Electrical steel (lamination steel, Non Grain Oriented Steel, silicon steel, relay steel, transformer steel) is a special steel tailored to produce specific magnetic properties: small hysteresis area leading to low power loss per cycle, low core loss, and high permeability.
Electrical steel is generally produced in cold-rolled strips lower than 2 mm thick. These strips are cut to shape to make laminations which are stacked together to make the laminated cores of transformers, and the stator and rotor of electric motors. Laminations could be cut for their finished shape by a punch and die or, in smaller quantities, may be cut with a laser, or by wire EDM.
Electrical steel is surely an iron alloy which can have from zero to 6.5% silicon (Si:5Fe). Commercial alloys normally have silicon content as much as 3.2% (higher concentrations usually provoke brittleness during cold rolling). Manganese and aluminum could be added as much as .5%.
Silicon significantly increases the electrical resistivity from the steel, which decreases the induced eddy currents and narrows the hysteresis loop in the material, thus decreasing the core loss. However, the grain structure hardens and embrittles the metal, which adversely affects the workability of the material, specially when rolling it. When alloying, the concentration levels of carbon, sulfur, oxygen and nitrogen has to be kept low, since these elements indicate the presence of carbides, sulfides, oxides and nitrides. These compounds, even during particles no more than one micrometer in diameter, increase hysteresis losses while 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 leading to an increase in power loss as time passes. Because of this, the carbon level is kept to .005% or lower. The carbon level can be reduced by annealing the steel in a decarburizing atmosphere, such as hydrogen.
Non-oriented Hot Dip Galvanized Steel Wire (image created using magneto-optical sensor and polarizer microscope)
Electrical steel made without special processing to manage crystal orientation, non-oriented steel, usually includes a silicon degree of 2 to 3.5% and has similar magnetic properties in every directions, i.e., it is isotropic. Cold-rolled non-grain-oriented steel is usually abbreviated to CRNGO.
Grain-oriented electrical steel usually includes a silicon degree of 3% (Si:11Fe). It is processed in a manner that this optimal properties are developed in the rolling direction, as a result of tight control (proposed by Norman P. Goss) of the crystal orientation relative to the sheet. The magnetic flux density is increased by 30% within the coil wnhsva direction, although its magnetic saturation is decreased by 5%. It really is employed for the cores of power and distribution transformers, cold-rolled grain-oriented steel is often abbreviated to CRGO.
CRGO is normally provided by the producing mills in coil form and needs to be cut into “laminations”, which are then used to create a transformer core, which can be an important part of any transformer. Grain-oriented steel is used in large power and distribution transformers and in certain audio output transformers.
CRNGO is less expensive than CRGO. It is actually used when price is more important than efficiency as well as for applications where direction of magnetic flux will not be constant, like electric motors and generators with moving parts. It can be used should there be insufficient space to orient components to take advantage of the directional properties of Electro Galvanized Steel.