EP2918688A1 - Method for desulphurisation of hot metal, and flux-cored wire for implementing same - Google Patents

Abstract

A process for the continuous desulphurization of a liquid iron, in which a flux-cored wire having a shell and a lining is injected into the liquid iron flowing continuously in a liquid-iron processing plant, said lining comprising a desulphurising product, characterized in that said desulfurizing product is metallic calcium, in that the lining also comprises at least one neutral element with respect to the cast iron, and in that the calcium content of the lining is such that the calcium content filled wire is between 30 and 120 g / m of cored wire. Filled wire for its implementation.

Description

The invention relates to the field of processes for the production in the liquid state of cast iron for the production of molded parts. It concerns mainly, but not exclusively, cast irons whose carbon is in the form of graphite, both vermicular graphite cast irons, and flake graphite cast irons, and spheroidal graphite cast irons known as GS fonts or ductile cast irons.

From a metallurgical point of view, a cast iron is an alloy of iron and carbon containing from 2 to 7% by weight of carbon approximately, and containing various other elements as alloying elements or impurities. In practice, the carbon content of a cast iron whose carbon is in the form of graphite is in most cases around 3.0 to 3.8% by weight. All percentages that will be given later will be percentages by weight.

Cast irons are commonly developed using facilities called "cupolas". They are vertical furnaces fed with scrap and coke. The melting takes place at high temperature and produces a carbon-rich liquid metal. At the end of these cupolas, the cast also contains elements such as silicon (from 0.5 to 2.5%), manganese (from 0.4% to 1.0%) and sulfur whose content can be as high as 0.12%. This sulfur comes from the use of coke, which contains it as an impurity.

This sulfur content is incompatible with the final applications of a graphitic cast iron, in particular those of spheroidal graphite cast irons. Excess sulfur before the spheronization and then inoculation treatments leads to overconsumption of spheroidising agents such as magnesium, with an increased risk of manufacturing poor quality molded parts (undesired graphite structure). Indeed, magnesium combines with other dissolved sulfur.

It is therefore absolutely necessary to reduce the sulfur content of the molten iron leaving the cupola furnace if it is intended to become graphitic cast iron after adjustment of its composition, in order to carry out the subsequent metallurgical treatments. Of course, if the liquid iron is destined to become a cast of another type, but in which, for a reason any, the sulfur content must be reduced compared to that of the iron leaving the cupola, the same problem arises and requires a priori the same type of solutions.

• CaC₂ : carbure de calcium
• S : soufre dissous
• CaS : sulfure de calcium
• C : carbone dissous

The desulphurization of the iron at the end of the cupola is generally achieved through the addition of calcium carbide CaC ₂ . The desulfurization reaction is as follows:

(1) CaC ₂ + S → CaS + 2 C

or :

• CaC ₂ : calcium carbide
• S : dissolved sulfur
• CaS: calcium sulphide
• C : dissolved carbon

The calcium sulphides thus created form a slag on the surface of the liquid iron which is enclosed in a container called a "pocket". It is then necessary to "unravel" it, that is to say to remove it by means of tools such as a plane, controlled manually or by a machine. The use of CaC ₂ results in the formation of a dry, solid slag, supernatant on the surface of the cast iron. This is therefore regularly cleaned by the operator to avoid irreparable deterioration of the refractory lining the inside of the pocket and with which the liquid iron and slag are in contact.

• soit de manière continue au sortir du cubilot dans une poche de type « siphon » alimentée en permanence par de la fonte liquide sortant d'un cubilot, et de laquelle sort, également en permanence, un flot de fonte liquide désulfurée se déversant dans un autre récipient ;
• soit en mode discontinu dans une poche classique.

In a standard desulfurization process, the operation is carried out in a pocket containing the cast iron:

• either continuously on leaving the cupola in a "siphon" type bag permanently fed with molten iron coming out of a cupola, from which also a stream of desulphurized liquid iron discharging into another container;
• either in batch mode in a conventional pocket.

The addition of calcium carbide in a conventional ladle is usually done in bulk, with CaC ₂ in the bottom pocket and casting the cast iron on. In a siphon bag, this addition is effected in a continuous manner, by a feeding system which brings the powder into the pouring jet of the cupola, or even within the liquid metal passing through the pocket, by a lance powder injection, or a flux-cored wire having a metal shell which contains CaC ₂ .

In the latter case, it is known to use cored wires containing about 20% CaC ₂ mixed with 80% CaCO ₃ calcium carbonate. The latter makes it possible to promote an energetic stirring of the liquid metal bath by decomposition of CaCO ₃ into CaO + CO _2gaz . The mixture of CaC ₂ with the liquid iron and the efficiency of the desulfurization are thus increased.

This energetic stirring can also be obtained, in addition to or in place of the use of Ca carbonate, by an injection of neutral gas (generally nitrogen) by means of a porous plug disposed in the bottom of the pocket. An electromagnetic stirring technique, described in the patent US-A-4,412,801 , also allows such a brewing of the cast iron.

Magnesium can also be used in admixture with other products for the desulfurization of cast iron. So the document EP-A-0 514 294 describes a product comprising a mixture of calcium carbide and magnesium grains: the magnesium is coated with calcium carbide. The mixture may further contain a slag of composition 2CaO.SiO ₂ . Again, the desulfurization process requires the use of CaC ₂ .

The use of calcium carbide poses a great deal of environmental and occupational hygiene problems, and from this point of view, the process just mentioned has serious economic and ecological disadvantages.

Indeed, particularly in the case of continuous processes where it is necessary to carefully control the agitation of the bath, all the CaC ₂ introduced into the melt does not react according to the reaction (1), and a significant amount of CaC ₂ is found in the slag slag. The evacuation of these polluting slags implies expensive additional treatments which, if they are not carried out, involve the classification of slags as "special wastes" unsuitable for use in public works and cement factories, as are often the case. classic slags of blast furnaces and steel mills. They must be stored in particularly supervised landfills. In addition, calcium carbide is an extremely flammable and water-reactive product leading to to the formation of explosive gases such as acetylene. Its transport is therefore highly regulated. It is finally particularly irritating for the skin.

The object of the invention is to completely prevent the use of calcium carbide for the desulphurization of cast iron, in particular cast irons whose carbon is in the form of graphite, for economic and ecological reasons, while maintaining an effective less comparable to desulphurization compared to processes using calcium carbide.

For this purpose, the subject of the invention is a continuous desulphurization process for a liquid iron, in which a flux-cored wire comprising an envelope is injected into the liquid iron flowing continuously in a liquid-iron processing plant. and a lining, said lining comprising a desulfurizing product, characterized in that said desulfurizing product is metallic calcium, in that the lining also comprises at least one neutral element with respect to the melt, and in that the content the metal calcium of the packing is such that the content of metallic calcium of the cored wire is between 30 and 120 g / m of cored wire.

Said neutral element present in the lining may be iron.

Said lining may also contain aluminum, representing up to 2% by weight of the lining.

Said cast iron may be a cast iron whose carbon is in the form of graphite.

The invention also relates to a cored wire for the metallurgical treatment of liquid metals, comprising a lining of a product intended to dissolve and react optionally with the liquid metal and a casing surrounding said lining, characterized in that said lining comprises calcium and at least one neutral element with respect to a cast iron, said content of the metallic calcium thread in the assembly formed by the packing and the casing being between 30 and 120 g / m of cored wire .

Said neutral element may be iron.

Said lining may also contain aluminum, representing up to 2% by weight of the lining.

Said lining may be in powder form.

Said packing may be in the form of an extruded bar.

As will be understood, the invention consists in carrying out the desulphurization of a cast iron, in particular of a graphitic cast iron, by a continuous process in which is added by means of a cored wire, as a desulfurizing product, a mixture of calcium base, a neutral element for cast iron such as iron, and possibly aluminum.

• Conservation d'une bonne efficacité de la désulfuration dans le cadre d'un procédé continu qui, économiquement, est préférable à un procédé discontinu ;
• Absence de contraintes environnementales liées à la nécessité d'un retraitement et/ou d'un stockage soigneux des laitiers de désulfuration ;
• Amélioration du coût du traitement de désulfuration par rapport aux procédés continus existants ;
• Décrassage aisé des poches par les opérateurs ;
• Intégration simple de ce nouveau procédé dans l'environnement du cubilot et d'une installation de désulfuration existante.

The development of the desulphurization process of the cast iron according to the invention was guided by several criteria that it was imperative (particularly for the first two) or preferentially respect to respond advantageously to the problem posed:

• Preservation of a good efficiency of desulfurization in the context of a continuous process which, economically, is preferable to a batch process;
• Absence of environmental constraints related to the need for careful reprocessing and / or storage of desulphurisation slags;
• Improved desulphurization treatment cost over existing continuous processes;
• Easy disconnection of the pockets by the operators;
• Simple integration of this new process into the cupola environment and an existing desulphurization plant.

Given the first two of these criteria, the use of calcium carbide was completely banned from the outset. It did not seem possible to find a technical solution that would increase the reaction rate of CaC ₂ to 100% and thus avoid finding it in desulphurisation slags. This is particularly true in the case where the desulfurization is carried out in the context of a continuous process, since the siphon pockets do not allow sufficient stirring of the metal-dairy assembly to obtain a reaction rate. close to 100%.

Similarly, the use of magnesium, although recognized as desulphurizing element of cast iron (in the steel industry), has been ruled out, mainly for reasons of reactivity of Mg in the liquid metal. Indeed, the boiling point of Mg is 1090 ° C while the working temperature of the cast iron at the output of the cupola is around 1450-1520 ° C. It is well known that the addition of magnesium in cast iron produces strong reactions. For the same reasons that prevent the design of bath stirring sufficient to obtain a total consumption of CaC ₂ , it therefore did not appear from the outset desirable to use Mg in the continuous desulfurization process.

One could think of using pure metallic calcium to constitute the desulfurizing agent. Calcium is a widely used element in the iron and steel industry for the pocket treatment performed before pouring. It makes it possible to modify the chemical composition of the inclusions, and consequently their morphology and their properties, for example in order to improve the flowability of the steels by transforming the solid alumina inclusions into inclusions of calcium aluminates (CaO, Al ₂ O ₃ ), liquids at usual casting temperatures, or making inclusions of plastic oxides during rolling. It is also known that the addition of calcium in the liquid steel can play a secondary role of desulfurization (the metal-dairy reactions, if it has a fairly strongly basic composition, is as much as possible in the state liquid possibly through the addition of fluxes such as CaF ₂ , and if the liquid steel is deoxidized, has a predominant role).

On the other hand, given the boiling point of calcium (1484 ° C), its addition to the liquid steel also causes strong reactions, because the treatment temperature of the liquid steel during the metallurgical operations in the pocket ( 1580 ° C-1600 ° C) being substantially higher than the boiling point of calcium. Finally, the low solubility of calcium in liquid steel makes it impossible to envisage its use as a desulfurizing agent without taking other measures in addition to the addition of calcium to ensure desulfurization.

It could be expected that these reactions would be substantially less, if not absent, in the case of liquid iron, whose temperature may be of the order of 1450 ° C. However, the cost of using pure calcium to desulfurize cast iron would not, a priori, achieve an economically satisfactory solution to the technical problem. This economic aspect was, for those skilled in the art, clearly dissuasive vis-à-vis the use of metallic calcium.

One could possibly imagine using a mixture of metallic calcium with a synthetic slag based on CaO and Al ₂ O ₃ . This type of slag is used in iron and steel industry in the pocket metallurgy, in order to promote the metal / slag exchanges with a very low melting point of the order of 1250 ° C, if the respective proportions of CaO and Al ₂ O ₃ are well controlled, thus favoring desulphurisation. Introduced in the form of flux-cored wire within the iron ladle, such a mixture would in principle have the advantage of multiplying the sulfur capture sites, in addition to the expected reaction of the metallic calcium with the sulfur dissolved in the iron. The inventors have actually experimented with this path.

However, it will be seen from the tests carried out that the solution chosen by the inventors has been different.

It consists in mixing judiciously selected calcium metal with at least one "neutral" element for the liquid iron (that is to say iron and / or an alloying element whose presence in the products manufactured in from the desulfurized iron thus produced would be desirable or tolerable within certain limits).

It appeared to the inventors that the most relevant criterion to be respected as regards the composition and morphology of the cored wire is its metric weight of metallic calcium. This takes into account not only the Ca content in the desulphurizing product, but also the diameter and compactness of the wire lining, to determine how much Ca, and therefore what length of wire, must be added to obtain the desulfurization depending on the sulfur content of the initial melt.

• La figure 1 qui montre schématiquement une installation pouvant être utilisée pour la mise en oeuvre du procédé selon l'invention et dont les éléments essentiels sont une poche siphon et une installation de déroulement de fil fourré ;
• La figure 2 qui est une photographie de la surface du contenu de la poche lors du traitement de désulfuration selon l'invention.

The invention will be better understood on reading the description which follows, given with reference to the following appended figures:

• The figure 1 which schematically shows an installation that can be used for carrying out the method according to the invention and whose essential elements are a siphon pocket and a flux cored wire installation;
• The figure 2 which is a photograph of the surface of the contents of the pouch during the desulfurization treatment according to the invention.

• La partie du laitier issue du cubilot décante alors que la fonte qui la renferme se trouve dans le premier compartiment 5, cette décantation étant accentuée par le brassage intense de la fonte liquide 2 dû, d'une part, au jet de fonte liquide 2 s'écoulant de la rigole 3, et d'autre part au jet gazeux 8 issu d'un bouchon poreux 9 placé dans le fond de la poche 1 ; le gaz injecté est un gaz dont la présence dans la fonte liquide n'entraîne pas d'effets indésirables sur la composition de celle-ci, typiquement de l'azote, voire de l'argon ; une telle injection est parfaitement classique, et le bouchon poreux 9 pourrait être remplacé par une lance plongeant dans la fonte liquide 2 présente dans le premier compartiment 5, ou une tuyère placée dans la paroi latérale de la poche 1 face au premier compartiment 5 ;
• Et les additions qui sont susceptibles d'apporter des éléments formant le laitier 6 sont réalisées dans le premier compartiment 5 et non dans le deuxième compartiment 7.
• De telles additions peuvent, classiquement, être réalisées à partir d'un fil fourré 10 déroulé à partir d'une bobine 11 par une machine d'injection 12 et guidé notamment par des galets 13 de façon à pénétrer et à fondre dans la fonte liquide 2 présente dans le premier compartiment 5. Ce fil fourré est constitué tout à fait habituellement par un garnissage du matériau à ajouter, sous forme de poudre ou d'un barreau extrudé, renfermé dans une enveloppe métallique.

The figure 1 shows very schematically a "siphon" pocket 1 known in itself for the continuous metallurgical treatment of a flow of molten iron 2 flowing from a channel 3 connected to an unillustrated cupola from which the liquid iron 2 comes out. The siphon bag 1 has a partition wall 4 which divides it into two compartments communicating in the lower part of the bag. 1. A first compartment 5 contains liquid iron 2 on the surface of which supernatant a slag 6 resulting from the melting of the iron 2, metallurgical reactions that occur in the pocket 1 and additions of materials made in the pocket 1 A second compartment 7, substantially narrower than the first 5, is almost free of such a slag on the surface of the cast iron that it contains, because:

• The part of the slag from the decanter decant while the cast iron which contains it is in the first compartment 5, this decantation being accentuated by the intense mixing of the molten iron 2 due, on the one hand, to the liquid cast iron 2 s flowing from the channel 3, and secondly to the gas jet 8 from a porous plug 9 placed in the bottom of the pocket 1; the injected gas is a gas whose presence in molten iron does not cause undesirable effects on the composition thereof, typically nitrogen or even argon; such an injection is perfectly conventional, and the porous plug 9 could be replaced by a lance immersed in the molten iron 2 present in the first compartment 5, or a nozzle placed in the side wall of the pocket 1 facing the first compartment 5;
• And the additions which are capable of providing elements forming the slag 6 are carried out in the first compartment 5 and not in the second compartment 7.
• Such additions can, conventionally, be made from a cored wire 10 unwound from a coil 11 by an injection machine 12 and guided in particular by rollers 13 so as to penetrate and melt in the molten iron 2 is present in the first compartment 5. This cored wire is quite usually constituted by a lining of the material to be added, in the form of a powder or an extruded bar, enclosed in a metal casing.

The advantages of the cored wire 10 containing the material to be added to the liquid metal with respect to an addition of the same material in the form of bulk powder or pieces are well known, including in the field of foundry: addition of a quantity of well controllable material and a depth certainly adequate, all this being achieved by adjusting the speed of injection of the wire, and the thickness of the metal shell which determines its melting speed. This technique is well known to those skilled in the art, it is not necessary to describe it further. The invention is based not on the use of the technique of addition of substances to liquid flux cored wire, known in itself, but on the nature of the substances thus added for the desulfurization of the liquid iron continuously.

The desulphurized liquid cast iron 14 flows continuously out of the siphon bag 1 by overflow over the side wall of the second compartment 7 and flows into a container (not shown) which will transport it to the site where it will be poured, while that the excess slag flows continuously, also by overflow through a scrubber opening 16 formed in the side wall of the pocket 1, and, if necessary, also using a plane controlled manually or automated, out the first compartment 5 of the siphon pocket 1 to pour into a container where it solidifies. This corresponds to the usual operation of conventional siphon 1 bags. Only a small quantity of slag flows out of the pocket 1 by accompanying the liquid desulfurized iron 2, thanks to an adequate dimensioning of the compartments 4, 5 associated with the stirring conditions of the molten iron 2.

In order to compare the efficiencies of the various technical solutions tested, the inventors have defined a quantity which will be called "desulfurization efficiency" E _d . This quantity represents the amount of desulphurizer required per tonne of liquid iron to reduce the sulfur content of the liquid metal by 1 ppm. Thus, E _d , expressed in g (t.ppm) ^-1 , is defined as follows:

(2) E _d = Cp / ([ S _f ] - [ S _i ])

• Cp : Consommation de produit désulfurant par tonne de fonte liquide (exprimée en gramme par tonne) ;
• [S _i] : teneur initiale en soufre dans la fonte (avant traitement, exprimée en ppm) ;
• [S _f] : teneur en soufre dans la fonte (pendant traitement, exprimée en ppm).

Or :

• Cp: Consumption of desulfurizing product per tonne of liquid iron (expressed in grams per tonne);
• [ S _i ]: initial sulfur content in the pig iron (before treatment, expressed in ppm);
• [ S _f ]: sulfur content in the pig iron (during treatment, expressed in ppm).

The lower E _d is, the more effective the desulfurization is since it means that less desulfurizing product is needed to remove a given amount of sulfur from the melt.

Reference Example 1 : Addition of CaC ₂ in bulk

This example is representative of the known prior art for the desulfurization of cast iron flowing continuously in a siphon pocket.

• Débit de fonte : 18 tonnes par heure ;
• Capacité de la poche siphon : 2 à 2,5 tonnes
• Température de la fonte dans la poche siphon : 1480 - 1520°C (fonction du débit de la fonte)
• Teneur initiale moyenne en soufre sortant du cubilot : 0,060% ;
• Consommation de CaC₂ : 3,9 kg par tonne de fonte
• Teneur moyenne en soufre obtenue par le traitement : 0,015%.

A loose calcium carbide powder is introduced into the cast iron stream 2 flowing out of the channel 3. The operating conditions are as follows:

• Cast iron flow: 18 tons per hour;
• Siphon pocket capacity: 2 to 2.5 tons
• Melting temperature in the siphon pocket: 1480 - 1520 ° C (function of the flow rate of the iron)
• Average initial sulfur content coming out of the cupola: 0.060%;
• CaC ₂ consumption: 3.9 kg per tonne of iron
• Average sulfur content obtained by the treatment: 0.015%.

The contents of the cupola pour into a siphon pocket identical to that shown schematically in FIG. figure 1 .

The sulfur content obtained in the pocket during the calcium carbide treatment is particularly low (0.015%) with a powder consumption of 3.9 kg of product per tonne of processed iron, ie a desulphurization efficiency E _d of 8.7 g ./t.ppm.

The slag formed is characterized by dry dross that can be removed from the siphon pocket without particular difficulties. The quantity produced is on average between 10 and 20 kg of slag per tonne of processed iron. But this slag contains about 4.5% by weight of unprocessed calcium carbide, which makes it imperative to reprocess it to remove calcium carbide, or to store it in a special landfill to meet current environmental standards or, presumably, to come into the different country. The overall cost of the desulfurization process is therefore not satisfactory, which is mainly the source of the problem that the invention aims to solve.

Reference Example 2 : cored wire of CaC ₂ and lime

Tests were conducted on the plant described in Example 1, and under comparable operating conditions relating to the flow of liquid iron, its temperature and its initial sulfur content.

• Fil fourré :
  • Diamètre 13,6 mm
  • Epaisseur de feuillard d'acier : 0,35mm
  • Poids métrique de feuillard : 147 g/m
  • Proportion de carbure de calcium : 20% en poids
  • Proportion de chaux : 80% en poids
  • Poids métrique de poudre : 180 g/m
• Débit de fonte : 18,3 tonnes par heure
• Température de la fonte dans la poche siphon : 1495°C
• Teneur initiale moyenne en soufre de la fonte sortant du cubilot : 0,060%
• Consommation de poudre de CaC₂ contenue : 2,5 kg par tonne de fonte
• Teneur moyenne en soufre obtenue par le traitement : 0,025%

A steel-lined cored wire containing 20% of calcium carbide and 80% of lime is introduced into this siphon pocket 1. The following are the characteristics of this reference desulfurization process, using calcium carbide:

• Filled wire:
  • Diameter 13.6 mm
  • Thickness of steel strip: 0.35mm
  • Metric weight of strap: 147 g / m
  • Proportion of calcium carbide: 20% by weight
  • Proportion of lime: 80% by weight
  • Metric weight of powder: 180 g / m
• Cast iron flow: 18.3 tons per hour
• Melting temperature in the siphon pocket: 1495 ° C
• Initial average sulfur content of the iron leaving the cupola: 0.060%
• Consumption of CaC ₂ powder contained: 2.5 kg per tonne of pig iron
• Average sulfur content obtained by the treatment: 0.025%

Under these conditions, the desulfurization efficiency E _d for the reference wire is 7.1 g./t.ppm if only the addition of material due to the powder is taken into account. But it is preferable, because more rigorous, to also take into account the mass of iron brought by the envelope of the wire for the calculation of E _d . In this case, E _d is 13.0 g./t.ppm The powder consumption is lower than the reference method of Example 1, and the desulfurization efficiency E _d is therefore better.

Between 5 and 10 kg of slag are produced per tonne of processed iron. This slag can be cleared without particular difficulties. However, given the CaC ₂ content of the cored wire which is necessary to obtain this good desulphurization, this slag still contains residual calcium carbide in a quantity sufficient to cause a problem, even if this amount is reduced compared to the addition. in bulk of Example 1.

Reference Example 3 : Thick wire of metallic calcium and synthetic slag

• Diamètre : 13,6mm
• Epaisseur de feuillard d'acier : 0,35mm
• Proportion de calcium métallique : 30% en poids
• Proportion de laitier synthétique: 70% en poids
• Poids métrique de poudre : 191 g/m

This yarn contains a mixture of calcium metal powders and synthetic lime aluminate slag CaO, Al ₂ O ₃ containing CaO and Al ₂ O ₃ in substantially equal parts (by weight). The physicochemical characteristics of this CW1 yarn are the following :

• Diameter: 13,6mm
• Thickness of steel strip: 0.35mm
• Proportion of metallic calcium: 30% by weight
• Proportion of synthetic slag: 70% by weight
• Metric weight of powder: 191 g / m

This wire was introduced into the siphon bag under the same conditions as those of the reference example 2.

The initial sulfur content at the start of the test was 0.085% with a flow rate of 20 tonnes per hour. During the first hour of use, the injection speed of the cored wire was 4.2 m / min, ie a consumption of 2.4 kg of powder per ton of processed iron. The sulfur content of the pig iron in the bag during the treatment was 0.038%, ie a desulphurization efficiency E _d of 5.1 g./t.ppm, or 9.0 g./t.ppm if the intake is taken into account. in iron from the wrap of the wire. This result was therefore at this stage better than that of the reference example 2.

These results then deteriorated. The sulfur content in the iron contained in the pocket 1 has risen to 0.065% despite an increase in the injection speed to 5 m / min (with a flow rate of iron and an initial sulfur content unchanged). The desulfurization efficiency E _d has thus deteriorated 14.3 g./t.ppm, or 25.4 g./t.ppm if the iron content of the wire casing is taken into account, and the test was discontinued since it was clearly Inconclusive.

It was found that the slag formed above the cast iron ladle was very liquid, and thus had a melting temperature significantly lower than usual. The slag was particularly difficult because the slag adhered to the pocket refractories. An explanation of the degradation of the desulphurization results could therefore be the recovery of sulfur in the liquid iron by a chemical exchange with the insufficiently cleaned slag remaining above the bath. This slag could not be easily evacuated, it is thought that it accumulated excessively on the surface of the liquid iron, thus causing a resulfuration of the liquid bath by new chemical exchanges.

The use of the mixture of a synthetic slag CaO, Al ₂ O ₃ and metallic calcium in a cored wire intended for the desulfurization of the melt therefore did not cause the expected effects, both with regard to the efficiency desulfurization but also vis-à-vis the ease of slagging.

However, the inventors have persevered in exploring the possibilities of using calcium as a desulfurizing agent for pig iron.

They then continued the tests by mixing a pure calcium powder with a neutral element that can reduce the unit cost of the cored wire.

"Neutral element" means an element which does not adversely affect the desulphurisation, in particular in that it does not have a negative impact on the composition and the physical properties of the slag, and the dissolution of which in the Liquid iron, in the amount present in the desulphurization product, would be tolerable with respect to future uses of the pig iron.

Typically, iron is such a neutral element since it constitutes the basic element of all melting and the casing of the cored wire, and an additional addition of iron to the liquid bath accompanying that of the desulfurization product would not have been possible. no chemical consequences, positive or negative. However, one could consider using other elements, in addition to or in place of iron, if their presence in the powder does not lead to give the cast a content in these elements that would be incompatible with its future use planned or possible. The desulphurization could also constitute all or part of the operation of addition of an element that one would like to find at a minimum level in the final cast iron, and this would make it possible to benefit from the reliability linked to the addition of an element by cored wire, of which we previously spoke.

The inventors have thus first tested a mixture of metal calcium and iron powders, in proportions of about 30% / 70% by weight, respectively. Another 40% / 60% ratio was also tested. Considering the dimensions of the wire and the compactness of the mixture, the metric weight of Ca was respectively 81 and 102 g / m.

In order to optimize the desulfurization treatment, they have also introduced into the cored wire a deoxidizing element, in this case aluminum.

By definition, the cast iron is not deoxidized at the outlet of the cupola. The dissolved oxygen content is certainly not very high, due to the presence typically of between 3.50% and 3.70% of carbon, but it is not zero. Measurements carried out show dissolved oxygen contents of about 20 to 40 ppm at the outlet of the cupola. The addition of aluminum in the cast iron reduces the oxidation of the bath according to the reaction:

(3) 2 Al + 3 O → Al ₂ O ₃

• Al : aluminium ajouté par le fil fourré ;
• O : oxygène dissous dans la fonte liquide ;
• Al₂O₃ : inclusions d'alumine.

Or :

• Al: aluminum added by the cored wire;
• O : dissolved oxygen in liquid iron;
• Al ₂ O ₃ : inclusions of alumina.

The simultaneous addition of calcium and aluminum allows a rapid transformation of the alumina inclusions thus formed into calcium aluminas (CaO, Al ₂ O ₃ ) which are sulfur-rich, and thus contribute to improving the efficiency of the desulfurization treatment.

Nevertheless, one must be aware that aluminum is generally considered harmful in fonts. It is therefore necessary to control the aluminum content in the flux-cored wire and consequently in the cast iron in order to maintain optimum quality of the molded parts. For example, for lamellar graphite cast iron, a content of 30 to 50 ppm of aluminum helps to germination, but it is recommended not to exceed a maximum aluminum content of 100 to 150 ppm in liquid iron. Depending on the final composition intended for the cast iron, it will be possible to choose to use either an aluminum-free wire (with impurities close) or a wire closing a known quantity of aluminum, the wire manufacturer being able to propose a whole range of products with various levels of aluminum.

Depending also on the residual aluminum content provided by the raw materials used in the cupola furnace and the flow rate of the iron to be treated, the person responsible for making the cast iron may therefore plan to use flux cored wires of various aluminum contents. , so useful, to systematically add an effective amount but not excessive aluminum in the melt. Typically, the aluminum content in the filling of the cored wire can vary from 0 to 2% by weight.

Examples 4.1 and 4.2 according to the invention : flux-cored wire based on metallic calcium and iron

Two cored wires according to the invention were tested. Table 1 shows the physical characteristics, where the thickness of the steel sheath and the calcium content vary. Table 2 presents the metallurgical results for each wire in three series of tests. These results are compared with those obtained by the application of the conventional method described in the example and those obtained by the application of Example 2 (reference cored wire). Table 1: Characteristics of cored wires according to the invention Tested wires Example 4.1 Example 4.2 External diameter (mm) 13.6 13.6 Thickness of the steel strip (mm) 0.35 0.50 Metric weight of strip (g / mm) 147 211 Calcium content (%) 30 40 Aluminum content (%) 1.2 0.8 Iron content (%) rest rest Metric weight of powder (g / m) 270 255 Metric weight of Ca (g / m) 81 102 Tested wires Example 4.1 Example 4.2 Trial 1 2 3 Melt flow (t / h) 18.4 17.0 19.4 Melting temperature (° C) 1535 1494 1528 Initial sulfur content of the cast iron (%) 0.077 0.058 0,056 Sulfur content of cast iron during pouch treatment (%) 0,015 0,019 0,009 Quantity of powder injected (kg / t) 2.9 1.9 2.0 Desulfurization efficiency E _d (g./t.ppm) 4.7 4.9 4.2 E _d (g./t.ppm) taking into account the contribution to Fe of the envelope 7.2 7.5 7.8

In terms of the desulfurization efficiency E _d , if the iron provided by the yarn casing is not taken into account, the cored wires 4.1 and 4.2 according to the invention give much better results than the reference cored wire of the invention. Example 2 which contains CaC ₂ : 30 to 40% improvement, a fortiori that the reference wire 1 (between 44 and 52% improvement). If one takes into account the iron brought by the envelope of the wire, the improvement seems smaller, but is nevertheless significant. The validity of the idea underlying the invention is therefore fully confirmed.

Concerning the mass consumptions of desulphurising product per tonne of cast iron (second test of thread 4.1 and thread 4.2), these can be reduced by half compared to the addition of calcium carbide in bulk and by 20% compared to the wire. filled reference of Example 2. If we consider the weight of wire required to obtain a given desulphurization, given that the thicknesses of the envelope are identical or of the same order of magnitude for Example 2 reference and the examples according to the invention, the invention therefore does not impose the use of coils that would have a weight greater than that of cored wire cores CaC ₂ . The transport costs of the coils according to the invention are not higher, or even lower, than those of cored wire coils containing other desulphurizing products. Similarly, there is no reason to modify a pre-existing wire injection machine to make possible the use of cored wire according to the invention. This can therefore be used in any foundry already equipped with a commercial thread injection machine, or can receive one.

Whatever the version of wire 4.1 or 4.2, the behavior of the formed slag is perfectly adapted to the deslagging of the siphon pocket. The formed slag does not adhere to the refractory walls of the ladle, unlike that of Example 3. For both versions of flux-cored wire, the quantity of slag generated by the desulfurization treatment is less than 5 kg per tonne of iron treated. This slag does not contain, of course, any trace of calcium carbide. It can therefore be removed from the foundry in the form of ordinary waste, without danger to the environment, and be used for the same purposes, for example, of the cement works and public works, as, for example, the classic slags of high -flowers and steel mills.

In this regard, unexpectedly and not observed so far, the inventors have systematically found with flux-cored wires according to the invention that the slag resulting from the desulfurization carried out by means of a cored wire according to the invention took a form very particular and advantageous. This was not found during the reference trials.

At the start of the desulfurization process, the surface of the cast iron is loaded with very fine slag pellets. These beads will then grow in diameter over time. By stirring the resulting liquid iron, for example, the gas injection by the porous plug, these beads are spontaneously and easily removed from the pocket, rolling on each other. It has been observed that the process was self-sustaining: the balls, which have become sufficiently large, with a diameter of the order of 10 cm, are evacuated by the scrubber orifice 16, are replaced by new ones. very small balls that will grow in turn, and so on. The scrubbing operation is thereby greatly facilitated. The figure 2 shows the surface of the slag 6, with the cored wire 10 which crosses it. We can see very well that the slag is very much in the form of small and large logs.

These balls 17, 18 are each composed of an alternation of thick concentric layers of slag with high proportions of sulfur (approximately 1 to 4 mm) and thinner metal layers (of the order of a millimeter). The mechanisms of formation of these beads 17, 18 are, for the moment, unknown, but this phenomenon is systematically observed.

The layers of slag containing sulfur being thus trapped in layers of solid metal, the sulfur which they contain can no longer be taken up in the liquid metal thus limiting the possibility of a partial recovery by the liquid melting of the sulfur which has extracted by the desulfurization mechanism.

The following Table 3 summarizes the technical and economic performance of the various processes used in the described tests, for the desulphurization of the iron at the end of a cupola. Board. 3: Technico-economic balance of the tested processes Addition of CaC ₂ in bulk Example 1 Fluxed wire Example 2 Filled wire Example 3 Filled wire Example 4.1 (invention) Filled wire Example 4.2 (invention) Chemical analysis 100% CaC ₂ 80% CaO 30% Ca 30% Ca 40% Ca 20% CaC ₂ 70% CaO.Al ₂ O ₃ 1.2% Al 0.8% Al Rest Fe Rest Fe Desulfurization efficiency E _d (powder only) 8.7 7.1 5.1 (start) to 14.3 (end) 4.8 4.2 E _d (powder + envelope) - 13.0 9.0 (start) to 25.4 (end) 7.4 7.8 Easy to clean good good bad Very good Very good Volume of slag generated per ton 10-20 kg / t 5-10kg / t - <5kg <5kg CaC _{2 content} in the evacuated slag 4.5% <1% 0% 0% 0% Environmental risk strong moderate - no no Cost of treatment * of one ton of cast iron 100 130 - 120 100 * the cost of processing the standard process is taken as a reference on a 100 basis

The flux-cored wire according to the version of example 3 is to be avoided since it does not generate slags that are easily removable, which leads to very poor desulphurization once past the beginning of the treatment. It was therefore not considered useful to evaluate several of the mentioned parameters, since even if it does not use calcium carbide, it is not a satisfactory technical solution on important points.

Although more expensive because of its conditioning, the reference cored wire of Example 2 (20% CaC ₂ - 80% CaO) is advantageous vis-à-vis the addition of CaC ₂ bulk. Indeed, the evacuated slag containing much less calcium carbide, it generates a lower environmental risk. Nevertheless, this risk exists, and one can think that the coming years will irremediably see the total prohibition of the use of the calcium carbide in foundry in all its forms. The inventors' approach is a priority in this context of respect for the environment, while taking into account, of course, the technical results to be achieved.

The flux-cored wire of Example 4.1 according to the invention (81 g / m of Ca) makes it possible to achieve a desulfurization efficiency of more than 40% higher than that of the CaC ₂ reference flux-cored wire, with an economic balance sheet as well. favorable (8% gain). Economically more expensive by 20% compared to the bulk addition, this example 4.1 has the considerable advantage of not generating any waste containing residual calcium carbide, in addition to being more efficient from a metallurgical point of view. For all these reasons, the immediate additional cost compared to the bulk addition of CaC ₂ , as long as it is legally possible, can be offset, or at least greatly reduced, by savings on recycling or storage of slag.

The version of Example 4.2 (102 g / m Ca) has a desulfurization efficiency quite comparable (40%) to that of the yarn of Example 4.1. As with the latter, the waste generated contains no trace of calcium carbide.

Note also the small amount of slag produced by the process according to the invention, which also helps to limit the problem of recycling or disposal.

Economically, the flux-cored wire according to the invention makes it possible to respond optimally technically and economically to the problem of desulfurization of the iron at the end of a cupola, while controlling the environmental constraints.

In general, the cored wire according to the invention is characterized by a composition of the packing, whether in the form of a powder or in the form of an extruded bar, calculated so that, taking into account the material of the casing of the wire filled, the material introduced into the cast iron, taken as a whole, contains between 30 and 120 grams of metallic Ca per meter of cored wire, the rest of the packing and the envelope of the wire which dissolves in the molten iron being constituted by at least one element whose behavior towards the cast iron can be described as "neutral" in the sense that has been seen, and which in most cases will be iron. These values were obtained by the following calculation.

In the reference example 3 and in examples 4.1 and 4.2 according to the invention, the addition of metallic Ca varies from 1.4 to 1.7 g per tonne of melt and per ppm of sulfur to be removed. This amount of sulfur to be removed is generally from 300 to 700 ppm, and thus, theoretically, 420 to 1190 g of metallic Ca per tonne of melt are required. It is considered that the optimum speed of introduction of the cored wire into the pocket, to obtain a melting of the wire at the right depth, is 2 to 5 m / min.

Thus, if we have a melting flow out of the cupola and through the 9t / h bag and if we want to reduce its sulfur content of 300 ppm, for a Ca addition of 1.4 g / t.ppm and a flow rate of wire 2 m / min, it takes 31.5 g / m of Ca. In the hypothesis where there is a melting flow rate of 30 t / h, where it is desired to reduce the sulfur content by 700 ppm and where there is a Ca addition of 1.7 g / t.ppm and a wire flow of 5 m / h min, 119 g / m of Ca is required. Therefore, a metal Ca content of 30 to 120 g / m of yarn is suitable to cover the most common cases of desulfurization of a cast iron leaving a cupola.

Claims (9)

A process for the continuous desulphurization of a liquid iron, in which a flux-cored wire comprising an envelope and a lining is injected into the liquid iron flowing continuously in a liquid-iron treatment plant, said lining comprising a desulfurizing product, characterized in that said desulfurizing product is metallic calcium, in that the lining also comprises at least one neutral element with respect to the cast iron, and in that the metal calcium content of the lining is such that the content of metal calcium of the cored wire is between 30 and 120 g / m of cored wire. Process according to claim 1, characterized in that said neutral element present in the packing is iron. Method according to one of claims 1 or 2, characterized in that said lining also contains aluminum, representing up to 2% by weight of the lining. Method according to one of claims 1 to 3, characterized in that said cast iron is a cast iron whose carbon is in the form of graphite. Filled wire for the metallurgical treatment of liquid metals, comprising a lining of a product intended to dissolve and react optionally with the liquid metal and a casing surrounding said lining, characterized in that said lining comprises calcium and at least one element neutral to a cast iron, said content of the metal calcium wire in the assembly formed by the lining and the casing being between 30 and 120 g / m of cored wire. Filled wire according to claim 6, characterized in that said neutral element is iron. Filled yarn according to claim 5 or 6, characterized in that said lining also contains aluminum, representing up to 2% by weight of the lining. Filled wire according to one of claims 5 to 7, characterized in that said lining is in powder form. Filled wire according to one of claims 5 to 7, characterized in that said lining is in the form of extruded bar.

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