Analysis of ferrite grain growth mechanisms during γ−α transformation in a niobium alloyed steel using EBSD

The austenite to ferrite phase transformation was studied in a C–Mn–Nb steel after different hot deformation schedules, leading to deformed and recrystallized austenite. The mechanisms of nucleation and growth of ferrite grains were investigated by means of the electron back scattered diffraction (EBSD) technique. The ferrite microstructures were characterised in terms of the misorientation angles between ferrite grains and the deviations from an ideal Kurdjumov–Sachs orientation relationship with the austenite. The results show that the grain refinement produced by the accumulation of the deformation in the austenite is limited to a certain extent by the ferrite grain coarsening taking place behind the transformation front. Both coalescence and normal grain growth have been observed to contribute to this coarsening. Coalescence is enhanced as a result of the variant selection taking place in transformation from a recrystallized austenite. The accumulation of the deformation in the austenite results in ferrite–ferrite boundaries of higher misorientation, causing coalescence in this case to be less favoured, as compared with normal grain growth.