Effect of ausforming and cooling rate on the distribution of high angle boundaries in low carbon bainitic structure

High Mn-high Nb micro-alloyed steels were investigated by electron back-scattering diffraction (EBSD) to determine the effect of cooling rate and ausforming on the characteristic of misorientation and the distribution of high angle boundaries (HABs) in low carbon bainitic microstructure. The results indicate that there is a dispersion of HABs interspacing from prior austenite grain boundaries to the grain interior. Both fast cooling and ausforming raise the drive force of coherent transformation, weaken variants selection, and are helpful for the increasing of HABs density near the boundaries and in the grains. Moreover, a larger deformation below the non-recrystallization temperature not only can flatten the austenite grains fully, but also can generate incoherent nucleation of fine ferrite grains at the prior austenite boundaries. Also the fine ferrite grains have significant misorientations with adjacent microstructure.