JPH1170615A - Precoat steel sheet excellent in hardness, processability and corrosion resistance, and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance hardness, processability and corrosion resistance by forming a topcoat film by applying a coating compsn. based on a compd. represented by a specific chemical formula and adding a compd., polyol and a curing agent to a resin solid in specific wt. ratios and adding specific strontium chromate to an undercoat film. SOLUTION: A topcoat film is composed of 1-15 wt.% of a resin solid of a compd. represented by specific chemical formula (wherein n is an integer of 2-10; and R is dibenzoyloxy-p-phenylene or biphenyl). As polyol, 40-90 wt.% of the resin solid of an acrylic resin or a polyester resin is used. As a curing agent, 9-50 wt.% of a blocked polyisocyanate compd. or an amino resin is used. Further, the content of strontium chromate added to an under coat film is set to 0.06-0.26 g/m<2> per film thickness of 1 μm. By this constitution, coating film hardness, processability and corrosion resistance can be enhanced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a precoated steel sheet having excellent hardness, workability and corrosion resistance.
The precoated steel sheet of the present invention is suitable for, for example, home electric appliances and building materials, and can also be used for automobiles.

[0002]

2. Description of the Related Art Many properties such as high hardness, high workability, stain resistance, chemical resistance, water resistance and corrosion resistance are required for paints for precoated steel sheets. Hardness, which is an important property,
The required level of workability and corrosion resistance after processing is high. The term "workability" as used herein refers to less damage to the coating film in the process of bending, cutting, drawing, etc. when processing a flat steel sheet into various shapes, elongation and flexibility of the coating film itself. It can be said that the larger the degree is, the better the moldability is. In addition, corrosion resistance means that after being molded as a product,
It refers to little or no peeling of the coating film or rusting from the object to be coated in the use environment.

Conventionally, polyester resin or epoxy resin has been mainly used as a paint for a precoated steel sheet on the outer surface and a back paint, and polyester, acrylic, vinyl or the like has been used as a paint on the outer surface. ing. The most common outer surface top coating is a polyester resin coating composed of a combination of a polyester resin (main component) and a melamine compound (curing agent).
Hardness is obtained by a melamine compound. However, this outer surface top coat is a pre-coated steel plate with strict performance requirements,
In particular, the performance of precoated steel sheets for home appliances and the like is not always sufficient, and improvement in performance has been desired.

[0004] In response to such a demand, for example, Japanese Unexamined Patent Publication No.
No. 234,752, an attempt has been made to obtain a coating film which satisfies both flexibility and coating film hardness at a high level by reacting a polyester resin with a melamine resin (curing agent) imparted with orientation by modification. I have. JP-A-7-316498 also discloses a coating composition containing a polyol as a main component and an isocyanate compound and / or an amino resin as a curing agent for the purpose of obtaining a coating film having a high degree of workability and coating film hardness. There has been proposed a coating composition for a precoated steel sheet to which a compound having a specific structure represented by the following general formula is added.

Embedded image

[0005]

However, among these prior arts, the technique disclosed in Japanese Patent Application Laid-Open No. Hei 6-234,752 involves a long process of synthesizing a modified melamine resin. Is difficult. Also, Japanese Patent Application Laid-Open
In the technology of -316498, no consideration is given to improving the performance of the coating film including the undercoat coating film, and a coating film which sufficiently satisfies the coating performance including the corrosion resistance after the forming process is obtained. It is not possible.

Accordingly, an object of the present invention is to solve the above-mentioned problems of the prior art, exhibit a high coating film hardness in which molding flaws and handling flaws are less likely to occur at the time of molding and transportation, and even in severe molding processing. An object of the present invention is to provide a pre-coated steel sheet having excellent workability in which a coating film is not easily cracked or the like and exhibiting excellent corrosion resistance in a working environment after the processing.

[0007]

Means for Solving the Problems The inventors of the present invention have solved the above-mentioned problems and have repeatedly studied to obtain a precoated steel sheet having excellent performance. As a top coating film, a polyol is used as a main component, a curing agent, and a coating film formed by adding a compound having a specific chemical structure as shown in the above general formula, and a strontium chromate as an undercoat film. It has been found that by forming a blended coating film, a precoated steel sheet excellent in both hardness, workability, and corrosion resistance after working can be obtained.

[0008] The present invention has been made based on such knowledge, and the characteristic configuration thereof is as follows. [1] A precoated steel sheet having an undercoat film and an overcoat film on the surface of a galvanized steel sheet which has been subjected to a chemical conversion treatment, wherein the overcoat film is a coating composition mainly comprising the following a) to c) A) a coating film formed by applying

Embedded image A) Main component of repeating unit: 1 to 15% by weight in resin solid content b) Polyol except for the above a): 40 to 90% by weight in resin solid content c) Curing agent: in resin solid content 9 to 50% by weight of the undercoat film has a thickness of 1 .mu.m with strontium chromate.
A precoated steel sheet excellent in hardness, workability and corrosion resistance, characterized by containing 0.06 to 0.26 g / m ² per m.

[2] In the precoated steel sheet of the above [1],
A precoated steel sheet having excellent hardness, workability and corrosion resistance, characterized in that a curing agent in a coating composition for forming a top coat is a blocked polyisocyanate. [3] The hardness and workability of the precoated steel sheet according to [1] or [2], wherein the undercoating film is a coating film formed by applying a coating composition containing a polyester resin as a main component. Pre-coated steel sheet with excellent resistance and corrosion resistance. [4] The precoated steel sheet according to any one of the above [1] to [3], wherein the undercoat film has a thickness of 2 to 12 μm, and is excellent in hardness, workability and corrosion resistance.

[5] A coating composition for an undercoat coating film containing strontium chromate is applied to the surface of the galvanized steel sheet which has been subjected to the chemical conversion treatment, and then baked at a sheet temperature of 180 to 260 ° C. Then, after applying a coating composition for an overcoating film containing the following components a) to c) as main components, a) the general formula (1)

Embedded image A) Main component of repeating unit: 1 to 15% by weight in resin solid content b) Polyol except for the above a): 40 to 90% by weight in resin solid content c) Curing agent: in resin solid content Baking treatment at a plate temperature of 180 to 260 ° C., and strontium chromate in the undercoat film in an amount of 0.0
A method for producing a precoated steel sheet having excellent hardness, workability and corrosion resistance, characterized by obtaining a precoated steel sheet containing 6 to 0.26 g / m ² .

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The precoated steel sheet of the present invention is a coated steel sheet in which an undercoat film and an overcoat film are formed on the surface of a galvanized steel sheet which has been subjected to a chemical conversion treatment. The zinc-based galvanized steel sheet used as the base steel sheet includes a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, an electro-galvanized steel sheet,
n-55% Al alloy plated steel sheet, hot-dip Zn-5% Al alloy plated steel sheet, Zn-Ni alloy plated steel sheet, Zn-Co
Gold-plated steel sheet, Zn-Cr alloy plated steel sheet, a composite zinc-plated steel sheet (for example, electric Zn-SiO ₂ dispersion plating steel plate) it is possible to use various galvanized steel sheet or the like.
Further, particularly for applications requiring corrosion resistance, molten Zn-5
It is preferable to use a 5% Al alloy plated steel sheet or a hot-dip Zn-5% Al alloy plated steel sheet.

The surface of such a zinc-based plated steel sheet is subjected to a chemical conversion treatment such as a phosphate treatment and a chromate treatment, and an undercoat film and an overcoat film are sequentially formed thereon.
By forming a coating film on the plated steel sheet surface which has been subjected to the chemical conversion treatment as described above, the adhesion of the coating film to the steel sheet surface is improved, and the corrosion resistance is also improved.

[0013] The top coat of the precoated steel sheet of the present invention is:
A) applying a coating composition mainly containing a specific compound, b) a polyol excluding the above a), and c) a curing agent,
It is formed by baking. First, the compound of the above a) is a compound represented by the following general formula (1).

Embedded image

The compound (a) is a bifunctional compound in which a hydroxyl group is bonded to both ends of a rigid dibenzoyloxy-p-phenylene or biphenyl skeleton via a methylene chain, and n (methylene group) Is preferably 2 to 10. When n is 1 or 0, synthesis is difficult, while when n exceeds 10, sufficient coating film performance cannot be obtained.

The compound (a) exhibits anisotropy when observed with a polarizing microscope. A coating film containing such a compound can achieve toughness of the coating film even after baking by an action such as alignment which is a characteristic of the liquid crystal compound, and as a result, a coating film having high hardness and high workability can be obtained. Think. The compounding amount of the compound (a) is 1 to 15% by weight, preferably 2 to 13% by weight, more preferably 3 to 15% by weight in the resin solid content.
10% by weight. If the amount of the compound is less than 1% by weight, the effect of improving the performance of the coating film is not remarkable, while if the amount exceeds 15% by weight, the flexibility of the coating film decreases.

Next, an acrylic resin or a polyester resin can be used as the polyol of the above item b). The acrylic resin as the polyol (b) is not particularly limited as long as it is a compound having at least two hydroxyl groups in one molecule and having a number average molecular weight of 1500 to 12,000. Is preferably from 1700 to 10000. Hydroxyl groups in the acrylic resin molecule are randomly arranged in the acrylic resin main chain, and when the number average molecular weight is less than 1500, the processability is significantly reduced. On the other hand, if the number average molecular weight exceeds 12,000, the viscosity becomes high, so that an excess of a diluting solvent is required, and the proportion of the resin in the coating material is reduced, so that an appropriate coating film cannot be obtained. Further, the compatibility with other compounding components is significantly reduced. The number average molecular weight of the acrylic resin is determined by gel permeation chromatography (hereinafter, referred to as “gel permeation chromatography”).
GPC) as measured in terms of polyester.

The acrylic resin is a copolymer obtained by subjecting an acrylic monomer or methacrylic monomer having a hydroxyl group to an acrylate or methacrylate and the like by a heat reaction in a known manner. As the acrylic monomer and methacryl monomer having a hydroxyl group, for example, 2-hydroxyethyl methacrylate, 2-hydroxyethyl acrylate, hydroxypropyl methacrylate, hydroxypropyl acrylate, and the like can be used. Also,
Examples of acrylates and methacrylates include methyl acrylate, ethyl acrylate, isopropyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate,
Ethyl methacrylate, isopropyl methacrylate,
For example, n-butyl methacrylate, 2-ethylhexyl methacrylate, or the like can be used. Commercially available acrylic resins include "Almatex" (manufactured by Mitsui Toatsu Chemicals, Inc.), "Desmophen" (manufactured by Sumitomo Bayer Urethane Co., Ltd.), and "Dianal" (manufactured by Mitsubishi Rayon Co., Ltd.). There is.

The polyester resin as the polyol (b) is not particularly limited as long as it has at least two hydroxyl groups in one molecule and has a number average molecular weight of 1,000 to 8,000. The preferred number average molecular weight range is from 1200 to 7000, more preferably 15
00 to 6000. The hydroxyl group in the molecule of the polyester resin may be at either the terminal or the side chain in the molecule. If the number average molecular weight of the polyester resin is less than 1,000, the processability will be significantly reduced. On the other hand, if the number average molecular weight exceeds 8000, the viscosity becomes high, so that an excess of a diluting solvent is required, and the proportion of the resin in the coating material is reduced, so that an appropriate coating film cannot be obtained. Further, the compatibility with other compounding components is significantly reduced. The number average molecular weight of the polyester resin is a molecular weight in terms of polystyrene measured by GPC.

The polyester resin is a copolymer obtained by heating and reacting a polybasic acid component and a polyhydric alcohol by a known method. As the polybasic acid component, for example, phthalic anhydride, isophthalic acid, terephthalic acid, trimellitic anhydride, maleic acid, adipic acid, fumaric acid and the like can be used. Examples of the polyhydric alcohol include, for example, ethylene glycol, diethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, triethylene glycol, Glycerin, pentaerythritol, trimethylolpropane, trimethylolethane and the like can be used. Commercially available polyester resins include “Almatex” (manufactured by Mitsui Toatsu Chemicals, Inc.), “alkynol” (manufactured by Sumitomo Bayer Urethane Co., Ltd.), “desmophen” (manufactured by Sumitomo Bayer Urethane Co., Ltd.), Byron (manufactured by Toyobo Co., Ltd.) and the like. The amount of the polyol (b) is 40 to 90% by weight in terms of the resin solid content. If the amount of the polyol is less than 40% by weight, the workability of the coating film cannot be sufficiently ensured, while if it exceeds 90% by weight, the hardness of the coating film becomes insufficient.

As the curing agent (c), a blocked polyisocyanate compound and / or an amino resin can be used. As the polyisocyanate compound, a polyisocyanate compound obtained by a general production method can be used. Among them, phenol, cresol, an aromatic secondary amine, Polyisocyanate compounds blocked with blocking agents such as secondary alcohols, lactams and oximes are preferred. By using this blocked polyisocyanate compound, storage with one liquid becomes possible, and use as a paint for a precoated steel sheet is facilitated.
Further, more preferred polyisocyanate compounds include non-yellowing hexamethylene diisocyanate (hereinafter abbreviated as HDI) and its derivatives, tolylene diisocyanate (hereinafter abbreviated as TDI) and its derivatives,
4,4'-diphenylmethane diisocyanate (hereinafter, referred to as
MDI) and derivatives thereof, xylylene diisocyanate (hereinafter abbreviated as XDI) and its derivatives, isophorone diisocyanate (hereinafter abbreviated as IPDI) and its derivatives, trimethylhexamethylene diisocyanate (hereinafter abbreviated as TMDI) and its derivatives, Examples include hydrogenated TDI and its derivatives, hydrogenated MDI and its derivatives, hydrogenated XDI and its derivatives, and the like.

Further, commercially available isocyanate compounds such as "Sumidur" (manufactured by Sumitomo Bayer Urethane Co., Ltd.), "Desmodur" (manufactured by Sumitomo Bayer Urethane Co., Ltd.) and "Coronate" (manufactured by Nippon Polyurethane Co., Ltd.) Can also be used. When an isocyanate compound is used as a curing agent, the compounding ratio [NCO / OH] of the isocyanate group of the isocyanate compound to the compound of the above item a) and the hydroxyl group of the polyol of the above item b) is from 0.8 to 1.2 in terms of a molar ratio. It is preferable to set the range of 0.90 to 1.10. If the molar ratio of [NCO / OH] is less than 0.8, curing of the coating film is insufficient, and desired coating film hardness and strength cannot be obtained. On the other hand, if the molar ratio of [NCO / OH] exceeds 1.2, a side reaction between excess isocyanate groups or between the isocyanate group and the urethane compound occurs, and the processability of the coating film is reduced.

As the amino resin as a curing agent, urea,
Resins obtained by the reaction of benzoguanamine, melamine, etc. with formaldehyde, and those obtained by alkyl etherification of these with alcohols such as methanol and butanol can be used. Specifically, methylated urea resin, n-
Butylated benzoguanamine resin, methylated melamine resin, n-butylated melamine resin, iso-butylated melamine resin and the like can be mentioned.

Further, "Cymel" (manufactured by Mitsui Cyanamid Co., Ltd.) and "Uban" (manufactured by Mitsui Toatsu Chemical Co., Ltd.)
"Sumimar" (Sumitomo Chemical Co., Ltd.) "Melan"
Commercially available amino resins such as (manufactured by Hitachi Chemical Co., Ltd.) can also be used. The compounding ratio (weight ratio of solid content) of the amino resin to the compound of a) and the polyol of b) is [compound] + [polyol] / [amino resin]: 95 to 65.
/ 5 to 35, preferably 90 to 75/10 to 25. The compounding amount of the curing agent in the above item (c) is 9 to 50% by weight in terms of the resin solid content. If the amount of the curing agent is less than 9% by weight, the hardness of the coating film is insufficient, while if it exceeds 50% by weight, the processability is insufficient.

In addition, depending on the purpose and application, a curing catalyst such as p-toluenesulfonic acid, tin octoate, dibutyltin dilaurate and 2-ethylhexoate lead; Kaolin, clay,
Pigments such as titanium oxide, talc, barium sulfate, mica, red iron oxide, manganese blue, carbon black, aluminum powder, and pearl mica; and other various additives such as an antifoaming agent and a flow stopper can be appropriately compounded. The thickness of the overcoat film is preferably 10 to 20 μm. The film thickness is 1
If it is less than 0 μm, there is a possibility that the overall coating performance as an overcoating film may not be sufficiently obtained, while if the film thickness exceeds 20 μm, the coating film hardness decreases.

Next, the undercoat film of the precoated steel sheet of the present invention will be described. There are no particular restrictions on the main agent of the coating composition for forming the undercoat coating film, but from the viewpoint of processability and adhesion between the topcoat coating film and the chemical conversion coating film, a polyester resin, an epoxy resin or a urethane resin. It is preferable to use a coating composition containing a resin as a main component. Also,
These resins may be used as a mixture of two or more kinds.

As a curing agent used in a coating composition for an undercoat film, a polyisocyanate compound or an amino resin can be used. These may be used in combination of two or more. Examples of the polyisocyanate compound used as a curing agent include aromatic diisocyanates such as xylylene diisocyanate, tolylene diisocyanate, and 4,4'-diphenylmethane diisocyanate;
Aliphatic diisocyanates such as hexamethylene diisocyanate and trimethylhexamethylene diisocyanate;
Alicyclic diisocyanates such as isophorone diisocyanate; or a polymer of these diisocyanates or an adduct thereof with a polyhydric alcohol; and a blocking agent (for example, phenolic, lactam, alcohol, mercaptan, imine, (Amine-based, imidazole-based or oxime-based blocking agents) and the like. Further, as a dissociation catalyst for these blocked polyisocyanate compounds, tin octoate, dibutyltin dilaurate, 2-ethylhexoate lead and the like can be used.

The amino resin used as a curing agent includes, for example, a condensate of formaldehyde or paraformaldehyde alkylated with a lower alcohol and urea, dicyandiamide, aminotriazine and the like. Methylol urea, methoxylated methylol dicyandiamide, methoxylated methylol melamine, methoxylated methylol benzoguanamine, butoxylated methylol melamine, butoxylated methylol benzoguanamine, and the like. As the curing catalyst, acids such as hydrochloric acid, monoalkyl phosphate, and P-toluenesulfonic acid, or salts of these acids with tertiary amines or secondary amine compounds can be used.

In the present invention, in addition to improving the adhesion between the undercoat and the overcoat and the chemical conversion coating, the undercoat contains a rust-preventive pigment in order to improve the corrosion resistance of the entire coating. As the rust-preventive pigment to be contained in the undercoat film, strontium chromate, zinc chromate, fine powder of a hardly soluble chromate such as barium chromate may be used, but in the present invention, the viewpoint that long-term corrosion resistance can be maintained. Use strontium chromate.

The content of strontium chromate added to the undercoat film is 0.06 to 0.2 per μm of film thickness.
6 g / m ² . If the content of strontium chromate is less than 0.06 g / m ² per 1 μm of film thickness, sufficient corrosion resistance will not be obtained, while if it exceeds 0.26 g / m ² , workability will be reduced. The thickness of the undercoat film is preferably 2 to 12 μm. If the film thickness is less than 2 μm, the effects of improving workability and corrosion resistance cannot be sufficiently obtained.
If it exceeds μm, both the processability and the hardness of the coating film will decrease.

In actually using the coating composition for forming the top coat and the undercoat, these are dissolved in an organic solvent before use. Examples of the organic solvent to be used include methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, Solvesso 100 (manufactured by Exxon Chemical), Solvesso 150 (manufactured by Exxon Chemical),
Solvesso 200 (manufactured by Exxon Chemical), toluene, xylene, methyl cellosolve, butyl cellosolve, cellosolve acetate, butyl cellosolve acetate, carbitol, ethyl carbitol, butyl carbitol, ethyl acetate, butyl acetate, petroleum ether, petroleum naphtha and the like. . In preparing the coating composition, an ordinary dispersing machine or kneading machine such as a sand grind mill, a ball mill, and a blender can be selected and used, and the respective components can be blended.

The method of applying the coating composition (each coating composition for undercoating and topcoating) for producing the precoated steel sheet of the present invention is not particularly limited, but is preferably roll coater coating, curtain flow. It is preferable to apply by a method such as painting. After the coating composition is applied to both the undercoat film and the topcoat film, the coating film is baked by a heating means such as hot air heating, infrared heating, induction heating or the like, and the resin is crosslinked to obtain a cured coating film. The baking treatment at the time of heat-curing the undercoating film is usually performed at a plate temperature of about 180 to 260 ° C. for about 30 seconds to 1 minute. The baking treatment for heating and curing the top coat is usually performed at a plate temperature of about 180 to 260 ° C. for about 30 seconds to 3 minutes. Incidentally, the pre-coated steel sheet of the present invention,
Further coating on top coat (eg clear coat)
And may be used in three coats and three bake.

[0032]

EXAMPLE A 0.5 mm thick hot-dip galvanized steel sheet (coating weight per side: 30 g / m ² ), which is a base steel sheet, was degreased and then coated with a chromate-based treating agent (“Surf” manufactured by Nippon Paint Co., Ltd.). Chromate treatment with NRC300 ") coat, and then form an undercoat with the coating composition for undercoat shown in Table 1, and further have a coating composition for overcoat shown in Tables 2 and 3 An overcoat film was formed from the material to obtain precoated steel sheets of the present invention and comparative examples. Tables 4 to 8 show the performance of these precoated steel sheets together with the production conditions. The coating compositions F-1 and D-1 for the top coat shown in Tables 2 and 3 were prepared as follows.

[Coating composition for top coat] (1) Preparation of compound In 200 ml of dry THF, 8.9 parts by weight of sodium hydride and 27.2 parts by weight of 6-chloro-1-hexanol were used. 8 parts by weight of benzyl bromide were added dropwise with stirring. A large excess of water was added to the reaction mixture, which was extracted with methylene chloride. The extract was washed with water, dried over anhydrous magnesium sulfate, distilled off using an evaporator, and the residue was distilled under reduced pressure to obtain compound 1-a (colorless liquid). Next, 27.6 parts by weight of p-hydroxybenzoic acid and 37.6 parts by weight were added to a reaction vessel equipped with an oil bath, a stirrer, a reflux device, and a thermometer.
0 parts by weight of sodium hydroxide and 0.5 parts by weight of potassium iodide were added, and dissolved in 100 ml of ethanol and 30 ml of water. To this solution was added 25 parts by weight of the compound 1-a.
Was added dropwise and heated to reflux. After completion of the reaction, ethanol was distilled off with an evaporator, water was added to the residue, and the mixture was extracted with methylene chloride. Thereafter, the extract was dried over anhydrous magnesium sulfate, and a compound 1-b (white crystals) was obtained using an evaporator. 26.3 parts by weight of compound 1-b and 0.1 ml of pyridine were dissolved in 75 ml of thionyl chloride, and this was heated to reflux, and after completion of the reaction,
The methylene chloride was distilled off under reduced pressure to obtain compound 1-c.
8.67 parts by weight of the compound 1-c was dissolved in chloroform, added dropwise to a solution of 1.65 parts by weight of hydroquinone in dry pyridine, reacted under a nitrogen atmosphere for 60 hours, then added with water and extracted with chloroform. The extract was washed with an alkali solution and water in this order, dried over anhydrous magnesium sulfate, and then evaporated to obtain a jelly-like compound 1-d.
I got 12.5 parts by weight of compound 1-d, 1.9 parts by weight of 10% palladium / charcoal, and 150 ml of ethanol were charged into an eggplant flask connected to a reflux condenser equipped with a three-way cock, and the inside of the reaction system was filled with hydrogen gas. The reaction was performed at room temperature and pressure for 24 hours. After completion of the reaction, the filtrate was removed with an evaporator to obtain a white crystalline compound. Field desorption ionization mass spectrum of this compound (hereinafter F
D-MS) was 550.65.

The chemical structural formula of this compound is shown below.

Embedded image

(2) Production of a coating composition for a top coat A polyester resin, a curing agent, a pigment, a curing catalyst and an additive are mixed with the compound obtained in the above (1) in a mixing ratio shown in Table 2. After that, the mixture was dispersed for about 30 minutes using a sand mill containing glass beads having a diameter of about 1 mm. Further, cyclohexanone was added to adjust the nonvolatile content to 60%, thereby producing a coating composition F-1. Further, as a coating composition for a top coat for a comparative example, after blending a polyester resin, a curing agent, a pigment, a curing catalyst and an additive in a blending ratio as shown in Table 3, glass beads having a diameter of about 1 mm were added. The mixture was dispersed for about 30 minutes using a sand mill. Further, cyclohexanone was added to adjust the nonvolatile content to 60%, thereby producing a coating composition D-1.

The test method and the evaluation method of the performance test of the precoated steel sheet are described below. (1) Appearance The quality of the surface properties of the coating film after baking was visually judged. (2) Pencil hardness JIS K5400 using Mitsubishi pencil "Uni".
The test was carried out according to 4.2 and showed a hardness limit at which the coating was not damaged.

(3) Workability (Bendability) At room temperature of 20 ° C., JIS G 3312 12.2.
After bending the steel plate in advance using the bending vise specified in 2 (Bending test), the steel plate was bent 180 °, and the coating surface of the processed part was observed with a 30-fold loupe. The presence or absence of (cracks) was examined and evaluated according to the following criteria. A: No fine pinholes or cracks were observed on the coating film surface. ：: 1 to 5 fine pinholes / cm were observed on the coating film surface. B: 6 to 20 minute pinholes / cm were observed on the coating film surface. C: More than 20 pinholes / cm were observed on the coating film surface, and cracks were also observed.

(4) Adhesion (cross-cut peeling) test (JIS
K 5400 8.5 (adhesiveness)) 10 mm at 1 mm intervals on the surface of the cured coating using a cutter knife.
100 squares were cut in a range of mm × 10 mm, the adhesive tape was pressed on the squares, and then peeled off vigorously. The state of peeling of the coatings on the squares was observed and evaluated according to the following criteria. ：: No peeling of the coating film ○: Number of remaining cross-cuts of 90 or more, less than 100 △: Number of remaining cross-cuts of 80 or more, less than 90 ×: Less than 80 of cross-cut remaining

(5) Flat plate corrosion resistance (JIS K 5400)
9.1 (in accordance with salt spray resistance))
A test piece was prepared so that the cut ends on the right and left sides would be upper burrs and lower burrs, respectively. After performing a salt spray test (SST) for 240 hours, a tape peeling test was performed on the cut ends of the test pieces. The corrosion resistance was evaluated according to the following criteria. ：: Peeling of the coating from the cut end is less than 3 mm ：: Peeling of the coating from the cut end is 3 mm or more and less than 5 mm ：: Peeling of the coating from the cut end is 5 mm or more and less than 10 mm ×: Cut end 10mm or more of film peeling from

[0040]

[Table 1]

[0041]

[Table 2]

[0042]

[Table 3]

[0043]

[Table 4]

[0044]

[Table 5]

[0045]

[Table 6]

[0046]

[Table 7]

[0047]

[Table 8]

[0048]

As described above, the precoated steel sheet of the present invention is excellent in all of the coating film hardness, formability and corrosion resistance after processing, and is also excellent in coating film appearance and adhesion.
For this reason, it is extremely useful as a precoated steel sheet used for parts requiring high coating film hardness, molding workability, and corrosion resistance in applications such as home appliances, building materials, and automobiles.
Further, the precoated steel sheet of the present invention is industrially extremely useful in that it can be manufactured by a simpler method than before.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI B05D 7/24 302 B05D 7/24 302R C09D 167/00 C09D 167/00 175/04 175/04 C23C 28/00 C23C 28/00 C // C08G 18/80 C08G 18/80 (72) Inventor Masaaki Yamashita 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside Nihon Kokan Co., Ltd. (72) Inventor Yasuyuki Kajita 1-chome, Marunouchi, Chiyoda-ku, Tokyo 1-2 1-2 Nihon Kokan Co., Ltd. (72) Inventor Keiichi Kotani 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Katsuyoshi Tanaka 4 Minamishinagawa, Shinagawa-ku, Tokyo Chome 1-115 Japan Paint Co., Ltd. Tokyo Office

Claims (5)

[Claims] 1. A pre-coated steel sheet having an undercoat film and an overcoat film on the surface of a galvanized steel sheet that has been subjected to a chemical conversion treatment, wherein the overcoat film is mainly composed of the following paints (a) to (c): A coating film formed by applying the composition, i) a general formula (1) A) Main component of repeating unit: 1 to 15% by weight in resin solid content b) Polyol except for the above a): 40 to 90% by weight in resin solid content c) Curing agent: in resin solid content 9 to 50% by weight of the undercoat film has a thickness of 1 .mu.m.
A precoated steel sheet excellent in hardness, workability and corrosion resistance, characterized by containing 0.06 to 0.26 g / m ² per m. 2. The precoated steel sheet having excellent hardness, workability and corrosion resistance according to claim 1, wherein the curing agent in the coating composition for forming the overcoat film is a blocked polyisocyanate compound. 3. The hardness, workability and corrosion resistance according to claim 1, wherein the undercoat film is a film formed by applying a coating composition containing a polyester resin as a main component. Excellent pre-coated steel sheet. 4. The hardness according to claim 1, wherein the undercoat film has a thickness of 2 to 12 μm.
Pre-coated steel sheet with excellent workability and corrosion resistance. 5. After applying a coating composition for an undercoat coating film containing strontium chromate on the surface of a galvanized steel sheet subjected to a chemical conversion treatment, baking treatment is performed at an ultimate plate temperature of 180 to 260 ° C. Then, after applying a coating composition for a top coat having the following main components a) to c) as a main component, a) a general formula (1) A) Main component of repeating unit: 1 to 15% by weight in resin solid content b) Polyol except for the above a): 40 to 90% by weight in resin solid content c) Curing agent: in resin solid content Baking treatment at a plate temperature of 180 to 260 ° C., and strontium chromate in the undercoat film in an amount of 0.0
A method for producing a precoated steel sheet having excellent hardness, workability and corrosion resistance, characterized by obtaining a precoated steel sheet containing 6 to 0.26 g / m ² .