Effect of Hot Band Annealing on the Microstructure and Mechanical Properties of Low Carbon Electrical Steels
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Effect of Hot Band Annealing on the Microstructure and Mechanical Properties of Low Carbon Electrical Steels E. Gutiérrez-Castañedaa, A. Salinas-Rodríguez b. Centro de Investigación y de Estudios Avanzados del IPN, P.O. Box 663, Saltillo, Coahuila, México, 25000. [email protected], [email protected] Keywords: Electrical steels, hot band annealing, microstructure, mechanical properties.
ABSTRACT Magnetic properties of grain non-oriented low-C electrical steels are improved when the hot rolled strip is annealed (HBA) prior to cold rolling and final annealing treatments. This improvement results from development of a {100} texture in the large grained ferrite microstructure produced during final annealing. HBA at 800-850 °C results in rapid decarburization and elimination of carbide particles which have caused concerns about the suitability of the mechanical properties in the final product. In this work, samples taken from a hot rolled electrical steel coil are subjected to HBA during 150 minutes at 850 °C, cold rolled and finally annealed three minutes at temperatures between 700 and 1000 °C. The resulting tensile properties are compared with those of samples subjected to a similar processing route but without the HBA treatment and samples of industrially semi-processed grain non-oriented electrical steel decarburized 16 hours at 750 °C. It is shown that the yield strength of samples with and without HBA depends on the final grain size according to the Hall-Petch relationship; the final grain size depends strongly on annealing temperature. However, the HBA treatment causes the strength to decrease by a factor of about 2.5 and the ductility to increase by a factor of about 1.5. It is observed that the microstructure and tensile properties of the semi-processed electrical steel subjected to a final decarburization annealing are identical to those observed in material subjected to HBA in the present work. These results indicate that the HBA treatment not only improves the magnetic properties but also leads to a significant reduction of production time for grain non-oriented electrical steels.
INTRODUCTION Grain non-oriented (GNO) electrical steel occupies the first place among magnetic materials in terms of both total tonnage and market value [1]. This material is used for the manufacture of cores of electrical machinery such as motors, generators, transformers, etc. For these applications, low energy losses and high permeability are required [2]. Along with good magnetic quality, the ability of the steel to be shaped by stamping is also increasingly important. This property to a large extent determines the service life of the expensive dies used in this operation, and increasing the life of the dies is an efficient way of reducing the cost of electrical equipment. Hence, mechanical properties of these materials are as important as magnetic properties [3, 4]. It is well known that properties of electrical steels are influenced strongly by the thermo-mechanical history [5, 6]. Conventional
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