| 摘要 |
PROBLEM TO BE SOLVED: 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. SOLUTION: 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.
|
