2002 Volume 42 Issue 12 Pages 1461-1467
The microstructural evolution during isothermal holding at 590-750°C after heavy deformation was examined in Nb steel. The strain induced ferrite transformation of Nb steel was significantly retarded compared with that of plain C-Mn steel, when Nb was mostly dissolved. When the grain boundary ferrite was present before deformation and strain free ferrite was formed during the following isothermal holding, abnormal grain growth occurred at the regions near the deformed ferrite by strain-induced boundary migration (SIBM). The SIBM was caused by the energy unbalance at the boundaries between deformed and strain free ferrite grains transformed from the deformed austenite. This rapid growth was not inhibited by the strain-induced NbC precipitates, which means the driving force for the abnormal growth was greater than the pinning force by the precipitates. However, the abnormal grain growth could be prevented by isothermal heat treatment in either fully transformed or untransformed structure. The ultrafine and polygonal ferrite grains were obtained by the recrystallization of ferrite and the grain growth was inhibited by strain-induced NbC precipitates during isothermal annealing at 650°C after deformation.