Note: Descriptions are shown in the official language in which they were submitted.
CA 02220468 1997-11-10
METHOD AND DEVICE FOR IMPROVING TAPPING OF LIQUID STEEL FROM A
SMELTING FURNACE INTO A LADLE
BACKGRO~ND OF THE lN V~ oN
1. Field of the Invention
The present invention relates to a method and a device for
improving tapping of liquid steel during the entire tapping
period of the steel from a smelting furnace into a ladle.
2. Description of the Related Art
When tapping crude steel or ingot steel, for example, from a
converter or an electric furnace, the liquid steel drops freely
through the air into a ladle along a distance of about 3-6m.
During this free fall, the liquid steel flow, due to its lowered
static pressure Pot relative to the atmospheric pressure of one
bar, entrains air and takes air along into the liquid steel
column rising in the ladle. The pressure conditions in the steel
poured into the ladle, which can be separated into P8t (< 1 bar)
and P~m.l can be described with the following equation (1)-
Ptotal = P.t + P~m = 1 bar (1)
Consequently, this quasi steel jet pump having a surface anda high drop height inevitable introduces oxygen and nitrogen into
CA 02220468 1997-11-10
the steel melt in the ladle by entraining air, so that the
elements N2 and ~2 are physically and chemically dissolved in the
melt in the ladle. This charging of the steel melt with nitrogen
and oxygen in the form of dissolved elements and in the form of
nitrites and oxides results in an inferior property of the
material and an inferior quality of the final steel product; in
addition, a large quantity of alloying elements and long
treatment periods, particularly in the area of secondary
metallurgy, are required.
Based on experience in the steel mill, it has been found
that, when steel drops between furnace and ladle bottom in a
pouring jet of about 180mm diameter, approximately 1 m3 per ton
of steel is entrained. This leads to an oxygen intake of about
210 Nl/t steel or about 300g oxygen or to an Al consumption of
about 340g for the deoxidation; or to about 637g alumina
deoxidation products which are partially separated in the ladle
slag (raffination) or to a reduction of the oxidic degree of
purity. Simultaneously, 7901 nitrogen/t steel or 987.5 g-
nitrogen/t steel are introduced into the steel, wherein the
nitrogen is partially dissolved in the steel and results in an
increase of the nitrogen content of the steel which, in turn,
negatively influences the quality of the steel products.
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S~MMARY OF THE lNv~llON
Therefore, it is the primary object of the present invention
to provide a method and a device of the above-described type in
which the aforementioned disadvantages are avoided. In
particular, the increased processing and alloying requirements,
for example, the use of aluminum for the deoxidation or the use
of a degasification stage in the area of secondary metallurgy,
are to be reduced or even entirely avoided.
In accordance with the present invention, the above object
is met in a surprising and simple manner by adjusting between the
tap hole of the furnace and the ladle bottom or the steel in the
ladle an oxygen-free atmosphere with a pressure of > 1 bar.
Consequently, by adjusting an oxygen-free atmosphere,
preferably by introducing argon, with an excess pressure for
substantially shielding the liquid steel over the drop height
thereof relative to the ambient air, the disadvantageous -
entrainment of air and, thus, large quantities of oxygen, can be
avoided.
The significant advantages provided by the invention are:
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- reducing the requirement of deoxidation agents in an
amount of 340g Al/t steel at a total consumption of
about 1 kg Al/t steel in the case of normal Al killed
steels;
- improving the oxidic degree of purity by a lower
content of oxides, such as alumina, r~m~;n;ng in the
steel; and
- reducing the nitrogen content increase of the steel,
which is particularly significant for electric steel
mills, because in electric steel mills a relatively
high content of nitrogen reaches the melt in the area
of the electrodes.
In the device for carrying out the method, a ladle dome is
arranged between the furnace tap and the ladle and the ladle is
connected to at least one gas supply.
The ladle dome serves to encapsulate the oxygen-free
atmosphere adjusted in the ladle relative to the ambient air
underneath the smelting furnace, i.e., between the furnace tap-of
the smelting vessel to the ladle edge; in accordance with an
advantageous features, for additionally shielding the atmosphere
CA 02220468 1997-11-10
in the ladle in order to protect against reoxidation, a pipe,
preferably of refractory material, extends from the furnace tap
hole into the ladle dome, wherein the pipe conducts the liquid
steel in a bundled manner into the ladle dome or the ladle.
The gas supply, for example, by means of rinsing stones
which may be in the form of pipes or by means of rinsing pipes,
ensures the oxygen-free atmosphere, wherein the ladle
advantageously is already flooded with argon or other comparable
gases already before being filled with the liquid steel and the
excess pressure of the oxygen-free atmosphere is adjusted. For
this purpose, it is proposed that the pressure of the inflowing
gas quantity is controlled or regulated and kept constant during
the entire tapping period; alternatively, the gas quantity is
adjusted and kept constant by means of a pressure regulator.
The various features of novelty which characterize the
invention are pointed out with particularity in the claims
annexed to and forming a part of the disclosure. For a better
understanding of the invention, its operating advantages,
specific objects attained by its use, reference should be had to
the drawing and descriptive matter in which there are illustrated
and described preferred embodiments of the invention.
~ CA 02220468 1997~ 10
BRIEF DESCRIPTION OF THE DRAWING
In the drawing:
The single figure of the drawing is a schematic illustration
of a direct current electric furnace with the reoxidation
protection between furnace tap and ladle in accordance with the
present invention.
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DESCRIPTION OF THE PREFERRED EMBODIMENTS
The figure of the drawing shows the present invention in a
direct current electric furnace. However, the invention is not
only suitable for an electric furnace, which may be constructed
as a direct current furnace or an alternating current furnace,
but also for a converter or induction furnace.
Crude steel is melted in a furnace 1 by means of an
electrode 2. The crude steel melt 3 covered by a slag layer 4 is
in a melting vessel which has on the tap side thereof a recess or
bay 5. In the area of the recess 5, the vessel has in the bottom
thereof a furnace tap 6 which can be closed by means of a closure
7 in the form of a flap or slide. For effecting slag-free
tapping, the melting vessel of the electric furnace 1 is mounted
on a swinging device 9 which can be actuated by means of a
cylinder 9.1.
A ladle 11 is arranged underneath the furnace tap 6. The
ladle 11 has a ladle closure 12 and, in the illustrated
embodiment, a rinsing stone 13 and 13' each in the ladle bottom
as well as in the ladle wall for introducing argon or other
gases; in addition, the drawing shows a rinsing pipe 13.2
extending from the ladle edge to the interior 11.1 of the ladle.
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The ladle 11 rests on a ladle carriage 22 which includes a device
23 for lifting and lowering the ladle 11, wherein the device 23
makes it possible to adjust a short distance between the furnace
tap or the bottom closure 7 and an opening 10.2 of a ladle dome
10 which encloses the ladle 11 from the ladle edge toward the
top.
The ladle dome 10 which is mounted on the ladle edge and
bridges the distance to the furnace tap 6 can be swung
horizontally and/or vertically; the ladle dome 10 has an opening
18 for adding alloying and deoxidation agents 17 during and after
the tapping procedure. Prior to opening the furnace tap 6 and,
thus, before starting emptying of the furnace 1, the interior
11.1 of the ladle 11 is advantageous flooded with argon or
another gas through the rinsing stones 13 in the bottom or the
rinsing stone 13.1' arranged in the ladle wall near the ladle
bottom and/or through the rinsing pipe 13'' and, consequently, an
oxygen-free atmosphere is adjusted with an excess pressure which
is built up to the dome space 10.1. When the furnace tap 6 is
opened, the liquid steel 16 flowing into the ladle 11 in the form
of a pouring jet 14 cannot entrain any oxygen.
An additional reoxidation protection of the pouring jet 14
during emptying the steel from the electric furnace 1 into the
- _ CA 02220468 1997-11-10
ladle 11 is achieved by a short pipe 8 of refractory material
which conducts the steel from the furnace tap 6 in a bundled
manner into the ladle dome 10 and the ladle 11. The oxygen-free
excess pressure atmosphere is maintained during the entire
tapping procedure above the ladle bottom or above the
metallurgical slag 15 of the liquid bath of liquid steel 16
building up in the ladle 11. Consequently, the liquid steel
drops through an air-free gas phase into the ladle during the
entire tapping period.
The excess pressure in the dome space 10.1 and the ladle
space 11.1 is measured by means of a pressure measuring device 19
and the gas flow (NL/min) of argon or gas is controlled or
regulated in such a way that the desired excess pressure of , 1
bar prevails in the entire ladle area up to the upper opening
10.2 of the ladle dome 10. This excess pressure can also be
adjusted and kept constant through a pressure regulator 21 which
controls the gas quantity 20 during the entire tapping period of
the crude steel 3 from the furnace 1 into the ladle 11.
While specific embodiments of the invention have been shown
and described in detail to illustrate the inventive principles,
it will be understood that the invention may be embodied
otherwise without departing from such principles.
