1. Field of the Invention
The present invention relates to a steel sheet having its surface prepared for paint coating
2. Description of the Prior Art
Such a steel sheet is useful as construction material or material for household electrical appliances. Galvanized steel sheets with their surface treated by phosphate treatment or chromate treatment, have been commonly used.
Chromate treated steel sheets have good corrosion resistance with passivation provided by chromium. However, the chromate treatment gives rise to problems due to the toxicity of chromium, particularly in the waste water treatment. On the other hand, the phosphate treatment provides a good substrate surface for painting. However, in order to impart corrosion resistance as well, it is common to subject the phosphate treated steel sheets to pickling with chromic acid as after-treatment. This after-treatment gives rise to the same problems as in the case of the chromate treatment. Further, a large amount of sludge produced by the treatment creates another problem for its disposal. Further, such conventional products do not necessarily have a satisfactory quality in their properties required for a substrate for painting, such as corrosion resistance after paint coating, paintability and degrease resistance. Especially, they tend to undergo property degradation when subjected to alkaline degreasing treatment at the user's end, i.e., they are inferior in the degreasing resistance.
There have been some proposals for improvement in an attempt to solve the above mentioned problems. Japanese Patent Publication No. 34406/74 discloses a method in which a silicate composite material comprising silica and an acrylic copolymer as the major components is used and certain improvements over this method are disclosed in Japanese Laid-Open Applications No. 77635/79 and No. 62972/80.
From the results of experiments conducted by the present inventors in which the composite materials proposed in the above mentioned prior art are applied to galvanized steel sheets, it has been found that the coatings thereby obtained do not fully satisfy the requirements for corrosion resistance after paint coating, paintability and degrease resistance, which are essential for steel sheet substrates for painting, and that there is still room for improvement.