Microstructural and textural evolution of double stage cold rolled non-grain oriented electrical steel

Non-grain oriented electrical steels (NGOES) are typically used in electric applications with rotating magnetic
fields and thus essential for the production of electric motors. The increasing number of electric vehicles results
in a growing demand for electrical steel sheets used in the stacked laminations of rotor and stator components. Emobility applications require the best possible magnetic properties for optimum performance and efficiency,
especially at high frequencies. In addition, increased mechanical strength is necessary to withstand the centrifugal forces resulting from high revolution speeds. Chemical composition, microstructure and crystallographic
texture strongly influence the magnetic properties, whereas the mechanical properties are mainly affected by the
chemical composition and grain size. Optimizing the texture is a possible method for improving the magnetic
properties without significantly compromising the mechanical strength. In this work a special processing route
for the production of 3.25% Si NGOES sample material has been studied, using laboratory cold rolling and
annealing facilities. Compared to the classical processing route of single stage cold rolling and annealing, this
alternative method involves two cold rolling steps with an intermediate annealing treatment between the first
and second rolling sequence and a final annealing treatment after the second rolling step. The samples were
produced with different intermediate thicknesses, changing the amount of cold rolling deformation of each
sample variation. The degree of deformation affects the recrystallization and grain growth behavior and thus the
resulting microstructure and texture. The microstructural and textural evolution was investigated in the hot
rolled, intermediate annealed and final annealed state using electron backscatter diffraction (EBSD). Finished
samples were additionally examined using magnetic testing and tensile testing. The results indicate a correlation
between the intermediate thickness and the final grain size of the steel samples. Furthermore, the highly grain
size dependent mechanical strength and magnetic losses are influenced. EBSD-data were used to interpret the
texture and to calculate a parameter describing the magnetic quality of the texture. The results show a strong
improvement of magnetically favorable texture components along with increasing intermediate thickness,
enabling significantly better polarization values in rolling direction and transversal direction. However,
anisotropy of the magnetic properties increases as well due to the textural changes.