JPH0120060B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a zinc-rich weldable coated steel sheet that causes less powdering during processing such as press working. In recent years, in the case of automobile bodies, weldable coated steel sheets, which are cold-rolled steel sheets coated with zinc-rich paint through a chromate-based coating, have been used for parts such as undercarriages and doors that require corrosion protection. Conventionally, the zinc-rich coating film of this type of painted steel sheet contained only zinc powder, but its anticorrosion properties are superior to those of hot-dip galvanized steel sheet and electrogalvanized steel sheet. However, since the zinc-rich coating film contains a large amount (usually 80% by weight or more) of zinc powder to provide electrical conductivity, it has the disadvantage of causing powdering during press processing. The present invention provides a zinc-rich weldable coated steel sheet that has improved the powdering problem of the zinc-rich coating film. The present invention was developed as a result of various studies aimed at improving the powdering resistance of zinc-rich coatings containing high amounts of zinc powder.The results show that adding zinc-magnesium alloy powder to zinc powder improves the powdering resistance and also improves corrosion resistance. This was done based on the knowledge that zinc powder and zinc-
A zinc-rich coating film containing 60% by weight or more of magnesium alloy powder is formed. The present invention will be explained in detail below. The coated steel sheet of the present invention, as shown in the attached drawing, has a chromate-based film 2 formed on the surface of a steel sheet 1, and further contains zinc powder 3 and zinc-magnesium alloy powder 4 thereon. A zinc-rich coating film 5 is formed. As the steel plate 1, a cold-rolled steel plate is usually used, but one with a roughened surface is used. This increases the amount of chromate film 2 deposited to improve corrosion resistance, shortens the distance between the welding machine tip and steel plate 1 to make it easier to conduct electricity, and even provides an anchor effect to the zinc-rich coating 5. It is intended for purposes such as granting. For this surface roughening, an average surface texture Rz of 4 to 20 microns is appropriate. If it is less than 4μ, the above-mentioned effects cannot be obtained, and if it exceeds 20μ, the coating surface may become rough and the anti-corrosion properties may decrease, or the coating may become distorted during processing, concentrating on convex portions and causing the coating to peel off. It becomes easier to wake up. The chromate-based film 2 may be formed by a known chromate treatment method such as a reactive chromate treatment (for example, chromate alone or an etching agent added thereto), a no-rinse coating type chromate treatment, or an electrolytic chromate treatment. However, after various studies, it is preferable to use a coating type chromate treatment liquid having the composition shown below in terms of corrosion protection and coating adhesion. Treatment liquid 1 (a) 10 parts by weight of chromium trioxide, 40-50% reduced to trivalent state (b) 3-4 parts by weight of phosphoric acid (100% H ₃ PO ₄ ) (c) 4-4 parts by weight of polyacrylic acid 5 parts by weight (d) Acrylic emulsion polymer solid content 17-20 parts by weight (e) 200-4000 parts by weight of water for making an aqueous solution Treatment liquid 2 In the composition of the treatment liquid 1, (a) chromium trioxide A treatment liquid in which all or part of the hexavalent chromium is reduced to a trivalent state, and the ratio of the amount of hexavalent chromium to the amount of trivalent chromium is 0 to 2.3. In the case of the present invention, the amount of deposited chromate film 2 is controlled by the total amount of chromium contained in the film, and the total amount of chromium is controlled to be 10 to 50 mg/m ² . this is
If it is less than 10 mg/m ² , corrosion resistance will be poor and coating film adhesion will not be stable, making it impossible to always obtain good adhesion. On the other hand, if it exceeds 50 mg/m ² , corrosion resistance improves, but the adhesion of the coating decreases and it becomes easy to peel off during press working. Zinc-rich coating film 5 has improved powdering resistance and corrosion resistance during processing by adding zinc-magnesium alloy powder to the conventional zinc-rich coating film containing only zinc powder, and 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 the zinc powder is set at 50% is that even if the amount exceeds 50%, no further improvement in powdering properties or corrosion resistance can be expected. On the other hand, the lower limit was set at 2% because if it is less than 2%, there is no effect of addition, and the powdering resistance and corrosion resistance are almost the same as when using zinc powder alone. In addition, if the total content of zinc powder and zinc-magnesium alloy powder in the zinc-rich coating 5 is less than 60%, the electrical conductivity will decrease and the electric weldability will deteriorate.
Contain 60% 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 paint film (dry paint film) is less than 5μ, corrosion resistance will be poor, so the thickness should be 5μ or more. However, if it exceeds 50μ, the conductivity will not be improved even if the surface assemblage is increased, so it should be kept below 50μ. Regarding corrosion resistance, if the zinc powder and zinc-magnesium alloy powder alone do not provide sufficient corrosion resistance and a higher level of corrosion resistance is required, it is also possible to add a rust preventive pigment. Suitable anti-rust pigments include strontium chromate, zinc chromate, and calcium leadate.
%Added. If it is added in an amount exceeding 5%, the elution of hexavalent chromium etc. from the coating film surface becomes significant, the anticorrosive effect is saturated, and the effect of increasing the amount added is small. 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, powdering resistance is due to zinc-
This is thought to be due to the fact that the hardness of 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 anti-corrosion property is thought to be due to the galvanic effect suppressing effect of the zinc-magnesium alloy on zinc. Magnesium is electrochemically less base than zinc, but when placed in a corrosive environment it produces stable corrosion products and also alleviates the galvanic effect on zinc. Therefore, although the details are unknown, it is presumed that the zinc-magnesium alloy also exhibits the same effects as magnesium. Therefore, it is thought that when a zinc-magnesium alloy is contained, the self-consuming positive galvanic action of zinc is alleviated, and the elution of zinc more than necessary is suppressed, thereby improving corrosion protection. In the case of the present invention, when the zinc-rich coating film 5 contains zinc powder and zinc-magnesium alloy powder as described above, the powdering resistance and corrosion resistance are improved compared to the case where only zinc powder is used, but when it is desired to further improve the corrosion resistance, Add anti-rust pigments as described above.
The action of adding 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 also has the action of suppressing excessive elution of zinc. In the case of the present invention, the magnesium content of the zinc-magnesium alloy is not particularly limited;
~5% of the products that can be manufactured on an industrial scale are
Favorable quality. This is because, in the case of zinc, when magnesium is added, the hardness increases rapidly as shown in FIG. 2, and alloys with arbitrary components can be used in relation to corrosion resistance. Next, the effects of the present invention will be explained with reference to Examples. Table 1 shows a cold-rolled steel plate (thickness 0.8 mm) whose surface has been roughened (Rz = 12 μ) by Dull skin pass, which is degreased to form a film amount of 30 mg/m ² (total chromium amount) on the surface. This is a summary of the coating composition and performance of a 15μ thick zinc-rich coating formed on top. The formation of the chromate-based film and the zinc-rich coating was conducted under the following conditions. (1) Chromate film formation conditions: 10 parts by weight of chromium trioxide, 3 parts by weight of phosphoric acid, 5 parts by weight of polyacrylic acid, 18 parts by weight of acrylic emulsion polymer solids, 2000 parts by weight of water, Cr ⁺⁶ /
Apply a paint-type chromate treatment solution consisting of Cr ⁺³ = 1.4 using the roll coating method. (2) Conditions for forming zinc-rich coating film: 250℃ (board temperature) after coating by roll coating method
Bake for 60 seconds.

【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. Peel off cellophane tape and evaluate 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 diaphragm test was performed with the painted 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 and bent 180 degrees at 4t.
The powdering resistance was tested for 500 hours and 240 hours based on JIS Z 2371, respectively, and evaluated according to the following criteria.

【table】

[Table] (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 1, even if the powder content is the same, those containing zinc-magnesium alloy powder have better electric weldability and corrosion resistance. Corrosion resistance can also be improved by adding antirust pigments. As described above, according to the present invention, the powdering resistance of zinc-rich coated steel sheets is improved, and the corrosion resistance is also improved accordingly.

[Brief explanation of drawings]

Figure 1 is a schematic sectional view of the painted steel plate of the present invention, Figure 2
The figure is a graph showing the relationship between Mg content and hardness of Zn-Mg alloy. 1... Steel plate, 2... Chromate film, 3...
Zinc powder, 4...Zinc-magnesium alloy powder,
5... Zinc-rich coating film.

Claims (1)

[Claims] 1. A zinc-rich coating film containing 60% by weight or more of zinc powder and zinc-magnesium alloy powder is formed on a steel plate with a roughened surface via a chromate-based film. Weldable painted steel sheet with excellent powdering resistance. 2. Claim 1, characterized in that the ratio of zinc powder/zinc-magnesium alloy powder is 50/50 to 98/2, and the total content of both in the coating film is 60 to 91% by weight. A weldable coated steel sheet with excellent powdering resistance described in . 3. The weldable coated steel sheet with excellent powdering resistance according to claim 1, characterized in that a rust preventive pigment is added to the zinc-rich coating film.