Grain oriented Electrical
Steel CRGO is undoubtedly the most important soft magnetic
material in use today. Wheather in small transformer,
distribution transformer or in large transformer & generator,
grain oriented electrical steel
CRGO is a must for the
production of energy saving electrical machines.
Grain oriented Electrical Steels are iron-silicon
alloys that provide low core loss and high permeability needed
for more efficient and economical electrical transformers.
CRGO Grain oriented grades of electrical steel are typically
used for transformer cores and large generators.
Non-oriented Electrical steel CRNGO fully processed
steels are iron-silicon alloys with varying silicon contents
and have similar magnetic properties in all directions in plan
of the sheet. Non-oriented Electrical steel are principally
used for motors, generators, alternators, ballasts, small
Transformers and a variety of other electromagnetic
applications.
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 Grain oriented Electrical steel
CRGO are used, mainly in power and distribution transformers.
However, it has not
supplanted non-oriented Electrical 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 Electrical steels (CRGO)
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 CRGO Electrical 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, Electrical 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 Electrical Steels use MnS as the
grain growth inhibitor, but other compounds, such as carbides,
oxides, or nitrides, are also effective.
CRGO Making
and using Grain oriented Electrical steel :
Grain Oriented Electrical Steel (CRGO) 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 Electrical
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 CRGO Grain oriented Electrical 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 Electrical steel CRGO 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.
Grain oriented Electrical steel CRGO 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 crgo being stress relief annealed, the cores are cut,
and the crgo 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 CRGO
electrical 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 CRGO 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 CRGO wound cores are utilized in
distribution transformer applications of 25 to 500 kVA.
Making and
using CRNGO non-oriented Electrical steels
Non-oriented electrical 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 electrical steel 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 (CRNGO) non-oriented
electrical steel grades, but is generally much smaller than
for the oriented electrical steel CRGO 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.
CRNGO Fully processed non-oriented Electrical steels are
generally used in applications in which:
Quantities are too small to warrant stress relieving by the
consumer, or
CRGO Laminations are so large that good physical shape
would be difficult to maintain after an 843°C stress relief
anneal.
Non-oriented steels CRNGO 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 electrical steels (CRNGO) 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 electrical steel CRGO 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 CRGO non-oriented electrical 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 electrical 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.
