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Silicon steel

Silicon steel, also called lamination steel, silicon electrical steel, electrical steel or transformer steel, is specialty steel tailored to produce certain magnetic properties, such as a small hysteresis area (small energy dissipation per cycle, or low core loss) and high permeability.

The material is usually manufactured in the form of cold-rolled strips less than 2 mm thick. These strips are called laminations when stacked together to form a core. Once assembled, they form the laminated cores of transformers or the stator and rotor parts 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 erosion.

Metallurgy
Electrical steel is an iron alloy which may have from zero to 6.5% silicon (Si:5Fe). Silicon significantly increases the electrical resistivity of the steel, which decreases the induced eddy currents and thus reduces the core loss. Manganese and aluminum can be added up to 0.5%.

Increasing the amount of silicon inhibits eddy currents and narrows the hysteresis loop of the material, thus lowering the core losses. However, the grain structure hardens and embrittles the metal, which adversely affects the workability of the material, especially when rolling it. When alloying, the concentration levels of carbon, sulfur, oxygen and nitrogen must be kept low, as these elements indicate the presence of carbides, sulfides, oxides and nitrides. These compounds, 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 steel in a decarburizing atmosphere, such as hydrogen.

Grain orientation
There are two main types of electrical steel: grain-oriented and non-oriented.

Grain-oriented electrical steel usually has a silicon level of 3% (Si:11Fe). It is processed in such a way that the optimum 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. Due to the special orientation, 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 high-efficiency transformers, electric motor and generators.

Non-oriented electrical steel usually has a silicon level of 2 to 3.5% and has similar magnetic properties in all directions, which makes it isotropic. It is less expensive and is used in applications where the direction of magnetic flux is changing, such as electric motors and generators. It is also used when efficiency is less important or when there is insufficient space to correctly orient components to take advantage of the anisotropic properties of grain-oriented electrical steel.

Lamination coatings
Electrical steel is usually coated to increase electrical resistance between laminations, 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.[1] 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. Former 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. [2]

Magnetic properties
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 test 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. Semi-processed electrical steels are delivered in a state that, after punching the final shape, a final heat treatment develops the desired 150-micrometer grain size. The fully processed steels are usually delivered with insulating coating, full heat treatment, and defined magnetic properties, for applications where the punching operation does not significantly degrade the material properties. Excessive bending, incorrect heat treatment, or even rough handling of core steel can adversely effect its magnetic properties and may also increase noise due to magnetostriction [2]

Amorphous steel
For certain transformers, cores made of amorphous steel are used. This material is a metallic glass prepared by pouring molten alloy steel on a rotating cooled wheel, which cools the metal so quickly (a rate of about one million degrees per second) that crystals do not form. The resulting amorphous metal transformers may have losses due to the core material only one-third that of conventional steels. However, its high cost (about twice that of conventional steel) and lower mechanical properties make use of amorphous steel economical only for certain distribution-type transformers. [3]

Practical concerns
Core steel is much costlier than the mild steel used for apparatus tanks, generator frames, etc.—in 1981 it was more than twise the cost ea weight.[4]

The size of magnetic domains in the sheet can be reduced by gravan the surface of the sheet with a laser, or mechanically. This greatly reduces the hysteresis losses in the assembled core. [5]
　

电工钢

    电工钢亦称硅钢片，是电力、电子和军事工业不可缺少的重要软磁合金，亦是产量最大的金属功能材料，主要用作各种电机、发电机和变压器的铁心。它的生产工艺复杂,制造技术严格,国外的生产技术都以专利形式加以保护,视为企业的生命。
　　电工钢板的制造技术和产品质量是衡量一个国家特殊钢生产和科技发展水平的重要标志之一。目前我国冷轧电工钢数量、质量、规格牌号,还不能满足能源(电力)工业发展的需求,在生产技术、设备、管理及科研等方面与日本相比,存在较大差距。
　　全世界电工钢的总产量约700万吨。近年来，特别这几年随着中国电力、电器工业的迅猛发展，中国的硅钢片需求量快速增加，2004年的消费量几乎已占全世界硅钢片产量的一半，导致中国的硅钢产量进入了一个快速发展时期，但仍然无法满足国内需求，2004年进口硅钢片164万吨。