Low yield ratio high-strength annealed steel sheet having good ductility and resistance to secondary cold-work embrittlement

摘要

The invention relates to a method of manufacturing a high- strength steel sheet by annealing the steel sheet after cold rolling. In order to obtain the low-yield ratio, high-strength steel sheet having high strength and good ductility, resistance to secondary cold-work embrittlement and spot weldability at low cost, the steel sheet containing 0.03-0.15% of P and specified amounts of C, Mn and Al as basic components and optionally containing, as a selective component, at least one element selected from a group of Si, Cr, Mo and B and a group of Nb, Ti, and V in such amounts as to meet the relation formula restricting the total content of Mn, Si, P, Cr and Mo is subjected to annealing under the conditions that the sheet is heated at a temperature of from Ac.sub.1 transformation point to 950° C. for from 10 seconds to 10 minutes and cooled in such a control manner that an average cooling rate between 600° C. and 300° C. is not less than a specified critical cooling rate CR pertaining to the chemical composition and within a range of 15°-200° C./sec (FIGS. 1 and 4). The invention is suitable for the production of bumper, door guard bar, and the like in the automotive vehicles.