JPH07300683A - Chromating method excellent in low-temperature baking property - Google Patents

Abstract

PURPOSE:To apply chromating excellent in baking property at a low temperature to a steel sheet by applying the aq. chromate-contg. resin composition contg. a chromate soln. of specified composition and an acrylic polymer emulsion in a specified ratio on a galvanized steel sheet, etc., and forming a film under specified conditions. CONSTITUTION:A chromate soln. with the weight ratio of chromium(III) ion to chromium(VI) ion (Cr<3+>/Cr<6+>) controlled to 1-4 and the weight ratio of phosphate ion to total chromium (PO<-3>4/total chromium) to 2-4 and an acrylic polymn. emulsion are mixed to prepare an aq. chromate-contg. resin composition with the weight ratio of the solid in emulsion to total chromium controlled to 5-40. The composition is applied on a galvanized steel sheet and an aluminized steel sheet to form a chromate-contg. resin film with the total film weight controlled to 0.5-3.5g/m<2> and the chromate weight in the film to 50-200mg/m<2>.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a chromate-containing aqueous resin coating method for coating the surfaces of zinc-based and aluminum-based plated steel sheets. The chromate-containing organic resin-coated steel sheet obtained by the present invention has almost no color on the appearance, and has no sticking between the coated steel sheets, and has excellent corrosion resistance, coating adhesion and fingerprint adhesion resistance. Have.

[0002]

2. Description of the Related Art Conventionally, a coating type chromate treatment containing hexavalent chromium and trivalent chromium for the purpose of improving the corrosion resistance of zinc-based and aluminum-based plated steel sheets is easy to maintain and manage, and various plating species can be treated. Since it has the advantage of being able to follow changes in line speed, it has been widely adopted. Further, for the purpose of improving coating film adhesion, anti-fingerprint adhesion, etc., a method of coating with an organic resin after reactive chromate treatment disclosed in JP-A-58-224175, and JP-A-63-63. A method for coating a chromate-containing water-based resin disclosed in Japanese Patent No. 145785 is used. Among these coating methods, chromate-containing water-based resin coating is promising as a method capable of following the recent increase in production line speed and imparting various functions.

In recent years, in the case of a coated steel sheet which is used without coating, represented by a 55% Al-Zn plated steel sheet, it retains excellent corrosion resistance even after being subjected to a forming process and has a surface without impairing its appearance. There is a desire for a method of coating
Further, in order to apply the plated steel sheet to the field of home electric appliances, a high degree of alkali resistance is required. Against this background, the above required performance is satisfied unless 55% Al-Zn plated steel sheet is coated with the chromate-containing aqueous resin solution disclosed in JP-A-63-145785 and then baked at a high temperature of 180 ° C or higher. I knew I couldn't.

[0004]

DISCLOSURE OF THE INVENTION The present invention provides a chromate treatment method that improves corrosion resistance and alkali resistance after processing without impairing the plating appearance even when low-temperature baking is performed on an existing high-speed line and does not cause sticking. It is intended to be provided.

[0005]

DISCLOSURE OF THE INVENTION As a result of intensive investigations by the present inventors on the above-mentioned conventional problems, the trivalent chromium ion and hexavalent chromium ion ratio and the weight ratio of the phosphate ion amount to the total chromium amount were specified. The composition containing the specified chromium liquid and the acrylic polymer emulsion in a specific mixing ratio with respect to the total chromium amount is applied on a zinc-based and aluminum-based plated steel sheet, and the total coating amount and the chromium adhesion amount are specified. By doing so, even with a chromate-containing resin film baked at a low temperature of 100 ° C or less, functions such as corrosion resistance, alkali resistance, coating adhesion and fingerprint adhesion resistance can be imparted without sticking to each other between the organic coated steel sheets. The present invention has been completed by finding out that it is a coating method.

That is, according to the present invention, (a) the weight ratio (Cr ³⁺ / Cr ⁶⁺ weight ratio) of trivalent chromium ions to hexavalent chromium ions is 1 to 4 and the amount of phosphate ions with respect to the total amount of chromium is a chromate solution weight ratio (PO ₄ ^3- / total Cr ratio by weight) is 2 to 4, (b) mixing ratio of containing an acrylic polymer emulsion, and the (a) (b) is total chromium A chromate-containing water-based resin composition having a solid content weight ratio (emulsion solid content / total Cr weight ratio) of the acrylic polymer emulsion of 5 to 40 relative to the amount is coated on a zinc-based and aluminum-based plated steel sheet, When dried, the total coating amount is 0.5
~ 3.5 g / m ² , the amount of chromium deposited in the film is 50 ~ 20
Provided is a chromate treatment method excellent in low-temperature bakeability, which comprises forming a chromate-containing resin film of 0 mg / m ² .

The present invention will be described in detail below.
The zinc-based and aluminum-based plated steel sheets targeted by the present invention include hot-dip aluminized steel sheets, hot-dip galvanized steel sheets, Al—Zn-plated steel sheets with hot melt 5% and 55%, alloyed hot-dip galvanized steel sheets, and electrogalvanized steel sheets. Etc., and is also applied to plated steel sheets to which at least one of chromium, silicon, cobalt, zirconium, tin, titanium, iron, lead, nickel, magnesium, manganese, molybdenum, phosphorus, etc., has been added as an alloying element. It is possible.

The chromate solution (a) which is a constituent of the chromate-containing water-based resin composition used in the present invention is a weight ratio (Cr ³⁺ / Cr) of trivalent chromium ions to hexavalent chromium ions.
^{6 +} weight ratio) (hereinafter abbreviated as chromium ratio) is 1 to 4, and
Weight ratio of phosphate ion to total chromium (PO ₄ ^3- /
It is necessary that the total Cr weight ratio) be 2 to 4. If the chromium ratio is less than 1, a yellow film appearance will be obtained and the plating appearance will be impaired. If it exceeds 4, the chromate solution will precipitate and the sticking resistance and alkali resistance will be unfavorable.

If the weight ratio of the amount of phosphate ions to the total chromium is less than 2, not only the yellow film appearance tends to occur, but also the chromate solution tends to precipitate. If it exceeds 4, a slightly green film appearance is exhibited, which is not preferable. It is desirable to use chromic anhydride and orthophosphoric acid in the chromate solution of the present invention, and to adjust the chromium ratio, a reducing agent such as ethanol, methanol, saccharose, or starch.

As the acrylic polymer emulsion (b) which is a constituent of the chromate-containing aqueous resin composition used in the present invention, a known acrylic resin emulsion is used. For example, an acrylic polymer emulsion obtained by emulsion polymerization using a nonionic emulsifier such as a polyoxyethylene-polyoxypropylene block polymer system described in Japanese Patent Application No. 4-314039 is stable in miscibility with chromate. Is desirable in terms of.

In the present invention, the acrylic polymer emulsion (b) has a solid content weight ratio (emulsion solid content / total Cr weight ratio) of 5 to 4 with respect to the total amount of chromium.
Must be 0. When it is less than 5, corrosion resistance and coating film adhesion are deteriorated, and when it exceeds 40, sticking resistance, alkali resistance and coating film adhesion are deteriorated, which is not preferable.

The (a) chromate solution of the present invention may further contain at least one metal ion selected from the group consisting of cobalt ion, nickel ion, zirconium ion, manganese ion and zinc ion as a metal ion. it can. These metal ions combine with chromic acid during film formation to form a poorly water-soluble chromic acid metal salt,
To further improve alkali resistance. It is desirable to supply the metal ions as carbonates, hydroxides, oxides, or phosphates, and sulfates, chlorides, nitrates, etc. inhibit the formation of a sparingly soluble salt with chromic acid. , Not preferable.

If necessary, the chromate solution (a) of the present invention may contain fluorine ions for improving the adhesion between the plating surface and the coating. Fluorine ions include hydrofluoric acid, zirconium hydrofluoric acid, hydrosilicofluoric acid,
It can be supplied in the form of titanium hydrofluoric acid, borohydrofluoric acid, or the like.

The chromate-containing water-based resin composition of the present invention may further contain the polyethylene wax water-based dispersant disclosed in Japanese Patent Application No. 4-314039 for improving lubricity.

In the present invention, the chromate-containing aqueous resin composition on the plated steel sheet has a total coating amount of 0.5 to
3.5 g / m ² , the amount of chromium deposited in the film is 50 to 200
It is preferable to apply and dry so that the amount may be mg / m ² . If the total coating amount is less than 0.5 g / m ² , fingerprint resistance, corrosion resistance and coating adhesion will be reduced, and if it exceeds 3.5 g / m ² , sticking resistance will be reduced. If the amount of chromium deposited in the coating is less than 50 mg / m ² , the corrosion resistance will be poor, and 200 mg / m ²
If it exceeds, the appearance of a yellow film will be impaired and the plating appearance will be impaired.

As a coating method, a roll coater method, spray coating, dipping air knife squeezing or the like can be applied. It should be noted that the drying is sufficient if the water splashes, but a more preferred ultimate plate temperature is 70 ° C. or higher.

[0017]

[Function] Unless the chromate-containing water-based resin composition is dried (baked) at a high temperature of 180 ° C. or higher, the performance such as sticking resistance, alkali resistance, and corrosion resistance after processing can be well balanced without impairing the plating appearance. Is difficult The present invention provides a non-colored appearance by controlling the chromium ratio and the amount of phosphate ion of the chromate-containing water-based resin composition, and the hexavalent chromium ion by controlling the compounding ratio of the acrylic polymer emulsion and chromium. It was also found that crosslinks between trivalent chromium ions and an acrylic polymer can be easily formed, and as a result, performances such as alkali resistance, sticking resistance, and coating adhesion are improved, and the present invention was completed. Came to do.

[0018]

EXAMPLES In order to more specifically explain the effects of the present invention, examples in which the present invention is applied to various plating materials will be given below together with comparative examples. Since the performance items required for each material are different, a predetermined evaluation test was performed on each plated steel sheet.

[Method for producing test material] A. In the case of electrogalvanized steel sheet Electrogalvanized steel sheet (Amount of coating: 20 / 20g / m
² , oil-coated plate) was degreased with alkali, washed with water and dried, and then the chromate-containing aqueous resin compositions of Examples 1 to 4 and Comparative Examples 1 to 5 shown in Table 1 were applied, and the dried steel sheet was used as a test material. did. These test materials were evaluated for sticking resistance, fingerprint adhesion resistance, bent portion corrosion resistance and alkali resistance a (chromium fixing rate).

B. In the case of hot-dip 55% Al-Zn alloy-plated steel sheet Hot-melt 55% Al-Zn alloy-plated steel sheet (Amount of coating:
A chromate-containing aqueous resin composition of Examples 5 to 8 and Comparative Examples 6 to 9 shown in Table 2 was applied to a 60/60 g / m ² , uncoated oil plate), and a dried steel plate was used as a test material. For these test materials, sticking resistance, corrosion resistance at bent parts,
An evaluation test for yellowness and alkali resistance b (blackening resistance) was performed.

C. In the case of alloyed hot-dip galvanized steel sheet Hot-dip galvanized steel sheet (coating weight: 90/90
(g / m ² , oil-coated plate) is degreased with alkali, washed with water and dried,
The chromate-containing aqueous resin compositions of Examples 9 to 12 and Comparative Examples 10 to 12 shown in Table 3 were applied and dried steel sheets were used as test materials. These test materials were evaluated for sticking resistance, alkali resistance a (chromium fixing rate), coating solution stability, flat surface corrosion resistance, and coating adhesion.

D. In the case of hot dip aluminum plated steel plate Hot dip aluminum plated steel plate (coating weight: 60/6
Examples 13 to 1 shown in Table 4 on 0 g / m ² , uncoated plate)
6, the chromate-containing water-based resin compositions of Comparative Examples 13 to 15 were applied and dried steel sheets were used as test materials. These test materials were evaluated for sticking resistance, yellowing resistance, alkali resistance b and coating adhesion.

2. Evaluation test method Sticking resistance The test material was cut into a size of 70 × 150 mm, both burrs were taken, the target surfaces were overlapped with each other, a load of 70 kgf · cm was applied with a torque wrench, and the test piece was placed in an atmosphere of 50 ° C for 24 hours.
It was exposed to time and evaluated according to the following evaluation criteria. ○: No sticking between target surfaces △: Some sticking between target surfaces ×: Strong sticking between target surfaces

Fingerprint adhesion resistance The surface of the test material was strongly pressed with a finger, and the state of traces of fingerprints was visually evaluated according to the following evaluation criteria. ○: The fingerprint trace is not visible △: The fingerprint trace is slightly visible ×: The fingerprint trace is clearly visible

Corrosion resistance of bent portion The test material was bent (4T bending) at intervals of four plates, and a salt spray test according to JIS-Z-2371 was performed for 200 hours. The bent portion was evaluated according to the following evaluation criteria. It was visually evaluated. ◎: No white rust ○: White rust occurrence area ratio less than 10% △: White rust occurrence area ratio 10 to less than 30% ×: White rust occurrence area ratio 30% or more

Alkali resistance a (chromium fixing rate) The test material was sprayed with an alkaline degreasing agent CL-N364S (manufactured by Nippon Parkerizing) at a concentration of 2% at 60 ° C for 2 minutes, washed with water and dried. From the results of measuring the amount of chromium adhered by fluorescent X-rays before and after degreasing, the chromium fixing ratio (chromium fixing ratio (%) = chromium adhering amount after degreasing / adhering amount before degreasing x 100) was calculated and evaluated according to the following evaluation criteria. . ◯: Chromium fixing rate of 95% or more Δ: Chromium fixing rate of 80% to less than 95% ×: Chromium fixing rate of less than 80%

The b value of the test material was measured with a yellowness color difference meter, and the difference from the steel sheet not subjected to the surface treatment (Δb: the larger the value, the stronger the yellow tint) was evaluated according to the following evaluation criteria. ◯: Yellowness (Δb) less than 3 Δ: Yellowness (Δb) 3 to less than 6 ×: Yellowness (Δb) 6 or more

Alkali resistance b (blackening resistance) The test material was immersed in a Ca (OH) ₂ saturated solution at 50 ° C., and the number of days when the difference in lightness (ΔL) before and after immersion was 15 or more was measured by a color difference meter. The following evaluation criteria were evaluated. ◎: 30 days or more ○: 20 to less than 30 days △: 10 to less than 20 days ×: less than 10 days

Coating liquid stability Coating liquid shown in Table 3 (chromate-containing aqueous resin composition)
Was exposed to an atmosphere of 40 ° C., the number of days when gelation or precipitation occurred was investigated, and evaluated according to the following evaluation criteria. ○: 2 weeks or more △: 1 to less than 2 weeks ×: less than 2 weeks

Corrosion Resistance of Flat Surface The test material was subjected to a salt spray test for 200 hours and visually evaluated according to the following evaluation criteria. ◎: No white rust ○: White rust occurrence area ratio less than 10% △: White rust occurrence area ratio 10 to less than 30% ×: White rust occurrence area ratio 30% or more

Coating Adhesion A melamine alkyd paint (Delicon 700 white manufactured by Dainippon Paint Co., Ltd.) was baked and dried as a test material so that the coating thickness would be 20 to 25 μm. After dipping this coated plate in boiling water for 2 hours, 100 squares of 1 mm square were cut as a coating adhesion test, and after extruding 5 mm with an Erichsen tester, tape peeling was performed to determine the number of remaining coating marks. It measured and evaluated by the following evaluation criteria. ⊚: Remaining coating film count of 100 ◯: Remaining coating film count of 95 or more and less than 100 Δ: Remaining coating film count of 90 to less than 95 ×: Remaining coating film count of less than 90

3. Evaluation test results In the case of electrogalvanized steel sheet shown in Table 1, molten 55% A in Table 2
In the case of the 1-Zn alloy plated steel sheet, Table 3 shows the evaluation test results of Examples and Comparative Examples in the case of alloyed hot dip galvanized steel sheet and Table 4 in the case of hot dip aluminum plated steel sheet. The acrylic emulsions used in these Examples and Comparative Examples are those described in JP-A-63-145785.

The conditions for preparing the test plates of Examples 1 to 16 are as follows: Chromate ratio of the chromate solution within the scope of the present invention is 1 to 4, weight ratio of phosphate ion amount to total chromium amount is 2 to 4, The solid content weight ratio of the acrylic polymer emulsion to the amount of chromium is 5 to 40, and the total amount of the film after coating and drying is 0.5 to
3.5 g / m ² , the amount of chromium deposited in the film is 50 to 200
It was mg / m ² and showed excellent performance. Comparative example 1
The test plate production conditions of Nos. 15 to 15 are inferior in performance in the examples that do not correspond to the present invention. Examples 6 and 7 are examples in which zinc ions and cobalt ions are further contained as metal ions.
Especially, it was excellent in alkali resistance. Examples 10 and 11
Is an example in which a fluorine ion is further contained, but the coating film adhesion was particularly excellent. Example 4 is an example in which a polyethylene wax is further contained, and particularly the corrosion resistance of the bent portion was excellent.

[0034]

The chromate-containing water-based resin composition according to the present invention is applied to the surfaces of zinc-based and aluminum-based plated steel sheets and baked at a low temperature to dry them, thereby preventing ticking resistance, alkali resistance, and corrosion resistance.
It is possible to satisfy well-balanced performance such as corrosion resistance after processing. Thus, by the method of treating the surface of the zinc-based aluminum-plated steel sheet of the present invention, the process saving,
A high-performance surface-treated steel sheet that is energy-saving and has excellent high-speed productivity can be obtained.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

[Table 3]

[0038]

[Table 4]

Claims (4)

[Claims] 1. A weight ratio of (a) trivalent chromium ions to hexavalent chromium ions (Cr ³⁺ / Cr ⁶⁺ weight ratio) of 1 to 4, and a weight ratio of phosphate ion amount to total chromium amount ( PO ₄
^3- / total Cr weight ratio) is 2 to 4;
(B) Acrylic polymer emulsion, and the mixing ratio of (a) and (b) is such that the acrylic polymer emulsion has a solid content weight ratio (emulsion solid content / total Cr weight ratio) with respect to the total amount of chromium. ) Is 5 to 40, and a chromate-containing water-based resin composition is applied to a zinc-based and aluminum-based plated steel sheet and dried to give a total coating amount of 0.5 to 3.5 g.
/ M ² , the amount of chromium deposited in the film is 50 to 200 mg / m ²
A chromate treatment method excellent in low-temperature bakeability, which comprises forming a chromate-containing resin film of. 2. The low temperature bakeability according to claim 1, wherein the chromate solution further contains, as metal ions, at least one metal ion selected from cobalt ion, nickel ion, zirconium ion, manganese ion and zinc ion. Chromate treatment method. 3. The chromate treatment method according to claim 1, wherein the chromate solution further contains fluorine ions. 4. The chromate treatment method excellent in low-temperature bakeability according to claim 1, 2 or 3, wherein the chromate-containing aqueous resin composition further contains a polyethylene wax aqueous dispersion.