EP0064280A1 - Process and apparatus for the continuous horizontal casting of metals - Google Patents

Abstract

1. A method for the horizontal continuous casting of metals and their alloys wherein the elongate workpiece is drawn off continuously, intermittently or in the pilgrim step mode, characterised in that the continuous drawing stroke or the intermittent one or the one effected in the batchwise mode is or are carried out at a drawing off speed which always fluctuates above the zero value in an oscillating mode.

Description

The invention relates to a method for the horizontal continuous casting of metals and their alloys and a continuous casting plant therefor.

It is known that in the horizontal or vertical continuous casting of metals and their alloys between the mold and the strand there is generally a changing relative movement in order to avoid the strand being torn off and to achieve qualitative advantages. This changing relative movement extends to a temporary standstill or even leads to a reversal of movement, depending on the respective operational practice and the technical possibilities of the system.

In the case of an oven-independent mold and continuous movement of the strand, this sequence of movements is usually achieved by a special movement device for the mold, and in the case of an oven-dependent mold, by non-continuous removal of the strand. This non-continuous removal in the go / stop or pilgrim step process. Hitherto required very complex drive units, of which very precise adherence to certain pre-selected extraction steps is expected. The state of the art is to solve this demanding drive task with complex electrohydraulic systems. Numerous, in particular horizontal, continuous casting plants are in operation in which such drives are successfully used. It is easily possible to convey strands with a round, square or rectangular cross-section, as well as hollow profiles, in the required motion sequences. In the case of round bars, for example, the diameter is in the range from 15 to 400 mm, the lifting speed being adapted to the solidification speeds in the mold and being, for example, between 6 and 120 mm / s. In order to carry out the correct coordination between the forward stroke / holding period I / reverse stroke / holding period II, such systems can carry out up to 1000 switching operations per minute. Recently, the use of a step-by-step direct current motor with corresponding electrical control for withdrawing strands from horizontal continuous casting molds has also become known (cf. DE-PS 23 4o 636).

The expansion of the continuous casting to thin strands with Dmr. ≤ 15 mm, for example 3 mm φ, as is necessary for the production of strands from hard alloys, magnetic alloys and stainless steels, but makes a significant increase in the take-off speed to, for example, 82o mm / s desirable and then leads to a reduction in switching times and thus to increase the number of switching operations / min. If the usual pull-off speed is retained, the casting performance drops accordingly. A certain remedy is provided by the design of the system as a multi-line system, which, however, involves great additional effort.

The object of the present invention is to be able to achieve take-off speeds of up to about 1000 mm / s in a horizontal continuous casting installation of the type mentioned at the outset, in particular for the continuous casting of bars with diameters <15 mm, preferably in the range from 3 to 10 mm.

This object is achieved in the context of the method according to the invention in that the continuous pulling stroke or the intermittent pulling strokes or those taking place in the pilgrim step are carried out with a pulling speed which oscillates over the value zero.

The pulling device thus gives the strand an oscillating movement between a maximum and a minimum value within each stroke. At least one AC-powered step motor is preferred as the drive unit of the pulling device, which enables up to 5,000 switching operations per minute.

In contrast to the known go / stop or pilgrim step methods, in which the strand withdrawal speed increases from 0 to a maximum value within one stroke and drops again to O and in which pronounced stopping times are provided between the strokes, oscillates, i.e. the strand withdrawal fluctuates in the invention according to the continuous caster in the course of a continuous movement between a maximum and minimum value within each stroke. The horizontal continuous casting installation according to the invention can work with continuous strand extraction but also discontinuous, such as go / stop or pilgrim step processes.

The electric (alternating current) stepper motor provided according to the invention has the advantage over the previously usual complex hydraulic extraction systems, without being able to manage all mechanical transmission systems such as gears, clutches, chains or the like. The drive roller is placed directly on the stub shaft of the stepper motor in a suitable manner. There is an optimal adaptation of the diameter of the drive roller to the diameter of the strand, whereby it must be taken into account that the take-off speed increases with a decreasing rod diameter with a constant casting performance. A compensation can be done without changing the operating frequency of the stepping motor by increasing the diameter of the driving pulley, but only to the extent that thereby increase ate the rotational moment of inertia of the drive pulley and the load moment of inertia of abzufördernden strand in a still permissible _M. If the adjustment is incorrect, the stepper motor will not operate and will stop. On the basis of the characteristic curves valid for the specific stepper motor, it is readily possible for the person skilled in the art to correctly select the drive pulley diameter for the respective take-off speed and to select the rod diameter taking into account the rotational and translational moments of inertia of the various masses to be accelerated. For example, for the stepper motor with a maximum torque of 26 Nm for strands with a diameter of 3 mm, a favorable disc diameter of 2oo mm and for strands of 8 mm diameter, a diameter of 5o mm was determined.

A constant strand exit temperature is crucial for trouble-free operation of the continuous casting process and for products of constant quality. In order to be able to comply with this, an electronic control system is provided according to the invention for regulating the operating frequency of the stepping motor or motors as a function of the strand outlet temperature. The strand exit temperature is measured continuously. In the event of deviations from the target value, the operating frequency of the stepping motor or motors is changed with the aim of reducing the strand outlet temperature from the actual value to the target value. The previously known regulation of the change in the amount of cooling water is unsuitable at the high line take-off speeds because the response speed is too low.

In order to exclude a disruption of the withdrawal movement when passing through the transition point cold strand to warm strand by the pulling device, at least two separate stepping motors are arranged one behind the other in the strand passage direction, which alternate the strand hold until the transition point from cold strand to warm strand has passed the pulling device. As soon as the transition point reaches the first stepper motor, the pair of rollers driven by it lifts off and the stepper motor following in the direction of the strand pulls the strand with the pair of rollers driven by it. As soon as the transition point, cold strand to warm strand, has passed the first stepping motor, its take-off rollers come into operation again, and the pair of rollers driven by the second stepping motor lifts off in order to let the transition point run through. Given the high take-off speed, this change also requires a suitable control, which according to the invention is implemented by an electronic control system that synchronizes the stepper motors, that is, their oscillating movement within each stroke, each motor being assigned its own power output stage, so that an independent control every motor is guaranteed. This design of the horizontal continuous casting installation according to the invention enables a high take-off speed to be maintained even when starting up, enables improved straight guidance of the strand and has a wear-reducing effect.

The pulling behavior of the strand from the mold can be favorably influenced by pairs of rollers of the pulling device positioned obliquely to the longitudinal axis of the strand. However, uneven wear in the mold is also avoided because the movement of the strand within the mold is low-friction. However, this version is limited to strands with a circular cross section.

The advantage of quasi-continuous operation is that continuous processing steps such as shaping, cutting and heat treatment can be connected without the need for complex synchronization with the lifting movement. The strand can be calibrated by calibration rollers in the same cross-sectional shape or in a different cross-sectional shape, e.g. from round to oval.

In this way, a densification of the strand with removal of residual porosity or even a change in cross-section can be achieved.

The part of the drawing device that is only required at the beginning of the continuous casting process is the starting rod as a connecting link between the melt in the outlet channel of the holding vessel and the continuous conveyor rollers. When starting, the start command must be correctly timed in relation to the introduction of the melt into the outlet channel. This problem could be solved by using a tubular start-up rod, in the front opening of which two insulated trigger contacts are attached, which are conductively connected to one another when the melt enters and thus provide a signal that can be sent via a preselectable time relay according to one of O to The start command for the trigger device gives the start command for any length of time. At the same time, the design of the starting rod ensures that the strand shell is securely gripped, which shows advantages over the usual design of the starting rod as a tip, with regard to a good welded joint it is also described in US-PS 39 o8 747. In addition, it is advantageous to achieve an additional positive connection between the starting rod and the casting strand by means of a pin inserted transversely in the front part of the starting rod and around which the melt flows. It has proven to be advantageous to insert the starting rod up to the inlet opening of the mold at the beginning of the casting process and to press it against the ceramic outlet stone with a seal made of insulating felt. As a result, the melt can penetrate unhindered into the interior of the tubular starting rod and weld to it to form a durable bond.

On the other hand, the seal prevents melt from getting between the start-up rod and the mold, which would otherwise lead to a jamming of the start-up rod in the mold, which could either prevent the rod from being pulled off or cause severe mold wear.

When using conventional discharge systems, it was often difficult to find the correct duration of the start delay. If it is chosen too short, the start-up rod is removed without being adequately welded to the strand shell, and there is a risk of the melt breaking through. If it is too long, the solidification can progress into the outlet channel, which makes it difficult to pull it off, or eventually even freeze the melt in the outlet stone. It is much better to equip the discharge stone with a tubular extension, approximately 1o to 3o mm in length, with an outer diameter of 1o to 25 mm 0 snorkel-like protrudes freely into the vessel through the opening of a nozzle stone inserted into the casting vessel wall. Here, thanks to the convection of the melt surrounding the snorkel, the outlet stone heats up to such an extent that a build-up in front of the opening can be avoided with certainty. However, high demands are placed on the compatibility of the snorkel material with the melt. When casting hard alloys, the initially used boron nitride was insufficient and the graphite used as an alternative was not sufficient. The use of the Alundum mass, on the rough surface of the outflow channel, which has resulted in severe fretting with the strand, has also not proven successful, so that no withdrawal movement could be carried out. Only the transition to stabilized or partially stabilized zirconium oxide as the material for the run-out stone brought about a significant improvement in durability, which also led to a satisfactory service life of several hours when casting hard alloys.

An embodiment characterizing the principle of the invention is described below with reference to the drawings.

• Fig. 1. eine grafische Darstellung des Verlaufes der Abzugsgeschwindigkeit des Stranges;
• Fig. 2 eine Abbildung eines mit dem Bewegungsablauf gemäß Fig. 1 abgezogenen Strangstückes;
• Fig. 3a, b, c die Ziehvorrichtung von drei Seiten;
• Fig. 4 ein Blockschaltbild für die elektronische bzw. elektrische Ansteuerung der Schrittmotore der Ziehvorrichtung, und
• Fig. 5 Anfahrsystem bestehend aus Ziehvorrichtung, Anfahrstab und Auslaufstein.

They represent:

• 1 shows a graphic representation of the course of the withdrawal speed of the strand;
• FIG. 2 shows an illustration of a strand piece drawn off with the movement sequence according to FIG. 1;
• 3a, b, c, the pulling device from three sides;
• Fig. 4 is a block diagram for the electronic or electrical control of the stepper motors of the pulling device, and
• Fig. 5 starting system consisting of pulling device, starting rod and outlet stone.

The oscillating sequence of movements within a stroke of approximately 40 mm is shown graphically in FIG. 1 as the angular velocity over the conveyor path. After the stroke of 40 mm, in which the movement oscillates or fluctuates between approximately 2 and 6 s- ¹ , a holding time of 0.15 s follows.

When using the horizontal continuous casting installation according to the invention, fine markings can be observed on the surface of the cast strand, as shown in FIG. 2. It is an illustration of the movement sequence shown in FIG. 1 in the course of stroke-wise strand removal with an oscillating movement sequence within one stroke. This mode of operation leads to the fact that the temperature of the solid strand shell at the contact points between the solid strand shell and the liquid melt in the inlet area of the mold drops far less than with stroke-wise conveying in the go / stop or pilgrim step method and furthermore that the volume shrinkage during solidification is distributed more evenly over the entire length of the strand.

Fig. 3 shows the pulling device. In a torsion-resistant profile frame construction 21, two stepping motors 2o are used. The wear-resistant drive roller 11 is placed directly on each of their shaft ends. The drive roller is provided with a groove-shaped incision which can additionally have radially extending notches on its flanks in order to ensure a good frictional connection between the rod and the drive roller. On the side opposite the motor, an additional radial bearing 15 is arranged, which also absorbs the forces exerted by the pressure roller 10. The respectively wear-resistant cylindrical pressure roller 10 arranged above the two drive rollers is held in a vertical guide by the piston of a pneumatic pressure cylinder 1.

By actuating the pressure cylinders with compressed air, which is controlled by appropriate valves, the drive systems on the line to be discharged are activated alternatively or jointly.

The mode of operation of the control of the two stepper motors can be seen from the block diagram of FIG. 4. The most important components are summarized in a high-speed translator. It provides the mains-powered power supply and the switchable control of the output stages for the stepper motor feed, which allows both independent and joint operation of both motors. The functional sequence is specified by the control unit: After the start command, the stepper motor turns clockwise Direction the digitally set preselection I, which causes a corresponding forward stroke of the strand, the size of which is determined by the diameter of the drive roller and the preselected number of steps of the motor. For example, a step number of 51 with a stepper motor with a step angle of α1.8 ° when using a drive roller with a diameter of 5o mm φ leads to a stroke of 4o mm, accordingly only a step number of 13 is required for the same stroke if the Drive roller diameter is 2oo mm ß. After positioning, the output signal COINZ I is used to address the timing element for the holding time II that can be digitally preselected from O to 1o s in steps of o.o1 s , which leads to a corresponding reverse stroke of the strand. After positioning, the output signal COINZ II is used to address the timer for the holding time I, which can also be digitally preselected from 0 to 10 s, after which the cycle begins again with the input signal INDEX I.

The operating frequency that determines the speed of the stepper motors is set using a speed controller. An increase in the operating frequency would lead to an increase in the strand withdrawal speed and thus, with otherwise unchanged operating conditions, to an increase in the temperature of the strand emerging from the mold and vice versa. With the help of a suitable, adjusted controller, the temperature of the emerging strand can be determined Change in the strand withdrawal speed can be kept constant.

5 shows the interaction of the three system components pulling device, starting rod and spout at the start of the casting process.

The tubular starting rod 34 is gripped by the drive roller or rollers 11 and pressure rollers 10. It is inserted into the mold 33 up to the entry opening where it rests with the annular seal 37 on the snorkel 31 in order to prevent melt from entering the narrow annular gap between the mold 33 and the start rod 34. In the front opening of the tubular start-up rod 34 are a transverse pin 36 which, in addition to welding, creates a positive connection between the casting strand 38 and the start-up rod 34, and the two trigger contacts 35 which, when they come into contact with the melt which has penetrated, provide the signal for the time delay of the start delay deliver, housed. The formation of the solidification front 40 between the solidified casting strand 38 and the liquid melt 39 caused by the conical shape of the inner bore of the outlet stone 31 prevents the strand from freezing in the outlet channel and enables the solidified strand to be pulled off easily.

Claims (12)

1. A process for the horizontal continuous casting of metals and their alloys, in which the strand is drawn continuously, intermittently or in the pilgrim step, characterized in that the continuous pulling stroke or the intermittent or in the pilgrim step pulling strokes with a pulling speed oscillating above the value zero is or will be executed. 2. The method according to claim 1, characterized in that the withdrawal speed fluctuates at a frequency of 15o Hz. 3. Horizontal continuous casting plant for metals and their alloys, consisting of a holding vessel with a horizontal outlet, a cooled mold (33) arranged downstream of the holding vessel and a drawing device (1 ₀ , 11, 2 ₀ ) that detects the strand (38), can be driven intermittently or in a pilgrimage step , characterized in that the pulling device (1 ₀ , 11, 2 ₀ ) can be driven in such a way that the continuous pulling stroke or the pulling strokes which take place intermittently or in the pilgrimage step are carried out with a pulling speed which oscillates over the value zero. 4. Continuous casting installation according to claim 3, characterized in that the drive for the pulling device (1 ₀ , 11, 2 ₀ ) has at least one AC stepper motor (2o) for high step frequency. 5. Continuous casting installation according to claim 4, characterized in that an electronic control system for regulating the step frequency of the stepping motor (s) (20) is provided as a function of the outlet temperature of the strand (38). 6. Continuous caster according to claim 5, with at least two stepper motors (20) arranged one behind the other in the continuous passage direction, characterized in that the electronic control system is designed to synchronize the stepper motors (20), each stepper motor (20) being assigned its own power output stage. 7. Continuous casting installation according to one of claims 3 to 6, characterized in that pairs of rollers (10, 11) of the pulling device (10, 11, 2 ₀ ) are placed obliquely to the longitudinal axis of the strand (38). 8. Continuous caster according to one of claims 3 to 7, characterized in that the pairs of rollers (1 ₀ , 11) are equipped with means (1) for exerting a pressure (38) plastically deforming pressure. 9. Continuous casting installation according to one of claims 3 to 8 with a start-up strand (34), characterized in that the start-up strand (34) in the area of the mold (33) has a tubular section sealingly against the outlet (31), in which trigger contacts ( 35) are arranged. 1 ₀ . Continuous casting installation according to Claim 9, characterized in that anchoring elements (36), for example a transversely inserted pin, knobs or the like, are provided in the tubular section of the starting strand (34). 11. Continuous casting installation according to one of claims 3 to 1 ₀ , characterized in that the outlet (31) has a pipe section extending into the holding vessel with an outwardly flared inner bore. 12. Continuous casting installation according to claim 11, characterized in that the tube section consists of stabilized or partially stabilized zirconium oxide.

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