Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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MAGNESIUM ALLOY SHEET AND ITS PRODUCTION
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
s 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. In
to some instances, 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.
In that approach, 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
is 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. When necessary to maintain the temperature above the 400°C
minimum, the alloy is reheated between passes.
2o 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. In each of up to
about 10 passes, 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.
2s 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. In
this, 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
3o 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.
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2
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.
However, the intermediate hot rolling necessitates a substantial consumption
of
s 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. Also, the intermediate hot rolling can
result in surface and edge cracking of the strip, and a resultant reduction in
metal
yield. These problems in the intermediate hot rolling are exacerbated in the
finish
io warm rolling and, while this is not the case in finish cold rolling, there
is the added
cost of a larger number of passes necessary in the cold rolling.
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
is temper by heating at about 260°C for one hour; or an H26 temper by
heating at
about 150°C for one hour. However, there is ample scope for variation
of the final
anneal to achieve resultant sheet having the mechanical properties desired for
different applications.
The time and energy consumption for the production of magnesium alloy
2o 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.
2s There have been proposals for the production of magnesium alloy plate
and strip by twin roll casting (TRC). The 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. Despite this, TRC suggests a
possible alternative to the above described process which has the benefit of
3o 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 oufiput 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. However, the TRC
strip
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3
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
s 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
io 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.
We have found that TRC magnesium strip, with a suitable microstructure
which enables the production of sheet, can be obtained by control over the
is conditions under which the strip is produced. A suitable microstructure is
found to
be related to the secondary dendritic arm spacing and the am~unt 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. We also
have
found that with attainment of a suitable microstructure, the as-cast TRC strip
after
2o a homogenization heat treatment is substantially more amenable to being
rolled
and annealed to produce suitable magnesium alloy sheet.
Thus according to the present invention, there is provided a method of
producing magnesium alloy strip, suitable for use in the production of
magnesium
alloy sheet by rolling reduction and heat treatment, wherein the method
includes
zs the steps of:
(a) casting magnesium alloy as strip, using a twin roll casting installation;
and
(b) controlling the thickness and temperature of the strip exiting from
between
rolls of the installation whereby the strip has a microstructure characterised
by a
primary phase having a form selected from deformed dendritic, equiaxed
dendritic
3o and a mixture of deformed and equiaxed dendritic forms.
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
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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
s 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. For thicker strip such as about 4mm to 5mm thick, the
equiaxed dendritic microstructure tends to be obtained at a temperature of at
least
io 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. 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. For thinner strip, the equiaxed dendritic
is 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. a
The equiaxed dendritic microstructure has primary phase grains which,
rather than exhibiting a shape reflecting dendritic crystal growth, are
somewhat
2o 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. However, the deformed primary grains are of
an
elongate flattened form extending in the rolling direction, substantially
parallel to
major surfaces of the strip.
2s The deformed dendritic microstructure is preferred. It is amenable to the
production of magnesium alloy sheet by a more simple form of the invention.
Also, 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
3o boundaries.
In the present invention, 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
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homogenized strip then is rolled to produce magnesium alloy sheet of a
required
gauge, and the sheet is subjected to a final anneal.
Thus, the present invention also provides a method of producing
magnesium alloy sheet, wherein the method includes the steps of:
s (a) casting magnesium alloy as strip, using a twin roll casting
installation;
(b) controlling the thickness and temperature of the strip exiting from
between
rolls of the installation whereby the strip has a microstructure characterised
by a primary phase having a form selected from deformed dendritic,
equiaxed dendritic and a mixture of deformed and equiaxed dendritic
io forms;
(c) subjecting the strip to a homogenizing heat treatment to achieve full or
partial recrystallization of the microstructure to a required grain size;
(d) rolling the homogenized strip to produce magnesium alloy sheet of a
required gauge; and
is (e) annealing the sheet produced by step (d).
The as-cast magnesium alloy strip preferably has a thickness of not more
than 5mm. The fihickness 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
2o 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. However, 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
2s 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
3o 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. However, even if a relatively high temperature of 400°C to
500°C is
desirable, it can be beneficial for the strip to be held for a period at an
intermediate temperature, such as at about 340°C to 360°C,
before heating to the
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higher temperature, as the intermediate temperature hold enables the level of
segregation in some alloys, such as AZ series alloys, to be reduced by
secondary
phase being taken into solid solution.
The period of time required for the homogenization heat treatment
s decreases with increasingly higher heat treatment temperature, but differs
with the
microstructure. With, for example, the deformed dendritic microstructure, the
heat
treatment results in recrystallization. At a temperature of about
420°C, 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,
to 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. After 16
hours at 420°C, the final microstructure obtained by heat treatment of
the
deformed dendritic microstructure is more uniform and consists of fine grains
of
is about 10~cm to 15~,m in size. In addition to this microstructural
transformation, it is
found that the segregation in some alloys, such as the AZ series alloys, is
able to
be almost eliminated after the annealing for 2 hours at 420°C, except
for a few
particles.
The relatively rapid elimination of segregation in the heat treatment of the
2o TRC magnesium alloy strip is in marked contrast to experience with TRC
aluminium alloys in which segregafiion 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
2s 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
3o is different with TRC magnesium alloy having the equiaxed dendrite
microstructure. In contrast to the microstructure having the deformed
dendritic
structure, the larger grains of the equiaxed microstructure do not
recrystallize into
smaller grains. Rather, the homogenizing heat treatmenfi results in a final
microstructure containing mainly large grains of about 50~,m to 200p.m in
size.
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After the homogenizing heat treatment, 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
s 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
to 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
is 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.
With the equiaxed dendritic grain structure, it is necessary to distinguish
between TRC strip produced with a roll exit temperature in the lower and upper
parts, respectively, of the temperature range of 230°C to 260°C.
For at least
2o some magnesium alloys, 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. However, again for at least some alloys, scalping is found not to be
zs necessary for strip produced with a higher roll exit temperature, such as
from
about 250°C to 260°C.
The need for scalping of strip which, as cast, had an equiaxed dendritic
microstructure produced at a lower roll exit temperature such as from
230°C to
240°C, arises from the surface defects in the strip which are not cured
by the
3o homogenization heat treatment. Both large (40%) and small (5%) reductions
per
pass in the hot rolling are found to produce cracks in the surface of the
strip. We
have observed that cracks appear after just one pass at the large reduction
setting and after only two passes at the small reduction setting. However,
suggestive of surface defects, it is found that the detrimental effects of the
surface
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cracks can be minimised by scalping, as indicated above. Moreover, strip cast
with a higher exit temperature, such as from about 250°C to
260°C, 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.
s 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%. To illustrate this, test samples of AZ31 B strip 330mm long,
120mm wide and 4.7mm thick (after scalping when necessary), were prepared
from TRC strip which, as-cast, had an equiaxed dendritic microstructure and
io 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. In the first pass, the reduction setting for the
mill was
is 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. In the subsequent three passes, the reduction setting was further
2o 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.
2s In a final four rolling passes, 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.
Further trials were conducted with samples from TRC AZ31 B alloy, but
3o produced from TRC strip having a deformed, rather than equiaxed, dendritic
as-cast microstructure. Some of the test samples were 200mm long, 50mm wide
and 2.6mm thick, while other larger samples were as detailed in the above
trials
with equiaxed microstructures. With each sample size, two sets of samples were
subjected to an homogenizing heat treatment by an overnight anneal, one set at
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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. For the smaller samples a reduction of
s 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
to cold rolling processing. In the sample annealed at 420°C, the large
grains were
more uniformly distributed. Hot rolling at an initial temperature of
350°C also
introduced the "banded" microstructure.
Decreasing the duration of the pre-rolling anneal from about 18 hours to 2
hours was found to not affect the rolling reduction and the surface quality.
The
is microstructures, however, contained significant amount of bands of large
grains.
o~educing the interval anneal time from 15 to 30 minutes to 7 to 15 minutes
between rolling passes was able to be achieved without reducing the
workability.
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
2o and width of the clusters of large grains were increased, but they did not
form
lengthy bands.
All samples produced by all the condifiions had an averaged grain size of
about 10~,m. These finer grains were contributed by the smaller starting
microstructures of the "deformed" dendrites.
2s The "band" microstructure can be detrimental to the ductility of the
finished
sheet along the rolling direction. The formation of this microstructure is
related to
the activation of a twinning deformation mechanism during the rolling process
that
introduces minor and major deformation zones that later recrystallize into
alternate bands of large and fine grains, respectively, during the final
anneal.
3o Normally, twinning is the main mode of deformation in magnesium alloys when
the deformation temperature is below about 320°C. 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
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pre-heating temperature and/or the roll speed are not sufficiently high to
prevent
the formation of the "banded" microstructure.
After the finish hot rolling, the resultant strip is subjected to a finish
cold
rolling stage. However, as detailed above, the finish hot rolling can be
omitted, if
s required, for TRC strip. In each case, we have not found direct evidence to
correlate the degree of grain refinement during recrystallization with the
size and
distribution of secondary particles in the TRC magnesium alloys. 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
to energy to induce recrystallization on subsequent heat treatment.
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%. However, in the process of the present, invention, the
finish
is cold rolling stage is not subject to such constraint. That stage in the
present
invention, with TRC strip which hays either the equia~a~ed or deformed
dendritic
microstrucfiure in its as cast condition, enables reduction levels of from 15%
to
25% in each pass.
In trials with 120mm wide and 0.7 to 0.75mm thick sheet, produced by hot
2o rolling at 420°C from homogenized TRC strip which, as-cast, had an
equiaxed
dendritic microstructure, the sheet was heat treated at not more than 30
minutes
at 420°C and then cold rolled. During the cold rolling, the rolling
mill was set such
that there was no gap between the rolls, and the total reduction after three
rolling
passes was 15%. In other trials a total reduction of 25% was obtained after
three
2s cold rolling passes. In the latter case, the microstructure consisted of
finer grains
with a size down to about 3p,m and larger grains with a size up to 12wm and an
average grain size of 7p,m. In a further trial, a reduction of 20% was
obtained in a
single cold roll pass, to provide a microstructure with finer grains of less
than
10p,m, and coarser grains up to 25~,m. The less uniform grain size after the
single
3o pass indicates that it is preferable to use multiple passes instead of a
single pass
to achieve a given total reduction.
It is indicated above that at hot rolling temperatures below 320°C, a
banded
microstructure can result. While this is undesirable, it is found that its
effect is
reduced by cross cold rolling to produce a regular "checkerboard"
microstructure.
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With samples similar to those detailed for cold rolling of sheet from
equiaxed dendritic TRC strip, but with sheet of 0.7mm to 0.75mm obtained from
deformed dendritic TRC strip, comparable results were obtained. Thus, with
respective samples subjected to three cold rolling passes, a total reduction
of 20%
s was obtained in one instance, while a reduction of 30% was obtained in the
other.
The increase in reduction from 20% to 30% was accompanied by a reduction in
average grain size from 7~,m to 4~m. However, there were more clusters of
large
grains in the samples reduced by 30%.
Further samples derived from TRC strip which, as-cast, had a deformed
to dendritic microstructure, exhibited bands of larger grains produced as a
consequence of hot rolling at 350°C. These bands were found to persist
after six
cold rolling passes. However, it is found that cold rolling could eliminate
most of
the bands of large grains indicated above as being formed by a reduction in
pre-hot rolling anneal time (such as from about 18 hours to 2 hours).
is Still further samples derived from TRC strip of both deformed and equiaxed
dendritic microstructures were subjected to rolling at room temperature with a
degree of reduction between each pass at a constant level between 1 % and 27%.
These samples, as cast, were subjected to a homogenizing anneal at
350°C or
420°C for 12 to 18 hours and then to the cold rolling, without an
intervening hot
2o rolling stage. The samples were 200mm long, 50mm wide and 2.6mm thick. At
greater than 20°/~ reduction per pass, a single pass was sufficient to
introduce
edge cracking. At a cold reduction of 14% per pass, two passes (for a total
reduction of 24%) caused edge cracking. At a cold reduction of 10% to 13% per
pass, three passes (for a total reduction of 35%) was able to be tolerated
without
2s edge cracking. At a cold reduction of 1 % to 2% per pass, 30 passes could
be
conducted (for a total reduction of 46%) before edge cracks appeared. However,
after reaching the maximum total reduction for any of these rolling sequences,
annealing of the strip such as at 350°C for 60 minutes or 420°C
for 30 minutes
enabled cold rolling to recommence at similar rolling reductions without
adverse
3o effects.
The difference in the reduction per cold roll pass does not affect the final
microstructure. For sheet produced with a thickness of 0.7mm, and then
annealed at 350°C for 60 minutes, the microstructure can exhibit fine
grains of
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12
3~,m in size, clusters of larger grains of up to 10~,m and an average grain
size of
5~,m.
Following the finish cold rolling, the as-rolled sheet is subjected to a
finish
anneal sufficient to achieve recrystallization. The duration of the anneal
s 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. However
ductility in the transverse direction is not adversely influenced by this
difference.
to In large part, the foregoing results have been established with trials
conducted with AZ31 B, AZ61, AZ91 and AM60 alloys. However, comparable
results are indicated for magnesium alloys in general. For such alloys, the
invention is expected to facilitate more simple, lower cost production of
magnesium alloy sheet, with the process of the invention requiring equipment
is which has a substantially lower capital cost than is necessary in ingot
based
processing.
Finally, it is to be understood fihat 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.
25
