EP0008692B1 - Method to avoid damage to cast guide elements in a continuous steel casting assembly - Google Patents

Abstract

A method for preventing damage to the strand guide elements of strand guide arrangements or roller aprons of a continuous casting installation for metals, especially steel, wherein it is possible to withdraw a strand which is immobile in the strand guide arrangement or roller apron, by means of a powerful withdrawal unit, without the risk of damaging elements of the strand guide arrangement. At pre-selected locations of the strand guide arrangement there are determined the withdrawal forces effective at such locations and acting upon the elements of the strand guide arrangement. Upon exceeding maximum permissible values the strand withdrawal operation is interrupted.

Description

The invention relates to a method for avoiding damage to strand guide elements of a continuous casting plant for steel.

When steel is continuously cast, undesirable deformations of the strand sometimes occur, especially when operational disruptions force the casting to be interrupted. If such an interruption in the pouring process takes a long time, the still thin strand crust is not able to pass along unsupported sections, e.g. B. between pairs of guide rollers to withstand the ferrostatic pressure of the liquid strand core. In such a case, the strand crust bulges. If a too long time interval elapses before the system starts up again, during which the solidification of the strand continues, it is often no longer possible to press such bulges back in with the help of the guide rollers against the resistance of the partially solidified strand. As a result, the strand may become stuck between the guide rollers while the strand is being pulled out, in connection with overloading of the same or of supporting structure parts. If the pull-out force remains the same, this leads to a deformation or destruction of these guide rollers or of the supporting structure parts.

Another disruption to normal casting operation, which cannot always be prevented, is a breakthrough, usually in a region of the strand which has solidified only peripherally, immediately below the mold. This is usually associated with unpleasant side effects, since a large amount of steel often flows into the secondary cooling zone of the system and solidifies in the system components located there, such as support and guide rollers, support structure parts, etc. This almost always results in significant, costly repairs causing damage. On top of that, a breakthrough in the vast majority of cases results in the casting operation being shut down.

The consequences of such a termination are usually, in addition to time-consuming elimination of the direct consequential damage of the breakthrough, in particular irreparable deformations of strand guide rollers due to one-sided, local overheating of the rollers in contact with the stationary, hot strand. When restarting, these deformations result in the rollers running out of round and, due to uneven loading of the roller bearings, can destroy them, etc. and lead to roller breakage. On top of that, the uneven running of the guide rollers leads to periodically uneven force exerted on the strand due to the same deformation of opposing rollers. At such points, reinforced pressure can result in rolling, combined with crack formation in the strand.

In practice, attempts are made to avoid the disadvantages described above which result from the termination of the pull-out process by the following procedure. If the supply of steel to the mold is interrupted, the strand is pulled out completely against the forces hindering the pull-out process, caused by irregularly shaped strands or strands welded with system parts, without interruption. This can result in the fact that due to very strong welds of the strand to the strand guiding elements such as rollers, grids, etc., which can no longer be detached, the large pull-out forces act directly on the support and bearing elements of the strand guiding and there are permanent deformations due to overloading and cause damage. For example, in a so-called first zone of the strand guide cast in by overflowing steel, such an overload can lead to a deformation of the support arms and the support pins of the 1st zone and thus to deviations in the geometry of the strand guide. These require expensive, time-consuming repairs. An advantage of the method described above, which wants to prevent the strand from coming to a standstill in the event of a breakthrough, compared to conventional casting practice, in which the strand is allowed to cool, is therefore no longer given in such a case. Deviations, which are not always discovered immediately, also lead to cracks in the cast product and thus to quality losses.

The present invention, when using the method of conveying a broken strand without interrupting the pulling process or when pulling out a strand bulged during a standstill, avoids the disadvantages associated with possible overloading of system parts, as described above. It has the task of preventing the destruction of guide elements such as rollers or deformations of elements of the support structure leading to deviations in the strand guide geometry and the associated quality-reducing influences on the cast product during the pull-out process. This object is achieved according to the invention in that when undesired deformed strands or strands welded by spilled steel with strand guide parts occur at selected points of the conveying strand guide, the deformation forces occurring there are measured and the measured values are compared with predetermined maximum values which prevent damage to the strand guide, with if it is exceeded, an impulse to interrupt the pull-out process is given.

With this procedure, the risk of loading Damage to elements of the strand guide due to excessive stress is eliminated. It is possible to use strongly dimensioned drivers to pull out a strand that is stuck in the strand guide or tends to get stuck, while avoiding overloading of system parts at risk. The pull-out force of these drivers can be set immediately and risk-free to the highest possible values, matched to the maximum permissible load index of the material used. A dynamic, if necessary jerky, tearing away of a strand that is welded with system parts is made possible.

However, if tearing is prevented due to excessive welding and the specified maximum values are exceeded, an impulse is given to interrupt the pull-out process. In such a case, the welds which hinder the pull-out process are removed as quickly as possible by hand using suitable devices and the pull-out is continued. The fact that a normal conveying of the strand, which makes it impossible to convey it out, avoids costly measures to clear the strand guide, such as cutting the strand by flame cutting, etc. The consequences of a breakthrough are cleared up much faster and the period until the system is ready for operation again is decisively shortened . After an advantageous step of the method according to the invention, the stresses occurring in the support arm of the first zone of the strand guide are measured.

• Diese Zeichnung zeigt eine Stranggiessanlage, bestehend aus einer Durchlaufkokille 1 mit nachgeordneter, in einzelne Segmente unterteilter, gebogener Strangführung und einem anschliessenden Treibrichter 2 mit angetriebenen und nicht angetriebenen Rollen 3. Die erste Zone 4 dieser Strangführung setzt sich zusammen aus einem Kühlgitter 5 und einem aus mehreren Rollenpaaren bestehenden Abschnitt 6 der Rollenführung. Sie ist mittels Lagerzapfen 7 in Lagerschalen 8 eines an der Stützkonstruktion 9 der Strangführung befestigten Tragarmes 10 gelagert.

The method is described in more detail using a schematic drawing of an example:

• This drawing shows a continuous casting installation, consisting of a continuous mold 1 with a downstream strand guide subdivided into individual segments, and a subsequent drive funnel 2 with driven and non-driven rollers 3. The first zone 4 of this strand guide is composed of a cooling grid 5 and one Section 6 of the roller guide consisting of several pairs of rollers. It is supported by means of journals 7 in bearing shells 8 of a support arm 10 fastened to the support structure 9 of the strand guide.

Molten steel is poured into the mold 1, and a partially solidified strand 11 is continuously guided and supported in the strand guide and pulled out of the mold 1 by the driven rollers 3.

Does the strand have deformations that hinder undisturbed conveying out and generate forces directed against the direction of extraction, or is the strand broken due to hardened steel with strand guide parts, e.g. B. connected to the cooling grid 5 and thus immobile, it comes to a slipping of the drive rollers while pulling out with ordinary drivers. This is avoided with a larger dimensioned leveling unit, and the forces required to break the connection can be entirely transferred to the strand.

With the help of strain gauges 12 attached to the upper side (tension side) of the support arm 10, tensile stresses occurring therein are determined, the measured values are conducted via a line 13 to a measuring and control device 14 and there with predetermined maximum values which are below those values, which could lead to a plastic deformation of the support structure. In the event that the resistance of the stuck or deformed strand is so great that the forces to be applied would be above these maximum values, the control and regulating element 14 gives an impulse to interrupt the pull-out process. This prevents damage to strand guide parts. Of course, instead of being aborted by this impulse, an optical and / or acoustic alarm signal can also be given, as a result of which the operating personnel are made aware of a possible overload situation and are prompted to take action. Instead of strain gauges 12, other voltage measuring devices, such as. B. load cells find use.

The arrangement of a strain gauge is not limited to the top of the load arm 10 of the first zone 4, but can be freely selected wherever an overload of strand guide parts and / or support guide elements leading to damage must be feared. So z. B. all pairs of guide rollers are secured against overloading by bulging strands in this way. In the event of an impending overload, z. B. the employment of the roles can be controlled with the help of the overload protection described above. Other geometrical deviations which lead to damage in the continuous casting installation, due to exceptional thermal distortion, can also be indicated and appropriate countermeasures taken as a result. It should be pointed out that the invention is not only applicable to the continuous slab caster shown in the figure, but can be used in any continuous caster with continuous molds.

Claims (2)

1. A method of avoiding damage to the strand- guide elements of an installation for the continuous casting of steel, characterized in that upon the occurrence of undesirably deformed strands, or strands that have become fused on to strand- guide parts by escaping steel, the deformation forces occurring at selected points on the strand guide, still performing its conveying function, are determined by a measuring technique, and the measured values are compared with predetermined maximum values at which damage to the strand guide is avoided, and an impulse is released for interrupting the extraction process when these maximum values are exceeded.

2. A method according to Claim 1, characterized in that stresses occurring in a carrier arm of the first zone of the strand guide are measured.

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