Use of Magnetic Techniques for Characterisation of the Microstructure Evolution during the Annealing of Low Carbon Steels

In this paper, how magnetic hysteresis loop and magnetic Barkhausen noise (MBN) measurements can provide non destructive information about the evolution of the microstructure during the annealing of cold rolled low carbon steel sheets is shown. The effect of recovery and recrystallization processes on hysteresis loop and MBN signal measurements is analysed and related to the microstructural changes produced due to these softening processes, which have been previously characterised by optical and transmission electron microscopy. The results obtained prove that the coercive field and the remanent induction from hysteresis loop measurements, as well as the total number and average amplitude of pulses of the MBN signal could successfully be employed to non-destructively monitor the kinetics of recovery and recrystallization processes in cold rolled low carbon steels. These parameters are sensitive to the reduction in the dislocation density taking place during recovery and to the combined effect of the decrease in the dislocation density and the average grain refinement during recrystallization. Additionally, some correlations are considered between parameters derived from hysteresis loops, and from MBN measurements that are satisfied during recovery, and can also be used to distinguish recovery from recrystallization.