Ductility, high strength steel plate and high-strength electroplated steel sheet excellent in stretch flange formability and shock absorption characteristics and methods for their preparation

摘要

PROBLEM TO BE SOLVED: To provide a high strength steel sheet and a high strength electroplated steel sheet which have a tensile strength of 400 to 540 MPa, excellent ductility and stretch-flanging properties, and excellent impact absorbing properties, and to provide a production method therefor.;SOLUTION: The steel sheets have a componential composition containing, by mass, 0.02 to 0.08% C, ≤0.50% Si, 0.5 to 1.5% Mn, ≤0.04% P, ≤0.005% S, ≤0.1% Al, and ≤0.025% N, and further containing Cr and Mo so as to satisfy Cr+1.2Mo≥0.2%, and have a structure in which the fraction of a ferritic structure as the main phase is ≥85% in an area ratio, and the ratio of a martensitic structure is 2 to 10% in an area ratio, and the total fraction of the second phase containing the above martensite is ≤15% in an area ratio.;COPYRIGHT: (C)2003,JPO