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
|
-
|