EP0110809B1 - Process and installation for the treatment of steel in the ladle - Google Patents

Abstract

A process and apparatus for the desulphurization, deoxidation and purification of a metal melt is presented. The process is particularly well suited for use in a ladle during the refining of steel. The present invention comprises means for defining at the surface of a melt at least a first working zone preferably via a plunger tube. Thereafter, combustible materials and oxygen are delivered to the working zone at a single impact point, preferably by a plurality of lances or by a multiple flow lance. As a result, a reactive high temperature slag is produced thereby. Finally, the melt is mixed so as to evenly distribute the heat preferably by permeable elements and/or at least one auxiliary lance.

Description

The present invention relates to a method for the metallurgical treatment of ladle steel, as well as to an installation for implementing this method.

• 1. Les procédés de déphosphoration ;
• 2. Les procédés de désulfuration par injection au moyen d'un gaz neutre ou réducteur de matières désulfurantes sous forme métallique telles que p. ex. le calcium-silicium, ou sous forme de laitiers, tels des laitiers Perrin ou des laitiers constitués p. ex. de chaux et de spath-fluor etc. ; d'autres matières telles que le magnésium ou le carbure de calcium, peuvent également être utilisées ;
• 3. Les procédés de désoxydation par addition ou par injection d'aluminium, de ferro-silicium, de calcium-silicium etc. ; les processus de désulfuration et de désoxydation peuvent d'ailleurs être combinés ;
• 4. Les procédés de réchauffage par arc électrique ou par induction ;
• 5. Les procédés d'affinage ou de purification de l'acier, soit par le vide, soit par simple barbotage avec ou sans ajoutes de laitier.

For several years, and in particular due to the introduction of continuous casting, more and more steelworks have been carrying out metallurgical treatments in the pocket intended for the transport of steel from the converter or from the electric furnace to the casting installations. . These treatments are of different natures. The most commonly used processes are:

• 1. The dephosphorization processes;
• 2. Desulfurization processes by injection using a neutral or reducing gas of desulfurizing materials in metallic form such as p. ex. calcium-silicon, or in the form of slags, such as Perrin slags or constituted slags p. ex. lime and fluorspar etc. ; other materials, such as magnesium or calcium carbide, can also be used;
• 3. Deoxidation processes by addition or injection of aluminum, ferro-silicon, calcium-silicon etc. ; the desulfurization and deoxidation processes can moreover be combined;
• 4. Heating processes by electric arc or by induction;
• 5. The refining or purification processes of steel, either by vacuum or by simple bubbling with or without the addition of slag.

Patent application DE-A-1 959 173 describes a treatment of ladle steel by means of slag obtained by combustion of metallic elements, such as AI, CaSi or non-metallic, such as CaO, AI ₂ 0 ₃ , Na _Z C0 ₃ , NaN0 ₃ , using a mixture of AI powder with Na nitrate.

According to patent application FR-A-2 204 692 and also from patent US-A-3 971 655, devices are known for delimiting zones of steel baths free of slag, through which are introduced treatment materials, combined with means for introducing into the baths streams of mixing gas.

The object of the invention is to create a method and an installation for carrying out desulfurization, deoxidation, purification and heating of the metal in a single operation.

This object is achieved by the method as characterized in claim 1 and by the installation as characterized in claim 4. Preferential variants are described in the sub-claims.

Combustible materials such as metallic aluminum, calcium carbide, calcium-silicon, calcium-aluminum and possibly scorifying materials, such as lime or lime, are added to the liquid metal by means of a lance. spath-fluorine, carried by a neutral or reducing carrier gas, and oxygen is injected at the same point of impact, while adjusting the amounts of additions and oxygen so as to form the slag having the desired composition . The process simultaneously leads to the formation of a very hot and particularly reactive slag and to a substantial increase in the temperature of the steel contained in the ladle, provided that the convection of the heat created through the bath is favored. This is done by the introduction of a neutral or reducing auxiliary gas, injected either by one or more permeable elements housed in the bottom of the bag, substantially below the point of impact of the injection lance, or by an auxiliary lance also discharging substantially below this same point of impact.

Another characteristic of the process according to the invention consists in the simultaneous use of means with a view to avoiding immediate contact between the slag which covers the metal, with the slag formed during the process. These means which will be described in more detail below, are essentially a dip tube which allows access to the metal in the pocket, while avoiding the presence in the working zone of the slag entrained during the transfer of the metal into the pocket .

It is also planned, according to the invention, to start the process only after having deposited at the point of impact, below the lance head, a sufficient quantity of starting slag. Indeed it is not recommended to start a reaction of the type proposed on a bare metal surface, but to provide a slag cushion which can be p. ex. lime, prefabricated solid or liquid slag, or a self-deepening such as exothermic powder.

The slag which covers the metal after pouring into a ladle is removed as far as possible. To mitigate the effects of slag residue, the surface of the bath is covered with a protective layer which is preferably lime. Then a lid which is part of the installation according to the invention is lowered on the edges of the pocket and the treatment process is started, possibly after having protected the part of the surface of the bath which is free from the slag / lime mixture, by starting slag, as described above.

The installation according to the invention consists of a metallurgical ladle which is characterized in that it comprises means for delimiting a working zone in the bath, the said means cooperating with means for injecting into said zone solid and gaseous materials and that means are provided for introducing into the bath an upward flow of bubbling gas, directed onto said zone. The means delimiting the working area essentially consist of a dip tube, known per se, which is provided tipping of a cap to allow penetration through the slag layer and the exposure of an area of the bath. This dip tube can be either a single compartment tube or a tube subdivided into several compartments. The means for injecting solid and gaseous materials are one or more vertical lances discharging from above, while the means intended for supplying the bubbling gas consist of permeable elements housed in the bottom of the bag and / or by a submerged lance.

Other advantages of the method and of the installation according to the invention will emerge from the description of the schematic drawings which represent possible embodiments of the installation.

Fig. 1 shows a section through an embodiment of the installation, using a single compartment dip tube. Fig. 2 shows a section through a dip tube with two compartments.

In fig. 1 there is a pocket 1 provided with a refractory layer not shown; in the bottom of the pocket is housed a permeable element 2 used to inject the inert bubbling gas or reducing agent G. A single element 2 has been shown for reasons of convenience. We also distinguish the cover 4 which covers the pocket to avoid heat loss and to prevent the entry of ambient air and which crimps the dip tube 5; the latter has at its base a cap 5e made of thin sheet metal, intended to pass through the layer of slag which swims over the steel 3 in the pocket to prevent these slag from entering the tube 5. After the tube 5 has been inserted into the bath 3, c. to d. after fitting the cover 4 on the pocket 1, the cap 5e having passed through the layer of slag, melts in the liquid metal. The bubbling obtained by a gas injected from below makes it possible in many cases to avoid the use of the cap 5e by creating a zone free of slag in the region of the tube 5. The tube 5 is provided with a cover 5c which has 5d gas exhausts. This cover crimps a lance 6 used for the introduction of solid and gaseous materials. The tube 5 also has a coating of. interior and exterior protection (not shown).

During tests it was indeed found that the dip tube was subjected to a relatively significant wear caused on the one hand by the high temperature prevailing inside the tube and on the other hand by the attack of the compounds particularly reagents to which it is exposed. To increase the service life of these tubes, it is obviously possible to cover them on their internal surface with special refractory coatings containing p. ex. zirconium or chromium-magnesia compounds. These coatings are not only expensive, but also difficult to remove. In addition, given the conditions of use of these tubes, such coatings must not only withstand high temperatures, be insensitive to slag with high reactivity, but also adhere well to the tube during falls resp. sudden temperature rises. It has proven to be particularly advantageous to use a coating composed of materials which give off either under the action of high temperatures or by reaction with the metallic bath respectively the slag and / or with combustion oxygen, a gas capable of reducing respectively to eliminate the contact between container and content. In principle, any material having the qualities described above can be used which, as a result of their decomposition, does not result in the introduction of undesirable materials into the metal bath. It is obviously advantageous to choose or include materials which partially decompose into compounds having a beneficial effect on the metallurgical treatment in progress.

Adequate protective coatings can be obtained by depositing calcium carbonate, magnesium, soda, etc. mixed with binders, which will be sprayed according to known methods on the surface of the metallurgical container or of the device which will be introduced into the liquid metal. These materials, in contact with the heat of the reactions of the metal bath or slag respectively, will release their CO ₂ , while causing an endothermic reaction which counteracts the corrosive effect of the high temperatures generated by the reaction.

The protective coating may also consist of combustible materials such as wood, wood agglomerates and / or cardboard. It is not beyond the scope of the invention to form protective coatings by means of a mixture of combustible materials and carbonates by using p. ex. of agglomerated wood respectively of cartons added and / or impregnated with carbonates preferably basic.

Either a single lance can be used, which must therefore be a multiflux machine capable of delivering through separate channels solid and gaseous materials which may not be compatible and the contact of which before leaving the lance must be avoided. Indeed, one can easily imagine that it is necessary to prevent the metallic aluminum powder and the oxygen from coming into contact in the insufflation lance itself. To avoid the use of a complex lance, it is also possible to provide two lances, directed towards the same point of impact and one of which will be used to inject the combustible materials conveyed by inert gas and the other to provide the oxygen.

In fig. 1 there is shown a single lance 6 multiflux, supplied with oxygen by a pipe 11 and combustible materials by a pipe 7. These materials are stored in tanks 8 provided with cellular metering devices 9; line 7 is connected to a source of inert gas G which can be a neutral or reducing gas. There is also shown a lance 10 intended to supply bubbling gas G and which can replace the permeable element 2 or supplement it in the case where the flow rate of the permeable element or elements 2 turns out to be insufficient to distribute

the heat created chemically through the bath and to effect a valid purification of the bath by extensive contact between the metal and the purifying slag created in situ. Finally, the pocket 1 is provided with a tap hole system 20.

Fig. 2 shows a dip tube 50, the main feature of which is that it is subdivided into two compartments 5a and 5b. The compartment 5a delimits the work area into which solids and oxygen are introduced via the lance 6. As for the compartment 5b, it communicates with the compartment 5a for the opening 30 which is formed in the partition between the two compartments. The compartment 5a is provided at its base with a bottom 12 which has a small opening 12a, while the compartment 5b is open downwards. A distinction is also made between the interior 51 and exterior 52 thermal protection coating.

The purpose of the embodiment shown in fig. 2 is to obtain as complete a combustion as possible of the materials injected into compartment 5a, the operation of heating and refining of the metal taking place essentially in compartment 5b, which contains only the slag heating and refining at very low deoxidizing content. This double tube is used when one wants to manufacture steels with a very low content of deoxidizing materials. In this case, it will be ensured that the permeable elements 2 respectively the lance 10, supplying the bubbling gas G, are located below the compartment 5b, in which the refining operation takes place as well as the heat transmission. Thus the slag formed in 5a overflows in part 5b where there is an intense bubbling between the metal and the hot slag.

In the case of the use of the double tube, it is also not necessary to inject the heating and refining mixture by means of a lance and. a carrier gas in zone 5a; indeed it suffices in principle to let this mixture flow, p. ex. in free fall in zone 5a and to inject the oxygen necessary for the combustion of thermogenic elements in this zone, taking care to create sufficient turbulence there.

Below are some examples which are particularly suitable for use with the installations shown in FIG. 1 or 2.

120 tonnes of steel are in a pocket. After careful cleaning, the slag is replaced by powdered lime. The temperature of the metal is 1,610 ° C; the sulfur content is 0.015%. We want to lower this content and heat the metal to 1,670 ° C to be able to continuously cast the metal for 50 minutes.

The blown thermogenic element will be aluminum in the form of powder or granules, mixed with calcium carbide, also in powder or granules, in a proportion of 1.6 kg of CaC ₂ to 0.74 kg of Al, as well as possibly 3 to 10% of CaF2 intended to improve the fluidity of the slag.

By injecting per kg of steel 1 kg of this mixture with 0.7 to 0.9 m ³ of oxygen, the temperature of the metal bath is increased by 18 °. To achieve a temperature increase of 60 ° C, 3.3 kg of mixture must be injected per tonne of steel, i.e. 400 kg of AI CaC ₂ mixture, without taking into account the 20 kg of calcium fluoride intended for adjust the viscosity of the slag. The 400 kg are injected in 20 minutes, which corresponds to a temperature increase of 3 ° C per minute. Throughout the duration of the operation, an intense current of argon is injected by the permeable element 2 and / or by the lance 10. The injection of argon is continued for 5 minutes after the injection. After this operation, the target temperature of 1,670 ° C was reached and the metal contained no more than 0.004% sulfur, while having excellent micrographic purity.

The proportions as well as the quantities to be added obviously depend on the materials to be injected. Thus, to manufacture according to the invention a Perrin slag in situ, for the purpose of purification and simultaneous heating, a mixture of 35% AI and 65% CaO can be added. The temperature increases by around 20 ° C per kg of aluminum added per tonne of steel. 0.6-0.7 Nm ₃ of oxygen must be provided per kg of aluminum added.

It is also possible to use a Ca-AI alloy to manufacture a purifying slag and simultaneously heat the steel. In this case, it is necessary to provide for the addition of a mixture titrating substantially 50% of Ca-AI and 50% of CaO, with an addition of 5 to 10% of fluorspar as fluidizer. This mixture will provide an increase of 16 to 20 ° C per kg of Ca-AI added per tonne of steel, while giving rise to a steel titrating less than 0.004% of S and having a high micrographic purity.

Claims (6)

1. A process for the treatment of steel in the ladle using reactive slags with a composition known per se, the slags being formed in a tube which is immersible in the metal contained in the ladle, by implementing metallothermic reactions, wherein oxygen is blown into the tube by means of a lance, metal fuels are introduced into the tube and react with the oxygen to form the slag, whilst a neutral or reducing bubbling gas is injected at the bottom of .the tube in which the steel treatment takes place.

2. A process according to claim 1, characterised in that the fuel materials are suspended in a neutral or reducing carrying gas and blown into the tube by means of a lance.

3. A process according to claim 1 or 2, characterised in that before the supply of solid and gaseous substances begins, a starting slag cushion is placed at the oxygen impact point and is, e. g., a prefabricated slag, limestone or a self-fluxing material.

4. An installation for the treatment of steel in the ladle by means of reactive slags of a composition known per se, the treatment being intended to form the said slags in a tube which is immersible in the metal contained in the ladle, by implementing metallothermic reactions, wherein the metallurgical ladle (1) comprises an immersible tube (50) mounted in a cover (4) and subdivided into a first compartment (5a), receiving the end of the lance (6) and a second compartment (5b) communicating with the first compartment (5a) via an aperture (30) in the wall separating the two compartments, the second compartment (5b) is open at the bottom, and means (2, 10) for providing a rising current of neutral or reducing bubbling gas are disposed below the second compartment (5b), the lance (6) being mounted in a cover (5c) covering the immersible tube (50) and supplied with oxygen via a first duct (11) and with solid materials via a second duct (7).

5. An installation according to claim 4, characterised in that the means for supplying the bubbling gas are formed by permeable elements (2) housed in the bottom of the ladle, or by an immersed lance (10).

6. An installation according to claim 4, characterised in that the tube (50) is covered at least partially by materials which are the source of a gas discharge when they are in contact with slags or molten metals.

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