Effect of Nb on grain growth of ferrite in C-Mn steel during isothermal holding after severe deformation

Abstract

The effect of Nb on grain growth of ferrite during isothermal holding after severe deformation was investigated. For Nb and C-Mn steels, the deformation was carried out in the temperature range 645-772°C, encompassing ferrite transformation temperature Ar ₃, after reheating at 900°C and 1250°C. After reheating at 900°C, the strain induced ferrite grains with an average grain size of 2 μm readily formed up to the equilibrium fraction at all deformation temperatures. Grain growth of the strain induced ferrite in the Nb steel was significantly inhibited during the following isothermal holding because of the grain boundary pinning by NbC precipitates. After reheating at 1250°C, the strain induced ferrite transformation was apparently reduced by solute drag of Nb so that strain free ferrite grains were formed during the subsequent isothermal holding. When strain free ferrite formed near the deformed ferrite grains, abnormal grain growth by strain induced boundary migration (SIBM) occurred in the Nb steel, as well as in the C-Mn steel. The SIBM occurred owing to the stored energy difference between the deformed ferrite and the strain free ferrite regardless of the presence of fine NbC precipitates or partially recrystallised grains.