A Study On Microstructure And Property Of X70 Pipeline Steel With Strain-Based Design

The pipeline used in areas where great ground movements may occur such as frozen ground, activities of seismic and unstable slopes should have high plastic capacity, in addition to requirement of high strength and toughness. The strain-based design method was posed. The pipeline steel with strain-based design is usually characterized by high initial work hardening rate, good ductility, and low yield stress to tensile strength ratio. So this type of pipeline steel can be able to maintain the existing transmission capacity under high deformation behavior. With the development of oil and gas field in remote regions where the pipeline service environment became worsening, the demand for pipeline steel with strain-based design is gradually increase. X70 grade pipeline steel was studied in this paper, the effect of the intercritical annealing parameters on structure and properties was systematically researched, the effect of the relaxation in controlled rolling and cooling process on the microstructure of specimens and the corrosion of specimens with different intercritical annealing were studied. The main contents are given as follows:Using scanning electron microcopy, optical microscopy, hardness testing and tensile testing machine were analyzed the influence of intercritical annealing parameters on microstructure and mechanical properties of X70 pipeline steel with strain-based design. The results indicate that through intercritical annealing, a good combination of strength and ductility of X70 pipeline steel with strain-based design is obtained. The volume fraction of second phase, strength and the work hardening rate increase by increasing the soaking temperature. When the soaking temperature exceeds 790℃, there is sufficient mobile dislocation created by austenite to martensite transformation during cooling process, resulting decline in the yield strength. When the soaking time exceeds 10min, the specimen can obtain stable structure. All specimens between 790℃～880℃for 10min quenched in different media can get low yield ratio, high uniform elongation and work hardening rate, when the specimens cooled in air, the hardness induces with increasing the soaking temperature because of formation of epitaxial ferrite formed and the induction of volume fraction of second phase.The experiments on thermal-mechanical simulator indicate that, when the specimen slowing cooling at 0.7℃/s to 700℃after deformation, there are many kind of eutectoid ferrite formed during relaxation. When the specimen slowing cooling at 0.9℃/s to 700℃after rolling, the volume fraction of ferrite has a sharp reduction and the grain becomes smaller with decreasing the relaxation time. The samples with the relaxation time of 63.6s followed by 9℃/s quick cooling at 700℃may obtain fine ferrite grains and equally distribution of second phase. When the relaxation time is 77s, the grain of acicular ferrite is large in size. When the relaxation time is 100s, there are many kind of eutectoid ferrite formed before quick cooling, so the second phase content is low.The corrosion tests results show that:The corrosion resistance of specimens with intercritical annealing is lower than X70 pipeline steel. When the soaking temperature at 740℃-780℃, the corrosion resistance is the highest, when the soaking temperature at 800℃, the corrosion resistance decreases slightly, and the corrosion resistance increases with increase the soaking temperature when the soaking temperature exceed 820℃. When the soaking temperature is 820℃, the corrosion resistance gradually increases with increasing the soaking time, the specimens soaking 7min or 10min have the highest corrosion resistance. When the specimens soaking at 820℃for 10min, and quenching in water, oil and air respectively, the specimen quenched in water has the highest corrosion resistance.