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NONMETALLIC inclusions are unavoidable during the steelmaking process, and the residue inclusions are frequently reported as the origins of steel performance issues such as fracture and fatigability.[1–3] Although much effort has been made in the utmost removal of inclusions, they still exist in products.[4–9] In contrast, the idea of using fine oxide inclusions as the heterogeneous nucleation sites has been brought to the forefront with the development of oxide metallurgy. It is proposed that the small oxide inclusions with homogenized distribution can induce intragranular ferrite effectively, which will improve the performance of steel.[10,11] Aluminum is commonly applied as a deoxidizer in the modern steelmaking process due to its strong deoxidization ability and low price. Nevertheless, the generated Al2O3 inclusions tend to aggregate and form large clusters in the molten steel. These inclusions are detrimental to the mechanical properties of products and easily cause nozzle clogging.[12,13] Therefore, calcium LINZHU WANG, SHUFENG YANG, JINGSHE LI, and SHUO ZHANG are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, P.R. China. JIANTAO JU is with the School of Metallurgical and Ecological Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, P.R. China. Contact e-mail: [email protected] Manuscript submitted August 26, 2016. Article published online February 1, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS B

treatment is widely used to modify solid Al2O3 inclusions into low-melting calcium aluminates, which is an effective method to prevent the inclusions from clogging the nozzles.[14–16] Yet, residual calcium aluminates with a large size have been still detected in products.[17,18] Hence, steelmakers were attempting to find new ways to solve this problem,[19–21] and it is found that Mg treatment has a remarkable improvement on the steel performance.[22] Many scholars advocated that the positive effects of Mg addition in the steel are attributable to the evenly distributed MgO-containing inclusions, and it can be concluded to three points: (1) The generated MgO-containing inclusions present a much weaker tendency to form clusters, and they can prevent the aggregation of aluminum inclusions by forming around their periphery.[23,24] (2) Numerous inclusions with small size precipitate in the steel with Mg treatment. Jiang et al. observed that inclusions decreased from 1.91 to 1.29 lm and the number density increased from 2.96 9 104 to 5.62 9 104 mm3 in the steel with the addition of Mg, compared with the steel without Mg addition.[25] (3) The fine MgO-containing inclusions have a facilitating effect on the formation of equiaxed crystallization and refinement of microstructure.[26–28] The experimental research indicates that the finer the inclusion particle is and the more the unit volume number is, the stronger the pinning effect is and the finer the grain is.[25] These studies indicate that it is of crucial importance to obta