Studied On The Recrystallisation Behavior Of Nb Micro-alloyed Steel

In hot deformation process, the relationship between the austenite microstructure evolution and the recrystallization behaviour is so closed, the recrystallization mechanism controls the microstructure evolution during the hot rolling of steels. With the development of micro-alloying technology, Nb is used widely in the production of high-strength steel to improve the mechanical properties. Because the solution drag effect of micro-alloyed element Nb and the pinning effect of fine precipitates such as Nb(CN), the recrystallization behaviour for the Nb steel is postponed and more difficult in comparison with the recrystallization behaviour for the C-Mn steel. Investigation of the recrystallization behaviour of deformation austenite for the Nb steel is the basis of determining rolling schedule, and it plays an important role in formulating the rolling parameters in industrial conditions.In this paper, the method of hot simulation test and microstructure observation is used to study the recrystallization behaviour of the Nb steel and the C-Mn steel. By the analysis of true stress-strain curves at different deformation stages, the dynamic recrystallization(DRX) of two steels is studied. The hot deformation equations of austenite, the DRX critical conditions and the influence of deformation parameters on recrystallization grain size for the two steels are determined. By the method of stress relaxation, the static recrystallization(SRX) of the Nb steel and the C-Mn steel is studied. The factors which influence the SRX and the austenite SRX kinetic models for two steels are discussed. Furthermore, the role of micro-alloyed element Nb on the recrystallization behaviour for the Nb steel is studied.The results show that Nb in solid solution retards the occur of SRX and DRX, increases the deformation activation energy and the apparent activation energy dramatically. The austenite grain size of DRX of the Nb steel is refined by decreasing the deformation temperature and improving the stain rate. The increase of strain accelerates the kinetic of SRX. The precipitation and recrystallization competes for stored energy of deformation. If deformation temperature is less, SRX of austenite of the Nb steel is retarded and precipitation of Nb(CN) takes place. By comparation, the austenite recrystallization behaviour of C-Mn steel is easer.