Ferry M. Direct Stripcasting of Metals and Alloys: Processing, Microstructure and Properties
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Continuous
casting
processes
and
the
emergence
of DSC
73
Table
3.1.
Some major commercial-scale
TSC
operations for steel, adapted from Table I
and
II
of Brimacombe and Samarasekera
(1995).
Process*
Mould
type
Casting
Slab Slab
surface
Typical
grades
speed
thickness
temperature
cast
(m/min) (mm)
eC)
CSPTM
Funnel-shaped
4.5-6.0
50
1050
Plain
carbon
steel
mould
outside
peritectic
range
Stainless
steels
ISPTM
Straight
mould
+ 4.5-5.0
60-80
1200
Unalloyed
and
subsequent
roller
alloyed
structural
segment
steels
Stainless
steels
CONROLUM
Parallel
mould
+ 2.4-3.7
80
1150
Stainless
steels
in-line
rolling
Carbon
steels
Thin
Slab
Lens-shaped
1.5-6.0
30-70
1175
Carbon
and
Conticaster™
mould
microa1loyed
steels
KH
Twin
belts
+
in-line
8-13
30
680
Low to
medium
rolling
without
carbon
steels
slab
reheating
*
CSP
=
compact
strip
production;
ISP
=
in-line
strip
production;
KH =
Kawasaki
Steel
and
Hitachi
Some
features
of
the
commercially available thin
slab
casters
for
steel
are
given
in
Table
3.1.
Most
of
these processes
incorporate
stationary
moulds
relying
on
mould
oscillation
to
create relative
movement
of
the
slab
within
the
confines
of
the
mould
and
to
provide
adequate
lubrication
between
the
mould
wall
and
the
solidifying slab (Brimacombe
and
Samarasekera
1995).
The
so-called
compact
strip
production
(CSPTM)
process
operates
with
a
funnel-shaped
copper-base
alloy
mould
to
produce
45 m
lengths
of
50
mm
thick
slab.
The
in-line
strip
production
(ISPTM)
process
produces
a
slightly
thicker
as-cast
slab
through
a
vertical-bow
type
mould
with
flat
parallel
sides;
there
is a
subsequent
reduction
of
the
slab
by
in-line rolling
below
the
mould
to
produce
15
mm
thick
plate.
Deformation
occurs
in
two
stages;
the
first
reduction
is
achieved
when
the
core
of
the
slab
is
liquid
with
the
final
reduction
occurring
in
the
solid-state
by
shaping
strands.
Other
TRC processes
such
as
CONROLLTM etc.
involve
casting
through
a
straight
mould
with
horizontal
sides
followed
by
in-line
bending
and
re-straightening
while
other
developments
have
involved
lens-
shaped
curved
moulds
(Thin Slab Conticaster™). As-cast slabs
produced
by
these
processes
have
a thickness
ranging
from
20
to
140
mm
with
subsequent
rolling
operations
usually
carried
out
in-line
or
by
hot
charging
the
slabs.
74 Direct strip casting
of
metals and alloys
Nozzle
Guide
roll
Side
plate
Figure
3.8.
The vertical twin belt caster developed
by
Kawasaki Steel for the
production of thin slab, after Tozawa
et
al.
(1990).
A variation of
TSC,
not
involving stationary moulds, is the twin belt caster
which may be configured from the vertical to horizontal position. Figure
3.8
shows the vertical twin belt caster developed
at
Kawasaki Steel showing the V
bell mouth that facilitates liquid metal feeding through a submerged entry
nozzle. This process is included in Table
3.1
which shows that casting speed
and as-cast slab thickness are different to the stationary mould casters
and
that
the
30
mm
thick slab is in-line
hot
rolled.
Rolls
Top
belt
\
Pinch
Rolls
\
Figure
3.9.
Schematic diagram of a typical Hazelett twin belt caster showing
the configuration of the belts and the liquid metal sump.
Continuous
casting
processes
and
the
emergence
of
DSC
75
Figure
3.9
shows one of the more notable types of belt caster, the
Hazelett
caster,
which was developed initially for the aluminium industry
but
is
now
also used
for steel, copper, zinc
and
lead alloys (Hazelett 1966; Hazelett
and
Szczypiorski
2003;
Hazelett
2006).
In
this process, the metal is cast between two moving belts
(the mould) that allow for casting velocities some
2 - 3 x higher than that
generated
by
·stationary moulds. There are notable attractions for using the
Hazelett process for thin slab (and strip) casting (Emley
1976; Hazelett 1966).
These include:
(i)
higher production rates; (ii) lower cost; (iii) the ability to
readily change section size;
(iv)
reduced maintenance,
and
(v)
the ability to cast
thin-gauge materials. Belt casters minimise friction between the slab (strand)
and
the mould
by
moving the cooling belts continuously
at
the same speed as
the strand. However, the moving mould requires considerable attention in
terms of special cooling and coating of the belts to minimise belt distortion and
surface defects of the cast slab
(§4.3.2.2).
~:4
_______
Sump
______
~.:
depth
Solidified
ingot
Figure 3.10. Schematic diagram of
an
articulated chill block caster for
the
prodt;.ction
of 10-40 rom thick slabs of aluminium.
Another version of the twin belt caster used in the aluminium industry is the
articulated
chill
block
caster shown schematically
in
Figure
3.10.
This caster uses
counter-rotating sets of chilling blocks that solidify the metal between the
blocks to produce
10
to
40
mm
thick slab that is usually in-line
hot
rolled before
coiling. The articulated chill block caster is considered to be more versatile than
the Hazelett caster as it produces a wider range of alloys, is more robust and
has greater flexibility for altering casting conditions (Emley
1976). The careful
control of
the rate of heat extraction through the chilling surface enables a wide
range of aluminium alloys to be produced with properties comparable to
Dee
ingots (Dean 1986).
A number of alloys can be produced
by
the aforementioned
TSe
processes
including aluminium, copper, zinc, lead and various types of iron alloys (Emley
76
Direct
strip
casting
of
metals
and
alloys
1976;
Brimacombe
and
Samarasekera
1994,
1995;
Hazelett
2006).
Iron alloys
include plain carbon steels outside the peritectic range (i.e.
0.065
- 0.15 wt.% C);
highly alloyed steels; microalloyed steels; stainless steels
and
deep drawing
grade steels. There is a difficulty associated with high-speed
casting-of steels
in
the peritectic range (Emi
2003).
This is associated with the peritectic reaction
which involves the transformation of liquid
+ delta-iron to austenite
(L
+
t5
-+
r)
and is accompanied
by
a volume contraction that causes a depression of the
initially solidified shell
in
the mould. This problem becomes significant for
high speed casting of large-width medium carbon steel slab, as transverse
surface cracks are often generated.
350
.--------.,.---------,
300
250
~
200
-
Bottom
face
of
slab
I
iO
0:'0
oP~
..
Top
face
ofslab
J : 0.
....
..................
~
:ft.....
.
...
.
~
150
••
?,.'<>
......
.
@
~
•..
v I
••••
~
<'..
: , :
100
~
:
I;N:
~
Edge
of
~.~:
~
Edge
of
50
sample
V i
sample
; i :
I I j ! j I I
O~-~-~~-~~~--~-~
1W
M ~ 0
~
80
120
Distance from sample centre (mm)
Figure 3.11. Secondary dendrite arm spacing through the
th~ckness
of a thin slab
compared to a conventionally cast slab, after Flemming et
al.
(1988).
Figure
3.11
shows a typical example of the refinement of as-cast grain size
in
TSC
material
by
comparison with conventional slab casting. The secondary
dendrite arm spacing
(SDAS)
of low carbon steel is -
20-40
J.Ull
at
the slab
surface which increases to
50-100
11m
at
the centre. As
in
conventional casting,
micro-
and
macrosegregation, surface cracking, inclusions
and
porosity are
present to some degree
in
the as-cast slabs.
3.2.3
Direct strip casting
(d
T
<
50
mm)
The alloys produced
by
CCC and
TSC
must
undergo a number of secondary
processing stages such as slab reheating, multiple-pass
hot
rolling
and
cold
rolling
and
annealing to obtain the desired final thickness. In contrast, the
Continuous
casting
processes
and
the
emergence
of
DSC
77
processing route to produce thin-gauge sheet
by
DSC is vastly simplified as
it
often allows strip to be cast
and
coiled in a single, continuous process. The
simple setup of a strip caster makes small plants
and
a low overall investment
in capital possible (§3.4.2). The DSC process is also capable of quickly changing
the casting conditions to generate a particular combination of properties in the
finished product.
It
is pertinent to note
at
this stage that some of the casting
configurations discussed in this section
may
be better described as thin slab
casters (i.e. belt casters)
but
the distinction between
TSC
and
DSC becomes
increasingly blurred
at
thicknesses less than -20 mm. For this reason, both
TSC
and
DSC processes are described.
Process
variations
and
types
of strip
caster
Some of the major technological directions
in
DSC involving moving
moulds
are
shown schematically in Figure
3.12;
this is
an
extended classification scheme
based
on
that proposed
by
Hendricks
(1995)
and
is divided into three major
groups for strip (and thin slab) casters:
o
Group
I - This
group
employs the use of travelling conveyor belts (Figure
3.12a). The metal is either cast onto the belt (Figure 3.12a(i))
or
sprayed onto
one side of the belt (Figure 3.12a(ii)). In some cases the liquid metal
may
be
introduced between two belts either
at
an
inclined angle (Figure 3.12a(iii))
or
vertically (Figure 3.12a(iv)). Figure 3.12a(iii) is of the same design as the
Hazelett twin belt caster shown in Figure 3.9. Belt casting is probably better
described as a
TSC
process as
it
produces thick-gauge strip
that
is often in-
line
hot
rolled
with
up
to three passes to produce the desired gauge
(Riechelt 1997).
o
Group
II - This
group
consists of a single roll to produce thin-gauge strip,
Figure 3.12b. Several metal feed systems are possible which further classify
the process into planar flow casting, melt
drag
and
melt over-flow casting
(Shibuya
and
Ozawa 1991). In the single roll process, the strip is
made
by
dragging the liquid melt in contact
with
the rotating wheel
due
the
momentum of the wheel
and
the surface tension of the liquid.
o
Group
III
- Twin roll casters incorporate either equal-sized rolls (Figures
3.12c(i) to 3.12c(iv))
or
unequal-sized rolls (Figure 3.12c(v)). Depending
on
the alloy to be cast, these processes
may
be
configured horizontally,
vertically
or
inclined.
It
is pertinent to note that TRC of steel is focused
mainly
on
vertical casting technology whereas the aluminium industry has
focused
on
horizontal
or
near-horizontal technology. A major difference
between these configurations is the molten metal feed system which is
described in further detail in §4.3.2.3. The horizontal TRC process has
recently been combined
with
spray forming using atomised droplets
(termed spray rolling) to produce a range of high strength aluminium sheet
products
at
high productivity (McHugh et
al.
2004).
78
Direct
strip
casting
of
metals
and
alloys
(a)
(b)
(c)
(i)
(ii)
(i) (ii)
GROUP I
GROUP
II
(iii)
GROUP III
(iii)
(iv) (v)
Figure 3.12. Possible classification scheme for direct strip casters,
adapted
from
Hendricks
(1995).
(a) Strip casters using conveyor belts (Group I):
(i)
single belt caster, (ii) single belt
spray caster, (iii)
twin
belt inclined caster
and
(iv)
twin
belt vertical caster.
(b)
Single
roll casters showing two possible feed systems (Group II).
(c)
Twin roll casters
(Group
III):
(i)
vertical, (ii) horizontal (2-High), (iii) horizontal (4-High),
(iv) inclined
and
(v)
twin
roll asymmetric configuration.
In
the vertical TRC process, liquid metal is
poured
from a tundish between the
two counter-rotating rolls
with
the liquid pool usually contained
by
side dams
(see §4.3.1.2). The rolls are made of a conducting metal
or
a combination of
metals
and
are rotated
at
a constant speed
and
cooled internally
by
water. The
rolls provide the cooling needed for solidification and, depending
on
alloy type,
may
contribute to a small reduction in thickness of the strip (Li 1995a). The
molten metal
and
the emerging
hot
strip are usually protected from oxidation
by
an
inert gas shroud (§4.3.1.2). The entire casting process is fully automated
Continuous
casting
processes
and
the
emergence
of
DSC
79
in terms of roll separation forces, temperature, cooling water, casting speed
with feedback control loops optimising the process
(§4.2).
An
important point
to note is the micron level roll gap control required in TRC of steel to ensure the
desired thickness accuracy in the as-cast product (Cramb 1995).
Horizontal TRC is
under
intense development for high productivity casting of
thinner gauge aluminium strip (see §3.3.3)
but
there are various other types of
continuous thin-slab/strip casters used in the aluminium industry. These
include: wheel and band casters, e.g. Southwire and Mann configurations; twin-
band casters, e.g. Hazelett configuration shown in Figure 3.9, and articulated
chill block casters, e.g. Hunter-Douglas and Alusuisse caster
II
types (Figure
3.10). These casters are used for a wide range of alloy types and products and
produce strip
in
a range of widths and thicknesses (Emley 1976).
Alloy 1 Alloy 2
Figure 3.13. Schematic diagrams of a single roll
and
twin roll casters for producing
clad sheet products, adapted from Haga
and
Suzuki
(2003).
An
interesting
new
development in single roll and twin roll casting has been
proposed for the production of aluminium alloy clad sheet products (Haga and
Suzuki
2003). The single roll process in Figure 3.13 involves two nozzles
attached to a roll whereby one strip is overlayed
on
another during casting.
The thickness of the strips can be controlled
by
the roll speed
and
contact length
between the melt
and
the roll. Two possible versions of the twin roll caster are
given in Figure
3.13. In the vertical process, a solid shell of a given alloy is
produced
on
each of the rolls which are pressed together between the roll bite
i)1.
either the semi-solid
or
solid states to produce the clad strip;
it
is desirable
that the contact surface of each shell is at least in the semisolid state to ensure
good bonding. In contrast to single-roll casting, this process generates sound
external surfaces
due
to their contact with both rolls. The horizontal process is
a combination of a melt drag twin roll caster with a
downward
melt drag twin
roll caster where the latter is used to cast the overlay strip onto the base. These
80
Direct
strip
casting
of
metals
and
alloys
processes are suitable for producing clad strips of a range of alloys and opens
up
the possibility of generating clad strip products of different metals.
The previous discussion describes strip casting using moving moulds
but
there
is also the possibility of producing as-cast strip at high speed from stationary
moulds; one of the first reported cases is the production of
10
mm
thick cast
aluminium strip (Moritz
1969).
A commercially-viable
DSC
process for copper
alloys is shown
in
Figure
3.14
for the production of -20
mm
thick strip of width
of
600-1000
mm. The liquid metal flows through a polished graphite mould
that extends into a holding furnace. Heat is extracted
from the mould by a
water-cooled copper cooler, into which the mould is pressed. The rate of strip
production is
-100-150
mm/min with intermittent strip withdrawal required to
avoid welding between the mould
and
the shell. Another notable continuous
casting processes for producing copper alloy strip is the Hazelett caster (see
§3.2.2).
Induction heated
graphite tundish
/
Insulation
Cooling
Gr~lOhite
mould
Solidified
ingot
Figure 3.14. Schematic diagram of a stationary mould strip caster for copper alloys,
adapted from Renschen
et
al.
(1989).
Thyssen Stahl and collaborators have developed the casting-pressing-rolling
technique which uses a stationary mould for the production of alloyed
and
unalloyed carbon steels, silicon steels
and
stainless steels (Kruger and Streubel
1995).
The plant setup is given
in
Figure
3.15
and slab of - 50
mm
thickness is
cast in a stationary funnel-shaped mould where it is subsequently sqeezed to
-
20
mm
by a set of rolls located just below the mould.
At
this stage, the slab
undergoes complete solidification. The as-cast thickness is further reduced to
-15
mm
in the pressing rolls located beneath the squeezing rolls. The solidified
strip then moves through a reheating induction furnace, a de scaling facility (not
shown) and a 4-high, in-line rolling mill where reductions of
up
to
60%
are
Continuous
casting
processes
and
the
emergence
of
DSC
81
possible. The strip of final thickness
4-10
mm
is then passed through a cooling
section then coiled in readiness for cold rolling.
Stationary mold
Squeezing rolls
In-line rolling
Figure 3.15. Schematic diagram of the
plant
setup of the casting-pressing-rolling
process showing the casting, rolling
and
coiling stages,
adapted from Kruger
and
Streubel (1995).
3.2.4 Melt quenching processes (d
T
« 1 mm)
'c.
The processes shown in Figure
3.16
are essentially modified versions of
DSC
whereby molten material is poured onto a rotating wheel, two counter-rotating
wheels (similar to
TRC)
or
a moving belt to produce a continuous or semi-
continuous filament
or
thin strip. However, they differ from the more
conventional
DSC
processes in that the dimensions of the final material are
usually less than
200
mm
in width with a thickness ranging from - 1
mm
to -20
~m.
The ribbon length is determined
by
the amount of molten metal
per
cast
but
can be greater than several kilometres. Furthermore, solidification occurs at
very high rates
(10
4
-
10
7
Kls) which are generated
by
the rapid casting rate
(often greater than 5
m/s)
and the good contact between the melt
and
moving
mould which results
in
a very high rate of heat transfer
at
the chilled boundary.
These high cooling rates usually generate solidification microstructures that are
far-from-equilibrium and may include phases
with
extended solubility,
metastable crystalline phases or amorphous alloys (Katgerman
1989).
It
is now useful to describe the processes in Figure 3.16 in more detail:
o Chill block melt spinning. This process is similar to single roll casting
(Figure 3.12b)
and
involves the formation of a molten jet impinging
oli.
a
rotating disk to produce rapidly-quenched ribbon
in
a continuous manner.
82
Direct
strip
casting
of
metals
and
alloys
The quench rate and ribbon geometry depend
on
the characteristics of the
melt puddle. A stable melt
puddle
during melt spinning results in good
quality and uniform ribbon. Ribbons of
up
to
200
rom
in
width
and several
kilometres in length may be produced
by
this process.
o
Planar flow casting. This process can produce wider tape than melt
spinning and differs from the latter in that the nozzle which feeds the
molten alloy is situated very close to the moving substrate (Katgerman
1989)
. The melt
puddle
is confined between the substrate
and
nozzle wall
which results in improved ribbon quality. Ribbon thicknesses
and
cooling
rates are similar to melt spinning and produce comparable microstructures.
o Melt drag casting. This is comparable to planar flow casting but, in this
case, the slot nozzle is situated horizontally. The metal is removed from the
slot nozzle by contact of the meniscus with the rotating wheel. Tapes with
widths
up
to
300
rom have been produced by applying either a slot
or
multiple nozzles.
o
Twin
roll quenching. This is similar to
TRC
(Figure 3.12c). A liquid jet
impinges between fast moving counter-rotating rolls
but
unlike TRC, this
technique is not readily used to produce thin ribbons.
(a)
(c)
Solidified
/'"
ribbon
(b)
(d)
Solidified strip
Figure 3.
16
. Melt quenching processes showing: (a) chill block melt spinning,
(b)
planar
flow casting,
(c)
melt drag casting
and
(d) double roll
quenching, adapted from Katgerman (1989).