JPH0230392B2 - YOSETSUSEITOSOKOHAN - 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 anti-corrosion steel sheet is required to have weldability and paintability on the uncorrosion-treated surface, similar to 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. The inventors of the present invention have conducted various studies in order to satisfy the above demands and solve the drawbacks, and have found that the desired purpose can be achieved by forming an iron phosphate film between the steel sheet and the undercoat layer. I found out. 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 an iron phosphate film formed on the surface of this steel plate 1. 3 is a chromate-based undercoat layer formed on this iron phosphate film, and 4 is an overcoat layer containing zinc powder further formed on this 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 thickness is too large, the thickness of the product coating film, including the iron phosphate coating 2, will become uneven, stress will be concentrated in specific areas during processing, and coating cracks will occur in those areas, as well as surface roughness. The thickness of the coating film on the convex portions 5 becomes extremely thin and the corrosion resistance deteriorates, so it is necessary to reduce the thickness to 20 μm 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. There is a problem that it is difficult to control the concentration of the etching solution, and the latter method also has problems such as environmental pollution and difficulty in recovering the grid. Therefore, in order to industrially form a uniform and dense surface roughness, it is preferable to carry out skin pass rolling using dull rolls. The iron phosphate film 2 formed on the surface of the steel plate 1 widens the range of the ratio of hexavalent to trivalent chromium in the chromate-based undercoat layer 3 formed on the iron phosphate film, thereby improving the metal surface coating. This facilitates the management of a stable aqueous solution for use, and maintains good adhesion of the top coat layer 4 even if the amount of the undercoat layer 3 is increased. 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
It was necessary that ~50% of the chromium trioxide be reduced to trivalent, but if an iron phosphate film is formed on the underside of the undercoat layer, the 6% concentration of chromium trioxide can be reduced without changing other compositions. It was found that even if the ratio of valent chromium amount/trivalent chromium amount was changed and expanded from 0 to 2.3, there was no problem in terms of performance. Therefore, the undercoat layer in the weldable coated steel sheet of the present invention is a trivalent chromium in which all or a part of hexavalent chromium is reduced to trivalent chromium, and the ratio of hexavalent chromium amount/trivalent chromium amount is 0 to 2.3. 10 parts by weight of chromium oxide, 3 to 4 parts by weight of phosphorus (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 water for making an aqueous solution. A stable aqueous solution for coating metal surfaces containing 200 to 4000 parts by weight is coated and dried, resulting in a composition in which the amount of trivalent chromium is greatly increased compared to conventional products. 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 amount) is up to 50mg/if no iron phosphate film has been formed.
m ² or more, there was a problem that the adhesion of the top coat would deteriorate if the amount was increased beyond this value. For this reason, even if conventional attempts were made to improve the corrosion resistance of the undercoat layer, the amount of coating was reduced by 50%.
Although it was difficult to increase the coating amount to more than 100 mg/m ² , it was found that by forming an iron phosphate film under the undercoat layer, the coating amount of the undercoat layer could be increased to 100 mg/m ² .
In addition, the lower limit of the coating amount of the undercoat layer is based on the relationship with conventional corrosion resistance.
Although the corrosion resistance was 10 mg/m ² , it was found that the same corrosion resistance as before could be maintained even if the amount was reduced to 5 mg/m ² due to the improved corrosion resistance provided by the iron phosphate coating. Figure 2 shows the coating adhesion and corrosion resistance of weldable coated steel sheets with and without an iron phosphate film formed on the underside of the undercoat layer in relation to the coating amount of the undercoat layer. As shown, curves 10 and 11
10a and 1 indicate the coating adhesion and corrosion resistance when an iron phosphate film is formed, respectively.
1a shows the coating film adhesion and corrosion resistance when no iron phosphate film is formed, respectively. The weldable painted steel plates were prepared under the conditions shown in Table 1, and the coating adhesion was determined by a 180-degree close bending Sellotape peel test, and the corrosion resistance was determined by JIS Z 2371.
It was subjected to a 240-hour salt spray test in accordance with .

【table】

[Table] The iron phosphate film formed by the present invention has a film amount of 1
If it is less than 500mg/ ^m2 , a uniform film will not be formed and it will not be possible to compensate for the decrease in adhesion of the topcoat layer due to the increase in the amount of undercoat layer applied, and if it exceeds 500mg/ ^m2 , the electrical conductivity will decrease. Since this decreases the electrical weldability, the appropriate coating amount is 1 to 500 mg/m ² . The iron phosphate coating may be formed using a commercially available iron phosphate coating treatment solution, such as percolide.
40A, Bonderide 901 (all manufactured by Nippon Parker), Diuridine (manufactured by Nippon Paint), etc. may be used. Note that even if the iron phosphate coating 2 is formed on the surface of the steel plate 1 with surface roughness as shown in FIG. 1, the iron phosphate coating 2 will be formed along the surface shape of the steel plate 1. Convex portions 5 and concave portions 6 of surface roughness are present. Therefore, when a stable aqueous solution for metal surface coating is applied on this iron phosphate film 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 layer 3 The amount of coating is thinner in the convex portions 5 and thicker in the concave portions 6, and when the distribution of the amount of chromium in the layer is investigated using an X-ray microanalyzer, the concave portions 6 become 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 can be used at temperatures of 200 to 260℃ without the addition of curing agents or curing accelerators.
A suitable material has a molecular weight of 10,000 to 100,000, which can be baked and dried in a short time at a plate temperature of . Further, the film thickness is set to 10 to 50 μm, because if it is less than 10 μm, the anticorrosion properties will be poor, and if it exceeds 50 μm, the electrolyte properties will not be improved even if the steel plate surface roughness is increased. Example 1 A cold-rolled steel plate (thickness: 0.8 mm) with a surface roughness of 4 μ was coated with a 190 mg/m ² iron phosphate film using a commercially available iron phosphate treatment solution, and one without. Then, a stable aqueous solution for metal surface coating is used to form an undercoat layer with a varying ratio of hexavalent chromium to trivalent chromium and coating amount, and on top of this an undercoat layer containing 85% by weight of zinc powder in a dry coating film. Epoxy resin topcoat layer
The coating adhesion and corrosion resistance of weldable coated steel sheets with a thickness of 15μ 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. Table 3 shows the investigation results when the ratio of hexavalent chromium/trivalent chromium was kept constant.

[Table] (Note) Corrosion-resistant products marked with an asterisk (*) often cause blisters.

【table】

[Table] Example 2 Various amounts of iron phosphate films were formed on one side of a cold-rolled steel plate (thickness: 0.8 mm) with a surface roughness of 20μ, and the metal surface coating used in Example 1 was applied thereon. By a stable aqueous solution (however, the ratio of hexavalent chromium/trivalent chromium amount is 1.4)
A 74 mg/m ² undercoat layer was further formed on this undercoat layer, and an overcoat layer similar to that in Example 1 was formed 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, since the weldable coated steel sheet of the present invention has an iron phosphate film formed on the underside of the undercoat layer, the undercoat layer The ratio range of the amount of hexavalent and trivalent chromium can be widened, and the stable aqueous solution for coating metal surfaces can be easily managed 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 plate of the present invention, and Fig. 2 shows a weldable coated steel plate in which the undercoat layer is directly formed on the steel plate and in which the undercoat layer is formed on the steel plate via an iron phosphate coating. 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. Figure 3 shows the distribution of chromium in the undercoat layer formed on a steel plate through an iron phosphate film, investigated using an X-ray microanalyzer, and the area indicated by dots. is the area with a large amount of chromium in the concavity of the steel plate surface roughness. DESCRIPTION OF SYMBOLS 1... Steel plate, 2... Iron phosphate film, 3... Undercoat layer, 4... Top coat layer, 5... Convex part, 6... Concave part, 1
0... Film adhesion of the weldable coated steel plate when an iron phosphate film is formed, 10a... Paint film adhesion of the weldable coated steel plate when the iron phosphate film is not formed, 11... ...Corrosion resistance of a weldable coated steel plate when an iron phosphate film is formed, 11a...corrosion resistance of a weldable coated steel plate when an iron phosphate film is not formed.

Claims (1)

[Claims] 1 An iron phosphate 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 the hexavalent chromium is in a trivalent state on this iron phosphate film. 10 parts by weight of chromium trioxide which has been reduced to a ratio of hexavalent chromium/trivalent chromium of 0 to 2.3, 3 to 4 parts by weight of phosphoric acid (100% H ₃ PO ₄ ), and 4 to 5 parts by weight of polyacrylic acid. A stable aqueous solution for metal surface coating containing 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 is coated and dried, resulting in an undercoat layer with a total chromium content of 5 to 100 mg. /m ² , and a weldable coated steel sheet characterized in that an overcoat layer of resin containing zinc powder is formed on the undercoat layer with a thickness of 10 to 50 μm.