Plated steel sheet

Abstract

Provided is a plated steel sheet characterized by comprising a base-material steel sheet, and a plating layer formed on a surface of the base-material steel sheet, wherein: the plating layer has a prescribed chemical composition; in a cross section of the plating layer, the length L of the interface between the plating layer and the base-material steel sheet and the length L0 of the surface of the base-material steel sheet satisfy (L-L0) / L0×100 ≥ 3; the plating layer includes an Fe-Al phase; and the thickness of the Fe-Al phase is 4-50 μm.

Description

[Technical Field]

[0001] The present invention relates to a plated steel sheet. [Background technology]

[0002] Zn-based coated steel sheets are known to exhibit sacrificial corrosion protection and have excellent corrosion resistance. On the other hand, many proposals have been made for coated steel sheets having a coating layer containing other elements instead of or in addition to Zn.

[0003] For example, Patent Document 1 discloses a steel sheet comprising: a substrate steel sheet; a first alloy plating layer having a thickness of 3 to 30 μm and having a composition containing, in mass %, 40 to 70% Fe, 0.3 to 10% Mn, and the balance being Al and unavoidable impurities, on at least one surface of the substrate steel sheet; a second alloy plating layer having a thickness of 0.10 to 10 μm and having a composition containing, in mass %, 5 to 50% Fe, 5 to 40% Mn, and the balance being Al and unavoidable impurities, on the first alloy plating layer; and a coating weight of 0 to 1000 mg / m2 deposited on the surface of the second alloy plating layer. 2 Patent Document 1 also describes an Al-based plated steel sheet characterized by having unalloyed Al of 1000 mg / m2. Patent Document 1 also describes a method of forming two Al-Fe-Mn alloy plating layers, each having a different Mn content, on the surface of a substrate steel sheet by hot-dip plating an Al-Mn alloy on the substrate steel sheet, and a method of forming two Al-Fe-Mn alloy plating layers, each having a different Mn content, on the surface of the substrate steel sheet. 2 It is taught that by limiting the content to the following ranges, it is possible to achieve both corrosion resistance after painting and resistance spot weldability in an environment similar to the corrosive environment of an automobile exterior panel. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2020-122205 Summary of the Invention [Problem to be solved by the invention]

[0005] In order to ensure corrosion resistance after painting, an alloying treatment is generally required for Al-based plated steel sheets such as those described in Patent Document 1. However, because the alloyed Al-based plating is relatively hard, the cold workability of the plated steel sheet may be reduced.

[0006] Therefore, an object of the present invention is to provide a plated steel sheet having an Al-containing plating layer, which has improved corrosion resistance and cold workability after painting. [Means for solving the problem]

[0007] As a result of investigations conducted by the present inventors to achieve the above object, they found that improved corrosion resistance and cold workability after painting can be achieved by optimizing the chemical composition of the Al-containing plating layer and appropriately controlling the thickness of the Fe—Al phase contained in the Al-containing plating layer and the morphology of the interface between the Al-containing plating layer and the base steel sheet, and thus completed the present invention.

[0008] The present invention, which has achieved the above object, is as follows. (1) A steel plate having a base steel sheet and a plating layer formed on the surface of the base steel sheet, The plating layer is composed of, in mass %, Fe: 20.0-55.0% Mg: 0-10.0% Si: 0 to 10.0% Zn: 0 to 30.0% and further comprising Ni: 0 to 1.000%, Ca: 0-4.000%, Sb: 0 to 0.500% Pb: 0~0.500%, Cu: 0-1.000%, Sn: 0 to 1.000%, Ti: 0 to 1.000%, Cr: 0 to 1.000%, Nb: 0 to 1.000%, Zr: 0 to 1.000%, Mn: 0 to 1.000%, Mo: 0 to 1.000%, Ag: 0~1.000%, Li: 0~1.000%, La: 0 to 0.500%, Ce: 0 to 0.500% B: 0~0.500%, Y: 0~0.500%, Sr: 0 to 0.500%, In: 0~0.500%, Co: 0 to 0.500%, Bi: 0 to 0.500% P: 0~0.500%, W: 0 to 0.500%, and V: 0 to 0.500% Contains at least one of the following in a total amount of 5.000% or less, The balance has a chemical composition consisting of 20.0% or more Al and impurities, In a cross section of the coating layer, an interface length L between the coating layer and the base steel sheet and a length L0 of a surface of the base steel sheet satisfy (L-L0) / L0×100≧3, A plated steel sheet, wherein the plated layer contains an Fe—Al phase, and the thickness of the Fe—Al phase is 4 to 50 μm. (2) The plated steel sheet according to (1) above, characterized in that (L-L0) / L0×100≧5. (3) The plated steel sheet according to (2) above, characterized in that (L-L0) / L0×100≧7. (4) The plated steel sheet according to any one of (1) to (3) above, wherein the Mg content in the plated layer is 0.2% or more. (5) The chemical composition is in mass%: Mg: 0.3 to 10.0%, and The plated steel sheet according to any one of the above (1) to (4), characterized by containing Si: 0 to 1.0%. (6) The plated steel sheet according to any one of the above (1) to (5), wherein the thickness of the Fe—Al phase is 12 to 50 μm. (7) In the cross section of the coating layer, the projected length T of the Fe-Al-Si phase in the coating layer i The surface length L0 of the base steel plate is ΣT i The plated steel sheet according to any one of the above (1) to (6), wherein / L0×100≦20 is satisfied. (8)ΣT i The plated steel sheet according to (7) above, wherein / L0×100≦1. (9) The chemical composition contains, in mass%, Mg: 0.3 to 10.0%; the plating layer further includes an Mg-containing phase, The plated steel sheet according to any one of (1) to (8) above, wherein the Mg-containing phase has a surface coverage of 20 to 100% in a cross section of the plated layer. (10) The plated steel sheet according to (9) above, wherein the Mg-containing phase has a surface coverage of 60 to 100%. (11) The plated steel sheet according to any one of (1) to (10) above, wherein the Mg content in the plated layer is 2.4% or less. (12) The plated steel sheet according to any one of (1) to (11) above, wherein the Si content in the plated layer is 0.2% or more. (13) The plated steel sheet according to any one of (1) to (12) above, wherein the area ratio of the MgZn2 phase in the plated layer is less than 10%. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a plated steel sheet having an Al-containing plating layer, which has improved corrosion resistance and cold workability after painting. [Brief explanation of the drawings]

[0010] [Figure 1]1 is a cross-sectional schematic view of a plated steel sheet according to an embodiment of the present invention, showing the interface length L between the plated layer and the base steel sheet and the surface length L0 of the base steel sheet. [Figure 2] 1 is a cross-sectional schematic view of a plated steel sheet according to a preferred embodiment of the present invention, showing the projected length Ti of the Fe—Al—Si phase and the length L0 of the surface of the base steel sheet. [Figure 3] FIG. 2 is a cross-sectional schematic view of a plated steel sheet according to another preferred embodiment of the present invention, illustrating the surface coverage of the Mg-containing phase. DETAILED DESCRIPTION OF THE INVENTION

[0011] <Plated steel sheet> A plated steel sheet according to an embodiment of the present invention comprises a base steel sheet and a plating layer formed on a surface of the base steel sheet, The plating layer is composed of, in mass %, Fe: 20.0-55.0% Mg: 0-10.0% Si: 0 to 10.0% Zn: 0 to 30.0%, and further comprising Ni: 0 to 1.000%, Ca: 0-4.000%, Sb: 0 to 0.500% Pb: 0~0.500%, Cu: 0-1.000%, Sn: 0 to 1.000%, Ti: 0 to 1.000%, Cr: 0 to 1.000%, Nb: 0 to 1.000%, Zr: 0 to 1.000%, Mn: 0 to 1.000%, Mo: 0 to 1.000%, Ag: 0~1.000%, Li: 0~1.000%, La: 0 to 0.500%, Ce: 0 to 0.500% B: 0~0.500%, Y: 0~0.500%, Sr: 0 to 0.500%, In: 0~0.500%, Co: 0 to 0.500%, Bi: 0 to 0.500% P: 0~0.500%, W: 0 to 0.500%, and V: 0 to 0.500% Contains at least one of the following in a total amount of 5.000% or less, The balance has a chemical composition consisting of 20.0% or more Al and impurities, In a cross section of the coating layer, an interface length L between the coating layer and the base steel sheet and a length L0 of a surface of the base steel sheet satisfy (L-L0) / L0×100≧3, The plating layer contains an Fe—Al phase, and the thickness of the Fe—Al phase is 4 to 50 μm.

[0012] As mentioned above, Al-based plated steel sheets generally require alloying treatment to ensure corrosion resistance after painting. However, because alloyed Al-based plating is relatively hard, the cold workability of the plated steel sheet may be reduced. For example, the alloyed Al-based plating may peel off into powder form (also known as powdering) during cold working, which may reduce powdering resistance. Therefore, it is generally difficult for plated steel sheets having a plating layer made of Al-based plating to achieve both corrosion resistance after painting and cold workability, particularly powdering resistance.

[0013] Therefore, the present inventors conducted research, focusing particularly on the chemical composition, structure, and morphology of the coating layer, in order to achieve both corrosion resistance after painting and cold workability in a coated steel sheet having a coating layer made of an Al-based coating. As a result, the present inventors found that both corrosion resistance after painting and cold workability can be significantly improved by optimizing the chemical composition of the coating layer and appropriately controlling the thickness of the Fe-Al phase contained in the coating layer and the morphology of the interface between the coating layer and the base steel sheet.

[0014] More specifically, the present inventors first found that by setting the Fe content in the coating layer to 20.0 mass% or more and controlling the thickness of the Fe-Al phase contained in the coating layer to 4 μm or more, the coating layer can be sufficiently alloyed, thereby improving the corrosion resistance after painting of the coated steel sheet. On the other hand, the present inventors also found that by controlling the thickness of the Fe-Al phase to 50 μm or less, excessive hardening of the coating layer can be suppressed, thereby improving the cold workability of the coated steel sheet.

[0015] Next, the present inventors investigated the morphology of the coating layer in order to further improve the cold workability of the coated steel sheet. As a result, the present inventors discovered that the cold workability of the coated steel sheet can be significantly improved by controlling the interface shape between the coating layer and the base steel sheet to have greater irregularities, more specifically, by controlling the interface shape to have greater irregularities such that the interface length L between the coating layer and the base steel sheet and the surface length L0 of the base steel sheet satisfy the relationship (L - L0) / L0 × 100 ≥ 3. FIG. 1 is a cross-sectional schematic diagram of a coated steel sheet according to an embodiment of the present invention, showing the interface length L between the coating layer and the base steel sheet and the surface length L0 of the base steel sheet. Referring to FIG. 1, the coated steel sheet 1 according to an embodiment of the present invention includes a base steel sheet 2 and a coating layer 3 formed on the surface of the base steel sheet 2, and the coating layer 3 contains an Fe-Al phase 4. In Fig. 1, the interface length L between the coating layer 3 and the base steel sheet 2 and the corresponding length L0 of the surface of the base steel sheet 2 satisfy the relationship (L-L0) / L0 x 100 ≥ 3, i.e., the interface length L is at least 3% longer than the length L0 of the surface of the base steel sheet 2. It can therefore be seen that the interface between the coating layer 3 and the base steel sheet 2 is controlled to have a shape with greater irregularities.

[0016] Without intending to be bound by any particular theory, it is believed that when the interface between the coating layer 3 and the base steel sheet 2 has a shape with greater irregularities as shown in Fig. 1 , the hard coating layer 3 can dig into the base steel sheet 2 from the irregularities at the interface during cold working such as bending, thereby deforming the base steel sheet 2 and allowing the cold working to proceed. As a result, it is possible to significantly suppress the occurrence of powdering due to bending, etc., and in other words, it is possible to significantly improve the cold workability of the coated steel sheet 1. On the other hand, if the interface between the coating layer 3 and the base steel sheet 2 has a flat shape or a flatter shape with less irregularities, it is not possible to allow the hard coating layer 3 to dig into the base steel sheet 2 during cold working such as bending, thereby allowing the cold working to proceed, and therefore it is not possible to sufficiently suppress the occurrence of powdering.

[0017] The inventors have also found that increasing the alloying rate during alloying of the coating layer 3 is effective in creating an interface shape with greater irregularities between the coating layer 3 and the base steel sheet 2. Explaining this in more detail, first, excessive Si and Mg content in the coating layer 3 may adversely affect the alloying of the coating layer 3, and therefore the Si and Mg contents in the coating layer 3 must each be controlled to 10.0 mass% or less. Additionally, in order to increase the alloying rate of the coating layer 3, it is necessary to appropriately control the metallographic structure of the base steel sheet 2 during the alloying treatment. More specifically, by making the base steel sheet 2 during the alloying treatment have a metallographic structure that is appropriately decarburized and contains a larger amount of austenite phase, the reaction between the coating layer 3 and the austenite phase in the base steel sheet 2 during the alloying treatment is promoted, which means that the alloying rate can be significantly increased. As will be described in detail later in connection with the manufacturing method of the plated steel sheet 1, the inventors have found that it is possible to create a metallographic structure of the base steel sheet 2 that is moderately decarburized and contains a larger amount of austenite phase by appropriately controlling the annealing process, cooling process, and plating process of the base steel sheet 2. As a result, it is possible to realize an interface shape with greater irregularities such that the interface length L between the plated layer 3 and the base steel sheet 2 and the corresponding length L0 of the surface of the base steel sheet 2 satisfy the relationship (L - L0) / L0 × 100 ≥ 3, thereby making it possible to significantly improve the cold workability of the plated steel sheet 1.

[0018] In particular, the inventors have now discovered for the first time that controlling the Fe-Al phase 4 in an appropriately alloyed coating layer 3 to within a range of 4 to 50 μm ensures sufficient corrosion resistance after painting and improves cold workability, and that controlling the interface shape between the coating layer 3 and the base steel sheet 2 to a shape with greater irregularities that satisfies the relationship (L-L0) / L0×100≧3 significantly improves the cold workability of the coated steel sheet 1. Therefore, the coated steel sheet according to the embodiment of the present invention is particularly useful in the automotive field, where both corrosion resistance after painting and cold workability are required.

[0019] Hereinafter, a plated steel sheet according to an embodiment of the present invention will be described in more detail. In the following description, the unit of content of each element, "%," means "mass %" unless otherwise specified. Furthermore, in this specification, unless otherwise specified, the symbol "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits.

[0020] [Plating layer] According to an embodiment of the present invention, a plating layer is formed on the surface of a base steel sheet, for example, on at least one surface, preferably both surfaces of the base steel sheet. The plating layer has the following chemical composition:

[0021] [Fe: 20.0 to 55.0%] When plated steel sheet is alloyed, Fe from the base steel sheet diffuses into the coating layer and alloys with Al, etc., so the coating layer inevitably contains Fe. To ensure corrosion resistance after painting, the plated steel sheet must be appropriately alloyed, and therefore the Fe content is set to 20.0% or more. The Fe content may be 25.0% or more, 30.0% or more, 35.0% or more, or 40.0% or more. On the other hand, if the Fe content is too high, excessive alloying of the coating layer may result in reduced cold workability. Therefore, the Fe content is set to 55.0% or less. The Fe content may also be 52.0% or less, 50.0% or less, 48.0% or less, or 45.0% or less.

[0022] [Mg: 0-10.0%] Mg is an element effective in improving the corrosion resistance of the coating layer, particularly its chemical conversion treatability. While the Mg content may be 0%, to achieve this effect, the Mg content is preferably 0.2% or more. The Mg content may be 0.3% or more, 0.5% or more, 0.8% or more, 1.0% or more, 1.5% or more, or 2.0% or more. On the other hand, excessive Mg content may slow the alloying rate during alloying treatment of the coating layer, making it impossible to obtain the desired interface shape between the coating layer and the base steel sheet. Therefore, the Mg content is set to 10.0% or less. The Mg content may be 8.0% or less, 6.0% or less, 5.0% or less, 4.0% or less, 3.0% or less, less than 2.5%, 2.4% or less, or 2.2% or less.

[0023] [Si: 0-10.0%] Si is an element effective in improving the adhesion of the coating layer. The Si content may be 0%, but to fully obtain this effect, the Si content is preferably 0.1% or more. The Si content may be 0.2% or more, 0.3% or more, 0.5% or more, 0.6% or more, or 0.8% or more. On the other hand, excessive Si content may slow down the alloying rate during alloying treatment of the coating layer, making it impossible to obtain the desired interface shape between the coating layer and the base steel sheet. Therefore, the Si content is set to 10.0% or less. The Si content may be 8.0% or less, 6.0% or less, 4.0% or less, or 2.0% or less. Further reducing the Si content can significantly suppress or reduce the formation of Fe-Al-based intermetallic compounds containing relatively large amounts of Si, more specifically, Fe-Al-Si phases containing 3% or more by mass of Si. If a relatively large amount of the Fe-Al-Si phase is present, galvanic corrosion may occur between the Fe-Al phase (containing less than 3% of elements other than Fe, Al, and Zn). Therefore, from the viewpoint of further improving corrosion resistance, the Si content is preferably 1.0% or less.

[0024] [Zn: 0-30.0%] Zn has a sacrificial corrosion protection effect and is an effective element for improving the corrosion resistance of the coating layer. The Zn content may be 0%, but to fully obtain this effect, the Zn content is preferably 1.0% or more. The Zn content may be 3.0% or more, 5.0% or more, 10.0% or more, 12.0% or more, 15.0% or more, or 18.0% or more. On the other hand, excessive Zn content may cause significant Zn melting during welding of the coated steel sheet, and the molten Zn may penetrate the steel and cause liquid metal embrittlement (LME) cracking. Therefore, the Zn content is preferably 30.0% or less. The Zn content may be 28.0% or less, 25.0% or less, 22.0% or less, or 20.0% or less.

[0025] Furthermore, the plating layer may optionally contain Ni: 0-1.000%, Ca: 0-4.000%, 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 The alloy may contain at least one of the following optional elements: Cr: 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 V: 0-0.500%. Although the amount of these optional elements is not particularly limited, it is preferable that the total amount is 5.000% or less. The optional elements may total up to 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%. These optional elements are described in more detail below.

[0026] [Ni: 0-1.000%] Ni is an element effective in improving the corrosion resistance of the coating layer. The Ni content may be 0%, but to achieve this effect, the Ni content is preferably 0.001% or more. The Ni content may be 0.003% or more, 0.005% or more, 0.008% or more, 0.010% or more, or 0.020% or more. While there is no particular upper limit, from the viewpoint of production costs, etc., the Ni content may be 1.000% or less, for example, 0.500% or less, 0.400% or less, 0.300% or less, 0.100% or less, 0.050% or less, or 0.030% or less.

[0027] [Ca: 0-4.000%] Ca is an element effective in ensuring wettability of the coating bath. The Ca content may be 0%, but to achieve this effect, the Ca content is preferably 0.001% or more. The Ca content may be 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, excessive Ca content may form a large amount of hard intermetallic compounds in the coating layer, making the coating layer brittle and reducing adhesion to the steel sheet. Therefore, the Ca content is preferably 4.000% or less. The Ca content may be 3.000% or less, 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.

[0028] [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%, I n:0~0.500%, Co:0~0.500%, Bi:0~0.500%, P:0~0.500%, W:0~0.500% and V:0~0.500%] Sb, Pb, Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, Li, La, Ce, B, Y, Sr, In, Co, Bi, P, W, and V may not be present in the coating layer, but may be present in the coating layer in amounts of 0.0001% or more, 0.001% or more, or 0.01% or more. These elements do not adversely affect the performance of the coated steel sheet as long as they are within the specified content range. However, excessive content of each element may reduce corrosion resistance. Therefore, the content of Sb, Pb, La, Ce, B, Y, Sr, In, Co, Bi, P, W, and V is preferably 0.500% or less, and may be, for example, 0.300% or less, 0.100% or less, 0.050% or less, or 0.020% or less. Similarly, the contents of Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li are preferably 1.000% or less, and may be, for example, 0.800% or less, 0.500% or less, 0.100% or less, 0.050% or less, or 0.020% or less.

[0029] The remainder of the plating layer other than the above elements consists of 20.0% or more Al and impurities. The Al content may be, for example, 25.0% or more, 30.0% or more, 35.0% or more, 40.0% or more, 45.0% or more, or 50.0% or more. Similarly, the Al content may be, for example, 80.0% or less, 75.0% or less, 70.0% or less, 65.0% or less, or 60.0% or less. Impurities in the plating layer refer to components that are mixed in due to various factors in the manufacturing process, including raw materials, when producing the plating layer.

[0030] [Measurement of the chemical composition of the plating layer] The chemical composition of the plating layer is determined as follows. First, the plating layer is stripped and dissolved from the plated steel sheet using an acid solution containing an inhibitor that suppresses corrosion of the base steel sheet, and the resulting acid solution is measured by ICP (inductively coupled plasma) emission spectroscopy to determine the chemical composition (average composition) of the plating layer. The acid species is not particularly limited, and any acid capable of dissolving the plating layer may be used. Note that the chemical composition of the plating layer in this embodiment is the average of measurements taken on three samples.

[0031] [(L-L0) / L0×100≧3] In an embodiment of the present invention, the morphology of the coating layer is controlled so that, in the cross section of the coating layer, the interface length L between the coating layer and the base steel sheet and the surface length L0 of the base steel sheet satisfy the relationship (L - L0) / L0 × 100 ≥ 3, i.e., the interface length L is longer than the surface length L0 of the base steel sheet by 3% or more. As described above with reference to FIG. 1 , by having the interface between the coating layer and the base steel sheet have a shape with greater irregularities that satisfies the relationship (L - L0) / L0 × 100 ≥ 3, during cold working such as bending, the hard coating layer can penetrate into the base steel sheet from the interfacial irregularities as a starting point, deforming the base steel sheet while the cold working proceeds. As a result, it is possible to significantly suppress the occurrence of powdering due to bending, etc., and therefore significantly improve the cold workability of the coated steel sheet. To further enhance this effect, it is preferable to control the interface shape between the coating layer and the base steel sheet to have greater irregularities, i.e., to increase the value of (L - L0) / L0 × 100. More specifically, the value of (L-L0) / L0×100 is preferably 4 or greater, and may be, for example, 5 or greater, 6 or greater, 7 or greater, or 8 or greater. There is no particular upper limit, but the value of (L-L0) / L0×100 may be, for example, 30 or less, 20 or less, 15 or less, 12 or less, or 10 or less.

[0032] [Fe-Al phase thickness: 4-50 μm] In an embodiment of the present invention, the coating layer contains an Fe—Al phase, and the thickness of the Fe—Al phase is 4 to 50 μm. In the present invention, the Fe—Al phase refers to a phase having a chemical composition, in mass %, of 40 to 70% Fe, 30 to 60% Al, 0 to 20% Zn, and less than 3% other elements (i.e., Fe, Al, and Zn total exceed 97%). By controlling the Fe content in the coating layer to 20.0 mass % or more as described above and the thickness of the Fe—Al phase contained in the coating layer to 4 μm or more, the coating layer can be sufficiently alloyed, thereby improving the corrosion resistance after painting of the coated steel sheet. From the viewpoint of further improving the corrosion resistance after painting, the thicker the Fe—Al phase, the more preferable it is, and it may be, for example, 6 μm or more, 8 μm or more, 10 μm or more, 12 μm or more, 14 μm or more, or 16 μm or more. On the other hand, if the Fe-Al phase is too thick, it may lead to excessive hardening of the plating layer and reduce the cold workability of the plated steel sheet. Therefore, the thickness of the Fe-Al phase is set to 50 μm or less, and may be, for example, 40 μm or less, 30 μm or less, 25 μm or less, or 20 μm or less.

[0033] [Fe-Al-Si phase] [ΣT i / L0×100≦20] According to a preferred embodiment of the present invention, in a cross section of the plating layer, the projected length T of the Fe—Al—Si phase in the plating layer i The surface length L0 of the base steel plate is ΣT i / L0×100≦20. The Fe-Al-Si phase is an Fe-Al-based intermetallic compound containing a relatively large amount of Si. More specifically, in the present invention, the Fe-Al-Si phase refers to a phase having a chemical composition consisting of, by mass%, 30-70% Fe, 30-60% Al, 3-20% Si, and less than 3% other elements. Therefore, if the Fe-Al-Si phase is present in a relatively large amount in the coating layer, galvanic corrosion may occur between the Fe-Al phase and the coating layer. Therefore, in a preferred embodiment of the present invention, the Fe-Al-Si phase is dispersed in the coating layer, i.e., the projected length T of the Fe-Al-Si phase in the coating layer is iThe surface length L0 of the base steel plate is ΣT i By controlling the content so as to satisfy / L0×100≦20, it is possible to further improve the corrosion resistance of the plated steel sheet after painting.

[0034] FIG. 2 is a cross-sectional schematic view of a plated steel sheet according to a preferred embodiment of the present invention, showing the projected length T i 2, the plated steel sheet 1 includes a base steel sheet 2 and a plating layer 3 formed on the surface of the base steel sheet 2, and the plating layer 3 includes an Fe-Al phase 4 and an Fe-Al-Si phase 5. Here, the projected length T i The sum of ΣT i (In Figure 2, ΣT i =T1+T2), and the surface length L0 of the base steel plate 2 is ΣT i / L0×100≦20 (i.e., the projected ratio of the Fe-Al-Si phase is 20% or less), and therefore it can be seen that the Fe-Al-Si phase 5 is dispersed and present in the Fe-Al phase 4. As is clear from the projected length T1 in FIG. 2, when the projected lengths of multiple Fe-Al-Si phases 5 partially overlap, the entire projected length including the overlapping portions is determined as a single projected length. Unlike the case shown in FIG. 2, if the Fe-Al-Si phase 5 is present in a layered form in the Fe-Al phase 4, when galvanic corrosion occurs at the contact portion between the layered Fe-Al-Si phase 5 and the Fe-Al phase 4, it is thought that corrosion will progress along the contact interface. As a result, the corrosion resistance of the coating layer 3 will be significantly reduced. In contrast, in the coated steel sheet 1 according to a preferred embodiment of the present invention, ΣT i By dispersing the Fe-Al-Si phase 5 in the Fe-Al phase 4 so as to satisfy / L0×100≦20, even if galvanic corrosion occurs at the contact area between one or more Fe-Al-Si phases 5 and the surrounding Fe-Al phases 4, the corrosion will not progress from the contact area to other Fe-Al-Si phases 5, thereby further improving the corrosion resistance of the plated steel sheet 1, particularly the corrosion resistance after painting.

[0035] By controlling the Si content in the coating layer, the projection ratio of the Fe-Al-Si phase, ΣT i For example, by controlling the Si content in the plating layer to 1.0% or less, the ΣT i / L0×100≦20 can be reliably satisfied. From the viewpoint of further enhancing the corrosion resistance improvement effect, ΣT i The lower the value of / L0×100, the more preferable, and may be, for example, 15 or less, 10 or less, 5 or less, 3 or less, 2 or less, or 1 or less. From the viewpoint of further improving corrosion resistance, the Fe—Al—Si phase 5 may be prevented from being present in the coating layer. That is, ΣT i The lower limit of / L0×100 may be 0. Although not particularly limited, for example, ΣT i The value of / L0×100 may be 0.1 or more, 0.2 or more, or 0.3 or more.

[0036] [Surface coverage of Mg-containing phase: 20-100%] According to another preferred embodiment of the present invention, the Mg content in the coating layer is 0.3 to 10.0%, and in this regard, the coating layer further contains a Mg-containing phase, and the surface coverage of the Mg-containing phase is controlled to 20 to 100% in the cross section of the coating layer. In the present invention, the Mg-containing phase refers to a phase having a chemical composition, in mass %, consisting of 0.5 to 90% Mg, 10 to 99.5% Al, 0 to 70% O, 0 to 3% Fe, and less than 3% other elements. As is clear from this chemical composition, the Mg-containing phase does not include the MgZn2 phase described below.

[0037] Fig. 3 is a cross-sectional schematic view of a plated steel sheet according to another preferred embodiment of the present invention, illustrating the surface coverage of the Mg-containing phase. Referring to Fig. 3, the plated steel sheet 1, like the case of Figs. 1 and 2, comprises a base steel sheet 2 and a plated layer 3 formed on the surface of the base steel sheet 2, and the plated layer 3 contains an Fe-Al phase 4. In Fig. 3, the plated layer 3 further contains an Mg-containing phase 6 in its surface portion. Here, the length M of each Mg-containing phase 6 is i Total of ΣM i (In Figure 3, ΣM i=M1+M2+M3), and the surface length L0 of the base steel plate 2 is ΣM i It can be seen that the relationship / L0×100≧20 is satisfied, i.e., the surface coverage of the Mg-containing phase is 20% or more. By controlling the surface coverage of the Mg-containing phase to 20% or more and allowing a relatively large amount of Mg to be present on the surface of the coating layer, the reaction can be accelerated by the action of Mg during chemical conversion treatment, thereby improving the adhesion of the chemical conversion coating to the coated steel sheet. From the viewpoint of further enhancing the effect of improving chemical conversion treatability, a higher surface coverage of the Mg-containing phase is preferable, and may be, for example, 30% or more, 40% or more, 50% or more, 60% or more, 70% or more, or 80% or more. There is no particular upper limit, and the surface coverage of the Mg-containing phase may be 100%. For example, the surface coverage of the Mg-containing phase may be 95% or less or 90% or less. To increase the surface coverage of the Mg-containing phase, it is preferable to increase the Mg content in the coating layer. More specifically, the Mg content in the coating layer is preferably 0.3% or more, and more preferably 0.6% or more. However, since the surface coverage of the Mg-containing phase does not depend solely on the Mg content, the Mg content in the plating layer may be appropriately determined according to the desired surface coverage, taking into consideration the manufacturing conditions, etc.

[0038] [Area ratio of MgZn2 phase: less than 10%] In an embodiment of the present invention, in relation to the upper limit of the Zn content in the coating layer being 30.0%, the MgZn2 phase may be formed in the coating layer in an area percentage range of less than 10%. The MgZn2 phase may or may not be present in the coating layer. When the MgZn2 phase is present in the coating layer, it may contribute to improving sacrificial corrosion protection. The area percentage of the MgZn2 phase may be, for example, 9% or less, 7% or less, 5% or less, or 3% or less. Similarly, the area percentage of the MgZn2 phase may be 0%, or may be, for example, 1% or more, or 2% or more.

[0039] [Plating layer analysis] The plating layer is analyzed as follows. First, a 15 mm x 20 mm sample is taken from the surface of the plated steel sheet so that the cross section of the plating layer can be observed. The sample is then embedded in resin and polished. Next, a backscattered electron image (BSE image) is obtained from the resulting mirror-polished sample using a scanning electron microscope with an electron probe microanalyzer (SEM-EPMA) within a field of view of 80 μm in the thickness direction and 100 μm perpendicular to the thickness direction, and the plating layer is identified from the BSE image. Next, the composition of each phase in the identified plating layer is analyzed by point analysis. From the obtained composition, the following phases were identified: Fe-Al phase (Fe: 40-70%, Al: 30-60%, Zn: 0-20%, and other elements: less than 3%), Fe-Al-Si phase (Fe: 30-70%, Al: 30-60%, Si: 3-20%, and other elements: less than 3%), Mg-containing phase (Mg: 0.5-90%, Al: 10-99.5%, O: 0-70%, Fe: 0-3%, and other elements: less than 3%), and MgZn2 phase. The specific measurement conditions for the above field of view using EPMA are as follows: Equipment: JEOL Ltd. JXA-8500 Accelerating voltage: 15 kV Irradiation current: 5×10 -7 A Irradiation time: 50ms

[0040] (L-L0) / L0×100 is determined as follows. First, the mirror-polished sample obtained above is observed using an SEM in a field of view of 80 μm in the thickness direction and 100 μm perpendicular to the thickness direction to obtain a BSE image. The BSE image is then measured using the "Analyze" function of the image analysis software "ImageJ" to measure the interface length between the coating layer and the base steel sheet (interface length L between the coating layer and the base steel sheet shown in Figure 1). The above procedure is performed for five fields of view, and the average value is calculated to determine the interface length L. Next, (L-L0) / L0×100 is determined from the obtained interface length L and the corresponding surface length L0 of the base steel sheet, i.e., the length of the long side of the observation field: 100 μm. The resolution of the SEM image is 2560 × 1920. To measure L0, use the "Find edge" function in the "Process" section of the image analysis software "ImageJ," then binarize it with the "Binary" function, and then use the "Measure" function in "Analyze" to read the "Perim."

[0041] The thickness of the Fe-Al phase is determined as follows: First, the thickness of the Fe-Al phase identified above is measured at five different points in the field of view using the "Analyze" function of the image analysis software "ImageJ," and then the thickness of the Fe-Al phase is determined by averaging the thicknesses measured at the five points.

[0042] ΣT i The projection ratio of the Fe-Al-Si phases is determined as follows: First, the Fe-Al-Si phases identified above are projected onto the surface of the base steel sheet using the image analysis software "ImageJ." The projection length T i The sum of ΣT i (In Figure 2, T1 + T2) is calculated. Specifically, using the toolbar "Straight" in ImageJ, draw a line in the horizontal direction of each Fe-Al-Si phase, and read the value displayed in "Length" on the toolbar to calculate T i Then, the calculated ΣT i and the corresponding surface length L0 of the base steel plate (length of the long side of the observation field: 100 μm), ΣT iDetermine / L0 × 100 (the projection ratio of the Fe-Al-Si phase).

[0043] The surface coverage of the Mg-containing phase is determined as follows: First, the length M of the Mg-containing phase present on the surface of the coating layer among the Mg-containing phases identified above is determined. i Total of ΣM i (In Figure 3, ΣM i M = M1 + M2 + M3) is calculated using the "Analyze" function of the image analysis software "ImageJ." Specifically, when drawing a horizontal line at both ends of each Mg-containing phase using the toolbar "Straight" in ImageJ, M is calculated by reading the value displayed in "Length" on the toolbar. i Then, the calculated ΣM i and the corresponding surface length L0 of the base steel plate (length of the long side of the observation field: 100 μm), ΣM i / L0 × 100 (surface coverage of the Mg-containing phase) is determined.

[0044] The area ratio of the MgZn2 phase is measured from the element distribution image of the mapping image obtained for the above sample using the image analysis software "ImageJ." Specifically, the area containing 25-45 at% Mg and 50-75 at% Zn (Mg + Zn: 90-100 at%) in the element mapping is binarized using the "Binary" function of "ImageJ," and the area ratio is measured using the "Analyze" function.

[0045] The plating layer may be any plating layer having the above-mentioned chemical composition, Fe—Al phase, Fe—Al—Si phase, Mg-containing phase, and / or MgZn2 phase, and is not particularly limited, but may be, for example, an alloyed hot-dip plating layer.

[0046] [Preferred chemical composition of base steel plate] As described above, the present invention aims to provide a plated steel sheet having an Al-containing coating layer and exhibiting improved corrosion resistance and cold workability after painting. This objective is achieved by optimizing the chemical composition of the coating layer, controlling the thickness of the Fe-Al phase contained in the coating layer to within a range of 4 to 50 μm, and controlling the interface shape between the coating layer and the base steel sheet to satisfy the relationship (L-L0) / L0×100≧3. Therefore, it is clear that the chemical composition of the base steel sheet itself is not an essential technical feature for achieving the objective of the present invention. Preferred chemical compositions of the base steel sheet used in the plated steel sheets according to embodiments of the present invention will be described in detail below. However, these descriptions are intended merely as examples of preferred chemical compositions of the base steel sheet and are not intended to limit the present invention to those using base steel sheets having such specific chemical compositions.

[0047] In an embodiment of the present invention, for example, the base steel plate contains, in mass%, C: 0.01 to 0.50%, Si: 0.001 to 3.000%, Mn: 0.10 to 3.00%, Al: 0.0002 to 2.000%, P: 0.100% or less, S: 0.1000% or less, N: 0.0100% or less, Nb: 0 to 0.15%, Ti: 0 to 0.15% V: 0~0.15%, Mo: 0-1.0% Cr: 0 to 1.0%, Cu: 0-1.0% Ni: 0 to 1.0% B: 0~0.0100%, W: 0 to 1.000%, Hf: 0 to 0.050%, Mg: 0 to 0.050% Zr: 0 to 0.050%, Ca: 0 to 0.010% REM: 0~0.30%, Ir: 0 to 1.000%, and Remainder: Fe and impurities It is preferable that the metal has a chemical composition consisting of the following: Each element will be described in more detail below.

[0048] [C: 0.01 to 0.50%] C is an element that inexpensively increases tensile strength and is an important element for controlling the strength of steel. To fully obtain this effect, the C content is preferably 0.01% or more. The C content may be 0.05% or more, 0.10% or more, 0.15% or more, 0.20% or more, 0.30% or more, or 0.35% or more. On the other hand, excessive C content may result in a decrease in elongation. For this reason, the C content is preferably 0.50% or less. The C content may be 0.45% or less or 0.40% or less.

[0049] [Si: 0.001 to 3.000%] Si acts as a deoxidizer and is an element that suppresses the precipitation of carbides during the cooling process during annealing of cold-rolled sheets. To fully obtain this effect, the Si content is preferably 0.001% or more. The Si content may be 0.010% or more, 0.100% or more, or 0.200% or more. On the other hand, excessive Si content may increase the steel strength but decrease the elongation. For this reason, the Si content is preferably 3.000% or less. The Si content may be 2.500% or less, 2.000% or less, 1.500% or less, or 1.000% or less.

[0050] [Mn: 0.10~3.00%] Mn is an element that improves the hardenability of steel and is effective in increasing strength. To fully obtain this effect, the Mn content is preferably 0.10% or more. The Mn content may be 0.30% or more, 0.50% or more, 1.00% or more, or 1.30% or more. On the other hand, excessive Mn content may increase the steel strength but decrease the elongation. For this reason, the Mn content is preferably 3.00% or less. The Mn content may be 2.80% or less, 2.50% or less, or 2.00% or less.

[0051] [Al: 0.0002~2.000%] Al acts as a deoxidizer for steel and has the effect of improving the soundness of steel. To fully obtain this effect, the Al content is preferably 0.0002% or more. The Al content may be 0.001% or more, 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, excessive Al content may generate coarse Al oxides, reducing the elongation of the steel sheet. For this reason, the Al content is preferably 2.000% or less. The Al content may be 1.500% or less, 1.000% or less, 0.800% or less, or 0.500% or less.

[0052] [P:0.100% or less] P is an element that segregates at grain boundaries and promotes embrittlement of steel. The lower the P content, the better, and ideally it is 0%. However, excessive reduction in the P content may result in a significant increase in costs. For this reason, the P content may be 0.0001% or more, or may be 0.001% or more, or 0.005% or more. On the other hand, excessive P content may result in embrittlement of steel due to grain boundary segregation, as described above. Therefore, the P content is preferably 0.100% or less. The P content may also be 0.050% or less, 0.030% or less, or 0.010% or less.

[0053] [S:0.1000% or less] S is an element that generates nonmetallic inclusions such as MnS in steel, reducing the ductility of steel parts. Since a lower S content is preferable, ideally 0%. However, excessive reduction in the S content can result in a significant increase in costs. For this reason, the S content may be 0.0001% or more, or may be 0.0002% or more, 0.0010% or more, or 0.0050% or more. On the other hand, excessive S content can cause cracks to occur originating from nonmetallic inclusions during cold forming. Therefore, the S content is preferably 0.1000% or less. The S content may be 0.0500% or less, 0.0200% or less, or 0.0100% or less.

[0054] [N:0.0100% or less] N is an element that forms coarse nitrides in steel sheets and reduces the workability of the steel sheets. Since a lower N content is preferable, ideally it is 0%. However, excessive reduction in the N content may result in a significant increase in manufacturing costs. For this reason, the N content may be 0.0001% or more, or may be 0.0005% or more, or 0.0010% or more. On the other hand, excessive N content may form coarse nitrides as described above, reducing the workability of the steel sheets. Therefore, the N content is preferably 0.0100% or less. The N content may also be 0.0080% or less, or 0.0050% or less.

[0055] The base steel sheet preferably has the basic chemical composition described above. Furthermore, the base steel sheet may contain, as necessary, one or more elements selected from the group consisting of Nb: 0-0.15%, Ti: 0-0.15%, V: 0-0.15%, Mo: 0-1.0%, Cr: 0-1.0%, Cu: 0-1.0%, Ni: 0-1.0%, B: 0-0.0100%, W: 0-1.000%, Hf: 0-0.050%, Mg: 0-0.050%, Zr: 0-0.050%, Ca: 0-0.010%, REM: 0-0.30%, and Ir: 0-1.000%, in place of a portion of the remaining Fe. The content of each of these elements may be 0.0001% or more, 0.0005% or more, 0.001% or more, or 0.01% or more.

[0056] The remainder of the base steel plate other than the above elements is composed of Fe and impurities. The impurities in the base steel plate are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when the base steel plate is industrially manufactured.

[0057] The chemical composition of the base steel sheet can be measured using a common analytical method. For example, the chemical composition of the base steel sheet can be measured by first removing the coating layer by mechanical grinding, and then measuring the chips using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) in accordance with JIS G 1201:2014. Specifically, for example, a 35 mm square test piece can be obtained from the base steel sheet at approximately half the thickness position, and the components can be determined by measuring them using a Shimadzu ICPS-8100 or similar measuring device under conditions based on a pre-established calibration curve. C and S, which cannot be measured by ICP-AES, can be measured using the combustion-infrared absorption method, N using the inert gas fusion-thermal conductivity method, and O using the inert gas fusion-non-dispersive infrared absorption method.

[0058] [Base steel plate thickness] The thickness of the base steel plate is not particularly limited, and may be, for example, 0.2 mm or more, 0.3 mm or more, 0.6 mm or more, 1.0 mm or more, or 2.0 mm or more. Similarly, the thickness of the base steel plate may be, for example, 6.0 mm or less, 5.0 mm or less, or 4.0 mm or less.

[0059] <Method of manufacturing plated steel sheets> Next, a preferred method for producing a plated steel sheet according to an embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for producing a plated steel sheet according to an embodiment of the present invention, but is not intended to limit the plated steel sheet to one produced by the production method described below.

[0060] The plated steel sheet according to an embodiment of the present invention can be manufactured by, for example, carrying out a casting process in which molten steel having an adjusted chemical composition is cast to form a steel billet, a hot rolling process in which the steel billet 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, a pretreatment process, an annealing process in which the pretreated cold-rolled steel sheet is annealed, a cooling process in which the annealed cold-rolled steel sheet is cooled, and a plating process in which a plating layer is formed on the obtained base steel sheet. Alternatively, the base steel sheet may be pickled after the hot rolling process without being coiled, and then directly subjected to the cold-rolling process. Each process will be described in detail below.

[0061] [Casting process] The conditions for the casting process are not particularly limited. For example, after melting in a blast furnace or electric furnace, various secondary smelting processes may be carried out, and then casting may be carried out by a conventional method such as continuous casting or ingot casting.

[0062] [Hot rolling process] A hot-rolled steel sheet can be obtained by hot-rolling the cast steel slab. The hot-rolling step is carried out by reheating the cast steel slab directly or after cooling it once, and then hot-rolling it. When reheating is carried out, the heating temperature of the steel slab may be, for example, 1100 to 1250°C. In the hot-rolling step, rough rolling and finish rolling are usually carried out. The temperature and reduction ratio of each rolling step can be appropriately determined depending on the desired metal structure and plate thickness. For example, the end temperature of finish rolling may be 900 to 1050°C, and the reduction ratio of finish rolling may be 10 to 50%.

[0063] [Winding process] The hot-rolled steel sheet can be coiled at a predetermined temperature. The coiling temperature can be appropriately determined depending on the desired metal structure, etc., and may be, for example, 500 to 800°C. The hot-rolled steel sheet may be recoiled before or after coiling and subjected to a predetermined heat treatment. Alternatively, the hot-rolled steel sheet may be pickled after the hot rolling step and then subjected to the cold rolling step described below without performing the coiling step.

[0064] [Cold rolling process] After subjecting the hot-rolled steel sheet to pickling or the like, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The reduction ratio in cold rolling can be appropriately determined depending on the desired metal structure and sheet thickness, and may be, for example, 20 to 80%. After the cold-rolling step, the sheet may be air-cooled to room temperature, for example.

[0065] [Pretreatment process] Next, a predetermined pretreatment process may be carried out before annealing the cold-rolled steel sheet. Such a pretreatment process may include a degreasing process. The degreasing process may include, for example, passing an electric current through the cold-rolled steel sheet in a solution having a pH of 8.0 or higher (electrolytic treatment). The current density during the current passing is 1.0 to 8.0 A / dm 2 The current application time may be 5 to 10 seconds.

[0066] [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 780 to 900°C in an atmosphere with a dew point of -10 to 10°C and holding the temperature for 10 to 300 seconds. By performing the annealing process under these conditions, it is possible to adequately decarburize the surface layer of the cold-rolled steel sheet. In this case, the reaction between the coating layer and the base steel sheet is promoted during the alloying treatment in the subsequent coating process, that is, the alloying rate can be increased. As a result, it is possible to achieve an interface shape with greater irregularities such that the interface length L between the coating layer and the base steel sheet and the corresponding surface length L0 of the base steel sheet satisfy the relationship (L - L0) / L0 × 100 ≥ 3, thereby significantly improving the cold workability of the coated steel sheet.

[0067] If the dew point is lower than -10°C, the annealing temperature is lower than 780°C, and / or the annealing time is shorter than 10 seconds, the decarburization of the surface layer of the cold-rolled steel sheet is insufficient, making it impossible to obtain a sufficient alloying rate during the alloying treatment of the coating layer. As a result, it becomes impossible to achieve an interface shape that satisfies the relationship (L-L0) / L0×100≧3 between the coating layer and the base steel sheet. On the other hand, if the dew point is higher than 10°C, the heating temperature is higher than 900°C, and / or the annealing time is longer than 300 seconds, an outer oxide layer may form on the surface of the base steel sheet, resulting in reduced galvanic properties, or excessive decarburization may result in reduced strength of the finally obtained coated steel sheet. The atmosphere in the annealing step may be a reducing atmosphere, more specifically, a reducing atmosphere containing nitrogen and hydrogen, for example, a reducing atmosphere of 1 to 10% hydrogen (e.g., 3% hydrogen and the balance nitrogen).

[0068] [Cooling process] The cold-rolled steel sheet whose surface layer has been decarburized in the annealing step needs to be appropriately cooled in the subsequent cooling step to obtain a desired surface layer structure. Specifically, the cooling step involves cooling from the heating temperature (annealing temperature) in the annealing step to a controlled temperature of 500 to 750°C at an average cooling rate of 5°C / s or more. This will be explained in detail below.

[0069] Typically, the annealed cold-rolled steel sheet is then cooled to a temperature below 500°C, for example, to a temperature of approximately 200°C, and then reheated and subjected to a plating process. However, this temperature history results in the transformation of the austenitized metal structure into a structure such as bainite or martensite in the annealing process. Therefore, in the subsequent plating process, the metal structure such as bainite or martensite is alloyed with the plating layer. However, because the alloying rate between these metal structures and the plating layer is relatively slow, the final plated steel sheet cannot achieve an interface shape that satisfies the relationship (L-L0) / L0×100≧3 between the plating layer and the base steel sheet. Therefore, in the cooling process of the present manufacturing method, it is extremely important to immerse the metal structure of the cold-rolled steel sheet, whose surface layer has been decarburized in the annealing process, in a plating bath while still containing a large amount of austenite phase, thereby directly alloying the austenite phase with the plating layer. In this regard, in this cooling step, by cooling from the annealing temperature to a controlled temperature of 500 to 750°C at an average cooling rate of 5°C / s or more, the metal structure of the cold-rolled steel sheet can be maintained in a state containing a large amount of austenite phase. As a result, in the subsequent plating step, it is possible to achieve an alloying rate sufficient to directly alloy the austenite phase with the plating layer and realize a desired interface shape.

[0070] While not intending to be bound by any particular theory, it is believed that the combination of decarburization and austenite phase increases the alloying rate, resulting in unevenness in the alloying rate between areas with and without austenite grain boundaries. This unevenness in the alloying rate is responsible for the formation of irregularities at the interface between the coating layer and the base steel sheet. If the controlled temperature is less than 500°C, the austenite phase transforms into bainite or martensite, making it impossible to achieve a sufficient alloying rate in the subsequent coating process. Furthermore, if the average cooling rate from the annealing temperature to the controlled temperature of 500 to 750°C is less than 5°C / s, the transformation to ferrite becomes significant, making it impossible to achieve a sufficient alloying rate in the subsequent coating process. As a result, in either case, it becomes impossible to achieve an interface shape that satisfies the relationship (L-L0) / L0 × 100 ≥ 3 between the coating layer and the base steel sheet. On the other hand, if the controlled temperature exceeds 750°C, the temperature becomes higher than is suitable for the subsequent coating process, and the desired coating layer may not be obtained. From the viewpoint of realizing an interface shape with greater irregularities, the higher the average cooling rate from the annealing temperature to the control temperature of 500 to 750°C, the more preferable, and for example, 15°C / s or more is preferable. Although the upper limit is not necessarily limited, the average cooling rate is preferably, for example, 30°C / s or less.

[0071] [Plating process] Next, in the galvanizing process, a coating layer is formed on at least one, preferably both, surfaces of the cold-rolled steel sheet (base steel sheet). More specifically, the galvanizing process involves immersing the cold-rolled steel sheet, cooled to the above-mentioned controlled temperature, in a coating bath (coating bath temperature: e.g., 680 to 750°C) having a predetermined chemical composition while maintaining a state in which the austenite phase is abundant, and then heat-treating the cold-rolled steel sheet at an alloying temperature of 680 to 750°C for 0.5 to 20 seconds. By performing the alloying process under these conditions, the coating layer is appropriately alloyed so that the Fe-Al phase has a thickness of 4 μm or more, and a sufficient alloying rate can be achieved based on the combination of decarburization and austenite phase. As a result, an interface shape with large irregularities can be achieved, in which the interface length L between the coating layer and the base steel sheet and the corresponding surface length L0 of the base steel sheet satisfy the relationship (L - L0) / L0 × 100 ≥ 3, thereby significantly improving the cold workability of the coated steel sheet.

[0072] If the alloying temperature is lower than 680°C, the coating layer solidifies without sufficient alloying, resulting in a low Fe content in the coating layer and / or making it impossible to obtain the desired Fe-Al phase thickness. As a result, the corrosion resistance of the coated steel sheet after painting is reduced. Furthermore, if the alloying treatment time is shorter than 0.5 seconds, the coating layer is insufficiently alloyed, making it impossible to create an uneven shape at the interface between the coating layer and the base steel sheet and / or making it impossible to obtain the desired Fe-Al phase thickness. As a result, the cold workability and / or corrosion resistance of the coated steel sheet after painting are reduced. On the other hand, if the alloying temperature is higher than 750°C or the alloying treatment time is longer than 20 seconds, excessive alloying of the coating layer occurs, resulting in a flatter interface with fewer unevenness. This may cause the final coated steel sheet to fail to satisfy the relationship (L-L0) / L0×100≧3. In this case, the cold workability of the coated steel sheet is reduced. From the viewpoint of ensuring the desired alloying, the alloying treatment time is preferably set to 5 to 20 seconds.

[0073] The plating step is carried out by, for example, hot-dip plating. The plating step is not limited to hot-dip plating, and may be electroplating, vapor deposition plating, thermal spraying, cold spraying, or the like. Other conditions for the plating step may be appropriately set taking into consideration the thickness and coating weight of the coating layer. For example, a cold-rolled steel sheet is immersed in a coating bath, then pulled out, and immediately sprayed with N2 gas or air by gas wiping, followed by cooling. This allows the coating weight of the coating layer to be adjusted within a predetermined range, for example, such that the thickness of the Fe-Al phase is 4 to 50 μm.

[0074] [Cooling after plating] Finally, the base steel sheet with the coating layer attached thereto is cooled to obtain a coated steel sheet according to an embodiment of the present invention. The cooling after coating is not particularly limited and can be carried out under any appropriate conditions known to those skilled in the art. For example, the cooling after coating can be carried out at an average cooling rate of 10°C / s or more. The cooling stop temperature is also not particularly limited and may be set appropriately within the range of 100 to 350°C, for example.

[0075] According to this manufacturing method, it is possible to produce a plated steel sheet having a coating layer in which the chemical composition of the coating layer is optimized within a predetermined range, i.e., by mass: 20.0-55.0% Fe, 0-10.0% Mg, 0-10.0% Si, and 20.0% or more Al, the thickness of the Fe-Al phase contained in the coating layer is controlled within a range of 4-50 μm, and the interface shape between the coating layer and the base steel sheet is controlled to satisfy the relationship (L-L0) / L0×100≧3. Therefore, due to the appropriately alloyed Fe-Al phase in the coating layer, sufficient corrosion resistance after painting can be ensured and cold workability can be improved. In addition, by controlling the interface shape between the coating layer and the base steel sheet to have a shape with greater irregularities, even when subjected to cold working such as bending, the hard coating layer can penetrate into the base steel sheet from the irregularities at the interface, deforming the base steel sheet as the cold working progresses. As a result, it is possible to significantly suppress the occurrence of powdering due to bending or other processes, thereby significantly improving the cold workability of the plated steel sheet. In addition, by appropriately controlling the Si content in the coating layer, the Fe-Al-Si phase can be dispersed within the Fe-Al phase, thereby further improving the corrosion resistance of the plated steel sheet after painting. Furthermore, by appropriately controlling the Mg content, mainly in the coating layer, it is possible to increase the surface coverage of the coating layer by the Mg-containing phase, thereby significantly improving the chemical treatability of the plated steel sheet. Therefore, such plated steel sheet can achieve superior corrosion resistance and cold workability after painting compared to conventional plated steel sheets. This can contribute to industrial development by improving productivity in the use of plated steel sheets for automobiles and building materials.

[0076] The present invention will be described in more detail below with reference to examples, but the following examples are merely illustrative of the present invention and are not intended to limit the present invention in any way. It goes without saying that the present invention can be modified as desired without departing from the gist of the present invention. [Example]

[0077] In the following examples, plated steel sheets according to the embodiments of the present invention were produced under various conditions, and the properties of the produced plated steel sheets were investigated.

[0078] First, molten steel was cast by continuous casting to form a slab having a chemical composition, by mass, of 0.20% C, 0.012% Si, 1.30% Mn, 0.030% Al, 0.005% P, 0.0020% S, and 0.0030% N, with the balance consisting of Fe and impurities. The slab was cooled, reheated to 1200°C, hot-rolled, and then coiled at a temperature of 600°C or less. Hot rolling was performed by rough rolling and finish rolling, with the finish rolling ending at a temperature of 900 to 1050°C and a reduction ratio of 30%. Next, the obtained hot-rolled steel sheet was pickled and then cold-rolled at a reduction ratio of 50% to obtain a cold-rolled steel sheet having a thickness of 0.8 mm. Next, the obtained cold-rolled steel sheet was subjected to a 5.0 A / dm 2 A pretreatment (degreasing treatment) was performed by passing a current through the specimen for 8 seconds at a current density of 1000 kJ / cm2.

[0079] Next, each cold-rolled steel sheet was cut into a size of 100 mm × 200 mm and then subjected to annealing treatment under the conditions shown in Table 1 (annealing atmosphere: 3% hydrogen and balance nitrogen). Next, the cut steel sheet samples were cooled from the annealing temperature to the controlled temperature at the average cooling rate shown in Table 1, and then immersed in a hot-dip galvanizing bath having a predetermined bath composition (galvanizing bath temperature: 680 to 750°C) and subjected to alloying treatment under the conditions shown in Table 1. After immersion in the galvanizing bath, the steel sheet samples were pulled out and subjected to N2 gas wiping to adjust the coating weight. Finally, the base steel sheet with the coating layer attached was cooled at an average cooling rate of 10°C / s or more to obtain coated steel sheets in which coating layers were formed on both sides of the base steel sheet.

[0080] The physical properties and characteristics of the resulting plated steel sheets were measured and evaluated by the following methods.

[0081] [Chemical composition analysis of plating layer] The chemical composition of the plating layer was determined by immersing a sample cut to 30 mm x 30 mm in a 10% HCl solution containing 0.04% Ivit 710K (manufactured by Asahi Chemical Industry Co., Ltd.) as an inhibitor, pickling the plating layer, and then measuring the plating components dissolved in the solution using ICP emission spectroscopy. The results are shown in Table 1.

[0082] [Evaluation of cold workability] The cold workability was evaluated as follows. First, a 100 x 50 mm x 0.8 mm sample of the plated steel sheet was subjected to a 90° bending test with an R of 2 mm, then ultrasonically cleaned, and the sample mass was measured. The difference between the sample mass before and after the 90° bending test was measured as the amount of powdering, and the cold workability, particularly the powdering resistance, was evaluated as follows. AAA: 6mg or less AA: More than 6~12mg A: More than 12~24mg B: More than 24mg

[0083] [Evaluation of corrosion resistance after painting] Corrosion resistance after painting was evaluated as follows. First, a 50 mm x 100 mm plated steel sheet sample was treated with zinc phosphate (SD5350 system: Nippon Paint Industrial Coating standard). Then, electrocoating (PN110 Powernics Gray: Nippon Paint Industrial Coating standard) was applied to a thickness of 20 μm and baked at 150°C for 20 minutes. Next, a cut was made in the center of the sample, reaching the steel substrate (base steel sheet). Next, a cyclic corrosion test according to JASO (M609-91) was performed for 180 cycles, and the width of the paint blister was measured. Corrosion resistance after painting was evaluated as follows. AAA: 2mm or less AA: over 2~3mm A: More than 3~4mm B: More than 4mm

[0084] [Evaluation of chemical conversion treatment properties] Chemical conversion treatability was evaluated as follows. First, a 50mm x 100mm sample of plated steel sheet was treated with zinc phosphate (SD5350 system: Nippon Paint Industrial Coating standard) to form a chemical conversion coating. Next, the sample surface was observed using secondary electron images from an SEM, and the area ratio of the area where the chemical conversion coating was not formed, commonly known as "clear," was measured. The chemical conversion treatability of the plated steel sheet was evaluated according to the area ratio of the clear coating using the following evaluation criteria. AA: Clear area rate 0-5% A: Clear area ratio: over 5% to 15% B: Over 15% of the surface area

[0085] Plated steel sheets that were rated AAA, AA, or A for cold workability and AAA, AA, or A for corrosion resistance after painting were evaluated as having improved corrosion resistance and cold workability after painting. The results are shown in Table 1.

[0086] [Table 1-1]

[0087] [Table 1-2]

[0088] Referring to Table 1, in Comparative Examples 30 and 31, the Mg and Si contents in the coating layers were high, which is considered to have prevented a sufficient alloying rate from being achieved during the alloying treatment of the coating layer. As a result, the value of (L-L0) / L0×100 at the interface with the base steel sheet was less than 3, i.e., the interface with the base steel sheet had a flatter shape with fewer irregularities, resulting in poor cold workability. In Comparative Example 32, the annealing temperature was low, which is considered to have prevented insufficient decarburization of the surface layer of the cold-rolled steel sheet, which is considered to have prevented a sufficient alloying rate from being achieved during the alloying treatment of the coating layer. As a result, the value of (L-L0) / L0×100 was less than 3, resulting in poor cold workability. In Comparative Example 33, the annealing time was short, which is considered to have similarly prevented insufficient decarburization of the surface layer of the cold-rolled steel sheet, which is considered to have prevented a sufficient alloying rate from being achieved during the alloying treatment of the coating layer. As a result, the value of (L-L0) / L0×100 was less than 3, resulting in poor cold workability. In Comparative Example 34, the dew point in the annealing process was low, which similarly resulted in insufficient decarburization of the surface layer of the cold-rolled steel sheet, and it is believed that a sufficient alloying rate could not be obtained during the alloying treatment of the coating layer. As a result, the value of (L-L0) / L0×100 was less than 3, and cold workability was reduced. In Comparative Example 35, the average cooling rate from the annealing temperature to the control temperature of 500 to 750°C was slow, which resulted in significant transformation from austenite to ferrite in the metal structure of the cold-rolled steel sheet, and it is believed that a sufficient alloying rate could not be obtained in the subsequent coating process. As a result, the value of (L-L0) / L0×100 was less than 3, and cold workability was reduced.

[0089] In Comparative Examples 36 and 37, the controlled temperature in the annealing process was low, which resulted in significant transformation from austenite to bainite or martensite in the metal structure of the cold-rolled steel sheet, and it is believed that this prevented a sufficient alloying rate from being achieved in the subsequent plating process. As a result, the value of (L-L0) / L0×100 was less than 3, resulting in poor cold workability. In Comparative Example 38, the alloying temperature of the coating layer was low, which resulted in the coating layer solidifying in an insufficient alloying state. As a result, the Fe content in the coating layer decreased, and the desired Fe-Al phase thickness could not be achieved, resulting in poor corrosion resistance after painting. In Comparative Example 39, the alloying temperature of the coating layer was high, which resulted in excessive alloying of the coating layer. As a result, the value of (L-L0) / L0×100 was less than 3, which meant that the interface with the base steel sheet was flatter with fewer irregularities, resulting in poor cold workability. In Comparative Example 40, the alloying treatment time of the coating layer was too short, resulting in insufficient alloying of the coating layer, and the interface between the coating layer and the base steel sheet could not be formed into an uneven shape. In other words, the value of (L-L0) / L0×100 was less than 3, and the desired Fe-Al phase thickness could not be obtained. As a result, the cold workability and corrosion resistance after painting were deteriorated. In Comparative Example 41, the alloying treatment time of the coating layer was too long, which is thought to have caused excessive alloying of the coating layer. As a result, the value of (L-L0) / L0×100 was less than 3, resulting in deterioration of cold workability. In Comparative Example 42, the thickness of the Fe-Al phase was too thick, resulting in excessive hardening of the coating layer and therefore deterioration of the cold workability of the coated steel sheet.

[0090] In contrast, in all of the plated steel sheets according to the present invention, the chemical composition of the coating layer was optimized within the specified ranges, i.e., by mass: 20.0-55.0% Fe, 0-10.0% Mg, 0-10.0% Si, and 20.0% or more Al. The thickness of the Fe-Al phase contained in the coating layer was controlled within a range of 4-50 μm. Furthermore, the interface shape between the coating layer and the base steel sheet was controlled to satisfy the relationship (L-L0) / L0 × 100 ≥ 3. This significantly improved both the post-painting corrosion resistance and cold workability of the resulting plated steel sheets. In particular, in Examples 8 to 11, in which the (L-L0) / L0 value was controlled to 5 or more, the cold workability was evaluated as AA, demonstrating further improvement in cold workability. Similarly, in Examples 12 to 29, in which the (L-L0) / L0 value was controlled to 7 or more, the cold workability was evaluated as AAA, demonstrating further improvement in cold workability. In addition, the thickness of the Fe-Al phase is set to 12 μm or more, and ΣT i In Examples 14 to 29, in which the ratio of SiO2 to SiO2 was controlled to 1 or less (i.e., the projection ratio of the Fe-Al-Si phase was 1% or less), the corrosion resistance after painting was evaluated as AAA, and very high corrosion resistance after painting was achieved. In addition, in Examples 8 to 11, in which the surface coverage of the Mg-containing phase was controlled to 20% or more, the chemical conversion treatability was evaluated as A, and similarly, in Examples 12 to 24 and 26 to 29, in which the surface coverage of the Mg-containing phase was controlled to 60% or more, the chemical conversion treatability was evaluated as AA, and very high chemical conversion treatability was achieved. [Explanation of symbols]

[0091] 1. Plated steel sheet 2 Base steel plate 3 plating layer 4. Fe-Al phase 5 Fe-Al-Si phase 6 Mg-containing phase L: Interface length between the coating layer and the base steel sheet L0: Length of the surface of the base steel plate

Claims

1. A steel plate having a base steel sheet and a plating layer formed on a surface of the base steel sheet, The plating layer comprises, in mass %, Fe: 20.0 to 55.0%, Mg: 0-10.0%, Si: 0 to 10.0%, Zn: 0-30.0% and further comprising Ni: 0-1.000%, Ca: 0-4.000%, Sb: 0 to 0.500%, Pb: 0 to 0.500%, Cu: 0 to 1.000%, Sn: 0-1.000%, Ti: 0 to 1.000%, Cr: 0-1.000%, Nb: 0 to 1.000%, Zr: 0 to 1.000%, Mn: 0 to 1.000%, Mo: 0-1.000%, Ag: 0-1.000%, Li: 0 to 1.000%, La: 0 to 0.500%, Ce: 0-0.500%, B: 0 to 0.500%, Y: 0 to 0.500%, Sr: 0-0.500%, In: 0 to 0.500%, Co: 0 to 0.500%, Bi: 0-0.500%, P: 0 to 0.500%, W: 0 to 0.500%, and V: 0~0.500% Contains at least one of the following in a total amount of 5.000% or less, The balance has a chemical composition consisting of 20.0% or more Al and impurities, In the cross section of the coating layer, the interface length L between the coating layer and the base steel sheet and the surface length L of the base steel sheet 0 is 20≧(LL 0 ) / L 0 × 100≧3 is satisfied, The plated steel sheet is characterized in that the plated layer contains an Fe—Al phase, and the thickness of the Fe—Al phase is 4 to 50 μm.

2. 20≧(LL 0 ) / L 0 The plated steel sheet according to claim 1, wherein x 100≧5.

3. 20≧(LL 0 ) / L 0 The plated steel sheet according to claim 2, wherein x 100≧7.

4. The plated steel sheet according to claim 1 or 2, wherein the Mg content in the plated layer is 0.2% or more.

5. The chemical composition is, in mass %, Mg: 0.3 to 10.0%, and The plated steel sheet according to claim 1 or 2, characterized by containing Si: 0 to 1.0%.

6. The plated steel sheet according to claim 1 or 2, wherein the thickness of the Fe—Al phase is 12 to 50 μm.

7. In the cross section of the plating layer, the projected length T of the Fe—Al—Si phase in the plating layer i and the length L of the surface of the base steel plate 0 is ΣT i / L 0 The plated steel sheet according to claim 1 or 2, wherein x 100≦20 is satisfied.

8. ΣT i / L 0 The plated steel sheet according to claim 7, wherein x 100≦1.

9. The chemical composition contains, in mass%, Mg: 0.3 to 10.0%; the plating layer further includes an Mg-containing phase, 3. The plated steel sheet according to claim 1, wherein the Mg-containing phase has a surface coverage of 20 to 100% in a cross section of the plated layer.

10. The plated steel sheet according to claim 9, wherein the Mg-containing phase has a surface coverage of 60 to 100%.

11. The plated steel sheet according to claim 1 or 2, wherein the Mg content in the plated layer is 2.4% or less.

12. The plated steel sheet according to claim 1 or 2, wherein the Si content in the plated layer is 0.2% or more.

13. In the plating layer, MgZn 2 The plated steel sheet according to claim 1 or 2, wherein the area ratio of the phase is less than 10%.

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