Hot-stamp molded body

Abstract

Provided is a hot-stamp molded body characterized by comprising a steel base material and a plating layer disposed on the surface of the steel base material, and in that: the plating layer has a prescribed chemical composition; the plating layer includes, in order from the steel base material side, an Fe2Al5 phase and an Fe-Al phase having an Fe content of 20-40 mass% the Zn content of the Fe-Al phase being 1.5-15.0 mass%; in a cross section of the plating layer, the projected length Li of Fe-Al-Si phases which have an equivalent circle diameter of 2 µm and in which the Si content in the Fe2Al5 phase is 5-20 mass%, and the length L0 of the surface of the steel base material satisfy the expression ΣLi / L0 ≤ 0.70; and the adhesion amount of the plating layer is 20 g / m2 or more per one surface.

Description

Hot stamped compact

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

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

[0003] Hot stamping is known as a press forming technique for 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 and then formed. With this technique, the steel is soft and has good formability during forming because the material is heated and then formed. Therefore, even high-strength steel can be formed with high precision into complex shapes. In addition, since the steel is quenched simultaneously with forming using a press die, the steel after forming is known to have sufficient strength.

[0004] In this regard, various studies have been conducted on hot stamped steel bodies that have been subjected to Al-containing plating and plated steel sheets for such hot stamped steel bodies.

[0005] For example, Patent Document 1 describes a steel sheet coated with a metal coating containing 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 wherein the Al / Zn ratio exceeds 2.9. Patent Document 1 also teaches that metal coatings with an Al / Zn ratio exceeding 2.9 have high sacrificial protection.

[0006] Patent Document 2 describes an aluminum alloy plated steel sheet including a base steel sheet and an aluminum alloy plated layer formed on the base steel sheet, the aluminum alloy plated layer containing, by weight %, 21 to 35% Zn, 1 to 6.9% Si, 2 to 12% Fe, the balance being Al and other inevitable impurities. Patent Document 2 also teaches that weldability and corrosion resistance can be ensured by controlling Al / (Zn+Si) of the aluminum alloy plated layer to 1.3 to 2.6.

[0007] Special table 2018-528324 publication Special table 2022-550142 publication

[0008] For example, when a plated steel sheet as described in Patent Documents 1 and 2, more specifically a plated steel sheet containing a relatively large amount of Al as a component in the plated layer, is used in hot stamping, the plated layer and the base steel (base steel sheet) may be alloyed during heating in the hot stamping, resulting in a decrease in corrosion resistance.

[0009] Therefore, an object of the present invention is to provide a hot stamped product having an Al-containing plating layer that has improved corrosion resistance even after hot stamping.

[0010] As a result of investigations conducted by the present inventors to achieve the above-mentioned object, they found that excellent corrosion resistance can be achieved in a hot stamped steel by forming a plating layer made of an Al-Zn-Si based plating in a coating weight equal to or greater than a predetermined amount, and by appropriately controlling the chemical composition and / or morphology of the FeAl phase in the plating layer formed after hot stamping and the Fe-Al phase formed on the FeAl phase and having a relatively low Fe content, and thus completed the present invention.

[0011] The present invention, which has achieved the above object, is as follows. (1) A steel alloy comprising a steel base material and a plating layer disposed on a surface of the steel base material, wherein the chemical composition of the plating layer is, in mass%, Zn: 2.5 to 40.0%, Si: 0.05 to 15.0%, Fe: 25.0 to 40.0%, Ni: 0 to 0.500%, Mg: 0 to 3.000%, Ca: 0 to 3.000%, Sb: 0 to 0.500%, Pb: 0 to 0.500%, Cu: 0 to 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 to 1.000%, Li: 0 to 1.000%, La: 0 to 0.500%, Ce: 0 to 0.500%, B: 0 to 0.500%, Y: 0 to 0.500%, Sr: 0 to 0.500%, In: 0 to 0.500%, Co: 0 to 0.500%, Bi: 0 to 0.500%, P: 0 to 0.500%, W: 0 to 0.500%, and the balance: Al and impurities, the total content of Ni, Mg, Ca, Sb, Pb, Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, Li, La, Ce, B, Y, Sr, In, Co, Bi, P, and W is 5.000% or less, the coating layer includes, in order from the steel base metal side, an Fe2Al5 phase and an Fe-Al phase having an Fe content of 20 to 40 mass%, the Zn content of the Fe-Al phase is 1.5 to 15.0 mass%, and in a cross section of the coating layer, the Si content in the Fe2Al5 phase is 5 to 20 mass% and the projected length L of the Fe-Al-Si phase having an equivalent circle diameter of 2 μm or more is i The length L0 of the surface of the steel base material is ΣL i / L0≦0.70, and the coating weight of the plating layer is 20 g / m per side. 2 (2) The hot-stamped product according to (1) above, characterized in that the plating layer has a chemical composition containing, in mass %, Zn: 5.0 to 35.0%, and Si: 0.25 to 6.0%, and the Zn content of the Fe—Al phase is 3.0 to 15.0 mass %. (3) ΣLi The hot-stamped steel according to (1) or (2), wherein / L0≦0.50.

[0012] According to the present invention, it is possible to provide a hot stamped product having an Al-containing plating layer that has improved corrosion resistance even after hot stamping.

[0013] 1 is an example of a cross-sectional schematic diagram of a surface portion of a hot-stamped steel according to an embodiment of the present invention, showing the projected length L of the Fe—Al—Si phase. i and the length L0 of the surface of the steel base material.

[0014] <Hot-stamped body> A hot-stamped body according to an embodiment of the present invention includes a steel base material and a coating layer disposed on a surface of the steel base material, and the coating layer has a chemical composition, in mass %, of Zn: 2.5 to 40.0%, Si: 0.05 to 15.0%, Fe: 25.0 to 40.0%, Ni: 0 to 0.500%, Mg: 0 to 3.000%, Ca: 0 to 3.000%, Sb: 0 to 0.500%, Pb: 0 to 0.500%, Cu: 0 to 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 to 1.000%, Li: 0 to 1.000%, La: 0 to 0.500%, Ce: 0 to 0.500%, B: 0 to 0.500%, Y: 0 to 0.500%, Sr: 0 to 0.500%, In: 0 to 0.500%, Co: 0 to 0.500%, Bi: 0 to 0.500%, P: 0 to 0.500%, W: 0 to 0.500%, and the balance: Al and impurities, the total content of Ni, Mg, Ca, Sb, Pb, Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, Li, La, Ce, B, Y, Sr, In, Co, Bi, P, and W is 5.000% or less, the coating layer includes, in order from the steel base metal side, an Fe2Al5 phase and an Fe-Al phase having an Fe content of 20 to 40 mass%, the Zn content of the Fe-Al phase is 1.5 to 15.0 mass%, and in a cross section of the coating layer, the Si content in the Fe2Al5 phase is 5 to 20 mass% and the projected length L of the Fe-Al-Si phase having an equivalent circle diameter of 2 μm or more is i The length L0 of the surface of the steel base material is ΣL i / L0≦0.70, and the coating weight of the plating layer is 20 g / m per side. 2 The present invention is characterized by the above.

[0015] As mentioned above, when a plated steel sheet containing a relatively large amount of Al in the coating layer is used in hot stamping, the coating layer may alloy with the base steel (base steel sheet) during heating in the hot stamping process, resulting in reduced corrosion resistance. More specifically, for example, when a plated steel sheet having a coating layer made of a conventional Al-Si-based coating is hot stamped, the coating layer and the base steel sheet alloy during high-temperature heating in the hot stamping process (e.g., heating at approximately 900°C or higher), resulting in the formation of a coating layer primarily containing an FeAl phase after hot stamping. However, in addition to the FeAl phase, a relatively large amount of Fe-Al-based intermetallic compounds containing relatively large amounts of Si, particularly an Fe-Al-Si phase containing 5% or more by mass of Si, may also be formed in the coating layer. In such cases, bimetallic corrosion (galvanic corrosion) may occur between the FeAl phase and the Fe-Al-Si phase, resulting in reduced corrosion resistance of the hot stamped steel.

[0016] Therefore, the present inventors conducted studies, focusing particularly on the chemical composition and morphology of the Al-Si-based coating layer before hot stamping, in order to provide a hot-stamped product having an Al-containing coating layer that exhibits excellent corrosion resistance even after hot stamping. As a result, the present inventors discovered that by appropriately producing an Al-Zn-Si-based coating layer in which Zn is further added to an Al-Si-based coating and appropriately selecting heat treatment conditions for hot stamping, it is possible to suppress the formation of an Fe-Al-Si phase that may cause galvanic corrosion between the FeAl phase and the coating layer formed after hot stamping, and further, to sufficiently dissolve Zn, which has a sacrificial anticorrosive effect, in the FeAl phase that is formed on the FeAl phase and has a relatively low Fe content, thereby achieving excellent corrosion resistance in the hot-stamped product.

[0017] More specifically, the inventors first determined that the coating weight of the Al-Zn-Si based plating layer was 20 g / m per side. 2In addition, the inventors have found that by appropriately selecting the heat treatment conditions for hot stamping, an Fe-Al phase having a lower Fe content than the Fe2Al5 phase can be formed on the Fe2Al5 phase on the steel base material side, without alloying the entire coating layer to the Fe2Al5 phase during high-temperature heating in hot stamping. More specifically, during high-temperature heating in hot stamping, Fe from the steel base material diffuses into the coating layer and is alloyed with Al, but by increasing the coating weight of the coating layer relatively and weakening the degree of alloying appropriately, a sufficiently alloyed Fe2Al5 phase is formed on the steel base material side of the coating layer, but an Fe-Al phase having a lower Fe content than the Fe2Al5 phase, i.e., an Fe-Al phase having an Fe content of 20 to 40 mass %, can be formed on the surface side of the coating layer.

[0018] Next, the present inventors discovered that, as will be described in detail later in connection with the manufacturing method of a hot-stamped steel sheet, by incorporating predetermined amounts of Si and Zn in the Fe—Al phase corresponding to the interfacial alloy layer formed at the interface between the base steel sheet and the coating layer before hot stamping, more specifically, by incorporating 3.0 mass % or more of Si and 2.0 mass % or more of Zn in the interfacial alloy layer and controlling the shape of the interfacial alloy layer to be flatter with fewer irregularities, it is possible to suppress the formation of a relatively large Fe—Al—Si phase in the Fe2Al5 phase in the coating layer formed after hot stamping, and to enable sufficient Zn to be solid-solved in the Fe—Al phase having an Fe content of 20 to 40 mass %. More specifically, the present inventors discovered that, in the coating layer after hot stamping, by having a relatively large Fe—Al—Si phase dispersed in the Fe2Al5 phase formed on the steel base metal side of the coating layer, i.e., by having a Si content of 5 to 20 mass % and a projected length L of the Fe—Al—Si phase having an equivalent circle diameter of 2 μm or more, i The surface length L0 of the steel base material is ΣL i It has been found that by controlling the ratio of SiO2 to L0 so as to satisfy / L0≦0.70, the corrosion resistance of the plating layer after hot stamping can be significantly improved.

[0019] FIG. 1 is an example of a cross-sectional schematic diagram of a surface portion of a hot-stamped steel according to an embodiment of the present invention, and shows the projected length L of the Fe—Al—Si phase. i and the length L0 of the surface of the steel base material. Referring to Fig. 1, a hot-stamped steel 1 according to an embodiment of the present invention comprises a steel base material 2 and a coating layer 3 disposed on the surface of the steel base material 2. The coating layer 3 includes, in order from the steel base material 2 side, an Fe2Al5 phase 4 located on the steel base material 2 and an Fe-Al phase 5 located on the Fe2Al5 phase 4, more specifically, an Fe-Al phase 5 having an Fe content of 20 to 40 mass %. Also, in Fig. 1, an Fe-Al-Si phase 6 having an Si content of 5 to 20 mass % and an equivalent circle diameter of 2 µm or more is present in the Fe2Al5 phase 4. Here, the projected length L i The sum of ΣL i (In Figure 1, ΣL i = L1 + L2 + L3), and the surface length L0 of the steel base material 2 is ΣL i / L0≦0.70 is satisfied, and therefore, a relatively large Fe—Al—Si phase 6 is dispersed in the Fe2Al5 phase 4. When the projected lengths of multiple Fe—Al—Si phases 6 partially overlap, the entire projected length including the overlapping portions is determined as a single projected length. If the Fe—Al—Si phase 6 is present in a layered form in the Fe2Al5 phase 4, when galvanic corrosion occurs at the contact portion between the layered Fe—Al—Si phase 6 and the Fe2Al5 phase 4, it is thought that corrosion will progress along the contact interface. As a result, the corrosion resistance of the coating layer 3 will be significantly reduced. In contrast, in the hot-stamped steel 1 according to the embodiment of the present invention, ΣL iBy dispersing the Fe—Al—Si phase 6 in the Fe2Al5 phase 4 so as to satisfy / L0 ≦ 0.70, even if galvanic corrosion occurs at the contact point between one or more Fe—Al—Si phases 6 and the surrounding Fe2Al5 phases 4, the corrosion will not start at the contact point and progress to other Fe—Al—Si phases 6, thereby significantly improving the corrosion resistance of the coating layer 3. Additionally, in the hot stamped steel 1 according to the embodiment of the present invention, the sacrificial corrosion protection effect of Zn can be effectively exerted by including 1.5 to 15.0 mass % of Zn in the Fe—Al phase 5, thereby significantly improving corrosion resistance, particularly red rust resistance.

[0020] Therefore, with the hot-stamped steel according to the embodiment of the present invention, excellent corrosion resistance can be achieved by suppressing the formation of the Fe—Al—Si phase 6, which may cause galvanic corrosion between the Fe—Al phase 4 and the Fe—Al—Si phase 6. More specifically, the corrosion progression suppression effect achieved by dispersing and forming relatively large Fe—Al—Si phases 6 is combined with the corrosion resistance improvement effect achieved by dissolving Zn in the Fe—Al phase 5. In particular, the present inventors have now discovered for the first time that the corrosion resistance of the hot-stamped steel 1 can be improved by forming the coating layer 3 from the Fe—Al phase 4 and the Fe—Al phase 5 having a relatively low Fe content and further appropriately controlling the chemical composition and / or morphology of the Fe—Al phase 4 and the Fe—Al phase 5 as described above. Therefore, the hot-stamped steel according to the embodiment of the present invention is particularly useful in the automotive field, where hot stamping is relatively common.

[0021] 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 term "to" indicating a numerical range means that the numerical values ​​before and after the range are included as the lower and upper limits.

[0022] [Plating Layer] According to an embodiment of the present invention, the plating layer is disposed on the surface of the 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 plating layer is disposed directly on the surface of the steel base material, but also the case where the plating 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 plating layer. The plating layer has the following chemical composition:

[0023] [Zn: 2.5 to 40.0%] Zn has a sacrificial corrosion protection effect and is an effective element for improving the corrosion resistance of the coating layer. To fully obtain this effect, the Zn content is set to 2.5% or more. The Zn content may be 3.5% or more, 5.0% or more, 8.0% or more, 10.0% or more, 12.0% or more, 15.0% or more, 18.0% or more, 20.0% or more, or 22.0% or more. On the other hand, excessive Zn content may cause significant melting of Zn during high-temperature heating in hot stamping, and the molten Zn may penetrate into the steel, causing liquid metal embrittlement (LME) cracking. Therefore, the Zn content is set to 40.0% or less. The Zn content may be 38.0% or less, 35.0% or less, 32.0% or less, 30.0% or less, or 28.0% or less.

[0024] [Si: 0.05 to 15.0%] Si is an element effective in improving the adhesion of the plating layer. To fully obtain this effect, the Si content is set to 0.05% or more. The Si content may be 0.1% or more, 0.2% or more, 0.25% or more, 0.3% or more, 0.5% or more, 0.8% or more, 1.0% or more, 3.0% or more, or 5.0% or more. On the other hand, excessive Si content may significantly increase the formation of an Fe—Al—Si phase in the Fe2Al5 phase, resulting in reduced corrosion resistance. Therefore, the Si content is set to 15.0% or less. The Si content may also be 12.0% or less, 10.0% or less, 8.0% or less, or 6.0% or less.

[0025] [Fe: 25.0 to 40.0%] When a plated steel sheet is heated during hot stamping, Fe from the steel base material diffuses into the plated layer and alloys with Al, etc., so the plated layer inevitably contains Fe. Therefore, the Fe content is 25.0% or more, and may be, for example, 26.0% or more, 28.0% or more, 30.0% or more, or 32.0% or more. On the other hand, if the Fe content is too high, excessive alloying of the plated layer may result in a decrease in red rust resistance. Therefore, the Fe content may be 40.0% or less, and may be, for example, 38.0% or less, 36.0% or less, or 34.0% or less.

[0026] Furthermore, the plating layer may optionally contain Ni: 0 to 0.500%, Mg: 0 to 3.000%, Ca: 0 to 3.000%, Sb: 0 to 0.500%, Pb: 0 to 0.500%, Cu: 0 to 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 At least one of the following may be contained: 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%. The total content of these optional elements (i.e., the total content of these elements) is 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, 1.000% or less, 0.800% or less, 0.500% or less, 0.100% or less, or 0.050% or less. The inclusion of these elements is not essential, and the total content of these elements may be 0%. If necessary, the lower limit of the total content of these elements may be 0.001%, 0.010%, 0.050%, or 0.080%. These optional elements will be described in detail below.

[0027] [Ni: 0 to 0.500%] 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 0.001% or more. The Ni content may be 0.003% or more, 0.005% or more, 0.008% 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 0.500% or less, for example, 0.400% or less, 0.300% or less, 0.100% or less, 0.050% or less, or 0.030% or less.

[0028] [Mg: 0 to 3.000%] Mg is an element effective in improving the corrosion resistance of the coating layer. The Mg content may be 0%, but to achieve this effect, the Mg content is preferably 0.001% or more. The Mg content may be 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, from the viewpoint of improving workability, the Mg content may be 3.000% or less. The Mg content may be 2.000% or less, 1.000% or less, 0.500% or less, 0.300% or less, 0.100% or less, 0.050% or less, or 0.020% or less.

[0029] [Ca: 0 to 3.000%] Ca is an element effective in ensuring the wettability of the plating bath. The Ca content may be 0%, but to achieve this effect, the Ca content is preferably 0.001% or more. The Ca content may be 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, excessive Ca content may form a large amount of hard intermetallic compounds in the plating layer, making the plating layer brittle and reducing adhesion to the steel sheet. Therefore, the Ca content is preferably 3.000% or less. The Ca content may be 2.000% or less, 1.000% or less, 0.500% or less, 0.300% or less, 0.100% or less, 0.050% or less, or 0.020% or less.

[0030] [Sb: 0 to 0.500%, Pb: 0 to 0.500%, Cu: 0 to 1.000%, Sn: 0 to 1.000%, Ti: 0 to 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 to 1.000%, La: 0 to 0.500%, Ce: 0 to 0.500%, B: 0 to 0.500%, Y: 0 to 0.500%, Sr: 0 to 0.500%, In: 0 to 0.500%, Co: 0 to 0.500%, Bi: 0 to 0.500%, P: 0 to 0.500% and W: 0 to 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 contained in the plating layer, but may be present in the plating layer in an amount 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 a predetermined content range. However, excessive content of each element may reduce corrosion resistance. Therefore, the contents of Sb, Pb, La, Ce, B, Y, Sr, In, Co, Bi, P, and W are preferably 0.500% or less, and may be, for example, 0.300% or less, 0.100% or less, 0.050% or less, or 0.020% 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, 0.100% or less, 0.050% or less, or 0.020% or less.

[0031] The remainder of the plating layer other than the above elements consists of Al and impurities. Impurities in the plating layer include components that are mixed in due to various factors in the manufacturing process, including raw materials, when the plating layer is produced. The Al content does not need to be particularly specified, but may be 10.0 to 72.45%. If necessary, the Al content may be 20.0% or more, 25.0% or more, 30.0% or more, 35.0% or more, 38.0% or more, 40.0% or more, or 42.0% or more, or may be 70.0% or less, 65.0% or less, 60.0% or less, 57.0% or less, 54.0% or less, 51.0% or less, or 48.0% or less.

[0032] [Measurement of Chemical Composition of 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. That is, the plating layer is dissolved in an acidic aqueous solution at room temperature containing an inhibitor that inhibits steel dissolution, for example, an acidic aqueous solution containing 10% hydrochloric acid and 0.04% IBIT 710K (manufactured by Asahi Chemical Industry Co., Ltd.), and the resulting acidic aqueous solution is analyzed by ICP (inductively coupled plasma atomic emission spectroscopy), to obtain the chemical composition of the plating layer.

[0033] [Fe2Al5 phase] [ΣL i In an embodiment of the present invention, the coating layer includes an FeAl phase on the steel base material side, and the Si content in the FeAl phase is 5 to 20 mass % and the projected length L of the Fe—Al—Si phase has an equivalent circle diameter of 2 μm or more. i The surface length L0 of the steel base material is ΣL i / L0≦0.70. As mentioned above, an Fe—Al—Si phase with a Si content of 5 to 20 mass % can cause galvanic corrosion between the Fe—Al—Si phase and the Fe2Al5 phase. In particular, if the Fe—Al—Si phase is relatively large and exists continuously in a layered or nearly layered form, corrosion will progress along the contact interface between the Fe—Al—Si phase and the Fe2Al5 phase, significantly reducing the corrosion resistance of the coating layer. Therefore, in order to suppress the progression of such corrosion and improve the corrosion resistance of the coating layer, it is necessary to have Fe—Al—Si phases with a circle equivalent diameter of 2 μm or more dispersed in the Fe2Al5 phase. Therefore, in an embodiment of the present invention, as shown in FIG. 1, the projected length L of each Fe—Al—Si phase with a circle equivalent diameter of 2 μm or more projected onto the surface of the steel base material is i The sum of ΣL i The surface length L0 of the steel base material 2 is ΣL i The corrosion resistance of the coating layer is improved by dispersing the Fe-Al-Si phase in the Fe2Al5 phase so as to satisfy the condition ΣL i The lower the value of / L0, the more preferable, and may be, for example, 0.65 or less, 0.60 or less, 0.55 or less, 0.50 or less, 0.45 or less, or 0.40 or less.

[0034] On the other hand, even if corrosion occurs in an Fe—Al—Si phase having an equivalent circle diameter of less than 2 μm, the corrosion does not proceed further from the corrosion to other Fe—Al—Si phases. Therefore, even if an Fe—Al—Si phase having an equivalent circle diameter of less than 2 μm exists, it does not necessarily have a negative effect on the corrosion resistance of the hot stamped steel. In this regard, from the viewpoint of further improving corrosion resistance, it is also acceptable to prevent an Fe—Al—Si phase having an equivalent circle diameter of 2 μm or more from existing in the Fe2Al5 phase. That is, ΣL i The lower limit of / L0 may be 0. Although not particularly limited, for example, ΣL i / L0 may be 0.05 or more, 0.10 or more, 0.15 or more, or 0.20 or more.

[0035] [Fe—Al Phase with 20 to 40 Mass% Fe] [Zn Content of Fe—Al Phase: 1.5 to 15.0 Mass%] In an embodiment of the present invention, the coating layer includes an Fe—Al phase on the surface side that has a lower Fe content than the Fe2Al5 phase (Fe content: over 40 to 60 mass%), i.e., an Fe—Al phase with an Fe content of 20 to 40 mass%, and the Zn content of the Fe—Al phase is 1.5 to 15.0 mass%. The Fe—Al phase with an Fe content of 20 to 40 mass% is less alloyed than the Fe2Al5 phase, which may contribute to improved red rust resistance. The Si content of the Fe—Al phase is less than 5 mass%. Note that in this specification, the Fe—Al phase with an Fe content of 20 to 40 mass% may be simply referred to as the Fe—Al phase. In an embodiment of the present invention, the corrosion resistance, particularly red rust resistance, of the coating layer can be significantly improved by including 1.5% by mass or more of Zn, which has a sacrificial corrosion protection effect, in the Fe—Al phase. From the viewpoint of further enhancing the corrosion resistance improvement effect, the higher the Zn content in the Fe—Al phase, the more preferable it is, and it may be, for example, 3.0% by mass or more, 4.0% by mass or more, 5.0% by mass or more, 6.0% by mass or more, or 8.0% by mass or more. On the other hand, excessive Zn content in the Fe—Al phase may saturate the effect and result in increased manufacturing costs. Therefore, the Zn content in the Fe—Al phase is set to 15.0% by mass or less, and may be, for example, 12.0% by mass or less, 10.0% by mass or less, or 8.0% by mass or less.

[0036] The plating layer may have an oxide film on at least a portion of its surface due to exposure to high temperatures during hot stamping.

[0037] [Analysis of the Coating Layer] The analysis of the coating layer is performed as follows. First, five samples are taken from the hot-stamped body so that the cross section of the coating layer can be observed. Next, for each sample, a rectangular area measuring 80 μm in the thickness direction and 100 μm in the direction perpendicular to the thickness direction is defined as one field of view. Images of the five fields of view for the five samples are then taken at a magnification of 1500x using SEM-EDS or EPMA to obtain mapping images. From the element distribution images in these mapping images, the locations of the Fe—Al phase having an Fe content of 20 to 40 mass%, the Fe2Al5 phase, and the Fe—Al—Si phase present within the Fe2Al5 phase, having a Si content of 5 to 20 mass% and a circle equivalent diameter of 2 μm or more, are determined. More specifically, element distribution images are obtained using SEM-EDS or EPMA, with the target elements being Fe, Al, Si, and Zn. In this element distribution image, the region satisfying 20 to 40 mass% Fe and 58 to 78 mass% Al is designated as the Fe-Al phase having an Fe content of 20 to 40 mass%. The Fe-Al phase region is identified by setting the concentration range of a color bar, which indicates the concentration of each element in the element distribution image by color tone, to 20 to 40 mass% Fe and 58 to 78 mass% Al, and overlaying the element distribution images of Fe and Al with the color bar adjusted. In this element distribution image, the region satisfying more than 40 to 60 mass% Fe, 40 to less than 60 mass% Al, 0 to less than 5 mass% Si, and 0 to 15 mass% Zn is designated as the Fe2Al5 phase (reference numeral 4 in FIG. 1). The region having a Si content of 5 to 20 mass % and a circle equivalent diameter of 2 μm or more, which is surrounded by the FeAl phase, is defined as the Fe-Al-Si phase (reference numeral 6 in FIG. 1). Next, the Fe-Al-Si phase is projected onto the surface of the steel base material using image analysis software (for example, the "Analyze" function of "ImageJ"), and the projected length L of each Fe-Al-Si phase is calculated. i Finally, the sum of the L obtained for the five samples is calculated. i Calculate the average of the total of ΣL i and the ratio ΣL iHere, L0, which is the length of the long side in the field of view, is also the distance between both ends of the measurement region (field of view) (however, the distance in the direction parallel to the surface of the hot-stamped solid 1), as shown in FIG.

[0038] The Zn content in the Fe—Al phase having an Fe content of 20 to 40 mass% is determined as follows. First, for each of the five samples, the Fe—Al phase having an Fe content of 20 to 40 mass% is identified by the above-mentioned method, and the concentration of elements constituting the Fe—Al phase (specifically, the Zn content) is measured using SEM-EDS or EPMA. More specifically, for each sample, the Zn content is measured at five measurement points spaced 50 μm apart in a direction parallel to the surface of the hot-stamped body at the center of the thickness of the Fe—Al phase, and the average value is taken as the measured value for each sample. The measured values ​​of the target element concentration for the five samples are obtained, and the average value of the measured values ​​is taken as the Zn content.

[0039] The plating layer having the above chemical composition, FeAl phase, and Fe—Al phase may be a hot-dip plating layer. Although it is not necessary to exclude plating layers other than hot-dip plating layers, it may be limited to hot-dip plating layers.

[0040] [Plating layer deposition amount: 20 g / m per side 2 In the hot stamped steel according to the embodiment of the present invention, the coating weight of the plating layer is 20 g / m per side. 2 Generally, a coating layer may be alloyed 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, the coating weight of the coating layer is relatively large, specifically, 20 g / m per side. 2By satisfying the above requirements and appropriately weakening the degree of alloying, a fully alloyed Fe2Al5 phase is formed on the steel base metal side of the coating layer, while an Fe-Al phase with a lower Fe content than the Fe2Al5 phase, i.e., an Fe-Al phase with an Fe content of 20 to 40 mass %, is formed on the surface side of the coating layer. Furthermore, by appropriately controlling the chemical composition and / or morphology of these phases as described above, excellent corrosion resistance can be achieved due to the presence of the resulting coating layer. On the other hand, if the coating weight of the coating layer is low, the alloying of the coating layer may progress too much during high-temperature heating in hot stamping, making it impossible to form an Fe-Al phase with a relatively low Fe content. In this case, the improved red rust resistance resulting from the formation of the Fe-Al phase cannot be obtained. In addition, if the coating weight of the coating layer is low, the effect of adding Zn, which has a sacrificial corrosion protection effect, to the coating layer may not be fully obtained, resulting in reduced corrosion resistance after hot stamping. From the perspective of improving corrosion resistance, the coating weight of the coating layer is preferably 30 g / m per side. 2 Above, 40g / m 2 or more than 50 g / m 2 More preferably, 60 g / m 2 More preferably, 70 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 170 g / m 2 It may be the following:

[0041] [Measurement of 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 × 30 mm sample is taken from the hot-stamped body, and the coating layer is then dissolved in an acidic aqueous solution at room temperature containing an inhibitor that suppresses steel dissolution, for example, an acidic aqueous solution containing 10% hydrochloric acid plus 0.04% Ivit 710K (manufactured by Asahi Chemical Industry Co., Ltd.), and the coating weight of the coating layer is determined from the change in mass of the sample before and after dissolution of the coating layer.

[0042] [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.

[0043] 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 0.04% Ivit 710K (manufactured by Asahi Chemical Industries Co., Ltd.), at room temperature. Next, using a high-frequency glow discharge optical emission spectrometer (GDS), the surface of the hot-stamped body is placed in an Ar atmosphere, and a voltage is applied to generate glow plasma. The surface of the hot-stamped body is then analyzed in the depth direction while sputtering. The elements contained in the material are 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, a region in the depth direction where the Al concentration exceeds 1.00 mass % and is 1.10 times or more the Al content of the steel base material is determined to be a solid solution layer.

[0044] [Preferable Chemical Composition of Steel Base Metal] As described above, the present invention aims to provide a hot stamped body having an Al-containing coating layer that has improved corrosion resistance even after hot stamping. The present invention aims to optimize the chemical composition and coating weight of the coating layer made of an Al-Zn-Si based coating, and to provide a hot stamped body having an FeAl phase formed on the steel base metal side of the coating layer, in which the Si content is 5 to 20 mass % and the projected length L of the FeAl-Si phase having an equivalent circle diameter of 2 μm or more is 5 to 20 mass %. i The surface length L0 of the steel base material is ΣL i The objective is achieved by controlling the Zn content so as to satisfy the relationship / L0 ≦0.70, and further by including 1.5 to 15.0 mass % of Zn in the Fe—Al phase formed on the surface side of the coating layer. Therefore, it is clear that the chemical composition of the steel base material itself is not an essential technical feature for achieving the objective of the present invention. Preferred chemical compositions of the steel base material used in the hot-stamped steel according to embodiments of the present invention will be described in detail below, but these descriptions are intended to be mere examples of preferred chemical compositions of steel base materials for hot-stamped steels and are not intended to limit the present invention to those using steel base materials having such specific chemical compositions.

[0045] In an embodiment of the present invention, for example, the steel base material has, in mass %, C: 0.13 to 0.50%, Si: 0.001 to 3.000%, Mn: 0.30 to 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 to 0.15%, Mo: 0 to 1.0%, Cr: 0 to 1.0%, Cu: 0 to 1.0%, Ni: 0 to 1.0%, B: 0 to 0.0100%, W: 0 to 1.000%, Hf: 0 to 0.050%, It is preferable that the alloy has a chemical composition consisting of Mg: 0 to 0.050%, Zr: 0 to 0.050%, Ca: 0 to 0.010%, REM: 0 to 0.30%, Ir: 0 to 1.000%, and the balance: Fe and impurities. Each element will be described in more detail below.

[0046] [C: 0.13 to 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 while decreasing 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 to 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 to 2.000%] Al acts as a deoxidizer for steel and has the effect of improving the soundness of steel. To fully obtain 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 generate coarse Al oxides, reducing the elongation of the steel sheet. 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. Since a lower P content is preferable, the ideal P content is 0%. However, excessive reduction in the P content may result in a significant increase in costs. Therefore, the P content may be 0.0001% or more, 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 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 non-metallic inclusions such as MnS in steel, resulting in a decrease in the ductility of steel parts. Since a lower S content is preferable, ideally 0%. However, excessive reduction in the S content may result in a significant increase in costs. Therefore, the S content may be 0.0001% or more, 0.0002% or more, 0.0010% or more, or 0.0050% or more. On the other hand, excessive S content may cause cracks originating from non-metallic 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 sheets and reduces the workability of the steel sheets. Since a lower N content is preferable, the ideal N content is 0%. However, excessive reduction in the N content may result in a significant increase in manufacturing costs. Therefore, the N content may be 0.0001% or more, 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 sheets. 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 contain, as necessary, one or more elements selected from the group consisting of Nb: 0-0.15%, Ti: 0-0.15%, V: 0-0.15%, Mo: 0-1.0%, Cr: 0-1.0%, Cu: 0-1.0%, Ni: 0-1.0%, B: 0-0.0100%, W: 0-1.000%, Hf: 0-0.050%, Mg: 0-0.050%, Zr: 0-0.050%, Ca: 0-0.010%, REM: 0-0.30%, and Ir: 0-1.000%, in place of a portion of the remaining Fe. 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 may be measured by a common analytical method. For example, the chemical composition of the steel base material may be measured by first removing the plating 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 may be obtained from the steel base material at approximately half the plate thickness position, and the composition may be determined by measuring the composition using a Shimadzu ICPS-8100 or similar (measuring device) under conditions based on a pre-created calibration curve. C and S, which cannot be measured by ICP-AES, 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.

[0056] [Thickness of Hot-Stamped Steel] The thickness of the hot-stamped steel is not particularly limited, and may be, for example, 0.2 mm or more, 0.3 mm or more, 0.6 mm or more, 1.0 mm or more, or 2.0 mm or more. Similarly, the thickness of the hot-stamped steel may be, for example, 6.0 mm or less, 5.0 mm or less, or 4.0 mm or less.

[0057] <Method for Manufacturing Hot-Stamped Steel> Next, a preferred method for manufacturing the 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 the hot-stamped steel according to an embodiment of the present invention, but is not intended to limit the hot-stamped steel to those manufactured by the manufacturing method described below.

[0058] 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.

[0059] [Casting Step] The conditions for the casting step are not particularly limited. For example, after melting in a blast furnace or an 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.

[0060] [Hot Rolling Process] A hot-rolled steel sheet can be obtained by hot-rolling a cast slab. The hot rolling process is carried out by reheating the cast slab directly or after cooling it once, and then hot-rolling it. When reheating is carried out, the heating temperature of the slab may be, for example, 1100 to 1250°C. In the hot rolling process, 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%.

[0061] [Coiling step] 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.

[0062] [Cold Rolling Step] 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 steel sheet may be cooled to room temperature, for example, by air cooling.

[0063] [Annealing Step] Next, the obtained cold-rolled steel sheet is annealed. The annealing step involves heating the cold-rolled steel sheet to a temperature of 730 to 900°C in an atmosphere with a dew point of -50 to 20°C and holding the temperature for 10 to 300 seconds. In particular, by performing the annealing step under conditions of a relatively high dew point of -10 to 20°C, the surface layer of the cold-rolled steel sheet can be appropriately decarburized. In this case, although the reason is not necessarily clear, it becomes possible to make the Fe-Al-Si phase more dispersed in the FeAl phase in the coating layer of the finally obtained hot-stamped steel. As a result, the ΣL i / L0 can be reduced, for example, i The annealing atmosphere may be a reducing atmosphere, more specifically, a reducing atmosphere containing nitrogen and hydrogen, for example, a reducing atmosphere containing 1 to 10% hydrogen (for example, 4% hydrogen and the balance being nitrogen).

[0064] [Cooling Step] The cooling step may be carried out at a cooling rate suitable for obtaining a desired metal structure from the heating temperature of the annealing step to the entry sheet temperature of the subsequent galvanizing step. Although not particularly limited, for example, the cooling step may be carried out at an average cooling rate of 10°C / s or more from the heating temperature of the annealing step to the entry sheet temperature of the galvanizing step.

[0065] [Plating Step] Next, in the plating step, a plating layer is formed on at least one, preferably both, surfaces of the cold-rolled steel sheet (steel base material). More specifically, the plating step is carried out using a plating bath having a predetermined chemical composition, for example, a plating bath containing 5.0 to 50.0% Zn, 0.1 to 18.0% Si, and the balance being Al and impurities (plating bath temperature: 650°C or higher, e.g., 650 to 680°C), such that the steel base material enters the plating bath at a temperature of 500°C or lower, and is then cooled to 300°C at an average cooling rate of 10°C / s or lower. The plating step is important for obtaining the desired chemical composition and morphology of the Fe—Al phase, which corresponds to the interfacial alloy layer formed at the interface with the steel base material, and will be described in detail below.

[0066] Generally, the galvanizing process is carried out so that the entry temperature of the steel sheet is the same as or approximately the same as the temperature of the galvanizing bath. However, if the difference between the entry temperature of the steel sheet into the galvanizing bath and the temperature of the galvanizing bath (i.e., galvanizing bath temperature - entry temperature of the steel sheet) is less than 150°C, if the entry temperature is higher than 500°C, or if the galvanizing bath temperature is lower than 650°C, it becomes impossible to form an appropriate Fe—Al phase. Specifically, in these cases, the Si content of the Fe—Al phase becomes less than 3.0%, and similarly, the Zn content becomes less than 2.0%. As a result, when such a galvanized steel sheet is subjected to high-temperature heating (hot stamping), the formation of the Fe—Al—Si phase in the Fe2Al5 phase cannot be sufficiently suppressed during the high-temperature heating, resulting in a ΣL iThe value of / L0 becomes large, and / or Zn, which has a sacrificial corrosion protection effect, cannot be sufficiently solid-dissolved in the Fe—Al phase having an Fe content of 20 to 40% by mass formed on the FeAl phase. This significantly reduces the corrosion resistance of the coating layer after hot stamping. Therefore, in this manufacturing method, the coating bath temperature is controlled to 650°C or higher and the entry sheet temperature is controlled to 500°C or lower, ensuring that the entry sheet temperature is at least 150°C lower than the coating bath temperature. This allows the interface alloy layer (Fe—Al phase) to crystallize at a low temperature and converts the interface alloy layer into a phase suitable for containing 3.0% by mass or more of Si and 2.0% by mass or more of Zn. It is believed that by containing 3.0% by mass or more of Si in this interface alloy layer, much of the Si present in the coating layer before hot stamping can be trapped in the interface alloy layer. Since the Fe-Al-Si phase after hot stamping is an intermetallic compound containing a relatively large amount of Si, it is believed that by trapping most of the Si in the interface alloy layer before hot stamping, it is possible to suppress the formation of the Fe-Al-Si phase in the Fe2Al5 phase during high-temperature heating in hot stamping. As a result, the Fe-Al-Si phase in the Fe2Al5 phase becomes ΣL i It is believed that this makes it possible to produce a hot-stamped product in which Zn is dispersed so as to satisfy the relationship / L0≦0.70. Furthermore, by including 2.0% by mass or more of Zn in the interface alloy layer, Zn, which has a sacrificial anticorrosion effect, can be solid-dissolved in the Fe—Al phase having an Fe content of 20 to 40% by mass, which is formed by alloying during high-temperature heating in hot stamping, and this makes it possible to improve the corrosion resistance of the coating layer after hot stamping.

[0067] Generally, the entry sheet temperature is controlled to be the same as or approximately the same as the coating bath temperature, as described above. Even if the entry sheet temperature is different from the coating bath temperature, the entry sheet temperature is generally controlled within a range of ±20°C relative to the coating bath temperature. Reasons for this include the fact that, even though the sheet temperature rises to the coating bath temperature during immersion in the coating bath, there is no need to incur production costs by cooling and controlling the entry sheet temperature to a lower temperature, and the fact that the coating bath may solidify due to the entry sheet temperature being lower than the coating bath temperature. Therefore, it is quite unexpected and surprising that the corrosion resistance of the coating layer after hot stamping can be improved by controlling the coating bath temperature to 650°C or higher and the entry sheet temperature to 500°C or lower, as in the present manufacturing method, thereby ensuring that the entry sheet temperature is at least 150°C lower than the coating bath temperature. The lower limit of the entry sheet temperature is not particularly limited, but for example, the entry sheet temperature of the steel base material into the coating bath may be 380°C or higher. Conventional wisdom holds that controlling the entry sheet temperature to 500°C or less may cause the plating bath to solidify due to the composition of the plating bath, etc. However, in this embodiment, it has been discovered that solidification of the plating bath can be prevented by setting the difference between the entry sheet temperature and the plating bath temperature to 150°C or more, which is significantly greater than conventional wisdom, i.e., by setting the entry sheet temperature to 500°C or less and increasing the plating bath temperature to a high temperature of 650°C. In order to prevent solidification of the plating bath, it is preferable to convect the plating solution by stirring it in the plating bath.

[0068] The plating step is carried out by, for example, hot dip plating. Other conditions of the plating step may be appropriately set taking into consideration the thickness and coating weight of the plating layer, etc. 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 a gas wiping method, and then cooled, so that the coating weight of the plating layer can be kept within a predetermined range, for example, 20 to 200 g / m per side. 2 can be adjusted within the range.

[0069] [Cooling after plating] As described above, cooling after plating is carried out to 300°C at an average cooling rate of 10°C / s or less. By cooling the plated steel sheet at such a relatively slow average cooling rate, it is possible to reduce the frequency at which Fe—Al phase nuclei are generated from the molten plating (nucleation frequency). In this case, the Fe—Al phase can be grown relatively slowly, which makes it possible to coarsen the Fe—Al phase and form an interfacial alloy layer with a relatively flat shape with few irregularities at the interface with the steel base material.

[0070] In relation to this, the inventors have discovered that in order to obtain desired characteristics in the coating layer after hot stamping, it is important to control the unevenness of the interface alloy layer before hot stamping to a flatter shape. During high-temperature heating in hot stamping, the interface alloy layer grows within the coating layer as the coating layer and the steel base material are alloyed. Therefore, if the degree of unevenness in the interface alloy layer is large, an Fe—Al phase may grow from the interface alloy layer into a needle-like shape within the coating layer during high-temperature heating in hot stamping, and the needle-like grown Fe—Al phase may protrude from the surface of the coating layer. Since the coating layer according to the embodiment of the present invention contains a relatively large amount of Al, its surface is covered with an oxide film composed of Al-containing oxides and the like. However, the Fe—Al phase that grows into a needle-like shape during high-temperature heating in hot stamping may destroy this oxide film. In such cases, Zn contained in the coating layer evaporates from the destroyed portion of the oxide film. Zn has a relatively low boiling point of approximately 907°C, which poses a problem of its tendency to evaporate during heating at approximately 900°C or higher during hot stamping. Covering the surface of the coating layer with an oxide film composed of Al-containing oxides or the like can suppress or reduce the evaporation of Zn in the coating layer, even when the coating layer is applied to hot stamping. However, if the oxide film is destroyed, Zn evaporates significantly from the destroyed portion of the oxide film. As a result, the Zn content of the entire coating layer after hot stamping and the amount of solute Zn in the Fe—Al phase having an Fe content of 20 to 40 mass% decrease, resulting in the Zn content of the Fe—Al phase becoming less than 1.5 mass%. As a result, the corrosion resistance of the hot-stamped steel sheet is reduced.

[0071] In contrast, by controlling the average cooling rate from after plating to 300°C to 10°C / s or less, as described above, the frequency at which Fe—Al phase nuclei are generated from the molten plating (nucleation frequency) is reduced, allowing the Fe—Al phase to grow relatively slowly. This makes it possible to coarsen the Fe—Al phase and form an interfacial alloy layer with a relatively flat shape with few irregularities at the interface with the steel base material. Therefore, needle-like growth of the Fe—Al phase from the interfacial alloy layer within the plating layer during high-temperature heating in hot stamping can be suppressed. As a result, destruction of the oxide film present on the surface of the plating layer can be suppressed or minimized, allowing the oxide film to significantly suppress or reduce Zn evaporation in the plating layer. This makes it possible to control the Zn content of the Fe—Al phase having an Fe content of 20 to 40 mass% in the plating layer after hot stamping to 1.5 to 15.0 mass%. On the other hand, if the average cooling rate from after plating to 300°C is faster than 10°C / s, the nucleation frequency of the Fe—Al phase increases, and therefore the growth of the Fe—Al phase from the molten plating becomes faster. Rapid growth of the Fe—Al phase results in the formation of an interfacial alloy layer with greater irregularities at the interface with the steel base material. In relation to this, during high-temperature heating in hot stamping, the Fe—Al phase may grow needle-like from the interfacial alloy layer, destroying the oxide film present on the surface of the plating layer. In such cases, Zn contained in the plating layer evaporates from the destroyed oxide film. As a result, the Zn content of the Fe—Al phase having an Fe content of 20 to 40 mass% in the plating layer after hot stamping becomes less than 1.5 mass%, resulting in a decrease in the corrosion resistance of the hot-stamped body. To further flatten the irregularities in the interfacial alloy layer before hot stamping, the average cooling rate from after plating to 300°C is preferably 5°C / s or less.

[0072] [Hot Stamping Process] Finally, the resulting plated steel sheet is hot stamped in a hot stamping process to produce a hot-stamped product containing the desired coating layer. From the perspective of obtaining the desired coating layer, 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, 900°C, hold it in the furnace for 5 to 50 seconds. Holding times of 50 seconds or longer may result in excessive alloying of the coating layer, making it difficult to sufficiently form an Fe—Al phase with a relatively low Fe content. 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 may be controlled in these gases. Examples of heating methods include electrical heating, high-frequency heating, and induction heating. After heating and holding in the furnace, the plated steel sheet is removed from the furnace, and after the plated steel sheet reaches a predetermined temperature, for example, a temperature of 850°C or less, it can be hot stamped under ordinary conditions. 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, although this is not particularly limited.

[0073] According to this manufacturing method, the chemical composition of the entire plating layer can be controlled within a predetermined range, and the deposition amount of the plating layer can be adjusted to 20 g / m per side. 2 At the same time, the Fe-Al-Si phase in the Fe2Al5 phase formed on the steel base material side is ΣL i This method can produce a hot-stamped product having a coating layer in which the Fe—Al—Si phase is dispersed so as to satisfy the relationship .gtoreq. / L0 ≦0.70, and the Zn content of the Fe—Al phase formed on the surface side is 1.5 to 15.0 mass %. Therefore, the corrosion resistance of the hot-stamped product can be significantly improved by combining the corrosion progression inhibition effect achieved by dispersing and forming the Fe—Al—Si phase in the Fe2Al5 phase with the corrosion resistance improvement effect achieved by dissolving Zn in the Fe—Al phase. Therefore, this hot-stamped product can achieve superior corrosion resistance compared to conventional hot-stamped products. This can contribute to industrial development by extending the life of automotive and building material applications.

[0074] The present invention will be described in more detail below with reference to examples, but the following examples are merely illustrative of the present invention and are not intended to limit the present invention in any way. It goes without saying that the present invention can be modified as desired without departing from the gist of the present invention.

[0075] 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.

[0076] First, molten steel was cast by a continuous casting method to form a slab having a chemical composition consisting of, in mass%, C: 0.20%, Si: 0.012%, Mn: 1.30%, Al: 0.030%, P: 0.005%, S: 0.0020%, and N: 0.0030%, with the balance consisting of Fe and impurities. The slab was once cooled, reheated to 1200 ° C, hot rolled, and then coiled at a temperature of 600 ° C or less. Hot rolling was carried out by performing rough rolling and finish rolling, the finish rolling end temperature was 900 to 1050 ° C, and the finish rolling reduction was 30%. Next, the obtained hot-rolled steel sheet was subjected to pickling, and then cold-rolled at a reduction ratio of 50% to obtain a cold-rolled steel sheet having 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 1 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 cooled at an average cooling rate of 10°C / s or more to produce a steel base material.

[0077] Next, the produced steel base material was cut into 100 mm × 200 mm pieces and plated using an in-house batch-type hot-dip galvanizing test apparatus. More specifically, the produced steel base material was first introduced into a coating bath having various chemical compositions and the coating bath temperature shown in Table 1 at the entry sheet temperature shown in Table 1 and immersed for approximately 3 seconds. It was then pulled up at a pulling speed of 20 to 200 mm / s, and the coating layer thickness was adjusted to the value shown in Table 1 by N2 gas wiping. Next, the steel base material with the coating layer attached was cooled to 300°C using nitrogen gas as the cooling gas at the average cooling rate shown in Table 1, 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.

[0078] [Chemical Composition Analysis of the Coating Layer, etc.] The chemical composition of the coating layer after hot stamping was determined as follows. First, the coated steel sheet was heated by electrical heating. After reaching a temperature of 900°C, the temperature was maintained for 10 seconds. Next, the coated steel sheet was removed from the furnace, sandwiched between flat dies at approximately room temperature, and quenched. This resulted in a hot-stamped product with a coating layer containing, from the base steel side, an FeAl phase and an Fe-Al phase with an Fe content of 20 to 40 mass%. After heating and quenching, the hot-stamped product was cut into 30 mm x 30 mm samples. The coating layer of each sample was dissolved in a room-temperature acidic aqueous solution containing 10% hydrochloric acid and 0.04% Ivit 710K (manufactured by Asahi Chemical Industries). The resulting acid solution was analyzed by ICP emission spectroscopy to determine the chemical composition of the coating layer. The mass of each sample was measured before and after pickling, and the coating weight of the coating layer was determined from the change in mass. The results are shown in Table 1.

[0079] [Structural analysis of the coating layer] Using the method described above, the Fe2Al5 phase, the Fe-Al phase having an Fe content of 20 to 40 mass%, and the Fe-Al-Si phase having an Si content of 5 to 20 mass% and an equivalent circle diameter of 2 μm or more were identified, and the Zn content of the Fe-Al phase was determined using the same method described above. i The value of / L0 was determined. Except for the example in which there was no Fe—Al phase having an Fe content of 20 to 40 mass % (specifically, Comparative Example 37), the FeAl phase and the Fe—Al phase having an Fe content of 20 to 40 mass % were present in this order from the steel base metal side.

[0080] [Red Rust Resistance] Red rust resistance after hot stamping was evaluated as follows. First, the plated steel sheet was heated by electrical heating. After the temperature of the plated steel sheet reached 900°C, it was held for 10 seconds. Next, the plated steel sheet was removed from the furnace and clamped between flat molds at approximately room temperature for quenching. A 50 mm x 100 mm sample of the hot stamped body after heating and quenching was 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 introduced in the center of the sample, reaching the base steel (steel base material). Next, a neutral salt spray cycle test specified in 8.1 of JIS H 8502:1999 was carried out to measure the number of cycles until red rust appeared at the cut portion, and the corrosion resistance was evaluated as follows: AA: More than 10 cycles A: 3 to 10 cycles B: Less than 3 cycles

[0081] [Paint Blistering] Paint blistering was evaluated as follows. First, a 50 mm x 100 mm sample of the hot stamped body after heating and quenching in the same manner as in the case of red rust resistance was subjected to zinc phosphate treatment (SD5350 system: Nippon Paint Industrial Coating standard), followed by electrodeposition coating (PN110 Powernics Gray: Nippon Paint Industrial Coating standard) to a thickness of 20 μm and baking at 150°C for 20 minutes. Next, a cut reaching the base steel (steel base material) was introduced into the center of the sample. Next, a neutral salt spray cycle test specified in 8.1 of JIS H 8502:1999 was performed for 120 cycles, the paint blister width was measured, and corrosion resistance was evaluated as follows. AAA: 2 mm or less AA: More than 2 to 3 mm A: More than 3 to 4 mm B: More than 4 mm

[0082] Hot-stamped articles with an Al-containing plating layer having improved corrosion resistance even after hot stamping were evaluated as having red rust resistance ratings of AA and A and coating blister ratings of AAA, AA and A. The results are shown in Table 1.

[0083]

[0084]

[0085] Referring to Table 1, it is believed that in Comparative Example 32, the desired Si content was not obtained in the interface alloy layer in the coating layer before hot stamping because the entry temperature in the coating process was high. As a result, the formation of the Fe-Al-Si phase in the FeAl phase in the coating layer after hot stamping became significant, resulting in the ΣL i The value of / L0 increased, and corrosion resistance after hot stamping deteriorated. In Comparative Example 33, the average cooling rate from after plating to 300°C was fast, which is thought to have increased the frequency of nucleation of the Fe-Al phase from the molten coating, resulting in the formation of an interfacial alloy layer with greater irregularities at the interface with the steel base material. In relation to this, it is thought that the oxide film on the surface of the coating layer was destroyed by the Fe-Al phase that grew into needle-like shapes during high-temperature heating in hot stamping, causing much of the Zn contained in the coating layer to evaporate. As a result, the Zn content of the Fe-Al phase decreased, and corrosion resistance after hot stamping deteriorated. In Comparative Example 34, the Zn content in the coating layer was low, which also reduced the Zn content of the Fe-Al phase, preventing the coating layer from fully exerting its sacrificial corrosion protection effect. As a result, red rust resistance, particularly after hot stamping, deteriorated. In Comparative Example 35, the Si content in the coating layer was high, resulting in significant formation of the Fe-Al-Si phase, resulting in a large increase in ΣL iThe value of / L0 increased, and the corrosion resistance after hot stamping deteriorated. In Comparative Example 36, the low Si content in the coating layer reduced the adhesion of the coating layer, resulting in reduced corrosion resistance after hot stamping. In Comparative Example 37, the coating layer had an insufficient adhesion, causing excessive alloying of the coating layer during high-temperature heating in hot stamping, and failing to form an Fe—Al phase with an Fe content of 20 to 40 mass%. As a result, the corrosion resistance improvement effect resulting from the solid solution of Zn in the Fe—Al phase was not obtained, and the corrosion resistance after hot stamping deteriorated. In Comparative Example 38, the coating bath temperature was low, causing the coating bath to solidify when the steel base material was immersed in the coating bath, preventing the coating layer from being properly formed. For this reason, subsequent production was stopped, and analysis and performance evaluation were not performed.

[0086] In contrast to this, in the hot stamped steel sheets according to all the examples, the chemical composition of the entire plating layer was controlled within a predetermined range, and the coating weight of the plating layer was 20 g / m per side. 2 At the same time, the Fe-Al-Si phase in the Fe2Al5 phase formed on the steel base material side is ΣL iBy dispersing the Fe—Al—Si phase so as to satisfy the relationship / L0 ≦0.70, and further controlling the Zn content of the Fe—Al phase formed on the surface side to 1.5 to 15.0 mass%, the corrosion resistance of the hot-stamped steel sheet could be significantly improved even when exposed to high temperatures of 900°C, due to the combination of the corrosion progression inhibition effect obtained by dispersing and forming an Fe—Al—Si phase in the Fe2Al5 phase and the corrosion resistance improvement effect obtained by dissolving Zn in the Fe—Al phase. Furthermore, when the element concentrations of the cross sections of the coating layers of the plated steel sheets according to all Examples before hot stamping were measured by EPMA, the Si content in the interface alloy layer was higher than that of Comparative Example 32, being 3.0 mass% or more in all cases, and the Zn content was 2.0 mass% or more in all cases. Furthermore, when the interface alloy layer of the plated steel sheets according to all Examples before hot stamping was identified from the element distribution images of the mapping images obtained using EPMA, the shape of the interface alloy layer was found to be flatter with less irregularities than that of Comparative Example 33. In particular, the chemical composition of the entire plating layer contains Zn: 5.0 to 35.0% and Si: 0.25 to 6.0%, the Zn content of the Fe—Al phase is 3.0 to 15.0 mass%, and further ΣL i In Examples 27 to 31, in which the relationship of / L0≦0.50 was satisfied, the red rust resistance was evaluated as AA and the coating blistering was evaluated as AAA, thereby achieving further improvement in corrosion resistance. Furthermore, when GDS measurements were performed on each hot-stamped steel, the presence of a solid solution layer containing an Fe—Al solid solution layer, an Fe—Zn solid solution layer, and / or an Fe—Al—Zn solid solution layer was confirmed between the steel base material and the coating layer in all of the hot-stamped steels according to the examples.

[0087] REFERENCE SIGNS LIST 1 Hot stamped body 2 Steel base material 3 Coating layer 4 Fe2Al5 phase 5 Fe-Al phase having an Fe content of 20 to 40 mass % 6 Fe-Al-Si phase having an Si content of 5 to 20 mass % and an equivalent circle diameter of 2 μm or more

Claims

1. A steel substrate comprising a steel base material and a plating layer disposed on a surface of the steel base material, the plating layer having a chemical composition, in mass%, of Zn: 2.5-40.0%, Si: 0.05-15.0%, Fe: 25.0-40.0%, Ni: 0-0.500%, Mg: 0-3.000%, Ca: 0-3.000%, 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%, W: 0-0.500%, and the balance: Al and impurities, the total content of Ni, Mg, Ca, Sb, Pb, Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, Li, La, Ce, B, Y, Sr, In, Co, Bi, P, and W is 5.000% or less, the plating layer includes, in order from the steel base material side, an Fe2Al5 phase and an Fe-Al phase having an Fe content of 20 to 40 mass%, the Zn content of the Fe-Al phase is 1.5 to 15.0 mass%, and in a cross section of the plating layer, a projected length L of an Fe-Al-Si phase in which the Si content in the Fe2Al5 phase is 5 to 20 mass% and the circle equivalent diameter is 2 μm or more, i and the length L0 of the surface of the steel base material is ΣL i / L0≦0.70, and the coating weight of the plating layer is 20 g / m per side. 2 A hot stamped product characterized by the above.

2. The hot stamped product according to claim 1, characterized in that the chemical composition of the plating layer contains, in mass%, Zn: 5.0 to 35.0%, and Si: 0.25 to 6.0%, and the Zn content of the Fe-Al phase is 3.0 to 15.0 mass%.

3. ΣL i 3. The hot stamped steel according to claim 1 or 2, wherein / L0≦0.50.

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