WO2005037461A1

WO2005037461A1 - Method and apparatus for controlling the steel bath in an ingot mould

Info

Publication number: WO2005037461A1
Application number: PCT/EP2004/052341
Authority: WO
Inventors: Stefano De Monte
Original assignee: Ergoline's Lab S.R.L.
Priority date: 2003-10-06
Filing date: 2004-09-28
Publication date: 2005-04-28
Also published as: ITPN20030067A1

Abstract

The present invention covers a novel approach to the identification of breakdown events in an ingot mould, so as to anticipate and avoid the possibility that, if damaged inside the ingot mould, the skin of the solidified outer layer (envelope) of the molten steel bath causes leaked molten steel to smear the underlying rollers when the ingot comes out of the mould, thereby causing the entire plant to stop. The invention is based on the generation of a high-frequency mechanical oscillation on the outer and inner surface of the ingot mould, so as to give rise to so-called surface or Rayleigh-type waves, and the analysis of how such oscillation is absorbed by the surface of the strand being in permanent contact with the ingot mould. Should the size or extension of this surface vary following a breakdown of the solidified outer layer, there occurs an abrupt increase in the contact with the copper walls and, as a result, an immediate variation of the acoustical impedance of the copper surface of the ingot mould.

Description

METHOD AND APPARATUS FOR CONTROLLING THE STEEL BATH IN AN INGOT MOULD

DESCRIPTION The present invention refers to a specific apparatus and a related method that are capable of enabling the meniscus level in a continuous casting process in an ingot mould to be determined in a very accurate and reliable manner, at a high measurement frequency. Although reference will in a preferential manner be made in the following description - for reasons of greater descriptive convenience - to the cooling-down and solidifying phase of a continuous casting of molten steel in an ingot mould, the present invention shall anyway be understood as generally applying also to the measurement of the level of a bath of molten metal in any appropriate container, as far as this falls within the scope as defined in the appended claims.

It is widely known in the art that, during a continuous casting process, the determination of the level of the meniscus of the molten steel and the point of separation of the liquid phase from the ingot mould, i.e. the point at which the outer solidified surface layer of the steel bath starts, is one among the most difficult and critical tasks in view of an effective and timely, quick-responding process control. In fact, slightly below said level, on the wall of the ingot mould, owing to the forced cooling-down of the latter there forms the starting point of the so-called "outer solid envelope", that is the almost cylindrical, sealed envelope of solidified metal, which tends to increase its thickness as it extends downwards along the ingot mould, and which contains in its interior the remaining portion of the steel bath still in its molten state.

If such level fails to be constantly and precisely monitored, in order to regulate the flow of molten steel and the extraction rate of steel ingot accordingly, the surface level of the molten steel bath may be subject to even quite rapid variations; these variations are largely known to give most easily rise to break-down effects at the surface of the above- mentioned outer solid envelope, and these breakdowns practically discontinue the capability of the same envelope of containing the molten steel thereinside without leakages.

Usually these breakdowns do not repair or seal by themselves as the steel moves downwards along the ingot mould, so that they are found again in the outer surface of the ingot when the latter comes out from the underside of the mould.

When this occurs, it unavoidably entrains a sensible degradation of the cast product.

Through these breakdowns of the outer solid envelope, therefore, a small amount of molten steel is capable of leaking to end up by scattering all along the subsequent portions of the steel-making plant, in particular by depositing onto the conveyance rollers, thereby causing the plant to have to be stopped, with obvious disadvantages of a technical-operational character.

In addition, the constancy of the upper level of solidification of the envelope is a basic pre-requisite in view, among other things, of preventing casting powders (slag) deposited on top, and which are aimed at avoiding oxidation of the molten steel bath, from being incorporated thereinto. The function of such casting powders is a fundamental one, owing also to the fact that they must melt when in contact with the steel and must wet the meniscus at the side, thereby lubricating it.

Anyway, the most serious drawback originating from an inaccurate measurement of the level of the molten steel bath in the ingot mould in a continuous casting process lies in the fact that, owing to the rate of extraction of the ingot from the mould having to be constant, the need arises for also the flow of molten steel from the tanks located thereabove to be constant, i.e. a condition that can be only achieved through a most accurate and practically continuous control of the steel level. A failure in obtaining such result would cause a steel to be produced, which lacks the required uniformity in its characteristics, thereby causing all of the resulting barstock to be rejected with a huge economic loss, which shall anyway not be dealt with here any further, considering that the pertaining technical and economic aspects of this particular issue are well-known to those skilled in the art.

Various methods that can be used to determine the level of molten steel in the ingot mould are currently known in the art. One of these methods, which is also the most widely practiced one nowadays, calls for the use of a radioactive source, usually cobalt, and a related radiation detector adapted to detect and indicate the presence of the radiations emitted by said radioactive source. Owing to both the radioactive source and the radiation detector being arranged at the opposite sides of the zone in which the level of molten steel that is to be controlled is located, when this level rises above a value to practically cause the detector to be "masked", i.e. hidden from the radioactive source by the molten steel bath, the molten steel bath itself can be automatically inferred as having reached such level.

Another largely known method implies the use of a plurality of sound- emitting sources, preferably working at an ultrasonic frequency, and a plurality of sound detectors arranged in an appropriate manner, or even suitably arranged arrays of thermocouples or magnetic elements.

Since the working principle on which this method is based is the same as the one of the afore-described method based on the use of a radioactive source, no further detailed description of this method shall be given here for reasons of greater simplicity.

Known from the disclosure in the patent publication JP 11304566A2 is also the use of a level detector in a molten steel bath, said detector comprising an electric coil that is submerged in said bath and through which an AC current is caused to flow. In the molten steel bath there is also submerged a second coil, which is coupled electromagnetically to the first coil via said molten steel bath. The level of the molten steel bath itself is inferred experimentally from the characteristics of said electromagnetic coupling.

All these methods are effective and variously used in the art. However, they share a major drawback in that, if an accurate measurement of the level and the variations thereof is to be reached, the need arises for a plurality of devices to be used, which have to be variously arranged and distributed all along or even inside the steel bath, and this most obviously introduces a considerable extent of both construction-related and operating complications with a corresponding increase in overall plant costs.

Furthermore, an additional drawback shared by the above-mentioned methods lies in the fact that they usually require frequent maintenance, with all related plant downtime problems and the need for a number of plant parts to be disassembled accordingly. Most clearly therefore appears the quite remarkable overall expensiveness connected with the use of these kinds of arrangements for the measurement of discrete levels of the surface level of the molten steel bath in the ingot mould.

Based on the above considerations, it therefore is a main object of the present invention to provide an improved arrangement for the measurement of the level of the meniscus of the molten steel in the ingot mould in a continuous casting process, as well as a related method of use of said arrangement, both said arrangement and said method being capable of doing away with the above-indicated drawbacks of prior-art approaches.

In addition, the arrangement according to the present invention shall be capable of being implemented and operated with the use of readily available and, therefore, low-cost materials and component parts.

According to the present invention, this aim, along with other features of the invention, is reached in an apparatus made and operating as recited in the appended claims.

The present invention may be implemented according to a preferred, although not sole embodiment, a detailed description of which is given below by way of non-limiting illustrative example with reference to the accompanying drawings, in which:

- Figure 1 is a symbolical cross-sectional view of an ingot mould according to the prior art; - Figure 2 is an enlarged cross-sectional view of a portion of a vertical section of an ingot mould provided with the arrangement according to the present invention; - Figure 3 is a symbolical view of the time intervals in the propagation of the pulses of transmitted waves vs. the pulses of reflected waves in an ingot mould provided with an arrangement according to the present invention;

- Figure 4 is an enlarged view of a second embodiment of the ingot mould illustrated in Figure 1;

- Figure 5 is a view of an example of pulses of oscillatory waves in the transmission and reception mode of an arrangement according to the present invention;

- Figure 6 is a symbolical view illustrating a time interval diagram similar to the one shown in Figure 3, but representing in particular the echoes of the second and subsequent contacts of the molten steel bath with the ingot mould.

With reference to Figure 1, a look may be taken now to a cross- sectional view of an ingot mould, where following details and items can be noticed:

- the ingot mould 1, - the casting tube 2, - the covering casting powder 3, - the solid outer layer 4 of solidified steel, - the liquid-state molten steel bath 5, - the meniscus 6, - the outer jacket 7 for the cooling medium 8 to flow therethrough. The present invention is essentially based on the application of the peculiar behaviour of vibratory surface waves of a mechanical nature, as generated at an appropriately high frequency and impressed upon the surface of a solid body, so that they may propagate on the surface of this solid body in a manner that is substantially tangential thereto.

It is widely known that if the surface of a solid body, in particular a metal body, is subjected to a high-frequency mechanical vibratory stress, this causes a sequence of waves to be generated, which tend to propagate in a uniform manner while moving away from the source of application. The surface waves in this sequence have characteristics and properties that differ from the ones of the so-called volumetric waves, i.e. the waves that propagate through the solid. In particular, this sequence of surface waves and/ or Rayleigh, shear horizontal, Lamb waves, propagate at a velocity that depends on both the nature of the solid on the surface of which the waves propagate, and the frequency of the impressed waves.

It shall be appreciated that, whenever reference is made from now on to surface-type waves, these shall be understood to extend to mainly include the kinds of waves indicated just above, so that there will arise no need for all these kinds of waves to be named each time.

What has been noted above is anyway widely known to all those skilled in the art, and has been only set forth to the purpose of enabling the present invention to be more readily understood.

Also well-known in the art is the fact that the above-mentioned surface waves propagate in a manner that is uniform and substantially diverging from the source of the oscillation being impressed, while transmitting itself, i.e. travelling along the crystalline structure of the metal body. As long as said crystalline structure extends in a uniform manner, said surface wave will propagate in a uniform manner, too, without any appreciable variations or reflections. However, and this is actually the basic feature which the present invention is based on, when and where the crystalline structure of the surface zone of the stressed body undergoes sensible modifications due to any cause or reason whatsoever, the surface wave propagating therealong will also be altered to a quite remarkable extent, while being in particular reflected back towards the oscillation source itself.

It has furthermore been found, and verified experimentally in the course of exhaustive test runs, that the presence of the meniscus brought about by the contact of the still liquid steel bath with the inner wall of the ingot mould and, more precisely, of the upper level of the liquid steel bath with said inner wall, causes the temperature along said inner wall to quickly undergo a most clear, very high rise, from To to Ti, as shown by way of illustrative example in Figure 1.

Such temperature rise brings in turn about - as this can on the other hand be most readily imagined - a resulting abrupt alteration in the crystalline structure of the surface and, if a train of high-frequency mechanical waves are travelling along the respective surface, such alteration of the crystalline structure generates a corresponding modification of said train of waves and, in particular, causes a part of the energy of said incident waves to be reflected, so that it converts into a secondary train of reflected waves, part of which move again towards the primary source of oscillation.

As this has on the other hand been verified experimentally, the propagation velocity of these reflected waves will once again depend on the frequency and both the chemical and physical nature of the surface involved. Anyway, if these conditions are known, it is possible for the alteration zone of the crystalline nature to be identified by measuring the length of the time required for a particular wave, or more preferably a short pulse comprising a train of waves, to cover the travelling path constituted by the first distance from the oscillation source to the alteration zone of the crystalline structure, and the second distance from said alteration zone of the crystalline structure to an appropriate device provided to receive and detect said reflected oscillation. It therefore becomes fully possible, using a radar-like technique, for a zone of alteration of the crystalline structure and, as a result, for the meniscus point, i.e. any point of contact of the molten steel bath with the inner wall of the ingot mould, even below the meniscus itself, to be most precisely identified and located.

With reference to Figure 3, the ingot mould illustrated in this Figure is provided with a device 9 adapted to generate a train of waves that propagate on the outer surface of the ingot mould itself, along the path 10, which, further to the possible outer side of the ingot mould, also comprises the upper edge 30 thereof and the outer surface 31 of the inner side.

Most obviously, further to said surface-wave generating device, said ingot mould must be provided also with a corresponding device 11 adapted to receive and detect the reflected waves (echoes). It will of course be readily appreciated that this receiving and detecting device 11 can be most easily and effectively integrated in the surface-wave generation device 9 itself, provided that said device is capable of performing in accordance with two selectively reversible operating modes, such as this is for instance the case of piezoelectric-crystal transducers.

Since the operating, driving and control principles, which these transducers are based on, are well known to those skilled in the art, and since these transducers are not by themselves relevant to the purposes of the present invention, for the sake of brevity they shall not be described here any further.

From the illustration in Figure 2, the position selected for the location of said device 9, 11 for the generation and the reception/ detection of surface oscillation waves along the walls of the ingot mould appears quite clearly, wherein it should anyway be appreciated that other positions might be preferred as well, such as for instance the position illustrated in Figure 4, which shows the combined wave generating and receiving device 9, 11 located in a position selected inside the cooling jacket 7, wherein the vibrating portion thereof is anyway placed in contact with the inwards facing surface 12 of the ingot mould.

Fully apparent will at this point be the way in which the above- described arrangement according to the present invention works: with reference to Figure 5, which is a diagrammatical view of a two- dimensional coordinate representation in which the abscissa is the time and the ordinate is the amplitude (although it would be more appropriate to speak about "presence" in the particular case considered here) of the surface wave being transmitted and received, the pulses Al, A2 and A3 represent a sequence of transmitted surface-wave pulses, whereas Bl, B2 and B3 represent the respective reflected pulses, or "echoes".

If tl, t2 and t3 represent the reception time of each "echo" pulse from the respective transmitted pulse, and the velocity of displacement of the surface wave being known, the distance of the alteration point of the crystalline structure from said combined device 9, 1 1 can most easily and readily be calculated. Ultimately, since the position of said combined device is known, the exact position of said alteration point and, therefore, of the meniscus 6 can be most easily and readily calculated, too.

In an advantageous manner, the above-mentioned devices can be appropriately associated to suitable processing means, the implementation and embodiment of which are well within the ability of those skilled in the art, and which are adapted to deliver in an almost immediate and continuous manner an electric, digital or analogue signal that is representative of the just measured position of the meniscus.

As this has already been indicated earlier in this description, it can be readily appreciated that such signal indicative of the level of the meniscus can be similarly used as an automatic control and command signal for driving the casting and filling means of the ingot mould.

The present invention may further be embodied in a still more effective manner if the risk is avoided for the various pulses being received Bl, B2, B3 to be confused, in the sense that they are not associated to the respective transmitted pulse, but rather to a non-corresponding pulse.

For this risk to be effectively avoided, said transmission and reception devices may be associated to respective automatic devices, generally known as such in the art, which are adapted to command a given transmission pulse A2 to be only emitted upon the reception of a pulse B 1 relating to a formerly released transmission pulse Al having been duly recorded. The above-described invention is effective in reaching its basic aim, i.e. the capability of effectively and continuously identifying the level reached by the meniscus, in a fully satisfactory manner. However, such capability is by no way preclusive of the possibility for other problems to be able to occur. It may in fact quite frequently happen that the outer solid layer, or "envelope", breaks down at one or more points, all of them located of course below the level of the meniscus, and - as a result - the liquid steel bath is able to partially spill out of said solid outer envelope and come into contact with the inner wall of the ingot mould. Such occurrence, which is well known as "breakdown" to and certainly most feared by steelmaking industry operators in general, can be brought about by a number of causes, the most typical and frequent ones among them being: - a too hot steel bath, - a too rapid downwards flow of the steel bath, - inclusion of casting powders, or slag, tending to modify the properties of the steel bath, - distortion of the ingot mould.

Regardless of the actual cause at the base of it, a breakdown of the solid outer layer causes part of the molten steel bath to abruptly leak out and fall onto the various parts of the plant lying therebelow, under resulting stoppage of the whole plant for cleaning and restoring - as needed - the parts that have been so contaminated and damaged by the leaked liquid steel. In order to be able to effectively prevent such problems from occurring, considering that they may have catastrophic consequences from a process economics point of view, it is absolutely necessary to be most timely and accurately informed on the occurrence of such breakdowns, along with the number thereof and the respective level of occurrence.

The present invention enables such result to be obtained, in the form of a continuous monitoring function enabling any possible occurrence of a breakdown to be timely and accurately identified, since it has been found experimentally that a contact of the molten steel bath with the wall of the ingot mould causes the latter to undergo an abrupt temperature rise and, as a result, a variation in the crystalline structure thereof, which is exactly of the same kind, although of a different intensity, as the one enabling the level of the meniscus to be identified. It therefore becomes most obvious, and readily verifiable experimentally, that the same propagation of a pulse of a train of mechanical vibratory waves used for identifying and locating the level of the meniscus, may also be used to identify the occurrence and the presence of possible breakdowns and resulting multiple contacts between the molten steel bath and the ingot mould.

With reference to Figure 6, this can be noticed to symbolically show:

- the pulse H of waves being transmitted along the wall of an ingot mould, this pulse propagating from the top downwards; - the pulse K of waves being reflected from the zone of the ingot mould that is in contact with the meniscus; - further pulses K₂, K3, etc., relating to reflected waves originated by accidental contacts between the molten steel bath and the surface of the ingot mould; - and, finally, the pulse Ke corresponding to the so-called "background echo" pulse, i.e. the almost always present reflected pulse that is due to the discontinuity of the lower end portion of the ingot mould.

Since the contact occurring between the liquid steel bath and the ingot mould in the event of such a breakdown is generally quite broad, even the respective reflected pulse will be correspondingly protracted, such as this is exemplified by the pulse K₂ in Figure 6, and this circumstance proves particularly useful, actually, in order to enable not only the level, but also the extension of possibly occurring contacts to be determined.

Since the detection of the contacts of the liquid steel bath with the surface of the ingot mould can be performed in a practically continuous manner, this possibility enables an arrangement according to the present invention, which is adapted to detect a plurality of both transmitted and received pulses, to be automatically connected to the process control and regulation means, so as to obtain a fully integrated and automated plant control and management system.

Claims

1. Apparatus for measuring the surface level or meniscus (6) of a bath (5) of molten metal in a cooling container, particularly adapted for use in conjunction with a cooling container that is constituted by an ingot mould (1) in a continuous casting process, said ingot mould (1) being provided with an upper edge (30) and an inner surface (31), characterized in that said apparatus comprises at least a generator (9) of high-frequency mechanical oscillation waves that is affixed rigidly on one of the walls of said ingot mould, said generator (9) being adapted to generate a mechanical vibratory stress that is transmitted superficially, or in the way of Rayleigh waves, shear horizontal waves or Lamb waves, along a propagation trajectory (10) that includes at least one of following paths: a) a portion of the outer surface of said ingot mould, up to the upper edge thereof, b) the outer surface of said upper edge (30), c) a portion of the inner surface (31) of said ingot mould, said portion being comprised between said upper edge (30) and said meniscus (6).

2. Apparatus according to claim 1, characterized in that said apparatus also comprises a device (11) for the reception and detection of waves having the same mechanical oscillation frequency and travelling along a propagation trajectory that includes at least one of the paths indicated in Claim 1 above.

3. Apparatus according to claim 1 or 2, characterized in that said surface mechanical-oscillation waves transmitted by said generator means are emitted with a pre-settable and defined duration as a sequence of wave pulses (Al, A2, A3) having said duration, said pulses being spaced from each other by respective time intervals, during which the emission of said waves is discontinued.

4. Apparatus according to claim 3, characterized in that said wave generator and said wave receiver/ detector comprise a same transducer, preferably of the piezoelectric type.

5. Apparatus according to claim 4, characterized in that said generator (9) is enabled to only emit a pulse of surface waves (A2) whatsoever upon said receiver/ detector (11) having recorded the reception of an echo-pulse (Bl) corresponding to a formerly transmitted pulse (Al).

6. Apparatus according to any of the preceding claims 2 to 5, characterized in that there are associated means adapted to record and calculate the lengths of time (tl, t2, t3) elapsing between the moment at which individual pulses of surface waves (Al, A2, A3) are transmitted, and the moment at which the respective echo-pulses (Bl, B2, B3) are received.

7. Apparatus according to any of the preceding claims 2 to 6, characterized in that it is associated to means for processing said time lengths (tl, t2, t3), which are adapted to correspondingly issue an electric signal that is representative of the position of said meniscus.

8. Apparatus according to any of the preceding claims 2 to 7, characterized in that said wave generator (9) and said wave receiver/ detector (11) are at least partially contained within an outer jacket (7) for the cooling medium of said ingot mould to flow therethrough, and are arranged on an outer wall of said ingot mould.

9. Apparatus according to any of the preceding claims 2 to 7, characterized in that it is adapted to record and calculate the length of time elapsing from the moment at which an individual pulse (H) is transmitted, to the moment at which a plurality of reflected pulses (K2,

K3) relating to said individual pulse (H) are received.

10. Apparatus according to claim 9, characterized in that it is adapted to record the duration of said reflected pulses (K2, K3), and to represent the values thereof through appropriate electric signals.