Influence of intermediate annealing temperature on the microstructure and texture of double stage cold rolled non-grain oriented electrical steel

Non-grain oriented electrical steels (NGOES) are commonly used as stacked laminations of rotors and stators in electrical vehicles. In order to achieve the best performance and efficiency for the motor, NGOES need the best possible magnetic properties of high polarization and low magnetic losses. Two important aspects influencing these magnetic properties are the grain size and the crystallographic texture of the material. Grain size and texture of the finished steel strongly depend on the production route and the processing parameters. Another important aspect besides the magnetic properties is the mechanical strength, which is needed to withstand the centrifugal forces, accounted with high rotational speed in high frequency applications. 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 cold rolling and annealing, this alternative method involves two cold rolling steps with an intermediate annealing treatment between the first and second rolling step and a final annealing treatment after the second rolling sequence. The samples were heat treated at different intermediate and final annealing temperatures, influencing the recrystallization and grain growth behavior and thus the resulting microstructure and texture. Finished samples were examined using magnetic testing, tensile testing, light imaging microscopy and by electron backscatter diffraction (EBSD). The results indicate a correlation between the two annealing treatments (intermediate and final) and the final grain size of the steel samples. Furthermore the highly grain size dependent magnetic losses are influenced. EBSD-data was 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 annealing temperatures, enabling significantly better magnetic properties.