EP2490843B1 - Method and apparatus for the lateral guidance of the melt during strip casting - Google Patents

Description

The present invention relates to a method and apparatus for lateral flow guidance in strip casting by the Coanda effect.

Out EP 0 635 323 B1 is a nozzle for continuous casting known. In this case, a pouring tip for a nozzle of a continuous casting apparatus will be described. Out US 4,526,223 is a continuous casting with two cooling drums known. Out EP 0 859 675 For example, a method and apparatus for casting a metal strip is known. Out WO 2008/087002 A1 For example, a method and apparatus for potting non-ferrous molten metals, particularly copper or copper alloys, is known.

u ist die Fließgeschwindigkeit des Gerinnes, g die Erdbeschleunigung und h_hyd die hydraulische Höhe des Gerinnes.When tape casting according to the Belt Casting technology, the hot, liquid melt from the distributor (tundish) must be placed on the moving sub-belt. The problem here is the combination of the uniform, horizontal distribution of the melt over the entire casting width, which can be up to two meters, and the simultaneous realization of high casting speeds of up to 30 m / min. For this purpose, special pouring or homogenizing devices are used, which are also referred to as snouts. The casting spouts can be closed (closed snout) or open (open snout). In an open spout, the hot melt is in direct contact with the surrounding gas (air, inert gas). On the casting spout, the melt coming from the distributor forms an open channel flow, which can be either subcritical (Froude number <1) or supercritical (Froude number> 1). The Froude number for open channels is defined as $\mathrm{Fri.}=\frac{\mathrm{u}}{\sqrt{\mathrm{G}{\mathrm{H}}_{\mathrm{hyd}}}};$

u is the flow rate of the channel, g is the gravitational acceleration and h _hyd the hydraulic height of the channel.

At the exit of the pouring spout, the melt leaves the refractory material and forms a melting case with a certain drop height, which depends on the position of the pouring spout over the moving strip. On the moving belt, the melt cools and is transported away. The initially still liquid melt on the conveyor belt is extremely sensitive from a fluid and heat technical point of view. Long-lasting flow patterns are retained during solidification and produce an undesirable microstructure, both microscopically and macroscopically.

The melt laterally constricted as a result of the interfacial tension during the overflow from the pouring spout onto the lower strip forms a hot strand, which generates undesired turbulences when impinging on the conveyor belt or also ensures that the lateral area on the lower belt is not even charged or filled with melt , The constriction of the melt occurs even when the spout is completely filled with melt until it exits. The strand induces turbulence and turbulence extending to the narrow but critical side region of the melt pool.

When tape casting copper, aluminum or zinc, for example with a Hazelett Caster, the situation is similar. Here, the melt between two inclined, circulating conveyor belts (upper and lower belt) is poured, the distance of the conveyor belts determines the casting thickness. When the melt passes from the casting spout into the melt pool located between the ribbons, lateral strands also form which penetrate deep into the melt pool and produce undesirable vortex structures. The axes of these vortex structures are oriented essentially normal to the lower belt. The strands remain intact in the melt pool and, under certain circumstances, attach themselves downstream to the side sealing, thus deep in the melt pool. In this case, the melt can even pass upstream against the transport direction of the strip. As a result, relatively stable, disturbing flow patterns are generated near the side seal, which in the continuous solidification and deteriorate the quality of the band edges deteriorate.

In the case of a continuously expanding by a defined angle open spout (eg JP 62248543 A ) the melt flow may already dissolve on the spout. In the edge area, the melt constricts and thickens. This forms a hot strand. The hot strand emerges from the spout and strikes the underlying moving sub-band with high kinetic energy. On the tape it comes to intense, unwanted vortexes. The lateral flow generally has a higher momentum and higher kinetic energy than the mid-band flow. The melt therefore penetrates laterally with high momentum onto the conveyor belt, whereby the typical strands structure is retained and a non-uniform velocity profile is induced.

In the case of parallel side walls of the casting spout (eg EP 0 962 271 A1 ), the melt remains on the side walls until it leaves the casting spout. There is therefore no flow separation on the spout instead. However, as soon as the melt leaves the pouring spout, it constricts due to lack of adhesion forces and then dominating cohesive forces and in turn forms the already described strand with the known flow pattern on the conveyor belt. Laser-optical speed measurements on the water model illustrate this effect.

Additional measures, such as inductive stirring, must counteract this vortex formation. Strip casting plants, which operate according to the above-described method, require an inductive stirrer for homogenizing the melt.

The object of the present invention is to provide a method and a device for lateral flow guidance of a molten metal during strip casting, whereby the stranding of the melt in the overflow reduced or completely suppressed by the casting spout on the moving belt and the associated undesirable microstructure in the band.

The object of the present invention is achieved by a method for lateral, assisting flow guidance of a molten metal during strip casting according to claim 1. Preferred embodiments will become apparent from the dependent claims 2-7.

The present invention also relates to a method for the lateral, passive flow guidance of a molten metal during strip casting by utilizing the Coanda effect. The lateral, directed into the melt interior constriction of an open melt jet in the distribution to a moving conveyor belt (strip casting technology) is inventively reduced. As a result, the solidifying melt is distributed more uniformly over the casting width on the conveyor belt. The characteristic undesirable flow patterns at the strip edges are prevented.

The melt is passed over a casting spout, in which the last stretch of the refractory side wall is specially curved three-dimensionally (convex, concave), specifically in the form of a wing-like, hereinafter referred to as Coanda profile. The Coanda effect describes the property of a fluid jet or jet (gaseous, liquid) to attach to and follow a nearby convex wall. In this way, the fluid jet changes its original direction of propagation. The effect of the Coanda effect is based on the superposition of several physical mechanisms, essentially the Bernoulli effect, the molecular ones Forces between wall and fluid jet, the flow boundary layer and the pressure gradient in the flow direction.

In the present case, the contour of the Coanda profile already begins in or on the spout and is specially shaped beyond the spout and at the same time also in the direction of the treadmill. It is a three-dimensional, wing-shaped contour of refractory material, such as SiC, MgO formed. Due to the Coanda effect, the melt follows the contour of the Coanda profile and at the same time is deflected outwards away from the center of the strip and downwards in the direction of the moving bath. This counteracts the constriction of the melt jet, this is therefore avoided. The velocity profile in the pool is clearly homogenized over the entire casting width.

The special, geometric shape at the outlet of a pouring spout causes the outflowing melt to expand horizontally. This reduces the striation. This ensures u. a. the Coanda effect for an expansion of the melt flow and thus for a homogenization of the melt across the casting width.

The kinetic energy must be as low as possible when the melt overflows from the casting spout onto the strip. Thus, on the one hand, the introduction of gas bubbles into the melt and, on the other hand, the generation of flow patterns is minimized. The lateral stratification during the overflow of the melt on the moving belt of a strip casting is reduced and improves the microstructure.

The essential advantage of the invention is that the passive flow guidance through the Coanda profile supports the homogenization process on the basis of the magnetic stirrer. The new method is thereby inexpensive. The refractory form of the distributor between the continuous casting distributor and conveyor belt only needs to be changed. The dimension of the inductive stirrer is reduced with simultaneous use of the Coanda profile.

The U. a. The principle based on the Coanda effect can be applied wherever free pouring streams are to be influenced by a passive flow without external influence only by shaping measures. The method can also be applied to strip casters for non-ferrous metals.

The object of the present invention is further achieved by a device for strip casting with lateral flow guidance of a molten metal with a casting spout according to claim 8. The outlet of the spout is preferably wing-shaped and simultaneously formed in the form of an ellipsoid of revolution or wing (2) in the direct casting direction x.

The exact geometry of the Coanda profile depends, among other things, on the flow velocity of the melt on the casting spout. The widths of the pouring device and the conveyor belt are usually between 1.0 m and 2.0 m. The length of the pouring device is about 1 m. The distance from the spout to the treadmill, therefore, the drop height of the melt from the spout to the idler belt, is about 20 mm to 80 mm. The speed of the treadmill is up to 30 m / min. The angle between the ladle and the treadmill is 0 ° to 20 °. The temperature of the melt depends on the steel composition. For Low Carbon Steel the temperature is 1550 ° C, for high alloy steels it is 1450 ° C. The process can also be applied to copper, aluminum or zinc strip casting, except on steel grades be applied. The kinematic viscosity of the melt is approximately ν = 1x10 ^-6 m ² / s. The melt is copper, aluminum, zinc, low carbon steel or high-alloy steel. The refractory coating contains MgO or SiC.

In a strip casting plant, the melt flows from the ladle into a continuous casting distributor. Below the distributor, another vessel can be located, which represents the actual task and distribution device, in particular casting spout, the melt on the moving sub-belt. From the distributor, the melt flows through the so-called Submerged Entry Nozzle (SEN) into the feeding device, which calms and distributes the melt.

For the method according to the invention, the shaping at the lateral outlet of the pouring spout is decisive. The last stretch of the refractory side wall at the exit area of the casting spout, which can be up to about 30 cm long, is designed to optimize flow. The contour is wing-shaped in the direct casting direction. Due to this special contour, the melt follows a relatively long time, therefore without separation along the contour. This behavior is supported by the Coanda effect. The Coanda profile is similar to a wing on the aircraft flows from the front. At the same time, the melt is also continued after leaving the casting spout. The melt is transported as quietly as possible, at low speed and with low turbulence to the running sub-belt. The wing sticks out over the spout. By combining these two conditions, the Coanda profile has the form of a flow-optimized ellipsoid of revolution.

Fig. 1a

zeigt eine Draufsicht auf eine stetig erweiternde Gießschnauze nach dem Stand der Technik,

Fig. 1b

zeigt eine Draufsicht auf eine erfindungsgemäße Gießschnauze,

Fig. 1c

zeigt eine seitliche Ansicht einen erfindungsgemäßen Austritt der Gießschnauze,

Fig. 2

zeigt anhand einer Draufsicht auf eine Gießschnauze die typische Strömungsablösung, wenn der Öffnungswinkel der Gießschnauze zu groß ist,

Fig. 3a

zeigt anhand einer Draufsicht auf eine Gießschnauze das Ergebnis einer quantitativen laseroptischen Geschwindigkeitsmessung im Pool einer stehenden Kokille (Hazelett-Caster) für den Fall ohne Coanda-Profil,

Fig. 3b

zeigt anhand einer Draufsicht auf eine Gießschnauze das Ergebnis einer quantitativen laseroptischen Geschwindigkeitsmessung im Pool einer stehenden Kokille (Hazelett-Caster) für den Fall mit Coanda-Profil,

Fig. 4

eine perspektivische Darstellung der erfindungsgemäßen Gießschnauze,

Fig. 5

eine Vergrößerung des erfindungsgemäßen Coanda-Profils aus Fig. 4.

The invention will be explained in more detail with reference to a drawing and an example. Show it:

Fig. 1a

shows a plan view of a continuously expanding spout according to the prior art,

Fig. 1b

shows a plan view of a casting spout according to the invention,

Fig. 1c

FIG. 2 shows a side view of an outlet according to the invention of the pouring spout, FIG.

Fig. 2

shows from a plan view of a spout the typical flow separation, when the opening angle of the spout is too large,

Fig. 3a

shows a top view of a casting spout the result of a quantitative laser-optical speed measurement in the pool of a standing mold (Hazelett-Caster) for the case without Coanda profile,

Fig. 3b

shows a top view of a casting spout the result of a quantitative laser-optical speed measurement in the pool of a standing mold (Hazelett-Caster) for the case with Coanda profile,

Fig. 4

a perspective view of the casting spout according to the invention,

Fig. 5

an enlargement of the Coanda profile according to the invention Fig. 4 ,

Fig. 1 a shows the basic situation at the outlet of the casting spout with the underlying pool flow with an open pouring spout 1 which widens steadily by a defined angle (0 ° <α <20 °). The melt stream 7 already releases on the pouring spout 1 at the point A. , In the edge region, the melt 3 constricts, thickening and it forms a strand that emerges from the spout 1 and strikes with high kinetic energy to the underlying, moving sub-belt 4 with side sealing. The lower band 4 moves in the x direction. The strand leads in the pool on the conveyor belt 4 to intensive, unwanted turbulence 5 or to areas that are not acted upon by melt 3. The development of turbulence 5 is via the flow separation A and constriction 6 in Fig. 2 , here by example shown a water model. In general, the lateral flow 7 has a higher momentum and a higher kinetic energy than the flow 7 in the middle of the band. The melt 3 therefore penetrates deeper into the melt pool of the conveyor belt 4, wherein the typical strands structure is maintained and a non-uniform velocity profile 8 is induced. Downstream of the point of impingement on the belt 4, the strand is applied to the side sealing (insulating block chain) in the re-attachment point or stagnation point B. This procedure is based on the laser-optical measurements in the mold pool of a Hazelett-Caster water model during the outflow without Coanda profile in Fig. 3a shown. Here, the melt 3 can even upstream against the transport direction x of the belt 4, whereby near the side seal relatively stable, disturbing flow patterns are generated, which may be retained in the continuous solidification under certain circumstances. Observations on a Hazelett caster in operation have shown that it is a highly transient process, with recovery point B moving upstream and downstream. This results in an uneven velocity profile 8.

In the case of parallel walls (α = 0 °), the melt 3 remains up to the outlet of the pouring spout 1 on the side walls. There is therefore no replacement on the spout 1 instead. However, as soon as the melt 3 leaves the pouring spout 1, it constricts due to lack of adhesion forces and then dominating cohesive forces and in turn forms the already described strand with the known flow pattern. Laser-optical speed measurements illustrate this effect, as in Fig. 3a will be shown.

The spout 1 according to Fig. 1b shows the general shape according to the invention at the side exit. The last stretch of the refractory side wall is wing-shaped. It is essential that the Coanda profile 2 is formed over the spout 1 in the y-direction and at the same time also in the direction of the treadmill in the x-direction. It therefore becomes a three-dimensional, Tragflügelförmige and thus streamlined wing contour 2 created from refractory material. In this case, the melt 3 follows the wing contour 2 as a result of the Coanda effect and is simultaneously deflected outwards in the z direction and downwards in the y direction, therefore away from the center of the band, as is additionally shown in FIG Figures 1b . 1c and 4 will be shown. As a result, the constriction 6 is avoided and the velocity profile 8 homogenized over the entire casting width. A uniform velocity profile 8 is obtained. Fig. 3b shows a laser optical speed measurement in the water model, in comparison to Fig. 3a only a Coanda profile 2 was installed. It becomes clear that the streaking is reduced.

LIST OF REFERENCE NUMBERS

1

Pouring Device / Distributor / Pouring Spout / Open Snout

2

Coanda profile / wing contour / wing profile / wing / ellipsoid of revolution

3

Melt / melt flow

4

Conveyor belt / belt / moving sub-belt

5

Vortex / turbulence

6

constriction

7

flow

8th

velocity profile

A

flow separation

B

Restoration point / stagnation point

Claims (9)

1. Method for lateral flow guidance of a metal melt during strip casting, wherein

   a) the melt (3) flows along adjacent fixed boundaries at the outlet of a distributor device, particularly a pouring spout (1), and at the same time is deflected outwardly and downwardly and

   b) the melt (3) is guided by way of the distributor device, particularly pouring spout (1), onto a moved belt (4), wherein

   the melt at the outlet region of the pouring spout (1) is guided by way of a three-dimensional contour, which is present at the sides of the pouring spout, of a ellipsoid of revolution or aerofoil (2) and the contour projects beyond the pouring spout so that the melt continues to be guided after leaving the pouring spout.
2. Method according to claim 1, wherein the melt flow is expanded in the outlet of the pouring spout (1).
3. Method according to one of claims 1 and 2, wherein the melt (3) is guided from the pouring spout (1) onto the running transport belt (4) over a distance of 20 millimetres to 80 millimetres.
4. Method according to any one of claim 1 to 3, wherein the speed of the transport belt (4) is up to 30 m/min.
5. Method according to any one of claims 1 to 4, wherein the angle from the pouring spout (1) to the transport belt (4) is 0° to 20°.
6. Method according to any one of claim 1 to 5, wherein the temperature of the melt (3) is from 660° C to 1550° C.
7. Method according to any one of claims 1 to 6, wherein the melt contains copper, aluminium, zinc, steels with low carbon content, high-alloyed steels and mixtures thereof.
8. Device for strip casting with lateral flow guidance of a metal melt, with a pouring spout (1) by way of which the melt (3) flows onto a moved belt (4) or between two moved belts (4), wherein the refractory side wall at the outlet region of the pouring spout (1) has at the sides thereof a flow-promoting three-dimensional contour of a ellipsoid of revolution or an aerofoil (2), which projects beyond the pouring spout.
9. Device according to claim 8, wherein the outlet of the pouring spout (1) is wingshaped and at the same time constructed in the form of a ellipsoid of revolution or an aerofoil (2) in direct casting direction x.

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