JP7854038B2 - Fluorine resin coated steel sheet - Google Patents

Description

This invention relates to a painted steel sheet with excellent aging processability.

Hot-dip Al-Zn plated steel sheets (hereinafter sometimes referred to as "55% Al-Zn plated steel sheets"), which have a standard composition containing approximately 55% by mass of Al and approximately 1.6% by mass of silicon in the plating layer, with the remainder being zinc, exhibit excellent corrosion resistance, and in recent years, demand for them has been increasing, mainly in the building materials sector.
Furthermore, fluororesin-coated steel sheets, which are 55% Al-Zn plated steel sheets coated with fluororesin, are used as metal siding for residential and commercial buildings, combining intricate molded patterns that mimic stone or wood using embossing rolls with interlocking shapes, due to the excellent workability and weather resistance of fluororesin.

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

Furthermore, for example, Patent Documents 2 to 4 disclose a technology that improves scratch and abrasion resistance while maintaining excellent processability by using materials such as glass flakes, ceramic fibers, silicon carbide powder, acrylic polymer beads, and tetrafluoroethylene resin powder as the materials constituting the topcoat layer.

However, while these fluororesin-coated steel sheets offer some degree of improved workability and weather resistance, as mentioned above, they suffer from a problem where the fluororesin crystallizes over time, leading to a deterioration in workability (the problem of workability over time).

Therefore, various technologies are being considered to solve the problem of the aging processability of fluororesin-coated steel sheets.
For example, Patent Document 5 discloses a technique that uses fine fluororesin particles of 5 μm or less as a material constituting the topcoat layer to promote compatibility with acrylic resin and suppress crystallization of fluororesin.
However, while the technology described in Patent Document 5 provides a certain degree of processability improvement, it fails to sufficiently suppress crystallization in the fluororesin topcoat layer after about three months, resulting in cracks in the topcoat layer in areas with a high degree of processing during embossing roll processing in siding molding. Therefore, the development of a technology that can improve processability over time has been desired.

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

In light of these circumstances, the present invention aims to provide a painted steel sheet with excellent processability over time, while maintaining good other physical properties such as processability and weather resistance.

The inventors of this invention conducted research to solve the above-mentioned problems regarding fluororesin-coated steel sheets, which have a chemical conversion coating, a primer layer, and a topcoat layer containing a fluororesin formed on a molten Al-Zn plated steel sheet. As a result, they found that by specifying the mass ratio of polyvinylidene fluoride resin constituting the topcoat layer within a specific range, and by including an inorganic filler effective as a crack propagation inhibitor and an organic aggregate effective as a stress relaxant in the topcoat layer, it is possible to suppress crack propagation and ensure flexibility in the topcoat layer regardless of whether the fluororesin crystallizes. Therefore, they discovered that excellent processability over time can be achieved while maintaining good other physical properties such as processability and weather resistance.

This invention is based on the above findings, and its gist is as follows.
1. A painted 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 contains a resin component comprising polyvinylidene fluoride resin and acrylic resin, wherein the mass content of the polyvinylidene fluoride resin is 50 to 80%.
Coloring pigments,
An inorganic filler with an average particle size of 1 to 30 μm and an aspect ratio of 5 or more, comprising 0.1 to 20% by mass,
Organic aggregate with an average particle size of 1 to 35 μm and a micro-Vickers hardness of 50 HV _0.05 or less, comprising 0.1 to 15% by mass,
A fluororesin-coated steel sheet characterized by containing [the specified ingredient].

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

3. The fluororesin-coated steel sheet according to claim 1 or 2, characterized in that the plating layer of the molten Al-Zn plated steel sheet contains Al: 20 to 95% by mass, Si: 1 to 3% by mass, and optional additives: 5% by mass or less, with the remainder being Zn and unavoidable impurities.

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

5. The fluoropolymer-coated steel sheet according to item 3 or 4, 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 HV _0.1.

According to the present invention, it is possible to provide a painted steel sheet with excellent processability over time while maintaining good other physical properties such as processability and weather resistance.

This is a schematic cross-sectional view showing a cross-section of one embodiment of the fluororesin-coated steel sheet of the present invention.

The embodiments of the fluororesin-coated steel sheet of the present invention will be described below, with reference to the drawings as needed. Note that the components shown in the drawings are schematic representations for the sake of explanation, and 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 treatment layer 20, a primer layer 30, and a topcoat layer 40 are formed on a molten Al-Zn plated steel sheet 10, as shown in Figure 1.
Furthermore, the top coat layer 40 contains a resin component 41 which contains polyvinylidene fluoride resin and acrylic resin, with the mass percentage of the polyvinylidene fluoride resin being 50 to 80%.
Coloring pigment (not shown),
Inorganic filler 42 having an average particle size of 1 to 30 μm and an aspect ratio of 5 or more is used in a mixture of 0.1 to 20% by mass.
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, is used in a mixture of 0.1 to 15% by mass.
It is characterized by including.

The topcoat layer contains an inorganic filler 42 having an average particle size of 1 to 30 μm and an aspect ratio of 5 or more, thereby allowing the inorganic filler 42 to act as a crack propagation inhibitor. Furthermore, the topcoat layer contains an organic aggregate 43 having an average particle size of 1 to 35 μm and a micro-Vickers hardness of 50HV _0.05 or less, thereby allowing the organic aggregate 43 to act as a stress relaxant. As a result, when the coated steel sheet is bent, the propagation of cracks generated from the surface of the topcoat 40 can be suppressed, and the tensile stress applied to the topcoat layer 40 can be dispersed, thereby suppressing the occurrence of cracks. Consequently, the fluororesin-coated steel sheet of the present invention achieves excellent aging resistance.
Furthermore, by including a certain amount of polyvinylidene fluoride resin (50-80% by mass in the base resin) in the top coat layer 40, good processability and weather resistance can also be maintained.

The following describes each component that makes up the fluororesin-coated steel sheet of the present invention.
As described above, the fluororesin-coated steel sheet of the present invention has a chemical conversion treatment layer 20, a primer layer 30, and a topcoat layer 40 formed on a molten Al-Zn plated steel sheet 10.

(Hot-dip Al-Zn plated steel sheet)
The fluororesin-coated steel sheet of the present invention uses a hot-dip Al-Zn plated steel sheet as the steel sheet. By using a hot-dip Al-Zn plated steel sheet, high corrosion resistance can be ensured.
Here, the type of hot-dip Al-Zn plated steel sheet is not particularly limited and can be appropriately selected according to 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 a 55% Al-Zn plated steel sheet or the like can be used.

The aforementioned 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 provides good corrosion resistance. For example, the plating layer of the hot-dip Al-Zn plated steel sheet may have a composition containing Al: 20-95% by weight, Si: 1-3% by weight, and optional additives: 5% by weight or less, with the remainder being Zn and unavoidable impurities. By having the above-described composition, the plating layer can form dendrite phases and interdendrite phases surrounding the dendrite phases in a network-like structure, thereby improving corrosion resistance.

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

Here, the Al content in the plating layer is set to 20 to 95% by mass, preferably 50 to 60% by mass, based on a balance between corrosion resistance and operational surface. If the Al content of the plating layer is at least 20% by mass, sufficient dendrite solidification of Al occurs. As a result, the main layer mainly contains supersaturated Zn and consists of a portion where Al has dendrite-solidified (α-Al dendrite phase) and the remaining portion of the dendrite gaps (interdendrite phase), and the dendrite phase is stacked in the direction of the plating layer thickness, thereby realizing a structure with excellent corrosion resistance.
Furthermore, the more α-Al dendrites are layered, the more complex the corrosion pathway becomes, making it more difficult for corrosion to reach the underlying steel plate, thus improving corrosion resistance. On the other hand, if the Al content in the plating layer exceeds 95% by mass, the amount of Zn, which has a sacrificial corrosion protection effect against Fe, decreases, and corrosion resistance deteriorates. For this reason, the Al content in the plating layer should be 95% by mass or less.

Furthermore, the Si in the plating layer is added to the plating bath to improve corrosion resistance and workability, with the aim of suppressing the growth of the interfacial alloy layer formed at the interface with the underlying steel sheet, and is therefore inevitably contained in the main layer. In the case of the molten Al-Zn plated steel sheet used in the painted steel sheet of the present invention, when molten plating is performed with Si contained in the plating bath, the underlying steel sheet is immersed in the plating bath, and at the same time, the Fe on the surface of the steel sheet reacts with the Al and Si in the bath to form an alloy, generating an alloy consisting of Fe-Al and/or Fe-Al-Si compounds. The formation of this Fe-Al-Si interfacial alloy layer can suppress the growth of the interfacial alloy layer. When the Si content in the plating 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 plating layer exceeds 3% by mass, the workability of the plating layer decreases, and the Si phase that becomes the cathode site is more likely to precipitate. For this reason, the Si content in the plating layer should be 3% by mass or less.

The aforementioned plating layer contains Zn as its main component. By including Zn in the plating layer, a sacrificial corrosion protection effect can be obtained, thereby improving corrosion resistance. On the other hand, when the Zn content is 80% by mass or less, it is preferable because the Al content can be ensured, and the corrosion resistance provided by the dendrite and interdendrite phases described above can be achieved.

Furthermore, in addition to the Al, Si, and Zn mentioned above, the plating layer may contain 5% by mass or less of an optional additive component.
Here, the optional additive components can be appropriately selected according to the performance required for the plating layer. Examples 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.
While these optional additives can provide benefits such as improved corrosion resistance, they may reduce the workability of the plating layer and worsen the limit elongation of the painted steel sheet. Therefore, it is preferable that the content of optional additives be 5% by mass or less.

The plating layer may contain Mg and/or Ca as optional additives from the viewpoint of corrosion resistance. When the plating layer corrodes, Mg and/or Ca will be included in the corrosion products, improving the stability of the corrosion products and delaying the progression of corrosion, resulting in improved corrosion resistance. The total content of Ca and/or Mg is not particularly limited as long as it is 5% by mass or less, but it is preferably 0.01 to 5% by mass. A content of 0.01% by mass or more provides a sufficient corrosion delay effect, while a content of 5% by mass or less prevents saturation of the effect, suppresses increases in manufacturing costs, and facilitates composition control of the plating bath.
Furthermore, it is preferable that the plating layer contains at least Mg. This is because the inclusion of Mg in the plating layer allows for the formation of _Mg₂Si together with the Si mentioned above, thereby providing a corrosion delay effect. Here, the Mg content in the plating layer is preferably 0.01 to 5% by mass, and more preferably 2 to 4.9% by mass.

Furthermore, similar to the alkaline earth metals Ca and Mg as optional additives, the plating layer may also contain, as an optional additive, one or more elements selected from Mn, V, Cr, Mo, Ti, Sr, Ni, Co, Sb, and B, in a total amount of 5% by mass or less, preferably 0.01 to 5% by mass.

The plating layer contains components of the underlying steel sheet incorporated into the plating through the reaction between the plating bath and the underlying steel sheet during the plating process, as well as unavoidable impurities in the plating bath. The underlying steel sheet components incorporated into the plating may include up to approximately 2% Fe. Examples of unavoidable impurities in the plating bath include Fe, Cu, and Zr. It is not possible to distinguish and quantify the Fe in the plating layer from that incorporated from the underlying steel sheet and that present in the plating bath. While there are no particular limitations on the total content of unavoidable impurities, 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, be 1% by mass or less.

The aforementioned interface alloy layer is a layer present at the interface between the plating layer and the underlying steel sheet. As described above, it is an Fe-Al and/or Fe-Al-Si compound that is inevitably formed by the alloying reaction between Fe on the steel sheet surface and Al and Si in the bath. Because this interface alloy layer is hard and brittle, if it grows too thick, it can become a starting point for crack formation during processing; therefore, it is preferable to make it as thin as possible.

The means for forming the plating layer on the base steel sheet are not particularly limited, and a conventional continuous hot-dip galvanizing system can be used. For example, the base steel sheet is heated to a predetermined temperature in an annealing furnace maintained in a reducing atmosphere. After annealing, rolling oil and other substances adhering to the steel sheet surface are removed, and the oxide film is reduced and removed. Then, the sheet is immersed in a molten zinc plating bath containing predetermined concentrations of Al and Zn, passing through a snout with its lower end submerged in the plating bath. Subsequently, the immersed steel sheet is lifted above the plating bath via a sink roll. The amount of plating deposited is then adjusted by spraying pressurized gas onto the surface of the steel sheet from a gas wiping nozzle positioned above the plating bath. Finally, it is cooled by a cooling device to form the plating layer.

Furthermore, if the plating layer does not contain any optional additives, the limit elongation of the molten Al-Zn plated steel sheet before coating formation can be improved to about 20% or more by applying a heat treatment, for example, at 200°C for 24 hours. This is thought to be because the plating layer softens as the supersaturated Zn dissolved in the aluminum-rich dendrite phase is expelled by the above heat treatment.
On the other hand, in the case of a plating layer having a composition containing optional additives, the effect of such heat treatment on improving processability is small. For example, when 2 to 4.9% of Mg is added as the optional additive, the limiting elongation rate after heat treatment remains below 5%. The reason for this is still unclear, but it is presumed that factors such as the high solubility of Mg in Al causing it to remain as a solid solution element in the aluminum-rich dendrite phase even after heat treatment are influencing the result.

Furthermore, the structure of the plating layer contains a dendrite phase and an interdendrite phase, and it is preferable that the Vickers hardness of the dendrite phase is 10 to 110 Hv _0.01 . By reducing the Vickers hardness of the dendrite phase to 10 to 110 Hv _0.01 , the workability of the painted steel sheet can be improved and the corrosion resistance after processing can be further enhanced. If the Vickers hardness of the dendrite phase exceeds 110 Hv _0.01 , sufficient workability may not be obtained, while if the Vickers hardness of the dendrite phase is less than 10 Hv _0.01 , the scratch resistance of the surface of the plating layer may be reduced. From a similar viewpoint, it is preferable that the Vickers hardness of the dendrite phase be 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 of fluororesin-coated steel sheet with 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 HMV-G21). The measurement method conforms to JIS Z 2244, and for the plating layer, the test is conducted under conditions of an indentation load of 0.98 mN (0.1 gf) (Hv _0.1 ).

(Chemical conversion coating)
The fluororesin-coated steel sheet of the present invention has a chemical conversion treatment film formed on the plating layer of the hot-dip galvanized steel sheet. By forming the chemical conversion treatment film on the plating layer, the corrosion resistance of the fluororesin-coated steel sheet can be further enhanced.

The type and formation conditions of the aforementioned chemical conversion coating are not particularly limited and can be appropriately selected according to the required performance.
For example, the coating can be formed by chromate treatment or chromate-free chemical treatment, which involves applying a chromate treatment solution or a chromium-free chemical treatment solution and then drying it at a steel plate temperature of 80 to 300°C without rinsing with water. Furthermore, when considering the working environment, it is preferable to form the chemical treatment film by chromate-free treatment (i.e., the chemical treatment film should not contain chromium).

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

The thickness of the primer layer is not particularly limited and can be adjusted as appropriate according to the required performance.
For example, from the viewpoint of achieving both rust prevention and workability, 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 it is 15 μm or less, sufficient workability can be ensured.

Here, the primer layer may contain a rust inhibitor from the viewpoint of improving the rust prevention of the painted steel sheet. The rust inhibitor can be either a chromate-type containing chromate salts or a chromate-free type without chromate salts. However, when considering the working environment, a chromate-free type (i.e., one that does not contain chromium in the primer layer) is preferred, and the following description will focus on the chromate-free type primer layer.

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

Furthermore, it is preferable that the polyester resin mainly consists of a polyester resin having urethane bonds.
Here, as the polyester resin having the urethane bond, known resins can be used, such as those obtained by the reaction of a polyester polyol with a diisocyanate or polyisocyanate having two or more isocyanate groups.
In addition, a resin obtained by reacting a polyester polyol with a diisocyanate or polyisocyanate having two or more isocyanate groups in an excess of hydroxyl groups (urethane-modified polyester resin), and then curing it with a blocked polyisocyanate, can also be used.

The aforementioned polyester polyol can be obtained by a known method utilizing a dehydration condensation reaction between a polyhydric alcohol component and a polybasic acid component.
Examples of polyhydric alcohols include glycols and polyhydric alcohols with a valency of three or higher. 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 polyhydric alcohols with a valency of three or higher include glycerin, trimethylolethane, trimethylolpropane, pentaerythritol, and dipentaerythritol. These polyhydric alcohols can be used individually or in combination of two or more types.
The aforementioned polybasic acids are usually polycarboxylic acids, but monovalent fatty acids may be used in combination as needed. Examples of polycarboxylic acids 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, etc., and their acid anhydrides, as well as 1,4-cyclohexanedicarboxylic acid, isophthalic acid, tetrahydroisophthalic acid, hexahydroisophthalic acid, hexahydroterephthalic acid, etc. These polybasic acids can be used alone or in combination of two or more types.

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); cyclic aliphatic diisocyanates such as isophorone diisocyanate, hydrogenated XDI, hydrogenated TDI, and hydrogenated MDI; and their adducts, biuretes, and isocyanurates. These polyisocyanate compounds can be used individually or in combination of two or more.

The polyester resin having urethane bonds possesses both flexibility and strength, and when processed, it can suppress the occurrence of cracks in the primer layer, among other effects. Furthermore, it has high affinity with chemical conversion coatings containing urethane resin, and contributes particularly to improving the corrosion resistance of the processed area.
Here, the hydroxyl value of the polyester resin having the urethane bond is preferably 5 to 120 mg KOH/g, more preferably 7 to 100 mg KOH/g, and even more preferably 10 to 80 mg KOH/g, from the viewpoint of solvent resistance, processability, etc.

Furthermore, the number-average molecular weight of the polyester resin having the urethane bond is preferably 500 to 15,000, more preferably 700 to 12,000, and even more preferably 800 to 10,000, from the viewpoint of solvent resistance, processability, etc.

Furthermore, it is preferable that the polyester resin is contained in the primer layer at a concentration of 40 to 88% by mass. Below 40% by mass, the binder function of the primer layer is reduced, and above 88% by mass, the function of the inorganic substances, such as inhibitory activity, 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 vanadium compounds that act as inhibitors include vanadium pentoxide, metavanadic acid, ammonium metavanadate, vanadium oxytrichloride, vanadium trioxide, vanadium dioxide, magnesium vanadate, vanadyl acetylacetonate, and vanadium acetylacetonate. 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 as or different from the vanadium compound added to the chemical conversion coating. It is believed that the vanadic acid compound reacts with ions on the surface of the zinc-plated steel sheet, gradually dissolving vanadate ions in response to moisture entering from the outside, forming a highly adhesive passive film that protects exposed metal and exhibits rust prevention.

Furthermore, the vanadium compound content in the primer layer is preferably 4 to 20% by mass. Below 4% by mass, the inhibitory effect may decrease, potentially leading to reduced corrosion resistance. Above 20% by mass, the moisture resistance of the primer layer may decrease.

Regarding the type of phosphate compound that acts as the inhibitor, for example, phosphate, ammonium phosphate, alkali metal phosphate, alkaline earth metal phosphate, etc., can be used. Among these, alkali metal phosphates such as calcium phosphate are preferred.

Furthermore, the content of the phosphate compound in the primer layer is preferably 4 to 20% by mass. Below 4% by mass, the inhibitory effect may decrease, potentially leading to a reduction in corrosion resistance. Above 20% by mass, the moisture resistance of the primer layer may decrease.

The magnesium oxide acting as an inhibitor has the effect of stabilizing the products generated 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. Below 4% by mass, the above effect may decrease, potentially leading to a reduction in corrosion resistance. Above 20% by mass, the flexibility of the primer layer may decrease, potentially reducing the corrosion resistance, especially in the processed area.

Furthermore, 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 those obtained by blocking the isocyanate group of a polyisocyanate compound with, for example, alcohols such as butanol, oximes such as methyl ethyl ketoxime, lactams such as ε-caprolactam, diketones such as acetoacetate diester, imidazoles such as 2-ethylimidazole, or phenols such as m-cresol.

(Top coat layer)
The painted steel sheet of the present invention further comprises a topcoat layer in addition to the hot-dip Al-Zn plated steel sheet, chemical conversion coating, and primer layer described above.
By optimizing the topcoat layer, it is possible to obtain painted steel sheets with good corrosion resistance, workability, weather resistance, etc.

The topcoat layer comprises a resin component as a matrix, and this resin component contains polyvinylidene fluoride resin (PVdF resin) and acrylic resin.
Furthermore, the mass percentage of polyvinylidene fluoride resin in the resin component must be 50 to 80%. Including polyvinylidene fluoride resin in the resin component makes it possible to improve the processability, weather resistance, and stain resistance of the fluororesin-coated steel sheet.

By having a mass percentage of polyvinylidene fluoride resin in the resin component of 45% or more, sufficient performance as a topcoat layer (processability, weather resistance, stain resistance, durability, etc.) can be imparted. From a similar viewpoint, it is preferable that the mass percentage of polyvinylidene fluoride resin be 50% or more.
Furthermore, by having a mass percentage of polyvinylidene fluoride resin in the resin component of 85% or less, a decrease in the adhesion of the topcoat layer can be suppressed. From a similar viewpoint, it is preferable that the mass percentage of polyvinylidene fluoride resin be 80% or less.

Regarding the polyvinylidene fluoride resin, it is preferable to use one with a weight-average molecular weight of 150,000 to 700,000 and a melting point of 150 to 180°C. Examples of such polyvinylidene fluoride resins include Arkema's "Kynar 500" (weight-average molecular weight: 350,000, melting point: 160 to 165°C).

The acrylic resin to be mixed with the polyvinylidene fluoride resin is preferably one with a weight-average molecular weight of 50,000 to 200,000. Furthermore, the acrylic resin can be obtained by polymerizing (or copolymerizing) at least one of the monomers (i) to (iv) listed below (provided that at least one acrylic monomer is included) by a conventional method.

(i) Ethylene monomers having a hydroxyl group, such as hydroxymethyl (meth)acrylate, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, and hydroxybutyl (meth)acrylate.
(ii) Ethylene monomers having a carboxyl group, such as (meth)acrylic acid, crotonic acid, itaconic acid, fumaric acid, and maleic acid.
(iii) Ethylene monomers copolymerizable with the above 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 alkyl (meth)acrylate.
(iv) Styrene derivatives such as α-methylstyrene, o-methylstyrene, m-methylstyrene, and p-methylstyrene.
By using monomers that have functional groups such as hydroxyl groups or carboxyl groups, crosslinking reactions with other reactive components are possible.

Among the monomers (i) to (iv) described above, it is preferable that 80% by mass or more of the constituent monomer units are at least one selected from alkyl acrylate monomers or alkyl methacrylate monomers having 1 to 8 carbon atoms in the alkyl group, 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, reducing processability. The reason why such alkyl acrylate monomers or alkyl methacrylate monomers are preferred is that they have excellent compatibility with polyvinylidene fluoride resin and also exhibit excellent weather resistance.

Furthermore, while the acrylic resin does not necessarily need to be self-crosslinkable, if it is to be self-crosslinkable, it should contain a so-called crosslinkable monomer having two or more radically polymerizable unsaturated bonds in its 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 crosslinkable monomer can be added up to 20% by mass of the acrylic resin.

The resin component may consist solely of the polyvinylidene fluoride resin and the acrylic resin, but may also include other resins such as polyester resins as needed.
The polyester resins include, for example, polyester resins, silicone-modified polyester resins, and acrylic-modified polyester resins, and as curing agents for the main resin, polyisocyanate compounds and/or amino resins can be used.

Furthermore, the acrylic resin and polyester resin are used in combination with a curing agent. Polyisocyanate compounds and amino resins can be used as the curing agent.

As the polyisocyanate compound, any isocyanate compound obtained by a general manufacturing method can be used, but it is particularly preferable to use a polyisocyanate compound that has been blocked with a blocking agent such as phenol, cresol, aromatic secondary amine, tertiary alcohol, lactam, or oxime, which can be used as a one-component paint. Using this blocked polyisocyanate compound allows for storage as a single component, making it easier to use as a paint.

Furthermore, 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.

Furthermore, commercially available isocyanate compounds such as Sumijoule (trade name, manufactured by Sumika Covestro Urethane Co., Ltd.), Desmodule (trade name, manufactured by Sumika Covestro Urethane Co., Ltd.), and Coronate (trade name, manufactured by Tosoh Corporation) can also be used.

When a polyisocyanate compound is used as the curing agent, the mixing 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 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 topcoat layer may not harden sufficiently, 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] exceeds 1.2, side reactions may occur between excess isocyanate groups or between isocyanate groups and the urethane compound, which may reduce the processability of the topcoat layer.

As for the amino resin used as the curing agent, resins obtained by the reaction of urea, benzoguanamine, melamine, etc. with formaldehyde, and those obtained by alkylating these with alcohols such as methanol and butanol, can be used.
Specifically, examples include methylated urea resin, n-butylated benzoguanamine resin, methylated melamine resin, n-butylated melamine resin, and iso-butylated melamine resin.

Furthermore, commercially available amino resins such as Cymel (product name, manufactured by Ornex Co.), Yuban (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 mixing ratio (weight ratio of solids) of the amino resin to the base resin is preferably (95/5) to (65/35) in terms of base resin/amino resin, and more preferably (90/10) to (75/25).

Furthermore, the amount of the curing agent is preferably 9 to 50% by mass in proportion to the resin solid content. Below 9% by mass, the hardness of the topcoat layer may be insufficient, while above 50% by mass, the processability may be insufficient.

The top coat layer contains a coloring pigment in addition to the resin component.
The aforementioned coloring pigments can be appropriately selected according to the desired color and additional properties such as heat shielding. Examples of such coloring pigments include titanium dioxide, red iron oxide, and carbon black, and these can also be used in mixtures.
Furthermore, the aforementioned coloring pigments shall exclude those that fall under the category of inorganic fillers, as described later.

The content of the coloring pigment in the topcoat layer is not particularly limited and can be adjusted as appropriate depending on the type of coloring pigment and the required performance. For the sake of the durability of the topcoat layer, the total amount of the coloring pigment is preferably 10 to 50% by mass, and more preferably 20 to 40% by mass.

The top coat layer, in addition to the resin component and the coloring pigment, contains an inorganic filler having an average particle size of 1 to 30 μm and an aspect ratio of 5 or more.
Such inorganic fillers can exert a crack propagation inhibitory effect in the topcoat layer, thereby improving the processability over time of the fluororesin-coated steel sheet of the present invention.

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

The average particle size of the inorganic filler must be 1 to 30 μm; however, if the average particle size of the inorganic filler is less than 1, a sufficient crack propagation suppression effect cannot be obtained. From a similar viewpoint, it is preferable that the average particle size of the inorganic filler be 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. Therefore, 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 cross-sectional observation of the topcoat layer using an electron microscope. The average particle size can be obtained by measuring the longest diameter of each inorganic filler present in multiple observation fields (e.g., five fields) and averaging them.

The aspect ratio of the inorganic filler must be 5 or greater; however, if the aspect ratio of the inorganic filler is less than 5, a sufficient crack propagation suppression effect cannot be obtained. From a similar viewpoint, it is preferable that the aspect ratio of the inorganic filler be 10 or greater.
On the other hand, if the aspect ratio of the inorganic filler exceeds 75, there is a risk of it being destroyed during paint formation and coating; therefore, it is preferable that the aspect ratio of the inorganic filler be 75 or less.
The aspect ratio of the inorganic filler is the ratio of the average particle size to the average thickness of the inorganic filler (average particle size / average thickness). The average particle size and average thickness of the inorganic filler can be obtained, for example, by cross-sectional observation of the topcoat layer using an electron microscope, as described above, and the aspect ratio can be calculated from the average particle size and average thickness of the inorganic filler present in multiple observation fields (e.g., five fields).

The type of inorganic filler is not particularly limited as long as it satisfies the particle size and aspect ratio described above. For example, natural mica, synthetic mica, vermiculite, talc, silicon nitride, kaolin, walnutite, titanate, alumina powder, graphite, glass flakes, potassium titanate fibers, sepiolite, gypsum fibers, carbon fibers, aramid fibers, larus fibers, zonolite, etc., and surface-treated products such as metal coatings and silane couplings of these can be used. These inorganic fillers can be used individually or in combination of multiple types.

The top coat layer, in addition to the resin component, the coloring pigment, and the inorganic filler, contains 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 organic aggregates can exert a stress-relaxing effect in the topcoat layer, thereby improving the processability over time 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 average particle size of the organic aggregate must be 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 a similar viewpoint, the average particle size of the organic aggregate is preferably 2 μm or more, and more preferably 3 μm or more.
On the other hand, if the average particle size of the organic aggregate exceeds 35 μm, the organic aggregate tends to detach from the topcoat layer during bending. Therefore, 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 and longest diameters of the organic aggregate. The shortest and longest diameters of the organic aggregate can be obtained, for example, by cross-sectional observation of the topcoat layer using an electron microscope, and the average particle size can be calculated by measuring the shortest and longest diameters of the organic aggregate present in multiple observation fields (e.g., five fields).

The micro-Vickers hardness of the organic aggregate must be 50HV _0.05 or less. If the micro-Vickers hardness of the organic aggregate exceeds 50HV _0.05 , a sufficient stress relaxation effect cannot be obtained. From a similar 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, sufficient durability of the topcoat layer may not be obtained. Therefore, it is preferable that the micro-Vickers hardness of the organic aggregate be 10HV _0.05 or higher.
The micro-Vickers hardness of the organic aggregate can be determined by embedding a sample of fluororesin-coated steel plate with room-temperature drying resin, polishing it, selecting the 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 HMV-G21). The measurement method conforms to JIS Z 2244, and the test is conducted under conditions of 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 requirements. For example, beads (particles) of acrylic resins, urethane resins, fluororesins, urea resins, acrylic triblock copolymers, butadiene rubbers, or surface-treated products of these materials with added functional groups can be used. These organic aggregates can be used individually or in combination of multiple types.

Furthermore, the total content of the inorganic filler and 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 organic aggregate is 0.2% by mass or more, sufficient crack propagation suppression and stress relaxation effects are obtained, resulting in superior processability over time. On the other hand, when the total content of the inorganic filler and organic aggregate is 35% by mass or less, the deterioration of the durability of the topcoat layer can be reliably suppressed.

In addition to the resin component, coloring pigment, inorganic filler, and organic aggregate, the topcoat layer may also contain other additives depending on the purpose and application.
Examples of the aforementioned other additives include metallic pigments such as aluminum powder, pigments such as carbonates and sulfates, various fine particles such as silica fine particles for gloss adjustment, waxes, defoamers, and leveling agents.
It is preferable that the total content of these and other additives in the topcoat layer be 10% by mass or less. This is to avoid hindering 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 for the topcoat layer can be applied by methods such as roll coater coating or curtain flow coating. After coating the coating composition, the topcoat layer can be formed by baking using heating means such as hot air heating, infrared heating, or induction heating. The baking temperature is usually set to a maximum plate temperature of approximately 240 to 260°C. In the case of hot air heating, the hot air velocity at the plate surface should preferably be high, at 1 to 10 m/second, more preferably 5 to 10 m/second.

Furthermore, while the average film thickness of the topcoat layer is not particularly limited, it is preferably 10 to 50 μm, and more preferably 12 to 30 μm. If the average film thickness of the topcoat layer is less than 10 μm, the durability of the topcoat layer may be reduced. If the average film thickness of the topcoat layer exceeds 50 μm, pinhole-like defects may easily occur due to the sudden boiling of the 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 in any way to the examples described below.

<Examples 1-4 of the present invention, Comparative Examples 1-3>
(Example 1 of the present invention)
A hot-dip Al-Zn plated steel sheet was prepared, with a thickness of 0.35 mm, a plating adhesion amount of 150 g/ ^m² per side, and a plating layer with a composition of Zn-55%Al-1.6%Si, which was subjected to a skin pass treatment.
On the molten Al-Zn plated steel sheet described above, a vanadium-based chromate-free chemical conversion treatment solution (using urethane resin as the resin component and containing an organic vanadium compound chelated with acetylacetone as the rust-preventive component) was applied to the molten Al-Zn plated steel sheet using a bar coater, dried at a steel sheet temperature of 90°C for a baking time of 10 seconds, and a chemical conversion treatment film was formed with an adhesion amount of 0.2 g/ ^m² .
Subsequently, a primer layer coating, based on an epoxy-modified urethane resin and using vanadium phosphate as a rust-preventive pigment, was applied to the above-mentioned chemical conversion treatment layer using a roll coater. The steel plate was then baked at a temperature of 210°C for 25 seconds to form a primer layer with a film thickness of 5 μm after baking.
Then, after the steel plate with the primer layer formed on it was water-cooled and dried, a topcoat coating was prepared by mixing a resin component consisting of 30% by mass of PVdF resin and 15% by mass of acrylic resin, totaling 45% by mass, with 40% by mass of iron-chromium composite oxide (Shepherd Color's "Black411") as a black pigment (average particle size 1.1 μm), 0.5% by mass of titanium dioxide as a white pigment (average particle size 500 nm), 3% by mass of synthetic mica as an inorganic filler (average particle size 15 μm, aspect ratio 10), and 3% by mass of acrylic beads as an organic aggregate (average particle size 20 μm, micro-Vickers hardness 20 HV _0.05 ). Subsequently, a topcoat coating was applied to the primer layer, and the topcoat layer was formed by baking it for 50 seconds at an ambient temperature of 400°C and an average hot air velocity of 5 m/s on the plate surface in an oven to a final plate temperature of 250°C, resulting in an average film thickness of 20 μm after drying. A sample coated steel plate (fluororesin coated steel plate) was then prepared.
The conditions for the topcoat layer of the sample painted steel sheet are also shown in Table 1. Furthermore, the average particle size and aspect ratio of the inorganic filler, as well as the average particle size and micro-Vickers hardness of the organic aggregate in the topcoat layer, were pre-controlled to the above ranges.

(Example 2 of the present invention)
A sample coated steel sheet 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 of 10 HV _0.05 ) was used as the organic aggregate in the topcoat layer instead of acrylic beads.
The conditions for the topcoat layer of the sample painted steel sheet are also shown in Table 1.

(Example 2 of the present invention)
A sample coated steel sheet was prepared under the same conditions as in Example 1 of the present invention, except that 1% by mass of polytetrafluoroethylene (PTFE) aggregate (average particle size 18 μm, micro-Vickers hardness of 10 HV _0.05 ) was used as the organic aggregate in the topcoat layer instead of acrylic beads.
The conditions for the topcoat layer of the sample painted steel sheet are also shown in Table 1.

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

(Example 4 of the present invention)
A sample coated steel sheet was prepared under the same conditions as in Example ₁ of the present invention, except that prior to the formation of the chemical conversion treatment layer, a molten Al-Zn plated steel sheet was annealed at an ambient temperature of 200°C for 12 hours to set the Vickers hardness of the dendrite phase in the plating layer to a range of 10 to 110 HV 0.1.
The conditions for the topcoat layer of the sample painted steel sheet are also shown in Table 1.

(Comparative Example 1)
A sample painted steel sheet 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 organic aggregate and synthetic mica as inorganic filler.
The conditions for the topcoat layer of the sample painted steel sheet are also shown in Table 1.

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

(Comparative Example 3)
A sample coated steel sheet was prepared under the same conditions as in Example 1 of the present invention, except that 1% by mass of polyimide aggregate (average particle size 20 μm, micro-Vickers hardness of 60 Hv _0.05 ) was used as the organic aggregate in the topcoat layer instead of acrylic beads.
The conditions for the topcoat layer of the sample painted steel sheet are also shown in Table 1.

<Evaluation>
Each sample of painted steel sheet obtained as described above was evaluated as follows.

(1) Workability and Workability over Time A bending test was conducted on each sample of painted steel sheet in accordance with JIS G 3322. The inner spacing when bending a 75 mm wide test piece 180 degrees was expressed by the number of sheets, and the limit number of sheets (T) at which no cracks occurred in the surface coating of the bent sheet was defined as No-Crack T. A smaller No-Crack T indicates better workability.
This bending test was performed immediately after sample preparation and after 72 hours of storage at 50°C (accelerated aging), equivalent to 3 months of summer storage. Table 1 shows the no-crack T results for both immediately after sample preparation and after accelerated aging.
Furthermore, for the bending test after accelerated aging, the aging processability was evaluated according to the following criteria, and the evaluation results are shown in Table 1.
○: 6T or less, practical for siding applications, etc. ×: Over 6T

From the results in Table 1, it can be seen that each sample of the present invention exhibits superior time-dependent processability compared to each sample of the comparative example.

According to the present invention, it is possible to provide a painted steel sheet with excellent processability over time while maintaining good other physical properties such as processability and weather resistance.

Claims (3)

A painted 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 contains a resin component comprising polyvinylidene fluoride resin and acrylic resin, wherein the mass content of the polyvinylidene fluoride resin is 50 to 80%.
Coloring pigments in a mass of 10-50% ,
An inorganic filler with an average particle size of 1 to 30 μm and an aspect ratio of 5 or more, comprising 0.1 to 20% by mass,
Organic aggregate with an average particle size of 1 to 35 μm and a micro-Vickers hardness of 50 HV _0.05 or less, comprising 1 to 12 % by mass,
Includes,
The fluororesin-coated steel sheet is characterized in that the plating layer of the molten Al-Zn plated steel sheet contains Al: 20 to 95% by mass, Si: 1 to 3% by mass, and optional additives: 5% by mass or less, with the remainder being Zn and unavoidable impurities . The fluororesin-coated steel sheet according to claim 1, characterized in that the total content of the inorganic filler and the organic aggregate in the top coat layer is 1.1 to 30 % by mass. The fluoropolymer-coated steel sheet according to claim 1 or 2 , 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 HV _0.1.