The effects of pre-existed austenite fraction on the phase constitution of martensite and retained austenite in intercritical-annealed medium Mn steel

Medium Mn steels have an attractive balance of the strength and ductility due to the TRansformation Induced Plasticity (TRIP) effect by the martensitic transformation of the retained austenite. However, the ferrite + austenite intercritical annealing for a long time is required for obtaining more fraction of retained austenite by enrichment of C and Mn because of the slow partitioning of Mn. To obtaining more retained austenite by short intercritical annealing, the effect of the pre-existed austenite fraction (6%~35%) before intercritical annealing on the fraction of the martensite and retained austenite phases was investigated in the 0.16C-0.5Si-3.5Mn (mass%) steel annealed at 690 °C for 180 s . The fraction of the austenite during annealing increased with increasing the fraction of pre-existed austenite, however the most retained austenite of 21% was obtained in the sample with 6% pre-existed austenite annealed at 690 °C for 180 s, not in the sample with 35% pre-existed austenite. The significant partitioning of the Mn in the retained austenite with thin acicular shape was confirmed in the samples with 6% pre-existed austenite by STEM-EDS, while the gradual gradient of Mn content from the ferrite/austenite interface to inside the austenite in the retained austenite with thick lath-shape could be recognized in the sample with 35% pre-existed austenite. This result indicates that a small amount of thin pre-existed austenite is effective to shorten the diffusion distance in austenite for Mn and enrich Mn into austenite during intercritical annealing.