HIGH TENSILE STRENGTH HOT ROLLED STEEL SHEET AND HIGH TENSILE STRENGTH PLATED STEEL SHEET HAVING EXCELLENT BAKING HARDENABILITY

摘要

PROBLEM TO BE SOLVED: To provide a high tensile strength hot rolled steel sheet and a high tensile strength plated steel sheet which have exceedingly improved baking hardenability without requiring the further refining of crystal grains and the further increase of a solid solution N content, and which have excellent cold aging resistance as well. ;SOLUTION: The steel sheet has a composition containing, by mass, 0.05 to 0.12% C, ≤0.5% Si, 1.2 to 3.0% Mn, ≤0.05% P, 0.001 to 0.1% Al and 0.005 to 0.02% N, and the balance Fe with inevitable impurities, and has a composite steel structure containing a low temperature transformed ferritic phase and a polygonal ferritic phase, and in which the ratio of the low temperature transformed ferritic phase to the whole of the structure is 50% in terms of an area ratio, and the average crystal grain size of the low temperature transformed ferritic phase and the polygonal ferritic phase is ≤8 μm.;COPYRIGHT: (C)2003,JPO