【作者】 徐雪霞；

【导师】 白秉哲；

【作者基本信息】 清华大学 ， 材料科学与工程， 2008， 博士

【摘要】 近年来模拟方法在材料的成分和工艺设计方面获得广泛应用,对大型工件更具优势。本论文采用数值模拟、人工神经网络和小试样物理模拟结合的方法,对贝氏体钢大型厚壁锻件、厚截面铸件和1500MPa级高强钢的成分和工艺设计进行指导,探索应用模拟方法研制高强韧贝氏体钢的途径,在此基础上对所研究贝氏体钢的强韧化机制和1500MPa级高强钢的超高周疲劳行为进行了研究。研究表明,采用NSHT热处理数值模拟程序计算热处理冷却过程的温度场分布,通过ANN模型预测或实测CCT图,结合小试样控制冷却的热处理工艺模拟,可以较好预测所研究贝氏体钢的组织和性能,有效指导成分和工艺设计。(1)对400mm*800mm*800mm反应堆压力容器(RPV)特厚大锻件在淬火过程中的温度场分布进行了计算,获得不同部位的冷却曲线;在微调SA508-3成分基础上通过W合金化进行初步成分设计,采用人工神经网络预测实验钢的CCT图;二者结合预测实验钢对应锻件不同位置的组织和性能。并通过小试样控制冷却模拟了大锻件不同位置的冷却过程,获取典型位置材料的组织和性能数据。在此基础上对RPV特厚大锻件的成分和工艺进行了优化设计。结果表明,对SA508-3进行适量微调和W合金化,经淬火+高温回火后可以获得良好的综合力学性能和截面内性能的均匀性,满足RPV锻件的技术条件要求。(2)对60mm厚的Mn-Si-Cr贝氏体钢板类铸件奥氏体化后的空冷进行了数值模拟,结合CCT图预测了不同成分铸件空冷后的组织,同时通过小试样热处理模拟预测了性能,二者结合预测了铸件典型位置的空冷淬透性。在此基础上合理设计成分与工艺,达到了淬透性要求。预测结果与实测结果一致。(3)采用小试样模拟不同尺寸钢棒空冷,建立了1500MPa级无碳化物贝氏体/马氏体(CFB/M)高强钢的改型CCT图;对不同形变温度的形变热处理进行了实验模拟。对其连续冷却转变规律进行了研究,探讨了冷却速度和形变温度对组织和性能的影响,为获得高强韧CFB/M组织提供了工艺指导。在根据实验结果获得CFB/M组织的基础上,研究了CFB/M高强钢的超高周疲劳行为。更多还原

【Abstract】 Simulation methods are applied widely for composition and process design during recent years. In the present work, combination of simulation methods including numerical simulation, artificial neural network and small-sample physical simulation were applied to guide composition and process design of bainite forging and casting with large size and 1500MPa grade super high strength steel, aiming at investigating the application of simulation methods on developing bainite steels with good strength and toughness. In addition, the strengthening and toughening mechanism of studied materials and ultra high cycle fatigue behaviors were studied based on the experimental results.The experimental results show that the structure and properties of studied bainite steels can be predicted correctly based on temperature field distribution during heat treatment calculated by NSHT heat treatment numerical simulation program, CCT diagram predicted by artificial neural network and heat treatment process simulation results obtained by small sample control cooling. Then the design of composition and heat treatment procedure can be guided adequately on the basis of above prediction results.The temperature field distribution of 400mm*800mm*800mm heavy section steel forging for reactor pressure vessel was numerical simulated by NSHT program. Experimental materials were originally designed by tungsten alloying on the adjusted composition of SA508-3 and their CCT diagrams were predicted by artificial neural network. Combing above results the structure and properties corresponding to arbitrary sites of the forging were predicted. Small sample control cooling was applied to simulate quenching of the typical sites of the forging to obtain information of structure and properties. Composition and heat treatment process were optimized based on above results. Experimental results show that satisfactory properties and property homogeneity with the forging section were obtained by tungsten alloying on SA508-3 after quenching and high temperature tempering, satisfying standards for RPV large size forging.The air cooling procedure after austenizing was numerical simulated for Mn-Si-Cr bainite steel plate cast with 60mm thickness. The structure of studied steels with different composition was predicted based on combination of numerical simulation results and CCT diagram, and the properties of typical site of the cast were predicted by small sample simulating heat treatment. Combining above results the hardenability of experimental materials for 60mm-thick cast were analyzed. According to prediction results the composition and heat treatment procedure were designed and good hardenability was obtained for studied steels. The prediction results are in consistence with measurement ones.For 1500MPa grade high strength steel with duplex phase of carbide free bainite/martensite(CFB/M) microstructure, the modified CCT diagram was established by small samples to simulate the air cooling of steel rods with different size. Thermal-mechanical treatments under different deformation temperature then control cooling were carried out by small samples. Continuous cooling transformation mechanism of experimental steel was studied, and the influences of cooling rate and deformation temperature on structure and properties were discussed. The results provide guidance for heat treatment design to obtain CFB/M structure with high strength and toughness. The CFB/M microstructure was obtained according to experimental results and the ultra high cycle fatigue behavior was investigated.更多还原