Pipeline And Properties Of Niobium Kang Da Deformation Of Tissue Containing High-grade Thick Gauge

High deformability pipeline steel is a new pipeline steel developed for strain-based design. It is applied mainly in geologies conditions scurviness areas with earthquakes and landslides et al., and requires having lower yield ratio, higher work hardening index and better uniform elongation et al. as well as high strength and toughness. Polygonal ferrite-bainite ("PF-B") dual phase pipeline steel has been the focus of research because of excellent strain capacity and better microstructures controllability. For "PF-B" dual phase pipeline steel, only should polygonal ferrite balance a certain volume fraction and fine size could get higher strength-toughness-ductility. The emphases and difficulties in production, adhibition and generalization as follows:How to control and refine ferrite grains and make the fine ferrite rapid precipitation, relationship of technical microstructures-mechanical properties, mechanism of strength-toughness-ductility, weldability, properties chang after work hardening and strain ageing all have been solved by technology different from traditional X80pipeline steel and composition optimization, and proposed solutions to improve strength, toughness and plasticity.According to the "controlled rolling-slow cooling relaxation-accelerate cooling" technology, ca-st billet reheat and hot deformation and controlled cooling simulation, pilot controlled rolling an-d cooling, industrial trial, welding thermal simulation, Thermo-Calc thermodynamic computing, optical metallographic, scanning electron microscope with EDAX and EBSD, transmission ele-ctron microscopy were employed to research austenitizing and recrystallization dynamics of dif-ferent Nb bearing experimental steels, relationship among Nb content-strain parameter-relaxati-on parameter-ferrite transformation dynamics, relationship among start cooling temperature-fer-rite-mechanical properties of steel plate and mechanical properties relationship of plate-pipe-pi-pe after strain aging, microstructur-es’ and the reasons of softening and embrittlement of HAZ and the solutions.At the tested temperature range, austenitic grains grow up as reheat temperature increases obvio-usly. When Nb content is less than0.051%, the austenitic grain size and reheat temperature foll-ow Arrhenius equation-In D=A-Q/RT, but it didn’t when Nb content is increased to0.080%be-cause of the pinning effect of (Ti, Nb)(C, N). The austenitic grain size and soaking time follow exponential relationship-D=ktn, and the index is less than0.1, its does not significantly inc- rease when the soaking time is more than20minutes. Increasing Nb content can repress growth of austenite grain in the reheating process. Reheat temperature>1150℃, the average size of aus-tenitic grain could be controlled within100μm when Nb content increased to0.08%. If the Ti element percent in the tested steels is around0.01%, the average size of austenitic grain can be repressed within100μm when undissolved Nb is more than about0.0047%. Deformation and holding process at high temperature, austenitic grain size reach minimum when happen comple-tely static recrystallization, it is controlled within40μm, the minimum size does not change wi-th the strain temperature and holding time. Increasing Nb content can reduce the austenite recry-stallization grain size, and maintain refining effect effectively, the austenite recrystallization gra-in can be controlled within30μm in0.08%-Nb steel.Deformation in austenite non-recrystallization region, as strain temperature<800℃and deform-ation>50%, it could speed up ferrite nucleation rate and refine polygonal ferrite grain. A lot of ferrite crystal nucleus can grow up when slow cooling rate<1℃·s-1and start cooling temperature≤690℃, the volume fraction and grain size of ferrite increased significantly. The ferrite transfor-mation can be restrained and refined when Nb content increases. The strain induced ferrite trans-formation is inhibited obviously when Nb increases to0.08%during the deformation process, its volume fraction is less than10%. And ferrite fraction and grain size decrease significantly when the Nb increased from0.031%to0.08%during the slow cooling relaxation process, the volume fraction only is15%, but the average grain size can be controlled within3μm.Reducing the start cooling temperature can improve ferrite transformation, but the ferrite grain would be excessively coarse as the start cooling temperature lowest. The ferrite grain can be refined under4μm, and ferrite transformation dynamics slow down evident when Nb increased to0.080%. The larger strain resistance capability performance requirement can be satisfied as the ferrite volume fraction is controlled in the range from30%to75%and its average size is repressed within5.5μm.The relational expression of start cooling temperature-ferrite fraction-mechanical properties, plate-pipe mechanical properties for26.4mm "PF-B" pipeline steel have been established. For steeel plate, the tensile strength changes is not obvious, the yield strength decreases significantly, the uniform elongation increases, and the impact toughness decreases as the volume fraction of ferrite increases, respectively. High movable dislocation density in ferrite grain is the key point to reduce yield ratio and increase uniform elongation, there is a little substructure in ferrite grain, and improving impact property depend on ferrite grain boundary. The more the ferrite fraction, the more obvious the work hardening and strain ageing phenomenons, the yield strength could be increased because of work hardening, and the uniform elongation could be lost caused by strain ageing, lead to properties of "PF-B" high deformability linepipe no sensitive to ferrite fraction. The key point to improve strength and toughness is refining ferrite grain.The impact toughness of coarse grain HAZ same as base metal when t8/3is less than some value, but the impact value will decrease sharply because of a lot of MA island as t8/3is too long. The softening phenomenon will appear because of a lot of fine ferrite as the peak temperature is about800℃. The impact toughness is the worst at750℃because of precipitation of alloying elements such as Nb and a lot of larger size MA components. The key factors to reduce impact toughness of HAZ include coarseness of microstructures and MA components. Increasing Nb can reduce MA island and refine microstructures, increase welding heat input, ease softening and improve impact toughness obviously of HAZ.For "PF-B" high strength and deformability pipeline steel, the ferrite fraction and average size could be controlled about30%and within5μm especially, and increasing the ferrite/bath bainite interfacial area to further improve the strength, toughness, ductility. To get finer ferrite grain as well as guarantee faster ferrite transformation dynamic, the suitable Nb content is about0.05%in steel and the appropriate technological parameters as follows:cast billet reheat to1150～1200℃for2-4hours; austenite recrystallization and strain temperature of1000～960℃, deformation≥25%, rolling pass interval time≥30s; austenite non-recrystallization region, strain temperature of800～760℃, deformation≥50%; start cooling temperature of700～660℃.