Method for manufacturing high strength hot-rolled steel sheet having excellent stretch flangeability and fatigue resistance

摘要

A high strength hot-rolled steel sheet has a tensile strength of not less than 780 MPa and exhibits excellent stretch flangeability and excellent fatigue resistance. A steel 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%.

主权项

1. A method of manufacturing high strength hot-rolled steel sheets with excellent stretch flangeability and fatigue resistance comprising:
heating steel at 1150 to 1350° C. which has a composition comprising, 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 0.005%, Ti at 0.05 to 0.15%, Al at 0.005 to 0.10% and N at 0.002 to 0.007%, wherein content of dissolved Ti is not less than 0.02%, the balance being represented by Fe and inevitable impurities, hot rolling the steel into a hot-rolled sheet which is terminated at a finishing temperature of 850 to 950° C., cooling the steel sheet to 530° C. at an average cooling rate of not less than 30° C./s and less than 55° C./s, cooling the steel sheet to a coiling temperature of 300 to 500° C. at an average cooling rate of not less than 100° C./s, and coiling the steel sheet at the coiling temperature such that the high-strength hot-rolled steel sheets comprise a bainite single phase microstructure having an average grain diameter of more than 3.0 to 5 μm.