[Technical Field of the Invention]
[0001]
The present invention relates to a non-oriented electrical steel sheet and a
method of manufacturing a non-oriented electrical steel sheet.
[Related Art]
[0002]
In recent years, the frequency control of a drive power supply has been
capable of being performed with the development of a drive system for a motor.
Along with this, there is an increasing demand for motors that operate at a variable
speed or rotate at a high speed above commercial frequencies. The centrifugal force
acting on the rotor of a motor, which rotate at a high speed, increases in proportion to
the square of the rotation speed. For this reason, a steel material used for the rotor of
a high-speed motor gradually has required high strengthening.
[0003]
Further, in an embedded magnet type inverter control motor that is
increasingly employed for a drive motor of a hybrid vehicle or an electric vehicle, a
compressor motor of an air conditioner, or the like, a slit is provided at the outer
circumferential portion of a rotor and a magnet is embedded in the slit. For this
reason, stress is concentrated on a narrow bridge portion, such as a portion between the
outer circumference of the rotor and the slit, due to a centrifugal force during the highspeed
rotation of the motor. Accordingly, a core material used for the rotor requires
strength that allows the rotor to not be deformed and fractured by a centrifugal force.
- 1 -
[0004]
In addition, eddy current is generated in a high-speed rotary motor by highfrequency
magnetic flux, so that the efficiency of the motor is lowered and heat is
generated. Since the magnet embedded in the rotor is demagnetized in a case where
the amount of generated heat is increased, low iron loss in a high frequency range is
also required.
[0005]
Methods using solid solution strengthening, precipitation hardening, crystal
grain refinement strengthening, composite structure strengthening, and the like are
known as a method of improving the strength of a steel sheet. However, since
magnetic characteristics deteriorate and workability during cold rolling deteriorates in
many of these methods, it is generally extremely difficult to achieve both strength and
magnetic characteristics at a high level.
[0006]
Against this background, attempts to realize excellent magnetic characteristics
and high strength are made in, for example, Patent Documents 1 to 3.
[Prior Art Document]
[Patent Document]
[0007]
[Patent Document 1] Japanese Unexamined Patent Application, First
Publication No. 2004-300535
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. 2007-186791
[Patent Document 3] Japanese Unexamined Patent Application, First
Publication No. 2012-140676
- 2 -
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0008]
However, in order to realize the energy-saving characteristics required for
motors for electric vehicles or hybrid vehicles in recent years, low iron loss has been
insufficient and high cost has been required in the techniques disclosed in Patent
Documents 1 to 3.
[0009]
The present invention has been made to solve the above-mentioned problems,
and an object of the present invention is to stably provide a non-oriented electrical steel
sheet, which has high strength and excellent magnetic characteristics, at a low cost.
[Means for Solving the Problem]
[0010]
The gist of the present invention is the following non-oriented electrical steel
sheet and a method of the non-oriented electrical steel sheet.
[0011]
(1) A non-oriented electrical steel sheet including, as a chemical composition
of a base metal, by mass%:
C: 0.0050% or less;
Si: 3.8% to 5.0%;
Mn: exceeding 0.2% and less than 2.0%;
P: 0.030% or less;
S: 0.0030% or less;
Al: 0.005% or more and less than 0.050%;
N: 0.0005% to 0.0030%;
- 3 -
element.
Ti: less than 0.0050%;
Nb: less than 0.0050%;
Zr: less than 0.0050%;
V: less than 0.0050%;
Cu: less than 0.20%;
Ni: less than 0.50%;
Sn: 0% to 0.10%;
Sb: 0% to 0.10%; and
a remainder: Fe and impurities,
in which Expression (i) is satisfied, and
the average grain size of the base metal is in a range of 10 to 80 11m,
Si + 0.5 x Mn 2: 4.3 ... (i)
here, an element symbol in Expression (i) is a content (mass%) of each
[0012]
(2) The non-oriented electrical steel sheet according to (1),
in which the tensile strength is 650 MPa or more.
[0013]
(3) The non-oriented electrical steel sheet according to (1) or (2),
in which the chemical composition contains, by mass%, one or two selected
from Sn: 0.005% to 0.10% and Sb: 0.005% to 0.10%.
[0014]
(4) The non-oriented electrical steel sheet according to any one of (1) to (3 ),
further including:
an insulation coating that is provided on a surface of the base metal.
- 4 -
[0015]
(5) A method of manufacturing the non-oriented electrical steel sheet
according to any one of (1) to (4), the method including:
performing hot rolling on a steel ingot that has the chemical composition
according to any one of (1) to (3) and then performing the following Step 1 or 2,
(Step 1) performing cold rolling and final annealing in order without
annealing of a hot-rolled sheet, and
(Step 2) performing annealing of a hot-rolled sheet at a temperature of 950°C
or lower and then performing the cold rolling and the final annealing in order,
in which the final annealing is performed in a state where a soaking
temperature is in a range of 750°C to 1000°C and a soaking time is in a range of 1 to
300 s.
[Effects ofthe Invention]
[0016]
According to the present invention, it is possible to stably obtain a nonoriented
electrical steel sheet, which has high strength and excellent magnetic
characteristics, at a low cost.
[Embodiments of the Invention]
[0017]
As the result of diligent examination performed by the present inventors in
order to solve the above-mentioned problems, the present inventors have obtained the
following knowledge.
[001 8]
Si, Mn, and Al are elements that have an effect of increasing the electric
resistance of steel to reduce eddy-current loss. Further, these elements are elements
- 5 -
that also contribute to the high-strengthening of steel.
[0019]
Among the above-mentioned elements, Si is an element that most efficiently
contributes to an increase in electric resistance and is an element that also most
efficiently contributes to an increase in strength. Like Si, Al also has an effect of
efficiently increasing electric resistance. However, there is a problem that the
toughness of steel is reduced and workability deteriorates in a case where a large
amount of Al and a large amount of Si are added. In contrast, Mn has a lower effect
of increasing the electric resistance than Si and Al, but has an advantage that
workability is less likely to deteriorate.
[0020]
Accordingly, in this embodiment, the Si content and the Mn content are
adjusted to be in appropriate ranges, so that workability is ensured while highstrengthening
and the improvement of magnetic characteristics are achieved.
[0021]
In addition, it is also important to control grain sizes to order to achieve highstrengthening
and the improvement of magnetic characteristics. In terms of high
strengthening, it is desirable that crystals in steel are fine particles.
[0022]
Further, iron loss, particularly, iron loss in a high frequency range needs to be
improved in the magnetic characteristics of a non-oriented electrical steel sheet used as
the material of cores of drive motors for electric vehicles and hybrid vehicles and
motors for compressors of air conditioners. Iron loss mainly consists of hysteresis
loss and eddy-current los s. Here, it is preferable that crystal grains are coarsened in
order to reduce hysteresis loss and it is preferable that crystal grains are refined in
- 6 -
order to reduce eddy-current loss. That is, there is a trade-off relationship between
both hysteresis loss and eddy-current loss.
[0023]
Accordingly, as the result of examination further performed by the present
inventors, the present inventors have found that there is the range of a preferred grain
size for achieving high-strengthening and a reduction in high-frequency iron loss and
the Al content and N content need to be adjusted to be in appropriate ranges.
[0024]
The non-oriented electrical steel sheet according to this embodiment is made
on the basis of the above-mentioned knowledge. Each requirement for the nonoriented
electrical steel sheet according to this embodiment will be described in detail
below.
[0025]
1. Overall configuration
Since having particularly high strength, the non-oriented electrical steel sheet
according to the embodiment of the present invention is suitable for a rotor. Further,
since also being excellent in magnetic characteristics, the non-oriented electrical steel
sheet according to the embodiment of the present invention is also suitable for a stator.
Furthermore, it is preferable that the non-oriented electrical steel sheet according to the
embodiment of the present invention includes an insulation coating on the surface of a
base metal to be described below.
[0026]
2. Chemical composition of base metal
The reason why each element is to be limited is as follows. In the following
description, "%" related to the content means "mass%". Further, in this embodiment,
- 7 -
a numerical range expressed using "to" means a range that includes numerical values
written in the front and rear of "to" as a lower limit and an upper limit.
[0027]
C: 0.0050% or less
C (carbon) is an element that causes the deterioration of iron loss. In a case
where the C content exceeds 0.0050%, the deterioration of iron loss occurs in the nonoriented
electrical steel sheet. For this reason, good magnetic characteristics cannot
be obtained. Accordingly, the C content is set to 0.0050% or less. It is preferable
that the C content be 0.0040% or less and it is more preferable that the C content be
0.0035% or less. C contributes to the high-strengthening of the steel sheet.
Accordingly, in a case where a user wants to obtain the effect thereof, it is preferable
that the C content be 0.0005% or more and it is more preferable that the C content be
0.0010% or more.
[0028]
Si: 3.8% to 5.0%
Si (silicon) is an element that increases the electric resistance of steel to
reduce eddy-current loss and to improve high-frequency iron loss. Further, since Si
has large solid solution strengthening capability, Si is an element also effective for the
high-strengthening of a steel sheet. On the other hand, in a case where the Si content
is excessive, workability significantly deteriorates and it is difficult to perform cold
rolling. Accordingly, the Si content is set to be in a range of 3.8% to 5.0%. It is
preferable that the Si content be 3.9% or more and it is more preferable that the Si
content be 4.0% or more. Further, it is preferable that the Si content be 4.8% or less
and it is more preferable that the Si content be 4.5% or less.
[0029]
- 8 -
Mn: exceeding 0.2% and less than 2.0%
Mn (manganese) is an element that is effective to increase the electric
resistance of steel to reduce eddy-current loss and to improve high-frequency iron loss.
However, in a case where the Mn content is excessive, magnetic flux density is
significantly reduced. Accordingly, the Mn content is set to exceed 0.2% and be less
than 2.0%. It is preferable that the Mn content be 0.3% or more, it is more preferable
that the Mn content is 0.4% or more, it is still more preferable that the Mn content
exceeds 0.5%, and it is even more preferable that the Mn content be 0.6% or more.
Further, it is preferable that the Mn content be 1.8% or less, it is more preferable that
the Mn content be 1.7% or less, it is still more preferable that the Mn content be less
than 1.5%, it is even more preferable that the Mn content be 1.4% or less, it is yet more
preferable that the Mn content be 1.2% or less, and it is a lot more preferable that the
Mn content be 1.0% or less.
[0030]
In the present invention, the Si content and the Mn content are appropriately
controlled to ensure the electric resistance of steel. For this reason, in addition to
each of the Si content and the Mn content being in the above-mentioned ranges, it is
necessary to satisfy the following expression (i). It is preferable that the left value of
Expression (i) be 4.4 or more and it is more preferable that the left value of Expression
(i) be 4.5 or more.
Si + 0.5 x Mn ~ 4.3 ... (i)
Here, an element symbol in Expression (i) is a content (mass%) of each
element.
The purpose of Expression (i) is as follows.
As described above, in this embodiment, the Si content and the Mn content
- 9 -
are adjusted to be in appropriate ranges, so that workability is ensured while highstrengthening
and the improvement of magnetic characteristics are achieved. First,
focusing on Si, Si is an element that increases the electric resistance of steel to reduce
eddy-current loss and to improve high-frequency iron loss. Further, since Si has large
solid solution strengthening capability, Si is an element also effective for the highstrengthening
of a steel sheet. On the other hand, in a case where the Si content is
excessive, workability significantly deteriorates and it is difficult to perform cold
rolling. From this point of view, the Si content is set to be in a range of 3.8% to 5.0%.
[0031]
Comparing the range of the Si content with Expression (i), Expression (i) is
satisfied regardless of the Mn content in a case where the Si content is in a range of
4.3% to 5.0%. Accordingly, in this case, good magnetic characteristics are obtained
(on the premise that other requirements of the non-oriented electrical steel sheet
according to this embodiment are satisfied). In addition, since the Si content satisfies
the requirement of this embodiment, the non-oriented electrical steel sheet has high
strength. On the other hand, Si is an element disadvantageous in terms of workability,
but workability is also good since the Si content is at least 5.0% or less.
[0032]
On the other hand, in a case where the Si content is 3.8% or more and less
than 4.3%, Expression (i) is not satisfied by only the Si content. That is, there is a
possibility that desired magnetic characteristics are not obtained from only Si.
Accordingly, Mn is used to compensate for the lack of magnetic characteristics. That
is, the Mn content increased in a range exceeding 0.2% and less than 2.0% so that
Expression (i) is satisfied. Accordingly, the magnetic characteristics of the nonoriented
electrical steel sheet are improved. On the other hand, since Si is 3.8% or
- 10 -
more, the strength of the non-oriented electrical steel sheet is also increased. With
regard to workability, since the Si content is less than 4.3%, workability tends to be
improved as compared to the above-mentioned case (that is, a case where the Si
content is 4.3% or more). Here, since it is difficult for Mn to affect workability,
workability is less likely to deteriorate even if the Mn content is increased so that
Expression (i) is satisfied. Further, an increase in the Mn content is effective at
increasing strength though it is not as effective as Si.
[0033]
As described above, in this embodiment, the Si content and the Mn content
have values in the above-mentioned numerical ranges and are set so that Expression (i)
is satisfied. Accordingly, workability can be ensured while the high-strengthening of
the non-oriented electrical steel sheet and the improvement of the magnetic
characteristics of the non -oriented electrical steel sheet are achieved.
[0034]
P: 0.030% or less
P (phosphorus) is included in steel as an impurity, and the ductility of the steel
sheet is significantly reduced in a case where the P content is excessive. Accordingly,
the P content is set to 0.030% or less. It is preferable that the P content is 0.025% or
less and is it more preferable that the P content is 0.020% or less.
[0035]
S: 0.0030% or less
S (sulfur) is an element that forms the fine precipitate ofMnS to increase iron
loss and to cause the magnetic characteristics of the steel sheet to deteriorate.
Accordingly, the S content is 0.0030% or less. It is preferable that the S content is
0.0025% or less and it is more preferable that the S content is 0.0020% or less. Since
- 11 -
there is a concern that an extreme reduction in the S content causes an increase in
manufacturing cost, it is preferable that the S content be 0.0001% or more, it is more
preferable that the S content be 0.0003% or more, and it is still more preferable that the
S content be 0.0005% or more.
[0036]
Al: 0.005% or more and less than 0.050%
Al (aluminum) is an element that is effective at stably refining crystal grains
when combined with Nand forming AlN. It is necessary to contain 0.005% or more
of Al in order to exhibit this effect. On the other hand, in a case where 0.050% or
more of Al is contained, an effect of refining crystal grains is reduced. Accordingly,
the Al content is set to 0.005% or more and be less than 0.050%. It is preferable that
the Al content be 0.008% or more, it is more preferable that the Al content be 0.010%
or more, it is still more preferable that the Al content be 0.015% or more, and it is even
more preferable that the Al content be 0.020% or more. Further, it is preferable that
the Al content be 0.048% or less and it is more preferable that the Al content be
0.045% or less. In this specification, the Al content means the total Al content
included in a base metaL
[0037]
N: 0.0005% to 0.0030%
N (nitrogen) is an element that is effective at stably refining crystal grains
when combined withAl and forming AlN. On the other hand, in a case where a large
amount of N is contained, excessive AlN is formed and the deterioration of iron loss is
caused. Accordingly, theN content is set to be in a range of 0.0005% to 0.0030%.
It is preferable that the N content be 0.0007% or more and it is more preferable that the
N content be 0.0010% or more. Further, it is preferable that the N content be
- 12 -
0.0027% or less and it is more preferable that theN content be 0.0025% or less.
[0038]
Ti: less than 0.0050%
Ti (titanium) is an element that is unavoidably mixed and may form
precipitates (carbide or nitride) by combining with carbon or nitrogen. In a case
where carbide or nitride is formed, these precipitates themselves cause magnetic
characteristics to deteriorate. In addition, these precipitates inhibit the growth of
crystal grains during final annealing to cause magnetic characteristics to deteriorate.
Accordingly, the Ti content is set to be less than 0.0050%. It is preferable that the Ti
content be 0.0040% or less, it is more preferable that the Ti content be 0.0030% or less,
and it is still more preferable that the Ti content be 0.0020% or less. Since there is a
concern that an extreme reduction in the Ti content causes an increase in
manufacturing cost, it is preferable that the Ti content be 0.0005% or more.
[0039]
Nb: less than 0.0050%
Nb (niobium) is an element that forms precipitates (carbide or nitride) by
combining with carbon or nitrogen to contribute to high-strengthening, but these
precipitates themselves cause magnetic characteristics to deteriorate. Accordingly,
the Nb content is set to be less than 0.0050%. It is preferable that the Nb content be
0.0040% or less, it is more preferable that the Nb content be 0.0030% or less, and it is
still more preferable that the Nb content be 0.0020% or less. Lower Nb content be
more preferable and it is preferable that the Nb content be equal to or lower than a
measurement limit.
[0040]
Zr: less than 0.0050%
- 13 -
Zr (zirconium) is an element that forms precipitates (carbide or nitride) by
combining with carbon or nitrogen to contribute to high-strengthening, but these
precipitates themselves cause magnetic characteristics to deteriorate. Accordingly,
the Zr content is set to be less than 0.0050%. It is preferable that the Zr content be
0.0040% or less, it is more preferable that the Zr content be 0.0030% or less, and it is
still more preferable that the Zr content be 0.0020% or less. A lower Zr content is
more preferable and it is preferable that the Zr content equal to or lower than a
measurement limit.
[0041]
V: less than 0.0050%
V (vanadium) is an element that forms precipitates (carbide or nitride) by
combining with carbon or nitrogen to contribute to high-strengthening, but these
precipitates themselves cause magnetic characteristics to deteriorate. Accordingly,
the V content is set to be less than 0.0050%. It is preferable that the V content be
0.0040% or less, it is more preferable that the V content be 0.0030% or less, and it is
still more preferable that the V content be 0.0020% or less. A lower V content be
more preferable and it is preferable that the V content be equal to or lower than a
measurement limit.
[0042]
Cu: less than 0.20%
Cu (copper) is an element that is unavoidably mixed. The intentional
addition of Cu causes an increase in the manufacturing cost of the steel sheet.
Accordingly, in the present invention, Cu does not need to be actively added and may
be added at a level of an impurity. The Cu content be set to be less than 0.20% that is
the maximum value of Cu to be unavoidably mixed in a manufacturing step. It is
- 14 -
preferable that the Cu content be 0.15% or less and it is more preferable that the Cu
content be 0.10% or less. The lower limit of the Cu content is not particularly limited,
but there is a concern that an extreme reduction in the Cu content causes an increase in
manufacturing cost. For this reason, it is preferable that the Cu content be 0.001% or
more, it is more preferable that the Cu content be 0.003% or more, and it is still more
preferable that the Cu content be 0.005% or more.
[0043]
Ni: less than 0.50%
Ni (nickel) is an element that is unavoidably mixed. However, since also
being an element that improves the strength of the steel sheet, Ni may also be
intentionally added. Here, since Ni is expensive, the Ni content is set to be less than
0.50% in a case where Ni is to be intentionally added. It is preferable that the Ni
content be 0.40% or less and it is more preferable that the Ni is 0.30% or less. The
lower limit of the Ni content is not particularly limited, but there is a concern that an
extreme reduction in the Ni content causes an increase in manufacturing cost. For
this reason, it is preferable that the Ni content be 0.001% or more, it is more preferable
that the Ni content be 0.003% or more, and it is still more preferable that the Ni
content be 0.005% or more.
[0044]
Sn: 0% to 0. 10%
Sb: 0% to 0.10%
Sn (tin) and Sb (antimony) are elements that are useful to ensure low iron loss
by being segregated on the surface and suppressing oxidation and nitriding during
annealing. Further, Sn and Sb also have an effect of improving a texture by being
segregated at grain boundaries and an effect of increasing magnetic flux density. For
- 15 -
this reason, at least one of Sn and Sb may be contained as necessary. However, in a
case where the contents of these elements are excessive, there is a possibility that the
toughness of steel is reduced, which makes it difficult for cold rolling to be performed.
Accordingly, each of the Sn content and the Sb content is set to 0.10% or less. It is
preferable that each of the Sn content and the Sb content be 0.06% or less. In a case
where a user wants to obtain the above-mentioned effects, it is preferable that at least
one of the Sn content and the Sb content be set to 0.005% or more and it is more
preferable that at least one of the Sn content and the Sb content be set to 0.010% or
more.
[0045]
In the chemical composition of the base metal of the non-oriented electrical
steel sheet according to the embodiment of the present invention, a remainder is Fe and
impurities. Here, "impurities" are raw materials, such as ore and scrap, and
components to be mixed due to various factors of manufacturing steps in a case where
steel is to be industrially manufactured, and mean materials that are allowed within a
range where the present invention is not adversely affected.
[0046]
The Cr content and the Mo content are not particularly specified as impurity
elements. Even if the non-oriented electrical steel sheet according to this
embodiment contains 0.5% or less of these elements, the effects of the present
invention are not particularly affected. Further, even if each of Ca and Mg is
contained in a range of 0.002% or less, the effects of the present invention are not
particularly affected. Even if rare earth elements (REM) are contained in a range of
0.004% or les s, the effects of the present invention are not particularly affected.
[0047]
- 16 -
0 is also an impurity element, but the effects of the present invention are not
affected even if 0 is contained in a range of 0.05% or less. Since 0 may be mixed in
an annealing step, the effects of the present invention are not particularly affected even
if 0 is contained in a range of 0.01% or less in the content of a slab stage (that is, a
ladle value).
[0048]
Further, elements, such as Pb, Bi, As, B, and Se, may be contained in addition
to the above-mentioned elements, but the effects of the present invention do not
deteriorate in a case where each of the contents thereof is in a range of 0.0050% or les s.
[0049]
The chemical composition of the above-mentioned base metal may be
measured using a general analysis method. For example, the composition of steel
may be measured using inductively coupled plasma-atomic emission spectrometry
(ICP-AES). C and S may be measured using a combustion-infrared absorption
method, N may be measured using an inert gas fusion-thermal conductivity method,
and 0 may be measured using an inert gas fusion-nondispersive infrared absorption
method.
[0050]
3. Grain size
As described above, in terms of high strengthening, it is desirable that crystals
in steel be fine particles. In addition, it is preferable that crystal grains be coarsened
in order to reduce hysteresis loss, and it is preferable that crystal grains be r efined in
order to reduce eddy-current loss.
[005 1]
In a case where the average grain size of the base metal is less than 10 Jlm,
- 17 -
hysteresis loss significantly deteriorates and it is difficult for magnetic characteristics
to be improved. On the other hand, in a case where the average grain size exceeds 80
11m, the strength of steel is reduced. Accordingly, the average grain size of the base
metal is set to be in a range of 10 to 80 11m. It is preferable that the average grain size
be 12 11m or more and it is more preferable that the average grain size be 141-!m or
more. Further, it is preferable that the average grain size be 70 11m or less and it is
more preferable that the average grain size be 60 11m or less.
[0052]
In the present invention, the average grain size of the base metal is obtained
according to JIS G 0551 (2013) "steel-grain size microscopic test method".

What is claimed is:
CLAIMS
1. A non-oriented electrical steel sheet comprising, as a chemical composition of a
base metal, by mass%:
element.
C: 0.0050% or less;
Si: 3.8% to 5.0%;
Mn: exceeding 0.2% and less than 2.0%;
P: 0.030% or less;
S: 0.0030% or less;
Al: 0.005% or more and less than 0.050%;
N: 0.0005% to 0.0030%;
Ti: less than 0.0050%;
Nb: less than 0.0050%;
Zr: less than 0.0050%;
V: less than 0.0050%;
Cu: less than 0.20%;
Ni: less than 0.50%;
Sn: 0% to 0.10%;
Sb: 0% to 0.10%; and
a remainder: Fe and impurities,
wherein Expression (i) is satisfied, and
an average grain size of the base metal is in a range of 10 to 80 11m,
Si + 0.5 x Mn 2: 4.3 ... (i)
here, an element symbol in Expression (i) is a content (mass%) of each
- 32 -
2. The non-oriented electrical steel sheet according to claim 1,
wherein a tensile strength is 650 MPa or more.
3. The non-oriented electrical steel sheet according to claim 1 or 2,
wherein the chemical composition contains, by mass%, one or two selected
from Sn: 0.005% to 0.10% and Sb: 0.005% to 0.10%.
4. The non-oriented electrical steel sheet according to any one of claims 1 to 3,
further comprising:
an insulation coating that is provided on a surface of the base metal.
5. A method of manufacturing the non-oriented electrical steel sheet according to any
one of claims 1 to 4, the method comprising:
performing hot rolling on a steel ingot that has the chemical composition
according to any one of claims 1 to 3 and then performing the following Step 1 or 2,
(Step 1) performing cold rolling and final annealing in order without
annealing of a hot-rolled sheet, and
(Step 2) performing annealing of a hot-rolled sheet at a temperature of 950°C
or lower and then performing the cold rolling and the final annealing in order,
wherein the final annealing is performed in a state where a soaking
temperature is in a range of 75oac to 1000oc and a soaking time is in a range of 1 to
300 s.