Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
2000-09-18
2002-10-29
Elve, M. Alexandra (Department: 1725)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S486000
Reexamination Certificate
active
06470959
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the control of heat flux in continuous metal casters, particularly (although not exclusively) those used for the continuous casting of aluminum and aluminum alloys. More particularly, the invention relates to a process of casting a molten metal to form a cast metal strip ingot while exerting control over the rate of withdrawal of heat from the cast metal to avoid surface defects and distortions of the casting cavity. The invention also relates to apparatus used in the process.
Continuous casters, such as twin belt casters and recirculating block casters, are commonly used for producing strip ingots (continuous metal strips) from molten metals, particularly aluminum alloys. In casters of this kind, a casting cavity is formed between continuously moving casting surfaces and molten metal is introduced into the casting cavity on a continuous basis. Heat is withdrawn from the metal via the casting surfaces and the metal solidifies in the form of a strip ingot that is continuously withdrawn from the casting cavity by the moving casting surfaces. The heat flux (or heat extracted from the solidifying metal) must be carefully controlled to achieve cast strip ingot of good surface quality and to avoid distortion of the casting cavity. Different metals (e.g. aluminum alloys) require different levels of heat flux for proper casting on a continuous basis, so it is important to be able to control the casting apparatus to provide the required levels of heat flux for a particular metal being cast.
The primary heat flux control is usually achieved by applying cooling water to the casting surfaces. In most belt casters this is done on the back face of the belt passing though the casting cavity. Other caster designs apply cooling water at positions remote from the casting cavity. However, the heat flux is often adjusted more precisely by additional means. For example, belt casters have been provided with porous ceramic coatings over the metal belts. Such coatings may optionally be partially or completely filled with a high conductivity inert gas, such as helium, to provide further refinement. In such cases, the expense of maintaining a consistent ceramic coating and the cost of the inert gas have made such procedures economically unattractive.
It is also known to apply a layer of a non-volatile liquid, e.g. an oil, to the casting surfaces before they come into contact with the molten metal. This layer is often referred to as “belt dressing” or as a “parting layer”. The thickness of the layer can be varied to provide for control of heat flux to the underlying casting surfaces. However, the use of such oils may adversely affect the surface quality of the cast strip ingot (particularly ingots made from aluminum alloys containing high levels of magnesium), and may give rise to environmental issues, particularly when excessive applications are required in order to achieve the desired degree of heat flux control.
An example of a continuous casting apparatus requiring heat flux control is described in U.S. Pat. No. 4,593,742 which issued on Jun. 10, 1986 to Hazelett et al., and was assigned to Hazelett Strip-Casting Corporation. The apparatus of the patent is a twin belt caster employing a flexible nozzle for introducing molten metal into the casting cavity formed between the belts. Heat flux is withdrawn through the casting belts by means of a high velocity moving layer of liquid coolant traveling along the reverse surfaces of the belts. In this patent, mention is made of the supply of a non-reactive (inert) protective gas to the inlet of the casting cavity to protect the molten metal from chemical attack.
U.S. Pat. No. 3,630,266, which issued on Dec. 28, 1971 to Leonard Watts, and was assigned to Technicon Corporation, also discloses a continuous caster having a casting nozzle introducing molten metal into a cooled casting cavity. In this case, a gas is supplied to the region of the cavity inlet to insulate the casting nozzle and to prevent the formation of solidified metal bridges between the nozzle and the cavity.
BRIEF SUMMARY OF THE INVENTION
An object of the invention is to facilitate the control of heat flux in continuous casting apparatus used for producing metal strip ingots from molten metals, particularly aluminum and aluminum alloys.
Another object of the invention is to enable the production of high surface quality metal strip ingots from continuous casting apparatus under changing operational conditions.
In the present invention, mention is made of the “region of the meniscus”. This is the open region (i.e. not containing molten metal) within the casting apparatus where the molten metal first engages a casting surface (forming a meniscus) and is therefore adjacent to the meniscus and is generally in gaseous communication with the exterior of the casting apparatus.
The present invention uses a controlled source of water vapour (steam) to create a stream of gas (usually air) of known and easily controllable humidity, which is used to flood the area of the caster in the region of the meniscus. It has been found that this produces an effect on the heat flux that is much larger than would be expected based on the change in the thermal conductivity of the gas brought about by the addition of the moisture. This can be used as a convenient and relatively inexpensive way of avoiding thermal distortion by controlling heat flux from the caster in way that, in particular, may be used with existing casting equipment with minor modification.
In one preferred aspect, the present invention provides a process of casting a molten metal to form a cast metal strip ingot, in which good heat flux control may be provided. The process continuously supplies molten metal to a casting cavity formed between a pair of moving continuous casting surfaces that withdraw heat from the molten metal to cause metal solidification, and continuously withdraws a resulting cast strip ingot from the casting cavity. The molten metal at an inlet of the casting cavity forms at least one meniscus at a position where the molten metal first contacts the casting surfaces. The invention involves supplying a gas containing water vapour substantially without liquid water to the inlet of the casting cavity in the region of the meniscus (a region containing the meniscus(es)) to control the heat withdrawal by the casting. Preferably there is sufficient space between the casting surface and the solidifying metal strip such that gas can penetrate the space during casting.
As an example of typical equipment and processor to which the present invention may be applied, there may be mentioned Sivilotti U.S. Pat. No. 4,061,177 incorporated herein by reference.
The heat withdrawal may be controlled to a single value by measuring the heat flux or temperature at some point along the casting cavity and comparing the measurement to a target parameter, or may be controlled to a predetermined function along the casting cavity by means of multiple heat flux or temperature measurements. Temperature measurements may include slab temperature measurements, including measurements at the exit of the casting cavity or temperature measurements at points behind the casting surface within the casting cavity. Heat fluxes, for example, may be determined by measuring the temperature increase of the coolant used to cool the casting surface in one or more locations and the flow rate of that coolant.
The gas containing water vapour may be obtained in a number of ways. It may be created, for example, by mixing a dry gas and steam externally of the region of the meniscus or within the region of the meniscus. For example, the gas may be supplied by providing a porous block or similar device adjacent to the region of the meniscus so that the porous block becomes heated by the molten metal, injecting liquid water into the interior of the porous block so that the liquid water is vapourized within the heated porous block and thereby forms a mixture of gas containing water vapour in the regions of the meniscus. However, it is
Desrosiers Ronald Roger
Fitzsimon John
Larouche André
Alcan International Limited
Cooper & Dunham LLP
Elve M. Alexandra
Tran Len
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