Effect of Hot Band Annealing on Microstructure of Semi-Processed Non-Oriented Low Carbon Electrical Steels
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Mater. Res. Soc. Symp. Proc. Vol. 1243 © 2010 Materials Research Society
Effect of Hot Band Annealing on Microstructure of Semi-Processed Non-Oriented Low Carbon Electrical Steels Emmanuel J. Gutiérrez Castañeda and Armando Salinas Rodriguez Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Saltillo Coahuila, P.O Box 663, México 25900. ABSTRACT Effects of hot band annealing on the final microstructure and magnetic properties of cold rolled and annealed non-oriented grain Si-Al electrical steel strips are investigated. Microstructures are characterized using optical and scanning electron microscopy and magnetic properties are determined using a vibrating sample magnetometer. It is shown that annealing of hot rolled bands at temperatures between 800 and 850 °C causes rapid decarburization and development of a microstructure consisting of large columnar ferrite grains free of secondary particles. This microstructure leads, after cold rolling and a fast annealing treatment, to large grain microstructures similar to those observed in production scale, fully processed strips. It is observed that the final grain size increases with the final annealing temperature, leading to a significant improvement of the magnetic properties. Therefore, hot band annealing technology can be an attractive alternative processing route for the manufacture of non-oriented grain low carbon Si-Al processed electrical steel strips. INTRODUCTION Electrical steel is a soft magnetic material used for cores of electrical generators, transformers and motors [1]. For these applications, low core loss and high permeabiity are required [2]. Magnetic properties are mainly dependent on two microstructural parameters: grain size and texture [3]. Therefore, to achieve low core loss in materials that are used after stress relief annealing, grain growth is necessary [2]. Some processing routes have been reported for production of high permeability electrical steel grades, such as optimized hot rolling schedule [4], hot-band annealing [5] and cold rolling in two steps with intermediate annealing [6]. Some authors have studied the effect of hot-band grain size on magnetic properties of non-oriented electrical steels [7]. They found that hysteresis losses decreased with increasing hot-band grain size. According to this, hot-band annealing could change the grain structure by enhancing grain growth, which can help to improve magnetic properties of these materials. In the present investigation, the effect of hot band annealing on microstructure of semi-processed non-oriented low C electrical steels has been investigated. EXPERIMENTAL PROCEDURE Hot rolled bands, 2.7 mm in thickness, of commercial grain non-oriented (GNO) low C electrical steel are obtained from a local steelmaker. Table I lists the chemical composition of the hot band as determined by optical emission spectrometry. This material is produced by hot rolling of 50 mm thick continuous cast thin slabs. Rectangular, 30 mm wide by 60 mm long samples are cut from the hot
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