JPH10193508A - Pre-coated steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a pre-coated steel plate without increasing cost and labour hours that is difficult to cause pressure marks and which has high workability. SOLUTION: In this pre-coated steel plate, a resin coating on the upper-most layer thereof is formed by coating a paint composition containing as essential ingredients 10 parts by weight of copolymer polyester resin having a glass transition temperature of less than 40 deg., 10 to 45 parts by weight of alkyl etherificated amino formaldehyde, 1 to 3 parts by weight of an acid catalyst and 1 to 5 parts by weight of organic resin beads having a Vickers hardness of 6 or more and a grain size 1 to 2 times larger than the thickness of the resin coating on the uppermost layer and thereafter baking the resin coating so formed in a heated air oven controlled so as to produce a wind velocity of 0.5 to 3.0m/sec on the plate surface.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a precoated steel sheet having excellent pressure mark resistance and workability, and a method for producing the same. The precoated steel sheet of the present invention is suitable for use in household electric appliances, indoor and outdoor equipment, and the like.

[0002]

2. Description of the Related Art A pre-coated steel sheet coated with a resin by applying a paint to the steel sheet in advance and drying or baking the coating film is formed immediately by a user without performing coating work which is problematic in terms of work and environment. Because it is made and has a beautiful glossy surface, it is often used in household electrical appliances used indoors such as refrigerators, washing machines and microwave ovens, and electrical appliances used outdoors such as air conditioner outdoor units and vending machines. Used. In the precoated steel sheet used for such applications, a relatively thick film having a smooth surface and a beautiful gloss is formed on the front side facing the outer surface, and a relatively thick film for imparting corrosion resistance is formed on the back surface. Often, a thin film is applied (therefore the surface smoothness of the coating is inferior).

[0003] Precoated steel sheets are usually steel strips (coils).
Or, they are shipped in cut sheets, but are generally stored for several days or months between production and use.
For example, in the case of a coil, it is often cut and stored with a coil weight of about 1 to 5 tons.
A pressure mark (transfer pattern in which the unevenness of the back surface coating is transferred to the front surface) is generated on the front side coating film under its own weight under the coil during storage. In the case of cut sheets, it is common to stack around 2 tons per lot, but also in this case, the front side coating film of the cut sheets stacked under the lot,
A similar pressure mark may occur. Hereinafter, a coating film (usually a front coating film) in which a pressure mark is a problem is simply referred to as a coating film.

[0004] The precoated steel sheet having the pressure mark has no particular problem in performance, but its commercial value is significantly reduced. This is because the pressure mark loses the beautiful gloss of the coating film originally possessed by the pre-coated steel sheet, and the pressure mark is clearly distinguished from the healthy part even to the naked eye, so strict appearance quality is required as in the above-mentioned applications. This is because it cannot be used for the intended use.

As a measure to prevent the pressure mark, there is a method of applying a protective film having a thickness of about 60 μm to the surface of the coating film of the pre-coated steel sheet. However, the cost is increased, and the protective film is removed after the forming process by the user and discarded. There is a drawback in that a time-consuming step of performing the process is added.

Although not a measure to prevent the occurrence of a pressure mark, there is a method of restoring the original gloss by heating after the generation of the pressure mark. However, this method also has a problem that the cost is increased due to the addition of the heating step, and the restoration of the gloss is not perfect.

[0007] Increasing the hardness of the coating film can fundamentally prevent pressure marks. However, this method is unavoidable because another important performance required for precoated steel sheets, that is, deterioration in workability, cannot be avoided. Can not be adopted.

JP-A-8-53646 discloses a polyester resin having a glass transition point of 40 to 65 ° C. and a number average molecular weight of 15,000 to 30,000, a cross-linking agent comprising a melamine resin or an isocyanate resin, and a resin having an average particle diameter of 2 to 50 μm. There is disclosed a coating composition for a precoated steel sheet having good pressure-mark resistance, comprising an organic resin powder and a lubricant. The pre-coated steel sheet formed from this coating composition does not have sufficient low-temperature workability and cannot reliably improve the resistance to pressure marks.

[0009]

As described above, it is difficult to achieve both workability and resistance to pressure marks, and in fact, satisfactory measures for preventing the occurrence of pressure marks have not yet been established.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of the conventional pressure mark countermeasure, to reduce the occurrence of pressure marks without increasing the cost and labor, and to improve the processability. It is an object of the present invention to provide a precoated steel sheet having the same and a method for producing the same.

[0011]

As described above, the decrease in gloss of the coating film due to the pressure mark is an increase in surface roughness resulting from the transfer of the unevenness of the back coating film. In order to prevent this, as described above, it is effective to increase the indentation hardness of the bulk (the entire coating film), but this method inevitably reduces the workability.

The present inventors have studied means for highly balancing indentation hardness and processability, and have found that a coating film obtained by curing a copolymerized polyester resin with an alkyl etherified aminoformaldehyde resin as a crosslinking agent is used. It has been previously proposed that a precoated steel sheet having excellent pressure mark resistance can be obtained by concentrating the surface of the steel (Japanese Patent Application No. 7-312678). However, even with this method, there is a slight decrease in workability, and the improvement in pressure mark resistance is not completely satisfactory.

Therefore, as a result of further study, a copolymerized polyester resin having a low glass transition point was used as a base resin, and in addition to the crosslinking agent, an acid catalyst and hard organic resin beads were blended. It has been found that the workability and the resistance to pressure marks can be further improved, and the above object can be achieved.

Here, the present invention is characterized in that the uppermost resin film provided on the steel plate is composed of 100 parts by weight of a copolymerized polyester resin having a glass transition point of less than 40 ° C., 10 to 45 parts by weight of an alkyl etherified aminoformaldehyde resin, 1 to 5 parts by weight of an acid catalyst, and organic resin beads 1 to 5 having a Vickers hardness of 6 or more and a particle size of 1 to 2 times the thickness of the uppermost resin film.
A pre-coated steel sheet having excellent pressure mark resistance and workability, characterized in that it is a baked film formed from a coating composition containing, as an essential component, parts by weight. In the present specification, all amounts of resin are weights based on solid content.

The Vickers hardness of an ordinary bake-curable polyester resin coating film is as low as about 4 or less. Inclusion of hard organic resin beads having a Vickers hardness of 6 or more in such a coating film is equivalent to adding solid columns, so that the strength of the entire coating film can be increased. Therefore, the incorporation of the hard resin beads leads to reinforcement of the indentation hardness of the coating film, thereby improving the pressure mark resistance.

The workability can be improved by using a polyester resin having a low glass transition point of less than 40 ° C. and excellent workability as a base resin. The crosslinking agent used in the present invention easily migrates to the outer surface side of the coating film during baking, and the crosslinking density on the outer surface side increases as compared with the inner surface side of the coating film. As a result, the crosslink density on the inner surface side does not increase so much, and the original good workability of the polyester resin is maintained. At the same time, since the indentation hardness increases on the outer surface side where the crosslink density increases, it is possible to improve the pressure mark property by adding organic resin beads while maintaining good workability. The surface concentration of the cross-linking agent can be controlled in an appropriate range by adjusting the air volume of a hot air oven used for baking.

Accordingly, from another aspect, the present invention provides a method for applying the above-mentioned coating composition, wherein the wind speed on the plate surface is 0.5 to 3.0.
A method for producing a precoated steel sheet having excellent pressure-mark resistance and workability, characterized in that a coating film is formed by baking a coating film in a hot-air oven controlled to m / sec to form an uppermost resin film.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. (1) Base Material Steel Sheet The type of steel sheet used as the base material of the precoated steel sheet of the present invention is not particularly limited. Surface-treated steel sheets such as cold-rolled steel sheets, galvanized steel sheets and aluminum-coated steel sheets are preferably used. Stainless steel plates can also be used.

(2) Undercoat Treatment Although the undercoat treatment is not essential, it is preferable to perform the undercoat treatment for the purpose of improving corrosion resistance and coating film adhesion. The undercoating treatment may be the same as that conventionally used for the precoated steel sheet, but usually includes a chromate treatment selected from a coating type chromate treatment, an electrolytic chromate treatment, and a reaction type chromate treatment, and / or a phosphate treatment. Will be implemented. In chromate treatment, 15 to 100 mg / m on one side in metal Cr equivalent
² is preferable, and in the case of phosphate (zinc phosphate) treatment, the amount of adhesion on one side is preferably 0.2 to 1.5 g / m ² .

(3) Undercoat Resin Film (Primer) The undercoat resin film is not always essential, but is preferably provided for improving adhesion, hiding, or protecting corrosion resistance. The dry film thickness is preferably about 1 to 10 μm.

The resin system used for the undercoat resin film is not particularly limited, but a polyester resin system, a polyurethane resin system, an epoxy resin system, or a combination system of these resins is preferable from the balance between workability and adhesion.

(4) Topcoat Resin Film The topcoat resin film can be formed from one or more resin layers. A coating composition comprising a copolymerized polyester resin, an alkyl etherified aminoformaldehyde resin, an acid catalyst, and organic resin beads as defined in the present invention; Formed from Normally, the top coating resin film is 1
Only a layer (ie, a baked film formed from the coating composition as defined in the present invention) is sufficient. Thus, by forming the uppermost resin film from the coating composition specified in the present invention, a precoated steel sheet having both workability and pressure-mark resistance can be obtained.

Copolyester Resin As the base resin of the coating film, a copolyester resin having excellent workability and having a glass transition point of less than 40 ° C. is used. The copolyester resin is a polyester resin synthesized from a polybasic acid and a polyhydric alcohol, and various types of copolyester resins are manufactured and commercially available.

Japanese Patent Application Laid-Open No. 8-53646 describes that when the glass transition point of the polyester resin is lower than 40 ° C., the workability is good, but the pressure mark resistance is poor. In the present invention, by using an acid catalyst in combination with a crosslinking agent, the crosslinking reaction is promoted, and the cross-linking density on the outer surface side of the coating film is increased by a gradient function described later, whereby the polyester having a glass transition point of less than 40 ° C By using a resin, good pressure mark resistance can be ensured.

The glass transition point of the copolymerized polyester resin is
If the temperature is as high as 40 ° C. or higher, workability, particularly low-temperature workability required for processing in winter, etc., is insufficient. Conversely, if the glass transition point is extremely low, blocking (a phenomenon in which painted surfaces stick to each other) may occur. Therefore, the glass transition point of the copolymerized polyester resin is preferably 0 ° C. or higher.

The number average molecular weight of the copolymerized polyester resin is not particularly limited, but the preferred number average molecular range is
9,000-25,000. If it is less than 9,000, workability may be insufficient, and if it exceeds 25,000, the viscosity of the paint will increase,
Paintability decreases. It is more preferably from 9,000 to 15,000 from the viewpoints of workability and coatability.

Alkyl-etherified aminoformaldehyde resin (crosslinking agent) Alkyl-etherified aminoformaldehyde resin which has self-condensability and easily causes surface concentration is used as a crosslinking agent which reacts with the copolymerized polyester resin in the baking step. Other crosslinking agents, for example, isocyanate compounds, do not have self-condensing properties, do not easily exhibit a gradient function due to surface concentration, and are unsuitable for the purpose of the present invention.

Examples of the alkyl etherified aminoformaldehyde resin include alkyl etherified methylol melamine resins, for example, methoxylated methylol melamine, methoxylated butoxylated mixed methylol melamine, and methoxylated melamine or imino group having a relatively small amount of alkyl etherification. Melamine or the like is used, and these may be used alone or in combination of two or more.

When a coating film of a coating material in which this alkyl etherified amino formaldehyde resin is blended with a copolymerized polyester is baked and cured, the alkyl etherified amino formaldehyde having a relatively small surface free energy is obtained due to the difference in surface free energy and compatibility between the two. The formaldehyde resin tends to migrate toward the surface (outer side) of the coating film, which becomes hotter during baking, and concentrate on the surface.

The alkyl etherified aminoformaldehyde resin used as a crosslinking agent has a self-condensing property. (Completely methoxylated methylolmelamine itself has a small self-condensing property, but the self-condensation reaction is accelerated by using an acid catalyst in combination. Therefore, the alkyl etherified amino formaldehyde resin concentrated on the surface can be cross-linked by cross-linking between cross-linking agents even if the amount of the copolyester resin is low and the condensation reaction cannot be performed with the resin. The crosslink density in the vicinity increases. In this manner, the coating film is provided with a gradient function in which the resin composition (the ratio of the polyester resin to the crosslinking agent) and the crosslinking density change in the thickness direction of the coating film.
As described above, if the cross-linking agent does not have self-condensability, the tilt function is not provided much.

Due to the dense crosslinked structure near the surface,
A gloss change such as a pressure mark can be suppressed. On the other hand, since the concentration of the cross-linking agent is low inside the coating film, the cross-linking of the copolyester resin does not occur so much, and the good processability of the copolyester resin is maintained.

The alkyl etherified aminoformaldehyde resin as a crosslinking agent is used in an amount of 10 to 45 parts by weight, preferably 15 to 40 parts by weight, more preferably 20 to 30 parts by weight, based on 100 parts by weight of the copolymerized polyester resin. use. If this amount is less than 10 parts by weight, secondary adhesion and corrosion resistance will be reduced due to insufficient crosslinking, and if it exceeds 45 parts by weight, workability will be reduced.

Acid Catalyst An acid catalyst is added to promote self-condensation of the alkyl etherified aminoformaldehyde resin as a cross-linking agent, and to exhibit the above-mentioned gradient function to a higher degree. Thereby, even when using an alkyletherified aminoformaldehyde resin having a small self-condensation property such as fully methoxylated methylol melamine, the gradient function by the self-condensation, that is, the internal concentration of the crosslinking agent and the self-condensation, While keeping the crosslink density low, the crosslink density near the surface can be increased, and the effect of suppressing the gloss change due to the pressure mark can be reliably exhibited.

As the acid catalyst, strong organic acids, especially sulfonic acids such as p-toluenesulfonic acid and dodecylbenzenesulfonic acid are preferred. The amount of the acid catalyst is in the range of 1 to 3 parts by weight, preferably 1.5 to 2.5 parts by weight, based on 100 parts by weight of the copolymerized polyester resin. If the amount is less than 1 part by weight, self-condensation of the alkyl etherified aminoformaldehyde resin is unlikely to occur, and the surface thickening amount may be insufficient to reduce the pressure mark resistance. If the amount exceeds 3 parts by weight, crosslinking may occur. Progresses too much, resulting in a reduction in workability.

Organic Resin Beads Hard organic resin beads having a Vickers hardness of 6 or more are contained in the coating in order to reinforce the entire coating and improve the resistance to pressure marks. Here, the Vickers hardness is the hardness defined by the load and the surface area of the depression when a diamond indenter having a regular square pyramid (136 ° facing angle) is pushed into the sample, and in the present invention, the load is 10 g. The values measured are used.

Even if resin beads having a Vickers hardness of less than 6 are used, a sufficient effect of improving the pressure mark property cannot be obtained. The Vickers hardness of the resin beads is preferably 6.
5 or more.

The material of the organic resin bead is not particularly limited as long as it satisfies the condition of hardness, but may be acrylic, polyurethane, polyester, polyamide, polyethylene, polypropylene, silicone, polyvinylidene fluoride, or the like. Examples thereof include polyfluorotetrafluoroethylene-based resins.

The shape of the beads is not particularly limited, but a spherical shape or a shape close thereto is preferable. The particle size of the beads is desirably about the same as or slightly larger than the thickness of the uppermost layer film containing the beads. When the thickness is smaller than the thickness of the uppermost film, the above-mentioned reinforcing effect is insufficient, and when the thickness is extremely large, the adhesiveness is insufficient and the film tends to fall off. Therefore,
The particle size of the beads is 1 to 2 times the film thickness of the top layer film,
In particular, it is preferably 1 to 1.5 times.

The amount of beads added is 1 to 5 parts by weight, preferably 2 to 4 parts by weight, per 100 parts by weight of the copolymerized polyester resin.
Parts by weight. If the amount is less than 5 parts by weight, the resistance to pressure mark is insufficient, and if it exceeds 5 parts by weight, deterioration of workability is inevitable.

The coating composition used for forming the uppermost film is as follows:
In addition to the above-mentioned essential components, other components commonly used in paints may be contained. Such other components include a solvent,
Pigments, other resins, defoamers, surface hardeners, matting agents,
And a wax component. Usually, solvents and pigments are always used.

Examples of suitable solvents include ketone solvents such as cyclohexanone, isophorone and N-methyl-2-pyrrolidone, Solvesso and the like, which can dissolve both the copolymerized polyester resin and the alkyl etherified aminoformaldehyde resin. Those that do not substantially dissolve the organic resin beads to be used are selected.

(5) Coating baking method The coating method of the above-mentioned coating composition for forming the uppermost layer film may be any method that can be used for producing a precoated steel sheet. For example, a roll coating method, a curtain flow coating method, a spray method, or the like can be used. The film thickness of the top layer film is
The dry film thickness is preferably in the range of 5 to 30 μm, particularly 7 to 25 μm. Usually, coating is performed only on one side, and another simple coating (service coat, for example, only the above-described primer) is often applied on the back side, but the coating composition may be applied on both sides.

The baking after painting is performed in a hot air oven (ie,
Hot air heating), and the speed of the hot air at that time,
It is possible to control the amount of the alkyl etherified aminoformaldehyde resin concentrated on the surface of the uppermost film. Specifically, the hot air velocity measured on the plate surface is 0.5 to 3.0 m / sec.
It is preferable to perform baking in a hot air oven under the following conditions.

If the wind speed is less than 0.5 m / sec, the amount of the alkyl etherified aminoformaldehyde resin concentrated on the surface is small, and the pressure mark resistance may be insufficient.
On the other hand, if the wind speed exceeds 3 m / sec, the surface concentration of the alkyl etherified aminoformaldehyde resin becomes excessive, and the surface cross-linking proceeds excessively, making the resin brittle, and the workability may be reduced.

The measurement of the wind speed on the plate surface can be performed by using a conventional measuring device such as a hot wire current meter, which is a kind of thermal current meter, or a thermistor current meter. The baking temperature may be the same as the baking temperature of a normal pre-coated steel sheet, and is not particularly limited, but PMT (maximum reached plate temperature) 200 to 250 ° C
Often done on a degree.

[0046]

Example: A hot-dip galvanized steel sheet having a thickness of 0.6 mm (amount of coating: 60 g / m ^{2 on} one side) was used as a base steel sheet, the surface of which was cleaned by an ordinary method, and immersed in a zinc phosphate solution as a base treatment. As a result, a zinc phosphate conversion coating (a zinc phosphate coating weight of 0.8 g / m ² ) was formed. Then, as a primer, a primer paint PB10P manufactured by Dainippon Ink and Chemicals was used and applied to one side of the steel sheet with a roll coater so that the dry film thickness became 7 μm, and the maximum temperature of the steel sheet (PMT) reached 210 ° C. To be
Bake hardened in 50 seconds. On this undercoat resin film,
A precoated steel sheet was produced by forming an overcoat resin film as described below.

The coating composition used for forming the overcoat resin film was prepared by appropriately adding the crosslinking agent shown in Table 2, the acid catalyst shown in Table 3, and the resin beads shown in Table 4 to 100 parts by weight of the copolymerized polyester resin shown in Table 1. They were prepared by adding them in the amounts (parts by weight) shown in Table 5 or Table 6 in combination. In Tables 1, 2 and 4, those marked with * are comparative materials outside the scope of the present invention. This coating composition contained cyclohexanone as a solvent and titanium oxide as a pigment.

The coating composition was applied by a roll coater so that the dry film thickness became 18 μm, and the
(Plate surface) was placed in a hot-air oven controlled at 0.3 to 6 m / s, and baked and hardened so that the PMT would be 230 ° C. in a baking time of 50 seconds to obtain a precoated steel sheet.

[0049]

[Table 1]

[0050]

[Table 2]

[0051]

[Table 3]

[0052]

[Table 4]

Using the test pieces of the prepared precoated steel sheets, their performance was evaluated in the following manner. The test results are also shown in Tables 5 and 6.

(1) Pressure Mark Resistance Another pre-coated steel sheet (B) was prepared by assuming that the coating surface of the test piece of the pre-coated steel sheet (A) prepared in the example was the reverse side of the pre-coated steel sheet. ) Was brought into surface contact with the coating film of the test piece under pressure to evaluate the resistance to pressure marks. The pre-coated steel sheet B is coated with a base material (zinc phosphate-treated sheet) in the same manner as described above on the same base steel sheet, and then dried using an epoxy-based paint used for painting the back surface (service coat) of the pre-coated steel sheet. And painted and baked.

The precoated steel sheets A and B were overlaid so that the coating surfaces of the test specimens of the same dimensions were in surface contact with each other, and 100 kg at 40 ° C.
/ cm ² × 24 hours, and the surface of the coating film of the test piece of the precoated steel sheet A after the hot pressing was visually observed, and the occurrence of the pressure mark was evaluated according to the following criteria.

：: No pressure mark was generated at all, ：: Pressure mark was generated but very slight, Δ: Moderate pressure mark was generated, ×: Pressure mark was significantly generated.

(2) Bending workability A test piece of a precoated steel sheet was subjected to 180 ° bending while changing the number of sandwiched sheets with the coating surface outside, and the bent portion was observed with a 10-fold loupe. To determine the presence or absence of cracks in the coating film. The bending workability was indicated by the minimum number of sandwiched sheets where no crack was observed.

For example, 0T indicates that close bending is possible, and 1T
Means that the number of sandwiched plates is one, that is, the interval between
This indicates that bending can be performed without cracking until a sheet having the same thickness is obtained. Therefore, the larger the displayed numerical value, the lower the workability. The bending workability depends on the temperature, and generally tends to decrease at low temperatures. Here, the bending test was performed at 0 ° C. (ice temperature).

[0059]

[Table 5]

[0060]

[Table 6]

As can be seen from Table 5, the precoated steel sheet of the present invention does not show any pressure mark.
Even if it occurred, it was very slight, excellent in resistance to pressure mark, and at the same time, the bending workability was good within 3T, and often as good as 1T or 2T.

On the other hand, the comparative examples shown in Table 6 show that the acid catalyst was not added (No. 35) or the amount of the alkyl etherified aminoformaldehyde resin or resin bead as the crosslinking agent was too small. (Nos. 39 and 41), when the Vickers hardness of the resin beads was too small (No. 44), the pressure mark resistance was reduced. On the other hand, whether the compounding amount of the acid catalyst, the crosslinking agent, or the resin beads is too large (No. 36, 40, 42)
On the other hand, when the glass transition point of the copolymerized polyester resin was higher than 40 ° C. (No. 43), the processability was lowered. In addition, when another crosslinking agent was used, improvement in pressure mark resistance could not be obtained.

In Test No. 45, since the particle size of the resin beads was too large, and the beads fell off, the film properties were not evaluated. Even if the particle size of the resin beads is too small (No. 4
6), the resistance to pressure mark was reduced. If the oven wind speed during baking after application is too low (No. 37), the surface concentration of the crosslinking agent does not sufficiently occur, and the pressure-mark resistance tends to decrease.If the wind speed is too high (No. 37). 38),
The surface concentration of the cross-linking agent occurred too much, and the workability was slightly insufficient.

[0064]

According to the precoated steel sheet of the present invention, a self-condensable crosslinkable agent capable of concentrating the surface and an acid catalyst are combined with a hard organic resin bead on a copolyester resin having good workability and a low glass transition point. By forming the uppermost layer from the coating composition blended in the above, it is possible to almost completely prevent the occurrence of a pressure mark, which is a drawback of this type of precoated steel sheet, without reducing workability. Therefore, the precoated steel sheet of the present invention is required to have a beautiful glossy appearance, and when the gloss is reduced, the commercial value is greatly impaired.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI C09D 161: 32)

Claims (2)

[Claims] 1. The resin coating of the uppermost layer provided on a steel plate,
Copolyester resin 100 with glass transition point less than 40 ° C
Parts by weight, 10 to 45 parts by weight of an alkyl etherified aminoformaldehyde resin, 1 to 3 parts by weight of an acid catalyst, and an organic resin having a Vickers hardness of 6 or more and a particle size of 1 to 2 times the thickness of the uppermost resin film. A precoated steel sheet having excellent pressure mark resistance and workability, characterized in that it is a baked film formed from a coating composition containing 1 to 5 parts by weight of beads as an essential component. 2. After applying the coating composition according to claim 1, the coating film is baked by a hot air oven controlled so that the wind speed on the plate surface is 0.5 to 3.0 m / sec, and the uppermost resin layer is formed. A method for producing a precoated steel sheet having excellent pressure mark resistance and workability, characterized by forming a film.