GB2116091A - Extraction of horizontally cast continuous metal strands - Google Patents

Abstract

A strand 1 is extracted from a continuous casting mould in a 'pilgrim- step' motion wherein the extraction- stroke has a length which is a multiple of 1 mm, while the retraction stroke has a length of a multiple of 0.1 mm, while the duration of each stroke is less than 1 second. Extraction apparatus further disclosed comprises clamping devices 9 supported on carriers 7 movable along guides 4 by piston and cylinder units 14. Fluid is supplied to the piston and cylinder units by way of servo- valves 15 controlled by a programmable electronic device 24. <IMAGE>

Description

SPECIFICATION Extraction of horizontally cast coninuous metal strands Whereas in vertical or curved continuous casting of metal strands, the strand is caused to issue from the mould partly under the effect of gravity and partly by causing the mould to oscillate vertically, it is necessary to apply traction to a strand which emerges horizontally from the mould.

It is proposed in Swiss Patent No. 446622 to grip the strand using a pair of clamping members and draw it from the mould in a step-wise movement.

The clamping members are advanced by a series of very small strokes, as many as possible of which takes place per unit time. This mode of operation is achieved by using several sets of clamping members, one set of which grips the strand and moves from a rearward to a forward position in order to advance the strand, while another set is returned from its forward to its rearward position ready to grip the strand in preparation for the next succeeding stroke. With this known method and device, the strand is either stationary or performing an advancing step.

In the horizontal continuous casting of steel it is very important that welding planes within the strand should not be evident at the strand surface. In addition, it is necessaryto prevent the formation of internal cracks and shrinkage cavities. If these defects are to be avoided, it is not sufficient to extract the strand in a series of steps and hold it firmly between these steps. It has also been found to be necessary to apply to the stand a retraction stroke.

The previously proposed method therefore fails to take into account the time needed forthe individual welding planes to crystallize. In other words, a rapid casting rate in order to achieve high production does not meet the requirements for satisfactory continuous casting.

The known device also has the drawback that the extraction strokes are transmitted very coarsely to the strand because the overlapping operation of several pairs of clamping members does not com pensate for play occuring within the hydraulic operating system. Such play can always occur in hydraulic circuits connected in parallel and thus neutralise the desired rigidity of the system.

The present proposal seeks to overcome these defects and permit defined, extremely fine strokes to be performed in very small cycle times, and also to modify the equipment employed in orderto reduce the play by a substantial extent.

Accordingly, there is provided a method for the horizontal continuous casting of a metal strand, for example steel, wherein the strand is extracted from a mould in a pilgrim-step motion using traction and retraction strokes, the length of each traction stroke being a multiple of 1 mm, and the length of each retraction stroke being a multiple of a fraction of 1 mm, the strokes taking place with cycle times of less than 1 second. Optionally, a pause or holding time of between 0 and 10 seconds may be interposed between one traction stroke and the next succeeding retraction stroke and/or between one retraction stroke and the next succeeding traction stroke. By adopting these measures, very fine movements may take place in very short periods of time, so that a strand having a surface of especially high quality is obtained.The lengths of the traction and retraction strokes may be adjusted depending upon the material to be cast, so that for example a retraction stroke in the 0.1 mm range at cycle times in th range of fractions of one second are associated with a traction stroke in the mm range. Furthermore, the depth of the welding planes within the cast strand is advantageously reduced compared with the methods previously known.

The pauses between successive strokes are advantageously adjustable in steps of 0.01 seconds, such small time steps contributing decisively to the welding of the individual zones of the strand as it cools. It is furthermore advantageous for the clamping time and/or the clamping force to be controlled according to a predetermined programme, thereby to permit automation and economy. Preferably, the clamping time is greater than the duration of the extraction or retraction times, so fixing the position once adopted by the cast strand in order to ensure a subsequent flow of like interval of the molten metal within the horizontal mould.

In order to carry out the method, there is furthermore proposed apparatus for extracting a continuously cast strand from a horizontal continuously casting mould, comprising at least one clamping device supported on a carrier guided for movement along a guide extending parallel to the direction of intended movement of the strand, and a piston and cyclinder unit for displacing the carrier along said guide, said unit being connected directly with the carrier and being supplied with fluid by way of a servo-valve operated by a programmable control means. Apparatus having this construction avoids the coarse control elements and operating devices used in the known apparatus, such as cam operated switches which act upon valves, cylinders with the usual seals, piping systems, limit switches and the like, and thereby avoids the undesirably large amount of play present in the known apparatus.The apparatus in accordance with the present proposal therefore achieves the desired quality of required fineness of movement, smaller by a factor of substantially 10 compared with the values achieved with the known extraction apparatus, while furthermore permitting the attainment of short cycle times.

The piston and cylinder unit which displaces the carrier is preferably operated by a hydraulic circuit to which is connected a hydraulic accumulator. In this way, a high pressure potential with a corresponding high flow rate is always available to the servo-valve.

The clamping device is desirably actuated by means of a differential piston and cylinder unit to permit an especially simple fixing of the clamping levers.

In the embodiment shortly to be described in more detail, there are two clamping devices, each mounted on a carrier displaceable by a respective piston and cylinder unit with its own attendent servo-valve. The valves are controllable synchronouslywith adjustable parameters programmed into the electronic control means, such parameters being the process variables of the horizontal continuous casting plant.

The drawing is a diagrammatic side view of an extraction apparatus in accordance with the present proposal showing the associated hydraulic and electrical circuits.

Referring to the drawing, a strand 1 emerges from a horizontal continuous casting ingot mould in the direction of arrow 2, and follows the axis 3. The strand is supported on roller beds and passes to a machine in which it is cut into lengths. Neither the mould, nor the roller beds, nor the cutting machine is shown in the drawing.

Movable along guides 4 are a pair of carriers for clamping devices 5 and 6 comprising a pair of clamping arms 9. Each of the carriers 7 consists of a wagon supported on flanged wheels 7a, 7b arranged to run on rails constituting the guide 4. Each of the carriers is equipped with a cover, also not shown in the drawing, to prevent dirt accumulating on the rails. Each of the carriers supported a pair of the arms 9 which pivot about respective shafts 8 and support clamping jaws 10. The levers on each carrier are connected by a differential piston and cylinder unit which is actuated to adjust the relative positions of the arms and cause the jaws to be applied to, and removed from, the strand.

Each carrier is displaceable along the guide by means of a servo-unit 12 connected directly to the carrier. Each servo-unit includes a piston and cylinder unit 14, the piston rod 13 of which is fast with the carrier and extends into the cylinder 14 mounted on the floor of the plant. A servo-valve 15 supplied fluid to the piston and cylinder unit with the desired degree of sensitivity and accuracy. Play cannot take place between the carrier and piston rod attached to it. The differential piston and cylinder units 11 which operate the clamping jaws are also supplied with fluid through an electronically controlled hydraulic supply unit.

The hydraulic supply circuit includes a motor M which drives a pump 16 which draws hydraulicfluid from a reservoir 17 and supplies itto lines 21 and 22 by way of a hydraulic accumulator 19 to maintain constant flow and pressure conditions, connected to the lines by way of a safety device 18. A relief valve 20 is connected to the line and opens in the event of unacceptably high pressures to dump excess fluid back into the reservoir 17. To each of the lines 21 and 22 is connected a servo-valve 15 and a control valve 23.

The valves 15 and 23 are controlled from a programmable, electronic control device 24 which applies signal to lines 25 and 26 to control the valves 23 and to the electric lines 27 and 28 to control the valves 15.

Using the low-elasticity and substantially play-free extraction apparatus described above, the method proposed herein may be carried out in the following exemplary cycle: A traction stroke 29 of an order or magnitude between 1 mm and 99 mm is followed by a pause or holding time of 0.3 seconds, after which a retraction stroke 30 commences and has a duration of 2.3 mm, for example. The retraction stroke is followed by a further holding pause of 0.5 seconds. The cycle is then repeated. These operations are carried out under the control of the device 24, which also operates the cylinders 11 to clamp and unclamp the jaws 10. The limiting positions of the traction stroke 29 and retraction stroke 30 are indicated in broken lines. The clamping devices may be operated alternately, so that the device shown at the right of the Figure returns to its rearward position during the holding pause in which that shown at the left of the Figure is in its forward position and holding the strand.

Claims (12)

1. A method for the horizontal continuous casting of a metal strand, for example steel, wherein the strand is extracted from a mould in a pilgrim-step motion under the application oftraction forces applied alternately in extraction and retraction strokes, the length of each extraction stroke being a multiple of 1 mm, and the length of each retraction stroke being a multiple of a fraction of 1 mm, the strokes taking place with cycle times of less than 1 second.

2. A method according to claim 1, wherein a pause of up to 10 seconds is interposed between one extraction stroke and the succeeding retraction stroke, or between one extraction stroke and the succeeding extraction stroke.

3. A method according to claim 2, wherein the pauses are multiples of 0.01 seconds.

4. A method according to any preceding claim, wherein the periods in which clamping jaws are applied to the strand for the purpose of extracting it from the mould, and/or the force applied to the strand by the jaws is controlled according to a predetermined programme.

5. A method according to claim 4, wherein the duration of the clamping period exceeds the duration of the extraction or retraction strokes.

6. Apparatus for extracting a continuously cast strand from a horizontal continuously casting mould, comprising at least one clamping device supported on a carrier guided for movement along a guide extending parallel to the direction of intended movement of the strand, and a piston and cylinder servo-unit for displacing the carrier along said guide, said unit being connected directly with the carrier and being operated by a programmable control means.

7. Apparatus according to claim 6, wherein a circuit through which fluid is supplied to the piston and cylinder unit is connected to a hydraulic accumulator.

8. Apparatus according to claim 6 or claim 7, wherein the clamping device is operated by means of a differential piston and cylinder unit.

9. Apparatus according to any of claims 6 to 8, including two or more clamping devices, each movable by means of a piston and cylinder unit supplied with fluid through a servo-valve in the form of a multi-way valve, the valves being synchronously controllable using parameters programmed into said control means.

10. A method substantially as hereinbefore described with reference to the drawing.

11. Apparatus substantially as hereinbefore described with reference to and as illustrated in the drawing.

12. Apparatus for the horizontal continuous cast ing of a metal strand, comprising a clamping device supported on a carrier reciprocable by a piston and cylinder unit along a track parallel to the direction in which the strand issues from a mould, and a control means for adjusting the length of extraction and retraction strokes of the piston and cylinder unit such that the extraction stroke may be set to an accuracy of 1 mm and the retraction stroke to an accuracy of one tenth of a mm.