大截面超洁净双性能汽轮机转子真空电渣重熔制备的科学基础

In order to improve the cleanness, the alloying element homogeneity, and the grain fineness to meet the requirement of the large size and ultra clean turbine rotor steel with two performance， the present project proposes the technology of the vacuum electroslag remelting for preparation the ingot for turbine rotor. Furthermore, with the help of the vacuum, the size of the residual inclusions in the large size and ultra clean turbine rotor ingot would decrease and the distribution also become more uniformity. The tiny and homogeneous residual inclusions are supposed to improve the performance of turbine rotor. According to the metallurgical thermodynamics, the vacuum electroslag remelting owns a greater ability to manufacture high quality steel, because it is able to reduce the residual oxygen content in the ingot as well as promote the homogeneity of the distribution of the residual inclusion. However, some phenomena are still cannot be understood such as the varying of the slag properties, the physical chemistry mechanisms of the deoxidation as well as the inclusion trapping during the process. Moreover, the inclusion behavior in the molten slag and liquid metal pool during the directional solidification should be focused on. It is our duty to reveal the sizes varying and the distribution of the inclusions trapped by the solidified ingot. In order to accurately control the vacuum electroslag remelting process, the evolution of the metal pool should be illustrated. Therefore, we want to clearly point out the physical chemistry mechanisms of the deoxidation as well as the inclusion removing. The relationship between the oxygen content of the steel and the slag compositions as well as the vacuum degree ranging would be demonstrated. On the other hand, for the sake of macrosegregation prediction, the physical fields and the redistribution of the elements in the vacuum electroslag remelting furnace would be presented. At last, we will construct a theory which can help us to produce high performance turbine rotor ingot.

为满足大截面超洁净双性能汽轮机转子钢洁净度、残余夹杂物与合金成分均匀度和组织精细度的控制要求，本项目提出真空电渣重熔制备汽轮机转子钢锭方法。基于冶金热力学分析，真空电渣重熔具有冶炼高洁净度钢的有利条件。但尚不明确真空条件下电渣重熔渣系性能变化及超洁净双性能汽轮机转子钢脱氧与夹杂物去除的物理化学机理，对残余夹杂物在定向凝固过程中的行为及凝固后夹杂物的尺寸、数量与空间分布也缺乏认识，并且存在重熔过程液态金属熔池形状的动态控制问题。研究真空电渣重熔条件下脱氧过程与夹杂物去除的物理化学规律，建立钢中氧含量与渣系组分、脱氧元素和真空度范围之间的关系。研究电渣工艺中定向凝固对非金属夹杂物的形态、尺寸和分布的影响。研究非稳态电渣重熔系统多物理场的分布及变化规律。研究真空电渣重熔凝固过程的合金元素再分配规律，建立宏观偏析预测模型。发展高洁净度和高均匀度合金钢冶炼工艺理论，提升我国汽轮机转子制造技术。

为满足大截面超洁净双性能汽轮机转子钢洁净度、合金成分均匀度与组织精细度的控制要求，本项目提出真空电渣重熔制备汽轮机转子钢锭方法。.本项目研制了国内首台套真空-气氛保护电渣重熔炉，可生产直径200mm、重量250kg以下的电渣锭。通过热重分析研究了常用渣系在1400℃的挥发行为，气相沉积结果显示挥发产物为AlF3，通过提升CaO/Al2O3比例和降低CaF2含量有利于减少氟化物的挥发。优化真空电渣重熔渣系，并提出了一种真空电渣重熔炉冶炼高洁净度钢锭的工艺方法。.基于相间反应热力学，发展了电渣重熔过程氧、硫传递的动力学模型，反应速率受限于离子在相内和相间的迁移速率，并与磁流体传热模型进行瞬态耦合计算。在交流电渣重熔过程中主要考虑了熔渣组成对传质过程的影响；在直流电渣重熔过程中，电化学反应起主导作用。.探明了真空电渣重熔过程脱氧机理。实验电极含有较高的碳含量(0.45%)，具备发生碳脱氧([C]+[O]=(CO)(g))的条件。根据C-Al热力学平衡可知，当炉内压力为常压时不能生成CO气体，当炉内压力为10kPa时能生成CO气体。作为脱氧产物的CO气体极其稳定且易于从熔体中逸出。真空度的提高也会导致熔渣挥发加重及熔炼稳定性降低。.采用欧拉-拉格朗日双向耦合方法，揭示了电渣重熔过程非金属夹杂物的去除、运动及捕获行为。根据夹杂物粒子在熔体中的复杂受力分析，夹杂物的去除主要发生在电极端部熔滴的形成与滴落过程，主要依赖熔渣的机械性刷洗。引用粒子在糊状区的捕获机制，钢锭中夹杂物分布沿径向增加，最大值位于0.9半径附近。.发展了电渣重熔过程合金元素成分再分配理论与模型。使用网格动态生成技术追踪电渣重熔中铸锭持续生长的过程，采用各向异性多孔介质模型模拟糊状区的动量衰减，采用Lever算法计算溶质再分配行为。分析了合金元素含量及电参数对碳元素宏观偏析的影响，为减轻转子钢电渣重熔过程中宏观偏析现象提供新思路。.通过提出内棒外管式双极串联和旋转电极的电渣重熔新方法，可望有效解决大截面超洁净电渣重熔汽轮机转子钢锭质量问题。