Microstructural evolution of commercial pure iron during directional annealing

Microstructural evolution during the directional annealing of the cold-rolled pure iron has been investigated. Columnar grains with the largest aspect ratio of 23.6 were produced in the specimen cold-rolled for 70% of thickness reduction at the hot zone temperature (HZT) of 850 °C and a temperature gradient (TG) of 200 °C/cm with an optimum drawing velocity of 5 μm/s. It was found that the columnar grain structure was developed due to abnormal grain growth. There is a critical coarsening temperature, below which the secondary recrystallization does not occur during the annealing; consequently no columnar grain structure can be produced. No obvious evidence shows that the grain size after primary recrystallization has an influence on the microstructural evolution. Due to the strong crystallographic texture of the small grains ahead of the hot zone, the columnar grains have small aspect ratios. Most of the boundaries of the columnar grain are high angle coincidence site lattice (CSL) grain boundaries. The grains with twin or low angle grain boundaries are apt to form island grains in the columnar structure.