| Along with the development of oil and gas pipelines,more and more rigorous requirements are put forward on the pipeline steel,it is still at the beginning on oil and gas production storage and transportation in China,developing pipeline steel resisting oil and gas transmission corrosion is the urgent need.In this paper,the microstructure,mechanical properties and corrosion properties of H2S stress corrosion resistant pipeline steel X70MS?X80MS and high Mn acid resistant pipeline steel are studied.The main work and research results of this thesis as follows:(1)The X70MS and X80MS are designed with a composition system of low c and low Mn,adding anti-acid corrosion alloy elements,and controlling the low S content.The microstructure evolution of X70MS and X80MS during cooling was studied by expansion ·method and metallographic observation,and the continuous cooling transition curve wasestablished.The results show that the metallographic structure with acicular ferrite as the main component can be obtained when the cooling rate of X70MS is 10?20?/s and the cooling rate of X80MS is 10?25?/s under undeformed condition.Under the condition of deformation,the microstructure obtained when the cooling rate of X70MS is 15?25?/s,and the cooling rate of X80MS is 15?35?/s is mainly acicular ferrite,with fine and uniform grains.As the cooling rate increases,the phase transition point decreases and the phase transition regionnarrows.When the cooling rate is constant,with the increase of deformation,the ferrite content increases,the bainite content decreases,and the grain size decreases.this trend decreases with the increase of cooling rate.With the increase of cooling rate,the microstructure changes from eutectoid ferrite and pearlite to acicular ferrite and bainite.The content of ferrite and pearlite decreases with the increase of cooling rate,while the content of acicular ferrite and bainite increases and the grain size decreases.(2)Through the research on the control process of the organization and properties the preparation process route and process parameters of X70MS and X80MS are determined.The research shows that X70MS has fine and uniform acicular ferrite microstructure when the final rolling temperature is 790?810?,the final cooling temperature is 500?550? and the cooling speed is 20?25?/s,and X80MS has good comprehensive mechanical properties when the final rolling temperature is 790?810?,the final cooling temperature is 450?500? and the cooling speed is 25?30 ?/s.Its strengthening mechanism mainly includes fine grain strengthening,dislocation strengthening,precipitation strengthening and phase transformation strengthening.In a certain range,as the final cooling temperature decreases and the cooling speed increases rapidly,the grains of X70MS and X80MS are refined,the proportion of acicular ferrite in the microstructure increases,and the strength and low temperature toughness increase.However,when the cooling rate is too fast,a large number of lath bainite and M-A components appear in the microstructure,and the toughness at low temperature obviously decreases.(3)Through the research of electrochemical corrosion,hydrogen induced cracking,stress corrosion cracking and high temperature and high pressure H2S/CO2 corrosion behaviors,the damage mechanism of experimental steel in acidic environment was analyzed.The results show that acicular ferrite has smaller grain size,smaller active area,slow corrosion rate and good electrochemical corrosion resistance.All samples of X70MS and X80MS meet the specified requirements of EFC-16 and have good HIC and SSCC resistance.The decreasing order of SSCC resistance of different microstructure types was AF>B>F+P.The main components of inclusions in cracks are oxides of Al,Si,and Mg.Under high temperature and high pressure H2S/CO2 environment,the initial corrosion rate of X80MS slowed down with the increase of time,and the corrosion membrane structure gradually became compact and complete.The main corrosion products are tetragonal pyrite and ferrous sulfide.(4)The corrosion mechanism and influencing factors of X70MS and X80MS in H2S environment were systematically analyzed.Hydrogen generated by the corrosion reaction continuously enters the matrix,accumulates at the hydrogen trap and causes hydrogen pressure,and finally forms hydrogen-induced cracks at the hydrogen trap.Uniform dispersion of reversible hydrogen traps can reduce the number of irreversible hydrogen traps such as phase boundaries,inclusions and grain boundaries,which is beneficial to improve the anti-H2S corrosion ability.HIC crack sensitivity of ferrite+pearlite structure is higher.The higher the proportion of acicular ferrite in acicular ferrite+polygonal ferrite structure,the lower the crack sensitivity.The crack sensitivity of HIC increases with the increase of grain size.The thermodynamically balanced and stable fine grain structure is the ideal structure to resist hic and sscc.The anti-H2S corrosion ability of pipeline steel can be improved by reducing the content of S and P,lowering the final cooling temperature,increasing the cooling speed,improving the uniformity of composition and structure,refining crystal grains,reducing the grade of banded structure,spheroidizing inclusions,making the distribution of M-A components more uniform,smaller in size,and tending to spheroidize.(5)Industrial trial-manufacture of X70MS was carried out on the production line of a steel plant in China,and the mechanical properties and corrosion properties of X70MS pipeline steel were tested.the microstructure mainly consisted of polygonal ferrite and acicular ferrite,and the mechanical properties reached the requirements of API Spec 5L for X70 pipeline steel,which showed good HIC and SSCC resistance.(6)The welding structure,mechanical properties and H2S corrosion resistance of experimental steel prepared by different controlled rolling and controlled cooling processes were systematically studied,and the hydrogen-induced cracking and stress corrosion cracking corrosion mechanism of the welding zone of corrosion resistant steel were revealed.The research shows that the weld zone is dominated by fine and uniform acicular ferrite,and the heat affected zone is mainly granular bainite structure.The yield strength and yield strength ratio after welding are higher than those of the base metal,the elongation rate and low temperature toughness are lower than those of the base metal.Reducing original austenite grain size in the coarse grain region and improving the morphology of the M-A component are beneficial to improving the low temperature toughness of the coarse grain region.The crack sensitivity rate,crack length rate and crack thickness rate of all sample welded joints meet the requirements of API spec 5L for pipeline steel under acidic service conditions.Hydrogen bubbling and HIC cracks were not found on the surface of the weld seam.Hydrogen bubbling and hydrogen-induced cracks appear in the heat affected zone,and the crack propagation mode is mainly transgranular cracking.SSCC test results are good,and the heat affected zone is a weak corrosion zone.It is beneficial to improve the anti-H2S corrosion performance of welded joints to refine the grains in the HAZs and reduce the proportion of M-A components.(7)The chemical composition of the anti-H2S high-manganese pipeline steel was designed by using the composition system of high C and high Mn.The preparation process route andprocess parameters were determined through the microstructure and property control technology.the electrochemical,HIC and SSCC corrosion behaviors of the experimental steel were also studied.The research shows that the microstructure of the experimental steel is single-phase austenite with a large number of dislocations,stacking faults and twins.The mechanical properties meet the requirements of API spec 5L for X70 and X80 pipeline steel,with extremely low yield ratio and excellent plastic deformation capability.HIC experimental results show that the crack sensitivity,crack length and crack thickness of the sample are all zero,and the samples have good hydrogen-induced cracking resistance.The results of SSCC experiment showed that all samples were not fractured and no hydrogen-induced crack was found in the working section.Because high manganese pipeline steel has extremely high hydrogen dissolving capacity,dislocations move with hydrogen clusters,and hydrogen is not enriched in inclusions,it has good H2S corrosion resistance. |
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