JP2000070843A - One-coat precoated steel panel excellent in molding processability and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide one-coat precoated steel panel hardly generating a crack or the like in a coating film even if severe molding processing is applied and made possible in high speed operation at a time of production. SOLUTION: A solvent type coating 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 KOHmg/g and an acid value 0-10 KOHmg/g, an epoxy resin with epoxy equivalent of 180-1,000 and a melamine resin are compounded as main components in a specific ratio, especially pref., a solvent type coating compsn. wherein 20-70 mol.% of a polyhydric alcohol component of a polyester resin is an ethylene oxide adduct to bisphenol A represented by the formula (wherein R1 and R2 are respectively H or CH3 and (m) and (n) are respectively 1-6) and/or a propylene oxide adduct to bisphenol A, is applied to the surface of galvanized steel subjected to chemical forming treatment and baked to form a coating film with a dry film thikness 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, automobile parts, and the like.

[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 made of 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 specific melamine resin (curing agent), a rust-preventive pigment, organic polymer fine particles, and the like is applied to form a one-coat processability and corrosion resistance. A coated steel sheet that satisfies adhesiveness, impact resistance, scratch resistance, and design properties has 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. Further, the surface of the coating film has an uneven finish, and cannot be used for applications requiring glossiness.

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. In addition, a direct coating of a solvent-type coating composition mainly containing a polyester resin having specific properties, a specific epoxy resin for improving adhesion to a base steel sheet, and a melamine resin as a curing agent. It has been found that by forming a film, a one-coat precoated steel sheet excellent in formability and high-speed operability 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
A solvent-based coating composition containing 3 to 25 parts by weight (solid content) of epoxy resin (B) and 5 to 30 parts by weight (solid content) of melamine resin (C) of 1000 to 1000 is applied and baked. A one-coat precoated steel sheet excellent in formability, characterized by having a coating film having a dry film thickness of 10 μm or less.

[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.

Embedded image

[3] In the one-coat precoated steel sheet of the above-mentioned [1] or [2], the polyester resin (A) in the solvent-type coating composition is obtained by polycondensation between a polybasic acid and a polyhydric alcohol.
An acid value in which a part of hydroxyl groups is modified with carboxylic acid is 3 to 10K
A one-coat precoated steel sheet having excellent moldability, characterized by being an OH mg / g polyester resin.

[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. And a solvent-based coating composition having a melamine resin (C) as a curing agent as a main component is directly applied to form a coating film.

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 treatment 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.

Hereinafter, the polyester resin (A), epoxy resin (B) and melamine resin (C) which are the main components of the solvent-type coating composition for forming the coating film will be described. 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 reduced, and for example, sufficient forming 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, scratch resistance, and post-forming corrosion resistance also decrease. on the other hand,
If the glass transition temperature Tg exceeds 80 ° C., the workability is reduced. The hydroxyl value of the polyester resin (A) is 3 KOHmg
If the hydroxyl value is less than 30 KOHmg / g, the processability will be reduced if the hydroxyl value exceeds 30 KOHmg / g. If the acid value of the polyester resin (A) exceeds 10 KOHmg / g, the compatibility with other compounding components will decrease.

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 it is low, a part of the hydroxyl groups of the polyester resin (A) is modified with a carboxylic acid, and the acid value is increased to 3 KOH mg / g or more (however, 10 KOH mg / g).
/ G), it is possible to further improve the adhesion to the base and further increase the curing speed. Representative examples of the polybasic acid include terephthalic acid, isophthalic acid, phthalic acid, succinic acid, adipic acid, sebacic acid, malonic acid, oxalic acid, trimellitic acid, and the like, or lower alkyl esters or acid anhydrides thereof. It is mentioned as a thing.

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.

Embedded image

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 coatings such as phenol novolak type epoxy resin and orthocresol novolak type epoxy resin can be used.

Melamine resin (C): The melamine resin (C) blended as a curing agent in the solvent-type coating composition is obtained by condensing a part or all of the methylol groups of a product obtained by condensing melamine and formaldehyde. It is a resin etherified with lower alcohols such as methanol, ethanol and butanol.

The mixing ratio of the polyester resin (A), the epoxy resin (B) and the melamine resin (C) in the coating composition is 3 to 25 parts by weight of the epoxy resin based on 100 parts by weight of the polyester resin in terms of solid content. , Preferably 5 to 20 parts by weight, melamine resin 5 to 30 parts by weight, preferably 10 to 25 parts by weight. Polyester resin 100
If the amount of the epoxy resin is less than 3 parts by weight, the properties such as coating film hardness, processing adhesion, scratch resistance, and corrosion resistance after molding are reduced. On the other hand, if the amount exceeds 25 parts by weight, processability, Characteristics such as processing adhesion and post-forming corrosion resistance are reduced. Further, if the amount of the melamine resin is less than 5 parts by weight based on 100 parts by weight of the polyester resin, the coating film hardness,
On the other hand, when the amount exceeds 30 parts by weight, characteristics such as workability, work adhesion, and corrosion resistance after forming work are deteriorated.

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. Examples of the curing catalyst that can be used include an acid or a neutralized product thereof.For example, p-toluenesulfonic acid, dodecylbenzenesulfonic acid, dinonylnaphthalenesulfonic acid, dinonylnaphthalenedisulfonic acid, and a neutralized product of these amines are given below. It is mentioned as a typical thing. By using these curing catalysts, crosslinking can be performed in a short time, and operability during production is improved.

The compounding amount of the curing catalyst is 0.1 to 0.1 parts by weight based on the total of 100 parts by weight of the polyester resin (A), the epoxy resin (B) and the melamine resin (C).
A range of 2 parts by weight is suitable. Further, if necessary, the coating composition used in the present invention may appropriately contain pigments, lubricants, dispersing agents, antioxidants, leveling agents, defoaming agents, and the like, which are usually used in the field of coatings. it can.

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, a conventional dispersing machine or kneading machine such as a sand grind mill, a ball mill or a blender is appropriately selected and used. And each component can be mix | 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 obtained 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.

[0033]

EXAMPLES Examples 1 to 8 of the present invention and Comparative Examples 1 to 7 Polyester resins, epoxy resins, melamine resins, curing catalysts, pigments, solvents, etc. were kneaded and dispersed in accordance with the formulations shown in Tables 1 to 4, and the degree of pigment dispersion was determined. Is grind gauge A method 25μ
m or less was prepared. 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. It was painted and baked under the conditions shown below. Next, the coating composition was applied and baked on the front side of the steel sheet under the same conditions as those on the back side to obtain one-coated precoated steel sheets of Examples 1 to 8 of the present invention and Comparative Examples 1 to 7.

Conventional Example 1 Hot-dip galvanized steel sheet having a thickness of 0.4 mm that has been subjected to a coating-type chromate-based chemical conversion treatment (amount of plating area / back = 30/3)
0 g / m ² ) is coated with a polyester-based primer (“V-nit 160 primer” manufactured by Dainippon Paint Co., Ltd.) used for a high-work type precoated steel sheet (2 coats) using a bar coater according to the conditions shown in Table 4 below.・ Bake,
On top of that, a top coat used for a high-work-type pre-coated steel sheet (two coats) was painted and baked. Next, a polyester-based primer and a top coat were applied and baked on the front side of the steel sheet under the same conditions as on the rear 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 Using a specular gloss meter, the 60-degree specular reflectance (6
0 ° gloss). (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) Scratch resistance The edge of a coin-shaped copper piece was strongly pressed against the coating surface of the pre-coated steel plate at an angle of about 50 degrees, and was pulled about 1 cm while maintaining the angle and load, and the coating surface was scratched. The degree of the scratch when the mark was attached was evaluated. The evaluation criteria are as follows. ○: No underlying metal surface is observed. △: A little underlying metal surface is seen. ×: The coating film is easily peeled off, and the underlying metal surface is clearly seen.

(5) Workability The test piece was bent at 180 ° at 20 ° C. with the test surface facing outward, and the T number was 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. (6) Work adhesion After the test piece used in the evaluation of the workability test was immersed in boiling water for 2 hours, an adhesive tape was stuck to the bent (1T) portion, and the peeled state after peeling off at a stretch was shown. evaluated. The evaluation criteria are as follows. ：: No abnormality △: Slight peeling of coating film ×: Many peeling of coating film

(7) Solvent resistance At 20 ° C., a gauze soaked with xylene was applied
It was reciprocated with a load of kg / cm ² . The number of reciprocations until the base of the steel sheet was visible was measured, and those for which the base was not visible even after more than 50 times were rated as> 50. (8) Corrosion resistance after forming process 1 After 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

(9) Corrosion resistance after forming process 2 After performing 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

Table 5 shows the results of the above tests. According to the table, the one-coated precoated steel sheets of the present invention have excellent moldability, appearance, work adhesion, solvent resistance,
It has corrosion resistance after processing. 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. The pre-coated steel sheet of the present invention has a dry film thickness of 10 μm or less,
The performance is almost the same as that of the conventional two-coat precoated steel sheet of Conventional Example 1 in which the total dry film thickness of the primer and the overcoat is 20 μm.

On the other hand, in Comparative Example 1 in which the coating film did not contain an epoxy resin and the polyester resin did not use an ethylene oxide adduct of bisphenol A, the pencil hardness, working adhesion, scratch resistance, and solvent resistance Poor properties and corrosion resistance after molding. In Comparative Example 2 in which no epoxy resin was blended, the pencil hardness, working adhesion, scratch resistance, and corrosion resistance after molding were the same.
Comparative Example 3 exceeding 5 parts by weight had processability, processing adhesion and corrosion resistance after molding, and Comparative Example 4 containing more than 30 parts by weight of melamine resin had processability, processing adhesion and corrosion resistance after molding, respectively. Inferior.

In Comparative Example 5 using a polyester resin having a number average molecular weight of less than 5000, the comparative example using a polyester resin having a glass transition temperature Tg of less than 20.degree. Sample No. 6 is inferior in pencil hardness, processing adhesion, scratch resistance, and corrosion resistance after molding. In Comparative Example 7 in which the dry film thickness of the coating film exceeds 10 μm, the high-speed coating property is low, and the appearance is particularly poor.

* 1 to * 11 in the following Tables 1 to 4 represent:
The following contents are shown. * 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 equivalent 500) * 6 methyl etherification * 7 dodecylbenzene sulfone Morpholine block of acid * 8 Titanium dioxide * 9 Strontium chromate * 10 Mixed solution of Solvesso @ 150, cyclohexanone and isophorone 50:40:10 * 11 Ultimate plate temperature

[0045]

[Table 1]

[0046]

[Table 2]

[0047]

[Table 3]

[0048]

[Table 4]

[0049]

[Table 5]

[0050]

As described above, the pre-coated steel sheet of the present invention is a one-coat pre-coated steel sheet in which the coating composition is directly applied to the surface of a galvanized steel sheet subjected to a chemical conversion treatment without using a primer. The thickness of the film is 10μ
m, excellent molding processability, appearance,
It has processing adhesion, solvent resistance, and corrosion resistance after processing. 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.
High-speed operation and resource saving are possible, and from this point of view, it is industrially very useful.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor, Toshimi Ito 1-2-1, Marunouchi, Chiyoda-ku, Tokyo, Japan Inside Nihon Kokan Co., Ltd. (72) Inventor Masaaki Yamashita 1-2-1, Marunouchi, Chiyoda-ku, Tokyo, Japan (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 (72) Inventor Shinka Kato 281-479, 1-ku, Nishinasuno-cho, Nasu-gun, Tochigi

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 a dry film thickness formed by applying and baking a solvent-type coating composition containing 5 to 30 parts by weight (solid content) of melamine resin (C) as a main component and having a dry film thickness of 10 μm.
1 coated pre-coated steel sheet having excellent formability, characterized by having a coating film of not more than m. 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-coat 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.