HIGH STRENGTH HOT ROLLED STEEL SHEET EXCELLENT IN ELONGATION AND STRETCH-FLANGE PROPERTY AND HAVING TENSILE STRENGTH OF amp;ge;780 MPa, AND METHOD FOR PRODUCING THE SAME

摘要

PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet having excellent elongation properties and stretch-flange properties and a method for producing the same, wherein TS of ≥780 MPa is stably obtained at a low cost, and a method for producing the same.;SOLUTION: The high strength hot rolled steel sheet with a tensile strength of ≥780 MPa has a composition composed of, by mass, 0.06 to 0.15% C, ≤1.2% Si, 0.5 to 2.0% Mn, ≤0.04% P, ≤0.005% S, ≤0.05% Al and 0.03 to 0.15% Ti, and the balance Fe with inevitable impurities, and has a microstructure in which the volume ratio of the total of a bainite phase and a bainitic ferrite phase occupied in the whole of the structure is ≥50%, and the volume ratio of the total of the bainite phase, bainitic ferrite phase and polygonal ferrite phase is ≥95%, wherein the total of the content of the Ti in precipitates with the average diameter of 20 nm is ≥50% of the total of the content of the Ti in all the precipitates precipitated in the steel.;COPYRIGHT: (C)2011,JPO&INPIT