【作者】 李阳华；

【导师】 李红英；

【作者基本信息】 中南大学 ， 材料科学与工程， 2013， 博士

【摘要】 随着油井深度增加,井内温度和压力相应提高,固井完井用油套管服役的地质环境发生了显著变化,对油套管的综合力学性能、使用性能和寿命提出了更高要求,特别是对强韧性匹配提出了极高的要求,现有美国API标准的高钢级油套管难以胜任。本文开展超深井用石油套管的成分设计、热变形、热矫直、热处理工艺及相关基础研究,为企业开发高强高韧的V150油套管提供技术支撑。根据超深井用V150油套管用钢的目标性能要求,利用人工神经网络技术开展了合金成分设计,确定了高强高韧V150钢的成分范围。对实验钢的热变形行为进行了全面而系统的研究,补偿了应变、摩擦以及变形热对流变应力的影响,基于2种不同模型构建了本构方程,绘制出了实验钢的热加工图。系统研究实验钢在连续冷却过程中的相变行为,测定了实验钢的CCT图,综合利用CCT图、温度场模拟、微观组织观察,分析了不同冷却方案的组织转变。系统研究了高强高韧油套管用钢的热处理工艺和强韧化机理,获得0℃横向冲击功达到130J的调质工艺。运用ANSYS/LS-DYAN软件建立三维有限元模型,实现无缝钢管热矫直过程的三维动态模拟。研究常规淬火+充分回火、常规淬火+不充分回火、亚温淬火+回火对油套管低温韧性及韧脆转变温度的影响。主要结论如下：1.确定V150钢的成分范围(质量分数,%)：0.25～0.28C,0.25～0.30Si,0.90～1.10Cr,0.9～1.10Mn,0.5～0.65Mo,0.07～0.10V,0.0015～0.005Ca, Cu≤0.1,Ni≤0.2,A1≤0.03,P<0.01,S<0.005。预测的性能值为：Rt0.7=1064MPa,Rm=1127MPa,A=19.7%；CVN=115J,完全满足超深井用油套管的强韧性要求。2.基于strain-compensated Arrhenius模型构建的流变应力方程,考虑了应变、摩擦力和温度效应对流变应力影响,可准确预测材料在实验范围内的流变应力,预测值与实验值的相关系数R=0.99456,AARE=4:730%。综合流变应力等高线图以及加工图,得出材料的最佳热变形工艺区间：变形温度为1110℃～1200℃,变形速率为0.03～2.4s-1,在该区域内热变形会发生动态再结晶,能量耗散系数峰值为37.5%。3.测得了实验钢的CCT图,Ac1为778.4℃,Ac3为828.2℃,当冷却速度为0.05Φ0.5℃/s时,转变产物为多边形铁素体、珠光体和少量贝氏体的混合组织,在1℃/s～5℃/s的冷却速度范围,转变产物为贝氏体,当冷却速度大于5℃/s时,转变产物为马氏体。合适的分级控冷工艺会对减小钢管淬火应力产生很好的效果,能够抑制孪晶马氏体,得到B/M复相组织、M/A复相组织,可较好抑制裂纹形成和扩展。根据CCT图分析得出的组织转变与实际淬火组织一致,表明所测CCT图较为准确,可用于指导高强高韧油套管的热处理工艺。4.奥氏体化温度和回火工艺对实验钢的组织性能影响较大,在890℃保温30min水淬,经1次和2次650℃/45min回火后,在强度满足150ksi钢级要求的前提下,0℃横向冲击功分别达到100J、110J。亚温淬火可以显著提高实验钢的冲击韧性,800℃/30min亚温淬火后,经1次和2次640℃/45min回火的0℃横向冲击功分别为120J、130J,强度仍然满足150ksi钢级要求。5.矫直管经过第1对矫直辊时,横截面的应力应变呈轴对称分布,经过第2、3对矫直辊时,钢管横截面的应力应变分布对称性较差。内表面周向残余应力的有限元模拟值在-130～-480MPa间波动,实测值在-189～-489MPa间变化,模拟值与实验值吻合较好。第2对矫直辊的压扁量为最重要的压扁矫正控制因子,3对辊压扁量的较优组合依次为1.4mm、4.0mm、2.6mm,椭圆度少于0.4%。压弯量为最重要的矫直控制因子,压弯量、压扁量、倾斜角的最优组合分别为45mm、4.0mm、31°,采用优化的矫直参数后,矫直管的不平度大大降低,管端1m内≤1/1000mm,管体≤1/1500mm,完全能满足高尺寸精度产品要求。6.结合能量法和形貌分析法确定常规淬火+充分回火、常规淬火+不充分回火、亚温淬火+回火等3种典型热处理工艺对应的低温韧性及韧脆转变温度,分别为-37℃、-2℃、-73℃。亚温淬火组织经过回火后,碳化物均匀析出,大部分α铁素体等轴化,少量未溶铁素体的存在,不仅未降低材料的整体强度,还能抑制微裂纹的产生和扩展,因此有较好的低温韧性。更多还原

【Abstract】 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.更多还原