【作者】 宁保群；

【导师】 刘永长；

【作者基本信息】 天津大学 ， 材料学， 2007， 博士

【摘要】 T91(9Cr-1Mo-V-Nb-N)钢是高Cr铁素体耐热钢的代表钢种,广泛用于超临界发电厂锅炉管道上;同时也是开发更高使用温度的铁素体耐热钢材的研究基准。随着能源短缺和环境污染问题的日益突出,提高锅炉管用钢的耐热温度以及提高电厂热效率的研究势在必行。目前对T91钢的研究大都是从合金化、工艺性能、化学性能和工程使用上进行的,较少有关于该钢相变过程、组织控制及强化机理的报导。为深入地研究T91钢相变过程及机理、组织形成与演化规律以及探索铁素体耐热钢进一步强化的成型新工艺,本文综合采用线膨胀测量和现代材料分析方法,进行T91钢加热过程中奥氏体化、连续冷却转变以及冷却过程中等温停留与施加微小应力变形等情况下的T91钢相变过程的研究,以期系统地澄清T91钢的相变过程及规律,阐明其影响机理。并在此基础上进行T91钢强化工艺的热机械变形模拟,论证通过形变热处理进行组织细化,第二相的诱导析出来进一步强化T91钢的可行性。在大量实验研究与理论分析的基础上,取得了如下结论:(1)通过对升温过程中加热速度对T91钢奥氏体形成影响的研究,绘制了连续加热奥氏体化过程转变曲线,实验结果表明:加热速度显著影响T91钢的奥氏体形成开始温度( )和结束温度( ),加热速度愈大,和愈高,奥氏体形成速率愈快;奥氏体化速率的峰值与加热速度符合如下关系:(其中为奥氏体化速率,V为加热速度)。与此同时,碳化物溶解与加热速度密切相关,加热速度较低时,奥氏体中大量M23C6型碳化物溶解,改变了C浓度分布,从而促进MX型碳化物的形成,形成的MX型碳化物可有效避免奥氏体晶粒粗大;加热速度大到一定程度时,M23C6型碳化物在奥氏体加热过程中溶解得很少或是不溶,而是被高的加热速度推迟到奥氏体区保温阶段溶解;此外,较快的加热速度可使奥氏体初始晶粒度细化,但不利于奥氏体的均匀化。df /dtmax(2)澄清了T91钢连续冷却过程中的相变规律,确定了该钢发生马氏体转变的临界冷却速度,绘制了连续冷却转变曲线,研究表明:T91钢的连续冷却过程中只存在铁素体和马氏体转变区,10℃/min和2℃/min分别为奥氏体向马氏体转变的上临界冷却速度和下临界冷却速度。不同淬火速度对T91钢马氏体开始转变温度(Ms)有较大的影响,其不同于随冷速增加而相变点升高的经典理论。淬火速度通过碳原子气团、淬火空位和内应力的形成来影响过冷奥氏体状态,从而影响相变点;随淬火速度的增加,过冷奥氏体转变后的组织呈细化趋势,但当速度大于1000℃/min时,冷速的变化对组织形貌几乎没有影响。(3)通过对T91钢过冷奥氏体冷却过程中,在Ms点附近等温停留的奥氏体稳定化研究发现:在Ms点以上较高温度等温停留时,该钢表现出明显的反稳定化现象,即马氏体转变开始温度高于Ms点,并随保温时间延长反稳定化程度增加;在Ms点以上较低温度等温停留时,T91钢的马氏体转变开始温度低于Ms点;且随保温时间的延长继续降低;其最终组织仍为完全马氏体,属于伪稳定化。在Ms点以下附近温度(400℃)保温时,马氏体继续转变温度降低,但因转变温度较高,而且机械稳定化作用太小,所以室温组织仍没有残余奥氏体出现,亦属伪稳定化现象。当在380℃以下等温停留时,T91钢的奥氏体稳定化趋势显著,马氏体继续转变温度显著降低,发生不完全马氏体转变,组织中存在残余奥氏体,此时奥氏体稳定化过程受机械稳定化和热稳定化的双重因素影响。(4)针对T91钢过冷奥氏体冷却过程中,施加微小压应力变形的研究发现:T91钢在850℃温度以上施加应力不会促进马氏体的形成。在此温度以下施加应力将促进马氏体的形成,提高马氏体相变开始温度。应力作用下的T91钢存在两种转变机制:施加应力温度较高时,其转变机制属应变诱发马氏体,组织呈细化趋势,晶界形态趋于不规则;在施加应力温度较低时,属应力诱发马氏体转变,其形态与热诱发马氏体相似。440℃是应力诱发马氏体相变的开始温度,150～200MPa存在着440℃应力诱发马氏体相变的临界应力。(5)利用T91钢宽的奥氏体未再结晶区和含有Nb、V等元素的特点,进行了形变热处理诱导Nb/V碳氮化物析出,从而使其性能进一步强化的探索性研究,研究发现:形变热处理后T91钢显微组织发生显著的均匀细化,更重要的是该工艺可以为MX型Nb/V碳氮化物颗粒的析出提供更多的形核位置,从而来生成更多、更细小的弥散分布的MX型碳氮化物。通过拉伸实验表明:形变热处理可以明显提高T91钢的强度,使其进一步强化,说明了采用形变热处理工艺来提高铁素体耐热钢的使用温度是切实可行的。更多还原

【Abstract】 T91(9Cr-1Mo-V-Nb-N)is the representative of high Cr ferritic heat-resistant steels, which has been widely used in high temperature structural components such as main steam pipe, superheater tube and resuperheater tube in advanced power plants in view of good mechanical properties, excellent oxidation-resistant properties, outstanding thermal properties relative to other elevated temperature alloys. So it is regarded as research benchmark for the development of new ferritic heat-resistant steels with higher application temperature. Furthermore,under the pressure of energy shortage and environment pollution,the study on the thermal efficiency of generating station and heat-resistant temperature of the boiler tube is also imperative.There are lots of papers which deal with alloying, processing property, chemistry property and potential engineering applications, etc, while few investigations on phase transformation and structural evolution of T91 steel are developing. In order to clarify the phase transformation process and mechanism, structural evolution and explore new forming process of T91 ferritic heat-resistant steel, the possible influence on the properties of T91 steel, such as austenization, continuous cooling transformation, isothermal holding, and applied stress were systematically characterized by means of high-resolution linear differential dilatometry and modern materials analysis methods. Controlled Rolling and Cooling process of T91 products was simulated in labs to demonstrate the feasibility of improving the mechanical properties through structural refinement and induced precipitation of secondary phase, results as following:(1) Austenization is not only the first stage for the heat treatment of iron and steels,but also the governing factor the state of undercooled austenite, which determines the final structure of steels. The curves of continuous heating transformation state the effect of heating rating on the austenization of T91 steel as following:Heating rate has a significant effect on the Ac1 temperature for the onset of austenization and the Ac3 temperature for the end of austenization. The thus determined temperatures of Ac1 and Ac3 increase with heating rate increasing. In case austenization rate evidently increase, the periods of austenization also shorten remarkably. The relation between the peak of transformation rate curve and the heating rate accords with the formulation:(With transformation rate and V heating rate). It is also found that the dissolution of carbonization is correlative to the heating rate. When heating rate is low, substantive M23C6 carbide dissolves. As a result, the formation of MX carbide profits from C concentration altering. Owing to the separation of MX carbide,the grains of austenite keep fine even at low heating rate. When the heating rate increase to some extent, most M23C6 carbide don’t dissolve until isothermal holding segment is adopted. The fast heating rate keeps initial grain size fine,but does harm to uniformity of austenite grains. df /dt(2) The rule of phase transformation of T91 steel during continuous cooling is clarified in this part, the martensite critical cooling speed is determined, and the continuous cooling transformation (CCT) curve is obtained. It follows:There reserve only ferritic and martensite during the process of continuous cooling of the explored T91 steel,10℃/min and 2℃/min are the upper and nether critical cooling rates for the phase transformation from austenite to martensite. The quenching rate has a significant effect on the onset temperature of Martensite transformation Ms. The result is different from the classical theory that claimed the transformation point falls when cooling rate reduces. The quenching rate determines the transform point by affecting the carbon atomic group, quenching vacancy and internal stress. With the quenching rate increasing, the structure has a tendency of fining, whereas the cooling rate beyond 1000℃/min, it has nothing on the morphology of the structure.(3) Study on the isothermal stabilization behaviors of austenite localizing near to Ms temperature during the cooling process of undercooled Austenite of T91 steel indicates:When isothermal holding temperature beyond upper Ms, T91 steel possess obvious anti-stabilization characterization. The tendency of anti-stabilization is strong with holding time prolonging. On the other hand, when isothermal holding temperature just beyond Ms, the martensite start temperature of T91 steel is less than Ms, and it decrease continuously with holding time prolonging, but ultimate structure keeps martensite without any residual austenite remaining,what is called false stabilization. While rest on 400℃under Ms, owing to the high transformation temperature,room temperature microstructure of T91 samples does not contain the residual austenite on action of mechanical stabilization, it can still be attributed to false stabilization. When isothermal transformation takes place under 380℃, austenite stabilization of the samples boosts up, the continued transformation temperature of austenization demand reduces, partial transformation of austenite occurs. There exists residual austenite in the final microstructure. At the moment, stabilization of austenite be enslaved to mechanical and heat stabilization treatments.(4) Study on the undercooled austenite applied external stress during cooling, the results demonstrate:In case of the temperature applied external stress over 850℃, the evidence of martensite formation is not observed in T91 steel, While it under 850℃, the result indicates the applied external stress not only facilitates the formation of martensite, but also enhances the onset temperature of the martensite transformation. It is summarized that there are two transformations occurring at this situation: when the temperature applied external compressive stress is high, the mechanism of strain-induced Martensite transformation acts on,as a result, the microstructure inclines to be fine, the grain boundary tends to be irregular. On the other hand, when the temperature applied external compressive stress is low, stress-induced Martensite transformation takes place, the morphology is similar to that of heat-induced martensite. The experiment data reveal that 200MPa is the critical stress for the explored T91 steel and 440℃is critical temperature of the start of the stress-induced martensite transformation.(5) The experiment of Thermal-Mechanical Control Process simulation have been preformed in the wide austenite non-recrystallization region of the explored T91 steel. The principle are verified the induced carbonitride precipitates by thermomechanical treatment can improve the properties of ferritic heat-resistant steel due to abundance Nb, V elements in T91 steel, the results show:The microstructure of T91 steel appears to be refined after thermomechanical treatment, furthermore, the process offers more nucleation positions that facilitate the formation of more nano-sized dispersing MX carbonitride precipitates.In addition, the tension experiment indicated thermomechanical treatment has a significant influence in improving the strength of T91 steel, all of these prove the tentative using thermomechanical treatment to enhance the working temperature of T91 steel is feasible.更多还原