Recent development of non-oriented electrical steel sheet for automobile electrical devices

This paper describes non-oriented electrical steel sheet for automobile motors and reactors. Electrical steel sheets for energy efficient motors show high magnetic flux density and low iron loss. They are suitable for HEV traction motors and EPS motors. A thin-gauge electrical steel sheet and a gradient Si steel sheet show low iron loss in the high-frequency range. Therefore, the efficiency of high-frequency devices can be greatly improved. Since a 6.5% Si steel sheet possesses low iron loss and zero magnetostriction, it contributes to reduce the core loss and audible noise of high-frequency reactors.     

Lean non-oriented electrical steel grades

A “Lean Grade Policy” has been adopted involving reduced losses of a given steel composition and supplied grades with typical losses closer to the guarantees. Increased final annealing temperatures and modified hot rolling have resulted in reduced losses and improved permeability. A “lean grade” will have higher heat conductivity and permeability compared to a “standard grade”. The policy could result in less variability and better quality control.     

Effect of initial grain size on texture evolution and magnetic properties in nonoriented electrical steels

The magnetic properties of nonoriented electrical steels are influenced by the grain size and crystallographic texture. The technologies used to control the grain size in nonoriented electrical steels are approaching their limits. However, there is still some room for improvement of the magnetic properties through texture control. Hot-band annealing is known to be one of the most effective processing stages for texture modification. In this study, two types of initial grain sizes prior to cold rolling are obtained by different hot-band annealing. The effect of initial grain size on texture evolution and magnetic properties in nonoriented electrical steels containing 2% Si is examined. The specimens having different initial grain sizes have significantly different textures in the cold-rolled state and the annealed state. During the recrystallization stage, new grains formed in the coarse-grained specimens have stronger Goss but weaker γ-fibre texture than those in the fine-grained specimens. During the grain growth after complete recrystallization, the coarse-grained specimens still have weaker γ-fibre texture than the fine-grained specimens. The magnetic induction of the coarse-grained specimens is always higher at the same temperature than that of the fine-grained specimens. The core loss of the coarse-grained specimens is lower at the same temperature than that of fine-grained specimens. However, the improvement of the core loss becomes less pronounced as the annealing temperature increases.     

On the correlation between microstructure and magnetic losses in electrical steel

The magnetic properties of electrical steel such as magnetization curves, magnetization behavior and specific magnetic losses are related to the microstructure and the texture of the steel. A quantitative model, which describes the effect of microstructure and texture and their interplay on the magnetic losses P, is still missing. Based on experimental data for nonoriented electrical steels and FeSi-samples with high (Si, Al)-content, a more general formula is proposed for the dependence of P, at a given value of magnetic induction B, as a function of the mean value of the grain size d of the material and of the intensities of the relevant magnetic texture components.     

Magnetic losses evolution of a semi-processed steel during forced aging treatments

An ultra-low carbon steel (30 ppm after decarburization) containing Al and Si was aged for distinct soaking times at 210 °C. The core loss increased continuously until around 24 h. After that, only slight changes were verified. It was found that only the hysteresis loss component changed during the aging treatment. By internal friction test and transmission electron microscopy it was seen that carbon precipitation caused the magnetic aging. By scanning electron microscopy it could be concluded that the increase of aging index was attributed to the high number of carbides larger than 0.1 μm.     

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.     

Effect of nitriding on grain oriented silicon steel bearing aluminum (the second study)

All kinds of high-permeability GO are manufactured using AlN as the main inhibitor. From a purely metallurgical viewpoint, three types of inhibitor preparation for high-permeability GO have been shown. They include a complete solution without nitriding, a complete precipitation with nitriding and a partial precipitation with nitriding. In this study, another possibility, i.e., a complete solution method with nitriding, was investigated to avoid the extra high-temperature slab reheating and to examine the effect of nitriding on GO bearing Al. This method can also provide the sharp Goss texture, and nitriding is shown to be very useful for changing the inhibitor intensity, depending on the primary grain size.     

Curie temperatures and crystallization behavior of amorphous Fe81−xNixZr7B12 (x=10, 20, 30, 40, 50, 60) alloys

Curie temperature, crystal structure and crystallization behavior of amorphous alloys with the stoichiometry Fe_81−xNi_xZr₇B₁₂ (x=10–60) have been studied by X-ray diffractometry (XRD), differential scanning calorimetry (DSC) and AC-magnetization (TMAG) measurements as functions of temperature. The thermal stability of long-range magnetic order, T_C vs. Ni content in as-quenched amorphous alloys exhibits maximum at 352 °C for x=40. The primary crystallization has been detected during annealing at the first crystallization stage of all ribbons investigated.     

Measurement of the stress sensitivity of magnetostriction in electrical steels under distorted waveform conditions

Electrical steel laminations used in the construction of transformer cores are subject to stresses introduced during their construction and analysis of the effect of this on the magnetostriction of the lamination has been investigated previously. It has been shown that higher harmonics of magnetostriction are of greater importance than the fundamental when considering transformer noise. Whereas previous studies have concentrated on the magnetostriction harmonics generated by sinusoidal magnetization, this investigation seeks to understand the relationship between harmonics present in the magnetization waveform and those in the magnetostriction waveform. A measurement system has been designed based on a similar principle to one previously described. In this case, a single Labview Virtual Instrument (VI) is used for the control of the applied stress, controlled magnetization and measurement of magnetostriction together with other magnetic parameters such as specific total loss, specific apparent power, permeability, coercivity and remanence. An adaptive digital feedback algorithm is utilized for control of arbitrary waveform which may be constructed from discrete harmonics or read from an input waveform. As well as measuring peak magnetostriction the software utilizes an FFT to calculate the harmonics of magnetostriction at each stress point. The effect of harmonics introduced into the magnetization waveform on the magnetostriction harmonics will be shown at various applied stresses. A harmonic, Harm_B in the flux density waveform is shown to have the effect of producing a dominant harmonic in the magnetostriction given by (Harm_B+1)/2.     

Grain growth process of two-phase nanocrystalline soft magnetic materials

In order to clarify the origin of the high thermal stability of the microstructure in bcc-Fe/amorphous two-phase nanocrystalline soft magnetic materials, we have investigated the changes in the magnetic and microstructural properties upon isothermal annealing at 898 K for an Fe₈₉Zr₇B₃Cu₁ alloy by means of transmission electron microscopy, Mössbauer spectroscopy and DC magnetometry. The mean grain size was found to remain almost unchanged at the early stage of annealing. However, rapid grain coarsening was evident at an annealing time of 7.2 ks where the intergranular amorphous phase begins to crystallize into Fe₂₃Zr₆. The grain growth process with a kinetic exponent of 1.6 is observed for the growth process beyond this annealing time, reflecting the disappearance of the intergranular amorphous phase. Our results confirm that the thermal stability of the bcc-Fe/amorphous two-phase nanocrystalline soft magnetic alloys is governed by the residual amorphous phase.     

Effect of atomic environment on the Fe hyperfine structure and the thermal stability of magnetic order in L10 ordered Fe-Pt alloys

The ⁵⁷Fe Mössbauer effect measurements were made for the L1₀ ordered Fe-Pt alloys with 39-62 at% Pt and the effect of local atomic environment on the hyperfine structure was investigated. Furthermore, the thermal stability of magnetic order was investigated for the alloys with high Pt concentration. From the analyses of the observed Mössbauer spectra, we found that dipole-field-like anisotropic transferred hyperfine fields are mainly responsible for the large difference in hyperfine field between Fe-site and Pt-site in the Fe-rich alloys. In the Pt-rich region far from stoichiometry, the existence of many Fe-sites occupied by excess Pt atoms causes a distribution of exchange fields. Therefore, the iron atoms in different local environments may have their several hyperfine fields with different temperature dependence. The anomalous temperature dependence of the averaged hyperfine field and line broadening observed for the 61, 62 at% Pt alloys can be understood from the co-existence of various sub-spectra with different temperature dependence. As a result, the thermal stability of magnetic order is largely reduced as the Pt concentration exceeds 60 at%.     

Study of the magnetic properties of the permanent magnets by Mössbauer spectroscopy

This paper presents methods that can be used in order to determine the relative remanent magnetization and uniaxial magnetic anisotropy constant of particles in powder-based permanent magnets using Mössbauer spectroscopy. The methods were verified on the permanent magnet barium ferrite.     

Magnetic behaviour of Fe–Cr nanoparticle systems

The low-temperature magnetic properties of Fe_1−xCr_x nanoparticles with various chromium content (x=2.4−83.0 at%) have been studied. The multiphase particles (α-FeCr, σ-FeCr and Fe/Cr oxides) have a core (metallic)–shell (oxide) structure. The magnetic properties of the Fe–Cr systems depend on the chromium content as well as on the types and relative contributions of the constituent crystalline phases but, in particular, they are determined by long-range interparticle dipolar interactions. The role of the weakly magnetic σ-FeCr phase is also discussed.     

Study of domain structure of magnetic powder particles by Mössbauer spectroscopy

The determination technique of monodomain state of the magnetic powder particles using the Mössbauer spectroscopy is described. The method was verified on the particles of gadolinium iron garnet near the compensation temperature. It has also shown that using the Mössbauer spectroscopy we can evaluate the domain wall energy in ferrites possessing the compensation point.     

An intercomparison of rotational loss measurements in non-oriented Fe–Si alloys

We report a comparison of rotational energy loss measurements in the same non-oriented Fe–Si laminations carried out by two laboratories Istituto Nazionale di Ricerca Metrologica (INRiM) in Torino, Italy and Wolfson Centre for Magnetics (WCM) in Cardiff, United Kingdom. The measurements were performed on disk samples at magnetizing frequencies between 5 and 200 Hz with controlled circular flux density loci ranging between 0.2 and 1.9 T. Energy loss was measured applying both the fieldmetric and the rate-of-rise of temperature methods. The latter, exploiting the rate of rise of temperature under quasi-adiabatic conditions, is conveniently adopted on approaching magnetic saturation. Results from the two laboratories agree well up to 1.4 T, despite the different physical principles of the fieldmetric vs. rate-of-rise of temperature methods and the different size of sample and measuring areas. The rate-of-rise of temperature method seems to be the natural approach at high induction values.     

Magnetic and magnetization properties of electrodeposited fcc CoPt nanowire arrays

Magnetic and magnetization properties of fcc Co_1−xPt_x (x?0.3) alloy nanowires fabricated by electrodeposition into self-synthesized anodic alumina templates are investigated. Magnetization curves, measured for varying wire geometries, show a crossover of easy axis of magnetization from parallel to perpendicular to the nanowire axis as a function of the diameter and length. The measured values of coercivity (H_c) and remanent squareness (SQ) of CoPt nanowire arrays, as a function of angle (θ) between the field and wire axis, support the crossover of easy axis of magnetization. The curling mode of the magnetization reversal process is observed for CoPt nanowire arrays. At low temperatures, the easy axis for magnetization of the nanowires is observed to deviate from the room-temperature orientation.     

Magnetic losses at high flux densities in nonoriented Fe–Si alloys

We present and discuss power loss measurements performed in Fe–(3.5 wt%)Si nonoriented laminations up to very high flux densities. The results are obtained on disk samples using a 1D/2D single-sheet tester, where the fieldmetric and the thermometric methods are applied upon overlapping polarization ranges. The power loss in the highest polarization regimes (e.g. J_p>1.8 T) is measured, in particular, by the rate of rise of temperature method, both under controlled and uncontrolled flux density waveform, the latter case emulating the conditions met in practical unsophisticated experiments. Lack of control at such extreme J_p levels is conducive to strong flux distortion, but the correspondingly measured loss figure can eventually be converted to the one pertaining to sinusoidal induction at the same J_p values. This is demonstrated as a specific application of the statistical theory of magnetic losses, where the usual formulation for the energy losses in magnetic sheets under distorted induction is exploited in reverse fashion.     

Hysteresis loss subdivision

Assuming that different energy dissipation mechanisms are at work along hysteresis, a hysteresis loss subdivision procedure has been proposed, using the induction at maximum permeability (around 0.8 T, in electrical steels) as the boundary between the “low-induction” and the “high-induction” regions. This paper reviews the most important results obtained in 10 years of investigation of the effect of microstructure on these components of the hysteresis loss. As maximum induction increases, the “low-induction loss” increases linearly up to 1.2 T, while the “high-induction loss” is zero up to 0.7 T and then increases as a power law with n=5. Low-induction loss behavior is linearly related to H_c between 0.4 and 1.2 T. Grain size has a larger influence on low-induction losses than on high-induction losses. Texture has a much stronger influence on high loss than on low-induction loss, and it is related to the average magnetocrystalline energy. 6.5%Si steel shows smaller hysteresis loss at 1.5 T than 3.5%Si steel only because of its smaler high-induction component. The abrupt increase in hysteresis loss due to very small plastic deformation is strongly related to the high-induction loss component. These results are discussed in terms of energy dissipation mechanisms such as domain wall movement, irreversible rotation and domain wall energy dissipation at domain nucleation and annihilation.     

Advances in amorphous and nanocrystalline magnetic materials

Recent advances made in the area of amorphous and nanocrystalline alloys exhibiting high saturation inductions are reviewed. A new chemical composition was identified that achieves a saturation induction of 1.64 T in an iron-based amorphous alloy. This alloy, when used in electrical transformers, shows a much improved performance over the existing amorphous alloy. Nanocrystalline FeCoCuNbSiB alloys are found to have saturation induction levels reaching 1.7 T. These materials are suited for use in sensors and inductors carrying large currents. Some of these nanocrystalline alloys show a BH squareness ratio exceeding 90%, which can be utilized in pulse power devices. Recent developments in the applications of these materials are also pointed out.     

Effect of dc-Joule-heating thermal processing on magnetoimpedance of Fe72Al5Ga2P11C6B4 amorphous alloy

The effect of multistep dc-Joule-heating thermal processing on magnetoimpedance (MI) of Fe₇₂Al₅Ga₂P₁₁C₆B₄ ribbons is presented. After material optimization significant increase of MI response up to value ΔZ/Z≈55% as well as sensitivity of about 6%/kA/m (for H?3–4 kA/m), were recorded in still amorphous samples at driving frequencies 2–3 MHz. On-line and post-annealing electrical resistivity together with Mössbauer spectra analysis and frequency dependence of the penetration depth were used for characterization of MI improvement.