铸轧高磁感取向Fe-6.5%si钢特殊晶界与组织性能控制

Grain-oriented 6.5%Si steels are the ideal core material for electrical devices used at medium or high frequency due to their low core losses and near zero magnetostriction. However, such a potential material have not been widely used in industry since the high silicon content led to poor workability at room temperature. Therefore, the suitable deformation route and suitable secondary recrystallization route are the focus of 6.5%Si steel research in recent years. In this project, the grain-oriented 6.5%Si steel will be produced by a novel strip casting route and the emphasis are the evolution and formation mechanism of Σ3 boundary. The effect of these boundaries on plasticity improving will be investigated including the grain refinement during strip casting and the inhibition of crack extension during subsequent rolling. In addition, the plasticity of the steel will also be improved by the decreasing of long-range order degree by the solution element. Based on these two aspects, a theory for plasticity improving of strip-cast grain-oriented 6.5%Si steel will be proposed. The evolution of Σ3 boundaries during rolling and recrystallization is going to be characterized and the relationship of Σ3 boundaries, primary recrystallization and secondary recrystallization will be summarized. The controlling strategy of microstructure and properties in grain-oriented 6.5%Si steel based on strip casting and grain boundary engineering will be proposed. This project lay the foundation for the control of Σ3 boundaries in grain-oriented 6.5%Si steel.

取向6.5%Si钢具有超低铁损，且磁滞伸缩为0，是最为理想的中高频低噪声电器的软磁铁芯材料。但6.5%Si钢室温脆性造成无法通过常规轧制工艺制备，其塑性加工和完善二次再结晶获得一直是研究热点与难点。本项目以铸轧取向6.5%Si钢为研究对象，分析亚快速凝固过程中大量Σ3晶界的形成机理。探索亚快速凝固过程中低能特殊晶界细化和稳定凝固组织的物理冶金学原理。并分析特殊晶界在后续变形过程中对位错运动和阻碍裂纹发生/扩展的影响规律，以及利用固溶合金元素降低长程有序度，揭示Σ3特殊晶界与有序相调控综合增塑机制。系统表征Σ3晶界在变形-热处理过程中的演化过程，获得Σ3晶界遗传性、组织稳定性和二次再结晶的内在联系。阐明亚快速凝固与晶界工程相结合的铸轧取向6.5%Si钢特殊晶界及组织性能调控工艺机理，解决取向6.5%Si钢特殊晶界控制的关键科学问题，为取向6.5%钢制备提供理论支撑。

在薄带铸轧、金属3D打印等亚快速凝固过程中的形核、长大过程展现了一些特殊的物理冶金现象，如凝固孪晶界形成、晶粒异常长大等。如何理解和解释这些现象，成为研究深过冷条件下的凝固行为及晶体形核和生长过程的关键。本项目研究了取向Fe-6.5%Si钢亚快速凝固过程中大量Σ3晶界的形成机理及其遗传和演化过程对位错运动和阻碍裂纹发生/扩展的影响规律，揭示Σ3晶界形成原因、遗传性、组织稳定性和二次再结晶的内在联系。结果表明：（1）凝固过程中形成的大量Σ3晶界由{112}/<111>孪晶构成，是典型的BCC金属孪晶结构，其宏观晶界的弯曲特性是由大量局部共格+非共格孪晶界形成，其结构显著区别于传统的退火孪晶和形变孪晶范畴，是凝固过程中形成的“生长型”孪晶结构。（2）通过对比分析其凝固参数以及液相中Fe相粒子附着实验，证明凝固孪晶界的形成机制为熔池中大量晶核间的取向“取向选择”作用。从而提炼出凝固孪晶的关键影响因素。（3）Σ3等低能晶界具有较强高温稳定性，具有较低的迁移速率，降低加工硬化速率且对裂纹扩展有明显阻碍作用，从而提高基体塑性加工性能。（4）Σ3晶界能够协调塑性变形，并且在轧制变形和退火过程中遗传下来，在二次再结晶退火过程中显著提高基体热稳定性，从而为Goss取向晶粒异常长大过程提供必要条件。该项目在亚快速凝固与晶界工程相结合的铸轧取向6.5%Si钢特殊晶界及组织性能调控工艺机理的基础上，解决取向6.5%Si钢特殊晶界控制的关键科学问题，为取向6.5%钢制备提供理论支撑。