AlN precipitates and microstructure in non-oriented electrical steels produced by twin-roll casting process

Abstract

Aluminum nitride (AlN) precipitates and microstructure of 4 wt.% (Si+Al) non-oriented electrical steel were investigated. The 2.0 mm thick cast strips with three different silicon/aluminum (Si/Al) ratios were produced by twin-roll casting process, then the strips were reheated, warm rolled, cold rolled and annealed. The microstructure and AlN precipitates were characterized using optical microscopy, scanning electron microscopy and transmission electron microscopy. The results showed that with the increase of Si/Al ratio, on the one hand, the casting microstructure changed from columnar grains to equiaxed grains, and the uniformity of annealed microstructure was improved; On the other hand, the number of AlN precipitates in cast strips reduced meanwhile the distribution became dispersed. By the reheat treatment, the size and distribution of the AlN precipitates can be changed. Moreover, the grain size of the annealed strips is in the range of 20–50 µm, at the same time, many AlN precipitates were located at grain boundaries. Therefore, controlling the Si/Al ratio is a simple method to obtain desired microstructure. Then AlN precipitates in non-oriented electrical steel prepared by twin-roll casting process hinder markedly the recrystallized grains growth. A compatible reheat treatment can be an approach worth exploring to control the behavior of AlN precipitates.