Ferry M. Direct Stripcasting of Metals and Alloys: Processing, Microstructure and Properties
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Secondary
processing
and
fabrication
203
strip-cast
AISI
304 where the high
(Cr
/
Ni)eq
ratio promotes the formation of
skeletal ferrite in the form of a 3-D network structure, Figure 6.6a. The effect of
temperature on the rate of dissolution of this phase is shown in Figure 6.6b
where it can be seen that dissolution is almost complete within a few minutes at
temperatures above
1l00°C. The strip-cast material has also been found to
exhibit a more rapid rate of dissolution of delta-ferrite compared
with
slab cast
material (Kim
et
al.
2003).
Such behaviour is to be expected since volume
diffusion of Cr
in
iron is the major contributor to the rate of dissolution of delta-
ferrite and this phase is more finely-dispersed
in
strip-cast steel.
6.4
Hot rolling processes
A possible means of improving the surface profile of strip-cast metals and
altering the as-cast structure is through
hot
rolling either directly after casting
or
by
reheating as-cast coils.
Hot
rolling is generally carried
out
at
high
homologous temperatures, i.e.
T >
0.5Tm
where
Tm
is the absolute melting
temperature of the alloy. Many microstructural changes occur during
hot
rolling: the original grains change shape
and
an internal substructure forms,
texture changes take place, precipitation may occur, dynamic recovery or
dynamic recrystallization processes are possible, the constituent particles may
fracture
and
redistribute and some homogenisation of the as-cast structure is
possible. The most notable material factors that affect the deformation
microstructure include crystal structure, stacking fault energy
(SFE),
initial
grain size and shape and the size, shape
and
volume fraction of any second
phase (Humphreys
and
Hatherly
2004).
Deformation also becomes more
homogeneous with increasing temperature
with
the frequency of
microstructural inhomogeneities such as microbands, transition bands
and
shear bands considerably reduced.
6.4.1
Conventional hot rolling
Although it is economically more viable to
hot
roll as-cast strip
in
a continuous
process (in-line rolling), situations may arise where this is
not
feasible and coil
reheating followed by
hot
rolling is then a viable alternative for altering the
strip dimensions, surface integrity
and
cast structure.
Hot
rolling of reheated
strip breaks
up
the cast structure with dynamic recrystallization and/or static
recrystallization further altering the microstructure. In alloys where a phase
transformation occurs below the rolling temperature (carbon steels and
titanium alloys), there is the possibility of significantly modifying the final
microstructure and properties. Since austenitising of low carbon steel
transforms the as-cast microstructure to austenite to produce a grain size that
depends on alloying additions, temperature
and
holding time, subsequent hot
rolling will deform
and
possibly recrystallize the austenite grain structure.
Cooling at various rates will generate microstructures ranging from fine
polygonal ferrite to bainite/martensite (Honeycombe
and
Bhadeshia
1995).
204
Direct
strip
casting
of
metals
and
alloys
6.4.2 In-line hot rolling
A number of ferrous and non-ferrous industries utilise in-line hot rolling in their
strip casting operation
but
published reports on the control of the hot rolling stage
and its effect
on
the final strip properties are very limited. The addition of an in-
line hot rolling mill to a Hazelett strip-casting plant
is
illustrated in Figure
3.22
for
the continuous production of copper sheet. In-line rolling has obvious
econontic
benefits in both the ferrous and non-ferrous industriesrsince:
(i)
strip profile,
surface integrity and thickness and
(ii)
microstructure 'can be modified in an
uninterrupted process.
0.8
)
S 0.6
~
u
~
c:
0.4
o
n
!!!
LL
0.2
600
800
1000
1200
Initial
Austenite
Grain
Size
(I-Lm)
Figure
6.7.
Recrystallization kinetics of coarse grained austenite
in
low carbon steels
following deformation
at
1150°C, after Korchynsky (1999).
A further obvious benefit of in-line hot rolling of strip-cast carbon steel
is
the
capability of altering the coarse columnar austenite structure generated
by
casting
(Table
5.2).
However, casting and hot rolling variables need to be carefully
controlled as these will influence the degree of structural refinement prior to
transformation to ferrite. Figure 6.7 illustrates, for low carbon steel, the effect of
austenite grain size and rolling reduction at
1150°C
on
the kinetics of
recrystallization
at
the same temperature. This figure is significant for hot
direct rolling of strip-cast low carbon steel. For an austenite grain size of
-500
I!ffi
(the approximate length of austenite grains produced
by
TRq,
Figure 6.7
indicates that a reduction of at least
30%
is required to ensure that
recrystallization is complete within a few seconds. In fact, Blejde
et
al.
(2000a)
Secondary
processing
and
fabrication
205
report that
hot
reductions below -15% are
not
sufficient to modify the as-cast
microstructure of low carbon steel
and
reductions of
at
least
30%
are needed to
significantly modify the microstructure. The deformation temperature has also
been found to influence the final ferrite morphology in low carbon steel
(Mukunthan
et
al.
2000).
In-line rolling
at
1050
0
e generated polygonal ferrite in
the range of
10
to
40
f.l.m
whereas rolling
at
860°C generated ultra fine ferrite of
2-4
f.l.m
at
the strip surface
with
the strip core consisting of polygonal ferrite in
the range of
10
to
20
f.l.m.
This low temperature rolling procedure generated
both high strength
and
ductility, as shown in Figure 6.2a.
Figure
6.8.
Optical micrographs
of:
(a)
twin roll cast
AA5182
after hot rolling
to
2 mm
gauge strip and direct chill cast
AA5182
after homogenisation and hot rolling
to
2.6
mm gauge strip, after Cheng and Morris
(2002a)
(with kind permission
of
Elsevier Limited).
In the non-ferrous industries, belt/block casters are commonly used
and
these
are generally coupled with an in-line
hot
rolling mill (see
e.g
. Figure 3.22)
whereby 15-40
mm
thick as-cast strip is directly reduced to 2.5-7.0
mm
by hot
rolling
(Uu
et
al
. 2005). Twin roll casting generates thinner gauge strip which
can either be
hot
rolled
or
directly fed to a cold rolling mill. In alloys that
do
not
undergo gross structural changes
by
solid-state transformation, the
microstructure developed
by
in-line rolling are usually typical of
hot
working
(Uu
and
Morris 2003,
Uu
and
Morris
2004;
Uu
et
al.
2005). Figure 6.8a shows
the
hot
deformation microstructure of a strip-cast AI-Mg alloy (AA5182) in
which elongated grains
and
shear banding are clearly evident; in contrast,
conventional hot-band produced
by
DeC
shows a fully recrystallized
microstructure (Figure 6.8b). The difference in hot-band microstructure also
has a small influence
on
the texture after cold rolling
and
annealing (§6.5.2.2).
Depending
on
the type of alloy and rolling conditions, as-cast strip may also
undergo recrystallization during
or
after rolling to generate either a partly
or
fully recrystallized microstructure.
206
Direct
strip
casting
of
metals
and
alloys
6.4.3 Ferrite grain refinement
by
controlled
rolling
Various rolling procedures have been devised for producing ferrite
with
an
average grain size below - 2
Jlm
in carbon
and
micro alloyed steels. In most of
the reported cases, thick continuously cast slabs are used, followed
by
various
combinations of
hot
and
cold rolling, austenitisation
and
annealing to generate
a fine ferrite grain size (see
e.g.
Matsumura
and
Yada 1987; Beynon
et
al.
1992).
A simple process devised
by
Hodgson and co-workers (Hickson
and
Hodgson
1999;
Hodgson
et
al.
1999; Hurley
et
al.
1999)
involved
hot
rolling of low carbon
steel strip
in
a single pass to produce a
1-2
Jlm
surface grain size
with
a range of
core microstructures.
It
has been demonstrated that single-pass
hot
rolling of strip-cast low carbon
steel in the vicinity of the
Ar3
temperature can generate ultra-fine polygonal
ferrite of average grain size below
2-4
Jlm,
Figure 6.9a (Ferry
and
Page
2001).
In
this work, the fine grain structure was produced throughout the thickness of
the as-cast strip by:
(i)
initial heat treatment of the strip prior to rolling to
produce a fine
(-20
Jlm)
austenite grain size,
(ii)
single-pass deformation in the
vicinity of
Ar3
(-870°C)
and
(iii)
rapid cooling. However, the fine-grained
microstructure required careful control of rolling
and
cooling conditions.
Figure 6.9b shows the strong influence of roll-bite entry temperature
on
ferrite
grain size;
it
can be seen that the production of fine-grained ferrite is restricted
to a narrow processing window. The cooling rate after rolling also affects the
final microstructure
with
bainitic/martensitic structures (due to rapid
quenching) often produced
(Page 1997).
10
(b)
oct
,
·:f
.....
p
•.......
~
~'\
l
...
.
\o!
~
• Surface
o Centre
OL-
______
~
________
L-
____
~
700
800
900
1000
Rolling temperature CC)
Figure
6.9.
(a)
Fine ferrite microstructure in strip-cast
0.06
wt.% C steel following
single-pass
hot
rolling to
60%
reduction.
(b)
Influence of roll bite entry
temperature
on
the final grain size, after Ferry
and
Page (2001).
Secondary
processing
and
fabrication
207
6.5 Low temperature rolling processes
6.5.1 Microstructural development during cold rolling
Cold deformation is generally restricted to low homologous temperatures
(T
de
! <
0.3Tm)
and, unlike
hot
deformation, results in a marked increase in
dislocation density
and
work hardening. The grains also subdivide in a
complex way to produce features such as a cellular substructure, microbands,
deformation bands, deformation twins and larger-scale heterogeneities such as
shear bands (Humphreys
and
Hatherly
2004).
Figure 6.
10.
Microstructural development of twin roll cast low carbon steel after: (a)
cold rolling showing
non
uniform deformation,
and
(b)
warm
rolling showing
both
non-uniform deformation
and
coarse, irregular-shaped
grains, after Ferry
and
Page (2001).
Low
carbon
steel
The as-cast microstructure of strip-cast low carbon steel usually contains a
mixture of phases
and
an inhomogeneous distribution of solute elements
(§5.3
.1.1). A typical microstructure of cold rolled low carbon steel strip
produced
by
TRC
is shown in Figure 6.10a. The deformation microstructure is
inhomogeneous; polygonal ferrite is elongated in the direction of rolling
whereas acicular ferrite is more resistant to deformation. Regions of localised
shear are also evident,
and
shear bands traverse the microstructure
at
an
acute
angle to
RD.
The hardness is variable throughout the microstructure; this is a
manifestation of the various transformation products produced
during
strip
casting. The
hard
acicular ferrite
and
bainite phases are more resistant to
deformation compared
with
polygonal ferrite
and
large reductions are required
to remove areas of the original as-cast structure. In contrast, conventional hot-
band
usually consists of fine-grained, polygonal ferrite with a low dislocation
density
and
a more uniformly deformed microstructure (Ray
et
ai.
1994).
208
Direct
strip
casting
of
metals
and
alloys
Stainless
steel
As shown
in
§5.3.2.1, these steels usually develop a columnar grain structure
on
casting
with
austenitic stainless steels containing residual delta-ferrite in either
cellular
or
skeletal morphology. During cold rolling of austenitic steels such as
AISI
304,
there is a tendency to form
strain-induced
martensite which results in a
microstructure of deformed austenite
and
-20% martensite (see
e.g.
Raabe
(1997)
and
Kim
et
al.
(2001». Due to the uniform
and
fine distribution of delta-
ferrite in the as-cast microstructure of AISI
304,
martensite nucleates in the
vicinity of ferrite
during
rolling which in
tum
produces a uniform distribution
of martensite. This type of structure is quite different to that produced in
conventional cold rolled material since the latter contains a low fraction of
delta-ferrite
«
5%)
due
to homogenisation
and
hot
rolling. This tends to
generate larger packets of less uniformly dispersed martensite
during
rolling.
Non
ferrous
alloys
For aluminium
and
copper alloys, microstructural evolution
during
cold rolling
is similar to that observed
in
other
fcc
strip-cast alloys
and
is
dependent
on
the
morphology
and
texture of the grain structure produced
by
casting.
It
is
pertinent to note, however, that the deformation microstructure is highly
dependent
on
SFE
of the alloy with this parameter exerting the strongest
influence
on
microstructural development. In general, cold rolling will tend to
elongate the as-cast grain structure along
RD
with
any
constituent phases
produced
by
casting broken
up
and
redistributed. However, segregation is
not
affected greatly
by
cold rolling (Gras
et
al.
2005)
and
subsequent annealing
behaviour is highly dependent
on
the as-cast
or
pre-heat-treated
microstructure. Compared
with
wrought
alloys, deformation of cast structures
is more inhomogeneous
and
there is a greater tendency for strain localisation in
the form of shear banding (Carlsson
et
al.
1990;
Cheng and Morris 2002a).
For hcp metals such as Ti
and
Mg alloys, established deformation theory would
indicate that the modes of deformation are similar to their
wrought
alloy
counterparts although, for strip-cast magnesium, very little has been published
on
microstructural development during cold rolling. Nevertheless, this
material should deform in a similar manner to
wrought
magnesium alloys.
In
general, there is very little published data on the deformation behaviour of
most types of strip-cast metals and further work is needed.
6.5.2 Rolling textures
The development of a preferred orientation or texture is a natural consequence of
cold rolling. The particular type of rolling texture depends mainly
on
the crystal
structure and
SFE
of the metal,
but
the starting microstructure, texture
and
degree
of cold rolling are also important variables
(§BA). The cold rolled microstructure
and texture are important as they influence the recrystallization texture
on
annealing and the mechanical behaviour of the final material
(§6.7.1.3).
Secondary
processing
and
fabrication
209
6.5.2.1
bee
metals
In bcc metals such as low carbon steel and ferritic stainless steel, slip is the
principal deformation process during cold rolling and occurs in the close packed
<111>
directions by pencil glide; the slip plane may
be
any of the planes
{110},
{112}
or
{123}
(see
e.g.
Hutchinson
1984;
Humphreys and Hatherly
2004).
Both
cold rolling
and
annealing textures in bcc metals are commonly described in
terms of certain orientation fibres in Euler space (see §B.3.3). The orientation
ranges of two of the more important fibres are given
in
Figure
B.7:
(i)
a-fibre
running from
{001}<110>
to
{111}<110>
along <110>//RD
and
(ii) J4fibre running
from
{111}<110>
to
{111}<112>
along <l11>//ND. Other texture components will
be present depending
on
the initial microstructure
and
the degree of cold
rolling (Ray
et
al.
1994).
(a) (b) (c)
Figure 6.11.
qJ2
=
45°
section of Euler space for
70%
cold rolled
low
carbon steel produced
by: a)
twin
roll casting
and
(b)
slab casting followed
by
hot
rolling.
(c)
70%
warm
rolled strip-cast steel
(1,
2
...
x random),
after
Page
(1997).
The textures of
70%
cold rolled low carbon steel produced by
IRC
and
by
a
conventional route are compared in Figure
6.11
as
qJ2
=
45°
sections of Euler space.
Cold rolling generates a well-developed
J4fibre plus a partially-developed
a-
fibre
and
a rotated cube component
({001}<110».
However, for
an
equivalent
rolling strain, the deformation texture of the as-cast strip «
4x
random) is
slightly weaker than strip rolled from conventional hot-band
(4
- 6 x random).
The equiaxed, fine-grained ferrite of conventional hot-band deforms more
uniformly than the complex microstructure observed
in
the strip-cast steel. In
the latter, the microstructure contains acicular ferrite/bainite separated
by
regions of polygonal ferrite and the strain distribution during rolling is
inhomogeneous
with
the formation of shear bands after moderate strains.
These factors
in
combination are expected to retard the rotation of grains
towards stable
end
orientations thereby retarding the development of the
classic bcc rolling texture (Ray
et
al.
1994).
210
Direct
strip
casting
of
metals
and
alloys
6.5.2.2 fcc
metals
In
contrast to bcc metals, slip and twinning are possible deformation processes
during cold rolling of
fcc
metals and generally occur
on
close packed
{1I1}
planes
(Humphreys and Hatherly
2004).
The cold rolling textures
in
fcc
metals are also
commonly described
in
terms of orientation fibres
in
Euler space
(§B.4.2).
However, these materials differ from bcc metals
due
to the crystallographic
nature of deformation and two principal orientation tubes termed the
a
fre
- and
fJ-fibre
form, Figure
B.6.
The strength of the rolling texture is affected
by
a
combination of processing and microstructural parameters similar to that found
for bcc metals. For example, heavily cold rolled high
SFE
metals such as
aluminium usually generate a texture containing
{1I2}<11I>
(Copper) and
{123}<634>
(S)
components but, with decreasing
SFE,
the texture gradually
changes to a strong
{1I0}<1I2>
(Brass)
component, as observed in austenitic
stainless steel and various copper alloys. A more complete coverage of these
textures and the transition from the copper-type to brass-type rolling texture in
fcc
alloys is given by Humphreys and Hatherly
(2004).
There are only a limited number of published studies
on
the rolling textures in
strip-cast
fcc
metals (Carlsson
et
al.
1990;
Takuda
et
al.
1991;
Raabe
1997;
Cheng
and Morris
2002a;
Liu and Morris
2003;
Gras
et
al.
2005;
Liu
et
al.
2005).
Stainless
steel and many types of copper alloys have a low
SFE
(YSFE<
80
mJ/m2)
whereas
this value is high for aluminium
(YSFE>120
mJ/m2).
Hence, differences in the
rolling textures are expected and indeed observedin these materials.
{O11}
<100>
12
8
f(9)
6
4
2
{O11}
{O11}
<211>
<111>
(Xrcc-fibre
{O11}
{112}
<011>
<111>
q>2=0
12
<I>
= 45°
10
8
f(9)
6
4
{123}
<634>
J3-fibre
{011}
<211>
As-cast
-_--'
o~----~~----~------~
OL-----~------~------~
o
30
60
90
45
75
90
Figure
6.12.
Effect of cold rolling reduction
on
the development
of
the
arcc-
and
,B-fibres
in
single-roll strip-cast AISI 304 (solid lines)
and
conventional hot-band AISI 304
(dashed lines), after Raabe
(1997)
(with kind permission of Elsevier Limited).
Secondary
processing
and
fabrication
211
Raabe
(1997)
compared the cold rolling textures
in
AISI 304 stainless steel strip
produced
by
slab casting
and
thermomechanical processing
or
strip casting.
The conventional
hot
band
contained equiaxed grains of average size 35
f..1m
with
a low faction of delta-ferrite whereas the strip-cast material consisted
predominantly of a columnar microstructure
with
a central equiaxed zone
and
a higher fraction of delta-ferrite. Despite the difference
in
starting
microstructures, the austenite
in
both
materials developed a rolling texture
characterised
by
a
fcc
-
and
,B-fibres
that are weaker
in
the strip-cast material for
a given level of strain, Figure
6.12.
In
both
materials, there are strong
{110}<211>
(Brass)
and
{110}<001>
(Goss) components
with
the latter increasing
in
intensity
at
high
strain levels. The results described here are broadly similar
to other studies
on
cold rolling of strip-cast AISI 304 (Takuda
et
al.
1991, 1993).
It
is pertinent to note that the as-cast strip
studied
by
Raabe
(1997)
exhibited a
weak
{OOI}<uvw>
texture throughout the thickness of the strip
and
a strongly
textured strip is likely to develop a weaker rolling texture.
The rolling textures of strip-cast aluminium
and
copper are
somewhat
similar
to
that
produced in conventional hot-band material
with
a characteristic
copper-type rolling texture usually generated (Carlsson
et
al.
1990;
Cheng
and
Morris 2002a; Gras
et
al.
2005). However, a reduction
in
SFE
in
copper
by
alloying generates the brass-type rolling texture similar to
that
produced
in
austenitic stainless steel. Figure
6.13
shows a
111
pole figure for a strip-cast
and
hot
rolled AA5182 Al alloy where a strong copper-type rolling texture has
formed
with
a maximum intensity of 9.8x random. Although
not
shown,
further cold rolling simply intensifies this rolling texture (Cheng
and
Morris
2002a). Gras
and
co-workers
(2005)
have shown that, despite the marked
difference
in
texture between the core
and
surface of twin roll cast AA3015 Al
alloy (Figure 5.37), cold rolling to high strains
(>
50%
reduction) generates a
relatively homogeneous rolling texture
throughout
the strip, as
shown
in
Figure
6.14.
CD
~~#-----+-----~~~TD
Figure 6.13.
111
pole
figure of strip-cast AA5182 after
hot
rolling
to
2
mm
gauge
strip
(contours:
1,2
... x random), after
Cheng
and
Morris (2002a)
(with
kind
permission of Elsevier Limited).
212
Direct
strip
casting
of
metals
and
alloys
• S
{132}<634>
...
B
s
{1110}<112>
•
Cu
{1112}<111>
0
a-fibre
{hkO<011>
1::..
y-fibre
(111}<uvw>
15
(a)
--.
c
t
12
/
...........................
.
-_i
:
::::<::::,"
,/f:::-:::.....~
,,..'
.,1..,
o 3
~.'
,,..
.....
"
0:::
.....•..
.
~~~>r.:·
...
o
><
0
.~~~~~;;~~~~::::::::::.....
"-i::::
~
-.:
-.:
-.:
-
-.:
-:
~
o
20
40
60
80
Rolling
reduction
(%)
15
(b)
........•
.'
--.
'#-
'Q;'
12
OJ
J9
~
9
~
Q)
c.
~
6
::J
X
2
o 3
_-----------.-.--
.•
-
....
-.'./1
•
=::=::::::::::::::.-::::~::;:~:.
c::
______
0-
__
_
><
0
::::::::::::------tl.---_:&::::~::~
o
20
40
60
Rolling
reduction
(%)
Figure
6.14.
Evolution
of
volume fraction
of
main texture components during cold
rolling
of
twin roll cast
AA3015:
(a)
strip surface and
(b)
strip
core,
after
Gras et
al.
(2005)
(with kind permission
of
Elsevier Limited).
6.5.2.3
hcp
metals
80
Cold rolling textures of hcp metals are influenced
by
the cia ratio of the hexagonal
crystal structure
and
the particular slip/twinning systems that operate
(Humphreys
and
Hatherly 2004). For metals with cia close to the ideal value of
1.633
(i.e.
cia = 1.623 for Mg), slip
on
the
{OOOl}
basal plane generates a rolling
texture with a strong
{OOOl}<OOl1>
component whereas metals with higher cia
ratios (Zn
and
Cd), deformation occurs by a combination of slip
and
twinning
which results in the basal planes becoming tilted 20-30° about
TD.
For metals
with cia < 1.633, the basal planes are again rotated
out
of the rolling plane by
an
angle of 30-40°
but
this time the rotation axis is
<1010>
and
parallel to
RD.
For
cold rolled titanium (c/a = 1.577), the texture usually shows major components
along the
{hk.l}<10l0>
fibre (Inoue
and
Inakazu
1988).
Very little has been published
on
the development of cold rolling textures in
strip-cast magnesium alloys. Despite the columnar structures
that
tend to form
by
DSC (§5.4.2.1),
it
is expected that cold rolling will generate a weakened
{0001}<0011>
rolling texture
with
more equiaxed solidification structures
expected to deform similar to conventional
wrought
alloys. Weaver
and
Garmestani
(1998)
have investigated the cold rolling textures of as-cast titanium
strip produced
by
single-roll strip casting (MORST) (see §5.4.3). Cold rolling
breaks
up
the as-cast microstructure and,
at
strains as
low
as 40% reduction,
shear
bands
develop
at
an
acute angle to RD. The rolling texture consists of a
split
{0002}
component
with
ODF analysis indicating
that
the basal poles are
concentrated
at
30°
from ND towards
TD.
The rolling texture after 40%
reduction is slightly different to that observed
in
wrought
alloys
with
the main