Influence of Crystallographic Features of GO Electrical Steel on the Magnetic Domain Structure

Abstract

Grain-oriented (GO) electrical steel is mainly used in the core materials for power transformers requiring high permeability and low losses. The predominant {110}<001> orientation of the steel meets the stringent loss requirements for the application due to the development of the sharp texture. For the further reduction of loss, the control of the magnetic domain structure is increasingly becoming a vital factor. In the present work, the magnetic domain structure and domain wall (DW) motion during the magnetization process of GO steel were studied in relation to its crystallographic characteristics.Within the framework of the polycrystalline bulk GO steel, three microstructural features are closely related to the crystallography, i.e. the crystal orientation of the grains, the grain boundary (GB) geometry between two grains, and the three-dimensional (3D) structure through the sample thickness. These are considered to be the main features which determined the magnetic microstructure. The crystal orientation was measured by means of electron backscatter diffraction (EBSD) and the Laue method. Magnetic domain structure and DW motions were observed by Kerr microscopy as a function of the applied field. By linking the crystallographic features of GO steel to the magnetic microstructure, a better understanding of the magnetization mechanism and associated magnetic properties was obtained.In the first part of this thesis, the anisotropic magnetic properties of GO steel obtained for different applied field directions were elucidated by investigating the magnetization curves of (110) Fe-3%Si steel sheet for various directions of axial magnetization in connection with the observed domain structure. The magnetic loss of the (110) oriented steel sheet were measured in an applied field making an angle  with the [001] axis. The angle  was varied from 0° to 90°. In off-[001] directions, the magnetization resulted in a highly structured domain pattern and DW displacements, which could be related to the shape of the magnetization curves. The magnetization curves could be divided in four stages, with each stage related to a specific domain structure. The anisotropic properties of the GO steel were shown to be caused by this different rearrangement of magnetic domains during the magnetization along the various crystal orientations.In the second part of the present work, the influence of the GB characteristics on the configuration and behavior of domains in GO electrical steel was discussed. Two types of GBs were defined with an identical misorientation according to their geometry and the DW motions were observed around the GBs. The GB geometry had a pronounced influence on the ability of DWs to penetrate the GB during the magnetization process. The results show that different external field strengths are required for the magnetic saturation of the regions within GO steel having GBs with different characteristics. This observation implies that a specific type of GBs affects the properties of GO electrical steels less negatively.In the last part of the thesis, the actual 3D magnetic microstructure of a FeSi alloy was analyzed. This has never been observed owing to the lack of appropriate experimental methods. The 3D domain structure of a Fe-6.58%Si alloy could be observed with a 1 m spatial resolution. The images were obtained by using Libovický’s precipitation method and automated serial sectioning. With this precise observation of the 3D magnetic structure, the orientation of 180° and 90° DWs were separately investigated for the first time. The structure and orientation of these DWs differ from the predictions of theoretical models.