Effect of stress relief annealing temperature and atmosphere on the microstructure and magnetic properties of non-oriented electrical steels

Non-oriented electrical steels are widely used as core material of electrical machinery such as motors and generators. Manufacturing process such as punching and shearing is needed to produce the core with non-oriented electrical steels, but the stress induced unavoidably during the manufacturing process makes the magnetic property of core is severely deteriorated. Stress relief annealing treatment, therefore, is commonly applied to remove the harmful effect of internal residual stress in the processed core. In this paper, the effect of stress relief annealing temperature and atmosphere on microstructure and magnetic properties of fully processed non-oriented electrical steels with different grades were investigated. Cut samples were prepared by shearing and submitted to stress relief annealing with various temperature (650 similar to 850 degrees C) and H-2 gas ratio in atmosphere (0 similar to 30%). It was found that core loss of sample is improved as temperature increases from 650 to 850 degrees C, while the magnetic flux density is deteriorated with increase of annealing temperature. Decrease of core loss was reasoned by grain size coarsening and shows some different tendency between samples, while decrease of magnetic flux density was caused by texture changes with annealing temperature. Increase of annealing temperature makes the coating layer is damaged and oxide layer is formed at interface between coating layer and parent metal. Increase of H-2 gas ratio in annealing atmosphere also affects the magnetic properties weekly by formation of internal oxides.