Synergistic Regulation of Inclusion Characteristics by La–Ce Mixed Rare Earth and Its Impact on the Mechanical Properties of U71Mn Steel

This study investigates the synergistic effects of the O/S ratio and La–Ce mixed rare earths on the inclusion occurrence state and mechanical properties of U71Mn steel. Thermodynamic calculations based on the Wagner model and Factsage reveal the formation sequence and stability of rare‐earth inclusions, indicating that La exhibits higher activity than Ce and preferentially forms fine and stable (La, Ce) 2 O 2 S inclusions under low O/S conditions. When O/S ≤ 0.75 and the rare earth content ≥ 0.015%, MnS can be completely modified, and the inclusions transform from elongated or composite types into refined and spherical morphologies, significantly improving steel cleanliness. Microstructural observations show that rare earths promote grain refinement and reduce interfacial stress concentration. Mechanical tests demonstrate that rare earth modification increases hardness and reduces its fluctuation, enhances impact toughness by 22%–112%, and improves yield and tensile strengths by 7%–13% and 1%–8%, respectively. The friction coefficient decreases by 10%–40%, indicating improved wear resistance. These results confirm that regulating the O/S ratio and La/Ce proportion enables controlled inclusion evolution and comprehensive improvement of the strength, toughness, and wear performance of U71Mn steel.