Development of high performance non-oriented electrical steels

Abstract

Non-oriented electrical steel (NOES) is a critical soft magnetic material used in electric motors and generators. Its manufacturing involves casting, hot rolling, cold rolling, and annealing to produce thin sheets with reduced eddy current losses. Achieving optimal magnetic properties requires precise control of crystallographic texture and grain size throughout these processing stages. Among them, hot rolling is the most influential in determining the final microstructure, as it involves complex metallurgical phenomena such as strain hardening, recovery, recrystallization, precipitation, and potential phase transformation. Despite its importance, limited studies have focused on the detailed characterization and modeling of the hot deformation behavior of NOES.

The first part of this thesis investigates the high-temperature deformation behaviour of NOES grades with and without phase transformation (1.3 and 3.2 wt.% Si). Hot compression tests were conducted, and the resulting flow curves were modeled using several constitutive equations. The Hensel-Spittel model provided the best overall agreement with the experimental data across all deformation conditions. A deep neural network (DNN) model was also developed, which produced significantly lower prediction errors compared to conventional constitutive formulations. Microstructural characterization of the hot deformed samples revealed distinct deformation and recrystallization behaviors between the two steels, driven mainly by the presence or absence of the α-γ phase transformation.

The second part of this thesis focuses on the development of high silicon NOES (~6.5 wt.% Si), which offers high permeability, near-zero magnetostriction, and low high-frequency core losses. Direct processing of such high Si steels is challenging due to the formation of brittle ordered intermetallic phases (B2 and DO3). To overcome this limitation, a modified processing route was evaluated, wherein 3.5 wt.% Si NOES sheets were produced through conventional hot and cold rolling, followed by silicon enrichment via hot dipping in an Al-Si alloy and subsequent diffusion annealing. This process successfully increased the Si content to ~6.5 wt.% while producing magnetic properties comparable to commercially siliconized materials, despite the latter’s higher cost and environmental impact. Electron Backscatter Diffraction (EBSD) analysis confirmed the development of magnetically favourable textures (e.g., //ND and Goss), contributing to improved magnetic performance.