Plated steel sheet

Abstract

A plated steel sheet provided with a base steel sheet and a plating layer formed on the surface of the base steel sheet, the plating layer having a prescribed chemical composition and containing an Fe-Al phase located at the interface with the base steel sheet and a main layer located on the Fe-Al phase, the plating layer being characterized in that: in the cross section of the plating layer, the Fe-Al phase is a Fe-Al phase; the length L of contact between the Fe-Al phase and the main layer and the length L0 of the plating layer satisfy L / L0 < = 4.0, the Fe-Al phase contains 3-15% by mass of Si and 2-15% by mass of Zn, and the amount of the plating layer deposited per surface is 20 g / m2 or more.

Description

Technical Field

[0001] This invention relates to coated steel sheets. Background Technology

[0002] Hot stamping (hot pressing) is a known technique for stamping materials that are difficult to form, such as high-strength steel sheets. Hot stamping is a thermoforming technique where the material to be formed is heated before being formed. 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 high precision. Furthermore, because quenching is performed simultaneously with forming using a stamping die, the formed steel has sufficient strength.

[0003] In this regard, various studies have been conducted on galvanized steel sheets used for hot stamping.

[0004] For example, Patent Document 1 describes a steel plate coated with a metal coating comprising 2.0 to 24.0 wt% zinc, 7.1 to 12.0 wt% silicon, any 1.1 to 8.0 wt% magnesium, and any additional element selected from Pb, Ni, Zr, or Hf, wherein the weight content of each additional element is less than 0.3 wt%, and the balance is aluminum and any unavoidable impurities and residual elements, wherein the Al / Zn ratio exceeds 2.9. Furthermore, in Patent Document 1, the metal coating with an Al / Zn ratio exceeding 2.9 is taught to provide high sacrificial protection.

[0005] 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 balance being Al and other unavoidable impurities. Furthermore, Patent Document 2 teaches that by controlling the Al / (Zn+Si) ratio of the aforementioned aluminum alloy coating to 1.3-2.6, weldability and corrosion resistance can be ensured.

[0006] 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

[0007] The problem that the invention aims to solve

[0008] For example, when using the coated steel sheet described in Patent Documents 1 and 2 in hot stamping, or more specifically, a coated steel sheet containing a large amount of Al as a component in the coating, the coating alloys with the iron base (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 coated steel sheet having an Al-containing coating that exhibits improved corrosion resistance even when applied to hot stamping.

[0010] means for solving problems

[0011] In order to achieve the above-mentioned objective, the inventors conducted research and found that by forming a coating composed of Al-Zn-Si system plating with an adhesion amount of more than a specified amount, and by appropriately controlling the chemical composition and morphology of the Fe-Al phase in the coating formed at the interface with the base steel plate, excellent corrosion resistance can be achieved even after hot stamping. Thus, the present invention was completed.

[0012] The present invention, which achieves the above objectives, is described below.

[0013] (1) A galvanized steel sheet, characterized in that it comprises a base steel sheet and a coating formed on the surface of the base steel sheet. The chemical composition of the coating, expressed as a percentage by mass, is: Zn: 5.0~40.0%, Si: 0.1~15.0% Fe: 0.5~25.0%, 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 Balance: 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 comprises a Fe-Al phase at the interface with the base steel plate and a main layer above the Fe-Al phase. In the cross-section of the coating, the contact length L between the Fe-Al phase and the main layer and the length L0 of the coating satisfy L / L0≤4.0. The Fe-Al phase contains, by mass%, 3.0–15.0% Si and 2.0–15.0% Zn. The coating adhesion amount is 20g / m² per single side. 2 above.

[0014] (2) The plated steel sheet according to (1) above, characterized in that, The chemical composition of the coating, expressed as a percentage by mass, contains... Zn: 10.0~35.0%, and Si: 0.1~6.0%, L / L0≤3.0.

[0015] (3) The plated steel sheet according to (1) or (2) above is characterized in that, from the interface between the base steel sheet and the coating, the depth of the base steel sheet in the depth direction is 0.5 μm or more, where the C concentration is less than 0.10% by mass.

[0016] Invention Effects

[0017] According to the present invention, a coated steel sheet having an Al-containing coating can be provided, which exhibits improved corrosion resistance even when applied to hot stamping. Attached Figure Description

[0018] Figure 1This is a schematic cross-sectional view of the coated steel sheet according to an embodiment of the present invention, showing the contact length L between the Fe-Al phase and the main layer and the length L0 of the coating. Detailed Implementation

[0019] <Coated steel sheet>

[0020] The galvanized steel sheet according to embodiments of the present invention is characterized in that it comprises a base steel sheet and a coating formed on the surface of the base steel sheet. The chemical composition of the coating, expressed as a percentage by mass, is: Zn: 5.0~40.0%, Si: 0.1~15.0% Fe: 0.5~25.0%, 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 Balance: 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 comprises a Fe-Al phase at the interface with the base steel plate and a main layer above the Fe-Al phase. In the cross-section of the coating, the contact length L between the Fe-Al phase and the main layer and the length L0 of the coating satisfy L / L0≤4.0. The Fe-Al phase contains, by mass%, 3.0–15.0% Si and 2.0–15.0% Zn. The coating adhesion amount is 20g / m² per single side. 2 above.

[0021] As described above, when using coated steel sheets containing a high proportion of Al as a component in the coating during hot stamping, the coating alloys with the iron base (base steel sheet) during heating in the hot stamping process, sometimes resulting in reduced corrosion resistance. More specifically, for example, when hot stamping is performed on coated steel sheets having a conventional Al-Si based coating, during high-temperature heating in the hot stamping process (e.g., heating at approximately 900°C or higher), the coating alloys with the base steel sheet, and after hot stamping, a coating mainly containing the Fe2Al5 phase is formed. However, in addition to the Fe2Al5 phase, Fe-Al based intermetallic compounds containing a significant amount of Si, particularly Fe-Al-Si phases containing 5% by mass or more of Si, are sometimes also formed in the coating. In such cases, dissimilar metal contact corrosion (galvanic corrosion) sometimes occurs between the Fe2Al5 phase and the Fe-Al-Si phase, reducing the corrosion resistance of the hot-stamped body.

[0022] Therefore, in order to provide coated steel sheets that exhibit excellent corrosion resistance even when applied to hot stamping, the inventors specifically studied the chemical composition and morphology of the coating. As a result, the inventors found that in coatings composed of Al-Zn-Si systems with further added Zn to Al-Si coatings, it is important to appropriately adjust the chemical composition and adhesion amount, and to appropriately control the chemical composition and morphology of the Fe-Al phase in the coating formed at the interface with the base steel sheet. More specifically, the inventors first found that by controlling the overall Si content of the coating of the coated steel sheet to 0.1 to 15.0% by mass, the coating adhesion can be improved in hot-stamped bodies, and the formation of the Fe-Al-Si phase, a Fe-Al intermetallic compound containing a large amount of Si, can be suppressed. Similarly, the inventors found that by controlling the overall Zn content of the coating to 5.0 to 40.0% by mass, and further controlling the coating adhesion amount to 20 g / m², the coating adhesion can be improved. 2 The above demonstrates that Zn can effectively perform its sacrificial corrosion protection function in the coating, thereby improving the corrosion resistance of the hot-stamped body. Furthermore, the inventors discovered that, in addition to appropriately controlling the overall Si and Zn content of the coating as described above, the Fe-Al phase formed at the interface with the base steel sheet also contains Si and Zn in specified amounts. More specifically, this Fe-Al phase contains 3.0–15.0% Si and 2.0–15.0% Zn by mass%, thereby further suppressing the formation of the Fe-Al-Si phase and enabling Zn to dissolve in the Fe2Al5 phase formed after hot stamping, thus significantly improving the corrosion resistance of the hot-stamped body.

[0023] While not intending to be bound by any specific theory, it is believed that by including 3.0–15.0% by mass Si in the Fe-Al phase, most of the Si present in the coating is trapped within this Fe-Al phase. Since the Fe-Al-Si phase is an intermetallic compound containing a relatively high amount of Si, it is thought that by trapping most of the Si in the Fe-Al phase, the formation of the Fe-Al-Si phase during the high-temperature heating of hot stamping can be suppressed. On the other hand, by including 2.0–15.0% by mass Zn in the Fe-Al phase, Zn, which has a sacrificial corrosion-resistant effect, can be dissolved in the Fe2Al5 phase formed by alloying during the high-temperature heating of hot stamping, thereby improving the corrosion resistance of the coating after hot stamping. Therefore, according to the embodiments of the present invention, the coated steel sheet having a coating containing, by mass percent, 3.0 to 15.0% Si and 2.0 to 15.0% Zn in the Fe-Al phase formed at the interface with the base steel sheet, even when applied to hot stamping, can achieve excellent corrosion resistance in the formed body after hot stamping by combining the effect of suppressing the formation of the Fe-Al-Si phase that can cause galvanic corrosion between the Fe2Al5 phases with the effect of improving the corrosion resistance of the coating containing the Fe2Al5 phase due to the solid solution of Zn.

[0024] However, in subsequent studies by the inventors, it was found that simply by appropriately controlling the overall chemical composition of the coating, the amount of coating, and the Si and Zn content in the Fe-Al phase, Zn sometimes could not be sufficiently dissolved in the Fe2Al5 phase in the hot-stamped body. In such cases, the corrosion resistance of the hot-stamped body could not be sufficiently improved. Therefore, the inventors conducted further research focusing on the morphology of the Fe-Al phase formed at the interface with the base steel sheet. As a result, the inventors discovered that by controlling the Fe-Al phase, which is equivalent to the interface alloy layer in the coating, to have a flatter shape with less unevenness at the contact surface with the main layer, and more specifically, by controlling the morphology of the Fe-Al phase in such a way that the contact length L between the Fe-Al phase and the main layer above the Fe-Al phase and the length L0 of the coating satisfy the relationship L / L0≤4.0, Zn could be sufficiently dissolved in the Fe2Al5 phase in the hot-stamped body, thereby significantly improving the corrosion resistance of the hot-stamped body.

[0025] While not intending to be bound by a specific theory, it is believed that by controlling the Fe-Al phase in the coating, which is equivalent to the interface alloy layer, to have a flatter shape with fewer bumps, the evaporation of Zn during the high-temperature heating of hot stamping can be reliably suppressed or reduced. As a result, Zn can be fully dissolved in the Fe2Al5 phase in the formed body after hot stamping. A detailed explanation follows.

[0026] Figure 1This is a schematic cross-sectional view of a coated steel sheet according to an embodiment of the present invention, showing the contact length L between the Fe-Al phase and the main layer and the length L0 of the coating. (Refer to...) Figure 1 According to an embodiment of the present invention, the coated steel sheet 1 includes a base steel sheet 2 and a coating 3 formed on the surface of the base steel sheet 2. The coating 3 includes an Fe-Al phase 4 located at the interface with the base steel sheet 2 and a main layer 5 located on the Fe-Al phase 4 (i.e., on the surface side of the coated steel sheet 1). Figure 1 It is understood that the contact length L between Fe-Al phase 4 and the main layer 5 and the length L0 of the corresponding coating 3 satisfy the relationship L / L0≤4.0, thus the contact surface of Fe-Al phase 4 with the main layer 5 is controlled to have a relatively flat shape. On the other hand, although not shown, it is known that when L / L0 exceeds 4.0, the unevenness of Fe-Al phase 4 at the contact surface with the main layer 5 becomes larger. During the high-temperature heating of hot stamping, Fe-Al phase 4 grows on the main layer 5 side along with the alloying of the coating and the base steel sheet. Therefore, when the unevenness of Fe-Al phase 4 at the contact surface with the main layer 5 is large, during the high-temperature heating of hot stamping, Fe-Al phase 4 grows in a needle-like manner on the main layer 5 side, and sometimes the needle-like Fe-Al phase 4 protrudes to the surface of the coating 3. The coating 3 of the coated steel sheet 1 in the embodiment of the present invention contains more Al, so its surface is covered by an oxide film composed of Al oxides, etc. However, the needle-like Fe-Al phase 4 that grows during the high-temperature heating of hot stamping can sometimes damage the oxide film. In such cases, Zn contained in the coating 3 evaporates from the damaged portion of the oxide film. Since Zn has a low boiling point of approximately 907°C, it is prone to evaporation when heated to approximately 900°C or higher during hot stamping. Because the surface of the coating 3 is covered by an oxide film composed of Al oxides, Zn evaporation in the coating 3 can be suppressed or reduced even when applied to hot stamping. However, when this oxide film is damaged, the evaporation of Zn 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. As a result, the corrosion resistance of the hot-stamped body decreases.

[0027] In contrast, according to the embodiment of the present invention, the coated steel sheet 1 having an Fe-Al phase 4 with an L / L0 ratio controlled to be 4.0 or less, since the Fe-Al phase 4 has a relatively flat shape at the contact surface with the main layer 5, it is possible to suppress the needle-like growth of the Fe-Al phase 4 on the side of the main layer 5 during the high-temperature heating of hot stamping. Therefore, it is possible to suppress the destruction of the oxide film present on the surface of the coating 3, or to minimize the destruction of such an oxide film, and thus the evaporation of Zn in the coating 3 can be significantly suppressed or reduced by the oxide film. Therefore, the coated steel sheet according to the embodiment of the present invention, even when applied to hot stamping, can make Zn sufficiently dissolved in the Fe2Al5 phase formed after hot stamping. By combining the corrosion resistance improvement effect caused by the solid solution of Zn in such Fe2Al5 phase with the corrosion resistance improvement effect caused by the suppression of the formation of the Fe-Al-Si phase as previously described, the corrosion resistance of the hot-stamped body can be significantly improved. In particular, the fact that the corrosion resistance of the hot-stamped body can be improved by appropriately controlling the chemical composition and morphology of the Fe-Al phase in the coating as described above to suppress the formation of the Fe-Al-Si phase and promote the solid solution of Zn in the Fe2Al5 phase is clarified for the first time by the inventors. Therefore, the coated steel sheet of the present invention is particularly useful in the automotive field, where hot stamping is widely used.

[0028] The coated steel sheet according to embodiments of the present invention will now be described in more detail. In the following description, unless otherwise specified, the unit of content for each element, "%", refers to "mass%". Furthermore, in this specification, the "~" indicating a numerical range is used to mean the lower and upper limits of the values ​​described before and after it, unless otherwise specified.

[0029] [Coating]

[0030] According to an embodiment of the present invention, the coating is formed on the surface of a base steel plate, for example, on at least one side of the base steel plate, preferably on both sides. The coating has the following chemical composition.

[0031] [Zn: 5.0~40.0%]

[0032] Zinc (Zn) is an element that sacrifices its anti-corrosion properties to effectively improve the corrosion resistance of the coating. To achieve this effect, the Zn content is set at 5.0% or more. The Zn content can be 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, when the Zn content is excessive, the melting of Zn during the high-temperature heating of hot stamping becomes significant, and sometimes the molten Zn penetrates into the steel, causing liquid metal embrittlement (LME) cracking. 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.

[0033] [Si: 0.1~15.0%]

[0034] Si is an effective element for improving the adhesion of coatings. To achieve this effect, the Si content is set to 0.1% or more. The Si content can be 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, when the Si content is excessive, the formation of the Fe-Al-Si phase in the coating after hot stamping 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.

[0035] [Fe: 0.5~25.0%]

[0036] Fe can be included in the coating, for example, by leaching from the base steel sheet into the plating bath, or by reacting with Al during the plating process to form a Fe-Al phase at the interface between the base steel sheet and the coating. Therefore, the Fe content is set to 0.5% or more, for example, 1.0% or more, 3.0% or more, 5.0% or more, 8.0% or more, 10.0% or more, or 12.0% or more. On the other hand, Fe is sometimes included in the coating up to about 25.0%, but if it is within this range, it will not adversely affect the plating steel sheet in the embodiments of the present invention. Therefore, the Fe content is set to 25.0% or less, for example, 22.0% or less, 20.0% or less, 18.0% or less, 15.0% or less, or 12.0% or less.

[0037] Furthermore, the coating can arbitrarily contain the following components: 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%. At least one of the following: Mo: 0~1.000%, Ag: 0~1.000%, Li: 0~1.000%, La: 0~0.500%, Ce: 0~0.500%, B: 0~0.500%, Y: 0~0.500%, Sr: 0~0.500%, In: 0~0.500%, Co: 0~0.500%, Bi: 0~0.500%, P: 0~0.500%, and W: 0~0.500%. The total content of these arbitrarily selected elements (i.e., the total content of these elements) is 5.000% or less. The total content of any chosen element 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 also be 0%. As needed, the lower limit for the total content of these elements can also be set to 0.001%, 0.010%, 0.050%, or 0.080%. The following provides a detailed explanation of these chosen elements.

[0038] [Ni: 0~0.500%]

[0039] 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.

[0040] [Mg: 0~3.000%]

[0041] 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.

[0042] [Ca: 0~3.000%]

[0043] Ca is an effective element for ensuring the wettability of the plating bath. The Ca content can be 0%, but to achieve this effect, a Ca content of 0.001% or more is preferred. The Ca content can be 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, excessive Ca content can sometimes lead to the formation of large amounts of hard intermetallic compounds in the coating, making the coating brittle and 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.

[0044] [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%]

[0045] 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 at a concentration 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 coated steel sheet. 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.

[0046] In the coating, the balance other than the elements mentioned above consists of Al and impurities. Impurities in the coating refer to components, represented by the raw materials, that are mixed in 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 15.0% to 94.4%. Depending on the needs, the Al content can be 20.0% or more, 30.0% or more, 40.0% or more, 50.0% or more, 60.0% or more, or 65.0% or more, and can be below 94.0% or less, 92.0% or less, 90.0% or less, 85.0% or less, 80.0% or less, 75.0% or less, or 71.0% or less.

[0047] [Determination of the chemical composition of the coating]

[0048] The chemical composition of the coating is determined as follows. First, the coating is peeled off and dissolved from the coated steel sheet using an acid solution containing an inhibitor that inhibits corrosion of the base steel sheet (e.g., Ibit 710K manufactured by Asahi Chemical Industry). The resulting acid solution is measured by ICP (inductively coupled plasma) luminescence spectrophotometry, thereby determining the chemical composition (average composition) of the coating. The type of acid is not particularly limited; any acid capable of dissolving the coating can be used. For example, a 10% aqueous solution of hydrochloric acid with a concentration of 0.04% of Ibit 710K can be used as the acid containing the inhibitor.

[0049] [Fe-Al phase] [Si: 3.0~15.0% and Zn: 2.0~15.0%]

[0050] In embodiments of the present invention, the coating comprises a Fe-Al phase located at the interface with the base steel sheet, the Fe-Al phase containing, by mass%, 3.0-15.0% Si and 2.0-15.0% Zn. As described above, it is believed that by including 3.0-15.0% Si in the Fe-Al phase, most of the Si present in the coating can be captured in the Fe-Al phase. It is believed that the Fe-Al-Si phase that can be formed after hot stamping is an intermetallic compound containing a relatively high amount of Si, particularly an intermetallic compound containing 5% or more Si by mass. Therefore, by capturing most of the Si in the interfacial alloy layer, i.e., the Fe-Al phase, before hot stamping, the formation of the Fe-Al-Si phase during the high-temperature heating of hot stamping can be suppressed. When a large amount of Fe-Al-Si phase is formed, galvanic corrosion may also occur between it and the Fe2Al5 phase formed by alloying during the high-temperature heating of hot stamping. Therefore, suppressing the formation of the Fe-Al-Si phase is very effective in improving the corrosion resistance of the coating after hot stamping. On the other hand, by containing 2.0 to 15.0% Zn in the Fe-Al phase, Zn with sacrificial corrosion protection can be dissolved in the Fe2Al5 phase after hot stamping. By combining this with the corrosion resistance enhancement effect caused by the inhibition of the Fe-Al-Si phase formation, the corrosion resistance of the hot stamped body can be significantly improved.

[0051] To further enhance the inhibition effect of Fe-Al-Si phase formation, the Si content in the Fe-Al phase is preferably 5.0% or more, for example, 6.0% or more, 8.0% or more, or 10.0% or more. On the other hand, even if the Fe-Al phase contains excessive Si, the above effect becomes saturated. Therefore, the Si content in the Fe-Al phase is set to 15.0% or less, for example, 14.0% or less, or 12.0% or less. Similarly, to further enhance the corrosion resistance improvement effect caused by Zn solid solution in the Fe2Al5 phase, the Zn content in the Fe-Al phase is preferably 3.0% or more, for example, 5.0% or more, 6.0% or more, 8.0% or more, or 10.0% or more. On the other hand, even if the Fe-Al phase contains excessive Zn, the above effect becomes saturated. Therefore, the Zn content in the Fe-Al phase is set to 15.0% or less, for example, 14.0% or less, or 12.0% or less.

[0052] [Main Layer]

[0053] In embodiments of the present invention, the coating comprises a main layer situated on the Fe-Al phase. As described above, the object of the present invention is to provide a coated steel sheet that exhibits improved corrosion resistance even when applied to hot stamping. This is achieved by appropriately optimizing the chemical composition and adhesion amount of the coating, and by controlling the morphology of the Fe-Al phase formed at the interface with the base steel sheet to contain Si: 3.0~15.0% and Zn: 2.0~15.0% by mass, with the contact length L between the Fe-Al phase and the main layer and the length L0 of the coating satisfying the relationship L / L0≤4.0. Therefore, for example, the microstructure of the main layer of the coating is not particularly limited except for the Fe-Al phase, and it is obvious that this is not a necessary technical feature for achieving the object of the present invention. In fact, during the high-temperature heating of hot stamping, the coating alloys with the base steel sheet, mainly forming a coating comprising the Fe2Al5 phase. Therefore, in the coated steel sheet of the embodiments of the present invention, in order to suppress the formation of the Fe-Al-Si phase in the coating after hot stamping and thereby promote the solid solution of Zn into the Fe2Al5 phase, it is extremely important, not as the main layer, to properly control the chemical composition and morphology of the Fe-Al phase corresponding to the interface alloy layer. By properly controlling them, the purpose of the present invention can be reliably achieved.

[0054] Without particular limitation, the main layer can contain at least one of the following: α-Al phase, η-Zn phase, and Si phase. For example, the main layer can contain the α-Al phase. The area fraction of the α-Al phase in the main layer is not particularly limited; for example, it can be 30% or more, 40% or more, 50% or more, or 60% or more. Similarly, the area fraction of the α-Al phase in the main layer can be 100%, for example, it can be less than 90%, less than 80%, or less than 70%. It should be noted that the area outside the Fe-Al phase in the coating, i.e., the non-Fe-Al phase, is the main layer; therefore, the coating consists of the Fe-Al phase and the main layer.

[0055] [L / L0≤4.0]

[0056] In embodiments of the present invention, the morphology of the Fe-Al phase is controlled such that the contact length L between the Fe-Al phase and the main layer and the length L0 of the coating satisfy the relationship L / L0≤4.0. As described above, by controlling the degree to which the Fe-Al phase in the coating, which corresponds to the interface alloy layer, satisfies the relationship L / L0≤4.0 to a flatter shape with less unevenness, the needle-like growth of the Fe-Al phase during the high-temperature heating of hot stamping can be suppressed. Therefore, the destruction of the oxide film present on the surface of the coating due to the needle-like growth of the Fe-Al phase can be suppressed, or the destruction of such oxide film can be minimized, thus significantly suppressing or reducing the evaporation of Zn in the coating through this oxide film. As a result, in the hot-stamped body, Zn can be sufficiently dissolved in the Fe2Al5 phase, thereby significantly improving the corrosion resistance of the hot-stamped body. To further improve this effect, it is preferable that the contact surface between the Fe-Al phase and the main layer is flatter, i.e., the L / L0 value is smaller. More specifically, L / L0 is preferably 3.8 or less, for example, it can be 3.5 or less, 3.2 or less, 3.0 or less, 2.8 or less, 2.5 or less, 2.2 or less, or 2.0 or less. There is no particular limitation on the lower limit, for example, L / L0 can be 1.0 or more, 1.2 or more, 1.5 or more, 1.7 or more, or 1.9 or more.

[0057] [Analysis of the coating]

[0058] The coating analysis was conducted as follows. First, five samples were taken from the coated steel sheet to observe the cross-section of the coating. Next, for each sample, a rectangular area of ​​80 μm in the thickness direction and 100 μm in the direction perpendicular to the thickness direction was defined as one field of view. For all five samples, five fields of view were captured using SEM-EDS or EPMA at 1500x magnification to obtain a mapping image. The location of the Fe-Al phase was determined based on the elemental distribution image of this mapping image. More specifically, the target elements 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, regions satisfying Fe: 25–65 wt% and Al: 30–70 wt% were identified as the Fe-Al phase. The Fe-Al phase region is defined by setting the concentration range of the color bars in the elemental distribution image, which represent the concentration of each element in the image, to Fe: 25~65 wt% and Al: 30~70 wt%. The Fe-Al phase is identified by overlapping the elemental distribution images of Fe and Al with the adjusted color bars. Figure 1 Symbol 4 in the image). In this elemental distribution image, the region where Fe exceeds 65% by mass is identified as the base metal steel plate ( Figure 1 Symbol 2 in the figure identifies the area outside the Fe-Al phase in the coating as the main layer. Figure 1(Symbol 5 in the text). Next, the contact length between the main layer and the Fe-Al phase was determined using image analysis software (such as the "Analyze" function of "ImageJ"). Figure 1 The contact length L between the Fe-Al phase and the main layer is shown. Finally, the average contact length obtained for the five samples is calculated as the contact length L, and the ratio L / L0 is calculated to the corresponding coating length L0 (the length of the long side in each field of view: 100 μm). Here, the length of the long side in the field of view, L0, is as follows: Figure 1 As shown, this is also the interval between the two ends of the contact length L (where the interval is in the direction parallel to the surface of the plated steel plate 1).

[0059] The Si and Zn contents in the Fe-Al phase were determined as follows. First, for the five samples mentioned above, the Fe-Al phase was identified using the method described above, and then the elemental concentrations (specifically, Si and Zn contents) constituting the Fe-Al phase were determined using SEM-EDS or EPMA. More specifically, in each sample, at the thickness position at the center of the Fe-Al phase thickness, the Si and Zn contents were measured at five measurement points spaced 50 μm apart in a direction parallel to the surface of the coated steel sheet, and the average value was taken as the measured value for each sample. The measured values ​​of the five samples for which the elemental concentration was to be determined were calculated, and the average value of these measured values ​​was taken as the elemental content (Si or Zn content). In addition, the microstructure in the main layer was identified based on the elemental distribution image of the mapping image obtained in one of the samples mentioned above. When calculating the area ratio of the microstructure in the main layer, the area ratios of the α-Al phase, η-Zn phase, and Si phase were determined by measuring the elemental distribution images of the mapping images obtained from the five samples mentioned above, and the average of the five fields of view was calculated to determine the microstructure. Here, in the elemental distribution images of SEM-EDS or EPMA, the α-Al phase is defined as the region satisfying Fe: 0~10 wt%, Al: 20~90 wt%, and Zn: 0~80%, and the η-Zn phase is defined as the region satisfying Fe: 0~10 wt%, Al: 0~less than 20 wt%, and Zn: more than 80 wt%. On the other hand, the Si phase is defined as the region satisfying Si: more than 80 wt%.

[0060] As a coating having the above-mentioned chemical composition, Fe-Al phase, and main layer, a molten coating may be included. While it is not necessary to exclude coatings other than molten coatings, they may be limited to molten coatings.

[0061] Coating adhesion amount: 20g / m² per single side 2 above]

[0062] In an embodiment of the present invention, the coating adhesion amount is 20 g / m per single side. 2The above. Typically, the coating sometimes alloys with the base steel sheet during the high-temperature heating process in hot stamping, resulting in reduced corrosion resistance. However, according to embodiments of the present invention, it is considered that by increasing the coating adhesion amount, specifically controlling it to 20 g / m² per single side... 2 The above ensures sufficient overall Zn content in the coating formed after hot stamping and sufficient dissolved Zn content in the Fe2Al5 phase, resulting in excellent corrosion resistance. Conversely, insufficient coating coverage prevents the full realization of the corrosion resistance improvement effect due to Zn, sometimes even leading to decreased corrosion resistance after hot stamping. From the viewpoint of improved corrosion resistance, a coating coverage of 30 g / m² per side is preferred. 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.

[0063] [Determination of Coating Adhesion]

[0064] The coating adhesion amount is determined as follows. First, a 30mm × 30mm sample is taken from the coated steel sheet. Then, the coating is peeled off and dissolved from the sample using an acid solution containing an inhibitor that inhibits corrosion of the base steel sheet (e.g., Ibit 710K manufactured by Asahi Chemical Industry). The coating adhesion amount is determined based on the change in mass of the sample before and after peeling and dissolution. There is no particular limitation on the type of acid; any acid capable of dissolving the coating can be used. For example, as the acid containing the inhibitor, an aqueous solution of 0.04% Ibit 710K and 10% hydrochloric acid can be used.

[0065] [Depth of the base steel plate where the C concentration in the depth direction is less than 0.10% by mass: ≥ 0.5μm]

[0066] According to an embodiment of the present invention, it is preferable that the depth of the C concentration of 0.10% by mass or less in the depth direction of the base steel plate from the interface between the base steel plate and the coating is 0.5 μm or more. By varying the C concentration in the surface portion of the base steel plate, the alloying behavior of the coating and the base steel plate during the high-temperature heating of hot stamping can be changed. While the reason may not be immediately clear, the inventors have found that by providing a region with a low C concentration in the surface portion of the base steel plate, and more specifically, by making the depth of the C concentration of 0.10% by mass or less 0.5 μm or more, the formation of the Fe-Al-Si phase can be further suppressed, thereby further improving the corrosion resistance after hot stamping. From the viewpoint of improved corrosion resistance, a greater depth of C concentration of 0.10% by mass or less is preferred; for example, it can be 0.8 μm or more, 1.0 μm or more, 1.2 μm or more, 1.5 μm or more, or 2.0 μm or more. There is no specific upper limit. For example, the depth for a C concentration of 0.10% by mass or less can be less than 15.0 μm, less than 10.0 μm, or less than 5.0 μm.

[0067] [Determination of C concentration at depths below 0.10% by mass in the base steel plate]

[0068] The depth of C concentration below 0.10% by mass in the depth direction of the base steel sheet from the interface between the base steel sheet and the coating is determined using a high-frequency glow discharge luminescence analyzer (GDS) as follows. Specifically, the method is as follows: the surface of the coated steel sheet is exposed to an Ar atmosphere, and while generating glow plasma by applying a voltage, analysis is performed in the depth direction while sputtering the surface of the coated steel sheet. Then, based on the characteristic emission spectrum wavelength of the element emitted by the excited atoms in the glow plasma, the elements contained in the material are identified, and the emission intensity of the identified elements is estimated. The data in the depth direction 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 emission intensity is converted to mass% by constructing a standard curve. Thus, when performing GDS measurements on coated steel sheets, the location where the Al concentration in the depth direction is less than 1.0% by mass is determined as the interface between the base steel sheet and the coating, and the distance from this interface to the location where the C concentration in the depth direction is 0.10% by mass or more is determined as "the depth from the interface between the base steel sheet and the coating where the C concentration in the depth direction of the base steel sheet is less than 0.10% by mass".

[0069] [Preferred chemical composition of the base steel plate]

[0070] As described above, the object of the present invention is to provide a coated steel sheet that exhibits improved corrosion resistance even when applied to hot stamping. This is achieved by appropriately optimizing the chemical composition and adhesion amount of the coating, forming a Fe-Al phase at the interface with the base steel sheet, containing, by mass percent, Si of 3.0–15.0% and Zn of 2.0–15.0%, and controlling the morphology of the Fe-Al phase such that the contact length L between the Fe-Al phase and the main layer and the length L0 of the coating satisfy the relationship L / L0 ≤ 4.0. Therefore, it is clear that the chemical composition of the base steel sheet itself is not an essential technical feature for achieving the object of the present invention. Hereinafter, a preferred chemical composition of the base steel sheet used in the coated steel sheet according to embodiments of the present invention will be described in detail. However, these descriptions are intended only to illustrate a preferred chemical composition for base steel sheets used in hot stamping and are not intended to limit the present invention to base steel sheets having such a specific chemical composition.

[0071] In embodiments of the present invention, for example, the base steel plate preferably has a chemical composition comprising, by weight percent: 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 Balance: Fe and impurities. The following provides a more detailed explanation of each element.

[0072] [C: 0.13~0.50%]

[0073] Carbon (C) is an inexpensive element that increases tensile strength and is important for controlling the strength of steel. To achieve this effect, the C content is preferably 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 0.50% or less. The C content can be 0.45% or less, or 0.40% or less.

[0074] [Si: 0.001~3.000%]

[0075] Si acts as a deoxidizer, suppressing the precipitation of carbides during the cooling process in the annealing of cold-rolled steel sheets. To achieve this effect sufficiently, 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.

[0076] [Mn: 0.30~3.00%]

[0077] Manganese (Mn) is an element that improves the hardenability of steel and is effective in increasing strength. To fully 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.

[0078] [Al: 0.0002~2.000%]

[0079] Al acts as a deoxidizer in steel, contributing to its overall health. To achieve this effect, the Al content is preferably 0.0002% or more. The Al content can 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 can sometimes lead 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.

[0080] [P: below 0.100%]

[0081] 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, excessive reduction in P content can sometimes lead to a significant increase in cost. Therefore, the P content can be 0.0001% or more, or 0.001% or more, or 0.005% or more. On the other hand, excessive P content, as mentioned above, can sometimes lead to embrittlement of the steel due to grain boundary segregation. 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.

[0082] [S: Below 0.1000%]

[0083] 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, and ideally, it should be 0%. However, excessive reduction in S content can sometimes result in a significant increase in cost. Therefore, the S content can be 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 cracking 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.

[0084] [N: below 0.0100%]

[0085] Nitrogen (N) is an element that forms large nitrides in steel sheets, reducing their workability. Lower N content is preferred, and ideally, it should be 0%. However, excessive reduction in N content can sometimes lead to a significant increase in manufacturing costs. Therefore, the N content can be 0.0001% or more, or 0.0005% or more, or 0.0010% or more. On the other hand, excessive N content, as mentioned above, can sometimes form large nitrides, 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.

[0086] The preferred basic chemical composition of the base steel plate is as described above. Furthermore, the base steel plate 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 balance Fe. These elements may be 0.0001% or more, 0.0005% or more, 0.001% or more, or 0.01% or more, respectively.

[0087] In the base steel sheet, the balance other than the aforementioned elements consists of Fe and impurities. Impurities in the base steel sheet refer to components that are mixed in during the industrial manufacturing of the base steel sheet, such as raw materials like ores and waste, due to various reasons in the manufacturing process.

[0088] The chemical composition of the base steel sheet can be determined using general analytical methods. For example, the chemical composition of the base steel sheet can be determined by first removing the coating through mechanical grinding, and then by using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) for the cut powder 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 base steel sheet, 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 determined by ICP-AES, can be determined using the combustion-infrared absorption method, N using the inactive gas melting-thermal conductivity method, and O using the inactive gas melting-non-dispersive infrared absorption method.

[0089] [Thickness of the base steel plate]

[0090] There is no particular limitation on the thickness of the base steel plate; 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 base steel plate can be, for example, 6.0 mm or less, or 5.0 mm or less, or 4.0 mm or less.

[0091] <Manufacturing Method of Coated Steel Sheet>

[0092] Next, a preferred manufacturing method for the galvanized steel sheet according to an embodiment of the present invention will be described. The following description is an example of a characteristic method for manufacturing the galvanized steel sheet according to an embodiment of the present invention, and is not intended to limit the galvanized steel sheet to the galvanized steel sheet manufactured by the manufacturing method described below.

[0093] The coated steel sheet 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; and a coating process in which a coating is formed on the obtained base steel sheet. Alternatively, after the hot rolling process, the sheet may not be coiled, but pickled and the cold rolling process may be performed directly. Each step will be described in detail below.

[0094] [Casting Process]

[0095] There are no particular restrictions on the conditions of 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, ingot casting, or slab casting methods.

[0096] [Hot rolling process]

[0097] Hot-rolled steel sheets are obtained by hot rolling cast slabs. The hot rolling process involves directly or temporarily cooling the cast slab before reheating and hot rolling. When reheating, the slab heating temperature can be, for example, 1100–1250°C. The hot rolling process typically includes roughing and finishing rolling. The temperature and reduction rate of each rolling pass can be appropriately varied according to the desired microstructure and plate thickness. For example, the finishing rolling end temperature can be 900–1050°C, and the finishing rolling reduction rate can be 10–50%.

[0098] [Winding process]

[0099] Hot-rolled steel sheets can be coiled at a specified temperature. The coiling temperature can be appropriately determined based on 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 the specified heat treatment. Alternatively, the coiling process can be omitted, and pickling followed by the cold rolling process described later can be performed.

[0100] [Cold rolling process]

[0101] After pickling and other processes, hot-rolled steel sheets are cold-rolled to obtain cold-rolled steel sheets. The reduction rate of cold rolling can be appropriately determined according to 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.

[0102] [Annealing process]

[0103] 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, in the final coated steel sheet, the depth of the C concentration in the depth direction of the base steel sheet from the interface between the base steel sheet and the coating can be controlled within a range of 0.5 μm or more. As a result, the corrosion resistance after hot stamping can be further improved compared to the case where the annealing process is performed under conditions with a dew point of less than -10°C. The atmosphere in the annealing process can be a reducing atmosphere, and more specifically, it can be a reducing atmosphere containing nitrogen and hydrogen, such as a reducing atmosphere of 1 to 10% hydrogen (e.g., 4% hydrogen and nitrogen balance).

[0104] [Cooling Process]

[0105] The cooling process can be carried out at a cooling rate suitable for obtaining the desired metal structure from the heating temperature of the annealing process to the penetration temperature of the next plating process. There are no particular limitations; for example, the cooling process can be carried out from the heating temperature of the annealing process to the penetration temperature of the plating process at an average cooling rate of 10°C / s or higher.

[0106] [Plating Process]

[0107] Next, in the plating process, a coating having the chemical composition and morphology described above is formed on at least one, preferably both, surfaces of the cold-rolled steel sheet (base steel sheet). More specifically, the plating process uses a plating bath whose composition has been adjusted to be within the range described above, for example, a plating bath containing Zn: 5.0~50.0%, Si: 0.1~18.0%, and the balance: Al and impurities (plating bath temperature: 650°C or higher, for example 650~680°C), and is carried out such that the immersion temperature of the base steel sheet into the plating bath is 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 formed at the interface with the base steel sheet, which will be described in detail below.

[0108] Typically, the plating process is carried out with the immersion temperature and the plating bath temperature being the same or equivalent. However, when the difference between the immersion temperature and the plating bath temperature (i.e., plating bath temperature - immersion temperature) of the base steel sheet is less than 150°C, the immersion temperature is higher than 500°C, or the plating bath temperature is lower than 650°C, the Fe-Al phase of a suitable type 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, when such a coated steel sheet is subjected to high-temperature heating (for hot stamping), the formation of the Fe-Al-Si phase in the Fe2Al5 phase cannot be suppressed, and / or the sacrificial corrosion-resistant Zn cannot be sufficiently dissolved in the Fe2Al5 phase. Therefore, the corrosion resistance of the coating after hot stamping is significantly reduced. Therefore, in this manufacturing method, by reliably controlling the plating bath temperature to above 650°C and the immersion plate temperature to below 500°C, making the immersion plate temperature at least 150°C lower than the plating bath temperature, the Fe-Al phase crystallizes at a low temperature, and the Fe-Al phase is transformed into a phase type suitable for containing 3.0~15.0% Si and 2.0~15.0% Zn. In this way, by trapping most of the Si in the Fe-Al phase, the formation of the Fe-Al-Si phase in the Fe2Al5 phase during the high-temperature heating of hot stamping can be suppressed. Furthermore, by including Zn in the Fe-Al phase within the aforementioned range, Zn, which has a sacrificial corrosion-resistant effect, can be dissolved in the Fe2Al5 phase formed by alloying during the high-temperature heating of hot stamping, thereby improving the corrosion resistance of the coating after hot stamping.

[0109] Generally, the immersion temperature is controlled to be the same as or equal to the plating bath temperature, as described above. Even when the immersion 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, it is not necessarily necessary to incur manufacturing costs to cool and control the immersion temperature to a lower level; and sometimes the plating bath solidifies due to the immersion temperature being lower than the plating bath temperature. Therefore, the fact that, as in this manufacturing method, by reliably controlling the plating bath temperature to above 650°C and the immersion temperature to below 500°C, making the immersion temperature at least 150°C lower than the plating bath temperature, can improve 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 temperature; for example, the immersion temperature of the base steel plate into the plating bath can be above 380°C. It should be noted that, in conventional wisdom, it was believed that controlling the immersion plate temperature below 500°C could lead to solidification of the plating bath due to its composition. However, in this embodiment, it was found that by setting the difference between the immersion plate temperature and the plating bath temperature to be significantly higher than the conventionally accepted 150°C or more—that is, setting the immersion plate temperature below 500°C and the plating bath temperature to a high temperature such as 650°C—solidification of the plating bath could be prevented. It should also be noted that, to prevent solidification of the plating bath, it is preferable to use agitation within the plating bath to induce convection of the plating solution.

[0110] The plating process can be performed, for example, by melt 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 up 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 20~200g / m² per side. 2 Within the range.

[0111] [Cooling after plating]

[0112] Cooling after plating, as described above, is performed up to 300°C at an average cooling rate of 10°C / s or less. By cooling the plated steel sheet at such a slow average cooling rate, the frequency of Fe-Al phase nucleation from the molten plating (nucleation frequency) can be reduced. In this case, the Fe-Al phase can grow more slowly, thus forming a coarser Fe-Al phase with a flatter shape at the interface with the base steel sheet, resulting in less unevenness; more specifically, the L / L0 of the Fe-Al phase is controlled to be 4.0 or less. As a result, needle-like growth of the Fe-Al phase on the main layer side can be suppressed during the high-temperature heating of hot stamping. Therefore, the destruction of the oxide film present on the surface of the coating can be suppressed, or the destruction of such oxide film can be minimized, thus significantly suppressing or reducing the evaporation of Zn in the coating through this oxide film.

[0113] On the other hand, when the average cooling rate from coating to 300°C is faster than 10°C / s, the nucleation frequency of the Fe-Al phase also increases, thus accelerating the growth of the Fe-Al phase from the molten coating. This accelerated growth results in a more uneven Fe-Al phase with an L / L0 exceeding 4.0 at the interface with the base steel sheet. Consequently, during the high-temperature heating of hot stamping, the Fe-Al phase grows into needle-like structures, sometimes damaging the oxide film present on the coating surface. In such cases, Zn contained in the coating evaporates from the damaged portion of the oxide film. To further reduce the L / L0 value and achieve a flatter Fe-Al phase at the interface with the main layer, the average cooling rate from coating to 300°C is preferably set to 5°C / s or less.

[0114] According to this manufacturing method, the overall chemical composition of the coating can be controlled within a specified range, and the coating adhesion amount can be 20 g / m² per single side. 2The above describes a method for manufacturing coated steel sheets, where the Fe-Al phase formed at the interface with the base steel sheet contains, by mass percent, 3.0–15.0% Si and 2.0–15.0% Zn, and the morphology of the Fe-Al phase is controlled to ensure that the contact length L between the Fe-Al phase and the main layer and the length L0 of the coating are L / L0 ≤ 4.0. Therefore, even under high temperatures such as those encountered during hot stamping, the corrosion resistance of the hot-stamped body is significantly improved through a combination of the corrosion resistance enhancement effect resulting from the solid solution of Zn in the Fe2Al5 phase and the corrosion resistance enhancement effect resulting from the inhibition of the formation of the Fe-Al-Si phase in the coating. Therefore, when used as a hot-stamping coated steel sheet, this method achieves superior corrosion resistance compared to conventional coated steel sheets. Thus, its extended service life contributes to industrial development when used as coated steel sheets for automotive and construction materials.

[0115] 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.

[0116] Example

[0117] In the following embodiments, coated steel sheets according to embodiments of the present invention were manufactured under various conditions, and the characteristics of the manufactured coated steel sheets were investigated.

[0118] 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%, and N: 0.0030%, with the balance 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 rolling, with the finishing temperature at 900–1050°C and a reduction rate of 30%. Next, the resulting hot-rolled steel sheet is pickled and then cold-rolled with a reduction rate 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 a base steel sheet.

[0119] Next, the manufactured base steel sheet was cut into 100mm × 200mm pieces, and coated using the company's intermittent melt coating test apparatus. More specifically, first, the manufactured base steel sheet was immersed in a coating bath with various chemical compositions and coating bath temperatures shown in Table 1 at the immersion temperatures shown in Table 1 for approximately 3 seconds. Then, it was lifted at a pulling speed of 20~200mm / s, and the coating adhesion was adjusted to the values ​​shown in Table 1 by wiping with N2 gas. Next, the coated base steel sheet 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 base steel sheet. The plate temperature was measured using a thermocouple spot-welded to the center of the base steel sheet.

[0120] The physical properties and characteristics of the obtained coated steel sheets were determined and evaluated using the following methods.

[0121] [Chemical composition analysis of the coating, etc.]

[0122] The chemical composition of the coating was determined as follows: Samples cut into 30mm × 30mm pieces were immersed in a 10% HCl aqueous solution containing 0.04% Ibit 710K as an inhibitor. After acid pickling and peeling, the coating components dissolved in the aqueous solution were determined by ICP emission spectroscopy. Furthermore, the mass of the samples before and after acid pickling and peeling was measured, and the coating adhesion amount was determined based on the change in mass. The results are shown in Table 1.

[0123] [Microstructure analysis of the coating]

[0124] The Fe-Al phase and the main layer were identified using the methods described above. Similarly, the Si and Zn contents of the Fe-Al phase were determined using the same methods. Furthermore, the L / L0 value and the depth at which the C concentration in the depth direction of the base steel plate was below 0.10% by mass were determined using the aforementioned methods. It should be noted that in all embodiments, the Fe-Al phase exists at the interface between the coating and the base steel plate, and a non-Fe-Al phase, i.e., the main layer, exists above the Fe-Al phase (on the surface side of the coated steel plate).

[0125] [Deepness of iron-based corrosion]

[0126] The iron-based corrosion depth 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 mold at approximately room temperature, and rapidly cooled. Samples of the heated and rapidly cooled coated steel sheet, 50 mm × 100 mm, were treated with Zn phosphate (SD5350 system: standard manufactured by Nippon Paint Industrial Coating Co., Ltd.). Then, electrodeposition coating (PN110 POWERNICS GRAY: standard manufactured by Nippon Paint Industrial Coating Co., Ltd.) was applied at 20 μm, followed by sintering at 150°C for 20 minutes. Next, a cut was made to reach the iron base (base steel sheet) towards the center of the sample. Finally, a neutral brine spray cycle test of 360 cycles as specified in JIS H 8502:1999, 8.1, was performed. Next, the electrodeposited coating was removed using a descaling agent, and the coating was removed using a 10% HCl aqueous solution with added inhibitor. The iron-based corrosion depth was then measured using a laser meter to evaluate the corrosion resistance as follows. AA: Iron-based corrosion depth is less than 0.3 mm A: The depth of iron-based corrosion exceeds 0.3~0.5mm. B: The iron-based corrosion depth exceeds 0.5mm.

[0127] [Coating swelling]

[0128] The coating swelling was evaluated as follows. First, similar to the case of iron-based corrosion depth, 50mm × 100mm samples of coated steel sheets, after heating and rapid cooling, were treated with Zn phosphate (SD5350 system: standard manufactured by Nippon Paint Industrial Coating Co., Ltd.). Next, an electrodeposition coating (PN110 POWERNICS GRAY: standard manufactured by Nippon Paint Industrial Coating Co., Ltd.) was applied to a depth of 20 μm, followed by sintering at 150°C for 20 minutes. Then, a cut was made towards the center of the sample, reaching the iron substrate (base steel sheet). Next, 120 cycles of neutral salt spray testing, as specified in JIS H 8502:1999, 8.1, were performed to determine the coating swelling width, and corrosion resistance was evaluated as follows. AAA: Less than 2mm AA: More than 2~3mm A: More than 3~4mm B: More than 4mm

[0129] The cases where the iron-based corrosion depth was rated as AA and A, and the coating swelling was rated as AAA, AA, and A, were evaluated as coated steel sheets that could exhibit improved corrosion resistance even when applied to hot stamping. The results are shown in Table 1.

[0130]

[0131]

[0132] Referring to Table 1, in Comparative Example 32, the high penetration temperature of the plating process resulted in the unsatisfactory Si and Zn content in the Fe-Al phase, leading to reduced corrosion resistance after hot stamping. In Comparative Example 33, the rapid average cooling rate from plating to 300°C resulted in a higher nucleation frequency of the Fe-Al phase, with the L / L0 value exceeding 4.0 at the interface with the base steel plate, indicating the formation of a more uneven Fe-Al phase. Relatedly, it is believed that the growth of needle-like Fe-Al phases during the high-temperature heating of hot stamping disrupts the oxide film on the coating surface, causing most of the Zn in the coating to evaporate. Consequently, corrosion resistance decreased after hot stamping. In Comparative Example 34, due to the low Zn content in the coating, the sacrificial corrosion protection effect could not be fully utilized, resulting in increased iron-based corrosion depth and reduced corrosion resistance after hot stamping. In Comparative Example 35, due to the high Si content in the coating, the formation of the Fe-Al-Si phase in the Fe2Al5 phase of the coating after hot stamping became significant. As a result, the coating swelling increased after hot stamping, and the corrosion resistance decreased. In Comparative Example 36, the Si content in the coating was low, thus the adhesion of the coating decreased, and the corrosion resistance after hot stamping decreased. In Comparative Example 37, the coating adhesion was insufficient, thus the corrosion resistance after hot stamping decreased. In Comparative Example 38, the plating bath temperature was low, so the plating bath solidified when the base steel sheet was immersed in the plating bath, and the coating could not be properly formed. Therefore, subsequent manufacturing was stopped, and no analysis or performance evaluation was performed.

[0133] In contrast, by controlling the overall chemical composition of the coating within a specified range in all embodiments of the coated steel sheet and achieving a coating adhesion amount of 20 g / m² per single side, 2The above describes a method for controlling the morphology of the Fe-Al phase at the interface with the base steel plate, where the Fe-Al phase contains 3.0-15.0% Si and 2.0-15.0% Zn by mass, and the contact length L between the Fe-Al phase and the main layer and the length L0 of the coating satisfy the relationship L / L0≤4.0. Even when applied to hot stamping, this method significantly improves the corrosion resistance of the hot-stamped body through a combination of the corrosion resistance enhancement effect caused by the solid solution of Zn in the Fe2Al5 phase and the corrosion resistance enhancement effect caused by the inhibition of the formation of the Fe-Al-Si phase in the coating. In particular, in Examples 6-21, where the overall chemical composition of the coating contains 10.0-35.0% Zn and 0.1-6.0% Si, and the morphology of the Fe-Al phase is controlled to satisfy the relationship L / L0≤3.0, the evaluation of iron-based corrosion depth and coating swelling are AA, respectively, further improving corrosion resistance. Furthermore, in Examples 27-31, where the overall chemical composition of the coating contains Zn: 10.0~35.0% and Si: 0.1~6.0%, and the morphology of the Fe-Al phase is controlled in a manner that satisfies the relationship L / L0≤3.0, thereby controlling the depth of C concentration in the surface layer of the base steel plate to be less than 0.10% by mass to be more than 0.5 μm, in addition to the evaluation of iron-based corrosion depth being AA, the evaluation of coating swelling is AAA, which can further improve corrosion resistance. In addition, the main layer of each coated steel plate was analyzed, and the results showed that in all the coated steel plates of the examples, the main layer contained at least one of α-Al phase, η-Zn phase and Si phase, and in particular, in all the coated steel plates of the examples, the main layer contained α-Al phase of more than 50% by area.

[0134] Explanation of reference numerals in the attached figures 1. Coated steel sheet 2. Base material steel plate 3. Coating 4 Fe-Al phase 5. Main Layer Contact length between L Fe-Al phase and main layer Length of L0 coating

Claims

1. A galvanized steel sheet, characterized in that, It comprises a base steel plate and a coating formed on the surface of the base steel plate. The chemical composition of the coating, expressed as a percentage by mass, is: Zn: 5.0~40.0%, Si: 0.1~15.0% Fe: 0.5~25.0%, 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 Balance: 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 comprises a Fe-Al phase at the interface with the base steel plate and a main layer above the Fe-Al phase. In the cross-section of the coating, the contact length L between the Fe-Al phase and the main layer and the length L0 of the coating satisfy L / L0≤4.

0. The Fe-Al phase contains, by mass%, 3.0–15.0% Si and 2.0–15.0% Zn. The coating adhesion amount is 20g / m² per single side. 2 above.

2. The plated steel sheet according to claim 1, characterized in that, The chemical composition of the coating, expressed as a percentage by mass, contains... Zn: 10.0~35.0%, and Si: 0.1~6.0%, L / L0≤3.

0.

3. The plated steel sheet according to claim 1 or 2, characterized in that, From the interface between the base steel plate and the coating, the depth of the C concentration of 0.10% by mass or less in the depth direction of the base steel plate is 0.5 μm or more.

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