Characterization of the Mechanical and Microstructural Properties of Thin Non-Orientated Electrical Steel Sheets

Abstract

With the rise and growth of electric vehicles, enhancing the design and implementation of electric engines in transportation vehicles is essential. Electric motors are an essential part of this enhancement. The material from which the stator and rotor cores of electric motors are made out of are required to be highly permeable with low magnetic losses and magnetostriction. Non-Oriented Electrical steel is a cost-effective choice of material that meets these requirements. In this thesis, three grades of non-oriented electrical steel sheets namely: 27PNX1350F, 25SW1250 and 20SW1200 with sheet thicknesses of 0.27mm, 0.25mm and 0.20mm respectively were categorized mechanically and microstructurally at room and an elevated temperature of 150°C.

The initial microstructure of the non-oriented electrical steel sheets was first categorized to confirm its chemical composition, present phases and texture. Then, quasi-static tensile tests at both temperatures were run after which the data was analyzed and the tensile properties were extracted and modeled. Then, fractured surfaces of the samples were examined under a scanning electron microscope to investigate the failure mechanisms under the tensile loading condition. After the complete characterization of the tensile properties, cyclic tests at both temperatures were run after which the data was analyzed and the fatigue properties were extracted and modeled. The failure mechanisms under cyclic loading were also investigated using scanning electron microscopy.

The sheets were confirmed to contain 3% silicon content with ferritic iron present. The steel sheets were also confirmed to have a body centered cubic crystal structure as well as a random grain orientation and texture. The tensile properties at room temperature and 150° C were calculated as well as modeled using the Ramberg-Osgood equation. The obtained elastic modulus for all three grades of non-oriented electrical steel sheets were found to be below the typically expected properties of structural steel but well within the range of electrical steels found in literature. The tensile properties along the rolling direction, transverse direction and 45° orientation were studied and similar results were obtained for the rolling and transverse directions while the results along the 45° orientation were noticeably different. The fracture surface revealed a mixture of ductile and brittle failure fracture mechanisms. A comparison between the three grades of non-oriented steel sheets and a comparison between the tensile properties at room temperature and 150°C was presented. It was found that the three grades of non-oriented electrical steel sheets had comparable tensile properties and there was no conclusive evidence that the sheet thickness had any effect on the tensile properties. It was also found that the increase in temperature had an adverse effect on the tensile properties of the steel sheets which was more significant in the yield strength and elongation.

The fatigue properties at room temperature and 150° C were also examined as well as modeled using the Basquin equation. In addition, design curves with reliability and confidence levels of 90% were obtained using the Owen's tolerance method. The fracture surface revealed a mixture of cyclic deformation and brittle failure fracture mechanisms. A comparison between the fatigue properties of the three grades was presented. Comparable lives at the similar stress amplitudes were observed and with this observation, the conclusion that the sheet thickness did not have an effect on the fatigue life was reached. The effect of mean stress, temperature and cutting method on the fatigue life of non-oriented steel sheets were also investigated and presented. Similar to the tensile properties, increasing the temperature also had an adverse effect on the fatigue life by yielding lower lives at the same stress amplitudes at the elevated temperature. Extra tests performed at different load ratios showed that increasing the mean stress did not significantly affect the life at the same stress amplitudes but lowered the expected endurance strength of the material. Finally, the fatigue life of specimens cut with water-jet and CNC machining were compared. It was found that water-jet cutting deteriorates the edge roughness of the specimens which in turn adversely affects the fatigue life significantly.