Effect of Deformation Mode on Microstructure Evolution in Nb-Microalloyed Steel

During most metal forming processes, metal undergoes a complex deformation history that may introduce high inhomogeneity of both deformation and microstructure. For example, recent observations show that the strain reversal in a Nb-microalloyed steel affects significantly, and in a complex way, not only dynamic and static recrystallisation kinetics, but also strain-induced precipitation, whereas the austenite-ferrite phase transformation kinetics seem to be rather insensitive to the change of the deformation mode. It is already known that the recrystallisation stagnation is a consequence of the competition between the driving pressure for recrystallisation and the pinning pressure caused by the strain-induced precipitation of Nb(C, N) precipitates. Both of these parameters depend in turn on the local dislocation density. Thus, it is expected that a variation of the local dislocation density due to reversal of the strain will affect at the same time, the local driving and the pinning pressures, which will cause the difference in the hardening levels. In the present paper, the influence of strain mode change on microstructure evolution and mechanical behaviour in microalloyed steels will be studied. The purpose-built Arbitrary Strain Path (ASP) machine will be used to carry out forward/reverse torsion tests at elevated temperatures to study the effect of complex deformation modes on the phenomena occurring during microstructure evolution of Nb-microalloyed steel. This will provide data for the modification of existing constitutive equations that mostly do not account for the strain path changes. Better through-process modelling will enable to achieve better properties and quality of the final product.