US3537505A

US3537505A - Method of controlling continuous casting

Info

Publication number: US3537505A
Application number: US816850*A
Authority: US
Inventors: Armin Thalmann; Richard Moser
Original assignee: Concast AG
Current assignee: SMS Concast AG
Priority date: 1965-12-30
Filing date: 1968-12-23
Publication date: 1970-11-03
Anticipated expiration: 1987-11-03

Description

United States Patent Inventors Armin Thalmann Qster and Richard Moser, Dietikon, Switzerland 816,850

Dec. 23, 1968 Division of Ser. No. 606,147, Dec. 30, 1966, abandoned mix 1970 Concast A G,

Zurich, Switzerland Dec. 30, 1965 Switzerland Appl. No. Filed Patented Assignee Priority METHOD OF CONTROLLING CONTINUOUS CASTING 8 Claims, 20 Drawing Figs.

US. Cl 164/4, 164/155 Int. Cl B22d 11/10, B22d l/02, B22d 17/32 Field of Search 164/4, 82,

[56] References Cited UNITED STATES PATENTS 2,145,438 H1939 Thulin 164/152 3,300,820 l/1967 Tiskus et al 164/155 3,344,841 10/1967 Rys et al 164/154X 3,349,834 10/1967 Wilson 164/155 3,358,743 12/1967 Adams 164/154 FOREIGN PATENTS 1,373,146 4/1964 France 164/154 Primary Examiner-J. Spencer Overholser Assistant Examiner- R. Spencer Annear Attorney-Sandoe, Neill, Schottler & Wikstrom ABSTRACT: In a continuous casting plant sensing elements a responsive to the metal supply to the mold, to the level of metal in the mold or in the partly solidified casting, and/or to a breakthrough of liquid metal from the casting generate signals that are automatically analyzed and applied to control the supply of metal to the mold, the rate of withdrawal of the cast-v ing from the mold and/or the cutting of the casting.

Patented Nov. 3, 1970 Sheet 1 ATTORNEYS Patented Nov. 3, 1970 3,537,505

Sheet 3 01 6 49 '49 FIGZ) 52 50 I 5/ 68 66 69 29 l 60 30 RICHARD MOSER u lkamcwa wwfl ATTORNEYS Patented Nov. 3, 1970 Shoot F|G.H

Ill

ATTORNEYS Patented Nov. 3, 1970 I 3,537,505

Shoot 6 Of 6 INVENTORS ARMIN THALMANN RICHARD MOSER ATTORNEYS METHOD OF CONTROLLING CONTINUOUS CASTING This is a division of application Ser. No. 606,147 filed Dec. 30,1966, now abandoned.

A signal indicating a fall in the level of molten metal in a continuously cast strand or in the molten metal in the continuous casting mold is fed to a switching network which analyzes the type of fault and automatically initiates control signal for plant correcting control.

The present invention relates to a method of controlling a continuous casting plant when a fault in operation arises which results in a fall in the level of the metal pool in the mold.

Continuous casting plant are usually operated by manual control means. Automatic controls that have been proposed include arrangements for weighing the metal for the purpose of checking the level of the metal in pouring vessels, as well as arrangements for monitoring the level of the metal pool in the mold. The first of these devices is used for controlling the rate of metal supply and the second for controlling the rate of withdrawal of the casting from the mold.

Some of the work of manual control is undertaken by electrical elements. In order further to reduce the work involved in an manual control in favour of automatic control, means must be provided for monitoring and indicating operational faults which may occur whilst casting proceeds.

The most troublesome fault during continuous casting is an eruption or breakthrough of liquid metal which may be due to unpredictable causes of a metallurgical and mechanical kind.

Moreover, during continuous casting troubles may arise in connection with the supply of liquid metal to the casting mold. One type of fault, caused for instance by a gating valve jamming in open position, may lead to an excessive supply of liquid metal into the mold. It has already been proposed to provide monitoring means which generate a signal when the level of the liquid pool in the mold rises beyond a fixed maximum level and to use the signal for activating an alarm.

In a different group of faults, due for instance to a partial blockage of the ingate by a fragment off the plug, the supply of metal to the mold is insufficient.

A known arrangement for monitoring the level of the liquid pool in the mold seeks to indicate such a deficiency by generating a signal when the level of the pool falls below a minimum level, the signal being similarly used to activate an alarm or to reduce the rate of withdrawal.

However, it has not yet been recognised that a fall in the level of the liquid pool may also be due to other causes which call for immediate remedial action. A fall in the level of the liquid metal also occurs when there is a breakthrough of metal through the crust of the partly solidified casting. Moreover. when casting begins and when casting ends liquid metal is ubsent at the minimum level. Consequently the signal derived from such a minimum level monitoring device has no unique meaning, in other words it does not indicate the nature of the fault or the operating conditions causing the indicated condition.

It is therefore the object of the present invention to provide control means for a continuous casting plant which respond to a breakthrough of metal as well as to a deficiency in the rate of metal supply in such manner that the appearance of a corresponding signal will permit immediate corrective controlling action to be initiated.

According to the invention this is achieved in that at least one element is provided which generates a signal characteristic of the type of fault that has occurred and that by reference to said signal the plant is controlled in the manner required for dealing with the particular fault.

The system for performing this method is characterised by electrical switching elements which evaluate the signal of the element indicating the fault, and which are associated with storage elements.

For unambiguously indicating the occurrence of breakthrough of metal the signal reporting the fault may be derived from an element which responds to it fail in the level oithe liquid metal in the partly solidified casting. Alternatively this signal may also be drived from an element which responds to the liquid metal issuing from the partly solidified casting.

For unambiguously indicating a deficiency in metal supply to the mold the fault signal may be derived from an element which responds to a fall in the level of the metal pool in the mold. Alternatively this signal may be derived from an element which monitors the size of the teeming metal jet.

For generating a signal indicating a fall in the level of the metal in the partly solidified casting or in the level of metal pool in the mold three possibilities are available:

a. A level indicator may be arranged to supply an analogous signal. The rate at which the level of the metal falls may then decide which kind of fault signal must be generated.

b. A level monitoring means may be arranged to supply signals at two levels. The signal magnitudes at these levels may then be used to decide the nature of the fault generating the signal.

c. Digital signals may be generated at two levels. The nature of the fault signal is then determined by the interval in time between the signals at the two levels in relation to a predetermined time interval.

The signals generated by the proposed elements may also be subject to the effects of other signals, as will be later described, for bridging the operating states which arise at the beginning and end of the casting process and for avoiding unwanted fault signuls during these phases.

Other features of the invention will be understood from the following description of embodiments of the invention with reference to the drawings in which:

FIG. 1 is an elevation cross sectioned view of a continuous casting plant, with sensing and control circuitry in schematic form;

FIG. 2 is a cross section of a continuous casting mold with monitoring means providing two level indications, and comprising a radiation source and radiation detectors;

FIG. 3 is a cross section of a continuous casting mold with monitoring means providing two level indications, and comprising a transmitting and detecting system for electromagnetic waves;

FIG. 4 is a cross sectioned view of a cast strand having two elements responsive to flowing metal from a breakthrough;

FIG. 5 is a side elevation of the system according to FIG. 4;

FIG. 6 is a cross section view of a continuous casting plant with a pyrometer responsive to the size of the teeming jet;

FIG. 7 is a cross sectioned view of a continuous casting plant with a magnetizing coil responsive to the size of the teeming jet; 7

FIG. 8 is a schematic diagram of the electrical system for evaluating the signal from one level according to FIG. 2;

FIG. 9 is a plot of the wave forms at different points of the circuit of FIG. 8 illustrating the form of signal obtained when a breakthrough occurs;

FIG. 10 is a schematic diagram of the electrical system for evaluating the signals derived from two levels according to FIG. 2;

FIG. 11 is a plot of wave forms at different points in the circuit of FIG. 10 illustrating the form of the signals obtained in the event of a breakthrough;

FIG. 12 is a plot of wave forms at different points in the circuit of FIG. 10 illustrating the form of the signals obtained in the event of a deficient metal supply;

FIG. 13 is a schematic diagram of an electrical system for evaluating signals received from elements according to FIG. 3;

FIG. 14 is a plot of wave forms at different points of the circuit of FIG. 13 illustrating the form of the signals in the event of a breakthrough;

FIG. 15 is a plot of wave forms at different points of circuit of FIG. [3 illustrating the form of the signals in the event of a deficient metal supply;

FIG. I6 is u schematic diagram of an electrical system for evaluating the signal from temperature-responsive elements according to FIGS. 4 and 5;

FIG. 17 is a schematic diagram of an electrical system for evaluating signals from fusible members according to FIGS. 4 and 5;

H0. 18 is a schematic diagram of an electrical system for evaluating signals obtained from a pyrometer according to FIG. 6;

FIG. 19 is a schematic diagram of an electrical system for evaluating signals obtained from a magnetizing coil according to FIG. 7; and

FIG. 20 is a schematic diagram of an electrical circuit layout for controlling the drive means and the servo elements which regulate the supply of steel.

In FIG. 1 there is shown a continuous casting plan plant with a curved mold and a curved casting guide, comprising a pouring ladle 1 with a plug 2 controlled by servo means 3. The metal flows from the ladle into a further intermediate pouring vessel or tundish 4. The gate of the tundish 4 is controlled by a slide valve 5 operable by servo means 6. From the tundish 4 the metal flows into a curved water-cooled continuous casting mold 7 in which a continuous casting 8 with a liquid core forms. This casting -8 is withdrawn from the mold 7 by withdrawing rollers 12 and is assisted by rollers 10 through the casting guide in which the casting is further cooled by sprayers 11. After having passed between the withdrawing rollers 12 which are driven by drive means 13 the casting 8 is cut into lengths by a cutting device. The cutting device 14 is associated with synchronising gear 15 which moves the cutting device at the same speed as a casting during the cutting operation. The synchronising gear 15 is driven by drive means 16. The cutting device 14 is also equipped with traversing gear 17 which guides and moves the cutter burner across the axis of the cast- The servo means 3, for example in the form of a motordriven lifting gear for raising and lowering the plug 2, is controlled by a controller 25. The tundish 4 is preferably fitted with weighing equipment in the form of load gauges 26. The load gauges 26 permit the level of the metal surface in the tundish 4 to be inferred from the weight of the metal. Preferably the load gauges are associated with means which respond to three levels of the metal surface, namely a maximum level, a minimum level and an absolute minimum level. Response to these levels affects the controller and results in actuation ofthe plug 2. At maximum level the plug 2 is closed, at minimum level it is opened. The effect of the absolute minimum level will be later described.

The servo means 6, which may be a hydraulic actuator for operating the slide valve 5 is controlled by a controller 27. The top of the mold 7 is fitted with level monitoring means 28, preferably comprising a radioactive source in the form of a rod and radiation detectors. The level monitoring means 28 respond to two levels ofthe metal pool in the mold 7, namely a maximum level and a minimum level. These two level indications are applied as signal 030 to an input of the controller 27 and cause the slide valve 5 to be operatedpAt maximum level the valve is closed and at minimum level it is opened. However, level monitoring means comprising rod sources are also suitable for continuously checking the volume of metal.

The drive means 13 for the withdrawing rollers 12 are controlled by a controller 31. This controller 31 which determines the normal rate of withdrawal of the casting can be influenced by information 032 which need not here be specified in detail, such as information relating to the thickness of the crust, the temperature ofthe casting and so forth.

At 33 is a further controller which controls the two drive means 16 and 18 associated with the cutter device 14. This controller 33 which determines the normal operation of the cutter operates by reference to information 034 which need likewise not here be described in detail, such as the length of the portions to be cut off, the temperature of the casting and so forth.

For the generation of signals indicating the presence of faults FIG. 1 illustrates three arrangements feasible for such applications:

d. For indicating a breakthrough of metal temperature responsive elements 41 such as thermoelements may be provided between the two cooling members marked 9; Should any metal break through the crust of the partly solidified casting these elements generate a signal 041.

In additionally or alternatively fusible members 42, such as metal bandstensioned by springs may be provided at this point. if liquid metal breaks through, these likewise generate a signal 042.

e. Near the bottom end of the mold 7 are two

level indicators

43 and 46 comprising a radioactive source in rod form and a radiation detector associated with each level. When the level of the metal in the partly solidified casting or the level of the metal pool falls below the levels of these

indicators

43 and 46

signals

045 and 048 are generated.

Further level indicators in the form of a transmitting and detecting system 49 for electromagnetic waves are provided above the mold 7. lf the level of the liquid metal in the mold falls to critical levels at which the rays are reflected at the metal surface so that they fall into one of the detectors, the latter will generate a

signal

051 or 052 respectively.

f. A deficiency of metal supplied to the mold 7 may be detected by monitoring the size of the teeming metal jet. A deficient supply may be due to one of several reasons. For instance, the gate may freeze, the flow of metal through the gate may be obstructed by a fragment of the plug, a slide valve may freeze and so forth. For monitoring the size of the teeming jet a pyrometer 53 is provided above the mold 7, which responds to the light radiated by the jet. This pyrometer generates a signal 053.

Alternatively, as indicated in H05. 7 and 19, atthe same point above the mold a magnetizing coil 54- may be provided if the gate is provided with a spout extending to the surface of the pool. A signal 054 can then be generated which reflects the magnitude of the eddy current loss in the liquid metal.

The manner in which use can be made of these three possibilities in principle of generating signals will be later described in greater detail with reference to Us. 2 to 7. According to the arrangements provided, the

signals

041, 042, 045, 048, 051, 052, 053, and 054 (FIG. T9) are fed to a safety controller 55.

Other information may also be fed to this controller at the same time. lnformation of such a kind may comprise:

g. A signal 026 indicating the absolute minimum level of metal in the feed head; i

h. A signal 020 indicating that casting is in progress in the plant; and

i. A signal 030 derived from the minimum level indicator of the pool monitoring means.

The signals enumerated in points g, h and i will be referred to later.

The

controllers

25, 27, 31 and 33 operate under the control of the safety controller 55 which in the event of a fault also activates an alarm 56 when there is a breakthrough of metal or an alarm 57 where there is a deficiency of metal supply.

FlG. 2 illustrates the provision, at the lower end of the mold 7, of two

level indicators

43 and 46, comprising a rod-shaped source 44 and two

radiation detectors

45 and 48. Each

detector

45, 48 may consist of a cadmium sulphide crystal which varies its electrical resistance according to the radiation dose which it receives. In normal operation the level of the pool of metal in the partly solidified casting 8 should be as indicated by the dashed line 60. Since the radiation is absorbed by the metal at the

detector levels

43 and 46 the resistances of the detectors will be high. However, if, as a consequence of a breakout of liquid metal, the level of the metal in the partly solidified casting 8 falls to the level 61 below the two

detector levels

43 and 46, the absence of liquid metal in the partly solidified casting 8 in the path of the rays results in the detectors being exposed to a higher dose of radiation with a resultant drop in resistance. The

detectors

45 and 48 thus generate

signals

045 and 048 which are processed to form a breakthrough signal in a manner that will be later described.

The same arrangement may also be used for the generation of a signal when there is adeficiency in the rate of metal supply. in such a case the surface of the pool will fall below the

detector level

43 or 46 without leaving a solidified marginal zone. Consequently the resistances of the crystals in the

detectors

45 and 48 will in such an event be still lower. Alternative ly the detector 28 which monitors the level of the pool may take the place of one of the two

detectors

45 or 48.

The arrangement according to FlG. 3 consists of a trans mitting and detecting arrangement 49 for electromagnetic waves, for example, for radar, laser or other beams. in normal operation the level of the pool in the mold 7 will be as indicated by the dashed line at 60. The electromagnetic beam 66 projected by a transmitter 50 in the direction towards the surface of the pool is reflected at the surface level 60, and the reflected beam will be situated as indicated by the dashed line 67. This reflected beam does not affect either of two

detectors

51 and 52. However, if as a result of a deficiency in metal supply the level 60 of the surface of the pool falls to the socalled first critical level indicated by the dashed line 62, then the beam 66 will be reflected at the surface of the pool in the direction marked 68 and the reflected beam will now fall on the detector 51 with a given angle of incidence. A signal 051 will therefore be generated.

if the surface of the pool continues to fall until it reaches the second critical level 63 then the beam 66 will be reflected at the surface of the pool in the direction 69 and fall on the second detector 52 with a given angle of incidence. When this occurs a signal 052 will therefore be generated.

The same elements will generate the

same signals

051 and 052 if the level of the metal in the partly solidified casting falls as the result of a breakthrough of metal. As will be later described the time interval between these signals decides whether they will be processed to form a signal for indicating a breakthrough or a deficiency in metal supply.

Instead of providing transmitting and detecting means for electromagnetic waves, ultrasonic devices could be used in an analogous way.

For the sake of completeness FIG. 3 also shows monitoring means 28 for the level of the pool in the form of a rod source 29 and a detector 30.

N05. 4 and 5 illustrate two methods of generating signals when liquid metal breaks through the crust of the partly solidified casting 8. a the mold is a secondary cooling zone comprising cooling members 9. In the regions where breakout of liquid metal could occur, i.e. in the zones 75 and 76, tempcrature responsive elements 41 and/or fusible members 42 may be provided. Since the temperature responsive elements 41 can monitor only part ofthe circumference of the casting 8 several such elements are distributed around the casting periphery.

in the event of a breakout of liquid metal some of the tem perature responsive elements 41 will be contacted by the flowing liquid metal and they will then generate a signal 041 which will be further discussed with reference to HQ. 16.

Similarly, any flowing metal will contact some part of the fusible member 42 and cause the generation of a signal 042 which will be further discussed with reference to FIG. 17. Preferably the fusible members 42 are kept under tension by springs 77 to ensure that the electrical circuit will be actually ruptured when the fusible members melt. For monitoring the entire periphery of the casting in the zone 76 the fusible members may have the form of loops.

FIG. 6 illustrates the method of detecting a deficiency in the rate of metal supply by monitoring the size of the jet 80. The emission of light by the metal which corresponds to the size of the jet 80 affects the resistance of the cadmium sulphide crystui in u pyrometer 53. A network associated with this pyromctcr transforms this resistance into a proportional voltage. The voltage obtuinedia therefore a measure of the ap proximate size of the teeming jet 80 and consequently represents the rate at which metal enters the mold. This voltage is the previously mentioned signal 053 which indicates a deficiency in metal supply.

FIG. 7 illustrates an arrangement for monitoring the size of the teeming jet with the aid of a magnetizing coil 54. The coil surrounds a gate 82 which extends from the tundish 4 to the level 60 of the pool. An alternating current which flows through the coil 54 varies in magnitude in proportion to the cross section of the metal flowing through the gate in accordance with the eddy current loss experienced in the metal. The current is transformed into a proportional voltage in a discriminator. These voltages may similarly provide the signal 054 which represents a deficiency in metal supply.

FlG. 8 illustrates the circuit in which the signal 045 is processed to form a signal 093 indicating a breakthrough of metal. A current driven by a voltage source 85 in the detector 45 flows through the cadmium sulphide crystal 86 and a resistor 87. The voltage drop across the resistor 87, i.e. the signal 045, is fed to a differentiating member 88 which delivers a signal 088. This signal is applied to a Schmitt comparator circuit 8 if the signal 088 which has been differentiated with respect to time, rises above a given threshhold 90 (FIG. 9), i.e. if the rate at which the liquid metal surface falls exceeds the threshold 90 which is determined by the rate of withdrawal, then signal 089 will appear in the output of the comparator circuit. lnorder to make allowance for variations in the rate of withdrawal the theshhold value 90 is preferably arranged to change in accordancewithany such variations. The signal 089 is taken to an amplifier 91. The amplifier output is connected to a storageelement in the form of a self-holding relay 92. The relay is therefore capable of responding to and of retaining the signal 089 which normally appears in the form of a pulse. The

signal 093 for indicating the occurrence of a metal breakthrough is generated by the closure of a contact 93 operated by the relay 92. A cancellation device not shown in the drawing permits this signal 093 to be extinguished.

FIG. 9 is a diagram illustrating the signals as a function of time that are generated by a breakthrough of liquid metal in solid line; the signals due to a deficiency in supply are also plotted in dashed lines for a comparison.

As can be seen from the plot of signals, for clearly indicating a deficiency in metal supply the illustrated arrangement is unsuitable because the rate of change of the signal 045 is then likely to be slow. Under normal casting conditions the rate of change would be nil. A discrimination between the two operating conditions is therefore difficult. In order to overcome this difficulty use is made of a level indicator employing a temperature responsive detector, which incidentally is analogously unable to discriminate between a breakthrough and normal casting conditions because of the presence of the solidified crust on the mold walls. However, clearly differentiable signals indicating a deficient metal supply can be obtained because the rate of change of the signal when the level of the metal pool sinks is still fast enough. The signal generated by the temperature responsive detector is processed in substantially the same way as that described with reference to FIGS; 8 and 9.

The circuit illustrated in FlG. 10 comprises two

detectors

45 and 48. The detector 48 is in effect exactly equal to the detector 45 which has been described with reference to FIG. 8. 94 is a source of potential, 95 is a cadmium sulphide crystal and 96 is a resistor. The

signals

045 and 048 are each taken to a

Schmitt comparator circuit

97 and 99 respectively. A

signal

097 or 099 will therefore appear in the output of one of the

comparator circuits

97 and 99 whenever one of the

signals

045 or 048 exceeds a predetermined threshhold value 105 (FIG. 11). This threshhold value 105 roughly corresponds to the magnitude of the signal which would be generated by the detector in the presence of a crust and in the absence of liquid metal. A signal 098 or 0100 will appear in the output of the corresponding comparator circuit 98 or when the

signal

045 or 048 exceeds a higher threshhold value 106 (FIG. 12). This latter threshhold 106 roughly corresponds to the mag nitude of the signal which the detector would generate in the absence of both a crust and liquid metal. From the magnitudes of the signals generated at the level of the two detectors by reference to the two threshholds it is possible clearly to differentiate between a breakthrough and a deficiency in metal supply.

The two

detectors

45 and 48 must be suitably spaced to ensure that intermediate conditions will not lead to faulty indications.

In the event of a metal breakthrough the signals appearing in the outputs of the two comparator circuits 9'7 and 99 associated with the lower threshholds consecutively affect an AND-gate 107 which delivers a signal 0107. This signal is further processed to provide a signal 093 indicating the occurrence of a breakthrough as has been described with reference to FIG. 8.

If the signals result from a deficiency in metal supply the signals appearing in the outputs of the

comparator circuits

98 and 100 associated with the higher threshholds will consecutively affect a second AND-gate 108 which delivers a signal 0108. In order to avoid the generation of an unwanted deficiency signal when casting begins and ends, a signal 026 representing the absolute minimum level of the metal in the tundish 4 and a blocking signal 0114 are likewise fed to further inputs of the gate 108.

For generating the signal 0114 a storage element 112 is provided which comprises NEITHER-NOR

gates

113 and 114. The output ofgatc 113 is taken to the first input of gate 114, whereas the output of gate 114 is taken, on the one hand, as already mentioned, to one input of the gate 103 and, on the other hand, to the first input of gate 113. The second input of the gate 114 receives a signal from a NOT gate 115 of which the input receives the signal 020 indicating that casting is in progress. This signal may either be triggered by the operator or it may be given by automatic means, and it continues so long as casting proceeds. The signal 030 from the minimum level of the pool level monitoring means is fed to the second input of the gate 113. The signal 020 thus serves to trigger the blocking signal 0114, whereas signal 030 operates to extinguish the blocking signal.

The signal 0108 is further processed to provide the metal deficiency signal 0111 in the same way as described with reference to FIG. 8, excepting that in the

drawing elements

109, 110 and 111 have'been substituted.

The FIGS. 11 and 12 are graphs of the locally generated signals which appear in the event ofa breakthrough or a metal deficiency.

FIG. 13 illustrates the generation of fault signals 093, 0111 from two different levels at which closely consecutive signals (time intervnl in the order of seconds) 051 and 052 are generated. The signal 051 is applied, on the one hand, to the input of a time delay element 120 and, on the other hand, to the input of a pulse generating element 121. The signal 052 is applied to a further pulse generating element 122. The signal 051 therefore appears in the form of a signal 0120 in the output of element 120 after a period of delay 127 (FIG. 15) and, on the other hand, as a signal 0121 in the form of a pulse of duration 125 (FIG. 14) in the output of element 121. Furthermore, the signal 052 appears in the output of element 122 as a signal 0122 in the form ofa pulse of duration 126.

The time 127 comprises the two

periods

125 and 126. The period 126 represent the response time of the

relays

92 and 110.

The period 125 is decisive for discriminating between the type of the fault inasmuch as at a given spacing of the two monitored levels a limiting value for the rate of fall of the metal surface is introduced. This limiting value may with advantage be derived from the rate of withdrawal of the casting because it varies with the casting parameters: quality of the steel, format, casting temperature and so forth. If the signals from the two monitored levels appear consecutively with an intervening interval of say 128 within the period 125, then this means that the metal level is falling at a rapid rate in relation to the limiting value, Le. the rate ofwithdrawal, and that there has been a breakthrough of metal. On the other hand, ifthe interval between the two signals is say 129 and exceeds the period 125, then this means that the fall of the metal surface is slow in relation to the limiting value, i.e. that the rate of supply of metal is insufficient.

The two signals 0121 and 052 are both applied to the input of an AND-gate 124 which delivers a signal 0124. This signal is further processed to provide the breakthrough signal 093 in the same way as already described with reference to FIG. 10.

The two

signals

0120 and 0122 as well as the signal 026 indicating the absolute minimum level of the metal in the tundish 4 are applied to the three inputs of an AN D-gate 123 which delivers a signal 0123. This signal 0123 is again processed to provide a metal deficiency signal 0111 in the same way as described with reference to FIG. 10.

The two FIGS. 14 and 15 are graphs illustrating the sequence in time of the signals.

FIG. 16 illustrates the electrical circuit used for the generation of a breakthrough signal 093 from the signal obtained from temperature responsive elements 41. As soon as one or more of the thermoelements 41 are contacted by liquid metal, a low voltage appears at the input of an amplifier 130. After amplification this voltage is applied to a Schmitt comparator circuit 131. A Schmitt comparator circuit has the property of being able to convert a continuously varying voltage, such as that generated by thermoelemcnts, into two different states 0 or I by reference to a given coincidence level. A signal 0131 is thus formed in the output of the comparator circuit 131.

This signal 0131 is further processed to provide the breakthrough signal 093 in the manner described with reference to FIG. 10.

FIG. 17 illustrates the generation of a breakthrough signal 093 from the response of fusible members 42. A current is driven by voltage source 132 through a resistor 133 and the fusible members 42. The input of a contact protection circuit 134 is electrically short-circuited by the fusible members 42 which are connected in parallel. In the event of a breakthrough, as described with reference to FIGS. 4 and 5, the electrical path through at least one of the fusible members will be at least temporarily broken by the liquid metal. A voltage pulse will therefore be applied to the input of circuit 134 and a signal 0134 will appear in the output. This signal is further processed to provide a breakthrough signal as described with reference to FIG. 10.

FIG. 18 illustrates the manner in which a signal 053 from the pyrometer 53 is processed to provide a deficiency signal 0111 in conjunction with the signal 030 obtained from the minimum level of the pool level monitoring device. The signal 053 is first applied to a Schmitt comparator circuit 141. The adjustable reference value of the comparator circuit 141 corresponds to a minimum permissible size of teeming jet when the slide valve 5 is wide open. A signal 0141 appears in the output of the comparator circuit. Moreover, the signal 030 is taken to a time delay element which delivers a signal 0140. During the time determined by the element 140 the valve has the opportunity of opening fully. The

signals

0140, 0141, 026 and 0114 are all applied to the inputs of AND-gate 142 which delivers a signal 0142. This signal 0142 is further processed to provide a deficiency signal 0111 in the same way as described with reference to FIG. 10.

FIG. 19 illustrates the evaluating circuit for a signal representing the size of the teeming jet obtained by means of the magnetizing coil 54. An AC voltage is fed to the coil 54 from a source of AC 145 through a resistor 14.6. The voltage drop across the resistor 146 is converted into a DC voltage 0148 in a rectifier bridge 147 and a smoothing capacitor 148. This DC signal 0148 is applied to an amplifier 149 which delivers the signal 054. This latter signal 054 is fed into the circuit shown in FIG. 18 in place of the signal 053 in order to generate the deficiency signal 0111.

FIG. 20 illustrates the circuitry in the main safety controller 55 which controls the drive means and the servos regulating the supply of steel. A suitable source of voltage is connected to the terminals and 156. The diagram represents the state of readiness for operation, i.e. the

serves

3 and 6 are energised in position "closed" and the drive means 13, 16 and 18 are without current. Moreover, control elements 160 to 164 for manual control have not been operated.

During the normal casting process the

servo

3 or 6 can be operated by

controller

25 or 27 respectively through a

contact

165 or 166 respectively for setting them into the positions "open" or closed" according to the level of the metal. Drive member 13 has been started by controller 31 via a contact 167. As soon as the desired length of casting has passed under the cutter unit 14 the drive means 16 and 18 respectively are started in the direction feed" via a

contact

168 and 169 respectively operated by controller 33. As soon as the casting has been cut the drive means 16 and 18 respectively are changed to return? by a limit switch not shown in the drawings. As soon as the return has been completed another limit switch controls the drive means 16 and 18 respectively to stop".

Should there be a breakthrough the relay 92 operates, as already described, and closes its contact 93. This causes the breakthrough signal 093 to be delivered, which starts the alarm 56. At the same time a relay 170 is energised which operates contacts Hi to l74. These disconnect the

controllcrs

25, 27. 31 and 33 and render them ineffective. Hence the

servos

3 and 6 move into position closed" and the drive means 13 is stoppedlf the cutter is in operation when the breakthrough signal appears, then the drive means 16 is likewise inactivated, whereas the drive means 18 is not affected by the breakthrough signal so that the cutting operation can be completed.

The changeover of the contacts 171 to 174 simultaneously brings the control elements 160 to 164 for manual operation into circuit. The plant is therefore now set up for manual control and the operator can seiectably activate the

servos

3 and 6 and the drive means 13, 16, 18 by operating the contacts of the

control elements

160, 161, 162, 163.

if in the cutting region the casting is not supported by rollers which travel together with the moving casting a cutting operation that has already begun must not be completed if the flame of the cutter would then damage a stationary roller. in order to cope with this situation a roller contact 177 is provided which responds for instance when the roller is excessively heated.

As already described, the relay '110 operates when a deficiency of metal supply occurs and closes a contact 111. The deficiency signal 0111 is thus generated and indicated by the alarm 57. At the same time a relay 175 which operates with delay is energised. After a selectable period of delay, in the course of which a temporary deficiency in supply could be rectified, the relay 175 closes a contact 176. This in turn results in the operation of the relay 170 and the plantis controlled in the same way as when a breakthrough occurs.

in horizontal casting a fall in the level of the metal in the tundish can be monitored for the purpose of generating a breakthrough signal. The breakthrough signal can be used to separate the tundish from the mold by hydraulic actuating means. The liquid steel is then discharged into an emergency vessel located underneath the point of separation.

The above examples describe only several means for generating and using fault signals. However, the invention is not intended to be limited in scope to these solutions, since more precise fault signals can be generated by suitably combining the above described principles. More particularly an arrangement may be provided comprising a rod source and several radiation detectors whereby any changes in the level of the metal can be indicated by making use of the varying signal strengths supplied by the detectors and the time intervals at which consecutive signals appear. Rapid and accurate information relating to the nature of any fault can thus be obtained. For example, a breakthrough may thus be identified in its initial stages from a relatively small change in level. By immediately intensifying the cooling effects such a breakthroughmay possibly even be healed in good time. if the change in level is appreciable, that is to say if the breakthrough has had time to progress and is therefore incapable of being healed, then a second breakthrough signal may be arranged to increase the rate of withdrawal sufficiently to permit the billet to be withdrawn from the plant before it has frozen completely tight.

This invention may be variously modified and embodied within the scope of the subjoined claims.

We claim:

1. A method of controlling a continuous casting plant during fault conditions which cause the level of the liquid metal surface in the mold to fall, comprising the steps of issuing an output fault signal which indicates a fall of metal level unusual to normal casting conditions beyond a level limit which is given by normal casting conditions and controlling the plant with this fault signal according to requirements to finish unusual casting conditions, and including the step of generating a breakthrough signal when a breakthrough occurs, said breakthrough being generated when there is a fall of level of liquid metal in the partly solidified casting.

2. A method of controlling a continuous casting plant during fault conditions which cause the level of the liquid metal surface in the mold to fall, comprising the steps of issuing an output fault signal which indicates a fall of metal level unusual to normal casting conditions beyond a level limit which, is given by normal casting conditions and controlling the plant with this fault signal according to requirements to finish unusual casting conditions, and including issuing a signal indicating an absolute minimum level of metal in a pouring vessel, which later signal is used for bridging fault signals at the end of the casting operation.

3. A method according to claim 1, wherein a deficiency signal is generated when there is a fall in the level of the liquid metal pool unusual to normal casting conditions.

4. A method according to claim 1, wherein a teeming jet is formed when the liquid metal is poured into the mold, and wherein said deficiency signal is derived according to the size of the teeming jet.

5. A method according to claim I, wherein said deficiency signal stops the supply of metal to the mold after a period of delay.

6. A method according to claim 1, wherein said continuous casting plant has withdrawing rollers and associated drive means for withdrawing the casting, and wherein said deficiency signal stops the drive means of the withdrawing rollers after a period of delay.

7. A method according to claim 1, wherein said continuous casting plant has a cutting device for cutting the casting, said v cutting device being equipped with a synchronizing gear and a traversing gear, and wherein said deficiency signal stops the synchronizing gear of the cutter after a period of delay but permits a proceeding cutting operation to be completed by the traversing gear.

8. A method according to claim 1, wherein said deficiency signal changes over the control of the plant to manual control after a period of delay.