Influence of peritectic phase transformation on hot ductility of high aluminium TRIP steels containing Nb

Increasing Al from 0.05 to 1% in Nb containing transformation induced plasticity steel resulted in deepening and considerable widening of the hot ductility trough. Further increase in the Al level to 1.5% produced a trough similar to the low Al steel but having better ductility in the temperature range of 650-800 degrees C. This improved ductility could be ascribed to its finer austenite grain size. Nb( CN) was able to precipitate readily in these steels and was important in influencing the hot ductility of the 0.05 and 1.5% Al steel in the temperature range of 750-1000 degrees C, with ductility improving as the particle size increased with test temperature. No AlN was found in 0.05% Al containing steel, and there was no significant dendritic precipitation of AlN in 1.5% Al containing steel, although precipitation of AlN in plate form was readily observed. In 1% Al steel, copious dendritic precipitation of AlN was present at the c grain boundaries, leading to rock candy fracture. The poor ductility shown in 1% Al containing steel is due to a combination of this dendritic precipitation, which took place only in a steel of peritectic carbon composition, and its coarse grain size. Both low and 1.5% Al containing steels had compositions outside the peritectic range. It is strongly advised that for this type of steel, the composition should be designed to fall outside the peritectic carbon range.