EP2917377A1

EP2917377A1 - Cored wire for the metallurgical treatment of a bath of molten metal and corresponding method

Info

Publication number: EP2917377A1
Application number: EP13792869.3A
Authority: EP
Inventors: Alain Markwitz; Olivier BAHUON; Sébastien GERARDIN
Original assignee: Affival SA
Current assignee: Affival SA
Priority date: 2012-11-09
Filing date: 2013-11-08
Publication date: 2015-09-16
Anticipated expiration: 2033-11-08
Also published as: WO2014072456A1; PL2917377T3; FR2997963A1; FR2997963B1; ES2743496T3; EP2917377B1; US20150267272A1

Abstract

Cored wire (1; 100) intended to be introduced into a bath of molten metal to carry out a metallurgical treatment, the cored wire comprising: a filling (2) extending locally along a longitudinal axis (L), the filling containing at least one active substance for treating the molten metal; and an external wrapper (4) longitudinally around the filling; characterized in that the filling comprises: an extruded rod (8) extending longitudinally and containing the active substance; and an intermediate layer (10) extending longitudinally between the extruded rod and the outer wrapper and containing a powder containing one or more of the following: a metal, a mixture of metals, a metal oxide, a mixture of metal oxides. Corresponding method.

Description

Filled wire for metallurgical treatment of a bath of molten metal and corresponding process

The invention relates to a cored wire for introduction into a bath of molten metal for metallurgical treatment, the cored wire comprising:

a lining extending locally along a longitudinal axis, the lining comprising at least one active substance for treating the molten metal; and

an outer casing longitudinally around the lining.

The molten metal is, for example, cast iron or steel. The objective of the metallurgical treatment is, for example, to add to the molten metal at least one substance intended to regulate the composition of the molten metal and / or the composition of the precipitates or non-metallic inclusions it contains.

In metallurgy, it is known to provide such a substance by means of "cored wires" in the form of coils. The flux-cored wire is generally composed of a lining comprising the active substance in pulverulent form, contained in a metallic envelope made of a metal whose composition is compatible with that of the molten metal to be treated. In the case of the treatment of a molten steel, this envelope is itself advantageously made of steel.

The flux-cored wire is introduced into the molten metal bath by means of an injection device, generally automatic, introducing a precise length of cored wire at an appropriate speed.

For example, in the field of foundry, for the manufacture of spheroidal graphite cast iron, it is known to use a cored wire to perform a nodularization treatment. It is a matter of adding magnesium to change the shape of the graphite particles of the cast iron, from lamellar to spheroidal. The added substance is usually a ferrosilicon-magnesium alloy powder.

In another example, in the field of iron and steel, it is known to treat steels with calcium. This treatment aims, for example, to modify the chemical composition of endogenous inclusions of the alumina type, in order to obtain inclusions of the aluminate type of lime. These inclusions, dispersed in the molten steel, are liquid at the casting temperature. Thus, they can not cling to the walls of the nozzles of a pocket or the distributor of a continuous casting plant. Flowability is improved, as is the final quality of the steel produced.

There are many types of cored wires whose packing consists of a pure calcium powder or calcium alloy, or a mixture of calcium powders and iron, or even aluminum. The alloy commonly called CaSi (calcium disilicide) or the A mixture of calcium and iron powders (commonly known as CaFe), for example, are widely used packings.

Although the introduction of a cored wire into the molten metal bath is an ingenious means for adding the active substance to the molten metal, the effectiveness of the introduction is sometimes limited. For example, for the CaFe powder-based cored wires used in the steel industry, the calcium addition yield, defined as the amount of calcium found in the steel after the injection of the cored wire divided by the amount of calcium introduced by the cored wire consumed is generally of the order of 10% to 15%, sometimes much less. The low efficiency of calcium comes mainly because of its low vaporization temperature. Indeed, of the order of 1480 ° C, the latter is generally lower than the working temperature of the liquid steel so that the calcium vaporizes while it is introduced into the liquid steel.

An object of the invention is to provide a cored wire performing more effectively the metallurgical treatment, while remaining of a competitive cost.

For this purpose, the subject of the invention is a cored wire of the type described above, in which the lining comprises:

an extruded bar extending longitudinally and comprising the active substance; and

an intermediate layer extending longitudinally between the extruded bar and the outer envelope and comprising a powder comprising one or more of: a metal, a mixture of metals, a metal oxide, a mixture of metal oxides.

According to particular embodiments, the device may comprise one or more of the following characteristics, taken in isolation or in any technically possible combination:

the lining further comprises a thermally insulating layer extending longitudinally between the extruded bar and the intermediate layer;

the extruded bar has an outer equivalent diameter D1 in a transverse plane substantially perpendicular to the longitudinal axis, the intermediate layer having an outer equivalent diameter D2 in the transverse plane, with D2 lying in the range 1.3 times to 6.2 times D1; ;

the outer casing comprises a strip of steel, aluminum, copper, nickel, or zinc, or an alloy of two or more thereof;

the extruded bar mainly comprises magnesium; the powder of the intermediate layer mainly comprises an alloy of iron and silicon further comprising calcium and / or barium and / or one or more rare earths;

the extruded bar mainly comprises calcium;

the powder of the intermediate layer comprises an iron powder or a mixture of iron powder and aluminum powder and / or magnesium powder and / or slag powder;

the outer envelope is metallic;

the outer envelope mainly comprises one or more of: steel, copper, aluminum, nickel, zinc;

the outer envelope has a transverse thickness of between 0.2 and 0.7 mm;

the extruded bar has an external equivalent diameter of between 2 and 10 mm;

the cored wire has an equivalent diameter of between 6 and 21 mm;

the extruded bar is substantially cylindrical with a circular base;

- The intermediate layer has a general shape of hollow cylinder, the cylinder being circular base;

the outer casing has a substantially tubular general shape with a circular base.

By "equivalent diameter" of an element is meant the diameter of a surface disk equal to the surface presented by the sectional element in a transverse plane. If the given element has a circular cross-section along the transverse plane, the equivalent diameter is equal to the ordinary diameter.

When the notion of equivalent diameter is used for an element, it is implicit that the element is locally substantially cylindrical, but not necessarily circular.

"Metallurgical treatment" means, for example:

^■ a change in the chemical composition of molten metal, and / or ■ a modifying metal properties obtained after solidification due molten metal, for example, changing the composition of inclusions or precipitates present before treatment, or the creation, as a result of the treatment, of such inclusions or precipitates, and / or

^■ a modification of the inclusion population present in the liquid metal in order to improve its development (improvement of flowability continuous casting). The invention further relates to a method of metallurgically treating a bath of molten metal, the method comprising the step of introducing a flux-cored wire as described above into the molten metal bath.

According to particular embodiments, the method may comprise one or more of the following characteristics, taken in isolation or in any technically possible combination:

the molten metal is cast iron and the filled cored wire is as described above;

the molten metal is steel and the filled cored wire is as described above. The invention will be better understood on reading the description which follows, given solely by way of example, and with reference to the appended drawings, in which:

- Figure 1 shows schematically, in perspective, a cored wire according to the invention;

- Figure 2 shows schematically, in cross section, the cored wire shown in Figure 1; and

3 represents a graph illustrating, in a particular application, the slowing down of the fading of the active substance (magnesium) when the flux-cored wire shown in FIGS. 1 and 2 is introduced into a bath of molten metal (liquid iron ).

Referring to Figures 1 and 2, there is described a cored wire 1 extending locally along a longitudinal axis L. Only a portion of the cored wire 1 is shown. The portion shown extends along the longitudinal axis L. This does not mean that all the cored wire 1 extends along the longitudinal axis L. Indeed, the cored wire 1 may have a certain curvature, for example s' it is rolled up to take up less space.

Similarly, a transverse plane T perpendicular to the longitudinal axis L is defined. It will be understood that the transverse plane T is transversal for the portion of the cored wire 1 represented, that is to say, locally transverse.

The cored wire 1 is for example intended to be introduced into a bath of molten iron (not shown).

The cored wire 1 comprises a lining 2 and an outer envelope 4, both extending longitudinally.

The outer casing 4 forms a peripheral portion of the cored wire 1, intended to be in contact with the molten metal bath when the cored wire 1 is introduced into the molten metal bath. The outer casing 4 is advantageously constituted by a metal strip 6 folded on itself around the longitudinal axis L.

The strip 6 is for example steel, copper, aluminum, nickel, or zinc, or a mixture of two or more of these elements.

The strip 6 advantageously comprises two longitudinal folds 6a, 6b (FIG.

2) stapled to one another to close the strip 6 on itself along the longitudinal axis L. The strip 6, thus folded, has a generally tubular shape which envelops the lining 2. Advantageously, the shape tubular is substantially cylindrical with a circular base and has an equivalent diameter D. D is advantageously between 6 and 21 mm. For example, D is 13 mm.

The lining 2 comprises an extruded bar 8 extending longitudinally and an intermediate layer 10 extending longitudinally between the extruded bar 8 and the outer casing 4.

The extruded bar 8 is advantageously substantially cylindrical with a circular base. The extruded bar 8 has a diameter D1 in the transverse plane T, with D1 advantageously between 2 and 10 mm, for example 8 mm.

The extruded bar 8 comprises an active substance for carrying out a treatment of molten iron. The active substance is, for example, magnesium. Advantageously, the extruded bar 8 mainly comprises magnesium.

By "predominantly" is meant that the extruded bar 8 comprises at least

50% by weight of magnesium, preferably at least 90% by weight of magnesium.

In the example, the extruded bar 8 is made of magnesium of industrial purity, for example 99.8% by weight.

The extruded bar 8 is not a simple cluster of compacted powder material during the closure of the cored wire 1, nor even an agglomerate of powder grains (powder material) bonded together by a binder of any kind. The extruded bar 8 is for example obtained by extrusion of a solid cylinder (billet) of material through a die through a press. The bar 8 can also be obtained directly by a continuous casting method, the liquid material being solidified in the form of a continuous bar. The porosity of the extruded bar 8 is considered almost zero, the apparent density of the bar being close to the true density of the material.

The intermediate layer 10 extends for example in the space between the extruded bar 8 and the outer shell 4.

The intermediate layer 10 has an equivalent external diameter D2. D2 is for example such that the ratio D2 / D1 is 1, 3 and 6.2. The intermediate layer 10 is advantageously constituted by a powder. The intermediate layer 10 may also comprise a thermally insulating layer covering the bar 8.

The intermediate layer 10 also comprises, mainly in the sense defined above, an active substance in the metallurgical treatment, for example a ferrosilicon alloy. Advantageously, the intermediate layer 10 can also comprise up to 12% by weight of calcium, barium and rare earths (lanthanum, cerium).

The composition of the powder of the intermediate layer 10 depends of course on the type of metallurgical treatment to be performed. It can be neutral, that is to say without any metallurgical effect on the bath of liquid metal to be treated, in this chamber, the powder plays a unique thermal insulation of the bar 8. It can also participate directly in the treatment metallurgical and thus assume a dual role of thermal insulation and active treatment element.

We will now describe the operation and use of the cored wire 1, to perform a nodularization and inoculation of the molten iron in order to obtain spheroidal graphite cast iron.

The cast iron to be treated is in the form of a bath of molten metal, for example contained in a container such as a pocket.

The cored wire 1 is introduced into the molten melt bath in a manner known per se.

The introduction of magnesium into the melt bath leads to a series of physical and chemical transformations, some simultaneous and others successive:

- a magnesium melting begins at 657 ^<Ό (for pure Mg),

a boiling point of magnesium at 1053 ° C. (for pure Mg).

The desulfurization reaction is as follows:

Mg + FeS -> Fe + MgS.

Simultaneously, an intense mixing of the melt bath by the Mg gas occurs, as well as a deoxidation of the melt. Indeed, magnesium is an energetic deoxidant. Once the desulfurization and deoxidation are complete, the remaining magnesium is incorporated into the melt. Of course, throughout the duration of the process, part of the magnesium vapor formed escapes to the surface of the melt and is oxidized in pure loss in the slag or in the atmosphere, giving rise for example to the formation of magnesia.

The magnesium addition yield is defined as the ratio between, on the one hand, the difference between the Mg content actually found in the melt after the introduction of the cored wire 1 and the Mg content in the melt before the introduction of the cored wire 1. introduction of the thread filled 1 and, on the other hand, the theoretical Mg content in the cast iron introduced by means of the cored wire 1 if 100% of the added Mg was actually to be found in the cast iron.

The magnesium contained in the extruded bar 8 has a nodulising role, that is to say that it makes it possible to obtain spheroidal graphite particles in the cast iron.

Thanks to the characteristics described above, the cored wire 1 performs the metallurgical treatment more efficiently, in the example, a nodularization of the cast iron, while remaining at a competitive cost.

The intermediate layer 10 acts as a thermal protection for the extruded bar 8, slowing the rise in temperature of the magnesium contained in the extruded bar 8. The location of the intermediate layer 10 around the extruded bar 8 protects the latter. When the flux-cored wire 1 is injected into the molten cast-iron bath, the temperature rise of the magnesium is delayed by a slow heat transfer. The cored wire 1 can therefore be introduced deeper into the molten iron column. This increases the contact time of the magnesium gas with the liquid iron and thus improves the magnesium addition efficiency.

In addition, in the extruded bar 8, magnesium has a reduced surface area of heat exchange compared to that which would present a packing of simple grains of powder. Indeed, the specific surface is no longer the surface of the grains, but the lateral surface of the extruded bar 8. This slows the vaporization of the magnesium, which improves the addition efficiency and moderates the reaction of the Mg with the molten cast iron .

The intermediate layer 10 has, in the example, also a metallurgical role. The intermediate layer 10 comprises a second active substance in the metallurgical treatment, ie for example a ferrosilicon alloy. This second active substance acts as an inoculant. Inoculation, as is known, regenerates the graphitic germination potential after the magnesium treatment. This makes it possible to avoid the formation of cementite and to contribute to obtaining the desired level of spheroidal graphite. For ferritic cast iron grades, this also promotes the formation of ferrite by increasing the density of spherules.

In addition, because the substance active against nodularization, magnesium, is contained in the extruded bar 8, it is possible to introduce the active substance more regularly than with son filled with the state of the art containing magnesium powder, whose density and compaction are difficult to control. The small mass variation of magnesium per meter in the flux-cored wire 1, advantageously of the order of +/- 2%, makes it possible to reduce the dispersion of the residual magnesium in the treated cast iron and, consequently, to reduce the consumption. of flux-cored wire for the same quantity of Mg actually introduced into the cast iron, as well as an improvement in the quality of the castings from the treated cast iron, in particular by reducing the porosities and / or the oxide sails. The metric weight of magnesium in the cored wire of the invention is much more accurate than the metric weight of magnesium contained in standard flux-cored wires, that is, made from magnesium powder.

Finally, in order to confer maximum metallurgical treatment efficiency to the cored wire 1, the ratio of diameters D2 / D1 is between 1.3 and 6.2. This interval was determined from the following criteria.

In order for the insulating intermediate layer to be sufficiently effective, it must be sufficiently thick. It is therefore necessary that the space between the extruded bar and the outer sheath is large enough to contain the powder. A ratio D2 / D1 greater than or equal to 1, 3 guarantees the minimum space for the thermal protection of the powder of the intermediate layer is sufficient.

A ratio D2 / D1 of less than or equal to 6.2 is based on both metallurgical and economic considerations. It ensures a minimum proportion of active substance (extruded bar 8) relative to the insulating substance. Too much imbalance generates significant thermal losses of the liquid metal bath to be treated (too much powder supply with respect to the supply of active substance), but also an increase in the cost price of the cored wire.

Example 1

Samples P1 to P8 were prepared and tested. The test results are given in Tables 1, 2a and 2b below. The purpose of these tests was to prove the interest of the cored wire 1 according to the invention in terms of the magnesium addition yield during the treatment of a liquid iron.

The samples P1 to P4 are conventional filled cored wires, filled with powdery material whose characteristics are as follows:

flux-cored wire containing FeSiMg, FeSi and FeSiTR powders mixed with pure magnesium powder,

- metric weight of the mixture in the yarn: 212.2 g / m,

- metric weight of magnesium in the yarn: 63.4 g / m or 29.9%,

metric weight of silicon in the wire: 82 g / m, ie 38.6%,

- Metric weight of Rare Earth in the yarn: 7.6 g / m or 3.6%, and

- steel strip thickness: 0.39 mm. The samples P5 to P8 come from a cored wire according to the invention and whose characteristics are as follows:

bar 8 in magnesium of purity 99.8%,

diameter D1 = 6.6 mm,

intermediate layer consisting of a mixture of FeSiBa, FeSiTR and CaSi powders,

diameter D2: 12.8 mm,

metric weight of the intermediate layer 10: 272 g / m,

- metric weight of magnesium: 58 g / m or 17.6%,

- metric weight of silicon: 123.4 g / m or 37.4%,

- metric weight of Rare Earth: 19.8 g / m or 6%, and

- steel strip thickness: 0.40 mm.

All the samples were tested under the same conditions, namely in cast iron pockets whose height to diameter ratio is 0.8. The injection speeds were adapted for each son to maintain the same amount of magnesium introduced per unit of time. For each sample, the magnesium addition yield was calculated (Tables 2a and 2b).

Table 1: Injection conditions

Table 2a: Technical Results of the Reference Wire % If adding% If adding Yield

% Residual Mg% sulfur oven% S final

Theoretical Magnesium

P5 0.25% 0.033 0.014 0.013 0.23 25.8

P6 0.25% 0.034 0.014 0.012 0.23 26.9 Thread according to

P7 0.25% 0.037 0.014 0.01 1 0.20 28.9 the invention

P8 0.25% 0.032 0.014 0.008 0.25 25.3

average 26.7

extent 3.6

Table 2b: Technical results of the yarn according to the invention

The magnesium addition yield obtained with the samples P5 to P8 according to the invention is on average 26.7%, against 19.3% with the samples P1 to P4, an improvement of + 38%. In Tables 2a and 2b, "range" is the difference between the highest addition yield and the lowest addition yield. For samples P5 to P8, the range is reduced by 42% compared to samples P1 to P4. This improves the predictability of the magnesium injection and more certainly makes it possible to avoid a second injection due to an insufficient addition of Mg, thus to consume less active product and not to prolong the elaboration. This demonstrates that beyond the positive impact on the magnesium yield during an injection, the use of an extruded wire allows a better regularity of the metric weight of magnesium, the treatment is more regular and the results in magnesium residual after treatment less dispersed. This lowers the target nominal magnesium content and therefore, in addition to the economic benefit of the reduction in lead consumption, the quality level of the cast iron will be improved, as it is recognized that a high level of magnesium, nevertheless necessary for obtaining the GS structure, has negative side effects such as increasing the tendency to shrinkage.

The intermediate layer 10 can also play a major metallurgical role and makes it possible, for example, to limit the fading of magnesium over time, that is to say the decrease in the magnesium content of the molten iron after introduction of the wire. thicket. Since the solubility of magnesium in liquid iron is limited and this solubility is a function of temperature, its concentration decreases continuously with time.

Magnesium is also a powerful deoxidizer and desulphurizer. Mg has a tendency to combine with oxygen to form MgO inclusions, becoming less and less effective with respect to the nodularization of graphite spherules. The effect of spheronization fades to sometimes become insufficient. The graphite then changes from a perfectly spheroidal shape to an irregular and jagged shape and finally vermicular if the magnesium content is too low. It is said that there is degeneration of the cast iron.

To counteract this fading of magnesium in liquid iron and to improve the spheronization of graphite, several technical solutions have been implemented. The intermediate layer 10 may thus contain other deoxidizing elements than magnesium, for example cerium, but also the chemical elements of groups IIA and NIA of the periodic table, and / or inoculant elements such as silicon. The multiplication of the sites of germination of the graphite nodules makes it possible to obtain many more spherules and thus to limit the degeneration affecting essentially the big nodules of graphite.

Example 2

Table 3 shows the samples tested and reference is made to Chart 3, which presents the results in which the curves C1 to C4 correspond to the evolution, as a function of time, of the proportion of Mg remaining in the cast, relative to the Mg introduced with the cored wire:

- reference: curve C1,

- PFT25: curve C2,

- PFT32: curve C3,

- PFT40: C4 curve.

Curve C5 corresponds to a lower limit below which it is not desired to go down in order to guarantee optimum quality of the castings.

Table 3: composition of the intermediate layer 10

The four types of cored wires were tested under the same operating conditions (15 wire treatments), namely: a cylindrical treatment pocket whose height-to-diameter ratio of the metal column is 1, 5,

the mass of metal treated is 2.5 tons,

the temperature of the melt is approximately 1470 to 1495 ° C,

the melting composition: 3.70% C; 2, 40% Si; 0.006 - 0.013% S.

It is found that the cored wires PFT25, PFT32 and PFT40 according to a preferred variant of the invention promote a slower fading of magnesium compared to the reference. The addition of 6% of barium in the intermediate layer thus makes it possible to extend the service life of the treated cast iron (residual value of Mg guaranteeing the quality of the castings) by 15 minutes (compared to the reference which does not is only 20 minutes).

The intermediate layer 10 surrounding the extruded bar 8 significantly reduces the fading of magnesium over time. It has thus been shown that an intermediate layer 10 consisting of a powder comprising a combination of the cerium, calcium and barium elements makes it possible to obtain a longer residence time for the magnesium in the molten iron.

This thus ensures a homogeneous quality of all cast parts from the cast iron so treated, from the first to the last. Indeed, the casting process can last several tens of minutes, it is important that the magnesium content is above the limit value when the last piece is manufactured, with the same cast iron pouch.

According to a variant not shown, the cored wire 1 may comprise an insulating layer extending longitudinally between the extruded bar 8 and the intermediate layer 10.

The insulating layer includes, for example, paper, moistened paper, metallized paper or metal. The insulating layer makes it possible to adjust the overall heat transfer coefficient between the molten metal bath and the extruded bar 8. Advantageously, the insulating layer makes it possible to retard the complete melting of the cored wire 1.

Referring to Figures 1 and 2, there will now be described a cored wire 100 which is a variant of the cored wire 1 described above. Unless otherwise specified, the cored wire 100 is similar to the cored wire 1, so that it is the same Figures 1 and 2 which illustrate the cored wire 1 and the cored wire 100.

Fluxed wire 100 differs mainly in its chemical composition and in its use. The cored wire 100 is for example intended to be introduced into a bath of molten steel (not shown).

In the flux-cored wire 100, the outer shell 4 is made of steel. Alternatively, it can be aluminum, nickel, zinc or copper.

The extruded bar 8 mainly comprises calcium. Preferably, the extruded bar 8 is made of calcium of industrial purity of 98.5%. According to a variant (not shown), the extruded bar 8 may be wrapped with a thermally insulating layer extending longitudinally.

The intermediate layer 10 comprises an iron powder. Alternatively, it may comprise powders of aluminum, magnesium and / or oxides such as slag.

For example :

the outer envelope 4 has a thickness of about 0.4 mm,

the iron powder has a metric weight of approximately 300 g / m 2,

- The extruded bar 8 has a metric weight of 85 g / m and a diameter of about 8.5 mm.

The cored wire 100 is implemented in a manner similar to the cored wire 1, for example to treat a molten steel bath with calcium.

An interest of the cored wire 100 is that it develops the same metric weight of calcium as a standard flux cored wire CaFe 30% (mixture of calcium and iron powders in the proportions: 30% Ca - 70% Fe) . It can be used as a direct replacement for CaFe's standard flux-cored wires, with increased performances in terms of calcium treatment efficiency in the liquid steel ladle and a reduced standard deviation of efficiency, ie better predictability. Example 3

The standard calcium treatment of molten steel ladles is done by injecting a CaFe cored wire of the state of the art. A pocket of 245 tons was used. A calcium content of the steel, before continuous casting, of 27 ppm was targeted.

The outer casing of the cored wire has a thickness of 0.4 mm. The filling of the cored wire consists of a mixture of calcium and iron powders in the proportion by mass 30/70. The metric weight of the powder mixture is 275 g / m 2.

The average length of cored wire injected is 620 m, at an injection speed of 290 m / min.

The average addition yield is 12.9%. The standard deviation obtained in the tests is 7.6% (absolute percentage). Then, 152 pockets of the same molten steel were treated with the cored wire 100 according to the invention.

The casing 4 of the cored wire 100 has a thickness of 0.4 mm. The lining of the cored wire 100 consists of a bar 8 of calcium whose diameter D1 is 8.5 mm and whose metric weight is 85 g / m and of an iron powder surrounding this bar 8 whose weight metric is 300 g / m.

The diameter D of the cored wire 100 is similar to that of the standard cored wire, namely 13.6 mm.

The average length of cored wire 100 injected was about 374 m, for the same injection speed of 290 m / min.

An average addition yield of 20.8% was obtained, with a standard deviation of 4.3% (absolute percentage).

The treatment time of the steel was reduced thanks to the flux-cored wire 100. On average, the treatment lasted less than 80 seconds with the flux-cored wire 100, compared with 130 seconds with the CaFe wire of the state of the art.

Substantially higher mean addition efficiencies were obtained: from 12.9% to 20.8%, an improvement of about + 60%, with a lower standard deviation of 43%.

Reducing the amount of cored wire 100 consumed represents a significant saving in the cost of metallurgical treatment.

Finally, it has been observed a decrease in the agitation of the liquid metal in the pocket during the injection of the cored wire 100. This decrease makes it possible to more easily handle pockets whose guard height (distance between the upper edge of the pocket and the surface of the liquid metal) is weak, without the risk of splashing metal. The flux-cored wire 100 also makes it possible to reduce the maintenance frequency of the lid of the pouch, since less metal remains attached to the walls as a result of splashes of molten metal. It also makes it possible to reduce the hydrogen recovery of the liquid steel and its reoxidation during the calcium treatment, again thanks to the slight stirring of the liquid metal which reduces its exposure to the ambient atmosphere.

In a conventional manner, a cored wire contains a powder or a mixture of powders whose purpose is to control the metric weight and the composition throughout the product. A conventional process for obtaining a cored wire comprises in particular the following steps:

- dosage of each of the powders according to the composition of the desired lining,

mixing the powders in a mixing device, and - Removal of the mixture to form a lining within the cored wire.

These three steps determine the quality of the cored wire obtained.

The dosage step of each of the powders makes it possible to respect the final proportion in each of the chemical elements in the packing. However, depending on the nature of the powders, this dosage is easily disturbed. For example, when using a supply treadmill, an overdose in one of the powders is possible by a falling effect. When the treadmill stops, the powder at the end can continue to flow due to its inertia. This effect is all the more marked as the powder has better flowability.

The mixing step is the most complex. Most of the mixers present on the cored wire manufacturing lines are of the "plowshares" type. Blades integral with a central rotary axis mix the different powders dosed upstream. However, mixers of this type easily induce segregation phenomena powders they are supposed to mix. Depending on the densities of the powders considered relative to each other, some powders tend to accumulate in dead zones of the mixer, which locally changes the composition of the mixture. In addition, demixing phenomena between the powders can also take place.

Finally, the removal of the mixture in the cored wire sometimes causes heterogeneities in the mixture. Indeed, during the removal of the mixture, segregation sometimes occurs, due in particular to the different trajectories of the powder particles or to the phenomenon of elutriation.

The implementation of an extruded bar according to the invention reduces the risk of a wrong dosage of the powders and a bad mixture. The metric weight of the extruded bar is much better controlled. Thus, the metric weight of the active substance is much better controlled. For example, this metric weight is independent of the density variations of the powders used.

In the present application, the term "thermally insulating layer" means an additional layer around the extruded bar. The additional layer makes it possible to delay the heat transfer from the outside of the cored wire towards its core when the cored wire is introduced into a bath of liquid metal. The additional layer is adapted to form an additional thermal barrier between the outer medium to the flux-cored wire (liquid metal) and the extruded bar. The propagation of heat is slowed down by the presence of the additional layer. The rise in temperature of the extruded bar is delayed. The effectiveness of the thermally insulating layer varies in particular according to its nature. Examples of thermally insulating layers are provided in the application FR-A-2871477 of the applicant.

The fact that the thermally insulating layer is advantageously located on the extruded bar and surrounding it for example completely further improves the thermal protection of the extruded bar.

Claims

1. A cored wire (1; 100) for introduction into a bath of molten metal for metallurgical treatment, the cored wire (1; 100) comprising:

- a lining (2) extending locally along a longitudinal axis (L), the lining (2) comprising at least one active substance for treating the molten metal; and

an outer envelope (4) extending longitudinally around the lining (2); characterized in that the lining (2) comprises:

an extruded bar (8) extending longitudinally and comprising the active substance; and

an intermediate layer (10) extending longitudinally between the extruded bar (8) and the outer casing (4) and comprising a powder comprising one or more of: a metal, a mixture of metals, a metal oxide, a mixture of metal oxides.

2. cored wire (1; 100) according to claim 1, characterized in that the lining (2) further comprises a thermally insulating layer extending longitudinally between the extruded bar (8) and the intermediate layer (10).

3. cored wire (1; 100) according to claim 1 or 2, characterized in that the extruded bar (8) has an outer equivalent diameter D1 in a transverse plane

(T) substantially perpendicular to the longitudinal axis (L), the intermediate layer (10) having an outer equivalent diameter D2 in the transverse plane (T), with D2 between 1.3 times and 6.2 times D1.

4. cored wire (1; 100) according to any one of claims 1 to 3, characterized in that the outer casing (4) comprises a strip (6) of steel, aluminum, copper, nickel, or zinc, or an alloy of two or more of these elements.

5. cored wire (1) according to any one of claims 1 to 4, characterized in that the extruded bar (8) comprises predominantly magnesium.

6. cored wire (1) according to claim 5, characterized in that the powder of the intermediate layer (10) comprises predominantly an alloy of iron and silicon further comprising calcium and / or barium and / or one or more rare earth.

7. cored wire (100) according to any one of claims 1 to 4, characterized in that the extruded bar (8) comprises predominantly calcium.

8. cored wire (100) according to claim 7, characterized in that the powder of the intermediate layer (10) comprises an iron powder, or a mixture of iron powder and aluminum powder and / or powder of magnesium and / or slag powder.

A method of metallurgically treating a bath of molten metal, the method comprising the step of introducing a flux-cored wire (1; 100) according to any one of the preceding claims into the molten metal bath.

10. Process according to claim 9, characterized in that the molten metal is cast iron and in that the filled cored wire (1) is as described in claim 5 or

6.

1 1. Process according to claim 9, characterized in that the molten metal is steel and the filled cored wire (100) is as described in claim 7 or 8.