Difference in recrystallization behavior between martensite and deformed ferrite

Microstructural change and softening behavior during annealing were investigated for deformed ferrite and lath martensite in an ultralow carbon 1.5mass%Mn-0.0018mass%B steel, and then the difference in recrystallization behavior between the materials was discussed in terms of the nucleation site of recrystallized grains. The ferritic and martensitic materials were obtained by furnace-cooling and water-quenching, respectively, after solution treatment. The ferritic material was cold-rolled at a reduction of 80% to give the same dislocation density as of the martensitic material. The deformed ferritic material contains a large number of geometrically necessary boundaries with large misorientations, while the martensitic material does only contain original grain boundaries such as prior austenite grain boundaries, packet boundaries and block boundaries. The recrystallization during annealing is markedly retarded in the martensitic material compared with the deformed ferritic material. As a result, the time for completing the recrystallization was roughly a hundred times longer in the martensitic material than in the deformed ferritic material. This is due to the difference in nucleation site of recrystallized grains, that is, the geometrically necessary (GN) boundaries introduced by the deformation for the ferritic material, and only the original grain boundaries for the martensitic material.