JP2011104988A - Precoated steel sheet and process for producing same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a precoated steel sheet preventing a degradation in aesthetic appearance caused by processing unevenness or interference color streaks by forming the photocatalyst layer (coating film) as the outermost layer in a discontinuous (network) layer, and having various excellent characteristics such as uniform appearance, workability, high weatherability, appearance durability, decomposition resistance for the photocatalyst, required for the precoat steel sheet, and a process for producing the same. <P>SOLUTION: The precoated steel sheet includes, at least on one side of a steel sheet base, a clear coating layer (A) comprising a silicon compound as a principal component and containing water-repellent fine particles dispersed therein, and a clear coating layer (B) superposed on the clear coating layer (A), and containing, as a principal component, fine titanium oxide particles demonstrating photocatalytic activity. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a precoated steel sheet that is excellent in workability, weather resistance, aesthetic durability, environmental cleanability, and the like, and is used as an exterior material for electrical equipment products, interior / exterior materials for buildings, road members, and the like. It is.

  Pre-coated steel sheets do not need to be painted after forming, and can be omitted from processing manufacturers and work environments can be improved, so various materials for electrical equipment products and exterior materials such as roofs and walls of buildings, It is used in a wide range of interior materials such as partitions.

  Such pre-coated steel sheets are usually used after being subjected to various processing including 90 ° bending and 180 ° bending by press forming, roll forming or emboss forming, and therefore require excellent workability and coating film durability. Is done. For this reason, as the resin for the coating film used for the pre-coated steel sheet, a thermosetting polyester resin that forms a cured coating film by baking at 200 ° C. or higher, a fluorine resin having excellent weather resistance, and the like are used. Yes.

  In recent years, the need for high performance and high functionality for pre-coated steel sheets has increased. In particular, aesthetic durability, that is, adhesion of fungi, algae, mold, etc., airborne soot in the atmosphere, and various oily mists over a long period of time. There is also a demand for measures against deteriorating the aesthetics of the pre-coated steel sheet due to adhesion of combustion products such as exhaust gas, carbon black, and other substances.

  For pre-coated steel sheets that have been individually provided with functions such as antibacterial / anti-fungal functions and anti-contamination functions, there are those that have been developed so far, but for pre-coated steel sheets that function in combination with other functions, The fact is that it has not been developed yet.

  Precoated steel sheets with antibacterial and antifungal functions include those in which a compound containing silver, copper, or the like is dispersed in the coating film. It suppresses or kills the growth of fungi and algae. On the other hand, the pre-coated steel sheet with anti-contamination function is to keep the coating surface as hydrophilic as possible so that the contaminated components attached to the surface can be washed away with rain water to suppress the deposition of deposits. It is.

  In particular, as a pre-coated steel sheet having antibacterial function and anti-contamination function, it utilizes a method of decomposing organic matter attached by a photocatalyst, and by dispersing a substance having photocatalytic activity in the coating film on the steel sheet surface, The organic substances adhering to the active oxygen produced are decomposed. However, the technology using this photocatalyst decomposes surface deposits such as organic substances adhering to the surface of the steel sheet and at the same time gradually decomposes the coating film itself as a matrix. In addition, there is a problem that wear of the coating film is promoted.

  In this regard, conventionally, proposals have been made to use an inorganic material, a fluororesin, or an organic-inorganic composite that is not easily decomposed as a matrix coating film of a photocatalyst (Patent Documents 1 to 3). Further, a method of forming a photocatalyst layer containing a photocatalyst and a fluororesin wax on at least one surface of a metal plate via a protective film formed of an inorganic paint (Patent Document 4), silica on a substrate, etc. as a matrix There is also a proposal for a member (Patent Documents 5 and 6) in which a surface layer is formed by dispersing a fluorine resin having a lubricating effect and a buffering effect on the photocatalytic coating film.

  In general, since the photocatalyst film layer has a high refractive index, it tends to exhibit an interference color due to unevenness in the amount of the photocatalytically active substance attached. In particular, the surface of the base substrate to be treated is highly glossy or has a surface roughness. Smaller and flatters have the problem of deteriorating aesthetics, such as noticeable processing unevenness and interference colors appearing streaks.

  In this regard, the photocatalytic film is dispersed on the base material in an island-like manner and deposited in a discontinuous manner, so that it is excellent in design and durability, and cracks do not occur, and the photocatalytic layer is forever Methods (Patent Document 7 and Patent Document 8) that can be kept stable have been proposed.

JP 2007-55207 A JP 2006-233073 A JP 2006-192716 A JP 2007-181951 A JP 2001-64539 A WO97 / 45502 JP 2000-246115 A JP 2009-131987

  However, with respect to the technique disclosed in Patent Document 1, although a photocatalytically active substance is dispersed in an inorganic coating film, the coating film itself has poor flexibility although it has excellent weather resistance such as discoloration. Therefore, when the coating film is applied to the precoated steel sheet as it is, there is a big problem that when the bending process or the drawing process is performed, cracks are easily generated and the coating film is peeled off.

  Further, the technology disclosed in Patent Document 2 relates to a steel plate in which a photocatalytically active substance is dispersed in a fluororesin coating film. However, there is a disadvantage that the coating film as a matrix is decomposed by the photocatalyst. This drawback can be solved by increasing the thickness of the coating film, but increasing the film thickness results in poor flexibility, resulting in new problems such as poor workability. End up.

  The technique disclosed in Patent Document 3 relates to a surface-treated metal obtained by dispersing a photocatalytically active substance in a coating film made of an organic-inorganic composite, but compared with the inorganic coating film of Patent Document 1. Although it is slightly excellent in workability, in order to satisfy the level of workability required for the precoated steel sheet, it is necessary to reduce the film thickness, resulting in a problem that the photocatalytic function is lowered.

  Furthermore, the coated metal plate of Patent Document 4 also has a problem that the coating film as a matrix is decomposed by the photocatalyst when this is applied to an exterior material.

  In addition, the techniques disclosed in Patent Documents 5 and 6 are effective in preventing scratches and scratches, but the photocatalytic film is likely to be scratched when processing involving deformation such as press molding is performed. Therefore, application as a precoated steel sheet is difficult.

  In addition, the technique disclosed in Patent Document 7 requires a complicated method for forming a photocatalytic film in an island shape, and requires a special apparatus for stable film formation. Further, Patent Documents 7 and 8 When processing such as press molding or roll molding is performed, damage such as scratches on the photocatalyst film is unavoidable, and therefore it has been difficult to apply to precoated steel sheets.

  As described above, all of the conventional techniques are difficult to apply to pre-coated steel sheets, or workability, weather resistance, aesthetic durability utilizing photocatalytic function, and matrix for photocatalysts, which are required as characteristics of pre-coated steel sheets. It did not satisfy the decomposition resistance of a certain coating film, and did not fully consider treatment unevenness and interference color countermeasures.

  Therefore, the object of the present invention is to solve the above-mentioned problems of the conventional technology and to make the photocatalyst layer (film), which is the outermost layer, discontinuous (network-like), so that processing unevenness and interference color can be reduced. In addition to preventing appearance deterioration due to the appearance of streaks, in addition to performance such as appearance uniformity and workability required as a pre-coated steel sheet, various properties such as high weather resistance, aesthetic durability, and decomposition resistance to photocatalysts An object of the present invention is to provide an excellent precoated steel sheet and a method for producing the same.

  The inventors have proposed the new technique according to the following gist configuration as a method effective for realizing the above-mentioned object by solving the above-mentioned problems of the prior art.

  The present invention includes a clear coating layer (A) in which the main component is composed of a silicon compound and contains water-repellent fine particles dispersedly on at least one surface of a steel plate substrate, and the main component laminated on the clear coating layer (A). And a clear film layer (B) containing titanium oxide fine particles exhibiting photocatalytic activity as a pre-coated steel sheet.

In the precoated steel sheet of the present invention,
(1) At least a part of the water-repellent fine particles in the clear coating layer (A) protrudes from the surface of the clear coating layer (B) and is dispersed in an island shape;
(2) The contact angle between the surface of the clear coating layer (B) and water is 22 to 60 degrees,
(3) At least a part of the water-repellent fine particles in the clear coating layer (A) has a particle diameter larger than the film thickness of the clear coating layer (A).
(4) The water-repellent fine particles in the clear coating layer (A) are made of fluorine resin particles, fluorine resin copolymer resin particles, silicon resin particles, or particles obtained by coating these resins on the surface. ,
(5) The silicon compound in the clear film layer (A) is an acrylic silicon resin,
(6) titanium oxide particles in the clear coating layer (B), it adhered amount per one side, is 10mg / m ² ~2000mg / m ² in terms of TiO _2,
(7) The clear coating layer (B) is mainly composed of a mixture of anatase type crystalline titanium oxide fine particles and amorphous titanium oxide,
(8) The steel sheet substrate has a chemical conversion film and a resin film formed thereon on at least one surface of a zinc-based plated steel sheet, and the resin film contains a rust preventive pigment. A primer layer made of a thermosetting resin, and a colored resin layer,
(9) The colored resin layer is mainly composed of an organosol-based baking type fluororesin having a mass ratio of polyvinylidene fluoride and acrylic resin of 85:15 to 50:50,
Is a more preferable solution.

  In the present invention, a clear coating layer (A) mainly composed of a silicon compound and a clear coating layer (B) mainly composed of titanium oxide fine particles exhibiting photocatalytic activity are sequentially laminated on at least one surface of a steel plate substrate. In the method for producing a precoated steel sheet, the water-repellent fine particles are dispersed and contained in the clear coating layer (A), and the coating amount of the clear coating layer (B) on the portion where the water-repellent fine particles are present and in the vicinity thereof A method for producing a pre-coated steel sheet is proposed in which at least a part of the water-repellent fine particles is dispersed from the surface of the clear coating layer (B) in an island shape by suppressing the above.

  In the method for producing a precoated steel sheet of the present invention, at least a part of the water-repellent fine particles in the clear coating layer (A) may have a particle size larger than the film thickness of the clear coating layer (A). This is a more preferable solution.

According to the precoated steel sheet of the present invention configured as described above and the manufacturing method thereof, the following effects can be expected.
(1) A clear coating layer (A) containing water-repellent particles as a base layer is laminated on the surface of a steel plate substrate, and further, titanium oxide fine particles having photocatalytic activity are further dispersed on the clear coating layer ( By laminating B), the clear coating layer (B) which is the outermost layer is not formed in the portion where the water-repellent particles of the clear coating layer (A) are present and becomes discontinuous (network-like). When the pre-coated steel sheet is subjected to bending or drawing, cracks are less likely to occur in the clear coating layer (B).
(2) Moreover, it is possible to prevent a decrease in aesthetics due to processing unevenness, interference color, and the like of the clear coating layer (B). In addition, since the clear coating layer (A) is exposed on the surface of the precoated steel sheet in the portion of the clear coating layer (A) where the water-repellent particles are present, various organic substances adhering to the surface of the precoated steel sheet Is repelled by the clear film layer (A) (water-repellent particles) exposed on the outermost layer, and decomposed and cleaned by the clear film layer (B) (photocatalyst layer), thereby maintaining the aesthetics of the surface. Can be maintained.
(3) As described above, the water-repellent fine particles in the clear coating layer (A) exposed on the surface of the precoated steel sheet have a function as a lubricant or a buffer, and therefore when subjected to bending or drawing, Generation | occurrence | production of the crack etc. to the steel plate surface can be suppressed.

(4) Since the clear coating layer (A) having a silicon compound as a main component is provided between the colored resin layer on the steel plate substrate surface and the clear coating layer (B) which is the outermost layer, it is clear. The coating layer (B) is effectively decomposed against the photocatalyst, and the lower colored resin layer is prevented from being decomposed and deteriorated by direct contact with the photocatalyst, and has good workability, weather resistance and aesthetic durability. It can be set as the precoat steel plate which has.

It is sectional drawing which shows one Embodiment of the precoat steel plate which concerns on this invention. It is sectional drawing and top view which show other embodiment of the precoat steel plate which concerns on this invention. It is explanatory drawing which shows the evaluation method of the bending workability of a precoat steel plate. It is sectional drawing and the top view which show the structure of the precoat steel plate of the comparative example 3 of an Example. It is sectional drawing and a top view which show the structure of the precoat steel plate of the comparative example 6 of an Example. It is sectional drawing and a top view which show the structure of the precoat steel plate of the comparative example 7 of an Example. It is sectional drawing and a top view which show the structure of the precoat steel plate of the comparative example 8 of an Example.

  The basic structure of the precoated steel sheet of the present invention is that a clear film layer (A) containing water repellent fine particles dispersed and containing a silicon compound as a main component on at least one surface of a steel sheet substrate, and the clear film layer (A) A clear coating layer (B) containing titanium oxide particles having photocatalytic activity is sequentially laminated thereon.

  In the present invention, in particular, the clear coating layer (A) is characterized in that water-repellent fine particles are dispersed and contained, whereby the clear coating layer (B) as an upper layer is contained in the clear coating layer (A). The water-repellent fine particles are repelled to form a discontinuous state (network), and at least a part of the water-repellent fine particles protrudes from the surface of the clear coating layer (B) and is dispersed in an island shape. As a result, when processed as a pre-coated steel sheet, cracks and the like are less likely to occur in the clear coating layer (B), which is the outermost layer, and the appearance is aesthetically pleasing due to processing irregularities and interference colors of the clear coating layer (B). It can be prevented from lowering.

  Moreover, in the portion where the water-repellent fine particles are present as described above, the clear coating layer (A) is exposed on the surface of the steel sheet, and the water-repellent fine particles serve as a lubricant and a buffer. When bending or drawing the steel sheet, it is possible to suppress the occurrence of scratches on the steel sheet surface.

Hereinafter, the precoated steel sheet of the present invention will be described in detail with reference to the drawings.
The precoated steel sheet of the present invention has a chemical conversion coating 2 on at least one surface of a zinc-based plated steel sheet 1 and a resin film (primer layer) formed thereon, as shown in FIG. 3, a colored resin layer 4) and two types of clear coating layers (clear coating layer (A) 5 and clear coating layer (B) 6) are sequentially laminated.

  The zinc-based plated steel sheet 1 preferably used in the present invention can be formed, for example, an electrogalvanized steel sheet, a hot-dip galvanized steel sheet, a hot-dip zinc-5 mass% aluminum alloy-plated steel sheet, a hot-dip zinc-55 mass% aluminum alloy. This includes plated steel sheets. The thickness of the plated steel sheet is not particularly limited, but is preferably about 0.2 to 1.6 mm from the viewpoint of forming.

  In the present invention, the chemical conversion treatment film 2 is first formed on the one surface or both surfaces of the galvanized steel sheet 1 prior to forming the primer layer 3. This chemical conversion coating 2 is formed in order to strengthen the adhesion between the zinc-based plated steel sheet 1 and the primer layer 3 and to provide corrosion resistance.

  Examples of the chemical conversion treatment film 2 suitably used in the present invention include a phosphate treatment film, a chromate treatment film, a silica-containing chromate treatment film, a chromate-free rust preventive comprising various metal oxides, and an organic resin alone. Or it consists of a composite, and in order to improve adhesiveness and corrosion resistance in particular, it is preferably composed of a thin film containing an organic resin and a metal oxide, a phosphoric acid compound and / or chromic acid. In this case, as the organic resin, it is desirable to use a water-dispersible resin, for example, an acrylic resin or a urethane resin dispersed with an emulsifier.

Among such chemical conversion treatment film components, chromic acid acts as a binder for obtaining adhesion between the chemical conversion treatment film and the zinc-based plating layer. On the other hand, the organic resin acts to improve adhesion between the chemical conversion film and a primer layer described later. In addition, it is preferable that chromium content in a chemical conversion treatment film is 20 mg / m < ² > or more in metal chromium conversion amount. This is because when the chromium content is less than 20 mg / m ² , sufficient corrosion resistance as a precoated steel sheet cannot be obtained.

Further, the chromate-free rust preventive agent used as the chemical conversion coating component is silicic acid and / or silicic acid compound, calcium or calcium compound, zirconic acid and / or zirconic acid compound, vanadic acid and / or vanadic acid compound, molybdic acid. It is preferable that it contains 1 or more types of a compound, phosphoric acid, and / or a phosphoric acid type compound, and content is 0.3-3.0 g / m < ² >. The reason for making the amount of this chromate-free rust inhibitor 0.3 to 3.0 g / m ² is 0.3
If it is less than g / m ² , the corrosion resistance and adhesion as a pre-coated steel sheet cannot be obtained, and if it exceeds 3.0 g / m ² , workability will be reduced. Further, as the binder, for example, a water dispersible acrylic resin or urethane resin may be used.

  In the precoated steel sheet of the present invention, the primer layer 3 is first formed as a first film on the surface of the chemical conversion film 2 formed on at least one surface of the zinc-based plated steel sheet 1 as described above. The primer layer 3 is made of a thermosetting resin containing a rust preventive pigment, and includes a chemical conversion treatment film 2 and an upper film described later (colored resin layer 4, clear film layer (A) 5, clear film layer (B) 6). It is a layer formed in order to obtain adhesion and corrosion resistance.

  The layer thickness of the primer layer 3 is preferably 2 μm or more and 10 μm or less. If the layer thickness is less than 2 μm, sufficient rust prevention property cannot be obtained. On the other hand, if it exceeds 10 μm, the workability deteriorates. This is not preferable.

  The thermosetting resin that is the main component of the primer layer 3 is not particularly limited, but it is preferable to use one or more of polyester resins, epoxy resins, and the like. A polyester resin or the like can be used. In addition, some epoxy resins are partially replaced with urethane resins, phenol resins, amino resins, etc., and epoxy compounds comprising bisphenols such as bisphenol A, bisphenol F, bisphenol AD, and epihalohydrin or β-methyl epihalohydrin. Alternatively, copolymers of these can be used. Furthermore, monocarboxylic acid or dicarboxylic acid modified products, mono, di or polyalcohol modified products, mono or diamine modified products, mono, di or polyphenol modified products of these epoxy compounds can also be used as epoxy resins.

  In addition, the reason for limiting the main body of the primer layer 3 to the thermosetting resin is that when a room temperature drying resin or a thermoplastic resin is used, when an upper film (colored resin layer, clear film layer) is formed thereon, This is because deformation and deterioration accompanying softening occur depending on the baking temperature, and there is a problem that strong adhesion to the upper layer film cannot be obtained.

  As a curing agent for the primer layer 3, a polyisocyanate compound and / or an amino resin can be used. As the polyisocyanate compound, hexamethylene diisocyanate (hereinafter referred to as HDI) and derivatives thereof obtained by a general production method, tolylene diisocyanate (hereinafter referred to as TDI) and derivatives thereof 4.4′-diphenylmethane diisocyanate (hereinafter referred to as MDI) and Its compounds, xylylene diisocyanate (hereinafter referred to as XDI) and derivatives thereof, isophorone diisocyanate (hereinafter referred to as IPDI) and derivatives thereof, trimethylhexamethylene diisocyanate (hereinafter referred to as TMDI) and derivatives thereof, hydrogenated TDI and derivatives thereof, hydrogenated MDI and Compounds such as derivatives thereof, hydrogenated XDI and derivatives thereof can be used. In particular, 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-pack type paint. This blocked polyisocyanate compound can be stored in a single liquid and can be easily used as a paint.

  Examples of the amino resin used for the curing agent include resins obtained by reacting urea, benzoguanamine, melamine and the like with formaldehyde, and those obtained by alkyl etherification with alcohols such as methanol and butanol. Specific examples include methylated urea resins, n-butylated benzoguanamine resins, methylated melamine resins, n-butylated melamine resins, and iso-butylated melamine resins.

  The mixing ratio of the curing agent of the primer layer 3 is preferably 9 to 50% by mass in the resin solid content, and this is less than 9% by mass, and the coating film hardness is insufficient, and 50% by mass. This is because if the thickness is too high, the processability is insufficient.

  Furthermore, a curing catalyst such as p-toluenesulfonic acid, tin octoate, dibutyltin dilaurate or the like may be used for the primer layer 3 depending on the purpose and application. Further, extender pigments such as calcium carbonate, kaolin, and clay, color pigments such as titanium oxide, dial, mica, carbon black, and aluminum powder, and various additives such as antifoaming agents and leveling agents may be added.

  In the present invention, in the primer layer 3, when the pre-coated steel sheet is formed, or when it is used outdoors for a long period of time, coating bulge and plating corrosion from a cut edge portion or a coating scratch portion are prevented. In order to prevent it, 5 mass%-50 mass% of rust preventive pigments are contained. When the content of the rust preventive pigment is less than 5% by mass, there is no effect of improving the corrosion resistance, and when it exceeds 50% by mass, the workability deteriorates with a decrease in the flexibility of the coating film, which is not preferable. .

  As rust preventive pigments, chromate, silica pigment, phosphite pigment, calcium compound, aluminum oxide, zirconic acid or / and zirconic acid compound, vanadic acid or / and vanadic acid compound, molybdate compound, It is preferable to use one or a mixture of two or more of phosphoric acid and / or phosphoric acid compounds. Among them, it is preferable to use chromate from the viewpoint of further improving rust prevention.

  Next, the colored resin layer 4 will be described. The colored resin layer 4 is formed on the primer layer 3 described above. As the resin of the colored resin layer 4, a resin excellent in adhesiveness with the primer layer 3 and capable of continuous coating in a pre-coated steel sheet manufacturing facility, for example, a polyester resin, a fluorine resin, an acrylic resin, a urethane resin, Resins in which these components are combined are advantageously suitable.

  In particular, in the present invention, in order to improve the workability of the pre-coated steel sheet and the durability (resistance to weathering) outdoors, polyvinylidene fluoride and acrylic resin as the main resin of the colored resin layer 4 are 85: It is preferable to use an organosol baking type fluororesin blended in a ratio of 15 to 50:50, more preferably 85:15 to 75:25. This is because, by using an organosol-type baking type fluororesin that is a mixture of polyvinylidene fluoride and an acrylic resin, it is possible to suppress crystallization of polyvinylidene fluoride, which is a crystalline resin, and durability (resistance to weathering). ) And processability can be improved, and furthermore, the adhesion between the primer layer 3 as the lower layer of the colored resin layer 4 and the clear film layer (A) 5 as the upper layer can be improved. is there.

  Note that when the amount of the polyvinylidene fluoride of the organosol-based baking type fluororesin exceeds 85% by mass, the coating property is lowered and the crystallization of the polyvinylidene fluoride is advanced, so that the workability is lowered. On the other hand, when the amount is less than 50% by mass, the durability, particularly weather resistance, of the polyvinylidene fluoride is significantly lowered, which is not preferable.

  As the acrylic resin contained in the organosol-based baking type fluororesin, a thermoplastic acrylic resin and a thermosetting acrylic resin can be used alone or in combination. As thermoplastic acrylic resins, from the viewpoint of compatibility with polyvinylidene fluoride, methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, butyl (meth) acrylate, (meth) acrylic One or two or more monomer polymers such as acid amyl, or a copolymer of these monomers with acrylic acid, styrene, or the like can be used. On the other hand, the thermosetting acrylic resin is composed of an acrylic resin having a crosslinkable functional group such as a hydroxyl group, a carboxyl group, a glycidyl group or an isocyanate group, and a curing agent such as an alkylated melamine, a polyol or a polyamide. Can be used.

  The colored resin layer 4 controls the color tone of the appearance of the precoated steel sheet, and titanium oxide, petal, mica, carbon black, fired black, titanium yellow, yellow iron oxide, phthalocyanine blue, phthalocyanine are included in the coating film. Blends various color pigments such as green. This color pigment is contained in an amount of about 15% by mass to 60% by mass. The reason is that the target hue can be formed as a stable and concealed coating film within the range of this content. In addition, additives such as a gloss adjusting agent such as synthetic silica, an antifoaming agent for improving coating workability, a surface adjusting agent, and an anti-scratch agent for a coating film can be blended as necessary.

  The colored resin layer 4 is preferably 8 to 25 μm thick. If the layer thickness is less than 8 μm, the color tone becomes unstable as the concealing property decreases, and sufficient durability, workability and corrosion resistance of the processed part cannot be obtained. On the other hand, if the layer thickness exceeds 25 μm, film defects such as occurrence of cracks (foaming) are likely to occur, and further cracks occur due to an increase in internal strain at the time of processing, which is not preferable.

  The colored resin layer 4 is applied to the surface of the primer layer 3 of the galvanized steel sheet by a normal method for producing a precoated steel sheet, for example, a chemical solution diluted with an organic solvent, using a roll coater or a curtain coater. It can be formed continuously by baking at a temperature of about 0C.

  Next, the clear coating layer (A) 5 will be described. The clear coating layer (A) 5 is formed on the colored resin layer 4 and provides mutual adhesion between the clear coating layer (B) 6 as an upper layer and the colored resin layer 4 as a lower layer. It has a function to improve. Further, the clear coating layer (A) 5 functions to suppress the colored resin layer 4 from being decomposed and deteriorated over time by the photocatalytic action of the titanium oxide fine particles blended in the clear coating layer (B) 6. Also have.

  The main component of the clear coating layer (A) 5 is a silicon compound such as acrylic silicon resin, trialkoxysilane and its condensate, acrylic silicate and its condensate, organohydroxysilane and its condensate, or a composition thereof and silica. It is particularly preferable to use an acrylic silicon resin. As the acrylic silicone resin, it is possible to use a composite, copolymerized or cross-linked with another cross-linking agent, or a cross-linked one using an acrylate or silicon-containing cross-linking agent. it can.

  The acrylate component of the acrylic silicone resin functions to improve the flexibility of the clear coating layer (A) 5 and to improve the adhesion with the colored resin layer 4 as the lower layer. The acrylate component includes alkyl-containing acrylic monomers and oligomers or polymers such as methyl acrylate and methyl methacrylate, hydroxyl-containing acrylic monomers and oligomers or polymers such as 2-hydroxyethyl acrylate, and ethylenic unsaturated compounds such as acrylic acid. One or more of carboxylic acid monomers, oligomers or polymers, acrylic monomers, oligomers or polymers containing amide groups or glycidyl groups can be used.

  On the other hand, the silicon component of the acrylic silicon resin strengthens the adhesiveness with the clear coating layer (B) 6 formed in the upper layer, and the clear coating layer (A) by the photocatalyst contained in the clear coating layer (B) 6. 5 itself has a function of suppressing decomposition and deterioration.

  Examples of the silicon component include alkoxysilane compounds such as methyltriethoxysilane, dimethyldiethoxysilane, phenylmethyldiethoxysilane, and γ-glycidoxypropyltrimethoxysilane, hydrolysates thereof, or one of condensates thereof. Two or more types can be used.

  The layer thickness of the clear coating layer (A) 5 is preferably 0.2 μm or more, more preferably 0.5 μm or more. On the other hand, the upper limit thickness of the clear coating layer (A) 5 is not particularly defined, but when the composition itself is poor in flexibility, it is preferably a thinner film, particularly 1 μm, in order to suppress deterioration of workability. By setting it as the following, favorable workability can be obtained irrespective of the kind of composition.

  As described above, the present invention is characterized in that the clear film layer (A) 5 contains the water-repellent fine particles 7 dispersed therein. That is, the clear film layer (B) 6 as an upper layer is repelled by the water-repellent fine particles 7 in the clear film layer (A) 5 and is not covered in and around the portion where the fine particles 7 are present. The coating layer (B) 6 is not formed in a continuous film shape. As a result, when processed as a pre-coated steel sheet, the occurrence of cracks and the like in the clear coating layer (B) 6 is suppressed, and the deterioration of aesthetics due to processing unevenness, streak-like interference color, etc. can be prevented. it can.

  In addition, as the water-repellent fine particles 7 suitable for exhibiting the above effect, fluorine-based resin particles such as polytetrafluoroethylene, polychlorotrifluoroethylene, or perfluoroalkoxy fluororesin, tetrafluoroethylene / hexafluoride, etc. Fluorine resin copolymer resin particles such as polypropylene copolymer or ethylene / tetrafluoroethylene copolymer, silicon resin particles, or particles coated with these resins on the surface, especially fine particles made of fluorine resin Is preferred.

  Further, in the present invention, for example, water-repellent fine particles 7 made of polytetrafluoroethylene are used, and fine particles 7 having a particle diameter larger than the film thickness are formed in the clear coating layer (A) 5 as shown in FIG. When included, the fine particles 7 are exposed on the surface of the clear coating layer (A) 5, that is, the surface of the precoated steel plate, and the fine particles 7 are used as a lubricant or a buffer when the precoated steel plate is bent. It can act more effectively and suppress the occurrence of scratches and the like. Examples of such fluororesin fine particles 7 include Dionion TF micro powder manufactured by Sumitomo 3M Limited.

  The particle diameter of the water-repellent fine particles 7 is preferably (average) having a particle diameter of 0.3 to 10 μm. The reason is that if the particle diameter is less than 0.3 μm, the water-repellent effect is lowered and the clear coating layer (B) 6 is formed in a continuous film shape, causing problems such as uneven appearance (interference color) and lowering of workability. On the other hand, when the thickness exceeds 10 μm, the clear coating layer (A) 5 has a problem. In order to disperse, it is necessary to make the thickness of the clear coating layer (A) 5 sufficiently thick, which is disadvantageous in terms of cost and it becomes difficult to stably form the clear coating layer (A) 5 itself. It is.

  Moreover, about the compounding quantity of the water-repellent fine particle 7 in the clear membrane | film | coat layer (A) 5, about 0.5-20 mass% is preferable by solid content ratio, More preferably, it is 1-10 mass%. If the blending amount is less than 0.5% by mass, the above effect cannot be obtained, and if it exceeds 20% by mass, the film becomes cloudy or difficult to paint, which is not preferable.

  Various additives such as silica fine particles, a dispersant, a coupling agent, or an ultraviolet absorber may be added to the clear coating layer (A) 5 within a range that does not change the color tone. As a method for forming the clear coating layer (A) 5 in a continuous coating line, there is a method in which a solution diluted in an organic solvent is applied and baked by a curtain coater, a roll coater or a die coater. In addition, when using an acrylic silicon resin as a main component of the clear membrane | film | coat layer (A) 5, it is preferable to bake at the temperature of about 150-200 degreeC after application | coating.

  Next, the clear coating layer (B) 6 that is the outermost layer of the precoated steel sheet of the present invention will be described. The clear coating layer (B) 6 is formed on the clear coating layer (A) 5. In the present invention, since the water-repellent fine particles 7 are contained in the lower clear coating layer (A) 5, the clear coating layer (B) 6 is used as an aqueous dispersion, for example, by a roll coating method in a continuous coating line. When applied, the clear coating layer (B) 6 is not coated on or near the water-repellent fine particles 7 and is formed in a discontinuous state (network shape). As a result, when processed as a pre-coated steel sheet as described above, cracks or the like hardly occur in the clear coating layer (B) 6 that is the outermost layer, and the clear coating layer (B) 6 is formed in a continuous film shape. The streak-like interference color generated at the time of occurrence is also suppressed, and a decrease in surface aesthetics can be prevented.

  As described above, the clear coating layer (B) is not covered in the vicinity of the water-repellent fine particles 7 contained and dispersed in the clear coating layer (A) and in the vicinity thereof, and the water-repellent fine particles are not clear. Although exposed on the surface of the coating layer (B), the contact angle between the surface of the clear coating layer (B) and water is 60 degrees or less, preferably 22 to 50 degrees, particularly preferably 25 to 50 degrees. is there. This is because when the contact angle of water exceeds 60 degrees, the self-cleaning effect (the effect of self-cleaning rain-stained dirt outdoors by rain) cannot be obtained. Clearly obtained at 50 degrees or less. Moreover, when the contact angle of water is less than 22 degrees, the effect (formability as a pre-coated steel plate, wiping off attached dirt) due to the water-repellent fine particles contained and dispersed in the clear coating layer (A) may be reduced. Is not preferable.

  And this clear membrane | film | coat layer (B) 6 decomposes | disassembles and cleans various organic substances adhering to the surface, when using a precoat steel plate as a new function, ie, an internal / exterior material, in a precoat steel plate, It has an effective function to maintain the aesthetics of forever. This function is exhibited by the titanium oxide fine particles having photocatalytic activity contained in the coating layer (B) 6.

  In the present invention, it is preferable to use anatase type crystalline titanium oxide as the titanium oxide fine particles having a high level of photocatalytic activity. As anatase type crystalline titanium oxide, those having a particle diameter of about 0.001 to 0.2 μm are optimal in terms of paintability and photocatalytic properties. As such anatase type crystalline titanium oxide, for example, Ishihara Sangyo ( ST-01, ST-21, ST-30L, etc. manufactured by Co., Ltd. are mentioned.

  In general, anatase type crystalline titanium oxide exhibits activity with respect to light having a short wavelength in the ultraviolet region, and therefore, when a precoated steel sheet is used as an exterior member, it can exhibit photocatalytic properties by ultraviolet rays contained in sunlight. it can. On the other hand, when a pre-coated steel sheet is used as an interior member, it is preferable to use a visible light responsive anatase type crystalline titanium oxide. As this visible light responsive titanium oxide, an oxide doped with nitrogen, sulfur or carbon is used. Titanium, oxygen-deficient titanium oxide, dye-sensitized titanium oxide, metal-supported titanium oxide, or the like can be used.

  The clear coating layer (B) 6 may be blended with amorphous titanium oxide together with the above anatase type crystalline titanium oxide. In this case, the mixing ratio of the anatase type crystalline titanium oxide and the amorphous titanium oxide is preferably 10:90 to 80:20 by mass ratio. Amorphous titanium oxide has the effect of improving the stability of the coating chemical and improving the action of the film as a binder, and the workability and durability of the film.

The clear coating layer (B) 6 is the titanium oxide particles deposited per galvanized steel sheet one side is 10mg / m ² ~2000mg / m ² in terms of TiO ₂ weight, more preferably 50 mg / m ² 2000 mg a / m ^2. When the adhesion amount is less than 10 mg / m ² , the photocatalytic properties are greatly deteriorated or a uniform film cannot be formed. On the other hand, when it exceeds 2000 mg / m ² , various forming processes are applied as a pre-coated steel sheet. When this occurs, cracks and peeling occur in the film, and in some cases, the cracks may propagate to the lower colored resin layer 4.

  The clear coating layer (B) 6 includes, in addition to the fine titanium oxide particles, one or more of metal oxides and compounds such as silicon, aluminum and zirconium, or phosphate compounds as binders and additives for forming the coating. An inorganic compound may be contained.

  The clear coating layer (B) 6 may contain an antibacterial agent such as a silver compound and an adsorbent such as activated carbon or zeolite as long as the color tone is not changed. It can be further increased.

  As a method of forming the clear coating layer (B) 6 on the clear coating layer (A) 5 which is a lower layer in a continuous coating line, water or There is a method in which a chemical solution diluted in an organic solvent is applied by a roll coater or a die coater and baked at a temperature of about 180 to 250 ° C.

As described above, the present invention is provided with the clear coating layer (A) 5 containing a water-repellent substance and the clear coating layer (B) 6 containing photocatalyst particles on one or both sides of a steel plate substrate. As shown in FIG. 2 (a) sectional view and (b) plan view, the clear coating layer (B) 6 as an upper layer is in the clear coating layer (A) 5 as shown in FIG. The water repellent fine particles 7 are repelled, and the portions where the fine particles 7 are present and the vicinity thereof are not covered, and the water repellent fine particles 7 are exposed from the surface of the precoated steel plate (clear coating layer (B) 6). It exists so as to straddle both the clear coating layer (A) 5 and the clear coating layer (B) 6. Thereby, in the precoat steel plate of this invention, the aesthetic durability by a photocatalyst, the decomposition-proof performance with respect to a photocatalyst, and the outstanding external appearance uniformity and workability can be obtained.
Moreover, by providing the above-described primer layer 3 and the colored resin layer 4 through the chemical conversion coating 2 as the steel plate base material, the above-described various characteristics required as a pre-coated steel plate are obtained by the interaction of each layer. Can do.

  Each of the coating layers is formed on at least one side of the steel plate substrate, but the same coating layer may be formed on the other side (back side), and after the chemical conversion treatment on the back side, You may form what is called a service coat of 1 coat and 1 bake used in a normal precoat steel plate.

The invention is illustrated by the following examples.
(Invention Example 1)
In Example 1 of the present invention, a molten 55% aluminum-zinc alloy having a thickness of 0.5 mm obtained by applying a silica-containing coating-type chromate treatment as a steel plate substrate so that the amount of adhesion is 40 mg / m ² in terms of metal chromium. A plated steel sheet (the amount of plating adhesion was 80 g / m ² on one side) was used. This hot dip galvanized steel sheet is coated with a melamine curable polyester resin paint “Precolor Primer FX-2 (manufactured by BASF Coating Japan)” containing about 20% by mass of strontium chromate as a rust preventive pigment. The primer layer (a-1) was formed by coating with a bar coater so as to have a thickness of 5 μm and baking at about 230 ° C. for 60 seconds.

  Next, on this primer layer, as a coloring pigment, an organosol-based baking type fluororesin “Precolor No. 8800 (manufactured by BASF Coating Japan Co., Ltd.)” having a weight ratio of polyvinylidene fluoride and acrylic resin of 80:20 is used. A paint (b-1) in which titanium oxide and calcined black are mixed with calcined silica for surface adjustment is applied with a bar coater so that the dry coating thickness is 22 μm, and then baked at about 240 ° C. for 60 seconds to be colored. A resin layer (gloss value 10 by 60 degree gloss method) was formed.

  Next, an acrylic silicon resin solution having a weight average molecular weight of about 20,000 comprising n-butyl acrylate and methyl methacrylate as an acrylic component and γ-methacryloxypropyltriethoxysilane as a silicon component is formed on the colored resin layer. A coating material (c-1) containing 5% by mass of polytetrafluoroethylene fine particles having an average particle size of 4 μm (Dinion TF9207 manufactured by Sumitomo 3M Limited) was applied with a bar coater so that the dry film thickness was 2 μm. Thereafter, the film was dried at about 150 ° C. for 30 seconds to form a clear film layer (A).

Furthermore, on the clear coating layer (A), an amorphous titanium oxide dispersion “Nippon Parkerizing Co., Ltd.” is used so that ST-21 manufactured by Ishihara Sangyo Co., Ltd. as anatase-type titanium oxide has a solid content ratio of 20 mass%. The aqueous dispersion (d-1) mixed with “PTI-5600” was applied with a roll coater so that the adhesion amount was 150 mg / m ^{2 in} terms of TiO ₂ , and then dried at about 200 ° C. for 20 seconds. A clear coating layer (B) was formed to obtain a precoated steel sheet of the present invention (Invention Example 1).

(Invention Examples 2-8, Comparative Examples 1-8)
Except for changing the primer layer, colored resin layer, clear coating layer (A), and clear coating layer (B) of Invention Example 1 to various conditions shown in Table 1, the same procedure as in Invention Example 1 was followed. Precoated steel sheets (Invention Examples 2 to 8) and comparative precoated steel sheets (Comparative Examples 1 to 8) were prepared. In addition, about the comparative example 7, the aqueous dispersion liquid of the said kind (d-1) was apply | coated with the spray on the clear membrane | film | coat layer (A), and it coat | covered so that a photocatalyst might be disperse | distributed to island shape.

The type (b-2) of the colored resin layer in Table 1 is a melamine curable polyester resin in which titanium oxide and carbon black are mixed as a color pigment ("Precolor No. 3800" (manufactured by BASF Coatings Japan). ), A thickness of 15 μm, and a gloss value of 60.
Also, the type (c-2) of the clear coating layer (A) is obtained by adding polytetrafluoroethylene fine particles (Sumitomo 3M Dyneon TF9205) having an average particle size of 8 μm to the same acrylic silicon resin solution as the type (c-1). The solid content ratio is 3% by mass. Type (c-3) is the same acrylic silicon resin solution as type (c-1) and does not contain water-repellent fine particles. (C-4) is an acrylic resin emulsion paint, and the type (c-5) is 30% of the powder of the type (d-1) with respect to the solution of the type (c-3) in a solid content ratio. Further, water-repellent fine particles (Sumitomo 3M Dionine TF9207) are mixed so that the solid content ratio is 3%.
Further, the type (d-2) of the clear coating layer (B) is obtained by removing the anatase-type titanium oxide from the type (d-1), and the type (d-3) is the type (d-1). Water-repellent fine particles (Sumitomo 3M Dionine TF9207) are mixed in an aqueous dispersion so that the solid content ratio is 3%.

All the pre-coated steel sheets (Invention Examples 1 to 8, Comparative Examples 1 to 58) produced according to the above were subjected to the following various tests, and the results are shown in Table 1.
(Test and evaluation method)
(1) Appearance uniformity The appearance of the obtained precoated steel sheet was evaluated visually. The evaluation criteria were as follows.
◎: No interference color and good appearance ○: Interference color is slightly generated, but there is no occurrence of unevenness, discoloration, etc. Appearance is good △: Unevenness, discoloration (interference color, etc.) occurs slightly ×: Unevenness, Discoloration (interference color, etc.) clearly occurs

(2) Bending workability In the room (20 ° C), the same two steel plates are stacked as shown in Fig. 3 and the upper end portions of these steel plates are sandwiched between 180 ° pre-coated steel plates, and the bent projections The state of the film (presence or absence of cracks) at the part (upper end) was visually evaluated. The evaluation criteria were as follows.
◎: Cracks are not recognized ○: Cracks are slightly observed △: Cracks are clearly generated ×: Cracks are severely generated

(3) Processing scratch property Press bending was performed so that the coating surface was convex, and the appearance change and scratched state of the coating surface were evaluated. The bending process was performed in a 20 ° C. room with an inner radius of 0.8 mm.
○: No flaws △: Slight flaws appear on the part where the press die hits, and discoloration occurs ×: Clear flaws appear on the coating film

(4) Weather resistance An accelerated weather resistance test was conducted for 1000 hours under the following conditions to evaluate the appearance. The evaluation criteria were as follows.
A: Almost no change in color tone or gloss.
○: Slight changes in color tone and gloss are recognized.
Δ: Color tone and gloss change are clearly recognized.
X: Significant changes in color tone and gloss such as severe chalking.
<Accelerated weather resistance test conditions>
Number of sunshine carbon arc lamps: 1 (no filter used)
Power supply voltage: Single-phase AC 180-230V
Light-off-irradiation cycle: 60-60 minutes Irradiation conditions Temperature indicated by black panel thermometer: 63 ± 3%
Relative humidity: 50 ± 5%
Light-off conditions Air temperature: 30 ° C
Relative humidity: 98% or more Cooling water temperature to the back of the test piece: Approximately 7 ° C
Injection of water onto the surface of the test piece: Not performed Irradiance received on the surface of the test piece: 285 ± 50 W / m ^{2 for} 300 to 700 mm

(5) Aesthetic durability The pre-coated steel sheets of the present invention example and the comparative example cut into 200 mm × 300 mm squares,
(A) The wall of the building adjacent to the road (about 1m above the ground)
(B) Hut wall adjacent to paddy field (contacts the ground)
The samples were left standing for 4 months in a vertically standing state, and the subsequent appearance was evaluated. The evaluation criteria were as follows.
○: There is almost no deposit on the surface, and the aesthetics are maintained. △: The surface is slightly covered with deposits, and the initial aesthetic is almost lost. ×: Vigorous deposits that cannot be removed by washing (algae and other organisms, Carbon etc.), and the initial aesthetic disappears

(6) Contact angle of water The surfaces of the precoated steel sheets of the present invention and comparative examples were irradiated with a 20 W black light (FL20S-BLB manufactured by Toshiba Lilac Co., Ltd.) for 24 hours under the condition of ultraviolet intensity of 0.2 mW / cm ² . Thereafter, water was dropped on the surface, and the contact angle of water after 3 seconds was measured with an automatic contact angle meter (DM-500 manufactured by Kyowa Interface Science Co., Ltd.). The results are shown in Table 1.

From the results shown in Table 1, in Examples 1 to 8 of the present invention, excellent performance was obtained in all evaluation tests.
On the other hand, in Comparative Example 1, since the clear film layer (A) was not formed, deterioration was observed in all of the processing scratch resistance, weather resistance and aesthetic durability.
Moreover, in the comparative example 2, since the clear membrane | film | coat layer (B) was not formed, the effect by a photocatalyst was not acquired but deterioration was recognized in aesthetic durability.
The precoated steel sheet of Comparative Example 3 is an upper layer because the clear coating layer (A) 5 does not contain water-repellent fine particles as shown in FIG. 4 (a) sectional view and (b) surface view. The clear coating layer (B) 6 was formed in a continuous film shape, and processing scratches were generated on the surface.
In Comparative Example 4, since the clear coating layer (B) does not contain titanium oxide, the aesthetic durability effect by the photocatalyst cannot be exhibited. In Comparative Example 5, the clear coating layer (A) is an acrylic resin emulsion paint. Since the silicon compound and water-repellent fine particles were not contained, the clear coating layer (A) was decomposed and deteriorated by the photocatalyst, and deterioration in weather resistance and workability was recognized.

Furthermore, in Comparative Example 6, since the water-repellent fine particles were mixed with the coating solution of the clear coating layer (B), the water-repellent fine particles 7 were as shown in FIG. 5A cross-sectional view and FIG. It exists only in the clear coating layer (B) 6. Therefore, although the clear coating layer (B) 6 is discontinuous by the water-repellent fine particles 7 and excellent appearance uniformity can be shown, the fine particles 7 are held in the clear coating layer (B) 6. The force was weak, scratches were likely to occur during processing, and most of them dropped off.
In Comparative Example 7, as shown in (a) cross-sectional view and (b) surface view of FIG. 6, the photocatalyst layer 8 is dispersed in an island shape (discontinuous) on the surface of the clear coating layer (A) 5. (The surface coverage is 90% by area), and the water-repellent fine particles 7 are not included in the coating layer. Therefore, when processing such as press molding or roll molding is performed, the surface is scratched. .
Comparative Example 8 is an example in which the photocatalyst particles are interposed together with the water-repellent fine particles in the clear coating layer (A). As shown in FIG. 7 (a) sectional view and (b) surface view, the water-repellent fine particles 7 is exposed from the surface of the clear coating layer (A) 5. The pre-coated steel sheet of Comparative Example 8 showed excellent bending workability due to the effect of the water-repellent fine particles 7, but the photocatalyst particles 8 penetrated into the clear coating layer (A) 5, so that the weather resistance decreased. I have.

  Moreover, all of Comparative Examples 1-8 resulted in the contact angle of water outside the range of 22-60 degrees.

  The pre-coated steel sheet of the present invention and the production method thereof are excellent in workability, weather resistance, aesthetics, etc., and of course are suitable as exterior materials for electrical equipment products and interior / exterior materials for buildings. Can also be used in applications where the above performance is required.

DESCRIPTION OF SYMBOLS 1 Plated steel plate 2 Chemical conversion coating 3 Primer layer 4 Colored resin layer 5 Clear coating layer (A)
6 Clear film layer (B)
7 Water-repellent fine particles 8 Photocatalyst particles, photocatalyst layer

Claims (12)

On at least one side of the steel plate substrate,
A clear coating layer (A) containing a silicon compound as a main component and containing water-repellent fine particles dispersed therein;
A clear coating layer (B) containing titanium oxide fine particles showing photocatalytic activity as a main component, laminated on the clear coating layer (A);
A precoated steel sheet characterized by comprising:   2. The precoated steel sheet according to claim 1, wherein at least part of the water-repellent fine particles in the clear coating layer (A) protrudes from the surface of the clear coating layer (B) and is dispersed in an island shape. .   The precoated steel sheet according to claim 1 or 2, wherein a contact angle between the surface of the clear coating layer (B) and water is 22 to 60 degrees.   4. At least a part of the water-repellent fine particles in the clear coating layer (A) has a particle diameter larger than the film thickness of the clear coating layer (A). The precoated steel sheet according to 1.   The water-repellent fine particles in the clear coating layer (A) are composed of fluorine resin particles, fluorine resin copolymer resin particles, silicon resin particles, or particles obtained by coating these resins on the surface. The precoated steel sheet according to any one of claims 1 to 4.   The precoated steel sheet according to any one of claims 1 to 5, wherein the silicon compound in the clear coating layer (A) is an acrylic silicon resin. Titanium oxide particles of the clear coating layer (B), the amount of deposition per side are either of claims 1 to 6, characterized in that in terms of TiO ₂ is ^{10mg / m 2 ~2000mg / m 2} 1 The precoated steel sheet according to item.   The pre-coated steel sheet according to any one of claims 1 to 7, wherein the clear coating layer (B) is mainly composed of a mixture of anatase type crystalline titanium oxide fine particles and amorphous titanium oxide. . The steel plate base material has a chemical conversion treatment film and a resin film formed thereon on at least one surface of a zinc-based plated steel sheet,
The resin film is
The precoated steel sheet according to any one of claims 1 to 8, comprising a primer layer made of a thermosetting resin containing a rust preventive pigment and a colored resin layer.   10. The precoated steel sheet according to claim 9, wherein the colored resin layer is mainly composed of an organosol-type baking type fluororesin having a mass ratio of polyvinylidene fluoride and acrylic resin of 85:15 to 50:50. A pre-coated steel sheet in which a clear film layer (A) mainly composed of a silicon compound and a clear film layer (B) mainly composed of titanium oxide fine particles exhibiting photocatalytic activity are sequentially laminated on at least one surface of a steel sheet substrate. In the manufacturing method,
The water-repellent fine particles are dispersed in the clear coating layer (A), and the amount of the water-repellent fine particles is suppressed by suppressing the coating amount of the clear coating layer (B) on the portion where the water-repellent fine particles are present and in the vicinity thereof. A method for producing a precoated steel sheet, characterized in that at least a part of the steel sheet is dispersed from the surface of the clear coating layer (B) in an island shape.   The production of the precoated steel sheet according to claim 11, wherein at least a part of the water-repellent fine particles in the clear coating layer (A) has a particle diameter larger than the film thickness of the clear coating layer (A). Method.

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