Simulations for Pearlite-to-Austenite Transformation in a DP590 Steel for Improved Mechanical Properties

Abstract

The present research work deals with the processing of third generation DP steels with lean chemistry. The main objective of the present dissertation was to understand the kinetics of formation of carbon rich austenite (i.e. austenite obtained through dissolution of pearlite only; avoiding dissolution of pro-eutectoid ferrite present in the as-received steel) to facilitate formation of carbon rich martensite (of better hardness) during processing of dual phase steels (DP steels) for enhanced mechanical performance. DICTRA (27) was used to predict the holding time at a given annealing temperature for complete pearlite dissolution in the given hot rolled steel. The simulations were done for both equilibrium heating conditions and non-equilibrium heating conditions. The simulations for equilibrium heating rates conditions predicted that with increase in austenization temperature, the holding time required for transformation of pearlite into austenite decreases. Further, as the annealing temperature increases, the mean variation of carbon in austenite also increases i.e. heterogeneity of carbon in austenite increases. For the non-equilibrium conditions, it was predicted that as the annealing temperature increases, the mobility of carbon also increasing and also the time required for heating increases, and thus formation of stable austenite takes place. Subsequent experimentation was done to validate the results obtained through DICTRA simulations. The experimental results were found in close agreement with the simulation results and thus, establishing that the simulations done on DICTRA software were correct for the hot rolled steel. Further, experiments were performed to investigate if the simulation results hold good for cold rolled specimens also (or not). Experiments were conducted separately for the hot rolled and cold rolled steel specimens. It was noted that the volume fractions of austenite under a given processing condition in cold rolled specimens, was higher as compared to hot rolled steel (and simulation results). The size distribution was measured using ‘analySIS FIVE’ software for both hot rolled and cold rolled steel. It was observed that ferrite and martensite phases in cold rolled annealed specimens were finer compared to the hot rolled specimens. Finally, tensile testing was performed to establish that annealing of the hot rolled steel (annealed with time periods as predicted through simulations) led to complete dissolution of pearlite (of the initial microstructure) and resulted in improved combinations of strength and ductility in the annealed steel (by formation of carbon rich austenite and hence martensite). A considerable improvement in strength-percent elongation combinations was observed for the hot rolled steels.