MXPA01006135A - 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

METHOD FOR DIVIDING STEEL CASTED DURING ITS PRODUCTION DESCRIPTION OF THE INVENTION The present invention is concerned with the domain of the manufacture of steels of low nitrogen content. It is advantageously applied to the production of grades of low and very low carbon content. It is known that the presence of nitrogen in steel can be proved 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 thus its ability to stamping or if the nitrogen is present in the form of aluminum nitrides, as as a consequence of a limitation of the welding capacity due to a solution in the nitrogen in the ZAC (area affected by heat) and the resulting local mechanical embrittlement. However, the presence of nitrogen may also be undesirable due to its impact on the same stages of production rows, such as an increase in cracks bound to the ductility saucer to the direct current or the decrease in the suitability of the product. obtained to be profiled. The manufacturing processes or the grade of certain steels therefore require very low nitrogen content in the final product obtained, for example, to fix the ideas, from 15 to 20 ppm for the sheets intended for Ref: 130272 the automotive construction or for steels for packaging, 50 ppm approximately for plates of marine platforms or 40 to 60 ppm for the threads or cords of reinforcement of tires, etc. These nitrogen contents are obtained in the steel works, in all the stages of elaboration of molten metal, after the electric furnace or after the converter until its solidification in the direct current. It is known that the elaboration in electric furnace in particular, is distinguished by a strong nitrogen contamination of the metal, due to the catalytic pyrolysis of the nitrogen molecule of the air in the thermal zone of the electric arc that facilitates its transfer to the liquid metal. This phenomenon is known to be an important factor that prevents the elaboration by means of the "electric row", of a part of degrees made nowadays by the "row of foundry" (Reduction-melting of iron ores in melting in a blast furnace after refining in oxygen in a pneumatic converter) for which the lowest nitrogen contents, of the order of 20 ppm, are currently obtained. The physical-chemical mechanisms that record the evolution of the nitrogen content of liquid steel are well known (see for example the article by Ch. Gatellier in H. Gaye that appears in the REVUE of METALLURGIE, CIT of January 1986, pages 25 a 42). Nitrogen undergoes a "metal-gas" chemical equilibrium that can be expressed by the formula N or N2 (gas). The equilibrium constant of this reaction, which is written as KN = aN / (PN2)? , it depends weakly on the temperature in the operation domain of the reactors concerned (1500 to 170 ° C). aN is the activity of dissolved nitrogen, which can be assimilated to the nitrogen content of the metal in the case of weakly allied carbon steels and PN2 is the partial pressure of the nitrogen of the gas in contact with the liquid metal. This means that in the presence of atmospheric N2, the nitrogen content of the metal will continuously increase towards its solubility limit, which is situated in the vicinity of 430 ppm at the temperature of the melting steel (1600 ° C). The denitruración of the metal is, as far as itself, obtained when circulating to the liquid metal a washing gas that does not contain nitrogen (PN2 = 0) in order to displace the previous reaction towards the defecha (effect of washing). Industrially, this gas can be argon or injected helium, but at a low cost and high cost or carbon monoxide formed in situ by decarburizing the metal after oxygen injection, which is conventionally practiced in gaseous form or in particles (see for example the article by K. Shinme and T. Matsuo, "Acceleration of nitrogen removal with decarburization by powdered oxidizer blowing under reduced pressure", which appears in the Japanese magazine ISIJ in 1987). The limit to this injection practice of 02 is linked to the carbon content of the metal at the beginning of the decarburization, which will impose the volume of CO emitted over time and consequently the possible nitriding and thus whatever the nitrogen content initial and contemplated of the metal to elaborate. This physical-chemical process must be completed by the role played by the metal surfactants, namely oxygen and sulfur, which both have the effect of blocking the transfer of nitrogen between the metal and the gas. Due to this, beyond a certain activity in dissolved oxygen, corresponding to an upper limit of the carbon content (which is of the order of 0.1% by weight for carbon steels), the denitrification by means of washing gas can be totally inhibited. It is thus understood all the interest to be able to develop a technique of denitruración of the liquid metal that allows in particular to elaborate by means of the "electric" route steels where the content of nitrogen is similar to those obtained by the "fusion" route, that is to say of the order of 20 ppm, even less in the final product obtained. The object of the present invention is precisely to promote a denitrification of the melting metal which takes better advantage of the denitrification potential of the washing gas on the one hand and which, on the other hand, makes it possible to control the final nitrogen content regardless of the initial carbon content of the metallic bath, which is currently the case with a classic decarburization. For this purpose, the object of the invention is a process for the denitrification of melting steel in the course of processing by means of oxygen insufflation, characterized in that it consists of also providing carbon in an insuflable form (powdery carbon) and because the carbon and oxygen are injected together but separately to the breast of the same zone of the metallic bath (to some 20 cm of distance between them for example). Thus, in the zone of carbon and oxygen supply, conditions favorable to denitriding are locally created. Indeed, in the case of a simple injection of oxygen (in the case of classical decarburization), the injection zone (spearhead) will be translated quickly by carbon depletion that will slow the formation of CO and by a activity - of dissolved oxygen correlatively high that, as is known, will counteract the denitruración of the metal by the bubbles of CO formed. The joint contribution of carbon to this same zone will allow a faster formation of CO bubbles by reaction between the carbon and oxygen contributed and a reduction of the local activity of the dissolved oxygen. This results in a better efficiency of the denitriding by the emitted CO, which thus replaces the natural tendency of the nitrided steel to contact the nitrogen of the air on the surface and thus lead overall to a decrease in the nitrogen content of the metal . It is certainly remembered that in an arc furnace, as well as in any steelmaking reactor that composes the route or trajectory of metal processing, the surrounding wall is not and can not be strictly hermetic with respect to the external atmosphere. Consequently, the final nitrogen content of the product obtained necessarily results from an exchange between the nitrogen recoveries (air pollution for example) and the denitriding applied during the processing in the liquid state. On the other hand, by regulating the contributions in a stoichiometric manner (namely a kilogram of C per 0.9 Nm3 of 02), the carbon content of the metal bath is not modified. This is done - a CO emission of "constant bath carbon content" and thus the duration can then be adapted to the desired denitrification '(nitrogen content contemplated in relation to the initial nitrogen content).

The invention will be better understood and other aspects and advantages will appear in view of the following description given with reference to the accompanying drawings in which: Figure 1 is a graph showing the comparative evolution of the nitrogen weight content of a water bath electric steel that contains more than 0.15% carbon by weight, depending on the volume of CO emitted in the bath, from a single injection of oxygen (curve a) and from a co-injection of carbon-oxygen from according to the invention (curve b); Figure 2 is a graph analogous to that of the preceding figure, but in a decarburized bath, that is in the case where the carbon content of the metal bath is reduced, namely less than 0.1%; Figure 3 is a graph showing the comparative evolution of the weight content of nitrogen 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 injected carbon. The co-injection technique according to the invention has been tested and applied in industrial conditions in a small 6-ton capacity furnace, simultaneously providing carbon and oxygen by means of two independent injection lances where the exit tips are placed side by side. side at the same level to the breast of the steel bath of fusion to treat, to a score of centimeters to each other. The contribution of carbon has been made through a carbon of low content of sulfur and nitrogen (content by weight less than 0.1% for these two elements) and using either argon, nitrogen as carrier gas. The supply of oxygen is done either by injection of 02 gaseous, either by injection of iron ore (equivalent to 0.2 Nm3 of 02 for a 1 kilogram of ore). The quantitative results obtained are presented first in figures 1 and 2, where the carbon-oxygen co-injection (curve b) is compared with a simple decarburization (curve a) and which represents the evolution of the nitrogen content of the metal in function from the volume of CO emitted to the bath, for a steel respectively to more than 0.15% carbon (figure 1) and less than 0.10% (figure 2). As can be seen, for relatively low decarburized steels, the dissolved 02 content is always too weak to block the diffusion of dissolved nitrogen into the bubbles of the wash gas and that, either from the decarburizing CO of the bath (curve a) or the CO generated by reaction between the carbon and the oxygen supplied to the bath according to the invention (curve b). In fact, a quite similar step is observed of these two denitriding kinetic curves, also neighboring each other, given as a function of the cumulative amount of CO that is released from the bath over time, although a slightly better efficiency can be noted. order of 5 ppm, in favor of the mixed injection according to the invention. On the other hand, for decarburized or low carbon steels - where the border will be placed at 0.10% by weight to fix the ideas, because it is known that below this threshold it is not possible to denude by the usual simple game of the decarburization -, it is observed in figure 3 that the kinetics of denitriding in case of co-injection (curve b) also has a step that in the preceding case and that is therefore independent of the initial carbon content of the bath. On the other hand, in the classic case of the mono-injection of 02 only (curve a), a systematic recovery of nitrogen is observed which increases regularly throughout the emission of decarburizing CO. This phenomenon of nitrogen recovery that, as already explained is the result of two mechanisms that act simultaneously but in the opposite direction, clearly show that in the case of low carbon content, denitrification by CO decarburization is blocked by local formation, in the vicinity of gas bubbles, of oxidized phases of high activity and that consequently the recoveries of atmospheric nitrogen are the dominant mechanism, all the more powerful besides the surface of the bath is then agitated by the bubbles that arrive and explode (curve a). On the contrary, as shown in curve b of 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 means of CO of washing, regardless of the initial carbon content, therefore the same for the very low carbon content. The influence of the carbon transport gas on the results obtained is given in figure 3. It can be seen that with a carbon injection under nitrogen flow (curve 1), the kinetics of denitriding is slower and leads to a limit metal nitrogen content (flat part p) below which it can not be accessed, higher than in the case of injection under argon flow. It is nevertheless possible to obtain a denitriding in this case which may be compatible with an "average" objective in the contemplated nitrogen content (flat part p at 35 ppm in the present case for example). The method of denitrification of the invention proves to be sufficiently flexible to allow multiple variants of application, where some examples are mentioned below: Use of any type of contribution of carbon and oxygen It can in fact be used as an oxygen donor oxidizing gas or any oxidizing powder (iron ore, but also manganese ore, silica powder, etc.). Likewise, any type of carbon product could be used for the purpose of carbon input. It would also be possible to use products containing both these elements, for which the local contribution is thus effected in known ways by automated means, including mixtures prepared in advance (coal / iron ore mixture for example).

-Use of any contribution technology that ensures the "local" conditions contemplated herein. In fact, classic injection lances, cooled or not, could be used; Partial submerged nozzles or any other form of injectors, either of the "separate injections" type for oxygen and carbon or of the "single injection" type of concentric or adjacent tubes.

-Use of this technique in any type of steel mill: The co-injection according to the invention can be practiced without particular difficulties in an electric furnace, but also in a converter with a blowing of 02 on the top (type LD, AOD) or by the background (type OBM, LWS); in furnace-enclosure or in installations under vacuum, type RH or it could also benefit from the vacuum effect in the denitriding (weak PN2 above the metal bath).

-Modification of the carbon / oxygen ratio in relation to stoichiometry The advantage of regulating the contributions of 02 and C to stoichiometry has been seen previously. Of course, it is also possible to maintain the nitrite conditions in the spear tip, slightly modifying this carbon-oxygen ratio in order to, for example, continue a metal decarburization at the same time as instead of the denitrurizing phase. Among the notable advantages of the invention will be noted in particular: the possibility of denitriding at low carbon contents. Due to the modification of local conditions (carbon content, dissolved oxygen activity) this technique allows, as we have seen, to denitrate the metal when the average carbon content of the metal bath is lower than 0.1% (limit below which no more denitrura with a simple decarburization). Phases of nitriding by emission of CO of "constant bath carbon content" have thus been made for an average carbon content of the bath comprised between 0.05 and 0.1% by weight. -the ease and flexibility of application of the process The technique does not need a strong investment. In the case of the electric furnace in particular, the necessary installations are in general already available in the plant, namely: an oxygen feed network coupled to a metal injection device (ordinarily already present for decarburization) and a powder distributor associated with a device for injecting carbon into metal (already present in general for the injection of carbon into the slag). This latter device must nevertheless be duplicated if a simultaneous injection of carbon and oxygen into the metal is sought, when a foaming scum develops in the metal bath at the same time. In the case of other processing reactors, it may be necessary to provide a carbon delivery device to the same zone as the injected oxygen. The cost of the practice - this denitrification technique is then summarized to that of consumables: Carbon and oxygen supply products and transport gas in the case of solid product injection. -a possible denitrification in "masked time" This technique can be particularly interesting in the case of a double-bowl electric furnace where the denitrification phase by simultaneous supply of carbon and oxygen could be done in masked time when the fusion is operated of a new metal charge in the other tank put under tension. For this, the denitriding operation will be done at the end of the elaboration of a face, out of electrical tension, the electrical power is transferred in the other tank for the fusion of the next load, without loss of productivity for the steel work. It will be evident that the method according to the invention can present multiple equivalents or variants of embodiment insofar as it is with respect to its definition given in the appended claims. It is noted that, in relation to this date, the best method known to the applicant to carry out the aforementioned invention is that which is clear from the present description of the invention.

Claims (4)

1. CLAIMS Having described the invention as above, the content of the following claims is claimed as property: 1. A process of denitrification of a melting steel bath during processing, by introducing oxygen, characterized in that it consists in providing the bath with carbon as well in insuflable form and because carbon and oxygen are injected jointly but separately into the same zone of the metal bath.
2. 2. The process according to claim 1, characterized in that the carbon and oxygen contributions are regulated in a stoichiometric manner.
3. 3. The process according to claim 1, characterized in that the carbon is injected in the powdery solid state with the aid of a transport gas.
4. 4. The process according to claim 1, characterized in that it is applied in a double-tub electrical steel work installation.