Annealing of electrical steel

Annealing of electrical steel, while classified as a heat treating operation and subject to conventional CQI-9 audit principles, is almost exactly the opposite of most other heat treating operations. One of the primary reasons for heat treating of steel is to harden or strengthen it via carburizing, nitriding or similar where performance is measured in terms of hardened case depth. Conversely, annealing of electrical steel is designed to remove carbon (decarburizing) from the bulk of the steel with concurrent grain growth and stress relief. For electrical steel the annealing performance is therefore measured in terms of reduction in electrical losses. Data are presented showing that core loss in electrical steel is increased by any form of plastic and elastic stress. Furthermore, core loss is also highly dependent on residual carbon, therefore any additional reduction in carbon beyond that achieved in the manufacturing process utilized by the steel mill, even in the parts per million (ppm) range, further reduces core loss. As a result, annealing can enable significant electrical performance benefits by reducing core loss and exciting current by as much as a steel grade level or more. The chemistry and thermodynamic conditions for the decarburization reaction must be carefully controlled so that decarburization occurs without deleterious subsurface oxidation. Examples are presented for both annealing of semi-processed as well as fully processed electrical steels. Even with highly alloyed, thin electrical steels, that are used in high frequency applications, such as for Hybrid Electric Vehicle (HEV) and Electric Vehicle (EV) motor designs, annealing provides a material performance benefit, but the property improvement is limited to frequencies under 800 Hz, after which thickness and resistivity become the dominant factors in controlling losses.