Research On The Flexible Rolling Technology Of X90 Pipeline Steel

Since the 12 th Five-Year Plan was implemented,China had adjusted the energy strategy so that the oil and gas industry has been developping at top speed these years.In the West-to-East Gas Pipeline Project,China has mastered the technology of X70 and X80 pipeline steel,for further high efficiency,it is necessary to upgrad the grade of pipeline steel.So further experimental research of X90 pipeline steel is particularly meaningful.In this paper,X90 pipeline steel is chosen as the object of the research and this article starts from the thought of optimization in rolling with the series of products about the Flexible Rolling Technology.The solid solution experiment has been done under the condition of different heating temperatures and holding time,and the change law of microstructure about the tested steel has been discovered through the metallographic observation experiment.The rule of continuous cooling transformation process and microstructures during this process is investigated through thermal simulation experiment and metallographic observation experiment.On that basis,the influence of different rolling process parameters on microstructures of tested steel is studyed by the thermal simulation experiment.The main results are as follows:1.The results show that there is a gradual growing trend of X90 pipeline steel austenite grain with heating temperature and holding time increasing in the heating process.A mathematical austenite grain growth model for X90 pipeline steel based on linear-regression is established by means of simulation and calculation and it is:Finally,in industrial pipeline steel production process,heating temperature should be limited within 1140~1200 ℃ and holding time should be chosen at about 30~40 min for a comprehensive consideration of micro-alloying fully solution and fine uniform original austenite grain.2.The continuous cooling experiment results indicate that, under the condition of un-deformation,there is mainly Granular Bainite transformation when the cooling rate is between 5~15 ℃ /s while the microstructure is mianly Bainite Ferrite when the speed is higher than 20 ℃/s.Deformation makes CCT curve move to the left upper side and expands the phase transformation range,at the same time,deformation improves the beginning temperature of transformation and also refines the final microstructure.Under the condition of deformation,Granular Bainite transformation could occur at the cooling rate of 3 ℃/s,and it proves that deformation promotes the Acicular Ferrite transformation.As the cooling rate is accelerated,the content of M-A island group is increasing and the shape turns to acicular or film from granular.Summarizing the above results,X90 pipeline steel will have high thoughness Acicular Ferrite containing ideal tissue fine M-A island group by not less than 20 ℃/s in the cooling process after controlled rolling.3.The experiment turns out that higher roughing rolling temperature is not conductive to microstructure refinement.And reduce of the finish rolling temperature is helpful to the transformation of uniform fine Granular Bainite.Increasing of finish rolling deformation benefits the microstructure refinement and also facilitates the Acicular Ferrite transformation,however,oversize of increasing would improve the transformation temperature leading to high temperature phase product which is not expected.And the range of 350~400 ℃ about final cooling temperature is the best condition for typical Granular Bainite transformation occurrence,besides,reduce of final cooling temperature is a useful way to refine the microstructure.4.According to the experimental results,optimizated rolling parameters of X90 pipeline steel in industrial production are observed as follows:Austenitizing temperature is limited between 1140~1200 ℃ holding 30~40min.Roughing rolling is chosen at austenitic recrystallization zone with deformation of 25% at the temperature of 1000~1050 ℃.Finish rooling is chosen at austenitic non-recrystallization zone with deformation of 30~40% at the temperature of800~830 ℃.Final colling temperature should be within the range of 300~350 ℃ and cooling rate should be not less than 20~25 ℃/s after controlled rolling.