Calcium-based flux-cored wire for metal bath metallurgical treatment and corresponding method

Abstract

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

Description

[0001] The present application relates to a flux-cored welding wire intended for introduction into a molten metal bath for metallurgical treatment, the flux-cored welding wire comprising a core extending locally along a longitudinal axis, and a sheath extending longitudinally around the core.

[0002] The present application also relates to a metallurgical processing method using such flux-cored welding wire.

[0003] The molten metal is, for example, steel. The purpose of the metallurgical treatment is, for example, to add 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 by means of flux-cored welding wires in the form of coils. Flux-cored welding wires generally consist of a core containing a powdered active substance enclosed in a metal shell made of a metal whose composition is compatible with that of the molten metal to be processed. In the case of processing molten steel, the shell itself is advantageously made of steel.

[0005] The flux-cored wire is introduced into the molten metal bath using an injection device (usually automatically) to introduce a precise length of flux-cored wire at an appropriate speed.

[0006] For example, it is known to treat steel with calcium. The specific purpose of this treatment is to change the chemical composition of the alumina type endogenous inclusions resulting from the deoxidation of the steel, so as to obtain inclusions that are liquid at the casting temperature. These liquid inclusions do not adhere to the walls of the ladle nozzle or to the distributors of the continuous casting equipment. As a result, the castability is improved, as is the final quality of the steel produced, which depends on the micro-inclusion cleanliness of the steel, which can be measured by the cumulative surface area of ​​the micro-inclusions in a cross section of the steel.

[0007] There are many types of flux-cored welding wires, the core of which is composed of pure calcium powder or calcium alloy, or a mixture of calcium powder and iron powder, or even aluminum. For example, an alloy commonly known as CaSi (calcium disilicide) or a mixture of calcium powder and iron powder (commonly known as CaFe) is a widely used core.

[0008] Although the introduction of a flux-cored wire into a molten metal bath is an ingenious way to add active substances to the molten metal, the efficiency of the introduction is sometimes limited. For example, for CaFe-based flux-cored wires used in steelmaking, the calcium addition yield, defined as the amount of calcium found in the steel after the flux-cored wire is injected divided by the amount of calcium introduced by the consumed flux-cored wire, is usually around 10% to 15%, and sometimes even lower. The low efficiency of calcium is mainly due to its low vaporization temperature. In fact, at around 1480°C, this vaporization temperature is usually lower than the working temperature of liquid steel, which causes the calcium to vaporize when it is introduced into the liquid steel.

[0009] To at least partially solve this problem and improve the calcium addition yield, a core has been proposed comprising an extruded rod containing mainly calcium and an intermediate layer extending longitudinally between the extruded rod and the outer shell, the intermediate layer comprising a powder containing a metal, a metal mixture, a metal oxide, or a mixture of metal oxides. Document EP 2917377 describes a flux-cored welding wire of this type.

[0010] However, it has been observed that immiscible micro-inclusions remain in the steel at casting temperature, which creates a risk of nozzle clogging and limits the final quality of the steel.

[0011] The object of the present application is to provide a flux-cored welding wire for calcium treatment with a good addition yield while further reducing the risk of nozzle clogging and improving the final quality of the steel.

[0012] To this end, the present application relates to a flux-cored welding wire intended to be introduced into a molten metal bath for metallurgical treatment, the flux-cored welding wire comprising a core extending locally along a longitudinal axis, and a sheath extending longitudinally around the core, the core comprising:

[0013] - extruded bars mainly containing calcium, and

[0014] - an intermediate layer extending longitudinally between the extruded rod and the outer shell, the intermediate layer comprising a powder containing one or more of the following: a metal, a metal mixture, a metal oxide, a mixture of metal oxides,

[0015] The powder contains at least 10% by mass of lime aluminate, and the lime aluminate contains at least a dodecalcium heptaaluminate phase.

[0016] According to a specific embodiment, the flux-cored welding wire includes one or more of the following features, which may be considered individually or in any technically feasible combination:

[0017] - the lime aluminate also contains one and / or the other of a tricalcium aluminate phase and a monocalcium aluminate phase;

[0018] - the powder contains at least 50% by mass of lime aluminate;

[0019] - the lime aluminate contains at least 5% by mass of dodecalcium heptaaluminate;

[0020] - the lime aluminate contains at least 50% by mass of dodecalcium heptaaluminate;

[0021] - the core further comprises a thermal insulation layer extending longitudinally between the extruded rods and the intermediate layer;

[0022] The extruded rod has an outer equivalent diameter D1 in a transverse plane substantially perpendicular to the longitudinal axis, and the intermediate layer has an outer equivalent diameter D2 in the transverse plane, D2 being 1.3 to 6.2 times greater than D1;

[0023] - strips whose outer shell comprises steel, aluminium, copper, nickel or zinc, or alloys of two or more of these elements; and

[0024] - The powder further comprises one or more of the following: iron powder, fluorine powder and iron-silicon alloy powder.

[0025] The "equivalent diameter" of an element is the diameter of a disk whose area is equal to the area presented by the element in cross section according to a transverse plane. If a given element has a circular cross section according to a transverse plane, the equivalent diameter is equal to the ordinary diameter.

[0026] When the concept of equivalent diameter is applied to a component, it is implied that the component is substantially cylindrical in part, but does not necessarily have a circular base.

[0027] "Metallurgical processing" means, for example:

[0028] - changes in the chemical composition of the molten metal, and / or

[0029] - changes in the properties of the metal obtained after solidification of the molten metal, for example due to a change in the composition of inclusions or precipitates present before treatment, or the creation of such inclusions or precipitates after treatment, and / or

[0030] - Modification of the inclusion population present in the liquid metal to improve its processing properties (increase castability in continuous casting).

[0031] The present application also relates to a method for metallurgical treatment of a molten metal bath, the method comprising the step of introducing a flux-cored welding wire into the molten metal bath.

[0032] According to a specific embodiment, the molten metal is steel.

[0033] The present application will be better understood by reading the following description which is given by way of example only and by referring to the accompanying drawings, in which:

[0034] - Figure 1 schematically shows a perspective view of a flux-cored welding wire according to the present application, and

[0035] - Figure 2 Schematically shows Figure 1 Cross-section of a flux-cored wire is shown.

[0036] refer to Figure 1 and Figure 2 , a flux-cored welding wire 1 according to the present application is described.

[0037] The flux-cored welding wire 1 partially extends along the longitudinal axis L. Only a portion of the flux-cored welding wire 1 is shown. The portion shown extends along the longitudinal axis L. This does not mean that the entire flux-cored welding wire 1 extends along the longitudinal axis L. In fact, the flux-cored welding wire 1 may have a certain curvature, for example if it is coiled to take up less space.

[0038] Likewise, a transverse plane P is defined that is perpendicular to the longitudinal axis L. It will be appreciated that the transverse plane P is transverse to the illustrated portion of the flux-cored welding wire 1 , ie partially transverse.

[0039] The flux-cored welding wire 1 is for example intended to be introduced into a bath of molten steel (not shown).

[0040] The flux-cored welding wire 1 comprises a core 2 and a sheath 4, both extending longitudinally.

[0041] The outer shell 4 forms a peripheral portion of the flux-cored welding wire 1 and is intended to come into contact with the molten metal bath when the flux-cored welding wire 1 is introduced into the molten metal bath.

[0042] The casing 4 is advantageously made of a metal strip 6 folded on itself around the longitudinal axis L.

[0043] The thickness of the housing 4 is, for example, approximately 0.4 mm.

[0044] The strip 6 is made of, for example, steel, copper, aluminum, nickel or zinc, or a mixture of two or more elements thereof.

[0045] The strip 6 advantageously comprises two longitudinal folds 6a, 6b ( Figure 2 ), which are stapled to each other to close the strip 6 itself along the longitudinal axis L. The strip 6 folded in this way has a generally tubular shape that wraps around the core 2. Advantageously, the tubular shape is substantially cylindrical with a circular bottom and has an equivalent diameter D. D is advantageously between 6 and 21 mm. For example, D is about 13 mm.

[0046] The core 2 comprises an extruded rod 8 extending longitudinally, and an intermediate layer 10 extending longitudinally and radially between the extruded rod 8 and the outer shell 4 .

[0047] The extruded rod 8 is substantially cylindrical with a circular base. The extruded rod 8 has, in the transverse plane P, a diameter D1 which is advantageously between 2 and 10 mm, for example 8 mm.

[0048] The extruded rod 8 contains calcium. Advantageously, the extruded rod 8 contains mainly calcium.

[0049] "Predominantly" means, for example, that the extruded rod 8 contains at least 50% by mass of calcium, preferably at least 90% by mass of calcium.

[0050] In this embodiment, the extruded rod 8 is made of technically pure (eg 98.5 wt. %) calcium.

[0051] The extruded rod 8 is not a simple mass of powder material compacted during the closing process of the flux-cored welding wire 1, nor is it even an agglomeration of powder particles (powder material) bound together by any type of binder. The extruded rod 8 is obtained, for example, by extruding a solid cylinder (blank) of material through a die using a press. The extruded rod 8 can also be obtained directly by a continuous casting method, and the liquid material solidifies in the form of a continuous rod. The porosity of the extruded rod 8 is considered to be almost zero, and the apparent density of the rod is close to the true density of the material.

[0052] For example, the extruded rod 8 has a metric weight of about 85 g / m and a diameter D1 of about 8.5 mm.

[0053] The intermediate layer 10 extends, for example, in the space between the extruded rod 8 and the outer shell 4 .

[0054] The intermediate layer 10 has an outer equivalent diameter D2. D2 is such that the ratio D2 / D1 is between 1.3 and 6.2, for example.

[0055] The intermediate layer 10 is advantageously made of powder.

[0056] The intermediate layer 10 is made of, for example, a powder containing one or more of the following: a metal, a metal mixture, a metal oxide, a mixture of metal oxides.

[0057] The powder contains at least 10% by mass of lime aluminate, which contains at least a dodecalcium heptaaluminate phase and optionally one and / or the other of a tricalcium aluminate phase and a monocalcium aluminate phase.

[0058] Lime aluminate (or calcium aluminate) is generally obtained by calcining a mixture of calcium oxide CaO and aluminum oxide Al2O3. Under normal temperature and pressure conditions (101325Pa, 25°C), depending on the mass fraction of Al2O3 in the initial mixture, lime aluminate can exist in the following stable phases:

[0059] - tricalcium aluminate: 3CaO·Al2O3, also known in metallurgy as C3A, where C=CaO and A=Al2O3;

[0060] - 12CaO·7Al2O3 or C12A7, also known as mayenite;

[0061] - Calcium aluminate: CaO·Al2O3 or CA;

[0062] - Calcium dialuminate: CaO·2Al2O3 or CA2;

[0063] - Calcium hexaaluminate: CaO·6Al2O3 or CA6.

[0064] According to the binary Al2O3-CaO diagram, the stable regions of the C3A+C12A7, CAC12A7 and C12A7+CA phases correspond to Al2O3 mass fractions between about 38% and 64% in the initial mixture. For these ratios, the temperature at which the first drop of liquid appears (solidus) is below 1500°C at atmospheric pressure.

[0065] The presence of these phases is advantageously determined by X-ray diffraction, with a relative accuracy of + / -5% for mass quantification.

[0066] Advantageously, the powder contains at least 50% by mass of lime aluminate. In a specific case, the powder consists of lime aluminate, advantageously only in the form described above.

[0067] Advantageously, the lime aluminate comprises at least 5% by mass of dodecalcium heptaaluminate, preferably at least 50% by mass of dodecalcium heptaaluminate.

[0068] In a specific embodiment, the lime aluminate contains at least 80 mass % of dodecalcium heptaaluminate.

[0069] For example, the intermediate layer 10 further comprises one or more of the following: iron powder, fluorine powder and iron-silicon alloy powder, advantageously forming a complement up to 100%.

[0070] According to a particular embodiment, among these powders, the intermediate layer 10 comprises at least iron powder and optionally one or two other powders.

[0071] The iron powder improves the rigidity of the flux-cored welding wire 1, making it easier to inject into the steel bath, in particular by making it easier to penetrate the slag layer.

[0072] The fluorine powder advantageously lowers the solidus temperature (the temperature at which the first drop of liquid appears) and / or the liquidus temperature (the temperature at which the lime aluminate completely melts) of the lime aluminate contained in the intermediate layer 10 .

[0073] The iron-silicon alloy powder advantageously reduces the reactivity of the calcium in the extruded rod 8 with the steel bath.

[0074] Finally, in order to give the flux-cored welding wire 1 the maximum metallurgical processing efficiency, the diameter ratio D2 / D1 is between 1.3 and 6.2. This range is determined based on the following criteria.

[0075] In order for the intermediate layer 10 to be sufficiently insulating, it must be thick enough. Therefore, the space between the extruded rod 8 and the outer shell 4 must be large enough to accommodate the powder. A D2 / D1 ratio greater than or equal to 1.3 ensures that the powder in the intermediate layer 10 has sufficient minimum thermal protection space.

[0076] The D2 / D1 ratio of less than or equal to 6.2 is based on both metallurgical and economic considerations. It ensures a minimum ratio of active material (extruded rod 8) relative to the insulating material. Excessive imbalance will lead to significant heat losses of the liquid metal bath to be processed (too much powder input compared to the active material input) and will also increase the cost of the flux-cored wire.

[0077] The core 2 may also include a thermal insulation layer 12 covering the rods 8 .

[0078] In the present application, "insulating layer" refers to an additional layer surrounding the extruded rod 8. This additional layer slows down the heat transfer from the outside of the flux-cored wire 1 to its core when the flux-cored wire is introduced into the liquid metal bath. This additional layer is suitable for forming an additional thermal barrier between the environment outside the flux-cored wire (liquid metal) and the extruded rod. Due to the presence of the additional layer, the conduction rate of heat is slowed down. Therefore, the temperature rise of the extruded rod is delayed.

[0079] The insulating layer 12 may comprise, for example, paper, wet 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 rod 8. Advantageously, the insulating layer 12 delays the complete melting of the flux-cored welding wire 1.

[0080] The applicant provides an example of a thermal insulation layer in application FR-A-2871477.

[0081] For example, the fact that the thermal insulation layer is advantageously located on the extruded rod 8 and completely surrounds it further improves the thermal protection of the extruded rod.

[0082] The flux-cored welding wire 1 is for example intended to be introduced into a bath of molten steel (not shown).

[0083] Example 1

[0084] Handling of 245 tonne molten steel ladles.

[0085] Flux-cored wire with a diameter of 13.6 mm, including:

[0086] - Industrial purity calcium extruded rods, 8.5mm diameter, metric weight 85g / m,

[0087] an intermediate layer consisting of a lime aluminate powder comprising a tricalcium aluminate phase and a dodecalcium heptaaluminate phase, or a dodecalcium heptaaluminate phase and a monocalcium aluminate phase, in both cases the mass percentage of dodecalcium heptaaluminate being at least 50%,

[0088] - Steel strip with a thickness of 0.40 mm.

[0089] Example 2

[0090] Processing of 320 tonne molten steel ladles, for example, with steel grades having limited silicon content (%Si < 300 ppm).

[0091] Flux-cored wire with a diameter of 13.6 mm, including:

[0092] - Industrial purity calcium extruded rods, 7.5mm diameter, metric weight 70g / m,

[0093] an intermediate layer consisting of a mixture of 50% by mass of lime aluminate powder containing 75% by mass of a dodecacalcium heptaaluminate phase and 25% by mass of one or more other lime aluminate phases, and 50% by mass of iron powder,

[0094] - Steel strip with a thickness of 0.50 mm.

[0095] The microinclusion count results were obtained using an automated scanning electron microscope and an EDS analyzer (Energy Dispersive Spectroscopy) on samples taken from the dispenser with a 30 mm diameter surface area. 2 The chemical composition of each inclusion detected by image analysis was determined on a polished section of the weld metal. The results showed that the number and size of oxysulfide inclusions decreased compared to the same experiment using a 13.6 mm diameter flux-cored wire, including:

[0096] - Industrial purity calcium extruded rods, 8.5mm diameter, metric weight 85g / m,

[0097] - an intermediate layer consisting of iron powder (without lime aluminate powder), and

[0098] - Steel strip with a thickness of 0.50 mm.

[0099] More specifically, in the case where the intermediate layer of the flux-cored welding wire contains lime aluminate powder, the number of oxysulfide micro-inclusions is 20 inclusions / mm 2 , and the maximum size of the detected inclusions was 5 μm, while the number of oxysulfide micro-inclusions was 40 inclusions / mm in the case of the intermediate layer of the flux-cored wire not containing lime aluminate powder 2 , and the maximum size of the detected inclusions is 8 μm.

[0100] These results demonstrate the effect of lime aluminate powder on improving the micro-inclusion cleanliness of steel processed using the flux-cored wire described in Example 2.

[0101] Advantages

[0102] Due to the above characteristics, in particular the presence of lime aluminate, which contains at least a dodecalcium heptaaluminate phase and optionally contains one and / or the other of a tricalcium aluminate phase and a monocalcium aluminate phase, advantageously, dodecalcium heptaaluminate is the majority in the lime aluminate by mass fraction, the flux-cored welding wire 1 allows calcium treatment of the steel and reduces the accumulated surface area of ​​micro-inclusions (non-metallic particles that are immiscible in the steel at the processing temperature of the liquid steel). This improves the final quality of the steel and reduces the risk of nozzle clogging by non-liquid particles during production.

Claims

1. A flux-cored welding wire (1) intended to be introduced into a molten metal bath for metallurgical treatment, the flux-cored welding wire (1) comprising a core (2) extending locally along a longitudinal axis (L), and a shell (4) extending longitudinally around the core (2), the core (2) comprising: - extruded rods (8) containing mainly calcium, and - an intermediate layer (10) extending longitudinally between the extruded rod (8) and the outer shell (4), the intermediate layer (10) comprising a powder containing one or more of the following: a metal, a metal mixture, a metal oxide, a mixture of metal oxides, The invention is characterized in that the powder contains at least 10 mass % of lime aluminate, and the lime aluminate contains at least dodecacalcium heptahaluminate phase.

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

3. The flux-cored welding wire (1) according to claim 1 or 2, wherein the powder contains at least 50% by mass of lime aluminate.

4. The flux-cored welding wire (1) according to any one of claims 1 to 3, wherein the lime aluminate contains at least 5% by mass of dodecacalcium heptaaluminate.

5. The flux-cored welding wire (1) according to claim 4, wherein the lime aluminate comprises at least 50% by mass of dodecacalcium heptaaluminate.

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

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

8. The flux-cored welding wire (1) according to any one of claims 1 to 7, wherein the outer shell (4) comprises a strip of steel, aluminum, copper, nickel or zinc, or an alloy of two or more of these elements.

9. The flux-cored welding wire (1) according to any one of claims 1 to 8, wherein the powder further comprises one or more of the following: iron powder, fluorine powder and iron-silicon alloy powder.

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

11. The method according to claim 10, characterized in that The molten metal is steel.

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