电渣重熔Mn18Cr18N奥氏体不锈钢热变形过程组织演变的研究

发布时间：2018-11-15 13:50

【摘要】：Mn18Cr18N高氮奥氏体不锈钢因其优良的耐蚀性、塑韧性被广泛用于发电机护环生产。由于冶金技术的制约,该钢种的传统铸造组织晶粒粗大且缺陷严重,电渣重熔工艺的运用极大提高了铸坯的组织质量。虽然组织缺陷大大减少,但是晶粒组织基本由粗大的柱状晶组成。Mn18Cr18N钢是非相变钢,无法通过热处理手段优化组织性能。所以,细化晶粒成为改善组织性能的关键技术手段。护环加工所用坯料的组织状态大部是分锻态的,由于锻造过程中加工工序繁多,工艺复杂,研究电渣重熔Mn18Cr18N钢的铸态组织的热变形行为,对优化护环生产工艺具有重要的现实意义。本文研究内容如下:首先,针对电渣重熔Mn18Cr18N奥氏体不锈钢进行单向热压缩实验,获得了不同变形条件下的应力-应变曲线,分析了该钢种的热变形规律。结果表明,该钢种对应变速率非常敏感,具有明显的正应变速率敏感性。其次,研究了Mn18Cr18N奥氏体不锈钢铸态组织在不同变形条件下的动态再结晶规律。分析获得了温度、应变、应变速率对动态再结晶软化行为的影响,并构建了该钢种的本构方程及动力学模型。并借助金相显微镜(OM)的组织观测,获得了变形组织的动态再结晶百分数,通过origin数据拟合软件,建立了动态再结晶百分数模型和晶粒尺寸模型。研究结果表明:动态再结晶晶粒主要以“项链”状模式从原始晶界处产生。随着温度的升高、应变速率的降低,动态再结晶百分数逐渐增大。对比锻态组织发现,铸态组织变形激活能较高,而动态再结晶激活能较低,说明了铸态组织较难发生塑性变形,却更易发生动态再结晶。然后,分析了Mn18Cr18N奥氏体不锈钢微观变形组织在动态再结晶机制下的能量耗散规律,基于动态材料模型,建立了该钢种在不同应变下的热加工图。研究发现,增大应变可以提高该钢的热加工性能。热加工图的建立有助于分析热加工工艺参数对微观组织的影响,为优化护环加工工艺提供理论依据。最后,针对Mn18Cr18N奥氏体不锈钢在Gleeble-1500D热模拟实验机双道次热压缩试验,研究了不同温度、保温时间、应变对该钢种静态再结晶规律的影响。研究结果表明,温度越高,道次间隔保温时间越长,静态再结晶百分数越大。
[Abstract]:Mn18Cr18N high-nitrogen austenitic stainless steel is widely used in generator ring production because of its excellent corrosion resistance and ductility. Due to the restriction of metallurgical technology, the traditional casting microstructure of the steel is coarse in grain size and serious in defect, and the application of electroslag remelting technology greatly improves the microstructure quality of the cast billet. Although the microstructure defects are greatly reduced, the grain structure is basically composed of coarse columnar crystals. Mn18Cr18N steel is not a phase change steel and can not be optimized by heat treatment. Therefore, grain refinement has become a key technology to improve microstructure and properties. The microstructure state of the blanks used in the ring protection machining is mostly in the split forging state. Due to the numerous working procedures and the complex process during the forging process, the hot deformation behavior of the as-cast structure of the Mn18Cr18N steel remelted by electroslag is studied. It has important practical significance to optimize the production process of protective ring. The main contents of this paper are as follows: firstly, unidirectional thermal compression experiments were carried out for electroslag remelted Mn18Cr18N austenitic stainless steel. The stress-strain curves under different deformation conditions were obtained, and the hot deformation law of the steel was analyzed. The results show that the steel is very sensitive to strain rate and has obvious positive strain rate sensitivity. Secondly, the dynamic recrystallization of Mn18Cr18N austenitic stainless steel under different deformation conditions was studied. The effects of temperature and strain rate on the dynamic recrystallization softening behavior were analyzed and the constitutive equation and kinetic model of the steel were established. The dynamic recrystallization percentage of deformed microstructure was obtained by means of microstructure observation of metallographic microscope (OM). The dynamic recrystallization percentage model and grain size model were established by origin data fitting software. The results show that the dynamic recrystallization grains are mainly formed in the "necklace" pattern from the original grain boundary. With the increase of temperature and the decrease of strain rate, the percentage of dynamic recrystallization increases gradually. Compared with the forged microstructure, the deformation activation energy of as-cast microstructure is higher, but the dynamic recrystallization activation energy is lower, which indicates that the as-cast microstructure is more difficult to produce plastic deformation, but more prone to dynamic recrystallization. Then, the energy dissipation law of the microstructure of Mn18Cr18N austenitic stainless steel under dynamic recrystallization mechanism was analyzed. Based on the dynamic material model, the hot working diagram of the steel was established under different strain. It is found that the hot working properties of the steel can be improved by increasing the strain. The establishment of the hot working diagram is helpful to analyze the influence of the process parameters on the microstructure and to provide the theoretical basis for optimizing the processing technology of the protective ring. Finally, the effects of different temperature, holding time and strain on the static recrystallization of Mn18Cr18N austenitic stainless steel in Gleeble-1500D thermal simulation machine were studied. The results show that the higher the temperature, the longer the holding time and the higher the percentage of static recrystallization.
【学位授予单位】：太原科技大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG142.1

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