| 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. |
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