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The modification mechanism of lanthanum on alumina inclusions in a nonoriented electrical steel at 1600 °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 LaSx shell is preferentially generated at the outside of Al2O3 and forms multiphase inclusions. With time increasing, the La2O2S or LaSx shell gradually reacts with the Al2O3 core and generates stable lanthanum‐containing inclusions. At the initial stage of lanthanum treatment, many transient inclusions including LaSx, 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 → LaSx, which is consistent with thermodynamic analysis. [ABSTRACT FROM AUTHOR]

• Both micro-sized inclusions and fine precipitates were statistically analyzed. • Proper La addition reduced inclusions, and enhanced grain size and texture factors. • The steel with 0.0006 wt% La possessed the best magnetic properties. • Excess rare earth sulfides deteriorated grain size and textures of steels. The effect of rare earth elements (REMs) on the magnetic properties of non-oriented electrical steels was studied using industrial trials. The variation of the magnetic properties was discussed in terms of the effect of inclusions, precipitates, grain size, and textures in the steel with and without REMs. Inclusions of MgO·Al 2 O 3 -CaS and (Mg, Mn, Ca)S in REMs-free steel were modified into composite ones of MgO·Al 2 O 3 -LaAlO 3 , La 2 O 2 S-MgO and (La, Ca)S in the steel with 0.0006% lanthanum, and to (La, Ce) 2 O 2 S-CaS-MgO, (Ce, La)S in the steel with 0.0007% lanthanum and 0.0015% cerium. The number density of fine MnS was remarkably decreased by the addition of REMs, leading to an increase in the average grain size. The steel containing 0.0006 wt% lanthanum possessed the best combination of core loss and magnetic induction due to the decrease of micro-sized inclusions and fine MnS, the increase of grain size, and the optimization of recrystallization textures. For the steel with excess REMs addition, the large number density of rare earth sulfides with the size of approximately 1 μm tended to deteriorate the grain size and textures. [ABSTRACT FROM AUTHOR]