STRAND GUIDING DEVICE AND METHOD FOR GUIDING A METAL STRIP
The invention relates to a strand guiding device and a method for guiding a metal strip, in particular a metal strip that has not yet solidified right through, in a continuous casting installation. Known strand guiding devices comprise a segment frame and at least one pair of opposing guiding rollers, between which the metal strip is guided. At least one of the guiding rollers takes the form of at least two part-rollers 122, 124 arranged next to each other. The part-rollers are mounted on the segment frame 110 by means of two outer bearings 132, 134 and at least one common intermediate bearing 133. To provide at least partial compensation for the segment frame 110 springing up or bending, caused indirectly by the ferrostatic pressure in the interior of the metal strip that is not yet solidified right through and occurring in particular in the region of the intermediate hearing 133 during the transport of the metal strip between the guiding rollers 120, three different means are proposed according to the invention, means which can also to used in combination will one another. These means are a bowing of the intermediately mounted guiding roller and/or a more yielding form of the outer bearings 132, 134 in comparison with the intermediate bearing 133 and/or a greater distance A1 between the segment frame 110 and the centre axis M of the part-rollers 122, 124 in the case of the intermediate bearing 133 in comparison with the outer bearings 132, 134.
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The invention relates to a strand-guiding apparatus and a method of guiding a metal strip that has not yet completely solidified, in particular, a thin slab in a continuous-casting installation.
The profile of a slab, in particular, of a thin slab, must meet strict requirements in terms of its bowing or thickness taper when the slab leaves a continuous-casting installation and is passed to a rolling mill. For example, the required tolerances for a profile camber in a thin slab that is to be sent to a compact-strip-production (CSP) finishing train are in the range of 0.5% to 1% relative to the slab thickness. This means that the profile camber, e.g. in a 50 mm thick slab, must only measure between 0.25 mm and 0.5 mm. In addition, this profile camber should be as constant as possible over the entire length of the slab.
The reason for the profile camber in metal strips is what is called ferrostatic pressure that is present inside the metal strips that have not yet completely solidified and that presses from inside against the strand shell, thereby causing an outward crowning of the strand shell. It is true that this ferrostatic pressure is essentially constant inside the liquid part of the strand; however, the pressure increases as the liquid part of the metal strip becomes longer. This bulging of the strand shell caused by the ferrostatic pressure results in a loading of the guide rollers guiding the metal strip in the strand-guiding apparatus and transmit this loading through their bearings to a section frame on which the guide rollers are mounted by means of bearings. The load being transmitted typically results in a deflection or spring-back of the section frame, in particular, in the area of center bearings in the case of divided guide rollers. This undesirable spring-back of the section frame typically results in an undesirable change in the roller gap geometry, and thus in particular in an undesirably large profile camber in the metal strip guided in the strand-guiding means. Due to the undesirable profile camber, the metal strip often as a result no longer meets the requirements of the downstream rolling mill.
These problems are known in the art and are discussed, e.g. in EP 1,043,095 [U.S. Pat. No. 6,568,460]. Here it is emphasized that the most critical factor in maintaining the above-mentioned norms is to precisely control the roller gap geometry in the area of residual solidification in light of the above-referenced problems. For this purpose, this European patent teaches an approach whereby a force-exerting means in the form of a hydraulic cylinder is provided in the center region of the section frame, i.e. in the area of the center bearings so as to compensate for the above-referenced undesirable spring-back of the section frame.
These hydraulic cylinders are, however, very costly both in terms of acquisition as well as maintenance, and additionally entail ongoing operating costs, e.g. due to the regular consumption of electrical power.
Based on this prior art, the basic object to be attained by the invention is to provide alternative means of implementing an at least partial compensation of section spring-back for a known strand-guiding apparatus and for a known method of guiding a metal strip, in particular, one not yet completely solidified.
This object is attained by the features of claim 1. This is characterized in that the means for at least partially compensating for the section spring-back is designed either in the form of a bowing of the intermediate-mounted guide roller and/or in the form of a more yielding design of the outer bearings as compared with the center bearing and/or in the form of a greater distance between the section frame and the center axis of the subrollers at the center bearing than at the outer bearings.
The invention takes the described spring-back of the section frame in the area of the center bearings under load as a given; no attempt is made to modify the extent of the spring-back by another design, in particular, by stiffening the section frame.
Instead, all three claimed proposals effect an at least partial compensation of the section spring-back by an approach whereby, despite the spring-back of the section frame the roller gap, geometry is not modified, or is modified only within tolerable limits relative to a load situation without the claimed means.
All three claimed means can be implemented relatively cost effectively; in particular, they do not require any ongoing operating costs for continuously consumed operating resources such as electric power or oil.
The following description of the invention differentiates between an “unloaded” state and a “loaded state” for the strand-guiding apparatus.
The term “unloaded state” of the strand-guiding apparatus means that no metal strip is being passed through the roller gap.
Conversely, “loaded state” denotes the situation in which a metal strip, in particular a metal strip that is not yet completely solidified, passes through the roller gap. As has already been described in the introduction, an internal ferrostatic pressure is present in the incompletely solidified metal strip, which pressure forces the strand shell of the metal strip outward, thereby basically causing a profile camber of the metal strip. The ferrostatic pressure also acts indirectly through the strand shell on the guide rollers of the strand-guiding apparatus, and also in turn though the guide rollers on the section frame. Ultimately, the pressure on the section frame causes a spring-back of the section frame, in particular, in the area of the center bearings.
What is important is the fact that all three of the claimed means according to the invention for compensating for the section spring-back are designed and present irrespective of whether the strand-guiding apparatus is considered under load or in the unloaded state. This does not conflict with the fact that the cross-section of the roller gap changes in each case as a function of load.
The claimed means for (partially) compensating for the section spring-back advantageously provide a limitation or adjustment of the undesirably large profile camber of the metal strip caused by the ferrostatic pressure down to a permissible threshold value.
Special embodiments of the means are described in the dependent claims.
If a plurality of guide roller pairs is disposed one after the other in the travel direction of the metal strip, it is advantageous if at least some of the means according to the invention for compensating for the spring-back of the section frame in the travel direction of the metal strip are calibrated so as to tend to be increasingly stronger at the guide rollers.
The aim of this feature is to solve the following set of problems: The position of the low point of the liquid core, and thus the length of the incompletely solidified region in a metal strip in a strand-guiding apparatus is significantly determined by the casting parameters: casting rate, superheating, and amount of secondary cooling. Basically, the still-liquid part of the metal strip increases in length as the casting rate becomes faster and cooling is reduced. The longer the still-liquid part of the strand, however, the greater is the ferrostatic pressure inside the metal strip. The claimed increasingly stronger design of the means for compensating for the spring-back advantageously effects a necessarily greater counter-pressure on metal strips with especially long regions that have not yet solidified completely. The greater prevailing ferrostatic pressure is then counteracted to a sufficiently large degree by the claimed design of the means, in particular, in the area of final solidification of the metal strip. Advantageously, the claimed compensation of the section spring-back, which tends to become stronger or increase in the travel direction of the metal strip, allows for the formation of a desirable, at least approximately constant profile camber over the entire length of the metal strip, and specifically and advantageously independently of the level of the prevailing casting parameters in operation, such as casting rate, superheating, or level of the secondary cooling.
The above problem is furthermore solved by a method of in particular guiding an incompletely solidified metal strip. The advantages of this method correspond to the advantages of the embodiment discussed in the last paragraph.
The invention is described with reference to four figures in which:
The following discussion describes in more detail the invention in the form of illustrated embodiments with reference to the above-mentioned figures. In the individual figures, identical elements are denoted by identical reference numbers.
In all three views—
In terms of a first means for at least partially compensating for this section spring-back, the invention proposes a bowing of the centrally supported guide rollers, as illustrated in
All of the claimed means according to the invention for an at least partial compensation of the spring-back of the section frame can be employed not only singly but also in any desired combination with one another.
The breaks in the lines shown in
Finally, the bold lines illustrate the extent of spring-back when the means according to the invention for partially compensating for the spring-back are employed. It is evident that the spring-back of the section frame resulting when the means according to the invention are used is significantly smaller than the spring-back of the section frame represented by the lines at the outer right without the means according to the invention; compare distance B with distance A. Finally, it is also evident in
Claims
1. A strand-guiding apparatus of a continuous-casting installation for guiding a metal strip that has not yet completely solidified, in particular, a thin slab, comprising: wherein the means is designed
- a section frame;
- at least one pair of opposing guide rollers that span a variable roller gap through which the metal strip is passed, wherein at least one of the guide rollers is designed in the form of two adjacent aligned subrollers;
- two outer bearings and at least one common center bearing supporting the at least two subrollers on the section frame; and
- means for at least partially compensating for a spring-back of the section frame in the area of the center bearing when the metal strip is conveyed through the roller gap;
- in the form of a bowing of the centrally supported guide roller; or
- in the form of a more yielding design of the outer bearings as compared with the center bearing; or
- in the form of a greater distance between the section frame and the center axis of the subrollers at the center bearing than at the outer bearings.
2. The strand-guiding apparatus according to claim 1 wherein in order to effect the bowing of a single centrally supported guide roller the two subrollers are each designed as tapered with a straight or convex shape, and are each supported by their thicker ends on the common center bearing.
3. The strand-guiding apparatus according to claim 1 wherein the bowing of the subrollers or of the guide roller is designed to be parabolic or as determined by a polynomial function.
4. The strand-guiding apparatus according to claim 1 wherein the more-yielding design of the outer bearings as compared with the center bearing is implemented by a comparatively softer suspension at the outer bearings.
5. The strand-guiding apparatus according to claim 1 wherein when multiple center bearings are present the center bearings are suspended in increasingly firmer fashion toward the center of the metal strip.
6. The strand-guiding apparatus according to claim 5 wherein the center bearing is/are designed as rigid in the area of the center of the metal strip.
7. The strand-guiding apparatus according to claim 1 wherein when a plurality of guide roller pairs disposed one after the other in the travel direction of the metal strip is present at least certain of the means according to the invention for compensating for the spring-back of the section frame are calibrated to be increasingly stronger at the guide rollers in the travel direction of the metal strip.
8. The apparatus according to claim 7, further comprising control means whereby the counter-pressure exerted by the strand-guiding apparatus on the strand shell tends to become increasingly larger so as to compensate a spring-back of the section frame in the travel direction of the metal strip.
9. In combination with a continuous-casting unit producing a strand extending and traveling in a longitudinal travel direction and having a solid shell and a liquid core, a strand-guiding apparatus comprising:
- a plurality of longitudinally spaced section frames;
- respective pairs of roller assemblies carried on the frames, transversely flanking and bearing transversely on the strand, each pair forming a gap through which the strand passes, each roller assembly being formed by two aligned subrollers;
- a respective center bearing between the subrollers of each roller assembly and bracing inner ends of the respective subrollers transversely against the respective frame;
- respective outer rollers at outer ends of the subrollers of each roller assembly bracing the respective outer ends transversely against the respective frame, the subrollers being shaped or being supported on the respective frame such that in an unloaded condition the gap is transversely narrower at the center bearing than at the outer bearing.
10. The apparatus defined in claim 9 wherein the subrollers have outer surfaces engageable with the strand and centered on respective axes.
11. The apparatus defined in claim 10 wherein the subrollers are tapered axially away from the center bearing.
12. The apparatus defined in claim 10 wherein the subrollers are substantially cylindrical and the axes of the subrollers of each roller assembly meet at a large acute angle at the respective center axis.
13. The apparatus defined in claim 10 wherein the outer bearings include cushioned supports by means of which they are more transversely deflectable than the respective center bearings.
14. The apparatus defined in claim 9 wherein the subrollers are shaped or supported so as to offer increasing resistance to transverse deflection downstream from the continuous-casting unit.
Type: Application
Filed: Aug 15, 2007
Publication Date: Dec 24, 2009
Applicant: SMS DEMAG AG (Duesseldorf)
Inventors: Markus Reifferscheid (Korschenbroich), Juergen Seidel (Kreuztal)
Application Number: 12/375,542
International Classification: B22D 45/00 (20060101); B22D 11/20 (20060101);