US3817311A

US3817311A - Method and apparatus for controlling a continuous casting machine

Info

Publication number: US3817311A
Application number: US00297523A
Authority: US
Inventors: W Pellinat
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1972-10-13
Filing date: 1972-10-13
Publication date: 1974-06-18
Anticipated expiration: 1991-06-18

Abstract

A method and apparatus for controlling a multistrand continuous casting machine having a ladle with a device for controlling the pour rate thereof, a tundish with a plurality of discharge ports, a plurality of open-ended water cooled molds for receiving strands of melt from the tundish discharge ports, spray water cooling for each strand as it leaves its associated mold and a plurality of strand feeding devices for withdrawing the strands from the molds. The control method and apparatus detects which strand is the fastest strand, causes this strand to be withdrawn at a constant predetermined maximum casting speed and controls the mold level for this strand by manipulating the device for controlling the ladle pour rate. The mold levels for the remaining strands are controlled by manipulating the speed of the strands. If one of the remaining strands becomes faster than the leading strand, the controls are switched whereby the new leading strand is withdrawn at the constant maximum allowable casting speed and its mold level is controlled by manipulating the pour rate. The control over the previous leading strand is switched and its mold level is now controlled by varying its speed.

Description

United States Patent 1191 Pellinat June 18, 1974 METHOD AND APPARATUS FOR [57] ABSTRACT CONTROLLING A CONTINUOUS CASTING A method and apparatus for controlling a multistrand MACHINE continuous casting machine having a ladle with a de- [75] Inventor: Wemer Karl Pellinat Taylors, Calif. vice for controlling the pour rate thereof, a tundish with a plurality of discharge ports, a plurality of open- Asslgneei lntelnaIlQnaI Business Machines ended water cooled molds for receiving strands of Corp0rat10n,Arm0nk,N-Y melt from the tundish discharge ports, spray water I 13, 72 cooling for each strand as it leaves its associated mold [221 fled Oct 19 and a plurality of strand feeding devices for withdraw- PP 297,523 ing the strands from the molds. The control method and apparatus detects which strand is the fastest [52] U S Cl 164/4 164/82 164/155 strand, causes this strand to be withdrawn at a con- [511 B22d 11/10 stant predetermined maximum casting speed and con- [58] Fieid 155 273 M trols the mold level for this strand by manipulating the device for controlling the ladle pour rate. The mold levels for the remaining strands are controlled by ma- [56] References Cited mpulatmg the speed of the strands. If one of the re- UNITED STATES PATENTS maining strands becomes faster than the leading Adams trand the controls are witched whereby the new fi leading strand is withdrawn at the constant maximum a 6 a allowable casting speed and its mold level is controlled 3,605,862 9/1971 Schultz 164/155 by manipulating the pour mm The control Over the leading strand is switched and its mold level Przmary Exammer-R. Spencer Annear Prevlous Attorney, Agent, or Firm-Donald F. Voss is now controlled by varymg its Speed 9 Claims, 11 Drawing Figures DETERMINE ACTUAL STRAND 1 SPEEDS bsi I IDENTIFY LARGEST es 202 es1=rsc-bsi I I 7 SWITCH SWITCH REMAINING STRAND WITH STRANDS INTO "206 203 LARGEST 8S NON-LEAD INTO LEAD STATUS STATUS I I 205 VARY SPEED OF I REMAINING MAKE SPEED CONTROL STRANDS TO 'ZOI 204 OF LEADING LADLE STOPPER CONTROL STRAND EQUAL ROD TO CONTROL MOLD LEVELS TO ISC LEAD STRAND MOLD LEVEL PATENIEMMH BM 3.817311 sum 10? 8 DETERMINE ACTUAL "STRAND -'-201 SPEEDS bsi IDENTIFY L RGEST es; 202

es|=rsc-bs| .SWITCH SWITCH REMAINING I STRAND WITH STRANDS INTO 206 ,1 205 LARGEST es NON-LEAD INTO LEAD STATUS STATUS 1 a 1 205 VARY SPEED OF 1 1 J REMAINING -MAKE SPEED CONTROL STRANDS TO OF LEADING LADLE STOPPER CONTROL STRAND EQUAL ROD TO CONTROL MOLD LEVELS TO rsc LEAD STRAND MOLD LEVEL V S--* A PAIENTEDJIIII Ia IIIII :3 81 7'; 31 1 SHEEY 2 II 8 (EN IER SET UP FIG. 20 FIG. 2b FlG.2c

COUNTER A151 TO N-I I 1 I FIG.2

EVALUATE MoLD I A LEVEL ERRoR FOR IDEN A152 I REMAINING S D. elI=r-l-b|I MoLD LEvEL YES HIGHER HAN v IS MOLD LEVEL 1 154 LowER THAN 9 N0 CALCULATE NEW cALcuLATENEw l REF SPEED rsl REF SPEED rsl CALCULATE CALCULATE DECREMENT AND EVALUATE AND EVALUATE COUNTER SPE D ERRo SP EDERRo eSI=rS|-bsI Sl=f$|-b$l CALCULATE ms CALCULATE ms EQg J DECREASE INCREASE SPEED OF SPEED OF P REMAINING REMAINING STRAND STRAND I I PATENTEDJIIII Ia IIITT SHEET 7 OT 8 SET-UP COUNTER TO N COMPUTE ERROR 881 OF FIRST STRAND STORE IN ERROR TBL DECREMENT COUNTER BY I IS COUNTER COMPUT ERRoR esI OF NEXT sTRAND *"REMAINING IDEN THIS STRAND AS STRAND" OVERLAY NEXT SPEED IN ERROR TABLE.

TOTAL NUMBER OF OPERATIONAL STRANDS TO BE MONITORED.

ACTUAL SPEED bsl IS SToRED IN ERRoR TABLE.

es1=rsc-bs1 EXAMPLE=-I20I50=I0 ERROR TABLE= REF SPEED ISC SPEED ERROR SIGN/MAGN AcT. MACII SPEED bsI STRAND I20 I I30 IDENTIFY STRAND. 1:

IN ERRoR TABLE As "LEADING STRAND" LAST ENTRY IN ERROR TABLE REPRESENTS FASTEST STRAND AND THIS STRAND BECOMES LEADING STRAND.

YES-

IF NEXT ERROR esI IS LESS POSITIVE 0R MORE NEGATIVE THAN PREVIOUS ERRoR esI, NEXT STRAND SPEED IS FASTER THAN PREvIous oNE.

NExT STRAND SPEED IS SLOWER THAN PREvIou ONE. N0 ENTRY IN ERRoR TABLE IS MADE.

FIG. 5

METHOD AND APPARATUS FOR CONTROLLING A CONTINUOUS CASTING MACHINE BACKGROUND OF THE INVENTION 1. Field of the lnvention The invention relates to a method and apparatus for controlling a continuous casting machine and more particularly to such method and apparatus for controlling a multistrand continuous casting machine and still more particularly to such method and apparatus for controlling a multistrand continuous casting machine where the continuous casting machine does not have pouring devices in the tundish for the regulatory control of the metal flow.

2. Description of the Prior Art In a multistrand continuous casting machine, molten metal flows from a ladle into a tundishand from the tundish through discharge nozzles into water cooled open-ended molds beingeither of vertical or curved design. lnitially, dummy bars close the bottom ends of the molds. The molten metal entering the molds starts to solidify in the molds and it adheres to the dummy bars which extend into sets of pinch rolls, one set for each bar, for withdrawing the dummy bars and consequently, the solidifing strands of metal from the associated molds. Roller aprons with water spray nozzles are located between the molds and sets of pinch rolls to reshape and further cool the solidifying strands of metal. The sets of pinch rolls then feed the strands of metal through straightening rolls and from there the strands pass to utilization devices such as equalizing furnaces, rollingmills and shears for cutting the strands into metal billets or slabs.

During the continuous castingoperation', it is essential to maintain the level of molten metal in the molds at a predetermined constant height so that a solid skin or shell is formed within the molds without mold overflow at the top of the molds or molten metal breakout at the bottom of the molds. In the past, it has been common to control the height of the molten metal in the mold by varying the speed of the pinch rolls withdrawing the cast strand from the mold and by controlling the rate at which molten metal is supplied to the mold. Such a control system is described in US. Pat. No. 3,300,820 and in improvement US. Pat. No. 3,349,834. In US. Pat. No. 3,300,820, a single error signal is employed to simultaneously control the pinch roll drive motor speed and a hydraulic system that in turn controls the stopper rod of the ladle and thus the pour rate into the mold. The improvement US. Pat. No. 3,349,834 has a dual mode of operation. The primary mode of operation controls the direction and speed of the motor driving the pinch rolls for withdrawing the casting from the mold as a function of the polarity and magnitude of a first error signal indicating deviation from a desired liquid level. The secondary mode of operation adds a control over the pour rate of metal into the mold when the punch roll drive speed becomes either greater or less than certain predetermined limits. The secondary mode of operation responds to a second error signal having a polarity and a magnititude that represent direction and magnititude of deviation in casting withdrawal rate from a desired predetermined rate. This second error signal is effective to change the rate at which liquid is poured from the ladle into the mold only when the casting withdrawal rate deviates from optimum by a predetermined excessive amount.

The continuous casting machines in both of the above mentioned patents are single mold machines and do not employ a tundish. Multistrand continuous casting machines having a tundish with a plurality of discharge nozzles introduce additional parameters to be controlled. This is because molten metal can flow from the uncontrolled tundish discharge nozzles at different rates unpredictably.

Some prior art control systems for multistrand continuous casting machines control the flow of molten metal from the tundish discharge nozzles into the molds by tundish stopper rods. This type of control is mentioned as an alternate method in US. Pat. No. 3,52 l ,696. The mold levels are then controlled by mold level gages and controls that adjust the tundish stopper rods to regulate tundish metal flow in response to mold level errors. The problem with such a control system is that the tundish stopper rods tend to perturb the mo]- ten metal flowing into the molds and this can cause the solid skin or shell in the molds to wash out.

Another form of control set forth in US. Pat. No. 3,521,696 is to maintain selective mold level by changing the withdrawal or casting speed in proportion to a change of level. With constant mold level, casting speed is proportional to the inflow rate of metal into the mold. This inflow rate is then stated to be proportional to the height of metal in the tundish. lt is further stated that casting speed can be changed by changing the height of metal in the tundish. The control system opens and closes the ladle stopper rod in response to a change in tundish level. Thus, the tundish level is controlled to maintain a desired average casting speed. Also, to keep the speed of any one strand from becoming excessive, an override control is provided.

The present invention does not change casting speed by controlling the height of metal in the tundish. The present invention recognizes that the molten metal can flow from the uncontrolled tundish discharge nozzles at different rates unpredictably. Therefore, in order to maximize throughput constistent with good quality, the fastest or leading strand is identified and the set of associated pinch rolls are operated at a constant predetermined maximum casting speed. Although this speed is constant, different predetermined speeds may be used as maximum casting speeds during the entire course of casting as metal conditions such as temperature, viscosity, or volume of metal in the ladle change. These different casting speeds are calculated as maximum allowable reference speeds via a heat transfer model or are looked-up from tables representing standard casting practice alternatives. When computing reference speeds, limiting control parameters, such as minimum allowable shell thickness, minimum allowable strand surface temperature, and maximum allowable crater depth, which is the physical location of the end point of solidification, are taken into account together with actual measurements of the prime variables, such as temperatures, machine speeds and spray water flow rates. Details about the applied control philosophy are contained in the IBM Data Processing application manual titled Process Control System for the Continuous Casting of Steel," GE200339-l, copyrighted 1971, which is incorporated herein by reference. The mold level of the mold associated with the leading strand is then maintained at a predetermined height by manipulating the metal pour rate from the ladle into the tundish as the position of the ladle pouring device is varied. The mold levels for the remaining strands are maintained at predetermined heights by varying the speeds of the associated sets of pinch rolls. Whenever a new leading strand is identified by determining the largest relative error among all of the strands, the manipulated variable for that strand, i.e., casting speed, becomes the controlled variable in that it is controlled at the predetermined maximum casting speed. The speed of the previous leading strand then becomes a manipulated variable along with the other remaining strands.

SUMMARY OF THE INVENTION The principal objects of the invention are to provide an improved method and apparatus for controlling a multistrand continuous casting machine without pouring control devices in the tundish which: (a) continuously provides advances regulatory control; (b) is relatively flexible and adapts to changes in casting speeds of the individual strands; (c) tends to maximize throughput rates without sacrifice of quality; and (d) requires only a minimum number of process variables easily measured with instruments which have been proven as being reliable in the casting mills hostile environment.

These objectives are achieved by continuously detecting the fastest or leasing strand and causing the casting speed for this strand to operate at a predetermined maximum allowable casting speed. The mold level for this leading strand is then maintained at a predetermined desired level by controlling the pour rate from the ladle into the tundish. The speeds of the remaining strands are varied so as to control the mold levels for the remaining strands at the desired levels. Thus, one control strategy is provided for the leading strand and another control strategy for the remaining strands. The strand which is the fastest strand always becomes the leading strand. The speed of the strands is continuously monitored so as to continuously identify the fastest strand. The newly identified fastest or leading strand is then caused to be withdrawn at a constant predetermined maximum allowable casting speed by switching control strategies.

FIG. I is a diagram illustrating the control system and method of the invention;

FIGS. 2a, 2b and 2c taken together as shown in FIG. 2 are a block diagram illustrating the invention;

FIG. 3 is a block diagram showing conventional representations of process control variables, to illustrate the behavior of a system;

FIG. 4 is a block diagram showing the logical functions performed by the digital computer shown in FIG. 2b;

FIG. 5 is a flow diagram illustrating speed error determination and leading strand identification;

FIG. 6 is a flow diagram illustrating pinch roll speed control for the leading strand;

FIG. 7 is a flow diagram illustrating pinch roll speed control for remaining strands; and

FIG. 8 is a flow diagram illustrating ladle stopper rod reference logic and conversion logic for ladle stopper rod position.

With reference to the drawings, a multistrand continuous casting machine 10 is shown in FIGS. 2a and 2b as including a ladle for holding molten material.

Ladle 15 is shown schematically and is a conventional bottom pour ladle where the flow of melt from the ladle is controlled by a reciprocating stopper rod 20. The vertical position of stopper rod 20, either on or off," determines the rate of flow of melt 16 from bottom opening 17 in ladle l5. Beneath ladle 15 is a conventional tundish 25 for receiving molten material as it passes from ladle 15 via opening 17.

Tundish 25 is provided with a plurality of discharge nozzles 26, one for each mold 30. In this particular example, there are n molds 30. The flow of melt from the tundish 25 via nozzles 26 into molds 30 is uncontrolled because no regulatory devices such as stopper rods are provided in tundish 25. However, tundishes with control devices such as stopper rods could be used, but then the stopper rods would be operated only for initiating metal supply, full on, or for completely stopping metal supply, full off.

The discharge nozzles 26 are aligned with molds 30 so that the streams of melt flow precisely into the center of the molds. The molds 30 which are conventional water cooled, open-ended molds are suitably lubricated to prevent the solidifying molten material from sticking to the walls. The solidifying molten material takes the shape of the mold and is in the form of a strand 31 having an outer skin. This skin is thick enough to contain the still liquid melt and separates from the lower portion of the mold walls as it emerges from the bottom of the mold. A starter or dummy bar, not shown, initially closes the bottom of the mold and as the molten material solidifes in the mold, it adheres to the starter bar. The starter bar is withdrawn from the mold and carries the solidifying strand of molten material with it.

The solidifying strand of molten material then moves through roller aprons 40 provided with water spray nozzles for further reshaping and cooling the strands 31. The starter bars and strands are actually withdrawn from the mold 30 by sets of pinch rolls 50 driven by motors 51 which are controlled in a manner to be described shortly. The strands 31 are fed by the associated sets of pinch rolls 50 through sets of bender rolls 60 which bend the strands horizontally. The strands then move through sets of straightening rolls 65, only one set shown. for correcting the curvature to cutting devices 70, only one shown, for cutting the strands into predetermined lengths. FIG. 2 illustrates a typical continuous casting machine using a stopper rod in the ladle and having vertical molds and vertical cooling. the invention could also be applied to continuous casting machines having ladles with sliding gate mechanisms, curved moldings, curved cooling, or any kind of melt such as ferrous, non-ferrous, or non-metals.

In order to maintain a continuous operation. it is necessary to insure that molten material is supplied to molds 30 at such a rate so as to prevent overflow at the top and molten material break out at the bottom of the mold. The actual level of molten material in the molds is detected by mold level detectors 35 of the type well known in the art. Such mold level detectors 35 include a radiation source positioned outside of the mold along one side thereof to pass radiation through the mold in a divergent band and a radiation detector positioned outside of the mold on the opposite side thereof to receive the divergent band of radiation. The radiation detector generates an analog electrical output signal which varies proportionally to the amount of radiation received. The molten material in the mold absorbs radiation from the source in varying amounts according the various molten material levels. The higher the level of molten material, the more radiation is absorbed and the radiation detector receives less radiation and consequently generates a lower level electrical signal.

The electrical signals generated by mold level detectors 35 are passed to termination and calibration circuit 101 of digital process control computer 100 of the type such as an IBM 1800 Data Acquisition and Control System or an IBM System/7. As it will be seen shortly, the height of molten material in the molds is maintained substantially constant at predetermined levels for any range of casting speeds. The withdrawal rate of the strands, i.e., the speed of the pinch rolls, is defined as casting speed. Computer 100 provides output control signals to speed control circuits 52 for controlling the speeds of motors 51 which drive corresponding sets of pinch rolls 50. The actual speeds of the sets of pinch rolls 50 are measured by tachometers 53. Tachometers 53 can be of a DC. generator type and driven by the sets of pinch rolls 50 or directly by the motor shaft. The speed signals generated by tachometers 53 are applied to the termination and calibration circuit 101 along with the signals from the mold level detectors 35 and a stopper rod position sensor 19. Other measured input signals such as spray water rate and temperature signals, etc., which are necessary to properly control the continuous casting process but not essential to the understanding of this invention would also be applied to the termination andcalibration circuit 101.

The analog input signals are passed in parallel by the.

termination and calibration circuit 101 to a signal conditioningcircuit 102 which for example converts current input signals to voltage levels; attenuates voltages; and performs low pass, passive filtering to reject normal mode A.C. noise. Conditioned input signals are then passed in parallel to multiplexor circuit 103. The function of multiplexor circuit I03 is to present the input signals at predetermined'sampling rates, one at a time, to analog to digital (A/D) conversion circuit 104. The output of the A/D circuit 104 is a digital signal which is presented to the input/output interface 105 of computer system 100. The input signal is then processed by central processing unit 106 according to programs stored in main and auxiliary storages 107 of computer system 100. The digital signals resulting from the processing are transferred to input/output interface l08 which in turn passes some of the digital signals to digital to analog (D/A) converter 109. The output signals from D/A converter 109 are analog signals which are applied to ladle stopper rod actuator 21 and speed control circuits 52. Depending upon the type of actuator used, output signals can also be in digital or pulse form. Details of the IBM I800 Data Acquisition and Control System and IBM System/7 are contained in the IBM manuals IBM 1800 System Summary, GA2- 6-6920, IBM I800 Functional Characteristics, GA2- 6-5918-8, copyrighted I966, 1969, IBM System/7 System Summary, GA34-0002 copyrighted I970, IBM System/7 Functional Characteristics, GA34-0003, copyrighted I970, 197 l, which are incorporated herein by reference.

The heights of molten material in the molds 30, the speeds of the set of pinch rolls 50 and the pour rate of molten material from ladle ISare related according to this invention in a manner as illustrated in FIGS. 1 and 4.,Molten metal flow from the uncontrolled tundish discharge nozzels 26 at different rates unpredictably. In order to maximize throughput, the fastest or leading strand is identified and the set of associated pinch rolls are operated at a constant predetermined maximum casting speed. All other strands are then defined as remaining strands and the speed of the pinch rolls associated with the remaining strands are varied so as to maintain the levels of molten material in the associated molds at a predetermined height. The molten material level of the mold associated with the leading strand is maintained at the predetermined height by manipulating the metal pour rate from ladle 15 by varying the position of the ladle stopper rod.

Before explaining in detail the logical functions performed by the digital computer, as shown in FIG. 2b, the functional systems diagram of FIG. 3 will be described. This figure is elementary and aids in understanding the behavior of a system and in describing the action of the systems variables.

In FIG. 3, a single rectangular box represents a dynamic function such that the output variable (the pointer is leaving the box) is a function of time and also a function of the input variable (the pointer is entering the box). The circle represents an algebraic function of addition. The shaded blocks represent the elements of the process itself and include the continuous casting machine (G2), the external measuring equipment (H), and the process load elements (N), which are generated by process disturbances. The blocks and circles without shading represent the functional elements of the digital computer controller and include the input elements (A), which are in most cases the computers operator consoles; the control elements used within a feedback control loop (GI) and within a feedforeward control loop (G3); and the circles. The elements that are circles can be considered as representations of equivalent computer hardware, logic, and control algorithms in the form of coded programs. The technique and symbols shown in FIG. 3 are used as the basis for defining abbreviations and subscripts of frequently used variables. The following lists variables, elements, subscripts and basic mathematical equations used:

VARIABLES Set point r Reference input e Actuating signal In Manipulated variable 0 Controlled variable ELEMENTS A Input elements GI Computer control elements within a feedback control loop G2 Process system elements G3 Computer control elements within a feedforeward control loo H Feedback elements, measuring elements N Process load elements Subscripts used in connection with the symbols listed on page 12:

2 position of pouring device I level of molten metal or height in the copper mold b Feedback variable n Load variable (disturbance) Manipulated variable m 61 *e 03* u In order to identify the leading and remaining strands, the actual strand speeds are measured as illustrated by block 201 in FIG. 1. In FIG. 4, the strand speed signals from tachometers 53 for strands I to n are applied to block 110 which represents the

circuits

101, 102, 103 and 104 of FIG. 2 respectively. Block 110 converts the signals from tachometers 53, one at a time, to provide a seris of digitized measurements bsi. Then, for each strand, a speed error signal esi is computed under control of a program in computer system 100 according to the formula esi rsc bsi. The letter i represents any strand number of n strands. The maximum allowable casting speed is labeled rsc and represents a reference which is either calculated via a mathematical heat transfer model or looked-up from tables containing standard casting practice alternatives. The numerical value itself is stored in digital form in computer system 100 and represented by block 111 in FIG. 4. The algebraic computation of rsc-bsi is performed by the central processing unit 106 in FIG. 2 and is represented by error detector 112 in FIG. 4.

The fastest or leading strand is the strand which results in the largest relative error among all the strands. All other strands are designated as remaining strands. However, the speed of the strands is continuously monitored and if one of the remaining strands yields the largest relative speed error, then it is switched into the leading strand status and the previous leading strand is switched into a remaining strand status. In order to determine the largest relative speed error esi, the sign and magnitude of each error esi is evaluated by the computer system 100. This evaluation is represented by block 202 in FIG. I and by blocks 113 and 114 in FIG. 4. Block 113 determines the sign and magnitude of the error for each strand. Block 114 represents the logic or routines for identifying the leading and remaining strands. The computer operations for performing the functions represented by blocks 113 and 114 are set forth in FIG. 5.

The program in computer system 100 for performing the functions represented by blocks 113 and 114 initializes a counter by setting it to a value of n where n represents the number of strands. This is represented by block 120 in FIG. 5. The program then causes a computation to take place for computing the speed error esl for the first strand. This computation is represented by block 12]. The results of the computation are stored in an error table represented by block 122. The counter represented by block 120 is then decremented by one. This is represented by block 123. A test is then made to determine if the counter is equal to zero. This test is represented by block 124. If the counter is equal to zero, the strand number in the error table 122, is identified as a leading strand and the routine is complete. This operation is represented by block 125. If the counter is not equal to zero, the error of the next strand is computed. This operation is represented by block 126. A test is then made to determine if the error of the next strand, i.e., the next error, is greater than the error for the previous strand. This determination is represented by block 127. If the next error is not greater than the previous error, then the strand associated with the next error is identified as a remaining strand. This operation is represented by block 128. If the next error were greater than the previous error, then the speed error for that strand is entered into the error table 122. This operation is represented by block 129. The foregoing operations provide for the identification of all strands as either leading or remaining strands.

The leading and remaining strand identification data is used for controlling switching schematically represented by block in FIG. 4. Switching 130 includes a switch for each strand and can be either hardware, for example a set of relays, electronic switches such as flipflops, or by means of a programmed control logic. Preferred and depicted in FIGS. 2 and 4 is a logical switch being incorporated in the program. Each switch is setable to one of two positions. One of the positions leads to a block in FIG. 4 for developing a predetermined maximum casting speed control signal. The other position leads to a block 150 for developing variable casting speed control signals.

The function performed by block 140 takes place in computer system 100 and the operation is represented in FIG. 6. Since the leading strand has already been identified, the computer 100 calculates and evaluates the speed error esi for the leading strand. This operation is represented by a block 141. A test is then made to determine if the speed of the leading strand is greater than the predetermined maximum allowable casting speed as represented by reference rsc. The speed of the leading strand should be equal to the predetermined maximum allowable casting speed. This provides the maximum throughput because the speed of the leading strand will then be kept substantially constant at the predetermined maximum allowable casting speed. This is possible because as it will be seen shortly, the pour rate is manupulated so that the height of the molten material in the mold 30 associated with the leading strand is controlled around the predetermined height level.

If the speed of the leading strand is greater than the reference rsc, a calculation is made for a speed correction signal ms and the value of the correction signal ms is of such magnitude and polarity that it will decrease the speed of the leading strand. This, of course. is accomplished by applying the error control signal ms to the speed control circuit 52 associated with the leading strand. If the speed of the leading strand is not greater than the reference rsc as determined by block 142, then a test is made as represented by block 144 to determine if the speed of the leading strand is less than the reference. If the speed of the leading strand is less than the reference, the calculation is made as represented by block 145 for the speed correction signal ms and the speed of the leading strand is increased by providing the proper control signal ms to speed control 52 associated with the leading strand. Of course, if the speed of the leading strand is not greater or less than the reference rsc, it must be equal to this reference and no speed error control signal ms is provided to speed control 52.

The function performed by block in FIG. 4 is also carried out by computer system 100 and the operation is represented in FIG. 7. Since a casting speed must be determined for each remaining strand, a counter is set up in computer system 100 to n-I, i.e., a value equal to the number of remaining strands. This is represented by block 151 in FIG. 7. Further, the casting speed for each remaining strand is varied depending upon the deviation of the height of molten material in the associated mold 30 from its setpoint. Thus, the next opera- 9 tion is to-calculate and evaluate mold level error, for the identified remaining strand according to the equation eli r1 bli. The predetermined mold level height reference value is represented by rl. The actual height of molten material in the mold for any one of the remaining strands, is represented by bli. The mold level error is then represented by eli.

The operation for calculating and evaluating mold level error for an identified remaining strand is represented by block 152. The next operation is to determine if the mold level for the identified remaining strand is higher than the reference level rI. This determination is represented by block 153. If the mold level is higher than the reference level, a new reference speed rs! is calculated and the speed error esi for that remaining strand is calculated and evaluated. This operation is represented by block 154. The speed control signal ms is then calculated for increasing the speed of the remaining strand by applying this control signal to the associated speed control circuit 52. This operation is represented by block 155.

If the height of the molten material in the mold was not higher than the reference height, a test would be made to determine if the mold level was lower than the reference as represented by block 156. If the mold level for that particular remaining strand is lower than the reference, a new reference speed rsI would be calculated and the speed error esi would be calculated and evaluated. This operation is represented by block 157. The speed error control signal ms would be calculated for decreasing the speed of the associated remaining strand. The speed error control signal ms would be applied to speed control circuit 52 associated with the identified remaining strands. This operation is represented by block 158.

[f the mold level for the particular identified remaining strand was not higher or lower than the reference, it must have been equal to the reference and no speed control signal ms would have been applied to the associated speed control circuit 52. The counter is then decremented by one as represented by block 159 and a test is made to determine if the counter is equal to zero as represented by block 160. If the counter is not equal to zero. the mold level error for the next identified remaining strand is calculated and evaluated and the operation repeats in the manner just described. Thus, a mold level error is calculated for each identifled remaining strand and the speed of the identified remaining is increased. decreased or left unchanged depending upon the mold level error.

The mold level crror determination takes place within computer system 100 and requires input signals from mold level detectors 35. These signals are applied to block 110 which for convenience is repeated in FIG. 4 in dotted line form. The actual mold level signal bl for one of the remaining strands is applied to error detector 161 together with the mold level reference signal r1. The mold level reference signal r1 is a value represented by digital signals provided by computer system 100 and represented by block 162 in FIG. 4. The mold level error signals e1 are shown as being stored by block 163. The mold level error signals are then applied to the block 150 via the mold level switching 165. The switches of mold level switching 165 are operated in the same manner as the strand speed switches 130 and each switch has two positions. There is a switch for each strand. Thus, one of the positions can be considered a leading strand position and the other position in a remaining strand position. The switches in the remaining strand positions apply the mold level errors from block 163 to block 150. The mold level error signal associated with the leading strand is applied via its associated switch 165 to block 170. The mold level error for the leading strand is determined in the same manner as mold level errors are determined for remaining strands.

The mold level error el for the leading strand, is used for developing a control signal for energizing ladle stopper rod actuator 21. The analog signal from stopper rod position sensor 19 is also applied to block 110. The digitized measurement coming from block 110 is the signal bp. This signal is applied to error detector 171 together with signal rp from block to develop a position error signal ep. The position error signal ep is converted to a position control signal mp by block 172. The position control signal mp is applied to ladle stopper rod actuator 21 for moving stopper rod 20.

The calculation for determination of the ladle stopper rod position error ep is performed by computer system 100 and the details of the operation are shown by flow diagram in FIG. 8. The mold level error for the leading strand is evaluated and the operation is represented by block 175. Block 176 represents the operation of determining if the mold level is greater than the reference rl. If it is, a new reference rp is calculated for the pouring device position and the pouring device error ep is calculated and evaluated as represented by block 177. The signal mp for energizing ladle stopper rod actuator 21 is calculated so as to decrease the opening of the pouring device, i.e., stopper rod 20 is moved toward opening 17.

If the mold level for the leading strand is not greater than the reference, a test is made to determine if it is less than the reference. This test is represented by block 179. If the mold level is less than the reference, a new reference rp is calculated for the ladle stopper rod position and the ladle stopper rod position error ep is calculated and evaluated. This operation is represented by block 180. The signal mp for controlling ladle stopper rod actuator 21 is then calculated, as represented by block 181, so as to increase the opening of the pouring device. i.e., move stopper rod 20 upward and further away from opening 17. If the mold level for the leading strand is not greater or less than the reference, it must be equal to the reference and therefore there is no need to calculate a new reference rp or to calculate and evaluate the stopper rod position error ep and in turn calculate the control signal mp.

ln HO. 1, it is seen that block 202 represents the function of identifying the strand with the largest speed error es. The strand with the largest speed error es is switched in to the leading strand status as represented by block 203. The speed of the leading strand is then made equal to the predetermined maximumallowable casting speed rsc. This is represented by block 204. The strand speed is then kept constant at the predetermined maximum allowable casting speed rsc and the ladle stopper rod 20 is manipulated so that the mold level is controlled around its predetermined height rl. All other strands are remaining strands and are switched into the non-lead or remaining strand status as represented by block 206. The casting speeds for these remaining strands are manipulated so that the mold levels are controlled around their preset height rI. It should be noted that the computer system 100 provides flexible control over the continuous casting machine 10. This is accomplished by utilizing the computer system 100 as a digital computer controlled utilizing the known principles of control technology, called Direct Digital Control (DDC). By consolidating all controls into the computer system, it is possible to use known control programs such as load change compensation and feedforeward control, investigation of process dynamics, and deadtime control. These known control programs are used in a practical application of the invention. Since the speed of the leading strand is kept constant at the predetermined maximum allowable casting speed and the ladle pour rate is manipulated so that the mold level of the associated mold is controlled around its preset height; dead time control provides compensation for distance velocity lag to prevent mold overflow or drainage.

The computer system 100 facilitates the calculation of a different casting speed rsc if a load change occurs such as near the end of the casting process due to a temperature drop of the molten material. The new load change can be introduced either as a process load element at a summing point within block 172 of FIG. 4 or as a control element within a feedforeward control loop. If the load change is introduced as a process load element, it would cause appropriate change in the manipulated position of the stopper rod 20, i.e., a change in the value mp from block 172. If the load change is introduced as a control element within a feedforeward control loop, the change is introduced into the calculation for the new reference rp from block 170.

Another type of load change is caused by irregular operations of the casting machine. These include irregularities in the machine speed of the leading strand caused by a strand shut-down; irregularities in the pouring rate caused by a plugged nozzle; irregularities in the machine speed of the leading strand caused by a break-out of metal below the mold. The computer system 100 facilitates these process disturbances by utilizing the method of load change compensation similar to the one caused by a temperature drop of the molten material. The new load change due to a strand shut down, plugged nozzle, or break-out, is introduced as a process load element at a summing point within block 172 of FIG. 4 and would cause appropriate change in the manipulated position of the stopper rod 20, i.e., a change in the value mp from block 172.

From the foregoing, it is seen that the invention provides improved control for a multistrand continuous casting machine without control devices in the tundish. It is further seen that there is a control strategy for the leading strand and a control strategy for remaining strands. Whenever a new leading strand is identified, it is switched into the leading strand status and its control strategy is changed appropriately. The previous leading strand is switched into a remaining strand status and its control strategy is also switched appropriately. The speed of the leading strand is kept constant at a predetermined maximum allowable casting speed so as to maximize throughput. The mold level for the leading strand is controlled around its preset height by manipulating the pour rate. The machine speeds for the remaining strands are manipulated so that the associated ploying a minimum number of monitored process variables.

What is claimed is:

l. A control system for a multistrand continuous casting machine having a ladle, pour rate control means for said ladle, tundish means having a plurality of discharge ports and positioned relative to said ladle to receive melt therefrom, a plurality of open ended molds located to receive strands of melt from said tundish means discharge ports, a pluralityof strand feeding devices for withdrawing strands from said molds, a plurality of speed control elements for controlling the speeds of said plurality of strand feeding devices, a plurality of mold level detectors for generating mold level signals proportional to the level of melt in said plurality of molds, a plurality of speed detectors for generating signals indicating the speeds of said strand feeding devices, the improvement comprising:

means for providing a predetermined reference speed signal, means for generating speed error signals from said speed signals and said reference speed signal,

means responsive to the largest speed error signal for generating a first speed correction control signal to cause one of said plurality of strand feeding devices to feed at a constant predetermined speed,

means for applying said first speed correction control signal to one of said speed control elements, means for providing a predetermined reference mold level signal,

means for generating mold level error signals from said mold level signals and said reference mold level signal, one of said mold level error signals being for said strand being fed at a constant predetermined speed,

means under control of speed error signals other than said largest speed error signals and responsive to mold level error signals other than said one mold level error signal for generating second speed correction control signals,

means for applying said second speed correction control signals to the other of said speed control elements,

means under control of said one mold level error signal for generating a pour rate control signal, and means for applying said pour rate control signal to said pour rate control means.

2. A control system for a multistrand continuous casting machine having a ladle, pour rate control means for said ladle, tundish means having a plurality of discharge ports and positoned relative to said ladle to receive melt therefrom, a plurality of open ended molds located to receive strands of melt from said tundish means discharge ports, a plurality of strand feeding devices for withdrawing strands from said molds, a plurality of speed control elements for controlling the speeds of said plurality of strand feeding devices, a plurality of mold level detectors for generating mold level signals proportional to the level of melt in said plurality of molds, a plurality of speed detectors for generating signals indicating the speeds of said strand feeding devices, the improvement comprising means for providing speed error signals for said speed signals,

means responsive to said speed error signals for generating a leading strand signal for the largest speed error signal and remaining strand signals for the other speed error signals,

means for generating a speed correction control signal for operating said strand feed devices at a'predetermined maximum speed, means for generating mold level error signals, means responsive to mold level error signals for generating speed correction control signals for operating said strand speed devices at variable speeds,

first switching means responsive to said leading strand signal for applying said speed correction control signal for operating said strand feeding devices at a predetermined maximum speed to the strand feeding device having said largest speed error and responsive to said remaining strand signals for applying speed correction control signals for operating said feeding devices at variable speeds to the other strand feeding devices,

means for generating a pour rate correction signal in response to the mold level error signal for the mold of said leading strand,

means for applying said pour rate correction signal to said pour rate control means, and

second switching means responsive to said leading strand signal for applying the mold level error signal for the mold of said leading strand to said pour rate correction signal generating means and responsive to said remaining strand signals for applying the other mold level error signals to said means for generating variable speed correction control signals.

3. The control system of claim 2 wherein said speed error signals are provided at a predetermined rate for repetitive generation of leading and remaining strand signals.

4. The control system of claim 2 further comprising means for changing said predetermned maximum speed.

5. The control system of claim 4 further comprising means for changing said pour rate correction signal as said predetermined maximum speed is changed.

6. A control system for a multistrand continuous casting machine having a ladle, pour rate control means for said ladle, tundish means having a plurality of discharge ports, a plurality of openended molds for receiving strands of melt from said tundish means discharge ports, a plurality of strand feeding devices for withdrawing strands from said molds and control devices for each strand, the improvement comprising:

a leading strand control system including means for generating pour rate correction signals and means for generating predetermined maximum speed control signals,

5 a remaining strand control system including means for generating variable speed control signals, means for identifying leading and remaining strands,

and means for applying leading and remaining strand control systems to controls for said identified leading and remaining strands.

7. The control system of claim 6 further comprising means for switching application of said leading and remaining strand control systems whereby the leading strand control system is applied to the controls for a newly identified leading strand and the remaining strand control system is applied to the controls for the previously identified leading strand.

8. A method for controlling a continuous multistrand casting machine having a ladle, pour rate control means for said ladle, tundish means having a plurality of discharge ports, a plurality of open-ended molds located to receive strands of melt from said tundish means discharge ports, a plurality of strand feeding devices for withdrawing strands from said mold, comprising the steps of:

determining which strand is being withdrawn from its associated mold at the fastest rate,

controlling the feed means for the strand withdrawn at the fastest rate to operate at a constant predetermined maximum rate,

controlling the pour rate control means for said ladle so as to control the melt discharge from said ladle into said tundish and thereby control the mold level in the mold associated with the strand being withdrawn at the fastest rate and varying the rate of the strand feeding devices for the other strands to control the mold levels in the associated molds.

9. The method of claim 8 further comprising the steps of continuously determining the strand being withdrawn at the fastest rate and switching control whereby the strand feeding device associated with the newly identified fastest strand is operated at a predetermined maximum rate and the strand feeding device for the previous fastest strand is operated at a variable speed to control the mold level in its associated mold. 1:

Claims

1. A control system for a multistrand continuous casting machine having a ladle, pour rate control means for said ladle, tundish means having a plurality of discharge ports and positioned relative to said ladle to receive melt therefrom, a plurality of open ended molds located to receive strands of melt from said tundish means discharge ports, a plurality of strand feeding devices for withdrawing strands from said molds, a plurality of speed control elements for controlling the speeds of said plurality of strand feeding devices, a plurality of mold level detectors for generating mold level signals proportional to the level of melt in said plurality of molds, a plurality of speed detectors for generating signals indicating the speeds of said strand feeding devices, the improvement comprising: means for providing a predetermined reference speed signal, means for generating speed error signals from said speed signals and said reference speed signal, means responsive to the largest speed error signal for generating a first speed correction control signal to cause one of said plurality of strand feeding devices to feed at a constant predetermined speed, means for applying said first speed correction control signal to one of said speed control elements, means for providing a predetermined reference mold level signal, means for generating mold level error signals from said mold level signals and said reference mold level signal, one of said mold level error signals being for said strand being fed at a constant predetermined speed, means under control of speed error signals other than said largest speed error signals and responsive to mold level error signals other than said one mold level error signal for generating second speed correction control signals, means for applying said second speed correction control signals to the other of said speed control elements, means under control of said one mold level error signal for generating a pour rate control signal, and means for applying said pour rate control signal to said pour rate control means.

2. A control system for a multistrand continuous casting machine having a ladle, pour rate control means for said ladle, tundish means having a plurality of discharge ports and positoned relative to said ladle to receive melt therefrom, a plurality of open ended molds located to receive strands of melt from said tundish means discharge ports, a plurality of strand feeding devices for withdrawing strands from said molds, a plurality of speed control elements for controlling the speeds of said plurality of strand feeding devices, a plurality of mold level detectors for generating mold level signals proportional to the level of melt in said plurality of molds, a plurality of speed detectors for generating signals indicating the speeds of said strand feeding devices, the improvement comprising means for providing speed error signals for said speed signals, means responsive to said speed error signals for generating a leading strand signal for the largest Speed error signal and remaining strand signals for the other speed error signals, means for generating a speed correction control signal for operating said strand feed devices at a predetermined maximum speed, means for generating mold level error signals, means responsive to mold level error signals for generating speed correction control signals for operating said strand speed devices at variable speeds, first switching means responsive to said leading strand signal for applying said speed correction control signal for operating said strand feeding devices at a predetermined maximum speed to the strand feeding device having said largest speed error and responsive to said remaining strand signals for applying speed correction control signals for operating said feeding devices at variable speeds to the other strand feeding devices, means for generating a pour rate correction signal in response to the mold level error signal for the mold of said leading strand, means for applying said pour rate correction signal to said pour rate control means, and second switching means responsive to said leading strand signal for applying the mold level error signal for the mold of said leading strand to said pour rate correction signal generating means and responsive to said remaining strand signals for applying the other mold level error signals to said means for generating variable speed correction control signals.

6. A control system for a multistrand continuous casting machine having a ladle, pour rate control means for said ladle, tundish means having a plurality of discharge ports, a plurality of open-ended molds for receiving strands of melt from said tundish means discharge ports, a plurality of strand feeding devices for withdrawing strands from said molds and control devices for each strand, the improvement comprising: a leading strand control system including means for generating pour rate correction signals and means for generating predetermined maximum speed control signals, a remaining strand control system including means for generating variable speed control signals, means for identifying leading and remaining strands, and means for applying leading and remaining strand control systems to controls for said identified leading and remaining strands.

8. A method for controlling a continuous multistrand casting machine having a ladle, pour rate control means for said ladle, tundish means having a plurality of discharge ports, a plurality of open-ended molds located to receive strands of melt from said tundish means discharge ports, a plurality of strand feeding devices for withdrawing strands from said mold, comprising the steps of: determining which strand is being withdrawn from its associated mold at the fastest rate, controlling the feed means for the strand withdrawn at the fastest rate to operate at a constant predetermined maximum rate, controlling the pour rate control means for said ladle so as to control the melt discharge from said ladle into said tundish and thereby control the mold level in the mold associated with the strand being withdrawn at the fastest rate and varying the rate of the strand feeding dEvices for the other strands to control the mold levels in the associated molds.

9. The method of claim 8 further comprising the steps of continuously determining the strand being withdrawn at the fastest rate and switching control whereby the strand feeding device associated with the newly identified fastest strand is operated at a predetermined maximum rate and the strand feeding device for the previous fastest strand is operated at a variable speed to control the mold level in its associated mold.