Note: Descriptions are shown in the official language in which they were submitted.
PROCESS AND APPARA~US FOR TH~ INGOT OR CONTINUOUS C~STING
OF METALS
_________________________________________________________________
The invention relates to a process for the ingot or continuous
casting of metals in a mould having heat-dissipating walls and
also to an apparatus for the performance of the process.
In both ingot casting and continuous casting, peripherally
extending depressions occur at variously regularly intervals on
the surface of the cast product. In ingot casting they are known
as casting marks and in continuous casting as oscillation ~arks.
Due to their notching effect these marks can encourage the
occurrence of cracks in the surface of the strand, thus
contributing towards crack formation during hot rolling of both
ingot and continuously cast material. Investigations have shown
that the formation of these undesirable marks on the surface of
~he cast ingot or strand are due to the fact that, due to the
surface tension of the melt, the melt is convexly curved in the
zone adjoining the heat-dissipating walls of the mould and
solidifies to for~ a meniscus shell at that place.
~he solidlfied menLscus shell forms with the heat-dissipating
walls a gap which can only be partially filled by the after-
~ ~ 3 ~
flowing molten metal during continuous casting, since due tocooling, such after-flowing metal rapidly solidifies on the
walls.
Various proposals have been made to at least reduce the formation
of such casting or oscillation marks on the surfaces of cast
ingots or strands, but such proposals are either unsuitable for
practical operation or have failed to give a satisfactory result
in practical operation.
One prior art proposal to reduce the oscillation mar~s in
continuous casting consists in selecting high frequencies
(f > = 110 min~1) for the oscillation of the mould and the small
oscillatory movements (S = 3 - 10 mm). Since the result of a
casting process using these conditions was unsatisfactory, an
attemp-t was made so to improve the oscillatory conditions by a
lubricating effect, improved by means of pulverulent additives,
between the melt and the cooled mould walls that no defor~ation
of the surface of the molten metal can take place during the
oscillatory movements as a result of slag lying on the surface
of the metal. It is not known whether such a process has
been adopted and found satisfactory in practical operation
(DE 3~ 13 611 A1).
In ano~her prior art process the cooling intensity of the cooled
walls of the mould on the melt in the zone of its surface is
reduced by means of an insert of lower thermal conductivity
disposed in the cooled ~ould wall adjoining the surface of the
melt. This step has proved unsatisfactory in practical operation,
_ 3 _ 2 ~ $ ~
because after only a few castings the insert is cracked and/or
worn, so that faults may occur on the strand surface or even
strand break-outs (~P 0 030 308 A1).
It is an object of the invention to provide a process for the
ingot or continuous casting of metals and also an apparatus
suitable for the perormance of the process, by means o~ both of
which ingots or strands can be cast whose surfaces are as free as
possible from casting or oscillation marks.
This problem is solved in the process according to the invention
by the feature that the metal cast into the mould is kept molten
by the supply of heat in a zone of the surface of the molten
metal adjoining the wall.
In contrast with one of the aforementioned prior art processes,
in the process according to the invention ~he cooling intensity
of the walls on the melt ~or the formation of a solidified shell
is not reduced, but an appropriate supply of heat at the critical
place in the zone of the melt surface adjoining the cooled mould
walls substantially reduces, or even prevents the formation of a
solidified meniscus shell starting from the cooled walls. In
practice, therefore, casting or oscillation marks are no longer
~ormed. Since according to the invention the cooling effect of
the mould wall, for example by the heating or insulating inserts,
ls not reduced and certainly not cancelled out, there is no
prevention of the desirable growth of th~e strand shell increasing
as quickly as possi.ble. Co~parative investigations of the
conventional continuous casting procsss and the process according
~8~3~ ~
_ 4 21421-255
to the invention have shown that using the process according to
the invention the depth of the oscillation marks can be reduced
by more than one power of ten up to two powers of ten.
In an experimental per~ormance of the process accord-
ing to the invention, satisfactory results were achieved when the
depth of penetration of the supplied heat into the melt was up to
15 mm. The width of the zone kept liquid by the supply of heat
should extend at a distance from the mould wall from practically
0 mm to approximately 15 mm. Of course, the determination of the
individual dimensions depends on the cooling intensity of the
walls, the mould cross-section, casting speed and the material of
the melt. Thus, the depth may possibly need to be up to about
50 mm and the width may possibly need to be up to about 30 mm.
The supply of heat overheats the melt to 15C above
the melting point of the metal, so that when the meniscus washes
over on the walls, the gap between the meniscus and the walls is
filled to the maximum extent possible.
According to a preferred feature of the invention, heat
is supplied to the melt from above. An induction heating can
be used for this purpose. Alternatively, however, use can also
be made of a gas burner. To substantiall~ avoid oxidation of the
surface of the molten metal by the gas burner flame, the flame
must be of an only weakly oxidizing nature. Oxidation oE the
steel melt can be reliably prevented iE heat is supplied in-
directly by the heating of the surface of the melt by means of
an .inert gas as intermediate carrier Eor the energy. Heat might
also be supplied indirectly by the heating of casting slag
.
~ ' .
- 5 ~ s~
lying on -the surface of the molten metal. In any case, the
supply of heat must be so devised that only a predetermined
narrow zone of the surface of the metal along the mould walls is
heated, but there is no direct heating of the cooled mould walls.
If a gas burner is used, care must be taken that if the level of
the metal is covered with casting oil or llquid ca~ting slag or
casting powder, the casting oil or casting slay is not displaced
in the zone adjacent the wall, since otherwise the lubricating
effect is reduced at that place or undesirable oxidations occur.
If according to another feature of the invention, the ~as burner
is directed at the melt at an inclination from above with a
component directed against the mould wall, the casting oil or
casting slag remains even in the zone adjacent the wall, so that
the afore-described proble~s do not crop up.
According to another feature of the invention, which is
particularly advantageously used in continuous casting, the melt
is heated in the zone of its surface by electromagnetic induction
fields penetrating laterally into the melt. A suitable selection
of the frequency of the current driving the electromagnetic
induction fields results in a high specific heating power bein~
in-troduced exclusively into the surface zones of the molten metal
which adjoin the wall, but not into the mould walls.
In an apparatus suitable for the performance of the process,
heating devices operative laterally of or above the surface of
the metal are provided in the mould in the zone adjacent the
q ~
- 6 -
wall. The heating devices are ~referably an induction heating or
gas burners directed at the surface o~ the molten metal. In
continuous casting use is preferably made of a heating device
consistin~ of a number of induction coils incorporated in the
mould walls and suitably disposed at the level of the surface of
the melt or of the slag layer.
Embodiments of the invention will now be described in greater
detail with reference to the diagrammatic drawings, wherein:
Fig. 1 is an axial section of a portion of the wall zone of an
ingot casting mould, and
Fig. 2 is an axial section of a portion of the wall zone of a
continuous casting mould.
During the casting of molten metal into a mould 1 having heat-
dissipating walls, due to the surface tension of the molten metal
at the liquid level 3 the melt 2 forms a convexly curved meniscus
in the zone adjacent the wall. The melt solidifies at the place
where the level 3 of the melt rising in direction 7 contacts the
heat-dissipating wall 1. A further supply of molten metal washes
over this solidified zone 4, the melt 2 only partially filling
the gap between the meniscus shell 4 and the mould wall 1, since
the melt is prevented from penetrating the narrow gap by the
ccoling effect of the mould wall 1. Every time the meniscus
shell is washed over by liquid melt, therefore, a linear
depression 5 of the ingot surface is produ~ed at the periphery of
the ingot.
To ensure that the melt solidi~ying on the surface 3 grows into
the inside of the mould as little as possible, disposed above the
level 3 of the melt is a heating device in the form of gas
burners 6 whose jet is directed at the surface 3 of the molten
metal. One component of the jet should also be directed at the
wall 1. Since during ingot castlng the level 3 of the ~olten
metal constantly rises, the heating device 6 must be lifted
synchronously with the rise of the level of the metal. In this
way conditions are maintained identical during ingot casting.
As shown in Fig. 2, during the casting of molten metal 2 into a
continuous casting mould 1, the casting level is constantly
covered with casting powder. This powder melts by contact with
the li~uid steel.
Similarly as in ingot casting, in this case also a solidifying
convex meniscus shell 4 is formed. During the oscillating motion
of the mould, accompanied by the withdrawal of the strand in the
direction 7 and the further supply of molten metal, the meniscus
shell is washed over by metal, which can only partially fill the
gap between the meniscus shell 4 and the mould wall 1, since it
is prevented from flowing into the narrow gap by the heavy
cooling of the wall 1. In every cycle of oscillation of the
mould, there~ore, a linear depression 5 is produced at the
periphery of the strand.
To ensure that the melt solidifying at the surrace 3 grows as
little as possible into the inside of the mould and fills the gap
as completely as possible with the afore-mentioned washing-over
- 8 ~ 2 ~
of the melt, an lnductive heating device 9 is disposed in the
mould wall 1 in the zone of the surface 3 of the molten metal.
The height of the inductor 9 is approximately 30 to 100 mm. The
inductor 9 can be installed fixed in the mould wall 1 since
during continuous casting, due to the continuous withdrawal of
the strand and the continuous supply of molten metal, the surface
of the melt is maintained at the same level.
In another embodiment (not shown) of the invention for continuous
casting, the inductor is disposed not in the wall 1 of the mould,
but immediately in front of said wall, close above the casting
powder ~ covering the surface 3 of the melt.
Experiments have shown that the specific heating power to be
provided must amount to several thousand kW/m2. With a heating
power of approximately 4000 to 8000 kW/m2 and a speed of rise and
casting of continuously cast melt of 0.15 m/min, a depth of
penetration of 15 mm can be achieved for steel. The depth of the
casting marks occurring was less than 0.01 mm. Clearly, with the
higher casting speeds customarily used in continuous casting, due
to the short dwell time of the meniscus in the zone of action of
the heating device 6, a higher specific heating power must be
selected. For continuous casting, therefore, specific heating
powers ~etween ~000 and 30 000 kW/m2 will be required. In a
typical example of continuous casting, the casting speed is
approximately 1 m/min. In that case a specific heating power of
aooo kW/m2 is needed. In the case of a zone at a di~tance of up
~$~
g
to 20 mm from the mould wall 1 to be acted upon with this heating
power, the following powers are obtained for different sizes:
slab size 2000 mm x 260 mm : 720 kW
bloom size 380 mm x 260 mm : 205 kW.
