Billet control method in a horizontal continuous casting system

A method for controlling the billet pushing back length for a horizontal continuous casting system is disclosed wherein, for allowing stable formation of the billet shell, the billet is pushed or forced back towards the mold during the transient stop or halt cycle of the intermittent billet extracting process. The pushing or forcing back pressure executed by the pinch rolls in controlled for each extracting cycle in order that the billet is pushed or forced back towards the mold a preset length.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a billet control method in a horizontal 
continuous casting system. More particularly it relates to a method for 
controlling the length of pushing back of the billet towards the mold with 
the aid of the pinch rolls during the top cycle of the intermittent billet 
extraction process in order to promote stable growth of the newly formed 
billet shell. 
2. Description of the Prior Art 
In a conventional horizontal continuous casting system, billet extraction 
is by an intermittent process according to which the billet is drawn out 
of the mold at preset speed a preset length and halted for some time so as 
to allow for stable growth of the newly formed shell, the billet being 
again extracted after termination the growth of the shell. During the stop 
cycle, the billet is contracted in size, thus possibly causing rupture of 
the newly formed shell. In the conventional process, the following methods 
(a) or (b) are used so as not to obstruct free billet contraction between 
the pinch rolls and the mold. (a) The method of opening the pinch rolls 
apart during the stop cycle, according to which the pinch rolls are opened 
apart during the stop cycle to permit free contraction of the extracted 
billet between the pinch rolls and the mold so as to prevent cracking or 
rupture of the newly formed shell. 
In the method (a), the previously formed shell and the newly formed shell 
are affixed to each other only slowly so that it is not possible to 
elevate the speed of the extraction cycle. In addition, the large-size 
billet tends to be ruptured on account of the higher frictional resistance 
between the billet and the rolls. Also, the extraction resistance in the 
mold is markedly changed with the mold profile, the temperature of the 
molten steel or the steel type, resulting in breakouts and obstruction of 
stable casting due to obstruction of free contraction of the billet and 
rupture of the newly formed shell. (b) The method of applying a reset 
pressure to the billet forcing back side of the driving hydraulic motor 
during the billet forcing back step. According to this method the billet 
forcing back step is provided in continuation to the stop cycle of the 
intermittent extraction process. During the billet forcing back step, a 
preset pressure is applied to the billet forcing back side of the pinch 
rolles driving hydraulic motor for pushing the billet back towards the 
mold for positively promoting billet contraction so as to prevent rupture 
of the newly formed shell. With the present method (b), however, the 
billet is forced back with a constant preset force despite fluctuations in 
the roll and/or mold resistances. The result is that occasionally the 
forcing back pressure is insufficient thus causing billet breakouts due to 
shell rupture, or the forcing back pressure becomes too strong thus again 
causing billet breakouts due to buckling of the newly formed shell. 
With the above described conventional methods (a), (b) consisting in 
opening the pinch rolls apart or applying the preset pressure to the 
billet forcing back side of the driving hydraulic motor, it is not 
possible to successfully deal with fluctuations in the resistance inside 
the mold or the roll resistance, thus causing rupture or buckling of the 
newly formed shell and resulting breakouts. 
SUMMARY OF THE INVENTION 
The present invention has been made in order to obviate the above described 
deficiencies and to provide a method for controlling the billet forcing 
back length in the horizontal continuous casting system so as to provide 
for stable growth of the billet shell. 
The control method of the present invention resides in that the forcing 
back pressure exerted by the pinch rolls is controlled on the cyclic basis 
in order that the billet being cast is forced back towards the mold a 
preset length at a preset location during the cycle time that the billet 
extraction is momentarily stopped. 
In this manner, a pressure is exerted to the billet forcing back side of 
the pinch roll driving hydraulic motor in an amount corresponding to the 
preset length by which the billet is contracted. The billet is forced back 
by such forcing back pressure by a length corresponding to the billet 
contraction caused so as to prevent rupture of the newly formed shell, the 
pushing back length is measured by the extraction length measurement unit 
and compared with the command value and a plurality of unusual setting 
values in the control circuit. The result of the comparison is used for 
correcting the billet extraction speed or forcing back pressure for the 
next cycle for reducing the error caused by changes in the mold resistance 
or the mold resistance so as to be within a preset allowable range. 
According to the present invention, it is unnecessary to take account of 
delicate changes in the resistance between the billet and the pinch rolls 
due to changes in billet size or the centering error. In addition, when it 
is found during a given cycle that the actual forcing back length is 
lesser than the command value or setting for the presently applied 
pressure, the pressure can be increased during the next cycle so that the 
forcing back length closer to the command value is reached. It is seen 
from above that present invention provides for stable growth of the newly 
formed shell and hence for stable horizontal continuous casting without 
rupture or buckling of the shell. 
The objects and advantages of the present invention will become more 
apparent from the following detailed description of the present invention, 
especially when read in conjunction with the accompanying drawings.

DESCRIPTION OF A PREFERRED EMBODIMENT 
In the control system shown in FIG. 1, a billet B from an extracting mold 2 
is extracted by a pair of pinch rolls 2. These pinch rolls 2 are driven by 
a hydraulic motor 3 in the normal direction or in the reverse direction. 
The hydraulic pressure supplied to the motor 3 is sensed by a pair of 
pressure transmitters 4a, 4b. The operation of the hydraulic motor 3 is 
controlled by a servo valve 5. The hydraulic pressure of an oil pressure 
source 6 is transmitted via servo valve 5 to the motor 3 for driving the 
motor. Since the oil pressure from servo valve 5 drives motor 3 and since 
pressure transmitters 4a and 4b sense the input and output pressures on 
the input and output lines to the motor, the differential between the 
signals from each of the transmitters 4a and 4b is indicative of the 
driving force delivered by the motor to pinch roll 2. A measurement roll 7 
is driven in rotation in contact with the billet B. A length sensor 8 
issues pulse signals as a function of rotation of the measurement roll 7, 
while a length counter 9 counts the number of pulses supplied from the 
length sensor 9. 
A pinch roll forcing-back pressure control circuit 10, hereafter referred 
to as control circuit, compares the actual forcing back length as measured 
by the length counter 9 and the respective setting values for issuing 
control command values. A differential amplifier 11 detects the pressure 
difference on the basis of the signal difference between the signals from 
the transmitter 4a and those from the transmitter 4b. The amplifier 12 
performs a control arithmetic operation with the output of the control 
circuit 10 as setting value and with the output of the amplifier 11 as 
actual or measured values. The amplifier 12 controls the servo valve 5 on 
the basis of the results of the control arithmetic operation. 
The sequential steps of the control process is now explained. In the 
horizontal continuous casting, an intermittent extraction system is 
adopted in which the step of extracting the billet B from the mold 1 by 
the pinch rolls 2 at a preset speed and the step of halting the extraction 
for allowing the growth of the shell of the extracted billet B are 
repeated cyclically. In order that the newly formed shell is not ruptured 
due to contraction of the billet B, a billet forcing-back step is provided 
in continuation to the halting step for positively assisting contraction 
of the billet B (see FIG. 1). Thus, upon expiration of a preset halting 
time interval, the motor 3 is driven in reverse by way of the servo valve 
5 for forcing back the billet B towards the mold 1 under the reverse 
driving force of the pinch rolls 2. The reverse driving torque or force of 
the hydraulic motor 3 at this time is controlled by operation of the servo 
valve 5 by the control signals from the control amplifier 12 on the basis 
of the forcing back pressure setting of the control circuit 10 
corresponding to the forced back length referenced to the contraction of 
the billet B. During this time, the pressure difference obtained at the 
differential amplifier 11 from the signal difference between the pressure 
transmitters 4a, 4b is fed back to the control amplifier. The actual value 
of the forced back length caused by the forcing-back pressure is counted 
by the length counter 9 through the measurement roll 7 and the length 
sensor 8. The resulting signal representative of the actual forced-back 
length is introduced into the forcing-back pressure control circuit 10 
sets a command value of the forced-back length as a function of such 
factors as roll resistance due to billet size mold profile, molten steel 
temperature, and the mold resistance, which itself is a function of the 
steel type, for outputting the corresponding command pushing-back pressure 
value to the control amplifier 12. The control circuit 10 also receives 
the actual or measured pushing-back length from the length counter, 
compares these input length signals with the unusual setting values I, II 
and high and low setting values, and performs the following operations 
under the conditions wherein the unusual setting I(LabI)&lt;unusual setting 
value II (LabII)&lt;highsetting (Lh)&lt;command pushing-back length (Lob)&lt;low 
setting (Ll). 
(a) Actual pushing-back length Li.gtoreq.unusual setting (LabI). 
In this case, the pushing back operation is in excess and hence the billet 
B is likely to undergo excess buckling. Hence it is necessary to 
transiently stop the extraction of the billet B to promote new shell 
growth. 
(b) Unusual setting (LabII).ltoreq.actual pushing-back length (Li)&lt;unusual 
setting (LabI). 
In this case, the billet B is likely to undergo some buckling, thus 
resulting in breakouts. Therefore, the billet extracting speed is 
decelerated during the next cycle to promote new shell growth. 
(c) Actual pushing back length (Li).gtoreq.high setting (Lh), the relation 
occurring repeatedly. 
In this case, the pushing-back pressure setting is decremented by a preset 
value during the next cycle so that the actual pushing-back length is 
reduced to a value within the range of high setting. 
(d) Actual pushing-back length (Li).ltoreq.low setting (Ll). 
For preventing rupture of the newly formed shell, the pushing-back pressure 
setting (command or object value) is incremented by a preset value during 
the next cycle until the setting is reduced to a value within the command 
or object pushing-back length. 
(e) Low setting (Ll)&lt;measured pushing-back length (Li)&lt;high setting(Lh). 
The case (e) comprises any other cases not falling under the above 
described cases (a) to (d). In the case (e), the current pushing back 
pressure setting is maintained for the next control cycle. 
By the above described cases (a) to (e), any error caused by changes in the 
mold or roll resistance can be reduced to a value within a preset range in 
such a manner that the measured pushing-back length Li is coincident with 
the command or object pushing-back length Lob.