| Reasonable matching of strength and plasticity or toughness is the key content in steel material research. X80 pipeline steel is widely used in oil and gas pipeline projects, its construction and service condition are relatively poor which put forword high strength but also higher plasticity and toughness requirements. In this paper, in order to provide theoretical guidance and experimental reference for the optimization of X80 pipeline steel mechanical property, taking hot-rolled X80 pipeline steel as research object, its plastic deformation behavior and organization evolution were studied, the mechanism of plastic deformation and damage, fracture were revealed from the perspective of mesomechanics.By means of OM, SEM(EBSD), TEM and other analysis technologies, X80 pipeline steel microstructure morphology were studied:frequency distribution of effective grain size of less than 1 μm in transverse was greater than in longitudinal or 45° direction; dimples were uniformly distributed in transverse and 45° direction and their section undulatinged, then the tensile fracture morphology indirectly reflected the sort of fracture energy:45° direction>transverse>longitudinal direction; copper type texture, cube texture and cube texture were in transverse, longitudinal and 45° direction, and{111} texture intensity was higher, where the type of main texture was{110}<112>, and <111>//TD grains accounted for the most that resulted in its mechanical properties anisotropy. Grain boundary strengthening mechanism was analyzed with the tilt grain boundary model composed of edge dislocations, then the results showed that: interaction force FABâ†'C between grain boundary and dislocation increased with misorientation θ; at the beginning, the force rised rapidly; until θ to 15°, the force growth dropped significantly, basically stable at a maximum, which was consistent with the relationship of grain boundary energy-misorientation.Study on the welding zone mechanical properties of X80 pipeline steel showed that:the region was columnar grain structure and owned the comprehensive feature of solidification structure and phase transition organization, then its impact energy sorted:parent metal>HAZ>welded seam. The welding crack formation process were as follows:under tensile stress caused by solidification cooling, the crack propagated along columnar crystal boundary, then there were non-metallic inclusions inside it, most of its fracture area was cleavage characteristic then fracture edge area was quasi cleavage characteristic.Analysis of tensile mechanical behavior of X80 pipeline steel showed that: strain-hardening exponent n in transverse was the largest, followed by longitudinal,45° direction; with n increasing, the uniform elongation increased, and strain-hardening coefficient K and n or σb were a positive correlation; its strain-hardening behavior had three stages:platform, slow decline and rapid decline; analysis of strain-hardening behavior based on C-J method showed that its two rapid decline processes corresponded to two uneven deformation stages: yield and necking.Effective grain size in transverse was the smallest and its greatest size was in 45° direction, then grain refinement make quantity of grain boundaries increase and was helpful for plasticity of X80 pipeline steel; small-angle grain boundary contributed to plastic deformation but strength; high-angle grain boundary strengthening may improve stress at the grain boundary then prompt deformation of neighboring grains. Strain-hardening behavior analysis of X80 pipeline steel quick cooling microstructure showed that:with the ferrite volume fraction increasing, uniform elongation increased, and yield ratio was negatively correlated with strain-hardening exponent, while its work hardening behavior undergone three processes:rapid decline, slow decline and again rapid decline, which turning points corresponded to yield and tensile strength. When the ferrite volume fraction reached 22.8%, its work hardening rate decreased with a fast stage and then a slow stage, then MA islands involved in deformation to ensure its strain-hardening ability.Impact fracture behavior analysis of X80 pipeline steel showed that its impact toughness exhibited anisotropy at room temperature:transverse was optimal and 45° direction was the worst, mainly because of fine grain toughening mechanism whose core elements were quantity and misorientation of grain boundaries; the lower the temperature was, the weaker the ability of crack growth was, the more remarkable the brittle fracture characteristics were; inclusions (MnS, All2O3, etc.) were the starting points of dimples forming, then crack was hindered at interface between inclusion and matrix while its path was more tortuous, and the crack propagation region had deformation bands.Two-dimensional finite element simulation of the damage-fracture mechanism of inclusion to matrix in Charpy impact test showed that:damage at inclusion interface and fracture of inclusion itself may be the source of crack; the stress triaxiality factor Rσ at 45° and 135° of round inclusion interface (two corner of square inclusion) was most serious, and damage at 90° of the interface was the lowest, which agreed with analysis of its plastic strain energy density; the maximum of equivalent strain and plastic strain energy density of inclusion interface were generally positive linear correlation with its size and the values of square inclusion were higher; plastic strain energy density at the interface of square inclusion with sharp corners was greater and the damage area was more concentrated, then its dimple was smaller than round inclusion, which absorbed less impact energy; the second highest damage region appeared in surrounding matrix of inclusion and the damage of round inclusion was lower and more uniform; plastic strain energy density at small inclusion interface was lower that resulting in a relatively uniform damage distribution and which area was relatively larger, then the dimple was relatively larger and more energy was absorbed. |
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