Formation and Deformation Mechanism of Al 2 O 3 -CaS Inclusions in Ca-Treated Non-Oriented Electrical Steels
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NON-ORIENTED electrical steels are widely used as core materials in motors and generators, which serve in a rotating magnetic field.[1] Magnetic properties, including low core loss and high magnetic induction, are the most significant requirements for the demands to save energy and improve efficiency. Inclusions in non-oriented electrical steels can impede the domain wall moving and exert the largest pinning force when the size of inclusions is equal to that of the domain wall, which is approximately 120 nm.[2] In addition, inclusions can pin at grain boundaries and retard the recovery and grain growth during the recrystallization process. Based on the Zener theory,[3] the pinning force is proportional to the volume fraction of the inclusions QIANG REN, WEN YANG, and ALBERTO N. CONEJO are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing (USTB), Beijing 100083, China. Contact email: [email protected] LIN CHENG is with the Research and Development Department, Shougang Zhixin Qian’an Electromagnetic Material Co. Ltd., Qian’an 064400, China. LIFENG ZHANG is with the State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China. Contact email: [email protected] Manuscript submitted June 8, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
and is inversely proportional to the mean size of inclusions. Thus, inclusions in non-oriented electrical steels are usually expected to be less and larger. However, since nuclei with h111i orientations occur most rapidly during the recrystallization of the steel,[4] large inclusions are effective in inducing a heterogeneous nucleation of grains with unfavorable orientations, which deteriorates the magnetic properties of the steel. Besides, inclusions with a large aspect ratio lead to a larger inclusion-steel interface compared to spherical ones, which also has a detrimental effect on the magnetic properties.[5,6] Sulfur is a major detrimental residual element and tends to form MnS and CuxS particles, worsening the steel magnetic properties.[7–12] Calcium treatment is one effective method for modifying the composition and morphology of oxides and sulfides in Al-killed steels.[13–17] Calcium can modify high-melting-point Al2O3 into liquid calcium aluminates, thus decreasing nozzle clogging and improving the continuous steel castability. Calcium can also react with sulfur and generate undeformable CaS inclusions, thus reducing stringy MnS particles and promoting steel isotropy.[18] There have been many studies investigating the modification of inclusions by calcium treatment in non-oriented electrical steels.[19–21] It has been reported that Al2O3-CaS inclusions are the main ones in the final products of Ca-treated non-oriented electrical steels.[20]
As for the formation of Al2O3-CaS inclusions in Ca-treated steels, Verma et al.[22] and Ren et al.[23] proposed that Al2O3-CaS inclusions were transient after the steel was calcium treated. Gradually, the reaction between CaS
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