GALVANIZED STEEL SHEET HAVING EXCELLENT FORMABILITY

摘要

PURPOSE:To improve the formability and corrosion resistance of a galvanized steel sheet by forming two layers of different chromate films on the surface of said steel sheet. CONSTITUTION:The electrolytic chromate film is first formed on the surface of the above-mentioned steel sheet with a known chromate liquid and electrolyzing conditions to coat the above-mentioned plating surface with the dense film which consists essentially of Cr<3+> and in which metallic Cr exists partly to improve the galling resistance in the stage of press forming. The coating type chromate film which consists essentially of Cr<6+> and has a self-restoration effect is formed by spraying of or immersion in a known chromate liquid on the upper layer of the electrolytic chromate film to improve the corrosion resistance. The lubricity is improved by the presence of the electrolytic chromate film and the exfoliation by the galling of the coating type chromate film itself in the pressing process is decreased if the steel sheet is constituted in the above- mentioned manner. The formability is thus improved.