5th International High Manganese Steel Conference - HMNS 2022

Medium Mn steels is a family of new steels which present ground-breaking properties due to their ultra-fine ferritic matrix and a high amount of retained austenite when processed through intercritical annealing. The high fraction of austenite is enabled thanks to carbon and manganese partition between austenite and ferrite at high temperature. Controlling the enrichment and the stability of austenite is crucial for the steel to show an efficient TRIP (transformation induced plasticity) effect.
The kinetics of enrichment in Mn and C of austenite depends on annealing conditions (intercritical annealing temperature and heating rate), the initial microstructure (cold-rolled or as quenched martensitic as well as the fraction of cementite) as well as the nominal composition of the steel. Moreover, hydrostatic stresses generated in austenite during the final cooling (due to the final martensite transformation or to the difference in thermal expansion coefficient between austenite and ferrite) also affects its apparent stability.

The purpose of this work is to decouple the effect of each aforesaid parameters on the stability of austenite. To do so, a single alloy (0.2C-4Mn-0.8Al-1.5Si) have been annealed at four intercritical temperatures, ranging from 680°C to 750°C after a controlled heating stage. Those temperatures have been chosen after thermodynamic equilibrium calculations. Before annealing, the plates have been manufactured in two different ways to vary the initial microstructures. They have been either simply cold-rolled or cold-rolled, fully annealed and quenched.
These thermal treatments have been performed on synchrotron beamline to follow in operando the phase transformations mechanisms using High Energy X-Ray Diffraction (HEXRD). The high energy (100 keV) coupled with a high-performance detector enable high acquisition rates (10 Hz) and thus time-resolved investigations even during the heating stage. HEXRD correlated with post-mortem wavelength dispersive spectroscopy allows an simultaneous tracking of the phase transformation sequences, the chemical partitioning and the internal stresses at phase scale.