Ferry M. Direct Stripcasting of Metals and Alloys: Processing, Microstructure and Properties
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Secondary
processing
and
fabrication
213
texture component near (1013)[2021] rather than
{hk.l}<1010>.
This small
discrepancy is probably associated with the low rolling strain, the starting
microstructure
and
texture.
6.5.3
Warm
rolling
of
strip-cast
low
carbon steel
Compared with
hot
rolling of carbon steel,
warm
rolling is a processing mode
carried
out
in
the ferrite phase field (i.e. <
Arl
in
Figure 1.4). The potential for
direct
warm
rolling of strip-cast low carbon steel has been discussed
by
Page
(1997);
this procedure would allow further process rationalisation through the
possible elimination of cold rolling and annealing. This process would also
improve roll life, reduce energy consumption and produce minimum scale
thereby retaining good surface quality of the strip (Barnett
and
Jonas 1997a).
6.5.3.1
Microstructural
evolution
A study of
warm
rolling of strip-cast low carbon steel was carried
out
by
Ferry
and
Page
(2001)
whereby the strip was reheated to
700°C,
held for
600s
and
rolled to
70%
reduction. The as-cast microstructure was
not
significantly
altered
by
the initial heat treatment although the volume fraction of polygonal
ferrite increased slightly together with a decrease in the carbide volume fraction
due
to particle dissolution. Warm rolling generated a microstructure similar to
that produced
by
cold rolling with shear bands present, together with the
preferred deformation of polygonal ferrite (Figure
6.10b). However,
warm
rolling produced -10% volume fraction of irregularly-shaped coarse ferrite
grains of considerably lower hardness compared with the surrounding
deformed regions, Figure
6.10b. These low-hardness (white) regions indicate
that some recrystallization has occurred either during or directly after rolling,
that is,
by
dynamic, metadynamic or static recrystallization (Ferry
and
Page
2001).
Observations of the cold rolled microstructure indicate that acicular
ferrite essentially retains its original morphology during deformation
and
since
this phase is also present prior to
warm
rolling,
it
is likely to be a contributing
factor
in
the development of coarse, irregular shaped grains.
6.5.3.2
Warm
rolling
textures
The deformation texture of warm-rolled strip-cast low carbon steel described
in
§6.4.3.1
is similar to that produced
by
cold rolling
and
shows a well-developed
r-fibre, partially-developed a-fibre and a rotated cube component (Figure
6.11c).
The
warm
rolling texture is slightly stronger than
th~t
produced
by
cold
rolling, which is consistent with observations on conventionally-processed low
carbon steel (Barnett
and
Jonas 1997a). A high deformation temperature is
expected to lower the dislocation density in the non-equilibrium phases both
before
and
during rolling resulting
in
more uniform strain distribution and less
grain fragmentation
by
shear banding. This allows easier rotation of grains
towards their stable end orientations.
214
Direct
strip
casting
of
metals
and
alloys
6.6
Static annealing processes
The principal material factors that influence static annealing include
composition, initial grain size
and
texture, second phase precipitates
and
solute
distribution. Some important processing variables include the deformation
mode, strain, strain rate and deformation temperature. The driving pressure for
softening
by
recovery
and
recrystallization (either dynamic
or
static) is the
stored energy of deformation. Recovery reduces the stored energy of the
material and, hence, the driving force for recrystallization,
by
mechanisms such
as the annihilation of dislocations
and
their re-arrangement into low angle grain
boundaries. A thorough treatment of recrystallization
and
related phenomena
is beyond the scope of this book
but
an extensive coverage of these processes is
given
by
Humphreys
and
Hatherly
(2004).
Figure 6.15 is a schematic diagram showing the progress of recrystallization in
terms of the volume fraction of the microstructure recrystallized as a function of
annealing time. Static recrystallization commences with a nucleation stage in
which strain-free grains develop at microstructural inhomogeneities such as
transition bands, shear bands and grain boundaries (Humphreys and Hatherly
2004).
Further growth of these nuclei consumes the remaining deformation
substructure to eventually produce a fully recrystallized microstructure. An
understanding of the nucleation and growth stages of recrystallization is
extremely important; they affect the kinetics of the process, the final grain size and
distribution and produce a recrystallization texture which has a large influence on
sheet formability
(§6.7.1.3).
Nucleation
~
\
Complete
recrystallization
o
•
\
Growth
& impingement
Log
time
Figure
6.15.
Schematic diagram showing the progress of static recrystallization
during isothermal annealing
(to
is the incubation time reqUired
for nucleation of recrystallizing grains).
6.6.1 Annealing methods
6.6.1.1 Batch
annealin~
Secondary
processing
and
fabrication
215
Batch annealing generally involves reheating as-rolled coils of metal
in
a
furnace that generates slow heating
and
cooling rates
due
to the thermal mass
of the coil. This results in
an
annealing cycle of several hours to days.
However, a microstructure
with
a highly desirable combination of mechanical
and physical properties can be generated. Several reports
on
batch annealing of
strip-cast
and
subsequently cold-rolled low carbon steel have demonstrated the
ability of the process for producing commercial quality strip for drawing
applications (Guillet
et
ai.
1996).
6.6.1.2
Continuous
annealin~
Continuous annealing is a far more rapid process
than
batch annealing. As-
rolled strip passes through a furnace in a continuous process
and
rapidly
heated to the desired temperature; recrystallization occurs within a few
seconds. The annealing furnace may use induction, radiation
or
molten
bath
heating to generate heating rates of the order of
10
to 1000 K/s
and
cooling rates
of similar magnitude. The heating rates in continuous annealing are therefore
several orders of magnitude greater than in batch annealing
and
the total
annealing time is reduced from days to minutes. The
rapid
throughput
of cold
rolled strip has obvious advantages in the overall strip casting process although
adequate mechanical properties of the annealed sheet
may
be more difficult to
achieve (Shiang
and
Wray 1991).
6.6.2 Quantifying recrystallization
The kinetics of static recrystallization after
hot
or
cold rolling are often
quantified using the classical nucleation
and
growth theory of phase
transformations. The Johnson-Mehl-Avrami-Kolmogorov
(JMAK)
equation
relates fraction recrystallized
(Xv)
to annealing time (t):
Xv
=
1-
exp[-B(t -
to)"]
(6.2)
where
to
is the incubation period needed to form a nucleus, n the JMAK
exponent
and
B a function of nucleation rate
(N)
and
growth rate of nuclei (G).
Since N
and
G increase rapidly
with
temperature, the kinetics of
recrystallization are expected to be a strong function of annealing temperature.
Furthermore,
N
and
G are both influenced
by
the stored energy of
deformation and, hence, the rate of recrystallization is also expected to be
dependent
on
deformation conditions
and
initial microstructure. Relationships
of the form of Eq.
6.2 are shown to predict reasonably the static recrystallization
kinetics of cold
and
hot
deformed metals (Humphreys
and
Hatherly 2004).
216
Direct
strip
casting
of
metals
and
alloys
6.6.3 Recrystallization
behaviour
of strip-cast metals
6.6.3.1
Low
carbon
steel
Recrystallization
kinetics
and
microstructural
development
The kinetics of recrystallization
and
development of microstructure
during
cold
rolling
and
annealing are important in strip-cast low carbon steel
but
only a
small
number
of investigations have been reported (Shiang
and
Wray 1989,
1991;
Ferry
and
Page
2001).
The significant influence of as-cast microstructure
on
the rate of recrystallization has been clearly demonstrated
by
Xu
(2006).
A
conventional 2
mm
gauge hot-band was austenitised to generate a coarse
austenite grain size
and
then cooled rapidly to produce a microstructure of
Widmanstatten/acicular ferrite similar to that produced
in
strip-cast steel.
While
both
starting microstructures are very similar, the rate of recrystallization
in the strip-cast material is sluggish, as illustrated in Figure
6.16.
This
study
shows that the as-cast microstructure is highly resistant to recrystallization
with
higher annealing temperatures required to achieve full recrystallization in a
reasonable time.
10000
:::R
0
0
LO
...
.E
10
Q)
E
1=
1
40
b· .
•••••••.•••••••••
Q
...........................
0'.
' .
....................
.................
• Acicular ferrite
o Strip cast structure
60 80 100
Rolling reduction (%)
Figure
6.16.
Time for
50%
recrystallization in cold rolled
low
carbon steel exhibiting
an
acicular microstructure produced
by
twin
roll casting or
heat
treatment of 2
mm
gauge
hot
band, from Xu (2006).
The recrystallization kinetics
and
final grain size of strip-cast low carbon steel
are strongly influenced
by
the deformation temperature (Ferry
and
Page
2001).
The kinetics of recrystallization
at
650°C of the cold rolled
and
warm
rolled
steels described earlier are given in Figure
6.17 which shows, for
both
processes,
Secondary
processing
and
fabrication
217
typical sigmoidal recrystallization behaviour
but
the time for
50%
recrystallization
(to.5)
following
warm
rolling is several orders of magnitude
greater than for cold rolling. After cold rolling, nucleation occurs initially at
shear bands (Figure 6.18a) with further recrystallization occurring slowly to
produce a recrystallized microstructure of average grain size of 7
J..Lm
and grains
ranging
in
size from 2 to
20
J..Lm
(Figure 6.18b). Despite the fine grain size
produced in the strip-cast steel, the size distribution is
not
as uniform as that
found in conventional
hot
band.
The recrystallization behaviour of low carbon steel after
warm
rolling is
different to that found for cold rolling. Recrystallization commences both in the
core of the strip
and
at
the strip surface (Figure
6.18c);
these grains grow slowly
to produce a coarse grained recrystallized microstructure of average grain size
of
34
J..lffi
with grains ranging in size from 5
J..Lm
to over
100
J..lffi
(Figure 6.18d).
The retarded recrystallization and coarse final grain size
in
warm
rolled strip-
cast steel is similar to that found
in
studies
by
Barnett
and
Jonas
(1997b)
on
conventional hot-band material. The high deformation temperature associated '
with
warm
rolling is expected to lower the stored energy of the matrix and
reduce both the driving force for recrystallization
and
the nucleation site
density.
1.0,------------------------,
'>
C
"t:l
0.8
Q)
J:!
lO.6
(.)
!!?
0.4
c:
o
ts
~
0.2
LL
..........
,.
....
...
..
,
......
.
"'/~Old
rolled
.:::
0·····
Dc?
.'
..
···0
.'
,f
p
.....
/l
..
'
••••
.'
••••••••
Warm
rolled
0······0
............
o
O.g·~···
.
..........
.
OL--~--~--~---L--~--~-~
0.1
10
100 1000 10000 100000 1000000
Time
(5)
Figure
6.17.
Effect of rolling temperature
on
the recrystallization kinetics of twin roll
cast low carbon steel, after Ferry
and
Page
(2001)
(with
kind
permission
from Institute of Materials, Minerals
and
Mining, UK).
The JMAK exponent (n-value) in
Eq.
6.2
is often used as
an
indicator of the
mechanistic processes associated with a particular phase transformation
(Christian
2002)
and is obtained from the gradient of the plot of
Inlog(l/l-X
v
)
218
Direct
strip
casting
of
metals
and
alloys
as a function of In(t - to). For both the cold and
warm
rolled strip-cast steels,
n-
values of - 0.5 were obtained from Figure 6.17 which is lower than that found in
conventionally produced low carbon steel
(n
= 1.0
-1.5)
(Barnett
et
al.
1997b).
Hence, the mode of recrystallization
in
strip-cast steel is different to that in
conventional
hot
band.
Inthe
former, annealing results
in
profuse nucleation
within shear bands (Figure 6.18a),
but
further growth is sluggish.
Several factors can retard the migration of grain boundaries during
recrystallization such as solute drag
and
particle pinning (Humphreys and
Hatherly
2004). Strip casting promotes supersaturation
and
microsegregation
due to the high solidification rates associated with the process. Strip-cast low
carbon steel may also contain a relatively high level of
S that can combine with
Mn
to produce a dispersion of very fine MnS particles during casting. The
aforementioned factors may result in the slow rate of recrystallization
in
strip-
cast steel.
Figure
6.18.
Partly and completely recrystallized microstructures
in
twin
roll
~ast
low
carbon steel following:
(a)
and
(b)
cold rolling to
70%
reduction
and
(c)
and (d)
warm
rolling to
60%
reduction, after Ferry
and
Page
(2001).
Secondary
processing
and
fabrication
219
Recrystallization textures
The rolling texture that forms in low carbon steel is generally independent of
processing route despite the possibility of widely varying processing histories
(CCC
or
strip-casting). This is in marked contrast to the recrystallization texture
which is strongly affected
by
a host of parameters pertaining to steel chemistry
and
the entire processing history (Hutchinson 1984; Hutchinson
et
al.
1990; Ray
et
al. 1994; Hutchinson
and
Ryde 1997). In low carbon steel, the most desirable
nucleation sites are grain boundaries since the nuclei that form have orientations
along the <111>//ND (y-fibre) (Ray
et
al.
1994).
This is
important
since a strong
<l11>//ND texture with a balance of
<112>
and
<110>
parallel to the rolling
direction results
in
superior formability of the annealed steel sheet
(§6.7.1.3).
The
strength
and
distribution of the texture components along the y-fibre is affected
by
several processing parameters (Ray
et
al.
1994): degree of cold rolling,
lubrication
during
rolling, annealing temperature
and
heating rate as well as
material parameters including
hot
band
microstructure
and
texture (dependent
on
hot
deformation history). Alloying additions
and
their distribution
in
the
matrix
and
precipitation
during
annealing are also significant.
(a) (b)
Figure 6.19.
((J2
=
45°
section of Euler space for
70%
cold rolled
and
recrystallized low
carbon steel
produced
by: (a)
twin
roll casting
and
(b) slab casting
followed
by
hot
rolling
(1,2
... x random).
The recrystallization texture of cold rolled strip-cast steel is generally
weak
with
a maximum intensity of
-2-3x
random, Figure 6.19a. This texture is weaker
than
that
produced
in
conventional hot-band, Figure 6.19b, which often
develops a moderate <l11>//ND texture with a weaker
{110}<001>
component
(Ray
et
al.
1994). The mixed microstructure of as-cast low carbon steel is vastly
different to the fine, equiaxed ferrite microstructure of conventional hot-band,
and
these differences
may
explain the weaker texture of the strip-cast steel. The
220
Direct
strip
casting
of
metals
and
alloys
principal nucleation sites in conventional hot-band are grain boundaries which
are associated with nucleation of
{lll}<uvw>
grains whereas the nucleation sites
in the as-cast strip are less well-defined. Therefore, selective nucleation and
growth of recrystallized grains of favourable orientation is unlikely
in
rolled as-
cast strip resulting
in
a weak annealing texture.
6.6.3.2 Stainless steel
Annealing
behaviour
The microstructure of cold rolled strip-cast AISI 304 generally contains
deformed austenite and some strain-induced martensite dispersed uniformly
throughout the microstructure (Kim
et
ai.
2001).
It
was shown that subsequent
annealing at
llOOoe results in the transformation of martensite to austenite,
recrystallization of deformed austenite and dissolution of delta-ferrite. Figure
6.20
shows the influence of initial austenite microstructure
and
the fraction of
delta-ferrite prior to cold rolling on the austenite grain size during grain
coarsening.
It
is clear that delta-ferrite has a significant influence
on
annealing
behaviour. In the early stages of annealing, austenite recrystallization
and
the
martensitic transformation generate a fine equiaxed microstructure with little
grain growth due to grain boundary pinning
by
the residual ferrite. In the
conventional
hot
band (0 =
0.5%)
and the pre-heat treated as-cast strip (0 -
0),
there is little resistance to grain coarsening and a larger recrystallized grain size
is generated with the former material producing a bi-modal grain size
distribution.
60
.---------------------------~
E40
-
::t
-
ru
'(i)
C
.
~
20
-
(!)
• Strip cast
t:..
Strip cast + 3min/1100"C
• Strip cast + 3h/11
OO"C
l'
.........
0
o Hot rolled
...
::,.:.
.......
.
~
...
v
O··t
••••
••••• A
• •
••
Ll
0'.
.
......
..
..
'
....
.
............
.
.' .'
..
'
,.-
..•..
.
......
.
..
'
....•...
o·
¢.
............
.
.'
..
'
"-6..'
..
I'~"
i:r
O~--------~I--------~I------~
. 0
10
20
30
Time
(sl2)
Figure 6.20. Evolution of grain size
during
recrystallization of strip-cast AISI304,
after
Kim
et
al.
(2001) (with kind permission of Iron
and
Steel Society, USA).
Secondary
processing
and
fabrication
221
The effect of annealing
on
the mechanical properties of the stainless steels
investigated
by
Kim
et
al.
(2001)
is summarised
in
Table
6.2.
It
can be seen that
the properties are similar for both types of material
in
agreement with other
studies of cold rolling
and
annealing of austenitic stainless steel (Takuda
et
al.
1993).
Table
6.2.
Properties of
70%
cold rolled
and
annealed AISI 304 strip,
adapted from
Kim
et
al.
(2001).
Material
Hot
band
Strip cast
Starting austenite
microstructure
Equiaxed grains
Delta ferrite =
0.4%
(CrINi)eq
= 1.87
Columnar
grains
and
central equiaxed zone
Delta ferrite =
3.0%
(CrINi)eq
= 1.94
* Limiting
drawing
ratio (see §6.7.12)
Recrystallization
textures
Microstructural
development
during
CRA
Md
to equiaxed r
Ydef
to
equiaxed r
Final grain size = 50
J.lm
Md
to equiaxed r
Ydef
to
equiaxed r
Final grain size =
30
J.lm
Tensile
Formability
properties parameters
(§6.7.1.3)
YS
=239 MPa LDR*=2.02
TS=
628 MPa
Ilr=
-0.54
%e =
63.1
YS
= 241 MPa LDR = 2.01
TS
= 626 MPa
Ilr
= -0.20
%e = 61.2
There are only a few published studies of the recrystallization textures that
develop
in
cold rolled strip-cast stainless steel. Takuda
et
al.
(1991,
1993)
twin
roll cast AlSI
304
stainless steel to 3
mm
gauge strip
and
carried
out
a complex
deformation
and
annealing cycle involving three annealing treatments with two
intermittent cold rolling stages to generate fully recrystallized 0.4
mm
gauge
strip. This processing route generated a very weak texture from the strongly
textured starting material with a maximum of 1.6x random located
in
the
vicinity of the
{110}
<
112
> texture component.
It
was noted that the
formability of this material was comparable to that of conventionally processed
material.
6.6.3.3
Aluminium
alloys
Annealing
behaviour
Secondary processing of thicker gauge
(>
5 mm) strip-cast aluminium alloys
by
rolling
and
annealing is a common industrial procedure since substantial
reductions are needed to achieve the required final gauge (Emley
1976).
As
in
ferrous alloys, the non-equilibrium as-cast microstructure
C
after DSC should
result in a markedly different response to annealing after cold rolling compared
with conventionally-produced materials. For example, as-cast aluminium strip
exhibits a dendritic structure with micro-
and
macrosegregation, supersaturated
elements,
and
a fine
but
non-uniform dispersion of intermetallic particles
and
other phases. The strip-cast material may need to undergo homogenisation
and
222
Direct
strip
casting
of
metals
and
alloys
hot
rolling stages prior to cold rolling
and
these processes will increase the
overall cost of the process.
In the absence of homogenisation, the recrystallization kinetics of cold-rolled
strip-cast Al alloys are generally slower
than
for material
produced
by
conventional casting
and
secondary processing (Odok
and
Thym 1974; Emley
1976;
Nes
and
Slevolden
1979;
Althoff
1980;
Zhao
et
al.
2004). This behaviour is
illustrated
in
Figure
6.21
for AA1050 alloy which shows
that
cold rolled as-cast
strip recrystallizes
at
a higher temperature
and
over a slightly
wider
temperature range
than
conventional cold rolled hot-band material.
........
~
c
o
ti
::J
-0
~
40
Direct strip cast
6mm gauge
Cl
.!:
"0
a::
20
250
300
350
400
450
500 550
600
Temperature
(DC)
Figure
6.21.
Range of recrystallization as a function of degree of cold rolling
in
AA1050
alloys
produced
by
TRC
and
DCC, followed
by
homogenisation
and
hot
rolling, adapted from Mondolfo (1976).
A
more
recent example of the effect of strip cast microstructure (produced
by
Hazelett belt casting)
on
the reduced rate of recrystallization after cold rolling
was
reported
by
Zhao
et
al.
(2004)
for AA5052
and
AA5182 Al alloys (Figure
6.22).
It
is interesting to note that, prior to cold rolling, these workers found a
more elongated recrystallized grain structure in strip-cast alloys compared
with
DCC-produced material. This difference
in
grain structure
may
have some
influence
on
the rate of recrystallization. They also observed
that
Mg content
has a small effect
on
the rate of recrystallization
with
highly alloyed material
(AA5182) recrystallizing more rapidly
due
to the higher stored energy of the
deformation microstructure (Humphreys
and
Hatherly 2004).
The influence of homogenisation
on
recrystallization of strip-cast Al alloys is
illustrated
in
Figure 6.23 for a cold-rolled AI-Mn alloy. The unhomogenised