Systematic Study on Large‐Size Alarm Inclusions in U75V Heavy Rail Steel Using Industrial CT and Interfacial Reaction Analysis

The formation of large‐sized alarm inclusions during production of heavy rail steel severely impairs its service performance. Using industrial CT and electrolytic extraction, these inclusions in U75V steel were identified as exogenous Al 2 O 3 –SiO 2 –CaO–MgO system containing MgO·Al 2 O 3 spinel, often exceeding 3000 μm in length. Erosion of MgO‐based refractory by the tundish‐covering flux significantly increased the MgO content of the covering flux, leading to the formation of high‑MgO and spinel‑type inclusions. They reacted with the submerged entry nozzle coating and were subsequently scoured off by steel flow, thereby forming alarm inclusions. The effects of flux and refractory compositions on erosion were simulated, and the results demonstrated that appropriately increasing the MgO content in the covering flux or enhancing the purity of the refractory effectively mitigated erosion. Consequently, the increase of MgO content in flux was suppressed, resulting in Al 2 O 3 –SiO 2 –CaO–MgO inclusions with lower MgO content. Thermodynamic calculations indicated that when the MgO content in inclusions was reduced to 7.6%, the spinel formation was prevented. Upon further reduction to 4.6%, only liquid inclusions attached to the nozzle surface. This study demonstrates that controlling erosion behavior in the tundish is of practical significance for reducing the formation of large‐sized alarm inclusions in heavy rail steel.