Formation mechanism of cerium-based inclusions and effect on microstructure and impact toughness in Q355ME Bildungsmechanismus von Einschlüssen auf Cer-Basis und Auswirkung auf Gefüge und Kerbschlagzähigkeit in Q355ML

The study explored the formation mechanisms of cerium-based inclusions and effects on the microstructure and impact toughness of industrial Q355ME steel through cerium addition. Formation of the inclusions was governed by both thermodynamic and kinetic factors. The results revealed that inclusions in cerium-free steel primarily consisted of magnesium aluminum spinel (with lesser manganese sulpfide), calcium aluminates (CaAl₄O₇, CaAl₁₂O₁₉), and calcium sulpfide with lesser aluminum oxide. Conversely, in cerium-containing steel, the inclusions predominantly included manganese sulpfide (with lesser magnesium aluminum spinel and a combination of cerium (III) sulpfide and cerium oxysulpfide), cerium (III) sulpfide-cerium oxysulpfide-calcium sulpfide (with lesser magnesium aluminum spinel) and calcium sulpfide. The introduction of cerium led to the disappearance of calcium aluminum oxide inclusions, producing smaller, more numerous, and spherical or ellipsoidal complex cerium-based inclusions. Furthermore, cerium addition resulted in a reduction in both the quantity and size of manganese sulpfide inclusions. Cerium-bearing complex inclusions contributed to grain refinement. Both transverse and longitudinal toughness values for cerium-containing steel were similar at identical temperatures, with longitudinal toughness surpassing transverse toughness in samples without cerium. Under identical conditions, especially at −80 °C, the impact toughness values of steel with cerium were significantly higher than those without cerium.