JPS6277498A - Post-treatment of cold rolled steel sheet - Google Patents

Abstract

PURPOSE:To remarkably improve the corrosion resistance of a cold rolled steel sheet by successively forming a metallic Cr film and a chromate film on the surface of the steel sheet by electrolysis and by forming a resin film on the chromate film. CONSTITUTION:A metallic Cr film of 0.03-1mum thickness and a chromate film are successively formed on the surface of a colfd rolled steel sheet by electrolysis. The chromate film is formed by 20-50mg/m<2> (expressed in terms of Cr). An aqueous soln. contg. a water soluble resin and/or an emulsion resin is then applied to the chromate film and dried to form a resin film of 0.5-4mum thickness. The especially preferred resin is polyacrylic resin or polyacrylonitrile resin.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention is a post-treatment method for cold-rolled steel sheets that improves corrosion resistance while maintaining the metallic luster of cold-rolled steel sheets, and that reduces restrictions on paint selection during painting. Regarding the method.

(Prior art) As a post-treatment method for cold-rolled steel sheets, cold l! ! :g11 A conventional method has been used to sequentially form metallic chromium and chromate films on the sheet surface by electrolysis to impart corrosion resistance while maintaining the metallic luster of the cold rolled steel sheet.

(Problem to be solved by the invention) However, since the corrosion resistance of products treated by this method is still insufficient, red rust may occur if the corrosion JEI boundary is severe or the period of use is long. Ta.

Corrosion resistance can be improved by using the above post-treatment methods, but if they are increased, the surface becomes yellowish or darkened, the metallic luster is lost, and the appearance is impaired. Met. In addition, even if the electrolytic electricity +41 is increased, the amount of chromate film does not increase proportionally, so increasing it costs a lot of money, and if it increases too much, the adhesion of the film will decrease. Met.

Generally speaking, if there is a limit to improving corrosion resistance with electrolytic treatment alone, painting can be applied, but regular painting with paint will result in a somewhat thick coating, so
Welding is no longer possible, and the selectivity of two-coat paints is also limited, so unless a specific top-coat paint is used,
Corrosion resistance after C is a problem.

(Means for Solving Problem 2α) Therefore, the present inventors have investigated various post-treatment methods for cold-rolled steel sheets that improve corrosion resistance while maintaining gloss and have fewer restrictions on topcoat selectivity. As a result, water-soluble resin or emulsion 04 after the electrolytic chromate treatment
It has been found that it is sufficient to form an O (oil film) on the chromate film by treating it with an aqueous solution containing 11ff or both of these.

That is, the post-treatment method of the present invention coats the surface of a cold-rolled steel sheet with metallic chromium and chromium %f with a gloss of 0.03 to 0.1μ.

After sequentially forming a chromate film with a weight of 20 to 50 g/l112, a water-soluble resin or an emulsion resin, or an aqueous solution containing both of these, is applied and dried to form a chromate film with a film amount of 0.5 to 4 μW. The vI characteristic is the formation of a resin film.

In the present invention, the amount of metallic chromium deposited by electrolysis is set to 0.03 to 0.1 μIII.
If it is less than 0.1 μm, corrosion resistance will be insufficient, and if it exceeds 0.1 μm, problems such as decreased workability due to hydrogen embrittlement, coloration, and increased cost will occur.

Also, the amount of chromate film is 20 to 5 as the amount of chromium.
0 vessel Fi/l112 is 20 years old g/m2
If it is less than 50IIIg/l1, the corrosion resistance will be insufficient.
This is because if it exceeds 12, the adhesion of the coating will decrease when applied.

The precipitation of metallic chromium and the formation of a chromate film may be performed by a known one-step method or a two-step method. For example, in order to export 0.03 to 0.1μ of metallic chromium in a two-step method, a chromic anhydride-sulfuric acid system in which chromic anhydride is 80 to 2509/e and sulfuric acid is 1775 to 1/125 of chromic anhydride is used. Using an electrolytic bath, bath temperature 40-50°C
5 to form a chromate film at a current density of 20 to 50^/dJ;
~409/e electrolytic bath, bath temperature 20~50℃, current density 5
It may be performed at ~50Δ/dlI12.

When applying an aqueous solution containing water-soluble resin, emulsion resin, or both, the resin film after drying is 0.5
Apply so that it becomes ~4 μR. This is because cold-rolled steel sheets usually have a surface roughness of 1 to 3 μm, so the resin film has a surface roughness of 0.5 μm.
This is because if it is less than μm, it is difficult to form a uniform film on such a rough surface. In particular, when an aqueous solution has a viscosity of several epS and is applied to a plate, it flows along the convexities of the surface roughness, and the four parts become liquid pools, and the convex parts are either extremely thin or the base of the steel plate is partially exposed. Therefore, sufficient corrosion resistance cannot be exhibited. On the other hand, if the resin film exceeds 4μ, as shown in the attached drawings, spot welding becomes difficult and costs increase.

The aqueous solution can be applied by any known method (e.g. roll foot method, roll ripple method, etc.), and drying after application can be done by any known method (e.g. hot air drying oven, infrared lamp, etc.) until the steel plate temperature is 60 to 250°C. Just let it dry.

Water-soluble resin and emulsion contained in aqueous solution 04
It is preferable that Jl has good adhesion to the chromate film, has no restrictions on paint selectivity when applying a paint, and has excellent paint film adhesion.

Examples of such resins include polyacrylic resins, polyacrylonitrile resins, polyethyleneimine O fats, and modified 84 fats containing these as main components.

It is also possible to add additives that improve edibility, film adhesion and weldability. As this additive, water-soluble or water-dispersible inorganic substances are preferable. For example, to improve corrosion resistance and weldability, metal sols (alumina sol, zirconia sol, magnesia sol, titania sol, silica sol, etc.) are used, and chromate coatings and Silica colloid, titanium dioxide, aluminum silicate, silane coupling agent, titanium coupling agent, barium carbonate, calcium carbonate, clay, talc, barium sulfate are used to improve the adhesion of the paint, the adhesion of the paint film after painting, and the corrosion resistance. ,
Suitable materials include diatomaceous earth, powdered silica, and anti-rust pigments.

Suitable rust-preventive pigments include chromates, tungstates, and molybdates. Among these, chromates include ammonium chromate, magnesium chromate, potassium chromate, potassium or sodium dichromate, Calcium chromate, zinc chromate, manganese chromate, nickel chromate, cobalt chromate,
Strontium chromate, etc., and tungstates such as sodium tungstate, potassium tungstate, ammonium tungstate, etc.
Examples of molybdates include potassium molybdate, potassium molybdate, and ammonium molybdate.

Further, it is also possible to further add a solid lubricant to the resin within a range that does not impair processability. Adding a solid lubricant may be necessary depending on the application, such as roll 7 ohming or press molding. This is to prevent it from peeling off or peeling off.

The reason for adding a solid lubricant here is that liquid lubricants such as synthetic resin oil and semi-solid lubricants such as wax can also be used, but these are not suitable for lubricating. effect, long-term storage stability of aqueous solution, dispersibility in aqueous solution,
This is because they are inferior to solid lubricants in terms of stability.

Preferably, the solid lubricant has good water dispersibility, is inexpensive, and does not reduce corrosion resistance, adhesion, weather resistance, etc. even when incorporated into the resin film. Examples of such solid lubricants include molybdenum disulfide, tungsten disulfide, boron nitride, graphite, graphite 7], and these solid lubricants may be used alone or in combination.

Next, the present invention will be explained with reference to examples.

(Example) A cold-rolled steel plate with a thickness of 0.6 mm was subjected to electrolysis using the chromic acid electrolyte and the electrolytic conditions shown in Table 1 to deposit metallic chromium and form a chromate film thereon. The O (fat-containing aqueous solution shown in No. 1) was applied using a roll coater and dried to form a resin film.

Thereafter, test pieces were taken from this cold-rolled steel plate and tested for corrosion resistance (salt spray test, wet test) and weldability, and the coefficient of dynamic friction and roll 7 ohming property were investigated. In addition, the test pieces were painted and tests for corrosion resistance and paint film adhesion were also conducted. The results are shown in Table 2.

Among the coating aqueous solutions in Table 1, the polyacrylonitrile aqueous solution was obtained by dissolving a commercially available polyacrylonitrile resin in water, but the other solutions were prepared as follows.

(1) Polyacrylic O (fat aqueous solution A).

Acrylic emulsion obtained by emulsion polymerization of 50% by weight of methyl methacrylate, 40% by weight of butyl acrylate, 9.8% by weight of acrylic acid, and 0.2% by weight of styrene [resin content: 40% by weight, pH 2.3] , viscosity 25 c
ps (25°C, B type viscometer), acid value per solid content 7
The aqueous solution of Item 1 of No. 8 was designated as A1.

(2) Polyacrylic resin aqueous solution A2 Silica sol 2009/e is added to the aqueous solution A1, (vinyl-tri-β-methoxy) silane 29/e is added to the aqueous solution A1, silane 29/e is added to the aqueous solution A1, and tetra(2009/e) is a titanate coupling agent. ,2diallyloxymethyl-1-,butyl)bis(di-tridecyl)phosphite titanate 0.
59/e, ammonium chromate 159/e, solid lubricant [molybdenum disulfide type, product name Oil Hi] C3
07, Nippon Graphite Industries Co., Ltd.] 409/e was added, and this aqueous solution was designated as A2.

(3) Polyethyleneimine resin aqueous solution B.

Ethyl orthosilicate 5 after hydration with 85% phosphoric acid 59
09 Ohabi γ-7 Mi-7 Probyltriethoxysilane 59
was slowly added under stirring, hydrolyzed at a temperature of -130°C, and water was added to bring the total volume to 308 Ioe.
The alkyl silicate hydrolyzate and the alkoxysilane hydrolyzate are each converted to h Si 02 te 46.89
A cloudy aqueous solution containing 4.49/e and 4.49/e was obtained.

Next, this aqueous solution 1009 was reacted with the following cationic resin aqueous dispersion, that is, epoxy polybutadiene with a molecular weight of about 1000 and polyethyleneimine with a molecular weight of about 300, and then neutralized with acetic acid. Then pl with phosphoric acid
A water-dispersible epoxy resin aqueous dispersion containing cationic nitrogen @ 2.3 mmol, adjusted to about 2.5 l, was added as a solid content of 109, and then water was added to bring the total amount to 200.
I changed it to m9.

The aqueous solution prepared here contains alkyl silicate hydrolyzate and alkoxysilane hydrolyzate.
io2 conversion Te approx. 23.49 /6 Oyohi 2.29 /
Contains all e, also contains cationic resin 509/e, p
H was 2.3.

Furthermore, compared to this cationic resin-containing aqueous solution 500+a8, a zwitterionic 0 (fatty aqueous dispersion, that is, an acrylic trichloride with a molecular weight of about 200,000 consisting of 40% methyl methacrylate, 50% butyl acrylate, and 10% acrylic acid) After reacting the original copolymer with epoxy polybutadiene and ethylene amine, a water-dispersible acrylic resin was neutralized with phosphoric acid (cationic nitrogen: 1.2 mmol/9, carboxyl group: 0.2 mlj) 709% of the solid content of the aqueous dispersion (containing mol/9) was added, and the condensate was added to bring the total amount to 90%.
The dose was set to 0 mg.

The aqueous solution prepared here contains an alkyl silicate hydrolyzate and an alkoxysilane hydrolyzate, respectively.
Contains approximately 13.097e and 1.29/e in terms of conversion, and a total of 13.097e and 1.29/e of cationic resin and zwitterionic resin.
It contained 05,597g. Moreover, pll was 2.5. This aqueous solution was designated as B.

(4) Polyethyleneimine resin aqueous solution B2 The titanate coupling agent tetra(2,2
Noaryloxymethyl-1-butyl)bis(di-tridecyl)phosphite titanate 19/e, ammonium chromate 29/e1 solid lubricant [graphite-based, trade name
Aldais F33G, Nippon Graphite Industries Co., Ltd.! i! ] to 2
09i was added to bring the aqueous solution to 82.

Moreover, each test in Table 2 was conducted and evaluated in the following manner.

(+) As-post-treated (1) Salt water spray test After 24 hours of testing according to JIS 22371, the test piece was observed with the naked eye, and the area where red rust occurred was evaluated using the following criteria.

Symbol Area where red rust occurs ◎ 0 to less than 3% 0 3% or more to less than 10% △ 10% or more to less than 30% × 30% or more (2) Humidity test left in an atmosphere of 50°C and 98% humidity for 72 hours Thereafter, the test piece was observed with the naked eye, and the area where red rust occurred was evaluated using the same criteria as in the salt spray test.

(3) Weldability test Using an electrode with a shape of 4iamφ-CF, welding force of 15
0Kg, t1110 cycles when energized, spot f8 welding at welding current 7 to 8, weldable, tensile strength 200
The evaluation was rated as ○ if a normal flattened F was formed at Kg, and × as if no current could be applied or welding was possible.

(4) Roll 7 ohming test After forming the roll 7 ohm, cut out a test piece including the processed part and conduct a salt spray test for 12 hours as described in (1) above. was evaluated using the following criteria.

Symbol Area where red rust occurs 0 0 to less than 3% 0 3% or more to less than 10% Δ 10% or more to less than 30% ) and epoxy-polyester paint (2 coats, 2 bakes)
The two types were painted in the following manner.

(a) Painting melamine alkyd paint After applying the paint to a dry film thickness of 30 μm using the roll coating method, 1
It was dried at 20°C for 2 minutes.

(b) Painting with epoxy-polyester paint After applying epoxy paint to a dry film thickness of 5μ,
Dry at 0°C for 30 seconds, apply polyester paint on top to a dry film thickness of 15μ, and dry at 250°C.
and dried for 45 seconds.

Next, the following tests were conducted on the thus coated material.

(5) Paint film adhesion test (a) Grid test After making cuts in the paint film at intervals of 1 m in a grid pattern based on JIS G 3321 for colored galvanized iron plates, apply cellophane tape and peel it off at 8 speed. The state of the peeled coating film was evaluated based on the following criteria.

Symbol: Paint film peeling state O: No paint film peeling at all Δ: 1 to 10% of the paint film peeling: X: 11% or more of the paint film peeling off (b) Erichsen test in the same manner as above, making cuts at 211 II 11 intervals After placing it in the shape of a grid, it was extruded using an Erichsen test stone, and the extruded portion was peeled off with cellophane tape in the same manner as above. Evaluation of paint film peeling was performed in the same manner as the goblin test.

(6) Salt spray test Erichsen test failure and extrusion of 6+u++
), and the area of red rust on the Erichsen extrusion was evaluated using the following criteria. The salt water spray n test was conducted for 300 hours in the case of the melamine alkyd paint coated board and for 500 hours in the case of the epoxy-polyester paint coated board.

Symbol Area where red rust occurs 0 Less than 1% 0 1% or more and less than 3% 0 3% or more and less than 10% Δ 10% or more and less than 30% × 30% or more As is clear from Table 2, the post-treatment of the present invention Cold-rolled steel sheets treated with this method have excellent corrosion resistance and can be welded.

It also has excellent paint film adhesion and has no limitations on paint selectivity. Furthermore, if a solid lubricant is contained in the aqueous solution, molding processing can also be applied.

(Effects) As explained above, the post-treatment method for cold-rolled steel sheets of the present invention has excellent corrosion resistance, so even when used in applications with severe corrosive environments or used for long periods of time, conventional post-treatment methods cannot It is less prone to red rust than treated products.

[Brief explanation of drawings]

The attached drawing is a graph showing the relationship between the thickness of the fat film formed by the post-treatment method of the present invention and spot weldability.

Claims (5)

[Claims] (1) 0.03 to 0.1 μm on the surface of cold rolled steel sheet by electrolysis
After sequentially forming metallic chromium of A method for post-treatment of cold-rolled steel sheets, characterized by forming a resin film of 0.5 to 4 μm. (2) The method for post-treatment of a cold rolled steel sheet according to claim 1, characterized in that the aqueous solution further contains silica sol, a rust preventive pigment, and a solid lubricant. (3) The method for post-processing a cold rolled steel sheet according to claim 1, wherein the resin contained in the aqueous solution is a polyacrylic resin. (4) The method for post-treatment of a cold rolled steel sheet according to claim 1, wherein the resin contained in the aqueous solution is a polyacrylonitrile resin. (5) The method for post-treatment of a cold rolled steel sheet according to claim 1, wherein the resin contained in the aqueous solution is a polyethyleneimine resin.