Research On Continuous Cooling Transformation Behavior Of Supercooled Austenite Of X80Pipeline Steel

With the rapid development of economy, the demand for oil and natural gas hasincreased dramatically in our country.How to transport oil and natural gas of remote areaslong-distance conveying to the eastern coastal areas where are developed has become anurgent problem.Therefore, the high strength and high toughness pipeline steel is the key tosolve the problem.X80pipeline steel is a high strength and high toughness micro-alloyed steel, the basiccharacteristics are high cleanliness, high homogeneity and ultra-fine grain microstructure.It can be obtained high strength and high toughness in the relatively economic processingconditions.Under the condition of certain purity and micro-alloyed steel, the control ofmicrostructure is the key to obtain high strength and high toughness.Super-cooled austenitecontinuous cooling transformation (CCT) curves can be systematically described thevariation of phase transition points,the speed of phase,the change of microstructure, theaustenite transformation process in the course of continuous cooling transformationand the structure and property of its product were analyzied by CCT curves.According tothe continuous cooling transformation curve,we can choose the most appropriate processspecification to obtain needed microstructure,thus improving the strength and plasticityand preventing the formation of welding cracks, etc.In this paper,the phase transition behavior in the process of continuous coolingtransformation of X80pipeline steel has been studied,and the influence of Rare Earth(RE)to the CCT curves of X80pipeline steel has also been studied.The CCT curves weredrawn by measuring thermal expansion curves of tested steels under different cooling ratesby RITA quenching thermal expansion instrument,combining with metallographicmethod.The microstructure of different cooling rates were observed by OpticalMicroscopy.The micro-hardness of tested steels under different cooling rates weremeasured by Vickers.By Transmission Electron Microscopy (TEM) and Scanning ElectronMicroscopy (SEM),the fine structure of microstructure under different cooling rates were studied. The influence of adding RE to X80pipeline steel on the CCT curve andmicrostructure hardness at the same cooling rate.On this basis,the continuous coolingtransformation model of X80pipeline steel was established.The results showed that:The variation of microstructure of X80pipeline steel, in thecontinuous cooling transformation process, was ferrite, pearlite, bainite and martensite.Thetested steel with RE had no pearlite transformation.The addition of RE had a certainrefinement on the microstructure of bainite and martensite.The phase mode of cooling rates(from0.08℃/s to10℃/s)to phase transition temperature was established by using ORIGINsoftware.The fitting degree of the simulation results with the tested value was very high,basically reflecting the trend of tested value, having a very high regression precision.