Development Of Acid-resistant Steel Q80S For Armor Layer Of Oceanic Flexible Pipe

In recent years,with the rapid development of China's economy,the country's demand for energy has increased.Under the circumstances,the exploitation and use of oil and gas on land has increased,and resources have been depleted.This has led to the exploitation of oil and gas from land to sea,and from shallow sea to deep sea.The pipelines for transporting oil and gas are mostly hard tubes of carbon steel,and the drawbacks of carbon steel tubes continue to emerge with the extension of time.For example,there are many welded joints of hard tubes,and the risk of corrosion failure and fatigue failure is easy to occur at the weld.The marine flexible hose can avoid these defects well and can be laid for thousands of meters without the welded joints,making it the first choice in the modern oil and gas industry.The tensile armor layer among the multi-layer structure of the marine hose is the most important structure and has the effect of resisting tensile stress.In China,the production of armor layer materials started later than foreign countries,and the strength of the produced materials is relatively low.Therefore,it is urgent to develop a material with high strength to achieve weight reduction of the hose and improve the softness of the hose.It also ensures that the material is insensitive to corrosion and fatigue,while guaranteeing intensity.In this paper,the production process of armor layer steel for oceanic flexible pipe is carried out.After composition design of alloy,smelting,casting,hot rolling,cold working and heat treatment,we need to obtain the high-strength steel with the mechanical properties of ReL?600MPa,Rm?700MPa,A?5%.Under the premise of satisfying the mechanical properties,the material also needs to meet the resistance to hydrogen induced cracking(HIC),sulfide stress corrosion cracking(SSCC),hydrogen embrittlement(HE),seawater corrosion and fatigue fracture.The main research work and results of this thesis are as follows:(1)According to the low carbon and low manganese alloying system,the composition ratio of Q80S experimental steel is designed.The phase transition point of the experimental steel was measured by phase change instrument during the heating and cooling process,and draw the static and dynamic CCT curves.The results show that under the condition of undeformed,the addition of Cr promotes the ferrite transformation process of the experimental steel at a lower cooling rate.Under the deformation condition,the austenite distortion can be increased and promotes the transformation of austenite to ferrite,increases the ferrite phase transition temperature and enhances the stability of the ferrite phase.(2)The laboratory rolling and heat treatment process research on armor layer steel of oceanic flexible pipe was carried out to obtain the best heat treatment process parameters.The results show that the hot rolling is selected in the recrystallized zone,the heating temperature is 1200?,the final rolling temperature is 955?,and the final cooling temperature is 345?.The yield strength of the experimental steel is lower,the elongation is higher,and the microstructure is composed of ferrite,bainite and a small amount of pearlite,meets the requirements of the cold rolling process;In the quenching and tempering heat treatment process,the high-strength steel is quenched at 900? for 30min and tempered at 650? for 30min to obtain martensite structure,and the comprehensive mechanical properties are optimal.(3)According to the requirements of the standard,HIC,SSCC and HE experiments were carried out on the experimental steel after heat treatment(warm rolling,quenching and tempering and annealing),and the behavior of the experimental steel with different processes was analyzed under different corrosion conditions.The results show that the experimental steel after quenching,annealing and warm rolling treatment has good resistance to hydrogen induced cracking corrosion and hydrogen embrittlement resistance.Quenching and tempering(quenching 900? for 30min+tempering 650? for 30min)process steel in the sulfide stress corrosion test meets the standard requirements,with good resistance to hydrogen induced corrosion fracture.(4)Using the full immersion method to simulate the seawater corrosion effect of experimental steel in different periods,and carry out research on its corrosion behavior and mechanism.The results show that the whole process of corrosion consists of three stages:rapid corrosion,gentle corrosion and stable corrosion.The corrosion rate of the experimental steel decreases with time and eventually stabilizes.The corrosion product formed during the corrosion process is Fe3O4,and Fe3O4 is a stable phase to reduce the degree of corrosion.The content of Cr on the surface of the substrate is more,and the distribution on the rust layer is relatively uniform.(5)The fatigue fracture properties of the experimental steel were studied by pull-pull high-frequency fatigue test.The microstructure,crack orientation and propagation law of fatigue fracture were analyzed by metallographic microscope,scanning electron microscope,electron probe and EBSD technique.The results show that the fatigue crack is initiated at the maximum stress concentration on the surface of the specimen,and the region has a fan-shaped expansion trajectory.There are a large number of secondary cracks in the crack propagation region,which can reduce the fatigue crack diffusion rate.The fatigue crack in the transient region is characterized by ductile fracture,and has good plasticity and toughness.Under the interaction of ferrite with high angle grain boundaries and bainite with high strength,the expansion of fatigue cracks is limited.In a tempered martensite structure in which the grain orientation has a diversity of diffuse distribution characteristics,the crack propagation is also limited by the uniformity of the microstructure.