ULTRA LOW CARBON HIGH STRENGTH COLD ROLLED STEEL SHEET FOR WEIGHT REDUCTION IN AUTOMOTIVE APPLICATION AND PROCESS OF PRODUCING THE SAME.

摘要

ABSTRACT TITLE: ULTRA LOW CARBON HIGH STRENGTH COLD ROLLED STEEL SHEET FOR WEIGHT REDUCTION IN AUTOMOTIVE APPLICATION AND PROCESS OF PRODUCING THE SAME. The present invention relates to ultra low carbon high strength cold rolled continuously annealed steel sheet with tensile strength of atleast 450 MPa having composition comprising in terms of weight %: C: =0.003, Mn: 0.75-0.95, Si:=0.03, P: 0.06-0.09, Nb: 0.01-0.03 , V: 0.003-0.02, N:= 0.004 and balance being Fe and other unavoidable impurities wherein elemental composition relation having [Mn+Si]/[P+Nb+V] must be in the range of 6 to 13 and %[Nb+V] / %[C+N] must be in the range of 2.5 to 8.5, where [X] is weight % of steel element X. Cold rolled continuously annealed steel further contains Ti and B in an amount such that 5 = {[Nb+Ti+V] /%[N+C+B]}=20, and [Ti+B]=[3N +0.8N] Where N =(14/48) x [Ti] wt%. Further the steel sheet provided has excellent drawability with r-bar value =1.5, Excellent phosphatability and surface property with phosphate crystal size of 4 µm or less and phosphate coating weight of 1.5-2.5 g/m2 after zinc phosphate chemical conversion coating treatment on said steel sheet surface, Bake hardening index of less than 5 MPa, superior room temperature aging resistance with No YPE after accelerated aging and DBTT less than -50 0C.