X12CrMoWVNbN10-1-1不锈钢铸锭组织均匀化和细化研究

发布时间：2018-07-05 18:11

本文选题：X12CrMoWVNbN10-1-1 + 组织均匀化和细化　；参考：《上海交通大学》2015年博士论文

【摘要】：X12CrMoWVNbN10-1-1马氏体不锈钢由于具有优良的高温强度、抗腐蚀性能、抗疲劳性能以及低的热膨胀系数,被广泛的用作超超临界发电机组高中压转子材料。近年来,随着人类能源需求的急剧增长,发电机组的装机容量不断提高,高中压转子的尺寸越来越大。尽管高中压转子尺寸的增大增加了电站运行的安全性,但是却给制造带来了巨大的困难。高中压转子的制造是一个融合了铸造、机械加工、热处理、无损检测等多个工序的复杂过程,其中铸造工序为后续加工提供原始坯料。因此为了保证高中压转子的质量,首先必须生产出高质量的大型铸锭。本文以X12CrMoWVNbN10-1-1(以下简称X12)高中压转子钢材料为研究对象,采用热场控制法,分别从两种不同的路径(一是采用砂铸型,通过熔体过热处理,在慢的冷却速度下,使铸锭整体温度场均匀分布、同时凝固;二是采用金属型,通过消除固—液界面间隙,增强冷却速度,使铸锭迅速凝固)实现了X12钢铸锭组织均匀化和细化,并在此基础上研究了不同工艺参数对铸锭组织的影响机理。主要结论如下:研究了砂型铸造过程中,不同工艺参数对X12钢铸锭宏观组织和微观组织的影响。在本实验条件下,砂型铸造的最佳工艺参数是:熔体过热温度1650°C,铸型温度800°C,熔体浇铸温度1600°C,在该条件下得到的铸锭,宏观组织几乎全部由直径1.1mm左右的等轴晶组成,微观组织由马氏体和δ铁素体组成,且其δ铁素体不超过2%。在砂型铸造中,铸型预热温度是影响宏观组织中等轴晶比例的主要因素;熔体过热温度和熔体浇铸温度是影响晶粒大小的主要因素;熔体过热温度是影响δ铁素体的主要因素;研究了不同熔体过热温度对X12钢组织的影响,并对X12钢的熔体过热细化机理进行了探讨。实验发现,经过1650°C的熔体过热处理后,X12钢的凝固组织都是由细小的等轴晶组成,且冷却速度对它的影响不大。通过对合金的微观组织和XRD物相组成的分析,排除了由于熔体内高熔点团簇分解增殖引发异质形核作用增强而导致组织细化的可能性;通过对凝固过程中过冷度的比较,排除了由于熔体变均匀而引发均质形核导致组织细化的可能性。考虑到X12钢凝固过程是包晶反应,在此基础上提出了X12钢熔体过热细化机理。研究认为,FeC合金中的液—液相转变在X12钢熔体过热细化中发挥着决定性的作用,当熔体过热温度为1650°C时,X12钢熔体正好处在液—液相转变区域,它的液相结构中既有类δ结构的团簇,又有类γ结构的团簇,类δ结构的团簇为δ铁素体的形核提供了更多的质点,类γ结构的团簇降低了包晶反应进行所需要的动力,因此短时间内在溶液中形成了大量的晶粒,从而得到了细小的铸态组织。通过对传统金属铸型的改造,提出了一种新的铸造方法—无间隙金属型铸造,该方法的特点就是在原来的金属铸型内侧放置了一个薄壁的可熔金属铸型,该可熔铸型是由低熔点的金属制成。通过对传统铸型铸造和无间隙金属型铸造的X12钢铸锭进行比较,发现新的凝固方法可以极大地提高整个凝固过程中的冷却速度进而细化铸锭组织。通过对凝固过程中可熔铸型的温度变化曲线的综合分析并结合数值模拟,揭示了无间隙金属铸型提高冷却速度的原因:当金属液被浇入到铸型之后,在铸型附近会立即形成一个凝固壳,此时热量通过凝固壳传递到可熔铸型,可熔铸型受热熔化成液体并填充在铸型和铸锭之间;在随后的凝固过程中,金属液始终保持液体状态直到铸锭的凝固彻底结束,这样就消除了由于铸锭凝固收缩和铸型受热膨胀所产生的空气间隙,使得传统铸型凝固中的复杂的传热模式转变成单一的热传导,进而大大地提高了冷却速度并细化了铸锭组织。采用数值模拟,模拟了X12钢无间隙金属型铸造过程中的铸锭的温度场和固相体积分数的变化,获得了影响无间隙金属型铸造冷却速度的主要因素以及无间隙金属型铸造的优化工艺条件,并在该工艺条件下实现了X12钢铸锭组织的均匀化和细化。模拟结果表明,可熔铸型材料的物理性质对无间隙金属型铸造过程中的冷却速度有着重要的影响。在其他性质相同的情况下,可熔铸型材料的熔点越低,热导率越高,凝固过程中可熔铸型的起始熔化时刻就越早;可熔铸型材料的熔化潜热越高,凝固过程中可熔铸型从起始熔化到完全熔化所需要的时间就越久,在选择可熔铸型材料的时候应该综合考虑各种物理性能的影响。尽管较高的浇铸温度可以使可熔铸型提前熔化,获得较大的冷却速度,但是由于系统内引入的热量过多,并不会使铸锭的凝固速度加快,因此在浇铸的时候应该选取较低的浇铸温度。在本文所考察的范围内,X12钢的最佳铸造工艺条件是:可熔铸型材料为6061铝合金,浇铸温度为1560°C。采用无间隙金属型铸造,在最佳工艺条件下对X12钢进行铸造,结果发现X12钢铸锭的宏观组织全部是由均匀细小的等轴晶组成,从铸锭表面到铸锭中心,晶粒尺寸仅从80μm增大到110μm,微观组织都是由马氏体组成。
[Abstract]:X12CrMoWVNbN10-1-1 martensitic stainless steel is widely used as the high school pressure rotor material for super supercritical power generator because of its excellent high temperature strength, corrosion resistance, fatigue resistance and low coefficient of thermal expansion. In recent years, with the rapid growth of human energy demand, the installed capacity of the generator set is constantly improved, high school pressure rotor The size of the high - pressure rotor increases the safety of the power plant, but it has brought great difficulties to manufacturing. The manufacture of the high - pressure rotor is a complex process that combines casting, machining, heat treatment, nondestructive testing and many other processes, in which the casting process provides the original billet for subsequent processing. Therefore, in order to ensure the quality of the high pressure rotor, it is necessary to produce high quality large ingot. This paper uses X12CrMoWVNbN10-1-1 (hereinafter referred to as X12) high and high pressure rotor steel material as the research object, using the heat field control method, from two different paths (one is using sand casting, through melt superheat treatment, at slow cooling rate. " At the same time, the whole temperature field of the ingot is uniformly distributed and solidified. Two is the use of metal type, by eliminating the gap between the solid and liquid interface and strengthening the cooling speed, making the ingot solidified rapidly) to realize the homogenization and refinement of the ingot structure of X12 steel. On this basis, the influence mechanism of different process parameters on the ingot structure is studied. The main conclusions are as follows: In the sand mold casting process, the influence of different process parameters on the macro structure and microstructure of X12 steel ingot is made. Under this experimental condition, the optimum parameters of sand mold casting are as follows: melt superheating temperature 1650 C, casting temperature 800 C, melt casting temperature 1600 C, and the macroscopic microstructure of the cast ingot under this condition is almost entirely from the diameter 1.1mm left The microstructure of the right ISO axis is composed of martensite and delta ferrite, and the delta ferrite is not more than 2%. in sand casting. The preheating temperature of the mold is the main factor affecting the medium axis crystal proportion of macrostructures. The melt superheating temperature and the melt casting temperature are the main factors affecting the grain size, and the melt superheat temperature is the influence of the delta ferrite. The influence of different melt superheating temperature on the microstructure of X12 steel was studied. The mechanism of melt superheating refinement of X12 steel was discussed. The experiment found that after the melt superheat treatment of 1650 degree C, the solidification structure of X12 steel was made up of fine equiaxed grains, and the cooling rate had little effect on it. The analysis of the composition of tissue and XRD phase excludes the possibility of microstructure refinement due to the enhancement of the heterogeneous nucleation caused by the decomposition and proliferation of high melting point clusters in the melt. By comparing the supercooling degree of the solidification process, the possibility of microstructure refinement caused by the homogeneous nucleation caused by the melting of the melt is excluded. Considering the solidification of the X12 steel The process is a peritectic reaction. On this basis, the superheating refining mechanism of X12 steel is proposed. It is considered that the liquid liquid phase transition in the FeC alloy plays a decisive role in the melt superheating refinement of the X12 steel. When the melt superheating temperature is 1650 C, the melt of X12 steel is in the liquid liquid phase transition region, and the liquid phase structure of the molten steel has a delta junction. Clusters of structured clusters and clusters of gamma like structures and clusters of delta structures provide more particles for the nucleation of delta ferrite. Clusters of gamma like structures reduce the power needed for the peritectic reaction. Therefore, a large number of grains are formed in the solution in a short time, and thus the fine cast microstructure is obtained. A new casting method, non gap metal casting, was put forward, which was characterized by a thin wall of molten metal cast on the inside of the original metal mold, which was made of low melting metal. By comparing the X12 steel ingot of the traditional casting and the non gap metal casting, the new casting was found to be new. The solidification method can greatly improve the cooling rate of the whole solidification process and then refine the ingot structure. Through the comprehensive analysis of the temperature change curve of the molten casting type in the solidification process and the numerical simulation, it reveals the reason that no gap metal casting can improve the cooling speed: when the metal is poured into the mold, the mold is attached to the mold. In the near future a solidifying shell is formed, when the heat passes through the solidified shell to the molten cast mold, and the molten cast heat is melted into liquid and filled between the mold and the ingot; in the subsequent solidification the liquid keeps the liquid state until the solidification of the ingot is completely finished, thus eliminating the ingot solidification and shrinkage and the casting mold. The air gap produced by thermal expansion makes the complex heat transfer mode in the traditional casting solidification transform into a single heat conduction, and then greatly improves the cooling rate and refines the ingot structure. Numerical simulation is used to simulate the change of the temperature field and the solid volume fraction of the ingot in the X12 steel without gap metal casting. The main factors affecting the cooling rate of the non gap metal casting and the optimum technological conditions for the non gap metal casting have been achieved, and the homogenization and refinement of the X12 steel ingot are realized under the conditions. The simulation results show that the physical properties of the molten cast materials are important for the cooling rate in the process of non gap metal casting. The lower the melting point of the casting type material, the higher the thermal conductivity, the earlier the melting time of the melt cast type in the solidification process, the higher the melting latent heat of the casting type material, the longer the time it takes for the melting casting from the beginning melting to the complete melting in the solidification process, and the choice of melting casting in the choice of melting casting. The effect of various physical properties should be taken into consideration when the material is made. Although the high casting temperature can make the molten casting melt in advance and get a larger cooling rate, the casting temperature should not be accelerated because of the excessive heat introduced in the system, so the lower casting temperature should be selected in the casting. In the scope of the study, the optimum casting conditions for X12 steel are: the molten cast material is 6061 aluminum alloy and the casting temperature is 1560 C., the X12 steel is cast under the best technological conditions. The results show that the macroscopic microstructure of the X12 steel ingot is all composed of uniform and fine equiaxed grains, from the surface of the ingot. To the center of the ingot, the grain size increases from only 80 m to 110 m, and the microstructure is composed of martensite.
【学位授予单位】：上海交通大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG260

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