Study Of Mechanical Properties And Resistance To Acid On The Steels Of Transportation And Storage For Oil And Gas

With the rapid development of world economy, energy demand is increasing for human. Therefore, steel demand is increasing for transportation and storage of oil and gas resources. At present, petroleum and natural gas were discovered and exploited in some remote areas, such as desert, polar, marine and so on. And most of oil and gas resources transported and stored contain some corrosive substances, such as H2 S, CO2 and so on. Therefore, pipeline steel and container steel should not only have high strength and toughness, but also excellent resistance to hydrogen induced cracking(HIC) and sulfide stress corrosion cracking(SSCC) will be required. Steel plates with high grade and big thickness were developed to satisfied to transportation and storage of oil and gas resources with efficiently, economically, safely. The uniformity of microstructure is difficult to be controlled for large thickness steel plate in the thickness direction, which affect strong toughness, resistance to HIC and SSCC of steel plates. Therefore, the study of effection for the organization and the inclusion of large thickness steel plate on its mechanical properties and resistance to acid is crucial.In this paper, Large thickness plates, such as X80 M pipeline steel plates with large strain, X80 M pipeline steel plates, deep sea X65 pipeline steel plates and acid container plates SA516Gr60N- HIC normalized were main materials studied. The affection of microstructure and inclusion of steel plates in the thickness direction on tensile property, Charpy impact toughness, Drop weight tear toughness and resistance of acid were studied by means of scanning electron microscopy, metallographic microscope, energy disperse spectroscopy. The results of the study are as follows:Mechanical properties of X80 M pipeline steel plates strain-based with 26.4mm thickness were studied. The results show that, X80 M pipeline steel plates strain-based is double phase microstructure of ferrite and bainite. The average ferrite volume fraction is about 60% for the whole thickness of plates, and ferrite grain size is, on average, about 5 microns in diameter. Comparative study of microstructure wascarried out for whole thickness of steel plates, which shows that the microstructure of head, middle and tail steel plate of the same rolled piece is slightly difference, and the mechanical properties of the various parts, such as strength, deformation properties,toughness at low-temperature, shape of stress-strain curve and so on, can satisfy the requirements of the standard performance and has excellent uniformity.Mechanical properties of X80 M pipeline steel plates with 33 mm thickness were studied. The results show that, microstructure of X80 M pipeline steel plates is made of bainite, quasi-polygonal ferrite, M/A island, degradation of pearlite. All kinds of mechanical properties of the steel plates with the acicular ferrite microstructure meet the requirements of standards on mechanical properties when average volume fraction of lath bainite phase less than 10%.The study of resistance of acid for deep-sea X65 pipeline steel plates shows that grain boundary, phase boundary and rigid nonmetallic inclusions are hydrogen trapping sites in steel. Size, morphology and type of the inclusions affect HIC sensitivity of the pipeline steel. HIC is not liable to form in steel with the small size of spherical inclusions. However, the steel with a microstructure consisting of the long strip and sharp corners shape inclusions is more sensitive to HIC. HIC can also propagate along the grain boundary or through the grain boundary. And rigid bainite microstructure is sensitive to HIC.Resistance to acid of normalized container steel plates SA516Gr60N-HIC were studied. The results shows that, the microstructure of normalized SA516Gr60N-HIC container steel plates is ferrite and a small amount of pearlite, and both type and level of inclusions can satisfy the requirements of the standard, which ensure that normalized container steel plates SA516Gr60N-HIC have good resistance to HIC and SSCC. Through comparing study of microstructure-property between pipeline steel plates and container steel plates, pipeline steel plates with ferrite of volume fraction from 30% to 65% have good strength, toughness and resistance to acid. When the volume fraction of ferrite is more than 90%, normalized SA516Gr60N-HIC container steel plates have excellent resistance to HIC and SSCC.