Note: Descriptions are shown in the official language in which they were submitted.
7~
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METHOD FOR PRODUCING A GRAIN-ORIENTED
ELECTRICAL STEEL SHEET HAVING
A HIGH MAGNETIC FLUX DENSITY
,
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a method for
producing a grain-oriented electrical steel sheet having
a high magnetic flux density.
Description of the Prior Art
Grain-oriented electrical steel sheet is a soft
magnetic material composed of crystal grains having a so
called Goss texture, expressed by {lOQ}<001> by the
Miller index in which the crystal orientation of the
sheet plane is the {110} plane and the crystal
orientation of the rolling direction is parallel to the
<001> axis. Grain-oriented electrical steel sheet is
used for cores of transformers, generators, and other
electrical machinery and devices.
Grain-oriented electrical steel sheet must have
excellent magnetization and watt loss characteristics.
The magnetization characteristic is defined by the
magnitude of the magnetic flux density lnduced in the
grain-oriented electrical steel sheet by a predetermined
magnetic field~ Here, Blo is used~ Soft magnetic
material having a high magnetic flux density, i.e.~ a
good magnetization characteristic, can advantageouslv
7~
-- 2 --
reduce the size of the electrical machinery and devices.
Watt loss is defined as powar lost due to
consumption as thermal energy in a core when it is
energized by an alternating magnetic field ha~ing a
5 predetermined intensity. Here, W17/50 is used- As is
known, the watt loss characteristic is influenced by the
magnetic flux density, sheet thickness, the impurities,
resistivity, and grain size of the grain-oriented
electrical steel sheet. Insreased demand has arisen for
grain-oriented electrical steel sheet having a low watt
loss along with the trend toward enegy conservation.
Grain-oriented electrical steel sheet is produced
by hot-and-cold rolling a slab to the desired final
sheet thickness and then finally annealing the resultant
steel strip to realize selective growth of the {110}<001>
oriented primary-recrystallized grains, i.e., to realize
so-called secondary recrystallization~
To realize secondary recrystallization, fine
precipitates, such as MnS and AlN, must be finely and
~0 uniformly dispersed in phases in the steel, while the
steel is subjected to processes prior to the final high
temperature annealing, so as to suppress growth of
primary recrystallized grains having orientations other
than the {110}<001> orientation during the final high
temperature annealing (inhibitor effect) controlling the
secondary recrystallization, it is possible to increase
the proportion of the accurately {110}<001> oriented
grains in the crystal grains, thereby increasing the
` ~Z~67~
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magnetic flux denslty of the grain-oriented electrical
steel sheet and, thus, reducing the watt loss. It is
important to develop production techniques allowiny
control of the secondary recrystallization.
Japanese Examined Patent Publication (Kokoku)
No. 40-15644 (Taguchi et al) and Japanese Examined
Patent Publication ~Kokoku) No. 51-13469 (Imanaka et al)
disclose basic techniques fox producing a grain-oriented
electrical steel sheet having a high magnetic flux
desity and decreased watt loss.
The basic techniques disclosed in the above two
Japanese examined patent publications however suffer
from some fundamental problems. In the method disclosed
in Japanese Examined Patent Publication No. 40-15644, it
is di~ficult to achieve overall optimum production
condition and to stably produce grain-oriented electrical
steel sheets having high magnetic flux density. As a
result, the method is not appropriate for the stable
production of products having the best magnetic
properties~
The method disclosed in Japanese Examined Patent
Publication No. 51-13469 involves double cold rolling
and use of an expensive element, such as Sb or Se. This
method therefore involves high production costs.
Alsot both the prior art methods require high slab
heating temperatures, disadvantageous from the viewpoint
of the eneryy used for heating the slab, decreased yield
due to slag generation and increased repair costs of
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slab-heating furnaces.
When heating a slab to make it rollable, one must
raise the slab heating temperature high enough to
solid-dissolve MnS, and other inhibitor elements. These
later precipitate as MnS, AlN, and the like, when the
steel is hot-rolled or subjected to hot-strip annealing.
The greater the degree of orientation desired, the
larger the amount of MnS, AlN, and other fine
precipitates that must be present in the steel and,
therfore, the higher the necessary slab heating
temperature. Japanese Unexamined Patent Publication
No. 48-51852 discloses an improvement of the method of
Japanese Examined Patent Publication No. 40-15644. In
this method, the Si content of the starting material is
increased. A high silicon content however, narrowly
restricts the conditions under which AlN can be ensured
in the hot-rolled strip~ Also, since the silicon
content is high, the temperature range at which AlN
precipitates during hot-rolling in an appropriate manner
for the secondary recrystallization shits higher,
requiring a higher slab heating temperature.
The adoption of continuous casting has created
additional problems in the production of grain-oriented
electrical steel sheet. In continuous casting linear,
secondary-recrystallization-incomplete portions, referred
to as streaks, are occasionally generated in the steel.
This impairs the magnetic properties of the steel~ The
problem of streaks is greatly aggravated by a high Si
~ 2~
-- 5
content. When the Si content exceeds 3.0%, stable
production of grain-oriented electrical steel sheet
becomes extemely difficult. Japanese Unexamined Patent
Publication No. 48-53919 (M.F. Littman) discloses to
remove the problem of streaks by subjecting a
continuously cast steel strand at double hot-rolling
steps when producing a hot rolled strip. Japanese
Unexamined Patent Publication No. 50-37009 (Akira
Sakakura ~t al) discloses a method for producing grain-
-oriented electrical steel sheet wherein a hot-rolled
steel strip is produced by double hot-rolling steps.
These two prior art methods, however, do not fully
utilize the advantages of continuous casting, i.e.,
omission of rough rolling. Two later publications,
Japanese Unexamined Patent Publication No. 53-19913
(Morio Shiozaki et al) and Japanese Unexamined Patent
Publication No. 54-120214 (Fumlo Matsumoto et al),
disclose how to employ single hot-rolling to produce
grain-oriented electrical steel sheet using a
2Q continuously cast strand. These proposals, however~
necessitate reconstruction of a casting or rolling
installation and still do not completely solve the
problem of streak generation.
SUMMARY OF THE INVENTION
I~ is an object of the present invention to provide
a method for producing a grain-oriented electrical steel
sheet having a magnetic flux density Blo of 1.89 Tesla
or more using a single cold-rolling step, wherein a
~Z~L06~70
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stable secondary recrystallization can be obtained under
less strict condition than in prior art, even at a low
slab heating temperature, at a high Si content than in
the prior art, and/or using a continuously cast slab.
The essence of the method according to the present
invention resides in the steps of: preparing a slab
which has a temperature of 1430C or less and which
consists of from 0.025% to 0.075% of C, from 3.0~
to 4.5% of Si, from 0.010~ to 0.060~ of acid soluble
aluminum, from 0.0030~ to 0.0130% of N, not more than
0.007% of S, from 0.08% to 0.45~ of Mn, and from 0.015%
to 0.045% of P, the balance being Fe; hot rolling the
slab to form a hot-rolled strip; annealing the hot-rolled
strip at a temperature in the range of from 850C to
1200C for a short period of time; heavily cold-rolling
the annealed strip at a reduction of not less than 80~,
thereby obtaining the final sheet thickness; continuously
decarburization-annealing the obtained cold-rolled strip
in a wet hydrogen atmosphere and then applying an
annealing separator on the strip; and, carrying out a
final high temperature annealing.
One of the features according to the present
invention is the sulfur content of 0.007% or less. In
the prior art, as disclosed in Japanese Examined Patent
Publication Nos. 30-3651, 40-15644, and 47-25250, sulfur
is believed useful for producing grain-oriented
electrical steel sheet since sulfur forms MnS, one of
the indispensable precipitates for generating secondary
``~ 12~6~70
recrystallization. According to these publications, the
effect of sulfur is most prominent in a certain range of
content which is determined by the amount of solute MnS
brought into solid solution during the slab-heating
processu AlN also forms precipitates believed useful
for producing a grain-oriented electrical steel sheet.
Conventionally, both MnS and AlN pxecipitates were used
as inhibitors.
The present inventors investigated in detail the
precipitation behavior of MnS and AlN. They discovered
that when a slab having the composition of an electrical
steel sheet is heated and then hot-rolled and when a
hot-rolled strip is annealed, MnS first precipitates at
a high temperature and AlN then precipitates at a low
temperature. Since MnS is already prPsent in the steel
when AlN precipitates, AlN tends to precipitate around
the MnS, resulting in complex precipitaion. Thus, the
size ~nd dispersion state or AlN are influenced by the
precipitation states of MnS. That is, when the amount
of MnS precipitated is great, the AlN is large sized and
is dispersed non-unifomly.
As known from Japanese Unexamined Patent Publication
No. 48 51852, a fundamental metallurgical concept for
producing a grain-oriented electrical steel sheet having
a high magnetic flux density with a single cold-rolling
process is to create an appropriate dispersion state of
AlN by u~ilizing the ~ ~ y transformation which occurs
during hot-rolling or annealing. When the Si content is
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high, the ~ ~ y transfromation is disadvantageously
changed, so that dispersion of AlN is impaired. In the
case of continuous casting, this is believed to result
in generation of streaks.
Based on the above-described discoveries and
consideration of the u ) y transformation, the present
inventors decreased the precipi-tation amount of MnS.
They ~hen discovered that, even with a high content of
Si in the steel, the dispersion state of AlN can be kept
uniform and ~he AlN precipita~es be kept small in size.
One of the features according to the present
invention therefore resides in the point that the sulfur
content is lower than in the prior art. Even with this,
the precipitation of AlN can be controlled appropriately
and the generation of streaks in continuous casting
which may occur when the Si content is high, can be
prevented.
Since the sulfur content is low, the precipitation
amount of MnS according to the present invention is less
than in the prior art. The decrease in the precipitation
amount of MnS means the total amount of the inhibitors
is decreased, which tends to a decrease in the magnetic
flux density. To compensate for the decrease in the
magnetic flux desity, Mn and P are added into steel in
appropriate amount.
Another feature according to the present invention
resides in the fact that the Mn and P added to the steel
do not change the inhibitors but render the primary
3LZ~ 7~3
g
recrystallization texture appropriate before the
secondary recrystallization. That is, they compensate
for the above-mentioned decrease in magnetic flux
density and even increase in the magnetic flux density
by texture control. The crystal grains are refined and
have a uniform size, with the result that the second
recrystalli~ation is stabilized.
Another feature according to the present invention
is that Si content in the starting material is at least
3.0%, while stabilizing the secondary recrystallization
and thus preventing the generation of streaks. This
results in one of tke lowest watt losses and highest
magnetic flux density available in the high grade
grain oriented electrical steel sheet.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be described in further
detail with reference to the drawingsl wherein:
Fig. lA through lD show photographs of the
crystal grain-macrostructures of the products produced
using steels containing 0.004%, 0.007%~ 0.015%, and
0.025% of S;
Fig, 2A through 2D show photographs of the
crystal grain-macrostructures of the products produced
using continuously cast strands containing 0.004%,
0.007~, 0.012%, and, 0.030% of S;
Fig. 3 is a graph of the influence of Mn and P
upon magnetic flux density Blo;
FigO 4 is a graph of the influence of Mn and
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P upon Blo regarding a product produced by using a
continuously cast slab containing 0.0090% of ~;
Fig. 5 is a graph of the magnetic properties
of the products produced under the same conditions as in
Fig. 3 but at slab heating temperatures of bo~h 1150C
and 1350C.
Fig. 6 is a graph of the influence of a
slab-heating temperature upon the magnetic flux density
of products;
Fig. 7 is a graph of the relationship between
the magnetic flux density (Blo) of products and the
heating rate in a temperature range of from 700C
to 1100C, such heating being carried out during a final
high temperature annealing;
Fig. 8 is a graph of the relationship between
the magnetic flux ensity, the watt loss, and Cr
content; and
FigO 9 is a graph similar to Fig. 5 regarding
Cr-containing steels.
DESCRIPTION OF PREFERRED EMBODIMENTS
Four steels, in which the S contents were 0.004~,
0.007%, 0.015%, and 0.025%, respectively, and which
contained 0.030% of C, 3.45~ of Si, 0.030% of acid-
-soluble aluminum, and 0~0085% of nitrogen, were prepared
in the form of 40 mm thick small samples. They were
heated to 1200C in a furnace and then withdrawn from
the furnace, allowing them to cool in an ambient air
down to the temperature of 1000C. The four steels were
67~
then held in a furnace for 30 seconds at 1000C. The
four steels having the temperature of 1000C were
hot-rolled by three passes to form 2.3 mm thick hot-
-rolled sheets. Then, the following processes were
successively carried out: contunuous annealing at
1100C for 2 minutes; cold-rolling to form a 0.30 mm
cold-rolled sheet; decarburization-annealing in a wet
hydrogen atmosphere; application of MgO; and final high
temperature annealing at 1200C for 20 hours.
As is apparent from the crystal-grain macro-
structures of the products shown in Figs lA through
lD no incomplete secondary recrystallization occurs when
the S content is 0.007% or less. Also, according to
experiments of the present inventors, no incomplete
secondary recrystallization occurs when the Si content
was 4O5% or less and when the S content was 0.007% or
less. Accordingly, the S content is limited to 0.007%
or less in the present invention. The S content is
desirably decreased in the molten stage of melting steel
because the desulfurization treatment during the final
high temperature annealing can be facilitated. According
to the present melting techniques for decreasing sulfur,
the S content which can be easily attained without
incurring cost increases is usually 0.001% or more.
Four continuously cast slabs, in which the S
contents were 0.004%, 0.007%; 0.012%, and 0.030~,
respectively, and which contained 0.055% of C, 3.30~ of
Si, 0.25% of Mn, 0.030% of acid-soluble aluminum, and
~2~(~67~
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0.0080% of N, were heated to 1410C in a furnace and
were hot-rolled to foxm 2.3 mm thick hot-rolled sheets.
Then, the following process were successiv~ly carried
out: continuous annealing at 1150C for 2 minutes;
cold-rolling to form a 0.30 mm cold-rolled strip;
decarburization-annealing in a wet hydrogen atmosphere;
application of MgO as annealing separator; and final
high temperature annealing at 1200C for 20 hours.
Also as is apparent from the crystal-grain
macrostructures shown in Figs. 2A through 2D, streaks
are less likely to generate when the content is lower,
and streaks do not generate at all when the S content is
0.007~ or less.
Continuous cast slabs in which the Mn and P contents
were varied, and which contained 0.050~ of C, 3.40% of
Si, 0.002% of ~, 0.030% of acid-soluble aluminum, and
0.0080~ of nitrogen, were prepared in the form of 40 mm
thick small samples. They were heated to 1150C in a
furnace and were hot-rolled by three passes to form a
2.3 mm thick hot-rolled sheets. The finishing
temperature of hot rolling was approximately 820C.
Then, the following processes were successively
carried out: continuous annealing at 1100C for 2
minutes; cold-rolling to form a 0.30 mm cold-rolled
sheet, decaburization-annealing in a wet hydrogen
atmosphere; application of MgO; and final high
temperature annealing at 1200C for 20 hours.
The ma~netic flux density Blo of the products is
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shown in Fig. 3. In Fig. 3 x corresponds to Blo < 1.80
Tesla, ~ corresponds to 1.80 < Blo ~ 1.89 Tesla, o
corresponds to l.B9 < Blo < 1.91 Tesla, and corresponds
to 1.91 Tesla < Blo. As is apparent from Fig. 3, when
the Mn content is low, the secondary recrystallization
becomes unstable, and when the Mn content is high, the
magnetic flux density Blo is high. When Mn is added in
more than a certain content, however, it is ineffective
for enhancing the maynetic flux density Blo and is
uneconomical since the amount of additive alloy becomes
disadvantageously great.
When the P content is too low, the magnetic flux
density Blo is low and the generation of incomplete
secondary recrystallization is increased. When the P
lS content is too high, the frequency of cracking during
cold rolling is increased.
Thus, the Mn content is limited to th~ range of
from 0.08~ to 0.45%, and the P content is limited to the
range of from 0.015% to 0.045% according to the present
invention. In these ranges the magnetic flux density
Blo is 1.89 Tesla or more, the secondary recrystalli-
zation is stable, and the problem of cracking is not
significant.
Continuously cast slabs, in which the Mn and P
contents were varied, and which contained 0.060% of C,
3~45% o Si, 0.004% of S, 0.033~ of acid-soluble
aluminum, and 0.0090% of nitrogen, were h~ated to 1410C
and were then hot-rolled to form a 2.3 mm thick hot-
6~C~
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-rolled strips~ Then, the following processes were
suc~essively carried out: continuous annealing at 850C
for 2 minutes; cold-rolling to form 0.30 mm cold-rolled
strips; decarburization-annealing in a wet hydrogen
atmosphere; application of MgO as annealing separakor;
and final high temperature annealing at 1200C for 20
hours.
The magnetic flux density Blo of the product is
shown in Fig. 4, wherein x corresponds to Blo < 1.80
Tesla, ~ corresponds to 1.80 _ B1o < 1.89 Tesla,
o corresponds to 1.89 c Blo < 1.92 Tesla, corresponds
to 1.92 Tesla < Blo < 1.93 Tesla, and ~ corresponds to
1.93 < Blo. As is apparent from Fig. 4, when the Mn
content is too low, the secondary recrystallization
becomes uns~able, and when the Mn content is high, the
magnetic flux density Blo is high. When Mn is added in
more than a certain content, however, it is ineffective
for enhancing the magnetic flux density Blo and is
uneconomical since the amounts of additive alloy becomes
disadvantageously great.
When the P content is low, the magnetic flux
density Blo is too low and the generation of incomplete
secondary recrystallization is increased. When the P
content is too high, the frequency of cracking during
25 cold rolling is increased.
Thus, the Mn content is limited to the range of
from 0.08% to 0.45%, and the P content is limited to the
range of from 0.015% to 0.045~ according to the present
~Z~LS~6~7~
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invention. In ~hese ranges, the magnetic flux density
Blo is 1.89 Tesla or more, the secondary recrystalli~
zation is stable, and the problem of crackiny is not
significant.
Regarding the other components, steel which is
subjected to the processes according to the present
invention may be melted in a converter, electric furnace,
or open-hearth furnace, provided that the cornposition of
steel falls within the ranges described hereinafter.
lQ The C content is thus at least 0.025%. At a C
content of less than 0.025%, secondary recrystallization
is instable. Even if secondary recrystallization
occurs, the magnetic flux density is low ~Blo is 1.80
Tesla at the highest). On the other hand, the C content
is 0.075% at the highest, since the decarburization
annealing time is long and thus uneconomical when the C
content exceeds 0.075%.
The Si content is 4.5% at the highest. At an Si
content exceeding 4.5~, numerous cracks occur during the
cold-rolling. The Si content is at least 3.0~,
preferably at least 3.2%~ At an Si contQnt less than
3.0~, the highest grade watt loss, i.eO, Wl7/50 of
1.05 w/kg at the sheet thickness of 0.30 mm, cannot be
obtained.
Since in the present invention AlN is employed for
the precipitates indispensable for the secondary
recrystallization, the minimum amount of AlN must be
ensured by providing an acid-soluble Al content and N
"` 3L2~670
- 16 -
content of at least 0.010~ and 0.0030%, respectively.
The acid ~oluble Al content is 0.060% at the highest.
At an acid-soluble Al content exceeding 0.060%, the AlN
does not disperse uniformly in the hot rolled strip,
thereby resulting in poor secondary recrystallization.
The N content is 0.0130% at the highest. At an N
content exceeding 0.0130%, the blisters form on the
surface of the steel sheet.
When the steel has the composition as described
above, a high slab heating temperature excePding 1300C,
accepted as conventional practice, is not necessary.
More surprisingly~ when the present inventors heated two
slabs to a high temperature and a low temperature,
respectively, and then subjected them to the processes
for producing grain-oriented electrical steel sheets,
they found that two obtained products having an identical
magnetic flux density will have a considerably lower
watt loss when obtained by low-temperature slab heating
than that when obtained by high-temperature slab heating.
Thus, low-temperature slab heating not only enables
production cost reductions and easy used of a
continuously cast strand as the starting material, but
also a watt loss reduction.
Fig. 5 illustrates the magnetic properties of
products produced under the same conditions as those of
Fig. 3 but at slab heating temperatures of both 1150C
and 1350C. From the comparison of the two products
(1150C and 1350C~, it i5 apparent that a lower slab-
` :~L23~670
- 17 -
-heating temperature can drastically decrease the watt
loss for the same magnetic flux density.
When the slab-heating temperature i5 1280C or
less, slag does not form at all during the slab heating.
In addition, when the slab-heating temperature is 1280C
or less and when the Si content is 3.0~ or less, the
highest grade product, i.e., a product which exhibits a
watt loss W17/50 of 1.50 w/kg or less at a sheet
thickness of 0.30 mm, can be obtained.
The lowest slab heating temperature is not
specifically limited, but is desirably 1050C, since at
a temperature lower than 1050C, a great driving force
is required for hot-rolling and the shape quality of
steel strip is impaired. ~he lowest slab temperature of
1050C is therefore preferred from the viewpoint of the
industrial production of the steel.
The slab used may be any slab produced by rough
rolling or continuous casting. A continuously cast slab
is preferable due to the inherent labor saving and
yield-enhancement features of continuous casting.
Furthermore, continuous casting ensures a uniform
chemical composition in a slab, resulting in uniform
magnetic properties in the longitudinal direction of the
product.
As is described in Japanese Unexamined Patent
Publication No. 53-19913, if a continuously cast slab is
heated to a high temperature, such as approximately
1320~C, streaks generate and thus stable production
~0~7~
- 18
becomes impossible. However, since the slab-heating
temperature can be 1280C or less according to the
present invention, no incomplete secondary recrystalli-
zation occurs at all. The present invention therefore
makes it possible to provide the highest-yrade watt loss
while employing low-temperature slab heating comparable
to that of carbon steels.
Recent advances in continuous casting techniques
have raised the productivity of continuous casting
machines to equal the capacity of continuous hot-rolling
mills. Continuous casting machines can therefore now be
directly combined with continuous hot-rolling mills.
When steels are supplied from a continuous casting
machine directly to a continuous hot-rolling mill, the
continuous hot-rolling mill can carry out rolling
without a waiting period. Therefore, according to one
advantageous hot rolling method which can be used in the
present invention, a slab is not cooled after continuous
casting and i5 directly hot-rolled while utilizing the
sensible heat of the slab. Alternatively, according
another advantageous hot-rolling method, a slab is
loaded in a recuperator furnace when the temperature of
the slab, especially the surface temperature, declines
slightly. The slab is subsequently heated in a very
compact heating furnace for carbon steels for a short
period of time and then hot rolled.
These hot-rolling methods are in active use for
producing carbon steels~ By using these methods for
12~6~
- 19 ~
producing grain-oriented electrical steel sheet, a high
hot-rolling efficiency comparable to that of carbon
steels can be obtained.
When a continuous casting machine is directly
combined with a continuous hot rolling mill, formation
of internal cracks can be advantageously prevented.
slab which contains a large amount of silicon has low
heat conductivity and, therefore, a great temperature
difference. Thus, thermal stress is created between the
surface and inner portions of the slab. If it is cooled
after continuous casting, internal cracks are formed in
the slab and thus yield is lowered. However, since a
slab i5 not cooled according to the advantageous hot-
-rolling method, formation of internal cracks can be
advantageously prevented, which is an advantage
specifically realized for hot-rolling silicon steels.
According to a conventional high--temperature slab
heating method, a slab usually has a thickness of from
150 mm to 300 mm and is hot-rolled by a rough-rolling
mill to form a 30 to 70 mm thick intermediate product~
The intermediate product is then hot-rolled by a
plurality of continuous finishing mills, to form a hot
rolled strip having a predetermined thickness.
According to such a conventional method, a slab
having a small thickness cannot be used, because the
slab in deformed in a slab-heating furnace due to high
temperature, with the result that the slab cannot be
withdrawn from the furnace, or because a slab-heating
7~)
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furnace must be extremely long.
According to the low-temperature slab heaking
method, a thin cast slab can be used, because a cast
slab can be directly hot-rolled. In addition, a thin
cast slab can be directly finishing rolled while omitting
the rouyh hot-rolling, thereby carrying out the hot
rolling very effectively. If a slab is too thin,
however, the production efficiency is low in continuous
casting. On the other hand, if a slab is too thick, the
load applied to a finishing hot~rolling mill is extremely
great. A slab thickness is thus preferably from 30 mm
to 70 mm.
The magnetic flux density is strongly influenced by
a slab-heating temperature. Continuously cast slabs
which Gontained 0.057% of C, 3O50% of 5i, 0.25~ of Mn,
0.039~ of P, 0.033% of acid-soluble Al, and 0~0093~ of N
were heated and hot-rolled by the single hot rolling
method to form 205 mm thick hot-rolled strips. Then,
the following processes were successively carried ou :
continuous annealing at 1120C for 2 minutes; cold-
-rolling to form 0.30 mm cold-rolled sheets;
decabuxization-annealing at 850C for 2 minutes in a wet
hydrogen atmosphere; application of MgO as annealing
saparator; and final high temperature annealing at
120~C for 20 hours.
The magnetic flux density Blo of the products are
shown in Fig. 6. As is apparant from Fig. 6 a higher
magnetic flux density Blo may be obtained with a slab
~23L(~670
- 21 -
heating temperature exceeding 1280C. In many cases,
such a higher magnetic flux density is specifically
desired. For example, a high magne~ic flux densiky is
de.sirable especially when a laser-beam irradiation
technique for reducing the watt loss is utilized for the
grain-oriented electrical steel sheet produced by the
method according to the present invention, because the
watt loss reduction is greater at a higher magnetic flux
density. Such a technique is effective for providing
especially low watt loss.
If a slab heating temperature i5 extremely high, a
heating installation cannot withstand a high temperature
from an industrial point of view. The highest slab-
-heating temperature should be 1430C.
In the method according to the present invention,
the hot-rolled strip is annealed at a temperature of
from 850C to 1200C for a short period of time and then
rapidly cooled to control the precipitation state of
AlN. If the annealing temperature is lower than B50C,
a high magnetic flux density cannot be obtained. On the
other hand, if the annealing temperature is higher than
1200C, the secondary recrystallization becomes
incomplete. An annealing time of 30 seconds or longer
is sufficient for attaining the object of annealing, and
an annealing time longer than 30 minutes is economically
disadvantages. The annealing time is usually from 1 to
30 minutes.
The annealing hot-rolled strip, which may be
,:
~ZlQ~i7~1
- 22 -
referred to as a hot-coil, is then cold-rolled. Heavy
cold-rolling with a reduction a degree or draft of at
least 80% is necessary in the cold~rolling for producing
a grain-oriented electrical steel sheet having a high
magnetic flux density.
The cold-rolled strip is then decarburization-
-annealed. The aims of the decaburization annealing are
to decarburize and primary-recrystallize a cold-rolled
strip and slmultaneously to form on it an oxide layer
10 which is necessary as an insulating film.
An annealing separatorl which is necessary for
forming an insulating film on the product, is applied on
the surface of decaburization-annealed cold-rolled
strip. The annealing separator is mainly composed of
15 MgO and may additionally comprise, if necessary, one or
more of TiO2 , A12O3 t CaO, B-compound, S-compound, and
N compound.
Subsequently, final high-temperature annealing is
carried out. The aims of the final high-temperature
20 annealing are to secondary-recrystallization and purify
a decarburization-annealed strip and form an insulating
film mainly composed of forsterite. Final high-
-temperature annealing is usually carried out at a
temperature of 1100C or more in a hydrogen atmosphere
25 or a mixture atmosphere containing hydrogenO The
temperature is then usually elevated to approxlmately
1200C and purification annealing is carried out so as
to reduce N and S in steel to a level as small as
-~ lZ1~67~
- 23 -
possible.
After the final high temperature annealing, a
coating liquid mainly composed of, for example,
phosphoric acid, chxomic acid ~nhydride, and aluminum
phosphate is applied on the steel strip, and annealing
for flattening is carried out. Due to the coating film,
the insulating film in further strenythened and can
generate a high tension. An insulating film which
essentially consists of MgO-SiO2 is finally formed.
Regarding the conditions of the final high-
-temperature annealing, the heating rate at a temperature
range where the secondary recrystallization occrus is
preferably slow, because this is effective for at~aining
a stable high magnetic flux density. In metallurgical
terms, in slow heating in a secondary recrystalli~ation
temperature range, the secondary recrystallized grains
having a smaller inclination from the {110}<001>
orientation generate at a lower temperature. Therefore,
the slow heating can enhance the volume proportion of
the secondary recrystallization grians which are
generated at a low temperature and thus are close to the
{110}~001> orientation, with the result that the magnetic
flux density is enhanced. In addition, since the growth
of crystal grains is less liable to be suppressed due to
fine MnS particles in the present invention, in which
the S content is low and thus the inhibiting effect due
to fine MnS is small, as compared with the conventional
methods, in which the amount of MnS is great, the grain
- ~21()G70
~ 24 -
growth occurs relatively actively at a low temperature.
Thus, slow heating is particularly ef-fective in the case
of low S steel for increasing the volume proportion of
the secondary recrystallized grains which are generated
at a low temperature and are thus close to the {110}<001>
orientation, and thus enhancing the magnetic flux
density.
From Fig. 7, it will be understood how the magnetic
flux density Blo is influenced by the heating rate in a
temperature range of from 700C to 1100C~
Molten steel which contained 0.060% of C, 3.35% of
Si, 0~25~ of Mn, 0.033% of acid-soluble Al, 0.030% of P,
0.005% of ~, and 0.0085~ of N, was continuouly cast to
form a strand. Slabs cut form a strand were heated to
1400C and then hot-rolled to form a 2.3 mm thick
hot-rolled strips. Then, the following processes were
successively carried out: continuous annealing at
1200C for 2 minutes; cold-rolling to form 0.30 mm
cold-rolled sheets; decarburization-annealing at 850C
for 2 minutes in a wet hydrogen atmosphere; application
of annealing separator; and final high tmeperature
annealing at 1200C for 20 hours. The heating rate was
varied in the final high temperature annealing.
As is apparent from Fig. 7, the magnetic flux
densi~y Blo is higher when the heating ra~e is lower.
The magnetic flux density Blo is particularly high when
the heating rate is 15C/hour or less.
During slow heating is at a temperature range of
~ ~ .
67~
- 25 -
from 700C to 1100C, the secondary recrystallization is
completed. At a heating rate lower than 15C/hour, the
magnetic flux density does not greatly vary depending
upon temperature, but the value-dispersion of the
magnetic flux density decreases at a low heating rate.
The minimum heating rate is desirably 7C/hour in the
light of economic efficiency. The temperature i5 then
usual]y elevated to approximately 1200C and purification
annealing is carried out so as to reduce N and S in
steel to a level as small as possible.
The grain-oriented electrical steel sheet may
contain, in addition to the above described elements, a
minor amount of one or more additive elements, for
example, Cr.
Continuous casting slabs which contained 0.06%
of C, 3.33~ of Si, 0.30% of Mn, 0.035% of P, 0.030~ of
acid-soluble Al, 0.0085% of N, 0.004~ S, and varying
contents of Cr were heated to 1350C and hot~rolled to
form 2.3 mm thick hot-rolled sheets. Then, the following
processes were successively carried out: continuous
annealing at 1120C for 2 minutes; cold-rolling to form
0.30 mm cold-rolled strips; decarburization annealing in
a wet hydrogen atmosphere; application of MgO as
annealing separator; final high temperature annealing at
1200C for 20 hours. Cr can advantageously broadened
the range of the acid-soluble Al at which a high magnetic
flux density i5 obtained. From Fig. 8, it will be
understood that Cr can also decrease the wa~t loss for
: .
670
26 -
indentical magnetic flux densities. A Cr content
exceeding 0.25%, however, is inappropriate because the
effects of Cr are not enhanced and the decarburization
rate is lowered in the decaburization annealing.
Continuously cast slabs which contained 0.06% of C,
3 33% of Si, 0.004% of S, 0.033% of P, 0.032~ of acid-
-soluble Al, 0.0090~ of N, and 0.15% of Cr, were heated
to 1150C and 1350C and then hot-xolled to form 2.3 mm
thick hot-rolled strips. Then, the following processes
were successively carried out: continuous annealing at
1150C for 2 minutes; cold-rolling to form 0.30 mm
cold-rolled strips; decaburization-annealing at 850C
for 2 minutes in a wet hydrogen atmosphere; application
of MgO as annealing separator; and final high temperature
annealing at 1200C for 20 hours. A tension coating
which was mainly composed of colloidal silica was
applied on the products. The magnetic properties, shown
in Fig. 9, are those measured after forming the tension
coating.
As is apparent from Fig. 9~ the slab-heating
temperature exert an influence upon the magnetic
properties, i.e., lower slab-heating temperature allows
a lower loss with the same magnetic flux density.
The present invention is now further described by
way of examples.
Example 1
Molten steel which contained 0.053% of C, 3.30% of
Si, 0.25~ of Mn, 0.030% of P, 0.006% of S, 0.027~ of
~Z~67~
- 27 -
acid-soluble Al, and 0.0090% of N, was cast into an
ingot. The ingot was rough hot-rolled to form a 250 mm
thick slab. The slab was heated to 1150C and then hot
rolled to form 2.3 mm thick hot-rolled sheets. Then,
the following processes were successively carried out:
continuous annealing at 1080C for 2 minutes; cold-
rolling to form a 0.30 mm cold-rolled sheet;
decarburization-annealing at 850C in a wet hydrogen
a~mosphere; application of MgO; and final high
temperature annealing at 1200C for 20 hours.
The magnetic properties of the product in the
rolling direction were as follows:
1. 91 Tesla
Wl7/50 = 1.01 w/kg.
No incomplete secondary recrystallization occurred.
Example 2
Molten steel which contained 0.058% of C, 3.45% of
Si, 0.20~ of Mn, 0~035~ of P, 0.005% of S, ~.026% of
acid-soluble Al, and 0.0090% of N, was cast into a
250 mm thick strand by continuous casting followed by
cooling down to 250C. The cut slab was heated to
1200C and then hot rolled to form 2.3 mm thick hot-
-rolled sheets. Then, the following processes were
successively carried out: annealing at 1080C for
2 minu~es; cold-rolling to form a 0.30 mm cold-rolled
sheet; decarburization-annealing at 850C in a wet
hydrogen atmosphere; applica~ion of MgO; and final high
temperature annealing at 1200C for 20 hours.
7~
- 28 -
The magnetic properties of the product in the
rolling direction were as follows:
Blo = 1.91 Tesla
W17/50 = 0.97 wtkg.
No incomplete secondary recrystallization occurred.
Example 3
Molten steel which contained 0.055~ of C, 3.35~
of Si, 0.20% of Mn, 0.035% of P, 0.006% of S, 0.027% of
acid-soluble Al, 0.009% of N, was cast by continuous
castin~ using a mold having a 250 mm thick mold cavity.
After solidification of molten steel, cut slabs were
loaded quickly without cooling in a car bottom type
heat-reserving furnace. When the temperature of the
slab was homogenized so that the average temperature of
lS the slab was approximately 1130C, the hot-rolling was
carried out, to form 203 mm thick hot-rolled sheets.
Then the following processes were successively carried
out: annealing at 1080C for 2 minutes~ cold-rolling to
form a 0.30 mm cold-rolled sheet, decarDurization-
-annealing at 850C in a we~ hydrogen atmosphere;
application of MgO; and final high temperature annealing
at 1200C for 20 hoursO
The magnetic properties of the product in the
rolling direction were as follows:
Blo = 1.90 Tesla
W17/50 = 1.04 w/kg.
No incomplete secondary recrystallization occurred.
Example 4
1;~ i70
- 29 -
Molten steel which contained 0.060% of C, 3.35%
of Si, 0.15% of Mn, 0.030% of P, 0.002% of S, 0.028% o~
acid-soluble Al, 0.0090% of N, was cast by continuous
casting using a mold with a 250 mm thick mold cavity.
During the continuous casting, heat-insulation was
carried out in a continuous casting machine. And one
end surface of the strand, which was liable to cool, was
gas-heated for a short period of time, so as to decrease
the cooling to a level as small as possible, such
cooling occurring after solidification of the molten
steel. The strands, i.e., slabs, were quickly
transferred to the inlet side of a hot-rolling mill, and
the hot-rolling was initiated when the cross sectional
central part and surface part of slabs had a temperature
of approximately 1200C, and approximately 1050C.
The slabs were hot-rolled to form a 2.3 mm thick
hot-rolled sheets. Then~ the following processes were
successively carried out: annealing at 1030C for
2 minutes; cold-rolling to form a 0.30 mm cold-rolled
sheet; decarburization-annealing at 850C in a wet
hydrogen atmosphere; application of MgO; and final high
temperature annealing at 1200C for 20 hours.
The magnetic properties of the product in the
rolling direction were as follows:
Blo = 1.89 Tesla
W17/50 = 1.05 w/kg.
Example 5
Molten steel which contained 0.060% of C, 3.30%
~2~(~67~
- 30 -
of Si, 0.20% of M~ 0.035% of P, 0.006% of S, 0.030% of
acid-soluble Al, and 0.0080~ of N, was by contlnuous
casting to form slabs. Slabs were heated to 1380C and
then hot rolled to form 2.3 mm thick hot-rolled sheets.
S Then, the following processes were successively carried
out: annealing at 1130C for 2 minutes; cold-rolling to
form a 0.30 mm cold-rolled sheet; decarburization-
~annealing at 850C in a wet hydrogen atmosphere;
application of MgO; and final high temperature annealing
at 1~00C ~or 20 hours. The heating rate at a
temperature of from 700C to 1100C was 10C!hour in the
final high-temperature annealing. Flattening annealing
was carried out and then a tension film mainly composed
of chromic oxide anhydride was applied on the sheet
surface.
The magnetic properties of the product in the
rolling direction were as follows:
Blo = 1.93 Tesla
W17/50 = 1.02 w/kg.
The product was then irradiated with a laser beam
to form spot-like irradiation regions in the C direction
(perpendicular to the rolling direction).
The magnetic properties of the laser-irradiated
product were excellent as follows.
Blo = 1.93 Tesla
W17/50 = 0.91 Tesla
Example 6
Molten steel which contained 0.057% of C, 3.45
:
Q67~
- 31 -
of Si, 0.29% of Mn, 0.039~ of P, 0.003% of S, 0.032% of
acid-soluble Al, and 0.0090% of N, was GaSt by continuous
casting to form slabs. Slabs were heated to 1380C and
then hot rolled to form 2.3 mm thick hot rolled sheets.
Then, the following processes were successively carried
out: annealing at 1130C for 2 minutes; cold-rolling to
form 0.30 mm cold-rolled strips; decarburization-
annealing at 850C in a wet hydrogen atmosphere;
application of MgO; and final high temperature annealing
at 1200C for 20 hours. The heating rate at a
temperature of from 700 to 1100C was 20C/hour in the
final high-temperature annealing. Flattening annealing
was carried out and then a tension film mainly composed
of chromic oxide anhydride was applied on the sheet
surface.
The magnetic properties of the product in the
rolling direction were as follows.
Blo = 1.92 Tesla
W17/50 = 1.05 w/kg.
Example 7
Molten steel which contained 0.060% of C, 3.38~
of Si, 0.20% of ~1, 0.04~% of P, 0.005% of S, 0.033% of
acid-soluble Al, 0.0085% of N, and 0.1~% of Cr, was by
continuous casting to form slabs. Slabs were heated to
1400C and then hot rolled to form 2.3 mm thick hot-
-rolled strips. Then, the following processes were
successively carried out: annealing at 1120C for
2 minutes; cold-roiling to form 0.30 mm cold-rolled
67~
- 32 -
strips; decarburization-annealing at 850C for 2 minu~es
in a wet hydrogen atmosphere; application of MgO; and,
final high temperature annealing at 1200C for 20 hours.
The heating rate at a temperature of from 700 to
1100C was 10C/hour in the final high-temperature
annealing. Flattening annealing was carried out and
then a tension film mainly composed of chromic oxide
anhydride was applied on the sheet surface.
The magnetic properties of the product were as
0 follows in the rolling direction.
Blo = 1.93 Tesla
W17/50 - 0.99 w/kg.
The product was then irradiated with a laser beam
to form spot-like irradiation regions in the C direction
(perpendicular to the rolling direction).
The magnetic properties of the laser-irradiated
product were excellent as follows.
Blo = 1.93 Tesla
W17/50 = 0.88 Tesla
Example 8
Molten steel which contained 0.053~ of C, 3.35%
of Si, 0.25~ of Mn, 0.035% of P, 0O003~ of S, 0.029~ of
acid-soluble Al, 0.0080~ of N, and 0.15% of Cr, was cast
by continuous casting to form slabs. Slabs were heated
to 1150C and then hot rolled to form 2.3 mm thick hot-
rolled sheets. Then, the following processes were
successively carried out: annealing at 1080C for
2 minutes; cold-rolling to foxm 0.30 mm cold-rolled
~Z~6
-- 33 --
strips; decarburization-annealing at 850C for 2 minutes
in a wet hydrogen atmosphere; application of MgO; and,
final high temperature annealing at 1200C or 20 hours,
The heating rate at a temperature of from 700 to 1100C
was 20C/hour in the final high-temperature annealing.
A tension film mainly composed of chromi,c oxide anhydride
was applied on the sheet surface.
The magnetic properties of the product were as
follows in the rolling direction.
B1o = 1.91 Tesla
W17/50 = 0.97 w/kg.
Example 9
Molten steel which contained 0.053% of C, 3.45%
of Si~ 0.23~ of Mn, 0.037~ of P, 0.003% of S, 0,027% of
acid-soluble Al, 0.0090% of N, and 0.20% of Cr, was cast
by continuous casting to form slabs by using a mold
having a 253 mm thick mold cavity. Slabs were heated to
1130C and then hot rolled to form 2.3 mm thick hot-
-rolled sheets. Then, the following processes were
successively carried out: annealing at 1080C for
2 minutes; cold-rolling to form 0.30 mm cold-rolled
sheets; decarburization-annealing at 850C for 2 minutes
in a wet hydrogen atmosphere; application of MgO; and~
final high temperature annealing at 1200C for 20 hours.
The heating rate at a temperature of from 700 to
1100C was 10C/hour in the final high-temperature
annealing. A tension film mainly composed of chromic
oxide anhydride was applied on the sheet surface.
2~67~
- 34 -
The magnetic properties of the product were as
follows in the rolling direction.
Blo = 1.90 Tesla
Wl7/50 = 1.01 w/~g.
Example 10
Molten steel which contained 0.053~ of C, 3.45%
of Si, 0.23% of Mn, 0~037% of P, 0O003~ of S, 0.027~ of
acid-soluble Al, 0.0090% of N, and 0.20~ of Cr was cast
by continuous casting to form slabs. During continuous
casting, heat-insulation was carried out in a continuous
casting machine, and one end surface of a strand, which
was liable to cool down, was gas-heated, for a short
period of time, so as to decrease cooling to a level as
small as possible, such cooling occurring after
solidification of molten steel. Cut strands, i.e.,
slabs, were quickly transfPrred to the inlet side of a
hot-rolling mill, and the hot rolling was initiated when
the cross sectional central part and surface part of
slabs had a temperature of approximately 1200C, and
approximately 1050C, respectively. 2.5 mm thick
hot-rolled strips were formed by hot-rolling. Then, the
following processes were successively carried out:
annealing at 1080C for 2 minutes; cold~rolling to form
0.30 mm cold-rolled sheets; decarburization-annealing at
850C in a wet hydrogen a-tmosphere; application of MgO;
and, final high temperature annealing at 1200C for
20 hours.
The magnetic properties of the product were as
Q670
- 35 -
ollows in the rolling direction.
Blo = 1.90 Tesla
W17/50 = 1.03 w/kg.
