Abstract: Microstructure and properties of HAZ in an Ultr-low Carbon Bainitic(ULCB) steel were investigated by welding simulation on a Gleeble-1500 thermal simulator.Results indicated that the bainitic structure of excellent toughness can be obtained under different welding conditions in the HAZ of Nb and B microalloyed steel.It was also found that the fracture behaviour at low temperatures is defferent due to the microstructural disparity derived from the different welding condition.By analyzing the behaviour of boron segregation,further discussion about the effect of t_8/5 on microstructural morphology and the effect of microstructure on crack propagation is also given.