Calcium-based flux-cored wire for metallurgical processing of metal baths and corresponding method

Abstract

The present invention relates to a flux-cored wire (1) intended for introduction into a bath of molten metal for metallurgical processing, comprising a core (2) extending locally along a longitudinal axis (L) and an outer casing (4) extending longitudinally around the core, the core comprising: an extruded rod (8) containing primarily calcium; and an intermediate layer (10) extending longitudinally between the extruded rod and the outer casing, the intermediate layer comprising a powder containing one or more of a metal, a mixture of metals, a metal oxide, or a mixture of metal oxides. The powder contains at least 10% by mass of calcium aluminate, the calcium aluminate containing at least a dodecacalcium heptaaluminate phase.

Description

[Technical Field]

[0001] The present invention relates to a cored wire intended for introduction into a bath of molten metal for metallurgical processing, the cored wire comprising a core extending locally along a longitudinal axis and an outer casing extending longitudinally around the core.

[0002] The present invention also relates to a metallurgical process for implementing such cored wire. [Background technology]

[0003] The molten metal is, for example, steel. The metallurgical treatment aims, for example, at adding to the molten metal at least one substance intended to adjust the composition of the molten metal and / or the composition of the precipitates or non-metallic inclusions it contains.

[0004] In metallurgy, it is known to introduce such substances using a cored wire provided in the form of a coil. The cored wire generally consists of a core containing the active substance in powder form, surrounded by a metal casing made of a metal whose composition is adapted to that of the molten metal to be treated. In the case of treating molten steel, this casing is advantageously itself made of steel.

[0005] The cored wire is typically introduced into the bath of molten metal using an automatic injection device to introduce the correct length of cored wire at the appropriate speed.

[0006] For example, it is known to treat steel with calcium. This treatment is intended in particular to modify the chemical composition of endogenous alumina-type inclusions resulting from the deoxidation of the steel in order to obtain liquid inclusions at the casting temperature. These liquid inclusions do not adhere to the walls of the nozzle of the ladle or the distributor of the continuous casting plant. This improves the castability and also the final quality of the steel produced, which depends on the microinclusion cleanliness of the steel, which can be measured by the cumulative surface area of ​​the microinclusions in the steel cross section.

[0007] There are many types of cored wires, the core of which is made of pure calcium powder or calcium alloys, or a mixture of calcium powder and iron powder, or even aluminum. For example, the alloy commonly called CaSi (calcium disilicide) or a mixture of calcium powder and iron powder (commonly called CaFe) are widely used cores.

[0008] The introduction of cored wire into a molten metal bath is an elegant means of adding active materials to the molten metal, but the introduction efficiency can be limited. For example, for CaFe-based cored wires used in steelmaking, the calcium addition yield, defined as the amount of calcium found in the steel after the cored wire injection divided by the amount of calcium introduced by the consumed cored wire, is typically around 10% to 15%, and in some cases much lower. The low calcium efficiency is primarily due to the low vaporization temperature, in fact, around 1480°C, which is generally lower than the working temperature of the molten steel, causing the calcium to vaporize during its introduction into the molten steel.

[0009] To at least partially remedy this problem and improve the calcium loading yield, a core has been proposed which comprises an extruded rod containing mainly calcium and an intermediate layer extending longitudinally between the extruded rod and the outer casing, the intermediate layer comprising a powder containing a metal, a mixture of metals, a metal oxide or a mixture of metal oxides. Document EP 2917377 describes this type of cored wire.

[0010] However, it has been observed that at the casting temperature immiscible microinclusions remain in the steel, creating a risk of nozzle clogging and limiting the final quality of the steel. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] EP2917377 [Patent Document 2] FR-A-2871477 Summary of the Invention [Problem to be solved by the invention]

[0012] The object of the present invention is to provide a cored wire for calcium treatment with good addition yield, while further reducing the risk of nozzle clogging and improving the final quality of the steel. [Means for solving the problem]

[0013] To this end, the invention provides a cored wire intended to be introduced into a bath of molten metal for metallurgical processing, comprising a core extending locally along a longitudinal axis and an outer casing extending longitudinally around the core, the core having: - extruded rods containing mainly calcium, and an intermediate layer extending longitudinally between the extrusion rod and the outer casing, the intermediate layer comprising a powder comprising one or more of a metal, a mixture of metals, a metal oxide, and a mixture of metal oxides; Including, The powder comprises at least 10% by weight of calcium aluminate, the calcium aluminate comprising at least a dodecacalcium heptaaluminate phase.

[0014] According to a particular embodiment, the cored wire has the following characteristics: - the calcium aluminate further contains one and / or the other of a tricalcium aluminate phase and a monocalcium aluminate phase; - the powder contains at least 50% by weight of calcium aluminate; - the calcium aluminate contains at least 5% by weight of dodecacalcium heptaaluminate; - the calcium aluminate contains at least 50% by weight of dodecacalcium heptaaluminate; - the core further includes a thermal insulating layer extending longitudinally between the extruded rod and the intermediate layer; - the extruded rod has an equivalent outer diameter D1 in a cross section substantially perpendicular to the longitudinal axis, and the intermediate layer has an equivalent outer diameter D2 in the cross section, D2 being 1.3 to 6.2 times D1; - the outer casing comprises strips of steel, aluminum, copper, nickel, or zinc, or an alloy of two or more of these elements; and The powder further comprises one or more of iron powder, fluorine powder, and iron-silicon alloy powder.

[0039] The present invention includes one or more of the following, alone or in any technically possible combination:

[0015] The "equivalent diameter" of an element means the diameter of a disk having an area equal to the area subtended by the element in cross-section. If a given element has a circular cross-section, the equivalent diameter is equal to the nominal diameter.

[0016] When the concept 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.

[0017] "Metallurgical processing" means, for example, - Altering the chemical composition of the molten metal, and / or - modification of the properties of the metal obtained after solidification of the molten metal, for example due to a modification of the composition of inclusions or precipitates present before the treatment or the formation of such inclusions or precipitates after the treatment; and / or - Modifying the total number of inclusions present in the liquid metal to improve its refining properties (improving castability in continuous casting) means.

[0018] The present invention also relates to a method for metallurgically treating a bath of molten metal, the method comprising the step of introducing a cored wire into the bath of molten metal.

[0019] According to a particular embodiment, the molten metal is steel.

[0020] The invention will be better understood on reading the following description, given purely by way of example, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a schematic perspective view of a cored wire according to the present invention; [Figure 2] FIG. 2 is a schematic cross-sectional view of the cored wire shown in FIG. 1. DETAILED DESCRIPTION OF THE INVENTION

[0022] A cored wire 1 according to the present invention will now be described with reference to FIGS.

[0023] The cored wire 1 extends 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 the entire cored wire 1 extends along the longitudinal axis L. In fact, the cored wire 1 may have some curvature, for example if it is wound to take up less space.

[0024] Similarly, a transverse plane P is defined that is perpendicular to the longitudinal axis L. It is understood that the transverse plane P is transverse, i.e., locally transverse, to a portion of the cored wire 1 shown.

[0025] The cored wire 1 is intended to be introduced, for example, into a bath of molten steel (not shown).

[0026] The cored wire 1 includes a core 2 and an outer casing 4, both of which extend in the longitudinal direction.

[0027] The outer casing 4 forms the periphery of the cored wire 1 and is intended to come into contact with the molten metal bath when the cored wire 1 is introduced into the molten metal bath.

[0028] The outer casing 4 is advantageously made of a metal strip 6 folded over on itself around a longitudinal axis L.

[0029] The outer casing 4 has a thickness of, for example, about 0.4 mm.

[0030] The strip 6 is made, for example, of steel, copper, aluminum, nickel or zinc, or a mixture of two or more of these elements.

[0031] The strip 6 advantageously includes two longitudinal folds 6a, 6b (FIG. 2) stapled together to close the strip 6 on itself along the longitudinal axis L. The strip 6 thus folded has an overall tubular shape encasing the core 2. Advantageously, the tubular shape is substantially cylindrical with a circular base and has an equivalent diameter D, which is advantageously between 6 and 21 mm. For example, D is about 13 mm.

[0032] The core 2 includes a longitudinally extending extruded rod 8 and an intermediate layer 10 extending longitudinally and radially between the extruded rod 8 and the outer casing 4 .

[0033] The extrusion rod 8 is advantageously substantially cylindrical with a circular base. The extrusion rod 8 has a diameter D1 in the cross section P, which is advantageously between 2 and 10 mm, for example 8 mm.

[0034] The extruded rod 8 contains calcium. Advantageously, the extruded rod 8 contains mainly calcium.

[0035] "Predominantly" means, for example, that the extruded rod 8 contains at least 50% by weight of calcium, preferably at least 90% by weight of calcium.

[0036] In the example, the extruded rod 8 is made of calcium of technical purity, for example 98.5% by weight.

[0037] The extruded rod 8 is not a simple mass of powder material compressed during the closing of the cored wire 1, nor is it an agglomerate of powder grains (powder material) bound together by any kind of binder. The extruded rod 8 is obtained, for example, by extruding a solid cylinder (billet) of material through a die using a press. The extruded rod 8 can also be obtained directly by a continuous casting process, where a liquid material is solidified in the form of a continuous rod. The extruded rod 8 is expected to have little porosity, and the apparent density of the rod is close to the true density of the material.

[0038] The extrusion rod 8 has, for example, a metric mass of about 85 g / m and a diameter D1 of about 8.5 mm.

[0039] The intermediate layer 10 extends, for example, into the space between the push rod 8 and the outer casing 4 .

[0040] The mid layer 10 has an equivalent outer diameter D2, where D2 is such that the D2 / D1 ratio is, for example, 1.3 to 6.2.

[0041] The intermediate layer 10 is advantageously made of powder.

[0042] The intermediate layer 10 is made of, for example, a powder containing one or more of a metal, a mixture of metals, a metal oxide, or a mixture of metal oxides.

[0043] The powder contains at least 10% by weight of calcium aluminate, which contains at least a dodecacalcium heptaaluminate phase, and optionally one and / or the other of a tricalcium aluminate phase and a monocalcium aluminate phase.

[0044] Lime aluminate (i.e. calcium aluminate) is generally obtained by calcining a mixture of calcium oxide, CaO, and aluminum oxide, Al2O3. Under normal conditions of temperature and pressure (101325 Pa, 25 °C), depending on the mass fraction of Al2O3 in the initial mixture, calcium aluminate forms the following stable phases: - Tricalcium aluminate 3CaO·Al2O3, also written as C3A in metallurgy, where C=CaO and A=Al2O3; - Dodecacalcium heptaaluminate: 12CaO·7Al2O3 or C12A7, also called mayenite; - Monocalcium aluminate: CaO·Al2O3 i.e. CA; - Monocalcium dialuminate: CaO·2Al2O3 i.e. CA2; - Monocalcium hexaaluminate: CaO·6Al2O3 or CA6 It can exist in

[0045] According to the Al2O3-CaO binary system, the stable domains of the C3A + C12A7, CA12A7, and C12A7 + CA phases correspond to mass fractions of Al2O3 of approximately 38% to 64% in the initial mixture. At these fractions, the temperature at which the first droplets appear (solidus) is below 1500 °C at atmospheric pressure.

[0046] The presence of these phases is advantageously determined by X-ray diffraction, with a relative accuracy of mass determination of + / - 5%.

[0047] Advantageously, the powder contains at least 50% by weight of calcium aluminate. In certain cases, the powder consists of calcium aluminate, advantageously exclusively in the above-mentioned form.

[0048] Advantageously, the calcium aluminate contains at least 5% by weight of dodecalcium heptaaluminate, preferably at least 50% by weight of dodecalcium heptaaluminate.

[0049] In certain cases, the calcium aluminate contains at least 80% by weight of dodecalcium heptaaluminate.

[0050] For example, the intermediate layer 10 further comprises one or more of iron powder, fluorine powder, and iron-silicon alloy powder, advantageously forming a total amount of 100%.

[0051] According to one particular embodiment, of these powders, the intermediate layer 10 comprises at least iron powder and possibly one or two other powders.

[0052] The iron powder improves the rigidity of the cored wire 1 and facilitates its injection into the steel bath, in particular by facilitating its crossing of the slag layer.

[0053] The fluorine powder advantageously reduces the solidus temperature (temperature at which the first droplets appear) and / or liquidus temperature (temperature at which the calcium aluminate is completely melted) of the calcium aluminate contained in the intermediate layer 10 .

[0054] The iron-silicon alloy powder advantageously reduces the reactivity of calcium in the extruded rod 8 with the steel bath.

[0055] Finally, the diameter ratio D2 / D1 is 1.3 to 6.2 in order to provide the cored wire 1 with maximum metallurgical processing efficiency. This interval is determined based on the following criteria.

[0056] To provide sufficient thermal insulation, the intermediate layer 10 must be thick enough. Therefore, the space between the extrusion rod 8 and the outer casing 4 must be large enough to accommodate the powder. A D2 / D1 ratio of 1.3 or greater ensures a minimum space within the intermediate layer 10 that is sufficient for thermal protection of the powder.

[0057] A D2 / D1 ratio of 6.2 or less is based on both metallurgical and economic considerations. It ensures a minimum ratio of active material (extruded rod 8) to insulating material. An imbalance not only leads to significant heat losses in the liquid metal bath being processed (too much powder input compared to the active material input), but also increases the cost of the cored wire.

[0058] The core 2 may also include a thermal insulating layer 12 covering the rods 8 .

[0059] In this application, "thermal insulation layer" refers to an additional layer around the extrusion rod 8. The additional layer retards heat transfer from the outside of the cored wire 1 to its core when the cored wire is introduced into a liquid metal bath. The additional layer is adapted to provide an additional thermal barrier between the external environment of the cored wire (liquid metal) and the extrusion rod. The presence of the additional layer slows down the propagation of heat. Therefore, the temperature rise of the extrusion rod is retarded.

[0060] The insulating layer 12 comprises, for example, paper, wet paper, metallized paper, or metal. The insulating layer makes it possible to adjust the overall heat transfer rate between the bath of molten metal and the extrusion rod 8. Advantageously, the insulating layer 12 delays the complete melting of the cored wire 1.

[0061] Examples of thermal insulating layers are provided in the applicant's application FR-A-2871477.

[0062] The fact that the thermal insulation layer is advantageously arranged on the push rod 8 and completely surrounds it, for example, further improves the thermal protection of the push rod.

[0063] The cored wire 1 is intended to be introduced, for example, into a bath of molten steel (not shown). [Example]

[0064] Example 1 Processing of a 245 tonne molten steel ladle. - extruded rods of technical purity calcium, with a diameter of 8.5 mm and a metric mass of 85 g / m; - an intermediate layer consisting of a calcium aluminate powder containing either a tricalcium aluminate phase and a dodecalcium heptaaluminate phase, or a dodecalcium heptaaluminate phase and a monocalcium aluminate phase, with at least 50% by weight of dodecalcium heptaaluminate in each of these two cases, - 0.40mm thick steel strip Includes 13.6mm diameter cored wire.

[0065] Example 2 For example, processing a 320 tonne molten steel ladle with a grade with limited silicon content (Si%<300ppm). - extruded rods of technical purity calcium, 7.5 mm in diameter and 70 g / m metric mass; - an intermediate layer consisting of a mixture of 50% by weight of calcium aluminate powder and 50% by weight of iron powder, the calcium aluminate comprising 75% by weight of dodecalcium heptaaluminate and 25% by weight of calcium aluminate of one or more other phases, - 0.50mm thick steel strip Includes 13.6mm diameter cored wire.

[0066] The results of the counting of the microinclusions were obtained by counting the analyzed surface of 30 mm of the sample taken from the distributor using an automated scanning electron microscope with an EDS analyzer (Energy Dispersive Spectroscopy) to determine the chemical composition of each detected inclusion by image analysis. 2 The results were obtained from the polished cross section of - extruded rods of technical purity calcium, with a diameter of 8.5 mm and a metric mass of 85 g / m; - an intermediate layer consisting of iron powder (not including calcium aluminate powder), and - 0.50mm thick steel strip The results show a reduction in the number and size of oxysulfide inclusions compared to the same experiment performed using a 13.6 mm diameter cored wire containing

[0067] More specifically, for cored wires that do not contain calcium aluminate powder in the intermediate layer, 1 mm 2 In the case of the cored wire with calcium aluminate powder in the interlayer, the number of oxysulfide microinclusions was 1 mm, compared with 40 inclusions per wire and the maximum size of the inclusions detected was 8 μm. 2 There are 20 inclusions per sample, and the maximum size of the inclusions detected is 5 μm.

[0068] These results demonstrate the effectiveness of calcium aluminate powder in improving the fine inclusion cleanliness of steel treated with the cored wire described in Example 2.

[0069] advantage Due to the above features, in particular the presence of calcium aluminate containing at least dodecalcium heptaaluminate phase and optionally one and / or the other of tricalcium aluminate and monocalcium aluminate phases, advantageously with dodecalcium heptaaluminate being the majority by mass fraction in the calcium aluminate, the cored wire 1 allows calcium treatment of steel with a reduction in the cumulative surface area of ​​microinclusions (non-metallic particles that are immiscible in steel at steel liquid treatment temperatures), thereby improving the final quality of the steel and reducing the risk of nozzle clogging by non-liquid particles during production. [Explanation of symbols]

[0070] 1 Cored wire 2 cores 4 outer casing 6 Strips 8 Extrusion Rod 10. Middle Class 12 Thermal insulation layer, heat insulating layer L longitudinal axis P cross section

Claims

1. A cored wire (1) intended for introduction into a bath of molten metal for metallurgical processing, comprising a core (2) extending locally along a longitudinal axis (L) and an outer casing (4) extending longitudinally around the core (2), wherein the core (2) is - extruded rods (8) containing mainly calcium, and an intermediate layer (10) extending longitudinally between the extrusion rod (8) and the outer casing (4), the intermediate layer (10) comprising a powder containing one or more of a metal, a mixture of metals, a metal oxide, a mixture of metal oxides; In the cored wire (1), A cored wire (1) characterized in that the powder contains at least 10% by weight of calcium aluminate, and the calcium aluminate contains at least a dodecacalcium heptaaluminate phase.

2. 2. The cored wire (1) according to claim 1, wherein the calcium aluminate further contains one and / or the other of a tricalcium aluminate phase and a monocalcium aluminate phase.

3. 3. The cored wire (1) according to claim 1 or 2, wherein the powder contains at least 50% by weight of calcium aluminate.

4. 4. The cored wire (1) according to any one of claims 1 to 3, wherein the calcium aluminate contains at least 5% by weight of dodecacalcium heptaaluminate.

5. 5. The cored wire (1) of claim 4, wherein the calcium aluminate comprises at least 50% by weight of dodecacalcium heptaaluminate.

6. 6. The cored wire (1) according to any one of claims 1 to 5, wherein the core (2) further comprises a thermal insulating layer (12) extending longitudinally between the push rod (8) and the intermediate layer (10).

7. 7. The cored wire (1) according to any one of claims 1 to 6, wherein the extruded rod (8) has an equivalent outer diameter D1 in a cross-section (P) substantially perpendicular to the longitudinal axis (L), and the intermediate layer (10) has an equivalent outer diameter D2 in the cross-section (P), D2 being 1.3 to 6.2 times D1.

8. 8. The cored wire (1) according to any one of the preceding claims, wherein the outer casing (4) comprises a strip (6) of steel, aluminium, copper, nickel or zinc, or an alloy of two or more of these elements.

9. The cored wire (1) according to any one of the preceding claims, wherein the powder further comprises one or more of iron powder, fluorine powder, and iron-silicon alloy powder.

10. 10. A method for metallurgical treatment of a bath of molten metal, comprising the step of introducing a cored wire (1) according to any one of claims 1 to 9 into the bath of molten metal.

11. 11. The method of claim 10, wherein the molten metal is steel.

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