Patent ID: 8784577

Claim:
A high-tensile-strength hot-rolled steel sheet having a composition which contains by mass % 0.02 to 0.08% C, 0.01 to 0.50% Si, 0.5 to 1.8% Mn, 0.025% or less P, 0.005% or less S, 0.005 to 0.10% Al, 0.01 to 0.10% Nb, 0.001 to 0.05% Ti, and Fe and unavoidable impurities as a balance, wherein the steel sheet contains C, Ti and Nb in such a manner that a following formula (1) is satisfied, and the steel sheet has a structure where a primary phase of the structure at a position 1 mm away from a surface of the steel sheet in a sheet thickness direction is one selected from a group consisting of a ferrite phase and a mixture structure of a ferrite phase and tempered martensite, a primary phase of the structure at a sheet thickness center position is formed of a ferrite phase, a difference ΔV between a structural fraction (volume %) of a secondary phase at the position 1 mm away from the surface of the steel sheet in the sheet thickness direction and a structural fraction (volume %) of a secondary phase at the sheet thickness center position is 2% or less, and a difference ΔD between an average grain size of the ferrite phase at the position 1 mm away from the surface of the steel sheet in the sheet thickness direction and an average grain size of the ferrite phase at the sheet thickness center position is 2 μm or less; wherein (Ti+(Nb/2))/C<4; and Ti, Nb, C: contents of respective elements (mass %).