To increase the strength and ductility of a steel sheet by composing it of steel with a composite structure contg. C, Si, Mn, P, Al, B, V, Nb and Ti and allowing each constitutional structure to have specified Vickers hardness.
This steel is the one having a compsn. contg., by mass, 0.003 to 0.04% C, 0.01 to 1.0% Si, ≤3.0% Mn, ≤0.05% P, 0.005 to 0.1% Al and 0.0002 to 0.003% B and moreover contg. one or more kinds among ≤0.1% V, ≤0.1% Nb and ≤0.1% Ti, in which the compositional value of each element satisfies the inequality as well. Moreover, in the internal metallic structure, the structure of ≥50 vol.% is composed of polygonal ferrite having ≤110 Vickers hardness Hv1, and the other structure is composed of the one of martetnsite and bainite having ≥1250 Vickers hardness Hv2. Then, the ratio of the Vickers hardness, i.e., Hv2/Hv1 is controlled to ≥2.2.
IKEDA CHIKAYUKI
HASHIMOTO SHUNICHI
KAJIMA TAKAHIRO
SHINTO YOSUKE
