JP2001009368A - One-coat precoated steel panel excellent in molding processability and production method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a one-coat precoated steel panel less prone to generate a crack in a coating film even in severe molding processing and enabling high speed operation at a time of production. SOLUTION: The surface of a galvanized steel panel to which chemical forming treatment is applied is coated with a solvent type paint compsn. wherein a polyester resin with a number average mol.wt. of 5,000-25,000, a glass transition temp. of 20-80 deg.C, a hydroxyl number of 3-30 KOH mg/g and an acid value of 0-10 KOH mg/g, an epoxy resin with an epoxy equivalent of 180-1,000 and a curing agent consisting of a blocked polyisocyanate compd. and a melamine resin are compounded as main components in a specific ratio and, especially pref., 20-70 mol% of the polyhydric alcohol of the polyester resin is an ethylene oxide adduct of bisphenol A and/or a propylene oxide aduct of bisphenol A represented by the formula and the coated steel panel is baked to form a coating film with a dry film thickness of 10 μm or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a one-coat precoated steel sheet having excellent formability and high-speed operability during production, and a method for producing the same. The precoated steel sheet of the present invention can be used as a material for home electric appliances such as refrigerators and fan heaters, building materials, and automobile parts.

[0002]

2. Description of the Related Art Normally, pre-coated steel sheets are coated with two coats. In this type of pre-coated steel sheet, adhesion to the underlying steel sheet and corrosion resistance are secured by mainly using a modified polyester resin or epoxy resin for the outer surface undercoat, and polyester or acrylic paint is used for the outer surface top coat. When used, it mainly imparts stain resistance, design properties, scratch resistance, barrier properties, and the like. On the other hand, the two-coat pre-coated steel sheet requires many steps for painting and baking, and it takes a lot of time. Therefore, from the viewpoint of streamlining the painting work and saving resources, it is necessary to make the coating one coat and thinner. Is desired.

[0003] However, when a conventional prepainted steel plate paint is directly used as one coat in a solvent type paint,
The undercoating paint alone is insufficient in stain resistance and design properties, and the overcoating paint alone is insufficient in adhesion to a base steel sheet and corrosion resistance. In addition, although one-coated steel sheet has already been manufactured as a powder coating, the powder coating has a disadvantage that the film thickness is large and it takes time to cure. Therefore, in order to adopt a one-coat pre-coated steel sheet with a solvent-type paint in consideration of rationalization of coating work and resource saving, etc., both functions of the undercoat layer and the overcoat layer of the two-coat pre-coated steel sheet are combined, and in a short time. A design of a curable thin film coating is required.

By the way, precoated steel sheets are required to have many properties such as high hardness, high workability, stain resistance, chemical resistance, water resistance, and corrosion resistance. Above all, formability is a very important performance for precoated steel sheets that are formed after painting and baking. Formability, as used herein, refers to less damage to the coating film in the process of bending, drawing, cutting, etc. when processing a flat metal plate into various shapes, and relatively gentle bending In molding processing such as, the greater the degree of elongation and flexibility of the coating film itself, the better the workability, but in severe molding processing such as drawing, not only the elongation and flexibility of the coating film, but also deformation Also, the strength to withstand stress during processing and the scratch resistance are important.

[0005] In response to the required characteristics of such a pre-coated steel sheet, for example, Japanese Patent Application Laid-Open No.
A paint composition containing a polyester resin, an amino resin (curing agent), etc. with a specific component composition, and a coated steel sheet using the same, in order to obtain a coating film having excellent stain resistance, weather resistance, and low temperature processability. Has been proposed. In Japanese Patent Application Laid-Open No. Hei 9-111183, a coating composition containing a polyester resin, a melamine resin (curing agent), a rust-preventive pigment, organic polymer fine particles, and the like is applied to form one coat, thereby improving workability, corrosion resistance, and adhesion. Resistance, impact resistance, scratch resistance,
Painted steel sheets satisfying the design have been proposed.

[0006]

However, among these prior arts, the technique disclosed in Japanese Patent Application Laid-Open No. Hei 9-176567 does not use an epoxy resin, and thus has a sufficient adhesion to the base steel sheet without an undercoat. Cannot be secured. Further, the technology disclosed in Japanese Patent Application Laid-Open No. 9-111183 is not designed so that a polyester resin can provide a coating film having a strength capable of withstanding stress during severe molding such as drawing with one coat. Therefore, sufficient moldability cannot be obtained.

Accordingly, an object of the present invention is to solve the above-mentioned problems of the prior art, to prevent the coating film from being cracked or the like even in severe forming work such as drawing workability, and to perform high-speed operation during manufacturing. It is an object of the present invention to provide a one-coated precoated steel sheet and a method for manufacturing the same.

[0008]

Means for Solving the Problems The inventors of the present invention have repeatedly studied to solve the above-mentioned problems and obtain a one-coat pre-coated steel sheet having excellent performance. The mixed weight of solid content of a polyester resin having specific properties, a specific epoxy resin for improving adhesion to a base steel sheet, and a blocked polyisocyanate compound (C) and a melamine resin (D) Forming processability and high-speed operability by forming a coating film directly coated with a solvent-based coating composition mainly containing a curing agent having a ratio (C) / (D) of 100/0 to 15/85. It has been found that a 1-coat pre-coated steel sheet excellent in quality can be obtained.

The present invention has been made on the basis of such knowledge, and the characteristic configuration thereof is as follows. [1] On the surface of the galvanized steel sheet subjected to the chemical conversion treatment, the number average molecular weight is 5,000 to 25,000, the glass transition temperature is 20 to 80 ° C., the hydroxyl value is 3 to 30 KOHmg / g, and the acid value is 0 to 10 KOHmg / g. g of polyester resin (A)
100 parts by weight (solid content ratio) and an epoxy equivalent of 180
3 to 25 parts by weight (solid content ratio) of the epoxy resin (B) to 1000, and the mixing weight ratio (C) / (D) of the blocked polyisocyanate compound (C) and the melamine resin (D) at the solid content. A coating having a dry film thickness of 10 μm or less formed by applying and baking a solvent-based coating composition containing 3 to 20 parts by weight (solid content) of a curing agent composed of 100/0 to 15/85. 1-coated pre-coated steel sheet having excellent formability, characterized by having a film.

[2] The one-coat precoated steel sheet according to [1], wherein the polyester resin (A) in the solvent-type coating composition is used.
Characterized in that 20 to 70 mol% of the polyhydric alcohol component of formula (1) is an ethylene oxide adduct of bisphenol A and / or a propylene oxide adduct of bisphenol A represented by the following formula (1): Excellent one-coat pre-coated steel sheet.

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[3] In the one-coat precoated steel sheet according to the above [1] or [2], the polyester resin (A) in the solvent-based coating composition has a hydroxyl group after polycondensation between a polybasic acid and a polyhydric alcohol. Acid value partially modified with carboxylic acid is 3 to 10 KO
A one-coat precoated steel sheet having excellent formability, characterized by being a polyester resin of Hmg / g.

[4] A method for producing a one-coated precoated steel sheet according to any one of the above [1] to [3], wherein a solvent-type coating composition is applied to a surface of a zinc-based galvanized steel sheet which has been subjected to a chemical conversion treatment. A method for producing a one-coated precoated steel sheet excellent in formability, characterized in that a hardened coating film is formed by performing short-time baking with a reached plate temperature of 200 to 250 ° C and a baking time of 20 to 90 seconds.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The details of the present invention and the reasons for limitation will be described below. In the one-coat precoated steel sheet of the present invention, a polyester resin (A) having specific properties and an epoxy resin (B) for improving the adhesion between the base steel sheet and the surface of the zinc-coated steel sheet subjected to the chemical conversion treatment. , Blocked polyisocyanate compound (C) and melamine resin (D)
Weight ratio (C) / (D) at a solid content of 100/0
To 15/85 to form a coating film directly coated with a solvent-type coating composition containing a curing agent as a main component.

Examples of the galvanized steel sheet serving as a base steel sheet of the precoated steel sheet of the present invention include a hot-dip galvanized steel sheet, an electrogalvanized steel sheet, an alloyed hot-dip galvanized steel sheet,
Aluminum-zinc alloy coated steel sheet (for example, hot-dip zinc-55% aluminum alloy-coated steel sheet, hot-dip zinc-5%
Various zinc-based plated steel sheets such as an aluminum alloy-plated steel sheet), an iron-zinc alloy-plated steel sheet, and a nickel-zinc alloy-plated steel sheet can be used.

The surface of the galvanized steel sheet needs to be subjected to a chemical conversion treatment as a pretreatment for coating in order to improve the coating film adhesion and corrosion resistance. Examples of the chemical conversion treatment agent include a phosphate type, a chromate type, and the like, and these form a chemical conversion film on the surface of the plating film. In the pre-coated steel sheet of the present invention, a specific solvent-type coating composition is applied to the surface of the galvanized steel sheet subjected to the chemical conversion treatment and baked to form a one-coat coating film.

The polyester resin (A), epoxy resin (B), blocked polyisocyanate compound (C) and melamine resin (D) which are the main components of the solvent-based coating composition for forming the coating film are described below. explain. Polyester resin (A): The polyester resin (A) blended in the solvent type coating composition has a number average molecular weight of 5,000.
２５25,000, preferably 10,000-22,000, and a glass transition temperature Tg of 20-80 ° C., preferably 30-7.
0 ° C., the hydroxyl value is 3 to 30 KOH mg / g, preferably 4 to 20 KOH mg / g, and the acid value is 0 to 10 KOH mg / g.
g, preferably 3 to 9 KOH mg / g.

If the number average molecular weight of the polyester resin (A) is less than 5,000, elongation of the coating film becomes insufficient, and workability decreases. In addition, properties such as processing adhesion and post-forming corrosion resistance are also reduced. On the other hand, when the number average molecular weight exceeds 25,000, the coating composition becomes high in viscosity, so that an excessive diluting solvent is required, and the proportion of the resin in the coating decreases, so that an appropriate coating film cannot be obtained. Further, the compatibility with other compounding components is significantly reduced.

If the glass transition temperature Tg of the polyester resin (A) is less than 20 ° C., the toughness of the coating film is lowered, and for example, sufficient molding workability with a one-coat coating film in press molding or the like cannot be obtained. In addition, properties such as coating film hardness, processing adhesion, and post-forming corrosion resistance are also reduced. On the other hand, when the glass transition temperature Tg exceeds 80 ° C., the processability decreases. If the hydroxyl value of the polyester resin (A) is less than 3 KOH mg / g, the crosslinking reaction becomes insufficient and the coating film hardness decreases, while if the hydroxyl value exceeds 30 KOH mg / g, the processability decreases. The acid value of the polyester resin (A) is 10 KOH
If it exceeds mg / g, the compatibility with other compounding components decreases.

The polyester resin (A) can be obtained by polycondensation of a polybasic acid and a polyhydric alcohol by a conventional method, but the resulting polyester resin (A) has very few free carboxyl groups and an acid value of When the value is low, a part of the hydroxyl groups of the polyester resin (A) is modified with a carboxylic acid to increase the acid value to 3 KOH mg / g or more (however, 10 KOHm / g).
g / g), it is possible to further improve the adhesion to the base and further increase the curing speed. Examples of the polybasic acid include terephthalic acid, isophthalic acid, phthalic acid, succinic acid, adipic acid, sebacic acid,
Typical examples thereof include malonic acid, oxalic acid, trimellitic acid and the like, and lower alkyl esters and acid anhydrides thereof.

In the range of 20 to 70 mol% of the polyhydric alcohol component of the polyester resin (A), an ethylene oxide adduct of bisphenol A and / or propylene of bisphenol A represented by the following formula (1): By using the oxide adduct, a tougher coating film having a higher elastic modulus can be obtained, and the moldability and solvent resistance can be further improved.

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Here, if the polyhydric alcohol component represented by the above formula (1) is less than 20 mol%, a tough coating film cannot be obtained, and the moldability and solvent resistance deteriorate. On the other hand, if it exceeds 70 mol%, the coating film becomes hard and the workability is reduced. Typical examples of the polyhydric alcohol component other than the above include ethylene glycol, 1,4-butanediol, 1,6 hexanediol, diethylene glycol, neopentyl glycol, and the like.

Epoxy resin (B): The epoxy resin (B) blended in the solvent-type coating composition is blended to improve the adhesion to the substrate, and has an epoxy equivalent of 1
80 to 1000, preferably 200 to 900 are used. The epoxy equivalent of the epoxy resin (B) is 180
If less than 10, the coating film hardness decreases, while the epoxy equivalent is 10
If it exceeds 00, the compatibility with the polyester resin decreases. As the epoxy resin (B), bisphenol A-based epoxy resin, bisphenol F-based epoxy resin,
Epoxy resins commonly used for paints such as phenol novolak type epoxy resin and orthocresol novolak type epoxy resin can be used.

Blocked polyisocyanate compound (C): The blocked polyisocyanate compound (C) compounded as a curing agent in the solvent type coating composition is hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, tolylene diisocyanate, 4,4 −
Diphenylmethane diisocyanate, xylylene diisocyanate, isophorone diisocyanate, naphthylene-1,5-diisocyanate, hydrogenated tolylene diisocyanate, hydrogenated 4,4-diphenylmethane diisocyanate, hydrogenated xylylene diisocyanate, 4,4-methylene bis (cyclohexyl isocyanate), , 4,6
-One or more polyisocyanate compounds selected from polyisocyanates and their derivatives represented by triisocyanatotoluene and the like, phenol, cresol,
Compounds blocked with blocking agents such as aromatic secondary amines, tertiary alcohols, lactams and oximes.
By using this blocked polyisocyanate compound, storage with one liquid becomes possible, and use as a paint for a precoated steel sheet is facilitated.

Melamine resin (D): The melamine resin (D) optionally added as a curing agent to the solvent-type coating composition is a part of the methylol group of the product obtained by condensing melamine and formaldehyde. Alternatively, it is a resin in which all are etherified with a lower alcohol such as methanol, ethanol, or butanol.

In the present invention, the mixing weight ratio (C) / (D) of the blocked polyisocyanate compound (C) and the melamine resin (D) blended as a curing agent is 100/0 to 100/0 in terms of solid content. 15/85, preferably 100
/ 0 to 50/50. If the mixing weight ratio (C) / (D) of the blocked polyisocyanate compound (C) and the melamine resin (D) is less than 15/85, 180
It is not preferable because workability under severe processing conditions such as bending is reduced.

The mixing weight ratio (C) / (D) of the polyester resin (A), the epoxy resin (B), and the solid content of the blocked polyisocyanate compound (C) and the melamine resin (D) in the coating composition. ) Is 100 / 0-15 /
The mixing ratio of the curing agent consisting of 85 is from 3 to 100 parts by weight of the polyester resin based on the solid content,
25 parts by weight, preferably 5 to 20 parts by weight, and the mixing weight ratio (C) / (D) of the blocked polyisocyanate compound (C) and the melamine resin (D) at a solid content of 100/0 to 0:
The curing agent consisting of 15/85 is used in an amount of 3 to 20 parts by weight, preferably 5 to 18 parts by weight. If the blending amount of the epoxy resin is less than 3 parts by weight with respect to 100 parts by weight of the polyester resin, properties such as working adhesion and corrosion resistance after forming processing are deteriorated, while if it exceeds 25 parts by weight, workability, working adhesion and molding are reduced. After processing, properties such as corrosion resistance deteriorate. Further, the mixing weight ratio (C) / (D) of the blocked polyisocyanate compound (C) and the melamine resin (D) in the solid content is 100/0 to 15/85 with respect to 100 parts by weight of the polyester resin. When the compounding amount of the curing agent is less than 3 parts by weight, the properties of the coating film hardness, stain resistance and the like are deteriorated due to insufficient curing of the coating film. After molding, properties such as corrosion resistance deteriorate.

In the coating composition used in the present invention, a curing catalyst can be used if necessary in order to accelerate the crosslinking reaction of the resin. Usable curing catalysts include p
-Toluenesulfonic acid, dodecylbenzenesulfonic acid,
Acids represented by dinonylnaphthalenesulfonic acid, dinonylnaphthalenedisulfonic acid and their neutralized amines or neutralized products thereof, trimethylenediamine, tin octoate dilaurate, dibutyltin laurate, lead 2-ethylhexoate, etc. Are typical examples. By using these curing catalysts, crosslinking can be performed in a short time, and operability during production is improved.

The amount of the curing catalyst is based on the solid content of the polyester resin (A), epoxy resin (B), and curing agent (blocked polyisocyanate compound (C) and melamine resin (D)) in the proportion of the active ingredient. Weight ratio (C) /
The curing agent is suitably in the range of 0.05 to 2 parts by weight based on 100 parts by weight of (D) a curing agent comprising 100/0 to 15/85). Further, the coating composition used in the present invention,
If necessary, pigments, lubricants, dispersants, antioxidants, leveling agents, defoamers, and the like, which are usually used in the field of paints, can be appropriately compounded.

In actually using the above coating composition, it is used by dissolving it in an organic solvent. As the organic solvent to be used, various solvents that are usually used for paints can be used.For example, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, 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. The compounding amount of the organic solvent is set so that the paint viscosity is 40 to 200 seconds (Ford Cup N
o. 4 / room temperature) is appropriate.

The composition of the coating composition used in the present invention has been described above. In preparing the coating composition, an ordinary dispersing machine or kneading machine such as a sand grind mill, a ball mill or a blender is appropriately selected. And each component can be blended. The pigment dispersity of the paint compounded in this manner is suitably adjusted to a grind gauge A method of 25 μm or less.

A cured coating film is formed by coating and baking the above coating composition.
0 μm or less. The coating of the precoated steel sheet of the present invention is 1
A coating film for a coated thin film is designed, and has a coating film performance enough to withstand severe molding processing even with a single coating and a film thickness of 10 μm or less. Further, such a thin coating film is very advantageous in terms of cost. Although the lower limit of the dry film thickness of the coating film is not particularly limited, it is generally preferably 2 μm or more.

Next, the method for producing the one-coated precoated steel sheet of the present invention will be described. The precoated steel sheet of the present invention is prepared by subjecting the surface of a zinc-coated steel sheet to be coated to the above-mentioned chemical conversion treatment. It is manufactured by painting and baking the solvent-type coating composition.

The method of applying the solvent-type coating composition is not particularly limited, but it is preferable to apply the composition by a roll coater. After the application of the solvent-type coating composition, the coating film is baked by a heating means such as hot-air drying, infrared heating or induction heating to crosslink the resin to obtain a cured coating film. The baking condition is a baking temperature of 20.
It is preferable to carry out baking for a short time at 0 to 250 ° C. (attained plate temperature) and for a baking time of 20 to 90 seconds, thereby forming a cured coating film and manufacturing one precoated steel sheet.

If the baking temperature is lower than 200 ° C., the crosslinking reaction does not proceed sufficiently, so that sufficient coating film performance cannot be obtained. On the other hand, when the baking temperature exceeds 250 ° C., deterioration of the coating film due to heat occurs, and the coating film performance decreases. If the baking time is less than 20 seconds, the crosslinking reaction does not proceed sufficiently, so that sufficient coating film performance cannot be obtained. On the other hand, if the baking time exceeds 90 seconds, the production cost is disadvantageous. The precoated steel sheet of the present invention is manufactured by a one-coat one-bake method, and it is preferable to apply the coating composition to the backside of the steel sheet in the same manner in order to further enhance the corrosion resistance of the precoated steel sheet.

[0035]

EXAMPLES Examples 1 to 10 of the present invention and Comparative Examples 1 to 11 According to the formulations shown in Tables 1 to 6, kneading polyester resin, epoxy resin, blocked polyisocyanate compound, melamine resin, curing catalyst, pigment, solvent, etc. It was dispersed to prepare a coating composition having a pigment dispersion degree of 25 μm or less by a grind gauge A method. The coating composition was coated with a bar coater on the back side of a hot-dip galvanized steel sheet (plate weight basis: 30/30 g / m ² ) having a thickness of 0.4 mm that had been subjected to a coating type chromate conversion treatment. Paint under the conditions shown in 6.
Baked. Next, the coating composition was painted and baked on the steel sheet front side under the same conditions as the rear side, and the present invention examples 1 to
No. 10 and one-coat precoated steel sheets of Comparative Examples 1 to 11 were obtained.

Conventional Example 1 Plate thickness 0.4 mm treated with a coating type chromate chemical conversion treatment
Hot-dip galvanized steel sheet (coating weight table / back = 30 /
30 g / m ² ) on the back side of a high-working pre-coated steel sheet (2
A polyester primer ("V-nit # 160 primer" manufactured by Dainippon Paint Co., Ltd.) used for the coating was applied and baked by a bar coater under the conditions shown in Table 6. The front side of the steel plate was also coated and baked with a polyester primer under the same conditions as the back side, then
An overcoat used for a high-work-type precoated steel sheet (two coats) was applied and baked on the polyester-based primer on the back side. Further, on the front side of the steel sheet, a top coat was applied and baked under the same conditions as those for the back side, to obtain a two-coated precoated steel sheet of Conventional Example 1.

Various tests were performed on the precoated steel sheet obtained as described above. The evaluation method of the test performed in this example is described below. (1) Appearance The quality of the surface properties of the coating film after baking was visually judged.
The evaluation criteria are as follows. ：: Color unevenness, gloss unevenness, citron skin, etc. are not observed. △: Color unevenness, gloss unevenness, citron skin, etc. are slightly observed. ×: Color unevenness, gloss unevenness, yuzu skin, etc. are observed over the entire surface.

(2) Gloss The specular reflectance (60 ° gloss) of the coated film surface was measured using a specular gloss meter. (3) Pencil Hardness Using a Mitsubishi Pencil Uni, evaluation was performed on scratches generated on the coating film according to the method of JIS K5400.

(4) Workability The test piece was bent at 180 ° with the test surface facing outward at 5 ° C., and evaluated by the T number at which cracking did not occur at the bent portion. The T number is 180 without holding anything inside the bent part.
0T when bending was performed and 1 plate of the same thickness as the test piece
1T, 2T, 3T and 3T, respectively. (5) Process adhesion After the test piece used in the evaluation of the processability test was immersed in boiling water for 2 hours, an adhesive tape was stuck to the bent (1T) portion, and the stripped state after peeling it off at a stretch was measured. evaluated. The evaluation criteria are as follows. ：: No abnormality △: Slight peeling of coating film ×: Many peeling of coating film

(6) Solvent resistance At 20 ° C., a gauze soaked with xylene was applied
It was reciprocated under a load of kg / cm ² . The number of reciprocations until the underlying metal surface was seen was measured, and those that did not show the underlying metal surface even after more than 50 times were> 50. (7) Corrosion resistance after forming 1 After performing actual press forming (cylindrical drawing), JIS K54
A salt water spray test was performed for 240 hours according to Example 00, and an adhesive tape was stuck on the side surface of the cylinder, and the peeled state after being peeled off at a stretch was evaluated. The evaluation criteria are as follows. ：: No abnormality △: Slight peeling of coating film ×: Many peeling of coating film

(8) Corrosion resistance after forming process 2 After conducting a draw bead test involving deformation and sliding, JIS
Perform a salt spray test for 240 hours according to K 5400,
An adhesive tape was stuck on the test surface, and the peeled state after being peeled off at a stretch was evaluated. The evaluation criteria are as follows. ：: No abnormality △: Slight peeling of coating film ×: Many peeling of coating film

Tables 7 and 8 show the evaluation results of the above tests. According to the table, the one-coat precoated steel sheets of the present invention all have excellent moldability, appearance, pencil hardness, work adhesion, solvent resistance, and post-formation corrosion resistance.
In addition, sufficient performance was obtained even when baking was performed in a short period of time, and it was found that this was very suitable for high-speed operation during production. In addition, the precoated steel sheet of the present invention has almost the same performance as the conventional two-coated precoated steel sheet of Conventional Example 1 in which the total dry film thickness of the primer and the overcoat is 20 μm while the dry film thickness of the coating film is 10 μm or less. Is shown.

On the other hand, in Comparative Example 1 in which the coating film did not contain the epoxy resin and the polyester resin did not use the ethylene oxide adduct of bisphenol A, the pencil hardness, the working adhesion, the solvent resistance and the shaping property were obtained. Poor post-corrosion resistance. Comparative Example 2 not containing an epoxy resin
Comparative Example 3 has a pencil hardness, processing adhesion and corrosion resistance after molding, and an epoxy resin compounding amount of more than 25 parts by weight has a workability, processing adhesion and corrosion resistance after molding, and shows that the blocked polyisocyanate compound and the melamine resin Comparative Example 4 in which the amount of the mixed resin exceeds 20 parts by weight is inferior in workability, work adhesion, and corrosion resistance after forming.

In Comparative Example 5 using a polyester resin having a number average molecular weight of less than 5,000, workability, working adhesion and corrosion resistance after molding were evaluated, and Comparative Example 5 using a polyester resin having a glass transition temperature Tg of less than 20 ° C. Sample No. 6 is inferior in pencil hardness, processing adhesion and post-forming corrosion resistance. Comparative Example 7 in which the dry film thickness of the coating film exceeds 10 μm
Has a low high-speed paintability and is particularly poor in appearance.

Comparative Example 8 in which the baking temperature was less than 200 ° C.
Comparative Example 9 has pencil hardness, processing adhesion, solvent resistance and corrosion resistance after molding processing, and the baking temperature exceeds 250 ° C. Comparative Example 9 has appearance, workability, processing adhesion and corrosion resistance after molding processing, and the baking time is less than 20 seconds. Comparative Example 10 is inferior in pencil hardness, processing adhesion, solvent resistance, and corrosion resistance after molding. Comparative Example 11 in which the mixing weight ratio (C) / (D) of the blocked polyisocyanate compound (C) and the melamine resin (D) in the curing agent at a solid content of less than 15/85 was poor in processability. Also, the processing adhesion is inferior.

Note that * 1 to * 13 in Tables 1 to 6 below
Indicates the following contents. * 1 Polyester resins No. 1 to No. 7 use terephthalic acid, isophthalic acid and adipic acid as polybasic acids, and ethylene glycol and BPA as polyhydric alcohols, respectively. Modified with melitic acid. In addition, polyester resin No. 8
Used terephthalic acid, isophthalic acid and sebacic acid as polybasic acids, and ethylene glycol and neopentyl glycol as polyhydric alcohols, respectively, and polycondensed these. * 2 Mn: number average molecular weight (ASTM D-3536)
* 3 Tg: glass transition temperature (JIS K 7121)
4.2 (2) [Measured based on [heat flux differential scanning calorimetry]) * 4 BPA: ethylene oxide adduct of bisphenol A * 5 bisphenol A type epoxy resin (epoxy equivalent 500) * 6 methyl ethyl ketone oxime of hexamethylene diisocyanate Block * 7 Methyl etherification * 8 Morpholine block of dodecylbenzenesulfonic acid * 9 Dibutyltin laurate * 10 Titanium dioxide * 11 Strontium chromate * 12 A mixed solution consisting of Solvesso # 150, cyclohexanone and isophorone 50:40:10 * 13 Ultimate plate temperature

[0047]

[Table 1]

[0048]

[Table 2]

[0049]

[Table 3]

[0050]

[Table 4]

[0051]

[Table 5]

[0052]

[Table 6]

[0053]

[Table 7]

[0054]

[Table 8]

[0055]

As described above, the pre-coated steel sheet of the present invention is a one-coat pre-coated steel sheet in which a coating composition is directly applied to the surface of a zinc-coated steel sheet subjected to a chemical conversion treatment without using a primer. The film thickness is 10
Despite being less than μm, it has excellent moldability, appearance, coating film hardness, process adhesion, solvent resistance, and corrosion resistance after molding. For this reason, it is extremely useful as a pre-coated steel sheet used in a part where formability is required in applications such as home appliances. Furthermore, the pre-coated steel sheet of the present invention has a coating thickness of 10 μm or less in one coat and can be baked for a short time, so that the coating process can be simplified, high-speed operation, and resource saving can be achieved. From this point, it is very useful industrially.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) B32B 31/26 B32B 31/26 C09D 161/28 C09D 161/28 163/00 163/00 167/00 167 / 00 175/04 175/04 (72) Inventor Keiji Yoshida 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Japan Nippon Kokan Co., Ltd. (72) Inventor Masaaki Yamashita 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Sun (72) Inventor Hiroyuki Kato 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Japan In-tube (72) Inventor Yasuyuki Kajita 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Japan (72) Inventor Susumu Ogawa 3-3359-37 Mihara, Otawara-shi, Tochigi Prefecture (72) Inventor Shinka Kato 281-479, 1-ku, Nishinasuno-cho, Nasu-gun, Tochigi F-term (reference) 4D075 BB26Z BB93Z BB95Z CA02 CA03 CA13 DA03 DB05 DC10 DC11 DC18 EA07 EB33 EB35 EB38 EB55 EB56 EC37 4F100 AB03A AB18A AK36B AK41B AK51B AK53B AK54B BA02 CA02B CC00B EH112 EH462 EH71A EJ082 EJ422 EJ681 GB48 JA05B JA07B JB02 JB07 JB20B JK06 JK12 JK14 JL01 JL02 YY00B 4J038 DA162 DB062 DB072 DD061 DD071 DD081 DD121 DD161 DG112 DG162 DG302 GA06 KA03 KA06 NA12 NA23 PA19 PB05 PB07 PB09 PC02

Claims (4)

[Claims] 1. The surface of a galvanized steel sheet subjected to a chemical conversion treatment has a number average molecular weight of 5,000 to 25,000, a glass transition temperature of 20 to 80 ° C., and a hydroxyl value of 3 to 30 KOHm.
g / g, 100 parts by weight of polyester resin (A) having an acid value of 0 to 10 KOH mg / g (solid content ratio), and 3 to 25 parts by weight of epoxy resin (B) having an epoxy equivalent of 180 to 1000 (solid content ratio) And 3 to 20 parts by weight of a curing agent having a mixing weight ratio (C) / (D) of the blocked polyisocyanate compound (C) and the melamine resin (D) in solid content of 100/0 to 15/85 ( 1-coat precoated steel sheet excellent in formability, characterized in that it has a coating film having a dry film thickness of 10 μm or less formed by coating and baking a solvent-type coating composition containing (solid content ratio) as a main component. . 2. 20 to 70 mol% of the polyhydric alcohol component of the polyester resin (A) in the solvent type coating composition.
Is an ethylene oxide adduct of bisphenol A and / or a propylene oxide adduct of bisphenol A represented by the following formula (1): 1-coated precoated steel sheet having excellent formability according to claim 1. . Embedded image 3. The polyester resin (A) in the solvent-type coating composition, after condensation polymerization of a polybasic acid and a polyhydric alcohol, has a partially modified hydroxyl group with a carboxylic acid to have an acid value of 3 to 10 KO.
The one-coat precoated steel sheet having excellent moldability according to claim 1 or 2, which is a polyester resin of Hmg / g. 4. The method for producing a one-coated precoated steel sheet according to claim 1, 2 or 3, wherein the surface of the zinc-coated steel sheet which has been subjected to the chemical conversion treatment is coated with a solvent-type coating composition and then reached. A method for producing a one-coated precoated steel sheet excellent in formability, characterized by forming a cured coating film by performing short-time baking at a temperature of 200 to 250 ° C and a baking time of 20 to 90 seconds.