The Study On The Influence Law Of Tissue And Performance Of Controlled Cooling Process For V140 Oil Casing

With the further development of oil and gas development,oil and gas wells are facing more and more complex geological conditions,to play a key role in the well of oil casing is becoming more and more special and demanding,the current API steel grade oil casing properties such as strength and toughness index has been difficult to meet the demand.Therefore,it is very important to develop the technology of producing non-API steel grade oil casing with high strength and toughness.The on-line controlled cooling technology of hot rolled seamless steel tube is beneficial to realize the regulation of primary microstructure and improve the strength and toughness of casing.In this paper,combined with the hot rolled seamless steel tube control cooling system development project carried out in cooperation with Baosteel,the composition design,continuous cooling phase change behavior research,thermal deformation behavior research and laboratory rolling and heat treatment experiment of V140 oil casing steel were conducted to study the influence of controlled cooling process on the microstructure and performance law.The main research contents and results of this paper are as follows:(1)Based on Cr-Mo alloy,low C and Mn elements are adopted to control the content of P and S.Meanwhile,alloy elements such as Nb and V are added to design the composition.According to the static phase-change experimental results,under the condition of static continuous cooling transformation,the microstructure of 08Nb experimental steel is composed of F+B at a cooling rate of 0.1?/s.When the cooling rate was 0.5?2?/s,all the tissues were B.When the cooling rate is greater than 5?/s,martensitic transformation begins to occur in the tissues,and the microstructure is composed of B+M.When the cooling rate exceeds 20?/s,the microstructure consists of M.(2)The dynamic CCT curve of 08Nb experimental steel was obtained by means of metallographic observation and full-automatic phase-change meter experiment.After thermal deformation of experimental steel,ferrite phase transition region expanded.When the cooling rate was 2?/s,some ferrites were still present in the structure.The initial temperature of the phase transition of the experimental steel was increased,and the transition region of martensite and bainite was also expanded to some extent.(3)The thermal deformation behavior of 08Nb experimental steel was studied by high temperature single-pass compression experiment.The experimental temperature ranged from 800?to 1100?,and the strain rate was 0.1?10s-1.The true stress-true strain curve of the experimental steel was measured,and the dynamic recrystallization behavior during the thermal deformation of the experimental steel was analyzed under different strain rates and deformation temperatures.The deformation behavior and microstructure evolution of experimental steels during high temperature compression were studied.The activation energy of thermal deformation was calculated to be 367.668kJ/mol,and the constitutive equation of high temperature of experimental steel was established and the mathematical model of deformation resistance was regressed.this can be used as a theoretical reference for subsequent experimental thermal processing.(4)Experimental results of laboratory rolling and heat treatment show that:the rolling process of post-rolling air-cooling off-line tempering 08Nb experimental steel is heating temperature 1200?,rolling temperature 850??870?,heat treatment process is heating quenching temperature 910?,heat preservation for 40min,tempering temperature 620?,heat preservation for 90min.After rolling,off-line quenching and tempering of 08Nb experimental steel,the final rolling temperature is 850??870?,and the quenching temperature is 910?.The tempering temperature is 620?,and the temperature is 90min.The mechanical properties of the experimental steels under the two processes can meet the performance requirements of domestic V140 steel grade oil casing,and the final microstructure of the two processes is tempered sostenitic microstructure.(5)The effects of three different post-rolling cooling methods on microstructure and properties of 08Nb experimental steel were studied under the same heat treatment conditions:post-rolling cooling,post-rolling quenching and post-rolling controlled cooling to 650? and post-rolling cooling.From the experimental results,it can be seen that the effective grain size of the experimental steel after rolling quenching process is the smallest and the ratio of large angle grain boundary is the highest(0.858nm and 36.3%),and its strength and toughness are relatively good.At the same time,the strengthening mechanism of post-rolling quenching process under the conditions of 910? reheating quenching and 620? tempering heat treatment was studied.