The evolution of austenite fraction and solute partitioning (Mn, Al, and C) during intercritical annealing was calculated for a medium-Mn steel containing 11 pct Mn. Austenite growth takes place in three stages. The first stage is growth under non-partitioning local equilibrium (NPLE) controlled by carbon diffusion in ferrite. The second stage is growth under partitioning local equilibrium (PLE) controlled by diffusion of Mn in ferrite. The third stage is shrinkage of austenite under PLE controlled by diffusion of Mn in austenite. During PLE growth, the austenite is progressively enriched in Mn. Compositional spikes evolve early during NPLE growth and broaden with annealing temperature and time.The 3rd generation, medium-Mn, and advanced high-strength steels are under intense investigation as a substitute to 1st (low alloy) and 2nd generation (high-Mn) steels. These steels aim at improved combinations of strength and ductility. [1][2][3][4] In medium-Mn steels, the Mn content is reduced, relative to the high-Mn steels, in the range between 3 and 12 pct and the microstructure consists of an ultrafine ferrite-austenite mixture. The transformation-induced plasticity (TRIP) of the retained austenite is responsible for the enhanced formability in these steels. Several processing routes have been developed in order to stabilize the austenite phase for optimum TRIP interactions. For steels containing between 3 and 8 weight pct Mn, the quench and partitioning (Q&P) process has been proposed. [5,6] In this process, austenite is stabilized by carbon partitioning from martensite to austenite, the partitioning taking place between the M s and M f temperatures. For steels containing 5 to 12 pct Mn, intercritical annealing, following the cold rolling of the martensitic microstructure, is investigated as a means of stabilizing the austenite by carbon and Mn partitioning. [7][8][9][10][11][12] The retained austenite fraction and stability depend, therefore, on the intercritical annealing temperature and time.Solute partitioning during intercritical annealing in medium-Mn steels has been investigated recently. Most of the research work is concerned with experimental determination of austenite volume fraction and composition. [13][14][15][16] Mn partitioning during intercritical annealing has been studied by atom probe tomography [13,17] and experimental results have been compared with simulation predictions. Mn partitioning has also been studied by TEM [14,[18][19][20] indicating that partitioning of Mn from ferrite to the austenite is slow. Thorough thermodynamic analyses of solute partitioning during intercritical annealing have been performed [21,22] indicating the role of alloying elements. However, kinetic analysis of the partitioning process is limited to relatively few alloy systems, 3 pct Mn, [23] 5 pct Mn [24,25] , and 12.2 pct Mn. [17] These works considered a specific intercritical annealing temperature and examined the partition of alloying elements at specific stages of annealing. Furthermore, the evolution of compos...
