Abstract: The effects of nitrogen content on austenite grain size at high temperature and impact toughness of simulated welding HAZ and actions of second phase particles,(Ti,Nb) N in series low carbon Ti-Nb microalloyed weldable steels were studied.The simulated weld with great heat input had been carried out.The austenite grain size at high temperature was measured and impact toughness after welding was tested.Morphology and distribution characteristics of second phase particles,(Ti,Nb) N in typical specimens had been observed by using transmission electron microscope by means of carbon extraction replicase technique.The results showed that there is good correspondence among Ti and N content,size of particles,(Ti,Nb) N,austenite grain size and impact toughness.Toughness after welding is improved because of formation of fine and dispersive second phase particles,(Ti,Nb) N which refines austenite grain size at high temperature.There is a scope of rational N content in low carbon Ti-Nb microalloyed weldable steels.