PURPOSE: To obtain the objective steel sheet excellent in powdering resistance and press formability as well as excellent in the coating properties of Fe upper layer plating by specifying bath conditions and the sheet temp. on the outlet side of a high frequency induction heating furnace and executing skin pass under prescribed conditions.
CONSTITUTION: Plating is applied to a steel sheet in a galvanizing bath contg. Al and the balance Zn. After that, its coating weight is regulated, and it is subjected to allaying treatment in a heating furnace so as to regulate the Fe content in a film into 8 to 12%, is subjected to skin pass and is thereafter applied with Fe upper layer plating. In this method, the plating is executed under the conditions in which the Al content ([Al%]) in the bath is regulated to 0.05 to <0.13%, the temp. of the bath is regulated to ≤460°C and the Al content in the bath and the impregnating sheet temp. (T°C) of the steel sheet into the bath satisfy the inequality. In this way, the Fe-Zn alloying reaction in the bath can be suppressed. After the plating, it is heated in a high frequency induction heating furnace so as to regulate the sheet temp. on the outlet side of the furnace into >495 to 520°C, is held for prescribed time, is thereafter cooled, is subjected to skin pass by a work roll having 0.4 to 0.9μ surface roughness Ra at ≥0.2Ton/mm draft and is thereafter applied with Fe upper layer plating.
MIYAGAWA YOICHI
MATSUBAYASHI KOJI
NAKAMURA AKIHIKO
