Incomplete Recrystallization and Subsequent Microstructural Evolution during Intercritical Annealing in Cold-rolled Low Carbon Steels

Microstructural evolution during intercritical annealing after rapid heating of cold rolled low carbon steels was investigated by EBSD and TEM. When the specimens were heated to above the Ac1 temperature before the completion of the primary recrystallization of α and then annealed, remarkable retardation of the primary recrystallization was observed, resulting in non-uniform grain size distribution of α. Because of the retardation, subgrains formed in non-recrystallized α are not consumed by recrystallized α and are thus able to gradually grow with annealing, and finally the grain size distribution of α becomes uniform. The analysis based on the rn−r0n=kt equation has shown that the subgrain growth in non-recrystallized α from the holding period of 100 to 1000 s follows n=3, indicating that the subgrain growth is controlled by the volume diffusion of Mn at α and γ interfaces. The transition of n from 3 to 2, observed at the holding time of longer than 1000 s, may be due to the gradual disappearance of the smaller γ phase at subgrain boundaries.