Influence of annealing atmosphere on oxide removability in a 3.2% Si non-grain oriented electrical steel

Abstract

In this work, the effects of gas composition, elapsed time of reaction and temperature during annealing on scale removability were investigated for a (3.2 wt.% Si) non-grain oriented electrical steel, and the results were discussed from the viewpoint of oxide morphology. The annealing tests were carried out under conditions similar to those of industrial annealing operations. For this purpose, steel samples with their original as-received tertiary scale were annealed over the temperature range 900-1 000°C in O2-CO2-H2O-Ar-N2 and N2-H2 gas mixtures and in pure N2. After annealing, oxide/steel samples were cooled in air and some were water quenched to study the effect of thermal shock on oxide scale removability. During the annealing tests, four types of oxide scales were observed: oxide scale without idiomorphic growth (Type I), oxide scale with idiomorphic growth (Type II), neutral scale (Type III) and reduced scale (Type IV). The experiments showed that the annealing atmosphere, annealing time, temperature and cooling media influence the morphology and removability of oxide. In general, the experiments indicated that a reducing atmosphere during annealing and water cooling at the end of annealing are the ideal conditions for oxide removal.