Niobium-containing phase precipitating in liquid steel during solidification in (220 mm×260 mm) continuous casting bloom of Nb microalloying F40MnVS steel (adding 0.029% Nb) is studied in the paper. The results show that there are many micro-sized large particles of NbC(/%:80-94Nb

1.2-7. 1V

1.6- 17. 1Ti) in bloom. The large NbC is distributed between the dendrites its morphologies can be block shape

long strip shape

or eutectic shape with steel matrix

part attached to sulfide. NbC at the edge position in bloom is mostly less than 5 μm

the morphology is mostly dotted while that at middle and center position is large and mostly long with size even up to tens of microns. The large NbC still exists in hot- rolled bar. According to calculated results by Thermo-Calc software

the NbC precipitates in liquid steel during solidification process when solid fraction reaches 0.961

and the existence of element Ti may be the mutual dissolution result of NbC and TiN. By increased the heating temperature of bloom before rolling to1200 ℃

the size of large NbC could be obviously decreased.