Effect of Rare‐Earth La Addition on Inclusion Modification Mechanism in 42CrMo Steel

Rare‐earth lanthanum (La) improves the strength–ductility synergy of steel through inclusion modification. This study investigates 42CrMo steels containing 0–0.013 wt% La using SEM–EDS, thermodynamic calculations, and first‐principles calculations, clarifying the mechanism of La in improving the strength and ductility. With increasing La content, inclusions gradually transform from elongated MnS and MnS/Al 2 O 3 composites to La‐containing inclusions. At 0.009 wt% La, the inclusions are effectively modified into fine, spherical La 2 O 3 –La 2 O 2 S–La 3 S 4 composites, whereas excess La (0.013 wt%) causes coarsening and agglomeration into chain‐like or irregular morphologies. Combined SEM–EDS observations and thermodynamic calculations suggest an evolution pathway of La 2 O 3 → La 2 O 2 S → LaAlO 3 → La 3 S 4 , while preformed Al 2 O 3 is difficult to modify by La. The dominant La‐modified inclusions exhibit lower elastic moduli and hardness and better matrix compatibility. Fine and dispersed La‐containing inclusions refine prior‐austenite grains from 32.15 to 17.21 μm through heterogeneous nucleation and Zener pinning, while optimized inclusion morphology alleviates local stress concentration. At 0.009 wt% La, the steel exhibits optimal comprehensive properties, with yield strength, tensile strength, and elongation increasing by 11.2%, 14.4%, and 37.4%, respectively, compared with La‐free steel. This study elucidates La‐induced inclusion modification and strength–ductility synergy mechanisms, providing a theoretical basis for high‐performance steel design via rare‐earth microalloying.