Development Of High Performance Steel Plate For Oil Storage Tanks

The development of the national strategic oil reserve base construction increases the requirement of large oil storage tank steel plates. The construction of large oil storage tanks with volume more than100,000m3increases the combination properties requirement of tank materials:high strength, high toughness, high uniformity and high stability, meanwhile, it has better high heat input welding ability. So far, high strength steel plates for large oil storage tank construction in China almost all depend on imports, it’s expensive and can not ensure date of delivery. The high performance steel plates for large oil storage tank were developed according to the production status of high strength oil storage tank steel plates inland, and integrating with the Capital Steel Co. project of "The Research and Development Center of High Additional Value Wide and Heavy Plate". The main works and results of this paper are as follows:(1)According to the performance characteristics of large oil storage tank steel plate, chemical compositions of high performance steel plate for large oil storage tank has been determined by decreasing carbon, manganese and molybdenum, adding micro-content niobium and titanium, and strictly controlling Pcm, sulphur and phosphorus, which is based on GB19189-2003chemical compositions requirement.(2)Using the MMS-300thermal simulator, high temperature quenching experiment and continuous cooling transformation experiment were performed to determine critical transition temperature of tested steel, draw static and dynamic CCT curves. The effect of heating temperature、hot deformation and cooling process parameters on microstructure evolution has been investigated. The results showed that uniform and fine austenite grain structures can be produced when the heating temperature at the range of1150～1200℃, deformation increases the beginning transformation temperatures and martensite and bainite can be obtain when the cooling velocity is between25℃/s and30℃/s.(3)The microstructure and mechanical properties variation during quenching and tempering of oil storage tanks steel have been systematically studied in this thesis. It is found that the steel has excellent combination mechanical properties when the range of quenching temperature and tempering temperature are850～910℃and640～680℃, respectively. The results showed that the length of martensite laths obviously increased with the increasing of quenching temperature, while the width of martensite laths had no significant change. There were increasing trend of strength, decreasing slightly of elongation ratio and impact energy of tested steel with the increase of quenching temperature. It is showed that with the increase of tempering temperature, the sub grains were formed by rearranging of dislocations in the martensite lath, and carbides began to aggregate and coarsen when the tempering temperature is higher than680℃. The strength of tested steel first slow, quick decreased, on the contrary, impact energy and elongation ratio were rising as the tempering temperature increase.(4) The effect of total reduction in non-recrystallization region, direct quenching temperature and tempering temperature on microstructure and mechanical properties of oil storage tank steel has been investigated during controlled rolling and simulation on-line direct quenching after hot rolling. There was excellent combination mechanical properties of tested steel when the total reduction in non-recrystallization region is60%, direct quenched at the range of85O～910℃, tempered at the range of640～680℃(5)The microstructure and mechanical properties of direct quenching steel and reheat quenching steel have been compared. The direct quenching steel has already precipitated fine and dispersed carbides during quenching due to thermomechanical treatment effect, which leads to higher strength than those of reheat quenching one. The tensile strength and yield strength of direct quenching steel were obviously higher than those of reheat quenching steel, the percentage elongation was approximate and the impact energy of them was more than180J at-20℃. The strength reduce of the former is lower than the latter, both impact energy and percentage elongation of them increased significantly through the same tempering treatment, which Shows a good tempering resistance of direct quenching steels. It is proved that on-line direct quenching process is an effective method to improve the strength and toughness properties of oil storage tank steel.(6)The weldability of oil storage tank steel was investigated deeply and thoroughly by Y-groove type cracking experiment, welding thermal simulation experiment and gas-electric vertical position welding experiment.The effect of different heat input and peak temperature on microstructure and mechanical properties of heat affected zone was obtained through thermo mechanical simulation experiments.The results showed that the developed oil storage tank steel has small weld cold cracking tendency. On condition that the large heat input was less than100kJ/cm, the welded joints of steel and base metal had excellent mechanical properties and good low temperature toughness.(7)Based on the results of laboratory studies, the chemical compositions of SG610E steel has been designed, and the corresponding system of rolling and heat treatment process has been determined. Industry trial-production of oil storage tank steel with different thickness specifications was done successfully. The mechanical properties and using performance met or exceeded the national standard requirement and successfully passed the National Boiler and Pressure Vessel Standardization Technical Committee for technical review.