大圆坯模具钢连铸过程溶质元素跨尺度传输及偏析成因研究

Substituting continuous casting for mold casting can achieve high efficiency and high yield of die steel production. However, the central segregation and porosity of solute elements in the large cross-section of round billet continuous casting process is a bottleneck problem of die steel quality improvement. Solidification is a process of thermo and solute transport at the phase interface, involving the macro-scale thermosolutal convection and micro-scale solute precipitation, local solute redistribution, etc., which directly results in internal defects of the casting round billet. In view of the macro-micro multi-scale transport phenomenon during the large cross-section of round billet casting process, a series of visual transparent model solidification experiments are used to reveal the relationship between macro-scale thermosolutal convection in the liquid pool and the micro-scale diffusion movement at the liquid-solid mushy zone. The effect of the columnar crystal and equiaxial crystal growth processes on the micro-scale diffusion of solute elements is also studied. A macro-micro multi-scale transmission model in the continuous casting solidification process is developed based on the volume average method. The macro-scale convection movement in the liquid pool, micro-scale diffusion movement at the mushy zone, and the microstructure evolution of solid phase are coupled in this model. Then the model is used to reveal the effects of thermosolutal convection, feeding flow and crystal sedimentation on the macrosegregation, and the combined effects of solidification shrinkage, precipitation pressure of gas solutes and surface tension on the formation and growth of porosity. The theoretical foundation for the improvement of the internal quality of large cross-section of round billet continuous casting die steel is built.

用连铸代替模铸能够实现模具钢制备的高效率和高收得率。然而，大截面圆坯连铸过程溶质元素中心偏析和疏松是制约模具钢质量提升的瓶颈问题。凝固过程是相界面热和溶质传输的过程，涉及宏观尺度的热溶质对流和微观尺度的溶质扩散、局部溶质再分配等不同尺度间的传输，直接导致连铸坯的内部缺陷。针对大圆坯连铸过程中的宏-微观跨尺度传输现象，开展可视化的透明模型凝固实验，揭示液相宏观热溶质对流与液固模糊区微观扩散运动的作用规律，同时研究凝固结晶方式即柱状晶与等轴晶生长过程对溶质元素微扩散的影响。基于体积平均法建立连铸坯凝固过程的多相宏-微观跨尺度传输模型，实现液相宏观对流、模糊区微观扩散及固相微观组织演化等不同尺度凝固传输现象的耦合。揭示热溶质对流、补缩流及晶粒沉降对大截面连铸坯宏观偏析的影响规律。揭示凝固收缩、气相溶质析出压力及表面张力共同作用下的疏松形成与长大规律。为大圆坯模具钢内部质量提升奠定理论基础。

大截面圆坯连铸过程溶质元素中心偏析和疏松是制约模具钢质量提升的瓶颈问题。针对大圆坯连铸过程中的宏-微观跨尺度传输现象，基于相似理论建立了连铸冷却条件下的可视化凝固实验装置，研究了连铸坯凝固过程中的传输行为，包括凝固推进过程中的流变行为和凝固微观组织演变规律（枝晶形貌、柱状晶/等轴晶生长、柱状晶向等轴晶的转变、等轴晶比例等）；并研究了冷钢带喂入对连铸坯凝固组织的影响，发现向结晶器内插入冷钢带能有效抑制柱状晶生长，提高等轴晶比例，并细化等轴晶粒。基于体积平均法建立了液相-柱状晶-等轴晶三相凝固模型，并对凝固实验过程进行了模拟，再现了实验中观察到的凝固组织演变过程，获得了实验无法准确量化的柱状晶、等轴晶体积分数、溶质浓度分布以及冷钢带喂入参数与凝固组织中等轴晶率的关系。建立了磁-热-流多场耦合大涡模拟模型，研究了电磁搅拌作用下的大截面连铸圆坯内的空气-保护渣-钢液三相瞬态流动行为，获得了大圆坯结晶器内的电磁场分布规律、凝固坯壳对结晶器流场的影响规律、渣/金界面的波动规律以及氩气泡/夹杂物在结晶器内的分布规律。为了消除或减轻大截面连铸圆坯的中心偏析和疏松等缺陷，提出采取结晶器喂钢带工艺，利用冷钢带的熔化降低连铸坯中心位置的过热度，促进等轴晶形核，进而扩大等轴晶区、减小柱状晶区，密实大截面连铸圆坯的内部组织结构。本项目的顺利实施为促进大截面连铸圆坯的内部质量提升奠定了理论基础。项目实施期间共发表学术论文38篇，其中ESI高被引论文1篇，SCI检索论文21篇，EI检索论文3篇。共授权发明专利5项。相关成果获得辽宁省优秀博士学位论文、辽宁省优秀自然科学学术成果一等奖、沈阳市“十大”优秀自然科学学术成果等。此外，得益于本项目的资助，项目负责人入选了2018年度中国科协青年人才托举工程。