Abstract: Calcium treatment can convert Al₂O₃ into liquid calcium aluminate, effectively mitigating nozzle clogging risks and improving steel quality. The effect of Ca content on inclusions was investigated. Results show that in untreated steel, inclusions are primarily solid Al₂O₃. At 0.000 8 wt.% Ca, small Al₂O₃ inclusions are partially modified into calcium aluminate, while large Al₂O₃ inclusions exhibit only partial surface modification, forming layered composite inclusions of Al₂O₃ and calcium aluminate. At 0.001 5 wt.% Ca, the inner Al₂O₃ layer in composite inclusions decreases and is converted into an outer layer ofcalcium aluminate indusions. At 0.002 4 wt.% Ca, inclusions are fully modified into calcium aluminate. At 0.003 9 wt.% Ca, CaS begins to attach to the calcium aluminate surface, forming CaO-Al₂O₃-CaS composite inclusions. At 0.005 1 wt.% Ca, the CaS content in CaO-Al₂O₃-CaS inclusions increases significantly. With increasing Ca content, the inclusion composition shifts from Al₂O₃-rich to calcium aluminate; the average diameter first decreases, then increases, and then decreases slightly, while sphericity first increases and then decreases. At 0.001 5 wt.% Ca, inclusions achieve the smallest average diameter and the highest sphericity. Inclusions in the steel with 0.001 5 wt.% Ca still undergo significant changes during cooling and solidification. From 1 600 to 1 480 ℃, the inclusion composition remains primarily calcium aluminate, but the size increases notably. Between 1 350 and 1 200 ℃, CaS begins to precipitate on the calcium aluminate surface, with the amount of precipitation increasing as the temperature decreases, forming core-shell CaO-Al₂O₃-CaS composite inclusions. From 1 600 to 1 200 ℃, the average inclusion diameter first increases and then decreases, while the number density first decreases and then increases. At 1 350 ℃, the average diameter reaches its maximum and the number density reaches its minimum.