Note: Descriptions are shown in the official language in which they were submitted.
123~S70
METHOD OF CONTROLLING THE temperature OF
STEEL STRIP IN THE COOLING ZONE OF A CON-
TENUOUS ANNEALING FURNACE
BACKGROUND OF THE INVENTION
_ .
1. Field of the Invention:
This invention relates to a method of controlling
the temperature of steel strip in the cooling zone of a
continuous annealing furnace.
2. Description of the Prior Art:
A continuous annealing furnace is an apparatus
which performs the heat treatment of a cold rolled steel
strip in accordance with a heat cycle as shown by way of
example in FIGURE 1 in order to improve its workability.
The heat treatment according to the heat cycle of FIGURE
1 is effected by a furnace having a fast cooling zone and
a final cooling zone.
There is known a system for cooling a steel strip
in the cooling zone of a continuous annealing furnace by
a combination of roll cooling and gas jet cooling. FIGURE
2 shows the cooling system disclosed in Japanese Patent
Publication No. 10973/1981. A steel strip 1 travels in
contact with four cooling rolls 2 to 5 in which a coolant
flows, and its heat is transferred to the cooling rolls.
Four plenum chambers 6 to 9 are provided in front of the
cooling rolls 2 to 5, respectively, for blowing jets of
I.`,
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cooling gas against the strip 1. Each plenum chamber
has a plurality of nozzles through which the cooling gas
is jetted out of the plenum chamber. The cooling gas
having an elevated pressure is supplied through blowers
12 and 13 and blown against the strip 1 through the plenum
chambers 6 to 9 not only for cooling the strip 1, but also
for bringing it into intimate contact with the cooling
rolls 2 to 5, as the failure of the strip to contact the
cooling rolls intimately due to the distortion of its edge
or central portion results in the lack of cooling uniform
mitt. FIGURE 2 also shows a pair of deflector rolls 10
and 11, devices 22 for moving the cooling rolls to control
the angle at which the strip is brought into contact with
the cooling rolls, pressure controllers 23 for controlling
the pressure of the cooling gas in the plenum chambers,
sensors 24 for detecting the pressure of the cooling gas
in the plenum chambers and dampers 25.
The coolant is circulated through the cooling rolls
2 to 5 by a circuit which is shown by way of example in
FIGURE 3 as disclosed in Japanese Patent Application No.
130457/1982, which was laid open under Laid-Open No. 23826/1982.
FIGURE 3 shows only one of the cooling rolls at 2 for the
sake of simplicity. The coolant is returned from the
cooling roll 2 to a coolant tank 15 through a discharge
line 14. It is delivered by a pump 16 from the
tank 15 to a heat exchanger 17 in which it is cooled, and
supplied to the cooling roll 2 by a supply line 18. The
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temperature of the coolant entering the cooling roll is
controlled by a coolant temperature controller lo. More
specifically, the coolant temperature is detected by a
sensor 20 and the flow of cooling water entering the heat
exchanger 17 is controlled by a control valve 21 to obtain
an appropriate coolant temperature set by the temperature
controller lo.
The cooling system employing the combination of
roll cooling and gas jet cooling as hereinabove described
lo effects the control of the temperature of the strip 1 by
controlling three variable factors, i.e., the length of
a strip portion contacting each cooling roll or in other
words the angle at which the strip is brought into contact
with each cooling roll, the coolant temperature and the
flow rate of the cooling gas blown against the strip. The
control of the coolant temperature has already been de-
scribed. The angle at which the strip contacts the cool-
in rolls is controlled by the roll moving devices 22.
The flow rate of the cooling gas is controlled by adjust-
in the position of each damper 25 so that the pressure of
the cooling gas detected by the pressure sensors 24 may
reach a predetermined level.
In order to cool the strip to a predetermined final
temperature, it is necessary to control the three factors,
i.e., (l) the flow rate of the cooling gas, (2) the angle
at which the strip is brought into contact with the cooling
lZ3~570
rolls and (3) the coolant temperature, by taking into
account not only the target s-trip temperature, but also
other conditions of the annealing operation such as the
dimensions of the strip and the speed of its travel. In-
solar as there exist a plurality of factors to be con-
trolled, however, it is impossible to determine each India
visual factor in a definite fashion. It is necessary to
develop some procedure which enables an appropriate control
of those factors.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a
method of controlling the temperature of a steel strip in
the cooling zone of a continuous annealing furnace so that
a high yield of production may be achieved.
According to one aspect of this invention, a pro-
determined strip temperature is obtained in the cooling
zone by controlling the flow rate of the cooling gas, the
angle at which the strip is brought into contact with the
cooling Lolls and the coolant temperature in the order
mentioned.
According to another aspect of this invention,
its object is attained by controlling first the flow rate
of the cooling gas and the angle of the strip simultane-
ouzel and the coolant temperature thereafter.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a graph showing by way of example a
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heat cycle which is employed in a continuous steel strip
annealing furnace;
FIGURE 2 is a schematic diagram of a system for
cooling strip by a combination of roll cooling and gas jet
cooling;
FIGURE 3 is a schematic diagram of a circuit for
the circulation of a coolant through cooling rolls;
FIGURE 4 is a graphical representation of the
control of three factors during the cooling of strips have
in a gradually increasing gauge;
FIGURE 5 is a graphical representation of the
control of the three factors during the cooling of strips
having a gradually decreasing gauge;
FIGURE 6 is a graphical representation of the
control of the three factors during the cooling of strips
having a gradually increasing gauge and a gradually degrees-
in gauge;
FIGURE 7 is a flow chart showing by way of example
the sequence of calculations for setting the three factors
to control the temperature of each strip to be cooled; and
FIGURE 8 is a schematic diagram showing another
example of the cooling system employing the combination of
roll cooling and gas jet cooling.
DETAILED DESCRIPTION OF THE INVENTION
According to an essential feature of this invention,
the control of the strip temperature is effected by varying
1~3957~)
the three control factors or parameters in a specific
order of preference. This order of preference is deter-
mined by the quickness in the response of control. Into-
far as the strip takes only several seconds to pass through
the cooling zone, the quickness in the response of control
has the most important bearing on an improved yield of
production in the event there is any change in the condo-
lions of the annealing operation, for example, during the
passage of a welded joint between two strips which are
different in gauge. The three control factors or pane-
meters differ from one another in response time, as follows:
(1) Flow rate of cooling gas - About 10 seconds;
(21 Angle of strip relative to the cooling rolls -
About two minutes; and
(3) Coolant temperature - About 10 minutes.
The inventors of this invention have, therefore,
thought of establishing a specific order of preference or
priority when varying the three factors in order to obtain
an appropriate heat cycle suited for the annealing cord--
lions and thereby an appropriate final cooling temperature
for the strip. According to a first aspect of this invent
lion, priority is in the order of (1) flow rate of cooling
gas, (2) angle of the strip relative to the cooling rolls
and (3) coolant temperature. This is exactly the order
of quickness in response and enables a high yield of product
lion irrespective of any change in the annealing conditions.
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According to a second aspect of the invention, the flow
rate of cooling gas and the angle of the strip relative
to the cooling rolls are first controlled simultaneously,
and the control of the coolant temperature having a longer
response time is delayed. This delay ensures an improved
yield of production. The simultaneous control of the
flow rate of the cooling gas and the angle of the strip
has the advantage of achieving a large change in the cool-
in conditions at a time within a relatively short length
of time.
The invention will now be described by way of
example with reference to FIGURES 4 to 7 of the drawings.
FIGURE 4 is a graphical illustration of the control of the
three factors for the continuous cooling of a plurality
of strips which gradually increase in gauge, while the
other annealing conditions remain the same for all the
strips. The points marked o in FIGURE 4 are the starting
points. As long as there is any room for an increase in
the flow rate of cooling gas, it is increased with an in-
crease in strip gauge as shown at A in FIGURE 4, since its
control has a faster response than the control of the other
two factors. If the flow rate of cooling gas has reached
its maximum level, the angle of the strip relative to the
cooling rolls is, then, increased with an increase in
strip gauge as shown at B in FIGURE 4, as long as there is
any room for such increase in the angle. If it has, then,
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become impossible to increase the angle to any further
extent, the coolant temperature is lowered, as shown at
C in FIGURE 4, in order to increase the cooling capacity
of the cooling system, though it shows a slower response
to control than the other two factors.
FIGURE 5 is a graphical illustration of the control
of the three factors for the continuous cooling of a
plurality of strips which gradually decrease in gauge.
With a gradual reduction in strip gauge, the flow rate of
cooling gas is first decreased as shown at A' in FIGURE 5,
then the angle of the strip relative to the cooling rolls
is decreased as shown at B', and finally the coolant them-
portray is raised as shown at C'.
FIGURE 6 is a graphical illustration of the control
of the three factors for the continuous cooling of a
plurality of strips having first a gradually increasing
gauge and then a gradually decreasing gauge. The control
under this situation may be effected by a combination of
the procedures shown in FIGURES 4 and 5. It will, however,
be noted that FIGURE 6 shows an example of operation in
which the coolant temperature is not varied irrespective
of the change in strip gauge.
In order to ensure stabilized or reliable control
of strip temperature, it is important to leave some allow-
ante for each of the maximum and minimum values to be set
for each of the three factors. Referring, for example,
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to the flow rate of cooling gas, its maximum value may
be 90% of the Blair capacity and its minimum value may
be the minimum required for avoiding nonuniform cooling
plus, say, 10~ of the blower capacity. The minimum flow
rate required for avoiding nonuniform cooling can be ox-
twined experimentally.
FIGURE 7 is a flow chart showing by way of example
the calculations which are performed for setting the three
factors at optimum levels for the cooling of each strip.
In FIGURE 7, the following symbols have the following
meanings:
To : Final strip temperature as measured;
To : Target value of final strip temperature;
Sue: Upper limit of final strip temperature
( g ' SUE So I);
SLY: Lower limit of final strip temperature
(e.g., SLY = TO - 20);
P : Pressure of cooling gas in the plenum chambers;
: Angle of strip relative to the cooling rolls.
The cooling curve, flow rate of cooling gas, angle
of strip relative to the cooling rolls and coolant tempera-
lure may be calculated as will hereinafter be described.
(i) Calculation for the Cooling Curve:
The characteristics of strip cooling by the system
shown in FIGURE 2 may be expressed by the following equal
lion:
~23957(3
do
do Tots + KIT -T ) (1)
where To strip temperature (C);
Tug: temperature of cooling gas at the nozzles (C);
T : average temperature of the coolant in the cool-
in rolls (C);
: coefficient of heat transfer between strip and
cooling gas ~Kcal/m2hC);
K : coefficient of heat transfer between strip and
coolant in the cooling rolls (Coulomb hC);
c : specific heat of strip ~Kcal/kgC);
y : specific gravity of strip (kg/m );
v : velocity of travel of strip (m/h);
d : strip gauge em );
x : strip length em).
The cooling curve for strip can be obtained by the integral
lion of equation (1) by the length of the strip portion cooled by
the cooling rolls and the cooling gas. The relationship
between the coefficient of heat transfer to the cooling
gas and the flow rate of the cooling gas yin the case of
cooling by the system of FIGURE 2, the pressure P of the
cooling gas in the plenum chambers) may be determined
experimentally as expressed, for example, by the following
equation:
P = at- 2 a I
where P: pressure of the cooling yes in the plenum chambers;
at, a and a: constants.
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(ii) Calculation of the Flow Rate of Cooling Gas:
The pressure P of the cooling gas in the plenum
chambers which makes it possible to obtain the target value
of the final strip temperature may be determined in
S accordance with equations (1) and (2) by taking into account
the specific values of the angle of the strip relative to
the cooling rolls and the coolant temperature Two which are
known.
(iii) Calculation of the Angle of Strip Relative to
the Cooling Rolls:
The angle which makes it possible to obtain the
target value of the final strip temperature may be deter-
mined in accordance with equations (1) and (2) by taking
into account the specific values of the flow rate of the
cooling gas or the pressure P of the cooling gas in the
plenum chambers and the coolant temperature Two which are
known. The angle may be expressed by the following equal
lion:
Strip length Angle of strip Outside
cooled by the = relative to x 3 x dia. of
cooling rolls em) the rolls (deg.) 60 the rolls
(m)
(iv) Calculation of the Coolant Temperature:
The coolant temperature Two which makes it possible
to obtain the target value of the final strip temperature
may be determined in accordance with equations (1) and (2)
by taking into account the flow rate of the cooling gas or
the pressure P of the cooling gas in the plenum chambers
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and the angle of the strip relative to the cooling rolls
which are known.
The calculations set forth at (i) to tip) above
are easy to perform by an electronic computer based on
the information on the annealing of each particular strip,
such as the strip dimensions, line speed and target heat
cycle.
The foregoing description has been based on the
use of the cooling system shown in FIGURE 2. This invent
lion is also applicable to the cooling of strip by other
cooling apparatus. Another cooling system is shown by
way of example in FIGURE 8. It includes gas jet coolers
26 and 27 by which the strip is cooled before it is cooled
by the cooling rolls 2 to 5. The gas jet coolers can
alternatively be disposed downstream of the cooling rolls.
It is also possible to combine the cooling systems of
FIGURES 2 and 8 by, for example, disposing the gas jet
coolers of FIGURE 8 upstream of the cooling system of
FIGURE 2. In the event the combined cooling system of
FIGURES 2 and 8 is utilized, it will be appropriate to
employ four control factors, namely:
(1) Flow rate of the cooling gas blown by the gas jet
coolers of FIGURE 8;
I Flow rate of the cooling gas blown by the gas jet
coolers facing the cooling rolls as shown in FIGURE
2;
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(3) Angle of the strip relative to the cooling rolls;
and
(4) Coolant temperature.
The foregoing description of FIGURES 4 to 8 has
been based on the control of one of the factors at a time
in accordance with the order of priority. According to
this invention, it is also possible to control the flow
rate of cooling gas and the angle of the strip relative
to the cooling rolls simultaneously as herein before set
forth. In this case, it is necessary to find experimentally
the proportions of the contributions which those two factors
make to the cooling of the strip, and it it, then, possible
to determine the values in accordance with equations (1)
and (2).
As is obvious from the foregoing description, this
invention is most saliently characterized by effecting the
preferential control of one or two factors or parameters
having a better response to control than the other factor
or factors, while keeping the other factor or factors in-
tact as far as practically feasible. Thus, the method of
this invention makes it possible to achieve the best posy
sidle yield of production irrespective of any change in
the annealing conditions.
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