Hi-B钢二次再结晶退火中异常长大Goss取向晶粒的三维形貌表征

Three-Dimensional Morphologies of Abnormally Grown Goss Oriented Grains in Hi-B Steel During Secondary Recrystallization Annealing

利用定量逐层研磨和计算机辅助重建及可视化技术,并结合电子背散射衍射(EBSD)技术,研究了Hi-B钢二次再结晶退火中异常长大Goss取向晶粒的三维形貌,并探讨了其长大规律和特征。研究表明,在三维尺度上,异常长大Goss取向晶粒呈现"饼形"晶粒形貌,异常长大过程中遵循"饼形"长大规律,即处在次表层的二次再结晶晶核在中间层快速长大取得尺寸优势后,反向沿厚度方向长大到样品表面,并在表面能的作用下继续沿板面方向异常长大,最终使得在板面方向的尺寸远大于厚度方向的尺寸;在Goss取向晶粒异常长大过程中,一些基体大尺寸晶粒由于尺寸优势会阻挡Goss取向晶粒长大,从而暂时保留在晶粒内部形成"岛状"晶粒。而在长大前沿,由于基体晶粒尺寸的不均匀性,特别是遇到一些大尺寸晶粒无法在短期内被吞噬掉,或者是2个异常长大的Goss取向晶粒相遇后造成某些方向长大停止,而一些基体晶粒被包裹进来成为"岛状"或"半岛状"晶粒,还有可能是不同取向晶粒晶界迁移率存在明显的差异性等方面的影响,使得Goss取向晶粒在某些方向长大受阻,从而表现出晶界前沿参差不齐,长大呈现典型的各向异性特征。

Grain-oriented silicon steel is mainly used as the core material in electrical transformers,and its magnetic properties are closely related to the sharpness of Goss texture（{110}001） formed by secondary recrystallization during high-temperature annealing. However, the mechanism of the abnormal growth of Goss oriented grains is still disputed in the literatures. It is well know that microstructure characterization is important to study the relevant mechanism and improve the properties of materials. Usually,the microstructures are characterized only using a single two-dimensional plane of polished or thin foil specimen. Much information on the morphologies is lost owing to the fact that a large part of microstructures is embedded beneath the polished surface, or removed during specimen preparation. Recently, computer-aided three-dimensional morphologies have been developed which can visualize microstructures in metals. The three-dimensional visualization promotes a better understanding of the actual information of polycrystalline materials, especially when the grain morphologies and size are required in three dimensions. In this work, the three-dimensional morphologies of abnormally grown Goss oriented grains in Hi-B steel during secondary recrystallization annealing were investigated by a combination of serial sectioning, computer-aided reconstruction and visualization, and electron back-scattered diffraction technique, then the rules and features of abnormal grown Goss oriented grains were also discussed. The results show that the abnormally grown Goss oriented grains have a pancake-shape grain structure in three-dimensional scale, and follow corresponding grain growth behavior. That is, the secondary recrystallization nuclei of the Goss oriented grains in the subsurface grow quickly into the center layer with a grain size advantage, and further extend back to the surface, then continue to grow abnormally along the plate plane direction with the help of surface energy. Finally, the dimension in the plate plane direction is much larger than that of the thickness direction. During abnormal growth of Goss oriented grains, some largesize grains will prevent Goss grains growth, and temporarily retain in the grains to become ＇island＇ grains.On the other hand, the growth front is quite ragged because Goss oriented grains growth is blocked in some directions, showing typical anisotropic growth features, and there are three possible reasons to account for this phenomenon. One reason is that Goss oriented grains may encounter some large-size grains due to inhomogeneity of matrix grains size, and these large-size grains will block the growth. Another is that two abnormally grown Goss oriented grains which meet together may cause the stagnation of grains growth in some directions, and at the same time some matrix grains are encapsulated to form＇island＇ or ＇peninsula＇ grains. Furthermore, it is also possible that there are obvious differences in grain boundary mobilities between different oriented grains and Goss oriented grains.