Effect of Cooling Rate on Order Degree of 6.5 wt.% Si Electrical Steel After Annealing Treatment

The effect of cooling rate on order degree of 6.5 wt.% Si electrical steel after annealing treatment was investigated by using transmission electron microscopy, the cooling rates were selected to be 1°C/s, 30°C/s, 70°C/s, 108°C/s, 1276°C/s, and 3878°C/s, respectively, the first three cooling rates were obtained by the gaseous nitrogen cooling in the full-automatic transformation test instrument, while the latter three were obtained by the liquid nitrogen, water, and ice saltwater quenching, respectively. The relationship between the cooling rate and the order degree had been established. The results showed that with an increasing of the cooling rate, the order degree decreased, the morphology of B₂ ordered phase transformed from saddle-like to point-like, but for the DO₃ ordered phase, the morphology was still point-like until it disappeared. The B₂ ordered phase was almost impossible to be suppressed completely by the heat treatment method, but the DO₃ ordered phase could be suppressed completely. For the liquid nitrogen quenching, the average size of the B₂ ordered domain was greater than that of the water-quenched specimen. The main reason for this phenomenon was that the liquid nitrogen, which was contacted with the hot surface of the specimen, was vaporized and the air layer became the obstacle between the hot specimen and the quenching medium during the quenching process, thus the cooling rate decreased.