HIGH STRENGTH ULTRA-LOW CARBON STEEL HAVING EXCELLENT BAKE HARDENABILITY, IMPROVED AGEING RESISTANCE, EXCELLENT FORMABILITY, SURFACE FINISH AND METHOD THEREFOR.

摘要

A cold-rolled bake hardenable steel sheet is provided having tensile strength of 340MPa or less together with excellent aging resistance with YPE less than 0.15% after accelerated aging, excellent drawability and superior bake hardenability is provided together with its method of manufacture. Cold rolled bake hardenable steels has an improved steel chemistry comprising in terms of weight % :- C: 0.001-0.003 wt% , Mn: 0.35-0.55 wt% , P: 0.035-0.06 wt, N: 0.004 wt% or less, Ti: 0.008-0.018 wt% , B: 0.0007- 0.0012 wt%, V: 0.005-0.02 wt%, having [Ti+B] wt% = [2.64 N +0.78N] and the balance being Fe and other associated or inevitable impurities essentially satisfying the following relations to achieve YPE after accelerated aging less than 0.15 confirming aging guarantee of minimum 6 months 69.25Ti + 174.1B +16.9V+0.0131 SPM = 1, Where N = N wt% fixed by Ti, N=maximum N wt% which can be added, SPM= skin pass elongation % after continuous annealing. A cold rolled continuously annealed steel with YS 180 MPa or more and BH Index of 30 MPa or more at lower yield point has an improved texture with a mean value of X-ray random intensity ratio of group of {111} to {001} orientation more than 3.5 and mean value of X ray random intensity for {111} orientation component is at least 2.5 multiple than that of {112} orientation component.