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, in order from the steel base material side, a Fe2Al5 phase, an Fe-Al phase having an Fe content of 20-40% by mass, and a surface oxide film, and in a cross section of the plating layer, the thickness of the Fe-Al phase is less than or equal to 20 [mu] m, and the thickness of the Fe-Al phase is less than or equal to 20 [mu] m, and the thickness of the surface oxide film is less than or equal to 30 [mu] m. 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, the Mg content in the surface oxide film is 0.05 at% or more, and the deposition amount of the 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 objectives, the inventors have conducted research with particular focus on the chemical composition and microstructure of the coating. As a result, the inventors have discovered that by forming a coating consisting of an Al-Zn-Mg-Si system coating with an adhesion amount of more than a specified amount, and by making the coating formed after hot stamping mainly consist of a Fe2Al5 phase, a Fe-Al phase formed on the Fe2Al5 phase and having a low Fe content, and a Mg-modified surface oxide film, and by appropriately controlling the morphology of the Fe2Al5 phase, 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: 2.5~40.0%, Si: 0.05~15.0% Fe: 25.0~40.0%, Mg: 0.005~3.000% Ni: 0~0.500% 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, 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 aforementioned coating, starting from the steel substrate side, sequentially comprises a Fe2Al5 phase, a Fe-Al phase with a Fe content of 20-40% by mass, and a surface oxide film. 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 Mg content in the above-mentioned surface oxide film is above 0.05 atomic%. 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: 5.0~35.0%; and Si: 0.25~6.0%, The Mg content in the above-mentioned surface oxide film is above 0.30 atomic%.

[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 article 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: 2.5~40.0%, Si: 0.05~15.0% Fe: 25.0~40.0%, Mg: 0.005~3.000% Ni: 0~0.500% 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, 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 aforementioned coating, starting from the steel substrate side, sequentially comprises a Fe2Al5 phase, a Fe-Al phase with a Fe content of 20-40% by mass, and a surface oxide film. 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 Mg content in the above-mentioned surface oxide film is above 0.05 atomic%. 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 on and studied the chemical composition and morphology of the Al-Si based coating before hot stamping. As a result, the inventors discovered that by appropriately preparing a coating consisting of an Al-Zn-Mg-Si based coating in which Zn and Mg are further added to the Al-Si based coating, and by appropriately selecting the heat treatment conditions for hot stamping, the formation of the Fe-Al-Si phase, which may cause galvanic corrosion between the Fe2Al5 phase and the hot-stamped body, can be suppressed in the coating formed after hot stamping, thereby improving the corrosion resistance of the hot-stamped body. Furthermore, the inventors discovered that by forming a Fe-Al phase with a low Fe content, more specifically 20-40% by mass, on the Fe2Al5 phase, the resistance of the hot-stamped body to red rust can be improved. Furthermore, the inventors have discovered that the addition of Mg can appropriately modify the surface oxide film in the coating after hot stamping, thereby improving the chemical conversion processability of the hot stamped body.

[0020] More specifically, firstly, the inventors discovered that by setting the adhesion amount of the coating composed of Al-Zn-Mg-Si system coatings to 20 g / m per side, 2 Therefore, by appropriately selecting the heat treatment conditions for hot stamping, the coating is not entirely alloyed to the Fe2Al5 phase during the high-temperature heating in hot stamping. Instead, a Fe-Al phase with a lower Fe content than the Fe2Al5 phase is formed on the Fe2Al5 phase on the steel base side. More specifically, during the high-temperature heating in hot stamping, Fe from the steel base diffuses into the coating and alloys with Al. However, by increasing the coating adhesion and moderately reducing the degree of alloying, although a fully alloyed Fe2Al5 phase is formed on the steel base side of the coating, a Fe-Al phase with a lower Fe content than the Fe2Al5 phase, i.e., a Fe-Al phase with a Fe content of 20-40% by mass, can also be formed on the surface side of the coating. While not intending to be bound by any particular theory, it can be considered that the improved resistance to red rust brought by the Fe-Al phase is due to the fact that the Fe-Al phase has a lower degree of alloying compared to the Fe2Al5 phase.

[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 predetermined amount of Si 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 Si in the interface alloy layer, the formation of the larger Fe-Al-Si phase in the Fe2Al5 phase can be suppressed in the coating formed after hot stamping. 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 on the base steel side of the coating, i.e., by having a Si content of 5 to 20% by mass and an equivalent circle diameter of 2 μm or more, the projection length L of the Fe-Al-Si phase is... 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, from the steel base material 2 side, sequentially includes an Fe2Al5 phase 4 located on the steel base material 2, an Fe-Al phase 5 located on the Fe2Al5 phase 4, more specifically, an Fe-Al phase 5 having a Fe content of 20-40% by mass, and a surface oxide film 6. Furthermore, in Figure 1 In the Fe2Al5 phase 4, there exists a Fe-Al-Si phase 7 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 7 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 7 exists dispersedly in the Fe2Al5 phase 4. When the projected lengths of multiple Fe-Al-Si phases 7 partially overlap, the overall projected length including the overlapping portion is defined as one projected length. It is assumed that even if the Fe-Al-Si phase 7 exists in a layered manner in the Fe2Al5 phase 4, galvanic corrosion occurs at the contact between the layered Fe-Al-Si phase 7 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 7 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 7 and the surrounding Fe2Al5 phase 4, corrosion will not proceed to other Fe-Al-Si phases 7 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 embodiment of the present invention, in association with the addition of Zn to the coating, Zn is dissolved in both the Fe2Al5 phase 4 and the Fe-Al phase 5. As a result, the corrosion resistance is further improved through the substitution corrosion protection effect of Zn.

[0023] In addition, Figure 1In this embodiment, Mg is present in the surface oxide film 6 at a concentration of 0.05 atomic percent or more. The presence of Mg allows for modification of the surface oxide film, improving the chemical conversion treatability of the hot-stamped body. More specifically, Mg generally exists in the coating in the form of Mg₂Si phase, etc. On the other hand, the coating 3 of the hot-stamped body 1 in this embodiment contains a significant amount of Al, and therefore its surface is covered by an oxide film composed of Al oxides, etc. Here, the Mg₂Si phase has a high melting point, making it difficult to oxidize even during the high-temperature heating of hot stamping. Therefore, it is considered impossible to modify the oxide film composed of Al oxides, etc. In contrast, the inventors have discovered that, as detailed later in connection with the method for manufacturing hot-stamped articles, by appropriately controlling the cooling after plating, a Mg-Al-Zn-Si intermetallic compound phase, which has a lower melting point and is more easily oxidized than the Mg₂Si phase, can be formed in the plating layer before hot stamping. More specifically, a Mg-Al-Zn-Si intermetallic compound phase with an Al content of 10% by mass or more can be formed, and the presence of this Mg-Al-Zn-Si intermetallic compound phase is effective for modifying the surface oxide film. As described above, the Mg-Al-Zn-Si intermetallic compound phase has a lower melting point and is more easily oxidized than the Mg₂Si phase. Therefore, during the high-temperature heating of hot stamping, this Mg-Al-Zn-Si intermetallic compound phase dissolves, and the dissolved Mg can be introduced into the Al-containing oxide present on the surface at an amount of 0.05 atomic% or more. As a result, it is believed that Mg can be used to appropriately modify the oxide film formed on the surface of the formed body during hot stamping, thereby improving the chemical conversion treatability of the resulting hot stamped body. Generally, if the chemical conversion treatability is reduced, areas of untreated film, known as uncovered areas, are sometimes formed, resulting in reduced corrosion resistance after coating. Conversely, by improving the chemical conversion treatability of the hot stamped body, the corrosion resistance of the hot stamped body after coating can be improved.

[0024] Therefore, the hot-stamped body according to an embodiment of the present invention achieves excellent corrosion resistance by suppressing the formation of the Fe-Al-Si phase 7, which may cause galvanic corrosion between the Fe2Al5 phase 4 and the Fe2Al5 phase 4; more specifically, by combining the corrosion suppression effect obtained from the dispersed formation of a relatively large Fe-Al-Si phase 7, the improved resistance to red rust due to the formation of the Fe-Al phase 5, and the improved chemical conversion treatment effect due to the modification of the surface oxide film by Mg. In particular, the fact that the corrosion resistance of the hot-stamped body 1 can be improved by appropriately controlling the morphology of the Fe2Al5 phase 4 by forming the coating 3 from the Fe2Al5 phase 4, the Fe-Al phase 5 having a relatively low Fe content, and the Mg-modified surface oxide film 6, is a fact that was first clarified by the inventors. Therefore, the hot-stamped body according to the embodiment of the present invention is particularly useful in the automotive field, where hot stamping is widely used.

[0025] The hot-stamped formed articles according to embodiments of the present invention will now be described in more detail. Furthermore, in the following description, unless otherwise specified, the unit "%" for the content of each element refers to "mass%". Moreover, in this specification, unless otherwise specified, the "~" indicating a numerical range is used to mean both a lower and upper limit value, encompassing the values ​​described before and after it.

[0026] [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.

[0027] [Zn: 2.5~40.0%] Zinc (Zn) is an element that provides corrosion resistance and is effective in improving the corrosion resistance of coatings. Furthermore, during chemical conversion treatment, Zn reacts with the treatment solution to form a chemical conversion film on the surface. To fully achieve these effects, 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, 20.0% or more, or 22.0% 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 sometimes the molten Zn seeps into the steel, causing liquid metal embrittlement (LME) cracks. Therefore, the Zn content is set at 40.0% or less. The Zn content can be 38.0% or less, 35.0% or less, 32.0% or less, 30.0% or less, or 28.0% or less.

[0028] [Si: 0.05~15.0%] 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 15.0% or less. The Si content can be 12.0% or less, 10.0% or less, 8.0% or less, or 6.0% or less.

[0029] [Fe: 25.0~40.0%] If the coated 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 25.0% or more, 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, over-alloying of the coating can sometimes reduce its resistance to red rust. Therefore, the Fe content is set to 40.0% or less, for example, 38.0% or less, 36.0% or less, or 34.0% or less.

[0030] [Mg: 0.005~3.000%] Mg is an effective element for improving the corrosion resistance of coatings, especially their chemical conversion treatability. To achieve this full effect, the Mg content is set at 0.005% or more. Mg content can be 0.010% or more, 0.020% or more, 0.030% or more, 0.050% or more, 0.080% or more, 0.100% or more, or 0.300% or more. On the other hand, from the viewpoint of improving processability, the Mg content is set at 3.000% or less. Mg content can be 2.500% or less, 2.000% or less, 1.500% or less, 1.000% or less, 0.800% or less, 0.600% or less, or 0.400% or less.

[0031] In addition, the coating may optionally contain Ni: 0~0.500%, 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. At least one of the following: 0.000%, 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.

[0032] [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.

[0033] [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.

[0034] [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.

[0035] In the coating, the remainder besides the elements mentioned above consists of Al and impurities. Impurities in the coating refer to components, represented by the raw materials, that are introduced during the manufacturing process due to various reasons. Furthermore, the Al content does not need to be specifically specified, but can range from 10.0% to 72.45%. Depending on the requirements, the Al content can 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, and can be less than 70.0%, less than 65.0%, less than 60.0%, less than 57.0%, less than 54.0%, less than 51.0%, or less than 48.0%.

[0036] [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.

[0037] [Fe2Al5 phase] [ΣL i / L0≤0.70] In an embodiment of the present invention, the coating contains a Fe2Al5 phase on the steel substrate side, 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. iThe 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 improving corrosion resistance and enhancing the overall effect, 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.

[0038] 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.

[0039] [Fe-Al phase with 20-40% by mass Fe content] In embodiments of the present invention, the coating includes a Fe-Al phase with a lower Fe content than the Fe2Al5 phase (Fe content: more than 40-60% by mass) on the surface side of the Fe2Al5 phase, specifically a Fe-Al phase with a Fe content of 20-40% by mass. Compared to the Fe2Al5 phase, the Fe-Al phase with a Fe content of 20-40% by mass exhibits a lower degree of alloying, which, consequently, contributes to improved resistance to red rust. The Si content in the Fe-Al phase is less than 5% by mass. It should be noted that, in this specification, the Fe-Al phase with a Fe content of 20-40% by mass is sometimes simply referred to as the Fe-Al phase.

[0040] [Mg content in the surface oxide film: ≥0.05 atomic%] In embodiments of the present invention, such as Figure 1 As shown, the coating comprises a surface oxide film on its surface, and the Mg content in this surface oxide film is controlled to be 0.05 atomic% or more. The coating of the hot-stamped formed body according to embodiments of the present invention contains a significant amount of Al, and therefore its surface is covered by an oxide film composed of Al oxides or the like. By introducing Mg into such a surface oxide film at an amount of 0.05 atomic% or more, the surface oxide film can be appropriately modified, thereby improving the chemical conversion processability of the hot-stamped formed body. From the viewpoint of further improving the chemical conversion processability, a higher Mg content in the surface oxide film is preferred; for example, it can be 0.10 atomic% or more, 0.20 atomic% or more, 0.30 atomic% or more, 0.50 atomic% or more, 0.80 atomic% or more, 1.00 atomic% or more, or 1.50 atomic% or more. On the other hand, even if the surface oxide film contains excessive Mg, the above-mentioned effect is saturated. Therefore, there are no particular limitations, but the Mg content in the surface oxide film is preferably set to 6.00 atomic% or less, for example, it can be 5.50 atomic% or less, 5.00 atomic% or less, 4.50 atomic% or less, or 4.00 atomic% or less.

[0041] [Analysis of the coating] The analysis of the coating was performed 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 width 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 Fe-Al phase with a Fe content of 20-40% by mass, the Fe2Al5 phase, the Fe-Al-Si phase with a Si content of 5-20% by mass and an equivalent circle diameter of 2 μm or more present inside the Fe2Al5 phase, and the surface oxide film 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 this elemental distribution image, the region satisfying Fe: 20~40 wt% and Al: 58~78 wt% is defined as the Fe-Al phase with a Fe content of 20~40 wt%. Regarding this Fe-Al phase region, the concentration range of the color bar representing the concentration of each element in the elemental distribution image is set to Fe: 20~40 wt% and Al: 58~78 wt%. The elemental distribution images of Fe and Al with adjusted color bars are overlaid to determine the Fe-Al phase with a Fe content of 20~40 wt%. Figure 1 Symbol 5 in the image. In this elemental distribution image, the region satisfying Fe: more than 40~60 mass%, Al: 40~less than 60 mass%, Si: 0~less than 5 mass%, and Zn: 0~15 mass% is designated 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 7 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 As shown, the long side in the field of view, i.e., the length L0, 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 stamped body 1).

[0042] The Mg content in the surface oxide film was determined by averaging the elemental concentrations constituting the surface oxide film in the five samples using SEM-EDS or EPMA. More specifically, in each sample, the regions satisfying O: 30–70% by mass, Al: 30–70% by mass, and Mg: 0–2.5% by mass were considered as surface oxides (in SEM-EDS or EPMA measurements). Figure 1 (Symbol 6 in the text). Next, in each sample, at the center of the thickness of the region identified as surface oxide, the Mg content was measured at five points spaced 50 μm apart in a direction parallel to the surface of the hot-stamped body, and the average value of these measurements was taken as the Mg content of each sample. The average Mg content of the five samples was taken as the Mg content in the surface oxide film.

[0043] As a coating having the above-mentioned chemical composition, Fe2Al5 phase, Fe-Al phase, and surface oxide film, a hot-dip coating may be included. While it is not necessary to exclude coatings other than hot-dip coatings, they may be limited to hot-dip coatings.

[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... 2 The above, and by moderately reducing the degree of alloying, although a fully alloyed Fe2Al5 phase is formed on the steel base side of the coating, a Fe-Al phase with a lower Fe content than the Fe2Al5 phase, i.e., a Fe-Al phase with a Fe content of 20-40% by mass, can be formed on the surface side of the coating. On the other hand, if the coating adhesion is low, the alloying of the coating will over-enhance during the high-temperature heating in hot stamping, and sometimes the Fe-Al phase with a low Fe content cannot be formed. In this case, the improved resistance to red rust due to the formation of the Fe-Al phase cannot be obtained. Alternatively, if the coating adhesion is low, the effect of adding Zn, which has a substitution effect for corrosion resistance, to the coating cannot be fully obtained, and in this case, the corrosion resistance after hot stamping is reduced. From the viewpoint of improving corrosion resistance, the coating adhesion 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 / m2 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 with an Al-containing coating that exhibits improved corrosion resistance even after hot stamping. In the Al-Zn-Mg-Si based coating, the chemical composition and adhesion amount are appropriately optimized. This coating mainly consists of a Fe2Al5 phase, a Fe-Al phase formed on the Fe2Al5 phase with a relatively low Fe content, and a Mg-modified surface oxide film. Furthermore, the projected length L of the Fe-Al-Si phase, with a Si content of 5-20% by mass in the Fe2Al5 phase and an equivalent circle diameter of 2 μm or more, is... i The length L0 of the surface of the steel base material satisfies ΣL i This objective is achieved by controlling the chemical composition of the steel sheet to be ≤0.70. Therefore, it is clear that the chemical composition of the steel sheet itself is not an essential technical feature for achieving the objectives 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 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, in the plating process, a plating bath with a specified chemical composition is used, for example, a plating bath containing Zn: 5.0~50.0%, Si: 0.1~18.0%, Mg: 0.010~3.000%, and the remainder: Al and impurities (plating bath temperature: 650°C or higher, for example 650~680°C), and the process is carried out such that the immersion temperature of the steel base material in the plating bath is 500°C or lower. After plating, the material is cooled once to 480°C at an average cooling rate of 20°C / s or higher, and then cooled a second time from 480°C 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 of the Fe-Al phase corresponding to the interfacial alloy layer formed at the interface with the steel base material, and further important for forming a Mg-Al-Zn-Si intermetallic compound phase in the coating. This will be explained 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%. 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. In this case, ΣL iThe value of / L0 increases, thus significantly reducing the corrosion resistance of the coating after hot stamping. 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. 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. Since the Fe-Al-Si phase after hot stamping is an intermetallic compound containing a relatively large amount of Si, by capturing most of the Si in the interfacial alloy layer before hot stamping, the formation of the Fe-Al-Si phase in the Fe2Al5 phase during the high-temperature heating of hot stamping can be suppressed. As a result, it is believed that the Fe-Al-Si phase in the Fe2Al5 phase can be manufactured to satisfy ΣL i Hot stamped parts dispersed in a manner where / L0≤0.70.

[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] The coated steel sheet, as described above, is cooled to 480°C in one cooling cycle at an average cooling rate of 20°C / s or higher, followed by a second cooling from 480°C to 300°C at an average cooling rate of 10°C / s or lower. By cooling the coated steel sheet using this combination of a primary cooling at a relatively fast average cooling rate (20°C / s or higher) and a secondary cooling at a relatively slow average cooling rate (10°C / s or lower), a Mg-Al-Zn-Si intermetallic compound phase can be formed in the coating. More specifically, when Mg is added to the coating, the Mg2Si phase generally crystallizes and precipitates easily. However, by rapidly cooling the coated steel sheet to 480°C at an average cooling rate of 20°C / s or higher in the primary cooling cycle, the crystallization and precipitation of the Mg2Si phase can be suppressed, and the composition of the liquid phase can be made suitable for the crystallization and precipitation of the Mg-Al-Zn-Si intermetallic compound phase. Next, in the secondary cooling process, by slowly cooling the steel sheet from 480°C to 300°C at an average cooling rate of less than 10°C / s, the Mg-Al-Zn-Si intermetallic compound phase can be stably and slowly crystallized and precipitated, forming a coating containing an Al content of 10% by mass or more and an Al content of 0.010% or more by area. The Mg-Al-Zn-Si intermetallic compound phase with an Al content of 10% by mass or more has a relatively low melting point and is easily oxidized. Therefore, by containing an Mg-Al-Zn-Si intermetallic compound phase of 0.010% or more by area in the coating, and dissolving this Mg-Al-Zn-Si intermetallic compound phase during the high-temperature heating of hot stamping, the dissolved Mg can be used to appropriately modify the oxide film formed on the surface of the formed body during hot stamping. As a result, the Mg content in the surface oxide of the obtained hot stamping form is above 0.05 atomic%, which can improve the chemical conversion processability of the hot stamping form.

[0074] On the other hand, if rapid cooling continues between 480°C and 300°C, or at an average cooling rate exceeding 10°C / s, the liquid phase composition becomes suitable for the crystallization of the α-(Al, Mg, Zn) phase. Specifically, Mg is dissolved in the α-Al phase, preventing the crystallization of the Mg-Al-Zn-Si intermetallic compound phase in the desired amount. In this case, it is impossible to modify the oxide film formed on the surface of the formed body during the high-temperature heating of hot stamping using Mg. As a result, the Mg content in the surface oxide is less than 0.05 atomic%, and the desired chemical conversion treatment properties cannot be obtained. There is no particular upper limit to the average cooling rate of the first cooling step; for example, it can be below 50°C / s. Similarly, there is no particular lower limit to the second cooling step; for example, it can be above 3°C / s.

[0075] [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, such as 900°C, hold it in the furnace for 5~50 seconds. If the holding time is longer than 50 seconds, the alloying of the coating will be over-alloyed, and sometimes it will be impossible to form an Fe-Al phase with a low Fe content. The heating atmosphere is not particularly limited and can be any ordinary condition, such as atmospheric conditions, a gas combustion atmosphere with a controlled air-to-fuel ratio, a nitrogen atmosphere, or a dew point controlled in these gases. Examples of heating methods include electric heating, high-frequency heating, and induction 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, such as 850°C or lower, 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.

[0076] According to this manufacturing method, it is possible to manufacture the following hot-stamped formed body: 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 coating is primarily composed of a Fe2Al5 phase, a Fe-Al phase formed on the Fe2Al5 phase that effectively improves resistance to red rust, and a Mg-modified surface oxide film. Furthermore, the Fe-Al-Si phase within the Fe2Al5 phase satisfies ΣL iThe dispersion is such that / L0≤0.70. Therefore, the combination of the corrosion suppression effect obtained by dispersing the Fe-Al-Si phase within the Fe2Al5 phase, the improved resistance to red rust due to the formation of the Fe-Al phase, and the improved chemical conversion treatment effect due to the modification of the surface oxide film by Mg, significantly improves the corrosion resistance of the hot-stamped body. Therefore, according to this hot-stamped body, superior corrosion resistance can be achieved compared to conventional hot-stamped bodies. Thus, its extended service life in automotive and building materials applications can contribute to industrial development.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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 480°C once using nitrogen as a cooling gas at the average cooling rate shown in Table 1, and then cooled a second time from 480°C to 300°C at the average cooling rate shown in Table 1, thereby obtaining a coated steel sheet with a coating 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.

[0081] [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 heated by electric heating to 900°C and held for 10 seconds. Then, the coated steel sheet was removed from the furnace, clamped in a flat mold at approximately room temperature, and rapidly cooled to obtain a hot-stamped body with a coating consisting of, sequentially from the steel base material side, a Fe2Al5 phase, a Fe-Al phase with 20-40% by mass of Fe, and a surface oxide film. For the coating of samples cut into 30mm × 30mm pieces from the heated and rapidly cooled hot-stamped body, the chemical composition of the coating was determined by dissolving the coating in a room-temperature acidic aqueous solution of 10% hydrochloric acid with 0.04% IBIT 710K (manufactured by Asahi Chemical Industry) added. The acid solution was measured 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.

[0082] [Microstructure analysis of the coating] Using the above method, Fe2Al5 phase, Fe-Al phase with 20-40% by mass Fe content, Fe-Al-Si phase with 5-20% by mass Si content and an equivalent circle diameter of 2 μm or more, and surface oxide film were identified. For the surface oxide film, the Mg content was determined using the same method. Furthermore, ΣL was determined using the above method. i The value of / L0. It should be noted that, except for the example without the Fe-Al phase having a Fe content of 20 to 40% by mass (specifically, Comparative Example 40), the above-mentioned steel base material side sequentially contains the Fe2Al5 phase, the Fe-Al phase having a Fe content of 20 to 40% by mass, and the surface oxide film.

[0083] [Resistance to Red Rust] The resistance to red rust after hot stamping was evaluated as follows. First, the coated steel sheet was heated by electric heating to 900°C and held for 10 seconds. Then, 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 was treated with Zn phosphate (SD5350 system: Nipponpaint Industrial Coatings standard), followed by a 20μm electrodeposition coating (PN110 Powernics Gray: Nipponpaint Industrial Coatings standard), and baked 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 salt spray cycle test as specified in JIS H 8502:1999, 8.1, was performed, and the number of cycles in which red rust formed from the cut was measured. The corrosion resistance was evaluated as follows.

[0084] A: More than 3 cycles B: Less than 3 cycles [Chemical transformation treatment] The chemical conversion treatability was evaluated as follows. First, the coated steel sheet was heated by electric heating to 900°C and held for 10 seconds. Then, the coated steel sheet was removed from the furnace, clamped in a flat mold 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) to form a chemical conversion coating. Next, the phosphorus intensity was measured by fluorescence X-ray analysis, and the amount of chemical conversion coating adhesion was calculated using a standard curve. Based on the amount of chemical conversion coating adhesion, the chemical conversion treatability of the hot-stamped body was evaluated according to the following evaluation criteria.

[0085] AA: Adhesion amount is 0.9g / m 2 above A: Adhesion amount is 0.1~less than 0.9g / m 2 B: Adhesion amount less than 0.1g / m 2 [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). Then, 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. Next, a groove was made in the center of the sample to reach the base metal (steel substrate). Then, 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 following cases, rated A for resistance to red rust, AA and A for chemical conversion treatment, and AAA, AA and A for coating swelling, are considered as hot-stamped bodies with Al-containing coatings that exhibit improved corrosion resistance even after hot stamping. The results are shown in Table 1.

[0087] Referring to Table 1, in Comparative Example 32, it was considered that 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 iA larger / L0 value leads to decreased corrosion resistance after hot stamping. In Comparative Example 33, it was believed that due to the slow average cooling rate during the first cooling from plating to 480°C, a large amount of Mg2Si phase was formed in the plating before hot stamping, preventing the sufficient formation of the Mg-Al-Zn-Si intermetallic compound phase. As a result, Mg could not be adequately introduced into the surface oxide after hot stamping, reducing chemical conversion treatability. In Comparative Example 34, it was believed that the fast average cooling rate during the second cooling from 480°C to 300°C resulted in most of the Mg in the plating before hot stamping being dissolved in the α-Al phase, again preventing the sufficient formation of the Mg-Al-Zn-Si intermetallic compound phase. As a result, Mg could not be adequately introduced into the surface oxide after hot stamping, reducing chemical conversion treatability. In Comparative Example 35, due to the low Zn content in the plating, the reaction with phosphoric acid was insufficient during chemical conversion treatment, reducing chemical conversion treatability. In Comparative Example 36, the Si content in the coating was high, and the formation of the Fe-Al-Si phase became significant, ΣL i A higher / L0 value leads to decreased corrosion resistance after hot stamping. In Comparative Example 37, the low Si content in the coating reduces coating adhesion and consequently, corrosion resistance after hot stamping. In Comparative Example 38, the low Mg content in the coating prevents sufficient Mg introduction into the surface oxides after hot stamping, reducing chemical conversion treatability. In Comparative Example 39, insufficient coating adhesion reduces corrosion resistance after hot stamping. In Comparative Example 40, excessive alloying results in a high Fe content in the coating, preventing the formation of an Fe-Al phase with a Fe content of 20-40% by mass. Consequently, resistance to red rust decreases after hot stamping. In Comparative Example 41, the low plating bath temperature causes the plating bath to solidify when the steel base material is immersed, preventing proper coating formation. Therefore, further 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 coating is primarily composed of Fe2Al5 phase, Fe-Al phase, and a Mg-modified surface oxide film, thereby ensuring that the Fe-Al-Si phase within the Fe2Al5 phase satisfies ΣL iThe dispersion of / L0≤0.70 significantly improves the corrosion resistance of the hot-stamped body, even when exposed to high temperatures of 900°C. This is due to the combination of corrosion suppression from the dispersed formation of the Fe-Al-Si phase within the Fe2Al5 phase, improved resistance to red rust from the formation of the Fe-Al phase, and improved chemical conversion properties from the modification of the surface oxide film by Mg. Furthermore, for the coated steel sheets before hot stamping in all embodiments, elemental concentration measurements of the coating cross-section using EPMA revealed that the Si content in the interfacial alloy layer was higher than that in Comparative Example 32, exceeding 3.0% by mass. Additionally, for the coatings before hot stamping in all embodiments, elemental distribution measurements using EPMA-obtained mapping images showed that the Mg-Al-Zn-Si intermetallic compound phase with an Al content of 10% by mass or more was present in the coating at an area fraction exceeding 0.010%. On the other hand, in Comparative Examples 33 and 34, the coating before hot stamping did not contain the Mg-Al-Zn-Si intermetallic compound phase. Specifically, the coating contained Zn: 5.0~35.0% and Si: 0.25~6.0% in its overall chemical composition, and the Mg content in the surface oxide film was 0.30 atomic% or more, thus satisfying ΣL... i In Examples 27-31, where the relationship was controlled by L0 ≤ 0.50, in addition to the evaluation of red rust resistance being A and chemical conversion treatment being AA, the evaluation of coating swelling was AAA, further improving corrosion resistance. Furthermore, GDS measurements were performed on each hot-stamped body, and the results confirmed that in all examples of 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 phase with a Fe content of 20-40% by mass 6: Surface oxide film 7: 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: 2.5~40.0% Si: 0.05~15.0% Fe: 25.0~40.0%, Mg: 0.005~3.000% Ni: 0~0.500% 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, 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, starting from one side of the steel base material, sequentially comprises an Fe2Al5 phase, an Fe-Al phase with a Fe content of 20-40% by mass, and a surface oxide film. In a cross section of the plated 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 i and the length L0 of the surface of the steel base material satisfies ΣL i / L0≤0.70, The Mg content in the surface oxide film is 0.05 atomic% or more. The plating layer has an adhesion amount of 20 g / m per one side 2 The 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: 5.0~35.0%; and Si: 0.25~6.0%, The Mg content in the surface oxide film is above 0.30 atomic%.

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

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