EP0209059A2 - Process and apparatus for driving a cast strand in a continuous-casting unit - Google Patents

Abstract

When steel is continuously cast, the cast strand is pulled out of a continuous mold and a secondary cooling zone by means of a drive device. In the event of malfunctions in the steel delivery or when changing the intermediate vessel, the steel feed into the mold and the strand pull-out movement must be stopped, which can lead to strand faults and machine damage. In order to eliminate these disadvantages, it is proposed to apply an oscillating movement to the cast strand (4) by alternately pulling out and pushing back when the steel supply is interrupted, and a relative movement between the strand surface and strand guide (7) and drive rollers (8, 9) essentially without interruption to create.

Description

The invention relates to a method and an apparatus for the continuous casting of steel, in particular for driving a cast strand, the strand being pulled out of a continuous mold and a secondary cooling zone by means of a drive device.

In order to pull the strand out of a mold and a subsequent strand guide, drive units are provided in each continuous casting installation, which generally have drive rollers. Driven guide rods are also known as drives as "walking beams". The drive unit is further assigned the task of inserting a starting strand into the mold before the start of casting.

During the continuous casting, the cast strand is pulled out of the mold and from the strand guide with the most uniform possible extraction speed. In the event of malfunctions and long sequence castings with intermediate vessel changes, it is inevitable to interrupt the steel supply to the mold and switch off the strand removal. At the end of casting, the bath level in the mold is usually slagged off with the strand at a standstill or moved at a creep speed and solidified by water cooling. The strand is then pulled out of the mold with a slow strand movement.

Every strand standstill and every creep motion brings significant disadvantages for the cast product and for the continuous caster itself. Depending on the previous casting speed, strand format, roller spacing in the strand guide, strand bulges occur, caused by the ferrostatic pressure. After the drive rollers have been restarted, such strand bulges are rolled flat, which leads to internal cracks in the strand.

Strong bulges can reduce the size of the bulges when the strand is at a standstill. However, this leads to local overcooling, while neighboring strand elements are not or only weakly cooled. Such differences in strand cooling create surface cracks on the cast strand. Furthermore, the straightening and bending rollers and their bearings are subjected to multiple stresses when the supercooled strand sections are transported through the bending or straightening unit.

However, as already mentioned, line stoppages are also very harmful to the continuous caster itself, especially to the rolls. The strong and, depending on the standstill, long heating in the area of the contact surface of the roll with the hot strand leads on the one hand to roll surface cracks and the cooling of the half of the roll directed away from the heat radiation of the strand on the other hand leads to roll deformations and roll deflections. This creates a non-circular running of the roll, also called roll run, which leads to bearing and segment overloads. Similar to the rolling of bulges, the roll impact is also harmful to the strand itself. Bent rollers furthermore lead to roller alignment errors and to overloading of rollers and their drive motors, which can force castings to be broken off. To avoid such malfunctions, strand damage and reel damage min To be able to partially avoid them, multi-supported rollers, large drive motors or even additional strand pulling stands are built. In order to make the rolls insensitive to surface cracks, expensive roll designs and roll coatings have been proposed which make the plant more expensive and increase the maintenance costs. Such measures are expensive, in particular in the case of slab lines with a large strand width and in blooming lines with long support guides.

The invention has for its object to eliminate the aforementioned shortcomings. In particular, in the event of unavoidable interruptions in the steel supply to the mold and in the event of pouring damage to the strand and to the continuous casting installation. In addition, however, the invention is also intended to enable sequence interruptions of 10 or more minutes, which, according to the prior art, led to casting breaks or loss of quality in the product and damage to the machine. The measures mentioned are intended to increase the productivity and profitability of the plant.

According to the invention, this object is achieved by the characterizing features of claim 1.

The device according to the invention is characterized in that the drive control device is provided with a drive program for predetermined oscillatory movements of the cast strand.

The procedure according to the invention and the device according to the invention make it possible to improve the strand quality in the strand interior and on the surface and at the same time to reduce machine wear, in particular the service life of the rolls and the maintenance extension of the roller alignment. This reduces maintenance costs. The reduced machine load by avoiding strand stoppages can lead to a more favorable design of the system. For example, the number of bearings per roll can be reduced.

A strand oscillation can also be switched on immediately after a strand breakthrough, in order to shear the so-called run-out bear at a still high strand temperature, for example on a cooling grill, or to roll it in by means of strand guide rollers.

The stroke length of the oscillation can be matched to the largest roller spacing and can be, for example, half a roller spacing. In order to support the entire strand surface cyclically, the length of the oscillating movement of the cast strand can be adapted approximately to the length of the largest roller spacing in the zone with strand shell support in the sense of a further method step. The stroke length of the oscillation movement can be further optimized if it corresponds to approximately half the average circumference of all rollers in the area of the strand shell support.

After a further process step, the maximum stroke length of the strand oscillation movement can be limited using the formula H = KL-250 mm, where H is the stroke length and KL is the measure of the entire mold length in mm.

Depending on the average roll spacing in the zone of the strand shell support, the oscillation speed of the cast strand can be selected so that the oscillation speed is increased accordingly with a larger roll spacing. According to an additional Characteristic is proposed to oscillate the cast strand at a speed that corresponds at least to the target casting speed before the interruption of the steel supply. Any speed curve can be selected between the two dead centers of the oscillating movement of the cast strand. With larger slabs, large masses must be accelerated and decelerated during the oscillation movement. It is therefore proposed to oscillate the cast strand with a sinusoidal speed curve or linear speed increase / decrease.

With short interruptions in the steel supply and few oscillating movements of the strand, the secondary cooling can be kept unchanged. In the event of longer interruptions or a high specific amount of cooling water, it is recommended according to an additional process step to reduce or switch off the secondary cooling during the oscillating movement of the cast strand. According to a further proposal, it is also possible to switch off the secondary cooling step by step during the oscillating movement of the cast strand according to a predetermined program, for example first in the driver and lastly below the mold.

In order to rule out any damage to the mold walls in the target bath level, it is additionally proposed to oscillate the strand within the mold in an area below the target bath level.

It is possible to keep the mold oscillation in operation at the same time as the strand oscillation. In order for the strand crust to form an optimally rounded edge in the area of the bath level, it is recommended according to a further process step that the mold oscillation be before or to stop or stop at the latest during the first reverse stroke of the cast strand.

A thin slag, preferably with exothermic additives, can be applied to maintain or strengthen the lubrication during the oscillation movement.

To control and / or control the oscillation movement, according to an additional embodiment, a position transmitter can continuously measure the movement of the strand end within the mold. In terms of control, such a position transmitter is connected to the drive control device. It is particularly advantageous if the top dead center is measured by the bath level control device itself and only a single additional measuring device is necessary for the measurement at bottom dead center.

In bow systems, it is difficult to evenly adjust the strand cooling on the inside and outside of the strand. In order to be able to avoid scratch marks on the inside of the mold when oscillating slightly deformed strands, it is recommended according to an additional embodiment to arrange the mold so that it can move across the strand movement direction through the oscillating strand. In the case of longer interruptions, plate molds can also be opened, i.e. the mutual distance between the two broad sides is increased.

In systems with an arcuate secondary cooling section, straightening rollers are arranged in front of the horizontal strand outlet to straighten the strand. An oscillating strand passes through these straightening rolls several times in both directions, ie the strand is straightened with every forward movement and bent again with every backward movement. According to a further advantageous embodiment is particularly recommended when casting steel types prone to cracking, along the bending or. Straightening path to provide the pairs of rollers transversely displaceable through the strand itself. The bending or straightening work can be significantly reduced during the strand oscillation.

In the strand shell of a cast strand, the tensile and compressive forces in the strand movement direction can be reduced during the oscillating movement of the strand and, at the same time, slippage between the drive rollers and the strand can be avoided if drive roller pairs are arranged at several points along the secondary cooling section.

In modern systems, warning devices for strand shell cracks are often installed in the mold today, which generate breakthrough warning signals and automatically stop the strand movement and interrupt the steel supply. Such line standstills also cause the disadvantages already mentioned, both for the line and for the machine. Particularly in systems that work at high casting speeds, strong strand bulges appear after short, sudden downtimes. In order to eliminate the disadvantages mentioned, it is recommended in the sense of an additional method variant to interrupt the steel inflow and the strand pull-out movement after the arrival of a signal from a breakthrough warning device and to initiate an oscillatory movement of the strand by means of a reverse stroke.

In the following, the invention will be explained on the basis of pictures and diagrams.

• Fig. 1 eine Seitenansicht einer schematisch dargestellten Stranggiessanlage,
• Fig. 2 ein Diagramm "Stahlzufluss in die Kokille in Abhängigkeit der Zeit",
• Fig. 3 ein Diagramm "Oszillationsgeschwindigkeit des Stranges in Abhängigkeit der Zeit",
• Fig. 4 ein Diagramm "Badspiegelhöhe in der Kokille in Abhängigkeit der Zeit" und
• Fig. 5 ein Blockdiagramm für die Steuerung.

Show it:

• 1 is a side view of a schematically illustrated continuous casting plant,
• 2 shows a diagram "steel inflow into the mold as a function of time",
• 3 shows a diagram "Oscillation speed of the strand as a function of time",
• Fig. 4 is a diagram "Bath level height in the mold as a function of time" and
• Fig. 5 is a block diagram for the controller.

In Fig. 1, 2 is a continuous mold with an oscillation drive 3 in a continuous casting system. A strand 4 is pulled out of a strand guide 7 and the mold 2 with a drive device 6. Until the end of the drive device 6, i.e. the liquid strand core extends to the drive roller pair 8, 8 '. A strand shell support is therefore necessary from the mold exit to the pair of rollers 8, 8 'in order to prevent strand bulges. The mold 2 is fed via a steel feed 10. By means of a bath level measuring device 12, 12 ', e.g. a radioactive radiator / receiver, a bath level 13 is measured and automatically maintained at a target bath level by means of an inflow control. A second measuring device 14, 14 'is arranged at the end of the mold exit side.

In the event of an interruption in the steel supply 10, the bath level 13 in the mold 2 drops with continued strand extraction. The target speed or a slower speed can be preprogrammed for this lowering movement. When the bath level reaches the measuring range of the lower measuring device 14, 14 ', the drive device 6 is switched from pulling out to pushing back so that the strand moves backwards. This forward and backward movement is now repeated several times, as long as the steel feed 10 is interrupted. An oscillatory movement is thus applied to the strand 4, with two rule the strand surface and the strand guide 7 and the drive rollers 8, 9 a relative movement is generated substantially without interruption.

The length of the oscillation movement of the strand 4 can be set to the length of the largest roller spacing 16 of the strand support zone or more. When determining the stroke length of the continuous oscillation, the length of the mold 2 can also be taken into account, the stroke H being advantageously limited according to the formula: H = KL-250 mm. KL represents the length of the mold in the direction of the strand.

With optimal coordination of all parameters when determining the stroke length, the stroke length is approximately half the average circumference of the rollers in the area of the strand shell support.

With a mold length KL of 900 mm, the max. Stroke length is therefore 650 mm. The maximum roll spacing for a slab line in the strand support area is usually 400 - 600 mm and half the roll circumference for a roll of 400 nm diameter is 628 mm. With a stroke length of 630 mm, the three described stroke parameters (mold length, roller spacing, roller diameter) are optimally met. With a smaller average roll diameter of e.g. A stroke length of 550 mm is sufficient for a stroke of 350 mm. When the stroke length is determined, the length 19 of the stroke movement within the mold is determined. The strand should not leave the mold during the oscillation, but should generally not oscillate in the bath level area either.

After initiation of the strand oscillation movement, a secondary cooling 17 is reduced or switched off. Depending on the steel quality, the secondary cooling 17 during the Os zillation movement according to a predetermined program, for example from the driver to the mold, can be switched off gradually. The oscillation movement of the strand can also be coordinated with the oscillation movement of the mold 2 in order to prevent wear due to scratch marks.

So that the strand 4 in the area of a bending or straightening section 15 does not have to be completely straightened or bent again with each stroke, the roller pairs can be arranged transversely displaceably in the direction of the arrow 20 within this straightening section 15. This displaceability can play in normal casting operation or can only be released by remote control during a string oscillation.

With 9 'a pair of drive rollers is shown approximately in the middle of the secondary cooling section.

Rollers 25 indicate that the mold can be displaced transversely to the direction of strand movement 50 according to arrow 51 and can adjust itself to the oscillating strand. A first zone connected to the mold can also be arranged displaceably together with the mold.

In the diagram in FIG. 2, a steel inflow curve as a function of time is shown at 21, for example in the case of an intermediate vessel change. In this example, the steel inflow remains interrupted during the time period 22. The steel inflow is shut off in the bend 52 of the curve 21.

In FIG. 3, curve 23 shows the course of the speed of the strand oscillation as a function of time. After the interruption 52 of the steel inflow (FIG. 2), the strand is initially pulled out for a period of time 24 until it reaches the bottom dead center of its oscillatory movement Has. Then it turns after the curve 23 oscillates with a sinusoidal speed curve along the route 19 (FIG. 1). The speed curve could also be set according to a dashed line 26, 26 '.

In Fig. 4, the position change of the bath level or the end of the strand in the mold during the oscillation movement is shown in a path-time (t) diagram. At bottom dead center 27 and at top dead center 28, the direction of movement changes in each case. The top dead center 28 lies below the target bath level 29 in the mold. The zero coordinate mark represents the lower limit of the mold.

In Fig. 5 the control-related relationships are shown schematically. A computer 30 continuously receives a signal 31 from the plug or slide control or the steel inflow control. 32 shows a continuous input of a signal from the bath level measuring devices 12, 14 (FIG. 1) and 33 shows the input of the height of the oscillating mold. If the system is equipped with a breakthrough warning device, which is based, for example, on the mold wall temperature measurement, this signal 34 is also fed to the computer 30. The inputs 35 and 36 are program inputs for the intended drive movement and for the intended cooling of the strand during the oscillating movement. By means of this input, the computer 30 can control the continuous caster in a process-appropriate manner automatically or at the push of a button if the steel supply is interrupted or in the event of a fault. Lines 40-43 are connected to the driving machine 45, the secondary cooling 46, the stopper control 47 and the mold oscillation 48, which are controlled by the computer 30 according to the process.

Claims (18)

1. A method for the continuous casting of steel, in particular for driving a cast strand, the strand being pulled out of a continuous mold and a secondary cooling zone by means of a drive device, characterized in that if the steel supply (10) is interrupted, the cast strand (4) is alternately Extending and pushing back an oscillating movement is applied in accordance with the requirement that a relative movement is generated between the strand surface and strand guide (7) and drive rollers (8, 9) essentially without interruption. 2. The method according to claim 1, characterized in that the length (19) of the oscillating movement of the cast strand (4) is set to a length of the largest roller spacing (16) in the zone of the strand shell support. 3. The method according to claim 1 or 2, characterized in that the length H of the oscillation movement is additionally matched to the mold length according to the formula H = KL-250 mm, where KL represents the total length of the mold. 4. The method according to any one of claims 1-3, characterized in that the length of the oscillating movement of the cast strand (4) additionally corresponds to approximately half the average circumference of the rollers (7, 8, 9) in the region of the strand support. 5. The method according to any one of claims 1-4, characterized in that the cast strand (4) is oscillated at a speed which corresponds at least to the previous target casting speed. 6. The method according to any one of claims 1-5, characterized in that the strand (4) is oscillated with a speed curve (26 ') which has a linear speed increase or decrease. 7. The method according to any one of claims 1-6, characterized in that the secondary cooling (17) is reduced or switched off during the oscillatory movement of the cast strand (4). 8. The method according to claim 7, characterized in that the secondary cooling (17) during the oscillating movement of the cast strand (4) according to a predetermined program is gradually switched off from the driver (6) to the mold (2). 9. The method according to any one of claims 1-8, characterized in that the strand within the mold (2) is oscillated in an area below the target bath level (13, 29). 10. The method according to any one of claims 1-9, characterized in that the mold oscillation is stopped at least briefly before the first backward stroke of the cast strand (4). 11. The method according to any one of claims 1-10, characterized in that the mold oscillation is stopped at the latest during the first backward stroke of the cast strand (4). 12. The method according to any one of claims 1-11, characterized in that a low-viscosity lubricating slag is preferably added with exothermic additives. 13. The method according to any one of claims 1-12, characterized in that the steel inflow (10) is interrupted by a signal from a breakthrough warning device, the strand withdrawal movement is stopped and the oscillatory movement of the strand (4) is initiated by a reverse stroke. 14. Device for the continuous casting of steel, in particular for driving a cast strand (4), with a strand guide (7) having drive rollers (8, 9) with a drive control device downstream, characterized in that the drive control device with a drive program for predetermined oscillatory movements of the cast Stranges (4) is provided. 15. The apparatus according to claim 14, characterized in that position sensors (12, 12 '; 14, 14') continuously measure the position of the strand end within the mold (2) and these position sensors (12, 12 '; 14, 14') in terms of control are connected to the drive control device. 16. Device according to one of claims 14 or 15, characterized in that the mold (2) through the oscillating strand (4) is movable transversely to the strand movement direction (50). 17. Device according to one of claims 12 - 16, characterized in that in systems with an arcuate secondary cooling section, a bending or straightening section (15) is provided with pairs of rollers (9) which can be displaced transversely through the strand (4). 18. Device according to one of claims 14 - 17, characterized in that drive roller pairs (9 ') are arranged at several locations along the secondary cooling section.

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