| 摘要 |
The invention provides a high strength hot rolled steel sheet which exhibits a tensile strength of not less than 780 MPa as well as excellent bendability. The steel sheet has a composition including, in terms of mass%, C at 0.05 to 0.15%, Si at 0.2 to 1.2%, Mn at 1.0 to 2.0%, P at not more than 0.04%, S at not more than 0.0030%, Al at 0.005 to 0.10%, N at not more than 0.005% and Ti at 0.03 to 0.13%, the balance being represented by Fe and inevitable impurities, includes surface regions having an area fraction of bainite of less than 80% and an area fraction of a ferrite phase with a grain diameter of 2 to 15 µm of not less than 10%, the surface regions extending from both surfaces of the steel sheet each to a depth of 1.5 to 3.0% relative to a total sheet thickness, as well as an inner region other than the surface regions having an area fraction of a bainite phase of more than 95%, and has a tensile strength of not less than 780 MPa. |
