SURFACE TREATED STEEL SHEET EXCELLENT IN STABILITY OF EXTERNAL APPEARANCE, WELDABILITY, AND ADHESION OF PAINT, AND ITS PRODUCTION

摘要

PROBLEM TO BE SOLVED: To obtain a surface treated steel sheet having weldability and adhesion of paint while securing stable external appearance by coating the surface of a steel sheet with granular tin of specific grain size at specific area ratio, coating a specific range of the above surface with specific amounts of metallic chromium, and then coating the surface of the metallic chromium with specific amounts of chromium oxide. SOLUTION: Tin plating is applied to a steel sheet by using an acidic tin plating bath where either of ethoxylated alpha naphthol or ethoxide alpha naphtholsulfonic acid is added as a brightener additive by >0.05 g/l to <0.20 g/l. The surface of the plated steel sheet is coated with granular tin of 0.4 to 2.4μm grain size at 2 to 20% area ratio. Electrolytic treatment is performed within 2 sec after the insertion of the steel sheet into a chromium plating bath. The steel sheet surface, free from tin coating, is coated with (30 to 150) mg/m<2> of metallic chromium, in the range of <=30μm radius around the part coated with granular tin. Further, the surface of the metallic chromium is coated with chromium oxide by (2 to 20) mg/m<2> expressed in terms of metallic chromium.