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Thermal deformation structure and behavior of silicon steel

19 Mar 2024
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In order to refine the microstructure of silicon steel ingot and improve its machinability, the effect of low voltage pulse magnetic field on the solidification microstructure of silicon steel was studied, and the thermal deformation behavior and microstructure of different solidification microstructure were compared. The results show that the traditional columnar crystal structure is transformed into equiaxed crystal structure when pulsed magnetic field is applied in the solidification process of silicon steel. With the increase of magnetic field frequency, the equiaxed crystal ratio (equiaxed crystal area/whole area) first increases and then decreases, and the equiaxed grain size first decreases and then increases. With the increase of excitation voltage, the ratio of equiaxed crystal increases, and the grain size of equiaxed crystal increases continuously. The optimal process parameters of pulsed magnetic field are obtained. The magnetic field frequency is 5 Hz and the excitation voltage is 200 V. Under the process parameter of pulsed magnetic field, the equiaxed crystal size in the solidification structure of silicon steel is refined to 1/2 of that without magnetic field, and the equiaxed crystal ratio is increased to 100%. Hot compression tests (950 ℃, strain rates 0.01 s-1 and 0.1 s-1) and hot rolling tests (1 100 ℃) were carried out on the cylindrical crystal structure without pulsed magnetic field and equiaxed crystal structure with pulsed magnetic field frequency of 5 Hz and excitation voltage of 200 V. The microstructure results show that a small amount of dynamic recrystallization occurs in the solidification structure without pulsed magnetic field during the hot compression process, and there are deformed grains in the structure which are difficult to eliminate, which affects the subsequent machining performance. The fine and uniform recrystallization microstructure was obtained under the same thermal compression condition under pulsed magnetic field, and the average grain size was 670 μm. Compared with the microstructure and texture results of hot-rolled sheet, it is also found that the equiaxed crystal solidification structure is more prone to dynamic recrystallization during hot deformation, and the rolling texture strength and anisotropy of sheet are weaker.

As an important magnetic material, silicon steel is widely used as the iron core of various motors, generators and transformers. Silicon steel is mostly produced by continuous casting and rolling, but when the silicon mass fraction is greater than 2%, the surface of the silicon steel sheet often produces uneven "corrugated" defects, which affects the lamination coefficient, leading to the decline of electromagnetic properties and the reduction of insulation film resistance. The main factor affecting corrugating defects is the uniformity of microstructure of hot rolled plate. Improving the microstructure uniformity of hot rolled sheet is beneficial to uniform deformation during rolling process, and has the effect of reducing corrugating defects. The microstructure uniformity of hot rolled plate can be improved by adjusting chemical composition, refining casting grain, adjusting hot rolling process and regularization after hot rolling. Among them, the most effective measures are to control the solidification process, refine the solidification structure, improve the equiaxed crystal rate, and increase the microstructure uniformity of hot and cold rolled silicon steel, so as to obtain the sheet with excellent electromagnetic properties and lamination coefficient.
CRGO Cold Rolled Grain Oriented Electrical Steel
In 1990, Nakada M et al. found that the application of pulsed electric field during metal solidification could spheroidize the dendritic primary phase generated. Then researchers will be strong pulse magnetic field (excitation voltage is not less than 2 000 V) on the solidification process of stainless steel, aluminum alloy and other metal materials. The results show that the equiaxed crystal transformation can be achieved by adjusting the processing parameters of high intensity pulsed magnetic field, so as to achieve the purpose of thinning the as-cast structure. In order to develop pulsed magnetic field solidification technology with more industrial application prospects, Yang Yuansheng et al proposed a new method of low voltage pulsed magnetic field (LVPMF) solidification (excitation voltage less than 300 V). The microstructure of magnesium alloy with low melting point and superalloy with high melting point has been refined remarkably. The new technology of low voltage pulse magnetic field solidification has been widely concerned because of its advantages such as low excitation current, safe operation process, small magnetic field device, strong penetration and easy industrial application. Hua Junshan et al. used Wood alloy with low melting point to study the effect of power frequency pulsed magnetic field on the liquid surface fluctuation behavior of metal. Teng Yuefei et al. studied the effect of rectangular section width to thickness ratio on the grain refinement behavior of K4169 superalloy during the solidification process under the action of low-pressure pulsed magnetic field. Zhang Jianwei et al. studied the grain thinning behavior of H13 steel during directional solidification under the action of pulsed magnetic field, and found that the low voltage pulsed magnetic field can significantly reduce the secondary dendrite wall spacing. Ji Huanming et al. studied the effect of low-pressure pulsed magnetic field on the semi-continuous casting process of AZ80 magnesium alloy, and found that the grain size was reduced by 46% after the application of pulsed magnetic field. However, at present, most studies focus on the effect of low-voltage pulsed magnetic field in the solidification process and the mechanism of microstructure, and few studies on the thermal deformation behavior and microstructure of low-voltage pulsed magnetic field solidified alloy after grain refinement.
 
The effect of low pressure pulse magnetic field on the solidification process of silicon steel can promote the isoaxial structure of as-cast columnar crystal and increase the axial crystal ratio. The optimal process parameters of pulsed magnetic field are obtained, namely, the excitation voltage is 200 V and the magnetic field frequency is 5 Hz. Under the action of pulsed magnetic field, the average grain size of medium-axis crystals in solidification microstructure is refined from 2.37 mm to 1.21 mm, and the ratio of equiaxial crystals can reach 100%. The flow stress of the as-cast equiaxed crystal structure refined by pulsed magnetic field is greater in the process of hot compression, and a fine uniform complete dynamic recrystallization structure is obtained after hot compression. The columnar crystal structure without pulsed magnetic field is elongated deformed grain after hot compression. When the as-cast equiaxed crystal structure is refined by pulsed magnetic field, almost complete dynamic recrystallization occurs during hot rolling deformation. The difference of yield strength and tensile strength along TD direction and RD direction is reduced, and the anisotropy is weakened.
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