Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention pertains to the art of electro-
metallurgy and more particularly to methods for electroslag re-
melting of metals. The invention may be employed in installations
for producing ingots by electroslag remelting either of one con-
sumable electrode, or of a plurality of groups of such electrodes.
It is common knowledge that the consumable electrode
melting-down rate, and consequently the ingot build-up rate, is
dependent upon the energy released in the slag bath: the higher
the current and the voltage, the greater the ingot build-up rate.
This means that increasing the ingot build up rate entails a rise
in the electric power consumption.
Various methods have been proposed to reduce the
electric power consumption at a predetermined ingot build-up
rate in the course of electroslag remelting. One of these in-
volves an action upon the slag bath and metal pool with the use
of an electromagnetic means, with ~he result that, at the same
electric power consumption, the consumable electrode melting-down
rate is accelerated as against that without such action. Employed
as the electromagnetic means in this method are inductance coils
installed outside the mould, as disclosed in ~ritish patent
number 1,335,383.
In other prior art electroslag remelting methods, ac-
celerating the consumable electrode melting-down rate without
increasing the electric power consumption is attained by imparting
to the electrode, apart from its lowering as it melts down, an
additional motion in the slag, which additional motion may be a
rotary motion around the electrode axis, a vertical (U.S. Pat.
No. 3,565,994) or a horizontal (British Pat. ~o. 1,202,192)
oscillatory motion, or a motion along a closed path in a horizontal
or a vertical plane. Another prior art method employs for the
same purpose power modulation by varying the voltage and the cur-
rent at definite time intervals, as disclosed in British patent
number 1,188,028.
The use of the methods referred to above calls for re-
latively complicated apparatus and hence entails a higher cost of
the equipment and of the process of electroslag remeltinq of con-
sumable electrodes as a whole.
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U.S. patent number 3,776,294 discloses a method for
electroslag remelting of metals, consisting in that a gas or a
gas mixture is blown into the slag bath in the course of melting
of an electrode or electrodes. According to the method, a gas or
a gas mixture is blown into the liquid slag in such a manner that
the gas bubbles upwards through the liquid slag, thereby upgrading
the metal refining quality. Alternatively, the gas may be blown
in a jet directed with respect to the mould wall at an angle
other than a right angle, causing the slag to rotate around the
electrode or electrodes and thereby improving the slag-metal
interaction.
The velocity of such a slag motion in the zone adjacent
the electrode or electrodes, however, is comparable with that
of the convective motion and thus fails to appreciably affect
the electrode melting-down rate and to marke~ly reduce the
electric power consumption at the predetermined ingot build-up
rate.
In the electroslag remelting b~ the prior art methods
of a plurality of consumable electrodes differing in cross-section,
the melting-down rates of the electrodes greatly differ from one
another, which is particularly pronounced when such electrodes are
connected to opposite poles of a current source. This impairs
the metal refining quality~ Resoxting to various electrical-
engineering means, such as an equalizing wire connecting the
bottom plate to the centre point of the transformer secondary
winding, has shown these to be inadequate for equalizing the
melting-down rates of electrodes differing in cross-section.
It is an object of the present invention to provide a
method for electroslag remelting of metals, which enables the
metal melting-down rate to be varied over a wider range than has
heretofore been possible.
A further object of the invention is to provide a
method for electroslag remelting of metals, which allows the
electric power consumption at a predetermined melting-down rate
be reduced.
Still further object of the invention is to provide
a method for electroslag remelting of metals, which makes it
possible to attain an uniform melting-down of electrodes when
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two or more electrodes or a plurality of groups thereof are used.
Yet further object of the invention is to provide a
method for electroslag remelting of metals, which makes it poss-
ible to either increase or decrease the melting-down rate without
respectively increasing the electric power consumption and im-
pairing the ingot surface quality.
The above-mentioned and other objects of the invention
are attained by providing a method for electroslag remelting of
metals, which comprises melting down at least one consumable
electrode in a liquid electrically conductive slag and blowing
a gas into said slag in at laast one jet directed toward the
consumable electrode portion immersed in the slag with sufficient
force to penetrate the slag and to create a flow of said slag
around the slag-immersed portion of said consumable electrode
and thereby accelerate the melting-down thereof.
When a gas, such as argon or nitrogen, is blown into
the liquid slag in a jet directed towards the consumable elec-
trode portion immersed into the slag, the energy of the gas jet
is spent for agitating the liquid slag and creating flows therein,
which flows speed up the rates of heat transfer in the liquid
slag. An electrode placed into the region of vigorous liquid
slag flows melts down at a more rapid rate. An intense direction-
al action of such slag or gas-slag mixture flows makes it poss-
ible either to cut down the electric power consumption at a pre-
determined ingot build-up rate or to reduce the melting-down rate
without impairing the ingot surface quality, and, when two or
more electrodes are used, to attain an uniform melting-down
thereof.
It has been experimentally established that to create ,r"
such a slag flow, the gas jet velocity at the nozzle outlet is
preferably set to be at least of 0.5 m/s per every millimeter
of distance from the nozzle to the electrode. At a lower gas
velocity, the slag flow created by the gas jet fails to reach
the consumable electrode and hence to accelerate its melting-
down. The upper limit of the gas velocity is not critical, it
is selected depending on the required electrode melting-down
rate with account for ensuring stability of the remelting process.
In the electroslag remelting of two or more consumable
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electrodes, a difference or inbalance in their melting-down rates
may arise, accompanied by a dif~erence in the depths o~ immersion
of the electrodes into the slag bath, with the result that some
of the electrodes may eventually come into contact with the metal
pool and hence their melting will terminate.
To eliminate the inbalance in the melting-down of
at least two consumable electrodes or of at least two groups
thereof, a gas is simultaneously blown into the li~uid slag
in the direction towards every consumable electrode or electrode
group, the flow rate of the gas blown in the direction towards
an electrode or electrode group deeper immersed into the slag
being higher and proportional to the immersion depth thereof.
It has been experimentally established that the diff-
erence in the flow rates of the gas blown in towards electrodes
immersed to different depths is preferably set to be at least
1 percent of the predetermined flow rate per every millimeter
of the difference in the electrode immersion depths.
It is most efficient to maintain the gas flow rate
proportional to an electrode immersion depth automatically with
the aid of a flow controller adapted for this purpose.
In order to reduce the consumption of a gas in accom-
plishing the method of the invention, the gas may be blown into
the liquid slag in an intermittent or pulsating jet with a
pulse duration ranging ~rom 0.02 to 2 minutes.
The above gas pulse duration limits are based on
practical considerations of avoiding the use of complex and costly
means to effect an intermittent gas feed, inasmuch as shortening
the pulse duration below 0.02 minute complicates the construction
of means for feeding a gas into the slag, whereas continuing a
pulse over 2 minutes lowers the efficiency of the blowing process.
Such an intermittent or pulsating gas feed may be
accomplished according to any schedule, however, such one is pre-
ferable when the gas is blown in pulses of equal duration at inter-
vals of the same duration between the pulses.
Like the electric power modulation/ the pulsating gas
feed makes it possible to reduce the electrode melting-down rate
without impairing the ingot surface quality.
With the aim to attain a more uniform heat distribution
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in the slag bath when at least two consumable electrodes or two
groups thereof are melted, it is advantageous to phase the gas
pulses so that the maximum ~low rate of the gas blown into the
slag towards the slag-immersed portion of an electrode or
electrode group coincides in time with the minimum (or zero) flow
rate of the gas blown into the slag towards the slag-immersed
portion of an adjacent electrode or electrode group~
To attain the highest efficiency in employing the
method of the invention, the gas is preferably blown into the
slag towards every consumable electrode in at least one jet per
every 100 mm of the width or diameter of the electrode.
- The method of the invention will now be explained in
greater detail with reference to specific examples of practicing
thereof and illustrated in the accompanying drawings, wherein:
Figure 1 is a diagrammatic view of an apparatus for
electroslag remelting of metals, wherein four consumable electrodes
are remelted by the method of the invention
Figure 2 is a plan view of the apparatus illustrated
in Figure 1.
In as much as the method of the invention may be most
advantageously utili2ed in multiple-eIectrode electroslag re-
melting furnaces, an example of carrying out the method in a
furnace wherein four electrodes are remelted is outlined below.
Figure 1 diagrammatically illustrates an electroslag
remelting furnace comprising a movable rectangular-section mould
1 supported during the initial stage of operation on a bottom
plate 2 which subsequently, as the mould is moved upwards, serves
as a support for the ingot. The bottom plate 2 may be installed
on a carriage (not shown) for conveying or removing the finis~ed
ingot beyond the area of production thereof. In the case under
consideration, four consumable electrodes 3 are remelted, con-
nected by the "electrode-electrode" circuit, the electrodes are
designated by reference numerals 3a, 3b, 3c and 3d ~Figure ~).
Being well known in the art, the electrode connection circuit is
not shown in the drawings.
In carrying out the method of the invention, a gas is
first blown into the slag in the direction towards the portion,
immersed into the slag, of a pair of electrodes, e.~. 3a and 3b,
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if~these melt down at a slower rate than does the pair of elec-
trodes 3c and 3d. This intensifies the melting-~own of the pair
of electrodes 3a and 3b, i.e. a greater amount of metal flows
down into the metal pool under this pair of electrodes than under
the other pair of electrodes.
If an inbalance in the electrode melting-down rates has
a risen, the melting-down rate of the deeper immersed electrodes
can be increased by blowing the gas into the slag towards the
slag-immersed electrode portions, at different flow rates. As
the dif~erence in the electrode immersion depths diminishes, the
gas blowing-in rates are equalized~ The existence of a difference
in the electrode immersion depths is judged from the readings of
voltmeters (not shown) connected across every electrode and the
ingot being formed.
~hen remelting two or more electrodes connected by the
"electrode-electrode" circuit, the voltages across the melting end
of every electrode and the mould body (if the mould is widened in
its top portion) or the bottom plate (if the mould has no widened
portion) are measured. The difference in the voltages makes it
possible to judge of the extent of immersion of electrodes with
respect to an electrode whose immersiQn depth is assumed as normal.
In a multiple-electrode electroslag remelting of metals, the
majority of electrodes are generally under the same potential
with respect to the mould body (~ottom plate), so the immersion
depth of these electrodes is used as a reference for determining
that of other electrodes. An immer.sion depth corresponding to
an lV voltage difference at a constant level of energy released
in the slag bath is usually determined for a specific ingot.
The method of the invention was tested in an applicant's
experimental plant, where four 40x200 mm cross-section consumable
electrodes were remelted in a 150x500 mm cross-section, 400 mm
high mould. Multiple-passage nozzles where through argon was
blown into the slag bath were installed in the mould walls 300mm
from the bottom end of the mould. Every electrode was spaced
at 20 mm from the mould wall, and the slag bath level was 80 mm
from the top end of the mould. It was experimentally found that
a drastic increase in the melting-down rate of these electrodes
was attained at an argon velocity at the noz~le outlet of 10 m/s
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and over. This was accompanied by a 15-percent increase in the
electrode melting-down rate at the same level of energy released
in the slag bath.
The method of the invention was further tested in
commercial electroslag remelting installations using electrodes
of different cross-section. The testing showed that thicker
electrodes melted down a slower rate. In the course of a trial
melting, the difference in the depths of immersion of the elect-
rodes into the slag reached ~0 mm li.e. 30 mm and 10 mm), and
the argon flow rate, when it was blown into the slag bath, a-
mounted to 0.5 l/min.for every electrode. To eliminate the dif-
ference in the depths of electrode immersion into the slag, the
flow rate of argon blown in the direction towards the electrodes
immersed to a depth of 30 mm was set at 0.6 l/min~, and towards
those immersed to a depth of 10 mm, at 0.4 l/min. In three min-
utes of gas blowing into the slag, the difference in the electrode
immersion depths was diminished to 5 mm. When the difference in
the argon flow rates was automatically set proportional to the -
electrode immersion depth, the difference in the electrode
immersion depths was eliminated in 0.5 to 1 minute.
With the object to reduce the argon consumption, blowing
it in a pulsating jet was tested, the gas feed being shut off
with the aid of a solenoid valve. The gas pulse duration was
from 0.02 to 2 minutes with intervals of the same duration be-
tween pulses, and the frequency was from 0.5 to 50 pulses of
equal duration per minute with intervals of the same duration be-
tween pulses. This allowed a higher electrode melting-down rate
be a~tained at an argon consumption only half as high as that
without a pulsating gas feed. When the intervals between pulses
were lengthened, with the same pulse duration, the effectiveness
of the method of the invention lowered, whereas shortening the
intervals, i.e. increasing the pulsation frequency, called for
the use of more complex gas feed means. Reducing the pulsation
frequency resulted in impairing the ingot quality.
In the above-described example, the gas was blown into
the slag bath, according to the invention, through nozzles whose
number opposite every consumable electrode was two, which provided
for a jet per every 100 mm of electrode width~ When the gas was
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blown in through only one no7zle, both the ingot surface quality
and the effectiveness of the method were impaired.
While particular embodiments of the invention have been
shown and described, it is not intended that the invention be
limited to the disclosed embodiments or to the details thereof,
since various modifications may be made in the invention without
departing from the scope as defined in the appended claims.
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