Investigation On Microstructures And Mechanical Properties For High Strength Pressure Vessel Steel

With the development of economy, the consumption and import of petroleum are increased year by year. The building for strategic oil reserves is relevant to the national economic security. We have to product large floating roof tanks with a single storage volume greater than100,000m3in order to construct for bascal national strategic reserve, in which requires steels with high strength, good toughness and combination with good welding property. For companies and our country advantages, the National Development and Reform Commission urges to product by domestic oil storage tank steels in China. On the hand, appeasing for domestic markets, the strategic oil reserves are constructed for national security. On the other hand, complementing the technologies and theories for heavy plate, the investigations help for elevation technical level of enterprises.In the studies, four types of steels were investigated by using hot rolling and quenching and tempering techniques. The influence of microstructures and mechanical properties were analyzed by alloy elements, heat treatment, precipitates and inclusions in order to design a rational alloy elements and heat treatment technology. Based on the analyses, theories and experiments data were composed for large scale manufacture and application. These can offer gist of theory and experiment for commercial products. The main conclusions in these studies as following:1. Designed four types steel were hot-rolled, quenching at950℃, quenching plus tempering at950℃and630℃, respectively. The composition of C and Mo influence on microstructures and mechanical properties were analyzed for optimal properties. The results indicated that hot rolled microstructures with bulk or long island (M/A island) were increased with increasing carbon content, and the microstructures became finer and more homogeneous after heat treatment. With increasing of molybdenum content, yield strength and ductility significantly increased, but toughness drastically decreased. As analysis above, the composition with0.2%molybdenum was best for mechanical properties.2. Selected1#and3#with0.2%molybdenum for quenching and tempering (1#:0.08C-1.50Mn-0.29Si-0.28Cr;3#:0.10C-1.39Mn-0.27Si-0.26Cr), the effects of heat treatment on microstructures and mechanical properties were studied in different heat treatment conditions. Microstructures of quenched and tempered were composed with tempering sorbite and martensite. The result indicated that1#tested steel with950℃quenched plus690℃tempered appeared the best mechanical properties with yielding strength of665MPa, tensile strength of720MPa and higher elongation of25.9%. When3#had been quenched at910℃and tempered at690℃, respectively, the yielding strength of605MPa, tensile strength of670MPa and combination ductility of28.6%were achieved. Depending on the results, impact energy of1#and3#test steel can meet the requirement of GB19189-2003.3. Studying on precipitates and inclusions of1#and3#test steel, the result showed that: the intragranular precipitates were composted with Cr、V and Mo, and the precipitates at grain boundaries mainly composted with Cr and V compound. Tensile properties of ductility and toughness were improved by precipitates and achieved high level properties. The main impurity was MnS compounds, which was drastically affected on impact toughness in cross direction for oil storage tank steels.