Effect of Mn and Al contents on hot ductility of high alloy Fe-xMn-C-yAl austenite TWIP steels

Effect of Mn (14.94,18.21, and 23.6 wt%) and Al (0.002,0.75, and 1.47 wt%) contents on hot ductility of five high alloy Fe-xMn-C-yAl austenitic Twinning induced plasticity (TWIP) steels were investigated by Gleeble-3500 thermo-mechanical simulator in the temperature range 700-1200 degrees C under a constant strain rate of 3 x 10(-3) s(-1). The results indicated that the hot ductility of different Mn-containing TWIP steels are not appreciable with all the reduction of area (RAs) values lower than 30%, and RAs would be further decreased as the Mn content increased. The matrix of TWIP steel is inhomogeneous with severe Mn microsegregation in the interdendritic zone. Moreover, the C microsegregation ratio increases from 0.85 to 1.16, 0.76-1.22, to 0.74-1.32 when Mn concentration increases from 14.94 wt%, 18.21 wt%, to 23.6 wt%, respectively. Additionally, the microstructure and the true stress-true strain curves suggested that dynamic recrystallization (DRX) took place in 14.94 wt% Mn bearing TWIP steel, while the fraction of DRX grains decreased dramatically with increasing Mn content. Therefore, it is inferred that the high Mn addition inhibited DRX, together with the acceleration effect of C microsegregation by Mn addition should be the most predominant factor of the hot ductility loss with Mn content increases in TWIP steels. On the other hand, Al addition to TWIP steels resulted in a dramatic increase of AIN particles content. The AIN particle accounted for nearly 64% of the total precipitate content for the 1.59 wt % Al containing TWIP steel. Compared with Al-free TWIP steel, the excessive number of fine AIN particles in the 1.59 wt% Al containing steel effectively pinned the austenite grain boundaries, which inhibited the occurrence of DRX and simultaneously promote grain boundary sliding, resulting in the deterioration of hot ductility.