Coating composition for steel sheets having zinc and zinc alloy coating layer, method for forming coating layer using the coating composition and steel sheet having the coating layer formed thereof

摘要

A coating composition for zinc- and zinc alloy-coated steel sheets includes: 1 to 30% by weight of hypophosphorous acid, 0.1 to 10% by weight of manganese, 0.01 to 5% by weight of a polyvinyl compound, zinc oxide and the balance of water, and further comprises 20% by weight or less of alcohol, when necessary, wherein the zinc oxide is included in such a content that, when 50 Ml (milliliters) of the coating composition is neutralized and titrated with 0.1N NaOH, the NaOH used for the neutralization titration is present in an amount of 10.0 Ml (milliliters) or less, and a content of free acid in the coating composition is adjusted to such an amount that, when 50 Ml (milliliters) of the coating composition is neutralized and titrated with 0.1N NaOH, the NaOH used for the neutralization titration is present in an amount of 10.0 Ml (milliliters) or less. The coating film formed on the steel sheet using the coating composition shows excellent properties such as lubricating property, adhesion and coating removal property, and also has a fine surface appearance.