Patent ID: 9284618
Filing Date: 2016-03-15
CPC Classification: C21D,C22C

Claim Text:
1. A high strength hot-rolled steel sheet with a tensile strength of not less than 590 MPa which has a chemical composition comprising, in terms of mass %, C at 0.040 to 0.10%, Si at not more than 0.3%, Mn at 1.7 to 2.5%, P at not more than 0.030%, S at not more than 0.005%, Al at not more than 0.1% and N at 0.006 to 0.025%, with the balance being represented by Fe and inevitable impurities, and has a microstructure in which a bainite phase represents not less than 60% on an area ratio basis, a total of a ferrite phase and a pearlite phase represents not more than 10% on an area ratio basis, and the bainite phase includes grains among which cementite grains have been precipitated at not less than 1.4×10 4 grains/mm 2 and the cementite grains have an average grain diameter of not more than 1.5 μm.