Unlocking the Mechanisms behind Austenite Formation of Cold‐Rolled Ferrite–Martensite Dual‐Phase Steels: The Role of Ferrite Recrystallization

Abstract

In this study, the austenite formation behavior is investigated for the ferrite–pearlite initial microstructure, considering the influence of ferrite recrystallization and cementite distribution. Special attention is also given to the effect of recrystallization state on cementite distribution for varied heating routes. Austenite nucleation for varied recrystallization state is elucidated in terms of activation energy evaluation and post microstructural characterization, and the austenite transformation kinetics are modeled and experimentally validated. In the uncrystallized state, austenite forms on the original pearlite colonies quickly due to the short carbon diffusion path. The high activation energy leads to the exceptionally difficult nucleation at the ferrite–cementite interface inside ferrite matrix. Once recrystallization occurs partially, priority of austenite nucleation is on those ferrite boundaries (both recrystallized and unrecrystallized boundaries) decorated by cementite. With complete recrystallization, preferential nucleation sites are found to be junctions of ferrite–cementite interface on recrystallized ferrite boundaries. Austenite transformation kinetics is well predicted by considering the varied nucleation density due to different recrystallization state, showing a good agreement with experimental data. It is demonstrated directly by experimental evidence that proceeding of recrystallization postpones the austenite transformation kinetics, while getting rid of the interference of heating rate and nucleation location.