ZINC ALLOY PLATED STEEL SHEET EXCELLENT IN PRESS FORMABILITY AND ELECTRODEPOSITION COATING PROPERTY AND HAVING PLURAL PLATING LAYERS AND ITS PRODUCTION

摘要

PURPOSE:To produce a zinc alloy plated steel sheet excellent in press formability and electrodeposition coating property by subjecting a steel sheet to hot-dip galvanizing and to alloying and then exerting plating in an acid electroplated zinc coating bath containing specific amounts of silica grains and nitric acid ions. CONSTITUTION:A steel sheet is passed through a hot-dip galvanizing bath, by which a galvanizing layer is formed on the surface of the steel sheet. Subsequently, heating is applied to the steel sheet to alloy the above galvanizing layer and this steel sheet, by which an alloyed hot-dip iron-zinc alloy coating layer as a lower layer having prescribed coating weight is formed on the surface of the steel sheet. Then, electroplating is applied to this steel sheet by using an acid electroplated zinc coating bath of pH1-4.5 containing 0.1-10g/l of silica grains and 100-20000ppm of nitric acid ions. By the above procedure, a zinc-silica dispersion coating layer as an upper layer in which 0.01-2wt.% of silica grains are uniformly dispersed and coating weight is regulated to 1-10g/m2 is formed on the above lower layer. By this method, the zinc alloy plated steel sheet excellent in press formability and electrodeposition coating property and having plural plating layers can be obtained.