JP2011026662A - Surface-treated steel sheet and method for manufacturing the same, and coating method using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface-treated steel sheet which obtains corrosion resistance and coating film adhesiveness after being coated, equal to or higher than those in the case where a coating substrate treatment is carried out, even when coating, especially electrodeposition coating is applied to the surface-treated steel sheet without performing a coating substrate treatment such as a zinc phosphate chemical conversion treatment or a zirconium-based chemical conversion treatment, and which is excellent in removability of rust preventive oil, and to provide a method for manufacturing the same. <P>SOLUTION: The surface-treated steel sheet is characterized in that particulate Bi is stuck on the surface of a steel sheet and rust preventive oil is applied on the particulate Bi in an amount of 0.5-2.0 g/m<SP>2</SP>. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention is excellent in corrosion resistance after rust-preventing oil coating, excellent in alkali degreasing properties of rust-preventing oil, and applied without coating base treatment such as zinc phosphate chemical conversion treatment or zirconium-based chemical conversion treatment, particularly electrodeposition. The present invention relates to a surface-treated steel sheet capable of obtaining post-coating corrosion resistance and coating film adhesion equivalent to or higher than those in the case where a coating ground treatment is applied, a method for producing the same, and a coating method using these.

  The steel sheet may be subjected to zinc plating in order to improve its rust prevention property. Therefore, a steel sheet is a steel sheet not plated (hereinafter referred to as a steel sheet), such as a cold-rolled steel sheet and a hot-rolled steel sheet, a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, and an electrogalvanized steel sheet. These can be broadly classified into galvanized steel sheets, but both are the targets here.

  In the case of coating a zinc-based plated steel sheet and a steel sheet, it is necessary to perform a coating ground treatment before coating. This is because the coating performance (corrosion resistance after coating and coating film adhesion) can be improved by the coating base film formed by the coating base treatment. Zinc phosphate-based chemical conversion treatment and zirconium-based chemical conversion treatment are generally used as the coating base treatment. When coating a sheet metal structure such as an automobile body, the process of cutting the steel sheet, the cut steel sheet After passing through the step of pressing and the step of joining the pressed steel plates, a coating ground treatment is applied. This is because, when the conventional coating base treatment is performed before press working, the coating performance is remarkably deteriorated due to film defects caused by the press working. Moreover, it is because weldability will fall remarkably by the electrical resistance of a paint ground film when the conventional paint ground treatment is given before welding simultaneously. Therefore, the coating base treatment is not performed in advance on the steel sheet normally used for manufacturing the sheet metal structure.

However, both the zinc-based plated steel sheet and the steel sheet may be subjected to a surface treatment to compensate for the defects of the respective steel sheets. For example, if it is a steel plate, as patent document 1, after passing through at least one process of pickling, continuous annealing, and grinding | polishing, 1 type of metal salt of Ti, Mn, Ni, Co, Cu, Mo, and W will be on the steel plate surface. Alternatively, a method for producing a cold-rolled steel sheet excellent in phosphate-treating property, characterized by subjecting the metal to 0.001 to 0.5 g / m ² by performing a cathodic electrolytic treatment for a short time in an aqueous solution containing two or more kinds. Is described. This is only for the purpose of improving the processability of the zinc phosphate-based chemical conversion treatment to be performed later, and does not act as a coating ground treatment film.

Moreover, if it is a zinc system plating steel plate, as patent document 2, a ZnO type oxide will be produced | generated on the surface of a plating layer, and Mn-Zn-OH-P type crystalline oxide will be 0.1 to 0.1 Mn as the upper layer. Zinc-based plated steel sheet excellent in lubricity, chemical conversion property, adhesive compatibility, and weldability, characterized by producing 100 mg / m ² and 1 to 100 mg / m ² as the amount of P, zinc-based as Patent Document 3 Press forming characterized by having an amorphous reaction product of phosphoric acid and zinc in the range of 20 mg / m ² or more and 350 mg / m ² or less in terms of phosphorus on the surface of the plated layer of the plated steel sheet A zinc-based plated steel sheet having excellent properties is described.

  All of these relate to a lubricated film for improving press workability, which is a weak point of galvanized steel sheets, and does not affect the coating performance.

Japanese Patent Publication No. 58-37391 JP-A-8-296016 Japanese Patent Laid-Open No. 9-170084

  As described above, when the method of Patent Document 1 is applied to a steel plate, the processability of the zinc phosphate chemical conversion treatment is improved, but the zinc phosphate chemical conversion treatment is a prerequisite for satisfying the coating performance. This cannot be omitted.

  When the methods of Patent Document 2 and Patent Document 3 are applied to the zinc-based plated steel sheet, the press workability of the zinc-based plated steel sheet is improved, but in order to satisfy the coating performance, the zinc phosphate-based chemical conversion treatment, etc. It is premised on the coating surface treatment of this, and this cannot be omitted.

  In addition, when the zinc phosphate chemical conversion treatment or the zirconium-based chemical conversion treatment is performed on the surface of the steel plate in advance, not only the film is damaged by the subsequent press work, but also the weldability is lowered due to its own film resistance. Furthermore, when trying to perform zinc phosphate chemical conversion treatment or zirconium-based chemical conversion treatment before press working or welding, it is necessary to treat the sheet-shaped steel sheet, but these usually required for several tens of seconds or more It is difficult to perform the process on the sheet in terms of production efficiency.

Means for Solving the Invention

The present inventor has intensively studied for the purpose of solving the above problems, and has found a solution.
That is, the present invention includes the following (1) to (5).

(1) A surface-treated steel sheet characterized in that particulate Bi adheres to the steel sheet surface, and rust preventive oil is applied on the steel sheet in an amount of 0.5 to 2.0 g / m ² .

(2) The surface according to (1) above, wherein there is no rusting in the wet environment corrosion resistance evaluation test described in the specification, and there is no water splashing in the alkali degreasing evaluation test described in the specification. Treated steel sheet.

(3) Using an aqueous solution containing dissolved Bi, particulate Bi is adhered to the steel plate surface by cathodic electrolysis and / or displacement plating, and then 0.5 to 2.0 g of rust preventive oil is applied to the surface. A method for producing a surface-treated steel sheet, wherein / m ² is applied.

(4) A coating method characterized by cutting the surface-treated steel sheet of (1) or (2), performing press working, joining them by welding, performing degreasing and washing with water, and then painting.

(5) The coating method according to (4), wherein the surface-treated steel sheet is a zinc-based plated steel sheet, the welding is spot welding, and the coating is electrodeposition coating.

  According to the present invention (1) to (5), it is excellent in corrosion resistance after coating with a rust-preventing oil, excellent in alkali degreasing properties of the rust-preventing oil, and coated with a base such as zinc phosphate chemical conversion treatment or zirconium-based chemical conversion treatment. Even if coating, particularly electrodeposition coating, is performed without performing the treatment, it is possible to obtain post-coating corrosion resistance and coating film adhesion that are equal to or higher than those obtained when the coating ground treatment is performed. Furthermore, about this invention (5), there also exists an effect that the spot weldability at the time of spot-welding a zinc-based plated steel plate improves.

  Examples of the steel sheet of the present invention include steel sheets such as cold-rolled steel sheets and hot-rolled steel sheets, and galvanized steel sheets such as hot-dip galvanized steel sheets, galvannealed steel sheets, and electrogalvanized steel sheets.

The surface-treated steel sheet of the present invention is characterized in that particulate Bi adheres to the surface of the steel sheet, and rust preventive oil is applied on the surface of 0.5 to 2.0 g / m ² .

The precipitated Bi is in the form of particles. The average particle diameter of Bi particles is not particularly limited, but is preferably 5 to 200 nm, more preferably 10 to 150 nm, and most preferably 15 to 100 nm. If the Bi particle diameter is too fine, the particles may be stacked and the coating film adhesion may be reduced. If the particles are too coarse, the Bi coverage on the substrate surface may be reduced, and the corrosion resistance after coating may be reduced. When Bi precipitates as a continuous film rather than in a particulate form, the effect of improving the coating film adhesion obtained by the particulate Bi is not exhibited, so that the superiority as a coating base is lost. The particulate Bi may or may not contain other substances as long as it contains Bi, but usually the part that becomes the core of the particulate contains the metal Bi and has the effect of surface contamination. In consideration, it is considered that a layer containing Bi hydroxide (Bi (OH) ₃ or the like) or Bi oxide (Bi ₂ O ₃ or the like) is formed in the outermost layer.

Bi coating weight is not particularly limited, but is preferably 1 to 100 mg / ^{m 2,} more preferably 5-80 mg / ^{m 2,} and most preferably 10 to 50 mg / ^{m 2.} When the amount of Bi attached is insufficient, the alkali degreasing property of the rust preventive oil applied to the steel sheet is deteriorated, and not only the press workability of the galvanized steel sheet is lowered, but also the corrosion resistance after coating is lowered. When it is excessive, the coating film adhesion decreases.

The particulate Bi on the steel sheet has not only a function as a coating base film or a lubricating film, but also an effect of improving the spot weldability of the zinc-based plated steel sheet and an effect of improving the degreasing property of the steel sheet. Although there are many unclear points regarding the mechanism of expression of these effects, the inventors consider as follows. The decrease in spot weldability with respect to the zinc-based plated steel sheet is thought to be due to the diffusion of Zn into the copper electrode. However, when Bi is present on the zinc-based plated surface, Cu / Bi is rapidly formed on the surface of the copper electrode. It is considered that an alloy layer is formed and the diffusion of Zn is suppressed.
It is considered that the improvement in degreasing properties for the steel sheet is due to the effect of suppressing excessive adsorption of the rust preventive additive contained in the rust preventive oil to the steel sheet surface. In addition, there is no prior art having such an effect, and only Bi is effective among various metals.

The surface-treated steel sheet of the present invention is coated with 0.5 to 2.0 g / m ² of rust preventive oil on the particulate Bi. If the lower limit is not reached, a problem of rusting will occur, and if the upper limit is exceeded, the degreasing property will be reduced. This range is suitable as a rust preventive oil adhesion amount that does not cause surface deterioration such as rusting and can quickly remove oil before coating.

  The structure of the surface-treated steel sheet according to the present invention has been described above. Next, the properties of the surface-treated steel sheet according to the present invention will be described. As described above, the surface-treated steel sheet of the present invention has post-coating corrosion resistance and coating film adhesion that are equal to or higher than those in the case where a coating base treatment such as zinc phosphate chemical conversion treatment or zirconium-based chemical conversion treatment is performed. Furthermore, the surface-treated steel sheet of the present invention has a predetermined amount (predetermined range set from the viewpoint of rust prevention and oil removal property) of the surface of the steel sheet, but the rust prevention oil is excellent in rust prevention oil. While maintaining performance, it also has oil removal performance (alkali degreasing performance) applied later. Rust prevention performance and alkali degreasing performance after coating with a rust preventive oil are evaluated by the following wet environment corrosion resistance evaluation test and alkali degreasing evaluation test, and it is necessary to pass the test.

<< wet environment corrosion resistance evaluation test method >>
The wet test (39 ° C., 95% RH) described in JIS K 2246 is performed for 480 hours, and the rusting state on the steel sheet surface is visually determined. “No rusting” means “good”, spot rusting or surface rusting means “good”, and “good” means only “good”.
<< Alkaline degreasing evaluation test method >>
An alkali degreasing solution containing sodium metasilicate nonahydrate: 10 g / L, sodium bicarbonate: 10 g / L, polyoxyethylene alkyl ether (HLB = 14): 2 g / L is maintained at 40 ° C. Degrease for 120 seconds. As a stirring condition in the dipping method, the liquid is stirred so that a flow of 10 cm / sec hits the surface of the test plate. After degreasing, the film is immersed in city water for 30 seconds and washed, and the water wettability after standing in the air for 30 seconds after washing is visually evaluated. When the water wetted area rate is 100%, ○, when water splashing is confirmed, ×, and only ○ is acceptable.

  The present invention relates to a method for depositing Bi on the surface of a steel sheet by a cathodic electrolysis method and / or a displacement plating method, but the treatment solution needs to be an aqueous solution containing dissolved Bi. The presence of insoluble Bi compounds such as hydroxylated Bi, oxidized Bi, and metal Bi in the treatment solution does not impede the effects of the present invention, but the deposition method is limited to cathodic electrolysis and displacement plating, so reduction deposition The possible form, that is, dissolved ionic Bi must be dissolved.

  As a method of supplying Bi, in addition to a method of dissolving Bi sulfate Bi or Bi nitrate Bi in an aqueous solution, a method of dissolving Bi hydroxide Bi or oxide Bi in an inorganic acid or an organic acid can be used. As the inorganic acid, sulfuric acid, hydrochloric acid, nitric acid and the like can be used, and as the organic acid, acetic acid, citric acid, malic acid, glycolic acid, cystine, catechol, tyrone, EDTA, NTA, HEDTA and the like can be used. In addition, when an organic acid that exhibits a sufficient chelating ability for Bi, such as an aminopolycarboxylic acid such as EDTA, is used, the pH can be adjusted with an alkali metal or ammonia.

  The pH of the treatment liquid is not particularly defined and can be arbitrarily adjusted to pH 1-13. However, when applying the displacement plating method, it is necessary to etch the material, and therefore pH 1 to 4 is preferable. The temperature of the treatment liquid is not particularly specified but is preferably 25 to 60 ° C. However, when the displacement plating method is applied, it is necessary to etch the material as in the case of pH, and therefore, 40 to 60 ° C. is preferable.

  The mass concentration, which is the content of Bi atoms in the aqueous solution, is not particularly specified, but is preferably 100 to 10,000 ppm, more preferably 200 to 7000 ppm, and most preferably 500 to 5000 ppm.

  Cathodic electrolysis and / or displacement plating is used as a method for depositing Bi. Fine particulate Bi can be deposited only by reducing and precipitating Bi ions by cathodic electrolysis and / or displacement plating. In a dry deposition method such as dry plating, Bi is not in the form of particles but is deposited as a continuous film, and sufficient coating film adhesion cannot be obtained.

In the case of the cathodic electrolysis method, a method is adopted in which a steel plate is immersed in the treatment liquid and a voltage is applied between the insoluble anode. A platinum electrode, a stainless steel electrode, a lead electrode, etc. can be used as an insoluble electrode. The current density is particularly non but preferably 0.01~50A / dm ² limited, more preferably 0.05~20A / dm ^2, and most preferably 0.1 to 10 A / dm ^2.

  In the case of the displacement plating method, the treatment can be performed by bringing the treatment liquid into contact with the steel sheet. Specifically, as in the case of cathodic electrolysis, a method of immersing a steel plate in the treatment liquid and a method of spraying the treatment liquid on the steel plate are employed.

  Cathodic electrolysis and displacement plating may be combined. Specifically, the effect of the present invention is not impaired even if the cathode electrolytic treatment is performed after the steel plate is immersed in the treatment solution and electrolessly subjected to substitution plating.

The steel plate to which Bi is attached needs to be coated with rust preventive oil on its surface. Although Bi adhering to the surface of the steel plate functions as a coating base, it is indispensable to apply anti-rust oil because the anti-rust effect on the steel plate before coating is poor. Even if the anti-rust oil is applied to the steel plate to which Bi is attached, components derived from the anti-rust oil are less likely to remain in the subsequent degreasing step. As for the adhesion amount of rust prevention oil, 0.5-2.0 g / m < ² > is preferable. If it is less than 0.5 g / m ² , the rust preventive effect of the rust preventive oil is lost, and the steel sheet surface is rusted before being coated. If it exceeds 2.0 g / m ² , when removing oil in the degreasing step before coating, the removability is lowered. As the rust preventive oil to be applied, known rust preventive oils can be used. For example, rust preventive additives such as petroleum sulfonate sodium salt, petroleum sulfonate barium salt, petroleum sulfonate calcium salt, and fatty acid barium salt. And mineral oil composed of petroleum hydrocarbons. In particular, alkaline earth metal salts such as calcium salts and barium salts have high rust-preventing power, but generally have a tendency to deteriorate alkaline degreasing properties. . In addition, commercially available rust preventive oils may contain low-boiling solvents, and the amount of rust preventive oil attached tends to decrease after oil coating. It is a residue after the volatile matter is not included.

  Finally, the method for coating the surface-treated steel sheet according to the present invention will be described. First, in the process up to the coating process of the surface-treated steel sheet of the present invention, there is no particular limitation except that a degreasing process for removing the rust preventive oil adhering to the surface-treated steel sheet of the present invention is included. For example, a cutting step, a pressing step, and a joining step by welding may or may not exist. Also, the painting process is not particularly limited. However, it is preferable that the steel sheet of the present invention is cut, pressed, joined by spot welding, degreased and washed with water, and then electrodeposited. By adopting this process, it is possible to omit the coating base treatment process from the manufacturing process of the sheet metal structure and the coating process. The reason why the electrodeposition coating is selected as the coating is that it is excellent as a coating method for coating the entire sheet metal structure having a complicated structure, for example, a bag structure, with good throwing power.

  In addition, a process liquid is a process liquid used for the process of this invention. Also, known techniques can be used for press working, welding, degreasing and washing with water, and electrodeposition coating.

  The contents of the present invention will be specifically described below with reference to examples and comparative examples.

Preparation of test plate Cold rolled steel plate: SPCC SD (JIS G 3141), alloyed hot dip galvanized steel plate: SGCD3 SMO F06 (JIS G 3302), hot dip galvanized steel plate: SGCD3 ZSMO Z08 (JIS G 3302), Electrogalvanized steel sheet: SECE MO E24 (JIS G 3313) was used, and the surface was previously degreased using an alkaline degreasing agent. After the degreasing treatment, it was washed with spray water, immersed in the treatment liquid shown in Compositions 1 to 5, and treated under the electrolytic conditions shown in Conditions 1 to 4. However, among the processing conditions, the processing time is shown in Table 1. The test plate after the treatment was immediately washed with spray water for 30 seconds, air blow dried after roll squeezing. The test plate for analysis was taken out here, the amount of Bi adhered was measured by fluorescent X-ray spectroscopic analysis (XRF), and the Bi particle diameter and coverage were determined using a field emission scanning electron microscope (FE-SEM). . Other test plates were roll-coated immediately after air blow drying, and rust preventive oil: Daphne Oil Coat Z3 (Idemitsu Kosan Co., Ltd. rust preventive oil, barium salt used as a rust preventive additive) 1.0 g / m ² after 24 hours at room temperature. Oil was applied so that

Treatment liquid composition 1 Bi ₂ O _{3 3} g / L
50% H ₂ SO ₄ 30 g / L
18% NH ₃ pH adjustment
pH 1.7
Temperature 40 ℃
(After adjusting Bi ₂ O ₃ in 50% H ₂ SO ₄ , pH adjustment using 18% NH ₃ )
Composition 2 Bi ₂ O ₃ 10 g / L
50% H ₂ SO ₄ 50 g / L
18% NH ₃ pH adjustment
pH 1.5
Temperature 40 ℃
(After adjusting Bi ₂ O ₃ in 50% H ₂ SO ₄ , pH adjustment using 18% NH ₃ )
Composition 3 Bi ₂ O ₃ 5 g / L
HEDTA 30g / L
18% NH ₃ pH adjustment
pH 4.5
Temperature 40 ℃
(After Bi ₂ O ₃ is dissolved in HEDTA aqueous solution, pH is adjusted using 18% NH ₃ )
Composition 4 NiSO ₄ · 6H ₂ O 3 g / L
(NH ₄ ) ₂ SO ₄ 15 g / L
pH 4.7
Temperature 40 ℃
Composition 5 Potassium permanganate 20 g / L
Monopotassium phosphate 50g / L
Phosphoric acid 15g / L
Sulfuric acid 5g / L
Temperature 40 ℃

Treatment condition 1 Cathodic electrolysis Current density: 0.1 Adm ⁻²
Condition 2 Cathodic electrolysis Current density: 2.0 Adm ⁻²
Condition 3 Electroless 2 seconds (displacement plating), then cathode electrolysis Current density: 0.1 Adm ⁻²
Condition 4 Electroless (Substitution plating only)

Corrosion resistance was evaluated by the evaluation test method described in the wet environment corrosion resistance evaluation specification. No rusting was evaluated as ○, spot rusting or surface rusting was evaluated as ×, and only ○ was acceptable.

Degreasing was evaluated by the evaluation test method described in the alkaline degreasing evaluation specification. A water wetted area rate of 100% was evaluated as ◯, a case where water splashing was confirmed as ×, and only ○ as a pass.

Lubricity Evaluation A test plate left indoors for 10 days was punched into a disk shape having a diameter of 115 mm, and the lubricity was evaluated using a high-speed deep drawing tester. The test was performed under the following conditions, and the limit wrinkle pressure was measured. In addition, lubricity evaluation was performed only with respect to the zinc-plated steel plate.
Punch diameter: 50.0mm Punch shoulder: 5mmR Die shoulder: 5mmR
Surface finish: # 1200 Punch speed: 60 m / min Drawing ratio: 2.3
The measurement results were evaluated based on the following criteria.
Less than 0.5 ton: ×
0.5 ton or more and less than 2.0 ton: △
2.0 ton or more and less than 3.0 ton: ○
3.0ton or more: ◎

Spot Weldability Evaluation The spot welding test was conducted by investigating the number of continuous dots during spot welding under the following welding conditions. As the electrode, a Cu—Cr electrode having a tip diameter of 4.5 mmφ, a tip angle of 120 degrees, and an outer diameter of 13 mmφ was used. Ten cycles of energization were performed using a 50 Hz power source. Pressurization was performed at a pressure of 1.7 kN for 30 cycles before energization, 10 cycles after energization, and without an up-down slope.

In addition, the welding current value in the continuous spot property investigation is the average of the current value I1 (kA) and the welding current value I2 (kA) at which the nugget diameter indicated by 4√t when the plate thickness is t (mm) is obtained. Using the value, the maximum number of hit points in which the nugget diameter of 4√t was maintained was obtained.
The measurement results were evaluated based on the following criteria.
120% or more of the number of continuous hit points in the material without surface treatment: ◎
80% or more and less than 120% of the number of continuous hits in materials without surface treatment: ○
Less than 80% of the number of continuous hit points in the material without surface treatment: ×

Applying the treatments of the coating examples and comparative examples, degreased using an alkaline degreasing agent, spray-washing the coated test plates, and without applying coating ground treatment such as zinc phosphate chemical conversion treatment Electrodeposition coating was performed using a cationic electrodeposition paint “GT-10HT” manufactured by Kansai Paint Co., Ltd. The temperature of the electrodeposition paint was 28 ° C., and the voltage application condition was linearly increased to 200 V over 30 seconds, and then maintained at 200 V for 150 seconds. The painted cup-shaped product was washed with water and baked for 30 minutes at 170 ° C. using an electric oven.

Evaluation of corrosion resistance after coating A cross-cut was put into a test plate after coating using a cutter, and a salt spray test was conducted for 1000 hours in accordance with JIS Z 2371. The maximum rust width on one side from the cut after the salt spray test was measured and evaluated according to the following evaluation criteria.
Criteria for cold-rolled steel sheet Rust width less than 1.0 mm: ◎
Rust width 1.0 mm or more and less than 1.5 mm: ○
Rust width 1.5 mm or more and less than 2.0 mm:
Rust width 2.0 mm or more: ×
Criteria for galvanized steel sheet Rust width less than 0.5 mm: ◎
Rust width 0.5 mm or more and less than 1.0 mm: ○
Rust width 1.0 mm or more and less than 1.5 mm: Δ
Rust width 1.5mm or more: ×

Evaluation of coating film adhesion The test plate after coating was immersed in boiling water of deionized water for 1 hour, and thereafter, 11 parallel lines with a 1 mm interval were drawn at right angles using a cutter on the coating surface, resulting in 100 grids. Cut cuts. Next, a cellophane tape was applied to the cross-cut portion and the tape was peeled off, and then the number of cross-cut peeled coatings was evaluated according to the following evaluation criteria.
0: ◎
1 to 5: ○
6-10 pieces: △
11 or more: ×

Claims (5)

A surface-treated steel sheet characterized in that particulate Bi adheres to the steel sheet surface, and rust-preventive oil is applied on the steel sheet in an amount of 0.5 to 2.0 g / m ² .   The surface-treated steel sheet according to claim 1, wherein there is no rusting in the wet environment corrosion resistance evaluation test described in the specification, and there is no water repelling in the alkali degreasing evaluation test described in the specification. Using an aqueous solution containing dissolved Bi, particulate Bi is adhered to the surface of the steel sheet by cathodic electrolysis and / or displacement plating, and then rust preventive oil is applied to the surface at 0.5 to 2.0 g / m ^2. A method for producing a surface-treated steel sheet, characterized by applying oil.   A coating method comprising cutting the surface-treated steel sheet according to claim 1 or 2, performing press working, joining them by welding, performing degreasing and washing with water, and then coating.   The coating method according to claim 4, wherein the surface-treated steel sheet is a galvanized steel sheet, the welding is spot welding, and the coating is electrodeposition coating.