Speaker
Description
Electrical steels are Fe-Si alloys with a silicon content typically ranging from 0 to $6.5$ wt$\%$. They belong to soft magnetic materials and are commonly used as core materials in various electromagnetic applications for generating, distributing, and consuming electrical energy. The thin sheets of these steels are classified into Grain-Oriented (GO) and Non-Oriented (NO) electrical steels. GO steels have exceptional microstructures consisting of centimeter-sized grains with the crystal lattice having the so-called Goss orientation $\{110\}$<$001$>, which are manifested by low power losses and high permeability in the rolling direction. Therefore, they are predominantly employed for the transformers with high efficiency. The most desirable $\{110\}$<$001$> crystallographic orientation in GO steel is achieved through abnormal grain growth, forced by the appropriate morphology of secondary phase particles such as MnS, AlN, and MnS+AlN.
NO electrical steels exhibit nearly identical magnetic properties in all directions within the sheet and are primarily used as core materials in rotating equipment. The isotropic magnetic properties can be achieved through the so-called "rotating" cube texture, defined by the $\{100\}$<$0$vw> crystallographic orientation.
In the present work, we introduce an original concept of thermo-chemical treatment for NO silicon steel to achieve a composite microstructure through the cross-section of the steel sheet. The proposed microstructural design ensures a combination of high strength and relevant magnetic properties, making it suitable for the construction of rotor cores in electric and hybrid vehicles. Our approach is based on abnormal grain growth with an appropriate crystallographic orientation, achieved by strain-induced grain boundary migration mechanism in combination with the inhibiting effect of nano-precipitates, primarily distributed in the subsurface region. A layer of coarse-grained microstructure (grain size $\sim 150$ $\mu$m) with a strong cubic $\{100\}$<$0$vw> or Goss $\{110\}$<$001$> crystallographic orientation was obtained in the central part of the steel sheet cross-section. This microstructure provides the desired magnetic properties. Additionally, a fine-grained microstructure (grain size $\sim 15$ $\mu$m), formed by nano-precipitates, was achieved in both sub-surface regions to enhance the mechanical strength. The proposed composite microstructure of NO steel demonstrates magnetic losses of approximately $9$ W/kg, comparable to the magnetic properties of samples processed under conventional industrial conditions. Stress-strain tests show that our samples exhibit a more than $30\%$ increase in tensile strength compared to the same material without a composite microstructure.
Acknowledgements
The work was carried out within the research project funded by the EU NextGenerationEU through the Recovery and Resilience Plan for Slovakia under project No. 09I03-03-V04-00314.
