Formation and evolution of non-metallic inclusions in 20CrMnTiH gear steel during LF-RH-CC process
The formation and evolution of non-metallic inclusions in 20CrMnTiH gear steel during LF-RH-CC process were analyzed by industrial trials,and the transformation trend of non-metallic inclu-sions during reoxidation and cooling solidification in 20CrMnTiH gear steel was analyzed by thermody-namic calculations.During LF refining,inclusions experience evolutions from Al2O3 to MgO-Al2O3 and CaO-MgO-Al2O3.After calcium treatment,inclusions are modified into CaO-MgO-Al2O3 and CaO-Al2O3-CaS.After calcium treatment at the end of RH vacuum refining,inclusions are modified into CaO-Al2O3-CaS with high CaS contents and CaO-MgO-Al2O3.During continuous casting,the content of CaS in inclusions increase with the reoxidation of steel.Thermodynamic calculations reveal that when 0.000 3%to 0.000 5%[Ca]is added to the steel,MgO-Al2O3 would be formed in the steel.When 0.000 5%to 0.000 7%[Ca]is added to the steel,liquid calcium aluminates would be largely formed.When the con-tent of[Ca]exceeds 0.000 7%,CaS would be formed.Moreover,liquid inclusions would be gradually transformed into solid inclusions with the reoxidation of steel.With the decrease of temperature,the contents of CaO and CaS in inclusions decrease and the contents of MgO and Al2O3 in inclusions in-crease,leading to the formation of CaS-CaO-MgO-Al2O3 with high CaS contents eventually.