<p>Severe internal oxidation formed in advanced high-strength steels (AHSSs) during the hot-rolled coiling process compromises subsequent cold rolling and galvanizing processes. Herein, we report how Sn microalloying governs internal oxidation behavior and modulates iron oxide phase transition process. Sn addition significantly reduces the depth of grain boundaries oxidation and the area of internal oxidation, as well as retards the process of oxide scale transformation. Sn preferentially segregates at the iron oxide/substrate interface, forming a diffusion barrier that suppresses outward diffusion of alloying elements and inward oxygen transport. Concurrently, Sn enrichment at grain boundaries obstructs short-circuit oxygen diffusion pathways, significantly reducing the depth of oxidation at the grain boundaries. Furthermore, Sn segregation decreases the interfacial oxygen chemical potential and oxygen availability for selective oxidation reaction. The strategic incorporation of surface-active elements has emerged as a viable metallurgical approach to reduce internal oxidation in hot-rolled coils for AHSS applications.</p>