Patent Application
20060248706
The invention relates to a process for producing a cast metal strip using two casting rolls and two side plates, which together form a melt space and a casting gap, metal melt being fed into the melt space and in the melt space forming a melt bath with a bath surface which is open at the top, and a cast metal strip being delivered out of the melt space through the casting gap, and a delimited surface region for the collection of particles which are foreign to the melt being formed on the bath surface under the action of at least one gas jet, and to a two-roll casting device used for this process.
The invention preferably relates to a casting process for producing a continuously cast steel strip with a strip thickness of between 0.5 mm and 10 mm using a two-roll casting installation, with the cast steel strip being removed substantially vertically downward.
A two-roll casting device with a vertically delivered metal strip is generally known and comprises, as is diagrammatically illustrated in
With a continuous supply of metal melt into the melt bath through the submerged casting nozzle, which causes movement in the melt bath, nonmetallic particles which are foreign to the melt are entrained. These particles float to the surface of the bath, where they agglomerate, together with particles which are foreign to the melt and were generated in the mold melt bath by chemical reaction with refractory material or by reoxidation, and are incorporated in the strand shells predominantly at the contact line with the casting rolls directly at the lateral surface of the casting rolls, forming inclusions and seeds for macrocracks and microcracks at the surface and in the region close to the surface of the cast metal strip.
A two-roll casting installation and a casting process for casting a metal melt in accordance with the prior art described is known, for example, from JP-A 2001-314946, WO 02/083343 and JP-A 2-207946.
To keep particles which are foreign to the melt away from the contact line between the casting-roll surface and the bath level surface, it is proposed in JP-A 2001-314946 that gas jets be applied in the region of this contact line, causing the particles which are foreign to the melt to drift away toward the center of the melt pool. The gas jets cover part of the casting roll surface and an edge region of the bath level surface, but bath fluctuations and temperature fluctuations which influence the strand shell growth occur at the casting roll surface in a sensitive area depending on the intensity and temperature of the gas jets. Unfortunately, substantially uniform starting conditions for the formation of the strand shells in this region are particularly important for the end product.
According to WO 02/083343, drifting of particles which are foreign to the melt and have been entrained into the melt bath toward the contact line between the metal bath and the lateral surfaces of the casting rolls is avoided during casting operation by means of shields which are obliquely immersed in the metal bath and the lower edges of which are positioned below the level of the outlet openings of the submerged casting nozzle. The intention of this is to additionally create a melt pool in the melt space, in which the nonmetallic particles can be separated off. The metal strip which is produced continuously using the two-roll casting device is wound into coils, and at the end of the winding operation of each individual coil, the shields are removed from the metal bath and the particles which have been separated out at the surface of the bath are blown toward at least one of the casting roll surfaces using gas nozzles and in this way discharged together with a short piece of the metal strip. The main drawback of this process is that each cast coil produces a piece of scrap, which interrupts the continuous production process and increases the scrap rate of production. Furthermore, metal melt accumulates on the shields and solidifies each time the shield is raised. If the shield consists of refractory material, eroded particles of the refractory material are additionally introduced into the melt, or chemical reactions occur between the liquid steel and the refractory material, which produce additional impurities.
JP-A 2-207946 has disclosed a two-roll casting device in which the foreign particles floating on the bath surface are removed by being continuously scooped out using rotating cup mechanisms. Since these devices at the bath surface have to work at the melting point of the metal, there is likely to be a high number of operating faults in these mechanical devices. In addition, in the case of a steel bath, the bath surface has to be protected from contact with atmospheric oxygen, and consequently it is not feasible to use scoop devices of this type under these conditions.
Therefore, it is an object of the present invention to avoid the drawbacks of the prior art described and to propose a process for producing a cast metal strip and a two-roll casting device, in which the introduction of particles which are foreign to the melt at or into the surface or into the region close to the surface of the cast strip is substantially avoided, a contact line between the bath surface and the casting roll lateral surface which is substantially free of disruption and is delimited from the formation of any waves at the bath surface is achieved and at the same time contact of oxygen with the bath surface is as far as possible avoided.
Working on the basis of a process of the type described in the introduction, this object is achieved by virtue of the fact that the at least one gas jet is directed on to the bath surface with the gas jet axis at a distance from the contact line between the bath surface and the casting roll.
In this case, the at least one gas jet is shaped in such a way that no gaps through which particles which are foreign to the melt can escape remain along the delimited surface region. In general, the delimited surface region may be formed by a gas jet which forms a closed ring with any desired outer contour or by a plurality of successive gas jets. At the same time, in particular in the case of metal melts which have a high tendency to oxidation, such as steel, an inert or reducing shielding gas atmosphere is produced and maintained above the metal bath and within a melt space which is optimally closed off with respect to the ingress of external air, which virtually rules out reoxidation of the metal melt.
The at least one gas jet is directed directly on to the bath surface. This produces a calm edge strip, which remains substantially unaffected by the formation of waves at the bath surface, between the region of contact between the gas jet and the bath surface and the casting rolls and/or side plates which delimit the melt space. This measure greatly assists with a constant, uniform and undisturbed formation of strand shells at the lateral surfaces of the casting rolls which rotate in accordance with the casting speed, if the casting roll surfaces also run and function in an optimally stable and homogenously uniform way.
In this context, it is particularly expedient if the at least one gas jet is directed on to the bath surface at an angle from 25° to 145°, preferably at an angle of from 35° to 90°, based on a horizontal plane. In this case, the bath surface substantially corresponds to this horizontal plane.
Each gas jet is assigned a gas jet axis. Preferably, the at least one gas jet is directed on to the bath surface with the gas jet axis at a distance from the contact line between the bath surface and the casting roll and/or from the contact line between the bath surface and the side plate. This distance is preferably constant and in a range between 10 mm and 50 mm, measured on the bath surface.
Since the side plates, unlike the rotating casting rolls, are substantially stationary, the at least one gas jet can be directed on to the side plate surface at a distance from the contact line between the bath surface and the side plate, and at least a part-stream of the gas jet is effectively diverted on to the bath surface.
The gas jet or gas jets are preferably in the form of fan jets and emerge from a correspondingly shaped nozzle. It is expedient for a multiplicity of nozzles to be arranged in succession, so as to produce a continuous narrow gas jet, similar to that used in a gas meter.
To form a delimited surface region of any desired shape on the bath surface, the at least one gas jet is in the form of a partially curved fan jet.
Once it emerges from the gas jet nozzle, the gas jet diverges with an opening angle of between 10° and 35° in the direction of flow. For the uniform and stable formation of a strand shell, it is necessary for all of the diverging gas jet to strike the bath surface, rather than being partially directed on to the lateral surface of the casting roll. At the side plates, which may execute an oscillating movement, direct contact between the gas jet and the side plate is perfectly permissible, since the disadvantageous effects encountered at the lateral surfaces of the casting rolls do not occur here.
According to a preferred embodiment, between the two side plates, if appropriate leaving clear a distance with respect to the side plates, the at least one gas jet acts on the bath surface parallel or obliquely, without interruption, to the contact line between the bath surface and the casting roll. This ensures that the casting roll surface is continuously shielded from contact with particles which are foreign to the melt. Continuous discharge of the particles toward the side plates and therefore into the edge zone of the cast metal strip is possible and also desirable, since the cast metal strip, at least before it is wound in a downstream coiler, passes through a trimming station, which is not necessarily arranged within the actual two-roll casting installation, and therefore a controlled increase in the level of nonmetallic inclusions in this region does not cause any additional scrap material. Arranging the gas jet so as to run obliquely with respect to the contact line between the bath surface and the casting roll additionally promotes continuous discharge of particles which are foreign to the melt toward the side plates. Furthermore, leaving clear a distance with respect to the side plates avoids local cooling of a spatially restricted zone at the side plates by the gas jets.
Equally, between the two casting rolls, if appropriate leaving clear a distance with respect to the casting rolls, the at least one gas jet acts on the bath surface parallel, without interruption, to the contact line between the bath surface and the side plate. As a result, if no increase in particles foreign to the melt is desired even at the edges of the metal strip while casting operation is ongoing, suitable shielding is achieved. Leaving clear a distance with respect to the casting rolls avoids local cooling on the casting roll lateral surface along a circumferential strip and therefore different levels of strand shell growth along the contact line between the casting roll lateral surface and the bath surface.
A further improvement to the restricting of the particles foreign to the melt is achieved if at least in sections at least two gas jets act on the bath surface at a distance from one another. This measure improves the surface quality of the strip in particular along the contact line between the casting roll lateral surface and the bath surface. It is preferable for the two gas jets to be arranged equidistantly with respect to one another.
Components of the two-roll casting device which form the melt space or are arranged directly within it can be included when forming the delimited surface region with gas jets. In this case, the delimited surface region is formed in sections by at least one gas jet and in sections by sections of the side plates or the casting rolls or a submerged casting nozzle or other internal fittings.
It is preferable for the at least one gas jet which strikes the metal bath at an angle to form a gap-free bow wave, i.e. a swell at the bath surface which extends parallel to the direction of extent of a fan jet and encloses the delimited surface region at least in sections. The bow wave may be continuous and in this way form this delimited surface region, or may form a delimited surface region in combination with components of the two-roll casting device, such as sections of the side plates or of the casting rolls or of a submerged casting nozzle or of other internal fittings.
The bow wave formed by the gas jets is held substantially constant at a height of from 0.05 mm to 10 mm, preferably from 0.1 mm to 3 mm, above the normal level of the bath surface. This creates a collection tank for the particles which are foreign to the melt, and the particles are held there until they are discharged in a controlled way or until casting ends automatically.
An inert or reducing gas is used to form the gas jet, to ensure that there is no reoxidation of the metal melt at the bath surface in this region. Preferred gases which can be used include argon, nitrogen, N+H2 or mixtures of at least two of these gases.
In the starting phase of a casting process, the process according to the invention should only be deployed when an operating bath level has been reached and therefore the metal melt has been substantially stabilized and calmed in the melt space and in particular at the bath surface. Therefore, during the starting phase of the casting process, the action of at least one gas jet on the bath surface is expediently only switched on 10 sec to 2 min after the introduction of melt into the melt space has commenced (start of casting).
Over a prolonged casting period, particles which are foreign to the melt accumulate within the delimited surface region and have to be removed at least at periodic intervals. This is preferably done during interruptions to production for operation reasons, during which the melt space itself is completely emptied and then the installation is restarted and casting recommenced. If these time intervals are too long, the action of at least one gas jet on the bath surface is interrupted in sections in a time interval in order for accumulated particles which are foreign to the melt to be discharged from a delimited surface region. This is achieved by the action of at least one gas jet on the bath surface being interrupted either along the contact line between the bath surface and at least one of the two casting rolls or along the contact line between the bath surface and at least one of the two side plates, and preferably along the contact line between the bath surface and both side plates. The discharge of particles which are foreign to the melt toward the side walls and therefore into the edge region of the cast metal strip avoids the formation of inclusions close to the surface at the wide sides of the metal strip, and this edge strip with increased levels of inclusions is removed during the trimming of the strip, which takes place within a subsequent process step. The discharging of particles which are foreign to the melt via the contact surface between the casting rolls and the metal melt in the melt space expediently takes place in a time interval immediately after the coil weight of the cast metal strip has been reached.
The invention also proposes a two-roll casting device for producing a cast metal strip of the generic type described in the introduction, having two casting rolls driven in rotation and side plates, which bear against the end sides of the casting rolls, these casting rolls and side plates together forming a melt space for receiving a melt bath with a bath surface, and a casting gap. At least one gas jet nozzle with an outlet opening for a directed gas jet is arranged in the melt space or directed or projecting into the melt space, in such a way that a delimited surface region for collection of particles which are foreign to the melt is formed on the bath surface. A two-roll casting device formed in this way is characterized in that the outlet opening of the gas jet nozzle is directed directly on to the bath surface at a distance from the contact line between the bath surface and the casting roll.
At a distance above the bath surface, the melt space is protected from the ingress of air by a covering hood. The covering hood bears against the side plates and the casting rolls with a contact surface or a seal, or in particular is set at a narrow gap from the casting rolls, in which case shielding gas which is introduced into the melt space escapes through these gaps and in this way prevents external air from entering this melt space. At least the outlet openings of the gas jet nozzles project through the covering hood into the melt space and are preferably secured to the covering hood and oriented.
In general, the orientation of the outlet opening of the gas jet nozzles determines the direction of the emerging gas jet. To this extent, the orientation of the nozzle axis in the outlet cross section of the gas jet nozzle corresponds to the orientation of the gas jet axis of the gas jet in the cross section of the outlet opening. Since the outlet openings of the gas jet nozzle and therefore the defined nozzle axis in the outlet opening of the gas jet nozzle are directed directly on to the bath surface, the drifting of particles which are foreign to the melt into undesirable zones of the bath surface is avoided. Favorable conditions for this are achieved if the distance between the gas jet axis directed on to the bath surface and the contact line between the bath surface and the casting roll is in a range from 10 mm to 50 mm, measured on the bath surface. Favorable conditions likewise result if the outlet opening of the gas jet nozzle or the nozzle axis, in the outlet cross section of the outlet opening, is directed toward the bath surface at an angle of from 25° to 145°, preferably at an angle of from 35° to 90°, based on a horizontal plane. The bath surface in this case forms the horizontal plane.
To produce a very narrow but elongate gas jet, the gas jet nozzle is configured as a fan jet nozzle or slot nozzle with a slot-shaped outlet opening. Arranging a plurality of gas jet nozzles of this type in succession allows a delimited region of any desired shape to be enclosed on the bath surface using gas jets.
It is expedient for the outlet opening of the gas jet nozzle to be directed directly on to the bath surface at a distance from the contact line between the bath surface and the side plate.
A beneficial effect is produced if, between the two side plates, if appropriate leaving clear a distance with respect to the side plates, the outlet opening of the gas jet nozzle is directed on to the bath surface parallel to the contact line between the bath surface and the casting roll.
Excessive local cooling at the side plates under the action of a continuous gas jet is avoided if, between the two casting rolls, if appropriate leaving clear a distance with respect to the casting rolls, the outlet opening of the gas jet nozzle is directed on to the bath surface parallel to the contact line between the bath surface and the side plate. Excessive local cooling at the casting roll surface is avoided if, between the two casting rolls, if appropriate leaving clear a distance with respect to the casting rolls, the outlet opening of the gas jet nozzle is directed on to the bath surface parallel to the contact line between the bath surface and the side plate.
Improved shielding with respect to the particles which are foreign to the melt is achieved if a gas jet nozzle is equipped with two, substantially equidistant, outlet openings for targeted gas jets, or two gas jet nozzles each having one outlet opening are provided, in which case the outlet openings are arranged in such a way that a double-delimited surface region for the collection of particles which are foreign to the melt is formed on the bath surface.
A continuous, delimited region for the collection of particles which are foreign to the melt is achieved if the outlet openings of at least one gas jet nozzle are directed on to the bath surface in such a way that, under the action of gas jets, they form a delimited surface region on the bath surface. However, this is also possible if the outlet openings of at least one gas jet nozzle are directed on to the bath surface in such a way that, together with sections of the casting rolls or of the side plates or of other internals in the melt bath, and under the action of gas jets in sequence, they form a delimited surface region on the bath surface.
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 appended figures, in which:
The basic structure of a two-roll casting device has already been described in the summary of the prior art with reference to
In particular for stainless steel grades, particularly high demands are imposed on the surface quality of the strips produced, since even minor inclusions of foreign substances, such as slags, metal oxides and the like, at the surface or in the region close to the surface form seed cells for microcracks and macrocracks, with noticeable adverse consequences for the surface condition.
The principle on which the process according to the invention is based is illustrated in
Gas jet nozzles 16 are arranged at a distance from the bath surface 8, with their outlet openings 17 or their nozzle axes 18 in the outlet cross section of the outlet opening 17 directed obliquely toward the bath surface 8. The gas jets 20 which emerge with the gas jet axes 21 produce a bow wave 24 of a certain height on the bath surface 8, this height also being determined to a significant extent by the flow velocity of the gas jets and the pressure with which they strike the bath surface. Particles which are foreign to the melt and float on the melt bath accumulate between opposite bow waves 24 or within the surface region 30 which delimited by a bow wave. The gas jet nozzles 16 are connected to supply lines 26, through which they are supplied with an inert or reducing gas. A multiplicity of gas jet nozzles are connected to the supply lines, which preferably form a circular pipeline.
In
A multiplicity of gas jets which are generated by gas jet nozzles arranged in a row produce a delimited surface region on the bath surface, within which surface region the particles which are foreign to the melt are accumulated.
One possible embodiment for the incorporation of gas jet nozzles into the covering hood 9 which shields the melt bath from the ingress of external air is illustrated in
A further advantageous embodiment for forming a delimited surface region 25 is illustrated in
However, the invention is not restricted to the embodiments illustrated and described, but rather can be modified in numerous ways. It is also possible for gas jets which follow one another and form a delimited surface region, as well as the associated gas jet nozzles, to be arranged in such a way that the gas jets are directed directly toward the bath surface in one peripheral section of the delimited surface region and are directed on to the casting roll surface or the side plates in a further section.
| Number | Date | Country | Kind |
|---|---|---|---|
| A 772/2003 | May 2003 | AT | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP04/04947 | 5/19/2004 | WO | 11/18/2005 |
