Patent Publication Number: US-6701999-B2

Title: Method and device for producing slabs of steel

Description:
PRIORITY CLAIM 
     This is a U.S. national stage of application No. PCT/DE98/01198, filed on Apr. 27, 1998. Priority is claimed on that application: 
     Country: Germany, Application No: 197 20 768.5, Filed: May 7, 1997. 
    
    
     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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An example of the invention is represented in the attached drawing, in which: 
     FIG. 1 shows the diagram of the continuous casting installation; 
     FIG. 2 shows the dependence of the gap or the actuating force over time; 
     FIG. 3 shows the dependence of the actuating force over the gap; 
     FIG. 4 shows the formation of the hysteresis; and 
     FIG. 5 shows stands in various operating states. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows, in the upper part of the image, the diagram of a continuous casting installation with a permanent mold  11 , at the mouth of which a slab B emerges and is guided by stands  21 . 1  to  21 . 5 . In the slab, the strand shell of which gradually solidifies, there is a crater S up to a lowest point S s . For the sake of simplicity, adjusting elements  31  are represented only in the case of the stand  21 . 4 . 
     Presented in FIG. 1 b  is the diagram of a stand  21 , which has an upper frame  22  and a lower frame  23 , which determine by means of adjusting elements  31  the gap between the guide rollers  24  arranged on them. One of the guide rollers is a drive roller  25 , the function of which will be described in further detail in FIG.  5 . 
     The adjusting elements have a tie rod  32 , which as a rule is fastened in the lower frame  23  and has at its opposite end a piston  33 , which is guided in a cylinder  34 . The individual stands  21  have at least four adjusting elements  31 , the cylinders  34  of which are in connection with an actuator  35 . 
     In the left-hand part of the diagram, the adjusting element  31  is equipped with a distance-measuring element  41 , which is in connection with a distance-measuring pick-up  42 , which is connected in terms of measuring technology to a computer  45 . 
     In the right-hand part of the diagram, the cylinder  34  is equipped with a pressure-measuring element  43 , which is connected to a pressure pick-up  44 , which is likewise connected in terms of measuring technology to the computer  45 . The computer  45  cooperates in control terms with the actuator  35 . 
     In addition, the actuator is connected to an oscillator  46 . 
     In FIG. 2, in the upper part of the image, the gap is plotted over time. By means of an oscillator, the gap “s” is changed by the slab thickness aimed (center line c). In the present case, it is a sinusoidal oscillation. The frequency (f) of the gap oscillation is 0.05 to 5.0 Hertz. However, other modes of oscillation are also possible and envisaged. 
     In the lower part of the image, the actuating force F is plotted over time. In the left-hand part of the image, the actuating force has a relatively small amplitude. In the right-hand part, the amplitude of the actuating force has increased distinctly. 
     In FIG. 3, the dependence of the actuating force over the gap is represented (F=f(s)). It is evident that, in first approximation, two curves, or in the greatest simplification two straight lines, to be precise F 1 =a−m 1 ·s and F 2 =b−m 2 ·s, can be represented by means of a computer. Since the two curves have different slopes, they intersect at a point P. 
     In a further approximation, the actuating force relation F/gap s shows a hysteresis which has substantially the form of an angle with two sides, with an apex point P. The optimum gap is expected in the region of the point. 
     Should it become evident in the evaluation during operation that the hysteresis curve is migrating along one side F 1  or F 2 , measures are to be taken to the effect that both sides are of approximately the same size and that their point of intersection and the break point of the hysteresis are in the region of the point P, in other words close to the optimum of the gap. 
     Should the image evaluation show that the hysteresis no longer has a break point and consequently has migrated out along one side of the angle F 1  or F 2 , measures are to be taken in the form and direction of the gap in order that the hysteresis is as uniform as possible on both sides of the point P. 
     In FIG. 4, the dependence of the actuating force over the gap has been refined even further. In dependence of the size of the crater, the hysteresis develops from type α through type β to solidified-through type γ. 
     Thus, the crater of type α has a thin shell with a crater of low viscosity, type β has a distinctly thicker shell and at the same time a crater with high viscosity and type γ has altogether solidified through. 
     The image representations presented here show a uniform distribution for the hystereses and consequently the optimum gap, either s α  or else s β . 
     The actual forms of the hystereses detectable during operation consequently allow the deviation from the optimum gap to be detected and the correct measures to be adapted in dependence on the degree and direction of the adjustment of the gap. Furthermore, conclusions can be drawn as to the degree of solidification. 
     FIG. 5 shows a stand in three different operating states. The item numbers correspond to those already presented in the images above. In FIG. 5 a  the image is normal casting operation, in which a position control is carried out on all cylinders. In the present example, a drivable guide roller is provided at the stand inlet on the upper frame. 
     In FIG. 5 b , operation when the strand has solidified through is represented. Here, the cylinders for the adjusting elements arranged in the region of the drivable guide roller are pressure-controlled and the cylinders represented downstream with respect to the strand are position-controlled. 
     In FIG. 5 c , for transporting the cold strand, the upper frame of the stand is inclined in such a way that the drive roller has direct contact with the cold strand by means of the adjusting elements arranged in the vicinity of said roller, by pressure control of the cylinders, and the cylinders of the adjusting elements which are arranged away from the drive roller are position-controlled. In this case, their position is set such that during the transport of the cold strand they do not have any contact with the latter.