Influence of Cold Rolling on Austenite Formation in Plain Low-Carbon Steel Annealed in Intercritical Region

Since intercritical annealing is the main processing stage for the production of dual-phase steels (DPS), studies on the formation of austenite are of great importance. In this research, the effects of the cold rolling and intercritical annealing temperature on the formation of austenite in plain low carbon steel have been investigated. For this purpose, the steel sheets were cold rolled for different amounts of reductions and annealed at different intercritical temperatures followed by quenching into an ice brine solution. In order to investigate the microstructural evolution as well as the effect of controlling parameters on the nucleation and growth processes of austenite (martensite at room temperature) formation, the samples were quenched at various time intervals during intercritical annealing and their microstructures were analyzed using optical and scanning electron microscopes. The results showed that cold rolling and other processes occurring during the formation of austenite significantly influences the initial microstructure and the nucleation and growth of austenite phase. In un-deformed and 50% cold rolled specimens, austenite formed from the pre-existing pearlite colonies. However, in the 70 cold rolled specimens, cementite spheroidization within the deformed pearlite colonies caused austenite to be formed from a microstructure consisting of cementite particles embedded in a matrix of ferrite. In this situation, nucleation and growth of austenite phase took place mainly on the grain boundaries of ferrite matrix. Furthermore, it was found that the temperature of intercritical annealing treatment has a strong effect on the kinetics of austenite formation.