| 摘要 |
PURPOSE: To obtain excellent dent resistance in a steel sheet by specifying the compsn. of a low phosphorous carbon aluminum killed steel and imparting suitable work hardenability and baking hardenability to the steel sheet. CONSTITUTION: The compsn. of a steel is composed of the one contg., by weight, 0.01 to 0.04% C, 0.05 to 0.4% Si, 0.05 to 0.30% Mn, 0.04 to 0.10% P, 0.003 to 0.02% S, 0.015% Al and <=0.004% N, furthermore contg., at need, 0.0002 to 0.0020% B, and the balance Fe with inevitable impurities. This steel is treated by a process including ordinary hot rolling, cold rolling, continuous annealing, skinpass rolling and surface treatment to regulate the density (ρ, pieces/mm<2> ) of transgranular carbides in the steel sheet structure toρSi0.5×10<5> , by which the nonaging baking hardening galvanized steel sheet in which the amt. to be worked after 2% stretch working is regulated to 40MPa and the amt. to be hardened by baking is regulated to 30 to 60MPa is produced. Moreover, at the time subjecting forming to this steel sheet, press forming is executed in the state of being applied with <=10% prestrain by extensional strain or while being applied with >=20% stretch forming.
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