Mechanical Properties And Corrosion Behavior Of Dual-phase X80 Pipeline Steel

With the increasing status of oil and gas energy,its storage and transportation are more and more important.Compared with traditional X80 pipeline steel,dual-phase X80pipeline steel is gradually popularized due to its high strength,deformation ability,corrosion resistance and low cost.In this paper,the rolled dual-phase X80 pipeline steel is used as research material.By analyzing the metallographic structures and their contents of horizontal and section planes,the morphologies and proportions of the phases for different angles?0°?45°?90°,0°is parallel to rolling direction?are investigated.The relation between the dual-phase's microstructure and the sampling angle is clarified.Through the tensile test,hardness test,slow strain tensile test,polarization experiment and immersion test,the mechanical properties and corrosion behavior of the dual-phase X80 pipeline steel of different sampling angles are studied.The relation between microstructure and performance is proposed.The results are as follows:?1?The microstructure of the rolled dual-phase X is B+F,which is mainly consisted of polygonal ferrite,bainite and a small amount of M/A islands.For the microstructure parallel to the rolling direction,F is equiaxed shape when B is strip-shaped along the rolling direction,the content of which is about 53.6%.For the microstructure perpendicular to the rolling direction,both F and B are flattened strips.With the sampling angle increases,the length of B strip become longer and its content is increased from44.8%to 53.3%so that the microstructure exhibits obvious anisotropy.The ductility of X80 pipeline steel at 0°sampled is best.Its F phase content,hardness,strength and elongation are 55.2%,207.7HV,635MPa,and 24%respectively.The main reason is that the B phase content is increased with the sampling angle.The increased internal dislocation density of F phase reduces the deformation ability thus two-phase coordination ability is lowered.?2?The strength of the dual-phase X80 pipeline steel in simulated seawater and soil solutions is approximately equal to that in the air.Cracks is initiated after the maximum force and no low-stress brittle fracture occurred,which indicating its low SCC sensitivity.The elongation and section shrinkage for 0°sample are 25.35%and 48.6%in the soil solution,which are both higher than those in the seawater.The current density and mass loss of X80 pipeline steel in soil medium are 6.69×10^-4mA/cm² and 5.28×10^-6g/mm²respectively,which are higher than those in seawater medium.Therefore,the corrosion in seawater is more serious.The main reason is that the ion species,content and pH are different from different corrosive media.The high concentration of Cl^-in seawater easily causes pitting.The SO₄^2-and Cl^-ratio in the soil medium is larger than that in the seawater.The adsorption of SO₄^2-is strong,which can effectively hinder the adsorption and penetration of Cl^-so as to the corrosion tendency is slowed down.In addition,the higher pH of the soil medium will lead to thickening of the passivation film on the surface of X80pipeline steel,which resists the penetration of Cl^-and protects the steel.?3?For the slow strain tensile test,the samples of different angles in the same medium are still anisotropic.Compared with that of ordinary rapid tension,the strength is reduced about 20MPa meanwhile the elongation and section shrinkage increased 1%and3%.There are sufficient time for to dislocation to release due to the decrease of tensile rate and the slow accumulation of dislocations.The deformation resistance is declined.The increase of two-phase coordination ability cause ductility improved.?4?Combined the rules between microstructure and performance,the rolled dual-phase X80 pipeline steel could well meet the requirements of large deformation.The ductility of pipeline steel parallel to the rolling direction is the best.Perpendicular to the rolling direction can be selected as curl direction of the pipeline steel to ensure greater deformation ability.Compared with the soil medium,the two-phase X80 pipeline steel is more sensitive to seawater corrosion.The change of elongation is more obvious than that of strength,yet no low-stress brittle fracture occurs.At a result,the two-phase X80pipeline steel has large deformation capacity and excellent corrosion resistance.It can be applied to wider fields of complex terrain and variable environment compared with the original X80 pipeline steel.