Abstract
The microstructure of a low Si, ultra-low-C, hot-rolled electrical steel strip is modified by annealing at T < To, the α→γ transformation temperature. This heat treatment causes the abnormal anisotropic growth of surface grains which consumes the original hot-rolled microstructure. The growth of the surface grains first takes place parallel to the rolling direction and then in a columnar form parallel to the normal direction until grains growing in opposite directions from the surfaces impinge at the center of the strip. It is shown that cold rolling and a short annealing treatment at temperatures between 700 and 800 °C leads to microstructures which result in iron energy losses that can be as much as 30% lower than those observed in the same material not subjected to the annealing prior to cold rolling. The magnitude of the reduction in energy losses depends on strip thickness and processing parameters. The major effect is observed in material annealed at 710 °C and the relative effect (with respect to material that is not annealed prior to cold rolling) decreases as the strip thickness decreases. It is shown that these effects can be attributed to the effect of the processing conditions on texture and grain size. The maximum reduction in energy losses is observed when the final microstructure consists of ferrite grains ~1.5 times larger than those obtained if the material is not annealed prior to cold rolling.
Highlights
Grain non-oriented (GNO) electrical steel is a soft magnetic material used for the manufacture of cores of electrical machinery, such as motors, generators, alternators, etc., where low energy losses and high permeability are required [1,2,3]
The major effect is observed in the material annealed at 710 ◦ C and the relative effect decreases as the strip thickness decreases
The size of the columnar grains is about the strip thickness
Summary
Grain non-oriented (GNO) electrical steel is a soft magnetic material used for the manufacture of cores of electrical machinery, such as motors, generators, alternators, etc., where low energy losses and high permeability are required [1,2,3]. Previous work [19] has demonstrated that the intercritical annealing of low C, low Si-Al hot-rolled electrical steel strips at temperatures between Ac1 and Ac3 causes rapid decarburization and the development of large columnar ferrite grains free of carbide particles which, after cold rolling and a short annealing treatment, leads to carbide-free, large ferrite grain microstructures with magnetic and mechanical properties at least 30% superior to those observed typically in the same steel in the industrially fully processed conditions These results have been attributed to the increment in grain size and to the development of a {100} fiber texture during the final annealing at temperatures up to 850 ◦ C. The effects of the magnitude of the deformation (65–95% reduction in thickness by rolling) and the temperature of a fast, 3-min annealing treatment applied at temperatures between 700 and 800 ◦ C on the resulting microstructure (texture and grain size) and iron energy losses were investigated
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