An extra low carbon steel was cold rolled to 85% reduction and annealed at 680 °C to generate a microstructure containing ~2 % recrystallized grains. A partly recrystallized volume was analyzed using 3-D FIB-EBSD tomography. The results show that nucleation and subsequent growth of recrystallizing grains is more complex processes than that revealed using 2-D metallographic techniques. In the present steel, it was found that subgrains were found to be the origin of nucleation and these grains exhibit an internal structure similar to the surrounding deformation substructure. However, a certain subgrain keeps expanding to a stage where some part or parts of the boundary reach(es) and consume(s) a high stored energy deformation zone(s) to form (a) local dislocation free zone(s) having an orientation similar to the subgrain. After this stage, the residual dislocations in the original subgrain are annihilated and nuclei enter a well-defined growth stage. The overall growth of recrystallization nuclei was found to be controlled by the variation in both the stored energy and orientation of the surrounding deformation substructure that results in heterogeneous growth by so-called orientation pinning.
Microstructural evolution, recovery and recrystallization kinetics of isothermally annealed ultra low carbon steel
