Chemical and Structural Characterization of Oxide Layers Formed during Decarburization in Grain-Oriented Electrical Steel

Abstract

Electrical steel is highly specialized for core materials that carry the magnetic flux in electrical machines. It is important to make high efficiency electrical steel due to the global movement of saving energy to improve environmental protection. Electrical steel, which is fabricated via a complicated process, is primarily comprised of iron matrix with about 3 mass% silicon. Decarburization annealing is required during the production of electrical steel to remove the contained carbon. Oxide layers are formed on the surface during the decarburizing process. Generally, the oxide layers including forsterite (Mg2SiO4), which is a reaction product of decarburization, and magnesia (MgO) are used as an insulating coating. The thickness and adhesion of the insulating coating are important in order to obtain electrical steel with good magnetic and surface properties. The coating layers obtain their characteristics from those of the oxide layers formed in the decarburization step, which depend on several oxidation conditions such as annealing temperature, gas atmosphere, and the chemical composition of the electrical steel. Consequently, it is very important to know the chemical and structural properties of the oxides in order to understand and control high temperature oxidation for the manufacture of electrical steel. The aim of this dissertation is to improve the efficiency of electrical steel

this was achieved by investigating the oxide layers formed during decarburization by various analytical methods and novel techniques, and studying the behavior of high temperature oxidation thermodynamically and by chemical and structural characterization.To characterize the oxide layers formed during decarburization annealing, the oxide layers were investigated by various methods of transmission electron microscopy (TEM), as shown in Chapter II. The main oxides formed in the subscales during decarburization annealing were fayalite (Fe2SiO4), iron oxides (FeO and F3O4), and silica (SiO2), which were identified by nanobeam electron diffraction (NBD), energy dispersive X-ray spectrometry (EDS), and electron energy loss spectrometry (EELS). The crystalline fayalites were found both in the surface region within several tens of nanometers and in the region within a micrometer of the surrounding silica. The atomic configuration in the unit cell of the fayalite was presented. Amorphous silica was formed both in the upper region of the subscales, where it had a spherical shape, and in the interface between the spherical silica and the iron matrix, where it had a lamellar shape.In Chapter III, a combination of glow-discharge optical-emission spectroscopy (GD-OES), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, electron backscatter diffraction (EBSD) and TEM with a sampling method using glow discharge sputtering was successfully employed to characterize the chemical information and microscopic features of the oxide layers formed during decarburization annealing in the direction of depth with high-depth resolution. The discontinuous surface oxides were made up of SiO2, (Fe,Mn)SiO3/(Fe,Mn)2SiO4, and Fe-O oxides. SiO2 was embedded in the (Fe,Mn)2SiO4 at the surface. The dispersed oxides were identified as a mixture of SiO2 and (Fe,Mn)2SiO4 at a depth of ~0.5 μm from the surface, whereas the layered oxide with a lamellar shape at a depth greater than ~1 μm solely comprised silica. The prolonged discharge sputtering gave rise to enhanced surface faceting on the substrate surface.The oxidation layers on the surfaces are affected by the temperature and oxidation potential during the decarburizing process. Knowledge of the chemical and structural properties of the surface oxide layer subscales permits the control of the high temperature oxidation processes during the manufacturing of electrical steel. In Chapter IV, the oxide layers were characterized by FTIR and GD-OES. The main oxide compounds formed within the subscales during decarburization annealing of the electrical steel were fayalite and silica. The contents of these oxides were quantitatively determined by wet analysis via the galvanostatic electrolysis method, which were compared with the fayalite content determined by FTIR spectrometry and the silica content determined by GD-OES. The results determined by the rapid methods and wet analysis showed good agreement. The present findings show that FTIR spectrometry and GD-OES measurements may be used for the rapid quantitative analysis of fayalite and silica in the oxide layers during the manufacturing of electrical steel.In Chapter V, the high temperature oxidation behaviors were studied under various oxidation conditions. The oxidation layers were investigated by SEM, TEM, GD-OES, and FTIR spectrometry as a function of the annealing temperature and time and the oxygen potential to survey the internal oxidation behavior during decarburization annealing. The distribution and morphology of oxides in the oxide layers are closely related with the oxygen potential and annealing temperature. The internal oxidation layers began to form at ~400 °C and the iron manganese silicates, such as (Fe,Mn)2SiO4 and (Fe,Mn)SiO3, appear in the outermost surface above 700 °C in an H2O/H2 = 0.66 atmosphere. Under insufficient oxidation conditions, iron manganese silicates form an amorphous state on the surface. At 850 °C, crystalline fayalite appears in the outermost surface. Thermodynamic calculations of selective oxidation were also examined, and the results were compared with the experimental results. In the Fe-Si-Mn-O system with a high oxygen potential and temperature, the internal oxidation behaviors were clearly explained. In light of the results of the characterization techniques used in this dissertation, the formation of oxides and behavior of high temperature oxidation during decarburization annealing were critically reviewed and explained. This information is useful for controlling and understanding high temperature oxidation for the manufacture of high-quality electrical steel.