Two grades of the Nb-containing steel, one modified with B, were examined in order to study the effect of B on the hot ductility. Since the hot ductility loss of the Nb-containing steel during the continuous casting is manifested as surface cracking, the surface thermal schedule including in situ melting was taken into account. The results revealed that addition of B improves the hot ductility of austenite. Two mechanisms for such improvement are proposed that are not based on the presence of intragranular ferrite. These mechanisms concern with depletion of strengthening elements in the austenite lattice through faster kinetics of grain boundary precipitation and non-equilibrium segregation of precipitants.