EP0156706A1 - Process for refining metals by injection - Google Patents

Abstract

A process for refining pig iron from below the surface of a molten metal bath, and the resulting product, in which an oxidizing gas is injected in the bath and, during the last refining stage, a mixture of oxidizing gas and inert gas is further injected. The inert gas content x of the mixture is made to vary according to a law corresponding to an oxidizing gas dilution curve that is located in an area determined by two envelope curves, to wit, a first maximum dilution curve defined by the straight line portions: x=-766.7 %C+168.7 for 0.16<%C<0.22 x=-550 %C+134 for 0.1<%C<0.16 x=-233 %C+102 for %C<0.1 and a second minimum dilution curve defined by the straight line portions: x=-1500 %C+255 for 0.14<%C<0.17 x=-400 %C+101 for 0.1<%C<0.14 x=-230 %C+84 for 0.05<%C<0.1 x=-72.5 for %C<0.

Description

The present invention relates to a process for refining cast iron in which an oxidizing gas, for example industrially pure oxygen, is injected to remove oxidizable impurities, such as carbon, and more particularly the processes in which all or part oxidizing gas is injected under the surface of the molten metal. Such processes are mainly known under the names of OBM, QBOP, LWS for those in which most of the oxygen is blown from the bottom, under the names of LD-OB, LD-OTB, STB for those in which only a small part of the oxygen is injected under the surface of the bath.

• 1.- l'oxygène traversant le bain métallique brasse de façon plus intense le bain et permet une meilleure approche des conditions d'équilibre et,
• 2.- la quantité de fumées d'oxyde de fer produites est beaucoup plus faible car la réaction d'oxydation du carbone se situe au sein même du métal contrairement aux procédés d'affinage par le dessus où cette réaction à lieu à l'interface laitier-métal. Il en découle que les procédés d'affinage par le haut sont impropres pour l'obtention, dans de bonnes conditions, des aciers à basses et très basses teneurs en carbone.

In the most commonly used pneumatic steelmaking processes, oxygen is blown through a lance above the charge so that the oxygen jet penetrates the melt and forms a very slag oxidized which, in contact with cast iron, reacts with carbon to form carbon monoxide. With bottom blowing processes, oxygen is injected under the surface of the bath through nozzles located at the bottom or near the bottom of the converter. A shielding gas, generally a hydrocarbon or a non-oxidizing gas (which may be in liquefied form) is used to surround the oxygen stream in order to reduce the very significant wear of the nozzles as well as of the refractories at the bottom of the converter. One of the appreciable advantages of these latter methods compared to the preceding ones is the possibility of obtaining higher yields of metal. These yields are obtained mainly because:

• 1.- the oxygen crossing the metal bath more intensely stirs the bath and allows a better approach of the equilibrium conditions and,
• 2.- the quantity of iron oxide fumes produced is much lower because the carbon oxidation reaction is located within the metal itself unlike refining processes from above where this reaction takes place at the interface slag-metal. It follows that the refining processes from above are unsuitable for obtaining, under good conditions, steels with low and very low carbon contents.

New methods have attempted to overcome this drawback: these are the LBE and LDAB methods, for example, in which a neutral gas is injected from the bottom which promotes the mixing of the metal, however without reach the processes in which part of the oxygen is injected from the bottom. However, these bottom refining processes have not so far made it possible to obtain, with the oxygen converter, steels with low and very low carbon contents which do not have high contents of mainly dissolved gas. oxygen.

Nevertheless, bottom refining processes are those for which the dissolved oxygen content is lowest, compared to top refining processes.

The presence of dissolved oxygen in the liquid metal is particularly troublesome. When the metal solidifies, this oxygen reacts with the oxidizable elements and more particularly with the residual carbon to form CO. This results in a lower carbon content of the solid metal, inhomogeneity due to the presence of cavities containing carbon monoxide and above all, for extra-mild steels, the presence of metal oxides.

There are several methods for overcoming these drawbacks. The first of these techniques is called calming. Is added to the liquid metal, before ingot ingot or continuous casting, highly oxidizable elements such as aluminum, silicon and other metalloids or mixtures thereof which react with the dissolved oxygen to form oxides which decant and are trapped by the cover slag. However, there is always a fair amount of these oxides in the metal when it solidifies, but the morphology of the inclusions is better controlled.

Another technique, used in the converter, is the purging of the metal using a neutral gas, mainly nitrogen or argon. It has the drawback of being moderately efficient and of varying the carbon content of the bath, hence a greater dispersion of the carbon contents during the casting.

• - gros investissements
• - coûts de fonctionnement et d'entretien élevés dus aux techniques d'obtention du vide
• - pertes de température nécessitant soit une surchauffe à la coulée, soit un système de réchauffage de la masse en fusion
• - temps de traitement long.

The latest techniques, which can be grouped under the generic term of vacuum processing techniques, are very effective, but have the following drawbacks:

• - large investments
• - high operating and maintenance costs due to vacuum techniques
• - temperature losses requiring either overheating during casting, or a system for heating the molten mass
• - long processing time.

• 1.- formation d'un microlaitier contenant principalement de l'oxyde de fer selon la réaction :
  
• 2.- décantation et réduction de ce microlaitier : en remontant à travers la masse métallique ce laitier réagit avec le carbone du bain selon la réaction :
  

In the processes in which an oxygen-containing gas is blown through a nozzle situated under the surfacing of the bath, the refining takes place in two stages:

• 1.- formation of a microlaitier containing mainly iron oxide according to the reaction:
  
• 2.- decantation and reduction of this microlaitier: by going up through the metallic mass this slag reacts with the carbon of the bath according to the reaction:
  

• l.- une première période durant laquelle le bain contient suffisamment de carbone pour que tout l'oxyde de fer produit soit réduit : ce qui se passe pour les teneurs en carbone du bain supérieures à une certaine valeur C^*.
• 2.- une seconde période durant laquelle le carbone contenu dans la masse métallique est trop faible pour réduire tout l'oxyde de fer produit au nez de la tuyère, ce qui entraîne une baisse notoire du rendement en fer de l'affinage et une augmentation de la quantité d'oxyde de fer contenu dans le laitier.

During the ripening, two periods can be determined:

• l.- a first period during which the bath contains enough carbon so that all the iron oxide produced is reduced: this happens for the carbon contents of the bath greater than a certain value C ^*.
• 2.- a second period during which the carbon contained in the metallic mass is too weak to reduce all the iron oxide produced at the nose of the nozzle, which leads to a noticeable drop in the yield of iron from refining and an increase the amount of iron oxide in the slag.

US Patent 3,930,843 describes a bottom refining process in which a mixture of oxygen and argon is introduced through the bottom of the converter into the bath of molten steel, when the carbon content of this steel is less than 0.25%. This introduction is carried out according to a process comprising three successive stages of dilution of oxygen with argon as a function of the carbon concentration in the metal bath. This patent gives no indication for obtaining the desired steels while reducing the refining time and the consumption of Argon.

Furthermore, in French patent FR-A-2,448,572, a method for refining low carbon steel in a converter is described in which argon is introduced with the oxidizing gas from a predetermined value. carbon content, in this case 0.02%. However, such a value is too low to obtain low levels of dissolved oxygen. For such a value, the oxygen concentration dissolved is very important and an injection of neutral gas cannot effectively lower this content.

The object of the present invention is to obtain steels having both a low carbon content (mild and extra-mild steels) and a low oxygen content in the converter. The object of the invention is to obtain these steels "with the converter", that is to say directly in the converter and not after a certain number of stages, such as calming ("killing" according to the English name -saxonne) with aluminum, silicon, etc ...

The present invention relates to a method for re ₇ Edier the aforementioned drawbacks and to obtain mild steel and extra-soft converter having dissolved oxygen levels below 200 ppm in the case of mild steel (0.08 <% C <0.03) and less than 300 ppm in the case of extra mild steels (% C <0.035).

tandis que la seconde courbe de dilution minimale est définie par les portions de droites :

dars lesquels x est le pourcentage de gaz inerte injecté dans le mélange et %C est la teneur en carbone du bain métallique à l'instant considéré.To this end, this process for refining the cast iron by the bottom in which is injected, into the molten metal bath, an oxidizing gas such as industrially pure oxygen, and is injected, during the last period of the refining, that is to say from a predetermined value of the carbon content of the bath, a mixture of oxidizing gas and inert gas ensuring the dilution of the oxidizing gas, the content of the mixture of inert gas varying according to of the carbon content of the bath, is characterized in that the content of the mixture of inert gas is varied, as a function of the carbon content of the bath, according to a law corresponding to a dilution curve of the oxidizing gas which is located in an area determined by two envelope curves, namely a first maximum-dilution curve defined by the portions of lines:

while the second minimum dilution curve is defined by the line portions:

in which x is the percentage of inert gas injected into the mixture and% C is the carbon content of the metal bath at the instant considered.

This process makes it possible to keep the dissolved oxygen content of the bath substantially constant throughout the decarburization and thus to minimize the amount of slag iron oxide. In addition, unexpectedly, this process is more economical for the intended purpose, allowing both to reduce the amount of Argon used while minimizing the amount of iron oxide present in the slag in the bath. According to a preferred embodiment, the total flow rate of the gaseous mixture (oxidizing gas and inert gas) injected through the bottom remains substantially constant throughout the last period of ripening. This flow is preferably the maximum flow compatible with a "calm" refining of the bath, that is to say without significant projections of the bath.

Finally, unlike the teaching of US Pat. No. 3,930,843, argon is used in the present invention as a dilution gas, the injection of which is controlled to decrease the CO concentration, which makes it possible to obtain, unexpectedly , a concentration of oxygen dissolved in the metal bath substantially constant throughout the duration of the process.

The inert dilution gas injected during the last refining period can be chosen from the group comprising nitrogen, argon, helium, neon, krypton, xenon or any mixture of these.

• La figure 1 est un diagramme illustrant la variation de la teneur en oxygène dissous en fonction de la teneur en carbone du bain métallique, à la coulée obtenue selon les différents procédés d'affinage connus et du procédé suivant l'invention.
• La figure 2 est un diagramme donnant deux lois de variation du pourcentage de gaz inerte injecté dans le mélange en fonction de la teneur en carbone du bain métallique, dans le cas de deux exemples de mise en oeuvre du procédé suivant l'invention, et l'étendue de la plage de variation de la loi précitée.
• La figure 3 est un diagramme illustrant la variation de la teneur en oxygène dissous en fonction de la teneur en carbone du bain métallique, respectivement dans le cas d'un procédé connu et de deux exemples de mise en oeuvre du procédé suivant l'invention.

Various embodiments of the present invention will be described below, by way of non-limiting examples, with reference to the appended drawing in which:

• FIG. 1 is a diagram illustrating the variation of the dissolved oxygen content as a function of the carbon content of the metal bath, with the casting obtained according to the various known refining methods and of the method according to the invention.
• FIG. 2 is a diagram giving two laws of variation of the percentage of inert gas injected into the mixture as a function of the carbon content of the metal bath, in the case of two examples of implementation of the method according to the invention, and l extent of the range of variation of the above-mentioned law.
• FIG. 3 is a diagram illustrating the variation of the dissolved oxygen content as a function of the carbon content of the metal bath, respectively in the case of a known method and of two examples of implementation of the method according to the invention.

We will first refer to the diagram in FIG. 1 which illustrates how the dissolved oxygen content, expressed in ppm on the ordinate, varies as a function of the carbon content of the metal bath in the case of different refining processes. . Zone A corresponds to known methods of refining from the top, zone B to known methods of refining from the bottom, zone C to known methods of refining from the bottom with purging, zone D to known methods of mixed refining and the zone E is a zone which can be reached using the method according to the invention. On this diagram is also plotted an equilibrium curve C, O at 1600 ° C for a carbon monoxide pressure of one bar.

It can already be seen from the diagram in FIG. 1 that the method according to the invention makes it possible to obtain dissolved oxygen contents much lower than all the refining methods known previously.

We will now describe, by means of the examples which follow, various modes of implementation of the refining process according to the invention and we will compare the results obtained with those of a conventional refining process from the bottom.

Comparative example 1

A bottom-blowing converter model equipped with an injection nozzle is produced in the laboratory. 600 kg of liquid iron containing 1.5% carbon and 1550 ° C. are loaded into this converter. Pure oxygen is then injected at a flow rate of 15 Nm ³ / h until the carbon content of the bath falls to 0.03% (point la of curve I in FIG. 3 corresponding to a content of dissolved oxygen of 1280 ppm). From this instant, industrially pure argon is injected into the bath, together with oxygen, at a constant flow rate of 15 _N m ³ / h. Metal samples are taken at regular intervals to determine the change in the dissolved oxygen content of the bath. After 3 minutes, that is to say after a consumption of 1.25 Nm ³ of argon / tonne of steel produced, it can be seen that the carbon content of the bath has been lowered to 0.01 % and that the dissolved oxygen content of the bath is then 750 ppm (point Ib of curve I in FIG. 3).

Example 2

The same converter is loaded with 600 kg of liquid iron containing 1.5% carbon. Industrially pure oxygen is injected at a flow rate of 15 Nm ³ / h until the bath has a carbon content of 0.212%, the temperature then being 1647 ° C. From this instant, the oxygen injected with argon is diluted by following the law corresponding to curve II of FIG. 2, the total flow rate of the injected gas (inert gas + oxygen) being kept constant. From this moment, the dissolved oxygen content, as a function of the carbon bath content, varies according to curve II of the diagram in FIG. 3. After 12.5 minutes, that is to say after consumption of 3.2 Nm ³ of argon / ton of steel produced, the carbon content of the bath is lowered to 0.01% while this dissolved oxygen content is 250 ppm (point 2b on curve II of FIG. 3). In other words, a dissolved oxygen content lower than 500 ppm is obtained compared to the case of Example 1.

Example 3

The same converter is loaded with 600 kg of liquid iron containing 1.5% carbon. As previously injected, oxygen is injected at a flow rate of 15 Nm ³ / h until a carbon content of 0.19% is obtained. The temperature of the bath is 1600 ° C. From this moment, the oxygen injected is diluted with argon, the argon content of the injected mixture varying, as a function of the carbon content of the bath, according to curve III of FIG. 2. The oxygen content dissolved varies then, depending on the carbon content of the bath, according to curve III of the diagram in Figure 3. After 9 minutes, that is a consumption of 2.95 Nm ³ / tonne of steel produced, the carbon content of bath is 0.02% and its dissolved oxygen content is 180 ppm (point 3b of curve III in Figure 3).

It can be noted, from curves II and III of FIG. 3, that in Examples 2 and 3 in which the method according to the invention is implemented, the content of dissolved oxygen in the bath does not exceed 200 ppm up to '' at a carbon content of 0.02%. This fact is very advantageous because it makes it possible to stop the refining to the desired carbon content and to obtain a well deoxidized metal bath.

In addition, it is well known in conversion plants that a low dissolved oxygen content in the bath promotes the purging of dissolved gases such as nitrogen and hydrogen. By using an inert gas having a very low dissolving power in steel, such as argon for example, it is possible to obtain nitrogen and hydrogen contents which are clearly lower than those obtained by the processes of conversion known to date.

Claims (5)

1. - Process for refining cast iron by the bottom into which an oxidizing gas such as industrially pure oxygen is injected into the molten metal bath, and is injected during the last refining period c that is to say from a predetermined value of the carbon content of the bath, a mixture of oxidizing gas and inert gas ensuring the dilution of the oxidizing gas, the content of the -mixture of inert gas varying depending on the carbon content of the bath, characterized in that the content of the mixture of inert gas is varied, as a function of the carbon content of the bath, according to a law corresponding to a curve of dilution of the oxidizing gas which is located in a area determined by two envelope curves, namely a first maximum dilution curve defined by the straight portions:

while the second minimum dilution curve is defined by the line portions:

in which x is the percentage of inert gas injected into the mixture and% C is the carbon content of the metal bath at the instant considered. 2. - A method of refining cast iron according to claim 1, characterized in that the total flow rate of the gaseous mixture (oxidizing gas and inert gas) injected through the bottom remains substantially constant throughout the last period of refining. 3. - Method according to claim 1 or 2, characterized in that the inert gases used belong to the group consisting of nitrogen, argon, helium, neon, krypton, xenon and any mixture of these gases . 4. - metal product produced according to the method according to one of claims 1 to 3, characterized in that said product has a dissolved oxygen content of less than 200 ppm for a carbon content of between 0.03% and 0.08% and less than 300 ppm for a carbon content less than 0.035%. 5. - Product according to claim 4, characterized in that it has a dissolved oxygen content greater than 100 ppm.

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