Influence of cerium content on inclusions modification and as-cast structure refinement in high-strength automotive beam steel

High-strength automotive beam steel serves as a critical component for supporting vehicle mass and external loads, requiring high strength, high toughness, and excellent cold formability. However, hard inclusions such as Al₂O₃, magnesium-aluminum spinel, and calcium aluminate present in the steel do not readily deform during rolling. Improper control will damage product performance. Cerium (Ce) plays a role in modifying inclusions and refining grain structure in steel. It can transform Al₂O₃, Mg-Al-O and Ca-Al-O inclusions into rare-earth inclusions such as CeAlO₃ and Ce₂O₂S. These cerium-containing inclusions can act as nucleation sites, refining the solidification structure of the steel. Through melting and casting experiments using a high-temperature tube furnace, combined with scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and thermodynamic calculations, the effects of different Ce content on inclusion modification and as-cast structure refinement in high-strength beam steel were systematically studied. The results show that as the Ce content in the steel increases, the inclusion modification sequence follows CeAl₁₁O₁₈→CeAlO₃→Ce₂O₃→Ce₂O₂S→CeS, with the final modified product depending on the Ce content. When the mass fraction of Ce is 0.005 5%, the modified inclusions are Ce-Al-O types, exhibiting polygonal or (near-)spherical morphologies due to genetic effects and modification-induced spheroidization. When the mass fraction of Ce reaches 0.018 0%, the inclusions are further modified into Ce-O-S types with spherical morphologies, showing characteristics of aggregation and growth, while the originally angular Mg-Al(-Ti)-O inclusions disappear. Ce effectively reduces the average size of inclusions and decreases the number of large-sized inclusions. As the Ce content increases, the number of inclusions first decreases and then increases, with the smallest average size and lowest quantity observed at a Ce mass fraction of 0.005 5%. Additionally, Ce exhibits a grain-refining effect in steel. Both Ce-Al-O and Ce-O-S inclusions promote solidification nucleation, with the most significant refinement of the as-cast microstructure achieved at a Ce mass fraction of 0.018 0%. Thermodynamic calculations further elucidate the formation mechanisms of the relevant inclusions.