DE202014101693U1 - Ring-shaped tool - Google Patents

Abstract

Annular tool (1) with at least one radially outwardly directed working area (4) with high wear resistance and a clamping part (5) closer to the axis, in particular a roller bit or cutting ring for rock, in particular for tunnel boring machines, characterized in that the tool consists of a material, which is formed from an iron-based alloy as a matrix with embedded hard material particles, the hard material particles from carbide and / or nitride and / or oxide and / or boride, optionally as carbonitride or oxicarbonitride with boron content of at least one of the elements, or in mixed form of the elements, the groups 4 and 5 of the periodic table and have a density at room temperature of greater than 7400 kg / m3, preferably greater than 7600 kg / m3.

Description

The invention relates to an annular tool with at least one radially outwardly directed working area with high wear resistance and an axially closer clamping part, in particular a roller bit or cutting ring for rock, in particular for tunnel boring machines.

State of the art

Drilling rigs for rock formations or rock and the like, are usually equipped for larger diameter with annular tools, which have an outwardly directed working area and roll under pressure on the rock bottom and thereby cause a removal or a break of this.

Tunnel boring machines, for example, have a large dish-shaped tool holder, in which a plurality of so-called roller bits or cutting rings are mounted rotatably mounted. When propulsion of the tool holder is rotated and pressed with high force to the mountains, the same on different radii arranged roller bits in the respective areas rock are effectively breaking and the excavated rock or the so-called cuttings is discharged behind the tool holder.

According to the mechanical requirements, the annular tool should have a tapering, radially outwardly directed working region, in particular a high wear resistance and high hardness and high toughness of the material.

In most cases, the tool raw part is shrunk onto an axis, wherein tensile stresses inevitably arise in the clamping region, which are superimposed in heavy, the hard rock breaking operation, respectively, the required compressive stresses of the material and give no substantially stationary loads of the tool material.

Roller bits should thus have a working range with the highest possible wear resistance and a clamping range with sufficiently high hardness and high toughness and overall superior breaking resistance of the material with varying mechanical stress, because a failure of a tool causes costly repair work with a standstill of the drill.

The cutting rings are usually made of a tool steel. The shaping is generally carried out by a forging process, wherein the desired material properties are achieved by a subsequent heat treatment. It is known to the person skilled in the art that the greatest possible wear resistance in tool steels can only be achieved with a high degree of hardness of the structure. Here it must be accepted that with increasing hardness the tenacity of the structure decreases. In order to achieve the best properties for tool steels in terms of hard use as a cutting ring, a compromise between maximum wear resistance and high toughness must be compromised.

Various attempts have been made to extend the life of the cutting rings by combining extremely wear resistant materials with tough but tough materials. DE 10 2005 039 036 B3 describes, for example, a steel roller chisel having segments welded in the work area, these tungsten carbide hard metal particles containing. Out JP 2000001733 A a similar cutting ring is known, which has a hard metal ring applied on the outer circumference on a base body made of ductile iron. Furthermore, from the scriptures JP 2007138437 A . GB 1188305 . GB 1379151 . DE10300624A1 and DE 101 61 825 A1 Cutting rings for tunnel boring machines known which have arranged on the outer circumference segments, or even cylindrical and otherwise specially shaped parts made of hard metal, which are connected by soldering, pressing or pouring with the body. Also will be in CA 2 512 737 A1 a cutting ring described in which segments of hard metal between two discs are axially clamped. All of these known approaches to solutions involve either a very complex and difficult production or lead for example by high thermal stresses during use or by the softening of the solder for premature failure of the cutting rings in use. In JP 59144568 A A method of manufacturing cutting rings is described in which a melt containing tungsten carbide-based cemented carbide particles is poured into a rotating mold, whereupon the cemented carbide particles concentrate in the exterior of the casting. This method has the disadvantage that the hard metal particles added to the melt are partially dissolved by the melt and, during solidification, can form undesirable, brittle microstructural constituents in the microstructure of the tool. Also, the minimum size of the added cemented carbide particles is limited by the dissolution process.

Object of the invention

The invention has for its object to provide a generic, annular tool, which allows an increased operating time in hard, rock-breaking operation.

Furthermore, it is an object of the invention to provide a method of the type mentioned for the production of annular tools, which according to the respective stresses have an optimal material structure.

The above-mentioned object to provide a generic, annular tool, which allows in hard, rock-breaking operation an increased service life, is achieved by the tool consists of a material which is formed from an iron-based matrix alloy with embedded therein hard material particles. The hard material particles may be formed of carbide, nitride, oxide or boride, but also as compounds of these, such as carbonitride, carboboride or Oxikarbonitrid with boron content. Depending on the application, it may be advantageous that mixtures of these different types of hard materials are included in the tool. The metal content in the hard material particles essentially comes from groups 4 and 5 of the periodic table (Ti, Zr, Hf, V, Nb, Ta), whereby here too only individual elements from these groups or mixtures thereof can be contained in the hard materials. Compared with the hard materials used in iron metallurgy, the metallic components of which come from Group 6 of the Periodic Table (eg tungsten carbide), have hard metals from Group 4 and 5 metals the advantage that this is common in practice melting and casting temperatures of iron-based alloys up to 1650 ° C have only a low solubility in a molten iron melt.

It is known that hard materials which are formed or precipitated during the solidification of an iron base melt and during the further cooling of the resulting workpiece preferably form eutectic microstructures or precipitate at grain boundaries. The hard materials thus formed can significantly reduce the toughness of the structure. The advantage of the low solubility of the above hard materials in a molten iron melt lies in the fact that on the one hand large amounts of these hard materials may be contained as solid particles in the melt, on the other hand, only a small amount of additional amounts in the solidification of the melt and in the further cooling of the workpiece Hard material particles are formed or eliminated in the microstructure. These small amounts of brittle hard materials negatively affect the toughness of the structure only to a small extent. However, these can increase toughness even when the precipitated particles are sufficiently fine to reduce grain growth of the matrix during heat treatment.

In order to achieve a high wear resistance and long service life of the roller bits, both a minimum proportion of hard particles in the structure should be present as well as the hard particles are distributed inhomogeneous in the cutting ring so that there is a high proportion of these radially outwardly directed working area of the roller bit. For a volume fraction of the wear-resistant working range of about 8% by volume (% by volume) considered sufficient, a proportion of hard material of at least 5% by volume, based in each case on the entire workpiece, has proven suitable. At least 8% by volume of hard material particles are necessary if heavy working conditions are foreseen for the cutting ring. The possible service life of the roller bits can be increased with a larger volume proportion of the work area. Thus, the proportion of the work area can be increased to about 25% by volume and above, to allow long operating times in difficult operating conditions.

The desired distribution of the hard material ponds in the cutting ring is achieved if their density is higher than the density of the melt and they move outward in the centrifugal casting process. Experiments have shown that good results are already achieved if the density of the hard material particles at room temperature is greater than 7400 kg / m ³ . A desired, high concentration of hard material ponds in the working area is achieved if they have a density greater than 7600 kg / m ³ at room temperature. Hard materials with this density are, for example, carbides, nitrides and carbonitrides of niobium, which have proven themselves in tests. It has also been shown that a slight addition of vanadium to these niobium hard materials can favorably influence the growth and the properties of the particles, but with the addition of vanadium the density of the particles decreases. A ratio of Nb atom -% / V atomic%> 5 should in any case be complied with for niobium-vanadium mixed carbides, which may optionally also be carbonitrides. Higher concentrations of these particles in the working range are achieved with a ratio Nb atom% / V atom%> 10.

It is known to the person skilled in the art that the wear resistance of a microstructure depends not only on the hardness of the matrix and the embedded hard material particles, but also on their quantitative ratio, but also on the size distribution of the hard material particles. In the following, all structural constituents are understood as a matrix, which are not the above-mentioned hard material particles. If the particles of hard material are too small, they can be removed from the matrix in the form of entire particles as a result of their fearsome wear, without particularly increasing the wear resistance. However, if the particles are too large, they may break under the high pressure load during the rock-breaking operation and thereby also not increase the wear resistance sufficiently. In the present case the roller chisel has It has been shown that best results can be achieved if at least 60% by volume, preferably at least 75% by volume, of the hard material particles having a size of less than 70 μm are formed.

In addition to the properties of the hard material particles, the properties of the matrix are of crucial importance in order to achieve high wear resistance in the working area of the roller bits. In particular, the properties of the matrix are crucial in order to allow a sufficient toughness of the structure both in the working area, as well as in the clamping area. The properties of the matrix are mainly based on their chemical composition and possible heat treatment. Carbon is the most important alloying element and above all affects the hardenability of the steel, with about 0.28% C being considered as the lower limit for sufficient hardenability of the steel for the present application. With a carbon content of more than 1.2% in the matrix, a carbide network can form in the structure which reduces its toughness. Silicon increases the strength and wear resistance as well as the castability of the melt, but should not exceed 2% in the matrix. Manganese reduces the critical cooling rate to form the martensite and, with a sufficient amount of up to 2%, enables air-hardening of the cutting rings. Higher manganese contents of up to 25% can significantly increase the solubility of carbon in austenite and influence the transformation properties of the austenite during cooling or mechanical stress. With manganese contents up to 25%, the carbon content in the matrix can also be up to 2.3%. Like manganese, chromium also increases the hardenability of the steel and forms secondary and tertiary carbides which are precipitated from the austenite and increase wear resistance, with excessive chromium contents resulting in a chromium carbide network in the structure. The chromium content should therefore not be higher than 6.0%. Like manganese and chromium, nickel also promotes martensite formation and, in addition, increases the toughness of the matrix. For nickel, a content of 2.5% appears as an upper limit in the matrix to achieve the necessary properties as sufficient. To set a low critical cooling rate, a combination of Mn, Cr and Ni has been demonstrated. Molybdenum increases the strength of the matrix up to about 2.2% and increases the wear resistance through the formation of carbides. Tungsten forms together with Nb and V mixed carbides and mixed nitrides and can thus increase the density of these hard materials. However, the content of W in the melt is adjusted so that after ejection of the primary hard materials formed in the matrix only a content of max. 1.5%, since otherwise a network of W-Mo mixed carbides may be formed together with Mo. For this reason, 1.5 × Mo + W should not exceed 3.5%. Due to the high affinity of Nb and V to C or N of these in the matrix remain only small amounts of less than max. 0.8% back. Like Nb and V, Ti, Zr, Hf, and Ta also remain low in the matrix. Cobalt may be included in the matrix to increase the high temperature strength of highly stressed cutting rings up to a content of 3%. For the deoxidation of the melt is often added to Al, which can remain partially dissolved in the matrix after solidification. Higher levels of Al can reduce the density of the melt and thus increase the density difference to the hard material particles. An Al content of up to 3% in the matrix is possible.

As a base composition for the matrix are particularly the alloys of alloyed tool steels, as described in the Standard DIN 10020 are described. Both cold work tool steels, hot work tool steels and high speed steels can be used as the base composition for the matrix. To avoid eutectic carbides, it is sometimes necessary to reduce the carbon content of the high-speed steels compared to the standard composition. With these matrix alloys, by a suitable heat treatment, which generally consists of a hardening process and a tempering process, the hardness required for a trouble-free use of the cutting rings of at least 44HRC can be achieved. It has been found that a particularly good wear resistance is achieved if the matrix of the cutting rings has a hardness of 50HRC and above. This hardness is required when drilling in hard, particularly abrasive rock formations. The heat treatment of the cutting rings must always be adapted to the particular application of the application in order to achieve a balance between hardness and toughness of the structure.

If the matrix composition is selected according to a manganese-hard steel, the advantage of a particularly tough and impact-resistant basic structure together with a pressure-hardening and therefore wear-resistant surface can be utilized. In "Houdremont, Handbuch der Sonderstahlkunde, Springer Verlag, 1956" and other literatures, such manganese hard steels, which according to their inventor are also called Hadfield steels and whose structure is austenitic manganese tool steels, are described. These steels have a manganese content of about 8 wt% to 15 wt%, exceptionally 6 to 25 wt%, and a carbon content of about 0.8 to 2.3 wt%. The ratio of wt% Mn to wt% C is about 10: 1. Manganese hard steels are characterized by a corresponding heat treatment in that their structure of a metastable, very tough austenite. By compressing the surface, the metastable austenite can be transformed into a hard and wear-resistant martensite, yielding a hard-surfaced, tough-core component. Depending on the proportion of Mn and C in the steel and their quantitative ratio to each other, the conversion behavior can be influenced. For the formation of the hard, martensitic surface alone, the stress during use may be sufficient. If the pressure load during use is insufficient to cause the required transformation of the microstructure in the area of the surface, then the surface area to be hardened can be hardened, for example by hammering or another mechanical treatment, before use. The composition of the matrix alloy can also be adjusted so that the surface or the entire tool body can be at least partially converted into martensite by a cooling below room temperature, preferably by means of liquid nitrogen.

Roller bits or similar annular tools described above, which contain at least one radially outwardly directed working area and an axially closer clamping part, and of an iron-based alloy as a matrix, in which hard material particles, such as carbides and / or nitrides and / or carbonitrides and / or borides, optionally incorporated in a mixed form of the elements of groups 4 and / or 5 of the periodic table, are made in which in a first step, a base alloy, for example in an induction furnace, melted and heated to a temperature of 1350 ° C to 1630 ° C. becomes. This base melt is used to bring most of the alloying elements for the subsequent finished alloy in the melt. Depending on the desired matrix composition and, depending on the choice of execution of the subsequent second step, the base melt may have the following composition in% by weight: Carbon (C) to 2.5 Silicon (Si) 12:01 to 3.0 Manganese (Mn) 12:05 to 28.0 Chrome (Cr) to 9.0 Nickel (Ni) to 4.3 Molybdenum (Mo) to 3.5 Tungsten (W) to 2.2 (1.5 × Mo + W) to 5.1 Vanadin (V) to 6.0 Niobium (Nb) to 35.0 Aluminum (Al) to 3.5 possibly Titanium (Ti) to 2.0 Zircon (Zr) to 3.0 Hafnium (Hf) to 1.0 Tantalum (Ta) to 5.0 Cobalt (Co) to 3.5 Iron (Fe) and impurity elements as rest.

If the metallic constituents of the hard material particles to be formed later are already contained in the base melt (elements from groups 4 and 5) and at the same time the proportion of C, N and B is kept as low as possible, in a second step carbon and / or nitrogen and Boron or introduced into the base melt, whereupon these elements with the already in the base melt elements of Group 4 and / or 5 of the Periodic Table to Hardstoffteilchen, which have a higher density than the melt connect. The hard materials formed have the structure M _x (C + N + B) _y , wherein the sum of carbon, nitrogen and boron in the hard materials formed is between 0.4 and 0.55 atomic components, or the ratio x: y is between 1.5 and 0.8. The amount of alloyed carbon is to be selected such that a carbon content of 0.3 to 2.3 wt% C remains in the residual melt. This provides sufficient carbon for the formation of martensite in the matrix during the subsequent heat treatment. The amount of the other alloying elements, except those of the 4th and 5th group, depends on the desired properties of the matrix surrounding the hard material particles, wherein the formation of a eutectic carbide network to achieve the highest possible toughness should be avoided. Particular attention should be paid here to the heat treatment properties of the matrix.

A rapid formation of hard materials with low wear of the melting vessel is obtained when the temperature of the base melt is maintained between 1550 ° C and 1630 ° C. The alloying of carbon, nitrogen and boron can be carried out by solid substances such as coke, high carbon ferrochrome, silicon carbide, ferro-nitrogen and ferroboron or by addition of carbon and / or nitrogen and / or boron-containing melts or gases. This component or components may or may also contain other alloying elements. Depending on the carbon, nitrogen and boron content of the added component or components, very large amounts of these may be necessary to achieve the desired carbon, nitrogen and boron levels in the final melt. The amount The added carbon, nitrogen and boron carriers can thus be significantly greater than the amount of base melt, with which the alloying element shares in the base melt can assume very high levels, eg niobium up to 35% by weight.

The melting of an alloy comprising elements of the 4th and / or 5th main group with low contents of carbon, nitrogen and boron has the advantage that the ferroalloys, by way of which the elements of the 4th and 5th groups are generally alloyed , dissolve quickly. If the contents of carbon, nitrogen and boron in the melt are too high, a layer of hard material can form on the surface of the ferro alloy pieces used, which severely hampers the dissolution. It has been shown in experiments that the proportion of carbon in the base melt in the above case should be less than 0.6 wt .-%.

It is also possible in the first step to adjust the composition of the base melt such that it does not contain the elements for forming the hard material particles and in the second step the hard material particles, by means of a solid or liquid, metallic pre-melt or by means of a similar mixture of metal and hard material particles , added and homogeneously distributed in the base melt. These hard particles may be carbides and / or nitrides and / or oxycarbonitrides and / or borides, optionally as carbonitrides and / or oxycarbonitrides with borane moieties, at least one of the elements or in a mixed form of the elements of groups 4 and 5 of the periodic table. The homogeneous distribution of the hard material particles in the base melt can be assisted by mechanical processes, for example by stirring, or also by the injection of gases in the lower region of the melting vessel.

Depending on the composition of the composition and the composition of the hard particles formed or incorporated, it may be advantageous, for example to prevent oxidation of constituents in the melt, the method steps 1 and or 2 in the whole or even partially under a protective gas atmosphere or under reduced ambient pressure.

After the homogeneous distribution of the hard material particles in the second step, the matrix melt with the hard material particles contained therein is poured in a third step into a rotating mold and allowed to solidify. Caused by the rotational movement about the longitudinal axis of the mold and thereby acting on the melt and the hard particles centrifugal force, the hard particles migrate outward into the later work area of the roller bit, where they form a very rich in hard materials microstructure. At the same time, a microstructure forms in the interior, which has only small contents of the primary precipitated or introduced hard materials. The resulting proportion of exterior hard materials is determined predominantly by the process parameters of the speed of the mold, the density difference between the hard material particles and the melt, the size distribution of the hard material particles and the cooling rate of the melt in the rotating mold. In order to achieve a high concentration of hard material particles in the outer region and thus a high wear resistance, the speed of the mold and thus the force acting on the melt and on the hard particles centrifugal acceleration should be as high as possible. Centrifugal accelerations of 700 m / s ² and above, measured at the outer diameter of the casting, have been proven. A high density difference between the hard material particles and the melt can be achieved above all by high proportions of niobium, tantalum and hafnium in the hard materials. For cost reasons, niobium-rich hard materials, in particular niobium-vanadium mixed carbides, have proven to be favorable for achieving a high proportion of hard material. In any case, the hard-substance particles precipitated or added in the second step should have a density which is greater than that of the matrix melt at a temperature of 50 ° C. above its liquidus temperature.

The migration of the hard material particles to the outside requires different time depending on the dimension of the casting and to achieve a maximum possible concentration of hard materials in the outer structure, the time between the time of pouring the melt into the mold and the solidification of the melt should be as large as possible. Preheating the mold to several 100 ° C can bring slight benefits here. The speed of solidification can be particularly reduced if the mold as a whole or in parts facing the casting is made of a material which conducts the heat very poorly. Above all, quartz sand and molded materials based on aluminum-silicate ceramics should be mentioned here. A heat-insulating ceramic-based or carbon-based coating on the inside of the mold also brings advantages here.

After the blank has been cast, it can be removed from the mold at a temperature of up to 1000 ° C. in order to keep the stresses in the ring low, the temperature can be equalized over the entire ring in an oven and then cooled down so slowly that the matrix structure is in a soft state at room temperature. The cooling rate depends here on the alloy composition of the matrix. If the later conditions of use of the roller chisel require it, eg drilling into particularly hard rock, so after emptying the blank from the mold this can be brought in an oven to the appropriate forging temperature and this are then plastically deformed in the drop forging process in one or more stages. Through this process, the toughness of the structure can be significantly increased. The forging process then includes the controlled cooling to room temperature. Thereafter, the blank can be mechanically preprocessed by, for example, turning, followed by heat treatment of the ring. This may, in the case of a matrix composition similar to a tool steel, consist of a hardening process and at least one annealing process. In the case of a matrix composition similar to a manganese-hard steel, a rapid cooling generally takes place after an annealing treatment, in order to achieve a metastable, austenitic structure. After the heat treatment, the mechanical finishing of the cutting ring is followed by, for example, turning and / or grinding.

Subsequently, the invention will be described with reference to an example carried out.

A pre-melt with 0.28% C, 1.3% Si, 0.9% Mn, 1.34% Cr, 2.2% Ni, 0.1% Mo, 0.8% V and 10.0% Nb was melted in an induction furnace, brought to a temperature of 1590 ° C at maintained at this temperature for 5 minutes and then brought to a carbon content of 2.35% with petroleum coke at the same temperature. After carburizing the temperature of the finished melt was lowered to 1570 ° C, held there for 3 minutes and then poured in a centrifugal casting process. As a centrifugal casting mold, a steel mold was used, in which a core of bonded silica was inserted. This core was previously coated on the inside surface with 1mm thick zirconia base size. The casting was removed at about 800 ° C from the mold and after a compensation phase of 60min in the oven in this cooled to room temperature, then preprocessed and brought by hardening and two tempering to a hardness of 53 HRC in the clamping range.

1 shows an example of a cut annular roller chisel 1 with the cross section 2 , The enriched with hard particles Part 3 includes the outside diameter of the ring 1 lying work area 4 , The clamping range 5 lies on the inner diameter of the ring 1 and contains only a small proportion of hard materials.

2 shows an example of the structure in the work area 4 , where the hard material particles are bright and the matrix is darkened. The hard material content is about 20%.

3 shows for comparison the microstructure in the clamping area 5 with only a small proportion of hard materials.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102005039036 B3 [0008]
• JP 2000001733 A [0008]
• JP 2007138437 A [0008]
• GB 1188305 [0008]
• GB 1379151 [0008]
• DE 10300624 A1 [0008]
• DE 10161825 A1 [0008]
• CA 2512737 A1 [0008]
• JP 59144568 A [0008]

Cited non-patent literature

• Standard DIN 10020 [0017]

Claims (7)

Ring-shaped tool ( 1 ) with at least one radially outwardly directed working area ( 4 ) with high wear resistance and an axially closer clamping part ( 5 ), in particular roller bit or cutting ring for rock, in particular for tunnel boring machines, characterized in that the tool consists of a material which is formed from an iron-based alloy as a matrix with embedded hard material particles, wherein the hard material particles of carbide and / or nitride and / or oxide and / or boride, optionally as carbonitride or oxycarbonitride with boron content of at least one of the elements, or in a mixed form of the elements of groups 4 and 5 of the Periodic Table, and a density at room temperature greater than 7400 kg / m ³ , preferably greater than 7600 kg / m ³ . Tool ( 1 ) according to claim 1, characterized in that the hard material particles are present in the tool to an extent of at least 5% by volume, in particular of more than 8% by volume, the hard material particles being located above the tool cross-section (FIG. 2 ) are distributed inhomogeneously and in the work area ( 4 ) have a higher volume fraction. Tool ( 1 ) according to claim 1 or 2, characterized in that the working area ( 4 ) has a volume fraction of at least 8.0%, preferably of at least 14.0%, in particular of approximately 20 to 25%, of the tool ( 1 ), in which working range more than 60% by volume, preferably more than 75% by volume, of the hard material particles having a size of less than 70 μm are formed. Tool ( 1 ) according to one of claims 1 to 3, characterized in that the hard material particles are substantially as niobium-vanadium mixed carbides, optionally with a nitrogen content, formed and a ratio of At .-% Nb to At .-% V of greater than 5 , preferably greater than 10, have. Nb [At .-%] / V [At .-%]> 5, preferably> 10 Tool ( 1 ) according to one of the preceding claims, characterized in that the matrix alloy has a chemical composition within the limits of in% by weight Carbon (C) 12:28 to 2.3 Silicon (Si) 12:01 to 2.0 Manganese (Mn) 12:05 to 25.0 Chrome (Cr) to 6.0 Nickel (Ni) to 2.5 Molybdenum (Mo) to 2.2 Tungsten (W) to 1.5 (1.5 × Mo + W) to 3.5 Vanadin (V) to 0.8 Niobium (Nb) to 0.4 Cobalt (Co) to 3.0 Aluminum (Al) to 3.0 Possibly Titanium (Ti) to 0.2 Zircon (Zr) to 0.2 Hafnium (Hf) to 0.1 Tantalum (Ta) to 0.25 Having iron (Fe) and impurity elements as the remainder. Tool ( 1 ) according to claim 5, characterized in that the matrix alloy consists of tool steel having a hardness of greater than 44 HRC, preferably 50 HRC and higher. Tool ( 1 ) according to claim 5, characterized in that the matrix alloy of manganese steel with a manganese concentration of 6 to 25 wt .-% Mn, preferably from 8 to 15 wt .-% Mn.

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