Research On Oxide Metallurgy Technology For Pressure Vessel Steel

As the development of pressure vessel industry and the application of high heat input welding technology, more excellent performance such as high strength, high toughness and good weldabilty, are required for pressure vessel steel. In high heat input welding condition, grain coarsening and induced cracks occur in welding heat affected zone (HAZ) of plate, which degrade toughness severely and restrict the application of high heat input welding technology. Oxide metallurgy is the most effective technology to enhance the toughness in HAZ, which refine the microstruture in HAZ by particles’pinning effect for austenite growing and promoting effect for intragranular ferrite growing.Based on oxide metallurgy technology, six furnace steel deoxidized by Ti/Zr/Mg were made in vacuum induction furnace. The effect of different deoxidation process on inclusion, precipitated phase, microstructure, mechanical properties and welding performance were studied. Then industrial experiment was conducted based on the laboratory research, and the mechanical properties and welding properties of industrial experimental steel were studied. The main conclusions are as following:(1) B element has great influence on inclusion’s average diameter and distribution. After adding B, the average diameter of inclusions decreases and the number of unit area inclusions increases. When B was added in the steel deoxidized by Ti-Zr-Mg, the average diameter inclusions decreased from0.85μm to0.69μm, and the number of unit area inclusions increased from2878mm-2to7914mm-2.(2) Thermodynamic calculation shows that the main precipitates in steel are MnS, Ti(CN), Nb(CN), BN, MC, M7C3etc. When C mass fraction in steel increases, the austenite phase range increases, the temperature of austenite transforming to ferrite decreases, the precipitation temperature of MnS, Ti(CN) decreases, and the precipitation temperature of Nb(CN) increases. (3) Among the six group experiments,3#steel with Ti-Zr-Mg deoxidation process, has the best match of strength and toughness. The tensile strength, yield strength and impact absorbed energy at-20℃are680MPa,520MPa and142.3J respectively.(4) Deoxidation process have great effect on the toughness of steel after simulated welding. The steel deoxidized by Ti-Zr-Mg has good welding property. When the heat input energy reaches100kJ/cm, the impact absorbed energy is greater than100J, which meets the requirement of national standards for pressure vessel steel.(5) On the condition of high heat input welding, the products deoxidized by Ti-Zr-Mg can pin austenitic grain and promote acicular ferrite formation, thus refine HAZ microstructure and ehance low temperature toughness.(6) Steel prepared by industrial experiment, has great mechanical properties and welding performance. Its tensile strength, yield strength and low temperature inpact energy respectively achieve to605MPa,465MPa and213.7J. After150kJ/cm heat input welding simulation, its low temperature impact toughness still achieve to52.1J, meeting the requirement of national standards for pressure vessel steel.