sheets and hot stamped products

Abstract

The present invention provides a steel sheet for hot stamping, the steel sheet having a specific chemical composition, while having a metal structure that contains, in area ratios, 10% or more of ferrite and 10% or more of pearlite, with the sum of ferrite and pearlite being 80% or more, and has a dispersion index of pearlite of 0.50 or more. The present invention also provides a hot stamp molded body which has a specific chemical composition, while having a metal structure that contains, in an area ratio, a total of 90% or more of at least one of martensite, bainite and tempered martensite, wherein the standard deviation of the hardness distribution of prior austenite grains at a position corresponding to 1 / 4 the sheet thickness is 150 Hv or less.

Description

[Technical Field]

[0001] The present invention relates to a steel sheet for hot stamping and a hot-stamped product produced using the same. [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 weight reduction and crashworthiness, and this has led to the development of high-strength steel sheets. However, increasing the strength of steel sheets reduces their formability, making it generally difficult to achieve both strength and formability in steel sheets.

[0003] In this regard, Patent Document 1 describes a cold-rolled steel sheet having a predetermined chemical composition, and a metal structure containing, by area ratio, 40.0% or more and less than 60.0% polygonal ferrite, 30.0% or more bainitic ferrite, 10.0% or more and 25.0% or less retained austenite, and 15.0% or less martensite, wherein 80.0% or more of the retained austenite has an aspect ratio of 2.0 or less, a long axis length of 1.0 μm or less and a short axis length of 1.0 μm or less, 80.0% or more of the bainitic ferrite has an aspect ratio of 1.7 or less and a mean crystal orientation misorientation value of 0.5° or more and less than 3.0° in regions surrounded by grain boundaries with a crystal orientation misorientation of 15° or more, and wherein the connectivity D value between the martensite, the bainitic ferrite, and the retained austenite is 0.70 or less. Furthermore, Patent Document 1 describes that the above configuration makes it possible to provide a high-strength cold-rolled steel sheet that is suitable for use as a structural member of an automobile or the like, has a tensile strength of 980 MPa or more, a 0.2% yield strength of 600 MPa or more, and is excellent in punching fatigue properties, elongation, and hole expandability. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2016 / 136810 Summary of the Invention [Problem to be solved by the invention]

[0005] 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 steel is soft and has good formability at the time of forming because the material is heated before being formed. Therefore, even high-strength steel can be formed with high precision into complex shapes. In addition, since the steel is quenched at the same time as forming using a press die, the steel after forming is known to have sufficient strength.

[0006] Hot-stamped steel sheets with such high strength can 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. On the other hand, the automotive industry and other industries are also demanding further weight reduction of steel materials. To achieve such weight reduction, steel materials must be strengthened 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 higher levels than conventional steel materials.

[0007] Therefore, an object of the present invention is to provide a hot-stamped steel sheet having a novel structure, which has high strength and is capable of suppressing hydrogen embrittlement, and a steel sheet for hot stamping for producing such a hot-stamped steel sheet. [Means for solving the problem]

[0008]

[0006] In order to achieve the above object, the inventors conducted studies focusing on the metallographic structures of a steel sheet before hot stamping and a hot-stamped steel sheet after hot stamping. As a result, the inventors found that by uniformly dispersing pearlite, which serves as the origin of austenite grains, during heating for hot stamping in a steel sheet before hot stamping, the prior austenite grains in the finally obtained hot-stamped steel sheet are homogenized, thereby reducing the variation in hardness of the prior austenite grains in the metallographic structure of the hot-stamped steel sheet. Furthermore, the inventors found that reducing the variation in hardness of the prior austenite grains in the metallographic structure of the hot-stamped steel sheet can suppress a local increase in hardness, thereby significantly improving hydrogen embrittlement resistance despite 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%, Si: 0.010 to 1.300%, Mn: 0.60-3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0200% or less, Al: 0.0010~0.5000%, Nb: 0.0010~0.100%, Ti: 0.010 to 0.200%, B: 0.0005~0.0200%, Cr: 0.010~0.80%, Mo: 0.0010 to 1.000%, Co: 0-2.00%, Ni: 0-3.00% Cu: 0-1.00% V: 0~1.00%, W: 0 to 1.000%, Ca: 0 to 0.010% Mg: 0 to 1.000%, REM: 0 to 1.000%, Sb: 0 to 1.000%, Zr: 0 to 1.000%, Sn: 0 to 1.000%, As: 0 to 0.100%, and The balance has a chemical composition consisting of Fe and impurities, In terms of area ratio, Ferrite: 10% or more, and Perlite: 10% or more The total of ferrite and pearlite is 80% or more, A steel sheet for hot stamping, having a metal structure in which the dispersion index of pearlite is 0.50 or more. (2) The chemical composition is in mass%: Co: 0.001 to 2.00%, Ni: 0.001 to 3.00%, Cu: 0.001 to 1.00%, V: 0.001 to 1.00%, W: 0.001 to 1.000%, Ca: 0.0001 to 0.010%, Mg: 0.0001 to 1.000%, REM: 0.0001 to 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 steel sheet for hot stamping according to (1) above, comprising one or more selected from the group consisting of: (3) In mass%, C: 0.40~0.70%, Si: 0.010 to 1.300%, Mn: 0.60-3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0200% or less, Al: 0.0010~0.5000%, Nb: 0.0010~0.100%, Ti: 0.010 to 0.200%, B: 0.0005~0.0200%, Cr: 0.010~0.80%, Mo: 0.0010 to 1.000%, Co: 0-2.00%, Ni: 0-3.00% Cu: 0-1.00% V: 0~1.00%, W: 0 to 1.000%, Ca: 0 to 0.010% Mg: 0 to 1.000%, REM: 0 to 1.000%, Sb: 0 to 1.000%, Zr: 0 to 1.000%, Sn: 0 to 1.000%, As: 0 to 0.100%, and The balance has a chemical composition consisting of Fe and impurities, At least one of martensite, bainite and tempered martensite in terms of area ratio: 90% or more in total; A hot stamped steel having a metal structure in which the standard deviation of the hardness distribution of prior austenite grains at the 1 / 4 position of the plate thickness is 150 Hv or less. (4) The chemical composition is in mass%: Co: 0.001 to 2.00%, Ni: 0.001 to 3.00%, Cu: 0.001 to 1.00%, V: 0.001 to 1.00%, W: 0.001 to 1.000%, Ca: 0.0001 to 0.010%, Mg: 0.0001 to 1.000%, REM: 0.0001 to 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 (3) above, comprising one or more selected from the group consisting of: [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, and a steel sheet for hot stamping for producing such a hot-stamped steel sheet. DETAILED DESCRIPTION OF THE INVENTION

[0011] <Steel sheets for hot stamping> The steel sheet for hot stamping according to the embodiment of the present invention comprises, in mass%, C: 0.40~0.70%, Si: 0.010 to 1.300%, Mn: 0.60-3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0200% or less, Al: 0.0010~0.5000%, Nb: 0.0010~0.100%, Ti: 0.010 to 0.200%, B: 0.0005~0.0200%, Cr: 0.010~0.80%, Mo: 0.0010 to 1.000%, Co: 0-2.00%, Ni: 0-3.00% Cu: 0-1.00% V: 0~1.00%, W: 0 to 1.000%, Ca: 0 to 0.010% Mg: 0 to 1.000%, REM: 0 to 1.000%, Sb: 0 to 1.000%, Zr: 0 to 1.000%, Sn: 0 to 1.000%, As: 0 to 0.100%, and The balance has a chemical composition consisting of Fe and impurities, In terms of area ratio, Ferrite: 10% or more, and Perlite: 10% or more The total of ferrite and pearlite is 80% or more, It is characterized by having a metal structure in which the dispersion index of pearlite is 0.50 or more.

[0012] As mentioned above, it is known that hydrogen embrittlement cracking is more likely to occur as the strength of a steel material increases. Therefore, the present inventors conducted studies focusing on the metallographic structures of a steel sheet before hot stamping and a hot-stamped steel sheet after hot stamping, with the aim of reducing or suppressing regions that can serve as initiation sites for hydrogen embrittlement cracking in such high-strength steel materials. More specifically, the present inventors first found that when the prior austenite grain size in the metallographic structure of a hot-stamped steel sheet varies greatly, the hardness increases in regions with smaller prior austenite grain sizes, and such localized high-hardness regions can serve as initiation sites for hydrogen embrittlement cracking. In response to this, the present inventors found that reducing the variation in the prior austenite grain size, thereby reducing the variation in hardness among the prior austenite grains, and more specifically, controlling the standard deviation of the hardness distribution of the prior austenite grains to 150 Hv or less, can reliably suppress such localized increases in hardness.

[0013] While not intending to be bound by any particular theory, it is believed that the onset temperature of martensitic transformation during hot stamping varies depending on the grain size of the austenite grains. More specifically, it is believed that austenite grains with larger grain sizes have a higher onset temperature of martensitic transformation than austenite grains with smaller grain sizes, resulting in lower hardness. Austenite grains with smaller grain sizes undergo martensitic transformation at a lower temperature than larger grains, resulting in increased hardness. Therefore, in order to suppress or reduce such localized increases in hardness, it is important to reduce the variation in the austenite grain size before martensitic transformation. In other words, it is believed that reducing the variation in the austenite grain size before martensitic transformation can reduce the variation in the prior austenite grain size after martensitic transformation, which in turn can reduce the variation in the hardness of the prior austenite grains in the metallographic structure of a hot stamped body. For these reasons, it is believed that by reducing the variation in hardness of prior austenite grains by controlling the standard deviation of the hardness distribution of prior austenite grains in the metallographic structure of a hot-stamped body to 150 Hv or less, it is possible to significantly suppress local increases in hardness due to differences in the timing of martensitic transformation. Since the presence of locally high hardness regions is thought to be highly likely to cause hydrogen embrittlement cracking, particularly at the interfaces between prior austenite grains where there is a difference in hardness, reducing the variation in hardness of prior austenite grains in the metallographic structure of a hot-stamped body is extremely effective in improving hydrogen embrittlement resistance.

[0014] In this regard, as will be explained in detail later regarding the method for producing a steel sheet for hot stamping, the present inventors have focused on the metallographic structure of a steel sheet before hot stamping, for example, a hot-rolled steel sheet, and have found that by uniformly dispersing pearlite in the metallographic structure, it is possible to reduce the variation in the prior austenite grain size in the final metallographic structure of a hot-stamped steel sheet, and in this regard, it is possible to control the standard deviation of the hardness distribution of the prior austenite grains to 150 Hv or less. As the strength of steel materials increases, relatively large amounts of Mn are sometimes added to improve the hardenability of the steel materials. However, research by the present inventors has found that with such a high Mn content (e.g., 0.60 mass% or more), pearlite is relatively easily formed, and therefore it is very difficult to uniformly disperse the pearlite formed in large amounts in the metallographic structure of a hot-rolled steel sheet compared to when the Mn content is low, and as a result, the variation in the prior austenite grain size in the metallographic structure after hot stamping becomes greater. However, the present inventors have found that, in response to such problems, applying a relatively high reduction in the final stage of finish rolling and then appropriately controlling the subsequent cooling can enable uniform dispersion of pearlite in the metal structure of the hot-rolled steel sheet, and as a result, can reduce the variation in the prior austenite grain size in the final metal structure of the hot-stamped steel, thereby significantly reducing the variation in the hardness distribution of the prior austenite grains. More specifically, the inventors have found that it is important to control the dispersion index of pearlite, which is obtained by dividing the number of A / B boundaries (boundaries between ferrite and pearlite phases) in an electron microscope image of the metallographic structure of a steel sheet for hot stamping, which is mainly composed of ferrite and pearlite, by the sum of the number of A / A boundaries (boundaries between ferrite and ferrite phases), the number of B / B boundaries (boundaries between pearlite and pearlite phases), and the number of A / B boundaries (boundaries between ferrite and pearlite phases), to 0.5 or more, and that by controlling the dispersion index of pearlite to 0.5 or more, it is possible to achieve a standard deviation in the hardness distribution of prior austenite grains of 150 Hv or less in the final metallographic structure of a hot-stamped body due to the uniform dispersion of pearlite.

[0015] The present inventors have now discovered for the first time that by hot stamping a steel sheet for hot stamping in which pearlite is uniformly dispersed within a predetermined range, it is possible to control the variation in hardness distribution of prior austenite grains within a predetermined range in the metallographic structure of the resulting hot-stamped steel. In addition, with the hot-stamped steel according to the embodiment of the present invention, controlling the variation in hardness distribution of prior austenite grains within a predetermined range significantly suppresses local increases in hardness, thereby enabling significantly improved hydrogen embrittlement resistance despite having a high tensile strength, for example, a high tensile strength of 2200 MPa or more.

[0016] Hereinafter, a steel sheet for hot stamping according to an embodiment of the present invention will be described in more detail. In the following description, "%", which is the unit of content of each element, means "mass%" unless otherwise specified. Furthermore, 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.

[0017] [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, 0.43% or more, 0.44% or more, 0.45% or more, or 0.46% 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.

[0018] [Si: 0.010~1.300%] Silicon is an element that improves the strength of a hot-stamped steel sheet through solid solution strengthening. If the Si content is less than 0.010%, the desired strength cannot be obtained. Therefore, the Si content is set to 0.010% or more. The Si content is preferably 0.050% or more, 0.100% or more, 0.200% or more, more than 0.250%, 0.255% or more, 0.260% or more, 0.270% or more, 0.280% or more, 0.300% or more, or 0.400% or more. On the other hand, if the Si content exceeds 1.300%, the amount of ferrite increases in the steel sheet for hot stamping, and the desired metal structure may not be obtained. Therefore, the Si content is set to 1.300% or less. The Si content is preferably 1.200% or less, 1.000% or less, 0.800% or less, 0.600% or less, or 0.500% or less.

[0019] [Mn: 0.60~3.00%] Mn is an element that promotes the transformation from austenite to pearlite in a hot-rolled steel sheet during the production of a hot-stamped steel sheet, and contributes to controlling the pearlite dispersion index in the steel sheet for hot stamping and the hardness distribution of prior austenite grains in the hot-stamped steel sheet. To achieve the pearlite dispersion index and the standard deviation of the hardness distribution of prior austenite grains within desired ranges, the Mn content is set to 0.60% or more. The Mn content is preferably more than 0.60%, 0.70% or more, 0.80% or more, 1.00% or more, or 1.30% or more. On the other hand, if the Mn content exceeds 3.00%, the transformation from austenite to pearlite in the hot-rolled steel sheet is excessively promoted, and the dispersion index of pearlite and the standard deviation of the hardness distribution of prior austenite grains cannot be within the desired range. Therefore, the Mn content is set to 3.00% or less. Preferably, the Mn content is 2.90% or less, 2.70% or less, 2.50% or less, 2.30% or less, or 2.00% or less.

[0020] [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.

[0021] [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.

[0022] [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.

[0023] [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.

[0024] [Al: 0.0010~0.5000%] Al is an element that has the effect of deoxidizing molten steel and improving the soundness of the steel. If the Al content is less than 0.0010%, deoxidation is insufficient, and coarse oxides are generated, deteriorating hydrogen embrittlement resistance. Therefore, the Al content is set to 0.0010% or more. The Al content is preferably 0.0030% or more, 0.0050% or more, 0.0100% or more, or 0.0300% or more. On the other hand, if the Al content exceeds 0.5000%, coarse oxides are formed in the steel, which reduces the hydrogen embrittlement resistance of the hot-stamped steel. Therefore, the Al content is set to 0.5000% or less. The Al content is preferably 0.4000% or less, 0.3000% or less, 0.2000% or less, 0.1500% or less, or 0.1000% or less.

[0025] [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. If the Nb content is less than 0.0010%, the desired strength cannot be obtained. Therefore, the Nb content is set to 0.0010% or more. The Nb content is preferably 0.005% or more, 0.009% or more, or 0.015% or more. On the other hand, if the Nb content exceeds 0.100%, a large amount of carbonitrides is formed in the steel, 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.

[0026] [Ti: 0.010~0.200%] Ti is an element that forms carbonitrides in steel and improves the strength of the hot-stamped body through precipitation strengthening. If the Ti content is less than 0.010%, the desired strength cannot be obtained. Therefore, the Ti content is set to 0.010% or more. The Ti content is preferably 0.015% or more, 0.020% or more, or 0.025% or more. On the other hand, if the Ti content exceeds 0.200%, a large amount of carbonitrides is formed in the steel, 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, 0.080% or less, 0.060% or less, or 0.050% or less.

[0027] [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 intermetallic compounds 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, 0.0080% or less, 0.0060% or less, or 0.0040% or less.

[0028] [Cr: 0.010~0.80%] Cr is an element that dissolves in prior austenite grains during heating before hot stamping, thereby increasing the strength of the hot stamped steel. If the Cr content is less than 0.010%, the desired strength cannot be obtained. Therefore, the Cr content is set to 0.010% 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 0.80%, 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 0.80% or less. The Cr content is preferably 0.70% or less, 0.60% or less, 0.50% or less, or 0.40% or less.

[0029] [Mo: 0.0010~1.000%] Mo is an element that improves the hardenability of steel. If the Mo content is less than 0.0010%, the desired strength cannot be obtained. Therefore, the Mo content is set to 0.0010% or more. The Mo content is preferably 0.005% or more, 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, if the Mo content exceeds 1.000%, coarse intermetallic compounds are formed in the hot-stamped steel, and the hydrogen embrittlement resistance of the hot-stamped steel deteriorates. Therefore, the Mo content is set to 1.000% or less. The Mo content is preferably 0.800% or less, 0.600% or less, 0.500% or less, or 0.300% or less.

[0030] The basic chemical composition of the steel sheet for hot stamping according to the embodiment of the present invention is as described above. Furthermore, the steel sheet for hot stamping may contain at least one of the following optional elements, as needed, in place of a portion of the remaining Fe. For example, the steel sheet for hot stamping may contain at least one element selected from the group consisting of Co: 0-2.00%, Ni: 0-3.00%, Cu: 0-1.00%, V: 0-1.00%, and W: 0-1.000%. The steel sheet for hot stamping may also contain at least one element selected from the group consisting of Ca: 0-0.010%, Mg: 0-1.000%, and REM: 0-1.000%. The steel sheet for hot stamping may also 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 steel sheet for hot stamping may also contain 0 to 0.100% of As. These optional elements will be described in detail below.

[0031] [Co: 0-2.00%] 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 Co is added in a large amount, the Co content is preferably 2.00% or less. The Co content may be 1.80% or less, 1.50% or less, 1.00% or less, 0.80% or less, or 0.60% or less.

[0032] [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. On the other hand, since the above effects are saturated even when Ni is contained in a large amount, the Ni content is preferably 3.00% or less. The Ni content may be less than 3.00%, 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.

[0033] [Cu: 0-1.00%] 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 1.00% or less. The Cu content may be 0.80% or less, 0.60% or less, 0.50% or less, or 0.30% or less.

[0034] [V:0~1.00%] 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 V is contained in a large amount, the V content is preferably 1.00% or less, and may be 0.80% or less, 0.60% or less, 0.50% or less, or 0.30% or less.

[0035] [W:0~1.000%] W is an element that improves the hardenability of steel. The W content may be 0.001% or more, but to ensure this effect, the W content is preferably 0.005% or more or 0.010% or more. On the other hand, since the above effects are saturated even when W is added in a large amount, the W content is preferably 1.000% or less. The W content may be 0.800% or less, 0.600% or less, 0.500% or less, or 0.300% or less.

[0036] [Ca: 0~0.010%] 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 contained in a large amount, the Ca content is preferably 0.010% or less. The Ca content may be 0.008% or less, 0.006% or less, 0.004% or less, 0.003% or less, or 0.002% or less.

[0037] [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.

[0038] [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.

[0039] [Sb: 0-1.000%] Sb is an element that suppresses the formation of oxides. To reliably obtain this effect, the Sb content is preferably 0.001% or more. On the other hand, since the above effects are saturated even when Sb is added in a large amount, 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, or 0.100% or less.

[0040] [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 effects are saturated even if Zr is added in a large amount, 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, or 0.100% or less.

[0041] [Sn: 0~1.000%] Sn is an element that suppresses the formation of oxides. To reliably obtain this effect, the Sn content is preferably 0.001% 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 1.000% or less. The Sn content may be 0.800% or less, 0.500% or less, 0.200% or less, or 0.100% or less.

[0042] [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. 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.

[0043] In the steel sheet for hot stamping 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 steel sheet for hot stamping is industrially manufactured.

[0044] The chemical composition of the above-mentioned steel sheet for hot stamping may be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Note that 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 a plated layer is provided on the surface of a steel sheet for hot stamping, the plated layer may be removed by mechanical grinding before analyzing the chemical composition.

[0045] [Ferrite: 10% or more, Pearlite: 10% or more, Total of Ferrite and Pearlite: 80% or more] The metal structure of a steel sheet for hot stamping according to an embodiment of the present invention contains, in area percentage, 10% or more of ferrite and 10% or more of pearlite, with the total of ferrite and pearlite being 80% or more. Pearlite serves as the origin of austenite grains during heating in hot stamping, so it is necessary for the area percentage to be 10% or more in the metal structure. Furthermore, in this embodiment, the inclusion of pearlite in combination with ferrite enables uniform dispersion of pearlite. The area percentages of ferrite and pearlite may each be any value of 10% or more, as long as their total is 80% or more. For example, they may each independently be 20% or more, 30% or more, 40%, 50% or more, or 60% or more. While there are no particular upper limits, the area percentages of ferrite and pearlite may each independently be 85% or less, 80% or less, or 70% or less. The total area percentage of ferrite and pearlite may be 85% or more, 90% or more, or 95% or more. The upper limit is not particularly limited, but the sum of the area ratios of ferrite and pearlite may be 100%, for example, 99% or less or 98% or less. The remaining structure is not particularly limited, but may consist of at least one of bainite, martensite, retained austenite, and carbide. The carbide may be, for example, Fe carbide, and may be generated in small amounts at the interface between ferrite phases. The area ratio of the remaining structure is 20% or less, for example, 17% or less, 15% or less, 12% or less, 10% or less, 8% or less, 5% or less, or 3% or less.

[0046] [Pearlite dispersion index: 0.50 or more] In the metallographic structure of the steel sheet for hot stamping according to an embodiment of the present invention, pearlite must be uniformly dispersed. In this embodiment, such uniform dispersion of pearlite is achieved by controlling the pearlite dispersion index to 0.50 or more. The pearlite dispersion index is obtained by dividing the number of A / B boundaries (boundaries between ferrite and pearlite phases) in an electron microscope image of the metallographic structure of the steel sheet for hot stamping, which is mainly composed of ferrite and pearlite, by the sum of the number of A / A boundaries (boundaries between ferrite and pearlite phases), the number of B / B boundaries (boundaries between pearlite and pearlite phases), and the number of A / B boundaries (boundaries between ferrite and pearlite phases). A high pearlite dispersion index means that the proportion of A / B boundaries is high in the metallographic structure mainly composed of ferrite and pearlite, i.e., the number of ferrite and pearlite boundaries is large. Therefore, by controlling the pearlite dispersion index to a high value, it is possible to reduce the number of connected pearlite particles and achieve more uniform pearlite dispersion. According to this embodiment, by controlling the pearlite dispersion index to 0.50 or more, the uniform dispersion of pearlite allows the standard deviation of the hardness distribution of prior austenite grains to be 150 Hv or less in the final metallographic structure of the hot-stamped steel sheet. As a result, the variation in hardness of prior austenite grains in the metallographic structure of the hot-stamped steel sheet can be reduced, and localized increases in hardness can be significantly suppressed. This allows for significantly improved hydrogen embrittlement resistance despite high tensile strength, for example, 2200 MPa or more. The higher the pearlite dispersion index, the more preferable it is. For example, it may be 0.52 or more, 0.55 or more, 0.58 or more, 0.60 or more, 0.62 or more, or 0.65 or more. While the upper limit of the pearlite dispersion index is not particularly limited, it may be, for example, 0.80 or less, 0.75 or less, or 0.70 or less.

[0047] The pearlite dispersion index is determined as follows. First, an electron channeling contrast image is obtained using a scanning electron microscope in a cross section perpendicular to the surface, centered at the 1 / 4 thickness position, covering a range of 35 μm in the direction perpendicular to the thickness direction and 10 μm in the thickness direction. This measurement can be performed using an EBSD analyzer consisting of a thermal field emission scanning electron microscope and an EBSD detector. For example, an EBSD analyzer consisting of a JEOL JSM-7001F and a TSL DVC5 detector can be used. Next, ten lines perpendicular to the thickness direction are drawn at 1 μm intervals on the obtained electron channeling contrast image. Next, the phase boundaries intersecting these lines are classified into A / A boundaries (ferrite-ferrite boundaries), B / B boundaries (pearlite-pearlite boundaries), and A / B boundaries (ferrite-pearlite boundaries), and the intersection points of each boundary are calculated. Next, the percentage of A / B boundaries in that field of view is obtained by dividing the number of A / B boundary intersections by the total number of intersections, i.e., the sum of the number of A / A boundary intersections, the number of B / B boundary intersections, and the number of A / B boundary intersections. The same procedure is repeated for five fields of view on the same sample, and the average value of the percentage of A / B boundaries in the five fields of view is determined as the pearlite dispersion index.

[0048] Plate Thickness The steel sheet for hot stamping according to the embodiment of the present invention is not particularly limited, and may have a thickness of, for example, 0.1 to 4.0 mm. The thickness may be 0.2 mm or more, 0.4 mm or more, 0.6 mm or more, 0.8 mm or more, or 1.0 mm or more. Similarly, the thickness may be 3.6 mm or less, 3.2 mm or less, 2.8 mm or less, 2.4 mm or less, or 2.0 mm or less. When the steel sheet for hot stamping is a hot-rolled steel sheet, the thickness may be, for example, 1.0 to 4.0 mm. On the other hand, when the steel sheet for hot stamping is a cold-rolled steel sheet, the thickness may be, for example, 0.1 to 2.0 mm.

[0049] [Plating] The steel sheet for hot stamping according to the embodiment of the present invention may have a plating layer on its surface. By having a plating layer on its surface, corrosion resistance can be improved after hot stamping. Examples of the plating layer include an aluminum plating layer, an aluminum-zinc plating layer, an aluminum-silicon plating layer, a hot-dip galvanized layer, an electrogalvanized layer, a galvannealed hot-dip galvanized layer, a zinc-nickel plating layer, and an aluminum-magnesium-zinc based plating layer.

[0050] <Hot stamped compact> In addition to the above-mentioned steel sheet for hot stamping, the present invention further provides a hot-stamped steel produced using the steel sheet for hot stamping. Hereinafter, the hot-stamped steel according to the embodiment of the present invention will be described in more detail. The hot-stamped steel comprises, in mass %, C: 0.40~0.70%, Si: 0.010 to 1.300%, Mn: 0.60-3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0200% or less, Al: 0.0010~0.5000%, Nb: 0.0010~0.100%, Ti: 0.010 to 0.200%, B: 0.0005~0.0200%, Cr: 0.010~0.80%, Mo: 0.0010 to 1.000%, Co: 0-2.00%, Ni: 0-3.00% Cu: 0-1.00% V: 0~1.00%, W: 0 to 1.000%, Ca: 0 to 0.010% Mg: 0 to 1.000%, REM: 0 to 1.000%, Sb: 0 to 1.000%, Zr: 0 to 1.000%, Sn: 0 to 1.000%, As: 0 to 0.100%, and The balance has a chemical composition consisting of Fe and impurities, At least one of martensite, bainite and tempered martensite in terms of area ratio: 90% or more in total; The steel sheet is characterized by having a metal structure in which the standard deviation of the hardness distribution of prior austenite grains at the 1 / 4 position of the plate thickness is 150 Hv or less.

[0051] [Chemical composition of hot stamped compact] Since the chemical composition is not substantially changed by hot stamping, the chemical composition of the hot-stamped steel sheet is the same as that of the steel sheet for hot stamping described above. Therefore, the explanations of the elements and the balance related to the chemical composition of the steel sheet for hot stamping described above apply not only to the steel sheet for hot stamping but also to the hot-stamped steel sheet.

[0052] [At least one of martensite, bainite and tempered martensite: 90% or more in total] The metal structure of the hot-stamped steel sheet contains at least one of martensite, bainite, and tempered martensite in a total area fraction of 90% 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 10% 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 90% or more in the hot-stamped steel sheet, 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 92% or more or 94% or more, 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%.

[0053] [Identification of metal structure and calculation of area ratio] The identification of the metallographic structure and calculation of the area ratio in the hot-stamped steel and the steel sheet for hot stamping described above are carried out as follows. First, a sample is cut out 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 the 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.

[0054] 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.

[0055] 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.

[0056] Next, the remaining region (the region where the "Grain Average Misorientation" exceeds 0.5°) is taken as the total area fraction of martensite, tempered martensite, and bainite. Carbides are determined as carbides by determining the bright contrast areas with granular shapes in secondary electron images taken using a scanning electron microscope in the same field of view as the EBSD observation, and the area fraction of the corresponding regions is calculated to obtain the area fraction of carbides. 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, and the area fraction of carbides from 100%.

[0057] [Standard deviation of hardness distribution of prior austenite grains at 1 / 4 of the plate thickness: 150Hv or less] In an embodiment of the present invention, the standard deviation of the hardness distribution of prior austenite grains at the 1 / 4 thickness position of the hot-stamped steel sheet is 150 Hv or less. Large variations in the hardness of the prior austenite grains can lead to localized increases in hardness, which can cause hydrogen embrittlement cracking. According to an embodiment of the present invention, by controlling the standard deviation of the hardness distribution of prior austenite grains at the 1 / 4 thickness position of the hot-stamped steel sheet to 150 Hv or less and reducing the hardness variation of the prior austenite grains, it is possible to reliably suppress localized increases in hardness that can be the initiation point of hydrogen embrittlement cracking. Preferably, the standard deviation is 140 Hv or less, 130 Hv or less, 120 Hv or less, or 110 Hv or less. Although there is no particular lower limit, the standard deviation of the hardness distribution of prior austenite grains at the 1 / 4 thickness position of the hot-stamped steel sheet may be, for example, 50 Hv or more, 60 Hv or more, or 80 Hv or more.

[0058] In an embodiment of the present invention, as described above, it is important to reduce the variation in hardness of the prior austenite grains by controlling the standard deviation of the hardness distribution of the prior austenite grains at the 1 / 4 position in the thickness direction of the hot-stamped steel sheet (not the hardness distribution of the entire metallographic structure at the 1 / 4 position in the thickness direction, but the hardness distribution of the prior austenite grains present in the metallographic structure) to 150 Hv or less, and therefore it is not necessary to control the hardness of the prior austenite grains themselves within a specific range. Therefore, the hardness of the prior austenite grains at the 1 / 4 position in the thickness direction of the hot-stamped steel sheet is not particularly limited, and may be, for example, 500 Hv or more and / or 1000 Hv or less. The hardness of the prior austenite grains at the 1 / 4 position in the thickness direction of the hot-stamped steel sheet refers to the average of all hardness measurements measured in the method for determining the standard deviation of the hardness distribution of prior austenite grains described below.

[0059] The standard deviation of the hardness distribution 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 so that a cross section perpendicular to the surface (thickness cross section) can be observed. The sample size, depending on the measurement device, should be large enough to allow observation of 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. Vickers hardness measurements are then performed on the mirror-finished cross section at a depth of 1 / 4 of the thickness from the surface using a micro-Vickers hardness tester in a direction parallel to the surface, with a load of 1 gf, at intervals at least three times the indentation distance, for a total of 100 or more measurements. Next, the same sample is measured using an EBSD analyzer, and the obtained microstructural analysis results are used to select only measurement points where the indentation is within the prior austenite grain (i.e., the indentation does not overlap the grain boundary). Finally, the standard deviation obtained based on the Vickers hardness measurements for at least 20 different extracted prior austenite grains is determined as the standard deviation of the hardness distribution of the prior austenite grains at the 1 / 4 position in the thickness direction of the hot stamped body. EBSD measurements can be performed using an EBSD analyzer consisting of a thermal field emission scanning electron microscope and an EBSD detector, for example, an EBSD analyzer consisting of a JEOL JSM-7001F and a TSL DVC5 detector. The vacuum level inside the EBSD analyzer is 9.6 x 10 -5 The acceleration voltage may be 15 kV or less, and the probe current level may be 13. A crystal orientation map of prior austenite grains is created by the method described in Acta Materialia, 58 (2010), 6393-6403, and the prior austenite grains are identified based on the crystal orientation map.

[0060] [Plating] The hot-stamped product according to this embodiment may have a plating layer on its surface. Having a plating layer on its surface can improve corrosion resistance. Examples of the plating layer include an aluminum plating layer, an aluminum-zinc plating layer, an aluminum-silicon plating layer, a hot-dip galvanized layer, an electrogalvanized layer, a galvannealed hot-dip galvanized layer, a zinc-nickel plating layer, and an aluminum-magnesium-zinc based plating layer.

[0061] [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 or 2600 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.

[0062] 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.

[0063] <Manufacturing method> Next, a preferred method for manufacturing a steel sheet for hot stamping and 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 manufacturing a steel sheet for hot stamping and a hot-stamped steel according to an embodiment of the present invention, but is not intended to limit the steel sheet for hot stamping and the hot-stamped steel to those manufactured by the manufacturing method described below.

[0064] <Method of manufacturing steel sheets for hot stamping> In the method for producing a steel sheet for hot stamping according to an embodiment of the present invention, it is particularly effective to control the finish rolling conditions and the subsequent cooling conditions. Specifically, in the method for producing a steel sheet for hot stamping according to an embodiment of the present invention, A process of hot rolling a slab having the chemical composition described above in relation to the steel sheet for hot stamping, which comprises heating the slab and then finish rolling the slab, and the rolling reduction in the final stage of the finish rolling is 40% or more (hot rolling process); A step of quenching the obtained hot-rolled steel sheet within 1.0 second after the end of finish rolling, and then cooling it at an average cooling rate of 90 ° C. / second or more (cooling step); A step of winding the hot-rolled steel sheet at a temperature of 500 to 700 ° C. (winding step) Each step will be described in detail below.

[0065] [Hot rolling process] [Slab heating] First, a slab having the chemical composition described above in relation to the steel sheet for hot stamping 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 method before hot rolling is not particularly limited, but since the slab used contains a relatively large amount of alloying elements in order to obtain a high-strength steel sheet, the slab may be heated to a heating temperature of 1100°C or higher before being subjected to hot rolling in order to dissolve the alloying elements in the slab.

[0066] [Rough rolling] In this method, for example, the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc. The conditions for rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.

[0067] [Finishing rolling] The heated slab, or the slab that has been subjected to rough rolling as needed, is then subjected to finish rolling. In this method, it is important that the reduction ratio in the final stage of finish rolling is 40% or more. By using a reduction ratio in the final stage of finish rolling of 40% or more, pearlite is uniformly dispersed in the hot-rolled steel sheet after rolling. This pearlite serves as the starting point for austenite during heating in the hot stamping process, which will be described in detail later in connection with the manufacturing method of a hot-stamped steel sheet. Therefore, uniform dispersion of pearlite can reduce the variation in prior austenite grain size in the hot-stamped steel sheet. As a result, the variation in hardness of prior austenite grains in the metal structure of the hot-stamped steel sheet can be reduced, thereby significantly suppressing localized increases in hardness. Therefore, despite having a high tensile strength, for example, a high tensile strength of 2200 MPa or more, it is possible to significantly improve hydrogen embrittlement resistance. More preferably, the reduction ratio in the final stage of finish rolling is 45% or more or 50% or more.

[0068] In steel sheets for hot stamping, the amount of Mn added tends to be increased to ensure high hardenability, for example, 0.60% or more. In relation to this, research by the present inventors has revealed that with such a high Mn content, pearlite tends to be relatively connected in the hot-rolled steel sheet, and therefore it is very difficult to uniformly disperse pearlite in the metallographic structure of the hot-rolled steel sheet compared to a steel sheet with a low Mn content. Therefore, when such a steel with a high Mn content is finish-rolled at a relatively low reduction of less than 40%, it is thought that the presence of connected pearlite in the metallographic structure will be particularly noticeable.

[0069] However, by setting the reduction rate in the final stage of finish rolling to 40% or more, it is possible to achieve a sufficiently dispersed distribution of pearlite in the hot-rolled steel sheet after the hot-rolling process and the subsequent cooling and coiling processes, despite the high Mn content of 0.60% or more. Therefore, in the metallographic structure of a hot-rolled steel sheet subjected to such reduction, there are no or sufficiently reduced portions of connected pearlite, so that the variation in the prior austenite grain size in the structure after hot stamping can be reduced. As a result, the variation in hardness of prior austenite grains in the metallographic structure of a hot-stamped steel can be reduced. There is no particular upper limit to the reduction rate in the final stage of finish rolling. Even in steel materials with such a high Mn content, by appropriately controlling the reduction rate, particularly in the final stage of finish rolling, it is possible to achieve a sufficiently dispersed distribution of pearlite in the metallographic structure of the hot-rolled steel sheet, thereby reducing the variation in the prior austenite grain size and suppressing local increases in hardness.

[0070] The morphology of such a metal structure is dominated by the reduction ratio in the final stage of finish rolling, the average cooling rate in the subsequent cooling process, and the coiling temperature in the coiling process, and is not significantly affected by, for example, optional cold rolling or subsequent annealing. This is because if a hot-rolled steel sheet is formed with a final reduction ratio in finish rolling of 40% or more, even if the hot-rolled steel sheet is cold-rolled and then annealed at a relatively high temperature, a metal structure in which carbides, grain boundaries, and retained austenite, which serve as austenite initiation sites, are dispersed and formed after cooling. In general, if the reduction ratio in the final stage of finish rolling is too high, there is a concern that the steel sheet may crack during rolling. In particular, in the case of a high-strength steel sheet having a C content of 0.40% or more, if the reduction ratio in the final stage is too high, in addition to the concern of cracking of the steel sheet, the rolling load on the rolling mill also increases significantly. For this reason, finish rolling with a final-stage reduction of 40% or more has not been performed on steel materials having the same chemical composition as the steel sheet for hot stamping according to the embodiment of the present invention. Therefore, it has not been known that pearlite, which serves as the starting point for austenite grains during heating in hot stamping, can be uniformly dispersed during heating in hot stamping by setting the final-stage reduction of 40% or more in finish rolling and further appropriately controlling the average cooling rate in the subsequent cooling step and the coiling temperature in the coiling step in combination. Therefore, naturally, in relation to this, it has also not been known that prior austenite grains are homogenized in the finally obtained hot-stamped steel, thereby reducing the variation in hardness of the prior austenite grains in the metallographic structure of the hot-stamped steel. These facts have now been revealed for the first time by the present inventors.

[0071] [Cooling process] Next, the finish-rolled hot-rolled steel sheet is quenched within 1.0 second after the end of finish rolling. Because ferrite generally forms from the grain boundaries of austenite grains, as the austenite grains become larger, the number of grain boundaries that serve as the origin of ferrite decreases. In such cases, it becomes difficult to prevent pearlite from linking together and uniformly disperse it. Therefore, quenching the hot-rolled steel sheet immediately after the end of finish rolling, specifically within 1.0 second, preferably within 0.8 seconds, after the end of finish rolling is extremely important for suppressing the growth of austenite grains and generating uniformly dispersed pearlite in the hot-rolled steel sheet. The average cooling rate and cooling time during quenching are not particularly limited, but for example, the average cooling rate is preferably 200 to 1000°C / second, and the cooling time is preferably 0.2 to 2.0 seconds.

[0072] Next, the hot-rolled steel sheet after quenching is cooled at an average cooling rate of 90°C / s or more. If the cooling rate is slow, a large amount of bainite is generated or austenite remains as retained austenite, making it impossible to form a metal structure composed mainly of ferrite and pearlite. As a result, it becomes difficult to control the arrangement of ferrite and pearlite, and in particular, it becomes difficult to achieve uniform dispersion of pearlite. On the other hand, by cooling at an average cooling rate of 90°C / s or more, it is possible to form a metal structure composed mainly of ferrite and pearlite, more specifically, a metal structure in which the total area ratio of ferrite and pearlite is 80% or more. The average cooling rate is preferably 95°C / s or more. Although there is no particular upper limit, the average cooling rate may be, for example, 200°C / s or less or 150°C / s or less.

[0073] [Winding process] Next, the finish-rolled hot-rolled steel sheet is coiled at a temperature of 500 to 700°C. If the coiling temperature is high, grain growth may occur, which may hinder uniform dispersion of pearlite. On the other hand, if the coiling temperature is low, bainite or martensite may be generated, making it impossible to form a metal structure mainly composed of ferrite and pearlite. In contrast, by controlling the coiling temperature to 500 to 700°C, grain growth can be suppressed and ferrite can be prevented from being linked and arranged in the hot-rolled steel sheet after rolling, thereby enabling uniform dispersion of pearlite. Preferably, the coiling temperature is 505 to 650°C or 550 to 650°C. 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.

[0074] [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.

[0075] [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.

[0076] [Annealing process] For example, after the cold rolling step, annealing may be optionally performed to adjust the metal structure and / or properties. The heating temperature in the annealing step is not particularly limited, but may be, for example, 800°C or lower.

[0077] [Plating process] For the purpose of improving corrosion resistance, etc., a plating treatment may be applied to the surface of a hot-rolled steel sheet or a 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 plating treatment may involve hot-dip galvanizing of the steel sheet, or hot-dip galvanizing may be followed by an alloying 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 based plating layer. Specific conditions for the plating treatment and the alloying treatment are not particularly limited and may be any appropriate conditions known to those skilled in the art.

[0078] [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.

[0079] <Method of manufacturing hot-stamped body> Next, a method for manufacturing a hot-stamped steel sheet according to an embodiment of the present invention will be described. Specifically, the method is characterized by comprising a step of hot stamping a steel sheet for hot stamping obtained by the method for manufacturing a steel sheet for hot stamping described above, in which the steel sheet for hot stamping is heated to a temperature range of 800°C to 1000°C and then held at that temperature for 60 to 600 seconds.

[0080] [Hot stamping process] A hot stamped steel sheet for hot stamping is hot stamped in a hot stamp forming process to produce a hot stamped steel sheet having a desired chemical composition and metallographic structure. In this embodiment, austenite is generated from pearlite uniformly dispersed in the metallographic structure of the steel sheet during heating in hot stamp forming. Subsequent forming and cooling operations produce a hot stamped steel sheet having a desired hard structure and a desired prior austenite grain size distribution with reduced variation, thereby reducing variation in the hardness of the prior austenite grains. From the viewpoint of obtaining such a desired hard structure and hardness distribution of the prior austenite grains, it is preferable to heat the steel sheet for hot stamping to a temperature range of 800°C to 1000°C and hold it in this temperature range for 60 to 600 seconds. If the heating temperature is less than 800°C, austenitization will be insufficient, and the desired area ratio of the hard structure (at least one of martensite, bainite, and tempered martensite) will not be obtained, which may result in a deterioration in tensile strength. On the other hand, if the heating temperature exceeds 1000°C, the austenite grains grow excessively, making it impossible to obtain the desired prior austenite grain size distribution. As a result, the desired hardness distribution of the prior austenite grains cannot be obtained, and hydrogen embrittlement resistance may deteriorate. If the holding time is less than 60 seconds, austenitization may be insufficient, as in the case of a heating temperature of less than 800°C, making it impossible to obtain the desired area ratio of hard structures (at least one of martensite, bainite, and tempered martensite), and tensile strength may deteriorate. If the holding time exceeds 600 seconds, the austenite grains grow excessively, making it impossible to obtain the desired prior austenite grain size distribution. As a result, the desired hardness distribution of the prior austenite grains cannot be obtained, and hydrogen embrittlement resistance may deteriorate.

[0081] 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.

[0082] 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.

[0083] The hot-stamped product according to this embodiment can be obtained by the above method. After hot stamping, a tempering treatment at 130 to 600°C or a bake hardening treatment after painting can be performed. Furthermore, a part of the hot-stamped product can be tempered by laser irradiation or the like to provide a partially softened region.

[0084] 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]

[0085] 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.

[0086] 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 was Fe and impurities. These slabs were heated to a temperature of 1100°C or higher and rough-rolled under predetermined conditions. Then, finish rolling, cooling, and coiling were performed under the conditions shown in Table 2. In all inventive and comparative examples, the average cooling rate during quenching after finish rolling was in the range of 200 to 1000°C / s, and the cooling time was in the range of 0.2 to 2.0 seconds. Next, the obtained hot-rolled steel sheets were cold-rolled at a predetermined reduction of 30 to 80%. Next, some of the steel sheets were annealed, plated, or temper-rolled under predetermined conditions. Next, the obtained steel sheets were 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 products were tempered or partially softened.

[0087] [Table 1-1]

[0088] [Table 1-2]

[0089] [Table 1-3]

[0090] [Table 1-4]

[0091] [Table 1-5]

[0092] [Table 1-6]

[0093] [Table 1-7]

[0094] [Table 1-8]

[0095] [Table 2-1]

[0096] [Table 2-2]

[0097] [Table 2-3]

[0098] [Table 2-4]

[0099] [Table 2-5]

[0100] [Table 2-6]

[0101] The properties of the hot stamped steel sheets thus obtained were measured and evaluated by the following methods.

[0102] [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.

[0103] [Hydrogen embrittlement resistance] The hydrogen embrittlement resistance of hot-stamped steel was evaluated as follows. First, test pieces measuring 1.2 mm thick x 7.0 mm wide x 68 mm long were prepared. Various strains (stresses) were applied to the test pieces using a four-point bending jig. The test pieces were then immersed in hydrochloric acid (room temperature, pH = 4) for 48 hours to determine the critical strain at which cracking occurred. Critical strains of 0.6% or greater were evaluated as pass (◯), and critical strains of less than 0.6% were evaluated as fail (×).

[0104] 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 results are shown in Table 3. Table 3 shows the area fractions of ferrite and pearlite and the dispersion index of pearlite in the hot-stamping steel sheet after the coiling process. The remaining structure other than ferrite and pearlite was bainite, martensite, retained austenite, and / or trace amounts of carbides. Similarly, Table 3 shows the area fraction of hard structure in the hot-stamped steel sheet and the standard deviation of the hardness distribution of prior austenite grains (prior γ grains) at the 1 / 4 position in the sheet thickness. The area fraction of hard structure means the sum of the area fractions of martensite, bainite, and tempered martensite. The remaining structure other than the hard structure was ferrite, retained austenite, and / or pearlite.

[0105] [Table 3-1]

[0106] [Table 3-2]

[0107] Table 3-3

[0108] Table 3-4

[0109] Table 3-5

[0110] Table 3-6

[0111] Referring to Table 3, in Comparative Example 1, the tensile strength was reduced due to the low C content. In Comparative Example 13, the strength was too high due to the high C content, and hydrogen embrittlement resistance was reduced. In Comparative Example 14, the tensile strength was reduced due to the low Si content. In Comparative Example 25, the ferrite content increased in the hot stamping steel sheet due to the high Si content, and the desired metal structure was not obtained, resulting in a pearlite dispersion index of less than 0.50. As a result, the standard deviation of the hardness distribution of prior austenite grains in the hot stamped steel sheet could not be controlled within the desired range, and hydrogen embrittlement resistance was reduced. In Comparative Example 26, the pearlite dispersion index in the hot stamping steel sheet and the standard deviation of the hardness distribution of prior austenite grains in the hot stamped steel sheet could not be controlled within the desired range due to the low Mn content, and hydrogen embrittlement resistance was reduced. In Comparative Example 41, the high Mn content is thought to have excessively promoted the transformation from austenite to pearlite in the hot rolled steel sheet. As a result, the dispersion index of pearlite in the hot stamping steel sheet and the standard deviation of the hardness distribution of prior austenite grains in the hot stamped steel sheet could not be controlled within the desired range, resulting in poor hydrogen embrittlement resistance. Comparative Examples 50, 59, 68, 77, 78, and 90 had poor hydrogen embrittlement resistance due to inappropriate P, S, N, O, or Al contents. Comparative Examples 91, 103, 115, 127, and 139 had low Nb, Ti, B, Cr, and Mo contents, respectively, which prevented sufficient improvement in strength and resulted in poor tensile strength. Comparative Examples 102, 114, 126, 138, and 150 had high Nb, Ti, B, Cr, and Mo contents, respectively, which presumably resulted in the formation of large amounts of carbonitrides or coarse intermetallic compounds in the steel, resulting in poor hydrogen embrittlement resistance.

[0112] In Comparative Example 259, the reduction ratio in the final stage of finish rolling in the hot rolling process was low, which is thought to have prevented pearlite from being uniformly dispersed in the hot-rolled steel sheet after rolling. As a result, the pearlite dispersion index in the steel sheet for hot stamping and the standard deviation of the hardness distribution of prior austenite grains in the hot-stamped steel sheet could not be controlled within the desired ranges, resulting in reduced hydrogen embrittlement resistance. In Comparative Example 270, the time from the end of finish rolling to the start of quenching was long, which was thought to have prevented austenite grain growth from being sufficiently suppressed, resulting in ferrite being linked and arranged, preventing pearlite from being uniformly dispersed. As a result, the pearlite dispersion index in the steel sheet for hot stamping and the standard deviation of the hardness distribution of prior austenite grains in the hot-stamped steel sheet could not be controlled within the desired ranges, resulting in reduced hydrogen embrittlement resistance. In Comparative Example 271, the average cooling rate after quenching in the cooling process was slow, which prevented the formation of a desired metallographic structure in the steel sheet for hot stamping. As a result, the dispersion index of pearlite in the steel sheet for hot stamping and the standard deviation of the hardness distribution of prior austenite grains in the hot-stamped steel sheet could not be controlled within the desired ranges, and hydrogen embrittlement resistance deteriorated. In Comparative Example 276, the coiling temperature was low, so a desired metallographic structure could not be formed in the steel sheet for hot stamping. As a result, the standard deviation of the hardness distribution of prior austenite grains in the hot-stamped steel sheet could not be controlled within the desired ranges, and hydrogen embrittlement resistance deteriorated. In Comparative Example 284, the coiling temperature was high, so it is believed that grain growth occurred, inhibiting uniform dispersion of pearlite. As a result, the dispersion index of pearlite in the steel sheet for hot stamping and the standard deviation of the hardness distribution of prior austenite grains in the hot-stamped steel sheet could not be controlled within the desired ranges, and hydrogen embrittlement resistance deteriorated.

[0113] In contrast, all of the hot stamping steel sheets and hot stamped steel sheets according to the present invention had predetermined chemical compositions and metallographic structures, and by controlling the pearlite dispersion index in the hot stamping steel sheets to 0.50 or more and controlling the standard deviation of the hardness distribution of prior austenite grains in the hot stamped steel sheets to 150 Hv or less, hydrogen embrittlement could be reliably suppressed despite a high tensile strength of 2200 MPa or more. Furthermore, in all of the hot stamped steel sheets according to the present invention, the hardness of the prior austenite grains at the 1 / 4 position in the plate thickness direction (the average of all hardness measurements in each example) was controlled to be in the range of 500 to 1000 Hv.

Claims

1. In mass%, C: 0.40-0.70%, Si: 0.010-1.300%, Mn: 0.60-3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0200% or less, Al: 0.0010-0.5000%, Nb: 0.0010 to 0.100%, Ti: 0.010-0.200%, B: 0.0005-0.0200%, Cr: 0.010-0.80%, Mo: 0.0010-1.000%, Co: 0-2.00%, Ni: 0-3.00%, Cu: 0 to 1.00%, V: 0 to 1.00%, W: 0-1.000%, Ca: 0-0.010%, 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, In terms of area ratio, Ferrite: 10% or more, and Perlite: 10% or more The total of ferrite and pearlite is 80% or more, It has a metal structure in which the dispersion index of pearlite is 0.50 or more, The dispersion index of pearlite is obtained by dividing the number of A / B boundaries (boundaries between ferrite phases and pearlite phases) in the electron channeling contrast image of the metal structure by the sum of the number of A / A boundaries (boundaries between ferrite phases), the number of B / B boundaries (boundaries between pearlite phases and pearlite phases), and the number of A / B boundaries (boundaries between ferrite phases and pearlite phases).

2. The chemical composition is, in mass %, Co: 0.001 to 2.00%, Ni: 0.001 to 3.00%, Cu: 0.001 to 1.00%, V: 0.001 to 1.00%, W: 0.001-1.000%, Ca: 0.0001-0.010%, 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 steel sheet for hot stamping according to claim 1, comprising one or more selected from the group consisting of:

3. In mass%, C: 0.40-0.70%, Si: 0.010-1.300%, Mn: 0.60-3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0200% or less, Al: 0.0010-0.5000%, Nb: 0.0010 to 0.100%, Ti: 0.010-0.200%, B: 0.0005-0.0200%, Cr: 0.010-0.80%, Mo: 0.0010-1.000%, Co: 0-2.00%, Ni: 0-3.00%, Cu: 0 to 1.00%, V: 0 to 1.00%, W: 0-1.000%, Ca: 0-0.010%, 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, At least one of martensite, bainite, and tempered martensite in terms of area ratio: 90% or more in total; A hot stamped steel having a metal structure in which the standard deviation of the hardness distribution of prior austenite grains at a 1 / 4 position in the plate thickness is 150 Hv or less.

4. A hot stamped body as described in claim 3, wherein the standard deviation of the hardness distribution of prior austenite grains at the 1 / 4 position in the plate thickness is 50 to 150 Hv or less.

5. A hot stamped body as described in claim 4, wherein the standard deviation of the hardness distribution of prior austenite grains at the 1 / 4 position in the plate thickness is 60 to 150 Hv or less.

6. A hot stamped body as described in claim 5, wherein the standard deviation of the hardness distribution of prior austenite grains at the 1 / 4 position in the plate thickness is 80 to 150 Hv or less.

7. The chemical composition is, in mass %, Co: 0.001 to 2.00%, Ni: 0.001 to 3.00%, Cu: 0.001 to 1.00%, V: 0.001 to 1.00%, W: 0.001-1.000%, Ca: 0.0001-0.010%, 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 any one of claims 3 to 6, comprising one or more selected from the group consisting of:

Images