Calcium-based cored wire for metallurgical treatment of a metal bath and corresponding process

Abstract

Calcium-based cored wire for metallurgical treatment of a metal bath and corresponding method Cored wire (1) intended to be introduced into a molten metal bath to carry out a metallurgical treatment, the cored wire comprising a lining (2) extending locally along a longitudinal axis (L), and an outer casing (4) extending longitudinally around the lining, the lining comprising: - an extruded bar (8) comprising mainly calcium, - an intermediate layer (10) 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, and optionally one and / or the other of the tricalcium aluminate and monocalcium aluminate phases. Figure for abstract: Figure 1.

Description

Title of the invention: Calcium-based cored wire for metallurgical treatment of a metal bath and corresponding method

[0001] 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.

[0002] The invention also relates to a metallurgical treatment method using such a cored wire.

[0003] The molten metal is, for example, steel. The metallurgical treatment has the objective, for example, of adding 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.

[0004] 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 comprising the active substance in powder form, enclosed in a metal envelope made of a metal whose composition is compatible with that of the molten metal to be treated. In the case of the treatment of molten steel, this envelope is itself advantageously made of steel.

[0005] The cored wire is introduced into the molten metal bath by means of an injection device, generally automatic, introducing a precise length of cored wire at an appropriate speed.

[0006] For example, it is known to treat steels with calcium. This treatment aims in particular to modify the chemical composition of the endogenous inclusions of the alumina type resulting from the deoxidation of the steel, in order to obtain inclusions which are liquid at the casting temperature. These liquid inclusions do not adhere to the walls of the nozzles of a ladle or the distributor of a continuous casting installation. The flowability is improved, as is the final quality of the steel produced, dependent on the micro-inclusionary cleanliness of the steel, measurable by the cumulative surface area of ​​the micro-inclusions in a cutting plane of the steel.

[0007] 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.

[0008] Although the introduction of a cored wire into the molten metal bath is a means ingenious to add the active substance to the molten metal, the efficiency of the introduction is sometimes limited. For example, for the CaFe powder-based cored wires used in steelmaking, the calcium addition efficiency, defined as the quantity of calcium found in the steel after the injection of the cored wire divided by the quantity of calcium introduced by the consumed cored wire, is generally of the order of 10% to 15%, sometimes much less. The low efficiency of calcium comes mainly from its low vaporization temperature. Indeed, at around 1480°C, the latter is generally lower than the working temperature of the liquid steel, which means that the calcium vaporizes while 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 external envelope, 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 blockage and limits the final quality of the steel.

[0011] An object 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 comprising mainly calcium, and

[0014] - an intermediate layer extending longitudinally between the extruded bar and the outer shell, 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,

[0015] 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.

[0016] According to particular embodiments, the cored wire comprises one or more of the following characteristics, taken in isolation or in all technically possible combinations:

[0017] - the powder contains at least 50% by mass of lime aluminate;

[0018] - the lime aluminate contains at least 5% by mass of dodeca hepta-aluminate- calcium;

[0019] - the lime aluminate comprises at least 50% by mass of hepta-aluminate of dodeca-calcium;

[0020] - the filling further comprises a thermally insulating layer extending along horizontally between the extruded bar and the intermediate layer;

[0021] - the extruded bar has an equivalent external diameter DI in a plane transverse 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 DI;

[0022] - the outer casing comprises a strip of steel, aluminum, copper, nickel, or zinc, or an alloy of two or more of these elements; and

[0023] - the powder further comprises one or more of: an iron powder, a fluorine powder and an iron and silicon alloy powder.

[0024] By "equivalent diameter" of an element is meant the diameter of a disc of surface equal to the surface presented by the element in section along a transverse plane. If the given element has a circular section along the transverse plane, the equivalent diameter is equal to the ordinary diameter.

[0025] 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.

[0026] By “metallurgical treatment” we mean for example:

[0027] - a modification of the chemical composition of the molten metal, and / or

[0028] - a modification of the properties of the metal obtained after solidification of the metal in melting due, for example, to the modification of the composition of inclusions or precipitates present before the treatment, or to the creation, following the treatment, of such inclusions or precipitates, and / or

[0029] - a modification of the inclusion population present in the liquid metal in with a view to improving its production (improving flowability in continuous casting).

[0030] The invention further relates to a method for metallurgical treatment of a molten metal bath, the method comprising the step of introducing a cored wire into the molten metal bath.

[0031] According to a particular embodiment, the molten metal is steel.

[0032] The invention will be better understood on reading the following description, given solely by way of example, and made with reference to the appended drawings, in which:

[0033] [Fig-1] [Fig.l] schematically represents, in perspective, a cored wire according to the invention, and

[0034] [Fig.2] [Fig.2] schematically represents, in cross-section, the cored wire shown in [Fig. 1].

[0035] With reference to Figures 1 and 2, a cored wire 1 according to the invention is described.

[0036] 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.

[0037] Similarly, a transverse plane P is defined perpendicular to the longitudinal axis L. It is understood that the transverse plane P is transverse for the portion of the cored wire 1 represented, that is to say locally transverse.

[0038] The cored wire 1 is for example intended to be introduced into a bath of molten steel (not shown).

[0039] The cored wire 1 comprises a filling 2 and an outer casing 4, both extending longitudinally.

[0040] 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.

[0041] The external envelope 4 is advantageously made of a metal strip 6 folded back on itself around the longitudinal axis L.

[0042] The external envelope 4 has, for example, a thickness of approximately 0.4 mm.

[0043] The strip 6 is for example made of steel, copper, aluminum, nickel, or zinc, or a mixture of two or more of these elements.

[0044] The strip 6 advantageously comprises two longitudinal folds 6a, 6b ([Fig.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 lining 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.

[0045] The lining 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.

[0046] The extruded bar 8 is advantageously substantially cylindrical with a circular base. The extruded bar 8 has a diameter DI in the transverse plane P, with DI advantageously between 2 and 10 mm, for example 8 mm.

[0047] The extruded bar 8 comprises calcium. Advantageously, the extruded bar 8 comprises mainly calcium.

[0048] By "predominantly" is meant for example that the extruded bar 8 comprises at least less than 50% by mass of calcium, preferably at least 90% by mass of calcium.

[0049] In the example, the extruded bar 8 is made of calcium of industrial purity, for example 98.5% by weight.

[0050] 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.

[0051] The extruded bar 8 has, for example, a metric weight of approximately 85 g / m and a diameter DI of approximately 8.5 mm.

[0052] The intermediate layer 10 extends for example in the space located between the extruded bar 8 and the external envelope 4.

[0053] 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.

[0054] The intermediate layer 10 is advantageously made of a powder.

[0055] 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.

[0056] 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.

[0057] Lime aluminate (or calcium aluminate) is generally obtained by calcining a mixture of calcium oxide CaO and aluminum oxide A12O3. Under normal temperature and pressure conditions (101325 Pa, 25°C), depending on the mass fraction of Al2O3 in the initial mixture, lime aluminate can be present in the following stable phases:

[0058] - tricalcium aluminate: 3CaOAl2O3, also noted C3A in the field of metallurgy, where C = CaO and A = A12O3;

[0059] - dodeca-calcium hepta-aluminate: 12CaO-7Al2O3 or C12A7, also called mayenite;

[0060] - monocalcium aluminate: CaOAl2O3 or CA;

[0061] - monocalcium dialuminate: CaO-2Al2O3 or CA2;

[0062] - monocalcium hexa-aluminate: CaO-6Al2O3 or CA6.

[0063] According to the binary diagram Al2O3-CaO, the stability domains of the phases C3A+C12A7, CA12A7 and C12A7+CA correspond to mass fractions of Al2O 3 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.

[0064] The presence of these phases is advantageously determined by X-ray diffraction, with a relative precision of + / - 5% for mass quantification.

[0065] Advantageously, the powder contains at least 50% by mass of lime aluminate. According to a particular case, the powder consists of lime aluminate, advantageously only in the form described above.

[0066] Advantageously, lime aluminate comprises at least 5% by mass of dodeca-calcium hepta-aluminate, preferably at least 50% by mass of dodeca-calcium hepta-aluminate.

[0067] According to a particular case, lime aluminate contains at least 80% by mass of dodeca-calcium hepta-aluminate.

[0068] 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%.

[0069] According to a particular embodiment, among these powders, the intermediate layer 10 comprises at least the iron powder, and optionally one or two of the other powders.

[0070] The 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.

[0071] The fluorine powder makes it possible to advantageously lower the solidus temperature (temperature at which the first drop of liquid appears) and / or the liquidus temperature (temperature at which the lime aluminate is completely melted) of the lime aluminate contained in the intermediate layer 10.

[0072] The iron and silicon alloy powder advantageously reduces the reactivity of the calcium of the extruded bar 8 with the steel bath.

[0073] Finally, in order to confer maximum metallurgical treatment 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.

[0074] 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 ratio D2 / D1 greater than or equal to 1.3 guarantees the minimum space so that the thermal protection of the powder of the intermediate layer 10 is sufficient.

[0075] A D2 / D1 ratio less than or equal to 6.2 is based on considerations both metallurgical and economical. It guarantees a minimum proportion of active substance (extruded bar 8) compared to the insulating substance. Too great an imbalance causes significant thermal 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.

[0076] The lining 2 may also comprise a thermally insulating layer 12 covering the bar 8.

[0077] 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.

[0078] 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.

[0079] Examples of thermally insulating layers are provided in the applicant's application FR-A-2871477.

[0080] 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.

[0081] The cored wire 1 is for example intended to be introduced into a bath of molten steel (not shown).

[0082] Example:

[0083] Processing of a 245-tonne ladle of molten steel.

[0084] Cored wire with a diameter of 13.6 mm comprising:

[0085] - an extruded bar of industrial purity calcium, 8.5 mm in diameter, of metric weight 85 g / m,

[0086] - an intermediate layer consisting of a lime aluminate powder comprising either the tricalcium aluminate and dodecacalcium hepta-aluminate phases, or the dodecacalcium hepta-aluminate and monocalcium aluminate phases, with at least 50% by mass of dodecacalcium hepta-aluminate in each of these two cases,

[0087] - a steel strip with a thickness of 0.40 mm.

[0088] 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 outer casing (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. A cored wire (1) according to claim 1, wherein the powder contains at least 50% by mass of lime aluminate.

3. A cored wire (1) according to claim 1 or 2, wherein the lime aluminate contains at least 5% by mass of dodeca-calcium hepta-aluminate.

4. A cored wire (1) according to claim 3, wherein the lime aluminate comprises at least 50% by mass of dodeca-calcium hepta-aluminate.

5. Cored wire (1) according to any one of claims 1 to 4, wherein the lining (2) further comprises a thermally insulating layer (12) extending longitudinally between the extruded bar (8) and the intermediate layer (10).

6. Cored wire (1) according to any one of claims 1 to 5, in which the extruded bar (8) has an equivalent external diameter DI 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 Dl.

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

8. A cored wire (1) according to any one of claims 1 to 7, wherein the powder further comprises one or more of: iron powder, fluorine powder, and iron-silicon alloy powder.

9. 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 the preceding claims into the molten metal bath.

10. A method according to claim 9, characterized in that the molten metal is steel.

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