To study the effect of Mg addition on inhibiting weld heat affected zones(HAZ)austenite grain growth of Ti-bearing low carbon steels,two steels with and without Mg treated were prepared using a laboratory vacuum.The welding testing was simulated by Gleeble 3500 thermomechanical simulator.The performance of HAZ was investigated that the toughness was improved from 33 to 185 J by adding 0.005% Mg(in mass percent)to the steel,and the fracture mechanism changed from cleavage fracture to toughness fracture.Through in-situ observation by a confocal scanning laser microscope,a significant result was found that the austenite grain of the steel with Mg treated was still keeping fine-grained structure after holding at 1400 ℃ and lasting for 300 s.This inhibition of austenite grain growth was mainly attributed to the formation of pinning particles after the addition of Mg.The obtained results propose a potential method for improving HAZ toughness of structure steels. To study the effect of Mg addition on inhibiting weld heat affected zones (HAZ) austenite grain growth of Ti-bearing low carbon steels, two steels with and without Mg treated were prepared using a laboratory vacuum. The welding testing was simulated by Gleeble 3500 thermomechanical simulator. The performance of HAZ was investigated that the toughness was improved from 33 to 185 J by adding 0.005% Mg (in mass percent) to the steel, and the fracture mechanism changed from cleavage fracture to toughness fracture. Through in-situ observation by a confocal scanning laser microscope, a significant result was found that the austenite grain of the steel with Mg treated was still keeping fine-grained structure after holding at 1400 ° C and lasting for 300 s. This inhibition of austenite grain growth was primarily attributed to the formation of pinning particles after the addition of Mg. The obtained results propose a potential method for improving HAZ toughness of structure steels.