Plated steel sheet

Abstract

This plated steel sheet comprises a steel sheet and a plating layer disposed over a surface of the steel sheet. The chemical composition of the plating layer contains, in mass%, 10.0-30.0% Al, 3.0-15.0% Mg, 0.01-2.0% Fe, greater than 0 to 2.0% Si, and 0.05-2.0% Ca, as well as one or two elements selected from the group consisting of group A and group B, with the remainder being Zn and impurities. The number density of Ca-Zn phases with an equivalent circle diameter of 1 μm or greater exposed on the surface of the plating layer is 0-10 per 10,000 μm2 area, and the number density of Al-Si-Zn-Ca phases with an equivalent circle diameter of 1 μm or greater exposed on the surface of the plating layer is 1-50 per 10,000 μm2 area.

Description

Technical Field

[0001] The present invention relates to a plated steel sheet. This application claims priority based on Japanese Patent Application No. 2022-100352 filed in Japan on June 22, 2022, and incorporates its content herein by reference.

Background Art

[0002] A Zn-Al-Mg based hot-dip plated steel sheet having a hot-dip Zn plating layer containing Al and Mg has excellent corrosion resistance. Therefore, the Zn-Al-Mg based hot-dip plated steel sheet is widely used as a material for structural members that require corrosion resistance, such as building materials.

[0003] For example, Patent Document 1 describes a plated steel material having a steel material and a plating layer including a Zn-Al-Mg alloy layer disposed on the surface of the steel material, wherein the Zn-Al-Mg alloy layer has a Zn phase and contains a Mg-Sn intermetallic compound phase in the Zn phase, and the plating layer has a chemical composition consisting of, by mass%, more than 65.0% Zn, more than 5.0% and less than 25.0% Al, more than 3.0% and less than 12.5% Mg, 0.1% to 20.0% Sn, and impurities, and satisfying the following Formulas 1 to 5. Formula 1: Bi + In < Sn Formula 2: Y + La + Ce ≦ Ca Formula 3: Si < Sn Formula 4: O ≦ Cr + Ti + Ni + Co + V + Nb + Cu + Mn < 0.25 Formula 5: O ≦ Sr + Sb + Pb + B < 0.5

[0004] Patent Document 2 describes a plated steel sheet having a steel material and a plating layer disposed on the surface of the steel material and containing a Zn-Al-Mg alloy layer. In the cross section of the Zn-Al-Mg alloy layer, the area fraction of the MgZn2 phase is 45 to 75%, the total area fraction of the MgZn2 phase and the Al phase is 70% or more, and the area fraction of the Zn-Al-MgZn2 ternary eutectic structure is 0 to 5%. The plating layer consists of, by mass, more than 44.90% to less than 79.90% of Zn, more than 15% to less than 35% of Al, more than 5% to less than 20% of Mg, 0.1% to less than 3.0% of Ca, and impurities. When element group A is Y, La, and Ce, element group B is Cr, Ti, Ni, Co, V, Nb, Cu, and Mn, element group C is Sr, Sb, and Pb, and element group D is Sn, Bi, and In, the total content of the elements selected from element group A is 0% to 0.5%, the total content of Ca and the elements selected from element group A is less than 0.1% to 3.0%, the total content of the elements selected from element group B is 0% to 0.25%, the total content of the elements selected from element group C is 0% to 0.5%, and the total content of the elements selected from element group D is 0% to 20.00%. A plated steel sheet having such a chemical composition is described.

[0005] In recent years, for hot-dip plated steel sheets used in building materials applications such as roofs and wall materials, both the planar corrosion resistance of the plating layer itself and the adhesion of the coating film (coating adhesion) when the plating surface is coated are required. On the other hand, a technique for achieving both high-level planar corrosion resistance and coating adhesion has not been studied.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] The present invention has been made in view of the above circumstances, and an object thereof is to provide a plated steel sheet excellent in both planar corrosion resistance and coating adhesion.

Means for Solving the Problems

[0008] In order to solve the above problems, the present invention adopts the following configuration. [1] A steel sheet and a plating layer disposed on the surface of the steel sheet, wherein the chemical composition of the plating layer is, by mass%, Al: 10.0 to 30.0%, Mg: 3.0 to 15.0%, Fe: 0.01 to 2.0%, Si: more than 0 to 2.0%, Ca: 0.05 to 2.0% is contained, furthermore, one or two selected from the group consisting of the following Group A and Group B are contained, the balance consists of Zn and impurities, the number density of Ca-Zn phases having an equivalent circle diameter of 1 μm or more exposed on the surface of the plating layer is 10,000 μm 2 per area of 0 pcs and Among the chemical composition of the plating layer, Al, Mg and Si are Al: 15 to 25% by mass, Mg: 4.5 to 8% by mass, Si: 0.1 to 2% by mass, the number density of Al-Si-Zn-Ca phases having an equivalent circle diameter of 1 μm or more exposed on the surface of the plating layer is 1 to 50 per 10,000 μm 2 area. A plated steel sheet characterized by this. [Group A] Ni: 0 to 1.0% [Group B] Sb: 0 to 0.5%, Pb: 0 to 0.5%, Cu: 0 to 1.0%, Sn: 0 to 2.0%, Ti: 0 to 1.0%, Cr: 0 to 1.0%, Nb: 0 to 1.0%, Zr: 0 to 1.0%, Mn: 0 to 1.0%, Mo: 0 to 1.0%, Ag: 0 to 1.0%, Li: 0 to 1.0%, La: 0 to 0.5%, Ce: 0 to 0.5%, B: 0 to 0.5%, Y: 0 to 0.5%, P: 0 to 0.5% and Sr: 0 to 0.5%, Co: 0 to 0.5%, Bi: 0 to 0.5%, In: 0 to 0.5%, V: 0 to 0.5%, W: 0 to 0.5% in total of one or more kinds is 0 to 5% 2 before ​​The number density of Mg-Si-Zn-Al phases with a major axis of 2 μm or more exposed on the surface of the plating layer is 5 to 150 per 10,000 μm 2 per area, as described in [1] to The plated steel sheet described in 3 Among the chemical compositions of the plating layer, Sn is 0.05 to 0.5% by mass, and in the X-ray diffraction measurement of the plating layer, an Mg2Sn phase is detected in the plating layer, as described in [1] or [2] The plated steel sheet described in 4 The plating layer has a chemical composition containing the group A by mass, as described in [1]. The plated steel sheet described in 5 The plating layer has a chemical composition containing the group B by mass, as described in [1]. The plated steel sheet described in 6 The number density of Ca-Zn phases with an equivalent circle diameter of less than 1 μm exposed on the surface of the plating layer is 1 or more per 10,000 μm 2 per area, as described in [1]. The plated steel sheet described in

Advantages of the Invention

[0009] According to each of the above aspects of the present invention, a plated steel sheet excellent in both planar corrosion resistance and coating adhesion can be provided.

Brief Description of the Drawings

[0010]

Figure 1

Modes for Carrying Out the Invention

[0011] ​​​​When a Ca-Zn phase precipitates in the plating layer and is exposed on the surface of the plating layer, it may accelerate the corrosion of the structure or phase surrounding the Ca-Zn phase (especially the Ca-Zn phase with an equivalent circle diameter of 1 μm or more). Therefore, when a coating film is formed on the plating layer where the Ca-Zn phase is exposed, the coating adhesion may decrease due to the influence of the corrosion products generated around the Ca-Zn phase. The inventors of the present invention have found that in order to improve the coating adhesion, it is necessary to prevent the precipitation of Ca-Zn phases with an equivalent circle diameter of 1 μm or more that accelerate the corrosion of the surrounding metal structure or phase on the surface of the plating layer as much as possible.

[0012] In order not to precipitate a Ca-Zn phase with an equivalent circle diameter of 1 μm or more, the Ca content of the plating layer can be reduced. On the other hand, if the plating layer contains Ca, an improvement in flat corrosion resistance is expected. Therefore, it is expected that both the flat corrosion resistance and the coating adhesion can be improved if the precipitation of Ca-Zn phases with an equivalent circle diameter of 1 μm or more can be suppressed on the surface of the plating layer while containing Ca.

[0013] Therefore, the inventors of the present invention intensively studied to improve both the flat corrosion resistance and the coating adhesion of the plating layer containing Al, Mg, Ca, and Zn, and found that it is effective to form a large amount of the Ai-Si-Zn-Ca phase in order to suppress the precipitation of the Ca-Zn phase with an equivalent circle diameter of 1 μm or more. It is speculated that when a large amount of the Ai-Si-Zn-Ca phase is formed, the Ca contained in the plating layer is consumed during the formation of this phase, and the precipitation of the Ca-Zn phase with an equivalent circle diameter of 1 μm or more is reduced.

[0014] Furthermore, when the inventors of the present invention studied further, they succeeded in reducing the Ca-Zn phase with an equivalent circle diameter of 1 μm or more by precipitating a large amount of the Ai-Si-Zn-Ca phase on the surface of the plating layer by adjusting the manufacturing conditions of the plating layer. By reducing the number density of the Ca-Zn phase on the surface of the plating layer, the coating adhesion is improved, and the flat corrosion resistance can also be improved by containing Ca.

[0015] Hereinafter, the plated steel sheet which is an embodiment of the present invention will be described. The plated steel sheet of this embodiment includes a steel sheet and a plating layer disposed on the surface of the steel sheet. The chemical composition of the plating layer contains, in mass%, Al: 10.0 to 30.0%, Mg: 3.0 to 15.0%, Fe: 0.01 to 2.0%, Si: more than 0 to 2.0%, Ca: 0.05 to 2.0%, further contains one or two selected from the group consisting of the following Group A and Group B, and the balance consists of Zn and impurities. The number density of Ca-Zn phases with an equivalent circle diameter of 1 μm or more exposed on the surface of the plating layer is 0 to 10 per 10000 μm 2 per area, and the number density of Al-Si-Zn-Ca phases with an equivalent circle diameter of 1 μm or more exposed on the surface of the plating layer is 1 to 50 per 10000 μm 2 per area.

[0016] [Group A] Ni: 0 to 1.0% [Group B] One or more of Sb: 0 to 0.5%, Pb: 0 to 0.5%, Cu: 0 to 1.0%, Sn: 0 to 2.0%, Ti: 0 to 1.0%, Cr: 0 to 1.0%, Nb: 0 to 1.0%, Zr: 0 to 1.0%, Mn: 0 to 1.0%, Mo: 0 to 1.0%, Ag: 0 to 1.0%, Li: 0 to 1.0%, La: 0 to 0.5%, Ce: 0 to 0.5%, B: 0 to 0.5%, Y: 0 to 0.5%, P: 0 to 0.5%, Sr: 0 to 0.5%, Co: 0 to 0.5%, Bi: 0 to 0.5%, In: 0 to 0.5%, V: 0 to 0.5%, W: 0 to 0.5% in total 0 to 5%

[0017] In the following description, the “%” indication of the content of each element in the chemical composition means “mass%”. The content of the element in the chemical composition may be expressed as an element concentration (for example, Zn concentration, Mg concentration, etc.). “Flat corrosion resistance” indicates the property that the plating layer (specifically, the Zn-Al-Mg alloy layer) itself is difficult to corrode. “Paint adhesion” indicates the property that when a paint film is formed on the plating layer, the paint film is difficult to peel off. The “plating layer” means a plating film manufactured by a so-called hot dip plating process.

[0018] As shown in FIG. 1, the plated steel sheet 1 according to the present embodiment has a steel sheet 11. The shape of the steel sheet 11 is not particularly limited. Further, the steel sheet 11 may be a base steel sheet formed into, for example, a steel pipe, a civil engineering and construction material (such as a fence channel, a corrugated pipe, a drain cover, a sand prevention plate, a bolt, a wire mesh, a guardrail, a water stop wall, etc.), a home appliance member (such as a housing of an outdoor unit of an air conditioner), an automobile part (such as a chassis member, etc.). The forming process may be various plastic processing methods such as press working, roll forming, bending working, etc.

[0019] There is no particular limitation on the material of the steel sheet 11. The steel sheet 11 can be various steel sheets such as general steel, Al-killed steel, extra low carbon steel, high carbon steel, various high-tensile steels, and some high alloy steels (such as steels containing strengthening elements such as Ni and Cr). The steel sheet 11 may be a hot-rolled steel sheet, a hot-rolled steel strip, a cold-rolled steel sheet, a cold-rolled steel strip, etc. described in JIS G 3302:2010. There are also no particular limitations on the manufacturing method of the steel sheet (hot rolling method, pickling method, cold rolling method, etc.) and its specific manufacturing conditions, etc.

[0020] As described later, a steel sheet 11 with adjusted surface roughness is used as the plating base plate. The adjustment of the surface roughness of the steel sheet can be performed, for example, by making the surface of the rolling roll or the roll for skin pass have a predetermined surface roughness and transferring the surface shape of the roll during rolling or skin pass.

[0021] The plated steel sheet 1 according to the present embodiment has a plating layer 12 disposed on the surface of the steel sheet 11. The plating layer 12 of the plated steel sheet 1 according to the present embodiment is mainly composed of a Zn-Al-Mg alloy layer due to the chemical composition described later. Further, the plating layer 12 of the plated steel sheet 1 according to the present embodiment may include an interfacial alloy layer mainly composed of Fe and Al between the steel sheet 11 and the Zn-Al-Mg alloy layer. That is, the plating layer 12 may have a single-layer structure of a Zn-Al-Mg alloy layer or a laminated structure including a Zn-Al-Mg alloy layer and an interfacial alloy layer.

[0022] The chemical composition of the plating layer according to this embodiment is composed of Zn and other alloying elements. The chemical composition of the plating layer will be described in detail below. Note that elements for which the lower limit of the concentration is described as 0% are optional elements that are not essential for solving the problems of the plated steel sheet according to this embodiment, but may be included in the plating layer for the purpose of improving characteristics and the like.

[0023] <Al: 10.0 to 30.0%> Al contributes to the improvement of planar corrosion resistance, paint adhesion, and workability. Therefore, the Al concentration is 10.0% or more. The Al concentration may be 11.0% or more, 12.0% or more, or 15.0% or more. On the other hand, when Al is excessive, the Mg concentration and the Zn concentration relatively decrease, and the paint adhesion deteriorates. Therefore, the Al concentration is 30.0% or less. The Al concentration may be 24.0% or less, 22.0% or less, or 20.0% or less.

[0024] <Mg: 3.0 to 15.0%> Mg is an essential element for ensuring planar corrosion resistance and paint adhesion. Therefore, the Mg concentration is 3.0% or more. The Mg concentration may be 4.0% or more, 5.0% or more, or 6.0% or more. On the other hand, when the Mg concentration is excessive, workability, particularly powdering property, deteriorates, and planar corrosion resistance may further deteriorate. Therefore, the Mg concentration is 15.0% or less. The Mg concentration may be 10.0% or less or 8.0% or less.

[0025] <Fe: 0.01% to 2.0%> The concentration of Fe may be 0%, but Fe may be contained in the plating layer at 0.01% or more. If the Fe concentration is 2.0% or less, it has been confirmed that there is no adverse effect on the performance of the plating layer. The Fe concentration may be, for example, 0.05% or more, 0.1% or more, 0.5% or more, or 1.0% or more. The Fe concentration is 2.0% or less. The Fe concentration may be 1.8% or less or 1.5% or less. Since Fe may be mixed from the base steel sheet, the Fe concentration may be 0.05% or more.

[0026] <Si: More than 0% to 2.0%> Si contributes to the improvement of planar corrosion resistance. It is also necessary for precipitating the Al-Si-Zn-Ca phase. Therefore, the Si concentration may be more than 0%, 0.01% or more, 0.02% or more, or 0.06% or more. On the other hand, if the Si concentration is excessive, the planar corrosion resistance and paint adhesion deteriorate. Therefore, the Si concentration is set to 2.0% or less. The Si concentration may be 1.8% or less, 1.6% or less, 1.2% or less, or 1.0% or less.

[0027] <Ca: 0.05% - 2.0%> Ca is an element that contributes to the improvement of planar corrosion resistance and is an element that can adjust the optimal Mg elution amount for imparting planar corrosion resistance. Ca is also necessary for precipitating the Al-Si-Zn-Ca phase. Therefore, the Ca concentration is 0.05% or more. Ca may be 0.10% or more, or 0.20% or more. When Ca is 0.10%, the density of the Ca-Zn phase with a circle equivalent diameter of less than 1 μm is likely to be 1 or more. On the other hand, if the Ca concentration is excessive, the paint adhesion deteriorates. Therefore, the Ca concentration is set to 2.0% or less. The Ca concentration may be 1.0% or less.

[0028] Furthermore, the plating layer of the present embodiment may contain one or two selected from the following Group A or Group B.

[0029] [Group A] Ni: 0 - 1.0% [Group B] One or more of Sb: 0 - 0.5%, Pb: 0 - 0.5%, Cu: 0 - 1.0%, Sn: 0 - 2.0%, Ti: 0 - 1.0%, Cr: 0 - 1.0%, Nb: 0 - 1.0%, Zr: 0 - 1.0%, Mn: 0 - 1.0%, Mo: 0 - 1.0%, Ag: 0 - 1.0%, Li: 0 - 1.0%, La: 0 - 0.5%, Ce: 0 - 0.5%, B: 0 - 0.5%, Y: 0 - 0.5%, P: 0 - 0.5%, Sr: 0 - 0.5%, Co: 0 - 0.5%, Bi: 0 - 0.5%, In: 0 - 0.5%, V: 0 - 0.5%, W: 0 - 0.5% in total 0 - 5%

[0030] <Ni: 0 - 1.0%> The concentration of Ni as Group A may be 0%. On the other hand, Ni contributes to the improvement of paint adhesion. Therefore, the Ni concentration may be 0.05% or more, 0.08% or more, or 0.1% or more. On the other hand, if the Ni concentration is excessive, the planar corrosion resistance deteriorates. Therefore, the Ni concentration is set to 1.0% or less. The Ni concentration may be 0.8% or less, 0.6% or less, or 0.5% or less.

[0031] Furthermore, the plating layer according to the present embodiment may contain, as Group B, one or more elements selected from Sb: 0 to 0.5%, Pb: 0 to 0.5%, Cu: 0 to 1.0%, Sn: 0 to 2.0%, Ti: 0 to 1.0%, Cr: 0 to 1.0%, Nb: 0 to 1.0%, Zr: 0 to 1.0%, Mn: 0 to 1.0%, Mo: 0 to 1.0%, Ag: 0 to 1.0%, Li: 0 to 1.0%, La: 0 to 0.5%, Ce: 0 to 0.5%, B: 0 to 0.5%, Y: 0 to 0.5%, P: 0 to 0.5%, Sr: 0 to 0.5%, Co: 0 to 0.5%, Bi: 0 to 0.5%, In: 0 to 0.5%, V: 0 to 0.5%, and W: 0 to 0.5%. The total of these elements is 0 to 5%. If the total exceeds 5%, the planar corrosion resistance or paint adhesion may decrease.

[0032] <Sb, Pb: 0 to 0.5% each> The concentrations of Sb and Pb may be 0%. On the other hand, Sb and Pb contribute to the improvement of paint adhesion. Therefore, the concentrations of Sb and Pb may each be 0.05% or more, 0.10% or more, or 0.15% or more. On the other hand, if the concentrations of Sb and Pb are excessive, the planar corrosion resistance deteriorates. Therefore, the concentrations of Sb and Pb are each set to 0.5% or less. The concentrations of Sb and Pb may each be 0.4% or less, 0.3% or less, or 0.25% or less.

[0033] <Cu, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li: 0 to 1.0% each> The concentrations of Cu, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li may each be 0%. On the other hand, these contribute to the improvement of paint adhesion. Therefore, the concentration of each of Cu, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li may be 0.05% or more, 0.08% or more, or 0.10% or more. On the other hand, when the concentrations of Cu, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li are excessive, the planar corrosion resistance deteriorates. Therefore, the concentration of each of Cu, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li is 1.0% or less. The concentration of each of Cu, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li may be 0.8% or less, 0.7% or less, or 0.6% or less.

[0034] <Sn: 0 to 2.0%> The Sn concentration may be 0%. On the other hand, Sn is an element that forms an intermetallic compound with Mg and improves the paint adhesion of the plating layer. Therefore, the Sn concentration may be 0.05% or more, 0.1% or more, or 0.2% or more. However, when the Sn concentration is excessive, the planar corrosion resistance deteriorates. Therefore, the Sn concentration is 2.0% or less. The Sn concentration may be 1.0% or less, 0.8% or less, or 0.5% or less.

[0035] <La, Ce, B, Y, P, and Sr: each 0 to 0.5%> The concentrations of La, Ce, B, Y, P, and Sr may each be 0%. On the other hand, La, Ce, B, Y, P, and Sr contribute to the improvement of paint adhesion. Therefore, the concentration of each of La, Ce, B, Y, P, and Sr may be 0.10% or more, 0.15% or more, or 0.20% or more. On the other hand, when the concentrations of La, Ce, B, Y, P, and Sr are excessive, the planar corrosion resistance deteriorates. Therefore, the concentration of each of La, Ce, B, Y, P, and Sr is 0.5% or less. The concentration of each of La, Ce, B, Y, P, and Sr may be 0.4% or less, 0.3% or less.

[0036] <Co, Bi, In, V, W: each 0 to 0.5%> The concentration of each of Co, Bi, In, V, and W may be 0%. On the other hand, Co, Bi, In, V, and W contribute to the improvement of the coating adhesion. Therefore, the concentration of each of Co, Bi, In, V, and W may be 0.10% or more, 0.15% or more, or 0.20% or more. On the other hand, when the concentration of Co, Bi, In, V, and W is excessive, the planar corrosion resistance deteriorates. Therefore, the concentration of each of Co, Bi, In, V, and W is set to 0.5% or less. The concentration of each of Co, Bi, In, V, and W may be 0.4% or less, 0.3% or less.

[0037] <Remainder: Zn and impurities> The remainder of the components of the plating layer according to this embodiment is Zn and impurities. Zn is an element that provides planar corrosion resistance and coating adhesion to the plating layer. Impurities refer to components contained in the raw materials or components mixed in during the manufacturing process, and are not components intentionally contained. For example, in the plating layer, due to the mutual atomic diffusion between the base steel plate and the plating bath, trace amounts of components other than Fe may be mixed in as impurities.

[0038] The chemical composition of the plating layer is measured by the following method. First, an acid solution containing an inhibitor that suppresses the corrosion of the steel plate is used to strip and dissolve the plating layer to obtain an acid solution. Next, the obtained acid solution is subjected to inductively coupled plasma (ICP) emission spectrometry. Thereby, the chemical composition of the plating layer can be obtained. The acid type is not particularly limited as long as it can dissolve the plating layer. The chemical composition measured by the above means is the average chemical composition of the entire plating layer.

[0039] Next, the metal structure of the plating layer will be described. On the surface of the plating layer according to this embodiment, a Ca-Zn phase with an equivalent circle diameter of 1 μm or more should not be crystallized as much as possible. The number density of the Ca-Zn phase with an equivalent circle diameter of 1 μm or more allowed on the surface is 10000 μm 2It is 0 to 10 per unit area. When a Ca-Zn phase with an equivalent circle diameter of 1 μm or more is exposed on the surface of the plating layer, corrosion products are formed around the Ca-Zn phase with an equivalent circle diameter of 1 μm or more at the initial stage of corrosion of the plating layer, and the paint adhesion is reduced by this corrosion product. Therefore, the number density of the Ca-Zn phase with an equivalent circle diameter of 1 μm or more is preferably low, and the number density is 0 (pieces / 10000 μm 2 ) is most preferable. When the number density exceeds 10 per 10000 μm 2 , the paint adhesion is reduced, which is not preferable. The number density of the Ca-Zn phase with an equivalent circle diameter of 1 μm or more may be 5 or less, or 3 or less when the unit is (pieces / 10000 μm 2 ).

[0040] For the identification of the Ca-Zn phase on the surface of the plating layer, an electron probe microanalyzer (EPMA) is used. The surface of the plating layer is observed with a scanning electron microscope attached to the EPMA to identify the intermetallic compound to be analyzed. Then, by performing elemental analysis on the identified intermetallic compound, it is determined whether the intermetallic compound is a Ca-Zn phase. The Ca-Zn phase is defined as an intermetallic compound containing 35 to 65 atomic% of Ca and 35 atomic% or more of Zn. The Ca-Zn phase may contain Mg, Al, Si, and Fe in the range of 10 atomic% or less, respectively.

[0041] Since the Ca-Zn phase with an equivalent circle diameter of 1 μm or more has an adverse effect on the paint adhesion, the crystallization of the Ca-Zn phase with an equivalent circle diameter of 1 μm or more is restricted. On the other hand, the Ca-Zn phase with an equivalent circle diameter of less than 1 μm has almost no influence on the paint adhesion, so it is not a problem in this embodiment. The Ca-Zn phase with an equivalent circle diameter of less than 1 μm is less likely to cause corrosion of the surrounding metal structure and phase. Therefore, it is preferable that the Ca-Zn phase with an equivalent circle diameter of less than 1 μm is formed on the surface of the plating layer. Therefore, on the surface of the plating layer according to this embodiment, the number density of the Ca-Zn phase with an equivalent circle diameter of less than 1 μm is 1 per 10000 μm 2 or more per unit area is preferable. The number density of the Ca-Zn phase with an equivalent circle diameter of less than 1 μm may be 5 or less, or 3 or less when the unit is (pieces / 10000 μm 2When [[ID=]], it may be 100 or less, or may be 50 or less. The number density of Ca-Zn phases with an equivalent circle diameter of less than 1 μm, which is more preferable, is 10 pieces / 10,000 μm 2 or less.

[0042] The equivalent circle diameter of the Ca-Zn phase is obtained by determining the individual area of the Ca-Zn phase when observing the Ca-Zn phase with a scanning electron microscope, and the diameter of the circle having that area is defined as the equivalent circle diameter of the Ca-Zn phase.

[0043] The number density of Ca-Zn phases on the surface of the plating layer may be affected by the average chemical composition of the plating layer. Among the chemical compositions of the plating layer, when Mg and Si are Mg: 4.5 to 8% by mass and Si: 0.1 to 2% by mass, the number density of Ca-Zn phases with an equivalent circle diameter of 1 μm or more exposed on the surface of the plating layer is 10,000 μm 2 It may be 0 to 3 per area.

[0044] Also, among the chemical compositions of the plating layer, when Al, Mg, and Si are Al: 15 to 25% by mass, Mg: 4.5 to 8% by mass, and Si: 0.1 to 2% by mass, the number density of Ca-Zn phases with an equivalent circle diameter of 1 μm or more exposed on the surface of the plating layer is 10,000 μm 2 It may be 0 per area.

[0045] Next, Al-Si-Zn-Ca phases with an equivalent circle diameter of 1 μm or more are exposed on the surface of the plating layer according to this embodiment. The number density of Al-Si-Zn-Ca phases on the surface is 1 to 50 per 10,000 μm 2 By the Al-Si-Zn-Ca phase being exposed on the surface of the plating layer, Ca contained in the plating layer is consumed for the formation of this phase, and the crystallization of Ca-Zn phases with an equivalent circle diameter of 1 μm or more is suppressed. As a result, the paint adhesion is improved. In this way, the planar corrosion resistance and paint adhesion of the plating layer can be improved.

[0046] To identify the Al-Si-Zn-Ca phase in the plating layer, an electron probe microanalyzer (EPMA) is used. The surface of the plating layer is observed with a scanning electron microscope attached to the EPMA to identify the intermetallic compound to be analyzed. Then, by performing elemental analysis on the identified intermetallic compound, it is determined whether the intermetallic compound is the Al-Si-Zn-Ca phase. The identification of the Al-Si-Zn-Ca phase is based on an intermetallic compound containing 20 to 80 atomic% of Zn, 1 to 10 atomic% of Si, 5 to 25 atomic% of Ca, with the balance being Al (Al: 10 atomic% or more) and 0 to 5 atomic% of other elements as the Al-Si-Zn-Ca phase. As other elements other than Al, Si, Zn, and Ca, any element contained in the plating layer may be used.

[0047] If the size of the Al-Si-Zn-Ca phase is small, the precipitation of the Ca-Zn phase cannot be effectively suppressed. Therefore, the size of the Al-Si-Zn-Ca phase that limits the number density needs to be 1 μm or more in terms of the equivalent circle diameter. When the number density of the Al-Si-Zn-Ca phase with an equivalent circle diameter of 1 μm is 1 (piece / 10000 μm 2 ) or more, the precipitation of the Ca-Zn phase with an equivalent circle diameter of 1 μm or more can be sufficiently suppressed. The equivalent circle diameter of the Al-Si-Zn-Ca phase is obtained by determining the individual area of the Al-Si-Zn-Ca phase when observing the Al-Si-Zn-Ca phase with a scanning electron microscope, and the diameter of the circle having that area is defined as the equivalent circle diameter of the Al-Si-Zn-Ca phase.

[0048] The number density of the Al-Si-Zn-Ca phase on the surface of the plating layer is set to 1 to 50 per 10000 μm 2 . If the number density is less than 1 (piece / 10000 μm 2 ), the precipitation of the Ca-Zn phase cannot be suppressed and the paint adhesion becomes insufficient. On the other hand, even if the number density of the Al-Si-Zn-Ca phase exceeds 50 (piece / 10000 μm 2 ), the effect of improving the paint adhesion saturates. Therefore, the upper limit is set to 50 (piece / 10000 μm 2 ). The number density of the Al-Si-Zn-Ca phase is expressed in units of (piece / 10000 μm 2When [[ID=]], it may be 3 or more, or may be 5 or more. Also, the number density of the Al-Si-Zn-Ca phase may be 40 or less, may be 30 or less, or may be 20 or less.

[0049] Next, on the surface of the plating layer according to the present embodiment, Mg-Si-Zn-Al phases having a major axis of 2 μm or more may be exposed. The number density of the Mg-Si-Zn-Al phases on the surface is 10,000 μm 2 and there are 5 to 150 per. When the Mg-Si-Zn-Al phases are exposed on the surface of the plating layer, the Mg-Si-Zn-Al phases are corroded at the initial stage of corrosion of the plating layer, whereby dense corrosion products of Mg, Si, Zn, and Al are formed. By forming this corrosion product, the planar corrosion resistance of the plating layer is further improved.

[0050] To identify the Mg-Si-Zn-Al phases of the plating layer, an electron probe microanalyzer (EPMA) is used. The surface of the plating layer is observed with a scanning electron microscope attached to the EPMA to identify the intermetallic compound to be analyzed. Then, by performing elemental analysis on the identified intermetallic compound, it is determined whether the intermetallic compound is an Mg-Si-Zn-Al phase. The identification of the Mg-Si-Zn-Al phase is based on an intermetallic compound containing Mg: 20 to 45 atomic%, Si: 15 to 40 atomic%, Zn: 15 to 40 atomic%, and Al: 5 to 20 atomic% as the Mg-Si-Zn-Al phase.

[0051] The number density of the Mg-Si-Zn-Al phases on the surface of the plating layer is affected by the average chemical composition of the plating layer. In order for the number density of the Mg-Si-Zn-Al phases to be 5 to 150 per 10,000 μm 2 in the chemical composition of the plating layer, it is preferable that Al, Mg, and Si are Al: 15 to 25 mass%, Mg: 4.5 to 8 mass%, and Si: 0.1 to 2%.

[0052] The shape of the Mg-Si-Zn-Al phase preferably has a major axis of 2 μm or more, more preferably a needle shape with a major axis of 2 μm or more. Furthermore, the aspect ratio is preferably 2 or more. By making the shape of the Mg-Si-Zn-Al phase have a major axis of 2 μm or more, the Mg-Si-Zn-Al phase is more likely to be dissolved at the initial stage of corrosion, and more dense corrosion products can be formed, thereby enhancing the planar corrosion resistance.

[0053] The major axis of the Mg-Si-Zn-Al phase is defined as the maximum length of the Mg-Si-Zn-Al phase when observed with an electron microscope. The aspect ratio is the ratio of the major axis to the minor axis (major axis / minor axis). The minor axis is the length in the direction perpendicular to the major axis direction, and more specifically, it is the maximum length within a range of ±5° with respect to the direction perpendicular to the major axis direction.

[0054] The number density of the Mg-Si-Zn-Al phase on the surface of the plating layer is preferably 5 to 150 per 10000 μm 2 ². By setting the number density to 5 (pieces / 10000 μm 2 ²) or more, the planar corrosion resistance can be further improved. On the other hand, even if the number density of the Mg-Si-Zn-Al phase exceeds 150 (pieces / 10000 μm 2 ²), the effect of improving the planar corrosion resistance saturates, so the upper limit is set to 150 (pieces / 10000 μm 2 ²). The number density of the Mg-Si-Zn-Al phase may be 10 or more, or 15 or more when the unit is (pieces / 10000 μm 2 ²). Also, the number density of the Mg-Si-Zn-Al phase may be 120 or less, 100 or less, 80 or less, 70 or less, 50 or less, or 30 or less.

[0055] Note that the Mg-Si-Zn-Al phase may be present at a number density of more than 0 to less than 5 per 10000 μm 2 ² of the surface area of the plating layer.

[0056] A method for measuring the number density of the Ca-Zn phase, Al-Si-Zn-Ca phase, and Mg-Si-Zn-Al phase will be described. On the surface of the plating layer, a measurement region in the shape of a 50-μm square is provided. There are eight measurement regions, and the eight measurement regions are randomly arranged on the surface of the plating layer. The measurement regions are spaced apart so as not to overlap each other. Intermetallic compounds are confirmed by observing the set measurement regions with a scanning electron microscope. Then, by means of EPMA, the composition of the intermetallic compounds is analyzed to distinguish the Ca-Zn phase, Al-Si-Zn-Ca phase, and Mg-Si-Zn-Al phase. Further, the number of each of the Ca-Zn phase with an equivalent circle diameter of 1 μm or more, the Al-Si-Zn-Ca phase with an equivalent circle diameter of 1 μm or more, and the Mg-Si-Zn-Al phase with a major axis of 2 μm or more in each measurement region is counted. The measurement conditions of EPMA are, for example, an acceleration voltage of 15 kV, a current of 0.05 μA, and an irradiation time of 50 ms. As the EPMA, for example, JXA-8230 manufactured by JEOL Ltd. is used.

[0057] For the Ca-Zn phase, Al-Si-Zn-Ca phase, and Mg-Si-Zn-Al phase, it is possible that a part of each phase is within the measurement region and the remainder of each phase is outside the measurement region, but such phases are also included in the objects to be counted for the number.

[0058] Also, when the Mg-Si-Zn-Al phase is needle-shaped, a plurality of Mg-Si-Zn-Al phases may overlap each other. In such a case, when the major axis directions of the respective phases are oriented in different directions and overlap, each overlapping phase is an object to be counted for the number. For example, when two Mg-Si-Zn-Al phases overlap and their respective major axis directions are different, the number is counted as two.

[0059] Then, based on the number of each of the Ca-Zn phase with an equivalent circle diameter of 1 μm or more, the Al-Si-Zn-Ca phase with an equivalent circle diameter of 1 μm or more, and the Mg-Si-Zn-Al phase with a major axis of 2 μm or more measured in the eight measurement regions, and the total area of the measurement regions, per 10,000 μm 2 The number is taken as the number density.

[0060] Also, when the plating layer contains 0.05 to 0.5% by mass of Sn, it is preferable that the plating layer contains the Mg2Sn phase. Since the amount of the Mg2Sn phase is small, its presence is detected and confirmed by X-ray diffraction measurement. By containing the Mg2Sn phase in the plating layer, the corrosion resistance of the plating layer is further improved. Whether the Mg2Sn phase is contained in the plating layer is determined by whether a diffraction peak specific to Mg2Sn appears. Here, the diffraction peak specific to Mg2Sn refers to, for example, a peak that appears at a diffraction angle 2θ of 23.4 ± 0.3 degrees.

[0061] The deposition amount per side of the plating layer may be, for example, in the range of 20 to 150 g / m 2 . By setting the deposition amount per side to 20 g / m 2 or more, the planar corrosion resistance and paint adhesion of the plated steel sheet can be further enhanced. On the other hand, by setting the deposition amount per side to 150 g / m 2 or less, the workability of the plated steel sheet can be further enhanced.

[0062] Next, the manufacturing method of the plated steel sheet of the present embodiment will be described. However, the manufacturing method of the plated steel sheet according to the present embodiment is not particularly limited. For example, according to the manufacturing conditions described below, the plated steel sheet according to the present embodiment can be obtained.

[0063] The manufacturing method of the plated steel sheet of the present embodiment is to anneal a steel sheet with adjusted surface roughness in a reducing atmosphere, immerse the steel sheet immediately after annealing in a molten plating bath and then lift it up to form a plating layer on the surface of the steel sheet. Next, cooling is performed by blowing a cooling gas until the temperature of the plating layer reaches 300°C or lower from the bath temperature. The gas flux when blowing the cooling gas is such that the gas flux from the bath temperature to the controlled cooling temperature (the gas flow velocity in the temperature range of not lower than the controlled cooling temperature and not higher than the bath temperature) is in the range of 100 to 5000 L / min / m 2 , and the gas flux from the controlled cooling temperature to the cooling stop temperature (in this embodiment, 300°C or lower) (the gas flow velocity in the temperature range of not lower than the cooling stop temperature and lower than the controlled cooling temperature) is in the range of 10000 to 80000 L / min / m 2 .

[0064] The controlled cooling temperature shall be in the range of -10°C to -80°C with respect to the crystallization temperature of the Al-Si-Zn-Ca phase.

[0065] The roughness of the surface of the steel sheet serving as the plating base plate is such that the curve length L of the roughness curve per reference length L0 p is made 1.0 or more, and the arithmetic mean roughness Ra is made 0.1 μm or more. If outside this range, a large amount of the Al-Si-Zn-Ca phase may crystallize near the interface between the plating layer and the steel sheet, and the number density of the Al-Si-Zn-Ca phase on the surface of the plating layer may decrease. (L p / L0) preferably has an upper limit of 3.0 or less, may be 2.5 or less, or may be 2.0 or less. The upper limit of the arithmetic mean roughness Ra is preferably 4.0 μm or less, and may be 3.5 μm or less. The adjustment of the roughness of the steel sheet surface is not particularly limited. For example, it may be adjusted by rolling the plating base plate with a rolling roll having the surface of the roll adjusted to a desired roughness or a roll for temper rolling to transfer the surface shape of the roll. Also, it may be adjusted by pickling. p The upper limit of (L

[0066] (L p / L0) and the measurement of the arithmetic mean roughness are performed, for example, using a shape measurement laser microscope (model number: VK-8700) manufactured by Keyence Corporation. As the measurement conditions, for example, measurement mode: laser confocal, measurement quality: high precision, pitch: 0.75 μm, double scan: ON, optical zoom: 1×, objective lens name: Plan, γ coefficient: 0.45, offset: 0%, the measurement is carried out. Note that the measuring device used for the measurement of (L p / L0) and the arithmetic mean roughness is not limited to the above example. In accordance with JIS B 0601:2013, a roughness curve was obtained by sequentially applying contour curve filters with cut-off values λc and λs to the measured cross-sectional curve. Specifically, from the obtained measurement results, components with a wavelength λc of 0.001 mm or less and components with a wavelength λs of 0.2 mm or more were removed to obtain a roughness curve. Based on the obtained roughness curve, (L p / L0) and the arithmetic mean roughness were calculated.

[0067] Annealing of the steel sheet to be the plating master is carried out in a reducing atmosphere. The reducing atmosphere and annealing conditions are not particularly limited. By this annealing, oxides present on the surface of the steel sheet are removed as much as possible.

[0068] Next, the steel sheet immediately after annealing is immersed in a molten plating bath. The chemical composition of the molten plating bath may be appropriately adjusted so that the chemical composition of the plating layer described above is obtained. Also, the temperature of the molten plating bath is not particularly limited, and a temperature at which molten plating can be carried out can be appropriately selected. For example, the plating bath temperature may be a value about 20°C or higher than the melting point of the plating bath.

[0069] Next, the steel sheet is pulled out from the molten plating bath. The deposition amount of the plating layer can be controlled through the control of the pulling-out speed of the steel sheet. If necessary, the steel sheet with the plating layer attached may be wiped to control the deposition amount of the plating layer. The deposition amount of the plating layer is not particularly limited, and for example, it can be within the range described above.

[0070] Next, the plating layer is cooled. Cooling is performed by blowing cooling gas onto the steel sheet immediately after being pulled out from the molten plating bath. Cooling by blowing cooling gas is continuously performed until the temperature of the steel sheet reaches 300°C from the bath temperature. The cooling conditions below 300°C are not particularly limited, and cooling by continuously blowing cooling gas may be performed, or natural air cooling may be performed.

[0071] In the cooling by blowing cooling gas, it is performed by arranging a cooling zone along the conveyance path of the steel sheet. The cooling zone is provided with a plurality of spraying nozzles for cooling gas. The shape of the gas nozzle from which the cooling gas jets out is, for example, in the range of 1 to 50 mm in diameter. The angle formed by the tip of the gas nozzle and the steel sheet is, for example, in the range of 70 to 110°, more preferably 90° (right angle). The distance between the tip of the gas nozzle and the steel sheet is in the range of 30 to 1000 mm. Note that the shape, angle, and distance of the gas nozzle are merely examples and are not limited to the above ranges.

[0072] The cooling gas to be sprayed is not particularly limited and may be a non-oxidizing gas such as nitrogen, an inert gas such as argon, or air, or a mixed gas thereof.

[0073] In this embodiment, the gas flux when spraying the cooling gas is controlled in two stages. That is, based on the temperature of the steel plate, the gas flux from the plating bath temperature to the controlled cooling temperature (a temperature in the range of -10 to -80 °C with respect to the crystallization temperature of the Al-Si-Zn-Ca phase) is 100 to 5000 L / min / m 2 in the range, preferably 500 to 5000 L / min / m 2 in the range, and the gas flux from the controlled cooling temperature to 300 °C or lower is 10,000 to 80,000 L / min / m 2 in the range. The controlled cooling temperature is a temperature estimated to be the crystallization start temperature of the Al-Si-Zn-Ca phase.

[0074] When the gas flux is in the range of 5000 L / min / m 2 or less, it is possible to suppress applying vibration to the steel plate during cooling. On the other hand, when the gas flux is in the range of 10,000 L / min / m 2 or more, it becomes possible to apply vibration to the steel plate during cooling.

[0075] And by setting the gas flux from the plating bath temperature to the controlled cooling temperature in the range of 100 to 5000 L / min / m 2 in the range, preferably 500 to 5000 L / min / m 2 in the range, it is possible to promote the nucleation of Ca-containing phases other than the Al-Si-Zn-Ca phase without applying vibration to the steel plate and concentrate Ca and Si in the liquid phase in the unfrozen state. Next, by setting the gas flux from the controlled cooling temperature to 300 °C or lower in the range of 10,000 to 80,000 L / min / m 2 in the range, it is possible to apply vibration to the surface of the liquid phase in the unfrozen state and crystallize a large amount of the Al-Si-Zn-Ca phase on the surface of the plating layer. When the range of the gas flux deviates from the above range, it becomes difficult to crystallize a large amount of the Al-Si-Zn-Ca phase on the surface of the plating layer.

[0076] Since the crystallization temperature of the Al-Si-Zn-Ca phase varies depending on the chemical composition of the plating layer, it is calculated using a calculated phase diagram. Specifically, a calculated phase diagram database that accumulates thermodynamic data of intermetallic compound phases, metal phases, etc. that can be contained in an Al-Mg-Zn alloy system is constructed, and calculations are performed by the CALPHAD method (CALculation of PHAse Diagram) to determine the crystallization temperature of the Al-Si-Zn-Ca phase for each chemical composition of the plating layer. More specifically, the crystallization temperature of the Al-Si-Zn-Ca phase can be estimated by using the thermodynamic equilibrium calculation software "Thermo-Calc" (registered trademark, manufactured by Thermo-Calc Software). Note that the thermodynamic equilibrium calculation software used for the calculation is not limited to "Thermo-Calc" (registered trademark), and other software may be used. For the obtained crystallization temperature of the Al-Si-Zn-Ca phase, the temperature within the range of -10 to -80 °C is set as the controlled cooling temperature.

[0077] In the above manufacturing method, by adjusting the surface roughness of the steel sheet surface in advance, the nucleation of the Al-Si-Zn-Ca phase is suppressed, and thereby the crystallization of the Al-Si-Zn-Ca phase inside the plating layer is suppressed. For such a steel sheet, hot dip plating is performed, and further, by controlling the cooling conditions after plating as described above, a large amount of the Al-Si-Zn-Ca phase is crystallized on the surface of the plating layer. As a result, it is presumed that a large number of Al-Si-Zn-Ca phases with an equivalent circle diameter of 1 μm or more are formed on the surface of the plating layer, and the crystallization of the Ca-Zn phase with an equivalent circle diameter of 1 μm or more can be suppressed.

[0078] Note that as long as the requirements shown in the present invention are satisfied, the manufacturing method of the plated steel sheet is not limited to the above content, and instead of the hot dip plating method, an electroplating method, a vapor deposition plating method, a thermal spraying method, a cold spray method, etc. may be adopted.

Example

[0079] Hereinafter, embodiments of the present invention will be described. However, the conditions in the embodiments are merely one set of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to this one set of conditions. The present invention can adopt various conditions as long as it does not deviate from the gist of the present invention and can achieve the object of the present invention.

[0080] For the plating substrate, a cold-rolled steel sheet (0.05C - 0.1Si - 0.2Mn) with a thickness of 1.2 mm was used. The surface roughness of a part of the plating substrate was controlled using a skin pass mill or the like. Annealing was performed on the steel sheet with the adjusted surface roughness. After annealing, the steel sheet was immersed in various molten plating baths and then pulled up to deposit a plating layer on the steel sheet surface. Subsequently, various plated steel sheets were manufactured by cooling with cooling gas from immediately after pulling up the plating bath until the plating layer reached 300°C.

[0081] The surface roughness of the steel sheet surface serving as the plating substrate was such that the curve length L of the roughness curve per reference length L0 p had a ratio (L p / L0) of 1.1 to 2.6, and the arithmetic mean roughness Ra was in the range of 0.6 to 3.7 μm.

[0082] (L p / L0) and the measurement of the arithmetic mean roughness were measured using a shape measurement laser microscope (model number: VK-8700) manufactured by Keyence Corporation. As the measurement conditions, the measurement mode was laser confocal, the measurement quality was high precision, the pitch was 0.75 μm, double scan was ON, the optical zoom was 1x, the objective lens name was Plan, the γ coefficient was 0.45, and the offset was 0%, and the measurement was performed.

[0083] When annealing the steel sheet in a reducing atmosphere, the annealing conditions were a soaking temperature of 600°C and a soaking time of 10 seconds. The annealing atmosphere was a reducing atmosphere composed of a mixed gas of 5% hydrogen and the balance nitrogen. Then, the annealed steel sheet was air-cooled with nitrogen gas, and after the immersion plate temperature reached the bath temperature + 20°C, it was immersed in the molten plating bath and then pulled up. The pulling speed was 20 to 200 mm / second.

[0084] The chemical composition of the molten plating layer was as shown in Table 1. The manufacturing conditions were as shown in Table 2. Also, the metal structure of the plating layer was evaluated, and the results are shown in Table 3. Furthermore, the planar corrosion resistance and paint adhesion of the plated steel sheet were evaluated, and the results are shown in Table 3.

[0085] The evaluation of the chemical composition of the plating layer and the metal structure of the plating layer was carried out by the means described above. For the Ca-Zn phase, those with an equivalent circle diameter of 1 μm or more and those with an equivalent circle diameter of less than 1 μm were taken as the measurement targets. For the Al-Si-Zn-Ca phase, those with an equivalent circle diameter of 1 μm or more were taken as the measurement targets. Also, for the Mg-Si-Zn-Al phase, those with a major axis of 2 μm or more were taken as the measurement targets. Note that all the measured Mg-Si-Zn-Al phases had an aspect ratio of 2 or more.

[0086] The evaluation of the planar corrosion resistance was carried out as follows. The obtained plated steel sheet was cut into 100 mm × 50 mm and subjected to a planar corrosion resistance evaluation test. The evaluation of the planar corrosion resistance was carried out by a corrosion acceleration test defined in JASO-CCT-M609, and after 150 cycles, the corrosion weight loss was compared. The evaluation criteria were as follows, and "AAA", "AA", and "A" were considered qualified.

[0087] AAA: Corrosion weight loss < 50 g / m 2 less than AA: Corrosion weight loss ≥ 50 g / m 2 less than 90 g / m 2 less than A: Corrosion weight loss ≥ 90 g / m 2 less than 120 g / m 2 less than B: Corrosion weight loss ≥ 120 g / m 2 or more

[0088] The evaluation of the paint adhesion was carried out as follows. The obtained plated steel sheet was cut into 100 mm × 50 mm and subjected to a paint adhesion test. After forming a coating film layer on the test piece, the end face and the back face were sealed with a silicone resin.

[0089] Next, cut defects reaching the substrate were imparted to the front surface with a cutter knife. The cut defects were imparted in a grid pattern at intervals of 1 mm. In this way, 100 regions of 1 square millimeter partitioned by the cut defects were formed. Next, it was continuously immersed in a 5% NaCl aqueous solution at 50°C for 500 hours, and after immersion, it was washed with water and dried. Then, after sticking an adhesive tape to the cut defect part of the dried sample, tape peeling was performed, and the peel area ratio was measured.

[0090] <Coating film layer> A coating film layer was formed by applying the primer paint resin and topcoat paint resin described below on the surface of the plating layer. The thickness of the layer made of the primer paint resin was set to 5 μm, the thickness of the layer made of the topcoat paint resin was set to 15 μm, and the total was 20 μm.

[0091] <Film-forming components of the coating film layer> (1) Primer paint resin for the front and back surfaces Polyester / melamine + isocyanate combination curing type (FLC687 paint resin manufactured by Nippon Fine Coatings Co., Ltd.) (2) Topcoat paint resin for the front surface High molecular polyester / melamine curing type (FLC7000 paint resin manufactured by Nippon Fine Coatings Co., Ltd.) (3) Topcoat paint resin for the back surface Polyester / melamine curing type (FLC100HQ paint resin manufactured by Nippon Fine Coatings Co., Ltd.)

[0092] The evaluation criteria for coating adhesion are shown below. It was judged based on the scoring shown below. "AAA", "AA", and "A" were regarded as passing.

[0093] AAA: Peel area ratio less than 10% AA: Peel area ratio 10 - less than 20% A: Peel area ratio 20 - less than 30% B: Peel area ratio 30% or more

[0094] As shown in Tables 1 to 3, Examples 1 to 30 and 39 according to the present invention, in which the chemical composition and metal structure of the plating layer were appropriately controlled, were excellent in both planar corrosion resistance and paint adhesion. The deposition amount per side of the plating layer in the examples was in the range of 20 to 150 g / m 2 2.

[0095] In Comparative Example 31, the amount of Al in the plating layer was insufficient. Therefore, in Comparative Example 31, the crystallization temperature of the Ca-Zn phase increased and a large amount of Ca-Zn phases with a circle equivalent diameter of 1 μm crystallized. As a result, the planar corrosion resistance was insufficient.

[0096] In Comparative Example 32, the amount of Al in the plating layer was excessive. Therefore, in Comparative Example 32, a large amount of Ca-Zn phases with a circle equivalent diameter of 1 μm in which Al was solid-solved crystallized. As a result, the paint adhesion decreased.

[0097] In Comparative Example 33, the amount of Mg in the plating layer was insufficient. Therefore, in Comparative Example 33, the crystallization temperature of the Ca-Zn phase increased and a large amount of Ca-Zn phases with a circle equivalent diameter of 1 μm crystallized. As a result, the planar corrosion resistance and the paint adhesion decreased.

[0098] In Comparative Example 34, the amount of Mg in the plating layer was excessive. Therefore, in Comparative Example 34, the appearance of the plating layer deteriorated significantly.

[0099] In Comparative Example 35, the amount of Si in the plating layer was excessive. Therefore, in Comparative Example 35, Si crystallized as a Si phase instead of an Al-Si-Zn-Ca phase, and thus the crystallization of Ca-Zn phases with a circle equivalent diameter of 1 μm was not suppressed, and the planar corrosion resistance and the paint adhesion decreased.

[0100] In Comparative Example 36, the amount of Ca in the molten plating layer was excessive. Therefore, in Comparative Example 36, an excessive amount of Ca-Zn phases with a circle equivalent diameter of 1 μm crystallized on the surface of the plating layer, and the planar corrosion resistance and the paint adhesion decreased.

[0101] In Comparative Example 37, the cooling gas flux from the bath temperature to the controlled cooling temperature was excessive. Therefore, in Comparative Example 37, due to the influence of vibration, nucleation of Ca-Zn phase with an equivalent circle diameter of 1 μm progressed on the surface of the plating layer, resulting in a decrease in flat corrosion resistance and paint adhesion.

[0102] In Comparative Example 38, the cooling gas flux from the controlled cooling temperature to 300 °C was insufficient. Therefore, in Comparative Example 38, vibration was not sufficiently applied, the formation of the Al-Si-Zn-Ca phase was insufficient, and the Ca-Zn phase with an equivalent circle diameter of 1 μm crystallized excessively, resulting in a decrease in flat corrosion resistance and paint adhesion.

[0103]

Table 1

[0104]

Table 2

[0105]

Table 3

Industrial Applicability

[0106] The plated steel sheet of the present disclosure is excellent in both flat corrosion resistance and paint adhesion, so it has high industrial applicability.

Explanation of Reference Numerals

[0107] 1... plated steel sheet, 11... steel sheet, 12... plating layer.

Claims

1. A steel sheet and a plating layer disposed on the surface of the steel sheet, wherein the chemical composition of the plating layer is in mass %, Al: 10.0 to 30.0%, Mg: 3.0 to 15.0%, Fe: 0.01 to 2.0%, Si: more than 0 to 2.0%, Ca: 0.05 to 2.0% is contained, furthermore, it contains one or two selected from the group consisting of the following Group A and Group B, the balance consists of Zn and impurities, The number density of Ca-Zn phases with an equivalent circle diameter of 1 µm or more exposed on the surface of the plating layer is 0 per µm ２ of area, and among the chemical composition of the plating layer, Al, Mg and Si are Al: 15 to 25% by mass, Mg: 4.5 to 8% by mass, Si: 0.1 to 2% by mass, The number density of Al—Si—Zn—Ca phases with an equivalent circle diameter of 1 μm or more exposed on the surface of the plating layer is 1 to 50 per μm ２ of the surface area, and the plated steel sheet is characterized by this. [Group A] Ni: 0 to 1.0% [Group B] Sb: 0 to 0.5%, Pb: 0 to 0.5%, Cu: 0 to 1.0%, Sn: 0 to 2.0%, Ti: 0 to 1.0%, Cr: 0 to 1.0%, Nb: 0 to 1.0%, Zr: 0 to 1.0%, Mn: 0 to 1.0%, Mo: 0 to 1.0%, Ag: 0 to 1.0%, Li: 0 to 1.0%, La: 0 to 0.5%, Ce: 0 to 0.5%, B: 0 to 0.5%, Y: 0 to 0.5%, P: 0 to 0.5%, Sr: 0 to 0.5%, Co: 0 to 0.5%, Bi: 0 to 0.5%, In: 0 to 0.5%, V: 0 to 0.5%, W: 0 to 0.5%, one or more of which in total is 0 to 5%

2. The number density of Mg—Si—Zn—Al phases having a major axis of 2 μm or more exposed on the surface of the plating layer is 5 to 150 per μm ２ of the area, and the plated steel sheet according to Claim 1.

3. among the chemical composition of the plating layer, Sn is Sn: 0.05 to 0.5% by mass, In the X-ray diffraction measurement of the plating layer, Mg ２ The plated steel sheet according to claim 1 or 2, wherein an Sn phase is detected in the plating layer.

4. The plated steel sheet according to claim 1, wherein the plating layer has a chemical composition containing the Group A in mass %.

5. The plated steel sheet according to claim 1, wherein the plating layer has a chemical composition containing the Group B in mass %.

6. The number density of Ca-Zn phases with an equivalent circle diameter of less than 1 μm exposed on the surface of the plating layer is 1 or more per μm ２ of the area, and the plated steel sheet according to claim 1.

Images