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EP1141422A1 - Method for denitriding molten steel during its production - Google Patents

Method for denitriding molten steel during its production

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Publication number
EP1141422A1
EP1141422A1 EP99959490A EP99959490A EP1141422A1 EP 1141422 A1 EP1141422 A1 EP 1141422A1 EP 99959490 A EP99959490 A EP 99959490A EP 99959490 A EP99959490 A EP 99959490A EP 1141422 A1 EP1141422 A1 EP 1141422A1
Authority
EP
European Patent Office
Prior art keywords
carbon
denitriding
bath
oxygen
nitrogen
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
EP99959490A
Other languages
German (de)
French (fr)
Other versions
EP1141422B1 (en
Inventor
Jean-Christophe Mailhan
Daniel Pernet
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
USINOR SA
Original Assignee
USINOR SA
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Filing date
Publication date
Application filed by USINOR SA filed Critical USINOR SA
Publication of EP1141422A1 publication Critical patent/EP1141422A1/en
Application granted granted Critical
Publication of EP1141422B1 publication Critical patent/EP1141422B1/en
Anticipated expiration legal-status Critical
Expired - Lifetime legal-status Critical Current

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/04—Removing impurities by adding a treating agent
    • C21C7/072—Treatment with gases
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/0025—Adding carbon material
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/0037—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00 by injecting powdered material
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F27—FURNACES; KILNS; OVENS; RETORTS
    • F27D—DETAILS OR ACCESSORIES OF FURNACES, KILNS, OVENS OR RETORTS, IN SO FAR AS THEY ARE OF KINDS OCCURRING IN MORE THAN ONE KIND OF FURNACE
    • F27D3/00—Charging; Discharging; Manipulation of charge
    • F27D3/0025—Charging or loading melting furnaces with material in the solid state
    • F27D3/0026—Introducing additives into the melt
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F27—FURNACES; KILNS; OVENS; RETORTS
    • F27D—DETAILS OR ACCESSORIES OF FURNACES, KILNS, OVENS OR RETORTS, IN SO FAR AS THEY ARE OF KINDS OCCURRING IN MORE THAN ONE KIND OF FURNACE
    • F27D3/00—Charging; Discharging; Manipulation of charge
    • F27D3/18—Charging particulate material using a fluid carrier

Definitions

  • the present invention relates to the field of the production of steels with low nitrogen contents. It advantageously applies to the development of low and very low carbon grades.
  • the presence of nitrogen in steel can be undesirable for various reasons.
  • One of them is the impact of this element on the properties of use of steels, as a result of a reduction 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 a re-dissolution of the nitrogen in the ZAC (zone affected by heat) and the resulting local mechanical embrittlement.
  • 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 the cracks linked to the ductility pocket in continuous casting, or the decrease in the ability of the product obtained to be drawn.
  • These nitrogen contents are expected at the steelworks, at all the stages of the development of the molten metal, from the electric furnace, or from the converter until its solidification in continuous casting.
  • the preparation in the electric oven in particular, is distinguished by a strong nitrogen contamination 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.
  • 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
  • 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, the The nitrogen content of the metal will continuously increase towards its solubility limit, which is around 430 ppm at the temperature of the molten steel (approximately 1600 ° C).
  • 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 aim of the present invention is precisely to promote denitriding of the molten metal which makes the best use of the denitriding potential of the washing gas on the one hand, and which on the other hand allows the final nitrogen content to be controlled independently of the initial carbon content of the metal bath, whereas this is currently the case with conventional decarburization.
  • the subject of the invention is a process for denitriding molten steel during production by blowing in oxygen, characterized in that it consists in also providing carbon in an insufflable form (powdered carbon). , and in that carbon and oxygen are injected jointly but separately within the same zone of the metal bath (at a distance of about 20 cm from each other for example).
  • the carbon content of the metal bath is not modified.
  • FIG. 1 is a graph showing the comparative evolution of the weight content in nitrogen from a steel bath in an electric oven containing more than 0.15% carbon by weight, depending on the volume of CO emitted into the bath, from a solitary injection of oxygen (curve a) and from 'a carbon-oxygen co-injection according to the invention (curve b);
  • FIG. 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 gas transporting the injected carbon.
  • the co-injection technique according to the invention has been tested and implemented under industrial conditions in a small furnace of 6 tonnes of capacity, 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 bath of molten steel to be treated, about twenty centimeters from each other.
  • the carbon was supplied by a coal with low sulfur and nitrogen contents (weight contents of less than 0% for these two elements), and using either argon or nitrogen as the carrier gas.
  • 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 lkg of ore).
  • the denitriding method of the invention turns out to be flexible enough to allow multiple implementation variants, some examples of which are mentioned below: - Use of any type of carbon and oxygen supply.
  • Co-injection according to the invention can be carried out without particular difficulties in the electric furnace, but also in the converter blowing O 2 from above (type LD, AOD) or through the bottom (type OBM, LWS); in a pocket oven or in vacuum systems, type RH, where we can also benefit from the effect of vacuum on denitriding (P N2 low above the metal bath).
  • type LD type LD, AOD
  • OBM type OBM
  • LWS in vacuum systems
  • P N2 low above the metal bath type RH

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Mechanical Engineering (AREA)
  • General Engineering & Computer Science (AREA)
  • Treatment Of Steel In Its Molten State (AREA)
  • Refinement Of Pig-Iron, Manufacture Of Cast Iron, And Steel Manufacture Other Than In Revolving Furnaces (AREA)

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

PROCEDE DE DENITRURATION DE L'ACIER EN FUSION EN COURS D'ELABORATION PROCESS FOR DENITRURATION OF FUSED STEEL DURING DEVELOPMENT
La présente invention se rapporte au domaine de l'élaboration d'aciers à basses teneurs en azote. Elle s'applique avantageusement à l'élaboration des nuances à bas et très bas carbone.The present invention relates to the field of the production of steels with low nitrogen contents. It advantageously applies to the development of low and very low carbon grades.
On sait que la présence d'azote dans l'acier peut s'avérer indésirable pour différentes raisons. L'une d'entre-elles est l'impact de cet élément sur les propriétés d'emploi des aciers, par suite d'une diminution de la ductilité du métal et donc de son aptitude à l'emboutissage, ou, si l'azote est présent sous forme de nitrures d'aluminium, par suite d'une limitation de la soudabilité due à une remise en solution de l'azote dans la ZAC (zone affectée par la chaleur) et de la fragilisation mécanique locale résultante. Mais, la présence d'azote peut aussi être indésirable en raison de son impact sur les étapes même des filières de production, tel qu'une augmentation des criques liées à la poche de ductilité à la coulée continue, ou la diminution de l'aptitude du produit obtenu à être tréfilé.It is known that the presence of nitrogen in steel can be undesirable for various reasons. One of them is the impact of this element on the properties of use of steels, as a result of a reduction 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 a re-dissolution of the nitrogen in the ZAC (zone affected by 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 the cracks linked to the ductility pocket in continuous casting, or the decrease in the ability of the product obtained to be drawn.
Les processus de fabrication, ou la nuance de certains aciers, nécessitent donc parfois de très basses teneurs en azote sur le produit final obtenu, par exemple, pour fixer les idées, de 15 à 25 ppm pour les tôles destinées à la construction automobile ou pour les aciers pour emballage, de 50 ppm environ pour les plaques des plates-formes off-shore, ou de 40 à 60 ppm pour les fils de renforcement des pneumatiques, etc.. Ces teneurs en azote sont attendues à l'aciérie, à toutes les étapes de l'élaboration du métal en fusion, depuis le four électrique, ou depuis le convertisseur jusqu'à sa solidification à la coulée continue. On sait que l'élaboration au four électrique, notamment, se distingue par une forte contamination en azote du métal, due au cracking de la molécule d'azote de l'air dans la zone thermique de l'arc électrique qui facilite son transfert au métal liquide. Ce phénomène est connu pour être un facteur important qui empêche l'élaboration par la "filière électrique" d'une partie des nuances réalisées aujourd'hui par la "filière fonte" (réduction-fusion des minerais de fer en fonte au haut fourneau puis affinage à l'oxygène dans un convertisseur pneumatique) par laquelle des teneurs en azote plus basses, de l'ordre de 20 ppm, sont couramment obtenues.The manufacturing processes, or the grade 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 the automobile construction or for steels for packaging, of around 50 ppm for the plates of offshore platforms, or from 40 to 60 ppm for tire reinforcement wires, etc. These nitrogen contents are expected at the steelworks, at all the stages of the development of the molten metal, from the electric furnace, or from the converter until its solidification in continuous casting. We know that the preparation in the electric oven, in particular, is distinguished by a strong nitrogen contamination 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 elaboration by the "electric sector" of a part of the nuances carried out today by the "cast iron die" (reduction-fusion of the iron ores in cast iron in the blast furnace then oxygen refining in a pneumatic converter) by which lower nitrogen contents, of the order of 20 ppm, are commonly obtained.
Les mécanismes physico-chimiques qui régissent l'évolution de la teneur en azote de l'acier liquide sont bien connus (voir par exemple l'article de Ch. Gatellier et H. Gaye paru dans la REVUE de METALLURGIE, CIT de Janvier 1986, p. 25 à 42). L'azote suit un équilibre chimique "métal-gaz" qui peut s'exprimer par la formule N •s=*' lA N , z) LaThe physico-chemical 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 "metal-gas" chemical equilibrium which can be expressed by the formula N • s = * ' l AN, z) La
1/2 constante d'équilibre de cette réaction, qui s'écrit K^ = aN /(PN2) , dépend faiblement de la température dans le domaine de fonctionnement des réacteurs concernés (1550 à 1700°C). aN est l'activité en azote dissous, laquelle peut être assimilée à la teneur en azote du métal dans le cas des aciers au carbone faiblement alliés, et PN2 est la pression partielle d'azote du gaz en contact avec le métal liquide. Ceci signifie qu'en présence de N atmosphérique, la teneur en azote du métal va continuellement augmenter vers sa limite de solubilité, laquelle se situe au voisinage de 430 ppm à la température de l'acier en fusion (1600°C environ).1/2 equilibrium constant of this reaction, which is written K ^ = a N / (P N2 ), depends slightly 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, the The nitrogen content of the metal will continuously increase towards its solubility limit, which is around 430 ppm at the temperature of the molten steel (approximately 1600 ° C).
La dénitruration du métal est, quant à elle, obtenue en faisant circuler dans le métal liquide un gaz de lavage ne comportant pas d'azote (PN2 = 0) afin de déplacer la réaction ci- avant vers la droite (effet de lavage). Industriellement, ce gaz peut être de l'argon ou de l'hélium injecté, mais à faible débit et avec un coût élevé, ou du monoxyde de carbone formé in situ par la décarburation du métal lors de l'injection d'oxygène, laquelle se pratique classiquement sous forme gazeuse ou particulaire (voir par exemple l'article de K.Shinme et T. Matsuo, "Accélération of nitrogen removal with decarburization by powdered oxidizer blowing under reduced pressure", paru dans la revue japonaise ISIJ en 1987). La limite à cette pratique d'injection d'O2 est liée à la teneur en carbone du métal en début de décarburation, qui va imposer le volume de CO émis au cours temps et donc la dénitruration possible, et ce quelque soient les teneurs en azote initiale et visée du métal à élaborer.The denitriding of the metal is obtained by circulating in the liquid metal a washing gas which does not contain 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 possible denitriding, regardless of the contents. initial and target nitrogen of the metal to be produced.
Cette approche physico-chimique doit être complétée par le rôle joué par les éléments tensio-actifs du métal, à savoir l'oxygène et le soufre, qui ont tous deux pour effet de bloquer les transferts d'azote entre métal et gaz. De ce fait, au-delà d'une certaine activité en oxygène dissous, correspondant à une limite supérieure de la teneur en carbone qui est de l'ordre de 0,1% pondéraux pour les aciers au carbone), la dénitruration par gaz de lavage peut être totalement inhibée. On comprend ainsi tout l'intérêt à pouvoir développer une technique de dénitruration du métal liquide permettant notamment d'élaborer par la filière "électrique" des aciers dont les teneurs en azote soient similaires à celles obtenues par la filière "fonte", c'est-à-dire de l'ordre de 20 ppm, voire moins sur le produit final obtenu.This physico-chemical approach must be complemented by the role played by the surface-active elements of the metal, namely oxygen and sulfur, which both have the effect of blocking the transfer of nitrogen between metal and gas. Therefore, beyond a certain dissolved oxygen activity, corresponding to an upper limit of the carbon content which is of the order of 0.1% by weight for carbon steels), denitriding by washing can be totally inhibited. We can therefore understand the advantage of being able to develop a technique for denitriding liquid metal, making it possible in particular to develop steels whose nitrogen contents are similar to those obtained by the "cast iron" sector, using the "electric" sector. that is to say of the order of 20 ppm, or even less on the final product obtained.
Le but de la présente invention est précisément de promouvoir une dénitruration du métal en fusion qui exploite au mieux le potentiel dénitrurant du gaz de lavage d'une part, et qui permette, d'autre part, de contrôler la teneur finale en azote indépendamment de la teneur initiale en carbone du bain métallique, alors que c'est actuellement le cas avec une décarburation classique.The aim of the present invention is precisely to promote denitriding of the molten metal which makes the best use of the denitriding potential of the washing gas on the one hand, and which on the other hand allows the final nitrogen content to be controlled independently of the initial carbon content of the metal bath, whereas this is currently the case with conventional decarburization.
A cet effet, l'invention a pour objet un procédé de dénitruration de l'acier en fusion en cours d'élaboration par insufflation d'oxygène caractérisé en ce qu'il consiste à apporter également du carbone sous une forme insufflable (carbone pulvérulent), et en ce que carbone et oxygène sont injectés conjointement mais séparément au sein de la même zone du bain métallique (à quelques 20 cm de distance l'un de l'autre par exemple).To this end, the subject of the invention is a process for denitriding molten steel during production by blowing in oxygen, characterized in that it consists in also providing carbon in an insufflable form (powdered carbon). , and in that carbon and oxygen are injected jointly but separately within the same zone of the metal bath (at a distance of about 20 cm from each other for example).
Ainsi, dans la zone d'apport du carbone et de l'oxygène, on crée localement des conditions favorables à la dénitruration. En effet, dans le cas d'une injection simple d'oxygène (cas de la décarburation classique), la zone d'injection (nez de lance) va se traduire rapidement par un appauvrissement en carbone qui va retarder la formation de CO, et par une activité en oxygène dissous corrélativement élevée qui, on le sait, va contrarier la dénitruration du métal par les bulles de CO formées. L'apport conjoint de carbone dans cette même zone va permettre une formation plus rapide des bulles de CO par réaction entre carbone et oxygène apportés, et une réduction de l'activité locale en oxygène dissous. On obtient de ce fait une meilleure efficacité de la dénitruration par le CO émis, laquelle va ainsi supplanter la tendance naturelle de l'acier à se nitrurer au contact de l'azote de l'air en surface et conduire donc globalement à une diminution de la teneur en azote du métal.Thus, in the carbon and oxygen supply zone, conditions favorable to denitriding are created locally. In fact, in the case of a simple injection of oxygen (in the case of conventional decarburization), the injection zone (lance nose) will quickly result in a carbon depletion which will delay the formation of CO, and by a correspondingly high dissolved oxygen activity which, as we know, will counter the denitriding of the metal by the CO bubbles formed. The joint supply of carbon in this same zone will allow faster formation of CO bubbles by reaction between carbon and oxygen supplied, and a reduction of the local activity in dissolved oxygen. This results in a better efficiency of denitriding by the CO emitted, which will thus supplant the natural tendency of steel to nitride in contact with nitrogen from the surface air and therefore lead overall to a reduction in the nitrogen content of the metal.
On rappelle en effet que dans un four à arcs, comme d'ailleurs dans tout réacteur sidérurgique qui compose la filière d'élaboration du métal, l'enceinte n'est pas et ne peut pas être rigoureusement étanche à l'égard de l'atmosphère extérieure. En conséquence, la teneur finale en azote du produit obtenu résulte nécessairement d'un compromis entre les reprises en azote (contamination par l'air par exemple) et la dénitruration mise en oeuvre lors de l'élaboration à l'état liquide.It is recalled in fact that in an arc furnace, as elsewhere in any steel reactor which makes up the metal production process, the enclosure is not and cannot be strictly sealed with respect to the outdoor atmosphere. Consequently, the final nitrogen content of the product obtained necessarily results from a compromise between the nitrogen uptake (contamination by air for example) and the denitriding implemented during the preparation in the liquid state.
Par ailleurs, en réglant de préférence les apports de façon stoechiométrique (à savoir lkg de C pour 0,9Nm3 d'O2), on ne modifie pas la teneur en carbone du bain métallique. On réalise ainsi une émission de CO à "teneur en carbone du bain constante", et dont la durée peut alors être adaptée à la dénitruration souhaitée (teneur en azote visée par rapport à la teneur en azote initiale).Furthermore, by preferably regulating the intakes stoichiometrically (namely 1 kg of C for 0.9Nm 3 of O 2 ), the carbon content of the metal bath is not modified. This produces a CO emission with "constant carbon content of the bath", the duration of which can then be adapted to the desired denitriding (targeted nitrogen content relative to the initial nitrogen content).
L'invention sera bien comprise, et d'autres aspects et avantages apparaîtront au vu de la description qui suit donnée en référence aux planches de dessins jointes sur lesquelles: - 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);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: - Figure 1 is a graph showing the comparative evolution of the weight content in nitrogen from a steel bath in an electric oven containing more than 0.15% carbon by weight, depending on the volume of CO emitted into the bath, from a solitary injection of oxygen (curve a) and from 'a carbon-oxygen co-injection according to the invention (curve 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%;- 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%;
- 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. La technique de co-injection selon l'invention a été testée et mise en oeuvre dans des conditions industrielles dans un petit four de 6 tonnes de capacité, en apportant simultanément carbone et oxygène par deux lances d'injection indépendantes dont les extrémités de sortie étaient placées cote à cote au même niveau au sein du bain d'acier en fusion à traiter, à un vingtaine de centimètres l'une de l'autre. L'apport de carbone a été réalisé par un charbon à basses teneurs en soufre et azote (teneurs pondérales inférieures à 0J%o pour ces deux éléments), et en utilisant soit de l'argon, soit de l'azote comme gaz porteur. L'apport d'oxygène s'est fait soit par injection d'O2 gazeux, soit par injection de minerai de fer (équivalent de 0,2 Nm3 d'O2 pour lkg de minerai).- Figure 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 gas transporting the injected carbon. The co-injection technique according to the invention has been tested and implemented under industrial conditions in a small furnace of 6 tonnes of capacity, 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 bath of molten steel to be treated, about twenty centimeters from each other. The carbon was supplied by a coal with low sulfur and nitrogen contents (weight contents of less than 0% for these two elements), and using either argon or nitrogen as the carrier gas. 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 lkg of ore).
Les résultats quantitatifs obtenus sont d'abord ceux présentés aux figures 1 et 2 où l'on compare la co-injection de carbone et d'oxygène (courbe b) à une décarburation simple (courbe a) et ce, en représentant l'évolution de la teneur en azote du métal en fonction du volume de CO émis dans le bain, pour un acier respectivement à plus de 0J5 % en carbone (figJ) et à moins de 0.10 % (fig.2).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) with a simple decarburization. (curve a) and this, by representing the evolution of the nitrogen content of the metal as a function of the volume of CO emitted in the bath, for a steel respectively with more than 0J5% carbon (figJ) and less than 0.10% (fig.2).
Comme on peut le voir, pour les aciers relativement peu décarburés, la teneur en O2 dissous est toujours trop faible pour parvenir à bloquer la diffusion de l'azote dissous vers les bulles de gaz de lavage, et ce, que ce soit du CO de décarburation du bain (courbe a) ou du CO généré par réaction entre le carbone et l'oxygène apportés au bain conformément à l'invention (courbe b). On observe en effet une allure tout à fait similaire de ces deux courbes de cinétique de dénitruration, d'ailleurs voisines l'une de l'autre, données en fonction de la quantité cumulée de CO qui se dégage du bain avec le temps, encore que l'on peut noter une efficacité légèrement meilleure, de l'ordre de 5 ppm, en faveur de l'injection mixte selon l'invention.As we can see, for relatively low 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 it is 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 one another, given as a function of the cumulative amount of CO which is released 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.
En revanche, pour les aciers décarburés ou à bas carbone -dont on placera la frontière à 0J0 % en poids pour fixer les idées, car on sait qu'en dessous de ce seuil on ne parvient plus à dénitrurer par le simple jeu habituel de la décarburation-, on observe sur la figure 3 que la cinétique de dénitruration en cas de co-injection (courbe b) a bien même allure que dans le cas précédent, et qu'elle est donc indépendante de la teneur initiale en carbone du bain. En revanche, dans le cas classique de la mono-injection d'O2 seul (courbe a), on constate une reprise systématique d'azote qui croît régulièrement tout au long de l'émission du CO de décarburation. Ce phénomène de reprise d'azote qui, comme déjà explicité auparavant, est la résultante de deux mécanismes agissant simultanément mais en sens contraire, montre clairement que dans le cas des bas carbone, la dénitruration par le CO de décarburation est bloquée par la formation locale, au voisinage des bulles de gaz, de phases oxydées à activité élevée, et qu'en conséquence les reprises d'azote atmosphérique sont le mécanisme dominant, d'autant plus puissant d'ailleurs que la surface du bain est alors agitée par les bulles qui viennent y crever (courbe a). Par contre, à l'instar de ce que montre la courbe b de la fig. 1, dans le cas de la co-injection selon l'invention (courbe b de la figJ), le mécanisme dominant est toujours celui de la dénitruration par le CO de lavage, indépendamment de la teneur initiale en carbone, donc même pour les très bas carbone. L'influence du gaz de transport du carbone sur les résultats obtenus est donnée sur la figure 3. On peut y voir qu'avec une injection du charbon sous flux d'azote (courbe 1), la cinétique de dénitruration est plus lente et conduit à une teneur en azote du métal limite (palier p) en-dessous de laquelle on ne peut accéder, plus élevée que dans le cas d'une injection sous flux d'argon. Il est néanmoins possible d'obtenir une dénitruration dans ce cas là, qui peut être compatible avec un objectif "moyen" sur la teneur en azote visée (palier p à 35 ppm dans le cas présent par exemple).On the other hand, for decarburized or low carbon steels -whose we will place the border at 0J0% by weight to fix ideas, because we know that below this threshold we can no longer denitride by the simple usual game of decarburization, it is observed 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 recovery of nitrogen which increases regularly throughout the emission of the 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, 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. J), the dominant mechanism is always that of denitriding by the washing CO, independently of the initial carbon content, therefore even for the very low carbon. The influence of the carbon transport gas on the results obtained is given in FIG. 3. It can be seen there that with an injection of the coal under nitrogen flow (curve 1), the kinetics of denitriding is slower and leads at a nitrogen content of the limit metal (p level) below which one cannot reach, higher than in the case of an injection under argon flow. It is nevertheless possible to obtain denitriding in this case, which may be compatible with an "average" objective on the nitrogen content targeted (level p at 35 ppm in the present case for example).
La méthode de dénitruration de l'invention s'avère être suffisamment souple de réalisation pour permettre de multiples variantes de mise en oeuvre, dont on mentionne ci- après quelques exemples: - Utilisation de tout type d'apport de carbone et d'oxygène.The denitriding method of the invention turns out to be flexible enough to allow multiple implementation variants, some examples of which are mentioned below: - Use of any type of carbon and oxygen supply.
On pourra en effet utiliser comme apporteur d'oxygène tout gaz oxydant, ou toute poudre oxydante (minerai de fer, mais aussi minerai de Manganèse, poudre de Silice, etc..) De même on pourra utiliser tout type de produit carboné aux fins de l'apport de carbone.We can indeed use as oxygen supply any oxidizing gas, or any oxidizing powder (iron ore, but also manganese ore, Silica powder, etc.) Similarly we can use any type of carbon product for the purpose of carbon intake.
On pourra également utiliser des produits contenant à la fois ces deux éléments, pour lesquels l'apport local est alors effectué de manière connues par des moyens automatisés, voire des mélanges préparés à l'avance (mélange Charbon/Minerai de fer par exemple).It will also be possible to use products containing both of these two elements, for 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).
- Utilisation de toute technologie d'apport assurant les conditions "locales" visées ici. On pourra utiliser en effet des lances d'injection classiques, refroidies ou non; des tuyères pariétales immergées, ou toute autre forme d'injecteurs, qu'il soient du type "à injections séparées" pour l'oxygène et le carbone, ou du type à "injection unique", à tubes concentriques, ou adjacents.- Use of any input technology ensuring the "local" conditions referred to here. We can use conventional injection lances, cooled or not; immersed parietal nozzles, or any other form of injectors, whether they are of the "separate injection" type for oxygen and carbon, or of the "single injection" type, with concentric tubes, or adjacent.
- Utilisation de cette technique dans tout type de réacteur sidérurgique: La co-injection selon l'invention peut se pratiquer sans difficultés particulières au four électrique, mais également au convertisseur à soufflage d'O2 par le haut (type LD, AOD) ou par le fond (type OBM, LWS); au four-poche ou dans les installations sous vide, type RH, où l'on pourra de surcroît bénéficier de l'effet du vide sur la dénitruration (PN2 faible au dessus du bain métallique). - Modification du rapport carbone/oxygène par rapport à la stoechiométrie.- 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 converter blowing O 2 from above (type LD, AOD) or through the bottom (type OBM, LWS); in a pocket oven or in vacuum systems, type RH, where we 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.
On a vu précemment l'avantage de régler les apports d'O2 et de C à la stoechiométrie. Comme on le comprend, il est en donc également possible de maintenir des conditions dénitrurantes en nez de lance, tout en modifiant légèrement ce rapport carbone/oxygène, afin par exemple de poursuivre une décarburation du métal en même temps qu'a lieu la phase dénitrurante.We have already seen the advantage of regulating the contributions of O 2 and C to stoichiometry. As can be understood, 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. .
Parmi les avantages marquants de l'invention, on notera en particulier:Among the significant advantages of the invention, it will be noted in particular:
- la possibilité de dénitruration à basses teneurs en carbone.- the possibility of denitriding at low carbon contents.
Du fait de la modification des conditions locales (teneur en carbone, activité en oxygène dissous), cette technique permet, comme on l'a vu, de dénitrurer le métal alors que la teneur moyenne en carbone du bain métallique y est inférieure à 0,1% (limite en-dessous de laquelle on ne dénitrure plus avec une simple décarburation). Des phases de dénitruration par émission de CO à "teneur en carbone du bain constante" ont pu être ainsi réalisées pour une teneur en carbone moyenne du bain comprise entre 0,05 et 0,1 %> en poids. - la facilité et souplesse de mise en oeuyre du procédé.Due to the modification of the local conditions (carbon content, dissolved oxygen activity), this technique makes it possible, 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 one no longer denitrites with a simple decarburization). Denitriding phases by CO emission with "constant carbon content of the bath" could thus be carried out 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.
La technique ne nécessite pas d'investissement lourd. Dans le cas du four électrique notamment, les installations nécessaires sont généralement déjà disponible en usine, à savoir: un réseau d'alimentation en oxygène couplé à un dispositif d'injection dans le métalThe 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, namely: an oxygen supply network coupled to a device for injecting into the metal
(ordinairement déjà présent pour la décarburation), et un distributeur de poudre associé à un dispositif d'injection du charbon dans le métal (déjà présent généralement pour l'injection de charbon dans le laitier). Ce dernier dispositif devra néanmoins être dédoublé si l'on veut réaliser une injection simultanée de carbone et d'oxygène dans le métal, alors que l'on développe dans le même temps un laitier moussant sur le bain métallique. Dans le cas des autres réacteurs d'élaboration, il peut être nécessaire de prévoir un dispositif d'apport du carbone dans la même zone que l'oxygène injecté.(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 latter device will nevertheless have to be split if one wishes to carry out a simultaneous injection of carbon and oxygen into the metal, while at the same time a foaming slag is being developed on the metal bath. In the case of other production reactors, it may be necessary to provide a device for supplying carbon in the same zone as the oxygen injected.
Le coût de la pratique de cette technique de dénitruration se résume alors à celui des consommables: produits d'apport du carbone et de l'oxygène, et gaz de transport dans le cas d'une injection de produits solides. - une dénitruration possible en"temps masqué", Cette technique peut être particulièrement intéressante dans le cas d'un four électrique à double-cuve, où la phase de dénitruration par apport simultané de carbone et d'oxygène pourra se faire en temps masqué pendant que s'opère la fusion d'une nouvelle charge métallique dans l'autre cuve mise sous tension. Pour cela, l'opération de dénitruration se fera en fin d'élaboration d'une charge, hors tension électrique, la puissance électrique étant transférée sur l'autre cuve pour la fusion de la charge suivante, sans perte de productivité pour l'aciérie.The cost of practicing this denitriding technique then comes down to that of consumables: carbon and oxygen supply products, and transport gas in the case of an injection of solid products. - possible denitriding in "masked time", This technique can be particularly advantageous in the case of an electric double-tank oven, where the denitriding phase by simultaneous supply of carbon and oxygen can be done in masked time for that the fusion of a new metallic charge takes place in the other energized tank. For this, the denitriding operation will be done at the end of the preparation of a load, without electric voltage, the electric power being transferred to the other tank for the fusion of the next load, without loss of productivity for the steelworks. .
Il va de soi que la méthode selon l'invention peut présenter de multiples équivalents ou variantes de réalisation dans la mesure où est respectée sa définition donnée dans les revendications jointes. It goes without saying that the method according to the invention can have multiple equivalents or variant embodiments as long as its definition given in the appended claims is respected.

Claims

R E V E N D I C A T I O N SR E V E N D I C A T I O N S
1) Procédé de dénitruration d'un bain d'acier en fusion en cours d'élaboration par introduction d'oxygène, caractérisé en ce qu'il consiste à apporter au bain également du carbone sous une forme insufflable, et en ce que carbone et oxygène sont injectés conjointement mais séparément au sein de la même zone du bain métallique.1) Method for denitriding a bath of molten steel in the course of preparation by introduction of oxygen, characterized in that it consists in bringing to the bath also carbon in an insufflable form, and in that carbon and oxygen are injected jointly but separately within the same area of the metal bath.
2) Procédé selon la revendication 1 caractérisé en ce que les apports de carbone et d'oxygène sont réglés de façon stoechiométrique.2) Method according to claim 1 characterized in that the carbon and oxygen supplies are regulated stoichiometrically.
3) Procédé selon la revendication 1 caractérisé en ce que le carbone est injecté à l'état solide pulvérulent à l'aide d'un gaz de transport.3) Method according to claim 1 characterized in that the carbon is injected in the pulverulent solid state using a transport gas.
4) Procédé selon la revendication 1 caractérisé en ce qu'on le met en oeuvre dans une installation d'aciérie électrique à double cuve. 4) Method according to claim 1 characterized in that it is implemented in an electrical steel plant installation with double tank.
EP99959490A 1998-12-18 1999-12-17 Method for denitriding molten steel during its production Expired - Lifetime EP1141422B1 (en)

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FR9816082A FR2787468B1 (en) 1998-12-18 1998-12-18 PROCESS FOR DENITRURATION OF MOLTEN STEEL DURING DEVELOPMENT
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