Study And Development Of High Strength And High Toughness V150Grade Casing For Ultra-deep Well

With the increase of depth of oil well in the process of oil exploitation, temperature and pressure in the well increase correspondingly, which makes the service environment of casing, used for well cementing and completion, change greatly. These changes put forward higher requirements for the comprehensive mechanical properties, service performance and life of casing, especially for the combination of strength and toughness, which could not be satisfied by present high grade casing produced according to API standard. In this paper, chemical composition design, hot deformation behavior, hot straightening process, heat treatment process and related basic research of the casing for ultra-high wells were investigated systematically. These studies could provide the steel enterprises with technical support for developing and manufacturing Vl50grade casing tube with high strength and high toughness.According to the target performance of Vl50grade steel, the composition of experimental steel was designed by artificial neural network. The comprehensive and systematic investigation on hot deformation behavior of experimental steel was carried out considering the effects of strain, friction and deformation heat on flow stress. Constitutive equations were established on the basis of two different models, and the processing map was developed as well. The phase transformation behavior of experimental steel during continuous cooling process was studied systematically and CCT diagram was obtained based on the study. Microstructure transformation of the steel under different cooling schemes was analyzed by utilizing comprehensively the CCT diagram, simulation of temperature field and microstructure observation. Based on the systematic study on the effect of different heat treatment parameters on the strength and toughness of experimental steel, a quenching and tempering process was obtained by which lateral absorbed-energy at0℃of the steel could be increased to130J. Hot straightening process of seamless tubes was simulated dynamically by3D finite element model established using software ANSYS/LS-DYAN. Effects of three heat treatment processes such as conventional quenching followed by tempering completely, conventional quenching followed by tempering incompletely and subcritical quenching followed by tempering, on the low temperature toughness and the ductile to brittle transition temperature for the steel were investigated respectively. The main conclusions can be drawn as follows:1. The chemical composition of V150grade steel could be designed as (in wt.%):0.25～0.28C,0.25～0.30Si,0.9%～1.1%Cr,0.9～1.10Mn,0.5～0.65Mo,0.07-0.IV,0.0015～0.005Ca, Cu≤0.1,Ni≤0.2, Al≤0.03, P<0.01, S<0.005. The predicted mechanical properties of the steel with this composition are:Rt0.7=1064MPa,Rm=1127MPa,A=19.7%; CVN=115J. Obviously, the properties of the designed steel can completely satisfy the requirements for the strength and toughness of casing for ultra deep wells.2. Constitutive equation was established based on the strain-compensated Arrhenius model with considering the effects of strain, friction and deformation heat on flow stress. The established constitutive equation could predict the flow stress accurately over the entire experimental range of strain rates, temperatures and strains. The correlation coefficient(R) and average absolute relative error(AARE) for the developed model are0.99456and4.730%respectively. The optimum domains of the processing parameters were determined as1110℃～1200℃and0.03～2.4s-1by processing map and contour map. When the steel is hot deformed in this optimum domain, dynamic recrystallization can occur and the energy dissipation coefficient is the peak value37.5%.3. The CCT diagram of experimental steel was obtained and the critical temperatures of AC1and AC3were determined as778.4℃and828.2℃respectively by DSC. When the cooling rates are in the range of0.05-0.5℃/s, the transformation product consists of polygonal ferrite, pearlite and a small amount of bainite. When the cooling rate is ranging from1℃/s to5℃/s, the product is mainly composed of bainite. When the product. Using an appropriate multi-stage controlled cooling process is beneficial to decreasing the quenching stress. This is because the formation of twin martensite could inhibited by this process on one hand. On the other hand, the composite microstructure of B/M and M/A is generated which inhibit the formation and propogation of cracks. The quenched microstructure of the steel shows good agreement with the results analyzed with CCT diagram indicating the obtained CCT diagram is accurate enough to provide basis for the heat treatment process of28CrMnMoV steel.4. Austennizing temperature and tempering process have significant influence on the microstructure and properties of experimental steel. When the steel was austenitized at890℃for30min followed by water quenching and then tempered at650℃for45min one time and twice respectively, the corresponding lateral absorbed-energy at0℃could reach100J and110J with the strength meeting the requirement of V150grade steel. Subcritical quenching process could remarkably increase the toughness of the steel. When the steel was austenitized at800℃for30min followed by water quenching and then tempered at640℃for45min one time and twice respectively, the corresponding lateral impact energy at0℃could reach120J and130J on the premise of strength meeting the requirement of V150grade steel.5. When the tube passed through the first pair of straightening rollers, the distribution of stress and strain on the cross section is axisymmetric. However, when it passed through the second and third pairs of straightening rollers, the distribution is not that axisymmetric. The simulated value of circumferential residual stress on internal surface is in the range of-130MPa--480MPa, which shows good agreement with the measured value in the range of-189MPa--489MPa. Flattening rate of the second pair of straightening rollers is the most correction control factor, and the ovality of tubes could be controlled less than0.4%when the flattening rates for the three straightening rollers are1.4mm,4.0mm and2.6mm respectively. The intermesh is the most important straightening control factor. The optimum set of straightening parameters (the intermesh, flattening rate and slant angle) were simulated as45mm, 4.0mm and31°. Flatness of the tubes could be controlled less than1/1000mm within the scope of lm away from the pipe end and below1/1500mm in the section of pipe body by optimizing straightening process with these parameters indicating these can satisfy the requierement of high precision products.6. Combined with the energy approach and analysis of morphology, effects of three different heat treatment processes(including conventional quenching followed by complete tempering, conventional quenching followed by incomplete tempering and subcritical quenching followed by complete tempering) on low temperature toughness and the ductile to brittle transition temperature were investigated. In addition, the corresponding ductile to brittle transition temperatures of the steel by the three heat treatment processes were determined as-37℃,-2℃and-73℃, respectively. The experimental steel could obtain excellent low temperature toughness by subcritical quenching and tempering. This is mainly because carbides precipitate homogeneously and most a ferrite grains gradually become equiaxed during the heat treatment process. In addition, the existence of undissolved ferrite could not only decrease the strength, but also inhibit the formation and propogation of cracks, resulting in an increase of toughness.