大型铸锭均质化问题及其新解

Solutions in Improving Homogeneities of Heavy Ingots

铸锭存在严重非均质问题,直接影响最终产品的性能和材料利用率,并制约后续热加工方法和工艺窗口选择。提高铸锭的均质化水平对节能降耗、提高材料利用率、保障构件服役性能和重大装备建设具有重要意义。本文介绍了大型铸锭非均质化因素如成分偏析、夹杂、缩孔/疏松和晶粒不均匀问题及其在后续热加工中的演变,提出了铸锭均质化窗口的概念;介绍分析了大型铸锭凝固过程宏观偏析数值计算和模拟研究的新进展以及提高铸锭均质化水平的新方法;针对冷却速率不均、不可控多相流动和非平衡溶质分凝3个造成铸锭非均质的根本原因,提出了可以根据目标均质化窗口进行铸造过程预设计的层状铸造(layer casting)新方法,建立了层状单元分配和变成分浇注成分控制模型。数值计算模拟和实验验证表明,层状铸造在降低多相流速、均化冷却速率、改变溶质分凝方面效果显著,提高了铸锭的均质化水平,并可大幅度降低设备投资、节能降耗,有望应用于高品质大型铸锭制造。

The inhomogeneity in large ingots not only decides the final properties of the product, but also restricts downstream hot working processing severely. It is very important to improve the homogeneity of ingots for saving energy, improving material utilization ratio, increasing performance of component,and the construction of key equipment. In this paper, the general inhomogeneity problem in large ingots,such as macrosegregation, inclusion, shrinkage porosity, and large crystal have been introduced. The evolutions of this inhomogeneity in the subsequent hot working processing have also been discussed,based on which the concept of homogeneity window for large ingots has been proposed. The research progress of numerical simulation of macrosegregation in large ingots and some new methods for improving the homogeneity of large ingot have also been introduced and analyzed. Three fundamental reasonsfor the inhomogeneity of ingots were concluded, i.e. the uneven cooling rate, the uncontrollable multiphase flow, and the solute redistribution during solidification. Aiming at these three fundamental reasons,a novel casting method called layer casting（LC）, which has been proposed by our team recently, was introduced to modify the serious inhomogeneity problem in large ingots. In this method, molten alloy was poured into the mold separately and layer upon layer. As soon as the poured molten alloy solidified to a critical volume fraction range, the next layer amount of molten alloy was poured into the mold. For each layer, the mass, composition, and pouring temperature of poured molten alloy could be artificially designed and controlled based on the target homogeneity window. Both experiment and numerical simulated results shown that, in comparison with conventional ingot fabrication method, the LC method can significantly decrease the uncontrollable multiphase flow, uniform the cooling rate, and improve the solute redistribution, subsequently, improve the homogeneity of ingots. For large ingots fabrication, the LC method has the potential to substantially decrease the energy consumption, materials consumption, and the investment of large equipment. Its wide application prospect for high quality large ingots is also expected.