-
This invention concerns improvements in and relating to
casting, particularly, but not exclusively to methods and
apparatus for manipulating strand thickness by a sequential
rolling process, including the location of such changes and /
or timing of such changes, for instance when ending a casting
cycle.
-
There exists a variety of continuous casting processes in
which a strand exits a mould and then enters a series of
rolling processes. In general, the molten material partially
solidifies in the mould to form a relatively thin skin with a
still molten core. The molten core is still present when the
strand leaves the bottom of the mould. It is preferred to
employ a liquid core reduction stage in the initial set of
rollers and / or soft reduction stage in the latter sets of
rollers to reduce the strand thickness.
-
Liquid core reduction allows a slab to be formed of a
suitable size for subsequent rolling whilst allowing a meniscus
with a larger surface area than would otherwise be possible in
the mould. The liquid core reduction stage involves a series
of rollers with decreasing spacing, which, as a consequence,
reduce the separation between the solidified skins and squeeze
the liquid core out. In the prior art the position of this
liquid core reduction stage along the strand pass line is
fixed.
-
At start up of a casting line, it is necessary to
introduce a dummy strand of sufficient dimensions to block the
lower part of the mould during initial molten material
introduction. At the end of the casting stage it is necessary
to remove and handle a slab larger than the desired thickness
for the rolling stage, but which contains a molten core which
cannot be squeezed out merely by progress through a liquid core
reduction stage as this would push the molten material out over
the top of the end of the strand.
-
Providing the desired size of slab for the rolling stage,
whilst allowing ready introduction of a dummy strand and
handling the still molten core of the end of the strand during
the end of strand casting are significant problems.
-
The present invention seeks to overcome these problems by
manipulating the position of rollers in the initial rolling
stage of a sequential rolling process.
-
Soft reduction is used to improve internal quality and
properties of the cast product. An important criteria,
particularly affecting internal quality of high carbon steels
(used for example in off shore, heavy plate, line pipe and
other constructional steels), is the extent of central
segregation and porosity. High segregation results in nonuniform
properties across the thickness of the finished product
which may be detrimental to the weldability and resistance to
hydrogen induced cracking (HIC). Segregation arises during the
final stages of solidification. As the steel solidifies, the
volume change due to phase transformation generates voids
within the strand. The reduced pressure within the voids
creates a liquid flow or suction of liquid into the voids. On
cooling, the liquid steel may become enriched with alloying
elements such as carbon, manganese, phosphorous, sulphur and
the like. It is this enriched solution that fills the voids
generated by the shrinkage producing an undesirable segregated
structure. Soft reduction seeks to provide a solidification
bridge or barrier which restricts liquid flow and so the voids
cannot be re-filled.
-
In the prior art the position of soft reduction along the
strand pass line is fixed. Variations in a number of factors,
including the material being cast and cooling rates for the
strand, effect the appropriate position for optimum soft
reduction along the strand pass line. As the location of
application of soft reduction is fixed in the prior art,
however, such variations lead to the inappropriate location of
soft reduction application. The benefits thereof are thus lost
and in some cases great harm is done.
-
Thus, the present invention additionally seeks to
overcome these problems with the positioning of soft reduction,
and this fully addresses the problems associated with central
segregation and porosity by manipulating the position of the
rollers in the final (solidification) rolling stage of a
sequential rolling process.
-
According to a first aspect of the invention there is
provided a method of casting, the method involving a moulding
stage for forming a partially solidified strand with a molten
core, the partially solidified strand passing to a roller stage
on leaving the moulding stage, the method comprising, at least
during a portion of a casting cycle :-
- providing a first portion of the rolling stage with a
reducing separation between rollers, thereby defining a volume
occupied by the strand in that portion;
- changing the volume of the strand between the rollers in
that portion by changing the position of one or more rollers;
and
- providing a further portion of the rolling stage with a
reducing separation between rollers.
-
-
The volume of the strand between the rollers in that
portion may be increased. Such a method may be used in liquid
core reduction to lower the level of the meniscus with in the
solidifying strand. Such a method may be used in dynamic soft
reduction to move the location of the soft reduction zone away
from the moulding stage / along the direction of travel of the
strand.
-
The volume of the strand between the rollers in that
portion may be decreased. Such a method may be used in liquid
core reduction to increase the level of the meniscus within the
moulding stage. Such a method may be used in dynamic soft
reduction to move the location of the soft reduction zone
towards the moulding stage / against the direction of travel of
the strand. The further portion in such a case is provided
closer to the moulding stage than the first portion.
-
The further portion of the rolling stage may be a
different portion of the rolling stage to the first portion
and/or the first portion subsequent to its change in volume.
-
According to a second aspect of the invention we provide
a method of casting, the method involving a moulding stage for
forming a partially solidified strand with a molten core, the
partially solidified strand passing to a roller stage on
leaving the moulding stage, the method comprising, at least
during a portion of a casting cycle :-
- providing a first portion of the rolling stage with a
reducing separation between rollers, thereby defining a volume
occupied by the strand in that portion;
- increasing the volume of the strand between the rollers
in that portion by changing the position of one or more
rollers; and
- providing a further portion of the rolling stage with a
reducing separation between rollers.
-
-
Preferably the increase in volume is provided by
increasing the separation of one or more rollers on one side of
the strand from one or more rollers on the other side of the
strand. Preferably all of the rollers are increased to a
predetermined equivalent separation. Preferably the separation
conforms to the equivalent dimension of the mould outlet and /
or the initial rolling stage outlet.
-
Preferably the further portion of the rolling stage with
a reduced separation is a different portion to the first
portion / increased volume portion and is provided further, in
the direction of travel of the strand, than the first portion
/ increased volume portion.
-
The further portion of the rolling stage may be defined
with separations between rollers equivalent to those provided
in the initial reduced separation portion. Preferably the
separation of the initial rollers of this portion are
configured to the adjoining rollers of the increased volume
portion. Preferably the last rollers on this reduced portion
configured to the separation of adjoining rollers of subsequent
portions. Preferably substantially parallel rollers are
provided in subsequent portions of the rolling stage.
-
Preferably the method further comprises decreasing the
volume of the strand between the rollers in the further portion
of the rolling stage with a reduced separation between rollers.
Most preferably it still further provides, providing a yet
further portion of the rolling stage with a reduced separation
between rollers. The method may still further provide
increasing the volume of the strand between still further
portions of the rolling stage and/or providing still further
portions of the rolling stage with a reduced separation.
-
The increase in volume of the strand between rollers
and/or the provision of further portions of the rolling stage
with a reduced separation may be provided by varying blocks of
rollers simultaneously or by varying one or more rollers in a
sequential manner. Thus the gradual movement of the increased
volume portion and/or reduced separation portion down a line of
rollers is envisaged.
-
According to a third aspect of the invention we provide
a method of casting, the method involving a moulding stage for
forming a partially solidified strand with a molten core, the
partially solidified strand passing to a roller stage on
leaving the moulding stage, the method comprising, at least
during a portion of a casting cycle :-
- providing a first portion of the rolling stage with a
reducing separation between rollers, thereby defining a volume
occupied by the strand in that portion;
- decreasing the volume of the strand between the rollers
in that portion by changing the position of one or more
rollers; and
- subsequently providing the first portion of the rolling
stage with a reducing separation between rollers.
-
-
Preferably the decrease in volume of the strand between
the rollers generates an increased volume of material in the
moulding stage. The decrease of volume may therefore give a
rise in the level of molten material in the moulding stage.
-
Preferably the method provides for the advancement of the
strand with the rollers in the decreased volume position.
-
Preferably the portion with a reducing separation is
provided before the level of molten material reaches the outlet
of the mould.
-
Preferably the reducing separation portion is provided in
an equivalent configuration to the initial reduced separation
portion, most preferably in the same portion of the rolling
stage. Preferably the separation of the initial rollers of
this portion are configured to the adjoining rollers of the
preceding portion. Preferably the last rollers of this reduced
portion configured to the separation of adjoining rollers of
the subsequent portion. Preferably substantially parallel
rollers are provided in subsequent portions of the rolling
stage.
-
According to a fourth aspect of the invention we provide
a method of casting, the method involving a moulding stage for
forming a partially solidified strand with a molten core, the
partially solidified strand passing to a roller stage on
leaving the moulding stage, the method comprising, at least
during a portion of a casting cycle :-
- providing a first portion of the rolling stage with a
reducing separation between rollers, thereby defining a volume
occupied by the strand in that portion;
- changing the volume of the strand between the rollers in
that portion by changing the position of one or more rollers;
and
- providing a further portion of the rolling stage with a
reducing separation between rollers., the further portion of
the rolling stage being closer to the moulding stage than the
first portion.
-
-
The invention, including third and fourth aspects of the
invention, may also be provided with the following features and
/ or options.
-
The decrease in volume of the strand between the rollers
is preferably provided by reducing the separation of one or
more rollers relative to one or more rollers on the other side
of the strand. Preferably a reduction between opposing pairs
of rollers is provided. Preferably all of the rollers are
reduced to an equivalent separation. It is preferred that this
separation be substantially equivalent to the corresponding
dimension of the desired strand product.
-
Preferably the provision of a first portion of the
rolling stage with a reducing separation between rollers is
provided by increasing the separation between one or more
rollers in that portion. Preferably the rollers are moved
apart to a decreasing degree in the direction of strand travel.
-
The decrease in volume of the strand between rollers
and/or the provision of further portions of the rolling stage
with a reduced separation may be provided by varying blocks of
rollers simultaneously or by varying one or more rollers in a
sequential manner.
-
Preferably the method provides a strand which is of
equivalent thickness throughout its length. Preferably the
relative positions of rollers in the non-reducing separation
portion are maintained constant throughout.
-
According to a fifth aspect of the invention we provide
a method of casting, the method involving a moulding stage for
forming a partially solidified strand with a molten core, the
partially solidified strand passing to a roller stage on
leaving the moulding stage, the method comprising, at least
during a portion of a casting cycle :-
- providing a first portion of the rolling stage with a
reducing separation between rollers, thereby defining a volume
occupied by the strand in that portion;
- changing the volume of the strand between the rollers in
that portion by changing the position of one or more rollers;
and
- providing a further portion of the rolling stage with a
reducing separation between rollers, the reducing separation
portion preferably providing soft reduction, the reducing
separation portion position being moved relative to the
moulding stage according to casting conditions.
-
-
The first and/or second and/or third and / or fourth and
/ or fifth aspects of the invention may further include the
following possibilities, options and features.
-
Preferably the portion of the rolling stage with a
reduced separation is initially provided in proximity to the
mould outlet, for instance for liquid core reduction.
-
Preferably the reduced separation is provided by
decreasing the separation between rollers in the direction of
strand travel. The reduction in separation may occur in a
linear or non-linear manner. The rate of reduction of
separation may be reduced at the start and/or end part of the
portion compared with the mid part of that portion.
-
Preferably pairs of opposing rollers are provided.
-
Preferably the rollers on one side of the strand are
provided in a line, parallel to the direction of travel of the
strand. Preferably the rollers on one side of the strand are
inclined relative to the direction of travel of the strand.
-
Preferably the change in position is provided by moving
one or more rollers on one side of the strand, the rollers on
the other side of the strand remain in position. Preferably
all the rollers forming the portion of reduced separation are
varied to an equivalent separation. The separation may
correspond to the corresponding separation of the mould outlet
or to the corresponding separation of the desired product.
-
Preferably the portion of the rolling stage subsequently
provided with a reducing separation is provided with the same
configuration as the first reduced separation portion.
-
The moulding stage may involve linear or curved mould or
a mould incorporating both linear and curved portions. A mould
incorporating a funnel portion may be provided.
-
The rolling stage or one or more portions thereof may be
provided in a linear configuration and/or in a non-linear (eg
curved) configuration and/or in a combination of such
configurations. It is preferred that a first portion of the
rolling stage after the moulding stage be provided in a linear
configuration. Preferably this portion is followed by a
bending portion, with most preferably subsequent further linear
and / or non-linear portions being provided.
-
In an alternative preferred embodiment, the method of the
present invention may be used shortly before final
solidification, for instance in a final set of rollers of a
sequential rolling process in order to reduce strand thickness
(a method referred to hereinafter as "soft reduction").
-
Soft reduction may be achieved by any of the variations
in roller separation and volume described hereinbefore.
Preferably soft reduction may be achieved by providing a
tapered roller gap or a variable, roller gap in the area of
final solidification which serves to compensate for slab
shrinkage and alleviate the aforementioned disadvantages. The
position of the tapered roller gap and / or its variation may
be dynamically controlled.
-
Generally speaking, the extent of soft reduction required
to compensate for strand shrinkage is dependent upon steel
composition and initial strand thickness. It may be typically
of the order of 2% of strand thickness. The liquid steel flow
between the solidified dendrites is restricted after about 0.8
solid fraction has been reached. The voids or pores do not refill
but simply become re-shaped on reduction. Furthermore the
rolling loads become significantly greater when the strand
becomes solid. Conversely, applying the method of soft
reduction before the strand centre has attained some
solidification may limit the observed improvement in
segregation. Soft reduction is, therefore, more efficient when
used over the preferred solidification range of about 0.3 to
0.8 fraction solid.
-
The length of the soft reduction zone may, however, be
dependent on the rate of soft reduction. If the rate of
reduction at each roller or taper (mm/m) is too great,
excessive loads and strains may be experienced at the liquid-solid
interface leading to internal cracking. The optimum
characteristics of the soft reduction zone are preferably
determined by or according to the cooling/solidification rate
of the strand and strain limits of the steel being cast.
Preferably mathematical modelling may be performed to establish
a suitable location and / or length of the soft reduction zone.
For 0.3 to 0.8 fraction solid, reduction rates of around 0.6 to
1.5 mm/m may be utilised.
-
Thus the length and location of the appropriate soft
reduction zone at any point in time may be accurately
determined with the aid of heat transference solidification
calculations, in order to derive optimum benefit from the
strand reduction. Preferably, a computer thermal model is used
to assess machine layout, secondary cooling and solidification
for a range of casting conditions. Added to strain analyses,
the process models may be advantageously used to determine
optimum soft reduction characteristics, such as length,
location, route, etc.
-
At different casting conditions (i.e. steel grade,
casting speed, section size, etc), the final solidification
position and therefore optimum soft reduction zone may vary.
The dynamic soft reduction system described hereinafter allows
soft reduction to be advantageously variably applied over some
or all segments. Preferably the form of the dynamic soft
reduction varies with varying conditions and / or with time.
Alternatively, soft reduction may be applied as a heavy line
taper over the full support length. Both systems provide for
effective soft reduction although a dynamic system is preferred
to ensure that optimised reduction rates may be consistently
applied. Particularly preferably, the benefits of dynamic soft
reduction may be optimised by applying dynamic spray control
and thermal tracking.
-
Cooling (eg spray cooling) may be provided at one or more
portions of the rolling stage, for instance in liquid core
reduction and / or soft reduction. The level of cooling may be
varied over time to control the amount and hence level of the
molten core. Spray cooling applied to a part of the strand
having a molten core is preferred.
-
An important factor in the efficiency of the soft
reduction method is the shape of the solidification point
across the width of the strand (the sump profile). This is
mainly influenced by the position and cooling intensity of the
secondary cooling system. An even sump profile or "U-shape" is
preferred to ensure that soft reduction is applied in the
correct position over most of the slab width.
-
If spray cooling is not set correctly (eg too little
cooling at the edge of the strand) solidification does not take
place at the same time across the width. This may lead to a "V-shape"
or a "W-shape" sump leading to uneven fraction solid
across the slab width within the soft reduction zone.
-
The required withdrawal force may be calculated according
to the resistances acting on the strand resisting it moving
through the machine. The resistances include friction due to
the mould and roller bearings, as well as slab straightening
forces, roller eccentricities and misalignment, etc. When soft
reduction is used (or even excessive drive roller pressures),
significant rolling forces on the strand are created. These
forces increase the withdrawal resistance and thus the required
torque of the motors within the casting machine. Sufficient
drive capability must be made available to allow withdrawal of
the strand. Moreover the effect of increased loading due to
soft reduction on strand guide segments require that further
checks are made to inter alia segment frames, drive rollers and
non-drive rollers, roller bearings, roller gap settings and
packers, segment drive torques and segment drive motor
suitability.
-
The casting machine thickness position is normally set by
equipment units arranged along the ferro-static length of the
casting apparatus from the mould, through the top zone, bender
and segments. The setting of these units is normally achieved
by clamping frames together hydraulically onto thickness packs.
To perform liquid core reduction or soft reduction in
accordance with the invention, it is desirable to provide the
casting apparatus with a means for adjusting the rollers
dynamically.
-
Thus one advantageous apparatus for liquid core
reduction or soft reduction comprises setting packers for
adjusting the roller gap. The segments which may be typically
used for soft reduction may be calculated from or predetermined
based upon, the steel grade to be cast and steady state casting
speed. The method of the invention therefore further comprises
a step in which the roller gaps are adjusted by static clamping
onto preset packers. Subsequent segments are set to the new
roller gaps. The location may be optimised to the predicted
steady state casting conditions.
-
However, it is preferred to carry out the method of the
invention with an apparatus comprising remotely operable means
for adjusting the rollers dynamically, for example hydraulic
means or electromechanical means coupled with a dynamic control
system. Preferred dynamic control systems for the hydraulic
means are, for example, pulse systems using standard valves or
servo or high gain proportional valve systems. Each of these
may advantageously use a sensor transducer or similar device at
each cylinder to dynamically feedback the signal of the strand
position. Due to its location, it may be desirable to instal
the system in a pull-down cylinder or a separate piggy-back
type cylinder (e.g. an air cylinder). A control device (such
as a digital controller) may be used and may be linked to a
host computer.
-
The advantage of the pulse system using standards valves
is that it may be adapted to suit new and existing machinery.
The servo system on the other hand may use high gain
proportional valves which are less demanding than a true servo
valve and may be adaptable to existing hydraulic systems
(typically in conjunction with filtration means).
-
Thus viewed from a sixth aspect the present invention
provides a casting apparatus comprising remotely operable means
for dynamically or statically adjusting one or more rollers.
Preferably the apparatus comprises a position sensor for
determining the position of one or more rollers.
-
In a preferred embodiment, a hydraulic cylinder is
mounted to the equipment frame and arranged with a dynamic
control system and digital controllers. As casting commences,
the strand may be tracked with conditions set by a process
control computer and set points down-loaded for optimum roller
gap setting to the digital controllers that control the
movement of fluid through the control valves which in turn move
each hydraulic cylinder and set the roller gap. Feedback for
the roller gap may be taken from a sensor transducer or the
like monitoring the cylinder stroke movement and thus the
roller gap.
-
Thus in another embodiment, the method of soft reduction
is carried out dynamically by including a step in which one or
more rollers is adjusted dynamically. For dynamic adjustment,
this advantageously involves the automatic adjustment of
individual roller gaps to suit the location of final
solidification. Dynamic soft reduction may therefore usefully
compensate for non-steady state casting conditions. That is to
say, during casting, the slab speed varies depending upon
conditions and casting practice with the result that the
solidification point of liquid metal may move up or down the
machine length. It will be appreciated, therefore, that to
achieve maximum flexibility for strand roller gap adjustment
during casting and to meet the conditions for soft reduction,
an apparatus that may respond dynamically and be capable of
accurately positioning and setting the roller gap is preferred.
-
Thermal tracking provides real time thermal modelling of
the cast product with calculations of slab surface temperature,
shell thickness and solidification position, etc. To ensure
the necessary accuracy, the modelling should have grade
dependent thermal and physical properties and spray zone
dependent heat transfer coefficients. Such a model may provide
calculated slab temperatures and shell thicknesses throughout
the casting machine. Numerous real time variables may be
inputted into the thermal tracking model including steel grade,
casting temperatures, section size, casting speed, mould
cooling water flow rates and temperature rises, and spray
cooling water flow rates. Thermal tracking model results may
be used to provide inputs into the dynamic liquid core
reduction and soft reduction control systems to ensure that
optimum strand thickness reduction rates and positions are
correctly applied in relation to calculated solidification
position.
-
In accordance with the sixth aspect of the invention,
liquid core reduction and soft reduction may be performed
dynamically in new casting machines or by retro-fitting
appropriate means to existing machines. The benefits of the
apparatus of the invention are that liquid core reduction
allows thinner sections to pass to the rolling mill therefore
less power is required during rolling. Typically, liquid core
reduction takes place in the top zone and bender segment. Soft
reduction gives enhanced quality off the casting machine for
certain grades of material. The soft reduction stage typically
takes place downstream of the top zone and bender system in
segments 1 to 5 and segments 6 to 7. Reduction over each unit
will typically be 2 millimetres.
-
An explanation of the invention and various embodiments
thereof will now be described, by way of example only, and with
reference to the accompanying Figures, in which:-
- Figure 1 illustrates a liquid core reduction stage in
continuous casting;
- Figure 2 illustrates a start up procedure for continuous
casting;
- Figure 3 illustrates a potential problem with liquid core
reduction for the end of a casting strand;
- Figure 4 illustrates a first embodiment of the invention
for handling the end of a casting strand;
- Figure 5 illustrates a second embodiment of the invention
for handling the end of a casting strand;
- Figure 6 illustrates a typical casting machine suitable
for employing the present invention;
- Figure 7 illustrates a schematic representation of
dimensions and tolerances in a method of the invention in which
liquid core reduction precedes soft reduction;
- Figure 8a illustrates a typical segment for use in the
method of the invention in end on view;
- Figure 8b illustrates the segment of Figure 8a in side
view;
- Figure 9 illustrates a schematic representation of the
soft reduction method of a preferred embodiment of the
invention;
- Figure 10 illustrates a further example of thermal
modelling which may be used to highlight the solid fraction
positions and proposed location of a soft reduction zone;
- Figure 11 illustrates roller gap configurations for soft
reduction;
- Figure 12 illustrates a preferred embodiment of the
apparatus of the invention comprising means for setting a
roller gap;
- Figures 13a and 13b illustrate entry and exit hydraulic
cylinders in end and side view respectively in one form;
- Figures 14a and 14b illustrate entry and exit hydraulic
cylinders in side view; and
- Figure 15 illustrate dynamic liquid core reduction with
dynamic soft reduction and static liquid core reduction with
dynamic soft reduction.
-
-
Continuous casting of metal typically involves the
introduction into a mould 3 of molten material 5, the molten
material partially solidifying therein to form a strand 6
comprising a solidified thin skin 7 and a molten core 9.
-
To minimise fluctuations in meniscus level 11 in the
mould, with consequential improvements in product quality, it
is desirable to keep turbulence and other fluctuations to a
minimum. It has been found that this consideration is at its
best when as large a surface area for the meniscus is as is
possible provided.
-
To minimise the cost of the subsequent rolling equipment
stages 13, however, and also to provide products of the desired
thickness, it is desirable to produce slabs of a considerably
lower cross sectional area than is desirable for the meniscus
area. Thus the fully solidified strand is thinner than the
strand leaving the mould by virtue of active compression. This
can be achieved through the use of one or both of liquid core
reduction and soft reduction stages. Liquid core reduction is
frequently employed following the exit of the strand 6 from the
mould 3 when the strand has a significant liquid core. Soft
reduction is generally carried out at a later stage when
cooling has significantly reduced the level of liquid in the
strand. The reasons and benefits for these stages are
discussed elsewhere in this document.
-
In the context of liquid core reduction, as illustrated
in detail in Figure 1, the molten material 5 introduced into
the mould 3 forms a skin 7 of increasing thickness as the
temperature drops for the skin and solidification occurs.
However, by the time the strand 6 exits the mould 3 it still
possesses a sizeable liquid core 9.
-
Reduction of the strand 6 towards a more desirable
thickness X can be achieved in the initial rolling stage 13
using liquid core reduction by pushing one or both of the skins
7 towards the other. Thus, pairs of cylindrical rollers 15 are
provided with one set provided in a straight line, along the
pass line, and with the other set tapering towards the first so
as to gradually decrease the separation distance. In this way
the opposing solidified skins 7 are brought together and the
liquid core 9 is squeezed out. In general this squeezing
process leads to a flow of material upwards within the core.
A 90mm gap on exiting the mould may well be reduced to a 70mm
strand in such a stage.
-
During strand casting start up it is necessary to
introduce a dummy, solid strand 20 into the bottom of the mould
to form a plug whilst the molten material is initially
introduced. A start up scheme for such a continuous casting
mould is illustrated in Figure 2. The profile of the strand
and the accompanying rollers are illustrated in twelve
sequential representations, numbered 1 to 12.
-
In representation 1, the liquid core reduction (LCR) unit
22, bender stage 24 and rolling stages are all opened to the
width of strand corresponding to the mould outlet 26. To plug
the mould a dummy strand is inserted with the drive rollers 28
being sequentially moved into grip the thin main length 30 and
drive the dummy bar upwards. The dummy bar has an enlarged
head portion 32 to block the mould and thinner subsequent
portion 30 corresponding to the desired eventual slab
thickness.
-
Once blocked, representation 2, casting can be started by
introducing molten material 34 into the mould 36. As the
molten material freezes to the dummy bar head and the initial
skin for the strand forms in the mould, the strand drives 28
can be started and the strand withdrawn from the mould 36.
-
Initially the LCR unit and bender rollers are set at a
fully open separation to accommodate the dummy bar and the cast
strand attached to it, representation 3. After sufficient cast
strand 40 has passed through the LCR unit, the LCR unit rollers
can be reduced in separation to give the desired squeezing
effect to reduce the strand 40 down to the desired thickness,
representation 4. A tapered stage is thus provided.
-
The passage of the dummy bar and subsequent cast strand
40 down through the various stages with the rollers being
reduced in separation to conform to the reduced strand
thickness is then pursued, representations 5 to 11.
-
Eventually the dummy bar 20 exits the rolling stages and
a steady state is reached in which molten material is
introduced into the mould, is reduced in thickness through the
LCR unit and is then rolled in its reduced format in the
subsequent bender and other stages.
-
A significant problem is encountered when the pouring of
molten material into the mould finishes at the end of a casting
run. With no further in flow of molten material the level of
molten material in the mould drops and will eventually result
in a solidified skin and molten core leaving the mould outlet
and entering the LCR unit. If the LCR unit and subsequent
stages are maintained with the squeeze effect down to the
desired slab thickness then problems occur. As illustrated in
Figure 3 if this squeeze is implemented then still molten
material 50 from the core flows up from the squeeze stage and
spills over the ends 52 of the solidified strand 54 with
consequential problems. Because of this it is necessary to
allow the passage through the subsequent rolling stages of a
large end portion corresponding in dimension to the mould exit,
with the rollers having to be separated throughout to
accommodate the passage of this thicker material.
-
The present invention, amongst other things, aims to
solve this problem and increase the effective length of the
useable product obtained.
-
In the shut down scheme illustrated schematically in
Figure 4, a series of sequential representations, numbers 13 to
25 are provided representing various stages in the shut down
procedure.
-
In representation 13, the casting process is in a steady
state with molten material 60 being introduced to the mould 62,
withdrawn through LCR unit 64 to reduce its thickness and
processed in subsequent benders and other stages.
-
Following the stoppage of material being fed to the
mould, representation 14, the level 66 in the mould starts to
drop, representation 15.
-
To accommodate the still molten core, whilst achieving a
reduction in the slab thickness the rollers in the LCR unit 64
are opened to a width corresponding to that of the mould
outlet, representation 16. This increase in volume 68 of the
LCR unit results in a depression in the level of still molten
core so leading to a crater 70 in the top of the strand. By
promptly also opening the gap between the rollers in the
bending stage 72 to a tapered configuration this volume can be
further increased and the crater depth increased,
representation 17.
-
Progress of the strand through this revised roller
configuration leads to liquid core reduction in the bender
stage with a consequential up flow of molten material and
reduction in the crater depth 74 at the end of the strand,
representation 18.
-
Before the molten core reaches the top level of the
strand, however, the rollers of the bender stage 72 are opened
to the full width and the rollers of the first segment 76 are
increased in separation to provide a gradual taper down to the
desired slab thickness. Once again, therefore, an increase in
volume is provided with a consequential reduction of the level
of the molten core relative to the solidified skins, whilst
still achieving a reduction of the slab thickness down to the
desired level, representation 19.
-
Opening the rollers in the previously tapering stage and
gradually opening the rollers in the next stage down line to
provide a taper continues to have this effect throughout,
thereby avoiding overflow of the molten core over the ends of
the strand throughout, whilst achieving a reduction in the slab
length which has the undesirable wide thickness.
-
Eventually the completely solidified strand, with a
minimal distance at the full thickness exits the rolling stage,
representation 25.
-
An alternative regime for handling the same situation is
provided in Figure 5. Again a series of sequential
representations of the process are provided in representations
26 to 38.
-
As before, in representation 26, steady state casting is
occurring with a LCR unit thickness reduction stage 80. Upon
stopping in flow of molten material, representation 27, the
level 82 of molten material in the mould drops, representations
28 and 29.
-
In contrast to the previous embodiment, at this point in
time the separation of the rollers in the LCR unit 84 is
reduced down to that of the desired strand thickness,
representation 30, with the net effect that molten core
material is forced back up into the mould giving a molten
material level rise in the mould. This material can then be
used for slab casting with the result that the level in the
mould drops once more, representation 31.
-
As the meniscus approaches the bottom of the mould the
rollers of the LCR unit 80 are opened to their tapered
squeezing configuration once more providing an increased volume
86 between the skins and leading to cratering 88 of the strand
end. The strand end then passes into the LCR stage and is
reduced in thickness down to the desired extent, but without
overflow of the molten core as the increased volume is
sufficient to accommodate the up flow of molten material,
representation 32.
-
In this way, the entire strand is converted to the
desired thickness and that strand is gradually withdrawn from
the process stream, representations 33 to 37.
-
When it is desired to commence casting once more the
sequence can be repeated by presenting a dummy bar to the
rollers, representation 38.
-
Both embodiments of the invention described hereinabove,
therefore, involve varying the internal volume of the slab so
as to ensure that the molten core is always below the end level
of the strand. In this way spillage is avoided. The crater in
the meniscus will vary, it will not completely disappear.
-
The invention, therefore, allows a large meniscus area to
be employed in the mould whilst achieving products of the
desired, reduced thickness. Furthermore this is achieved
whilst minimising loss of material in an undesirable product
form at the end of any casting strand procedure.
-
Figure 6 illustrates schematically a typical casting
machine comprising mould 600 and top zone 601, bender 602 and
segment sections 603a and 603b. Section 603a represents
segments a to e and section 603b represents segments f to g.
-
The adjustment of the segments is normally achieved by
clamping frames together, hydraulically, onto thickness packs.
In accordance with the invention, dynamic hydraulic means allow
the roller gap in these segments to be advantageously adjusted
during casting.
-
With reference to Figure 6, the casting methods of the
invention may use any one of the following combination of
steps:
- 1) parallel strand thickness typically say 150
millimetres in top zone 601 and bender 602 and in support
segments a to e and f to g - no thickness reduction is
applied.
- 2) parallel strand thickness 150 millimetres in top
zone 601 and bender 602 and soft reduction in segments a
to e and f to g - soft reduction without liquid core
reduction.
- 3) liquid core reduction reducing strand thickness by
up to 25mm in top zone 601 and / or bender 602 and soft
reduction in segments a to e and f to g - liquid core
reduction with soft reduction.
- 4) liquid core reduction by up to 25mm in top zone 601
and bender 602 and parallel strand thickness in the
support segments - liquid core reduction without soft
reduction (as described above).
-
-
The purpose of liquid core reduction is to reduce the
strand thickness (roller gap) from say 150 millimetres to 125
millimetres. This may be achieved by setting the entry
dimension of the top zone segment to 150 millimetres and
tapering through to the exit bender segment dimension at 125
millimetres. This tapering reduction of the roller gap is
performed at a controlled rate by hydraulic cylinders, finally
clamping onto hard packs. Throughout the operation, a sensor
monitors the position of the cylinder stroke and thus the
roller gap position.
-
For
steps 1 and 2 above in which the parallel strand
thickness is 150mm in the top zone and bender, the strand is
set to hard packers. In
step 3, liquid core reduction is
achieved as follows:
- i) position strand roller gap to say 150 millimetres
(i.e. move hydraulic cylinders from open position of say
150 and stroke);
- ii) as the hot strand runs through the top zone and
bender, the units are loaded as per the forces in Table
1 below;
- iii) when the hot slab has passed through each unit, it
is closed at a control rate onto a set packer thickness
and pressure is held during casting;
- iv) at the end of cast the strand is brought back
parallel to 150 millimetres until cleared through the top
zone and bender;
- v) provision is required to move the entry cylinders
independently of the exit cylinders.
-
-
Soft reduction may be achieved in segments a to e (603a
and f to g (603b) and in conjunction with or separately to
liquid core reduction to control the final solidification of
the casting machine.
-
The roller gap position control is achieved through
hydraulic cylinders. Packers used in the case of an existing
damping method are removed and roller gap adjustment is made
dynamic.
-
Soft reduction may be achieved as follows:
- 1) outlet roller gap is set at say 150
millimetres or 125 millimetres from bender
(depending on whether liquid core reduction has
been applied or not);
- 2) the segment responds to instructions on set
points from the host computer,
- 3) each segment is capable of roller gap
positioning at a controlled rate within a stepless
range,
- 4) forces acting on the segment are defined in
Table 1,
- 5) provision is made to move the entry cylinders
independently of the exit cylinders.
-
-
A schematic representation of dimensions and tolerances
in a method of the invention in which liquid core reduction
precedes soft reduction is shown in Figure 7.
-
In Figures 8a and 8b, a typical strand segment is
illustrated for use in the method and apparatus of the
invention. The main valve manifolds 801 are located outside
the spray chamber area 802 adjacent to the equipment with hard
piping and hoses to a valve block 803 on each hydraulic
cylinder 804 in the spray chamber area 802. Adjusting the
hydraulics allows the roller gap 806 to be varied.
-
Figure 9 illustrates a schematic representation of the
progress of a soft reduction method with the roller positions
indicated for a soft reduction taper 902 in a soft reduction
zone 903 as compared with a rigid line taper 904 which
addresses contraction due to cooling only.
-
Figure 10 shows examples of thermal modelling and
calculation of fraction solids. The upper results shows the
increase in shell thickness for fraction solids ranging from
0.0 (liquidus line) through to 1.0 (solidus line). The lower
result shows the increase in fraction solid at the centre of
the slab versus distance from meniscus. This shows that the
optimum soft reduction position of typically 0.3 to 0.8fs
occurs at say 8.4 to 10 metres from the meniscus. This is
within the third (c) and fourth (d) curved support segments.
-
Based on the above mentioned calculations the appropriate
location under steady state can be determined and based on
suitable monitoring the appropriate locations with process
variations can be determined.
-
Roller gap configurations for a soft reduction process
are under a variety of conditions are shown in the resolutions
of Figure 11. The formats 1 to 8 show the final solidification
point (apex of the liquid core) at various positions along the
machine length. Set points down-loaded from the process
control computer tracking systems are sent to the position
control digital controllers and to the hydraulic cylinders for
roller gap setting according to where this solidification point
is determined as occurring. Sensor transducers are used to
monitor the roller gap position and provide feedback to the
control system.
-
A preferred embodiment of the apparatus for setting and
/ or controlling a roller gap is illustrated in Figure 12. The
apparatus comprises a hydraulic cylinder 1301 with a sensor
transducer 1302 which are mounted to the equipment frame,
together with position control valves 1303 and digital
controllers 1304. Power for the hydraulics is taken from the
main machine system. As casting commences, the strand is
tracked with conditions set by the process control computer
1305 and set points down-loaded for setting the optimum roller
gap. This is achieved using position control digital
controllers that control the movement of fluid through the
control valves which in turn move each hydraulic cylinder and
set the roller gap. Feedback for the roller gap is taken from
the sensor transducer 302 monitoring the cylinder stroke
movement 1306 and thus the roller gap.
-
The apparatus is designed to control the setting of
roller gaps for the following equipment:
- top zone - two exit cylinders to give dynamic
liquid core reduction;
- bender - two entry and two exit cylinders to give
dynamic liquid core reduction;
- segments - two entry and two exit cylinders to give
dynamic soft reduction.
-
-
Entry and exit hydraulic cylinders are arranged at the
corners of each unit as shown (as numeral 1401 in Figure 13a
and 13b and as numeral 1501 in 14a and 14b). The top zone
roller gap is controlled at the exit side by two hydraulic
cylinders and pivots at the entry side where the roller gap is
fixed. The bender and segments are controlled at the entry and
exit side by two hydraulic cylinders on each side (ie by
providing independent roller gap movement and setting).
-
As liquid core reduction is removed at the end of the
cast, the roller gap in the segments is designed to pivot by
movement of the hydraulic cylinders thus minimising the liquid
core reduction taper and maximising slab yield. Figure 14b
shows the segment roller gap pivot movement in the open
position as compared to the tapering position of Figure 14a.
-
There are a number of soft reduction and liquid core
reduction methods which may be used in accordance with the
present invention. Preferred embodiments are illustrated
schematically in Figure 15. Thus, parts a and b of Figure 15
illustrate dynamic liquid core reduction with dynamic soft
reduction and static liquid core reduction with dynamic soft
reduction. Figures c and d illustrate no liquid core reduction
and dynamic soft reduction. Figure e illustrates heavy line
taper.
-
Dynamic liquid core reduction with dynamic soft reduction
allows dynamic positioning of all strand containment units
below the mould. Set points for roller gaps are automatically
controlled to maximise accuracy. The control system includes
dynamic spray control and on-line thermal tracking together
with an apparatus for roller gap setting.
-
Static liquid core reduction with dynamic soft reduction
allows static positioning of the top zone and bender by
clamping onto preset thickness packers and dynamic positioning
of soft reduction for the segments.
-
No liquid core reduction with dynamic soft reduction
allows dynamic positioning of soft reduction for the segments.
This embodiment is appropriate if it is desirable not to have
the extra slab thickness flexibility offered by liquid core
reduction.
-
The most cost effective method of setting the strand
roller gap is to include line taper by static clamping onto a
preset packer thickness and adjusting these packers between
casts if required. This system ensures that some degree of
soft reduction (albeit at a reduced rate) is always applied
without requiring complex control systems and dynamic
movements. Table 2 summarises the various alternatives
relating to liquid core reduction and soft reduction giving
indications of likely effects on product quality.
| The effect of Liquid Core Reduction and Soft Reduction |
| Base = Base proposal which includes line taper only |
| i) Dynamic liquid core reduction with dynamic soft reduction DLCR + DSR |
| ii) Static liquid core reduction with dynamic soft reduction SLCR + DSR |
| iii) No liquid core reduction but with dynamic soft reduction DSR |
| iv) Heavy line taper HLT (Benefit - compared to Base case) |
| ITEM | BASE | I
DLCR + DSR | II
SCLR + DSR | III
DSR | IV
HLT |
| INTERNAL QUALITY | 0 | | | | |
| SURFACE QUALITY | 0 | X 1 | X 1 | 0 | 0 |
| SLAB EDGE SHAPE | 0 | XX | XX | 0 | 0 |
| THICKNESS FLEXIBILITY | 0 | | | | 0 |
| ROLL LOADS | 0 | XX | XX | X | 0 |
| MAINTENANCE | 0 | XX | XX | X | 0 |
| CAPITAL COST | 0 | XX | XX | X | 0 |
