| With the increasing demand for oil and natural gas, the exploitation of pipeline extends to the desert, polar and marine and other remote areas.During the running of pipeline,it may be subjected to earthquakes, landslides, soil structure changes, the role of submarine erosion and other geological disasters.They will make the position of pipeline changed.What’s more,it will even result in a large degree of deformation to some parts of the pipeline.Pipeline failure will no longer be controlled by stress,but by strain.We must develop a new type of pipeline with Round house stress-strain curves,relatively low yield ratio,high strain hardening index and high rate of uniform plastic deformation.This paper takes high deformability pipeline steel X70as the research object and makes research on recrystallization behavior and deformation behavior of austenite under deformation and continuous cooling,based on it,making research on microstructure and properties of high deformability pipeline steel X70in the rolling process. The main work and the main results are as follows:(1) Measure the dynamic CCT curve of high deformability pipeline steel X70by using thermal simulation testing machine MMS-300.The results indicated when the cooling rate was0.5℃/s, ferrite, pearlite and little bainite (F+P+B)was obtained;when the cooling rate was1-3℃/s, the organization was mainly composed of polygonal ferrite and bainite; when the cooling rate was3-5℃/s, the organization included polygonal ferrite, little acicular ferrite and bainite; when the cooling rate was greater than5℃/s,the organization was mainly composed of polygonal ferrite.(2) Measure deformation resistance of high deformability pipeline steel X70under different parameters by using thermal simulation testing machine, the results indicated with the deformation temperature increasing, resistance of deformation reduced, under high temperature and constant deformation, resistance of deformation increased as the strain rate increased; resistance of deformation increased as the deformation increased. With the deformation adding to a limitation to some entent, the dynamic recovery and dynamic recrystallization appeared and resistance of deformation maintained a definite value.(3) Grope for rolling, cooling process parameters by means of thermal simulation experiment and analyze their effects to the microstructure of HD X70,what’s more, selectively analyze effects of relaxtion to structure transformation of HD X70,the results showed that when the other factors were constant,with the relaxtion time increasing, more ferrite were produced, in addition, the grain size of ferrite was little larger, relaxtion termination temperature was a great parameter deciding the quantity and grain size of polygonal ferrite.(4) Use the experimental hot rolling mill in the laborary to roll the ingot into a thickness of13mm experimental steel plates,then in order to control the cooling rate, the experimental plates would go through laminar cooling device. The experimentally rolling process concludes TMCP and relaxation techniques. Analyze the mechanical properties of the experimentally rolling plates by means of tensile test and impact toughness test, results showed that main properties were meeting the criterion:the stress-strain curve was smooth, Rto.5>485MPa, Rm>570MPa, Rt0.5/R≤0.85, UEL≥10%,n>0.1. Observe the structures by means of OM and SEM, the results illustrated the structure was mainly composed of ferrite and bainite, combining with the tensile test and impact toughness test results, the best volume fraction of ferrite was60%-70%in the experiment. Observe the microstructures by means of EBSD and TEM, the results illustrated the deformation potential was obviously increased when small-angle boundary existed in bainite.relaxtion promoted the precipitation of the dissolved micro-alloying elements and enhanced the strength of HD X70, what’s more, made the grain refinement after phase transformation. |
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