Fabrication of high-performance low silicon non-oriented electrical steels by a new method: Low-finishing-temperature hot rolling combined with batch annealing

The grain size prior to cold rolling plays an important role in the microstructural, textural evolution and magnetic properties of non-oriented silicon steel sheets. In this work, we proposed a new way to obtain large grain size before cold rolling and applied it to a 1.0 wt% Si-0.2 wt% Al non-oriented silicon steel, i.e., appropriate low-finishing-temperature hot rolling combined with batch annealing. The grain growth process during batch annealing was investigated using a quasi in-situ electron backscattered diffraction (EBSD) technique. It was demonstrated that significant grain growth took place through a strain-induced boundary migration mechanism. Then a large number of in-grain shear bands were formed in the cold rolled microstructure and could provide nucleation sites for Goss ({1 10} (001)) and lambda-fiber ((001)//ND) recrystallized grains, thus resulting in a coarse-grained and homogeneous recrystallization microstructure with weakened gamma-fiber ((111)//ND) texture, strengthened Goss and lambda-fiber texture. Consequently, superior magnetic properties (B-50 = 1.763 T, P-1.5/50 = 4.30 W/kg) were obtained. For comparison, the conventional processing route without batch annealing was also investigated. A small-grained and inhomogeneous recrystallization microstructure with pronounced gamma-fiber texture was produced in the finally annealed sheet, which could significantly deteriorate both the magnetic induction and iron loss. This work provided a new way to enlarge the grains in hot rolled sheets by low temperature batch annealing instead of the conventional high temperature normalization. Thus a new processing route to fabricate low silicon non-oriented electrical steels with high permeability and low iron loss was established.

Automated summary

The grain size before cold rolling has a significant impact on the microstructural, textural evolution, and magnetic properties of non-oriented silicon steel sheets. This study proposes a new method to achieve a large grain size before cold rolling by low-finishing-temperature hot rolling combined with batch annealing. The grain growth during batch annealing is found to occur through strain-induced boundary migration mechanism, which leads to the formation of in-grain shear bands that serve as nucleation sites for recrystallized grains. The resulting microstructure exhibits superior magnetic properties.