Abstract:
An apparatus for continuous horizontal casting with a stationary mold and the casting method accomplished with such apparatus contemplates employing at least one gripper roll pair which directly engages a casting which is then withdrawn in steps or incrementally from the mold. At least one roll of each roll pair is driven by a stepping DC-motor to which it is directly coupled. Each casting withdrawal step can be divided into substeps which are controllable to produce a uniform preset speed pattern during each step.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part application of our copending U.S. application Ser. No. 319,917, filed Nov. 10, 1981, and entitled &#34;Apparatus For Horizontal Continuous Casting,&#34; now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method and apparatus for horizontal continuous casting with a stationary guide mold and one or a plurality of pairs of gripping rolls actuated or driven by a DC-motor for the stepwise or incremental extraction or withdrawal of the continuous casting or cast strand from the mold. 
     In horizontal continuous casting of metals, in particular of alloys having an elevated melting point, the casting or strand, which has not yet completely solidified, is withdrawn from a stationary or an oscillating mold. In such a case the withdrawal or extraction of the casting occurs discontinuously, i.e. stepwise or incrementally. Depending on the process, either there is a pause between the individual extraction steps or the casting is pushed back over a small distance. The pushing back of the casting is intended to take into account, on the one hand, the thermal expansion of the casting, and, on the other, to effect a solid welding or fusion of the individual casting skin or shell sections formed within the continuous casting mold. 
     Two different basic arrangements are known for the withdrawal of the casting or strand from the mold. The first arrangement consists of jaws that engage or grip the casting and move it over a small distance. When the grip of the jaws on the casting is released and the jaws are returned to their original position, they reengage the casting, whereupon the motion cycle is repeated. This type of drive is inaccurate in view of the fact that it is necessary to move large masses, i.e. the jaws, thus requiring correspondingly large gear drives or transmissions that have a rather substantial lost motion or relatively great amount of play. Moreover, there exists the problem that the jaws will not always grip the casting at exactly the same spot, thereby producing additional inaccuracies during the extraction of the casting. 
     The exact movement pattern or motion cycle during the withdrawal of the casting or strand is of particular significance, not only because of production rate considerations, but also for the achievement of a high-quality continuous-casting product. If the withdrawal of the casting occurs in such a way that there does not occur any welding or fusion between the freshly formed casting skin or shell ring and the balance of the casting skin or shell, and if the casting with its casting skin is extracted further, the skin will rupture and the molten mass will emerge or break-out from the mold, whereupon the continuous casting process has to be interrupted immediately. 
     A more accurate control or regulation of the extraction or withdrawal of the casting can be brought about if gripping rolls directly engage the casting. The increased accuracy is due to the fact that during such a withdrawal of the casting or strand, there exists a constant contact between the casting and the gripping rolls, which contact constitutes a prerequisite for the accurate extraction or withdrawal of the casting or strand. 
     From German Pat. No. 1,583,611 there is known a device for the intermittent removal of a metallic casting from a mold, wherein a pair of feed rolls driven by a transmission alternatingly engage the casting and additionally perform a reciprocating motion along the casting axis. To achieve reproducibility of the casting motion, there is additionally provided a clamping device for the casting. This device is of relatively complex design and does not permit a completely accurate motion cycle, so the drawbacks already referred to still arise. 
     In German Pat. No. 2,340,636 there is disclosed a device for the stepwise extraction of a casting from a horizontal continuous-casting mold in which a pair of gripper rolls is actuated by a gearing or transmission driven by a stepwise operating DC-motor. For control of the motor its speed is determined in an analog manner and the angle of rotation of the gripper rolls is determined in a digital manner. The motion of the gripper rolls cannot be inferred directly from the speed of rotation of the motor in view of the fact that the system for the transmission of forces between the motor and the gripper rolls has some free motion or play. Through digital determination of the movement of the gripper rolls, however, it is possible to determine the motion of the casting which, in turn, serves for adjustment of the motor speed. In addition, this device is designed so that when a predetermined torque is exceeded a pause in the casting withdrawal motion is initiated in order to wait for the detachment of the ring of the casting skin from the mold wall. This device therefore has a complex design and is thus markedly susceptible to trouble or malfunction. As a result, in many instances the required operational safety is not provided. 
     German Pat. No. 1,783,032 describes a device for continuous horizontal casting by means of a stationary guiding mold and a DC-motor-actuated or driven pair of gripper rolls for performing the stepwise or incremental extraction of the casting from the mold. The DC-motor is connected with a control device which, through programmed periods and sequences, transmits pulse sequences to the DC-motor in programmed durations and sequences. The power transmission proper takes place, however, not through the DC-motor, but through an electro-hydraulic stepping motor. The term &#34;electro-hydraulic stepping motor&#34; is to be understood to include the combination of an electric stepping motor and a hydraulic motor, with the electric stepping motor actuating a control valve that controls the supply of pressure medium to the hydraulic motor. By controlling the pressure with the valve there can be brought about a control of the speed of the hydraulic motor. While this arrangement has the necessary torque for the withdrawal of the casting, the required accuracy of extraction or withdrawal of the casting does not always exist because fluids are not fully incompressible, thus causing inaccurate step sequences. Also this equipment is very costly. 
     SUMMARY OF THE INVENTION 
     Therefore, with the foregoing in mind, it is a primary object of the present invention to provide a new and improved method of, and apparatus for, horizontal continuous casting which does not exhibit the aforementioned drawbacks and shortcomings of the prior art. 
     Another and more specific object of this invention is to provide an improved method and apparatus for the substantially horizontal extraction or withdrawal of a continuous casting or strand that is characterized by a particularly simple structure, yet ensures accurate extraction of the casting or strand from the substantially horizontal continuous casting mold. 
     Yet a further significant object of the present invention aims at providing a new and improved construction of an apparatus of the character described which is relatively simple in construction and design, extremely economical to fabricate, highly reliable in operation, not readily subject to breakdown or malfunction and requires a minimum of maintenance and servicing. 
     Now in order to implement these and still further objects of the invention, which will become more readily apparent as the description proceeds, the apparatus for horizontal continuous casting is manifested by the features that at least one gripper roll is arranged on an extraction motor shaft or is directly coupled thereto. In this way there is achieved not only a very simple structure, but also a direct relation between the angle of rotation of the motor and the motion of the casting. As a result, the casting motion can be supervised very simply and accurately. Also, control or regulation operations can be carried out very precisely and are always reproducible, thereby readily achieving accurate step or increment sequences. 
     In an illustrative embodiment of the invention a stepwise or incrementally operating DC-motor is connected or coupled directly to one roll of an extraction or withdrawal roll pair and is driven or regulated by a control device that transmits control signals in programmed durations and sequences. The control signals are arranged or configured to produce casting extraction steps or increments that are each subdivided into partial steps that are individually controllable or regulatable. 
     A pulse transmitter for supplying digital information concerning the angle of rotation of the motor and a forward/backward counter series-connected thereto, form part of the control device. The forward/backward counter which stores the motion position or angle of rotation information is also connected with a microprocessor which, in turn, is connected with a ROM (Read-Only Memory) that stores information concerning the preset pulses needed for each partial step or increment of the motor. By means of signals corresponding to the difference between the counter status or state (i.e. the motor position) and the preset pulses, the microprocessor, preferably via a digital-analog converter, controls a pulse-width modulating DC-current amplifier that supplies the driving or power signal to the DC-motor. 
     In this way, and at relatively low cost, a highly accurate control of the motor is achieved via a pulse-width modulating DC-current amplifier that produces pulses per unit of time, whose pulse widths are modulated by the control device in such a way that there is brought about a uniform change in the rate of speed of the casting or strand during each step or increment. Control of these pulses is effective since each casting withdrawal step or increment is subdivided into partial steps, whereby the desired speed profile in each step or increment can be reproducibly maintained. Consequently, an optimum casting or strand surface can be obtained, even in the area of abutment of the individual annular casting sections, and no troublesome notches are created in the surface of the casting. With this arrangement the possibility exists that, following each partial step, there can take place a correction of the casting withdrawal or extraction length of the withdrawn casting or strand for the subsequent partial step, which means that each step can be made to correspond precisely to the specified conditions. 
     For the automatic control of the start-up cycle it is advantageous if the microprocessor is connected with another ROM that stores the step sequences for the start-up cycle. 
     In order to adapt the apparatus or device in a simple manner to the processing of different alloys without requiring any specially trained personnel, provision has been made, in accordance with another characteristic of the invention, whereby the microprocessor is also connected via a time-signal generator to a time-interval selector which serves for presetting the casting withdrawal or extraction period for the selected alloys. Thus, the required adjustment can be brought about through the simple setting of a withdrawal time or time interval for a specific alloy without requiring any major intervention in the controls to achieve this purpose. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features and objects of an exemplary embodiment of the invention are explained below in greater detail with reference to the drawings wherein: 
     FIG. 1 shows a wiring block diagram or block circuit diagram of a control or regulation system according to the invention; and 
     FIG. 2 shows a diagram of the preset speed pattern produced by the system of FIG. 1 in the course of one casting withdrawal or extraction step. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Describing now the drawings, it is to be understood that, to simplify the showing thereof, only enough of the structure of the continuous casting apparatus for horizontal continuous casting of strands has been illustrated therein as is needed to enable one skilled in the art to readily understand the underlying principles and concepts of this invention. Turning now specifically to FIG. 1 of the drawings, the continuous casting apparatus illustrated therein by way of example and not limitation will be seen to perform an operation in which a continuous casting or strand 1 is drawn out substantially horizontally in steps or incrementally from a stationary guide or continuous casting mold 2 that is fixed to a melt container or tundish 3. In this operation, a pair of gripper or pinch rolls 4a, 4b directly engage the continuous casting or strand 1. The gripper or pinch roll 4a is directly coupled with a DC-motor 5. This DC-motor 5 is thus importantly devoid of any transmission or gearing and by directly coupling the gripper or pinch roll 4a with such transmission-less DC-motor 5, i.e. to its output shaft or output side, there is afforded an extremely accurate regulation of the withdrawal motion of the casting or strand 1 departing from the continuous casting mold 2, since there do not arise any slip errors as would be the case if an intermediate transmission or gearing were employed as was heretofore the case with the prior art equipment. As a result of the just-explained direct drive of the gripper or pinch roll 4a there is also not necessary any slip-compensation circuitry and the entire regulation operation and related apparatus is appreciably simplified in its construction and more accurate in operation. 
     Furthermore, the directly motor-coupled gripper or pinch roll 4a of the first roller pair 4a, 4b engaging the casting or strand 1 which has departed from the continuous casting mold 2, advantageously is positioned at the region of the emerging casting or strand 1 where there is still present an internal liquid metal core or sump, in other words relatively close to the continuous casting mold 2 and the melt container or tundish 3. As a result of this relatively close spacing of the gripper or pinch roll 4a from the mold 2 there is beneficially precluded or at least appreciably minimized the possibility that undesirable external effects, such as air drafts in the production hall or the like, will adversely act upon the emerging casting 1. These air drafts or currents otherwise might, for instance, unintentionally cool the emerging casting 1 and result in uncontrolled length changes thereof, with attendant reduction in the otherwise attainable precise casting withdrawal movement because of the previously explained beneficial direct coupling of the gripper roll 4a with the transmission-less DC-motor 5. 
     This DC-motor 5 is connected with an encoder E that transmits 20,000 pulses per revolution of such motor 5 to a forward/backward or up/down counter Z. Information or data concerning the preset pulses for each individual partial step or increment is stored in a ROM (Read-Only Memory) 1. A microprocessor M compares the preset pulse data with the encoder signal delivered to it via an interface or interface circuit I 1 , and generates a difference signal related to how far the encoder signal, and hence the motor 5, is away from the ideal speed curve determined by the preset pulse information or data. As a function of the movement of the casting or strand 1, either forward or backward, the microprocessor M either reduces or increases the width of the pulses supplied to the motor 5 in order to force this motor 5 into the desired speed pattern. The microprocessor M supplies the pulse-width modulated signal to the DC-motor 5 via a pulse generator including an interface or interface circuit I 2 , a digital-analog converter K for converting the digital output signals of the microprocessor to an analog signal, and a pulse-width modulating DC-amplifier V under the control of the analog signal from D/A-converter K. 
     The microprocessor M is also connected via a time-interval selector W with a time-signal generator T. By means of the time-interval selector W, it is possible to set the duration of the withdrawal period as a function of the alloy to be processed. 
     In a ROM (Read-Only Memory) 2 there is stored the exact motion sequence for the start-up process. In a RAM (Random-Access Memory) there is stored the number of forwardly and/or rearwardly required partial steps in each casting withdrawal step or increment. Starting of the process is brought about via the on-off switch S and the interface or interface circuit I 3  according to a program set by a program selector P. 
     The stepwise or incrementally operating DC-motor 5 need not necessarily have an output or drive shaft of its own. Instead in the exemplary embodiment for instance the rotor, which contains the windings that are supplied with the current pulses from amplifier V via four suitable not particularly shown brushes, has a cylindrical recess for receiving in a form-locking manner the shaft of the gripper or pinch roll 4a. The outer diameter of the rotor is about 27 inches (approximately. 70 cm), its inner diameter is about 18 inches (approximately. 50 cm). The stator has a permanent magnet element. Due to this design of the DC-motor 5, a high torque can be achieved even during the motor starting cycle. 
     The diagram represented in FIG. 2 illustrates the preset speed pattern repeated during each step or increment in the withdrawal of the casting. Movement is slowly imparted to the casting until a maximum rate has been attained, whereupon the speed is slowly reduced until the casting is stopped and then pushed back as represented by the negative portion of the curve. The step or increment as a whole takes 1.1 seconds and the various partial steps are spaced every 1/20th of a second or less. Because of the continuous change in the casting or strand speed, a shock-like motion of the liquid metal within the very thin and mechanically unstable skin of the casting is avoided, and a homogeneous build-up of the casting can be brought about. 
     While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.