薄带双辊铸轧凝固过程组织演变的数值模拟

发布时间：2018-04-30 00:14

本文选题：双辊铸轧 + 镁合金　；参考：《东北大学》2015年博士论文

【摘要】：薄带双辊铸轧技术作为冶金及材料加工领域内的一项前沿技术,因为其短流程及快速凝固的特点,不仅可以提高生产效率、降低成本,还能有效的改善带坯的性能,同时还可以生产常规工艺下难以加工的产品。由于双辊薄带铸轧过程中各工艺参数间的匹配比较复杂,如果只是采用实验方法,不仅耗时长、工作量大,还存在一定的盲目性。而通过计算机模拟则可以方便地找出铸轧过程中各工艺参数之间的联系,得到不同工艺参数对铸轧产品的微观组织以及铸轧工艺过程稳定性的影响,找出其中的规律性,对于优化铸轧生产工艺,提高薄带质量具有重要的理论意义。本文基于立式双辊薄带铸轧工艺凝固过程的特点,建立了薄带铸轧凝固过程微观组织演化的数理模型,并结合元胞自动机(CA)方法,实现了对薄带凝固过程的微观组织演化的数值模拟。本文的主要研究内容如下： (1)运用有限元法来求解铸轧过程熔池内部的三维宏观传输问题,基于铸轧熔池的形状特点,采用了智能网格划分技术,以AZ31B镁合金为对象研究了不同工艺条件下熔池内部的温度场及流场的分布情况。 (2)建立了基于溶质扩散控制的枝晶生长模型,并通过对KGT模型的改进,建立了对流条件下的枝晶尖端生长动力学模型,结合元胞自动机方法,提出了枝晶生长模型的数值算法。 (3)通过跟踪一个随辊面移动的微区作为微观组织模拟区域,该模拟区域内部的温度场由宏观温度场的结果通过插值的方法计算,而溶质扩散及晶粒的形核生长的计算则在微观的尺度上进行。随着模拟区域与铸辊同步的移动,实现了对铸轧凝固微观组织演化过程的宏微观耦合模拟。 (4)基于所建立的枝晶生长模型及宏微观耦合算法,编制了铸轧薄带凝固过程微观组织模拟程序,可以实现对枝晶形貌及晶粒组织演化的可视化模拟。 (5)运用编制的模拟程序,对镁合金单个等轴晶及多个等轴晶的生长进行了模拟研究,并与实验结果进行了对比,两者吻合较好。对不同工艺条件下,镁合金薄带的微观组织演化进行了模拟,研究了工艺参数对镁合金薄带凝固组织的影响规律。 (6)运用编制的模拟程序,对3%Si硅钢单个等轴晶及定向凝固过程的枝晶形貌进行了模拟计算,以此来研究硅钢凝固过程枝晶的生长特性。对硅钢凝固过程中柱状晶向等轴晶的转变(CET)进行了模拟,研究了温度梯度和冷却速度对凝固过程中柱状晶与等轴晶间竞争生长的影响规律。在此基础上对不同过热度条件下硅钢薄带的凝固组织进行了模拟,与实验结果的对比显示了两者具有很好的一致性。通过对不同工艺条件下铸轧硅钢薄带凝固组织的模拟,研究了工艺参数对薄带的枝晶间距、柱状晶区比例等形貌特征的影响规律,为铸轧硅钢薄带的工艺优化提供了理论依据。采用流场作用下的枝晶生长动力学模型,结合“偏心四边形”CA生长算法,对铸轧凝固过程中柱状晶沿着铸轧方向倾斜的现象进行了模拟,模拟结果与实验结果取得了很好的一致。
[Abstract]:As one of the frontier technologies in the field of metallurgy and material processing , the thin strip double - roll casting and rolling technology can not only improve the production efficiency and reduce the cost , but also can effectively improve the performance of the strip blank .

Based on the characteristics of solidification process of vertical twin - roll strip casting and rolling process , a mathematical model of microstructure evolution of thin strip casting and rolling solidification process is established , and the numerical simulation of microstructure evolution of thin strip solidification process is realized by combining the method of cellular automata ( CA ) . The main research contents are as follows :

( 1 ) Using the finite element method to solve the three - dimensional macroscopic transmission problem in the casting - rolling process pool , based on the shape characteristics of the casting - rolling pool , the distribution of the temperature field and the flow field in the molten pool under different process conditions is studied with AZ31B magnesium alloy as the object .

( 2 ) The dendrite growth model based on solute diffusion control is established , and the dynamic model of dendrite tip growth under convection conditions is established by the improvement of KGT model , and the numerical algorithm of dendrite growth model is proposed by combining the cellular automata method .

( 3 ) By tracking a micro - area moving along with the roll surface as the microstructure simulation area , the temperature field inside the simulation area is calculated by the interpolation method based on the result of the macroscopic temperature field , and the calculation of solute diffusion and nucleation growth of the crystal grains is carried out on the microscopic scale . As the simulation area moves synchronously with the casting roll , the macro - microscopic coupling simulation of the casting - rolling solidification microstructure evolution process is realized .

( 4 ) Based on the established dendrite growth model and the macro - micro - coupling algorithm , the microstructure simulation program for the solidification process of the casting and rolling thin strip is prepared , and the visualization simulation of dendrite morphology and grain structure evolution can be realized .

( 5 ) The growth of single isometric crystal and multiple axes of magnesium alloy was simulated and compared with experimental results . The microstructure evolution of magnesium alloy thin strip was simulated under different process conditions . The influence of process parameters on solidification microstructure of magnesium alloy thin strip was studied .

In this paper , the effects of temperature gradient and cooling rate on the growth of dendrite in the solidification process of Si - Si steel are simulated . The influence of temperature gradient and cooling rate on the growth of columnar crystals and equiaxial crystals in solidification process is simulated .

【学位授予单位】：东北大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG335.9

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