| Under the background that energy saving emission reducing has become an international consensus, in order to reduce the construction cost of oil-gas linepipe and increase the oil-gas transport efficiency, the research and development of high grade pipeline steel with higher transfer pressure become hot. So, it creates an unprecedented chance for the pipeline steel with higher strength, good crack-arrest property, anti-large-deformation ability and excellent field weldability. The previous studies on high grade pipeline steel shows that strength improving might lead to the significant decrease in low temperature toughness, and the crack-preventing property is closely related to the energy change during fracture process and the fracture morphology, whose deep mechanism still needs to be further studied.Taking pipeline steel X100project researched and developed by The State Key Lab of Rolling and Automation of Northeastern University and Laiwu Iron and Steel Group as background, through thermal simulation experiment and hot rolling experiment, the austenite re-crystallization law at high temperature and the phase transformation law of austenite after high-temperature deformation of0.20Mo-0.04Nb series and0.09Nb series steels were studied, and the correlationship among TMCP, tempering processes, microstructure and mechanical properties was studied. On this basis, the industrial pilot of0.20Mo-0.04Nb series pipeline steel X100was performed. In order to synthetically evaluate the strength-toughness and the crack-preventing property, aiming at TMCP+tempered plates, the influence law of charpy impact fracture behavior and microstructure on its DWTT crack-preventing property was considerably studied. The chief work and achievements are as follows:(1) By thermal simulation experiments, the evolution law of microstructure and property of pipeline steel X100was studied, and it has been experimentally found that,0.20Mo-0.04Nb series tested steel has a wide dynamic re-crystallization temperature range, and its dynamic re-crystallization easily occurs, but the dynamic re-crystallization temperature range of0.09Nb series tested steel is relatively narrow, and compared with0.20Mo-0.04Nb series tested steel, the final temperature of static re-crystallization for0.09Nb series tested steel is higher about50℃. For two tested steels, the transformation temperature range is about400～600℃. And the dynamic continuous cooling curves were obtained, the following analysis showed that, in order to get the microstructure mainly consisting of lath bainite, granular bainite and acicular ferrite, the cooling rate must be controlled within15～25℃/s, and with the cooling rate increasing, the microstructure density increased, the lath bainite increased in number.(2) Research on the hot rolling and hot treatment processes of the tested steels with two kinds of compositions was carried out, and it was found that, for0.20Mo-0.04Nb series tested steel, the multi-phase microstructure of LB, GB, AF and fine carbides could be obtained, and its mechanical properties could meet the standard requirements of pipeline steel X100. While for0.09Nb series tested steel, the microstructure of GB, QF and large M/A islands was obtained, its mechanical properties were lower and couldn’t satisfy the standard requirements of pipeline steel X100. The heat treatment in range of250～500℃for0.20Mo-0.04Nb series tested steel was carried out, and the results showed that, tempered at400℃, the microstructure of the tested steel was GB,AF and fine carbides, the comprehensive mechanical properties were good, the crack forming energy was large, and the crack-preventing property was excellent.(3)0.20Mo-0.04Nb series tested steel was selected to carry out the industrial pilot, and the relationship among production process, microstructure and property was concluded. The results showed that, when the starting cooling temperature at730～800℃, the finishing cooling temperature at400～520℃with the cooling rate18～27℃/s, the yield strength was704-805MPa, the yield ratio was about0.88～0.97, the impact energy at0℃was163-267J, the performance parameter of DWTT was86%～91%, so, the product properties met the requirements. With cooling rate increasing, the microstructure type was gradually changed from GB+AF+PF into LB+AF+little amount of GB, and M/A islands in matrix decreased.(4) In order to optimize the comprehensive mechanical properties of TMCP plates, the industrial trail of tempering heat treatment was carried out for0.20Mo-0.04Nb series trial-production plate. And the results showed that, tempered at450℃, grain coarsening could be observed, the width of deformation band formed during rolling process increased, and the precipitation quantity of M/A islands at boundaries of grains and deformation bands increased, distributed more diffusely. The yield strength and tensile strength were731MPa and931MPa, respectively, the impact energy at0℃was234J, and the yield ratio decreased from original0.88to0.79, then, the comprehensive mechanical properties were improved.(5) For industrial pilot plates, study of dynamic charpy impact test was conducted. And the results showed that, compared with the plate with finishing cooling temperature520℃, for the plate with finishing cooling temperature400℃, during the impact fracture at-60～20℃, the crack forming energy and propagation energy were higher, the propagation time changed little, and little layer in impact fracture occurred only at low temperature-60℃, thus the crack-preventing property was better. For the plate after tempering, the crack forming energy and propagation energy further increased, the crack-preventing property was also improved, moreover better for the plate with finishing cooling temperature400℃.(6) During DWTT fracture at0℃for industrial pilot plates, when existing the angle between the propagation path of main crack and the deformation band, the crack propagation is hindered significantly, and the propagation path became tortuous, so, the crack-preventing effect was obvious. The secondary cracks near main crack ended in AF of multi-phase microstructure. And in the pilot plates, the homogeneous and fine acicular ferrite, lath bainite, film-like and spot-like M/A islands were favorable for the impact toughness, especially reflected as good low temperature crack-preventing property. |
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