Investigation on evolution of inclusions in bearing steel during secondary refining

Composition, type and quantity of inclusions in bearing steel during LF refining and RH vacuum treatment were studied. The experimental results were analyzed and discussed in combination with thermodynamic calculation and interface parameters between inclusions and liquid steel. The inclusion analysis results showed that the deoxidization product Al₂O₃ disappeared after 25 min refining, and the inclusions in steel were mainly pure spinel, spinel containing small amount of CaO, CaO·2Al₂O₃ and CaO·Al₂O₃. Pure spinel, spinel containing small amount of CaO, CaO·2Al₂O₃ and CaO·Al₂O₃ were still the main inclusions in steel after 65 min refining to the end of LF refining. After RH vacuum treatment for 25 min, the total number of inclusions in steel was reduced by 75% compared with that after LF refining. The removal efficiency of pure spinel and spinel containing small amount of CaO inclusions was 99.5% and 93.2%, respectively, and that of CaO·2Al₂O₃ inclusion was 67%. The inclusions after RH treatment were mainly liquid calcium aluminate CaO·Al₂O₃ and 12CaO·7Al₂O₃. The size of spinel inclusions in the refining process was concentrated below 10 μm, and the inclusions above 20 μm were mainly calcium aluminates in the liquid phase, which had appeared in the early stage of LF refining. Solid inclusions pure spinel, spinel containing small amount of CaO and CaO·2Al₂O₃ whose contact angle with molten steel was greater than 90° were easy to remove in RH vacuum treatment, while liquid inclusions CaO·Al₂O₃ and 12CaO·7Al₂O₃ whose contact angle with molten steel was less than 90° were not easy to remove. Therefore, controlling the inclusions into solid inclusions after LF refining was beneficial to the high efficient removal of inclusions in RH vacuum treatment. The thermodynamic calculation results show that when the w(TO)is 0.001 0% and w(Mg) in steel exceeds 0.000 18%, the deoxidization product Al₂O₃ cannot be stable in thermodynamics. It is difficult to obtain solid Al₂O₃ inclusions under the condition of Al deoxidation and high basicity slag refining. To obtain fully solid spinel or high melting point calcium aluminate inclusions, w(Ca) in steel should be controlled below 0.000 1%。When w(Ca) in steel exceeds 0.000 2%, it will achieve the thermodynamic conditions for formation of liquid inclusions.