EP1141422B1 - Method for denitriding molten steel during its production - Google Patents

Abstract

The invention concerns a method which consists in injecting into a molten metal bath to be treated, jointly but separately into the same bath zone, oxygen and carbon in a form capable of being blown (powder carbon preferably) so as to generate locally in the bath CO bubbles from those two elements, which will then be loaded in denitriding nitrogen. A stoichiometric adjustment of the carbon and oxygen inputs enable a constant carbon denitriding in the bath. The method is preferably applicable to the production of low-carbon steel grades, in particular in an electric oven.

Description

The present invention relates to the field of production of low steels nitrogen contents. It is advantageously applied to the development of shades at low and very low carbon.

We know that the presence of nitrogen in steel can be undesirable for different reasons. One of them is the impact of this element on the properties of use of steels, as a result of a decrease in the ductility of the metal and therefore in its ability to stamp, or, if the nitrogen is present in the form of aluminum nitrides, due to a limitation of the weldability due to re-solution of nitrogen in the ZAC (zone affected by the heat) and the resulting local mechanical embrittlement. However, the presence of nitrogen can also be undesirable because of its impact on the very stages of the production chains, such as an increase in cracks associated with the ductility ladle in continuous casting, or the decrease in the ability of the product obtained to be drawn.

The manufacturing processes, or the nuance of certain steels, therefore sometimes require very low nitrogen contents on the final product obtained, for example, to fix ideas, from 15 to 25 ppm for sheets intended for automobile construction or for steels for packaging, around 50 ppm for plates on offshore platforms, or from 40 to 60 ppm for tire reinforcement wires, etc. These nitrogen contents are expected at the steelworks, at all stages of the development of molten metal, from the furnace electric, or from the converter until it solidifies with continuous casting. We know that the elaboration in the electric oven, in particular, is distinguished by a strong contamination in nitrogen of the metal, due to the cracking of the nitrogen molecule from the air in the thermal zone of the electric arc which facilitates its transfer to the liquid metal. This phenomenon is known to be an important factor which prevents the development by the "electrical sector" of a part of grades produced today by the "cast iron industry" (reduction-smelting of iron ores in blast furnace then refining with oxygen in a pneumatic converter) by which lower nitrogen contents, of the order of 20 ppm, are commonly obtained.

The physicochemical mechanisms which govern the evolution of the nitrogen content of liquid steel are well known (see for example the article by Ch. Gatellier and H. Gaye published in the REVUE de METALLURGIE, CIT of January 1986, p. 25 to 42). Nitrogen follows a chemical "metal-gas" balance which can be expressed by the formula N ⇄ ½ N _{2 (gas)} ,. The equilibrium constant of this reaction, which is written K _N = a _N / (P _N2 ) ^½ , depends weakly on the temperature in the operating range of the reactors concerned (1550 to 1700 ° C). a _N is the dissolved nitrogen activity, which can be assimilated to the nitrogen content of the metal in the case of low-alloy carbon steels, and P _N2 is the partial nitrogen pressure of the gas in contact with the liquid metal. This means that in the presence of atmospheric N ₂ , the nitrogen content of the metal will continuously increase towards its solubility limit, which is in the vicinity of 430 ppm at the temperature of the molten steel (approximately 1600 ° C.).

The denitriding of the metal is obtained by circulating in the liquid metal a washing gas containing no nitrogen (P _N2 = 0) in order to move the above reaction to the right (washing effect) . Industrially, this gas can be argon or helium injected, but at low flow rate and with a high cost, or carbon monoxide formed in situ by the decarburization of the metal during the injection of oxygen, which is conventionally practiced in gaseous or particulate form (see for example the article by K. Shinme and T. Matsuo, "Acceleration of nitrogen removal with decarburization by powdered oxidizer blowing under reduced pressure", published in the Japanese journal ISIJ in 1987). The limit to this practice of injecting O ₂ is linked to the carbon content of the metal at the start of decarburization, which will impose the volume of CO emitted over time and therefore the possible denitriding, whatever the contents are. initial and target nitrogen of the metal to be produced.

JP-A-63 206 421 relates to a process for advanced denitriding of a steel bath by means of a simultaneous and separate injection of gaseous hydrocarbon and powdery oxide.

This physico-chemical approach must be supplemented by the role played by metal surfactants, namely oxygen and sulfur, both of which have the effect to block nitrogen transfers between metal and gas. Therefore, beyond a certain activity in dissolved oxygen, corresponding to an upper limit of the carbon content which is 0.1% by weight for carbon steels), denitriding by washing gas can be totally inhibited.

We can therefore understand the advantage of being able to develop a denitriding technique liquid metal, making it possible in particular to develop steels with the "electrical" sector the nitrogen contents are similar to those obtained by the "pig iron" process, that is to say from around 20 ppm or less on the final product obtained.

The purpose of the present invention is precisely to promote denitriding of molten metal which exploits the denitriding potential of the washing gas on the one hand, and which allows, on the other hand, to control the final nitrogen content independently of the content initial carbon of the metallic bath, whereas this is currently the case with a classic decarburization.

To this end, the invention relates to a process for denitriding molten steel being developed by oxygen blowing as defined in claim 1. In particular, carbon and oxygen are injected jointly but separately and side by side at the same level of the metal bath (at a distance of about 20 cm from each other for example).

Thus, in the carbon and oxygen supply zone, locally created favorable conditions for denitriding. Indeed, in the case of a simple injection oxygen (in the case of conventional decarburization), the injection area (lance nose) will quickly translate into carbon depletion which will delay the formation of CO, and by a correspondingly high dissolved oxygen activity which, as we know, will thwart the denitriding of the metal by the CO bubbles formed.

The joint contribution of carbon in this same area will allow more training CO bubbles quickly by reaction between carbon and oxygen supplied, and a reduction in local dissolved oxygen activity. This results in better efficiency of the denitriding by the CO emitted, which will thus supplant the natural tendency of steel to nitriding on contact with nitrogen from the surface air and therefore overall leading to a decrease in the nitrogen content of the metal.

It is indeed recalled that in an arc furnace, as elsewhere in any reactor iron and steel making up the metal production chain, the enclosure is not and cannot be strictly airtight with respect to the outside atmosphere. Consequently, the content final nitrogen of the product obtained necessarily results from a compromise between recoveries in nitrogen (contamination by air for example) and the denitriding implemented during working out in the liquid state.

Furthermore, by preferably adjusting the intakes stoichiometrically (namely 1 kg of C for 0.9Nm3 of O ₂ ), the carbon content of the metal bath is not modified. This produces a CO emission with "constant carbon content in the bath", the duration of which can then be adapted to the desired denitriding (targeted nitrogen content relative to the initial nitrogen content).

• la figure 1 est un graphique montrant l'évolution comparée de la teneur pondérale en azote d'un bain d'acier au four électrique contenant plus de 0.15% de carbone en poids, en fonction du volume de CO émis dans le bain, à partir d'une injection solitaire d'oxygène (courbe a) et à partir d'une co-injection carbone-oxygène selon l'invention (courbe b);
• la figure 2 est un graphique analogue à celui de la figure précédente, mais sur bain décarburé, c'est-à-dire dans le cas où la teneur pondérale en carbone du bain métallique est faible, à savoir inférieur à 0.1%;
• la figure 3 est un graphe montrant l'évolution comparée de la teneur pondérale en azote en fonction du volume de CO émis dans le bain par co-injection carbone-oxygène selon la nature du gaz de de transport du carbone injecté.

The invention will be well understood, and other aspects and advantages will appear on reading the following description given with reference to the accompanying drawing plates in which:

• FIG. 1 is a graph showing the comparative evolution of the nitrogen content by weight of a steel bath in an electric oven containing more than 0.15% of carbon by weight, as a function of the volume of CO emitted in the bath, from a solitary injection of oxygen (curve a) and from a carbon-oxygen co-injection according to the invention (curve b);
• Figure 2 is a graph similar to that of the previous figure, but on a decarburized bath, that is to say in the case where the carbon content by weight of the metal bath is low, namely less than 0.1%;
• FIG. 3 is a graph showing the comparative evolution of the nitrogen content by weight as a function of the volume of CO emitted in the bath by carbon-oxygen co-injection according to the nature of the transport gas of the carbon injected.

The co-injection technique according to the invention has been tested and implemented under industrial conditions in a small oven with a capacity of 6 tonnes, by simultaneously supplying carbon and oxygen by two independent injection lances whose outlet ends were placed side by side at the same level within the molten steel bath to be treated, about twenty centimeters from each other. The carbon was supplied by a coal with low sulfur and nitrogen contents (weight contents lower than 0.1% for these two elements), and using either argon or nitrogen as carrier gas . The oxygen was supplied either by injection of gaseous O ₂ or by injection of iron ore (equivalent to 0.2 Nm ₃ of O ₂ per 1 kg of ore).

The quantitative results obtained are first of all those presented in Figures 1 and 2 where we compare the co-injection of carbon and oxygen (curve b) to a simple decarburization (curve a), by representing the change in the nitrogen content of the metal as a function of volume of CO emitted into the bath, for a steel with more than 0.15% carbon respectively (fig.1) and less than 0.10% (fig.2).

As can be seen, for relatively poorly decarburized steels, the dissolved O ₂ content is always too low to succeed in blocking the diffusion of the dissolved nitrogen towards the bubbles of washing gases, and this, whether CO decarburization of the bath (curve a) or of the CO generated by reaction between the carbon and the oxygen supplied to the bath in accordance with the invention (curve b). We observe indeed a very similar appearance of these two kinetics of denitriding curves, which are also close to each other, given as a function of the cumulative amount of CO which emerges from the bath over time, again that a slightly better efficiency can be noted, of the order of 5 ppm, in favor of the mixed injection according to the invention.

On the other hand, for decarburized or low carbon steels - which we will place the border at 0.10% by weight to fix ideas, because we know that below this threshold we can no longer denitritrate by the simple usual game of decarburization, it can be seen in FIG. 3 that the kinetics of denitriding in the event of co-injection (curve b) has the same appearance as in the previous case, and that it is therefore independent of the initial carbon content of the bath. On the other hand, in the classic case of mono-injection of O ₂ alone (curve a), there is a systematic uptake of nitrogen which increases regularly throughout the emission of decarburization CO. This nitrogen uptake phenomenon, which, as already explained above, is the result of two mechanisms acting simultaneously but in opposite directions, clearly shows that in the case of low carbon, the denitriding by decarburization CO is blocked by local formation. , in the vicinity of gas bubbles, of oxidized phases with high activity, and that consequently the recovery of atmospheric nitrogen is the dominant mechanism, all the more powerful since the surface of the bath is then agitated by the bubbles who die there (curve a). On the other hand, like what curve b in fig. 1, in the case of co-injection according to the invention (curve b of FIG. 2), the dominant mechanism is always that of denitriding by the washing CO, independently of the initial carbon content, therefore even for very low carbon.

The influence of the carbon transport gas on the results obtained is given on the figure 3. We can see there that with an injection of coal under nitrogen flow (curve 1), the kinetics of denitriding is slower and leads to a limit nitrogen content of the metal (level p) below which one cannot access, higher only in the case of a injection under argon flow. It is nevertheless possible to obtain denitriding in this case there, which can be compatible with an "average" objective on the targeted nitrogen content (level p to 35 ppm in this case for example).

The denitriding method of the invention turns out to be flexible enough to realization to allow multiple variants of implementation, which are mentioned below some examples:

- Use of any type of carbon and oxygen supply.

Any oxidizing gas, or any oxidizing powder (iron ore, but also manganese ore, silica powder, etc.) Similarly, any type of carbon product can be used for the purpose of adding carbon.

We can also use products containing both of these elements, to which the local supply is then carried out in a known manner by automated means, or even mixtures prepared in advance (Coal / Iron Ore mixture for example).

- Use of any input technology ensuring the "local" conditions referred to here.

It will be possible in fact to use conventional injection lances, cooled or not; of the immersed parietal nozzles, or any other form of injectors, whether of the "to separate injections "for oxygen and carbon, or of the" single injection "type, with tubes concentric, or adjacent.

- Use of this technique in any type of steel reactor:

Co-injection according to the invention can be carried out without particular difficulties in the electric furnace, but also in the O ₂ blowing converter from the top (type LD, AOD) or from the bottom (type OBM, LWS); in a pocket oven or in vacuum systems, type RH, where one can also benefit from the effect of vacuum on denitriding (P _N2 low above the metal bath).

- Modification of the carbon / oxygen ratio compared to stoichiometry.

We have already seen the advantage of regulating the contributions of O ₂ and C to stoichiometry. As we understand, it is therefore also possible to maintain denitriding conditions at the nose, while slightly modifying this carbon / oxygen ratio, in order for example to continue decarburization of the metal at the same time as the denitriding phase takes place. .

Among the significant advantages of the invention, it will be noted in particular:

- the possibility of denitriding at low carbon contents.

Due to the modification of local conditions (carbon content, activity in dissolved oxygen), this technique allows, as we have seen, to denitride the metal while the average carbon content of the metal bath is less than 0.1% (limit below which we no longer denitrit with a simple decarburization). Denitriding phases by emission of CO at "constant carbon content of the bath" could thus be achieved for an average carbon content of the bath of between 0.05 and 0.1% by weight.

- the ease and flexibility of implementing the process.

The technique does not require heavy investment. In the case of the electric oven in particular, the necessary installations are generally already available in the factory, at know: an oxygen supply network coupled to a metal injection device (usually already present for decarburization), and a powder distributor associated with a device for injecting carbon into the metal (already generally present for the injection of coal in the slag). This last device will nevertheless have to be split if you want carry out a simultaneous injection of carbon and oxygen into the metal, while at the same time develops a foaming slag on the metal bath. In the case of other production reactors, it may be necessary to provide a device for supplying the carbon in the same area as the injected oxygen.

The cost of practicing this denitriding technique then comes down to that of consumables: carbon and oxygen supply products, and transport gases in the case injection of solid products.

- possible denitriding in "masked time",

This technique can be particularly interesting in the case of an oven electric double-tank, where the denitriding phase by simultaneous supply of carbon and oxygen can be done in masked time while the merger of a new metallic charge in the other tank energized. For this, the denitriding operation will be done at the end of the development of a load, without electric voltage, the electric power being transferred to the other tank for the fusion of the next charge, without loss of productivity for the steelworks.

It goes without saying that the method according to the invention can have multiple equivalents or variant embodiments insofar as its definition given in the attached claims.

Claims (4)

1. Method for denitriding a bath of molten steel during its production by the introduction of oxygen, wherein carbon in a form capable of being blown is likewise introduced into the bath, the carbon and the oxygen being injected jointly but separately into the bath of molten steel to be treated, characterized in that the carbon and the oxygen are injected side by side at the same level of the bath of molten steel, so as to generate a reaction between the injected carbon and oxygen.
2. Method according to Claim 1, characterized in that the inputs of carbon and oxygen are adjusted in a stoichiometric manner.
3. Method according to claim 1 or 2, characterized in that the carbon is injected in the powdery solid state with the aid of a transport gas.
4. Method according to any one of claims 1 to 3, characterized in that it is carried out in an electrical steelworks installation with double vat.

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