The first example deals with the kinetics of austenite formation and solute partitioning in medium-Mn steels. These steels belong to the 3rd generation, advanced high-strength steels and are a substitute to 1st (low alloy) and 2nd generation (high-Mn) steels aiming at improved combinations of strength and ductility. In medium-Mn steels, the manganese content is reduced, relative to the high-Mn steels, in the range between 3 and 12 wt% 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 and several processing routes have been developed in order to stabilize the austenite phase for optimum TRIP interactions. For steels containing 5 to 12 wt% manganese, intercritical annealing, following the cold rolling of the martensitic microstructure, is investigated as a means of stabilizing the austenite by carbon and manganese partitioning. The retained austenite fraction and stability depend, therefore, on the intercritical annealing temperature and time. Simulations have been carried out on a Fe-0.18C-11Mn-3.8Al steel. The austenite fraction is shown in Fig.1a as a function of annealing time for several intercritical annealing temperatures. The evolution of austenite consists of three stages. In stage I, the initial rapid increase of austenite fraction is due to growth under no-partitioning local equilibrium conditions (NPLE mode), where the growth is controlled by carbon diffusion. In stage II the intermediate slow growth of austenite takes place under local equilibrium with partition of manganese and aluminum (PLE mode), controlled by Mn diffusion in ferrite. In stage III the final very slow equilibration is controlled by Mn diffusion in austenite and is associated with the shrinkage of austenite. Points B in Figure 1a indicate the NPLE to PLE transition, for each annealing temperature, points C mark the maximum austenite volume fraction corresponding to the transition between PLE growth controlled by diffusion of Mn in ferrite and PLE growth controlled by Mn diffusion in austenite. Points D mark the final stable volume fraction of austenite corresponding to the equilibrium volume fractions computed by Thermo-Calc (points E). Mn partitioning during austenite growth is shown in Fig.1b. Austenite growth under PLE mode (stage II) is controlled by Mn diffusion in ferrite. There is a significant enrichment of austenite in manganese at the interface with ferrite. This is attributed to the low diffusivity of manganese in austenite, which does not allow the accommodation of the Mn diffusive flux from ferrite. This Mn enrichment of austenite leads in austenite stabilization. The simulations presented above allow the selection of optimum heat treatment conditions in order to achieve the desirable microstructure in terms of austenite amount and stability for enhanced TRIP interactions in these steels.

Contributors: G.N. Haidemenopoulos and H. Kamoutsi

Reference: H. Kamoutsi, E. Gioti, G.N. Haidemenopoulos, Z. Cai and H. Ding, Kinetics of solute partitioning during intercritical annealing of a medium-Mn steel, Metallurgical and Materials Transactions A, 46, No.11, pp. 4841-4846, 2015