EP1812607B1 - Cored wire for treating molten metals - Google Patents

Description

The invention relates to the technical field of tubular envelopes containing compacted powdered or granular materials, these core envelopes being used for the treatment of liquid metals, especially steels, and being conventionally referred to as "filled cores".

The introduction into the liquid metal baths of these cored wires allows the refining, deoxidation, degassing, calming and / or modification of the composition of these baths.

For example, for the desulfurization of high sheath melts for the steel conversion, it is known to use cored son containing Mg and Ca C ₂ or alternatively Na ₂ CO _3, CaCO _3, CaO, MgO.

Flux-cored wires are typically used in secondary metallurgy of steels, among other means such as pocket stirring, powder injection, CAS (Composition Adjustment Sealed), arc pocket furnace, RH (Ruhrstahl Heraeus), tank vacuum.

The cored wires are used for the desulphurization of cast irons, for the production of GS cast irons, the inoculation of casting cast irons.

The inoculation of cast irons consists in introducing into the cast irons elements which favor the germination of graphite to the detriment of cementite, these elements being, for example, alkalis, alkaline earths (Ca) or bismuth, alloyed with silicon. Generally, desulfurization, nodulisation and inoculation are performed in order. Magnesium and silicon carbide are often used and the bath temperatures are of the order of 1300 to 1400 ° C, ie lower than those of the liquid steel bags.

The primary functions of flux cored wire are, for steels, deoxidation, desulfurization, inclusion control and shading.

The deoxidation operation consists in combining the dissolved oxygen in the liquid steel from the converter or the electric furnace (content of about 500 ppm or more) with a deoxidizing agent, a part of which will remain dissolved in the metal. liquid. Examination of dissolved oxygen activity curves in liquid iron at 1600 ° C., in equilibrium with various oxidizing elements, suggests that relatively modest additions of aluminum make it possible to reduce the residual dissolved oxygen contents very considerably, to form pure alumina, the aluminum being this is widely used as a deoxidizing agent for flat products.

The electric furnace flows into a pocket more or less decarburized metal, dephosphorized, but effervescent: given its dissolved oxygen content, the product% CO x% O is such that at the temperature considered, the reaction of formation of CO is spontaneous in the bath of liquid steel.

Deoxidation is so called calming, by reference to this effervescence of the primary liquid steel bath.

The deoxidizing agents contained in the cored wires are ferroalloys, most often (ferrosilicon, ferromanganese, aluminum). They lead to the formation of oxides (silica, manganese oxide, alumina) which, by moderate mixing of the pocket, decant in the slag.

Despite all the precautions taken, residual inclusions of alumina can cause the plugging of the casting nozzles or the appearance of defects on the end products of small section such as continuous casting casings in thin slabs.

As a result, the cored wires also conventionally contain calcium for aluminum-killed steels. The addition of calcium alloys to a liquid steel killed with aluminum allows a modification of the inclusions of alumina, by partial reduction by calcium. Calcium aluminates are liquid at the temperature of liquid steels, close to 1600 ° C., and therefore globular on product when their CaO content is between 40% and 60%. The amount of calcium in solution needed to achieve the change in inclusions depends on the aluminum content of the metal bath. Most of the calcium introduced by cored wire is therefore in the liquid metal in the form of liquid inclusions of lime aluminates, and does not exceed a few ppm.

In practice, it is difficult to avoid the violent bubbling of the liquid steel, caused by the sudden volatilization of the calcium contained in the cored wire. The vapor pressure of calcium is indeed about 1.8 atm at 1600 ° C. The bubbling, if it is too intense, can disturb the penetration conditions of the flux-cored wire in the steel bath and be accompanied by pollution of the bath, which oxidizes or rises. At the same time, splashes of liquid steel occur, passing through the slag layer and oxidizing in contact with the air before falling back. In addition, there is a risk of steel projection out of the pocket.

This may result in a rise in the levels of O ₂ , N ₂ and even H ₂ of the steel obtained. Boiling is reduced by introducing the calcium, not unalloyed, but as CaSi, with the major drawback of introducing silicon into the liquid steel, which is unfavorable for some steels such as deep drawing.

To overcome this drawback, it has been proposed to introduce calcium in the form of CaNi alloy, optionally mixed with a little CaSi alloy. Other solutions are presented in the document EP-0190089 .

To overcome this drawback, it has been possible to purge the volume located between the metal surface and the lid, by injecting argon in the case of steel with a low nitrogen concentration. In practice, since the furnaces are not waterproof, a strong argon stream causes air suction and a small argon flow implies a prohibitive inerting time of the gaseous volume above the liquid steel ladle.

It should also be noted that stirring or bubbling with argon through the porous plug of the pocket causes an intumescence of the slag surface, which further increases the calcium losses by evaporation or oxidation, during the simultaneous introduction of cored wire. intumescence causing direct contact of the liquid metal with the air.

The apparent yield of calcium addition is only a reflection of the inclusion cleanliness of the metal. This return is low, most of it calcium added by cored wire lost by evaporation and / or oxidation with the atmosphere, slags and refractories.

It is therefore very important, in order to minimize these side reactions, to carry out the calcium addition after careful decantation of the deoxidation inclusions and to adapt the addition to the desired conversion rates for these inclusions.

The exogenous oxide inclusions resulting from the contact of calcium with the refractories or the powders of the distributor are in fact difficult to eliminate before the solidification of the metal. These inclusions of alumina are solid and more harmful than the inclusions of calcium aluminate with regard to the capping of continuous casting nozzles, for example.

Calcium-cored wire treatment of aluminum-killed liquid steel can also result in the formation of calcium sulphide deposited in continuous casting nozzles for steels with low aluminum content and high sulfur content.

The control of the inclusional state by the addition of chemical components housed in flux-cored wires mainly concerns oxides and sulphides.

The addition of sulfur increases the amount of manganese sulphides and the machinability of the steel.

The addition of calcium, selinium or tellurium makes it possible to modify the composition, the morphology or the rheological behavior of the inclusions during subsequent deformations.

In particular, the control of the inclusion cleanliness is very important for bearing steels, free cutting steels, reinforcing steels or valve spring steels.

The deoxidation and the control of the inclusionary state of the steels, thanks to the chemical additions by flux-cored wire, are thus complex operations coming from the know-how of the steelmaker, operations for which the qualities of the cored wire are very important: regularity of composition , regularity of compaction in particular.

However, the manufacture and use of these cored wires pose a very large number of practical problems some of which will be discussed below.

Insufficient or irregular compaction

Irregular compaction of the material contained in the envelope results in an irregularity in the quantities of this material introduced, per unit time, in the bath of steel or liquid metal.

Insufficient compaction of the material contained in the flux-cored wire thus reduces the amount, per unit of time, of the material that can be introduced into the liquid metal by dipping the flux-cored wire into the liquid metal bath.

If compaction is insufficient, the pulverulent material can move inside the cored wire.

Excessive mechanical forces in unwinding

If the compacting operation required a large plastic deformation of the metal casing, the high rigidity, by hardening, of the casing of the cored wire causes considerable efforts in unwinding, in particular from small diameter drums, to small radius of curvature.

By drum, here is meant well so called dynamic packaging reels that the walls of so-called static packaging cages.

Insufficient stiffness of the cored wire

Some cored wires, especially of flattened section, have insufficient rigidity for their introduction deep into some high density metal baths, especially if these baths are covered by a high viscosity slag.

Spiral deformation during unwinding

It has been observed during the unwinding of the packed wire packed in a static cage a spiral deformation of the wire, so that the cored wire does not penetrate the liquid metal bath, but curls and remains on the surface.

Disagrafage of the casing of the cored wire

It has been observed, for certain products, during unwinding of the cored wire of its storage drum or its cage, or during the straightening of the wire preceding its introduction into the liquid bath, a désagrafage of the envelope of the cored wire .

The other techniques for closing the cored wire wrapping strips (edge to edge, overlap, welding) have other disadvantages: envelope thicknesses reducing the powder / sheath ratio, risk of deterioration of the powder during welding.

Reduction of the time required for the introduction into the bath of a given quantity of additives.

Increasing the speed of introduction of the yarn into the bath can lead to accidents if the yarn breaks against the bottom of the container or comes out of the bath before it has had time to melt.

Increasing the wire diameter leads to an increase in the winding radius, the coils necessary for winding such wires then becoming too large to be easily used in the reduced spaces available at the steel mill.

As an indication, to introduce 1 kg of CaSi per ton of steel into a 150 ton bag, ie 150 kg of a CaSi powder placed in a wire having a density of 240 g / m, a length of 625 m of cored wire is required, the introduction of this kilometer of wire at 2 m / s representing a working time of more than five minutes.

Premature destruction of the cored wire

If the casing of the cored wire is destroyed prematurely, by rapid fusion as soon as it enters the metal bath, the content of the wire is released in the vicinity of the surface of the bath.

Deformation of the U-shaped wire in the liquid metal bath

It is furthermore claimed in a document of the prior art that the cored wire can lose its rigidity and gradually bend in the liquid metal bath so that its end rises to the surface before the wire content is released. this rise being due in particular to the ferrostatic thrust, the apparent density of the wire being generally lower than that of the metal bath.

If the cored wire contains Ca, Mg, a shallow release of these elements in the bath of liquid metal results in very high yield losses, for example for the desulphurization of cast irons.

The massive release of calcium at a shallow depth in the liquid metal bath causes a violent reaction and splashes of liquid metal.

Depth of insufficient penetration of the flux-cored wire into the liquid metal bath

V étant la vitesse d'introduction du fil, comprise entre 3 et 30 m/mn pour des raisons de sécurité.For example, the document US 4,085,252 whose following relation between the penetration depth L, the thickness e of the metallic casing of the wire and the diameter d of a Cerium bar: $$\mathrm{The}=1.7\left(\mathrm{e}+0.35\mathrm{d}\right)\mathrm{v}{.10}^{-2}$$

V being the speed of introduction of the wire, between 3 and 30 m / min for safety reasons.

If the depth L is low, for example 30 cm, there is a high risk that the product contained in the cored wire does not come into contact with the supernatant slag, and thus be lost.

If the depth L is too low, there is also a risk of heterogeneity in the distribution of the chemical element (or elements) contained in the flux-cored wire in the liquid metal bath.

Reactivity of the powders contained in the wire and clogging of continuous casting installations

As indicated in the document US 4,143,211 , the chemical affinity of elements such as rare earths, Al, Ca, Ti, for oxygen leads to the formation of oxides which can adhere to the internal walls of the flow control nozzles of continuous casting plants and cause clogging .

It is therefore necessary to provide the steelmakers with cored wires facilitating the homogeneous introduction of the quantity just necessary for the desired result (deoxidation, inclusion control, mechanical strength, etc.) for the final steel product.

• demandes de brevet européen publiées sous les numéros : 0.032.874 , 0.034.994 , 0.044.183 , 0.112.259 , 0.137.618 , 0.141.760 , 0.187.997 , 0.236.246 , 0.273.178 , 0.277.664 , 0.281.485 , 0.559.589 ;
• demandes de brevet français publiées sous les numéros : 2.235.200 , 2.269.581 , 2.359.661 , 2.384.029 , 2.392.120 , 2.411.237 , 2.411.238 , 2.433.584 , 2.456.781 , 2.476.542 , 2.479.266 , 2.511.039 , 2.576.320 , 2.610.331 , 2.612.945 , 2.630.131 , 2.688.231 ;
• brevets américains publiés sous les numéros : 2.705.196 , 3.056.190 , 3.768.999 , 3.915.693 , 3.921.700 , 4.085.252 , 4.134.196 , 4.174.962 , 4.163.827 , 4.035.892 , 4.235.007 , 4.364.770 , 4.481.032 , 4.486.227 , 4.671.820 , 4.698.095 , 4.708.897 , 4.711.663 , 4.738.714 , 4.765.599 , 4.773.929 , 4.816.068 , 4.832.742 , 4.863.803 , 4.906.292 , 4.956.010 , 6.053.960 , 6.280.497 , 6.346.135 , 6.508.857 . Et demande de brevet japonais publié sous le numéro : 02-061006 .

In an attempt to solve at least one of these technical problems, a very large number of structures and processes for manufacturing cored wires have been proposed in the prior art, for example illustrated in the following documents:

• European patent applications published under the numbers: 0032874 , 0034994 , 0044183 , 0112259 , 0137618 , 0141760 , 0187997 , 0236246 , 0273178 , 0277664 , 0281485 , 0559589 ;
• French patent applications published under the numbers: 2235200 , 2269581 , 2359661 , 2384029 , 2392120 , 2411237 , 2411238 , 2433584 , 2456781 , 2476542 , 2479266 , 2511039 , 2576320 , 2610331 , 2612945 , 2630131 , 2688231 ;
• US patents published under the numbers: 2705196 , 3056190 , 3768999 , 3915693 , 3921700 , 4085252 , 4134196 , 4174962 , 4163827 , 4035892 , 4235007 , 4364770 , 4481032 , 4486227 , 4671820 , 4698095 , 4708897 , 4711663 , 4738714 , 4765599 , 4773929 , 4816068 , 4832742 , 4863803 , 4906292 , 4956010 , 6053960 , 6280497 , 6346135 , 6508857 . And Japanese patent application published under the number: 02-061006 .

For example, addition bodies of a treating agent similar to the wires are illustrated in FIG. FR2392126 and JP55-122834 .

The brief presentation of some of these earlier documents illustrates the great variety of technical solutions envisaged to respond to the various technical problems mentioned in the introduction.

The document EP-B2-0.236.246 discloses a cored wire comprising a metal envelope stapled by a circumferentially connected fold, closed on itself and whose edge is engaged inside the compacted mass forming the core of the cored wire.

The stapling is carried out along a generatrix of the envelope of the cored wire, possibly reinforced by crimping with transverse indentations over the entire width of the staple band. Compaction of the core of the cored wire is obtained by forming an open fold, opposite the staple zone, then closing this fold by radial pressure.

The casing of the cored wire is made of steel or aluminum and contains, for example, a powdery CaSi alloy containing 30% Ca by mass.

The document US 4163827 discloses a cored wire comprising a ferrosilicon core containing Ca, Al, powdered embedded in a resin or a polymeric binder such as polyurethane, this core being extruded before being wrapped by single or double winding, in a helix, d a thin strip of metal, plastic or paper with a thickness of 0.025 mm to 0.15 mm. Such a cored wire has many disadvantages. In the first place, the materials forming the resin are an unacceptable source of pollution for the liquid metal bath. Secondly, the mechanical strength and rigidity of the wire are very insufficient.

Thirdly, the ferrosilicon powder is practically unprotected with respect to the high temperature of the liquid metal.

The document EP-0032874 discloses a flux-cored wire comprising a metal thin-film sheath containing an additive at least partially surrounded by a casing of organic or metallic synthetic material in the form of a strip of thickness less than 100 microns. The wire has a flattened shape. The thin strip is made of polyethylene, polyester or polyvinyl chloride and form of sealing, possibly heat shrinkable. No manufacturing process is described for this flattened cored wire, whose design is more of a chimera than an industrial disclosure.

The document FR-2610331 of the applicant describes a cored wire comprising an axial zone containing a first powdery material or granular, surrounded by an intermediate metal tubular wall, and an annular zone, between this intermediate wall and the envelope of the cored wire, this annular zone containing a second powdery or granular material. The axial zone advantageously contains the most reactive materials with respect to the bath to be treated.

As long as the outer metal envelope of this cored wire is not destroyed, the material that fills the annular zone acts as lagging that reduces the rise in temperature of the intermediate wall, thus reducing the risk of bending of the wire which would prevent sinking into the bath, the intermediate wall retaining a certain rigidity.

The document US 3921700 discloses a cored wire to be wrapped in steel, containing a magnesium axial wire and an iron powder, of low thermal conductivity and high heat capacity, thus forming thermal insulation protecting the magnesium from too rapid heating when the cored wire is immersed in liquid steel. Alternatively, graphite or carbon is mixed with the iron powder.

Many of the technical problems with the use of flux cored wires result from the fact that it is virtually impossible to determine what actually happens to this wire when it is immersed in the bath of liquid metal, such as a steel pocket at 1600 ° C. In particular, the following questions are delicate: what is the shape of the wire in the bath (straight, curved in U), how deep is it destroyed by fusion. Only fragmentary and sometimes contradictory information can be found in this respect in the prior art.

• la température de fusion de l'inoculant contenu dans le fil va baisser ;
• la température de fusion de l'acier de la gaine de fil va baisser ;

le carbone diffusant au travers de la surface extérieure de la gaine de fil.Thus, the document FR-2384029 discloses an inoculation wire comprising a steel casing sheathing a powdered ferrosilicon compound packed to more than 65% by weight of silicon. According to this prior document, the silicon diffuses towards the steel casing of the wire, during its introduction into the liquid metal, so that:

• the melting temperature of the inoculant contained in the wire will drop;
• the melting temperature of the steel of the wire sheath will drop;

the carbon diffusing through the outer surface of the wire sheath.

According to this prior document, a cored wire comprising a mild steel sheath (melting temperature 1538 ° C.) containing a ferrosilicon with 75% silicon (melt temperature 1300 ° C.) will melt around 1200 ° C. when immersed for example in a gray cast iron at 1400 ° C, this fusion from the inner part of the sheath, due to the diffusion of silicon in the sheath which lowers the melting temperature of mild steel.

The document US 4174962 mentions, in addition to this diffusion of silicon, a dissolution of the outer wall of the cored wire sheath, by erosion and diffusion, even if the melting temperature of the sheath is greater than the temperature of the liquid metal bath.

The document US 4297133 describes a paper tube wound in layers, this tube being closed by metal caps. The burning time of the paper is indicated as three seconds when the tube is placed in a bath of liquid steel at 1600-1700 ° C.

The applicant has described in the publications FR-2821626 and FR-2810919 flux-cored wires comprising shells which, combustible without leaving troublesome residues, momentarily retard the propagation of heat towards the core of the wire, these envelopes being made of paper said for pyrotechnic application, fuel and thermal insulation.

According to these two prior documents of the applicant, by increasing the number of layers of paper, it delays the explosion of the cored wire containing calcium, or the vaporization of calcium and it is thus possible to introduce the cored wire at sufficient depth in the bath of liquid metal to avoid a surface reaction of the bath of the additive contained in the wire with the risks that would ensue: oxidation and / or bath renitruration, liquid metal projection, fumes, very low yield of the additive introduction operation by cored wire.

According to these two previous documents, the slow combustion of the so-called pyrotechnic paper does not cause the appearance of combustion residues affecting the composition of the liquid metal bath and does not produce inclusions modifying the behavior of the bath during casting. In the embodiment described by the document FR-2821626 , above this pyrotechnic paper envelope burning without leaving any harmful traces in the liquid metal bath, a metal protection is placed to prevent the layers of pyrotechnic paper from being damaged during winding on the drum of the cored wire or when the cored wire is unrolled from this drum.

The plaintiff was perplexed in finding that the cored wires described in the documents FR-2821626 or FR-2810919 did not always give a much higher yield to the cored wires without helical wound paper strips.

The applicant has endeavored to solve this technical problem, by providing, in addition, a cored wire whose life in the liquid metal bath is increased, compared to conventional son, so as to reach a predetermined depth in the bath of liquid metal.

1. 1) qu'il était important d'éviter toute combustion des enroulements de papier décrits dans les documents FR-2.821.626 et FR-2.810.919 , avant entrée du fil fourré dans le bain de métal liquide (zone de libre parcours du fil fourré) ;
2. 2) des moyens pour éviter cette combustion ;
3. 3) que le gain en durée de vie du fil fourré, était assuré lorsque la combustion du papier n'intervenait pas avant l'entrée du fil fourré dans le bain de métal liquide, le papier ne devant pas nécessairement être pyrotechnique, ou classé M1, ou à résistance à l'inflammation élevée, contrairement à ce qui est indiqué dans FR-2.821.626 ou FR-2.810.919 , le papier ne brûlant pas dans le bain de métal liquide, mais se pyrolysant pour se transformer en une matière dont les propriétés thermophysiques sont à ce jour inconnues de la demanderesse, cette pyrolyse n'étant obtenue que par le respect de certaines mesures qui seront détaillées dans la suite.

The plaintiff, after complex and long tests, discovered in particular:

1. 1) that it was important to avoid burning the paper windings described in the documents FR-2821626 and FR-2810919 before entering the flux-cored wire into the liquid metal bath (free-flow zone of the flux-cored wire);
2. 2) means to prevent this combustion;
3. 3) that the gain in service life of the cored wire was ensured when the burning of the paper did not intervene before the entry of the cored wire into the bath of liquid metal, the paper not necessarily having to be pyrotechnic, or classified M1 , or resistance to high inflammation, contrary to what is indicated in FR-2821626 or FR-2810919 , the paper not burning in the bath of liquid metal, but pyrolyzing to become a material whose thermophysical properties are unknown to the applicant, this pyrolysis being obtained only by the respect of certain measures which will be detailed in the following.

The Applicant has thus discovered inexpensive and safe means of increasing the service life of flux-cored wires in liquid metal baths, these means being compatible with all the structures described previously for flux-cored wires, these means thus bringing about a technical effect. further advantageous to each of the individual advantages of the different types of pre-filled wires.

The invention therefore relates to a flux-cored wire as specified in claim 1. It comprises powdered or compacted grains or embedded in a resin, at least one material selected from the group consisting of Ca, Bi, Nb, Mg, CaSi , C, Mn, Si, Cr, Ti, B, S, Se, Te, Pb, CaC ₂ , Na ₂ CO ₃ , CaCO ₃ , CaO, MgO, land rats, and it comprises an outer thermal barrier layer, enveloping a metal sheath, said outer thermal barrier layer being made of a pyrolyzing material upon contact with a bath of liquid metal. The pyrolyzing material is loaded with water or with a chemical compound with latent heat of high vaporization, especially greater than 2MJ / kg.

• le matériau pyrolysant est un papier kraft, un papier aluminisé ou un multicouches comprenant au moins une bande de papier kraft et au moins une couche de papier aluminisé ;
• le matériau pyrolysant est recouvert d'une feuille métallique mince ;
• la feuille métallique mince est en aluminium ou alliage d'aluminium ;
• le matériau pyrolysant présente une conductivité thermique comprise entre 0,15 et 4 W/m.K, avant pyrolyse ;
• le matériau pyrolysant présente une épaisseur radiale comprise entre 0,025 mm et 0,8 mm, avant pyrolyse ;
• le matériau pyrolysant présente une température de début de pyrolyse de l'ordre de 500°C ;
• le matériau pyrolysant comprend une couche de papier humidifiée ;
• le matériau pyrolysant est fixé par collage à une gaine métallique interne au fil fourré ;
• le matériau pyrolysant est placé entre une gaine métallique interne au fil et une enveloppe externe métallique ;
• l'enveloppe externe métallique est agrafée, le matériau pyrolysant étant placé, dans la bande d'agrafage, en interposition, de sorte à empêcher tout contact direct métal/métal dans la bande d'agrafage ;
• la gaine métallique interne est d'épaisseur radiale comprise entre 0,2 et 0,6 mm environ, l'enveloppe externe métallique étant d'épaisseur radiale comprise entre 0,2 et 0,6 mm environ ;
• le matériau pyrolysant est un papier kraft en mono ou multicouches, d'épaisseur comprise entre 0,1 et 0,8 mm.

According to various embodiments, the cored wire comprises the following additional characters, if necessary combined:

• the pyrolyzing material is a kraft paper, an aluminized paper or a multilayer comprising at least one strip of kraft paper and at least one layer of aluminized paper;
• the pyrolyzing material is covered with a thin metal sheet;
• the thin metal sheet is aluminum or aluminum alloy;
• the pyrolyzing material has a thermal conductivity of between 0.15 and 4 W / mK, before pyrolysis;
• the pyrolyzing material has a radial thickness of between 0.025 mm and 0.8 mm, before pyrolysis;
• the pyrolyzing material has a starting temperature of pyrolysis of the order of 500 ° C;
• the pyrolyzing material comprises a layer of moistened paper;
• the pyrolyzing material is fixed by gluing to a metal sheath internal to the flux-cored wire;
• the pyrolyzing material is placed between a metal sheath internal to the wire and a metal outer shell;
• the outer metal envelope is stapled, the pyrolyzing material being placed in the staple band in interposition so as to prevent any direct metal / metal contact in the staple band;
• the inner metal sheath is of radial thickness between about 0.2 and 0.6 mm, the outer metal sheath being of radial thickness between about 0.2 and 0.6 mm;
• the pyrolyzing material is a mono or multilayer kraft paper with a thickness of between 0.1 and 0.8 mm.

• la figure 1 est une représentation du principe d'introduction du fil fourré dans un bain d'acier liquide ;
• les figures 2 à 12 sont des courbes température fonction du temps, issues de simulation numérique ;
• les figures 13 à 21 sont des courbes température fonction du temps, issues de campagnes d'essais menées par la demanderesse.

Other objects and advantages of the invention will become apparent from the following description of embodiments, a description which will be made with reference to the accompanying drawings in which:

• the figure 1 is a representation of the principle of introduction of the cored wire in a bath of liquid steel;
• the Figures 2 to 12 are time-dependent temperature curves derived from numerical simulation;
• the Figures 13 to 21 are time-dependent temperature curves derived from test campaigns conducted by the applicant.

We first refer to the figure 1 , which is a representation of the principle of introduction of a cored wire into a ladle of liquid steel.

The cored wire 1 is extracted from a cage 2 such as, for example, described in the document FR-2703334 of the applicant, or else extracted from a drum 3, and introduced into an injector 4.

This injector 4 drives the wire in a bent guide tube 5, the cored wire coming out of this guide tube 5 at a height of the order of one meter to one meter and a half above the surface of the liquid steel bath 6 contained in a pocket 7.

• un premier milieu dans lequel le fil fourré est logé à l'intérieur du tube de guidage ;
• un deuxième milieu situé au dessus du bain d'acier liquide dans lequel le fil fourré est placé en contact direct avec l'atmosphère environnante ;
• un troisième milieu qui est le bain d'acier ou de métal liquide lui-même.

The cored wire 1 is therefore placed in three thermally very different media:

• a first medium in which the cored wire is housed inside the guide tube;
• a second medium located above the liquid steel bath in which the cored wire is placed in direct contact with the surrounding atmosphere;
• a third medium which is the bath of steel or liquid metal itself.

The Applicant wished, at first, to thermally simulate the path of the cored wire in order to limit the number of tests with instrumented cored wire.

For this model, the three-dimensional radiative exchanges between plane, opaque, gray and diffuse surfaces were simulated by calculating shape factors and transfer factors.

The form factors were calculated by the plane flow method, the transfer factors being calculated by the coating method taking into account diffuse multi-reflections.

Inside the guide pipe, the flux received is supposed to radiate from the tube wrapping the cored wire with a form factor equal to 1.

For the free passage of the cored wire after the exit of the guide tube 5 and before entering the liquid metal bath 6, the flow is considered radiative but coming from the liquid metal bath 6 and the walls of the pocket 7.

Inside the liquid metal bath 6, the transfer is considered as convective with an exchange coefficient of the order 50,000 W / m ² K, the surface temperature being imposed.

The total emissivity of the outer surface of the cored wire is considered equal to 0.8, that of the guide tube is equal to 1 while that of the bath is considered equal to 0.8.

avec :

• φ flux thermique échangé entre les deux surfaces en W/m²
• ε coefficient tenant compte des émissivités des deux surfaces,
• F facteur de forme prenant en compte les surfaces, les formes et l'orientation des deux surfaces l'une par rapport à l'autre,
• σ constante de STEFAN-BOLTZMANN égale à 5,67 x 10^-8 W/m²K
• T₁ et T₂ températures absolues en Kelvin des deux surfaces avec T₁ supérieur à T₂.

The radiative thermal flux exchanged, according to the law of STEFAN-BOLTZMANN is of the form: $$\mathrm{\phi }=\mathrm{\epsilon }\times \mathrm{F}\times \mathrm{\sigma }\times \left({\mathrm{T}}^4{}_1-{\mathrm{T}}^4{}_2\right)$$

with:

• φ heat flux exchanged between the two surfaces in W / m ²
• ε coefficient taking into account the emissivities of the two surfaces,
• F form factor taking into account the surfaces, the shapes and the orientation of the two surfaces with respect to each other,
• σ STEFAN-BOLTZMANN constant equal to 5.67 x 10 ^-8 W / m ² K
• T ₁ and T ₂ absolute Kelvin temperatures of both surfaces with T ₁ greater than T ₂ .

The figure 2 gives the variation of the transfer factor between the flux-cored wire and the bath of liquid metal (ε x F) as a function of the distance above this bath of liquid metal, the value zero on the x-axis corresponding to the surface of the bath of liquid metal.

The cored wire is considered to comprise three concentric cylindrical layers, namely a steel sheathed calcium core, this steel sheath being covered with paper.

For numerical simulation, the diameter of the core of calcium is 7.8 mm, the thickness of the steel sheath is 0.6 mm while the thickness of the paper can be set at different values, example 0.6 mm for eight layers of paper superimposed.

For the simulation, the cored wire is considered to be formed of a solid core made of interlocked calcium and in contact with the steel sheath which is itself nested and in contact with the paper.

où

• L1 est la longueur du tube de guidage 5 et,
• V est la vitesse de passage du fil fourré dans le tube 5

The guide tube 5 is represented by a hollow cylinder made of steel of constant temperature, giving an energy to the flux-cored wire during the time T1, such that: $$\mathrm{<figure-callout\; id="1"\; label="T"\; filenames="imgf0001.png"\; state="\{\{state\}\}">T</figure-callout>}1=\mathrm{The}1/\mathrm{V}$$

or

• L1 is the length of the guide tube 5 and,
• V is the speed of passage of the cored wire in the tube 5

The bath of liquid metal and the walls of the pocket 7 are represented in the numerical model by a volume of temperature equal to 1600 ° with radiation and convection to the cored wire depending on whether the wire is above the bath 6 or in this bath of liquid metal 6.

The heat exchange is convective with a very high exchange coefficient (50,000 W / m ² K) from the time T2 where the cored wire enters the liquid metal bath 6.

où :T2 is calculated as follows: $$\mathrm{<figure-callout\; id="2"\; label="T"\; filenames="imgf0002.png,imgf0004.png"\; state="\{\{state\}\}">T</figure-callout>}2=\mathrm{The}1+\mathrm{The}2/\mathrm{V}$$

or :

L2 is the distance between the lower end of the guide tube 5 and the surface of the liquid metal bath 6.

The speed of travel of the cored wire is equal to 2 m / s, the initial temperature of the cored wire being 50 ° C.

The free path of the cored wire beyond the guide tube 5 and before introduction into the bath of liquid metal is considered to be 1.4 m in length.

The yarn is considered destroyed when, by calculation, the surface of the calcium core has a temperature above 1400 ° C.

As it appears in figure 3 the modeling indicates that, for a reference wire devoid of thermal protection, the surface temperature of the calcium core increases by 70 ° C only during the free path and reaches the threshold of 1400 ° C at 0, 15 s after a run inside the liquid metal bath of only 30 cm for a speed of 2m / s.

The temperature gradient between the steel sheath and the calcium core does not exceed, by calculation, 65 ° C.

Thus, when the surface temperature of the core of calcium is 1400 ° C, that of the outer surface of the steel sheath is 1465 ° C, so that the steel sheath does not melt before the destruction of the flux-cored wire, the latent heat of fusion of this steel sheath is therefore not taken into account in the numerical simulation.

• 0,025 mm pour la courbe 4a,
• 0,05 mm pour la courbe 4b,
• 0,1 mm pour la courbe 4c,
• 0,6 mm pour la courbe 4d

The figure 4 gives four curves of temperature evolution of the calcium core surface of a flux-cored wire as a function of time, each of these four curves corresponding to a different thickness of protective paper, namely:

• 0.025 mm for the curve 4a,
• 0.05 mm for curve 4b,
• 0.1 mm for curve 4c,
• 0.6 mm for the 4d curve

The comparison of Figures 3 and 4 shows, by numerical simulation, a protective effect of the paper surrounding the steel sheath, this effect being all the more marked as the thickness of the paper is important.

The curves represented in figure 4 were obtained by considering that the layers of paper remain intact, without combustion.

According to this hypothesis, an insulation thickness of 0.025 mm would be sufficient to protect the cored wire to the bottom of the bath of liquid metal.

But the burning temperature of the paper is around 550 ° C.

A study of the temperature rise of the paper surface in the free path has been carried out neglecting the effect of convection with respect to radiation, which is in fact predominant.

In figure 5 is shown the evolution of the surface temperatures of the paper as a function of the conductivity of this paper, during the first second of free travel of the cored wire, the thickness of the paper being 0.6 mm, the running speed of the paper. cored wire being 2m / s.

Curve 5a corresponds to a conductivity of 0.1 W / K.m, curve 5b corresponds to a conductivity of 0.15 W / K.m and curve 5c corresponds to a conductivity of 0.2 W / K.m.

The figure 5 shows that the burning of paper is probable and the destruction of the paper in the free path of the cored wire is not excluded.

The figure 6 represents the evolution of the temperature of the paper surface for a thermal conductivity of this paper of 0.15 W / Km, a injection speed of the cored wire of 2m / s, the paper thickness being in curve 6a of 0.6 mm, in curve 6b of 0.2 mm and in curve 6c of 0.1 mm.

This figure 6 suggests that by decreasing the thickness of the paper, the surface temperature of this paper decreases and therefore the risk of burning of this paper during the free path of the flux-cored wire above the bath of liquid metal.

As is known to those skilled in the art, the surface of the bath of liquid metal such as steel is covered with a layer of slag which forms a heat shield, the figure 7 shows that the temperature of the paper covering the cored wire is largely affected by the variation of the temperature of the radiation source.

The curves 7a, 7b, 7c and 7d respectively correspond to emitting surface temperatures of 1500, 1400, 1300 and 1200 ° C.

For the simulation represented in figure 7 , the injection speed of the cored wire was 2m / s and the thermal conductivity of the paper 0.15 W / Km

By these numerical simulations, confirmed during experimental tests, the Applicant has been able to make the assumption that the variability of the results obtained during the implementation of a structure as described in the document FR-2810919 results from a burning of the paper during the free passage of the flux-cored wire above the bath of liquid metal, this paper no longer playing, therefore, its thermal protection effect of the cored wire, inside the liquid steel bath .

The Applicant has made the following additional hypothesis: the paper would not burn inside the liquid steel bath but would pyrolyze.

• une première conductivité qui est celle du papier d'origine (0,15 W/K.m), cette première conductivité étant maintenue jusqu'à une température de l'ordre de 500°C de début de pyrolyse ;
• une deuxième conductivité (300W/K.m), supposée atteinte lorsque la température du papier pyrolysé est de 600°C, la pyrolyse étant supposée terminée lorsque cette température de 600°C est atteinte.

The applicant then pursued numerical simulations by considering the paper as a body having two different thermal conductivities according to the temperature:

• a first conductivity which is that of the original paper (0.15 W / Km), this first conductivity being maintained up to a temperature of the order of 500 ° C of start of pyrolysis;
• a second conductivity (300W / Km), assumed to be reached when the temperature of the pyrolyzed paper is 600 ° C, the pyrolysis being assumed to be completed when this temperature of 600 ° C is reached.

Between 500 and 600 ° C, the change in conductivity from 0.15 W / K.m to 300 W / K.m is assumed to be linear in the simulation as a function of temperature.

The figure 8 gives the results of the numerical simulation for the surface temperature of the calcium contained in the flux-cored wire, the paper being supposed to be dissolved in the bath of liquid metal, just after its pyrolysis.

Curve 8a corresponds to the conventional cored wire, without protective paper.

Curve 8b corresponds to a cored wire provided with a protective paper having a thickness of 0.6 mm.

Curve 8c corresponds to a cored wire provided with a protective paper to a thickness of 1.2 mm.

The figure 8 suggests that if the paper disappears after pyrolysis, it is not possible to protect the cored wire so that it reaches the bottom of the liquid steel bath, even by doubling the thickness of the paper.

However, the applicant has found, in industrial trials, that the cored wire coated with protective paper sometimes reaches the bottom of the bath.

It is therefore likely that the paper does not disappear after pyrolysis inside the liquid steel bath.

Pyrolysis of Kraft paper was carried out by raising the temperature of the sheets of paper, protected from oxygen, to a temperature of about 600 ° C. and a measurement of the thermal conductivity of the paper was carried out before and after pyrolysis.

It emerges from this study that the thermal conductivity of the paper varies shortly after its pyrolysis.

The applicant has therefore taken over the numerical simulation by considering this time, in contrast with the hypothesis corresponding to the figure 8 that the paper does not disappear after pyrolysis, the conductivity of the paper after pyrolysis being considered to be 0.15, 1, 2, 4 W / Km for the curves 9a, 9b, 9c, 9d respectively. This simulation better reflects the test results as will appear below.

In order to avoid any combustion of the paper enveloping the steel sheath of the cored wire, the Applicant has conceived of absorbing the radiation or of reflecting it by moistening this paper or covering it with aluminum.

The figure 10 shows the results of the numerical simulation for the variations of surface temperature of the paper as a function of time, the curves 10a, 10b, 10c, 10d respectively corresponding to humidity of 0%, 59%, 89% and 118%.

For this simulation represented in figure 10 , the injection speed of the cored wire was 2m / s, the thermal conductivity of the paper being 0.15 W / Km

The figure 11 gives the result of the radiative calculation carried out by adding a very thin layer of aluminum in coating of the paper enveloping the steel sheath of the cored wire.

This figure 11 shows that the radiative transfer factor is reduced by a factor of 8 compared to that of paper whose emissivity is 0.8.

The figure 12 allows the comparison of surface temperature changes of the paper as a function of time with and without aluminum coating, the injection speed of the cored wire remaining of 2m / s and the thermal conductivity of the paper being 0.15 W / Km

The surface temperature of the paper increases very little, according to this numerical simulation, in the free path of the cored wire, the aluminum providing a very effective thermal protection for the paper of the cored wire.

To verify the hypotheses formulated by the applicant during the simulations presented above, tests were carried out by the plaintiff using filled cored wire.

• vidage du fil fourré ;
• positionnement de thermo-couples en contact avec la gaine interne en acier du fil fourré, à l'opposé de la zone d'agrafage ;
• remplissage du fil fourré avec la poudre.

Instrumented cored wire is manufactured in three stages:

• emptying the cored wire;
• positioning thermocouples in contact with the inner steel sheath of the cored wire, opposite the staple zone;
• filling the cored wire with the powder.

The electrical connections and connection wires of the thermocouples are protected by steel tubes.

The instrumented wire is introduced into a steel steel ladle and then reassembled after a predetermined downtime.

Since the baths are permanently stirred with argon, an inert atmosphere is created in the free path above the surface of the liquid steel bath, which limits the risk of accidental combustion of the paper of the cored wire.

On the Figures 13 to 21 point I corresponds to the entrance of the cored wire into the liquid steel ladle.

Initially, a reference test was carried out with a non-paper-coated cored wire, the variation of the temperature inside the reference cored wire, as a function of time, being given in FIG. figure 13 .

The temperature drop at point D of the figure 13 is related to the destruction of thermo-couples.

The figure 14 compares the results obtained with the reference wire (reference 14a) and a cored wire comprising a layer of Kraft paper placed between the calcium core and the steel sheath (reference 14b).

In view of this figure 14 , the placement of Kraft paper inside the cored wire can delay the rise in temperature by 0.4 seconds or a total time of 0.7 seconds before destruction.

The figure 15 compares the results obtained with the reference wire (curve 15a) and two instrumented son provided with two layers of external Kraft paper (curves 15b, 15c).

The temperature rise delay obtained is 0.8 and 1.2 seconds allowing the cored wire to reach the bottom of the pocket.

The abrupt rise in temperature of the curves 15b and 15c corresponds to the moment when the Kraft paper is totally degraded, the steel sheath of the cored wire coming into direct contact with the liquid steel bath.

The figure 16 compares the results obtained with the reference wire (curve 16a) and a cored wire protected by two layers of Kraft paper and two layers of aluminized paper (two curved tests 16b and 16c).

The curves of the figure 16 show that the presence of two layers of kraft paper and two layers of aluminized paper retard the rise in temperature by about 1 second, compared to a conventional reference wire.

In figure 17 The results obtained are presented with two samples protected by three layers of kraft paper and two layers of aluminized paper (curve 17b and 17c) to be compared with the values of the reference wire (curve 17a).

The figure 18 allows to compare the results obtained with six layers of kraft paper and two layers of aluminized paper (curves 18b and 18c), to be compared with the reference wire (curve 18a).

The rise in temperature is here delayed by more than 1.2 seconds.

Curve 19b of the figure 19 gives the results obtained for a cored wire protected with four layers of kraft paper and an aluminum layer, the delay of the rise in temperature being 0.6 seconds with respect to the reference wire, curve 19a.

Curve 20b of the figure 20 gives the result obtained with a cored wire protected by eight layers of kraft paper and an aluminum layer, the delay of the rise in temperature being 0.8 seconds relative to the reference wire, curve 20a.

Curve 20c corresponds to a test in which the cored wire dipped laterally into the slag and did not penetrate the molten steel, this test indirectly giving the temperature of the slag, ie 1200 ° C.

Curves 21b and c of the figure 21 give the results obtained for filled son protected by two layers of aluminized paper, the delay of the rise in temperature being about 0.7 seconds with respect to the reference wire, curve 21a, these results are to be compared with those of the figure 18 .

The numerical and experimental results that have been presented above with reference to Figures 2 to 12 confirm that the outer layers of paper with a cored wire constitute a thermal insulator to protect these cored wires for durations between 0.6 and 1.6 seconds, compared to a conventional cored wire.

The Applicant has discovered that this protective effect is obtained by the pyrolysis of the paper in the bath of liquid metal, the paper to be protected from any combustion especially during its free path above the bath of liquid metal in the pocket.

The risks of combustion can be limited by injecting argon above the liquid metal bag or by soaking the paper with water or covering the paper with a metal band.

The document FR-2810919 of the applicant describes the establishment of thermal insulation paper between a steel outer casing and a steel sheath containing the powdery or granular additive.

The outer steel sheath is designed to prevent the paper from being damaged during handling of the cored wire.

The applicant has discovered that these so-called hybrid yarns as described in the document FR-2810919 allowed a significant delay in temperature rise only if the paper is present in the staple or overlap zone so as to avoid any metal / metal contact in the staple zone, the paper being pyrolyzed in the bath of liquid metal.

The experimental work was carried out with the assistance of Armines, Center of Energetics, Ecole des Mines de Paris.

Claims (14)

1. Cored wire, comprising:

   - in powder or grains compacted or embedded in a resin, at least one material chosen from among the group consisting of Ca, Bi, Nb, Mg, CaSi, C, Mn, Si, Cr, Ti, B, S, Se, Te, Pb, CaC2, Na2C03, CaC03, CaO, MgO, and rare earths,

   - a metallic sheath, and

   - at least one external thermal barrier layer surrounding the metallic sheath, said external thermal barrier layer being made of a material that pyrolizes upon contact with a metal bath,

   characterized in that the pyrolizing material is soaked with water or a chemical compound with a high latent heat of vaporization, in particular higher than 2 MJ/kg.
2. Cored wire according to claim 1, characterized in that the pyrolizing material is Kraft paper, aluminized paper or a multiple layer comprising at least one strip of Kraft paper and at least one layer of aluminized paper.
3. Cored wire according to claim 2, characterized in that the pyrolizing material is covered with a thin metallic sheet.
4. Cored wire according to claim 3, characterized in that the thin metallic sheet is made of aluminum or aluminum alloy.
5. Cored wire according to any of claims 1 to 4, characterized in that the pyrolizing material has a thermal conductivity that is between 0.15 and 4 W/m.K inclusive, before pyrolysis.
6. Cored wire according to any of claims 1 to 5, characterized in that the pyrolizing material is of a radial thickness of between 0.025 mm and 0.8 mm inclusive, before pyrolysis.
7. Cored wire according to any of claims 1 to 6, characterized in that the pyrolizing material has a pyrolysis start temperature on the order of 500 C.
8. Cored wire according to any of claims 1 to 7, characterized in that the pyrolizing material comprises a layer of dampened paper.
9. Cored wire according to any of claims 1 to 8, characterized in that the pyrolizing material is affixed by gluing it to a metallic liner inside the cored wire.
10. Cored wire according to any of claims 1 to 9, characterized in that the pyrolizing material is placed between a metallic layer inside the rod and an external metallic enclosure.
11. Cored wire according to claim 10, characterized in that the external metallic enclosure is seamed, and the pyrolizing material is interposed inside the seaming strip, so as to prevent all direct metal/metal contact inside the seaming band.
12. Cored wire according to claim 10 or 11, characterized in that the internal metallic liner has a radial thickness of between approximately 0.2 and 0.6 mm, and where the external metallic enclosure is of a radial thickness of between about 0.2 and 0.6 mm.
13. Cored wire according to claim 12, characterized in that the pyrolizing material is a Kraft paper, either single or multi-layer, of a thickness of between 0.1 and 0.8 mm.
14. Cored wire according to any of claims 1 to 13, characterized in that it comprises, in powder or grains compacted or embedded in a resin, at least one material chosen from among the group consisting of Ca, Bi, Nb, Mg, CaSi, C, Mn, Si, Cr, Ti, B, S, Se, Te, Pb, CaC2, Na2C03, CaC03, CaO, MgO, and rare earths.

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