EP0064238B1 - Process and device for bending a strand in a continuous steel casting machine - Google Patents

Abstract

In a method and apparatus for bending a strand in a continuous casting installation for metals, especially steel, the ferrostatic pressure of the liquid core or pool of the cast strand is supported at a transition curve along a bending path between a circular arcuate-shaped roller apron path or track and a straight roller apron path or track by means of pairs of support rolls. In order to obtain uniform low elongation velocities at the strand skin or sheel throughout the entire transition curve for the purpose of improving the surface quality of the cast strand and for avoiding structural flaws at the solid-liquid interface as well as for simplifying the adjustment, alignment and maintenance work, the transition curve along the roller apron track is continuously accommodated to changing casting and/or strand parameters by the action of the strand itself.

Description

The invention relates to a method for bending a strand in a continuous casting installation, in which the strand is bent along a bending curve between an arc-shaped and a straight support guideway with increasing or decreasing curvature, and in addition to absorb the ferrostatic pressure of the strand having a liquid core Support roller pairs which can be displaced transversely to the strand running direction, each having a predetermined roller spacing, and a device for carrying out the method.

From DE-B 1'250'973 it is known to carry out the bending work in several subsequent steps in steel continuous casting plants, both when bending a straight strand into a circular arc and when straightening a curved strand, so that the strand in the bending area is opposite a single-point bending or Single point judging is less stressed. With step-by-step bending, the bending radius is gradually reduced and / or with step-by-step straightening, it is gradually increased. With a relatively small radius and with subsequent circular arc strand guidance and gradual straightening process, a low overall height of the system and, in practice, acceptable elongation values in the strand crust can be achieved in the straightening part of the strand guidance. However, the alignment of such support guides is very complex, because on the one hand the roles of both support tracks along the bending section and on the other hand the bending section must be precisely aligned with the curved and the straight support guide track.

From CH-A-414 957 a strand guide for a continuous casting installation is known which has rigid pairs of rollers which can be moved through the strand both perpendicularly and perpendicular to the surface of the strand. The rigid and the movable roller pairs are arranged along a straight or circular longitudinal axis defined by the mold shape. The moving pairs of rollers adapt to the current shape of the strand as it passes through the strand within a defined range. Precise alignment of the movable rollers on the straight or circular Sfrang track can be omitted without disadvantage to the strand quality due to this scope. In this system, the strand leaves the strand guide in the same geometric shape as when it entered it, i.e. in a straight or circular arc shape. In the case of the circular-arc strand guide, a drive straightening unit for straightening the strand is arranged downstream according to the conventional method. This drive leveling unit is due to the lack of support between the bending rollers. not suitable for slurry with a liquid core. Furthermore, this drive leveling unit works according to the single-point leveling concept with a correspondingly high elongation or change in elongation in the strand crust. This continuous caster is not suitable for strands with a high casting speed and the resulting liquid core in the straightening unit.

From DE-A-2'341'563, which forms the preamble, a strand guide for supporting and guiding as well as for deflecting an only partially solidified strand is known. In this support guide, a roller is rigidly arranged in the pair of rollers of a straight and an arcuate part of the support guide adjoining the bending curve. Along the bending curve, pairs of rollers are arranged to support the ferrostatic pressure, at least four of these rollers being rigidly attached as force-transmitting bending or straightening rollers and two of which are provided on the tension fiber side and at least two on the compression fiber side of the strand. With this bending device, too, the precise alignment of the rollers along the transition curve and with respect to the curved and the straight support guideway is difficult and time-consuming. The rate of expansion at the solidification front when bending the strand changes step by step along the bending curve part from zero to a maximum value and from this step by step again to zero. This bending characteristic extends the entire bend curve part by the swelling and the swelling part of the strain rate change along the bend curve. In order to achieve low expansion speed values on a bending curve part between a straight mold and an arc-shaped support guide part, a corresponding length for the bending curve part with a corresponding overall height of the system must therefore be provided, which has a negative impact on the system costs. The alignment work of such a bending curve part places high demands and is complex.

The invention has for its object to provide a method for bending a strand and a bending device, which overcome the disadvantages mentioned in the description of DE-A 2 341 563 and which stress the strand compared to this known bending method with lower elongation values and elongation rates on the one hand to avoid cracks in the surface crust surface and structural defects at the phase boundary solid-liquid and on the other hand to make the bending device simple and inexpensive to set up. The bending device, its adjustment, alignment and maintenance work along the transition curve are also to be significantly simplified.

In terms of the method, this object is achieved in that the strand is bent in the region of the bending curve by a constant bending moment, which is applied exclusively by rollers of the circular-arc-shaped and straight support guideway, and in that the displaceability of all the pairs of supporting rollers that take up the ferrostatic pressure is chosen along the bending curve, that their position adapts itself transversely to the bending curve to the respective casting and strand parameters of the bending strand.

A device for solving this problem is defined in claim 2.

With the method according to the invention or the bending device according to the invention, it can be achieved that a minimal constant rate of expansion occurs in the strand crust over the entire length of the bending curve and thus minimal stresses on the strand surface and on the solid-liquid phase boundary. A desired or permissible strain rate can be set by choosing the length of the bending curve. This results in strands that are free of cracks and structural defects, even with difficult-to-cast steel grades and / or at high casting speeds. All the rollers along the bending curve part between the curved and the straight part of the support guide do not transmit any transverse forces or no bending forces. The bending or straightening device can be built more simply and this results in cost savings. Additional advantages result from the fact that the adjustment and alignment work along the bending curve part is omitted and the maintenance work is thus simplified considerably. Alignment errors of the circular-arc-shaped and the straight support guide part to one another have practically no negative influence on the strand quality, because the transverse movable pairs of rollers along the bending curve part adjust themselves to a corresponding optimal transition curve. An extension of the bending curve to a length of 3 or more meters within the metallurgical support length is easily possible and practically without additional costs. Extremely low strand crust stresses are achieved, which result in the lowest roller bending stresses in the curved and in the straight support guideway. The method according to the invention further enables an optimal ongoing adaptation of the bending curve to changing casting and strand parameters. Special attention is paid to long-lasting sequence castings with format adjustment while the casting is in progress and major changes in the casting speed, cooling capacity and / or steel quality.

errechnet wird. R_o stellt dabei den gewählten Radius des Kreisbogens und L die gewählte Länge der Biegekurve dar.The connection point of the bending curve on the circular arc can be chosen as desired. If, for example, the height of a continuous caster is to be low and only a low ferrostatic pressure is desired in the horizontal part of the guide, the connection point of the bending curve to the circular arc in the straightening part can be brought close to an arc mold with a relatively small radius. Optimal conditions for the position of the connection point circular arc - bending curve result if the bending curve adjoins the circular arc at an angle cp _o , the angle cp _{o being} measured from the horizontal radius curve when bending and from the vertical radius line of the circular curve when straightening and according to the formula

is calculated. R _o represents the selected radius of the circular arc and L the selected length of the bending curve.

A distance Y _o between a connection point on the circular arc and an extension of the straight strand path can be calculated as follows:

R _o represents the selected radius of the circular arc and L the selected length of the bending curve.

In principle, it is possible to provide one or both rollers of a pair of cross-movable support rollers along the bending curve with a resilient or hydraulic overload protection. However, because these support rollers do not have to absorb any bending forces, it is particularly economical to arrange the support roller pairs in a freely movable manner while maintaining the mutual spacing of the rollers assigned to a pair of rollers. By omitting any overload protection in these pairs of rollers, the strand guidance along the bending curve can be made particularly simple.

Greater bending and bending reaction forces can occur on the rollers transversely to the direction of the strand running within the curved and the straight support guideway. Depending on the current strand temperature, the strand cross-section etc., such rolls can be stressed and damaged beyond the permissible load limit. It is therefore of particular advantage if, in the arcuate and in the straight part of the support guide, in front of or after the pair of rollers adjacent to the bending curve, pairs of rollers which are freely movable through the strand are also arranged transversely to the direction of the strand.

• Fig. 1 einen Ausschnitt einer schematisch dargestellten Stützführungsbahn im Biegeteil eines Stranges,
• Fig. 2 einen Ausschnitt einer schematisch dargestellten Stützführungsbahn im Richtteil eines gebogenen Stranges,
• Fig. 3 eine Skizze zur Erläuterung der geometrischen Grössen einer Biegelinie zwischen einem Kreisbogen und einer Geraden,
• Fig. 4 einen Schnitt durch ein Stützführungsgerüst,
• Fig. 5 eine Draufsicht auf das Stützführungsgerüst nach Fig. 4 und
• Fig. 6 eine Seitenansicht gemäss Pfeil VI der Fig. 4.

Examples of the subject matter of the invention are explained below with reference to figures. Show it:

• 1 shows a section of a schematically illustrated support guideway in the bent part of a strand,
• 2 shows a section of a schematically illustrated support guideway in the straightening part of a curved strand,
• 3 is a sketch for explaining the geometrical sizes of a bending line between a circular arc and a straight line,
• 4 shows a section through a supporting guide structure,
• Fig. 5 is a plan view of the support guide structure according to Fig. 4 and
• 6 shows a side view according to arrow VI of FIG. 4.

In Fig. 1 is a portion of a cast strand 1 with a liquid core 2 in a section of a support guide, which is composed of a straight part 4, a bending curve 16 and an arcuate part 6. All rollers 7-12 are arranged in such a way that they support the ferrostatic pressure of the liquid core 2 and thus can prevent the strand from bulging.

Roller pairs 7, 7 'and 8, 8' are adjacent to the bending curve 16. The pair of rollers 7, 7 'is the last in the straight part 4 of the support guide and the pair of rollers 8, 8' is the first in the arcuate part of the support guide. These rollers 8, 8 'and 7, 7' are usually rigidly attached to the machine stand. Along the bending curve 16 of the support guide, all pairs of support rollers 9, 10 are each freely movable in the direction of arrow 14 transversely to the strand running direction 13 through the strand 1. The strand 1 thus sets the pairs of rollers 9, 10 along the bending curve part 16 itself to an optimal bending curve. The pairs of support rollers 9, 10 can be rigidly connected by means of tabs 15 to keep the mutual distance. The tabs 15 or equivalent connection means absorb the force generated by the ferrostatic pressure. In order to limit overload forces at 110-120% of the supporting force for the ferrostatic pressure, the tabs 15 can be provided with appropriately designed known elastic overload safeguards.

Roller pairs 11, 11 'in the straight part 4 or roller pairs 12, 12' in the arcuate part 6 of the support guide, which in FIG. 1 in the strand running direction 13 before or after the roller pair 7, 7 'or 8 adjacent to the bending curve part 16, 8 'are arranged, if desired, can also be freely moved through the strand 1 transversely to the strand running direction 13. These roles are also relieved of bending or reaction forces. It is advantageous if these forces are absorbed by rollers which are arranged far from the pair of rollers 7, 7 'or 8, 8'.

The same reference numerals are used in FIG. 2 as in FIG. 1. The difference from FIG. 2 to FIG. 1 is that a straight strand 1 is bent along the bending curve 16 in FIG. 1 and along the bending curve 16 ′ in FIG 2 a curved strand 18 is straightened.

wobei R_o der gewählte Radius des Kreisbogens und L die gewählte Länge der Biegekurve darstellen.3 shows a bending curve 25 in the straightening area between connection points 20 and 21. A circular arc path with a radius R _{o is shown} between point 20 and a point 22 and a straight path is shown between point 21 and point 23. Y _o represents the height of the point 20 above the extension of the straight path, which also forms the X axis. An angle ϕ _o lies between a vertical radius line 24 and a straight line 26 running to the connection point 20. The position of the two connection points 20 and 21, given the radius R _o for the circular arc and the horizontal length L of the bending curve 25, can be calculated as follows: the angle (p _o , which is measured when bending from a horizontal radius line and when straightening from the vertical radius line 24 of the circular arc is calculated using the formula

where R _{o represents} the selected radius of the circular arc and L represents the selected length of the bending curve.

errechnet werden, wobei R_o der gewählte Radius des Kreisbogens und L die gewählte Länge der Biegekurve darstellen.The distance Y _o between the connection point 20 de bending curve on the circular arc and an extension of the straight path (X-axis) can be according to the formula

are calculated, where R _{o represents} the selected radius of the circular arc and L represents the selected length of the bending curve.

an einer Faser im Abstand a von der neutralen Mittelachse des Stranges errechnet sich wie folgt:

wobei Vs die Giessgeschwindigkeit in mm darstellt. secThe strain rate

on a fiber at a distance a from the neutral central axis of the strand is calculated as follows:

where Vs represents the casting speed in mm. sec

The following example shows numerical values that are common in practice. A continuous caster with an arc mold and with an arcuate strand guide is assumed.

The elongation values in this example are approximately a power of 10 lower than with a similar strand guide with a one-point straightening unit.

In FIGS. 4-6, rollers 40 and 41 are fastened to yokes 45 and 46 by means of their bearings 43 and 44. The rollers 40 and 41 form a pair of support rollers, which could be arranged along a bending curve, but also along a curved or straight support guide.

The yokes 45 and 46 are clamped together with force devices 50, 50 'against stops 51, 51', so that the pair of rollers 40, 41 forms a non-positively closed unit. Guides 47 are provided on both sides of the strand guide. These guide the yokes 45 and 46 so that the pair of rollers 40, 41 can be moved freely by strand 49 transversely to the strand running direction.

The force devices 50, 50 ′, which are designed as a hydraulic piston-cylinder unit, support the ferrostatic pressure of the line 49. They have no bending or straightening forces to absorb. Instead of the power devices, tie rods or spindles could also be provided.

The power device 50 'and the stop 51' are placed on the left side in FIG. 4 such that drive shafts 53 for the rollers 40, 41 can be brought out. On the right side in FIG. 4 the power device is shown in an example for non-driven rollers.

FIG. 6 schematically shows a weight compensation for each pair of rollers 40, 41 in the form of a spring 55. Such a weight balance, which is only necessary when the guides 47 are arranged vertically or at an angle, could also consist of a counterweight or a grease nut. The effectiveness of the weight compensation pond is usually only necessary until a start-up strand has entered the strand guide.

The described method and the device are not only suitable for casting slab formats. Special advantages can also be achieved when casting large bloom formats and pre-profiles for supports (dogbones) because such pre-profiles place particularly high demands on the straightening process with regard to freedom from cracks.

Claims (5)

1. A method for bending a strand in a continuous casting plant, in which the strand (1) is bent along a bending curve (16, 16', 25) between an arcuate and a straight supporting guideway (6 and 4 respectively) with increasing or diminishing curvature and in which additionally several pairs of backing rollers (9, 10) with predetermined distance between the rollersand displaceable at right angles to the direction of travel (13) of the strand are used to absorb the ferrostatic pressure of the strand (1) which comprises a liquid core (2), characterized in that the strand (1) is bent in the region of the bending curve (16, 16', 25) by a constant bending moment which is applied exclusively by rollers (7, 7', 8, 8') of the arcuate and straight supporting guideways*(6 and 4 respectively), and that the displaceability of all the pairs of backing rollers (9, 10), which absorb the ferrostatic pressure along the bending curve (16, 16', 25), is so selected that their position adapts continuously to the respective casting and strand parameters of the strand being bent (1), by moving at right angles to the bending curve.

2. A device for bending a strand (1) in a continuous casting plant with a bending curve portion (16, 16', 25) between an arcuate and a straight supporting guideway (6 and 4 respectively), in which bending curve portion several pairs of backing rollers (9, 10) with predetermined distance between the rollers and displaceable at right angles to the direction of travel (13) of the strand are arranged to absorb the ferrostatic pressure of the strand (1) which comprises a liquid core, characterized in that bending rollers (7, 7'; 8, 8') are placed exclusively on the arcuate and straight supporting guideways (6 and 4 respectively) and that only pairs of rollers (9, 10) for accomodating the ferrostatic pressure are arranged in the region of the bending curve portion (16, 16', 25), these roller pairs (9, 10) being displaceable by the strand (1) at right angles to the direction of travel (13) of the strand and corresponding to the path of the strand.

3. A device according to claim 2, characterized in that between the point (20) at which the bending curve (16, 16', 25) joins the arc (6) and an extension of the straight supporting guideway (4) there is provided a distance Y_o according to the formula

where R_o represents the selected radius of the arc (6) and L the selected length of the bending curve (16, 16', 25).

4. A device according to claim 2 or claim 3, characterized in that the bending curve (16, 16, 25) joins the arc (6, R_o) at an angle cp_o, the angle (p_o being measured from the horizontal radial line of the arc in the case of bending and from the vertical radial line of the arc in the case of straightening and being calculated according to the formula

where R_o represents the selected radius of the arc and L the selected length of the bending curve.

5. A device according to any one of claims 2 to 4, characterized in that in the arcuate and straight portions of the supporting guideway (6 and 4 respectively) pairs of rollers (12, 12'; 11, 11') are arranged respectively before or after the pairs of bending rollers (7, 7'; 8, 8') adjacent the bending curve (16, 16') and are freely movable by the strand at right angles to the direction of travel (13) of the strand while retaining their distance from each other.

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