NITROGENATION AND HOT EMBRITTLEMENT IN TI-ADDED ULTRA-LOW CARBON-STEELS

Hot ductility of Ti-added ultra low carbon steels has been studied with particular emphasis on the behavior of TiS. Hot ductility of low Mn low carbon steels initially solution-treated in the high temperature gamma region is reduced with intergranular fracture by high strain rate deformation at temperatures ranging from 900 to 1 100-degrees-C. This can be explained by the fact that hardening due to dynamic precipitation of Fe-rich sulfide within the matrix accelerates the void initiation at interface of grain boundary precipitate/matrix. In Ti-added steels, the ductility loss is considerably smaller compared to that of Ti-free low Mn steels, since Ti has a role of trapping S atoms as TiS particles in the same way as Mn. However, nitrogenation due to heating in the nitrogen gas atmosphere or in air changes TiS into the more stable TiN especially in the surface layer. As a result, the extra free S atoms formed by this reaction leads to hot embrittlement by the same mechanism.