Note: Descriptions are shown in the official language in which they were submitted.
CA 02308641 2000-08-03
t 1 .
HEARTH ROLLS FOR HEATING FURNACE AND SOAKING FURNACE OF
VERTICAL HEAT TREATING FURNACE AND VERTICAL HEAT
TREATING FURNACE INCLUDING HEARTH ROLLS
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to hearth rolls for a
heating furnace and a soaking furnace of a vertical heat
treating furnace. The present invention also relates to
a vertical heat treating furnace using the hearth rolls.
2. Description of Related Art
Vertical heat treating furnaces are ordinarily
divided into respective sections of a heating furnace, a
soaking furnace, and a cooling furnace, and a
predetermined heat treatment cycle is performed.
Hereinafter, the heating furnace and the soaking furnace
contained in the vertical heat treating furnace are
described as one set of equipment in the present
invention, and are referred to as "a heating/soaking
furnace." Further, the vertical heat treating furnace
includes a plurality of hearth rolls as transfer rolls
located on the upper portion and the lower portion of the
vertical heat treating furnace, and a metal strip is
passed while being suspended by these hearth rolls and
subjected to a necessary heat treatment in the process.
However, because metal strips to be passed are not
always flat and include a bent portion and a locally
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extended portion, a transfer problem such as meandering
and the like is liable to occur while they are being
passed.
In particular, large vertical heat treating furnaces
(of such class in which the distance between upper
rollers and lower rollers exceecls 15-20 m) have been
constructed in large numbers, and the prevention of
transfer problems is a leading problem to be solved in
these furnaces. To prevent problems in the transfer of
metal strip, the shape of hearth rolls has been variously
devised such as by the formation of a crown and the like
on the hearth rolls. However, when a large crown is
formed to prevent meandering as a transfer problem, there
is a possibility that a problem called "buckling" in
which a metal strip is buckled in a width direction, can
occur. This problem is significant, particularly when
the furnace temperature is high and a problem is caused
when a sheet is passed. Thus, buckling is one of leading
causes of lowered operating efficiency of equipment and
product yield.
There have been made various devices to effectively
prevent meandering and buckling, which are problems
caused when a metal strip is passed in a vertical heat
treating furnace.
Japanese Unexamined Patent Application Publications
Nos. 55-100919 and 57-137431, for example, disclose
controlling the crown of a roll using the thermal
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expansion in a hearth roll by devising the inner
structure of the hearth roll.
Further, Japanese Unexamined Patent Application
Publications Nos. 7-331335 and 3-47926 disclose
controlling the crown of a hearth roll by controlling its
temperature by applying heat to the hearth roll from the
outside.
Japanese Unexamined Patent Application Publications
Nos. 8-199247, 7-138656, 58-120739, and 52-136812
disclose conventional examples in which the shape of a
hearth roll itself is devised. These publications
disclose a hearth roll having a one-stepped taper, which
is arranged such that the central portion of the hearth
roll has a flat shape or a crown shape, with both sides
of the roll having a taper.
The applicant has disclosed in Japanese Unexamined
Patent Application Publication No. 59-116331 that a roll
having a two-stepped taper shape can be used together
with the above roll having a one-stepped taper shape and
a crown shape.
Recently, however, a steel sheet having width much
larger than that of a conventional steel sheet has been
required as a steel sheet for integrally forming an
automobile body.
Therefore, it has been required to pass a metal
sheet, in particular, a steel sheet having a wider range
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of sheet width, which is larger than a conventional
range, in a single vertical heat treating furnace.
Regarding a steel sheet for automobiles, a line was
conventionally operated with a sheet width ranging from
800 mm to 1500 mm. Recently, it has been required to
pass a steel sheet having a sheet width of about 800-
1500 mm, and sometimes a steel sheet having a sheet width
larger than this, in the same line.
When a range of sheet width is wide as described
above, a transfer problem cannot be sufficiently overcome
by simply devising only the roll shape of a one-stepped
taper roll as in the conventional technology.
An optimum roll shape has been known as to the one-
stepped taper roll and used as an effective means for
preventing meandering and buckling in the operation in
the conventional range of sheet width. However, the roll
shape cannot be used as it is in a wide range of sheet
widths having a ratio of maximum to minimum sheet width
of, for example, 2 or more.
The present inventors have discovered the one-
stepped taper roll has a problem in that while it can
effectively prevent the occurrence of buckling in a wide
metal strip when the inclination of a taper is reduced,
meandering is liable to occur in a narrow metal strip.
In contrast, when the inclination of the taper is
increased, while meandering can be effectively prevented
in a narrow metal strip, buckling is liable to occur in a
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wide metal strip, particularly when its thickness is
thin. Thus, it is impossible to follow a wide range of
sheet width and to cope with a problem caused by the wide
range of sheet width by the use of the one-stepped taper
roll.
Further, even if the method of controlling the crown
of a hearth roll by temperature control is applied, it is
impossible to follow the wide range of sheet thickness
and to cope with a problem caused by the wide range using
this method, and it is necessary to reconfigure equipment
on a large scale to follow the wide range of sheet width.
Further, a metal strip can be passed stably in a
vertical heat treating furnace even by a conventional
hearth roll to a certain extent when the metal strip is
in a steady state in which it is passed at an
approximately constant speed. However, when operating
conditions are varied in a furnace to treat metal strips
having a wide variety of sizes and various kinds of metal
strips, the sheet passing speed is often changed
considerably. Meandering and buckling often occur when
the speed is changed (by changes corresponding to 40-50%
of a steady speed). In the conventional hearth roll, it
is very difficult to achieve a stable sheet passing
property by taking even the change of sheet passing speed
into consideration, and further it is not easy to achieve
this property even by the use of the two-stepped taper
roll.
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In a continuous annealing furnace for steel strip,
for example, an ordinary sheet passing speed is about
200-400 m/min in a steady state.
SUMMARY OF THE INVENTION
The present invention can cope with the transfer of
a steel strip in a wide range of sheet width only by
simply optimizing a hearth roll at a low equipment cost
without the need for remodeling equipment on a large
scale. The present invention is preferable to a
heating/soaking furnace of a vertical heat treating
furnace for treating a steel strip having a widea range of
sheet width in which a rate of maximum to minimum sheet
width is 2 or more.
The inventors have discovered that meandering and
buckling can be prevented by optimizing the shape and
disposition of two-stepped taper rolls more effectively
than conventional taper rolls also in correspondence
particularly to a wide range of sheet width and to a
change in speed.
That is, the above-described problems of the known
apparatus have been solved by a hearth roll for a
heating/soaking furnace of a vertical heat treating
furnace, which comprises a flat section at a central
portion and two-stepped taper sections on both sides of
the flat section. The inclination R1 of each of first taper
sections continuous to the flat section is larger than
the inclination R2 of each of second taper sections further
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continuous to each of the first taper sections. The inclination R2 is within
the
range of 0.005 x 10-3 to 4.10-3. The length Lc (mm) of the flat section and
the
length L1 (mm) of each of the first taper sections are related according to
the
following formulas (1) and (2), and Wmin <_ Wmax-400.
Preferably, a convex curve section and a concave
curve section are formed at the boundary between the flat
section and each of the first taper sections, and at the
boundary between each of the first taper sections and
each of the second taper sections, respectively. Each of
the convex and concave sections has a radius of curvature
of at least 20 m.
Preferably, the inclination R1 of each of the first
taper sections is within the range of 0.2 x 10-3 to 10 x
10-3.
The above-described problems have been solved by a
vertical heat treating furnace using hearth rolls for a
heating/soaking furnace. The hearth rolls at the inlet
of the vertical heat treating furnace satisfy the
following formulas (3) and (4) and the hearth rolls from
the intermediate portion to the outlet of the vertical
heat treating furnace satisfy the following formulas (5)
and (6), as well as the lengths Lc and (Lc + 2 x L1) are
increased from the inlet to the outlet of the furnace,
stepwise or sequentially, in the hearth roll groups of
the respective ones of upper rolls and lower rolls
disposed side by side in the furnace.
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Further, the above-described problems have been
solved by a vertical heat treating furnace using hearth
rolls for a heating/soaking furnace as transfer rolls.
The hearth rolls at the inlet of the vertical heat
treating furnace satisfy the following formulas (3), (4),
(7) and (8), and the hearth rolls from the intermediate
portion to the outlet of the furnace satisfy the
following formulas (5), (6), (9) and (10), as well as the
langths Lc and (Lc + 2 x L1) are increased from the inlet
to the outlet of the furnace, stepwise or seque.rAtially,
in the hearth roll groups of the respective ones of upper
rolls and lower rolls disposed side by side in the
furnace and the inclinations Ri and R2 of the tapers are
reduced from the inlet to the outlet of the furnace
stepwise or sequentially.
0.5 Wmin s Lc S Wmin ,,,(1)
Wmin s Lc + 2 x L1 s Wmax - 400 ...(2)
0.5 Wmin s Lc s 0.7 Wmin ...(3)
Wmin <_ Lc + 2 x L1 <_ (Win + Wmax - 400) /2 ...(4)
0.7 Wmin 5 Lc s Wmin ...(5)
(Wmin + Wmax - 400) /2 s Lc + 2 x L1 s Wmax -- 400
...(6)
3. 0 x 10'3 5 Rl s 10 x 10-3 (7)
1.2 x 10'3 R2 s 4.0 10-3 (9)
0. 2 x 10'3 s Rl s 3. 0 10'' ..,(9)
0.05 X10-3 5 R2 5 1.2 x lO 3 ... (10)
where,
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.., Wmin is the minimum width (mm) of metal strip to be
subjected to heat treatment, and
Wmax is the maximum width (mm) of metal strip to be
subjected to heat treatment.
The furnace may be optionally partitioned into the
inlet portion, the intermediate portion and the outlet
portion of the furnace.
Further, the stepwise increase of the length means
that the value of Lc and the like is increased in the
next roll in adjacent rolls (when upper rolls and lower
rolls are handled as belonging to different roll systems,
adjacent rolls in each system) at least any one position
from the inlet to the outlet of the furnace, while the
same value may be sometimes set in the adjacent rolls.
This is also applicable to the case in which the
inclinations R1 and R2 are reduced stepwise. It is
contemplated as a typical case of the above arrangement
to separate the interior of the furnace into several
blocks and to change the value among the blocks.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 illustrates an exemplary embodiment of a
hearth roll of the present invention;
Fig. 2 is a schematic view of a vertical heat
treating furnace;
Fig. 3 is a graph showing conditions under which
meandering and buckling occur depending upon the width
and thickness of sheet;
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., ,
Fig. 4 is a graph showing the condition of Lc under
which meandering and buckling occur in a vertical heat
treating furnace;
Fig. 5 is a graph showing the condition of (Lc + 2 x
L1) under which meandering and buckling occur in the
vertical heat treating furnace;_
Fig. 6 shows an effect of the present invention of
reducing an operation rate resulting from meandering and
buckling; and
Fig. 7 shows an effect of the present invention of
reducing a speed achieving rate resulting from
meandering and buckling.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
First, a two-stepped taper roll, which is used as an
embodiment of a hearth roll of the present invention,
will be described with reference to Fig. 1.
The hearth roll 10 of the present invention has a
two-stepped taper structure, which is symmetrical on the
right and left sides of the hearth roll 10. The hearth
roll 10 includes a flat section 12 having a length of
Lc (mm) formed at the central portion of the hearth roll
10, first taper sections 14 each having a length of
L1 (mm) formed on both sides of the flat section 12, and
second taper sections 16 each having a length of L2 (mm)
formed on both sides of the first taper sections 14.
The flat section 12 may be approximately flat and,
for example, may be formed as a gentle curved surface
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having a radius of curvature of, for example, at least
100 m.
When the inclination of each the first taper
sections 14 (C1/L1) is represented by R1, and the
inclination of each of the second taper sections 16
(C2/L2) is represented by R2, then R1 > R2.
Further, it is preferable that the boundary 18
between the flat section 12 and each of the first taper
sections 14, and the boundary 20 between each of the
first taper sections 14 and each of the second taper
sections 16 are formed in a round shape without any
corner as a catching part. It is also preferable that
these boundaries 18, 20 are arranged as a convex curve
section 22 and a concave curve section 24, respectively.
However, because it is desirable to make the connecting
portions thereof as gentle as possible, it is preferable
to set the radii of curvature thereof to at least 20 m,
respectively. The two-stepped taper sections 14, 16 on
both sides of the flat section 12 are not necessarily
symmetrical and the inclinations of the two-stepped taper
sections 14, 16 on the right and left sides may be
varied, or the widths of the taper sections 14, 16 may be
varied.
Next, Fig. 2 schematically shows a typical vertical
heat treating furnace to which the present invention is
applied.
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., .
In the example shown in Fig. 2, the vertical heat
treating furnace 30 comprises a heating furnace 32 for
performing heating, and a soaking furnace 34 for
performing soaking, and these furnaces 32, 34 are
arranged continuously. A preheating furnace may be
disposed in front of the heatirig furnace 32. At the
time, however, the hearth rolls of the preheating furnace
can be regarded as the same as a group of hearth rolls on
the inlet side of the heating furnace.
A metal strip 36 enters the furnace from the inlet
of the vertical heat treating furnace 30. That is, the
metal strip 36 enters the heating furnace 32 and is
passed while being suspended by upper hearth rolls 38 and
lower hearth rolls 40 disposed on the upper side and the
lower side, respectively, of the furnace. The metal
strip 36, which is passed in the furnace, is heated by
heating elements 42. While the heating elements 42 are
shown only partly in Fig. 2 for simplicity, a plurality
of the heating elements 42 are disposed at desired
positions in the heating furnace 32 and the soaking
furnace 34. A radiant tube or the like can be used as a
heating element.
A shield plate 44 is conventionally interposed
between a hearth roll 10 and a heating element 42, such
as a radiant tube, so that the crown of the hearth roll
10 is not deformed by the radiant heat from the heating
element 42.
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The most effective position of the shield plate 44
also has been examined. As a result, it has been
confirmed that the effect of the shield plate 44 is not
significant in the latter half section of the heating
furnace 32 and in the soaking furnace 34 where the
temperature of the metal strip 36 approaches the
temperature in the furnace and the temperature of the
heating element 42. It has also been confirmed that the
shield plate 44 has a large effect in the former half
section of the heating furnace 32 where the temperature
of the metal strip 36 is considerably lower than the
temperature in the furnace and the temperature of the
heating element 42.
When the shield plate 44 is not used, both of the
ends of the hearth roll 10 located in the former half
section of the heating furnace 32 are heated by the
heating element 42, while the central portion of the
hearth roll 10 is kept at a low temperature by the metal
strip 36 having a low temperature. Accordingly, the
hearth roll 10 is liable to develop a concave crown,
whereby the metal strip 36 is liable to be meandered.
It has been found that the installation of the
shield plate 44 makes it difficult for both of the ends
of the hearth roll 10 to be heated by the heating element
42 so that the crown of the hearth roll 10 remains in a
normal state and meandering is reduced.
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.= Fig. 3 graphically illustrates how meandering and
buckling properties can be improved, and how operation
can be stabilized, by the two-stepped taper roll of the
present invention as compared to a conventional one-
stepped taper roll. In Fig. 3, the abscissa represents
the sheet width of a metal strfp passed in the furnace
and the ordinate represents the sheet thickness of the
metal strip.
The taper angle of the roll, the radius of curvature
of the taper boundary of the roll, and the like of the
two-stepped taper roll employed in Fig. 3 meet the above-
described preferred conditions according to the present
invention. Further, the occurrence of respective sheet
passing problems was determined depending upon whether
the problems were caused when the sheet passing speed was
lowered by 50% as compared with an ordinary sheet passing
speed (300 m/min). This also is applied likewise to Figs.
4 and 5 which are described below.
The conventional one-stepped taper roll can
approximately prevent meandering and buckling and
stabilize operation when the maximum/minimum ratio of
sheet width of a metal strip (Wmax/Wmin) is less than 1-
2, at most. When, Wmax/Wmin is 2 or more, the occurrence
of buckling cannot be completely prevented in a metal
strip having a large width and a small thickness even if
the length of the flat portion of the hearth roll, the
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.. f.
taper length, and the like thereof are variously
adjusted.
In contrast, in the two-stepped hearth roll of the
present invention, meandering and buckling can be
effectively prevented over a wide range in which
Wmax/Wmin is 2 or more, so long as satisfactory
conditions are used for the hearth roll.
The inventors have conducted more detailed studies
based on the above-described knowledge and completed the
present invention. The knowledge obtained as a result of
the studies carried out by the inventors are described
below.
First, the optimum value of the length Lc of the
flat portion of a hearth roll 10 having a two-stepped
taper as shown in Fig. 1 is determined based on the
minimum sheet width Wmin of a metal sheet to be passed,
and it is preferable to set the length Lc as follows:
0. 5 Wmin s Lc s Wmin ... (1)
When Lc is less than 0.5 Wmin, the width of a sheet
is ordinarily made too large at taper portions and
buckling is likely to occur.
To cope with this problem, it is preferable to vary
the value depending upon the position at which the hearth
roll is disposed in the vertical heat treating furnace 30
as shown in Fig. 2, and it has been found that it is most
preferable to set the value of Lc to satisfy the
relationship 0.5 Wmin s Lc s 0.7 Wmin at the inlet of the
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vertical heat treating furnace 30 (that is, at the inlet
of the heating furnace 32), to prevent the meandering of
a narrow metal strip, and to set the value of Lc to
satisfy the relationship 0.7 Wmin s Lc s Wmin at a
location from the intermediate portion of the furnace to
the outlet of the vertical heat treating furnace 30 (that
is, the outlet of the soaking furnace 34) because the
temperature of the metal strip is increased.
The shape of a sheet is ordinarily improved in the
latter half section of the vertical heat treating furnace
30 and meandering is unlikely to occur. Therefore, it
has been found that it is effective to set Lc to a larger
value within the range of 0.7 x Wmin or more to prevent
buckling.
As shown in Fig. 4, when Wmin is too large at the
inlet of the heating furnace, meandering often occurs
even if the tapers on both of the sides of the hearth
roll are variously adjusted, because the shape of a metal
strip is not yet completely restored, whereby problems
such as the reduction of speed and the like are caused.
On the contrary, unless Lc is set larger from the
central portion of the heating furnace 32 to the soaking
furnace 34 as compared with the inlet of the heating
furnace 32 as shown in Fig. 4, the problem of buckling is
often caused in a wide metal strip, even if the tapers on
both of the sides of the hearth roll are variously
adjusted. However, the maximum value of Lc does not
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,= ,
exceed Wmin. This is because if Lc is set larger than
Wmin, meandering is caused from the central portion of
the heating furnace 32 to the soaking furnace 34 in case
of Wmin, while the shape of the metal strip is corrected
from the central portion of the heating furnace 32 to the
soaking furnace 34.
Next, as a result of repeated studies and tests on
actually operating equipment also as to the width L1 of
each of the first taper sections, it has been found that
it is most preferable to form the hearth roll such that
Lc + 2 x L1 is sequentially increased from the inlet of
the heating furnace 32 to the outlet of the heating
furnace within the range of the following formula (2):
Wmin s Lc + 2 x Ll s Wmax - 400 ... (2)
Lc + 2 x Ll must be set larger than the minimum
width Wmin of a metal strip to prevent the meandering of
a narrow metal strip. Further, when Lc + 2 x L1 is
larger than the maximum width Wmax - 400, buckling is
likely to occur in-a wide metal strip even if any value
is selected as the inclinations Rl and R2 of the two-
stepped taper portions of the hearth roll.
Fig. 5 shows the optimum range of Lc + 2 x Li and
how meandering and buckling are caused when the optimum
range is not satisfied. It has been found that it is
difficult.to completely prevent the occurrence of
meandering and buckling unless Lc + 2 x L1 is set
properly, even if any value is selected as R1 and R2.
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Further, it has become apparent that the inclinations R1
and R2 of the taper portions are preferably set when the
relationship of R1 > R2 is achieved, Ri is set to a value
from 0.2 x 10'3 to 10 x 10'3 and R2 is set to a value from
0.05 x 10'3 to 4 X 10'3.
Further, it has been found-as to R1 and R2 that it
is more preferable for them to satisfy the following
relationships: 3.0 x 10-3 s R1 s 10 x 10-3 and 1. 2 x 10-3
R2 s 4.0 x 10'3, respectively on the inlet of the furnace,
and to satisfy the following relationships: 0.2 x 10'3
R1 5 3.0 x 10-3 and 0.05 x 10'3 s R2 s 1.2 x 10-3,
respectively, on the outlet of the furnace. The
inclinations R1 and R2 are set as described above because
it is preferable to put greater emphasis on the
prevention of buckling in the latter half section of the
furnace in design likewise in the case of Lc and other
parameters.
It is preferable that the respective values of L1
and Lc + 2 x L1 are sequentially increased from the inlet
to the outlet of the vertical heat treating furnace 30,
or, in some embodiments, made equal to each other. As a
method of sequentially increasing the values, the values
may be varied in the former half section and the latter
half section of the vertical heat treating furnace 30 by
dividing the interior the furnace into the two portions.
Otherwise, the values may be sequentially increased at
about three to five steps in some embodiments. Further,
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the values may be sequentially and continuously increased
in other embodiments. It also has been found that
several special rolls such as CPC (meandering correcting)
rolls and the like, which are ordinarily installed in a
S furnace, are not included in the scope of the roll shape
of the present invention because only a small number of
these special rolls are typically used, and the effects
of the present invention can be sufficiently obtained
even if they are arranged as, for example, flat rolls.
It also has become apparent that while hearth
rolls are disposed on the upper portion and the lower
portion in the interior of the vertical heat treating
furnace 30, it is preferable to sequentially increase the
above-described roll param,eters(L1,Lc+2 x Ll) in the
i_~.d=v=duai roll groups of the upper rolls and the lower
rolls because tension is differently imposed on a metal
strip on the upper portion and the lower portion of the
furnace due to the influence of gravity and other
factors.
Several examples of the actual shapes of hearth roll
(prescribed by Lc and L1) are exemplified in TABLE 1.
TABLE 1 shows the relationship between Lc and L1 of
a hearth roll applied in the vertical heat treating
furnace for the respective cases of metal strips which
are passed through the furnace and whose minimum and
maximum widths are Wmin and Wmax.
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TABLE 1 shows the minimum value (min), median value
(mid) and maximum value (max) of L1 to each of the
minimum value (min), median value (mid) and maximum value
(max) of Lc as a matrix with respect to the respective
cases of Wmin and Wmax. The values shown in the
parentheses are not actually used.
In the present invention, it is preferable to use
values in the ranges of the min and mid of Lc and the min
and mid of L1 at the inlet of the heating/soaking furnace
in the vertical heat treating furnace, and it is
preferable to use values within the ranges of the mid and
max of Lc and the mid and max of L1 from the intermediate
portion to the outlet of the heating/soaking furnace in
the vertical heat treating furnace.
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TABLE 1
case Wmin Wmax Wapax Lc (rnzn) Ll (mm) Transfer problem
(mm) (mm) Wn)in
min mid max meadering buckling
min:250 125 150 (175)
1 500 1000 2.0 mid:350 75 100 125 0 O
max:500 (0) 25 50
min:250 125 275 (425)
2 500 1500 3.0 mid:350 75 225 375 0 0
max:500 (0) 150 300
min:400 200 275 (350)
3 800 1500 1.9 mid:560 120 195 270 0 0
max:B00 (0) 75 150
min:400 250 350 (500)
4 800 1800 2.3 mid:560 150 270 420 a 0
max:800 (0) 150 300
min:400 250 400 (600)
5 800 2000 2.5 mid:560 150 300 520 0 0
max:800 (0) 200 400
min:500 250 400 (550)
6 1000 2000 2.0 mid;700 150 300 450 0 0
max:1000 (0) 150 300
min:700 400 450 500
7 800 1800 2.3 mids850 300 350 400 x x
max:1000 250 275 300
min:200 250 275 300
8 800 1800 2.3 mid:300 200 225 250 X X
max:400 150 175 200
Wmin <Lc + 2X L1 < Wmax - 400
Wmin: min width; Wmax: max width o: no problems
X: occurrence of problems
Lc: length of flat section;
L1: length,of first taper sections
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"In the case 7, formula (1) : 0.5 Wmin <_ Lc <_ Wmin and/or formula (2)
Wmin <_ Lc + 2 x L1 < Wman - 400 are not satisfied. That is to say, in a
narrow
metal strip wherein formula (1) is not satisfied, meandering was liable to
frequently
occur whereas, in a wide metal strip wherein formula (2) is not satisfied
because
Lc + 2 x Li increased too much and thereby, buckling was liable to frequently
occur.
By contrast, in the case 8, formula (1), which is a prerequisite for formula
(2), is not satisfied and therefore buckling frequently occurred. But in the
case 8,
the value of Lc + 2 x L1 was small, and thus, the capability of preventing the
occurrence of meandering became smaller and meandering frequently occurred.
The reduction of the operation rate of equipment and
the reduction of a speed achieving rate, which were caused
by meandering and buckling, could be greatly improved as
shown in Figs. 6 and 7 by the use of the present
invention, in addition to that the yield of product, which
was deteriorated by defective products resulting from the
stopping of a line, the reduction of the line speed and
the like, could be improved by an average of 0.2%.
Fig. 6 shows the reduction of the operation rate of
equipment caused by meandering and buckling when the
vertical heat treating furnace according to the present
invention is used and also when a conventional vertical
furnace is used. When a steel sheet is meandered a large
amount, or when it is greatly drawn, operation is finally
carried out at a lowered speed. However, when the degree
of meandering and buckling is greatly increased, the steel
sheet must be subjected to a countermeasure by st-opping
the operation and lowering the temperature of the furnace,
which reduces the operation rate of the equipment_ In
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this case, the reduction of the operation rate of the
equipment is represented by the rate between a time during
which the equipment is interrupted by meandering and
buckling and a working time. The reduction of the
operation rate, which was conventionally about 3%, can be
reduced to 0.5% or less by the employment of the present
invention. The working time is typically represented as a
possible operation time, which is determined by
subtracting the out of work time, the setup change time
and the like, from a calendar time.
Fig. 7 shows the reduction of the speed achieving
rate of equipment caused by meandering and buckling when
the vertical heat treating furnace according to the
present invention was employed and when the conventional
vertical furnace was employed. The speed achieving rate
is the rate between a speed calculated from a capacity of
equipment and an actual operation rate, and it serves as
an index representing a capacity in operation. When
meandering or buckling occurs in a steel sheet, while a
countermeasure is typically employed to continue operation
without the occurrence of serious disadvantages caused by
speed reduction, the speed achieving rate is finally
lowered and the desired amount of production cannot be
achieved. The reduction of the speed achieving rate,
which was conventionally about 7%, can be reduced to about
2% by the employment of the present invention.
23
CA 02308641 2000-08-03
Steel strips were passed through continuous annealing
furnaces (Nos. 1, 2, 4, 5, and 6) having roll arrangements
shown in TABLES 2-4 below and continuous galvanizing
furnaces (Nos. 3, 7 and 8) (distance between upper rolls
and lower rolls was 20 m and a steady sheet passing speed
was 300 m/min).
It has been confirmed in the present invention that
even if the sheet passing speed is varied by 40% or more
(50% under optimum conditions) in the vertical heat
treating furnace capable of coping with a wide range of
sheet width (Wmax/Wmin Z 2), no meandering and buckling
problems occur and sheets can be stably passed.
The shape and size of hearth rolls in the vertical
heat treating furnace, that is, in the heating/soaking
furnace can be optimized by the present invention and
operation can be stably conducted without the occurrence
of meandering and buckling in metal strips having a wide
range of sheet width. As a result, the occurrence of
problems such as the reduction of yield, line stopping,
the reduction of line speed, and other problems can be
prevented.
24
CA 02308641 2000-08-03
N m
N W N O O N M Q1 =-1 O U') m N O O O~
0 14 N m O O O11 1-1 .-1 O rl O O N N
O
H ~
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H til N
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CA 02308641 2000-08-03
N N C
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(V 10 O u) NLn f~ ~ N
m
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a O
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z ' i N nc IV LO l0 h o0
26
CA 02308641 2000-08-03
TABLE 4
block 5 (upper line:upper rolls; lower *2
line:lower rolls)
hearth Lc Ll Lc + R1 R2 meandering remarks
No. rolls (mm) (mm) 2 x L1 buckling
(No. ) (mm)
(10-3 )
1 >50$ present
2:50$ invention
2 12-20 750 400 1550 0.15 0.05 >50$ present
12-20 750 400 1550 0.2 0.1 250$ invention
3 >50$ present
>50% invention
4 >50$ present
>50$ invention
>50$ present
>50% invention
6 40% present
40% invention
7 40% present
40% invention
8 45% present
45% invention
*2: (sheet passing speed) -(steady sheet passing speed) x100$
(steady sheet passing speed)
27
