Analysis of the formation of slag groove defects during the continuous casting of the round billets of 38CrMoAl steel

38CrMoAl steel (Al = 0.8–1.1 wt%) is valued for its excellent physical properties but faces challenges due to its high-Al content, which can react with SiO₂ in mould slag during continuous casting, degrading slag performance. In view of the frequent occurrence of solid crusts in the slag pool and groove defects on the round billet surface, this study systematically investigated the in-service slag and elucidated the occurrence mechanism of groove defects. As the operation proceeded, the steel–slag reaction approached equilibrium: (SiO₂) content decreased to about 9 wt%, while (Al₂O₃) increased to approximately 38 wt% after the third heat. These significant compositional shifts induced a sharp increase in viscosity, break temperature and melting temperature of the slag. Concurrently, the simpler structures Q ⁿ (n = 0, 1, 2) gradually aggregated to form complex structures Q ⁿ (n = 3, 4, 5), and as a result, the melting and flow properties deteriorated significantly. Consequently, solid crusts formed in the slag pool and were transported into slag channels via liquid slag infiltration. During shell withdrawal, inadequate lubrication of the slag film caused by solidified crusts would eventually cause the formation of groove defects on the billet surface along the withdrawal direction. This issue could be mitigated by adding fluxing agents to stabilise post-reaction slag performance.