Detrimental effect of oxidation on magnetic properties of nonoriented electrical steel sheet

Abstract

The conventional final heat treatment of Si+Al alloyed nonoriented electrical steel sheet is a continuous two-stage process with decarburization in the first stage and high-temperature recrystallization annealing in the second. The present investigation shows that this practice has an important detrimental effect on core loss and thus prevents the full optimization of the final product. This is due to the use of wet gas in the first annealing stage which causes harmful oxidation processes and, if nitrogen-containing atmospheres are used, also facilitates subsurface nitration during annealing at high temperatures in the second stage. Therefore, the current development of N. O. grades with superior magnetic properties is based on improved steels with extremely low carbon content, thus avoiding the conventional decarburization annealing with its associated detrimental oxidation processes.