Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02383075 2007-08-24
1
INGOT MOULD FOR THE CONTINUOUS CASTING OF STEEL INTO BILLET
AND COGGED INGOT FORMATS
The invention relates to a chill mould for the continuous
casting of steel.
In the continuous casting of billet formats and small bloom
formats, nowadays use is made almost exclusively of tubular
chill moulds, the mould cavity of which is defined by a
chill-mould tube. Such chill-mould tubes consist, as a
rule, of a tube made of copper or a copper alloy having a
wall thickness of 8 - 25 mm, which is produced by a large
number of costly operations. Chill-mould tubes made of
copper or a copper alloy are, as a rule, cold-drawn in
order to achieve an increase in hardness that gives the
chill-mould tube the requisite strength. In addition to
the material costs, the measures for hardening the material
and for shaping, in particular, force up the costs of
production. The chill-mould tube is provided with a
casting cone in the mould cavity and is provided with a
smooth wall or with smooth walls on the outside. In many
cases the mould cavity is endowed with coverings made of
chromium and nickel which are applied by electrodeposition.
with a view to cooling such tubular chill moulds, water is
forced through at high speed, e.g. at 6 - 14 m/s, on the
outside of the copper tube in a water gap. For uniform
cooling of the copper tube, a water gap having a regular
width is required. The water gap is determined, on the one
hand, by the external dimension of the copper tube and, on
the other hand, by a water jacket that is matched to this
external dimension.
In the continuous casting of billet and bloom formats the.
copper tubes represent wearing parts which have to be
replaced after 120 - 200 castings on account of scratches,
warpage etc. With a view to increasing the economic
efficiency, various processes have become known which all
CA 02383075 2002-02-15
2
have as their objective making such costly copper tubes
suitable for use for a second and possibly a third time.
The wear pattern of such chill moulds is characterised, as
a rule, by warpage and cracking in the region of the
surface-level of the bath, caused by the high thermal
stress, and by abrasive wear and scratches in the lower
half of the chill mould. If such flaws in the mould cavity
are removed by machining, the mould cavity increases in
size and the cross-sectional dimension of the cast strands
becomes larger.
In order to avoid such enlargements of the cross-section of
the strand, the explosive deforming of chill-mould tubes on
a mandrel that is matched to the dimension of the mould
cavity has become known. Other pressing processes for
reforming the widened tubes have also become known. All
these reforming processes, such as explosion recalibration
or press recalibration, have as a common disadvantage a
reduction in the external cross-section of the chill-mould
tube. As a result of this reduction in cross-section, the
water gap between the chill-mould tube and the water jacket
becomes enlarged in uncontrolled manner, which in turn
exerts a disadvantageous influence on the cooling of the
chill mould.
The object underlying the invention is to eliminate the
disadvantages in the state of the art that have been
described and, in particular, to redesign the structure of
the chill mould for tubular chill moulds in such a way that
the costly production of billet chill moulds and bloom
chill moulds with cold-drawn tubes made of copper or copper
alloys can be avoided. A further objective is seen in a
chill-mould structure that has a substantially longer
lifespan and is capable of being brought back to desired
CA 02383075 2007-08-24
3
dimensions by recalibration in the region of the mould
cavity.
According to the invention, this object is achieved by a chill mould for the
continuous casting of steel in billet and bloom formats, consisting of an
inner
body forming the mould cavity, which is cooled by means of a cooling medium,
characterised in that the inner body comprises a coating substrate which is
manufactured from aluminium or an aluminium alloy and is provided on the
mould-cavity side with a coating which after its introduction into the mould
cavity
is brought to the dimension of the mould cavity by a processing operation.
With the chill mould according to the invention it is
possible to overcome the disadvantages in the state of the
art that have been described in the case of tubular chill
moulds and to avoid the costly production of billet chill
moulds and bloom chill moulds from cold-drawn copper tubes.
In the case where the coating is renewable, it is possible
to recoat the coating substrate as often as desired,
without thereby changing casting parameters such as strand
format or water gap. By virtue of the freedom that is
granted as regards the design and the choice of material of
the coating substrate, the heat output of the chill mould
can easily be adapted to specific requirements. The
coating, which is introduced in the form of a thick layer
and is brought to the desired dimension of the mould cavity
by a preferably metal-cutting processing operation, can
also be adapted with respect to cooling capacity and, if
desired, also with respect to wear to the specific
requirements in the course of continuous casting, depending
on the continuous-casting parameters, for example the
casting temperature or the composition of the steel. It is
presupposed that the coating exhibits an appropriate strength
at the casting temperature.
CA 02383075 2007-08-24
4
In the case of the tubular chill moulds the chill-mould tube
has to guarantee a high extraction of heat, on the one
hand, and the requisite stability under load, on the other
hand. The operating life during casting operation is
regarded as a measure of the stability under load. At
least two factors contribute to the stability of a chill-
mould tube. The stability of a chill-mould tube is
determined, on the one hand, by its capacity to withstand
the high thermal loading in casting operation, conditioned
by the contact with a melt on the inside, accompanied by a
simultaneous intense cooling on the outside. The stability
of a chill-mould tube is further determined by its capacity
to withstand the mechanical stresses in casting operation.
In order to enable a sufficient dimensional stability of
the chill-mould tube, its compressive strength has.to be
such that it withstands the pressure of the cooling water,
particularly since the pressure of the cooling water acts
practically on the entire outer jacket of the chill-mould
tube, whereas on the mould-cavity side above the casting
level no corresponding counterpressure is present and
merely a counterpressure that increases with the spacing
from the casting level is brought~ about by the melt.
Copper tubes which, despite the thermal and mechanical
loads in casting operation, are intended to display an
acceptable stability under load usually have - depending on
the casting format - wall thicknesses of 8 - 25 mm. With
increasing wall thickness the extraction of heat is reduced,
even in the case of materials displaying high thermal
conductivity. In the case of the chill mould according to
the invention there is freedom to optimise the requirements
with regard to the dissipation of heat and the stability of
the inner body forming the mould cavity independently of
one another through the choice of suitable materials for
the coating substrate, on the one hand, and for the
coating, on the other hand. For example, the coating
substrate may be designed in such a way that it provides
CA 02383075 2007-08-24
for a high mechanical strength of the inner body and
consequently guarantees the desired stability of the inner
body, whereas the coating can be suitably chosen with
regard to its thermal properties and its thickness in order
to optimise the dissipation of heat from the inner body. A
coating substrate that is manufactured from a material
having increased mechanical strength may exhibit a reduced
wall thickness and therefore enable an increase in the heat
extraction of the chill mould. Presupposing that the coating
is renewable, a substantially longer operating life of the
chill mould can then be obtained by repeated repair.
In accordance with the invention it is proposed to
manufacture the coating substrate from aluminium or an
aluminium alloy, for example from the alloy.AlMgSil which
is known as Anticorodal WN 6082. Aluminium or aluminium
alloys exhibit a thermal;conductivity in the range 130 -
220 W/mK. Since the coating substrate in casting operation
is always located at a finite spacing, determined by the
thickness of the coating, from a melt which has been
introduced into the mould cavity and the inner body is, in
addition, cooled, a coating substrate that is manufactured
from aluminium or an aluminium alloy can be maintained in
casting operation at a temperature at which aluminium or
aluminium alloys exhibit a particularly high strength.
Moreover, hardened mouldings made of aluminium or an
aluminium alloy can be produced relatively inexpensively,
for example by extrusion.
The coating can be adapted to the specific requirements in
the course of continuous casting, varied in the
longitudinal direction of the chill mould, and also with
regard to various grades of steel to be cast. A material
displaying high thermal conductivity, for example copper or
a copper alloy having a thermal conductivity of 200 - 400
CA 02383075 2007-08-24
6
W/mK, is preferably chosen for the coating, at least in the
upper region close to the surface-level of the bath. In
the lower region of the mould cavity, harder coatings, made
of nickel for example, are also conceivable.
In order to ensure that the coating substrate in casting
operation does not overheat and displays a high degree of
strength and dimensional stability, even under extreme
conditions, the coating is realised in the form of a thick
layer with a thickness of 0.5 - 5 mm, preferably 1 - 4 mm.
Such a coating can be produced by electrodeposition or by
cladding or by means of thermal spraying, for example flame
spraying or plasma spraying, and can be provided by a
processing operation with a surface that corresponds to the
desired shape of the mould cavity with the requisite
precision.
In addition to the heat extraction or the wear resistance etc,
questions concerning the lubrication of the strand that is
formed can also be taken into account when choosing the
material for the coating. Therefore, according to one
embodiment example, it is proposed to intercalate into the
coating a lubricant for lubricating the shell of the
strand. By way of lubricant, those based on molybdenum
and/or tungsten, preferably MoS2 and/or WSZ, are proposed.
Depending on the choice of materials for the coating
substrate and for the coating, heat extractions can be obtained
that are equal to or even higher than those in the case of
the classical chill mould pertaining to the state of the
art that has been described, even if the thermal
CA 02383075 2007-08-24
7
conductivity of the coating substrate is lower than the
thermal conductivity of the renewable coating. The wall
thicknesses, particularly of the coating substrate, that
are definitive for the transmission of heat can be made
relatively thin.
With a view to enlarging the surface that the coolant flows
around, according to one embodiment example the coating
substrate can be provided with cooling fins on the side
facing away from the mould cavity. With a view to
adjusting the cooling parameters, a spacing between the
cooling fins of, for example, 5 - 8 mm can be chosen. In
the case of such structural designs the wall thickness of
the coating substrate between the cooling fins may amount
to 2 - 10 mm, preferably 5 - 8 mm. Together with a copper
coating of 3 mm, for example, a coating substrate with such
thin wall thicknesses guarantees a high extraction of heat.
It is conceivable for the coating substrate to be produced
with appropriate cooling fins in one pressing operation
from a cold-pressable aluminium alloy. It is also possible
to assemble the coating substrate from several parts and
subsequently to coat it on the inside. Coating substrates
for chill moulds with a polygonal mould-cavity cross-
section can, for example, be assembled from several flat or
curved sheets which each form one of the side walls of the
chill mould bounding the mould cavity.
The materials that differ from the classical tubular chill
mould impart to the chill mould according to the invention,
given optimal choice of the wall thickness of the coating
substrate and of the thickness of the coating, a number of
properties which can be utilised with advantage with regard
to casting operation and the structural design of casting
installations. The chill mould according to the invention
affords advantages with regard to the use of an electro-
magnetic stirrer on the outside of the coating substrate.
CA 02383075 2007-08-24
7a
Given optimal selection of the material of the coating
substrate, in comparison with known chill moulds with an
identical stirrer an enhanced stirring capacity can be
achieved or a lower-power stirrer can be used for the
purpose of achieving the same stirring effect. For,
compared with copper or copper alloys, aluminium or
aluminium alloys result in a significantly lower
attenuation of the electromagnetic field generated by an
electromagnetic stirrer. On account of the use of
aluminium or an aluminium alloy for the coating substrate,
the chill mould according to the invention is relatively
lightweight in comparison with a corresponding chill mould
CA 02383075 2002-02-15
8
made of copper or a copper alloy. In the case of the chill
mould according to the invention, on account of the lower
weight the oscillation of the chill mould that is necessary
in casting operation can be implemented with simplified
means in comparison with a corresponding chill mould made
of copper or a copper alloy. The lower weight generally
results in easier handling of the chill mould according to
the invention, particularly in the course of interchange or
in the course of installation and removal and in the course
of transportation of the chill mould. All the measures
associated'with transportation of the chill mould can be
implemented by simplified means.
Moreover, aluminium acts absorptively to a lesser degree
than copper in respect of radioactive radiation. The chill
mould according to the invention therefore exhibits
increased transparency in respect of radioactive radiation
in comparison with a comparable chill mould made of copper
or a copper alloy. This property of the chill mould
according to the invention can be used with advantage with
regard to the layout of devices for measuring the level of
the bath surface of a melt which has been introduced into
the mould cavity of the chill mould. The level of the bath
surface of a melt is conventionally determined with the aid
of a measurement of the transmission of radioactive
radiation through the walls of the chill mould at right
angles to the casting direction. The chill mould according
to the invention permits such transmission measurements to
be carried out with enhanced sensitivity and optionally
permits working with weaker radioactive radiation sources
and/or with simpler measurement technology.
The invention is additionally elucidated in the following
on the basis of examples. Illustrated are:
CA 02383075 2002-02-15
9
Fig. 1 a vertical section through a chill mould,
Fig. 2 a horizontal section through the chill mould
along line I-I in Fig. 1 and
Fig. 3 a vertical section through another example
of a chill mould.
A billet chill mould or bloom chill mould 3 with a mould
=cavity 4 for the continuous casting of steel is represented
schematically in Figs. 1 and 2. Such chill moulds are
cooled intensely with a cooling medium, preferably with
cooling water. The direction of flow of the cooling water
is represented by arrows 5. The structure of the chill
mould is as follows. On the mould-cavity side a coating
substrate 6 bears a highly thermally conductive renewable
coating 7 made of copper or of a copper alloy having a
thermal conductivity of 200 - 400 W/mK. This coating 7 can
be applied onto the coating substrate 6 by electro-
deposition. But it can also be applied by thermal
spraying, for example flame spraying or plasma spraying, or
by cladding. After application of the coating 7 in a
thickness of 0.5 - 5 mm, preferably 2 - 4 mm, the mould
cavity 4 is brought to the desired dimension and to the
desired surface finish by a processing operation. For the
processing of the surface of the mould cavity, all
processes that are known in the state of the art can be
employed; particularly suitable are metal-cutting
processing operations such as milling, grinding, spark
erosion or processing operations involving laser beams. A
lower and an upper sealing plate of the chill mould are
represented by 10, 10' . A jacket is represented by 9.
The choice of the material of the coating substrate 6 is
oriented with priority towards the stability under load for
the purpose of performing the supporting function and
towards good dimensional stability at elevated temperature.
The strength of the coating substrate 6 should be higher
10
than that of the coating at the temperatures that are
realised in casting operation. Aluminum or aluminium
alloys enter into consideration as materials for the
coating substrate. In the production of a coating
substrate 6 the excellent properties of aluminium and
aluminium alloys in the course of pressing, for example,
can also be a decisive factor. Coating substrates 6 that
are assembled from several parts can also be used without
disadvantages, because the coating in the mould cavity
covers the seam points between the individual parts
seamlessly. The coating substrate can, for example, be
constructed from several parts which are held together by
means of welding, with the aid of suitable fastening means
such as screws or rivets, or in some other way.
The.coating substrate 6 in this example is provided with
cooling fins 11 on the side facing away from the mould
cavity 4. In order to obtain an appropriately large
cooling surface, the spacings between the cooling fins 11
amount to 5 - 8 mm. The wall thickness 12 of the coating
substrate 6 between the cooling fins 11 can also have a
thin dimension at 2 - 10 mm, preferably 5 - 8 mm.
In Fig. 3 a chill mould 20 with, by way of example, square
cross-section is provided with a stirring device 21. By
virtue of the different structure of the chill mould in
comparison with classical tubular chill moulds, the
stirring device 21 can be brought closer to the mould
cavity 22. It is also possible to optimise the material
for the coating substrate 23 and for the jacket 24 with
regard to the demands as regards operation of the
electromagnetic stirring device 21. For example, the
strength of the electromagnetic field that is generated by
the stirring device 21 in the mould cavity 22 can be
maximised by a suitable presetting in respect of the
electrical conductivity of the coating substrate 23. The
CA 02383075 2002-02-15
CA 02383075 2002-02-15
11
use of aluminum or an aluminium alloy in this context
affords advantages on account of the relatively low
electrical conductivity of these materials.
A coating 26 consisting of a highly thermally conductive
material is applied in the bath-level region 25 or in the
upper half of the chill mould, and a coating 28 consisting
of a material that is harder than copper, for example.
nickel, is applied in the lower part or the lower half of
the mould cavity.
Lubricants (indicated by dots) for lubricating a strand
crust are intercalated in the coatings 26 and 28.
Lubricants based on molybdenum and/or tungsten, preferably
MoS2 and/or WSz, can be intercalated in the course of
introduction of the coating, for example by flame spraying,
into a highly diverse range of coating materials. Other
lubricants known in the state of the art that are capable
of being intercalated into coatings are also included
within the spirit of the invention.
In the examples of Figures 1 - 3 only straight chill moulds
are represented. But the invention is not restricted to
such chill moulds having a straight mould cavity. All
chill moulds for the continuous casting of steel in billet
and bloom formats that exhibit a tubular coating substrate
fall within the subject-matter of the invention. The
geometry of the mould cavity can be chosen arbitrarily.
For certain steel alloys, in particular peritectic steels,
it can be advantageous if an interlayer 29 consisting of a
material having lower thermal conductivity than copper, for
example nickel, is applied in the region of the bath
surface-level 25 between the highly thermally conductive
coating 26 and the coating substrate 23.
fi CA 02383075 2002-02-15
12
In the course of application of the coating it is possible
to embed measuring probes, for example temperature sensors,
into the coating at selected points. Prior to application
of the coating the measuring probes to be embedded can be
arranged with great precision on or close to the surface of
the coating substrate to be coated and in the course of
application of the coating can be covered with the material
forming the coating. In this way the measuring probes can
be arranged within the coating without having to rely,
after application of the coating, on producing bores that
terminate in the coating and that are suitable for
receiving the measuring probes. As is generally known, the
positioning of measuring probes in bores can be controlled
only in relatively imprecise manner. Such inexactitudes,
which represent a cause of inaccuracies in measurements by
means of the measuring probes, are avoided if the measuring
probes - as described above - are embedded in the coating
in the course of production of the coating.
Aluminium is a relatively base metal. Parts made of
aluminium or an aluminium alloy therefore have a tendency
towards corrosion in the event of a connection to other
metals that is obtained via an electrolyte. The corrosion
resistance of the coating substrate of the chill mould
according to the invention can be achieved with known
means, for example by applying suitable protective layers
at exposed points.
