Study On Thermal Deformation Behavior And Microstructure Evolution Law Of X80 Seamless Pipeline Steel

Pipeline transportation is an economic,safe and effective mode of transportation for oil and gas.X80 seamless steel pipe body is complete,no weld,especially suitable for oil and natural gas transportation in harsh environment.In order to obtain high quality seamless steel tubes,it is necessary to select the best forming process in the hot working process.In the process of hot deformation,the microstructure is closely related to the performance of the material.Therefore,it is very important to study the microstructure variables of X80 seamless pipeline steel during hot deformation,such as recrystallization volume fraction,dislocation density and grain size.The main work of this paper is as follows:1)The flow stress characteristics of X80 seamless pipe line steel were studied by hot compression test.The true stress-true strain curve of the material was obtained.The critical strain of dynamic recrystallization of the material was determined according to the true stress-true strain curve.At the same time,in order to explore the optimal processing parameters of the material,the hot processing maps under different strains were established.It is concluded that the main softening mechanism of X80 seamless pipeline steel under experimental conditions is dynamic recrystallization.The best hot working process of X80 seamless pipe line steel is in the range of deformation temperature of 1100～ 1200 °C and strain rate of 0.04 ～ 5s-1.2)The effect of different deformation conditions on the grain refinement of X80 seamless tube steel was studied by hot compression experiment.The effects of different heating temperature and holding time on austenite grain growth were studied by heating furnace.Based on the experimental results,a static grain growth model was established.The results show that decreasing the strain rate and increasing the temperature will increase the grain size.3)The unified viscoplastic constitutive model of X80 seamless pipeline steel was established based on the internal variable method.The parameters of the model were obtained by using MATLAB genetic algorithm combined with thermal simulation experiment data.The results show that the unified viscoplastic constitutive model can accurately express the stress-strain relationship and microstructure evolution of X80 seamless pipeline steel during hot deformation.4)The unified viscoplastic constitutive model was embedded into DEFORM software by secondary development,and the finite element model of isothermal compression was established.Comparing the simulation and experimental data errors,the results show that the model can predict the hot deformation behavior and microstructure evolution of X80 seamless pipe line steel during hot compression with high accuracy.