Effect of grain size distribution on recrystallisation kinetics in a Fe-30Ni model alloy

The high strength and toughness of microalloyed steels are derived by controlled rolling schedules, using recrystallisation to give grain refinement. A number of relationships (based on the JMAK equations) for the rate of recrystallisation, with a range of Avrami parameter values, have been reported. This paper discusses the role of grain size distribution on the recrystallisation rates and Avrami values for an austenitic model steel (Fe-30 Ni). Cold deformation has been used to provide uniform macroscopic strain distributions (strains of 0.2 and 0.3, which are equivalent to 0.27 and 0.45 strain if deformed at 850°C), followed by recrystallisation during annealing at 850 – 950°C. It is shown that the Avrami parameter is directly related to the grain size distribution, with a lower Avrami exponent being seen for a larger average and wider grain size distribution. In-situ heating in an SEM with EBSD has shown the recrystallisation kinetics to be affected by both nucleation site and local strain heterogeneity.