Abstract: The industrial trials and thermodynamic calculation were carried out to investigate the composition evolution, crystallization phase, and crust mechanism of tundish cover powder (TCP) during continuous casting of ultra-low carbon steel. The results showed that the ladle slag had a relatively strong oxidation capacity at RH departure and the w(CaO)/w(Al₂O₃) in the slag was low. The top slag of RH refining needs to be modified. Due to the slag carryover, mix of ladle filler sands and TCP, the composition of TCP during continuous casting has been significantly changed. Furthermore, the MgO·Al₂O₃ spinel in liquid phase was first formed and it promoted the nucleation and growth of 2CaO·Al₂O₃·SiO₂ and CaO·TiO₂ phases, resulting in the crust of TCP. To avoid the crust of TCP during continuous casting, the ladle carryover at cast end should be strictly controlled, and the 
mass fraction of SiO₂ and MgO in the original TCP are reduced to (10±2.5) % and 5.0 %～7.5 % respectively, which will inhibit the precipitation of 2CaO·Al₂O₃·SiO₂  and MgO·Al₂O₃ spinel phases.

Key words: ultra-low carbon steel, tundish cover powder, crust mechanism, industrial trials, thermodynamic calculation