Abstract: Obtaining high-quality gear steel calls for strict requirements on the oxygen content, shape, composition, size, and distribution of nonmetallic inclusions to ensure high-quality steel and castability of the molten steel during the continuous casting process. Al₂O₃ inclusions with high melting point are easily generated in Al-deoxidized gear steel, and they easily result in nozzle clogging and deterioration in the properties of the steel. A reasonable calcium treatment can reduce the nozzle clogging and increase improving the castability of the molten steel, and thus it has been widely used in steel plants. Calcium treatment is often used to convert Al₂O₃ inclusions with high melting point to calcium aluminate with low melting point. The factors of calcium treatment on nonmetallic inclusions in gear steel were investigated through industrial trials. The dependency of calcium yield on various factors, namely, the amount of calcium addition, feeding speed of calcium wire, and feeding position and slag thickness were discussed. The effect of the amount of calcium addition on nonmetallic inclusions in the molten steel was studied at a feeding rate of 1.5 m·s⁻¹. With the initial mass fraction of T.Ca lower than 10×10⁻⁶ in the steel and a feeding rate of 1.5 m·s⁻¹, the calcium yield can be higher than 20% if the amount of calcium addition, clearance height, and slag thickness during the refining process are appropriate. Large numbers of nonmetallic CaS inclusions with high melting point are formed in the molten steel with the mass fraction of T.Ca higher than 17×10⁻⁶, and the inclusions are formed far away from the liquidus region. With increase in the T.Ca content in the steel, the average size and number density of the nonmetallic inclusions gradually increase. The effect of calcium treatment on the modification of nonmetallic inclusions studied by thermodynamic calculation results agrees well with the measurements taken via industrial trials.