PMO在特殊钢立式连铸过程中的物理与力学冶金行为

It is inevitable that some defects, including chemical segregations, shrinkage cavities and porosities, may form during the metal solidification process due to selective crystallization and volume shrinkages. These defects largely hinder the quality improvement of cast slabs, especially for some special steels with higher content of alloying elements. It has been demonstrated that grain refinement of solidified microstructure could overcome this bottleneck. In this regard, accumulated from our previous working experience in electric current pulsed (ECP) refinement during metal solidification, the proposers developed pulsed magneto-oscillation (PMO) technique to effectively refine grains. Based on the theoretical prediction, numerical simulation and experimentation, this project will be conducted through three unique pronged approaches, including inhomogeneous-nucleation interfacial tension thermal analysis, thermal simulation of continuous cast slab, and physical simulation of metal liquid flow. This research will reveal the solutal diffusion and distribution, microstructural features and stability, thermodynamics and dynamics for the inhomogeneous nucleation on the front of solid-liquid interface on the atomic scale. In addition, this research will clarify the distribution and influence of electric, magnetic and mechanical fields on the temperature and flow fields during the continuous casting process. Through this program, the grain refinement mechanism and activation condition of the PMO technique in continuous casting will be elucidated, enabling the theoretical foundation of solidification in electromagnetic fields for the PMO large-scale application.

由于选分结晶和体积收缩，金属在凝固过程中不可避免地会出现成分偏析和缩孔疏松缺陷。对于合金元素含量较高的特殊钢，这些缺陷尤为突出，已成为铸坯质量提升的瓶颈。细化凝固组织是解决这些凝固缺陷的有效办法。申请人及其团队在揭示了脉冲电流细化金属凝固组织机制的基础上，提出了脉冲磁致振荡（简称PMO）凝固细晶原创技术。本项目理论分析、数值模拟和实验研究相结合，采用PMO作用下金属异质形核DSC-润湿角联测、连铸坯枝晶生长热模拟和液态金属流动物理模拟等三项独具特色的实验手段，在微观上揭示PMO作用下铸坯固液界面前沿溶质扩散与分布规律、固液界面微观结构及其稳定性、钢的异质形核机制及其热力学和动力学条件，在宏观上揭示PMO作用下铸坯中电、磁、力分布规律及其对铸坯温度场和流场的影响，从而比较完整地阐述PMO细化连铸坯凝固组织的机制和条件，丰富电磁凝固理论，为高效经济地将PMO应用于商业化连铸生产奠定基础。

细化特殊钢连铸坯凝固组织是实现凝固均质化和获得高品质特殊钢的有效手段。本项目基于申请人原创的脉冲磁致振荡（PMO）凝固细晶均质化技术，聚焦研究决定PMO细晶效果的PMO力学冶金效应和物理冶金效应两个关键科学问题。得到如下重要结果：.（1）电磁场演变规律研究表明，PMO作用下单个脉宽期间，磁场方向改变一次，感应电流方向改变两次，电磁力方向改变三次，其中电磁力水平分量以向内的压力为主；熔体内部电磁场大小与PMO激励电流峰值呈正相关，与频率和铸坯拉速无关，与线圈匝数呈正相关，与线圈内径呈负相关；磁场大小与PMO脉宽呈正相关，感应电流和电磁力大小与PMO脉宽呈负相关。.（2）流场和温度场演变规律研究表明，PMO作用下熔体内部形成双环流流形；熔体流速随激励电流峰值、频率和线圈匝数的增大而增大，随线圈内径和铸坯拉速的增大而减小，随脉宽的增大先增大后减小，连铸二冷区环流分界面随拉速的增大而逐渐向下方移动；电磁力对流项是导致熔体形成双环流，以及熔体流动强度随各类参数演变规律的根本原因；PMO在金属熔体中产生的强制流动显著降低连铸坯熔体的径向温度梯度。.（3）异质形核研究表明，施加直流电、脉冲电流和PMO都能显著降低凝固点，增大过冷度，且随着电流强度的增大，过冷度增大。.（4）溶质分布研究表明，PMO对固液界面前沿溶质分布的影响远小于电磁搅拌。这主要是由于PMO在金属熔体中产生的强制流动远小于电磁搅拌。.本项目通过上述研究，本项目提出“电/磁致过冷形核”和强制流动引起的枝晶熔断是PMO细化凝固组织的两个重要原因，丰富和发展了结晶雨机制，并为PMO均质化技术的工业化推广提供了理论基础。.项目执行期出版专著1部，发表论文24篇，授权发明专利14项，获冶金科学技术一等奖和三等奖各1项，中国专利优秀奖1项，新增PMO应用企业1个，主办学术会议4次，做会议邀请报告19次，培养博士生3人，硕士生11人。