JP7698235B2 - Galvanized Steel Sheet - Google Patents

Description

The present invention relates to a plated steel sheet.
This application claims priority based on Japanese Patent Application No. 2022-100352, filed on June 22, 2022, the contents of which are incorporated herein by reference.

Zn-Al-Mg hot-dip plated steel sheets, which have a hot-dip Zn coating layer containing Al and Mg, have excellent corrosion resistance. For this reason, Zn-Al-Mg hot-dip plated steel sheets are widely used as materials for structural components that require corrosion resistance, such as building materials.

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, in which the Zn-Al-Mg alloy layer has a Zn phase and contains an Mg-Sn intermetallic compound phase in the Zn phase, and the plating layer has a chemical composition consisting of, in 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 satisfies 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

Patent Document 2 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, in which the area fraction of the MgZn ₂ phase is 45 to 75%, the total area fraction of the MgZn ₂ phase and the Al phase is 70% or more in a cross section of the Zn-Al-Mg alloy layer, and the Zn-Al-MgZn 2 phase is 45 to 75%. _2. The area fraction of the ternary eutectic structure is 0 to 5%, and the plating layer is composed of, in mass%, Zn: more than 44.90% and less than 79.90%, Al: more than 15% and less than 35%, Mg: more than 5% and less than 20%, Ca: 0.1% to less than 3.0%, and impurities, and the element group A is Y, La, and Ce, the element group B is Cr, Ti, Ni, Co, V, Nb, Cu, and Mn, the element group C is Sr, Sb, and Pb, and the element group D is Sn, Bi, and In. The document describes a plated steel material having a chemical composition in which the total content of 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 0.1% to less than 3.0%, the total content of elements selected from element group B is 0% to 0.25%, the total content of elements selected from element group C is 0% to 0.5%, and the total content of elements selected from element group D is 0% to 20.00%.

In recent years, hot-dip galvanized steel for use in building materials such as roofs and walls is required to have both flat surface corrosion resistance, which is the corrosion resistance of the plating layer itself, and good adhesion of the paint film (paint adhesion) when painted on the plated surface. However, no technology had been developed that could achieve high levels of both flat surface corrosion resistance and paint adhesion.

International Publication No. 2018/139619 International Publication No. 2018/139620

The present invention has been made in consideration of the above circumstances, and its objective is to provide a plated steel sheet that has both excellent flat-surface corrosion resistance and paint adhesion.

In order to solve the above problems, the present invention employs the following configuration.
[1] A steel plate and a plating layer disposed on a surface of the steel plate,
The chemical composition of the plating layer is, in mass%,
Al: 10.0-30.0%,
Mg: 3.0 to 15.0%,
Fe: 0.01-2.0%,
Si: more than 0 to 2.0%,
Ca: 0.05 to 2.0%;
Further, it contains one or two selected from the group consisting of the following Group A and Group B,
The balance is Zn and impurities,
the number density of Ca-Zn phases having a circle equivalent diameter of 1 μm or more exposed on the surface of the plating layer is 0 per 10,000 ^μm2 area;
The chemical composition of the plating layer is such that Al, Mg, and Si are 15 to 25 mass% Al, 4.5 to 8 mass% Mg, and 0.1 to 2 mass% Si,
A plated steel sheet, characterized in that a number density of Al-Si-Zn-Ca phases having an equivalent circle diameter of 1 μm or more exposed on the surface of the plated layer is 1 to 50 per 10,000 ^μm2 area.
[Group A] Ni: 0 to 1.0%
[Group B] 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% and Sr: 0-0.5%, Co: 0-0.5%, Bi: 0-0.5%, In: 0-0.5%, V: 0-0.5%, W: 0-0.5%, one or more of these, total 0-5%.
[ 2 ] The plated steel sheet according to [1], wherein the number density of Mg-Si-Zn-Al phases having a major axis of 2 μm or more exposed on the surface of the plated layer is 5 to 150 per 10,000 ^μm2 .
[ 3 ] In the chemical composition of the plating layer, Sn is 0.05 to 0.5 mass%,
The plated steel sheet according to [1] or [2] , wherein an Mg ₂ Sn phase is detected in the plated layer in an X-ray diffraction measurement of the plated layer.
[ 4 ] The plated steel sheet according to [1], wherein the plating layer has a chemical composition containing, in mass %, the Group A.
[ 5 ] The plated steel sheet according to [1], wherein the plating layer has a chemical composition containing, in mass %, the B group.
[ 6 ] The plated steel sheet according to [1], wherein the number density of Ca-Zn phases having an equivalent circle diameter of less than 1 μm exposed on the surface of the plated layer is 1 or more per 10,000 ^μm2 area.

According to each of the above aspects of the present invention, a plated steel sheet having both excellent planar corrosion resistance and paint adhesion can be provided.

1 is a cross-sectional schematic view of a plated steel sheet according to an embodiment of the present invention. FIG.

When a Ca-Zn phase crystallizes in a plating layer and is exposed on the surface of the plating layer, it may promote corrosion of the structure or phase surrounding the Ca-Zn phase (particularly a Ca-Zn phase with a circle equivalent diameter of 1 μm or more). For this reason, when a coating film is formed on a plating layer with an exposed Ca-Zn phase, the coating adhesion may be reduced due to the influence of the corrosion products formed around the Ca-Zn phase. The inventors have found that, in order to improve coating adhesion, it is necessary to prevent the crystallization of Ca-Zn phases with a circle equivalent diameter of 1 μm or more on the surface of the plating layer, which promotes corrosion of the surrounding metal structure or phase.

In order to prevent the crystallization of Ca-Zn phases with a circle equivalent diameter of 1 μm or more, it is sufficient to reduce the Ca content in the plating layer, but on the other hand, if the plating layer contains Ca, it is expected that the flat surface corrosion resistance will be improved. Therefore, if it is possible to suppress the crystallization of Ca-Zn phases with a circle equivalent diameter of 1 μm or more on the surface of the plating layer while containing Ca, it is expected that both flat surface corrosion resistance and paint adhesion can be improved.

Therefore, the inventors conducted extensive research to improve both the planar corrosion resistance and paint adhesion of a plating layer containing Al, Mg, Ca, and Zn, and discovered that forming a large amount of Ai-Si-Zn-Ca phase is effective in suppressing the crystallization of Ca-Zn phase with a circle equivalent diameter of 1 μm or more. It is presumed that if a large amount of Ai-Si-Zn-Ca phase is formed, the Ca contained in the plating layer is consumed during the formation of this phase, resulting in less crystallization of Ca-Zn phase with a circle equivalent diameter of 1 μm or more.

Further investigations by the inventors have revealed that by adjusting the manufacturing conditions of the plating layer, they have succeeded in crystallizing a large amount of Ai-Si-Zn-Ca phases on the surface of the plating layer and reducing the amount of Ca-Zn phases with a circular equivalent diameter of 1 μm or more. Reducing the number density of the Ca-Zn phases on the surface of the plating layer improves paint adhesion, and the inclusion of Ca also makes it possible to improve planar corrosion resistance.

Hereinafter, a plated steel sheet according to an embodiment of the present invention will be described.
The plated steel sheet of the present embodiment includes a steel sheet and a plating layer disposed on a surface of the steel sheet, and 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%, and Ca: 0.05 to 2.0%, and further contains one or two selected from the group consisting of Group A and Group B below, with the balance consisting of Zn and impurities, and the number density of Ca-Zn phases having a circle equivalent diameter of 1 μm or more exposed on the surface of the plating layer is 0 to 10 per 10,000 ^μm2 area, and the number density of Al-Si-Zn-Ca phases having a circle equivalent diameter of 1 μm or more exposed on the surface of the plating layer is 1 to 50 per 10,000 ^μm2 area.

[Group A] Ni: 0 to 1.0%
[Group B] 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%, one or more of these, total 0-5%

In the following explanation, the "%" indication of the content of each element in the chemical composition means "mass %". The content of an element in the chemical composition may be expressed as element concentration (e.g. Zn concentration, Mg concentration, etc.). "Plane corrosion resistance" refers to the property of the plating layer (specifically the Zn-Al-Mg alloy layer) itself to be resistant to corrosion. "Paint adhesion" refers to the property of a coating film formed on the plating layer that makes it difficult to peel off. "Plating layer" refers to a plating film produced by so-called hot-dip plating processing.

As shown in FIG. 1, the plated steel sheet 1 according to this embodiment has a steel sheet 11. There are no particular limitations on the shape of the steel sheet 11. The steel sheet 11 may also be a base steel sheet that has been formed into, for example, steel pipes, civil engineering and building materials (fences, corrugated pipes, drainage ditch covers, sand-flying plates, bolts, wire mesh, guardrails, water-stopping walls, etc.), home appliance components (casings for air conditioner outdoor units, etc.), and automobile parts (suspension components, etc.). The forming process is, for example, various plastic processing techniques such as pressing, roll forming, and bending.

There are no particular limitations on the material of the steel plate 11. The steel plate 11 can be, for example, various steel plates such as general steel, Al-killed steel, ultra-low carbon steel, high carbon steel, various high tensile steels, and some high alloy steels (steels containing strengthening elements such as Ni and Cr). The steel plate 11 may be a hot-rolled steel plate, hot-rolled steel strip, cold-rolled steel plate, and cold-rolled steel strip described in JIS G 3302:2010. There are also no particular limitations on the manufacturing method of the steel plate (hot rolling method, pickling method, cold rolling method, etc.) and the specific manufacturing conditions thereof.

As described later, a steel sheet 11 with adjusted surface roughness is used as the steel sheet to be plated. The surface roughness of the steel sheet can be adjusted, for example, by setting the surface of the rolling roll or the roll for the skin pass to a predetermined surface roughness and transferring the surface shape of the roll during rolling or the skin pass.

The plated steel sheet 1 according to this embodiment has a plated layer 12 disposed on the surface of the steel sheet 11. The plated layer 12 of the plated steel sheet 1 according to this embodiment is mainly composed of a Zn-Al-Mg alloy layer due to the chemical composition described below. The plated layer 12 of the plated steel sheet 1 according to this embodiment may also include an interface alloy layer containing Fe and Al as main components between the steel sheet 11 and the Zn-Al-Mg alloy layer. In other words, the plated layer 12 may have a single-layer structure of a Zn-Al-Mg alloy layer, or may have a laminated structure including a Zn-Al-Mg alloy layer and an interface alloy layer.

The chemical composition of the plating layer according to this embodiment is composed of Zn and other alloy elements. The chemical composition of the plating layer is described in detail below. Note that elements described as having a lower limit of concentration of 0% are optional elements that are not essential for solving the problems of the plated steel sheet according to this embodiment, but are allowed to be included in the plating layer for the purpose of improving characteristics, etc.

<Al: 10.0-30.0%>
Al contributes to improving the flat corrosion resistance, paint adhesion and workability. Therefore, the Al concentration is set to 10.0% or more. The Al concentration may be set to 11.0% or more, 12.0% or more, or 15.0% or more. On the other hand, if Al is excessive, the Mg concentration and Zn concentration are relatively decreased, and the paint adhesion is deteriorated. Therefore, the Al concentration is set to 30.0% or less. The Al concentration may be set to 24.0% or less, 22.0% or less, or 20.0% or less.

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

<Fe: 0.01% to 2.0%>
The Fe concentration may be 0%, but the plating layer may contain Fe at 0.01% or more. It has been confirmed that the performance of the plating layer is not adversely affected if the Fe concentration is 2.0% or less. 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 in from the base steel sheet, the Fe concentration may be 0.05% or more.

<Si: more than 0% to 2.0%>
Si contributes to improving the plane corrosion resistance. It is also necessary to crystallize 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 plane corrosion resistance and paint adhesion deteriorate. Therefore, the Si concentration is set to 2.0% or less. The Si concentration may be set to 1.8% or less, 1.6% or less, 1.2% or less, or 1.0% or less.

<Ca: 0.05% to 2.0%>
Ca is an element that contributes to improving the flat surface corrosion resistance, and can adjust the amount of Mg elution that is optimal for imparting flat surface corrosion resistance. Ca is also necessary to crystallize the Al-Si-Zn-Ca phase. Therefore, the Ca concentration is set to 0.05% or more. Ca may be 0.10% or more, or may be 0.20% or more. When Ca is set to 0.10%, the density of the Ca-Zn phase with a circle equivalent diameter of less than 1 μm tends 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 set to 1.0% or less.

Furthermore, the plating layer of this embodiment may contain one or two types selected from group A or group B below.

[Group A] Ni: 0 to 1.0%
[Group B] 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%, one or more of these, total 0-5%

<Ni: 0-1.0%>
The concentration of Ni in group A may be 0%. On the other hand, Ni contributes to improving 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 flat 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.

Furthermore, the plating layer according to this embodiment contains the following elements in group B: 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: One or more of the following elements may be contained: 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%, and W: 0-0.5%. The total of these elements is 0-5%. If the total exceeds 5%, the flat corrosion resistance or paint adhesion may decrease.

<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 improving paint adhesion. Therefore, the concentrations of Sb and Pb may 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 plane corrosion resistance deteriorates. Therefore, the concentrations of Sb and Pb are 0.5% or less. The concentrations of Sb and Pb may be 0.4% or less, 0.3% or less, or 0.25% or less.

<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 improving paint adhesion. Therefore, the concentrations of Cu, Ti, Cr, Nb, Zr, Mn, Mo, Ag and Li may each be 0.05% or more, 0.08% or more, or 0.10% or more. On the other hand, if the concentrations of Cu, Ti, Cr, Nb, Zr, Mn, Mo, Ag and Li are excessive, the plane corrosion resistance deteriorates. Therefore, the concentrations of Cu, Ti, Cr, Nb, Zr, Mn, Mo, Ag and Li are each 1.0% or less. The concentrations of Cu, Ti, Cr, Nb, Zr, Mn, Mo, Ag and Li may each be 0.8% or less, 0.7% or less, or 0.6% or less.

<Sn: 0-2.0%>
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, if the Sn concentration is excessive, the planar corrosion resistance deteriorates. Therefore, the Sn concentration is set to 2.0% or less. The Sn concentration may be 1.0% or less, 0.8% or less, or 0.5% or less.

<La, Ce, B, Y, P and Sr: 0 to 0.5% each>
The concentrations of La, Ce, B, Y, P and Sr may be 0%. On the other hand, La, Ce, B, Y, P and Sr contribute to improving coating adhesion. Therefore, the concentrations 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, if the concentrations of La, Ce, B, Y, P and Sr are excessive, the plane corrosion resistance deteriorates. Therefore, the concentrations of La, Ce, B, Y, P and Sr are 0.5% or less. The concentrations of La, Ce, B, Y, P and Sr may be 0.4% or less, 0.3% or less.

<Co, Bi, In, V, W: 0 to 0.5% each>
The concentrations of Co, Bi, In, V, and W may be 0%. On the other hand, Co, Bi, In, V, and W contribute to improving paint adhesion. Therefore, the concentrations 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, if the concentrations of Co, Bi, In, V, and W are excessive, the plane corrosion resistance deteriorates. Therefore, the concentrations of Co, Bi, In, V, and W are 0.5% or less. The concentrations of Co, Bi, In, V, and W may be 0.4% or less, or 0.3% or less.

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

The chemical composition of the plating layer is measured by the following method. First, an acid containing an inhibitor that suppresses corrosion of the steel sheet is used to peel off and dissolve the plating layer to obtain an acid solution. Next, the obtained acid solution is subjected to inductively coupled plasma (ICP) emission spectrometry. This makes it possible to obtain the chemical composition of the plating layer. There are no particular restrictions on the type of acid, so long as it is an acid that can dissolve the plating layer. Note that the chemical composition measured by the above-mentioned means is the average chemical composition of the entire plating layer.

Next, the metal structure of the plating layer will be described.
The surface of the plating layer according to this embodiment should not have Ca-Zn phases with a circle-equivalent diameter of 1 μm or more crystallized out as much as possible. The number density of Ca-Zn phases with a circle-equivalent diameter of 1 μm or more that is permissible on the surface is 0 to 10 per 10,000 μm ^2. When the Ca-Zn phase with a circle-equivalent 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 a circle-equivalent diameter of 1 μm or more in the early stage of corrosion of the plating layer, and the corrosion products reduce the paint adhesion. Therefore, it is preferable that the number density of Ca-Zn phases with a circle-equivalent diameter of 1 μm or more is low, and it is most preferable that the number density is 0 (pieces/10,000 μm ² ). If the number density exceeds 10 pieces per 10,000 μm ² , the paint adhesion is reduced, which is not preferable. The number density of Ca-Zn phases with a circle-equivalent diameter of 1 μm or more may be 5 or less, or 3 or less, when the unit is (pieces/10,000 μm ² ).

An electron probe microanalyzer (EPMA) is used to identify the Ca-Zn phase on the surface of the plating layer. The surface of the plating layer is observed using a scanning electron microscope attached to the EPMA to identify the intermetallic compound to be analyzed. Then, elemental analysis is performed on the identified intermetallic compound to determine whether the intermetallic compound is a Ca-Zn phase. The Ca-Zn phase is identified as an intermetallic compound containing 35 to 65 atomic % Ca and 35 atomic % or more Zn. The Ca-Zn phase may contain Mg, Al, Si, and Fe in the range of 10 atomic % or less each.

The crystallization of Ca-Zn phases having a circle-equivalent diameter of 1 μm or more is restricted because Ca-Zn phases having a circle-equivalent diameter of 1 μm or more adversely affect coating adhesion. On the other hand, Ca-Zn phases having a circle-equivalent diameter of less than 1 μm have almost no effect on coating adhesion, so they are not a problem in this embodiment. Ca-Zn phases having a circle-equivalent diameter of less than 1 μm are less likely to cause corrosion of the surrounding metal structure and phase. For this reason, it is preferable that Ca-Zn phases having a circle-equivalent diameter of less than 1 μm are formed on the plating layer surface. Therefore, it is preferable that the number density of Ca-Zn phases having a circle-equivalent diameter of less than 1 μm is 1 or more per 10,000 μm ² on the surface of the plating layer according to this embodiment. The number density of Ca-Zn phases having a circle-equivalent diameter of less than 1 μm may be 100 or less, or 50 or less, when the unit is (pieces/10,000 μm ² ). The number density of Ca-Zn phases having an equivalent circle diameter of less than 1 μm is more preferably 10 pieces/10,000 ^μm2 or less.

The circular equivalent diameter of the Ca-Zn phase is determined by determining the individual areas of the Ca-Zn phase when observed with a scanning electron microscope, and the diameter of the circle having that area is defined as the circular equivalent diameter of the Ca-Zn phase.

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. When the chemical composition of the plating layer contains Mg: 4.5 to 8 mass % and Si: 0.1 to 2 mass %, the number density of Ca-Zn phases with a circle equivalent diameter of 1 μm or more exposed on the surface of the plating layer may be 0 to 3 per 10,000 ^μm2 .

Furthermore, when the chemical composition of the plating layer contains Al, Mg, and Si in the range of 15 to 25 mass% Al, 4.5 to 8 mass% Mg, and 0.1 to 2 mass% Si, the number density of Ca-Zn phases having a circle equivalent diameter of 1 μm or more exposed on the surface of the plating layer may be 0 per 10,000 ^μm2 .

Next, an Al-Si-Zn-Ca phase having a circle equivalent diameter of 1 μm or more is exposed on the surface of the plating layer according to this embodiment. The number density of the Al-Si-Zn-Ca phase on the surface is 1 to 50 ^per 10,000 μm2. By exposing the Al-Si-Zn-Ca phase on the surface of the plating layer, Ca contained in the plating layer is consumed for the generation of this phase, and the crystallization of the Ca-Zn phase having a circle equivalent diameter of 1 μm or more is suppressed. This improves the paint adhesion. In this way, the planar corrosion resistance and paint adhesion of the plating layer can be improved.

An electron probe microanalyzer (EPMA) is used to identify the Al-Si-Zn-Ca phase in the plating layer. 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, elemental analysis is performed on the identified intermetallic compound to determine whether the intermetallic compound is an Al-Si-Zn-Ca phase. The Al-Si-Zn-Ca phase is identified as an intermetallic compound containing 20-80 atomic % Zn, 1-10 atomic % Si, 5-25 atomic % Ca, and the remainder being Al (Al: 10 atomic % or more) and 0-5 atomic % of other elements. The other elements other than Al, Si, Zn, and Ca may be any of the elements contained in the plating layer.

If the size of the Al-Si-Zn-Ca phase is small, the crystallization of the Ca-Zn phase cannot be effectively suppressed, so the size of the Al-Si-Zn-Ca phase that limits the number density needs to be 1 μm or more in terms of circle equivalent diameter. By having a number density of 1 (piece/10000 μm ² ) or more of the Al-Si-Zn-Ca phase with a circle equivalent diameter of 1 μm, the crystallization of the Ca-Zn phase with a circle equivalent diameter of 1 μm or more can be sufficiently suppressed. The circle equivalent diameter of the Al-Si-Zn-Ca phase is determined by determining the area of each 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 circle equivalent diameter of the Al-Si-Zn-Ca phase.

The number density of the Al-Si-Zn-Ca phase on the surface of the plating layer is 1 to 50 per 10,000 μm ^2. If the number density is less than 1 (piece/10,000 μm ² ), the crystallization of the Ca-Zn phase cannot be suppressed, resulting in insufficient paint adhesion. On the other hand, even if the number density of the Al-Si-Zn-Ca phase exceeds 50 (pieces/10,000 μm ² ), the effect of improving paint adhesion is saturated, so the upper limit is set to 50 (pieces/10,000 μm ² ). The number density of the Al-Si-Zn-Ca phase may be 3 or more, or 5 or more, when the unit is (pieces/10,000 μm ² ). The number density of the Al-Si-Zn-Ca phase may be 40 or less, 30 or less, or 20 or less.

Next, the surface of the plating layer according to this embodiment may have exposed Mg-Si-Zn-Al phases with a major axis of 2 μm or more. The number density of the Mg-Si-Zn-Al phases on the surface is 5 to 150 ^per 10,000 μm2. When the Mg-Si-Zn-Al phases are exposed on the surface of the plating layer, the Mg-Si-Zn-Al phases are corroded in the early stage of corrosion of the plating layer, forming dense corrosion products of Mg, Si, Zn, and Al. The formation of these corrosion products further improves the planar corrosion resistance of the plating layer.

An electron probe microanalyzer (EPMA) is used to identify the Mg-Si-Zn-Al phase in the plating layer. The surface of the plating layer is observed using a scanning electron microscope attached to the EPMA to identify the intermetallic compound to be analyzed. Then, elemental analysis is performed on the identified intermetallic compound to determine whether the intermetallic compound is an Mg-Si-Zn-Al phase. The Mg-Si-Zn-Al phase is identified as an intermetallic compound containing 20-45 atomic % Mg, 15-40 atomic % Si, 15-40 atomic % Zn, and 5-20 atomic % Al.

The number density of the Mg-Si-Zn-Al phase on the surface of the plating layer is affected by the average chemical composition of the plating layer. In order to set the number density of the Mg-Si-Zn-Al ^phase to 5 to 150 per 10,000 μm2, the chemical composition of the plating layer may be such that Al, Mg, and Si are 15 to 25 mass% Al, 4.5 to 8 mass% Mg, and 0.1 to 2 mass% Si.

The shape of the Mg-Si-Zn-Al phase is preferably 2 μm or more in length, and more preferably acicular with a long diameter of 2 μm or more. Furthermore, it is preferable that the aspect ratio is 2 or more. By making the shape of the Mg-Si-Zn-Al phase have a long diameter of 2 μm or more, the Mg-Si-Zn-Al phase is more likely to dissolve in the early stages of corrosion, and more dense corrosion products can be formed, thereby improving planar corrosion resistance.

The long axis of the Mg-Si-Zn-Al phase is the maximum length of the Mg-Si-Zn-Al phase when observed under an electron microscope. The aspect ratio is the ratio of the long axis to the short axis (long axis/short axis). The short axis is the length in the direction perpendicular to the long axis, and more specifically, the maximum length within a range of ±5° with respect to the direction perpendicular to the long axis.

The number density of the Mg-Si-Zn-Al phase on the surface of the plating layer is preferably 5 to 150 pieces per 10,000 μm ^2. By setting the number density to 5 (pieces/10,000 μm 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/10,000 μm ² ), the effect of improving the planar corrosion resistance is saturated, so the upper limit is set to 150 (pieces/10,000 μm 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/10,000 μm ² ). In addition, 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.

The Mg-Si-Zn-Al phase may be present on the surface of the plating layer at a number density of more than 0 and less than 5 per 10,000 ^μm2 area.

This section describes the method for measuring the number density of the Ca-Zn phase, Al-Si-Zn-Ca phase, and Mg-Si-Zn-Al phase. A square measurement area of 50 μm on each side is provided on the surface of the plating layer. Eight measurement areas are provided, and the eight measurement areas are randomly arranged on the surface of the plating layer. The measurement areas are spaced apart so as not to overlap each other. The set measurement areas are observed with a scanning electron microscope to confirm the presence of intermetallic compounds. Then, the composition of the intermetallic compounds is analyzed by EPMA to distinguish between Ca-Zn phase, Al-Si-Zn-Ca phase, and Mg-Si-Zn-Al phase. Furthermore, the number of Ca-Zn phases with a circle equivalent diameter of 1 μm or more, Al-Si-Zn-Ca phases with a circle equivalent diameter of 1 μm or more, and Mg-Si-Zn-Al phases with a major axis of 2 μm or more in each measurement area is counted. The measurement conditions for EPMA are, for example, an acceleration voltage of 15 kV, a current of 0.05 μA, and an exposure time of 50 ms. As the EPMA, for example, JXA-8230 manufactured by JEOL Ltd. is used.

It is possible that part of each of the Ca-Zn phase, Al-Si-Zn-Ca phase and Mg-Si-Zn-Al phase is within the measurement area and the remainder is outside the measurement area, but such phases are also included in the number measurement.

Furthermore, when the Mg-Si-Zn-Al phase is needle-shaped, multiple Mg-Si-Zn-Al phases may overlap each other. In such cases, when the major axis directions of the phases face in different directions, each of the overlapping phases is counted as one piece. For example, when two Mg-Si-Zn-Al phases overlap each other and their major axis directions face in different directions, the number of pieces is counted as two.

Then, based on the number of Ca-Zn phases with a circle equivalent diameter of 1 μm or more, Al-Si-Zn-Ca phases with a circle equivalent diameter of 1 μm or more, and Mg-Si-Zn-Al phases with a major axis of 2 μm or more measured in the eight measurement regions and the total area of the measurement regions, the number per 10,000 ^μm2 is determined as the number density.

In addition, when the plating layer contains 0.05 to 0.5 mass % Sn, it is preferable that the plating layer contains an Mg ₂ Sn phase. Since the amount of the Mg ₂ Sn phase is small, its presence is detected and confirmed by X-ray diffraction measurement. The inclusion of the Mg ₂ Sn phase in the plating layer further improves the corrosion resistance of the plating layer. Whether or not the plating layer contains an Mg ₂ Sn phase is determined by whether or not a diffraction peak specific to Mg ₂ Sn appears. Here, the diffraction peak specific to Mg ₂ Sn refers to, for example, a peak that appears at a diffraction angle 2θ of 23.4±0.3 degrees.

The coating weight of the plating layer per side may be, for example, within the range of 20 to 150 g/ ^m2 . By setting the coating weight per side to 20 g/m2 ^{or more} , the flat corrosion resistance and paint adhesion of the plated steel sheet can be further improved. On the other hand, by setting the coating weight per side to 150 g/m2 or ^less , the workability of the plated steel sheet can be further improved.

Next, a method for manufacturing the plated steel sheet according to this embodiment will be described, but the method for manufacturing the plated steel sheet according to this embodiment is not particularly limited. For example, the plated steel sheet according to this embodiment can be obtained according to the manufacturing conditions described below.

In the method for producing a plated steel sheet according to the present embodiment, a steel sheet with an adjusted surface roughness is annealed in a reducing atmosphere, and the steel sheet immediately after annealing is immersed in a hot-dip galvanizing bath and then pulled out to form a plated layer on the surface of the steel sheet. Next, cooling is performed by spraying a cooling gas until the temperature of the plated layer is reduced from the bath temperature to 300°C or less. The gas flux when spraying the cooling gas is set to a range of 100 to 5000 L/min/ ^m2 from the bath temperature to the controlled cooling temperature (gas flow rate in a temperature range from the controlled cooling temperature to the bath temperature) and a range of 10000 to 80000 L/min/ ^m2 from the controlled cooling temperature to the cooling stop temperature (300°C or less in this embodiment) (gas flow rate in a temperature range from the cooling stop temperature to less than the controlled cooling temperature).

The controlled cooling temperature is within the range of -10°C to -80°C relative to the crystallization temperature of the Al-Si-Zn-Ca phase.

The surface roughness of the steel sheet to be used as the base sheet for plating is such that the ratio (L _p /L ₀ ) of the curve length L _p of the roughness curve per the reference length L ₀ is 1.0 or more, and the arithmetic mean roughness Ra is 0.1 μm or more. Outside this range, a large amount of 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. The upper limit of (L _p /L ₀ ) is preferably 3.0 or less, and 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. There is no particular restriction on the adjustment of the surface roughness of the steel sheet, but it may be adjusted, for example, by rolling the base sheet for plating with a rolling roll or a roll for temper rolling whose roll surface has been adjusted to a desired roughness, and transferring the surface shape of the roll. It may also be adjusted by pickling.

The measurement of (L _p /L ₀ ) and the arithmetic mean roughness is performed, for example, using a shape measuring laser microscope (model number: VK-8700) manufactured by Keyence Corporation. The measurement is performed under the following measurement conditions: measurement mode: laser confocal, measurement quality: high accuracy, pitch: 0.75 μm, double scan: ON, optical zoom: 1x, objective lens name: Plan, γ coefficient: 0.45, offset: 0%. The measurement device used to measure (L _p /L ₀ ) 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 profile curve filters with cutoff values λc and λs to the cross-sectional curve obtained by measurement. Specifically, a roughness curve was obtained by removing components with a wavelength λc of 0.001 mm or less and components with a wavelength λs of 0.2 mm or more from the obtained measurement results. Based on the obtained roughness curve, (L _p /L ₀ ) and the arithmetic mean roughness were calculated.

Annealing of the steel sheet to be plated is carried out in a reducing atmosphere. There are no particular restrictions on the reducing atmosphere and annealing conditions. This annealing removes as much oxide as possible from the steel sheet surface.

Next, the steel sheet immediately after annealing is immersed in a hot-dip galvanizing bath. The chemical composition of the hot-dip galvanizing bath may be adjusted appropriately so as to obtain the chemical composition of the plating layer described above. The temperature of the hot-dip galvanizing bath is not particularly limited, and a temperature at which hot-dip galvanizing can be performed may be appropriately selected. For example, the temperature of the galvanizing bath may be set to a value approximately 20°C or more higher than the melting point of the galvanizing bath.

Next, the steel sheet is pulled out of the hot-dip galvanizing bath. The adhesion weight of the plating layer can be controlled by controlling the pulling speed of the steel sheet. If necessary, the adhesion weight of the plating layer may be controlled by wiping the steel sheet to which the plating layer is attached. The adhesion weight of the plating layer is not particularly limited and can be within the above-mentioned range, for example.

The plating layer is then cooled. Cooling is performed by spraying cooling gas onto the steel sheet immediately after it has been pulled out of the hot-dip plating bath. Cooling by spraying cooling gas is performed continuously until the temperature of the steel sheet drops from the bath temperature to 300°C. There are no particular limitations on the cooling conditions below 300°C, and cooling by spraying cooling gas may be continued or the steel sheet may be allowed to cool naturally.

Cooling by spraying cooling gas is performed by arranging a cooling zone along the steel plate transport path. The cooling zone is equipped with multiple spray nozzles for cooling gas. The shape of the gas nozzle from which the cooling gas is sprayed is, for example, in the range of 1 to 50 mm in diameter. The angle between the tip of the gas nozzle and the steel plate is, for example, in the range of 70 to 110°, and more preferably 90° (right angle). The distance between the tip of the gas nozzle and the steel plate 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.

There are no particular limitations on the cooling gas to be sprayed, and it may be a non-oxidizing gas such as nitrogen, an inert gas such as argon, or air, or a mixture of these.

In this embodiment, the gas flux when blowing the cooling gas is controlled in two stages. That is, based on the temperature of the steel sheet, the gas flux from the plating bath temperature to the controlled cooling temperature (a temperature in the range of -10 to -80°C relative to the crystallization temperature of the Al-Si-Zn-Ca phase) is set to a range of 100 to 5000 L/min/ ^m2 , preferably 500 to 5000 L/min/ ^m2 , and the gas flux from the controlled cooling temperature to 300°C or lower is set to a range of 10000 to 80000 L/min/ ^m2 . The controlled cooling temperature is a temperature estimated to be the crystallization start temperature of the Al-Si-Zn-Ca phase.

When the gas flux is in the range of 5000 L/min/m2 or ^less , it is possible to suppress vibration being applied to the steel sheet during cooling. On the other hand, when the gas flux is in the range of 10000 L/min/m2 or ^more , it is possible to apply vibration to the steel sheet during cooling.

Then, by setting the gas flux from the coating bath temperature to the controlled cooling temperature in the range of 100 to 5000 L/min/m ² , preferably in the range of 500 to 5000 L/min/m ² , nucleation of Ca-containing phases other than the Al-Si-Zn-Ca phase is promoted without vibrating the steel sheet, and Ca and Si are concentrated in the unsolidified liquid phase. Next, by setting the gas flux from the controlled cooling temperature to 300°C or less in the range of 10000 to 80000 L/min/m ² , vibration is applied to the surface of the unsolidified liquid phase, and a large amount of Al-Si-Zn-Ca phase can be crystallized on the surface of the coating layer. If the gas flux range is outside the above range, it becomes difficult to crystallize a large amount of Al-Si-Zn-Ca phase on the surface of the coating layer.

The crystallization temperature of the Al-Si-Zn-Ca phase varies depending on the chemical composition of the plating layer, and is therefore calculated using a calculated phase diagram. Specifically, a calculated phase diagram database is constructed that accumulates thermodynamic data on intermetallic compound phases and metal phases that may be contained in Al-Mg-Zn alloys, and calculations are performed using the CALPHAD (CALculation of Phase Diagram) method 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 using thermodynamic equilibrium calculation software "Thermo-Calc" (Thermo-Calc is a registered trademark of Thermo-Calc Software). The thermodynamic equilibrium calculation software used for the calculation is not limited to "Thermo-Calc" (registered trademark), and other software may be used. The controlled cooling temperature is set to a temperature within the range of -10 to -80°C with respect to the crystallization temperature of the obtained Al-Si-Zn-Ca phase.

In the above manufacturing method, the surface roughness of the steel sheet surface is adjusted in advance to suppress nucleation of the Al-Si-Zn-Ca phase, thereby suppressing crystallization of the Al-Si-Zn-Ca phase inside the coating layer. Hot-dip coating is performed on such a steel sheet, and the cooling conditions after coating are controlled as described above, causing a large amount of Al-Si-Zn-Ca phase to crystallize on the surface of the coating layer. It is presumed that this makes it possible to form a large number of Al-Si-Zn-Ca phases with a circle equivalent diameter of 1 μm or more on the surface of the coating layer, while suppressing the crystallization of Ca-Zn phases with a circle equivalent diameter of 1 μm or more.

As long as the requirements of the present invention are met, the manufacturing method of plated steel sheet is not limited to the above, and electroplating, vapor deposition plating, thermal spraying, cold spraying, etc. may be used instead of hot-dip plating.

Below, an embodiment of the present invention will be described. However, the conditions in the embodiment are merely one example of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to this one example of conditions. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

Cold-rolled steel sheets (0.05C-0.1Si-0.2Mn) with a thickness of 1.2 mm were used as the original sheets for plating. The surface roughness of some of the original sheets for plating was controlled using a skin pass mill or the like. The steel sheets with the adjusted surface roughness were then annealed. After annealing, the steel sheets were immersed in various hot-dip plating baths and then pulled out to attach a plating layer to the steel sheet surface. Next, various plated steel sheets were produced by cooling using cooling gas from immediately after pulling out of the plating bath until the plating layer reached 300°C.

The surface roughness of the steel sheet surface to be plated was set so that the ratio of the curve length _Lp of the roughness curve per reference length _L0 ( _Lp / _L0 ) was 1.1 to 2.6, and the arithmetic mean roughness Ra was in the range of 0.6 to 3.7 μm.

The (L _p /L ₀ ) and arithmetic mean roughness were measured using a shape measuring laser microscope (model number: VK-8700) manufactured by Keyence Corporation. The measurement conditions were as follows: measurement mode: laser confocal, measurement quality: high precision, pitch: 0.75 μm, double scan: ON, optical zoom: 1x, objective lens name: Plan, γ coefficient: 0.45, offset: 0%.

The annealing conditions for the steel sheet in a reducing atmosphere were a soaking temperature of 600°C and a soaking time of 10 seconds. The annealing atmosphere was a reducing atmosphere consisting of a mixed gas of 5% hydrogen and the balance nitrogen. The annealed steel sheet was then air-cooled with nitrogen gas until the immersion sheet temperature reached the bath temperature + 20°C, after which it was immersed in a hot-dip galvanizing bath and then pulled up. The pulling speed was 20 to 200 mm/sec.

The chemical composition of the hot-dip plating layer was as shown in Table 1. The manufacturing conditions were as shown in Table 2. The metal structure of the plating layer was also evaluated, and the results are shown in Table 3. Furthermore, the flat corrosion resistance and paint adhesion of the plated steel sheet were evaluated, and the results are shown in Table 3.

The chemical composition and metal structure of the plating layer were evaluated using the methods described above. The Ca-Zn phases measured had a circular equivalent diameter of 1 μm or more and those with a circular equivalent diameter of less than 1 μm. The Al-Si-Zn-Ca phases measured had a circular equivalent diameter of 1 μm or more. The Mg-Si-Zn-Al phases measured had a major axis of 2 μm or more. All of the measured Mg-Si-Zn-Al phases had an aspect ratio of 2 or more.

Plane corrosion resistance was evaluated as follows. The resulting plated steel sheets were cut into 100 mm x 50 mm pieces and subjected to a planar corrosion resistance evaluation test. Plane corrosion resistance was evaluated using an accelerated corrosion test specified in JASO-CCT-M609, and corrosion weight loss was compared after 150 cycles. The evaluation criteria were as follows, with "AAA", "AA" and "A" being considered pass.

AAA: Corrosion weight loss less than 50g/ ^m2 AA: Corrosion weight loss 50g/ ^m2 or more and less than 90g/ ^m2 A: Corrosion weight loss 90g/m2 or ^more and less than 120g/ ^m2 B: Corrosion weight loss 120g/m2 or ^more

Paint adhesion was evaluated as follows. The resulting plated steel sheets were cut into 100 mm x 50 mm pieces and used for paint adhesion tests. After a coating layer was formed on the test pieces, the end faces and back surface were sealed with silicone resin.

Next, cut scratches were made on the front surface with a cutter knife, reaching down to the base steel. The cut scratches were made in a lattice pattern with 1 mm intervals. In this way, 100 areas of 1 mm square separated by cut scratches were formed. The specimens were then continuously immersed in a 5% NaCl aqueous solution at 50°C for 500 hours, after which they were washed with water and dried. After drying, adhesive tape was applied to the cut scratch areas of the specimens and then peeled off, and the peeled area ratio was measured.

<Coating layer>
A coating layer was formed by applying the primer coating resin and topcoat coating resin described below to the surface of the plating layer. The thickness of the layer made of the primer coating resin was 5 μm, and the thickness of the layer made of the topcoat coating resin was 15 μm, for a total thickness of 20 μm.

<Film-forming components of coating layer>
(1) Primer paint resin on the front and back surfaces: Polyester/melamine + isocyanate combined curing type (Fine Coatings FLC687 paint resin)
(2) Topcoat paint resin on the front surface: High molecular weight polyester/melamine curing type (Fine Coatings FLC7000 paint resin)
(3) Back side top coat paint resin: polyester/melamine curing type (Fine Coatings FLC100HQ paint resin)

The evaluation criteria for paint adhesion are shown below. Evaluation was based on the following scoring system. "AAA", "AA" and "A" were considered pass.

AAA: Peeled area ratio less than 10% AA: Peeled area ratio 10-20% A: Peeled area ratio 20-30% B: Peeled area ratio 30% or more

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 coating weight of the plating layer per side in the examples was in the range of 20 to 150 g/ ^m2 .

In Comparative Example 31, the amount of Al in the plating layer was insufficient. As a result, in Comparative Example 31, the crystallization temperature of the Ca-Zn phase increased, and a large amount of Ca-Zn phase with a circle equivalent diameter of 1 μm crystallized. This resulted in insufficient flat surface corrosion resistance.

In Comparative Example 32, the amount of Al in the plating layer was excessive. As a result, in Comparative Example 32, a large amount of Ca-Zn phase with a circle equivalent diameter of 1 μm in which Al was dissolved crystallized. This resulted in a decrease in paint adhesion.

In Comparative Example 33, the amount of Mg in the plating layer was insufficient. As a result, in Comparative Example 33, the crystallization temperature of the Ca-Zn phase increased, and a large amount of Ca-Zn phase with a circle equivalent diameter of 1 μm crystallized. This resulted in a decrease in flat surface corrosion resistance and paint adhesion.

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

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 rather than an Al-Si-Zn-Ca phase, which meant that the crystallization of a Ca-Zn phase with a circle equivalent diameter of 1 μm was not suppressed, and the flat surface corrosion resistance and paint adhesion were reduced.

In Comparative Example 36, the amount of Ca in the hot-dip plating layer was excessive. As a result, in Comparative Example 36, Ca-Zn phases with a circle equivalent diameter of 1 μm were excessively crystallized on the surface of the plating layer, resulting in reduced planar corrosion resistance and paint adhesion.

In Comparative Example 37, the cooling gas flux from the bath temperature to the controlled cooling temperature was excessive. As a result, in Comparative Example 37, the nucleation of Ca-Zn phases with a circle equivalent diameter of 1 μm progressed on the surface of the plating layer due to the influence of vibration, and the flat corrosion resistance and paint adhesion decreased.

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

The plated steel sheet disclosed herein has excellent both planar corrosion resistance and paint adhesion, making it highly applicable industrially.

1...plated steel plate, 11...steel plate, 12...plated layer.

Claims (6)

The present invention comprises a steel sheet and a plating layer disposed on a surface of the steel sheet,
The chemical composition of the plating layer is, in mass%,
Al: 10.0-30.0%,
Mg: 3.0 to 15.0%,
Fe: 0.01-2.0%,
Si: more than 0 to 2.0%,
Ca: 0.05 to 2.0%;
Further, it contains one or two selected from the group consisting of the following Group A and Group B,
The balance is Zn and impurities,
the number density of Ca-Zn phases having a circle equivalent diameter of 1 μm or more exposed on the surface of the plating layer is 0 per 10,000 ^μm2 area;
The chemical composition of the plating layer is such that Al, Mg, and Si are 15 to 25 mass% Al, 4.5 to 8 mass% Mg, and 0.1 to 2 mass% Si,
A plated steel sheet, characterized in that a number density of Al-Si-Zn-Ca phases having an equivalent circle diameter of 1 μm or more exposed on the surface of the plated layer is 1 to 50 per 10,000 ^μm2 area.
[Group A] Ni: 0 to 1.0%
[Group B] 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%, one or more of these, total 0-5% 2. The plated steel sheet according to claim 1, wherein a number density of Mg—Si—Zn—Al phases having a major axis of 2 μm or more exposed on the surface of the plated layer is 5 to 150 per 10,000 ^μm2 . In the chemical composition of the plating layer, Sn is 0.05 to 0.5 mass %,
The plated steel sheet according to claim 1 or 2 , wherein an Mg ₂ Sn phase is detected in the plated layer by X-ray diffraction measurement of the plated layer. The plated steel sheet according to claim 1, wherein the plating layer has a chemical composition containing, by mass%, the A group. The plated steel sheet according to claim 1, wherein the plating layer has a chemical composition containing, by mass%, the B group. 2. The plated steel sheet according to claim 1, wherein a number density of Ca-Zn phases having an equivalent circle diameter of less than 1 μm exposed on the surface of the plated layer is 1 or more per 10,000 ^μm2 area.

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