JPH1170614A - Precoat steel sheet excellent in hardness, processability and corrosion resistance, and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance hardness 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 polyester 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 1-18 wt.% of a resin solid of a polyester compd. containing a main repeating unit represented by specific chemical formula [wherein n is an integer of 2-10; R is (CH2 )-m (wherein m is 1-10) and R' is 1,4-cyclohexane carboxylic acid]. 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, hardness, corrosion resistance or the like 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-7-316497 also discloses a coating composition containing 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 high workability and coating film hardness. In particular, there has been proposed a coating composition for a precoated steel sheet to which a polyester compound obtained by reacting 1,4-cyclohexanedicarboxylic acid and / or a lower alkyl ester thereof with an alcohol component is added.

[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 No. 316497, 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 molding 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. Coating film based on a coating composition comprising a polyol as a main component, a curing agent, and a polyester compound obtained by reacting 1,4-cyclohexanedicarboxylic acid and / or a lower alkyl ester thereof with an alcohol component. Further, it has been found that by forming a coating film containing strontium chromate as an undercoat coating film, a precoated steel sheet excellent in all of 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) a polyester compound having as a main repeating unit: 1 to 18% by weight in the resin solid content b) Polyol other than the above a): 40 to 90% by weight in the resin solid content c) Curing agent: resin solids 9 to 50% by weight in the medium ratio The undercoat film is made of strontium chromate with a film thickness of 1 μm.
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] After applying a coating composition for an undercoat coating film on the surface of the zinc-based galvanized steel sheet subjected to the chemical conversion treatment,
After baking at an ultimate plate temperature of 0 to 260 ° C., and then applying a coating composition for a top coat mainly containing the following (a) to (c), a) a general formula (1)

Embedded image A) a polyester compound having as a main repeating unit: 1 to 18% by weight in the resin solid content b) Polyol other than the above a): 40 to 90% by weight in the resin solid content c) Curing agent: resin solids 9 to 50% by weight in the medium The baking treatment is carried out at a plate temperature of 180 to 260 ° C., and strontium chromate is contained in the undercoating 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.

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:
It is formed by applying a coating composition containing a) a specific polyester compound, b) a polyol other than the above a), and c) a curing agent as main components and baking. First,
The polyester compound (a) is a polyester compound having the following general formula (1) as a main repeating unit.

Embedded image

The polyester compound (a) can be obtained by reacting 1,4-cyclohexanedicarboxylic acid as an acid component and / or a lower alkyl ester thereof with an alcohol component. 1,4-Cyclohexanedicarboxylic acid is mainly used as an acid component for obtaining the polyester compound of the above-mentioned a), and a part of them is terephthalic acid, isophthalic acid, naphthalene-2,7-dicarboxylic acid, diphenyl It can be replaced by a dicarboxylic acid such as a dicarboxylic acid, or a lower alkyl ester of these dicarboxylic acids. Examples of the lower alkyl ester include a methyl ester and an ethyl ester having 1 to 2 carbon atoms. However, even in this case, it is desirable that the main component of cyclohexane-1,4-dicarboxylic acid be 85 mol% or more of the entire acid component, and the proportion of other compounds to be replaced be suppressed to less than 15 mol%.

As the alcohol component for obtaining the above-mentioned polyester compound (a), a polyalkylene glycol having 10 or less carbon atoms, preferably a polyalkylene glycol having 6 or less carbon atoms can be used. Further, these may be used as a mixture. Also, as the alcohol component,
In addition to the glycol components described above, trihydric or higher polyhydric alcohols may be used. Examples of the polyhydric alcohol include triethylene glycol, glycerin, pentaerythritol, trimethylolpropane, and trimethylolethane.

The above-mentioned acid component and alcohol component react with each other to produce the above-mentioned polyester compound (a).
The polyester compound (a) used in the present invention can be obtained by a usual polyester production method such as a transesterification method or a direct esterification method. Usually, the acid component and the alcohol component react at a molar ratio of 1: 2, and therefore, in order to perform the reaction efficiently, it is desirable to mix the acid component and the alcohol component at a molar ratio as close as possible to 1: 2. Further, the polyester compound can be used either alone or in combination of two or more.

The polyester compound (a) shows an orientation characteristic of the thermotropic liquid crystal when observed alone while raising the temperature from room temperature to 200 ° C. or higher. It is thought that it has contributed to the improvement. The amount of the polyester compound (a) is from 1 to 18% by weight, preferably from 2 to 15% by weight, and more preferably from 3 to 10% by weight, based on the solid content of the resin. 1% by weight of the above polyester compound
When the amount is less than the above, the effect of improving the coating film performance is not remarkable, while when the amount is more than 18% by weight, the flexibility of the coating film decreases.

Next, an acrylic resin or a polyester resin can be used as the polyol in 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 acrylic ester or a methacrylic ester by a known method. 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 is a compound having at least two hydroxyl groups in one molecule and having 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] between the isocyanate group of the isocyanate compound and the hydroxyl group of the polyester compound of the above item a) and the polyol of the above item b) is 0.8 to 0.8 in molar ratio.
1.2, more preferably in the range of 0.90 to 1.10. When the molar ratio of [NCO / OH] is 0.8
If the amount is less than the above, the curing of the coating film is insufficient, and the 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 polyester compound of the above item a) and the polyol of the above item b) is preferably [polyester compound] + [polyol] / [amino resin]: 95 to 65/5 to 35. Is 90 ~
The ratio is preferably 75/10 to 25. C)
The amount of the curing agent is 9 to 50% by weight in terms of 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, lead 2-ethylhexoate, etc .; 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, condensates of formaldehyde or paraformaldehyde alkyletherified with a lower alcohol and urea, dicyandiamide, aminotriazine and the like. Examples include methylol urea, methoxylated methylol dicyandiamide, methoxylated methylol melamine, methoxylated methylol benzoguanamine, butoxylated methylol melamine, and butoxylated methylol benzoguanamine. 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 topcoat 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 to be 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.

[0034]

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 polyester compound In a reaction vessel equipped with a heating device, a stirrer, a rectification tower and a thermometer, 1,4-cyclohexanedicarboxylic acid dimethyl ester (trans Isomer ratio 20%, Towa Chemical Industry Co., Ltd.
200 parts by weight), 130 parts by weight of ethylene glycol, and 0.53 parts by weight of titanium tetra-n-butoxide. The reaction temperature was gradually increased to 240 ° C., and methanol was added in about 5 hours. The reaction was completed by distilling 64 parts by weight to obtain a polyester compound. This polyester compound was a white wax-like solid at room temperature, and had a number average molecular weight of 260 as determined by electrolytic desorption / ionization spectroscopy (FD-MS).

The chemical structural formula of this polyester 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 added to the polyester compound obtained in the above (1) in the compounding ratio shown in Table 2. After blending, the mixture was dispersed for about 30 minutes using a sand mill containing glass beads having a diameter of about 1 mm. Further, by adding cyclohexanone, the nonvolatile content becomes 60%.
% To prepare a coating composition F-1. Further, as a coating composition for a top coat for a comparative example, a polyester resin, a curing agent,
After blending pigment, curing catalyst and additives, diameter is about 1m
The mixture was dispersed for about 30 minutes using a sand mill containing m glass beads. 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 (Bending workability) 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

[0042]

[Table 1]

[0043]

[Table 2]

[0044]

[Table 3]

[0045]

[Table 4]

[0046]

[Table 5]

[0047]

[Table 6]

[0048]

[Table 7]

[0049]

[Table 8]

[0050]

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.

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI B32B 27/36 B32B 27/36 C08G 63/199 C08G 63/199 C23C 28/00 C23C 28/00 C (72) Inventor Masaaki Yamashita 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Yasuyuki Kajita 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Keiichi Otani Inventor (1-2) Inventor Katsuyoshi Tanaka 4-1-1, Minamishinagawa, Shinagawa-ku, Tokyo 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) a polyester compound having as a main repeating unit: 1 to 18% by weight in the resin solid content b) Polyol other than the above a): 40 to 90% by weight in the resin solid content c) Curing agent: resin solids 9 to 50% by weight in the medium ratio The undercoat film is made of strontium chromate with a film thickness of 1 μm.
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. A coating composition for an undercoat film is applied to the surface of a zinc-coated steel sheet which has been subjected to a chemical conversion treatment, and then applied to the surface thereof.
Baking treatment at a plate temperature of ~ 260 ° C, and then a)
After applying a coating composition for a top coat mainly composed of (c) to (c), a) a general formula (1) A) a polyester compound having as a main repeating unit: 1 to 18% by weight in the resin solid content b) Polyol other than the above a): 40 to 90% by weight in the resin solid content c) Curing agent: resin solids 9 to 50% by weight in the medium The baking treatment is carried out at a plate temperature of 180 to 260 ° C., and strontium chromate is contained in the undercoating 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 .