Influence of Initial Microstructure and Cold Rolling Reduction on Transformation Texture and Magnetic Properties of Industrial Low-Grade Electrical Steel
YANG Ping(), MA Dandan, GU Chen, GU Xinfu
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
Cite this article:
YANG Ping, MA Dandan, GU Chen, GU Xinfu. Influence of Initial Microstructure and Cold Rolling Reduction on Transformation Texture and Magnetic Properties of Industrial Low-Grade Electrical Steel. Acta Metall Sin, 2024, 60(3): 377-387.
Compared with high-grade electrical steel, low-grade electrical steel has the advantages of low cost and high production quantity but low profits. Therefore, researchers often focus on studying high-grade electrical steel without phase transformation. The microstructure evolution of low-grade electrical steel is more complicated compared to high-grade steel due to the three transformation stages— casting, hot rolling, and final annealing—that are present between austenite and ferrite during their processing. During continuous casting, the <100> columnar grains commonly formed in the low-grade electrical steel cast slabs with phase transformation illustrate the characteristics of the pronounced transformation delay and suppression. In such conditions, the change in hot rolling temperature will cause diversity in hot-rolled microstructures and textures and affect the subsequent cold rolling and annealing microstructure and texture. Based on the previous studies on the effect of hot rolling processes on the transformation texture of industrial low-grade electrical steel and the observation of the transformation delay and suppression of columnar grains in cast slabs, this work further investigates the influence of the initial microstructures before cold rolling and cold rolling reduction on the transformation texture and explores the law of texture inheritance. In particular, the idea of retaining {100} texture using metastable ferrite hot rolling is proposed to improve magnetic properties. The results show that there are more {100} deformed grains in the hot-rolled plate heated at low temperature, and the {100} texture inheritance is obvious after cold rolling and transformation annealing, which effectively improves the magnetic properties. The {100} transformation texture is weakened with the increase in rolling reduction because the initial {100} grains gradually disappear with increasing rolling reduction. An analysis shows that although the {100} transformation texture induced by the surface effect is hindered by the alloying Al and P elements in the used industrial electrical steel, the favorable initial {100} texture produced using low-temperature hot rolling promotes the memory-type transformation texture. In addition, the transformation texture obtained at a high annealing temperature is still better than the recrystallization texture obtained at a low annealing temperature. The significance of these results lies in the possible future practice of enhancing {100} texture in hot rolled plate by metastable ferrite rolling to improve magnetic properties in final annealed sheets.
Fig.1 Schematic of rolling and annealing process routes (Inset shows the schematic of microstructure of initial sample)
Fig.2 Inverse pole figure (IPF)-Z maps (a-d) and orientation distribution function (ODF) figures at φ2 = 45° section (e-h) for through-thickness cross section of hot rolling samples in processes A (a, e), B (b, f), C (c, g), and D (d, h), respectively[1] (φ1, Φ, φ2—Euler angles; RD and ND represent rolling direction and normal direction of the sheet, respectively; IPF-Z represents the projection of the grain orientations that are parallel to ND in the crystal coordinate system)
Fig.3 IPF-Z maps (a-e) and ODF figures (φ2 = 45°) (f-j) for through thickness cross section of annealed samples at 1100oC for 7 min after cold rolling (0.50 mm, 75% reduction) in the hot rolling processes A (a, f), B (b, g), C (c, h), D (d, i), and E (e, j), respectively
Fig.4 Average grain sizes of annealed samples at 1100oC for 7 min after cold rolling (0.50 mm, 75% reduction) in different hot rolling processes
Fig.5 IPF-Z maps (a-e) and ODF figures (φ2 = 45°) (f-j) for through thickness cross section of annealed samples at 1100oC for 7 min after cold rolling (0.20 mm, 90% reduction) in the hot rolling processes A (a, f), B (b, g), C (c, h), D (d, i), and E (e, j), respectively
Fig.6 Average grain sizes of annealed samples at 1100oC for 7 min after cold rolling (0.20 mm, 90% reduction) in different hot rolling processes
Fig.7 IPF-Z maps (a, b) and ODF figures (φ2 = 45°) (c, d) for through thickness cross section of annealed samples at 950oC for 5 min after different cold rolling processes in the hot rolling process C (a, c) 0.60 mm, 70% reduction (b, d) 0.20 mm, 90% reduction
Fig.8 IPF-Z maps (a, b) and ODF figures (φ2 = 45°) (c, d) for through thickness cross section of annealed samples at 950oC for 5 min after different cold rolling processes in the hot rolling process D (a, c) 0.60 mm, 70% reduction (b, d) 0.20 mm, 90% reduction
Fig.9 IPF-Z maps (a, b) and ODF figures (φ2 = 45°) (c, d) for through thickness cross section of annealed samples at 1000oC for 5 min after different cold rolling processes in the hot rolling process C (a, c) 0.60 mm, 70% reduction (b, d) 0.20 mm, 90% reduction
Fig.10 IPF-Z maps (a, b) and ODF figures (φ2 = 45°) (c, d) for through thickness cross section of annealed samples at 1000oC for 5 min after different cold rolling processes in the hot rolling process D (a, c) 0.60 mm, 70% reduction (b, d) 0.20 mm, 90% reduction
Fig.11 Iron loss (P1.5) (a, c) and magnetic induction (B50) (b, d) of transformation annealed samples at 1100oC for 7 min after different cold rolling processes in hot rolling processes A-E, respectively (TD—transverse direction) (a, b) 0.50 mm, 75% reduction (c, d) 0.20 mm, 90% reduction
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