Effect Of Heat Treatment On Microstructure And Properties Of X70Q Steel

At present,oil and gas resources are indispensable to industrial manufacturing and daily life.Pipeline transportation,which is low-cost,convenient,and widely used,has become the main transportation mode of oil and gas resources.Under the premise of increasingly scarce resources,in addition to pipeline steel widely used on land,it is also urgent to develop pipeline steel for alpine,deep sea,desert,and other environments.At present,pipes and accessories are mainly connected with flanges.To improve the performance,welding of flanges and pipes with the same material is an ideal way of connection.In this paper,the effect of heat treatment on the microstructure and properties of X70Q pipeline steel used as pipe or flange is studied to improve its performance.The critical phase transition temperature of X70Q steel was measured by DSC,and optical metallography microscope（OM）was used.Scanning electron microscope（SEM）,X-ray（XRD）,hardness tester,tensile tester,impact tester,electrochemical workstation,immersion corrosion test,and salt spray corrosion analysis test means such as heat treatment was studied in X70Q steel microstructure,dynamic,and corrosion resistance of the law and mechanism of the influence of heat treatment process was optimized.The results show that:The microstructure of X70Q steel is composed of polygonal ferrite and granular bainite,the hardness is HBW190,the tensile strength is 589MPa,the yield strength is 526MPa,the elongation after fracture is 19.6%,the area reduction is 29.7%,and the impact toughness is 243J/cm2.Austenite transformation takes place in the temperature range of680℃～740℃.After quenching at 920℃for 30min and tempering at 200℃～680℃for 2h,with the increase of tempering temperature,the quenched microstructure of X70Q steel is transformed into tempered martensite,tempered trostite and tempered sorbitite respectively.With the increase of quenching temperature,the hardness increases and the amount of ferrite in the structure increases gradually.When the quenching temperature exceeds 840℃,the quenched microstructure of X70Q steel is mainly low carbon martensite.With the increase of quenching temperature,the grain grade decreases gradually,and the hardness changes are not obvious.When the quenching temperature is 920℃,the tensile properties of X70Q steel reach the highest.After holding at 920℃for 10min to 60min,the grain size of X70Q steel decreased with the extension of holding time,and the martensite lat in the microstructure gradually tended to be uniform,and the ferrite content gradually increased.When holding time was 30min,the microstructure of X70Q steel was mainly lat martensite,the hardness was HRC39.5,the grain size was 8.7.After tempering at 200℃,its tensile strength and yield strength are 1359MPa and1289MPa respectively,showing the best comprehensive performance.After tempering for 2 hours in the range of 200℃～680℃,with the increase of tempering temperature,the quenched microstructure of X70Q steel is transformed into tempered martensite,tempered troosite and tempered sorbsite respectively.Its plasticity and toughness gradually increase,strength and hardness gradually decrease.After tempering at 300℃,it shows temper brittleness,and after tempering at 600℃,the comprehensive performance is the best.There is little difference between the electrochemical properties of X70Q steel in brine between the original and heat treated steels.The corrosion resistance of X70Q steel is the best after tempering at 650℃,while the corrosion resistance of X70Q steel is the worst after tempering at 400℃.Based on the above results,the optimal heat treatment process is as follows:heating and holding at 920℃for 30min,water quenching and then tempering at 600℃.The simulated seawater immersion experiment shows that comprehensive corrosion occurs at the beginning of immersion,and the surface corrosion gradually worsens with the extension of immersion time.The dense inner rust layer covers the surface of the matrix,effectively preventing further corrosion.The corrosion rate of X70Q steel decreases from0.334mm/a and 0.401mm/a after soaking for 1 week to 0.042mm/a and 0.041mm/a after soaking for 2 months.After corrosion in neutral salt spray,with the extension of corrosion time,the corrosion rates of X70Q steel in the original state and the best heat treatment state show an initial increase trend,and the corrosion rates begin to decrease after 48h,and reach a stable value of 0.016mm/a and 0.15mm/a after 480h.In the corrosion experiment,the surface corrosion products of X70Q steel in different states were the same,which were all composed of fibrousγ-Fe OOH,α-Fe OOH,Cr OOH and Fe（OH）₃.