HIGH-STRENGTH HOT-ROLLED STEEL SHEET HAVING EXCELLENT BENDING WORKABILITY AND METHOD FOR PRODUCING SAME

摘要

Provided is a high-strength hot-rolled steel sheet having tensile strength of 780 MPa or greater and excellent bending workability. The composition is, by mass%, C: 0.05 to 0.15%, Si: 0.2 to 1.2%, Mn: 1.0 to 2.0%, P: 0.04% or less, S: 0.0030% or less, Al: 0.005 to 0.10%, N: 0.005% or less, and Ti: 0.03 to 0.13%, with the balance being Fe and inevitable impurities. Surface layer regions from both surfaces of the steel sheet to a depth of 1.5 to 3.0% of the total sheet thickness have a bainite surface area percentage of less than 80% and a surface area percentage of a ferrite phase having a grain size of 2 to 15 µm of 10% or greater. An inner region excluding these surface layer regions has a bainite phase surface area percentage exceeding 95% and a tensile strength of 780 MPa or greater.