Patent ID: 8529829

Claim:
A hot-rolled steel sheet comprising: C: no less than 0.02% and no more than 0.15%, Si: no less than 0.2% and no more than 2.0%, Mn: no less than 0.5% and no more than 2.5%, Al: no less than 0.02% and no more than 0.15%, Cu: no less than 1.0% and no more than 3.0%, Ni: no less than 0.5% and no more than 3.0%, and Ti: no less than 0.03% and no more than 0.5% (% means mass % for chemical components hereinafter), wherein the hot-rolled steel sheet comprises bainitic ferrite, ε-Cu precipitates dispersed in the bainitic ferrite, and optionally granular bainitic ferrite; the hot-rolled steel sheet has a metallographic structure in longitudinal cross section such that the sum of bainitic ferrite and granular bainitic ferrite accounts for no less than 85% by area; the hot-rolled steel sheet has a tensile strength (TS) greater than 900 MPa; and the hot-rolled steel sheet has a balance between strength and stretch flangeability [tensile strength (TS)×bore expanding ratio (λ): MPa·%] and a balance between strength and ductility [tensile strength (TS)×total elongation (El): MPa·%] such that these values satisfy the formula: (TS×λ: MPa·%)≧146000−5.0×(TS×El: MPa·%).