Note: Descriptions are shown in the official language in which they were submitted.
CA 02718307 2010-09-10
a
METHOD OF CONTROLLING SLIDING NOZZLE DEVICE AND PLATE USED
THEREFOR
TECHNICAL FIELD
[0001] The present invention relates to a method of controlling a sliding
nozzle
device placed at a bottom of a ladle, which is used in continuous casting
facilities, and
also relates to plates used for the sliding nozzle device.
BACKGROUND ART
[0002] A molten steel flow discharged from a ladle to a tundish is controlled
by a
sliding nozzle device. The sliding nozzle device uses a plurality of sliding
nozzle plates
(hereinafter, "sliding nozzle plates" are simply referred to as "plates"),
each having a
nozzle bore and made of a refractory material. While the plurality of plates
are pressed
with a high pressure, at least one of the plates is slid for adjusting an
opening ratio of
the nozzle bore, thereby controlling the molten steel flow.
[0003] One of the known methods of controlling the sliding nozzle device
includes
measuring a weight of molten steel in the tundish, and adjusting an opening
ratio of the
nozzle bore based on (a) a deviation between a measured value and a reference
value of
the molten steel weight, or (b) a change rate of the molten steel weight. For
adjusting
the opening ratio of the nozzle bore, for example, two types of sliding
distances (control
parameters) of the plate, such as long and short, are set in advance, and
these two types
of output signals are transmitted from a control device as pulse signals
according to a
deviation level. An output cycle of the pulse signals is also controlled
according to a
preset value (control parameter), for example, 5 seconds.
[0004] When the opening ratio of the nozzle bore is controlled by the
above-described method, the plate may be slid frequently to maintain molten
steel at a
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constant level, regardless of operational conditions. For this reason,
corrosion of the
plates is accelerated, thereby severely limiting the number of use of the
plates.
[0005] Consequently, Patent Documents 1 and 2 disclose methods of controlling
a
sliding nozzle device, in which a position of a plate is maintained in the
case that a
direction of a change in the measured value is approaching to the reference
value, even
if a weight of molten steel in a tundish has a deviation between a measured
value and a
reference value thereof. With this method, the life of the plates can be
increased, and
further a stability of the weight of molten steel in the tundish can be
improved.
Therefore, it is considered that fluctuations in the weight or a surface level
of molten
steel due to disturbances can be further reduced.
Patent Document 3 discloses a method of controlling a sliding nozzle device by
adjusting an opening ratio of a nozzle bore, based on a relation among a
molten steel
head in a ladle, the opening ratio of the nozzle bore, and molten steel flow
discharged
from the ladle.
[0006] Recently, in accordance with requests to reduce costs and operator's
burden
of handling, there is a growing need for downsizing the plate. For example,
Patent
Document 4 describes that by defining a distance from an edge of a nozzle bore
of a
plate to an end of the plate, the plate can be economically shaped, but does
not cause a
leakage of molten steel.
[Patent Document 1 ] Japanese Unexamined Patent Application Publication No.
62-158556
[Patent Document 2] Japanese Unexamined Patent Application Publication No.
62-158557
[Patent Document 3] Japanese Unexamined Patent Application Publication No.
2
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2003-164951
[Patent Document 4] Japanese Unexamined Patent Application Publication No.
11-138243
DISCLOSURE OF INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[0007] If a steel grade is changed in casting operation, constituents of
molten steel
are changed. Accordingly, the nozzle bore may tend to be clogged, or on the
contrary,
corrosion of the nozzle bore may be accelerated, which may change an amount of
molten steel discharged from the plate. Additionally, if a bore diameter of
the nozzle is
changed, the amount of molten steel discharged from the plate is changed.
Specifically,
if an inclusion attaches to the nozzle bore and a cross-sectional area of the
nozzle bore
decreases, an initially-set sliding distance makes only a small change in
molten steel
flow, which may require two or three continuous slides of the plate in the
direction
where the area of the nozzle bore increases. As a result, the number of slides
becomes
excessive. On the other hand, if the bore diameter of the nozzle increases,
one slide of
the plate opens the nozzle bore too large, which requires the plate to be slid
in the
opposite direction shortly. This also results in an increase of the number of
slides.
[0008] However, since control parameters are fixed, the methods disclosed in
Patent Documents 1 and 2 are not adaptable to the variation in molten steel
flow caused
by some changes in the steel grade, the bore diameter of the nozzle, and so
on. Thus,
this method may not well control the weight of molten steel in the tundish.
What is
worse, corrosion of the plates increases with a rise in frequency of slides of
the plate,
which may shorten the life of the plates. In this case, a troublesome re-
tuning has to be
performed for a control system of the sliding nozzle device.
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[0009] If a drive mechanism of the sliding nozzle device is changed from a
link
drive system, linking a slide frame and a hydraulic cylinder via an arm, to a
direct drive
system, directly coupling the slide frame and the hydraulic cylinder, an
amount of
backlash in a linkage is decreased, so that the plate moves too much compared
to before.
Accordingly, corrosion of the plate increases with the rise in frequency of
slides of the
plate, which may shorten the life of the plate.
[0010] On the other hand, in the method described in Patent Document 3, if the
sliding distance of the plate is long, corrosion of the plate increases, which
may increase
a frequency in opening and closing the nozzle bore. This causes a problem that
the
number of use of the plates is limited severely.
[0011] In general, corrosion of the plates is broadly classified into edge
corrosion Q
(see FIG 10(a)), in which an edge of the nozzle bore is corroded at reducing
the molten
steel flow, and stroke corrosion R (see FIG. 10(b)), in which a sliding
surface of the
plate is corroded due to sliding movements of the plate. As the number of
slides or the
sliding distance of the plate is increased, these two types of corrosion are
accelerated.
Such an increase of corrosion of the plates may cause leakage of molten steel,
so
that the stroke length of the plate has to be twice or more the bore diameter
of the
nozzle, as disclosed in Patent Document 4. Therefore, the conventional method
has a
limitation on reducing a total length of the plate.
[0012] The present invention has been made in view of the above circumstances
and aims to provide a method of controlling a sliding nozzle device and plates
used
therefor, which are operable to automatically optimize sliding conditions of
the sliding
nozzle device and to increase life of the plates, even when operational
conditions are
changed.
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The present invention also aims to provide a method of controlling a sliding
nozzle device and plates used therefor, which are operable to decrease a
corrosion rate
of the plates and the stroke length of the plate.
MEANS FOR SOLVING THE PROBLEMS
[0013] A first aspect of the present invention provides a method of
controlling a
sliding nozzle device in order to control a flow of molten steel discharged
from a ladle
to a tundish in continuous casting, the method comprising measuring a weight
of molten
steel in the tundish, calculating a deviation between the measured value and a
reference
value of the weight of molten steel, and outputting control signals at a
predetermined
cycle, the control signals controlling a sliding distance of a plate based on
the deviation
and/or a change rate in the weight of molten steel in the tundish, wherein an
average
cumulative sliding rate (%/minute) of the plate is calculated, and if the
average
cumulative sliding rate (%/minute) of the plate is out of a preset control
range, a sliding
rate of the plate is changed within a predetermined setting range.
The term "sliding rate (%)" as used herein refers to a value calculated by
dividing a single sliding distance of the plate for controlling the molten
steel flow by the
bore diameter of the nozzle provided in the plate before use. This is because
the molten
steel flow per unit time varies with the bore diameters of the nozzle, and
accordingly the
sliding distance of the plate varies with the bore diameters of the nozzle,
when the
molten steel flow is controlled by the sliding nozzle device. Also, the term
"average
cumulative sliding rate of the plate" as used herein refers to an average
value of a
cumulative sliding rate of the plate per predetermined time, where the
cumulative
sliding rate of the plate is a product of the sliding rate and the number of
slides.
[0014] The inventors of the present invention found out the following: in the
sliding
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nozzle device control, the cumulative sliding rate of the plate, namely, the
product of the
sliding rate and the number of slides, greatly affected the life of the
plates; and it was
possible to automatically optimize the sliding conditions of the device by
controlling
this cumulative sliding rate within an optimal range, even when the
operational
conditions were changed. The first aspect of the present invention is based on
the above
knowledge.
[0015] In the method of controlling the sliding nozzle device according to the
first
aspect of the present invention, it is preferable that the control range for
the average
cumulative sliding rate of the plate is between 0.5 %/minute and 18 %/minute.
If the average cumulative sliding rate is less than 0.5 %/minute, a control
accuracy of the weight of molten steel in the tundish is reduced. If the
average
cumulative sliding rate exceeds 18 %/minute, the life of the plates is
reduced.
[0016] In the method of controlling the sliding nozzle device according to the
first
aspect of the present invention, it is also preferable that the predetermined
setting range
for the sliding rate of the plate is between 3 % and 20 %.
If the sliding rate of the plate is less than 3 %, the control accuracy of the
weight
of molten steel in the tundish is reduced. If the sliding rate of the plate
exceeds 20 %,
the life of the plates is reduced.
[0017] A second aspect of the present invention provides a method of
controlling a
sliding nozzle device used with a ladle, wherein an average sliding rate of a
plate
included in the sliding nozzle device is between 3 % and 20 %.
The term "average sliding rate (%)" as used herein refers to an average value
of
the sliding rate for 60 minutes. Specifically, the average sliding rate can be
expressed in
the following formula: an average sliding rate (%) = 100[(a total sliding
distance of a
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plate for 60 minutes/the number of slides for 60 minutes)/a bore diameter of a
nozzle
provided in the plate before use.]
[0018] As previously explained, when controlling the molten steel flow by the
sliding nozzle device, the molten steel flow per unit time varies with the
bore diameters
of the nozzle, and accordingly the sliding distance of the plate varies with
the bore
diameters of the nozzle. For this reason, the average value of the sliding
rate for 60
minutes is used as the control parameter, and the value of the control
parameter is
defined.
[0019] The average sliding rate is between 3% and 20%, more preferably between
5% and 15%. If the average sliding rate exceeds 20%, the stroke corrosion is
increased
and the life of the plates is reduced. Moreover, the number of slides is
increased, and
thus the stroke corrosion is increased and the life of the plates is reduced.
If the average
sliding rate is less than 3%, the weight of molten steel in the tundish
fluctuates widely,
which deteriorates flow controllability.
[0020] In the second aspect of the present invention, the average sliding rate
is set
between 3% and 20%, and the sliding distance of the plate is reduced to a
minimal
required, thereby reducing the corrosion rate of the plates. In addition, the
number of
slides can be reduced with a reduction of the average sliding rate, thereby
further
reducing the corrosion rate of the plates.
[0021] In the method of controlling the sliding nozzle device according to the
second aspect of the present invention, it is preferable that the number of
slides of the
plate is between 10 times and 60 times per 60 minutes.
The second aspect of the present invention aims to minimize a sliding amount
of
the plate and thereby to reduce the corrosion rate of the plates, by setting
the number of
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slides of the plate per 60 minutes between 10 times and 60 times, more
preferably
between 10 times and 30 times. If the number of slides of the plate for 60
minutes
exceeds 60 times, the corrosion rate of the plates is increased and the life
of the plates is
reduced. On the other hand, if the number of slides of the plate for 60
minutes is less
than 10 times, the weight of molten steel in the tundish fluctuates widely,
which
deteriorates flow controllability.
[0022] In the methods of controlling the sliding nozzle device according to
the first
and second aspects of the present invention, it is preferable that a stroke
length of the
plate is 1.5 times or more but less than 2 times a bore diameter of the nozzle
provided in
the plate.
If the stroke length of the plate is less than 1.5 times the bore diameter of
the
nozzle, the plate may not have an enough corrosion allowance, thereby reducing
the life
of the plates. On the other hand, if the stroke length is twice or more the
bore diameter
of the nozzle, the life of the plates has almost no difference, but the total
length of the
plate is increased.
[0023] The term "stroke length of the plate" as used herein refers to a
distance
between the center of the nozzle bore of the plate and a theoretical point
that is a center
of a nozzle bore of a counterpart plate theoretically projected on the plate,
at a position
where a distance between the center of the nozzle bore of the plate and the
center of the
nozzle bore of the counterpart plate contacting with the plate is the longest,
in the
sliding nozzle device for which the plate is used. In FIG 2, the plate is
positioned such
that the distance between the nozzle bores is the longest, in the sliding
nozzle device
using the plate. Also in the figure, the stroke length of an upper plate is a
distance S
between a center A of the nozzle bore of the upper plate and a theoretical
point B of the
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upper plate, in which the theoretical point B corresponds to the center of the
nozzle bore
of a lower plate.
[0024] In the methods of controlling the sliding nozzle device according to
the first
and second aspects of the present invention, it is also preferable that a
plate used in the
methods has the stroke length of 1.5 times or more but less than 2 times the
bore
diameter of the nozzle.
EFFECT OF THE INVENTION
[0025] In the method of controlling the sliding nozzle device according to the
present invention, when the average cumulative sliding rate of the plate is
out of the
control range, the sliding conditions of the sliding nozzle device can be
optimized
automatically by changing the sliding rate of the plate within the
predetermined setting
range, even if the operational conditions are changed. As a result, the
corrosion rate of
the plates is reduced, thereby improving a tolerance of the plates, and
further
downsizing the plates.
[0026] Also in the method of controlling the sliding nozzle device according
to the
present invention, by setting the average sliding rate between 3% and 20%, the
corrosion rate of the plates is reduced, and the life of the plates is
drastically improved.
Moreover, in the method of controlling the sliding nozzle device and the
plates used
therefor according to the present invention, the stroke length of the plate is
set 1.5 to 2
times the bore diameter of the nozzle, so that the plates can be downsized.
BRIEF DESCRIPTION OF DRAWINGS
[0027] FIG 1 is a schematic view showing a configuration of a sliding nozzle
device, using control methods according to first and second embodiments of the
present
invention.
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FIG 2 is a sectional side view showing plates of the sliding nozzle device.
FIG 3 is a control flowchart showing the method of controlling the sliding
nozzle
device according to the first embodiment of the present invention.
FIG 4 is an explanatory diagram showing a time history in a deviation between
a weight
of molten steel in a tundish and a reference value thereof.
FIG. 5 is a graph showing a relation between a life of a plate and an average
cumulative
sliding rate of the plate.
FIG. 6 is a graph showing a relation between a life of a plate and an average
sliding rate
of the plate.
FIG. 7 is a graph showing a relation between a life of a plate and a stroke
length divided
by a bore diameter of a nozzle.
FIG. 8 is a graph showing a relation between a life of a plate and an average
sliding
rate.
FIG. 9 is a graph showing a relation between a life of a plate and a stroke
length divided
by a bore diameter of a nozzle.
FIG. 10(a) is a sectional side view of plates showing edge corrosion of the
plates.
FIG. 10(b) is a sectional side view of plates showing stroke corrosion of the
plates.
DESCRIPTION OF REFERENCE NUMERALS
[00281 10: sliding nozzle device; 11: ladle; 12: tundish; 13: plate (sliding
nozzle
plates); 13u: upper plate; 13d: lower plate; 14u, 14d: nozzle bore; 15: upper
nozzle; 16:
lower nozzle; 17: slide frame; 18: upside frame; 19: suspending frame; 20:
hydraulic
cylinder; 20a: rod; 21: hydraulic unit; 22: control device; 23: load cell
BEST MODE FOR CARRYING OUT THE INVENTION
[00291 Embodiments of the present invention will be described referring to the
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accompanying drawings for a better understanding of the present invention.
Hereinafter,
a description will be given on sliding nozzle plates including two plates: an
upper plate
(fixed plate) and a lower plate (sliding plate). Basically the same
description can be also
applied to the sliding nozzle plates including three plates: an upper plate
(upper fixed
plate), a middle plate (sliding plate), and a lower plate (lower fixed plate).
[00301 [Configuration of a sliding nozzle device]
FIG 1 shows a configuration of a sliding nozzle device 10, using control
methods according to first and second embodiments of the present invention.
The sliding nozzle device 10 comprises a plate 13 (sliding nozzle plates) and
a
sliding means for sliding the plate 13.
[0031] The plate 13 includes an upper plate 13u and a lower plate 13d having a
nozzle bore 14u and a nozzle bore 14d, respectively. The upper plate 13u is
fixed at a
bottom of a ladle 11 via an upside frame 18, and an upper nozzle 15 is
connected to the
nozzle bore 14u. On the other hand, the lower plate 13d is fixed on a slide
frame 17
located inside of a suspending frame 19, which is openable and closable with
respect to
the upside frame 18. And, the lower plate 13d slides along a lower surface of
the upper
plate 13u. In addition, a lower nozzle 16 is connected to the nozzle bore 14d
of the
lower plate 13d.
[00321 The upside frame 18 extends in a sliding direction of the slide frame
17. At
one end of the upside frame 18 in the extending direction thereof, a hydraulic
cylinder
20 is placed. And, an end portion of a rod 20a of the hydraulic cylinder 20 is
connected
to one end of the slide frame 17.
[00331 A tundish 12 is placed immediately beneath the ladle 11. At the bottom
of
the tundish 12, load cells 23, 23 are provided for measuring a weight of
molten steel in
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the tundish 12. Outputs from the load cells 23, 23 are input to a control
device 22. The
control device 22 outputs control signals, corresponding to the output values
from the
load cells 23, 23, to a hydraulic unit 21. The hydraulic unit 21 activates the
hydraulic
cylinder 20 according to the control signals, and slides the slide frame 17.
[0034] [Method of controlling the sliding nozzle device according to a first
embodiment of the present invention]
Referring to a control flow chart in FIG 3, a description will be given on a
method of controlling the sliding nozzle device according to the first
embodiment of the
present invention.
[0035] (1) The control device 22 receives the output signals transmitted from
the
load cells 23, 23, which are placed at the bottom of the tundish 12 (S 1).
(2) The control device 22 performs a conventional automatic control of the
sliding nozzle device, in which a control force of the hydraulic cylinder 20
is calculated
based on a deviation between the output signals from the load cells 23, 23 and
the
reference value thereof. Then, the control device 22 outputs the control
signals to the
hydraulic unit 21, and the hydraulic unit 21 drives the hydraulic cylinder 20
based on
the control signals and slides the lower plate 13d, thereby controlling the
opening ratio
of the nozzle bore (S2). The opening ratio is controlled in the same method as
disclosed
in Patent Document 1 and so on. That is to say, predetermined ranges are set
between
the reference values and change rates of the molten steel weight as shown in
Table 1,
and types of the control signals are determined within each reference setting.
Also, an
output cycle of the control signals is set at 5 seconds.
[0036] [Table 1 ]
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Control Reference for
K : -A -A<K : 0 0<K:_5 +A +A<K
Molten Steel Weight
+W3 1.0 to 3.0% Stay Close small Close large Close large
+W2 0.5 to 1.0% Stay Stay Close small Close large
+W1 0 to 0.5% Stay Stay Stay Close small
-W1 -0.5 to 0% Open small Stay Stay Stay
-W2 -1.0 to -0.5% Open large Open small Stay Stay
-W3 -3.0 to -1.0% Open large Open large Open small Stay
[00371 In Table 1, "K" represents the change rate in the weight of molten
steel (kg/5
sec.), and "A" represents a constant. "Close small" indicates a pulse signal
for sliding a
sliding plate for a short distance, in a direction where an opening area of a
nozzle bore
becomes small. "Close large" indicates a pulse signal for sliding a sliding
plate for a
long distance, in a direction where an opening area of a nozzle bore becomes
small.
"Open small" indicates a pulse signal for sliding the sliding plate for a
short distance, in
a direction where the opening area of the nozzle bore becomes large. "Open
large"
indicates a pulse signal for sliding the sliding plate for a long distance, in
a direction
where the opening area of the nozzle bore becomes large. In this instance,
provided that
the sliding distances of the plate are 5 mm and 10 mm and the bore diameter of
the
nozzle is 85 mm, then the sliding rate in "Close small" and "Open small" is
each 6%,
and the sliding rate in "Close large" and "Open large" is each 12%. In a state
of "Stay,"
the sliding plate does not slide.
[00381 (3) After outputting the control signals in Table 1, to obtain an
optimal
control, the control device 22 adjusts the control parameter related to the
sliding
distance of the plate in the following procedures.
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First, an average cumulative sliding rate (%/minute) of the plate is
calculated
(S3).
[00391 The average cumulative sliding rate (%/minute) of the plate is
calculated
based on a cumulative sliding rate (%) of the plate and the number of slides
(number of
times) of the plate within a predetermined period. In this embodiment, the
cumulative
sliding rate (%) of the plate is calculated based on the types of the control
signals and
the number of outgoing control signals for sliding the plate, both of which
are initially
set. For example, in Table 1, if "Open large (12%)" is transmitted twice,
"Close large
(12%)" is transmitted once, "Close small (6%)" is transmitted once, and "Stay"
is
transmitted twice for last ten minutes, then the cumulative sliding rate for
ten minutes is
42%. And, the number of the outgoing control signals for sliding the plate in
this period
is four times, so that the average cumulative sliding rate of the plate for
ten minutes is
10.5%/minute.
[00401 As for the control signals for sliding the plate, the sliding rates of
the plate
can be calculated by taking an actual measurement on each control signal
(pulse signal)
and the sliding distance of the plate under the condition that the plate is
before use and
pressure is put on a surface of the plate. Alternatively, a position sensor
may be
provided in a drive device such as the hydraulic cylinder, and measured
results thereof
may be used as the sliding distance of the plate. Further alternatively, an
actual sliding
distance of the plate may be measured.
[00411 A calculation of this average cumulative sliding rate (%/minute) of the
plate
begins at least 5 minutes or more prior to the starting time of the
calculation (the time of
the control signal output). If less than 5 minutes, an accuracy of the average
cumulative
sliding rate (%/minute) is decreased. There is no particular upper limit on
the period for
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calculating the average cumulative sliding rate (%/minute), so that the
calculation
period can be a cumulative time from the start to the end of casting in the
ladle, for
example. In this case, the sliding distance and the number of slides of the
plate due to
the control signals are continuously counted since the control starts right
after a
beginning of casting, and then, in every output cycle (e.g., 5 seconds) of the
control
signals, the average cumulative sliding rate is calculated based on data that
has been
accumulated since the beginning of casting. In addition, an arbitrary
specified period,
namely, 5 minutes to 60 minutes prior to the time of the control signal
output, may be
determined as the calculation period.
[0042] (4) The average cumulative sliding rate of the plate is examined
whether or
not it is within a control range between 0.5%/minute and 18%/minute (S4).
(5) When the average cumulative sliding rate is less than 0.5%/minute, the
control parameter related to the sliding distance of the plate is changed to
increase the
sliding distance of the plate. When the average cumulative sliding rate
exceeds
18%/minute, the control parameter related to the sliding distance of the plate
is changed
to decrease the sliding distance of the plate (S6). When the average
cumulative sliding
rate is less than 0.5%/minute, the control accuracy of the weight of molten
steel in the
tundish is reduced, and when the average cumulative sliding rate exceeds
18%/minute,
the life of the plates is reduced.
[0043] In this regard, it is more preferable to set the sliding rate of the
plate in the
range between 3% and 20%. If the sliding rate of the plate is less than 3%,
the control
accuracy of the weight of molten steel in the tundish is reduced, and if the
sliding rate of
the plate exceeds 20%, the life of the plates is reduced. As for a setting of
the sliding
rate of the plate, if a plurality of the control signals is used, an average
value thereof can
CA 02718307 2010-09-10
be set as the sliding rate of the plate. For example, in Table 1, the sliding
rates of the
plate are 6% and 12% as the control signals. In this case, the average sliding
rate is 9%.
[0044] As for a sliding speed of the plate or the output cycle of the control
signals,
in addition to the sliding rate of the plate, the predetermined control range
is set and the
control is performed with variable control parameters, which can further
improve the
accuracy of the control method of this embodiment.
[0045] (6) If the average cumulative sliding rate of the plate is within the
control
range, it is examined whether or not a casting operation is completed (S5).
(7) If the casting operation is not completed yet, the above-described
procedure (1) and subsequent procedures are performed again from the step S I.
On the
other hand, if the casting operation is completed, the sliding nozzle device
10 is
stopped.
[0046] In the control method of this embodiment, in addition to the average
cumulative sliding rate (%/minute) of the plate, a cycle (minute) of the
weight of molten
steel in the tundish and/or the number of inflection points (number/minute) in
the
weight of molten steel in the tundish are controlled, which can improve the
accuracy of
the flow control.
[0047] FIG 4 shows a time history in the deviation between the weight of
molten
steel in the tundish and the reference value thereof. FIG. 4 shows results of
the control
performed by the control method shown in the flow of FIG. 3, using the sliding
nozzle
device of FIG. 1. Before this control method was used, the average cumulative
sliding
rate (%/minute) of the plate was 20%/minute, which was out of the range set in
this
embodiment. However, by changing the control parameter related to the sliding
distance
of the plate, namely, by changing the sliding rates of the plate into 12% and
6% (see
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Table 1), the average cumulative sliding rate of the plate became 9%/minute.
In other
words, the sliding distance of the lower plate was decreased, and what was
more, the
number of slides of the plate was reduced, and the life of the plates was
increased. Also,
in FIG 4, a fluctuation cycle of a weight deviation was extended after the
control turned
on.
[00481 FIG. 5 shows a relation between the life of the plates and the average
cumulative sliding rate of the plate. If the average cumulative sliding rate
of the plate is
18%/minute or less, the life of the plates is increased. Moreover, if the
average
cumulative sliding rate is 12%/minute or less, the life of the plates is
further increased.
If the average cumulative sliding rate exceeds 18%/minute, the edge corrosion
and the
stroke corrosion of the plate are increased, thereby reducing the life of the
plates.
[00491 FIG. 6 shows a relation between the life of the plates and the average
sliding
rate of the plate. Tests were conducted under the condition that the average
cumulative
sliding rate of the plate was 18% or less. If the average sliding rate of the
plate is 20%
or less, the life of the plates is increased. Moreover, if the average sliding
rate is 10% or
less, the life of the plates is further increased. If the average sliding rate
exceeds 20%,
the edge corrosion and the stroke corrosion of the plates are increased,
thereby reducing
the life of the plates.
[00501 FIG. 7 shows a relation between the life of the plates and the stroke
length
divided by the bore diameter of the nozzle. The tests of FIG.7 were performed
by the
control method shown in FIGS. 1 and 3, but only the stroke length of the plate
was
changed due to a setting change of the sliding nozzle device. The tests were
conducted
using three pieces of the plates with respect to each stroke length, and the
test results
were evaluated on an average value of the life of the plates. The test results
showed that
17
CA 02718307 2010-09-10
the life of the plates was rapidly reduced if the stroke length was less than
1.5 times the
bore diameter of the nozzle, but the life of the plates had no significant
change even if
the stroke length was 2 times or more the bore diameter of the nozzle.
[00511 The tests of FIGS. 5 to 7 were performed using the plate having a
length of
600 mm, a width of 260 mm, a thickness of 50 mm, and a nozzle bore diameter of
85
mm. The plate was made of tar-impregnated alumina-carbon material with 80% or
more
of A1203. At the tests, pressure acting on the surface of the plate was 100
kN, a casting
period was 45 to 55 minutes per one heat, and a ladle capacity was 300 ton.
[0052] The number of slides and the sliding distance (mm) were measured by an
operator at the side of the sliding nozzle device. The average sliding rate
was calculated
in the following formula: an average sliding rate (%) = 100[(a total sliding
distance of a
plate for 60 minutes/the number of slides for 60 minutes)/a bore diameter of a
nozzle
provided in the plate before use.] In addition, the sliding distance and the
number of
slides were measured for 60 minutes in the same ladle. For example, when one
heat for
a certain ladle was completed in 45 minutes, a further 15-minute heat for the
same ladle
was taken into account in order to measure the sliding distance and the number
of slides
for 60 minutes in total. Here, the sliding distance and the number of slides
excluded the
following: slides of the plate for setting a predetermined opening ratio of
the nozzle
bore provided in the plate, at the start of the slide; and slides of the plate
for stopping a
discharge of molten steel, at the end of casting and in an emergency. Also,
the tests of
FIGS. 5 and 6 were performed by changing the sliding speed of the plate, the
sliding
distance of the plate, a range of a dead zone that keeps the position of the
plate, the
output cycle, and so on.
[0053] [Method of controlling a sliding nozzle device according to a second
18
CA 02718307 2010-09-10
embodiment of the present invention]
Descriptions will be given on a method of controlling a sliding nozzle device
according to a second embodiment of the present invention.
[0054] Provided that an internal diameter of nozzle bores 14u, 14d is "D," a
sliding
distance of the plate 13 is 0.2D if an average sliding rate of the plate 13 is
20%, whereas
the sliding distance of the plate 13 is 0.03D if the average sliding rate of
the plate 13 is
3%. A lower plate 13d is controlled by pulses output from the control device
22. Thus,
when the lower plate 13d is controlled by two types of pulses, namely, large
pulses and
small pulses, the average sliding rate of the plate 13 becomes between 3% and
20%
theoretically if the sliding distance controlled by the large pulses is set at
0.2D or less,
and the sliding distance controlled by the small pulses is set at 0.3D or
more. The same
can be applied to the case of using a plurality of pulses. In other words, it
is only
necessary to set the sliding distance controlled by a maximum pulse at 0.2D or
less, and
the sliding distance controlled by a minimum pulse at 0.03D or more.
[0055] Next, a description will be given on results of a control test for a
sliding
nozzle device 10, using the average sliding rate as a parameter.
[0056] FIG. 8 is a graph showing a relation between a life of the plates and
the
average sliding rate. The life of the plates in a vertical axis of the graph
indicates the
number of heats where the plates were usable. An operator visually observed
the edge
corrosion and the stroke corrosion at the surface of the plate used, and
examined
whether or not the plate could be used again.
[0057] The tests were conducted using the plate having a length of 600 mm, a
width
of 260 mm, a thickness of 50 mm, and a nozzle bore diameter of 85 mm. The
plate was
made of tar-impregnated alumina-carbon material with 80% or more of A1203. In
19
CA 02718307 2010-09-10
addition, a means for sliding the plate in the sliding nozzle device 10 had a
stroke of
160 mm, and the plate 13 had a stroke length S of 160 mm (see FIG 2). At the
tests, the
pressure acting on the surface of the plate was 100 kN, a casting period was
45 to 55
minutes per one heat, and a ladle capacity was 300 ton.
[0058] In the tests, using the method of controlling the sliding nozzle device
disclosed in Japanese Unexamined Patent Application Publication No. 62-158556
(JP
62-158556), the number of slides of the plate 13 was also controlled.
According to the method of controlling the sliding nozzle device disclosed in
JP
62-158556, a position of the plate is maintained (a) when a measured value
according to
load cells 23, 23 is within a dead zone close to a reference value, or (b)
when the
measured value is out of the dead zone, but a deviation between the measured
value and
the reference value is within a predetermined range and the measured value is
approaching to the reference value.
[0059] In this test, the number of slides of the plate was controlled by
adjusting a
setting of the sliding speed of the plate, a setting of the sliding distance
of the plate, and
a range of the dead zone where the position of the plate was maintained.
However, a
control range of a weight of molten steel in a tundish was within 1 percent
by mass.
[0060] As for the sliding distance of the plate, two types of long and short
travel
distances were set. In the case that the plate slid in the same direction
twice or more, the
number of slides was counted as twice or more. The sliding distance was set
based on a
period and an oil quantity for exciting an electromagnetic valve in a
hydraulic system.
[0061] The number of slides and the sliding distance (mm) were measured under
the same conditions as the above-described method of controlling the sliding
nozzle
device according to the first embodiment of the present invention, and the
average
CA 02718307 2010-09-10
sliding rate was calculated in the above-described formula.
[00621 In FIG 8, the number of slides is divided into groups per 10 slides,
and the
life of the plates of each group is plotted on the average sliding rates.
According to FIG
8, it is obvious that the life of the plates is increased as the average
sliding rate is
decreased. Specifically, the life of the plates is drastically increased as
the average
sliding rate becomes 20% or less, whereas the life of the plates is extremely
decreased
as the average sliding rate exceeds 20%. In addition, the life of the plates
becomes
longer with lesser number of slides. Especially, the life of the plates became
the longest
when the number of slides was 10 to 30 times. When the number of slides
exceeded 60
times, the life of the plates became 7 times or less even if the average
sliding rate was
reduced.
In addition, when the average sliding rate was less than 3 % or when the
number
of slides was less than 10 times, the control range of the weight of molten
steel in the
tundish exceeded 3%, and the flow controllability was slightly decreased.
[00631 FIG. 9 shows a relation between the life of the plates and the stroke
length
divided by the bore diameter of the nozzle. As for the plate, the plate used
in FIG. 8 was
also used, but only the stroke length was changed due to a setting change of
the sliding
nozzle device. Test results were obtained under the same test conditions as
the
above-described method in FIG. 8, except for the conditions that the number of
slides
was within 21 to 30 times and the average sliding rate was within 10 to 15%.
The test
was conducted using three pieces of the plates with respect to each stroke
length, and
the test results were evaluated on an average value of the life of the plates.
The test results showed that the life of the plates was rapidly reduced if the
stroke length was less than 1.5 times the bore diameter of the nozzle, but the
life of the
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CA 02718307 2010-09-10
plates had no significant change even if the stroke length was 2 times or more
the bore
diameter of the nozzle.
[00641 A level fluctuation of molten steel in a mold adversely affects steel
quality,
so that a molten steel flow from a tundish to the mold is controlled with a
high accuracy.
Consequently, by reducing the number of slides of the plate in the control of
the molten
steel flow from a ladle to the tundish, even when a fluctuation in an amount
of molten
steel in the tundish increases in some degree, such fluctuation can be
absorbed by
controlling the molten steel flow from the tundish to the mold. Specifically,
a control
range of the weight of molten steel in the tundish is preferably within 3
percent by
mass, and more preferably within 1 percent by mass. These control ranges have
a
smaller influence on the level fluctuation of molten steel in the tundish, and
have no
harmful effects on a quality of steel to be a product.
[00651 While the embodiments of the present invention have been described
above,
the present invention is not limited to the above-described embodiments, and
other
embodiments and various modifications may be made without departing from the
scope
or spirit of the present invention.
INDUSTRIAL APPLICABILITY
[00661 The present invention can be used in a sliding nozzle device which
controls
a molten steel flow discharged from a ladle to a tundish. The present
invention can
automatically optimize sliding conditions of the sliding nozzle device even if
operational conditions are changed. Accordingly, a corrosion rate of a plate
is reduced,
and a life of the plates is drastically improved.
22
