AUTOMOTIVE HIGH STRENGTH STEEL SHEET EXCELLENT IN COLLISION RESISTING SAFETY AND FORMABILITY AND ITS PRODUCTION

摘要

PROBLEM TO BE SOLVED: To obtain an automobile high strength steel sheet combining excellent collision resisting safety and formability by specifying the occupying volume ratio of retained austenite in the microstructure after 5% forming in a steel sheet having a specified compsn. essentially consisting of C, Mn, Si, Al and Fe and specifying the strain hardening exponent in the characteristics of the steel sheet. SOLUTION: In a steel 1 sheet having a compsn. consisting essentially of, by weight, 0.05 to 0.20% C, 0.5 to 2.0% Mn, 0.5 to 4.0% Si+Al, and the balance Fe and furthermore contg., at need, prescribed amounts of Ca, rare earth metals, Cr, V, Mo, Cu, Ni, Nb, Ti and B, the content of C in the microstructure before forming is regulated to >=1.0%, the occupying ratio of retained austenite in which the average radius equivalent to a circle is regulated to <=5μm is regulated to >=5%, and the occupying ratio of retained austenite in the microstructure after 5% forming is regulated to >=3%. In this way, a steel sheet having >=130 strain hardening exponent, >=70 yield strength×strain hardening exponent and <=80% yield ratio is obtd. The microstructure of this steel sheet can be obtd. by regulating hot rolling, cooling treatment and coiling temp. or the like.