Note: Descriptions are shown in the official language in which they were submitted.
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~he invention relates to a method for melting steel in
a closed electrical-resistance furnace, a layer of slag being used
to cover the molten metal.
~ow tnat the production of sponge iron has been develop-
ed to the extent that large-scale production is possible, it is
possible to make steel in an electrical-resistance furnace.
Because of its content of slag-forming substances, and its high
porosity, sponge iron has a high electrical resistance. This
makes it possible to use large amounts of electrical power in the
~urnace and, at the same time, to obtain satisfactory distribution
of power.
In steel production, metallurgical treatments are being
carried out to an increasing degree outside the furnace. These
traatments are carried out on the steel upon tapping and in the
ladle which accepts the steel emerging from the furnace, it being
possible to subject the ladle to vacuum for the purpose of obtain-
ing specific qualities in the steel. It is also desirable, or
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necessary, in many cases to carry out further processing in other
furnace groups, e.g. in an induction channel-urnace.
If, however, the electrical-resistance furnace is batch-
operated, the analysis of the steel is subject to certain fluc-
tuations from one batch to the next, since the composition of the
burden also alters unavoidably. In batch-operation, therefore,
the course of the metallurgical process, which is largely dependent
upon the carbon, sulphur, and phosphorus content of the burden,
undergoes corresponding fluctuations.
It is thérefore the purpose of the invention to provide
an improved method for melting steel in a closed electrical-
resistance furnace. It is also sought to provide for the furnace
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operation to be continuous, so that the s-teel produced is of
substantially constant quality.
According to the present invention, in a method of
continuously producing steel in a closed electrical-resistance
furnace, in which an iron-bearing oxidizing burden having a
refining action is charged into the furnace, a deep bath of
the molten steel is maintained in the furnace, the volume of
the molten metal corresponding to between 5 hours and 2
days production, and a thick cover of slag is maintained
covering the molten steel, the slag having a decarburizing,
dephosphoriæing, and desulphurizing effect, the thickness of
the slag cover being between 250 and 1500 mm or corresponding
to 1 to 10 days production, the furnace being heated
continuously by electrodes immersed in the slag to bring
said slag to a temperature in excess of the temperature
at which the molten steel in the furnace is tapped.
The above-mentioned measures make it possible
to operate an electrical-resistance furnace continuously,
The FeO content in the slag required for decarburizing is
obtained, when sponge iron is used, by adding iron ores to
the charge or, if pre-reduced ores or agglomerates are used,
by adjusting the degree of pre-reduction. This also produces
steel of a constant composition, since differences in the
composition of the burden charged into the furnace are
compensated for by maintaining a thick covering of slag and ;
a large bath of metal. Continuous operation, as made possible ~ ;
by the method according to the invention, results in a
substantially longer life for the furnace lining, in comparison
with intermittent operation, since it eliminates the alternating
heat stresses occurring, during intermittent operation, between
the melting and charging operations. Continuous operation also
eliminates sharp fluctuations in the load on the power supply.
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Environmental pollution, as compared with smelting steel
in an arc furnace, is reduced because vapourization of metal on
so-calle~ ~Iburning areas~ is largely eliminated, and this results
in a sharp decrease in the volume of stack gas.
One particular advantage of the method according to the
invention is that or approximately the same investment costs per
ton/year, there is a sharp reduction in furnace-lining costs, as
compared with batch operation, since the high thermal stresses on
the roof and walls of the furnace, arising during conventional
furnace operation, are subRtantially reduced as a result of the
reduction in radiation from the electrodes which are immersed in
the layer of slag and covered by the burden lying on the bath of
metal. Destruction of the brickwork by temperature fluctuations,
which is unavoidable in batch-operation, is eliminated by the
substantially uni~orm temperature obtaining in the furnace. Since
radiation losses during the melting period are eliminated by
immersing the electrodes in the ~hick layer of slag, and since
radiation losses during the unavoidable dead periods arising in
batch-operation are also eliminated, the power consumed by the
method according to the invention per ton o~ Einished product is
less than that consumed by existing methods. According to a
conservative eskimate, the power consumption, which has hitherto ;
been 600 kWh/t of steel, would appear to drop to about 540 kWh/t
of steel, i.e. by about l~/o.
Since continuous operatio~ resulks in a more uniform ~ ~
load on the source o-f power, it is possible in most cases to ;
obtain electric power at a lower price, where power is obtained
from the outside. Experience shows that rates for an electrical-
resistance furnace may be up to 3~/O lower than ~or an arc furnace.
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The reason for this is that there are no peak loads, high utiliza-
tion gives continuous power consumption, and the operation of the
Eurnace is better adapted to the power supply.
Continuous operation also has a favourable effect on
subsequent processing equipment, such as -Eor example a continuous
casting unit, since the operation o-f such a unit is no longer
dependent upon the intermittent operation of a conventional urnace.
As a result of the thick layer o slag covering the bath
of metal, the steel picks up less nitrogen from the air, and this
is another advantage of the method according to the invention.
The method according to the invention also makes it possible to
use pellets of an inferior quality than heretofore, since the
necessary slag work can be carried out even when a large amount of
slag is present.
Advantageously in the method of the invention, cooling
of the furnace is increased in the vicinity of the slag level.
This results in a considerable decrease in slag attack on the
brickwork in this particularly highly-stressed area of the
furnace, since the said brickwork is protected by the cooled slag.
The method according to the invention also makes it
possible to charge some of the burden into the furnace through ;
hollow electrodes and, if necessary, to feed in an oxygen-contain-
ing gas or, under certain circumstances, pure oxygen, thus
influencing the refining effect.
In producing tonnage steel, the method according to the ;
invention makes it possible to add the alloying substances outside
the furnace, in a ladle. In producing special steels, use may be
made of separate refining and alloying units.
The invention also relates to a device for carrying out
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the method explained above, provision being made, in the case
of a closed electrical-resistance furnace, for the said furnace
to be equipped wi-th at least one tap for liquid metal and one
tap for slag, with the said taps being located at different
levels, and for the said furnace to be equipped, in the vicinity
of the slag level, with a device providing increased cooling
in that area, as compared with other areas.
The accompanying drawings shows a diagrammatic
representation of a furnace for carrying out the method
according to the invention.
In the drawing, a vessel 1 of an electrical-
resistance furnace has a furnace cover 2. Soderberg electrodes
3a, 3b extend into the furnace vessel 1. As compared with the
graphite electrodes used in arc furnaces, which for production
and load-carrying reasons cannot be more than 600 - 700 mm in
diameter at the most, Soderberg electrodes are less expensive
and, since they are not limited as to diameter, may have a
diameter of 2000 mm. Arranged between the said electrodes
are charging pipes 4a, b, c and 5a, b, c communicating with
bunkers 7 located above the furn~ce. A crude-gas line 6 is
also provided. A retaining device 8 for electrodes 3a, b is
arranged on a supporting structure 9 above the furnace.
The burden within the furnace is marked 10.12 is
the molten steel at the bottom of furnace vessel 1, the
molten metal being covered by a thick layer 11 of slag. The
depth of the bath of molten steel will depend upon the diameter
of the vessel 1, typically ranging from 200 mm in the case of
a small furnace to 1500 mm in the case of a large furnace. ~ ~`
The volume of molten steel will generally correspond to
between 5 hours and 2 days production, while the volume of
slag will correspond to between 1 and 10 days production.
The said furnace vessel is provided with additional cooling
in the vicinity of the slag level as indicated at 20, it also
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has two tap-holes 13, 14 arranged one above the other, upper
hole 13 being intended for slag and lower hole 14 for steel.
When the measures according to the invention are
used, refining is achieved by means of a burden which ensures
a constant ................................................
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FeO content oE between 15 and l~/o in the slag~ This is in accord-
ance with standard slagging practice in electric-steel plants.
~ he layer of slag, which is maintained at all times and
in which the electrodes are immersed without touching the bath of
metal, has two purposes. Firstly~ heat is transferred by convection
from the slag to the bath of metal. The thick layer of slag, which
is maintained by arranging the slag tap-hole 13 at an appropriate
height above the steel tap-hole 14, ensures not only a large heat
potential, but also satisfactory heat distribution. Secondly, the
thick layer o slag produces satisfactory desulphurizing and de- `
phosphorizing, since, in addition to the normal boundary-surface
reaction between the bath of metal and the layer of slag, pre-
dephosphorizing and pre-desulphurizing occurs as molten droplets
of iron on the layer of slag, and the pellets initially floating
thereon, descend through the layer of slag. The refining reaction
is improved accordingly.
The large specific surace of the metal droplets produces - -
an extremely satisfactory reaction. Since heat is transferred from
the slag, which is heated by the immersed electrodes, to the bath
20 of metal, and takes place entirely by convection, the temperature
of the slag must be slightly higher than the tapping temperature ~ ~
of the metal. For instance, if a steel tapping temperature of 1650 -
is required (with subsequent argon flushing), the slag temperature
must be 1670C. To this end, slags with a melting temperature of
about 1620& are required, and these are superheated by about 50C.
The composition o a slag of this kind may be, for example, 15%
Feo, 2~/o sio2, and 65% CaO.
The large bath of metal equalizes the analyses, which
otherwise tend to fluctuate, especially as regards the phosphorus
and sulphur contents.
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