Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PRODUCTION OF THIN STEEL STRIP
TECHNICAL FIELD
This invention relates to the production of thin
steel strip. It has particular application to the
production of steel strip by continuous casting and inline
hot rolling of the strip to a final gauge or thickness.
Steel strip produced by continuous thin slab
casting or by twin roll strip casting may be subjected to
inline hot rolling to reduce the strip to the final gauge.
In a twin roll caster 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 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.
TRhen casting steel strip in a twin roll caster
the strip leaves the nip at very high temperatures of the
order of 1400°C and if exposed to air, it suffers very
rapid scaling due to oxidation at such high temperatures.
It has therefore previously been proposed to install
descaling equipment to descale the strip immediately before
it enters the inline rolling mill. On the other hand,
there have been proposals to shroud the newly cast strip
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within an enclosure containing a non-oxidising atmosphere
up to the time that it enters the inline rolling mill so as
to reduce scaling and avoid the need to descale prior to
hot rolling. One such proposal is described in United
States Patent 5,762,126 according to which the cast strip
is passed through a sealed enclosure from which oxygen is
extracted by initial oxidation of the strip passing through
it thereafter the oxygen content in the sealed enclosure is
maintained at less than the surrounding atmosphere by
continuing oxidation of the strip passing through it so as
to control the thickness of the scale on the strip emerging
from the enclosure. By this means it a.s possible to allow
the strip to exit the enclosure and be presented to the hot
rolling mill with scale thickness no more than 10 microns.
According to the previous proposal as described
in United States Patent 5,762,126 the strip leaving the hot
rolling mill was subjected to water cooling and then
coiled. With this procedure, the strip may need to be
subsequently descaled or pickled prior to some end uses.
We have now determined that if the hot rolled strip is
subjected to descaling prior to coiling at temperatures in
the range 900°C to 600°C the rate of rescaling will be very
low. The strip can be coiled with very low scale of the
order of only 1 to 2 micron thick and there will be no
further appreciable scale development. The resulting coils
can be sent directly to end users for many applications
such as in the production of drums and any painted strip
products. In other applications, they will allow much
faster pickling than the material not subjected to
descaling immediately prior to coiling.
DISCLOSURE OF THE INVENTION
According to the present invention, there is
provided a method of producing steal strip comprising:
continuously casting a steel strip;
guiding the strip through an enclosure containing
an atmosphere which inhibits oxidation of the strip surface
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and consequent scale formation;
feeding the strip through a rolling mill in which
it is rolled as it is produced;
passing the rolled strip through a descaler so as
to remove scale therefrom as it leaves the rolling mill;
and
passing the rolled and descaled strip to a
coiler.
The strip may be passed out of said enclosure
prior to passing through the rolling mill.
Alternatively, the rolling mill may be disposed
within the enclosure so that the strip passes through the
rolling mill before leaving the enclosure.
Preferably, the strip is continuously cast by
supporting a casting pool of molten steel on a pair of
chilled casting rolls forming a nip between them and the
solidified strip is produced by rotating the rolls in
mutually opposite directions such that the solidified strip
moves downwardly from the nip.
The atmosphere in said enclosure may be formed of
inert or reducing gases or it may be an atmosphere
containing oxygen at a level lower than the atmosphere
surrounding the enclosure.
Preferably, the atmosphere in the enclosure is
formed by sealing the enclosure to restrict ingress of
oxygen containing atmosphere, causing oxidation of the
strip within the enclosure during an initial phase of
casting thereby to extract oxygen from the sealed enclosure
and to cause the enclosure to have an oxygen content less
than the atmosphere surrounding the enclosure, and
thereafter maintaining the oxygen content in the sealed
enclosure at less than that of the surrounding atmosphere
by continuous oxidation of the strip passing through the
sealed enclosure thereby to control the thickness of the
resulting scale on the strip.
Preferably, the rolling mill hot rolls the strip
at a temperature in the range of 750 to 1250°C.
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The invention further provides apparatus for
producing steel strip comprising:
a continuous caster for continuously casting
steel strip;
strip guide means to guide the strip through a
transit path away from the caster;
an oxidation inhibiting enclosure to confine the
strip throughout said transit path to control formation of
scale on the strip;
a rolling mill operable to roll the strip as it
is produced;
a descaler operable to descale the strip as it
leaves the rolling mill; and
a coiler to receive the rolled and descaled
strip.
Preferably, the continuous caster includes:
a pair of generally horizontal casting rolls
forming a nip between them;
metal delivery means to deliver molten steel into
the nip between the casting rolls to form a casting pool of
molten steel supported on the rolls;
means to chill the casting rolls; and means to
rotate the casting rolls in mutually opposite directions
whereby to cast said strip and to deliver a.t downwardly
from the nip.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the invention may be more fully
explained one particular embodiment will be described in
detail with reference to the accompanying drawings in
which:
Figure 1 is a vertical cross-section through a
steel strip casting and rolling installation constructed
and operated in accordance with the present invention;
Figure 2 illustrates essential components of a
twin roll caster incorporated in the installation;
Figure 3 is a vertical cross-section through part
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of the twin roll caster;
Figure 4 is a cross-section through end parts of
the caster.
Figure 5 is a cross-section on the line 5-5 in
Figure 4;
Figure 6 a.s a view on the line 6-6 in Figure 4;
and
Figure 7 is a diagrammatic view of part of a
modified installation constructed and operated in
accordance with the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The illustrated casting and rolling installation
comprises a twin roll caster denoted generally as 11 which
produces a cast steel strip 12 which passes in a transit
path 10 across a guide table 13 to a pinch roll stand 14.
Immediately after exiting the pinch roll stand 14, the
strip passes into a hot rolling mill 15 comprising roll
stands 16 in which it is hot rolled to reduce its
thickness. The thus rolled strip exits the rolling mill
and passes to a run out table 17 on which it may be force
cooled by water jets 18. In accordance with the present
invention the strip is then passed through a descaler 19
before passing between pinch rolls 20A of a pinch roll
stand 20 to a coiler 50.
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 into the nip 27
between the casting rolls 22. Hot metal thus delivered to
the nip 27 forms a pool 30 above the nip and this pool 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
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upper surface of pool 30 (generally referred to as the
"meniscus" level) may rise above the lower end of the
delivery nozzle so that the lower end of the delivery
nozzle is immersed within this pool.
Casting rolls 22 are water cooled so that shells
solidify on the moving roller surfaces and are brought
together at the nip 27 between them to produce the
solidified 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 stabilise. 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 35 to one side of the caster
outlet is swung across the caster outlet to guide the clean
and of the cast strip onto the guide table 13 which feeds
it to the pinch roll stand 14. Apron 34 is than retracted
back to its hanging position to allow the strip 12 to hang
in 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 kind 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.
The installation is manufactured and assembled to
form a single very large scale enclosure denoted generally
as 37 defining a sealed space 38 within which the steel
strip 12 is confined throughout a transit path from the nip
between the casting rolls to the entry nip 39 of the pinch
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roll stand 14.
Enclosure 37 is formed by a number of separate
wall sections which fit together at various seal
connections to form a continuous enclosure wall. These
comprise a wall section 41 which is formed at the twin roll
caster to enclose the casting rolls and a wall section 42
which extends downwardly beneath wall section 41 to engage
the upper edges of scrap box 33 when the scrap box is in
its operative position so that the scrap box becomes part
of the enclosure. The scrap box and enclosure wall section
42 may be connected by a seal 43 formed by a ceramic fibre
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. Cylinder units 40
are operable to lift the scrap box from carriage 45 when it
is in the operative position so that it is pushed upwardly
against the enclosure wall section 42 and compresses the
seal 43. After a casting operation the cylinder units 40
are released to lower the scrap box onto carriage 45 to
enable it to be moved to scrap discharge position.
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 14A against which the enclosure is sealed by
sliding seals 60. Accordingly, the strip exits the
enclosure 38 by passing between the pair of pinch rolls 14A
and it passes immediately into the hot rolling mill 15.
The spacing between pinch rolls 14 and the entry to the
rolling mill should be as small as possible so as to
control the formation of scale prior to entry into the
rolling mill.
Most of the enclosure wall sections may be lined
with fire brick and the scrap box 33 may be lined either
with fire brick or with a castable refractory lining.
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The enclosure wall section 41 which surrounds the
casting rolls is formed with side plates 51 provided with
notches 52 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 the enclosure. Seals 53 may be
formed of ceramic fibre rope.
The cylinder units 32 extend outwardly through
the enclosure wall section 41 and at these locations the
enclosure 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 ands 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 the enclosure through which the side plates are
initially inserted into the enclosure and into the holders
28A for application to the rolls. The top of the enclosure
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 whereby to limit
the supply of oxygen to the strip 12 as it passes from the
casting rolls to the pinch roll stand 14. Initially the
strip will take up all of the oxygen from the enclosure
space 38 to form heavy scale on the strip. However, the
sealing of space 38 controls the ingress of oxygen
containing atmosphere below the amount of oxygen that could
be taken up by the strip. Thus, after an initial start up
period the oxygen content in the enclosure space 38 will
remain depleted so limiting the availability of oxygen for
oxidation of the strip. In this way, the formation of
scale is controlled without the need to continuously feed a
reducing or non-oxidising gas into the enclosure space 38.
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In order to avoid the heavy scaling during the start-up
period, the enclosure space can be purged immediately prior
to the commencement of casting so as to reduce the initial
oxygen level within the enclosure and so reduce the time
for the oxygen level to be stabilised as a result of the
interaction of oxygen from the scaled enclosure due to
oxidation of the strip passing through it. The enclosure
may conveniently be purged with nitrogen gas. It has been
found that reduction of the initial oxygen content to
levels of between 5% to 10% will limit the scaling of the
strip at the exit from the enclosure to about 10 microns to
17 microns even during the initial start-up phase.
In a typical caster installation the temperature
of the strip passing from the caster will be of the order
of 1400°C and the temperature of the strip presented to the
mill may be about 1200°C. The strip may have a width in
the range 0.9 m to 2.0 m and a thickness in the range
0.7 mm to 2.0 mm. The strip speed may be of the order of
1.0 m/s. It has been found that with strip produced under
these conditions it is quite possible to control the
leakage of air into the enclosure space 38 to such a degree
as to limit the growth of scale on the strip to a thickness
of less than 5 microns at the exit from the enclosure space
38, which equates to an average oxygen level of 2% with
that enclosure space. The volume of the enclosure space 38
is not particularly critical since all of the oxygen will
rapidly be taken up by the strip during the initial start
up phase of a casting operation and the subsequent
formation of scale is determined solely by the rate of
leakage of atmosphere into the enclosure space though the
seals. It is preferred to control this leakage rate so
that the thickness of the scale at the mill entry is in the
range 1 micron to 5 microns. Experimental work has shown
that the strip needs some scale on its surface to prevent
welding and sticking during hot rolling. Specifically,
this work suggests that a minimum thickness of the order of
0.5 to 1 micron is necessary to ensure satisfactory
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rolling. An upper limit of about 8 microns and preferably
microns is desirable to avoid "rolled-in scale" defects
in the strip surface after rolling and to ensure that scale
thickness on the final product is no greater than on
5 conventionally hot rolled strip.
After leaving the hot rolling mill the strip
passes to run out table 17 on which it may be forced cooled
by water jets 18. In accordance with the present
invention, it then passes through a descaler 19 before
being coiled on coiler 20. Descaler 19 may be of
conventional construction. It may typically comprise upper
and lower water jet nozzles 61, 62 supplied with water from
a pressure pump through respective water jet manifolds 63,
64 so as to produce water jets 65, 66 directed against the
upper and lower surfaces of the strip to remove the scale
from both surfaces.
In a typical installation producing steel strip
to widths of the order of 1300 mm to 1700 mm there may be
an array of 15 to 20 upper nozzles 61 and a similar array
of 15 to 20 lower nozzles 62 spaced across the strip.
Typically the nozzles may each have a spray angle of the
order of 50° to 55° and be spaced to produce fanned jets
which overlap by about 10 mm. They may sit at a stand off
distance of 60 to 70 mm and be operated to produce a nozzle
flow rate of about 60 L/min at a pressure of 7 MPa to
lOMPa.
The water jets of the descaler further reduce the
temperature of the strip prior to coiling. Typically, the
strip may be descaled at a temperature of 750°C and coiled
at a temperature of around 600°C. In these circumstances
the strip in the coil will typically have scale of only 1
to 2 microns thick and there will be no appreciable further
scaling as the strip cools to ambient temperature. This is
a significant development and results in a commercially
valuable product in that for many applications such as
manufacture of drums or production of painted products the
lightly scaled strip can be used directly as a feed
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material without further treatment, thus eliminating the
cost of pickling or pre-treatment normally required with
strip formed by conventional processes. For more demanding
applications the lightly scaled strip can be pickled much
more rapidly than the strip which has not been descaled
immediately prior to coiling.
Figure 7 illustrates a modification by which the
enclosure 37 is extended to enclose the rolling mill 15 so
that the strip is rolled before it leaves the enclosure
space 38. In this case, the strip exits the enclosure
through the last of the mill stands 16 the rolls of which
serve also to seal the enclosure so that separate sealing
pinch rolls are not required.
It has been found that in order to avoid
excessive scale formation, the strip should preferably
leave enclosure 37 at a temperature of no more than 925°C.
Accordingly, with the arrangement illustrated in Figures 1
to 6, the strip may be hot rolled at a temperature in the
range 750 to 925°C whereas in an installation arranged as
illustrated in Figure 7 the strip may be hot rolled at
temperatures in the full range of 750° to 1250°C.
Although the preferred embodiment of the
invention employs a twin roll strip caster, this is not
essential in invention in its broadest aspect and the twin
roll caster could be replaced by a conventional continuous
thin slab caster producing an initial slab or strip with a
thickness of the order of 120 to 50 mm which may than be
rolled a.n the hot rolling mill down to a final thickness of
the order of 16.0 to 0.7 mm.
