Hot stamped compact

Abstract

Provided is a hot-stamped molded article characterized by comprising a steel base material and a plating layer disposed on a surface of the steel base material, wherein: the plating layer has a prescribed chemical composition; the average C concentration from the surface of the steel base material to 1 µm in the depth direction is 0.25 mass% or less; the steel base material contains 90% or more of martensite in terms of area ratio; the adhered amount of the plating layer is 30 g / m2 or more per one surface; the plating layer contains a Zn-containing phase comprising at least one of an η-Zn phase, Fe-Zn-based intermetallic compound phase, and Mg-Zn-based intermetallic compound phase; and the hot-stamped molded article has a Vickers hardness of 400 HV or more.

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 steel sheets used in structural components is one effective way to achieve both weight reduction and crashworthiness, and against this background, the development of high-strength steel sheets is underway.

[0003] 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.

[0004] In this regard, various studies have been conducted on hot stamped steel sheets that have been subjected to Zn-containing plating.

[0005] For example, Patent Document 1 describes a steel sheet coated with a metal coating having an Al / Zn ratio of more than 2.9, which contains 2.0 to 24.0 wt. % zinc, 7.1 to 12.0 wt. % silicon, optionally 1.1 to 8.0 wt. % magnesium, and optionally an additional element selected from Pb, Ni, Zr, or Hf, wherein the weight content of each additional element is less than 0.3 wt. %, and the balance is aluminum and any unavoidable impurities and residual elements, and the metal coating has an Al / Zn ratio of more than 2.9, and teaches that a part obtained by hot stamping the steel sheet exhibits high sacrificial corrosion protection.

[0006] Patent Document 2 discloses a method for manufacturing a steel sheet, comprising the steps of: A) providing a steel sheet pre-coated with a metal coating containing 2.0 to 24.0 wt. % zinc, 1.1 to 7.0 wt. % silicon, and optionally 1.1 to 8.0 wt. % magnesium when the amount of silicon is between 1.1 and 4.0 wt. %, and optionally an additional element selected from Pb, Ni, Zr, or Hf, the weight content of each additional element being less than 0.3 wt. %, with the remainder being aluminum, unavoidable impurities, and residual elements, wherein the Al / Zn ratio is greater than 2.9; and B) cutting the coated steel sheet to obtain blanks. A method for manufacturing a hardened part includes the steps of: C) heat treating the blank at a temperature between 840 and 950°C to obtain a fully austenitic microstructure in the steel; D) transferring the blank into a press tool; E) hot forming the blank to obtain a part; and F) cooling the part obtained in step E) to obtain a microstructure in the steel that is martensite or martensite-bainite, or is composed of at least 75% equiaxed ferrite, 5 to 20% martensite, and 10% or less bainite. Patent Document 2 also teaches that the above manufacturing method can produce hardened parts free of LME. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Special Publication No. 2018-528324 [Patent Document 2] Special Publication No. 2018-527462 Summary of the Invention [Problem to be solved by the invention]

[0008] For example, when a galvanized steel sheet as described in Patent Documents 1 and 2 is used in hot stamping, the coating layer after hot stamping may alloy with the base steel (steel material), resulting in reduced corrosion resistance. Furthermore, the hot-stamped product obtained by hot stamping a galvanized steel sheet is then joined using spot welding or the like. During this process, it is necessary to suppress liquid metal embrittlement (LME) cracking. This phenomenon occurs when Zn, which has been converted into a liquid phase by welding heat input, penetrates into the steel material along grain boundaries, embrittling the steel material, and then tensile stress generated by welding acts on the embrittled area. In this regard, Patent Document 2 teaches how to suppress LME cracking during hot stamping. However, Patent Document 2 does not necessarily provide sufficient consideration from the perspective of suppressing LME cracking during spot welding after hot stamping, or even from the perspective of achieving both suppression of LME cracking and improved corrosion resistance. Furthermore, it is known that LME cracking generally occurs more significantly in steel materials with relatively high strength, and that the higher the strength of a steel material, the greater the susceptibility to LME cracking. On the other hand, the automotive industry and other sectors are also demanding further weight reduction in steel materials, and to achieve such weight reduction, steel materials must be made stronger than ever before. Therefore, there is a high demand for steel materials, more specifically, hot stamped steel, that can solve the problem of LME cracking even when the strength is the same as or higher than conventional steel materials.

[0009] Therefore, an object of the present invention is to provide a hot stamped steel sheet that has high strength and maintains high corrosion resistance even after hot stamping, and is capable of suppressing LME cracking during spot welding. [Means for solving the problem]

[0010] As a result of investigations to achieve the above object, the inventors first found that forming a coating layer containing a specific Zn-containing phase at a coating weight of at least a predetermined amount can maintain sufficient corrosion resistance even when applied to hot stamping. In addition, the inventors discovered that even if a coating layer formed at such a coating weight has a high Vickers hardness of 400 HV or more, by appropriately modifying the surface layer structure of the steel base material before hot stamping so that the C concentration in the surface layer of the steel base material after hot stamping is relatively low, the occurrence of LME cracking during spot welding after hot stamping can be significantly suppressed or reduced. This finding led to the completion of the present invention.

[0011] The present invention, which has achieved the above object, is as follows. (1) A steel substrate and a plating layer disposed on the surface of the steel substrate, The plating layer is composed of, in mass %, Al: 0.5 to 30.0%, Mg: 0.50~15.00%, Si: 0 to 2.0%, and Fe: 15.0 to 70.0% and further comprising Ni: 0 to 1.000%, Ca: 0-3.0% Sb: 0 to 0.500% Pb: 0~0.500%, Cu: 0-1.000%, Sn: 0 to 1.000%, Ti: 0 to 1.000%, Cr: 0 to 1.000%, Nb: 0 to 1.000%, Zr: 0 to 1.000%, Mn: 0 to 1.000%, Mo: 0 to 1.000%, Ag: 0~1.000%, Li: 0~1.000%, La: 0 to 0.500%, Ce: 0 to 0.500% B: 0~0.500%, Y: 0~0.500%, Sr: 0 to 0.500%, In: 0~0.500%, Co: 0 to 0.500%, Bi: 0 to 0.500% P: 0 to 0.500%, and W: 0 to 0.500% Contains at least one of the following in a total amount of 5.000% or less, The balance has a chemical composition consisting of Zn and impurities, The average C concentration from the surface of the steel base material to 1 μm in the depth direction is 0.25 mass% or less, The steel base material contains martensite in an area ratio of 90% or more, The coating weight of the plating layer is 30 g / m per side 2 That's all, the plating layer contains a Zn-containing phase including at least one of an η-Zn phase, an Fe-Zn-based intermetallic compound phase, and an Mg-Zn-based intermetallic compound phase, A hot stamped product characterized by having a Vickers hardness of 400 HV or more. (2) The hot-stamped steel according to (1) above, characterized in that the average C concentration is 0.18 mass % or less. (3) The hot-stamped product according to (2) above, characterized in that the average C concentration is 0.10 mass % or less. (4) The hot-stamped body according to any one of (1) to (3) above, characterized in that the chemical composition contains, in mass%, Al: 6.0 to 30.0% and Mg: 3.00 to 15.00%, and the Zn-containing phase contains at least one of an η-Zn phase and an Fe-Zn-based intermetallic compound phase. (5) The hot-stamped body according to any one of (1) to (4), characterized in that the chemical composition contains, in mass%, Al: 20.0 to 30.0% and Mg: 5.00 to 15.00%, and the Zn-containing phase contains an Mg-Zn-based intermetallic compound phase. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a hot stamped steel sheet that has high strength and maintains high corrosion resistance even after hot stamping, and that can suppress LME cracking during spot welding. DETAILED DESCRIPTION OF THE INVENTION

[0013] <Hot stamped compact> A hot-stamped steel according to an embodiment of the present invention includes a steel base material and a plating layer disposed on a surface of the steel base material, The plating layer is composed of, in mass %, Al: 0.5 to 30.0%, Mg: 0.50~15.00%, Si: 0 to 2.0%, and Fe: 15.0 to 70.0% and further comprising Ni: 0 to 1.000%, Ca: 0-3.0% Sb: 0 to 0.500% Pb: 0~0.500%, Cu: 0-1.000%, Sn: 0 to 1.000%, Ti: 0 to 1.000%, Cr: 0 to 1.000%, Nb: 0 to 1.000%, Zr: 0 to 1.000%, Mn: 0 to 1.000%, Mo: 0 to 1.000%, Ag: 0~1.000%, Li: 0~1.000%, La: 0 to 0.500%, Ce: 0 to 0.500% B: 0~0.500%, Y: 0~0.500%, Sr: 0 to 0.500%, In: 0~0.500%, Co: 0 to 0.500%, Bi: 0 to 0.500% P: 0 to 0.500%, and W: 0 to 0.500% Contains at least one of the following in a total amount of 5.000% or less, The balance has a chemical composition consisting of Zn and impurities, The average C concentration from the surface of the steel base material to 1 μm in the depth direction is 0.25 mass% or less, The steel base material contains martensite in an area ratio of 90% or more, The coating weight of the plating layer is 30 g / m per side 2 That's all, the plating layer contains a Zn-containing phase including at least one of an η-Zn phase, an Fe-Zn-based intermetallic compound phase, and an Mg-Zn-based intermetallic compound phase, It is characterized by having a Vickers hardness of 400HV or more.

[0014] As mentioned above, when a hot-stamped product obtained by hot stamping a galvanized steel sheet is joined by spot welding, it is necessary to suppress liquid metal embrittlement (LME) cracking. Although the reason for this is not entirely clear, research by the present inventors and others has revealed that carbon contained in steel is an element that promotes such LME cracking. Therefore, it is believed that the occurrence of LME cracking can be suppressed or reduced by reducing the carbon concentration in the surface layer of the steel sheet, where LME cracking occurs, for example, by decarburization. However, in practice, when applied to hot stamping, the LME suppression effect based on such a reduction in the carbon concentration in the surface layer of the steel sheet is limited and may not necessarily be satisfactory.

[0015] As a result of various investigations, the inventors have found that, even if the carbon concentration in the surface layer of a steel base material before hot stamping is reduced by decarburization or the like in order to improve LME resistance, carbon contained in the bulk of the steel base material diffuses to the surface layer during high-temperature heating in hot stamping, and this recarburization in the surface layer eliminates or reduces the LME-inhibiting effect achieved by the initial low carbon concentration in the surface layer. Therefore, the inventors have conducted further investigations and found that, by incorporating a structure that can inhibit such recarburization into the surface layer of a steel base material before hot stamping, the LME-inhibiting effect achieved by the initial low carbon concentration in the surface layer can be fully exerted, and the occurrence of LME cracking during spot welding after hot stamping can be reliably inhibited or reduced, even when a predetermined coating weight of a Zn-containing coating layer is included to maintain sufficient corrosion resistance. More specifically, as will be described in detail later in connection with the method for producing a hot-stamped body, the present inventors have determined that the coating weight of the coating layer containing a specific Zn-containing phase, more specifically a Zn-containing phase containing at least one of an η-Zn phase, an Fe-Zn-based intermetallic compound phase, and an Mg-Zn-based intermetallic compound phase, is 30 g / m per side. 2 By satisfying the above, sufficient corrosion resistance is maintained even when applied to hot stamping, and by forming a structure in which the amount of pearlite in the surface layer of the steel base material is relatively small and the amount of coarse pearlite is reduced, the average C concentration from the surface of the steel base material to 1 μm in the depth direction can be suppressed to 0.25 mass% or less when applied to hot stamping, and it was found that in relation to this, LME cracking during spot welding after hot stamping can be reliably suppressed or reduced.

[0016] Without intending to be bound by any particular theory, it is believed that when a hot-stamped steel according to an embodiment of the present invention is obtained by hot stamping, the structure in the surface layer of the steel base material acts as follows to suppress or reduce the diffusion of carbon contained in the bulk of the steel base material to the surface layer and recarburization during high-temperature heating in hot stamping. More specifically, when the carbon concentration in the surface layer of the steel base material is reduced by decarburization or the like, the amount of pearlite formed in the microstructure in the surface layer of the steel base material is relatively small in association with this low carbon concentration. Forming such a region with relatively little pearlite in the surface layer of the steel base material makes it possible to fully exert the LME suppression effect based on the low carbon concentration. However, simply reducing the amount of pearlite is thought to result in the formation of a carbon diffusion path (i.e., a carbon recarburization path) through austenite along the grain boundaries when pearlite precipitates along the grain boundaries, as the pearlite transforms to austenite during high-temperature heating in hot stamping. During high-temperature heating in hot stamping, carbon in the bulk of the steel base material tends to diffuse toward the surface due to the concentration gradient between the high carbon concentration in the bulk and the low carbon concentration in the surface. If austenite-mediated carbon recarburization pathways exist along the grain boundaries, carbon in the bulk will diffuse toward the surface through these pathways, accelerating recarburization in the surface layer. As a result, the LME suppression effect achieved by initially achieving a low carbon concentration in the surface layer cannot be fully achieved. In contrast, according to an embodiment of the present invention, by further reducing the amount of coarse pearlite in the surface layer region, where the amount of pearlite is relatively small, austenite transformed from pearlite can be dispersed and present on grain boundaries even during high-temperature heating in hot stamping. This reliably disrupts the austenite-mediated carbon recarburization pathways.

[0017] More specifically, when pearlite transforms to austenite during high-temperature heating in hot stamping, a two-phase structure of ferrite and austenite is formed. In this case, the austenite present at the interface between ferrite and austenite extends to the surface of the steel base material, forming a carbon recarburization pathway. As a result, carbon diffusion from the bulk of the steel base material to the surface is promoted. In this regard, when obtaining a hot-stamped steel according to an embodiment of the present invention by hot stamping, it is important to reduce the amount of pearlite in the surface layer of the steel base material and the amount of coarse pearlite in the surface layer. This surface layer structure can reduce the amount of austenite transformed from pearlite even during high-temperature heating in hot stamping. Furthermore, this austenite can be dispersed at grain boundaries, reliably disrupting the carbon recarburization pathway through austenite. As a result, the average C concentration in the final hot-stamped steel from the surface to 1 μm in the depth direction can be suppressed to 0.25 mass% or less. In particular, when a zinc-containing coated steel sheet is hot stamped, interdiffusion of Fe in the steel base material and the coating layer occurs during high-temperature heating during hot stamping, resulting in alloying of the coating layer with Fe. However, because carbon in the steel base material is not contained in the coating layer, carbon is concentrated relative to Fe near the surface of the steel base material. Therefore, even if the carbon concentration in the surface layer of the steel base material is reduced by decarburization or other processes before hot stamping, the carbon concentration in the surface layer of the steel base material after hot stamping may be much higher than the carbon concentration in the bulk. In contrast, according to an embodiment of the present invention, the diffusion of carbon from the bulk to the surface layer is suppressed by disrupting the recarburization pathway as described above. This significantly suppresses or reduces carbon concentration near the surface of the steel base material compared to conventional coated steel sheets. More specifically, it is possible to suppress the average carbon concentration within 1 μm of the surface of the steel base material to 0.25 mass% or less.Therefore, according to embodiments of the present invention, even though a Zn-containing coating layer is provided in a sufficient coating weight to maintain corrosion resistance and therefore conditions are relatively favorable for LME, recarburization during high-temperature heating in hot stamping is significantly suppressed, thereby fully demonstrating the LME-inhibiting effect achieved by the initial low carbon concentration in the surface layer of the steel base material, and it is possible to reliably suppress or reduce the occurrence of LME cracking during subsequent spot welding. The fact that the occurrence of LME cracking can be suppressed or reduced as described above by appropriately modifying the surface layer structure of the steel base material before hot stamping in a hot stamped product provided with a Zn-containing coating layer was discovered for the first time by the present inventors. Therefore, hot stamped products according to embodiments of the present invention are particularly useful in the automotive field, where spot welding is relatively common.

[0018] 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.

[0019] [Plating layer] According to an embodiment of the present invention, a coating layer is disposed on the surface of a steel base material, for example, disposed on at least one, preferably both, surfaces of the steel base material. Here, the expression "disposed on the surface of the steel base material" includes not only the case where the coating layer is disposed directly on the surface of the steel base material, but also the case where the coating layer is disposed indirectly on the surface of the steel base material, for example, the case where a solid solution layer, which will be described later, is included between the steel base material and the coating layer. The coating layer has the following chemical composition:

[0020] [Al: 0.5 to 30.0%] Al is an element effective in improving the corrosion resistance of the coating layer. To fully obtain this effect, the Al content is set to 0.5% or more. The Al content may be 0.7% or more, 1.0% or more, 3.0% or more, 5.0% or more, 7.0% or more, 10.0% or more, or 15.0% or more. On the other hand, if the Al content is excessive, the amount of Zn necessary to impart sacrificial corrosion protection decreases. Therefore, the Al content is set to 30.0% or less. The Al content may be 27.0% or less, 25.0% or less, 22.0% or less, or 20.0% or less.

[0021] [Mg: 0.50~15.00%] Mg is an element effective in improving the corrosion resistance of the coating layer. To fully achieve this effect, the Mg content is set to 0.50% or more. The Mg content may be 0.51% or more, 0.52% or more, 0.53% or more, 0.55% or more, 0.60% or more, 0.80% or more, 1.00% or more, 1.50% or more, 2.00% or more, or 3.00% or more. On the other hand, excessive Mg content may cause coating blistering and rust flow due to excessive sacrificial corrosion protection. Therefore, the Mg content is set to 15.00% or less. The Mg content may be 12.00% or less, 10.00% or less, 9.00% or less, 8.00% or less, 7.00% or less, 6.00% or less, or 5.00% or less.

[0022] [Si: 0-2.0%] Silicon is an element effective in improving the corrosion resistance of the plating layer. The Si content may be 0%, but if necessary, Si may be contained in the plating layer in an amount of 0.01% or more, 0.05% or more, 0.1% or more, 0.2% or more, 0.4% or more, or 0.6% or more. On the other hand, from the viewpoint of improving the plating adhesion of the plating layer, the Si content may be 2.0% or less. The Si content may be 1.8% or less, 1.5% or less, 1.2% or less, 1.0% or less, or 0.8% or less.

[0023] [Fe: 15.0 to 70.0%] When a plated steel sheet is heated during hot stamping, Fe from the steel base material diffuses into the coating layer and alloys with Zn and other elements, resulting in the coating layer inevitably containing Fe. Therefore, the Fe content is 15.0% or more, and may be, for example, 20.0% or more, 25.0% or more, 30.0% or more, 35.0% or more, 40.0% or more, 45.0% or more, or 50.0% or more. On the other hand, according to an embodiment of the present invention, as will be described in detail later in connection with the method for producing a hot-stamped steel sheet, it is believed that by appropriately modifying the surface layer structure of the steel base material before hot stamping, it is possible to delay the alloying of Zn and other elements in the coating layer with Fe in the steel base material, although the reason is not entirely clear. Therefore, according to an embodiment of the present invention, the Fe content in the coating layer can be suppressed to a maximum of 70.0% or less. In this regard, it is preferable to set the coating weight of the coating layer to 30 g / m per side. 2 By controlling the above, a relatively large amount of the plating layer in which alloying has not progressed sufficiently can be present, and sufficient corrosion resistance can be maintained due to the presence of such a plating layer and further due to the presence of a specific Zn-containing phase in the plating layer, more specifically the presence of at least one of an η-Zn phase, an Fe-Zn intermetallic compound phase, and an Mg-Zn intermetallic compound phase. From the viewpoint of improving corrosion resistance, the lower the Fe content, the better, and it may be, for example, 67.0% or less, 65.0% or less, 62.0% or less, 60.0% or less, 57.0% or less, or 55.0% or less.

[0024] Furthermore, the plating layer may optionally contain Ni: 0-1.000%, Ca: 0-3.0%, Sb: 0-0.500%, Pb: 0-0.500%, Cu: 0-1.000%, Sn: 0-1.000%, Ti: 0-1.000%, Cr: 0-1.000%, Nb: 0-1.000%, Zr: 0-1.000%, Mn: 0-1.000%, Mo: 0-1.000%, At least one of Ag: 0-1.000%, Li: 0-1.000%, La: 0-0.500%, Ce: 0-0.500%, B: 0-0.500%, Y: 0-0.500%, Sr: 0-0.500%, In: 0-0.500%, Co: 0-0.500%, Bi: 0-0.500%, P: 0-0.500%, and W: 0-0.500% may be contained. The content of these optional elements is not particularly limited, but preferably a total of 5.000% or less. The total content of the optional elements may be 4.500% or less, 4.000% or less, 3.500% or less, 3.000% or less, 2.500% or less, 2.000% or less, 1.500% or less, or 1.000% or less. These optional elements are described in detail below.

[0025] [Ni: 0 to less than 1.000%] Ni is an element effective in improving the corrosion resistance of the plating layer. The Ni content may be 0%, but to achieve this effect, the Ni content is preferably 0.0001% or more. The Ni content may be 0.0004% or more, 0.001% or more, 0.005% or more, 0.010% or more, or 0.020% or more. While there is no particular upper limit, from the viewpoint of production costs, etc., the Ni content may be 1.000% or less, for example, 0.980% or less, 0.950% or less, 0.900% or less, 0.700% or less, 0.500% or less, 0.400% or less, 0.300% or less, or 0.100% or less.

[0026] [Ca: 0-3.0%] Ca is an element effective in ensuring wettability of the coating bath. The Ca content may be 0%, but to achieve this effect, the Ca content is preferably 0.01% or more. The Ca content may be 0.05% or more, 0.1% or more, 0.5% or more, or 1.0% or more. On the other hand, excessive Ca content may form a large amount of hard intermetallic compounds in the coating layer, making the coating layer brittle and reducing adhesion to the steel sheet. Therefore, the Ca content is preferably 3.0% or less. The Ca content may be 2.5% or less, 2.0% or less, or 1.5% or less.

[0027] [Sb:0~0.500%, Pb:0~0.500%, Cu:0~1.000%, Sn:0~1.000%, Ti:0~1.000%, Cr:0 ~1.000%, Nb:0~1.000%, Zr:0~1.000%, Mn:0~1.000%, Mo:0~1.000%, Ag:0~1.000 %, Li:0~1.000%, La:0~0.500%, Ce:0~0.500%, B:0~0.500%, Y:0~0.500%, Sr:0~ 0.500%, In: 0~0.500%, Co: 0~0.500%, Bi: 0~0.500%, P: 0~0.500% and W: 0~0.500%] Sb, Pb, Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, Li, La, Ce, B, Y, Sr, In, Co, Bi, P, and W may not be present in the coating layer, but may be present in the coating layer in amounts of 0.0001% or more, 0.001% or more, or 0.01% or more. These elements do not adversely affect the performance of the hot-stamped steel sheet as long as they are within the specified content range. However, excessive content of each element may reduce corrosion resistance. Therefore, the content of Sb, Pb, La, Ce, B, Y, Sr, In, Co, Bi, P, and W is preferably 0.500% or less, and may be, for example, 0.300% or less, 0.100% or less, or 0.050% or less. Similarly, the contents of Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li are preferably 1.000% or less, and may be, for example, 0.800% or less, 0.500% or less, or 0.100% or less.

[0028] The remainder of the plating layer other than the above elements consists of Zn and impurities. The impurities in the plating layer are components that are mixed in due to various factors in the manufacturing process, including raw materials, when the plating layer is produced. The Zn content of the remainder is not particularly limited, but may be, for example, 5.0% or more, 10.0% or more, 15.0% or more, or 20.0% or more.

[0029] [Measurement of the chemical composition of the plating layer] The chemical composition of the plating layer is determined by dissolving only the plating layer in an acidic aqueous solution and chemically analyzing it. Specifically, the plating layer is dissolved in an acidic aqueous solution at room temperature containing an inhibitor that suppresses the dissolution of steel, specifically, 10% hydrochloric acid plus 1% Hibilon (A-6) (manufactured by Sugimura Chemical Industry Co., Ltd.), and the resulting acidic aqueous solution is then analyzed by ICP (inductively coupled plasma atomic emission spectroscopy) to obtain the chemical composition of the plating layer.

[0030] The plating layer is not particularly limited and may be any plating layer having the above chemical composition and containing the Zn-containing phase described below, but may be, for example, a hot-dip plating layer, an alloyed hot-dip plating layer, or the like.

[0031] [Zn-containing phase] In an embodiment of the present invention, the coating layer includes a Zn-containing phase including at least one of an η-Zn phase, an Fe-Zn-based intermetallic compound phase, and an Mg-Zn-based intermetallic compound phase. According to an embodiment of the present invention, as will be described in detail later in connection with a method for producing a hot-stamped steel, by appropriately modifying the surface layer structure of the steel base material before hot stamping, it is believed that alloying of Zn and other elements in the coating layer with Fe in the steel base material can be delayed. Additionally, due to an Al oxide film formed on the surface of the coating layer, evaporation of Zn and / or Mg in the coating layer can be significantly suppressed or reduced, even during high-temperature heating in hot stamping. As a result, the coating layer after hot stamping can include at least one of an η-Zn phase, an Fe-Zn-based intermetallic compound phase, and an Mg-Zn-based intermetallic compound phase, which have excellent corrosion resistance, as a Zn-containing phase. In the present invention, the η-Zn phase refers to a phase mainly composed of Zn and containing other elements such as Fe in a solid solution state, and more specifically, refers to a phase having a Zn concentration of 97 atomic % or more, an Fe concentration of 3 atomic % or less, and other elements of 3 atomic % or less, as measured by a scanning electron microscope with an electron probe microanalyzer (SEM-EPMA). The Fe-Zn intermetallic compound phase is, for example, δ-FeZn 10 phase and Γ-Fe3Zn 10 For example, when the Zn-containing phase in the coating layer includes at least one of an η-Zn phase and an Fe-Zn-based intermetallic compound phase, the corrosion resistance of the hot stamped body can be improved. On the other hand, the Mg-Zn-based intermetallic compound phase can be, for example, an MgZn2 phase, an Mg2Zn3 phase, an MgZn phase, an Mg2Zn 11 phase, and Mg 21 Zn 25The Zn-containing phase may be at least one of the Mg-Zn intermetallic compound phases. The Mg-Zn intermetallic compound phase has higher corrosion resistance than the η-Zn phase and the Fe-Zn intermetallic compound phase. Therefore, in a preferred embodiment of the present invention, the Zn-containing phase in the coating layer includes an Mg-Zn intermetallic compound phase in addition to or instead of at least one of the η-Zn phase and the Fe-Zn intermetallic compound phase, thereby further improving corrosion resistance. In a more preferred embodiment of the present invention, the Zn-containing phase in the coating layer is composed solely of an Mg-Zn intermetallic compound phase. By constituting the Zn-containing phase solely from an Mg-Zn intermetallic compound phase, which has higher corrosion resistance, it is possible to more significantly improve the corrosion resistance of the coating layer. As will be described in detail later in connection with the manufacturing method, the η-Zn phase, the Fe-Zn intermetallic compound phase, and the Mg-Zn intermetallic compound phase can be produced in desired ratios by appropriately controlling the chemical composition of the coating layer in addition to forming an Al oxide film on the surface of the coating layer to suppress or reduce evaporation of Zn and / or Mg during high-temperature heating in hot stamping. For example, by making the chemical composition of the plating layer contain, in mass%, 6.0 to 30.0% Al and 3.00 to 15.00% Mg, it is possible to form at least one of an η-Zn phase and an Fe-Zn intermetallic compound phase as a Zn-containing phase in the plating layer. Similarly, by making the chemical composition of the plating layer contain, in mass%, 20.0 to 30.0% Al and 5.00 to 15.00% Mg, it is possible to form an Mg-Zn intermetallic compound phase as a Zn-containing phase in the plating layer, in particular, to form only an Mg-Zn intermetallic compound phase.

[0032] [Identification of Zn-containing phases] The Zn-containing phase is identified by X-ray diffraction (XRD). Specifically, XRD measurement is performed on the surface of the coating layer using a Cu tube as an X-ray source. If a peak is present within the range of 2θ = 42.9 to 43.6°, the Zn-containing phase in the coating layer is determined to include the η-Zn phase. For the Fe-Zn intermetallic compound phase and the Mg-Zn intermetallic compound phase, if a peak is present within ±0.3° from the diffraction peak with the highest intensity on the ICDD card, the Zn-containing phase in the coating layer is determined to include the Fe-Zn intermetallic compound phase and / or the Mg-Zn intermetallic compound phase.

[0033] [Plating layer adhesion: 30g / m per side 2 End] In the hot stamped steel according to the embodiment of the present invention, the coating weight of the plating layer is 30 g / m per side. 2 That's all. Generally, a coating layer may alloy with the steel base material during high-temperature heating in hot stamping, resulting in a decrease in corrosion resistance. However, according to an embodiment of the present invention, although the reason is not necessarily clear, it is believed that the alloying of the coating layer with the steel base material can be delayed due to the surface layer structure of the steel base material that has been appropriately modified before hot stamping, i.e., a surface layer structure in which the amount of pearlite is relatively small and the amount of coarse pearlite is reduced. Therefore, it is possible to set the coating weight of the coating layer to 30 g / m per side. 2 By controlling the above, it is possible to have a relatively large amount of the coating layer in which alloying has not progressed sufficiently after hot stamping, and it is believed that the presence of such a coating layer and the presence of a specific Zn-containing phase in the coating layer, more specifically the presence of at least one of the η-Zn phase, the Fe-Zn intermetallic compound phase, and the Mg-Zn intermetallic compound phase, makes it possible to maintain sufficient corrosion resistance. On the other hand, if the coating weight of the coating layer is small, the effects related to the delay in alloying as described above cannot be fully obtained, and the corrosion resistance after hot stamping may decrease. From the viewpoint of improving corrosion resistance, the coating weight of the coating layer is preferably 30 g / m per side. 2 or more than 40g / m 2 More preferably, 50 g / m2 or more, and even more preferably 60 g / m 2 More than 80 g / m 2 Although the upper limit is not particularly limited, the coating weight of the plating layer is, for example, 200 g / m 2 Below 190g / m 2 Below 180g / m 2 or less than 170g / m 2 It may be the following:

[0034] [Measurement of plating coating weight] The coating weight of the coating layer is determined by dissolving only the coating layer in an acidic aqueous solution. That is, a 30 mm x 30 mm sample is taken from the hot-stamped body, and the coating layer is dissolved in an acidic aqueous solution at room temperature containing an inhibitor that suppresses steel dissolution—specifically, 10% hydrochloric acid plus 1% Hibilon (A-6) (manufactured by Sugimura Chemical Industry Co., Ltd.). The coating weight of the coating layer is determined from the change in weight of the sample before and after dissolution of the coating layer.

[0035] [Average carbon concentration from the surface of the steel base material to a depth of 1 μm: 0.25 mass% or less] In the hot-stamped steel according to an embodiment of the present invention, the average C concentration from the surface of the steel base material to 1 μm in the depth direction is 0.25 mass% or less. As will be described in detail later in connection with the manufacturing method of the hot-stamped steel, the structure associated with the low carbon concentration in the surface layer of the steel base material before hot stamping, i.e., the surface layer structure having a relatively low amount of pearlite and a reduced amount of coarse pearlite, significantly suppresses recarburization during high-temperature heating in hot stamping. In connection with this, the average C concentration from the surface of the steel base material to 1 μm in the depth direction is reduced to 0.25 mass% or less in the finally obtained hot-stamped steel. As a result, the LME suppression effect due to the initial low carbon concentration in the surface layer of the steel base material can be fully exerted, making it possible to reliably suppress or reduce the occurrence of LME cracking during spot welding after hot stamping. Therefore, from the viewpoint of improving LME resistance during spot welding after hot stamping, the lower the average C concentration from the surface of the steel base material to 1 μm in the depth direction, the better, and it may be, for example, 0.22 mass% or less, 0.20 mass% or less, 0.18 mass% or less, 0.15 mass% or less, 0.12 mass% or less, 0.10 mass% or less, 0.08 mass% or less, or 0.06 mass% or less. Although there is no particular lower limit, for example, the average C concentration from the surface of the steel base material to 1 μm in the depth direction may be 0.001 mass% or more, 0.005 mass% or more, or 0.01 mass% or more.

[0036] [Measurement of the average carbon concentration from the surface of the steel base material to a depth of 1 μm] The average carbon concentration from the surface of the steel substrate to a depth of 1 μm is determined using a high-frequency glow discharge optical emission spectrometer (GDS) as follows. Specifically, the surface of the hot-stamped compact is placed in an Ar atmosphere, and a voltage is applied to generate glow plasma. The surface of the hot-stamped compact is then sputtered and analyzed in the depth direction. The elements contained in the material are then identified from the element-specific emission spectrum wavelengths emitted by excited atoms in the glow plasma, and the emission intensity of the identified elements is estimated. Depth data can be estimated from the sputtering time. Specifically, the relationship between sputtering time and sputtering depth can be calculated in advance using a standard sample, allowing the sputtering time to be converted to sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the surface of the material. The obtained emission intensity is converted to mass% by creating a calibration curve. When the hot stamped body is subjected to GDS measurement in this manner, the position where the Zn concentration in the depth direction becomes 0.1% or less is determined to be the surface of the steel base material, and the average C concentration in the region from the surface to 1 μm in the depth direction is determined to be the "average C concentration from the surface of the steel base material to 1 μm in the depth direction."

[0037] As described above, when hot stamping a plated steel sheet having a conventional Zn-containing coating, Fe in the steel base material diffuses into the coating layer during high-temperature heating during hot stamping, while C in the steel base material does not diffuse into the coating layer. This results in a relative enrichment of C relative to Fe near the surface of the steel base material. Therefore, even if the surface layer of the steel base material has a low carbon concentration by decarburization or the like before hot stamping, the C concentration in the surface layer of the steel base material after hot stamping may be much higher than the C concentration in the bulk. However, according to an embodiment of the present invention, the diffusion of C from the bulk to the surface layer is suppressed due to the suppression of recarburization, and therefore the enrichment of C near the surface of the steel base material can be significantly suppressed or reduced compared to conventional plated steel sheets. From the viewpoint of improving LME resistance, it is preferable that the average C concentration within 1 μm from the surface of the steel base material in the depth direction is equal to or lower than the C content of the steel base material. More specifically, the average C concentration from the surface of the steel base metal to 1 μm in the depth direction is less than 1.10 times the C content of the steel base metal, and may be, for example, 1.05 times or less, 1.00 times or less, 0.90 times or less, 0.80 times or less, 0.70 times or less, 0.60 times or less, or 0.50 times or less. The lower limit is not particularly limited, and, for example, the average C concentration from the surface of the steel base metal to 1 μm in the depth direction may be 0.05 times or more, 0.10 times or more, or 0.15 times or more the C content of the steel base metal. In the present invention, the "C content of the steel base metal" refers to a value measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES) of chips in accordance with JIS G 1201:2014 using a test specimen obtained from approximately halfway through the thickness of the steel base metal.

[0038] [Solid solution layer] In an embodiment of the present invention, the hot-stamped steel may further include a solid solution layer between the steel base material and the coating layer. Depending on the coating composition before hot stamping and the hot stamping conditions, a solid solution may be formed between Fe diffusing from the steel base material to the coating layer and Al and / or Zn in the coating layer, and a solid solution layer containing a solid solution of these elements may be formed between the steel base material and the coating layer after hot stamping. Therefore, when the hot-stamped steel further includes a solid solution layer, the solid solution layer contains Fe and one or both of Al and Zn. More specifically, the solid solution layer includes an Fe-Al solid solution layer, an Fe-Zn solid solution layer, and / or an Fe-Al-Zn solid solution layer.

[0039] The presence of a solid solution layer can be confirmed as follows. First, the plating layer alone is dissolved in an acidic aqueous solution containing an inhibitor that suppresses steel dissolution—specifically, 10% hydrochloric acid plus 1% Hibilon (A-6) (manufactured by Sugimura Chemical Industry Co., Ltd.)—at room temperature. Next, using a high-frequency glow discharge optical emission spectrometer (GDS), the surface of the hot-stamped compact is placed in an Ar atmosphere, and a voltage is applied to generate glow plasma. The surface of the hot-stamped compact is then analyzed in the depth direction while sputtering. The elements contained in the material are then identified from the element-specific emission spectrum wavelengths emitted by excited atoms in the glow plasma, and the emission intensity of the identified elements is estimated. Depth data can be estimated from the sputtering time. Specifically, the relationship between sputtering time and sputtering depth can be determined in advance using a standard sample, allowing the sputtering time to be converted to sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the surface of the material. The obtained emission intensity is converted to mass% by creating a calibration curve. When the hot stamped body is subjected to GDS measurement in this manner, the region in the depth direction where the Zn concentration exceeds 0.1% is determined to be the solid solution layer.

[0040] [Martensite area ratio of steel base material: 90% or more] The steel base material of the hot-stamped steel according to the embodiment of the present invention contains 90% or more martensite in terms of area fraction. The remaining structure is not particularly limited, but may consist of at least one of bainite, ferrite, retained austenite, and pearlite in an amount of 10% or less. Martensite is a very hard structure, and therefore, by containing 90% or more martensite in terms of area fraction in the hot-stamped steel, high strength, specifically a Vickers hardness of 400 HV, can be achieved. On the other hand, if the area fraction of martensite is low and the proportion of soft structures such as ferrite is high, it may be impossible to achieve a Vickers hardness of 400 HV. Therefore, a larger area fraction of martensite is preferable, and may be, for example, 92% or more, 94% or more, 96% or more, or 98% or more. The upper limit of the area fraction of martensite is not particularly limited and may be 100%.

[0041] [Identification of martensite and calculation of area fraction] The identification and calculation of martensite area fraction are performed as follows. First, a sample is taken so that the cross section of the hot-stamped steel sheet parallel to the rolling direction and thickness direction serves as the observation surface. The observation surface is then mirror-polished and etched with nital etchant, after which the microstructure is observed using a scanning electron microscope (SEM). A 300 μm × 300 μm area is photographed at 1000x magnification at a depth of 1 / 4 the thickness of the observation surface. The microstructure photograph is then binarized to black and white and then analyzed to identify pearlite, bainite, and ferrite. The total area fraction of these components is then calculated using a method based on the "Method for Microscopic Testing of Grain Size in Steels" specified in JIS G 0551:2020. Because retained austenite is difficult to distinguish from martensite using an SEM, the area fraction of retained austenite is measured using X-ray diffraction. Finally, the area fraction of martensite is determined by subtracting the total area fraction of pearlite, bainite, ferrite, and retained austenite obtained by the above method from 100%.

[0042] [Mechanical properties] The hot-stamped steel according to the embodiment of the present invention can achieve excellent mechanical properties, such as a Vickers hardness of 400 HV or more, more specifically, a Vickers hardness of 400 HV or more at a position halfway through the thickness of the steel base material. The Vickers hardness is preferably 500 HV or more, and more preferably 550 HV or more. There is no particular upper limit, but the Vickers hardness may be, for example, 650 HV or less or 600 HV or less.

[0043] [Hardness measurement] Vickers hardness is determined as follows. First, a test piece is cut from any position of the hot-stamped body, excluding the edge, so that a cross section perpendicular to the surface (thickness cross section) can be observed. The thickness cross section of the test piece 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 particle size of 1 to 6 μm is dispersed in alcohol or pure water. This thickness cross section serves as the measurement surface. Next, Vickers hardness is measured using a micro-Vickers hardness tester at a load of 1 kgf at intervals of at least three times the indentation. A total of 20 measurements are taken randomly around half the thickness of the steel base material, so as not to include the low-carbon surface layer, and the arithmetic average of these measurements is determined as the hardness of the hot-stamped body.

[0044] [Preferred chemical composition of steel base material] The chemical composition of the steel base material according to the embodiment of the present invention may be any chemical composition that is generally applied to hot stamped bodies and that can achieve a Vickers hardness of 400 HV or more. Preferred chemical compositions of the steel base material will be described in detail below, but these descriptions are intended to merely exemplify preferred chemical compositions of steel base materials that are suitable for achieving a Vickers hardness of 400 HV or more, and are not intended to limit the present invention to those that use steel base materials having such specific chemical compositions.

[0045] In an embodiment of the present invention, for example, the steel base material contains, in mass %, C: 0.13~0.50%, Si: 0.001 to 3.000%, Mn: 0.30-3.00%, Al: 0.0002 to 2.000%, P: 0.100% or less, S: 0.1000% or less, N: 0.0100% or less, Nb: 0 to 0.15%, Ti: 0 to 0.15% V: 0~0.15%, Mo: 0-1.0% Cr: 0 to 1.0%, Cu: 0-1.0% Ni: 0 to 1.0% B: 0~0.0100%, W: 0 to 1.000%, Hf: 0 to 0.050%, Mg: 0 to 0.050% Zr: 0 to 0.050%, Ca: 0 to 0.010% REM: 0~0.30%, Ir: 0 to 1.000%, and Remainder: Fe and impurities It is preferable that the metal has a chemical composition consisting of the following: Each element will be described in more detail below.

[0046] [C:0.13~0.50%] C is an element that inexpensively increases tensile strength and is an important element for controlling the strength of steel. To fully obtain this effect, the C content is preferably 0.13% or more. The C content may be 0.15% or more, 0.20% or more, 0.30% or more, or 0.35% or more. On the other hand, excessive C content may result in a decrease in elongation. For this reason, the C content is preferably 0.50% or less. The C content may be 0.45% or less, or 0.40% or less.

[0047] [Si: 0.001 to 3.000%] Si acts as a deoxidizer and is an element that suppresses the precipitation of carbides during the cooling process during annealing of cold-rolled sheets. To fully obtain this effect, the Si content is preferably 0.001% or more. The Si content may be 0.010% or more, 0.100% or more, or 0.200% or more. On the other hand, excessive Si content may increase the steel strength but decrease the elongation. For this reason, the Si content is preferably 3.000% or less. The Si content may be 2.500% or less, 2.000% or less, 1.500% or less, or 1.000% or less.

[0048] [Mn: 0.30~3.00%] Mn is an element that improves the hardenability of steel and is effective in increasing strength. To fully obtain this effect, the Mn content is preferably 0.30% or more. The Mn content may be 0.50% or more, 1.00% or more, or 1.30% or more. On the other hand, excessive Mn content may increase the steel strength but decrease the elongation. For this reason, the Mn content is preferably 3.00% or less. The Mn content may be 2.80% or less, 2.50% or less, or 2.00% or less.

[0049] [Al: 0.0002~2.000%] Al acts as a deoxidizer for steel and improves the quality of the steel. To fully achieve this effect, the Al content is preferably 0.0002% or more. The Al content may be 0.001% or more, 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, excessive Al content may produce coarse Al oxides, reducing the elongation of the steel material. For this reason, the Al content is preferably 2.000% or less. The Al content may be 1.500% or less, 1.000% or less, 0.800% or less, or 0.500% or less.

[0050] [P:0.100% or less] P is an element that segregates at grain boundaries and promotes embrittlement of steel. The lower the P content, the better, and ideally it is 0%. However, excessive reduction in the P content may result in a significant increase in costs. For this reason, the P content may be 0.0001% or more, or may be 0.001% or more, or 0.005% or more. On the other hand, excessive P content may result in embrittlement of steel due to grain boundary segregation, as described above. Therefore, the P content is preferably 0.100% or less. The P content may also be 0.050% or less, 0.030% or less, or 0.010% or less.

[0051] [S:0.1000% or less] S is an element that generates nonmetallic inclusions such as MnS in steel, reducing the ductility of steel parts. Since a lower S content is preferable, ideally 0%. However, excessive reduction in the S content can result in a significant increase in costs. For this reason, the S content may be 0.0001% or more, or may be 0.0002% or more, 0.0010% or more, or 0.0050% or more. On the other hand, excessive S content can cause cracks to occur originating from nonmetallic inclusions during cold forming. Therefore, the S content is preferably 0.1000% or less. The S content may be 0.0500% or less, 0.0200% or less, or 0.0100% or less.

[0052] [N:0.0100% or less] N is an element that forms coarse nitrides in steel and reduces the workability of the steel. The lower the N content, the better, and ideally it is 0%. However, excessive reduction in the N content may result in a significant increase in manufacturing costs. For this reason, the N content may be 0.0001% or more, or may be 0.0005% or more, or 0.0010% or more. On the other hand, excessive N content may form coarse nitrides as described above, reducing the workability of the steel. Therefore, the N content is preferably 0.0100% or less. The N content may be 0.0080% or less, or 0.0050% or less.

[0053] The preferred basic chemical composition of the steel base material is as described above. Furthermore, the steel base material may optionally contain, in place of a portion of the remaining Fe, one or more elements selected from the group consisting of 0-0.15% Nb, 0-0.15% Ti, 0-0.15% V, 0-1.0% Mo, 0-1.0% Cr, 0-1.0% Cu, 0-1.0% Ni, 0-0.0100% B, 0-1.000% W, 0-0.050% Hf, 0-0.050% Mg, 0-0.050% Zr, 0-0.010% Ca, 0-0.30% REM, and 0-1.000% Ir. The content of each of these elements may be 0.0001% or more, 0.0005% or more, 0.001% or more, or 0.01% or more.

[0054] The remainder of the steel base material other than the above elements consists of Fe and impurities. The impurities in the steel base material 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 base material is industrially manufactured.

[0055] The chemical composition of the steel base material can be measured using common analytical methods. For example, the chemical composition of the steel base material can be determined by first removing the coating layer by mechanical grinding, and then measuring the chips using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) in accordance with JIS G 1201:2014. Specifically, for example, a 35 mm square test piece is obtained from approximately half the thickness of the steel base material and measured using a Shimadzu ICPS-8100 or other measuring device under conditions based on a pre-established calibration curve. Carbon and sulfur, which cannot be measured by ICP-AES, can be measured using the combustion-infrared absorption method, nitrogen using the inert gas fusion-thermal conductivity method, and oxygen using the inert gas fusion-non-dispersive infrared absorption method.

[0056] <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.

[0057] The hot-stamped steel according to an embodiment of the present invention can be manufactured by, for example, a casting process in which molten steel having an adjusted chemical composition is cast to form a slab, a hot rolling process in which the slab is hot-rolled to obtain a hot-rolled steel sheet, a coiling process in which the hot-rolled steel sheet is coiled, a cold rolling process in which the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet, an annealing process in which the cold-rolled steel sheet is annealed, a cooling process in which the annealed cold-rolled steel sheet is cooled, a plating process in which a plating layer is formed on the obtained steel base material, and a hot stamp forming process in which the obtained plated steel sheet is hot-stamped. Alternatively, the steel may be pickled after the hot rolling process without being coiled, and then directly subjected to the cold rolling process. Each process will be described in detail below.

[0058] [Casting process] The conditions for the casting process are not particularly limited. For example, after melting in a blast furnace or electric furnace, various secondary smelting processes may be carried out, and then casting may be carried out by a conventional method such as continuous casting or ingot casting.

[0059] [Hot rolling process] A hot-rolled steel sheet can be obtained by hot-rolling the cast steel slab. The hot-rolling step is carried out by reheating the cast steel slab directly or after cooling it once, and then hot-rolling it. When reheating is carried out, the heating temperature of the steel slab may be, for example, 1100 to 1250°C. In the hot-rolling step, rough rolling and finish rolling are usually carried out. The temperature and reduction ratio of each rolling step can be appropriately determined depending on the desired metal structure and plate thickness. For example, the end temperature of finish rolling may be 900 to 1050°C, and the reduction ratio of finish rolling may be 10 to 50%.

[0060] [Winding process] The hot-rolled steel sheet can be coiled at a predetermined temperature. The coiling temperature can be appropriately determined depending on the desired metal structure, etc., and may be, for example, 500 to 800°C. The hot-rolled steel sheet may be recoiled before or after coiling and subjected to a predetermined heat treatment. Alternatively, the hot-rolled steel sheet may be pickled after the hot rolling step and then subjected to the cold rolling step described below without performing the coiling step.

[0061] [Cold rolling process] After subjecting the hot-rolled steel sheet to pickling or the like, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The reduction ratio in cold rolling can be appropriately determined depending on the desired metal structure and sheet thickness, and may be, for example, 20 to 80%. After the cold-rolling step, the sheet may be air-cooled to room temperature, for example.

[0062] [Annealing process] Next, the obtained cold-rolled steel sheet is annealed. The annealing process involves heating the cold-rolled steel sheet to a temperature of 730 to 900°C in an atmosphere with a dew point of -20 to 10°C and holding the temperature for 10 to 300 seconds. By performing the annealing process under such relatively high dew point conditions, it is possible to appropriately decarburize the surface layer of the cold-rolled steel sheet. This decarburization process makes it possible to form a region with a relatively low amount of pearlite in the surface layer of the steel base material in the plated steel sheet before hot stamping. More specifically, it is possible to form a depth region of 3 μm or more in which the area ratio of pearlite is 20% or less from the surface of the steel base material in the sheet thickness direction. By forming such a structure with a relatively low amount of pearlite in the surface layer of the steel base material, it is possible to reliably suppress the average C concentration from the surface of the steel base material to 1 μm in the depth direction in the finally obtained hot-stamped product to 0.25 mass% or less. If the dew point is lower than -20°C, the heating temperature is lower than 730°C, and / or the holding time is shorter than 10 seconds, decarburization in the surface layer of the cold-rolled steel sheet will be insufficient. As a result, it will be impossible to form a region with a relatively low amount of pearlite in the surface layer of the steel base material. On the other hand, if the dew point is higher than 10°C, the heating temperature is higher than 900°C, and / or the holding time is longer than 300 seconds, an outer oxide layer will form on the steel sheet surface, which may result in a decrease in platability or a decrease in the strength of the final hot-stamped steel sheet due to excessive decarburization. The dew point is preferably -10 to 5°C, more preferably -5 to 5°C. The atmosphere in the annealing step may be a reducing atmosphere, more specifically a reducing atmosphere containing nitrogen and hydrogen, for example, a reducing atmosphere of 1 to 10% hydrogen (e.g., 4% hydrogen and the balance nitrogen).

[0063] [Cooling process] The cold-rolled steel sheet whose surface layer has been decarburized in the annealing process must be appropriately cooled in the subsequent cooling process to obtain the desired surface layer structure. Specifically, the cooling process includes cooling from the heating temperature of the annealing process to a controlled temperature of 620 to 670°C at an average cooling rate of 20°C / s or more (primary cooling), and cooling from the controlled temperature to a coating bath temperature (e.g., the melting point of the coating bath + 20°C) at an average cooling rate of 10°C / s or less (secondary cooling). The primary cooling and secondary cooling will be described in more detail below.

[0064] [Primary cooling] In the primary cooling, it is important to suppress pearlite precipitation at high temperatures. More specifically, pearlite precipitates at high temperatures from the heating temperature of 730 to 900°C in the annealing process to the control temperature of 620 to 670°C. Because of its fast diffusion, it easily diffuses to grain boundaries and forms pearlite along the grain boundaries. This pearlite formed along the grain boundaries transforms to austenite during high-temperature heating in hot stamping, forming austenite-mediated carbon recarburization paths along the grain boundaries and promoting the recarburization of carbon in the bulk to the surface layer of the steel base material. Therefore, in the temperature range from the heating temperature in the annealing process to the control temperature, it is extremely important to suppress pearlite precipitation at high temperatures in the surface layer of the steel base material by cooling the cold-rolled steel sheet at a relatively fast average cooling rate of 20°C / s or more. If the average cooling rate is less than 20°C / s and / or the control temperature is higher than 670°C, pearlite precipitates at high temperatures where diffusion is fast, promoting the formation of pearlite along the grain boundaries. In relation to this, in the plated steel sheet before hot stamping, it becomes impossible to reduce the amount of coarse pearlite in the surface layer of the steel base material. More specifically, a large amount of coarse pearlite with a circle equivalent diameter of 5 μm or more forms along the grain boundaries in the surface layer of the steel base material. If the amount of such coarse pearlite formed along the grain boundaries increases, the formation of carbon recarburization paths by austenite along the grain boundaries is promoted during high-temperature heating in hot stamping. As a result, it becomes impossible to suppress the average C concentration in the steel base material from the surface to 1 μm in the depth direction to 0.25 mass% or less in the finally obtained hot-stamped product, and sufficient LME resistance cannot be achieved during spot welding.

[0065] [Secondary cooling] On the other hand, in the secondary cooling after the primary cooling, it is important to precipitate pearlite at a low temperature where diffusion is relatively slow. More specifically, pearlite can be precipitated by cooling from a controlled temperature of 620 to 670°C to the plating bath temperature (for example, the melting point of the plating bath + 20°C) at an average cooling rate of 10°C / s or less. Since the diffusion of pearlite precipitated in such a low temperature range below the controlled temperature is relatively slow, it is not formed in a form connected along the grain boundaries, but rather the pearlite can be dispersed and present on the grain boundaries. In such a structure, even during high-temperature heating in hot stamping, A c1 At or above this point, austenite transformed from pearlite can be dispersed and present on the grain boundaries in the same way, so it is possible to reliably cut off the recarburization pathway of carbon by austenite. On the other hand, if the average cooling rate is more than 10°C / s and / or the control temperature is less than 620°C, martensite and bainite will mainly precipitate instead of pearlite. Martensite and bainite have a faster transformation rate to austenite than pearlite, and c1 The ferrite-austenite two-phase structure is exposed to high temperatures for a longer period of time during hot stamping than pearlite. In such cases, recarburization paths are likely to form at the grain boundaries, making it difficult to achieve sufficient LME resistance.

[0066] [Plating process] Next, in the plating step, a plating layer is formed on at least one, preferably both, surfaces of the cold-rolled steel sheet (base steel). More specifically, the plating step is performed by hot-dip plating using a plating bath having a predetermined chemical composition (bath temperature: e.g., 420 to 480°C), and an alloying treatment may be performed after the hot-dip plating. The plating process is not limited to hot-dip plating, and may be electroplating, vapor deposition plating, thermal spraying, cold spraying, or the like. Other conditions for the plating step may be appropriately set taking into consideration the thickness and coating weight of the plating layer. For example, after immersing a cold-rolled steel sheet in a plating bath, it is pulled out and immediately sprayed with N2 gas or air by gas wiping, followed by cooling, thereby achieving a coating weight of the plating layer within a predetermined range, e.g., 30 to 200 g / m per side. 2 can be adjusted within the range.

[0067] [Cooling after plating] During cooling after plating, it is necessary to control the dew point of the cooling gas (e.g., nitrogen gas) within the range of −10 to 10°C. Cooling the plated steel sheet in such an atmosphere with a relatively high dew point allows a relatively thick Al oxide film to be formed on the surface of the plating layer. Due to the formation of this Al oxide film, evaporation of Zn and / or Mg in the plating layer can be significantly suppressed or reduced, even during high-temperature heating in hot stamping. As a result, the plating layer after hot stamping can reliably contain at least one of the η-Zn phase, Fe-Zn intermetallic compound phase, and Mg-Zn intermetallic compound phase, which have excellent corrosion resistance, as a Zn-containing phase. Furthermore, due to the suppression of evaporation of Zn and / or Mg in the plating layer, the Fe content in the plating layer of the obtained hot-stamped product can be relatively reduced, thereby further improving the corrosion resistance after hot stamping. By controlling the dew point of the cooling gas during cooling after plating within the range of −10 to 10°C and appropriately adjusting the chemical composition of the plating layer, it is possible to form a desired Zn-containing phase in the plating layer. For example, in addition to the dew point control, by making the chemical composition of the coating layer contain, by mass%, 6.0 to 30.0% Al and 3.00 to 15.00% Mg, it is possible to form at least one of an η-Zn phase and an Fe-Zn intermetallic compound phase as a Zn-containing phase in the coating layer. Similarly, by making the chemical composition of the coating layer contain, by mass%, 20.0 to 30.0% Al and 5.00 to 15.00% Mg, it is possible to form an Mg-Zn intermetallic compound phase as a Zn-containing phase in the coating layer, in particular, to form only an Mg-Zn intermetallic compound phase.

[0068] [Hot stamping process] Finally, the resulting plated steel sheet is hot stamped in a hot stamping process to produce a hot-stamped product having the desired surface layer composition and hard structure. From the perspective of obtaining the desired hard structure, it is preferable to load the plated steel sheet into a furnace at 800 to 1000°C, and after the temperature of the plated steel sheet reaches a predetermined temperature, for example, a furnace temperature of −10°C, hold it in the furnace for 60 to 600 seconds. If the heating temperature is less than 800°C and / or the holding time is less than 60 seconds, austenitization will be insufficient, making it impossible to obtain the desired area ratio of hard structure (i.e., an area ratio of martensite of 90% or more), and the final hot-stamped product may not achieve a Vickers hardness of 400 HV or more. 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. After being heated and maintained in the furnace, the plated steel sheet is removed from the furnace, and then, after the plated steel sheet reaches a predetermined temperature, for example, a predetermined temperature of 850° C. or less, hot stamp forming can be performed under normal conditions. After hot stamp forming, the plated steel sheet may be cooled, for example, to a temperature range of 250° C. or less at an average cooling rate of 20° C. / second or more, although this is not particularly limited.

[0069] 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]

[0070] In the following examples, hot stamped steel sheets according to the embodiments of the present invention were produced under various conditions, and the properties of the produced hot stamped steel sheets were investigated.

[0071] First, molten steel was cast by continuous casting to form a steel billet having the chemical composition shown in Table 1. The steel billet was once cooled, reheated to 1200°C, hot-rolled, and then coiled at a temperature of 600°C or less. Hot rolling was performed by rough rolling and finish rolling, with the finish rolling ending at a temperature of 900 to 1050°C and a reduction of 30%. Next, the obtained hot-rolled steel sheet was pickled and then cold-rolled at a reduction of 50% to obtain a cold-rolled steel sheet with a thickness of 1.6 mm. Next, the obtained cold-rolled steel sheet was subjected to an annealing process under the conditions shown in Table 2 in a mixed gas atmosphere of 4% hydrogen and the balance nitrogen in a furnace with an oxygen concentration of 20 ppm or less, and then a cooling process was similarly performed under the conditions shown in Table 2 to produce a steel base material.

[0072] Next, the produced steel base material was cut into 100 mm x 200 mm pieces and plated using a batch-type hot-dip plating test device manufactured in-house. More specifically, the produced steel base material was first immersed in a plating bath with a predetermined chemical composition for approximately 3 seconds, then pulled up at a pulling speed of 20 to 200 mm / s, and the coating weight of the coating layer was adjusted to the value shown in Table 2 using N2 gas wiping. Next, the steel base material with the coating layer attached was cooled from the plating bath temperature (approximately 420 to 480°C) to room temperature using nitrogen gas controlled to the dew point shown in Table 2 as a cooling gas, thereby obtaining a plated steel sheet with a coating layer formed on both sides of the steel base material. The sheet temperature was measured using a thermocouple spot-welded to the center of the steel base material.

[0073] [Chemical composition analysis of plating layer] The chemical composition of the coating layer was determined as follows. First, the coated steel sheet was placed in an atmospheric heating furnace at 900°C. After the temperature of the coated steel sheet reached the furnace temperature of -10°C, it was held there for 100 seconds. Next, the coated steel sheet was removed from the furnace and clamped between flat molds at approximately room temperature for quenching. After heating and quenching, the coated steel sheet was cut into 30 mm x 30 mm pieces. The cut samples were immersed in an acidic aqueous solution at room temperature, consisting of 10% hydrochloric acid and 1% Hibilon (A-6) (manufactured by Sugimura Chemical Industry Co., Ltd.). The coating layer was then pickled and removed. The coating components dissolved in the solution were then measured by ICP emission spectroscopy to determine the composition. The results are shown in Table 2.

[0074] [Evaluation of LME resistance during spot welding] First, a plated steel sheet was placed in an atmospheric heating furnace at 900°C. After the plated steel sheet reached a furnace temperature of -10°C, it was held there for 100 seconds. Next, the plated steel sheet was removed from the furnace and quenched by clamping it between flat molds at room temperature. Two 50mm x 100mm samples of the plated steel sheet after heating and quenching were prepared. These two plated steel sheet samples were spot welded to form welded joints using a dome radius welding electrode with a tip diameter of 8mm at a 2° impact angle, 4.0kN pressure, 0.5s welding time, and 12kA current. Next, the length of the LME crack that occurred directly below the electrode in the weld was measured, and LME resistance was evaluated as follows. AAA: 0 μm AA: More than 0~20μm A: Over 20 μm to less than 80 μm B:80μm or more

[0075] [Corrosion resistance evaluation] The corrosion resistance after hot stamping was evaluated as follows. First, the plated steel sheet was placed in an atmospheric heating furnace at 900°C. After the temperature of the plated steel sheet reached the furnace temperature of -10°C, it was held there for 100 seconds. Next, the plated steel sheet was removed from the furnace and clamped between flat molds at approximately room temperature for quenching. After heating and quenching, 50 mm x 100 mm samples of the hot stamped steel were treated with zinc phosphate (SD5350 system: Nippon Paint Industrial Coating Co., Ltd. standard) and then electrocoated (PN110 Powernics Gray: Nippon Paint Industrial Coating Co., Ltd. standard) to a thickness of 20 μm, followed by baking at 150°C for 20 minutes. Next, a cut was made in the center of the sample, reaching the base steel (steel material). A cyclic corrosion test was then conducted in accordance with JASO (M609-91) to measure the number of cycles required for red rust to form at the cut. Corrosion resistance was evaluated as follows. AAA: Over 240 cycles AA: 180-240 cycles A: 90 to less than 180 cycles B: Less than 90 cycles

[0076] [Evaluation of hardness after hot stamping (HS)] First, as in the corrosion resistance evaluation, a plated steel sheet was placed in an atmospheric heating furnace at 900°C. After the temperature of the plated steel sheet reached the furnace temperature of -10°C, it was held there for 100 seconds. Next, the plated steel sheet was removed from the furnace, sandwiched between flat molds at approximately room temperature, and quenched to obtain a hot-stamped product (HS condition A). On the other hand, a hot-stamped product of Comparative Example 41 was obtained (HS condition B) under the same conditions as HS condition A, except that the plated steel sheet was placed in an atmospheric heating furnace at 700°C. Test specimens were cut from any position of the obtained hot-stamped product, excluding the edge, so that a cross section perpendicular to the surface (thickness cross section) could be observed. The thickness cross section of the test specimen was polished using #600 to #1500 silicon carbide paper and then mirror-finished using a diluted solution such as alcohol or a liquid prepared by dispersing diamond powder with a particle size of 1 to 6 μm in pure water. This thickness cross section served as the measurement surface. Next, Vickers hardness was measured at intervals of at least three times the indentation using a micro Vickers hardness tester under a load of 1 kgf. A total of 20 measurements were taken at random points near half the thickness of the steel base material, so as not to include the surface layer with a reduced carbon concentration. The arithmetic mean of these measurements was determined as the hardness after hot stamping (HS), and was evaluated as follows. AAA: Hardness after HS is over 550HV AA: Hardness after HS is over 500 to 550HV A: Hardness after HS is 400-500HV B: Hardness after HS is less than 400HV

[0077] [Identification of Zn-containing phases] The Zn-containing phase was identified by X-ray diffraction (XRD). Specifically, XRD measurement was performed on the surface of the coating layer using a Cu tube as an X-ray source. If a peak was present within the range of 2θ = 42.9 to 43.6°, the Zn-containing phase in the coating layer was determined to include the η-Zn phase. Regarding the Fe-Zn-based intermetallic compound phase and the Mg-Zn-based intermetallic compound phase, if a peak was present within a range of ±0.3° from the diffraction peak with the highest intensity in the ICDD card, the Zn-containing phase in the coating layer was determined to include the Fe-Zn-based intermetallic compound phase and / or the Mg-Zn-based intermetallic compound phase. Table 2 only shows the presence or absence of at least one of the η-Zn phase and the Fe-Zn-based intermetallic compound phase, and the Mg-Zn-based intermetallic compound phase. However, for example, in Examples 1 and 13, the η-Zn phase and the δ-FeZn 10 phase and Γ-Fe3Zn 10 The presence of the Fe-Zn intermetallic compound phase was confirmed, and in Examples 2 and 14, the η-Zn phase and the Γ-Fe3Zn 10 The presence of the phases was confirmed. In Examples 5 and 17, the η-Zn phase and the MgZn2 phase were detected. In Examples 9 and 20, the MgZn2 phase was detected, and in Examples 12 and 19, the MgZn2 phase, the Mg2Zn3 phase, and the MgZn phase were detected.

[0078] Hot stamped steel sheets with a hardness rating of AAA, AA, or A after HS, LME resistance rating of AAA, AA, or A, and corrosion resistance rating of AAA, AA, or A were evaluated as having high strength, maintaining high corrosion resistance even after hot stamping, and being capable of suppressing LME cracking during spot welding. The results are shown in Table 2. In the hot stamped steel sheets shown in Table 2, the remaining structure other than martensite was bainite, ferrite, retained austenite, and / or pearlite.

[0079] [Table 1]

[0080] [Table 2-1]

[0081]

Table 2-2

[0082] Referring to Table 2, in Comparative Example 37, although the LME resistance and corrosion resistance were good, the hardness after HS was low, and therefore the desired high strength could not be achieved. In Comparative Example 38, the heating temperature in the annealing step was low, which is thought to have resulted in insufficient decarburization in the surface layer of the cold-rolled steel sheet. As a result, the average C concentration from the surface of the steel base material to 1 μm in the depth direction was high, and LME resistance was reduced. In Comparative Example 39, the holding time in the annealing step was short, which is thought to have similarly resulted in insufficient decarburization in the surface layer of the cold-rolled steel sheet. As a result, the average C concentration from the surface of the steel base material to 1 μm in the depth direction was high, and LME resistance was reduced. In Comparative Example 40, the dew point in the annealing step was low, which is thought to have similarly resulted in insufficient decarburization in the surface layer of the cold-rolled steel sheet. As a result, the average C concentration from the surface of the steel base material to 1 μm in the depth direction was high, and LME resistance was reduced. Cross-sectional observation of the plated steel sheets according to Comparative Examples 38 to 40 before hot stamping using an SEM revealed that the depth region where the area ratio of pearlite was 20% or less from the surface of the steel base material in the sheet thickness direction was less than 3 μm in all cases. In Comparative Example 41, the average cooling rate of the primary cooling in the cooling process was low, which is thought to have caused pearlite to precipitate at high temperatures and form along grain boundaries, thereby accelerating recarburization during high-temperature heating in hot stamping. As a result, the average C concentration was high from the surface of the steel base material to 1 μm in the depth direction, resulting in reduced LME resistance. In Comparative Example 42, the controlled temperature for the primary cooling in the cooling process was high, which is thought to have caused pearlite to precipitate at high temperatures and form along grain boundaries, thereby accelerating recarburization during high-temperature heating in hot stamping. As a result, the average C concentration was high from the surface of the steel base material to 1 μm in the depth direction, resulting in reduced LME resistance. Cross-sections of the plated steel sheets before hot stamping according to Comparative Examples 41 and 42 were observed by SEM, and a relatively large amount of coarse pearlite particles with a circle equivalent diameter of 5 μm or more was found to have formed in the surface layer of the steel base material. In Comparative Example 43, the controlled temperature of the secondary cooling in the cooling step was low, which is thought to have resulted in the precipitation of mainly bainite rather than pearlite, and thus promoted recarburization during high-temperature heating in hot stamping.As a result, the average C concentration from the surface of the steel base material to 1 μm in the depth direction increased, resulting in reduced LME resistance. In Comparative Example 44, the average cooling rate of the secondary cooling in the cooling process was fast, which similarly led to the precipitation of bainite rather than pearlite, presumably promoting recarburization during high-temperature heating in hot stamping. As a result, the average C concentration from the surface of the steel base material to 1 μm in the depth direction increased, resulting in reduced LME resistance. In Comparative Example 45, the hot stamping conditions were inappropriate, resulting in insufficient austenitization, making it impossible to achieve the desired martensite area ratio, and reducing hardness after HS. In Comparative Example 46, the coating weight of the coating layer was 30 g / m per side. 2 However, it is believed that the low dew point of the cooling gas after plating prevented the sufficient formation of an Al oxide film on the surface of the plating layer. As a result, the evaporation of Zn and / or Mg in the plating layer could not be sufficiently suppressed or reduced, and in connection with this, the specific Zn-containing phase could not be formed in the plating layer, resulting in a decrease in corrosion resistance after HS.

[0083] In contrast, in the hot stamped steel sheets according to all of the examples, the coating weight of the coating layer having the predetermined coating chemical composition and containing the specific Zn-containing phase was 30 g / m per side. 2In addition, by controlling the average C concentration from the surface of the steel base material to 1 μm in the depth direction to 0.25 mass% or less, even when applied to hot stamping, high strength and high corrosion resistance are maintained, and the LME suppression effect due to the initial low carbon concentration in the surface layer of the steel base material is fully exerted, making it possible to reliably suppress or reduce the occurrence of LME cracking during subsequent spot welding. When the cross-sections of the plated steel sheets before hot stamping according to all Examples were observed by SEM, the depth region from the surface of the steel base material in the sheet thickness direction where the area ratio of pearlite was 20% or less was 3 μm or more in all cases, and the proportion of coarse pearlite with a circle equivalent diameter of 5 μm or more in the surface layer of the steel base material was sufficiently low compared to Comparative Examples 41 and 42. In particular, in Examples 5 to 8, 16 to 18, and 24 to 28, in which the Al and Mg contents in the coating layer were 6.0 mass% or more and 3.00 mass% or more, respectively, the Zn-containing phase in the coating layer included at least one of an η-Zn phase and an Fe-Zn-based intermetallic compound phase, and as a result, the corrosion resistance was evaluated as AA, which was further improved compared to Example 1, etc., in which the corrosion resistance was evaluated as A. Similarly, in Examples 9 to 12, 19, 20, and 29 to 32, in which the Al and Mg contents in the coating layer were 20.0 mass% or more and 5.00 mass% or more, respectively, the Zn-containing phase in the coating layer included an Mg-Zn-based intermetallic compound phase, and in particular, included only an Mg-Zn-based intermetallic compound phase, and as a result, the corrosion resistance was evaluated as AAA, which was further improved. Furthermore, GDS measurements were performed on each hot-stamped body, and the presence of a solid solution layer including an Fe-Al solid solution layer, an Fe-Zn solid solution layer, and / or an Fe-Al-Zn solid solution layer was confirmed in all hot-stamped bodies according to the examples.

Claims

1. A steel base material and a plating layer disposed on a surface of the steel base material, The plating layer comprises, in mass %, Al: 0.5-30.0%, Mg: 0.50-15.00%, Si: 0 to 2.0%, and Fe: 15.0-70.0% and further comprising Ni: 0-1.000%, Ca: 0-3.0%, Sb: 0 to 0.500%, Pb: 0 to 0.500%, Cu: 0 to 1.000%, Sn: 0-1.000%, Ti: 0 to 1.000%, Cr: 0-1.000%, Nb: 0 to 1.000%, Zr: 0 to 1.000%, Mn: 0 to 1.000%, Mo: 0-1.000%, Ag: 0-1.000%, Li: 0 to 1.000%, La: 0 to 0.500%, Ce: 0-0.500%, B: 0 to 0.500%, Y: 0 to 0.500%, Sr: 0-0.500%, In: 0 to 0.500%, Co: 0 to 0.500%, Bi: 0-0.500%, P: 0 to 0.500%, and W: 0~0.500% Contains at least one of the following in a total amount of 5.000% or less, The balance has a chemical composition consisting of Zn and impurities, The average C concentration from the surface of the steel base material to 1 μm in the depth direction is 0.25 mass% or less, The steel base material contains martensite in an area ratio of 90% or more, The coating weight of the plating layer is 30 g / m per side 2 That's all, the plating layer includes a Zn-containing phase including at least one of an η-Zn phase, an Fe-Zn-based intermetallic compound phase, and an Mg-Zn-based intermetallic compound phase; A hot stamped product characterized by having a Vickers hardness of 400 HV or more.

2. The hot-stamped steel according to claim 1, wherein the average C concentration is 0.18 mass% or less.

3. The hot-stamped steel according to claim 2, wherein the average C concentration is 0.10 mass% or less.

4. The hot-stamped steel according to any one of claims 1 to 3, characterized in that the chemical composition contains, in mass%, Al: 6.0 to 30.0% and Mg: 3.00 to 15.00%, and the Zn-containing phase contains at least one of an η-Zn phase and an Fe-Zn-based intermetallic compound phase.

5. 4. The hot-stamped steel according to claim 1, wherein the chemical composition contains, in mass%, Al: 20.0 to 30.0% and Mg: 5.00 to 15.00%, and the Zn-containing phase includes an Mg-Zn-based intermetallic compound phase.

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