Effects of High-Content Rare Earth Ce Addition Methods on Modification of TiN Inclusions in 20CrMnTi Gear Steel

Ti microalloying can effectively improve the comprehensive properties of 20CrMnTi gear steel, but it also readily leads to the formation of large-sized TiN inclusions with sharp angular edges, which act as preferential initiation sites for fatigue cracks. Rare earth Ce is capable of modifying these coarse TiN inclusions in a targeted manner, and the addition strategy of Ce directly determines the final modification performance. This work systematically compares the inclusion modification effects between the conventional Ce-Fe alloy addition process and the synchronous co-addition process of Al, Ce and Ti. The results show that in the Ce-free steel, most TiN inclusions are large, angular particles with an average size of 4.27 μm. After Ce introduction, the original coarse complex inclusions Al2O3-TiN and MnS-TiN are transformed into fine composite inclusions, and are composed of Ce-Al-O-rich or Ce-O-S-rich core phases and TiN. Combined with EDS analysis, thermodynamic calculations and reported crystallographic investigations, the core regions match the characteristic signatures of CeAlO3 and Ce2O2S. Therefore, these phases are tentatively assigned as the two aforementioned inclusions. The synchronous co-addition of Al, Ce and Ti delivers superior modification performance, which can refine the average size of TiN to 2.43 μm. Thermodynamically, CeAlO3 and Ce2O2S possess lower formation Gibbs free energy than Al2O3, demonstrating that their formation is thermodynamically favorable after Ce addition. These rare earth phases can act as heterogeneous nucleation cores for subsequent TiN precipitation, thereby achieving the refinement and modification of large-sized TiN inclusions.