GB1577416A - Method of making a steel ingot and to apparatus for carrying out the method - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO A METHOD OF MAKING A STEEL INGOT AND TO APPARATUS FOR CARRYING OUT THE METHOD (71) We, VEREINIGTE EDELSTAHL WERKE AKTIENGESELLSCHAFT (VEW), a Company organised and existing under the laws of Austria, of 12, Elisabethstrasse, 1010 Vienna, Austria, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:: The invention relates to a method of making a steel ingot having low hydrogen and sulphur contents, by the electroslag remelting process, in which the electrode to be remelted is inserted through an opening in a lid closing the mould and a flushing gas is introduced into the interior of the mould and into the annular gap between the opening in the lid and the electrode; the invention also relates to apparatus for carrying out this method.

From Austrian Patent No. 314,107 it is known to blow agents into the slag and into the melt when carrying out an electroslag remelting process, in order to remove undesired component parts. Such agents are for instance CaF2, together with an inert gas, such as argon, helium, nitrogen.

Furthermore, from German Offenlegungsscrift No. 2,308,321 it is known to guide inert gas during the remelting process through an annular box surrounding the electrode into the interior of the mould and to simultaneously cause a curtain-like sealing of the interior of the mould by a jet of air conducted to the electrode.

These measures, however, do not enable production of ingots which have a low hydrogen content as well as a low sulphur content; according to these known methods, it is also impossible to avoid the disadvantage that the bottom, or foot, 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 method used hitherto, 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 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 cost 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 to avoid the above described disadvantages and difficulties and its object is to provide an improved electroslag remelting method by which it is possible to produce ingots low in hydrogen and sulphur by preventing access of humidity from the air of the environment and by creating optimal conditions for desulphurization, which ingots have a very uniform composition over their height from the foot to the crop.

According to a first aspect of the invention there is provided a method of making a steel ingot having low hydrogen and sulphur contents, by the electroslag remelting process, in which process an electrode to be remelted is inserted through an opening in a lid closing a mould, an annular gap being defined between the opening in the lid and the electrode, and a flushing gas is introduced into the mould and into the annular gap, the method further comprising the following steps: (a) lowering the hydrogen content of the slag before the remelting process starts, (b) introducing dry air into the mould above the slag during the remelting process, and (c) simultaneously creating a gas curtain of dry air in said annular gap to prevent air containing water-vapour from enter ing the mould.

Advantageously, before beginning the remelting process, dry air is introduced into the slag that has been introduced into the mould, whereby the partial pressure of the hydrogen is lowered and the hydrogen is flushed out.

Suitably, the slag may be heated by a graphite electrode during flushing of ffie slag and to maintain the temperature of the slag at a value slightly above the liquidus temperature.

Preferably, a remelting slag having a CaO-content suitable for the formation of CaO-SiO2-complexes is used, i.e. a CaOcontent of approximately 3 to 40% by weight and an SiO2-content of approxi mately 3 to 25% by weight; this has also proved favourable for a reduction of the hydrogen content.

The combination of the steps in the method according to the invention has the effect that the remelting slag is very low in hydrogen; that by producing a gas curtain of dry air no water vapour can be brought in from which hydrogen would be produced; and that, by introducing dry air into the interior of the mould, a good desulphurization effect of the slag is ensured. By reaction with oxygen in the air, SO, forms, which is conducted away in gaseous form.

According to a further aspect of the invention, there is provided apparatus for carrying out the method of the first aspect of the invention, which apparatus comprises an electroslag remelting mould having a lid with an opening therein, said lid being divided into parts which are pivotable relative to one another between open and closed positions, an electrode extending into the mould through said opening in the lid, an annular gap being formed between said opening in the lid and said electrode, and means for supplying flushing gas into the mould and into said annular gap.

It thus becomes possible to fuse a plurality of consumable electrodes successively to form a single block and to obtain high operational safety and accuracy as regards the desired low hydrogen and sulphur contents.

Advantageously, there is provided on each pivotable lid part at least one conduit segment, which segments have separate supply means for dry air and which complement each other to form an annular conduit when the lid parts are in the closed position.

Suitably, each conduit segment is provided with outflow openings, which openings are arranged above one another.

The apparatus according to the said further aspect of the invention furthermore comprises a blowing lance adanted for immersion in the slag and a graphite elec trode adanted for nositioning above the slag, whereby the hydrogen content of the slag may be reduced. The lance may be water-cooled and/or made of graphite.

An embodiment of the invention shall now be described, by way of example only, and with reference to the acompanying drawings, wherein: Fig. 1 is a front view of the apparatus during the slag flushing step of the method, Figs. 2 and 3 are a front view and a horizontal projection of the apparatus during the remelting step of the method.

Referring to the Figures, there is shown a cooled bottom plate 1 and a liftable mould 2. Slag 3, which has been premelted outside the apparatus 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 9 to be remelted is inserted so as to leave an annular gap 10 between the lid 8 and electrode 9. The rid 8 is designed in two parts 81, 81l which can be pivoted between open and closed positions about bolts 11, 111 (Fig. 3).Between the rims of the lid 8 and the mould 2, a highly refractory insulation 12, advantageously consisting of a fibre mat, is inserted. The lid parts 81, 81l are provided with coolant conduits 13. On each lid part 81, 811 and above the annular gap 10 is arranged at least one conduit segment 16 for dry air; each segment has a supply conduit 15. When the lid parts are pivoted inwardly towards each other into the closed position, the segments on the lid parts complement one another to form an annular conduit 14. The segments 16, and hence the annular conduit 14, are provided with outflow openings 17 directed towards the electrode 9. Advantageously, these openings are in the form of slits 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 any air containing water-vapour entering the annular gap. Branch conduits 18 lead from the conduits 15 to penetrate the lid and lead- into the interior of the mould. By introducing dry air with overpressure into the interior of the mould, favourable conditions for desulphurization are created; furthermore the sealing effect of the curtain of dry air created in the annular gap is further improved. The apparatus according to the invention ensures that an atmosphere free of water vapour is maintained, even when slightly conical electrodes are used and when electrodes are exchanged.

The method of the invention is illustrated by the following examples.

An electroslag remelting ingot having a diameter of 1,000 mm and a length of 4 m was produced from quenched and 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 furnance and having a composition of 20% SiO,, 20% CaO, 30% CaF2 and 30% Awl203 had a hydrogen content of 32 ppm. This slag was poured into the mould while the lid was pivoted outwardly 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 1000C above the liquidus temperature, i.e. at 15000C; 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 conduits 18 and producing the air curtain in the annular gap 10 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 bath in the region which was to form the foot end of the electroslag 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 after completion of 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 step for reducing the hydrogen content of the forged ingot was not necessary.

Comparative meltings had the following results. When the same consumable elec trodes were used for producing an ingot without flushing of the slag and without the introduction of dry air into the interior and into the annular gap there resulted a hvdro gen content of 5.8 pDm in the foot region, 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 non-uniformity between the foot and crop regions requires annealing of the forged ingot for 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 bath, one hour after the beginning of the remelting procedure, amounts 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 a slag having equal portions of alumina, lime and fluorspar is used under otherwise equal conditisr,s as described in the example, an ingot is obtained having a hydrogen content of 2.6 ppm in the foot region and of 2.4 ppm in the crop region.

If a basic slag is used without slag flushing and a gas curtain of inert gas such as nitrogen or argon is produced in the annular gap between the opening in the lid and the electrode, the hydrogen content of the steel bath, which corresponds to the content of the ingot in the foot region, amounts to 4.5 ppm one hour after the beginning of the remelting procedure, 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 has a slight dissolving power for sulphur at the beginning of the remelting procedure, which vanished while the procedure continued, due to saturation.

Also, if neutral slag is used under otherwise equal conditions, the hydrogen and sulphur contents are unsatisfactory; 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 it can be seen that the steps 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.

WHAT WE CLAIM IS:- 1. A method of making a steel ingot having low hydrogen and sulphur contents, by the electroslag remelting process, in which process an electrode to be re

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (12)

**WARNING** start of CLMS field may overlap end of DESC **. An electroslag remelting ingot having a diameter of 1,000 mm and a length of 4 m was produced from quenched and 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 furnance and having a composition of 20% SiO,, 20% CaO, 30% CaF2 and 30% Awl203 had a hydrogen content of 32 ppm. This slag was poured into the mould while the lid was pivoted outwardly 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 1000C above the liquidus temperature, i.e. at 15000C; 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 conduits 18 and producing the air curtain in the annular gap 10 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 bath in the region which was to form the foot end of the electroslag 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 after completion of 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 step for reducing the hydrogen content of the forged ingot was not necessary. Comparative meltings had the following results. When the same consumable elec trodes were used for producing an ingot without flushing of the slag and without the introduction of dry air into the interior and into the annular gap there resulted a hvdro gen content of 5.8 pDm in the foot region, 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 non-uniformity between the foot and crop regions requires annealing of the forged ingot for 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 bath, one hour after the beginning of the remelting procedure, amounts 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 a slag having equal portions of alumina, lime and fluorspar is used under otherwise equal conditisr,s as described in the example, an ingot is obtained having a hydrogen content of 2.6 ppm in the foot region and of 2.4 ppm in the crop region. If a basic slag is used without slag flushing and a gas curtain of inert gas such as nitrogen or argon is produced in the annular gap between the opening in the lid and the electrode, the hydrogen content of the steel bath, which corresponds to the content of the ingot in the foot region, amounts to 4.5 ppm one hour after the beginning of the remelting procedure, 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 has a slight dissolving power for sulphur at the beginning of the remelting procedure, which vanished while the procedure continued, due to saturation. Also, if neutral slag is used under otherwise equal conditions, the hydrogen and sulphur contents are unsatisfactory; 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 it can be seen that the steps 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. WHAT WE CLAIM IS:-

1. A method of making a steel ingot having low hydrogen and sulphur contents, by the electroslag remelting process, in which process an electrode to be re

melted is inserted through an opening in a lid closing a mould, an annular gap being defined between the opening in the lid and the electrode, and a flushing gas is introduced into the mould and into the annular gap, the method further comprising the following steps: (a) lowering the hydrogen content of the slag before the remelting process starts, (b) introducing dry air into the mould above the slag during the remelting process, and (c) simultaneously creating a gas curtain of dry air in said annular gap to prevent air containing water-vapour from enter ing the mould.

2. A method as claimed in Claim 1, wherein the step of lowering the hydrogen content of the slag before the start of the rernelting process is effected by flushing the slag in the mould with dry air.

3. A method as claimed in Claim 1, wherein the step of lowering the hydrogen content of the slag before the start of the remelting process is effected by flushing the slag in the mould with dry air, and the slag is simultaneously heated by a graphite electrode and is maintained at a temperature slightly above the liquidus temperature.

4. A method as claimed in any preceding claim, wherein the slag has a CaOcontent of between 3 and 40% by weight and a SiO2-content of between 3 and 25% by weight to be suitable for forming CaO SiO2-complexes.

5. Apparatus for carrying out the method of Claim 1, comprising an electroslag remelting mould having a lid with an opening therein, said lid being divided into parts which are pivotable relative to one another between open and closed positions, an electrode extending into the mould through said opening in the lid, an annular gap being formed between said opening in the lid and said electrode, and means for supplying flushing gas into the mould and into said annular gap.

6. Apparatus as claimed in Claim 5, further comprising on each pivotable part of the lid at least one conduit segment, the segments having separate supply conduits for dry air, which conduit segments complement each other to form an annular conduit when the lid parts are in the closed position.

7. Apparatus as claimed in Claim 6, wherein said conduit segments are provided with outflow openings arranged above one another.

8. Apparatus as claimed in Claim 5, further comprising a blowing lance adapted for immersion in the slag and a graphite electrode adapted for positioning above the slag, whereby the hydrogen content of the slag may be reduced.

9. Apparatus as claimed in Claim 8, wherein said blowing lance is water-cooled.

10. Apparatus as claimed in Claim 8 or Claim 9 wherein said blowing lance is made of graphite.

11. A method substantially as hereinbefore described with reference to Figs. 1 to 3 of the accompanying drawings.

12. Apparatus substantially as hereinbefore described with reference to Figs. 1 to 3 of the accompanying drawings.