Influencing the Evolution of Inclusions and Microstructure Refinement in U71Mn Heavy Rail Steel with Mixed Rare Earth Elements

U71Mn heavy rail steel is prone to fatigue failure due to large composite inclusions and strong anisotropy induced by irregular MnS and Mn–Al–Si–O inclusions. In this work, the modification behavior of inclusions by mixed rare earth La–Ce treatment in U71Mn steel was systematically investigated using vacuum induction melting, SEM–EDS characterization, and thermodynamic calculations. The results show that La and Ce additions initially generate numerous fine rare-earth inclusions, which are gradually reduced through interfacial reactions and flotation, leading to effective melt purification. Rare-earth content significantly affects inclusion characteristics. At La 0.0082% and Ce 0.0045%, inclusions mainly consist of rare-earth-modified MnS together with RE–O–S and RE–S phases. Increasing La and Ce to 0.016% and 0.010% eliminates MnS inclusions and promotes the formation of spherical rare-earth inclusions. La–Ce treatment exhibits a pronounced modification effect on small inclusions (D ≤ 5 μm), markedly reducing MnS aspect ratio and transforming their morphology toward ellipsoidal or spherical shapes. The inclusion system evolves from MnS–silicate dominance to rare-earth oxysulfide dominance, and a phase-dominated transition mechanism is proposed. Combined thermodynamic analysis, automated inclusion statistics, and Zener pinning modeling demonstrate that appropriate La–Ce additions enhance grain boundary pinning, refine grains and pearlite lamellae, and improve the comprehensive performance of U71Mn steel.