Patent ID: 8657970

Claim:
A hot-rolled steel sheet, comprising: in terms of mass %, C: 0.015% or more to less than 0.035%; Si: less than 0.05%; Mn: 0.9% or more to 1.8% or less; P: less than 0.02%; S: less than 0.01%; Al: less than 0.1%; N: less than 0.006%; Ti: 0.05% or more to less than 0.11%; and either one or both of Cu: 0.01% or more to 1.5% or less and Ni: 0.01% or more to 0.8% or less, with the remainder being Fe and inevitable impurities, wherein Ti/C is in a range of 2.5 or more to less than 3.5, Nb, Zr, V, Cr, Mo, B and W are not included, a microstructure comprises a mixed microstructure of polygonal ferrite and quasi-polygonal ferrite in a proportion of greater than 96%, martensite and residual austenite are not included, a maximum tensile strength is in a range of 520 MPa or more to less than 720 MPa, an aging index AI is in a range of more than 15 MPa, a product of a hole expansion ratio (λ) % and a total elongation (El) % is in a range of 2350 or more, and a fatigue limit is in a range of 200 MPa or more.