Effect of Continuous Annealing Thermal Pattern on Strength and Ductility of Cold Rolled Dual Phase Steel

Abstract

Strength and ductility of cold rolled dual phase steels produced by two types of intercritical continuous annealing were investigated on three kinds of steels different in carbon, silicon and manganese contents. Over-aging temperatures of water quench aging ( QA) type annealing and direct aging (DA) type annealing were varied from as water quenched to 500°C. Ductility of steel was evaluated by elongation and hole expanding ratio associated with stretch formability and stretch flangeability, respectively. Although the steel with the least chemistry showed no significant ductility difference, at the same strength level, between both types of annealing, the steels with the higher chemistry resulted in conspicuous difference between the two types, i.e., high elongation was given by DA type annealing and high hole expanding ratio, by the other. The increased elongation of DA type steel was explained by the high work hardening rate in the relatively wide range of tensile strain, which is due to the high hardness second phase as well as the formation of retained austenite. The increased hole expanding ratio of QA type steel mainly resulted from softening of martensite which increases the resistance for the crack initiation at the interface of ferrite and martensite by decreasing their hardness difference.