Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
1999-11-08
2004-03-09
Elve, M. Alexandra (Department: 1725)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S478000, C164S416000, C164S454000, C164S154100
Reexamination Certificate
active
06701999
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for producing slabs from steel, in which the strand leaves a permanent mold with liquid melt enclosed by the strand shell and, in a downstream strand guiding assembly, the gap between guide rollers mounted in stands is set infinitely variably by adjusting elements connecting lower and upper frames, and relates to an associated apparatus for this.
2. Discussion of the Prior Art
DE 26 12 094 C2 discloses an apparatus for changing the distance between parts of a frame or stand of a strand guiding assembly lying opposite one another in pairs and connected by tie rods, in which bushes which can be turned with the aid of pressure cylinders are provided. The movable frame parts are connected by pressure cylinders, exchangeable spacers being insertable between the movable frame part and the inner bushes for the purpose of setting a pre-determinable roller spacing. In this embodiment, an infinitely variable setting of the spacing between the guide rollers can also be carried out.
In a disadvantageous way, the adjustment of the gap by the turning of the bushes is possible only over a very limited distance. In addition, considerable mechanical wear must be expected during the adjusting operation. With these known hydraulic clamping cylinders, it is not possible for the clamping force to be deduced, since part of the clamping force is absorbed by the so-called spacers.
U.S. Pat. No. 3,891,025 discloses continuous casting stands which are hydraulically adjustable and the gap of which is recorded by position sensors and a servo unit can be set.
The essential object of the subject matter of this patent is merely to apply adequate pressing force, or set the gap, for transporting the strand.
DE-A-24 44 443 discloses a process for continuously casting a steel melt in which the change in thickness of the casting is determined and compared with a specific reference value, in order in this way to control the drawing rate and/or the amount of secondary cooling water.
Practice has shown that such a method of detecting the lowest point of the liquid crater can be used only in the case of a geometrically ideal installation and a quite specific casting rate and cooling. In the hostile conditions of a metallurgical plant, however, an installation cannot be set up exactly with respect to the gap, or else thermal deformations occur in the segments or the installation operates in an inexact way, with the consequence that the changes in thickness determined are subject to considerable variations, in particular in the region of the lowest point of the liquid crater.
SUMMARY OF THE INVENTION
Cognizant of the difficulties mentioned above, the object of the invention is to provide a process and an apparatus with which the gap can be set exactly over the entire strand guiding assembly by simple means and, in addition, the current position of the lowest point of the liquid crater within the slab can be determined. Furthermore, while being of a simple construction, the apparatus is to be capable of reliably guiding the cold strand.
According to the invention, the gap is changed by an oscillation about a predeterminable center line of the slab thickness aimed for. In this case, an oscillation value which keeps to a negligible level the dynamic influences on the strand shell, which is still relatively thin after leaving the mold, is chosen. The amplitude of the oscillating gap is set to a value which prevents plastic deformation of the strand shell.
The current value of the gap is recorded by means of distance measuring elements and is fed to a computer. At the same time, the actuating force of the adjusting elements for the infinitely variable changing of the gap is determined and likewise fed to the computer. By means of a computing program, the amplitude is monitored and, when the amplitude of the actuating force increases, the gap is set to a pre-determinable value and/or the gap between the guide rollers is pressure-controlled by means of one of the adjusting elements setting the gap in an infinitely variable manner.
The amplitude of the actuating force is in this case a measure of the degree of solidifying of the strand. That is to say, a relatively small amplitude of the actuating force is encountered when the strand shell is still thin and there is a large liquid crater. The amplitude reaches its greatest value when the strand is solidified through.
Consequently, recording the amplitude of the actuating force provides a reliable measure for recording the current position of the lowest point of the liquid crater and carrying out a dynamic soft reduction.
The computer also establishes a relationship between the gap and the actuating force. It has been found in this case that, if the gap deviates from its optimum value, the following situation arises:
if the gap is smaller than the optimum, the edge pressure of the slab increases, with the consequence that the actuating force increases
if the gap is larger than the optimum, no edge pressure occurs and the strand bulges, the actuating force assuming a lower overall value.
In the case of quasi-static measurement, in first approximation this can be represented by two simple curves F
1
and F
2
, which represents overall the form of an angle with two sides. At the optimum gap, the optimum pressure distribution over this strand shell and the liquid crater enclosed by it is also to be encountered.
Recording the current actuating force allows the optimum gap to be set by detecting from the oscillation whether the trend away from the optimum gap is toward the larger or smaller gap, in order then to take specific measures to counteract this.
In the case of dynamic measurement, the actuating force F behaves with respect to the gap s in the form of a hysteresis curve. The deformation work of a segment during the stroke, i.e. the area within the hysteresis curve, can be calculated by evaluation software and the strand consistency can be deduced. The hysteresis curve has a relatively small area overall when the shell is still thin and the crater is relatively large. The hysteresis curve has a relatively large area when the shell is continuing to grow and the crater volume is decreasing. The hysteresis assumes a particularly slender form when the strand has solidified right through.
The invention achieves an optimization of the production performance from qualitative and quantitative aspects, to be precise with respect to qualitative optimization by a soft reduction which is always carried out optimally (seen locally, dynamic soft reduction) and with respect to quantitative optimization of the production performance by the possibility of being able to maximize utilization of the machine length, with high operational reliability at the same time.
Moreover, if displacement-controlled hydraulics are used, no further mechanical components are required.
In addition, any so-called thermal tracking software there may be is considerably improved in its accuracy.
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Schubert Ingo
Von Wyl Horst
Cohen & Pontani, Lieberman & Pavane
Elve M. Alexandra
Mannesmann AG
Tran Len
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