Electrical steel
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Electrical steel (E-steel, lamination steel, silicon electrical steel, silicon steel, relay steel, transformer steel) is speciality steel used in the cores of electromagnetic devices such as relays, electric motors, electric generators, and transformers primarily to reduce power loss. It is an iron alloy with silicon as the main additive element (instead of typically carbon).
Metallurgy
[edit]Electrical steel is an iron alloy which may have from zero to 6.5% silicon (Si:5Fe). Practical commercial alloys usually have silicon content up to 3.2%(higher concentrations result in brittleness during cold rolling). Manganese and aluminum can be added up to 0.5% to improve workability and corrosion resistance.[2]
Silicon increases the electrical resistivity of iron by a factor of about 5; this resistivity change decreases induced eddy currents and narrows the hysteresis loop of the material, thus lowering core loss by about three times compared to conventional steel.[2][3] However, the grain structure hardens and embrittles the metal; this change adversely affects the workability of the material, especially when rolling. When alloying, contamination must be kept low, as carbides, sulfides, oxides and nitrides, even in particles as small as one micrometer in diameter, increase hysteresis losses while also decreasing magnetic permeability.
The presence of carbon has a more detrimental effect than sulfur or oxygen. Carbon also causes magnetic aging when it slowly leaves the solid solution and precipitates as carbides, thus resulting in an increase in power loss over time. For these reasons, the carbon level is kept to 0.005% or lower. The carbon level can be reduced by annealing the alloy in a decarburizing atmosphere, such as hydrogen.[2][4]
Physical properties examples
[edit]- Melting point: ~1,500 °C (example for ~3.1% silicon content)[5]
- Density: 7,650 kg/m3 (example for 3% silicon content)
- Resistivity (3% silicon content): 4.72×10−7 Ω·m (for comparison, pure iron resistivity: 9.61×10−8 Ω·m)
- Fatigue Strength: 250-350 MPa
- Tensile Strength: 300-450 MPa
- Elongation: 2-4%
Standards
[edit]Electrical steel can be specified by various standards:
- ASTM A677 High Permeability (non-oriented)[6]
- IEC 60404-8-7 Reduced Coercivity (cold rolled, grain oriented)[7]
- JIS C2552 Reduced Coercivity (cold rolled, non-oriented)[8]
Grain orientation
[edit]
Non-Grain Oriented
[edit]Cold-rolled non-grain-oriented (CRNGO) electrical steel made without special processing to control crystal orientation, non-oriented steel, usually has a silicon level of 2 to 3.5% and has similar magnetic properties in all directions, i.e., it is isotropic.
Grain Oriented
[edit]Grain-oriented electrical steel (GOES) usually has a silicon level of 3% (Si:11Fe). It is processed in such a way that the optimal properties are developed in the rolling direction, due to a tight control (proposed by Norman P. Goss) of the crystal orientation relative to the sheet.[9][10] The magnetic flux density can be increased by 30% in the coil rolling direction, although its magnetic saturation is typically decreased by 5%.
Cold-rolled grain-oriented (CRGO) silicon steel is usually supplied by the producing mills in coil form and has to be cut and converted into "laminations", which can be used to form a transformer core, which is an integral part of any transformer. Grain-oriented steel is used in large power and distribution transformers and in certain audio output transformers.
Magnetic domains and domain walls in oriented silicon steel (image made with CMOS-MagView)
Magnetic domains and domain walls in oriented silicon steel (image made with CMOS-MagView)
Magnetic domains and domain walls in non-oriented silicon steel (image made with CMOS-MagView)
Amorphous
[edit]Amorphous metal, or metallic glass, used for electric steel can prepared by pouring molten alloy onto a rotating cooled wheel, which cools the metal at a rate of about one megakelvin per second, so fast that crystals do not form. Amorphous ribbon coil can be cast to any specific width under roughly 13 inches and easily sheared.
Amorphous electric steel ribbon can be made extremely thin, 13-25 μm, for use in electrical transformer cores.
Amorphous electric steel ribbon is limited to about 50 μm when stamped such as in motor lamentation applications, due to material fragility.
Cost
[edit]CRNGO is less expensive than CRGO.[citation needed].
In 2019, amorphous steel outside the US cost approximately $0.95/pound compared to HiB grain-oriented steel (CRGO) which costs approximately $0.86/pound.
Lamination
[edit]Electrical steel is usually manufactured in cold-rolled strips less than 2 mm thick. These strips are cut to shape to make laminations which are stacked together to form the laminated cores of transformers, and the stator and rotor of electric motors. Laminations may be cut to their finished shape by a punch and die or, in smaller quantities, may be cut by a laser, or by wire electrical discharge machining.
Electrical steel is usually coated to increase electrical resistance between laminations, reducing eddy currents, to provide resistance to corrosion or rust, and to act as a lubricant during die cutting. There are various coatings, organic and inorganic, and the coating used depends on the application of the steel.[11] The type of coating selected depends on the heat treatment of the laminations, whether the finished lamination will be immersed in oil, and the working temperature of the finished apparatus. Very early practice was to insulate each lamination with a layer of paper or a varnish coating, but this reduced the stacking factor of the core and limited the maximum temperature of the core.[12]
ASTM A976-03 classifies different types of coating for electrical steel.[13]
| Classification | Description[14] | For Rotors/Stators | Anti-stick treatment |
|---|---|---|---|
| C0 | Natural oxide formed during mill processing | No | No |
| C2 | Inorganic, glass like thin film, high resistance | No | No |
| C3 | Organic enamel or varnish coating | No | No |
| C3A | As C3 but thinner and more weldable | Yes | No |
| C4 | Coating generated by chemical and thermal processing, moderate resistance | No | No |
| C4A | As C4 but thinner and more weldable | Yes | No |
| C4AS | Anti-stick variant of C4 | Yes | Yes |
| C5 | High-resistance similar to C4 plus inorganic filler | Yes | No |
| C5A | As C5, but thinner and more weldable | Yes | No |
| C5AS | Anti-stick variant of C5 | Yes | Yes |
| C6 | Inorganic, thick coating for high resistance & strength properties | Yes | Yes |
Magnetic properties
[edit]The typical relative permeability (μr) of electrical steel is 4,000-38,000 times that of vacuum, compared to 1.003-1800 for stainless steel.[15][16][17]
The magnetic properties, which can be measured by the Epstein frame or ring core methods, of electrical steel are dependent on heat treatment, as increasing the average crystal size decreases the hysteresis loss. Hysteresis loss for common grades of electrical steel, may range from about 2 to 10 watts per kilogram (1 to 5 watts per pound) at 60 Hz and 1.5 tesla magnetic field strength.
Electrical steel can be delivered in a semi-processed state so that, after punching the final shape, a final heat treatment can be applied to form the normally required 150-micrometer grain size. Fully processed electrical steel is usually delivered with an insulating coating, full heat treatment, and defined magnetic properties, for applications where punching does not significantly degrade the electrical steel properties. Excessive bending, incorrect heat treatment, or even rough handling can adversely affect electrical steel's magnetic properties and may also increase noise due to magnetostriction.[12]
The size of magnetic domains in sheet electrical steel can be reduced by scribing the surface of the sheet with a laser, or mechanically. This greatly reduces the hysteresis losses in the assembled core.[18]
Applications
[edit]Non-grain-oriented electrical steel (NGOES) is mainly used in rotating equipment, for example, electric motors, generators and over frequency and high-frequency converters.
Grain-oriented electrical steel (GOES), on the other hand, is used in static equipment such as transformers.[19] Transformers with amorphous electric steel cores can reduce core losses by up to 70% as compared to that of conventional electrical steels cores depending on the load.
CRNGO is typically used over CRGO when
- cost is more important than CRGO's added efficiency
- direction of magnetic flux is not constant, as in electric motors and generators with moving parts
- sufficient orientation space is unavailable to take advantage of the directional properties of higher cost CRGO.
See also
[edit]- Ferrosilicon, starter material for silicon steel
- Mumetal
- Permalloy
- Supermalloy
- Metglas
References
[edit]- ↑ Metallography.org. "Etching Procedures". Metallography.org. Retrieved 5 September 2026.
- 1 2 3 Tong, Colin (2018). Introduction to Materials for Advanced Energy Systems. Springer. pp. 400–. ISBN 978-3-319-98002-7.
- ↑ Buschowl, K.H.J. et al. ed. (2001) Encyclopedia of Materials:Science and Technology. Elsevier. pp. 4807–4808. ISBN 0-08-043152-6
- ↑ Sidor, Y.; Kovac, F. (2005). "Contribution to modeling of decarburization process in electrical steels" (PDF). Вісник Львівського університету. Серія фізична. 38: 8–17. Archived from the original (PDF) on 17 January 2021. Retrieved 30 July 2020.
- ↑ Niazi, A.; Pieri, J. B.; Berger, E.; Jouty, R. (1975). "Note on electromigration of grain boundaries in silicon iron". Journal of Materials Science. 10 (2): 361–362. Bibcode:1975JMatS..10..361N. doi:10.1007/BF00540359. S2CID 135740047.
- ↑ "Standard Specification for Nonoriented Electrical Steel Fully Processed Types". store.astm.org. Retrieved 5 September 2026.
- ↑ "IEC 60404-8-7:2020". webstore.iec.ch. Retrieved 5 September 2026.
- ↑ "JSA - JIS C 2552 - Cold-rolled non-oriented electrical steel strip and sheet delivered in the fully-processed state | GlobalSpec". standards.globalspec.com. Retrieved 5 September 2026.
- ↑ Fu, Yujie; Fan, Lifeng (28 February 2025). "Microstructure and Texture Evolution of High Permeability Grain-Oriented Silicon Steel". Metals. 15 (3): 268. doi:10.3390/met15030268. ISSN 2075-4701. Archived from the original on 5 April 2025.
- ↑ Wang, Liu, Song, Liu, Shen, Jin, Wang, Yin-Ping, Hai-Tao, Hong-Yu, Jia-Xin, Hui-Ying, Yang, Guo-Dong (15 April 2018). "Ultra-thin grain-oriented silicon steel sheet fabricated by a novel way: Twin-roll strip casting and two-stage cold rolling". Magnetism and Magnetic Materials. 452: Pages 288-296 – via Elsevier Science Direct.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ↑ Fink, Donald G. and Beatty, H. Wayne (1978) Standard Handbook for Electrical Engineers 11th ed. McGraw-Hill. pp. 4–111. ISBN 978-0070209749
- 1 2 Jump, Les (March 1981) Transformer Steel and Cores, Federal Pioneer BAT
- ↑ "ASTM A976 – 03(2008) Standard Classification of Insulating Coatings by Composition, Relative Insulating Ability and Application". ASTM A976 – 03(2008). ASTM.
- ↑ "Classification of Insulating Coating for Electrical Steel" (PDF). Archived from the original on 5 March 2016. Retrieved 28 February 2024.
{{cite web}}: CS1 maint: bot: original URL status unknown (link) - ↑ "10.2: Permeability of Some Common Materials". 25 April 2019.
- ↑ "Permeability".
- ↑ "Magnetic Properties of Electrical Steel, Power Transformer Core Losses and Core Design Concepts".
- ↑ de Lhorbe, Richard (June/July 1981), Steel No Lasers Here, Federal Pioneer BAT
- ↑ Electrical Steel Market Outlook. Commodity Inside. 15-02-2020.