【作者】 王晓峰；

【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】 摘要：随着电力机组蒸汽参数的不断提高,高压锅炉管的服役环境发生显著的变化,同时也对高压锅炉管用耐热钢的服役性能与寿命提出了更高的要求,特别是对高温力学性能和抗氧化性能提出了很高的性能指标,传统的耐热钢已无法满足使用要求。本文开展超临界火电机组用耐热钢的成分设计、热变形、热处理工艺、高温性能及相关基础研究,为企业开发高性能耐热钢提供技术支撑。根据超临界火电机组用耐热钢的目标性能,利用人工神经网络技术开展了成分设计,确定了实验钢的成分范围。对实验钢的热变形行为进行了全面的研究,基于多种不同模型构建了本构方程,对Johnson Cook模型进行了改进。研究了T24钢在连续冷却过程中的相变行为,测定了相应的CCT图,利用解析化方法绘制了CCT图,分别与相近成分和不同成分的CCT图进行了比较。研究了实验钢的热处理工艺,在此基础上确定了实验钢的服役温度范围。对耐热钢的显微组织进行了分析,计算了晶粒度,利用EBSD手段分析了实验钢在不同状态的晶粒和晶界的变化。研究了实验钢在高温条件的氧化行为,分析了氧化膜的生长和剥落的过程,并测定了实验钢的高温物理性能。主要结论如下：1.利用BP人工神经网络模型确定T24钢的成分范围(Wt,%)为：0.07C,0.21Si,0.47Mn,0.09Cu,0.04Ni2.41Cr,1.03Mo,0.06Ti,0.24V,0.1Al,P<0.01,S<0.005。2.基于modified Zerilli-Armstrong、strain-compensated Arrhenius等模型,构建了本构方程准确预测材料在实验范围内的流变应力。Johnson CookR原始模型预测的相关系数=0.962, AARE=9.41%。通过引入温度与应变速率耦合影响因子对Johnson Cook原始模型进行改进,改进后模型预测的相关系数R=0.991, AARE=5.37%。3.测得T24实验钢的CCT图,Ac1为773℃,Ac3为963℃,当冷却速度小于0.1℃/s时,发生高温转变和中温转变,转变产物为铁素体、珠光体和贝氏体的混合组织,在0.5℃/s～10℃/s的冷却速度范围,转变产物为粒状贝氏体,当冷却速度大于20℃/s时,转变产物为马氏体。当冷却速度为0.03℃/s、0.05℃/s和0.1℃/s时,过冷奥氏体先在高温区发生组织转变,转变量分别为49%、25%和22%,之后在中温区发生组织转变。相近成分T23钢的CCT图及临界点与T24钢相似,Ac1和Ac3分别为777℃、963℃,成分有较大差别的T91钢的CCT图及临界点与T24相差较大,Ac1和Ac3分别为758℃、871℃。4.T24实验钢较优的奥氏体化工艺为1000℃/30min,较优回火工艺为750℃/70min。实验钢经1000℃/30min奥氏体化在750℃回火70min后,室温抗拉强度为615MPa,屈服强度为564MPa,伸长率为22.3%,在570℃的高温抗拉强度为497MPa,屈服强度为447MPa,伸长率为10.9%,服役温度不能超过580℃。5.T24钢热处理后组织为粒状贝氏体,且铁素体基体上分布有岛状颗粒,组织晶粒度级别数为5～6。T23钢的组织与T24钢相似。T9钢和T91钢的组织相似,主要为回火马氏体,基体分布有较多的析出物。T24钢在不同的状态下,组织中小角度晶界的比例较高,尤其以2°～3°晶界为主,大角度晶界以60°为主,且规律为随着取向差角的增加,小角度晶界比例迅速递减,当取向差角大于50°,大角度晶界比例显著增长,在60°附近达到峰值。6.T24钢在570℃高温氧化时,0～120h内的氧化速率为6.675×10-5,120～1700h内的氧化速率为5.996×10-8，1700～2600h内的氧化速率为2.686×10-6,2600～6200h内的氧化速率为4.513×10-7。T24钢在600℃高温氧化时,0～2000h内的氧化速率为1.161×10-5,2000～10000h内的氧化速率为4.162×10-6。T23钢在600℃的高温氧化时,0～1500h内的氧化速率为1.174×10-5,1500～5000h内的氧化速率为5.985×10-6,5000～10000h内的氧化速率为1.759×10-8。T91钢在625℃高温氧化时,0～1000h内的氧化速率为2.313×10-6,1000～10000h内,氧化速率为3.171×10-11。T9钢在600℃高温氧化时,0～500h内的氧化速率为3.367×10-5,500～6000h内的氧化速率为2.752×10-12,6000～10000h内的氧化速率为1.978×10-7。7.T24钢和T23钢的导热性能均高于T9钢和T91钢,在常温下,T9钢的导热系数为26.5W/m·K,略低于T91钢,当温度升高到800℃时,T91钢的导热系数为21.6W/m·K,比T9钢的高2.9W/m·K。在100～500℃的温度区间内,T23钢和T24钢的线膨胀系数大小差别不大,高于T9钢的线膨胀系数,T91钢的线膨胀系数最小。在温度低于500℃的情况下,T24钢的弹性模量最大,T91钢次之,T23钢的弹性模量最小。更多还原

【Abstract】 Abstract:With the increase of steam parameters of generator sets in the power plants in recent years, service environment of high pressure boiler tube changed greatly correspondingly, which put forward higher requirements for service performance and life of heat-resistant steel. Because traditional heat-resistant steels can not satisfy these requirements, it is necessary to develop new steels with excellent service performance, especially for high temperature mechanical properties and oxidation resistance. In this paper, chemical composition design, hot deformation behavior, heat treatment process, high temperature properties and related basic research of the heat-resistant steel were investigated systematically. These studies could provide the steel enterprises with technical support for developing and manufacturing high pressure boiler tubes.According to the target performance of heat-resistant steels used for supercritical units, the composition of experimental steel was designed by BP artificial neural network. The comprehensive and systematic investigation on hot deformation behavior of experimental steel was carried out, and the constitutive equations were established based on several models. Hot compression test data was used to modify the Johnson Cook model. The phase transformation behavior of T24steel during continuous cooling process was studied systematically and the CCT diagram was obtained based on the study. The obtained CCT diagram was compared with other heat-resistant steels’. The CCT diagram of T24steel was also obtained by mathematical analysis method. Heat treatment process of the experimental steel was studied systematically and the service temperature range was determined based on this investigation. Microstructure of the steels was analyzed, and grain size was also evaluated using three methods respectively. Variations of grains and grain boundaries of the experimental steel in different states were investigated by means of EBSD. High temperature oxidation behavior of the steel was studied. In addition, growth process and exfoliation of oxidation layer were also analyzed. High temperature physical properties of the experimental steels were determined and analyzed. The main conclusions can be drawn as follows:1. By utilizing artificial neural network, the chemical composition of T24steel could be designed as (in wt.%):0.07C,0.21Si,0.47Mn,0.09Cu,0.04Ni,2.41Cr,1.03Mo,0.06Ti,0.24V,0.1A1, P<0.01, S<0.005.2. Constitutive equations was established based on the modified Zerilli-Armstrong, strain-compensated Arrhenius and ANN models, and the flow stress could be predicted accurately. The original Johnson Cook model was modified by introducing a variable factor which considers the couple effect of temperature and strain rate. The correlation coefficient(R) and average absolute relative error(AARE) for modified model is0.991and5.37%respectively, which is higher than the values0.962and9.41%of the original model.3. The CCT diagram of T24steel was obtained and the critical temperatures of AC1and AC3were determined as773℃and963℃respectively by DSC. When the cooling rates are below0.1℃/s, the transformation product consists of ferrite, pearlite and bainite. When the cooling rates are in the range of0.5～10℃/s, transformation product is mainly composed of granular bainite. When the cooling rates is above20℃/s, martensite is the main transformation product. High temperature transformation occurs at first when the super-cooled austenite is cooled at the rates of0.03℃/s,0.05℃/s and0.1℃/s, and then middle temperature microstructure transformation begins to take place. The percentages of transformation product in high temperature ranges are49%,25%and22%correspondingly. Difference in compositon between T23steel and T24steel is small and therefore the CCT diagrams of the two steels are similar. The critical temperatures Ac1and AC3of T23steel are777℃and963℃respectively. There are considerable differences in CCT diagram and critical temperatures for T91steel T24steel due to the big difference in composition and the AC1and AC3of T91steel are758℃and871℃respectively.4. Based on the experiment and analysis, the optimized austenitizing process and tempering technology can be determined as1000℃/30min and750℃/70min respectively. When the experimental steel is heat treated applying this process, room temperature tensile strength, yield strength and elongation of the steel are615MPa,564MPa and22.3%respectively. High temperature(570℃) tensile strength, yield strength and elongation are497MPa,447MPa and10.9%using the same tempering process。The service temperature of T24steel should be controlled below580℃.5. The main microstructure of the heat-treated T24and T23steel is granular bainite with some island-shaped particles on it, and the grain size grade of T24steel are evaluated as5-6. The main microstructure of the heat-treated T9and T91steel is tempered martensite with some precipitates in matrix. Low-angle boundaries always account for a large proportion of the total in the microstructure of T24steel in many states, especially for the boundaries with angles of2°～3°. Boundaries with angles of60°are the main wide-angle boundaries. With the increase of misorientation angle, the proportion of low-angle boundary decrease sharply. When the misorientation angle is higher than50°, the boundaries with angles of60°account for maximum propotion.6. When T24steel is heated isothermally in the air at570℃, the oxidation rate is6.675×10-5within120h, and5.996×10-8in the time range of120-1700h. Oxidation rate are2.686×10-6and4.513×10-7respectively for the time range of1700-2600h and2600-6200h. When the heating temperature increases to600℃, the oxidation rates are1.161×10-5within2000h and1.161×10-5in the time range from2000h to10000h. Under the same heating conditions(600℃),the oxidation rates of T23steel are1.174×10-5,5.985×10-6and1.759×10-8corresponding to the time ranges of0-1500h,1500-5000h and5000-10000h respectively. As for T9steel, the oxidation rates are determined as3.367×10-6,2.752×10-12and1.978×10-1rcorresponding to the time range0～500,500-6000h, and6000-10000h respectively. When T91steel is heated in the air at625℃, the oxidation rate are2.313×10-6and3.171×10-11respectively corresponding to the time ranges of0-1000h and1000-10000h.7. Heat-conducting property of T23andT24steel is higher than that of T9and T91steel. Heat conductivity coefficient of T9steel is26.5W/m-K at room temperature, which is a bit lower than that of T91steel. Heat conductivity coefficient of T91steel is21.6W/m-K at800℃,2.9 W/m-K higher than that of T9steel. The difference in linear expansion coefficient between T23andT24steel is very small in the temperature of100～500℃, and the values are higher than that of T91and T9steel. The minimum value of inear expansion coefficient is gained from T91steel. Elasticity modulus of T24steel is higher than that of other steels, and the minimum value of it can be obtained from T23steel.更多还原