Note: Descriptions are shown in the official language in which they were submitted.
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CASTING STEEL STRIP
BACKGROUND OF THE INVENTION
This invention relates to continuous casting of steel strip in a strip
caster, particularly a twin roll caster.
In a twin roll caster, molten metal is introduced between a pair of
counter-rotated horizontal casting rolls which are internally 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. The
term
"nip" is used herein to refer to the general region at which the casting rolls
are closest
together. The molten metal may be poured from a ladle into a smaller vessel
from
which molten metal flows through a metal delivery nozzle located above the
nip,
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 casting rolls to dam the two ends of the casting pool
against
outflow, although alternative means such as electromagnetic barriers have also
been
proposed.
When casting steel strip in a twin roll caster, the strip leaves the nip at
very high temperatures of the order of 1400°C and can suffer very rapid
scaling due to
oxidation at such high temperatures. Such scaling may result in a significant
loss of
steel product. For example, 3% of a 1.55 mm thicl~ strip (typical scale
thiclffless 23
microns) can be lost from oxidation as the strip cools. Moreover, such scaling
results
in the need to descale the strip prior to further processing by picl~ling to
avoid surface
quality problems such as rolled-in scale, and causes significant extra
complexity, cost
and environmental concerns. For example, the hot strip material may be passed
directly to a rolling mill in line with the strip caster and thence to a run
out table on
which it is cooled to coiling temperature before it is coiled. However,
scaling of the
hot strip material emerging from the strip caster progresses so rapidly that
it may be
necessary to install descaling equipment to descale the material immediately
before it
enters the in line rolling mill. Even in cases when the strip is cooled to
coiling
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temperature without hot rolling, it will generally be necessary to descale the
strip
either before it is coiled or in a later processing step.
To deal with the problem of rapid scaling of strip emerging from a
twin roll strip caster, it has been proposed to enclose the newly formed strip
within a
sealed enclosure, or a succession of such sealed enclosures, in which a
controlled
atmosphere or atmospheres is maintained in order to inhibit oxidation of the
cast strip.
The controlled atmosphere can be produced by charging the sealed enclosure or
successive enclosures with non-oxidizing gases. Such gases can be inert gases
such
as nitrogen or argon or exhaust gases from fuel burners.
United States Patent 5,762,126 discloses an alternative relatively cheap
and energy efficient way of limiting exposure of the high temperature strip to
oxygen.
The strip is caused to pass through an enclosure where oxygen is extracted
from the
atmosphere by the formation of scale. The enclosure is substantially sealed so
as to
control the ingress of oxygen into the enclosure atmosphere and control the
extent of
scale formation. In this method of operation, it is possible to rapidly reach
a steady
state condition in which scale formation is brought to low levels without the
need to
deliver a non-oxidizing or reducing gas into the enclosure.
U. S. Patent 5,816,311 discloses a way of controlling the extent of
scale formation by providing downstream a chamber where groups of nozzles
spray a
quenching medium onto the strip. The quenching medium was a methyl alcohol,
water, or mixture of methyl alcohol and another quenching medium liquid at
room
temperature. It was expected that water spraying in a nitrogen atmosphere
would lead
to unacceptable levels of oxidation as water contains dissolved oxygen and the
breakdown of water (steam) to oxygen and hydrogen would provide further
oxidation;
however, it was surprisingly and unexpectedly found as described in the '311
patent
that it was possible to Iimit the thickness of oxide on the strip to no more
than 0.5
microns. Additionally, it was surprisingly found that these levels of oxide
were
tolerable for cold rolling without pickling and then metal coating of the
strip. This
quenching of the steel strip was found, however, to result in uneven cooling
of the
steel strip introducing stresses and other defects in the strip.
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International Patent Application PCT/AU00/01478, on which parent
application serial no. 10/121,567 is based, discloses how a substantially non-
oxidizing
atmosphere can be cheaply and effectively produced within a downstream
enclosure,
through which the hot cast steel strip passes, by introducing water in a fine
mist spray
to generate steam within the enclosure. The steam generation increases the
gaseous
volume within the enclosure so as to produce a positive pressure in the
enclosure
which substantially prevents the ingress of atmospheric air. It can also
produce an
increased level of hydrogen gas within the enclosure to significantly reduce
the
oxygen level in the enclosure and reduce the rate of oxidation of the strip.
In the
disclosure of International Application PCT/AU00/01478, it was considered
necessary to isolate the enclosure in which steam is generated from the
enclosure to
which the casting rolls are exposed so as to avoid the risl~ of exposure of
the casting
pool to water or steam. We have now found, surprisingly, that by the
introduction of
water in a fine mist spray, the conversion of the water to steam and the
production of
hydrogen gas is so effective that it is possible to generate increased levels
of hydrogen
gas in an enclosure to which the casting rolls are exposed, either by allowing
communication with gas flow between that enclosure and the downstream
enclosure
into which the fine mist spray is introduced and/or by direct introduction of
a fine
mist spray into the enclosure to which the casting rolls are exposed. By
direct
introduction of the fine mist spray into the enclosure to which the casting
rolls are
exposed, it is also possible to omit the separate downstream enclosure.
SUMMARY OF THE INVENTION
The present invention provides a method of continuously casting steel
comprising:
(a) forming a casting pool of molten steel on chilled casting
surfaces of at least one casting roll;
(b) moving the chilled casting surfaces to produce a solidif ed steel
strip moving away from the casting pool;
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(c) guiding the solidified strip through first enclosure adjacent the
casting roll surfaces, and optionally thereafter second enclosure, as it moves
away
from the casting pool;
(d) sealing the first enclosure and, if present, the second enclosures
against ingress of atmospheric air separately or with an intercommunication
between
said enclosures permitting gas flow from the second enclosure to the first
enclosure;
and
(e) introducing water into at least one of said enclosures in form of
a fine mist to produce an increased level of hydrogen gas within the first
enclosure
while tending to avoid liquid water contact with the steel strip and the
casting surfaces
of the casting roll or rolls.
A "fine mist" herein is a water spray where, in general, the water
evaporates and is converted to steam before reaching the surface of the strip.
There
may still be the odd water droplets that reach the strip, but the intention is
to avoid
contact of the liquid water with the strip. Too much liquid water on the strip
can
cause uneven cooling of the strip. The precise droplet size and range of sizes
of the
water in the fine mist will be dependent on the temperature of the strip in
the
enclosure where the fine mist is sprayed, and the location of the spray
nozzles within
the enclosure and their distance from the strip. Notably, the location in
relation to the
droplet size and range is sensitive where the fine mist is sprayed in the
first enclosure
to avoid contact of the liquid water with the casting surfaces of the casting
roll or
rolls. The droplet size and range of the fine mist should be selected for the
particular
embodiment according to the geometry to provide flexibility in operation, and
for the
generation hydrogen gas while avoiding contact of the liquid water with the
strip and
the casting surfaces.
The step of introducing water in the form of fine mist to generate
steam also produces a positive pressure in the enclosure where it is
introduced,
namely, either the first enclosure and the second enclosure. However, if the
fine
water mist is sprayed into the second enclosure, and not into the first
enclosure, the
first and second enclosures are directly interconnected or spaced from each
other by
one or more chambers, with a passageway therebetween, through which gas can
flow
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from the second enclosure to the first enclosure. This passageway may be the
same or
a different passageway from the passageway through which the cast strip moves
from
the first enclosure to the second enclosure. In any event, the sealing of the
first
enclosure and/or the second enclosures need not be complete, but only
sufficient to
provide a positive atmosphere within the first enclosure, and if present the
second
enclosure, with a reduced level of oxygen and an increased level of hydrogen
gas in
relation to the external atmosphere.
In an embodiment where the fine mist is sprayed into the second
enclosure to produce hydrogen gas therein and flows into the first enclosure
through a
connecting passageway, water may in addition be introduced into the first
enclosure
in form of a fine mist to generate steam therein and to increase the level of
hydrogen
gas therein while tending to avoid liquid water contact with the steel strip
and the
chilled casting surfaces of the casting roll or rolls.
In an alternative embodiment, the first enclosure and the second
enclosure may be separately sealed against ingress of atmospheric air, and
water may
be introduced into the first enclosure in form of a fine mist to produce an
increased
level of hydrogen gas therein while tending to avoid liquid water contact with
the
steel strip and the casting surfaces of the casting roll or rolls. Such water
introduced
as a fine mist also generates steam within the first enclosure to produce a
positive
pressure therein and avoid egress of atmospheric air into the first enclosure.
In this
embodiment, water may additionally be introduced into the second enclosure in
form
of a fine mist to produce an increased level of hydrogen gas andlor to
generate steam
producing a positive pressure therein, while tending to avoid liquid water
contact with
the steel strip.
In any embodiment, the cast strip may be guided through the first
enclosure and into the second enclosure on a transit path through said
connecting
passageway. Alternatively, the strip may be guided from the first enclosure
into the
second enclosure along a transit path through a second passageway and/or
through a
connecting chamber or chambers separated from said first passageway through
which
gas flows between the enclosures.
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The invention further provides apparatus for casting steel strip
comprising:
(a) a pair of generally horizontal-positioned casting rolls forming a
nip between them;
(b) metal delivery system to deliver molten steel above the nip
between the casting rolls to form a casting pool of molten steel supported on
the rolls;
(c) a cooling system to chill the casting rolls;
(d) a drive system to counter-rotate the casting rolls in opposite
directions;
(e) said casting rolls having chilled casting surfaces to produce a
cast strip delivered downwardly from the nip;
(f) a first enclosure adjacent the casting rolls through which the
cast strip passes on a transit path away from the nip;
(g) optionally a second enclosure through which the cast strip
passes after the strip has passed through the first enclosure;
(h) enclosure seals sealing the first enclosure and, if present,
second enclosures separately or with an intercommunication between the first
and
second enclosures permitting flow of gas between said enclosures; and
(i) one or more water sprays operable to spray water in form of a
fine mist into at least one of said enclosures to produce an increased level
of hydrogen
gas witlun the first enclosure while tending to avoid liquid water contact
with the steel
strip and the casting surfaces of the casting rolls.
The fine mist water spray fixrther may generate steam within one or
both of the first and second enclosures.
The apparatus for casting steel strip also may have strip guides to
guide the strip delivered downwardly from the nip through a transit path in
the first
enclosure and through a transit path in the second enclosure.
The first enclosure and the second enclosure may be interconnected by
a connecting passageway capable of permitting flow of gas therebetween, and
the
water sprays may comprise one or more water spray nozzles mounted in the
second
enclosure operable to spray a fine mist into that enclosure adjacent the steel
strip
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while tending to avoid liquid water from contacting the steel strip, to
generate steam
and increase the level of hydrogen gas in both enclosures.
In the described method, the cast steel strip may be delivered to a hot
rolling mill in which it is hot rolled as it is produced. The strip may exit
the second
enclosure before entering the rolling mill, and in this embodiment, may
comprise a
pair of mill rolls between which the strip passes to exit the second
enclosure.
However, the strip may remain within the second enclosure as it enters into
the rolling
mill, or the rolling mill may be positioned between the first and second
enclosures.
This positioning of the rolling mill may be achieved by sealing the second
enclosure
against mill rolls or a housing of the rolling mill.
DESCRIPTION OF THE DRAWINGS
In order to more fully explain, particular embodiments will be
described in detail with reference to the accompanying drawings in which:
Fig. 1 is a vertical cross-section through a steel strip casting and rolling
installation constructed and operated in accordance with the present
invention;
Fig. 2 illustrates essential components of a twin roll caster incorporated
in the installation and including a first hot strip enclosure;
Fig. 3 is a vertical cross-section through the twin roll caster;
Fig. 4 is a cross-section through end parts of the caster;
Fig. S is a cross-section on the line 5-5 in Fig. 4;
Fig. 6 is a view on the line 6-6 in Fig. 4;
Fig. 7 illustrates a section of the installation downstream from the
caster which includes a second strip enclosure and an in-line rolling mill;
and
Fig. 8 illustrates a modified embodiment which incorporates additional
water mist sprays.
DETAILED DESCRIPTION
The casting and rolling installation illustrated in Figs. 1 to 7 comprises
a twin roll caster denoted generally as 11 that produces a cast steel strip 12
which
passes in a transit path 10 across a guide table 13 to a pinch roll stand 14.
After
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exiting the pinch roll stand 14, the strip passes to a hot rolling mill 16 in
which it is
hot rolled to reduce its thickness. The rolled strip exits the rolling mill
and passes to a
run out table 17 on which it may be force cooled by a fine mist from water j
ets 18 and
thence to a coiler 19.
Twin roll caster 11 comprises a main machine frame 21 which
supports a pair of parallel casting rolls 22 having casting surfaces 22A.
Molten metal
is supplied during a casting operation from a ladle 23 through a refractory
ladle outlet
shroud 24 to a tundish 25 and thence through a metal delivery nozzle 26 above
the nip
27 between the casting rolls 22. Molten metal thus delivered forms a casting
pool 30
supported on the casting surface 22A of the casting rolls 22. This casting
pool 30 is
confined at the ends of the rolls by a pair of side closure dams or plates 28
which are
applied to stepped ends of the rolls by a pair of thrusters 31 comprising
hydraulic
cylinder units 32 connected to side plate holders 28A. The upper surface of
casting
pool 30 (generally referred to as the meniscus level) may rise above the lower
end of
the delivery nozzle 26 so that the lower end of the delivery nozzle is
immersed within
this casting pool.
Casting rolls 22 are internally water cooled so that metal shells solidify
on the moving casting surfaces of the casting rolls and are brought together
at the nip
27 between the rolls to produce the cast strip 12, which is delivered
downwardly from
the nip between the rolls.
At the start of a casting operation, a short length of imperfect strip is
produced as the casting conditions stabilize. After continuous casting is
established,
the casting rolls are moved apart slightly and then brought together again to
cause this
leading end of the strip to break away in the manner described in Australian
Patent
Application 27036/92 so as to form a clean head end of the following cast
strip. The
imperfect material drops into a scrap box 33 located beneath caster 11, and at
this
time, a swinging apron 34, which normally hangs downwardly from a pivot to one
side of the caster outlet, is swung across the caster outlet to guide the
clean end of the
cast strip onto the guide table 13 from where it is fed to the pinch roll
stand 14.
Apron 34 is then retracted back to its hanging position to allow the strip 12
to hang in
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a loop beneath the caster before it passes to the guide table 13 where it
engages a
succession of guide rollers 36.
The twin roll caster may be of the bind which is illustrated and
described in some detail in granted Australian Patents 631728 and 637548 and
United
States Patents 5,184,668 and 5,277,243 and reference may be made to those
patents
for appropriate constructional details which form no part of the present
invention.
Between the casting rolls and pinch roll stand 14, the newly formed
steel strip is enclosed within a first enclosure denoted generally as 37
defining a
sealed space or atmosphere 38 adjacent the casting surfaces 22A of casting
rolls 22.
First enclosure 37 is formed by a number of separate wall sections which fit
together
at various seal connections to form a continuous enclosure wall. The enclosure
37 is
comprised of a wall section 41 which is formed at the twin roll caster to
enclose the
casting rolls, and an enclosure wall 42, which may extend downwardly beneath
wall
section 41, to engage the upper edges of scrap box 33 when the scrap box is in
its
operative position. The scrap box and enclosure wall 42 may be connected by a
seal
43 formed by a ceramic fiber rope fitted into a groove in the upper edge of
the scrap
box and engaging flat sealing gasket 44 fitted to the Lower end of wall
section 42.
Scrap box 33 may be mounted on a carriage 45 fitted with wheels 46 which run
on
rails 47 whereby the scrap box can be moved after a casting operation to a
scrap
discharge position. Screw jack units 40 are operable to lift the scrap box
from
carriage 45 when it is in the operative position so that it is pushed against
the
enclosure wall 42 and compresses the seal 43. After a casting operation the
jack units
40 are released to lower the scrap box onto carriage 45 to enable it to be
moved to the
scrap discharge position.
First enclosure 37 further comprises a wall section 48 disposed about
the guide table 13 and connected to the frame 49 of pinch roll stand 14 which
includes
a pair of pinch rolls 50 against which enclosure 37 is sealed by sliding seals
60.
Accordingly, the strip exits the first enclosure 37 by passing between the
pair of pinch
rolls 50 and passes into a second enclosure denoted generally as 61 through
which the
strip passes to the hot rolling mill 16., Most of the first enclosure wall
sections may be
lined with fire brick and the scrap box 33 may be lined either with fire brick
or with a
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castable refractory lining. Alternatively, all or parts of the first enclosure
wall
sections may be formed by internally water cooled metal panels. The enclosure
wall
section 41 which surrounds the casting rolls is formed with side plates 51
provided
with notches S2 shaped to snugly receive the side dam plate holders 28A when
the
side dam plates 28 are pressed against the ends of the rolls by the cylinder
units 32.
The interfaces between the side plate holders 28A and the enclosure side wall
sections
51 are sealed by sliding seals 53 to maintain sealing of first enclosure 37.
Seals 53
may be formed of ceramic fiber rope.
The cylinder units 32 extend outwardly through the enclosure section
41 and at these locations first enclosure 37 is sealed by sealing plates 54
fitted to the
cylinder units so as to engage with the enclosure wall section 41 when the
cylinder
units are actuated to press the side plates against the ends of the rolls.
Thrusters 31
also move refractory slides 55 which are moved by the actuation of the
cylinder units
32 to close slots 56 in the top of first enclosure 37 through which the side
plates are
initially inserted into the enclosure and into the holders 28A for application
to the
rolls. The top of first enclosure 37 is closed by the tundish, the side plate
holders 28A
and the slides 55 when the cylinder units are actuated to apply the side dam
plates
against the rolls. In this way the complete enclosure 37 is sealed prior to a
casting
operation to establish the sealed space 38 adjacent the casting surfaces 22A
of casting
rolls 22.
The second enclosure 61 may be separate from the first enclosure 37,
where the strip can be held in a separate atmosphere in second enclosure 61 up
to the
hot rolling mill 16. Rolling mill 16 contains a series of pass line rollers 62
to guide
strip horizontally through second enclosure 61 to the worlc rolls 63 of
rolling mill 16
which are disposed between two larger backing rolls 64. Second enclosure 61 is
sealed at one end against pinch rolls 50 by sliding seals 65, and at the other
end, it is
sealed against the working rolls 63 of rolling mill 16 by sliding seals 66.
The sliding
seals 65 and 66 could be replaced by rotary sealing rolls to run or the strip
in the
vicinity of the pinch rolls and reduction rolls, respectively.
Second enclosure 61 is fitted with a pair of water spray nozzles 67 and
68 that are each operable to spray a fine mist of water droplets adjacent the
surface of
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the steel strip as it passes through the second enclosure, and thereby to
generate steam
within the second enclosure while tending to avoid liquid water contact with
the steel
strip. Spray nozzle 67 is mounted in the roof of enclosure 61 downstream from
the
pinch roll stand 14. Nozzle 68 is located at the other end of enclosure 61 in
advance
of the rolling mill 16. The nozzles 67 and 68 may be standard commercially
available
mist spray nozzles operable with a gas propellant to produce a fine mist of
water. In
the illustrative method of the present invention the gas propellant may be an
inert gas
such as nitrogen. In a typical installation the nozzles will be operated under
nitrogen
at a pressure of around 400 ld'a. The water may be supplied at around 100-500
kPa
pressure, although the pressure of the water is not critical. The nozzles are
set up to
produce a fine mist spray across the width of the strip to generate steam
within the
second enclosure 61.
In operation of the illustrated caster, both of first enclosure 37 and
second enclosure 61 may initially be purged with nitrogen gas prior to
commencement of casting. Prior to casting, the water sprays are activated so
that as
soon as the hot strip passes into second enclosure 61 steam is generated
within that
enclosure so as to produce a positive pressure preventing ingress of
atmospheric air.
The supply of nitrogen may be terminated after commencement of casting.
Initially
the cast strip will take up all of the oxygen from the first enclosure 37 to
form heavy
scale on the strip. However, the sealing of space 38 of first enclosure 37
controls the
ingress of oxygen containing atmosphere below the amount where substantial
amounts of oxygen are taken up by the steel strip. Thus, after an initial
start up period
the oxygen content in the first enclosure 37 will remain depleted and limiting
the
availability of oxygen for oxidizing of the strip. In this way, the formation
of scale on
the cast strip is controlled without the need to maintain a supply of nitrogen
to space
38 of the first enclosure 37.
As previously described, pinch roll 14 is provided with sliding seals
60, 65 to slide on the pinch rolls SO at the division between first and second
enclosures 38 and 61. The pinch rolls and seals are effective to prevent a
back flow
of liquid water from second enclosure 61 but pinch roll stand 14 provides a
gas flow
passageway around the two ends of the pinch rolls 50 by which gas can flow
from the
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second enclosure 61 to the first enclosure 38. It has been found in operation
of the
apparatus that the intercommunication between the two enclosures by this
interconnecting passageway is quite sufficient to permit increased levels of
hydrogen
to flow from the second enclosure 61 into the first enclosure 37. This is
shown by the
following results obtained by operation of a twin roll casting and rolling
installation
as illustrated in the drawings and testing with and without the operation of
the fine
mist water sprays 67 and 68. Gas sampling of the atmosphere within both the
first
enclosure 37 and the second enclosure 61 was carried out at the locations A, B
and C
indicated in Fig. 1 with the following gas analyses reported in Table 1 below.
The
remainder of the gas in the atmospheres analyzed is nitrogen gas (Na).
Table 1
Oxygen Water Hydrogen Carbon Carbon
(O2) (H20) (H2) Monoxide Dioxide
(CO) (CO2)
(vol (vol %) (vol %) (voI %) (vol %)
%)
Casting 0.2 0.11 0.10 0.05 < 0.01
Pool
AFirstEncl 2.5-1.0 2.25-0.6 0.4-0.15 0.2-0.0 0.36-0.06
BFirstEncl 3.0-1.0 2.1-0.3 0.4-0.1 0.13-0.0 0.2-0.0
CSec'dEncl 0.5 1.6-0.7 0.5-0,31 0.08-0.0 0.01-0.0
It will be seen that the levels hydrogen within the first enclosure 37,
although smaller than the levels in the second enclosure 61, are still
increased
substantially by operation of the fine mist water nozzles in the second
enclosure 61.
The increased levels of hydrogen in both the first and second enclosures 37,
61 are
associated with a marked reduction in oxygen content and dramatically reduce
scale
formation. It is further seen that there are elevated humidity levels in both
the first
and second enclosures indicating the presence of steam, and both enclosures
are under
positive pressure by the presence of steam. The increased hydrogen level may
be
explained by catalytic reaction of water molecules in the fine mist under the
high
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temperature conditions surrounding the steel strip within the second enclosure
to form
hydrogen gas. Oxygen gas simultaneously formed from water molecules is taken
up
by oxidizing of the strip during initial passage of the strip through the
second
enclosure, so that a substantial quantity of hydrogen gas is generated.
Subsequent
oxidation of the strip is suppressed by the hydrogen gas and the positive
pressure
within the second enclosure which limits ingress of atmospheric air, but is
sufficient
to maintain the hydrogen content in the second enclosure and to produce a very
thin
layer of scale on the strip which has been found to be desirable on hot
rolling to avoid
sticlcing in the roll bite. It has been found that the very thin layer of
scale produced in
the extremely moist atmosphere in second enclosure 61 serves as a strongly
adherent
lubricant which minimizes roll wear and operational difficulties at the
rolling mill. At
the same time, because the fine mist spray is generated into steam in the
second
enclosure, contact of the steel strip with liquid water tends to be avoided
and the
prospect of uneven cooling of the strip is substantially reduced if not
eliminated.
Fig. 8 illustrates a modification to the casting and rolling installation
by which additional water spray nozzles 71, 72 are arranged to generate a fine
water
mist spray in the first enclosure 37. Apart these additional spray nozzles,
the
installation illustrated in Fig. 8 is the same as previously described.
Accordingly, the
other components have been identified in Fig. 8 by the same reference numerals
as in
Fig. 1. Spray nozzles 71, 72 are similar to the nozzles 67, 68 and may be
operated in
similar fashion and under the same conditions to spray a fine water mist
adjacent the
surface of strip 12 while attending to avoid liquid water contact with the
strip.
Further, spray nozzles 71, 72 are positioned toward the exit end of enclosure
37 to
minimize the possibility of liquid water coming into contact with the casting
surfaces
22A of casting rolls 22. A curtain gate seal may be installed at a location
between the
spray nozzles 71, 72 and the casting rolls as indicated at 73 to further
minimize this
risk.
It is also shown from Fig. 8 that the operation with increased levels of
hydrogen gas in first enclosure 37 can be achieved by the fine mist spray from
nozzles
71, 72 without the operation of nozzles 67, 68 in the second enclosure 61, and
without
the presence of second enclosure 61.
