JPH07171497A - Precoated steel sheet having excellent corrosion resistance - Google Patents

Abstract

PURPOSE:To provide the precoated steel sheet which is made of a cold-rolled steel sheet as the blank and has excellent corrosion resistance and also superior antifoulancy and surface hardness. CONSTITUTION:In this precoated steel sheet, at least one surface of a cold-rolled steel sheet has a phosphate coating film or chromate coating film on it and an undercoating material and an overcoating material applied on the undercoating material are baked on this coating film. The undercoating material contains at least one base resin selected from among epoxy, polyester and acrylic resins and at least one curing agent selected from among melamine resins, urea resins and block isocyanate resins in amounts to provide (base resin)/(curing agent), by weight ratio, of 10/0.5 to 10/8 and further contains a zinc powder added to this mixture of the base resin and the curing agent in an amount to provide ((base resin)+(curing agent))/(zinc powder), by weight ratio, of 1/1 to 1/9. Alternatively, the zinc powder and a chromium based rust preventive pigment are added to the mixture of the base resin and the curing agent in amounts to provide ((base resin)+(curing agent))/((zinc powder)+(chromium based rust preventive pigment)), by weight ratio, of 1/1 to 1/9 and (chromium based rust preventive pigment)/(zinc powder), by weight ratio, of 0.02/1 to 0.2/1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a coated steel sheet used for home appliances, building materials, automobile parts and the like, and more particularly to a precoated steel sheet made of a cold rolled steel sheet and having excellent corrosion resistance.

[0002]

2. Description of the Related Art In recent years, due to environmental problems, labor shortages, cost reduction, etc., in the field of home appliances, building materials, and more recently automobile parts, conventional post-processing coating (hereinafter referred to as post-coating) has been replaced. Therefore, coating before processing (hereinafter referred to as precoat) is rapidly becoming popular. As the performance required for the precoated steel sheet, not only primary physical properties such as workability, scratch resistance, and stain resistance but also corrosion resistance are extremely important performances. And, in order to satisfy these performances, high processability polyester is generally used as the coating material, and from the viewpoint of corrosion resistance, the material used is generally zinc plating, zinc alloy plating or the like. Is.

However, recently, the demand for cost reduction of these precoated steel sheets has become very strict. In response to these cost reduction requests, various measures are currently being taken, such as reducing the coating weight of the material used, changing to cheaper paints, and improving coating productivity, but no major effect can be expected. .

On the other hand, if a cold-rolled steel sheet is used as a raw material, the cost will be greatly reduced, but in a pre-coated steel sheet made of a cold-rolled steel sheet, red rust is likely to occur in the initial stage in a corrosive environment. Therefore, especially in the case of pre-coated steel sheets that have a white appearance in most cases, they are rarely used except in fields where the red rust is prominent and some environments are very mild.

On the other hand, a rust preventive pigment represented by a chromium-based pigment is added to the undercoat paint for the purpose of improving the corrosiveness, and its immobilizing action is generally used. Although the occurrence can be delayed, the effect is small. On the other hand, there are many techniques in which zinc dust is added to the paint to improve the corrosion resistance and the sacrificial corrosion resistance is utilized.
For example, zinc metal (trade name) and JP-A-54-
61277, JP-A-1-14867, JP-A1-2
A cold-rolled steel sheet, which is seen in No. 0059, is coated with a zinc dust coating to impart weldability and at the same time has improved corrosion resistance, and an undercoat for realizing high corrosion resistance such as Al-plated and Zn-Al-plated steel sheets. A steel sheet for exterior building materials, etc., which is obtained by applying zinc dust paint to the above and then applying a top coat, can be mentioned.

However, even if the zinc dust coatings found in these techniques are applied as they are to cold-rolled steel sheets and top-coating is applied, in most cases it is effective in suppressing the occurrence of red rust in the cold-rolled steel sheets. Particularly important surface properties of precoated steel sheets, such as stain resistance and surface hardness, are significantly impaired and almost never used.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems and provide a precoated steel sheet made of a cold rolled steel sheet, which is excellent in corrosion resistance, stain resistance and surface hardness.

[0008]

The present invention has a phosphate coating or a chromate coating on at least one surface of a cold-rolled steel sheet, and is further selected from an epoxy resin, a polyester resin and an acrylic resin. One or two or more base resins and one or more curing agents selected from melamine resins, urea resins and blocked isocyanate resins are contained, and the weight ratio of the base resin to the curing agent is base resin / curing agent = 10 / 0.5 to 10/8, and zinc powder was added at a weight ratio of (base resin + curing agent) / zinc powder = 1/1 to 1/9 to the total amount of base resin and curing agent. It is intended to provide a precoated steel sheet excellent in corrosion resistance, which is characterized in that an undercoat paint and an overcoat paint are further baked on the undercoat paint.

The undercoat paint comprises one or more base resins selected from epoxy resins, polyester resins and acrylic resins, and one or more hardeners selected from melamine resins, urea resins and blocked isocyanate resins. The weight ratio of base resin / curing agent = 10 / 0.5 to 10 /
8 and further, zinc powder and chromium-based rust preventive pigment (base resin + curing agent) / (zinc powder + chromium-based rust preventive pigment) = 1 /
1-1 / 9, and chromium-based rust preventive pigment / zinc powder = 0.02
It is preferably an undercoat paint added in a weight ratio of ˜0.2.

The amounts of the phosphate coating and the chromate coating attached are 0.1 to ² g / m ² and 5 to 100 mg / m, respectively.
It should be ² . The chromium anticorrosive pigment is preferably strontium chromate, zinc chromate or calcium chromate. Further, the film thickness after curing of the undercoat paint and the topcoat paint is 1 to 15 μm and 1 respectively.
It is preferably 0 to 30 μm.

[0011]

The present inventors have conducted extensive studies to solve the above-mentioned conventional problems, and as a result, the sacrificial anticorrosive action of the zinc powder in the undercoat paint further causes the chromium-based rust preventive pigment to be immobilized. The action improves the corrosion resistance of the cold-rolled steel sheet, and at the same time,
This is a basic performance requirement for precoated steel sheets. Focusing on the fact that processability, stain resistance, and surface hardness are greatly affected by the crosslink density of the resin in the undercoat paint, solving the problems with the basic idea of optimizing the amount of curing agent in the undercoat paint Came to do.

That is, according to the present invention, a cold rolled steel sheet is used as a starting material, and after the formation of a chromate or phosphate coating, zinc powder or zinc powder and a chromium-based rust preventive pigment are contained, and the amount of the curing agent in the resin is The present invention provides a precoated steel sheet, which is obtained by applying an undercoating paint optimized to the above, and further applying an overcoating paint on it, which has excellent corrosion resistance, stain resistance, surface hardness, and workability.

The present invention is described in detail below. A cold rolled steel sheet is used as a starting material, and a chemical conversion treatment coating of a phosphate coating or a chromate coating is formed on the cold rolled steel sheet for the purpose of improving the adhesion between the steel sheet and the undercoat paint. As the phosphate coating, usual ones such as zinc phosphate and iron phosphate can be used.
The amount of the phosphate coating adhered is not particularly limited,
0.1 to 2.0 g / m ² is preferable. If it is less than 0.1 g / m ² , the adhesion between the undercoat paint and the steel sheet is insufficient, so
Blisters are likely to occur in a corrosive environment. If it exceeds ² g / m ² , the adhesion between the undercoat paint and the steel sheet is sufficiently maintained, but since the chemical conversion treatment film is electrically insulating, the sacrificial anticorrosion current of zinc does not act on the cold rolled steel sheet and red rust occurs. The control effect becomes insufficient. In addition, cracks are easily formed in the processed coating layer even during bending, which adversely affects the workability of the top coating film.

[0014] On the other hand, the adhesion amount of the chromate is also not particularly ^{limited, 5mg / m 2 ~100mg / m} 2 is preferred. 5
If it is less than mg / m ² , the adhesion between the undercoat paint and the steel sheet is still insufficient, so blisters are likely to occur in a corrosive environment.
If it exceeds 100 mg / m ² , the sacrificial anticorrosion current of zinc powder is less likely to act on the cold-rolled steel sheet due to the electrical insulation of the chromate layer, and the effect of suppressing red rust is reduced, which is not preferable.

Next, after the chemical conversion treatment of the steel sheet, zinc powder, or an undercoat paint containing zinc powder and a chromium-based rust preventive pigment is applied. The undercoat contains one or more base resins selected from epoxy resins, polyester resins and acrylic resins, and one or more hardeners selected from melamine resins, urea resins and blocked isocyanate resins. Then, the weight ratio of the base resin and the curing agent must be base resin / curing agent = 10 / 0.5 to 10/8. If the amount of the curing agent is less than the base resin / curing agent = 10 / 0.5, the cross-linking density of the resin becomes insufficient, which reduces the stain resistance and surface strength of the top coating composition, which is not preferable. Further, when the amount of the curing agent is larger than that of the base resin / curing agent = 10/8, the crosslink density becomes too high and the workability of the undercoating material is remarkably lowered, which adversely affects the workability of the topcoating material. Therefore, it is not preferable. Further, a catalyst may be used if necessary in order to enhance the reactivity between the base resin and the curing agent.

Further, the undercoat paint contains zinc powder or zinc powder and a chromium-based anticorrosive pigment in addition to the base resin and the curing agent. The blending amount must be (base resin + curing agent) / zinc dust = 1/1 to 1/9 in weight ratio. Alternatively, the weight ratio should be (base resin + curing agent) / (zinc dust + chromium rust preventive pigment) = 1/1 to 1/9.
If the amount of zinc powder is less than (base resin + curing agent) / zinc dust = 1/1, the effect of zinc on sacrificial anticorrosion to suppress red rust is insufficient, and (base resin + curing agent) / zinc dust = 1 /
When the amount of zinc powder is more than 9, the workability of the undercoating material is remarkably deteriorated, which adversely affects the processing of the top coating film, which is not preferable.

Similarly, if the amount of zinc dust and chromium-based rust preventive pigment is less than (base resin + curing agent) / (zinc dust + chrome rust preventive pigment) = 1/1, the sacrificial anticorrosive action of zinc and the chromium-based rust preventive The effect of immobilizing rust pigment on red rust is insufficient, and (base resin + curing agent) / (zinc dust + chrome-based rust preventive pigment) = 1/9 If it is too much, the workability of the undercoat paint will be significantly reduced,
This adversely affects the workability of the top coating film and is not preferable.

A weight ratio of chromium-based rust preventive pigment / zinc dust is preferably 0.02 to 0.2. Chrome type rust pigment /
If the weight ratio of zinc dust is less than 0.02, the passivation effect of the chromium-based rust preventive pigment is small, and if it exceeds 0.2, the amount of zinc dust is relatively small, so the zinc dust is reduced. The sacrificial anticorrosion effect of is reduced. Any of these is not preferable because it causes deterioration of corrosion resistance. Further, strontium chromate, zinc chromate, and calcium chromate are effective chromium-based rust preventive pigments.

The thickness of these undercoat layers is 1 to 15 μm.
m is suitable, but 3 to 10 mμm is particularly preferable.
When it is less than 1 μm, sufficient corrosion resistance cannot be obtained, and when it exceeds 15 μm, corrosion resistance is sufficient but workability is deteriorated, which is not preferable.

An overcoat layer is further formed on the undercoat layer. A suitable film thickness is 10 to 30 μm.
If it is less than 10 μm, the hiding property of the paint is insufficient and stable color is not produced. If it exceeds 30 μm, not only is it economically disadvantageous, but the processability is also deteriorated.

The resin used for the overcoat is not particularly limited, but a polyester resin is preferable in view of processability. If necessary, for example, a melamine resin, a benzoguanamine resin, a urea resin, a blocked isocyanate resin, or the like may be added to these top-coat paints, and a coloring pigment may be added thereto.

As the coating method of the top coat and the undercoat, for example, natural roll coat, reverse roll coat,
It can be performed by a usual method such as curtain flow coating. As the curing method, heat, electron beam, ultraviolet ray, infrared ray,
Either far infrared rays may be used.

As described above, according to the present invention, after forming a phosphate coating or a chromate coating on at least one surface of a cold-rolled steel sheet, zinc powder or zinc powder and chromium-based rust preventive agent is added in addition to the base resin and the curing agent. To obtain an inexpensive pre-coated steel sheet that contains pigments and has an appropriate blending amount, and then coats an overcoating material with excellent corrosion resistance, workability, stain resistance, surface hardness, etc. You can

[0024]

EXAMPLES The present invention will be specifically described below based on examples.

Example 1 (Invention Examples 1 to 27, Comparative Example 1)
-8) First, invention examples and comparative examples using zinc powder as one of the constituent requirements of the undercoat coating composition will be described below.
After degreasing a 0.5 mm-thick cold-rolled steel sheet, the conditions of chemical conversion treatment, undercoating, and topcoating were changed as shown in Table 1 to obtain samples of Invention Examples 1 to 27. The coating conditions are as follows. The film thickness is a dry film thickness. Undercoat coating conditions: Film thickness 0.5 to 20 μm Baking conditions after 60 seconds PMT = 230 ° C. Topcoat coating conditions: Film thickness 20 μm Baking conditions after 60 seconds PMT = 230 ° C. Pretreatment of the precoated steel sheet thus obtained, undercoat coating The tests of the composition of the top coating composition and the coating film performance thereof were conducted as follows, and the results are shown in Table 1. Further, Comparative Examples 1 to 8 are also shown in Table 2 similarly to the invention examples.

The chemical conversion treatment was carried out as shown in Table 1. Of the undercoat paints, the epoxy resin is Epicoat 1010 (molecular weight: 5500) manufactured by Yuka Shell Co., Ltd.
Byron 200 (Molecular weight: 25,000, Tg: 80 ° C) manufactured by Toyobo Co., Ltd. was used as the polyester resin, and Dianal BR-73 (Molecular weight: 85,000, Tg: 100 ° C) manufactured by Mitsubishi Rayon Co., Ltd. was used as the acrylic resin. As the curing agent, melamine resin was Cymel 303 manufactured by Mitsui Cyanamid Co., Ltd., urea resin was UFR-65 manufactured by Mitsui Cyanamid Co., Ltd., and blocked isocyanate resin was Coronate 2513 manufactured by Nippon Polyurethane Co., Ltd.

(Example 2) (Invention Examples 28 to 55, Comparative Examples 9 to 18) Next, as one of the constituent requirements of the undercoat coating composition, an invention example using zinc powder and a chromium-based rust preventive pigment and A comparative example will be described below. After degreasing a 0.5 mm-thick cold-rolled steel sheet, the conditions of chemical conversion treatment, undercoating and topcoating were changed as shown in Table 1 to obtain samples of Invention Examples 28 to 55. The coating conditions are as follows. The film thickness is a dry film thickness. Undercoat coating conditions: film thickness 0.5 to 20 μm baking conditions after 60 seconds PMT = 230 ° C. overcoat coating conditions: film thickness 20 μm baking conditions after 60 seconds PMT = 230 ° C. pretreatment of the precoated steel sheet thus obtained, undercoat coating,
Tests of the composition of the top coating composition and the coating film performance thereof were conducted as follows, and the results are shown in Table 3. Further, Comparative Examples 9 to 18 are also shown in Table 4 like the invention examples.

The chemical conversion treatment was carried out as shown in Table 1. Of the undercoat paints, the epoxy resin is Epicoat 1010 (molecular weight: 5500) manufactured by Yuka Shell Co., Ltd.
Byron 200 (Molecular weight: 25,000, Tg: 80 ° C) manufactured by Toyobo Co., Ltd. was used as the polyester resin, and Dianal BR-73 (Molecular weight: 85,000, Tg: 100 ° C) manufactured by Mitsubishi Rayon Co., Ltd. was used as the acrylic resin. As the curing agent, melamine resin was Cymel 303 manufactured by Mitsui Cyanamid Co., Ltd., urea resin was UFR-65 manufactured by Mitsui Cyanamid Co., Ltd., and blocked isocyanate resin was Coronate 2513 manufactured by Nippon Polyurethane Co., Ltd.

The evaluation methods of corrosion resistance, workability, surface hardness and stain resistance are as follows. (Corrosion resistance) After drawing a cross cut reaching the base steel sheet to scratch the coating film, a salt spray test (JIS-Z-231
7) is 120 for Examples 1-27 and Comparative Examples 1-8.
It was carried out for 24 hours for Examples 28 to 55 and Comparative Examples 9 to 18, and the maximum peeling width of the coating film from the cross cut portion and the red rust occurrence state were evaluated. The evaluation was made in three levels according to the following criteria. Maximum peeling width ○: Blister width of 1 mm or less △: Blister width of one side 1 to 5 mm ×: Blister width of one side of 5 mm or more Red rust occurrence situation ○: Red rust did not occur △: Partial red rust occurred ×: Significant red rust occurred

(Workability) After performing a bending process at 20 ° C. and 180 ° with a spacer having a thickness of 0.5 mm interposed therebetween, the processed portion was observed with a 30-fold magnifying glass. In the evaluation, the number of spacers was changed, and the number of spacers when cracks did not occur was displayed. The display method is 0T when there is no spacer,
When it was one, it was like 1T.

(Stain resistance) After 24 hours, traces of the magic ink on the test piece were wiped off with ethanol and evaluated. The evaluation was made in three levels according to the following criteria. ○: No trace of the magic ink △: Some trace of the magic ink remains ×: Trace of the magic ink is remarkable

(Surface Hardness) The surface hardness was evaluated by a scratch method using Mitsubishi Uni manufactured by Mitsubishi Pencil Co., Ltd. The evaluation is expressed by the hardness of a pencil, which is the limit of scratches.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

[0036]

[Table 4]

[0037]

[Table 5]

[0038]

[Table 6]

[0039]

[Table 7]

[0040]

[Table 8]

[0041]

As described above, the precoated steel sheet of the present invention has excellent corrosion resistance while using cold-rolled steel sheet as a raw material, and has excellent stain resistance and workability.
The present invention is very effective in reducing the cost of precoated steel sheets.

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Claims (6)

[Claims] 1. A cold-rolled steel sheet having a phosphate coating or a chromate coating on at least one surface thereof, on which one or more base resins selected from epoxy resins, polyester resins and acrylic resins are provided. And one or more curing agents selected from melamine resin, urea resin and blocked isocyanate resin, and the weight ratio of the base resin to the curing agent is base resin / curing agent = 10 / 0.5 to
10/8, and zinc powder is (base resin + curing agent) / zinc powder = 1/1 to 1 with respect to the total amount of base resin and curing agent.
A precoated steel sheet with excellent corrosion resistance, characterized in that an undercoat paint added in a weight ratio of 9/9 and a topcoat paint are further baked thereon. 2. The undercoat paint is one or more base resins selected from epoxy resins, polyester resins and acrylic resins and one or two base resins selected from melamine resins, urea resins and blocked isocyanate resins.
One or more curing agents are contained, the weight ratio of the base resin and the curing agent is base resin / curing agent = 10 / 0.5 to 10/8, and zinc powder and chromium-based rust preventive pigment (base resin + (Curing agent) / (Zinc dust + chrome-based rust preventive pigment) = 1/1 to 1/9
The precoated steel sheet having excellent corrosion resistance according to claim 1, which is an undercoat paint added in a weight ratio of chromium-based rust preventive pigment / zinc dust = 0.02-0.2. 3. The phosphate coating or chromate coating
Adhesion amount is 0.1-2g / m ², 5-100mg
/ M² The precoat excellent in corrosion resistance according to claim 1.
Steel plate. 4. The precoated steel sheet excellent in corrosion resistance according to claim 2 or 3, wherein the chromium-based rust preventive pigment is strontium chromate, zinc chromate or calcium chromate. 5. The precoated steel sheet excellent in corrosion resistance according to claim 1, wherein the film thickness of the undercoat paint after curing is 1 to 15 μm. 6. The film thickness of the top coating composition after curing is 10 to 30 μm.
The precoated steel sheet excellent in corrosion resistance according to any one of claims 1 to 5.