Method of controlling tensions in continuous annealing furnace and system therefor

Helper rolls in a continuous annealing furnace are divided by a master speed hearth roll serving as the boundary into a plurality of control blocks disposed forwardly and rearwardly of the master speed hearth roll, and the speed of rotation of the master speed hearth roll is used as a reference speed. Tension of a steel strip are continuously controlled on the basis of values detected by a tension meter in the plurality of control blocks towards the inlet of the furnace for the helper rolls disposed forwardly of the master speed hearth roll and towards the outlet of the furnace for the helper rolls disposed rearwardly of the master speed hearth roll.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of controlling tension in a 
continuous annealing furnace provided therein with tension control means, 
and a system therefor. 
2. Description of the Prior Art 
Recently, annealing processes for rendering predetermined processability, 
deep drawing properties and the like to cold-rolled steel strips have been 
carried out by continuous annealing furnaces. These continuous annealing 
furnaces each comprise a heating zone for heating the steel strip to a 
predetermined temperature, a soaking zone for holding the steel strip at a 
predetermined soaking temperature and a cooling zone for cooling the steel 
strip to substantially room temperature. The cooling zone further includes 
a rapid cooling zone for rapidly cooling the steel strip at a 
predetermined cooling rate, a slow cooling zone for slowly cooling the 
steel strip or holding same at a predetermined temperature to effect 
overaging treatment, and the like. Consequently, the above-described 
continuous annealing furnace generally forms a long continuous line, and 
therefore, it is necessary to render appropriate tension to the steel 
strip in the furnace in order to maintain stabilized operating conditions 
in the furnace. 
FIG. 1 is an explanatory view showing a general example of the conventional 
continuous annealing furnace. As shown in FIG. 1, the continuous annealing 
furnace comprises a heating zone 1, a soaking zone 2, a first cooling zone 
3, a second cooling zone 4, and a third cooling zone 5, bridle rolls 6a, 
6b are provided in front and behind the furnace, and further, a tension 
control unit 7 is interposed between the bridle roll 6a and the heating 
zone 1. A steel strip 10 is loaded in order of the zones in the 
abovedescribed arrangement, and subjected to heat treatment. Namely, the 
steel strip is heated to a predetermined temperature in the heating zone 
1, held at a predetermined temperature in the soaking zone 2, thereafter, 
passes through the first cooling zone 3, the second cooling zone 4 and the 
third cooling zone 5 while being successively cooled. The cooling rates in 
the respective cooling zones may be varied depending upon the compositions 
of the steel strip material to be treated and the intended characteristics 
of the material quality thereof. 
Now, to control the steel strip tension in the furnace in this conventional 
example, tensions of the steel strip at the inlet and the outlet of the 
furnace are generally set. The actual adjustment of the tension is 
performed by means of a dancer roll provided between the bridle rolls 
disposed at the inlet of the furnace and the outlet of the furnace and 
with this arrangement the tension of the steel strip in the respective 
blocks in the furnace is not controllable. Consequently, proper tension 
has not been given to the steel strip in the respective cooling zones, 
thus presenting problems such as buckling in a non-aligned fashion, and 
slip, all of which are caused by unfitness and instability in tension of 
the steel strip. In order to obviate such problems, for example, in 
Japanese Patent Application Publication No. 30928/77, there has been 
disclosed such a method that the interior of a continuous heat treating 
furnace is divided into a plurality of blocks, and tension on the steel 
strip in the respective blocks are controlled in association with tension 
of the steel strip in the preceding and succeeding blocks. Namely in FIG. 
1, tension meters 8a, 8b, 8c, 8d and 8e are provided in the furnace for 
detecting the tension of the respective sections of the steel strip. 
Output signals of tension meters represent the detected tensions in the 
respective sections, and are fed to steel strip tension control means 9a 
and 9g for controlling motors TM and M1 to M20. Each of the motors M1 to 
M19 drives each helper roll H1 to H19 for guiding the steel strip, 
individually from each other. The torque motor TM operates the tension 
control unit 7, and the motor M20 operates the bridle rolls 6b. The bridle 
rolls 6a are operated by a motor M21. More specifically, output signals 
from the tension meter 8a are fed to the steel strip tension control means 
9a, 9b and 9c, outputs from the tension meter 8b to the steel strip 
tension control means 9b, 9c and 9d, outputs from the tension meter 8c to 
the steel strip tension control means 9c, 9d and 9e, outputs from the 
tension meter 8d to the steel strip tension control means 9d, 9e and 9f, 
and outputs from the tension meter 8e to the steel strip tension control 
means 9e, 9f and 9g. As described above, the respective tension meters 
feed their outputs to the groups of the steel strip tension control means 
of the block in question and the groups of the steel strip tension control 
means in the blocks preceding and succeeding the block in question. In 
addition, the tension command signals TS.sub.1 to TS.sub.5 are fed to the 
respective steel strip tension control means 9b to 9f for setting optimum 
tension in the respective sections of the steel strip. Furthermore, a 
tension setting signal TSC for setting the tension of the tension control 
unit 7 is fed to the steel strip tension control means 9a for driving the 
tension control unit 7. 
In the arrangement of FIG. 1, deviation of tensions value between the 
detected tension value and the set tension value are obtained for each 
zone in the furnace, and the deviation tension values thus obtained are 
combined with detected tension values in the preceding and succeeding 
zones or a detected tension value in the preceding or succeeding zone to 
be used for controlling the torque of a motor or motors for a roll or 
rolls. With the arrangement as described above, it becomes possible that a 
preset distribution of tension in the furnace is maintained and the set 
tension values in the respective zones in the furnace can be automatically 
switched successively or simultaneously. 
Nevertheless, the conventional control means present the following 
disadvantages. 
(1) In the case a line speed, which is given as the master speed for the 
furnace, is based on the bridle roll 6b, the speed of the bridle roll 6b 
at the outlet of the furnace is varied depending upon the tension of the 
steel strip of the final cooling zone, whereby the speed of the bridle 
unit at the outlet is varied. 
(2) Since a tension command signal for each zone is calculated from the 
detected tensions of the steel strip in the preceding and/or succeeding 
zones, the change of the tension command signal in a given zone affects 
the tension command signals in other zones, whereby the tension control is 
not stabilized. Also a fluctuation in deflection between the set value and 
the detected value of the tension affects the tension command signals in 
whole of the zones. 
(3) Since the steel strip is given tension a high temperature in the 
heating and soaking zones, the steel strip is elongated due to plastic 
deformation depending upon the dimensions and temperature of the steel 
strip. 
In this case, in principle, it suffice to hold the tension only in the 
heating zone and soaking zone at proper values. However, in the example 
shown in FIG. 1, the control of tension in the blocks preceding and 
succeeding the block in question, which are principally irrelevant to the 
block in question, are subject to the influence of the tension in the 
block in question, so that a stable tension cannot be obtained. 
Particularly, the influence is high in the case of the materials to be 
annealed at high temperature. For this reason, even in the case proposed 
as above, such problems have not been obviated as the movement in a 
non-aligned fashion, buckling, slip and the like of the steel strip, all 
of which are caused by unbalance in tension generated in the steel strip. 
SUMMARY OF THE INVENTION 
The present invention has as its object the provision of a method of 
controlling tension of a steel strip in a furnace, wherein the speed of a 
continuous annealing line is controlled on the basis of a master speed 
hearth roll provided in the furnace, and the tension of the steel strip is 
continuously controlled towards both the inlet and outlet of the furnace 
from the master speed hearth roll as the boundary. 
The present invention contemplates to achieve the abovedescribed object in 
such a manner that a master speed hearth roll provided at a predetermined 
position in a continuous annealing furnace is driven at a preset speed, on 
the basis of which the speed of the continuous annealing line is 
controlled, the master speed hearth roll insulates the tension of the 
steel strip in front of the master speed hearth roll from the tension of 
the steel strip behind the master speed hearth roll so that the inside of 
the furnace is divided into a plurality of tension control blocks. All of 
the signals including steel strip tension detecting signals for 
self-control blocks through steel strip tension detecting signals for 
control blocks adjacent to the master speed hearth roll are applied to 
tension control means provided corresponding to the respective control 
blocks, and tensions of a steel strip in the plurality of control blocks 
is continuously controlled in sequence towards both the inlet and outlet 
of the furnace from the master speed hearth roll.

DETAILED DESCRIPTION OF THE INVENTION 
Here, the speed of the master speed hearth roll, serving as the reference 
of the speed, is set so as to satisfy the following conditions. Namely, 
the master speed hearth roll is set to serve as the boundary which divides 
the interior of the furnace into two regions for controlling the tension 
of the steel strip including one region in which elongation of the steel 
strip due to thermal expansion or due to plastic deformation caused by the 
tension of the steel strip in the furnace and an other region in which 
thermal shrinkage due to cooling is generated and elongation due to 
plastic deformation caused by the tension of the steel strip is very small 
in value. More specifically, in the case of the continuous annealing 
furnace, since the speed is controlled in the respective zones in most 
cases, it is desirable to provide the master speed hearth roll at a 
portion where the steel strip is at high temperature of about 400.degree. 
C. or above, for example, the boundary between the soaking zone and the 
rapidly cool zone. Furthermore, it is desirable to control the master 
speed hearth roll in a manner that the master speed hearth roll is formed 
to be a dull roll having an average surface roughness of 1 to 7 microns to 
thereby increase the coefficient of friction with the steel strip. 
FIG. 2 is a block diagram showing a preferred embodiment of the present 
invention. The arrangement of the furnace shown in FIG. 2 is similar to 
that illustrated in FIG. 1, and therefore, a detailed description will be 
omitted. 
As shown in FIG. 2, for example, a master speed hearth roll 20 controlled 
by an automatic speed regulator (ASR) and serving as the reference for the 
line speed is provided at the center of the furnace, i.e., between a 
soaking zone 2 and a first cooling zone 3, and further, a tension control 
unit 11 is provided at the outlet of furnace. Furthermore, tension meters 
8a through 8e are provided in the respective zones of the furnace, and 
control blocks are provided forwardly and rearwardly of the master speed 
hearth roll 20 serving as the boundary. More specifically, an output from 
the tension meter 8a is fed to steel strip tension control means 9a and 
9b, and an output from the tension meter 8b is fed to steel strip tension 
control means 9a, 9b and 9c. While, an output from the tension meter 8c is 
fed to steel strip tension control means 9d, 9e, 9f and 9h, and output 
from the tension meter 8d is fed to steel strip tension control means 9e, 
9f and 9h, and further, the output from the tension meter 8e is fed to 
steel strip tension control means 9f and 9h. Output signals from position 
detectors, (not shown) provided in tension control unit 7 and 11, are 
adapted to control dancer rollers 7R and 11R of the tension control units 
7 and 11 to settle in place. 
Tension command signals TS.sub.1 to TS.sub.5 similar to those in the prior 
art are fed as the command values to the steel strip tension control means 
9b 9f, and tension setting signals TSC.sub.1 and TSC.sub.2 are fed as the 
command values to the steel strip tension control means 9a and 9h for 
controlling torque motors TM. Furthermore, a line speed setting signal SS 
is fed to the abovedescribed ASR. 
If the tension changes, e.g., the tension of the steel strip in the first 
cooling zone decreases, under the normal condition of the tension control, 
the tension command signal TS3 is first of all changed. This change causes 
a deviation in value between the output from the tension meter 8c and the 
tension command signal TS.sub.3. The tension control is fed back to the 
steel strip tension control means 9d, de, 9f and 9h at a preset gradient 
in proportion to the deviation value. As a result, motors 3M, 4M and 5M 
for driving helper rolls in the first, second and third cooling zones 3, 4 
and 5 are decreased in rotational speed, the output of the torque motor TM 
for the tension control unit 11 is decreased, and the tension of the steel 
strip in the first cooling zone 3 is decreased. At this time, the dancer 
roll 11R of the tension control unit 11 is raised, however, an output from 
the position detector of the dancer roll 11R increases the speed of the 
bridle roll 6b, to thereby control the dancer roll 11R to settle in place. 
As described above, the tension forwardly and rearwardly of the master 
speed hearth roll 20 are continuously controlled on the basis of the 
master speed hearth roll 20. The master speed hearth roll 20 functions 
only as the reference for speed and is separated from a system of 
controlling the tension and hence, there occurs no interference 
therebetween. In addition, the basic patterns of tension are developed by 
optionally setting the tension command signals TS1 to TS5 independently 
from each other. 
FIG. 3 is a characteristic curve diagram of the steel strip in the 
embodiment shown in FIG. 2. 
It is apparent from FIG. 3 that the tension of the steel strip is varied 
from the master speed hearth roll 20 as the boundary toward both the inlet 
and the outlet of the furnace, thereby providing a stabilized control. In 
the example shown in FIG. 3, the varied values of tension of the steel 
strip range from 0.4 kg/mm.sup.2 to 2.0 kg/mm.sup.2 depending upon the 
sheet thickness, grade of steel, line speed and the like. 
FIG. 4 shows another embodiment of the present invention showing the 
continuous annealing furnace in which bridle devices for controlling the 
tension of the steel strip are provided both at the inlet and the outlet 
of the first cooling zone 3 and the tension of the steel strip in the 
first cooling zone 3 only can be decreased by both bridle devices at the 
inlet and the outlet of the first cooling zone 3. A roll 211 disposed at 
the center in a bridle device 21 provided at the inlet of the first 
cooling zone 3 is selected as the roll for the reference speed 
(corresponding to the master speed hearth roll 20), and the control of 
tension of the steel strip is effected towards both the inlet and the 
outlet of the furnace from the roll 211 at the center as the boundary. In 
this case, a bridle device 22 at the outlet of the first cooling zone 
functions as a boundary of control blocks as well. Except for the 
arrangement of these bridle devices, the method and arrangement for 
controlling the tensions of the steel strip in the respective zones of the 
furnace are identical with those shown in the embodiment of FIG. 2, and 
therefore, their illustration and description will be omitted. In 
addition, in the case the temperature of the steel strip at the outlet of 
the first cooling zone 3 is 400.degree. C. or above, one of the rolls in 
the bridle device 22 at the outlet may be selected as the roll for the 
reference speed. 
According to the present invention, the adverse effects in fluctuation of 
the tension of the steel strip due to the thermal expansion and elongation 
caused by the plastic deformation of the steel strip are eliminated, so 
that stabilized control of tension of the steel strip can be effected, 
thereby avoiding movement in a non-aligned fashion, buckling, slip and the 
like of the steel strip. 
It should be apparent to one skilled in the art that the abovedescribed 
embodiment is merely illustrative of but a few of the many possible 
specific embodiments which can represent the applications of the 
principles of the present invention. Numerous and varied other 
arrangements can be readily devised by those skilled in the art without 
departing from the spirit and scope of the present invention.