U.S. patent application number 10/547607 was filed with the patent office on 2007-04-05 for continuous casting method.
Invention is credited to Hubert Sommerhofer, Peter Sommerhofer.
Application Number | 20070074846 10/547607 |
Document ID | / |
Family ID | 32749075 |
Filed Date | 2007-04-05 |
United States Patent
Application |
20070074846 |
Kind Code |
A1 |
Sommerhofer; Hubert ; et
al. |
April 5, 2007 |
Continuous casting method
Abstract
The invention concerns the continuous casting of metals of all
kinds by direct cooling with liquid metal or ionic liquid as
coolant, for direct cooling of the strand (4). According to the
invention, the coolant is directed to the strand (4) in at least
one jet (10) and flows in a turbulent flow in this jet.
Inventors: |
Sommerhofer; Hubert; (Graz,
AT) ; Sommerhofer; Peter; (Graz, AT) |
Correspondence
Address: |
KOLISCH HARTWELL, P.C.
200 PACIFIC BUILDING
520 SW YAMHILL STREET
PORTLAND
OR
97204
US
|
Family ID: |
32749075 |
Appl. No.: |
10/547607 |
Filed: |
February 24, 2004 |
PCT Filed: |
February 24, 2004 |
PCT NO: |
PCT/EP04/01794 |
371 Date: |
October 25, 2006 |
Current U.S.
Class: |
164/486 ;
164/444; 164/487 |
Current CPC
Class: |
B22D 11/1241
20130101 |
Class at
Publication: |
164/486 ;
164/487; 164/444 |
International
Class: |
B22D 11/124 20060101
B22D011/124 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 28, 2003 |
EP |
03450060.3 |
Claims
1. A process for continuous casting of metals, whereby liquid metal
or ionic liquid is used as coolant for direct cooling of the
strand, characterized in that the coolant is forcibly directed at
the strand in at least one jet with turbulent flow.
2. The process according to claim 1, characterized in that the
coolant is chosen from the group consisting of: lead, tin, bismuth,
gallium, indium or alloys thereof.
3. The process according to claim 1 characterized in that the
coolant has, in Centigrade Celsius, a melting point which is lower
or equal to 60% of the melting point of the casting material in
Centigrade Celsius.
4. The process according to claim 1, characterized in that the at
least one jet streams into coolant filling an entire gap-like room
between a surface of the strand and an inner surface of a pipe
which surrounds the strand.
5. The process according to claim 1, characterized in that the
coolant flows essentially in the direction into which the strand is
moving.
6. The process according to claim 1, characterized in that the jet
has the form of a conical wall jet and in that its Reynolds number
amounts to at least 15 000 and preferably to over 25 000.
7. The process according to claim 1, characterized in that the jet
comprises a plurality of individual jets with circular cross
section and in that its Reynolds number amounts to at least
5,000.
8. The process according to claim 1, characterized in that oxygen
or an oxygen containing gas, is supplied to the strand upstream of
the point where the jet(s) hit(s) the strand.
9. A device for a process according to claim 1, with a storage tank
for the cooling medium, a heating element and a pump, with pipes
which connect the storage tank with a cooling device for the strand
and eventually a heat exchanger which is located in the pipe
transporting the coolant from the cooling device to the storage
tank, characterized in that the cooling device includes at least
one nozzle which directs the cooling liquid directly onto the
strand, preferably in near vicinity of the mould exit, and a
coolant collecting unit.
10. The device according to claim 9, characterized in that the
cooling device (5) includes a pipe (12) arranged around the strand
(4) or its path, respectively, and forming a gap-like room around
the strand (4) which is filled with coolant (8).
11. The device according to claim 9, characterized in that at least
one, coolant distribution unit is provided in near vicinity of the
mould exit, combined with a pipe arranged in some distance from the
at least one coolant distribution unit (6) in the direction of the
movement of the strand and having a second coolant distribution
unit on the upstream end of pipe.
12. The device according to claim 9, characterized in that a
cleaning unit for the surface of the strand is provided.
13. The device according to any of the claim 9, characterized in
that the mould is an insulated mould.
14. The device according to claim 9, characterized in that the
nozzle has the form of a ring-like slit surrounding the strand.
15. The device according to claim 9, characterized in that a
plurality of nozzles are arranged along a ring-like line
surrounding the strand.
16. Device The device according to claim 9, characterized in that
an inlet for oxygen or an oxygen containing gas, is provided
between the mould exit and the nozzle(s) for the coolant.
17. The device according to claim 12, characterized in that the
cleaning unit is outside the cooling device.
Description
[0001] The invention concerns the continuous casting of metals of
all kinds where liquid metal is used as coolant for directly
cooling the strand and a device for such a cooling method according
to the introducing parts of claims 1 and 9, respectively.
[0002] There are several documents in the prior art which show such
a process:
[0003] The U.S. Pat. No. 3,430,680 A discloses a cooling method,
with a vertically oriented pipe, through which the coolant flows
due to gravity. A nozzle is centrally inserted into this pipe, the
metal to cast runs through this nozzle in a liquid state and the
same velocity as the coolant, in order to avoid any shear forces or
other disturbances of the liquid-liquid contact surface. The metal
to cast solidifies from this contact surface towards the centre of
the strand and is finally hard and tough enough to be separated
from the coolant. Beside the enormous difficulties to come to
identical velocities for two separate liquids in a stream
consisting of two components flowing in purely a laminar state, the
cooling efficiency is rather poor for exactly this reasons!
[0004] The U.S. Pat. No. 3,874,438 A discloses a cooling bath of
liquid metal which has its surface underneath the (shape providing)
crucible outlet. The temperature situation is very tricky, the melt
reaches its solidification point at the area of the outlet, shortly
before entering the cooling bath. The cooling bath is provided in a
cylinder and cooled by heat transfer through the side walls of the
cylinder. Special provisions, namely an additional recipient for
cooling liquid are provided for keeping the surface of the coolant
at the chosen height. It is extremely difficult to keep the
solidification temperature within the outlet, one has to take into
account that the strand is still liquid over the greatest part of
its cross section, that thermal energy is set free by the
solidification and that the cooling process along the strand is
changing during the cast, because the coolant gets warmer and
warmer.
[0005] The U.S. Pat. No. 5,344,597 A discloses a very sophisticated
process for the manufacturing of thin sheets of steel out of the
liquid state: A thin layer of molten cast is brought to the surface
of a coolant consisting of molten metal (e.g. led) and swims on it
to a roller bed where it is kept, guided and moved away. The
coolant comes into contact with the lower side of the cast only.
The coolant is transferred from the surface region of the bath to a
cooler and brought back to the bottom area of the bath by a pump.
Due to the one-side-only cooling, due to the fact that the coolant
is underneath the cast and due to the parallel movement of the
sheet to the surface, the heat transport and therefore the cooling
is rather poor and asymmetrical, leading to stress and distortions
of the product.
[0006] A similar process is described in U.S. Pat. No. 4,955,430 A:
The main difference is, how the liquid steel is brought onto the
led and out of it again, the very casting process is the same and
has the same setbacks.
[0007] Two very similar devices and processes, respectively, are
disclosed in U.S. Pat. No. 4,510,989 A and in U.S. Pat. No.
4,751,959 A, respectively: Molten cast, which already has a thin
solid layer on its surface, is brought into a cooling bath
consisting of molten metal and moved in along a bow-like path. The
volume of the bath is much, much bigger than the volume of the
immersed strand therefore, an enormous amount of coolant is
necessary for the process. The coolant is, at its best, randomly
agitated by circulation pumps mounted on the bottom of the vessel.
Great problems arise due to the big surface area of the coolant and
the dangerous vapours and gases resulting from the various
processes in the bath. An other problem lays with the fact that the
strand has to be brought into a curved form and than into an
elongated form again. This causes structural problems and flaws in
the quality of the final product.
[0008] U.S. Pat. No. 2,363,695 A shows an interesting idea: Molten
material, in most cases steel is fed through an thermally insulated
pipe with U-shape to a nozzle which is directed upwards in a vessel
filled with liquid led as coolant and then drawn vertically
upwards. The coolant is kept in the vessel without stirring or
agitating, therefore only moved by the strand and the convection
movement due to temperature differences, meaning without remarkably
movement. This, and the co-movement of the strand and the led
raising due to its warming brings problems with uniform cooling and
long lasting operation cycles.
[0009] SU 863 161 A discloses the casting of a pipe in two steps:
In the first step, a water cooled mould is used to produce a thin
layer of solidified metal on the surface of the strand, in the
second step, the strand is, along a curved path, further cooled by
direct contact with liquid metal. The liquid metal is kept in a
ring-like slit around the cast and is cooled indirectly with water.
Beside problems with the toroidal form of the mould, problems with
the uniformity of complicated heat transfer exist: The heat goes
from the molten core through the solidified surface area to the
liquid metal which is used as coolant, further into the wall of the
mould and into the water which circulates in channels in this wall.
It is nearly impossible to come to a defined and uniform cooling
scheme with such an arrangement.
[0010] U.S. Pat. No. 3,128,513 A discloses a casting process where
molten salt is used as coolant. Therefore, the strand has a higher
density than the coolant and sinks to the bottom of the vessel. The
pressure of the liquid interior of the strand is used to form the
cross section of the strand, but this also brings a lot of problems
and even dangerous risks (outbreak of molten metal, etc.). The
coolant is simply kept in the vessel without any agitating, in some
embodiments, where movable moulds for the strand are used, the
contact between the surface of the strand and the coolant is
hampered.
[0011] JP 62101353 A discloses a conventional casting process for
pipes. In order to form the interior surface correct and smoothly,
a hollow core is inserted at the nozzle. The hollow core is cooled
on its inside with molten metal instead of water, in order to
prevent any danger of direct contact of water and molten metal
(steam explosion) in case of an accident. There is no direct
contact between the strand and the liquid metal.
[0012] Principally speaking, the introduction of continuous casting
is a very effective way of producing semi-finished products.
Products are rolling ingots, extrusion billets, strips and wires,
sometimes also pipes, furthermore forging feedstock and in small
amounts also thixotropic pre-material. Cast materials are
aluminium, copper, magnesium, nickel and their alloys as well as
steel. The high number of parameters influencing the casting
process has led to the development of many different designs of
moulds.
[0013] Usually the casting melt is cooled indirect by a mould as
far as it is necessary to solidify a shell strong enough to carry
the stresses at the mould exit and to resist a breakout of liquid
casting melt. Short behind the mould exit the strand is cooled
directly by water realised as film cooling or as spray cooling or a
two phase cooling with water and air. The direct cooling stage
ensures the solidification of the liquid core of the strand.
Sometimes the second cooling stage is followed by a third one, a
submerging in a water bath or a soft cooling stage by a flow of
air.
[0014] The process variant with an indirect cooling stage in the
mould and a following direct cooling stage with water or water and
air is state of the art, but the disadvantage of this cooling
concept is that friction between mould and strand causes damage of
the new formed surface. Furthermore, the reheating of the
strandshell induced by the arising air gap between mould and strand
as consequence of solidification shrinkage is disadvantageous too.
These two events are leading to defects like surface cracks (in
case of too high friction), segregation and cell size variations,
shell bending significant for the subsurface layer of a continuous
cast strand. In any case the subsurface is very different from the
core of the strand and therefore has to be machined off especially
from rolling ingots. This means that an additional process step is
necessary which leads to additional costs. One approach to reduce
the thickness of the subsurface layer is the application of
lubricant. Many different lubrication systems have been developed
applying lubricant but also lubricant/gas mixtures to reduce the
friction and the heat withdrawal in the mould, but a fully
elimination of the subsurface layer was not possible. Another
approach was to reduce the mould length in order to decrease the
thickness of the subsurface layer requiring a better and therefore
costlier process control system.
[0015] For the production of single crystals, a heated mould is
used in the so called Ohno continuous casting process (OCC), the
mould temperatures are higher than the melting point of the cast
material in order to prevent nucleation at the mould wall and to
ensure axial directional solidification. The necessary heat removal
for this process is realised by direct cooling at only one position
at a defined distance from the mould exit. Strands produced in this
process are always single crystals with a very smooth surface. But
the production of single crystals is not the aim of usual
continuous casting, as the produced strands should be formable by
rolling, extruding or forging or other cold or hot working process
with isotropic properties.
[0016] Between these two process types (casting with cooled mould
and casting with heated mould) lays the possibility to work with an
insulating mould and to cool the strand by direct cooling only.
This also promises a smooth, subsurface layer free strand when
working with correct process parameters. As the active mould length
is very short, this process requires a very fast, accurate process
control system.
[0017] One feature most described processes have in common, the use
of water as coolant, induces a more or less stable steam film on
the cooled surface, depending on the surface temperature and the
cooling water supply density. This leads to a very strong variable
heat transfer coefficient during the direct cooling stage.
Depending on the cooling concept, the properties of the cast
material, the roughness of the strand surface, water supply density
and water velocity as well as water temperature are decisive. But
these parameters are influencing hot tearing, surface crack
development and possible casting rate. Because parameters may
change during the casting process, also the quality of the product
may change.
[0018] The EP 063 832 discloses a concept for the "casting" of a
probe which gets solidified in its mould and is therefore no real
casting process, even less a continuous casting process.
[0019] The DE 41 27 792 discloses to cast a problematic probe into
a pre-heated mould with special geometric properties, where a
special form of solidification takes place. This is a casting
process, but has nothing to do with a continuous casting
process.
[0020] As one may see, there exists a great interest in a simple,
reliable continuous casting process and device which avoids the
mentioned disadvantages without loosing the advantages of the known
processes.
[0021] In order to achieve this aim, the invention proposes to use
one or more jets or streams of liquified metal or ionic liquids as
cooling medium with turbulent flow and, advantageously, an
insulated mould. This makes sure that no water steam film exists at
the surface of the strand and that the coolant hits the strand in a
defined way after a defined treatment. This guarantees that the
cooling properties and characteristics are well defined and
controllable.
[0022] Ionic liquids or designer liquids is the name for a group of
salts composed of organic kations and mostly inorganic anions which
have a melting point below 100.degree. C. They may be used with the
invention as long they do not decompose at the maximal working
temperature of the process or react with the strand under the given
circumstances. In the following description, they are in most cases
not mentioned expressively, but always included when the term
"molten metal" or "coolant" or the like is used.
[0023] The mould consists preferably of an insulating mould, which
enables a solidification of the strand shell in the near vicinity
of the mould exit. This is responsible for the prevention of many
surface defects and the prevention of an unwanted subsurface layer.
Solidification occurs by the influence of the direct cooling. The
direct cooling uses a liquid metal like lead, tin, bismuth,
gallium, indium or alloys of them as well as other liquid metals or
alloys being liquid below the solidification temperature of the
cast metal or alloy.
[0024] The feature of direct cooling in continuous casting with
liquid metal ensures a very constant cooling behaviour, prevents,
it this is wanted, oxidation of the new formed strand surface and
eliminates the danger of explosions as a consequence of the use of
water as coolant fully. Furthermore the hot tearing and cold
tearing may be eliminated by the choice of the cooling metal and
cooling metal temperature at the cooler entry and cooler exit. The
produced strand is substantially free of the well known subsurface
layer usually found in conventional continuous casting processes.
The grain structure of the produced strands can be controlled by
adjusting the coolant temperature.
[0025] In some cases, par example when casting aluminium, or some
other metals, oxidation of the surface may be advantageous, because
it gives a very well defined border to the coolant with respect to
reactions and interactions between the coolant and the strand. In
such cases, air or oxygen may be inserted at the downstream end of
the mould, the mould exit (coquille), but upstream of the place(s)
where the jet(s) hit the surface of the strand. A very simple way
to achieve this is (when vertically casting occurs) to let a small
annular slot between the coquille and the coolant distribution unit
which slot has a connection to the ambient air. If necessary, more
sophisticated supplies may be used.
[0026] Furthermore, it is possible to attach the casting machine to
a rolling unit, since the strand exit temperature can be adjusted
and hence will safe energy costs for reheating. In this process
type no lubricant is necessary, this makes the process easier,
cheaper but also increases the quality of the produced strand, as
it is known that the lubricant also interacts and reacts with the
hot strand surface leading to hydrogen enrichment and other surface
defects.
[0027] The liquid metal as coolant can be directed onto the hot
strand surface as continuous film or jet or as drops. The coolant
distribution unit can be realised by a continuous slot around the
strand perimeter but also may consist of slotted segments at
different angles to the strand withdrawal direction. In order to
increase the heat withdrawal it is possible to add direct cooling
stages to ensure a higher heat transfer area leading to higher
possible casting rates. The mould itself can have any cross section
and be cylindrical or conical getting wider in casting direction.
For lower casting rates it is also possible to realise the direct
cooling step by submerging the coolant distribution unit and the
hot strand into a bath of liquid cooling metal.
[0028] In general it also is possible to operate a conventional
mould with a first indirect cooling step and a secondary direct
cooling step with liquid metal as coolant, but in order to prevent
known surface defects and the inhomogeneous subsurface layer the
cooled mould length has to be very short.
[0029] It is possible to use the casting process according to the
invention for vertical and horizontal continuous casting (or in any
other desired angle). The invention was successful applied for
casting of copper, magnesium and aluminium showing that it can
applied for all non-ferrous metals and alloys as well as for
steel.
[0030] Advantages of the new cooling concept are: [0031] Easier
cooling control, since the heat transfer number is very constant in
comparison to that of a direct cooling with water and in comparison
with the undefined submerging in stagnant coolants consisting of
liquid metals or salts. [0032] No oxidation of the new formed
strand surface, if this is wanted [0033] Smooth surface without
surface defects [0034] No or only a negligible inhomogeneous
subsurface layer of the cast strand exists - (no machining off
necessary) [0035] Grain structure can be controlled by adjusting
the coolant temperature [0036] Hot and cold tearing can be
eliminated by adjusting and controlling the coolant temperature in
the different stages of the cooling stages as well as by the choice
of the liquid metal (or alloy) as coolant [0037] Inline rolling of
the cast strand is possible and would safe energy costs for
reheating [0038] No lubricant necessary [0039] Easier mould
design
[0040] The invention will be described in greater detail under
reference to the drawing. The drawing shows in:
[0041] FIG. 1 a mould according to the invention in a vertical
cross section,
[0042] FIG. 2 an other embodiment of the invention in a similar
view,
[0043] FIG. 3 a third embodiment of the invention in a similar
view,
[0044] FIG. 4 a fourth embodiment of the invention in a similar
view,
[0045] FIG. 5 a fifth embodiment of the invention in a similar
view,
[0046] FIG. 6 a sixth embodiment of the invention in a similar
view,
[0047] FIG. 7 a principal view of the cooling system and
[0048] FIG. 8 a principal view of an strand cooler.
[0049] FIG. 1 shows a strand with vertical withdrawal direction.
The cooling is done in a totally new way, using a complete filled
strand cooler which is, in some ways, operated similar to heat
exchanger known from chemical industry. The melt 1 is sucked from
the tundish 2 (which can be heated) into the mould 3 and solidifies
at the mould exit since the strand 4 is cooled by a liquid metal
coolant 8 over the entire length of a cooling unit. The coolant 8
fills the entire gap-like room 11 between the surface of the strand
4 and the inner surface of a pipe 12 which surrounds the strand.
The temperature of the strand 4 decreases during its movement
through the strand cooler until its end is reached. A strand
cleaning unit 7 ensures the slip off of the coolant from the strand
4.
[0050] But contrary to common heat exchangers, the cold coolant is
fed into the strand cooler 5 and is distributed as it is required
for the cast shape by a coolant distribution unit 6. The coolant 8
leaves the coolant distribution unit 6 either through a slit which
has the form of a ring (depending on the cast shape) and is
directed to the surface of the strand 4 or through a plurality of
openings or nozzles which are arranged along a closed line and are
directed to the surface of the strand too. The variant with the
slit forms a closed, conical "wall" of flowing coolant 8, the
variant with the openings a plurality of jets 10 of coolant 8. In
both cases, it is important that the velocity of the coolant 8,
when leaving the coolant distribution unit 6, is high enough to
make the flow turbulent. The reason for this is, that a turbulent
flow has a much greater capacity of heat transport in the direction
normal (away from the strand) to the flow direction than a laminar
flow.
[0051] The situation in FIG. 1, with the wall or jets 10 flowing in
their very media, means that in some distance from the coolant
distribution unit 6 the combination of the movement of the strand 4
with the movement of the coolant- 8 due to its circulation, forced
by a pump as described later, overcome the jets 10 or wall and the
flow pattern in the coolant 8 induced by them.
[0052] From the mould exit to a coolant collecting unit 9, the
coolant 8 takes up heat from the hot strand 4, thereby heating up.
The coolant collecting unit 9 ensures the required coolant
distribution along the strand perimeter. This process type enables
highest cooling rates but needs an accurate pressure control in the
coolant feed.
[0053] In order to explain the discrimination between laminar and
turbulent flow, reference is made to FIG. 8, which shows a slit of
circular form between two concentric, circular walls, the inner
cylinder having an diameter of "d", the outer, hollow, cylinder an
inner diameter of "D", the hydraulic diameter "D.sub.H" of the slit
is calculated by: D.sub.H=D-d
[0054] The hydraulic diameter "D.sub.H" of an annulus and of
noncircular channels is equivalent to the diameter of a circle with
the same cross section. Hydraulic diameters for different shapes of
the cross section are listed for example in Robert H. Perry (Ed.):
"Perry's Chemical Engineers' Handbook" (Sixth Edition 1984). Pages
5-25 and 5-26 of this publication are hereby incorporated by
reference. If a liquid medium flows through the slit with the
hydraulic diameter "D.sub.H" (given in meters) in the direction
normal to the plane of the drawing, having a kinematic viscosity v
(given in m.sup.2/s) and the mean velocity v (given in m/s), the so
called Reynolds number Re may be calculated by: Re=(D.sub.H
V)/V
[0055] The point, where laminar flow turns over to turbulent flow
is not only dependent on the cross-section of the channel but also
on the shape of the cross sectional area. For Reynolds numbers
(which have, by definition, no dimension) greater than about 12 000
in the case of channels in form of annular slits, the flow is
usually turbulent. According to the invention, the Reynolds number
in case of a "wall jet" should be at least 15 000 and preferably
over 25 000. It has to be mentioned, that the slit in the coolant
distribution unit 6 is not cylindrical, but conical, but the
differences are small enough to be disregarded.
[0056] In a preferred embodiment the coolant distribution unit 6
comprises several individual parts, which can be adjusted against
each other preferably by means of a thread, in order to change the
width of the conical slits in the coolant distribution unit 6. This
enables the operator to change easily the width of the slits, and
thus the Reynolds number, even during operation.
[0057] In case of discreet openings or nozzles with the free
diameter d (in meters), the Reynolds number is defined by: Re=(d
V)/V
[0058] The change from laminar to turbulent flow occurs with such a
geometry at a point between 2 600.ltoreq.Re .ltoreq.4000, depending
of hard to define second order effects. The Reynolds number in case
of such individual jets should be therefore at least 5 000,
preferably over 7500.
[0059] For all usable coolants liquid metals as well as ionic
liquids, the kinematic viscosity may be found in the data sheets or
chemical or metallurgical textbooks, the velocity is given by the
known cross section area (in m.sup.2) of the slit and the volume of
coolant (in m.sup.3) passing per second, the width of the slit
(which is half its hydraulic diameter) is known from the
construction, therefore, with this description at hand, there
exists no problem for the man skilled in the art to come to the
turbulent flow which is used by the invention.
[0060] FIG. 2 represents a process type, in which the cast strand 4
may be cooled softer than in the process type of FIG. 1. The
casting melt 1 is sucked from the tundish 2 (which can be heated)
into the mould 3 and solidifies at the mould exit as the heat is
withdrawn by the coolant in direct contact with the strand 4.
Instead of a pipe 12, a cooling box 13 is provided around the area
where the strand 4 solidifies during its movement. The cooling box
13 serves to collect the hot coolant. At the bottom end of the
cooling box 13 a strand cleaning unit 7 is fixed, it ensures that
no coolant (in a technical sense) is remaining on the strand
surface. The "cold" coolant is distributed along the strand
perimeter as required for the cast strand shape by a coolant
distribution unit 6. After getting in contact with the strand 4,
the now hot coolant flows down to the bottom of the cooling box 13
and then leaves it through the coolant outlet.
[0061] FIG. 3 represents a casting process according to the
invention, and mould, respectively, with a heat withdrawal rate,
which is substantially higher than that of the aforementioned
casting processes shown in FIG. 2. Due to two consecutive cooling
steps a high rate of heat flow away from the strand 4 to the
coolant 9 is achieved. Thereby separate coolant feeds are provided
for each cooling step. The casting melt 1 in the tundish 2 (which
can be heated) is sucked into the mould 3 and solidifies at the
mould exit. The axial heat removal in the strand 4 is, in a first
cooling stage, similar to that according to FIG. 2 but gets
increased by a second cooling stage in an additional cooling unit,
which is similar to the cooling unit shown in FIG. 1. The device
for the first cooling stage consists of a coolant distributor 6
which produces a coolant film 14. The device for the second cooling
stage consists of a coolant distribution unit 6' and an attached
pipe 12, acting as a heat exchanger tube, which ensures a higher
heat removal than cooling stage one. The strand 4 is cleaned
(technical clean) from the remaining coolant 8 on the surface by
the cleaning unit 7. A cooling or collecting box 15 encloses the
whole cooling unit.
[0062] The FIGS. 4, 5 and 6, respectively, show devices similar to
those shown in FIGS. 1, 2 and 3, respectively, but with horizontal
withdrawal of the strand. Continuous casting with horizontal
withdrawal is well known in the art, for the person skilled in the
art, there is no problem to adapt the invention to this version of
casting. The only difference that should be mentioned is, that the
liquid metal has a much higher density than the water which has
mostly been used in the prior art. Therefore, the free applied
coolant in the devices according to FIG. 5 and the first cooling
stage of FIG. 6 must be differently pressurised on the top-side and
the down-side of the strand 4.
[0063] FIG. 7 shows the flow sheet for the whole casting plant: The
liquid metal used as coolant is stored in a tank 16, which needs to
be heated by a heating unit 17 before starting the casting process.
The liquid coolant is pumped by the pump 18 into the cooling unit
5. In the cooling unit 5 it picks up heat from the hot strand 4,
then the hot coolant leaves the cooling unit and gives up this heat
in the heat exchanger 19. Then the cold coolant flows back into the
coolant tank 16. The heat withdrawn in heat exchanger 19 can be
used for different things in any case it may help to safe costs for
energy in a firm. The coolant tank 16 as well as the whole cooling
system needs to be free from air and especially from oxygen, this
is ensured by flushing the coolant tank 16 and the cooling unit 5
with inert-gas 20. As inert-gas 20, all such gases known in the art
are usable, they have to stay inert at the given temperatures at
contact with the coolant and the material of the strand. It is, of
course, advantageous to use the same inert-gas in the storage tank
16 and the cooling unit 5. The whole casting plant can further
comprise a strand withdrawal unit 25 and a Flying saw 26 for
cutting the strand 4 in pieces of certain length.
[0064] In order to come to defined and repeatable conditions in the
cooling unit 5, it is preferred to have sensors for the temperature
(TIC) 21, 22, sensors for the flow rate (FIC) 23 and sensors for
the pressure (PIC) 24 at least near the entrance of the cooling
agent into the cooling unit 5. It is of course advantageous to have
further measuring points within this system.
[0065] The invention is not restricted to the shown and described
embodiments.
[0066] Coolant can be a liquid metal like lead, tin, bismuth,
gallium, indium or alloys of them as well as metals or alloys,
which are having a melting point lower equal 60% of the melting
point of the casting material. Further, it is possible to use
non-metallic liquids, namely any liquid medium, which does not
react with the material of the strand at the relevant temperatures
and which stays in a liquid state at all temperatures involved in
the cooling process. This may be some organic compounds, especially
for strands of low-melting alloys.
[0067] It is not necessary that the storage tank 16 is arranged at
lower level than the mould 3, but for safety reasons, this
arrangement is preferred. If an other arrangement is provided, the
pump 18 and other armatures have to be put to other positions, but
this brings no problem to the man skilled in the art.
[0068] The pipes, the pump 18, the armatures, the sensors 21, 22,
23, 24, the cooling device 5, the pipe-like heat exchanger and
other equipment for the coolant are, given the disclosure of the
invention, readily available for the man skilled in the art of
casting metal, may it be ferrous or not.
[0069] Some additional features and advantages of the invention
are: The casting process can apply one or more direct cooling
steps. The use of liquid metal as coolant prevents, if this is
wanted, the formation of oxide layers on the strand surface. The
adjustment of the coolant feed temperature and coolant flow rate
allows good control of the cooling rate and hence the formation of
grain structure. The use of an insulating mould or, more precisely,
a low heat removal in the mould, prevents the formation of surface
defects and inhomo- geneous subsurface layers. The use of liquid
metal for the direct cooling in continuous casting eliminates the
danger of explosions known from the conventional process using
water as coolant. This increases the safety in cast shops enormous.
For this continuous casting process no lubricant is necessary.
Applying one of the above described process types in horizontal
continuous casting enables inline rolling of the cast ingots in
order to safe energy costs for the reheating of the ingot. The
process eliminates hot tearing and cold tearing when operating at
optimum process parameter (coolant temperatures at different stages
of the cooling unit). The process has no restrictions concerning
the shape of the cast strand or the number of parallel cast
strands.
[0070] The existing plants may easily be adapted to the invention,
existing cooling systems using water my be stripped and replaced by
the new system. The mould itself hardly needs any adaptation, it is
only necessary to have the freezing area at the end of the mould,
therefore, insulated moulds or very short cooled moulds may be best
used.
* * * * *