Effects of Nb on strain induced ferrite transformation in C-Mn steel

Effects of the Nb addition on the strain induced ferrite transformation just above Ar3 temperature were investigated. Hot compression tests were performed with varying the true strain up to 1.6 (80% reduction) using Gleeble 1500. After the hot deformation, samples were immediately water-quenched to examine ferrite formation characteristics. The grain boundary misorientation angles were measured by electron backscatter diffraction in order to observe evolution of the ferrite grains. For reheating temperatures such as 900 and 1000 deg. C, where Nb was mostly precipitated as NbC, strain induced ferrite grains of 1-2 μm were formed homogeneously within the austenite grain in Nb steel. In the cases of higher reheating temperatures 1100 and 1250 deg. C, where most of Nb was dissolved, the strain induced ferrite transformation was remarkably reduced and the ferrite morphology was changed to elongated grains. It was considered that the ferrite transformation during deformation was retarded by both the solute drag effect of Nb and the consumption of strain energy for the dynamic precipitation of NbC