HIGH-TENSION HOT-ROLLED STEEL SHEET HAVING HIGH BAKING HARDENABILITY AND EXCELLENT FORMABILITY FOR EXTENSION FLANGE AND METHOD FOR MANUFACTURING THE SAME

PROBLEM TO BE SOLVED: To provide a high-tension hot-rolled steel sheet that has high baking hardenability, is excellent in formability for extension flange and shows no problem in aging at room temperature. SOLUTION: The high-tension hot-rolled steel sheet has a composition containing, by mass, 0.05-0.12% C, ≤0.5% Si, 1.2-3.0% Mn, ≤0.05% P, 0.001-0.1% Al, 0.005-0.02% N and the balance being Fe and unavoidable impurities and has a microstructure composed of a low temperature transformation ferrite phase (AF phase) and a polygonal ferrite phase (PF phase); wherein the total area of the two phases accounts for ≥90% of the entire structure; the area of the AF phase accounts for 10-50%; the average crystal grain size is ≤8 μm in the two phases; and the AF phase has a ratio of Hv(max)/Hv(min) of ≤2.2. Here, Hv(max) and Hv(min) represent the maximum hardness and minimum hardness, respectively, of the AF phase determined through observation under SEM of 15 visual fields consisting of 5 visual fields per site at 3 sites in the width direction at the center of the steel sheet in a longitudinal direction. COPYRIGHT: (C)2011,JPO&INPIT