Note: Descriptions are shown in the official language in which they were submitted.
CA 02351744 2001-06-26
ROLLS FOR DISPOSING AT ENTRY SIDE OR EXIT SIDE OF
QUENCHING ZONE OF CONTINUOUS ANNEALING FURNACE
AND QUENCHING ZONE UNIT USING ROLLS
BACKGROUND OF THE INVENTION
Field of Invention
The invention relates to rolls disposed before and/or after a quenching zone
of
a continuous annealing furnace for continuously heat-treating strip and to
quenching
zone unit including the rolls.
2. Description of Related Art
As the size of automotive vehicles has increased in recent years, the width of
steel strips has also increased. Moreover, from the point of view of
preventing global
warming, high-strength steel plates are increasingly employed to achieve a
vehicular
weight reduction by down-gauging the steel strips.
High-strength steel strips having increased widths and reduced thicknesses are
being produced using continuous annealing furnaces. Continuous annealing
furnaces
are now required to treat a steel strip having an increased width ranging from
600 mm
to 1850 mm and to 2100 mm in some cases.
Moreover, the annealing temperature of steel strips is further elevated, and,
consequently, the steel strip passing through the furnace is further softened,
resulting
in increased amounts of defective products. For quality control, more precise
control
of the rapid cooling operation after annealing is required.
Because of these reasons, conventional processes are no longer sufficient for
achieving stable operations of continuous annealing furnaces.
As shown in Fig. 4, an upright continuous annealing furnace for annealing
steel strip comprises a heating zone 2 for heating a steel strip to a
predetermined
temperature to perform annealing, a soaking zone 3, and a cooling zone for
cooling
the high-temperature material, i.e., the steel strip l, to a room temperature.
The cooling zone normally comprises a plurality of furnace zones, namely, a
quenching zone 4 (or "primary cooling zone" ) for rapid cooling the high-
temperature
3 0 steel strip, an over-aging zone 5, and a secondary cooling zone 6.
Before and after the quenching zone 4, i.e., at the entry side and the exit
side
of the quenching zone 4, hearth rolls or bridle rolls for feeding the steel
strip 1 are
CA 02351744 2001-06-26
provided. Moreover, bridle roll units 8 for preventing fluttering of the steel
strip 1
inside the quench zone 4 are provided in many cases.
Herein, the term "quenching zone unit" includes the quenching zone 4 and the
rolls, such as the bridle roll unit 8 disposed before and after the quenching
zone 4.
Although the upright continuous annealing furnace shown in Fig. 4 includes
the over-aging zone S and the secondary cooling zone 6, the over-aging zone 5
and the
secondary cooling zone 6 may be omitted when applied to, for example, a molten
metal plating line.
By employing the process of quenching a high-temperature metal strip in a
quenching zone, the quality of the steel strip can be adequately controlled
and the
resulting products have both sufficient formability and sufficient strength.
An
exemplary steel strip being steel plates for vehicular bodies having a baking
hardening
property.
As processes for quenching the steel strips, a gas jet cooling process
comprising cooling the atmospheric gas in the quenching zone using a heat
exchanger,
circulating the gas, and blowing cooled gas jet streams at high speeds to the
steel
strips; a roll cooling process comprising cooling rolls by placing cooling
media into
the rolls and pressing the rolls against a steel strip to quench the steel
strip; a water
quenching process using water as a cooling medium; a mist cooling process, and
the
like, are known.
Among these processes, the gas jet cooling process advantageously provides
steel strips having satisfactory appearance and shapes after cooling.
Moreover, the
cost for the cooling equipment is relatively low. Thus, a high-speed gas jet
cooling
process, in which a temperature range of 300°C or more is quenched
using a
quenching zone including gas jet cooling equipment having a heat transfer
coefficient
of 170 W/(mz~°C) or more per surface, is now being performed. Herein,
the phrase "a
temperature range of 300°C or more is rapidly quenched" means that the
temperature
of the steel strip that is quenched is 300°C or more at the entry side
of the quenching
zone.
However, in the high-speed gas jet cooling process, the cooling air hitting
the
steel strip surfaces reaches connection sections, for example, the bridle roll
unit,
disposed between the heating zone and the cooling zone, thereby over-cooling
the
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CA 02351744 2001-06-26
edge portions of the hearth rolls or the bridle rolls installed in the
connection sections
and generating large thermal crowns in the centers of these rolls.
Consequently, the
steel strip suffers from buckling in the width direction.
The following publications disclose means for solving this problem. Japanese
Unexamined Patent Application Publication No. 56-65942 discloses that
fluttering of
steel strip is reduced by providing inner-furnace bridle rolls at the
quenching zone
entry side and increasing the tension of steel strip at a gas jet nozzle unit.
However,
operational experiences demonstrate that the buckling of steel strip at the
inner-
fmnace bridle rolls disposed at the quenching zone entry side cannot be
completely
prevented.
Japanese Unexamined Patent Application Publication No. 60-40463 discloses
a seal for preventing gas leakage from the connecting portion. However, when
the
seal for preventing gas leakage is applied to a high-speed gas jet cooling
unit, the
cooling gas hitting the steel strip surfaces leaks from the connecting units
provided
I S before and after the quenching zone, thereby generating a temperature
distribution in
the rolls disposed in the bridle roll units installed at the entry side and
exit side of the
quenching zone and resulting in the buckling of the steel strip. Accordingly,
the
bridle roll units require additional means for solving this problem.
Otherwise, the
edge portions of the rolls inside the bridle roll unit are excessively cooled,
large
thermal crowns are developed in the centers of these rolls, and buckling
occurs in the
width direction of the steel strip.
Japanese Unexamined Patent Application Publication No. 6-93347 discloses
that kinetic energy is reduced by disposing a sealing apparatus at an upper
portion of a
gas jet chamber of a quenching zone and injecting, from the sealing apparatus,
a
stream which flows in a direction opposing the stream at the surface of the
steel strip.
However, the sealing apparatus requires installation of a counter-stream
injection
apparatus and a seal roll, resulting in increased costs. Moreover, the
operation thereof
is complicated.
Japanese Unexamined Patent Application Publication No. 9-268324 discloses
that in a quenching zone, the angle of a roll crown is adjusted so as to
control the
buckling threshold tension to be larger than the tension of the steel strip.
However, in
this continuous heat treating process, the rolls having a roll angle of the
disclosed
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CA 02351744 2001-06-26
range do not come into satisfactory contact with the steel strip, resulting in
slipping
between the rolls and the steel strip.
SUMMARY OF THE INVENTION
Accordingly, objects of the invention are to solve the above-described
problems and to achieve a high-speed gas jet cooling process without causing
defects,
such as buckling and meandering, even when rapid cooling is performed in a
temperature range of 300°C or more and at a quenching zone having a jet
cooling unit
having a heat transfer coefficient per surface of 170 W/(m'-~ ° C). The
invention can
achieve a reliable operation of a continuous annealing furnace by allowing a
steel strip
of a reduced gauge and an increased width to stably pass through the line. The
invention also can solve problems such as decrease in yield, decrease in line
speed,
and shutdown.
In order to achieve these goals, an exemplary embodiment of the invention
provides a roll to be disposed before or after a quenching zone of a
continuous
annealing furnace, satisfying the following relationships:
Lc z 0.7 X Wmin
R = -0.1 x 10'3 to +0.2 x 10'3; and
TR z 20
wherein Lc represents the length (nun) of a flat portion in the center of the
roll,
Wm;n represents the minimum width (mm) of a steel strip, R represents the
inclination
of tapered portions disposed at the two sides of the roll, and TR represents
radius of
curvature (m) of the boundaries between the flat portion and the tapered
portions.
Another exemplary embodiment of the invention provides a quenching zone
unit of a continuous annealing furnace, comprising at least one hearth roll
and/or at
least one bridle roll disposed at the entry side and/or the exit side of a
quenching zone.
The above-described roll comprises the hearth roll and/or bridle roll.
Another exemplary embodiment of the invention provides a quenching zone
unit of a continuous annealing furnace, including at least one bridle roll
unit having a
plurality of rolls. The at least one bridle roll unit is provided at the entry
side and/or
the exit side of a quenching zone. The above-described exemplary roll
comprises
each of these rolls.
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CA 02351744 2001-06-26
Preferably, the roll closest to the quenching zone is a flat roll satisfying
the
relationships (i) R = 0 and (ii) TR =~.
Preferably, at least one pair of seal rolls is disposed at the entry side
and/or the
exit side of the quenching zone.
BRIEF DESCRIPTION OF THE DRAW~TGS
Fig. 1 illustrates the vicinity of a quenching zone according to an exemplary
embodiment of the invention;
Fig. 2 illustrates the vicinity of a quenching zone according to another
exemplary embodiment of the invention;
Fig. 3 illustrates the vicinity of a quenching zone of a horizontal continuous
annealing furnace;
Fig. 4 illustrates the structure of a continuous annealing furnace;
Fig. 5 shows an exemplary embodiment of a roll according to the invention;
Fig. 6 is a graph showing the relationship between a hength Lc of the flat
portion of a roll and a trouble ratio;
Fig. 7 is a graph showing the relationship between an inclination R of the
tapered portion of a roll and a trouble ratio;
Fig. 8 is a graph showing the relationship between the length Lc of the flat
portion of the roll at the entry side and the exit side of the quenching zone
and
problems regarding feeding;
Fig. 9 is a graph showing the relationship between the inclination R of the
tapered portion at the entry side and the exit side of the quench zone and
problems
regarding feeding;
Fig. 10 is a graph comparing yield reduction rates of a conventional process
and the process of the invention; and
Fig. 11 is a graph comparing operational efficiency reduction rates of the
conventional process and the process of the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Preferred embodiments of the invention will be described with reference to the
drawings.
5
CA 02351744 2001-06-26
Although the rolls of the invention can be preferably applied to the
continuous
amealing furnace shown in Fig. 4, the invention is not limited to this
application.
Rather, rolls according to the invention can be applied to a wide variety of
continuous
armealing furnaces having quenching zones.
The stmctures of a typical quenching zone and quenching zone units disposed
before and after the quenching zone according to the invention will be
described with
reference to Fig. 1.
In Fig. l, a quenching zone 4 includes a gas jet cooling apparatus 12 and
performs quenching of a steel strip 1 fed into the quenclung zone 4. In order
to give a
target tension to the steel strip 1 and to prevent fluttering of the steel
strip 1 inside the
quenchiilg zone 4, bridle roll units 8 are disposed before and after the
quenching zone
4. Seal rolls 11 are disposed inside the respective bridle roll units 8 to
prevent the
cooling gas ejected in the gas jet cooling apparatus 12 from reaching entering
the
bridle roll units 8. Also, a heater 7 is provided in each of the bridle roll
units 8 to
prevent temperature drops inside the bridle roll units 8 and to maintain the
temperature at a predetermined temperature. A plurality of hearth rolls 9 and
bridle
rolls 10 are provided inside each of the bridle roll units 8.
In Fig. 1, two of the hearth rolls 9 and three of the bridle rolls 10 are
installed
inside the bridle roll unit 8 located near the entry side of the quenching
zone 4 and one
hearth roll 9 and three of the bridle rolls 10 are installed inside the bridle
roll unit 8
located near the exit side of the quenching zone.
The invention is, however, not limited to the particular configuration shown
in
Fig. 1. Other configurations can also be used as long as the steel strip 1 is
provided
with a target tension. For example, as shown in Fig. 2, the hearth roll 9 may
also
function as the bridle roll and the bridle roll unit 8 may be provided with
three bridle
rolls 10.
Furthermore, the invention can be applied to a horizontal continuous annealing
furnace as shown in Fig. 3.
Embodiments of the invention can optimize the thermal crowns of the hearth
rolls 9 and the bridle rolls 10 installed inside the bridle roll units 8 and
prevent the
steel strip 1 from slipping, buckling, and meandering by a desired tension
applied to
the steel strip.
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CA 02351744 2001-06-26
The rolls installed after and before the quenching zone according to the
invention can be applied to both the hearth rolls and the bridle rolls.
Next, a roll profile for achieving stable strip feeding without slippage of
the
steel strip will be described in detail.
As illustrated in Fig. 5, an exemplary roll 20 according to the invention can
be
disposed after or before the quenching zone. The roll 20 comprises a
substantially flat
portion 22 having a length Lc and tapered portions 24 having an inclination R.
The
flat portion 22 is sandwiched by the tapered portions 24, and the roll 20 is
thereby
symmetrical.
the roll crown has a convex shape. When the inclination R is negative, the
roll crown
has a concave shape. The inclination R is defined as the ratio of the length L
of the
tapered portion to the value C which equals half the difference in outer
diameters
between the beginning of the tapered portion and the end of the tapered
portion, i.e.,
R = C/L.
According to the invention, the flat portion 22 does not necessarily need to
be
exactly flat. For example, the flat portion may have a gently curved surface
having a
radius of curvature of 100 m or more.
In order to specify preferred ranges of the invention, the relationship
between
the length Lc of the flat portion of the roll and the ratio of problems caused
by
slippage between the roll surface and the steel strip was examined. The
results are
shovm in Fig. 6. In this examination, conditions such as the inclination R of
the roll
tapered portion and the radius of curvature at the boundary between the flat
portion
and the tapered portions were kept within preferred conditions according to
the
invention. Regarding cooling conditions, a temperature range of 300°C
or more was
quenched using a quenching zone including gas jet cooling equipment having a
capacity of 170 W/(m2~ °C) or more, reduced to a heat transfer
coefficient per steel
strip surface. The trouble ratio in the graph is normalized by the average of
conventional operational data.
The examination shows that the preferred length of the flat portion (Lc) of
the
roll relative to the minimum strip width Wm;~ of steel strip fed to the flat
portion
satisfies condition (1):
Lc z 0.7 x Wmin
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CA 02351744 2001-06-26
Next, the relationship between the inclination R of the tapered portion and
the
problem ratio caused by the slippage between the steel strip and the roll
surface was
examined. The results are shown in Fig. 7. Other conditions such as the length
Lc of
the flat portion of the roll, the radius of curvature of the boundary between
the flat
portion and the tapered portions, etc. were within the preferred conditions of
the
invention, and the cooling conditions were the same as those in the
examination
regarding Fig. 6. The trouble ratio is normalized by the average value of
conventional
operational data.
Accordingly, the range of the inclination R of the tapered portion preferably
satisfies condition (2):
R = -0.1 X 10'3 to +0.2 x 10'3
Furthermore, the boundary between the roll flat portion and the tapered
portions is preferably smooth and round without edges in order to prevent
slipping
and buckling of the steel strips. To smooth the boundary between the flat
portion and
the tapered portions of the roll, the radius of the curvature TR at the
boundary is
preferably 20 m or more. In other words, the curvature TR satisfies condition
(3):
TR z 20
Next, based on the above-described parameters, the conditions which prevent
buckling and meandering of the steel strip were investigated. The results are
shown in
Fig. 8. In Fig. 8, the horizontal axis indicates the entry side and the exit
side of the
quenching zone and the vertical axis indicates the length Lc of the roll flat
portion. R
and TR were set within the ranges of the invention. The graph shows whether
buckling and/or meandering occurred in the steel strip fed therein. The
cooling
conditions were the same as those in the examination regarding Fig. 6. In the
graph,
circles (O) indicate that the steel strip had no defect and squares (O)
indicate that
buckling was observed in the steel strip. The speed of the strip line was set
to a
nomnal speed (100 to 300 m/min). No meandering was observed in the examination
regarding Fig. 8.
Fig. 8 also demonstrates that condition (1):
Lc z 0.7 X Wmin
is preferable.
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CA 02351744 2001-06-26
Fig. 9 is a graph indicating the generation of buckling and meandering when a
steel strip was fed. The horizontal axis in the graph indicate the entry side
and the exit
side of the quenching zone. The veutical axis in the graph indicate the
inclination R of
the tapered portion. The conditions such as Lc and TR were set within the
preferred
range of the invention. The cooling conditions were the same as those in the
examination regarding Fig. 6. In the graph, circles (O) indicate the steel
strip had no
defect, triangles (D) indicate that meandering was observed, and squares (D)
indicate
that buckling was observed. The speed of the strip line was set to a normal
speed (100
to 300 m/min).
Fig. 9 also demonstrates that condition (2):
R = -0.1 x 10'3 to +0.2 X 10'3
is preferable. This range is actually wider than the preferred range for
preventing
problems caused by slippage as shown in Fig. 7.
In order to prevent the steel strip from vibrating inside the quenching zone,
bridle roll units are generally provided after and before the quenching zone
to
maintain the tension of the steel strip at the quenching zone at a high level.
The rolls
installed inside these bridle roll units are preferably the rolls within the
preferred
ranges of the invention so as to avoid problems, such as slipping, buckling
and
meandering, as described above. Under a high tension at the quenching zone,
meandering is likely to occur when rolls have concave-shaped crowns, R < 0,
whereas
buckling is likely to occur when rolls have convex-shaped crowns, R > 0.
The optimum shape of the roll for avoiding these problems is a flat roll of R
=
0 located substantially in the center of the preferred range shown in Fig. 9.
In the flat
roll, TR = ~ (radius of curvature = 0). Other advantages of the flat roll are
its ease of
manufacturing and low manufacturing costs. In the bridle roll unit, the
tension of the
steel strips is relatively low at the roll closest to the quenching zone
compared to the
rolls at the preceding positions. hi this respect, it is preferable that,
among these
bridle rolls, the one closest to the quenching zone be a flat roll of R = 0
and TR =
and the preceding rolls be the rolls satisfying the conditions of the
invention.
Preferably, the cooling gas injected into the quench zone is prevented from
reaching inside of the bridle roll unit as much as possible. When a large
amount of
cooling gas passes through the connecting portion between the quenching zone
and
9
CA 02351744 2001-06-26
the bridle roll unit and reaches the bridle roll unit, the edges of the rolls
inside the unit
are excessively cooled, generating remarkable thermal crowns in the central
portion of
the roll. Thus, when the gauge of the strip is reduced, the probability of
slipping and
buckling becomes high.
In order to solve this problem, at least one pair of seal rolls is preferably
installed in each of the connecting portions of the bridle roll units located
at the entry
side and the exit side of the quenching zone.
Table 1 shows profiles of the rolls of the invention as installed inside the
bridle roll units disposed before and after the quenching zone of the upright
continuous atuzealing fm~naces. The profiles are defined as Lc and R, and
shows
whether undesirable phenomena such as slipping, buckling, and meandering occur
or
not. In Table 1, "A" indicates neither slipping, buckling, nor meandering was
observed; "B" indicates slipping, buckling, or meandering was occasionally
observed;
and "C" indicates slipping, buckling, or meandering was frequently observed.
The quenching zone had a gas jet cooling unit having a capacity of 170
W/(m2~ ° C) or more reduced to a heat transfer coefficient per steel
strip surface and
was provided with a pair of seal rolls at the entry of the quenching zone. The
temperature range of 300°C or more was quenched.
CA 02351744 2001-06-26
SampleWidthWm~n
Wmd~cLengthInclinationCurvatureSlippingBucklingMeanderingReference
of of R of Radius
Strip Fiat TaperedTR
(mm) Portion
Portionat
) (X10'')Boundary
(m)
1 600 600 1600700 D.2 20 A A A Example
2 1000 600 1600700 0.2 20 A A A Example
3 1250 600 1600700 0.2 20 A A A Example
4 1600 600 1600700 0.2 20 A A A Example
600 600 1600700 -0.05 20 A A A Example
6 1000 600 1600700 -0.05 20 A A A Exarhple
7 1250 600 1600700 -0.05 20 A A A Example
8 1600 600 1600700 -0.05 20 A A A Example
9~ 1000 700 20001000 0.0 ~ ~ A A A Example
1250 700 20001000 0.0 ~ A A A Example
11 1600 700 20001000 0.0 ~ A A A Example
12 1850 700 20001000 0.0 ~ A A A Example
13 2000 700 20001000 0.0 ~ A A A Example
14 1000 700 2000800 -0.2 20 8 A A Comparative
Example
1
1250 700 2000800 -0.2 20 B A B Comparative
Example
1
16 1600 700 2000800 -0.2 20 C A C Comparative
Example
1
17 1850 700 2000800 -0.2 20 C A C Comparative
Example
1
18 1000 700 2000800 0.4 20 B B A Comparative
Example
2
19 1250 700 2000800 0.4 20 C B A Comparative
Example
2
1600 700 2000800 0.4 20 C C A Comparative
Example
2
21 1850 700 2000800 0.4 20 C C A Comparative
Example
2
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CA 02351744 2001-06-26
As can be understood from Table l, in samples 14 to 17 (Comparative
Example 1), the inclination R of the tapered portions of each of the rolls was
negative;
hence, significantly large concave crowns were formed, resulting in slipping
and
buckling of the steel strips.
In samples 18 to 21 (Comparative Example 2), the inclination R of the tapered
portions of each of the rolls was positive; hence, significantly large convex
crowns
were formed, resulting in slipping and meandering of the steel strips.
Samples 1 to 13 are rolls according to the invention.
EXAMPLES
Operations were conducted using an upright continuous annealing furnace
having a quenching zone unit including rolls of the invention disposed before
and
after a quenching zone.
The upright continuous annealing furnace had a Wm;" value of 700 mm and a
Wmax v~ue of 1850 mm. The quenching zone unit used was that shown in Fig. 1.
The
quenching zone included a gas jet cooling apparatus having a capacity of 170
W/(rn2~ °C) or more, reduced to a heat transfer coefficient per steel
strip surface.
An operation according to the invention satisfied all conditions (1) to (3).
That is, in the operation of the invention, all of the rolls used inside the
bridle roll
units were in conformity with conditions (1) to (3). The operations, each
satisfying
only one of conditions (1) to (3), were also performed and were compared to a
conventional process.
In the operation according to the invention satisfying all of conditions (1)
to
(3), the rolls disposed before and after the quenching zone had the following
profiles:
Lc = 1.0 X 700, R = 0.05 X 10'3, and TR = 50 m.
In the operation satisfying only condition (1), the rolls disposed before and
after the quenching zone had the following profiles: Lc = 1.0 x 700, R = 0.4 X
10'3,
and TR = 10 m.
In the operation satisfying only condition (2), the rolls disposed before and
after the quenching zone had the following profiles: Lc = 0.5 X 700, R = 0.05
x 10'3,
3 0 and TR = 10 m.
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CA 02351744 2001-06-26
In the operation satisfying only condition (3), the rolls disposed before and
after the quenching zone had the following profiles: Lc = 0.5 x 700, R = 0.4 x
10'3,
and TR = 50 m.
In the operation performed according to a conventional process, the rolls
disposed before and after the quenching zone had the following profiles: Lc =
0.5 X
700, R = 0.5 x 10'3, and TR = 8 m
Fig. 10 shows the r elationship between each of the operation conditions and
the decrease in yield of the products due to slipping, buckling, and
meandering
between the rolls and the steel strip. In the graph, the yield reduction rate
is
normalized by the amount of the defective product relative to the entire
production in
a conventional process.
When slipping, meandering, or buckling occur, the speed of the line must be
reduced, resulting in a reduced production yield.
Improvements compared to the conventional process can be attained by
satisfying one of conditions (1) to (3), but when all of these conditions are
satisfied in
combination as in the invention, decrease in yield can be significantly
improved to
approximately one-tenth of the conventional process.
Fig. 11 shows the relationship between each of conditions (1) to (3) and
operation efficiency reduction rate of the line caused by slipping, buckling,
or
meandering between the rolls and the steel strip.
Here, "operation e~ciency" is defined as the ratio of the line speed
calculated
from the capacity of the equipment to the actual operation speed and is an
indicator of
the capacity in operation. In the graph, the operational efficiency reduction
rate is
normalized by an average value of the difference between a theoretical line
speed
calculated from capacity and an actual line speed of a conventional process.
When slipping, buckling, or meandering occurs, the speed of the line is
decreased, resulting in a reduced treatment speed. It may be possible to
continue the
operation in such a state without major problems, but the operation efficiency
will
eventually be decreased, failing to achieve an expected production amount. If
the
problems are major, it becomes necessary to stop the line, decrease the
temperature of
the furnace, and dispose of the steel strips in the furnace, thus failing to
achieve a
predetermined production amount and decreasing the operation efficiency.
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CA 02351744 2001-06-26
By employing the rolls of the invention; the operation rate of the upright
continuous annealing furnace was improved by 0.1 % on average and the
operation
efficiency reduction rate was lowered to one-fifth compared to the
conventional
process.
Also, occurrence of line shutdown, decrease in the line speed, and so forth
due
to slipping, buckling, and meandering were maintained at minimum levels,
thereby
significantly improving the production yield and operation efficiency of the
furnace.
14
