Hot press molded body

Abstract

Provided is a hot-press molded body provided with a steel base material and a plating layer disposed on the surface of the steel base material, the plating layer having a prescribed chemical composition, the plating layer containing a Fe2Al5 phase, the Zn content of the Fe2Al5 phase being 1.5-15.0 mass%, and the Zn content of the plating layer being 0.5-1.5 mass% in a cross section of the plating layer. The projected length Li of the Fe-Al-Si phase having an Si content of 5-20 mass% in the Fe2Al5 phase and an equivalent circle diameter of 2 [mu] m or more and the length L0 of the surface of the steel base material satisfy [sigma] Li / L0 < = 0.70, and the amount of the deposited plating layer is 20 g / m2 or more per surface.

Description

Technical Field

[0001] This invention relates to a hot-stamped formed body. Background Technology

[0002] In recent years, the automotive industry has demanded lighter vehicle bodies from the perspective of improving fuel efficiency. To balance vehicle lightweighting and crash safety, increasing the strength of steel plates used in frame components is one effective method, and against this backdrop, the development of high-strength steel plates is underway.

[0003] Hot stamping is a known technique for forming materials that are difficult to shape, such as high-strength steel sheets. Hot stamping is a thermoforming technique that involves heating the material to be formed before shaping. In this technique, because the material is heated before forming, the steel is soft and has good formability during forming. Therefore, it is known that even high-strength steel can be formed into complex shapes with good precision. Furthermore, because quenching is performed simultaneously with forming using a stamping die, the formed steel has sufficient strength.

[0004] Relatedly, various studies have also been conducted on hot-stamped bodies with Al coating and the coated steel sheets used for such hot-stamped bodies.

[0005] For example, Patent Document 1 describes a steel sheet containing 2.0 to 24.0 wt% zinc, 7.1 to 12.0 wt% silicon, 1.1 to 8.0 wt% magnesium, and any additional elements selected from Pb, Ni, Zr, or Hf, with each additional element having a weight content of less than 0.3 wt%, and the remainder being aluminum and any unavoidable impurities and residual elements. The sheet is coated with a metal coating having an Al / Zn ratio greater than 2.9. Furthermore, Patent Document 1 teaches that a metal coating with an Al / Zn ratio greater than 2.9 provides high sacrificial protection.

[0006] Patent Document 2 discloses an aluminum alloy-coated steel sheet comprising a base steel sheet and an aluminum alloy coating formed on the base steel sheet. The aluminum alloy coating, by weight percent, comprises Zn: 21-35%, Si: 1-6.9%, Fe: 2-12%, with the remainder being Al and other unavoidable impurities. Furthermore, Patent Document 2 teaches that by controlling the Al / (Zn+Si) ratio of the aluminum alloy coating to 1.3-2.6, weldability and corrosion resistance can be ensured.

[0007] Existing technical documents Patent documents Patent Document 1: Japanese Patent Publication No. 2018-528324 Patent Document 2: Japanese Patent Publication No. 2022-550142 Summary of the Invention

[0008] The problem that the invention aims to solve For example, if a coated steel sheet as described in Patent Documents 1 and 2 is used in hot stamping, or more specifically a coated steel sheet containing more Al as a component of the coating, the coating alloys with the base metal (base steel sheet) during heating in the hot stamping process, which sometimes reduces corrosion resistance.

[0009] Therefore, the object of the present invention is to provide a hot-stamped body with an Al coating that has improved corrosion resistance even after hot stamping.

[0010] Methods for solving problems In order to achieve the above-mentioned objective, the inventors conducted research and found that by forming a coating composed of an Al-Zn-Si system coating with an adhesion amount of more than a specified amount and by appropriately controlling the chemical composition and / or morphology of the Fe2Al5 phase in the coating formed after hot stamping, excellent corrosion resistance can be achieved in the hot stamped body, thus completing the present invention.

[0011] The present invention, which achieves the above objectives, is described below.

[0012] (1) A hot-stamped formed body, characterized in that it comprises a steel base material and a coating disposed on the surface of the steel base material, The chemical composition of the above coating, expressed as a percentage by mass, is as follows: Zn: 2.5~30.0% Si: 0.05~12.0% Fe: 40.0~80.0% 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 remainder consists of Al and impurities, and... 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 less than 5.000%. The above coating contains the Fe2Al5 phase. The Zn content of the Fe2Al5 phase is 1.5~15.0% by mass. In the cross-section of the above coating, the projected length L of the Fe-Al-Si phase with a Si content of 5~20% by mass and an equivalent circle diameter of 2μm or more in the Fe2Al5 phase is... i The length L0 of the surface of the aforementioned steel base material satisfies ΣL i / L0≤0.70, The coating thickness is 20 g / m² per side. 2 above.

[0013] (2) The hot-stamped formed body according to (1) above, characterized in that the chemical composition of the coating, in mass %, contains: Zn: 5.0~25.0%, and Si: 0.25~6.0%, The Zn content of the above Fe2Al5 phase is 3.0~15.0 by mass.

[0014] (3) The hot-stamped formed body according to (1) or (2) above, characterized in that ΣL i / L0≤0.50.

[0015] Invention Effects According to the present invention, it is possible to provide a hot-stamped body with an Al coating that has improved corrosion resistance even after hot stamping. Attached Figure Description

[0016] Figure 1 This is an example of a cross-sectional schematic diagram of the surface portion of a hot-stamped formed article according to an embodiment of the present invention, showing the projected length L of the Fe-Al-Si phase. i The length L0 of the surface of the steel base material. Detailed Implementation

[0017] <Hot-stamped formed body> The hot-stamped formed body according to the embodiments of the present invention is characterized in that it comprises a steel base material and a coating disposed on the surface of the steel base material. The chemical composition of the above coating, expressed as a percentage by mass, is as follows: Zn: 2.5~30.0% Si: 0.05~12.0% Fe: 40.0~80.0% 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 remainder consists of Al and impurities, and... 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 less than 5.000%. The above coating contains the Fe2Al5 phase. The Zn content of the Fe2Al5 phase is 1.5~15.0% by mass. In the cross-section of the above coating, the projected length L of the Fe-Al-Si phase with a Si content of 5~20% by mass and an equivalent circle diameter of 2μm or more in the Fe2Al5 phase is... i The length L0 of the surface of the aforementioned steel base material satisfies ΣL i / L0≤0.70, The coating thickness is 20 g / m² per side. 2 above.

[0018] As described above, if a coated steel sheet containing a significant amount of Al as a component in the coating is used in hot stamping, the coating may alloy with the base metal (base steel sheet) during heating in the hot stamping process, sometimes resulting in reduced corrosion resistance. More specifically, for example, when a coated steel sheet with a conventional Al-Si based coating is hot stamped, the coating alloys with the base steel sheet during high-temperature heating in the hot stamping process (e.g., heating at approximately 900°C or higher), forming a coating primarily containing the Fe2Al5 phase after hot stamping. However, in addition to the Fe2Al5 phase, a significant amount of Fe-Al based intermetallic compounds containing Si, particularly the Fe-Al-Si phase containing 5% by mass or more of Si, may also form in the coating. In such cases, dissimilar metal contact corrosion (galvanic corrosion) may occur between the Fe2Al5 phase and the Fe-Al-Si phase, reducing the corrosion resistance of the hot-stamped body.

[0019] Therefore, in order to provide a hot-stamped body with an Al-containing coating that exhibits excellent corrosion resistance even after hot stamping, the inventors have focused particularly on the chemical composition and morphology of the Al-Si based coating before hot stamping. As a result, the inventors have discovered that by appropriately preparing a coating consisting of an Al-Zn-Si based coating obtained by further adding Zn to an Al-Si based coating, and by appropriately selecting the heat treatment conditions for hot stamping, the formation of the Fe-Al-Si phase, which can cause galvanic corrosion between the Fe2Al5 phase and the hot-stamped body, can be suppressed in the coating formed after hot stamping. Furthermore, the Zn with its anti-corrosion properties can be fully dissolved in the Fe2Al5 phase, thereby achieving excellent corrosion resistance in the hot-stamped body.

[0020] More specifically, firstly, the inventors discovered that by setting the adhesion amount of the coating composed of Al-Zn-Si system coatings to 20 g / m per side, 2 In summary, although at least part or all of the coating is alloyed with the Fe2Al5 phase during the high-temperature heating in hot stamping, the corrosion resistance after hot stamping can be improved by increasing the amount of coating adhesion, improving the coating adhesion by adding Si, and providing corrosion protection by adding Zn.

[0021] Next, the inventors discovered that, as will be described in detail later in connection with the method for manufacturing hot-stamped articles, by containing a specified amount of Si and Zn in the Fe-Al phase corresponding to the interface alloy layer formed at the interface between the base steel sheet and the coating before hot stamping, and more specifically, by containing 3.0% by mass or more of Si and 2.0% by mass or more of Zn in the interface alloy layer, and by controlling the shape of the interface alloy layer to be a flatter shape with less unevenness, the formation of the larger Fe-Al-Si phase in the Fe2Al5 phase can be suppressed in the coating formed after hot stamping, and Zn can be fully dissolved in the Fe2Al5 phase. More specifically, the inventors discovered that in the coating after hot stamping, by dispersing the relatively large Fe-Al-Si phase in the Fe2Al5 phase formed in the coating, that is, by having a projected length L of the Fe-Al-Si phase with a Si content of 5 to 20% by mass and an equivalent circle diameter of 2 μm or more. i The length L0 of the surface of the steel base material satisfies ΣL i Controlling the corrosion resistance of the coating by setting L0≤0.70 can significantly improve the corrosion resistance of the coating after hot stamping.

[0022] Figure 1 This is an example of a cross-sectional schematic diagram of the surface portion of a hot-stamped formed article according to an embodiment of the present invention, showing the projected length L of the Fe-Al-Si phase. i The length L0 of the surface of the steel base material. (Refer to...) Figure 1 The hot-stamped formed body 1 of the present invention includes a steel base material 2 and a coating 3 disposed on the surface of the steel base material 2, the coating 3 comprising Fe2Al5 phase 4. Furthermore, in Figure 1 In the Fe2Al5 phase 4, there exists a Fe-Al-Si phase 5 with a Si content of 5-20% by mass and an equivalent circle diameter of 2 μm or more. Here, the projection length L of each Fe-Al-Si phase 5 onto the surface of the steel base material 2 is... i The total ΣL i (exist Figure 1 The middle is ΣL i=L1+L2+L3) and the length L0 of the surface of the steel base 2 satisfy ΣL i / L0≤0.70, therefore, a relatively large Fe-Al-Si phase 5 exists dispersedly within the Fe2Al5 phase 4. When the projected lengths of multiple Fe-Al-Si phases 5 partially overlap, the total projected length including the overlapping portion is defined as one projected length. It is assumed that even if the Fe-Al-Si phase 5 exists in a layered manner within the Fe2Al5 phase 4, galvanic corrosion occurs at the contact between the layered Fe-Al-Si phase 5 and the Fe2Al5 phase 4, and corrosion proceeds along the contact interface. As a result, the corrosion resistance of the coating 3 is significantly reduced. In contrast, in the hot-stamped formed body 1 of the embodiment of the present invention, by satisfying ΣL... i The Fe-Al-Si phase 5 is dispersed within the Fe2Al5 phase 4 in a manner where L0 ≤ 0.70. Even if galvanic corrosion occurs at the contact points between one or more Fe-Al-Si phases 5 and the surrounding Fe2Al5 phase 4, corrosion will not proceed to other Fe-Al-Si phases 5 starting from the corrosion at those contact points. Therefore, the corrosion resistance of the coating 3 can be significantly improved. Furthermore, in the hot-stamped formed body 1 of the embodiments of the present invention, by containing 1.5 to 15.0% by mass of Zn in the Fe2Al5 phase 4, the substitution corrosion-resistant effect of Zn can be effectively exerted, thereby significantly improving corrosion resistance.

[0023] Therefore, the hot-stamped body according to embodiments of the present invention achieves excellent corrosion resistance through a combination of the corrosion suppression effect obtained by suppressing the formation of Fe-Al-Si phase 5, which can cause galvanic corrosion between Fe2Al5 phase 4 and Fe2Al5 phase 4, and more specifically by the dispersed formation of relatively large Fe-Al-Si phase 5, and the corrosion resistance improvement effect obtained by dissolving Zn in Fe2Al5 phase 4. In particular, the fact that the corrosion resistance of the hot-stamped body 1 can be improved by forming the coating 3 from Fe2Al5 phase 4 and appropriately controlling the chemical composition and morphology of the Fe2Al5 phase 4 as described above is a fact that has been clarified for the first time by the inventors. Therefore, the hot-stamped body according to embodiments of the present invention is particularly useful in the automotive field, where hot stamping is widely applied.

[0024] Hereinafter, the hot-stamped formed articles according to embodiments of the present invention will be described in more detail. In the following description, unless otherwise specified, "%" as the unit of content of each element refers to "mass%". Furthermore, in this specification, "~" indicating a numerical range is used to mean the lower limit and upper limit value including the values ​​described before and after it unless otherwise specified.

[0025] [Coating] According to an embodiment of the present invention, the coating is disposed on the surface of a steel base material, for example, on at least one, preferably both, surfaces of the steel base material. Here, the phrase "disposed on the surface of the steel base material" includes not only the case where the coating is directly disposed on the surface of the steel base material, but also the case where the coating is indirectly disposed on the surface of the steel base material, for example, where a solid solution layer, as described later, is included between the steel base material and the coating. The coating has the following chemical composition.

[0026] [Zn: 2.5~30.0%] Zinc (Zn) is an element that has a corrosion-resistant effect and is effective in improving the corrosion resistance of coatings. To achieve this effect, the Zn content is set at 2.5% or more. The Zn content can 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, or 20.0% or more. On the other hand, if the Zn content is excessive, the melting of Zn during the high-temperature heating of hot stamping becomes significant, and there is a possibility that molten Zn will seep into the steel, causing liquid metal embrittlement (LME) cracks. Therefore, the Zn content is set at 30.0% or less. The Zn content can be 28.0% or less, 26.0% or less, 25.0% or less, 24.0% or less, or 22.0% or less.

[0027] [Si: 0.05~12.0%] Si is an effective element for improving the adhesion of coatings. To achieve this effect, the Si content is set to 0.05% or more. The Si content can 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, if the Si content is excessive, the formation of the Fe-Al-Si phase in the Fe2Al5 phase becomes significant, sometimes reducing corrosion resistance. Therefore, the Si content is set to 12.0% or less. The Si content can be 10.0% or less, 8.0% or less, or 6.0% or less.

[0028] [Fe: 40.0~80.0%] If the steel sheet is heated during hot stamping, Fe from the base steel diffuses into the coating and alloys with Al and other materials, thus the coating inevitably contains Fe. Therefore, the Fe content is 40.0% or more, for example, more than 40.0%, 41.0% or more, 42.0% or more, 45.0% or more, 50.0% or more, 55.0% or more, or 60.0% or more. On the other hand, if the Fe content is too high, the corrosion resistance may decrease due to over-alloying of the coating. Therefore, the Fe content is set to 80.0% or less, for example, less than 75.0%, 70.0% or less, or 65.0% or less.

[0029] Furthermore, the coating may optionally contain 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%, M At least one of the following: o: 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%. The total content of these optional elements (i.e., the total content of these elements) is less than 5.000%. The total content of optional elements can be below 4.500%, 4.000%, 3.500%, 3.000%, 2.500%, 2.000%, 1.500%, 1.000%, 0.800%, 0.500%, 0.100%, or 0.050%. The presence of these elements is not mandatory, and their total content can be 0%. As needed, the lower limit of the total content of these elements can be set to 0.001%, 0.010%, 0.050%, or 0.080%. These optional elements are described in detail below.

[0030] [Ni: 0~0.500%] Ni is an effective element for improving the corrosion resistance of coatings. The Ni content can be 0%, but to achieve this effect, the Ni content is preferably 0.001% or higher. The Ni content can be 0.003% or higher, 0.005% or higher, 0.008% or higher, 0.010% or higher, or 0.020% or higher. There is no specific upper limit, but from a manufacturing cost perspective, the Ni content is set to 0.500% or lower; for example, it can be 0.400% or lower, 0.300% or lower, 0.100% or lower, 0.050% or lower, or 0.030% or lower.

[0031] [Mg: 0~3.000%] Mg is an effective element for improving the corrosion resistance of coatings. The Mg content can be 0%, but to achieve this effect, the Mg content is preferably 0.001% or more. The Mg content can be 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, from the viewpoint of improving processability, the Mg content can be 3.000% or less. The Mg content can 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.

[0032] [Ca: 0~3.000%] Ca is an effective element for ensuring the wettability of the plating bath. The Ca content can be 0%, but to achieve the aforementioned effect, the Ca content is preferably 0.001% or more. The Ca content can be 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, if the Ca content is excessive, a large amount of hard intermetallic compounds will form in the coating, making the coating brittle and sometimes reducing its adhesion to the steel sheet. Therefore, the Ca content is preferably 3.000% or less. The Ca content can 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.

[0033] [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: 0~1.00 0%, 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 included in the coating, but may be present in the coating in amounts of 0.0001% or more, 0.001% or more, or 0.01% or more. These elements, as long as they are within the specified content range, will not adversely affect the performance of the hot-stamped body. However, excessive amounts of any element can sometimes 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, 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, for example, 0.800%, 0.500%, 0.100%, 0.050% or less or 0.020% or less.

[0034] In the coating, the remainder besides the elements mentioned above consists of Al and impurities. Impurities in the coating refer to components, primarily the raw materials, that are introduced during the manufacturing process due to various reasons. It should be noted that the Al content does not need to be specifically specified, but it can be set to 5.0% to 42.55%. Depending on the requirements, the Al content can be 10.0% or more, 15.0% or more, 20.0% or more, 25.0% or more, or 28.0% or more, and can be below 42.0% or less, 40.0% or less, 37.0% or less, 34.0% or less, 32.0% or less, or 30.0% or less.

[0035] [Determination of the chemical composition of the coating] The chemical composition of the coating was determined by dissolving the coating in an acidic aqueous solution and performing chemical analysis. Specifically, the coating was dissolved in an acidic aqueous solution containing an inhibitor that inhibits the dissolution of steel, such as 0.04% IBIT 710K (manufactured by Asahi Chemical Industry) in 10% hydrochloric acid at room temperature. The resulting acidic aqueous solution was then analyzed by ICP (inductively coupled plasma atomic emission spectrometry) to obtain the chemical composition of the coating.

[0036] [Fe2Al5 phase] [ΣL i / L0≤0.70] In an embodiment of the present invention, the coating contains a Fe2Al5 phase, wherein the Si content in the Fe2Al5 phase is 5-20% by mass and the projected length L of the Fe-Al-Si phase has an equivalent circle diameter of 2 μm or more. i The length L0 of the surface of the steel base material is controlled to satisfy ΣL i / L0≤0.70. As described above, Fe-Al-Si phases with a Si content of 5-20% by mass can undergo galvanic corrosion with the Fe2Al5 phase. In particular, if the Fe-Al-Si phase is large and exists continuously in a layered or near-layered form, corrosion occurs along the interface between the Fe-Al-Si phase and the Fe2Al5 phase, significantly reducing the corrosion resistance of the coating. Therefore, to suppress such corrosion and improve the corrosion resistance of the coating, it is necessary to disperse the Fe-Al-Si phase with an equivalent circle diameter of 2 μm or more within the Fe2Al5 phase. Therefore, in embodiments of the present invention, as... Figure 1 As shown, the projection length L of each Fe-Al-Si phase with an equivalent circle diameter of 2 μm or more onto the surface of the steel base material is... i The total ΣL i The length L0 of the surface of the steel base material 2 satisfies ΣL i By using a / L0≤0.70 method, the Fe-Al-Si phase is dispersed within the Fe2Al5 phase, thereby improving the corrosion resistance of the coating. From the perspective of further enhancing corrosion resistance, ΣL i The lower the value of / L0, the better. For example, it can be below 0.65, below 0.60, below 0.55, below 0.50, below 0.45, or below 0.40.

[0037] On the other hand, even if the Fe-Al-Si phase with an equivalent circle diameter of less than 2 μm corrodes, it will not initiate further corrosion to other Fe-Al-Si phases. Therefore, the presence of Fe-Al-Si phases with an equivalent circle diameter of less than 2 μm does not necessarily have an adverse effect on the corrosion resistance of the hot-stamped body. Relatedly, from the viewpoint of further improving corrosion resistance, it is also possible to ensure that the Fe2Al5 phase does not contain Fe-Al-Si phases with an equivalent circle diameter greater than 2 μm. That is, ΣL i The lower bound of / L0 can be 0. While there are no specific restrictions, for example, ΣL i / L0 can be greater than 0.05, greater than 0.10, greater than 0.15, or greater than 0.20.

[0038] [Zn content in Fe2Al5 phase: 1.5~15.0% by mass] In embodiments of the present invention, the Zn content of the Fe2Al5 phase is 1.5 to 15.0% by mass. Besides the aforementioned dispersion control of the larger Fe-Al-Si phase in the Fe2Al5 phase, by including Zn, which has a substitution and corrosion-resistant effect, in the Fe2Al5 phase at an amount of 1.5% by mass or more, the corrosion resistance of the coating can be significantly improved. From the viewpoint of further improving corrosion resistance, a higher Zn content in the Fe2Al5 phase is preferred, for example, it can be 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, even if the Fe2Al5 phase contains excessive Zn, the effect saturates, which may lead to an increase in manufacturing costs. Therefore, the Zn content of the Fe2Al5 phase is set to 15.0% by mass or less, for example, 12.0% by mass or less, 10.0% by mass or less, or 8.0% by mass or less.

[0039] As is associated with exposure to high temperatures during hot stamping, the coating may have an oxide film on at least a portion of its surface.

[0040] [Analysis of the coating] The coating analysis was conducted as follows. First, five samples were collected from the hot-stamped body so that the cross-section of the coating could be observed. Next, for each sample, a rectangular area with a thickness of 80 μm in the thickness direction and a rectangular area with a thickness of 100 μm in the direction perpendicular to the thickness direction was taken as one field of view. For a total of five fields of view for the five samples, SEM-EDS or EPMA was used to capture images at 1500x magnification to obtain mapping images. Based on the elemental distribution images of these mapping images, the positions of the Fe2Al5 phase and the Fe-Al-Si phase containing Si content of 5-20% by mass and an equivalent circle diameter of 2 μm or more within the Fe2Al5 phase were determined. More specifically, the elements to be measured were set as Fe, Al, Si, and Zn, and elemental distribution images based on SEM-EDS or EPMA were obtained. In these elemental distribution images, the regions satisfying Fe: more than 40-60% by mass, Al: 40-less than 60% by mass, Si: 0-less than 5% by mass, and Zn: 0-15% by mass were defined as the Fe2Al5 phase (…). Figure 1 Symbol 4 in the text). Regions with a Si content of 5-20% by mass and an equivalent circle diameter of 2 μm or more, and which are surrounded by the Fe2Al5 phase, are designated as Fe-Al-Si phases. Figure 1 (Symbol 5 in the text). Next, using image analysis software (e.g., the "Analyze" function of "ImageJ"), the Fe-Al-Si phases are projected onto the surface of the steel base material, and the projection length L of each Fe-Al-Si phase is calculated. i The total ( Figure 1 (L1+L2+L3). Finally, calculate L for the 5 samples.i The average of the sums is taken as ΣL i Calculate the ratio ΣL to the length L0 of the corresponding steel base material surface (length of the long side in each field of view: 100μm). i / L0. Here, as Figure 1 The length L0 of the long side in the field of view is also the interval between the two ends of the measurement area (field of view) (where the interval is in the direction parallel to the surface of the hot stamping body 1).

[0041] The Zn content in the Fe2Al5 phase was determined as follows: First, for the five samples mentioned above, the Fe2Al5 phase was identified using the method described above. Then, the elemental concentration (specifically, Zn content) constituting the Fe2Al5 phase was determined using SEM-EDS or EPMA. More specifically, in each sample, the Zn content was measured at five points spaced 50 μm apart at the center of the Fe2Al5 phase thickness, parallel to the surface of the hot-stamped body. The average value of these measurements was taken as the measured value for each sample. The measured values ​​of the five samples representing the elemental concentration were calculated, and the average value of these measured values ​​was taken as the Zn content.

[0042] The coating of embodiments of the present invention may consist solely of the aforementioned Fe2Al5 phase, more specifically solely of the Fe2Al5 phase containing the Fe-Al-Si phase, or may include phases other than the Fe2Al5 phase. For example, a Fe-Al phase with a lower Fe content than the Fe2Al5 phase, particularly a Fe-Al phase having a Fe content of 20 to 40% by mass, may be included on the Fe2Al5 phase on the steel base material side.

[0043] As a coating having the above-mentioned chemical composition and Fe2Al5 phase, a hot-dip coating may be included. It is not necessary to exclude coatings other than hot-dip coatings, but hot-dip coatings may be the only option.

[0044] Coating adhesion: 20g / m² per side 2 above] In the hot-stamped formed body according to the embodiments of the present invention, the coating adhesion amount is 20 g / m per side. 2 The above. Generally speaking, the coating sometimes alloys with the steel base material during the high-temperature heating in hot stamping, which reduces its corrosion resistance. However, according to an embodiment of the present invention, by increasing the coating amount, specifically 20 g / m² per side... 2The above describes a coating containing appropriate amounts of Si and Zn. Although at least part or all of the coating alloys with the Fe2Al5 phase during high-temperature heating in hot stamping, the improved corrosion resistance after hot stamping is achieved through the increased coating amount, the improved coating adhesion due to the addition of Si, and the substitution corrosion protection effect of the addition of Zn. Furthermore, by appropriately controlling the chemical composition and morphology of the Fe2Al5 phase as described above, excellent corrosion resistance can be achieved due to the presence of the resulting coating. On the other hand, if the coating amount is low, the above effects may not be fully obtained. From the viewpoint of improving corrosion resistance, the coating amount is preferably 30 g / m² per side. 2 Above, 40g / m 2 Above or 50g / m 2 The above, more preferably 60g / m 2 The above is further preferred to be 70g / m 2 The optimal value is 80g / m³. 2 That's all. There's no specific upper limit; the coating adhesion amount can be, for example, 200g / m². 2 Below, 190g / m 2 Below, 180g / m 2 Below or 170g / m 2 the following.

[0045] [Determination of Coating Adhesion] The amount of coating adhesion was determined by dissolving the coating solely with an acidic aqueous solution. Specifically, a 30mm × 30mm sample was taken from the hot-stamped body, and the coating was then dissolved in an acidic aqueous solution containing an inhibitor that inhibits the dissolution of steel, such as 0.04% IBIT 710K (manufactured by Asahi Chemical Industry) added to 10% hydrochloric acid at room temperature. The amount of coating adhesion was determined by the change in mass of the sample before and after the coating was dissolved.

[0046] [Solid solution layer] In embodiments of the present invention, the hot-stamped formed body may further include a solid solution layer between the steel base material and the coating. Depending on the coating composition before hot stamping and the hot stamping conditions, solidification may sometimes occur between Fe diffusing from the steel base material to the coating and Al and / or Zn in the coating, forming a solid solution layer containing these elements between the steel base material and the coating after hot stamping. Therefore, when the hot-stamped formed body 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.

[0047] The presence of the solid solution layer can be confirmed as follows. First, the coating is dissolved using an acidic aqueous solution containing an inhibitor that suppresses the dissolution of steel, specifically a room-temperature acidic aqueous solution containing 0.04% IBIT 710K (manufactured by Asahi Chemical Industry) in 10% hydrochloric acid. Next, using a high-frequency glow discharge emission spectroscopy (GDS) device, with the surface of the hot-stamped body in an Ar atmosphere, glow plasma is generated by applying voltage, and analysis is performed in the depth direction while sputtering the surface of the hot-stamped body. Then, based on the characteristic emission spectrum wavelengths of the elements emitted by atoms excited in the glow plasma, the elements contained in the material are identified, and the luminescence intensity of the identified elements is estimated. The depth direction data can be estimated based on the sputtering time. Specifically, by pre-determining the relationship between sputtering time and sputtering depth using standard samples, the sputtering time can be converted into sputtering depth. Therefore, the sputtering depth converted from sputtering time can be defined as the depth from the surface of the material. The obtained luminescence intensity is converted into mass by constructing a standard curve. In cases where GDS measurements are performed on hot-stamped formed bodies like this, regions where the Al concentration in the depth direction exceeds 1.00% by mass and is more than 1.10 times the Al content of the steel base material are identified as solid solution layers.

[0048] [Preferred Chemical Composition of Steel Base Material] As described above, the object of the present invention is to provide a hot-stamped body having an Al-containing coating, which exhibits improved corrosion resistance even after hot stamping. This is achieved by appropriately adjusting the chemical composition and adhesion amount of the coating, which is composed of an Al-Zn-Si system coating, and by ensuring that the Si content in the Fe2Al5 phase formed on the steel substrate side of the coating is 5-20% by mass and the projected length L of the Fe-Al-Si phase has an equivalent circle diameter of 2 μm or more. i The length L0 of the surface of the steel base material satisfies ΣL i The chemical composition of the steel base material is controlled in a manner where L0 ≤ 0.70, thereby achieving the objective by containing 1.5 to 15.0% by mass of Zn in the Fe2Al5 phase. 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 this invention. Hereinafter, a preferred chemical composition of the steel base material used in the hot-stamped formed body according to embodiments of the present invention will be described in detail. However, this description is intended only to illustrate a preferred chemical composition of the steel base material used in the hot-stamped formed body, and is not intended to limit the present invention to the use of steel base materials having such a specific chemical composition.

[0049] In embodiments of the present invention, for example, the steel base material preferably has a chemical composition comprising: in mass %, C: 0.13~0.50% Si: 0.001~3.000% Mn: 0.30~3.00% Al: 0.0002~2.000% P: Below 0.100% S: Below 0.1000% N: below 0.0100% 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%, Ir: 0~1.000%, and The remainder consists of Fe and impurities. The following provides a more detailed explanation of each element.

[0050] [C: 0.13~0.50%] Carbon (C) is an inexpensive element that increases tensile strength and is an important element for controlling the strength of steel. To achieve this effect, the C content is preferably set at 0.13% or more. The C content can 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 can sometimes lead to a decrease in elongation. Therefore, the C content is preferably set at 0.50% or less. The C content can be 0.45% or less, or 0.40% or less.

[0051] [Si: 0.001~3.000%] Si acts as a deoxidizer, inhibiting the precipitation of carbides during the cooling process in the annealing of cold-rolled steel sheets. To achieve this effect, the Si content is preferably 0.001% or more. The Si content can be 0.010% or more, 0.100% or more, or 0.200% or more. On the other hand, excessive Si content can sometimes lead to an increase in steel strength and a decrease in elongation. Therefore, the Si content is preferably 3.000% or less. The Si content can be 2.500% or less, 2.000% or less, 1.500% or less, or 1.000% or less.

[0052] [Mn: 0.30~3.00%] Manganese (Mn) is an element that improves the hardenability of steel and is effective in increasing strength. To achieve this effect, the Mn content is preferably set at 0.30% or more. The Mn content can be 0.50% or more, 1.00% or more, or 1.30% or more. On the other hand, excessive Mn content can sometimes lead to an increase in steel strength and a decrease in elongation. Therefore, the Mn content is preferably set at 3.00% or less. The Mn content can be 2.80% or less, 2.50% or less, or 2.00% or less.

[0053] [Al: 0.0002~2.000%] Al acts as a deoxidizer in steel, contributing to its structural integrity. To achieve this effect, the Al content is preferably 0.0002% or more. The Al content can also 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 leads to the formation of coarse Al oxides, reducing the elongation of the steel sheet. Therefore, the Al content is preferably 2.000% or less. The Al content can be 1.500% or less, 1.000% or less, 0.800% or less, or 0.500% or less.

[0054] [P: below 0.100%] Phosphorus (P) is an element that promotes embrittlement of steel due to grain boundary segregation. Lower P content is preferred, and ideally, it should be 0%. However, excessively low P content can sometimes lead to a significant increase in cost. Therefore, the P content can be set to 0.0001% or more, or 0.001% or more, or 0.005% or more. On the other hand, if P is excessive, as mentioned above, grain boundary segregation can sometimes cause embrittlement of the steel. Therefore, the P content is preferably set to 0.100% or less. The P content can be 0.050% or less, 0.030% or less, or 0.010% or less.

[0055] [S: Below 0.1000%] Sulfur (S) is an element that forms non-metallic inclusions such as MnS in steel, leading to a decrease in the ductility of steel components. Lower S content is preferred, ideally 0%. However, excessively low S content can sometimes result in a significant increase in cost. Therefore, the S content can be set to 0.0001% or more, or 0.0002% or more, 0.0010% or more, or 0.0050% or more. On the other hand, excessive S content can sometimes lead to cracks originating from non-metallic inclusions during cold forming. Therefore, the S content is preferably set to 0.1000% or less. The S content can be 0.0500% or less, 0.0200% or less, or 0.0100% or less.

[0056] [N: below 0.0100%] Nitrogen (N) is an element that forms large nitrides in steel sheets, reducing their workability. Lower N content is preferred, ideally 0%. However, excessively low N content can sometimes lead to a significant increase in manufacturing costs. Therefore, the N content can be set to 0.0001% or more, or 0.0005% or more, or 0.0010% or more. On the other hand, if the N content is excessive, as mentioned above, large nitrides can sometimes form, reducing the workability of the steel sheet. Therefore, the N content is preferably set to 0.0100% or less. The N content can be 0.0080% or less, or 0.0050% or less.

[0057] The preferred basic chemical composition of the steel base material is as described above. Furthermore, the steel base material may, as needed, contain one or more elements selected from 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% to replace a portion of the remaining Fe. These elements may be present in amounts of 0.0001% or more, 0.0005% or more, 0.001% or more, or 0.01% or more, respectively.

[0058] In steel base material, the remaining portion besides the aforementioned elements consists of Fe and impurities. Impurities in steel base material refer to components that are mixed in during the industrial manufacturing of steel base material, typically from raw materials such as ores and scrap iron, due to various reasons in the manufacturing process.

[0059] The chemical composition of the steel base material can be determined using general analytical methods. For example, to determine the chemical composition of the steel base material, the coating is first removed by mechanical grinding, and then the chips are measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) according to JIS G 1201:2014. Specifically, for example, a 35 mm square test piece can be obtained from approximately half the thickness of the steel base material, and the composition can be determined using a Shimadzu ICPS-8100 or similar measuring device under conditions based on a pre-prepared standard curve. C and S, which cannot be measured by ICP-AES, can be measured using the combustion-infrared absorption method; N can be measured using the inactive gas melting-thermal conductivity method; and O can be measured using the inactive gas melting-non-dispersive infrared absorption method.

[0060] [Thickness of hot-stamped formed body] The thickness of the hot-stamped formed body is not particularly limited; for example, it can be 0.2 mm or more, or 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 formed body can be 6.0 mm or less, or 5.0 mm or less, or 4.0 mm or less.

[0061] <Manufacturing Method of Hot Stamped Formed Body> Next, a preferred manufacturing method for the hot-stamped formed article according to an embodiment of the present invention will be described. The following description is intended to illustrate a characteristic method for manufacturing the hot-stamped formed article according to an embodiment of the present invention, and is not intended to limit the hot-stamped formed article to being manufactured by the manufacturing method described below.

[0062] The hot-stamped formed body according to embodiments of the present invention can be manufactured, for example, by performing the following steps: a casting process in which molten steel with 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 coating is formed on the obtained steel base material; and a hot-stamping process in which the obtained coated steel sheet is hot-stamped. Alternatively, after the hot rolling process, the cold rolling process can be performed directly after pickling without coiling. Each step will be described in detail below.

[0063] [Casting Process] There are no particular restrictions on the conditions for the casting process. For example, after smelting in a blast furnace or electric furnace, various secondary smelting processes can be carried out, followed by casting using conventional continuous casting or ingot casting methods.

[0064] [Hot rolling process] Hot rolling of cast slabs yields hot-rolled steel sheets. The hot rolling process involves directly or temporarily cooling the cast slab before reheating it. When reheating, the slab's heating temperature can be, for example, 1100–1250°C. The hot rolling process typically includes roughing and finishing rolling. The temperature and reduction rate for each rolling pass can be appropriately determined based on the desired microstructure and sheet thickness. For example, the finishing rolling end temperature can be 900–1050°C, and the finishing rolling reduction rate can be 10–50%.

[0065] [Winding process] Hot-rolled steel sheets can be coiled at a specified temperature. The coiling temperature can be appropriately determined according to the desired metal structure, for example, it can be 500~800℃. Alternatively, the hot-rolled steel sheet can be uncoiled before or after coiling and subjected to a specified heat treatment. Alternatively, the coiling process can be omitted, and the cold rolling process (described later) can be performed after pickling following the hot rolling process.

[0066] [Cold rolling process] After pickling and other processes, hot-rolled steel sheets are cold-rolled to obtain cold-rolled steel sheets. The reduction rate during cold rolling can be appropriately determined based on the desired metal structure and sheet thickness, for example, it can be 20% to 80%. After the cold rolling process, the sheet can be cooled to room temperature, for example, by air cooling.

[0067] [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 -50 to 20°C and holding it for 10 to 300 seconds. In particular, by performing the annealing process under conditions with a higher dew point of -10 to 20°C, the surface layer of the cold-rolled steel sheet can be appropriately decarburized. In this case, the reason may not be clear, but in the coating of the finally obtained hot-stamped body, the Fe-Al-Si phase can be further dispersed in the Fe2Al5 phase. As a result, compared with the case where the annealing process is performed under conditions with a dew point below -10°C, ΣL can be reduced. i The value of / L0, for example, can be used to convert ΣL i Reducing L0 to below 0.50 can further improve the corrosion resistance after hot stamping. The atmosphere in the annealing process can be a reducing atmosphere, more specifically a reducing atmosphere containing nitrogen and hydrogen, such as a reducing atmosphere with 1 to 10% hydrogen (e.g., 4% hydrogen and nitrogen balance).

[0068] [Cooling Process] The cooling process can be carried out from the heating temperature of the annealing process to the immersion temperature of the next plating process at a cooling rate suitable for obtaining the desired metal structure. There are no particular limitations; for example, the cooling process can be carried out from the heating temperature of the annealing process to the immersion temperature of the plating process at an average cooling rate of 10°C / s or higher.

[0069] [Plating Process] Next, in the plating process, a coating is formed on at least one, preferably both, surfaces of the cold-rolled steel sheet (steel base material). More specifically, the plating process is carried out using a plating bath with a specific chemical composition, for example, containing Zn: 5.0~50.0%, Si: 0.1~18.0%, and the remainder: Al and impurities (plating bath temperature: 650°C or higher, for example 650~680°C), with the steel base material immersed in the plating bath at a temperature of 500°C or lower, followed by cooling to 300°C at an average cooling rate of 10°C / s or lower. The plating process is important for obtaining the desired chemical composition and morphology of the Fe-Al phase corresponding to the interfacial alloy layer formed at the interface with the steel base material, which will be described in detail below.

[0070] Generally, the plating process is carried out with the immersion plate temperature and the plating bath temperature being the same or similar. However, when the difference between the immersion plate temperature and the plating bath temperature (i.e., plating bath temperature - immersion plate temperature) is less than 150°C, when the immersion plate temperature is higher than 500°C, or when the plating bath temperature is lower than 650°C, the Fe-Al phase of suitable seed cannot be formed. Specifically, in these cases, the Si content of the Fe-Al phase is less than 3.0%, and similarly, the Zn content is less than 2.0%. As a result, if such a plating 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 this high-temperature heating. iThe value of / L0 increases, and / or the Zn with its anti-corrosion properties cannot be fully dissolved in the Fe2Al5 phase. Therefore, the corrosion resistance of the coating after hot stamping is significantly reduced. Therefore, in this manufacturing method, by controlling the plating bath temperature to 650°C or higher and the immersion plate temperature to 500°C or lower, the immersion plate temperature is reliably kept at least 150°C lower than the plating bath temperature. This allows the interfacial alloy layer (Fe-Al phase) to crystallize and precipitate at low temperature, transforming it into a phase suitable for containing at least 3.0% by mass of Si and at least 2.0% by mass of Zn. It is believed that by containing at least 3.0% by mass of Si in the interfacial alloy layer, most of the Si present in the coating before hot stamping can be captured in the interfacial alloy layer. It is believed that since the Fe-Al-Si phase after hot stamping is an intermetallic compound containing a large amount of Si, the formation of the Fe-Al-Si phase in the Fe2Al5 phase during the high-temperature heating of hot stamping can be suppressed by capturing most of the Si in the interfacial alloy layer before hot stamping. As a result, it is believed that the Fe-Al-Si phase in the Fe2Al5 phase can be manufactured to satisfy ΣL i The hot-stamped formed body is dispersed in a manner where L0 ≤ 0.70. Furthermore, by including 2.0% by mass or more Zn in the interfacial alloy layer, Zn with anti-corrosion properties can be dissolved in the Fe2Al5 phase, thereby improving the corrosion resistance of the coating after hot stamping.

[0071] Generally, the immersion plate temperature is controlled to be the same as or at the same level as the plating bath temperature, as described above. Even when the immersion plate temperature differs from the plating bath temperature, it is usually controlled within ±20°C relative to the plating bath temperature. Reasons for this include: although the plate temperature rises to the plating bath temperature during immersion, there is no need to incur manufacturing costs to cool and control the immersion plate temperature to a lower level; and sometimes the plating bath solidifies due to the immersion plate temperature being lower than the plating bath temperature. Therefore, the fact that, as in this manufacturing method, by controlling the plating bath temperature to 650°C or higher and the immersion plate temperature to 500°C or lower, the immersion plate temperature can be reliably kept at least 150°C lower than the plating bath temperature, thereby improving the corrosion resistance of the coating after hot stamping, is extremely unexpected and surprising. There is no particular limitation on the lower limit of the immersion plate temperature; for example, the immersion plate temperature of the steel base material into the plating bath can be 380°C or higher. It should be noted that, in conventional wisdom, it is believed that by controlling the immersion plate temperature below 500°C, the plating bath may solidify due to its composition. However, in this embodiment, it has been found that by making the temperature difference between the immersion plate and the plating bath significantly greater than the conventional 150°C or more—that is, by keeping the immersion plate temperature below 500°C and the plating bath temperature at a high temperature such as 650°C—solidification of the plating bath can be prevented. Furthermore, to prevent solidification, it is preferable to use agitation within the plating bath to induce convection of the plating solution.

[0072] The plating process can be performed, for example, by hot-dip plating. Other conditions in the plating process can be appropriately set by considering factors such as the thickness and adhesion of the coating. For example, after immersing the cold-rolled steel sheet in the plating bath, it can be lifted out and immediately blown with N2 gas or air using a gas wiping method, followed by cooling. This allows the coating adhesion to be adjusted to a specified range, such as 20~200 g / m² per side. 2 Within the range.

[0073] [Cooling after plating] Cooling after plating is performed as described above, with an average cooling rate of less than 10°C / s up to 300°C. By cooling the plating steel sheet at such a relatively slow average cooling rate, the frequency of Fe-Al phase nucleation (nucleation frequency) generated from the molten plating layer can be reduced. In this case, the Fe-Al phase can grow relatively slowly, thus coarsening the Fe-Al phase and forming an interface alloy layer with a relatively flat shape and fewer irregularities at the interface with the steel base material.

[0074] Relatedly, in order to obtain the desired characteristics in the coating after hot stamping, the inventors have found that it is important to control the unevenness of the interfacial alloy layer before hot stamping to a flatter shape. During the high-temperature heating of hot stamping, the interfacial alloy layer grows within the coating as the coating alloys with the steel base material. Therefore, if the unevenness of the interfacial alloy layer is large, the Fe-Al phase grows from the interfacial alloy layer into needle-like structures within the coating during the high-temperature heating of hot stamping, and these needle-like Fe-Al phases sometimes protrude to the surface of the coating. The coating of the embodiments of the present invention contains a large amount of Al, and therefore its surface is covered by an oxide film composed of Al oxides or the like. However, the needle-like Fe-Al phases that grow during the high-temperature heating of hot stamping sometimes damage this oxide film, and in this case, the Zn contained in the coating evaporates from the damaged portion of the oxide film. Since Zn has a relatively low boiling point of about 907°C, there is a problem that it easily evaporates when heated to about 900°C or higher during hot stamping. The surface of the coating is covered by an oxide film composed of Al oxides, which can suppress or reduce Zn evaporation even when applied to hot stamping. However, when this oxide film is damaged, Zn evaporation from the damaged portion becomes significant. Therefore, the overall Zn content of the coating after hot stamping and the amount of dissolved Zn in the Fe2Al5 phase decrease, with the Zn content in the Fe2Al5 phase falling below 1.5% by mass. Consequently, the corrosion resistance of the hot-stamped body decreases.

[0075] In contrast, by controlling the average cooling rate from coating to 300°C to below 10°C / s, as described above, the frequency of Fe-Al phase nucleation from the molten coating (nucleation frequency) can be reduced, allowing the Fe-Al phase to grow more slowly. Therefore, the Fe-Al phase can be coarsened, resulting in an interface alloy layer with a relatively flat shape and fewer irregularities at the interface with the steel substrate. Consequently, the growth of the Fe-Al phase from this interface alloy layer into needle-like structures within the coating during the high-temperature heating of hot stamping can be suppressed. As a result, the destruction of the oxide film present on the surface of the coating can be suppressed or minimized, thus significantly suppressing or reducing Zn evaporation in the coating. Relatedly, the Zn content of the Fe2Al5 phase in the hot-stamped coating can be controlled to 1.5~15.0% by mass. On the other hand, if the average cooling rate from coating to 300°C is higher than 10°C / s, the nucleation frequency of the Fe-Al phase also increases, and consequently, the growth of the Fe-Al phase from the molten coating accelerates. If the Fe-Al phase grows faster, a more uneven interfacial alloy layer is formed at the interface with the steel substrate. Relatedly, during the high-temperature heating of hot stamping, the Fe-Al phase grows from the interfacial alloy layer in needle-like shapes, sometimes damaging the oxide film present on the surface of the coating. In such cases, the Zn contained in the coating evaporates from the damaged oxide film. As a result, the Zn content of the Fe2Al5 phase in the coating after hot stamping is less than 1.5% by mass, and the corrosion resistance of the hot-stamped body decreases. To make the unevenness of the interfacial alloy layer before hot stamping more even, the average cooling rate from coating to 300°C is preferably set to 5°C / s or less.

[0076] [Hot stamping forming process] Finally, the obtained coated steel sheet is hot-stamped in a hot stamping process to produce a hot-stamped body containing the desired coating. From the viewpoint of obtaining the desired coating, it is preferable to: place the coated steel sheet into a furnace at 800~1000°C, and after the temperature of the coated steel sheet reaches a predetermined temperature, for example, a furnace temperature of -10°C, hold it in the furnace for 60~600 seconds, more preferably after reaching 900°C, and hold it in the furnace for 60~600 seconds. The heating atmosphere is not particularly limited and can be any ordinary condition, such as an atmospheric atmosphere, a gas combustion atmosphere with a controlled air-to-fuel ratio, a nitrogen atmosphere, or a dew point controlled in these gases. As a heating method, examples include furnace heating using an electric furnace, a gas furnace, etc., and flame heating. After holding the heat in the furnace, the coated steel sheet is removed from the furnace, and then, after the coated steel sheet reaches a predetermined temperature, for example, a predetermined temperature below 850°C, it can be hot-stamped under normal conditions. After hot stamping, there are no particular limitations; for example, it can be cooled to a temperature range below 250°C at an average cooling rate of 20°C / second or higher.

[0077] According to this manufacturing method, a hot-stamped formed body having the following coating can be manufactured, wherein the overall chemical composition of the coating is controlled within a specified range and the coating adhesion amount is set to 20 g / m² per side. 2 The above, and the Fe-Al-Si phase formed in the Fe2Al5 phase on the steel base material side satisfies ΣL i The Fe2Al5 phase is dispersed in a manner with L0 ≤ 0.70, resulting in a Zn content of 1.5~15.0% by mass. Therefore, the combination of the corrosion suppression effect obtained by dispersing the Fe-Al-Si phase within the Fe2Al5 phase and the corrosion resistance enhancement effect obtained by dissolving Zn in the Fe2Al5 phase significantly improves the corrosion resistance of the hot-stamped body. Thus, compared to conventional hot-stamped bodies, this type of body achieves superior corrosion resistance. Therefore, its extended service life in automotive and building material applications can contribute to industrial development.

[0078] The present invention will now be described in more detail through embodiments, but these embodiments are merely examples of the present invention, and the present invention is not limited to these embodiments in any way. The present invention can, of course, be modified in any way without departing from its spirit.

[0079] Example In the following embodiments, hot-stamped articles according to embodiments of the present invention were manufactured under various conditions, and the characteristics of the manufactured hot-stamped articles were investigated.

[0080] First, molten steel is cast using a continuous casting method to form a slab with a chemical composition (by mass%) consisting of C: 0.20%, Si: 0.012%, Mn: 1.30%, Al: 0.030%, P: 0.005%, S: 0.0020%, N: 0.0030%, and the remainder being Fe and impurities. This slab is temporarily cooled and then hot-rolled to 1200°C, followed by coiling at a temperature below 600°C. Hot rolling is performed by roughing and finishing, with the finishing temperature at 900–1050°C and a reduction of 30%. Next, the resulting hot-rolled steel sheet is pickled and then cold-rolled with 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 is subjected to an annealing process in a furnace with an oxygen concentration of less than 20 ppm and in a mixed gas atmosphere of 4% hydrogen and nitrogen balance, under the conditions shown in Table 1. Then, it is cooled at an average cooling rate of more than 10°C / s to produce steel base material.

[0081] Next, the manufactured steel base material was cut into 100mm × 200mm pieces, and coated using an intermittent hot-dip galvanizing test apparatus manufactured by our company. More specifically, firstly, the manufactured steel base material was immersed in a plating bath with various chemical compositions and plating bath temperatures shown in Table 1 at the immersion plate temperatures shown in Table 1, for approximately 3 seconds. Then, it was lifted at a lifting speed of 20–200 mm / s, and the coating adhesion was adjusted to the values ​​shown in Table 1 by wiping with N2 gas. Next, the coated steel base material was cooled to 300°C using nitrogen as a cooling gas at the average cooling rate shown in Table 1, thereby obtaining a coated steel sheet with coatings formed on both sides of the steel base material. The plate temperature was measured using a thermocouple spot-welded to the center of the steel base material.

[0082] [Chemical composition analysis of the coating, etc.] The chemical composition of the coating after hot stamping 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 -10°C in the furnace, it was held for 100 seconds. Next, the coated steel sheet was removed from the furnace, clamped in a flat die at approximately room temperature, and rapidly cooled to obtain a hot-stamped body with a coating containing the Fe2Al5 phase on the surface of the steel base material. The hot-stamped body after heating and rapid cooling was cut into 30mm × 30mm samples of the coating. The coating was dissolved in a room-temperature acidic aqueous solution of 10% hydrochloric acid with 0.04% IBIT 710K (manufactured by Asahi Chemical Industry). The chemical composition of the coating was determined by measuring the acid solution obtained by ICP emission spectroscopy. In addition, the mass of the sample before and after acid pickling was measured, and the coating adhesion amount was determined by the change in mass. The results are shown in Table 1.

[0083] [Microstructure analysis of the coating] Using the above method, the Fe2Al5 phase and the Fe-Al-Si phase with a Si content of 5-20% by mass and an equivalent circle diameter of 2 μm or more were identified. For the Fe2Al5 phase, the Zn content was similarly determined using the above method. Furthermore, the ΣL... i The value of / L0.

[0084] [Substrate metal corrosion depth] The corrosion depth of the base metal after hot stamping was evaluated 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 -10°C of the furnace temperature, it was held for 100 seconds. Next, the coated steel sheet was removed from the furnace, clamped in a flat die at approximately room temperature, and rapidly cooled. A 50mm × 100mm sample of the hot-stamped body after heating and rapid cooling was treated with Zn phosphate (SD5350 system: Nipponpaint Industrial Coatings standard). Then, an electrodeposition coating of 20μm (PN110 PowernicsGray: Nipponpaint Industrial Coatings standard) was applied, followed by baking at 150°C for 20 minutes. Next, a cut was made in the center of the sample to reach the base metal (steel substrate). Then, a neutral brine spray cycle test as specified in 8.1 of JIS H8502:1999 was performed for 360 cycles. Next, the electrodeposited coating was removed using a stripping agent (Discoat), and the coating was removed using a 10% HCl aqueous solution with added inhibitor. The corrosion depth of the base metal was then measured using a laser meter, and the corrosion resistance was evaluated as follows.

[0085] AA: The corrosion depth of the base metal is less than 0.3 mm. A: The corrosion depth of the base metal exceeds 0.3~0.5mm. B: The corrosion depth of the base metal exceeds 0.5 mm. [Coating swelling] The coating swelling was evaluated as follows. First, a 50mm × 100mm hot-stamped sample, subjected to the same heating and quenching process as for red rust resistance, was treated with Zn phosphate (SD5350 system: Nipponpaint Industrial Coatings standard). Next, an electrodeposition coating (PN110 PowernicsGray: Nipponpaint Industrial Coatings standard) was applied to a thickness of 20 μm, followed by baking at 150°C for 20 minutes. Then, a cut was made in the center of the sample to reach the base metal (steel substrate). Next, a neutral salt spray cycle test of 120 cycles as specified in JIS H8502:1999, 8.1, was performed, and the coating swelling amplitude was measured. Corrosion resistance was evaluated as follows.

[0086] AAA: Less than 2mm AA: More than 2~3mm A: More than 3~4mm B: More than 4mm The cases where the corrosion depth of the base metal was rated as AA and A, and the coating swelling was rated as AAA, AA, and A, were evaluated as hot-stamped bodies with Al-containing coatings that exhibited improved corrosion resistance even after hot stamping. The results are shown in Table 1.

[0087] Referring to Table 1, it is believed that in Comparative Example 32, due to the high immersion temperature of the plating process, the desired Si content could not be obtained in the interfacial alloy layer of the plating layer before hot stamping. As a result, in the plating layer after hot stamping, the formation of the Fe-Al-Si phase in the Fe2Al5 phase became significant, ΣL iThe larger the value of / L0, the lower the corrosion resistance after hot stamping. In Comparative Example 33, it was believed that the average cooling rate from coating to 300°C was fast, thus increasing the nucleation frequency of the Fe-Al phase in the molten coating and forming a more uneven interfacial alloy layer at the interface with the steel base. Relatedly, it was believed that during the high-temperature heating of hot stamping, the oxide film on the coating surface was destroyed by the growth of needle-like Fe-Al phases, and most of the Zn contained in the coating evaporated. As a result, the Zn content of the Fe2Al5 phase decreased, and the corrosion resistance after hot stamping decreased. In Comparative Example 34, due to the low Zn content in the coating, the Zn content of the Fe2Al5 phase also decreased, and the substitution corrosion protection effect in the coating could not be fully utilized. As a result, in particular, the corrosion depth of the base metal increased after hot stamping, and the corrosion resistance decreased. In Comparative Example 35, the high Si content in the coating led to a significant formation of the Fe-Al-Si phase, and ΣL i A higher / L0 value leads to decreased corrosion resistance after hot stamping. In Comparative Example 36, the low Si content in the coating resulted in reduced coating adhesion and consequently, decreased corrosion resistance after hot stamping. In Comparative Example 37, insufficient coating adhesion led to decreased corrosion resistance after hot stamping. In Comparative Example 38, the low plating bath temperature caused the plating bath to solidify during immersion of the steel base material, preventing proper coating formation. Therefore, subsequent manufacturing was halted, and no analysis or performance evaluation was performed.

[0088] In contrast, in all the hot-stamped formed articles of the embodiments, the overall chemical composition of the coating is controlled within a specified range, and the coating adhesion amount is set to 20 g / m² per side. 2 The above, and make the Fe-Al-Si phase in the Fe2Al5 phase satisfy ΣL iThe Fe2Al5 phase was dispersed in a manner with L0 ≤ 0.70, thereby controlling the Zn content to be 1.5~15.0% by mass. Thus, even when exposed to a high temperature of 900°C, the combination of the corrosion suppression effect obtained by dispersing the Fe-Al-Si phase in the Fe2Al5 phase and the corrosion resistance improvement effect obtained by dissolving Zn in the Fe2Al5 phase significantly improved the corrosion resistance of the hot-stamped form. Furthermore, for the coated steel sheets before hot stamping in all embodiments, the elemental concentration of the coating cross-section was measured using EPMA. The results showed that the Si content in the interface alloy layer was higher than that in Comparative Example 32, both exceeding 3.0% by mass, and similarly, the Zn content was both exceeding 2.0% by mass. Additionally, for the coated steel sheets before hot stamping in all embodiments, the interface alloy layer was determined based on the elemental distribution image obtained using SEPMA mapping. The results showed that the shape of this interface alloy layer was flatter with less unevenness compared to Comparative Example 33. In particular, the overall chemical composition of the coating contains 5.0-25.0% Zn and 0.25-6.0% Si, with the Zn content of the Fe2Al5 phase being 3.0-15.0% by mass, thereby satisfying ΣL i In Examples 27-31, where the relationship of / L0≤0.50 was controlled, the evaluation of the corrosion depth of the base metal was AA, and the evaluation of the coating expansion was AAA, further improving the corrosion resistance. Furthermore, GDS measurements were performed on each hot-stamped body, and the results confirmed that in all examples of the hot-stamped bodies, a solid solution layer containing Fe-Al, Fe-Zn, and / or Fe-Al-Zn solid solutions existed between the steel base material and the coating.

[0089] Explanation of reference numerals in the attached figures 1: Hot stamping formed body 2: Steel base material 3: Coating 4: Fe2Al5 phase 5: Fe-Al-Si phase with Si content of 5-20% by mass and equivalent circle diameter of 2 μm or more.

Claims

1. A hot-stamped formed body, characterized in that, It comprises a steel base material and a coating disposed on the surface of the steel base material. The chemical composition of the coating, expressed as a percentage by mass, is: Zn: 2.5~30.0% Si: 0.05~12.0% Fe: 40.0~80.0% 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 remainder consists of Al and impurities, and... 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 less than 5.000%. The coating contains the Fe2Al5 phase. The Zn content of the Fe2Al5 phase is 1.5~15.0% by mass. In the cross-section of the coating, the projected length L of the Fe-Al-Si phase with a Si content of 5-20% by mass and an equivalent circle diameter of 2 μm or more in the Fe2Al5 phase is... i The length L0 of the surface of the steel base material satisfies ΣL i / L0≤0.70, The coating adhesion amount is 20g / m² per side. 2 above.

2. The hot-stamped formed body according to claim 1, characterized in that, The chemical composition of the coating, expressed in % by mass, contains: Zn: 5.0~25.0%, and Si: 0.25~6.0%, The Zn content of the Fe2Al5 phase is 3.0~15.0 by mass.

3. The hot-stamped formed body according to claim 1 or 2, characterized in that, ΣL i / L0≤0.50。

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