Ferrite recrystallization and austenite formation during annealing of cold-rolled advanced high-strength steels: In situ synchrotron X-ray diffraction and modeling

Highlights: • Cold-rolled DP steels microstructure depends on the heating rate of final annealing. • HEXRD was used to investigate the start of ferrite recrystallization. • HEXRD was used to measure austenite formation at different heating rates. • Incomplete recrystallization leads to an acceleration of austenite formation. • A phenomenological model is developed to describe this interaction. -- Abstract: Ferrite recrystallization and austenite formation occurring during annealing of cold-rolled advanced high-strength steels are key mechanisms as they largely determine the final microstructure and mechanical properties. However, the influence of processing parameters on these mechanisms and their interactions is still not fully understood. This is particularly the case for Dual-Phase steels having an initial cold-rolled microstructure consisting of ferrite and martensite before annealing, which were scarcely investigated compared to ferrite-pearlite initial microstructures. In situ synchrotron X-ray diffraction experiments together with post-mortem metallographic analysis allowed clarifying both ferrite recrystallization and austenite formation during annealing of a ferrite-martensite initial microstructure depending on the process parameters of the annealing cycle. Results showed a major influence of recrystallization state on austenite formation, leading to an unexpected effect of heating rate on austenite formation kinetics. A modeling approach was undertaken to rationalize the influence of heating rate on austenite formation by taking into account the bi-phased ferrite-martensite initial microstructure and the effect of ferrite recrystallization state.