Post-dynamic recrystallisation after multiple peak dynamic recrystallisation in C–Mn steels

Post-dynamic softening after multiple peak dynamic recrystallisation in low C–Mn steels was investigated using interrupted torsion tests at 1100 and 900° C and a strain rate of 0·003 s−1. Under these conditions, the stress-strain curve exhibited oscillations characteristic of strain coarsening dynamic recrystallisation. The softening behaviour exhibited a two stage sigmoidal curve, the first curve consisting of an immediate softening up to about 10% of the total amount. The second was characterised by a gradual increase up to 80–90%. Complete softening was observed after unloading in the steady state but not after unloading at the first stress minimum. Because of the lower driving force for recrystallisation when interrupting in the first minimum, incomplete softening is attributed to the impingement of the new grains on the metadynamically recovered grains. The kinetics of post-dynamic recrystallisation after multiple peak dynamic recrystallisation was accurately predicted by the kinetics equation obtainedfor metadynamic recrystallisation at high strain rates. The apparent incubation timefor post-dynamic recrystallisation is attributed to the low driving force for recrystallisation at low strain rates. The gradual increase in softening during the second stage corresponded to the growth of new grains. The grain sizes after full softening were in reasonable agreement with the predicted value using the metadynamically recrystallised grain size equation obtained at high strain rates. Because the kinetics and grain size equations displayed a strong dependence on strain rate, a weaker dependence on temperature, and no dependence on strain, the second stage was interpreted as being due to metadynamic recrystallisation and not classical static recrystallisation. The low rate of work hardening in the reloading curve after a short interpass time indicates that recovery mainly took place during the first stage. When unloading after the first stress minima, the work hardening rate in the reloading curve after almost complete softening did not reproduce the first hit (deformation) due to the presence of the metadynamically recovered grains. However, when unloading in the steady state after long interpass times the reloading curve was similar to the first hit, indicating that full metadynamic recrystallisation had taken place.