Investigation On Microstructure Controlling And Fracture Behavior Of High-thickness X80 Pipeline Steel Under Ultra Fast Cooling

In order to meet the requirements of large diameter、high pressure transmission and security of pipeline transport,high-thickness,high toughness pipeline steel is the major development direction,and X80 pipeline steel will be one of the first choice for gas transmission pipeline in a fairly long period of time.But the relationship between microstructure and properties of high-thick X80 pipeline steel lacks the system characterization results,especially extreme specification X80 pipeline steel under the ultra fast cooling.In addition,the relationship between the fracture behavior and microstructure of high-thickness X80 pipeline steel is not very clear.This dissertation studies on the microstructure transformation characteristics of X80 pipeline steel,controling and system characterization the microstructure of the high-thickness X80 pipeline steel,and research on the relationship between the fracture behavior and microstructure,the main work and the research results are as follows:(1)The curves of static continuous cooling transformation were established by the application of thermal dilation method and metallographic analysis,and the microstructure transformation characteristics of pipeline steel under different isothermal condition was studied by thermal simulation experiment.The results show that this composition system pipeline steel can forme acicular ferrite and granular bainite microstructure in a wide scale of cooling rate.Acicular ferrite nucleate by shear mechanism,after the end of acicular ferrite transformation,carbon-rich austenite between the acicular ferrite transforms into polygonal ferrite.(2)The thermal simulation experiment is used to study the influence of different process parameters on the microstructure evolution and the properties of pipeline steel.Results show that with the increase of ultra fast cooling temperature,the content of acicular ferrite increases at first and then decreases,the Vikers hardness of spicimens decreases;the increasing of cooling rate makes the starting phase transformation tempera-ture reduce,and is benefit to the formation of acicular ferrite.Defects such as dislo-cation increases with the increasing of the deformation of austenitic non-recrystallizati-on area,and is benefit to the formation of acicular ferrite.(3)To get three different microstructure with deferent acicular ferrite percentage,and summarise the optimum microstructure of high-thickness X80 pipeline steel,hot rolling experiments are used at laboratory.Results show that the microstructure of acicular ferrite with a small amount of granular bainite is uniform along the thickness direction,acicular ferrite grain size is small and large angle grain boundary content is higher,has a large number of movable dislocation,the test steel’s yield strength is 576 MPa,the tensile strength is 720 MPa,the yield rato is 0.79,the elongation is 24.35%,and the impact absorbing energy under-20℃ is 299.5 J,fracture appearance transition temperature is about-60℃,the test steel has well match of strength and toughness.(4)A preliminary explore of the microstructure and performance of large deformation of I stage + ultra fast cooling +hot coiling process test steel.by laboratory hot rolling experiments.Results show that the acicular ferrite content of I stage large deformation + ultra fast cooling + hot coiling process test steel is 57%,fine polygonal ferrite content is 16%.The substructure of acicular ferrite is lath substructure,the width of the lath is uniform and has a high density of dislocation tangles,M/A islands are dispersed distribution.A large number of precipitate with the size of about 10nm diffuse distribution,makes up the decline in the strength due to more polygons ferrite,and has higher elongation at the same time.The test steel’s yield strength is 565 MPa,tensile strength is 702 MPa,the yield rato is 0.79,the elongation is 27.7%,the performance meet the requirements of API X80 pipeline steel.(5)A omparing of the influence on fracture behavior of three different microstructure with deferent percentage content of acicular ferrite through the Charpy v-notch impact test and the observation of the crack propagation path.Results show that the test steel with microstructure of acicular ferrite + a small amount of granular banite,the loss of the toughness with test temperature decreasing is slow,crack formation energy is not sensitive to the change of the test temperature.The crack extends serratedly in acicular ferrite microstructure,appears a sharp turn at the area with a large amount of little axiohitic MlA islands.The test steel with microstructure of acicular ferrite + granular banite + polygonal ferrite,the loss of the toughness with test temperature decreasing is dramatic,mainly reflect in the sharp decreasing in the crack propagation energy.The crack deflects at the grain boundary through polygonal ferrite.The test steel with microstructure of acicular ferrite + granular banite + polygonal ferrite + a little amount of degenerated perlite,brittle fracture tendency is bigger,crack initiation energy and extension energy are all fell sharply with the decreasing of temperature,the specimen easily crazes along the degenerated pearlite.