The effects of N on the hot ductility of low carbon steels have been studied with particular emphasis on the relation with Si. The ductility of Si, Al-killed steels is largely reduced by slow strain rate (10−3–10−4S−1) deformation at temperatures from low temperatureγtoγ/αduplex phase region (from 750 to 950°C in this case), accompanied by ductile intergranular fracture of austenite. The cause of the loss of ductility is found to be dynamic precipitation of hexagonal close packed (hep) (Si, Al)N both on theγgrain boundaries and within the grains, and the phenomenon is largely enhanced by either increasing Si or N content. Similar phenomena, i.e. precipitation hardening-like behaviour and dependencies both of deformation conditions and of Si and N contents, are also observed in Al-free Si-killed steels. The cause of this ductility loss should be ascribed to dynamic precipitation of some kind of silicon nitride, although the precipitation has not been detected directly in all the specimens examined.