The effect of Mn addition on a brittle-to-ductile transition (BDT) in Ti added ultra-low carbon steels was investigated comparing with the effect of Ni on it. The effects of Mn on the temperature dependences of 0.2% proof stress, activation volume and effective stress in Ti added ultra-low carbon steels were nearly the same as those of Ni. The decrease in the activation energy of dislocation glide with Mn content was also nearly the same amount as that with Ni, indicating that dislocation mobility relating to thermally activated process was increased with Mn addition. The temperature dependence of absorbed impact energy showed that the BDT temperature increases with the Mn content. The fraction of inter-granular fracture surface increased with Mn content suggesting that Mn decreases the surface energy for inter-granular fracture, which controls the BDT temperature. Those results suggest that Mn has a potential to improve low temperature toughness when grain boundaries are strengthened enough. The effect of Mn addition on the temperature dependence of absorbed fracture energy in no Ti-added ultra-low carbon steels was also presented.
