US3627021A

US3627021A - Continuous casting control system using vacuum vessel pressurization

Info

Publication number: US3627021A
Application number: US844949A
Authority: US
Inventors: Ronald G Schultz; Stevan N Vojnovic
Original assignee: United States Steel Corp
Current assignee: United States Steel Corp
Priority date: 1969-07-25
Filing date: 1969-07-25
Publication date: 1971-12-14
Anticipated expiration: 1988-12-14
Also published as: GB1317750A; FR2053166B3; BE753778A; DE2035610A1; ES382043A1; NL7010943A; FR2053166A7; AU1764370A

Abstract

A repressurization control system provides a uniform flow from a batch vacuum degassing vessel for continuous casting. A function generator provides a vessel pressure signal which maintains a uniform effective metal head during the cast and is controlled by the amount of metal cast. System sluggishness is overcome by feedback controls correcting the signal for actual vessel pressure and for departure from desired casting speed.

Description

United States Patent [72] Inventors Ronald G. Schultz [56] References Cited y Whine, Ohio; UNITED STATES PATENTS Stevan N. Vojnovlc. Monroeville Borough, 3 300 820 H1967 Tiskus et al I 64/1 55 Pa.

3,340,925 9/1967 Woodburn, Jr 164/155 X 2' Q l g r'gz 3,457,985 7/l969 Wilson 164/155 e d 14'1971 3,478,808 1 H1969 Adams 164/4 1 3,480,073 11 [I969 Wilson 164/155 [73] sa'ess'eelcmpm'im 3,521 696 7/1970 Lowman etal. 164/154 Primary ExaminerR. Spencer Annear [54] CONTINUOUS CASTING CONTROL SYSTEM Attorney-Rea c, H l

USING VACUUM VESSEL PRESSURIZATION 9 Claims, 2 Drawing Figs. 1

ABSTRACT: A repressurization control system provides a l l I I i I I l I l l i I i l l Unl orm 0W rom a atc vacuum e assln VCSSE Ol' CO- [52] U.S.Cl 2662012585, n f b h d g g If 51 I U and 11/10 tinuous casting. A function generator provides a vessel pres- 164/4 82 sure signal which maintains a uniform effective metal head l e 0 I 155 during the cast and is controlled by the amount of metal cast. System sluggishness is overcome by feedback controls correcting the signal for actual vessel pressure and for departure from desired casting speed.

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613 cocunx Warm/r7- 70 sub Aron aulrrm PL) e t 5 a .74 Pan-Am moron! 545512; 8 a on: re: COIITIOL L l l T Pom-unems: awn I 0 vuve Mm 1 P rnm- I lviwm Zliill 4/0010 u'vn. L/ou/n um /64 ran can! "at Parr/1m- PIOPORYIOIML 011mm :2 la .70 :2 r INTEGRAL 173'" m I ill-2g can "out" M mrh 1 t as 2a nuuuran 4a 5 $6 l 20 .50 J POTENTI- aware/r CONTENUUIUS CASTING CONTROL SYSTEM IUSIING VACUUM VESSEL PRESSURHZATHGN This invention relates to a continuous casting control system using vacuum vessel pressurization, and more particularly, to a control system where vessel pressure and volume of metal cast are used in feedback control to provide a uniform flow rate in the outflow of molten metal from a batch vacuum degassing vessel.

A single ladle of molten steel is emptied into a degassing vessel in batch degassing. A sliding gate at the bottom of the vessel is opened to allow the metal to flow through the gate nozzle into a mold. The rate of flow of metal from a vessel is determined from the equation F=Ak VF where F is the rate of flow A is the area of the vessel nozzle k is the vessel nozzle constant H is the effective head, the height of the metal above the vessel nozzle less the barometric height of molten metal due to difference in pressure inside and outside the vessel.

As soon as the gate is opened to allow metal to flow through the nozzle, F changes because H changes. As the vessel empties, H decreases to a value where F is too small for satisfactory continuous casting. Theoretically, F can be held relatively uniform during the cast by increasing A, either with an eroding nozzle or by changing to larger nozzles as the metal flows out of the vessel. However, eroding nozzles are difficult to control, and no satisfactory nozzle changing device has been developed.

When the ladle has been emptied into the degassing vessel, flow may be controlled either by maintaining the seal or by opening to atmospheric pressure, but these methods have too narrow a control range.

If the vessel nozzle is opened before the ladle is emptied into the vessel and the ladle nozzle is manipulated to hold H relatively constant, a desired uniform flow results until the ladle is empty. At that time pressurizing the vessel and increasing the pressure, based on casting speed, holds l-l relatively constant as metal flows out thereby providing a relatively uniform flow. This method has the disadvantages that the ladle must be in position throughout most of the cast and automatic control is present only at the end of the cast. in addition, manual control by ladle stopper manipulation is unsatisfactory because of a tendency towards over correction causing oscillations and because the sluggish response may not be able to handle system disturbances or provide fast enough action when highflow rate is required.

The present invention uses the method of sealing the vessel after degassing and then opening the gate. Gas is admitted into the vessel under sufficient pressure to maintain a uniform H. The amount of gas is programmed according to vessel size and amount of metal cast. The amount of gas admitted is corrected by feedback of vessel pressure and casting speed resulting in a fast acting control system.

It is, therefore, an object of our invention to provide a continuous casting control system with batch vacuum degassing which maintains a uniform flow rate.

Another object is to provide such a system with relatively short time constants resulting in fast correction of system disturbances.

Still another object is to provide a system which continuously controls the flow rate during the entire cast.

A still further object is to provide a system independent of vessel nozzle erosion.

These and other objects will become more apparent after referring to the following drawings and specification in which:

FIG. 11 is a schematic drawing of the control system of our invention; and

FIG. 2 is a schematic drawing of an alternative embodiment of our invention for providing a signal representing the volume of metal cast.

Referring now to FIG. l, reference numeral 2 indicates a vacuum degassing vessel for degassing molten steel 8. Vessel 2 has a cover 4, a discharge nozzle 6, a sliding gate 8 and a line connected to vacuum apparatus (not shown) through valve 12 for withdrawing gas released during the pouring operation. The metal 8 flows from vessel 2 into a mold M where it solidifies, and is continuously withdrawn by pinch rolls 116. A mold liquid level detector 1% is connected to a liquid level indicator circuit 20. Circuit 20 is connected to a controller circuit 22 which is connected to a pinch roll speed regulator circuit 24. Circuit 24 is connected to a pinch roll drive motor 26 which drives pinch rolls 116 and a tachometer generator 28. The electrical output 30 of generator 26 is connected to pinch roll speed regulator circuit 24L The parts thus far described are conventional with parts M to 30 constituting a casting control system 32, which may be the type described in Tisltus et al. US. Pat. No. 3,300,820 dated Jan. 31, 1967, but with no dead band in controller 22.

Tachometer generator output 30 is connected to

potentiometers

34 and 36. Potentiometer 34 is connected to an integrator amplifier 30 which may be a Model 19-407 manufactured by Consolidated Electrodynamics Corporation. Potentiometer 36 is connected to an amplifier All) which may be a Model l9-30l manufactured by Consolidated Corporation.

When casting on a multistrand system, such as four billets, there will be three additional

tachometer generator signals

30, 30", and 30" from the other casting lines (not shown) which will be connected in a similar manner to potentiometers 34', 1%", 341", 36, 36" and 36" which in turn are connected to amplifiers 38 and Ml, respectively.

integrator 36 is connected to a function generator 42, which may be a fixed diode function generator manufactured by Electronic Associates, lnc., West Long; Branch, New Jersey. Amplifier 410 and a potentiometer Ml are connected to a proportional integral controller 46 which may be a Model 18-402 manufactured by Consolidated Electrodynamics Corporation.

A conventional pressure transducer m located in vessel 2 provides an electrical pressure signal to one input of a comparator 50 which may be a Model 19-501 manufactured by Consolidated Electrodynamics Corporation. A potentiometer 52 and an amplifier 54 are connected to the other input of comparator 50. A relay coil 56 is connected to the output of comparator 50. Transducer 48, function generator a2 and proportional integral controller M are all connected through normally closed relay contacts 56C to a proportional electrical controller 58 which may be a Model l8-40l manufactured by Consolidated Electrodynamics Corporation.

The output of controller 5% is connected to the gate open contact of a single pole double throw gate limit switch 60. The gate closed contact of switch 60 is connected to a close valve power source 62. Switch 60 is connected to an actuator 64 which operates a gas valve 66 which may be an equal percentage valve manufactured by Leslie Company, Lyndhurst, New Jersey, to admit gas from a supply 6% into vessel 2 through line 70.

in operation, prior to a cast,

potentiometers

36, 36', 36", and 36" are set to UN where N is the number of lines in operation. Amplifier d0 will then supply a signal representing the average casting speed for multistrand operation.

Potentiometers

34, 34V, 34", and 341" are set to a value proportional to the mold cross-sectional area of each line. Potentiometer 52 is set to a value corresponding to the maximum allowable pressure in vessel 2. Potentiome'ter 44 is set to a value corresponding to the desired casting speed. Gate 0 is closed and cover 4 is replaced by the ladle and its pouring apparatus (not shown). Power source 62, through switch 60, sets valve actuator 6d to close valve 666. Vacuum valve 12 is opened and the vessel is pumped down to the proper vacuum level.

With the parts so prepared, molten metal is poured from the ladle into the vessel. When the ladle is empty, valve i2 is closed, the ladle and pouring apparatus are removed and cover d replaced to seal vessel 2. Gate 6 is then opened allowing molten metal to flow into mold ll4l. Switch 60 connects controller 50 to valve actuator 64. When the mold level reaches the proper height, control system 32 begins to function, providing tachometer generator output 30 as the casting speed signal.

Assuming single strand operation, the casting speed signal 30 is attenuated by and the resultant signal potentiometer 34 is integrated in integrator 38 to give a signal representing the volume of metal that has been cast. This signal is converted in function generator 42 to a signal which represents the pressure required in vessel 2 to maintain a steady casting rate as the metal flows out of vessel 2. The function of generator 42 is determined from the equation P H=H,..H,.+ (2) where I H is the total head,

H, is the metal head above sliding gate 8,

H is the barometric head, a constant,

PV is the vessel pressure,

d is the density of molten metal.

In order to maintain a uniform flow and casting speed, H must be held constant. H,, is determined from vessel shape and metal volume by subtracting the volume of metal cast as supplied from integrator 38 from the volume of metal in vessel 2 at the start of the cast. Pv is the output signal'from function generator 42 which represents the desired pressure in vessel 2 at any metal level beginning from the starting metal level. In normal operation, the output of generator 42 is zero at the start of a cast. and, as the volume of metal that has flowed from the vessel increases, the desired vessel pressure increases to maintain a constant flow.

The output of generator 42 is then connected to proportional controller 58 which provides a proportional signal to valve actuator 64, setting valve 66 to admit the amount of gas into vessel 2 for the desired vessel pressure.

The dynamics of the system thus far described are sluggish because of the long time constant associated with vessel 2, and in order to better correct disturbances in the system, it is advisable to provide a pressure feedback control. A signal from pressure transducer 48 is combined with the signal from generator 42. The desired vessel pressure is compared to the actual vessel pressure and the corrected value supplied to controller 58. This overcomes the sluggishness to a great degree.

If the metal volume at the start of the cast is above or below the nominal value set in function generator 42, there will be an error in the operation of controller 58, since the initial value of H. will be either too high or too low which will result in too high or too low a casting speed. If, during a cast, the nozzle erodes. casting speed will be too high, and casting speed will be too low if the nozzle freezes. To prevent undesirable casting speeds under these conditions, a third feedback control is provided.

Tachometer generator output 30 is connected through potentiometer 36 and amplifier 40 to proportional controller 46. Controller 46 then compares this signal with the setting of potentiometer 44 which is set for the desired casting speed. The output of controller 46 is an error signal to controller 58 to correct vessel pressure for casting speeds lower or higher than the desired reference speed. The combination of pressure feedback and casting speed feedback results in a fast response and corrective action for disturbances in the system or for calibration changes in load cells.

When vessel 2 is emptied, the control system is disconnected. If the pressure is vessel 2 rises above the value set in potentiometer 52, comparator 50 changes state, activating relay 56 and opening contacts 56C. This will maintain the position of gas valve 66 until pressure in vessel 2 falls below the allowable limit setting of potentiometer 52.

When more than one line is being cast, depending upon the number of lines in operation, signals 30, 30", and 30" may be active. With

potentiometers

30, 30, 30" and 30" set at UN where N is the number oflines operating, amplifier 40 will provide an average casting speed. Each of the

potentiometers

34, 34, 34" and 34 will be set for the area of its respective casting line, and integrator 38 will then have an output representative of the total volume cast on all the casting lines.

Referring now to the alternative embodiment shown in FIG. 2,

numerals

72, 74, 76 and 78 are four conventional load cells under the four supports (not shown) of vessel 2. The load cells are connected to an amplifier 80 which is connected to a potentiometer 82. Two

potentiometers

84 and 86 together with potentiometer 82 are connected to an amplifier 88 which may be a Model 19-301 manufactured by Consolidated Electrodynamics Corporation. The output of amplifier 88 is then connected to function generator 42 in place of the output of integrator 38.

in operation,

load cells

72, 74, 76 and 78 each provide a weight signal which is added in amplifier 80. Potentiometer 82 is set to a value that will transform weight into volume of metal. Potentiometer 84 is adjusted to provide a zero output from amplifier 88 prior to the cast to account for the tare weight of vessel 2. When vessel 2 is filled, potentiometer 86 is adjusted to provide zero output from amplifier 88, providing an initial volume setting. As metal flows from vessel 2, the output of amplifier 88 increases, thereby providing a measure of the volume of metal cast for generator 42. This method requires no recognition of multistrand casting nor a change from nominal in the volume of a cast.

While several embodiments of our invention have been described, it will be apparent that other adaptations and modifications may be made.

We claim:

1. Apparatus for providing a uniform flow rate of molten metal from a sealed bottom pour vessel into a continuous cast ing mold having means responsive to mold liquid level for controlling pinch rolls withdrawing the cast from the mold comprising a supply of pressurized gas, a line connecting the gas supply to the vessel, a valve in said line for controlling the admission of gas into the vessel, means for providing a signal representing the volume of metal cast, a function generator connected to the means for providing a signal representing volume for providing a signal representing the required gas pressure to maintain a uniform head during the cast, a vessel pressure transducer for providing a signal representing actual gas pressure in the vessel, and means connected to the function generator and the vessel pressure transducer for setting said valve in response to the combination of said actual vessel pressure signal and said required vessel pressure signal to maintain a uniform molten metal flow rate.

2. Apparatus according to claim 1 which includes means for providing a signal representing actual casting speed, a speed potentiometer for providing a signal representing desired casting speed, means for providing a signal representing the difference between actual and desired casting speeds expressed in vessel pressure, and means combining the signal representing casting speed difference and the signal representing required vessel pressure, thereby providing a corrected pressure signal to the valve setting means.

3. Apparatus according to claim 2 which includes means connected to said vessel pressure transducer for disconnecting the corrected pressure signal from the valve setting means when the vessel pressure reaches a predetermined level.

4. Apparatus according to claim 3 in which the means for providing a signal representing the volume of metal cast includes means for providing a signal representing the casting speed of each strand being cast, an area potentiometer for providing a signal representing the cross-sectional area of the mold of each strand being cast, and an integrator for combining the casting speed signals and the area signals during the cast.

5. Apparatus according to claim 4 in which the means for providing a casting speed signal is a tachometer generator connected to the pinch rolls; the means for disconnecting the pressure signal from the valve setting means includes a pressure potentiometer for providing a signal representing the maximum allowable vessel gas pressure, a comparator having its first input connected to the vessel pressure transducer and its second input connected to the pressure potentiometer, a relay connected to be energized by the output of the comparator when its second input is greater than its first input, and normally closed relay contacts of said relay connecting the corrected pressure signal to the valve setting means; and the means for setting the valve includes a proportional controller for providing a signal proportional to the corrected pressure signal and a valve actuator for setting the valve position in response to the proportional signal.

6. Apparatus according to claim which includes means for providing a signal representing the actual casting speed of each strand being cast, a proportioning potentiometer connected to each actual casting speed signal, and amplifier means connected to each proportioning potentiometer for providing an actual average casting speed signal to the means for providing a signal representing casting speed difierence by setting each proportioning potentiometer to UN where N is the number of strands being cast.

7. Apparatus according to claim 3 in which the means for providing a signal representing the volume of metal cast includes load cell means supporting the vessel for providing a signal representing weight, a converting potentiometer connected to the load cell means for converting the weight signal into a signal representing metal volume, a tare potentiometer for providing a signal representing vessel tare expressed in metal volume, means combining the signals from convening and tare potentiometers thereby providing a signal representing volume of metal cast, and a zeroing potentiometer connected to the signal combining means for adjusting the signal representing volume of metal cast to zero when the vessel is initially filled.

8. Apparatus according to claim 7 in which the means for providing a casting speed signal is a tachometer generator connected to the pinch rolls; the means for disconnecting the pressure signal from the valve setting means includes a pressure potentiometer for providing a signal representing the maximum allowable vessel gas pressure. a comparator having its first input connected to the vessel pressure transducer and its second input connected to the pressure potentiometer, a relay connected to be energized by the output of the comparator when its second input is greater than its first input, and normally closed relay contacts of said relay connecting the corrected pressure signal to the valve setting means; and the means for setting the valve includes a proportional controller for providing a signal proportional "to the corrected pressure signal and a valve actuator for setting the valve position in response to the proportional signal.

9. Apparatus according to claim which includes means for providing a signal representing the actual casting speed of each strand being cast, a proportioning potentiometer connected to each actual casting speed signal, and amplifier means connected to each proportioning potentiometer for providing an actual average casting speed signal to the means for providing a signal representing casting speed difierence by setting each proportioning potentiometer to l /N where N is the number of strands being cast.

UNITED STATES PATENT ormcE CERTIFICATE OF COREUMUN Patent No. 3, 7, Dated December 14, 1971 lnventofls) Ronald G, Schultz et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

, Column 2, line 23, after "consolidated" should read H Electrodynamics Column 3, line 4; after "attenuated by" should read potentiometer 34 same line, after "sign-all" cancel "potentiometer 3 Signed and sealed thio 7th day of November 1972.

(SEAL) Attest:

EDWARD MQFLETCHERJR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patenta RM P 4 (10-69) USCOMM-DC 50376-P69 fl' U.S. GOVERNMENT PRINTING OFFICE: I969 0-355-334,

Claims

1. Apparatus for providing a uniform flow rate of molten metal from a sealed bottom pour vessel into a continuous casting mold having means responsive to mold liquid level for controlling pinch rolls withdrawing the cast from the mold comprising a supply of pressurized gas, a line connecting the gas supply to the vessel, a valve in said line for controlling the admission of gas into the vessel, means for providing a signal representing the volume of metal cast, a function generator connected to the means for providing a signal representing volume for providing a signal representing the required gas pressure to maintain a uniform head during the cast, a vessel pressure transducer for providing a signal representing actual gas pressure in the vessel, and means connected to the function generator and thE vessel pressure transducer for setting said valve in response to the combination of said actual vessel pressure signal and said required vessel pressure signal to maintain a uniform molten metal flow rate.

6. Apparatus according to claim 5 which includes means for providing a signal representing the actual casting speed of each strand being cast, a proportioning potentiometer connected to each actual casting speed signal, and amplifier means connected to each proportioning potentiometer for providing an actual average casting speed signal to the means for providing a signal representing casting speed difference by setting each proportioning potentiometer to 1/N where N is the number of strands being cast.

7. Apparatus according to claim 3 in which the means for providing a signal representing the volume of metal cast includes load cell means supporting the vessel for providing a signal representing weight, a converting potentiometer connected to the load cell means for converting the weight signal into a signal representing metal volume, a tare potentiometer for providing a signal representing vessel tare expressed in metal volume, means combining the signals from converting and tare potentiometers thereby providing a signal representing volume of metal cast, and a zeroing potentiometer connected to the signal combining means for adjusting the signal representing volume of metal cast to zero when the vessel is initially filled.

8. Apparatus according to claim 7 in which the means for providing a casting speed signal is a tachometer generator connected to the pinch rolls; the means for disconnecting the pressure signal from the valve setting means includes a pressure potentiometer for providing a signal representing the maximum allowable vessel gas pressure, a comparator having its first input connected to the vessel pressure transducer and its secOnd input connected to the pressure potentiometer, a relay connected to be energized by the output of the comparator when its second input is greater than its first input, and normally closed relay contacts of said relay connecting the corrected pressure signal to the valve setting means; and the means for setting the valve includes a proportional controller for providing a signal proportional to the corrected pressure signal and a valve actuator for setting the valve position in response to the proportional signal.

9. Apparatus according to claim 8 which includes means for providing a signal representing the actual casting speed of each strand being cast, a proportioning potentiometer connected to each actual casting speed signal, and amplifier means connected to each proportioning potentiometer for providing an actual average casting speed signal to the means for providing a signal representing casting speed difference by setting each proportioning potentiometer to 1/N where N is the number of strands being cast.