To provide a TRIP type dual-phase steel sheet of a TPF steel type, having improved elongation and formability for an extension flange at room temperature by controlling a secondary-phase structure.
The dual-phase steel sheet comprises 0.02-0.12% C, 0.5-2.0% Si+Al, 1.0-2.0% Mn and the balance Fe with unavoidable impurities; and has a steel structure comprising, by a space factor, 80% or more of polygonal ferrite, 1-7% of retained austenite and the balance being bainite and/or martensite, wherein among the secondary phases of the retained austenite and the martensite in the dual-phase structure, such a coarse and massive secondary phase as to have an aspect ratio of 1/3 or less and an average particle diameter of 0.5 μm or larger is reduced. Thereby, the dual-phase steel sheet acquires the improved elongation and formability for the extension flange.
SUGIMOTO KOICHI
MUKAI YOICHI
AKAMIZU HIROSHI
SHINSHU TLO KK