Strip Casting

Abstract

The invention relates to a process and a device for strip casting using a travelling mould which is cooled by a liquid coolant. The invention is characterised in that the coolant is liquid metal or ionic liquid. Preferably, the coolant flows with turbulent flow through the cooling device attributed to the travelling mould.

Description

The invention concerns the strip casting of metals of all kinds with indirectly cooling the strip and a device for such a cooling method according to the introducing part of claim 1.

Such a device and method are disclosed, e.g., in FR 2 289 271 A, in GB 2 310 155 A, in U.S. Pat. No. 5,411,075 A, in U.S. Pat. No. 4,671,340 A and JP 63 192 541 A, whereby water is used as coolant. Other documents like the EP 1 452 252 A from the applicant deal with strip casting and disclose the use of more exotic coolants like liquid metals and ionic liquids.

The strip casting method exists in various forms, either with two or with one belt, in more recent cases with no belt at all, but with two rotating drums which form a gap between their mantle surfaces, and more exotic forms with one drum and a belt or a drum alone. All these variants have a so called travelling mould and are summoned in the description and the claims with the term “strip casting” and the invention covers all of them.

In the case of two drums, cast metal is fed by a special feeding device into this gap and solidifies in the area of this gap, at least on its surface, sufficient to bring the formed metal strip (or sheet) in manageable state. Cooling occurs only indirectly, this means that water is sprayed on the rear side of the belt(s) or the inner mantle surface of the drums and not on the strip directly. Beside these process variants exists the possibility to cast on one wheel with a cavity in it and one belt closing this cavity (Rotary machine) where the wheel is cooled inside by a water spray and the belt on its outer side also by water spray. In case of the single roll caster the casting melt is given on top of a single wheel without a cavity, the wheel is cooled inside, so the cooling starts only from one side of the strip. Beside the twin roll caster, this process may also be promising for the future.

Principally speaking, the introduction of strip casting as a continuous casting process is a very effective way of producing semi-finished or nearly finished products. Products are sheets or strips, thin strips, wires and rods. Cast materials are ferrous metals like steel as well as nonferrous metals like copper, aluminium, magnesium, nickel and so on as well as their alloys.

In this process there is used a travelling mould (system) to get nearly zero friction to reach very high casting velocities that are necessary for being economic.

Usually water is used as coolant in these processes. Water as coolant has some very important disadvantages like low operation temperature as a consequence of low evaporation temperature and there is also the danger of explosions when water gets in contact with hot casting metal directly. The strong cooling to low temperatures causes high temperature gradients in the travelling moulds reducing the life time of them as a consequence of crack formation after a short operation times.

In some strip casting machines, like the rotary strip casting machine, the strong cooling on the belt side by water causes shrinkage of the strip, which leads to the formation of an air gap between strip and mould. This air gap is blocking the heat withdrawal leading to lower casting rates and to casting defects like segregation, shell bending, surface cracks and cell size variations of a continuously cast strip. High casting rates require strong cooling but not down to very low temperatures.

Using water as coolant means that the process is not very stable, because of only a small process window—the low evaporation temperature of water is responsible for strong changes of the heat transfer number depending on the surface temperature of the surface to cool, even small changes in the feeding rate of cast metal result in great changes of the cooling velocity and therefore the metallurgic properties of the product.

The use of water as coolant implies high temperature gradients in the travelling mould systems, that are limiting the life time of the mould and if getting surface cracks, these cracks are to see on the cast product—so not all qualities can be produced with such systems or the life time of the mould in such systems is not very high.

The above mentioned disadvantages which are inherently connected to the use of water as coolant show that, there is a demand for a simple, reliable strip casting process and device which avoids the existing negative facts of the existing strip casting processes without loosing the advantages of the known processes.

In order to achieve this aim, the invention proposes the use of liquid metal or ionic liquids as cooling medium, preferably in a turbulent flow. This ensures that the cooling properties and characteristics are well defined and controllable.

Ionic liquids or designer liquids is the name for a group of salts composed of organic cations and mostly inorganic anions which generally have a melting point below 100° C. They may be used with the invention as long they do not decompose at the maximal working temperature of the process or react with the casting device under the given circumstances. In the following description, they are in most cases not mentioned expressively, but always included when the term “liquid metal” or “coolant” is used. By the way, water, as many liquids, contains a small amount of ions but is not a ionic liquid.

In order to distinguish between the metal to be cast and the metal used as coolant, the first is referred to as “cast metal” and the second as “liquid metal”, a somewhat artificial but practical distinction.

Solidification of the cast metal or alloy occurs by the influence of the indirect cooling. The indirect cooling uses, according to the invention, a liquid metal like lead, tin, bismuth, gallium, indium or alloys of them as well as other liquid metals or alloys being liquid below the solidification temperature of the cast metal or alloy.

The feature of indirect cooling in strip casting with liquid metal ensures a very constant and easy controllable cooling behaviour. The cooling intensity can be adjusted by changing the coolant flow rate and coolant feed temperature. This allows to realize higher temperatures on the cooling side of the mould leading to lower temperature gradients in the mould (longer life time, because lower stress induces less cracks of the mould on the side of the cast metal). The formation of an air gap between strip and travelling moulds can be prevented in the rotary type and single wheel process.

This leads to a more equal quality of the cast strip over the cross section, to higher casting rates and the formation of surface cracks can be prevent. The grain structure of the produced strip, thin strip, rod or wire can be controlled by adjusting the coolant temperature and coolant flow rate. Therefore, even alloys which tend to form cracks during solidification and subsequent cooling may be cast in good quality when using liquid metal as coolant. Furthermore, the danger of explosions as a consequence of the use of water as coolant is eliminated fully.

In general, it is possible to operate a conventional travelling mould system for strip casting with this liquid metal coolant when these systems are adapted with a suitable device for cooling the travelling mould with liquid metal and a device for heating up the coolant to process temperatures and to cool back the coolant after getting the dissipation heat from the strip as well as a device for ensuring a closed coolant loop.

It is preferred that the coolant has, in Centigrade Celsius, a melting point which is lower or equal 60% of the melting point of the casting material in Centigrade Celsius.

It is further preferred that the direction of the flow of the coolant in the area of the travelling mould is in counter flow to the direction of the movement of the cast strip.

A Cooling device for a process according to the invention has a storage tank for the cooling medium, a heating device and a pump, with pipes which connect the storage tank with the cooling device for indirectly cooling the strip of cast metal and a heat exchanger which is located in the backflow pipe transporting the coolant from the cooling device to the storage tank.

Preferably, the cooling device has internals like wings, changes in the size and/or form of the cross section of the fluid channels, deviation plates, etc. which bring the coolant in turbulent state and eventually to provide a pertinent flow pattern.

The invention is described in greater detail referring to the enclosed drawing, which shows in

FIG. 1, purely schematically, a twin roll casting machine according to the invention,

FIG. 2 a part of a closed coolant loop,

FIG. 3 a cross section along line III-III of FIG. 4 and

FIG. 4 a cross section along line IV-IV of FIG. 3.

In FIG. 1, a twin roll casting machine for vertical strip casting is shown in principal, adopted with the devices necessary for cooling the rolls. A difference to common twin roll casters is that the bearing of the rolls is arranged in that way, that the rolls are open, preferably on both ends, allowing a girder 9 to extend through the drums essentially parallel to their axis of rotation. This girder 9 is fixed on a bearing pedestal 10 on each side of the drum. Feeding pipes 2 and backflow pipes 6 for the coolant are provided.

FIG. 2 shows the coolant supply device with a coolant storage tank 12, a coolant pump 13 that is connected to the feeding pipes 2 in FIG. 1 and the backflow pipes 6 of the coolant. The backflow passes through an external heat exchanger 14, where the dissipation heat is transmitted to a secondary cooling cycle, and then back to the coolant storage tank.

FIG. 3 shows the cross section III-III of FIG. 4, being a cross section in the middle of the right drum in FIG. 1. Here, the arrangement of the cooling device for the twin roll shell can be seen in greater detail. The cooling device consists of the feeding pipe 2, which is connected to a distribution chamber 3, the distribution chamber 3 ensures a defined pressure loss and hence an even distribution of the coolant along the length of the drum. After passing the distribution chamber, the coolant flows up through the heat exchanger 4, where it takes up the dissipation heat of the strip from the hot drum shell 1. Leaving the heat exchanger 4, the coolant flows into the collection chamber 5, where it is collected and fed into the backflow pipe 6, which leads out of the drum to the external heat exchanger 14, shown in FIG. 2.

In the heat exchanger 14 the dissipation heat is transfused to a secondary cooling system, and may be used elsewhere in the mill for heating tasks. Finally, the coolant flows back into the coolant storage tank 12. It is, of course, possible and in some appliances advantageous to provide two or even more primary and or secondary cooling systems in order to provide adequate cooling in different areas of the drums, belts or the like.

The coolant is stored in the tank 12 and pumped into the feeding pipe 2 by a pump 13. This pump can be submerged in the coolant storage tank or be outside of the tank connected by a pipe, how it is shown in FIG. 2. These different possible installations of the pump are depending on the type of pump, which is used and the necessary coolant flow rate. The pump can be a mechanical pump but also an electromagnetic pump, depending on the value of the necessary coolant flow rate, pump efficiency, investment costs and the coolant itself. For the man skilled in the art of metal casting, the selection of the pump and its installation is, knowing the invention and the details of the plant, no problem.

Referring to FIG. 3, the internal heat exchanger 4 is pressed by hydraulic pistons 7 onto the inner surface of the caster shell 1. The rotation of the internal heat exchanger 4 is prevented by the girders 8 and 9. The hydraulic pistons 7 are supported by the girder 9. Additional provisions may be used to consider the thermal expansions during different stages of operation.

The internal heat exchanger 4 is designed with installations that allow very high turbulence of the coolant leading to very high heat transfer coefficients and to provide favourable fluid patterns near the various parts of the cooling surfaces.

FIG. 4 shows the longitudinal section IV-IV of FIG. 3, depicting the right drum of FIG. 1. One may see that the shell 1 of the drum is fixed on the hollow shafts 11, the position of the drum is fixed in the bearing 16. In order to ensure the rotation of the drum, a gear wheel 17 is provided, but the drive of the drums can also be installed in another way.

At 18, means for the tightening of the internal heat exchanger 4 against the shell 1 are provided. This may be done by one special packing ring, which is pressed between the circumference of the heat exchanger 4 and the inner side of the drum by the hydraulic pistons 7. A more complex design would be to arrange two packing rings along the circumference of the heat exchanger 4 in a certain distance of about 5 mm to each other, so that the small chamber between the two packing rings can be filled by inert gas and hence prevents liquid cooling metal leaking out of the heat exchanger 4. In any case the packing ring has to be made of a material, that allows higher temperatures and do not wear out too fast by the friction between caster shell and heat exchanger 4. In order to adapt the tightening device to the thermal expansion of the inner surface of the shell, springs or pneumatic pistons or the like may be provided.

The shell of the drum may consist of a copper alloy like Elbrodur B 95, or may comprise two hollow shells, where the outer shell is made of an copper alloy like Elbrodur B 95 or an other copper alloy allowing high heat withdrawal from the hot strip and the inner shell is a thin steel shell giving the drum the necessary stiffness. The shell can have a top layer of wear resistant material on its outer as well as on its inner surface in order to ensure a longer lifespan.

The storage tank for the coolant may be equipped with an inert gas inlet 15 in order to ensure an inert atmosphere in the tank. Such inert gas like Helium, Argon, and in some circumstances, depending on the chemical nature of the coolant, Nitrogen, may be used to bring out the air of the pipes and cooling areas prior to start the circulation of the coolant. Furthermore it may be advantageous to install a coolant refresh device that allows the regeneration of used, consumed coolant after a certain operation time.

It is possible to use the cooling system according to the invention for vertical and horizontal casting direction (or in any other desired angle). The invention is applicable for strip casting of ferrous metals like steel and for nonferrous metals like copper, aluminium, and other non-ferrous metals and their alloys. It is possible to adapt existing plants according to the invention without great problems; In order to do so, it is only necessary to mount suitable containments which prevent the exit of coolant along the inner surface of the drum or belt. The material of this casing has to withstand the temperature and chemical impact of the coolant as well as its dynamic forces, resulting from its density and flow. For the man skilled in the art of metal casting and knowing the invention, the selection of the necessary materials is no problem.

In order to ensure the preferred flow pattern and the required turbulent flow of the liquid metal, internals, like wings, changes in the size and/or form of the cross section of the fluid channels, deviation plates, etc. are provided. For the man skilled in the art of fluid thermodynamics, it is, in knowledge of the invention, no problem to design the layout of such a casing with its internals.

Depending on the used coolant in order to prevent the freezing of the device, it is preferred to heat all or at least most pipes and devices through which the coolant flows. Further, the pipes and devices should be mounted in inclined position. This ensures that, in case of a failure of the pump, the coolant flows by its weight back into the storage tank having a heating.

The advantages of the cooling concept according to the invention are: Easier and better cooling control, because the heat transfer number is very constant in comparison to that of cooling with water as coolant; Longer life time of the travelling mould, Higher casting rate, Smooth cast strip surface without surface defects; No or only a negligible inhomogeneous subsurface layer of the cast strip; Grain structure and segregation can be controlled by adjusting the coolant temperature and by coolant flow rate; Inline rolling of the cast strip, rod or wire is possible and safe energy costs for reheating.

Claims

1. A process for strip casting using a travelling mould which is cooled by a liquid coolant, wherein the coolant is liquid metal or ionic liquid.

2. The process according to claim 1, wherein the coolant is chosen from the group consisting of lead, tin, bismuth, gallium, indium, or alloys thereof.

3. The process according to claim 1, wherein the coolant has, in degrees Celsius, a melting point which is lower or equal 60% of the melting point of the casting material in degrees Celsius.

4. The process according to claim 1, wherein the direction of the flow of the coolant in the area of the travelling mould is counter to the direction of the movement of the cast strip.

5. The process according to claim 1, wherein the flow of the coolant in vicinity of the surface to be cooled is a turbulent flow.

6. The process according to claim 5, wherein the flow of the coolant in vicinity of the surface to be cooled is highly turbulent.

7. A cooling device for a process according to any one of the claims 1 to 6, the cooling device having a storage tank (12) for the coolant, a heating device and a pump (13), with pipes (2) which connect the storage tank (12) with the cooling device (4) for indirectly cooling a strip of cast metal and a heat exchanger (14) which is located in a backflow pipe (6) transporting the coolant from the cooling device to the storage tank.

8. The device according to claim 7, wherein the cooling device (4) includes one or more features configured to create a turbulent flow of coolant and/or create a predetermined flow pattern of coolant in a vicinity of a surface to be cooled.

9. The device according to claim 8, wherein the features include one or more of wing-shaped internal structures, changes in a size and/or form of a cross section of a fluid channel, and deviation plates.