Grinding sphere, method for manufacturing the grinding sphere, and method for grinding rocks in a semi-autogenous mill.

Abstract

GRINDING BALL, METHOD FOR MANUFACTURING THE GRINDING BALL AND METHOD FOR GRINDING ROCKS IN A SEMI-AUTOGENOUS MILL. The present invention relates to a grinding ball (19) comprising: - a carbon content of between 1.1 and 1.4% by weight, - a chromium content of between 10 and 14% by weight, - a manganese content of between 0.8 and 1.5% by weight, - a silicon content of between 0.6 and 1% by weight, - a molybdenum content of less than 1% by weight, - a nickel content of less than 1% by weight, - any impurities with a total content of less than 0.5% by weight, - iron being the remainder to obtain 100%, the grinding ball (19) being characterized by comprising a distinct distribution of chromium carbides (5) as opposed to a lattice distribution.

Description

1 / 18 “GRINDING BALL, METHOD FOR MANUFACTURING THE GRINDING BALL AND METHOD FOR GRINDING ROCKS IN A SEMI-AUTOGENOUS MILL” Subject matter of the invention

[0001] The present invention relates to cast iron grinding balls with a high chromium content, designed for semi-autogenous grinding. It also relates to a method for manufacturing said balls. Background of the invention

[0002] In the mining industry, grinding is designed to liberate valuable metallic mineral particles from gangue, which consists of worthless but often highly abrasive minerals. Plants consist of crushing stations, grinding stations, then sections to concentrate, usually by flotation, sulfide ores such as copper or lead and zinc, which are often associated.

[0003] In the grinding section of these factories, the present method is based on a semi-autogenous rotary mill and one or more rotary ball mills. This type of process line can be duplicated depending on the desired productivity or the types of ores present in the mine.

[0004] The semi-autogenous mill is characterized by an original design. The diameter is very large, generally more than five meters, with a proportionally short length. It is characterized by a length-to-diameter ratio that is usually less than 1, preferably between 0.5 and 1. The ore supply, which is continuous, originates directly from the mine or a crushing section. A variable amount of water is added to the ore blocks of different dimensions. Productivities are very high, often significantly greater than 1000 tons per hour.

[0005] These mills are protected by linings that allow the material to be ground to be lifted. Figures 1A and 1B show a mill. Petition 870250085888, dated 09 / 23 / 2025, page 8 / 34 2 / 18 Semi-autogenous 1. These mills comprise liners 2 with protruding parts called elevators 3, which allow for very intense lifting. When the mill is rotating around its horizontal geometric axis, the pieces of rock are lifted and fall back onto the rock bed at the bottom. Additionally, through relative movement between the blocks and impacts related to rotation, the size of the material is significantly reduced, which explains the term “autogenous grinding”.

[0006] For some very hard ores, the size of the rocks is no longer reduced once they reach a certain critical size and thus accumulate in the mill, decreasing its efficiency. To limit this effect, a small number of large spheres are added, generally occupying between 8 and 12% of the available volume in the mill. These spheres have dimensions greater than 100 mm, often 125 mm and sometimes 160 mm, and weigh up to 16 kg each. Driven by elevators, they will fall, after falling 5 to 7 m, onto the rocks and will help to crush hard and difficult-to-grind blocks, in the best-case scenarios. This methodology corresponds to the name "semi-autogenous grinding." The semi-autogenous mill is described in detail on the following websites: • https: / / www.911 metallurgist.com / blog / sag-mill-ball-size- evaluator-evaluation-factors • http: / / ffden-2.phys.uaf.edu- 211 fall2002.web.dir / keith palchikoff / grinding mill 2.html Material that is fine enough can exit the mill through a discharge screen and is sent to the following treatment stages.

[0007] The grinding balls used in semi-autogenous mills need to have good impact resistance as well as good wear resistance. In fact, the balls used in semi-autogenous mills are subjected to significant abrasion wear and many impacts. This Petition 870250085888, dated 09 / 23 / 2025, p. 9 / 34 3 / 18 is due to the combined action of very hard minerals in the form of large blocks, which often have sharp edges, and destruction by breakage and cracking, related to impact conditions within this equipment. The smaller worn or broken spheres are no longer effective in their role of crushing critically sized blocks that accumulate in the mill. These small spheres exit the mill through open holes in the discharge grid of the semi-autogenous mill.

[0008] To better combine the properties of wear resistance and impact resistance, two types of spheres are generally used.

[0009] On the one hand, there are low-alloy carbon steel spheres. These steels comprise, by weight, from 0.4 to 0.9% carbon, less than 1% manganese, chromium and silicon, as well as elements in smaller quantities such as molybdenum, vanadium, titanium, niobium, as well as more harmful impurities such as sulfur and phosphorus, for example. These spheres are formed by forging a bar derived from casting.

[0010] On the other hand, there are spheres made of chromium cast iron, with a chromium content greater than or equal to 5% by weight, which are formed directly by casting in a sand or metal mold. These alloys have the characteristic of including chromium carbides, referred to as primary carbides, which are formed during solidification by casting. These are M7C3 type carbides. During solidification, carbide-free austenite cells appear first. Subsequently, network carbides form around these austenite cells at the eutectic point. Figures 2A and 2B typically show the distribution of carbides in a cast iron formed by casting in a mold. Figure 2A shows the network distribution of carbides 5 that is formed between the austenite dendrites during solidification. Figure 2B schematically shows these same network carbides.A network of carbides 5 can thus be seen, distributed within a matrix 4 devoid of the almost continuous network of primary carbides. These carbides enable... Petition 870250085888, dated 09 / 23 / 2025, page 10 / 34 4 / 18 improves wear properties compared to the aforementioned steels, but its non-uniform and coarse distribution deteriorates impact resistance properties compared to those same steels.

[0011] Forging in chromium cast iron alloys has always been banned because these crude carbides initiate cracking during forging. Low-alloy steels, which by definition lack chromium carbides, do not present this problem, which has allowed the development of forging methods in these grades.

[0012] Thus, according to the previous technique, there are, on the one hand, low-alloy steels that have good impact resistance and medium wear resistance and, on the other hand, high-chromium cast irons that have good wear resistance but medium impact resistance.

[0013] As mentioned earlier, after the grinding section, there is a concentration section, generally by flotation, for sulfide ores such as copper or lead and zinc. Chromium enrichment in the cast iron spheres allows for the optimization of the flotation steps that occur during recovery in this section. The presence of chromium allows for a higher quality pulp to be obtained with, as a corollary, a reduction in the amount of reagent required. The chromium content, however, needs to be perfectly dosed to avoid excessive costs related to chromium addition. In parallel, the carbide content and therefore the carbon content in the cast irons also needs to be perfectly controlled to avoid friability of the material due to excess carbides.

[0014] Grinding balls forged from chrome white cast iron with different carbon and chromium contents are known from US documents 4,221,612, US 3,961,994 and CN 103,710,646. Petition 870250085888, dated 09 / 23 / 2025, page 11 / 34 5 / 18

[0015] Grinding balls forged from white chrome cast iron obtained from a bar manufactured by cold casting or continuous casting are thus known from US patent 4,221,612. The grinding balls have a carbon content by weight of between 1 and 3% and a chromium content of between 2 and 8%.

[0016] Grinding balls forged from white cast iron with a high chromium content, obtained from a bar manufactured by continuous casting, are known from US patent 3,961,994. The grinding balls have a carbon content by weight of between 1.5 and 3% and a chromium content of between 8 and 25%.

[0017] Grinding spheres obtained by molding are known from document CN 103,710,646. The grinding spheres have a carbon content by weight between 1.7 and 2.15% and a chromium content between 5.3 and 8%. Objectives of the invention

[0018] The present invention proposes a grinding sphere that has the advantages of both low-alloy steels and chromium cast irons, namely, that it has both good impact resistance and good wear resistance, while having a chromium content that is optimized for the concentration section. For this purpose, according to the invention, the composition and manufacturing method are optimized. The present invention proposes this type of sphere, in particular, for use in the context of a semi-autogenous grinding method. Summary of the invention

[0019] The present invention relates to a grinding sphere comprising, by weight: - a carbon content between 1.1 and 1.4%, - a chromium content between 10 and 14%, - a manganese content between 0.8 and 1.5%, Petition 870250085888, dated 09 / 23 / 2025, page 12 / 34 6 / 18 - a silicon content between 0.6 and 1%, - a molybdenum content of less than 1%, - a nickel content of less than 1%, - any impurities with a total content of less than 0.5%, - with iron being the remainder to obtain 100%, the said grinding sphere comprising a distinct distribution of chromium carbides as opposed to a network distribution, which provides the sphere with enhanced impact resistance properties.

[0020] The carbon content is maintained in the range of 1.1-1.4% by weight to obtain a sufficient, but not excessive, amount of carbides to avoid friability of the sphere. In conjunction, the chromium content is maintained in the range of 10-14% to obtain a matrix sufficiently rich in chromium for better recovery after milling while avoiding an excess cost related to chromium addition. Preferably, the carbon content and chromium content are correlated according to the following inequalities: 2.55 < Cr-5.42*C < 7.67 and 41.76 < Cr+28.66*C < 53.69.

[0021] Additionally, the carbides are finely distributed within the microstructure of the sphere. Preferably, they have an equivalent diameter of less than 100 pm, more preferably less than 50 pm, and even more preferably less than 20 pm.

[0022] The microstructure comprises a matrix in which chromium carbides are distributed. Preferably, the microstructure comprises martensite with a percentage greater than 50%, residual austenite with a percentage between 7 and 25%, a total fraction of pearlite and bainite between 2 and 10%, the remainder consisting of chromium carbides with a percentage less than or equal to 22%.

[0023] The present invention also relates to the method for manufacturing this grinding sphere which comprises the following steps: - Production, by continuous casting, of a bar that has the following composition Petition 870250085888, dated 09 / 23 / 2025, page 13 / 34 7 / 18 The aforementioned chemical process to obtain the distinct distribution of chromium carbides, - Shaping the bar by deforming it in one or more stages to obtain a rough piece that has the shape of the grinding sphere, - Heat treatment in one or more cycles of the rough piece to obtain the grinding sphere with a predominantly martensitic microstructure. Brief description of the figures

[0024] Figure 1A shows a schematic view of a semi-autogenous mill.

[0025] Figure 1B illustrates the grinding mechanism inside the semi-autogenous mill.

[0026] Figure 2A is an optical metallography of a sphere made of high-chromium cast iron formed by casting in a mold according to the previous technique. Figure 2B is a schematic illustration of the carbide distribution of Figure 2A.

[0027] Figure 3A shows two optical metallographies of a high-chromium cast iron sphere formed by forging after continuous casting according to the invention. Figure 3B is a schematic illustration of the carbide distribution of Figure 3A.

[0028] Figures 4A and 4B illustrate the method for measuring the number of grains measured respectively along the geometric axis x and the geometric axis y, allowing the evaluation of the average grain size.

[0029] Figure 5 is a schematic illustration of the continuous casting step implemented in the method according to the invention.

[0030] Figure 6 schematically illustrates, as a continuation of Figure 5, the optional rolling step of the bar obtained from continuous casting.

[0031] Figure 7 schematically illustrates, as a continuation of Figure 5 or Figure 6, the forging stage of the bar obtained from continuous casting or rolling.

[0032] Figure 8 illustrates the forging stage in greater detail. Petition 870250085888, dated 09 / 23 / 2025, page 14 / 34 8 / 18

[0033] Figure 9 illustrates the combined effect of carbon and chromium on matrix composition and carbon content. Caption 1. Semi-autogenous mill 2. Lining 3. Elevator 4. Matrix 5. Carbide 6. Induction furnace a. for casting b. for heating 7. Arch furnace 8. Pan 9. Cold mold 10. Extraction system 11. Magnetic stirring system 12. Bar a. Liquid part 13. Cutting equipment 14. Propeller-type furnace 15. Laminator 16. Forging press a. Stationary part b. Moving part 17. Knife 18. Ingot 19. Grinding sphere. Detailed description of the invention

[0034] The present invention relates to the method for manufacturing grinding balls and to grinding balls more specifically designed for Petition 870250085888, dated 09 / 23 / 2025, page 15 / 34 9 / 18 use in a semi-autogenous mill. Typically, this involves spheres that have a diameter between 90 mm and 150 mm.

[0035] The grinding ball is made of a high-chromium cast iron which has the following composition by weight: - a carbon content between 1 and 2%, - a chromium content between 7 and 16%, - a manganese content between 0.5 and 3%, - a silicon content between 0.2 and 1.5%, - a molybdenum content of less than 1.5%, - a nickel content of less than 1.5%, - any impurities / contaminations such as vanadium, niobium and titanium with a total content of less than 0.5%, - with iron being the remaining amount to reach 100%.

[0036] Preferably and as claimed, it has the following composition by weight: - a carbon content between 1.1 and 1.4%, - a chromium content between 10 and 14%, - a manganese content between 0.8 and 1.5%, - a silicon content between 0.6 and 1%, - a molybdenum content of less than 1%, - a nickel content of less than 1%, - any impurities such as vanadium, niobium and titanium with a total content of less than 0.5%, - with iron being the remaining amount to reach 100%.

[0037] More preferably, it has the following composition by weight: - Carbon: 1.2% - Chromium: 12%, - Manganese: 1.1% - Silicon: 0.8% Petition 870250085888, dated 09 / 23 / 2025, p. 16 / 34 10 / 18 - molybdenum: < 1.5%, - Nickel: < 1.5% - any impurities with a total content of less than 0.5%, - with iron being the remainder to obtain 100%.

[0038] According to the invention, the chromium content and the carbon content are jointly and respectively maintained in the range of 10-14% and 1.1-1.4%. In fact, as shown schematically in Figure 9, the carbon content and the chromium content are closely related. The dashed lines, called co-nodes, are lines that represent alloys that have the same matrix composition, i.e., among other things, the same chromium content in the matrix. Moving from one co-node to another following the solid arrow reflects an increase in the chromium content in the matrix. Conversely, moving along a co-node, the matrix composition remains unchanged, but the carbide content evolves and increases as one moves towards the dashed arrow. In fact, almost perpendicular to the co-nodes, lines of equal carbide content are also shown in Figure 9. When following a line of equal carbide content, the chromium carbide content is unchanged, but as one moves parallel to the continuous arrow, the matrix becomes richer in chromium.The lines of equal carbide content and the co-nodes are not parallel to the geometric axes C and Cr. This means that modifying only the C content or only the Cr content will modify the carbide content and also the chromium content in the matrix. Thus, it can be observed in Figure 9 that with an equal carbon content in the overall composition of the material in example 'Ex', an increase in the chromium content in the overall composition is accompanied by an increase in the chromium content in the matrix and an increase in the carbide content in the matrix. Therefore, there is a reason to achieve a balance between the carbon and chromium contents to obtain a sufficient, but not excessive, amount of carbides and chromium in the matrix. This balance is achieved with the aforementioned ranges of 10-14% and 1.1-1.4% by weight for chromium and carbon, respectively. Preferably, the carbon and chromium contents are... Petition 870250085888, dated 09 / 23 / 2025, page 17 / 34 11 / 18 correlated according to the two inequalities: 2.55 < Cr-5.42*C < 7.67 and 41.76 < Cr+28.66*C < 53.69.

[0039] In terms of microstructures, the sphere according to the invention has a primarily martensitic microstructure, i.e., with a martensite percentage greater than 50%, with a fine and uniform distribution of chromium carbides, called primary carbides, of the M7C3 type, within the matrix. Preferably, the primary carbides have an equivalent diameter less than 100 µm, more preferably less than 50 µm, and even more preferably less than 20 µm. The carbides are not perfectly circular. To calculate the equivalent diameter, the area A of the carbides is measured by image analysis, and an equivalent diameter Deq for a circular carbide of equal area is determined based on the formula Deq=2*(A / n)1 / 2. The average of the equivalent diameters is obtained based on measurements taken from at least three images. Typically, for the carbide size range according to the invention, measurements are, for example, taken on images that have a size of 660 pm x 495 pm.The size of the carbides is substantially uniform between the surface and the core of the sphere with the manufacturing method described below.

[0040] The method for manufacturing the grinding sphere according to the invention comprises the following steps: - A step in the continuous casting of the bar, which will also be described as a billet, with the aforementioned composition allowing this fine distribution of primary carbides to be obtained. - A step in shaping the bar by deformation in one or more stages, to obtain a raw piece in the shape of the grinding sphere. - A heat treatment step for the raw workpiece, in one or more cycles, to obtain a grinding sphere with a primarily martensitic microstructure. Petition 870250085888, dated 09 / 23 / 2025, page 18 / 34 12 / 18

[0041] The continuous casting stage is illustrated in Figure 5, more specifically for continuous horizontal casting. This technique favors solidification with fine grains by rapid cooling in a cold mold 9 cooled by circulating water.

[0042] The equipment comprises a liquid metal reservoir, called ladle 8, used as a buffer between the melting equipment, which is an induction furnace 6a or an arc furnace 7, and the continuous horizontal casting. Solidification (the liquid part is referenced by 12a) is initiated in the cold mold 9 made of a copper alloy that combines good heat conductivity and good resistance to wear by friction, optionally followed by a graphite part encased in a water-cooled copper shell and optionally followed by secondary cooling by water jets. The internal morphology of this composite or copper cold mold considers the specific shrinkage related to the alloy composition, which will go from the liquid to the solid state.

[0043] The bar 12 or billet, usually round, begins to solidify in this part of the equipment and then continues to solidify towards the center in ambient air with a movement exerted by an extraction system 10. Sometimes, some short movements in the opposite direction to the extraction are possible to improve the surface quality of the billet. The bar 12 is then subjected to a magnetic stirring system 11 before the cutting equipment 13, which divides the bar 12 into the chosen length. It will be specified that various magnetic stirring systems may, if applicable, be used in the continuous casting line.

[0044] Additionally, various means can be implemented depending on the alloy in order to ensure an absence of porosity related to solidification (shrinkage or gas blow holes).

[0045] A first parameter, well known to those skilled in the art, is the melting temperature, which needs to be as close as possible to Petition 870250085888, dated 09 / 23 / 2025, page 19 / 34 13 / 18 solidification temperature, but compatible with industrial production. Superheating at 5 to 40 °C above the solidification temperature will be the rule, although superheating at 10 to 15 °C is preferred. This technique makes it possible to ensure good internal quality of the billet by reducing shrinkage in the liquid metal. The water jets will be controlled to accelerate solidification while preventing the formation of surface cracks.

[0046] Additionally, the ejection speed and ejection step outside the cold mold need to be adapted to the cast alloy. Programming the ejection speed can be complex, with stops and jolts, or even accelerations and braking. As an example, the ejection step for a round billet measuring 90 mm will be between 4 and 12 mm, and preferably around 7 to 8 mm. The ejection speed will be between 50 and 250 steps per minute, and preferably around 150 steps per minute.

[0047] Additionally, magnetic stirrers can be placed in different locations to ensure the internal quality of the bar. In fact, solidification is dendritic and develops from the surface initially in contact with the cold copper mold. Subsequently, the dendrites continue to grow towards the center, and those corresponding to the bottom of the billet will grow more rapidly due to gravity; temperature gradients can also form in the volume, not yet solidified, of the solidifying billet, which sometimes increases the risk of central defects. A first electromagnetic stirrer can be positioned around the cold mold, allowing for a relatively low but uniform casting temperature. A second stirrer can be positioned at the end of the casting when the solidified thickness is about 20 mm.This allows, in addition to homogenizing the temperature of the liquid metal, the removal of excessively long dendrites that could prevent obtaining the desired internal structure. As an example, for... Petition 870250085888, dated 09 / 23 / 2025, page 20 / 34 14 / 18 a billet with a diameter of 90 mm, the electromagnetic stirrer could be placed at a distance corresponding to the solidification end of said billet, or about 7 m from the cold mold.

[0048] At the end of the continuous casting stage according to the invention, the structure comprises a fine distribution of chromium carbides, called primary carbides, of the M7C3 type, which are formed during eutectic solidification. Two optical microscopes and schematic representations thereof are shown in Figures 3A and 3B (after forging), respectively. Unlike the solidification structures of the prior art for a high-chromium cast iron cast to size in a mold (Figures 2A and 2B), the carbides 5 do not have the form of a network, but preferably a distinct distribution within the matrix. These periodically distributed primary carbides, or in other words, having a distinct distribution as opposed to a network distribution, confer improved abrasion resistance without deteriorating impact resistance properties.It will be observed that carbides can have a certain orientation which is given by the subsequent deformation steps.

[0049] Additionally, the solidification grain size is reduced due to the rapid and controlled solidification of the continuous casting step according to the invention as well as the use of the magnetic stirrer(s). This grain fineness also contributes, but to a lesser extent, to the improved impact resistance.

[0050] To evaluate grain size, the interpolation method is used. For a known length, the number of grains crossed in the X direction is counted as described in Figure 4A. A reference length is arbitrarily chosen, 200 pm, for example. The figures on the right show the number of intersections. This method is repeated in the other Y direction. In the illustrated example, an average value of 35 pm is obtained in X and an average value of 100 pm is obtained in Y, that is, a Petition 870250085888, dated 09 / 23 / 2025, page 21 / 34 15 / 18 average general time of 67 pm.

[0051] According to the invention, for a bar having a diameter or thickness greater than 85 mm, the solidification grain size is less than 90 µm, preferably less than 80 µm and particularly preferably between 30 and 70 µm, especially in the first 15 mm below the surface, preferably 20 mm or even 25 mm below the surface. In comparison, the grain size obtained by casting in a sand mold is 100 to 400 µm and 100 to 200 µm in a metal mold.

[0052] After continuous casting, the shaping stage occurs, which can be done by rolling and / or forging. This is illustrated using figures 6 to 8. It can be done by rolling in a series of grooved rolling mills that gradually form the sphere. More frequently, it is done using a press 16 to forge an ingot 18 cut from the bar 12 as illustrated in figures 7 and 8. It can also be planned to first perform rolling to reduce the diameter of the bar as illustrated in figure 6 and then shape the ingots obtained from the bar into a sphere in the forging press. It can also be planned, after forging in the press, to perform a rolling stage to improve the sphericity of the sphere that comes out of the press.

[0053] During the optional rolling stage in Figure 6, bar 12 is heated in a propeller-type furnace 14 or through a series of induction furnaces 6b in the austenitic range before being rolled in the rolling confines 15, to reduce the bar thickness and close any porosities. The rolled bar 12 is then reheated in these same types of furnaces 14, 6b in the austenitic range before being introduced into the forging press 16 (Figure 7). Typically, heating is done at a temperature between 950 and 1250 °C. Bar 12 is then cut by knife 17 into an ingot 18 which is Petition 870250085888, dated 09 / 23 / 2025, page 22 / 34 16 / 18 introduced into the press 16 which, in the illustrated example, comprises a stationary part 16a and a moving part 16b. The ingot 18 is deformed into a rough piece having the shape of a sphere 19 by the moving part 16b, which is moved towards the stationary part 16a. Optionally, as mentioned earlier, the sphericity of the rough piece can then be improved by passing it through two cylinders that have a shape similar to an Archimedes screw.

[0054] The spherical blank is then subjected to heat treatment in one or more cycles to obtain the final product. There is a first austenitizing and cooling cycle intended to form the primarily martensitic microstructure. Austenitizing is performed in a temperature range between 880 and 1075 °C for a period of between 30 minutes and 3 hours. Optionally, this cycle can be performed in several stages, with the first stage maintaining the temperature between 620 and 730 °C for a period of between 15 minutes and two hours, followed by a second stage maintaining the temperature between 880 and 1075 °C for a period of between 30 minutes and 3 hours. Next, the blank is cooled to a temperature below 220 °C in order to form martensite. Cooling can be done in oil, water, blown air, a polymer, etc.This austenitizing and cooling cycle can be followed by stress-relief quenching at a temperature between 150 and 400 °C for a period of between 30 minutes and 6 hours. This stress-relief quenching is intended to slightly reduce the internal stresses generated by the transformation of austenite into martensite.

[0055] It will be specified that the method described above can be done continuously in order to avoid or at least limit the heating phases between casting and forming, for example, or between forming and heat treatment.

[0056] At the end of the manufacturing process, a microstructure is obtained with Petition 870250085888, dated 09 / 23 / 2025, page 23 / 34 17 / 18 a matrix comprising a martensite percentage greater than 50%, preferably between 60 and 80%, a residual austenite percentage between 7 and 25%, and preferably between 10 and 20%, and a pearlite and bainite fraction between 2 and 10% in total. In addition to the aforementioned structures, the microstructure comprises primary carbides distributed in the matrix and optionally various secondary carbides of the M23C6 type, formed during the heat treatment cycles. The microstructure thus comprises, for a total percentage of 100%, the aforementioned structures with a remainder consisting of chromium carbides with a percentage that can reach 22%. The residual austenite fraction is measured by X-ray diffraction according to the ASTM E97513 standard, and the fractions of the other phases are measured by image analysis.The final properties are a hardness of 54 to 65 Rc and more generally closer to 60 Rc, where Rockwell C hardness is measured according to the ISO6508-1:2016 standard.

[0057] The grinding spheres according to the invention thus have excellent wear resistance conferred in a manner known to be due to the high hardness of the alloy obtained due to the presence of martensite and chromium carbides. However, surprisingly, this excellent wear resistance is combined with very good impact resistance properties due to the fine primary carbide distribution as well as the reduced size of the solidification grains.

[0058] The impact resistance properties were tested and compared with those of grinding balls made of high-chromium cast iron formed by casting according to the prior art. The test is based on a technical article by the US Bureau of Mines (R. Blickensderfer and JH Tylczak, Minerals & Metallurgical Processing, May 1989, pp. 60-66). The test consists of allowing, for each of the two types of balls, 46 balls with a diameter of 125 mm to fall from a height of 10 m. The test is performed in cycles with each of the Petition 870250085888, dated 09 / 23 / 2025, page 24 / 34 18 / 18 spheres are released successively and then reintegrated into the cycle to be released again. The spheres are weighed regularly. If the weight loss exceeds 50%, the test is stopped. For a forged carbon steel, the baseline specification is at least 60,000 impacts. For grinding spheres made of high-chromium cast iron formed by casting, the test was stopped after 47,000 impacts, which is a median result. For grinding spheres of the same composition formed by forging according to the invention, the maximum of 200,000 impacts was exceeded without reaching the 50% weight loss criterion.

[0059] The grinding balls according to the invention thus have excellent wear resistance with impact resistance properties at least equal to those of conventional forged carbon steels. Petition 870250085888, dated 09 / 23 / 2025, page 25 / 34

Claims

1 / 4 CLAIMS 1. Grinding sphere, consisting, by weight, of: - a carbon content between 1.1 and 1.4%, - a chromium content between 10 and 14%, - a manganese content between 0.8 and 1.5%, - a silicon content between 0.6 and 1%, - a molybdenum content less than 1%, - a nickel content less than 1%, - any impurities with a total content less than 0.5%, - the remainder being iron to obtain 100%, said grinding sphere (19) being characterized by the fact that it consists of a distinct distribution of chromium carbides (5) and has a microstructure comprising martensite with a percentage greater than 50%, residual austenite with a percentage between 7 and 25%, a total fraction of pearlite and bainite between 2 and 10% and chromium carbides with a percentage less than or equal to 22%.

2. Grinding sphere, according to claim 1, characterized in that it consists, by weight, of: - a carbon content of 1.2%, - a chromium content of 12%, - a manganese content of 1.1%, - a silicon content of 0.8%, - a molybdenum content of less than 1.5%, - a nickel content of less than 1.5%, - any impurities with a total content of less than 0.5%, - the remainder being iron to obtain 100%.

3. Grinding sphere, according to either of claims 1 or 2, characterized in that the carbon content and chromium content correspond to the following ratios: 2.55 < Cr - 5.42*C < 7.67 and 41.76 < Cr + 28.66*C < 53.

69. Petition 870260063801, dated 06 / 29 / 2026, page 8 / 15 2 / 4 4. Grinding sphere, according to any one of claims 1 to 3, characterized in that the chromium carbides (5) have an equivalent diameter less than 100 pm, more preferably less than 50 pm, even more preferably less than 20 pm, with the equivalent diameter Deq determined based on the formula Deq=2*(A / n)1 / 2 with A being the area of ​​the carbides measured by image analysis.

5. Grinding sphere, according to any one of claims 1 to 4, characterized in that it has a microstructure comprising martensite with a percentage between 60 and 80%, residual austenite with a percentage between 10 and 20%, and a total fraction of pearlite and bainite between 2 and 10%.

6. Grinding ball, according to any one of claims 1 to 5, characterized in that it has a Rockwell C hardness between 54 and 64, the Rockwell C hardness being measured in accordance with ISO6508-1:2016.

7. Grinding ball, according to any one of claims 1 to 6, characterized in that it has a diameter between 90 mm and 150 mm.

8. Method for manufacturing the grinding sphere as defined in any one of claims 1 to 7, characterized in that it includes the following steps: - producing, by continuous casting, a bar (12) having the defined chemical composition, for the grinding sphere, in any one of claims 1 or 2, wherein production by continuous casting makes it possible to obtain the distinct distribution of chromium carbides (5), - shaping the bar (12) by deforming it in one or more stages in order to obtain a blank that has the shape of the grinding sphere (19), - heat treating the blank in one or more cycles to obtain the grinding sphere (19) with a microstructure mainly Petition 870260063801, dated 06 / 29 / 2026, p.9 / 15 3 / 4 martensitic, wherein the heat treatment step includes an austenitizing cycle at a temperature between 880 and 1075 °C for a period of between 30 minutes and 3 hours, followed by cooling to a temperature below 220 °C to transform the austenite at least partially into martensite.

9. Method for manufacturing the grinding sphere, according to claim 8, characterized in that, for a bar (12) having a diameter or thickness greater than 85 mm, the solidification grain size at the end of the production step of the bar (12) by continuous casting is less than 80 pm in the first 15 millimeters below the surface of the bar (12), the grain size being evaluated with an interpolation method: for an arbitrarily chosen reference length, the number of grains passed in an X direction and a Y direction is counted, an average value of the grain size is obtained for the X direction and the Y direction and the average of said average values ​​in the X and Y directions is equal to the grain size.

10. Method for manufacturing the grinding sphere, according to claim 9, characterized in that the solidification grain size is between 20 and 75 μιτι in the first 15 millimeters below the surface of the bar (12).

11. Method for manufacturing the grinding sphere, according to claim 10, characterized in that the solidification grain size is between 30 and 70 μm in the first 15 millimeters below the surface of the bar (12).

12. Method for manufacturing the grinding sphere, according to any one of claims 8 to 11, characterized in that the continuous casting is carried out at a temperature of 5 to 40 °C, preferably 10 to 15 °C, above the solidification temperature.

13. Method for manufacturing the grinding sphere, according to any one of claims 8 to 12, characterized in that the solidification of the bar (12) is initiated in a cold mold (9) that is at least partially metallic and cooled.

14. Method for manufacturing the grinding sphere, according to any one of claims 8 to 13, characterized in that the solidification of the bar (12) is initiated in the presence of one or more magnetic stirrers (11).

15. Method for manufacturing the grinding sphere, according to any one of claims 8 to 14, characterized in that the forming step is done by rolling and / or forging.

16. Method for grinding rocks in a semi-autogenous mill, characterized by including the use of a grinding sphere (19), as defined in any one of claims 1 to 7. Petition 870260063801, dated 06 / 29 / 2026, page 11 / 15

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