Note: Descriptions are shown in the official language in which they were submitted.
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CASTING STEEL STRIP
TECHNICAL FIELD
This invention relates to the casting of steel
strip.
it is known to cast metal strip by continuous
casting in a twin roll caster. In this technique molten
metal is introduced between a pair of contra-rotated
horizontal casting rolls which are cooled so that metal
shells solidify on the moving roll surfaces and are brought
together at the nip between them to produce a solidified
strip product delivered downwardly from the nip between the
rolls. The term "nip" is used herein to refer to the
general region at which the rolls are closest together.
The molten metal may be poured from a ladle into a smaller
vessel or series of vessels from which it flows through a
metal delivery nozzle located above the nip so as to direct
it into the nip between the rolls, so forming a casting
pool of molten metal supported on the casting surfaces of
the rolls immediately above the nip and extending along the
length of the nip. This casting pool is usually confined
between side plates or dams held in sliding engagement with
end surfaces of the rolls so as to dam the two ends of the
casting pool against outflow, although alternative means
such as electromagnetic barriers have also been proposed.
Although twin roll casting has been applied with
some success to non-ferrous metals which solidify rapidly
on cooling, there have been problems in applying the
technique to the casting of ferrous metals. One particular
problem has been the achievement of sufficiently rapid and
even cooling of metal over the casting surfaces of the
rolls. in particular it has proved difficult to obtain
sufficiently high cooling rates for solidification onto
casting rolls with smooth casting surfaces and it has
therefore been proposed to use rolls having casting
surfaces which are deliberately textured by a regular
pattern of projections and depressions to enhance heat
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transfer and so increase the heat flux achieved at the
casting surfaces during solidification.
Our United States Patent 5,701,948 discloses a
casting roll texture formed by a series of parallel groove
and ridge formations. More specifically, in a twin roll
caster the casting surfaces of the casting rolls may be
textured by the provision of circumferentially extending
groove and ridge formations of essentially constant depth
and pitch. This texture produces enhanced heat flux during
metal solidification and can be optimised for casting of
steel in order to achieve both high heat flux values and a
fine microstructure in the as cast steel strip.
Essentially when casting steel strip, the depth of the
texture from ridge peak to groove root should be in the
range 5 microns to 50 microns and the pitch of the texture
should be in the range 100 to 250 microns for best results.
For optimum results it is preferred that the depth of the
texture be in the range 15 to 25 microns and that the pitch
be between 150 and 200 microns.
Although rolls with the texture disclosed in
United States Patent 5,701,948 have enabled achievement of
high solidification rates in the casting of ferrous metal
strip it has been found that they exhibit a marked
sensitivity to the casting conditions which must be closely
controlled to avoid two general kinds of strip defects
known as "crocodile-skin" and "chatter" defects. More
specifically it has been necessary to control crocodile-
skin defects by the controlled addition of sulphur to the
melt and to avoid chatter defects by operating the caster
within a narrow range of casting speeds.
The crocodile-skin defect occurs when 5 and y
iron phases solidify simultaneously in shells on the
casting surfaces of the rolls in a twin roll caster under
circumstances in which there are variations in heat flux
through the solidifying shells. The S and y iron phases
have differing hot strength characteristics and the heat
flux variations then produce localised distortions in the
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solidifying shells which come together at the nip between
the casting rolls and result in the crocodile-skin defects
in the surfaces of the resulting strip.
A light oxide deposit on the rolls having a
melting temperature below that of the metal being cast can
be beneficial in ensuring a controlled even heat flux
during metal solidification on to the casting roll
surfaces. The oxide deposit melts as the roll surfaces
enter the molten metal casting pool and assists in
establishing a thin liquid interface layer between the
casting surface and the molten metal of the casting pool to
promote good heat flux. However, if there is too much
oxide build up the melting of the oxides produces a very
high initial heat flux but the oxides then resolidify with
the result that the heat flux decreases rapidly. This
problem has been addressed by endeavouring to keep the
build up of oxides on the casting rolls within strict
limits by complicated roll cleaning devices. However,
where roll cleaning is non-uniform there are variations in
the amount of oxide build up with the resulting heat flux
variations in the solidifying shells producing localised
distortions leading to crocodile-skin surface defects.
Chatter defects are initiated at the meniscus
level of the casting pool where initial metal
solidification occurs. One form of chatter defect, called
"low speed chatter", is produced at low casting speeds due
to premature freezing of the metal high up on the casting
rolls so as to produce a weak shell which subsequently
deforms as it is drawn further into the casting pool. The
other form of chatter defect, called "high speed chatter",
occurs at higher casting speeds when the shell starts
forming further down the casting roll so that there is
liquid above the forming shell. This liquid which feeds
the meniscus region, cannot keep up with the moving roll
surface, resulting in slippage between the liquid and the
roll in the upper part of the casting pool, thus giving
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rise to high speed chatter defects appearing as transverse
deformation bands across the strip.
Moreover, to avoid low speed chatter on the one
hand and high speed chatter on the other, it has been
necessary to operate within a very narrow window of casting
speeds. Typically it has been necessary to operate at a
casting speed within a narrow range of 30 to 32 metres per
minute. The specific speed range can vary from roll to
roll but in general the casting speed must be well below 40
metres per minute to avoid high speed chatter.
We have now determined that it is possible to
produce a,roll casting surface which is much less prone to
generation of chatter defects and which enables the casting
of steel strip at casting speeds well in excess of what has
hitherto been possible without producing strip defects.
Moreover, the casting surface provided in accordance with
the invention is also relatively insensitive to conditions
causing crocodile-skin defects and it is possible to cast
steel strip without crocodile-skin defects.
DISCLOSURE OF THE INVENTION
According to the invention there is provided a
method of continuously casting steel strip comprising
supporting a casting pool of molten steel on one or more
chilled casting surfaces and moving the chilled casting
surface or surfaces to produce a solidified strip moving
away from the casting pool, wherein the or each casting
surface is textured by a random pattern of discrete
projections having pointed peaks with a surface
distribution of between 10 and 100 peaks per mm' and an
average height of at least 10 microns.
Preferably, the average height of the discrete
projections is at least 20 microns.
Preferably too, the strip is moved away from the
casting pool at a speed of more than 40 metres per minute.
It may, for example, be moved away at a speed of between 50
and 65 metres per minute.
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The molten steel may be a low residual steel
having a sulphur content of not more than 0.025%.
The method of the present invention may be
carried out in a twin roll caster.
Accordingly the invention further provides a
method of continuously casting steel strip of the kind in
which molten metal is introduced into the nip between a
pair of parallel casting rolls via a metal delivery nozzle
disposed above the nip to create a casting pool of molten
steel supported on casting surfaces of the rolls
immediately above the nip and the casting rolls are rotated
to deliver a solidified steel strip downwardly from the
nip, wherein the casting surfaces of the rolls are each
textured by a random pattern of discrete projections having
pointed peaks with a surface distribution of between 10 and
100 peaks per mm' and an average height of at least 10
microns.
The invention further extends to apparatus for
continuously casting steel strip comprising a pair of
casting rolls forming a nip between them, a molten steel
delivery nozzle for delivery of molten steel into the nip
between the casting rolls to form a casting pool of molten
steel supported on casting roll surfaces immediately above
the nip, and roll drive means to drive the casting rolls in
counter-rotational directions to produce a solidified strip
of metal delivered downwardly from the nip, wherein the
casting surfaces of the rolls are each textured by a random
pattern of discrete projections having pointed peaks with a
surface distribution of between 10 and 100 peaks per mm'
and an average height of at least 10 microns.
A textured casting surface in accordance with the
invention can be achieved by grit blasting the casting
surface or a metal substrate which is protected by a
surface coating to produce the casting surface. For
example the or each casting surface may be produced by grit
blasting a copper substrate which is subsequently plated
with a thin protective layer of chrome. Alternatively the
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casting surface may be formed of nickel in which case the
nickel surface may be grit blasted and no protective
coating applied.
The required texture of the or each casting
surface may alternatively be obtained by deposition of a
coating onto a substrate. In this case the material of the
coating may be chosen to promote high heat flux during
metal solidification. Said material may be a material
which has a low affinity for the steel oxidation products
so that wetting of the casting surfaces by those deposits
is poor. More particularly the casting surface may be
formed of an alloy of nickel chromium and molybdenum or
alternatively an alloy of nickel molybdenum and cobalt, the
alloy being deposited so as to produce the required
texture.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the invention may be more fully
explained the results of experimental work carried out to
date will be described with reference to the accompanying
drawings in which:
Figure 1 illustrates experimental apparatus for
determining metal solidification rates under conditions
simulating those of a twin roll caster;
Figure 2 illustrates an immersion paddle
incorporated in the experimental apparatus of Figure 1;
Figure 3 indicates heat flux values obtained
during solidification of steel samples on a textured
substrate having a regular pattern of ridges at a pitch of
180 microns and a depth of 60 microns and compares these
with values obtained during solidification onto a grit
blasted substrate;
Figure 4 plots maximum heat flux measurements
obtained during successive dip tests in which steel was
solidified from four different melts onto ridged and grit
blasted substrates;
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Figure 5 indicates the results of physical
measurements of crocodile-skin defects in the solidified
shells obtained from the dip tests of Figure 4;
Figure 6 indicates the results of measurements of
standard deviation of thickness of the solidified shells
obtained in the dip tests of Figure 4;
Figure 7 is a photomicrograph of the surface of a
shell of a low residual steel of low sulphur content
solidified onto a ridged substrate at a low casting speed
and exhibiting a low speed chatter defect;
Figure 8 is a longitudinal section through the
shell of Figure 7 at the position of the low speed chatter
defect;
Figure 9 is a photomicrograph showing the surface
of a shell of steel of low sulphur content solidified onto
a ridged substrate at a relatively high casting speed and
exhibiting a high speed chatter defect;
Figure 10 is a longitudinal cross-section through
the shell of Figure 9 further illustrating the nature of
the high speed chatter defect;
Figures 11 and 12 are photomicrographs of the
surfaces of shells formed on ridged substrates having
differing ridge depths;
Figure 13 is a photomicrograph of the surface of
a shell solidified onto a substrate textured by a regular
pattern of pyramid projections;
Figure 14 is a photomicrograph of the surface of
a steel shell solidified onto a grit blasted substrate;
Figure 15 plots the values of percentage melt
oxide coverage on the various textured substrates which
produced the shells of Figures 11 to 14;
Figures 16 and 17 are photomicrographs showing
transverse sections through shells deposited from a common
steel melt and at the same casting speed onto grit blasted
and ridged textured substrates;
Figure 18 plots maximum heat flux measurements
obtained on successive dip tests using substrates having
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chrome plated ridges and substrates coated with an alloy of
nickel, molybdenum and chrome;
Figures 19, 20 and 21 are photomicrographs of
steel shells solidified onto the different cooling
substrates;
Figure 22 is a plan view of a continuous strip
caster which is operable in accordance with the invention;
Figure 23 is a side elevation of the strip caster
shown in Figure 22;
Figure 24 is a vertical cross-section on the line
24-24 in Figure 22;
Figure 25 is a vertical cross-section on the line
25-25 in Figure 22;
FiQure 26 is a vertical cross-section on the line
26-26 in Figure 22;
Figure 27 represents a typical surface texture
produced according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Figures 1 and 2 illustrate a metal solidification
test rig in which a 40 mm x 40 mm chilled block is advanced
into a bath of molten steel at such a speed as to closely
simulate the conditions at the casting surfaces of a twin
roll caster. Steel solidifies onto the chilled block as it
moves through the molten bath to produce a layer of
solidified steel on the surface of the block. The
thickness of this layer can be measured at points
throughout its area to map variations in the solidification
rate and therefore the effective rate of heat transfer at
the various locations. it is thus possible to produce an
overall solidification rate as well as total heat flux
measurements. It is also possible to examine the
microstructure of the strip surface to correlate changes in
the solidification microstructure with the changes in
observed solidification rates and heat transfer values.
The experimental rig illustrated in Figures 1 and
2 comprises an induction furnace 1 containing a melt of
molten metal 2 in an inert atmosphere which may for example
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be provided by argon or nitrogen gas. An immersion paddle
denoted generally as 3 is mounted on a slider 4 which can
be advanced into the melt 2 at a chosen speed and
subsequently retracted by the operation of computer
controlled motors S.
immersion paddle 3 comprises a steel body 6 which
contains a substrate 7 in the form of a chrome plated
copper block measuring 40mm x 40mm. It is instrumented
with thermo-couples to monitor the temperature rise in the
substrate which provides a measure of the heat flux.
In the ensuing description it will be necessary
to refer to a quantitative measure of the smoothness of
casting surfaces. One specific measure used in our
experimental work and helpful in defining the scope of the
present invention is the standard measure known as the
Arithmetic Mean Roughness Value which is generally
indicated by the symbol Ra. This value is defined as the
arithmetical average value of all absolute distances of the
roughness profile from the centre line of the profile
within the measuring length lm. The centre line of the
profile is the line about which roughness is measured and
is a line parallel to the general direction of the profile
within the limits of the roughness-width cut-off such that
sums of the areas contained between it and those parts of
the profile which lie on either side of it are equal. The
Arithmetic Mean Roughness Value may be defined as
x j lm
Ra = 1/1, I y I dx
x = 0
Tests carried out on the experimental rig
illustrated in Figures 1 and 2 have demonstrated that the
sensitivity to chatter and crocodile-skin defects
experienced when casting onto a casting surface textured by
a regular pattern of ridges can be avoided by employing a
casting surface textured by a random pattern of discrete
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projections with pointed peaks. The random pattern texture
can be achieved by grit blasting and will generally result
in an Arithmetic Mean Roughness Value of the order of 5 to
Ra but, as explained below, the controlling parameters
5 are the surface density of the peak projections and the
minimum depth of the projections rather than the roughness
value.
The testing has further demonstrated that the
sensitivity of ridged textures to crocodile-skin and
10 chatter defects is due to the extended surfaces along the
ridges along which oxides can build up and melt. The
melted oxide flows along the ridges to produce continuous
films which dramatically increase heat transfer over
substantial areas along the ridges. This increases the
initial or peak heat flux values experienced on initial
solidification and result in a subsequent dramatic
reduction in heat flux on solidification of the oxides
which leads to crocodile-skin defects. With a casting
surface having a texture formed by a random pattern of
sharp peaked projections the oxides can only spread on the
individual peaks rather than along extended areas as in the
ridged texture. Accordingly, the melted oxides cannot
spread over an extended area to dramatically increase the
initial heat flux. This surface is therefore much less
sensitive to crocodile-skin defects and it has been also
shown that it does not need to be cleaned so thoroughly as
the ridged texture to avoid such defects.
The tests have also demonstrated that the random
pattern texture is much less prone to chatter defects and
permits casting of low residual steels with low sulphur
content at extremely high casting speeds of the order of 60
metres per minute. Because the initial heat flux on
solidification is reduced as compared with the ridged
texture low speed chatter defects do not occur. At high
speed casting, although slippage between the melt and the
casting surface will occur, this does not result in
cracking. It is believed that this is for two reasons.
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Firstly because the initial heat transfer rate is
relatively low (of the order of 15 megawatts/m' as compared
with 25 megawatts/m' for a ridged texture), the
intermittent loss of contact due to slippage does not
result in such large local heat transfer variations in the
areas of slippage. Moreover, the randomness of the pattern
of the texture pattern results in a microstructure which is
very resistant to crack propagation.
Figure 3 plots heat flux values obtained during
solidification of steel samples on two substrates, the
first having a texture formed by machined ridges having a
pitch of L80 microns and a depth of 60 microns and the
second substrate being grit blasted to produce a random
pattern of sharply peaked projections having a surface
density of the order of 20 peaks per mm'and an average
texture depth of about 30 microns, the substrate exhibiting
an Arithmetic Mean Roughness Value of 7 Ra. it will seen
that the grit blasted texture produced a much more even
heat flux throughout the period of solidification. Most
importantly it did not produce the high peak of initial
heat flux followed by a sharp decline as generated by the
ridged texture which, as explained above, is a primary
cause of crocodile-skin defects. The grit blasted surface
or substrate produced lower initial heat flux values
followed by a much more gradual decline to values which
remained higher than those obtained from the ridged
substrate as solidification progressed.
Figure 4 plots maximum heat flux measurements
obtained on successive dip tests using a ridged substrate
having a pitch of 180 microns and a ridge depth of 60
microns and a grit blasted substrate. The tests proceeded
with solidification from four steel melts of differing melt
chemistries. The first three melts were low residual
steels of differing copper content and the fourth melt was
a high residual steel melt. In the case of the ridged
texture the substrate was cleaned by wire brushing for the
tests indicated by the letters WB but no brushing was
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carried out prior to some of the tests as indicated by the
letters NO. No brushing was carried out prior to any of
the successive tests using the grit blasted substrate. It
will be seen that the grit blasted substrate produced
consistently lower maximum heat flux values than the ridged
substrate for all steel chemistries and without any
brushing. The textured substrate produced consistently
higher heat flux values and dramatically higher values when
brushing was stopped for a period, indicating a much higher
sensitivity to oxide build-up on the casting surface.
The shells solidified in the dip tests to which
Figure 4 refers were examined and crocodile-skin defects
measured. The results of these measurements are plotted in
Figure 5. it will be seen that the shells deposited on the
ridged substrate exhibited substantial crocodile defects
whereas the shells deposited on the grit blasted substrate
showed no crocodile defects at all. The shells were also
measured for overall thickness at locations throughout
their total area to derive measurements of standard
deviation of thickness which are set out in Figure 6. It
will be seen that the ridged texture produced much wider
fluctuations in standard deviation of thickness than the
shells solidified onto the grit blasted substrate.
Figure 7 is a photomicrograph of the surface of a
shell solidified onto a ridged texture of 180 microns pitch
and 20 micron depth from a steel melt containing by weight
0.05% carbon, 0.6% manganese, 0.3% silicon and less than
0.01% sulphur. The shell was deposited from a melt at
1580 C at an effective strip casting speed of 30m/min. The
strip exhibits a low speed chatter defect in the form of
clearly visible transverse cracking. This cracking was
produced during initial solidification and it will be seen
that.there is no change in the surface microstructure above
and below the defect. Figure 8 is a longitudinal section
through the same strip as seen in Figure 7. The transverse
surface cracking can be clearly seen and it will also be
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seen that there is thinning of the strip in the region of
the defect.
Figures 9 and 10 are photomicrographs showing the
surface structure and a longitudinal section through a
shell deposited on the same ridged substrate and from the
same steel melt as the shell as Figures 7 and 8 but at a
much higher effective casting speed of 60m/min. The strip
exhibits a high speed chatter defect in the form of a
transverse zone in which there is substantial thinning of
the strip and a marked difference in microstructure above
and below the defect, although there is no clearly visible
surface cracking in the section,of Figure 10.
Figures 11, 12, 13 and 14 are photomicrographs
showing surface nucleation of shells solidified onto four
different substrates having textures provided respectively
by regular ridges of 180 micron pitch by 20 micron depth
(Figure 11); regular ridges of 180 micron pitch by 60
micron depth (Figure 12); regular pyramid projections of
160 micron spacing and 20 micron height (Figure 13) and a
grit blasted substrate having a Arithmetic Mean Roughness
Value of 10 Ra (Figure 14). Figures 11 and 12 show
extensive nucleation band areas corresponding to the
texture ridges over which liquid oxides spread during
initial solidification. Figures 13 and 14 exhibit smaller
nucleation areas demonstrating a smaller spread of oxides.
Figure 15 plots respective oxide coverage
measurements derived by image analysis of the images
advanced in Figures 11 to 14 and provides a measurement of
the radically reduced oxide coverage resulting from a
pattern of discrete projections. This figure shows that the
oxide coverage for the grit blasted substrate was much the
same as for a regular grid pattern of pyramid projections
of 20 micron height and 160 micron spacing.
Figures 16 and 17 are photomicrographs showing
transverse sections through shells deposited at a casting
speed of 60m/min from a typical M06 steel melt (with
residuals by weight of 0.007% sulphur, 0.44% Cu, 0.009% Cr,
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0.003% Mo, 0.02% Ni, 0.003% Sn) onto a grit blasted copper
substrate with a chromium protective coating (Figure 16)
and onto a ridged substrate of 160 micron pitch and 60
micron depth cut into a chrome plated substrate (Figure
17). It will be seen that the ridged substrate produces a
very coarse dendrite structure as solidification proceeds,
this being exhibited by the coarse dendrites on the side of
the shell remote from the chilled substrate. The grit
blast substrate produces a much more homogenous
microstructure which is fine throughout the thickness of
the sample.
Examination of the microstructure produced by
ridged and grit blasted substrates shows that the ridged
substrates tend to produce a pattern of dendritic growth in
which dendrites fan out from nucleation sites along the
ridges. Examination of shells produced with the grit
blasted substrates has revealed a remarkably homogenous
microstructure which is much superior to the more ordered
structures resulting from regular patterned textures.
The randomness of the texture is very important
to achieving a microstructure which is homogenous and
resistant to crack propagation. The grit blasted texture
also results in a dramatic reduction in sensitivity to
crocodile-skin and chatter defects and enables high speed
casting of low residual steels without sulphur addition.
In order to achieve these results it is important that the
contact between the steel melt and the casting surface be
confined to a random pattern of discrete peaks projecting
into the melt. This requires that the discrete projections
should have a peaked formation and not have extended top
surface areas, and that the surface density and the height
of the projections be such that the melt can be supported
by the peaks without flowing into the depressed areas
between them. Our experimental results and calculations
indicate that in order to achieve this result the
projections must have an average height of at least 10
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microns and that the surface density of the peaks must be
between 10 and 100 peaks per mm'.
An appropriate random texture can be imparted to
a metal substrate by grit blasting with hard particulate
materials such as alumina, silica, or silicon carbide
having a particle size of the order of 0.7 to 1.4mm. For
example, a copper roll surface may be grit blasted in this
way to impose an appropriate texture and the textured
surface protected with a thin chrome coating of the order
of 50 microns thickness. Alternatively it would be
possible to apply a textured surface directly to a nickel
substrate with no additional protective coating.
It is also possible to achieve an appropriate
random texture by forming a coating by chemical deposition
or electrodeposition. In this case the coating material
may be chosen so as to contribute to high thermal
conductivity and increased heat flux during solidification.
it may also be chosen such that the oxidation products in
the steel exhibit poor wettability on the coating material,
with the steel melt itself having a greater affinity for
the coating material and therefore wetting the coating in
preference to the oxides. We have determined that two
suitable materials are the alloy of nickel, chromium and
molybdenum available commercially under the trade name
"HASTALLOY C" and the alloy of nickel, molybdenum and
cobalt available commercially under the trade name "T800".
Figure 18 plots maximum heat flux measurements
obtained on successive dip tests using a ridged chromium
substrate and in similar tests using a randomly textured
substrate of "T800" alloy material. In the tests using a
ridged substrate the heat flux values increased to high
values as the oxides build up. The oxides were then
brushed away after dip No 20 resulting in a dramatic fall
in heat flux values followed by an increase due to oxide
build up through dips Nos 26 to 32, after which the oxides
were brushed away and the cycle repeated. In the tests on
the "T800" substrate, the substrate was not cleaned and any
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oxide deposits were simply allowed to build up throughout
the complete cycle of tests.
it will seen that heat flux values obtained with
the ridged chromium substrate are higher than with the
"T800" substrate but exhibit the typical variations
associated with melting and resolidification as the oxides
build up which variations cause the crocodile-skin defects
in cast strip. The heat flux measurements obtained with
the "T800" substrate are lower than those obtained with the
ridged chrome surface but they are remarkably even
indicating that oxide build up does not create any heat
flux disturbances and will therefore,not be a factor during
casting. The "T800" substrate in these tests had an Ra
value of 6 microns.
it has also been shown that shells deposited on
randomly textured "T800" substrates are of much more even
thickness than those deposited on chrome substrates.
Measurement of standard deviation of thickness of shells
deposited on "T800" substrates have consistently been at
least 50% lower than equivalent measurements on shells
deposited on ridged chrome substrates, indicating the
production of shells of remarkably even thickness not
exhibiting any distortions of the kind which produce
crocodile-skin deformation. These results are confirmed by
microscopic examination of the test shells. Figure 19 is a
photomicrograph of the cross-section of a typical steel
shell solidified onto a ridged chromium substrate whereas
Figure 20 shows a photomicrograph of a shell as deposited
on a"T800 substrate in the same test. It will be seen
that the latter shell is of much more uniform cross-section
and also is of more uniform microstructure throughout its
thickness.
Results similar to those obtained with the "T800"
substrate have also been achieved with a randomly textured
substrate of "HASTALLOY C". Figure 21 is a photomicrograph
of a shell solidified onto such a substrate. This shell is
not quite as uniform or as thick as the shell deposited on
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the "T800" substrate as illustrated in Figure 20. This is
because the respective M06 steel exhibits slightly lower
wettability on the "HASTALLOY C" substrate than on the
"T800" substrate and so solidification does not proceed so
rapidly. In both cases, however, the shell is thicker and
more even than corresponding shells obtained with ridged
chromium surfaces and the testing has shown that the
solidification is not affected by oxide build up so that
cleaning of the casting surfaces will not be a critical
factor.
Figures 22 to 26 illustrate a twin roll
continuous strip caster which may be operated in accordance
with the present invention. This caster comprises a main
machine frame 11 which stands up from the factory floor 12.
Frame 11 supports a casting roll carriage 13 which is
horizontally movable between an assembly station 14 and a
casting station 15. Carriage 13 carries a pair of parallel
casting rolls 16 to which molten metal is supplied during a
casting operation from a ladle 17 via a distributor 18 and
delivery nozzle 19 to create a casting pool 30. Casting
rolls 16 are water cooled so that shells solidify on the
moving roll surfaces 16A and are brought together at the
nip between them to produce a solidified strip product 20
at the roll outlet. This product is fed to a standard
coiler 21 and may subsequently be transferred to a second
coiler 22. A receptacle 23 is mounted on the machine frame
adjacent the casting station and molten metal can be
diverted into this receptacle via an overflow spout 24 on
the distributor or by withdrawal of an emergency plug 25 at
one side of the distributor if there is a severe
malformation of product or other severe malfunction during
a casting operation.
Roll carriage 13 comprises a carriage frame 31
mounted by wheels 32 on rails 33 extending along part of
the main machine frame 11 whereby roll carriage 13 as a
whole is mounted for movement along the rails 33. Carriage
frame 31 carries a pair of roll cradles 34 in which the
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rolls 16 are rotatably mounted. Roll cradles 34 are
mounted on the carriage frame 31 by interengaging
complementary slide members 35, 36 to allow the cradles to
be moved on the carriage under the influence of hydraulic
cylinder units 37, 38 to adjust the nip between the casting
rolls 16 and to enable the rolls to be rapidly moved apart
for a short time interval when it is required to form a
transverse line of weakness across the strip as will be
explained in more detail below. The carriage is movable as
a whole along the rails 33 by actuation of a double acting
hydraulic piston and cylinder unit 39, connected between a
drive bracket 40 on the roll carriage and the main machine _
frame so as to be actuable to move the roll carriage
between the assembly station 14 and casting station 15 and
vice versa.
Casting rolls 16 are contra rotated through drive
shafts 41 from an electric motor and transmission mounted
on carriage frame 31. Rolls 16 have copper peripheral
walls formed with a series of longitudinally extending and
circumferentially spaced water cooling passages supplied
with cooling water through the roll ends from water supply
ducts in the roll drive shafts 41 which are connected to
water supply hoses 42 through rotary glands 43. The roll
may typically be about 500 mm diameter and up to 2000 mm
long in order to produce 2000 mm wide strip product.
Ladle 17 is of entirely conventional construction
and is supported via a yoke 45 on an overhead crane whence
it can be brought into position from a hot metal receiving
station. The ladle is fitted with a stopper rod 46
actuable by a servo cylinder to allow molten metal to flow
from the ladle through an outlet nozzle 47 and refractory
shroud 48 into distributor 18.
Distributor 18 is formed as a wide dish made of a
refractory material such as magnesium oxide (MgO). One
side of the distributor receives molten metal from the
ladle and is provided with the aforesaid overflow 24 and
emergency plug 25. The other side of the distributor is
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provided with a series of longitudinally spaced metal
outlet openings 52. The lower part of the distributor
carries mounting brackets 53 for mounting the distributor
onto the roll carriage frame 31 and provided with apertures
to receive indexing pegs 54 on the carriage frame so as to
accurately locate the distributor.
Delivery nozzle 19 is formed as an elongate body
made of a refractory material such as alumina graphite.
its lower part is tapered so as to converge inwardly and
downwardly so that it can project into the nip between
casting rolls 16. It is provided with a mounting bracket
60 whereby to support it on the roll carriage frame and its
upper part is formed with outwardly projecting side flanges
55 which locate on the mounting bracket.
Nozzle 19 may have a series of horizontally
spaced generally vertically extending flow passages to
produce a suitably low velocity discharge of metal
throughout the width of the rolls and to deliver the molten
metal into the nip between the rolls without direct
impingement on the roll surfaces at which initial
solidification occurs. Alternatively, the aozzle may have
a single continuous slot outlet to deliver a low velocity
curtain of molten metal directly into the nip between the
rolls and/or it may be immersed in the molten metal pool.
The pool is confined at the ends of the rolls by
a pair of side closure plates 56 which are held against
stepped ends 57 of the rolls when the roll carriage is at
the casting station. Side closure plates 56 are made of a
strong refractory material, for example boron nitride, and
have scalloped side edges 81 to match the curvature of the
stepped ends 57 of the rolls. The side plates can be
mounted in plate holders 82 which are movable at the
casting station by actuation of a pair of hydraulic
cylinder units 83 to bring the side plates into engagement
with the stepped ends of the casting rolls to form end
closures for the molten pool of metal formed on the casting
rolls during a casting operation.
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During a casting operation the ladle stopper rod
46 is actuated to allow molten metal to pour from the ladle
to the distributor through the metal delivery nozzle whence
it flows to the casting rolls. The clean head end of the
strip product 20 is guided by actuation of an apron table
96 to the jaws of the coiler 21. Apron table 96 hangs from
pivot mountings 97 on the main frame and can be swung
toward the coiler by actuation of an hydraulic cylinder
unit 98 after the clean head end has been formed. Table 96
may operate against an upper strip guide flap 99 actuated
by a piston and a cylinder unit 101 and the strip product
may be confined between a pair of vertical side rollers
102. After the head end has been guided in to the jaws of
the coiler, the coiler is rotated to coil the strip product
15 20 and the apron table is allowed to swing back to its
inoperative position where it simply hangs from the machine
frame clear of the product which is taken directly onto the
coiler 21. The resulting strip product 20 may be
subsequently transferred to coiler 22 to produce a final
20 coil for transport away from the caster.
Full particulars of a twin roll caster of the
kind illustrated in Figures 12 to 16 are more fully
described in our United States Patents 5,184,668 and
5,277,243 and International Patent Application
PCT/AU93/00593.
In accordance with the present invention the
copper peripheral walls of rolls 16 may be grit blasted to
have a random texture of discrete peaked projections of the
required depth and surface density and this texture may be
protected by a thin chrome plating. Alternatively, the
copper walls of the rolls could be coated with nickel and
the nickel coating grit blasted to achieve the required
random surface texture. In another alternative an alloy
such as HASTALLOY C or T800 alloy material may be
electrodeposited on the copper walls of the casting rolls.
Figure 27 represents a typical surface texture
produced according to the invention.
