HIGH STRENGTH STEEL SHEET HAVING EXCELLENT STRETCH FLANGE FORMABILITY AND FATIGUE PROPERTY, AND METHOD FOR REFINING MOLTEN METAL THEREOF

摘要

PPROBLEM TO BE SOLVED: To provide a high fatigue property-low yield ratio high strength steel sheet having excellent stretch flange formability. PSOLUTION: The high strength steel sheet having excellent stretch flange formability and fatigue properties has a composition containing, by mass, 0.03 to 0.20% C, 0.08 to 1.5% Si, 1.0 to 3.0% Mn, ≤0.05% P, 0.0005 to 0.02% S, 0.0005 to 0.01% N, ≤0.01% acid soluble Al, 0.08 to 0.2% acid soluble Ti and one or two kinds selected from Ce and La by 0.0005 to 0.04% in total, and, in which, by a mass base, a (Ce+La)/acid soluble Al ratio is ≥0.1, also, a (Ce+La)/S ratio is 0.4 to 50, and the balance iron with inevitable impurities, and the number density of inclusions with a circle equivalent diameter of ≤2 μm present in the steel sheet is ≥15 pieces/mmSP2/SP. PCOPYRIGHT: (C)2010,JPO&INPIT