TECHNICAL FIELD
[0001] This invention relates to the casting of metal strip. It has particular application
to the casting of metal strip by continuous casting in a twin roll caster.
[0002] 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 or series of smaller
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.
[0003] The setting up and adjustment of the casting rolls in a twin roll caster is a significant
problem. The rolls must be accurately set to properly define an appropriate width
for the nip, generally of the order of a few millimetres or less, and there must also
be some means for allowing at least one of the rolls to move outwardly against a biasing
force to accommodate fluctuations in strip thickness particularly during start up.
[0004] Usually, one of the rolls is mounted in fixed journals and the other is rotatably
mounted on supports which can move against the action of biasing means to enable that
roll to move laterally to accommodate fluctuations in strip thickness. The biasing
means may be in the form of helical compression springs or alternatively, may comprise
a pair of pressure fluid cylinder units.
[0005] A strip caster with spring biasing of the laterally moveable roll is disclosed in
Australian Patent Application 85185/98. In that case the biasing springs act between
the roll supports and a pair of thrust reaction structures, the positions of which
can be set by operation of a pair of powered mechanical jacks to enable the initial
compression of the springs to be adjusted to set initial compression forces which
are equal at both ends of the roll. The positions of the roll supports need to be
set and subsequently adjusted after commencement of casting so that the gap between
the rolls is constant across the width of the nip in order to produce a strip of constant
profile. However, as casting continues the profile of the strip will inevitably vary
due to eccentricities in the rolls and dynamic changes due to variable heat expansion
and other dynamic effects. Previously, there has been no means to provide dynamic
wedge or profile control to suppress strip profile fluctuations during casting. By
the present invention, it is possible to provide a very effective means for such dynamic
profile control.
DISCLOSURE OF THE INVENTION
[0006] According to the invention there is provided apparatus for continuously casting metal
strip comprising a pair of parallel casting rolls forming a nip between them; metal
delivery means to deliver molten metal into the nip between the rolls to form a casting
pool of molten metal supported on casting roll surfaces immediately above the nip;
pool confining means to confine the molten metal in the casting pool against outflow
from the ends of 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 at least one of the casting rolls is mounted on a pair of moveable roll carriers
which allow that one roll to move bodily toward and away from the other roll, wherein
there is a pair of roll biasing units acting one on each of the pair of moveable roll
carriers to bias said one roll bodily toward the other roll, and wherein each roll
biasing unit comprises a thrust transmission structure connected to the respective
roll carrier, a thrust reaction structure, compression spring means acting between
spring abutments on the thrust reaction structure and the thrust transmission structure
to exert a thrust on the thrust transmission structure and the respective roll carrier,
thrust reaction structure setting means operable to vary the position of the thrust
reaction structure, and control means to control operation of the setting means such
that movements of the thrust transmission structure are replicated as movements of
the thrust reaction structure whereby movements of the thrust transmission structure
do not significantly affect the biasing force imposed thereon by the compression spring.
[0007] Preferably, the setting means is a pressure fluid actuable means acting between the
thrust reaction structure and a fixed structure.
[0008] The pressure fluid actuable means may be provided by a fluid cylinder and piston
unit connected at one end to a fixed structure, the other end of that unit either
forming or being connected with the thrust reaction structure.
[0009] The control means may comprise a first position sensor to sense the position of the
thrust transmission structure, and means to operate the fluid pressure means such
that a movement sensed by the sensor is replicated by a movement of the thrust reaction
structure.
[0010] The roll carriers may comprise a pair of roll end support structures for each of
the rolls disposed generally beneath the ends of the respective roll.
[0011] Each pair of roll end support structures may carry journal bearings mounting the
respective roll ends for rotation about a central roll axis.
[0012] The casting rolls and roll carriers may be mounted on a roll module installed in
and removable from the caster as a unit. In that case, the thrust transmission structure
of each biasing unit may be disconnectable from the respective roll carrier to enable
the module to be removed without removing or dismantling the roll biasing units.
[0013] In apparatus in accordance with the invention both of the casting rolls may be biased
by respective pairs of biasing units. Alternatively, one of the rolls may be restrained
against lateral bodily movement and the other allowed to move laterally against spring
biasing forces in accordance with the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] In order that the invention may be fully explained one particular embodiment will
be described in some detail with reference to the accompanying drawings in which:
Figure 1 is a vertical cross section through a strip caster constructed in accordance
with the present invention.
Figure 2 is an enlargement of part of Figure 1 illustrating important components of
the caster.
Figure 3 is a longitudinal cross section through important parts of the caster.
Figure 4 is an end elevation of the caster;
Figures 5, 6 and 7 show the caster in varying conditions during casting and during
removal of the roll module from the caster;
Figure 8 is a vertical cross-section through a roll biasing unit incorporating a roll
biasing spring; and
Figure 9 is a schematic representation of essential components of the caster.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0015] The illustrated caster comprises a main machine frame 11 which stands up from the
factory floor (not shown) and supports a casting roll module in the form of a cassette
13 which can be moved into an operative position in the caster as a unit but can readily
be removed when the rolls are to be replaced. Cassette 13 carries a pair of parallel
casting rolls 16 to which molten metal is supplied during a casting operation from
a ladle (not shown) via a tundish 17, 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 and are brought together at the nip between them to produce a
solidified strip product 20 at the roll outlet. This product may be fed to a standard
coiler.
[0016] Casting rolls 16 are contra-rotated through drive shafts 41 from an electric motor
and transmission mounted on the main machine frame. The drive shaft can be disconnected
from the transmission when the cassette is to be removed. 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 strip product approximately the width of the rolls.
[0017] The ladle is of entirely conventional construction and is supported on a rotating
turret whence it can be brought into position over the tundish 17 to fill the tundish.
The tundish may be fitted with a sliding gate valve 47 actuable by a servo cylinder
to allow molten metal to flow from the tundish 17 through the valve 47 and refractory
shroud 48 into the distributor 18.
[0018] The distributor 18 is formed as a wide dish made of a refractory material such as
magnesium oxide (MgO). One side of the distributor 18 receives molten metal from the
tundish 17 and the other side of the distributor 18 is provided with a series of longitudinally
spaced metal outlet openings 52. The lower part of the distributor 18 carries mounting
brackets 53 for mounting the distributor onto the main caster frame 11 when the cassette
is installed in its operative position.
[0019] 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. Its upper part is formed
with outwardly projecting side flanges 55 which locate on a mounting bracket 60 which
forms part of the main frame 11.
[0020] 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 nozzle 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.
[0021] 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 cassette is in its
operative position. Side closure plates 56 are made of a strong refractory material,
for example boron nitride, and have scalloped side edges to match the curvature of
the stepped ends of the rolls. The side plates can be mounted in plate holders 82
which are movable 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.
[0022] During a casting operation the sliding gate valve 47 is actuated to allow molten
metal to pour from the tundish 17 to the distributor 18 and through the metal delivery
nozzle 19 whence it flows onto the casting rolls. The head end of the strip product
20 is guided by actuation of an apron table 96 to a pinch roll and thence to a coiling
station (not shown). Apron table 96 hangs from pivot mountings 97 on the main frame
and can be swung toward the pinch roll by actuation of an hydraulic cylinder unit
(not shown) after the clean head end has been formed.
[0023] The removable roll cassette 13 is constructed so that the casting rolls 16 can be
set up and the nip between them adjusted before the cassette is installed in position
in the caster. Moreover when the cassette is installed two pairs of roll biasing units
110, 111 mounted on the main machine frame 11 can be rapidly connected to roll supports
on the cassette to provide biasing forces resisting separation of the rolls.
[0024] Roll cassette 13 comprises a large frame 102 which carries the rolls 16 and upper
part 103 of the refractory enclosure for enclosing the cast strip below the nip. Rolls
16 are mounted on roll supports 104 which comprise a pair of roll end support structure
90 carrying roll end bearings 100 by which the rolls are mounted for rotation about
their longitudinal axis in parallel relationship with one another. The two pairs of
roll supports 104 are mounted on the roll cassette frame 102 by means of linear bearings
106 whereby they can slide laterally of the cassette frame to provide for bodily movement
of the rolls toward and away from one another thus permitting separation and closing
movement between the two parallel rolls.
[0025] Roll cassette frame 102 also carries two adjustable stop means 107 disposed beneath
the rolls about a central vertical plane between the rolls and located between the
two pairs of roll supports 104 so as to serve as stops limiting inward movement of
the two roll supports thereby to define the minimum width of the nip between the rolls.
As explained below the roll biasing units 110, 111 are actuable to move the roll supports
inwardly against these central adjustable stop means but to permit outward springing
movement of one of the rolls against preset biasing forces.
[0026] Each adjustable stop means 107 is in the form of a worm or screw driven jack having
a body 108 fixed relative to the central vertical plane of the caster and two ends
109 which can be moved on actuation of the jack equally in opposite directions to
permit expansion and contraction of the jack to adjust the width of the nip while
maintaining equidistance spacing of the rolls from the central vertical plane of the
caster.
[0027] The caster is provided with two pairs of roll biasing units 110, 111 connected one
pair to the supports 104 of each roll 16. The roll biasing units 110 at one side of
the machine are constructed and operate in accordance with the present invention.
These units are fitted with helical biasing springs 112 to provide biasing forces
on the respective roll supports 104 whereas the biasing units 111 at the other side
of the machine incorporate hydraulic actuators 113. These actuators are operable to
hold the respective roll supports 104 of one roll firmly against the central stops
and the other roll is free to move laterally against the action of the biasing springs
112 of the units biasing 110.
[0028] The detailed construction of biasing units 110 is illustrated in Figure 8. As shown
in that figure, the biasing unit comprises a spring barrel housing 114 disposed within
an outer housing 115 which is fixed to the main caster frame 116 by fixing bolts 117.
[0029] Spring housing 114 is formed with a piston 118 which runs within the outer housing
115. Spring housing 114 can be set alternatively in an extended position as illustrated
in Figure 8 and a retracted position by flow of hydraulic fluid to and from the cylinder
118. The outer end of spring housing 114 carries a pressure fluid operable means in
the form of an hydraulic cylinder unit 119 operable to set the position of a spring
reaction plunger 121 connected to the piston of unit 119 by a connecting rod 130.
[0030] The inner end of the spring 112 acts on a thrust transmission structure 122 which
is connected to the respective roll support 104 through a load cell 125. The thrust
structure is initially pulled into firm engagement with the roll support by a connector
124 which can be extended by operation of a hydraulic cylinder 123 when the biasing
unit is to be disconnected.
[0031] When biasing unit 110 is connected to its respective roll support 104 with the spring
housing 114 set in its extended condition as shown in Figure 8 the position of the
spring housing 114 and cylinder unit 119 is fixed relative to the machine frame and
the position of the spring reaction plunger 121 can be set to adjust the effective
gap between the spring abutments on the reaction plunger and the thrust transmission
structure 122. The compression of the spring 112 can thereby be adjusted to vary the
thrusting force applied to the thrust transmission structure 122 and the respective
roll support 104. With this arrangement the only relative movement during casting
operation is the movement of the roll support 104 and thruster structure 122 as a
unit against the biasing spring. Since the biasing unit acts to bias the roll support
104 inwardly against the stop it can be adjusted to preload the roll support with
a required spring biasing force before metal actually passes between the casting rolls
and that biasing force will be maintained during a subsequent casting operation.
[0032] In accordance with the present invention, dynamic wedge or profile control is achieved
by continuous operation of the hydraulic cylinder unit 119 to vary the position of
the spring reaction plunger to replicate movements of the thrust transmission structure
122 due to lateral movements of the roll support 104. Any inward or outward movement
of roll support 104 will cause a corresponding inward or outward movement of the cylinder
of cylinder unit 119 and therefore of spring reaction plunger 121 so as to maintain
a constant compression of the compression spring 112. Accordingly, it is possible
to maintain a constant biasing force at each end of the roll regardless of movements
of the roll mountings and dynamic wedge control can therefore be achieved. It is not
possible to achieve this result by endeavouring to control biasing forces generated
by a pressure fluid biasing system. Such control would be dependant on measured force
signals that would necessarily have errors which would feed back through the control
system. The use of springs in combination with a continuous setting means in accordance
with the present invention enables very accurate setting of constant biasing forces
which can be maintained throughout a casting operation. It is possible to use very
low stiffness springs and because the two compensation or control systems for the
two roll ends operate completely independently there is no cross-talk between the
two.
[0033] As illustrated diagrammatically in Figure 9, the control means can be comprised of
position sensors 150, sensing the position of the thrust transmission structures 122
and connected into a control circuit which controls the operation of the cylinder
unit 119 so that the movements of the thrust transmission structures 122 are replicated
by the cylinders of units 119. The control circuit may comprise controllers 151 connected
to the sensors 150 and to the cylinder units 119 to operate the cylinders 119 during
casting so as to replicate movements of the thrust transmission structures 122. Controllers
151 may also receive input signals from a logic device 152 to allow operation of the
cylinders for initial setting of the roll supports (input point 153) prior to casting
and subsequent adjustment for static wedge adjustment after casting (input point 154).
[0034] The control means may also cause cylinder unit 119 to be operated to impose additional
movements on the spring reaction plunger 121 to produce variations in the biasing
force to compensate for variations in strip thickness across the whole width of the
strip or at the corresponding edge of the strip due to deformation variations at the
ends of the rolls during casting. Variations in strip thickness can be sensed by X-ray
sensors which scans across the strip downstream from the caster and feed signals to
an input point 155 of the logic device 152 of the control circuit as also indicated
in Figure 9. The thickness variations due to roll deformation will be generally sinusoidal
in the longitudinal direction of the strip so as to produce sinusoidal control signals
which can be used to control operation of cylinder units 119 to impose a corresponding
and compensating sinusoidal movement on the spring reaction plunger 121. To achieve
appropriate strip thickness control, the control signals must be applied to the cylinder
units 119 in proper phase relationship with the rotation of the rolls, ie during each
rotation the pattern of the control signals must be matched with the pattern of roll
end movements caused by the roll deformations. Proper phase matching is achieved by
applying the signals at an initial phase relationship with a reference signal producing
one pulse per revolution of the rolls and then varying the phase relationship to produce
a minimisation of the amplitude of thickness variations. This may be achieved by tracking
or plotting an amplitude error signal.
[0035] The construction units of biasing units 111 forms no part of the present invention.
Full details of these units and the manner in which the roll cassette frame 102 can
be moved into and out of the casting machine are described in Australian Patent specification
85185/98.
1. Apparatus for continuously casting metal strip comprising a pair of parallel casting
rolls (16) forming a nip (16a) between them; metal delivery means (17, 18, 19) to
deliver molten metal into the nip between the rolls (16) to form a casting pool (30)
of molten metal supported on casting roll surfaces immediately above the nip (16A);
pool confining means (56) to confine the molten metal in the casting pool (30) against
outflow from the ends of the nip; and roll drive means (41) to drive the casting rolls
(16) in counter-rotational directions to produce a solidified strip of metal (20)
delivered downwardly from the nip (16A); wherein at least one of the casting rolls
is mounted on a pair of moveable roll carriers (104) which allow that one roll to
move bodily toward and away from the other roll and there is a pair of roll biasing
units (110) acting one on each of the pair of moveable roll carriers (104) to bias
said one roll bodily toward the other roll, characterised in that each roll biasing
unit (110) comprises a thrust transmission structure (122) connected to the respective
roll carrier (104), a thrust reaction structure (121), compression spring means (112)
acting between spring abutments on the thrust reaction structure (121) and the thrust
transmission structure (122) to exert a thrust on the thrust transmission structure
(122) and the respective roll carrier (104), thrust reaction structure setting means
(119) operable to vary the position of the thrust reaction structure (121), and control
means (151) to control operation of the setting means (119) such that movements of
the thrust transmission structure (122) are replicated as movements of the thrust
reaction structure (121) whereby movements of the thrust transmission structure (122)
do not significantly affect the biasing force imposed thereon by the compression spring
means (112).
2. Apparatus as claimed in claim 1, further characterised in that the setting means (119)
is a pressure fluid actuable means acting between the thrust reaction structure (121)
and a fixed structure (114).
3. Apparatus as claimed in claim 2, further characterised in that the fluid actuable
means (119) is provided by a fluid piston and cylinder unit connected at one end to
the fixed structure (114), the other end of that unit either forming or being connected
with the thrust reaction structure (121).
4. Apparatus as claimed in claim 2 or claim 3, further characterised in that the control
means comprises a position sensor (150) to sense the position of the thrust transmission
structure (122) and means to operate the fluid pressure means such that a movement
sensed by the sensor is replicated by a movement of the thrust reaction structure
(121).
5. Apparatus as claimed in any one of claims 2 to 4, further characterised in that the
fluid actuable means (119) is operable to impose additional movements on the thrust
reaction structure (121) to produce variations in the biasing force to compensate
for variations in strip thickness during casting.
6. Apparatus as claimed in claim 5, further characterised in that there is strip thickness
sensing means to produce signals indicative of strip thickness and the control means
(151) utilises those signals to cause the operation of the fluid actuable means to
impose (119) said additional movements on the thrust reaction structure (121).
7. Apparatus as claimed in any one of claims 1 to 6, further characterised in that the
roll carriers (104) comprise a pair of roll end support structures (90) for each of
the rolls disposed generally beneath the ends of the respective roll.
8. Apparatus as claimed in claim 7, further characterised in that each pair of roll end
support structures (90) carries journal bearings (100) mounting the respective roll
ends for rotation about a central roll axis.
9. Apparatus as claimed in any one of claims 1 to 8, further characterised in that the
casting rolls (16) and the roll carriers (108) are mounted on a roll module (13) installed
in and removable from the caster as a unit.
10. Apparatus as claimed in claim 9, further characterised in that the thrust transmission
structure (122) of each biasing unit (110) is disconnectable from the respective roll
carrier (104) to enable the module (13) to be removed without removing or dismantling
the roll biasing units (110).