High temperature straining behaviour of high FeSi electrical steel by torsion tests

Steel with an increased Si-content has better magnetic properties in electrical applications in terms of high electrical resistivity, reduced energy losses and low magnetostriction. Nevertheless, the oxygen affinity of this element at high working temperatures and the poor ductility observed at room temperature caused by order structures make the thermomechanical processing of these alloys rather difficult. Since these materials do not present a phase transformations from ferrite to austenite, a fundamental study of their workability using torsion tests will help to understand and to optimise their production process. Important critical temperatures in these materials are T-ord (the temperature above which the material is disordered), T-nr (the temperature below which static recrystallisation is not taking place any more) and other restoration temperatures appearing during processing. Fe-Si electrical steels, with silicon concentrations of 2, 3 and 4 wt.-%, were tested according to a multi-deformation torsion schedule under continuous of cooling conditions in 18 passes, with temperature ranges from 1150 to 810 degrees C, at a strain rate of 1 s(-1), the interpass time and the amount of plastic deformation were varied from 20 to 5 see and from 0.1 to 0.3, respectively. Different critical temperatures, important for the processing of these alloys, were calculated from the dependence of the mean flow stress (MFS) on inverse temperature, based on their changes of slope. The temperatures at which the restorations mechanism, the recrystallization and the recovery stops, T, were determined and can be described using the relation developed here, based on their dependence on composition, deformation parameters and cooling rate. The metallographic analysis of quenched samples is in good agreement with the critical temperatures obtained through the measurement of the MFS.