TWI903471B - Coated steel - Google Patents

Abstract

This invention employs a plated steel material having a plated layer disposed on the surface of a base steel material. The plated layer, by mass percent, contains Al: 5.0–40.0%, Mg: 0.5–15.0%, Fe: 5.0–40.0%, with the remainder being Zn and impurities. In a cross-section perpendicular to the surface of the plated steel material, a section of the plated steel material of a predetermined length parallel to the surface is taken as the cross-section. When observing the area, the length L of the boundary line between the coating and the base steel satisfies the following formula (1). The coating includes a first area with an Fe concentration of less than 5.0% by mass, a second area with an Fe concentration of 5.0% to less than 30.0% by mass, and a third area with an Fe concentration of 30.0% to less than 80.0% by mass. In the first area, there is more than 0% but less than 5% Al-containing phase by area. (L－ _L0 )/ _L0 ×100≧2.0(%)…(1) _L0 is the straight-line distance between one end and the other end of the boundary line in the observation area, and L is the length of the boundary line between one end and the other end.

Description

Coated steel

Invention field

This invention relates to a plated steel material.

Invention Background

For example, steel structures are used in civil engineering and infrastructure. Steel structures are exposed to harsh corrosive environments, such as coastal areas and areas exposed to desalination spray. Therefore, stainless steel is used to inhibit corrosion and maintain the steel structure over the long term.

On the other hand, stainless steel uses high-cost alloying elements such as Cr and Ni. Therefore, the construction of steel structures utilizing stainless steel presents a high-cost problem. Consequently, pre-coated products (e.g., Zn-Al-Mg based coated steel) are gradually being used to replace stainless steel.

However, the plating on large steel structures or tubular steel structures can sometimes disappear due to welding, or partially due to corrosion from the cut ends. Furthermore, steel parts such as bolts and washers are inherently difficult to manufacture from sheet metal.

In light of the above, large steel structures, tubular steel structures, bolts, washers, and other steel parts are generally manufactured by applying a post-plating treatment (so-called hot-dip galvanizing) to steel that has been processed into a specified shape.

Hot-dip Zn plating is widely used as a post-plating treatment, but recently, Zn-Al-Mg plating has also begun to be used to improve corrosion resistance.

Patent document 1 describes a plated steel material having steel and a plating layer; the plating layer comprises: a surface plating layer disposed on the surface of the steel and composed of a Zn-Al-Mg alloy layer with an Fe concentration of less than 3% by mass; and an intermediate plating layer disposed between the steel and the aforementioned surface plating layer and composed of a Zn-Al-Mg alloy layer with an Fe concentration of 3% by mass or more but less than 30% by mass, and the layer thickness being 3 μm or more; An interface alloy layer is disposed between the steel and the intermediate coating layer; the combined thickness of the surface coating layer and the aforementioned intermediate coating layer is 8 μm or more but less than 300 μm; the average chemical composition of the coating layer, by mass%, consists of Zn: more than 65.00%, Al: more than 6.5% but less than 22.5%, Mg: more than 3.0% but less than 12%, and impurities; the Mg concentration of the intermediate coating layer, by mass%, exceeds 3.0%.

[Previous Technical Documents]

[Patent Documents]

[Patent Document 1] Japanese Patent Application Publication No. 2021-4403

Invention Summary

Recently, research has been conducted on using hot-dip galvanized steel not only in civil engineering and infrastructure but also as a material for automotive parts. When using galvanized steel as a material for automotive parts, there is a need to improve the adhesion of the coating, post-coating corrosion resistance, base metal corrosion resistance, and sacrificial corrosion resistance.

The plated steel described in Patent Document 1 exhibits excellent resistance to red rust, but it makes no mention of the adhesion of the plating layer, post-coating corrosion resistance, corrosion resistance of the base steel, or sacrificial corrosion resistance.

Therefore, the present invention addresses the problem of providing a plated steel with excellent adhesion, post-coating corrosion resistance, red rust resistance, base iron corrosion resistance, and sacrificial corrosion resistance.

To solve the above problems, this invention adopts the following structure.

[1] A plated steel material having a base steel material and a plated layer disposed on the surface of the base steel material, wherein the chemical composition of the plated layer, by mass%, contains Al: 5.0~40.0%, Mg: 0.5~15.0%, Fe: 5.0~40.0%, Si: 0~2.0%, Ca: 0~2.0%, and further contains one or two of the groups selected from the following A group and B group, with the remainder being Zn and impurities. In a cross section perpendicular to the surface of the plated steel material, when a cross section of the plated steel material of a predetermined length parallel to the surface is used as the observation area, the length L of the boundary line between the plated layer and the base steel material satisfies the following formula (1). The plating layer comprises: a first region disposed on the aforementioned surface side of the plated steel and having an Fe concentration of less than 5.0% by mass; a second region adjacent to the first region and having an Fe concentration of 5.0% by mass or more but less than 30.0% by mass; and a third region disposed between the second region and the parent steel and having an Fe concentration of 30.0% by mass or more but less than 80.0% by mass. The thickness of the first region is 5 to 100 μm, the thickness of the second region is 5 to 100 μm, and the thickness of the third region is 5 to 100 μm. The first region contains 0% to 5% Al-containing phase by area, and the Al-containing phase contains Zn and 20 to 99% by mass of Al.

[Group A] Ni: 0~1.0%.

[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, totaling 0~5%).

(LL ₀ )/L ₀ ×100≧2.0(%)...(1)

In Equation (1), _L0 is the straight-line distance between one end and the other end of the aforementioned boundary line in the aforementioned observation area, and L is the length of the aforementioned boundary line between the aforementioned one end and the aforementioned other end.

[2] The plated steel described in [1] satisfies the following formula (2).

(LL ₀ )/L ₀ ×100≧4.0(%)...Equation (2)

In Equation (2), _L0 is the straight-line distance between one end and the other end of the aforementioned boundary line in the aforementioned observation area, and L is the length of the aforementioned boundary line between the aforementioned one end and the aforementioned other end.

[3] The plated steel as described in [1] satisfies the following formula (3).

(LL ₀ )/L ₀ ×100≧6.0(%)...Equation (3)

In equation (3), _L0 is the straight-line distance between one end and the other end of the aforementioned boundary line in the aforementioned observation area, and L is the length of the aforementioned boundary line between the aforementioned one end and the aforementioned other end.

[4] The plated steel as described in any of [1] to [3], wherein the thickness of the aforementioned first region is 15 μm to 100 μm.

[5] The plated steel described in any of [1] to [3], wherein the area fraction of the aforementioned Al-containing phase in the first region does not reach 1%.

[6] A plated steel as described in any of [1] to [3], wherein the first region described above contains: a Mg-Zn phase comprising Zn and 20 to 60% by mass of Mg, wherein the area fraction of the Mg-Zn phase in the first region is 2% or more.

[7] A plated steel as described in any of [1] to [3], wherein the aforementioned first region contains a binary eutectic structure of Zn phase and Mg-Zn phase, and the area fraction of the aforementioned binary eutectic structure in the aforementioned first region is 1% or more.

[8] The plated steel as described in any of [1] to [3], wherein the thickness of the aforementioned second region is 15 μm to 100 μm.

[9] A plated steel as described in any of [1] to [3], wherein the aforementioned second region contains: a Mg-Zn phase comprising Zn and 20 to 60% by mass of Mg, and the area fraction of the aforementioned Mg-Zn phase in the aforementioned second region is 5% or more.

[10] The plated steel as described in any of [1] to [3], wherein the aforementioned second region contains an Fe-Al alloy phase with an equivalent circular diameter of 15 μm or less and an aspect ratio of 2 or more, and the area fraction of the aforementioned Fe-Al alloy phase in the aforementioned second region is 5% or more.

[11] A plated steel as described in any of [1] to [3], wherein the aforementioned second region contains a binary eutectic structure of Zn phase and Mg-Zn phase, and the area fraction of the aforementioned binary eutectic structure in the aforementioned second region is 2% or more.

[12] The plated steel as described in any of [1] to [3], wherein the thickness of the aforementioned third region is 15 μm to 100 μm.

[13] A plated steel as described in any of [1] to [3], wherein the aforementioned third region contains: a Mg-Zn phase comprising Zn and 20 to 60% by mass of Mg, wherein the area fraction of the aforementioned Mg-Zn phase in the aforementioned third region is 10% or more.

[14] The plated steel as described in any of [1] to [3], wherein the chemical composition of the aforementioned plating layer contains Mg at a rate of 5.0% or more relative to the total amount of Zn and Mg (Mg/(Zn+Mg)(%)).

[15] The plated steel as described in any of [1] to [3], wherein the chemical composition of the aforementioned plating layer contains Mg at a rate of 6.5% or more relative to the total amount of Zn and Mg (Mg/(Zn+Mg)(%)).

[16] The plated steel as described in any of [1] to [3], wherein the aforementioned plated layer contains Sn at a concentration of 0.02 to 2.0% by mass, and the aforementioned plated layer contains _Mg2Sn phase.

According to the present invention, a plated steel with excellent adhesion, post-coating corrosion resistance, red rust resistance, base iron corrosion resistance, and sacrificial corrosion resistance can be provided.

1: Plated steel

1a: Surface

11: Mother Steel

12: Coating layer

12A: Area 1

12B: Area 2

12C: Region 3

13: Divider (Interface)

13a: One end

13b: One end

_L0 : Distance

L: Length

A: Analysis line

B: Analysis Line

F: Analysis Line

C: Analysis Line

D: Analysis Line Analysis Line

F5: Analysis Line

F30: Analysis Line

M1: Region

M2: Area

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

Figure 2 is a schematic cross-sectional view of the plated steel according to an embodiment of the present invention, and also shows the observation area.

Figure 3 is a schematic cross-sectional view of the plated steel according to an embodiment of the present invention, and also illustrates the method for determining the extent of the first and second regions.

Figure 4 is a schematic cross-sectional view of the plated steel according to an embodiment of the present invention, and also illustrates the method for determining the extent of the first and second regions.

Figure 5 is a schematic cross-sectional view of the plated steel according to an embodiment of the present invention, and is an enlarged view of region M1 in Figure 4.

Figure 6 is a schematic cross-sectional view of the plated steel according to an embodiment of the present invention, and is an enlarged view of region M2 in Figure 4.

The form in which the invention is implemented.

The inventors have conducted research to provide coated steel with excellent adhesion, post-coating corrosion resistance, red rust resistance, base iron corrosion resistance, and sacrificial corrosion resistance.

The plated steel described in Patent 1 sometimes has an interfacial alloy layer with a thickness of 1μm to 5μm. In the plated steel described in Patent 1, if a thicker interfacial alloy layer is formed, the adhesion or processability of the plated layer will be greatly reduced, thus limiting the thickness of the interfacial alloy layer to below 5μm. However, the inventors have discovered that layers containing more Fe, such as interfacial alloy layers, can improve the corrosion resistance of the base iron through a barrier effect. Therefore, the inventors have successfully improved the corrosion resistance of the base steel without reducing the adhesion of the coating by forming a thicker layer containing more Fe between the Mg-Al-Zn alloy layer and the base steel, and by controlling the interface morphology between the Fe-rich layer and the base steel. This further improves the post-coating corrosion resistance, red rust resistance, and sacrificial corrosion resistance.

More specifically, the inventors have discovered that by applying strain to the surface of the base steel before forming the coating layer using hot-dip galvanizing, and by controlling the cooling conditions after removal from the galvanizing bath to control the morphology of the interface between the base steel and the coating layer, a coated steel with excellent corrosion resistance, coating adhesion, red rust resistance, base iron corrosion resistance, and sacrificial corrosion resistance can be obtained. Furthermore, the inventors have discovered that by suppressing the precipitation of Al-containing phases on the surface of the coating layer and suppressing the Fe concentration to a lower level, the corrosion resistance and base iron corrosion resistance after coating can be further improved.

The following describes the coated steel according to an embodiment of the present invention.

The plated steel of this embodiment comprises a base steel and a plating layer disposed on the surface of the base steel. The chemical composition of the plating layer, by mass%, contains Al: 5.0~40.0%, Mg: 0.5~15.0%, Fe: 5.0~40.0%, Si: 0~2.0%, Ca: 0~2.0%, and further contains one or two of the elements selected from the groups A and B below, with the remainder being Zn and impurities. In a cross-section perpendicular to the surface of the plated steel, when a cross-section of the plated steel of a specified length parallel to the surface is used as the observation area, the length L of the boundary line between the plating layer and the base steel satisfies the following formula (1). The plating layer comprises: a first region disposed on the surface of the plated steel with an Fe concentration of less than 5.0% by mass; a second region adjacent to the first region with an Fe concentration of 5.0% by mass or more but less than 30.0% by mass; and a third region disposed between the second region and the parent steel with an Fe concentration of 30.0% by mass or more but less than 80.0% by mass. The thickness of the first region is 5 to 100 μm, the thickness of the second region is 5 to 100 μm, and the thickness of the third region is 5 to 100 μm. The first region contains 0% to 5% Al-containing phase by area, and the Al-containing phase contains Zn and 20 to 99% by mass of Al.

[Group A] Ni: 0~1.0%.

[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, totaling 0~5%).

(LL ₀ )/L ₀ ×100≧2.0(%)...(1)

In equation (1), _L0 is the straight-line distance between one end and the other end of the boundary line in the observation area, and L is the length of the boundary line between one end and the other end.

Furthermore, the plated steel in this embodiment should preferably meet the following formula (2).

(LL ₀ )/L ₀ ×100≧4.0(%)...Equation (2)

Furthermore, the plated steel in this embodiment should preferably meet the following formula (3).

(LL ₀ )/L ₀ ×100≧6.0(%)...Equation (3)

In equations (2) and (3) _{, L0} is the straight-line distance between one end of the boundary line and the other end in the observation area, and L is the length of the boundary line between one end and the other end.

Furthermore, the thickness of the first region should preferably be 15 μm to 100 μm.

Furthermore, the area ratio of Al-containing phases within region 1 should preferably be less than 1%.

Furthermore, the first region should preferably contain a Mg-Zn phase comprising Zn and 20-60% by mass of Mg, and the area fraction of the Mg-Zn phase in the first region should be 2% or more.

Furthermore, the first region should preferably contain a binary eutectic structure of Zn phase and Mg-Zn phase, and the area fraction of the binary eutectic structure in the first region should be 1% or more.

Furthermore, the thickness of the second region should preferably be 15μm to 100μm.

Furthermore, the second region should preferably contain a Mg-Zn phase comprising Zn and 20-60% by mass of Mg, and the area fraction of the Mg-Zn phase in the second region should be 5% or more.

Furthermore, the second region should preferably contain an Fe-Al alloy phase with an equivalent circular diameter of 15 μm or less and an aspect ratio of 2 or more, and the area fraction of the Fe-Al alloy phase in the second region should be 5% or more.

Furthermore, the second region should preferably contain a binary eutectic structure of Zn and Mg-Zn phases, and the area fraction of the binary eutectic structure in the second region should be 2% or more.

Furthermore, the thickness of the third region should preferably be 15 μm to 100 μm.

Furthermore, the third region should preferably contain a Mg-Zn phase comprising Zn and 20-60% by mass of Mg, and the area fraction of the Mg-Zn phase in the third region should be 10% or more.

Furthermore, in the chemical composition of the coating, the content of Mg relative to the total amount of Zn and Mg (Mg/(Zn+Mg)(%)) should preferably be 5.0% or more.

Furthermore, in the chemical composition of the coating, the content of Mg relative to the total amount of Zn and Mg (Mg/(Zn+Mg)(%)) should preferably be 6.5% or more.

Furthermore, the coating should preferably contain Sn at a concentration of 0.02% to 2.0% by mass, and the coating should contain the Mg2Sn phase.

In the following explanation, the "%" for the content of each element in the chemical composition means "mass %". The content of elements in the chemical composition is sometimes expressed as elemental concentration (e.g., Zn concentration, Mg concentration, etc.).

"Adhesion of the coating" refers to the property of the coating not easily peeling off.

"Post-coating corrosion resistance" refers to the property of the coating itself to resist corrosion when a coating has been applied to its surface.

"Red rust resistance" refers to the property of inhibiting the formation of red rust in the plating layer during corrosion.

"Base corrosion resistance" refers to the inherent resistance of the plating coating to corrosion.

"Sacrificial corrosion resistance" refers to the property of the base steel to suppress corrosion in exposed areas (such as the cut ends of the plated steel, cracks in the plating layer during processing, and areas exposed due to plating peeling).

As shown in Figure 1, the plated steel 1 of this embodiment has a master steel 11. The shape of the master steel 11 is not particularly limited; for example, examples of the master steel 11 include steel plates, angle steel with an L-shaped cross-section, and porous metals. Furthermore, the master steel 11 can also be, for example, a steel pipe, civil engineering materials (gratings, corrugated pipes, drainage ditch covers, anti-sand plates, bolts, wire mesh, guard rails, waterproof walls, etc.), household appliance components (air conditioner outdoor unit housings, etc.), automotive parts (chassis components, etc.), etc., formed and processed master steel. Additionally, the master steel 11 can also be a shaped product made by forming a steel plate into a specified shape. Furthermore, the mother steel 11 can also be manufactured by fusing two or more formed steel plates, each formed into a specified shape, together to create a shape such as that of an automotive part. Forming processes include various plastic forming methods such as stamping, pressing, and bending.

There are no particular restrictions on the material of the master steel 11. For example, the master steel 11 can be made of ordinary steel, all-static aluminum steel, ultra-low carbon steel, high carbon steel, various high-tensile steels, and some high-alloy steels (including steels containing strengthening elements such as Ni and Cr). The master steel 11 can also be manufactured into hot-rolled steel plates, hot-rolled steel strips, cold-rolled steel plates, and cold-rolled steel strips as described in JIS G 3302:2010. There are no particular restrictions on the manufacturing methods (hot rolling, pickling, cold rolling, etc.) and specific manufacturing conditions of the steel plates.

Furthermore, the master steel 11, which serves as the base plate for plating, can also be a pre-plated steel with a pre-plated coating formed on its surface. An example of a pre-plated steel is a Ni pre-plated steel with a Ni plating formed on its surface. The pre-plated steel can be obtained, for example, by electrolytic treatment or replacement plating. Electrolytic treatment is performed by immersing the master steel 11 in a sulfuric acid bath or chloride bath containing metal ions of various pre-plating components. Replacement plating is performed by immersing the master steel in an aqueous solution containing metal ions of various pre-plating components, with the pH adjusted by sulfuric acid, to perform metal replacement precipitation.

The plated steel 1 of this embodiment has a plating layer 12 disposed on the surface of a base steel 11. The plating layer 12 comprises, as described below, a first region 12A with an Fe concentration of less than 5.0% by mass, a second region 12B with an Fe concentration of 5.0% by mass or more but less than 30.0% by mass, and a third region 12C with an Fe concentration of 30.0% by mass or more but less than 80.0% by mass. The plating layer 12 comprising the first region 12A, the second region 12B, and the third region 12C has a chemical composition containing Zn and other alloying elements, and may further contain impurities. Furthermore, the plating layer 12 may also contain Zn and other alloying elements, with the remainder consisting of impurities.

The chemical composition of the coating is explained in detail below. Furthermore, the elements with a lower concentration limit of 0% are not essential for solving the problems of the coated steel of this embodiment, but are permissible elements included in the coating for purposes such as property enhancement.

<AI: 5.0~40.0%>

Al (Al) helps improve resistance to red rust, corrosion resistance of base metals, and sacrificial corrosion resistance. Therefore, the Al concentration is set at 5.0% or higher. Alternatively, the Al concentration can be set at 10.0%, 15.0%, or 20.0% or higher. On the other hand, when the Al concentration is excessive, the concentrations of Mg and Zn may decrease relatively, especially the sacrificial corrosion resistance. Furthermore, the appearance of the coating may sometimes be significantly reduced. Therefore, the Al concentration is set at 40.0% or lower. Alternatively, the Al concentration can be set at 35.0% or lower, or 30.0% or lower.

<Mg: 0.5~15.0%>

Mg is an essential element for ensuring corrosion resistance after coating, even at the expense of corrosion protection. It is also necessary for the crystallization of the MgZn2 phase. Therefore, the Mg concentration is set at 0.5% or higher. It can also be set at 2.0%, 3.0%, or 4.0% or higher. On the other hand, if the Mg concentration is excessive, processability, especially pulverability, may deteriorate, further reducing the corrosion resistance of the base metal. Furthermore, the appearance of the coating may be significantly reduced. Therefore, the Mg concentration is set at 15.0% or lower. It can also be set at 10.0% or lower, or 8.0% or lower.

<Mg/(Zn+Mg): 5.0% or more, or 6.5% or more>

In the chemical composition of the plating layer, the content of Mg relative to Zn and the total amount of Mg (Mg/(Zn+Mg)(%)) can be 5.0% or more, or even 6.5% or more. This further enhances the corrosion resistance, even at the expense of other properties. In (Mg/(Zn+Mg)), Mg and Zn represent the Mg concentration and Zn concentration in the plating layer, respectively.

<Fe: 5.0%~40.0%>

The first, second, and third regions of the coating in this embodiment contain a certain amount of Fe. Most of the Fe diffuses into the coating from the base steel, which serves as the base plate. It has been confirmed that when the Fe concentration is below 40.0%, its presence in the coating does not adversely affect performance. Most of the Fe exists in the second and third regions as an Al-Fe alloy phase; therefore, there is a tendency for a higher Fe concentration as the thickness of the second or third region increases. If the Fe concentration is less than 5.0%, the corrosion resistance of the base steel sometimes deteriorates in the later stages of coating corrosion. Therefore, the Fe concentration is set in the range of 5.0% to 40.0%. The Fe concentration can be set to 10.0% or higher, 15.0% or higher, or 20.0% or higher. Alternatively, the Fe concentration can be set to 30.0% or lower, or 28.0% or lower.

<Si: 0%~2.0%>

Si is an element that can be added arbitrarily, and can be 0%. However, by containing Si, _Mg₂Si phase, Al-Ca-Si-Zn phase, Mg-Al-Si-Zn phase, etc., can be formed in the coating, thereby improving the corrosion resistance of the coating. Therefore, the Si concentration can also be set to more than 0%, 0.1% or more, or 0.2% or more. On the other hand, if the Si concentration is too high, the corrosion resistance and sacrificial corrosion resistance of the base metal may sometimes deteriorate. Therefore, the Si concentration is set to 2.0% or less. Alternatively, the Si concentration can be set to 0.8% or less, or 0.6% or less.

<Ca: 0%~2.0%>

Ca is an optional additive element, and its concentration can be 0%. However, by including Ca, Al-Ca-Zn, Al-Ca-Si-Zn, and Ca-Zn phases can be formed in the plating layer, thereby improving the corrosion resistance of the plating layer. Furthermore, Ca can be adjusted to achieve the optimal Mg leaching amount that imparts the best corrosion resistance to the base iron. Therefore, the Ca concentration can be 0.05% or higher, or 0.1% or higher. On the other hand, if the Ca concentration is excessive, the corrosion resistance and processability of the base iron may sometimes deteriorate. Therefore, the Ca concentration is set to 2.0% or lower, or even 1.0% or lower.

Furthermore, the coating layer of this embodiment may also contain one or both of the groups selected from Group A and Group B below.

[Group A]Ni: 0~1.0%

[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% or more of these, totaling 0~5%.

<Ni: 0~1.0%>

The concentration of Ni, being a group A, can also be 0%. Ni helps improve sacrificial corrosion resistance. Therefore, the Ni concentration can also be set to 0.001% or higher. On the other hand, if the Ni concentration is excessive, the corrosion resistance of the base iron may sometimes deteriorate. Therefore, the Ni concentration is set to 1.0% or less. Alternatively, the Ni concentration can be set to 0.8% or less, 0.6% or less, or 0.5% or less, 0.1% or less, or 0.01% or less.

The coating of this embodiment contains 0-5% of one or more elements of group B mentioned above. If the total amount of elements of group B exceeds 5%, the corrosion resistance or sacrificed corrosion resistance of the base iron may sometimes decrease. The elements of group B will be explained below.

<Sb, Pb: 0~0.5% respectively>

The concentrations of Sb and Pb can also be 0%. Sb and Pb help improve sacrificial corrosion resistance. Therefore, the concentrations of Sb and Pb can be set to 0.001% or higher, 0.005% or higher, or 0.01% or higher, respectively. On the other hand, if the concentrations of Sb and Pb are excessive, the corrosion resistance of the base iron may sometimes deteriorate. Therefore, the concentrations of Sb and Pb are set to 0.5% or lower. Alternatively, the concentrations of Sb and Pb can be set to 0.3% or lower, 0.1% or lower, or 0.05% or lower, respectively.

<Cu, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li: 0~1.0% respectively>

The concentrations of Cu, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li can also be 0%. These elements help improve sacrificial corrosion resistance. Therefore, the concentrations of Cu, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li can be set to 0.005% or more, or 0.01% or more. On the other hand, if the concentrations of Cu, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li are excessive, the corrosion resistance of the base iron may sometimes deteriorate. Therefore, the concentrations of Cu, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li can be set to 1.0% or less. Alternatively, the concentrations of Cu, Ti, Cr, Nb, Zr, Mn, Mo, Ag, and Li can be set to 0.5% or less, 0.1% or less, or 0.05% or less.

<Sn：0~2.0%>

The Sn concentration can also be 0%. Sn is an element that forms intermetallic compounds with Mg, thus improving the corrosion resistance of the plating layer at the expense of corrosion resistance. Therefore, the Sn concentration can also be set to 0.01% or more, 0.02% or more, or 0.05% or more. However, if the Sn concentration is too high, the corrosion resistance of the base iron may sometimes deteriorate. Therefore, the Sn concentration is set to 2.0% or less. Alternatively, the Sn concentration can be set to 1.5% or less, 1.0% or less, 0.5% or less, or 0.2% or less. Due to the presence of the _Mg2Sn phase, the Sn content should preferably be between 0.02% and 2.0%.

<La, Ce, B, Y, P, and Sr: 0~0.5% respectively>

The concentrations of La, Ce, B, Y, P, and Sr can each be 0%. La, Ce, B, Y, P, and Sr help improve sacrificial corrosion resistance. Therefore, the concentrations of La, Ce, B, Y, P, and Sr can be set to 0.005% or more, or 0.01% or more, respectively. On the other hand, if the concentrations of La, Ce, B, Y, P, and Sr are excessive, the corrosion resistance of the base iron may sometimes deteriorate. Therefore, the concentrations of La, Ce, B, Y, P, and Sr are set to 0.5% or less. Alternatively, the concentrations of La, Ce, B, Y, P, and Sr can be set to 0.2% or less, 0.1% or less, 0.05% or less, or 0.02% or less, respectively.

<Co, Bi, In, V, W: 0~0.5% respectively>

The concentrations of Co, Bi, In, V, and W can each be 0%. Co, Bi, In, V, and W help improve corrosion resistance at the expense of corrosion. Therefore, the concentrations of Co, Bi, In, V, and W can be set to 0.001% or more, 0.002% or more, or 0.004% or more, respectively. On the other hand, if the concentrations of Co, Bi, In, V, and W become excessive, the corrosion resistance of the base iron may sometimes deteriorate. Therefore, the concentrations of Co, Bi, In, V, and W are set to 0.5% or less. Alternatively, the concentrations of Co, Bi, In, V, and W can be set to 0.1% or less, 0.05% or less, 0.02% or less, or 0.01% or less, respectively.

<Remaining components: Zn and impurities>

The remaining components of the plating layer in this embodiment are Zn and impurities. Zn is the element that imparts corrosion resistance to the base steel and sacrifices corrosion resistance. The Zn concentration is not particularly limited and can be 15.0% or more, 30.0% or more, or 50.0% or more. Impurities refer to components contained in the raw materials or components mixed in during the manufacturing process. For example, in the plating layer, sometimes due to atomic diffusion between the master steel and the plating bath, trace amounts of components other than Fe are also mixed in as impurities.

The chemical composition of the plating layer was determined using the following method. First, the plating layer was obtained by immersing the sample in an acid containing an inhibitor that inhibits corrosion of the base steel at room temperature for 20 minutes. Then, the obtained acid solution containing the plating layer was quantitatively analyzed by ICP emission spectrometry. This allowed the determination of the chemical composition of the plating layer. For example, a 10% hydrochloric acid solution containing 0.06% (by mass) of the inhibitor (manufactured by Asahi Chemical Industry Co., Ltd., IBIT 710K) could be used as the acid containing the inhibitor. Furthermore, the chemical composition determined by the above method represents the overall chemical composition of the plating layer.

The coating layer of this embodiment will then be explained in more detail.

In this embodiment of the coated steel, the interface between the coated layer and the base steel is an uneven surface. By making the interface between the coated layer and the base steel uneven, the adhesion of the coated layer is significantly improved.

Whether the interface between the coating and the base steel is an uneven surface that improves the adhesion of the coating can be confirmed by the following method: In a cross section perpendicular to the surface of the coated steel, as shown in Figure 2, when a cross section of the coated steel with a specified length, for example, 100 μm in length, parallel to the surface 1a of the coated steel is used as the observation area, whether the length L of the boundary line 13 between the coating 12 and the base steel 11 satisfies the following formula (1). Furthermore, in Figure 2, a cross section exceeding 100 μm in a direction parallel to the surface 1a of the coated steel was observed, but the length of the cross section is not limited to 100 μm and can be set to any length (specified length). Specifically, in a section perpendicular to the surface 1a of the coated steel, a section of coated steel with a length of 100 μm parallel to the surface 1a is designated as the observation area. Then, the straight-line distance _L0 between one end 13a and the other end 13b of the boundary line 13 in the observation area, and the length L of the boundary line 13 between one end 13a and the other end 13b are calculated, and the value of (( _LL0 )/ _L0 × 100) is calculated. As shown in Equation (1), ( _LL0 )/ _L0 × 100 is 2.0% or more, thereby improving the adhesion of the coating. The coated steel of this embodiment can also satisfy Equation (2) below instead of Equation (1), or it can satisfy Equation (3) below instead of Equation (1). (LL ₀ )/L ₀ ×100 does not require a specific upper limit. If it is too large, the surface smoothness of the coating will be reduced. Therefore, it can be below 40.0%, below 10.0%, or below 9.0%.

(LL ₀ )/L ₀ ×100≧2.0(%)...(1)

(LL ₀ )/L ₀ ×100≧4.0(%)...(2)

(LL ₀ )/L ₀ ×100≧6.0(%)...(3)

Furthermore, the observation area was set as a cross-section of the coated steel of a specified length, parallel to the surface 1a of the coated steel, observed using a SEM (JEOL's "JSM-7000F", accelerating voltage: 15kV) at a magnification of 1000x or higher. The resolution of the observation area was set to at least 2560 pixels horizontally and at least 1920 pixels vertically for a specified length of 100μm.

Furthermore, as shown in Figure 2, when the surface 1a of the plated steel is not planar in the microscopic region (approximately 100 μm) of the observation area, the direction parallel to the surface in a wider region of the plated steel (e.g., a region of several ^mm² or more) (e.g., the direction in which the plate extends when a plate is placed) can be set as the direction of surface 1a's extension. Based on this direction, the "section perpendicular to surface 1a" and the "direction parallel to surface 1a" can be identified.

Furthermore, as shown in Figure 2, one end 13a and the other end 13b of the dividing line 13 are the intersection points of the dividing line 13 and the straight line dividing the observation area. _L0 is set as the length of the straight line connecting one end 13a and the other end 13b. L is the length of the dividing line 13 from one end 13a to the other end 13b. The length L of the dividing line 13 can be measured, for example, using ImageJ, a common-domain image processing software. An image observed using SEM at a magnification of 1000x or higher is set as image data at the above-mentioned resolution. The length of the dividing line 13 is measured using the measurement function of ImageJ for this image data.

Next, the microstructure of the deposited layer is described.

The proportions of phases and microstructures contained in the coating of this embodiment will affect the red rust resistance, base iron corrosion resistance, and sacrificial corrosion resistance of the coated steel. Even for coatings with the same composition, the phases or microstructures contained in the metal structure will change due to different manufacturing methods, resulting in different properties. The metal structure of the coating can be easily confirmed by mirror polishing a section perpendicular to the surface of the coated steel and analyzing the section using an electron beam probe microanalyzer with a scanning electron microscope (SEM-EPMA, EPMA measuring device: JEOL JXA-8230, accelerating voltage: 15kV, current: 0.05μA, irradiation time: 50ms). In this embodiment, the coating thickness is approximately 10~300μm. In the SEM, the magnification is set to 200~5000x, and the cross-section of the coating in a ^36000μm² region is confirmed within this field of view. In this embodiment, SEM may only observe partial views of the coating. Therefore, to obtain average information about the coating, 25 views are selected from the aforementioned cross-sections as average information. That is, the metal structure is observed in a total of 25 × 36000 ^μm² views to determine the area fraction of the phases or structures constituting the coating. Multiple cross-sections can be measured if necessary. Furthermore, the cross-section should preferably be located near the center of the planar portion of the object (sample) being measured.

To identify each phase, point analysis was used in elemental analysis performed using EPMA to determine the phase composition. Phases with similar compositions were identified based on elemental distribution readings. In point analysis, based on the EPMA resolution results, Al, Zn, Mg, Fe, and Si were analyzed at 250 pixel × 250 pixel grid points. The electron beam diameter was set to less than 1 μm. Phases with similar compositions could be identified by using elemental distribution to distinguish phases with approximately the same composition. That is, to identify each phase, in areas other than the layered structure (the binary eutectic structure of Zn/MgZn ₂₎ , point analysis was used in SEM-EPMA analysis to determine the phase composition, and phases with approximately the same composition were identified based on elemental distribution readings. The area of each phase was measured using the image analysis software "Image J (version 1.54f)". The ratio of the total area of each phase to the total area of the observation field was set as the area ratio (%) of each phase.

Furthermore, the area ratio (area proportion) of each phase in the field of view corresponds to the volume proportion of that phase in regions 1, 2, and 3.

The plating layer of this embodiment, as described above, includes a first region with an Fe concentration of less than 5.0% by mass, a second region with an Fe concentration of 5.0% by mass or more but less than 30.0% by mass, and a third region with an Fe concentration of 30.0% by mass or more but less than 80.0% by mass. The phases/microstructures contained in any one or more of the first, second, and third regions will be described below.

Al-containing phase

The Al-containing phase in this embodiment is a phase containing Zn and 20-99% by mass of Al. The Zn content in the Al-containing phase can also be 1-80% by mass. Furthermore, the Al-containing phase may also contain other elements totaling less than 5% by mass. These other elements include Mg, Si, etc. The Al-containing phase can be a form in which Zn is dissolved in Al, or it can be an aggregate of fine Zn phase with a particle size less than 1 μm and fine Al phase with a particle size less than 1 μm. The Al-containing phase can be identified by using EPMA elemental distribution, thus clearly distinguishing it from other phases or structures.

Al-containing phases are sometimes present in Region 1. However, coatings containing a large amount of Al-containing phase in Region 1 exhibit reduced corrosion resistance after application; therefore, it is ideal to minimize the presence of Al-containing phases in Region 1.

Mg-Zn phase

The Mg-Zn phase is a phase containing Zn and 20-60% by mass of Mg. The Zn content in the Mg-Zn phase can also be 40-80% by mass. Furthermore, the Mg-Zn phase may also contain other elements totaling less than 5% by mass. These other elements are, for example, Si. Specifically, examples of the Mg-Zn phase include the _MgZn2 phase or the _{Mg2Zn11 phase} . The Mg-Zn phase can be identified by using the elemental distribution of EPMA, thus clearly distinguishing it from phases containing Al and [binary eutectic structures of Zn/MgZn2 _] .

The Mg-Zn phase is contained in region 1. Furthermore, the Mg-Zn phase is sometimes contained in regions 2 and 3. By including the Mg-Zn phase in the plating layer, red rust resistance and corrosion protection of the base iron are improved, and corrosion protection is further enhanced at the expense of other properties.

[Binary eutectic structure of Zn/MgZn ₂ ]

The binary eutectic structure _of Zn/MgZn2 is a eutectic structure composed of the η-Zn phase and the _MgZn2 phase. In SEM reflectance electron images, the layered structure of the Zn/ _MgZn2 binary eutectic structure composed of the η- _Zn phase and the MgZn2 phase is clearly distinguishable from Al-containing phases, Mg-Zn phases, Zn phases, and Fe-Al phases. Furthermore, the identification of structures other than the Zn/ _MgZn2 binary eutectic structure, such as the Al phase and Mg-Zn phase, only requires examining the remaining portions excluding the layered structure of the Zn/ _MgZn2 binary eutectic structure.

The binary eutectic structure _of Zn/MgZn2 is sometimes present in region 1 or region 2. By ensuring that the binary eutectic structure of Zn/ _MgZn2 exists to a certain extent, the corrosion resistance after coating can be improved.

Fe-Al alloy phase

The Fe-Al alloy phase is a phase containing Al and 20-60% by mass of Fe. The Al content in the Fe-Al alloy phase can also be 40-80% by mass. Furthermore, the Fe-Al alloy phase may also contain other elements totaling less than 5% by mass. These other elements include Si, etc. The Fe-Al alloy phase is primarily composed of _Al₅Fe . Additionally, the Fe-Al alloy phase can also contain AlFe, _Al₃Fe , _Al₅Fe₂ , _etc. , in addition to _Al₅Fe . Furthermore, when the Fe-Al alloy phase contains Si in the plating layer, it sometimes forms a Fe-Al-Si compound phase. As Fe-Al-Si compound phases to be identified, there is the AlFeSi phase, and isomers such as α, β, q₁, and q₂-AlFeSi phases exist.

The Fe-Al alloy phase is mainly contained in regions 2 and 3. The Fe-Al alloy phase exhibits a certain degree of corrosion resistance to Fe. Furthermore, as an intermetallic compound phase, it possesses high insulation properties and is not easily corroded.

Other intermetallic compounds may also be present in the coating as a residue. Examples of other intermetallic compounds include _Mg₂Si phase, _Mg₂Sn phase, Zn phase, Al-Ca-Zn phase, Al-Ca-Si-Zn phase, Ca-Zn phase, and Mg-Al-Si-Zn phase.

_Mg2Si phase

When the plating layer contains Si, Si sometimes precipitates in the form of the _Mg₂Si phase. _{The Mg₂Si} phase exhibits excellent corrosion resistance. The presence of _Mg₂Si can be determined by SEM/EPMA as described below. The phase satisfying Mg: 50~70 at% and Si: 30~50 at% can be identified as the _Mg₂Si phase.

_Mg2Sn phase

By incorporating the _Mg₂Sn phase into the coating, the corrosion resistance of the coated steel and the corrosion protection of the base iron are further enhanced. The _Mg₂Sn phase is present in small amounts, and its presence was confirmed by X-ray diffraction. Due to the presence of the _Mg₂Sn phase, the Sn content in the chemical composition of the coating should ideally be 0.02% to 2.0%. The presence of the _Mg₂Sn phase was determined by X-ray diffraction using Cu-Kα radiation at an X-ray output of 50kV and 300mA. The measurement range was set to 2θ = 10~30°, and the scanning step was set to 0.02° increments. Furthermore, when a diffraction peak was detected at 23.4±0.3°, it was determined that the _Mg2Sn phase was present.

Zn phase

The Zn phase contains more than 80% by mass of Zn, and may also contain less than 20% by mass of Al, and may contain less than 5% by mass of other elements such as Si and Mg. The Zn phase is sometimes present in region 1 or region 2. The presence of the Zn phase further enhances corrosion resistance. The Zn phase appears white in SEM reflectance electron microscopy images, thus clearly distinguishing it from other phases or structures.

Al-Ca-Zn phase, Al-Ca-Si-Zn phase, Ca-Zn phase, Mg-Al-Si-Zn phase

If the plating bath contains Ca or Si, these intermetallic compound phases may precipitate in the plating layer. These intermetallic compound phases exhibit superior corrosion resistance compared to the aforementioned _Mg₂Si phase. In addition to being confirmed by SEM and EPMA, the presence of these intermetallic compound phases can also be confirmed by X-ray diffraction. The X-ray diffraction conditions can be the same as those for the _Mg₂Sn phase.

The plating layer of this embodiment, as described above, includes a first region with an Fe concentration of less than 5.0% by mass, a second region with an Fe concentration of 5.0% by mass or more but less than 30.0% by mass, and a third region with an Fe concentration of 30.0% by mass or more but less than 80.0% by mass. The boundaries between the first, second, and third regions are determined as follows.

First, a section perpendicular to the surface of the coated steel is exposed. This exposed section is mirror-polished. The Fe concentration is determined using point analysis with an electron beam microanalyzer (EPMA). Furthermore, the analytical results obtained using the EPMA can be used to identify the various phases of the aforementioned coating.

First, as part of the first stage of measurement, as shown in Figure 3, a plurality of straight analytical lines are set on the cross-section of the coating 12, parallel to a direction orthogonal to the thickness direction of the coating 12. Ten analytical lines are set at equal intervals along the thickness direction of the coating. Furthermore, when the coating thickness exceeds 100 μm, the interval between the analytical lines is set to 10 μm. Ten measurement points are set at 10 μm intervals along the analytical lines in a direction orthogonal to the thickness direction. If the length of the analytical line is less than 90 μm and it is not possible to set 10 measurement points at 10 μm intervals, the interval can be appropriately reduced, or the number of measurement points can be decreased. In Figure 3, the single-point chain represents the analysis line, and the black dots on the analysis line represent the measurement points. Furthermore, the analysis lines are set to maximize the number of analysis lines across the cross-section of the coating 12. When the thickness of the coating 12 is less than 10 μm, the measurement in the first stage is omitted, and the measurement in the second stage described below is performed.

Point analysis was used to determine the Fe concentration (mass %) in the coating at each measurement point. The electron beam output for point analysis was set to 15 kV, 4 × ^10⁻⁷ A, and the dot diameter at the front of the electron beam was set to 0.2 μm. The average Fe concentration at 10 measurement points was calculated for each analysis line, and this average value was set as the Fe concentration for each analysis line. Then, analysis line A, where the Fe concentration was less than 5.0% and closest to 5.0%, and analysis line B, where the Fe concentration exceeded 5.0% and closest to 5.0%, were identified. Furthermore, the analysis line C, where the Fe concentration is less than 30.0% and the Fe concentration is closest to 30.0%, and the analysis line D, where the Fe concentration is more than 30.0% and the Fe concentration is closest to 30.0%, were identified.

Next, as part of the second stage of measurement, as shown in Figures 4 and 5, Fe concentration analysis was performed in the region between analytical lines A and B on the cross-section of the coating 12. Furthermore, as shown in Figures 4 and 6, Fe concentration analysis was also performed in the region between analytical lines C and D. Figure 4 is a diagram showing analytical lines A, B, C, and D. Figure 5 is an enlarged view of region M1 between analytical lines A and B in Figure 4. Figure 6 is an enlarged view of region M2 between analytical lines C and D in Figure 4.

In the second stage of the measurement, as shown in Figures 4-6, multiple analytical lines are set at 1 μm intervals along the thickness direction of the coating in the region between analytical lines A and B, and in the region between analytical lines C and D. Ten measurement points are set at 10 μm intervals along each analytical line. Figures 4 and 5 show, as an example, a portion of the analytical lines set between analytical lines A and B. Figures 4 and 6 also show a portion of the analytical lines set between analytical lines C and D. At each measurement point, the Fe concentration (mass %) is measured using point analysis. The electron beam output and the spot diameter of the electron beam tip during point analysis are set the same as in the first stage of the measurement.

For each analytical line, the average Fe concentration at 10 measurement points is calculated, and this average value is set as the Fe concentration for each analytical line. Then, the analytical line with the Fe concentration closest to 5.0% is identified as the "Fe concentration 5.0% analytical line" (e.g., analytical line F5 in Figure 5). Similarly, the analytical line with the Fe concentration closest to 30.0% is identified as the "Fe concentration 30.0% analytical line" (e.g., analytical line F30 in Figure 6). Using the Fe concentration 5.0% by mass analytical line F5 as the dividing line, the area closer to the surface 1a of the plated steel (analytical line A side) than analytical line F5 is designated as the first region 12A. Furthermore, using the analytical line F30 (Fe concentration 30.0% by mass) as the dividing line, the region closer to the mother steel than analytical line F30 is designated as Region 3, 12C. Further, the region between analytical lines F5 and F30 is designated as Region 2, 12B.

That is, as shown in Figures 4-6, the first region 12A refers to the region between the surface 1a of the plated steel and the analytical line F5 with an Fe concentration of 5.0% by mass. The second region 12B refers to the region between the analytical line F5 with an Fe concentration of 5.0% by mass and the analytical line F30 with an Fe concentration of 30.0% by mass. The third region 12C refers to the region between the analytical line F30 with an Fe concentration of 30.0% by mass and the interface 13 between the plated layer 12 and the parent steel 11. The first region 12A, the second region 12B, and the third region 12C are formed in layers throughout the entire plated layer 12. Region 12A is disposed in coating 12 on the surface 1a of the plated steel 1, closer to it than Region 12B. Region 12B is disposed in coating 12 between Region 12A and Region 12C. Region 12C is disposed in coating 12 adjacent to the parent steel 11 and closer to it than Region 12B.

<Region 1>

The first region is a region located on the surface of the coated steel where the Fe concentration is less than 5.0% by mass. The surface above the first region constitutes the surface of the coated steel. By including the first region, the red rust resistance, base iron corrosion resistance, and sacrificial corrosion resistance of the coated steel are improved. Furthermore, by including the first region, which has a lower area fraction of Al phase, the corrosion resistance after coating is improved.

Zone 1 sometimes contains Al-containing phases at a rate of less than 5% by area. Ideally, the area percentage of Al-containing phases should be less than 1%, and even better, 0%. The higher the Al phase content, the worse the corrosion resistance after coating. Therefore, it is ideal for Zone 1 to contain no Al-containing phases, or if it does, ideally, it should not exceed 5%. If the area percentage of Al-containing phases in Zone 1 is more than 5%, the corrosion resistance after coating will decrease.

Furthermore, the first zone may also contain 2% or more of the Mg-Zn phase by area. Compared to Al-containing phases, the Mg-Zn phase offers superior corrosion resistance and sacrificial corrosion protection after coating. Therefore, to improve the corrosion resistance and sacrificial corrosion protection of coated steel, the first zone should preferably contain 2% or more of the Mg-Zn phase. The area percentage of the Mg-Zn phase in the first zone should preferably be 5% or more, 10% or more, 15% or more, 20% or more, 30% or more, or 40% or more. There is no specific upper limit for the Mg-Zn phase in the first zone; for example, it should preferably be below 95%, or it can be below 90%.

Furthermore, the first region may contain at least 1% of the [Zn/MgZn ₂ binary eutectic structure] by area. By containing the [Zn/MgZn ₂ binary eutectic structure] in the first region, the corrosion resistance and sacrificial corrosion protection of the coated steel are further improved. The area ratio of the [Zn/MgZn ₂ binary eutectic structure] in the first region should preferably be 3% or more, 5% or more, or 10% or more. There is no specific upper limit for the [Zn/MgZn ₂ binary eutectic structure] in the first region; for example, it should preferably be below 90%, but it can also be below 80%, 60%, 50%, or 40%.

In the first region, Zn phase, _Mg2Si phase, _Mg2Sn phase, Al-Ca-Zn phase, Al-Ca-Si-Zn phase, Ca-Zn phase, Mg-Al-Si-Zn phase, etc., may also be present as a remainder. Furthermore, in the first region, Fe-Al alloy phase may also be present at an area fraction of less than 3%.

The thickness of the first zone is 5~100μm. By making the thickness of the first zone 5μm or more, the corrosion resistance, red rust resistance, base iron corrosion resistance, and sacrificial corrosion resistance after coating are improved. On the other hand, it is sometimes difficult to make the thickness of the first zone exceed 100μm in manufacturing, so the upper limit is set to below 100μm. The thickness of the first zone can be below 70μm, below 50μm, or below 40μm. Furthermore, the thickness of the first zone can be above 10μm or above 15μm. The thickness of the first zone adopts the average thickness. Regarding the average thickness, in the SEM images obtained above, the distance along the thickness direction of the coating from the coating surface to the analytical line F5 (5.0% Fe concentration) was measured at 10 locations with intervals of more than 10 μm, and the arithmetic mean was used.

<Region 2>

The second region is located between the first and third regions and contains 5.0% by mass or more but less than 30.0% by mass of Fe. The second region contains Fe at a rate of less than 30.0% by mass, and further contains Zn, Al, and Mg. Therefore, the second region sometimes contains an Fe-Al alloy phase or a Mg-Zn phase. The second region preferably contains more Mg-Zn phase than the Fe-Al alloy phase. Furthermore, the second region may also contain more Fe-Al alloy phase than the Mg-Zn phase, but the difference in their content by area ratio should be less than 12%, preferably less than 10%, and more preferably less than 5%. The Mg-Zn phase improves resistance to red rust, corrosion resistance of the base iron, and sacrificial corrosion resistance, while the Fe-Al alloy phase improves the corrosion resistance of the base iron.

By incorporating this second region into the coating, the corrosion resistance and red rust resistance of the base metal are enhanced, and the corrosion resistance is further improved after coating. The effect of utilizing the second region to improve the corrosion resistance and red rust resistance of the base metal becomes apparent in the later stages of corrosion.

Furthermore, the second region sometimes contains a binary eutectic structure of Zn/MgZn _2. This improves the corrosion resistance after coating.

The Fe-Al alloy phase contained in the second region should preferably be needle-like, with an equivalent circular diameter of less than 15 μm and an aspect ratio of 2 or greater when the cross-section of the second region is observed. The Fe-Al alloy phase helps improve the corrosion resistance of the base iron through its barrier effect. On the other hand, the Fe-Al alloy phase easily becomes the starting point for red rust formation during corrosion, but the Fe-Al alloy phase contained in the second region is needle-like, thus the formation of red rust is relatively suppressed.

Regarding the morphology of the Fe-Al alloy phase that may be present in the second region, when confirming the proportion of phases and microstructures contained in the coating layer of this embodiment using a electron beam probe microanalyzer (SEM-EPMA), it is sufficient to identify needle-shaped Fe-Al alloy phases with an equivalent diameter of less than 15 μm and an aspect ratio of more than 2.

The area fraction of the Fe-Al alloy phase in the second region can also be 5% or more. This further enhances the corrosion resistance of the base iron. The area fraction of the Fe-Al alloy phase in the second region can also be 10%, 20%, or 30% or more. There is no specific upper limit for the Fe-Al alloy phase in the second region; for example, it is preferable to be below 80%, 70%, or 60%.

The area fraction of the Mg-Zn phase in the second region can also be 5% or more. This further enhances the sacrificial corrosion resistance, in addition to improving the corrosion resistance and red rust resistance of the base iron. Compared to Fe-Al, the Mg-Zn phase offers higher sacrificial corrosion resistance. Therefore, from the perspective of sacrificial corrosion resistance, the more Mg-Zn phase in the second region, the better. The area fraction of the Mg-Zn phase in the second region can also be 10%, 20%, 30%, or 40% or more. There is no specific upper limit for the Mg-Zn phase in the second region; for example, it is preferable to be below 70%, 60%, or 50%.

The area fraction of the [Zn/MgZn ₂ binary eutectic structure] in the second region can also be 2% or more. This further enhances the corrosion resistance and sacrificial corrosion protection of the coated steel. The area fraction of the [Zn/MgZn ₂ binary eutectic structure] in the second region can also be 5% or more, 8% or more, or 10% or more. There is no specific upper limit for the [Zn/MgZn ₂ binary eutectic structure] in the second region; for example, it should preferably be below 90%, but it can also be below 80%, 50%, 30%, or 20%.

In the second region, there may also be residual phases such as Al phase, _Mg2Si phase, _Mg2Sn phase, Zn phase, Al-Ca-Zn phase, Al-Ca-Si-Zn phase, Ca-Zn phase, and Mg-Al-Si-Zn phase. The area ratio of these residual phases should preferably be less than 10% in total.

The thickness of the second region is set to 5-100 μm. By setting the thickness of the second region to 5 μm or more, the corrosion resistance, sacrificial corrosion resistance, and red rust resistance of the base iron are improved. Ideally, it should be set to 15 μm or more. On the other hand, it is sometimes difficult to make the thickness of the second region exceed 100 μm in manufacturing; therefore, the upper limit is set to below 100 μm. The thickness of the second region can be below 70 μm, 50 μm, 40 μm, or 30 μm. The thickness of the second region is set as the average thickness.

Regarding the average thickness, in the images obtained using SEM observation, the distance in the thickness direction of the coating between the analytical line for 5.0% Fe concentration and the analytical line for 30.0% Fe concentration was measured at 10 locations with intervals of more than 10 μm, and the arithmetic mean was used.

<Region 3>

The third region is located between the second region and the base steel and contains 30.0% to 80.0% Fe by mass. Because the third region contains 30.0% to 80.0% Fe by mass, it contains a relatively large amount of Fe-Al alloy phase. The Fe-Al alloy phase in the phase or microstructure of the third region occupies the largest area. Furthermore, the third region sometimes contains Mg-Zn phase in approximately the same amount as the Fe-Al alloy layer. By having the third region, containing 30% or more Fe by mass, present in the plating layer, the corrosion resistance of the base steel of the plating layer is improved. The reason for the improved corrosion resistance of the base steel is speculated as follows: Because the third region contains more than 30.0% by mass of Fe, it contains a higher concentration of the Fe-Al alloy phase, which functions as a barrier layer in the parent steel.

Furthermore, in the third region, as mentioned above, in addition to the Fe-Al alloy phase, a Mg-Zn phase may also be present. The area fraction of the Mg-Zn phase in the third region can also be 10% or more, 15% or more, 20% or more, or 30% or more. This further enhances corrosion resistance. Compared to Fe-Al, the Mg-Zn phase offers higher corrosion resistance. Therefore, from the perspective of corrosion resistance, the more Mg-Zn phase in the third region, the better. There is no specific upper limit for the _MgZn2 phase in the third region; for example, it is preferable to be below 50%.

In the third region, as a remaining portion, it may also contain Al phase, _Mg₂Si phase, _Mg₂Sn phase, Zn phase, Al-Ca-Zn phase, Al-Ca-Si-Zn phase, Ca-Zn phase, Mg-Al-Si-Zn phase, etc. The area ratio of these remaining phases should preferably be less than 10% in total.

The thickness of the third region is set to 5-100 μm. By setting the thickness of the third region to 5 μm or more, the corrosion resistance of the base metal is improved. On the other hand, it is sometimes difficult to make the thickness of the third region exceed 100 μm in manufacturing; therefore, the upper limit is set to below 100 μm. The thickness of the third region can be below 70 μm, below 50 μm, below 40 μm, or below 30 μm. Furthermore, the thickness of the third region can exceed 5 μm or exceed 15 μm. The thickness of the third region is set as the average thickness. Regarding the average thickness, in the SEM images obtained above, the distance in the coating thickness direction between the analysis line of 30.0% Fe concentration and the boundary line between the coating and the base steel was measured at 10 locations with intervals of more than 10 μm, and the arithmetic mean was used.

The thickness of the coating is the sum of the thicknesses of regions 1, 2, and 3. That is, the coating thickness should preferably be 15 μm to 300 μm.

Furthermore, in the plated steel of this embodiment, the parent steel and the third region should preferably be in direct contact. That is, there should preferably be no alloy layer different from that of the third region between the parent steel and the third region.

Next, the manufacturing method of the plated steel according to this embodiment will be explained.

The method for manufacturing plated steel according to this embodiment includes: a shot blasting step, which performs shot blasting treatment on the surface of the base steel; a flux coating step, which applies flux to the base steel after the shot blasting step; a first plating step, which immerses the base steel after the flux coating step in a first plating bath and then lifts it out; a second plating step, which immerses the base steel after the first plating step in a second plating bath and then lifts it out; and a cooling step, which cools the plating layer after the second plating step. Thus, the plated steel of this embodiment is manufactured using a so-called two-stage plating method.

The first plating bath in the first plating step is a plating bath containing Zn, and the second plating bath in the second plating step is a plating bath containing Al, Mg, and Zn. The plating method used in both the first and second plating steps is the so-called hot-immersion plating method.

In the shot blasting step, the surface of the base steel is shot-blasted. This applies strain to the surface of the base steel. The conditions for shot blasting are described below. Various shapes and materials of shot can be considered, but it is preferable to use steel shot according to JIS Z 0311:2004, especially spherical shot. The center particle size of the shot can be in the range of 40~450μm. The hardness can be Hv390~510. Specifically, for example, steel shot (TSH-30) manufactured by WINOA IKK JAPAN Co., Ltd. can be used.

In the blasting process, according to the General Rules for Blasting Treatment of Green Billets (JIS Z 0310:2016), centrifugal force or air pressure is used to propel the blasting material against the surface of the base steel. The amount of blasting material applied can range from 5 to 400 kg/ ^m² . The amount of blasting material applied can be 10 kg/ ^m² or more, or 15 kg/ ^m² or more. Alternatively, the amount of blasting material applied can be less than 100 kg/ ^m² , or less than 50 kg/ ^m² .

The surface of the master steel is strained by a spraying step. If the strained master steel is subjected to the first and second coating steps of hot-dip galvanizing, when the master steel is immersed in the second coating bath, the reaction between the Fe in the Fe-Zn layer formed in the first coating step and the Al in the coating bath becomes more active, resulting in a relatively large amount of Fe-Al alloy phase. Furthermore, by applying strain, the reaction rates between the base steel and Zn in the first plating step are distributed, thereby making the interface between the plating layer and the base steel more prone to becoming uneven, thus obtaining a plating steel that satisfies the relationship in equation (1) above. This improves the adhesion of the plating layer.

Next, in the flux application step, for example, the base steel is immersed in an aqueous flux solution at 80°C for 30 seconds and then removed and dried in an atmospheric environment at 150°C. The flux is a _ZnCl₂ -based solution containing various salts such as NaCl, KCl, NaF, _SnCl₂ , _SnCl₄ , and _BiCl₃ , as well as surfactants, and hydrochloric acid may be added as needed. By applying flux to the base steel before plating, oxides on the surface of the base steel are removed, stabilizing the plating reaction. As a flux, for example, a flux can be prepared by dissolving NaCl in water at a concentration of 0~100g/L, _KCl at a concentration of 0~100g/L, SnCl2 at a concentration of 0~20g/L, _and ZnCl2 at a concentration of 100~300g/L.

Next, in the first plating step, the flux-coated steel is immersed in the first plating bath and then lifted out. The first plating bath is a zinc plating bath with Zn as the main component.

A zinc plating layer is formed by immersing the base steel in a first plating bath. Ideally, an Fe-Zn alloy layer should be formed on the base steel side of the zinc plating layer. Specifically, the zinc plating layer should preferably consist of an Fe-Zn alloy layer and an n-Zn layer formed sequentially from the base steel side. The thickness of the Fe-Zn alloy layer in the zinc plating layer is not particularly limited. Furthermore, the Zn plating layer can also be an alloyed zinc plating layer where the alloying of Fe and Zn extends to the surface of the plating layer.

The thickness of the zinc plating or alloyed zinc plating is preferably 5-150 μm, and more preferably 20-60 μm. If the thickness of such plating is less than 5 μm, the thickness of the third region will also be less than 5 μm, reducing the corrosion resistance of the base metal. Furthermore, it is more difficult to manufacture such plating with a thickness exceeding 150 μm.

The immersion time in the first plating bath can, for example, be in the range of 10 to 600 seconds. This allows an Fe-Zn alloy layer to form within the zinc plating. If the immersion time is less than 10 seconds, the Fe-Zn alloy layer will not form sufficiently, and the interface between the final plating layer and the base steel will become less smooth, thus reducing the adhesion of the plating layer. On the other hand, if the immersion time exceeds 600 seconds, the Fe-Zn alloy layer may grow excessively, which in turn reduces the adhesion of the plating layer.

The temperature of the first plating bath can be in the range of 420~480℃. If the bath temperature is high, the growth of the Fe-Zn alloy layer may accelerate, leading to excessive formation of the Fe-Zn alloy layer and consequently reducing the adhesion of the plating layer. At low bath temperatures, plating defects such as incomplete plating and foreign matter adhesion become more likely to occur.

After being lifted from the first coating bath, the coating adhesion amount is adjusted as needed, by wiping with nitrogen gas or by adjusting the speed at which the steel plate is lifted from the coating bath.

Next, in the second coating step, the master steel with a zinc coating or an alloyed zinc coating is immersed in the second coating bath and then removed. The composition of the second coating bath can be substantially the same as the chemical composition of the coating. Alternatively, the composition of the second coating bath can be appropriately adjusted to fall within the range of the chemical composition of the coating of this embodiment. Specifically, it is preferable to increase the amount of Mg, Al, and other alloying elements other than Zn to approximately 1.01 to 1.20 times the target value of the chemical composition of the coating, so as to fall within the range of the chemical composition of the coating of this embodiment.

By immersing a zinc-plated master steel in a second plating bath, the dissolution of Zn from the η-Zn layer contained in the zinc plating occurs, and the substitution of Zn with Al in the Fe-Zn alloy layer takes place, thereby forming a third region containing an Fe-Al alloy phase.

Furthermore, if the master steel with the alloyed zinc plating layer is immersed in the second plating bath, the substitution of Zn with Al occurs in the Fe-Zn alloy layer within the alloyed zinc plating layer, forming a third region containing the Fe-Al alloy phase.

The immersion time in the second plating bath can, for example, range from 5 to 300 seconds. This promotes the growth of the second and third plating zones. If the immersion time is less than 5 seconds, the second and third zones will not form sufficiently, and the interface between the plating layer and the base steel will become less smooth, reducing the corrosion resistance of the base steel and the adhesion of the plating layer. On the other hand, if the immersion time exceeds 300 seconds, the second and third zones will overgrow, making the plating layer itself more prone to cracking.

The bath temperature for the second plating bath can be in the range of 400℃~680℃, or 460~670℃. If the bath temperature is high, the growth of the Fe-Al alloy phase may be accelerated, potentially leading to excessive formation of the Fe-Al alloy phase and the creation of another interfacial alloy layer between the Fe-Al alloy phase and the base steel in the third region. This also increases the wear and tear on the plating equipment. At low bath temperatures, plating defects such as incomplete plating and foreign matter adhesion are more likely to occur.

After being lifted from the second coating bath, the coating adhesion amount is adjusted as needed by methods such as nitrogen blowing and wiping, or by adjusting the speed at which the mother steel is lifted from the coating bath.

Next, as a cooling step, the coating is cooled (controlled cooling). During the cooling step, cooling is performed at an average cooling rate of less than 5.0°C/second, preferably less than 2.0°C/second, until the temperature of the coating rises from the bath temperature to 330°C, which is the controlled cooling stop temperature. The controlled cooling stop temperature should preferably be below 330°C. Cooling can be performed, for example, by blowing cooling gas. When cooling is performed by blowing cooling gas, a plurality of cooling gas blowing nozzles are arranged along the transport path of the base steel, and cooling gas is simply blown from these nozzles. The cooling gas can be air, nitrogen ( _N2 ), argon, etc., with nitrogen ( _N2 ) being the most suitable.

By cooling at an average cooling rate of less than 5.0 °C/s, the alloying reaction between unreacted Fe and Al directly remaining on the surface of the plating layer progresses, leading to the consumption of Al and Fe in the plating layer. As a result, a first region with an Fe concentration of less than 5.0% by mass is formed. Furthermore, Al is consumed in the reaction with Fe, resulting in less precipitation of Al-containing phases, and the area fraction of Al-containing phases in the first region also decreases. Moreover, by cooling from the bath temperature to a controlled cooling stop temperature, such as 330 °C, at an average cooling rate of less than 5.0 °C/s, the precipitation of the Mg-Zn phase proceeds. Furthermore, the eutectic reaction of the Zn phase and the _MgZn2 phase proceeds, thereby promoting the formation of the binary eutectic structure of the η-Zn phase and the _MgZn2 phase (the binary eutectic structure of Zn/ _MgZn2 ).

On the other hand, on the parent steel side of the plating layer, Fe diffusing from the parent steel reacts with Al in the plating bath, thereby forming a second region with an Fe concentration of 5.0% or higher. In this second region, in addition to a greater amount of the Fe-Al alloy phase, a Mg-Zn phase sometimes also forms.

The plated steel of this embodiment can be manufactured using the above method.

Furthermore, the plated steel of this embodiment can also be formed using methods such as vapor deposition, melt spraying, and cold spraying, achieving the same effect as that formed using melt deposition.

[Implementation Example]

The effects of this invention will be specifically explained below through examples.

As the master steel, master steel of shapes A, B, and C as described below (SS400 as specified in JIS G 3101) shall be used.

A: Steel plate with dimensions of 200mm in length, 100mm in width, and 3.2mm in thickness.

B: An L-shaped angle steel bar (length 200mm, short side length 50mm, plate thickness 3.2mm) obtained by bending the steel plate of A at a 90° angle at the center of the plate width.

C: Porous metals (XS-62 (3.2mm thickness) as specified in JIS G 3351:1987). In the case of porous metals, "the surface of the plated steel" refers to the surface corresponding to a plane orthogonal to the thickness direction in the overall plate shape having a plurality of through holes.

Furthermore, in the cases of materials B and C mentioned above, due to the difficulty in observing, measuring, and evaluating the cross-sectional microstructure and coating adhesion, in the test examples where the parent steel is material B or C, the parent steel is changed to material A for the manufacture of the coated steel. The coated steel using material A as the parent steel is then observed, measured, and evaluated.

The aforementioned mother steel was subjected to a blasting process. In this process, steel shot (TSH-30) manufactured by WINOA IKK JAPAN Co., Ltd. was used as the blasting material. According to the General Rules for Blasting Treatment of Green Billets (JIS Z 0310:2016), the blasting material was propelled onto the surface of the mother steel by air pressure. The amount of blasting material projected is shown in Tables 2A and 2B. The projection conditions were set as follows: ejection pressure: 0.5 MPa, nozzle-to-sample (mother steel) distance: 800 mm, projection angle: 90°.

Next, a flux coating step is performed on the mother steel after the spraying step. In this step, the material is immersed in an aqueous flux solution ( _ZnCl2 /NaCl/ _SnCl2 = 220g/20g/10g/L) at 80°C for 30 seconds, then lifted out and thoroughly dried in a drying oven at 150°C.

Next, the base steel after flux application is immersed in the first plating bath and then lifted out, followed by immersion in the second plating bath and then lifted out. The temperature of the first plating bath is set to 460°C, and the temperature of the second plating bath is as recorded in Tables 2A and 2B. The immersion time in the first plating bath is set to 180 seconds, and the immersion time in the second plating bath is as recorded in Tables 2A and 2B. The first plating bath is a zinc plating bath containing more than 95% Zn. The zinc plating layer formed by immersion in the first plating bath has a thickness ranging from 5 to 150 μm and includes an Fe-Zn alloy layer and an n-zinc layer. The second plating bath is a Zn-Al-Mg plating bath containing Al, Mg, other alloying elements, and Zn. The composition of the second plating bath is appropriately adjusted to be substantially the same as, or to be a target value for, the chemical composition of the final plating layer. Specifically, the amounts of Mg, Al, and other alloying elements (excluding Zn) are increased by 1.2 times compared to the target value for the chemical composition of the plating layer.

Next, as a cooling step, compressed air is blown as a cooling gas onto the mother steel lifted from the second coating bath, while cooling is performed at a controlled rate from the coating bath temperature down to 330°C, which is the controlled cooling stop temperature. The cooling rates are as recorded in Tables 2A and 2B. Cooling is carried out within a temperature range below 330°C. This process produces the coated steel.

For samples cut into 30mm × 30mm pieces, ICP emission spectrometry was used to quantitatively analyze the elements dissolved into the hydrochloric acid solution using the above method, thereby determining the composition of the coating.

Confirm, as follows, whether the boundary between the plating layer and the base steel constitutes a surface that satisfies the irregular shape of equation (1).

First, small samples measuring 20mm × 15mm × 3.2mm were collected from the coated steel. After resin embedding, the samples were ground until mirror-polished, exposing a cross-section perpendicular to the surface of the coated steel. The sample was observed using a field emission scanning electron microscope (SEM) (JEOL JSM-7000F, accelerating voltage: 15kV) to obtain image data. A 100μm section of the coated steel, parallel to the surface of the coated steel and viewed at SEM magnification of 1000x or higher, was designated as the observation area. The image resolution was set to at least 2560 pixels horizontally and at least 1920 pixels vertically.

For the image data, _L0 and L are obtained using the above method. Furthermore, the coating and the base steel are distinguished based on the Fe concentration of 90% obtained from the above EPMA analysis, and the portion with a Fe concentration of 90% or higher is designated as the base steel. In addition, the relationship between _L0 and L is evaluated to see if it satisfies Equation (1). The calculation results (values) on the left side of Equation (1) are shown in Tables 3A and 3B.

Furthermore, the boundaries between the first, second, and third regions of the coating layer are determined as follows:

A small sample (20mm × 15mm × 3.2mm) was collected from the plated steel. After resin embedding, it was ground until mirror-polished, exposing a section perpendicular to the surface of the plated steel. The sample was observed using an electron beam probe microanalyzer with scanning electron microscopy (SEM-EPMA, EPMA measuring device: JEOL JXA-8230, accelerating voltage: 15kV, current: 0.05μA, irradiation time: 50ms), and the Fe concentration was determined using point analysis.

First, as part of the first stage of measurement, as shown in Figure 3, straight analytical lines, parallel to the thickness direction of the exposed coating, are established on the cross-section of the coating. Ten analytical lines are evenly spaced along the thickness direction of the coating. Ten measurement points are then set at 10 μm intervals along each analytical line, in a direction orthogonal to the thickness direction of the coating. In Figure 3, the single-point chain represents the analytical line, and the black dots on the analytical line represent the measurement points.

At each measurement point of the analytical line, the Fe concentration (mass %) in the plating layer was determined by point analysis. The measurement conditions for point analysis were as described above. The average Fe concentration at 10 measurement points was calculated for each analytical line, and this average was set as the Fe concentration for that analytical line. Then, analytical line A, where the Fe concentration was less than 5.0% and closest to 5.0%, and analytical line B, where the Fe concentration exceeded 5.0% and closest to 5.0%, were identified. Further, analytical line C, where the Fe concentration was less than 30.0% and closest to 30.0%, and analytical line D, where the Fe concentration exceeded 30.0% and closest to 30.0%, were identified.

Next, as the second stage of measurement, the analytical line for 5.0% Fe concentration and the analytical line for 30.0% Fe concentration were identified using the above method. Using the 5.0% Fe concentration analytical line as the dividing line, the area closer to the surface of the plated steel than this analytical line was designated as Region 1. Similarly, using the 30.0% Fe concentration analytical line as the dividing line, the area closer to the parent steel than this analytical line was designated as Region 3. Furthermore, the area between the 5.0% Fe concentration analytical line and the 30.0% Fe concentration analytical line was designated as Region 2.

Next, the area ratio of the phase/structure in regions 1, 2, and 3 was identified as follows: Using the samples used to identify regions 1, 2, and 3, cross-sections of the coating in the thickness direction perpendicular to the surface of the base steel were observed. The SEM magnification was set to 200–50,000x to confirm the cross-section of the coating in a region of 36,000 ^μm² . Since SEM may reveal localized views of the coating, 25 views were selected from the aforementioned cross-sections to obtain average information about the coating. That is, the metal structure in a total field of view ^of 36000×25μm2 is observed to determine the area fraction of the phase or structure of the metal structure that constitutes the coating.

First, the eutectic structure consisting of Zn and _MgZn2 phases in a layered manner is defined as the [Zn/ _MgZn2 binary eutectic structure]. For regions other than the [Zn/ _MgZn2 binary eutectic structure], the phase containing Zn and 20-99% by mass of Al is defined as the Al-containing phase. Furthermore, the phase containing Zn and 20-60% by mass of Mg is defined as the Mg-Zn phase. Further, the phase containing Al and 20-60% by mass of Fe is defined as the Fe-Al alloy phase. Then, the area fraction of these phases/structures is determined.

Furthermore, the presence of the _Mg₂Sn phase was confirmed using the above method. The presence of the _Mg₂Sn phase was determined when a diffraction peak was detected at 23.4 ± 0.3°. The results are shown in Tables 3A and 3B.

The coating adhesion was evaluated as follows: The coated steel was subjected to a stone impact test. Using a gravelometer, at an air pressure of 400 kPa and room temperature, 400g of No. 6 gravel (JIS A500, particle size 5~13mm) was thrown at a distance of 50cm to impact the test piece. The projection angle was set to 90° with the surface of the base steel. Subsequently, the test piece was ultrasonically cleaned (room temperature, pure water, 10 minutes). The coating adhesion was evaluated based on the difference (weight reduction) between the weight per unit area (g/ ^m² ) of the test piece before ultrasonic cleaning and the weight per unit area (g/ ^m² ) of the test piece after ultrasonic cleaning. The evaluation criteria are as follows, with AAA, AA, and A set as qualified.

AAA: Weight reduction less than 10g/ ^m²

AA: Weight reduction of 10g/ ^m² or more but less than 60g/ ^m²

A: Weight reduction to 60g/ ^m² or more but less than 120g/ ^m²

B: Weight reduced to 120g/ ^m² or more

The corrosion resistance after coating was evaluated as follows: A primer, intermediate coating, and topcoat were applied to the surface of the plating layer on the coated steel. The primer was formed by applying an epoxy resin coating for galvanizing (trade name "HI-PON 20 DECHLO") manufactured by Nippon Paint Co., Ltd., with a thickness of 50 μm. The intermediate coating was formed by applying "HI-PON 30 MASTIC MIDDLE COAT K" manufactured by Nippon Paint Co., Ltd., with a thickness of 30 μm. The topcoat was formed by applying "HI-PON 50 Top Coat" manufactured by Nippon Paint Co., Ltd. of Japan, with a film thickness of 30μm. For the coated steel, a cutting mark was made extending to the base steel using a cutting tool, and the corrosion acceleration test specified in JASO-CCT-M609 was performed. After 360 cycles, the corrosion resistance after coating was determined based on the maximum expansion width on one side of the cutting mark. The evaluation criteria are as follows, with AAA, AA, and A considered acceptable.

AAA: Below 0.5mm

AA: ≥0.5mm but <1mm

A: More than 1mm but less than 2mm

B: Exceeding 2mm

Resistance to red rust is evaluated as follows: The coated steel is subjected to the corrosion acceleration test specified in JASO-CCT-M609. The number of cycles leading to red rust is then determined. Based on the number of cycles leading to red rust, the following evaluation criteria are used for assessment.

AAA: The red rust formation cycle is over 1200 cycles.

AA: The red rust formation cycle is more than 900 cycles but less than 1200 cycles.

A: The red rust formation cycle is between 540 and 900 cycles.

B: The red rust formation cycle has not reached 540 cycles.

The corrosion resistance of the base steel was evaluated as follows: Samples of the plated steel, 150 mm in length and 70 mm in width, were subjected to the corrosion acceleration test specified in JASO-CCT-M609. The evaluation was then conducted by measuring the corrosion depth (μm) in the samples after 1560 cycles. During the measurement, a 30 mm section was taken at half the length (75 mm) and the center of the width direction of the sample, and observed. During observation, the section embedded in the resin and mirror-polished was examined at 40x magnification using an optical microscope, and the observed field of view was photographed. The maximum corrosion depth (μm) was measured at 30 mm in the width direction of the section. The evaluation criteria are shown below, with AAA, AA, and A designated as passing.

AAA: less than 50μm

AA: 50μm or larger but less than 200μm

A: Above 200μm but below 800μm

B: Above 800μm

Sacrificial corrosion resistance is evaluated as follows: Using a precision cutter, the coated steel is cut perpendicular to its surface, exposing the cut end. That is, at the cut end, the cross-section of the coating and the cross-section of the base steel are exposed. A neutral saline spray test as specified in JIS Z2371:2015 is performed on this cut end to determine the time (h) until red rust forms on the cut end. The evaluation criteria are as follows, with AAA, AA, and A considered acceptable.

AAA: 2400h and above

AA: 1500h or more but less than 2400h.

A: 720h or more but less than 1500h

B: Not 720h reached

As shown in Tables 1A to 4B, the chemical composition of the coating, the thickness of the first region, the area fraction of the Al-containing phase in the first region, the thickness of the second region, and the thickness of the third region in Examples 1 to 35 are within the scope of this invention. Therefore, the coating exhibits excellent adhesion, post-coating corrosion resistance, red rust resistance, base metal corrosion resistance, and sacrificial corrosion resistance.

Furthermore, in the first region of Examples 1-35, as a remainder, one or more of the following phases are present: _Mg₂Si phase, _Mg₂Sn phase, Zn phase, Al-Ca-Zn phase, Al-Ca-Si-Zn phase, Ca-Zn phase, and Mg-Al-Si-Zn phase. Further, in a portion of the first region of the examples, Fe-Al alloy phase is present at an area fraction of 3% or less. Also, in the third region of Examples 1-35, Fe-Al alloy phase is present as a remainder. Further, sometimes one or more of the following phases are present as a remainder in the second and third regions: _Mg₂Si phase, _Mg₂Sn phase, Zn phase, Al-Ca-Zn phase, Al-Ca-Si-Zn phase, Ca-Zn phase, and Mg-Al-Si-Zn phase.

On the other hand, as shown in Tables 1A to 4B, in Comparative Examples 36 to 46, any one of the following—the chemical composition of the coating, the value (%) on the left side of equation (1), the thickness of the first region, the area fraction of the Al-containing phase in the first region, the thickness of the second region, and the thickness of the third region—is outside the scope of the present invention. Consequently, at least one of the following properties of the coating—adhesion, post-coating corrosion resistance, red rust resistance, base metal corrosion resistance, and sacrificial corrosion resistance—is poor.

In Comparative Example 36, the Al content of the plating layer was insufficient. Therefore, the thickness of regions 2 and 3 was insufficient, resulting in poor resistance to red rust and poor corrosion protection of the base iron.

In Comparative Example 37, the Al content in the plating layer was excessive. Therefore, the flux reaction became unsuccessful, and the appearance of the plating layer deteriorated significantly.

In Comparative Example 38, the Mg content in the coating was insufficient, while the Fe content was excessive. Therefore, the corrosion resistance, red rust resistance, base iron corrosion resistance, and sacrifice corrosion protection were poor after coating.

In Comparative Example 39, the amount of Mg in the plating layer was excessive. This caused the flux reaction to become unsuccessful, resulting in a significantly deteriorated appearance.

In Comparative Example 40, the Si content of the plating layer was excessive, while the Fe content was insufficient. Therefore, the thickness of regions 2 and 3 was insufficient, resulting in poor resistance to red rust and poor corrosion protection of the base iron.

In Comparative Example 41, the Ca content in the plating layer was excessive. Furthermore, the Fe content was insufficient. Consequently, the flux reaction became unsuccessful, resulting in a significantly deteriorated appearance.

In Comparative Example 42, the amount of spraying in the spraying step was insufficient. Therefore, it did not meet equation (1), resulting in poor adhesion of the coating layer.

In Comparative Example 43, the cooling rate during the cooling step was too fast. Therefore, the area fraction of the Al-containing phase in Region 1 fell outside the scope of this invention, resulting in poor corrosion resistance after coating.

In Comparative Example 44, the immersion time in the plating bath was insufficient. Therefore, the thickness of region 2 was insufficient, resulting in poor resistance to red rust.

In Comparative Example 45, the immersion time in the plating bath was insufficient. Therefore, the thickness of region 3 was insufficient, resulting in poor corrosion resistance of the base metal.

Comparative Example 46 was not shot-blasted. Therefore, sufficient strain could not be introduced into the pre-plating base steel, resulting in substandard adhesion of the plating layer (1).

[Industry-level applicability]

The coated steel of this invention exhibits excellent adhesion, post-coating corrosion resistance, red rust resistance, base metal corrosion resistance, and sacrificial corrosion resistance. Therefore, it has potential applications in civil and infrastructure fields, or in automotive parts.

(without)

Claims (16)

A plated steel material has a base steel material and a plated layer disposed on the surface of the base steel material. The chemical composition of the plated layer, by mass%, contains Al: 5.0-40.0%, Mg: 0.5-15.0%, Fe: 5.0-40.0%, Si: 0-2.0%, Ca: 0-2.0%, and further contains one or two of the elements selected from the group consisting of group A and group B, with the remainder being Zn and impurities. In a cross-section perpendicular to the surface of the plated steel material, when a cross-section of the plated steel material of a predetermined length parallel to the surface is used as the observation area, the length L of the boundary line between the plated layer and the base steel material satisfies the following formula (1). The aforementioned plating layer comprises: a first region disposed on the aforementioned surface side of the plated steel and having an Fe concentration of less than 5.0% by mass; a second region adjacent to the aforementioned first region and having an Fe concentration of 5.0% or more but less than 30.0% by mass; and a third region disposed between the aforementioned second region and the aforementioned mother steel and having an Fe concentration of 30.0% or more but less than 80.0% by mass. The thickness of the aforementioned first region is 5 to 100 μm, the thickness of the aforementioned second region is 5 to 100 μm, and the thickness of the aforementioned third region is 5 to 100 μm. The aforementioned first region contains 0% or more but less than 5% Al-containing phase by area, and the Al-containing phase contains Zn and 20 to 99% by mass of Al; [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% of one or more of the following: totaling 0-5%; (L- _L0 )/ _L0 ×100≧2.0(%)…(1) In Equation (1), _L0 is the straight-line distance between one end and the other end of the aforementioned boundary line in the aforementioned observation area, and L is the length of the aforementioned boundary line between the aforementioned one end and the aforementioned other end. For example, the plated steel in Request 1 satisfies the following formula (2); (L－ _L0 )/ _L0 ×100≧4.0(%)…Formula (2) Wherein, _L0 in Formula (2) is the straight-line distance between one end and the other end of the aforementioned boundary line in the aforementioned observation area, and L is the length of the aforementioned boundary line between the aforementioned one end and the aforementioned other end. For example, the plated steel in Request 1 satisfies the following formula (3): (L－ _L0 )/ _L0 ×100≧6.0(%)…Formula (3) Wherein, _L0 in Formula (3) is the straight-line distance between one end and the other end of the aforementioned boundary line in the aforementioned observation area, and L is the length of the aforementioned boundary line between the aforementioned one end and the aforementioned other end. For example, the plated steel of any of claims 1 to 3, wherein the thickness of the aforementioned first region is 15 μm to 100 μm. For any of the steel plates requested in items 1 to 3, the area ratio of the Al-containing phase in the aforementioned first region does not reach 1%. For example, the plated steel of any one of claims 1 to 3, wherein the aforementioned first region contains: a Mg-Zn phase comprising Zn and 20 to 60% by mass of Mg, and the area fraction of the aforementioned Mg-Zn phase in the aforementioned first region is 2% or more. For example, the plated steel of any one of claims 1 to 3, wherein the aforementioned first region contains a binary eutectic structure of Zn phase and Mg-Zn phase, and the area ratio of the aforementioned binary eutectic structure in the aforementioned first region is 1% or more. For example, the plated steel of any of the claims 1 to 3, wherein the thickness of the aforementioned second region is 15 μm to 100 μm. For example, the plated steel of any one of claims 1 to 3, wherein the aforementioned second region contains: a Mg-Zn phase comprising Zn and 20 to 60% by mass of Mg, and the area fraction of the aforementioned Mg-Zn phase in the aforementioned second region is 5% or more. For any of the claims 1 to 3, the plated steel contains an Fe-Al alloy phase with an equivalent circular diameter of less than 15 μm and an aspect ratio of more than 2 in the aforementioned second region, and the area fraction of the aforementioned Fe-Al alloy phase in the aforementioned second region is more than 5%. For example, the plated steel of any one of claims 1 to 3, wherein the aforementioned second region contains a binary eutectic structure of Zn phase and Mg-Zn phase, and the area ratio of the aforementioned binary eutectic structure in the aforementioned second region is 2% or more. For example, the plated steel of any of the claims 1 to 3, wherein the thickness of the aforementioned third region is 15 μm to 100 μm. For any of the claims 1 to 3, the plated steel contains a Mg-Zn phase comprising Zn and 20 to 60% by mass of Mg in the aforementioned third region, and the area fraction of the aforementioned Mg-Zn phase in the aforementioned third region is 10% or more. For any of the coated steels in claims 1 to 3, the content of Mg relative to the total amount of Zn and Mg in the aforementioned coating chemical composition (Mg/(Zn+Mg)(%)) is 5.0% or more. For any of the coated steels in claims 1 to 3, the content of Mg relative to the total amount of Zn and Mg in the aforementioned coating chemical composition (Mg/(Zn+Mg)(%)) is 6.5% or more. For example, the plated steel of any one of claims 1 to 3, wherein the aforementioned plating layer contains Sn at a concentration of 0.02 to 2.0% by mass, and the aforementioned plating layer contains _Mg2Sn phase.