JP2000185259A - Coated steel plate having high durability and weathering performance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a coated steel plate in which a thick coating film can be formed without using a vinyl chloride resin and which has high scratch resistance and corrosion resistance and has improved machinability and weathering properties as well. SOLUTION: On a steel plate plated with zinc or aluminum, to which a leveling treatment (zinc phosphate treatment, chromate treatment etc.), is applied, a leveling coating film having a thickness of 10-80 μm is formed from a coating material containing a polyamine, a block isocyanate, nonvolatile matters, and a chromate rust proof pigment of 5-50 wt.% with respect to the total amount of the polyamine, block isocyanate and nonvolatile matters, following which a face coating film having a thickness of 15-30 μm is formed from a fluorine plastic resin, in which at least one kind of fillers of 1-20 wt.% with respect to the total amount of nonvolatile matters selected from the group consisting of glass fibers, alumina fibers, a boron nitride, and thermoplastic resin beads may be contained.

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 which is resistant to scratching, has excellent durability and weather resistance, and is particularly suitable for use in outdoor building materials, and a method for producing the same.

[0002]

[Prior Art] Coated steel sheet (pre-coated steel sheet) which has been coated in advance and does not need to be coated after processing
Has been widely used for indoor and outdoor building materials. Typical painted steel sheets for building materials are classified by the type of resin used for painting.Polyester steel, also known as color steel, fluorine-based steel with high weather resistance, thick film, excellent in corrosion resistance and durability And vinyl chloride.
As a primer (primer) for these paints, epoxy paints are generally used.

[0003] Polyester-based coated steel sheets have a molecular weight of 3000
It is a coating of about 5000 polyester resin as a main component with a coating thickness of about 15 μm. Although it is inexpensive, the thickness of the coating film is so small that if it is scratched, it will be generated from the scratched part, resulting in insufficient corrosion resistance. Also, the workability is not satisfactory.

[0004] A typical fluorine-coated steel sheet is a steel sheet coated with a paint containing polyvinylidene fluoride and an acrylic resin as main components. This coated steel sheet is excellent in weather resistance, but since the total thickness of the undercoat (primer) + overcoat (topcoat) is as thin as about 25 μm, rust is easily generated from scratches as in the above. If scratches occur, they can be dealt with by touch-up (repair by partial coating), but it is time-consuming and uncertain because there are oversights.

[0005] A PVC coated steel sheet, which is abbreviated as a PVC steel sheet, is obtained by applying a sol paint (plastisol) of polyvinyl chloride (PVC) to a thickness of about 150 to 300 µm.
Since the thickness of the coating film is very large as compared with a polyester-based or fluorine-based coated steel sheet, even if it is scratched, it is difficult for the scratch to reach the base material, so that it shows excellent corrosion resistance. In addition, since the coating film is made of a thermoplastic resin and also contains a plasticizer, it is excellent in workability.
Since (OT) is also possible, it has been widely used for building materials.

[0006]

However, in recent years, it has become difficult to use polyvinyl chloride resin, and the situation is the same for painted steel sheets. One reason for this is the concern over the generation of dioxins when disposing of polyvinyl chloride. It is considered that there is no problem if the waste is disposed at a high temperature because the resin is completely decomposed, but if the incinerator temperature is low, harmful substances may be generated.

[0007] Another reason is the possibility of environmental pollution by the plasticizer contained in the polyvinyl chloride resin. Generally, a plasticizer is added to the polyvinyl chloride resin in order to improve processability. General plasticizers are high-boiling solvents represented by dioctyl phthalate (DOP), dibutyl phthalate (DBP) and the like, which are considered to evaporate gradually from the coating film. Although the effect of the plasticizer thus volatilized on the human body is not clear at present, coated steel sheets that do not use such substances are required in the market.

The present invention has the advantages of having a thick film, high corrosion resistance, and excellent workability of a vinyl chloride-based coated steel sheet, and has a low possibility of environmental pollution, durability and weather resistance. It is an object to provide an excellent coated steel sheet.

[0009]

As a precoated steel sheet which can be made into a thick film, JP-A-10-130572 discloses a resin system comprising a specific alicyclic diamine and a specific polyisocyanate blocked with ε-caprolactam. It has been proposed to form a thick urea resin-based undercoat layer by using it as a primer. Although it is possible to increase the thickness of this primer, the undercoat layer still has insufficient corrosion resistance, and cannot prevent rusting, particularly when the scratch reaches the base material or at the end face.

The present inventors have repeatedly studied to develop a coated steel sheet which can be made thicker, has high corrosion resistance, and also has high durability and weather resistance. As a result, it has been found that the following means are effective. 1) Use a urea resin paint capable of forming a thick film as a primer to prevent rust from being generated from scratches during handling; 2) Add a chrome-based rust-preventive pigment to the primer coating to Enhance corrosion resistance; 3) Use a fluorine-based paint as the top coating to exhibit weather resistance; 4) Add inorganic fillers and / or thermoplastic resin beads to the fluorine-based paint to increase abrasion resistance, The present invention has been completed on the basis of the above findings, and "at least one surface of a zinc-based or aluminum-based plated steel sheet that has been subjected to coating pretreatment,
A first coating layer having a thickness of 10 to 80 μm containing about 5 to 50% by weight of a chromate-based rust preventive pigment in a urea resin comprising a reaction product of a polyamine and a polyisocyanate, and a fluororesin formed thereon And a second coated film layer having a thickness of 15 to 30 μm and comprising, as a main component, a coated steel sheet excellent in durability and weather resistance ”.

The second coating layer of the coated steel sheet preferably contains 1 to 20% by weight of at least one selected from the group consisting of inorganic fillers and thermoplastic resin beads. Thereby, the abrasion resistance of the coating film, and hence the scratch resistance, is improved.

The coated steel sheet of the present invention can be applied to a zinc-based or aluminum-based coated steel sheet which has been subjected to a pre-coating treatment, in an amount of 5 to 50% by weight of a chromate-based protective coating with respect to the total amount of polyamine, blocked isocyanate and nonvolatile components. Applying a paint containing rust pigment and baking at a maximum temperature of 180 to 240 ° C, then applying a fluororesin paint to the maximum temperature
Baking at 200 to 240 ° C. The fluororesin-based paint to be used contains at least one inorganic filler and / or thermoplastic resin bead in an amount of 1 to 20 with respect to the total amount of nonvolatile components.
It may be contained in an amount of wt%.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION In the coated steel sheet of the present invention, a zinc or aluminum plated steel sheet having excellent corrosion resistance is used as a base material. The zinc-coated steel sheet includes a zinc-coated steel sheet and a zinc alloy-coated steel sheet, and similarly, the aluminum-coated steel sheet includes an aluminum-coated steel sheet and an aluminum alloy-coated steel sheet.

As a specific example of such a plated steel sheet,
Although not limited to these, electro-galvanized steel sheet, hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet,
Examples include an electric zinc-nickel alloy-plated steel sheet, a hot-dip 5% Al-Zn alloy-plated steel sheet, a hot-dip 55% Al-Zn alloy-plated steel sheet, and a hot-dip aluminum-plated steel sheet.

The plated steel sheet of the base metal is subjected to a pre-coating treatment (that is, a base treatment) according to a conventional method in order to improve corrosion resistance and coating film adhesion. This coating pretreatment is generally a chemical conversion treatment such as chromate treatment or phosphate treatment (typically zinc phosphate treatment). As the chromate treatment, an electrolytic type or a reactive type chromate treatment can be used, but a coating type chromate treatment is preferable from the viewpoint of work efficiency.

The pretreatment for coating may be carried out in the same manner as in the prior art, and the amount of adhesion may be the same as in the prior art. As a guideline, if the chromate treatment 20 to 80 mg / m ² about by Cr terms of metal, a zinc phosphate treatment would be about 0.3~1.2 g / m ^2. The coating pretreatment can be performed using a commercially available product as instructed.

As the undercoat paint, a paint containing a blocked isocyanate and a polyamine as main resin components is used because a thick film can be formed. The reason for using a blocked isocyanate obtained by reacting an isocyanate group of a polyisocyanate with a blocking agent to block the paint is to use a one-pack type.

In this paint, when heated, the blocking agent is dissociated to regenerate the isocyanate group (-NCO), which reacts with the amine group (-NH ₂ ) in the polyamine to form a urea group (-NHCONH).
-) Polymerizes by forming Therefore, the resin in the coating film formed by baking is a urea resin composed of a reaction product of polyisocyanate and polyamine. However, with this urea resin coating alone, sufficient corrosion resistance cannot be ensured, and it is not possible to prevent the formation of rust on the damaged part or end face that has reached the base material. Therefore, in the present invention, the undercoat paint contains a considerably large amount of a rust-preventive pigment.

The blocked isocyanate used for the undercoat in the present invention has been developed as a one-pack type polyurethane-based paint and can be obtained by reacting a polyisocyanate with an appropriate blocking agent. Polyisocyanates are roughly classified into aromatic and aliphatic ones. Aromatic ones are excellent in reactivity, and aliphatic ones are excellent in weather resistance. Therefore, in the case of polyurethane-based paints, aliphatic polyisocyanates capable of forming a coating film having excellent weather resistance are mainly used, and
In the case of a liquid paint, a blocked isocyanate obtained by reacting an aliphatic polyisocyanate with a blocking agent is mainly used.

However, in the present invention, the blocked isocyanate is used in a primer. Therefore, as the polyisocyanate, an aliphatic system containing an alicyclic (eg, hexamethylene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, etc.) can also be used,
Aromatic polyisocyanates having excellent reactivity are more preferable. Examples of the polyisocyanate suitable for use in the present invention include aromatic polyisocyanates such as tolylene diisocyanate, diphenylmethane diisocyanate, polymethylene polyphenyl polyisocyanate, naphthalene diisocyanate, and tolidine diisocyanate. Among them, tolylene diisocyanate is preferred from the viewpoint of performance and economy (price).

The polyisocyanate is not a monomer, and derivatives such as prepolymers, adducts (adducts such as trimethylolpropane), isocyanurates, and biurets can also be used. Further, two or more kinds of polyisocyanates or derivatives thereof may be used in combination.

The blocking agent for the polyisocyanate preferably has a dissociation temperature of 150 ° C. or higher, which is the temperature at which the blocking agent dissociates from the blocked isocyanate, in order to enable a thick film. Blocking agents with a dissociation temperature below 150 ° C (eg cresol with a dissociation temperature of 120 ° C, dissociation temperature of 140 ° C
The block isocyanate using methyl ethyl ketone oxime of the formula (1) tends to generate bubbles (bubbles) at the time of baking after coating, so that thick film coating may be difficult.
If the dissociation temperature is too high, the baking temperature must be very high or the baking time must be lengthened. Therefore, the dissociation temperature is preferably 200 ° C or less, particularly 180 ° C or less. An example of a preferred blocking agent is ε- having a dissociation temperature of 170 ° C.
Caprolactam, but other blocking agents having a dissociation temperature of 150 to 200 ° C, especially 160 to 180 ° C, can also be used.

The reaction between the polyisocyanate and the blocking agent may be the same as in the prior art, and is generally carried out by reacting the polyisocyanate with an equivalent or slightly excess blocking agent in a solvent under heating. The blocked isocyanate preferably has a number average molecular weight in the range of 1,000 to 4,000. Since various products are commercially available as the blocked isocyanate, an appropriate one can be selected from commercially available products.

Polyamines used in undercoat paints are also aliphatic.
Either an alicyclic (including alicyclic) type or an aromatic type may be used, but an alicyclic polyamine is preferred. As the alicyclic polyamine, for example, those used as a curing agent for an epoxy resin can be used. As a specific example, 1-
Cyclohexylamino-3-aminopropane, diaminocyclohexanes, bis (4-aminocyclohexyl) methane, bis (4-aminocyclohexyl) sulfone, 3,3 ′
-Dimethyl-4,4'-diaminodicyclohexylmethane,
And isophorone diamine. Most preferred is 3,3 '
-Dimethyl-4,4'-diaminodicyclohexylmethane. Two or more polyamines may be used.

The mixing ratio of the blocked isocyanate and the polyamine in the undercoat is preferably such that the molar ratio of isocyanate group / amino group is in the range of 0.6 to 2.0. This molar ratio is more preferably 0.8-1.
2

A coating containing a blocked isocyanate and a polyamine is suitable for a coated steel sheet because it can be applied as a thick film, but it alone has insufficient corrosion resistance on an end face or the like. Therefore, in the present invention, a rust-preventive pigment is further added to this paint, and this paint is used as an undercoat (primer).

As the rust preventive pigment, a chromate (chromate) type rust preventive pigment having excellent rust preventive properties is used. Specific examples include zinc chromate, strontium chromate, barium chromate, calcium chromate and the like, and one or more of them can also be used. The amount of the chromate-based rust preventive pigment to be added is in the range of 5 to 50% by weight based on the total amount of nonvolatile components in the paint (the blocking agent portion in the blocked isocyanate is not included in the nonvolatile components). If the amount is smaller than this, the corrosion resistance of the cut end face cannot be secured, and if the amount is larger than this, the workability decreases, and the secondary adhesion of the coating film (long-term adhesion)
Also decrease. The more preferable addition amount of this rust preventive pigment is 20 to
40 wt%.

The undercoat paint can be prepared by dissolving the blocked isocyanate and the polyamine in an appropriate solvent, adding a rust preventive pigment and uniformly dispersing the same.
The solvent is generally an organic solvent, for example, toluene,
Examples include hydrocarbon solvents such as xylene, ester solvents such as ethyl acetate and butyl acetate, ether solvents such as cellosolves, and ketone solvents such as methyl isobutyl ketone, methyl ethyl ketone, isophorone and cyclohexanone.

Optional components that may be optionally added to the paint include extenders (eg, silica, alumina, talc, calcium carbonate, titania, etc.) added for the purpose of adjusting the physical properties of the coating film and reducing costs. is there. When an extender is added, the amount of addition is 10% with respect to the total amount of nonvolatile components.
It is preferably at most wt%. The undercoat further includes an antifoaming agent, a pigment dispersant, an anti-sagging agent, a leveling agent,
Various additives such as silane coupling agents, and catalysts for the reaction between polyisocyanate and polyamide (eg,
(Organotin compound) may be added in a small amount.

The undercoat is applied to a base material plated steel sheet which has been subjected to a pre-coating treatment. The painting is a roll coat,
It can be carried out by any appropriate method such as curtain flow coating and spray coating. In the case of continuous coating on a coiled steel plate, roll coating is generally used. Painting is 10 ~ 80μm
Is performed so as to obtain a coating film thickness of If the coating thickness is less than 10 μm, rust generation due to scratching cannot be prevented,
When the thickness is more than 80 μm, the workability is reduced. The thickness of the coating is preferably from 20 to 60 μm. The undercoat paint used in the present invention can form a coating film having the above film thickness by one application, but may be applied two or more times.

The baking after the application of the undercoat is performed so that the maximum temperature is 180 to 240 ° C. In this temperature range, the baking time is preferably in the range of 30 to 70 seconds. If the temperature is lower than this, curing of the resin becomes insufficient, and desired film characteristics cannot be obtained. On the other hand, when heated to a temperature higher than 240 ° C., the resin may be decomposed, and the film properties such as workability deteriorate. The preferred baking temperature (maximum temperature) is 200-220 ° C.

By this baking, the blocking agent is dissociated from the blocked isocyanate in the coating film, and the generated free polyisocyanate reacts with the polyamine to form a urea resin. Therefore, the formed undercoat film (first layer film)
The urea resin produced by the reaction between the polyisocyanate and the polyamine has a structure in which the rust preventive pigment and the extender pigment, if present, are dispersed, and the amount of the rust preventive pigment in the coating film is 5 to 50 wt.
%.

On this undercoat film, a topcoat material is applied and baked to form an overcoat film (second layer film). As the top coating, a fluororesin coating excellent in weather resistance and durability is used. This paint may be the same as that conventionally used for a fluorine-based coated steel sheet. A typical example thereof is a paint containing polyvinylidene fluoride (PVDF) and an acrylic resin as a base resin. An example of a commercially available product of such a paint is Precolor No. 8800 made by NOF Corporation. It is preferable that the ratio of the fluororesin in the resin in the fluororesin paint is 70% by weight or more.

The overcoating material contains at least one selected from the group consisting of inorganic fillers and thermoplastic resin beads in an amount of 1 to 20% by weight, preferably 3 to 20% by weight, based on the total amount of nonvolatile components.
You may make it contain in the amount of 10 wt%. As a result, unevenness is formed on the coating film surface, the wear resistance of the coating film is improved, and the coating film is hardly damaged. Examples of inorganic fillers include glass fiber,
Alumina fibers, boron nitride, and the like are included. Although the resin type of the thermoplastic resin beads is not particularly limited, for example, acrylic resin, nylon, polyethylene resin and the like can be used. These fillers or beads (hereinafter simply referred to as fillers) preferably have an average particle size (average fiber length in the case of fibers) of about 3 to 50 μm.

[0035] In addition to the above, the top coat usually contains a coloring pigment. Solvents and other additives may be the same as those described for the undercoat. The extender can be used within a range that does not lower the processability.

The top coat can be applied in the same manner as the undercoat. The coating is performed so that the thickness of the coating film is 15 to 30 μm, preferably 20 to 25 μm. If the top coat is too thin, the hue will not be stable and the corrosion resistance and weather resistance will be insufficient.
If this coating film is too thick, the processability will decrease. After baking after painting, the maximum temperature is 200-240 ° C, preferably 220-240 ° C.
Perform at 240 ° C. The baking time depends on the temperature,
Usually, it is good to be in the range of 40 to 90 seconds. As a result, a top coat having a structure containing a fluororesin as a main component and, in some cases, the filler dispersed therein is obtained.

The thus obtained coated steel sheet of the present invention comprises, in order from the bottom, a zinc-based or aluminum-based plating layer, a base treatment layer (coating pretreatment layer), and a urea resin containing a rust-preventive pigment. It has a structure having four coating layers of a first coating layer and a second coating layer made of a fluorine-based resin which may contain a filler.

The resin coating layer is not limited to the above-mentioned first coating layer and second coating layer. If it is permissible to perform the coating three or more times, another coating layer may be formed above or below the first coating layer. However, in order to ensure weather resistance, the uppermost layer is a fluororesin coating film.
For example, an epoxy resin-based coating that has been used as a primer of a coated steel sheet was formed as a lowermost coating under a urea-based coating, or as an intermediate coating on a urea-based coating. It can also be a three-layer coating film.
In that case, the chromate-based rust-preventive pigment can be added to the epoxy resin-based coating film in addition to the urea resin-based coating film.

The at least two resin-based coatings described above
Although it may be formed on both sides of the base material plated steel sheet, it is usually formed on only one side. In this case, the opposite surface may be a plated surface, a ground treatment, or another resin-based coating.

The coated steel sheet of the present invention is excellent in weather resistance because the overcoat film is a fluororesin-based film, and is hard to be scratched because the undercoat film can be made thicker. The corrosion resistance of the cut end surface is also excellent due to the added rust-preventive pigment, and rust hardly occurs even if a scratch reaches the base material. Furthermore, it has excellent workability. Therefore, this coated steel sheet is particularly suitable for outdoor building materials (eg, roofing materials, wall materials), but for other applications used outdoors (eg, air conditioner outdoor units, and vending machine outer panels). Household appliances). In addition, it is naturally applicable to building materials used indoors and the like.

[0041]

[Example 1] Hot-dip galvanized steel sheet (sheet thickness 0.6 m
m, the plating weight is 90 g / m ^{2 on} one side)
After the degreasing treatment, both surfaces were treated with zinc phosphate using a commercially available treating solution (the amount of adhesion was 1 g / m ^{2 on} one surface).

The undercoat paint was polyamine (Lalomin, 3,3'-dimethyl-4,4'-diaminodicyclohexylmethane, manufactured by BASF) and blocked isocyanate (Desmodur BL1100, manufactured by Sumitomo Bayer Urethane, ε-caprolactam with a dissociation temperature of 170 ° C.). Blocked tolylene diisocyanate) was dissolved in cyclohexanone as a solvent at a ratio such that the ratio of amino groups to isocyanate groups was 1: 1.2, and the resulting resin solution was subjected to the ratio shown in Table 1 (based on the total amount of nonvolatile components). In addition, strontium chromate or zinc chromate (both made by Kikuchi Color Co., Ltd.) are added together with 5 wt% of the extender pigment silica to the total amount of non-volatile components.
It was prepared by uniformly dispersing by appropriate means.

On one side of the pre-treated hot-dip galvanized steel sheet, the undercoat was applied with a bar coater to a coating thickness of 40.
μm, and baked for about 60 seconds at the maximum temperature shown in Table 2 to form an undercoat film.

On this undercoat film, a commercially available weather-resistant fluororesin paint (Nippon Oil & Fat Co., Ltd.
No. 8800, the base resin was polyvinylidene fluoride and acrylic resin = 7: 3, containing white pigment) and a short glass fiber (average fiber length 30μm) added with 5wt% of the total non-volatile content. 20μm coating thickness with a bar coater
At the maximum temperature shown in Table 2
After baking for 2 seconds, a top coat was formed.

The performance of the coated steel sheet obtained was evaluated as follows, and the test results are also shown in Table 2. (1) Workability The coated steel sheet was subjected to 180 ° bending at 23 ° C. with the painted surface outside and bending. The occurrence of cracks in the portion was observed with a 10-fold loupe, and evaluated by the minimum number of sandwiched plates where no cracks were observed (T, 0T: close bending). 2T or less is good.

(2) Corrosion resistance A salt spray test specified in JIS Z 2371 was used. After a cross cut reaching the base of the plated steel plate was put on the test piece, the test piece was subjected to a salt spray for 500 hours, and the swollen width (maximum value) of the coating film from the cut portion was measured. If the blister width is 2 mm or less, the corrosion resistance is good.

(3) Moisture Resistance A cross-cut test piece reaching the base material of a plated steel sheet was exposed to a moist atmosphere of 50 ° C.-98% RH or more for 1000 hours, and then the coating film swollen due to the generation of rust in the cross-cut portion. Blister width
(Maximum value) was measured. If the blister width is 2 mm or less, the wet resistance is good.

(4) Scratch resistance Evaluation was made by making a scratch on the coating surface of the test piece using a diamond needle. While changing the load of the diamond needle, the needle was moved at a speed of about 1 cm / s, and the coating was peeled off, and the evaluation was made at the limit load at which the plated steel base material was exposed. The greater the load, the more the coating film is tough and harder to peel off. If this load is 100 g or more, the scratch resistance is good.

(5) Weather Resistance Using a sunshine weather meter, the black panel temperature was set to 63 ° C., the water spray time on the test piece was set to 12 minutes / 60 minutes, and the glossiness of the coating film after 3000 hours was measured. The evaluation was based on the gloss retention relative to the initial gloss. If the gloss retention is 90% or more, the weather resistance is good.

[0050]

[Table 1]

[0051]

[Table 2]

As can be seen from Table 2, since the undercoat was applied in a thick film of 40 μm in all the examples, the scratch resistance was excellent including the comparative examples. Furthermore, the coated steel sheet under the conditions according to the present invention had good workability of 0 to 2T, and was excellent in corrosion resistance and wet resistance. In addition, because the top coat is fluorine-based,
All weather resistance was good.

However, when the chromate-based rust preventive pigment was not added, the corrosion resistance was significantly deteriorated, and the wet resistance was lowered. Further, when the addition amount of the chromate-based rust-preventive pigment exceeded 50 wt%, the processability deteriorated. On the other hand, if the baking temperature of the undercoat or topcoat was too low, the processability was reduced and the scratch resistance was also slightly reduced. Even if the baking temperature of the undercoat or topcoat was too high, the workability decreased.

Example 2 Hot-dip galvanized steel sheet (sheet thickness 0.6
mm, the coating weight is 120 g / m ² ) on one side as a base material.After degreasing, both sides are chromated with a commercially available coating type chromate treatment solution (the coating weight is 40 in terms of metal Cr).
mg / m ² ).

As the undercoat paint, the paint of C shown in Table 1 of Example 1 was used, and this was applied to one side of a base material-plated steel sheet which had been subjected to a chromate treatment using a bar coater.
μm, and baked at a maximum temperature of 210 ° C. for about 60 seconds to form an undercoat film.

Thereafter, the same fluororesin paint as used in Example 1 except that the type and amount of the filler to be added was changed as shown in Table 3 was applied with a bar coater so that the coating film thickness became about 20 μm. It was applied and baked at a maximum temperature of 240 ° C. for about 60 seconds to form an overcoat. Three types of fillers were used: alumina fibers (average fiber diameter: 30 μm), boron nitride (particle diameter: 3 to 5 μm), and nylon beads (particle diameter: 5 to 10 μm). Each performance of the obtained coated steel sheet was evaluated in the same manner as in Example 1, and the results are shown in Table 3.

[0057]

[Table 3]

In the case where the overcoat film contains no filler,
Compared to o.1, it can be seen that the inclusion of the filler improves the scratch resistance, and in the case of boron nitride, the corrosion resistance and the wet resistance are also improved. However, when the filler was alumina fiber or boron nitride, the workability was reduced. This reduction in workability is hardly seen in glass fibers or thermoplastic resin beads. Also, when the filler was a thermoplastic resin bead, the weather resistance was somewhat reduced as the amount of addition increased.

[0059]

According to the present invention, an undercoating film is formed by blending a chromate-based rust-preventive pigment with an undercoating material based on a blocked isocyanate and a polyamine capable of forming a thick film. Even if scratches or the like occur, the scratches rarely reach the base material or plating, so that rust or the like from the scratches is less likely to occur. Further, the rust-preventive pigment ensures the corrosion resistance when the flaw reaches the steel plate or plated surface or at the cut end surface. Therefore, the durability is remarkably superior to that of the conventional coated steel sheet.

Further, by forming a coating film of a fluororesin paint as an overcoat, weather resistance is imparted and rust can be prevented even in a wet environment. Conventional fluororesin-based coated steel sheets have weaknesses in scratch resistance, and rusting is likely to occur from scratches during construction, but in the present invention, this weakness can be overcome by increasing the thickness of the undercoat coating film, Furthermore, the corrosion resistance was further improved by the rust preventive pigment. In addition, since neither the undercoat nor the overcoat is a vinyl chloride coating, there is no environmental concern.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Akito Yoshioka 4-33, Kitahama, Chuo-ku, Osaka City Inside Sumitomo Metal Industries Co., Ltd. (72) Inventor Sachio Matsuo 2nd Dejima Nishimachi, Sakai City, Osaka Sumitomo Metal Building Materials Co., Ltd. Sakai Works Co., Ltd. (72) Inventor Shigeyoshi Sugiyama 2nd Dejima Nishimachi, Sakai City, Osaka Sumitomo Metal Construction Materials Co., Ltd. F-term in Sakai Plant (reference) 4D075 AE03 BB21Y BB21Z BB93Y BB93Z CA02 CA32 DA06 DB07 DC02 EA02 EA17 EB16 EB32 EB38 EB56 EC13 EC15 EC24 EC54 4F100 AA00C AA00H AA22B AA22A ABABAAB ABABABAABAA EH462 EH71A EJ422 GB07 JB02 JB16C JB16H JL00 JL09 YY00B 4J038 DG061 DG261 EA012 KA08 MA02 NA03 PA07 P B05 PC02

Claims (4)

[Claims] 1. A chromate-based rust preventive of about 5 to 50% by weight in a urea resin comprising a reaction product of a polyamine and a polyisocyanate on at least one surface of a zinc-based or aluminum-based plated steel sheet which has been subjected to a coating pretreatment. Thickness containing pigment
Excellent durability and weather resistance, characterized by having a first coating layer having a thickness of 10 to 80 μm and a second coating layer having a thickness of 15 to 30 μm mainly composed of a fluororesin formed thereon. Painted steel plate. 2. The coated steel sheet according to claim 1, wherein the second coating layer contains at least one selected from the group consisting of inorganic fillers and thermoplastic resin beads in an amount of 1 to 20% by weight. 3. A zinc-based or aluminum-based plated steel sheet which has been subjected to a pre-coating treatment, containing a polyamine, a blocked isocyanate and a chromate-based rust-preventive pigment in an amount of 5 to 50% by weight based on the total amount of non-volatile components. A process of applying a paint to be applied and baking at a maximum temperature of 180 to 240 ° C, followed by a process of applying a fluororesin paint and baking at a maximum temperature of 200 to 240 ° C. Method for producing coated steel sheet with excellent heat resistance and weather resistance. 4. The coated steel sheet according to claim 3, wherein the fluororesin paint contains at least one selected from inorganic fillers and thermoplastic resin beads in an amount of 1 to 20% by weight based on the total amount of nonvolatile components. Manufacturing method.