JP7614046B2 - Fluororesin coated steel sheet - Google Patents

Description

The present invention relates to a coated steel sheet with excellent workability over time.

Hot-dip Al-Zn coated steel sheets (hereinafter sometimes referred to as "55% Al-Zn coated steel sheets"), which have a standard composition with the coating layer containing approximately 55 mass% Al, approximately 1.6 mass% silicon, and the remainder being zinc, have shown excellent corrosion resistance and, in recent years, their demand has been increasing mainly in the field of building materials.
Furthermore, fluororesin-coated steel sheets, which are made by coating 55% Al-Zn-plated steel sheets with a fluororesin coating, are used as wall materials for houses and stores as metal siding, combining complex patterns shaped with interlocking shapes to imitate stone or wood using an embossing roll, due to the excellent workability and weather resistance of fluororesin.

For example, Patent Document 1 discloses a fluororesin-coated steel sheet, which has a topcoat layer containing a specific vinylidene fluoride resin formed on the uppermost surface of a metal sheet having a chemical conversion treatment layer on the surface and a primer layer on top of that. For example, as described in Patent Document 1, vinylidene fluoride resin is a thermoplastic resin, and by suppressing crystallization by rapid cooling after baking and making it amorphous, it is possible to achieve excellent processability.

For example, Patent Documents 2 to 4 disclose techniques that use glass flakes, ceramic fibers, silicon carbide powder, acrylic polymer beads, tetrafluoroethylene resin powder, and the like as materials that make up the topcoat layer, thereby improving scratch and abrasion resistance while maintaining excellent processability.

However, although such fluororesin-coated steel sheets have a certain degree of effect in improving workability and weather resistance as described above, there is a problem in that the fluororesin crystallizes over time, causing a deterioration in workability (workability over time problem).

Therefore, various techniques have been investigated to solve the problem of the aging processability of fluororesin-coated steel sheets.
For example, Patent Document 5 discloses a technique in which fine fluororesin particles of 5 μm or less are used as a material constituting the topcoat layer to promote compatibility with acrylic resin and suppress crystallization of the fluororesin.
However, although the technology of Patent Document 5 can improve processability to a certain extent, crystallization cannot be sufficiently suppressed in the fluororesin topcoat layer after about three months, and cracks occur in the topcoat layer at the locations where the process is severe during embossing roll processing in siding molding. Therefore, there is still a need to develop a technology that can improve processability over time.

Patent No. 1853338 JP 2001-137775 A Publication No. 2003-71980 Japanese Patent Application Publication No. 9-221621 Japanese Patent Application Publication No. 6-262139

In view of these circumstances, the present invention aims to provide a coated steel sheet that has excellent workability over time while maintaining other physical properties such as workability and weather resistance.

The present inventors conducted research to solve the above problems with respect to fluororesin-coated steel sheets in which a chemical conversion coating and a primer layer, as well as a topcoat layer containing a fluororesin, are formed on a hot-dip Al-Zn-plated steel sheet. As a result, they discovered that by specifying the mass ratio of the polyvinylidene fluoride resin constituting the topcoat layer within a specific range and by incorporating an inorganic filler that acts as a crack propagation inhibitor and an organic aggregate that acts as a stress relaxation agent in the topcoat layer, it is possible to suppress crack propagation in the topcoat layer and ensure flexibility, regardless of whether the fluororesin crystallizes, and therefore it is possible to achieve excellent workability over time while maintaining other physical properties such as workability and weather resistance.

The present invention has been made based on the above findings, and the gist of the present invention is as follows.
1. A coated steel sheet having a chemical conversion coating, a primer layer and a topcoat layer formed on a hot-dip Al-Zn-plated steel sheet,
The top coat layer comprises a resin component containing a polyvinylidene fluoride resin and an acrylic resin, the polyvinylidene fluoride resin being present in an amount of 50 to 80% by mass;
A color pigment;
0.1 to 20 mass% of an inorganic filler having an average particle size of 1 to 30 μm and an aspect ratio of 5 or more;
0.1 to 15 mass% of organic aggregate having an average particle size of 1 to 35 μm and a micro Vickers hardness of 50 HV _0.05 or less;
A fluororesin-coated steel sheet comprising:

2. The fluororesin-coated steel sheet according to 1, characterized in that the total content of the inorganic filler and the organic aggregate in the topcoat layer is 0.2 to 35 mass%.

3. The fluororesin-coated steel sheet according to 1 or 2, characterized in that the coating layer of the hot-dip Al-Zn-coated steel sheet contains 20-95% by mass of Al, 1-3% by mass of Si, and 5% by mass or less of optional added components, with the remainder being Zn and unavoidable impurities.

4. The fluororesin-coated steel sheet according to 3, characterized in that the plating layer contains 50-60 mass% Al, 1-3 mass% Si, and 5 mass% or less of optional added components, with the remainder being Zn and unavoidable impurities.

5. The fluororesin-coated steel sheet according to item 3 or 4 above, characterized in that the plating layer has a dendritic phase and an interdendritic phase, and the Vickers hardness of the dendritic phase is 10 to 110 _HV0.1.

The present invention provides coated steel sheets that have excellent workability over time while maintaining other physical properties such as workability and weather resistance.

1 is a cross-sectional view that illustrates a cross section of one embodiment of a fluororesin-coated steel sheet of the present invention.

Below, an embodiment of the fluororesin-coated steel sheet of the present invention will be described with reference to the drawings as necessary. Note that each component shown in the drawings is shown diagrammatically for ease of explanation, and some parts may differ from the actual dimensions and shapes.

The fluororesin-coated steel sheet of the present invention (hereinafter, sometimes referred to as "the coated steel sheet of the present invention") is a coated steel sheet in which a chemical conversion layer 20, a primer layer 30 and a top coat layer 40 are formed on a hot-dip Al-Zn-plated steel sheet 10, as shown in FIG.
The top coat layer 40 includes a resin component 41 containing polyvinylidene fluoride resin and acrylic resin, and the mass ratio of the polyvinylidene fluoride resin is 50 to 80%;
A color pigment (not shown);
0.1 to 20 mass % of inorganic filler 42 having an average particle size of 1 to 30 μm and an aspect ratio of 5 or more;
0.1 to 15 mass% of organic aggregate 43 having an average particle size of 1 to 35 μm and a micro Vickers hardness of 50 HV _0.05 or less;
The present invention is characterized by comprising:

The topcoat layer contains inorganic filler 42 having an average particle size of 1 to 30 μm and an aspect ratio of 5 or more, so that inorganic filler 42 can act as a crack propagation inhibitor. Also, the topcoat layer contains organic aggregate 43 having an average particle size of 1 to 35 μm and a micro Vickers hardness of 50 HV _0.05 or less, so that organic aggregate 43 can act as a stress relaxation agent. As a result, when the coated steel sheet is bent, the propagation of cracks generated from the surface layer side of topcoat 40 can be suppressed, and the tensile stress applied to topcoat layer 40 can be dispersed to suppress the generation of cracks, so that the fluororesin coated steel sheet of the present invention can achieve excellent aging resistance.
Furthermore, by including a certain amount of polyvinylidene fluoride resin in the topcoat layer 40 (50 to 80% by mass in the base resin), it is possible to maintain good processability, weather resistance, and the like.

Hereinafter, each member constituting the fluororesin-coated steel sheet of the present invention will be described.
As described above, the fluororesin-coated steel sheet of the present invention has a chemical conversion layer 20, a primer layer 30 and a top coat layer 40 formed on a hot-dip Al-Zn-plated steel sheet 10, and has a chemical conversion layer 20, a primer layer 30 and a top coat layer 40 formed on the hot-dip Al-Zn-plated steel sheet 10.

(Hot-dip Al-Zn coated steel sheet)
The fluororesin-coated steel sheet of the present invention uses a hot-dip Al-Zn-plated steel sheet as the steel sheet, which can ensure high corrosion resistance.
Here, the type of the hot-dip Al-Zn-plated steel sheet is not particularly limited and can be appropriately selected depending on the required performance and application. For example, when the fluororesin-coated steel sheet of the present invention is used as a building material, high durability is required, so that a 55% Al-Zn-plated steel sheet or the like can be used.

The hot-dip Al-Zn-plated steel sheet has a plating layer formed on a base steel sheet. The composition of the plating layer is not particularly limited as long as it has good corrosion resistance. For example, the plating layer of the hot-dip Al-Zn-plated steel sheet may have a composition containing 20-95% by weight of Al, 1-3% by weight of Si, and 5% by weight or less of optional added components, with the balance being Zn and unavoidable impurities. By having the plating layer have the above-mentioned composition, a dendritic phase and an interdendritic phase that surrounds the dendritic phase in a mesh-like shape can be formed in the plating layer, thereby improving corrosion resistance.

From a similar perspective, it is preferable that the plating layer has a composition of "plating bath components" as defined in JIS G 3321 (2019) 5.1, specifically, a composition containing Al: 50-60 mass%, Si: 1-3 mass%, and optional added components: 5 mass% or less, with the remainder consisting of Zn and unavoidable impurities.

Here, the Al content in the plating layer is 20 to 95 mass%, preferably 50 to 60 mass%, in consideration of the balance between corrosion resistance and operability. If the Al content in the plating layer is at least 20 mass%, dendritic solidification of Al occurs sufficiently. As a result, the main layer mainly contains Zn in a supersaturated state, and is composed of a portion where Al is solidified into dendrites (α-Al dendritic phase) and the remaining portion between the dendrites (interdendritic phase), and the dendritic phase is laminated in the thickness direction of the plating layer, thereby realizing a structure with excellent corrosion resistance.
In addition, the more the α-Al phase dendrites are stacked, the more complex the corrosion path becomes, making it more difficult for corrosion to reach the base steel sheet, improving corrosion resistance. On the other hand, if the Al content in the coating layer exceeds 95 mass%, the content of Zn, which has a sacrificial corrosion protection effect against Fe, decreases, deteriorating corrosion resistance. For this reason, the Al content in the coating layer is set to 95 mass% or less.

In addition, the Si in the coating layer is added to the coating bath for the purpose of suppressing the growth of an interfacial alloy layer formed at the interface with the base steel sheet, and for the purpose of improving corrosion resistance and workability, and is inevitably contained in the main layer. In the case of the hot-dip Al-Zn-coated steel sheet used in the coated steel sheet of the present invention, when the hot-dip coating process is performed with Si contained in the coating bath, the Fe on the steel sheet surface reacts with the Al and Si in the bath to form an alloy consisting of Fe-Al and/or Fe-Al-Si compounds as soon as the base steel sheet is immersed in the coating bath. The formation of this Fe-Al-Si-based interfacial alloy layer can suppress the growth of the interfacial alloy layer. And, when the Si content in the coating layer is 1% by mass or more, the growth of the interfacial alloy layer can be sufficiently suppressed. On the other hand, when the Si content in the coating layer exceeds 3% by mass, the coating layer is easily precipitated with a Si phase that reduces workability and becomes a cathode site. For this reason, the Si content in the coating layer is set to 3% by mass or less.

The plating layer contains Zn as the main component of the plating layer. By including Zn in the plating layer, a sacrificial corrosion protection effect can be obtained, and it is possible to improve corrosion resistance. On the other hand, when the Zn content is 80 mass% or less, it is preferable in that the Al content can be secured and the above-mentioned corrosion resistance due to the dendrite phase and interdendrite phase can be realized.

Furthermore, the plating layer may contain up to 5 mass % of optional additive elements in addition to the above-mentioned Al, Si, and Zn.
The optional additive components can be appropriately selected depending on the performance required for the plating layer, and examples thereof include alkaline earth metals such as Ca and Mg, and additive components such as Mn, V, Cr, Mo, Ti, Sr, Ni, Co, Sb, and B.
Although these optional components have the effect of further improving corrosion resistance, they may reduce the workability of the plating layer and deteriorate the ultimate elongation of the coated steel sheet, so the content of these optional components is preferably 5 mass% or less.

The plating layer may contain Mg and/or Ca as the optional additive component from the viewpoint of corrosion resistance. When the plating layer corrodes, Mg and/or Ca are contained in the corrosion product, which improves the stability of the corrosion product and retards the progress of corrosion, resulting in an effect of improving corrosion resistance. The total content of Ca and/or Mg is not particularly limited as long as it is 5 mass% or less, but is preferably 0.01 to 5 mass%. By setting the content to 0.01 mass% or more, a sufficient corrosion retardation effect can be obtained, while by setting the content to 5 mass% or less, the effect does not saturate, an increase in production costs can be suppressed, and the composition of the plating bath can be easily controlled.
The plating layer preferably contains at least Mg. When the plating layer contains Mg, Mg ₂ Si can be produced together with Si, and a corrosion retardation effect can be obtained. The content of Mg in the plating layer is preferably 0.01 to 5 mass%, and more preferably 2 to 4.9 mass%.

Furthermore, like the alkaline earth metals Ca and Mg as optional additives, the plating layer may further contain one or more of Mn, V, Cr, Mo, Ti, Sr, Ni, Co, Sb, and B as optional additives in a total amount of 5 mass% or less, preferably 0.01 to 5 mass%, since these have the effect of improving the stability of corrosion products and retarding the progression of corrosion.

The plating layer contains components of the base steel sheet that are incorporated into the plating due to the reaction between the plating bath and the base steel sheet during plating processing, and unavoidable impurities in the plating bath. The base steel sheet components that are incorporated into the plating may contain up to about 2% Fe. Examples of the types of unavoidable impurities in the plating bath include Fe, Cu, Zr, etc. Fe in the plating layer cannot be quantified separately as being incorporated from the base steel sheet and being in the plating bath. There are no particular limitations on the total content of unavoidable impurities, but from the viewpoint of maintaining the corrosion resistance and uniform solubility of the plating, it is preferable that the total amount of unavoidable impurities excluding Fe is 1 mass% or less.

The interfacial alloy layer is the layer of the plating layer that exists at the interface with the base steel sheet, and as described above, is an Fe-Al and/or Fe-Al-Si compound that is inevitably produced by an alloying reaction between the Fe on the steel sheet surface and the Al and Si in the bath. This interfacial alloy layer is hard and brittle, and if it grows too thick it will become the starting point for cracks during processing, so it is preferable to make it as thin as possible.

The means for forming the coating layer on the base steel sheet is not particularly limited, and a normal continuous hot-dip coating facility can be used. For example, the base steel sheet is heated to a predetermined temperature in an annealing furnace maintained in a reducing atmosphere, and while annealing, rolling oil and the like adhering to the steel sheet surface are removed, and the oxide film is reduced and removed. The steel sheet is then immersed in a hot-dip galvanizing bath containing a predetermined concentration of Al and Zn through a snout whose lower end is immersed in the coating bath. The steel sheet immersed in the coating bath is then pulled up above the coating bath via a sink roll, and the coating amount is adjusted by spraying pressurized gas from a gas wiping nozzle arranged above the coating bath toward the surface of the steel sheet, and then cooled by a cooling device to form the coating layer.

Furthermore, when the coating layer does not contain any optional additives, the critical elongation of the hot-dip Al-Zn coated steel sheet before the coating is formed can be improved to about 20% or more by, for example, subjecting it to a heat treatment at 200°C for 24 hours. This is believed to be because the Zn supersaturated in the aluminum-rich dendritic phase is expelled by the heat treatment, softening the coating layer.
On the other hand, in the case of a coating layer having a composition containing optional added elements, the effect of improving workability by such heat treatment is small, and for example, when 2 to 4.9% of Mg is added as the optional added element, the limit elongation after heat treatment remains at less than 5%. The reason for this is not yet clear, but it is presumed that it is influenced by the fact that Mg has a high solubility in Al and therefore remains as a solid solution element in the aluminum-rich dendritic phase even after heat treatment.

Furthermore, the structure of the coating layer contains a dendritic phase and an interdendritic phase, and the Vickers hardness of the dendritic phase is preferably 10 to 110 Hv _{0.01. By reducing the Vickers hardness of the dendritic phase to 10 to 110 Hv 0.01 ,} the workability of the coated steel sheet can be improved and the corrosion resistance after processing can be further improved. If the Vickers hardness of the dendritic phase exceeds 110 Hv _0.01 , there is a risk that sufficient workability cannot be obtained, while if the Vickers hardness of the dendritic phase is less than 10 Hv _0.01 , there is a risk that the scratch resistance of the coating layer surface will be reduced. From the same viewpoint, the Vickers hardness of the dendritic phase is preferably 20 to 100 Hv _0.01 , and more preferably 30 to 90 Hv _0.01 .

The Vickers hardness of the dendrite phase can be measured by embedding a sample fluororesin-coated steel sheet in a room-temperature drying resin, polishing it, selecting the dendrite phase of the plating layer from the exposed cross section, and measuring the Vickers hardness of the selected dendrite phase using a microhardness tester (e.g., Shimadzu Corporation's HMV-G21 microhardness tester). The measurement method complies with JIS Z 2244, and in the case of the plating layer, the test is performed under an indentation load of 0.98 mN (0.1 gf) (Hv _0.1 ).

(chemical conversion coating)
In the fluororesin-coated steel sheet of the present invention, a chemical conversion coating is formed on the plating layer of the hot-dip galvanized steel sheet. By forming a chemical conversion coating on the plating layer, the corrosion resistance of the fluororesin-coated steel sheet can be further improved.

The type and forming conditions of the chemical conversion coating are not particularly limited, and can be appropriately selected depending on the required performance.
For example, it can be formed by a chromate treatment or chromate-free chemical conversion treatment in which a chromate treatment liquid or a chromium-free chemical conversion treatment liquid is applied and then dried without rinsing at a steel sheet temperature of 80 to 300° C. When the working environment and the like are taken into consideration, it is preferable that the chemical conversion coating be formed by a chromate-free treatment (i.e., the chemical conversion coating does not contain chromium).

(Primer layer)
In the fluororesin-coated steel sheet of the present invention, a primer layer is formed between the chemical conversion coating and the topcoat layer. By forming a primer layer between the chemical conversion coating and the topcoat layer, the adhesion of the topcoat layer described below to the hot-dip Al-Zn-plated steel sheet can be further improved, and the corrosion resistance and rust prevention properties can also be further improved.

The thickness of the primer layer is not particularly limited, and can be appropriately adjusted depending on the required performance.
For example, from the viewpoint of achieving both rust prevention and processability, the thickness of the primer layer can be set to 2 to 15 μm. When the thickness is 2 μm or more, sufficient rust prevention is obtained, while when the thickness is 15 μm or less, sufficient processability is ensured.

Here, the primer layer may contain a rust inhibitor from the viewpoint of improving the rust resistance of the coated steel sheet. The rust inhibitor may be either a chromate type that contains chromate salts or a chromate-free type that does not use chromate salts. However, when considering the working environment, etc., a chromate-free type (i.e., the primer layer does not contain chromium) is preferable, and the following describes a chromate-free type primer layer.

The resin constituting the matrix component of the primer layer is not particularly limited, but it is preferable to use a polyester resin and/or an epoxy resin from the viewpoints of workability, corrosion resistance, rust prevention, etc.

The polyester resin preferably contains a polyester resin having a urethane bond as a main component.
Here, as the polyester resin having a urethane bond, a known resin such as a resin obtained by reacting a polyester polyol with a diisocyanate or polyisocyanate having two or more isocyanate groups can be used.
Also usable is a resin obtained by reacting a polyester polyol with a diisocyanate or polyisocyanate having two or more isocyanate groups in a state of excess hydroxyl groups (urethane-modified polyester resin) and curing the resin with a blocked polyisocyanate.

The polyester polyol can be obtained by a known method utilizing a dehydration condensation reaction between a polyhydric alcohol component and a polybasic acid component.
The polyhydric alcohol includes glycols and trihydric or higher polyhydric alcohols. Examples of glycols include ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, polyethylene glycol, polypropylene glycol, neopentyl glycol, hexylene glycol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2-butyl-2-ethyl-1,3-propanediol, methylpropanediol, cyclohexanedimethanol, and 3,3-diethyl-1,5-pentanediol. Examples of trihydric or higher polyhydric alcohols include glycerin, trimethylolethane, trimethylolpropane, pentaerythritol, and dipentaerythritol. These polyhydric alcohols can be used alone or in combination of two or more.
As the polybasic acid, a polyvalent carboxylic acid is usually used, but a monovalent fatty acid or the like can be used in combination as necessary. Examples of the polyvalent carboxylic acid include phthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, 4-methylhexahydrophthalic acid, bicyclo[2,2,1]heptane-2,3-dicarboxylic acid, trimellitic acid, adipic acid, sebacic acid, succinic acid, azelaic acid, fumaric acid, maleic acid, itaconic acid, pyromellitic acid, dimer acid, and the like, and acid anhydrides thereof, 1,4-cyclohexanedicarboxylic acid, isophthalic acid, tetrahydroisophthalic acid, hexahydroisophthalic acid, hexahydroterephthalic acid, and the like. These polybasic acids can be used alone or in combination of two or more.

Examples of the polyisocyanate compounds include aliphatic diisocyanates such as hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, and dimer acid diisocyanate, aromatic diisocyanates such as xylylene diisocyanate (XDI), metaxylylene diisocyanate, tolylene diisocyanate (TDI), and 4,4-diphenylmethane diisocyanate (MDI), and further cyclic aliphatic diisocyanates such as isophorone diisocyanate, hydrogenated XDI, hydrogenated TDI, and hydrogenated MDI, as well as adducts, biurets, and isocyanurates thereof. These polyisocyanate compounds can be used alone or in combination of two or more.

The polyester resin having a urethane bond has both flexibility and strength, and has the effect of suppressing the occurrence of cracks in the primer layer when processed, etc. In addition, it has a high affinity with chemical conversion coatings containing urethane resins, and contributes to improving the corrosion resistance of processed parts in particular.
Here, the hydroxyl value of the polyester resin having a urethane bond is preferably 5 to 120 mgKOH/g, more preferably 7 to 100 mgKOH/g, and even more preferably 10 to 80 mgKOH/g, from the viewpoints of solvent resistance, processability, and the like.

In addition, the number average molecular weight of the polyester resin having urethane bonds is preferably 500 to 15,000, more preferably 700 to 12,000, and even more preferably 800 to 10,000, from the viewpoints of solvent resistance, processability, etc.

The polyester resin is preferably contained in the primer layer in an amount of 40 to 88% by mass. If it is less than 40% by mass, the binder function of the primer layer is reduced, and if it exceeds 88% by mass, the functions of the inorganic substances described below, such as the inhibitor effect, may be reduced. The inorganic substances contained in the primer layer may include vanadium compounds, phosphate compounds, magnesium oxides, etc., which function as inhibitors.

Examples of the vanadium compound acting as the inhibitor include vanadium pentoxide, metavanadate, ammonium metavanadate, vanadium oxytrichloride, vanadium trioxide, vanadium dioxide, magnesium vanadate, vanadyl acetylacetonate, vanadium acetylacetonate, etc. Among these, it is preferable to use a tetravalent vanadium compound or a tetravalent vanadium compound obtained by reduction or oxidation as the vanadium compound. The vanadium compound added to the primer layer may be the same or different from the vanadium compound added to the chemical conversion coating. It is believed that the vanadate ions that gradually dissolve in moisture entering from the outside react with the ions on the surface of the zinc-based plated steel sheet to form a passive coating with good adhesion, protecting the exposed metal parts and exerting a rust-preventing effect.

The vanadium compound content in the primer layer is preferably 4 to 20% by mass. If it is less than 4% by mass, the inhibitor effect may decrease, resulting in a decrease in corrosion resistance, and if it exceeds 20% by mass, it may result in a decrease in the moisture resistance of the primer layer.

As for the type of phosphate compound that acts as the inhibitor, for example, phosphoric acid, ammonium salts of phosphoric acid, alkali metal salts of phosphoric acid, alkaline earth metal salts of phosphoric acid, etc. can be used. Among these, it is preferable to use alkali metal salts of phosphoric acid such as calcium phosphate.

The content of the phosphate compound in the primer layer is preferably 4 to 20% by mass. If it is less than 4% by mass, the inhibitor effect may decrease, resulting in a decrease in corrosion resistance, and if it exceeds 20% by mass, it may result in a decrease in the moisture resistance of the primer layer.

The magnesium oxide acting as an inhibitor has the effect of stabilizing the products produced by initial corrosion as sparingly soluble magnesium salts. The amount of magnesium oxide added to the primer layer is preferably 4 to 20% by mass. If it is less than 4% by mass, the above effect may decrease, leading to a decrease in corrosion resistance, and if it exceeds 20% by mass, the flexibility of the primer layer may decrease, leading to a decrease in corrosion resistance, especially in the processed parts.

The crosslinking agent used in forming the primer layer reacts with the polyester resin having the urethane bond to form a crosslinked coating film, and is preferably a blocked polyisocyanate compound. Examples of the blocked polyisocyanate include polyisocyanates in which the isocyanate groups of the polyisocyanate compound are blocked with, for example, alcohols such as butanol, oximes such as methyl ethyl ketoxime, lactams such as ε-caprolactams, diketones such as acetoacetic diester, imidazole, imidazoles such as 2-ethylimidazole, or phenols such as m-cresol.

(Topcoat layer)
The coated steel sheet of the present invention further comprises a topcoat layer in addition to the above-mentioned hot-dip Al-Zn plated steel sheet, chemical conversion coating and primer layer.
By optimizing the topcoat layer, a coated steel sheet having good corrosion resistance, workability, weather resistance, etc. can be obtained.

The top coat layer contains a resin component as a matrix, and the resin component contains a polyvinylidene fluoride resin (PVdF resin) and an acrylic resin.
The content by mass of the polyvinylidene fluoride resin in the resin component must be 50 to 80%. By including the polyvinylidene fluoride resin in the resin component, it is possible to improve the processability, weather resistance, antifouling properties, etc. of the fluororesin-coated steel sheet.

When the content by mass of the polyvinylidene fluoride resin in the resin component is 45% or more, the performance (processability, weather resistance, antifouling property, durability, etc.) required for the top coat layer can be sufficiently imparted. From the same viewpoint, the content by mass of the polyvinylidene fluoride resin is preferably 50% or more.
In addition, by making the content ratio of the polyvinylidene fluoride resin in the resin component 85% or less by mass, it is possible to suppress a decrease in adhesion of the top coat layer. From the same viewpoint, it is preferable that the content ratio of the polyvinylidene fluoride resin in the resin component is 80% or less by mass.

It is preferable to use polyvinylidene fluoride resin with a weight average molecular weight of 150,000 to 700,000 and a melting point of 150 to 180°C. An example of such a polyvinylidene fluoride resin is "Kynar 500 (weight average molecular weight: 350,000, melting point: 160 to 165°C)" manufactured by Arkema.

The acrylic resin to be mixed with the polyvinylidene fluoride resin preferably has a weight average molecular weight of 50,000 to 200,000. The acrylic resin can be obtained by polymerizing (or copolymerizing) at least one of the following monomers (i) to (iv) (including at least one acrylic monomer) by a conventional method.

(i) Ethylenic monomers having a hydroxyl group, such as hydroxymethyl (meth)acrylate, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, and hydroxybutyl (meth)acrylate.
(ii) Ethylenic monomers having a carboxyl group, such as (meth)acrylic acid, crotonic acid, itaconic acid, fumaric acid, and maleic acid.
(iii) Ethylenic monomers copolymerizable with the above-mentioned monomers (1) and (2), such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl acrylate, n-propyl acrylate, n-butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, n-octyl acrylate, and (meth)acrylic acid alkyl esters.
(iv) Styrene and styrene derivatives such as α-methylstyrene, o-methylstyrene, m-methylstyrene and p-methylstyrene.
Among these monomers, by using a monomer having a functional group such as a hydroxyl group or a carboxyl group, a crosslinking reaction with other reactive components is possible.

Among the above-mentioned monomers (i) to (iv), it is preferable that 80% by mass or more of the constituent monomer units is at least one selected from acrylic acid alkyl ester monomers or methacrylic acid alkyl ester monomers having an alkyl group of 1 to 8 carbon atoms, such as methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, n-propyl acrylate, n-propyl methacrylate, isopropyl acrylate, isopropyl methacrylate, n-butyl acrylate, n-butyl methacrylate, isobutyl acrylate, and isobutyl methacrylate.

However, the amount of methyl methacrylate in the acrylic resin is preferably less than 80% by mass. If the amount of methyl methacrylate is 80% by mass or more, the topcoat layer becomes too hard and its processability decreases. The reason why such alkyl acrylate monomers or alkyl methacrylate monomers are suitable is that they have excellent compatibility with polyvinylidene fluoride resin and also have excellent weather resistance.

The acrylic resin does not need to be self-crosslinking, but if it is to be self-crosslinking, it contains a so-called crosslinking monomer having two or more radically polymerizable unsaturated bonds in the molecule. Examples of radically polymerizable monomers include polymerizable unsaturated compounds such as ethylene glycol diacrylate, ethylene glycol dimethacrylate, triethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, 1,4-butanediol diacrylate, neopentyl glycol diacrylate, 1,6-hexanediol diacrylate, pentaerythritol diacrylate, pentaerythritol trimethacrylate, pentaerythritol tetramethacrylate, glycerol dimethacrylate, glycerol diacrylate, diallyl terephthalate, diallyl phthalate, glycidyl acrylate, and glycidyl methacrylate. The crosslinking monomer can be added in an amount of up to 20% by mass of the acrylic resin.

The resin component may be composed of only the polyvinylidene fluoride resin and the acrylic resin, but may also contain other resins such as polyester-based resins, if necessary.
The polyester resin includes, for example, polyester resin, silicon-modified polyester resin, and acrylic-modified polyester resin, and as a curing agent for the base resin, a polyisocyanate compound and/or an amino resin can be used.

The acrylic resin and polyester resin are used in combination with a curing agent. The curing agent used here can be a polyisocyanate compound or an amino resin.

As the polyisocyanate compound, an isocyanate compound obtained by a general manufacturing method can be used, but it is preferable to use a polyisocyanate compound blocked with a blocking agent such as phenol, cresol, aromatic secondary amine, tertiary alcohol, lactam, or oxime, which can be used as a one-liquid paint. The use of this blocked polyisocyanate compound makes it possible to store it in one liquid form, making it easy to use as a paint.

More preferred polyisocyanate compounds include HDI and its derivatives, TDI and its derivatives, MDI and its derivatives, XDI and its derivatives, IPDI and its derivatives, TMDI and its derivatives, hydrogenated TDI and its derivatives, hydrogenated MDI and its derivatives, and hydrogenated XDI and its derivatives.

In addition, commercially available isocyanate compounds such as Sumidur (product name, manufactured by Sumika Covestro Urethane Co., Ltd.), Desmodur (product name, manufactured by Sumika Covestro Urethane Co., Ltd.), and Coronate (product name, manufactured by Tosoh Corporation) can also be used.

When a polyisocyanate compound is used as the curing agent, the compounding ratio [NCO/OH] of the isocyanate group of the polyisocyanate compound to the hydroxyl group in the base resin is preferably 0.8 to 1.2 in terms of molar ratio, and more preferably in the range of 0.90 to 1.10.
If the molar ratio of [NCO/OH] is less than 0.8, the curing of the topcoat layer is insufficient, and the desired hardness and strength of the topcoat layer may not be obtained. On the other hand, if the molar ratio of [NCO/OH] is more than 1.2, side reactions between the excess isocyanate groups or between the isocyanate groups and the urethane compound may occur, and the processability of the topcoat layer may decrease.

As the amino resin as the curing agent, resins obtained by reacting urea, benzoguanamine, melamine or the like with formaldehyde, and those obtained by alkyl etherifying these with alcohols such as methanol and butanol can be used.
Specific examples include methylated urea resins, n-butylated benzoguanamine resins, methylated melamine resins, n-butylated melamine resins, and iso-butylated melamine resins.

In addition, commercially available amino resins such as Cymel (product name, manufactured by Allnex), U-Ban (product name, manufactured by Mitsui Chemicals, Inc.), and Melan (product name, manufactured by Hitachi Chemical Co., Ltd.) can also be used.

When an amino resin is used as the curing agent, the compounding ratio of the amino resin to the base resin (weight ratio of solids) is preferably (95/5) to (65/35) (base resin/amino resin), and more preferably (90/10) to (75/25).

The amount of the curing agent is preferably 9 to 50% by mass in terms of the ratio of the resin solids. If it is less than 9% by mass, the hardness of the topcoat layer may be insufficient, and if it exceeds 50% by mass, the processability may be insufficient.

The top coat layer contains a color pigment in addition to the resin component.
The color pigment can be appropriately selected depending on the desired color, additional performance such as heat insulation, etc. Examples of the color pigment include titanium oxide, red iron oxide, carbon black, etc., and these can also be used in combination.
Incidentally, the color pigments mentioned above do not include those which fall under the category of inorganic fillers, which will be described later.

The content of the color pigment in the top coat layer is not particularly limited and can be adjusted appropriately depending on the type of color pigment and the required performance. In view of the fastness of the top coat layer, the total content of the color pigment is preferably 10 to 50 mass %, and more preferably 20 to 40 mass %.

The top coat layer contains, in addition to the resin component and the color pigment, an inorganic filler having an average particle size of 1 to 30 μm and an aspect ratio of 5 or more.
Such an inorganic filler can act as a crack propagation inhibitor in the topcoat layer, and can improve the aging workability of the fluororesin-coated steel sheet of the present invention.

The content of the inorganic filler in the topcoat layer is 0.1 to 20% by mass. From the viewpoint of obtaining an excellent crack propagation suppression effect, the content of the inorganic filler in the topcoat layer must be 0.1% by mass or more, and is preferably 1% by mass or more. Furthermore, from the viewpoint of maintaining the durability and processability of the topcoat layer, the content of the inorganic filler in the topcoat layer must be 20% by mass or less, and is preferably 15% by mass or less.

The inorganic filler must have an average particle size of 1 to 30 μm, but if the average particle size of the inorganic filler is less than 1, a sufficient crack propagation suppression effect cannot be obtained. From the same viewpoint, the inorganic filler preferably has an average particle size of 2 μm or more.
On the other hand, if the average particle size of the inorganic filler exceeds 30 μm, the appearance may deteriorate during roll coating, so the average particle size of the inorganic filler must be 30 μm or less, preferably 20 μm or less, and more preferably 10 μm or less.
The average particle size of the inorganic filler can be obtained, for example, by observing the cross section of the topcoat layer using an electron microscope, and the average particle size can be determined by measuring the longest diameter of each of the inorganic fillers present in multiple observation fields (for example, five fields) and averaging them.

The aspect ratio of the inorganic filler must be 5 or more, but if the aspect ratio of the inorganic filler is less than 5, a sufficient crack propagation suppression effect cannot be obtained. From the same viewpoint, the aspect ratio of the inorganic filler is preferably 10 or more.
On the other hand, if the aspect ratio of the inorganic filler exceeds 75, there is a risk of destruction during preparation and application of the paint. Therefore, the aspect ratio of the inorganic filler is preferably 75 or less.
The aspect ratio of the inorganic filler is the ratio of the average particle diameter to the average thickness of the inorganic filler (average particle diameter/average thickness). As described above, the average particle diameter and average thickness of the inorganic filler can be obtained by, for example, observing the cross section of the topcoat layer using an electron microscope, and the aspect ratio can be calculated from the average particle diameter and average thickness of the inorganic filler present in multiple observation fields (for example, five fields).

The type of inorganic filler is not particularly limited as long as it satisfies the above-mentioned particle size and aspect ratio. For example, natural mica, synthetic mica, vermiculite, talc, silicon nitride, kaolin, wollastonite, titanate, alumina powder, graphite, glass flakes, potassium titanate fiber, sepiolite, gypsum fiber, carbon fiber, aramid fiber, laras fiber, zonolite, etc.; or surface-treated products of these with metal coating, silane coupling, etc. can be used. These inorganic fillers can be used alone or in combination.

The top coat layer contains, in addition to the resin component, the color pigment, and the inorganic filler, an organic aggregate having an average particle size of 1 to 35 μm and a micro Vickers hardness of 50 HV _0.05 or less.
Such an organic aggregate can act like a stress relaxant in the topcoat layer, and can improve the aging workability of the fluororesin-coated steel sheet of the present invention.

The content of the organic aggregate in the topcoat layer is 0.1 to 15% by mass. From the viewpoint of obtaining an excellent stress relaxation effect, the content of the organic aggregate in the topcoat layer must be 0.1% by mass or more, preferably 1% by mass or more, and more preferably 2% by mass or more. Furthermore, from the viewpoint of maintaining the durability and strength of the topcoat layer, the content of the organic aggregate in the topcoat layer must be 15% by mass or less, preferably 12% by mass or less, and more preferably 10% by mass or less.

The organic aggregate must have an average particle size of 1 to 35 μm, but if the average particle size of the organic aggregate is less than 1, a sufficient crack propagation suppression effect cannot be obtained. From the same viewpoint, the organic aggregate preferably has an average particle size of 2 μm or more, and more preferably has an average particle size of 3 μm or more.
On the other hand, if the average particle size of the organic aggregate exceeds 35 μm, the organic aggregate is likely to come off from the top coat layer during bending, so the average particle size of the organic aggregate must be 35 μm or less, preferably 30 μm or less, and more preferably 20 μm or less.
The average particle size of the organic aggregate is the average of the shortest diameter and the longest diameter of the organic aggregate. The shortest diameter and the longest diameter of the organic aggregate can be obtained, for example, by observing the cross section of the topcoat layer using an electron microscope, and the average particle size can be calculated by measuring the shortest diameter and the longest diameter of the organic aggregate present in a plurality of observation fields (for example, five fields).

The micro Vickers hardness of the organic aggregate is required to be 50HV _0.05 or less, but if the micro Vickers hardness of the organic aggregate exceeds 50HV _0.05 , a sufficient stress relaxation effect cannot be obtained. From the same viewpoint, the aspect ratio of the organic aggregate is preferably 45HV _0.05 or less, and more preferably 40HV _0.05 or less.
On the other hand, if the micro Vickers hardness of the organic aggregate is less than 10, there is a risk that sufficient durability of the top coat layer cannot be obtained, so the micro Vickers hardness of the organic aggregate is preferably _10HV0.05 or more.
The Vickers microhardness of the organic aggregate can be measured by embedding a sample fluororesin-coated steel plate in a room temperature drying resin, polishing it, selecting an organic aggregate from the cross section of the exposed topcoat layer, and measuring the Vickers hardness of the selected organic aggregate using a microhardness tester (e.g., Shimadzu Corporation's HMV-G21 microhardness tester). The measurement method is in accordance with JIS Z 2244, and the test is performed under a load of 0.49 mN (0.05 gf) (HV _0.05 ).

The type of organic aggregate is not particularly limited as long as it satisfies the above-mentioned particle size and micro Vickers hardness. For example, beads (particles) of acrylic resin, urethane resin, fluororesin, urea resin, acrylic triblock copolymer, butadiene rubber, etc., or surface-treated products such as those with functional groups added thereto, can be used. These organic aggregates can be used alone or in combination.

The total content of the inorganic filler and the organic aggregate in the topcoat layer is preferably 0.2 to 35% by mass, and more preferably 1 to 30% by mass. When the total content of the inorganic filler and the organic aggregate is 0.2% by mass or more, the crack propagation suppression effect and the stress relaxation effect are sufficiently obtained, and therefore, better processability over time can be obtained. On the other hand, when the total content of the inorganic filler and the organic aggregate is 35% by mass or less, the decrease in durability of the topcoat layer can be reliably suppressed.

In addition to the resin component, the color pigment, the inorganic filler, and the organic aggregate, the top coat layer may contain other additives depending on the purpose and application.
Examples of the other additives include metallic pigments such as aluminum powder, pigments such as carbonates and sulfates, various fine particles such as silica fine particles for adjusting gloss, wax, antifoaming agents, and leveling agents.
The total content of these other additives in the top coat layer is preferably 10% by mass or less, so as not to impair the effects of the present invention, such as processability over time.

The coating conditions for forming the topcoat layer are not particularly limited. For example, the coating composition that is the material for the topcoat layer can be applied by a method such as roll coater coating or curtain flow coating. After the coating composition is applied, the topcoat layer can be formed by baking using a heating means such as hot air heating, infrared heating, or induction heating. The temperature of the baking process is usually set to a maximum plate temperature of about 240 to 260°C. In addition, in the case of hot air heating, it is preferable to set the hot air speed at the plate surface to be as fast as 1 to 10 m/s, preferably 5 to 10 m/s.

Furthermore, the average thickness of the topcoat layer is not particularly limited, but is preferably 10 to 50 μm, and more preferably 12 to 30 μm. If the average thickness of the topcoat layer is less than 10 μm, the durability of the topcoat layer may be reduced, and if the average thickness of the topcoat layer exceeds 50 μm, pinhole defects called "popping" may easily occur due to bumping of thinner in the paint during baking.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

<Inventive Examples 1 to 4, Comparative Examples 1 to 3>
(Example 1 of the invention)
A hot-dip Al-Zn plated steel sheet having a thickness of 0.35 mm, a coating weight of 150 g/m ² per side, and a coating layer having a composition of Zn-55% Al-1.6% Si that was subjected to skin pass treatment was prepared.
A vanadium-based chromate-free chemical conversion treatment solution (a chemical conversion treatment solution that uses urethane resin as the resin component and contains an organic vanadium compound chelated with acetylacetone as the rust-preventive component) was applied to the above-mentioned hot-dip Al-Zn-plated steel sheet with a bar coater, and the steel sheet was dried at an attained temperature of 90°C for a baking time of 10 seconds to form a chemical conversion coating so as to give a coating weight of 0.2 g/ ^m2 .
Thereafter, a primer layer paint having an epoxy-modified urethane resin as a base and vanadium phosphate as an anti-rust pigment was applied onto the above chemical conversion coating layer using a roll coater, and the paint was baked at a steel plate temperature of 210°C for a baking time of 25 seconds to form a primer layer with a film thickness of 5 μm after baking.
The steel plate on which the primer layer was formed was then water-cooled and dried, and a topcoat layer paint was then prepared by blending 30% by mass of PVdF resin and 15% by mass of acrylic resin (total of 45% by mass) with 40% by mass of iron-chromium composite oxide (Shepherd Color's "Black 411") (average particle size 1.1 μm) as a black pigment, 0.5% by mass of titanium oxide (average particle size 500 nm) as a white pigment, 3% by mass of synthetic mica (average particle size 15 μm, aspect ratio 10) as an inorganic filler, and 3% by mass of acrylic beads (average particle size 20 μm, micro Vickers hardness 20 HV _0.05 ) as an organic aggregate. Thereafter, a paint for the topcoat layer was applied onto the primer layer, and the topcoat layer was formed by baking for 50 seconds at a plate temperature of 250°C in an oven with an atmospheric temperature of 400°C and an average hot air speed on the plate surface of 5 m/sec, so that the average film thickness after drying would be 20 μm, and a sample coated steel plate (fluororesin coated steel plate) was produced.
The conditions of the topcoat layer of the sample coated steel sheets are also shown in Table 1. The average particle size and aspect ratio of the inorganic filler, and the average particle size and micro Vickers hardness of the organic aggregate in the topcoat layer were controlled in advance to be within the above-mentioned ranges.

(Example 2 of the invention)
A sample coated steel plate was prepared under the same conditions as in Example 1 of the present invention, except that 1% by mass of PTFE aggregate (average particle size 18 μm, micro Vickers hardness 10 _HV0.05 ) was used as the organic aggregate in the top coat layer instead of acrylic beads.
The conditions of the topcoat layer of the sample coated steel sheets are also shown in Table 1.

(Example 2 of the invention)
A sample coated steel plate was prepared under the same conditions as in Example 1 of the present invention, except that 1 mass% of polytetrafluoroethylene (PTFE) aggregate (average particle size 18 μm, micro Vickers hardness 10 _HV0.05 ) was used as the organic aggregate in the top coat layer instead of acrylic beads.
The conditions of the topcoat layer of the sample coated steel sheets are also shown in Table 1.

(Example 3 of the invention)
A sample coated steel sheet was prepared under the same conditions as in Example 1 of the present invention, except that 3 mass% of talc (average particle size 20 μm, aspect ratio 10) was used as the inorganic filler in the top coat layer instead of synthetic mica.
The conditions of the topcoat layer of the sample coated steel sheets are also shown in Table 1.

(Example 4 of the invention)
Prior to the formation of the chemical conversion coating layer, the hot-dip Al-Zn-plated steel sheet was annealed at an atmospheric temperature of 200°C for 12 hours to set the Vickers hardness of the dendritic phase in the plating layer to a range of 10 to 110 _HV0.1 . Except for this, a coated steel sheet sample was produced under the same conditions as in Invention Example 1.
The conditions of the topcoat layer of the sample coated steel sheets are also shown in Table 1.

(Comparative Example 1)
A coated steel sheet sample was prepared under the same conditions as in Inventive Example 1, except that the topcoat layer did not contain acrylic beads as an organic aggregate and synthetic mica as an inorganic filler.
The conditions of the topcoat layer of the sample coated steel sheets are also shown in Table 1.

(Comparative Example 2)
A sample coated steel plate was prepared under the same conditions as in Example 1 of the present invention, except that the top coat layer did not contain acrylic beads as an organic aggregate or synthetic mica as an inorganic filler, but contained 3 mass% synthetic silica (average particle size 4 μm).
The conditions of the topcoat layer of the sample coated steel sheets are also shown in Table 1.

(Comparative Example 3)
A sample coated steel plate was prepared under the same conditions as in Example 1 of the present invention, except that 1 mass% of polyimide aggregate (average particle size 20 μm, micro Vickers hardness 60 _Hv0.05 ) was used as the organic aggregate in the top coat layer instead of acrylic beads.
The conditions of the topcoat layer of the sample coated steel sheets are also shown in Table 1.

<Evaluation>
Each of the samples of coated steel sheets obtained as described above was evaluated as follows.

(1) Workability and workability over time A bending test was carried out on each sample of coated steel sheet in accordance with JIS G 3322. When a test piece with a width of 75 mm was bent 180 degrees, the inner distance was expressed by the number of sheets, and the limit number of sheets (T) at which no cracks occurred in the bent surface coating was taken as No-Crack T. The smaller this No-Crack T, the better the workability.
This bending test was carried out immediately after the sample was made and after it was kept at 50°C for 72 hours (accelerated aging), which is equivalent to storing the sample for 3 months in summer. The results of no crack T immediately after the sample was made and after accelerated aging are shown in Table 1.
In addition, in the bending test after accelerated aging, the aged workability was evaluated according to the following criteria. The evaluation results are shown in Table 1.
〇: 6T or less, practical for siding applications, etc. ×: Over 6T

The results in Table 1 show that the samples of the present invention have superior processability over time compared to the samples of the comparative examples.

The present invention provides coated steel sheets that have excellent workability over time while maintaining other physical properties such as workability and weather resistance.

10: hot-dip Al-Zn-plated steel sheet 20: chemical conversion coating layer 30: primer layer 40: top coat layer 41: resin component 42: inorganic filler 43: organic aggregate

Claims (5)

A coated steel sheet having a chemical conversion coating, a primer layer and a top coat layer formed on a hot-dip Al-Zn-plated steel sheet,
The top coat layer comprises a resin component containing a polyvinylidene fluoride resin and an acrylic resin, the polyvinylidene fluoride resin being present in an amount of 50 to 80% by mass;
10 to 50 mass of color pigment;
0.1 to 20 mass% of an inorganic filler having an average particle size of 1 to 30 μm and an aspect ratio of 5 or more;
1 to 12 mass% of organic aggregate having an average particle size of 1 to 35 μm and a micro Vickers hardness of 50 HV _0.05 or less;
Including,
The fluororesin-coated steel sheet is characterized in that the inorganic filler is at least one selected from the group consisting of natural mica, synthetic mica, vermiculite, talc, silicon nitride, kaolin, wollasnite, titanate, alumina powder, graphite, glass flake, potassium titanate fiber, sepiolite, gypsum fiber, carbon fiber, aramid fiber, laras fiber, zonolite, and surface-treated products thereof . 2. The fluororesin-coated steel sheet according to claim 1, wherein a total content of the inorganic filler and the organic aggregate in the topcoat layer is 1.1 to 30 mass %. The fluororesin-coated steel sheet according to claim 1 or 2, characterized in that the coating layer of the hot-dip Al-Zn-based coated steel sheet contains 20-95% by mass of Al, 1-3% by mass of Si, and 5% by mass or less of optional added components, with the remainder being Zn and unavoidable impurities. The fluororesin-coated steel sheet according to claim 3, characterized in that the plating layer contains 50-60 mass% Al, 1-3 mass% Si, and 5 mass% or less of optional added components, with the remainder being Zn and unavoidable impurities. 5. The fluororesin-coated steel sheet according to claim 3, wherein the plating layer has a dendritic phase and an interdendritic phase, and the dendritic phase has a Vickers hardness of 10 to 110 _HV0.1.

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