High-strength hot-rolled steel sheet and a manufacturing method thereof having excellent stretch-flange formability and the fatigue characteristics

The invention provides a high strength hot-rolled steel sheet having a tensile strength of not less than 780 MPa and exhibiting excellent stretch flangeability and excellent fatigue resistance. A steel which has a composition containing 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 0.005%, Ti at 0.05 to 0.15%, Al at 0.005 to 0.10% and N at not more than 0.007% is hot-rolled in such a manner that the steel is heated to 1150 to not more than 1350°C and thereafter hot rolled by hot rolling which is terminated at a finishing temperature of 850 to 950°C. After the completion of the hot rolling, the steel sheet is cooled to 530°C at an average cooling rate of not less than 30°C/s, and is subsequently cooled to a coiling temperature of 300 to 500°C at an average cooling rate of not less than 100°C/s. The steel sheet is then coiled at the coiling temperature. In this manner, a high strength hot-rolled steel sheet having a tensile strength of not less than 780 MPa as well as excellent stretch flangeability and fatigue resistance is obtained which contains dissolved titanium at not less than 0.02% and includes a bainite single phase microstructure having an average grain diameter of not more than 5 µm, preferably more than 3.0 µm, or a microstructure which includes such a bainite phase at an area ratio of not less than 90% and a second phase other than the bainite phase having an average grain diameter of not more than 3 µm.