MANUFACTURE OF HIGH STRENGTH MOLTEN CALVANIZED STEEL SHEET FOR ALLOYING EXCELLENT IN PLATING ADHESIVENESS AND STRETCH-FRANGING PROPERTY

摘要

PURPOSE:To manufacture this steel sheet excellent in plating adhesiveness and strentch-flanging properties by subjecting a hot rolled steel sheet in which the compsn. of the surface layer part and the inside is specified to heating treatment in specified conditions, galvanizing it and executing heating treatment in specified conditions. CONSTITUTION:A steel slab in which the components of the surface layer part are regulated to, by weight, 0.04 to 0.20% C, =0.3% Si, 0.5 to 2% Mn, =0.03% P,=0.005% S, 0.01 to 0.10% Al and the balance Fe with inevitable impurities and those of the other inside are regulated to 0.04 to 0.20% C, 0.5 to 2.0% Si, 0.7% Mn, to 2.0% =0.03% P, =0.005% S, 0.01 to 0.10% Al and the balance Fe with inevitable impurities, is hot-rolled. Next, this slab is heated to =800 deg.C, is thereafter cooled to 400 to 500 deg.C at =30 deg.C/sec cooling rate, is held to the above temp. range for 40 to 500sec, and is thereafter galvanized. The slab is, furthermore, rapidly heated to 500 to 600 deg.C, is held to the above-mentioned temp. range for =10sec and is thereafter cooled to a room temp.