JP3293870B2 - Highly durable coated steel sheet having both scratch resistance and workability, and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a steel plate such as a zinc-plated steel plate, an aluminum-zinc alloy-plated steel plate, and the like, which is mainly used for construction materials by using a fluororesin-based paint as a coating material. The present invention relates to a highly durable fluororesin-based coated steel sheet having excellent weather resistance and corrosion resistance.

[0002]

2. Description of the Related Art Painted steel sheets are JIS G 3302 hot-dip galvanized steel sheets, JI
SG 3313 electro-galvanized steel sheet, JIS G 3
317 5% Al-Zn hot-dip galvanized steel sheet , 55% A
After surface treatment of various steel sheets such as l-Zn hot-dip galvanized steel sheet , various paints are continuously applied by a roll coater or the like, and weather resistance, corrosion resistance, chemical resistance, stain resistance, workability and design property The quality of the material is stable and uniform, and the use of this coated steel plate eliminates the need for processing or post-construction coating processes. Its use is expanding.

[0003] Among these coated steel sheets, the performance required for building materials is weather resistance, corrosion resistance, adhesion, workability, scratch resistance and the like. However, in the field where the use environment is severe, maintenance free is required. Therefore, its performance requirements are extremely high. From this point, as a paint to be coated, a fluororesin-based paint is considered to be the most excellent in weather resistance, corrosion resistance, chemical resistance, stain resistance, etc. due to its physically and chemically stable properties, and is attracting attention. .

However, even with these fluororesin-based coated steel sheets having excellent performance, the thickness of the coating film to be coated is usually as thin as about 25 to 35 μm and the hardness of the coating film is low. It is easy to be damaged during molding, forming or processing such as forming, etc. In addition, the corrosion resistance of the damaged part and the processed part may be inferior, use a protective film or repair the damaged part after processing or construction It is common. This is a problem of the fluororesin-based coated steel sheet.

[0005] As a means for solving these problems, conventionally, while maintaining the advantages of the fluororesin-based paint, the hardness of the coating film is increased and the hardness is increased to improve the scratch resistance and abrasion resistance. Means that do not impair workability and coating film adhesion have been studied, and metal plates such as Japanese Patent Publication No. Sho 60-7946, "Method of manufacturing highly durable coated metal plate" and Japanese Patent Application Laid-Open No. 61-237636, "Painted steel plate". A method has been proposed in which a fibrous E glass or C glass is added to a fluororesin-based paint to be coated on a substrate. However, these findings suggest that the glass to be added to the paint is preferably fibrous glass from the viewpoint of coating film performance, and the addition of flake or bead glass to the fluororesin paint has not been studied in detail. Also,
From the knowledge that the addition of glass fibers to a fluororesin-based paint increases the water permeability (water permeability) and reduces the corrosion resistance, surface treatment of the added glass fibers improves the wettability to fluororesins and pigments. Research and proposals have been made to reduce the water permeability (water permeability) of the coating film by using the method.

However, since these fluororesin-based coated steel sheets have a thin film thickness of 25 to 35 μ in the first place, even if the coating film strength is high and the coating film hardness is high, it is completely used during processing and construction work. It does not mean that you can prevent damage. That is, the actual situation is that a so-called flowering phenomenon occurs during actual exposure of the fluororesin-based coated steel sheet. The flowering phenomenon is a phenomenon in which the coating film of the fluorine-based resin has high strength, high internal stress, and low water permeability. This is a phenomenon in which the adhesion of the coating film around the portion is reduced and the film is turned up in a circular shape. When the peeling of the coating film occurs due to the flowering phenomenon, local corrosion of the galvanization proceeds, and finally red rust is generated. Fluororesin-coated steel sheets are inherently very weather-resistant, but if they are damaged, corrosion will progress locally in a short period of time (3-4 years), greatly impairing their life. It is the current situation.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and in a fluororesin-based coated steel sheet, even if the coating film is damaged during processing, construction, etc., the coating film may not be damaged. An object of the present invention is to obtain a fluororesin-based coated steel sheet which is excellent in corrosion resistance of a flaw in which red rust does not occur in a short period of time.

[0008]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and its gist is as described in the claims.

Hereinafter, the present invention will be described in detail.

[0010] The coated steel sheet according to the present invention comprises a steel sheet such as a galvanized steel sheet or an aluminum-zinc alloy-plated steel sheet, a chemical conversion treatment layer as a lowermost layer, and a rust-preventive pigment mainly containing strontium chromate thereon. An epoxy resin-based undercoat layer, a fluororesin-based intermediate layer on which glass beads surface-treated with a silane-based coupling agent are dispersed and blended, and an acrylic-modified vinylidene fluoride resin-based overcoat layer thereon. Features
In addition, the production method of the present invention comprises applying an epoxy resin paint containing a rust-preventive pigment containing strontium chromate as a main component, drying it to form an undercoat film, and then applying a silane-based coupling agent on the undercoat film. A fluororesin-based paint to which the treated glass beads were added was applied and dried to form an intermediate coating film, and then an acrylic-modified vinylidene fluoride resin-based paint was applied thereon and dried to form a topcoat film, It is characterized by forming a coated steel plate of three coats and three bake.

[0011] The original sheet and the steel sheet as the material are galvanized steel sheet, aluminum-galvanized steel sheet and the like, and those having various aluminum concentrations can be used.
A chemical conversion treatment is performed on the surface of these original sheets. The chemical conversion treatment is carried out by a spray or dipping type zinc phosphate type or a coating type chromic acid type treating agent.

Next, an undercoat is applied to the surface of the base plate subjected to the chemical conversion treatment and dried. As the undercoat paint, an epoxy resin paint containing a rust preventive pigment containing strontium chromate as a main component is used. Since the present invention intends to utilize the action of the rust preventive pigment, the incorporation of the rust preventive pigment into the undercoat layer is an essential condition. The thickness of the undercoat layer is preferably 3 to 10 μm in terms of the thickness of the dried coating film.

Next, a fluororesin-based paint containing glass beads is applied to the surface of the undercoat film thus formed and dried to form an intermediate coat film. The thickness of the dried coating film of the intermediate coating layer is preferably in the range of 10 to 25 µ. Vinylidene fluoride paint or vinyl fluoride paint is used as the fluororesin paint. In particular, an acrylic-modified vinylidene fluoride-based paint is recommended because of its excellent adhesion and processability of the coating film. The mixing ratio of the acrylic resin and the vinylidene fluoride resin is preferably 1.5: 8.5 to 6: 4.

The glass beads used are those subjected to a silane coupling treatment. Even when glass beads that have not been subjected to silane coupling treatment are blended, many of the glass beads settle during storage or during coating, and the glass beads have poor wettability, poor compatibility with fluoroplastics and pigments, and moisture permeability. And the corrosion resistance of the fluororesin-coated steel sheet decreases, and the desired effect of the present invention cannot be obtained.

That is, one of the important features of the present invention is that the glass beads are dispersed and blended in the intermediate coating layer, so that the moisture permeability of the coating film of the intermediate coating layer is appropriately adjusted, and thus sufficient moisture is removed. The purpose is to supply the undercoat layer with the rust preventive action of the rust preventive agent in the undercoat layer, to alleviate the internal stress of the coating film, and to suppress the expansion of the scratches and the progress of corrosion at the scratched portion.

According to the study of the present inventor, in order to obtain such an effect while maintaining the above-mentioned advantages of the fluororesin in the fluororesin-based coating film, and to satisfy the workability, Contrary to conventional wisdom regarding aggregates added to resin, it is not possible with fibrous or flaked glass treated with a silane coupling agent after surface treatment with a titanate coupling agent, but not with a silane coupling agent. It has been found that it must be treated spherical glass, ie glass beads.

That is, by adding and blending the silane-coupling glass beads to the fluororesin, the hardness of the fluororesin coating film is increased and the coating film is relatively hardly damaged. However, since the moisture permeability of the coating film can be appropriately adjusted, the rust preventive action of the rust preventive agent in the undercoat layer can be brought out, and the internal stress of the coating film can be reduced, so that the corrosion resistance of the scratched portion is improved. It is considered that the anti-flary ring property can be improved.

It is preferable that the blending amount of the glass beads in the intermediate fluororesin paint is set to 1 to 30% by weight.
If the amount is less than 1% by weight, the moisture permeability of the coating film is low and stress cannot be alleviated, so that the effect of improving the corrosion resistance of the damaged part and the resistance to flowering cannot be obtained. Naturally, the effect of scratch resistance due to the improvement of the hardness of the coating film by the addition of glass beads cannot be obtained. On the other hand, when the content exceeds 30% by weight, the coating workability deteriorates, and the bending property of the coating film also deteriorates.

As the glass, E glass and A glass can be used, but the diameter of the glass beads is preferably set to 1 to 40 μm. This is because if the diameter of the glass beads is less than 1 μm, the moisture permeability becomes too high, resulting in poor corrosion resistance. The effect of scratch resistance by improving the hardness of the coating film is also insufficient. If it exceeds 40 μm, the dispersion of the glass beads in the paint cannot be performed uniformly, and the glass beads settle on the paint pan, drum, etc. at the time of painting, and the workability becomes extremely poor.

The coated steel sheet of the present invention has an acrylic-modified vinylidene fluoride resin-based overcoat layer on such an intermediate coat layer. The second invention of the present application is characterized in that a matting agent is blended into the acrylic-modified vinylidene fluoride resin-based overcoat layer. This may be achieved by applying and drying the above-mentioned intermediate coating composition, and then applying and drying an acrylic-modified vinylidene fluoride resin-based coating composition or an acrylic-modified vinylidene fluoride resin-based coating composition containing a matting agent as a top coating composition. The thickness of the coating film is preferably in the range of 10 to 25 µ.

In the present invention, it is important to use an acrylic-modified vinylidene fluoride resin paint for the overcoat layer. By using an acrylic-modified vinylidene fluoride resin resin coating for the overcoat layer, it is possible to ensure sufficient moisture permeability for the overcoat layer coating film. If sufficient moisture permeability cannot be ensured in the coating film of the overcoat layer, the effect of the combination of the undercoat layer and the intermediate coat layer described above cannot be exerted. This is because the effect of improving the properties cannot be obtained. Further, by using an acrylic-modified vinylidene fluoride resin-based paint for the overcoat layer, excellent adhesion and processability of the coating film can be obtained. As for the acrylic-modified vinylidene fluoride, the mixing ratio of the acrylic resin and the vinylidene fluoride resin in the overcoat layer is preferably 1.5: 8.5 to 6: 4.

The matting agent blended in such an acrylic-modified vinylidene fluoride resin-based overcoat layer includes silica, white carbon, talc and the like, and any of them can be used. In the present invention, the addition of the matting agent is also important in increasing the moisture permeability of the coating film by making the coating film of the overcoat layer porous. From the viewpoint of improving the moisture permeability of the coating film, as the matting agent, silica fine powder, spherical silica fine particles are preferable,
The average particle diameter is preferably 1 to 8 μm, and the blending amount is preferably 2 to 10 parts with respect to 100 parts of the fluororesin of the overcoat layer. More preferably, the average particle diameter is 2 μm, and the amount is 2 to 8 parts based on 100 parts of the fluororesin.

As described above, the present invention provides a standard two-coat two-bake coating method by treating glass beads with a silane-based coupling agent, blending the mixture into a three-coat fluororesin-coated steel plate intermediate coating, and coating. The steel sheet has a water permeability that is not too low as compared with a steel sheet or a three-coated three-baked steel sheet, and can suppress the internal stress of the coating film to a moderately low level.

As described in Japanese Patent Publication No. Sho 60-7946, "Method of manufacturing a highly durable coated steel sheet" and Japanese Patent Application Laid-Open No. 61-237636, "Coated steel sheet", a coating film containing glass fiber or flaky glass is added. Painted steel plate has high coating film hardness,
Since the water permeability is low and the internal stress is high, the corrosion resistance of the damaged portion cannot be obtained, and the flowering phenomenon in which the wound portion is intensively corroded to generate a circular scratch cannot be avoided. The reason is that the moisture permeability (moisture permeability,
g / m ² . day) and internal stress (Кg / cm ² ) are shown in Table 2.
This is as shown in the comparative example.

[0025]

EXAMPLES Examples of the present invention will be described below together with comparative examples.

Example 1 A coating type chromate manufactured by Kansai Paint Co., Ltd. (trade name: Cosmer 1) having a thickness of 0.35 mm of a galvanized steel sheet according to JIS G 3302 and a zinc adhesion amount of Z27.
50) was coated with a roll and dried in a hot-air drying oven at 100 ° C. to perform a chromate-based pretreatment, and an epoxy resin containing a strontium chromate-based rust-proof pigment manufactured by Dainippon Ink and Chemicals, Ltd. was applied to the surface. Paint (Product name:
FG64P) was applied so that the thickness of the dried coating film was 5 μm, and dried under the conditions of a maximum plate temperature of the material of 180 ° C. and a drying time of 45 seconds to obtain an undercoat coating film.

Next, an average particle size of 20 μm
15% by weight of glass beads with a silane coupling treatment on the surface of an acrylic-modified vinylidene fluoride paint (trade name: Fluoropone) manufactured by Dainippon Ink and Chemicals, Inc.
The thickness of the dry coating film is 15μ.
Paint so that the maximum plate temperature of the material is 210 ℃,
Drying was performed under the condition of a drying time of 60 seconds to form an intermediate coating film.

Further, an acrylic-modified vinylidene fluoride brown paint (trade name: Fluoropone) manufactured by Dainippon Ink and Chemicals, Inc., containing no aggregate such as glass fiber, was applied as a top coat on the intermediate coat. Dry film thickness is 1
0μ, the maximum plate temperature of the material is 250
It dried at 60 degreeC and the conditions of 60 second of drying time, and formed the overcoat film, and manufactured the fluororesin-type coating galvanized steel plate.

Example 2 A coating type chromate manufactured by Kansai Paint Co., Ltd. (trade name: Cosmer 1) having a thickness of 0.35 mm of a galvanized steel sheet according to JIS G 3302 and a zinc adhesion amount of Z27.
50) was coated with a roll and dried in a hot-air drying oven at 100 ° C. to perform a chromate-based pretreatment, and an epoxy resin containing a strontium chromate-based rust-proof pigment manufactured by Dainippon Ink and Chemicals, Ltd. was applied to the surface. Paint (Product name:
FG64P) was applied so that the thickness of the dried coating film was 5 μm, and dried under the conditions of a maximum plate temperature of the material of 180 ° C. and a drying time of 45 seconds to obtain an undercoat coating film.

Next, an average particle size of 20 μm
Glass beads with a silane coupling treatment on the surface of the mixture were mixed and dispersed at 15% by weight in an acrylic-modified vinylidene fluoride resin-based paint (trade name: Fluoropone) manufactured by Dainippon Ink and Chemicals, Ltd. to form a dry coating. Thickness 1
Painted to 5μ, the maximum plate temperature of the material is 210
The coating was dried at a temperature of 60 ° C. for a drying time of 60 seconds to form an intermediate coating film.

Further, an acrylic-modified vinylidene fluoride resin-based brown paint (trade name: Fluoropone) manufactured by Dainippon Ink and Chemicals Co., Ltd., in which no aggregate such as glass fiber is blended, as an overcoat paint on the intermediate coating film. 2μ average particle size
Silica fine powder of acrylic modified vinylidene fluoride resin 10
Add 3 parts to 0 parts, mix and disperse, and dry film thickness is 1
0μ, the maximum plate temperature of the material is 250
It dried at 60 degreeC and the conditions of 60 second of drying time, and formed the overcoat film, and manufactured the fluororesin-type coating galvanized steel plate.

Comparative Example 1 As an intermediate coating material, glass fiber having an average diameter of 10 μm and an average length of 35 μm, which was subjected to a titanate coupling treatment and then a silane coupling treatment, was subjected to an acrylic modified fiber manufactured by Dainippon Ink and Chemicals, Inc. A vinylidene fluoride-based paint (trade name: Fluoropone) was used in the same manner as in Example 2 except that 15% by weight of the paint was used.
Fluororesin coated steel sheet was manufactured.

Comparative Example 2 Fluorine was prepared in the same manner as in Example 1 except that an acrylic-modified vinylidene fluoride resin paint (trade name: Fluoropone) manufactured by Dainippon Ink and Chemicals, Inc., which did not contain glass beads, was used as the intermediate paint. A resin-coated galvanized steel sheet was manufactured.

Comparative Example 3 An acrylic-modified vinylidene fluoride resin-based paint (trade name: Fluoropone) manufactured by Dainippon Ink and Chemicals, Inc., containing no glass beads, was used as the intermediate coating so that the thickness of the dried coating film became 10 μm. And dried under the conditions of a maximum plate temperature of 210 ° C. and a drying time of 60 seconds to form an intermediate coating film.
A scale-like aluminum powder surface-treated with μ acrylic resin was applied to an acrylic-modified vinylidene fluoride-based paint (trade name: Fluoropone) manufactured by Dainippon Ink and Chemicals, Inc.
1% by weight of the dry coating film
Painted to 5μ, the maximum plate temperature of the material is 250
A metallic resin-coated fluororesin-coated steel sheet was manufactured in the same manner as in Example 1 except that the coating was dried at 60 ° C. and a drying time of 60 seconds to form a top coat.

Comparative Example 4 Using an acrylic-modified vinylidene fluoride resin-based paint (trade name: Fluoropone) manufactured by Dainippon Ink and Chemicals, Inc., which does not contain glass beads as an intermediate coating, so that the thickness of the dried coating film becomes 10 μm. And dried under the conditions of a maximum plate temperature of 210 ° C. and a drying time of 60 seconds to form an intermediate coating film.
A glass fiber having an average length of 0 μm and an average length of 35 μm, which is subjected to a titanate-based coupling treatment and then a silane-based coupling treatment is used to produce an acrylic-modified vinylidene fluoride-based paint manufactured by Dainippon Ink and Chemicals, Inc.
And 15% by weight of the mixture were dispersed and blended.
μ so that the maximum plate temperature of the material is 250
A fluororesin-coated steel sheet was manufactured in the same manner as in Example 1 except that the coating was dried at a temperature of 60 ° C. and a drying time of 60 seconds to form a top coat.

Comparative Example 5 The average diameter was 10
μ, glass fiber having an average length of 35μ, titanate-based coupling treatment and then silane-based coupling treatment were applied to an acrylic-modified vinylidene fluoride-based paint (trade name: Fluoropone) manufactured by Dainippon Ink and Chemicals, Inc. % By weight, dispersed and blended, and dried so that the thickness of the dried coating film was 22 μm. The material was dried under the conditions of a maximum plate temperature of 250 ° C. and a drying time of 60 seconds to form a top coating film. In the same manner as in Example 1, a fluororesin-based coated steel sheet was manufactured.

COMPARATIVE EXAMPLE 6 The average particle size was 20 as the top coating without using the intermediate coating.
Use a mixture of 15% by weight of glass beads treated with silane coupling on the surface of μ in acrylic-modified vinylidene fluoride-based paint (trade name: Fluoropone) manufactured by Dainippon Ink and Chemicals, Inc. and dried. Paint so that the coating thickness is 22μ, and the maximum plate temperature of the material is 25
A fluororesin-coated steel sheet was manufactured in the same manner as in Example 1 except that the coating was dried under the conditions of 0 ° C. and a drying time of 60 seconds to form a top coat.

Comparative Example 7 The dry coating film thickness of the top coat was 22 μm without using the intermediate coat.
A fluororesin-coated galvanized steel sheet was obtained in the same manner as in Example 1 except that the coating was performed so that

Comparative Example 8 A fluororesin-coated galvanized steel sheet was prepared in the same manner as in Example 2 except that the intermediate coat was not used and the dry coating thickness of the top coat of Example 2 was applied to be 22 μm. Obtained.

Comparative Example 9 The undercoating film was dried at a maximum plate temperature of 190 ° C. and a drying time of 25 seconds. The intermediate coating was not used, and the topcoat was used as a matting agent having an average particle diameter of 2 μm. Using 6% by weight of silica fine powder dispersed in melamine-modified polyester (trade name: F38) manufactured by Nippon Paint Co., Ltd.
Painted so that the dry film thickness becomes 15μ, the maximum plate temperature of the material is 220 ° C, and the drying time is 30 seconds.
A coated galvanized steel sheet was manufactured in the same manner as in Example 1 except that an overcoat film was formed.

Table 1 shows the types of raw materials, chemical conversion treatment, undercoating, intermediate coating, and topcoating for Examples 1 and 2 and Comparative Examples 1 to 9. Moreover, about an Example and Comparative Examples 1-9,
The performance of the coating on each coated steel plate was tested. Table 2 shows the results.
Shown in

The test method, test conditions, and evaluation criteria will be described below.

The moisture permeability was measured according to JIS Z-0208 (cup method). However, the temperature is 50 ° C and the relative humidity is 9
Performed at 8%.

The initial and secondary internal stresses were measured by the method of Inoue (Kogaku Kagaku Magazine [46] 2p148-151). The secondary internal stress was measured one week after ultraviolet irradiation.

The evaluation of the flowering was carried out by scratching the surface of the coating with an NT cutter (office cutter), and after 5 years from the actual exposure test, flowering of the coating from the damaged part, the occurrence of red rust and the progress of red rust. I went in. ○ is red rust,
No occurrence of flowering, Δ indicates slight red rust and flowering observed, and X indicates red rust and flowering observed over the entire wound.

The corrosion resistance was evaluated by the salt spray test for 3,000 hours, where the flat portion was evaluated as ○ when no blistering occurred, Δ when slight blistering occurred, and × when significant blistering occurred, and the cross cut portion was evaluated. For, the width of blistering is expressed in mm. Regarding the processed part, を indicates that no rust was generated in the 2T processed part, Δ indicates that some rust was generated, and 著 し い indicates that significant rust was generated. Was evaluated as x.

The scratch resistance was measured by a pencil hardness test and a coin scratch test. Pencil hardness test is JIS D-020
The coin scratch test was performed based on No. 2 and the coating film was scratched with a coin, and the top coating film without peeling was evaluated as ○, the top coating was peeled and the intermediate coating was exposed, and the material was exposed. What was evaluated was evaluated as x.

Vise 0T, 2T, 4T in workability test
Example: Using a hot-dip galvanized steel sheet of 0.35 mm, the coated steel sheet of the comparative example was bent 180 degrees with a vise, and the cracks of the coating film of the processed part in each case of 0T, 2T, and 4T and The presence or absence of peeling of the coating film was evaluated. The evaluation criteria were 5: no crack, 4: crack but no peeling of the coating, 3: peeling of less than 30% of the coating, 2: 30% or more and 7
Less than 0% peeling of the coating film, 1 means 70% or more peeling of the coating film. The evaluations under / are evaluations before and after forced peeling with cellophane tape.

The chemical resistance was evaluated as ○ when the coating film surface was immersed in a chemical for 500 hours and there was no change in the coating surface, Δ when the coating film was partially blistered on the surface of the coating film, and B on the entire surface. Those were evaluated with x.

The weather resistance was 1 in the EMMAQUA test.
After the elapse of one million Langley, the discoloration / fading / gloss retention of the coating film was evaluated.

[0051]

[Table 1]

[0052]

[Table 2]

As is evident from the results shown in Table 2, the coated steel sheet using the fluororesin paint of the present example was compared with the conventional standard fluororesin coated steel sheets as in Comparative Examples 2 and 7. In addition, since the coating film has high moisture permeability and low internal stress, it has excellent corrosion resistance at the scratched portion, and does not cause local red rust even if the coating film is damaged. In addition, the glass fiber and the flaky glass blended like Comparative Example 4 and Comparative Example 5 have low moisture permeability of the coating film, and when a flaw occurs, the coating film from the flawed portion is similar to a standard fluororesin coated steel sheet. From the progress of local rust to the occurrence of red rust.

Since the coated steel sheet of the present invention uses the acrylic-modified fluororesin itself for the top coat, it can make use of the inherent properties of the fluororesin, such as weather resistance and workability. Although the coating film hardness is slightly lower than that to which glass fiber is added, it is hard to be damaged as compared with Comparative Examples 2 and 7. When a coated steel sheet is used for building materials, it is impossible to handle it, form processing, and do not damage it at all from the construction environment.According to the fluororesin coated steel sheet of the present invention, even if the steel sheet is damaged, It can be seen that the occurrence of local corrosion in the damaged portion in a short period can be prevented.

[0055]

As described above, according to the present invention, in a fluororesin-coated steel sheet, the moisture permeability and internal stress of the coating film can be appropriately adjusted, and the generation of red rust from the damaged portion in a short period of time. It is possible to obtain a fluororesin-coated steel sheet which is excellent in corrosion resistance not to scratches and satisfactory in workability, and of course also excellent in strength and hardness of coating film, adhesion, weather resistance, abrasion resistance and corrosion resistance,
The industrial effect is extremely large.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI B32B 27/18 B32B 27/18 Z 27/20 27/20 AZ 27/30 27/30 D 27/38 27/38 C09D 127 / 12 C09D 127/12 127/24 127/24 163/00 163/00 // B32B 15/08 B32B 15/08 G (58) Field surveyed (Int. Cl. ⁷ , DB name) B05D 1/00- 7/26 B32B 27/18 B32B 27/20 B32B 27/30 B32B 27/38 C09D 127/12 C09D 127/24 C09D 163/00 B32B 15/08

Claims (7)

(57) [Claims] 1. A steel sheet surface, a chemical conversion treatment layer as a lowermost layer, an epoxy resin-based undercoat layer containing a rust-preventive pigment containing strontium chromate as a main component, and a surface treatment with a silane-based coupling agent thereon. A highly durable coated steel sheet having both scratch resistance and workability, characterized by having a fluorine resin-based intermediate layer in which glass beads are dispersed and blended, and an acrylic-modified vinylidene fluoride resin-based upper layer thereon. 2. The highly durable coated steel sheet according to claim 1, wherein a matting agent is blended in the acrylic-modified vinylidene fluoride resin-based overcoat layer. 3. The matting agent content of the acrylic-modified vinylidene fluoride resin-based overcoating layer is 2 to 10 parts with respect to 100 parts of the fluorine-based resin. Durable painted steel plate. 4. The highly durable coated steel sheet according to claim 2, wherein the matting agent of the acrylic-modified vinylidene fluoride resin-based overcoat layer is silica fine powder. 5. The highly durable coated steel sheet according to claim 1, wherein the diameter of the glass beads dispersed and mixed in the fluororesin-based intermediate coating layer is 1 to 40 μm. 6. The highly durable coated steel sheet according to claim 1, wherein the blending amount of the glass beads in the fluororesin-based intermediate coating layer is 1 to 30% by weight. 7. After a chemical conversion treatment is applied to the surface of a steel sheet, an epoxy resin-based paint containing a rust-preventive pigment containing strontium chromate as a main component is applied thereon as an undercoat and dried, and then a silane-based paint is applied thereon as an intermediate coat. Corrosion resistance and processability characterized by coating and drying a fluororesin-based paint to which glass beads surface-treated with a coupling agent has been added, and then applying and drying an acrylic-modified vinylidene fluoride resin-based paint as an overcoat A method for manufacturing highly durable coated steel sheets that combines