1982 Volume 68 Issue 9 Pages 1276-1282
Dual phase (DP) steels with low yield to tensile strength ratio of 45% and extra deep drawing high strength (EDDH) steels with high r value above 2.0 have been successfully produced in a multipurpose continuous annealing line with cooling rate of 40°C/s by gas jet.
Dent resistance of trunk lid panel made from 1.2Mn-0.5Cr DP steel sheets of 0.70 mm thickness is higher than that made from Al killed mild steels of 0.90 mm thickness. In order to obtain tensile strength between, 40 to 46kgf/mm2, it is effective to control C content within 0.0250.040%, and to adjust annealing temperature precisely just above 740 °C.
Excellent deep drawabilities are attained by reducing C content down to 0.004% or less in 0.03Nb-0.06P-0.05Al EDDH steels. In actual production, the extremely low carbon content is attained by the combination of decarburizing with bottom blown vessel (down to 0.015% C) and subsequently with reformed RH degasser in about 15 min (less than 0.003%).
The EDDH steels have lower yield strength than 20 kgf/mm2 and higher elongation than 42%.
In the multipurpose continuos annealing line, the strip tensions in soaking section and rapid cooling section in the furnace are regulated in dependently to avoid heat buckling at 830°C and fluttering caused by gas jet, respectively.
