Inclusions in 304 stainless steels are transformed from manganese silicate to manganese chromite (spinel) upon heating in the range of 1000-1350 C, while this transformation is not observed when heating temperature is 1400 C. Thermodynamic analysis with software FactSage TM is applied in interpreting the transformation. Manganese chromite spinel, with light color under SEM, is deemed to be formed on the surface of MnO-SiO 2 inclusions by the reaction between Cr in steel matrix and MnO-SiO 2 in inclusions. Heating experiments with different time at 1235 C (rolling insulation temperature) are carried out to confirm the kinetics of the transformation reaction. Mass transfer coefficients of inclusions' composition, SiO 2 , MnO, and Cr 2 O 3 are ranging from 5 Â 10 À7 to 9 Â 10 À7 m s À1 , depending on diameters and components of inclusions.

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The evolution of inclusions during the electroslag remelting (ESR) process, especially the inclusions in droplets is investigated by an industrial trial which remelts a plain carbon steel billet in the slag of CaF2–Al2O3–CaO–MgO. Results show that oxide inclusions change from SiO2–MnO to Al2O3–MnO during the ESR process. The transfer of aluminum from the slag to the steel increases the aluminum content in the steel and causes the reaction between (MnO), (SiO2) in inclusions with the [Al] in the steel to generate (Al2O3) inclusions. Thermodynamic results show that inclusion evolution is mainly influenced by the temperature and the total Al content in the steel. With the increase in temperature at the electrode tip, the MnO content in inclusions increases rapidly. Meanwhile, as the aluminum transfers from the slag to the steel, the Al2O3 inclusion increases clearly with the corresponding decrease in MnO and SiO2 inclusions. In addition, from the electrode to the ingot, the number density of inclusions decreases from 208.82 to 33.49 # mm−2, whereas the average size increases from 3.79 to 5.76 μm. Both the chemical modification and the physical absorption of slag mainly occur at the liquid film and droplet stage.

Industrial trials and thermodynamic calculation are carried out to systematically investigate the variation of steel cleanliness and inclusions during the reoxidation of molten steel, steady and unsteady continuous casting process of GCr15 bearing steels. Comparing with inclusions in molten steel, the content of Al2O3, CaO, CaS, and MgO of inclusions in the steady bloom is increased by 2.17%, −18.47%, 14.01%, and 2.27%, respectively, due to the reoxidation of molten steel will inhibit the transformation of inclusions. As the casting length in the first bloom increases from 2 to 9 m, the total oxygen (T.O) content reduced from 12.9 to 7.2 ppm and the number density of inclusions decreases from 15.65 to 10.97 # mm−2 with a gradual decrease in Al2O3 content. Based on thermodynamic consideration, in molten steel, the lower content of T.O, the lower Al2O3 content of inclusions. For solid steel, with the increase in content of T.O, mainly the Al2O3 content of inclusions gradually increases and the CaS content gradually decreases when the T.O content is between 4 and 8 ppm. However, the Al2O3 and CaO content of inclusions gradually increases, then the MgO and CaS contents gradually decreases when the T.O content is between 8 and 25 ppm.