Metal founding – Means to shape metallic material – Continuous or semicontinuous casting
Reexamination Certificate
2000-09-22
2003-12-09
Elve, M. Alexandra (Department: 1725)
Metal founding
Means to shape metallic material
Continuous or semicontinuous casting
C164S428000, C164S480000, C164S444000
Reexamination Certificate
active
06659164
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a continuous casting plant for the continuous casting of a thin strip, in particular a steel strip having a thickness of below 20 mm, preferably between 1 and 12 mm, comprising a mold provided with two casting rolls, with a strip-like strand being united from two half-shells and exiting in a vertically downward direction at the nip formed by the casting rolls of said mold, wherein below the nip a deflecting-supporting means is provided for deflecting the strand emerging vertically from the mold into a roughly horizontal direction, and a method for continuous casting.
To keep the structural height and hence the costs low, the strip cast on a continuous casting plant of this kind has to be deflected from the vertical into the horizontal direction of strand discharge as gently as possible. It is also advantageous to support the strip in order to keep down the tensile load acting inside the newly solidified strip at the nip as a result of its own weight.
On the one hand, this gentle deflection must afford protection to the product. This means that excessive bending stress in the outer fiber or excessive plastic deformation in the cooling-down strip must be avoided and that sliding friction along hard, rough surfaces or pointed edges must be avoided in order to eliminate scratching or possible adhering, mainly in the region of the strip edges.
On the other hand, deflection must also involve the least possible damage to the casting process taking place upstream. Usually, a controlling device (e.g. a driver) located downstream provides for the strip extracted from the mold to be carried off without damaging the product. This driver appropriately acts by means of a position control. This means that changes in the casting speed which manifest themselves in a change of the position of the strip (loop formation increases or decreases) must be corrected by the above-mentioned driver (master-slave control principle). These corrective actions of the driver must by no means interfere with the casting operation progressing upstream, e.g. by inducing tensile stress, compressive strain or buckling strain in the hot strip just leaving the nip. Tension control is not suitable in view of the danger of rupturing the still very hot strip (low tensile strength).
To keep down sliding friction and avoid scratching or possible adhering of the strip to the deflecting-supporting means it is known to employ skids for deflection, said skids providing only linear support to the strip, which as a rule has completely solidied upon leaving of the mold, namely linear support in the longitudinal direction of the strip.
A continuous casting plant of the initially described kind is known from JP-A 63-30158. There, the strand exiting the nip of the casting-roll mold in the vertical direction—this narrowing is also referred to as the “kissing point”— is supported on both sides by a support means formed by two support means of conveyor-belt-type construction arranged parallel to each other, e.g. endless chain belts etc., and its movement is constrained over a predetermined vertical region. Subsequent to this constrainment, a guide of arcuate design extending roughly over a quarter circle is provided which serves for deflecting the strand from the vertical direction into a roughly horizontal direction.
According to JP-A 63-30158 it is difficult to ensure that deflection will damage neither the product nor the casting process, especially since the conveyor-belt-like support means arranged directly below the mold as well as subsequently arranged pinch rolls or rolls provided directly on-line exert an influence on the extraction of the strand. A further serious disadvantage is to be seen in that tension control cannot be realized between the mold and the endless chain belts on account of the danger of rupturing the strip, and on account of the danger of buckling of the strip position control cannot be realized, either. Moreover, it is not possible to ensure uniform sliding friction along the arcuate guide deflecting the strand from the vertical into the horizontal. Thus, the strand is exposed to a fluctuation of forces which influences the casting process within the mold in an unforeseeable manner and may cause disturbances during the casting operation.
Further it is known, namely from JP-A 56-119607, to provide a roller table with motor-driven rolls, in imitation of conventional continuous slab casting technology. However, this solution is disadvantageous in that the driven roller table entails high costs, especially since the rolls must not only be driven but must also be provided with internal cooling. Moreover, it is necessary that all of the rolls move synchronously with the casting rolls so as to avoid undesirable relative movements between the rollers and the strip and thus avoid any damage to the strip that might be caused thereby, and this necessitates great expenditures in terms of control engineering, an expensive drive mechanism and strong motors and hence entails additional costs. Furthermore, minor speed differentials may occur even with the most rapid response behavior of the rolls, and it is difficult to maintain the strip in a geometrically precisely defined position in order to actually achieve the optimum supporting effect.
It is further known (EP-B 0 540 610, EP-A 0 726 112 and EP-A 0 780 177) for a loop of strip which is freely suspended during the continuous casting operation to be provided between the pair of casting rolls and the first pair of pinch rolls that conveys the strip onward, resulting in the advantage that at the start of casting the size of the loop of strip will adjust automatically as a function of the casting conditions. Yet, a drawback of this method is the fact that the strand has no supporting device whatsoever; entirely unsupported, the entire weight of the strand is suspended by its hottest and hence weakest strip cross section, which is located at the nip, i.e. the kissing point. This results in a high risk of cracking or rupturing of the strip. In addition, start-up of such a plant is unfavorable, since this can only be done using a dummy bar head. To be able to start up the plant without a dummy bar head it is necessary to have start-up skids, such as those described in EP-A 0 780 177 and EP-A 0 726 112.
From U.S. Pat. No. 5,350,009 it is known in a continuous casting plant according to the preamble of claim
1
to allow the strand to get on a supporting belt moving along with the strand in the extraction direction, which belt is wound with the strand and separated again from the same later on. In order to guide the strand to the supporting belt, it is also known from that document to arrange an arcuate runner below the mold and direct the beginning of the strand toward the supporting belt. As soon as the strand is moved along with the supporting belt, the runner is placed in a resting position remote from the strand. A continuous casting plant of that type is complex in terms of construction and cumbersome to handle, the more so as a supporting belt moving along with the strand has to be provided, which must have at least the length of the continuously cast strand. That belt not only must be moved synchronously along with the strand, but it is also necessary to wind that supporting belt on and off several times in order to separate it from the strand. A continuous casting plant comprising such a supporting belt involves not only high investment costs, but also high operating costs. Furthermore, an arcuate runner is difficult to manufacture, in particular if such a runner is to be provided with a cooling means. In addition, runners of that type do not offer a large area support such that the very thin hot cast strand strip is not offered a good support, which constitutes a problem, in particular in the starting phase, in which the arcuate runners are employed according to that document.
The invention aims at avoiding the above disadvantages and difficulties and providing a continuous casting plant of the initially describe
Hohenbichler Gerald
Pellissetti Stefano
Schertler Armin
Acciai Speciali Terni S.p.A.
Ostrolenk Faber Gerb & Soffen, LLP
Tran Len
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