Patent Application
20050236135
This invention relates to twin roll casting of magnesium and magnesium alloys (herein generally referred to collectively as “magnesium alloy”).
The concept of twin roll casting of metals is old, dating back at least to inventions by Henry Bessemer in the mid-1900's. However, it was not until about 100 years later that interest in possible commercial use of twin roll casting began to be investigated. The concept as proposed by Bessemer was based on the production of strip using a metal-feeding system in which molten metal was fed upwardly through a bite defined between two laterally spaced, parallel rolls. More recent proposals were based on a downwards feed of molten metal to the rolls. However it has become accepted that the preferred arrangement is with the rolls spaced vertically, rather than horizontally as in those earlier proposals, with the alloy feed being substantially horizontal. While the rolls are spaced vertically, their axes preferably are in a plane which is inclined at a small angle of up to about 15° to the vertical. With this inclination, the lower roller is displaced downstream, relative to the upper roller, with respect to the direction of alloy feed to and beyond the bite.
While there has been some commercial use of twin roll casting, this has been limited in its extent. It also has been limited in the range of alloys to which it is applied, since use essentially has been restricted to suitable aluminium alloys. To this stage, there has been limited success in establishing a suitable process for twin roll casting of magnesium alloys.
In achieving a practical process for successfully twin roll casting of magnesium alloys, such as on a substantially continuous or a semi-continuous basis, there are several problems which need to be overcome. A first of these is that magnesium alloy melts tend to oxidise and catch fire, while moisture from any source presents a potential risk of explosion. There are established procedures based on use of a suitable flux or a suitable atmosphere to prevent oxidation and risk of fire, while moisture is able to be excluded. Also, magnesium and some magnesium alloys that do not contain or have only low additions of beryllium, such as AZ31, can have a high tendency to oxidise in the melt state, such that conventional flux or the atmosphere control is not adequate during the twin roll casting operation. However, overcoming these problems adds to the complexity of processes for twin roll casting such that the complexity is a problem.
A further problem is that magnesium alloys have a thermal capacity such that, relative to aluminium alloys, they tend to freeze quickly. Also, again relative to aluminium alloys, some magnesium alloys such as AM60 and AZ91 have a considerably larger freezing range, or temperature gap between the solidus and liquidus temperatures. The range or gap may be about 70 to 100° C. or higher for magnesium alloys, compared with about 10 to 20° C. for many aluminium alloys. The large freezing range or gap gives rise to surface defects and internal segregation defects in twin roll cast sheet in the as-cast condition.
Importantly, there is the problem of the continuous requirement to reduce operating costs, including costs for consumables and casting preparation and thereby make twin roll casting more competitive with alternative technology, more flexible for both short operating periods (e.g. one day) and long operating periods (e.g. weeks), and enable its range of application to be extended. This is a general problem for twin roll casting technology, but is more severe for the casting of magnesium alloys in view of other problems discussed above. Also, there is a problem in extending twin roll casting technology in order to enhance the physical properties of strip material produced. While this also is a general problem for the technology, it is particularly acute in the case of magnesium alloys due to problems in producing substantially crack-free strip which has good surface quality and is substantially free of internal segregation defects.
The present invention is directed to providing a process for the twin roll casting of magnesium and magnesium alloys which, at least in preferred forms, enables one or more of the above problems to be ameliorated.
The present invention is directed to providing an improved process for twin roll casting of magnesium alloys, to produce magnesium alloy strip of a required thickness and width. The process of the invention enables the width of the strip to be up to and beyond about 300 mm, such as up to about 1800 mm, as required. In general, the thickness of the strip can range from about 1 mm or less, up to about 15 mm, but preferably the thickness is from about 3 mm to about 8 mm.
The process of the present invention provides for the casting of magnesium alloy by supplying molten alloy to a chamber formed between a nozzle and a pair of oppositely rotating, substantially parallel rolls which are internally fluid cooled and which are spaced generally one above the other to define a bite there between. The process includes introducing molten magnesium alloy through the nozzle, and cooling the magnesium alloy by heat energy extraction therefrom by the cooled rolls whereby substantially complete solidification of the magnesium alloy is achieved in the chamber, prior to the magnesium alloy passing through the bite defined between the rolls.
These general features of the process of the present invention are the same as those required for twin roll casting of aluminium alloys. However, this essentially is the extent of similarity between respective processes for magnesium alloys and for aluminium alloys. Indeed despite the indicated similarity, the process for casting of aluminium alloys provides little if any guidance as to a process suitable for magnesium alloys. Also, to the extent that twin roll casting has been attempted with other alloys, these are found to necessitate processes which are similar to that required for aluminium alloys and which also provide little if any guidance as to a process suitable for magnesium alloys.
Thus, according to the invention, there is provides a process for the production of magnesium alloy strip, by twin roll casting, wherein the process includes the steps of:
In the process of the invention, the magnesium alloy may be supplied to an inlet end of the nozzle, for flow therethrough to enter the chamber through an outlet end of the nozzle, from a feed device comprising a tundish to which the alloy is supplied from a suitable source of molten alloy. However, a float box or other alternative form of feed device can be used in place of a tundish. It is required that the feed device provides a controlled, substantially constant melt head for the molten magnesium alloy. That is, molten alloy in the tundish, float box or the like is required to be maintained at a depth such that the surface of the molten alloy therein is at a controlled, substantially constant height (or melt head) above the intersection between a horizontally extending central plane of the nozzle and a plane containing the axes of the rolls. Relative to that intersection, which substantially corresponds to the centre line of the bite of the rolls in that plane, the melt head for casting magnesium alloy of the above-indicated strip thickness provided by the invention, preferably is from 5 mm to 22 mm. The melt head may be from 5 mm to 10 mm for magnesium and magnesium alloys with lower levels of alloy element addition, such as commercial pure magnesium and AZ31, and from 7 mm to 22 mm for magnesium alloys with higher levels of alloy element addition, such as AM60 and AZ91.
The melt head of 5 to 22 mm required by the present invention is in marked contrast to requirements for twin roll casting of aluminium alloys. In the latter case, the melt head generally is kept to a minimum of about 0 to 1 mm. This difference, significant in itself, is inter-related with a number of other important differences, as will become apparent from the following description.
In the process of the invention, the magnesium alloy supplied to the tundish or other feed device is superheated above its liquidus temperature. The extent of superheating may be to a temperature of from about 15° C. to about 60° above the liquidus temperature. In general, the lower end of this range, such as from 15° C. to about 35° C., preferably from about 20° C. to 25° C., is more appropriate for magnesium and alloys with lower levels of alloy element additions. For alloys with higher levels of alloy element additions, the upper end of the range, from about 35° C. to about 50° C. to 60° C., generally is more appropriate.
The extent of superheating necessary in twin roll casting of magnesium alloys is similar to that required for aluminium alloys. With twin roll casting of aluminium alloys, superheating is to a level of about 20° C. to 60° C., usually about 40° C., above the alloy liquidus, compared to the 15° C. to 35° C. for magnesium alloys with lower levels of additions or 35° C. up to 50° C. to 60° C. for magnesium alloys with higher levels of additions required for the invention. Despite this similarity, there are important fundamental dissimilarities between the two distinct aluminium and magnesium alloy types. An important dissimilarity between the aluminium alloys and magnesium alloys, particularly magnesium alloys with higher levels of alloy element addition, is indicated by the respective temperature gap between liquidus and solidus temperatures. Thus, whereas aluminium alloys usually have a liquidus/solidus temperature gap of about 10° C. to 20° C., that gap for at least magnesium alloys with higher levels of alloy element addition is more usually from about 70° C. to 100° C., but can be substantially in excess of that range. Even where the freezing ranges for aluminium alloys and the magnesium alloys are similar, such as with magnesium alloys with lower levels of alloy element addition, the magnesium alloys have much better castability than aluminium alloys.
In the twin roll casting of magnesium alloys with higher levels of alloy element addition, full solidification of the molten alloy must be controlled to be within a relatively narrow region between the outlet of the nozzle and the bite of the rolls. In view of this, it is surprising that significant superheating above the alloy liquidus is appropriate. It will be appreciated that such superheating significantly increases the quantity of heat energy which needs to be extracted from the molten alloy in order to achieve full solidification of the alloy. As also will be appreciated, the relatively wide liquidus/solidus temperature gap of magnesium alloys, such as with higher levels of alloy element addition, also makes full solidification control difficult to attain. However, in general, the required control is able to be achieved where the casting is conducted under conditions providing for alloy strip exiting from the rolls to have a surface temperature within a required range. In particular, it is necessary that alloy strip exits from the rolls with a surface temperature below about 400° C.
With twin roll casting of magnesium alloys, full solidification of the molten alloy again must be controlled to be within a relatively narrow region between the outlet of the nozzle and the bite of the rolls. The zone is not as narrow for alloys with lower levels of alloy element addition as it is for alloy with a high level of alloy element addition. Despite this and the lower level of superheating appropriate for alloys with the low levels of alloy element addition, the level of superheating these alloys again is surprising, even if more acceptable, given the narrower freezing range applicable. Again, the required control is able to be achieved where the casting is conducted under conditions providing for strip exiting from the rolls to have a surface temperature below about 400° C. However, the temperature preferably is substantially below 400° C., such as from about 180° C. to about 300° C., for alloys with low levels of alloy element addition.
As indicated above, a strip surface temperature of below about 400° C. is necessary. However, the extent to which it is desirable for the temperature to be below that level varies with the level of alloy element addition. For magnesium alloys with higher levels of alloy element addition, a surface temperature of from about 300° C. to 400° C. alloy strip exiting from the rolls is necessary to enable the production of crack-free strip with good surface finish. For alloy with a lower level of alloy element addition, a lower surface temperature ranging from 300° C. down to about 180° is necessary for production of crack-free strip of good surface finish.
At progressively higher temperatures, the likelihood of cracks, surface defects and ultimately hot spots, increases. However, attaining such temperatures in strip exiting from the rolls necessitates a very high level of heat energy extraction, particularly with alloy having lower levels of alloy element addition. As will be appreciated, the heat energy extraction needs to be such as to allow for the heat energy due to superheating, the level of heat energy necessary to bridge the temperature gap between the liquid and solidus for the alloy, and the need to reach a surface temperature substantially below the solidus temperature. However, the surface temperature to be attained in the overall range of 180° C. to 400° C. depends on the solidus temperature for a given alloy. It also can decrease with increasing strip thickness since the surface temperature is to be such as to give rise to a suitable temperature below the solidus at the centre of the strip.
The indicated upper limit of 400° C. for strip surface temperature is at a level which is from about 40° C. to 190° C. below the solidus temperatures for magnesium casting alloys. To ensure that the temperature at the centre of the strip is at a suitable level, the surface temperature preferably is not less than about 85° C. below the solidus temperature for a given alloy. The need for this is not simply to ensure that the strip has solidified throughout. Rather, it is to ensure that throughout its thickness the alloy strip has sufficient strength to enable its production without cracking or surface defects, under the specific load necessarily applied to the rolls.
The need to attain a surface temperature in the indicated range below 400° C., in the production of magnesium alloy strip is a feature distinguishing the process of the invention from a process for producing aluminium alloy strip. With the aluminium alloys, it is necessary only that the strip has solidified throughout its thickness, such that the centre of the strip is able to be just below the solidus temperature. Under such conditions, the aluminium alloy strip has sufficient strength to enable it to be hot rolled. However, with magnesium alloy strip, it is necessary that substantially the full thickness is sufficiently below the solidus temperature in order that the strip can be subjected to hot rolling.
The level of the specific load is a further feature by which the present invention differs significantly from a process for production of strip of aluminium alloy. The specific load applied to the rolls in the process of the present invention for magnesium alloys is from about 2 kg to about 500 kg per mm of roll length. The range preferably is from 100 to 500 kg/mm. However, the range can be as low as about 2 to about 20 kg/mm and hence the specific load in the process of the present invention can be more than an order of magnitude lower than the specific loads used in producing aluminium alloy strip by twin roll casting. For aluminium alloys, a specific load of from about 300 to about 1200 kg/mm is usual. In each case, there is resultant hot rolling of the alloy moving to and passing through the bite of the rolls. The level of specific load used for aluminium alloys results in hot rolling giving rise to a thickness reduction of from about 20% to about 25%. In contrast, the specific load required for the present invention results in a thickness reduction of from about 4% to about 9% in magnesium alloy strip being produced.
As with the alloy strip surface temperature range of 180° C. to 400° C., the level of applied load and resultant thickness reduction are to facilitate production of magnesium alloy strip which is substantially free of cracks and has good surface quality. At higher levels of applied load and thickness reduction, production of strip which is substantially free of cracks is more difficult to achieve, while surface defects also become more likely to arise.
To allow for the liquidus/solidus gap and also to avoid segregation, it is necessary that heat energy extraction from the molten and solidifying magnesium alloy of proceeds relatively rapidly. Alloy contacting the surface of each roll drops rapidly in temperature to below the solidus but, as solidification proceeds through to the centre of strip being formed, cooling is less rapid. As the strip being formed is advancing towards the bite between the rolls, lines in longitudinal sections through the thickness of the strip showing alloy at the liquidus temperature have V-shape form, pointing in the direction of strip advance and extending from points at which the alloy contacts each roll. Lines in those sections showing alloy at the solidus temperature also have a V-shape form, pointing in that direction and extending from those contact points, but with the arms of the V-shape having a larger included angle. Thus, the temperature gap between those lines for alloy at the liquidus and the solidus, increases in the direction of travel with distance from each roll surface to the centre of the forming strip. It is required that the increase in this gap be kept to a minimum. In general, it is found that this is achieved if the strip exhibiting from the bite of the rolls has a surface temperature below about 400° C., such as within the range of from 300° C. to 400° C.
In the chamber formed between nozzle and the rolls, cross-sections parallel to a plane through the axes of the rolls decrease in area, through to a minimum at the bite between the rolls, due to the curved surfaces of the rolls. The distance from the nozzle outlet to that plane is referred to as the “set-back”. In its flow over the distance of the set-back, molten magnesium alloy issuing from the outlet travels a short initial part of the set-back distance before making contact with the rolls. The contact with each roll is along a longitudinal line on its surface. The distance from the outlet to the respective contact line of each roll is dependent upon the width of lips of the nozzle defining the outlet, the closeness of fitting of the nozzle between the rolls and the diameter of the rolls. In the process of the invention the set-back, which also varies with the diameter of the rolls, may be in the range of about 12 mm to about 17 mm for rolls having a diameter of about 185 mm. The set-back increases or decreases with increase or decrease in the diameter of the rolls and, for example, for rolls having a diameter of about 255 mm, the set-back most preferably is from about 28 to about 33 mm, such as about 30 mm.
The initial part of the set-back, from the outlet of the nozzle to the above-mentioned line at which the alloy makes contact with the surface of each roll, is lo dependent upon the diameter of the rolls and the set-back. However, the initial part of the set-back most preferably is such that factors including the surface tension of the magnesium alloy and the melt head are able to maintain a convex meniscus at each of the upper and lower molten metal surface over the length of that initial part. Depending on the thickness of strip to be produced, that initial part may be up to 35%, such as from about 10% to 30% of the set-back, with solidification of alloy to be achieved in the remainder of that length and in advance of the bite of the rolls. From the lines of contact the convex meniscus of alloy makes with the rolls, full solidification of the alloy between upper and lower surfaces preferably proceeds in advance of the final 5% to 15% of the set-back which immediately precedes the bite of the rolls. Thus, full solidification of the alloy throughout the thickness of strip being formed may need to be achieved in not more than about 50% of the set-back distance. However, some cooling from the superheat temperature will occur in the nozzle and in the initial part of the set-back.
The features of the present invention for twin roll casting of magnesium alloys enable a practical benefit relative to standard practices in relation to aluminium alloys. This is in relation to start-up for commencement of a casting cycle. The procedures enabled by the present invention enable start-up in not more than a few minutes, such as from 0.5 up to 3 to 5 minutes for the invention compared with up to 50 minutes for standard practices for aluminium alloys.
In the standard practices for twin roll casting of aluminium alloys, there is used either a lay-off or a hard-sheet start-up. In a lay-off start-up, the rolls are rotated substantially in excess of production speed, such as by 40%, when a casting cycle commences. The molten alloy is unable to fill the chamber defined between the nozzle and the rolls at the higher roll speed. Thus, only broken sheet, which is thinner and narrower than required is produced, although the width progressively increases. When full width is achieved, the roll speed is gradually reduced, enabling the thickness of the sheet to increase progressively. Eventually, the chamber is full and stable operation at production roll speed is established.
For the hard-sheet start-up, roll speed initially is substantially lower, such as by 40%, than production speed. The lower speed enables filling of the chamber defined by the nozzle and the rolls, and quick commencement of production of “hard sheet” of full thickness and width. Gradually the roll speed is increased to attain stable operation at production roll speed.
The substantial period of time necessary to attain production roll speed with each of these forms of standard practice for twin roll casting of aluminium alloys obviates the need for effective and efficient temperature stabilization. Thus, production start-up is by superheated molten alloy being supplied to a tundish, for flow from the latter to the nozzle. Heating of the tundish and nozzle by incoming alloy is gradual and it necessarily takes a substantial period to attain equilibrium operating temperatures throughout the casting apparatus.
In the present invention, it is found that equilibrium operating temperatures are able to be attained efficiently, in a short period of time, by preheating the tundish, or other feed device, and the nozzle. For this, hot air preferably is blown into and through the tundish, and then through the nozzle so as to exit from the nozzle outlet. The hot air is at a temperature sufficient to heat the tundish quickly to close to its required operating temperature, and may be from about 500° C. to 655° C., such as from 550° C. to 600° C. In the short time for this to be achieved, the nozzle is heated to a sufficient temperature ranging down to about 200° C. to 400° C. along the nozzle outlet. Where, for example, the nozzle has internal guide members for directing alloy to each end of the outlet, to achieve uniform alloy flow along the length of the outlet, the nozzle temperature may be about 400° C. at each end of the outlet and, due to hot air being impeded by the guide members, about 200° C. at a central region of the outlet.
The preheating used in the process of the present invention enables equilibrium operating temperatures to be established in not more than a few minutes, such as about 3 to 5 minutes. Thus, the lay-off procedure gives rise to a substantial risk of molten alloy not being solidified before passing through the bite of the rolls such that, with magnesium alloys, there is a substantial fire risk. Also, while the hard-sheet procedure more readily ensures that all alloy is solidified before passing through the rolls, there is a fire-risk arising from there being an increased possibility of molten alloy flooding from the chamber, between the nozzle and the rolls. The present invention obviates the need for either of these protracted start-up procedures used for twin roll casting of aluminium alloys, since the short time required for temperature equilibrium to be obtained enables start-up with close to full operational roll speed. Thus, the output of full thickness, full width sheet or strip is able to be quickly established.
In the course of twin roll casting, in accordance with the present invention, it is found that there can be considerable temperature variation across the width of strip or sheet exiting from the bite or gap of the rolls. The variation is such that a central region of the strip is hotter than edge regions. The variation in temperature can be up to about 70° C., and generally is in excess of about 20° C. The temperature variation can introduce a surface defect referred to as hot-line, and/or can result in the strip twisting due to thermal stress. Similar temperature variation and consequences can be encountered in alloys other than magnesium alloys.
We have found that the temperature variation can at least be reduced by use of a modified form of nozzle. The modified nozzle has a top plate and a bottom plate, with the lateral extent of the outlet of the nozzle being defined by a respective edge of each of the plates. Over a central region of at least one of the plates, that edge is set back relative to end regions of the edge. The central region of the edge has a length and location corresponding to the central region of strip or sheet to be cast. While a central region of each plate may be set back, it is preferred that only the top plate has such set back central region.
The set-back preferably is substantially uniform across the central region, although the set-back may be of concave arcuate form. The set-back preferably is less than about 7 mm, such as from 2 to 4 mm. With such set-back aligned with a region of the strip at which a relatively higher temperature would prevail but for the set-back, the temperature difference across the width of the strip is able to be substantially reduced or eliminated. Thus, hotline is reduced or prevented, while twisting of the strip is reduced or prevented.
It is indicated above that, with the twin roll casting of magnesium alloys, there are several problems which need to be overcome. The first of these is the risk of oxidation and fire. The present invention does not obviate the need for use of the established procedures based on the use of a suitable flux and atmosphere. However it does enable this risk to be still further reduced. Thus, the efficient start-up procedures enabled by the present invention substantially avoids the risk of fire from molten alloy not being solidified full before passing through the rolls or from molten alloy flooding from the chamber between the nozzle and the rolls. Also, the low roll load of about 2 to 500 kg/mm and corresponding low level of rolling reduction, combined with limited superheating and rapid solidification in advance of the bite between the rolls, further reduce the risk of molten alloy passing through the bite and being exposed to the atmosphere by cracking or surface defects.
As indicated, the invention does not obviate the need for use of a suitable atmosphere to control fire risk. However, an important preferred form of the invention provides an improvement on established procedures. In relation to fire risk control, it is common practice to use a mixture of sulphur hexafluoride in dry air. The SF6/dry air mix is not suitable for magnesium alloys high in aluminium, while it is not always reliable at start-up or at the end of a casting run. In each case, we have found that substantial improvement is possible by adding to the mixture a few percent, such as from about 2 to 6 volume %, of a hydrofluorocarbon. The compound 1,1,1,2-tetrafluoroethane, referred to by the designation HFC-134a, is particularly preferred. However, other gases can be used with or without SF6/HFC-134a.
During a casting operation, a protective atmosphere of SF6/dry air or other suitable atmosphere is maintained to protect against the risk of a fire. Where the alloy being cast is one for which that mixture provides limited protection, the mixture as supplied also contains the hydrofluorocarbon, preferably HFC-134a. This significantly improves the protection against fire risk. However, for alloys for which the SF6/dry air mixture generally is effective, it generally is necessary to add the hydrofluorocarbon for a short period at start up and at termination of a casting operation.
The problem of premature freezing is substantially overcome by the rapid establishment of equilibrium operating temperatures and high speed, assisted by the good castability of magnesium alloys. Significant factors enabling this are preheating such as described above, and quick attainment of roll speed and, hence, other operating conditions.
Difficulties arising from wide freezing range of magnesium alloys with high levels of additions are addressed by features of the present invention which also facilitate the enhancement of the physical properties of magnesium alloy strip produced by the invention. There is a number of inter-related features which are relevant to these matters.
With aluminium alloys, rapid solidification is able to be achieved by the good contact quality between the molten alloy and the surface of the rolls due to the large rolling reduction of about 20% to 25%. However, with magnesium alloys, such level of rolling reduction is not suitable as it will introduce surface defects, such as surface cracking. However, achieving a convex meniscus maintains an optimised contact of molten magnesium alloy with each roll, and establishes a uniform solidification front enabling sufficiently rapid solidification. The convex menisci are achieved by the substantial melt head required by the present invention, while the contact between the alloy and the rolls still is enhanced by the lower level of rolling reduction necessary to avoid surface defects, such as cracks. With aluminium alloys, the high level of rolling reduction and small, if any, melt head substantially preclude convex menisci, and produce menisci which are concave or vary between concave and convex.
With the rapid solidification enabled by the present invention for the production of magnesium alloy strip, it is found that a number of practical benefits are able to be achieved. Thus, the strip can have a microstructure having the secondary dendritic arm spacing of primary magnesium refined to about 5 to 15 μm, compared with 25 to 100 μm for magnesium alloy microstructures resulting from conventional casting technologies. This refinement leads to uniform distribution of intermetallic secondary phases, thereby facilitating improvement in mechanical properties by cold working of the strip.
Also, the rapid solidification refines the size of particles of intermetallic secondary phases to about 1 μm, compared to up to 25 to 50 μm for magnesium alloy microstructures from conventional casting technologies. This refinement minimises crack initiation around those particles, further facilitating improvement in mechanical properties by cold working of the strip.
Moreover, the rapid solidification can be controlled for achieving equi-axed growth of alpha magnesium dendrites across the thickness of strip being formed, by variation in the cooling rate from initial to final solidification through to the middle of the strip thickness. This, together with melt treatment such as grain refining, minimizes detrimental centre-line segregation, while maintaining the integrity of the as-cast magnesium alloy strip. This is not an issue in the twin roll casting of aluminium alloys as the alpha aluminium dendrites are always columnar-like, as there is no segregation problems for these alloys.
Additionally, the magnesium alloy strip produced by the present invention is well suited to processing for controlling its microstructure and properties. Thus, hot rolling and final heat treatment can be carried out on the as-cast strip to refine the microstructure and enhance the mechanical properties of resultant final gauges. Typical requirements for a range of applications necessitate the refinement of primary magnesium grain size and substantially uniform properties in both longitudinal and transverse directions. We have established that one or two longitudinal cold rolling passes, followed by suitable heat treatment, can refine the primary magnesium grains by recrystallization. Also, applying controlled transverse strain and suitable heat treatment, both after one or two longitudinal cold rolling passes, enables refinement of primary magnesium grains, as well as substantially uniform transverse and longitudinal mechanical properties.
As to operating costs, it will be appreciated that the ability to achieve stable solidification and establishment of production within a few minutes is particularly significant. Establishing stable thermal distributions is of importance in this regard. Sufficient magnesium melt protection during the production of strip reduces the preparation time between operations, and allows cost-effective small and medium sized operation.
In order that the invention may more readily be understood, reference now is directed to the accompanying drawings, in which:
FIGS. 6 to 8 show alternative modular nozzle arrangements suitable for an installation as in
In the schematic representation of
A form of tundish 26 for enclosure 14 is shown in
Installation 10 also includes a nozzle 30 and a roll arrangement 32. Nozzle 30 extends forwardly from wall 26a of tundish 26, and into a gap between upper and lower rolls 32a and 32b of arrangement 32. The rolls 32a, 32b extend horizontally and are vertically spaced to define a bite or nip 34 therebetween. Arrangement 32 also includes an exit table or conveyor 35 on the side of rolls 32a,32b remote from nozzle 30.
The arrangement of
With use of installation 10, tundish 26 and nozzle 30 initially are pre-heated to temperature levels detailed earlier herein. For this purpose, a hot air gun 46 (shown in
From tundish 26, the alloy flows at a controlled rate through opening 40 to cavity 39 of nozzle 30. From cavity 39, the alloy discharges through the length of outlet 42, into chamber 44, and then through the bite or nip 34 between rolls 32a,32b. The rolls 32a,32b are internally water-cooled and rotated in unison in the respective directions shown by arrows X. The molten alloy progressively solidifies in chamber 44 due to the cooling effect of rolls 32a,32b, to form magnesium alloy strip 50 (as shown in
Turning now to
The arrangement of
Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2002951075 | Aug 2002 | AU | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/AU03/01097 | Aug 2003 | US |
| Child | 11068514 | Feb 2005 | US |
