ES2743496T3 - Tubular wire for metallurgical treatment of a molten metal bath and corresponding procedure - Google Patents

Abstract

Cored wire (1; 100) intended to be introduced into a molten metal bath to carry out a metallurgical treatment, the cored wire (1; 100) comprising: - a filler (2) that extends locally along a longitudinal axis ( L), the filler (2) comprising at least one active substance to treat the molten metal; and - an outer casing (4) extending longitudinally around the filling (2); - an extruded bar (8) extending longitudinally and including the active substance; and - an intermediate layer (10) extending longitudinally between the extruded bar (8) and the outer shell (4) and comprising a powder that includes one or more of: a metal, a mixture of metals, a metal oxide, a mixture of metal oxides,

Description

DESCRIPTION

Tubular wire for the metallurgical treatment of a molten metal bath and corresponding procedure [0001] The invention relates to a tubular wire intended to be introduced into a molten metal bath for metallurgical treatment, the tubular wire comprising:

- a filling that extends locally along a longitudinal axis, the filling comprising at least one active substance for treating molten metal; Y

- an outer wrap longitudinally around the filling.

[0002] The molten metal is, for example, cast iron or steel. The objective of the metallurgical treatment can be, 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.

[0003] In metallurgy, it is known to provide such a substance by means of "tubular wires" which are presented in the form of coils. The tubular wire is generally composed of a filler comprising the active substance in powder form, packaged in a metal shell formed of metal whose composition is compatible with that of the molten metal to be treated. In the case of molten steel treatment, this wrap is advantageously made of steel.

[0004] The tubular wire is introduced into the molten metal bath using a generally automatic injection device, which introduces a precise length of tubular wire at a suitable speed.

[0005] For example, in the foundry sector, for the manufacture of spheroidal graphite pig iron, it is known to use a tubular wire to perform a nodularization treatment. This is, by an addition of magnesium, to change the shape of the graphite particles of pig iron, to laminate to spheroidal. The added substance is generally a ferrosilicon magnesium alloy powder.

[0006] According to another example, in the steel industry, the treatment of steels with calcium is known. For example, this treatment is intended to modify the chemical composition of endogenous alumina inclusions, in order to include calcium aluminate inclusions. These inclusions, dispersed in molten steel, are liquid at the casting temperature. Therefore, they cannot adhere to the bucket walls of a bucket or a distributor of a continuous casting facility. The molding capacity is improved, as is the final quality of the steel produced.

[0007] There are numerous types of tubular wires that have a filler consisting of pure calcium or calcium alloy powder, or a mixture of calcium and iron powders, including aluminum. For example, the alloy commonly called CaSi (calcium disilicide) or the mixture of calcium and iron powders (usually called CaFe) are widely used fillers.

[0008] Although the introduction of a tubular wire into a molten metal bath is a smart way to add the active substance to the molten metal, the effectiveness of the introduction is sometimes limited. For example, for tubular wires based on CaFe powders used in steel, the performance of calcium addition, defined as the amount of calcium found in the steel after the injection of the tubular wire divided by the amount of calcium introduced by the consumed tubular wire, generally it is of the order of 10% to 15%, sometimes much less. The low efficacy of calcium is essentially due to its low vaporization temperature. In fact, on the order of 1480 ° C, the latter is generally lower than the working temperature of liquid steel, which causes calcium to vaporize while it is introduced into liquid steel.

[0009] US 3921 700 describes a tubular wire adapted to treat a bath of molten ferrous material.

[0010] It is an object of the invention to provide a tubular wire that performs the metallurgical treatment more efficiently while still having a competitive cost.

[0011] For this purpose, the object of the invention is a tubular wire according to claim 1.

[0012] According to the particular embodiments of the invention, the device may comprise one or more of the following characteristics, taken in isolation or according to all technically possible combinations:

- the filling may further comprise a thermally insulating layer that extends longitudinally between the extruded bar and the intermediate layer;

- the outer shell comprises a strap made of steel, aluminum, copper, nickel or zinc, or an alloy of two or more of these elements;

- the extruded bar mainly comprises magnesium;

- the intermediate layer mainly comprises an iron and silicon alloy that also includes calcium and / or barium and / or one or more rare earths;

- the extruded bar mainly comprises calcium;

- 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 shell is metal;

- the outer casing mainly contains one or more elements between: steel, copper, aluminum, nickel, zinc; - the outer shell has a transverse thickness between 0.2 and 0.7 mm;

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

- the tubular wire has an equivalent diameter between 6 and 21 mm;

- the extruded bar is substantially cylindrical with a circular base;

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

- the outer shell has a generally substantially tubular shape of circular base.

[0013] By "equivalent diameter" of an element, the diameter of a disk is understood to have a surface equal to the surface presented by the element in section according to a transverse plane. If the given element has a circular section according to the transverse plane, the equivalent diameter is equal to the ordinary diameter.

[0014] When the notion of equivalent diameter is used for an element, it is implied that the element is locally substantially cylindrical, but does not necessarily have a circular base.

[0015] "Metallurgical treatment" means, for example:

■ a modification of the chemical composition of molten metal; I

■ a modification of the properties of the metal obtained after solidification of the molten metal due, for example, to the modification of the composition of the inclusions or precipitates existing before the treatment, or to the creation, after the treatment, of said inclusions or precipitates , I

■ a modification of the population of inclusions present in the liquid metal in order to improve its production (improvement of the casting capacity in continuous casting).

[0016] The invention further relates to a method of metallurgical treatment of a molten metal bath, the process comprising the step of introducing a tubular wire as described above in the molten metal bath.

[0017] According to the particular embodiments, the method may comprise one or more of the following characteristics, taken in isolation or according to all technically possible combinations:

- The molten metal is cast iron and the tubular wire introduced is as described above;

- The molten metal is made of steel and the tubular wire introduced is as described above.

[0018] The invention will be better understood by reading the description given below, given by way of example only and made in reference to the accompanying drawings, in which:

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

- Figure 2 schematically shows, in cross section, the tubular wire shown in Figure 1; and - figure 3 shows a graph illustrating, in a particular application, the slowing down of the fading of the active substance (magnesium) when the tubular wire shown in figures 1 and 2 is introduced into a molten metal bath (liquid pig iron) .

[0019] With reference to Figs. 1 and 2, a tubular wire 1 is described that extends locally along a longitudinal axis. Only a portion of the tubular wire 1 is shown. The portion shown extends along the longitudinal axis L. This does not mean that all the tubular wire 1 extends along the longitudinal axis L In fact, the tubular wire 1 can have a certain curvature, for example, if it is rolled to occupy less space.

[0020] Similarly, a transverse plane T is defined perpendicular to the longitudinal axis L. It is understood that the transverse plane T is transverse for the illustrated portion of tubular wire 1, that is, locally transverse.

[0021] The tubular wire 1 is intended, for example, to be introduced into a cast iron bath (not shown).

[0022] The tubular wire 1 comprises a filler 2 and an outer sheath 4, both extending longitudinally.

[0023] The outer shell 4 forms a peripheral portion of the tubular wire 1, intended to be in contact with the molten metal bath when the tubular wire 1 is introduced into the molten metal bath.

[0024] The outer shell 4 is advantageously constituted by a metal strip 6 folded on itself around the longitudinal axis L.

[0025] The strip 6 is made of steel, copper, aluminum, nickel or zinc, for example, or of a mixture of two or more of these elements.

[0026] The strap 6 advantageously comprises two longitudinal folds 6a, 6b (figure 2) stapled together to achieve a closure of the strap 6 on itself along the longitudinal axis L. The strap 6, thus folded, has a tubular shape generally wrapping the filling 2. Advantageously, the tubular shape is substantially cylindrical with a circular base and has an equivalent diameter D. Advantageously, D is between 6 and 21 mm. For example, D is 13 mm.

[0027] The filling 2 comprises a longitudinally extending extruded bar 8 and an intermediate layer 10 extending longitudinally between the extruded bar 8 and the outer shell 4.

[0028] Advantageously, the extruded bar 8 is substantially cylindrical with a circular base. The extruded bar 8 has a diameter D1 in the transverse plane T, D1 being advantageously between 2 and 10 mm, for example, 8 mm.

[0029] The extruded bar 8 comprises an active substance for treating the cast iron. The active substance is magnesium, for example. Advantageously, the extruded bar 8 contains mainly magnesium.

[0030] By "mainly" it is meant that the extruded bar 8 comprises at least 50% by weight of magnesium, preferably at least 90% by weight of magnesium.

[0031] In the example, the extruded bar 8 is constituted by magnesium having industrial purity, for example, 99.8% by weight.

[0032] The post-treatment extruded bar 8 is not a mere cluster of compacted powder material at the time of the closure of the tubular wire 1, nor an agglomerate of powder grains (powder material) joined together by a binder independently of the kind. The extruded bar 8 is obtained, for example, by extrusion of a solid cylinder of material (billet) through a die by means of a press. The bar 8 can also be obtained directly by a continuous casting process, the liquid material solidifying in the form of a continuous bar. The porosity of the extruded bar 8 is considered practically nil, the apparent density of the bar being close to the actual density of the material.

[0033] The intermediate layer 10 extends, for example, in the space between the extruded bar 8 and the outer shell 4.

[0034] The intermediate layer 10 has an equivalent external diameter D2. For example, D2 is such that the ratio D2 / D1 is between 1.3 and 6.2.

[0035] The intermediate layer 10 is advantageously constituted by a powder. The intermediate layer 10 may also include a thermally insulating layer that covers the bar 8.

[0036] The intermediate layer 10 also mainly comprises, as 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).

[0037] The powder composition of the intermediate layer 10 obviously depends on the metallurgical treatment to be performed. It can be neutral, that is, it does not have a metallurgical effect on the liquid metal bath to be treated, in this case, the powder only acts as a thermal insulator of the bar 8. It can also participate directly in the metallurgical treatment and, therefore, Assume a double function of thermal insulation and active treatment element.

[0038] The operation and use of tubular wire 1 to perform a nodularization and inoculation of the liquid pig iron to obtain spheroidal graphite pig iron will now be described.

[0039] The pig iron to be treated is in the form of a molten metal bath contained, for example, in a container such as a spoon.

[0040] Tubular wire 1 is introduced into the cast iron bath according to modalities known per se.

[0041] The introduction of magnesium in the pig iron bath leads to a series of physical and chemical conversions, some simultaneous and others successive:

- a fusion of magnesium starting at 657 ° C (for pure Mg),

- a boil of magnesium at 1053 ° C (for pure Mg).

[0042] The desulfurization reaction is as follows:

Mg FeS -> Fe MgS.

[0043] Simultaneously, an intense mixture of the pig iron bath by the gaseous Mg takes place, as well as a deoxidation of the pig iron. In fact, magnesium is an energetic deoxidizer. Once desulfurization and deoxidation is complete, the remaining magnesium is incorporated into the pig iron. Obviously, during the entire duration of the process, a part of the formed magnesium vapors escapes to the pig iron bath surface and is completely lost by oxidation in the slag or atmosphere, resulting, for example, in the formation of magnesia.

[0044] The performance of magnesium addition is defined as the ratio between, on the one hand, the difference in Mg content that is effectively found in pig iron after the introduction of tubular wire 1 and the Mg content in the pig iron before the introduction of the tubular wire 1, and on the other hand, the theoretical content of Mg in the pig iron introduced by means of the tubular wire 1 if 100% of the Mg added in the pig iron must actually be found.

[0045] The magnesium contained in the extruded bar 8 plays a nodularizing role, that is, it allows obtaining spheroidal graphite particles in the pig iron.

[0046] Thanks to the characteristics described above, the tubular wire 1 more effectively achieves a metallurgical treatment, in the example, the nodularization of the pig iron, while maintaining a competitive cost.

[0047] The intermediate layer 10 acts as a thermal protection for the extruded bar 8, slowing the increase in the temperature of the magnesium contained in the extruded bar 8. The positioning of the intermediate layer 10 around the extruded bar 8 allows the latter to be protected. At the time of the injection of the tubular wire 1 in the cast iron bath, the increase in the magnesium temperature is delayed through the slowed thermal transfer. Therefore, the tubular wire 1 can be introduced deeper into the liquid pig iron column. The contact time of the magnesium gas with the liquid pig iron is lengthened and, therefore, improves the performance of the magnesium addition.

[0048] In addition, in the extruded bar 8, magnesium has a specific surface area of reduced heat exchange with respect to which a filler consisting of simple grains of powder will present. In fact, 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 magnesium, which improves the performance of the addition and moderates the reaction of Mg with molten pig iron .

[0049] In the example, intermediate layer 10 also plays a metallurgical role. The intermediate layer 10 comprises a second active substance for metallurgical treatment, namely, a ferrosilicon alloy, for example. This second active substance plays the role of inoculant. As is known, inoculation regenerates the potential for graphite germination after magnesium treatment. This allows the formation of cementite to be avoided and helps to obtain the desired spheroidal graphite rate. For ferritic pig iron grades, it also promotes the formation of ferrite by increasing the density of the spherules.

[0050] Furthermore, since the active substance with respect to nodularization, that is, magnesium, is contained in the extruded bar 8, it is possible to introduce the active substance more regularly than with prior art tubular wires containing Powdered magnesium, whose density and compaction are difficult to control. The small variation in the weight of magnesium per meter in the tubular wire 1, advantageously of the order of / -2%, allows to reduce the dispersion of residual magnesium in the treated pig iron and, consequently, to reduce the consumption of tubular wire for the same Amount of Mg effectively introduced into pig iron, as well as an improvement in the quality of castings from treated pig iron, particularly through a reduction in porosities and / or oxide films. The weight per meter of magnesium in the tubular wire of the invention is much more precise than the weight per meter of magnesium contained in standard tubular wires, that is, made of magnesium powder.

[0051] Finally, in order to impart the maximum efficiency of metallurgical treatment to tubular wire 1, the ratio of diameters D2 / D1 is between 1.3 and 6.2. This interval was determined from the following criteria.

[0052] For the intermediate insulating layer to be efficient enough, it must be thick enough. Therefore, the space between the extruded bar and the outer shell will be wide enough to contain the dust. A ratio D2 / D1 greater than or equal to 1.3 guarantees the minimum space so that the thermal protection of the dust of the intermediate layer is sufficient.

[0053] A D2 / D1 ratio of less than or equal to 6.2 is based on both metallurgical and economic considerations and allows to guarantee a minimum proportion of active substance (extruded bar 8) with respect to the insulating substance. If the imbalance is too large, this will generate large thermal losses in the bath of liquid metal to be treated (too much dust taking into account the contribution of active substance), but also an increase in the cost of production of the tubular wire.

Example 1:

[0054] Samples P1 to P8 were prepared and tested. The test results are given in tables 1,2a and 2b below. The objective of these tests was to demonstrate the interest of the tubular wire 1 according to the invention in terms of magnesium addition performance during the treatment of liquid pig iron.

[0055] Samples P1 to P4 are conventional commercially available tubular wires, filled with powder material whose characteristics are as follows:

- tubular wire containing FeSiMg, FeSi and FeSiTR powder in a mixture with pure magnesium powder; - weight per meter of the mixture in the wire: 212.2 g / m,

- weight per meter of magnesium in the wire: 63.4 g / m, that is, 29.9%,

- weight per meter of silicon in the wire: 82 g / m, that is, 38.6%,

- weight per meter of rare earth in the wire: 7.6 g / m, that is, 3.6%, and

- steel strap thickness: 0.39 mm.

[0056] Samples P5 to P8 were taken from a tubular wire according to the invention and whose characteristics are the following:

- bar 8 of magnesium with a purity of 99.8%;

- diameter D1 = 6.6 mm;

- intermediate layer 10 consisting of a mixture of FeSiBa, FeSiTR and CaSi powders,

- diameter D2: 12.8 mm,

- weight per meter of intermediate layer 10: 272 g / m,

- weight per meter of magnesium: 58 g / m, that is, 17.6%,

- weight per meter of silicon: 123.4 g / m, that is, 37.4%,

- weight per meter of rare earths: 19.8 g / m, that is, 6%, and

- thickness of steel strip: 0.40 mm.

[0057] All samples were analyzed under the same conditions, namely, in cast steel spoons that have a height ratio with respect to the diameter of 0.8. The injection rates were adapted for each of the wires in order to preserve the same amount of magnesium introduced per unit of time. For each sample, the performance of magnesium addition was calculated (tables 2a and 2b).

(continuation)

[0058] The yield of the addition of magnesium obtained with samples P5 to P8 according to the invention is an average of 26.7%, compared to 19.3% with samples P1 to P4, that is, an improvement of 38 In Tables 2a and 2b, the "range" is the difference between the highest addition performance and the lowest addition performance. For samples P5 to P8, the range was reduced by 42% compared to samples P1 to P4. This improves the predictability of the magnesium injection and allows a second injection to be avoided with greater certainty due to insufficient Mg addition and, therefore, allows a lower consumption of active product and avoids prolonging the processing. This shows that, in addition to the positive impact on the performance of magnesium at the time of injection, the use of an extruded wire provides a better regularity of the weight of magnesium per meter, the treatment is more regular and the results for residual magnesium after of the treatment are less dispersed. This makes it possible to reduce the nominal magnesium content and, therefore, in addition to the economic advantage of a reduction in wire consumption, the quality level of pig iron will improve, since it is known that a high level of magnesium, however necessary to obtain the GS structure, it has negative side effects, such as increasing the tendency to shrink.

[0059] The intermediate layer 10 can also play an important metallurgical role and allows, for example, to limit the fading of magnesium over time, that is, a decrease in the magnesium content of the liquid pig iron after the introduction of the tubular wire. Since the solubility of magnesium in liquid pig iron is limited and that this solubility is a function of temperature, its concentration decreases continuously as a function of time.

[0060] Magnesium is also a potent deoxidizer and desulfurizer. Mg has a tendency to combine with oxygen to form MgO inclusions, so it becomes less and less efficient for nodularization of graphite spherules. The effect of spheroidization decreases, sometimes until it becomes insufficient. The graphite then changes from a perfectly spheroidal shape to an irregular and uneven shape and finally to a vermicular shape if the magnesium content is too low. The term pig iron degeneration is then used.

[0061] To counteract this fading of magnesium in liquid pig iron and improve graphite spheroidization, several technical solutions were applied. Therefore, the intermediate layer 10 may contain deoxidizing elements other than magnesium, for example, cerium, but also chemical elements of Groups IIA and IIIA in the Periodic Table and / or inoculation elements such as silicon. The multiplication of the germination sites of graphite nodules allows to obtain many more spherules and, therefore, limits the degeneration that essentially affects large graphite nodules.

Example 2:

[0062] Table 3 presents the samples analyzed and reference is made to figure 3, which provides the results in which curves C1 to C4 correspond to the evolution, as a function of time, of the proportion of Mg remaining in the pig iron, with respect to the Mg introduced with the tubular wire:

- reference: curve C1,

- PFT25: curve C2,

- PFT32: C3 curve,

- PFT40: curve C4.

[0063] Curve C5 corresponds to a lower limit below which it is not desired to descend to ensure optimum quality of the castings.

[0064] The four types of tubular wires were tested in the same operating conditions (15 wire treatments), namely:

- a cylindrical treatment spoon having a height-diameter ratio of the metal column of 1.5;

- the mass of the treated metal is 2.5 tons;

- the temperature of pig iron is approximately 1470 at 1495 ° C,

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

[0065] PFT25, PFT32 and PFT40 tubular wires according to a preferred variant of the invention were found to promote a slower magnesium fading than the reference. The addition of 6% barium to the intermediate layer allows, therefore, to extend the useful life of the treated pig iron (limited residual value of Mg that guarantees the quality of the castings) in 15 minutes (compared to the reference one that it's only 20 minutes).

[0066] The intermediate layer 10 surrounding the extruded bar 8 allows a significant reduction in the fading of magnesium over time. Therefore, it was demonstrated that an intermediate layer 10 constituted by a powder comprising a combination between the elements cerium, calcium and barium, allows to obtain a longer residence time of magnesium in the liquid pig iron.

[0067] This also allows to guarantee a homogeneous quality of all the castings from the cast iron bath treated in this way, from the first to the last. In fact, the casting process can last several tens of minutes and, therefore, it is important that the magnesium content is above the limit value when the last piece is manufactured with the same treated cast iron spoon.

[0068] According to a variant not shown, the tubular wire 1 may comprise an insulating layer that extends longitudinally between the extruded bar 8 and the intermediate layer 10.

[0069] The insulating layer may comprise, for example, paper, moistened paper, metallized paper or metal. The insulating layer allows the adjustment of the overall thermal transfer coefficient between the molten metal bath and the extruded bar 8. Advantageously, the insulating layer allows the complete fusion of the tubular wire 1 to be delayed.

[0070] With reference to Figures 1 and 2, a description is now given of a tubular wire 100 which is a variant of the tubular wire 1 described above. Unless stated otherwise, the tubular wire 100 is similar to the tubular wire 1, which means that the same figures 1 and 2 are used to illustrate the tubular wire 1 and the tubular wire 100.

[0071] Tubular wire 100 differs mainly in its chemical composition and use.

[0072] The tubular wire 100 is intended, for example, to be introduced into a bath of molten steel (not shown).

[0073] In the tubular wire 100, the outer shell 4 is made of steel. As a variant, it can be aluminum, nickel, zinc or copper.

[0074] Extruded bar 8 contains mainly calcium. Preferably, the extruded bar 8 is formed by calcium having an industrial purity of 98.5%. According to a variant (not shown), the extruded bar 8 may be wrapped by a thermally insulating layer that extends longitudinally.

[0075] The intermediate layer 10 comprises iron powder. As a variant, it can comprise powders of aluminum, magnesium and / or oxides such as slag.

[0076] For example:

- the outer envelope 4 has a thickness of approximately 0.4 mm;

- iron powder has a weight per meter of approximately 300 g / m,

- the extruded bar 8 has a weight per meter of approximately 85 g / m and a diameter of approximately 8.5 mm.

[0077] Tubular wire 100 is implemented similarly to tubular wire 1, for example, to treat a bath of molten steel with calcium.

[0078] An advantage of tubular wire 100 is that it develops the same weight per meter of calcium as a conventional 30% CaFe tubular wire (mixture of calcium powder and iron in the proportions: 30% Ca - 70% Fe ). Therefore, it can be used as a direct replacement for conventional CaFe tubular wires, with a higher level of performance in terms of the performance of the calcium treatment in the liquid steel bucket and a reduced standard deviation of the yield, i.e. Better predictability

Example 3:

[0079] Standard calcium treatment of a molten steel spoon was performed by injecting a prior art CaFe tubular wire. A 245 ton spoon was used. It is contemplated that the calcium content of the steel, before being sent for continuous casting, was 27 ppm.

[0080] The outer sheath of the tubular wire has a thickness of 0.4 mm. The filling of the tubular wire is constituted by a mixture of calcium and iron powder in a weight ratio of 30/70. The weight per meter of the powder mixture is 275 g / m.

[0081] The average length of the injected tubular wire is 620 m, at an injection rate of 290 m / min.

[0082] The average yield of addition was 12.9%. The standard deviation obtained in the tests is 7.6% (absolute percentage).

[0083] Next, 152 spoons of the same molten steel were treated with the aid of the tubular wire 100 of the invention.

[0084] The sheath 4 of the tubular wire 100 has a thickness of 0.4 mm. The filling of the tubular wire 100 consists of a calcium bar 8 with a diameter D1 of 8.5 mm and a weight per meter of 85 g / m, as well as an iron powder surrounding this bar 8 with a weight per meter 300 g / m

[0085] The diameter D of the tubular wire 100 is similar to that of the standard tubular wire, namely 13.6 mm

[0086] The average length of the injected tubular wire 100 was approximately 374 m, for the same injection speed of 290 m / min.

[0087] An average yield of 20.8% was obtained with a standard deviation of 4.3% (absolute percentage).

[0088] The treatment time of the steel was reduced by means of the tubular wire 100. On average, the treatment lasted less than 80 seconds with the tubular wire 100, compared to 130 seconds for the prior art CaFe wire.

[0089] Much higher average addition yields were obtained: from 12.9% to 20.8%, that is, an improvement of approximately 60%, with a lower standard deviation of 4.3%.

[0090] The reduction in the amount of tubular wire consumed 100 represents a significant saving in the cost of metallurgical treatment.

[0091] Finally, a reduction in the agitation of the liquid metal in the spoon was observed at the time of the injection of the tubular wire 100. This decrease allows the spoons to be treated more easily, whose guard height (distance between the upper edge of the spoon and liquid metal surface) is low, without risk of metal projections. The tubular wire 100 also allows a reduction in the frequency of maintenance of the spoon cover, since less metal remains attached to the walls after the projection of liquid metal. In addition, it allows reducing the reuptake of hydrogen in liquid steel and its reoxidation during treatment with calcium, again due to the lower agitation of the liquid metal that reduces its exposure to the surrounding atmosphere.

[0092] Conventionally, a tubular wire contains a powder or a mixture of powders for which the weight per meter and the composition should be controlled during production. A conventional method for obtaining a tubular wire particularly comprises the following steps:

- dose each powder according to the desired filling composition;

- mix the powders in a mixing device; Y

- deposit the mixture to form a filling inside the tubular wire.

[0093] These three stages determine the quality of the tubular wire obtained.

[0094] The dosing stage of each of the powders allows respecting the final proportion for each of the chemical elements that form the filling. However, depending on the type of powder, this dosage can be easily altered. For example, when a conveyor belt is used for delivery, it is possible that one of the powders may be added in excess through the dropping effect. When the conveyor stops, the dust at its end can continue to flow due to its inertia. This effect is even more marked since the powder has better fluidity.

[0095] The mixing stage is the most complex. Most of the mixers present in the tubular wire production lines are of the "plow fence" type. The blades secured to a central rotating shaft mix the different powders provided upstream. However, mixers of this type easily induce segregation phenomena of the powders that are intended to be mixed. Depending on the density of the powders considered with each other, some powders tend to accumulate in the dead spots of the mixer, which locally modifies the composition of the mixture. In addition, mixing phenomena between powders can also take place

[0096] Finally, depositing the mixture in the tubular wire occasionally causes heterogeneities in the mixture. In fact, during the deposition of the mixture, segregation sometimes occurs, particularly due to the different paths of the dust particles or the elutriation phenomenon.

[0097] The implementation of an extruded bar according to the invention reduces the risk of incorrect dosing of the powders and a bad mixture. The weight per meter of the extruded bar is controlled much better. Therefore, the weight per meter of the active substance is controlled much better. For example, this weight per meter is independent of the variations in the density of the powders used.

[0098] In the present application, "thermally insulating layer" means an additional layer around the extruded bar. The additional layer allows the heat transfer from the outside of the tubular wire to its core to be delayed when the tubular wire is introduced into a bath of liquid metal. The additional layer is adapted to form a complementary thermal barrier between the outer medium of the tubular wire (liquid metal) and the extruded bar. Heat propagation slows down due to the presence of the additional layer. Therefore, the increase in the temperature of the extruded bar is delayed.

[0099] The effectiveness of the thermally insulating layer varies particularly depending on its nature. Examples of thermally insulating layers are given in application FR-A-2871477 of the Applicant.

[0100] The fact that the thermally insulating layer is advantageously located in the extruded bar and, for example, completely surrounds it, further improves the thermal protection of the extruded bar.

Claims (10)

1. Tubular wire (1; 100) intended to be introduced into a molten metal bath for metallurgical treatment, including tubular wire (1; 100): 5 - a filler (2) that extends locally along a longitudinal axis (L), the filler (2) comprising at least one active substance for treating molten metal; Y - an outer envelope (4) extending longitudinally around the filling (2); - an extruded bar (8) extending longitudinally and including the active substance; Y 10 - an intermediate layer (10) extending longitudinally between the extruded bar (8) and the outer shell (4) and comprising a powder that includes one or more of: a metal, a mixture of metals, a metal oxide, a mixture of metal oxides, characterized in that the extruded bar (8) has an equivalent external diameter D1 in a transverse plane (T) 15 substantially perpendicular to the longitudinal axis (L), the intermediate layer (10) having an equivalent external diameter D2 in the transverse plane (T) , D2 being between 1.3 times and 6.2 times D1. 2. Tubular wire (1; 100) according to claim 1, characterized in that the filling (2) further comprises a thermally insulating layer that extends longitudinally between the extruded bar (8) and the intermediate layer 20 (10). 3. Tubular wire (1; 100) according to claim 1 or 2, characterized in that the outer shell (4) comprises a strap (6) made of steel, aluminum, copper, nickel or zinc, or an alloy of two or more of these elements. 25 4. Tubular wire (1) according to any one of claims 1 to 3, characterized in that the extruded bar (8) mainly comprises magnesium. 5. Tubular wire (1) according to claim 4, characterized in that the powder of the intermediate layer (10) 30 mainly comprises an alloy of iron and silicon which also includes calcium and / or barium and / or one or more rare earths. 6. Tubular wire (100) according to any one of claims 1 to 3, characterized in that the extruded bar (8) mainly comprises calcium. 35 7. Tubular wire (100) according to claim 6, 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 magnesium powder and / or powder of scum. Method for metallurgically treating a molten metal bath, the method comprising the step of introducing a tubular wire (1; 100) according to any of the preceding claims into the molten metal bath. Method according to claim 8, characterized in that the molten metal is pig iron and that the tubular wire (1) introduced is as described in claim 4 or 5. Method according to claim 8, characterized in that the molten metal is made of steel and that the tubular wire (100) introduced is as described in claim 6 or 7.