大型汽轮机转子钢电渣重熔铸锭黑斑形成机理及趋势评价研究

Freckle (Channel segregation) formation becomes more significant in the electroslag remelting process (ESR) ingot of rotor steel for large steam turbine and limits the enlargement of steam turbine in order to improve the efficiency further. Unfortunately, there is lack of systemic study on the freckle formation of ESR ingot for rotor steel and controversies on the freckle formation and criterion are still existed. Therefore, in this study, a mathematical model of macroscopic multi-physic fields is firstly developed to simulate the electromagnetic effect, heat transfer and multiphase flow in the ESR system. Secondly, a multi-component dendritic growth model coupled with fluid flow and thermosolutal diffusion is developed to characterize the dendrite structure morphology of mushy zone and predict the dendritic growth, solute segregation and non-stable interdendritic flow in mushy zone of ESR ingot. Base on the mentioned studies above, the effects of non-stable interdendritic flow on the interface imbalance of dendritic growth and solute enrichment are investigated by the solute macro/micro-segregation model using the multi-scale numerical simulation method, and the mechanism of freckle formation and evolution is proposed. Finally, the evaluation criterion of freckle formation is established, based on the quantitative prediction of dendrite structure morphology of mushy zone, and provides the theoretical guidance for the process optimization to avoid the freckle formation in ESR ingot.

大型汽轮机用转子锻件电渣重熔铸锭母材制备过程中面临黑斑（孔道偏析）形成趋势越加明显的难题，成为了转子大型化提高汽轮机效率的限制性环节。鉴于目前国内外转子钢电渣重熔铸锭黑斑形成机理研究尚不系统深入的现状，本项目拟首先建立电渣重熔过程宏观多物理场数学模型，系统阐述转子钢电渣重熔过程电磁效应、热量传递、多相流流动等宏观传输行为。然后，建立流动和热溶质扩散协同作用下多组元枝晶生长数学模型，定量表征金属熔池糊状区枝晶凝固组织形貌和特征参数，阐明金属熔池糊状区枝晶生长、溶质偏析和钢液非稳定流动行为。在此基础上，采用宏微观多尺度数值模拟方法建立电渣重熔过程宏微观多尺度溶质偏析数学模型，研究金属熔池糊状区非稳态钢液流动下枝晶生长界面失衡和枝晶间浓缩钢液汇集行为，揭示电渣重熔钢锭孔道偏析形成和演变规律。最后，建立基于凝固枝晶组织定量预测的黑斑形成趋势评价标准，为抑制铸锭黑斑缺陷优化电渣重熔工艺提供理论指导。

转子作为汽轮机核心部件之一，服役过程中常年经受高温热冲击、离心载荷、扭转和弯曲等外力作用，对其综合力学性能要求较为苛刻。转子钢母材一般需要电渣重熔制备技术生产，但面临黑斑缺陷的问题。本研究基于CFD商业软件FLUENT建立了电渣重熔过程多物理场耦合数学模型，揭示了高中压转子钢30Cr1Mo1V电渣重熔体系内电渣重熔体系内电磁场、流场、温度场和凝固场规律，以及自耗电极熔化滴落行为。基于Cellular Automaton理论建立了电渣重熔铸锭凝固晶粒组织生长数学模型，揭示了高中压转子钢30Cr1Mo1V电渣重熔体系内电渣重熔铸锭凝固组织演变规律，以及电渣重熔工艺对铸锭凝固组织的影响规律。在此基础上，采用特征单元法，基于Cellular automaton理论建立二维枝晶生长数学模型，揭示了钢液温度和熔体流动对枝晶生长行为的影响规律。当电渣重熔电流强度和填充比分别为8000A和0.56时，电极熔化速度为230kg/h，金属熔滴平均直径为10mm，金属熔池深度115mm，铸锭凝固组织为表面向中心倾斜生长的柱状晶，枝晶前沿流速较低，柱状晶生长受影响较小。基于上述宏微观多尺度数值模拟方法建立了电渣重熔铸锭凝固组织生长规律与电渣重熔工艺之间的定量关系，可为高级别大型汽轮机转子钢母材制备提供理论指导。