Effects of Magnesium on Secondary Phases and Banded Structures in H13 Hot‐Work Die Steel

H13 hot‐work die steel is the most widely utilized hot‐work tooling material globally. This study systematically investigates the effects of magnesium on secondary phases, elemental segregation, and hardness in H13 steel using optical microscopy, scanning electron microscopy, energy‐dispersive spectroscopy, electron probe microanalysis, and micro‐Vickers hardness testing. Key findings demonstrate that magnesium modifies inclusions by transforming Al 2 O 3 –CaO–SiO 2 complex inclusions into discrete AlMgO 4 (spinel) and MgO particles. This modification significantly increases the population of inclusions smaller than 3 μm while virtually eliminating inclusions larger than 8 μm, concurrently reducing chained inclusion clusters. The steel exhibits banded microstructures parallel to the rolling direction, characterized by alternating bright and dark bands. Elemental mapping confirms pronounced segregation of C, Mo, and Cr within dark bands, where V‐rich primary MC‐type carbides preferentially form. Magnesium addition refines these dark bands, yielding a more uniform distribution while simultaneously reducing the quantity and size of primary carbides. Crucially, magnesium enhances both the magnitude and uniformity of matrix microhardness. These improvements stem from Mg's ability to mitigate elemental segregation, thereby optimizing carbide distribution and ultimately elevating the steel's hardness and service performance.