2004 Volume 44 Issue 3 Pages 618-622
Semi-processed, non-oriented electrical steels used in motor and transformer cores have their magnetic properties optimized by means of a heat treatment performed under a moistened nitrogen atmosphere containing small amounts of hydrogen. The oxidation of carbon on the steel surface by the oxidizing gases present in this atmosphere decreases the carbon content of the steel. The consequent reduction in the amount of soluble carbon and in the volume fraction of carbides reduces magnetic losses and the susceptibility of the steel to magnetic aging. In this work, a cold-rolled, semi-processed electrical steel was submitted to six thermal treatments covering the most usual conditions employed in industry, combining two types of atmosphere (mixtures of nitrogen with 5% and 10% H2) and three dew points (10, 20 and 30°C). The treatments were carried out in a pilot electrical oven especially adapted to simulate the thermal cycle and the moistened atmospheres employed in industry. The results obtained show that, at the beginning of the process, the decarburization rate is controlled by the chemical reaction of carbon on the steel surface. Later, an oxide layer may build up, depending on the humidity level of the atmosphere, reducing the rate of decarburization.
