【作者】 王耀华；

【导师】 袁泽喜；

【作者基本信息】 武汉科技大学 ， 材料学， 2004， 硕士

【摘要】 微合金钢是近几十年来发展迅速的一类钢。它能够在保证有良好韧性和塑性的同时，极大的提高了钢材的强度，并且满足成本的需求。本论文是在试验钢中加入了0.123wt-％的微合金元素Nb，研究了预变形对Nb的碳氮化物析出行为的影响。 本文采用时效硬度法测试了试验钢的析出动力学曲线，用光学显微镜和TEM等检测手段，观察了试验钢的金相显微组织和第二相粒子的析出行为，研究了第二相粒子析出行为对试验钢的显微组织、力学性能和焊接性能的影响。 研究结果表明，未变形试验钢和预变形试验钢的时效动力学曲线都具有时效硬度谷值和峰值。但预变形促进了Nb的碳氮化物的析出，即在相同的时效温度下，到达时效硬度谷值和峰值的时间缩短。未变形试验钢在640℃时效2小时，组织为回火索氏体和再结晶的铁素体晶粒，这是因为未溶的NbN为再结晶提供了形核位置，促进了未变形板条马氏体的再结晶；预变形试验钢在640℃时效5分钟，组织为保持马氏体位向的回火索氏体，没有再结晶的铁素体晶粒，这是时效析出的Nb的细小弥散的碳氮化物阻止了变形板条马氏体再结晶。通过TEM观察，Nb的碳氮化物析出的形貌主要有方形、球形和圆片形。预变形使试验钢第二相析出的质点更细小，分布更弥散，这种细小弥散的第二相对强度的贡献也更大。未变形试验钢的第二相强化量为83MPa，而预变形试验钢的第二相强化量为148MPa。 本文还考察了试验钢的焊接性能。试验钢碳当量Cq为0.138％，裂纹敏感性组成Pcm为0.0795，焊接热影响区的最高硬度值HV10为214，可认为试验钢具有较好的焊接性能，可在不预热的条件下进行焊接。更多还原

【Abstract】 Microalloyed steels have received considerable interest over many years and continue to gain wider industrial applications, because of low capital investment and high productivity. Microalloying can get combination of strength-toughness and increase strength greatly. The precipitation behavior of Nb carbonitride and its effect on properties of microalloy by addition of 0.123 wt-% Nb were studied in this work.The ageing-harding method was used to get the dynamics curve of aging. By means of Optical microscopy, TEM and other experiments, the influence of precipitation on the microstructure, mechanical properties and welding performance were studied. The results showed that the dynamics curve of aging have the peak value and valley value. The deformation boosts the precipitation of Nb carbonitride. At the same aging temperature, deformation makes the time of arriving the peak point and valley shorten. At 640℃ aging for 7200s, undeformed steels occurred partly recrystallization and microstructure were tempered sorbite and granular ferrite. Unsolution NbN offered nucleation site and accelerate recrystallization. At 640℃ aging for 300s, the microstructure of the deformed steel was tempered sorbite keeping exposure of the lath martensite without recrystallized granular ferrite. Fine and disperse precipitation of Nb carbonitride prohibited recrystallization of deformation lath martensite. TEM results showed that Nb carbonitride pattern had three kinds: square, globular and disc. Deformation makes pricipitation Nb (CN) became finer and disperser. These precipitate particles, former at grain boundary and dislocation are very small in size and result in large strengthening. The precipitation strengthening of undeformation steel is 83MPa, while deformation steel is 148MPa.The carbon equivalent of steel was 0.138%, crack-sensitivity index was 0.0795 and the highest hardness of heat-affected zone was 214, so the steel has good welding peformance and can be welded without preheating.更多还原