Low yield ratio high strength steel sheet having large work hardening ability and excellent uniform elongation and weldability and method for producing the same

摘要

PROBLEM TO BE SOLVED: To provide a high-strength steel sheet which shows a low yield ratio, from the standpoint of improvement of earthquake-proof safety, is high in work hardenability and excellent in uniform elongation, weldability and weld crack resistance and allows reduction of preheating even in small-heat-input welding.SOLUTION: A low-yield-ratio high-strength steel sheet high in work hardenability and excellent in uniform elongation and weldability meets a specified steel composition and has a specified Ceq of 0.40-0.47% and a specified Pcm of 0.220% or lower. The structure at the position with a depth of 1/4 of the sheet thickness consists of a diplophase of a soft phase and a hard phase and meets conditions of a fraction of the hard phase of 10-20 area%, a hardness Hv of the hard phase of 310-370 and a hardness Hv of the soft phase of 160-190. In the soft phase, the total amount of Nb, V and Mo occupying in the precipitation is greater than 50 mass%, and carbide and carbonitride of circle-equivalent diameters of 5 nm or greater are present so as to meet conditions of an average circle-equivalent diameter of 50 nm or smaller and a number density of 10 particles/μmor higher.