Intercritical annealing of cold-rolled ferrite-pearlite steel: Microstructure evolutions and phase transformation kinetics

The manufacturing of Dual-Phase steels includes as a crucial step the annealing of a cold-rolled ferrite-pearlite (F/P) microstructure, which involves numerous and interacting metallurgical mechanisms, namely recovery/recrystallization of ferrite, globularization, manganese enrichment, coarsening of cementite and finally austenite transformation. Present study focuses on the austenite transformation considering its interaction with the ferrite recrystallization and the influence of the chemical composition of the cementite. The behavior of a cold-rolled F/P microstructure is studied at three heating rates to induce weak and strong interactions between the mechanisms, in particular using post mortem microstructure observations but also in situ High Energy X-Ray Diffraction experiments on a synchrotron beamline. Slow heating leads to a necklace austenite distribution whereas fast heating conducts to a banded topology. This particular microstructure morphogenesis is explained by the presence of numerous intergranular (or isolated) carbides inside the ferrite matrix, inherited from the hot-rolling. Thermokinetic analysis accounting for the cementite composition shows that the pearlite islands transformation necessarily involves the partition of substitutional elements. Conversely, the dissolving isolated carbides undergo a partition/partitionless transition on heating. After the dissolution of the cementite, a final ferrite/austenite transformation takes place. The phase transformation kinetics increases with increasing heating rates, despite the thermal-activated nature of the austenite growth process. This is interpreted thanks to kinetic simulations with DICTRA software, which allow to analyze the austenite growth regimes involving or not the partition of the alloying elements.