Description

The invention concerns a stopper rod and a method for inducing a rotational flow of a molten metal.

In continuous casting of molten metal, in particular molten steel in a continuous casting plant, molten metal is provided in a vessel, in particular in a vessel in the form of a ladle or a tundish.

An outlet is provided in the bottom of the vessel in which the molten metal is provided, through which the molten metal in the vessel can be casted into a downstream mould located under the vessel.

In the bottom of a tundish such an outlet is provided in the form of a tundish nozzle. Such a tundish nozzle can be provided in the form of a submerged entry nozzle (SEN) or a submerged entry shroud (SES). Molten metal from the tundish can be casted through the tundish nozzle into the mould. Stopper rods (also called stoppers) are provided to control the amount of molten metal flowing through the outlet, in particular through a tundish nozzle.

These stopper rods have a rod-shaped stopper body which is vertically aligned above the outlet, e.g., above the tundish nozzle. At its upper end, a metal rod is attached to the stopper rod, whereby the metal rod is in turn connected to a lifting device (which is part of the stopper rod control system) via which the stopper rod can be lifted and lowered vertically. At its lower end, the stopper rod has a nose, also known as the “stopper nose”. By lowering the stopper rod, the nose can be guided against the outlet in such a way that the outlet can be completely closed by the nose and no more molten metal can flow through the outlet. Furthermore, the stopper rod can be lifted vertically so that it opens the outlet in a controlled way and molten metal can flow through the outlet. Accordingly, the flow rate of molten metal through the outlet, e.g., the tundish nozzle, can be controlled by means of the stopper rod.

During casting, non-metallic particles present in the molten metal may be deposited on the refractory material, in particular on the stopper rod, the outlet or the immersion nozzle downstream of a tundish nozzle. More specifically, these particles can be alumina particles present in the molten metal. This deposition of non-metallic material on the refractory material is also known as “clogging” and detrimentally impacts its performance. In order to suppress clogging, it is known that an inert gas, especially argon or nitrogen, can be introduced into the molten metal in the area of the nose of the stopper rod.

For example, generic stopper rods with a gas outlet in the nose area are described in EP 2 067 549 A1 , EP 2 189 231 A1 , EP 2 233 227 A1 or EP 3 705 204 A1 , EP 1 736260 A1 , JP H03 110048 A, EP 3 903 963 A1 , EP 3 939 717 A1 , KR 2014 0082497 A, CN 113 547 112 A, EP 3 928 891 A1 , CN 112 743 070 A, KR 101 794 598 B1.

Apart from suppressing clogging, a further objective in continuous casting is to achieve high quality steel without non-metallic inclusions and defects. It has been shown that inducing a rotational movement (swirling motion or swirling flow) of the molten metal inside e.g., a tundish nozzle can improve quality of the cast steel. Such rotational flow is regularly achieved by means of a magnetic field (see e.g., EP 3 597 328 A1 ). However, this approach requires a high investment in such apparatuses, further requires constant maintenance and consumes significant amounts of energy during operation. Other attempts are known to achieve a rotational flow by means of a spiral structure on a stopper rod (see e.g., KR101499201 B1). A device acting as a swirl generator which needs to be positioned in the inlet region of a nozzle is disclosed in EP 1 106 286 A1 . The inventors have realized that, in order to achieve a proper rotational flow induced by a stopper rod, at least one channel, such as a spiral I helical shaped channel, must be present on the surface of a stopper rod in the area of the stopper rod nose and that there must be a gas injection directly into the spiral shaped channel.

Therefore, it is an object of the invention to provide a stopper rod and a method for inducing a rotational flow of a molten metal.

The object is achieved by a stopper rod according to claim 1 and by a method according to claim 13.

The core idea of the invention is based on the finding, that a gas injection through at least one channel on an exterior surface of a stopper rod that is at least partially provided on a nose of a stopper rod, which runs at least partially around the central longitudinal axis and progresses along the direction of the longitudinal axis (that is, the channel is oblique to the longitudinal direction), induces a rotational flow in the molten metal flowing inside a tundish nozzle.

In a first aspect of the invention the object is achieved by a stopper rod for controlling the flow of molten metal and for inducing a rotational flow of molten metal, said stopper rod comprising: a rod-shaped stopper body, said rod-shaped stopper body extending along a central longitudinal axis (L) from a first end to a second end, said rod-shaped stopper body comprising a nose at said second end, wherein said nose provides an exterior surface; a chamber, said chamber extending along said central longitudinal axis (L) within said stopper body from said first end towards said second end and ending at a distance from said second end; at least one channel (i.e. , one or more channels), preferably a plurality of channels (i.e. , more than one channel I at least two channels),

(each of said at least one channels I each of said one or more channels) being provided on an exterior surface of said rod-shaped stopper body and at least partially on said exterior surface of said nose, and

(each of said at least one channels I each of said one or more channels) at least partially running around said central longitudinal axis (L), and

(each of said at least one channels I each of said one or more channels) progressing along the direction of said central longitudinal axis (L); gas supply connecting means, said gas supply connecting means connecting said chamber and each of said at least one channel (in other words: each of said one or more channels).

An essential element of such a stopper rod for controlling the flow of molten metal and for inducing a rotational flow of molten metal is the at least one (that is one or more) channel provided on an exterior surface of said rod-shaped stopper body and at least partially on said exterior surface of said nose, and at least partially running around said central longitudinal axis (L), and progressing along the direction of said central longitudinal axis (L), e.g., on a helical path, or along an otherwise tilted I oblique path. The at least one channel is provided on an exterior surface of said rod-shaped stopper body and at least partially on said exterior surface of said nose, that means (each of) said at least one channel has a portion aligned on said exterior surface of said nose, while the rest of each respective channel may align on the exterior surface of the stopper body (in a region adjacent to the nose). Preferably a plurality of channels is provided. It is particularly preferable to provide an even number of channels wherein pairs of channels are arranged opposed to each other in respect to the longitudinal axis. From here on the number of channels may also be referred to as channels, which is not restricted to a plurality of channels. For avoidance of doubt, a number of channels includes any number of channels such as one or more channels.

The respective gas supply connecting means are used to induce gas from the (central) chamber of the stopper rod into each of said at least one channel. The gas supply connecting means can be of direct permeability, such as e.g., a distinct hole I distinct holes or a pipe I pipes leading through said rod-shapes stopper body. Alternatively, they can also be of indirect permeability, such as a porous insert, e.g., in the form of a rotationally symmetric insert, e.g., a cylindrical insert. The gas supply connecting means can also be a mixture of direct and indirect permeability. Each of the said gas supply connecting means may comprise a plurality of direct permeability elements such as a plurality of distinct holes or pipes, preferably the said direct permeability elements feature a maximum dimension (such as a radius or a width) in the region where said direct permeability elements lead into the respective channels, which is below 0,2 mm, preferably below 0,1 mm.

Since said at least one channel is at least partially located on said exterior surface of said nose, a gas can be introduced into the molten metal at or near said nose. Since the said at least one channel is at least partially running around said central longitudinal axis (L) and progressing along the direction of the longitudinal axis (L) it was observed by the inventors, that a gas is initially accelerated within the channels into an oblique direction with respect to said central longitudinal axis (L) of the stopper rod. When the gas exits said at least one channel into the molten metal, this oblique gas entry induces a rotational flow of the molten metal near and below said nose.

In an outward direction of said at least one channel, i.e. , on the side of said at least one channel facing away from the stopper body, said at least one channel is preferably completely open. This has the advantage that the gas inside said at least one channel already induces a rotational movement to the molten metal along the whole length of said at least one channel.

Said at least one channel may be bordered by walls (preferably except on the side of the channel facing away from the stopper body, optionally sections of the channel might also be bordered partially on the side facing away from the stopper body). This has the advantage that gas can be guided and directed into a certain direction as the gas will be guided by the walls. Said at least one channel preferably comprise a first channel wall, limiting the channel in a direction towards said first end, and may preferably comprise a second channel wall limiting the channel in a direction towards said second end. Said at least one channel preferably comprises a first channel wall, limiting the channel in a direction towards said first end, and may not comprise a second channel wall limiting the channel in a direction towards said second end. This allows guiding and directing the gas into a certain direction (as defined by this first I upper wall) and further inducing the gas in a more uniform way into the molten metal. Therefore, according to a preferred embodiment it is provided that each of said at least one channel comprises a first channel wall, limiting the channel in a direction towards said first end, wherein said first channel wall and said exterior surface of said nose form a first edge, and wherein said first edge has the shape of a sharp edge, preferably said first edge has a radius not above 2 mm, preferably not above 1 mm. This further improves guiding and directing the gas into a certain direction as more gas is retained inside the channel. Said at least one channel may comprise a channel bottom which may be aligned to said first channel wall, and which may be a linear area, or a curved area or a line. The channel cross-section may be of different geometrical forms, such as preferably a V-shaped cross-section, or a II- shaped cross-section. Preferably each of said gas supply connecting means leads into each of said at least one channel at said bottom of said channel.

Preferably (each of) said at least one channel defines a channel direction, wherein (each of) said channel directions are aligned at an angle (c to the said central longitudinal axis (L). The angle (c is defined as the smaller angle between said channel direction and said central longitudinal axis (L), in case of skew lines, the smaller angle between any two intersecting lines parallel to said channel direction and said central longitudinal axis (L).

Preferably (each of) said angle(s) (c is (are) in the range between 30° to 60°, preferably in the range of 40° to 50°. The inventors have realized that this range allows inducing a rotational flow in a stable way, e.g., with low fluctuations.

Outside these ranges the induced rotational flow was drastically reduced. Preferably all of said angles (c are the same, which results in a homogenous flow of the molten metal. Preferably all of said plurality of channels are aligned such that they induce a rotational flow into a same rotational flow direction. Said induced rotational flow direction can either be a clockwise direction or an anticlockwise direction, as seen along said central longitudinal axis (L) from said first end to said second end.

It is preferred to use a stopper rod according to the invention comprising said at least one channel, whereas (each of) said at least one channel induces a rotational flow into a rotational flow direction, whereas preferably the rotational flow direction is a clockwise direction or an anti-clockwise direction, as seen along said central longitudinal axis (L) from said first end to said second end. Preferably a clockwise direction is used for applications stationed at the norther hemisphere while an anti-clockwise direction is used for applications stationed at the southern hemisphere.

Preferably (each of) said at least one channel runs along a helical or spiral path on said exterior surface of said nose. This has shown to achieve the best acceleration of the gas guided inside the channels and has improved the rotational flow of the molten metal.

In the area of the first end, which forms the upper end of the stopper body in the functional position of the stopper rod, i.e., with the central longitudinal axis aligned vertically, means may be provided on the stopper body by which the stopper body can be attached to a device for lifting and lowering the stopper rod vertically. These means may be designed according to the state of the art. For example, fasteners with an internal thread into which a metal rod with an external thread can be screwed may be provided. This metal rod in turn can interact with a lifting device in such a way that the stopper rod can be lifted and lowered via the metal rod.

In the area of its second end, being opposite the first end and being the lower end of the stopper body in the functional position of the stopper rod, the exterior surface (i.e. , the outer contour) of the stopper body has the shape of a nose or stopper nose, as known from the state of the art. Preferably, the exterior surface of the nose is rotationally symmetrical in relation to the longitudinal axis. Preferably the stopper nose is to be understood as the shape that resembles a rounded cone or that is dome-shaped, while the rest of the stopper body may be of a circular cylindrical outer contour rotationally symmetrical to the central longitudinal axis L.

The exterior surface of the nose preferably expands from the second end into the direction towards the first end. According to a preferred embodiment, the exterior surface of the nose expands from the second end in the direction towards the first end conically or is formed as a cone. According to a particularly preferred design, the exterior surface of the nose is dome-shaped.

As known from the state of the art, the stopper rod comprises a chamber which extends along the central longitudinal axis into said stopper body from the first end towards said second end and ends in the stopper body at a distance from the second end. This chamber may preferably be rotationally symmetrical in relation to the central longitudinal axis and, for example, have a circular-cylindrical shape. The stopper rod according to the invention comprises gas supply connecting means leading from said chamber through said rod-shaped stopper body into said at least one channel. Thus, gas introduced into the chamber, in particular inert gas such as argon or nitrogen, can be passed through the gas supply connecting means into said at least one channel. To supply gas to the chamber, the chamber can be connected to a gas supply system. This gas supply system can be provided, as known from the state of the art, to supply gas especially in the area of the first end of the stopper body.

The gas supply connecting means are designed in such a way that gas can be passed from the chamber through the stopper body into said at least one channel.

Preferably (each of) said at least one channel is designed such that a gas supplied by a gas supply system via said chamber through said gas supply connecting means to said at least one channel of said stopper rod exits said at least one channel into the molten metal at an oblique angle with respect to said central longitudinal axis, such that a rotational flow of the molten metal is induced.

Generally, a rotational flow can be induced into a clockwise or anti-clockwise direction, when viewed from said first end to said second end.

Generally rotational flow may be measured by the circulation C, which has the unit of m ² /s. The circulation C is defined as the line integral over a closed loop of the component of the velocity vector V that is locally tangent to the contour (that is the line segment dl), so that C = $V ■ dl. Preferably the rotational flow Rot(t) is defined by the circulation (Rot(t)=C). Preferably the target for the rotational flow is defined by a target for the circulation (Roto=Co). Preferably the rotational flow is defined by the swirl number. Preferably the target for the rotational flow is defined by a target for the swirl number.

Gas supply elements, such as gas supply connecting means or channels, may either be of direct permeability or of indirect permeability depending on its properties in terms of letting gas flow through the element. A direct permeability element allows for a gas to flow through a discrete conduit (such as e.g., a hole or a pipe), while an indirect permeability element allows for a gas to pass a permeable structure, e.g., in the sense of a porous structure. Such a porous structure or element may, for example, have a porosity known from porous purging plugs for gas purging of molten metal in ladles.

Preferably said stopper rod comprises auxiliary gas supply connecting means, said auxiliary gas supply connecting means leading from said chamber and through said rod-shaped stopper body into an auxiliary channel, said auxiliary channel being provided on said exterior surface of said nose, said auxiliary channel being designed to introduce additional gas into the molten metal, without increasing the rotational induced flow. Preferably said auxiliary channel is ring- shaped or otherwise rotationally symmetric around said central longitudinal axis (L). Preferably said auxiliary gas supply connecting means are of direct permeability. Preferably said auxiliary channel is arranged on said exterior surface of said nose with a distance from said at least one channel in a direction towards said second end. In other words, said auxiliary channel is below (as in a use position) of said at least one channel. This allows additional gas introduction into the molten metal, without increasing the rotational induced flow by said at least one channel any further.

According to a particularly preferred embodiment, said auxiliary channel, especially if it is ring-shaped, is rotationally symmetrical in relation to said central longitudinal axis (L). This allows a shielding effect for said nose by a gas curtain around said nose arising from said auxiliary channel.

Preferably said at least one channel is a plurality of channels (that is at least two channels I more than one channel), each channel of the plurality of channels is arranged in a rotationally symmetric arrangement with respect to the said central longitudinal axis (L), such that the distance between each nearest neighboring channels is the same (each neighboring pair of said plurality of channels is equally spaced). The rotationally symmetric arrangement of said plurality of channels again improves the flow characteristics of the molten metal, but additionally also reduces forces and vibrations of the stopper rod itself. Preferably (each of) said at least one channel has a maximum depth of at least 3 mm, preferably 7 mm, more preferably 12 mm. Preferably (each of) said at least one channel has a maximum depth of at least 3 mm, preferably 7 mm, more preferably 12 mm along at least (any) 30%, preferably at least 50% and more preferably at least 70% of its channel length. It was found that with a depth of less than 3 mm a high share of the gas introduced into the channels is flushed out and the induced rotational flow is strongly decreased, while above a depth of 3 mm and even more at more than 7 mm and more than 12 mm enough gas is retained and guided inside the channel. Additionally, at more than 12 mm it seems that additionally a certain guidance of the molten metal is achieved, improving the rotational flow characteristics.

It is especially preferred that (each of) said at least one channel is extending from a first channel end to a second channel end, whereas said second channel end is closer to said second end of said stopper body than said first channel end. The distance between said first channel end and said second channel end defines a channel length. Preferably (each of) said at least one channel has a channel length in the range of 30 mm to 200 mm, preferably 50 mm to 150 mm, which has resulted in a good directional flow of the gas. Preferably (each of) said at least one channel has the same channel length, which reduces turbulence in the metal flow and reduces stopper vibrations.

Preferably (each of) said gas supply connecting means leads into (each of) said at least one channel at a position between 50 mm to 100 mm distance from said second end of the stopper body. In other words, the gas enters said at least one channel in a region where the flow velocity of the molten metal during casting is high. This ensures that the gas being sucked down by the stream of molten metal effectively and improves guiding of the molten metal and the gas and thus increases the rotational flow of the molten metal.

Preferably (each of) said gas supply connecting means leads into (each of) said at least one channel by means of indirect permeability. In other words, (each of) said gas supply connecting means allows the said gas to flow via a permeable structure into (each of) said at least one channel. This allows easy start of the gas flow, as clogging of the gas supply connecting means is reduced. Preferably said gas supply connecting means leads into (each of) said at least one channel by means of indirect permeability of said channel over at least the region between 50 mm to 100 mm distance from said second end of the stopper body. Preferably (each of) said gas supply connecting means leads into (each of) said at least one channel at said bottom of said channel. This ensures that the gas being sucked down by the stream of molten metal effectively and enables a good guidance, especially a good guidance length for the molten metal and the gas inside the channel. According to one embodiment, said gas supply connecting means may comprise or fully consist of a material based on indirect permeability, thus it may be at least one porous element. This at least one porous element has a porosity allowing gas to pass through the at least one porous element from the chamber to the channel.

Preferably (each of) said at least one channel provided on an exterior surface of said rod-shaped stopper body and at least partially on said exterior surface of said nose is aligned such that (each of) said at least one channel is provided at least over 50% of its channel length, preferably over at least 70% of its channel length, more preferably over at least 90% of its channel length, most preferably over its entire channel length on said exterior surface of said nose. The rest of the channel may be aligned on an exterior surface of said stopper body adjacent to said nose. This results in good force transmission between the gas and the molten metal and has shown to induce the rotational flow with a high yield.

The rod-shaped stopper body and the chamber extending along the central longitudinal axis in the stopper body may be designed according to the state of the art. In this respect, the rod-shaped stopper body can preferably be made of a refractory material, especially a ceramic refractory material. In particular, the rodshaped stopper body may be made of a refractory material based on alumina (AI2O3) and carbon, i.e. , a so-called alumina-carbon material. The inventive stopper rod can be manufactured using state-of-the-art technologies for the production of stopper rods. In this respect, the stopper rod can be produced in the form of a monoblock stopper. The stopper body is preferably produced by isostatic pressing, as is known from the state of the art. In addition to isostatic pressing, the gas supply lines can be produced by drilling, for example. The channel can, for example, be milled out of the surface of the nose.

One object of the invention is the provision of a vessel for holding molten metal, comprising a bottom, wherein an outlet for discharging molten metal from said vessel is provided at said bottom, and wherein the amount of molten metal flowing through said outlet is controlled by the stopper rod according to the invention. The vessel for holding molten metal is preferably a tundish, preferably a tundish for receiving molten metal, even more preferably a tundish for receiving molten steel, in particular in a continuous casting plant. The outlet is preferably a tundish nozzle.

One object of the invention is the use of a stopper rod according to the invention in a vessel for holding molten metal (preferably a tundish, preferably a tundish for receiving molten metal, even more preferably a tundish for receiving molten steel, in particular in a continuous casting plant) for controlling the flow of molten metal through a nozzle, such as a tundish nozzle.

In a second aspect of the invention, the object is achieved by providing a method for controlling the flow of molten metal and for inducing a rotational flow of molten metal, comprising:

A stopper rod providing step for providing a stopper rod according to the invention, e.g., to a stopper rod control system with a lifting device, for controlling the flow of molten metal, such as a stopper rod comprising: a rod-shaped stopper body, said rod-shaped stopper body extending along a central longitudinal axis (L) from a first end to a second end, said rod-shaped stopper body comprising a nose at said second end, wherein said nose provides an exterior surface; a chamber, said chamber extending along said central longitudinal axis (L) within said stopper body from said first end towards said second end and ending at a distance from said second end; at least one channel, preferably a plurality of channels, being provided on an exterior surface of said rod-shaped stopper body and at least partially on said exterior surface of said nose, and at least partially running around said central longitudinal axis (L), and progressing along the direction of said central longitudinal axis (L); gas supply connecting means, said gas supply connecting means connecting said chamber and each of said at least one channel.

- a gas supply system providing step in which a gas supply system provides a gas to said chamber;

- a gas guiding step in which said gas is guided at a (e.g., predefined) gas flow (Q(t)) by said gas supply system via said chamber through said gas supply connecting means to said at least one channel of said stopper rod.

Preferably said stopper rod is a stopper rod according to the invention, more preferably said stopper rod is a stopper rod according to the first aspect of the invention. Preferably said stopper rod comprises an auxiliary gas supply connecting means, said auxiliary gas supply connecting means leading from said chamber and through said rod-shaped stopper body into an auxiliary channel, said auxiliary channel being provided on said exterior surface of said nose, preferably said auxiliary channel is ring-shaped, preferably said auxiliary channel is of direct permeability, whereas direct permeability is achieved by at least one of a hole and a pipe, and whereas the step of guiding said gas, e.g., at a predefined gas flow, by said gas supply system via said chamber through said gas supply connecting means to said at least one channel of said stopper rod comprises setting a gas pressure of said gas, e.g., such that said predefined gas flow is guided into said at least one channel.

Preferably the method comprises additionally:

- a tundish nozzle providing step for providing a tundish nozzle and a rotational flow measurement unit, capable of measuring the rotational flow (Rot(t)) of the molten metal inside of said tundish nozzle;

- a rotational flow determination step for determining the rotational flow (Rot(t)) of the molten metal inside of the tundish nozzle by said rotational flow measurement unit;

- a target providing step for providing a target for the rotational flow (Roto) inside of the tundish nozzle;

- a gas increasing step for increasing said gas flow (Q(t)) in case said target for the rotational flow (Roto) is higher than said determined rotational flow (Rot(t));

- a gas decreasing step for decreasing said gas flow (Q(t)) in case said target for the rotational flow (Roto) is lower than said determined rotational flow (Rot(t)).

Preferably the rotational flow measurement unit can be integrated into the tundish nozzle. Alternatively, the rotational flow measurement unit may be aligned with the tundish nozzle. The rotational flow measurement unit might comprise coils, which measure a magnetic field displacement as a result of the rotational flow of the molten metal inside the tundish nozzle. The rotational flow (Rot(t)) and the target for the rotational flow (Roto) might be indicated by the swirl number. The rotational flow (Rot(t)) and the target for the rotational flow (Roto) might be indicated by the circulation C in m ² /s.

Exemplary embodiments of the invention are explained in more detail by means of illustrations:

Figure 1a, a cross-sectional view of a tundish comprising a stopper rod according to the invention, wherein in the bottom of the tundish there is provided an outlet in the form of a submerged entry nozzle;

Figure 1 b, a cross-sectional view of an alternative embodiment of a tundish comprising a stopper rod according to the invention, wherein in the bottom of the tundish there is provided an outlet in the form of a submerged entry shroud;

Figure 2 left, a front view of the stopper rod according to Fig. 1 a and 1 b;

Figure 2 right, a cross-sectional view of the stopper rod according to Fig. 1a and 1 b;

Figure 3a, a view of the nose of the stopper rod according to Fig. 1 a and 1 b; and Figure 3b, a channel of Figure 3a in more detail;

Figure 4, a view of the nose of the stopper rod according to a second embodiment of the invention;

Figure 5, a view of the nose of the stopper rod according to a third embodiment of the invention; Figure 6, a view of the nose of the stopper rod according to a fourth embodiment of the invention;

Figure 7 shows the result of a CFD simulation of a stopper rod according to Fig. 1a and 1 b with gas flow at different time steps;

Figure 8 shows the result of a CFD simulation of a stopper rod according to Fig. 1a and 1 b without gas flow at different time steps;

Figure 9 shows a graph depicting the gas flow Q and the induced circulation C as a function of the pressure p of a gas;

Figure 10 shows a diagram of method steps according to an embodiment of the invention.

In order to better illustrate the features of the embodiments shown in the figures, the figures do not reflect the proportions of the embodiments according to practice.

Figure 1a shows a tundish identified in its entirety by the reference sign 1 , which is part of a continuous casting plant for casting steel. Tundish 1 comprises, as is known from the state of the art, a metal vessel 3 lined on its inside with a refractory material 5. Molten metal can be provided in the space enclosed by the refractory material 5. In the bottom 7 of tundish 1 , a tundish nozzle 9 in the form of a submerged entry nozzle (SEN) is provided through which molten metal in tundish 1 can be cast into a mould (not shown). A vertically aligned central longitudinal axis L of a stopper rod 100 runs through the tundish nozzle 9.

Along the central longitudinal axis L a stopper rod 100 is arranged in its functional position. The stopper rod 100 is connected to a state-of-the-art lifting device (not shown) by means of which the stopper rod 100 can be lifted and lowered along the central longitudinal axis L. The stopper rod 100 comprises a stopper body 101 which comprises a stopper nose 103 at its second end / lower end. By means of the lifting device, the stopper rod 100 can be lifted into the second position shown in Figure 1a, in which the tundish nozzle 9 is open, so that a molten metal provided in the tundish 1 can be casted through the tundish nozzle 9 (here: submerged entry nozzle 9). Furthermore, the stopper rod 100 can be lowered by means of the lifting device into a first position (not shown in Figure 1a) in which the stopper nose 103 rests against the tundish nozzle 9 in such a way that it is closed by the stopper rod 100. Accordingly, the tundish nozzle 9 can be closed and opened by means of the stopper rod 100, thereby controlling the amount of molten metal flowing through the tundish nozzle 9. Further shown is a gas supply system 210 for providing a gas 200 to the stopper rod 100.

The tundish 1 shown in Figure 1 b is broadly identical to the tundish shown in Figure 1a and indicated with the same reference signs as far as the tundish 1 according to Figure 1a is identical to the tundish 1 according to Figure 1 b, only the gas supply system 210 for providing a gas 200 is not shown. The difference between the tundish 1 according to Figures 1a and 1 b lies in the fact that in the bottom 7 of tundish 1 according to Figure 1b there is provided a tundish nozzle 10 in the form of a submerged entry shroud (SES). As known from the art, submerged entry shroud 10 is comprised of an upper part 10.1 , located at the bottom 7 of tundish 1 , and a lower part 10.2, attached below upper part 10.1 such that the upper part 10.1 and the lower part 10.2 form a continuous chamber along the central longitudinal axis of submerged entry shroud 10.

Figure 2 shows a first embodiment of the stopper rod 100 as shown in Figure 1 in a front view (left) and in a cross-sectional view (right). The stopper rod 100 comprises a rod-shaped stopper body 101 , the outer circumferential surface of which is rotationally symmetrical to the central longitudinal axis L of the stopper rod 100. The stopper body 101 extends along the central longitudinal axis L from its first, upper end 105 in the functional position according to Figure 1 to its second, lower end 107 in the functional position according to Figure 1 . At its second end 107, the stopper body 101 comprises a nose 103. The nose 103 has its second nose end 104b aligned with the second end 107 of the stopper body 100. The outer surface of the stopper body 101 in a region between the first end 107 of the stopper body 100 until the first nose end 104a, has a circular cylindrical outer contour rotationally symmetrical to the central longitudinal axis L. The external surface of the nose 103, between a first nose end 104a and a second nose end 104b, is rotationally symmetrical to the longitudinal axis L and is generally dome-shaped.

The stopper body 101 has a chamber 109 which, as shown in Figure 2 (right), extends along the central longitudinal axis L from the first end 105 in a direction towards the second end 107 into the stopper body 101 and ends in the stopper body 101 at a distance from the second end 107. The chamber 109 is rotationally symmetrical in relation to the central longitudinal axis and has a circular-cylindrical shape along most of its height.

The stopper body 101 preferably is made of a refractory material in the form of an alumina-carbon material (AI2O3-C material). The stopper body 101 can be produced as a monoblock stopper body by means of isostatic pressing.

A gas supply system 200 provides a gas 210 to the stopper rod 100 at the first end 105, through which the gas 210, such as an inert gas such as argon or nitrogen, is led into the chamber 109. From chamber 109 the gas is further guided via gas supply lines 123 to the channels 111. In the channels 111 the gas 210 is accelerated along the channel direction 112 and finally exits the channels 111 into the molten metal and imposes a rotational flow to the molten metal.

In this embodiment six channels 111 are arranged on the outer surface of nose 103. All channels 111 follow a helical path around the central longitudinal axis L, that is they are at least partially running around said central longitudinal axis (L) (see Figure 3a), and progressing along the direction (z) of said central longitudinal axis (L); The channels 111 are completely open to the outside, i.e. on the side of the channel 111 facing away from the stopper body 101 , and are, according to its V-shaped cross-sectional area, limited by a first channel wall 113 and a second channel wall 115, which start from a common linear area which forms the channel bottom 117 of the channel 111 (see Figure 3b). Towards the outer surface of the nose 103, the first channel wall 113 and the second channel wall 115 diverge and finally merge into the outer surface of the nose 103. The first channel wall 113 is limiting the channel 111 in a direction towards the first end 105 and forms a first edge 119 with the outer surface of the nose 103. The second channel wall 115 is limiting the channel 111 in a direction towards the second end 107 and forms a second edge 121 with the outer surface of the nose 103. The first edge 119 and the second edge 121 each form a sharp edge with a radius well below 0.5 mm. The first and second edges 119 and 121 run equally spaced to each other around the longitudinal axis L, and progressing along the direction of the central longitudinal axis, resembling a helical path of the channel 111 on the exterior surface of the nose 103. The distance between the first and second edges 119, 121 defines the width of the channel mouth, i.e., the width of channel 111 in the area in which channel 111 merges into the outer surface of nose 103 and is 10 mm in the embodiment. The shortest distance between an imaginary plane that extends between the first and second edges 119, 121 and the channel bottom 117 defines the depth of channel 111 , which in the embodiment is 8 mm. This results in a cross-sectional area of channel 111 of 40 mm ² .

Each channel 111 defines a channel direction 112 and each of the six channel directions 112 are aligned at an angle (c to said central longitudinal axis (L). The angle (c is defined as the smaller angle between said channel direction and said central longitudinal axis (L), in case of skew lines, the smaller angle between any two lines parallel to said channel direction and said central longitudinal axis (L). Here all angles (c are the same with a _t = 45° for all i=1..6.

From chamber 109, gas supply connecting means 123 in the form of one gas supply line 123 per channel 111 lead through the refractory material of the stopper body 101 into the respective channel 111. The six gas supply lines 123 each have a straight course with a circular cross-sectional area and are arranged symmetrically with respect to the central longitudinal axis L and are evenly spaced from each other. Accordingly, the six gas supply lines 123 are spaced from each other by a rotational angle <p of 60° (i.e., at cp = 0°, 60°, 120°, 180°, 240°, 300°) with respect to the central longitudinal axis L. In accordance with their symmetry with respect to the central longitudinal axis L, the gas supply lines 123 lead into six channels 111 at six evenly spaced areas, which are also spaced from each other at a rotational angle <p of 60° (i.e., at cp = 0°, 60°, 120°, 180°, 240°, 300°) with respect to the central longitudinal axis L, as can be seen particularly clearly in Figure 3a, which is a view onto the stopper nose 103 1 onto the second end of the stopper body 107 (that is in its use position from below the stopper rod 100). Thus, the distance between each nearest neighboring channels 111 is the same (each neighboring pair of said plurality of channels 111 is equally spaced).

The gas supply lines 123 each extend along a respective longitudinal axis, with the six longitudinal axes of the gas supply lines 123 intersecting at a common point on the central longitudinal axis L. The six longitudinal axes of the gas supply lines 123 are each arranged at an angle of approximately 60° to the central longitudinal axis L of the stopper body 101 , this angle being the smaller angle included between the section of the longitudinal axes of the gas supply line 123 passing through the gas supply lines 123 and the section of the central longitudinal axis L of the stopper body 101 passing through the second end 107 of the stopper body 101.

The gas supply lines 123 lead into each of the respective six channels 111 by means of direct permeability as shown in Figure 3a and 3b. In other words, the gas supply lines 123 allow the gas to flow via a discrete conduit into each of the respective six channels 111.

Each of the channels 111 extends from a first channel end 111 u to a second channel end 111 d, each second channel end 111 d is closer to the second end 107 than the respective first channel end 111 u. The distance between a respective first channel end 111 u and said second channel end 111 d defines a channel length 1111. In this example, all channel lengths 1111 are the same, namely 160 mm (alternatively, as shown in the figures, all channel lengths 1111 are the same, namely 50 mm, and each second channel end 111d is arranged at a distance of 50 mm from the second end of the stopper body 107). Each gas supply line 123 leads into one of the six channels 111 at a position in said channel at a distance of 80 mm from the second end of the stopper body 107.

An alternative first embodiment (not shown in the Figures) differs from the first embodiment mentioned above in that no second channel wall 115 limits the channel 111 in a direction towards its second end 107, thus also no second edge 121 is present. Here the channel diverges continuously and smoothly into the surface of the nose 103 in the direction towards its second end 107.

A second embodiment of Figure 4 shows a stopper rod 100 with an auxiliary gas supply connecting means 133, which here is an auxiliary gas supply line, that leads from chamber 109 through the rod-shaped stopper body 101 into an auxiliary channel 131 with a ring-shaped geometry, the auxiliary channel 131 aligned on the exterior surface of the nose 103. The auxiliary channel 131 is arranged on the exterior surface of the nose 103 at a distance from the channels 111 in a direction towards the second end 107 (that is below the channels 111 in a use position). The auxiliary gas supply connecting means 133, which here is an auxiliary gas supply line, leads into the ring-shaped auxiliary channel 131 by means of direct permeability as shown in Figure 4. In other words, the auxiliary gas supply connecting means 133, which here is an auxiliary gas supply line allows the gas to flow via a discrete conduit into the auxiliary channel 131 . The ring-shaped auxiliary channel 131 is symmetrical in relation to said central longitudinal axis (L), that is, the central longitudinal axis passes through the center of the ring-shaped auxiliary channel 131. The radius of the auxiliary channel in this example is 15 mm, the slit of the ring-shaped auxiliary channel has a width of 0,5 mm.

A third embodiment of Figure 5 shows a stopper rod 100 like the first or second embodiment, with the difference that the six gas supply lines 123 are leading into six channels 111 by means of indirect permeability. In other words, the gas supply lines 123 allow the gas to flow via a permeable structure into each of the respective six channels 111. Here the indirect permeability I porous structure spans within the region between 50 mm to 100 mm distance from said second end of the stopper body 107, where the gas supply lines 123 lead into the channel 111 at the bottom 117 of the channels 111.

A fourth embodiment of Figure 6 is similar to the third embodiment of Figure 5, except that the indirect permeability I porous structure spans over the whole channel length 1111 from the first channel end 111 u to the second channel end 111d.

In another embodiment a stopper rod (100) according to one of the first, second, third, fourth or fifth embodiment is provided, a gas supply system 200 supplies a gas 210 to the chamber 101 at a gas flow rate of 5 l/min, the gas being argon, whereas the gas is guided at this predetermined gas flow via the chamber 109 through the gas supply lines 123 to the six channels 111. In the channels 111 the gas 210 is accelerated along the channel direction 112 and finally exits the channels 111 into the molten metal and imposes a rotational flow to the molten metal.

Computer-based numerical simulations based on the CFD (computational fluid dynamics) method were performed for the stopper rod 100 according to the first embodiment, both with and without injection of a gas 210.

The results are shown in Figures 7 and 8. The left image of Figure 7 shows the flow characteristics shortly after start of the metal flow and the gas injection (here at t=0, 1 s after start of the gas injection). The flow lines (shown as black lines) inside the tundish nozzle 9 show an onset of rotational flow. The circulation (determined at the height indicated by the ellipsis) is C=0,063 m ² /s. The right image shows the situation at t=11s, where a strong rotational flow of the molten metal is observed inside of the tundish nozzle 9 (see helical path of the flow lines shown as black lines in the tundish nozzle 9). The circulation at t=11 s (at the height of the ellipse) is C=0,298 m ² /s.

In Figure 8 the results for the case without injection of a gas 210 can be seen. Here, during the whole simulation a low rotational flow is experienced inside the tundish nozzle 9 (at t=1 Os, as shown in the left of Figure 8, the rotational flow is low with a circulation C=0,018 m ² /s, at t=21 s, as shown in the right of Figure 8, the rotational flow is still low with a circulation C=0,022 m ² /s,). Furthermore, the simulation shows that the flow is more unstable in case no argon is injected.

The result shows, that the stopper rods 100 of the embodiments are suitable for inducing a rotational flow into a molten metal by means of gas injection into the channels 111.

Figure 9 in the upper graph schematically shows the results achieved by a stopper rod according to the third embodiment: a graph is shown with the gas flow Q (in l/min) of a gas 210 as a function of the pressure (in bar). Below a certain threshold pressure pthres basically no gas flow Q is observed (Q«0). Above the threshold pressure pthres gas flow Q gradually rises, whereas all gas 210 is transported through the auxiliary gas supply connecting means 133, which here is an auxiliary gas supply line into the ring-shaped auxiliary channel 131. This is because the flow resistance of the six channels 111 of indirect permeability is higher than that of the ring-shaped auxiliary channel 131 of direct permeability. In this flow regime basically no or low rotational flow is induced into the molten metal (Rot(t)=C is small). Only above a saturation pressure p _sa t the gas 210 is starting to flow into the six gas supply lines 123 and into the six channels 111. By further increasing the pressure, more and more gas 210 is injected into the molten metal via the six channels 111 , thereby increasing the rotational flow (Rot(t)=C is increasing in this regime). This is shown schematically in the lower graph of Figure 9, where the induced rotational flow Rot (in this example in the form of the circulation, Rot(t)=C in units of m ² /s) is shown as a function of the pressure (in bar). Thus, it is possible to induce a certain rotational flow into the molten metal by guiding a gas 210 at a (e.g., predefined) gas flow by a gas supply system 200 via a chamber 109 through gas supply connecting means 123 to the six channels 111 of the stopper rod.

To set a certain rotational flow of the molten metal, an additional rotational flow measurement unit 20 can be used. This makes it possible to either increase the gas flow in case a higher rotational flow of the molten metal is desired or to decrease the gas flow in case a lower rotational flow of the molten metal is required. Thereby an active control (with a feedback) of the induced rotational flow can be achieved. This is shown in Figure 10, where a stopper rod 100 according to the invention is provided in a stopper rod providing step 300, a gas supply system 200 is provided in a gas supply system providing step 310, and a tundish nozzle 9, 10 comprising a rotational flow measurement unit 20 is provided in a tundish nozzle providing step 320. A gas 210 is guided in a gas guiding step 325 at a (e.g. predefined) gas flow (Q(t), at a certain time t) by the said supply system 200 via the chamber 109 through the gas supply connecting means 123 to the channels 111 of the stopper rod 100. The rotational flow (Rot(t) at time t, in this example the rotational flow is characterized by the circulation Rot(t)=C) of the molten metal inside of the tundish nozzle 9, 10 is determined in a rotational flow determination step 330 by the rotational flow measurement unit 20. A certain target for the rotational flow (Roto=Co) inside of the tundish nozzle 9, 10 is provided in a target providing step 340. In case the target for the rotational flow (Roto=Co) is higher than the determined rotational flow (Rot(t)=C), i.e. Roto > Rot(t), the gas flow (Q(t)) is increased in a gas increasing step 350, e.g. by a certain amount AQ, thus Q(t+1 )=Q(t)+AQ, which leads to an increase of rotational flow (Rot(t)=C). In case the target for the rotational flow (Roto=Co) is lower than the determined rotational flow (Rot(t)=C), i.e. Roto < Rot(t), the gas flow (Q(t)) is decreased in a gas decreasing step 360, e.g. by a certain amount AQ, thus Q(t+1 )=Q(t)-AQ, which leads to a decrease of rotational flow (Rot(t)=C). This allows to achieve and maintain a rotational flow (Rot(t)=C) at or at least close to the target for the rotational flow (Roto=Co), thus Rot(t)=C=Roto=Co.

List of reference numerals and factors: L Central longitudinal axis through stopper body r Radial distance (normal distance) from L z z-axis along the direction of L, in the direction from the first end of the stopper body to the second end of the stopper body

<p Rotational angle around L a Angle between i-th channel direction and z-axis

Q(t) Gas flow at a time t

Rot(t) Determined rotational flow at time t

Roto Target for the rotational flow

C Circulation in m ² /s

Co Target for the circulation in m ² /s

1 Tundish

3 Metal vessel

5 Refractory material

7 Bottom of tundish

9 Tundish nozzle in form a submerged entry nozzle (SEN)

10 Tundish nozzle in form a submerged entry shroud (SES)

10.1 Upper part of submerged entry shroud (SES)

10.2 Lower part of submerged entry shroud (SES)

20 Rotational flow measurement unit

100 Stopper rod

101 Stopper body

103 Nose of stopper body

104a First nose end

104b Second nose end

105 First end of stopper body

107 Second end of stopper body

109 Chamber

111 Channel

111 u First channel end

111d Second channel end 1111 Channel length

112 Channel direction

113 First channel wall

115 Second channel wall

117 Channel bottom

119 First edge

121 Second edge

123 Gas supply connecting means

131 Auxiliary channel

133 Auxiliary gas supply connecting means

200 Gas supply system

210 Gas

300 stopper rod providing step

310 gas supply system providing step

320 tundish nozzle providing step

325 gas guiding step

330 rotational flow determination step

340 target providing step

350 gas increasing step

360 gas decreasing step