Microstructure evolution during rod and bar rolling

The evolution of austenite microstructure during rod and bar rolling has been investigated using simulation by hot torsion. The simulation reproduced the strains and interpass times for each rolling schedule for finishing passes where the interpass times were less than 1 second, although strain rate was limited to 3 sec-1. Dynamic recrystallisation by strain accumulation can occur for interpass times less than 100ms for temperatures up to 1200°C. The kinetics of post dynamic softening (termed metadynamic recrystallisation) do not appear to be temperature-dependent, but are a function of strain rate. At a strain rate of 3 sec-1there is 50% softening within 200 to 300ms at temperatures from 900 to 1200°C. Mathematical models were developed for the metadynamic kinetics and the dynamic and metadynamic grain sizes. These predict that there will be extremely rapid metadynamic recrystallisation for strain rates that apply in the mill. Metadynamic recrystallisation produces a slightly coarser grain size than dynamic recrystallisation.