JPH0114868B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to improvements in weldable coated steel sheets in which the coating film is rendered electrically conductive by zinc powder. In recent years, one-sided anti-corrosion steel plates, which have undergone anti-corrosion treatment on one side in advance, have been used in the underbody of some automobiles to prevent corrosion from the back side. This single-sided corrosion-resistant steel sheet is required to have weldability and paintability on the uncorrosion-treated surface, just like the conventionally used cold-rolled steel sheet. Galvanized steel sheets such as galvanized steel sheets were used, but they had the drawback of being expensive due to low production efficiency and a large number of steps. For this reason, in recent years, single-sided coated steel plates in which weldability and anti-corrosion properties have been imparted to the coating film have been studied as single-sided anti-corrosion steel plates that can be manufactured at low cost. The present inventors applied 10 parts by weight of chromium trioxide, 40 to 50% of which has been reduced to a trivalent state, and phosphoric acid %H ₃ PO ₄ ) 3 to 4 parts by weight, polyacrylic acid 4 to 5 parts by weight, acrylic emulsion polymer solid content 17 to 20 parts by weight, and water for making an aqueous solution.
The undercoat layer, which is coated with a stable aqueous solution for metal surface coating containing 200 to 4000 parts by weight and dried, has a total chromium content of 10 to 4000 parts by weight.
50mg/m ² is formed, and a resin topcoat layer containing zinc powder is formed on this undercoat layer with a thickness of 10 to 50μ, and the coating amount of the undercoat layer is larger on the concave areas than on the convex areas due to the surface roughness. We proposed weldable painted steel sheets, which are becoming more and more popular. This coated steel sheet has poor electrical weldability because the undercoat layer does not contain conductive metal powder and contains an insulating resin as a binder. The adhesion of the coating layer is good, and conversely, the greater the amount of coating, the better the corrosion resistance.This problem has been solved by creating surface roughness on the steel plate, which makes it difficult to improve both properties at the same time. However, in view of the performance of the undercoat layer, the above-mentioned coated steel sheet had to be manufactured by strictly controlling the ratio of hexavalent and trivalent chromium in the aqueous solution.
However, due to the narrow scope of control, it was not easy to make adjustments. Furthermore, since there is a limit to the improvement in corrosion resistance by increasing the amount of zinc powder contained in the top coat layer, there has been a demand for improvement in corrosion resistance by using an undercoat layer. In order to satisfy the above demands and solve the drawbacks, the inventors of the present invention conducted various studies and found that zinc-iron phosphate [Zn ₂ Fe (PO ₄ ) ₂ 4H _{2 ]} was added between the steel sheet and the undercoat layer. O] It was discovered that the desired purpose could be achieved by forming a film. FIG. 1 schematically shows a cross section of a weldable coated steel plate of the present invention, where 1 is a steel plate with surface roughness, and 2 is a zinc-iron phosphate film formed on the surface of this steel plate 1. be. 3 is this zinc phosphate. A chromate-based undercoat layer 4 is formed on the iron coating 2, and a topcoat layer 4 containing zinc powder is further formed on the undercoat layer 3. The structures and compositions of these will be explained in detail below. First, regarding the steel plate 1, the surface of the steel plate 1 is rough, and its surface roughness (roughness measured by a surface roughness meter) is
Rmax) is 4 to 20μ. This surface roughness was created to shorten the distance between the tip of the welding machine that contacts the top coat layer 4 and the steel plate 1 during electric welding, and to improve weldability. Electric weldability is improved. However, if the size is too large, the thickness of the product coating film including the zinc phosphate-iron coating 2 will become uneven, stress will be concentrated in a specific area during processing, and coating film cracks will occur in that area. The thickness of the coating film on the convex portions 5 of surface roughness becomes extremely thin and corrosion resistance deteriorates, so it is necessary to reduce the thickness to 20μ or less. Furthermore, if the surface roughness is too small, no improvement in electric weldability can be expected, so it is necessary to set it to 4μ or more. The surface roughness of the steel plate 1 is most preferably formed by a chemical etching method (for example, etching with an aqueous ferric chloride solution) or a shot blasting method because it is uniform and dense. However, in industrial implementation, the former method is preferable. The latter method has the problem of difficulty in controlling the concentration of the etching solution, and the latter method also has problems such as environmental pollution and difficulty in grid recovery. Therefore, in order to industrially form a uniform and dense surface roughness, it is preferable to carry out skin pass rolling using dull rolls. Zinc phosphate formed on the surface of steel plate 1. iron coating 2
is this zinc phosphate. By widening the range of the ratio of hexavalent and trivalent chromium in the chromate-based undercoat layer 3 formed on the iron coating, it is possible to easily manage the stable aqueous solution for coating metal surfaces, and to apply the undercoat layer 3. Even if the amount is increased, the adhesion of the top coat layer 4 can be maintained well. The conventional undercoat layer 3 contains 10 parts by weight of chromium trioxide, 40 to 50% of which has been reduced to a trivalent state, and phosphoric acid (100 parts by weight).
%H ₃ PO ₄ ) 3 to 4 parts by weight, polyacrylic acid 4 to 4 parts by weight
5 parts by weight, acrylic emulsion polymer solids content 17
~20 parts by weight and 200 parts of water to make an aqueous solution
A stable aqueous solution for coating metal surfaces containing 4000 parts by weight was coated and dried, and its composition was the residue of the aqueous solution from which water had evaporated during drying. In the case of a conventional aqueous solution, as mentioned above, the hexavalent chromium of chromium trioxide is 40
~50% of chromium trioxide was required to be reduced to trivalent, but by forming a zinc-iron phosphate film on the underside of the undercoat layer, chromium trioxide can be reduced without changing other compositions. It has been found that there is no problem in terms of performance even if the ratio of the amount of hexavalent chromium to the amount of trivalent chromium is changed and expanded from 0 to 2.3. Therefore, in the undercoat layer of the weldable coated steel sheet of the present invention, all or a part of hexavalent chromium is reduced to trivalent chromium, and the amount of hexavalent chromium/3
Chromium trioxide with a valence chromium content ratio of 0 to 2.3
10 parts by weight, 3 to 4 parts by weight of phosphoric acid (100% H ₃ PO ₄ ), 4 to 5 parts by weight of polyacrylic acid, 17 to 20 parts by weight of acrylic emulsion polymer solids, and 200 parts by weight of water to make an aqueous solution. A stable aqueous solution for metal surface coating containing ~4000 parts by weight is coated and dried.
The composition has a significantly increased amount of trivalent chromium compared to the conventional composition. In addition, the coating amount of the undercoat layer has traditionally been controlled by the total chromium amount, which is the amount of all chromium contained in the layer, but the coating amount is based on the total chromium amount (hereinafter, the coating amount of the undercoat layer is the total chromium amount). (refers to the value of
Up to 50 mg/m ² , and if the amount is higher than this, there is a problem that the adhesion of the top coat decreases. For this reason, even if conventional attempts were made to improve the corrosion resistance of the undercoat layer, it was difficult to increase the coating amount to more than 50 mg/ ^m2 , but forming a zinc-iron phosphate film under the undercoat layer It was found that the coating weight of the coating layer could be increased up to 100/m ² . In addition, the lower limit of the coating amount of the undercoat layer was conventionally 10 mg/ ^m2 due to corrosion resistance, but zinc phosphate
It was found that the corrosion resistance improved by the iron coating made it possible to maintain the same corrosion resistance as before even when the amount was reduced to 5 mg/m ² . Figure 2 shows the coating adhesion and corrosion resistance of weldable coated steel sheets with and without a zinc phosphate-iron phosphate film formed on the underside of the undercoat, depending on the amount of undercoat applied. Curves 10 and 11 respectively indicate the coating adhesion and corrosion resistance when a zinc phosphate-iron coating is formed;
0a and 11a indicate coating film adhesion and corrosion resistance, respectively, when no zinc phosphate-iron coating was formed. The weldable painted steel plate was prepared under the conditions shown in Table 1, and the coating adhesion was 180.
Corrosion resistance was determined by a 240-hour salt spray test in accordance with JIS Z 2371.

【table】

[Table] If the amount of zinc-iron phosphate film formed in the present invention is less than 1 mg/ ^m2 , a uniform film will not be formed, and the adhesion of the top coat layer will decrease as the amount of undercoat layer increases. If the amount exceeds 500 mg/m ² , the electrical conductivity decreases and the electric weldability deteriorates, so the amount of the film is preferably 1 to 500 mg/m ² . For the zinc phosphate-iron coating, commercially available zinc phosphate coating treatment solutions [e.g. Granozin 46N-1,
DP#2000 (all manufactured by Nippon Paint) etc.]
If a steel plate is treated using a conventional method, it will be formed on the surface of the steel plate. Note that even if the zinc phosphate-iron coating 2 is formed on the surface of the steel plate 1 with surface roughness as shown in FIG. Therefore, the convex portions 5 and concave portions 6 of surface roughness exist. Therefore, when a stable aqueous solution for metal surface coating is applied on this zinc phosphate-iron coating 2 to form the undercoat layer 3, the aqueous solution flows down from the convex portions 5 to the concave portions 6 after application, so that the undercoat is The amount of coating of the layer 3 is such that the convex portions 5 are thin and the concave portions 6 are thick. When the distribution of the amount of chromium in the layer is investigated using an X-ray microanalyzer, the concave portions 6 are as shown in FIG. Here, as the thickness of the undercoat layer in the recesses 6 increases, the adhesion of the topcoat layer 4 decreases, but this decrease is compensated for by the anchoring effect due to the surface roughness and the increase in surface area. No decrease in adhesion was observed. The overcoat layer 4 formed on the undercoat layer 3 is a synthetic resin containing zinc powder, which is an anticorrosive and conductive substance. In the case of this overcoat layer, in order to increase the corrosion resistance and electrical conductivity of the coating film, it is preferable to minimize the amount of synthetic resin as a binder. Because I need it,
Zinc powder can only contain a maximum of 96% by weight. Further, the zinc powder is required to be at least 80% by weight in order to impart good electrical conductivity to the top coat layer.
The average diameter of the zinc powder is 1.5~1.5 in consideration of paintability.
A suitable value is 10μ, preferably 1.5 to 6μ. Various synthetic resins can be used, but epoxy resins with excellent adhesion, especially molecular weight 1, which can be baked and dried in a short time at a plate temperature of 200 to 260°C without the addition of hardeners or curing accelerators, are suitable. ~100,000 is appropriate. Further, the film thickness is set to 10 to 50μ, since corrosion resistance is poor if it is less than 10μ, and electric weldability is not improved even if the steel plate surface roughness is increased if it exceeds 50μ. Example 1 A zinc-iron phosphate film of 212 mg/m ² was formed on the surface of a cold-rolled steel plate (thickness: 0.8 mm) with a surface roughness of 4 μ using a commercially available zinc phosphate treatment solution, and one where it was not formed. On the other hand, a stable aqueous solution for metal surface coating was used to form an undercoat layer with varying ratios of hexavalent chromium to trivalent chromium and coating amounts, and on top of this, 85% by weight of zinc powder was applied as a dry coating. The coating adhesion and corrosion resistance of a weldable coated steel sheet with a 15μ thick epoxy resin topcoat layer were investigated. The composition of the stable aqueous solution for coating metal surfaces is as follows. Chromium trioxide 10 parts by weight Phosphoric acid 3 parts by weight Polyacrylic acid 5 parts by weight Acrylic emulsion Polymer solid content 18 parts by weight Water 2000 parts by weight Table 2 shows the investigation results when the coating amount of the undercoat layer was kept almost constant. Also, Table 3 shows the investigation results when the ratio of the amount of hexavalent chromium/the amount of trivalent chromium was kept constant.

[Table] (Note) Items marked with an asterisk (*) in corrosion resistance often cause blisters.

【table】

[Table] Example 2 Various amounts of zinc-iron phosphate films were formed on one side of a cold-rolled steel plate (thickness: 0.8 mm) with a surface roughness of 20μ,
On top of that, the stable aqueous solution for metal surface coating used in Example 1 (however, the ratio of hexavalent chromium amount/trivalent chromium amount is 1.4)
An undercoat layer of 79 mg/m ² was formed thereon, and an overcoat layer similar to that in Example 1 was formed on this undercoat layer to produce a coated steel sheet that could be welded on one side, and its spot weldability was investigated. Table 4 shows the results.

【table】

[Table] As described above, the weldable coated steel sheet of the present invention has a zinc phosphate-iron film formed on the underside of the undercoat layer, which makes it more durable compared to a case where a zinc phosphate-iron film is not formed. This makes it possible to widen the ratio range of the amount of hexavalent and trivalent chromium in the undercoat layer, making it easier to manage a stable aqueous solution for coating metal surfaces during production. Furthermore, since the coating amount of the undercoat layer can be greatly increased, the anticorrosion properties of the undercoat layer can also be improved.

[Brief explanation of drawings]

Fig. 1 is a schematic cross-sectional view of a weldable coated steel sheet of the present invention, and Fig. 2 shows a weldable coated steel plate in which an undercoat layer is directly formed on the steel plate and a case in which a zinc-iron phosphate coating is applied to the steel plate. This figure shows the relationship between the coating amount of the undercoat layer and the coating film adhesion and corrosion resistance when the undercoat layer is formed through the coating. Figure 3 shows the distribution of chromium in the undercoat layer formed on a steel plate through a zinc phosphate-iron film, investigated using an X-ray microanalyzer, and is indicated by dots. The area with a large amount of chromium is the concave part of the steel plate surface roughness. 1... Steel plate, 2... Zinc phosphate-iron film, 3...
...Undercoat layer, 4...Top coat layer, 5...Protrusions, 6...Concavities, 10...Coating film adhesion of weldable coated steel sheet when zinc phosphate-iron film is formed, 10a …
...Coating adhesion of weldable coated steel sheet when zinc phosphate-iron film is not formed, 11...Corrosion resistance of weldable coated steel sheet when zinc phosphate-iron film is formed, 11a... ...Corrosion resistance of weldable painted steel sheets when no zinc phosphate-iron film is formed.

Claims (1)

[Claims] 1 A zinc phosphate-iron film of 1 to 500 mg/ ^m2 is formed on the surface of a steel plate with a surface roughness of 4 to 20μ, and all or part of hexavalent chromium is added to the surface of the zinc phosphate-iron film. 10 parts by weight of chromium trioxide, which has been reduced to a valent state and has a ratio of hexavalent chromium/trivalent chromium of 0 to 2.3, and 3 to 4 parts of phosphoric acid (100% H ₃ PO ₄ ).
An undercoat coated with a stable aqueous solution for metal surface coating containing 4 to 5 parts by weight of polyacrylic acid, 17 to 20 parts by weight of acrylic emulsion polymer solids, and 200 to 4000 parts by weight of water to form an aqueous solution and dried. The layer has a total chromium content of 5 to 100 mg/m ² , and on top of this undercoat layer is a resin overcoat layer containing zinc powder of 10 to 100 mg/m 2 .
A weldable coated steel plate characterized by each having a thickness of 50μ.