Synergistic optimization of cleanliness and grain size via Mg–Ce treatment for enhanced mechanical properties in H13 steel

The effects of Mg–Ce treatment on the microstructure and mechanical properties of H13 steel were investigated through microstructural characterization and thermodynamic analysis. The results show that Mg–Ce treatment significantly improves steel cleanliness, reducing the O and S contents to 0.0003% and 0.0013%, respectively, and refining the average inclusion diameter from 1.84 μm to 1.31 μm. With the addition of 0.03 wt% Ce, the austenite grain size was refined from 44.4 μm to 29.6 μm, a reduction of 33%, attributed to the reduction in grain boundary energy by Ce and the grain boundary pinning effect of La 2 O 2 S. Excessive Ce (0.057 wt%), however, coarsened the grains due to weakened carbide pinning. In terms of mechanical properties, 0.03 wt% Ce increased the hardness by 4.4 HRC (+9%) and the impact energy by 4.73 J (+35%), owing to grain refinement, grain boundary purification, and inclusion refinement. Excessive Ce led to coarse inclusions and degraded toughness. In summary, Mg–Ce treatment achieves a synergistic enhancement in both strength and toughness of H13 steel through coordinated control of cleanliness and grain size, with the combination of approximately 0.003 wt% Mg and 0.03 wt% Ce identified as the optimal addition.