Process for automatic control of the startup of a continuous casting apparatus

At the latest before response of the measuring mechanism which senses the filling state of the melt tundish the opening rate of the outlet of the tundish is brought to near its standard opening rate and, after obtaining a clear signal from the filling state measuring mechanism or reaching a first predetermined filling height, the withdrawal mechanism is switched on and the withdrawal speed delayed for determination of the opening rate of the outlet. Advantageously the speed of the discharge rollers is measured by a tachometer which transmits a normalized signal which is then multiplied by a signal determined by the standard opening rate. The resultant signal controls the opening rate of the outlet. After the running stage of the withdrawal mechanism or after attaining a second predetermined filling height which is higher than the first filling height, only an I-governing branch of the control circuit controls the opening rate of the outlet and, after reaching a third predetermined filling height, which corresponds approximately with the standard desired filling height, control is transferred to an I-P regulation.

FIELD OF THE INVENTION 
My present invention relates to an automatic process and apparatus for the 
continuous casting of metal, e.g. steel, and, more particularly, to a 
process for controlling the startup of a continuous metal casting 
apparatus. 
BACKGROUND OF THE INVENTION 
A continuous casting apparatus can comprise a water cooled mold which is 
provided with a tundish having an adjustable outlet for a melt and an 
extraction, strand advancing or withdrawal mechanism for the solidified 
product or strand. 
The mold is provided with a measuring mechanism for measurement of the 
state of filling of the mold which controls the speed of the withdrawal 
mechanism, or its withdrawal rollers if it is so equipped during free 
running operation, or the opening of the tundish outlet. 
Before the start of casting operation the lower mold opening is closed with 
a starting "head" formed by a dummy bar; when the molten metal is then 
supplied the molten steel fuses with the head onto which cold scrap can be 
applied to facilitate the fusion. 
By the water cooled mold so much heat is drawn from the hot flowing metal 
that a rigid shell of the strand forms against the mold wall. As soon as 
the state of filling of the mold is such that the level of the metal has 
reached a predetermined height, the withdrawal mechanism is switched on 
and the cooled strand is drawn off while continuously solidifying. The 
continuous casting can then be subjected to classical continuous metal 
casting control process. 
An automatic casting process has problems since the measuring mechanism is 
mounted in the vicinity of the desired (setpoint) height of the bath level 
and useable measurements are only transmitted when the bath level 
approaches its desired level. 
During casting vigorous bath motion and splashing occurs, which can result 
in erroneous measurements of the state of filling or make difficult the 
interpretation of such measurements. 
An automatic startup of the casting process is described in German Patent 
Document No. 32 21 708. The process described there for filling the mold 
is characterized by an intermittent automatic opening of the outlet for 
dollopwise filling of the mold over a time interval from the beginning of 
melt feed until the filling of the space in the vicinity of the starting 
head and/or until the decay of the initial strong bath motions in the 
mold. 
From the Belgian Patent Document BE No. 704,306 it is known to fill the 
mold continuously in a first stage of a casting process and, when the 
height of the reservoir metal level is detected by the measuring mechanism 
to again close the outlet. Subsequently the withdrawal mechanism is 
started and the outlet is placed under control of a P-governing branch of 
an electronic control circuit, i.e. a proportional (P) controller. 
However one does not choose this kind of process when casting articles with 
a small cross sectional area. Here the volume filled by the steel is very 
small; the thermal inertia of the steel-mold system is drastically 
reduced. Also the stopper which closes the outlet is brought from the open 
to the closed positions in about 0.8 seconds by the customary adjusting 
drive. Thus during the measurement of a first value by the measuring 
mechanism, the outlet can only close half way in the available time. 
In dollopwise filling with very cool charges (say, about 20.degree. C. 
above the melting point) there is the great danger that with small stopper 
displacement the heat input or delivery to the outlet and mold per stopper 
stroke will become too small and the outlet or the downcomer or immersion 
tube will become clogged. 
In casting after changing the immersible tube, this danger is even greater 
since the immersible tube is not preheated. It can also be observed with 
very inferior grades of steel during a pause in a stroke of the stopper 
that the steel will congeal in the immersible tube and in the next stroke 
the immersible tube will be broken loose from the reservoir. To treat this 
condition the stopper stroke can be lengthened at the outlet so that the 
largest possible quantity of heat per stroke is delivered to the 
immersible tube. 
This approach is, however, limited since a lengthening of the open time 
automatically means an increase of the stopper stroke and a large opening 
at the outlet at the time and in the region of the level measurement which 
can lead to a steel overflow. Also in this process the indefinite stopper 
position on reaching the switchover point is not beneficial for adjustment 
of the apparatus from the initial pouring stage to the regulated stage. 
With charges having very high Al-content(without CaSi) these problems are 
still greater. Here the alumina builds up at the outlet on every closing 
of the stopper so that the stopper null position is displaced upwardly. 
Because of a nonuniform structure of the alumina deposit the stopper can 
no longer be tightly closed. An even higher energy input must be used here 
in order to avoid clogging the apparatus. 
OBJECTS OF THE INVENTION 
It is an object of my invention to provide an improved process for 
automatic control of the startup of a continuous casting apparatus. 
It is another object of my invention to provide an improved process for 
automatic control of the startup of a metal casting apparatus which can be 
used to cast products having small cross sectional areas without clogging 
either the outlet of the tundish or the immersible tube. 
It is also an object of my invention to provide an improved process for 
startup of a metal casting process which allows a reliable castin using a 
melt whose temperature is close to the melting point. 
SUMMARY OF THE INVENTION 
These objects and others which will become more readily apparent 
hereinafter are attained in accordance with my invention in a process for 
automatic control of the startup of a metal casting apparatus comprising a 
water cooled mold, which is provided with a tundish having an adjustable 
outlet for a melt and a withdrawal mechanism for the solidified strand. 
The mold is provided with a measuring mechanism for measurement of the 
state of filling of the mold which controls the speed of the withdrawal 
mechanism, or its withdrawal rollers, if it is so equipped, during free 
running operation or the opening of the outlet. 
According to my invention at least shortly before the response of the 
measuring mechanism to the state of filling the opening rate of the outlet 
is brought to about the standard opening rate and, after obtaining a 
clearly useable signal from the measuring mechanism or ween a first 
predetermined state of filling has been reached, the withdrawal mechanism 
is switched on and the speed of the withdrawal mechanism (controlled by 
the level in the mold in the usual feedback loop) determines or 
contributes to the determination of the opening rate of the oullet. 
Advantageously from the start of casting to either the obtaining of a 
clearly useable signal from the measuring mechanism or to the first 
predetermined state of filling thereof (which can be the same point in 
time), the opening rate of the outlet is near the standard opening rate. 
In another feature of my invention at the start of casting a dollop or 
portion of metal is permitted to run into the mold, and the opening rate 
of the outlet is larger than the standard opening rate during casting. 
Furthermore the opening rate of the outlet can be at least twice as large 
as the standard opening rate during casting. 
Advantageously one closes the outlet at least at a short time before the 
opening rate of the outlet is brought close to the standard opening rate. 
According to further features of my invention the withdrawal mechanism has 
a plurality of withdrawal rollers clamping the cast product and the speed 
of the withdrawal rollers is measured by means of a tachometer which 
transmits a normalized tachometer signal which is multiplied by a signal 
determining the standard opening rate and the resultant signal from the 
multiplication controls the opening rate of the outlet. 
Advantageously after the running stage of the withdrawal mechanism or after 
attaining a second predetermined filling height greater than the first 
filling height, control of the opening rate of the outlet is transferred 
completely to an I-governing branch of a control circuit, i.e. an integral 
controller or I controller). After reaching a third predetermined filling 
state which has a height approximately equivalent to the desired height in 
the desired filling state, the opening rate of the outlet is put under I-P 
regulation, i.e. integral-proportional or PI control. 
My invention provides a substantially more reliable casting process. The 
regulation and control of the process is provided with economical and 
tested electronic components. It also provides an opportunity to cast 
steel whose melt temperature is just over the melting point.

SPECIFIC DESCRIPTION 
As shown in FIG.1 the upper end of a water cooled continuous casting mold 1 
is supplied with steel melt 3 by an immersion or dip tube 2. The melt 3 can 
be found in an intermediate vessel or tundish 4 whose outlet 5 is closable 
by stopper 6. The position of the stopper 6 is determined by a hydraulic 
positioning member 17 such as a hydraulic servomotor. The configuration of 
the positioning member 17 depends on a regulating device 7. 
The lower side of the mold 1 is closed by starting head 8 on which cold 
scrap iron 11 is disposed; the cooled strand or continuous casting 
connected therewith is clamped into the outlet rollers 10. 
A radiation source 12, usually Co-60 extending over a height H, is 
positioned at the upper end of the mold 1. The radiation originating from 
the source 12 is received by a sensor 13. 
After suitable amplification and signal processing (circuit 14) the 
received signal is transmitted to the electrical control unit 15. The 
metal level and state of filling is determined from the attenuation of the 
radiation reaching the sensor 13. In the following description the filling 
height is reported as a percentage of the measuring range. Thus a nominal 
filling height of 70% means that height which is reached when an output 
signal of 70% of the maximum signal range arises. 
The electrical control unit 15 which is shown in more detail in FIG. 2 
receives the filling height signal from the amplifier-processor 14 and the 
position of the stopper 6 from the positioning member 17 and acts on the 
regulating device 7 and the tachometer control 16 of the withdrawal 
mechanism 9. The control unit 15 moreover receives the withdrawal speed of 
the (cold) strand by a tachometer 18 attached to a withdrawal roller 10. 
FIG. 2 shows the control unit 15 in greater detail. The configuration of 
the various switches prior to casting is shown. By using known electronic 
design features the switches change configuration only once, that is, 
during pouring to a give metal level the switches can not reverse to their 
initial configuration. 
The condenser 38 retains the instantaneous zero setting of the stopper 6. A 
voltage is developed at potentiometer 20 which from experience is known to 
lead approximately to the standard stopper position in casting, while the 
bath height in the mold is adjusted to its desired or set-point value by 
the potentiometer 30. 
The limiting indicator or threshold circuit 31 acts on the switching 
mechanisms or switches 24, 26, and 29 as well as the tachometer control 16 
as soon as the metal in the mold reaches a predetermined level. In the 
multiplying unit 25 the stopper position (whose zero setting position is 
allowed for by the condenser 38 and the amplifier 19) set by the 
potentiometer 20 is multiplied by a value proportional to the outlet speed 
of the product. Further the circuit also includes an I-governing branch 
(I-controller) 27 as well as a P-governing branch (P-controller) 28, 
whereby the governing branches not only are adjusted for control of the 
startup operation but also for the normal operation. 
The operation at startup is as follows: The switch 23 is closed and 
simultaneously the switch 22 is opened. The correct zero setting of the 
stopper 6 is retained fixed in condenser 38 and the desired set-point 
starting value fixed in the potentiometer is provided directly to the 
regulating device 7 by the adder 21 after a null correction. This desired 
starting value corresponds advantageously to the middle stopper position 
during casting. In any case the outlet 5 should neither be opened too 
much, because then a steel overflow occurs, nor should it be opened too 
little, because then the outlet becomes plugged. 
As soon as the casting level measuring mechanism reaches a 20% filled 
condition, the casting apparatus starts. Simultaneously the switching unit 
or switch 24 reverses; the desired stopper opening value is changed 
additionally by the multiplier block 25 depending on the actual pouring 
speed. 
The signal transmitted by the tachometer 18 is normalized, that is, the 
multiplier value is between 0 (casting apparatus idle) and 1 (desired 
speed attained). Because of that the stopper first travels a short 
distance toward the outlet 5 (this of course reduces the pouring or 
casting speed) and then is opened proportionally to the startup curve of 
the withdrawn mechanism. 
Should a breakdown occur in the withdrawn mechanism the outlet 5 is most 
quickly shut. The multiplier value can be dependent on the casting speed 
linearly or in a first speed range logarithmically then linearly or 
exponentially. Particularly for very small cross sections (smaller than 
100 mm.sup.2) one must make a careful determination of the mathematical 
dependence of the multiplier value on the casting speed by routine 
experiment to avoid a breakthrough or an overflow in casting. 
On reaching the 50% state of filling the necessary position control is 
replaced by an equalizing control and of course that control is switched 
on by reversal of the switching unit 26 with the exception of the I-branch 
27. It is understood that until this time the I-governing branch 27 should 
be reset for the above described control of the position of the stopper 6 
(see the dashed arrow and reference character 17 in FIG. 2), since it 
otherwise would have attained its saturation limit and would cause an 
extremely fast rise of the stopper 6. 
At an about 70% state of filling, which as described above corresponds to 
the desired filled condition, then by closing of the switch unit 29 of the 
P-governing branch 28 the control is switched on. Now the stopper position 
is subjected to a classical I-P regulation, in which the actual value 
transmitted by amplifier 14 is compared with a desired value set by 
potentiometer 30 and the stopper position is changed according to the 
difference. From experience a premature activation of the P-governing 
branch or branch 28 results in a violent variation in the metal flow rate 
as well as an accompanying steel overflow risk. In no case should the 
P-governing branch 28 be switched on before the I-governing branch 27. 
Instead of as described above to bring the opening rate of the outlet 5 
immediately to the standard opening rate, it is advantageous in another 
embodiment of my invention to provide an immediate input of a large 
quantity of heat: first a large portion of metal is allowed to flow into 
the mold and then the opening rate of the outlet 5 is returned to the 
standard opening rate. For this purpose instead of one potentiometer 20 
two are used (see the potentiometers 120 and 220 in FIG. 3 which shows the 
upper part of the block diagram of the circuit of FIG. 2 with some 
modifications). 
With the potentiometer 220 and with the potentiometer 120 as described in 
connection with FIG. 2 the opening rate corresponding to the large portion 
of added metal is adjusted to the central stopper position W.sub.c in 
casting. Components in the embodiment of FIG. 3 which are the same is in 
the embodiment of FIG. 2 are given reference numbers equal to the 
reference number used in FIG. 2 plus 100. 
In the embodiment of FIG. 3 before casting begins the switch 221 is closed 
and the switch 222 is opened. During casting the switch 123 is closed (at 
time t.sub.0 --see FIG. 4) and the stopper 6 moves from the null position 
W.sub.o until at the predetermined maximum value W.sub.M. After about 1 
second (time=t.sub.1) the switch 123 is opened (switch 122 is closed) and 
the stopper 6 moves back into its new null position required by the 
hardened metal. Approximately simultaneously with the switch 123 the 
switches 221 and 222 are reversed, so that the actual state of the 
potentiometer 120 controls the position of the stopper 6 as soon as the 
switch 123 (time=t.sub.3) is closed. The waiting time depends on the 
distribution, the heating of the melt, and the quality of the steel and 
amounts to between 0 and about 15 seconds. It has proved advantageous when 
a pause is not required to close the base outlet 5 completely in a very 
short time in order in subsequent process steps to be able to allow for 
the instantaneous null position of the stopper, W.sub.01. At time=t.sub.3 
the stopper 6 moves into a position W.sub.c, which depends on the voltage 
developed across the potentiometer 120 as well as the charge on the 
condensor 138. At time=t.sub.5 the 20% state of filling is reached, which 
starts the outlet mechanism starts. At the 30% state of filling the 
I-governing branch 27 takes control. 
It is important during the casting to move the stopper member into a 
position in which a closure of the pouring opening or outlet 5 still 
occurs when the measuring mechanism transmits a first useable signal. It 
is also important to be able to change the opening rate depending on the 
casting speed. The fastest possible running of the product at the desired 
speed is desirable and has proved advantageous in the controlling process 
according to my invention. 
Although the process according to my invention has been illustrated in 
detail by analog switches, naturally it is possible to perform the 
required automatic control and regulation with digital electronics as long 
as the digital electronics has a sufficiently short cycle time.