Method of manufacturing a low yield ratio high-strength steel and low yield ratio high-strength steel

摘要

PROBLEM TO BE SOLVED: To provide a low yield-ratio high tensile-strength steel capable of obtaining high strength and a low yield ratio simultaneously with excellent weldability and low temperature toughness, and to provide a method for producing the low yield-ratio high tensile-strength steel.;SOLUTION: The low yield-ratio high tensile-strength steel contains steel components of, by mass, 0.01 to 0.08% C, ≤0.4% Si, 1.0 to 2.0% Mn, ≤0.02% P, ≤0.01% S, 0.005 to 0.06% Nb, 0.005 to 0.025% Ti, ≤0.06% Al and 0.001 to 0.005% N, in which P_CM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B is 0.1 to 0.18%, and the balance iron with inevitable impurities. The total fraction of polygonal or quasi-polygonal ferrite and pearlite in the steel structure at a position of 1/4 thickness in the cross section in the sheet thickness direction is 50%, the Vickers hardness at a load of 98N in the bottom section of 1mm in the surface of the steel is ≤300 Hv, and the numerical value of a difference between the above Vickers hardness and the minimum value of the Vickers hardness measured at intervals of 1 mm to the cross-sectional direction in the sheet thickness of the steel is less than the numerical value 1.5 times the sheet thickness expressed by mm.;COPYRIGHT: (C)2007,JPO&INPIT