Enhancing Cleanliness and Refining Primary Carbides in H13 Hot‐Work Die Steel through Mg–La Treatment

The effect of La treatment (0–0.089 wt% La), Mg treatment (0–0.0038 wt% Mg), and Mg–La treatment on cleanliness and primary carbides in as‐cast H13 steel is systematically investigated via combined microstructural characterization and thermodynamic analysis. The results demonstrate that Mg–La treatment enhances deoxidation and desulfurization, achieving ultralow O (0.00012 wt%) and S (0.0007 wt%). Mg–La treatment transforms the original MgO·Al 2 O 3 , TiN, and MnS inclusions into La 2 O 3 , La 2 O 2 S, LaN, La–O–N, La–P–As, La–O–S–P–As, and Mg–La–O–S inclusions. With La content increasing from 0 to 0.089 wt%, the inclusion area and number density decrease to minima at 0.022 wt% La before increasing again, while the inclusion size exhibits a similar nonmonotonic trend, with its minimum occurring at 0.056 wt% La. In addition, as the La content increases, the area of primary carbides gradually decreases. This is because La treatment refines the secondary dendrite arm spacing (SDAS) in H13 steel, thereby inhibiting the precipitation of primary carbides. With the addition of 0.045 wt% La, SDAS is reduced by 30.0% and primary carbide area decreases by 26.1%. Consequently, the optimal addition levels of 0.0033 wt% Mg and 0.056 wt% La are found to simultaneously enhance steel cleanliness and refine primary carbides.