Properties Of Non-oriented Electrical Steels Research Articles

Effect of initial grain size on microstructure, texture and magnetic properties of non-oriented electrical steel

The effect of initial grain size prior to cold rolling on the microstructure, texture evolution, and magnetic properties of Fe-3.5 wt% Si non-oriented electrical steel (NOES) was investigated through the introduction of normalization. There were massive intragranular shear bands in {111}<110> and {111}<112> deformed grains in the cold-rolled sheet with large initial grain size, providing nucleation sites for new recrystallized grains with Goss ({110}<001>), λ-fiber, γ-fiber, and {113}<631> components. Subsequent annealing resulted in weaker γ-fiber ({111}<110>−{111}<112>), stronger α*-fiber concentrated at {113}<631>, along with Goss and λ-fiber ({100}<001>−{100}<012>) textures. In addition, appropriate final annealing temperature effectively reduced iron loss. The simultaneous optimization of texture and grain size by normalization and annealing treatments led to improved magnetic properties, with the superior magnetic induction (B50 = 1.719) T, and the lowest core loss (P15/50 = 2.69 W/kg, P10/400 = 16.337 W/kg).

The effect of thickness of the hot-rolled sheet on the magnetic properties of non-oriented electrical steel

Non-oriented (NO) electrical steel sheets with a thin thickness are drawing interest as potential materials for electric vehicle drive motor cores due to their ability to reduce high-frequency core loss. To produce electrical steel sheets with a thickness of 0.25 mm or less, it is beneficial to minimize the thickness of the hot-rolled sheets. However, it is well known that it is difficult to achieve hot-rolled coils with a thickness of 1.4 mm or less with a conventional hot rolling process. Therefore, the thin slab casting and rolling (TSCR) process is now used to produce thin hot-rolled coils that cannot be produced by conventional methods.In this study, we investigated the effects of hot-rolled sheet thickness produced by the TSCR process and cold-rolling reduction ratio on the crystallographic texture, microstructure, and magnetic characteristics of 3% Si NO steel. The results showed that the textures of the hot-rolled sheet and the annealed hot-rolled sheet are virtually unaffected by the hot-rolling reduction ratio. However, the cold-rolling reduction ratio substantially affects many aspects of NO electrical steels. The recrystallized grain size after the final annealing process decreases as the cold reduction ratio increases, while the magnetic flux density decreases with increasing cold reduction ratio, especially for reductions exceeding 82.8 %.In conclusion, the thickness of hot-rolled steel sheets manufactured by the TSCR process has little effect on the microstructure and texture of hot-rolled sheet. Therefore, reducing the cold rolling reduction ratio, which can be achieved by using thin hot-rolled sheets manufactured by the TSCR process, is a promising method for enhancing the magnetic characteristics of thin-gauge NO electrical steel sheets.

Diffusion of Alloying Cobalt Oxide (II, III) into Electrical Steel.

This paper aims to reduce power loss in electrical steel by improving its surface resistivity. The proposed approach involves introducing additional alloying elements through diffusion once the steel sheet reaches the desired thickness. Various effective techniques have been suggested and tested to enhance the resistivity of the strip. The method entails creating a paste by combining powdered diffusing elements with specific solutions, which are then applied to the steel's surface. After firing the sample, a successful transfer of certain elements to the steel surface is achieved. The amount and distribution of these elements can be controlled by adjusting the paste composition, modifying the firing parameters, and employing subsequent annealing procedures. This study specifically investigates the effectiveness of incorporating cobalt oxide (II, III) into non-oriented silicon iron to mitigate power loss. The experimental samples consist of non-oriented electrical steels with a composition of 2.4 wt% Si-Fe and dimensions of 0.305 mm × 300 mm × 30 mm. Power loss and permeability measurements are conducted using a single strip tester (SST) within a magnetic field range of 0.5 T to 1.7 T. These measurements are performed using an AC magnetic properties measurement system under controlled sinusoidal conditions at various frequencies. The research explores the impact of cobalt oxide (II, III) addition, observing successful diffusion into the steel through the utilization of a paste based on sodium silicate solution. This treatment results in a significant reduction in power loss in the non-oriented material, with power loss reductions of 14% at 400 Hz and 23% at 1 kHz attributed to the elimination of a porous layer containing a high concentration of the diffusing element. The formation of porosity in the cobalt addition was found to be particularly sensitive to firing temperature near the melting point. The diffusion process was examined through scanning electron microscopy (SEM) in combination with energy-dispersive X-ray spectroscopy (EDS). The results demonstrate improved power losses in the coated samples compared with the uncoated ones. In conclusion, this study establishes that the properties of non-oriented electrical steels can be enhanced through a safer process compared with the methods employed by previous researchers.

A new hysteresis simulation method for interpreting the magnetic properties of non-oriented electrical steels

The magnetic properties of non-oriented electrical steels (NOESs) are characterized using an analytical simulation method accounting for the microstructures in ferromagnetic materials. Complementary experimental data for thin sheet laminations, obtained using a standard single strip tester (SST), are employed with the hysteresis mechanism investigated in terms of the measurement system and Weiss Mean Field effects. It is shown that the magnetic hysteresis loops of NOESs of 3% SiFe can be generated with remarkable accuracy for a broad range of magnetization frequencies and peak flux densities. The simulation method is also suitable for performing an energy loss analysis with calculated energy losses, when compared to corresponding measured data, showing a strikingly accurate match with, in most cases, an error of less than 1%.

Effect of hot band annealing prior to cold rolling on the mechanical toughness and magnetic properties of non-oriented electrical steel

In this work, the effect of hot band annealing on subsequent microstructure, toughness and final magnetic properties of non-oriented electrical steel was investigated. It was found that the hot band annealing temperature plays an important role in the microstructure, toughness, texture and final magnetic properties. After hot band annealing, the grain size of hot-rolled and final annealed steels were increased, which made contribution to the reduction of iron loss. Besides that, hot band annealing could enhance the favorable θ-fiber texture and weaken the unfavorable γ-fiber texture of the final annealed steel. The magnetic induction B5000 of final annealed steel increased with the increasing of hot band annealing temperature within the testing range. In addition, the coarsening grain size caused by high hot band annealing temperature leaded to a sharp decrease in toughness of the hot-rolled steel. An increase in test temperature would improve the impact toughness of hot-rolled steel after hot band annealing. When the test temperature rose to 100 °C, ductile fracture occurred in all the hot-rolled steels under the hot band annealing condition of 850–950 °C.

Effect of Phosphorus Content on Magnetic and Mechanical Properties of Non-Oriented Electrical Steel.

The effect of target phosphorus (P) content on the precipitates, microstructure, texture, magnetic properties, and mechanical properties of low-carbon (C) and low-silicon (Si) non-oriented electrical steel (NOES) was investigated and the influence mechanism was clarified. The results indicate that the precipitates in the steels are mainly aluminum (Al)-manganese (Mn)-Si-bearing complex nitrides ((Al,Si,Mn)xNy) and P-bearing complex nitrides ((Al,Si,Mn)xNy-P). Increasing target phosphorus content in the steels decreases (Al,Si,Mn)xNy, and increases (Al,Si,Mn)xNy-P. The number density of the precipitates is the lowest, and the average size of the precipitates and grain size of the finished steel is the largest in the samples with target P content at the 0.14% level (0.14%P-targeted). The average grain size and microstructure homogeneity of the steels are influenced by the addition of phosphorus. The content of the {111}<112> component decreases, and the favorable texture increases after phosphorus is added to the steel. The magnetic induction of the steel is improved. Grain refinement and microstructure inhomogeneity lead to an iron loss increase after target phosphorus content increases in the steel. The best magnetic induction B50 is 1.765 T in the 0.14%P-targeted samples. The tensile strength and yield strength are improved owing to solid solution strengthening and the grain refinement effect of phosphorus added to the steels.

The effect of temper rolling and final annealing on microstructure, texture and magnetic properties of non-oriented electrical steels

In this paper, the effect of temper rolling and final annealing on microstructure, texture and magnetic properties of a 0.6 wt% Si non-oriented electrical steel, with 5.7% temper rolling reduction, were investigated via EBSD and XRD three-dimensional orientation distribution function (ODF) analysis. The results indicate that final annealing plays an essential role in improving the magnetic properties of non-oriented electrical steels. The core loss decreases with the increasing annealing time, reaches the lowest value at 10 min, and then tends to be stable. The magnetic induction quickly increased with prolonged annealing time, reached the highest value, then decreased slightly to a lower value after 15 min. The peak permeability increased rapidly with the increasing annealing time, reached the highest value after 3 min, and became stable after 10 min. The grains abnormally develop from the sample surface to the inner layer derived from gradient strain. Before and after temper rolling, the texture of the material is mainly dominated by {111}<112>. As annealing time increases, the intensity of {111}<112> decreases, emerging the weak Goss ({110}<001>) and other orientations. Finally, there are without preferred orientations when time exceeds 10 min.

Effect of rare earth elements on magnetic properties of non-oriented electrical steels

The effect of rare earth elements (REMs) on the magnetic properties of non-oriented electrical steels was studied using industrial trials. The variation of the magnetic properties was discussed in terms of the effect of inclusions, precipitates, grain size, and textures in the steel with and without REMs. Inclusions of MgO·Al2O3-CaS and (Mg, Mn, Ca)S in REMs-free steel were modified into composite ones of MgO·Al2O3-LaAlO3, La2O2S-MgO and (La, Ca)S in the steel with 0.0006% lanthanum, and to (La, Ce)2O2S-CaS-MgO, (Ce, La)S in the steel with 0.0007% lanthanum and 0.0015% cerium. The number density of fine MnS was remarkably decreased by the addition of REMs, leading to an increase in the average grain size. The steel containing 0.0006 wt% lanthanum possessed the best combination of core loss and magnetic induction due to the decrease of micro-sized inclusions and fine MnS, the increase of grain size, and the optimization of recrystallization textures. For the steel with excess REMs addition, the large number density of rare earth sulfides with the size of approximately 1 μm tended to deteriorate the grain size and textures.

Effects of hot rolling and coiling temperatures on microstructure, texture and magnetic properties of 1.6 wt% Si non-oriented silicon steel

Bigger grains prior to cold rolling are favorable to the magnetic properties of non-oriented electrical steels. However, it is hard to coarsen grains in hot rolled sheets of low silicon steels due to γ→α transformation. In the work, the influences of different hot rolling temperatures (820 °C, 840 °C and 875 °C) and coiling temperatures (680 °C and 750 °C) on the microstructural and textural evolution of 1.6 wt% Si steel were comparatively investigated in detail. The mechanism of grain growth during coiling was investigated using a quasi in-situ electron backscattered diffraction (EBSD) technique. The main finding was that pancake grains with an aspect ratio of 1.5–3.5 formed after hot rolling at 840 °C and further significantly grew after coiling at higher temperature 750 °C through a strain induced boundary migration mechanism. However, the significant grain growth did not occur after coiling at low temperature of 680 °C. By contrast, elongated deformed grains and equiaxed grains were produced after finish rolling at 820 °C and 875 °C, respectively, in which pronounced grain growth was not observed even after coiling at 750 °C. The significant grain growth promoted the formation of shear bands during cold rolling and led to weakened γ-fiber (<111 >//ND) texture, enhanced λ-fiber (<100 >//ND) and big grains in the annealed sheet. As a result, both magnetic induction and iron loss were significantly improved. This work provides a promising method to coarsen grains before cold rolling by design hot rolling process for low silicon non-oriented silicon steels.

Examination of magnetic properties of nonoriented electrical steels using ring-type specimens

The Epstein frame test is commonly used for investigating the magnetic properties of electrical steels because of the convenient sample preparation. This test requires the samples to be cut half-parallel and half-perpendicular to the direction of rolling to account for the magnetic anisotropy of the sheet. An alternative method to evaluate the magnetic properties of electrical steels is the ring core test. This test more closely represents the magnetic cores in rotating machinery such as motors and generators because the sample shape for this test resembles the practical rotating applications. The measured results may vary according to the test methods. In this study, the differences between the results of the Epstein frame tests and ring core tests were investigated for the various grades of NO steels. For the magnetic induction measurements, results of the ring core test showed a trend of lower values than in the results of the Epstein frame tests. The core losses measured by the ring core tests are generally greater than those of the Epstein frame tests. The difference in results due to the test methods can be attributed to the anisotropy parameter, A(h), which can be obtained from the crystallographic texture of the sheet sample. The difference becomes significant when an area fraction of the texture component shows a high 1-A(h) value by the Epstein test but a low value by the ring test.

Mechanical properties and crystallographic texture of non-oriented electrical steel processed by repetitive bending under tension

Improving the magnetic properties of non-oriented electrical steel (NOES) through the optimization of crystallographic texture has been an on-going research activity for decades. However, using traditional rolling and annealing procedures, the obtained final textures were usually very similar, i.e., exhibiting the {111} (γ) and <110> (α) fibres, which were not the desired {001} texture (θ-fibre) for optimal magnetic quality. In the current work, a 1.8 wt% Si NOES was processed using a new sheet metal deformation method, i.e., repetitive bending under tension (R-BUT), also known as continuous bending under tension (C-BUT), to modify the texture of the electrical steel. The hot-rolled and annealed NOES plates were repeatedly bent and unbent when they were pulled under tension. The deformed plates were then heat treated at different temperatures for various times. Neutron diffraction and electron backscatter diffraction (EBSD) characterisation of the macro- and micro-textures proved that the R-BUT process significantly reduced the undesired {111} texture while promoting the {001} texture. The cube texture, which rarely formed after conventional rolling and annealing, was also seen in the R-BUT samples after annealing. It was shown that, the shear plastic deformation (induced by R-BUT) played a significant role in promoting the desired textures. In addition, the results indicated that the NOES processed by R-BUT could be deformed beyond its common formability limit, which may provide a method to address the poor workability challenge of high silicon electrical steels.

Optimum Magnetic Properties of Non-Oriented Electrical Steel Produced by Compact Strip Production Process

Optimum grain size and effects of crystallographic textures on magnetic properties of Fe-0.65%Si non-oriented electrical steel produced by compact strip production (CSP) process were investigated by optical microscope, electron backscatter diffraction (EBSD), and X-ray diffraction (XRD) techniques. Magnetic induction and core loss show a decreasing trend with the increase of grain size, and grain sizes for optimal magnetic properties are in the range of 26–30 μm. Core loss would be mainly affected by grain size, whereas crystallographic texture would primarily affect magnetic flux density. Magnetic properties increase with increasing of texture factor (volume fraction ratio of {100}/{111}) and magnetic texture factor (volume fraction ratio of &lt;100&gt;/&lt;111&gt;), and increasing with the decrease of A-parameter (minimum angle between magnetization direction and the closest &lt;100&gt; direction) and A(h→), respectively. Simultaneously, with increasing of A-parameter and A(h→), a linear decrease of B50 was obtained.

Influence of Process Parameters on Grain Size and Texture Evolution of Fe-3.2 wt.-% Si Non-Oriented Electrical Steels.

The magnetic properties of non-oriented electrical steel, widely used in electric machines, are closely related to the grain size and texture of the material. How to control the evolution of grain size and texture through processing in order to improve the magnetic properties is the research focus of this article. Therefore, the complete process chain of a non-oriented electrical steel with 3.2 wt.-% Si was studied with regard to hot rolling, cold rolling, and final annealing on laboratory scale. Through a comprehensive analysis of the process chain, the influence of important process parameters on the grain size and texture evolution as well as the magnetic properties was determined. It was found that furnace cooling after the last hot rolling pass led to a fully recrystallized grain structure with the favorable ND-rotated-cube component, and a large portion of this component was retained in the thin strip after cold rolling, resulting in a texture with a low γ-fiber and a high ND-cube component after final annealing at moderate to high temperatures. These promising results on a laboratory scale can be regarded as an effective way to control the processing on an industrial scale, to finally tailor the magnetic properties of non-oriented electrical steel according to their final application.

Integrated Process Simulation of Non-Oriented Electrical Steel.

A tailor-made microstructure, especially regarding grain size and texture, improves the magnetic properties of non-oriented electrical steels. One way to adjust the microstructure is to control the production and processing in great detail. Simulation and modeling approaches can help to evaluate the impact of different process parameters and finally select them appropriately. We present individual model approaches for hot rolling, cold rolling, annealing and shear cutting and aim to connect the models to account for the complex interrelationships between the process steps. A layer model combined with a microstructure model describes the grain size evolution during hot rolling. The crystal plasticity finite-element method (CPFEM) predicts the cold-rolling texture. Grain size and texture evolution during annealing is captured by the level-set method and the heat treatment model GraGLeS2D+. The impact of different grain sizes across the sheet thickness on residual stress state is evaluated by the surface model. All models take heterogeneous microstructures across the sheet thickness into account. Furthermore, a relationship is established between process and material parameters and magnetic properties. The basic mathematical principles of the models are explained and demonstrated using laboratory experiments on a non-oriented electrical steel with wt.% Si as an example.

Effects of Cr Content on Electromagnetic Properties of Medium and Low Grade Non-Oriented Electrical Steel

This paper takes the medium and low grade non-oriented electrical steel as the research object. Through the data analysis in the production process, the corresponding relationship between the Cr content and the electromagnetic properties of the non-oriented electrical steel was studied. The results show that with the increase of Cr content, the core loss increases and the magnetic induction decreases, thus the magnetic properties are directly proportional to Cr content.

How non-conventional machining affects the surface integrity and magnetic properties of non-oriented electrical steel

Non-oriented electrical steel (NOES) laminations are commonly used to manufacture the rotor and stator core of electric machines. To achieve high machine efficiencies, it is desirable for these NOES laminations to be able to achieve a high saturation magnetisation whilst incurring minimal core losses. It is known that inappropriate machining of these laminations could cause significant deterioration in their magnetic properties. However, the mechanisms by which machining influences this deterioration are less understood. This study investigates the magnetic deterioration after four nonconventional machining methods: Abrasive Waterjet, Wire Electric Discharge Machining, Pulsed Laser, and Continuous Wave Laser.An in-depth investigation of surface integrity through a range of methods, i.e., surface topography, scanning electron microscopy (SEM), nanoindentation, electron backscatter diffraction (EBSD), and magnetic domain imaging, were conducted to study the mechanisms causing magnetic deterioration. The surface integrity after machiningusing conventional methods (e.g., microstructure and texture), was found to not be of high relevance unless this is combined with analysis on how machining affects the micro-magnetic domain structure. This paper, for the first time, highlights this aspect and attempts to make initial quantitative evaluations on how the magnetic domains are affected in the superficial layer that is the result of non-conventional machining.

Impact of residual stress evoked by pyramidal embossing on the magnetic material properties of non-oriented electrical steel

Targeted magnetic flux guidance in the rotor cross section of rotational electrical machines is crucial for the machine’s efficiency. Cutouts in the electrical steel sheets are integrated in the rotor sheets for magnetic flux guidance. These cutouts create thin structures in the rotor sheets which limit the maximum achievable rotational speed under centrifugal forces and the maximum energy density of the rotating electrical machine. In this paper, embossing-induced residual stress, employing the magneto-mechanical Villari effect, is studied as an innovative and alternative flux barrier design with negligible mechanical material deterioration. The overall objective is to replace cutouts by embossings, increasing the mechanical strength of the rotor. The identification of suitable embossing geometries, distributions and methodologies for the local introduction of residual stress is a major challenge. This paper examines finely distributed pyramidal embossings and their effect on the magnetic material behavior. The study is based on simulation and measurements of specimen with a single line of twenty embossing points performed with different punch forces. The magnetic material behavior is analyzed using neutron grating interferometry and a single sheet tester. Numerical examinations using finite element analysis and microhardness measurements provide a more detailed understanding of the interaction of residual stress distribution and magnetic material properties. The results reveal that residual stress induced by embossing affects magnetic material properties. Process parameters can be applied to adjust the magnetic material deterioration and the effect of magnetic flux guidance.

The Influence of Normalization Temperatures on Different Texture Components and Magnetic Properties of Nonoriented Electrical Steels

The effects of normalization on different texture components and magnetic properties in G50W600 nonoriented electrical steel are studied by electron backscatter diffraction (EBSD). The results show that normalization can coarsen surface {110}‐oriented grains and recrystallized rotated cube or {114}&lt;481&gt;‐oriented grains in the center. Moreover, the highest texture intensity change from original rotated cube texture to the {114}&lt;481&gt; texture with the increase of normalization temperature. Just after the completion of recrystallization in final sheets corresponding to hot‐rolled samples, {111}&lt;112&gt; is the main texture component, while that in final sheets corresponding to normalized samples is {114}&lt;481&gt;. The final sheets normalized at 920 and 980 °C have a higher iron loss and higher magnetic induction than the final sheets corresponding to hot‐rolled samples, which is caused by the fact that normalized samples show finer final grain size, higher volume faction of {100} and {110}, and lower volume fraction of γ‐fiber than hot‐rolled samples. With the increasing of annealing temperature and time, the magnetic induction and iron loss of normalized samples decrease. Furthermore, the final sheets normalized at 940 °C have the best magnetic properties. During final high temperature annealing, the non γ‐fiber oriented grains show a higher growth ability than γ‐fiber oriented grains.

Correlation between Magnetic Properties and Chemical Composition of Non-Oriented Electrical Steels Cut through Different Technologies

Due to worldwide regulations on electric motor manufacturing, the energy efficiency of these devices has to be constantly improved. A solution may reside in the fact that high quality materials and adequate cutting technologies should be carefully chosen. The magnetic properties of non-oriented electrical steels are affected by the cutting methods, through induced plastic, and thermal stresses. There is also an important correlation between chemical composition and different magnetic properties. In this paper, we analyze different industrial grades of non-oriented electrical steels, used in electrical machines’ core manufacturing as M800-65A, M800-50A, M400-65A, M400-50A, M300-35A, and NO20. The influence of the cutting methods on the normal magnetization curve, total energy loss and its components, and relative magnetic permeability is investigated in alternating currents using a laboratory single sheet tester. The chemical composition and grain size influence are analyzed and correlated with the magnetic properties. Special attention is devoted to the influence of the increased cutting perimeter on the energy losses and to the way it relates to each chemical alloy constituent. The final decision in what concerns the choice of the proper magnetic material and the specific cutting technology for the motor magnetic cores is imposed by the desired efficiency class and the specific industrial applications.

The influence of residual stress on flux-barriers of non-oriented electrical steel

In rotating electrical machines, cutouts in the rotor laminations control the magnetic flux density distribution in the d- and q-axis of the magnetic core. Guiding the magnetic flux by cutouts leads to very narrow bridges in the electrical steel. However, at the same time, this can limit the maximum speed of a rotor e.g. due to the material’s maximum allowed mechanical stress. Therefore, the centrifugal forces confine the achievable power density. The aim of this study is the examination of the effect of flux-barriers fabricated by mechanical embossing on the magnetic properties of non-oriented electrical steel. The embossing process causes a static residual stress distribution and thereby a reduction of the permeability resulting from Villari’s effect. The non-oriented electrical steel samples are embossed with varying distances between the barriers and measured at different angles to the barrier edges on a single-sheet tester. A comparison with respect to their directional and embossing geometry-dependent effectiveness as magnetic flux barriers is carried out. A correlation of the results with measurements of the local flux density, obtained by neutron-grating interferometry, is performed in order to enable a consideration of the mechanical embossing on the local magnetic flux density distribution.