Mold for Continuous Casting of Metal Strands

The present application is a National Stage Application of PCT International Application No. PCT/IB2023/053832 (filed on Apr. 14, 2023), under 35 U.S.C. § 371, which claims priority to European Patent Application No. 22168449.1 (filed on Apr. 14, 2022), which are each hereby incorporated by reference in their complete respective entireties.

The invention relates to the field of casthouse technology, and more precisely to machines for casting metal strands by direct chill (DC) casting. DC casting is a continuous (or more precisely, semi-continuous) casting process, and is of widespread use, in particular for casting certain non-ferrous metals and alloys. In particular the present invention relates to a novel mold for semi-continuous casting machines for casting strands or ingots in non-ferrous metals and alloys that are neither precious metals nor refractory metals, and in particular in aluminium and aluminium alloys.

The invention also relates to a method of casting a metal strand or ingot using such a mold, and to metal strands obtained by this method. The invention also relates to a method for maintaining or repairing said mould.

Casting machines for transforming molten metal into a solid, elongated body of metal are known for a long time. Open ended, cooled moulds with an inner perimeter wall are used for a so-called continuous (or rather semi-continuous) casting process. At the beginning of this casting process, liquid metal is poured through said open ended mold, which is closed by a starting block, also called bottom block. As the metal is solidifying in the neighborhood of the inner perimeter wall of the mould, the bottom plate advances into the casting direction, which may be vertical (and in this case downwards) or horizontal, while the emerging metal strand is cooled. Cooling of the emerging metal strand is usually achieved by water that is sprayed over the metal strand as it emerges from the mould outlet; for this reason, this process is also called «chill casting». When emerging from the mold outlet, the metal strand may still be liquid or semi-solid in its centre and will completely solidify in a few seconds under the effect of water-chilling as it moves away from the mould outlet. Such a process and related equipment are described in many patent documents, such as U.S. Patent Publication No. 2002/0148593 A1 and European Patent Publication No. EP 4 035 786 A1 (Rusal).

This process allows to produce metal strands with a substantially constant cross section; their length is limited by the dimensions of the equipment (namely the depth of the casting pit, in the case of vertical casting, or the length of the structure supporting the horizontally emerging metal strand), by the available quantity of liquid metal, and by the overall stability of the casting process. For increased productivity, multiple strands can be cast in one run from liquid metal: to this end, a plurality of molds is provided in a so-called casting table. For billet casting, casting tables with several dozens of molds are used, depending on the diameter of the billets.

Molds for continuous casting are usually complex tools made from chemically resistant materials with good thermal conductivity; they comprise a perimeter wall that is in contact with the liquid and solidifying metal, as well as cooling means. Cooling of the metal is usually achieved by water that is sprayed over the metal strand as it emerges from the outlet of mold.

Perimeter walls have a contact surface that experiences friction with the outer surface of the solidifying metal strand; they need to be lubricated in order to minimize wear and tear. When the surface state of a perimeter wall is terminally degraded such that the mold does no longer allow to cast billets with the required surface quality, the mold can be disabled in the casting table; this allows to continue the use of the casting table, but with decreased productivity.

Perimeter walls can be made from various materials. They can be made from metallic materials such as steel and will then need to be lubricated. They can also be made from graphite, which is a chemically inert, auto-lubricating material resisting to fairly high temperatures. Additional lubrication by appropriate lubricants can be provided, as graphite is porous and will let pass a certain amount of lubricant or gas, depending on the applied pressure, on the temperature, and on the effective contact surface. Such graphite perimeter walls can be made as inserts (sleeves) which can be replaced if necessary.

Certain prior art continuous billet casting systems for liquid aluminium are provided with a permeable or porous graphite perimeter wall through which lubricant and gas are distributed to the inside surface of the mold. This has been described in U.S. Pat. No. 4,598,763 (Wagstaff). Patent Publication No. WO 95/23044 (Wagstaff) discloses various embodiments of molds for casting rolling ingots; these molds have graphite rings provided with channels for cooling liquid sprayed onto the outer surface of the emerging metal strand. Conduits for delivery of oil or gas may be circumferential groove-shaped conduits, and/or they may be holes partially drilled through the perimeter wall. U.S. Patent Publication No. 2007/0163746 describes an improved perimeter wall lubrication system for molten aluminium molds using for billet casting, achieving a more even flow of lubricant through the peripheral wall. Permeable cavity rings, separated from each other by a gasket or sealing material, are described in Patent Publication No. WO 01/00353 (Norsk Hydro).

One objective of the present invention is to provide a mold for ingot casting that has improved lifetime, and that can be cleaned, repaired or renovated easily.

Another objective of the present invention is to provide a casting table and mold for multiple strand ingot casting that have improved lifetime.

Another objective of the invention is to improve the overall process stability of multiple strand ingot casting, while increasing casting speed (expressed, for a given cross section, in length of the metal strand per time unit), and in particular to improve the stability of the lubrication at the interface between the mold and the emerging metal surface.

Yet another objective of the invention is to improve the process control of multiple strand ingot casting.

Yet another objective of the invention is to decrease the quantity of lubricant used in multiple strand ingot casting, while ensuring sufficient and stable lubrication at the interface between the mold and the emerging metal surface.

According to the invention, these problems are solved by several means that cooperate with each other.

One essential feature of the invention is cooling of the mold body. According to prior art, during continuous casting a cooling fluid, usually water, is sprayed on the emerging strand of solidifying metal. As a result of the mold design according to the invention, there are at least two independent means of cooling.

Primary cooling is achieved through the mold body which is cooled from its inside during the casting operation. More precisely, the mold body is cooled by a cooling liquid circulating in cooling channels arranged in the wall of the mold; this allows efficient cooling of the peripheral wall of the mold which is in contact with the solidifying metal.

Secondary cooling is achieved by a water spray: The surface of the emerging metal strand is cooled by cooling fluid sprayed in an annular zone all over the surface of the strand.

In addition to these primary and secondary cooling means, the surface of the peripheral wall of the mold which is in contact with the solidifying metal is lubricated by an appropriate lubricant, such as oil or a mixture of oil and water. The use of a lubricant improves the surface quality of the emerging metal strand throughout the continuous casting process which proceeds under dynamic and steady state conditions.

Another essential feature of the invention is the use of a removable insert defining the perimeter wall which will be in contact with the solidifying metal. This allows efficient inspection, maintenance and repair of the mold, without disabling defective mold cavities in the case of multiple-strand molds. This insert may be a copper insert, but is advantageously made from aluminium or aluminium alloy. Surprisingly, the contact between the solidifying and moving aluminium skin of the metal strand being cast and the solid aluminium insert allows to obtain an excellent surface quality, especially if a lubricant is used.

The present invention is particularly useful for horizontal casting.

A first object of the invention is a mold for semi-continuous casting of metal strands from liquid metal, comprising a mold body provided with at least one mold cavity, said mold cavity comprising:

characterized in that said mold body is provided with a channel for cooling fluid, and in that said protective insert is removable, and is made from a metallic material.

During casting operation, liquid metal will be admitted to the mold through said nozzle plate and flows through said insert, which is arranged downstream of said liquid metal inlet. Said insert is cooled by a cooling fluid circulating in said channel. Said channel for cooling fluid is in fluid connection with a connection interface for cooling fluid. The liquid metal solidifies in contact with the cooled insert, and when leaving the mold outlet, at least the surface of the metal strand will be solid. At the mold outlet, cooling fluid is sprayed onto the surface of the metal strand as it leaves the mold.

Said nozzle plate is advantageously made from a non-metallic inorganic material, and preferably from a ceramic material.

Said insert is advantageously made from copper or copper alloy, or, preferably, from aluminium or aluminium alloy. These materials have a high thermal conductivity, are fully recyclable and easy to machine. Machinability is a significant asset here because it allows to provide the insert with a plurality of spraying nozzles by which the cooling fluid used for primary cooling of the insert is sprayed onto the merging metal strand. The inner perimeter wall of said insert, which is in contact with the cast metal, is advantageously made from copper or copper alloy, or, preferably, from aluminium or aluminium alloy. It has preferably a flat surface.

According to advantageous embodiments of the invention:

The mold according to the invention can further comprise a removable insert clamping plate, fixed onto said front plate and capable of clamping said insert against said nozzle plate.

Said insert can be clamped between said nozzle plate, using a nozzle clamping plate, and said mold body, using an insert clamping plate. This allows easy removal of the insert, for inspection, repair, maintenance and/or replacement. Said insert has a noncircular cross section, allowing to cast metal strands with a cross section that is noncircular, such as foundry ingots.

According to an advantageous embodiment, said mold further comprises at least one conduit provided in said insert which is in fluid connection with a connection interface for a lubricant fluid. In particular, said conduit is designed for admitting a lubricant fluid between said insert and said nozzle plate.

The mold according to the invention can comprise comprise a plurality of mold cavities, each of which showing the inventive features mentioned above. Said mold cavities can be identical or not. They are advantageously aligned on one single axis (i.e., arranged in a row), and in this case, the front plate forms advantageously one single piece, and the back plate advantageously forms one single piece, too. Said waterbox extends substantially over the whole mold.

In an advantageous embodiment the number of mold cavities is comprised between five and twenty, preferably arranged in a row.

In an advantageous embodiment each mold cavity has its own means of adjustment for the pressure and/or the throughput of said lubricant fluid.

Another object of the invention is the use of a mold according to any of the embodiments of the invention for casting of metal strands with a noncircular cross section, said metal strands being preferably foundry ingots. During casting, cooling fluid is circulated in said cooling channels and/or said waterbox, and/or cooling fluid is sprayed onto the metal strand emerging from the mold outlet using said nozzles, and/or lubricant fluid is admitted through said conduits surface of said metal strand.

Another object of the invention is a method for casting a metal strand from liquid metal, in particular a metal strand for use as a foundry ingot, using said mold according to the invention, with the proviso that said metal strand does not have a circular cross section.

In this method, the temperature of the cooling water is typically between about 5° C. and about 50° C., the volume of cooling water per cast mass of aluminium is between about 5 Nm/t and about 40 Nm/t, the volume of lubrication agent per cast mass of aluminium is comprised between about 5 mL/t and about 400 mL/t, the volume of lubrication agent per strand and hour is comprised between about 0.5 mL/h and 18.5 mL/h, and the casting speed was between about 0.1 mm/s and about 15 mm/s. Said lubrication agent can typically be used in a concentration of between about 0.05 L/mand about 2 L/mof water, preferably between about 0.1 L/mand about 0.8 L/mof water, and more preferably between about 0.14 L/mand about 0.6 L/mof water. If there is not enough lubrication, the surface of the metal strand shows increased surface roughness; if there is too much lubrication the surface of the metal strand may show ripples.

Another object of the invention is a method of maintaining or repairing a mold according to the invention, wherein said insert clamping plate is removed, said insert is removed for inspection and/or maintenance and/or repair and/or replacement, another or the same insert is inserted, and said insert calming plate is put in place again and fastened. Said maintenance or repair can comprise polishing of the perimeter surface prior to putting in place the insert again.

The term “aluminium” as used herein includes aluminium alloys, and the term “copper” as used herein includes copper alloys.

Typical molds according to the invention will be described here in relation with.

The moldshown onin a perspective front view comprises ten mold cavitiesarranged in a row.shows the corresponding perspective rear view. In this embodiment the mold cavitiesare identical, and in particular have an identical cross section, but in other embodiments they can have different cross sections and/or sizes. Each mold cavityhas a liquid metal inlet(visible on) and a mold outlet. During the continuous casting process, liquid metal will be introduced into the mould cavitythrough its inlet, and a solidified metal strand (not shown on the figures) will emerge from said mold outlet.

The moldcomprises several parts, namely a front plateand a back plate, arranged parallel to each other, and each manufactured from a single piece. Said front plateand back plateare pressed against a body, held together by appropriate meansfor fastening or assembling, such as bolts. The back platecomprises the liquid metal inlet, whereas the front platecomprises the mold outlet. Said body, front plateand back plateare typically made from metallic materials having a good thermal conductivity. As an example, the following metals and their alloys can be used: aluminium, copper, titanium, as well as stainless steel. As an example, if the moldis intended to be used for casting aluminium or aluminium alloys, the back plateis typically made from steel, and the front plateis typically made from copper or aluminium, whereas the bodyis advangeously made from copper.

The inner surface (called here «contact surface») of a mold is in contact with liquid metal and solid metal and is subject to friction and wear. According to the invention, at least part of the contact surface is the inner surface of a sleeve which can be replaced if needed. This sleeve takes the form of an insert, which protects at least part of the mold cavity. In particular, said insert forms the perimeter wallof the mold cavity, the inner, flat surfaceof said perimeter wall being in contact with the molten metal during the casting operation.

Said insertis substantially rectangular in cross section; its edges may be rounded. The insertis fixed with an insert clamping plateagainst the front plateusing fixing means such as bolts. This can be seen from, as well as fromwhich shows an enlarged exploded perspective front view of the mold of. As can be seen, the insertcan be removed by unlocking said fixing meansand removing the clamping plate.

According to the invention, any direct contact between the inner surface of the mold bodyand the liquid metal is avoided. Instead, it is the inner surfaceof the perimeter wall of the protective insertthat will be in contact with the liquid metal. Said protective insertis in thermal contact with a cooling fluid. Said protective insertbeing subject to wear, it can be replaced if necessary. Protective insertswith a cross section that is substantially rectangular are shown infor a first embodiment; protective inserts having a cross section that is substantially circular can be used for billet casting (not shown of the figures).

On the rear of the mold, the insertis completed by a nozzle plate. This can be seen fromwhich shows a perspective rear view of the mold of. The liquid metal enters the liquid metal inletthrough the nozzle platehaving a substantially circular perimeter wall. The nozzle plateis fixed against the back plateof the mold bodyby nozzle clamping plateusing a fixation meanssuch as bolts. Said nozzle clamping plateadvantageously has the same height as the mold boldly. Said nozzle plateand said nozzle clamping plateare typically arranged parallel to each other, and parallel with respect to the front plateand the back plate. As can be seen from, nozzle platecan be replaced very easily by removing the nozzle clamping plate.

shows an enlarged exploded perspective rear view of the mold of. Inserthas been introduced from the front side, and nozzle plateis inserted from the rear side, abutting against the insert; the back plateis then pressed against nozzle plateusing said fixation means.

This can be seen fromwhich shows a cross section of a mold cavitysimilar to that shown on. Said nozzle platehas an opening acting as the liquid metal inlet, delimited by a perimeter wall, and a substantially flat bottom portion, the front surfaceof which abuts against the rear surfaceof the insert. One or more O-ringscan be used to ensure tightness of this abutment.

According to the invention, the moldis provided with cooling means. Said cooling means are of at least two kinds: the mold itself is cooled by a cooling fluid that circulates in cooling channels(so-called primary cooling means), and a plurality of spray nozzlesare provided in the protective insertat the mold outletfor spraying a cooling fluid onto the emerging metal strand are provided (so-called secondary cooling means). This will now be explained in more detail.

Said primary and secondary cooling means are provided in the form of at least one cooling fluid circuit. A circuit for cooling fluid is provided in the mold body, said circuit comprising cooling fluid channelsand/or a cooling fluid box(as in the embodiment of). An appropriate connection interfaceis provided to connect said cooling fluid circuit to an external supply of cooling fluid. Said cooling circuit carries the cooling fluid through channelsarranged in the mold body, and in particular in the outlet part of the mold, in order to cool said mold bodyand in particular said outlet part. As the most frequently used cooling fluid is water, the cooling circuit will here also be called a «water circuit» and the cooling fluid boxwill here also be called a “water box”, which does however not imply that other fluids cannot be used (such as water-glycol mixtures).

shows the water box circuit, which ensures primary cooling of the mold. Said cooling fluid channel(called here also «water channel») is provided in vicinity of the mold bodyand preferably, as in, adjacent to the mold body, so as to ensure excellent thermal contact with the perimeter wall, said perimeter wallbeing itself in thermal contact with the hot metal being cast. The cooling fluid circulating in said fluid channelis therefore a means to cool the liquid metal during casting, as it passes through the insert: in contact with the perimeter wall the liquid metal will solidify at least in a certain thickness from its surface, said thickness in a given portion of metal being increasing as said metal portion is passing through the insert.