manufacturing the grain-oriented electrical
steel sheet.
【Background Art】
Electrical steel sheets have a high degree of
permeability and a low degree of core loss, and are thus
frequently used as materials for cores, etc. Electrical
steel sheets may be broadly categorized as grain-oriented
electrical steel sheets and non-oriented electrical steel
sheets.
Grain-oriented electrical steel sheets are
characterized by {110}<001> grains having a {110} plane
parallel to the rolled surface and a <001> axis (magnetic
easy axis) parallel to the rolling direction. Grainoriented
electrical steel sheets have superior magnetic
characteristics in a particular direction, and are thus
widely used as material for cores of devices that are used
at a fixed position, such as transformers, electric motors,
Page 2
generators, or other electric devices. The magnetic
characteristics of grain-oriented electrical steel sheets
may be expressed by magnetic flux density and core loss. A
grain-oriented electrical steel sheet having a higher
degree of magnetic flux density and a lower degree of core
loss is favored. In general, the magnetic flux density of
electrical steel sheets is expressed by B8 values measured
in a magnetic field of 800 Amp/m, and the core loss of
electrical steel sheets is expressed by W17/50 indicating
lost watts per kilogram at 50 Hz and 1.7 Tesla.
N.P. Goss developed an early technique for grainoriented
electrical steel sheets. According to the
technique, grains of steel are oriented in the {110}<001>
orientation (known as Goss orientation) by a cold rolling
method. Thereafter, the technology for grain-oriented
electrical steel sheets has been developed up to the
present level.
That is, it is necessary to increase the proportion
of grains having {110}<001> orientation or a similar
orientation so as to manufacture a grain-oriented
electrical steel sheet. A heating process is necessary to
induce recrystallization of grains of steel sheets, and
thus to orient the grains of the steel sheets. In an
annealing process, however, the growth of crystals
Page 3
generally occurs in random orientations. Therefore, a
particular method is necessary to obtain grain-oriented
electrical steel sheets having grains grown in a particular
direction.
In general, electrical steel sheets are annealed in
two steps: primary recrystallization annealing and
secondary recrystallization annealing. Primary
recrystallization occurs by using energy accumulated during
a cold rolling process as a driving force, and secondary
recrystallization occurs by using boundary energy of
primarily recrystallized grains as a driving force. During
the secondary recrystallization which is also called
"abnormal grain growth," grains grow to a size of several
millimeters (mm) to several centimeters (cm).
However, secondarily recrystallized grains have
different orientations depending on the temperature of
recrystallization. If the secondary recrystallization
occurs at a certain temperature, the proportion of grains
having Goss orientation increases, and thus an electrical
steel sheet having a low degree of core loss may be
obtained.
Therefore, it is necessary to suppress the secondary
recrystallization until the temperature reaches a certain
level at which grains having Goss orientation are
Page 4
obtainable and to start the secondary recrystallization at
a certain temperature. Generally, inhibitors are used for
this purpose. Inhibitors exist in the form of precipitates
in steel and suppress the movement of grain boundaries and
the formation of new grains. If a proper inhibitor is
selected, the inhibitor may not obstruct the growth of
grains at a recrystallization temperature at which the
grains recrystallize as grains having Goss orientation, for
example, because the inhibitor is dissolved or removed at
the recrystallization temperature, and thus the
recrystallization and growth of grains having Goss
orientation may markedly occur at the recrystallization
temperature.
Therefore, the selection of a proper inhibitor may be
a crucial factor in increasing the proportion of grains
having Goss orientation in electrical steel sheets and
reducing the core loss of the electrical steel sheets. An
MnS-based inhibitor, developed by ARMCO, USA, may be the
first inhibitor. However, in techniques in which MnS-based
inhibitors are used, since MnS exists as coarse particles
in steel slabs and thus does not function as an inhibitor,
MnS is first dissolved through a solid solution treatment
and is then precipitated as fine particles. To this end,
slabs are heated to 1350°C or higher to sufficiently
Page 5
dissolve MnS. However, the slab heating temperature is much
higher than a temperature to which steel slabs are
generally heated and thus may decrease the lifespan of a
heating furnace, thereby causing problems such as a
decrease in the lifespan of a heating furnace or corrosion
of a slab due to silicon oxides melting and flowing on the
surface of the slab. In addition, a method of manufacturing
non-oriented electrical steel sheets through two cold
rolling processes and an intermediate annealing process
therebetween has been proposed by ARMCO. However,
electrical steel sheets manufactured by the method thereof
do not have sufficient magnetic characteristics.
In 1968, Nippon Steel Corporation proposed a new
conceptual electric steel sheet product named "Hi-B." The
electric steel sheet product Hi-B uses AlN and MnS as
inhibitors and is producible through a single cold rolling
process. Although the electric steel sheet product Hi-B has
a high degree of magnetic flux density and a low degree of
core loss, a slab has to be heated to a high temperature
during a solid solution treatment process so as to dissolve
inhibitors.
JFE has proposed another electrical steel sheet using
MnSe and antimony (Sb) as inhibitors. However, the
electrical steel sheet is also disadvantageous in that a
Page 6
slab has to be heated to a high temperature.
To address problems of such high-temperature heating
methods of the related art, a low-temperature heating
method has been developed. According to the low temperature
heating method, inhibitors are not formed at the beginning
of a manufacturing process but are formed immediately
before secondary recrystallization so that the slab heating
temperature may be decreased to 1300°C or lower, or 1280°C
or lower. The core technology of the low-temperature
heating method is a nitriding annealing process in which
nitrogen (N) necessary for forming AlN functioning as an
inhibitor is added to steel by diffusing nitrogen gas at a
later stage of manufacturing. Therefore, a high-temperature
heating process is not necessary for dissolving aluminum
(Al) and nitrogen (N) and forming AlN. Thus, various
process problems of high-temperature heating methods could
be solved.
A method of increasing the specific resistance of
electrical steel sheets may be considered an important
method of decreasing the core loss of electrical steel
sheets. That is, as shown in Formula 1 below, the core loss
of steel sheets is reverse proportional to the specific
resistance of the steel sheets. Thus, particular elements
may be added to steel sheets to increase the specific
Page 7
resistance of the steel sheets.
[Formula 1]
Wec = (π2·d2·I2·f2)/(ρ·6)
where Wec: core loss, d: crystal diameter, I: current,
f: frequency, and ρ: specific resistance.
An exemplary element that increases the specific
resistance of electrical steel sheets is silicon (Si). That
is, the core loss of electrical steel sheets may be reduced
by adding as much silicon (Si) as possible to the
electrical steel sheets. However, if an excessive amount of
silicon (Si) is added to a steel sheet, the brittleness of
the steel sheet is increased, and thus cold-rolling
characteristics of the steel sheet are deteriorated.
Therefore, the method of adding silicon (Si) has practical
limitations. Like silicon (Si), phosphorus (P) may increase
the specific resistance of steel sheets. However, since
even a very small amount of phosphorus (P) increases the
brittleness of steel sheets, there is also a limit to
adding phosphorus (P).
【Disclosure】
【Technical Problem】
Aspects of the present disclosure may provide an
improved electrical steel sheet having superior magnetic
characteristics such as a low degree of core loss and
Page 8
designed to be manufactured by a low-temperature heating
method, and an improved method for manufacturing the
electrical steel sheet.
However, the present disclosure is not limited to the
above-mentioned aspects. The above-mentioned aspects and
other aspects of the present disclosure will be clearly
understood by those of skill in the art through the
following description.
【Technical Solution】
According to an aspect of the present disclosure, an
electrical steel sheet having a low degree of core loss may
include, by wt%, silicon (Si): 1.0% to 4.0%, aluminum (Al):
0.1% to 4.0%, and at least one rare earth element: 0.05% to
0.5% in total content.
The electrical steel sheet may further include carbon
(C): 0.003 wt% or less, manganese (Mn): 0.03 wt% to 0.2 wt%,
sulfur (S): 0.001 wt% to 0.05 wt%, and nitrogen (N): 0.01
wt% or less.
The electrical steel sheet may further include at
least one selected from the group consisting of phosphorus
(P): 0.5% or less, tin (Sn): 0.3% or less, antimony (Sb):
0.3% or less, chromium (Cr): 0.3% or less copper (Cu): 0.4%
or less, and nickel (Ni): 1% or less.
The rare earth element or a compound of the rare
Page 9
earth element may be used as an inhibitor.
According to another aspect of the present disclosure,
a method for manufacturing an electrical steel sheet having
a low degree of core loss may include: heating a slab to
1050°C to 1300°C, the slab including, by wt%, silicon (Si):
1.0% to 4.0%, aluminum (Al): 0.1% to 4.0%, and at least one
rare earth element: 0.05% to 0.5% in total content; hot
rolling the slab; cold rolling the slab; primarily
recrystallizing the slab; and secondarily recrystallizing
the slab.
The slab may further include carbon (C): 0.1 wt% or
less, manganese (Mn): 0.03 wt% to 0.2 wt%, sulfur (S):
0.001 wt% to 0.05 wt%, and nitrogen (N): 0.01 wt% or less.
After the hot rolling of the slab, the method may
further include at least one selected from: annealing the
hot-rolled slab; and pickling the hot-rolled slab.
The cold rolling may be performed at a reduction
ratio of 85% to 90%.
The cold rolling may be performed two or more times
with an intermediate annealing process therebetween, and a
reduction ratio of the final cold rolling may be 60% or
greater.
The primary recrystallizing may be performed within a
temperature range of 700°C to 950°C.
Page 10
The secondary recrystallizing may be performed by
heating the slab to a maximum temperature of 1100°C to
1300°C at a heating rate of 5°C/hr to 30°C/hr.
【Advantageous Effects】
As described above, according to the present
disclosure, rare earth metals (REMs) are used as inhibitors,
and a large amount of aluminum (Al) is added to a steel
sheet to increase the specific resistance of the steel
sheet, thereby markedly decreasing the core loss of the
steel sheet.
【Description of Drawings】
FIGS. 1A and 1B are images taken with a microscope to
show inhibitors formed in steel sheets to which rare earth
elements are added.
FIGS. 2A and 2B are graphs illustrating core loss
according to the total content of rare earth elements.
【Best Mode】
The present disclosure will now be described in
detail.
The inventors have conducted research into a method
of manufacturing an electric steel sheet having a low
degree of core loss by adding an inhibitor to increase the
number of particles having Goss orientation and the
specific resistance of the electric steel sheet without
Page 11
increasing the brittleness of the electric steel sheet. As
a result, the inventors have found that the objects as
described above could be achieved by adding a rare earth
metal (REM) (hereinafter referred to as a "rare earth
element") to an electrical steel sheet and increasing the
content of aluminum (Al) in the electrical steel sheet.
Based on this knowledge, the inventors have invented the
present invention.
That is, according to the present disclosure,
aluminum (Al) is added to an electrical steel sheet in an
amount of 0.1 wt% or greater. According to results of the
research conducted by the inventors, like silicon (Si),
aluminum (Al) has a significant effect on increasing the
specific resistance of a steel sheet but does not increase
the brittleness of the steel sheet if the content of
aluminum (Al) is within a certain range. Although silicon
(Si) is additionally added to a steel sheet to increase the
specific resistance of the steel sheet, the content of
silicon (Si) in the steel sheet is limited because silicon
(Si) may increase the brittleness of the steel sheet.
Therefore, aluminum (Al) may be used together with silicon
(Si) to increase the specific resistance of a steel sheet
without increasing the brittleness of the steel sheet. To
this end, it may be preferable that aluminum (Al) be added
Page 12
in an amount of 0.1 wt% or greater. However, if the content
of aluminum (Al) in the electrical steel sheet is
excessively high, the brittleness of the electrical steel
sheet is increased. Therefore, the content of aluminum (Al)
may be adjusted to be 4.0 wt% or less so as not to affect
cold rolling characteristics of the electrical steel sheet.
The above-mentioned aluminum (Al) content range is
much higher than the aluminum (Al) content range (for
example, 0.05 wt% or less) of general electrical steel
sheets using AlN as an inhibitor. That is, if aluminum (Al)
is added within the content range of the present disclosure,
it may be difficult to finely and uniformly distribute AlN
functioning as an inhibitor, and thus AlN may not
sufficiently function as an inhibitor for inducing the
formation of particles having Goss orientation.
Therefore, the present disclosure proposes a new
conceptual inhibitor instead of an AlN inhibitor, so as to
improve both the specific resistance and the crystal
orientation of electrical steel sheets. To this end, rare
earth elements are used as inhibitor forming elements in
the present disclosure. Rare earth elements are 17 elements
consisting of: scandium (Sc) and yttrium (Y) included in
group 3, and the 15 elements with atomic numbers 57 to 71
(the lanthanides) in the periodic table. The rare earth
Page 13
elements serve individually or in the form of a compound
with sulfur (S) or oxygen (O) to hinder the movement of
boundaries of primarily recrystallized grains but do not
hinder the growth of grains having Goss orientation at a
secondary recrystallization temperature, thereby having a
significant effect on increasing the proportion of
particles having Goss orientation. In addition, a compound
of a rare earth element is finely and uniformly distributed
in a cast slab, and thus it is unnecessary to perform a
solid solution treatment on the slab in a later process to
finely precipitate the compound. Owing to this, a slab
heating temperature may be adjusted within the range of a
general low-temperature heating method, and thus problems
of high-temperature heating methods may not occur.
One of the rare earth elements may be used, or two or
more of the rare earth elements may be used. For example,
the total content of rare earth elements in a steel sheet
may be adjusted to be 0.05% or greater so as to obtain
sufficient inhibitor effects. However, if the total content
of rare earth elements is excessively high, coarse
compounds may be formed. Thus, the upper limit of the total
content of rare earth elements is set to 0.5 wt%. Coarse
compounds may not have a sufficient effect on suppressing
the growth of grains during primary recrystallization.
Page 14
According to an exemplary embodiment of the present
disclosure, a rare earth element or rare earth elements may
be added to an electrical steel sheet in a total amount of
0.065% to 0.4% so as to further reduce the core loss of the
electrical steel sheet.
That is, an electrical steel sheet of the present
disclosure may include aluminum (Al) and at least one rare
earth element (REM) in addition to silicon (Si). In this
case, the content of silicon (Si) in the electrical steel
sheet may be adjusted to be within the range of 1.0 wt% to
4.0 wt% due to the following reasons.
That is, as described above, silicon (Si) may be
added in an amount of 1.0% or greater to increase the
specific resistance of the electrical steel sheet. As the
content of silicon (Si) increases, the specific resistance
of the electrical steel sheet increases, and thus the core
loss of the electrical steel sheet may decrease. That is, a
high content of silicon (Si) may be favored. However, since
electrical steel sheets are generally manufactured through
a cold rolling process, the content of silicon (Si) may be
adjusted to be 4.0 wt% or less by taking into consideration
cold-rolling characteristics.
Therefore, the electrical steel sheet of the present
disclosure may include, by wt%, silicon (Si): 1.0% to 4.0%,
Page 15
aluminum (Al): 0.1% to 4.0%, and at least one rare earth
element: 0.05% to 0.5% in total content.
In addition, the electrical steel sheet of the
present disclosure may further include additional elements
and inevitable impurities, and there is no particular limit
to such additional elements or impurities. For example,
elements such as carbon (C), manganese (Mn), sulfur (S), or
nitrogen (N) may be additionally added to the electrical
steel sheet of the present disclosure, and according to
some embodiments of the present disclosure, the contents of
the elements may be adjusted as follows.
Carbon (C): 0.003 wt% (30 ppm) or less
A large amount of carbon (C) may be present in a slab,
for example, due to the load of a decarbonizing process.
However, since carbon (C) causes magnetic aging, the
content of carbon (C) in a final product (electrical steel
sheet) may be adjusted to be low. That is, the content of
carbon (C) in the electrical steel sheet of the present
disclosure is limited to 0.003 wt% or less. As described
above, since carbon (C) is an undesirable impurity in a
final product, the content of carbon (C) in the electrical
steel sheet of the present disclosure does not have a
particular minimum limit.
Page 16
Manganese (Mn): 0.03 wt% to 0.2 wt%
Manganese (Mn) lowers a solid-solution temperature at
which precipitates dissolve during a reheating process and
prevents the creation of cracks in both ends of a steel
sheet during a hot rolling process. To obtain these effects,
manganese (Mn) may be added in an amount of 0.03% or
greater. However, if manganese (Mn) is added in excessively
large amounts, Mn oxides and MnS may be formed, and thus
the function of the rare earth element may be lowered to
result in a high degree of core loss. Therefore, it may be
preferable that the content of manganese (Mn) be within the
range of 0.03 wt% to 0.2 wt%.
Sulfur (S): 0.001 wt% to 0.05 wt%
Sulfur (S) may combine with the rare earth element to
form an inhibitor. To this end, it may be preferable that
sulfur (S) be added in an amount of 0.001 wt% or greater.
However, an excessively high content of sulfur (S) may lead
to the formation of a coarse sulfur compound which does not
properly function as an inhibitor suppressing the growth of
primarily recrystallized grains. Therefore, the upper limit
of the sulfur (S) content is set to be 0.05 wt%.
Page 17
Nitrogen (N): 0.01 wt% or less
If nitrogen (N) is added to some electrical steel
sheets, nitrogen (N) functions as an inhibitor. However,
since the electrical steel sheet of the present disclosure
does not actively use a nitride inhibitor, nitrogen (N) is
not actively added. In addition, if an excessive amount of
nitrogen (N) is added to steel, the steel may undergo
swelling called blisters. Therefore, the content of
nitrogen (N) in the electrical steel sheet of the present
disclosure is limited to 0.01 wt% or less.
In addition to the above-listed elements, the
electrical steel sheet of the present disclosure may
further include other elements such as phosphorus (P), tin
(Sn), antimony (Sb), chromium (Cr), copper (Cu), or nickel
(Ni) that are usually included in general electrical steel
sheets. The contents of such elements in the electrical
steel sheet of the present disclosure are not limited to
specific ranges as long as the contents of the elements are
within generally-acceptable ranges. For example, the
electrical steel sheet of the present disclosure may
further include one or more of phosphorus (P): 0.5% or less,
tin (Sn): 0.3% or less, antimony (Sb): 0.3% or less,
chromium (Cr): 0.3% or less, copper (Cu): 0.4% or less, and
Page 18
nickel (Ni): 1% or less.
As described above, the electrical steel sheet of the
present disclosure includes a large amount of aluminum (Al),
and at least one rare earth element or a compound of the
rare earth element is present as an inhibitor in the
electrical steel sheet. The aluminum (Al) may increase the
specific resistance of the electrical steel sheet, and the
inhibitor may increase the proportion of particles having
Goss orientation in the electrical steel sheet.
As a result, according to an exemplary embodiment,
the electrical steel sheet may have a high degree of
magnetic flux density within the range of 1.8 T or greater
in B8 and a low degree of core loss.
The electrical steel sheet of the present disclosure
may be manufactured by a method used to manufacture general
electrical steel sheets. That is, the electrical steel
sheet of the present disclosure is not limited to a
specific manufacturing method. However, an exemplary
embodiment is proposed by taking into consideration the
characteristic composition of the electrical steel sheet
and resulting behaviors of the inhibitor.
That is, the electrical steel sheet of the present
disclosure may be manufactured by a low-temperature heating
method including a primary recrystallization annealing
Page 19
process and a secondary recrystallization annealing process
after a hot rolling process and a cold rolling process.
Specific conditions thereof are as follows.
First, a slab is heated. In the present disclosure,
the slab has substantially the same composition as the
composition of the electrical steel sheet. However, since
carbon (C) is removed from the slab in a later
decarbonization annealing process, the content of carbon
(C) in the slab may be higher than the content of carbon
(C) (for example, 0.0003 wt% or less) in the electrical
steel sheet. If the content of carbon (C) in the slab is
excessively high, the load of a decarbonization process may
be increased, and thus productivity may be decreased.
Therefore, the content of carbon (C) in the slab for
forming the electrical steel sheet of the present
disclosure may be within the range of 0.10 wt% or less.
Since carbon (C) is an optional element, the minimum limit
of the content of carbon (C) in the slab may not be set.
However, if the content of carbon (C) in the slab is
excessively low, phase transformation may not sufficiently
occur in the slab during a hot rolling process, and thus
nuclei of {110}<001> Goss grains may not be sufficiently
formed. In this case, the magnetic characteristics of the
electrical steel sheet may be deteriorated. Therefore, the
Page 20
lower limit of the content of carbon (C) in the slab may be
set to be 0.01 wt%.
Furthermore, at least one rare earth element may be
added during a steel making process, and thus the
electrical steel sheet of the present disclosure may
include at least one rare earth element as described above.
In the case of adding two or more rare earth elements, the
rare earth elements may be added in the form of a
mischmetal in which rare earth elements are mixed. That is,
since rare earth elements have similar chemical properties
and are difficult to separate from each other, rare earth
elements may be smelted in a mixed state. For example,
depending on the kind of ore (such as moissanite or
bastnasite), a salt in which several rare earth elements
are mixed may be obtained. Such a mixed salt is reduced
with a reactive metal such as manganese (Mn), calcium (Ca),
or sodium (Na), or is electrolyzed so as to obtain a metal.
This metal includes a plurality of elements and is called a
"mischmetal." A mischmetal may be used to control the
contents of rare earth elements during a steel making
process, and if the total content of rare earth elements is
within the above-mentioned range of the present disclosure,
the composition or type of the mischmetal are not limited.
In the present disclosure, at least one rare earth
Page 21
element is used as an inhibitor forming element, and an
inhibitor formed of the rare earth element may be uniformly
and finely distributed in steel even though a solid
solution treatment necessary for the case of using other
inhibitors such as MnS or MnSe is not performed. Therefore,
a high-temperature heating process is not necessary. As
such, in the present disclosure, the slab may be heated to
1300°C or lower so as to lower the load of a heating
furnace and prevent silicon (Si) oxides formed on the
surface of the slab from melting. More preferably, the slab
may be heated to 1250°C or lower. However, when a later hot
rolling process is considered, it may be preferable that
the slab be heated to 1050°C or higher.
After the slab is heated as described above, the slab
may be hot rolled. The slab may be hot rolled by a general
method. According to an exemplary embodiment, the slab may
be hot rolled to obtain a hot-rolled steel sheet having a
thickness of 2.0 mm to 3.0 mm. In this case, the load of a
later cold rolling process may not be excessive, and a
sufficient reduction ratio may be obtained in the later
cold rolling process.
Then, the hot-rolled steel sheet may be subjected to
a hot band annealing process or a pickling process. However,
these processes are not essential.
Page 22
After the hot rolling process and the optional hot
band annealing process, the steel sheet may be subjected to
a cold rolling process. The cold rolling process may be
performed once, twice, or more times with an intermediate
annealing process therebetween. The cold rolling process is
important for texturing the steel sheet and may preferably
be performed at a reduction ratio of 85% to 90% (total
reduction ratio if performed two or more times). That is,
the reduction ratio of the cold rolling process may
preferably be 85% or greater so as to sufficiently texture
the steel sheet and thus induce the formation of a large
number of grains having Goss orientation after primary
recrystallization and secondary recrystallization. However,
if the reduction ratio of the cold rolling process is
excessively high, the load of the cold rolling process may
also be excessive. Thus, the upper limit of the reduction
ratio is set to 90%.
If the cold rolling process is performed two or more
times with an intermediate annealing process therebetween,
the reduction ratio of the final cold rolling process (for
example, the second time if performed twice) may be 50% or
greater.
Thereafter, the cold-rolled steel sheet may be
processed through a primary recrystallization annealing
Page 23
process. Preferably, the primary recrystallization
annealing process may be performed within the temperature
range of 700°C to 950°C for sufficient recrystallization.
According to an exemplary embodiment, another purpose of
the primary recrystallization annealing process may be
decarbonization as described later. If the primary
recrystallization annealing process is performed at 700°C
or lower, decarbonization may occur, and if the primary
recrystallization annealing process is performed at 950°C
or higher, primarily recrystallized grains may be coarse.
In this case, the driving force for secondary
recrystallization may be weak, and thus Goss grains may not
be fully developed.
The primary recrystallization annealing process may
be performed under a wet atmosphere of hydrogen and
nitrogen for decarbonizing the steel sheet. In this case,
the primary recrystallization annealing process may also be
called a "decarbonization annealing process." Conditions of
the decarbonization annealing process such as a gas mixing
ratio or a dew point are similar to those of a
decarbonization annealing process for general electrical
steel sheets, and thus there is no particular limit to the
conditions.
After the primary recrystallization annealing process,
Page 24
the steel sheet is additionally heated for the following
secondary recrystallization annealing process. In the
secondary recrystallization annealing process, the steel
sheet may preferably be heated at a heating rate of 5°C/hr
to 30°C/hr to a final temperature of 1100°C to 1300°C. If
the heating rate is 5°C/hr or lower, the productivity of
the secondary recrystallization annealing process may be
lowered due to a long annealing time. In addition, the
primarily recrystallized grains may become coarse before a
secondary recrystallization temperature, and thus the
driving force for secondary recrystallization may be weak.
On the contrary, if the heating rate is 30°C/hr or higher,
the inside and outside of a coil of the steel sheet may
have different temperatures, and thus secondary
recrystallization may non-uniformly occur, thereby
deteriorating magnetic characteristics of the steel sheet.
In addition, it may be preferable that the secondary
recrystallization annealing process be performed within the
temperature range of 1100°C to 1300°C for inducing the
recrystallization of most of the grains of the steel sheet.
Even if the maximum temperature of secondary
recrystallization is 1100°C, secondary recrystallization
may occur completely. However, small grains located inside
secondarily recrystallized grains may not be completely
Page 25
removed, and thus the core loss of the steel sheet may be
increased. If secondary recrystallization occurs at 1300°C
or higher, the coil of the steel sheet may undergo
deformation, and thus productivity may be lowered.
In some cases, the steel sheet may be coated with an
annealing separator before the secondary recrystallization
annealing process. Any material such as MgO or Al2O3 widely
used in the art to which the present disclosure pertains
may be used as the annealing separator.
In addition, any process not described in the above
but used to manufacture general electrical steel sheets may
be used for manufacturing the electrical steel sheet of the
present disclosure.
【Mode for Invention】
Hereinafter, the idea of the present disclosure will
be described more specifically through examples. However,
the following examples are for illustrative purposes only
and are not intended to limit the scope of the present
invention. That is, the scope of the present invention is
defined by the claims, and modifications and variations
reasonably made therefrom.
(Examples)
Example 1
Page 26
A molten steel producing process was performed to
obtain molten steel samples, each including carbon (C):
0.05 wt%, manganese (Mn): 0.07 wt%, sulfur (S): 0.007 wt%,
nitrogen (N): 0.006 wt%, and silicon (Si), aluminum (Al),
and at least one rare earth element as shown in Table 1 (in
which element contents are expressed in wt%). When the
molten steel samples were prepared, rare earth elements
were added individually or in the form of mischmetals
according to the compositions of the molten steel samples.
The molten steel samples were cast into slabs having a
thickness of 250 mm, and the slabs were heated to 1150°C.
Then, the slabs were subjected to a hot rolling process to
obtain hot-rolled steel sheets having a thickness of 2.3 mm.
Then, a hot band annealing process was performed by heating
the hot-rolled steel sheets to 1100°C, and the steel sheets
were cooled and pickled. Thereafter, a cold rolling process
was performed once on the pickled steel sheets to obtain
cold-rolled steel sheets having a thickness of 0.27 mm. The
cold-rolled steel sheets were heated to 830°C under a wet
atmosphere of hydrogen and nitrogen for primary
recrystallization and decarbonization up to a residual
carbon level of 30 ppm. Thereafter, the steel sheets were
heated to 1200°C at a heating rate of 15°C/hr for secondary
recrystallization, and then the steel sheets were cooled.
Page 27
In this manner, electrical steel sheets were prepared under
various conditions. In Table 1 below, B8 refers to magnetic
flux density, and W17/50 refers to core loss.
[Table 1]
NO. Si Al ***REEs (individually or as
a mischmetal)
B8(T) W17/50
(W/kg)
La Pr Ce others
*CSS 1 0.5% 4.5% 0.1% - 1.551 5.811
CSS 2 4.2% 0.5% 0.2% 1.580 4.512
**ISS 1 2% 3.0% 0.1% 1.905 0.895
ISS 2 2% 3.0% 0.1% 1.912 0.889
ISS 3 2% 3.0% 0.05% 0.05% 0.04% Nd 0.1% 1.903 0.891
CSS 3 2% 3% 0.6% 1.754 1.983
CSS 4 2% 3% 0.2% 0.2% 0.2% 1.789 2.208
ISS 4 1.8% 2.7% 0.2% 1.904 0.901
CSS 5 2.5% 1.5% 0.3% 0.2% 0.2% Y 0.1% 1.690 4.609
ISS 5 3.1% 1.0% 0.15% 1.913 0.867
ISS 6 3.1% 1.0% 0.15% 1.903 0.874
ISS 7 3.1% 1.0% 0.15% Nd 0.1% 1.919 0.888
CSS 6 3.1% 1.0% 0.4% 0.15% 1.760 2.471
ISS 8 3.1% 1.0% 0.15% Nd 0.2% 1.921 0.865
ISS 9 3.1% 1.0% 0.15% Y 0.1% 1.918 0.861
ISS 10 2.9% 1.5% 0.15% 1.900 0.881
ISS 11 2.9% 1.5% 0.15% 1.908 0.870
ISS 12 2.9% 1.5% 0.15% Nd 0.1% 1.910 0.866
CSS 7 2.9% 1.5% 0.4% 0.17% 1.800 1.498
ISS 13 2.9% 1.5% 0.15% Nd 0.2% 1.911 0.859
ISS 14 2.9% 1.5% 0.15% Y 0.1% 1.915 0.877
CSS 8 1.3% 3.5 1.489 4.352
CSS 9 3.1 1.0 0.01% 0.02% 1.540 1.761
*CSS: Comparative Steel Sample, **ISS: Inventive Steel
Sample, ***REEs: Rare Earth Elements
Page 28
Comparative steel sample 1 had a lower silicon (Si)
content and a higher aluminum (Al) content when compared to
the ranges recommended in the present disclosure. Due to
the excessive amount of aluminum (Al), Comparative steel
sample 1 had poor cold-rolling characteristics, a low
degree of magnetic flux density, and a high degree of core
loss. Comparative steel sample 2 having an excessive amount
of silicon (Si) had properties similar to those of
Comparative steel sample 1.
Comparative steel samples 3, 4, 5, 6, and 7 contained
excessive amounts of rare earth elements, and thus the
magnetic flux density and core loss thereof were
unsatisfactory.
Comparative steel sample 8 contained no rare earth
element but a large amount of aluminum (Al). Aluminum (Al)
added in large amounts was not so useful for the formation
of an inhibitor. Moreover, since a nitriding annealing
process was not performed, there was very little
possibility of formation of an inhibitor in Comparative
steel sample 8, and thus the magnetic flux density and core
loss of Comparative steel sample 8 were very unsatisfactory.
The total content of rare earth elements in Comparative
steel sample 9 was outside the range of the present
Page 29
disclosure, and thus the magnetic flux density and core
loss of Comparative steel sample 9 were unsatisfactory even
though they were superior to those of Comparative steel
sample 8.
However, all inventive steel samples having
compositions in accordance with the present disclosure had
a magnetic flux density of 1.9 T or greater and a core loss
of 0.901 W/kg or less.
Example 2
In this example, the mechanism of how added rare
earth elements function as inhibitors was checked by
preparing electrical steel slabs having modified
compositions. That is, the electrical steel slabs each
included carbon (C): 0.05 wt%, manganese (Mn): 0.07 wt%,
sulfur (S): 0.007 wt%, nitrogen (N): 0.006 wt%, silicon
(Si): 3.1 wt%, aluminum (Al): 1.5 wt%, and praseodymium
(Pr) (rare earth element): 0.08 wt% (refer to FIG. 1A) or
rare earth elements: 0.24 wt% in total content
(corresponding to Inventive steel sample 3 to which a
mischmetal was added). AS in Example 1, the electrical
steel slabs were subjected to a hot rolling process, a cold
rolling process, and a primary recrystallization process to
obtain primarily recrystallized steel sheets. Thereafter,
Page 30
inhibitors formed in the primarily recrystallized steel
sheets were photographed with a transmission electron
microscope by a replica method, and the captured images are
shown in FIGS. 1A and 1B.
As shown in FIGS. 1A and 1B, when praseodymium (Pr)
was added (refer to FIG. 1A), praseodymium (Pr) or a
compound of praseodymium (Pr) was detected as an inhibitor,
and when a mischmetal was added (refer to FIG. 1B), cerium
(Ce), lanthanum (La), neodymium (Nd), and praseodymium (Pr)
included in the mischmetal were detected as inhibitors.
That is, it could be checked that rare earth elements serve
as satisfactory inhibitors as described in the present
disclosure.
Example 3
Electrical steel sheets were prepared by the same
method as that in Example 1 by using slabs each including
carbon (C): 0.05 wt%, manganese (Mn): 0.07 wt%, sulfur (S):
0.007 wt%, nitrogen (N): 0.006 wt%, and silicon (Si): 3.1
wt% and aluminum (Al): 1.0 wt% (refer to FIG. 2A), or
silicon (Si): 3.1 wt% and aluminum (Al): 2.0 wt% (refer to
FIG. 2B). Subsequently, a relationship between core loss
and total content of rare earth elements of each electrical
steel sheet was plotted as shown in FIGS. 2A and 2B. As
Page 31
shown in FIGS. 2A and 2B, if the total content of rare
earth elements is within the range of the present
disclosure, core loss is relatively very low.
Therefore, advantageous effects of the present
disclosure could be confirmed.
Page 32

WE CLAIM:
【Claim 1】
An electrical steel sheet having a low degree of core
loss, the electrical steel sheet comprising, by wt%,
silicon (Si): 1.0% to 4.0%, aluminum (Al): 0.1% to 4.0%,
and at least one rare earth element: 0.05% to 0.5% in total
content.
【Claim 2】
The electrical steel sheet of claim 1, further
comprising carbon (C): 0.003 wt% or less, manganese (Mn):
0.03 wt% to 0.2 wt%, sulfur (S): 0.001 wt% to 0.05 wt%, and
nitrogen (N): 0.01 wt% or less.
【Claim 3】
The electrical steel sheet of claim 1 or claim 2,
further comprising at least one selected from the group
consisting of phosphorus (P): 0.5% or less, tin (Sn): 0.3%
or less, antimony (Sb): 0.3% or less, chromium (Cr): 0.3%
or less copper (Cu): 0.4% or less, and nickel (Ni): 1% or
less.
【Claim 4】
The electrical steel sheet of claim 1 or claim 2,
wherein the rare earth element or a compound of the rare
earth element is used as an inhibitor.
【Claim 5】
Page 33
A method for manufacturing an electrical steel sheet
having a low degree of core loss, the method comprising:
heating a slab to 1050°C to 1300°C, the slab
comprising, by wt%, silicon (Si): 1.0% to 4.0%, aluminum
(Al): 0.1% to 4.0%, and at least one rare earth element:
0.05% to 0.5% in total content;
hot rolling the slab;
cold rolling the slab;
primarily recrystallizing the slab; and
secondarily recrystallizing the slab.
【Claim 6】
The method of claim 5, wherein the slab further
comprises carbon (C): 0.1 wt% or less, manganese (Mn): 0.03
wt% to 0.2 wt%, sulfur (S): 0.001 wt% to 0.05 wt%, and
nitrogen (N): 0.01 wt% or less.
【Claim 7】
The method of claim 5, wherein after the hot rolling
of the slab, the method further comprises at least one
selected from:
annealing the hot-rolled slab; and
pickling the hot-rolled slab.
【Claim 8】
The method of claim 5, wherein the cold rolling is
performed at a reduction ratio of 85% to 90%.
Page 34
【Claim 9】
The method of claim 8, wherein the cold rolling is
performed two or more times with an intermediate annealing
process therebetween, and a reduction ratio of the final
cold rolling is 50% or greater.
【Claim 10】
The method of claim 5, wherein the primary
recrystallizing is performed within a temperature range of
700°C to 950°C.
【Claim 11】
The method of claim 5, wherein the secondary
recrystallizing is performed by heating the slab to a
maximum temperature of 1100°C to 1300°C at a heating rate
of 5°C/hr to 30°C/hr.