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TY  - THES
AU  - Heller, Martin
TI  - Multi-scale understanding of physical processes and material properties in electrical steel
PB  - Rheinisch-Westfälische Technische Hochschule Aachen
VL  - Dissertation
CY  - Aachen
M1  - RWTH-2023-08262
SP  - 1 Online-Ressource : Illustrationen, Diagramme
PY  - 2023
N1  - Veröffentlicht auf dem Publikationsserver der RWTH Aachen University
N1  - Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2023, Kumulative Dissertation
AB  - Non-grain-oriented electrical steel is a common core material in electric machines such as generators or electric motors. With its ferromagnetic properties, electrical steel reinforces and guides the induced magnetic field and is responsible for the high effective torque of these rotating applications. Various factors influence the magnetic properties, such as chemical composition, sheet thickness, grain size, residual stresses, and texture. Since there is no phase transformation along the process chain, each process step has an influence on the final microstructure and texture and thus on the final magnetic properties. The process chain usually consists of slab casting, hot rolling, cold rolling, heat treatment, shear cutting and final assembly. Often, the different process steps are only considered independently of each other, leaving synergy potential between different process parameters unused. Furthermore, texture is often neglected in the industry, even though it has been shown to affect the magnetic properties of non-grain-oriented electrical steel. Another, more fundamental problem is related to grain size. For a given ferromagnetic material, it is well known that the hysteresis loss decreases with increasing grain size (smaller grain boundary area) and lower dislocation density (residual stresses), as there are fewer pinning sites for domain wall movement. However, which deformation structures develop at which grain boundaries and how both the specific deformation structures and the grain boundaries influence certain magnetic properties is not well understood. In the context of this work, all the above-mentioned topics will be addressed. Along the process chain, the focus is on the heat treatment process, where the microstructure and texture are rebuilt through recovery and recrystallisation. Although recrystallisation is well understood on a mesoscopic scale, investigations on the microscopic scale remain a challenge. In addition, parameters such as the grain boundary mobility, which would be crucial for building a physically based simulation model for these phenomena, are largely unknown. With the recrystallisation scratch experiment an experimental approach to access grain boundary mobility with a good statistical background is provided, and first results underline the high mobility of grain boundaries with a misorientation angle of 30-40°. Complementary to this, the large-area quasi-in-situ EBSD measurements help to fully characterise the initial microstructure and texture with their differences along the sheet thickness, which is important because it allows process-microstructure-texture correlations to be made and differences after recrystallisation to be back-correlated. For example, these experiments have shown that nucleation is not random, but that there are specific relationships between the orientation of the matrix and the nuclei. Furthermore, it is elaborated that an application-specific design of electrical steel is of crucial importance, as one has to navigate a complex field of tension between material requirements (mechanical and magnetic), machine operating conditions, chemical composition as well as process parameters and microstructure and texture development. Thereby, the influence of texture should not be neglected or simplified, as it leads to anisotropic properties that are noticeable in the machine performance, although the term “non-grain-oriented” indicates an isotropic behaviour. Next, single and bi-crystals were studied to understand their deformation behaviour (under extreme conditions) and how different deformation structures and grain boundaries affect the magnetic properties. Using micropillar compression, it was found that the geometric transmission factor, which is easy to calculate, can be a good first indicator of whether dislocations can eventually overcome a grain boundary. However, when and where the deformation begins, and which slip systems are activated must be examined more closely in each individual case. For example, the activated slip systems changed for a  49.2° (6 ̅31) grain boundary, although the geometric transmission factor for the activated slip systems found in the single crystalline counterparts was already high. Subsequently, single and bi-crystals were grown on the macroscale and a Single-Sheet-Tester was miniaturised to test the magnetic properties of these crystals. Some exemplary results include the different pinning strengths of different deformation structures, the easy domain nucleation at a random 41° (4 9 (12) ̅) grain boundary, and the surprising finding that the <111> crystal is not the hardest to magnetize in the low polarization region. I believe that these results expand the current state of the art in several ways and are important for both industry and academia. Awareness of application-specific design and the relevance of texture could encourage various industries to not just buy electrical steel off the shelf, but actually optimise it for their application to increase efficiency and ultimately reduce energy consumption. A better understanding of recrystallisation could lead to process innovations that enable faster heat treatment and a beneficial final texture. Successful measurements of the grain boundary mobility could finally open the field for physically based heat treatment simulations, which are mostly empirical today, and thus save process development time and costs. Understanding the deformation behaviour of single and bi-crystals can help to better understand the shear cutting process, which today often involves cutting only one or two grains, and ultimately reduce wear. Furthermore, the interaction of dislocations with individual grain boundaries is not yet well understood. A better understanding could help to better predict the deformation behaviour of materials and indicate which grain boundaries should be avoided in the production process. Finally, a better understanding of the interrelationship between dislocation structures – from single dislocations to tangles, dense structures, twin boundaries and grain boundaries – and electromagnetic properties could foster research-based innovations to avoid certain dislocation structures, and the development of the miniaturised Single-Sheet-Tester opens up a new field of research to analyse the relationships between microstructure and magnetic properties in more detail.
LB  - PUB:(DE-HGF)11
DO  - DOI:10.18154/RWTH-2023-08262
UR  - https://publications.rwth-aachen.de/record/967843
ER  -