Coupled Enrichment of Cu and Sn at the Oxide/Steel Interface and Its Regulation by Si in Recycled Steels

The accumulation of residual elements such as Cu and Sn in recycled steels has become an increasingly critical issue, as their enrichment during high-temperature oxidation can lead to surface hot shortness and deterioration of surface quality. In this work, the coupled enrichment behavior of Cu and Sn at the oxide/steel interface and its regulation by Si were systematically investigated through high-temperature oxidation experiments and microstructural characterization. The results reveal that selective oxidation of Fe during high-temperature exposure leads to the rejection of Cu toward the oxide/steel interface, resulting in significant interfacial enrichment. The presence of Sn further intensifies this enrichment by lowering the melting point of the Cu-rich phase and promoting the formation of Cu-Sn liquid films along grain boundaries, thereby aggravating intergranular penetration and surface degradation. In contrast, the addition of Si effectively suppresses the interfacial enrichment of Cu and Sn. Microstructural analyses indicate that Si promotes internal oxidation and facilitates the formation of Si-containing oxides such as Fe₂SiO₄ within the oxide scale and near the interface, which modifies the interfacial structure and limits the diffusion and accumulation of Cu-rich phases. Consequently, the formation and penetration of Cu-Sn liquid are significantly inhibited. These findings clarify the coupling mechanism of Cu and Sn during oxidation and reveal an effective Si-based strategy for mitigating the detrimental enrichment of residual elements in recycled steels, providing guidance for improving the surface quality of steels produced from scrap-containing charges.