Influence of final annealing atmosphere on surface microstructure and iron loss in non-oriented electrical steel

Abstract

Reducing iron loss in non-oriented electrical steels is critical for enhancing the efficiency of highperformance electric machines. While bulk microstructural factors such as crystallographic texture and grain size are well-established contributors, the role of surface microstructures formed during final annealing has received less attention. This study examines the influence of final annealing atmosphere-100 % N2 , 50 % N2 + 50 % H2 , and 100 % H2 -on surface microstructural evolution and its correlation with iron loss. Although bulk crystallographic texture and grain size remained comparable across all conditions, significant variations were observed in surface layer composition, interfacial roughness, and near-surface inclusion morphology. Hydrogen-rich atmospheres promoted the formation of smooth, amorphous Al2 O3 layers with fine near-surface inclusions, whereas nitrogen-rich conditions yielded thick, crystalline AlN layers with coarse inclusions concentrated near the surface. Iron loss measurements before and after removal of surface microstructure confirmed that these surface features markedly increase loss, primarily through enhanced domain wall pinning. These findings demonstrate a direct relationship between annealing atmosphere and surface microstructure, and establish atmosphere control as a viable route to minimize iron loss in non-oriented electrical steels. (c) 2026 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.