TECHNICAL FIELD

The present disclosure generally relates to metallurgy, and in particular it relates to a method and controller for controlling the temperature of molten metal in a continuous casting process.

BACKGROUND

In continuous casting of metallic material, such as steel, solid material is smelted and treated in a furnace. The molten metal obtained from the solid material is tapped or poured into ladles. The molten metal is normally further treated in the ladles. After ladle treatment, the molten metal is sequentially poured from the ladles into a tundish which is a container from which the molten metal is tapped into one or more moulds for casting of the material. The period during which a ladle pours its molten metal into a tundish is herein termed a ladle tapping cycle. The tundish thus acts as a buffer which provides a continuous flow of molten metal into the mould(s) also during ladle change.

The quality of the finished metallic material is dependent of the temperature of the molten metal during the continuous casting process. The optimal ladle treatment temperature of molten metal is normally lower than the

temperature of the molten metal obtained from the furnace. In order to attain an optimal ladle treatment temperature of molten metal, the temperature of the molten metal in the ladle is typically reduced under controlled conditions. If there is a large ladle tonnage combined with a relatively slow casting speed, the temperature of the molten metal in the tundish gradually decreases as molten metal is tapped from the ladle into the tundish. Moreover, the temperature in the molten metal in the tundish is normally not evenly distributed because molten metal flowing from the ladle is hotter than the molten metal already in the tundish which has walls acting as cooling elements lowering the temperature in the vicinity of the walls.

SUMMARY

The inventors have realised that, by controlling the temperature of the molten melt in the tundish, an optimal temperature of the molten metal can be obtained also in the tundish. Moreover, it has also been realised that it is not sufficient to merely control a heating arrangement to heat molten metal in the tundish to obtain a satisfying result.

In view of the above, a general object of the present disclosure is to improve the quality of metallic material produced in a continuous casting process. In particular, it would be desirable to be able to provide solidified metallic material of constant high quality.

Hence, according to a first aspect of the present disclosure there is provided a method of controlling the temperature of molten metal in a tundish during a ladle tapping cycle in a continuous casting process, wherein the method comprises: a) obtaining a measure of a temperature of molten metal in the

tundish, b) comparing the obtained measure of temperature with a desired

tundish melt temperature, c) determining whether the measure of temperature is lower than the desired tundish melt temperature, and in case the measure of temperature is lower than the desired tundish melt temperature, d) controlling the temperature of the molten metal in the tundish by means of a heating arrangement which heats the molten metal in the tundish, and by means of an electromagnetic stirrer which stirs the molten metal in order to distribute heated molten metal in the tundish such that the temperature of the molten metal in the tundish approaches the desired tundish melt temperature.

An effect which may be obtainable thereby is that the cast material, i.e. the metallic material produced in the continuous casting process, can be produced at a constantly higher quality than has previously been possible. In particular, by controlling the temperature of the molten metal in the tundish, the temperature of the molten metal can be kept at a beneficial level throughout the casting process and by the provision of stirring the heated molten metal is evenly distributed in the tundish such that a homogenous or essentially homogenous melt temperature can be obtained in the tundish. The desired tundish melt temperature is an optimal tundish melt or molten metal temperature which is known from for example casting experiments, empirical tests or previous casting experience.

According to one embodiment step a) of obtaining a measure of a

temperature comprises estimating the temperature of the molten metal based on a model of the continuous casting process and on a ladle temperature of molten metal. The ladle temperature is the temperature of the molten metal when it is tapped from the ladle to the tundish.

According to one embodiment the ladle temperature is obtained from a control loop associated with control of molten metal temperature in the ladle.

According to one embodiment step a) of obtaining a measure of a

temperature comprises obtaining measurement values of the temperature of molten metal in the tundish. By stirring the molten metal in the tundish and thereby obtaining a homogeneous temperature, measurements of the temperature of the molten metal become more accurate. Thus, the stirring provides a homogenous melt temperature resulting in a higher quality final product as well as more accurate melt temperature measurements which facilitates the control of the melt temperature in the tundish.

The measure of the molten metal temperature in the tundish can be obtained either by estimation or via direct measurements, for example by means of consumable thermocouples. As an alternative, a combination of melt temperature estimation and direct measurement is also contemplated.

One embodiment comprises repeating steps a) to d) during the ladle tapping cycle. One embodiment comprises, prior to step b) of comparing, obtaining the desired tundish melt temperature.

The method as claimed in any of the preceding claims, wherein the desired tundish melt temperature is an optimal casting process temperature obtained from casting experiments. Thus, data obtained from previously performed casting experiments, empirical tests or casting experience are utilised to set the desired tundish melt temperature.

According to one embodiment the step of obtaining the ladle temperature involves obtaining the ladle temperature from a control loop associated with control of molten metal temperature in the ladle. According to one embodiment the desired tundish melt temperature is an optimal casting process temperature in the tundish.

According to a second aspect of the present disclosure there is provided a computer program comprising computer executable components which causes a controller to perform the method according to the first aspect when the computer-executable components are run on a processing unit included in the controller.

According to a third aspect of the present disclosure there is provided a controller for controlling the temperature of molten metal in a tundish during a ladle tapping cycle in a continuous casting process, wherein the controller comprises: a processing unit, and a memory comprising computer executable components which when run on the processing unit causes the controller to perform the method of the first aspect. The controller may advantageously be utilised in a tundish control system. Thus, according to a fourth aspect of the present disclosure there is provided a tundish control system comprising: a controller according to the third aspect, a heating arrangement, and an electromagnetic stirrer. The controller may thus control the heating arrangement and the stirrer such that the molten metal obtains the desired tundish melt temperature in the tundish.

According to one embodiment the heating arrangement comprises an oxy- fuel burner.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, etc. are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, etc., unless explicitly stated otherwise. BRIEF DESCRIPTION OF THE DRAWINGS

The specific embodiments of the inventive concept will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. l depicts a longitudinal section of a part of a continuous caster;

Fig. 2a is an elevated view of a tundish and electromagnetic stirrers; Fig. 2b is a longitudinal section of a tundish and a heating arrangement;

Fig. 3 is a schematic diagram of a controller for controlling the temperature of molten metal in a tundish during a ladle tapping cycle in a continuous casting process;

Fig. 4 is a schematic view of a tundish control system; Figs 5a-b illustrate molten metal temperature control in a tundish; and Fig. 6 is a flowchart of a method of controlling the temperature of molten metal in a tundish during a ladle tapping cycle in a continuous casting process.

DETAILED DESCRIPTION

The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplifying

embodiments are shown. The inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example, so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like numbers refer to like elements throughout the description.

Fig. l depicts a longitudinal section of an example of continuous caster l. Continuous caster ι comprises a ladle 3, a tundish 5, and moulds 7. The ladle 3 has a pipe 3a through which molten metal M may be tapped into the tundish 5. The exemplified tundish 5 has a lid 5a with an opening 5c through which the pipe 3a of the ladle 3 may discharge molten metal M. The tundish 3 further comprises nozzles 5b, e.g. submerged entry nozzles (SEN), through which molten metal M in the tundish 3 may be discharged to respective moulds 7. The flow direction of the continuous casting process is indicated by arrows A.

Ladle 3 is arranged in a fixed position during a ladle tapping cycle. Upon a ladle change the ladle 3 may be moved in one of the directions B, wherein ladle 3 is exchanged with another ladle filled with molten metal to be discharged into the tundish 5. Molten metal M can thereby continuously be tapped or discharged from the tundish 5 into the moulds 7.

It should be noted that the number of tundish nozzles, whether the tundish has a lid or not and the number of openings in the lid is not important for the purpose of the method presented herein. A method and controller for controlling the molten metal temperature in a tundish, such as tundish 3, during a ladle tapping cycle will now be described with reference to Figs 2-6.

Fig. 2a schematically shows an elevated view of a tundish 5 and

electromagnetic stirrers 9. The electromagnetic stirrers 9 may be moved towards and from the tundish side walls as shown by the arrows. It is envisaged that according to a variation hereof, at least one of the

electromagnetic stirrers could be fixed to a tundish side wall. It is also contemplated that only one electromagnetic stirrer could be used, either attached to a tundish side wall or moveable towards and from the tundish side wall. The electromagnetic stirrer(s) 9 is/are arranged to stir molten metal M in the tundish 5 as will be elaborated herebelow.

Fig. 2b is a longitudinal section of a tundish 5 and a portion of a heating arrangement 11 arranged to heat molten metal in the tundish 5. The heating arrangement 11 can for example comprise a set of one or more oxy-fuel burners to provide heat. According to one variation, the heating arrangement 11 is arranged to indirectly heat molten metal in the tundish 5, i.e. heat is provided to a heat transfer interface such as the lid 5a as shown in Fig. 2b, or to a side wall of the tundish. The heating arrangement 11 may for example be arranged on the lid. The heating arrangement 11 may thus for example be arranged on the inside of the lid or on the outside of the lid. Alternatively, the heating arrangement may be an independent structure, i.e. an arrangement detached from the tundish lid or tundish side wall. According to one variation, the heating arrangement may be arranged to heat the molten metal directly in the tundish, i.e. by providing heat directly to the molten metal without utilisation of a heat transfer interface.

It may be advantageous to heat the molten metal via a heat transfer interface instead of direct heating of the molten metal. If for example the heating arrangement comprises oxy-fuel burners, heating of a heat transfer interface instead of direct heating of the molten metal eliminates oxide contamination of molten metal in the tundish. Fig. 3 shows a schematic block diagram of a controller 13 for controlling the temperature of molten metal in a tundish during a ladle tapping cycle. The controller 13, which for example may be a programmable logic controller (PLC), comprises an input/output (I/O) unit 13a, a processing unit 13b and a memory 13c. The I/O unit 13a is arranged to communicate with an

electromagnetic stirrer 9 and a heating arrangement 11. According to one variation the I/O unit 13a may be arranged to receive a measure of the temperature of molten metal in a tundish in the form of process variable values obtained by one or more heat sensing means. The processing unit 13b is arranged to communicate with the I/O unit 13a and with the memory 13b. The memory 13c comprises computer-executable components which can be loaded into the processing unit 13b. When the computer-executable components are run on the processing unit 13b the controller 13 controls the temperature of molten metal in a tundish according to the method presented herein.

Fig. 4 shows a tundish control system 15 comprising a controller 13, a heating arrangement 11 and an electromagnetic stirrer 9. According to the example shown in Fig. 4, the heating arrangement 11 and the electromagnetic stirrer 9 are in position to influence the temperature of molten metal M in tundish 5 via the controller 13.

The operation of the tundish control system 15 will now be described with reference being made to Figs 5a-c and Fig. 6.

In Fig. 5a molten metal M having a ladle temperature Ti is discharged from the ladle 3 into the tundish 5. In a step a) the processing unit 13b of the controller 13 obtains a measure of a temperature T2 of molten metal M in the tundish 5. The obtained measure of temperature may for example be a measure of an average temperature of the molten metal M in the tundish 5. The measure of the temperature T2 can according to one variation of the method be obtained by estimating the temperature T2 of the molten metal M in the tundish 5. The estimation can for example be based on a model of the continuous casting process and on the ladle temperature Ti of molten metal M. By means of the model, approximate temperature values of molten metal at various points in time of a ladle tipping cycle may be obtained with the ladle temperature Ti as initial condition. The ladle temperature can for example be obtained from a control loop associated with control of molten metal temperature in the ladle 3. Alternatively, step a) may comprise obtaining measurement values of the temperature T2 of molten metal in the tundish. Temperature measurements may in this case for example be obtained from several locations in the molten metal to obtain a mean molten metal temperature in the tundish. After stirring of the molten metal, the temperature of molten metal in the tundish is more evenly distributed, facilitating temperature measurements of the molten metal.

In a step b) the obtained measure of temperature T2 is compared with a desired tundish melt temperature by the processing unit 13b. As earlier mentioned the desired tundish melt temperature can be determined in casting experiments, empirical tests or through extensive casting experience. The desired tundish melt temperature depends, among other things, on the desired quality of the casted product. In a step c) it is determined whether the measure of temperature T2 is lower than the desired tundish melt

temperature. According to one variation, the desired tundish melt temperature is obtained prior to step b). The desired tundish melt temperature may be higher than the ladle temperature, essentially equal to the ladle temperature, or lower than the ladle temperature, depending on the desired final product quality. If the desired tundish temperature is lower than the ladle temperature, then stirring may be performed without utilising the heating arrangement until the desired tundish melt temperature has been obtained. At that time, the heating arrangement may be controlled to heat the molten metal in the tundish to obtain the desired tundish melt temperature.

If the measure of temperature T2 is lower than the desired tundish melt temperature the controller 13 controls the temperature of the molten metal M in the tundish 5 in a step d) by means of a heating arrangement 11 which heats the molten metal in the tundish, and by means of electromagnetic stirrer 9 which stirs the molten metal in the tundish 5. As mentioned earlier, the heating arrangement may for example comprise oxy-fuel burners. This is depicted in Fig. 5b where oxy-fuel burners provide heat via flames F. By stirring, the heated molten metal is distributed in the tundish 5 such that the temperature of the molten metal M in the tundish 5 approaches the desired tundish melt temperature T3, as shown in Fig. 5b.

The present disclosure hence provides a method, a controller and tundish control system adapted for molten metal temperature control in a tundish in a continuous casting process for the production of for example billets, blooms or slabs of steel, aluminium or copper.

The inventive concept has mainly been described above with reference to a few examples. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.