Effects of Al–La alloy ratios on the modification behaviours of the TiN and sulphide inclusions in 20CrMnTi steel under composite deoxidations

The morphology of inclusion is one of the important causes of fatigue failure in 20CrMnTi gear steel. Strictly controlling the size and morphology of inclusion in steel is of great significance for improving the quality of the 20CrMnTi gear steel. The main inclusions in 20CrMnTi gear steel are TiN and sulphide. The effects of different ratios of Al–La composite deoxidiser on the modification behaviour of TiN and sulphide inclusions in 20CrMnTi steel are studied by experiments and thermodynamic calculations. The results show that the TiN inclusion in steel will be transformed into small size LaAlO₃–TiN inclusion under different deoxidation conditions of Al–La alloy. When the ratio of Al to La in Al–La alloy is 8:2, the modification effect of TiN inclusion in steel is the best, and the average size of TiN inclusion after modification is 1.86 μm. The sulphur oxides containing rare earth La will be formed in smaller size after the sulphide is modified, and the sulphur inclusions are mainly LaAlO₃–MnS–TiN composite inclusions. When the ratio of Al to La in Al–La alloy is 9:1, the sulphur-containing inclusion in steel has the best modification effect, and the average size of sulphur-containing inclusion after modification is 1.05 μm.