Calcium-based flux-cored wire for the metallurgical treatment of a metal bath, and corresponding method

Abstract

The invention relates to a flux-cored wire (1) intended to be introduced into a bath of molten metal to perform a metallurgical treatment, the flux-cored wire including a core (2) extending locally along a longitudinal axis (L), and an outer casing (4) extending longitudinally around the core, the core including: - an extruded bar (8) predominantly comprising calcium, - an intermediate layer (10) extending longitudinally between the extruded bar and the outer casing, the intermediate layer including a powder comprising one or more of: a metal, a mixture of metals, a metal oxide, a mixture of metal oxides. The powder contains at least 10% by weight of lime aluminate, the lime aluminate containing at least the dodecacalcium hepta-aluminate phase.

Description

[0001] CALCIUM-BASED FLUX-CORMED WIRE FOR METALLURGICAL TREATMENT OF A METAL BATH AND CORRESPONDING METHOD

[0002] The present invention relates to a cored wire intended to be introduced into a bath of molten metal to carry out a metallurgical treatment, the cored wire comprising a lining extending locally along a longitudinal axis, and an external envelope extending longitudinally around the lining.

[0003] The invention also relates to a metallurgical treatment method using such a cored wire.

[0004] Molten metal is, for example, steel. The metallurgical treatment, for example, aims 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.

[0005] In metallurgy, it is known to provide such a substance by means of cored wires in the form of coils. The cored wire is generally composed of a lining containing the active substance in powder form, enclosed in a metal casing made of a metal whose composition is compatible with that of the molten metal to be treated. In the case of the treatment of molten steel, this casing is itself advantageously made of steel.

[0006] The cored wire is introduced into the molten metal bath by means of an injection device, usually automatic, introducing a precise length of cored wire at an appropriate speed.

[0007] For example, it is known to treat steels with calcium. This treatment aims in particular to modify the chemical composition of endogenous inclusions of the alumina type resulting from the deoxidation of the steel, in order to obtain inclusions that are liquid at the casting temperature. These liquid inclusions do not adhere to the walls of the nozzles of a ladle or the tundish of a continuous casting installation. Castability is improved, as is the final quality of the steel produced, which depends on the micro-inclusionary cleanliness of the steel, measurable by the cumulative surface area of ​​the micro-inclusions in a section plane of the steel.

[0008] There are many types of cored wires whose filling consists of pure calcium powder or calcium alloy, or a mixture of calcium and iron powders, or even aluminum. The alloy commonly called CaSi (calcium disilicide) or the mixture of calcium and iron powders (generally called CaFe), for example, are widely used fillings. Although the introduction of a cored wire into the molten metal bath is an ingenious way to add the active substance to the molten metal, the efficiency of the introduction is sometimes limited. For example, for cored wires based on CaFe powders used in steelmaking, the calcium addition efficiency, defined as the quantity of calcium found in the steel after injection of the cored wire divided by the quantity of calcium introduced by the cored wire consumed, is generally in the order of 10% to 15%, sometimes much less.The low efficiency of calcium is mainly due to its low vaporization temperature. At around 1480°C, this is generally lower than the working temperature of liquid steel, which means that the calcium vaporizes as it is introduced into the liquid steel.

[0009] To at least partially overcome this problem, and to improve the calcium addition efficiency, packings have been proposed comprising an extruded bar comprising mainly calcium, and an intermediate layer extending longitudinally between the extruded bar and the outer casing, the intermediate layer comprising a powder comprising a metal, a mixture of metals, a metal oxide, or a mixture of metal oxides. Document EP 2 917 377 describes this type of cored wire.

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

[0011] An aim of the invention is to provide a cored wire for carrying out calcium treatment with good addition efficiency, while further reducing the risk of nozzle clogging and improving the final quality of the steel.

[0012] To this end, the invention relates to a cored wire intended to be introduced into a bath of molten metal to carry out a metallurgical treatment, the cored wire comprising a lining extending locally along a longitudinal axis, and an external envelope extending longitudinally around the lining, the lining comprising:

[0013] - an extruded bar containing mainly calcium, and

[0014] - an intermediate layer extending longitudinally between the extruded bar and the outer casing, the intermediate layer comprising a powder comprising one or more of: a metal, a mixture of metals, a metal oxide, a mixture of metal oxides, the powder contains at least 10% by mass of lime aluminate, the lime aluminate containing at least the dodeca-calcium hepta-aluminate phase. According to particular embodiments, the cored wire comprises one or more of the following characteristics, taken in isolation or in all technically possible combinations:

[0015] - the lime aluminate also contains one and / or the other of the phases tricalcium aluminate and monocalcium aluminate;

[0016] - the powder contains at least 50% by mass of lime aluminate;

[0017] - lime aluminate contains at least 5% by mass of dodeca-calcium hepta-aluminate;

[0018] - the lime aluminate comprises at least 50% by mass of dodeca-calcium hepta-aluminate;

[0019] - the filling further comprises a thermally insulating layer extending longitudinally between the extruded bar and the intermediate layer;

[0020] - the extruded bar has an equivalent external diameter D1 in a transverse plane substantially perpendicular to the longitudinal axis, the intermediate layer having an equivalent external diameter D2 in the transverse plane, with D2 between 1.3 times and 6.2 times D1;

[0021] - the outer casing comprises a strip of steel, aluminum, copper, nickel, or zinc, or an alloy of two or more of these elements; and

[0022] - the powder further comprises one or more of: an iron powder, a fluorine powder and an iron and silicon alloy powder.

[0023] The equivalent diameter of an element means the diameter of a disc with a surface area equal to the surface area presented by the element in cross-section along 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.

[0024] When the concept of equivalent diameter is used for an element, it is implicit that the element is locally substantially cylindrical, but not necessarily circular in base.

[0025] By “metallurgical treatment” we mean, for example:

[0026] - a change in the chemical composition of the molten metal, and / or

[0027] - a change in the properties of the metal obtained after solidification of the molten metal due, for example, to the change in the composition of the inclusions or precipitates present before the treatment, or to the creation, following the treatment, of such inclusions or precipitates, and / or

[0028] - a modification of the inclusion population present in the liquid metal with a view to improving its production (improving the flowability in continuous casting). The invention further relates to a method for the metallurgical treatment of a molten metal bath, the method comprising the step of introducing a cored wire into the molten metal bath.

[0029] According to a particular embodiment, the molten metal is steel.

[0030] The invention will be better understood by reading the following description, given solely by way of example, and made with reference to the appended drawings, in which:

[0031] - figure 1 schematically represents, in perspective, a cored wire according to the invention, and

[0032] - figure 2 schematically represents, in cross section, the cored wire shown in figure 1.

[0033] With reference to figures 1 and 2, a cored wire 1 according to the invention is described.

[0034] 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. Indeed, the cored wire 1 may have a certain curvature, for example if it is wound so as to occupy less space.

[0035] Similarly, we define a transverse plane P perpendicular to the longitudinal axis L. We understand that the transverse plane P is transverse for the portion of the cored wire 1 represented, that is to say locally transverse.

[0036] The cored wire 1 is for example intended to be introduced into a bath of molten steel (not shown).

[0037] The cored wire 1 comprises a filling 2 and an outer sheath 4, both extending longitudinally.

[0038] The outer casing 4 forms a peripheral portion of the cored wire 1, intended to be in contact with the bath of molten metal when the cored wire 1 is introduced into the bath of molten metal.

[0039] The external envelope 4 is advantageously made up of a metal strip 6 folded back on itself around the longitudinal axis L.

[0040] The outer casing 4 has, for example, a thickness of approximately 0.4 mm.

[0041] The strip 6 is, for example, made of steel, copper, aluminum, nickel, or zinc, or a mixture of two or more of these elements.

[0042] The strip 6 advantageously comprises two longitudinal folds 6a, 6b (figure 2) stapled to each other 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 filling 2. Advantageously, the tubular shape 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 approximately 13 mm.

[0043] The packing 2 comprises an extruded bar 8 extending longitudinally and an intermediate layer 10 extending longitudinally, and radially between the extruded bar 8 and the external envelope 4.

[0044] The extruded bar 8 is advantageously substantially cylindrical with a circular base. The extruded bar 8 has a diameter D1 in the transverse plane P, with D1 advantageously between 2 and 10 mm, for example 8 mm.

[0045] The extruded bar 8 comprises calcium. Advantageously, the extruded bar 8 comprises mainly calcium.

[0046] By "predominantly" is meant, for example, that the extruded bar 8 comprises at least 50% by mass of calcium, preferably at least 90% by mass of calcium.

[0047] In the example, the extruded bar 8 is made of calcium of industrial purity, for example 98.5% by weight.

[0048] The extruded bar 8 is not a simple mass of powdery material compacted during the closing of the cored wire 1, nor even an agglomerate of powder grains (powdery material) bound together by a binder of any kind. The extruded bar 8 is for example obtained by extruding a solid cylinder (billet) of material through a die using a press. The extruded 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 to be almost zero, the apparent density of the bar being close to the true density of the material.

[0049] The extruded bar 8 has, for example, a metric weight of approximately 85 g / m and a diameter D1 of approximately 8.5 mm.

[0050] The intermediate layer 10 extends, for example, in the space located between the extruded bar 8 and the external envelope 4.

[0051] The intermediate layer 10 has an equivalent external diameter D2. D2 is for example such that the ratio D2 / D1 is between 1.3 and 6.2.

[0052] The intermediate layer 10 is advantageously made of a powder.

[0053] The intermediate layer 10 is for example made up of a powder comprising one or more of: a metal, a mixture of metals, a metal oxide, a mixture of metal oxides.

[0054] The powder contains at least 10% by mass of lime aluminate, the lime aluminate containing at least the dodeca-calcium hepta-aluminate phase, and optionally one and / or the other of the tricalcium aluminate and monocalcium aluminate phases. Lime aluminate (or calcium aluminate) is generally obtained by calcining a mixture of calcium oxide CaO and aluminum oxide AI2O3. Under normal conditions of temperature and pressure (101325 Pa, 25°C), depending on the mass fraction of ALOs in the initial mixture, lime aluminate can be present in the following stable phases:

[0055] - tricalcium aluminate: 3CaO-AI2O3, also noted C3A in the field of metallurgy, where C = CaO and A = AI2O3;

[0056] - dodeca-calcium hepta-aluminate: 12CaO-7AI2O3 or C12A7, also called mayenite;

[0057] - monocalcium aluminate: CaO-AhOs or CA;

[0058] - monocalcium dialuminate: CaO-2AI2O3 or CA2;

[0059] - monocalcium hexa-aluminate: CaO-6AI2O3 or CA6.

[0060] According to the binary AhOs-CaO diagram, the stability ranges of the C3A+C12A7, CA12A7 and C12A7+CA phases correspond to mass fractions of AlpOs between approximately 38% and 64% in the initial mixture. For these proportions, the temperature at which the first drop of liquid appears (solidus) is less than 1500°C at atmospheric pressure.

[0061] The presence of these phases is advantageously determined by X-ray diffraction, with a relative precision of + / - 5% for mass quantification.

[0062] Advantageously, the powder contains at least 50% by mass of lime aluminate. In a particular case, the powder consists of lime aluminate, advantageously only in the form described above.

[0063] Advantageously, the lime aluminate comprises at least 5% by mass of dodeca-calcium hepta-aluminate, preferably at least 50% by mass of dodeca-calcium hepta-aluminate.

[0064] In a particular case, lime aluminate contains at least 80% by mass of dodeca-calcium hepta-aluminate.

[0065] For example, the intermediate layer 10 further comprises one or more of: an iron powder, a fluorine powder and an iron and silicon alloy powder, advantageously forming the complement to 100%.

[0066] According to a particular embodiment, among these powders, the intermediate layer 10 comprises at least the iron powder, and possibly one or two of the other powders.

[0067] Iron powder improves the rigidity of the cored wire 1 and makes it easier to inject into the steel bath, in particular by facilitating the crossing of the slag layer.

[0068] The fluorine powder makes it possible to advantageously lower the solidus temperature (temperature at which the first drop of liquid appears) and / or the liquidas temperature (temperature at which the lime aluminate is completely melted) of the lime aluminate contained in the intermediate layer 10.

[0069] The iron and silicon alloy powder advantageously reduces the reactivity of the calcium of the extruded bar 8 with the steel bath.

[0070] Finally, in order to give maximum metallurgical processing efficiency to the cored wire 1, the diameter ratio D2 / D1 is between 1.3 and 6.2. This interval was determined from the following criteria.

[0071] In order for the intermediate layer 10 to be sufficiently insulating, it must be sufficiently thick. The space between the extruded bar 8 and the outer casing 4 must therefore be large enough to contain the powder. A D2 / D1 ratio greater than or equal to 1.3 guarantees the minimum space so that the thermal protection of the powder in the intermediate layer 10 is sufficient.

[0072] A D2 / D1 ratio 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) compared to the insulating substance. Too great an imbalance causes significant heat losses from the liquid metal bath to be treated (too much powder input compared to the active substance input), but also an increase in the cost price of the cored wire.

[0073] The filling 2 may also comprise a thermally insulating layer 12 covering the bar 8.

[0074] In the present application, the term "thermally insulating layer" means an additional layer around the extruded bar 8. The additional layer makes it possible to delay the heat transfer from the outside of the cored wire 1 to its core when the cored wire is introduced into a bath of liquid metal. The additional layer is adapted to constitute an additional thermal barrier between the environment outside the cored wire (liquid metal) and the extruded bar. The propagation of heat is slowed down due to the presence of the additional layer. The rise in temperature of the extruded bar is therefore delayed.

[0075] The insulating layer 12 comprises, 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 12 makes it possible to delay the complete melting of the cored wire 1.

[0076] Examples of thermally insulating layers are provided in the application

[0077] FR-A-2871477 of the applicant. The fact that the thermally insulating layer is advantageously located on the extruded bar 8 and surrounds it, for example, completely further improves the thermal protection of the extruded bar.

[0078] The cored wire 1 is for example intended to be introduced into a bath of molten steel (not shown).

[0079] Example 1:

[0080] Processing a 245-ton ladle of molten steel.

[0081] 13.6 mm diameter cored wire including:

[0082] - an extruded bar of industrial purity calcium, 8.5 mm in diameter, with a metric weight of 85 g / m,

[0083] - an intermediate layer consisting of a lime aluminate powder comprising either the tricalcium aluminate and dodeca-calcium hepta-aluminate phases, or the dodeca-calcium hepta-aluminate and monocalcium aluminate phases, with at least 50% by mass of dodeca-calcium hepta-aluminate in each of these two cases,

[0084] - a 0.40 mm thick steel strip.

[0085] Example 2:

[0086] Processing of a 320-tonne ladle of molten steel, for example of a grade with restricted silicon content (% Si < 300 ppm).

[0087] 13.6 mm diameter cored wire including:

[0088] - an extruded bar of industrial purity calcium, 7.5 mm in diameter, with a metric weight of 70 g / m,

[0089] - an intermediate layer consisting of a mixture of 50% by mass of lime aluminate powder and 50% by mass of iron powder, the lime aluminate comprising 75% by mass of dodeca-calcium hepta-aluminate and 25% by mass of one or more other phases of the lime aluminate,

[0090] - a 0.50 mm thick steel strip.

[0091] The micro-inclusion counting results were obtained using an automated scanning electron microscope associated with an EDS analyzer (in English: Energy Dispersive Spectroscopy, i.e. energy dispersive spectroscopy) which allows the chemical composition of each inclusion detected by image analysis to be determined, on a polished section for an analyzed surface of 30 mm. 2, on samples taken from a distributor. The results show a reduction in the number and size of oxysulfide inclusions, compared to the same experiment carried out with a 13.6 mm diameter cored wire comprising:

[0092] - an extruded bar of industrial purity calcium, 8.5 mm in diameter, with a metric weight of 85 g / m,

[0093] - an intermediate layer made of iron powder (without lime aluminate powder), and

[0094] - a 0.50 mm thick steel strip.

[0095] In more detail, the number of oxysulfide micro-inclusions is 20 inclusions per mm 2 and the maximum size of the inclusions detected is 5 pm in the case of the cored wire including the lime aluminate powder in the intermediate layer, against 40 inclusions per mm 2and a maximum size of detected inclusions of 8 pm in the case of the cored wire not including the lime aluminate powder in the intermediate layer.

[0096] These results demonstrate the effect of lime aluminate powder on improving the micro-inclusion cleanliness of steels treated with the cored wire described in Example 2.

[0097] Benefits

[0098] Thanks to the characteristics described above, in particular the presence of lime aluminate containing at least the dodeca-calcium hepta-aluminate phase, and possibly one and / or the other of the tricalcium aluminate and monocalcium aluminate phases, advantageously with the dodeca-calcium hepta-aluminate being the majority mass fraction in the lime aluminate, the cored wire 1 allows calcium treatment of a steel with a reduction in the cumulative surface area of ​​micro-inclusions (non-metallic particles immiscible in the steel at the treatment temperature of the liquid steel). This improves the final quality of the steel, while reducing the risk of clogging of the nozzles by non-liquid particles during production.

Claims

CLAIMS 1. Cored wire (1) intended to be introduced into a bath of molten metal to carry out a metallurgical treatment, the cored wire (1) comprising a lining (2) extending locally along a longitudinal axis (L), and an external envelope (4) extending longitudinally around the lining (2), the lining (2) comprising: - an extruded bar (8) comprising mainly calcium, and - an intermediate layer (10) extending longitudinally between the extruded bar (8) and the outer casing (4), the intermediate layer (10) comprising a powder comprising one or more of: a metal, a mixture of metals, a metal oxide, a mixture of metal oxides, characterized in that the powder contains at least 10% by mass of lime aluminate, the lime aluminate containing at least the dodeca-calcium hepta-aluminate phase.

2. Cored wire (1) according to claim 1, in which the lime aluminate further contains one and / or the other of the phases tricalcium aluminate and monocalcium aluminate.

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

4. Cored wire (1) according to any one of claims 1 to 3, in which the lime aluminate contains at least 5% by mass of dodeca-calcium hepta-aluminate.

5. Cored wire (1) according to claim 4, in which the lime aluminate comprises at least 50% by mass of dodeca-calcium hepta-aluminate.

6. Cored wire (1) according to any one of claims 1 to 5, wherein the filling (2) further comprises a thermally insulating layer (12) extending longitudinally between the extruded bar (8) and the intermediate layer (10).

7. Cored wire (1) according to any one of claims 1 to 6, in which the extruded bar (8) has an equivalent external diameter D1 in a transverse plane (P) substantially perpendicular to the longitudinal axis (L), the intermediate layer (10) having an equivalent external diameter D2 in the transverse plane (P), with D2 between 1.3 times and 6.2 times D1.

8. Cored wire (1) according to any one of claims 1 to 7, in which the outer casing (4) comprises a strip (6) made of steel, aluminum, copper, nickel, or zinc, or an alloy of two or more of these elements.

9. A cored wire (1) according to any one of claims 1 to 8, wherein the powder further comprises one or more of: an iron powder, a fluorine powder and an iron and silicon alloy powder.

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

11. Method according to claim 10, characterized in that the molten metal is steel.

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