AU722408B2

AU722408B2 - Process for optimizing surface quality of continuous castings

Info

Publication number: AU722408B2
Application number: AU28857/97A
Authority: AU
Inventors: Gerd-Joachim Deppe; Hans Uwe Franzen; Lothar Parschat; Fritz-Peter Pleschiutschnigg; Hans Gunter Thurm
Original assignee: Mannesmann AG
Current assignee: Vodafone GmbH
Priority date: 1996-04-02
Filing date: 1997-04-02
Publication date: 2000-08-03
Anticipated expiration: 2017-04-02
Also published as: JPH11513936A; DE59703679D1; EP0907442A1; JP3130053B2; CN1215357A; BR9708495A; WO1997036706A1; CN1072067C; AU2885797A; UA44840C2; EP0907442B1; RU2163856C2; ES2157072T3; CA2250871A1; DE19614760A1; ATE201623T1

Abstract

The invention relates to a process in a casting mill for producing continuous castings, in particular continuously cast steel, in which the movement of the casting is determined and modified. The invention is characterised by the following process steps: (a) the melt surface is covered with a casting powder which forms a liquid slag to produce a lubricating film between the casting outer shell and the mould inner wall; (b) a measured value characterising the friction between the casting outer shell and mould inner wall is determined in the oscillation device and forwarded to the evaluation unit (in the form of a computer); (c) the signal which characterises the path-time behaviour of the casting is likewise forwarded to the computer; (d) the computer correlates and links the measured values or signals for the path-time behaviour of the casting and the friction of the casting in the mould to produce comparable values and compares them to a reference value; (e) the reference value is determined as a mean value of the casting speed from the path-time behaviour of the casting; (f) from the discrepancy between actual and reference values, a signal is generated to adjust the casting powder composition in order to reduce friction and/or mould vibration.

Description

Process for Optimizing Surface Quality of Continuous Castings The present invention relates to a process for optimizing the surface quality of continuously cast steel produced in a casting machine.

When steel is cast in a continuous casting machine, the continuously cast strand is generally conveyed at a constant withdrawal speed. The level of the melt surface in the mold is kept constant by controlling the flow of the melt from the distributor. Some casting machines, particularly billet and blooming machines, cast with a constant flow of melt from the distributor and regulate the level in the mold by adjusted the strand withdrawal speed. These two measures are so familiar to the expert that no special reference in the literature is needed.

In both of the aforementioned cases, somewhat more simply in the first than in the second, tests are made to measure the friction conditions between the continuously cast strand and the mold, and thus to determine anomalies in the mold environment. Such tests are designed to measure the force needed to move the mold during casting operation, and then to compare this value to the forces that occur during idle operation. In the past, both mechanical and hydraulic mold oscillation drives have been used and studied. For a mechanical mold drive, such systems are known from "Concast Standard News," Volume 30, 1/1991, pp. 4 to A suitable hydraulic mold drive system is known from DE 35 43 790 C2.

With respect to the formation and quality of the casting surface, it is known see Stahl u.

Eisen ["Steel and Iron"] 108 (1988), No. 3, pp. 1125 to 1127) that when oscillating continuous casting molds are used, great significance attaches to the casting powder used to form a lubricating film between the mold wall and the strand shell. Attempts have therefore been made (Stahl u. Eisen 107 (1987) No. 14, 15 pp. 673 to 677) to obtain information on strand behavior in the mold by measuring the drawing force during start-up. For this purpose, a suitable force measurement device is installed in the cold strand. Of course, this method is suitable for monitoring only during the start-up process. During the actual operational phase, the use of this measurement method is not possible.

In addition to the type of lubrication between the mold and the strand shell, factors such as the changes inside the mold due e.g. to oscillation parameters (stroke height, stroke frequency, curve shape), the grade of the steel itself, the strand withdrawal speed, the cooling conditions, the steel temperature and the type of strand guidance, are important, especially in cast-rolling, in forming the surface of the casting. It is therefore not possible, simply by comparing the mold oscillation curves during idling to those during casting, to draw conclusions that would justify direct intervention in the casting operation.

In all of these considerations, the speed of the casting in the mold is assumed to be uniform, probably because the strand withdrawal is carried out by means of uniformly rotating rollers.

However, the actual strand speed is substantially influenced by the friction conditions in the mold. This can be reflected by the up-and-down movement of the strand, sometimes visible to the naked eye (see Stahl u. Eisen (1987), No. 14, 15, pp. 673 to 677). From DE 38 06 583 Al, it is known to ascertain the movement of the casting in a region as soon as possible after leaving the mold. In this case, measurement signals are supplied by a diode line camera to an evaluation unit or a display unit. The process known from the aforementioned document 3 serves to take into account the intrinsic vibrations of the casting and the casting machine and is used to adjust the casting operation in such a way as to avoid critical regions.

Accordingly, the present invention provides a process in a casting machine for producing continuous castings, in which molten metal is introduced into a continuous mold and is withdrawn from the mold in a partially solidified state; the movement of the casting is determined in a region as directly as possible after leaving the mold; the determination of the movement takes place without delay and in a contact-free manner by means of sensors responding to radiation; and the sensors are designed and arranged in such a way as to produce an analyzable measurement signal concerning the path-time behavior of the casting, including: a) covering the melt surface with a casting powder that forms a liquid slag to produce a lubricating film between the strand shell and the mold inner wall; b) determining measurement values characterizing the friction between the strand shell and the mold wall; c) generating the measurement signal characterizing the path-time behavior of the casting; d) correlating the measurement values and signals to produce comparable variables and comparing said variables with a reference value determined as the mean value of the strand speed from the path-time behavior of the casting; and from the discrepancy between the actual and reference values, a signal is generated to adjust the casting powder composition so as to reduce the coefficient of friction and/or the mold oscillation.

25 Preferably the signal to adjust the mold oscillation is supplied to a control unit of an oscillation drive in such a way that the movement pulse transmitted from the mold to the casting is as small as possible or close to zero.

Preferably the measurement values characterizing the friction of the casting in the mold are formed, in the case of a hydraulic drive of the mold oscillation device, from the difference in pressure in the hydraulic cylinder between the idle and the operational states.

Alternatively the measurement values characterizing the friction of the casting in the mold, in the case of a mechanical drive, are taken from a load cell arranged in the oscillation rod.

Preferably the mixture ratio of different casting powders is adjusted relative to each other.

n mPreferably the aggregate state of the casting powder is modified before its contact with the S Smolten metal in the mold. .97.o 18/05/00 H :\ARymel'\Keep\Spec i\Andriiew\28857 .97 .d~oc 18/05/00 4 Preferably said casting powder is softened or liquefied by being heated before its contact with the molten metal in the mold.

Preferably the path-time behavior of the casting is ascertained optically by a diode line camera arranged laterally next to the casting on its narrow side, the orientation of which camera agrees with the casting direction.

In order that the present invention may be more clearly ascertained, a preferred embodiment will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of a casting process according to a preferred embodiment of the present invention; Figure 2 shows typical strand and mold velocity and displacement curves obtained from the process of Figure 1; and Figure 3 shows typical strand path curves for the process of Figure 1.

Figure 1 shows a continuously cast strand 11 that is guided, when leaving a mold 12, by rollers 14.

The mold 12 is connected to a casting powder feed 17, which is attached via blocking devices 16 to the casting powder container 15. The casting powder feed 17 also passes o through a heating device 18.

On the narrow side of the strand 11, a sensor is provided, here, a diode line camera that detects the diode line of the casting. The camera orientation agrees with the casting direction.

The diode line camera 21 is connected via a measurement line 28 to the measurement pickup 22 to determine the strand movement relative to the distance 24 and the speed 23.

H :\ARymer\Keep\SpOci \Anlirew\28857.97 .doc 18/05/00 The signals related to changes in the speed and path of the continuously cast strand are furnished to a computer 26 and, as applicable, a display device 25, in some cases, a printer.

In addition, further parameters 27 are sent to the computer 26.

On the output side, the computer 26 is connected via a control line 31 to a control component 32, actuators 16 and, in terms of control, blocking sliders of the casting power container and is also connected via a control component 33 to the heating device 18 for the casting powder.

Via a control line 34, the computer 26 is also connected to a control component 35 for controlling the oscillation 13.

Figure 2 shows a typical expression of measurement signals. The upper part shows an excerpt of the average speed of the mold, which, in the given example, oscillates in accordance with a sine curve.

Below this, the average speed of the total strand is shown; superimposed on this is the actual speed of the strand in the region directly below the mold. This drawing clearly shows the friction-dependent and, in some cases, short-term sticking of the strand shell to the mold inner wall of the strand in the vicinity of the mold. The sine curve below this shows the actual path of the mold. Under this, the actual path of the strand in the vicinity of the mold is shown.

Both the indicated strand speed and strand path are measurement values, not calculated values.

The illustrated curve is an example and shows characteristic actual shapes with analyzable points. From the arrangement of the minimum, maximum and turning points, the expert is able to draw conclusions about the actual behavior of the strand shell in the mold. For the purpose of evaluation, the position of the strand at timepoints T with the curve shapes at points A is used. The distance S is hereby derived directly from the speed V.

Figure 3 shows variations of the strand path. Here, the curve radii of the individual curves at points A as well as any existing direction changes should be taken into account. By modifying the mold oscillation and the casting powder, influence is exercised on the actual path of the continuously cast strand.

Claims

2. A process as claimed in claim 1, wherein the signal to adjust the mold oscillation is supplied to a control unit of an oscillation drive in such a way that the 25 movement pulse transmitted from the mold to the casting is as small as possible or close to zero.
3. A process as claimed in either claim 1 or 2, wherein the measurement values characterizing the friction of the casting in the mold are formed, in the case of a hydraulic drive of the mold oscillation device, from the difference in pressure in the hydraulic cylinder between the idle and the operational states.
4. A process as claimed in either claim 1 or 2, wherein the measurement values characterizing the friction of the casting in the mold, in the case of a mechanical drive, are 3 5 taken from a load cell arranged in the oscillation rod. S' A4 5. A process as claimed in any one of the preceding claims, wherein the mixture ratio C of different casting powders is adjusted relative to each other. KZf H:\ARymer\K.ep\Speci\Anriew\28U57.97 .doc 18/05/00 8
6. A process as claimed in any one of the preceding claims, wherein the aggregate state of the casting powder is modified before its contact with the molten metal in the mold.
7. A process as claimed in claim 6, wherein said casting powder is softened or liquefied by being heated before its contact with the molten metal in the mold.
8. A process as claimed in any one of the preceding claims, wherein the path-time behavior of the casting is ascertained optically by a diode line camera arranged laterally next to the casting on its narrow side, the orientation of which camera agrees with the casting direction.
9. A process for producing continuous castings substantially as herein before described with reference to figures 1 to 3 of the accompanying drawings. Dated this 19th day of May 2000 MANNESMANN AG By their Patent Attorneys GRIFFITH HACK Fellows Institute of Patent and Trade Mark Attorneys of Australia S* *i a* *q* o g o* oo* H: \ARymer\Keep\peci \Andrew\298 57. 97 .doc 18/05/00