Transition between static and metadynamic recrystallization kinetics in coarse Nb microalloyed austenite

The recrystallization behaviour of a Nb microalloyed steel, under deformation conditions corresponding to the initial stands of a thin slab hot rolling process which is characterized by coarse initial austenite grain sizes and large strains per pass, has been investigated. A range of strain levels were studied ranging from below the critical strain for the onset of dynamic recrystallization, εc, well into the steady state for dynamic recrystallization, ε>εss. There is a transition strain, εc<εT<εss, separating the post-dynamic softening strain-dependent range from that which is strain-independent. At strains above εT, the kinetics of recrystallization is independent of strain but strongly dependent on strain rate. Under these conditions, metadynamic recrystallization operates as the prime softening mechanism and the kinetics have been derived. At strains between εc and εT, both metadynamic and classical static recrystallization are involved in post-dynamic softening. A model is derived to predict the evolution of softening as a function of the holding time for different strain ranges, based on the contributions of the individual components to the overall softening.