| In modern society, the requirements of oil,natural gas was enlarged. Strength, low temperature impact toughness, Hydrogen Induced Cracking (HIC), Sulfide Stress Corrosion Cracking (SSCC) and Stress Corrosion Cracking (SCC) properties for pipeline steels were proposed for a higher requirement. To improve the strength and toughness of X80and higher grade pipeline steels, the effects of microstructure type, M-A islands, inclusions, etc, on acicular ferrite(AF) X80pipeline steel were investigated; the production process of bainite X100and bainite/martensite X120pipeline steels were investigated in laboratory. The main works involved as follows:(1) Thermal simulated experiments with different parameters was carried out in MMS-300thermal simulator, the effect of deformation, deformation temperature, and strain rate on the dynamic recrystallization behavior and static recrystallization behavior of austenite of X80and X100pipeline steels was investigsted. It shows that as strain rate increased, dynamic recrystallization occurred at a higher temperature, and the temperature range was reduced and it was difficult to be occurred for dynamic recrystallization. Zener-Hollomon parameter was introduced to confirm the effects of deformed temperature and strain rate on dynamic recrystallization. Zhou Ji-hua modulus was used to non-linear regression for actual data and the regression coefficient was obtained. Least square method was used to confirm the static recrystallization activation energy of X80and X100pipeline steels as308.7kJ/mol and340.8kJ/mol, respectively. Avrami formula was used to study the kinetics of static recrystallization, and kinetics formulas of static recrystallization of X80and X100were obtained.(2) The continuous cooling experiments were investigated, and continuous cooling transformation (CCT) was obtained.The results show that Start phase transformation point and finish point were lower for high Nb X100and X120pipeline steels. When the steel was deformed, the bainite transformation was restained and small sized M-A islands distributed more dispersively.(3) The trial-production was done in4300mm heavy plate mill for Nb micro-alloyed X80pipeline steel. When the finished rolling temperature, finished cooling temperature and cooling velocity was780～840℃,530～550℃and18℃/s, respectively, the microstructure was acicular ferrite (AF) and some polygonal ferrite (PF)/bainite, the comprehensive mechanical properties were better. When the steel was relaxation for a short time, the strength decreased, while DWTT properties were not improved. The formation and propagation of secondary crack was induced by inclusions neared to each other, and then the low temperature toughness was deteriorated. As volume fraction of M-A islands increased, CVN energy at-20℃increased. When cracks propagated to M-A islands, the propagation was forbidden, and crack propagation energy increased. M-A islands was mainly martensite with little austenite, which was analyzed by Selected-area diffraction pattern (SAD).(4) For Nb-Mo micro-alloyed X100pipeline steel, when the finished rolling temperature, finished cooling temperature and cooling velocity was800～850℃,400～450℃and20～35℃/s, respectively, or800～850℃,350～400℃and20～35℃/s, respectively, with60s relaxation time, the comprehensive mechanical properties were better in laboratory, while the finished rolled temperature of Nb micro-alloyed steel was20～50℃higher than that of Nb-Mo micro-alloyed steel.For given Nb-Mo micro-alloyed steels, by fine grain strengthening, solid solution strengthening, precipitation strengthening, dislocation strengthening, subgrain strengthening and so on, the yield strength of experimental steels was predicted as764MPa, which was well correspondence to actual ones. The orientation relation between martensite and retained austenite was[101]γ//[111]α,(-1-11)γ//(-101)α in TEM. The width of micro-twin distributed at grain boundaries or lath boundaries was2-40nm. For Nb-Mo micro-alloyed steel, with increasing precipitates size, the ratio of Nb/Ti increased, and the morphology of precipitates intended to regularization.(5) When the finished rolling temperature, finished cooling temperature and cooling rate was860～880℃350℃and50℃/s, respectively, the yield strength, tensile strength, elongation and CVN toughness at-20℃was890MPa,1009MPa,17.2%and203J, respectively, and the comprehensive mechanical properties of X120experimental steel were better. With increasing CVN toughness at-20℃, DWTT properties at-20℃increased. When the CVN energy at -20℃was over99J, DWTT properties at-20℃was higher than85%. For the steel finished rolled at760℃that was close to transformation temperature(Ar3),fracture separation was observed in DWTT samples.(6) The comprehensive mechanical properties of the steel reheated, quenched and tempered (RQT and tempered at500℃for60min) was much better than that of the quenched steel. For the steel tempered at650℃, polygonal ferrite (PF), in which finer precipitates distributed dispersively, was observed. With increasing tempering temperature, the morphology of carbides evolved from strip like to dots/sphericity. When the tempering temperature increased from350℃to650℃, the volume fraction of small angle grain boundaries (SAGBs) decreased from0.209to0.115. NbC precipitates was finer for the steel tempered at500℃,. |
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