EVOLUTION OF RECRYSTALLIZATION BY CHANGES IN MAGNETIC HYSTERESIS LOOP IN A NON-ORIENTED ELECTRIC STEEL COLD ROLLED

Abstract

Non‐oriented steels, with low carbon, are widely used in the fabrication of electrical motor nucleus. The performance of these motors is affected by the level of recrystallization. These steels can come from the steel plant in two different conditions: totally processed or semi‐processed. The semi‐processed steels have a partially deformed structure and are submitted to the final annealing process after reaching the end shape. An adequate annealing heat treatment is important to get an appropriate magnetic property. In the present study, samples of an electric steel, with the composition (0.05 wt% C, 1.28wt% Si, 0.29wt% Mn), cold rolled 50% in thickness, were withdrawn during the industrial heat treatment at temperatures of 575, 580, 600, 620 and 730 °C with the objective of evaluating the evolution of recrystalization with temperature. Magnetic properties were measured at room temperature in a vibrating sample magnetometer. Although the changes in magnetic hysteresis loop with temperature are difficult to observe, they have been identified by using pattern classification techniques, such as principal‐component analysis and Karhunen‐Loève expansion. These tools have been applied to vectors which are built from each hysteresis loop, properly renormalized, whose components correspond to amplitude of the loop at given equally spaced values of the renormalized field. The samples have been classified in four sets, namely, set A corresponding to temperatures 575/580, set B corresponding to temperatures 600/620, set C corresponding to the samples without annealing heat treatment, and set D corresponding to recrystallized samples. The results for the classification of the different microstructures have been obtained by using both techniques, and in particular a 100% success rate has been reached by using Karhunen‐Loève expansion.