U.S. patent number 3,670,089 [Application Number 05/068,637] was granted by the patent office on 1972-06-13 for apparatus for electroslag remelting of metals with molten slag introduction.
This patent grant is currently assigned to Institut Elektrosvarki Im. E. O. Patona. Invention is credited to July Georgievich Emelianenko, Mikhail Markovich Kljuev, Jury Vadimovich Latash, Boris Izrailevich Medovar, Boris Evgenievich Paton.
United States Patent |
3,670,089 |
Paton , et al. |
June 13, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
APPARATUS FOR ELECTROSLAG REMELTING OF METALS WITH MOLTEN SLAG
INTRODUCTION
Abstract
An electroslag plant or apparatus in which molten slag can be
introduced through the lower portion of a crucible device, either
through its sidewall or bottom plate, into the bottom of the
remelting zone, which occurs in an electroslag remelting process,
in an amount sufficient to achieve a predetermined depth in the
remelting zone. The plant can can use single or plural consumable
electrodes and in some embodiments has a bottom plate weld lug to
enhance efficient current flow. The achievement of the
predetermined depth is signalled when current flows as a result of
the bottom introduced slag contacting and closing a circuit through
an energized electrode pre-positioned in the remelting zone.
Inventors: |
Paton; Boris Evgenievich (Kiev,
SU), Latash; Jury Vadimovich (Kiev, SU),
Medovar; Boris Izrailevich (Kiev, SU), Emelianenko;
July Georgievich (Kiev, SU), Kljuev; Mikhail
Markovich (Elektrosal, SU) |
Assignee: |
Institut Elektrosvarki Im. E. O.
Patona (Kiev, SU)
|
Family
ID: |
3618863 |
Appl.
No.: |
05/068,637 |
Filed: |
September 1, 1970 |
Related U.S. Patent Documents
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Application
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Filing Date |
Patent Number |
Issue Date |
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592054 |
Nov 4, 1966 |
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61014 |
Jul 9, 1970 |
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592054 |
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10419 |
Feb 11, 1970 |
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10485 |
Feb 11, 1970 |
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592054 |
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Foreign Application Priority Data
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Oct 25, 1966 [AU] |
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A 9973/66 |
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Current U.S.
Class: |
373/46; 164/515;
164/492 |
Current CPC
Class: |
H05B
3/023 (20130101); H05B 3/60 (20130101); H05B
3/0009 (20130101); C22B 9/18 (20130101); F27D
11/04 (20130101); F27D 3/18 (20130101) |
Current International
Class: |
F27D
11/00 (20060101); F27D 3/00 (20060101); C22B
9/18 (20060101); C22B 9/16 (20060101); F27D
3/18 (20060101); F27D 11/04 (20060101); H05B
3/02 (20060101); H05B 3/60 (20060101); H05b
003/60 () |
Field of
Search: |
;13/9,9ES ;75/10
;249/79,109,106 ;164/348,252 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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47,457 |
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Jan 1909 |
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CH |
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1,162,094 |
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Jan 1964 |
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DT |
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Other References
"Potential of Electroslag Refinings," Metals 2(10), p 44-48, March
1967 .
"Electoslag Refining Technology Unit," Metals 1(1), p. 48 June 1966
.
"9000 LB. Ingot Produced by Electroslag Remelting," Light Metals
and Metal Industry 28(330), p. 49, Nov. 1965. .
"Electroslag Refining in the U.K.," Steel Times, p 92-97, July 15,
1966.
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Primary Examiner: Gilheany; Bernard A.
Assistant Examiner: Envall, Jr.; R. N.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation in part of abandoned
applications Ser. No. 592,054, entitled "a Method of Electroslag
Remelting of Metal and Plant Effecting Same" filed Nov. 4, 1966 and
Ser. No. 61,014 entitled "Method of Electroslag Remelting of Metal
and Plant Effecting Same" filed July 9, 1970 as a division of Ser.
No. 592.054. Also, this application is a continuation of Ser. No.
10,419 entitled "Method and Apparatus for Electroslag Remelting of
Metals" filed Feb. 11, 1970, now abandoned; and Ser. no. 10,485
entitled "Slag Introduction Method For Electoslag Remelting
Process" filed Feb. 11, 1970, also abandoned. These two abandoned
applications in turn being continuations in part of Ser. No.
592,054.
Claims
What is claimed and desired to be secured by Letters Patent is:
1. A plant for electroslag remelting of metal from consumable
electrodes comprising: crucible means including a bottom plate and
a separable crucible placed on said bottom plate; means for
disposing portions of consumable electrodes in said crucible means
with lower ends thereof located adjacent the bottom of said
crucible means; and means located in the lower part of said
crucible means enabling introduction of molten slag from outside of
said crucible means directly into the bottom portion of said
crucible means through a channel means having at least a portion
thereof formed by said separable crucible.
2. A plant for electroslag remelting of metal from at least one
consumable electrode comprising: a crucible means including a
bottom plate and a crucible device placed on said bottom plate; a
holder for said consumable electrode enabling the insertion of
portions of said electrode into said crucible device; a pouring
device for supplying molten slag into the lower part of said
crucible means; said pouring device communicating therewith through
a channel formed due to linking of said bottom plate with said
crucible device.
3. A plant as defined in claim 2, wherein said channel is formed by
an external boring provided in said bottom plate, covered from
above by end surfaces of said crucible device and said pouring
device.
4. A plant as defined in claim 3 wherein said external boring is of
trapezoidal cross-section.
5. A plant as defined in claim 2, wherein said channel is formed on
the lower end of said crucible device by a radial boring covered
from below by said bottom plate.
6. A plant as defined in claim 2, wherein said channel is formed by
borings provided in the lower end of said crucible device and
outside said bottom plate, said borings facing each other.
7. A plant as defined in claim 2, wherein said syphon pouring
device is made detachable along the longitudinal plane of its
channel.
8. In an electroslag remelting furnace the improvement comprising:
crucible means which comprises a bottom plate and a separable
crucible placed on said bottom plate and a conduit means adjacent
the lower portion of said crucible means for the introduction of
molten slag from outside said crucible means into the bottom
portion of said crucible means, said conduit means having at least
a portion thereof formed by said separable crucible.
9. A furnace as defined in claim 8, wherein said aperture is, at
least in part, formed through said crucible means at a point above
said bottom plate.
10. A furnace as defined in claim 8, wherein a consumable electrode
means extends into said crucible and wherein said aperture is
formed through said crucible means at a point below the lowermost
end of said electrode means.
11. In a furnace for electroslag remelting of metal from at least
one consumable electrode extending into a crucible means and having
means for causing relative motion between said electrode and said
crucible means and wherein said crucible means comprises a bottom
plate and a separable crucible placed thereon, the improvement
comprising: conduit means extending into the lower portion of said
crucible means and having at least a portion thereof formed by said
base plate and said separable crucible, and said conduit means
extending outwardly and upwardly thereof whereby sag poured into
the outer and upper portions of said conduit means is introduced
into the lower portion of said crucible means.
12. A furnace as defined in claim 11, wherein said conduit means
enters said crucible means at such a point to permit introduction
of slag into said crucible means at a point below the lowermost end
of said electrode.
13. A furnace as defined in claim 11, wherein said conduit means
enters said crucible means at a point above its said bottom
plate.
14. A furnace as defined in claim 11, wherein said crucible means
comprises a sidewall having an outwardly extending flange at the
bottom thereof mounted on said bottom plate and wherein at least a
portion of said conduit means comprises a recess in said bottom
plate extending beneath said flange.
15. A furnace as defined in claim 14, including upwardly extending
funnel means connected to said recess, said funnel means being
constructed to receive molten slag poured thereinto outside of said
crucible means and to deliver said slag under gravity flow through
said recess into said crucible.
16. A furnace as defined in claim 15, wherein said funnel means is
removably attached to said bottom plate.
17. A furnace as defined in claim 15, wherein said funnel means
extends upwards sufficiently to hold molten slag at a higher level
than the lowermost end of said electrode when it is pre-positioned
to a furnace start-up location.
18. A furnace as defined in claim 11, wherein a plurality of
electrodes extend into said crucible.
19. A furnace as defined in claim 11, wherein the cross sectional
area of said conduit means ranges from 6 to 120 square
centimeters.
20. A furnace as defined in claim 19, wherein said conduit means
extends into said crucible means a distance ranging from once to
twice the depth dimension of said conduit means.
21. A furnace as defined in claim 11, wherein the electrode is so
placed that the gap between the electrode and crucible wall ranges
from 5 to 100 millimeters.
22. A furnace as defined in claim 20, wherein said conduit means is
at least partially lined with a refractory material.
23. A furnace as defined in claim 14, wherein said recess has a
cross sectional area of 6 to 120 square centimeters at a point
beneath said flange and extends into the interior of said crucible
means beyond said sidewall a distance of once to twice its depth
beneath said flange.
24. A furnace as defined in claim 23, including upwardly extending
funnel means connected to said recess, said funnel means being
constructed to receive molten slag poured thereinto outside said
crucible means and to deliver said slag under gravity flow through
said recess into the interior of said crucible means.
25. A furnace as defined in claim 24, wherein said funnel means is
removably attached to said bottom plate.
26. A furnace as defined in claim 25, wherein funnel means extends
upwardly sufficiently to hold molten slag at a level higher than
the lowermost end of said electrode.
27. A furnace as defined in claim 26, wherein said funnel means
extends upwardly sufficiently to hold molten slag at a level
ranging from one-fourth the smallest transverse sectional dimension
of said ingot to be formed to twice the smallest transverse
sectional dimension of the ingot to be formed.
28. A furnace as defined in claim 14, and including a funnel means
having a funnel with a conduit extending downward therefrom and
having at the lower end of said conduit a flange attached to said
bottom plate and communicating with said recess.
29. A furnace as defined in claim 28, wherein said conduit extends
upwardly at an acute angle to the vertical.
30. A furnace as defined in claim 28, wherein said recess extends
from the interior to the exterior of said crucible means, passing
beneath said sidewall, said sidewall flange and said conduit flange
to communicate with said conduit.
31. An electroslag refining furnace system comprising: a water
cooled crucible means including a bottom plate and a crucible
device, said crucible means having consumable electrode means
extending into the top thereof and current supply means connected
into a circuit with said electrode means to initiate flow of
current when electrically conducting slag contacts said electrode
means; and fluid communicating means from exterior to interior of
said crucible means comprising conduit means in the lower portion
of said crucible means, said conduit means having at least a
portion thereof formed by said base plate and said crucible device,
whereby molten slag poured into said upwardly extending conduit
means flows by gravity through said conduit means into the bottom
of said crucible means to create in said crucible means a rising
pool of molten slag that reaches a level to contact said electrode
means whereby said current flow commences.
32. In a furnace for electroslag remelting of metal having a
crucible with wall means defining a furnace start-up remelting zone
by the lower portion thereof and at least one consumable electrode
adapted to depend into said crucible with its lowermost end in said
remelting zone and having means for applying electrical power to
said electrode to melt the same, the improvement comprising:
conduit means through said lower portion of said crucible
communicating with said remelting zone and including a slag
conducting aperture opening into said remelting zone; recess means
formed in said wall means of said crucible in the remelting zone
thereof; and weld lug means in and pressed against at least a
portion of said recess means to provide electrical contact between
said weld lug means and said crucible.
33. A furnace as defiend in claim 32, wherein said slag conducting
aperture is located in said crucible wall means at a point above
the lowermost boundary of said remelting zone.
34. A furnace as defined in claim 32, wherein said slag conducting
aperture is located in said crucible wall means at a point above
the lowermost end of said consumable electrode.
35. A furnace as defined in claim 32, wherein said crucible
includes a bottom plate at the lowermost end thereof and said slag
conducting aperture is located in said bottom plate.
36. A furnace as defined in claim 35, wherein a second aperture is
located in said crucible wall means.
37. A furnace as defined in claim 36, wherein said weld lug means
is of the same composition as the ingot to be formed in said
crucible.
38. A furnace as defined in claim 32, wherein a plurality of
consumable electrodes are adapted to depend into said crucible.
39. A furnace as defined in claim 32, including upwardly extending
funnel means connected to said slag conducting aperture, said
funnel means being constructed to receive molten slag poured
thereinto outside said crucible wall means and to deliver said slag
by gravity flow through said aperture into said remelting zone.
40. A furnace as defined in claim 32, wherein a plurality of
consumable electrodes are adapted to depend into said crucible and
wherein said crucible has at its lowermost end a bottom plate, and
said means for applying electrical power comprises an AC power
circuit including said electrodes and a current balancing line
connected to said bottom plate whereby current flowing through said
electrodes can be increased or decreased by said circuit in order
to maintain equal rates of melting of said electrodes.
41. A furnace as defined in claim 40, wherein said AC power circuit
includes the secondary winding of a transformer and wherein there
are two consumable electrodes, each connected to an opposite side
of said secondary winding and wherein said balancing line is
connected as a center tap from said secondary winding.
42. A furnace as defined in claim 40, wherein said weld lug means
is in direct electrical contact with both of said bottom plate and
said balancing line.
43. In a furnace for electroslag remelting of metal having a
crucible with wall means defining a remelting zone by the lower
portion thereof and plural consumable electrodes adapted to depend
into said crucible with their lowermost ends in said remelting zone
and having means for applying electrical power to said electrodes
to melt the same, the improvement comprising: conduit means
extending into said remelting zone of said crucible and including a
slag conducting aperture communicating with said remelting zone,
weld lug means pressed against a recess formed in the wall means of
said crucible in the remelting zone thereof to provide electrical
contact between said weld lug and said crucible, and an AC Power
circuit including said electrodes, said power circuit having a
current balancing line connected to said bottom plate whereby
current flowing through said electrodes is increased or decreased
by said current to maintain equal rates of melting of said
electrodes.
44. An electroslag refining furnace system as defined by claim 31,
including a slag melting apparatus adapted to melt slag and deliver
it to said fluid communicating means.
45. An electroslag refining furnace system as defined by claim 31,
wherein said current supply means is adapted to supply melting
current to said electrode means until an ingot of predetermined
size is formed in said water cooled crucible means and to then
terminate the supplied current, and wherein said system includes
electrode holder means for moving said electrode means with respect
to said crucible means.
46. An electroslag refining furnace system as defined in claim 31,
wherein said electrode means comprises at least two electrodes and
wherein said current supply means is adapted to supply melting
current between said electrodes from the time the electrically
conducting slag contacts said electrode means.
47. An electroslag refining furnace system as defined in claim 31,
wherein said current supply means delivers alternating current to
said electrode means.
48. An electroslag remelting furnace system comprising a receptacle
for forming an ingot, said receptacle including a water cooled
bottom plate and a separable water cooled crucible; a support
column; electrode holding and moving means disposed on said
supporting column adapted to support at least one electrode
extending within said crucible with the lower electrode and
initially disposed a predetermined distance from said bottom plate;
electrical means for supplying current to said electrode holding
means and an electrode when held by the same; fluid communicating
means extending from the exterior to the interior of said ingot
receptacle comprising channel means in the lower portion of said
ingot receptacle, at least a portion of said channel means formed
by an upper portion of said base plate and a lower portion of said
crucible; and an upwardly extending pouring device outside said
ingot receptacle in communication with said channel and extending
vertically from the base plate a relatively short distance
predetermined in relation to the spacing between said bottom plate
and the lower end of an electrode when held by said holding means;
said construction being such that molten slag poured into said
upwardly extending pouring device flows by gravity through said
channel via a short path into the lower portion of said ingot
receptacle creating therein a rising pool of molten slag that
reaches a level sufficient to contact an electrode held by said
holding and moving means and to cause current flow through the slag
bath and any electrode held in said holding means when voltage is
applied and when molten slag is poured from exterior to interior of
said ingot receptacle.
49. An electroslag remelting furnace system as defined by claim 48,
wherein at least a portion of said channel means is formed by a
depression in said bottom plate, said depression at least in part
covered by a lower portion of said separable water cooled
crucible.
50. An electroslag remelting furnace system as defined by claim 49,
wherein said channel means formed by said depression in said bottom
plate extends from an exterior position to the interior of said
ingot receptacle, said exterior position being spaced from the
outer surface of said separable water cooled crucible.
51. An electroslag remelting furnace system as defined by claim 48,
wherein at least a portion of said channel means is formed by an
opening in the lower portion of said separable water cooled
crucible.
52. An electroslag remelting furnace system as defined by claim 48,
wherein at least a portion of said channel means is formed by both
a depression in said base plate and an opening in said lower
portion of said separable water cooled crucible and wherein said
depression and said opening are in cooperating relationship.
53. An electroslag remelting furnace as defined by claim 48,
further comprising at least one consumable electrode held by said
electrode holding means.
54. An electroslag remelting furnace system as defined by claim 48,
further comprising recess means formed in an interior wall of said
ingot receptacle at the lower portion thereof; and weld lug means
in and pressed against at least a portion of said recess means to
provide electrical contact between said weld lug means and said
ingot receptacle.
Description
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus for electroslag
remelting of metal from consumable electrode means, and
particularly to such an apparatus having provision for introduction
of molten slag at the bottom portion of the remelting crucible.
In the electroslag remelting of metals, a bath of molten slag is
obtained in a remelting zone, for example, a crucible or a mold
(often referred to as a crystallizer). At least one consumable
electrode is disposed in said zone with its lowermost end immersed
in said molten slag bath. Electric current is caused to flow from
the electrode to and through the slag bath. The passage of the
current through the slag bath produces heat which causes the
electrode to melt. As the electrode melts, the remainder of the
electrode is lowered into the slag bath so that all of the
electrode is progressively melted. Because the metal in the
electrode has a density greater than that of the slag bath, a
molten pool of metal is formed below the slag bath. This molten
pool of metal progressively solidifies into an ingot of refined
metal.
Known in the prior art are apparatus for electroslag remelting of
metal, using consumable electrodes in a cooled crucible, disposed
on a bottom plate; for carrying out the remelting process, a pool
of molten slag is formed in said crucible.
The molten slag pool is obtained in the crucible in one case due to
the melting of a solid flux or a mixture of its charge constituents
during the remelting of a consumable electrode directly in the
crucible. In another case, non-consumable electrodes, carbon or
graphite, are employed for these purposes. This method is known as
the "dry start" method.
There is also employed a flux premelted in a separate unit or a
mixture of its charge constituents, followed by top pouring the
molten slag thus obtained into the crucible. This method is
referred to herein as "top pouring."
In the first two cases of preparing the molten slag pool, the time
as required for obtaining an ingot is increased by as much as 10 to
20 percent, since the melting of slag is carried out directly in
the crucible, which is likely to decrease the production rate of
the plant by as much as 10 to 20 percent.
Besides, when preparing the molten slag pool with the use of
consumable electrodes, there occurs an incomplete melting of the
flux in the peripheral zone of the crucible which is likely to
drastically impair the surface of the ingot being melted and to
increase the bottom discard to be cropped during the subsequent
processing of the ingot up to 10 percent.
Though the preparation of the molten slag pool in the crucible by
top pouring therein the molten slag is a progressive method, which
allows increasing the production rate of the plant and ensuring a
high quality of the bottom part of the ingot, this method possesses
its disadvantages, too.
When placing the consumable electrode in the crucible, the gap
therebetween is small, and the pouring of the molten slag therein
presents difficulties. The molten slag gets on the crucible walls
and consumable electrode, and is likely to produce slag sows or
lumps thereon. The falling off of the slag sows into the slag pool
during the melting process may result in marked variations of
electrical conditions of the melting process.
To eliminate said disadvantages requires that during the pouring of
the molten slag the consumable electrode should be outside the
crucible, for which reason the design of the plant must provide for
lifting the electrode clamped in the electrode holder over the
crucible so that the latter could be displaced from under the
electrode for pouring the slag therein.
The short electric circuit is elongated thereby, and consequently,
the losses of active energy increase therein, too, which results in
a reduction of the power factor of the plant (cos.phi.). After top
pouring the molten slag into the crucible, a voltage is applied to
the installation, and the consumable electrode is lowered at a
maximum speed into the crucible until it is brought into contact
with the slag. During this time, a crust or lining of the solid
slag may form on the crucible walls and on the cooled bottom plate
or a dummy bar, if it is to be placed on the bottom plate, which
crust is likely to insulate the molten slag pool from the bottom
plate and crucible, which results in a breaking occurring in the
current circuit, and the melting process may not start.
Disadvantages of the existing plants employed for effecting the
electroslag remelting of metal according to said method, consist in
their excessive height, which is connected with a necessity of
pouring the molten slag with the consumable electrode being raised,
and with considerable losses of time as required for effecting
auxiliary operation. Besides, there are required dummy bars or
sacrificial plates for protecting the bottom plate against the
burning through.
SUMMARY OF THE INVENTION
In conformity with the present invention, the apparatus enables the
slag pool to be introduced into the crucible by pouring the molten
slag into its bottom part or lower portion. The consumable
electrode (or electrodes) is inserted into the crucible until its
lower end is at a predetermined distance from the bottom plate, and
a voltage is applied to the plant simultaneously with the pouring
of slag into the bottom portion of the crucible. When the level of
slag in the crucible reaches the electrode, there occurs the
completion (closing) of the electric circuit of the plant, and the
process of remelting the consumable electrode beings.
Because of a rapid rising of the slag level in the crucible, there
is insufficient time for a crust or lining of solidified slag to
form on the bottom plate (or dummy bar) and on the crucible walls,
and current begins to flow in the electric circuit of the plant,
while the pouring of the molten slag continues to be carried out
while current flows, the pouring of slag being discontinued only
after the formation of the molten slag pool of a specified
depth.
As provided by the present invention, a novel electroslag remelting
plant includes an electrode holder complete with one or more
consumable electrodes, adjustably depending into a crucible placed
on a bottom plate, in the lower part or portion of the plant there
is disposed a pouring device for supplying the molten slag into the
crucible through a channel (access port). The access port or
channel for introduction of the slag can be constructed in many
ways such as a bore or aperture adjacent the lower portion of the
crucible through either or both of the crucible wall or the
crucible bottom plate. For convenience in cleaning out solidified
slag in the channel such channel can be made separable, e.g., it
can be formed by the linking of the bottom plate with the
crucible.
In one novel embodiment of the realization of the present
invention, the channel, through which the pouring device
communicates with the crucible, is formed by an external boring
(elongated recess) in the bottom plate, covered from above by the
end surfaces of the crucible wall and syphon pouring device.
In another novel embodiment of the invention, the channel or
passage for supplying the molten slag is formed in the lower end
portion of the crucible wall by a radial boring or groove, covered
from below by the bottom plate of the crucible. The channel or
passage for supplying the molten slag may be also formed by two
borings or grooves facing each other, said borings or grooves being
located in the lower end portion of the crucible wall and in and to
the outside of the bottom plate of the crucible, respectively.
In conjunction with the foregoing novel aspects, it is expedient to
design the pouring device detachable along the longitudinal plane
of its channel.
The engineering solutions set forth herein allow manufacturing a
plant for electroslag remelting of metal, said plant being simple
in operation and design.
According to a further novel feature of this invention, and in
conjunction with the novel aspect of bottom pouring, the base plate
is provided with a recess in which a piece of metal of the same
composition as the ingot is fitted. This piece of metal is referred
to as a weld lug. The weld lug extends up above the top surface of
the base plate and sideways pressure is exerted against the bottom
of the lug in the recess to press it firmly against the bottom
plate. When remelting begins in the molten slag bath, the top of
the weld lug extending into the bottom of the mold will melt and
weld to the ingot. Thus, excellent electrical contact will be
obtained between the base plate and the ingot.
In accordance with another novel embodiment of the present
invention, wherein provision is made to pour molten slag into the
mold through the bottom portion thereof as described above and
apparatus enable power to be applied between at least two
electrodes which are fed simultaneously as a unit into the mold as
the electrodes melt. The power applied between the electrodes
causes current to flow between the electrodes through the molten
slag thus heating the slag and melting the electrodes. This
technique of energizing the electrodes and supplying heat to the
molten slag greatly reduces the inductance of the system because
the leads supplying power to the system can be and are maintained
close together. As a result, the power factor of the system is made
considerably higher. Because the power factor is higher, much less
power is required to produce a given size ingot and a much lower
capacity transformer may be used to produce a given size ingot.
When a single electrode is used, the application of electrical
power between the electrode and bottom plate during pouring results
in the liquid slag being electrically connected directly to one
side of the power source. In this system embodiment of the present
invention, wherein the power is applied between two electrodes,
such a hazard is eliminated.
An object of the present invention is to provide unique apparatus
which eliminates the above-said disadvantages of the existing
plants for carrying into effect electroslag remelting. This object
is achieved by providing novel apparatus enabling supplying molten
slag to a remelting zone from the bottom part of the crucible
whereby the above disadvantages are overcome or minimized.
It is a further object of the present invention to provide novel
electroslag remelting plants or apparatus incorporating structure
enabling bottom pouring of molten slag to start the remelting
process.
These objects are realized by unique plant and system structure for
electroslag remelting of metal as discussed in the foregoing
"Summary."
DESCRIPTION OF THE DRAWINGS
The nature of the present invention will further become more fully
apparent from a consideration of the following description of an
exemplary embodiment thereof, taken in conjunction with the
accompanying drawings, in which:
FIG. 1 is a general side elevation view of a plant for electroslag
remelting of metal according to the present invention;
FIG. 2 is a front view of the plant shown in FIG. 1;
FIG. 3 is a vertical section detail view of the plant taken along
line A--A of FIG. 2 showing structural details of one embodiment of
the bottom pouring device;
FIG. 4 is a cross-section view, taken along line B--B of FIG.
1;
FIG. 5 is a vertical section view of a second embodiment of a
bottom pouring device according to the present invention;
FIG. 6 is a cross-section view showing the bottom of a furnace
similar to that illustrated in FIG. 4 and shows the use of a weld
lug;
FIG. 7 is a vertical section taken along line C--C of FIG. 6
showing details of the device for clamping the welding lug;
FIG. 8 is an elevation view partially in section schematically
illustrating the system of the present invention as applied to a
bifilar furnace; and
FIG. 9 is a sectional view taken horizontally through the mold
shown in FIG. 8, illustrating the spaced apart arrangement of the
electrodes in the mold.
The proposed plant for electroslag remelting of metal has a
supporting column 1 (FIGS. 1 and 2) complete with carriages 2 and 3
disposed thereon, said carriages being displaced progressively
relative to columns 1 along guides 4 by the aid of drives 5 and
6.
Attached to the carriage 2 is an electrode holder 7 of the clamp
type complete with a drive 8, designed to secure a consumable
electrode 9 moving relative to a crucible 10 during the progressive
motion of the carriage 2.
The crucible part 10, to be placed on its bottom plate 11, is
connected by a bracket 12 to the carriage 3 and during the
displacement thereof it can rise relative to the bottom plate 11,
placed, in its turn, on a carriage 13.
In the lower part of the plant, there is provided a pouring device
14, communicating with the crucible 10 and intended for supplying
therein the molten slag which is premelted in a separate unit, for
example, in an arc furnace.
The pouring device 14 communicates with the crucible 10 through a
channel or passage 15 (FIGS. 3 and 4) which terminates in an access
port, as formed by the linking of the bottom plate 11 with the
crucible 10. Thus, in the FIG. 3 embodiment the channel 15 has
boundaries determined by the top surface of bottom plate 11 and the
end surface of the sidewalls of crucible 10.
In the exemplary, preferred embodiment of the present invention,
represented in FIGS. 3 and 4, the access port in channel 15 is
formed by an external boring or aperture provided in the bottom
plate 11, and is covered from above by the lower end surfaces of
the crucible 10 and pouring device 14. This channel 15 has its
upper surface determined by the lower end surfaces of crucible 10
and pouring device 14 and its lower surface determined by an
extension of bottom plate 11.
To facilitate the removal of slag after the completion of melting,
it is desirable that the boring provided in the bottom plate 11
should have in its cross section a trapezoidal or segment-shaped
form. That is, for ease of removal of the slag from channel 15
after completion of remelting, it is desirable that the
longitudinal and transverse cross section of the channel should be
trapezoidal or segment shaped, as is apparent in FIGS. 3 and 4.
The channel or passage 15 into the lower portion of the crucible
may be formed by a radial boring or aperture (which is not shown in
the drawing) provided on the lower end of the crucible 10 and
covered from below by the bottom plate 11 or by borings or grooves
(not shown in the drawing) provided on the lower end of the
crucible 10 and the bottom plate 11 facing each other. The access
into the crucible can also be formed by spaced apart apertures (not
shown in the drawing), in one or the other or both of the bottom
plate 11 and the sidewall of crucible 10. All these embodiments of
the channel 15 provide for a rapid access thereto for cleaning it
from the slag after the completion of the melting process.
The pouring device 14 is provided in its upper part with a
receiving funnel 16, which may be made as a single piece integral
with it or detachable therefrom. It is expedient to make the
pouring device 14 detachable along the plane of its channel 17, if
the cleaning operation is to be effected immediately after the
pouring of the molten slag into the crucible 10. The pouring device
14 may be made non-detachable, if the cleaning of the channel 17
from the slag is effected after the completion of the melting
process; in this case, however, the channel 17 should have a slight
taper, as seen in FIG. 5, for instance, from 1 to 3 percent, with
the big end down.
The top end of receiving funnel 16 ordinarily is at a distance
above bottom plate 11 sufficient to insure an adequate head of slag
in funnel 16 so that the slag reaches its predetermined depth
inside crucible 10 and contacts the lowermost end of electrode 9.
It is desirable that the lateral end of the pouring device 14
should repeat the shape of the lateral surface of the lower flange
18 of the crucible 10. The pouring device 14 may be fastened to the
lower part of the crucible 10 or to the bottom plate 11, and may be
made of metal or with a lining of the internal channel 17 and
receiving funnel 16. Thus, when pouring device 14 has the same
shape as flange 18 of the crucible 10, the pouring device 14 can be
fastened to the flange 18 of crucible 10 or to bottom plate 11 or
to both flange 18 and plate 11.
Since the pouring device 14 can be made of metal, part or all of
the inside of channel 17, receiving funnel 16 and channel 15 can be
lined as at 20 in FIG. 5, to resist heat.
If desired, heating elements (not shown) can be placed on the
receiving funnel 16 and/or channel 17 in order to maintain or to
increase the temperature of the molten slag as it flows
therethrough.
Apart from the described component members, the plant or apparatus
is also provided with a system for supplying a cooling liquid to
the crucible 10 and bottom plate 11; a system for electric supply
(a transformer, bus bars, and flexible cables); a system for
exhausting gases evolving from the crucible during the melting
process; apparatus for controlling an adjusting the melting
operation, that are not described here in detail as being not
relevant to the essence of the present invention.
The proposed installation operates as follows.
The consumable electrode 9 (or electrodes) is introduced into the
electrode holder 7 and is clamped there by the aid of drive 8.
Then, due to a displacement of the carriage 2, the electrode 7 is
adjusted down into the crucible so that its lower end is disposed
at a distance from the bottom plate 11 somewhat smaller than the
thickness of layer of the molten slag to be poured into the
crucible 10. Hence, when the layer of slag in the crucible is equal
to, for example, 200 mm, the lower end of the electrode 9 should be
spaced from the bottom plate 11 at a distance of 190 mm.
The voltage is applied to the crucible by switching in the
transformer.
The molten slag is poured from a ladle into the receiving funnel 16
of the pouring device 14, and is supplied into the crucible 10
through channels 17 and 15. The pouring of the slag is discontinued
at the moment the level of the molten slag in the crucible reaches
the lower end of the electrode 9, which is evidenced by the current
flowing through the plant circuit.
Thereupon, desirable electrical conditions of the melting process
are preset by the aid of an appropriate apparatus, said electrical
conditions being maintained constant throughout the melting process
involving the building up of the ingot, or may vary according to
the present program, which is effected due to a variation in the
speed of feeding the electrode 9 by adjusting the rotational speed
of the drive 5, and to a variation of the voltage of the secondary
winding of the transformer intended to supply the plant. Thus, as
pointed out above, the electrode is lowered to a depth of immersion
to obtain the desired current flow to maintain the desired slag
temperature. As the electrode melts, it is fed into the mold to
maintain the end of the electrode immersed in the molten slag.
The ingot of a required height having been built up in the crucible
10, the melting process is discontinued, for which purpose feeding
of the electrode is stopped, the transformer switched off, and the
carriage 2 then raised into its upper position. The remaining stub
of the electrode 9 is thereafter removed from the electrode holder
7. Thereupon, the crucible part 10 is raised by the aid of the
carriage 3 until the built-up ingot is made to leave it completely,
whereupon the carriage 13 complete with the bottom plate 11 and
ingot are rolled out aside from the crucible part 10. The ingot is
then removed and the channels 15 and 17 are cleaned from the
solidified slag. Sometimes, with a view of saving time, the pouring
device 14 is to be cleaned from the slag in the course of the
melting process.
Subsequently, carriage 13 together with the bottom plate 11 is
again placed under the crucible part 10, which is lowered onto the
bottom plate. The pouring device 14 is connected thereto, and the
working procedure as described above is repeated.
The proposed plant may be made use of to manufacture ingots of a
round, oval, square, rectangular or any other cross section
depending upon the crucible shape.
The method and plant, realized according to the present invention,
provide for a maximum possible coefficient of utilization of the
working time; allow obtaining ingots with the bottom portion
thereof of a high quality, which permits practically to avoid
cropping the bottom discard; facilitate the rapid performance of
the operation of pouring the molten slag into the crucible and the
preparation of the plant before starting the subsequent melting
process. The greater the weight of the ingot being formed, the
greater is the efficiency of the instant apparatus.
The proposed plant is of a comparatively small height.
Aside from the above-mentioned advantages, the proposed plant
provides for carrying out the process of electroslag remelting
without the use of metallic dummy bars that are to be placed in the
existing units on the bottom plate with a view of preventing its
damage during the beginning of the melting process.
The utilization of the proposed plant proves to be more efficient
the greater the weight of ingots that are to be made therein. It is
also possible to employ one or a plurality of the consumable
electrodes for obtaining an ingot. Thus, one or a plurality of the
consumable electrodes clamped together without insulation between
and with power applied between electrodes and the bottom plate can
be employed to obtain an ingot. Other electrical arrangements can
be used when a plurality of electrodes are employed such as
designing the circuitry so that the applied electric current flows
between the ends of the electrodes when they are in contact with
the molten slag rather than from the electrodes to the bottom
plate. In such an arrangement the current can be caused to flow
between two or four electrodes as shown in FIGS. 5 and 6 of Belgian
Pat. No. 670,299 or between three electrodes as shown in FIG. 4 of
British Pat. No. 979,583 wherein a three phase transformer is used
for the electrical supply.
Referring to the access port of channel 15 as above described, this
access port and the radial cross section of channel 15 ordinarily
have the same area. These areas ordinarily range from 6 to 120 sq.
cm. for circular ingots of 65-1,500 mm diameter (or equivalent
non-round cross sections). The use of these cross sections assures
that the back pressure in channel 15 will not be excessive and that
slag will solidify in channel 15 blocking backflow through the
access port from the crucible and the remelting proceeds.
The above-described lining of the internal channel 17 and receiving
funnel 16, if desired, can be extended into channel 15, using
shields 20 of refractory material such as graphite as shown in FIG.
5, to prevent the molten flux from burning through the funnel wall.
The graphite shields may be 8-10 mm in thickness in a typical
installation.
When electrode 9, (see FIG. 1), is clamped in electrode holder 7,
it is adjustably lowered by means of carriage 2 so that its lower
portion moves into the crucible 10 until its lowermost end is
spaced above the bottom plate 11 a distance from 4 to 20 percent
less than the thickness (that is, the depth) of the layer of the
molten slag to be poured into crucible 10.
Illustrated in FIGS. 6 and 7, there is an exemplary use of a weld
lug. To provide a perfect electrical connection between the ingot
being formed in the mold and the bottom plate 21, a circular recess
22 is provided extending down into the bottom plate 21 from its top
surface which forms the bottom of the mold. A circular piece of
metal 23 of the same composition as the ingot which is to be formed
in the mold by the electroslag remelting process is fitted in
recess 22. FIG. 6 shows a pouring device 14 with a receiving funnel
16 and a connecting channel 17 attached to the mold 10 in a manner
similar to that hereinbefore described for FIG. 4.
A cylindrical passage or bore 24 is defined in the bottom plate 21
extending horizontally from recess 22 to the outer side of the wall
of bottom plate 21. Cylindrical passage 24 slidably mounts a spring
biased clamp pin 25, the inner end of which engages the weld lug
23. The outer end of pin 25 is slidably guided through a U-shaped
bracket 26 mounted on the sidewall of bottom plate 21. Pin 25 is
provided with an abutment collar 27 on the portion disposed between
bracket 26 and the bottom plate sidewall. A coil compression spring
28 surrounds the pin 25 between the collar 27 and the U-shaped
bracket 26 and applies a force against the collar 27 which urges
the pin 25 against the weld lug 23 which presses the weld lug 23
against the side of the recess 22. In this manner an excellent
electrical contact is obtained between the weld lug 23 and the
bottom plate 21. When the molten slag bath is first prepared in the
mold, the heat of the molten slag will cause the top of the weld
lug 23 extending up into the mold to melt and the molten pool,
which is initially formed in the bottom of the mold, will come in
contact with the melted upper portion of the weld lug. As a result,
when the ingot starts to solidify, weld lug 23 will be welded to
the bottom solidified portion of the ingot being formed and an
excellent electrical contact will be obtained between the weld lug
and the ingot and thus between the ingot and the bottom plate
21.
In an embodiment illustrated in FIGS. 8 and 9, a system in accord
with the present invention may comprise a mold 30 including a
bottom plate 32 and sidewalls 34. Although not shown in FIGS. 8 and
9, mold 80 is water cooled by conventional techniques, such as has
been hereinbefore described. A pair of electrodes 36 are positioned
over the mold 30 extending down into the open top of the mold. The
electrodes are supported by an electrode holder 38 mounted on a
carriage 40. Carriage 40 can be moved up and down a supporting
tower 42 to feed the electrodes 36 together as a unit into the mold
30 as the electrodes melt. The sidewalls 34 of the mold are mounted
on a second carriage 44 which also is movable up and down the tower
42. Drive motors, similar to drives 5 and 6 in FIG. 1 can provide
the motive force for the carriages 40 and 44.
As can be seen in FIG. 9, a channel 46 is defined by a groove in
the top surface of the bottom plate 32 extending from inside of the
mold sidewalls to outside thereof and extending into a tongue 48
formed on the bottom plate. The channel 46 is preferably positioned
as shown in FIG. 9 at a point half way between the two electrodes.
A funnel 50 provided with a base plate 52 rests on the tongue 48 of
the bottom plate and closes the top of the portion of the channel
46 which extends out into the tongue 48. The sidewalls are formed
with a flange 54 which abuts against the plate 52 so that the
portion of the channel 46 extending outside of the sidewalls 34 is
completely covered. The passage of funnel 50 connects with the
channel 46. As a result, a closed channel is provided between the
bottom of the interior of the mold and the mouth of the funnel
50.
Each of the two electrodes 36 is connected to an opposite side of
the secondary winding 56 of a transformer 58. The seconding winding
has a center tap to which the mold bottom plate 32 is connected,
preferably by means of a weld lug as hereinbefore described, but
not shown in this embodiment. In operation, the assembly of
electrodes 36 is first lowered into the mold 30 to a position
determined by the desired depth of the bath of molten slag to be
formed in the mold. AC power is applied between the electrodes from
the transformer 58. Then superheated molten slag is poured into the
bottom of the mold 30 through the funnel 50 and the channel 46.
When the molten slag in the mold 30 reaches a depth sufficient to
contact the two electrodes 36, current will begin to flow between
the electrodes through the molten slag thus heating the molten slag
and beginning to melt the electrodes 36. This flow of current will
be indicated by an indicator 60, which for example may be an
ammeter connected in the conductor between one of the electrodes
and the transformer 58. When the technician who is controlling the
pouring of the molten slag into the mold observes that current
begins to flow through the electrodes 36 as indicated by the
indicator 60, he immediately stops pouring the molten slag. In this
manner, by initially positioning the electrode at the proper depth
in the mold, the desired amount of molten slag in the mold is
readily obtained with precision. Because there will be some
reaction time between the indication provided by indicator 60 and
the time that pouring of slag actually stops, the slag will be
poured to a depth a little above the ends of the electrodes in the
mold. The desired amount of slag is nevertheless precisely obtained
by initially positioning the bottom ends of the electrodes just
below the desired level of slag in the mold.
As the electrodes 36 are melted, they are fed into the molten slag
by the carriage 40 moving on the tower 42 to maintain the
electrodes immersed at the desired depth in the molten slag. As the
electrodes melt, they will form a molten pool beneath the bath of
molten slag which will solidify into an ingot starting from the
bottom of the mold with a pool of molten metal being maintained
between the bath of molten slag and the solidified ingot. In FIG.
8, the solidified ingot in the mold is designated by the reference
number 62, the bath of molten slag is designated by the reference
number 64, and the pool of molten metal is designated by the
reference number 66. The connection between the bottom plate 32 and
the center tap of the secondary winding 56 serves to maintain the
melting rates of the two electrodes equal. Should one of the
electrodes melt slower than the other it will become more deeply
immersed in the molten slag. The resistance between this electrode
and the bottom plate 32 will be reduced relative to that between
the other electrode and the bottom plate. As a result, some current
will flow between the center tap and the more deeply immersed
electrode, thus increasing the current flow through the more deeply
immersed electrode relative to the other electrode. This action
results in the more deeply immersed electrode melting at a greater
rate until its immersion becomes less, the current decreases and
melting rate decreases. In this manner, the melting rates of the
electrodes tend to equalize. As the ingot is formed, the bath of
molten slag will rise in the mold 30. When the bath nears the top
of the mold, the melting of the electrodes is ended and the molten
pool of metal at the top of the ingot and the bath of molten slag
is allowed to solidify. After the solidification has taken place,
the sidewalls 34 are stripped from the ingot by moving the carriage
44 up on the tower 42. The sidewalls 34 are conical shaped with the
large end down as shown in FIG. 8 to facilitate stripping.
In this manner, a high quality ingot is produced by an electroslag
remelting system with a relatively high power factor. The ingot can
be produced by remelting of ferrous or nonferrous metals from the
consumable electrodes.
The "dry start" method of obtaining a molten slag bath, mentioned
above as prior art, is time consuming and increases the time for
producing finished product ingots by as much as 20 percent compared
to the time required when the slag is melted outside of the
remelting zone. In addition, the dry start method has the
disadvantage that the aforementioned arcing leads to oxygen release
from the slag whereby the first portion of the metal melted from an
electrode is out of specification. Furthermore, such arcing
ordinarily does not melt the slag at the periphery of the remelting
zone and as a result the heat produced by the current passing
through the slag at the beginning of the remelting process is not
sufficient to adequately refine the metal being produced. As a
result of the oxygen contamination and as a result of the initial
incomplete slag melting, the bottom portion of the formed ingot is
of inferior quality and is ordinarily trimmed or cropped from the
rest of the ingot and reprocessed or discarded. This bottom portion
can amount to up to 10 percent of the entire ingot.
The prior art "top puring" method overcomes the aforementioned
disadvantages of the "dry start" method but has disadvantages of
its own as follows.
If top pouring is carried out with the electrode removed from the
remelting zone, a crust of molten slag is often formed at the
bottom of the remelting zone during the time the electrode is being
lowered into the zone after pouring has been completed. This crust
insulates the bottom of the remelting zone so as to block current
flow whereby the electroslag remelting process is prevented from
starting. When this occurs, the crust-containing slag must be
removed from the remelting zone and a new batch of slag poured.
This phenomenon is referred to as a false start.
Moreover, if the apparatus is designed so that the electrode can be
positioned above the remelting zone, the apparatus is required to
be of greater height than otherwise, requiring more factory space,
and the lead attached to the electrode is required to be longer
whereby inductance is increased so that the power factor is lowered
requiring more power per pound of metal produced.
If top pouring is carried out with the electrode in place in the
remelting zone, then a long electrode of small cross-section must
be used in order to provide a sufficient gap between the electrode
and the sidewalls of the remelting zone so that the slag stream
does not contact and coat either the electrode or the walls with a
scale of solid slag. Moreover, such scale falls in solid form into
the molten slag during the remelting process and either can cause
marked variation in the current applied during remelting thereby
causing non-uniform results or else can be trapped within the metal
melted from the electrode so as to form undesirable inclusions in
the formed ingot.
The length of the electrode utilized in this method results in high
inductance and a lower power factor thereby raising production
costs. In addition, a tall tower must be provided for supporting
and feeding the electrode. Such a tower adds significantly to the
cost of the installation.
In order to ensure that the slag being top poured does not contact
either the electrode or the remelting zone walls, the pouring
stream must be of relatively small cross-section. As a result,
pouring of the slag to a required depth in the remelting zone takes
a significant amount of time so that often a crust of solidified
slag forms at the bottom of the remelting zone whereby a false
start occurs.
Both of the aforementioned techniques of top pouring have the
disadvantage that the slag during pouring reacts with nitrogen in
the air to form nitrides which dissolve in the metal being produced
lowering the quality of the finished product ingot. In addition,
moisture in the air dissolves in the slag during pouring and
disassociates into hydrogen and oxygen which dissolve in the metal
being produced; the hydrogen causes cracking to occur in the
finished product ingot. These chemical reactions are encouraged due
to the long period of time during which top pouring is carried out
and by the large surface area of slag presented during pouring.
Both of the aforementioned techniques of top pouring are dangerous.
The fact that the ladle from which the molten slag is poured during
top pouring is in an elevated position presents considerable danger
to personnel in case of accidental spilling. Moreover, energizing
of the electrode can result in a small explosion due to short
circuiting causing excess heating of the slag which explosion can
upset the ladle if it is still in position over the remelting
zone.
Furthermore, top pouring techniques have the very important
disadvantage that the electrode cannot be energized previous to the
completion of pouring. In top pouring with the electrode outside
the remelting zone, the electrode cannot be energized previous to
its insertion into the remelting zone for reasons of safety. In top
pouring with the electrode in place, depending into the mold, the
electrode or electrodes cannot be energized because slag splashing
against them during pouring causes short circuiting resulting in
explosion. Energizing of the electrode current circuit previous to
the completion of pouring would result in a significant time
savings so that the apparatus can be used more efficiently.
In addition to the foregoing, top pouring techniques have the
disadvantage of requiring special measuring equipment to determine
the slag level in the remelting zone at any particular time during
top pouring.
As stated hereinbefore under the "Summary of the Invention," the
slag pool is produced by pouring the molten slag into the bottom
part of the crucible 10. This bottom part of the crucible, as
defined by the crucible sidewalls and its separate bottom plate 11,
forms a remelting zone in which the electrode or electrodes, such
as shown in the embodiment of FIG. 8, are melted by the electrical
current as soon as the slag reaches its predetermined depth by
rising to contact the electrode (2). The pouring is sufficiently
fast and current flow begins sufficiently quickly after the pouring
is started that the formation of a slag crust on the bottom of the
remelting zone is prevented and thus false starts are eliminated.
In fact, the time required for obtaining molten slag in the
remelting zone with an energized electrode in place is minimized to
a matter of a few minutes or less.
Because the electrode is already in place and because bottom
pouring avoids pouring slag past the electrode, the gap between the
electrode and the remelting zone walls, i.e., the sidewalls of the
crucible, can be made very small thus permitting a large diameter
electrode to be used. As a result, a significantly shorter
electrode can be used and the height of the tower required is
accordingly reduced. With shorter electrodes the inductance of the
circuit is reduced and the power factor of the system is
accordingly increased. Since the slag is poured into the bottom of
the remelting zone, the possibility of formation of slag scale on
the remelting zone walls is entirely eliminated.
With this technique of pouring the molten slag into the bottom of
the remelting zone, the operator controlling the pouring can
determine very precisely when to stop pouring the molten slag. When
the molten slag reaches the electrode, current begins to flow in
the electrode and that current flow provides a condition indicating
to the operator that the slag bath has reached the predetermined
depth. Accordingly, pouring of the slag is discontinued when
current starts to flow through the electrode. Because the slag can
be poured quickly through a closed channel into the bottom of the
remelting zone, there is little opportunity for the slag to react
with nitrogen in the air or to dissolve moisture from the air.
Furthermore, because slag is poured into the bottom of the
remelting zone, the ladle is positioned near the base of the
furnace thus greatly reducing the danger to personnel.
DETAILED OPERATING PARAMETERS
The initial predetermined distance between the consumable electrode
and the bottom plate is essentially the same as the predetermined
initial depth which the slag achieves in the remelting zone. These
are not exactly the same because slight additional molten slag will
enter the remelting zone between the time when the circuit
completion is signalled and the time when the discontinuance of the
slag pouring is actually implemented. Accordingly, the bottom of
the electrode is positioned just below the desired slag level as
the molten slag is poured.
The predetermined initial depth of slag in the remelting zone
preferably ranges from one-fourth the transverse sectional
dimension of the ingot to be formed to twice the transverse
sectional dimension of the ingot to be formed. The transverse
sectional dimension is: the diameter, if the ingot cross-section is
circular; the length of a side, if the ingot cross-section is
square; the length of the shortest side, if the ingot cross-section
is rectangular; or the length of the shortest dimension across the
center of the ingot, if the ingot is any other shape.
If the initial slag depth is less than one-fourth the transverse
sectional dimension of the ingot to be formed, arcing can occur
whereby electroslag remelting no longer takes place. If the initial
slag depth is more than twice the transverse sectional dimension of
the ingot to be formed, a crust of solid slag can form interfering
with the current flow. As an illustrative example, for a
rectangular slab ingot of 9-15 tons whose smallest cross-sectional
dimension is 630 mm, the initial slag depth may advantageously be
250 .+-. 15 mm.
It is desirable that the molten slag be introduced (that is,
poured) into the remelting zone at a rate sufficiently fast and
under such conditions that no crust of solid slag is formed on the
bottom plate in the remelting zone. The rate of slag introduction
so that such a crust will not form is dependent upon the amount of
slag being utilized which in turn is dependent upon the size of the
ingot to be formed. In general, for ingots of circular cross
sections of 65-1,500 mm diameter (or rectangular or square ingots
of equivalent cross sectional area), slag pouring rates ranging
from 5 to 1,500 Kg/minute are usually utilized. As an exemplary
situation, for 9-15 ton slab ingots having cross sections of
693,000-900,900 square mm, a pour rate of 750 Kg/minute may be
advantageously used.
It is desirable that the rate of slag introduction should be
sufficient so that the mass of molten slag in the remelting zone at
any time has sufficient superheat to remelt any of the previously
added slag that has solidified or to prevent any slag from
solidifying that has lost its superheat.
It is also desirable in order to prevent crust formation that the
slag be introduced under turbulent flow. For the same ingots of
65-1,500 mm diameter ingots (or equivalent rectangular or square
ingots), such flow should be from 1 to 20 kg/sq.cm./min.
The achievement of the aforementioned flow rates is aided by the
utilization of molten slag superheated to possess a viscosity of no
greater than the viscosity of water at the standard reference
temperature of 68.degree. F. and preferably much less than this
viscosity. It is especially preferred that the slag be superheated
to possess a viscosity ranging from 0.1 to 0.5 centipoises. In
order to achieve such a viscosity, the slag for the bottom pour is
at a temperature at least 100.degree. C. higher than its melting
point but no higher than 100.degree. C. below the boiling point of
the slag. For the slags normally used for electroslag remelting
such as ANF-6 the acceptable temperature range is approximately
1,440.degree. to 2,100.degree. C.
The molten slag is advantageously introduced into the bottom of the
remelting zone utilizing a reservoir which interconnects via a
closed channel or conduit with the bottom of said remelting zone.
The head of slag in the reservoir is utilized to obtain the desired
depth of slag in the remelting zone. Thus, the electrode is
initially positioned in the remelting zone with its lowermost
portion below the top of the reservoir so that the slag can be
poured to a level to contact the electrode. Preferably the
reservoir extends above the level ultimately desired for the slag
so that there will be no overflow of slag from the reservoir. The
slag is conveniently introduced into the reservoir utilizing a
ladle.
The cross-sectional area of the stream of slag entering the
remelting zone, that is the cross-sectional diameter of the conduit
interconnecting the reservoir with the bottom of the remelting
zone, ranges from 6 to 120 sq. cm. for circular ingots of 65-1,500
mm diameter (or the equivalent rectangular or square ingots). With
inadequate conduit cross section, the back pressure on the stream
being introduced can be so great that introduction rate will
ordinarily be too slow and a false start may occur. With
excessively large conduit cross sections, slag will not solidify
completely in the conduit just previous to the access port where
the conduit opens into the remelting zone, in which case metal will
enter the conduit producing an ingot with a "side tongue." The
channel through which the slag flows in the bottom pour should
extend into the crucible or mold a distance equal to 1 to 2 times
the channel depth.
Owing to a rapid rising of the slag level in the remelting zone and
its superheated condition, a crust is not formed on the bottom
plate. This is extremely important with a monophase start in which
the circuit path is through the bottom plate.
Once slag pouring is discontinued and remelting has begin, the
depth of the immersion of the electrode is increased so as to cover
the conical point which is formed on the electrode during remelting
and to increase the current flow so as to maintain the desired slag
temperature so that proper remelting occurs.
All slag compositions used for electroslag remelting or refining
processes can be poured into the crucible according to the present
invention. Representative slag compositions are set out in B. I.
Medovar et al, Electroslag Remelting, N64-11419 U.S. Department of
Commerce, Office of Technical Services, Joint Publications Research
Service on Table 10, page 53.
EXAMPLE
The remelting of an electrode to produce a 4 ton ingot was carried
out by lowering an electrode into a mold or crucible of 4 ton
capacity of the type as shown in FIG. 1. Next, 300 kg. of ANF-6
flux from the above-referred to Table 10 is placed into a heated,
carbon-lined crucible ladle and heated by means of nonconsumable
carbon electrodes to a temperature of not less than about
1,750.degree. C. For this production cycle the diameter of the
crucible cavity was about 500 mm and the depth was 800 mm.
Simultaneously with the heating of the slag, cooling water is
circulated through the crucible sidewalls and the base plate. The
power transformer is then turned on to energize the electrode and
the base plate with a voltage of 70 to 90 v. The pouring lip of the
crucible ladle is then aligned with the receiving funnel 16 and the
ladle tilted to pour the molten slag into the remelting crucible in
approximately 0.5 to 1.0 minute. The pouring is terminated
immediately upon the appearance of a current, 1 kiloamp, in the
transformer electrode line. At such time the level of the molten
slag is 5-10 mm above the end of the electrode. The electric
current then maintains the temperature of the molten slag and the
consumable electrode commences remelting.
The start-up of this remelting process resulted in the absence of
arcing of the electric current and the production of an ingot with
a fully utilizable bottom portion. There were no start-up or
operational disturbances experienced due to slag solidifying onto
the electrodes or mold sidewalls as has been the case when the slag
is top poured into the mold.
In the preferred embodiment of the present invention, the access
port, through which the pouring device communicates with the
crucible or mold is formed by an aperture in the bottom plate. The
channel communicating with the access port has its top surface
defined by a flange extending from the crucible sidewall and an end
surface of the pouring device and its bottom surface defined by an
extension of said bottom plate.
As stated above, the access port for supplying molten slag is
formed by a radial aperture in the crucible sidewall just above the
bottom plate. The access port for supplying the molten slag can
also be formed by two apertures adjoining each other, one of these
apertures being located in the bottom plate and one in the crucible
sidewall.
It is expedient to utilize an electrode of sufficient size so that
the minimum gap between the electrode and the crucible sidewall is
greater than 5 mm and less than 100 mm; this embodiment has the
advantage of providing very little air space, thereby minimizing
reactions which degrade the ultimate ingot. This embodiment is made
feasible by the pouring method and apparatus of the present
invention.
The invention may be embodied in other specific forms without
departing from the scope, spirit or essential characteristics
thereof. The present embodiments are therefore to be considered in
all respects as illustrative and not restrictive, the scope and
spirit of the invention being indicated by the appended claims
rather than by the foregoing description, and all changes which
come within the meaning and range of equivalency of the claims are
therefore intended to be embraced therein.
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