EP0046721B1 - Nozzle for oxygen-injection lances used in the decarburisation of pig iron, in particular chromium pig iron - Google Patents

Abstract

A nozzle is disclosed for use in the decarburization of molten pig iron by means of a jet of oxygen from a lance provided with such nozzle. The nozzle according to the invention is characterized by a divergent portion which, beyond the neck, has a frustoconical part, with an apex angle of between 60 DEG and 70 DEG and preferably between 62 DEG and 66 DEG , and wherein an angle of 65 DEG is preferred. The nozzle according to the invention can be used in particular for the decarburization of chromium containing pig iron and makes it possible to achieve very high chromium and iron yields.

Description

The nozzle which is the subject of the invention relates in the most general way to the decarburization of cast irons by means of lances disposed above the level of the liquid iron, which emit through a nozzle a jet of oxygen in the direction of the surface of this font. The invention relates more particularly to the characteristics of the oxygen jets from injection lances equipped with this new nozzle and the influence of these characteristics on the conditions of interaction between these oxygen jets and the liquid iron. This new nozzle applies in particular, but not limited to, the decarburization of chrome cast irons.

A certain number of studies have been made on the characteristics which the nozzles mounted at the end of the oxygen injection lances used for decarburizing the irons must have. Thus, in the work which concerns the metallurgy of steel by means of basic converters, entitled “BOF Steelmaking”, which was published in 1976 in the United States by the Iron and Steel Society, precise indications in volume 3, chapter 8, pp. 150 to 153, on the characteristics of the nozzles mounted on the oxygen lances. It is stated, in particular at p. 150, lines 28 to 35, that a reasonably uniform supersonic flow can be obtained by means of a simple divergent conical part which follows the constriction. Fig. 8-5, p. 151, shows such a nozzle for an oxygen lance. Finally, p. 153, lines 7 to 17, precise information is given on the half-angle at the top which should be adopted for the divergent tapered part. Avoid too large angles which reinforce the shock waves causing the jet to disperse too quickly. It is proposed to choose the half angle at the top of the injection nozzle in the range between 2.5 and 10 °, a half angle of 5 ° being considered as a practical compromise.

Experience has shown that the nozzles thus produced give relatively satisfactory results in the general case of decarburization of ordinary cast irons, but, on the other hand, difficulties have been encountered in the treatment of cast irons with chromium.

We first studied a decarburization process for chromium or chromium nickel cast iron containing in weight percent 1.5 to 8 of C, 10 to 30 of Cr and up to 30 of Ni, which comprises, at less in the final phase of decarburization, the formation of a gas / liquid iron emulsion in which the carbon is oxidized directly by oxygen.

The conditions necessary for the production of this emulsion are critical. Indeed, it is known that the production of emulsions between a liquid pig iron, a slag and a gas phase is relatively easy. This is the case with the decarburization of cast irons by the LD process. It is then the viscosity characteristics of the slag which play the most important role, as shown in the article by A. Chatterjee, NO Lindfor and JA Wester: “Process metallurgy of LD Steelmaking” (“Ironmaking and steelmaking” 1976 ", No. 1). On the other hand, so that an emulsion between a gaseous phase and the liquid metal can form and be maintained in a stable manner, even in the almost total absence of slag, until the carbon content is lowered to a final level close to 0.2%, well-defined conditions of liquid iron temperature, carbon content, distance to the bath, flow rate and oxygen pressure must be achieved.

The tests have shown that, when the favorable conditions are met and the emulsion between the gas phase and the liquid iron is formed and is maintained satisfactorily, both rapid and extensive decarburization are obtained and, at the same time, a yield in particularly high chrome metal. Tests were thus carried out on unit quantities of chromium cast iron of approximately 60 kg. The nozzle used had a neck diameter of 2 mm, and a flow rate of 168 NI / min of oxygen.

The studies carried out and the numerous tests carried out for the development of this decarburization process for chromium cast irons have shown that it was not enough to adequately adjust the operating parameters which had been identified in order to obtain a reproducible establishment of a stable emulsion. In many cases, delays in the formation of the emulsion and a certain instability thereof were observed; these delays and / or this instability led to a fall in the yields of chromium and iron. In some cases even, the emulsion did not form at all.

We therefore sought a means of improving the reproducibility of the triggering of the gas / liquid iron emulsion and also of increasing the stability of this emulsion once formed.

The means which is the subject of the invention is a new nozzle for an oxygen injection lance with a supersonic jet, thanks to which it is possible to cause with much greater efficiency the formation of the gas / cast iron emulsion. liquid.

This new nozzle according to the invention is characterized by a divergent which comprises, beyond the neck or throttling, a frustoconical part whose angle at the top is between 60 and 70 ° and, preferably, between 62 and 66 °, the angle of 65 ° being close to the optimum under the conditions of the tests. This frustoconical part can be extended by a surface of revolution around the same axis, the generator of which has a concavity turned inwards, so as to reduce the dispersion of the jet.

The new nozzle according to the invention makes it possible to implement in a particularly effective manner the process for decarburizing chromium cast irons by a jet of supersonic oxygen which had been developed.

This new nozzle also applies in a much more general way to the decarburization of cast irons of all types thanks to a particular efficiency for causing the emulsion of liquid cast iron by means of the gas phase, and, possibly also, the emulsion of liquid dairy.

The example and the figures below make it possible to better understand the characteristics of the nozzle which is the subject of the invention as well as its application to the decarburization of chromium cast irons.

Fig. 1 shows a nozzle of known type for supersonic oxygen jet.

Fig. 2 represents the variation of the thrust of the jet as a function of the angle at the top of the diverging point of a frustoconical nozzle.

Fig. 3 shows a nozzle for a supersonic oxygen jet according to the invention.

Fig. 4 shows the variation of the chromium yield as a function of the angle at the top of the frustoconical diverging portion of the nozzle.

Fig. 5 shows the variation of the iron yield as a function of this same angle at the top. Fig. 6 shows a nozzle according to the invention with an improved profile.

In the first decarburization tests on chrome cast irons carried out, an oxygen lance was used comprising a nozzle, the throat or neck of which consisted of a cylindrical tubular zone of approximately 2 mm in diameter and 20 mm in length. The end of this zone bu bu Iaire on the outlet side was not extended by a divergent and the expansion of the oxygen jet was therefore absolutely not limited to the outlet of the constriction. Experience had shown that, under these conditions, the nozzle was covered with a layer of refractory oxides based on chromium oxide. This layer was deposited mainly during the period of formation and then maintenance of the gas / liquid iron emulsion.

An analysis of such a layer had given for example as composition:

We see from this example that this layer resulted essentially from the oxidation of the components of chromium cast iron, with the intervention of only a few percent of lime coming from the slag initially introduced - alumina and magnesia came from the lining. It was found that this deposit tended to surround the orifice of the nozzle and to form a more or less well-defined funnel around this orifice.

We had the idea of making a nozzle with a divergent, both to try to improve the efficiency of the jet and to avoid the risks of disturbance of this jet by refractory oxide deposits projected from the cast iron liquid. Nozzles were then produced, the general shape of which is shown in FIG. 1. These nozzles have an inlet 1 connected to the oxygen supply pipe, then a cylindrical neck 2 of approximately 2 mm in diameter and 20 mm in length and, finally, a frustoconical diverging portion 3 whose angle at the top a1 is approximately 10 ° according to the teaching of the book "BOF Steelmaking" cited above.

The test results for these nozzles have been negative. It was found, in fact, that there were no projections of oxides inside the divergent but it was not possible to form the gas / liquid iron emulsion in a reproducible and stable manner.

Tests were then undertaken in order to specify in particular the influence of the angle at the apex of the frustoconical divergence on the thrust of the jet at constant flow. Fig. 2 gives the results of such tests in the case of a series of eight nozzles with the same characteristics as those of FIG. 1, except the angle of the divergent. Nozzle 1 did not have a divergent, nozzles 2, 3, 4, 5, 6, 7 and 8 had divergent frustoconical 4 mm high whose angles at the top were respectively 41, 53, 61, 65, 69, 77 and 100 °.

Three series of tests were carried out for three different levels of oxygen flow: 172.181 and 193 NI / min. For each test, the N-shaped thrust exerted by the jet on the platform of a balance disposed at a distance of about 3 cm from the orifice was measured.

The curves 1, 2 and 3 plotted respectively for the flows of 193, 181 and 172 NI / min each time show the existence of the same singular point in the case of the divergence of 65 °.

Although this singular point corresponds to a minimum thrust, we had the idea of realizing according to the invention nozzles for the decarburization of cast irons by supersonic oxygen jet, comprising a frustoconical diverging whose angle at the top is between 60 and 70 ° and preferably between 62 and 66 °, the angle of 65 ° corresponding, to the accuracy of the measurements, to the minimum thrust, under the conditions of the test.

Fig. 3 shows a nozzle according to the invention. It comprises an inlet 4 and a neck 5 whose characteristics are similar to those of the inlet 1 and the neck 2 of FIG. 1. On the other hand, the frustoconical divergent 6 has an apex angle a2 of 65 °. It has been found that it is possible, by means of such a nozzle, to bring about a completely reproducible emulsion of chromium cast irons by means of a jet of supersonic oxygen, according to the process defined by the tests. mentioned above. In addition, this emulsion, once formed, has great stability and is maintained until final decarburization of the cast iron, that is to say until a carbon content close to 0.2%. It is possible, without being able to affirm it, that the particular efficiency of the diverging at an angle of 60 to 70 ° and, preferably between 62 and 66 °, is due to the particularly important turbulence caused in the flow oxygen through this form of divergence.

We then studied the influence of the angle at the top of the divergent on the average yields of Cr and Fe of the decarburization operation. The tests had in fact shown that, generally, great stability in the formation and maintenance of the gas / liquid iron emulsion was accompanied by very high yields of Fe and Cr. These yields are calculated in percent by weight by reporting the amounts of Fe and Cr contained in the steel obtained after decarburization to those initially contained in the cast iron.

The example below gives the results of a series of decarburization tests of cast iron carried out in an identical manner by varying only the angles of divergence of the nozzles.

A series of six nozzles of the same type as that of FIG. 3, but each having a tapered diverging angle with a different apex. The six apex angle values thus produced are: 41, 53, 61, 65, 69 and 77 °. Each of these nozzles has a cylindrical neck 2 mm in diameter and 20 mm long and the height of the divergent truncated cone is in all cases 4 mm. By means of each of these nozzles, three to nine castings of chromium cast iron were carried out according to a single procedure similar to that already described.

A batch of homogeneous pig iron was prepared having the following composition: Cr 17%, C 6%, Si 0.3%, Mn 0.3%, S <0.03%, P <0.03%.

For each test, a quantity of 60 kg of this cast iron is brought to 1380 ° C. in an oven comprising induction heating, the surface of the liquid cast iron being covered with 340 g of lime. Oxygen is then injected by means of a vertical lance with a flow rate of approximately 180 NI / min. One of the six nozzles just described is used and the vertical distance between the end of the nozzle and the bath is 26 mm. The oxygen thus ejected reacts with the bath and one can observe three successive phases. In a first phase, the oxygen reacts mainly on the surface of the iron bath, preferably oxidizing Cr, Si and Fe: as the oxides formed which contain mainly Cr ₂ 0 ₃ accumulate at on the surface of the bath, a secondary reaction of reduction of these oxides by carbon begins. The speed of this reduction reaction increases gradually at the same time as the temperature rises to about 1650 ° C around the tenth minute. The CO formed is released during this time and burns, giving flames.

In a second phase, from the eleventh minute, the reduction of oxides, mainly chromium oxide, by carbon becomes faster than the formation of these oxides. In this period of strong reaction, the temperature rises again, however less quickly. From about the fifteenth minute, the decarburization speed stabilizes, the carbon content, which is then about 4%, continues to decrease at a rate of about 0.3% per minute and, at the same time , a corresponding reduction in chromium oxide is observed. This mechanism continues until about the twentieth minute: the temperature of the bath then reaches approximately 1750 ° C., while the C content has dropped to approximately 2.9%. At the end of this second phase, the metal oxides formed initially are almost completely reduced.

Around the twentieth minute, the conditions are met for the start of a third phase, which will make it possible to lower the carbon content up to around 0.2%. At the start of this third phase, the temperature of the molten bath is very high. Under these conditions, while maintaining the conditions of oxygen flow and distance between the end of the nozzle and the melt bath unchanged, the formation, from the melt bath itself, of an emulsion between gas and cast iron which quickly covers the surface of the bath then develops in thickness until doubling the initial volume of the cast iron. Everything happens as if the iron itself, under the action of the jet of oxygen and the formation of CO, by direct reaction of the oxygen with the carbon contained in this iron, boiled in all its mass, thanks to the physico-chemical conditions achieved. Within the emulsion thus formed, the reaction rates are high, which allows decarburization to continue at a rapid rate, until a final carbon content of approximately 0.2% is reached. the twenty-ninth minute. The temperature is then around 1820 ° C. and the oxygen blowing is stopped.

The weighing of the steel obtained after solidification and the analysis of its iron and chromium content make it possible to determine the yields of corresponding Fe and Cr.

Figs. 4 and 5 make it possible to observe the variations in these yields as a function of the angle of the frustoconical diverging portion of the nozzle. It is very clearly seen that these yields go through a maximum for the nozzle which has an angle of 65 °. Although certain factors may slightly modify the optimal angle of the tapered diverging member according to the invention, it is, according to the tests carried out, between 62 and 66 ° in the field of the operating conditions which can be used.

With a nozzle with a 65 ° angle at the top, the deposits on the internal wall of the nozzle, due to projections of refractory oxides from the liquid metal, are low.

Additional tests have shown that these deposits can be further reduced and also reduce the dispersion of the jet by extending the frustoconical part of the divergent by a surface of revolution around the same axis, the generator of which has a concavity oriented inward. It is preferable that the tangent to the generating curve of this part of the nozzle is originally confused with the generator of the cone. At the end of the generator, the tangent can become parallel to the axis or close to parallelism.

Fig. 6 shows a nozzle according to the invention thus improved. We see the inlet 7, the cylindrical neck 8 and the frustoconical divergent angle 9 at the apex a2 which are similar to the corresponding parts of the nozzle according to the invention of FIG. 3. The part 10 which extends the frustoconical part 9 has a concavity facing the axis. At point 11 of connection between the truncated cone and the part 10, the tangent to the generatrix of the latter merges with the generatrix of the truncated cone. At point 12 at the end of the nozzle, the tangent to the generatrix of the concave part is almost parallel to the axis of the nozzle. Such an improved nozzle according to the invention has the non-negligible advantage, thanks to the shape of its outlet end, of eliminating the risks of penetration of oxide or even metal projections from the bath of molten metal.

The nozzle according to the invention can be made of various materials. It is often preferred to use copper. It is important to machine the interior surfaces properly and to give them a good surface finish to avoid catching particles of oxide or projected metal. In general, the nozzle is connected to a metal lance cooled by a suitable fluid.

Although the examples which have just been given relate to tests carried out on small quantities, it has been verified that the results obtained can be extrapolated on an industrial scale. Thus, the angles at the apex of the frustoconical divergences according to the invention also lead to optimal results in the case of large section nozzles used for the treatment of cast irons in industrial quantities.

In addition, although the nozzle which is the subject of the invention has been tested mainly in the case of decarburization of chromium cast irons, tests have shown that it could also be used advantageously for the decarburization of all types of fonts.

Thus, in the most general manner, it is possible to use oxygen lances equipped with nozzles according to the invention for the decarburization of cast irons in basic converter by methods such as LD or by similar methods.

In these different cases, the use of the nozzles according to the invention makes it possible to increase the efficiency and the speed of decarburization and also makes it possible to increase the iron yield.

Claims (4)

1. A nozzle for the decarburisation of cast irons by a supersonic oxygen jet comprising an inlet (4 or 7), a cylindrical neck portion (5 or 8) and a divergent portion having a frustoconical part (6 or 9) which is connected to the cylindrical neck portion, characterised in that the frustoconical part (6 or 9) of the divergent portion has an angle at the apex (a2) of between 60 and 70°.

2. A nozzle according to claim 1, characterised in that the frustoconical part (6 or 9) of the divergent portion has an angle at the apex (a2) of between 62 and 66° and preferably close to 65°.

3. A nozzle according to claim 1 or 2, characterised in that the divergent portion thereof comprises, extending the frustoconical part (9), a portion having a surface of revolution (10) about the same axis, the generating curve of which has an inwardly directed concavity.

4. Use of the nozzle according to one of claims 1 to 3 for the decarburisation of chromium cast irons.

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