Investigations of non-grain oriented Si steel sheets

Ionica Ioniță1, Broscăreanu Ionuț2, Aurora Anca Poinescu*2,Daniela Avram3, Elena Valentina Stoian2, Ana-Maria Hossu1

1 Department of Sciences and Advanced Technologies, Valahia University of Târgoviște, România2 Department of Material Engineering, Valahia University of Târgoviște, Faculty of Materials Engineering and Mechanics,Târgoviște, România3 Department of Food Engineering, Valahia University of Târgoviște, Faculty of Environmental Engineering and FoodScience, Târgoviște, România

Depending on the Si content, the magnetic and the electrical properties of steels for electrotechnical purpose are produced with oriented grains or contrarily with non-oriented grains. Usually, the Si content of such steels varies between 0.8 - 4.8%. Generally, steels with less than 3% Si are used for small and medium power electricmachines, while steels with higher Si content, i.e. above 3%, are used for high power electric machines and electric transformers. Si in steel increases the strength anddecreases the magnetic losses guaranteeing the development of large grains, which improves the magnetic properties of the laminated sheets. For the processing of thesheets, the magnetic core of the electric machine has to be considered in addition to the magnetic properties of the material and the energy losses, the evolution of thematerial’s microstructure in the cutting area, which may possibly need reconditioning operations. The aim of this study is oriented to the morphological, mechanicalelectrical and magnetic characterization of Si steel with non-oriented grains in order to be further employed as transformer cores.

The determination of the elemental composition in the steel sheet samples(dimensions 30 x 80 mm) was done with a spark-emission opticalspectrometer, Q4 -TASMAN, ASTM E415 method (Table 1).

For the microscopy analyzes, samples were cut from the M400-50A-C5silicon steel roller on the transverse direction (figure 1).

Corresponding author: poinescua@yahoo.com

Figure 1. Metallographic microstructure of M400-50A-C5 - enlarge x200, 4% Nital attack

Magnetic measurements were performed using a Unitole Tester - Brockhaus Messtechnik. For alternating current measurements the equipment from Brockhaus MesstechnikEpstein performs measurements according to IEC 60404 (figure 2).

Figure 2. Variation curve of the relative magnetic permeability µ in the function of magnetic polarization J

The hysteresis losses are influenced by the size of the recrystallization grains, the frequency ofthe current, the value of the maximum induction and the density of the material. Magneticlosses include losses through hysteresis and losses through eddy current (Table 2). They are inwatts per kilogram (W/kg) and are determined with the Epstein 700 device, at a certaininduction and at a certain frequency (figure 3).

Figure 3. The J-H-hysteresis curves for: Sample 1, Sample 2 and Sample 3

The magnetic measurements were performed using a Unitol Tester - BrockhausMesstechnik. For alternating current measurements the equipment from BrockhausMesstechnik Epstein performs measurements according to IEC 60404.

CONCLUSIONSThe magnetization process is influenced by the grain size, the thickness of the sheet, the degree of deformation, the flatness of the surface. One of the most commonly used methodsfor improving the properties of these types of materials is to remove impurities that act on the grain boundaries. In comparation with different types of steel, electrotechnical steelcan be considered a high purity material. The electrotechnical sheet and especially the textured sheet change their magnetic characteristics as a result of the technological processesof cutting or stamping. During the plastic deformation process the material absorbs energy, which is manifested by increasing the number of dislocations.The J polarization tends to decrease with the increase of the applied magnetic field, a phenomenon that has two explanations: firstly there is a saturation of the electromagnet poles,which represents both the source of the magnetic field's production and the concentration of the magnetic flux lines; a second cause of this unnatural phenomenon may berepresented by the commercial measuring device which prevents the user to have control over the measured sizes.The non-grain oriented sheets M400-50A-C5 are manufactured in different qualities, differing by the amount of silicon in the alloy are used as cores in transformers, generators andmotors, efficiently converting magnetic energy into electricity. The percentage of silicon in the alloy can vary from 1 to 1.9%, and aluminum is used to prevent the material from aging(0.2 ÷ 0.5)%. In order to increase the resistivity of the sheet metal, usually small amounts of manganese (0.1 ÷ 0.4)% are added. Through a proper choice of the composition of thesilicon sheet with non-oriented grains studied and of suitable metallurgical and thermal treatments, it can be obtained rollers with superior magnetic properties, thermal andstructural stability, sensitivity to mechanical stresses of magnetic parameters, mechanical properties and thermal conductivity.

The breaking resistance, according to table 5, for silicon sheet thicknesses of about 0.65mm varies from 467 MPa at 472 MPa. The resistance to breaking increases with thevariation of increase of the thickness of the analyzed sheet. The elongation is directlyproportional to the thickness increase. All the mechanical characteristics obtained areaccording to the standards.

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