JPH0148870B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to improvements in weldable coated steel sheets with zinc-rich coatings. In recent years, weldable coated steel sheets, which are cold-rolled steel sheets coated with zinc-rich paint, have been used for areas that require corrosion protection, such as the underside of automobile bodies and doors. However, in this weldable coated steel sheet, the base plate was a cold-rolled steel plate, and the zinc-rich coating film also contained zinc powder, so both the base coat and the coating film had insufficient corrosion resistance, and the coating film was powdered during processing. It had the disadvantage of causing In addition, since only a chromate film was present on the back surface, it had the disadvantage of easily rusting. Therefore, the present invention provides a weldable coated steel sheet that eliminates the drawbacks of conventional weldable coated steel sheets. The present invention uses a Zn-Fe alloyed hot-dip galvanized steel sheet (a steel sheet is hot-dip galvanized and then heat-diffused to make the plating layer Zn-Fe alloyed) as the base plate for coating, and also creates a gap between the chromate film and the zinc-rich paint film. A thin resin film is newly interposed on the zinc-rich coating, and zinc-magnesium alloy powder is added in addition to zinc powder to the zinc-rich coating to improve corrosion resistance through the plating layer, resin film, and zinc-magnesium alloy powder. The powdering resistance of the coating film is improved by using zinc-magnesium alloy powder. The resin film is formed by coating and drying an aqueous solution containing a hydrolyzate of an alkyl silicate, a hydrolyzate of a tri- and/or dialkoxysilane compound, a cationic resin and/or an amphoteric resin, and phosphoric acid. Uses resin film. That is, the present invention applies a hydrolyzate of an alkyl silicate, a hydrolyzate of a tri- and/or dialkoxysilane compound, a cationic resin, and/or an amphoteric resin to the surface of a Fe-Zn alloyed hot-dip galvanized steel sheet through a chromate film. A resin film is formed by applying and drying an aqueous solution containing an ionic resin and phosphoric acid, and a zinc-rich coating film containing 60% by weight or more of zinc powder and zinc-magnesium alloy powder is further formed on the resin film. The present invention relates to a weldable multi-layer coated steel sheet with excellent corrosion resistance. The present invention will be explained in detail below. As mentioned above, the Zn-Fe alloyed hot-dip galvanized steel sheet of the present invention is produced by hot-dip galvanizing the steel sheet and then subjecting it to heat treatment to cause thermal diffusion between the steel base and the plating layer. It is a Zn-Fe alloy, and there is no problem even if the plating layer contains Al, which is normally contained in the plating layer of hot-dip galvanized steel sheets. Further, a material to which corrosion-resistant elements such as Mg, Ti, and Cr are added may also be used. Since this steel sheet usually has a higher surface roughness than a cold-rolled steel sheet, a large amount of chromate film adheres to it, improving corrosion resistance and improving the adhesion of the zinc-rich coating. Furthermore, the convex portions of the surface roughness shorten the distance from the welding tip during spot welding, etc., thereby improving electrical conductivity. The chromate film may be any conventionally known film.
Generally known chromate films are those formed by reactive chromate treatment (including reinforced chromate treatment in which an etching agent is added to chromic acid), and those formed by Norrinse coating type chromate treatment (including those in which organic resins and inorganic substances are added). There are those formed by electrolytic chromate treatment and those formed by electrolytic chromate treatment, but any of these may be used, or a combination thereof may be used. However, since Zn-Fe alloyed hot-dip galvanized steel sheets have excellent corrosion resistance and poor reactivity of the plating layer, it is necessary to form them by reactive chromate treatment using 5 to 60 g of chromic anhydride and 0.2 to 20 g of cobalt sulfate or cobalt nitrate. The treatment is preferably carried out with an aqueous solution containing 0.03 to 10 g of fluorine ion or fluorine complex ion. The amount of chromate film is controlled by the total amount of chromium contained in the film, and for corrosion protection, the total amount of chromium is 10 to 120 mg/m ²
It is preferable to If it is less than 10 mg/m ² , corrosion protection will be insufficient, and if it exceeds 120 mg/m ² , coating film adhesion will decrease and it will be easy to peel off during press working or the like. The resin film is a hydrolyzate of alkyl silicate,
A film obtained by applying an aqueous solution containing a hydrolyzate of an alkoxysilane compound, a cationic resin and/or amphoteric resin, and phosphoric acid and drying it without washing with water, and which has good electric weldability. To ensure this, the film thickness should be 2.0μ or less. But 0.1μ
If it is less than 0.1μ, the anticorrosion properties will be reduced. The alkyl silicate used in this aqueous solution is generally an alkyl silicate having 1 to 8 carbon atoms and a partial condensate thereof, such as methyl orthosilicate and ethyl orthosilicate. Examples of alkoxysilane compounds include tri- and/or dialkoxysilane compounds in which one or two organic groups and three or two alkoxy compounds are bonded to a silicon atom, such as γ-
These include aminopropyltriethoxysilane, methyltriethoxysilane, and γ-methacryloxypropyltrimethoxysilane. Cationic resins are water-soluble or water-dispersible resins that have cationic nitrogen in their molecules, and are made by reacting polyalkylene polyamines and their derivatives, such as polyethyleneimine and polypropyleneimine, and polyamines with polyglycidyl compounds. epoxy resins, diallylamine polymers, etc. Further, an amphoteric ionic resin is a water-soluble or water-dispersible resin having a cationic nitrogen and an anionic carboxyl group in its molecule, and for example, the above-mentioned cationic resin can be prepared by a known method (for example, Resins with carboxyl groups introduced as anionic groups (using chromic acetic acid), acrylic and alkyd carboxyl group-containing resins, aziridine compounds such as ethyleneimine, propyleneimine or hydroxyethyleneimine, glycidylamine or its salts, etc. These include resins obtained by reaction with basic nitrogen-containing alkylating agents. In this aqueous solution, the cationic resin and/or
Or, since phosphoric acid is coordinated at one end of the amphoteric ionic resin, it reacts with trivalent chromium in the chromate film to form chromium phosphate, which firmly adheres to the chromate film and coated with zinc-rich paint. In some cases, it causes a condensation reaction with hydroxyl groups and carboxyl groups in the paint, resulting in a strong adhesion. Furthermore, since chromium phosphate formed by the reaction between phosphoric acid and the chromate film has excellent corrosion resistance, it further improves the corrosion resistance of the chromate film. It is also possible to improve the performance by adding additives to this aqueous solution within a range that does not impair the processability of the film. For example, metal sols (e.g. alumina sol, zirconia sol, magnesia sol, titania sol, etc.) are used to improve weldability and anti-corrosion properties, and silica colloids, titanium dioxide, aluminum silicate are used to increase the anchoring effect. , barium carbonate, calcium carbonate, clay,
Talc, barium sulfate, diatomaceous earth, powdered silica, etc. can be added. The zinc-rich coating film has improved powdering resistance and corrosion resistance during processing by adding zinc-magnesium alloy powder to the conventional zinc-rich coating film that only contains zinc powder.The mixing ratio of both powders is Zn. Powder/Zn-Mg alloy powder = 50/50~
Make it 98/2. The reason why the upper limit of the amount of zinc-magnesium alloy powder added to zinc powder was set at 50% was that even if it exceeds 50%, the improvement effect on powdering properties and anticorrosion properties is not much different than when it is 50%, and the cost increases. be. On the other hand, the lower limit was set to 2%.
This is because if the amount is less than %, the addition effect cannot be obtained, and the powdering resistance and anticorrosion properties are almost the same as those of zinc powder alone. In addition, if the total content of zinc powder and zinc-magnesium alloy powder in the zinc-rich coating is less than 60%, the electrical conductivity will decrease and the electric weldability will deteriorate.
% or more. However, if it exceeds 91%, the resistance to peeling due to processing decreases, so it should be kept below 91%. As the resin for the coating film containing both of the above powders, a linear epoxy resin having a molecular weight of 10,000 to 100,000 is preferable in terms of quality and workability. If the thickness of the coating film (dry coating film) is less than 2μ, corrosion resistance will be poor, so it should be 2μ or more. However, if it exceeds 50μ, the conductivity will decrease and the cost will increase.
Keep it below 50μ. Regarding corrosion resistance, if the zinc powder and zinc-magnesium alloy powder alone do not provide sufficient corrosion resistance and a higher degree of corrosion resistance is required, it is also possible to add a rust preventive pigment. Suitable antirust pigments include strontium chromate, zinc chromate, and calcium leadate.
%Added. If it is added in an amount exceeding 5%, the elution of hexavalent chromium from the surface of the coating film becomes significant, the anticorrosive effect is saturated, and the effect of increasing the amount added is small. In addition, if the application requires an appearance, it is also possible to add a pigment to color the product. In the case of the present invention, as mentioned above, when the zinc-rich coating film contains zinc powder and zinc-magnesium alloy powder, powdering resistance and corrosion resistance during processing are improved, and the reason is thought to be as follows. . First, the powdering resistance is improved, and this is thought to be due to the fact that the hardness of the zinc-magnesium alloy powder is significantly higher than that of zinc. In other words, in general, if the powder is hard during press working, less powder will stick to the mold, so if the zinc-magnesium alloy powder is harder than zinc powder, it will stick less to the mold, resulting in powder resistance. This is thought to improve performance. On the other hand, the improvement in corrosion resistance is thought to be due to the zinc-magnesium alloy suppressing the galvanic action of zinc in the plating layer. Although magnesium is electrically less base than zinc, it produces stable corrosion products when placed in a corrosive environment, and
Alleviates the self-consuming galvanic effect of zinc. Therefore, although the details are unknown, it is presumed that the zinc-magnesium alloy also has the same effect as magnesium. Therefore, it is believed that the addition of this zinc-magnesium alloy alleviates the self-consuming galvanic action of zinc, suppresses the elution of more zinc than necessary from the plated layer or coating, and improves the corrosion resistance. In the case of the present invention, when the zinc powder and zinc-magnesium alloy powder are included in the zinc-rich coating film as described above, the powdering resistance and corrosion resistance are improved compared to the case where only the zinc powder is used. Add anticorrosive pigments as described above. The additive action of this anti-rust pigment is similar to that of zinc-magnesium alloy powder, and in addition to the anti-corrosion action of the anti-rust pigment itself,
It has the effect of suppressing excessive elution of zinc. In the present invention, the magnesium content of the zinc-magnesium alloy powder is not particularly limited, but one containing 1 to 5%, which can be manufactured on an industrial scale, is preferable from the viewpoint of workability and quality. This is because in the case of zinc, when magnesium is added, the hardness increases rapidly as shown in the attached drawing, and there is no problem with weldability.
This is because any content can be used for anti-corrosion properties. The weldable coated steel sheet of the present invention is manufactured by forming a chromate film on a Zn-Fe alloyed hot-dip galvanized steel sheet by a known method, and then forming a resin film thereon. Aqueous solution 1 is used as an aqueous solution for forming a resin film.
Hydrolyzate of alkyl silicate per SiO ₂
0.5 to 250 g in terms of _SiO2 conversion, 0.1 to 250 g of hydrolyzate of alkoxysilane compound in terms of SiO2, 0.5 to 250 g of cationic resin and/or zwitterionic resin,
Aqueous solutions each containing 1 to 250 g of phosphoric acid are used. If the coating is less than these lower limits, the film thickness will be reduced.
It is difficult to increase the thickness to 0.1μ or more, and the effect of addition cannot be obtained. If the amount exceeds the upper limit, the film tends to become thicker, and the pH becomes lower, which may cause the chromate film to dissolve. This aqueous solution is applied for 50~
Dry at 250°C, preferably 50-120°C. When dried in this manner, the water evaporates and the residue forms a resin film. Next, a zinc-rich paint containing zinc powder and zinc-magnesium alloy powder is applied onto this resin film and baked and dried. However, when this zinc-rich coating film is not required on the back side, the resin film is formed. Next, the effects of the present invention will be explained with reference to Examples. Example A chromate film, a resin film, and a zinc-rich coating were formed on a Zn-Fe alloyed hot-dip galvanized steel sheet and a cold-rolled steel sheet (dull finish) with a thickness of 0.8 mm under the conditions shown in Table 1, and their performance was investigated. I went. The chromate film and resin film were formed under the following conditions. (1) Formation of chromate film (a) Formation method A Chromic anhydride 5-60g/, cobalt sulfate or cobalt nitrate 0.2-20g/, fluorine ion or fluorine complex ion 0.03-10g/
was sprayed at 70°C for 10 seconds. (B) Formation method B: 10 parts by weight of chromium trioxide, 40 to 50% of which has been reduced to a trivalent state, 3 to 4 parts by weight of phosphoric acid, 4 to 5 parts by weight of polyacrylic acid, and an acrylic emulsion. An aqueous solution consisting of 17 to 20 parts by weight of polymer solids and 200 to 4000 parts by weight of water is applied by roll coating without washing with water.
Dry at 120°C. (2) Formation of resin film After adding water to 5 g of 85% phosphoric acid, 50 g of ethyl orthosilicate and 5 g of γ-aminopropyltriethoxysilane were slowly added with stirring, hydrolyzed at a temperature of 30°C, and water was added. By adjusting the total volume to 308 ml, a slightly cloudy aqueous solution (hereinafter referred to as aqueous solution (1)) containing about 46.8 g/and about 4.4 g/SiO ₂ of hydrolyzate () and hydrolyzate (), respectively, was obtained. Ta. This aqueous solution
(1) Cationic nitrogen obtained by reacting 100 g of epoxy polybutadiene (trade name BF-1000, manufactured by Adeka Argus) with a molecular weight of about 1000 and polyethyleneimine with a molecular weight of 300 and partially neutralizing it with acetic acid. An aqueous dispersion of a water-dispersible epoxy resin containing 2.3 mmol/g (PH 6.8, solid content 20%) was adjusted to pH 2.5 with phosphoric acid, and then 10 g of the solid content was added, and then water was added. By adjusting the total volume to 200ml, the hydrolyzate ()
and the hydrolyzate () as SiO ₂ at approximately 23.4 g/ and approximately 2.2 g/, respectively, and the resin at 50
g/containing aqueous solution (2) of PH2.3 (hereinafter referred to as aqueous solution)
(2). ) was obtained. Molecular weight approx. 40% methyl methacrylate, 50% butyl acrylate and 10% acrylic acid.
Zwitterionic water-dispersible resin obtained by reacting 200,000 acrylic terpolymer with ethyleneimine and neutralizing it with phosphoric acid (1.2 mmol/g of cationic nitrogen and carboxyl 70 as solid content (contains 0.2 mmol/g)
% and added to 500 ml of the above aqueous solution (2), and then added water to make the total volume 900 ml, to give about 13.0 g/ and about 1.2 g of SiO ₂ of hydrolyzate () and hydrolyzate (), respectively. An aqueous solution having a pH of 2.5 and containing a total of about 105.5 g of a cationic resin and a zwitterionic resin was obtained.
This was designated as aqueous solution A. Further, 10 g of colloidal silica (trade name: Snotex, manufactured by Nissan Chemical Co., Ltd.) was added to the aqueous solution A as an inorganic substance. This was designated as aqueous solution B.
The aqueous solution A and the aqueous solution B obtained as described above were applied onto the chromate film by a squeeze roll method and dried at 100° C. for 10 seconds without washing with water. (3) Formation of zinc-rich coating film 240℃ (plate temperature) after coating by roll coating method
Bake for 60 seconds.

【table】

【table】

[Table] The coating film performance was also investigated in the following manner. (1) Paint film adhesion A bending test was conducted in accordance with the JIS G 3312 test method for colored galvanized iron plates. In the bending test, 180 degrees of close bending was performed using 0 sheets (0 t), 1 sheet (1 t), and 2 sheets (2 t) at intervals on the inside of the bend. After that, cellophane tape was pasted on the coating film of the processed part, and it was suddenly bent. The cellophane tape was peeled off and evaluated according to the following criteria.

[Table] (2) Powdering resistance After cutting the test specimen into blanks with a diameter of 360 mm, they were coated with anti-corrosion lubricant (Oil Coat Z2, manufactured by Idemitsu Kosan) and punched with a punch diameter of 200 mm using a 300-ton hydraulic press.
Punch R13mm, die R4mm, drawing height 65mm,
A cylindrical deep drawing test was performed with the coated surface facing outward under the conditions of a 22-ton presser. After the test, the powder adhering to the die mold was scraped off with abrasive paper, the amount was visually evaluated, and the next step was performed. It was evaluated based on the criteria.

[Table] (3) Corrosion resistance Test specimens were cross cut with a knife, 2t 180 degree close bending, and the cut end was exposed, and a salt spray test was conducted based on JIS Z 2371. The time required for red rust to appear on the parts was evaluated. (4) Weldability After performing spot welding under the following welding conditions, a tensile test was conducted to investigate the incidence of tensile shear strength of less than 350 Kgf.

【table】

[Table] As is clear from Table 2, the painted base plate is Zn-Fe
Corrosion resistance and powdering resistance are improved by using an alloyed hot-dip galvanized steel sheet, interposing a resin film between the chromate film and the zinc-rich coating, and adding zinc-magnesium alloy powder to the zinc-rich coating. Furthermore, even if there is no zinc-rich coating on the back surface, the presence of a plating layer or resin film improves corrosion resistance, so it can be used in applications where the back surface corrosion resistance is insufficient with the conventional chromate film alone. As described above, the weldable coated steel sheet of the present invention has excellent corrosion resistance and powdering resistance, and if a resin film is also formed on the back surface, the corrosion resistance will be improved.

[Brief explanation of drawings]

The attached drawing is a graph showing the relationship between the Mg content and hardness of Zn-Mg alloys.

Claims (1)

[Claims] 1. Hydrolyzate of alkyl silicate, hydrolyzate of tri and/or dialkoxysilane compound, cationic resin and/or
Alternatively, a resin film is formed by applying and drying an aqueous solution containing an amphoteric ionic resin and phosphoric acid, and a zinc-rich coating film containing 60% by weight or more of zinc powder and zinc-magnesium alloy powder on the resin film. A weldable multi-layer coated steel sheet with excellent anti-corrosion properties.