Grain-oriented Steel Research Articles

Development of Distribution Transformers Assembled of Composite Wound Cores

This paper proposes the manufacturing of distribution transformers using a novel type of magnetic core which is called composite wound core. A composite wound core is constructed of a combination of conventional and high magnetization grain-oriented steel. The main advantage of transformers assembled of composite wound cores over conventional transformers is the significant reduction of the manufacturing and operating cost. For the analysis of composite wound core transformers, a FE model considering anisotropy and high saturation conditions, and an advanced 3D hybrid FE-BE model have been developed.

Evaluation of a 2.5 mm×2.5 mm Sized Two-dimensional Vector Magnetic Sensor and Measurement of Magnetic Property Distribution of a Magnetic Circuit Model Core with Grain-oriented Electric Steel Sheets

This paper presents a 2.5×2.5mm sized two-dimensional vector magnetic sensor (V-H sensor). The V-H sensor consists of B-needle probes for measurements of localized magnetic flux density vector B and a double H-coil for measurements of magnetic field strength vector H. The H-coil is made in very small size and the outside dimension is 2mm×2mm and the distance between B-needles is 2.5mm. In this paper, accuracy of the B-needle probe and frequency characteristic of the H-coil are evaluated and detailed two-dimensional vector magnetic properties in a magnetic circuit model made of a grain-oriented electric steel sheet are measured with the developed V-H sensor and results are compared with the crystal grain structure.

Iron Loss Influenced by Magnetic Annealing in 0.1-mm-Thick Grain-Oriented High-Silicon Steels

The influence of magnetic annealing on iron loss has been investigated in 0.1-mm-thick grain-oriented high-silicon steels. To prepare such thin high-silicon steels, 0.3-mm-thick commercial electrical steels were mechanically ground to a specific thickness and then annealed at 1200°C under a high vacuum atmosphere. Due to much faster evaporation of Fe than Si, strip thickness decreased and silicon content increased. The 0.23-mm-thick 3% Si samples typically were thinned to a thickness of 0.10 mm, and silicon content increased to 6.5% after annealing for approximately 50 h. The high-silicon steels were further magnetically annealed at 770°C in a dc magnetic field, and an excellent iron loss of around 0.65 W/kg (at 1.5 T/60 Hz) was obtained. The iron loss behavior with both siliconizing and magnetic annealing processes is discussed in the light of microstructural evolution of B2 ordering phases and domain boundary drag effect of surface subsidiary domains.

Static and Dynamic Hysteresis Models for Studying Transformer Transients

Shortcomings of the classical approach in describing magnetization processes in grain-oriented transformer steel is analyzed. A noniterative method of accounting for hysteresis and dynamic properties of transformer core is proposed. Fitting the model to catalog data is described. The method is illustrated by calculating inrush current produced by subsequent transformer energizations.

Effect of an electrical insulating coating on the efficiency of laser treatment of grain-oriented electrical steel

Effect of an electrical insulating coating on the efficiency of laser treatment of an electrical grain-oriented steel has been studied. The similarity and distinctions of the mechanisms of the action of laser treatment and electrical insulating coating on the magnetic losses in steel have been analyzed. It has been shown, both on separate samples and on commercial parties that the coating reduces the efficiency of laser treatment and that the lower the tensile stresses produced by the coating in the metal, the more pronounced the effect of laser treatment. The results obtained also show the absence of the additivity upon the combined action of laser treatment and electrical insulating coating on the metal.

Effect of the grain orientation in the material used for the preparation of an ultrathin electrical steel on its texture and magnetic properties

The effect of initial orientation of samples on the magnetic properties and texture of ultrathin grain-oriented steel strips produced of them has been studied. In a number of cases, the samples with a larger initial deviation from the ideal (110)[001] orientation demonstrate a higher level of magnetic properties (if considering the total set of characteristics) in comparison with the samples with a small initial deviation. For the samples with a larger initial deviation, the final magnitudes of the magnetic induction obtained after the treatment exceed its initial magnitudes. Therefore, for producing an ultrathin grain-oriented electrical steel with high properties, a material with a scattered orientation of grains relative to the ideal (110)[001] orientation can be used.

Recent progress in high Bs Fe-based nanocrystalline soft magnetic alloys

High saturation magnetic flux density (high-Bs) alloy has been developed in an Fe-based nanocrystalline alloy system. A nanocrystalline phase with an average grain size of about 20 nm is obtained by annealing Cu-substituted and/or Cu-and-Si-complex-substituted Fe–B amorphous alloys. The alloy exhibits low coercivity of less than 7 A m−1 and a high Bs of more than 1.8 T. The iron loss at 50 Hz and 1.6 T for a toroidal core made of Fe80.5Cu1.5Si4B14 nanocrystalline alloy is 0.46 W kg−1, which is about 2/3 of that of grain-oriented Si steel. Moreover, the iron loss at 10 kHz and 0.2 T for a wound core made of this alloy is 7.5 W kg−1, which is about 25% of that of non-grain-oriented Si steel and about 60% of that of an Fe-based amorphous alloy. In addition, the cut cores made of the alloy show good superimposed dc-current characteristics and appear promising in applications such as power choke coils (at the high-frequency region).

Tracking the Evolution of Abnormal Grain Growth in Grain-Oriented Electrical Steels by Coercivity Measurements

The measurements of coercivity were utilised to investigate the main evolution stages of abnormal grain growth in grain-oriented electrical steels. The investigated grain-oriented steels were taken from industrial line after final cold rolling. The steels were subjected to short time annealing at elevated temperatures with high heating rates. The coercivity measurements carried out within the present work were confronted with microstructure and texture analysis in order to show the direct dependence of the measured coercivities on metallurgical characteristics of the investigated steel. It is shown that the coercivity measurements technique could capture main microstructural changes taking place in recrystallization.

AC loss density component in electrical steel sheets

This article proposes a new approach to the loss separation issue. The conventional loss separation into three components is abandoned. The dependence of the dynamic component of loss density versus frequency is assumed to be a power law. The model is verified for selected samples of nonoriented and grain-oriented steel.

Grain Boundary Penetration by Lancet Domains in Fe-3%Si Grain-Oriented Steel

We studied the influence of the grain boundary characteristics on the configuration and behavior of domains in the vicinity of a grain boundary in grain-oriented (GO) electrical steel. We evaluated the boundary misorientation and microstructure with electron backscatter diffraction and transmission electron microscopy. We used magneto-optic Kerr microscopy to investigate the magnetic domain structure and the domain wall motion at the grain boundary at various external field strengths. We found that the grain boundary geometry has a pronounced influence on the ability of domain walls to penetrate the grain boundary during the magnetization process. The results show that different external field strengths are required for the magnetic saturation of the regions within GO steel having grain boundaries with different characteristics. A tilted grain boundary with a boundary plane normal making an angle less than 10° to the rolling direction allowed for the easy penetration of the lancet domains during magnetization, suggesting that this type of grain boundary affects the properties of GO electrical steels less negatively.

Texture evolution in Fe–3% Si steel treated under unconventional annealing conditions

The present work investigates texture evolution stages in grain-oriented steel heat-treated using unconventional conditions. The Fe–3%Si steel taken after final cold rolling reduction from an industrial line was subjected to a laboratory isothermal annealing at different temperatures. The annealing temperatures were varied in a range of 850–1150 °C. During the annealing each specimen was heated at 10 °C/s and kept at the stated temperature for 5 min. Development of microstructure and texture in the annealed specimens were followed by the DC measurements of magnetic properties. The grain oriented steel, taken from the same industrial line after final box annealing was also analyzed and compared with the laboratory annealed specimens. It was shown that there is an optimal temperature region that, with combination of a fast heating rate, led to the best conditions of a drastically reduced development time of the {110} < 001 > crystallographic texture in the cold rolled grain-oriented steel. Materials heat treated below the optimum temperature region account for a primary recrystallization, while applying heat above this region leads to a secondary recrystallization without abnormal grain growth. Moreover, in the optimum temperature range, there was a particular temperature leading to the most optimal microstructure and texture. The magnetic properties, measured after the optimal heat treatment, were close to that measured on specimens taken after the final box annealing. The electron back scattered diffraction measurement technique revealed that sharpness of the {110} < 001 > crystallographic texture, developed at the optimum temperature is comparable to the steel taken after the industrial final box annealing. This fact is evidence that there is a temperature where the abnormal grain growth proceeds optimally.

Abnormal Growth of Goss Grains in Grain-oriented Electrical Steels

With the help of electron back scattering diffraction techniques and field emission microscope, the misorientation and the precipitation environment of Goss grains in conventional grain-oriented steel were observed and investigated at the initial stage of secondary recrystallization. It reveals that the abnormal Goss grains have a high fraction of high angle boundaries ranging from 25 to 40 deg. The most important observation is that some of {110} grains in matrix indicated higher particle density than their neighbor grains during final annealing at 875°C before secondary recrystallization, which could create a favorable environment for their abnormal grain growth. Based on misorientation and precipitation results, the abnormal growth mechanism of Goss grains was sketched.

Effect of Current Density on Composition and Microstructure of Si Diffusion Layer by Electrodeposition

A Si diffusion layer on grain-oriented low-silicon steel substrates was produced by pulse electrodeposition in KCl-NaCl-NaF-SiO2 molten salt and the effect of current density upon the composition and microstructure of the siliconized layer was investigated. The results showed that by glow discharge spectrometry (GDS), the change of Si content of siliconized layers was similar in the range of 20-60 mA/cm2. Si content in the surface was maximum, and then dropped sharply within the surface layer (&lt; 7 m). The Si content remained nearly constant in the middle part of the siliconized layer. The content of Si near to the substrate decreased relatively slowly. The Si content in the surface and the layer thickness increased with increasing current density. Cross-sectional observations revealed that the Si diffusion layers had a two-layer structure: the top layer composed of columnar grains grown perpendicularly to the substrate surface and a transition layer with equiaxed grains was close to the substrate. In addition, the thickness of the layer was too small when the current density was 20 mA/cm2, while the layer became more porous as the current increased from 40 to 60 mA/cm2 according to SEM observations. The optimum current density for deposition was 30 mA/cm2.

Three-Dimensional Polycrystal Magnetic Field Analysis of Thin Steel

The 3-D polycrystal magnetic field analysis method by means of coordinate transformation is proposed. The local coordinates are fixed in each crystal grain and the global coordinates are defined in the polycrystal steel. Though it is assumed that each crystal grain has the same magnetized characteristics as those of a single crystal grain, the crystal grain shape and its orientations of alpha, beta, and gamma angles are taken into account. When the proposed model is applied to the real grain oriented (GO) steel with 56 crystal grains in 80-mm × 80-mm, and with 0.35-mm thickness, the calculation results are compared with the grain orientations measured by X-ray diffraction. The GO steel has a higher degree of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(110)[001]</i> texture with about 3% silicon. The calculated results of higher magnetic flux density part and lower magnetic flux density part in the real GO steel are explained by the alpha angle distribution. The calculated inclination angle distribution and the measured alpha angle distribution seem to be in good agreement. The calculated normal component of the magnetic flux density reflects the beta angle distribution of the real GO steel. It is concluded that the proposed 3-D polycrystal magnetic field analysis is appropriate.

Measurement of Electrical Steels With Direct Field Determination

The objective of the presented work was to introduce the principles of direct field control into magnetic measurements. Different grades of nonoriented and grain-oriented steels were tested in dc magnetization regime. A vertical array of three Hall sensors was used to control the sample field. With small gradients of the sample surface field our results showed comparable measurement errors with the single sheet tester. Measurement of the hysteresis loss and the coercive force was mostly independent of the field determination method. However, for accurate estimation of the magnetic induction parameters the sample field should be determined directly. No unique dependence was found between the measured dc and 50 Hz Epstein/single sheet tester parameters except the remanent induction of the nonoriented steels.

Transformers Made of Composite Magnetic Cores: An Innovative Design Approach

Nowadays, transformers are made of conventional magnetic cores which are constructed of a single grainoriented or amorphous, magnetic steel. Even though, the transformer is the most efficient of electrical machines, with efficiencies typically above 90%, it is possible to improve transformer performance by using composite magnetic cores. Patents related to this simple and effective technique can be traced back to 1929. The specific technique can be applied to wound core distribution transformers. By using wound cores constructed with a combination of conventional and high permeability grain-oriented steel the total owing cost (TOC) of the transformer can be reduced effectively. This paper presents a brief review of patents on wound and composite magnetic cores and introduces a generalized technique for the determination of the optimum design variables of a new composite wound core design. Keywords: Composite magnetic cores, distribution transformers, finite element methods, grain-oriented steel, no load losses, optimization methods, total owing cost, wound cores

Generalization of the Classical Method for Calculating Dynamic Hysteresis Loops in Grain-Oriented Electrical Steels

We have studied the ability of two one-dimensional (1-D) time-stepping models, both based on the concept of magnetic viscosity, to reproduce dynamic loops and losses in grain-oriented (GO) electrical steels under arbitrary magnetization regimes. We found that GO steels (0.3 mm thick) can be modeled quite accurately at magnetizing frequencies up to 200 Hz by a thin sheet representation, which is applied to a bulk material. At higher frequencies, acceptable results can be obtained through a finite-difference solver of a 1-D penetration equation whose applicability to GO steels can be explained in terms of domain wall bowing. Because of the inertial effect introduced by the magnetic viscosity, the average error in the loss prediction is reduced from 40% for the conventional classical method to 5% for the methods we studied. We demonstrated the accuracy of the models using two GO steels whose losses and B-H characteristics were measured by computer-controlled Epstein and single-sheet testers.

Precipitates in electrical steels

Precipitates heavily influence the magnetic properties of electrical steels, either as a key controlled requirement as part of the manufacturing process or as an unwanted harmful residual in the final product. In this current work copper-manganese sulphides precipitates are the primary inhibitor species in the conventional grain-oriented (CGO) steels examined and grain boundary pinning is effective at a mean precipitate size of 30–70 nm. The growth of CuMnS has been studied and the results show that a precipitate size above ∼100 nm allows the onset of secondary recrystallisation in the heating conditions applied. The effect of precipitates on the magnetic properties of both grain-oriented and non-oriented steels in their final product form is then examined. Examples of grain-oriented material still containing large numbers of precipitates clearly show the detrimental effects with increases in total power loss of 40% or more. Loss deterioration by about 20% is also seen in samples of high silicon non-oriented material in which titanium carbo-nitride precipitates have been observed. In this case the precipitates are believed to have formed during cooling after final annealing. Finally a grain-oriented steel with a large number of very small precipitates, which do not seem to have any harmful effect on the magnetic properties, is demonstrated.

Texture measurement of grain-oriented electrical steels after secondary recrystallization

The measurement of the final Goss texture sharpness in grain-oriented electrical steels is a challenging task due to the immense grain size ranging from millimeters to centimeters. Although, it is widely claimed in the literature that the orientation deviations from the ideal Goss orientation lie in the range of about 7° for conventional grain-oriented steel and in the range of about 3° for high permeability grades, no precise investigation with an appropriate statistical relevance is known to the authors. In this work, X-ray diffraction and large-area EBSD-based orientation microscopy (EBSD: electron backscatter diffraction) were used for texture analysis and orientation determination in order to estimate the Goss orientation spread of different grades of grain-oriented steel. Two production routes for grain-oriented steel sheets are compared, the conventional route and a low heating route with lower inhibitor strength. The results of the texture measurement demonstrate that both routes deliver comparable values of orientation deviations. Furthermore, it can be shown that small differences of the magnetic properties can be correlated with the texture sharpness of the material.

Multiscale modeling of the magneto-mechanical behavior of grain-oriented silicon steels

The prediction of the magnetic and magnetostrictive behavior of grain-oriented (GO) silicon steels is discussed. An experimental procedure for the measurement of the magneto-mechanical quantities is first detailed. Experimental measurements of the anhysteretic magnetization and magnetostriction are compared to results from the literature. A multiscale model, based on an energetic approach and infinite medium hypothesis, is used. Significant discrepancies between experiments and predictions are highlighted. The specimen shape combined to large grain size induces some strong boundary (surface) conditions leading to a change in the definition of the local potential energy. A specific demagnetizing term is introduced in the definition of the potential energy, creating an initial heterogeneous distribution of the magnetic domains and saturating the magnetostriction along the rolling direction. This modification strongly increases the ability of the model to predict the magneto-mechanical behavior of GO steels.