Throughout this specification, the present invention will be described in relation to magnesium but it is to be understood that the present invention is applicable to other reactive metals.

BACKGROUND ART Direct chill casting is a process in which molten metal is fed to a water-cooled mould and a cast metal strand is withdrawn from the mould. Direct chill casting includes horizontal direct chill casting and vertical direct chill casting. The strand withdrawn from the mould consists of a solid shell surrounding a liquid core which subsequently solidifies. The strand is sprayed with cooling water as it exits the mould to both cool the strand and to extract additional heat from within the water-cooled mould. A casting lubricant is used in direct chill casting to reduce friction between the solidified shell and the face of the mould for the dual purposes of reducing mould wear and avoiding tearing of the shell.

Casting lubricants include casting oils and casting greases. Casting oils are typically continuously fed to the interface between the face of the mould and the metal by an arrangement which is either integrally formed with the mould or which is separately constructed to function with the mould. Casting greases may be applied manually by brushing the face of the mould prior to casting or by use of an automatic grease lubricating system such as that disclosed in International Publication No. WO 94/00258. Casting lubricants require high thermal stability and resistance to oxidative degradation at high

temperatures. Throughout this specification, the expression "casting lubricant" is to be understood to include such casting oils and greases and includes animal, vegetable, synthetic and mineral casting oils and greases such as vegetable shortening, lard used in baking, castor oil, rapeseed oil, esters, paraffins and synthetic liquids.

Magnesium is a highly reactive and thermodynamically unstable element with molten magnesium readily oxidising in ambient air. Three approaches have been used to inhibit the severe oxidation process. Salt cover fluxes may be sprinkled over the molten metal; oxygen may be excluded from contacting the molten metal by blanketing the molten metal with an inert gas such as helium, nitrogen or argon; or a protective cover gas may be used to blanket the molten metal. Protective cover gases typically comprise air and/or carbon dioxide and a small amount of an inhibiting agent which reacts/interacts with the molten metal to form a film/layer on the molten metal which protects it from oxidation. The mechanism by which inhibiting agents protect molten reactive metals is not well understood.

US 1972317 was filed in 1932 and teaches a method for inhibiting the oxidation of readily oxidisable metals (for example, magnesium) by maintaining a fluorine containing atmosphere in contact with the surface of the metal. Lines 70-74 of the patent state, "The present invention is not concerned with, nor do the claims cover, any step in which the inhibition of detrimental attack by atmospheric gases upon magnesium takes place within a casting mould." US 3034186 was filed in 1956 and teaches an aluminium casting lubricant consisting of a dispersion of solid boric acid (H3B03) in a suitable oil or oily based material. The casting lubricant is said to inhibit metal oxidation on ingot surfaces and prevent magnesium burning in aluminium alloys with high magnesium content. The

casting lubricant is believed to have never been commercially used.

US 4930566 teaches a method for continuous casting of an aluminium-lithium alloy through an open-ended mould which is lubricated by a casting lubricant. Pressurised gas is directed into the mould to contact the molten metal immediately prior to solidification to displace the point at which the metal contacts the mould. The gas comprises 1-15k by volume oxygen with the balance being an inert and incombustible gas such as nitrogen, argon, carbon dioxide, helium or a mixture thereof. A variation of the teaching of US 4930566 relates to the casting of magnesium in which the pressurised gas is carbon dioxide containing 10% sulphur hexafluoride (SF6). The gas and the casting lubricant are separately introduced into the mould.

CA 2047384 teaches a method for vertical direct chill casting in which a casting lubricant is used to lubricate the mould and a protective gas containing a high concentration of an inhibiting agent such as SF6 is drawn into the mould by a self-generating vacuum.

JP 2-277098 teaches a non-ferrous metal casting lubricant consisting of a plant oil lubricant containing 0.5-3k by weight of powdered boron nitride which is said to improve lubricating properties and reduce the amount of lubricant required as compared with plant oil lubricant alone.

SUMMARY OF THE PRESENT INVENTION In a first aspect, the present invention provides a mould lubricant comprising a casting lubricant containing a gaseous oxidation inhibiting agent.

Gaseous oxidation inhibiting agents include inert gases such as nitrogen (N2) and argon (Ar) and sulphur or fluorine containing gases such as sulphur dioxide (SO2), sulphur hexafluoride (SF6), boron trifluoride (BF3), sulphur and fluorine containing gases as described in US 1972317, and mixtures thereof.

The mould lubricant according to the first aspect of the present invention may be prepared by evacuating the casting lubricant and returning it to a desired pressure by introduction of the gaseous oxidation inhibiting agent.

In a second aspect, the present invention provides a method for direct chill casting of a reactive metal through a mould, the method comprising lubricating the mould with a mould lubricant according to the first aspect of the present invention.

The method according to the second aspect of the present invention can be applied to the casting of reactive metals in conventional horizontal or vertical direct chill casting equipment by substituting the conventional casting lubricant with a mould lubricant according to the first aspect of the present invention.

The mould lubricant may be introduced to the mould from a reservoir where it is maintained at, above, or below atmospheric pressure.

In a third aspect, the present invention provides a reactive metal cast by the method according to the second aspect of the present invention.

Although the precise mechanism is not well understood, it is believed that surface oxidation of a reactive metal cast according to the second aspect of the present invention is minimised by the presence of the gaseous oxidation inhibiting agent which reacts or interacts with the surface of the molten reactive metal.

Embodiments of the present invention will now be described by way of example only.

Example 1 A mould lubricant according to the first aspect of the present invention was prepared by dissolving SF6 in canola oil.

A 500ml sample of canola oil was placed in a clean dry vacuum flask and evacuated to 70kPa for approximately 14 hours. The evacuated oil was agitated and returned to atmospheric pressure by slowly bubbling SF6 into the oil

with an SF6 atmosphere being maintained above the oil.

The flask was weighed before evacuation and after SF6 addition with the weight increase being 3.78 grams.

Samples of the resulting mould lubricant and untreated canola oil were analysed by gas chromatography. The untreated sample was found to contain approximately 6.6k by weight dissolved air and no SF6. The mould lubricant was found to contain approximately 4.7W by weight SF6 and approximately 5.8W by weight air.

Example 2 A strand of magnesium was cast on a Hertwich horizontal direct chill casting machine fitted with a standard 100mm x 100mm square cross-section mould using a mould lubricant prepared according to Example 1 to lubricate the mould.

Prior to casting, the standard oil reservoir was thoroughly cleaned, new oil lines were fitted to the oil injectors and new refractory components were fitted to the mould. The mould lubricant was placed in the oil reservoir and SF6 was bubbled into the mould lubricant in the oil reservoir to maintain an SF6 blanket above the oil to minimise liberation of SF6 from the mould lubricant.

As is conventional, the mould lubricant was brought up to the mould from the oil reservoir via the oil lines and oil injectors and smeared over the face of the mould prior to the commencement of casting. A magnesium strand was cast at a casting speed of 300mm/min and the surface of the strand was bright and shiny immediately after casting indicating that the presence of SF6 in the mould lubricant had inhibited oxidation of the surface of the magnesium strand. The surface of the strand was analysed by X-ray Photoelectron Spectroscopy (XPS) which indicated the presence of a fluoride film on the surface of the strand.

Comparative Example 1 A comparative casting trial was performed which was identical in all respects to Example 2 except that the mould lubricant was replaced by untreated canola oil and SF6 was not bubbled into the oil reservoir. Immediately after casting, the resulting strand of magnesium had a discoloured surface featuring blackened areas which were believed to be due to oxidation.