| To reduce the cost of long-distance pipelines and improve transmission efficiency,large-diameter high-pressure transmission pipelines are the inevitable trend of pipeline engineering development,so it is necessary to use high-strength pipeline steels.Although pipelines above X100 have been researched,developed and laid test sections,X80 steel has become the preferred grade for high-strength pipeline steel in the world considering the high requirements of welding performance,longitudinal and circumferential crack arrestability,cost performance,safety and other comprehensive factors.The property requirements of X80 pipeline steels are also different due to the different application environments of pipeline engineering.For example,there are great differences in the requirements for their resistance to large deformation and low-temperature toughness in deep-sea areas and extremely cold areas.The service environments of pipeline steels studied in this paper are mainly the remote polar,frozen soil,plateau and other harsh environments.The pipelines will face the brittle fracture problem at extremely cold temperatures.Therefore,higher requirements are put forward for the low-temperature toughness of transmission pipelines.The international theory and technical means of improving the toughness of pipeline steels have been very perfect,and the toughness of existing commercial X80 pipeline steel has been at a high level.Whether we can make a further breakthrough on the basis of the existing theory and technology,and continue to improve the low-temperature toughness and crack arrestability of pipeline steels on the basis of the existing X80 acicular ferrite(AF)pipeline steels with high strength and high toughness is the research focus and difficulty of this paper.Focusing on the composite factors which affect the main properties like low-temperature toughness and the arrestability to cracks,such as material purity,main chemical compositions,components of complex microstructure,grain size and crystallographic orientation,based on the study of dynamic continuous cooling transformation rules of high strength and toughness pipeline steels,the paper regulated the microstructures and phases of complex acicular ferrite pipeline steels through the optimal design of chemical composition and complex microstructure,the factors affecting low-temperature impact toughness and the mechanisms were deeply studied.Based on the in-depth understanding of the relationship among different microstructure characteristics,crystallographic orientation characteristics and the crack arrestability of the matrix,the following results were obtained through the control of the whole process factors affecting toughness and the excavation of theory and technology:By adjusting the main parameters of thermo mechanical controlled processing(TMCP),the component proportion of complex acicular ferrite and the content of high angle grain boundaries(HAGBs)in X80 pipeline steels were controlled,and good low-temperature toughness was obtained,the specific rules are as follows:With the increase of the cooling rate,the volume fraction of polygonal ferrite(PF)or quasi-polygonal ferrite(QF)decreased but that of the bainitic ferrite(BF)increased.With the increase of the finishing rolling temperature,the grains were coarsened but the proportions of the components in AF were basically unchanged.When the finishing cooling temperature was raised to 550℃,a severely coarsened microstructure emerged and a large number of MA(Martensite-Austenite)islands with a sharp-angled shape which were harmful to the low-temperature toughness appeared.When the relaxed time was prolonged,both the grain size and the volume fraction of polygonal ferrite were gradually increased.According to the research results of the thermal simulation test and industrial test,a microstructure composed of fine QF,granular bainitic ferrite(GB)and a small amount of BF(QF+GB accounted for more than 90%)was designed for the X80 pipeline steel with high toughness for low-temperature application.In this microstructure,the content of HAGBs was more than 50%.Parameters for the industrial TMCP such as finishing rolling temperature,finishing cooling temperature and cooling rate were optimized to be 750℃,480℃and 20℃/s,respectively.By adjusting the contents of the key alloying elements Nb and Mo,the influence rules of different Nb and Mo contents on the phase transformations were studied.Under the same TMCP process,the relationship between the difference of mechanical properties and microstructure characteristics of the tested steels with different Nb and Mo contents was studied.The results indicated that increasing the Mo content was conducive to the refinement of microstructure and the formation of acicular ferrite,and the dynamic CCT curve moved to the lower right.With the decrease of the Nb content,the difficulty of acicular ferrite formation was increased,the grains were coarsened,and the dynamic CCT curve moved to the upper left.Under the same TMCP process,compared with the 0.2%Mo steel,the 0.3%Mo steel had higher strength due to the increases of BF volume fraction,and MA island size and number density.However,the increases of the size,volume fraction and shape irregularity of MA islands also led to the decrease of low-temperature impact energy of the 0.3%Mo steel.Under the same TMCP process,compared with the 0.08%Nb steel,the increase of grain size of the 0.04%Nb steel led to the decrease of strength,and its low-temperature impact energy also decreased due to the decrease of HAGB content.The relationship among crack propagation,crack arrestability of microstructure and microstructure characteristics at low temperatures was investigated,and the relationship among crystallographic orientation characteristics,fracture behavior and low-temperature impact toughness was also established.The results showed that HAGBs could make the cracks deviate greatly from the original direction.The AF microstructures had stronger arrestability to cracks because of their high density of HAGBs,which was reflected from more tortuous crack propagation paths and smaller cleavage fracture units on the fracture surface.Therefore,they had excellent impact toughness.Further study showed that crystallographic orientation characteristics had a great influence on the toughness of materials.The maximum content of {001} cleavage planes parallel to the fracture surface in the air-cooled steel after rolling also led to the highest DBTT(Ductile-brittle transition temperature).Compared with the steel directly and rapidly cooled to room temperature after rolling,the steel with air cooling+fast cooling+air cooling after rolling had more {100} cleavage planes parallel to the surface of the V-notch,resulting in larger and more secondary cracks which could significantly alleviate the stress concentration,it also exhibited higher intensities of the {332}<113>component.Therefore,its impact absorbed energy was higher.Moreover,a modified equation was used to quantitatively predict the DBTT of PF microstructure,however,it could be only used for an approximate prediction for AF due to its complex microstructure.It was found that even in ultra-cleaned steel,a small amount of rare earth(RE)elements had an effect on the microstructure and properties of pipeline steels.In the ultra-cleaned steel with O and S contents less than 10 ppm,the addition of RE would still cause an increase in the volume fraction of inclusions consisting of complicated RE oxysulfide and RE sulfide.More inclusions formed in the 112 ppm RE steel were harmful to the low-temperature toughness while few inclusions formed in the 47 ppm RE steel had almost no influence on the low-temperature toughness.Through comprehensive analysis of the effects of rare earth and TMCP on inclusions and microstructure,it was found that the low-temperature toughness of ultra-cleaned pipeline steel containing rare earth was controlled by the content of inclusions and HAGBs.For the steel with 112 ppm addition of rare earth,as the finishing cooling temperature was increased from 481℃ to 584℃ and the cooling rate was reduced from 20℃/s to 13℃/s,there was an obvious decrease in the low-temperature impact energy.The reduced impact energy(about 33 J)of the steel came from two parts including the influence of more inclusions formed due to the 112 ppm addition of RE(about 18 J)and the effect of the lower content of HAGBs(about 15 J). |
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