Organic composite coated steel strip having improved corrosion resistance and spot weldability

摘要

An organic composite coated steel strip having well-balanced corrosion resistance, coating properties including adaptability to electrophoretic coating and water resistant secondary coating adherence, and spot weldability is provided. The organic composite coated strip comprises a zinc or zinc base alloy plated steel substrate; a chromate layer on the substrate containing up to 70% by weight of Cr<6><+> based on the total chromium quantity, said chromate layer being coated in a weight of 5 to 500 mg/m<2> in terms of elemental chromium; and an organic composite layer on said chromate layer mainly comprising silica and an organic high molecular weight resin. In the organic composite coated steel strip, (a) silica has an average primary particle diameter of 5 to 30 nm; (b) silica is agglomerated into agglomerates of an average thickness of 0.2 to 2 mu m and an average apparent diameter of 5 to 300 mu m; and (c) the organic composite layer has been coated to a dry weight of 0.1 to 3 g/m<2>. The organic composite coated steel strip of the invention has various uses including automobile bodies.