Formation Mechanism of Finely Dispersed Core–Shell Inclusions Induced by Mg Treatment in 75Cr1 Steel

The effects of different Mg addition levels on the morphology, size, number, distribution, and composition of inclusions in 75Cr1 steel during refining were systematically investigated in this study. Compared with conventional Ca treatment, Mg treatment significantly increases the fraction of submicron inclusions and promotes a more uniform dispersion. The inclusions are modified into spherical core–shell particles, with MgO·Al 2 O 3 or MgO serving as the core. Classical nucleation theory suggests that Mg‐containing inclusions possess lower nucleation barriers and smaller critical nuclei, contributing to a higher nucleation density and finer particle size. Ostwald ripening further reveals that the slow growth kinetics of MgO facilitate maintaining this refined scale. Lattice mismatch analysis, supported by FactSage simulations, specifies that Ca–Al oxides preferentially nucleate on Mg‐containing cores, followed by CaS precipitation that forms the outer layer. These sequential reactions stabilize the multilayer core–shell structure. To sum up, establishing MgO formation as the inclusion core is crucial for achieving refined and uniformly dispersed inclusions during the refining of 75Cr1 steel.