US3457985A

US3457985A - Continuous casting apparatus with means automatically controlling the holding vessel discharge

Info

Publication number: US3457985A
Application number: US602319A
Authority: US
Inventors: James H Wilson
Original assignee: United States Steel Corp
Current assignee: United States Steel Corp
Priority date: 1966-12-16
Filing date: 1966-12-16
Publication date: 1969-07-29
Anticipated expiration: 1986-07-29

Description

July 29, 1969 J. H. wlLsoN 3,457,985

CONTINUOUS CASTING APPARATUS WITH MEANS AUTOMATICALLY CONTROLLING THE HOLDING VESSEL DSCHARGE Filed Dec. 16, 1966 3 Sheets-Sheet 1 35 .5f/Pa ya/144.4 A

1 GEA/wmf? July 29, 1969 1. H. wlLsoN 3,457,985

CONTINUOUS CASTING APPARATUS WITH MEANS AUTOMATICALLY CONTROLLING THE HOLDING VESSEL DISCHARGE Filed Dec. 16, 1966 5 Sheets-Sheet 2 mvzimok. JAMES ML150/y H. WILSON CONTINUOUS CASTING APPARATUS WITH MEANS AUTOMAT 3,457,985 ICALLY July 29, 1969 1 CONTROLLING TEE HOLDING vEssEL DISCHARGE 16. 1966 3 Sheets-Sheet 5 Filed Dec.

United States Patent O 3,457,985 CONTINUOUS CASTING APPARATUS WITH MEANS AUTOMATICALLY CONTROLLING THE HOLDING VESSEL DISCHARGE James H. Wilson, Franklin Township, Westmoreland County, Pa., assignor to United States Steel Corporation, a corporation of Delaware Filed Dec. 16, 1966, Ser. No. 602,319 Int. Cl. B22e 19/04, 25/00, 11/10 U.S. Cl. 164-155 5 Claims ABSTRACT OF THE DISCLOSURE Continuous casting apparatus including a holding vessel above a flow-through mold, in which automatic control of the discharge valve of the vessel is responsive to the height of the metal level in the vessel. When the metal level falls below the normal operating value, the discharge valve is automatically closed. A rise of the level effects opening of the valve. An electrical system providing a wide dead band produces the desired spacing between valveopen and valve-closed conditions.

This invention relates to continuous casting apparatus, and more particularly to continuous casting apparatus comprising a ladle for molten metal, a water-cooled continuous casting mold, and a holding vessel having a bottom discharge valve between the ladle :and the mold, in which provision is made for automatically controlling the opening and closing of the discharge valve in the holding vessel.

Apparatus for the continuous casting of metals and particularly steel nearly always includes -a ladle for molten metal, and an open-ended tubular water-cooled continuous casting mold in which a metal casting may be formed. Because of the necessity for maintaining a substantially uniform molten metal level in the mold and the difficulties in accurately controlling the pouring rate when teeming molten metal directly from a ladle into a mold, it is frequently advantageous to include a holding vessel between the ladle and the mold. This holding vessel provides a reservoir for molten metal, and as a consequence, a substantially uniform flow of metal from the holding vessel into the mold can be maintained even when there are marked fluctuations in the teeming rate from the ladle into the holding vessel.

The holding vessel may be either a tundish or a degassing vessel. The use of a tundish between the ladle and the mold in continuous steel casting is well known in the rart. The purpose of -a tundish is to smooth out of the flow rate of molten metal into the mold. A continuous in-line degassing vessel having a molten metal inlet at its upper end and a valve controlled discharge opening at its lower end performs the same function as a tundish in smoothing out the metal flow rate, `and more importantly, also removes considerable quantities of undesirable gases, par ticularly oxygen, from the molten metal before it is teemed into the mold.

When starting up a continuous casting apparatus of the type described, molten metal is first poured from the ladle into the holding vessel. It is desirable to establish :a predetermined operating level of molten metal in the holding vessel before metal is teemed from this vessel into the mold. When the holding vessel is a degassing vessel operated under high vacuum, it is necessary to provide a sufficient depth of molten metal to provide a barometric seal 3,457,985 Patented July 29, 1969 lCe and thereby prevent the passage of air from the atmosphere into the evacuated vessel. In order to establish the desired liquid level, it is necessary to keep the discharge valve and the holding vessel closed until this level has been reached. Thereafter this discharge valve should remain open throughout the casting operation, unless the level of molten metal in the holding vessel falls to some predetermined level, considerably lower than that at which the discharge valve is opened, below which further operation of the system is undesirable. In the case of a vacuum degassing vessel, the discharge valve should be shut in the event that the liquid level falls so low that there is danger of breaking the barometric seal. In the case of tundish, it is desirable to close the discharge valve when the tundish has been substantially emptied of molten metal, leaving mostly non-metallic impurities floating on the surface of the metal. By closing the discharge valve of the tundish before these non-metallic impurities are teemed into the mold, the accumulation of such impurities in the mold, where they would adversely effect the quality of the casting, is prevented.

In some instances it is desirable to provide a holding vessel having a pair of inlet openings :and a single discharge opening, so that casting operations may continue indefinitely, or in any case for longer than the length of time required to empty ya single ladle, by alternately teeming molten metal from a pair of ladles. It is desirable in such apparatus to provide means for switching from one ladle to the other, so that molten metal is teemed only one ladle at a time, and for automatically controlling the movements of the stopper rod of the teeming ladle in response to the level of molten metal in either the holding vessel or the mold.

It is an object of this invention to provide an apparatus for automatically opening the discharge valve of a holding Vessel located between the ladle and the mold of a continuous casting apparatus when a predetermined height of molten metal is reached.

It is a further object of this invention to automatically close the holding vessel discharge valve in the event that the molten metal level falls to some predetermined level which is substantially below that at which the discharge valve was opened, while keeping this valve open when the metal level falls only a slight degree below the level at which the valve was opened.

A further object of this invention is to provide an automatic liquid level responsive ladle stopper rod control for a continuous-continuons system having two ladles, which control system includes means for switching from one ladle to the other.

According to this invention, there is provided a continuous metal casting `apparatus including at least one ladle, a holding vessel therebelow having a bottom discharge opening controlled by a valve, :and a continuous casting mold below the holding vessel, in which the opening of the discharge Valve of the holding vessel is automatically controlled by -a signal which is ultimately derived from load cells which measure the amount of molten metal in the holding vessel and hence the metal level therein. The discharge valve is conveniently a hydraulically operated valve in which the hydraulic control system for operating the valve includes ya piston :and a solenoid-operated four-way valve which controls movements of the piston. An amplified signal from the load cells generates an error signal when the actual level of molten metal in the holding vessel differs from a pre-set level, the polarity of the error signal being determined by the direction of deviation and the amplitude of the error signal being determined by the magnitude of deviation. A bi-stable magnetic amplifier amplifies the error signal and thereby controls a relay which alternately energizes one or the other of the two solenoids controlling the four-way valve without ever energizing both solenoids simultaneously.

The present invention also provides means for automatically closing the discharge valve of the holding vessel when the molten metal level therein drops to a second predetermined level, considerably below the first level at which the discharge valve was opened, and below which further operation of the holding vessel is undesirable. Since there is considerable difference between the valve opening level and the valve closing level, the electrical system of this invention includes means providing a wide deadband, so that the discharge valve will not open until the predetermined opening level is reached, but will then remain opened unless the predetermined closing level is reached, in which case the discharge valve is closed to prevent further metal teeming.

This invention also provides a degassing vessel or tundish having a pair of inlet openings at its upper end and a single discharge opening at its lower end, each of the inlet openings being adapted to receive molten metal, so that molten metal may be teemed into the holding vessel from one ladle at a time and, when the first vessel is substantially empty, it may be shut off and removed and metal immediately teemed from the other ladle without interruption. This type of operation is known as continuous-Continous operation. More especially this invention provides means for switching from one ladle to the other and for automatically controlling the teeming of molten metal from the teeming ladle in response to the level of molten metal in either the holding vessel or the mold.

Referring now to the drawings:

FIG. 1 is a diagrammatic view of a preferred apparatus according to this invention, showing the elements of the continuous casting apparatus schematically and the elements of the electrical control and hydraulic control systems in block diagrams.

FIG. 2 is a schematic diagram of the electrical apparatus of this invention which controls the opening and closing of the holding vessel discharge valve.

FIG. 3 is a schematic electrical diagram of the apparatus for controlling the teeming of molten metal from a pair of ladles into the holding vessel.

Referring now to FIG. 1, 11 and 12 are a pair of bottom pour ladles having

stopper rods

13 and 14 respectively for controlling the teeming of molten metal therefrom. Molten metal is teemed from one ladle at a time into a vacuum degassing vessel 15 having a pair of inlet openings 16 and 17, a connection 18 to a source of high vacuum, and a bottom discharge opening 19 which is controlled by sliding gate valve 20, which has an orifice 21. The sliding gate valve may be that shown and described in the copending application of James T. Shapland, Ser. No. 453,730, filed May 6, 1965, now Patent No. 3,352, 465, issued Nov.14, 1967.

Below the discharge opening 19 of holding vessel 15 is an open-ended tubular water-cooled flow-through continuous casting mold 22 which is adapted to receive molten metal from holding vessel 15 at its upper end, and to discharge a partially solidified metal casting from its lower end (not shown).

The

stopper rods

13 and 14 of ladles 11 and 12 respectively are controlled by identical

hydraulic control systems

23 and 24 respectively, shown in -box diagrams in FIG. 1. These control systems may be similar to the hydraulic control system shown and descirbed in Carleton Patent No. 2,832,110, issued Apr. 29, 1958, and more especially in FIG. of that patent.

The sliding gate valve 20 may be controlled by .a hydraulic system which is essentially similar to that shown in FIG. 5 of the aforesaid Carleton Patent No. 2,832,110.

Referring to FIG. 1 herein, where the components of this system are shown diagrammatically, the sliding gate valve 20 is controlled by hydraulic cylinder 25 which has a reciprocating piston 26. The position of this piston is controlled by the aforesaid hydraulic system, which includes a hydraulic reservoir 27 for hydraulic fiuid, a pump 28, and a four-way valve 29 which is controlled by a dual solenoid operator 30 comprising a pair of solenoids, only one of which can be energized at a time.

This invention provides a novel control system for controlling the dual solenoid 30 in response to the level of molten metal in degassing vessel 15, as indicated by load cells 31. An on-ofl switch 31a, which is closed during normal operation of the apparatus herein described, opens and closes the circuit between load cells 31 and the system which controls dual solenoid operator 30. A load cell signal representing the average reading of the load cells 31 is amplified by amplifier 32 which drives a motor 33, which produces an error signal Ee in error signal generator 34. This error signal generator 34 may be like that shown in FIG. 4 of Milnes Patent No. 3,204,460, issued Sept. 7, 1965. The error signal Ee has a polarity which is determined by the direction of deviation between the actual liquid level and a desired preset liquid level in vessel 15, and a magnitude which is determined by the amount of deviation. When the actual liquid level is above the preset level, Ee is positive, and this error signal places bistable magnetic amplifier 35 in the on state, energizing relay 36 which controls the current flow in electrical system 37. This electrical system 37 controls the dual solenoid operator 30 which in turn controls the hydraulic system which actuates the movements of valve 2t).

Referring now to FIG. 2, the error signal generator 34 is shown in greater detail with reference to a schematic wiring diagram. The purpose of error signal generator 34 is to form an error signal voltage Ee whose polarity is determined by the direction of deviation between the actual liquid level and a preset liquid level in vessel 15, and whose amplitude is determined by the magnitude of this deviation. Error signal generator 34 includes a DC power source such as a battery, a pair of

slide wire resistors

41 and 42 which are in parallel and connected to the opposite poles of battery 40, and a bridge circuit which includes a pair of

conductors

43 and 44 having

contacts

45 and 46 respectively, which are slidable along resistors 4.1 and 42 respectively. The position of contact 45 and hence the potential of conductor 43 is governed by the level of molten metal in degassing vessel 15. The load cells 31 indicate the liquid level in degassing vessel 15 and this signal is amplified by amplifier 32 which supplies the power input to motor 33, which has a shaft 47 for moving contact 45 to a position along slide wire resistor 41 which corresponds to the actual liquid level of molten metal in vessel 15. A reference potential is established in conductor 44 by setting contact 46 at a desired position on slide wire 42 which corresponds to the liquid level desired in vessel 15. When

conductors

43 and 44 are at the same potential, the bridge is balanced and no current flows through either conductor. However, when

conductors

43 and 44 are at different potentials, an error signal voltage Ee is generated, and connection of

conductors

43 and 44 will cause a current to fiow therethrough. As indicated above, the polarity of error signal Ee indicates the direction of deviation between the actual liquid level and the desired preset liquid level, and the arnplitude of Ee is proportional to the extent of such deviation, as indicated by the difference in voltage between conductor 43 and conductor 44. A switch 48 in series with conductor 44 is provided for starting up the apparatus.

The electrical control system of this invention includes a bistable magnetic amplifier 35 having a single output which is either on or ofi Various bistable magnetic amplifiers are known in the art. The bistable magnetic amplifier 35 may be Type 100C008 made by the Norbatrol Electronics Corporation, Pittsburgh, Pa., and described in their Catalog CX60-3, dated 'October 1961. The bistable magnetic amplifier 35 includes a magnetic core 50, an AC power source 51 which is typically supplied with 110 volts AC, and a plurality of windings each having a pair of terminals for attachment of electrical components.

Conductor 43 is connected in series with the DC control Winding 52 of magnetic amplifier 35. The current through winding 52 is proportional to the error signal voltage Ee. Both are taken as positive when the actual liquid level in vessel is above the preset liquid level.

The current in DC control winding 52 is normally opposed by a bias current in Ibias winding 53. The bias circuit 54 which controls this current also includes a battery 55 and a variable resistor 56. The polarity is such that the bias current is always negative. That is, the bias current opposes the .primary DC control current in winding 52 when the actual liquid level is above the preset liquid level, i.e., when Ee and the control current in winding 52 are positive. Thus the magnitude of the error signal voltage E., must be large enough for the control current to overcome the bias current before the magnetic amplifier 35 is placed in the on state.

The magnetic amplifier 35 also includes a bandwidth control comprising a variable resistor 57 connected to a winding 58 for controlling the width of the deadband which in turn determines the difference between the liquid level at which valve opens and the level at which valve 20 closes. The width of the deadband increases as the resistance of resistor 57 increases. Generally the resistance of resistor 57 is chosen so that valve 20 closes at a liquid level considerably lower than the level at which it opens.

The output of magnetic amplifier 35 is a single output which includes a rectifier. A relay 36 is in the external circuit of the amplifier output. When the algebraic sum of the DC control current in winding 52 and the bias current in winding 53 is zero or negative, the magnetic amplifier 35 is in the ofi state and relay 36 is de-energized. But when the algebraic sum of these currents is positive, amplifier 35 is in the on state and relay 36 is energized.

To start up a continuous casting operation, switch 4S is closed, and molten metal is teemed from ladle 11 into degassing vessel 15 with the discharge valve 20 closed. At the start, the error signal voltage E., is negative. The current flowing through winding 52 is also negative. In this state, the negative current in winding 52 is summed with the bias current in winding 53, which in all cases shall be considered to be negative. The bistable magnetic amplifier 35 is in the off state as long as the resultant of the control current in winding 52 and the current in winding 53 is negative. Relay 36 remains closed while the amplifier 35 is in the o state.

As the molten metal level continues to rise, it reaches the point at which the potentials in the liquid level con trolled conductor 43 and the reference conductor 44 are equal so that the error signal Ee is zero. The resultant current is still negative and therefore the magnetic amplier 35 remains in the off state. As the liquid level further rises, the current in winding 52 becomes positive. When this current is sufficient to offset the negative current in bias winding 53, the core 50 is saturated with a polarity such that the magnetic amplifier 35 is thrown into the on state. This causes relay 36 to be energized. Energization of relay 36 opens Contact 36A and closes contact 36B in electrical system 37. Electrical system 37, 4which has an independent power supply (typically 110 volts AC), controls dual solenoid operator 30 (FIG. 1), which includes a pair of opposed solenoids 61 and `62 controlling valve 29. Energization of relay 36 deenergizes solenoid I61 and simultaneously energizes solenoid 62. This moves the four-way valve 29 to the position causing the movement of piston 26 in the direction which opens 6 discharge valve 20. Molten metal is then teemed into mold 22.

`Once valve 20 has been opened, it is caused to remain open as long as normal operation of the continuous casting apparatus continues, even though the actual liquid level Iin vessel 15 may from time to time fall below the level at which the valve 20 was opened. This is accomplished by means of variable resistor 57 which serves as a deadband control as already mentioned. As long as the molten metal level in degassing vessel 15 is sufficiently above the ybarometric height of molten metal so that there is no danger of breaking the vacuum in vessel 15, it is desirable to keep discharge valve 20 open. The height at which valve 20 must be closed is considerably lower than the height at which this valve is opened, and therefore a wide band Width is used.

Referring now to FIG. 3, the control system for operating the

stopper rods

13 and 14 of ladles 11 and 12 respectively will now be described. This control system is generally similar to the control system shown and described in the co-pending application of Tiskus et al., Ser. No. 353,696, now Patent No. 3,300,820, issued lan. 31, 1967, except that the system of said co-pending application has been modified herein so that a pair of ladle stopper rods may be controlled. Also, in a preferred ernbodiment, the control of the stopper rods is made in response to the level of molten metal in degassing vessel 15, rather than in response to the level of molten metal in mold 22.

FIG. 3 illustrates in `electrical schematic form a liquid level control circuit 70 which responds to the error signal Ee to produce a control signal Ec which controls the positions of

ladle stopper rods

13 and 14.

The stopper rod control 24 for stopper rod 13 of ladle 11 includes a pair of solenoids 71 and 72 which control a four-way valve, whose movements control the admission of hydraulic fluid to a hydraulic cylinder `which raises and lowers stopper rod 13 of ladle 11. The stopper rod control 23 for stopper rod 14 of ladle 12 works in exactly the same way. This stopper rod control 23 includes a pair of solenoids 73 and 74 which control stopper rod 14. Energization of solenoids 71 and 73 causes upward movement of

stopper rods

13 and 14 respectively, While energization of solenoids 72 and 74 causes downward movement of

stopper rods

13 and 14 respectively.

Metal is teemed from only one of the ladles 11 and 12 at a time. This may be accomplished by means of a multiple pole relay 80, whose operation is controlled by a switch 81 located at any convenient point, such as an operators console. IRelay includes three normally closed contacts 80A, 80B, and 80C, and three normally open contacts 80D, 80E and 80F. Contacts 80A and |80B control the flow of current through contacts 99B and 102B respectively to solenoids 71 and 72 respectively. Contacts 99B and 102B are controlled by relays 99 and 102 respectively, which will be described subsequently. Contact 80C controls the flow of current from power supply line 100 to solenoid 74. Contacts 80D and 80E control the fiow of current through contacts 99B and 102B respectively to solenoids 73 and 74 respectively, while contact SGF controls current from power line 100 to solenoid 72. When the contacts in multiple pole switch 80 are in their normal positions as shown, solenoid 74 is energized, holding stopper rod 14 of ladle 12 in the closed position, While stopper rod 13 of ladle 11 is free to move up and down in response to the alternate energizations of solenoids 71 and 72. By closing switch 81 the relay 80 (FIG. 3) is energized and the normally open closed contacts 80A, 80B and 80C are opened. This contacts 80D, 80E and 80F are closed, and the normally causes solenoid 72 to be energized, holding stopper rod 13 of ladle 11 in the closed position, While up and down movements of stopper rod 14 of ladle 12 are controlled by solenoids 73 and 74.

The error `signal Ee is fed to a liquid level control circuit 70, which responds to the error signal Ee, in order to provide a control signal Ec that adjusts the position of stopper rod 13. In this fashion a servo loop is obtained by which stopper rod 13 is adjusted as a function of the liquid level in degassing vessel 15.

The control signal Ec has an intermittent component Ecl that energizes the solenoid 71 to cause an increase in liquid fiow rate and an intermittent component EQ2 that energizes the solenoid 72 to cause a decrease in the flow rate. The control circuit 70 is designed so that the components Ecl and EQ2 cannot be on simultaneously, although both may be off at the same time.

The error signal Ee cannot simply be amplified in order to control the solenoid operated valve, since that simple a system would produce serious overshooting and hunting in the control of the liquid level in the vessel 15. The -control circuit 70 must anticipate the changes in liquid level in the vessel by also being responsive to to the rate of change of liquid level and modify the control signal Ec accordingly. Furthermore, the loop response time is such that it is desirable for the control signal Ecl plus EQ2 to be intermittent. The intermittent control system causes the changes in the position of stopper rod 13 to be made in small steps, thus permitting sufficient time to elapse between the steps so that the change in pour rate will be refiected in the error signal The liquid level control circuit 70 employs a dual output magnetic amplifier 90 to control an excitation of solenoids 71 and 72, or alternatively solenoids 73 and 74. The dual output magnetic amplifier 90 has an AC power source 91 and a pair of DC input control windings 92 and 93, so that whatever signal is developed across winding 92 is algebraically summed with whatever signal is developed across winding 93 to determine whether output A or output B or neither output is turned on. The output circuit of magnetic amplifier 90 includes a common negative conductor 94 and a pair of positive output conductors 95 and 96. The magnetic amplifier 90 can be turned on or off by operation of switch 97 in the AC supply 91. In the off state both relays 99 and 102 remain deenergized regardless of the error voltage amplitude or polarity.

When the magnetic amplifier is in state A, a 4closed loop is completed between common conductor 94 and conductor 95. When the amplifier is in state B a circuit is completed between conductor 94 and conductor 96. The logic of magnetic amplifier 90 is such that both outputs A and B cannot be on at the same time but both outputs can be off at the same time. The operation of this circuit can be more easily understood by starting at the back end first and comprehending how the output of the magnetic amplifier affects the solenoids 71, 72, 73 and 74.

When the magnetic amplifier is in state A, relay 99 is energized, thereby closing the normally open relay contacts 99A and 99B. The closing of contact 99B completes a circuit from the line 100, which may be an AC power source, through the now closed contact 99B, the solenoid 71, and through the normally closed contact `101A back to line 100. Thus the throwing of the magnetic amplifier 90 into state A causes an increase in the rate of molten metal flow from ladle 11 because of the energization of solenoid 71. At the same time the closing of contact 99A energizes the time delay relay 101. This time delay relay 101 is set to operate at a predetermined length of time, for example, approximately one-half second after it has been energized, so that one-half second after the contact has been closed, the time delay relay contact 74A opens, thus breaking this circuit to solenoid 71. In this manner, the stopper rod 13 is lifted only by the amount that would be determined by one-half second of flow of liuid from the hydraulic system. Thus only a small step change is made in the pour rate of molten metal from the ladle 11.

If the magnetic amplifier is in state B, then the relay '102 is energized, thereby closing contacts 102A and 102B. The closing of contact 102B energizes the solenoid 72, thereby lowering the stopper rod 13 and decreasing the flow of molten metal from ladle 11. The simultaneous closing of contact 102A energizes the time delay relay 103 and thereby causes the contact 103A to open in one-half second which then de-energizes solenoid 72. With the solenoid 72 de-energized, the entire system continues to function with a metal fiow rate into the degassing vessel 15 that has been decreased to an extent determined by the amount which the stopper rod descends in one-half second of operation of the hydraulic system.

When an error signal Ee exists, it charges the capacitor 104 at a rate which is controlled by the setting of the variable resistor 105. Once the variable resistor '105 has been set, the rate of charge of the capacitor 104 is then strictly a function of the magnitude of the error signal Ee. The polarity of charge of the capacitor 104 is a function of the polarity of the error signal Ee. The capacitor 104 is connected in series with the variable resistor 106, across the control winding 92 of the dual output magnetic amplifier 90. When the voltage across the capacitor 104 reaches a pre-selected value (the value being governed by the setting of the variable resistor 106) it produces a signal across the control winding which causes the magnetic amplifier 90 to provide an electrical output at either A or B, depending on the polarity of the charge across the capacitor 104. Thus either the relay 99 or the relay 102 is operated by the magnetic amplifier 90 depending entirely upon the polarity of Ee.

As has been described previously, the excitation of either relay 99 or relay 102 will excite the coil of time delay relay 101. The time delay relay 101 can be adjusted manually to operate over a range of time periods; the time period selected in the embodiment illustrated is approximately one-half second. When one-half second has elapsed not only does the contact 101A open to immediately de-energize whichever of the two solenoids 71 and 72 was energized, but in addition the contact 101B closes to discharge the capacitor 104. By thus discharging the capacitor 104, the signal across the control winding 92 is eliminated, the dual output magnetic amplifier 90 ceases to have an output, and the cycle may be started all over. As long as the error signal Ee persists, the capacitor 104 will conttinue to be charged to turn on the appropriate relay 99 or 102 to cause the appropriate half second excitation of the solenoid controlled hydraulic valve in stopper rod control 23. This valve in turn will cause the pour rate of molten metal from ladle 11 to vary in a direction tending to decrease the magnitude of error signal Ee. Thus the frequency of pulsing through control winding 92 is proportional to the error signal Ee, and, for a given signal Ee, `the frequency of pulsing can be adjusted by the setting of the variable resistor 105 which then sets the charging rate of the capacitor 104.

In order to avoid overshooting the mark when employing the control circuitry just described in connection with the response of the magnetic amplifier 90 to the er ror signal Ee, it is desirable to further regulate the pouring rate in accordance with the rate of change of the liquid level in the degassing vessel 15. If the liquid level, for example, is below the pre-selected level but is increasing at a rapid rate, it is desirable to decrease rather than increase the pour rate to prevent overshooting and to obtain accurate liquid level control and rapid correction of the error. The circuitry connected with the control winding 93 is employed to achieve this purpose of overriding the control dictated by the error signal Ee when the rate of change of the liquid level is sufficiently rapid and is in a direction tending to decrease the error signal Ee.

A potentiometer arrangement 110 is employed for controlling the current through DC control winding 93. This potentiometer arrangement includes a battery 111, a slide wire resistor 112, and a variable resistor 113 in series with battery 111 and slide wire resistor 112. A slider contact 114 is moved along a slide wire resistor 112 in such a fashion that the position of the contact 114 along the resistor 112 corresponds to the location of the liquid level in the degassing vessel 15. This may be achieved by having the sliding contact 114 controlled in the same fashion as is the slider arm 74 in FIG. 4 of the aforesaid Milnes Patent No. 3,204,460. If the contact 114 is stationary, a voltage is developed across capacitor 115 which is equal and opposite to the voltage on contact 114 so that no current flows through variable resistor 116 or winding 93, which are in parallel. However, as long as the contact 114 is moving, a current will iiow through the circuit that includes the variable resistor 116 in order to charge the capacitor 115. The current iiowing through the resistor 116 develops a signal across the control winding 93, which signal is summed with the signal across the control winding 92. The various parameters in this portion of the circuit, such as the magnitude of the battery 111, the capacitance of the capacitor 115, resistances of the

variable resistors

112 and 113, as well as the number of turns on the control winding 93, can be adjusted or selected so that the signal across the winding 93 will dominate the signal across the winding 92 and will be the controlling signal as long as there is an appreciable rate of change of the contact 114. Thus, for example, if the liquid level is below that desired and the level is rising rapidly, the signal developed across the control winding 93 will oppose that across the control winding 92 so as to cause the dual output magnetic amplifier 90 to be switched to state B instead of state A, thereby reducing rather than increasing the pour rate. If, however, the rate of change of liquid level is zero, or very small, then the signal across the winding 93 will be minor compared with the signal across the winding 92, and the state of the dual output magnetic amplifier 90 will be determined solely by the polarity of the error signal Ee.

The time delay relay 103 performs the same function for the circuit associated with the control winding 93 as does the time delay relay 101 for the control element 92. When either the relays 99 or 102 are energized, the time delay relay 103 becomes energized by the closing of either contact 99A or 102A. The time delay relay 103 is preset to have its contact 103A close a predetermined length of time after it has been energized, which in this case may be in the order of one-half second. Thus onehalf second after a change in the position ofnstopper rod 13 has been initiated, the relay contact 103A will close and short out the variable resistor 116, thereby dropping the signal on the control coil 93 to zero and turning off whichever state the dual output amplifier 90 may have been in.

It should be remembered that when one of the states (that is, state A or state B) is turned olf in this dual output amplifier 90, that the other state is not thereby automatically turned on, and thus the turning off of one of the states simply means that there is no output at all and thus no energization for either the relay 99 or the relay 102, which in turn means that the stopper rod 13 will remain in the position to which it was last moved.

The switch 97 is normally closed but may be opened when it is desired to disenable the stopper rod control circuit.

The description with reference to FIG. 3 has been with particular reference to the control of teeming in ladle 11. When ladle 11 is substantially empty, it is necessary to switch over from ladle 11 to ladle 12 in order to continue the teeming of molten metal into degassing vessel 15 without interruption. By manual closing of the switch 98, the normally closed contacts of relay 80 are opened,

and the normally open contacts are closed. When this is done, solenoid 73 will be actuated in place of solenoid 71 to increase the stopper rod opening of stopper rod 14 in ladle 12, and energization of solenoid 74 will cause partial closing of stopper rod 14 in ladle 12.

Instead of generating the error signal Ee from a signal from load cells 31, the error signal can be obtained from a signal which is obtained from a bank of thermocouples (FIG. l) and fed to a liquid level deviation detector 121, as more fully described in Milnes Patent No. 3,204,- 460. A switch 122 is placed in the liquid level detection circuit and is in the off position when the signal from load cells 31 is used to generate the error signal.

While this invention has beeny described with respect to specific embodiments thereof, it will be understood that variations can be made by those skilled in the art without departing from the scope and spirit of this invention.

What is claimed is:

1. In a continuous metal casting apparatus including a bottom pour ladle having a stopper rod controlling the discharge of molten metal therefrom; a holding vessel below said ladle having an upper inlet opening for receiving molten metal from said ladle, a bottom discharge opening, a discharge valve controlling said discharge opening, and valve operating means :for automatically controlling the opening and closing of said discharge valve; and an open-ended How-through continuous casting mold beneath the discharge opening of said holding vessel, the improvement comprising:

(1) means for continuously measuring the level of molten metal in said holding vessel;

(2) means for generating an error signal whose polarity indicates the direction of deviation lof the measured level of molten metal in said holding vessel yfrom a preset level and whose magnitude indicates the extent of such deviation;

(3) single output amplifier means for providing an output signal Kwhen the polarity of said error signal indicates lan actual liquid level above the preset level in said holding vessel and when said error signal exceeds a predetermined magnitude, and

(4) means for controlling said valve operating means 'in response to said output signal.

2. Apparatus according to claim 1 in which said amplitier means is a bistable magnetic amplier.

3. Apparatus according to claim 1 in which said amplifier means has a deadband whereby said discharge valve is opened when the level of molten steel exceeds a rst predetermined level and is closed when the level of molten steel falls below a second predetermined level which is lower than said first level.

4. Apparatus according to claim 1 in which said valve operating means comprises 4a hydraulic system having a source of fluid under pressure, a four-way valve, a doubleacting piston controlled by said four-way valve, and a pair oflsolenoids for controlling the position of said Ifour-way va ve.

5. Apparatus according to claim 1 in which said means for controlling said Valve operating means includes a circuit having a power source, a pair of opposed solenoids, one of which causes said discharge valve to open and the other of which causes said discharge valve' to close, a relay in the output circuit of said amplifier means and adapted to be energized by said output signal, and a pair of contacts operated by said relay for controlling the energization of said solenoids, one of said contacts being normally open and the other normally closed so that said solenoids cannot be simultaneously energized.

References Cited UNITED STATES PATENTS Re. 25,892 ll/l965 Tarukawa 222-56 X 2,381,505 8/ 1945 Lindholm 222-56 (Other references on following page) 11 i 12 3,133,726 5/1964 Tarukawa 222.-56 X J. SPENCER oVERHoLsER, Primary Examiner 2,905,989 9/1959 Black 164-155 3,300,820 1/1967 Tiskus e1 a1. 164-155 R- D. BALDWIN, Asslstant Exammer FOREIGN PATENTS 5 n U.s. c1. XP. 732,115 6/1955 Great Britain. 164-64, 281; 222-56 965,167 7/1964 Great Britain.