Patent ID: 9580782
Date: 2017-02-28
CPC Classifications: B21B,C21D,C22C

Claim:
1. 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 difference ΔV between a structural fraction (volume %) of a secondary phase at a position 1 mm away from a surface of the steel sheet in a sheet thickness direction and a structural fraction (volume %) of a secondary phase at the sheet thickness center position is 2% or less; wherein Ti, Nb, C: contents of respective elements (mass %); and wherein a primary phase of the structure at the position 1 mm away from the surface in the sheet thickness direction is formed of either a tempered martensite structure or a mixture structure of bainite and tempered martensite, the structure at a sheet thickness center position includes the primary phase formed of bainite and/or bainitic ferrite and the secondary phase which is 2% or less by volume %, and a difference ΔHV between Vickers hardness HV1 mm at the position 1 mm away from the surface in the sheet thickness direction and Vickers hardness HV1/2t at the sheet thickness center position is 50 points or less.