| With the gradual shift of oil and gas exploration and exploitation to deepwater sea area,the research and development of high-performance deep-sea pipeline steel has become a hot topic in pipeline manufacturing enterprises.Compared with the land pipeline,the deep-sea pipeline has more demanding requirements in both laying and service environment,which mainly include high strength,high plasticity,large wall thickness,small diameter and resistance to bending or crushing.The microscopic deformation behavior of dual-phase structure is the main factor affecting the strengthening and plasticizing mechanism of deep-sea pipeline steel during deformation process.However,the changes of steel tube after forming and expanding,such as wall thickness accuracy,ovality accuracy and the uniformity of plastic strain distribution,which have a great influence on the resistance to bending and crushing.Therefore,the simulation research on the deformation behavior of deep-sea pipeline steel has important guiding significance for the research and development of higher-grade deep-sea pipeline steel.This paper takes X70 high-strain deep-sea pipeline steel as the research object.In order to solve the difficulty of studying the strong and plasticizing mechanism at the microscopic scale,a finite element model of dual-phase microstructure was established by combining Voronoi algorithm and Python script based on the consideration of the dual-phase characteristics of pipeline steel.Based on this model,the deformation behavior of the dual-phase microstructure and the microstructure characteristic parameters such as bainite content and length-diameter ratio during room temperature tensile process were studied,which will affect the microstructure deformation behavior.At the same time,in view of complicated forming steps and intricate deformation process at the macroscopic scale,the finite element model of pipeline steel forming and expanding process was established based on linear motion hardening theory.On this basis,the evolution law of steel plate deformation,stress-strain distribution and accumulation during forming process procedure is revealed.The simulation results of dual-phasic microstructure deformation behavior show that the microstructure deformation of large-strain deep-sea pipeline steel is incongruity during the tensile process.With the exacerbation of deformation extent,a strain concentration zone of 45° in the direction of loading is formed in the microstructure.The strength of large-strain deep-sea pipeline steel follows the low of mixture,which means that it enhances linearly with the increase of bainite content VB.When the VB value is in the range of 40%-50%,the deformation difference between the two phases is minimal.The rise of strength is more obvious while the length-diameter ratio of bainite is less than 5.The slender bainite can effectively block the propagation of strain concentration band in the dual-phase structure,promote the plastic deformation of bainite,restrain the plastic deformation of ferrite,and reduce the difference of plastic deformation degree between the two phases.The simulation results of deformation behavior of large-strain deep-sea pipeline steel in the forming process show that the plastic deformation distributes symmetrically along the thickness of the central layer in the bending and U-forming process,and the plastic strain of the inner and outer walls is the largest.The overall plastic deformation of the steel tube is intensified after the expansion,especially in the straight wall section.In addition,the expansion can also effectively improve the distribution of residual stress and raise the accuracy of ellipticity.The wall thickness of the pipe changes with the forming process and decreases obviously after the expansion. |
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