EP0032350B1 - Method for refining a molten metal bath in a crucible with oxygen injection from above, and crucible used therefor - Google Patents

Abstract

Process of refining of a metal bath in a crucible with oxygen blast at the top and crucible used. The invention consists essentially of providing in the bottom (15) of the crucible agitating gas injectors (15) located at least in a peripheral circular ring situated in immediate proximity to the refractory side wall. The injectors are preferably concentrated opposite the journals. Secondary injectors are, also preferably, provided in the bottom zone, intermediate between the ring and the center of the crucible. The invention makes it possible, in relation to the prior injection practice, to reduce the rate of dissolution of the agitating gas in the molten metal and then makes it possible to use a low-cost agitating gas, such as nitrogen, without risk of excessive nitridation of the bath.

Description

The present invention relates to the production of metals, in particular steel. It relates more particularly to the pneumatic refining of a cast iron bath contained in a crucible with oxygen blown from above.

We know that a top blown oxygen refining crucible is a metal container lined with a thick refractory lining. The container essentially consists of a vertical cylindrical part (called side wall) which is on the one hand closed at its lower end by a recessed circular bottom and on the other hand extended at its upper end by a frustoconical part forming the spout. Through this penetrates a vertical lance for blowing one or more refining oxygen jets towards the surface of the metal bath contained in the crucible.

It is known to be able to widen the metallurgical possibilities of techniques for refining cast iron by blowing oxygen from above by injecting brewing gas from the bottom of the crucible. The latter is most often a chemically inert gas with respect to impurities in the metal, such as nitrogen, argon, etc., pure or mixed. This technique is notably described in French patent n ° 2 233 401. But it is not excluded to use a reactive gas, such as oxygen or C0 ₂ , pure or mixed with an inert gas and which, in addition of its brewing function, participates in the refining itself by combining its action with that of the oxygen blown from above. French patent 1,210,459 describes such stirring by the bottom of the crucible using injectors arranged in an outer ring so as to leave free a central part with a diameter at least equal to half the total diameter of the bottom.

lorsque, comme le décrit par exemple la demande de brevet luxembourgeois n° 81 207, le procédé est mis en oeuvre conjointement avec le maintien d'une atmosphère d'oxygène bien répartie sur la surface du bain.We know that the beneficial effects that can be expected are notably easier blowing, an improvement in the iron yield and the quantity of steel as well as a reduction in the consumption of deoxidizers and ferro-alloys. One can also benefit from an increase in the quantity of cooling additions (scrap or ore) thanks to an increase in the rate of secondary combustion.

when, as described for example in Luxembourg patent application No. 81 207, the process is implemented in conjunction with maintaining an oxygen atmosphere well distributed over the surface of the bath.

One of the main technological characteristics of the process is the incorporation into the bottom of the crucible of injectors enabling the mixing gas to be blown into the bath at any time during and / or after the top blowing of oxygen. refining. The injectors used, which must ensure a selective passage of the stirring gas to avoid infiltration of molten metal in the opposite direction, are air-tight refractory elements. Elements of this type now exist, whose lifespan is equal to that of the bottom of the crucible and which have been the subject of several patent applications formulated by the inventors.

This flexibility in the choice of the time, the duration and the intensity of the gas injection makes it possible, as it was quickly underlined above, to considerably improve the performance of crucibles with oxygen blown from above. by combining their own advantages with those of conventional bottom blowing oxygen processes.

This new technique has just conquered the industrial sanction under the name “LBE” process (Lance-Brassage-Equilibre) evoking the balance that it tends to achieve between metal and slag in the crucible.

The inventors have now discovered that the LBE process makes it possible to greatly expand the possibilities of refining with oxygen from above, not only thanks to the flexibility already mentioned in the choice of the time, the duration and the intensity of gas mixing, but also - and quite surprisingly - by a judicious choice of the geometric conditions which such mixing must meet. In addition, this choice can be implemented in a very simple manner from considerations relating only to the location of the injectors in the bottom of the crucible.

To this end, the invention relates to a method for refining a metal bath, in particular of cast iron, in a crucible with oxygen blowing from above and according to which a gas is injected from the bottom of the crucible. of nitrogenous stirring, characterized in that, as a function of the tendency for the stirring gas to dissolve in the metal bath, the stirring gas is injected by means of air-tight refractory elements disposed at the periphery of the bottom of the crucible in the immediate vicinity of its side wall when said tendency is found to be significant, and when the tendency to dissolve is found to be weak, the stirring gas is optionally injected through the central part of the bottom situated substantially midway between the center of the bottom and the side wall of the crucible , the peripheral injection then being maintained or eliminated.

The subject of the invention is also a particular crucible for implementing the method, characterized in that it has, incorporated into its refractory lining, injectors constituted by air-tight refractory elements for blowing the stirring gas and located in the bottom of the crucible, in a peripheral ring located in the immediate vicinity of the lateral refractory wall of said crucible, and possibly also of the secondary injectors located in the intermediate middle position between the center of the bottom of the crucible and the peripheral ring and in that means are designed to supply the neck injectors separately peripheral ring and injectors in the middle position.

In accordance with an advantageous embodiment, the injectors are located in the bottom of the crucible in a region lying between the two traces of the surface of the bath on the bottom of the crucible when the latter is placed respectively in its two tilted positions.

Until now, it was customary to distribute the injectors in the bottom of the crucible in a circular arrangement approximately halfway between the center of the bottom and the side wall. This arrangement representing a logical location for obtaining an optimal stirring effect, taking into account the jet or jets of oxygen blown from above, the points of impact of which are located in the central region of the surface of the bath.

The implementation of the invention, which, as will be understood, basically consists in pushing the blowing gas blowing zone as far as possible towards the periphery of the bottom, presents, compared to the usual technique, a remarkable result : stabilize the nitriding of the bath to completely acceptable contents for practically all the nuances developed to date. The advantage of this result will be better understood after having indicated that, in prior practice, blowing with nitrogen leads to fluctuating residual contents of this element, often going beyond the tolerances which imply, or the scrapping of the load, or subsequent corrective treatments which are always delicate and expensive, for example vacuum treatments.

The invention therefore has in particular the advantage of being able to use in all circumstances and throughout the refining operation a brewing gas by nature soluble in the liquid metal under certain conditions, for example nitrogen , without risk of excessive dissolution. This possibility makes it possible to reduce the cost price of the operation in appreciable portions since, as is known, among gases for industrial and steel use in particular, nitrogen is undoubtedly the most economical, in particular as byproduct of the production of oxygen from atmospheric air.

Furthermore, it remains possible, in accordance with a preferred variant of the invention, to combine this peripheral location with the traditional location by geometrically splitting the injection possibilities in two concentric circles on the bottom of the crucible, one at the periphery , the other approximately halfway between the center of the bottom and the side wall of the crucible.

Thus, it is possible to increase the overall flow rate of stirring gas while modulating it in order in particular to reserve peripheral insufflation at the most critical moments when, during the development of the refining, the conditions are such that nitriding of the bath is to be feared (in particular at the end of the decarburization period, when the bath reaches its maximum temperature and the washing with CO is less effective).

It is therefore possible, from a pneumatic power station supplying separately the two categories of injectors - respectively in peripheral position and in middle position - to be placed in optimal stirring conditions while avoiding excessive nitrogen uptake by the bath. This result can be easily achieved by any suitable means of measuring the tendency of the bath to nitriding.

As such, and according to a preferred implementation of the invention, during the refining and at a sufficient rate, the parameters indicative of the tendency towards nitriding of the metal bath are determined and the reaction is obtained according to the response obtained. effecting a change in the geometrical conditions of the insufflation of the stirring gas, in the sense that this insufflation is preferably carried out by the injectors situated at the periphery of the bottom of the crucible.

It is recalled that the parameters indicative of the tendency to nitriding the bath are essentially the temperature of the latter as well as the progress of the refining reactions over time and more practically the progress of the reaction. decarburization.

It is understood that these parameters are determined empirically according to the configuration and the dimensions of the refining installations and that specific factors such as the flow rate of the oxygen blown from above and the characteristics must be taken into account. the jet (s) from the blowing lance.

In accordance with a preferred embodiment variant of the crucible according to the invention, the injectors are not distributed circularly but located in places such that, during the tilting of the crucible, they are no longer in contact with the bath of molten metal.

The advantage of such an arrangement is in particular to be able to stop the blowing of the stirring gas when it is not or no longer necessary, without risking infiltration in return of molten metal in the injectors. The benefit obtained is not negligible in the case of air-tight refractory elements, at least for some of them, whose good behavior is conditioned by the maintenance of a gas flow, light but necessary permanently, even outside brewing periods to establish a pressure in the elements counterbalancing the ferro-static pressure.

In one embodiment, the injectors are located symmetrically on either side of the tilting plane vertical to the tilting axis (axis of the pins equipping the crucible), the crucible being assumed itself in the vertical position.

This provision is valid for the main injectors located at the periphery, but also, if necessary, for the secondary injectors provided in the middle position between the center and the bottom edge. However, if we want to better distribute the blowing zones in the bottom of the crucible, we will have these median injectors in the immediate vicinity of the tilting plane, on both sides. be and in a direction parallel or substantially parallel to the tilt axis and, in any case, sufficiently distant both from the central zone subjected to the influence of the oxygen jets blown from above and from the side wall to no longer be in contact with the bath once the crucible has been tilted.

• la figure 1 est une vue schématique en coupe verticale d'un creuset conforme à l'invention selon le plan BB de la figure 2,
• . la figure 2 est une vue en coupe horizontale selon le plan AA de la figure et montrant le fond du creuset.

The invention will be better understood and other aspects and advantages will appear more clearly in the light of the description which follows given with reference to the single sheet of drawings appended in which:

• FIG. 1 is a schematic view in vertical section of a crucible according to the invention according to the plane BB of FIG. 2,
• . Figure 2 is a horizontal sectional view along the plane AA of the figure and showing the bottom of the crucible.

In the figures, the same elements are designated by identical references.

The crucible shown consists mainly of a metal breastplate 1 of axial symmetry covered internally with a thick refractory lining 2. The assembly thus formed consists of a cylindrical part 3 forming the side wall of the crucible and extended at its upper end by a frusto-conical part, or spout, 4, and closed in line with its lower part by a recessed bottom 5 by means of an annular seal made of refractory rammed earth represented in 6.

The cylindrical part has, approximately halfway up the assembly, a usual reinforcement belt 7 provided with two diametrically opposite pins 8 and 8 ′ ensuring the support of the crucible and allowing its tilting around the axis of tilting symbolized in 9. Furthermore, a vertical lance 10 for blowing oxygen from above penetrates axially into the crucible through the opening of the spout.

As can be seen, the bottom 5 of the crucible is equipped with mixing gas injectors 15 and 18 which, in this case, are air-tight refractory elements incorporated in the refractory masonry of the bottom.

It will be recalled that these elements, which are not part of the specific object of the invention, usually consist of a metal receptacle 11 inside which is placed an air-tight refractory mass 12 designed so that it can be crossed by gas under pressure. The gas arrives through a supply pipe 13 provided at the base of the element and enters the bath through the opposite face 14 intended to be placed in contact with the latter.

It is also emphasized that, for reasons of obvious convenience, these elements replace ordinary bricks constituting the bottom and that in this respect, they advantageously have an identical shape and an identical format, or very slightly smaller, respectively than the shape and format of the bricks they replace.

• - une première catégorie composée des éléments 15, séparés en deux groupes équivalents diamétralement opposés et disposés à la périphérie du fond 5, au voisinage immédiat de la paroi latérale 3 selon un cercle imaginaire 16 centré sur le milieu du fond, indiqué en 17 sur les figures;
• - une deuxième catégorie constituée des éléments 18, également séparés en deux groupes équivalents diamétralement opposés et placés en position intermédiaire entre les éléments périphériques 16 et le centre 17, selon un cercle imaginaire 19, concentrique au premier cité.

In accordance with a characteristic of the invention, the airtight refractory elements are differentiated into two categories identifiable by their geometric location on the bottom 5:

• a first category composed of the elements 15, separated into two equivalent diametrically opposite groups and arranged at the periphery of the bottom 5, in the immediate vicinity of the side wall 3 according to an imaginary circle 16 centered on the middle of the bottom, indicated at 17 on the figures;
• - A second category consisting of the elements 18, also separated into two equivalent diametrically opposite groups and placed in an intermediate position between the peripheral elements 16 and the center 17, according to an imaginary circle 19, concentric with the first city.

Before proceeding, it is important to remember that, in accordance with the main object of the invention, only the peripheral elements 15 are essential; the elements 18 being provided only incidentally as complementary elements, give the process greater flexibility as to the range of total flow rates of stirring gas that can be injected and as to a possible modulation of the flow rates according to different zones d injection on the bottom 5.

As such, it can be seen that there is provided, under the bottom and at a short distance, two concentric circular feeders 20 and 21 supplying by the tubulars 13 suitably arranged, respectively the elements 15 and the elements 18 in mixing gas.

These manifolds are themselves supplied with gas by the main pipes 22 and 23 which rise along the cylindrical part and pass through the pin 8 pierced for this purpose, to reach a source of pressurized gas symbolized at 24. This this is provided with accessories for separately adjusting the gas flow rates in elements 15 and 18 and to which we will return in more detail later.

As can be clearly seen in FIG. 2, the peripheral elements 15 are not distributed regularly along the circumference 16, but are concentrated in line with the axis 9 of the pins.

• on a traçé en 25 et 26 deux droites perpendiculaires au plan de basculement symbolisé en 27 et qui correspondent concrètement à l'intersection de la surface du bain avec le fond 5 du creuset lorsque ce dernier est en position basculée, soit d'un côté pour le décrassage du laitier, soit de l'autre pour la coulée du métal.

Such a provision results from the following considerations:

• we have traced at 25 and 26 two straight lines perpendicular to the tilting plane symbolized at 27 and which correspond concretely to the intersection of the surface of the bath with the bottom 5 of the crucible when the latter is in the tilted position, or on one side for the slag removal, either on the other for the casting of metal.

These traces which, on the usual blown crucibles from above, are equidistant from the center 17, therefore define between them a region of the bottom which is no longer in contact with the molten bath when the crucible is in one either of its tilt positions.

Therefore, it is understood that if it is desired, in accordance with a preferred embodiment of the invention, to remove the elements 15 in contact with the bath during the operations of emptying the crucible, they must be located on the portions of the circle 16 delimited by traces 25 and 26.

It is understood that the positions of these traces cannot be determined in advance once and for all, but may vary empirically from one installation to another depending in particular on the size of the crucible, the shape of its beak or its degree of filling.

The above indications are also applicable to the secondary elements 18. However, since these do not necessarily have to be placed very far from the center 17, it follows that they can advantageously be arranged in a circle entirely comprised between traces 24 and 25, that is to say, as shown in FIG. 2, in a circle such as 19 whose diameter is slightly less than the width of the bottom strip defined by these two traces.

However, it is often sufficient, taking into account the requirements for the flow of stirring gas, to provide secondary elements 18 only on a portion of the circle 19.

In this case, it is preferable to arrange them as far as possible from the elements 15 so as to distribute the blowing zone as well as possible on the bottom 5.

Such an arrangement is well represented in FIG. 2, where it can be seen that the elements 18 have been placed in the vicinity of the tilting plane 27, in orientations substantially perpendicular to those of the primary elements 15.

We will now give, with reference to the table of values below, indications relating to trials of implementation of the invention on an industrial scale.

These tests were carried out on a conventional crucible with oxygen blowing from above according to the traditional technique of the "L.D." process. and having a capacity of 310 tonnes.

In normal operation (without blowing from the bottom), the metal after refining and before casting fairly regularly has a nitrogen content very close to 20 ppm (parts per million) - 1st line of the table -.

The bottom of the crucible was initially equipped with twelve air-tight refractory elements arranged according to the prior art, that is to say regularly distributed in a circle approximately halfway between the center of the bottom and the side refractory wall.

The results obtained after 250 charges show an average nitrogen content of the bath of 25 to 30 ppm (line 2 of the table). These charges related to steel grades with carbon contents in the range of 25 to 105 thousandths of a percent. The maximum dispersions of the nitrogen content were observed for the grades between 70 and 100 thousandths of carbon percent and amounted to almost 35 ppm above the value indicated above. Under these conditions, a significant number of flows had to be decommissioned due to excessive nitriding. Subsequently, the twelve air-tight refractory elements were mounted in the bottom according to a geometry conforming to the essential characteristic of the invention, that is to say according to a circular crown, in the immediate vicinity of the lateral refractory wall. All other things being equal and for the same operating conditions as above, the tests, which involved a much smaller number of charges, were nevertheless sufficient to obtain significant statistical results.

These results showed that the nitriding of the bath was constantly maintained at values of 20 ppm (line 3 of the table) with a reduced dispersion having not led to any derating.

Here we find results practically identical to those obtained without blowing the stirring gas (1st line of the table) and it can therefore be concluded that the invention makes the metal bath almost "transparent" to the stirring nitrogen.

A third series of tests was carried out while respecting the method of installing the twelve air-tight elements, as previously described with reference to the figures, that is to say according to an arrangement in two categories 15 and 18.

Following a gas injection carried out according to an operational diagram which will be specified later, the results showed a tendency to nitriding which remained limited to around 20 ppm (line 4 of the table) and this despite the activation of the air-tight elements 18 in median arrangement.

In fact, these results could be obtained thanks to a blowing diagram which consisted essentially in a modulation of the flow rates blown by the elements of the two categories, according to the state of progress of the refining operation. More precisely, only the peripheral elements were activated during the phase of end of decarburization of the bath, the other elements having also been used only after the completion of this phase.

Of course, such an operating scheme could be implemented thanks to a system allowing separate supplies of each category of elements.

A system of this type has been schematically represented on the right-hand side of FIG. 1. It comprises, between the single source 24 of nitrogen under pressure and the two main supply lines 22 and 23, an electropneumatic group composed, from the output from the source, from a general volumetric counter 28 debiting in two circuits in parallel, each equipped with a solenoid valve 29 (29 ') with "all or nothing" control, followed by a sub-assembly 30 (30') grouping conventional components for adjusting the flow rate from a fixed or variable setpoint.

The whole is connected to the conduits 22 and 23 by a sealed circular connection 31 with two concentric passages at the level of the pin 8 and allowing the immobilization of the supply system despite the movements of the crucible on its tilting axis.

With regard to the operating mode, it is useful to recall that the skilled person, by the only experience he has acquired in the practice of refining with oxygen blown from above, knows at all times the 'state of progress of metallurgical reactions inside the crucible and can therefore act manually on the controls 29 and 29' with full knowledge of the facts.

The fact remains, however, that the invention lends itself perfectly to its integration into an automatic control system assisted by a computer, as symbolized at 32 and controlling the solenoid valves 29 and 29 'from a sensor 33 measuring the parameters representative of the 'state of progress of the refining operation, therefore of the tendency to nitriding the bath, such as for example the temperature of the reaction gases and their composition, in particular carbon monoxide.

It goes without saying that the invention cannot be limited to the examples described with reference to the figures, but extends to all variants or to all equivalents insofar as the characteristics contained in the appended claims are reproduced.

In particular, the invention applies not only to the case of nitrogen, but to any other nitrogenous stirring gas capable of dissolving in the molten metal contained in the crucible and the final quality of which is not insensitive to its concentration in this gas.

Thus, the invention, in its variant embodiment with two categories of geometrically differentiated injectors, appears, not only as a technique for reducing the nitriding of a steel bath stirred with nitrogen, but also, and much more general, like a technique. for controlling the rate of dissolution of a nitrogenous stirring gas in a molten bath, whatever the respective natures of this gas and of this bath.

In this respect, the invention is not limited either to two categories of injectors but extends to a higher number, knowing that the tendency to dissolve increases when the injectors are activated near the center of the bottom and decreases when approaching the side refractory wall.

Furthermore, these injectors are not necessarily preformed functional entities, but may very well have no individual individuality as such and appear during the actual construction of the bottom of the crucible, for example by means of specially shaped refractory bricks of so as to form therebetween, once juxtaposed, interstices reserved for blowing the gas, as described in particular by US Pat. No. 2,456,798 (SLICK).

Similarly, it is clear that the source of pressurized stirring gas is not necessarily unique, but can be increased tenfold at will, and in particular as many times as the number of categories of injectors.

Claims (7)

1. Method of refining a bath of metal in a crucible with oxygen blown through the top and according to which a nitrogenous stirring gas is injected through the base of the crucible, characterised in that, according to the tendency of the stirring gas to dissolve in the bath of metal, the stirring gas is injected by means of refractory elements permeable to gas arranged at the periphery of the bottom of the crucible in the immediate vicinity of its lateral wall when said tendency is found to be considerable, and when the tendency to dissolve is found to be slight the stirring gas is injected if required through the central part of the bottom situated substantially half way between the centre of the bottom and the lateral wall of the crucible, peripheral injection being then maintained or suppressed.

2. Method as claimed in claim 1, characterised in that peripheral injection is accomplished through the part of the bottom of the crucible included between two straight lines representing the intersection of the bottom with the surface of the bath when the crucible is in its tilted positions.

3. Method as claimed in claims 1 or 2, characterised in that peripheral injection of the stirring gas is accomplished in the part of the bottom in the immediate vicinity of a point plumb with the axis of tilting of the crucible.

4. Method as claimed in claims 1, 2 or 3 and according to which the stirring gas is of nitrogen, characterised in that during the course of refining the parameters indicative of the tendency to nitriding of the bath are determined by a measurement of the state of advancement of the decarbonisation of the bath.

5. Refining crucible with oxygen blown through the top for the implementation of the method according to claim 1, characterised in that it has, incorporated in its refractory lining, injectors consisting of refractory elements permeable to gas for the insufflation of the stirring gas located in the bottom of the crucible, according to a peripheral ring situated in the immediate vicinity of the lateral refractory wall of said crucible, and if required also secondary injectors located in a median position intermediate between the centre of the bottom of the crucible and the peripheral ring and in that means are provided for supplying separately the injectors of the peripheral ring and the injectors in a median position.

6. Crucible as claimed in claim 5, characterised in that the injectors are located in the bottom of the crucible in a region included between the two outlines of the surface of the bath on the bottom of the crucible when the latter is placed in its two respective tilted positions.

7. Crucible as claimed in claim 6, characterised in that the injectors of the peripheral ring are concentrated in the vicinity of a point plumb with the axis of tilting and in that the injectors in a median position are concentrated in the immediate vicinity of the plane of tilting of the crucible according to an orientation substantially perpendicular to that of said injectors of the peripheral ring.

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