Abstract：Characteristics of inclusions in plate steel refined by LF-Calcium treatment-RH process was studied by industrial experiments. Thermodynamic calculation was performed to study the evolution mechanism of inclusions during calcium treatment. It was found that the mass percent of T[O] and number density of inclusions decreased,average size of inclusions increased during secondary refining processes. After calcium treatment，inclusions were in CaO-MgO-Al2O3-CaS quaternary system. After RH refining，inclusions were in CaO-MgO-Al2O3 ternary system. The mass percent of CaO of inclusions decreased and the Al2O3 content of inclusions increased during RH refining. Thermodynamic calculation results showed that CaS could form directly in molten steel as well as through the reaction between molten steel and calcium aluminate inclusions. However，the formation of CaS was not available after RH refining.
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