Note: Descriptions are shown in the official language in which they were submitted.
9~34
The invention relates to a method of making steel ingots
having low hydrogen and sulphur contents according to the
electric slag remelting process, in which the electrode to be
remelted is guided through an opening in a lid closing the
mould and a flush gas is introduced into the interior of the
mould and into the annular gap between the opening in the lid
and the electrode, as well as an arrangement for carrying out
this method.
According to Austrian Patent No. 314,107 it is known to
blow agents into the slag and into the melt when carrying out
an electric slag remelting method, in order to remove undesired
component parts. Such agents are e.g. CaF2 together with inert
gas, such as argon, helium, nitrogen.
Furthermore, according to German Offenlegungsschrift No.
2,308,321, it is known to guide inert gas during the remelting
procedure through an annular box surrounding the electrode into
the interior of the mould and to simultaneously cause a
curtain-like sealing of the in-terior of the mould by a jet of
air conducted to the electrode.
These measures, however, do not allow a production of
ingots which have a low hydrogen content as well as a low
sulphur content; according to these known methods it is also
not possible to avoid the disadvantage that the bottom part of
the ingot shows a higher degree of undesired impurities, in
particular hydrogen, than the middle and upper parts of the
ingot. This is due to the fact that according to the working
manner hitherto used, in which remelting has been started
immediately upon introduction of the pre-melted slag into the
mould, the slag takes up moisture from the environment while it
is poured in, the hydrogen created by decomposition merges by
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91934
diffusion into the steel phase and then is no longer given off. This hydrogen
content in the bottom or foot part of the ingot substantially determines the
time, work and costs necessary for further processing the ingot, in particular
its annealing time, irrespective of the composition in the remaining parts of
the ingot.
The invention aims at avoiding the above described disadvantages
and difficulties and has as its object to provide an improved electric slag
remelting method by which it is possible to produce ingots poor in hydrogen
and sulphur by preventing the access of humidity from the air of the
environment and by creating optimal conditions for desulphurization, which
ingots have a very uTIiform composition over their height from the foot to
the crop.
According to the invention there is provided in a method of making
steel ingots having low contents of hydrogen and sulphur, according to the
electric slag remelting procedure, in which an electrode to be remelted is
guided through an opening in a lid closing a mould, an annular gap being
provided between the opening in the lid and the electrode, and a flush gas is
introduced into the mould and into the annular gap, the improvement which is
characterized by a combination of the following steps
a) lowering the hydrogen content of the slag before the remelting procedure
starts by flushing the slag in the mould with dry air,
b) introducing dry air into the mould above the slag during the remelting
procedure, and
c) simultaneously creating a gas curtain of dry air in said annular gap to
prevent water-vapour-containing air from entering the mould.
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~091~3~
Advantageously, before the beginning of the remelting procedure,
dry air is introduced into the slag that has been brought into the mould,
whereby the partial pressure of the hydrogen is lowered and the hydrogen is
flushed out.
It is suitable to heat the slag by a graphite electrode during the
flushing of the slag and to maintain the temperature
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34
of the slag at a value slightly above the liquidus temperature.
According to a preferred embodiment, a remelting slag
having a CaO-content suitable for the formation of CaO-SiO2-
complexes is used, i.e. a CaO-content of approximately 3
to 40 % by weight and an SiO2-content of approximately 3 to
25 % by weight; this has also proved favorable for a reduction
of the hydrogen content.
The combination of measures according to the invention
has the effect that the remelting slag is very poor in hydrogen;
that by producing a gas curtain of dry air no water vapour can
be brought in from which again hydrogen would be produced; and
that by bringing dry air into the interior of the mould a good
desulphurization effect of the slag is sagefuarded. Under the
action of oxygen from the air, S02 forms, which is conducted
away in gaseous form.
The invention also comprises an arrangement for carrying
out the method with an electric slag remelting mould, which
can be closed by a lid in the remelting phase, wherein the
electrode is guided through the opening in the lid into the
interior of the mould and wherein means for supplying flush
gas into the interior of the mould and into the annular gap
between the opening of the lid and the electrode are provided.
According to the invention, the lid is designed
dividedly, and the lid parts are pivotable; thereby it
becomes possible to fuse down a plurality of consumable
electrodes successively to form a single block and to obtain
a high operational safety and accuracy as regards the desired
low hydrogen and sulphur contents.
~ dvantageously, on each pivotable lid part conduit segments
having separate supply means for dry air are provided, which
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34
complement each other to form an annular conduit when the lid
is closed.
According to a preferred embodiment, on each pivotable
lid part there are provided conduit segments provided with
outflow openings, which conduit segments are arranged one
above the other.
The arrangement according to the invention furthermore
is characterised in that in the slag flushing phase a water-
cooled blowing lance or a blowing lance of graphite is
immersible in the slag, wherein simultaneously a graphite
electrode can be positioned above the slag.
The arrangement according to the invention shall now be
described in more detail by way of an e~emplary embodiment and
with reference to the accompanying drawings, wherein:
Fig. 1 is a front view of the arrangement in the slag
flushing phase,
Figs. 2 and 3 are a front view and a horizontal projection
in the remelting phase.
In the Figures, a cooled bottom plate is denoted by 1
and the liftable mould is denoted by 2. Slag 3 which has been
premelted outside of the arrangement is introduced into the
mould and heated by a graphite electrode 4, in that an arc 5
is created between the surface of the slag and the electrode.
A water-cooled blowing lance 6, which is connected by a
conduit 7 to a source of dry air, is immersed in the slag and
air is introduced until the desired purification effect has
been achieved. In Fig. 2, the remelting phase is illustrated,
wherein the mould 2 is covered by a lid 8, which lid is provided
with a central opening through which the electrode to be
remelted 9 is guided by leaving open an annular gap 10. The
3~
lid 8 is dividedly designed, wherein the parts 8', ~" can be
pivoted into the closed position about the bolts 11, 11'.
Between the lid opening and the mould opening, a highly
refractory insulation 12, advantageously consisting of a fibre
mat, is inserted. The lid parts are provic~ed with coolant
conduits 13. Above the annular gap 10 on each lid part at
least one conduit segment 16 for dry air is provided; each
segment has a supply conduit 15. When pivoted in, the segments
of the lid parts supplement one another to an annular conduit
14. The segments or the annular conduit, respectively, are
provided with outflow openings 17 directed towards the jacket
of the electrode 9. Advantageously, these openings are designed
slit-like and lie above one another in two or more Layers, so
that when the dry air emerges from these slit openings a kind
of labyrinth sealing is formed which reliably prevents the
water-vapour-containing air from getting into the annular gap.
On the conduits 15, there are provided branch conduits 18
penetrating the lid and leading into the interior of the mould.
By introducing dry air with overpressure into the interior of
the mould favorable conditions for the desulphurization are
created; furthermore the sealing effect of the curtain of dry
air created in the annular gap is further improved. The
arrangement according to the invention safeguards that an
atmosphere free of water vapour is maintained even when
slightly conical electrodes are used and when electrodes are
exchanged.
The execution of the method of the invention is illustrated
by the following example:
An electric slag remelting ingot having a diameter of
1,000 mm and a length of 4 m was produced from quenched and
~ILQ91~L939L
tempered steel 28 NiCrMoV 8 5 (according to DIN) from a number
of electrodes having a diameter of 500 mm. The hydrogen content
of the degased consumable electrodes amounted to 2.1 ppm; the
sulphur content was 200 ppm; the humidity of the air at the
remelting plant amounted to 12 g water vapour per Nm3. Slag
pre-melted in a slag melting furnace and having a composition
of 20 % SiO2, 20 ~ CaO, 30 % CaF2 and 30 % A1203 had a
hydrogen content of 32 ppm. This slag was poured into the
mould while the lid was pivoted out and a graphite electrode,
as illustrated in Fig. 1, was positioned above the slag. After
ignition of the arc, the slag was maintained at a temperature
of approximately 50 to 100 above the liquidus temperature,
i.e. at 1500C; dried air was blown under the surface of the
slag by the water-cooled lance for 15 minutes, whereby the
hydrogen content of the slag was lowered to 13 ppm. Thereafter
the graphite electrode and the blowing lance were removed and
the lid parts together with the insulation and the supply
conduits were pivoted into the closed position, the electrode
to be remelted was introduced into the interior of the mould
and after blowing in dried air with an overpressure of
approximately 1.5 atmospheres overpressure into the interior
of the mould through the conduit 13 and producing the air
curtain in the annular gap between the electrode and the lid,
remelting was started. One hour after the beginning of the
remelting procedure, a sample was taken from the steel sump
corresponding to the foot region of the electric slag remelting
ingot to be produced, wherein a hydrogen content of 2.2 ppm
was found. Thus there was no noticeable increase in the hydrogen
content relative to the starting hydrogen content of the
remelting electrode. After two further hours of remelting and
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after completion of the remelting two further samples were
taken, which had a hydrogen content of 2.1 and 2.0 ppm. The
latter corresponds to the hydrogen content in the crop region
of the remelted ingot produced. The sulphur content of the
ingot amounted to 60 ppm, uniformly over the entire length of
the ingot. A reannealing for reducing the hydrogen content of
the forged ingot was not necessary~
Comparative meltings had the following results: When the
same consumable electrodes were used for producing an :ingot
without flushing of the slag and without the introduct:ion of
dry air into the interior and into the annular gap, there
resulted a hydrogen content of 5.8 ppm in the foot range and
3.5 ppm in the middle and in the crop of the ingot one hour
after the beginning of the remelting procedure. Such a high
hydrogen content and such a non-uniformity between the foot
and crop regions requires annealing of the forged ingot of
more than 100 hours in order to lower the hydrogen content to
an acceptable value.
If the slag is not flushed, but used as it comes from
the slag premelting furnace, the hydrogen content of the steel
sump, one hour after the beginning of the remelting procedure,
amounts to 4.7 ppm in spite of a gas curtain of dry air in
the annular gap, and then gradually decreases to 2.8 ppm. The
sulphur content of the ingot in such a case is 120 ppm.
If one uses a slag having equal portions of alumina, lime
and fluorspar under otherwise equal conditions as described in
the example, one obtains an ingot having a hydrogen content
of 2.6 ppm in the foot region and of 2.4 ppm in the crop
region.
If one uses basic slag without slag flushing and if one
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produces a gas curtain of inert gas such as nitrogen or argon
in the annular gap between the opening in the lid and the
electrode, the hydrogen content of the steel sump amounts to
4.5 ppm one hour after the beginning of the remelting procedure,
which corresponds to the content of the ingot in the foot
region, and then decreases to 3.0 ppm. The sulphur content in
the foot region of the ingot produced amounts to 120 ppm and
in the crop region to 200 ppm. This means that the slag had
a slight dissolving power for sulphur at the beginning of the
remelting procedure, which vanished while the procedure
proceeded due to saturation.
Also if neutral slag is used under otherwise equal
conditions, the hydrogen and sulphur contents are unsatis-
factory; the hydrogen content is slightly lower, approximately
4.0 to 3.0 ppm; the sulphur content is even higher, namely
150 ppm in the foot region and 200 ppm in the crop region
of the ingot.
From the comparative melts one can see that the inventive
measures of flushing the slag, creating the gas curtain in the
annular gap, introducing dry air into the interior of the
mould and the composition of the slag coact in an optimal
manner with the effect that the ingots produced according to
the invention have low contents of hydrogen as well as of
sulphur and have a very uniform composition over the entire
length of the ingot. Therefore they need not be annealed or
need be annealed only for a short time~
