WEB Investigation of microstructural evolution in medium-Mn steel during intercritical annealing: role of elemental partitioning in interface migrationTuesday (22.09.2020) 10:40 - 10:55 S: Structural Materials 1 Part of:
The growing demands of weight saving, emission reduction and passenger safety in the automotive industry have promoted new concepts for medium-Mn steels design to achieve an excellent combination of high strength and superior ductility. Tailoring the mechanical properties through different microstructural evolution has been intensively studied, yet the elemental partitioning behaviour during austenite reversion and its role in the interface migration is not fully understood.
In this study, cyclic partial phase transformation annealing in medium-Mn steels was applied to study the role of elemental partitioning in interface migration. Three thermal cycles between 700 °C and 850 °C were proceeded at the heating/cooling rate of 50 °C/s in the investigated Fe-5Mn-2Al-0.1C and Fe-5Mn-2Al-0.2V-0.1Nb-0.1C steels. The simulation by the classical diffusive transformation theory using the DICTRA software and the MOBFE2 kinetic database indicated Mn segregation in form of a spike in front of the migrating interface. The results show that during the heating stage of intercritical annealing, the interface moves toward the ferrite phase and no Mn-spike is predicted. The following cooling stage generates the Mn-spike with the typical width of 20 nm - 60 nm and makes the interface stagnant. Mn-spike reveals the content concentration of 10%, doubling the Mn concentration in matrix. By repeating the cyclic annealing between 700 °C and 850 °C, the previous Mn-spikes are eased to low concentration. The concentration profiles across the interface were determined by atom probe tomography (APT) technique, providing experimental validations to the simulation. Together with the dynamic monitoring of phase transformation by in-situ synchrotron X-ray diffraction (SynXRD), the austenite reversion kinetics is quantitatively investigated. The dynamic monitoring of interface migration by both numerical modelling and experiments reveals the effects elemental partitioning during the phase transformation. The role of partitioning in microstructural evolution is further discussed in detail.