Influence of S Content and Ti Addition on Hot Ductility of Ultra Low Carbon Steels at High Temperature

In the connection of crack formation in continuously cast slabs, brittleness of ultra low carbon steels caused by sulphur (S) and its recovery by titanium (Ti) addition were evaluated by the reduction area of samples pulled apart in high temperature tensile test and the followings were found: (1) In the case of without Ti addition, brittleness appears in the steels with S content more than around 0.01% due to formation of liquid iron sulphide (FeS) and brittleness does not appear at all with the S content less than 0.003%. (2) In the case of Ti addition, the brittleness in the steels with S content more than 0.009% is improved at the temperature higher than 1323 K but appears sometime at lower than 1273 K. (3) The above mentioned improvement is considered to be due to the shift from the liquid FeS to following precipitated solid titanium sulphide (TiS) and above mentioned appearance of the brittleness is considered to be due to TiS and titanium nitrides (TiN) precipitated along gamma grain boundary, which are based on the precipitate analysis in Ti added steels. (4) In the case of thermal history, cooling and reheating, in continuous casting, it is considered that the brittleness does not appears in the steels with Ti content over 0.016% and that it appears in the steels with S content over 0.011% without Ti addition.