Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/06—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of magnesium or alloys based thereon
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/06—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/001—Continuous casting of metals, i.e. casting in indefinite lengths of specific alloys
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/06—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars
  • B22D11/0622—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars formed by two casting wheels
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/16—Controlling or regulating processes or operations
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D21/00—Casting non-ferrous metals or metallic compounds so far as their metallurgical properties are of importance for the casting procedure; Selection of compositions therefor
  • B22D21/02—Casting exceedingly oxidisable non-ferrous metals, e.g. in inert atmosphere
  • B22D21/04—Casting aluminium or magnesium
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D27/00—Treating the metal in the mould while it is molten or ductile ; Pressure or vacuum casting
  • B22D27/20—Measures not previously mentioned for influencing the grain structure or texture; Selection of compositions therefor

Definitions

• This invention relates to magnesium alloy sheet and to a process for its production.
• the most common approach to the production of magnesium alloy sheet involves hot rolling of an ingot produced by pouring a melt of the alloy into a suitable mould.
• the ingot is subjected to a homogenizing soak at a suitable elevated temperature and then scalped to obtain clean, smooth surfaces.
• the scalped ingot is rolled to produce plate, then strip and finally sheet by a rough hot rolling treatment, followed by hot intermediate/finish rolling, and a final anneal.
• the hot intermediate rolling is followed by cold rolling to enable the reduction to the final gauge of the resultant sheet to be fine tuned.
• the ingot may for example be up to 1800mm long, 1000mm wide and up to 300mm thick.
• the homogenization heat treatment usually is from 400°C to 500°C for up to 2 hours.
• the scalping usually is to a depth of about 3mm.
• the rough hot rolling at from about 400°C to 460°C, is able to achieve a substantial reduction in each pass, such as from 15% to 40%, generally about 20%, in as many as 25 passes, to produce flat plate of about 5mm thick.
• the alloy is reheated between passes.
• the rough hot rolling usually is followed by intermediate hot rolling at
• 340°C to 430°C to reduce flat plate to strip of about 1 mm thick.
• a reduction of about 8% to 15%, generally about 10% is achieved.
• Reheating is necessary after each pass in order to maintain the temperature above the 340°C minimum.
• the intermediate hot rolling is followed by finish rolling, to reduce the strip to sheet of a final gauge of about 0.5mm, by either warm rolling or cold rolling.
• the finish warm rolling is conducted at from 190°C to 400°C.
• the strip is reduced in each of from 10 to 20 passes by from 4% to 10%, usually about 7%. Again, heating between each pass is necessary due to rapid cooling of the thin alloy. Care in reheating is necessary as overheating can result in excessive reduction and loss of control over the gauge.
• Cold rolling can be preferred to enable fine tuning to the final gauge, but this necessitates only 1 % to 2% thickness reduction in each pass and, hence, a larger number of passes to the final gauge.
• the rough hot rolling stage is quite efficient, despite the high number of passes, since there is only limited cooling between passes and the lower rate of heat loss necessitates reheating after only a small proportion of the passes.
• the intermediate hot rolling necessitates a substantial consumption of energy as a coil mill is employed in processing the 5mm plate down to 1 mm strip, and heat losses necessitate heating before each pass which significantly prolongs the overall process of producing sheet.
• the intermediate hot rolling can result in surface and edge cracking of the strip, and a resultant reduction in metal yield.
• the final anneal after the finish warm or cold rolling varies according to the intended application for the magnesium alloy sheet produced.
• the final anneal may be an O temper requiring heating at about 370°C for one hour; an H24 temper by heating at about 260°C for one hour; or an H26 temper by heating at about 150°C for one hour.
• O temper requiring heating at about 370°C for one hour
• H24 temper by heating at about 260°C for one hour
• an H26 temper by heating at about 150°C for one hour.
• the time and energy consumption for the production of magnesium alloy sheet by the above production stages is relatively large. As a consequence, the cost of production of the sheet is high relative to that for aluminium sheet, for example.
• the present invention seeks to provide a process for the production of magnesium alloy sheet which reduces the level of consumption of time and energy and thereby enables more cost effective production of the sheet.
• TRC process does not enable the direct production of magnesium alloy sheet, since the benefits of TRC do not favour producing strip thinner than about 1 to 2mm.
• TRC suggests a possible alternative to the above described process which has the benefit of eliminating the stages of ingot production, homogenizing heat treatment, scalping and the rough hot rolling stage by utilising TRC strip as the feed for subsequent processing to sheet. That is, in terms of gauge, the output from TRC ranges from being comparable to the plate obtained after that rough hot rolling stage down to strip resulting from the intermediate warm rolling stage.
• the TRC strip differs significantly from either of the plate resulting from the rough hot rolling, or the strip resulting from the intermediate warm rolling, of ingot alloy and is too variable in its microstructure to enable simple reliance on that alternative.
• the as-cast TRC strip is found to vary in its microstructure with its casting conditions. In addition to this overall variability, it is not completely uniform throughout its thickness. It contains dendrites of different sizes and discontinuous or a varying amount of segregation from the surfaces towards the centre. Also, the as-cast TRC strip is prone to the generation of surface cracks during rolling with even a small reduction, and any segregation adversely affects the ductility of the finished strip. Thus, a homogenization heat treatment is necessary prior to any rolling schedule, although this is found not to fully offset the variation in microstructure and the resultant difficulty in rolling.
• TRC magnesium strip with a suitable microstructure which enables the production of sheet, can be obtained by control over the conditions under which the strip is produced.
• a suitable microstructure is found to be related to the secondary dendritic arm spacing and the amount of rolling reduction achieved in producing the as-cast strip, with the suitable microstructure reflected by the temperature at which the strip exits from the rolls.
• the as-cast TRC strip after a homogenization heat treatment is substantially more amenable to being rolled and annealed to produce suitable magnesium alloy sheet.
• a suitable microstructure having "deformed” and/or “equiaxed” dendritic primary phase is able to be produced with a roll exit temperature of from about 200°C to 350°C, such as from about 200°C to 260°C.
• a deformed dendritic microstructure, substantially free of equiaxed dendritic particles, is obtained with a relatively low exit temperature which varies with the thickness of the strip. For thicker strip, such as about 4mm to 5mm thick, the deformed dendritic microstructure tends to be obtained at a temperature of from about 200°C to 220°C. For thinner strip, the deformed dendritic microstructure tends to be obtained at from about 200°C to 245°C, more usually above about 220°C.
• An equiaxed microstructure, substantially free of deformed dendritic particles, generally is obtained with a relatively high exit temperature which also varies with the strip thickness.
• the equiaxed dendritic microstructure tends to be obtained at a temperature of at least about 230°C and, for this microstructure and thickness, it is preferred that the exit temperature is at an intermediate level of from about 230°C to 240°C.
• the exit temperature is at higher exit temperatures for such thicker strip, particularly at a high level of from about 250°C to 260°C, there is increased segregation in grain boundaries near to the surfaces of the as-cast strip.
• the equiaxed dendritic microstructure tends to be obtained at exit temperatures higher than about 245°C, and with a lesser tendency for segregation in grain boundaries near to surfaces of the as-cast strip.
• the equiaxed dendritic microstructure has primary phase grains which, rather than exhibiting a shape reflecting dendritic crystal growth, are somewhat rounded and of substantially uniform size in all directions.
• the deformed dendritic microstructure has primary phase grains which have a shape which more clearly reflects dendritic crystal growth.
• the deformed primary grains are of an elongate flattened form extending in the rolling direction, substantially parallel to major surfaces of the strip.
• the deformed dendritic microstructure is preferred. It is amenable to the production of magnesium alloy sheet by a more simple form of the invention.
• the equiaxed dendritic microstructure is more prone to micro-cracking near the surfaces of the as-cast strip, particularly at exit temperatures of 240°C to 250°C, with the micro-cracking appearing in the segregation regions in grain boundaries.
• magnesium alloy TRC strip is produced to a suitable thickness of less than 10mm, under conditions providing a suitable microstructure.
• the strip then is subjected to a homogenization heat treatment to achieve full or partial recrystallization to an appropriate grain size.
• the homogenized strip then is rolled to produce magnesium alloy sheet of a required gauge, and the sheet is subjected to a final anneal.
• the present invention also provides a method of producing magnesium alloy sheet, wherein the method includes the steps of: (a) casting magnesium alloy as strip, using a twin roll casting installation;
• step (d) rolling the homogenized strip to produce magnesium alloy sheet of a required gauge; and (e) annealing the sheet produced by step (d).
• the as-cast magnesium alloy strip preferably has a thickness of not more than 5mm.
• the thickness most preferably is less than 5mm, such as down to about 2.5mm.
• the microstructure is one characterised by deformed dendritic and/or equiaxed dendritic primary phase.
• the primary phase may substantially comprise equiaxed dendritic primary phase produced by strip of 4mm to 5mm thickness exiting the twin rolls having a temperature of from 230°C to 260°C, preferably from 230°C to 240°C.
• the primary phase preferably substantially comprises deformed dendritic primary phase produced by the strip exiting the rolls at a temperature of from 200°C to 245°C for thin strip less than 3mm thickness and from 200°C to 220°C for strip thicknesses between 4mm and 5mm.
• the homogenization heat treatment preferably is at a temperature of from about 330°C to 500°C, preferably from about 400°C to 500°C.
• the strip preferably is subjected to the heat treatment sufficiently soon after exiting the twin rolls so as to minimise loss of heat energy from the as-cast strip, to thereby minimise the time and heat energy input required to obtain the homogenization temperature.
• the period of time required for the homogenization heat treatment decreases with increasingly higher heat treatment temperature, but differs with the microstructure.
• the heat treatment results in recrystallization.
• the recrystallization can be well advanced over a period of only about 2 hours, and tends to be preferentially in regions associated with finer cells.
• a few large, isolated equiaxed dendrites within the deformed dendrites become individual solid grains, although remnants of the dendritic structure are still visible within the grains. After 6 hours at 420°C, the large grains begin to recrystallize.
• the final microstructure obtained by heat treatment of the deformed dendritic microstructure is more uniform and consists of fine grains of about 10 ⁇ m to 15 ⁇ m in size.
• the segregation in some alloys, such as the AZ series alloys is able lo be almost eliminated after the annealing for 2 hours at 420°C, except for a few particles.
• TRC magnesium alloy strip The relatively rapid elimination of segregation in the heat treatment of the TRC magnesium alloy strip is in marked contrast to experience with TRC aluminium alloys in which segregation is very significant and not able to be removed by homogenization heat treatment. This is found to result from secondary particles precipitating in an early stage of solidification in the production of TRC magnesium alloys, such that those particles are relatively uniformly distributed over the entire strip cross-section. In contrast, secondary particles are formed in a later stage in the solidification of aluminium alloys and are relatively concentrated in the centre of the thickness of as-cast TRC aluminium alloy strip.
• the microstructural transformation during the homogenizing heat treatment is different with TRC magnesium alloy having the equiaxed dendrite microstructure.
• the larger grains of the equiaxed microstructure do not recrystallize into smaller grains. Rather, the homogenizing heat treatment results in a final microstructure containing mainly large grains of about 50 ⁇ m to 200 ⁇ m in size.
• the TRC strip can be subjected to further rolling finishing which is the same for each microstructure type. Where this is the case, the further processing includes stages of finish hot rolling, finish cold rolling and a final anneal. However, the finish hot rolling can be omitted for both the deformed and equiaxed dendritic microstructures.
• the finish cold rolling of the deformed microstructure can be further improved by using a larger rolling reduction between the interval anneals than for the equiaxed microstructures, to provide a most cost-effective form of the invention. Also, in the case of the equiaxed dendritic microstructure it can be beneficial, in at least some circumstances, to scalp the strip to remove a surface layer, before the finish hot rolling.
• the finish hot rolling may be conducted at a temperature at which the rolling causes continuous recrystallization, such that dislocations remain within the recrystallized grains. Generally this necessitates hot rolling temperatures above 200°C. However, the hot rolling usually is at a temperature of from about 350°C to 500°C, preferably from about 400°C to 500°C.
• strip produced with a lower roll exit temperature of from about 230°C to 240°C for example, is found not to be able to undergo finish hot rolling, even after an extended homogenization heat treatment, unless the strip first is scalped to remove a sufficient surface layer, such as to a depth of about 3mm.
• scalping is found not to be necessary for strip produced with a higher roll exit temperature, such as from about 250°C to 260°C.
• strip cast with a higher exit temperature is found after homogenization heat treatment to be able to be successfully subjected to a hot rolling reduction of up to 25% per pass without displaying surface cracks.
• the finish hot rolling particularly where conducted at a relatively high temperature, is able to achieve a relatively high actual reduction per pass, such as from 20% to 25%.
• test samples of AZ31B strip 330mm long, 120mm wide and 4.7mm thick were prepared from TRC strip which, as-cast, had an equiaxed dendritic microstructure and which was subjected to a homogenization heat treatment at about 420°C.
• Each sample was hot rolled at 420°C to produce sheeting to a total length of about 2000mm, a width of 120mm and a thickness of from 0.7 to 0.75mm.
• a mill speed of 18m/min was determined to be sufficient for the hot rolling, at the initial temperature of 420°C.
• the reduction setting for the mill was between 40% and 45% of the strip thickness, and this was increased to 50% for the second pass and to 60% for the third pass.
• the actual reduction achieved in the strip for each pass was between 20% and 25%.
• An intermediate anneal at 420°C for 30 minutes was conducted between passes one and two, and two and three.
• the reduction setting was further increased to between 70% and 90% until the mill gauge was between 0.13mm and 0.15mm (0.005" to 0.006"), with the work piece being re-heated to 420°C after each pass.
• the actual reduction in the subsequent three passes was in the order of 17%, which is less than the previous three passes, but it was considered that thinner sheet would lose heat more quickly, resulting in less rolling reduction.
• the mill gauge was maintained at between 0.13mm and 0.15mm until the sheet thickness reached between 0.7mm and 0.75mm. The actual amount of reduction per pass decreased from 15% to 8% as the sheet became thinner.
• test samples from TRC AZ31B alloy were 200mm long, 50mm wide and 2.6mm thick, while other larger samples were as detailed in the above trials with equiaxed microstructures.
• two sets of samples were subjected to an homogenizing heat treatment by an overnight anneal, one set at 350°C and the other at 420°C.
• the samples then were subjected to the same hot rolling schedule (with respect to the reduction settings for the mill) as described previously, but at two temperature levels of 350°C and 420°C, to reach a sheet thickness of between 0.7mm and 0.75mm.
• a reduction of between 21 % and 26% was measured per pass for each of the first four passes, followed by one more pass of between 17% and 19% reduction.
• the pre-rolling annealing temperature was found to influence the formation of a "banded" microstructure.
• the "banded" microstructure in the samples annealed at 350°C before rolling was obvious and persisted even after further cold rolling processing.
• the large grains were more uniformly distributed.
• Hot rolling at an initial temperature of 350°C also introduced the "banded" microstructure.
• the formation of the banded microstructure was slightly affected by the time reduction. In the samples rolled with 7 to 15 minutes internal anneal, the number and width of the clusters of large grains were increased, but they did not form lengthy bands.
• the rolling mill therefore preferably has the capability to heat the rolls so that the temperature of the work piece will not drop below 320°C during the rolling operation, at least if the pre-heating temperature and/or the roll speed are not sufficiently high to prevent the formation of the "banded" microstructure.
• the resultant strip is subjected to a finish cold rolling stage.
• the finish hot rolling can be omitted, if required, for TRC strip.
• the principal parameter appears to be the amount and the distribution of stored deformation energy. Cold rolling is an effective method for providing high levels of such stored energy to induce recrystallization on subsequent heat treatment.
• finish warm rolling stage As detailed above, conventional processing of magnesium alloy in the finish treatment for producing sheet frequently uses a finish warm rolling stage.
• a finish cold rolling stage can be used, but necessitates only a low level of reduction per pass of 1 % to 2%.
• the finish cold rolling stage is not subject to such constraint. That stage in the present invention, with TRC strip which has either the equiaxed or deformed dendritic microstructure in its as cast condition, enables reduction levels of from 15% to 25% in each pass.
• the difference in the reduction per cold roll pass does not affect the final microstructure.
• the microstructure can exhibit fine grains of 3 ⁇ m in size, clusters of larger grains of up to 10 ⁇ m and an average grain size of 5 ⁇ m.
• the as-rolled sheet is subjected to a finish anneal sufficient to achieve recrystallization.
• the duration of the anneal decreases with increase in temperature level, as indicated by the general suitability of for example 350°C for less than about 60 minutes or 420°C for less than about 30 minutes.
• Each of these treatments result in similar microstructures, although the latter treatment results in a larger grain size scatter.
• ductility in the transverse direction is not adversely influenced by this difference.
• the foregoing results have been established with trials conducted with AZ31B, AZ61 , AZ91 and AM60 alloys.
• comparable results are indicated for magnesium alloys in general.
• the invention is expected to facilitate more simple, lower cost production of magnesium alloy sheet, with the process of the invention requiring equipment which has a substantially lower capital cost than is necessary in ingot based processing.

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• Crystallography & Structural Chemistry (AREA)
• Materials Engineering (AREA)
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• 2003
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