The presence of retained austenite (RA) has been proven beneficial for the formability of Advanced High Strength Steels. Therefore, multiple thermal austenite stabilization methods have been researched, such as Mn-partitioning during intercritical annealing in medium Mn steels. T
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The presence of retained austenite (RA) has been proven beneficial for the formability of Advanced High Strength Steels. Therefore, multiple thermal austenite stabilization methods have been researched, such as Mn-partitioning during intercritical annealing in medium Mn steels. The Cyclic Partial Phase Transformation (CPPT) approach has been successful in obtaining localized Mn-enrichment at the ferrite/austenite interface through cyclic annealings at intercritical temperatures in a medium Mn steel. In this thesis, the possible application of CPPT heat treatments towards interfacial austenite stabilization at room temperature in medium Mn Dual Phase (DP) steels is studied. ThermoCalc modelling and dilatometry measurements were used to determine the optimal CPPT parameters for fully martensitic Fe-Mn-C-Si samples with varying concentrations of manganese. Scanning Electron Microscopy acquisitions of the resulting microstructure showed a strong influence from the initial microstructure. Samples which went through a full austenitization formed coarse martensitic islands and equiaxed ferrite grains. However, samples which were intercritically annealed from a fully martensitic state resulted in fine and elongated ferrite grains surrounded by martensite. X-Ray Diffraction analysis revealed that CPPT treated samples with 2wt.%Mn and 4wt.%Mn had a maximum RA volume fraction of 1wt.% and 4wt.% respectively. Up to 16wt.% of RA was obtained at room temperature through CPPT heat treatments for samples with 6wt.% Mn. This difference in results has been attributed to the concentration spike of Mn at the ferrite/austenite interface characterizing the NPLE austenite growth, as well as to the widening of the Mn-enriched zone through repeated isothermal intercritical annealing cycles. While a relatively high volume fraction of RA has been successfully obtained at room temperature in 6wt.% Mn DP steels, its interfacial morphology could not be confirmed due to the interference of the fine ferrite/martensite microstructure with the Electron Back-Scattered Diffraction analysis.