<p>Owing to its excellent properties, high manganese steel has become a promising candidate material for liquefied natural gas storage tanks. However, its relatively low thermal conductivity and high manganese content often lead to coarse macrostructure and severe microsegregation, respectively, which can ultimately degrade the mechanical performance. This study investigates the effects of rare earth Cerium (Ce) addition on the macrostructure and microsegregation behavior of high manganese steel. The results indicate that an optimal amount of Ce promotes the formation of AlCeO₃&#xa0;+&#xa0;Ce₂O₃ inclusions, which act as heterogeneous nucleation sites, thereby refining the macrostructure. In contrast, excessive Ce leads to the formation of CeS and coarsening of inclusions, diminishing the grain refinement effect. Moreover, while a moderate Ce content effectively reduces carbon microsegregation through dendrite arm refinement, an excessive amount induces carbon segregation at grain boundaries. This is attributed to a prolonged local solidification time and enhanced carbon diffusion, which collectively counteract the beneficial effect of microsegregation suppression.</p>