Effects of Heat Treatment under Strong Magnetic Field of 1T or Higher on Magnetic Properties of Non-oriented Electrical Steel Sheet

In order to improve magnetic properties of non-oriented electrical steel sheets, effects of heat treatment in a magnetic field to control crystallographic orientation were studied. In this paper, we report on the effects of the amplitude and direction of the magnetic field applied during the heat treatment in comparison to magnetic properties of samples, which the longitudinal direction is parallel to the rolling direction (RD) or rolling transverse direction (TD). To clarify the effects of heat treatment in a magnetic field on the magnetic properties of the electrical steel sheets, heat treatment without magnetic field was firstly carried out and a change rate, defined as a value obtained by dividing the iron loss value after the heat treatment by the iron loss value before the heat treatment, was used in the evaluation. Fig. 1 shows a schematic view of the heat treatment apparatus. The heat treatment apparatus consisted of an electric furnace at inner diameter of a superconducting magnet (10 T - CSM) and a quartz glass tube inserted into the furnace. The quartz tube was depressurized to less than $10 ^{-3}$ Pa with a turbo molecular pump. The heat treatment temperature was controlled at 1023 K, 1123 K, and 1273 K. 10 mm $\times 50$ mm sized samples were prepared by cutting a non-oriented electrical steel sheet, 50A470 (JIS C 2552), with electrical discharge machining. Samples were cut so that their longitudinal direction was parallel to the rolling direction or rolling transverse direction. The samples were placed at the center of the magnetic field in the quartz tube using a holding jig made of quartz in a way that the direction of the magnetic field would be parallel to the longitudinal direction of the samples. The strength of the applied magnetic field was set to a maximum of 10 T. Fig. 2 shows iron loss change rate of the samples heat treated at 1273 K with respect to iron loss before heat treatment. In Fig. 2, for example, “RD-10T” means the sample with longitudinal direction parallel to the rolling direction and applied magnetic field during the heat treatment of 10 T. The vertical axis represents the change rate and the horizontal axis represents the amplitude of the excitation magnetic flux density controlled by the sinusoidal wave of 50 Hz with a small-sized single sheet tester. As shown in Fig. 2, both the RD- and TD-samples heat-treated without magnetic field (0 T) under the temperature condition of 1273 K showed a remarkable change in the magnetic properties. The grain growth was accelerated by the heat treatment, the iron loss was reduced, and the permeability under low excitation conditions was improved. Contrarily, the permeability at 1.6 T or higher decreased. It was also found that the effect of the heat treatment on the reduction of the iron loss was greater in the TD-samples. As for the results of heat-treatment in magnetic field, the iron loss of the RD-sample was reduced after heat treatment in 10 T. On the other hand, the iron loss of the TD-sample increased after heat treatment in 10 T. It was also found that application of the strong magnetic field of 10 T throughout the heat treatment, i.e., during the heating and cooling, caused increase of the iron loss and decrease of the magnetic permeability regardless of the direction of the material. The cause of this phenomenon is possibly the contraction of the polycrystalline specimen in the strong magnetic field. The details will be discussed at the presentation and in the full version of this paper.