Influence of hot rolling reduction rate on the microstructure, texture and magnetic properties of a strip-cast Fe-6.5 wt% Si grain-oriented electrical steel

Abstract

Grain-oriented electrical steel containing 6.5 wt% Si is a promising soft magnetic material with very low iron losses for high-frequency applications. However, due to the ordered phases formed in the microstructure, the room-temperature ductility (formability) of this material is extremely low. It is essentially not possible to process this steel by conventional cold rolling. In this study, an Fe-6.5 wt% Si grain-oriented electrical steel was processed through strip casting (also known as twin-roll casting), hot rolling, warm rolling, cold rolling, primary and secondary annealing. The effect of hot rolling reduction rate (12–40%) on the microstructure, crystallographic texture and magnetic properties of the final steel sheets was investigated. The results showed that hot rolling increased the area fraction of the Goss ({1 1 0}〈0 0 1〉) grains, reduced the grain size, and promoted the precipitation of fine second-phase particles. After warm rolling, cold rolling and primary annealing, the recrystallized microstructure was the finest if the hot-rolling reduction rate was the highest (40%). After secondary annealing, the steel with the largest hot rolling reduction rate (40%) showed the most apparent abnormal grain growth, leading to the formation of the Goss texture. As a result, the magnetic flux density and the magnetic Barkhausen noise (MBN) also showed the largest values among all the samples. On the other hand, those with lower hot-rolling reduction rates (12–32%) showed no or less abnormal grain growth during secondary recrystallization, which resulted in lower magnetic flux density and weaker magnetic Barkhausen noise. Increasing the amount of hot rolling reduction was thus able to promote abnormal grain growth during secondary recrystallization, and optimize the magnetic properties of the steel.