Iron alloy optimized for magnetic properties

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Polycrystalline structure of grain oriented electrical steel after coating has been removed.

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 motors, generators, and transformers because it reduces power loss. It is an iron alloy with silicon as the main additive element (instead of carbon). The exact formulation is tailored to produce specific magnetic properties: small hysteresis area resulting in low power loss per cycle, low core loss, and high permeability.

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.

Metallurgy

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Electrical steel is an iron alloy which may have from zero to 6.5% silicon (Si:5Fe). 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%.[1]

Silicon increases the electrical resistivity of iron by a factor of about 5; this change decreases the induced eddy currents and narrows the hysteresis loop of the material, thus lowering the core loss by about three times compared to conventional steel.[1][2] 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.[1][3]

Iron-silicon relay steel

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Physical properties examples

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Grain orientation

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Non-oriented electrical silicon steel (image made with magneto-optical sensor and polarizer microscope)

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. Cold-rolled non-grain-oriented steel is often abbreviated to CRNGO.

Grain-oriented electrical steel 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. The magnetic flux density is increased by 30% in the coil rolling direction, although its magnetic saturation is decreased by 5%. It is used for the cores of power and distribution transformers, cold-rolled grain-oriented steel is often abbreviated to CRGO.

CRGO is usually supplied by the producing mills in coil form and has to be cut into "laminations", which are then 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.[10]

CRNGO is less expensive than CRGO. It is used when cost is more important than efficiency and for applications where the direction of magnetic flux is not constant, as in electric motors and generators with moving parts. It can be used when there is insufficient space to orient components to take advantage of the directional properties of grain-oriented electrical steel.

• 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 steel

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This material is a metallic glass 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 steel is limited to foils of about 50 μm thickness. The mechanical properties of amorphous steel make stamping laminations for electric motors difficult. Since amorphous ribbon can be cast to any specific width under roughly 13 inches and can be sheared with relative ease, it is a suitable material for wound electrical transformer cores. In the price of amorphous steel outside the US is approximately $.95/pound compared to HiB grain-oriented steel which costs approximately $.86/pound. Transformers with amorphous steel cores can have core losses of one-third that of conventional electrical steels.

Lamination coatings

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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

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You will get efficient and thoughtful service from Steelhighsen.

For Rotors/Stators Anti-stick treatment C0 Natural oxide formed during mill processing No No C2 Glass like film No No C3 Organic enamel or varnish coating No No C3A As C3 but thinner Yes No C4 Coating generated by chemical and thermal processing 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 more weldable Yes No C5AS Anti-stick variant of C5 Yes Yes C6 Inorganic filled organic coating for insulation properties Yes Yes

Magnetic properties

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The typical relative permeability (μr) of electrical steel is 4,000-38,000 times that of vacuum, compared to 1.003- for stainless steel.[15][16][17]

The magnetic properties of electrical steel are dependent on heat treatment, as increasing the average crystal size decreases the hysteresis loss. Hysteresis loss is determined by a standard Epstein tester and, 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 magnetic properties of electrical steel are tested using the internationally standard Epstein frame method.[18]

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.[19]

Applications

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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.[20]

See also

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• Ferrosilicon, starter material for silicon steel

References

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Essential Guide to Electrical Steels

Photo by Harrison Broadbent on Unsplash

Electrical steel is a unique product used in a broad range of industries, including energy, automotive, aerospace, manufacturing, and medical device. Also known as silicon steel and lamination steel, this material provides excellent magnetic properties, making it ideal for use in things, like generators, motors, and transformers. Electrical steel also cuts power losses, boosts electrical device efficiency, and reduces energy use. The exact formulation for electrical steel, however, must be tailored to produce specific magnetic properties for specific applications.

Electrical steel is an iron alloy with silicon as the main additive element rather than carbon. The silicon additive increases its electrical resistivity. That reduces eddy currents&#;the circular electrical currents induced in a conductor by a changing magnetic field. Eddy currents waste energy and heat the conductor. The silicon also helps create a finer grain structure in the steel, further reducing eddy currents. Electrical steels also have low hysteresis losses&#;the energy losses that occur when you magnetize or demagnetize ferromagnetic materials.  

Benefits of Electrical Steels

Electrical steels offer a range of benefits that make them ideal for use in electromagnetic devices. Electrical steel:

• Enhances efficiency: The material improves the efficiency of electromagnetic devices by reducing core losses. That can generate significant energy savings, especially in large electrical devices like motors and transformers.

• Reduces operating costs: The boost in efficiency produced by electrical steel often results in lower operating costs. A more efficient motor consumes less electricity, which saves money on energy bills.

• Smaller and lighter devices: Electrical steels enable manufacturers to produce smaller, lighter electromagnetic devices. Being highly efficient, electrical steel requires less material to produce the same magnetic field strength.

• Less environmental impact: Electrical steel helps reduce the impact of electromagnetic devices by reducing energy consumption and wasting heat, making the material ideal for environmentally-conscious manufacturers.

Types of Electrical Steel

Many types of electrical steels exist, each with its properties and applications. Two common types of electrical steels are grain-oriented (GO) steel and non-grain-oriented (NGO) steel. GO steel&#;s grain structure aligns in one direction. That gives them higher magnetic permeability and lower core losses than other electrical steel types. Manufacturers often use GO steel in static devices and equipment, like transformers, where energy efficiency is critical. GO steels, however, are expensive and difficult to manufacture.

NGO electrical steel lacks a preferred grain orientation. Plus, it has higher core losses than GO electrical steel. NGO steels are ideal for applications where manufacturability and cost are more important than efficiency. Often used in rotating devices and equipment, like electric motors, generators, and high-frequency converters, NGO electrical steel is less expensive and easier to manufacture than other electrical steels. 

Other types of electrical steel are high-silicon electrical steel and amorphous electrical steel. High-silicon electrical steels, which contain more silicon than other types of electrical steel, are typically used in high-performance applications, such as large transformers and motors. Amorphous electrical steels, which features a non-crystalline structure, are typically used in high-performance applications where the highest possible efficiency is required. Both steels are expensive and difficult to manufacture.

Properties and Applications of Electrical Steel

Electric steel&#;s properties make it ideal for a wide range of applications, including capacitors, inductors, relays, and solenoids. It is also used in the cores of household appliances, in the rotors and stators, and to step down voltage in electricity transmission and distribution systems. Plus, it&#;s used in the motors and other electrical components of industrial equipment and machinery, such as robots, CNC machines, and conveyor belts.

Electrical steel&#;s properties also make it well-suited for use in the cores of autotransformers to improve performance and efficiency and in current transformers to measure the current flowing in electrical circuits. They also make electric steel well-suited for use in the cores of magnetic switches and relays to measure the current flowing in an electrical circuit and in electrical ballast to improve their performance and efficiency.

This material&#;s properties include:

• High permeability: Electrical steel&#;s high permeability means it is easily magnetized and demagnetized&#;a property critical for electromagnetic devices, which must switch their magnetic field quickly and efficiently.

• Lower core losses: Electrical steels have low core losses. So, they waste less energy and are more efficient and environmentally friendly.

• High electrical resistivity: This property measures how well a material resists the flow of electricity. Electrical steel&#;s high electrical resistivity helps reduce the eddy currents that waste energy and heat conductors.

• Good mechanical properties: Electrical steels have good mechanical properties, such as strength and ductility, so that they can withstand a high degree of mechanical stress.

Manufacturing Process of Electrical Steel

The manufacturing process for electrical steel is complex and requires tight control to ensure the desired properties are achieved. The most critical steps in the process include melting, casting, reheating, and hot rolling. It also involves cold rolling, annealing, and finishing. However, the specific manufacturing process for electrical steel varies depending on the type produced and the properties desired. For instance, an NGO electrical steel material needs a less complex manufacturing process to achieve its unique properties than does GO steel does.

Ashton Henning