DESCRIPTION
TITLE OF INVENTION: METHOD OF MANUFACTURING GRAIN--
ORIENTED ELECTRICAL STEEL SHEET
TECHNICAL FIELD
[0001]
The present invention relates to a method of
manufacturing a grain-oriented electrical steel sheet
suitable for iron core and so forth of electric
appliances.
BACKGROUND ART
[0002]
A grain-oriented electrical steel sheet has been
used as a material for composing an iron core of electric
appliances such as transformer. It is important for a
grain-oriented electrical steel sheet to be excellent in
magnetization characteristics and iron loss
characteristics. in recent years, there has been a
growing demand for a grain-oriented electrical steel
sheet characterized by small energy loss and low iron
loss. Since a steel sheet having a large magnetic flux
density generally has low iron loss, and may be downsized
when used as an iron core, so that development thereof
has very strongly been targeted at.
[0003]
In order to improve a magnetic flux density of a
grain-oriented electrical steel sheet, it is important to
highly integrate the crystal grains to {110}<001>
1
orientation called Goss orientation. Orientation of
crystal grains is controlled making use of catastrophic
grain growth called secondary recrystallization.
Management. of a structure obtained by a primary
recrystallization before the secondary recrystallization
(primary recrystallization structure), and management of
fine precipitate called inhibitor such as A1N, or element
segregated in the grain boundary hold the key for control
of the secondary recrystallization. The inhibitor allows
crystal grains having {1l0}<001> orientation to grow
predominantly in the primary recrystallization structure,
so as to suppress growth of crystal grains with other
orientations,
[0009]
One of the known method of producing the inhibitor
is such as allowing AIN to deposit by nitriding conducted
before the secondary recrystallization (Patent Document
5, for example). Still another known method totally
different in mechanism is such as allowing AlN to deposit
during annealing (riot--rolled sheet annealing), which
takes place in the duration from hot rolling and cold
rolling, without relying upon the nitriding (Patent
Document 6, for example).
[0005]
It is, however, difficult to effectively improve the
magnetic flux density even with these techniques.
CITATION LIST
PATENT LITERATURE
2
[0006]
Patent Literature 1. Japanese Examined Patent
Publication No. 62-045285
Patent Literature 2a Japanese Laid-Open Patent
Publication No. H02-077525
Patent Literature 3: Japanese Laid-Open Patent
Publication No, S62-040315
Patent Literature 4: Japanese Laid-Open Patent
Publication No. H02---274812•
Patent Literature 5: Japanese Laid-Open Patent
Publication No. H04-297524
Patent Literature 6: Japanese Laid-Open Patent
Publication No. H10-121213
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0007]
It is therefore an object of the present invention
to provide a method of manufacturing a grain-oriented
electrical steel sheet, capable of effectively improving
the magnetic flux density.
SOLUTION TO PROBLEM
[0008]
Aiming at controlling the primary recrystallization
structure in the method of manufacturing a grain-oriented
electrical steel sheet involving the nitriding process,
the present inventors paid a special attention to
conditions of finish rolling in the hot rolling. While
3
the details will be given later, the present inventors
found out that it is important to set the finish
temperature in the finish rolling to 950°C or below;
start cooling within 2 seconds after completion of the
finish rolling; to set the cooling rate to 10°C/sec or
above; and to set coiling temperature to 700°C or below.
When these conditions are satisfied, recrystallization
and grain growth before annealing may be suppressed. The
present inventors also found out that, for the case where
the finish temperature in the finish rolling is set to
950°C or below,. it is important to set heating rate,
within a predetermined temperature range (800°C or above
and 1000°C or below) in the annealing (hot-rolled sheet
annealing) after the hot rolling, to 5°C/sec or above.
By the heating in this way, recrystallized grains may
effectively be refined. The present inventors reached an
idea that the {lil)<112> orientation which generates at
around the grain boundaries in the primary recrystallized
structure may be increased by combining these conditions,
thereby the degree of integration of the secondary
recrystallized grains with the {110}<001> orientation may
be increased, and the grain-oriented electrical steel
sheet excellent in the magnetic characteristics may be
manufactured. Note that, in the conventional. method of
manufacturing a grain-oriented electrical steel sheet
(Patent Document 5, for example) involving the nitriding
process, the heating rate in the hot-rolled sheet
annealing has been determined while giving priority on
productivity and stability, from the viewpoints of load
4
exerted on facility and difficulty in temperature
control.
[0009]
Summary of the present invention is as follows.
[0010]
(1)
A method of manufacturing a grain-oriented
electrical steel sheet including:
heating a silicon steel slab at 1280°C or below, the
silicon steel slab containing, in % by mass, Si: 0.8% to
70, and acid-soluble Al: 0,01% to 0.065%, with a C
content of 0.085% or less, a N content of 0.012% or less,
a Mn content of 1% or less, and a S equivalent Seq.,
defined by "Seq [S]+0.406x[Se]" where [S] being S
content (%) and [Se] being Se content (%), of 00015% or
less, and the balance of Fe and unavoidable impurities;
hot rolling the heated silicon steel slab so as to
obtain a hot-rolled steel strip;
annealing the hot-rolled steel strip so as to obtain
an annealed steel strip;
cold rolling the annealed steel strip so as to
obtain a cold-rolled steel strip;
decarburization annealing the cold-rolled steel
strip so as to obtain a decarburization-annealed steel
strip in which primary recrystallization is caused;
coating an annealing separating agent on the
decarburization-annealed steel strip; and
finish annealing the decarburization-annealed steel
strip so as to cause secondary recrystallization, wherein
5
the method further comprises performing a nitriding
treatment in which a N content of the decarburizationannealed
steel strip is increased between start of the
decarburization annealing and occurrence of the secondary
recrystallization in the finish annealing,
the hot rolling the heated silicon steel slab
comprises:
finish rolling with a finish temperature of 950°C or
below; and
starting cooling within 2 seconds after completion
of the finish rolling, and coiling at 700°C or below,
a heating rate of the hot-rolled steel strip within
the temperature range from 800°C to 1000°C in the
annealing the hot-rolled steel strip is 5°C/sec or above,
and
a cooling rate over a duration from the completion
of the finish rolling up to a start of the coiling is
10°C/sec or above.
(2)
The method of manufacturing a grain-oriented
electrical steel sheet according to (1), wherein a
cumulative reduction in the finish rolling is 93% or
above.
(3)
The method of manufacturing a grain-oriented
electrical steel sheet according to (1) or (2), wherein a
cumulative reduction in the last three passes in the
finish rolling is 40% or above.
(4)
6
The method of manufacturing a grain--oriented
electrical steel sheet according to any one of (1) to
(3), wherein the silicon steel slab further contains Cu:
0.4% by mass.
(5)
The method of manufacturing a grain-oriented
electrical steel sheet according to any one of (1) to
(4), wherein the silicon steel slab further contains,
in % by mass, at least one selected from the group
consisting of Cr: 0,30 or less, P: 0.5% or less, Sri: 003%
or less, Sb: 003% or less, Ni: 1% or less, Bi: 0001% or
less, B: 0°01% or less, Ti: 0.01% or less, and Te: 0.01%
or less.
ADVANTAGEOUS EFFECTS OF INVENTION
[0011]
According to the present invention, by combining the
various conditions, a structure of the hot-rolled steel
strip and so forth may be suitable for forming crystal
grains with the Goss orientation, and thereby the degree
of integration of the Goss orientation may be increased
through the primary recrystallization and the seconda y
recrystallization. As a consequence, the magnetic flux
density may be increased and the iron loss may be
decreased in an effective manner.
BRIEF DESCRIPTION OF DRAWINGS
[0012]
7
[FIG. 1] FIG. 1 is a flow chart illustrating a
method of manufacturing a grain-oriented electrical steel
sheet,
[FIG. 2] FIG. 2 is a chart illustrating results of a
first experiment; and
[FIG. 3] FIG. 3 is a chart illustrating results of a
second experiment.
DESCRIPTION OF EMBODIMENTS
[0013]
Embodiments of the present invention will be
detailed below, referring to the attached drawings. FIG„
I is a flow chart illustrating a method of manufacturing
a grain-oriented electrical steel. sheet.
[0014]
First, as illustrated in FIG. 1, in step Sl, a
silicon steel material (slab) with a predetermined
composition is heated to a predetermined temperature, and
in step S2, the heated silicon steel material is hot
rolled. As a result of the hot rolling, a hot-rolled
steel strip is obtained. Thereafter, in step S3, the
hot-rolled steel strip is annealed (hot-rolled sheet
annealing) to thereby homogenize the structure in the
hot-rolled steel strip and control precipitation of
inhibitor. As a result of the annealing (hot-rolled
sheet annealing), an annealed steel strip is obtained.
Subsequently, in step S4, the annealed steel strip is
cold rolled. The cold rolling may be conducted once, or
may be repeated multiple times while conducting
8
intermediate annealing in between. As a result of the
cold rolling, a cold-rolled steel strip is obtained. For
the case where the intermediate annealing is adopted, the
annealing of the hot-rolled steel strip before the cold
rolling is omissible, and instead the annealing may be
implemented in the intermediate annealing (step S3), In
other words, the annealing (step S3) may be effected on
the hot-rolled steel strip, or on the steel strip once
subjected to cold rolling and before the final cold
rolling.
[0015]
After the cold rolling, in step S5, decarburization
annealing of the cold-rolled steel strip is performed.
In the decarburization annealing, the primary
recrystallization occurs. As a result of the
decarburization annealing, a decarburization-annealed
steel strip is obtained. Then, in step S6, an annealing
separating agent containing MgO (magnesia) as a main
component is coated over the surface of the decarburized
steel strip, followed by finish annealing. During the
finish annealing, the secondary recrystallizati_on occurs,
a glass coating mainly composed of forsterite is formed
over the surface of the steel strip, and purification
proceeds. As a result of the secondary
recrystallization, a secondary recrystallization
structure with the Goss orientation is obtained. As a
result of the finish annealing, a finish-annealed steel
strip is obtained. A nitriding treatment in which a N
content of the steel strip is increased is performed,
9
between start of the decarburization annealing and
occurrence of the secondary recrystallization in the
finish annealing (step S7).
[0016]
The grain--oriented electrical steel sheet may be
obtained in this way.
[0017]
Reasons for limitation of the components of the
silicon steel slab used in this embodiment will now he
explained. In the description below, % means % by mass.
[0018]
The silicon steel slab used in this embodiment may
contain Si: 0.8% to 7%, and acid--soluble Al: 0.01% to
0°065°, a C content may be 0.0850 or less, a N content
may be 0.012% or Jess, a Mn content may be 1% or less,
and a S equivalent Seq., defined by "Seq.=[S]+0.406x[Se]`°
where [5] being S content (%) and [Se] being Se content
(%),may be 0.015% or less, and the balance may be Fe and
unavoidable impurities. Cu: 0.4% or less may further be
contained in the silicon steel slab. Also at least one
selected from the group consisting of Cr: 0.3% or less,
P: 0.5% or less, Sn: 0.3% or less, Sb: 0.3% or less, Ni:
16 or -less, Bi: 0.01% or less, B. 0.01% or less, Ti
0,01% or less, and Tea 0.01% or less may be contained.
[0019]
Si contributes to increase the electric resistance
and reduces the iron loss. Si content of less than 0.8%
would result in only insufficient levels of these
effects. Also the y transformation would occur during
10
the finish annealing (step S6), and thereby the crystal
orientation would not fully be controlled. If the Si
content exceeds 7%, the cold rolling (step S4) would be
very difficult, so that the steel strip would crack in
the process of cold rolling. Accordingly, the Si content
is set to 0.8% to 7%. Taking the industrial productivity
into account, the Si content is preferably 4.8% or less,
and more preferably 4.00 or less. Also taking the abovedescribed
effects into account, the Si content is
preferably 2.8% or above.
[0020]
The acid-soluble Al combines with N to form
(Al,Si)N, which serves as an inhibitor. The content of
acid-soluble Al of less than 0.01% would result in only
an insufficient amount of formation of inhibitor. The
content of acid-soluble Al exceeding 0.065% would
destabilize the secondary recrystallization.
Accordingly, the content of acid-soluble Al is set to
0,01% to 0.065 The content of acid-soluble Al is
preferably 0.0018% or above, more preferably 0°0220 or
above. The content of acid-soluble Al is preferably
0.035% or less.
[0021]
C is an element effective for controlling the
primary recrystallization structure, but adversely
affects the magnetic characteristics. The
decarburization annealing (step S5) is implemented for
this reason, wherein the C content exceeding 0.085% would
require a longer time for the decarburization annealing,
11
and would degrade the productivity. Accordingly, the C
content is set to 0.085% or less, and preferably 0,08% or
less. From the viewpoint of control of the primary
recrystallization structure, the C content is preferably
0.05% or above.
[0022]
N contributes to form A1N or the like which serves
as an inhibitor. The N content exceeding 0°012% would,
however, result in formation of void, called blister, in
the steel strip during the cold rolling (step S4)o
Accordingly, the N content is set to 00012% or less, and
preferably to 0.01% or less. From the viewpoint of
formation of the inhibitor, the N content is preferably
00004% or above.
[0023]
Mn contributes to increase the specific resistance
and to reduce the iron loss. Mn also suppresses crack in
the process of hot rolling (step S2)o The Mn content
exceeding 1% would, however, reduce the magnetic flux
density. Accordingly, the Mn content is set to 1% or
less, and preferably 0.8% or less. From the viewpoint of
reduction in iron loss, the Mn content is preferably
0005% or above. Mn also combines with S and/or Se,
thereby improve the magnetic characteristics.
Accordingly, with the Mn content (% by mass) denoted as
[Mn], a relation of "[Mn]/([S]+[Se])?4" preferably holds.
[0024]
S and Se exist in the steel strip as being combined
with Mn, and contribute to improve the magnetic
1.2
characteristics. However, if the S equivalent Seq.
defined by "Seq.=[S]+0.406x [Se]" exceeds 000150, the
magnetic characteristics are adversely affected.
Accordingly, the S equivalent Seq. is set to 0.0150 or
less,
[0025]
As described in the above, the silicon steel slab
may contain Cu. Cu may contribute forming an inhibitor.
However, if the Cu content exceeds 0.40, dispersion of
deposit would tend to be non-uniform, and thereby the
effect of reducing the iron loss would saturate.
Accordingly, the Cu content is set to 0.40 or less, and
preferably 0.30 or less. From the viewpoint of formation
of the inhibitor, the Cu content is preferably 0.05% or
above.
[0026]
As described in the above, the silicon steel. slab
may contain at least one selected from the group
consisting of Cr: 0.30 or less, P: 0.5% or less, Sn: 0.30
or less, Sb: 0.3° or less, Ni: 1% or less, Pie 0.01% or
less, B: 0.010 or less, Ti: 0.010 or less, and Tew 0,01.
[0027]
Cr is effective for improving an oxide layer formed
over the surface of the steel strip during the
decarburization annealing (step S5). If the oxide layer
is improved, the glass coating formed so as to originate
from the oxide layer in the process of finish annealing
(step S6) is improved. The Cr content exceeding 0.30
would, however, degrade the magne^ic characteristics.
13
Accordingly, the Cr content is set to 0.3% or less. From
the viewpoint of improving the oxide layer, the Cr
content is preferably 0002% or above.
[0028]
P contributes to increase the specific resistance
and reduce the iron loss. The P content exceeding 0.5%
would, however, make cold rolling (step S4) difficult.
Accordingly, the P content is set to 0,5% or less, and
preferably 0,3% or less. From the viewpoint of reducing
the iron loss, the P content is preferably 0.02% or
above.
[0029]
Sn and Sb are boundary segregation elements. In this
embodiment, since the silicon steel slab contains acidsoluble
Al, so that Al would be oxidized by water
released from the annealing separating agent depending on
conditions of the finish annealing (step S6), If Al is
oxidized, inhibitor strength would vary from site to site
in the coiled steel strip, and thereby the magnetic
characteristics would vary. In contrast, when the Sn
and/or Sb are contained as the boundary segregation
elements, the oxidation of Al may be suppressed, and
thereby the magnetic characteristics may be suppressed
from varying. The Sn content exceeding 043% would,
however, make the oxide layer less likely to be formed
during the decarburization annealing (step S5), and
thereby the glass coating would be formed only to an
insufficient degree. This would also make the
decarburization annealing (step S5) very difficult. The
14
same will apply also to the case where the Sb content
exceeds 0.3%. Accordingly, the Sn content and the Sb
content are set to 0.3% or less. From the viewpoint of
suppressing the oxidation of Al, the Sn content and the
Sb content are preferably 0.02% or above.
[0030]
Ni contributes to increase the specific resistance
and to reduce the iron loss. Ni is an effective element
also in view of.controlling the metal structure of the
hot-rolled steel strip, and improving the magnetic
characteristics. The Ni content exceeding 1% would,
however, destabilize the secondary recrystallization in
the process of finish annealing (step S6). Accordingly,
the Ni content is set to 1% or less, preferably 0..3% or
less. From the viewpoint of improving the magnetic
characteristics such as decreasing the iron loss, the Ni
content is preferably 0.020 or above.
[0031]
Bi, B, Ti, and Te contribute to stabilize the
deposit such as sulfide, and to enhance their functions
as the inhibitor. The Bi content exceeding 0.01% would,
however, adversely affect the formation of the glass
coating. The same will apply also for the case where the
B content exceeds 0.01%, where the Ti content exceeds
0.01%, and where the Te content exceeds 0.01%.
Accordingly, the Bi content, the B content, the Ti
content, and the Te content are set to 0.01% or less.
From the viewpoint of enhancing the inhibitor, the Bi
15
content, B content, Ti content, and Te content are
preferably 0.0005° or above.
[0032]
The silicon steel slab may further contain elements
other than those described in the above, and/or, other
unavoidable impurities, so long as the magnetic
characteristics will not be degraded.
[0033]
Next, conditions of the individual. steps in this
embodiment will be explained.
[0039]
In the heating of the slab in step Sl, the silicon
steel slab is heated at 1280°C or below. In other words,
the slab is heated by so-called low-temperature slab
heating in this embodiment. In an exemplary process of
manufacturing the silicon steel slab, a steel containing
the above-described components is melt in a converter or
electric furnace to thereby obtain a molten steel. Next,
the molten steel is degassed in vacuo as necessary, which
is followed by continuous casting of the molten steel,
or, ingot casting, blooming and rolling. Thickness of
the silicon steel slab is typically 150 mm to 350 mm, and
preferably 220 mm to 280 mm, The silicon steel slab may
alternatively be formed into a thin slab of 30 mm to 70
mm thick, When the thin slab is used, rough rolling
preceding the finish rolling in the hot rolling (step S2)
may be omissible.
[0035]
16
By setting the temperature of heating at 1280°C or
below, the precipitates in the silicon steel slab may
fully be precipitated, the geometry thereof may be made
uniform, and thereby formation of skid mark is avoidable.
The skid mark is a typical expression of an incoil
variation of the secondary recrystallization behavior.
By the strategy, also various problems associated with
heating at higher temperatures (so-called hightemperature
slab heating) are avoidable. Problems
associated with the high-temperature slab heating include
necessity of a dedicated heating furnace, and a large
amount of scale generated during melting.
[0036]
The lower the temperature of heating slab, the
better the magnetic characteristics. While the lower
limit value of the temperature of heating slab is
therefore not specifically limited, too low temperature
of heating would make the hot rolling, subsequent to the
heating of the slab, difficult and would thereby degrade
the productivity. Accordingly, the temperature of
heating slab is preferably set to 1280°C or below, taking
the productivity into account.
[0037]
In the hot rolling in step S2, for example, the
silicon steel slab is subjected to rough rolling, and
then subjected to finish rolling. For the case where the
thin slab is used as described in the above, the rough
rolling may be omissible. In this embodiment, the finish
temperature of finish rolling is set to 950°C or below.
17
By setting the finish temperature of the finish rolling
to 950°C or below, as clearly known from the results of a
first experiment described later, the magnetic
characteristics may be improved in an effective manner.
[0038]
(First Experiment)
Now, a first experiment will be explained. In the
first experiment, relation between the finish temperature
of the finish rolling in hot rolling and the magnetic
flux density B8 was investigated. The magnetic flux
density B8 herein is defined by the one observed when the
grain--oriented electrical steel sheet is applied with a
magnetic field of 800 Alm at 50 Hz.
[0039]
First, a silicon steel slab of 40 mm thick
containing, in % by mass, Si: 3.24%, C: 0.054%, acid---
soluble Al: 0.028%, N: 0.006%, Mn: 0.05%, and S: 0.007%,
and composed of the balance of Fe and unavoidable
impurities, was manufactured. Then, the silicon steel
slab was heated at 1150°C, and then subjected to hot
rolling to obtain a hot-rolled steel strip of 2.3 mm
thick. The finish temperature of the finish rolling
herein was varied in the range from 750°C to 1020°C. A
cumulative reduction in the finish rolling was set to
94.3%, and a cumulative reduction in the last three
passes in the finish rolling was set to 45%. The cooling
was started one second after the completion of the finish
rolling, and the steel strip was coiled at a coiling
temperature of 540°C to 560°C, Cooling rate over the
18
duration from the start of cooling up to the coiling was
set to 16°C/sec.
[0040]
Then, the hot-rolled steel strip was annealed. In
this annealing, the hot-rolled steel strip was heated at
a heating rate of 7,2°C/sec over the duration in which
the hot-rolled steel strip was in the temperature range
from 800°C to 1000°C, and kept at 1100°C. Thereafter,
the steel strip after the annealing was cold rolled down
to a thickness of 0423 mm, to thereby obtain a coldrolled
steel strip. Subsequently, the cold-rolled steel
strip was subjected to decarburization annealing at 850°C
so as to proceed the primary recrystallization, and then
further annealed in an ammonia--containing atmosphere for
nitiridinge By the nitriding, the N content of the steel
strip was increase up to Oo019o by mass.. Next, the steel
strip was coated with an annealing separating agent
containing MgO as a main component, and then subjected to
finish annealing at 1200°C for 20 hours, to thereby allow
the secondary recrystallization to proceed.
[0041]
The magnetic flux density B8 of the steel strip
after the finish annealing was measured as the magnetic
characteristic. In the measurement of magnetic flux
density B8, "Methods of measurement of the magnetic
properties of magnetic steel sheet and strip by means of
a single sheet tester" (SST test) specified by JIS C2556
was adopted, with a single sheet sample of 60 mmx300 mm,
Results are illustrated in FIG. 2. It is known from FIG.
19
2 that a magnetic flux density of as high as 1.91 T or
above may be obtained at a finish temperature of the
finish rolling of 950°C or below.
[0042]
While the reason why a large magnetic flux density
may be obtained by setting the finish temperature of the
finish rolling to 950°C or below is not fully clarified,
it is supposed as follows. If strain is accumulated in
the steel strip during the hot rolling, and if the finish
temperature of the finish rolling is set to 950°C or
below, the strain is maintained. As the strain
accumulates, in the process of decarburization (step S5),
the primary recrystal.lizat.ion structure (texture) which
contributes to generate crystal grains with the Goss
orientation is obtained. The primary recrystallization
structure contributive to generation of the crystal
grains with the Goss orientation is exemplified by a
texture with the (lll}<1l2> orientation.
[0043]
The lower the finish temperature of the finish
rolling, the better the magnetic characteristics.
Accordingly, while the lower limit value of the finish
temperature is not specifically limited, too low finish
temperature would make the finish rolling difficult to
thereby degrade the productivity. It is therefore
preferable to set the finish temperature to 950°C or
below taking the productivity into account. For example,
the finish temperature is preferably set to 750°C or
above, and 900°C or below.
20
[0044]
A cumulative reduction in the finish rolling is
preferably set to 93% or above. This is because, by
setting the cumulative reduction in the finish rolling to
93% or above, the magnetic characteristics may he
improved. The cumulative reduction in the last three
passes is preferably set to 40% or above, and more
preferably 45% or above. This is because, also by
setting the cumulative reduction in the last three passes
to 40% or above, and particularly 45% or above, the
magnetic characteristics may be improved. This is also
supposedly because the accumulation of strain introduced
by the hot rolling increases with the elevation of the
cumulative reduction. From the viewpoint of rolling
capacity and so forth, the cumulative reduction in the
finish rolling is preferably set to 970 or less, and the
cumulative reduction in the last three passes is
preferably set to 60% or less.
[0045]
In this embodiment, the cooling is started within 2
seconds after completion of the finish rolling. If the
interval from the end of finish rolling up to the start
cooling exceeds 2 seconds, the recrystallization would
tend to proceed nonuniformly, while being associated with
variation in temperature in the longitudinal direction
(rolling direction) and the width-wise direction of the
steel strip, and thereby the strain having been
accumulated increasingly by the hot rolling is
unfortunately released. Accordingly, the interval from
21
the end of finish rolling up to the start of cooling is
set to 2 seconds or shorter.
[0046]
In this embodiment, the steel strip is coiled at a
temperature of 700°C or below. In other words, the
coiling temperature is set to 700°C or lower. If the
coiling temperature exceeds 700°C, the recrystallization
would tend to proceed nonuniformly, while being
associated with variation in temperature in the
longitudinal direction (rolling direction) and the widthwise
direction of the steel strip, and thereby the strain
having been accumulated increasingly by the hot rolling
is unfortunately released. Accordingly the coiling
temperature is set to 700°C or lower,
[0047]
The lower the coiling temperature, the better the
magnetic characteristics. Accordingly, while the lower
limit value of the coiling temperature is not
specifically limited, too low coiling temperature would
increase the interval up to the start of coiling, to
thereby degrade the productivity. Accordingly, the
coiling temperature is preferably set to 700°C or below
taking the productivity into account. For example, the
coiling temperature is preferably set to 450°C or above,
and 600°C or below.
[0048]
In this embodiment, the cooling rate (for example,
average cooling rate) in the duration from the completion
of the finish, rolling up to the, start of the coiling is
22
set to 10°C/sec or above. If the cooling rate is smaller
than 10°C/sec, the _recrystallization would tend to
proceed nonuniformly, while being associated with
variation in temperature in the longitudinal direction
(rolling direction) and the width--wise direction of the
steel strip, and thereby the strain having been
accumulated increasingly by the hot rolling is
unfortunately released. Accordingly, the cooling rate is
set to 10°C/sec or above. While the upper limit value of
the cooling rate is not specifically limited, it is
preferably set to 10°C/sec or above, taking capacity of a
cooling facility and so forth into account.
[0049]
In the annealing in step S3, in continuous
annealing, for example, the heating rate (for example,
average heating rate) in the temperature range of the
hot-rolled steel strip from 800°C to 1000°C is set to
5°C/sec or above. By setting the heating rate in the
temperature range from 800°C to 1000°C to 5°C/sec or
above, the magnetic characteristics may be improved in an
effective manner, as will be clear from a second
experiment described in the next.
[0050]
(Second Experiment)
Now, a second experiment will be explained. In the
second experiment, relation between the heating rate in
the annealing (step S2) and the magnetic flux density B8
was investigated.
[0051]
23
First, a silicon steel slab of 40 mm thick
containing, in % by mass, Si: 3.25%, Ca 0.057%, acidsoluble
Al: 0.02V%, N: 0.004%, Mn: 0.06%, S: 0.011%, and
Cu: 0.1%, and composed of the balance of Fe and
unavoidable impurities was manufactured. Then, the
silicon steel slab was heated at 1150°C, and then
subjected to hot rolling to obtain a hot-rolled steel
strip of 203 mm thick. The finish temperature of the
finish rolling herein was set to 830°C. The cumulative
reduction in the finish rolling was set to 94.3%, and the
cumulative reduction in the last three passes in the
finish rolling was set to 45%. The cooling was started
one second after the completion of the finish rolling,
and the steel strip was coiled at a coiling temperature
of 530°C to 550°C. Cooling rate over the duration from
the start of cooling up to the coiling was set to
16°C/sec,
[0052]
Then, the hot... rolled steel strip was annealed. In
this annealing, the hot-rolled steel strip was heated at
a heating rate of 3°C/sec to 8°C/sec over the duration in
which the hot-rolled steel strip was in the temperature
range from 800°C to 1000°C, and kept at 1100°C.
Thereafter, the steel strip after the annealing was cold
rolled down to a thickness of 0023 mm, to thereby obtain
a cold-rolled steel strip. Subsequently, the cold-rolled
steel strip was subjected to decarburization annealing at
850°C so as to proceed the primary recrystallization, and
then further annealed in an ammonia-containing atmosphere
24
for nitiriding. By the nitriding, the N content of the
steel strip was increased up to 0.017% by mass. Then,
the steel strip was coated with an annealing separating
agent containing MgO as a main component, and then
subjected to finish annealing at 1200°C for 20 hours, to
thereby allow the secondary recrystallization to proceed.
[0053]
Then, similarly to the first experiment, the
magnetic flux density B8 of the steel strip after the
finish annealing was measured as the magnetic
characteristic. Results are illustrated in FIG. 3. It is
known from FIG, 3 that, by setting the heating rate of
the hot--rolled steel strip in the temperature range from
800°C to 1000°C of 5°C/sec or above, a magnetic flux
density B8 of as high as 1091 T or above may be obtained.
[0054]
While the reason why a large magnetic flux density
may be obtained by setting the heating rate to 5°C/sec or
above is not fully clarified, it is supposed as follows„
That is, by the rapid heating at 5°C/sec or above, it is
supposed that the strain accumulated during the hot
rolling may effectively be used for promoting refining of
the crystal grains, and thereby a texture contributive to
generation of the crystal grains with the Goss
orientation may be obtained.
[0055]
While the annealing temperature in step S3 is not
specifically limited, it is preferably set to 1000°C to
1150°C, in order to clear non--uniformity in the crystal
25
structure and dispersion of deposit due to difference in
temperature history caused in the hot rolling. The
annealing temperature exceeding 150°C would dissolve the
inhibitor. From these points of view, the annealing
temperature is preferably set to 1050°C or above, and is
also preferably set to 1100°C or below.
[0056]
It is preferable that the number of times of
repetition of the cold rolling in step S4 is
appropriately selected depending on required
characteristics and cost of the grain-oriented electrical
steel sheet to be manufactured. The final cold rolling
ratio is preferably set to 80% or above, This is for the
purpose of promoting orientation of the primary
recrystallized grains such as in {11l} in the process of
decarburization annealing (step S5), and of increasing
the degree of integration of the secondary recrystallized
grains with the Goss orientation.
[0057]
The decarburization annealing in step S5 is
proceeded in a moist atmosphere, for example, in order to
remove C contained in the cold-rolled steel strip.
During the decarburization annealing, the primary
recrystallization occurs. While temperature of the
decarburization annealing is not specifically limited,
setting it to 800°C to 900°C, for example, the grain
radius achieved in the primary recrystallization is
approximately 7 pm to 18 pm, which ensures more stable
expression of the secondary recrystallizat.ione In other
26
words, a more excellent grain--oriented electrical steel
sheet may be manufactured,
[0058]
The nitriding treatment in step S7 is proceeded
before the secondary recrystallization occurs during the
finish annealing in step S6. By the nitriding, N is
allowed to intrude into the steel strip, so as to form
(Al,Si)N, which functions as the inhibitor. By the
formation of (Al,S.i.)N, the grain-oriented electrical
steel sheet with a large magnetic flux density may be
manufactured in a stable manner. The nitriding may be
exemplified by a process of annea=ling, subsequent to the
decarburization annealing, in an atmosphere containing a
gas with a nitriding ability such as ammonia; and a
process of adding a powder having a nitriding ability
such as MnN to the annealing separating agent so as to
accomplish the nitriding during the finish annealing.
[0059]
In step S6, the annealing separating agent
containing magnesia as a main component, for example, is
coated over the steel strip, followed by the finish
annealing, to thereby allow the crystal grains with the
{110}<001> orientation (Goss orientation) to
predominantly grow by the secondary recrystallization,
[0060]
As described in the above, in this embodiment, the
finish temperature of the finish rolling in the hot
rolling (step S2) is set to 950°C or below, the cooling
is started within 2 seconds after the completion of the
27
finish rolling, the coiling is conducted at a temperature
of 700°C or below, the heating rate in the temperature
range of 800°C to 1000°C in the process of annealing
(step S3) is set to 5°C/sec or above, and the cooling
rate over the duration from the end of finish rolling up
to the start of coiling is set to 10°C/sec or above. By
combining these various conditions, an excellent level of
magnetic characteristics may be obtained. The reason
why, partially described in the above, is supposedly as
follows.
[0061]
By setting the finish temperature of the finish
rolling to 950°C or below, the interval up to the start
of cooling to 2 seconds or shorter, the cooling rate to
10°C/sec or above, and the coiling temperature to 700°C
or below, strains accumulated during the hot rolling is
maintained, and thereby recrystallization is suppressed
up to the start of annealing (step S3). In other words,
the rolling strain is maintained through work hardening
by rolling and suppression of recrystallization. In
addition, by setting the heating rate in the temperature
range from 800°C to 1000°C to 5°C/sec or above, refining
of the recrystallized grains is promoted. By the
continuous annealing, variation in temperature in the
longitudinal direction (rolling direction) and in the
widthwise direction may be suppressed, to thereby allow
a uniform recrystallization to proceed. In the process
of decarburization annealing (step S5) subsequent to cold
rolling (step S4), the primary recrystallization occurs,
28
in which crystal grains with the {ll1)<112> orientation
are likely to grow from the vicinity of the grain
boundary. The crystal grains with the {111}<112>
orientation contributes to predominant growth of crystal
grains with the {110}<001> orientation (Gass
orientation). In other words, a good primary
recrystallization structure may be obtained.
Accordingly, when the secondary recrystallization occurs
during the finish annealing (step S6), a structure
accumulated in the {110}<001> orientation (Goss
orientation) and very suitable for improving the magnetic
characteristics may be obtained in a stable manner.
EXAMPLE
[0062]
Next, experiments conducted by the present inventors
will be explained. Conditions in these experiments were
adopted merely for the purpose of confirming feasibility
and effects of the present invention, so that the present
invention is by no means limited thereto.
[0063]

In Example 1, silicon steel slabs of 40 mm thick
were manufactured using steels Si to S7 each containing
the components listed in Table 1, and composed of the
balance of Fe and unavoidable impurities. Next, each
silicon steel slab was heated at 1150°C, and then hotrolled
to obtain a hot-rolled steel strip of 2,3 mm
thick. In this process, the finish temperature of the
finish rolling was varied in the range from £345°C to
29
855°C. The cumulative reduction in the finish rolling
was set to 94%, and the cumulative reduction in the last
three passes in the finish rolling was set to 45%. The
cooling was started one second after the completion of
the finish rolling, and the steel strip was coiled at a
coiling temperature of 490°C to 520°C, The cooling rate
over the duration from the start of cooling up to the
coiling was set to 13°C/sec to 14°C/sees
[0064]
Then, each hot-rolled steel strip was annealed. In
this annealing, the hot rolled steel strip was heated at
a heating rate of 7°C/sec over the duration in which the
hot-rolled steel strip was in the temperature range from
800°C to 1000°C, and then kept at. 1100°C, Thereafter,
the steel strip after the annealing was cold-rolled down
to a thickness of 0.23 mm, to thereby obtain a cold--
rolled steel strip. Subsequently, the cold-rolled steel
strip was subjected to decarburizati.on annealing at 850°C
so as to allow the primary recrystallization to occur,
followed by annealing in an ammonium-containing
atmosphere for nitriding. By the nitriding, the N
content of the steel strip was increased up to 0.,016% by
mass. Next, the steel strip was coated with an annealing
separating agent containing MgO as main component, and
then subjected to finish annealing at 1200°C for 20
hours, to thereby allow the secondary recrystallization
to occur.
[0065]
30
Then, similarly as described in the first experiment
and the second experiment, the magnetic flux density B8
of the steel strip after the finish annealing was
measured as the magnetic characteristic. Results are
listed in Table 2.
[0066]
[Table 1]
TABLE I
S T EEL C HEMICAL COMPONENT ( SS )
C Si Mn ACID-SCLUELE Al N S Sc Se g. Cu Cr- P Sn Sys Ni Bi
s l 0.065 3.25j 0.11 0.026 0.007 1 0.003 - 0.008 0 2 - - - - - -
S2 0.061 3.25 0.11 0.02 7 0.007 0.007 0.007 0.1 - - - - - I'
S3 0.060 3.23 0.11 0.027 0.009 Y 0.007 - 0.007 - - 0 1 - - - -
S4 0.064 3.24 0.11 0.028 0.006 0.007 - 0.007 - -
.
- 0.1 - - -
S5
S6
0.061
0.059
3.23
3.25
0.11
0.11
0,026
0.025
0.008
0.007
0.006
0.007
0.005
-
0.008 -
0.007 -
-
-
-
-
-
-
0.^ I
-
-
0 2
-
-
S7 ` 0.062 3.24 0 i 1 0.027 0.008 0.007 - 0.001 - - - - -
.
- 0.006
NOTE) MEANS THECHE ICANLCOMPONENT IS NOT INTENTIONALLY ADDED
LE 2
CONDITIONS OF CONDITIONS OF
CONDITIONS OF FINISH ROLLING COOLING AFTER HOT-ROLLED STEEL
FINISH ROLLING COILING ANNEALING MAGNETIC
E YPLE
No
STEEL
CUMULATIVE
CUMULATIVE
REDUCTION
FINISH TIME TO AVERY GE
TEMPERATURE
HEATING ANNEALING
FLUX
. DENSITY
REDUCTION IN THE LAST TEMPS START OF COOLING (° RATE TEMPERATURE E3 (T )
THREE PASSES
RATURE
a
COOLING RATE
'°^•/SEC)
(SEC) /S EC)
s - S1 94 45 848 1 14 500 7 1100 1.932
` -2 S2 94 45 854 3 490 7 d 1 00 1 .929
r; -3 S3 94 45 651 1 3 520 7 1100 1.930
S4 94 45 347 1
f
14 500 7 1100 1.932
75 S5 94 45 855 1 13 510 7 1100 1.930
1 -0 S3 94 45 349 4 520 7 1100 1 .929
1-7 S7 94 9 45 852 6' 1 14 500 7 1 100 1.932
[0068]
As is known from Table 2, samples No, 1-1 to No. 1--
7, all satisfying the conditions specified by the present
invention, were found to show large values of magnetic
flux density B8.
[0069]

In Example 2, silicon steel slabs of 40 mm thick
were manufactured using a steel S11 containing the
components listed in Table 1, and composed of the balance
of Fe and unavoidable impurities. Then, each silicon
steel slab was heated at 1150°C, and then hot-rolled to
obtain a hot-rolled steel strip of 2.3 mm thick. In this
process, the cumulative reduction in the finish rolling,
the cumulative reduction in the last three passes, and
the finish temperature were set as listed in Table 4.
Each steel strip was started to cool after the elapse of
time listed in Table 4 after completion of the finish
rolling, and coiled at a coiling temperature listed in
Table 4. The interval from the start of cooling up to
the coiling was set to any of the values listed in Table
4,
[0070]
Then, each hot-rolled steel strip was annealed. In
this annealing, the heating rate over the duration in
which the hot-rolled steel strip was in the temperature
range from 800°C to 1000°C, was set to any of the values
listed in Table 4, and kept at 1100°C. Thereafter, the
steel strip after the annealing was cold rolled down to a
34
thickness of 023 mm, to thereby obtain a cold--rolled
steel strip. Subsequently, the cold-rolled steel strip
was subjected to decarburization annealing at 850°C so as
to proceed the primary recrystallization, and then
further annealed in an ammonia-containing atmosphere for
nitiriding. By the nitriding, the N content of the steel
strip was increase up to 0.016% by mass. Then, the steel
strip was coated with an annealing separating agent
containing MgO as a main component, and then subjected to
finish annealing at 1200°C for 20 hours, to thereby allow
the secondary recrystallization to occur.
[0071]
Then, similarly as described in Example 1, the
magnetic flux density B8 of the steel strip after the
finish annealing was measured as the magnetic
characteristic. Results are listed in Table 4, together
with the results of Example 1,
[0072]
[Table 3]
TABLE 3
STEEL
CHEMICAL COMPONENT (MASS%)
0- Si Mn ACI®-SOLUBLE Al N Se
S11 LL 0.062 '^.2& 0 .9 0.020 0.0083 0.007
[0073]
[Table 4]
35
TABLE /1
CONDITIONS OF FINISH ROLLING
CONDITIONS OF
COOLING AFTER
FINISH POLLING COILING
CONDITIONS OF
HOT-ROLLED STEEL
ANNEALING MAGNETIC
SAMPLE
No.
STEEL
CUMULATIVE
REDUCTION
(%)
CUMULATIVE
REDUCTION
IN THE LAST
THREE PASS
FINISH
TEMPERATURE
(°G
TIME TO
START OF
COOLING
(SEC)
AVERAGE
COOLI JG
RATE
(°G /SEC)
TE^9PERATURE
(`C)
HEATING
RATE
( 'G /SEC)
ANNEALING
TEMPERATURE
(°C)
FLUX
DENSITY
B8 (T)
1-1 61 94 45 846 14 500 7 1 100 1.932
1-2
1-3
S2
S3
94
94
45
45
654
851
1
1
13
13
490
520
7
7
1100
1100
1.929
1.930
1-4 S4 94 45 847 1 14 500 7 1100 1 .932
1-5 S5 94 45 855 1 13 510 7 1100 1.930
1-6 56 94 45 849 1 14 520 7 1100 1.929
1-7 ST 94 45 852 14 500 7 1100 1.932
A ,P E 2-1 S11 92 38 754 1 13 500 ^ L S 7 1100 1.935
2-2 51`1 92 38 947 14 .680 7 1100 1.912
2-3 S11 92 36 661 2 14 570 7 1100 1-91 5
2-4 Ell 92 38 822 1 10 650 7 1100 1.928
2-5 S11 l 92 38 906 1 g 11 700 7 1100 1.919
2-6 S11 92 38 875 1 14 640 5 1100 1.918
2-7 fi S11 93 38 818 1 14 540 7 1100 1.933
2-8 S11 94 40 821 1 13 550 7 1100 1.934
2-9 S11 94 45 757 1 14 510 7 1100 1.936
2-11 Ell 92 36 958 "1 1 14 680 7 1100 1.905
CO _IP .RAIN. 2-12 ^I S11 92 38 840 3 14 630 7 1100 1.888
L 2-13 611 92 38 901 1 7 680 7 1100 1.891
2-14 S1i 92 38 842 2 10 750 7 1100 1,897
2-15 S11 92 38 837 1 14 590 3 1100 1 1.904
[0074]
As is known from Table 4, samples No, 2-1 to N
9, all satisfying the conditions specified by the present
invention, were found to show large values of magnetic
flux density B8. On the other hand, samples No. 2-11 to
Noe 2-15, all do not satisfies any of the conditions
specified by the present invention, were found to show
small values of magnetic flux density B8.
[0075]
It should be noted that the above embodiments merely
illustrate concrete examples of implementing the present
invention, and the technical scope of the present
invention is not to be construed in a restrictive manner
by these embodiments. That is, the present invention may
be implemented in various forms without departing from
the technical spirit or main features thereof.
INDUSTRIAL APPLICABILITY
[0076]
The present invention is applicable, for example, to
industries related to manufacturing of electrical steel
sheet and industries using electrical steel sheet.
37

CLAIMS
[Claim 1] A method of manufacturing a grain-oriented
electrical steel sheet comprising:
heating a silicon steel slab at 1280°C or below, the
silicon steel slab containing, in % by mass, Si: 0.8%
7%, and acid-soluble Al: 0.01% to 0.065%, with a C
content of 0.085% or less, a N content of 0.012% or less,
a Mn content of 1 % or less, and a S equivalent Seq.,
defined by "Seq,-[S]+0.406x[Se]" where [S] being S
content (%) and [Se] being Se content (%), of 0.015% or
less, and the balance of Fe and unavoidable impurities;
hot rolling the heated silicon steel slab so as to
obtain a hot.-rolled steel strip;
annealing the hot--rolled steel strip so as to obtain
an annealed steel strip;
cold rolling the annealed steel strip so as to
obtain a cold-rolled steel strip;
decarburization annealing the cold-rolled steel
strip so as to obtain a decarburization-annealed steel
strip in which primary recrystallization is caused;
coating an annealing separating agent on the
decarburization-annealed steel strip; and
finish annealing the decarburization-annealed steel
strip so as to cause secondary recrystallization, wherein
the method further comprises performing a nitriding
treatment in which a N content of the decarburizationannealed
steel strip is increased between start of the
decarburization annealing and occurrence of the secondary
recrystallizaLion in the finish annealing,
38
the hot rolling the heated silicon steel slab
comprises:
finish rolling with a finish temperature of 950°C or
below; and
starting cooling within 2 seconds after completion
of the finish rolling, and coiling at '700°C or below,
a heating rate of the hot rolled steel strip within
the temperature range from 800°C to 1000°C in the
annealing the hot-rolled steel strip is 5°C/sec or above,
and
a cooling rate over a duration from the completion
of the finish rolling up to a start of the coiling is
10°C/sec or above.,
[Claim 2] The method of manufacturing a grain--oriented
electrical steel sheet according to Claim 1, wherein a
cumulative reduction in the finish rolling is 93% or
above.
[Claim 3] The method of manufacturing a grain-oriented
electrical steel sheet according to Claim 1, wherein a
cumulative reduction in the last three passes in the
finish rolling is 40% or above.
[Claim 4] The method of manufacturing a grain-oriented
electrical steel sheet according to Claim 2, wherein a
cumulative reduction in the last three passes in the
finish rolling is 40% or above.
[Claim 5] The method of manufacturing a grain-oriented
electrical steel sheet according to Claim 1, wherein the
silicon steel slab further contains Cu: 044% by mass.
39
[Claim 6] The method of manufacturing a grain-oriented
electrical steel sheet according to Claim 2, wherein the
silicon steel slab further contains Cu: 0.4% by mass.
[Claim 7] The method of manufacturing a grain-oriented
electrical steel sheet according to Claim 3, wherein the
silicon steel slab further contains Cu: 0.4% by mass.
[Claim 8] The method of manufacturing a grain oriented
electrical steel sheet according to Claim 4, wherein the
silicon steel slab further contains Cu: 0,.4% by mass,
[Claim 9] The method of manufacturing a grain--oriented
electrical steel sheet according to Claim 1, wherein the
silicon steel slab further contains, in % by mass, at
least one selected from the group consisting of Cr: 0,3%
or less, P: 005% or less, Sn: 003% or less, Sb : 043% or
less, Ni: 1% or less, Bi: 0.01° or less, B: 0001% or
less, Ti: 0001% or less, and Teo 0.01% or less.
[Claim 10] The method of manufacturing a grain-oriented
electrical steel sheet according to Claim 2 wherein the
silicon steel slab further contains, in % by mass, at
least one selected from the group consisting of Cr- 0.3%
or less, Bo 0,5% or less, Sn: 0.3% or less, Sb: 0.3% or
less, Ni: 1% or less, Bi: 0,01% or less, B: 0°01% or
less, Ti: 0.01% or less, and Tee 0.01% or less.
[Claim 11] The method of manufacturing a grain oriented
electrical steel sheet according to Claim 3 wherein the
silicon steel slab further contains, in % by mass, at
least one selected from the group consisting of Cr: 0.3%
or less, P. 0.5% or less, Sri, 003% or less, Sb: 003% or
40
less, Ni: 1% or less, Bi: 0.01% or less, B: 0.01% or
less, Ti: 00010 or less, and Teo 0.01% or less.
[Claim 12] The method of manufacturing a grain-oriented
electrical steel sheet according to Claim 4 wherein the
silicon steel slab further contains, in % by mass, at
least one selected from the group consisting of Cr: 0030
or less, P: 0.5% or less, Sn: 0.3% or less, Sb: 0430 or
less, Ni: 1% or less, Bi: 0°010 or less, B: 0.01% or
less, Ti: 0.01% or less, and Ten 0®0l% or less.
[Claim 13] The method of manufacturing a grain-oriented
electrical steel sheet according to Claim 5 wherein the
silicon steel slab further contains, in % by mass, at
least one selected from the group consisting of Cr: 003%
or less, P: 0,50 or less, Sn: 0030 or less, Sb: 0,3% or
less, Ni: 1% or less, Bi: 00010 or less, B: 0.01% or
less, Ti: 0.01% or less, and Te: 0>O1% or less.
[Claim 14] The method of manufacturing a grain-oriented
electrical steel sheet according to Claim 6 wherein the
silicon steel slab further contains, in % by mass, at
least one selected from the group consisting of Cr: 0.3%
or less, P: 0.5% or less, Sne 0.3% or less, Sb: 0.3% or
less, Ni: 1% or less, 00010 or less, B: 0°01% or
less, Ti: 0001% or less, and Teo 0.01% or less.
[Claim 15] The method of manufacturing a grain-oriented
electrical steel sheet according to Claim 7 wherein the
silicon steel slab further contains, in % by mass, at
least one selected from the group consisting of Cr: 0.3%
or less, P: 005% or less, Sn: 0930 or less, Sb: 003% or
41
less, Ni: 1% or less, Bi: 0,01% or less, B: 0.01% or
less, Ti:: 0.01% or less, and Teo 0..01% or less..
[Claim 16] The method of manufacturing a grain--oriented
electrical steel sheet according to Claim 8 wherein the
silicon steel slab further contains, in % by mass, at
least one selected from the group consisting of Cr: 0.3%
or less, P: 0.5% or less, Sn: 0.3% or less, Sb: 0.3% or
less, Ni: 1% or less, Bi: 0.01% or less, B: 0.01% or
less, Ti: 0.01% or less, and Te°. 0.01% or less.