The Effect of Strain and Annealing on the Growth of NbC Precipitates During Two-Pass Hot Deformation of a Fe-30Ni-Nb Model Microalloyed Steel

The present work investigates the effect of double-pass deformation and annealing on the evolution of NbC precipitates during hot deformation of a Fe-30Ni-Nb microalloyed model steel. The investigation has been performed using transmission electron microscopy. NbC precipitation mainly took place on the periodic dense dislocation networks constituting the microband (MB) walls that largely maintained their crystallographic alignment up to large strains. The double-pass deformation has markedly reduced the precipitate size compared to the equivalent single-pass straining despite twice as long post-deformation annealing time. This has largely been attributed to relocation of the precipitates at the second pass to solute-richer areas, which shifts the precipitation process back to the nucleation and growth stage, and to particle rotations that disrupt their (semi-)coherency with austenite and thus restrict their growth due to increased misfit stresses. The extended holding after two-pass deformation did not increase the precipitate size due to widespread dislocation annihilation through recovery processes, leading to coarsening and disintegration of the MB wall dislocation networks. This resulted in decreased efficacy of the pipe diffusion and complete loss of access to this type of diffusion for a large fraction of precipitates, which markedly hindered the precipitation growth and coarsening processes.