Investigation on the microstructural evolution in a Medium-Mn steel (X10Mn5) after intercritical annealing

TRansformation Induced Plasticity (TRIP)-assisted Medium-Mn steels, with Mn-contents in the range of 4-10 ma.-%, have recently gained a lot of interest due to their promising mechanical properties. This steel group contains ≥ 30 % of retained austenite, which is stabilized by Carbon and mainly Manganese during intercritical annealing. The present work investigated the influence of annealing temperature and cooling rate on the microstructural evolution by means of dilatometry. Two thermodynamical models for the prediction of optimal annealing temperature and maximum retained austenite content have also been thoroughly evaluated. For further characterization, scanning electron microscopy, EBSD, micro-hardness testing and X-ray diffraction were carried out. The investigations manifested a pronounced influence of both annealing temperature and cooling rate, on the phase fractions of ferrite, austenite and martensite, which must be taken into account by design of batch annealing route for Medium-Mn TRIP steels in order to obtain superior combination of strength and ductility.