【作者】 谭舒平；

【导师】 韩杰才； 程世长；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2009， 博士

【摘要】 S30432是目前世界上最先进的超超临界(USC)发电机组上使用的高等级耐热钢。本文通过晶间腐蚀试验、金相试验、力学性能试验和热模拟试验等方法以及XRD、SEM、TEM、HR-TEM等测试手段对S30432奥氏体耐热钢晶间腐蚀敏感性、不同热处理状态下的室温性能、高温性能、持久性能、高温应力状态下组织演变、高温强化机理以及热变形行为进行了详尽的研究,对S30432钢的国产化进程具有推进作用。研究结果表明,碳含量对S30432钢晶间腐蚀敏感性的影响比铌含量的影响更为显著,当铌含量与碳含量的比值大于5.8时,可以抑制晶间腐蚀的发生。随固溶温度升高,S30432钢的晶间腐蚀敏感性逐渐下降,当固溶温度不低于1100℃、碳含量不高于0.083%时,可以完全避免晶间腐蚀的发生。S30432钢管固溶后要快速冷却,要避免在840℃附近温度区域停留,以防止由于M23C6碳化物析出量增加而增大晶间腐蚀敏感性。固溶处理温度和时间对室温强度影响明显,如果固溶温度超过1150℃室温强度明显下降。冲击功随时效时间增加明显下降,但2000小时后趋于稳定。碳含量对室温强度影响明显,铌含量对室温强度影响不明显。S30432钢650℃4小时时效后持久强度均高于在700℃4小时进行时效处理。低碳中铌试验钢持久强度最高。元素Cu、Nb、C、N、和B的强化当量系数分别为1、1、4、20、337。MX相是S30432钢中最主要的强化相,对维持高温持久强度的贡献最大。MX相尺寸约50nm~120nm,通过与位错的强烈相互作用使钢的变形抗力增大。在650℃下同时效时间4小时相比,当时效时间达到2000小时时,MX相中的铌和氮的含量仅增加了5.50%和2.72%,这种缓慢变化有利于其强化效果的稳定性。细小的M23C6相在晶界、晶内弥散分布,起到重要强化作用,M23C6相的尺寸在0.3μm~0.8μm,大量分布在晶界上,在晶内也有弥散分布。在650℃下同时效时间4小时相比,当时效时间达到2000小时时,M23C6相析出量增加16.74倍,其尺寸也明显增大。少数大尺寸的M23C6相,不但对强化不起作用,甚至有危害作用。富铜相对S30432钢的强化有重要贡献。富铜相的尺寸在3nm~30nm,平均尺寸约10nm,弥散分布在晶内和晶界,阻碍位错运动使钢得到强化,同时富铜相还存在于亚结构和孪晶中阻止位错运动,使钢的强度得到提高。通过对S30432钢进行热模拟试验,获得了S30432钢在各个应变速率条件下不同变形温度时的流变曲线。确定了在不同变形条件下的峰值应力和峰值应变。求得了在0.005s-1~5s-1及800℃~1200℃变形条件下S30432钢的热变形激活能为485 kJ/mol。建立了S30432钢的热变形方程和动态组织状态图。确定了S30432钢的动态再结晶临界应力与其Z参数间的关系。确定了S30432钢动态再结晶晶粒尺寸D与其Z参数和参数A间的定量关系。建立了S30432钢的热加工图和热拉伸塑性图并对其中典型的变形机制进行了分析。更多还原

【Abstract】 S30432 is a high-class heat-resistant steel used for components of most advanced USC boiler in the world. By means of SEM, TEM, HR-TEM and thermal simulator, the intergranular corrosion resistance, the mechanical properties, the microstructure evolution under the stress condition at high temperature, the strengthening mechanism and the hot deformation behavior of S30432 steel were investigated in order to accelerate its domestic production.The results show that the effect of carbon content on intergranular corrosion secretiveness of S30432 steel is more remarkable than that of niobium content. When the ratio of niobium content and carbon content is above 5.8, intergranular corrosion will be suppressed.With increasing of solid-solution temperature, the intergranular corrosion sensitivity decreases gradually. In the case of solution temperature exceeds 1100℃and carbon content is lower than 0.083wt% , intergranular corrosion can be avoided. To prevent intergranular corrosion sensitivity caused by M23C6 phase precipitation, long hold-up time around 840℃must be avoided by rapid cooling after solid-solution treatment.Temperature and time of solid-solution treatment affects the strength at room temperature obviously. When solution temperature exceeds 1150℃, the strength decreases apparently. Impact toughness decreases with aging time increasing and get even after 2000 hours. Carbon content affects the strength obviously and niobium content has minor effect on strength at room temperature.Creep rupture strength of tested steels aged at 650℃for 4 hours is higher than that of tested steels aged at 700℃. The steel with low carbon content and middle niobium content possess the highest creep rupture strength. The strengthing equivalent parameters of copper, niobium, carbon, nitrogen and boron elements are 1, 1, 4, 20, and 337 respectively.MX phase is the predominant strengthening phase in S30432 steel, and makes largest contribution to elevated temperature strength. The size of MX phase is about 50~120nm, and it has strong interaction with dislocation causing the increasing of deformation resistance.Compared with aging at 650℃for 4 hours, niobium content and nitrogen content in MX phase increased only 5.50% and 2.72% respectively when aging for 2000 hours, and these slow changes are beneficial to maitain its strengthening effect.M23C6 phase phase is major strengthening phase in S30432 steel. The size of M23C6 phase is about 0.3μm ~0.8μm. Most of M23C6 phase distributed in grain boundary, and also distributed in innergrain. After aging for 2000 hours at 650℃, the precipitation amount of M23C6 increased about 16.74 times than that of 4 hours, and its size increased evidently. Large size M23C6 can’t play strengthening effect and becomes harmful to high temperature strength.Cu-rich phase plays important role in strengthening of S30432 steel. The size of Cu-rich is about 3nm ~30nm and average size is about 10nm. Cu-rich phase distributes dispersively in grain boundary and innergrain. Cu-rich phase strengthen the steel by hindering the movement of dislocation. Meanwhile, Cu-rich phase existing in sub-structure and twin grains can impede the movement of dislocation further to improve the strength of steel.The flow curves under various strain rates and different deformation temperature conditions were obtained through test by Gleeble3500 thermal-mechanical simulator and the peak stress and peak stain under different deformation conditions were obtained. Under the strain rate of 0.005 s-1~5 s-1 at the temperature range of 800℃to 1200℃, the hot deformation activation energy of the steel is 485kJ/mol and the hot deformation equation was got.The relationship between critical stress for dynamic recrystallization and Zener-Hollomon parameter was determined.The dynamic microstructure diagram of S30432 steel under the deformation temperature range of 800℃~1200℃and the strain rate range of 0.005s-1~5s-1 were established. The quantitative relationship between the size D of S30432 steel dynamic recrystallization grain and its Zener-Hollomon parameter and A parameter was determined.Processing maps for S30432 steel under different strain conditions were set up. Typical deformation mechanisms in processing maps were discussed and hot-tension ductility diagram was obtained.更多还原