| 摘要 |
A method of manufacturing a thick high-tensile-strength hot-rolled steel sheet which possesses both of high strength with TS of 510MPa or more and excellent ductility thus exhibiting the excellent strength-ductility balance, and further possesses excellent low temperature toughness is provided. To be more specific, a high-tensile-strength hot-rolled steel sheet has a composition which contains 0.02 to 0.08% C, 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 (Ti+(Nb/2))/C < 4 is satisfied, and the steel sheet has a structure where a primary phase of the structure at a position 1mm away from a surface in a sheet thickness direction is one selected from a group consisting of a ferrite phase, tempered martensite 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, and a difference AV between a structural fraction (volume%) of a secondary phase at the position lmm away from the surface in the sheet thickness direction and a structural fraction (volume%) of a secondary phase at the sheet thickness center position is 2% or less. |
