Related article "Silicon Steel and Their Applications"

Silicon steel is undoubtedly the most important soft magnetic material in use today. Applications vary in quantities from the few ounces used in small relays or pulse transformers to tons used in generators, motors, and transformers. Continued growth in electrical power generation has required development of better steels to decrease wasteful dissipation of energy (as heat) in electrical apparatus and to minimize the physical dimensions of the increasingly powerful equipment now demanded.

The earliest soft magnetic material was iron, which contained many impurities. Researchers found that the addition of silicon increased resistivity, decreased hysteresis loss, increased permeability, and virtually eliminated aging.

Substantial quantities of oriented steel are used, mainly in power and distribution transformers. However, it has not supplanted nonoriented silicon steel, which is used extensively where a low-cost, low-loss material is needed, particularly in rotating equipment. Mention should also be made of the relay steels, used widely in relays, armatures, and solenoids. Relay steels contain 1.25 to 2.5% Si, and are used in direct current applications because of better permeability, lower coercive force, and freedom from aging.

Important physical properties of silicon steels include resistivity, saturation induction, magneto-crystalline anisotropy, magnetostriction, and Curie temperature. Resistivity, which is quite low in iron, increases markedly with the addition of silicon. Higher resistivity lessens the core loss by reducing the eddy current component. Raising the silicon content will lower magnetostriction, but processing becomes more difficult. The high Curie temperature of iron will be lowered by alloying elements, but the decrease is of little importance to the user of silicon steels.

The magnetization process is influenced by impurities, grain orientation, grain size, strain, strip thickness, and surface smoothness. One of the most important ways to improve soft magnetic materials is to remove impurities, which interfere with domain-wall movement; they are least harmful if present in solid solution. Compared with other commercial steels, silicon steel is exceptionally pure. Because carbon, an interstitial impurity, can harm low induction permeability, it must be removed before the steel is annealed to develop the final texture.

The mechanism for the growth of grains with cube-on-edge orientation during the final anneal is not completely understood. The process involves secondary recrystallization, which, by definition, is characterized by accelerated growth of one set of grains in an already recrystallized matrix.

For secondary recrystallization, normal grain growth must be inhibited in some manner. As the temperature is raised, certain grains break loose from the inhibiting forces, and grow extensively at the expense of their neighbors. Producers know that, on a practical basis, appropriate cold rolling and recrystallization sequences must be carefully followed to obtain the desired secondary recrystallization nuclei and the correct texture. Today`s silicon steels use MnS as the grain growth inhibitor, but other compounds, such as carbides, oxides, or nitrides, are also effective.

Making and using oriented steel

Oriented silicon steel is more restricted in composition than non-oriented varieties. The texture is developed by a series of careful working and annealing operations, and the material must remain essentially single-phase throughout processing, particularly during the final anneal because phase transformation destroys the texture. To avoid the y loop of the Fe-Si phase system, today`s commercial steel has about 3.25% Si. Higher silicon varieties, which might be favored on the basis of increased resistivity and lower magnetostriction, are precluded by difficulties in cold rolling.

Temperature, atmosphere composition, and dew point are closely controlled to decarburize the strip without oxidizing the surface. During this treatment, primary recrystallization occurs, forming small, uniform, equiaxed grains. The coating of magnesium silicate glass which forms will provide electrical insulation between successive laminations when assembled in a transformer core. At this stage, the steel is graded by cutting Epstein samples from the coil; the samples are stress relief annealed and flattened at 790°C, and tested for core loss.

Applications for oriented silicon steel include transformers (power, distribution, ballast, instrument, audio, and specialty), and generators for steam turbine and water wheels.

Lay-up cores, in general, utilize the whole spectrum of grain oriented quality and gages. The gage and grade of material for a given application are determined by economics, transformer rating, noise level requirement, loss requirements, density of operation, and even core size. Because the strip must be flat to produce a good core, coils are flattened after the high temperature anneal. Then, the strip is coated with an inorganic phosphate for insulation. Samples from each coil end are graded after a laboratory stress relief anneal, as previously described. From such strip, the transformer manufacturer cuts his required length improves the insulation of the strip. Consequently, it decreases the eddy current losses and heat buildup, which is of particular importance in transformers which must withstand an impulse test.

As noted earlier, an important requirement in the manufacture of lay-up cores is minimizing transformer noise. Noise is a function of manufacturing and core design factors, the core material characteristic being one of the most important. The dependence of magnetostriction on silicon content has already been noted. In addition, magnetostriction is reduced by improving the texture and by introducing tensile stresses through application of glass-type insulation coatings. Because compressive stresses affect magnetostriction adversely, it is important that the lamination remains flat for assembly. Operating induction is also a factor that affects noise, and indeed affects the transformer`s general operating characteristics. Operating inductions of lay-up transformers are usually in the 10,000 to 17,000 G range; power ratings extend over the 500 to 1,000,000 kVA range.

Wound cores are wound toroidally with the [100] crystallographic direction around the strip. Processing steps are somewhat different from those used for lay-up transformers though the starting material is the same-large toroidally annealed coil coated with magnesium silicate, which usually provides sufficient insulation.

For wound core application, unreacted MgO powder is removed from the strip surface, and a sample from each coil end is cut into Epstein strips to be tested as before. After being graded, the coil is shipped to the transformer manufacturer either as slit multiples or as a full-width coil for subsequent slitting. The slit multiple, wound to the given core dimension, must be stress relief annealed at 790°C in a dry nonoxidizing atmosphere. Annealing trays and plates must be of low carbon steel to eliminate any carbon contamination, which can be very detrimental to quality.

After being stress relief annealed, the cores are cut, and the transformer core is assembled by lacing the steel around the copper (or aluminum) current-carrying coils. In the stress relief annealed condition, grain-oriented steel is sensitive to mechanical strain; therefore, cores must be assembled carefully. Regardless of how carefully assembly is accomplished, the final core quality is always poorer than it was in the stress-relief annealed, uncut condition.

The difference in quality, commonly referred to as the "destruction factor", is due to the relative strain sensitivity of the grain-oriented steel, the handling procedure in fabrication, and the uniformity and amount of air gap in the core. Being a function of the transformer design and fabrication, the latter two factors are controlled best by the manufacturer. Most wound cores are utilized in distribution transformer applications of 25 to 500 kVA.

Making and using non-oriented silicon steels

Non-oriented silicon steels do not use a secondary recrystallization process to develop their properties, and high temperature annealing is not essential. Therefore, a lower limit on silicon, such as is required for the oriented grades, is not essential.

Non-oriented grades contain between 0.5 and 3.25% Si plus up to 0.5% Al, added to increase resistivity and lower the temperature of primary recrystallization. Grain growth is very desirable in the nonoriented grades, but is generally much smaller than for the oriented grades.

Processing to hot rolled band is similar to that described for the oriented grade. After surface conditioning, the bands are usually cold rolled directly to final gage, and sold to the transformer manufacturer in one of two conditions fully-processed, or semi processed. After final cold rolling, the strip is annealed, decarburizing it to 0.005% C or lower and developing the grain structure needed for the magnetic properties. Samples are then taken from each coil end, and tested.

Fully processed nonoriented silicon steels are generally used in applications in which:

• Quantities are too small to warrant stress relieving by the consumer, or
• Laminations are so large that good physical shape would be difficult to maintain after an 843°C stress relief anneal.

Non-oriented steels are not as sensitive to strain as the oriented product. Consequently, shearing strains constitute the only strain effects, which should degrade the magnetic quality. Because laminations are generally large, these shearing strains can be tolerated. Most of the fully processed grades are used as stamped laminations in such applications as rotors and stators.

The non-oriented steels have a random orientation. They are commonly used in large rotating equipment, including motors, power generators, and AC alternators. Fully processed steels are given a "full" strand anneal (to develop the optimum magnetic quality), making them softer and more difficult to punch than semi-processed products. Grades with higher alloy content are harder and thus easier to punch.

Improved punchability can be provided in fully processed steels by adding an organic coating, which acts as a lubricant during stamping and gives some additional insulation to the base scale. If good inter-lamination resistance is required, fully processed material can be purchased with core plate.

Semi processed products are generally given a lower-temperature decarburizing anneal after the final cold rolling. Carbon is not necessarily removed to the same low level as in fully processed material. The transformer manufacturer will subsequently stress relief anneal the material in a wet decarburizing atmosphere to obtain additional decarburization and develop the magnetic properties. Samples are taken after the mill decarburization anneal, cut into specimens, decarburized at 843°C for at least one hour and tested to grade the coil.

Semi processed nonoriented silicon steels are used for applications in which the customer does the stress relief anneal. In general, such products have good punching characteristics, and are used in a variety of applications including small rotors, stators, and small power transformers. Semi processed steels can be purchased with a tightly adherent scale, or with an insulating coating over the oxide. The organic coating acts as a lubricant during punching, but it does not withstand stress relief annealing temperatures; therefore, it is not applied to semi-processed material.

Table 1. The most important silicon steel designations specified by different standards

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