KR100536811B1 - Coating Solution for Pre-sealed Steel Sheet Containing Clay-Polymer Nanocomposite and Method for Coating the Same on the Pre-sealed Steel Sheet - Google Patents

Abstract

The present invention relates to a resin composition for a pre-shielded steel sheet having improved corrosion resistance, weldability and electrodeposition coating resistance despite a thick resin coating film thickness of 2 μm or more and a method of coating the same on the pre-shielded steel sheet.

According to the present invention, 100 parts by weight of an epoxy resin having a number average molecular weight of 15,000 to 60,000, 5 to 30 parts by weight of a polymer-clay nanocomposite, 5 to 20 parts by weight of melamine resin, 1 to 5 parts by weight of wax, and a major axis of 0.5 to According to the present invention, there is provided a resin composition for pre-shielded steel sheets composed of 80 to 300 parts by weight of one or more metal powders selected from the group consisting of Al, Cu, Ni, and the like. Or providing a zinc or zinc alloy plated steel sheet treated with any one of SiO x layer deposition treatment, applying the resin composition to a steel sheet treated as described above so that a dry film thickness is 1.5 to 8 μm, and the resin Provided is a method of coating a resin composition on a pre-shielded steel sheet, which consists of baking and cooling the steel sheet coated with the composition at 200 to 250 ° C.

The resin film according to the present invention exhibits excellent corrosion resistance, chemical resistance, weldability, electrodeposition coating property and processability when coated on a galvanized or zinc alloy plated steel sheet.

Description

Coated Solution for Pre-sealed Steel Sheet Containing Clay-Polymer Nanocomposite and Method for Coating the Same on the Pre-sealed Steel Sheet}

The present invention relates to a resin composition for a pre-shielded steel sheet and a method for coating the same on a pre-shielded steel sheet, and more particularly, for a pre-shielded steel sheet having weldability and electrodeposition coating property with improved corrosion resistance despite thick resin film thickness of more than 2 μm. Resin composition and a method of coating the same on a pre-shielded steel sheet

Steel plate manufactured by steel company is subjected to phosphate treatment and painting after assembling the car body in automobile company. At this time, there are several places where the steel plate overlaps in the assembled body, for example, the hem (Hem) of the door of the car, for example. In this way, since the phosphate treatment solution or paint hardly penetrates the steel plate overlapping part of the vehicle body, the corrosion resistance becomes weak. In order to solve this problem, the automobile company is used to seal the overlapping parts of the steel sheet (Sealing) to block the corrosion factor access to the site. However, since this causes a decrease in productivity and a cost increase, a surface treated steel sheet which can omit the sealing process is required.

Accordingly, European steel companies produce steel sheets coated with galvanized steel sheets and then chromated and coated with organic coatings having a thickness of about 4 to 10 μm on them. This ensures that the sealing step can be omitted in the vehicle body manufacturing step in the automobile company. This is called pre-sealed steel sheet. However, since the thick shield film is formed in the pre-shielded steel sheet in order to secure corrosion resistance, the electrical resistance welding (spot welding) required in the subsequent step is lowered. In order to solve this problem, a metal powder having excellent electrical conductivity is mixed and used in the resin coating, but there is still a problem that electrodeposition paintability is poor due to the thick resin coating.

An object of the present invention is to provide a resin composition for a pre-shielded steel sheet and a method of coating the pre-shielded steel sheet to contain the polymer-clay nanocomposite to improve the corrosion resistance, chemical resistance and ensure adhesion, weldability and electrodeposition coating property.

According to the present invention, 100 parts by weight of an epoxy resin (topic resin) having a number average molecular weight of 15,000 to 60,000, 5 to 30 parts by weight of a polymer-clay nanocomposite, and 5 to 5 parts melamine resin based on 100 parts by weight of the epoxy resin (topic resin) At least one metal selected from the group consisting of 20 parts by weight, 1-5 parts by weight of wax, and Al, Cu, Ni, Zn, Fe ₂ P, Fe, Mn, Co, Ti, and Sn having a major axis of 0.5-4 μm Provided is a resin composition for a pre-shielded steel sheet composed of 80 to 300 parts by weight of powder. In this case, the polymer-clay nanocomposite may be an amine clay having 14 to 20 carbon atoms, a tallow ammonium clay substituted at a terminal with hydroxyl (OH-), benzyl-, or methyl-, and One or more clays selected from the group consisting of polyoxymethylene clays of which the terminals are substituted with amine groups are mixed in an epoxy or phenoxy resin in an amount of 5 to 30 parts by weight to make exfoliation. Preferably, 5-20 parts by weight of silica fume is added.

In addition, according to the present invention, a chromate treatment to attach chromium of 30 to 200 mg / m ² , chromium-free coating treatment to dry film thickness of 0.5 ~ 1.0㎛, or physical or chemical vapor deposition so that the coating thickness is 50 to 500 nm Providing a zinc or zinc alloy plated steel sheet treated with any one of SiOx layer treatment, applying the resin composition to the steel sheet so as to have a dry film thickness of 1.5 to 8 μm, and the resin composition coated steel sheet There is provided a method of coating a resin composition on a pre-shielded steel sheet, which is composed of baking and cooling to 200 to 250 ° C.

Hereinafter, the present invention will be described in detail.

The physical properties of the preshielded steel sheet are largely affected by the pretreatment layer and the resin layer. Especially, the resin in the upper part of the steel sheet is mainly due to the primary shielding effect against corrosion factors and the shielding effect to prevent rapid dissolution or loss of the lower pretreatment layer. It serves to secure the corrosion resistance of the free shielded steel sheet. If the corrosion resistance of the pre-shielded steel sheet is not secured, corrosion holes of the steel sheet may be generated along with corrosion of the vehicle body when used in automobiles, and the like, and the vehicle may be damaged.

The resin film constituting the upper layer of the pre-shielded steel sheet should be formed of a rigid film having excellent alkali resistance and solvent resistance as well as a protective function of the lower pretreatment layer. If the resin coating is not strong, such as corrosion resistance of the pre-shielded steel sheet, the resin coating does not endure in alkaline degreasing solution and acidic phosphate solution in the degreasing and phosphate treatment process during the vehicle car processing. Physical properties are not achieved.

In addition, corrosion resistance should be exerted in the hem of the vehicle body in which the pre-shielded steel sheet is not used for electrodeposition coating, but in other parts, electrodeposition coating should be performed smoothly such as automotive plated steel sheet or cold rolled steel sheet.

The vehicle body goes through a degreasing and phosphate treatment bath, and then enters the electrodeposition paint bath. In the electrodeposition paint bath, the paint particles having the properties of the anode are attached to the vehicle body having the cathode. When the electrodeposition paint is initially attached to the steel sheet, the electrodeposition paint adheres to the steel sheet as the pH rapidly changes from neutral to alkaline due to the generation of hydrogen gas on the surface of the steel sheet. The electrodeposition coating film formed by this process is no longer adhered to the electrodeposition coating particles due to the increased electrical resistance as the thickness increases, the electrodeposition coating is terminated. However, if the preshield resin does not have an advantageous hydrophilic structure for electrodeposition coating, the exchange of water or ions does not occur actively during electrodeposition coating, resulting in excessive hydrogen gas generation at a specific site due to localized current concentration. As a result, the electrodeposited coating film is formed on the surface of the coating film with hydrogen bubble marks, and the coating surface becomes rough. In addition, even in the middle and top coating processes, the non-uniform shape is expressed as it is, causing coating failure. Thus, in order for the resin film to have an advantageous structure for the electrodeposition coating, it is preferable that the structure of the resin constituting the resin film contains a structure having a hydrophilic group, that is, a hydroxyl group (-OH) and an amide group (-NH3). Therefore, in the present invention, an epoxy or epoxy-phenoxy mixed resin advantageous for electrodeposition coating may be used as the main resin.

As said epoxy resin, the epoxy resin of the average molecular weights 15,000-50,000, especially the diglycidyl ether type epoxy resin are used. When the average molecular weight is less than 15,000, the epoxy resin reacts with amines or amides rather than with a melamine resin, which hardens the metal particles to float and precipitates the resin. If the average molecular weight is 50,000 or more, it reacts with amine or amide, but the viscosity of the composition rises, making it difficult to uniformly coat, increasing the cleaning cost of the production equipment, and increasing the manufacturing cost. In order to limit the average molecular weight of the epoxy resin in the above range.

Polymer-clay nanocomposites include amine-based clays having 14 to 20 carbon atoms, tallow ammonium clays substituted at the ends with hydroxyl (OH-), benzyl-, or methyl- groups, and One or more clays selected from polyoxymethylene clays substituted with amine groups are used, and the powder is mixed so that the internal form becomes exfoliation by mixing 5 to 30 parts by weight in the epoxy resin. The powder is added in an amount of 5 to 30 parts by weight based on the epoxy resin (topic resin). If the clay is not peeled off, the protective role against the corrosion factor is reduced. If the content is less than 5 parts by weight, the corrosion resistance is not greatly improved. If the content is more than 30 parts by weight, the hardness of the entire resin increases, causing the film to swell. It is not preferable because the film becomes uneven, incompletely coated spots occur, and the resin film loses elasticity and is detached.

Melamine resin is used as a hardening | curing agent in the said epoxy resin. The melamine resin is added in an amount of 5 to 20 parts by weight based on the epoxy resin (topic resin). If the content of the curing agent is less than 5 parts by weight, the curing reaction of the coating film is not sufficient and the desired physical properties cannot be secured. If it exceeds 20 parts by weight, there is no effect of increasing the curing reaction by addition, and there is a fear that the added amount of the curing agent may lower the solution stability and the physical properties of the resin film.

Wax is used to improve the lubricity of the steel sheet. As the wax, various kinds of waxes such as polyamide-based, polyolefin-based, cannuber-based, teflon-based and silicone-based waxes may be used, but polyamides for uniform dispersion in solution of Teflon-based waxes or metal powders, which are effective even in small amounts, may be used. Preference is given to using waxes. The wax selected above may be added alone or in combination. The wax is added in an amount of 1 to 5 parts by weight based on the epoxy resin. Teflon-based wax is not preferable because the content is less than 1 part by weight, the lubricating performance is insufficient, and if the content is more than 5 parts by weight, the Teflon wax is inferior to the adhesion with the electrodeposition coating film applied to the upper portion. The polyamide wax is added in an amount of 1 to 5 parts by weight based on the epoxy resin. If the content is less than 1 part by weight, the lubrication performance is insufficient, it is difficult to prevent the sedimentation of the metal powder, and if it exceeds 5 parts by weight is cured through the reaction with the epoxy resin at room temperature is not preferable because it inhibits the storage stability.

In the resin of the present invention, a metal powder is added to improve weldability. When the metal powder is dispersed in the resin solution, it is preferable to have a plate shape or a needle shape rather than a sphere to prevent sedimentation in the resin solution and to protect the corrosion factor from penetrating, and the particle size of the metal powder is larger than the thickness of the resin film to be dried. It is good that the major axis is 0.5 ~ 4㎛ in plate shape. Under the same resin viscosity, the spherical spheres are much less buoyant than the needles or plates because they are buoyant. Therefore, when the major axis of the metal particles is less than 0.5 µm, the metal powders are connected in the resin coating. It is difficult to form an annular structure, which makes it difficult to flow electric current during welding. If the particle size exceeds 4 μm, the particles protrude on the resin coating film, which not only damages the surface appearance but also damages the mold during processing. It is not desirable because it causes problems.

As the metal powder, a metal powder selected from the group consisting of Al, Cu, Ni, Zn, Fe ₂ P, Fe, Mn, Co, Ti, and Sn is added alone or in combination. The metal powder is preferably a hard metal rather than a soft metal. The reason why hard metals are advantageous for weldability is that when pressing force is applied during welding, the soft metals do not remain intact and have the property of dispersing current by aggregating with the same metals or resins. This is because the efficiency of energizing the current by maintaining the shape of is large.

The content of the metal powder is 80 to 300 parts by weight based on 100 parts by weight of the epoxy resin (topic resin). If the content of the metal powder is less than 80 parts by weight, the metal powder is too small to be welded. If the content is over 300 parts by weight, the metal powder is too large in the resin film, so that the coating of the metal powder causes the powder to be powdered. This is because it comes off in the form of).

In addition, silica fume may be added in an amount of 5 to 20 parts by weight based on the epoxy resin in order to improve corrosion resistance and adhesion to the base steel sheet. When it exceeds 20 parts by weight, it was found that there is little effect of improving the corrosion resistance and adhesion by the addition of silica.

Hereinafter, a method of coating the resin composition on the steel plate for preshield will be described in detail.

Corrosion resistance, electrodeposition coating and weldability are applied by coating the resin composition of the present invention on a plated steel sheet subjected to reactive or coated chromate treatment, chromium free coating, or pretreatment of SiOx layer using physical or chemical vapor deposition on the coated steel sheet. A resin coated steel sheet for preshields containing excellent clay-polymer nanocomposites is produced.

Galvanized steel sheet or zinc alloy coated steel sheet may be used as the plated steel sheet. Various methods such as electroplating, hot-dip plating, and vacuum deposition may be used as a method of plating a zinc alloy mixed with other metals such as zinc, zinc, iron, nickel, cobalt, or the like on a steel sheet.

The plated steel sheet is treated with a chromate solution or a chromium free film for the purpose of improving corrosion resistance, or pretreated with a SiOx layer using physical or chemical vapor deposition. As the chromate treatment, reactive or coated chromate treatment may be used, and coated chromate is preferable from the viewpoint of improving corrosion resistance. The coated chromate treatment solution is usually composed of chromic anhydride, phosphoric acid, silica and silane dissolved in water. However, it is preferable to use a two-component type of chromium solution and silane because of the stability of the chromate solution. The amount of chromium deposited on the steel sheet during the chromate treatment is preferably 30 to 200 mg / m ² . It is because corrosion resistance is not enough when it is less than 30 mg / m ² , and coating film adhesion and processability are deteriorated when it is more than 200 mg / m ² . After coating so that the thickness of the baked dry coating is 0.5 ~ 1.0㎛ and baked at a steel plate temperature of 80 ~ 120 ℃ and air-cooled. This is because when the thickness of the dry coating film is less than 0.5 µm, physical properties such as corrosion resistance decrease, and when the thickness exceeds 1.0 µm, the production cost increases without improving the corrosion resistance. In the case of SiOx layers, a thickness of 50-500 nm is preferred, since this range is the most efficient in terms of corrosion resistance and economy.

The coating film of the resin composition according to the present invention is baked to have a thickness of 1.5 to 8.0 µm (resin amount of 2.0 to 10.0 g / m ² ) on the plated steel sheet pretreated with the chromate, chromium-free, or SiOx layer. . This is because when the dry coating thickness is less than 1.5 µm, physical properties such as corrosion resistance decrease, and when the thickness exceeds 8.0 µm, the thickness of the resin is high and there is a problem in welding.

After apply | coating a resin composition, it bakes at the steel plate temperature of 200-250 degreeC. If the baking temperature is less than 200 ° C., the curing reaction of the resin is insufficient, and the physical properties of the coating film are lowered. If the baking temperature is higher than 250 ° C., the curing reaction does not proceed any more with the change of the material of the steel sheet, which consumes only heat. not. After baking, the resin composition is completely coated on the preshielded steel sheet by cooling by a conventional method such as water cooling or air cooling.

Hereinafter, embodiments of the present invention will be described in detail.

Example 1

In this example, the solution stability of the resin solution and the physical properties of the resin coating were evaluated according to the content of the epoxy resin, the content of the curing agent, and the content of the polymer-clay nanocomposite.

Epoxy resin as the main resin, amide resin and melamine resin as the curing agent were used, and the amine polymer-clay nanocomposite having 18 carbon atoms was blended in the amounts shown in Table 1 below. By weight, 200 parts by weight of Fe ₂ P and zinc powder were formulated to prepare the resin compositions of Inventive Examples 1 to 5 and Comparative Examples 1-16.

Thereafter, solution stability, chemical resistance, electrodeposition coating, and corrosion resistance of the resin composition were evaluated. Solution stability was evaluated according to the presence or absence of gel formation in the resin composition after 30 days at room temperature in consideration of the sufficient time to actually use.

The case where a gel is not formed is shown by (circle), and the case where a gel is formed is shown by (triangle | delta) and the case where it forms partially.

Chemical resistance and electrodeposition coating properties were coated on the steel sheet of the resin compositions of the invention examples and comparative examples to prepare a specimen and evaluated for the resin coating formed on the specimen. The resin composition was dried on a zinc alloy electroplated steel sheet (holding steel sheet) coated with zinc alloy at a thickness of 30 g / m ^{2 on} a steel plate having a thickness of 0.7 mm and chromate treated with a Cr deposition amount of 50 mg / m ² , respectively. After coating by using a bar coater (Bar coater) to a thickness and baked at a steel plate temperature of 240 ℃ and then cooled to prepare a resin coated steel sheet specimens.

The treatment conditions for evaluating the chemical resistance and electrodeposition coating are preliminary degreasing using the automotive body treatment process (Chemical name: Pyroclean 442, concentration: 5% by weight, treatment time: 150 seconds, temperature: 50 ° C, Spray) Degreasing (Chemical name: Pyroclean 442, concentration: 5% by weight, treatment time: 150 seconds, temperature: 50 ° C, dipping) → washing with water (pure, treatment time: 150 seconds, temperature: room temperature, spraying) → phosphate Treatment (Chemical name: Bonderite 699D, Temperature: 45 ℃, Acidity: 20.5, Free acidity: 1.05, Acceleration: 2.5) → Electrodeposition coating (Chemical name: ED-1800Gray, Solid content: 20%, Temperature: 28 ℃, Treatment time: 180 seconds, voltage: 50V) → It carried out by baking (165 degreeCx 20 minutes).

Since painting process of automobile is made by the above process, chemical resistance evaluation is carried out to degreasing in the middle of electrodeposition coating (preliminary degreasing + main degreasing, PH of 13 or more as alkaline solution), and then discoloration and peeling degree of resin coating (coating film). The evaluation was divided into five stages. Evaluation criteria are as follows.

(Double-circle): No peeling of resin coating film, color difference (compared to color difference before processing) E1 or less

○: No peeling of the resin coating film, color difference (contrast color difference before treatment) E 2-3

□: No peeling of resin film, color difference (contrast color difference before treatment) E 5 or more

(Triangle | delta): Partial peeling of resin coating film (The peeling part is 30% or less of resin conductive cloth surface.)

×: The resin coating film is completely peeled off (the peeled portion is more than 50% of the resin conductive cloth surface)

Electrodeposition paintability was evaluated in three ways according to the surface state of the electrodeposition coating film after the last process of the above treatment conditions. If the degree of roughness is severe with the appearance of the electrodeposition coating, it is " × " If it looked smooth, and there was no blemish, it was "◎" and the intermediate grade was evaluated as "□".

Corrosion resistance was investigated at 35 ° C using a salt water spray tester (SST) and then measured the time of occurrence of white blue and blue red according to elapsed time.

◎: Red blue color occurs over 500 hours

○: Red blue color occurs at 300 ~ 500 hours

□: Red blue color occurs at 100 ~ 300 hours, white blue color occurs at 100 ~ 300 hours

△: Red blue color occurs within 100 to 300 hours, white blue color occurs within 100 hours

×: white rust occurs within 100 hours

TABLE 1

In Table 1, the nanocomposite is used and 5-20 parts by weight of the melamine resin is used as a curing agent. In the present invention, chemical resistance, solution stability, electrodeposition coating, and corrosion resistance are shown to have the best effect. On the other hand, even in the case of using a nanocomposite, when using an amide resin as a curing agent (see Comparative Example 5) it can be seen that in terms of chemical resistance and corrosion resistance compared to the case of using a melamine resin.

Example 2

In Example 2, the physical properties of the resin composition according to the content of the polymer-clay nanocomposite, metal powder, and wax (change amount to the subject resin) were evaluated. Melamine resin (methylated melamine formaldehyde; manufactured by USA Cytec), metal powder (needle type, particle size 2 ~ 4 μm, spherical shape, particle size 2 ~ 10 μm) as a curing agent to epoxy resin (topic resin) having a molecular weight of 30,000, Teflon wax To each compound in the amounts shown in Table 2 (butyl solvent solution used) to prepare a resin composition. After coating this resin composition on the steel plate by the method similar to Example 1, it evaluated about corrosion resistance, workability, electrodeposition coating property, and weldability.

The workability was evaluated by the planar friction coefficient, and the conditions of evaluating the planar friction coefficient were 1m / min withdrawal speed and 0.5 kgf / mm ² with pressing force.

◎: friction coefficient is 0.15 or less

□: Friction Coefficient 0.15 ~ 0.2

×: coefficient of friction 0.2 or more

Electrodeposition coating property was evaluated in the same manner as in Example 1.

In order to evaluate the weldability, each specimen was welded using an AC pneumatic spot welding machine under a pressurization of 250 kgf and an energization time of 16 cycles at a welding current of 8 kA. RWMA Class II of Cu—Cr alloy was used as a welding electrode. Weldability was evaluated by continuous RBI. The specimens for evaluation were overlapped to two points of 80 mm in length and 20 mm in width to the point of 20 mm from the end, and then welded one point to the center of the overlapped area, and the number of spots in which the welded part fell by the tensile tester was examined. This RBI was evaluated as a continuous RBI, and the evaluation criteria are as follows.

◎: 1000 ~ 1500 consecutive RBI

○: 500-1000 consecutive RBIs

□: Number of consecutive RBIs 300 ~ 500

△: Number of consecutive RBIs 100-300

×: No welding

In order to evaluate the adhesion, the specimens were collected in a size of 30 mm X 50 mm, immersed in boiling water for 30 minutes, dried, and then peeled off using a Scotch Tape. The evaluation was performed according to the degree of resin on the tape. The evaluation criteria are as follows.

◎: No peeling on the tape surface

□: Peeling within 5% of area ratio

△: peeling more than 5% of area ratio

×: All surface peeling occurred on the tape surface

[Table 2]

From Table 2, it can be seen that the various physical properties measured when including 5-30 parts by weight of polymer-clay nanocomposite, 1-5 parts by weight of wax, and 80-300 parts by weight of metal powder are the best. You can see that it gets worse.

Example 3

In this embodiment, the resin prepared by the composition of Inventive Example 24 of Example 2 was evaluated for the physical properties according to the baking temperature and the thickness of the dry coating film when forming the resin coating.

Electrodeposited after the resin coating at the baking temperature and dry coating thickness shown in Table 3 on the zinc alloy plated steel plate chromium-free or chromate treated to 0.6 ㎛ thickness in the same manner as in Example 1 using the comparative material and the invention material Tensile, weldability, corrosion resistance and adhesion were evaluated. The symbols used in the tables have the same meanings as described above.

TABLE 3

Table 3 shows that the baking temperature has a problem in corrosion resistance and adhesiveness at 200 ° C. or less, and problems in electrodeposition coating and corrosion resistance occur at temperatures exceeding 250 ° C. In addition, when the coating film thickness is less than 1.5 µm or more than 8 µm, problems have arisen in weldability, adhesiveness, and the like. In other words, the resin composition of the present invention is 1.5-8㎛ thickness when coated on the pre-shielded steel sheet, the baking temperature can be seen that the most preferred 200 ~ 250 ℃.

The resin coating coated on the galvanized steel sheet or the zinc alloy coated steel sheet according to the resin composition and processing conditions of the present invention shows excellent corrosion resistance, chemical resistance, weldability, electrodeposition coating and workability. It is suitable for application to areas where it is difficult to form phosphate coatings or electrodeposition coatings, such as heme sites.

1 is a view showing the structure of a polymer-clay nanocomposite in the form of peeling,

2 is a side sectional view of a pre-shielded steel sheet coated with the resin composition of the present invention.

Claims (4)

100 parts by weight of an epoxy resin (topic resin) having a number average molecular weight of 15,000 to 60,000, 5 to 30 parts by weight of polymer-clay nanocomposites, 5 to 20 parts by weight of melamine resin, and wax based on 100 parts by weight of the epoxy resin (topic resin). 80 parts by weight of one or more metal powders selected from the group consisting of 1 to 5 parts by weight and Al, Cu, Ni, Zn, Fe ₂ P, Fe, Mn, Co, Ti and Sn having a major axis of 0.5 to 4 μm Resin composition for pre-shielded steel sheet, characterized in that consisting of parts. 2. The polymer-clay nanocomposite according to claim 1, wherein the polymer-clay nanocomposite is selected from the group consisting of amine clays having 14 to 20 carbon atoms, tallow ammonium clays substituted at the ends of hydroxyl, benzyl or methyl groups and polyoxymethylene clays substituted at the amine groups A resin composition for pre-shielded steel sheet, characterized in that the selected one or more clays are mixed in an epoxy resin at 5 to 30 parts by weight to form a peeling type. The resin composition for pre-shielded steel sheet according to claim 1, wherein the resin composition contains 5 to 20 parts by weight of silica fume. Either chromate treatment to attach chromium of 30 to 200 mg / m ² , chromium-free coating to dry film thickness of 0.5 to 1.0 μm, or physical or chemical vapor deposition of SiO x layer to make film thickness of 50 to 500 nm. Providing a treated zinc or zinc alloy plated steel sheet; Coating the resin composition of any one of claims 1 to 3 on the steel sheet treated as described above so that the dry film thickness is 1.5 to 8 μm; And baking the steel sheet coated with the resin composition at 200 ° C. to 250 ° C., and cooling the steel sheet coated with the resin composition.