Study On Heat Treatment Of High Grade Pipeline Steel

With the increasing of the world energy demand, petroleum and natural gas werediscovered and exploited in some remote areas. The petroleum and natural gas wereaffected inevitably by areas with the harsh weather conditions and the complicatedgeological conditions for the long-distance transport. The pipeline steel will berequired to have good mechanical properties with high strength, high toughness,corrosion resistance and excellent deformability. Pipeline steel that has goodmechanical properties is generally produced by TMCP process, and heat treatmentcan further improve the mechanical properties of pipeline steel. This study, which hasestablished the static and dynamic CCT curve of low carbon high strength pipelinesteel by the Gleeble-3800thermal simulation testing machine, lays a theoreticalfoundation for the rolling process and heat treatment process. Then according to theCCT curve, the microstructure and mechanical properties of manganese-richedpipeline steel and low-manganese pipeline steel were studied by different temperatureof heart treatment. The main results were as follows:Drawing the static and dynamic CCT curve low carbon pipeline steel by theGleeble-3800thermal simulation testing machine, metallurgical microscopy, andvickers hardness tester. The results show that, when cooling rate is low, the staticsample microstructure is ferrite and bainite, the dynamic sample without ferrite. Withthe increase of cooling rate, hardness increases, lath bainite and martensitebecomes obvious, the phase transition temperature of the test steel point is reduced,and the martensitic transformation temperature rise. When the cooling speed afterrolling is10-30°C/s can get good organization of pipeline steel.The effects of quenching at950°C and tempering at560-640°C on themicrostructure and mechanical properties of C-Mn-Mo-Ni-Nb-Ti-V based low-carbonmanganese-riched pipeline steel were investigated by metallurgical microscope,scanning electron microscopy(SEM), tensile experiment, Charpy impact test machineand vickers hardness tester. The results show that, the rolling state steel has good strength and toughness, due to acicular ferrite (AF) containing a largeamount of fine and uniform granular bainite microstructure (GB). A multiphasemicrostructure with lath bainite and ferrite is achieved after quenching and tempering.With the increasing of tempering temperature, the recovery processes of lath bainiteare strengthened gradually, and adjacent lath is linked to combine, leading tomicrostructure coarsening. After quenching at950°C and tempering at640°C, thestrength of steel decreased, the plasticity and toughness obviously increased,and the elongation is26.9%, impact energy at-20°C is392J, the percentage ofCharpy shear area reaches100%.The effects of heat treatment at650-850°C on the microstructure andmechanical properties of low-carbon low-manganese pipeline steel were investigatedby the microstructure observation, mechanical experiment and energy spectrumanalysis. The results show that the rolling steelhas good strength and toughness, due to containing a large amount of acicular ferritemicrostructure. After tempering at650C, M/A islands decompose and spheroidize,bainite recover and recrystallize, a large number of precipitation occur, resulting instrength increasing and toughness decreasing slightly. The microstructure consists offerrite and irregular M/A islands or bainite after dual phase heat treatment (700-800C). After heat treatment at700C, tensile strength increases to778MPa,-40Cimpact energy is still as high as132J, tensile curve shows continuous yieldingbehavior. After heat treatment at750C, strength decreased slightly, the impactenergy is increased to302J, elongation rate is increased to22%, indicating goodplasticity and toughness. After normalizing treatment at850C, the strength of thesample decreased obviously. So after heat treatment at650-750C, the mechanicalproperties of TMCP steel are improved.