Study On Recrystallization Behavior And Microstructural Evolution Of X70 Pipeline Steel

Pipeline transportation is an economic,safe and effective way for oil and gas transportation.Recently,with the increased development of pipeline transportation technology,the development and research of pipeline steel has been promoted.X70 pipeline steel is a kind of controlled rolling and cooling,low-carbon high-strength micro-alloy steel.Due to its high strength,toughness and corrosion resistance,X70 pipeline steel has been widely used in the pipeline transportation.In recent years,pipeline steel has developed rapidly at home and abroad,the demand has increased substantially,and the requirement for pipeline steel has become higher and higher.In order to obtain the desired shape and size,the best forming technology needs to be selected during the process.During the process of thermal deformation,recrystallization can be used to refine the grain and improve the mechanical properties of the hot-rolled products.Therefore,it is very important to study on the recrystallization behavior and microstructure evolution of X70 pipeline steel during the hot rolling process.Based on the experiments,a series of high temperature deformation simulation experiments of X70 pipeline steel were carried out by Gleeble-1500 thermal-mechanical simulator.The mechanical properties and microstructure evolution of the material during the recrystallization and grain growth were analyzed.And the kinetics equation of recrystallization and grain growth for X70 pipeline steel were established to direct the development of the actual production process.The main contents are as following:Through the metallographic experimental method,the austenite grain evolution behavior of low carbon micro-alloyed X70 pipeline steel under different heating temperatures and holding times was studied,and Beck,Hillert and Sellars model were established.The results show that the austenite grain size increases when the temperature and the time increase,and the grain coarsens obviously above 1150°C.Through comparing and analyzing of the three models,Sellars model has a higher accuracy to predict X70 austenite grain growth behavior.Based on the experimental results of grain growth,the single pass compression experiments of X70 steel were carried out.The effects of different hot processing parameters on dynamic recrystallization behavior were analyzed,and dynamic recrystallization kinetics and recrystallization grain size model were developed.The results show that with the increase of strain rate and deformation temperature,the grain size of dynamic recrystallization become smaller.The dynamic recrystallization model obtained was suitable for describing the recrystallization behavior and grain evolution behavior of the X70 steel.Based on the equations,the hot processing maps were established.It is concluded that the most obvious feature of X70 steel was that it was not suitable for processing at low temperature(<1027?).According to the results of dynamic recrystallization,the two pass hot compression experiments of X70 steel were carried out,and the static recrystallization and metadynamic recrystallization were studied.The effects of the temperature,pre-strain and strain rate during the experiment on the grain size of static recrystallization were significant,while the initial grain size made little influence on the recrystallization behavior.The metadynamic recrystallization fraction and grain size increased with the increasing of temperature and deformation rate.The kinetics and microstructure evolution equations of static recrystallization and metadynamic recrystallization of X70 steel were established.The prediction results were in good agreement with the experimental ones.