| The appearance of Ultra Low Carbon Bainitic (ULCB) steel is the result of the society demands and development of modern metallurgy, with characteristics of super low carbon, super fine grain, high purity, high strength and toughness. It is greatly different from the widely used steel presently, so some new problems will appear in the Heat Affected Zone (HAZ). Then it is significant to study the influence of thermal process on the HAZ microstructure and properties of ULCB steel.Through the methods of thermal simulation and welding experiment, the microstructure and properties of two kinds of ULCB steel, the tensile strength of which is 700MPa and 610MPa separately, have been studied. ULCB700, which has a tensile strength of 700MPa, is mainly used in industrial machine, and ULCB610, which has a tensile strength of 610MPa, is mainly used in the building of storage tank.Welding heat cycle is a dominant data for thermal simulation. An analytical calculation model of welding heat cycle suitable for the ULCB steel in this paper is obtained through analyzing the usual models and comparing the models with the welding experiment results.The influence rule of heat process on the microstructure and property of ULCB700 steel HAZ is studied through the thermal simulation, and the SH-CCT chart is obtained, which reveals the phase transition of HAZ at different cooling rate. The influence rule of the heat cycle on the toughness of HAZ is studied systemically. Some welding experiments with different technology parameter have been taken, and the mechanical property of the welding joint has been tested. The prediction of ULCB700 steel HAZ microstructure and property under usual welding condition is realized.The Electro Gas Welding (EGW) is an important welding process in the building of storage tank. Based on the transient temperature distribution solution on the workpiece which is heated by a random movable point heat source, an analytical calculation model is derived for the EGW. In this model the welding heat input is approximately treated as a group of elementary point heat source along 3-D axes, and the distribution model of welding heat input, which is suitable for the EGW, is established. Through this model the formation of the weldpool is studied, and the temperature field and the heat cycle curves are calculated. Some welding experiment validates the reliable of the model.The influence rule of welding heat cycle parameter on the microstructure and toughness and the influence of welding technology parameter on the microstructure and property have been studied through systematical thermal simulation and welding experiment. The results of the experiment shows that the toughness of the HAZ decreases when the ULCB610 steel goes through the high heat input EGW. But the microstructure and property of the HAZ and the properties of the welding joint improved obviously when the parent metal goes through treatment of 950℃quench and 650℃temper.
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