Effect of normalization treatment of non-oriented silicon plate on microstructure evolution

- Jul 19, 2019-

The grain size of the final product of non-oriented silicon steel has an important influence on its magnetic properties, especially the important factors affecting its iron loss. The grain size of non-oriented silicon steel generally varies from 30 to 200 μm, and the grain size of 2.0% to 3.2% silicon steel to obtain optimal magnetic properties is 100 to 150 μm. Therefore, the final product of high grade non-oriented silicon steel should aim to achieve a uniform structure close to the optimal grain size. The non-oriented silicon steel of 3.0% silicon was used as the experimental material to carry out the high temperature hot rolling experiment with the finish rolling temperature greater than 850 °C, and the hot rolled plate was normalized at different temperatures to study the temperature at which the complete recrystallization was completed. A sample of 15 mm × 10 mm was cut in the middle of the hot rolled sheet, and normalization experiments were carried out in a heating furnace. The normalizing temperatures were set to 900 ° C, 950 ° C, and 1 000 ° C, respectively, and the normalization time was 4 min.

The microstructure of the hot-rolled sheet after normalization and normalization at 900 ° C, 950 ° C and 1 000 ° C is shown in Figure 1. After hot rolling, the microstructure of the steel strip from the surface layer to the center layer in the thickness direction shows significant unevenness. It is divided into three regions along the thickness direction: surface recrystallized structure, mixed structure of transition layer recrystallized structure and deformed structure, and central laminated flat elongated deformation structure. This is mainly due to the fact that the surface layer of the hot rolling process is subjected to the maximum friction between the roll and the steel strip, and has the highest deformation energy storage, and occurs more fully recrystallized; while the subsurface layer is mainly a small amount of coarse recrystallized grains; It is a tissue that has been flattened and stretched.

Observing the metallographic structure of different normalizing temperatures, the high-temperature hot-rolled sheet recrystallized completely at 950 °C. With the increase of the normalizing temperature, the grain structure of the microstructure gradually increased after the normalization, but when the normalization temperature exceeded After 1 000 ° C, some grains appeared to increase significantly, and the unevenness of the normalized structure increased.