Modification Mechanism of Lanthanum on Alumina Inclusions in a Nonoriented Electrical Steel

The modification mechanism of lanthanum on alumina inclusions in a nonoriented electrical steel at 1600 degrees C is investigated using laboratory experiments. Inclusions are analyzed using an automatic scanning electron microscope equipped with energy-dispersive spectrum at 1, 5, 10, and 30 min after lanthanum treatment. The contents of total oxygen (TO), total sulfur (TS), and total lanthanum (TLa) in steel are analyzed. Results show that Al2O3 inclusions are modified by lanthanum in two ways. One is that the dissolved lanthanum directly reduces the aluminum in Al2O3 inclusions and forms LaAl11O18 or LaAlO3 ones. The other is that a new La2O2S or LaS x shell is preferentially generated at the outside of Al2O3 and forms multiphase inclusions. With time increasing, the La2O2S or LaS x shell gradually reacts with the Al2O3 core and generates stable lanthanum-containing inclusions. At the initial stage of lanthanum treatment, many transient inclusions including LaS x , La2O2S, and La-P(-As) are formed due to the high concentration of lanthanum, while gradually redissolving with time increasing. With the increase in TLa content in steel, the formation sequence of stable inclusions is Al2O3 -> LaAl11O18 -> LaAlO3 -> La2O2S -> LaS x , which is consistent with thermodynamic analysis.