The effect of antimony on internal oxidation and magnetic properties of Si-Al electrical steels

Decarburization treatment performed by motor and transformer manufacturers often results in internal oxidation of the solute elements. The presence of subscale (zone affected by precipitation of Si-Al oxides) adversely affects magnetic properties, particularly permeability of the annealed steel. The permeability deterioration is proportional to subscale thickness. Though it is theoretically feasible to design an anneal such that complete decarburization will be achieved without internal oxidation, the practical realization of the above anneal will be restricted by time and/or cost limitations. An alternative approach to the problem of internal oxidation lies in searching for alloying elements which suppress the diffusion of oxygen into the steel. It was established that additions of .08 wt% Sb to Si-Al-bearing electrical steel resulted not only in the development of a better magnetic texture, but also in a substantial suppression of the internal oxidation and subsequent improvement in magnetic properties. It was shown that after 1 hour decarburizing anneal 1 wt% Si steel containing Sb developed subscale layer approximately 3 times thinner and had 200 units higher permeability than antimony free material. The mechanism by which Sb affects the rate of internal oxidation was also established.