Ultra-Fast Heating Treatment Effect on Microstructure, Mechanical Properties and Magnetic Characteristics of Non-Oriented Grain Electrical Steels

Abstract

This paper focuses on the effect of rapid annealing on Non-Grain Oriented Electrical Steel (NGO) in terms of microstructure, mechanical properties, and magnetic properties. The Ultra-Fast Heating (UFH) tests were performed by a transversal induction heater on NGO electrical steel samples (cold rolled down to 0.5 mm), varying the heating power (80 kW and 90 kW) and the speed of the strip through the induction heater. This allowed us to exploit heating rates (HR) in the range of 200–300 °C/s and targeting peak temperature (Tpeak) up to a maximum of 1250 °C. The comparison between the microstructure as obtained by conventional annealing and the ultra-fast heating process highlights a clear effect in terms of grain size refinement provided by the UFH. In particular, the average grain size as obtained by UFH ranges two/three times lower than by a conventional process. The results show the possibility of applying UFH to NGO steels, targeting mechanical properties such as those obtained by the standard process, combined with the benefits from this innovative heat treatment in terms of green energy and the minimization of CO2 emissions. Magnetic characterization performed by a single sheet tester (30 × 90 mm) showed that the values of core losses are comparable with conventional NGO grades.

Keywords

• non-grain oriented electrical steel
• ultra-fast heating
• conventional annealing
• thermal treatment
• grain refinement
• mechanical properties