Date of Award

3-24-2019

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

Core loss, Crystallography, Electrical steel, Electron backscatter diffraction, Texture, Thermomechanical processing

Supervisor

Afsaneh Edrisy

Supervisor

Youliang He

Rights

info:eu-repo/semantics/openAccess

Abstract

The aim of this research is to improve the magnetic properties of non-oriented electrical steels (NOES) for magnetic cores used in electric motors. This was done by controlling the grain size and the final texture of the electrical steel sheets, such that the magnetically favourable θ-fibre (//ND) components are promoted, while the magnetically unfavourable γ-fibre (//ND) components are suppressed. The origins of the magnetically favourable cube and Goss textures were investigated through all thermomechanical processing stages using electron backscatter diffraction (EBSD) techniques. Three types of Goss regions were found after cold rolling. The first two types were embedded inside deformed {111} grains, while the third type was located at grain boundaries. Similarly, the cube texture was also retained at grain boundaries, as well as inside the shear bands of deformed rotated Goss {110} grains. Partial recrystallization led to the preferential nucleation of the Goss and cube nuclei. The effect of annealing time and grain growth on the texture evolution of NOES was investigated. It was found that by simply changing the annealing time, the texture showed significantly different features, i.e. depending on the annealing time, the Goss ({011}), the θ-fibre (//normal direction, ND) or the γ-fibre (//ND) may dominate the texture. The formation of the various textures during annealing at different times was discussed against the oriented growth theory based on a statistic analysis of the grain boundary misorientation and grain size. In order to improve the final texture in non-oriented electrical steel, an unconventional cold rolling scheme was employed, in which the cold rolling was carried out at an angle (i.e. 30°, 45°, 60°, and 90°) to the hot rolling direction (HRD). It was found that cold rolling at an angle of 60° resulted in substantially improving the crystallographic texture of NOES. Furthermore, the effect of skin pass rolling at various reduction rates from 5–20% on the texture evolution was investigated. Finally, the effect of annealing temperature on the microstructure and texture of 3.2% Si steel was studied. It was found that having coarse grains after hot band annealing combined with high temperature annealing can result in significantly weakened γ-fibre components, and promoting the θ-fibre texture. The magnetic properties were measured using Epstein frame test and magnetic Barkhausen noise analysis (MBN). It was found that the DC core losses and MBNrms decreased with increasing grain size, due to the decrease in pinning sites. On the other hand, AC measurements revealed that the optimized texture and grain size was achieved after annealing at 850 °C for 60 minutes, with AC core losses comparable to those obtained from commercially available grain-oriented electrical steel (GOES) with the same Si content and sheet thickness.

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