Patent Application
20050067134
The invention relates to a tundish having a refractory lining for producing and transferring high-purity metal melt from a casting ladle into the permanent mold of a continuous casting installation, and to a process for producing a high-purity metal strand using a continuous casting installation.
During the continuous casting of metal strands, in particular during the continuous casting of steel, a tundish is usually fitted between the casting ladle and the continuous casting permanent mold, in order to compensate for fluctuations in the supply of melt and in the rate at which the metal strand is drawn off from the continuous-casting installation. Especially in the case of sequence casting, it is necessary to store a sufficient quantity of metal melt in the tundish to span the time required to change the ladle.
The melt is usually transferred from the tundish to the permanent mold of a continuous casting installation through an outflow opening in the tundish base, which is assigned a controllable closure member, such as a slide or a stopper, and also through a submerged casting pipe or a casting nozzle. The permanent mold may be designed in a very wide range of ways, for example it may be an oscillating tube or plate mold, a mold formed by a single casting roll or by two interacting casting rolls and side plates, or a mold formed by revolving belts or tracks.
In the case of multi-strand casting installations, this tundish is designed as a distributor vessel and, via a plurality of melt outlets, supplies a plurality of continuous casting permanent molds arranged next to one another. V-shaped distributor vessels are known for two-strand casting installations.
Furthermore, the tundish is usually used to calm the metal melt which flows in from the casting ladle and is supposed to allow slag particles and other nonmetallic inclusions to be separated out during the residence time of the metal melt in the tundish. To ensure that this is achieved to a sufficient extent, the flow properties of the metal melt are usually deliberately influenced by flow-guiding internal fittings in the tundish. Trough-like tundishes formed in this way are already known, for example from EP-B 804 306 and EP-A 376 523.
If the flow and temperature characteristics in a trough-shaped tundish, as has been used for decades in conventional steelmaking processes and continuous casting installations, are considered, liquid steel is introduced from the casting ladle, via a shroud, into a manifold vessel or tundish. The induced steel jet flows toward the tundish base, where it strikes the flat base of the tundish or a flow-diversion device, which diverts the jet of liquid toward the bath level surface and extracts kinetic energy through dissipation. In the inlet region, the flow generally returns to the bath level surface, migrates along the latter and is submerged again along the narrow back wall and along the side walls of the trough-shaped tundish. As a result, depending on the shape of the tundish, substantially two oppositely rotating recirculation rolls (upward flow in the longitudinal-center section), which migrate in the direction of the outlet opening, are induced. The jet temperature decreases in the direction of the outlet opening as a result of heat losses via the side walls and the bath level surface, with the temperature loss between the feed location and outlet location being dependent on the throughput.
The foreign substances in the metal melt, which are to be separated out as efficiently as possible, originate firstly from the steelmaking process, and are flushed out of the casting ladle into the tundish when the metal melt is transferred. Secondly, foreign substances are also introduced into the metal melt in the tundish itself. These foreign substances originate from the refractory lining material of the tundish and/or from the liquid steel covering slag which is generally used, and are abraded and suspended firstly through mechanical erosion as a result of wall shear stresses or through chemical erosion resulting from reoxidation processes. Furthermore, inclusions of slag are formed through resuspension on account of high bath level velocities and increased surface turbulence.
Therefore, it is an object of the present invention to avoid the drawbacks which have been outlined and to propose a tundish and a process for producing a metal strand in which the reintroduction of particles into the metal melt within the tundish is minimized and overall the maximum possible separation rate for all the inclusions which are present in the metal melt is achieved, so that a melt which is as pure as possible is fed to the permanent mold.
This object is achieved, in a tundish according to the invention with a refractory lining, by virtue of the fact that a refractory-lined interior space of the tundish, as a function of an operating bath level (h), satisfies the condition that a dimensionless ratio (κ) of the refractory-lined surface area (Aref) which is wetted by the metal melt to the filling volume (V) which is delimited by this refractory-lined surface area and the bath-level-dependent exposed surface area (ATop) and results from the relationship
be between 3.83 and 4.39.
It is preferable for these values for the dimensionless ratio κ to be between 3.83 and 4.2.
The dimensionless ratio K, which defines a volumetric wetting level, demonstrates that the contact surface area between lining and metal melt should be minimized in relation to the quantity of metal melt stored in the tundish. At the same time, however, the fact that a suitable separation surface area for maximum particle separation is required should not be disregarded. Analyses of a very wide range of tundish shapes have revealed that optimum particle separation rates can be achieved with tundish shapes in which the ratio κ is within the claimed range. The range limits indicated result from the geometry of a hemisphere
and the geometry of an upright cylinder, in which the radius of the circular base area is equal to the height of the cylinder (κ=3π1/3≅=4.39).
A high particle separation rate is established if, in addition, the refractory-lined interior space of the tundish, as a function of the operating bath level (h), satisfies the condition that the ratio (ζ) of the exposed surface area (ATop) to the refractory-lined surface area (Aref) wetted by the metal melt be between 0.45 and 1.0. The dimensionless ratio ζ, which places the exposed surface area, which acts as a particle separation surface, in a relationship to the wetted lining surface area, which acts as a particle-generating surface, shows that within the preferred range the contradictory effects balance one another out. An expedient particle separation rate is established with a ratio ζ of between 0.5 and 0.8.
The κ and ζ values determined above do not take account of any additional tundish internal fittings, such as flow diverters, weirs, etc.
To ensure a high particle separation rate, it is expedient for the operating bath level to be between 0.5 m and 1.5 m.
The demand for a high level of particle separation from the metal melt in the tundish is reliably ensured, in the case of sequence casting, even during the ladle change phase if the filling volume of the interior space of the tundish contains at least 5 times, preferably at least 7 times, the quantity of metal melt which is cast each minute in normal operation.
To realize expedient separation rates, the filling volume of the interior space of the tundish is at least 0.75 m3, but preferably at least 1.0 m3. Even these volumes ensure a sufficient residence time for the melt in the tundish at casting rates of from 60 to 100 t of steel per hour. Higher minimum volumes are recommended for higher casting rates.
The possible embodiments of a tundish which are claimed in accordance with the invention combine the following contradictary requirements:
Preferred forms of the tundish result if the refractory-lined interior space of the tundish is substantially formed by a generatrix which rotates about a vertical tundish axis. This produces rotationally symmetrical vessel interior spaces.
The optimum shape, which for a given tundish volume has a maximum surface area for separation of inclusions into the bath-covering slag and, at the same time, forms the smallest possible surface which is wetted by aggressive metal melt for mechanical and chemical erosion, is formed by a hemisphere or a segment of a hemisphere. For the hemisphere segment shape, it is possible to give a generally applicable relationship for the theoretically ideal area ratio of bath level surface area to wetted refractory lining:
where h/R≦1 in which h corresponds to the operating bath level and R corresponds to the bath level radius. If h/R=1, a hemisphere geometry is present and ζ is 0.5. If the h/R ratio is reduced, for example, to 0.6, for the same distributor volume the ratio of the bath level surface area to the lining surface area wetted with liquid steel is increased to ζ=0.73. Therefore, if a sphere segment geometry (h/R<1) is selected for a defined tundish volume, an additional increase in the purifying action is likely.
Further possible embodiments result if the refractory-lined interior space of the tundish is substantially formed by a generatrix which rotates about a vertical tundish axis at a fluctuating, preferably harmonically pulsating distance (r) from the vertical tundish axis. Therefore, cross sections which are elliptical in the direction which is normal with respect to the vertical tundish axis, but also cross sections with any other desired external contour, for example a square cross section with large rounding radii, or polygonal cross sections, are possible.
Suitable forms of tundish result if the tundish, at least in sections, has an interior space which is in the shape of a hemisphere, a truncated cone, a paraboloid of revolution or a cylinder and in this case the cross section of the tundish interior space, in a section plane taken normally to the vertical tundish axis, at least in sections, is circular or elliptical in form.
To allow optimum use to be made of the entire interior space of the tundish for particle separation, there is a submerged pipe which projects into the tundish in order to supply the melt, and a flow diverter is arranged on the tundish base beneath the submerged pipe and the outlet opening is arranged at a location on the tundish base which is spaced apart from the flow diverter by at least half the diameter of the base.
In particular if the tundish according to the invention is to be used to supply a plurality of strands, arranged next to one another, in a continuous casting installation with melt and the melt is therefore to be distributed between a plurality of permanent molds, the tundish comprises a melt feed tank and at least one melt discharge tank, with each melt discharge tank being separated from the melt feed tank by a transfer passage, preferably an overflow, and each melt discharge tank delimiting an interior space of the tundish. This type of tundish, in which the melt flows through two tanks arranged in series, means that the region where the melt is supplied from the casting ladle is separated from the region where the melt is discharged into the permanent mold not only spatially but also structurally, and therefore allows additional continuity to be achieved in the flow characteristics. The connecting region between melt feed tank and melt discharge tank can be produced by an overflow or by a transfer passage, which may also be arranged below the bath level. The geometric conditions described above relating to the configuration of the interior space must be satisfied at least by the melt discharge tank. An additional contribution is made to reducing the amount of foreign substances introduced from the lining of the tundish if the melt feed tank delimits an interior space of the tundish and satisfies the conditions of the dimensionless ratio (κ) and if appropriate also the dimensionless ratio (ζ). The melt feed tank is assigned a flow diverter, and the melt discharge tank is assigned at least one outlet opening.
To allow simple manipulation of the tundish according to the invention, in particular for it to be prepared for casting and positioned accurately above the permanent mold opening, the tundish is supported on a distributor carriage, which preferably has lifting and/or tilting devices, has a movement drive and is designed such that it can be displaced on a movement path between an operating position and a waiting position.
The advantages and effects described are also produced in a process for producing a high-purity metal strand, preferably a steel strand using a continuous casting installation, in which metal melt is passed from a casting ladle into a tundish and from the latter into a continuous-casting permanent mold, a melt volume (V) of a metal melt contained in the refractory-lined interior space of the tundish being set in such a way, as a function of the respective operating bath level, that a dimensionless ratio (κ) of the contact surface area (Aref) formed by the metal melt to the melt volume (V) which is delimited by this contact surface area (Aref) formed by the metal melt and the bath-level-dependent exposed surface area (ATop) and which results from the relationship
is between 3.83 and 4.39. It is preferable for this dimensionless ratio (κ) to be between 3.83 and 4.2.
A high degree of purity in the melt for the subsequent casting process is achieved if, in addition, a melt volume (V) of the metal melt contained in the interior space is set in such a way that the ratio (ζ) of the exposed surface area (ATop) formed by the metal melt to the contact surface area (Aref) formed by the metal melt is between 0.45 and 1.0, preferably between 0.5 and 0.8.
To realize favorable separation rates and therefore a high purity of the cast product, the operating bath level is set to between 0.5 m and 1.5 m. The melt volume which is located in the interior space of the tundish is in this case set to at least 0.75 m3, preferably at least 1.0 m3. The demands imposed with regard to a high level of particle separation are reliably ensured, in the case of sequence casting, even while the casting ladle is being changed, if the melt volume is set to at least 5 times, preferably at least 7 times, the quantity of metal melt which is cast each minute during normal operation.
In this case, the metal melt substantially takes up an interior space formed by a generatrix which rotates about a vertical tundish axis. Alternatively, the metal melt may also take up an interior space which is formed by a generatrix which rotates about a vertical tundish axis at a fluctuating, preferably harmonically pulsating distance (r) from the vertical tundish axis.
The melt is supplied below the metal bath level, in order not to disturb the slag-covered separation surface, and is guided in a defined way to the melt outlet.
The tundish according to the invention may also be operated in short-circuit mode, with the result that in particular the introduction of harmful particles from the tundish lining is kept at a low level. The term short-circuit mode is to be understood as meaning a procedure in which the metal melt which flows out of the casting ladle into the tundish or the interior space of a tundish flows through the latter over a short path and then flows back out of the outlet opening of the tundish or the interior space of the tundish. In this case, a flow profile in which a large proportion of the metal melt flowing in is not subject to any circulating flow within the tundish, but rather experiences only minor flow diversions on its substantially direct path from the melt inlet to the melt outlet, is established. This is achieved, in the method described, by virtue of the fact that the horizontal distance between the jet of metal melt which enters the melt volume substantially vertically and the jet of metal melt which emerges from the melt volume substantially vertically is set to less than half the base diameter of the interior space.
Further advantages and features of the present invention will emerge from the following description of non-restricting exemplary embodiments, in which reference is made to the following figures, in which:
a, 2b show the tundish according to the invention in the form of vertical and horizontal projections in accordance with a first embodiment,
a, 3b show the tundish according to the invention in the form of vertical and horizontal projections in accordance with a second embodiment,
a, 4b show the tundish according to the invention for a two-strand casting installation in the form of vertical and horizontal projections,
As shown in
A filling volume (V) in the interior space 14 of the tundish 1 is filled by the metal melt 17, with the exposed surface area (ATop) of the metal melt forming the bath level 8, which is at operating bath level (h) and is covered by a slag layer 22, into which foreign particles are continuously separated out of the metal melt. In the tundish 1, a partial region of the surface area of the refractory lining 16 is wetted by metal melt 17, and this wetted refractory-lined surface area (Aref) is exposed to particularly high thermal loads and chemical and mechanical erosion. Particles are continuously suspended from the refractory lining 16 into the metal melt 17 and discharged again to the slag layer 22 with the melt flow at the transition to this slag layer 22.
a and 3b show a further embodiment of a possible tundish, in which each cross-sectional area taken normally to the vertical tundish axis 20 is formed by an ellipse, as can be seen from the horizontal projection. The inner contour results in geometric terms from rotation of a generatrix (E) about the vertical tundish axis 20, with the radius distance (r) between the generatrix and the vertical tundish axis varying as a function of the rotation angle (φ). In this case too, the flow diverter 21 and the outlet opening 9 are arranged as far as possible away from one another, in order to create favorable flow conditions in the interior space 14 and to ensure a high particle separation rate.
The tundish may also be formed by a plurality of holding tanks for metal melt.
A flow diverter 21 is incorporated in the base of the refractory lining in the melt feed tank 25. In this case, in a similar manner to that illustrated in
As is already customary for conventional continuous casting installations, the tundish according to the invention, in the same way as has previously been the case for the conventional tundishes, is supported on a distributor carriage 30 in such a manner that its height can be adjusted by means of lifting and/or tilting devices 31 and if appropriate also tiltably, and can be displaced, generally on rails along a movement path 32, between an operating position, in which the submerged casting pipe projects into the permanent mold, and a waiting position, in which the tundish is heated and prepared for its use (
The tundish is usually closed off by a cover in order to substantially avoid cooling of the melt through thermal radiation. If necessary, additional internal fittings in the tundish are possible, with a beneficial effect on the melt flow. The metal melt can also be transferred between the adjacent melt tanks below the bath level of the melts which have been introduced through one or more tubular transfer passages, which has the advantage of the slag layer only being exposed to very minor flow motion.
| Number | Date | Country | Kind |
|---|---|---|---|
| A 1961-2001 | Dec 2001 | AT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP02/13687 | 12/4/2002 | WO |
