WO 2008/141398 PCT/AU2008/000738 METAL-COATED STEEL STRIP The present invention relates to strip, typically steel strip, which has a corrosion-resistant metal alloy 5 coating. The present invention relates particularly to a corrosion-resistant metal alloy coating that contains aluminium-zinc-silicon-magnesium as the main elements in 10 the alloy, and is hereinafter referred to as an "Al-Zn-Si Mg alloy" on this basis. The alloy coating may contain other elements that are present as deliberate alloying additions or as unavoidable impurities. Hence, the phrase "Al-Zn-Si-Mg alloy" is understood to cover alloys that 15 contain such other elements as deliberate alloying additions or as unavoidable impurities. The metal-coated strip may be sold as an end product itself or may have a paint coating applied to one or both surfaces and be sold as a painted end product. 20 The present invention relates particularly but not exclusively to steel strip that is coated with the above-described Al-Zn-Si-Mg alloy and can be cold formed (e.g. by roll forming) into an end-use product, such as 25 roofing products. Typically, the Al-Zn-Si-Mg alloy comprises the following ranges in % by weight of the elements aluminium, zinc, silicon, and magnesium: 30 Aluminium: 40 to 60 % by weight Zinc: 40 to 60 % by weight Silicon: 0.3 to 3% by weight Magnesium 0.3 to 10 % by weight 35 Typically, the corrosion-resistant metal alloy coating is formed on steel strip by a hot dip coating WO 2008/141398 PCT/AU2008/000738 -2 method. In the conventional hot-dip metal coating method, steel strip generally passes through one or more heat 5 treatment furnaces and thereafter into and through a bath of molten metal alloy held in a coating pot. The heat treatment furnace that is adjacent a coating pot has an outlet snout that extends downwardly to a location below an upper surface of the bath. 10 The metal alloy is usually maintained molten in the coating pot by the use of heating inductors. The strip usually exits the heat treatment furnaces via an outlet end section in the form of an elongated furnace 15 exit chute or snout that dips into the bath. Within the bath the strip passes around one or more sink rolls and is taken upwardly out of the bath and is coated with the metal alloy as it passes through the bath. 20 After leaving the coating bath the metal alloy coated strip passes through a coating thickness control station, such as a gas knife or gas wiping station, at which its coated surfaces are subjected to jets of wiping gas to control the thickness of the coating. 25 The metal alloy coated strip then passes through a cooling section and is subjected to forced cooling. The cooled metal alloy coated strip may 30 thereafter be optionally conditioned by passing the coated strip successively through a skin pass rolling section (also known as a temper rolling section) and a tension levelling section. The conditioned strip is coiled at a coiling station. 35 Depending on the end-use application, the metal coated strip may be painted, for example with a polymeric WO 2008/141398 PCT/AU2008/000738 -3 paint, on one or both surfaces of the strip. One corrosion resistant metal coating composition that is used widely in Australia and elsewhere for 5 building products, particularly profiled wall and roofing sheets, is a 55%Al-Zn coating composition that also comprises Si. The profiled sheets are usually manufactured by cold-forming painted, metal alloy coated strip. Typically, the profiled sheets are manufactured by 10 roll-forming the painted strip. After solidification, a 55%Al-Zn alloy coating normally consists of alpha-Al phase dendrites and beta-Zn phase in interdendritic regions. Silicon is added to the 15 coating alloy composition to prevent excessive alloying between the steel substrate and the molten coating in the hot-dip coating method. A portion of the silicon takes part in quaternary alloy layer formation but the majority of the silicon precipitates as needle-like, pure silicon 20 particles during solidification. These needle-like silicon particles are present in interdendritic regions. It has been found by the applicant that when Mg is included in a 55%Al-Zn-Si alloy coating composition, Mg 25 brings about certain beneficial effects on product performance, such as improved cut-edge protection, by changing the nature of corrosion products formed. However, it has also been found by the applicant 30 that Mg reacts with Si to form a Mg 2 Si phase and that the formation of Mg 2 Si phase compromises the abovementioned beneficial effects of Mg in many ways. In particular, the Mg 2 Si phase is more voluminous than Si, is brittle, and has a "Chinese script" morphology with sharp edges. At levels 35 of Mg less than around 3%, and depending on other elements added, the Mg 2 Si forms as a continuous film in interdendritic regions, often growing across the full WO 2008/141398 PCT/AU2008/000738 -4 thickness of the thin metallic coating. The Mg 2 Si phase provides crack initiation sites, crack propagation pathways and potential corrosion conduits to the base steel and is undesirable on that basis. 5 Patent literature reviewed by the applicant has highlighted that increasing the cooling rate during solidification can refine the dendrite solidification structure of a 55%Al-Zn-Si alloy coating and also the size 10 of Mg 2 Si phase in the structure. However, it is also the case that cooling a molten coating at rates high enough to sufficiently refine the Mg 2 Si phase is very difficult to achieve without damaging the soft coating surface and consequently has not been an option on this basis. 15 The above description is not to be read as an admission of the common general knowledge in Australia or elsewhere. 20 In general terms, the present invention provides -a method of forming a metal coated strip, such as a steel strip, that comprises solidifying a molten coating of an Al-Zn-Si-Mg alloy on the strip with a high rate of cooling only after a sufficiently strong, solid supporting network 25 of alpha-Al phase dendrites has been established in the coating and before a Mg 2 Si phase has started to form in the coating. According to the present invention there is 30 provided a method of forming a metal coated strip, such as a steel strip, that comprises: (a) passing the strip through a hot dip coating bath that contains Al, Zn, Si, and Mg and optionally other 35 elements and forming a molten Al-Zn-Si-Mg alloy coating on the strip, and WO 2008/141398 PCT/AU2008/000738 -5 (b) cooling the coated strip to solidify the molten Al-Zn-Si-Mg alloy on the strip, with the cooling step comprising a high rate of cooling the coated strip only after a sufficiently strong, solid supporting network 5 of alpha-Al phase dendrites has been established in the coating and before a Mg 2 Si phase has started to precipitate. The present invention is based on a realisation 10 that it is beneficial to solidify a molten coating of an Al-Zn-Si-Mg alloy on a strip, such as a steel strip, with a high rate of cooling only after a sufficiently strong, solid supporting network of alpha-Al phase dendrites has been established in the coating and before a Mg 2 Si phase 15 has started to form in the coating. The applicant has found that a high cooling rate during this stage of solidification disrupts the growth of the undesirable continuous Mg 2 Si phase film mentioned above 20 and results in a discontinuous (i.e. a fragmented) Mg 2 Si phase that, in overall terms, results in substantially improved properties for the coating compared to the properties of a coating of the same composition obtained by solidifying the molten coating at a conventional 25 cooling rate. Typically, a start temperature for the high rate of cooling is 500 0 C. 30 In any given situation, the start temperature depends on a number of factors. One important factor is the composition of the Al-Zn-Si-Mg alloy coating. However, in any given situation, a skilled person can readily determine by experimentation an appropriate start 35 temperature. Typically, the start temperature is in a range of WO 2008/141398 PCT/AU2008/000738 460-500 0 C. Typically, the start temperature is a temperature at which at least 50% by volume of the coating has 5 solidified as dendrites of an alpha-Al phase. The start temperature may be a temperature at which at least 60% by volume of the coating has solidified as dendrites of an alpha-Al phase. 10 Typically, the high rate of cooling is at least 150OC/sec. The high rate of cooling may be at least 15 200*C/sec. Typically, the high rate of cooling is in a range of 150-2000OC/sec. 20 Typically, a finish temperature for the high rate of cooling is in a range of 280-330*C. Typically, an average rate of cooling at the high rates of cooling is at least 250*C/s. 25 Typically, the cooling step includes an initial cooling step down to the start temperature, with the initial cooling step being at a lower rate than the later high rate of cooling. 30 Typically, the lower cooling rate is in a range of 30-150*C/sec. Typically, the coating is 5-30 microns in 35 thickness. Typically, the method further comprises forming a WO 2008/141398 PCT/AU2008/000738 -7 coating of a paint on the coated strip. According to the present invention there is also provided a strip, such as a steel strip, that has a 5 coating of an Al-Zn-Si-Mg alloy on the strip that is formed by the above-described method. Typically, the coating is in a range of 5-30 microns in thickness per side of the strip. 10 Typically, the coating has a microstructure that comprises alpha-Al phase dendrites, a Zn-rich phase (such as beta-Zn phase and/or MgZn 2 phase) in interdendritic regions, and fragmented Mg 2 Si phase in interdendritic 15 regions. The present invention is based on experimental work carried out by the applicant. 20 The experimental work was carried out on samples of steel strip that were coated with a 54%Al-42%Zn-1.5%Si 2%Mg-O.5%Fe alloy. 25 The samples were heated in an annealing furnace to a temperature and for a time that was sufficient to melt the alloy coatings and then cooled to ambient temperature at selected cooling rates. 30 Specifically: (a) one set of samples was cooled at a conventional cooling rate of 70*C/s to ambient temperature, and 35 (b) another set of samples was cooled at a conventional cooling rate of 70*C/s to a temperature of 500 0 C and then cooled at a significantly higher cooling WO 2008/141398 PCT/AU2008/000738 rate of 800 0 C/s to ambient temperature, in accordance with one embodiment of the method of the present invention. Figures 1 and 2 are images of the microstructures 5 of representative samples from items (a) and (b) above after the samples were cooled to ambient temperature. The microstructure in Figure 1 shows that the conventional cooling rate produced a coarse Mg 2 Si phase 10 identified by the arrows in the Figure - in an interdendritic region between alpha-Al phase dendrites. The lighter regions in the image are Zn-rich phases (including beta-Zn and/or MgZn 2 ) in interdendritic regions. 15 The microstructure in Figure 2 shows that the initial relatively slow cooling rate followed by the high cooling rate produced a microstructure having fine Mg 2 Si phase identified by the arrows in the Figure. The microstructure also comprises alpha-Al phase dendrites and 20 Zn-rich phases (including beta-Zn and/or MgZn 2 ) in interdendritic regions. The applicant found that cracking behaviour of the coating changed as a result of the above 25 microsctructural changes. With the microstructure in Figure 1, crack initiation occurs by micro-cracking of the brittle phases Mg 2 Si in the coating at quite low strains, nominally less than 2%, during bending. Crack propagation is facilitated by continuity of the brittle phases, being 30 easier for large continuous precipitates. However, with the microstructure in Figure 2, where the precipitates are finer and discrete, micro-cracking occurs within the precipitates and crack propagation occurs by flow localisation in the adjoining ductile matrix and crack 35 linking between precipitates. Crack propagation is retarded by a more ductile alpha-Al phase matrix, resulting in improvement in coating ductility. From the WO 2008/141398 PCT/AU2008/000738 -9 experience of the applicant, the above changes to the coating structure (refinement) may also retard the level of corrosion associated with a similar level of cracking. 5 Many modifications may be made to the present invention as described above without departing from the spirit and scope of the invention.