EP0093040A1 - Process for increasing the life of gas-permeable refractory elements in the bottoms of metallurgical vessels, especially for steel-basic-oxygen furnaces - Google Patents

Abstract

Method for improving the service life of permeable refractory elements fitted to the bottom of metallurgical refining vessels, in particular from top-blown oxygen blast steel converters, said elements serving for the controlled injection of a fluid stirring in the metal bath contained in the converter. The method is characterized in that, during the refining of any charge, a very fluid slag is formed which is kept in the converter after casting of metal by tilting the latter; in that the converter is then replaced in a vertical position so as to allow the deposition and spreading of the slag on the bottom, and in that the said slag is allowed to harden and ensure its setting against the bottom while maintaining a permanent flow of stirring fluid through permeable refractory elements. The invention makes it possible to extend the lifetime of the permeable refractory elements, and consequently, also to improve the lifetime of the bottom of the converter, while preserving, or even increasing, the permeability of these elements.

Description

The present invention relates to the field of the production of metals, in particular steel. More specifically, it relates to metallurgical refining vessels, in particular steel refinery converters with top-blown oxygen blowing, the bottom of which is provided with permeable refractory elements.

We know metallurgical treatments which consist in subjecting a bath of molten metal to a pneumatic stirring by controlled injection of a stirring fluid, usually an inert gas such as nitrogen or argon, through permeable refractory elements mounted in the usual refractory lining which forms the bottom of the container containing the bath (FR-A-2,322,202, US No. 3,259,484).

The application of such a brewing technique to a converter of steelworks to oxygen blown from above, is currently developing all over the world under the trade name "LBE process" (Lance - Brassage Equilibre). This process tends to achieve, as its name suggests, the balance between metal and slag and thus allows to combine to a large extent the respective advantages of conventional refining processes with top-blown oxygen and top-blown from the bottom.

Many solutions have already been proposed to give the refractory elements a sufficient selective permeability to ensure a satisfactory flow rate of stirring fluid, while avoiding penetration in the opposite direction of the molten metal. Among the known solutions, one can in particular quote that described in the European patent application published n ° 0021.861 and which consists in providing interstitial passages within a tight refractory mass, either using smooth-walled bodies incorporated in a monolithic refractory block, either by assembling juxtaposed refractory plates with interposition of calibrated spacers between them.

Furthermore, these elements, like any refractory material, inevitably wear out in contact with the molten metal. This wear is accelerated further by reason of the gas blowing causing convection of the very sensitive liquid metal at the blowing members, but the induced effects also make _one feel on the life of the vector surrounding refractory. In this With regard to this, experience shows that, in the case of traditional top-blown oxygen converters (LD type), the bottom generally wears out more slowly than the refractory on the wall, whereas it is rather the opposite which happens when these converters are provided with permeable refractory elements at the bottom.

If not to have to frequently repair the bottom and replace the used permeable elements, which would be highly penalizing, the problem which arises is therefore to know if it is possible to slow down the rate of wear of these elements, and this while preserving their permeability.

To this end, the subject of the invention is a method for improving the life of the permeable refractory elements equipping the bottom of metallurgical refining vessels, in particular the converters of steelworks with oxygen blowing from above, said elements serving for the controlled injection of a stirring fluid into the molten metal bath contained in the container, process characterized in that during the refining of any charge, a fluid slag is formed which is kept then in the converter after casting the molten metal by tilting said container; in that the container is straightened in a vertical position so that the slag settles and is distributed over the bottom; and in that said slag is allowed to dry and ensure its setting while maintaining in the permeable refractory elements a sufficient pressure to ensure a permanent flow of stirring fluid.

To fix the ideas, in the case of a steelworks converter with a capacity greater than 200 t, it will be possible to maintain, in the elements, a pressure ensuring a flow rate of stirring fluid of the order of 30 m ³ / h approximately per element, counted in m ³ gas.

If necessary, we can perfect the distribution of slag on the bottom by tilting the converter on either side of its vertical position.

Subsequently, it will be considered that the metallurgical refining container is a converter of steelworks to oxygen blown from above (type LD), it being understood that the invention also applies to any metallurgical container capable of tilting around 'a horizontal axis and in which the refining operation is accompanied by the formation of a slag.

The deposition technique according to the invention can be carried out systematically after each charge, or at least after each charge having led to exposure, even partial, of the permeable refractory elements.

The proposed method is very simple and poses no uncontrolled difficulty. It supposes only the precaution of forming, during, or more simply at the end of refining of a charge, a slag capable, by its fluidity, of flowing along the wall of the converter, then spreading over the bottom and stick to it while hardening. In practice, this boils down to a person skilled in the art to a visual verification of the slag's ability to meet these three requirements, which is moreover intended to be designated by the expression "a fluid slag is formed" used in the above formulation of the subject of the invention.

Thus, if the slag is not fluid enough, its spreading and bonding (mechanical grip) on the bottom are no longer guaranteed. To achieve this, we can add to it, at the end of ripening, thinning agents, such as fluorspar, or other usual flux, which lowers its melting temperature. Conversely, if the slag is too fluid, its hardening time is unnecessarily long. In addition, it is to be feared that its relatively too low melting point will cause its disappearance from the bottom as soon as the refining treatment of the next charge begins. In this case, thickening agents will be added to the slag, for example dolomite or any other refractory oxide or mixture of refractory oxides which is commonly used in steel works.

Once hardened and in mechanical grip on the bottom, the slag forms a refractory layer which subtracts the permeable elements from direct contact with the molten metal bath. Furthermore, the presence of these permeable elements at the bottom does not imply any other requirement during the hardening of the slag than that which consists in maintaining through them a low flow rate of brewing fluid, flow which can be qualified "safety" and which, moreover, does not penalize productivity in any way, since it takes place during the necessary hardening of the slag.

In addition, this flow which can be considered lost (that is to say not used for the treatment of the bath itself), increases only very slightly the overall cost of the operation, given its relatively low value compared to that used during the mixing of the bath (of the order of 150 m ³ / h). We can even say that the consequences on the cost are practically negligible, if we take care to choose a gas widely available on the market, like nitrogen for example, or where appropriate, a recovery gas produced in the plant itself, such as CO _Z.

Once the slag layer has hardened, the converter is ready for a new charge. It is noted from the start of the refining treatment, that not only the permeability of the bottom is preserved, but that it has even significantly increased compared to the level it had during the refining of the charges carried out without prior deposit of dairy on the bottom.

It is recalled for all practical purposes, that a possible indicator of the "level" of permeability can be constituted by the pressure / flow rate ratio of fluid in the supply line of the latter to the permeable refractory element. This ratio can be determined from a reference value taken when the element is in the new state by vacuum blowing, or during the refining of the first charge to the converter.

• - l'observation semble montrer que la préservation de la perméabilité est assurée par la présence d'un réseau de canaux reliant la face de soufflage de l'élément à la surface libre du fond au travers de la couche de laitier déposé, ce réseau se formant lors du séchage de ladite couche grâce au soufflage permanent du fluide de brassage,
• - quant à l'amélioration de cette perméabilité, il pourrait s'agir d'un phénomène interne à l'élément réfractaire perméable. On peut penser à priori que l'explication est d'origine thermique. Le refroidissement du fond, dont l'effet est accentué par le flux permanent de fluide de brassage, serait de nature à créer au sein des éléments de soufflage des contraintes mécaniques qui, en se libérant donnent naissance à un réseau de micro-fissures, s'amorçant préférentiellement en paroi des passages originels prévus pour le fluide de brassage.

With regard to the results obtained on permeability, the explanation is not yet fully understood:

• - the observation seems to show that the preservation of the permeability is ensured by the presence of a network of channels connecting the blowing face of the element to the free surface of the bottom through the layer of deposited slag, this network is forming during the drying of said layer thanks to the permanent blowing of the mixing fluid,
• - As for the improvement of this permeability, it could be a phenomenon internal to the permeable refractory element. One can think a priori that the explanation is of thermal origin. The cooling of the bottom, the effect of which is accentuated by the permanent flow of stirring fluid, would be likely to create within the blowing elements mechanical stresses which, when released, give rise to a network of micro-cracks, s '' priming preferably on the wall of the original passages provided for the stirring fluid.

One can also think of an explanation of an aeromechanical nature, the stirring fluid which can, for one part, circulate laterally in zones of lesser pressure drop which possibly form at the interface of the layer of slag deposited and of the pre-existing refractory bottom.

As already said, the technique according to the invention can be implemented at any time, between any two loads of the same campaign or even at the end of the first load, on a converter in the state new.

It should also be emphasized that, thanks to the improvement in the lifetime of the permeable elements obtained by the implementation of the invention, it is no longer the bottom behavior which limits the duration of a campaign. In other words, the bottom is no longer a problem for the wear of the converter, so that we find ourselves, on this plane, in the same situation as with a conventional converter with oxygen blowing from above. (type LD).

Furthermore, the invention applies whatever the type of permeable refractory elements used. It should be stressed, however, that excellent results have been obtained with the elements mentioned at the start and of which we can have more knowledge by referring to the detailed description given in European patent application No. 0021,861 already cited. .

Claims (5)

1) Method for improving the life of permeable refractory elements fitted to the bottom of steelworks converters with top-blown oxygen blowing, said elements serving for the controlled injection of a stirring fluid into the molten metal bath contained in the converter, a process characterized in that, during the refining of any charge, a fluid slag is formed, which is then kept in the converter after casting of the liquid metal by tilting of said converter; in that the converter is replaced in a vertical position so that the slag settles and is distributed over the bottom; and in that said slag is allowed to harden and ensure its setting on the bottom while maintaining in the permeable refractory elements a pressure sufficient to ensure a permanent flow of stirring fluid. 2) Method according to claim 1, characterized in that, in order to facilitate the distribution of slag on the bottom, the converter is tilted on either side of its vertical position. 3 °) Method according to claims 1 or 2, characterized in that, in order to increase the fluidity of the slag if necessary, is added to the converter, during or at the end of the refining, thinning agents intended to lower the slag melting temperature. 4 °) Method according to claims 1 or 2, characterized in that in order to reduce the fluidity of the slag if necessary, is added in the converter, during or at the end of ripening, thickening agents intended to increase the slag melting temperature. 5 °) Method according to claim 1, characterized in that, during the drying of the slag, is maintained in the permeable elements a sufficient pressure to ensure a permanent flow of stirring fluid of the order of 30 m ³ / h by element, counted in m gas.