JPH09150480A - Precoated steel panel excellent in pressure mark resistance and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inexpensively produce a precoated steel panel good in processability and pressure mark resistance without requiring an excessive process. SOLUTION: Topcoat paint containing copolyester with a number average mol.wt. of 8000-25,000 and an alkyl etherified aminoformaldehyde resin (AEAFA resin) in a wt. ratio of 90/10-70/30 is applied to a steel panel and the formed coating film is baked by hot air drying controlled so that the wind velocity on the surface of the steel panel becomes 0.5-3.0m/sec to form the uppermost resin layer of a precoated steel panel. This resin layer has the AEATA resin conc. on the surface thereof so that the concn. of the surface AEAFA resin calculated by formula [wherein N1 is the N-concn. (at%) of the uppermost layer measured by an X-ray photoelectronic spectral method of the resin layer and N0 is the N-concn. (at%) of the uppermost layer of a resin layer composed only of the AEAFA resin] becomes 30-80%.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a precoated steel sheet having excellent pressure mark resistance and a method for producing the same,
More specifically, the present invention relates to a precoated steel sheet having excellent pressure mark resistance that can be used for household electric appliances, indoor / outdoor appliances, and the like, and a manufacturing method thereof.

[0002]

2. Description of the Related Art A precoated steel sheet coated with a resin by coating a coating on the steel sheet in advance and drying or baking the coating film allows a user to immediately perform a forming process without performing a coating operation which causes a lot of problems in work and environment. Because it is made and its surface is beautiful, it is widely used for home appliances such as refrigerators, washing machines, and microwave ovens used indoors, and outdoor appliances such as air conditioner outdoor units and vending machines. It's starting to happen.

[0003] Precoated steel sheets are usually steel strips (coils).
Or it is shipped as a cut board, but until it is used after manufacturing,
It is generally stored for days or months. For example, in the case of a coil, it is often cut and stored at a coil weight of about 1 to 5 tons, but when it is rewound during use, the coating film on the front side of the part under the coil's own weight during storage is , Pressure marks (transfer pattern in which unevenness of the back surface coating film is transferred to the surface) may occur. In the case of cut plates, it is common to stack around 2 tons per lot, but in this case as well, similar pressure marks may occur in the front coating film of the cut plates that were stacked under the lot. is there.

The precoated steel sheet having the pressure mark has no particular problem in terms of performance, but the beautiful gloss inherent to the precoated steel sheet is lost, and it can be clearly distinguished from the healthy portion by the naked eye, and therefore, it cannot be used in home appliances. It cannot be used for applications with severe appearance quality. Therefore, the commercial value of the precoated steel sheet is significantly reduced.

As a measure for preventing the pressure mark, there is a method of coating the surface of the coating film of the precoated steel sheet with a protective film having a low thickness of 60 μm, but this is costly and the protective film is removed after the molding process by the user. However, there is a drawback that a time-consuming process is added.

Although not a countermeasure for preventing pressure marks, there is a method of restoring the original gloss by heating after the pressure marks are generated. However, even in this method, there is a problem that the cost is increased due to the addition of the heating step.

On the other hand, as a measure for preventing the occurrence of pressure marks by improving the coating film of a precoated steel sheet, Japanese Patent Laid-Open No. 59-
No. 118452 proposes a method of increasing the Tg (glass transition point) of the front surface coating film by 10 ° C. or more than that of the back surface coating film. However, by simply increasing the Tg of the coating film, deterioration of workability can be avoided. It is difficult to adopt because it is not possible.

In Japanese Patent Laid-Open No. 5-185030, Tg is provided on the back side.
Of 0 ° C. or higher, lower than the Tg of the surface coating film, and having a thickness of 3 to 40 μm, a clear coating film (containing no pigment) is disclosed. This method is to absorb and relieve the pressure applied to the surface coating film by the clear coating film, and this effect is reduced when a pigment is added, so that the method is limited to the clear coating film. However, on the back side of an actual pre-coated steel sheet, a colored coating of about 5 μm (containing a pigment) called a service coat is usually provided, and a clear coating exposes the underlying color,
Users tend to be shunned. Further, applying a clear coating film on the upper layer of the service coat, as described in this publication, results in high cost.

In Japanese Unexamined Patent Publication (Kokai) No. 6-170334, the difference in surface roughness between the front surface and the back surface is 0.7 μm or less in terms of center line average roughness Ra, and the coating film on the back surface is a nonmetallic compound having a Mohs hardness of 4 or less. At a ratio of 20 wt% or more with respect to the total pigment of the coating film,
Further, a precoated steel sheet has been proposed, which is characterized in that the back coating film has a thickness of 3 to 20 μm. This is intended to prevent the occurrence of pressure marks by the two points of reducing the difference in roughness between the front and back coating films and the cushioning effect due to the addition of the soft compound. With regard to (1), since the unevenness of the back surface coating film is transferred by the pressure mark, reducing the difference in roughness between the front surface and the back surface has some effect, but it does not lead to an essential improvement of the coating film. Further, as the soft non-metallic compound, examples of extender pigments such as calcium carbonate, kaolin, and talc are mentioned. However, when a large amount of such a soft material is contained in the coating film, the coating film hardness is naturally increased. It is difficult to say that it is a perfect countermeasure, because it will decrease and the defects such as scratches will easily occur.

[0010]

As described above, the present situation is that a method for preventing the occurrence of pressure marks has not yet been satisfactorily obtained. An object of the present invention is to solve the above-mentioned problems of the conventional countermeasures / improvement methods for pressure marks, and to suppress the occurrence of pressure marks without increasing cost and labor, pressure mark resistance. To provide an excellent precoated steel sheet and a method for producing the same.

[0011]

Means for Solving the Problems The present inventors have made extensive studies to achieve the above object. The decrease in gloss due to the pressure mark is caused by the change in the unevenness (surface roughness) of the front surface coating film due to the transfer of the unevenness of the back surface coating film as described above. To prevent this from the viewpoint of coating film physical properties, use the bulk
A method of increasing the indentation hardness of the entire coating film is generally used, but this method cannot avoid deterioration in workability.

The inventors of the present invention have made extensive studies as to a method for highly balancing surface indentation hardness and workability, and have found that this can be realized by providing a so-called tilt function in the thickness direction of the coating film. . That is, in the topcoat (topcoat) coating film that is the uppermost layer, the outermost surface layer has a higher crosslink density to have a structure that is hard enough to prevent the occurrence of pressure marks.
If the inner layer of the top coat on the side close to is made to have a resin structure that is relatively soft and has a good workability, a decrease in workability can be avoided.

The present invention based on this finding is the following precoated steel sheet and a method for producing the precoated steel sheet. The uppermost resin layer is a precoated steel sheet containing a copolyester resin having a number average molecular weight of 8,000 to 25,000 and an alkyl etherified aminoformaldehyde resin (abbreviated as AEAFA resin) in a weight ratio of 90/10 to 70/30. Therefore, the composition of the outermost layer of this resin layer measured by X-ray photoelectron spectroscopy was 30 to 80% in surface AEAFA resin concentration calculated by the following formula, and was excellent in pressure mark resistance. Precoated steel sheet.

[0014]

(Equation 2)

In the formula, N ₁ is the N concentration (at%) of the outermost layer of the resin layer measured by X-ray photoelectron spectroscopy, and N ₀ is AEAFA.
It is the N concentration (at%) of the outermost layer measured by X-ray photoelectron spectroscopy of the resin layer made of only the resin.

A precoated steel sheet for forming a resin layer of the uppermost layer from a coating material containing a copolymerized polyester resin having a number average molecular weight of 8,000 to 25,000 and an alkyl etherified aminoformaldehyde resin in a weight ratio of 90/10 to 70/30. In the manufacturing method, after applying the coating material, the coating film is baked by hot air drying in which the wind speed on the plate surface is controlled to be in the range of 0.5 to 3.0 m / sec. Excellent pre-coated steel sheet manufacturing method.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The constitution of the present invention will be described in detail below.

(1) Base material steel sheet The type of steel sheet used as the base material of the precoated steel sheet is not particularly limited. Cold-rolled steel sheets, surface-treated steel sheets such as zinc-based plated steel sheets and aluminum-based plated steel sheets are preferably used.

(2) Substrate treatment There is no particular limitation as long as it can be used for the pretreatment of ordinary pre-coated steel sheet, but usually, coating type chromate treatment, electrolytic chromate treatment, reactive type chromate treatment, phosphate treatment and the like are used. It is often done. At this time, if chromate treatment is applied, the amount of adhered metal Cr is 15 to 100 mg / m on one side.
² is preferable, and if it is a phosphate (zinc phosphate) treatment,
The amount of adhesion on one side is preferably 0.2 to 1.5 g / m ² .

(3) Undercoat resin coating layer (primer layer) The undercoat resin coating layer is not always essential, but is preferably provided for improving adhesion, hiding power, and ensuring corrosion resistance. The resin system used for the undercoat resin coating layer is not particularly limited, but a polyester resin system, a polyurethane resin system, an epoxy resin system or a mixture of these resin systems is preferable in view of the balance between processability and adhesion.

(4) Topcoat resin coating layer (uppermost resin layer) In order to prevent the generation of pressure marks, which is the object of the present invention, the physical properties of the topcoat resin coating layer which is the uppermost resin layer are important. In the present invention, as the base resin constituting the uppermost layer, using a copolymerized polyester resin excellent in processability,
As the cross-linking agent, an alkyl etherified aminoformaldehyde resin having self-condensation property and capable of surface thickening is used in combination, so that the uppermost resin layer has a gradient composition in which the cross-linking agent is thickened on the surface side. Thereby, the surface of the uppermost resin layer has a high crosslink density and a sufficiently high indentation hardness to prevent the occurrence of pressure marks, but the inside of this resin layer has a low crosslink density and is relatively soft, The excellent processability of the copolyester resin is retained.

Copolymerized Polyester Resin The copolymerized polyester resin used as the base resin for the uppermost coating film in the present invention is a polymer obtained by polycondensation of a polybasic acid component and a polyhydric alcohol component. As the polybasic acid component, terephthalic acid, isophthalic acid, phthalic acid, adipic acid, succinic acid, sebacic acid, maleic acid, fumaric acid, dicarboxylic acids such as cyclohexanedicarboxylic acid,
And ester-forming derivatives thereof (lower alkyl ester, acid anhydride, etc.). Polyhydric alcohols include ethylene glycol, 1,2-propanediol,
Examples thereof include 1,3-propanediol, 1,4-butanediol, neopentyl glycol, 1,6-hexanediol and diethylene glycol. Each component may be used alone or in combination of two or more. If desired, a polybasic acid having a valence of 3 or more (eg, trimellitic acid, pyromellitic acid, etc.) and / or a polyhydric alcohol having a valence of 3 or more (eg, pentaerythritol, trimethylolpropane, glycerin, etc.) can also be used.

The number average molecular weight of the copolyester resin is
Use 8,000 to 25,000. Number average molecular weight is 8,000
When it is less than 0, the processability of the resin film is lowered, and when it is more than 25,000, the viscosity of the paint is increased and the paintability is deteriorated. The number average molecular weight of the copolymerized polyester which is preferable from the viewpoint of processability and coatability is 10,000 to 20,000.

Alkyl etherified amino formaldehyde resin (cross-linking agent) An alkyl etherified amino formaldehyde resin (hereinafter abbreviated as AEAFA resin) is used as a cross-linking agent for curing the resin film by condensation reaction with the copolyester resin during baking. To do.

As the AEAFA resin, an alkyl etherified methylol melamine resin, a methoxylated methylol melamine, a methoxylated butoxylated mixed methylol melamine, and a methoxylated melamine or an imino group type melamine having a relatively small amount of alkyl etherification are used. These may be used alone or in combination of two or more.

Therefore, the coating material used for forming the uppermost resin layer is a copolyester resin of the base resin and A of the crosslinking agent.
It contains EAFA resin as an essential component. The ratio of copolymerized polyester resin / AEAFA resin is 90/10 to 70/30, preferably 85/15 to 75/25 in terms of solid content weight ratio.
And That is, based on the total amount of both, AEA of the cross-linking agent
The concentration of FA resin is 10 to 30% by weight as the average value of the entire resin layer.
Becomes If the proportion of the AEAFA resin is less than 10% by weight, the degree of crosslinking of the entire resin layer will be insufficient, resulting in a decrease in secondary adhesion and corrosion resistance. On the other hand, the proportion of AEAFA resin is 30% by weight
If it exceeds, the cross-linking agent becomes excessive, the hardness of the entire resin layer becomes high, and the workability deteriorates.

The paint used is a copolyester and A
In addition to the EAFA resin, it may contain other components commonly used in paints. Examples of such components include solvents, pigments, condensation reaction catalysts, other resins, defoaming agents, leveling agents, surface hardening modifiers, delustering agents, wax components, and the like. The solvent is preferably an organic solvent capable of dissolving both the copolyester resin and the AEAFA resin, and examples of suitable solvents include cyclohexanone, isophorone, sorbeso, N-methyl-2.
-Such as pyrrolidone. As the condensation reaction catalyst, an acid catalyst (eg, organic acid such as p-toluenesulfonic acid) is preferable. When another resin is used in combination, its type and amount are selected so that the surface AEAFA resin concentration described later can be obtained after baking and curing.

When an AEAFA resin is mixed with a copolyester resin and applied, and the coating film is baked and cured, the AEAFA resin having a relatively small surface free energy tends to migrate to the surface of the coating film during baking and become concentrated. There is. Also, A
The EAFA resin has a self-condensation property and can be crosslinked and cured by the condensation reaction between the AEAFA resins without the condensation reaction with the copolyester resin.

Due to the self-condensation property of this AEAFA resin,
During the baking of the coating film, A due to the above difference in surface free energy
The surface concentration of the EAFA resin becomes stronger, and the gradient function that the resin composition (the ratio of the polyester resin and the cross-linking agent) changes in the thickness direction of the coating film (the concentration of the AEAFA resin decreases from the surface to the inside) It is applied to the coating film. Other cross-linking agents such as isocyanate compounds do not have self-condensation properties, so the cross-linking agent must always react with the copolyester resin, and even if surface thickening occurs, the degree is small and It is difficult to develop the tilt function.

Thus, the AEAF used as the crosslinking agent
Resin A is self-condensable (fully methoxylated methylol melamine itself has a small self-condensation property, but the self-condensation reaction is promoted by the combined use of an acid catalyst), and the surface free energy is a copolyester. Since it is smaller, the cross-linking agent AEAFA resin is concentrated on the surface of the uppermost resin layer during baking and curing, so that the cross-linking density near the surface becomes high. Due to this dense cross-linking structure, only the vicinity of the surface of the resin layer is selectively made to have high hardness, and it is possible to suppress the generation of pressure marks due to the transfer of the unevenness of the back surface coating film. But,
On the inside of the uppermost resin layer, on the contrary, the concentration of the AEAFA resin as the cross-linking agent becomes low, and the copolyester is appropriately cross-linked by the AEAFA resin, so that secondary adhesion and corrosion resistance are secured while maintaining processability. To be done.

The surface concentration of the AEAFA resin is set to such an extent that the surface AEAFA resin concentration calculated by the following formula is 30 to 80%, preferably 40 to 60%. If this concentration is less than 30%, the cross-linking density (hence, indentation hardness) on the surface of the uppermost resin layer does not reach a level sufficient to prevent the occurrence of pressure marks. On the other hand, if this concentration exceeds 80%, the surface becomes brittle and becomes susceptible to cracking. The degree of surface concentration of the AEAFA resin can be controlled by the wind velocity of hot air during baking by hot air drying, as described later.

[0032]

(Equation 3)

In the formula, N ₁ is the N (nitrogen) concentration in the outermost layer of the resin layer measured by X-ray photoelectron spectroscopy, and N ₀ is AEAF.
It is the N concentration (the unit is at% in each case) of the outermost layer measured by X-ray photoelectron spectroscopy of the resin layer made of only the A resin. That is, the surface AEAFA resin concentration defined by the above equation means the occupation ratio (at% basis) of the AEAFA resin on the surface of the uppermost resin layer. As described above, the AE of the uppermost resin layer
The AFA resin concentration is 10 to 30% by weight on average. AEAF
No surface thickening of A resin (composition is uniform in thickness direction)
Assuming that, the AEAFA resin concentration defined by the above equation corresponding to the AEAFA resin concentration of 10 to 30% by weight is
It should be 10-30% calculated. Therefore, in the present invention, the concentration of the AEAFA resin in the outermost layer is higher than the average value of the entire resin layer, and the surface of the AEAFA resin is concentrated.

The X-ray photoelectron spectroscopy (XPS) measurement was carried out using ESCA3MKII manufactured by VG. The X-ray source is AlKα (hν = 1486.6 eV), and the analyzer has a binding energy of Au4f7 / 2 of pure gold of 83.7 eV (half-width 1.25 eV).
Calibrated as. Since the coating film is an insulator, it causes an increase in binding energy (charge up) during XPS measurement. For the correction, the binding energies of C1s and N1s, which were calibrated with Au 4f7 / 2 binding energy of 83.7 eV, were used by depositing gold on the individual coatings of the copolyester resin and the AEAFA resin, respectively. That is, polyester C--H group C1s
The XPS spectrum of the sample was corrected by setting the binding energy to 284.7 eV and the N1s binding energy of the AEAFA resin to 399.3 eV.

The measurement was carried out by fixing the photoelectron take-off angle, which is the angle between the sample surface and the detected photoelectrons, to 70 °. For the detection depth 3λ sin θ, C1s for kinetic energy of about 1200 eV in polyethylene by Ashley
Using the calculated value of the inelastically scattered electron mean free path λ of photoelectrons of 4 nm, it is estimated to be 11.3 nm.

The nitrogen concentration (at%) was calculated from the following formula from the measured area intensity of the XPS spectrum. C _N = (I _N / S _N ) / Σ (I _i / S _i ), where C _N is the atomic concentration of nitrogen, I _N is the peak area intensity of nitrogen, S _N is the relative sensitivity coefficient of nitrogen, I _i Is the peak area intensity of each element (C, N, O) other than hydrogen present in the resin layer, and S _i is the relative sensitivity coefficient of each element. The relative sensitivity coefficient of each element is [C] 1s: 1.0, [O] 1s: 2.6l,
[N] 1s: 1.71 was used.

(5) Coating Baking Method The coating method of the above coating material is not particularly limited as long as it is a method used in the production of ordinary precoated steel sheets. Any of a roll coating method, a Ringer roll method, a curtain flow coating method, a spray method and the like may be used.

Baking after coating is carried out in a hot air oven (ie,
Hot air drying), depending on the speed of the hot air at that time,
The amount of AEAFA resin concentrated on the surface of the uppermost resin layer is controlled. Specifically, the wind velocity of hot air on the board is 0.5 m to 3.
Bake the coating under the condition of 0 m / sec. Wind speed is 0.5
If it is less than m / sec, the surface concentration of the AEAFA resin does not sufficiently occur, and the above surface AEAFA resin concentration may not be obtained. When the wind speed exceeds 3m / sec, AEA
The surface concentration of the FA resin becomes excessive, and crosslinking proceeds excessively on the surface of this resin layer, resulting in brittleness and deterioration of moldability.

For the measurement of the wind speed, a conventional measuring instrument such as a hot-wire anemometer, a thermistor anemometer, which is one type of a thermal anemometer, may be used. The baking temperature may be the same as the baking temperature of a normal precoated steel sheet, and is not particularly limited, but PMT
It is generally sufficient if the (maximum plate temperature) is about 200 to 250 ° C.

[0040]

[Example] A hot-dip galvanized steel sheet having a thickness of 0.6 mm (coating coating weight: 60 g / m ^{2 on} one side) was used as a base steel sheet, and after cleaning the surface, first, by dipping in a zinc phosphate solution as a base treatment. A zinc phosphate conversion coating (zinc phosphate coating weight 0.8 g / m ² ) was formed. Then, as a coating for the undercoat resin coating, a primer coating PB10P manufactured by Dainippon Ink and Chemicals was applied on one side with a roll coater to a dry film thickness of 7 μm.
It was baked and cured in 50 seconds so that the PMT was 210 ° C.
An overcoat resin coating layer was formed on the undercoat resin coating layer as described below to prepare a precoated steel sheet.

As the coating material for coating the overcoat resin, the four types of copolymerized polyester resins shown in Table 1 below and Table 2 below.
A coating material was used in combination with the seven types of cross-linking agents shown in. This paint contained cyclohexanone as a solvent, titanium oxide as a pigment, and p-toluenesulfonic acid as an acid catalyst. After applying this coating material with a roll coater to a dry film thickness of 18 μm, hot air oven (hot air temperature 28
Bake-hardening was performed in 0 ° C) while controlling the velocity of the hot air (plate surface) to 0.3 to 6 m / sec. PMT at the time of baking
Was in the range of 220-240 ° C.

[0042]

TABLE 1 copolymerized polyester Symbol Number average molecular weight A 5,000 B 12,000 C 22,000 D 30,000

[0043]

[Table 2] Crosslinking agent symbol Name P Hexamethoxymethylol melamine Q Methoxylated butoxylated mixed methylol melamine R Butoxylated methylol melamine S Imino group type methylol melamine T Methoxylated methylol benzoguanamine U Hexamethylene diisocyanate V Diphenol of bisphenol A Glycidyl ether Of the above crosslinking agents, P to T are AEAFA resins,
U and V are comparative cross-linking agents other than AEAFA resin. The performance of the produced precoated steel sheet was evaluated as follows.

The coated surface of the precoated steel sheet (designated as A) prepared for pressure resistance was
The pressure mark resistance was evaluated by bringing the coated surface of another precoated steel sheet (referred to as B) prepared assuming the back side of the precoated steel sheet into surface contact under pressure. In particular,
Same as the above, one side of the steel sheet that was pretreated is coated with an epoxy coating film used for back surface coating (service coat) of a normal pre-coated steel sheet at a dry film thickness of 6 μm and baked, and another pre-coated steel sheet assuming the back side B was produced.

Pre-coated steel sheets A and B having the same size were placed on each other so that the coated surfaces were in surface contact with each other, and the test pieces were placed at 40 ° C. for 10 minutes.
A hot press of 0 kg / cm ² × 24 hours was performed, the coating film surface of the precoated steel sheet A after the hot press was visually observed, and the generation state of pressure marks was evaluated according to the following criteria. ◯: No pressure mark was generated at all, Δ: Pressure mark was slightly generated, × = Pressure mark was significantly generated.

Bending processability A test piece of a precoated steel sheet was bent 180 ° with the coated surface facing outward while the number of sandwiched plates was reduced, and then the coating film at the bending portion was observed with a 10-fold magnifying glass. The bending workability was indicated by the minimum number of plate sandwiches where cracks could not be confirmed.

For example, 0T indicates that contact bending is possible, and 1T
Indicates that the bent plate can be bent without causing cracks in the coating film until the distance between the bent plates becomes one plate having the same thickness. Therefore, the larger the displayed numerical value, the lower the workability. Although this bending workability depends on temperature, in this example, the test was performed at 23 ° C. (normal temperature). When the bending workability is 2T or less, particularly 1T or less, the bending workability is sufficiently good.

Paint workability The paint was put in a paint pan and the pick-up property of the paint was evaluated according to the following criteria. ◯: Liquid film is completely lifted, Δ: Liquid film is partially broken, but liquid film is completely transferred to the applicator roll, ×: Transferred liquid film is broken, so-called solo occurs.

The test results were obtained by determining the type of copolymerized polyester and crosslinking agent used in the coating material, the weight ratio of polyester / crosslinking agent in the coating material (based on solid content), the hot air velocity on the plate surface, and the surface obtained by the above method. It is summarized in Table 3 together with the AEAFA resin concentration.

[0050]

[Table 3]

As can be seen from Table 3, according to the invention,
The weight ratio of copolyester / AEAFA resin is 90 /
After applying the paint which is 10-70 / 30, the hot air velocity is 0.5-
Form a resin layer with surface-enriched AEAFA resin as a cross-linking agent so that the surface AEAFA resin concentration is within the range of 30-80% when the coating film is baked and cured by hot air drying in the range of 3.0 m / sec. I was able to. It is possible to increase the hardness only on the surface by thickening the surface of this cross-linking agent, and it is possible to prevent the occurrence of pressure marks on the precoated steel sheet while maintaining the excellent workability inherent to the copolyester resin film. .

[0052]

As described above, according to the present invention, the pressure mark resistance is improved while the workability and the scratch resistance are kept good without the need for a separate process which increases the cost. The precoated steel sheet can be obtained, and the precoated steel sheet can be suitably used for applications such as household electrical appliances, indoor and outdoor equipment, and the like.

Claims (2)

[Claims] 1. The uppermost resin layer has a number average molecular weight of 8,000.
A precoated steel sheet containing ˜25,000 copolyester resin and alkyl etherified aminoformaldehyde resin (abbreviated as AEAFA resin) in a weight ratio of 90/10 to 70/30, measured by X-ray photoelectron spectroscopy The composition of the outermost layer of this resin layer is the surface AE calculated by the following equation.
A pre-coated steel sheet with excellent pressure mark resistance, which has an AFA resin concentration of 30 to 80%. (Equation 1) In the formula, N ₁ is the N concentration (at%) of the outermost layer of the resin layer measured by X-ray photoelectron spectroscopy, and N ₀ is the outermost layer of the resin layer consisting of the AEAFA resin only, measured by X-ray photoelectron spectroscopy. N concentration (at%). 2. A precoat for forming the uppermost resin layer from a coating material containing a copolymerized polyester resin having a number average molecular weight of 8,000 to 25,000 and an alkyl etherified aminoformaldehyde resin in a weight ratio of 90/10 to 70/30. In the method for producing a steel sheet, after applying the coating material, the coating film is baked by hot air drying so that the wind speed on the sheet surface is controlled to be in the range of 0.5 to 3.0 m / sec. A method for producing a precoated steel sheet having excellent markability.