Related
article
"Silicon Steel and Their Applications"
Abstract:
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. |
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
IEC
404-8-4
(1986) |
EN
10106
(1995) |
AISI
|
ASTM
A677
(1989) |
JIS
2552
(1986) |
GOST
21427
0-75 |
- |
M235-50A |
- |
- |
- |
- |
250-35-A5 |
M250-35A |
M 15 |
36F145 |
35A250 |
2413 |
270-35-A5 |
M270-35A |
M 19 |
36F158 |
35A270 |
2412 |
300-35-A5 |
M300-35A |
M 22 |
36F168 |
35A300 |
2411 |
330-35-A5 |
M330-35A |
M 36 |
36F190 |
- |
- |
- |
M250-50A |
- |
- |
- |
- |
270-50-A5 |
M270-50A |
- |
- |
50A270 |
- |
290-50-A5 |
M290-50A |
M 15 |
47F168 |
50A290 |
2413 |
310-50-A5 |
M310-50A |
M 19 |
47F174 |
50A310 |
2412 |
330-50-A5 |
M330-50A |
M 27 |
47F190 |
- |
- |
350-50-A5 |
M350-50A |
M 36 |
47F205 |
50A350 |
2411 |
400-50-A5 |
M400-50A |
M 43 |
47F230 |
50A400 |
2312 |
470-50-A5 |
M470-50A |
- |
47F280 |
50A470 |
2311 |
530-50-A5 |
M530-50A |
M 45 |
47F305 |
- |
2212 |
600-50-A5 |
M600-50A |
- |
- |
50A600 |
2112 |
700-50-A5 |
M700-50A |
M 47 |
47F400 |
50A700 |
- |
800-50-A5 |
M800-50A |
- |
47F450 |
50A800 |
2111 |
- |
M940-50A |
- |
- |
- |
- |
- |
M310-65A |
- |
- |
- |
- |
- |
M330-65A |
- |
- |
- |
- |
350-65-A5 |
M350-65A |
M 19 |
64F208 |
- |
- |
400-65-A5 |
M400-65A |
M 27 |
64F225 |
- |
- |
470-65-A5 |
M470-65A |
M 43 |
64F270 |
- |
- |
530-65-A5 |
M530-65A |
- |
- |
- |
2312 |
600-65-A5 |
M600-65A |
M 45 |
64F360 |
- |
2212 |
700-65-A5 |
M700-65A |
- |
64F400 |
- |
2211 |
800-65-A5 |
M800-65A |
- |
- |
65A800 |
2112 |
- |
- |
M 47 |
64F500 |
- |
- |
1000-65-A5 |
M1000-65A |
- |
64F550 |
65A1000 |
- |
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