Development of grain-oriented electrical steel with excellent high frequency magnetic property

Abstract

Recent interest in sustainable development increased demand for efficient transformer to reduce energy consumption. Transformers with high operating frequencies are highly advantageous owing to their energy efficiency and compact sizes. In order to meet the high demand for efficient transformer, magnetic properties at high frequencies should be improved. To improve magnetic properties, we successfully fabricated grain-oriented (GO) electrical steel using a novel partial decarburization and multistage cold rolling method. Fe-2.0%Si-C alloys were cold rolled and subsequently partially decarburized in a wet hydrogen gas and these steps are repeated several times. In this process, the sheet is divided into surface layer (α ferrite) and interior (pearlite-like structure + α ferrite). It has been verified that this novel process has a vast potential for making GO electrical steel by continuous process. To improve the high frequency magnetic properties, the sheets were further cold rolled to 0.03 – 0.1 mm. The 0.05 mm-thick sheet showed <001> // rolling direction (RD) texture (78%) with uniform grain size and excellent magnetic properties at high frequency. It was verified that core loss could be controlled by the growth of <001> // RD grains, and grain size was affected by the sheet thickness and annealing time. The formation of {110}<001> appears to be inhibited by special bands composed of <111> // ND grains at the boundary of {111}<112> deformed grains during early annealing. Our specimen achieved a core loss (P0.5/20000) improvement of about 20% (4.54W/kg) and one of the highest magnetic flux densities (1.80T) compared with commercial electrical steel. Finally, to further improve the core loss, an Fe–Si–Al electrical steel was fabricated by an environment-friendly method involving Al diffusion through physical vapor deposition for high-frequency transformer applications. To improve the core loss of the sheet, the effects of Al diffusion patterns and frequency on the core loss of the electrical steel were theoretically and experimentally investigated. The core loss of our specimen at 20 kHz (8.36 W/kg) was approximately 15 % superior to that of a commercial electrical steel of the same thickness.