Effects Of Dynamic Continous Cooling Transformation Of Pipeline Steels For High Pressure Gas Transportation

In this thesis, to develop the high Nb X80 pipeline steel, the continuous cooling phase transformation behaviors of High-Nb X80 steels with different compositions under different hot deformation conditions were studied.The continuous cooling transformation (CCT) curves of three high-Nb X80 pipeline steels with different componention under the different hot deformation conditions were determined on a Gleeble-3500 thermal mechanical simulator. Subsequently, the microstructure of the samples was observed by means of optic microscopy (OM) and scanning electron microscopy (SEM), furthermore, the influences of hot deformation processes and cooling rate on microstructure were analyzed.The results show that the mixed microstructure of acicular ferrite with the high strength and toughness can be obtained at a wide range of cooling rate in high Nb pipeline steel. However, the hot deformation processes have a great effect on continuous cooling transformation and its microstructures after phase transformation. During the continuous cooling processes, the beginning phase transformation temperature of X80 pipeline steels decreases gradually with the increasing of cooling rate, and the microstructure changes from polygon ferrite to the acicular ferrite. As the single pass hot deformation is in recrystallization field, the austenitic grain can be refined by deformation, and the deformed storage energy of austenite increase, which results that the polygonal ferrite and acicular ferrite phase transformation are accelerated. Therefore, the CCT curves move upward and left, and the transformed microstructures can be refined. When the hot deformation is in the no-recrystallization field, the refined original austenitic grain becames elongated, and the deformed storage energy further increases, therefore, the phase transformation is promoted remarkably. Thecritical cooling rate of polygon ferrite further increases, and the CCT curves move upward and left observably, furthermore, the transformed microstructures are refined. Moreover, with the decreasing of deformed temperature, the effect of deformation become strongly. During the relaxation treatment after hot deformation in no-recrystallization field, although the deformed storage energy decreases becouse of the static recovery, the effect of deformed on phase transformation became weakly. Whenas, the content of solid solution Nb in austenite decreases becouse of NbC precipitation, which results that the effect of solution Nb on restraint phase transformation is weakened. Simultaneity, the particles of precipitated NbC can become the nucleation of ferrite to promote formation of polygonal ferrite, and refine grain of ferrite.