Research Of Enhancement Mechanism Of Plasticity Of X80Pipeline Steel Research Of Enhancement Mechanism Of Plasticity Of X80Pipeline Steel

As petroleum natural gas pipeline outspreading to polar, marine and geological instability region, X80pipeline steel not only requires high strength and high toughness, also needs high deformability in order to ensure the safe working. Grain refinement has greatly increased the strength and toughness of X80pipeline steel, but also resulted in a decline in the plastic deformation capacity, which brings potential safety hazard to the operation of pipeline steel. So it is needed to research the enhancement mechanism of plasticity of X80pipeline steel.There is a critical grain size for fine crystal reinforcing, when grain size is larger than critical grain size, plasticity increased as grain size decreased, when grain size is less than critical grain size, plasticity decreased suddenly as grain size decreased. A model of relationship between grain size and plastic deformation under given stress has been established based on the theory of dislocation pile-up; and the calculation of critical grain size has been discussed. According to the analysis based on the model, the factors that affect the macroscopic plastic deformation mainly includes:the effect of grain size on the pile-up number of dislocations in a single grain, the effect of grain size on the slipping distance of dislocations, and the effect of grain size on the activated probability of dislocation sources. The possible physical mechanism of the suddenly decrease of plasticity when grain size is less than critical grain size is the shortage of mobile dislocations as a result of the very low activated probability of dislocation sources. For further research, based on the model, using the plastic elongation corresponding fracture strength as parameters measuring the plastic performance, modeling of relationship between plastic elongation and grain size has been established. According to the calculation results of the model, with grain size decreased, the plasticity of polycrystalline material increased and then decreased when grain size is less than critical grain size, and the calculation result of the critical grain size of X80pipeline steel is about2.5μm, which is verified by the experimental data. The calculation of critical grain size proved a theoretical basis and reference range for increasing strength and plasticity by means of refining grain size.Distinctly only refining grain size can not enhance the plasticity of X80 pipeline steel, although multiphase design is an effective mean. The microstructure, tensile mechanical properties and texture of four X80pipeline steels manufactured by different steel mills have been observed and detected in order to analyze the relationship between microstructure and properties, and the relevant factors. The volume fraction, the grain size and the distribution of ferrite have been analyzed systematically. The analysis results indicate that bainite/ferrite two phases designed pipeline steel has more excellent strength and plasticity. The volume fraction, the grain size and the distribution of ferrite are important factors that affect the strength and plasticity of pipeline steel, it is worth noting that too small grain size of ferrite would reduce the plasticity of pipeline steel. The analysis of the effect of work hardening exponent of different stage on yield ratio indicates that the effect of the work hardening exponent of the second stage is more significant than that of the first stage. Improving the work hardening ability of the second stage should be emphasized in the microstructure design.Based on the microcosmic mechanism of plastic deformation, a plastic deformation model has been built which is appropriate for bainite/ferrite two phases designed pipeline steel. According to the analysis of the model, the yield strength and yield ratio of dual-phases microstructure decreased as ferrite volume fraction increased, while the grain size of ferrite has little effect on yield strength and yield ratio, the uniform elongation increased as ferrite volume fraction increased and grain size of ferrite increased, and the effect of ferrite volume fraction on uniform elongation decreased as grain size decreased. The microstructural parameters satisfied the stress-based design and strain-based design have been calculated by the model, which provides a scientific basis for the research and exploitation of X80pipeline steel both with high strength and excellent plasticity.