Ferry M. Direct Stripcasting of Metals and Alloys: Processing, Microstructure and Properties
Подождите немного. Документ загружается.
DSC
process
variables
and
cast
strip quality 133
4.5.2.2
Heat
flux
and
a
moving
mould
A
number
of workers have studied the time-dependence of
heat
flux
during
the
initial stages of solidification
in
DSC (Vangala
et
al.
1992; Kopp
et
al.
1998;
Guthrie
et
al.
2000). In the
work
of Guthrie
and
co-workers, thermocouples
were
embedded
in
one of the rolls to monitor temperature changes
during
vertical TRC
with
the
data used to calculate the interfacial
heat
flux as a
function of time using
Eq.
A.2
and
a similar calculation
method
to
that
of
Strezov
and
Herbertson (1998). Figure 4.28 shows the cyclic variation
in
heat
flux
through
the roll as
it
revolves
and
continually makes
and
breaks contact
with
the solidifying metal. During periods of non-contact
with
the melt pool,
the
heat
flux falls to zero.
4r---------------------------------------~
"E
~
3
~
X
:J
I;::
2
~
.r:::.
(ij
'u
~
M=
0.5s
~
O-···~·"~···~···~·
..
·~,,
..
b--,,·~···
I I I I
o
40
80
120 160
200
Time (s)
Figure 4.28. Variation
in
heat
flux
at
the roll/melt interface
during
twin
roll casting of
low carbon steel
at
4
m/min
to produce 6
mm
gauge strip, after Guthrie
et
al.
(2000)
(with kind permission of The Metals, Minerals
and
Materials Society, USA).
Guthrie
et
al.
(2000)
examined the heat flux history of Figure 4.28
in
detail to
assess heat flux changes
during
the time of contact between the solidifying steel
and
the rolls of the caster. Figure 4.29 shows schematically the process of
solidification
in
TRC
and
the variation
in
heat
flux between the initial contact of
the melt
with
one of the rolls
and
the exit
point
of the strip (roll nip).
Depending
on
casting conditions, there are
up
to six possible stages:
o Nucleation occurs
at
the meniscus region between the molten metal
and
the
moving roll surface to form a thin solid shell (stage
1).
For direct contact of
the shell
with
the roll, heat is transferred
by
conduction whereas
at
non-
contact points caused
by
the entrapment of gas between the shell
and
the
roll,
heat
is transferred through the gas layer
by
conduction and, to a lesser
extent, radiation. There is only a small heat flux
at
this stage since the
initial contact between the surfaces is expected to be poor.
134
Direct
strip
casting
of
metals
and
alloys
D As solidification proceeds (stage
2),
the shell thickens
and
there is a rise in
temperature of the roll causing
it
to expand. There is also
an
increase in the
metallostatic pressure over the solidifying shell
(P
= pgh)
due
to the
increasing height of the liquid metal above it. These factors increase the
contact pressure betwen the shell
and
the roll which also decreases the
thickness of any entrapped gas films (see §4.5.3.3). Susequently, this stage
is associated
with
a significant increase in the interfacial
heat
flux.
D Depending
on
the casting conditions (higher casting speeds
and
thinner
gauge strip for a given roll separating force), the heat flux
may
fall again
due
to solidification shrinkage of the thin shell which tends to separate
it
from the roll (stage
3)
. However, a second peak in
heat
flux
may
occur
(stages 4
and
5).
This was considered to be a result of the interaction
between the dendritic network structures of each solidifying shell which
increases the overall strength of the strip
and
subsequently raises the
contact pressure between the strip
and
the rolls.
It
is pertinent to note that
stages 4
and
5 were
not
found
at
low casting speeds since the thicker shells
produced
do
not
deform extensively in the rolling zone.
D As the strip approaches the exit point of the caster (stage
6),
the remaining
heat is transferred to the rolls which reduces the temperature gradients
at
the roll-strip interface. There is also further contraction of the more robust
shells from the rolls. These factors decrease the heat flux to zero as the strip
passes through the roll nip.
THIN STRIP
HIGH
SPEED
Radial distance
THICK STRIP
LOW
SPEED
Radial distance
Solidified shell
Fi
gure
4.29. Variation in heat transfer coefficient
during
twin
roll casting
of
low carbon
steel,
adapted
from Guthrie et
al
. (2000).
DSC
process
variables
and
cast
strip
quality
135
The stages outlined
in
Figure
4.29
also appear to be applicable to aluminium
alloys, despite the more extensive deformation that occurs during casting
(Vangala
et
al.
1992).
The heat flux profile can show two peaks
due
to initial
metal/mould contact, shrinkage of the semi-solid
and
subsequent separation
from the roll until the solid is strong enough to exert pressure
on
the rolls. This
leads to a higher rate of heat transfer that eventually removes the majority of
heat from the strip.
1000
,------------------------,
Q'
"'E
100
~
~
-
r::
.~
~
10
8
0.1
0.001
o
Thin
slab casting
'"
Single roll casting
.6.
Twin
roll
casting
• Melt spinning
¢ Planar flow casting
0.01
~1
1 10
Casting
velocity (m/s)
100
1000
Figure 4.30. Effect of substrate moving velocity
on
the interfacial heat transfer
coefficient
in
twin-belt, twin-roll, single-roll, free-jet melt spinning
and
planar
flow casting, adapted from Wang
and
Matthys
(1996)
and
Guthrie et
al.
(2000).
4.5.3 Factors influencing heat transfer
in
strip casting
4.5.3.1
Casting velocity
Wang
and
Matthys
(1996)
published a detailed review of some important
variables that influence heat transfer during rapid solidification. The effect of
mould velocity
on
average
interfacial heat transfer coefficient is given
in
Figure
4.30
for a wide range of casting configurations and materials (carbon steel,
stainless steel, silicon iron, Al alloys,
eu
alloys and Ni alloys).
It
can be seen
that an increase
in
substrate moving velocity from
0.01
to
10
mls increases the
heat transfer coefficient by over two orders of magnitude.
By
excluding the
data for two extremely large values of
11
(one for twin roll and free-jet melt
136
Direct
strip
casting
of
metals
and
alloys
spinning), Wang
and
Matthys
(1996)
showed that the experimental data
conform reasonably to the following empirical relation:
(4.15)
where
Ii
is the average interfacial heat transfer coefficient (kW/m2K)
and
Vs is
the substrate moving velocity (m/s). The
data
given Figure 4.30 are average
heat transfer coefficients compiled from a variety of sources
and
variations in
casting conditions for any particular experiment are expected to contribute to
the scatter. Nevertheless,
Eq.
4.15
is considered to be
an
adequate engineering
estimate of the average heat transfer coefficient in processes such as twin roll
strip casting (Wang
and
Matthys
1996).
The influence of casting velocity
on
heat transfer
during
solidification is
substantial
and
the processes involved
must
be understood.
It
has been argued
by
Strezov
and
Herbertson
(1998)
that an increase in heat flux
and
heat transfer
coefficient
at
high casting speeds is a result of
an
improvement
in
melt-
substrate contact
due
to changes in meniscus shape
and
dynamic wetting
but
this issue is yet to be resolved. Mizoguchi
and
Miyazawa
(1989)
postulated that
the increase in heat transfer
with
increasing roll velocity
in
TRC
was
due
to
reduction
in
thickness of the entrapped gas layer
at
the interface (as expected
from Eq. 4.14). Considering the importance of heat flux
in
DSC, more
work
is
needed to gain a more basic understanding of the effect of mould velocity
on
interfacial heat flux.
4.5.3.2
Structure
and
condition
of
the
mould
surface
Mould
surface
topography
Grosjean
et
al.
(1993)
demonstrated the importance of substrate topography
(roughness)
on
the average heat flux generated
during
TRC of stainless steel,
carbon steel
and
silicon steel. Using copper rolls, it
was
shown
that
an
increase
in
surface roughness resulted in a decrease in the average
heat
flux (Figure
4.31a). These results are consistent
with
the work of Wang
and
Matthys
(1996,
2002)
on
splat cooling of Ni onto various substrates (Figure 4.31b), Loulou
et
ai.
(1999)
on
splat cooling of Sn onto a Ni substrate,
and
Guthrie
et
ai.
(2000)
on
planar flow casting of aluminium. Taking
data
from various sources, Wang
and
Matthys
(1996)
proposed the following relationship between heat transfer
coefficient (kW/m2K)
and
the arithmetic average roughness,
Ra
(in !lm) of a
substrate surface:
(4.16)
Table
4.2
provides the coefficients in
Eq.
4.16 for various metals
and
substrates
with
c
2
;I.
0 indicating that heat flux reaches
an
asymptotic value
when
the
surface roughness becomes large, as shown in Figure 4.31. The trends given
here are consistent
with
the predictions of Eq. 4.14 in that
an
increase in surface
DSC
process
variables
and
cast
strip quality
137
roughness decreases Ac and, hence, reduces the heat transfer coefficient
due
to
the lower thermal conductivity through the interfacial voids. Hitchcock
et
aI.
(1981)
have also shown that an increase in
Ra
decreases the wettability of
liquid metals; this is expected to increase interfacial heat transfer (§4.5.3.4). The
foregoing results are
in
contrast to the work of Yukumoto
and
Yamane
(1995)
on
TRC
of a Ni alloy and the melt/substrate contacting experiments of Strezov
and Herbertson
(1998).
These workers observed that machined surface ridges
on
the mould led to an
increase
in
interfacial heat flux.
It
is clear that further
research is necessary to understand the effect of mould surface topography on
heat transfer during strip casting since this important parameter can be easily
controlled.
20r-----------------------.
100
o
''0 0
0·····
.•
0
b-
0
6
........................
0
ty
•••.••
0 0
o
.......................
.
(a)
~
E
~
::2:
"
0
.....
';('10
:::J
;;::
....
C\l
Q)
..r::
1ii
+=l
:5
(b)
o
\)
~\
/.l.
.....
4,.
00
0
..0......
c
b
···········0·
..
····
...
opper
.
~""-a.
inO
0
8,
"'0..
Steel
.~
•.•••
-<>.
Aluminium
OL---~--~----~--~--~
1
I
o
20
40
60
80
100
0.Q1
0.1
10
100
Roll roughness (Ilm)
Figure 4.31. Effect of
mould
surface roughness
on
interfacial
heat
transfer coefficient
for (a) TRC of steel, after Grosjean
et
al.
(1993)
(with
kind
permission of iron
and
Steel Society, USA),
and
(b) splat quenching of
Ni
(after
Wang
and
Watthys 2002) (with kind permission of Elsevier Limited).
Figure 4.32a shows transient heat flux data of Strezov
and
Herbertson
(1998)
for a range of substrate surface profiles (see
e.g.
Figure A.2). The time
dependence of the heat flux is strongly influenced
by
substrate topography and
shows that a smooth substrate
(R.
-
0.15
Ilm)
generates a markedly different
heat flux profile compared with a textured (ridged) substrate. The effect of
ridge pitch (for a constant depth of
30
Ilm)
on both the maximum heat flux and
heat flux after
50
ms immersion time is given
in
Figure
4.32.
It
is clear that
qmax
is higher for a ridged substrate with a maximum occurring
at
a pitch of
100-200
Ilm.
However, after
50
ms contact time the opposite trend can be seen (i.e.
qsnwoth
> q'idged)'
At
this latter stage, a solid shell has already formed
on
both
types of substrate and,
in
the case of ridged substrates, heat transfer occurs
both through the peaks and valleys with substantial gas entrapment
in
the
valleys expected to lead to a much lower heat flux
in
these regions.
It
is clear
138
Direct
strip
casting
of
metals
and
alloys
that
heat
transfer after substantial shell formation is
somewhat
different to
that
occurring
up
to the
point
of
nucleation « 10 ms).
It
has
been
proposed
by
Strezov et
ai.
(2000)
that
local
maximum
heat
fluxes
in
excess
of
-600
MW/m
2
occur
through
the
peaks
of specially-machined
pyramid-shaped
substrates
(see
inset
in
Figure
A.2); a
value
much
greater
than
the
spatial-averaged
heat
flux «
30
MW
1m
2
).
This implies
that
much
of
the interfacial
heat
flux is concentrated
through
the
peaks
of
either
the
ridges
or
pyramids.
Despite
the
large
transient
heat
fluxes associated
with
textured
substrates, Strezov
and
Herbertson
(1998)
pointed
out
that
the
time-averaged
heat
flux (lJ)
over
the
entire
immersion
period
is actually
greater
for a
smooth
substrate
leading
to
thicker as-cast
coupons; a
result
consistent
with
the
data
in
Table 4.2. This
work
highlights the
highly transient
nature
of
heat
transfer
during
the initial contact
period
between
molten
metal
and
the
mould
wall
and
illustrates
the
importance
of the
entire
heat
flux history
during
solidification.
Table
4.2.
Coefficients in
Eq.
(4.16)
for various casting conditions, after Wang and
Matthys
(1996)
(with kind permission
of
The Metals, Minerals
and Materials Society,
USA).
Material
Substrate
c
1
c
2
n Source
AI-5%Cu
Copper 0
9.1
0.13
Prates and Biloni
(1972)
Nickel Copper
67.0
0.7
2 Wang and Matthys
(2002)
Aluminium
33.8
1.98
2
Steel
19.7
0.42
2
Copper Copper
0
65.2
0.13
Wang and Matthys
(2002)
30~~------~======~
(a)
[1]
Smooth
[2]
30x75~m
[3]
30
x
100
~m
[4]
30
x
150
~m
[5]
30
x
200
~m
[6]
30
x
300
~m
25,-----------------------,
(b)
~
1
20
/~~~.·
..
··O"···
......
O
............................
.
~
15
..
6
...
~
~10V1
~
2
c
"""
--
....
"::.~~.~
"-
..
_
..
_
..
-
~
lO''::::(~,
p.
___
-
~
5
8;~~~
..
·-<>-·
....
···
.s
OL---~--~----~--~----~
o
10
20
30
40
50
Contact time (ms)
50
100
150
200
250
300
Ridge pitch (mm)
Figure
4.32.
(a)
Effect surface topography on interfacial heat flux during casting
of
AISI304
stainless steel onto copper substrates.
(b)
Effect of ridge spacing on
maximum heat flux and heat flux after
50
ms contact time, adapted
from Strezov and Herbertson
(1998).
DSC
process
variables
and
cast
strip quality 139
50,------------------------,
50
(a)
'"
(b) 9
..............
0 ....
0,
~
40
5
~
30
No
oxide melting Oxide melting
<+=
10
(J)
.I::.
20
iii
'0
C\l
't:
10
2
0 ..... 0 .....
0
....
0
..... 6
No
oxide melting
c:
0
I
10
20
30
40
50
0
2
4 6
8
10
Contact time (ms) Immersion number
Figure
4.33.
Solidification
of
AISI
304
onto a smooth copper substrate (R. -
0.15
!lm):
(a)
Heat
flux
histories
of
samples solidified with and without oxide melting;
(b)
Effect
of
oxide buildup on maximum heat
flux,
adapted from Strezov et
al.
(2000).
Mould
surface
oxidation
The influence of interfacial oxide formation
during
strip casting has a
substantial effect
on
interfacial heat flux
due
to the higher thermal resistance of
the oxide layer compared
with
interfacial gas films
or
direct metal/mould
contact (Prates
and
Biloni
1972;
Miyazawa
et
ai.
1990; Todoroki
et
ai.
1998;
Strezov
et
al.
2000). Strezov
et
ai.
(2000)
demonstrated a decrease
in
heat
flux
during
the consecutive immersion of a smooth copper substrate into a stainless
steel melt
due
to the
buildup
of manganese silicates
on
the substrate surface.
Above a certain oxide thickness, they observed a dramatic increase
in
heat
flux
followed
by
a
sharp
reduction (Figure 4.33). The sharp increase
was
attributed
to resistance heating
and
subsequent melting of the oxide which
improved
the
contact between the melt
and
substrate. The decrease thereafter
was
associated
with re-solidification of the oxide which also caused substantial reheating (and
localised remelting) of the solidifying shell below the oxide.
4.5.3.3
Melt
atmosphere
The gas atmosphere
in
contact
with
the melt
during
strip casting can also
influence heat transfer (Miyazawa
et
ai.
1990;
Strezov
and
Herbertson
1998;
Choo
et
ai.
2002). Strezov
and
Herbertson
(1998)
studied the effect of gas type
on
the maximum transient heat flux
during
solidification of austenitic stainless
steel
on
various types of copper substrate. Figure 4.34a shows
that
the
maximum
heat
flux increases by
-5
MW
1m2
as the argon atmosphere is
completely replaced
with
helium. With reference to Eq. 4.14, this behaviour is
expected if
heat
is conducted through the interfacial gas film since k
He
is 8 x
greater
than
kAr
in
the temperature range 300-1800K (Holman 2002).
Consistent
with
this work, Miyazawa
et
ai.
(1990)
calculated
that
the overall
140
Direct
strip
casting
of
metals
and
alloys
heat transfer coefficient for
TRC
should decrease with increasing gas film
thickness,
due
to gas entrapment between the solidifying metal
and
the
revolving roll surface ( Figure 4.34b).
It
is relevant to note that Figure 4.34a
shows a consistent rate of increase in heat flux with
He
addition despite the
markedly different substrate topography. This indicates that the wettability of
the melt with the substrate may also be improved
(§4.5.3.4).
30.-----------------------.
15.-----------------------,
(a)
""
.0
..............
.0
............. .
.€
.()
............
.
~
20
~
....
·········"O·············~idgedSubstrate
~
Ii=
••••••••••••••••
16
............
"¢
.•..•.........
~
~
10
)< ••
············~···············~mooth
substrate
:J
E
.~
::E
N~
E
~
C
C
10
Q)
·0
Ii:
8
~
5
Ul
r::
~
-
16
Q)
(b)
O~--~--~----~--_~I--~
~
O~--~--~----~--~--~
o
20
40
60
80
100
0
20
40
60
80
100
Helium
in
atmosphere (%) Thickness of
gas
film
(J,lm)
Figure 4.34. (a) Effect of gas atmosphere
on
interfacial
heat
flux
during
solidification of
AISI
304 stainless steel onto copper substrates, after Strezov
and
Herbertson (1998)
(with
kind
permission
of
the
Iron
and
Steel Institute
of
Japan). (b) Effect
of
gas film thickness,
15
g'
on
overall
heat
transfer coefficient
during
TRC
of steel,
adapted
from Miyazawa
et
al.
(1990) .
4.5.3.4
Melt
composition
The addition of surface active elements to the melt (such as
5,
Te,
Bi
and
5n
in
iron) may increase the interfacial heat flux
due
to enhanced wettability of
liquid metal with the mould surface (Mizukami
et
al.
1989;
Granasy
and
Ludwig
1991;
5trezov
et
al.
2000).
There have been reports, however, showing
the opposite behaviour (see
e.g.
Figure
4.36).
Figure 4.35a shows the significant
increase
in
maximum interfacial heat flux with increasing tellurium
concentration for solidification of stainless steel
on
a smooth copper substrate.
Each alloy concentration given in Figure 4.35a is associated with a certain
surface tension between the melt and the substrate
(Ogino
et
al.
1983;
5trezov
et
al.
2000).
Figure 4.35c shows the significant influence of surface tension
(inversely related to wettability) on interfacial heat flux. An interesting
part
of
this work was the negligible influence of Te
on
heat flux for substrates
containing ridges and pyramids (Figure 4.35b).
It
is clear that these types of
substrate have a dominant influence on heat flux with the tips of ridges and
pyramids probably becoming fully wetted in the initial stages of melt/substrate
DSC
process
variables
and
cast
strip quality 141
contact (Strezov
et
al.
2000). Since ridges provide a larger contact area
with
the
melt, a higher interfacial
heat
flux is expected, as
shown
in
Figure 4.35b.
25,------------------------,
25,------------------------,
(a) (b)
~
~
....
'O'
......
o
.......
Q .................................... 'O
.. ..
620
Ridges
x
::l
Ii:
10
Q)
~
15
......
<>
...............
0
.....................
<:>-
....
..
~
Flat top pyramids
.~
:2
10~--~L---~1-----L----_1~
o
0.02
0.04
0.06
O.OB
0.02 0.04 0.06
O.OB
Tellurium concentration (%) Tellurium concentration (%)
22.-----------------------~
C"
~
:2
~lB
x
::l
Ii:
10
Q)
..c
E
14
::l
E
'x
co
:2
10L-~
__
_L
__
_L
__
~
__
L_~
__
~
0.6
O.B
1.0 1.2 1.4 1.6
1.B
2.0
Surface tension (N/m)
Figure
4.35.
Effect of tellurium in AISI 304 stainless steel
on
interfacial heat flux during
solidification onto:
(a)
smooth
(Ra
-
0.15
J.lm)
and
(b)
textured copper substrates.
(c)
Effect of melt/substrate surface tension
on
maximum
heat
flux for the data
given
in
(a), after Strezov et
al.
(2000)
(with kind permission of
The Metals, Minerals
and
Materials Society, USA).
4.5.3.5 Melt superheat
There is some uncertainty
in
the literature concerning the influence of the melt
temperature
on
interfacial heat transfer. Strezov
and
Herbertson
(1998)
found
that
an
increase
in
superheat
during
initial stages of solidification resulted
in
a
decrease
in
maximum transient heat flux; a coarser columnar microstructure
was
also produced. However, other workers have observed the opposite trend
142
Direct
strip
casting
of
metals
and
alloys
for
both
heat
transfer
and
microstructural evolution (Miyazawa
et
al.
1990;
Gr<imisy
and
Ludwig 1991; Ozbayraktar
and
Koursaris 1996; Todoroki
et
ai.
1997; Loulou
et
ai.
1999; Wang
and
Matthys 2002). Figure 4.36 gives typical
results showing the increase
in
interfacial
heat
transfer
with
increasing melt
superheat for splat quenched AISI 304 stainless steel onto a copper substrate.
As expected from Eq. 4.8,
an
increase
in
heat flux is achieved
by
increasing the
melt superheat as this increases the thermal driving force
(l1T
i
) for
heat
flow
through
the interface. Furthermore,
an
increase
in
melt superheat
may
also
increase the wettability between the molten metal
and
the substrate (Todoroki
et
ai.
1997; Loulou
et
ai.
1999), although Strezov
and
Herbertson (1998) believe
the opposite to
be
true.
25,---------------------------.
20
40
60
80
100
Melt
superheat
(0C)
120 140
Figure
4.36.
Relationship between melt superheat and peak heat transfer coefficient
during splat quenching
of
AISI
304 onto a copper substrate, after Todoroki et
al.
(1997)
(with kind permission
of
The
Metals, Minerals and Materials Society,
USA).
4.5.3.6
Other
variables
Several other
important
variables are
known
to affect interfacial heat transfer
during
solidification. For example,
an
increase
in
contact pressure between the
solidifying strip
and
mould
wall is expected to increase interfacial heat transfer
by
decreasing the void surface area, A
v
'
as
shown
in
Eq. 4.14. The influence of
roll separating force
in
TRC
on
interfacial heat transfer has
not
been extensively
investigated
but
it
is reasonable to conclude from Figure 4.29
and
other
work
(Buchnor
and
Schmitz 1992; Kopp
et
ai1998) that the
heat
flux increases
when
higher pressures are exerted
at
the interface between the strip
and
rolls. Figure
4.37 shows the substantial increase
in
heat transfer coefficient
with
increasing
contact pressure for pressurized die casting
and
forging. While these processes