脉冲磁致振荡促进固液界面前沿异质形核机理

The Pulse Magneto-Oscillation (PMO) is an original technique which was proposed by the Center for Advanced Solidification Technology of Shanghai University. The high homogenizing continuous casting billet, with macrosegregation greatly decreased and solidification structure notably refined, can be achieved by PMO treatment. So far, it has been confirmed by both experiments and industrial application that PMO could stimulate the heterogeneous nucleation in front of the solid-liquid (S/L) interface, but the mechanism has not been revealed clearly. As the growth undercooling usually far smaller than nucleation undercooling, the heterogeneous nucleation in front of dendritic of billet will be very hard. In this project, we intend to calculate the electric, magnetic, thermal and force effects of PMO on solidification process, and then the distinctive “Thermal Simulating Apparatus for Dendritic Growth of Continuous Casting Billet” will be employed to study the effect of PMO on temperature gradient, distribution of solutes and undercooling in front of S/L interface. Finally, the mechanism of the heterogeneous nucleation in front of S/L interface under the stimulating of pulse magneto-oscillation will be revealed, and it will support the industrial application of PMO.

脉冲磁致振荡（Pulse Magneto-Oscillation ，简称PMO）是上海大学先进凝固技术中心开发的原创技术，能够显著细化连铸坯凝固组织、减小宏观偏析。前期研究和工业试验表明，PMO可促使固液界面前沿大量异质形核，从而细化凝固组织。由于晶体生长所需过冷度一般远小于形核所需过冷度，因此在铸坯枝晶生长前沿形核是非常困难的。本课题拟在电磁理论计算的基础上，利用独创的连铸坯凝固过程热模拟技术，研究PMO的电、磁、热、力效应及其对连铸坯固液界面前沿温度分布、溶质扩散和过冷状态的影响规律，揭示PMO促进固液界面前沿形核的机理，为PMO技术的工业应用提供理论支撑。

本项目通过理论分析、定向凝固、连铸坯凝固过程热模拟及工业试验，研究了脉冲磁致振荡（Pulse Magneto-Oscillation ，简称PMO）的电磁热力效应及其促进固液界面形核的机制。理论分析和定向凝固实验表明，PMO会造成固液界面温度梯度轻微增加，但变化幅度仅约为1 K/mm，对成分过冷区影响很小。但是，PMO作用下，固液界面前沿的溶质浓度梯度显著增加，溶质富集层减薄，从而造成成分过冷区及最大成分过冷度增加，利于固液界面前沿的异质形核。热模拟实验和工业试验表明，中碳钢铸坯在结晶器电磁搅拌和末端电搅作用下仍可以形成穿晶组织，而在二冷区施加PMO，等轴晶面积比例可以达到25%，该现象也表明PMO可以有效促进固液界面前沿形核，而非只通过强制对流作用。