Effect of Inter-critical Annealing Parameters on the Recrystallization, Austenite Formation and Stabilization in a Dual Phase Steel

Abstract

Advanced high strength steels (AHSS) have been developed in the last few decades especially for automotive applications due to their remarkable properties combining high strength and good ductility. DP steels are the most common grade of advanced high strength steels which are produced through inter-critical annealing of a low carbon steel followed by an accelerated cooling to obtain the ferrite-martensite (dual phase) microstructure. The strength of DP steels is manipulated by the amount of martensite and ductility by the size and distribution of this phase. The fraction of martensite formed depends upon the austenite amount available during the intercritical annealing process. So, it is always important to understand the kinetics of ferrite recrystallization and austenite formation during the annealing of cold rolled sheets. The present work focuses on understanding the effect of intercritical annealing parameters (viz. annealing temperature, soaking time periods etc.) on ferrite recrystallization, austenite formation and its stabilization during the production of dual phase steel from a low carbon cold rolled steel. The main effort of this research work has been to arrive at the combination of annealing process parameters to produce the desired ferrite-martensite structures. For the study, commercial software viz. Thermo-Calc (3.0) and JMat-Pro (7.0) have been used to predict the various annealing parameters. The predicted values have been compared with the experimental values and a significant variation has been observed in the results. It has been concluded that the annealing parameters clearly affect the phenomenon of ferrite recrystallization, and austenite formation and its stabilization during the production of dual phase steels. Hence, this study is very helpful in understanding the synergistic effect of ferrite recrystallization and austenite phase transformation behaviour in dual phase steels to tailor design the mechanical properties by microstructure control.