Magnetic properties of electrical steel with Si and Al concentration gradients

Electrical steel with a Si content up to 6.5 wt% is a good soft magnetic material, because the power losses are reduced through the increased electrical resistivity of the bulk material by the presence of alloying elements like Si and Al. Nevertheless, with the increasing of the alloying elements, the concentration of Fe atoms in the bulk is reduced and a lower magnetic saturation is reached. Samples are produced by hot dipping in a Si-Al hypereutectic bath, followed by annealing treatments for diffusion. Series of different concentration profiles have been obtained and the magnetic properties measured. A short immersion is sufficient to form a Si-Al enriched layer with D0₃ ordered structure (25at% Si-Al) in the steel surface: 10 μm of D0₃ or more can be obtained after 20 s immersion (dependent on substrate and experimental conditions). A short diffusion annealing lowers and broadens this level, e.g., to a classical level of 6.5 wt% of Si, while the center of the steel remains at the original level (e.g., 3 wt% Si). Similar gradients are obtained with the simultaneous presence of Si and Al. Magnetic measurements were performed at 50 and 400 Hz after different thermal cycles, leading to different gradients of Si and Al. The power losses are reduced more than 50% in the final material compared with the original. It appears that a short annealing after the dipping already gives low power losses, even before a homogeneous concentration profile is obtained over the sample thickness. A smooth concentration gradient is preferable, with a lower Si content in the center, in order to maintain a good magnetic saturation. This can also be shown through modeling of the magnetic behavior with the concentration gradient. The reduction of power losses is more pronounced at 400 Hz, proving the importance of the skin effect. A further advantage of this procedure is the shorter annealing time after hot dipping.