DESCRIPTION
Method for Manufacturing Grain-Oriented Electrical Steel Sheet
Having Excellent Magnetic Properties
Technical 5 cal Field
The present invention relates to a method for
manufacturing a grain-oriented electrical steel sheet having
excellent magnetic properties, and more particularly to a
method for manufacturing a grain-oriented electrical steel
10 sheet having excellent magnetic properties as a result of
applying a three-stage heating pattern consisting of ultrarapid
heating, rapid heating and general heating to a heating
process in primary recrystallization annealing.
15 Background Art
A grain-oriented electrical steel sheet is a soft
magnetic material having excellent magnetic properties in the
rolling direction as a result of the so-called Goss texture,
in which all the grains of the steel are oriented in the {110}
20 direction and the crystallographic orientation in the rolling
direction is parallel to the <001> axis.
This grain-oriented electrical steel sheet is manufactured
so as to show excellent magnetic properties by secondary
recrystallized grains obtained by inhibiting the growth of
25 primary recrystallized grains during final annealing following
2
primary recrystallization and selectively growing grains having
the {110}<001> orientation among the inhibited grains. Thus,
an inhibitor of growth of the primary recrystallized grains
(hereinafter referred to as the “inhibitor”) is very important.
The key of the technology for manufacturing grain-oriente5 d
electrical steel sheets is that grains having the {110}<001>
orientation among the inhibited grains can preferentially grow
to form secondary recrystallized grains.
Secondary recrystallization in the final annealing process
10 occurs when the inhibitor grows or is degraded with increasing
temperature to loss its function to inhibit primary
recrystallized grains, and in this case, grain growth occurs
within a relatively short time. The growth of all primary
recrystallized grains should be inhibited up to immediately
15 before secondary recrystallization in the final annealing
process occurs, and for this purpose, precipitates should be
uniformly distributed in a sufficient amount and a suitable
size, should be thermally stable, and should not be easily
decomposed up to a high temperature immediately before second
20 recrystallization occurs.
This {110}<001> texture can be obtained by a combination
of various processes. To obtain this texture, the slab
composition should be strictly controlled, and the conditions
of a series of processes, including slab heating, hot rolling,
25 hot-rolled sheet annealing, cold rolling, primary
3
recrystallization annealing, and final annealing (secondary
recrystallization annealing), should be strictly controlled.
As used herein, the term “primary recrystallization”
refers to general recrystallization in which new grains are
nucleated and grow at a specific temperature or higher. Th5 e
first recrystallization is generally performed either at the
same time as decarburization annealing after cold rolling or
immediately after decarburization annealing, and grains having
a uniform and suitable size are formed by the first
10 recrystallization. Generally, the orientation of grains in
grain-oriented electrical steel sheets are dispersed in several
directions, or orientations other than the Goss orientation
have textures arranged parallel to the surface orientation, and
the ratio of the Goss orientation to be finally obtained in the
15 grain-oriented electrical steel sheets is very low.
As technologies of improving magnetic properties by
controlling heating conditions in primary recrystallization
annealing, those that use rapid heating in a decarburization
annealing process are disclosed in Japanese Patent Laid-Open
20 Publication Nos. 2003-3213, 2008-1978, 2008-1979, 2008-1980,
2008-1981, 2008-1982, and 2008-1983.
Japanese Patent Laid-Open Publication No. 2003-3213
discloses a technology of manufacturing a grain-oriented
electrical steel sheet having high magnetic flux density by
25 controlling the amount of nitrification and controlling the
4
ratio of I[111]/I[411] in textures after annealing to 2.5 or
less. In addition, it discloses that the amounts of aluminum
and nitrogen and the heating rate in the decarburization
annealing process should be controlled in order to control the
5 textures.
Japanese Patent Laid-Open Publication Nos. 2008-1978,
2008-1979, 2008-1980, 2008-1981, 2008-1982, and 2008-1983
disclose methods of magnetic flux density by performing
decarburization during hot-rolled sheet annealing or
10 controlling the hot-rolled sheet annealing temperature to
control the lamellar distance while performing rapid heating in
the temperature range of 550~720 at 40 /sec or higher, and
preferably 75-125 /sec, during decarburization annealing.
These patent documents disclose that {411}-oriented grains
15 among primary recrystallized grains influence the preferential
growth of {110}-oriented secondary recrystallized grains, and
that grain-oriented electrical steel sheets are manufactured by
controlling the ratio of {111}/{411} in primary recrystallized
textures after decarburization annealing to 3.0 or less,
20 performing nitrification and enhancing the inhibitor.
However, in these patent documents, the temperature range
in which a great change in the textures is shown is 700~720 ,
and only a method of improving magnetic flux density by
performing rapid heating to the temperature range of 550~720
25 including the above temperature range (700~720 ) is suggested.
5
In addition, these patent documents have technical
limitations in that they do not attempt to directly increase
the ratio of grains having the Goss orientation, but attempt to
increase the ratio of {411}-oriented grains that have an
indirect influence on abnormal grain growth (second5 ary
recrystallization) in the Goss orientation in secondary
recrystallization annealing after decarburization annealing.
Even when the above prior patent documents are considered
together, these patent documents do not suggest a method for
10 manufacturing a grain-oriented electrical steel sheet, in which
the magnetic properties of the steel sheet can be improved by
controlling the density of Goss orientation in a decarburized
sheet through a three-stage heating pattern of ultra-rapid
heating + rapid heating + general heating (which means that the
15 heating rate differs between temperature zones) during first
recrystallization annealing.
(Prior art documents)
(Patent documents)
(Patent document 1) JP2003-3213 A (2003.1.08.)
20 (Patent document 2) JP2008-1978 A (2008.1.10.)
(Patent document 3) JP2008-1979 A (2008.1.10.)
(Patent document 4) JP2008-1980 A (2008.1.10.)
(Patent document 5) JP2008-1981 A (2008.1.10.)
(Patent document 6) JP2008-1982 A (2008.1.10.)
25 (Patent document 7) JP2008-1983 A (2008.1.10.)
6
Disclosure
Technical Problem
Accordingly, the present invention has been made in order
to solve the above-described problems occurring in the prior
art, and an object of the present invention is to provide 5 e a
novel method for manufacturing a grain-oriented electrical
steel sheet in which the magnetic properties of the steel
sheet can be improved by increasing the volume fraction of
grains having the Goss orientation (particularly the exact
10 Goss orientation) in primary recrystallization annealing using
a three-stage heating pattern consisting of ultra-rapid
heating, rapid heating and general heating, and increasing the
density of crystallographic orientations.
15 Technical Solution
In order to accomplish the above object, the present
invention provides a method for manufacturing a grain-oriented
electrical steel sheet, the method comprising: providing a
grain-oriented electrical steel sheet slab comprising, by wt%,
20 Si: 2.0-4.0%, C: 0.085% or less, acid-soluble Al: 0.015-0.04%,
Mn: 0.20% or less, N: 0.010% or less, S: 0.010% or less, and
the balance of Fe and inevitable impurities; heating the slab
to a temperature of 1280 or below; hot-rolling the heated
slab; optionally annealing the hot-rolled steel sheet;
25 subjecting the resulting steel sheet to one cold rolling or two
7
or more cold rollings with intermediate annealing therebetween;
subjecting the cold-rolled steel sheet to primary
recrystallization annealing, and subjecting the annealed steel
sheet to secondary recrystallization annealing, wherein the
primary recrystallization annealing sequentially comprises a5 n
ultra-rapid heating process for heating the steel sheet at an
average heating rate of 300 /sec or higher, a rapid heating
process for heating the steel sheet at a lower average heating
rate than the average heating rate of the ultra-rapid heating
10 process, but not lower than 100 /sec, and a general heating
process for heating the steel sheet at a lower average heating
rate than the average heating rate of the rapid heating
process.
The ultra-rapid heating process is performed by heating
15 the steel sheet at an average heating rate of 300 /sec or
higher from room temperature to Ts (), which is a temperature
of 500~600 before recrystallization, the rapid heating
process is performed by heating the steel sheet at an average
heating rate of 100~250 /sec from Ts () to 700 , and the
20 general heating process is performed by heating the steel sheet
at an average heating rate of 40 /sec or lower from 700 to
the decarburization annealing temperature.
In the method of the present invention, the number of
grains having a size of 35 or larger, measured when observing
25 the cross-section of the steel sheet after the primary
8
recrystallization annealing but before the secondary
recrystallization annealing, is less than 30.
In addition, in the method of the present invention, the
volume fraction of grains having an orientation of up to 15°
from the {110}<001> orientation is 2% or more when measured i5 n
a layer corresponding to 1/8 of the thickness from the surface
of the steel sheet after primary recrystallization annealing
but before secondary recrystallization annealing, and the
volume fraction of grains having an orientation of up to 5°
10 from the {110}<001> orientation is 0.09% or more when measured
under the above conditions.
In addition, in the inventive method for manufacturing the
grain-oriented electrical steel sheet, a β angle as the areaweighted
average of the absolute value of crystallographic
15 orientation, measured for the steel sheet after secondary
recrystallization annealing, is controlled in the range of 1.5-
2.6°, and a δ angle is controlled to 5° or less. Herein, the β
angle is an average angle of deviation from the {110}<001>
orientation in the direction perpendicular to the rolling
20 direction of the secondary recrystallized texture, and the δ
angle is an average angle of deviation between the <001>
orientation and the rolling direction in the secondary
recrystallized texture.
In the inventive method for manufacturing the grain25
oriented electrical steel sheet, the heating process in primary
9
recrystallization annealing may be controlled to a three-stage
pattern using a plurality of inducing heating furnaces.
Advantageous Effects
According to the present invention, a grain-oriente5 d
electrical steel sheet having high magnetic flux density and
low core loss can be manufactured by using a three-stage
heating pattern (ultra-rapid heating + rapid heating + general
heating) in primary recrystallization annealing to increase
10 the volume fraction of Goss orientation (particularly exact
Goss orientation) in the primary recrystallized steel sheet to
thereby increase the density of crystallographic orientations.
Mode for Invention
15 Hereinafter, the inventive method for manufacturing a
grain-oriented electrical steel sheet will be described in
detail.
The present inventors conducted studies on nucleation in
primary recrystallization of a grain-oriented electrical steel
20 sheet, particularly the behavior in primary recrystallization
of grains having Goss orientation ({110}<001>) that can grow
into nuclei in secondary recrystallization. As a result, the
present inventors could infer that the nucleation of Gossoriented
grains occurs in the shear band on which strain energy
25 is concentrated during primary recrystallization after
10
receiving a strong strain, the accumulated strain energy of the
shear band partially decreases by recovery in a heating region
during primary recrystallization annealing, and thus the
nucleation sites of Goss-oriented grains decrease.
Based on this inference, the present inventors conducte5 d
studies and experiments on the heating conditions in primary
recrystallization annealing, which can minimize the decrease in
the accumulated strain energy of the shear band, which is
caused by recovery, to increase the nucleation of Goss-oriented
10 grains. As a result, the present inventors could first find
that the volume fraction of Goss orientation, particularly
exact Goss orientation, can be significantly increased by
performing first recrystallization annealing using a threestage
heating pattern consisting of two-stage rapid heating
15 (ultra-raid heating + rapid heating) and general heating, in
which the two-stage heating includes an ultra-rapid heating
process of performing heating in a specific temperature region
at a much higher rate than conventional rate.
The present invention provides a method for manufacturing
20 a grain-oriented electrical steel sheet, the method comprising:
heating a grain-oriented electrical steel sheet slab
comprising, by wt%, Si: 2.0-4.0%, C: 0.085% or less, acidsoluble
Al: 0.015-0.04%, Mn: 0.20% or less, N: 0.010% or less,
S: 0.010% or less, and the balance of Fe and inevitable
25 impurities; heating the slab; hot-rolling the heated slab;
11
optionally annealing the hot-rolled steel sheet; subjecting the
resulting steel sheet to one cold rolling or two or more cold
rollings with intermediate annealing therebetween; subjecting
the cold-rolled steel sheet to primary recrystallization
annealing, and subjecting the annealed steel sheet to secondar5 y
recrystallization annealing, wherein the primary
recrystallization annealing employs a three-stage heating
pattern consisting of ultra-rapid heating, rapid heating and
general heating, in which the ultra-rapid heating process is
10 performed by heating the steel sheet at an average heating rate
of 300 /sec or higher in a region from room temperature to Ts
(), which is a temperature of 500~600 before
recrystallization, the rapid heating process is performed by
heating the steel sheet at an average heating rate of
15 100~250 /sec in a region ranging from Ts () to 700 , and the
general heating process is performed by heating the steel sheet
at an average heating rate of 40 /sec or lower in a range
ranging from 700 to the decarburization annealing
temperature.
20 According to the present invention, a novel three-stage
heating pattern (ultra-rapid heating + rapid heating + general
heating) is applied in primary recrystallization annealing.
Specifically, the steel sheet is heated from room temperature
to the pre-recrystallization temperature (500~600 ) at a rate
25 of 300 /sec or higher, and then heated at a rate of
12
100~250 /sec, whereby the decrease (i.e., recovery) in the
strain energy of Goss oriented grains in the shear band can be
minimized to maximize the nucleation of Goss–oriented grains to
thereby form good recrystallized grains.
The above-described Ts () is a temperature at which 5 the
ultra-rapid heating process is converted to the rapid heating
process. Because recrystallization is initiated at a
temperature of about 550~600, Ts is preferably 500~600, and
more preferably 550~600, and is preferably the
10 recrystallization initiation temperature or lower.
As used herein, the term “room temperature” refers to the
temperature of the steel sheet at a time point when the heating
process in primary recrystallization annealing is initiated.
In addition, the present invention was completed based on
15 a new finding that the fraction of exact Goss-oriented grains
as seeds capable of causing secondary recrystallization can be
increased by the ultra-rapid heating process to below the
recrystallization temperature in primary recrystallization
annealing, so that the nucleation of very highly dense Goss20
oriented grains can be induced, thereby maximizing the effect
of improving magnetic properties.
When a conventional heating pattern consisting of rapid
heating followed by general heating is applied in primary
recrystallization annealing, the volume fraction of
25 orientation, which is in 15° from the {110}<001> orientation,
13
is only about 1%. Unlike this, according to the present
invention, when primary recrystallization annealing is
performed by rapidly heating the steel sheet from room
temperature to about 550 or lower at a rate of 300 /sec or
higher (preferably 400 /sec or higher), rapidly heating 5 the
steel sheet from 570 or lower to 700 at a rate of
100~250 /sec (more preferably 120~180 /sec or higher), and
generally heating the steel sheet from 700 or higher to the
decarburization annealing temperature at a rate of 40 /sec or
10 lower, the volume fraction of grains having an orientation of
15° or less from the {110}<001> orientation can be controlled
to 2% or more, and particularly, the volume fraction of exact
Goss grains having an orientation of 5° or less from the
{110}<001> orientation can be controlled to 0.09% or more.
15 The present inventors measured the volume fraction of
grains, which are in the range of 5°, 10° and 15° from the
{110}<001> orientation, in a layer corresponding to 1/8 of the
thickness from the surface of a sample (at least 95%
recrystallized) immediately after rapid heating in primary
20 recrystallization annealing. As a result, it was observed that
the total Goss orientation was increased during rapid
annealing, and the fraction of exact Goss orientation, which is
in 5° from the {110}<001> orientation in recrystallized grains
formed by ultra-rapid heating + rapid heating + general
25 heating, was maximized.
14
As described above, when the rate of increase in the exact
Goss orientation closer to the {110}<001> orientation in
primary recrystallized structures is higher than the rate of
increase in orientations far from the {110}<001> orientation,
the exact Goss orientation acts as nuclei in sec5 ondary
recrystallization to directly increase the density of Gossoriented
grains that grow into secondary recrystallized grains,
thereby significantly improving the magnetic flux density and
core loss properties of the steel sheet.
10 However, if the rate of the rapid heating after the ultrarapid
heating is excessively high, the magnetic properties are
deteriorated rather than improved. This is believed to be
attributable to the following reasons. When two-stage rapid
heating (ultra-rapid heating + rapid heating) is applied in
15 primary recrystallization, the size distribution of grains is
uniform up to a specific heating rate, but if the rate of
heating from Ts() to 700 is higher than 250 /sec, the
uniformity of grains will increase so that the fraction of
grains having a size larger than 35 will excessively increase
20 and grains having undesired orientation will grow due to the
grain growth caused by the size advantage, and thus the
magnetic properties will be deteriorated rather than improved.
In addition, Goss-oriented grains have the highest strain
energy and thus are first recrystallized, and then grains
25 having the {111}<112> orientation and the {411}<148>
15
orientation are recrystallized. After Goss-oriented grains
have been first recrystallized, the fraction of orientations
such as {111}<112> and {411}<148> gradually increases during
grain growth, and the growth of orientations such as {111}<112>
and {411}<148> can reduce the growth of Goss-oriented 5 ed grains
during primary recrystallization. For this reason, the rate of
heating from 700 or higher needs to be increased, and the
rate of heating from 680 or higher is preferably 40 /sec or
lower.
10 Thus, in order to effectively improve the magnetic
properties by increasing the fraction of Goss-oriented grains,
heating in primary recrystallization annealing is performed by
ultra-rapidly heating the steel sheet from room temperature to
Ts at an average heating rate of 300 /sec or higher, then
15 rapidly heating the steel sheet to 700 at an average heating
rate of 100~250 /sec, and then heating the steel sheet from
700 or higher at an average heating rate of 40 /sec or
lower.
Furthermore, the present inventors measured the area20
weighted average of angles deviating from the {110}<001>
orientation of secondary recrystallized grains in a sample
using the three-stage heating pattern in primary
recrystallization annealing. The main characteristics of the
device used in the measurement are as follows. The measurement
25 was performed using an X-ray CCD detector based on the X-ray
16
Laue method. The positions of X-ray diffraction in the CCD
detector and the specimen and the slanted angle of the detector
were controlled in a unit of 1 , and the analysis of
orientation strain of single crystal was used, thereby
increasing the accuracy of measurement. The orientation 5 at
each position of the specimen was measured while moving the
sample, and the absolute angle of deviation of the measured
orientation from ideal Goss orientation was calculated. Then,
the area-weighted average of the angles at all the positions
10 was calculated to determine the area-weighted average of the
absolute values of the deviation angles.
The deviation angles were measured for four angles, an α
angle, a β angle, a γ angle, and a δ angle. The α angle is
defined as the average deviation angle from the {110}<001>
15 orientation in the normal direction (ND) of a secondary
recrystallized texture; the β angle is defined as the average
deviation angle from the {110}<001> orientation in the
transverse direction (TD) of a secondary recrystallized
texture; the γ angle is defined as the average deviation angle
20 from the {110}<001> orientation in the rolling direction (RD)
of a secondary recrystallized texture; and the δ angle is
defined as the average deviation angle between the <001>
orientation and rolling direction (RD) of a secondary
recrystallized texture.
25 The results of the measurement showed that, when the two17
stage rapid heating consisting of ultra-rapid heating and rapid
heating as described in the present invention was applied in
primary recrystallization annealing, all the deviation angles
were reduced. Particularly, the β angle was close to 2°, and
the δ angle was also rapidly reduced. When the β angle i5 s
close to 2°, the magnetic domain width is reduced to minimize
electromagnetic energy, and the disclosure magnetic domain is
reduced to improve magnetic properties.
According to the inventive method for manufacturing the
10 grain-oriented electrical steel sheet as described above, the
area weighted average of the absolute value of the β angle,
measured for a steel sheet after secondary recrystallization
annealing, can be controlled in the range of 1.5-2.6°, and
preferably 1.5-2.4°, and the area weighted average of the
15 absolute value of the δ angle can be controlled to 5° or less,
and preferably 4.5° or less.
Hereinafter, the reasons for the limitation of the
components of the grain-oriented electrical steel sheet that is
used in the present invention will be described.
20 Si serves to increase the resistivity of the grainoriented
electrical street sheet to reduce the core loss. If
the content of Si is less than 2.0 wt%, the resistivity will
decrease to increase the core loss, and if the content of Si is
more than 4.0 wt%, the brittleness of the steel will increase
25 to make cold rolling difficult, and the formation of secondary
18
recrystallized grains becomes unstable. For these reasons, the
content of Si is limited to 2.0-4.0 wt%.
Al is finally converted into nitrides such as AlN and
(Al,Si,Mn)N, which act as inhibitors. If the content of Al is
less than 0.015 wt%, it cannot show a sufficient inhibi5 tor
effect, and if the content of Al is excessively high, it will
adversely affect hot-rolling operation. For these reasons, the
content of Al is limited to 0.015-0.04 wt%.
Mn has the effect of increasing the resistivity to reduce
10 the core loss, like Si. Also, Mn reacts with nitrogen, which
is introduced for nitrification together with Si, to form a
precipitate of (Al,Si,Mn)N, and thus plays an important role in
inducing secondary recrystallization by inhibiting the growth
of primary recrystallized grains. However, if it is added in
15 an amount of more than 0.20 wt%, it will promote austenite
phase transformation during hot rolling to reduce the size of
primary recrystallized grains, and thus secondary
recrystallized grains become unstable. For these reasons, the
content of Mn is limited to 0.20 wt% or less.
20 When C is added in a suitable amount, it promotes the
austenite transformation of the steel to refine the hot-rolled
structure during hot rolling, thus facilitating the formation
of uniform microstructures. However, if the content thereof is
excessively high, coarse carbides will precipitate, and removal
25 of carbon during decarburization will be difficult. For these
19
reasons, the content of C is 0.085 wt% or less.
N is an element that reacts with Al and the like to refine
grains. When this element is suitably distributed, it can
suitably refine structures after cold rolling as described
above to make it easy to ensure primary recrystallized grain5 s
having a suitable grain size. However, if the content thereof
is excessively high, primary recrystallized grains will be
excessively refined, and thus a driving force for grain growth
in secondary recrystallization will increase due to fine grains
10 so that grains having undesirable orientations can also grow.
Also, if the content of N is more than 0.010 wt%, the
temperature of initiation of secondary recrystallization will
increase to deteriorate the magnetic properties of the steel
sheet. For these reasons, the content of N is limited to 0.010
15 wt% or less. When a treatment for increasing the amount of
nitrogen is performed between cold rolling and secondary
recrystallization annealing, the content of N in the slab may
also be 0.006% or less.
S is an element that has a high solid-solution temperature
20 and severely segregates, and the content thereof is preferably
reduced to the lowest possible level, but it is a kind of
inevitable impurity that is incorporated during steel making.
In addition, S forms MnS that affects the size of primary
recrystallized grains. For this reason, the content of S is
25 limited to 0.010 wt% or less, and preferably 0.006 wt%.
20
Any person skilled in the art will appreciate that, in
addition to the above components, various components that are
contained in grain-oriented electrical steel sheets may be used
as alloying elements in the electrical steel sheet of the
present invention. A combination of conventionally kno5 wn
components and the application thereof fall within the scope of
the present invention.
Hereinafter, a method for manufacturing a grain-oriented
electrical steel sheet having excellent magnetic properties
10 using a grain-oriented electrical steel sheet slab having the
above-described composition will be described in detail.
A grain-oriented electrical steel sheet having the abovedescribed
composition is reheated before hot rolling. Herein,
the slab is preferably heated to 1280 or lower, and more
15 preferably 1200 or lower, in order to partially dissolve
precipitates. This is because if the slab heating temperature
increases, the production cost of the steel sheet increases and
the surface portion of the slab can be melted to reduce the
service life of a heating furnace. Particularly, when the slab
20 is heated to 1200 or lower, the columnar structure of the
slab can be prevented from growing coarsely, and cracking can
be prevented from occurring in the width direction of the sheet
in a subsequent hot-rolling process, thereby increasing yield.
After the grain-oriented electrical steel sheet has been
25 reheated, it is hot-rolled. In the hot-rolling process, a hot21
rolled steel sheet having a thickness of 2.0-3.5 mm can be
produced. The produced hot-rolled steel sheet may, if
necessary, be annealed, and then is cold-rolled. If the hotrolled
steel sheet is annealed, it may be heated to 1000~1250 ,
and then homogenized at a temperature of 850~1000 , 5 followed
by cooling. The annealing of the hot-rolled steel sheet is
optionally performed and may also be omitted.
The cold rolling may be performed by subjecting the steel
sheet either to one cold rolling or to two cold rollings with
10 intermediate annealing therebetween. The cold rolled steel
sheet may have a final thickness of 0.1-0.5 mm, and preferably
0.18-0.35 mm.
The cold-rolled steel sheet is then subjected to primary
recrystallization annealing. As described above, according to
15 the present invention, ultra-rapid heating is introduced in the
heating process during primary recrystallization annealing.
Specifically, a three-stage heating pattern consisting of
ultra-rapid heating, rapid heating and general heating is
applied in the heating process during primary recrystallization
20 annealing.
In the ultra-rapid heating process of the three-stage
heating pattern, the steel sheet is heated from room
temperature to a temperature between 500 and 600 , preferably
a temperature (Ts) 550 and 600 , at an average heating rate
25 of 300 /sec or higher. In the rapid heating process, the
22
steel sheet is heated from the temperature (Ts) to 700 at an
average heating rate of 100~250 /sec. Then, the steel sheet
is heated from 700 or higher at an average heating rate of
40 /sec or lower. In this manner, the magnetic properties of
the grain-oriented electrical steel sheet can be improved, 5 d, and
the reasons therefor are as described above.
The method for heating in primary recrystallization
annealing is not specifically limited, may be performed using
an induction heating furnace or may be performed in a three10
stage heating pattern using a plurality of induction heating
furnaces. For example, it may be performed by ultra-rapidly
heating the steel sheet in a first induction heating furnace as
a rate of 300 /sec or higher, and preferably 400 /sec or
higher, rapidly heating the steel sheet in a second induction
15 heating furnace at a rate of 100~250 /sec, and more preferably
120~180 /sec, and generally heating the steel sheet in a third
induction heating furnace at a rate of 40 /sec or lower.
In the primary recrystallization annealing, the heated
steel sheet is subjected to decarburization and nitrification
20 annealing. The nitrification annealing may be performed after
or simultaneously with decarburization.
If nitrification annealing is performed simultaneously
with decarburization, it may be performed in a mixed gas
atmosphere of ammonia, hydrogen and nitrogen. If
25 decarburization is first performed after the heating process in
23
the primary recrystallization annealing, and then nitrification
annealing is performed, precipitates such as Si3N4 or (Si,Mn)N
are formed on the surface layer of the steel sheet, and such
precipitates are thermally unstable and thus easily decomposed,
and the diffusion of nitrogen also occurs very fast. For 5 these
reasons, in this case, the temperature of the nitrification
annealing should be controlled at 700~800 , and precipitates
such as thermally stable AlN or (Al,Si,Mn)N should be formed in
the final annealing process so that they can act as inhibitors.
10 Unlike this, when decarburization and nitrification annealing
are simultaneously performed, there is an advantage in that
precipitates such as AlN or (Al,Si,Mn)N are simultaneously
formed, and thus these precipitates can be used as inhibitors
in the final annealing process without having to transform
15 these precipitates so that a long treatment time is not
required. Accordingly, it is more preferable to perform
decarburization and nitrification annealing at the same tome.
However, the inventive method for manufacturing the grainoriented
electrical steel sheet is not limited to simultaneous
20 decarburization and nitrification annealing during the first
recrystallization annealing, and performing nitrification
annealing after decarburization is also effective in
manufacturing the inventive grain-oriented electrical steel
sheet having advantageous properties.
25 After an annealing separator has been applied to the
24
primary recrystallized steel sheet, the steel sheet is
subjected to final annealing to cause secondary
recrystallization, so that a {110}<001> texture is formed in
which the {110} plane is parallel to the rolling plane and the
<001> direction is parallel to the rolling direction. 5 The
annealing separator that is used herein is preferably based on
MgO, but is not limited thereto.
The purposes of the final annealing are generally to form
a {110}<001> texture by secondary recrystallization and to
10 impart insulating properties by forming a glassy layer by the
reaction of MgO with an oxide layer formed during
decarburization, and also to remove impurities that adversely
affect magnetic properties. In the final annealing process, in
the heating zone before secondary recrystallization occurs, the
15 steel sheet is maintained in a mixed gas atmosphere of nitrogen
and hydrogen so that secondary recrystallized grains are well
developed by protecting the grain growth inhibitor nitride, and
after the completion of secondary recrystallization, the steel
sheet is maintained in a 100% hydrogen atmosphere so that
20 impurities are removed.
Hereinafter, the present invention will be described in
further detail with reference to examples.
Example 1
A grain-oriented electrical steel sheet slab comprising,
25 by wt%, Si: 3.18%, C: 0.056%, Mn: 0.09%, S: 0.0054%, N: 0.0051%,
25
soluble Al: 0.028%, and the balance of Fe and inevitable
impurities, was heated at a temperature of 1150 for 210
minutes, and then hot-rolled to manufacture a hot-rolled steel
sheet having a thickness of 2.3 mm. The hot-rolled steel sheet
was heated to a temperature of 1100 or higher, maintained a5 t
910 for 90 seconds, quenched in water, pickled, and then
cold-rolled to a thickness of 0.30 mm.
The cold-rolled steel sheet was heated in the furnace, and
then subjected to simultaneous decarburization and
10 nitrification by maintaining the steel sheet at a temperature
of 845 for 160 seconds in a mixed gas atmosphere formed by
simultaneously adding 74.5% hydrogen, 24.5% nitrogen and 1% dry
ammonia gas and having a dew-point temperature of 65 . The
nitrogen content of the nitrified steel sheet was controlled
15 between 170 ppm and 210 ppm. In the heating process, the steel
sheet was heated from room temperature to 570 at various
heating rates of 30 /sec, 110 /sec, 420 /sec and 560 /sec,
and then from 570 to 700 at various heating rates of
30 /sec, 70 /sec, 110 /sec, 140 /sec, 190 /sec, 270 /sec
20 and 350 /sec, and then from 700 to 845 (decarburization
annealing temperature) at a rate of 30 /sec.
The annealing separator MgO was applied to the steel sheet
which was then subjected to final annealing in a coiled state.
In the final annealing, the steel sheet was maintained in a
25 mixed atmosphere of 25% nitrogen + 75% hydrogen until it
26
reached 1200 , and after the steel sheet reached 1200 , it
was maintained in a 100% hydrogen atmosphere for 10 hours or
more, and then cooled in the furnace. Magnetic properties
measured for each condition are shown in Table 1 below.
Table 5 1
Rate of heating
from room
temperature to
570 (/sec)
Rate of heating
from 570 to
700 (/sec)
Rate of heating
from 700 to
845 (/sec)
Magnetic flux
density (B10,
Tesla)
Core loss (W17/50,
W/kg)
Remarks
30 30 30 1.88 1.04 Comparative
material 1
30 140 30 1.92 0.96 Comparative
material 2
30 270 30 1.91 0.97 Comparative
material 3
30 350 30 1.90 1.00 Comparative
material 4
420 30 30 1.91 1.01 Comparative
material 5
420 70 30 1.91 0.98 Comparative
material 6
420 110 30 1.95 0.92 Inventive
material 1
420 140 30 1.96 0.90 Inventive
material 2
420 190 30 1.96 0.91 Inventive
material 3
420 270 30 1.92 0.97 Comparative
material 7
420 350 30 1.91 0.99 Comparative
material 8
560 30 30 1.91 1.00 Comparative
material 9
560 70 30 1.92 0.97 Comparative
material 10
560 110 30 1.94 0.92 Inventive
material 4
560 140 30 1.97 0.89 Inventive
material 5
560 190 30 1.96 0.91 Inventive
material 6
560 270 30 1.92 0.98 Comparative
material 11
560 350 30 1.91 1.00 Comparative
material 12
27
110 110 30 1.92 0.98 Comparative
material 13
As can be seen in Table 1 above, comparative materials 1
to 4, which were generally heated from room temperature to
570 at a rate of 30 /sec, had low magnetic flux density and
high core loss compared to the steel sheets which were ultra5 -
rapidly heated.
In addition, comparative material 13 which was subjected
to first stage rapid heating (two-stage heating pattern) by
heating the steel sheet from room temperature to 700 at a
10 rate of 110 /sec in primary recrystallization, showed a lower
magnetic flux density of 1.92 Tesla and a higher core loss of
0.98 W/kg than those of inventive materials 1 to 6.
On the contrary, it was shown that inventive materials 1
to 6 subjected to a three-stage heating pattern comprising two15
stage heating (ultra-rapid heating + rapid heating) conditions
during primary recrystallization showed a high magnetic flux
density of 1.94-1.97 Tesla and a low core loss of 0.89-0.91
W/kg.
Example 2
20 A grain-oriented electrical steel sheet slab comprising,
by wt%, Si: 3.25%, C: 0.048%, Mn: 0.07%, S: 0.005%, N: 0.0045%,
soluble Al: 0.027%, and the balance of Fe and inevitable
impurities, was heated at 1150 for 210 minutes, and then hotrolled
to produce hot-rolled steel sheets having thicknesses of
28
1.7 mm, 2.0 mm and 2.3 mm. These hot-rolled steel sheets were
heated to a temperature of 1100 or higher, maintained at
910 for 90 seconds, quenched in water, pickled, and then
cold-rolled to thicknesses of 0.23 mm, 0.27 mm and 0.30 mm.
The cold-rolled steel sheets were heated in 5 the furnace,
and then subjected to simultaneous decarburization and
nitrification by maintaining the steel sheets at a temperature
of 845 for 160 seconds in a mixed gas atmosphere formed by
simultaneously adding 74.5% hydrogen, 24.5% nitrogen and 1% dry
10 ammonia gas and having a dew-point temperature of 65 . The
nitrogen content of the nitrified steel sheets was controlled
between 170 ppm and 210 ppm. In the heating process, each
steel sheet was heated from room temperature to 570 at rates
of 30 /sec, 140 /sec, 160 /sec and 560 /sec, and from 570
15 to 700 at rates of 30 /sec, 140 /sec and 350 /sec. Then,
the steel sheets were heated from 700 to 845
(decarburization annealing temperature) at a rate of 25 /sec.
The annealing separator MgO was applied to each steel
sheet which was then subjected to final annealing in a coiled
20 state. In the final annealing, the steel sheet was maintained
in a mixed atmosphere of 25% nitrogen +75% hydrogen until it
reached 1200 , and after each steel sheet reached 1200 , it
was maintained in a 100% hydrogen atmosphere for 10 hours or
more, and then cooled in the furnace. Magnetic properties
25 measured for each condition are shown in Table 2 below.
29
Table 2
Thickness
(mm) of hotrolled
steel
sheet
Thickness
(mm) of coldrolled
steel
sheet
Rate of
heating from
room
temperature to
570 (/sec)
Rate of
heating from
570 to 700
(/sec)
Magnetic flux
density (B10,
Tesla)
Core loss
(W17/50, W/kg)
Remarks
1.7 0.23 30 30 1.91 0.90 Comparative
material 14
1.7 0.23 140 140 1.93 0.86 Comparative
material 15
1.7 0.23 560 30 1.92 0.88 Comparative
material16
1.7 0.23 560 140 1.96 0.75 Inventive
material 7
1.7 0.23 560 350 1.92 0.88 Comparative
material 17
2.0 0.27 30 30 1.91 0.96 Comparative
material 18
2.0 0.27 160 140 1.93 0.90 Comparative
material 19
2.0 0.27 560 30 1.91 0.95 Comparative
material 20
2.0 0.27 560 140 1.96 0.85 Inventive
material 8
2.0 0.27 560 350 1.93 0.93 Comparative
material21
2.3 0.30 30 30 1.89 1.03 Comparative
material 22
2.3 0.30 140 140 1.93 0.96 Comparative
material 23
2.3 0.30 560 30 1.91 1.00 Comparative
material 24
2.3 0.30 560 140 1.96 0.96 Inventive
material 9
2.3 0.30 560 350 1.92 0.97 Comparative
material 25
As can be seen in Table 2 above, when the thicknesses of
the cold-rolled steel sheets were 0.23 mm, 0.27 mm and 0.30 mm,
inventive materials 7 to 9 subjected to the heating patter5 n
comprising ultra-rapid heating followed by rapid heating all
showed excellent magnetic properties.
30
On the contrary, comparative materials 14, 18 and 22,
which were generally heated from room temperature to 570 at a
rate of 30 /sec, and comparative materials 15, 19 and 23
subjected to one-stage rapid heating (two-stage heating
pattern) by heating from room temperature to 700 at 5 a rate of
140~160 /sec, showed inferior magnetic properties compared to
inventive materials 7 to 9 subjected to ultra-rapid heating
followed by rapid heating.
Example 3
10 A grain-oriented electrical steel sheet slab comprising,
by wt%, Si: 3.25%, C: 0.052%, Mn: 0.105%, S: 0.0049%, N:
0.0048%, soluble Al: 0.028%, and the balance of Fe and
inevitable impurities, were heated at a temperature of 1150
for 210 hours, and then hot-rolled to produce a hot-rolled
15 steel sheet having a thickness of 2.3 mm. The hot-rolled steel
sheet was heated to a temperature of 1100 or higher,
maintained at 910 for 90 seconds, quenched in water, pickled,
and then cold-rolled to a thickness of 0.30 mm.
The cold-rolled steel sheet was heated in the furnace, and
20 then subjected to simultaneous decarburization and
nitrification by maintaining the steel sheet at a temperature
of 845 for 160 seconds in a mixed gas atmosphere formed by
simultaneously adding 74.5% hydrogen, 24.5% nitrogen and 1% dry
ammonia gas and having a dew-point temperature of 65 . The
25 nitrogen content of the nitrified steel sheet was controlled
31
between 170 ppm and 210 ppm.
In the heating process, the steel sheet was heated from
room temperature to 570 at rates of 30 /sec, 110 /sec and
560 /sec, and then from 570 to 700 at rates of 30 /sec,
110 /sec, 140 /sec, 190 /sec and 350 /sec, and then 700 5 00
to 845 (decarburization annealing temperature) at a rate of
25 /sec.
The annealing separator MgO was applied to the steel sheet
which was then subjected to final annealing in a coiled state.
10 In the final annealing, the steel sheet was maintained in a
mixed atmosphere of 25% nitrogen +75% hydrogen until it reached
1200 , and after the steel sheet reached 1200 , it was
maintained in a 100% hydrogen atmosphere for 10 hours or more,
and then cooled in the furnace. Magnetic properties measured
15 for each condition are shown in Table 3 below.
The fraction of Goss-oriented grains in a layer
corresponding to 1/8 of the thickness from the surface of the
decarburized steel sheet was measured at deviation angles of up
to 5° and 15° from the {110}<001> orientation. In addition,
20 the number of grains having a size of 35 in a cross-section
perpendicular to the rolling direction of the decarburized
steel sheet was measured, and the fraction of grains having an
orientation of up to 15° from the {411}<148> orientation was
measured. The results of the measurement are shown in Table 3
25 below. Herein, the size of grains was expressed as the average
32
between the longest length and the shortest length.
Table 3
Fraction of Goss orientation
Up to 15° Up to 5° (Exact goss)
Rate of
heating
from room
temperature
to 570
(/sec)
Rate of
heating
from
570 to
700
(/sec)
Magnetic
flux
density
(B10,
Tesla)
Core
loss
(W17/50,
W/kg)
Number
of
coarse
grains
(35
or
more)
Fraction (%)
of up to 15°
from
{411}<148>
Fraction
(%) of
orientation
Ratio of
increase
compared
to
comparative
material 26
Fraction
(%) of
orientation
Ratio of
increase
compared
to
comparative
material 26
Remarks
30 30 1.88 1.04 40 14.9 1.75 - 0.07 - Comparative
material 26
30 140 1.93 0.96 38 15.2 1.87 6.9 0.08 14.3 Comparative
material 27
30 350 1.91 0.99 42 14.5 1.92 9.7 0.08 14.3 Comparative
material 28
560 30 1.91 0.99 35 14.8 1.85 5.7 0.08 14.3 Comparative
material 29
560 140 1.97 0.89 22 14.7 2.18 24.6 0.13 85.7 Inventive
material 10
560 190 1.95 0.91 27 16.0 2.34 33.7 0.12 71.4 Inventive
material 11
560 350 1.92 0.97 42 16.0 2.34 33.7 0.12 71.4 Comparative
material30
110 110 1.92 0.97 41 15.1 1.91 9.1 0.08 14.3 Comparative
material 31
As can be seen in Table 3 above, comparative material 29,
which was heated at a high rate only in the temperature ra5 nge
from room temperature to 570 , comparative materials 27 and 28,
which were heated at a high rate only in the temperature range
from 570 to 700 , and comparative material 31 which was heated
at a high rate in both the temperature range from room
10 temperature to 570 and the temperature range from 570 to
700 , all showed a somewhat increase in the fraction of Gossoriented
grains compared to comparative material 26 which was
heated at a slow rate in primary recrystallization, but an
increase in the fraction of exact Goss grains having an
15 orientation of up to 5° from the {110}<001> orientation was as
33
low as 14.3%. This could be explained by the fact that there
was no great change in the fraction of grains having the
{411}<148> orientation of the {411} orientation in the primary
recrystallized grains. In other words, when the steel sheet
was heated from 570 or higher at a rate of 140 5 /sec, the
fraction of grains having the {411}<148> orientation somewhat
increased, but this increase was very low (less than 5%), and
it appears that the influence of the growth of the {411}<148>
Goss orientation on the exact Goss orientation is not so
10 significant.
On the contrary, in inventive materials 10 and 11, the
volume fraction of grains having an orientation of up to 15°
from the {110}<001> orientation was 2% or more, and
particularly the effect of directly increasing the fraction of
15 grains having the exact Goss orientation was very high. This
can be confirmed by the fact that the difference between the
inventive material and the comparative material was greater
when the tolerance angle (meaning an angle deviating from the
Goss orientation {110}<001>) was 5° or less, compared to when
20 the tolerance angle was 15°.
In other words, in inventive materials 10 and 11 which
were heated by two-stage rapid heating (ultra-rapid heating
from room temperature to 570 , and then rapid heating from 570
to 700 ) during primary recrystallization annealing, the
25 fraction of grains having an orientation of up to 5° from the
34
{110}<001> orientation was 0.09% or more, which was very
different from the fractions of Goss-oriented grains in
comparative materials 26 to 31.
Accordingly, it can be seen that, when the heating
conditions of the present invention are applied, the fractio5 n
of grains having an orientation very close to the Goss
orientation, that is, the exact Goss orientation having a
deviation angle of up to 5° from the {110}<001> orientation,
significantly increases, and thus nuclei capable of growing
10 into grains having the desired orientation increase, and these
grains grow so that the orientation of secondary recrystallized
grains is very close to the Goss orientation, thus improving
the magnetic properties of the steel sheet, because the Gossoriented
grains in the grain-oriented electrical steel sheet
15 grow even when the amount of the Goss-oriented grains in the
primary recrystallized grains is very small.
When the heating rate in the temperature range from 570 to
700 after ultra-rapid heating during primary
recrystallization annealing is higher than 250 /sec, the
20 fraction of Goss-oriented grains increases, and the effect of
improving the magnetic properties of the steel sheet is not
significant, because the number of large grains having a size
of 35 or larger when observing the cross-section of the steel
sheet before secondary recrystallization annealing after
25 primary recrystallization annealing excessively increases (30
35
or more; comparative material 25), and due to these large
grains, grains having orientations other than the Goss
orientation, which adversely affect the magnetic properties of
the steel sheet, are grown by the size advantage, and thus
orientations deviating from the {110}<001> orientation in 5 the
final steel sheet product increase.
Example 4
A grain-oriented electrical steel sheet slab comprising,
by wt%, Si: 3.13%, C: 0.057%, Mn: 0.095%, S: 0.0045% N: 0.0049%,
10 soluble Al: 0.029%, and the balance of Fe and inevitable
impurities, was heated at a temperature of 1150 for 210
minutes, and then hot-rolled to produce a hot-rolled steel
sheet having a thickness of 2.3 mm. The hot-rolled steel sheet
was heated to a temperature of 1100 or higher, maintained at
15 910 for 90 seconds, quenched in water, pickled, and then
cold-rolled to a thickness of 0.30 mm.
The cold-rolled steel sheet was heated in the furnace, and
then subjected to simultaneous decarburization and
nitrification by maintaining the steel sheet at a temperature
20 of 845 for 160 seconds in a mixed gas atmosphere formed by
simultaneously adding 74.5% hydrogen, 24.5% nitrogen and 1% dry
ammonia gas, and having a dew-point temperature of 65 . The
nitrogen content of the nitrified steel sheet was controlled
between 170 ppm and 210 ppm. In the heating process, the steel
25 sheet was heated from room temperature to 570 at various
36
rates of 30 /sec, 110 /sec and 560 /sec, and then from
570 to 700 at various rates of 30 /sec, 110 /sec,
140 /sec, 190 /sec and 350 /sec, and then from 700 to
845 (decarburization annealing temperature) at a rate of
25 5 /sec.
The annealing separator MgO was applied to each steel
sheet which was then subjected to final annealing in a coiled
state. In the final annealing, the steel sheet was maintained
in a mixed atmosphere of 25% nitrogen +75% hydrogen until it
10 reached 1200 , and after the steel sheet reached 1200 , it
was maintained in a 100% hydrogen atmosphere for 10 hours or
more, and then cooled in the furnace. Magnetic properties
measured for each condition are shown in Table 4 below.
After each specimen was subjected to secondary
15 recrystallization, the area-weighted average of angles
deviating from the {110}<001> orientation of grains was
measured, and the results of the measurement are shown in Table
4. The measurement was performed based on the X-ray Laue
method using an X-ray CCD detector while controlling the
20 position of the detector in units of 1 in order to increase
the accuracy of the measurement. While the specimen was moved,
the orientation at each position of the specimen was measured,
and for the orientation measured at each position, the absolute
value of the angle deviating from the ideal Goss orientation
25 was calculated, after which the area-weighted average of the
37
deviation angles at all the positions was determined.
Table 4
Area-weighted average of angles deviating from
{110}<001> orientation
Rate of
heating
from room
temperature
to 570
(/sec)
Rate of
heating
from 570
to 700
(/sec)
Magnetic
flux
density
(B10,
Tesla)
Core loss
(W17/50,
W/kg) α β γ δ
Remarks
30 30 1.89 1.02 4.99 3.14 6.1 6.57 Comparative
material 32
30 140 1.92 0.98 4.17 2.62 5.2 5.24 Comparative
material 33
30 350 1.91 0.99 3.89 2.84 4.52 5.27 Comparative
material 34
560 30 1.89 1.04 3.57 3.40 4.91 5.45 Comparative
material35
560 140 1.96 .090 3.48 2.2 3.7 4.25 Inventive
material 12
560 190 1.95 0.91 2.77 2.37 3.48 4.01 Inventive
material13
560 350 1.92 0.99 3.56 2.94 3.99 5.05 Comparative
material 36
110 110 1.92 0.98 3.49 2.64 4.03 5.01 Comparative
material 37
As can be seen in Table 4 above, in inventive materials 9
and 10 subjected to ultra-rapid heating followed by 5 rapid
heating, the area-weighted averages of deviation angles were
low as follows: α angle: 3.48° or less, β angle: 1.5-2.4°, γ
angle: 3.7° or less, and δ angle: 4.5° or less. Particularly,
the area-weighted average of the β angle and the δ angle were
10 rapidly lowered, suggesting that the magnetic properties of the
inventive materials were improved. This is related directly to
the principle of the present invention according to which
magnetic properties are improved. In other words, the width of
magnetic domains is minimized by the lowered β angle and δ
38
angle, and thus electromagnetic field energy is minimized while
disclosure magnetic domains that adversely affect magnetic
properties are minimized.
39
Claims
1. A method for manufacturing a grain-oriented
electrical steel sheet, the method comprising: heating a
grain-oriented electrical steel sheet slab; hot-rolling the
heated slab; optionally annealing the hot-rolled steel sheet5 ;
subjecting the resulting steel sheet to one cold rolling or two
or more cold rollings with intermediate annealing therebetween;
subjecting the cold-rolled steel sheet to primary
recrystallization annealing; and subjecting the annealed steel
10 sheet to secondary recrystallization annealing, wherein the
primary recrystallization annealing sequentially comprises an
ultra-rapid heating process of heating the steel sheet at an
average heating rate of 300 /sec or higher, a rapid heating
process of heating the steel sheet at a lower average heating
15 rate than the average heating rate of the ultra-rapid heating
process, but not lower than 100 /sec, and a general heating
process of heating the steel sheet at a lower average heating
rate than the average heating rate of the rapid heating
process.
20
2. The method of claim 1, wherein the grain-oriented
electrical steel sheet comprises, by wt%, Si: 2.0-4.0%, C:
0.085% or less, acid-soluble Al: 0.015-0.04%, Mn: 0.20% or
less, N: 0.010% or less, S: 0.010% or less, and the balance of
25 Fe and inevitable impurities.
40
3. The method of claim 2, wherein the ultra-rapid
heating process is performed by heating the steel sheet at an
average heating rate of 300 /sec or higher from room
temperature to Ts (), which is a temperature of 500~600
before recrystallization, the rapid heating process 5 ss is
performed by heating the steel sheet at an average heating rate
of 100~250 /sec from Ts () to 700 , and the general heating
process is performed by heating the steel sheet at an average
heating rate of 40 /sec or lower from 700 to a
10 decarburization annealing temperature.
4. The method of claim 2 or 3, wherein the grainoriented
electrical steel sheet has an N content of 0.006 wt%
or less, and a process for increasing the content of N in the
15 steel sheet is performed between the cold rolling and the
secondary recrystallization annealing.
5. The method of any one of claims 1 to 3, wherein the
ultra-rapid heating process is performed by heating the steel
20 sheet at an average heating rate of 400 /sec or higher from
room temperature to Ts (), which is a temperature of 500~600
before recrystallization, the rapid heating process is
performed by heating the steel sheet at an average heating rate
of 120~180 /sec from Ts () to 700 , and the general heating
25 process is performed by heating the steel sheet at an average
41
heating rate of 40 /sec or lower from 700 to the
decarburization annealing temperature.
6. The method of any one of claims 1 to 3, wherein the
number of grains having a size of 35 or larger, measured whe5 n
observing the cross-section of the steel sheet after the
primary recrystallization annealing, but before the secondary
recrystallization annealing, is less than 30.
10 7. The method of any one of claims 1 to 3, wherein the
grain-oriented electrical steel sheet is heated to 1280 or
lower before the hot rolling.
8. The method of any one of claims 1 to 3, wherein the
15 volume fraction of grains having an orientation of up to 15°
from the {110}<001> orientation is 2% or more when measured in
a layer corresponding to 1/8 of the thickness from the surface
of the steel sheet after the primary recrystallization
annealing but before the secondary recrystallization annealing.
20
9. The method of claim 8, wherein the volume fraction of
grains having an orientation of up to 5° from the {110}<001>
orientation is 0.09% or more when measured in a layer
corresponding to 1/8 of the thickness from the surface of the
25 steel sheet after the primary recrystallization annealing but
before the secondary recrystallization annealing.
42
10. The method of any one of claims 1 to 3, wherein a β
angle as an area-weighted average of an absolute value of
crystallographic orientation, measured for the steel sheet
after the secondary recrystallization annealing, is controlled
in the range of 1.5-2.6°, and a δ angle is controlled to 5° o5 r
less, wherein the β angle is an average angle of deviation from
the {110}<001> orientation in the direction perpendicular to
the rolling direction of the secondary recrystallized texture,
and the δ angle is an average angle of deviation between the
10 <001> orientation and the rolling direction in the secondary
recrystallized texture.
11. The method of claim 10, wherein the β angle measured
for the steel sheet after the secondary recrystallization
15 annealing is controlled to 2.4° or less, and the δ angle is
controlled to 4.5° or less.
12. The method of any one of claims 1 to 3, wherein the
heating process in the primary recrystallization annealing is
20 performed using a plurality of induction heating furnaces.