U.S. patent number 3,886,992 [Application Number 05/257,421] was granted by the patent office on 1975-06-03 for method of treating metal melts with a purging gas during the process of continuous casting.
This patent grant is currently assigned to Rheinstahl Huettenwerke AG. Invention is credited to Horst Abratis, Hermann Maas, Claus Raeune.
United States Patent |
3,886,992 |
Maas , et al. |
June 3, 1975 |
Method of treating metal melts with a purging gas during the
process of continuous casting
Abstract
A method of, and apparatus for, treating metal melts during
continuous casting, wherein the teeming metal is enclosed to a
point below the surface of the metal pool in the continuous casting
mold to avoid contact with the ambient atmosphere and the surface
of the pool is covered with a layer of slag. Small volumes of
purging gas are introduced in bubble form into the metal melt at
the location where such metal melt begins to form a teeming
jet.
Inventors: |
Maas; Hermann (Hagen Atw,
DT), Abratis; Horst (Hagen Atw, DT),
Raeune; Claus (Hattingen, DT) |
Assignee: |
Rheinstahl Huettenwerke AG
(Essen, DT)
|
Family
ID: |
5809217 |
Appl.
No.: |
05/257,421 |
Filed: |
May 26, 1972 |
Foreign Application Priority Data
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May 28, 1971 [DT] |
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2126606 |
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Current U.S.
Class: |
164/473; 164/437;
164/475 |
Current CPC
Class: |
C22B
9/05 (20130101); C21C 7/0043 (20130101); C21C
7/0068 (20130101); B22D 11/11 (20130101) |
Current International
Class: |
C21C
7/00 (20060101); B22D 11/11 (20060101); C22B
9/00 (20060101); C22B 9/05 (20060101); B22d
011/10 () |
Field of
Search: |
;164/55,56,66,82,259,266,281 ;222/DIG.11,DIG.21 ;266/38 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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228,418 |
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Jul 1963 |
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OE |
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1,425,163 |
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Dec 1965 |
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FR |
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445,034 |
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Feb 1968 |
|
CH |
|
1,945,276 |
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Mar 1971 |
|
DT |
|
443,705 |
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Jan 1949 |
|
IT |
|
Primary Examiner: Annear; R. Spencer
Attorney, Agent or Firm: Kleeman; Werner W.
Claims
Accordingly, what is claimed is:
1. A method of treating metal melts during continuous casting for
suppressing the formation of undesirable oxidic non-metallic
inclusions, comprising the steps of enclosing the teeming metal to
a point below the surface of the metal pool in a continuous casting
mold to avoid contact with the ambient atmosphere, covering the
surface of the pool with a layer of slag, and introducing small
volumes of purging gas in bubble form into the metal melt at the
location where such metal melt begins to form a teeming jet, so
that the injector-like action of the melt divides the gas into a
multiplicity of very small bubbles which are carried into the mold
by the melt.
2. The method as defined in claim 1, including the step of
introducing deoxidants and alloying elements together with the
purging gas into the metal melt at the location where such metal
melt begins to form a teeming jet.
3. The method as defined in claim 2, wherein the deoxidants and
alloying elements are introduced continuously at a rate
corresponding to the casting rate.
4. The method as defined in claim 2, wherein the deoxidants and the
alloying elements are added at a rate which progressively increases
as casting continues.
5. A method of treating metal melts during continuous casting in
order to reduce the formation of undesirable oxidic accumulations,
comprising the steps of confining the teeming metal so as to avoid
contact with the ambient atmosphere to a location below the surface
of the metal pool in a continuous casting mold, and introducing
small volumes of purging gas in bubble form into the metal melt
approximately at the location where the metal melt begins to form a
teeming jet, so that the injector-like action of the melt divides
the gas into a multiplicity of very small bubbles which are carried
into the mold by the melt.
6. The method as defined in claim 1, including the step of
introducing at least alloying elements together with the purging
gas into the metal melt at the location where such metal melt
begins to form a teeming jet.
7. The method as defined in claim 6, wherein the alloying elements
are introduced continuously at a rate corresponding to the casting
rate.
8. The method as defined in claim 6, wherein the alloying elements
are introduced at a rate which progressively increases as casting
continues.
9. The method as defined in claim 1, including the step of
introducing deoxidants together with the purging gas into the metal
melt at the location where such metal melt begins to form a teeming
jet.
10. The method as defined in claim 9, wherein the deoxidants are
introduced continuously at a rate corresponding to the casting
rate.
11. The method as defined in claim 9, wherein the deoxidants are
introduced at a rate which progressively increases as casting
continues.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a new and improved method of, and
apparatus for, treating metal melts during continuous casting while
enclosing the teeming metal to below the surface of the metal pool
in the mold to prevent contact with the ambient atmosphere,
covering the surface of the pool with a layer of slag and
introducing gases into the teeming metal.
A method of treating the metal with a purging gas during continuous
casting in a tundish fitted with a pouring pipe has been disclosed
in German petty Pat. No. 7,023,173. The arrangement disclosed in
such patent which consists of a blowing orifice in the bottom of
the tundish in the vicinity of the pouring nozzle has a very modest
purifying effect, since the depth of the bath in a tundish is only
about 30 to 40 cms. and therefore offers only a short path for the
passage of the gas through the steel melt. Hence, the purifying
effect regarding the purging of undesirable gaseous constituents in
the melt and the removal of non-metallic occlusions is very slight.
Moreover, when entering the tundish nozzle the steel may pick up
fresh oxidic impurities. The maintenance and insertion of porous
purging bricks in the tundish is time-consuming and expensive.
It has already been proposed according to U.S. Pat. No. 3,502,249
to deliver major volumes of gas, which as known are needed for
controlling the teeming rate, through the stopper rod to the outlet
nozzle of a ladle. As such these large volumes of gas cause the
metal jet to spatter and they have no significant purifying effect
because they do not enter the metal pool in the mold below its
surface. The spattering of the teeming jet entering the atmosphere
is intended to be prevented by the provision of an expansion
chamber.
In the published French Patent Application No. 2,035,336 it is
proposed to pass gas through the porous wall of a pouring nozzle to
prevent products of oxidation from being deposited and choking the
nozzle, particularly when pouring aluminium-killed steel. The
introduction of gas around the periphery of the teeming jet does
not result in an intimate comixture of gas and steel so that the
purifying effect is very limited. The porosity needed for the
admission of the gas weakens the brick and leads to premature wear.
Moreover, during service difficulties arise in the supply of the
gas.
U.S. Pat. No. 2,005,311 describes the passage of major volumes of
gas through the stopper rod of a ladle for the purpose of cooling
same. This arrangement also has no purifying effect.
Conventional methods of suppressing oxidic impurities during
continuous casting have been unable to prevent coarse and
principally fine impurities from being deposited, particularly in
the marginal zones to depths of about 11/2 cms., a circumstance
which in the further processing of ingots and slabs by rolling
necessitates the expense of subsequent scarfing and/or the
considerable loss of material involved in machining.
SUMMARY OF THE INVENTION
In view of the above-described state of the art it is a primary
object of the present invention to devise a method of, and
apparatus for, treating metallic melts in a manner enabling these
undesirable oxidic accumulations to be substantially reduced.
Another object of the invention is to bring about a satisfactory
dissolution and even distribution of any deoxidants and/or alloying
elements that may be also introduced into the melt.
Furthermore, when casting slabs of rectangular cross-section
another object of the invention is to reduce the frequency of
transverse cracking by controlling the pattern of flow in the
casting head and, when aluminium-containing steels are cast to
prevent the pouring pipes from being choked.
According to the invention these objects and others which will
become more readily apparent as the description proceeds are
achieved with the proposed method by introducing small volumes of
purging gases in bubble form into the melt where the teeming jet is
being formed. The proposed method of introducing the gas into the
center of the teeming jet as the jet forms, in conjunction with the
injector-like action of the teeming metal, results in the gas being
divided into a multiplicity of very small bubbles having diameters
not exceeding about 5 mm. which provide an intimate mixture between
gas and steel. The injector effect also assists in keeping the gas
exit opening clear. The proposed method produces continuous
castings which are substantially free from surface occlusions.
Moreover, macroscopic purity is greatly improved. The small bubbles
are particularly good for picking up oxidic impurities. By
introducing the gas at the point where the teeming jet is in the
course of formation they are forced to travel a long way through
the liquid steel and the probability of collisions between gas
bubbles and oxide impurities and the resultant purifying effects
are thus greatly improved. During the slow ascent of the gas
bubbles in the pool they readily take-up the non-metallic
occlusions and convey them to the slag on the surface of the liquid
pool. The small gas volume has the further advantage that the
surface of the pool is not violently agitated and that there is
avoided contamination of the casting by particles of slag from the
floating slag cover. The small bubbles affect the pattern of flow
in the mold in the direction of reducing descending components of
flow. The small bubbles are carried into the critical surface zones
where they float upwards in the descending steel and thus purge
this zone of the casting which is particularly liable to exhibit
faults. The lift imparted to the descending metal at the
solidification boundary by the rising bubbles affects metal flow
and enables the impurities to float up, so that the probability of
impurities being intercepted by the surface zones is greatly
diminished.
The accumulation of products of oxidation in the nozzles through
which the metal flows cannot occur when the proposed method is
adopted. It is a known fact that the entrainment by the teeming jet
of particles that have grown during periods of deposition leads to
the appearance of larger and injurious occlusions in the steel.
When casting aluminium-containing slabs the proposed method also
operates to prevent the flow orifice from being choked. In the
application of the proposed method to the casting of rectangular
sections the provision of the teeming jet with gas bubbles also
leads to a reduction of the velocity of flow towards the narrow
sides of the section. The thermal stressing of this zone which is
particularly liable to develop cracks is thus reduced and the
occurrence of transverse cracks can be reduced.
In detail the invention can be further developed in the following
advantageous ways.
By introducing, simultaneously with the purging gases, and at the
same point of delivery, deoxidants and/or alloying elements, a
satisfactory dissolution and uniform distribution of the alloying
elements in the casting can be achieved and the deoxidation
products can be precipitated by the action of the gas bubbles.
During continuous casting the velocity of flow is by no means
constant but is subject to fluctuations -- even disregarding the
slower pouring rate when starting. The velocity is adjusted by the
position of a closure means in the pouring vessel or in the
tundish. In order to achieve a very uniform distribution of
deoxidants and alloying elements, even when the pouring rates
fluctuate, the invention proposes to introduce the deoxidants
and/or alloying elements continuously at rates corresponding to the
existing pouring rate. For compensating losses of alloying elements
which have a high affinity for oxygen, such as aluminium and
titanium, losses which increase as casting continues, the invention
proposes to introduce the deoxidants and/or alloying elements at
progressively increasing rates as casting continues.
For performing the proposed method of continuous casting a pouring
vessel is particularly appropriate which contains a closure means
below which a pouring pipe extends to below the surface of the pool
in the mold, and in which these means in their interior contain a
longitudinal conduit which has its bottom end in the region where
the metal begins to form a teeming jet, whereas at its upper end it
is provided with one or more supply means. For example the closure
means may comprise a stopper rod fitted with a cone or ball head.
The purging gases as well as alloying elements, if these are to be
added, in such case are introduced from a supply at the top into
the stopper rod and issue from the exit end of the conduit at the
ceramic head of the rod.
The pouring vessel may in detail and with advantage comprise the
following features:
For the generation of small gas bubbles the exit opening for the
gas should have a cross-section of between 0.2 and 7 sq.mm.
A uniform introduction of alloying elements and deoxidants can be
achieved if at the upper end of the conduit there is provided,
besides the supply means for the gas, a second supply means
comprising a wire roll, drive means and a funnel for guiding the
wire into the conduit. The drive means permits the wire to be
introduced into the steel at the exit end of the conduit, either at
a constant or a controlled rate, the end of the wire being melted
as it makes contact with the flowing steel and being thus uniformly
distributed in the steel. By selecting a suitable rate of feed the
introduction of the alloying element can be at a constant rate.
Since the entry funnel embraces the wire without much clearance
blow-by through the wire supply funnel of a purging gas that is
being introduced is avoided. Purging gases which have proved to be
particularly satisfactory include the noble gases, e.g. argon.
Preferably the volumetric rate of gas introduction should be
between 3 litres (S.T.P.) and 18 litres (S.T.P.) per minute.
For controlling and regulating the rate of introduction of the
additives it is desirable to drive the wire roll by an electric
motor which is speed-controlled according to the position of the
stopper rod.
Moreover, it is also advantageous if the volume of gas introduced
matches the pouring rate. Experiments have shown that an excellent
purifying effect can be achieved when the ratio of the volume of
the poured steel to the volume of the argon gas introduced is
between 40 : 1 and 10 : 1. For adapting the gas volume to the
pouring rate the invention proposes to incorporate a regulating
valve in the gas supply and to control the valve according to the
position of the stopper rod. A conventional needle valve would be
an appropriate control valve for this purpose.
BRIEF DESCRIPTION OF THE DRAWING
An embodiment of the invention will be hereinafter described with
reference to the drawing which shows a tundish and the upper end of
a continuous casting mold in section.
DETAILED DESCRIPTION OF THE INVENTION
In the proposed method of purifying casting melts during the
process of continuous casting the teeming metal 18 is protected
from the ambient atmosphere by being enclosed in a pouring pipe or
tube 15 which extends to below the surface 16 of the pool of metal
in a permanent mold 17. The surface of the pool 16 is covered with
slag 19. Through a suitable device 1, which in the illustrated
embodiment is a stopper rod, a small volume of purging gas is
introduced into the pool 11 at the rate of 10 litres (S.T.P.) per
minute when teeming begins. The gas is admitted from a gas supply 6
which may incorporate a regulating valve 6a of conventional design,
and thus only schematically depicted, and controlled with reference
to the position of the stopper rod 1 by means of a suitable control
mechanism 20. The ratio of the teemed volume of steel to the volume
of gas i.e. argon introduced is 20 : 1. The stopper rod 1 contains
a conduit 2 extending axially along its length and ending at
location or exit opening 3 in the sealing cone 4 centrally outside
the seating surface 5 in the region where the metal enters the
pouring pipe. When exclusively gas is introduced the diameter of
the exit opening 3 is between about 0.5 and 3 mm. For example, a
diameter of 1 mm. for a pouring rate of 1.3 tons/minute has proved
to be most advantageous. When casting slabs for the production of
sheet material the surface faults were reduced to below 50 percent
of their usual frequency when using untreated melts. Faults in the
interior of the sheets in two rolling mills were found to be only
38 and 28 percent of the number of faults found to occur when using
untreated melts. Besides a purging gas alloying elements and
deoxidants in the form of wire 14 may also be introduced through
the conduit 2. In such a case the exit opening 3 should be such
that a free cross-section for the issuing gas of between 0.2 and 7
sq.mm. still remains. The wire 14 is introduced from a driven
supply roll 7 into the conduit 2 through a funnel 8 which surrounds
the wire with little clearance. When this arrangement is used for
pouring, the steel 11 which is covered by a protective layer of
slag 10 in the tundish 9 flows past the exit of the conduit 2 and
entrains the purging gas, such as argon, in the form of small
bubbles 13 and simultaneously melts away the end of wire 14 which
may be drawn from the supply roll 7 and fed down the conduit 2 by
standard drive means 21, such as a speed-controlled electric motor,
the speed of which may be controlled by the control mechanism 20 as
a function of the position of the stopper rod 1. The steel 11, the
bubbles of argon 13 and the dissolved alloying wire 14 travel down
the pouring pipe 15 with the exclusion of air to a point below the
surface of the metal pool 16 in the mold 17. The gas bubbles 13
rise in the mold 17 and precipitate non-metallic occlusions in the
covering slag layer 19.
Instead of the stopper shown in the illustrated embodiment, the
closure may have the form of a sliding gate nozzle. For introducing
the gas and any possible additions in such case the means 1 above
the sliding gate would be a lance instead of a stopper rod.
While there is shown and described present preferred embodiments of
the invention, it is to be distinctly understood that the invention
is not limited thereto but may be otherwise variously embodied and
practiced within the scope of the following claims.
* * * * *