ORGANIC COMPOSITE COATED STEEL STRIP HAVING IMPROVED CORROSION RESISTANCE AND WELDABILITY

摘要

Organic composite coated steel strips having improved corrosion resistance, weldability and coating adherence are obtained when an organic composite layer is applied to a chromate layer on a zinc or zinc base alloy plated steel substrate. The chromate layer containing up to 70% of Cr<6><+> based on the total chromium weight is coated in a weight of 5 to 500 mg/m<2> of Cr. The organic composite layer is formed to a dry weight of 0.2 to 3.0 g/m<2> from a coating composition comprising (A) a silica sol having silica dispersed in an organic solvent with a water content of up to 3.0%, the silica having an organic deposit on its surface in an amount of up to 5.0% of C based on the total silica weight, the silica having an average particle size of 0.05 to 3.0 mu m and a specific surface area of 50 to 800 m<2>/g, and (B) an organic resin binder having a number average molecular weight of at least 2,000. The coated strips are suitable for use as automobile bodies by press forming.