EP2222885B1 - Cobalt alloy, fibre-forming plate and method for producing mineral wool - Google Patents

Description

The present invention relates to a metal alloy for use at a very high temperature, particularly used in a process for manufacturing mineral wool by fiberizing a molten mineral composition, or more generally for the constitution of tools having high temperature strength. in an oxidizing medium such as molten glass, and cobalt-based alloys that can be used at high temperature, in particular for the production of articles for producing and / or heat-transforming glass or other mineral material, such as of mineral wool making machinery.

A fiber drawing technique, called internal centrifugation, consists in continuously dropping liquid glass inside a set of revolution parts rotating at a very high speed of rotation about their vertical axis. A centerpiece, called "plate", more often referred to in the art as the "spinner", receives the glass against a so-called "band" wall pierced with holes, which the glass passes under the effect of centrifugal force for to escape from all parts in the form of melted filaments. An annular burner located above the outside of the plate, producing a downward gas flow along the outer wall of the strip, deflects these filaments downwardly by stretching them. These then "solidify" in the form of glass wool.

The plate is a fiber-drawing tool that is very thermally stressed (thermal shocks during start-ups and shutdowns, and establishment in stabilized use of a temperature gradient along the part), mechanically (centrifugal force, erosion due to the passage of the glass) and chemically (oxidation and corrosion by the molten glass, and by the hot gases coming out of the burner around the plate). Its main modes of deterioration are: the deformation by hot creep of the vertical walls, the appearance of horizontal or vertical cracks, the erosion wear of the fiberizing orifices, which require the pure and simple replacement of the organs. Their constituent material must therefore withstand during a production time long enough to remain compatible with the technical and economic constraints of the process. For this purpose, materials with a certain ductility, creep resistance and resistance to corrosion and / or oxidation are sought.

Various known materials for the realization of these tools are superalloys based on nickel or cobalt reinforced by precipitation of carbides. Particularly refractory alloys are based on chromium, cobalt (refractory element which brings to the matrix of the alloy an intrinsic strength at high temperature improved) and nickel (to stabilize the face-centered cubic crystal lattice of the Co).

We thus know WO-A-99/16919 a cobalt-based alloy having improved mechanical properties at high temperature, comprising the following elements (in weight percent of the alloy): Cr 26 to 34% Or 6 to 12% W 4 to 8% Your 2-4% VS 0.2 to 0.5% Fe less than 3% Yes less than 1% mn less than 0.5% Zr less than 0.1% the remainder being constituted by cobalt and unavoidable impurities, the molar ratio of tantalum with respect to carbon being of the order of 0.4 to 1.

The selection of carbon and tantalum proportions is intended to form in the alloy a dense but discontinuous network of intergranular carbides consisting essentially of chromium carbides in Cr ₇ C ₃ and (Cr, W) ₂₃ C _{6 form} and by carbides tantalum TaC. This selection gives the alloy improved mechanical and oxidation resistance properties at high temperatures, allowing the drawing of a molten glass whose temperature is 1080 ° C.

We also know WO 01/90429 cobalt-based alloys likely to be used at even higher temperatures. These alloys have a good compromise between the mechanical strength and the oxidation resistance from 1100 ° C thanks to a microstructure whose intergranular zones are rich in precipitates of tantalum carbide. These carbides play on the one hand the role of a mechanical reinforcement by opposing the intergranular creep at very high temperature, and on the other hand have an effect on the oxidation resistance related to their oxidation in Ta ₂ O ₅ , which forms oxides occupying all the old volume of TaC carbides preventing the penetration of the aggressive medium (liquid glass, hot gases) in intergranular spaces.

More recently, it has been described in the application WO2005 / 052208 which does not form part of this invention, an alloy endowed with a high mechanical strength at high temperature in an oxidizing medium, on the basis of a cobalt matrix stabilized with nickel and containing chromium, reinforced by precipitation of carbides, in particular of titanium and tantalum.

The alloys described in the previous patent applications may especially be used in industrial conditions for the fibering of new glass compositions, in particular basaltic, whose melting temperature is higher than that of the compositions conventionally used in processes for obtaining wool of glass. Such compositions are described in the following description.

For example a fibering plate made from the alloy described in Example 6 of WO 2005/052208 can withstand, over relatively long periods, melting glass temperatures in the range of 1200 to 1240 ° C., corresponding to a metal temperature of between 1160 and 1210 ° C., depending on the profile of the plate.

The industrial production of glass fibers of the basaltic type can, however, be economically interesting only if the mechanical strength of the plate, and therefore of the constituent alloy, is sufficient at the fibration temperatures mentioned above. In particular, the life of the plate within the fiberizing device, which is one of the most important cost factors in the overall fiber drawing process, will be all the longer as the mechanical strength of the alloy , combined with its resistance to corrosion, will be important.

The present invention aims at providing still improved alloys whose high temperature mechanical resistance is increased, making it possible to work at a temperature (for the metal) of up to 1200 ° C., or even at higher temperatures, and having a duration improved life under such fiber drawing conditions.

In particular, the subject of the present invention is a cobalt-based alloy, further comprising chromium, and carbon, which contains the following elements (the proportions being indicated in percentage by weight of the alloy): Cr 23 to 34% Ti 0.2 to 5% Your 0.5 to 7% VS 0.2 to 1.2% Or less than 5% Fe less than 3% Yes less than 1% mn less than 0.5% the rest being cobalt and unavoidable impurities.

The alloy according to the present invention differs from the alloys incorporating carbides of Ti and Ta described in the application WO 2005/052208 (See in particular Examples 6 and 7), in that the nickel content is substantially lower than those described in this publication (8.7% by weight for the alloys of Examples 6 and 7). Until now, it was thought that the presence of such a quantity of nickel was necessary to extend the temperature stability domain of the face-centered cubic crystal structure of the cobalt matrix (see for example page 7 lines 18-21 of W02005 / 052208 or page 8 lines 29-32 and page 17 lines 25-30 of WO 2001/90429 ). In addition, tests conducted on alloys demand WO99 / 16919 have shown that the presence of a substantial amount of nickel appears preferable to limit the oxidation of such alloys when used in a high temperature fiber drawing process.

Unexpectedly, and even contrary to what could be expected, the The properties of the alloy compositions according to the present invention, that is to say having a much lower proportion of nickel than previously described, appeared to be greater than those of the previously described alloys. In particular, the lifetimes of the plates obtained from the alloys according to the invention during a high temperature fiber drawing process appeared very substantially improved.

We can refer to the request WO 2005/052208 for a complete description of the advantages and the microstructure present in the alloys according to the present invention. Indeed, the microstructures of the new alloys, observed by electron microscopy, are essentially identical to those already described in the application. WO 2005/052208 . In particular, there are mixed carbides of Ta and Ti (Ta, Ti) C arranged at the grain boundaries of the alloys, which have an improved microstructure at high temperature: less fragmentation and less rarefaction of carbides (Ta, Ti) vs. Better still, the addition of Ti to the carbides TaC stabilizes them so much at high temperature that fine secondary carbides (Ta, Ti) C, very useful for resistance to intragranular creep, precipitate spontaneously in the matrix (whereas generally the secondary precipitates obtained by special heat treatment tend to disappear under the same conditions). This stability vis-à-vis high temperatures, makes these carbides (Ta, Ti) C particularly advantageous.

It is advantageous to favor the carbides (Ta, Ti) C as the main hardening phase, while respecting a ratio of the atomic contents of the sum of the metals (Ta + Ti) to the carbon close to 1, but which can be greater, in particular of the order of 0.9 to 2. In particular a slight difference smaller than unity remains admissible in the sense that the few additional carbides that could be generated (chromium carbides) are not troublesome for all properties at all temperatures . An advantageous ratio range is generally from 0.9 to 1.5.

Carbon is an essential constituent of the alloy, necessary for the formation of metal carbide precipitates. In particular, the carbon content directly determines the amount of carbides present in the alloy. It is at least 0.2% by weight to obtain the minimum reinforcement desired, preferably at least 0.6% by weight, but preferably limited to at most 1.2% by weight to prevent the alloy from becoming hard and difficult to machine because of too high density of reinforcements. The lack of ductility of the alloy at such levels prevents it from accommodating without breaking an imposed deformation (for example of thermal origin) and withstanding sufficiently the propagation of cracks.

As already described, chromium contributes to the intrinsic mechanical strength of the matrix in which it is present in part in solid solution, and in some cases also in the form of carbides essentially Cr ₂₃ C _{6 type} in fine dispersion inside grains where they provide resistance to intragranular creep or in the form of Cr ₇ C ₃ or Cr ₂₃ C ₆ type carbides present at the grain boundaries, which prevent grain-on-grain slippage thereby contributing. also to the intergranular reinforcement of the alloy. Chromium contributes to the corrosion resistance as a chromium oxide precursor forming a protective layer on the surface exposed to the oxidizing medium. A minimal amount of chromium is required for the formation and maintenance of this protective layer. Too high a chromium content, however, is detrimental to strength and toughness at elevated temperatures, as it leads to too high rigidity and too low stress elongation that is incompatible with high temperature stresses.

In general, the chromium content of an alloy which can be used according to the invention is 23 to 34% by weight, preferably of the order of 26 to 32% by weight, advantageously of approximately 27 to 30% by weight. .

The nickel, present in the alloy in the form of a solid solution with cobalt, is present in an amount of less than 5% by weight of the alloy. Preferably, the amount of nickel present in the alloy is less than 4%, or even less than 3% or even less than 2% by weight of the alloy. Below 1% by weight of the alloy, the threshold at which the Ni is present only in the form of unavoidable impurities, excellent life values of plates, not yet observed so far, have also been obtained. By unavoidable impurities is meant within the meaning of the present invention that the nickel is not present intentionally in the composition of the alloy but that it is introduced in the form of impurities contained in at least one of the main elements of the alloy (or in at least one of the precursors of said main elements).

Most generally, the tests carried out by the applicant have shown that nickel was almost always present in the form of unavoidable impurities of at least 0.3% by weight and most often at least 0.5% by weight. or at least 0.7% by weight. However, percentages of nickel in the alloy less than 0.3% by weight must also be considered as part of the invention, but the cost generated by such purity would then make the cost of the alloy too expensive to allow the commercial viability of the fiber drawing process.

Titanium is a more common element and less expensive than tantalum, so it penalizes less the final cost of the alloy. The fact that this element is light can also be an advantage.

A minimum amount of titanium of 0.2 to 5% by weight of the alloy appeared preferable to produce a sufficient amount of TiC carbides, certainly due to the solubility of titanium in the cobalt cfc matrix. A titanium content of the order of 0.5 to 4% seems advantageous, especially 0.6 to 3%. Excellent results have been obtained for alloys comprising Ti contents between 0.8 and 2%.

In comparison with the alloys described in the application WO2405 / 052208 alloys according to the invention comprising mixed carbides of tantalum and titanium demonstrate a still improved high temperature stability, as will be described later.

The tantalum present in the alloy is partly in solid solution in the cobalt matrix of which this heavy atom locally distorts the crystal lattice and hinders or even blocks the progression of the dislocations when the material is subjected to mechanical stress, thus contributing to the intrinsic strength of the matrix. The minimum content of tantalum for the formation of mixed carbides with the Ti according to the invention is of the order of 0.5%, preferably of the order of 1% and very preferably of the order of 1, 5% or even 2%. The upper limit of the tantalum content can be chosen to be about 7%. The tantalum content is preferably of the order of 2 to 6%, in particular of 1.5 to 5%. The tantalum content is very preferably less than 5%, even 4.5% or even 4% and advantageously close to 3. A small amount of tantalum has the double advantage of substantially reducing the overall cost of the alloy but also to allow easy machining of said alloy. The higher the content of tantalum, the harder the alloy is, that is to say difficult to form.

• du silicium en tant que désoxydant du métal fondu lors de l'élaboration et du moulage de l'alliage, à raison de moins de 1% en poids ;
• du manganèse également désoxydant, à raison de moins de 0,5% en poids ;
• du fer, en une proportion pouvant aller jusqu'à 3% en poids sans altération des propriétés du matériau et de préférence en une proportion inférieure ou égale à 2% poids, par exemple inférieure ou égale à 1% poids ;
• la quantité cumulée des autres éléments introduits à titre d'impuretés avec les constituants essentiels de l'alliage (« impuretés inévitables ») représente avantageusement moins de 1% en poids de la composition de l'alliage.

The alloy may contain other elements in the form of unavoidable impurities. It generally includes:

• silicon as the deoxidizer of the molten metal in the production and molding of the alloy, less than 1% by weight;
• manganese also deoxidizing, less than 0.5% by weight;
• iron, in a proportion of up to 3% by weight without alteration of the properties of the material and preferably in a proportion of less than or equal to 2% by weight, for example less than or equal to 1% by weight;
• the cumulative amount of the other elements introduced as impurities with the essential constituents of the alloy ("unavoidable impurities") advantageously represents less than 1% by weight of the composition of the alloy.

The alloys according to the invention are preferably free of Ce, La, B, Y, Dy, Re and other rare earths.

The alloys that can be used according to the invention, which contain highly reactive elements, can be shaped by casting, in particular by inductive melting in at least partially inert atmosphere and casting in a sand mold.

The casting may optionally be followed by a heat treatment at a temperature that may go beyond the fiberizing temperature.

The invention also relates to a method of manufacturing an article by casting from the alloys described above as an object of the invention.

The process may comprise at least one cooling stage, after casting and / or after or during a heat treatment, for example by cooling in air, in particular with a return to ambient temperature.

The alloys that are the subject of the invention can be used to manufacture all kinds of parts mechanically stressed at high temperature and / or made to work in an oxidizing or corrosive medium. The invention also relates to such articles made from an alloy according to the invention, in particular by foundry.

Among such applications include the manufacture of articles used for the development or hot processing of glass, for example fiber plates for the manufacture of mineral wool.

Thus, the subject of the invention is also a process for the manufacture of mineral wool by internal centrifugation, in which a flow of molten mineral matter is poured into a fiber-drawing plate whose peripheral band is pierced with a multitude of orifices through which filaments of molten mineral material escape, which are then drawn into wool by the action of a gas, the temperature of the mineral material in the plate being at least 1200 ° C. and the fibering plate being made of an alloy as defined above.

The alloys according to the invention thus make it possible to fiberize glass or a similar molten mineral composition having a liquidus temperature T _liq of the order of 1130 ° C or more, for example from 1130 to 1200 ° C, in particular 1170 ° C or more.

In general, the fiberization of these molten mineral compositions can be carried out in a temperature range (for the melt composition reaching the plate) between T _liq and T _log2,5 where T _log2,5 is the temperature at which the composition The fondue has a viscosity of ^2.5 poise (dPa.s), typically of the order of 1200 ° C or higher, e.g. 1240-1250 ° C or higher.

Among these compositions of mineral material, it may be preferred compositions containing a significant amount of iron, which are less corrosive vis-à-vis the constituent metal of the fiberizing members.

de l'ordre de 0,1 à 0,3, notamment 0,15 à 0,20.Thus, the process according to the invention advantageously uses a composition of oxidizing mineral material, in particular with respect to chromium, capable of repairing or reconstituting the protective layer of Cr ₂ O ₃ oxide which is established on the surface. In this respect, compositions containing iron essentially in ferric form (Fe ₂ O ₃ oxide) may be preferred, in particular with a molar ratio of the degrees oxidation II and III, expressed in the ratio $\frac{\mathrm{FeO}}{\mathrm{FeO}+{\mathrm{Fe}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}}$

on the order of 0.1 to 0.3, especially 0.15 to 0.20.

Advantageously, the mineral material composition contains a high iron content allowing rapid kinetics of reconstitution of chromium oxide with a level of iron oxide (so-called "total iron" rate, corresponding to the total content of iron expressed conventionally in the form of Fe ₂ O ₃ equivalent) of at least 3%, preferably at least 4%, in particular of the order of 4 to 12%, in particular of at least 5%. In the redox range above, this corresponds to a ferric iron content Fe ₂ O ₃ alone of at least 2.7%, preferably at least 3.6%

Such compositions are known in particular from WO 99/56525 and advantageously comprise the following constituents: SiO ₂ 38-52%, preferably 40-48% Al ₂ O ₃ 17-23% SiO ₂ + Al ₂ O ₃ 56-75%, preferably 62-72% RO (CaO + MgO) 9-26%, preferably 12-25% MgO 4-20%, preferably 7-16% MgO / CaO ≥ 0.8, preferably ≥ 1.0 or ≥ 1.15 R ₂ O (Na ₂ O + K ₂ O) ≥ 2% P ₂ O ₅ 0-5% Total iron (Fe ₂ O ₃ ) ≥ 1.7%, preferably ≥ 2% B ₂ O ₃ 0-5% MnO 0-4% TO ₂ 0-3%

Other known compositions of WO-00/17117 are particularly suitable for the process according to the invention.

They are characterized by the following weight percentages: SiO ₂ 39-35% preferably 40-52% Al ₂ O ₃ 16-27% - 16-25% CaO 3-35% - 10-25% MgO 0-15% - 0-10% Na ₂ O 0-15% - 6-12% K ₂ O 0-15% - 3-12% R ₂ O (Na ₂ O + K ₂ O) 10-17% - 12-17% P ₂ O ₅ 0-3% - 0-2% Total iron (Fe ₂ O ₃ ) 0-15% - 4-12% B ₂ O3 0-8% - 0-4% TiO ₂ 0-3% MgO being between 0 and 5%, especially between 0 and 2% when R ₂ O ≤ 13.0%.

According to one embodiment, the compositions have iron oxide levels of between 5 and 12%, especially between 5 and 8%, which can make it possible to obtain a fire resistance of the mineral wool mattresses.

Although the invention has been described mainly in the context of the manufacture of mineral wool, it can be applied to the glass industry in general to produce elements or accessories oven, die, or feeder including for the production of textile glass yarns, glass packaging.

Outside the glass industry, the invention can be applied to the manufacture of very diverse articles, when they must have a high mechanical strength in an oxidizing and / or corrosive medium, particularly at high temperature.

In general, these alloys can be used to make any type of refractory alloy fixed or moving parts for operating or operating a high temperature heat treatment furnace (above 1200 ° C.), a heat exchanger or reactor of the chemical industry. It can thus be for example hot fan blades, cooking support, charging material, etc. They can also be used to produce any type of heating resistor intended to operate in a hot oxidizing atmosphere, and to realize Turbine components, used in land, sea or air vehicle engines or in any other application that does not target vehicles, eg power plants.

The invention thus relates to the use in an oxidizing atmosphere at a temperature of at least 1200 ° C of an article consisting of an alloy as defined above.

The examples which follow, in no way restrictive of the compositions according to the invention or of the conditions of the implementation of the fiberizing plates according to the invention, illustrate the advantages of the present invention.

EXAMPLE 1

By the inductive melting technique under an inert atmosphere (in particular argon), a molten charge is prepared for the following composition which is then formed by simply casting into a mold of sand: Cr 27.83% Or 1.33% VS 0.36% Your 3.08% Ti 1.34% Fe 2.00% mn <0.5% Yes <0.3% Zr <0,1% sum other impurities <1%,
the rest being cobalt.

The casting is followed by a heat treatment comprising a solution phase for 2 hours at 1200 ° C. and a secondary carbide precipitation phase for 10 hours at 1000 ° C., each of these stages ending with air cooling. to room temperature.

In this way, a 400 mm diameter fibering plate of conventional shape was manufactured.

EXAMPLE 2

According to a manufacturing method identical to Example 1, a second fibering plate 400 mm in diameter and having the same characteristics is prepared from a melted filler of the following composition: Cr 28.84% Or 0.78% VS 0.41% Your 2.95% Ti 1.21% Fe 0.66% mn <0.5% Yes <0.3% Zr <0,1% sum other impurities <1%, the rest being cobalt.

EXAMPLE 3 (Comparative)

According to the same conditions as for Examples 1 to 2 above, two plates of diameters 400 mm identical to the previous ones are prepared for comparison by their shape characteristics but obtained from the alloy composition according to Example 6 of US Pat. WO 2005/052208 : Cr 28.3% Or 8.7% VS 0.4% Your 3.0% Ti 1.5% Fe <2% mn <0.5% Yes <0.3% Zr <0,1% sum other impurities <1%, the rest being cobalt.

The capacity of the plates thus formed was evaluated in the fiberglass fiber application. More specifically the plates were placed on a industrial fiber drawing line of a basaltic glass composition: SiO ₂ Al ₂ O ₃ Total iron (Fe ₂ O ₃ ) CaO MgO Na ₂ O K ₂ O Various 45.7 19 7.7 12.6 0.3 8 5.1 1

It is a relatively oxidizing glass compared to a conventional glass because of its high iron content and a redox of 0.15. Its liquidus temperature is 1140 ° C.

The plates are used with two different shots of 10 and 12,5 tons per day until their decision is decided following the ruin of the plate, declared by a visible deterioration or by a quality of fiber produced become insufficiently good .

Apart from the variation of pulling, the fiberization conditions have remained identical from one plate to another: the temperature of the mineral composition arriving in the plate is of the order of 1200 to 1240 ° C. The temperature of the metal according to the profile of the plate is between 1160 and 1210 ° C.

The results of the lifetimes of the plates, according to their conditions of use, are reported in Table 1. In this table, for the sake of clarity and to facilitate an immediate comparison, the values of the lifetimes obtained for Plates according to the invention (Examples 1 and 2) were matched with the values obtained for the reference plates (Example 3), for identical conditions of the course.

It is seen in Table 1 that the plates according to the present invention always have, for comparable conditions of use, the longest life time.

The solidus temperature of the alloy constituting the plates is then measured according to conventional differential thermal analysis (DTA) techniques, after their use in the preceding fiberizing process.

By the term "solidus temperature" is meant in the sense of your present description, the melting temperature of the alloys in equilibrium. Due to a different method of analysis, it should be noted that the values obtained from the solidus temperatures reported in Table 2 differ somewhat from the values previously obtained in WO 2005/052208 . However, the relative differences in melting temperature between the alloys according to the invention and the reference alloy remain identical, whatever the method used.

The results obtained are reported in Table 2:

It can be seen that the solidus temperature of the alloys according to the invention is approximately 10 ° C. higher than the alloys of the prior art in all cases, which reflects a greater refractoriness. Because of the relative proximity between the operating temperature of the plate in the fiberizing process and the melting temperature of the alloy constituting the plate, such an improvement is extremely significant and could alone justify your superior properties. of high temperature strength, as observed on the present alloys.

The high temperature strength properties of the alloys of Example 1 according to the invention and of Example 3 according to the prior art were evaluated in tests of resistance to three-point bending at 1250 ° C. load of 31 MPa for a period of 200 hours. The tests were carried out for each alloy on a series of parallelepipedic specimens of 30 mm wide and 3 mm thick, the load being exerted in the middle of a center distance of 37 mm. The results are shown in Table 3. Table 3 shows the slope of the three-point creep curves obtained for each alloy, said slope illustrating the rate of deformation (in μm.h ^-1 ) of the specimen by creep.

Table 3 summarizes all the results obtained, giving for each alloy the average creep rates, as well as the maximum and minimum values observed over the entire set of test pieces. Table 3 Creep velocity in three-point bending (μm.h ^-1 ) Average value Minimum value Maximum value Alloy example 1 (according to the invention) 4.1 2.8 5.7 Alloy example 3 (comparative) 17.7 3.5 30.8

By comparing the data reported in Table 3, it is observed, for the alloy according to the invention, a resistance to creep under high temperature stress substantially improved. Combined with the increase of the solidus temperature of the alloys according to the invention, this improvement of the creep resistance leads to the increase in the life of a plate made from an alloy according to the invention when this is carried out on an industrial fiber-drawing line of a basaltic glass, as previously reported.

Claims (11)

1. An alloy, characterized in that it contains the following elements (the proportions being indicated in percentages by weight of the alloy): Cr: 23 to 34% Ti: 0.2 to 5% Ta: 0.5 to 7% C: 0.2 to 1.2% Ni: less than 5% Fe: less than 3% Si: less than 1% Mn: less than 0.5%, the balance consisting of cobalt and inevitable impurities.
2. The alloy as claimed in claim 1, characterized in that it comprises less than 4% Ni by weight, preferably less than 3% Ni by weight and very preferably less than 2% Ni by weight.
3. The alloy as claimed in claim 1 or 2, characterized in that it comprises at least 0.2% and preferably at least 0.6% carbon by weight.
4. The alloy as claimed in one of the preceding claims, characterized in that it comprises the metals Ti and Ta in a molar ratio to carbon (Ti+Ta)/C of around 0.9 to 2, in particular 0.9 to 1.5.
5. The alloy as claimed in one of the preceding claims, characterized in that it comprises 0.5 to 4% titanium by weight, preferably around 0.6 to 3% titanium by weight.
6. The alloy as claimed in one of the preceding claims, characterized in that the tantalum content is around 1 to 7%, in particular around 2 to 6%.
7. The alloy as claimed in one of the preceding claims, characterized in that the chromium content is around 26 to 32%, in particular around 27 to 30%.
8. An article for the manufacture of mineral wool made of an alloy as claimed in any one of claims 1 to 7, especially produced by casting.
9. A fiberizing spinner for the manufacture of mineral wool, made of an alloy as claimed in any one of claims 1 to 8, especially by casting.
10. A process for manufacturing mineral wool by internal centrifugation, in which a flow of molten mineral material is poured into a fiberizing spinner as claimed in claim 9, the peripheral band of which is perforated by a multitude of holes through which filaments of molten mineral material escape, said filaments being attenuated into wool through the action of a gas, the temperature of the mineral material in the spinner being at least 1200°C.
11. The process as claimed in claim 10, characterized in that the molten mineral material has a liquidus temperature of around 1130°C or higher, especially 1170°C or higher.