Study On Microstructure And Properties Of Pipeline Steel With Large Deformation Resistance

With the increasing demand for oil and gas,people have been inclined to use long-distance pipelines to transport oil and gas from remote oil and gas fields to cities.However,the long-distance oil and gas transmission pipeline will be damaged to varying degrees by various geographical environments,especially the movement of the crust,landslides and mudslides,which may cause large impact deformation to the pipeline.In order to increase the safety of pipeline operation in such areas,new technology based on strain design has been adopted internationally to allow steel pipes to bear a certain degree of longitudinal strain,so as to ensure the safety of steel pipes after deformation due to geological changes.At present,the development of large deformation resistant pipeline steel based on strain design in China is not mature,the product performance level fluctuates greatly,and the yield is very low.In order to improve the domestic production technology level of this kind of pipeline steel and adapt to the development trend of pipeline steel,it is necessary to carry out technological exploration and theoretical research on the manufacturing technology of large strain pipeline steel plate in the laboratory.In this paper,the austenite recrystallization and transformation kinetics of pipeline steel with two-phase structure are studied by means of thermal simulation experiment and industrial trial production experiment,metallographic microscope,scanning electron microscope,tensile experiment and impact experiment.The composition system and main process parameter control range of X70 and X80 grade pipeline steel with large deformation are determined The microstructure and properties of the steel plate produced by the steel plant were analyzed.The main conclusions are as follows:(1)When the experimental steel simulated the deformation of the final stand of rough rolling at about 1000?,with the increase of deformation temperature,the increase of deformation amount and the decrease of deformation rate,the tendency of dynamic recrystallization of austenite increased during the deformation process;after the deformation,with the extension of holding time,the volume fraction of static recrystallization of austenite increased;Nb can obviously inhibit the recrystallization process of austenite,expanding the deformation temperature range of the non recrystallization zone.(2)The results of CCT curve and phase transformation dynamics of experimental steel show that the increase of Nb content can obviously enlarge bainite phase region and narrow the formation region of polygonal ferrite.Therefore,in order to obtain a certain amount of dual phase structure of ferrite content in anti large deformation pipeline steel,the Nb content in the composition is best controlled below 0.05%.In the process of rolling and cooling,with the decrease of deformation temperature and deformation The volume fraction of polyferrite in the microstructure increases with the increase of the amount,the decrease of the slow cooling rate and the decrease of the open cooling temperature.(3)The results of industrial trial production show that with the decrease of open cooling temperature after slow cooling,the volume fraction of polygonal ferrite in the structure increases,the grain size increases,and the yield strength of the steel plate decreases accordingly;the decrease of red returning temperature during cooling is conducive to obtaining the lath bainite structure,increasing the hardness difference of soft and hard phases in the structure,and improving the tensile strength of the steel plate.(4)For X70 and X80 grade pipeline steel with large deformation resistance,the optimized chemical composition and the control range of main process parameters are proposed.