HIGH STRENGTH (UTS: 800MPA MIN.) DUAL PHASE COLD ROLLED STEEL SHEET HAVING VERY LOW YIELD RATIO, EXCELLENT SURFACE FINISH, BAKE HARDENING, WELDABILITY AND A METHOD FOR MANUFACTURING THE SAME.

摘要

Present invention relates to 800 MPa Tensile strength level dual phase cold rolled steel sheet comprising chemical elements in terms of mass percent: 0.121% to 0.15% of C, Si: 0.3% or less, Mn: 1.5% to 1.95 ,N: 0.006% or less, Cr: 0.4% to 0.6%, Ti: 0.005% to 0.03%, and the balance being Fe and other inevitable impurities, whereas (Mn+Si)/C is in a range of 10 to 18 for excellent combination of phosphatability and hole expansion ratio, and the steel further satisfies the following relations; 2.7 log10(%Fea-%Ms) = {(Mn+Si)/ (C+Cr+Ti)} and 0.9 = log10 [(SCS-RCS)/40] = 4.6 1.7[%Mn] -2.3 [%Cr]- 2[C%] (Where %Fea is total area percent of ferrite and %Ms is total area percent of martensite relative to a whole microstructure of said steel, and all compositional elements are in weight %). Cold rolled steel described in present invention has excellent surface property comprising a 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 along with good hole expansion ration of =40 % and BH index of =35 MPa.