Investigation of the relationship between electrical steel microstructures with their magnetic properties and the potential for EM sensing and characterisation

The excellent magnetic properties of non-grain oriented electrical steels (NGOES) arise from a uniform large (approximately 100-150 μm) grain structure with preferred texture, which is developed through hot rolling to coil (HRC), hot band annealing (HBA), cold rolling and final annealing. After the HRC and HBA varying microstructures through thickness can develop: in the HRC condition fine recrystallised grains with strong {110} texture in the surface layers and elongated large ‘streaky’ grains with strong {111} or {100} texture in the central region; in the HBA condition surface layers typically show a 60-100μm equiaxed grain structure with predominant {110} texture whilst the central region can have retained unrecrystallised streaky grains or a fully recrystallised equiaxed grain structure with {111} and {100} texture components. It is undesirable to have streaky grains in the HBA condition as they are difficult to remove during subsequent processing and therefore can adversely affect the final magnetic properties. The aim of the research was to determine if electromagnetic (EM) sensors can differentiate HBA microstructures that have streaky grains and/or HRC microstructures that can lead to retention of streaky grains. The microstructures in two commercial NGOES (2.3wt% and 3.0wt% Sicontent) in the HRC and HBA (including samples after lab annealing cycles) condition have been characterised. The magnetic properties (BH curves and permeability values) were characterised for individual microstructural regions (samples extracted from the surface and centre) and the overall sample using a single sheet tester, BH machine and EM sensors (using a COMSOLFE model to obtain permeability from the inductance value). It has been found that the different microstructures resulted in different magnetic properties, but differences in grain size, texture and dislocation density all affect the EM sensor signal making it difficult to isolate streaky grains within the microstructures where the other factors can also be variable. In addition, as the permeability values are strongly influenced by the magnetic field, even for relatively small magnetic field changes, measurements using a deployable EM sensor become complex as both the magnetic field and microstructure varies through thickness. Initial simulations using a COMSOLFE model taking into account a varying microstructure through thickness (layered approach) and the EM sensor geometry have been completed.