Annealing conditions and high magnetic induction in thin-gauged 3% Si-Fe alloy strips

In this study, the optimal combination of heating rate and annealing atmosphere for obtaining high magnetic induction is suggested in 3% Si-Fe alloy strips containing 70 ppm bulk content of sulfur. Strips of 0.1 mm thickness are prepared through vacuum induction melting, hot and multi-stage cold-rolling processes. Between each cold rolling process, an intermediate annealing is performed at 800°C for 20 min. In the high vacuum or flowing hydrogen of 5N purity, the strips are heated up to 1200 °C with various heating rates. At 1200°C, the strips are finally vacuum-annealed for 12 h. Results show that a trough in magnetic induction is observed in the case of vacuum annealing. Magnetic induction under hydrogen atmosphere decreased with increasing rate. From etch-pit observation, only the strip vacuum-annealed at 400°C/h is composed of {100} + {111} texture and the other strips consisted only mostly of {110} grains. At 25°C/h, a relatively high segregation profile is formed within the lower temperature range in comparison to that in the direct isothermal annealing. This condition results in the sluggish selective growth of {100} or {111} grains, due to strong grain boundary pinning effect of segregated sulfur within the lower temperature. The total number of {110} grains is found to be higher at higher heating and hydrogen flow rate. Such a difference in total number arises from the difference in selective growth kinetics of grains influenced by heating rate, hydrogen flow rate and the corresponding segregation concentration profile of sulfur.