The influence of tin on the hot ductility of a 0. 15wt%C steel is investigated using a continuous-casting thermal simulator with three cooling rates. Tin can apparently deteriorate the hot ductility of the steel. Non-equilibrium grain boundary segregation of tin occurs during cooling and plays an important role in reducing the hot ductility of the steel. There is a critical cooling rate for the Sn segregation being between 5 and 20K/s.