The origin and evolution of non-metallic inclusions for Al-killed casing steel during EAF-LF-VD-CC process were studied by industrial tracer experiments. The formations of Al-2O_3 and MgO-Al-2O_3 micro- and macro-inclusions from ladle slag were analyzed by theoretical calculations. The results show that micro-inclusions were formed during deoxidation and temperature decreasing in the process of casting, most of the macro-inclusions come from entrapped ladle and mold slags. In addition, most of the Al-2O_3 formed were modified into 12CaO-7Al-2O_3 and 3CaO-Al-2O_3 inclusions which are liquid at steelmaking temperature. MnS cannot be fully modified to CaS and it was only wrapped outside the alumina or calcium aluminate in the form of (Ca,Mn)S with the amount of Ca-Si wire fed during the test.
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