Note: Descriptions are shown in the official language in which they were submitted.
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The invention relates to a process for the continuous
casting of steel and employing an intermediate container with
bottom-pouring means and a pass-mould, the surface of the molten
metal in the mould being covered by a multi-phase mixture of
liquid inert gas and a particulate substance.
In the continuous casting of steel it is known to
place casting powder on the surface of the molten metal, that part
of the powder that is in direc~ contact with the liquid steel
melting and exercising a lubricating action between the forming
strand and the wall of the mould. When casting products of larger
size, a casting tube is customarily used in conjunction with cast-
ing powder, which tube protects the steel, is introduced into
the mould to extend below the level of the molten metal and, in
the zone of that level separates the casting powder or layer of
slag from the strand as it is being cast. When casting smaller
sizes of product, e.g. billets, it is however hardly possible to
use casting tubes since, because of the cross-sections in question
and the necessary wall-thickness of the casting tube, there is
too little space between the tube and the cooling wall of the
mould. This results in the formation at this point of a bridge
of unfused casting powder and/or of solidified steel which can
lead to rupture of the casting operation. If howe~er casting
is carried out using casting powder but without a casting tube,
there results the disadvantage that particles of slag are drawn
i~to the interior of the strand by the stream of molten steel,
and the steel is thus contaminated. ~herefore, smaller sizes of
product are normally cast using oil as the lubricant. In this
system, on the one hand the surface of the molten metal in the
mould is no longer protected against atmospheric oxygen, while on
the other hand this leads to an u~satisfactory lubricating effect,
par~icularly when casting free-cutting steels, since the oil is
decomposed by the high temperature, and the carbon-containing
residues do not provide sufficient lubrication. This results in
surface defects which lead ~o cracks, particularly during further
shaping operations such as rolliny and, possibly, drawing, and
result in products that are unusable or are capable of meeting
only lower quality standards. Furthermore, the effect of the oil
can result in the occurence of what are known as pin-prick pores
which likewise have a deleterious effect upon the surface of the
strand. A further disadvantage encountered in the casting of
free-cutting steels, especially lead-bearing steels, and when
using oil resides in the fact that during casting intermittent
small eruptions accompanied by spurts of hot mekal from the mould
can occur, so that the men operating the installation are at risk.
When casting powder and a casting tube are used in the
casting of larger sizes of product such as blooms and slabs
one of the functions of the casting powder is to protect the
surface of the molten metal in the mould against atmospheric
oxygen and to absorb impurities contained in the molten steel.
The movements of the surface of the molten metal necessitate, for
this purpose, a layer of casting powder having a thickness of
2 n approximately 1 cm. Only that part of the casting powder in
direct contact with the surface of the molten metal liquefies
and serves ~o carry out lubrication. A layer of caked casting
powder is still present between this liquid layer and the pulver-
ulent top layer. The oscillatory movement not only draws liquid
but also caked and pul~erulent casting powder into the gap
between the wall o the mould and the molten or solidified steel.
This mixture of casting powder has a less efficient lubricating
effect than liquefied casting powder, and this results in a strand
surface of poorer quality and having pronou~lced osciLlation marks
and casting powder inclusions. Oscillation marks promo-te the
formation of cracks and lead to increased scarfing losses.
In the case of stainless steels, casting powder inclusions lead
to increased grinding losses.
It is also known to use a homogeneous multi-phase
mixture when continuously casting a metal into a mould with
the poured stream of metal open i.e~ uncovered. In this p~ocedure
a particulate substance, for example particles of soot are
introduced into a lique~ied inert gas and are intended to improve
the lubricating effect along the wall o the mould. In this
system, the multi-phase mixture is applicd to the poured stream
of metal by what is known as a phase separator. By way o~ the
poured stream of metal, i.e. along this s~ream, the multi-phase
mixture reaches the surface of the molten metal in the mould and
protects the metal against the action of the atmosphere. Because
of the introduction of the multi-phase mixture into the mould by
way of the poured stream of metal and particularly when fairly
large quantities of particulate substance are used, the particles
are carried into the casting head by this poured stream of metal,
and this leads to impurities. Also, when using this procedure
for applying the multi-phase mixture to the poured stream of
metal, trouble can occur due to ex~ernal influences, such as
for example draught. The solid particles of the multi-phase mixture
block the relatively small openings in the nozzles o~ the phase
separator so that the protective effect of tle poured stream is
unsatisfactory and this has a disadvantageous effect on the
quality of the cast material. In the case of larger sizes of
product, even distribution of the multi-phase mixture and there~ore
of the solid bodies over the surface of the tnolten metal in the
mould by way of the poured stream is not ensured, and this leads
to uneven lubrication along the wall of the mould. Wavering of
the poured stream following bottom-pouring cannot be prevented
for various reasons, for example irregularities in the pouring
nozzles~ This can result in the phase separator being sprayed
with steel and this leads to trouble in supplying the multi-phase
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mixture and to stoppages in the castin~ operation.
The object of the present invention is to provide a
method of applying a multi-phase mixture that, on the one hand,
ensures trouble-free feed of the mixture, and, on the other hand,
results in a high ~egree of purity of the cast steel and an
improved surface.
This object is achieved in that during casting, the
- multi-phase mixture of liquid inert gas containing a suspension
of solid bodies, which fuse after separation from the mix-ture
when in contact with the molten steel, is applied directly to
the surface of the molten metal in the mould.
Surprisingly, it has been found that the application
of the multi-phase mixture directly to the surface of the molten
metal can largely prevent non-metallic particles fxom being carxied
into the casting head. The solid bodies only perform the function
of providing lubrication which is improved by the fusing of
these solid bodies. The inclusion of solid bodies in the surface
of the strand can be prevented. The multi-phase mixture takes
over the function oE providing protection against atmospheric
oxygen. Troubles at the feed apparatus due to spatter from the
poured stream can no longer occur.
According to one feature of the invention, in casting
with an open stream of poured rnetal the multi-phase mixture is
applied at a distance ~rom the poured stream that is greater than
half the diameter o this stream. When the poured stream strikes
the surface of the molten metal in the mould a wave is formed in
the zone around the poured stream, and the height and flow of
this wave form a small protective wall which prevents the solid
bodies from entering the zone in the i~nedaite vicinity of the
poured stream and from being carried ~y this stream into the
casting head. The open stream of poured material between the
intermediate conkainer and the surface of the molten metal is
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protected against atmospheric oxygen b~ the vaporizing inert gas.
The impact of the poured stream on the surface of the molten
metal eliminates a large part of t~le kinetic energy, so that the
depth of flow in the sump is reduced an~ this has a favourabl~
effect upon the quality of the cast steel.
When casting larger sizes of product and using an open
poured stream and the downwardly directed flow necessitated
thereby, pronounced cooling of the steel in the zone of the surface
of the molten metal in the mould cannot be prevented, and this
leads to the formation of a top crust and therefore to considera~le
reduction in quality in the cast strand~ According to the
invention, this disadvantage is eliminated in that the steel
emerging from the pouring orifice in the bottom of the intermediate
vessel is introduced into the mould below the surface of the
molten metal and is caused to flow towards and along said surface.
In acGordance with a further feature of the invention,
a quantity of 1~15 kg of liquid inert gas and a quantity of
0.01-0.2 kg of casting powder are provided as the multi-phase
miY~ture for each tonne of cast steel. The small quantity of
casting powder has the effect of permitting only liquid casting
powder to reach the zone between the wall of the mould and the
steeL, and this results in a good lubricating action. The gas
present in the zone of the wall of the mould prevents casting
powder, contained in the liquid nitrogen, from reaching the wall
of the mould so that the oscillatory movement does not cause any
unfused casting powder to become mixed with the liquid casting
powder, and this prevents the powder form ~eing included in the
surface of the strand. Thus, losses due to dressing operations,
such as for example grinding losses in the case of stainless
steel, are reduced. The liquid casting powder performs the
additional function of adsorbing non-metallic inclusions rising
to the surface of the molten metal. When casting billets it is
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possible to dispense with oil lubrication and this leads to
elimination of the above-descri~ed disa~vantages particularly
when casting lead-bearing free-cutting steels~
In order to achieve an even distribution of the multi
phase mixture over the su.rface of the molten metal when producing
products of larger size, it is preferred, in accordance wi-th the
invention, to apply the multi-phase mixture directly to the
surface of the molten metal at several places.
Nitrogen is advantageously used as the liquid inert gas.
The invention will now be descrihed by reference to some
embodiments and to the drawings illustrating them.
In the drawings:
Fig. 1 is a vertical section on line I-I of Fig 2 through a
mould wherein the multi-phase mixture is supplied to the surface
of the molten metal,
Fig. 2 is a plan view of the Fig. 1 arrangement, and
Fig. 3 is a section through part of an intermediate container
having a casting tube and through part of a mould of a second
embodiment.
Referrinq to Fig. 1, molten steel flows from an .-
intermediate container, not illustrated, as an open poured stream
2 into a billet mould 1. In the zone of the casting head 3 the
steel commences to solidify to form a crust 4, the thickness of
whlch increases along the length of the strand. During casting,
a multi-phase mixture is applied directly to thc surface 5 of the
molten metal by way of a pipe 6, this mixture consisting of
liquefied inert gas and, suspended therein, fusible solid bodies
which are advantageously in the form of casting powder. A mixture
of liquid inert gas with a little casting powder suspended therein
and of unliquef.ied inert gas forms on thc surface 5 of the molten
metal as a layer 7. Advantageously 1-15 kg of liquid inert gas,
for example nitrogen, is supplied or each tonne o steelO The
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suspended quantity of castiny powder is 0.01-0.2 kg in the
above-mentioned quantity of gas. The liquid nitrogen is fed to a
pipe 6 through a phase separator 30 as described in DT-OS
26 06 871. The casting powder is introduced into the pipe 6 with
the aid of a low-pressure powder distributor 31, a consistent
suspension being important. The multi-phase mixture to be applied
to the surface 5 of the molten metal is regulated with the aid of
a valve 32. Instead of casting powder, other fusible solid boclies
capable of forming a suspension, for example glass powder,
paraffin etc., can be added to the liquid inert gas. The outlet
of the pipe 6 is located just above the surface of the molte~
metal and at a di~tance from the poured stream 2 that is greater
than half the diameter of this stream in the zone of the surface
of the molten metal. Advantageously, the multi-phase mixture is
applied near the wall of the mould, say at a distan~e of 1 4 cm
therefrom.
As shown in Fig. 2, the poured stream 2 has a diameter ~.
It should be mentioned that the poured stream 2 does not always
have the ideal circular cross-section. When the poured stream
2 strikes the surface 5 of the molten metal, a wave 9 forms in the
zone around the area of impact of the poured stream, the zone 10
terminating at a di~tance fxom the poured s~ream 2 that is
approximately equal to half the diameter of said stream. This
wave, caused by the kinetic energy of the poured stream, and the
inert gas which vaporizes around the poured stream prevent the
casting powder from reaching the~zone immediately adjacent the
poured stream and from being carried into the molten casting head
3 by this stream. As the liquid inert gas vaporizes, casting
powder is deposited on the surface o~ the molten metal. As this
is happening, the heat given off by the molten casting head causes
the powder to fuse so that it can be drawn, as a film, into the
space between the solidified crust 4 and the wall of the mould.
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Since the unliquefied casting powder in the liquid nitrogen does
not reach the walls of the mould because of the vaporization,
the oscillatory movel~ent of the mould 1 cannot cause any
unliquefied casting powder to find its way ~etween the steel and
the wall of the mould. Thus it is possible, on the one hand,
to maintain an optimum lubricating action with the small quantity
of casting powder and, on the ~thcr hand, to prevent non-metallic
particles from being carried into the casting head 3. Furth~rmore,
the inert gas provides protection against atmospheric oxygen both
for the surface of the molten metal and, i~ i~s gaseous form,
for the poured stream issuing from the intermediate container.
Fig. 3 shows an intermediate container 15 having a
bottom-pouring orifice 16. The steel issuing from the orifice
16 is carried below the surface S of the molten metal in a slab
mould 20 by means of a casting pipe 17. The casting pipe 17 has
two upwardly directed bores 21 drilled therein which impart to the
steel a flow 22 which is directed towards and along the surface 5
of the molten metalO The liquid nitrog~n with the cas~ing powder
suspended therein is applied directly on to the surface 5 of the
molten metal through two pipes 6 and 6'. If required the layer 7
of the multi-phase mixture ca~ be applied at ad~itional places.
As a result of the flow 22 hot steel is always supplied to the
surface of the molten metal so that a top crust cannot form~ while
at the same time non-metallic inclusions can settle in the liquid
layer of slag lying on ~he surface of the molten metal.
