RU2819724C1 - Forged grinding balls for semi-automatic grinding mill - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to grinding ball for semi-automatic grinding mills. Ball has a composition containing in wt.%: carbon 1.1–1.4, chromium 10–14, manganese 0.8–1.5, silicon 0.6–1, molybdenum less than 1, nickel less than 1, any impurities with total content of less than 0.5, the rest is up to 100% iron. Grinding ball has discrete distribution of chromium carbides and has microstructure with percentage content of martensite of more than 50%.

EFFECT: ball has high impact strength and wear resistance.

17 cl, 9 dwg

Description

Object of the invention

[0001] The present invention relates to high chromium content cast iron grinding balls for semi-autogenous grinding. The invention also relates to a method for producing said balls.

Prior Art

[0002] In the mining industry, comminution is designed to liberate valuable metallic mineral particles from waste rock, which consists of worthless but often highly abrasive minerals. Plants have crushing stations, grinding stations, then areas for beneficiation, usually by flotation, of sulfide ores such as copper or lead-zinc, which are often combined.

[0003] The current process in the grinding sections of these plants is based on a rotating semiautogenous mill and one or more rotating ball mills. This type of production line can be duplicated depending on the desired productivity or the types of ores that are in the mine.

[0004] The semi-autogenous mill is characterized by an original design. The diameter is very large, usually 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 0.5-1. Continuous supply of ore is carried out directly from the mine or from the crushing area. Variable amounts of water are added to pieces of ore of different sizes. Productivity is very high, often well above 1000 tons per hour.

[0005] These mills are protected by linings that allow the material being ground to be lifted. Figures 1A and 1B show a semi-autogenous mill 1. These mills have liners 2 with projecting parts called lifters 3, which provide a very intense lift. As the mill rotates around its horizontal axis, pieces of rock are lifted and fall back onto the rocks at the bottom. Further, due to the relative movement between the pieces and the effects associated with rotation, the size of the material is significantly reduced, which explains the term "autogenous comminution".

[0006] For some very hard ores, the size of the rocks no longer decreases once they reach a certain critical size, and thus they accumulate in the mill, reducing its efficiency. To limit this effect, a small number of large balls are added, typically occupying 8-12% of the available volume in the mill. These balls measure over 100 mm, often 125 mm and sometimes 160 mm, and weigh up to 16 kg each. Driven by lifts, after a fall from 5-7 m, they will collide with rocks and, in the best cases, help crush hard and difficult to grind pieces. This method corresponds to the name “semi-autogenous grinding”. The SAG mill is described in detail on the following websites:

• https://www.911metallurgist.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

Sufficiently fine material can exit the mill through the discharge screen and is sent to the next processing stages.

[0007] Grinding balls used in semi-autogenous grinding mills must have good impact strength as well as good wear resistance. In fact, the balls used in semi-autogenous grinding mills are subject to significant abrasive wear and numerous impacts. This is due to the combined effect of very hard minerals in large pieces, often with sharp edges, and the fracture by breaking and splintering associated with the shock load operating conditions inside the equipment. Smaller, worn or broken balls are no longer effective in crushing the critical size pieces that accumulate in the mill. These small balls exit the mill through open holes in the discharge grate of the semi-autogenous mill.

[0008] To best combine wear resistance and toughness properties, two types of balls are typically used.

[0009] On the one hand, there are balls made of low-alloy carbon steel. These steels contain, by weight, 0.4-0.9% 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. These balls are formed by forging a rod obtained after casting.

[0010] On the other hand, there are balls made of chromium cast iron, with a chromium content greater than or equal to 5 wt.%, which are formed directly by sand or metal casting. These alloys have the option of including chromium carbides, called primary carbides, which are formed during casting solidification. These carbides are M ₇ C ₃ type carbides. During solidification, austenite cells free of carbides first appear. Network carbides then form around these austenite cells at the eutectic point. Figures 2A and 2B show the typical distribution of carbides in cast iron formed by mold casting. Figure 2A shows the network distribution of carbides 5 that forms between austenitic dendrites during solidification. Figure 2B schematically shows these same network carbides. Thus, a network of carbides 5 can be seen distributed within a matrix 4 devoid of a quasi-continuous network of primary carbides. Compared to the above-mentioned steels, these carbides improve wear-resistant properties, but their uneven and coarse-grained distribution worsens the impact toughness properties compared to the same steels.

[0011] Forging of chromium cast iron alloys has always been prohibited because these coarse carbides initiate cracking during forging. Lightly alloyed steels, which by definition are free of chromium carbides, do not have this problem, which has made it possible to develop forging methods for these grades.

[0012] Thus, according to the prior art, there are, on the one hand, low-alloy steels, which have good toughness and average wear resistance, and, on the other hand, high-chromium cast irons, which have good wear resistance but average impact toughness.

[0013] As mentioned previously, after the grinding section there is a beneficiation section, typically by flotation, for sulfide ores such as copper or lead-zinc. Enrichment of cast iron balls with chromium allows optimization of the flotation stages that take place during extraction in a given area. The presence of chromium makes it possible to obtain a better quality pulp with, as a consequence, a reduction in the amount of the required reagent. However, chromium content must be perfectly dosed to avoid cost overruns associated with chromium addition. In parallel, the carbide and therefore carbon content of cast irons must also be ideally controlled to avoid embrittlement of the material due to excess carbides.

[0014] From documents US 4221612, US 3961994 and CN 103710646, forged grinding balls made of white chromium cast iron with varying carbon and chromium content are known.

[0015] Thus, from US 4,221,612, forged grinding balls made of white chromium cast iron are known which are produced from a die-cast or continuous-cast core. The grinding balls contain a carbon content of 1-3 wt.% and a chromium content of 2-8 wt.%.

[0016] US 3,961,994 discloses forged grinding balls made of high chromium white cast iron produced from a continuous casting rod. The grinding balls contain a carbon content of 1.5-3 wt.% and a chromium content of 8-25 wt.%.

[0017] From document CN 103710646, grinding balls obtained by molding are known. The grinding balls contain a carbon content of 1.7-2.15 wt.% and a chromium content of 5.3-8 wt.%.

Objectives of the invention

[0018] The present invention provides a grinding ball having the advantages of low alloy steels as well as the advantages of chromium cast irons, i.e., having both good toughness and good wear resistance, while having a chromium content that is optimized for the beneficiation section. For this purpose, according to the invention, the composition and manufacturing method are optimized. The present invention proposes this type of ball, in particular for use in the context of a semi-autogenous grinding process.

Brief description of the invention

[0019] The present invention relates to a grinding ball containing, by weight:

- carbon content 1.1-1.4%,

- chromium content 10-14%,

- manganese content 0.8-1.5%,

- silicon content 0.6-1%,

- molybdenum content less than 1%,

- nickel content less than 1%,

- any impurities with a total content of less than 0.5%,

- the rest is up to 100% iron,

wherein said grinding ball has a discrete distribution of chromium carbides as opposed to a network distribution, which gives the ball improved toughness properties.

[0020] The carbon content is maintained in the range of 1.1-1.4 wt.% to provide sufficient, but not excessive amounts of carbides to avoid ball embrittlement. At the same time, the chromium content is maintained in the range of 10-14% to produce a matrix rich enough in chromium for better recovery after grinding, while avoiding the cost overruns associated with 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] In addition, the carbides are finely distributed within the microstructure of the ball. Preferably they have an equivalent diameter of less than 100 µm, more preferably less than 50 µm and even more preferably less than 20 µm.

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

[0023] The present invention also relates to a method for making this grinding ball, including the following steps:

- obtaining, by continuous casting, a rod having the above-mentioned chemical composition, obtaining a discrete distribution of chromium carbides,

- forming the rod by deforming it in one or more stages to obtain a workpiece having the shape of a grinding ball,

- heat treatment of the workpiece in one or several cycles to obtain a grinding ball with a predominantly martensitic microstructure.

Brief description of graphic materials

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

[0025] Figure 1B depicts the grinding mechanism within a semi-autogenous mill.

[0026] Figure 2A is an optical metallography result of a ball made of high chromium cast iron formed by casting into a mold according to the prior art. Figure 2B is a schematic representation of the carbide distribution of Figure 2A.

[0027] Figure 3A shows the results of two optical metallographs of a high chromium cast iron ball formed by forging after continuous casting in accordance with the invention. Figure 3B is a schematic representation of the carbide distribution of Figure 3A.

[0028] Figures 4A and 4B illustrate a method for measuring the number of grains measured along the x-axis and y-axis, respectively, to estimate the average grain size.

[0029] Figure 5 is a schematic representation of the continuous casting step carried out in the method according to the invention.

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

[0031] Figure 7 schematically depicts, as a continuation of Figure 5 or Figure 6, the forging step of a rod obtained by continuous casting or rolling.

[0032] Figure 8 illustrates the forging step in more detail.

[0033] Figure 9 depicts the combined effect of carbon and chromium on matrix composition and carbon content.

[0034] Conventions

1. SAG mill

2. Lining

3. Lift

4. Matrix

5. Carbide

6. Induction oven

a. for casting

b. for heating

7. Arc furnace

8. Ladle

9. Chill

10. Pull system

11. Magnetic stirring system

12. Rod

a. Liquid part

13. Cutting equipment

14. Oven with a pusher

15. Rolling mill

16. Forging press

a. Fixed part

b. Moving part

17. Knife

18. Blank for forging

19. Grinding ball

Detailed Description of the Invention

[0035] The present invention relates to a method for making grinding balls and to grinding balls more specifically intended for use in a semi-autogenous grinding mill. It usually includes balls with a diameter of 90-150 mm.

[0036] The grinding ball is made of high chromium cast iron having the following mass composition:

- carbon content 1-2%,

- chromium content 7-16%,

- manganese content 0.5-3%,

- silicon content 0.2-1.5%,

- molybdenum content less than 1.5%,

- nickel content less than 1.5%,

- any impurities/contaminants such as vanadium, niobium and titanium, with a total content of less than 0.5%,

- the rest is up to 100% iron.

[0037] Preferably and as stated, it has the following mass composition:

- carbon content 1.1-1.4%,

- chromium content 10-14%,

- manganese content 0.8-1.5%,

- silicon content 0.6-1%,

- molybdenum content less than 1%,

- nickel content less than 1%,

- any impurities such as vanadium, niobium and titanium, with a total content of less than 0.5%,

- the rest is up to 100% iron.

[0038] More preferably, it has the following mass composition:

- carbon: 1.2%,

- chrome: 12%,

- manganese: 1.1%,

- silicon: 0.8%,

- molybdenum: less than 1.5%,

- nickel: less than 1.5%,

- any impurities with a total content of less than 0.5%,

- the rest is up to 100% iron.

[0039] According to the invention, the chromium content and carbon content are simultaneously and respectively maintained in the range of 10-14% and 1.1-1.4%. Indeed, as schematically shown in Figure 9, carbon content and chromium content are closely related. The dotted lines, called co-nodes, are lines representing alloys that have the same matrix composition, i.e., among other things, the same chromium content in the matrix. The transition from one co-site to another, following the solid arrow, reflects an increase in the chromium content in the matrix. And, conversely, when moving along the co-node, the composition of the matrix remains unchanged, but the carbide content changes and increases as it moves in the direction of the dotted arrow. Figure 9 also shows lines with equal carbide content almost perpendicular to the co-nodes. Following the line of equal carbide content, the chromium carbide content remains unchanged, but when moving parallel to the solid arrow, the matrix becomes more enriched in chromium. Lines of equal carbide content and co-nodes are not parallel to the C and Cr axes. This means that changing only the C content or only the Cr content will change the carbide content as well as the chromium content of the matrix. Thus, Figure 9 shows that with equal carbon content in the overall composition of the material in the 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. Thus, there is a rationale for finding a compromise between carbon and chromium content in order to obtain sufficient, but not excessive amounts of carbides and chromium in the matrix. This trade-off is found with the aforementioned ranges of 10-14 wt.% and 1.1-1.4 wt.% for chromium and carbon, respectively. Preferably, the carbon and chromium contents are correlated according to two inequalities: 2.55 ≤ Cr - 5.42 × C ≤ 7.67 and 41.76 ≤ Cr + 28.66 × C ≤ 53.69.

[0040] In terms of microstructures, the ball according to the invention has a predominantly martensitic microstructure, i.e. with a percentage of martensite greater than 50%, with a fine and uniform distribution of chromium carbides, called primary carbides, of the M ₇ C ₃ type within the matrix. Preferably, the primary carbides have an equivalent diameter of less than 100 microns, more preferably less than 50 microns, and even more preferably less than 20 microns. Carbides are not perfectly round. To calculate the equivalent diameter, the area A of the carbides is measured by image analysis and the equivalent diameter D _eq for a round carbide of equal area is determined by the formula Deq=2 ×( A/π ) ^1/2 . The average of the equivalent diameters is obtained from measurements taken on at least three images. Typically, for the size range of carbides according to the invention, measurements are, for example, performed on images having a size of 660 μm x 495 μm. When applying the manufacturing method described below, the size of the carbides between the surface and the core of the ball is essentially constant.

[0041] The method for manufacturing a grinding ball according to the invention includes the following steps:

- A step of continuously casting a rod, which will also be referred to as a casting blank, with the above-mentioned composition, allowing to obtain the specified fine distribution of primary carbides,

- The stage of forming the rod by deformation in one or more stages to obtain a workpiece in the form of a grinding ball,

- The stage of heat treatment of the workpiece in one or several cycles to obtain a grinding ball with a predominantly martensitic microstructure.

[0042] The continuous casting stage is illustrated in Figure 5, more specifically continuous horizontal casting. This method promotes hardening with fine grains by rapid cooling in a mold 9 cooled by circulating water.

[0043] The equipment includes a liquid metal reservoir called a ladle 8 used as a buffer between the melting equipment, which is the induction furnace 6a or the arc furnace 7, and the continuous horizontal casting. Solidification (liquid portion designated 12a) is initiated in a copper alloy mold 9, which combines good thermal conductivity and good wear resistance, by friction, optionally followed by a graphite portion encased in a copper shell cooled by water, and optionally followed by secondary cooling by water jets. The internal morphology of this copper or composite die explains the specific contraction associated with the composition of the alloy that changes from a liquid to a solid state.

[0044] The core 12 or casting blank, usually rounded, begins to solidify in a given piece of equipment and then, in the surrounding air, continues to solidify toward the center as it is moved by the drawing system 10. Sometimes, some short strokes are possible to improve the surface quality of the casting blank. in the direction opposite to the stretch. The rod 12 is then subjected to a magnetic stirring system 11 before cutting equipment 13, which cuts the rod 12 to a selected length. Several magnetic stirring systems can be used on a continuous casting line if necessary.

[0045] In addition, depending on the alloy, various means may be used to ensure the absence of porosity associated with solidification (shrinkage or gas purge holes).

[0046] The first parameter, well known to those skilled in the art, is the casting temperature, which should be as close to the solidification temperature as possible, but compatible with industrial production. A superheat of 5-40°C above the curing temperature is the rule, but a superheat of 10-15°C is preferred. This method makes it possible to ensure good internal quality of the casting blank by reducing shrinkage in the liquid metal. To speed up hardening, water jets will be controlled, preventing cracks from forming on the surface.

[0047] In addition, the drawing speed and the drawing pitch from the die must be adapted to the cast alloy. Programming the pull speed can be complex, with stops and starts or even accelerations and decelerations. For example, the drawing pitch for a 90 mm round piece will be 4-12 mm and preferably about 7-8 mm. The pulling speed will be 50-250 steps per minute, preferably about 150 steps per minute.

[0048] In addition, magnetic stirrers can be placed in various locations to ensure the internal quality of the rod. Solidification occurs according to the dendritic type and develops from the surface initially in contact with the copper die. Next, the dendrites continue to grow towards the center, and those corresponding to the bottom of the casting blank will grow faster under the influence of gravity; Temperature gradients can also form in the uncured volume of the solidifying workpiece, sometimes increasing the risk of a central defect. The first electromagnetic stirrer may be located around the die, providing a relatively low but uniform casting temperature. A second stirrer may be located at the end of the casting when the solidified thickness is approximately 20 mm. It allows, in addition to homogenizing the temperature of the liquid metal, the removal of excessively long dendrites, which can interfere with obtaining the desired internal structure. For example, for a workpiece with a diameter of 90 mm, the electromagnetic stirrer can be placed at a distance corresponding to the end of solidification of the said workpiece, or approximately 7 m from the die.

[0049] At the end of the continuous casting stage according to the invention, the structure contains a fine distribution of chromium carbides, called primary carbides, of the M ₇ C ₃ type, which are formed during eutectic solidification. Two optical microscopes and their schematic images are shown in Figures 3A and 3B (after forging), respectively. Unlike the prior art solidification structures for high chromium iron cast to size in a mold (Figures 2A and 2B), the carbides 5 do not have a network shape, but rather a discrete distribution within the matrix. These primary carbides, when distributed periodically, or in other words having a discrete distribution, as opposed to a network distribution, impart improved abrasive wear resistance without compromising toughness properties. Carbides can have a specific orientation, which is determined by subsequent stages of deformation.

[0050] In addition, 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 magnetic stirrer(s). This fine grain also contributes, but to a lesser extent, to increasing impact strength.

[0051] An interpolation method is used to estimate the grain size. For a known length, the number of grains passing in the X direction is counted as described in Figure 4A. The reference length is chosen arbitrarily, for example, 200 μm. The numbers on the right indicate the number of intersections. This method is repeated in the other Y direction. In the illustrated example, an average of 35 µm is obtained in the X-axis and an average of 100 µm is obtained in the Y-axis, i.e., an overall average of 67 µm.

[0052] According to the invention, for a rod having a diameter or thickness greater than 85 mm, the solidification grain size is less than 90 microns, preferably less than 80 microns and in particular preferably 30-70 microns, especially in the first 15 millimeters below the surface, preferably 20 mm or even 25 mm below the surface. For comparison, the grain size obtained by sand casting is 100-400 microns, and 100-200 microns in a metal mold.

[0053] After continuous casting comes the forming stage, which can be performed by rolling and/or forging. It is illustrated using Figures 6-8. It can be made by rolling in rows of grooved rollers, gradually forming a ball. Most commonly, this is done by using a forging press 16 to forge a forging blank 18 cut from the rod 12, as shown in Figures 7 and 8. It may also be provided to first perform rolling to reduce the diameter of the rod, as shown in Figure 6, and then form the blanks for forging obtained from a rod into a ball shape in a forging press. Also, after forging, a rolling step may be provided in the press to improve the sphericity of the ball emerging from the press.

[0054] During the optional rolling step of Figure 6, rod 12 is heated in a pusher furnace 14 or a series of induction furnaces 6b in the austenitic range before rolling in mill stands 15 to reduce the thickness of the rod and close any porosities. The rolling rod 12 is then heated again in the same types of furnaces 14, 6b in the austenitic range before being introduced into the forging press 16 (Figure 7). As a rule, heating is carried out at a temperature of 950-1250°C. The rod 12 is then cut by a knife 17 into a forging blank 18, which is inserted into a press 16, including in the illustrated example a stationary portion 16a and a moving portion 16b. The forging workpiece 18 is deformed into a ball-shaped workpiece 19 by the moving part 16b, which moves towards the stationary part 16a. Then, optionally, as mentioned earlier, the sphericity of the workpiece can be improved by passing it through two cylinders having a shape similar to an Archimedean screw.

[0055] The ball-shaped preform is then heat treated in one or more cycles to obtain the final product. There is a first austenitization and hardening cycle designed to produce a predominantly martensitic microstructure. Austenization is carried out in the temperature range of 880-1075°C for a period of time from 30 minutes to 3 hours. Optionally, this cycle can be carried out in several stages, with the first stage to maintain a temperature of 620-730°C for a period of time from 15 minutes to two hours, followed by a second stage to maintain the temperature at 880-1075°C for a period of time from 30 minutes to 3 hours. The workpiece is then quenched to a temperature of less than 220°C to form martensite. Quenching can be done in oil, water, blown air, polymer, etc. The specified austenitizing and quenching cycle can be followed by stress-relieving quenching at a temperature of 150-400°C for a period of 30 minutes to 6 hours. This quenching Stress Reliever is designed to slightly reduce internal stresses caused by the transformation of austenite to martensite.

[0056] The method described above can be performed continuously to avoid or at least limit heating phases between casting and molding, for example, or between molding and heat treatment.

[0057] At the end of the manufacturing process, a microstructure is obtained with a matrix comprising a percentage of martensite greater than 50%, preferably 60-80%, a percentage of retained austenite of 7-25% and preferably 10-20%, and a percentage of pearlite and bainite fraction of 2-10%. In addition to the above structures, the microstructure includes primary carbides distributed in the matrix, and optionally some secondary carbides of the M ₂₃ C ₆ type formed during heat treatment cycles. Thus, the microstructure includes the above-mentioned structures with a balance of up to 100% consisting of chromium carbides with a percentage that can reach 22%. The residual austenite fraction is measured by RX diffraction according to ASTM E975-13, and the other phase fractions are measured by image analysis. The final properties are hardness of 54 to 65 Rc and more often close to 60 Rc, with Rockwell C hardness measured according to ISO6508-1:2016.

[0058] Thus, the grinding balls according to the invention have excellent wear resistance, which is ensured in a known manner by 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 toughness properties due to the fine distribution of the primary carbide as well as the reduced solidification grain size.

[0059] The toughness properties were tested and compared with the properties of grinding balls made from high chromium cast iron cast in accordance with the prior art. Testing is based on a technical paper from the US Bureau of Mines (R. Blickensderfer and J. H. Tylczak, Minerals & Metallurgical processing, May 1989, pp. 60-66). This test consists of allowing 46 balls of both types with a diameter of 125 mm to fall from a height of 10 m. The test is carried out in a cycle, with each of the balls being dropped in sequence and then reintegrated into the cycle to be dropped again. The balls are weighed regularly. If weight loss exceeds 50%, testing is stopped. For carbon steel formed by forging, the basic specification is at least 60,000 blows. For grinding balls made from high chromium cast iron, testing was stopped after 47,000 impacts, which is an average result. For grinding balls of the same composition, forged according to the invention, the 200,000 blow limit was exceeded without reaching the 50% weight loss criterion.

[0060] Thus, the grinding balls of the invention have excellent wear resistance with toughness properties at least equal to those of conventional forged carbon steels.

Claims (39)

1. Grinding ball (19), containing, by weight: - carbon content 1.1-1.4%, - chromium content 10-14%, - manganese content 0.8-1.5%, - silicon content 0.6-1%, - molybdenum content less than 1%, - nickel content less than 1%, - any impurities with a total content of less than 0.5%, - the rest is up to 100% iron, wherein said grinding ball (19) has a discrete distribution of chromium carbides (5) and has a microstructure with a martensite percentage of more than 50%. 2. Grinding ball (19) according to claim 1, containing, by weight: - carbon content 1.2%, - chromium content 12%, - manganese content 1.1%, - silicon content 0.8%, - molybdenum content less than 1%, - nickel content less than 1%, - any impurities with a total content of less than 0.5%, - the rest is up to 100% iron. 3. Grinding ball (19) according to claim 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. 4. Grinding ball (19) according to any one of claims. 1-3, characterized in that the chromium carbides (5) have an equivalent diameter of less than 100 μm, more preferably less than 50 μm, even more preferably less than 20 μm. 5. Grinding ball (19) according to any one of claims. 1-4, characterized by the fact that it has a microstructure including martensite with a percentage of more than 50%, retained austenite with a percentage of 7-25%, a total fraction of pearlite and bainite with a content of 2-10% and chromium carbides with a percentage of less than or equal to 22%. 6. Grinding ball (19) according to any one of claims. 1-5, characterized by the fact that it has a microstructure including martensite with a percentage of 60-80%, retained austenite with a percentage of 10-20% and a total fraction of pearlite and bainite with a content of 2-10%. 7. Grinding ball (19) according to any one of claims. 1-6, characterized in that it has a Rockwell C hardness of 54 to 64. 8. Grinding ball (19) according to any one of claims. 1-7, characterized in that it has a diameter of 90-150 mm. 9. A method for manufacturing a grinding ball (19) according to any one of claims. 1-8, including the following stages: - production by continuous casting of a rod (12) having the chemical composition according to claim 1 or 2, whereby production by continuous casting makes it possible to obtain a discrete distribution of chromium carbides (5), - forming the rod (12) by deforming it in one or more stages to obtain a workpiece having the shape of a grinding ball (19), - heat treatment of the workpiece in one or several cycles to obtain a grinding ball (19) with a predominantly martensitic microstructure, and the heat treatment stage includes an austenitization cycle at a temperature of 880-1075 ° C for a period of time from 30 minutes to 3 hours, followed by hardening to a temperature of less than 220 °C to transform austenite at least partially into martensite. 10. The method for manufacturing a grinding ball (19) according to claim 9, characterized in that for a rod (12) having a diameter or thickness of more than 85 mm, the solidification grain size at the end of the stage of producing the rod (12) by continuous casting is less than 80 μm in the first 15 millimeters below the surface of the rod (12). 11. A method for manufacturing a grinding ball (19) according to claim 10, characterized in that the solidification grain size is 20-75 μm in the first 15 millimeters below the surface of the rod (12). 12. A method for manufacturing a grinding ball (19) according to claim 11, characterized in that the solidification grain size is 30-70 μm in the first 15 millimeters below the surface of the rod (12). 13. A method for manufacturing a grinding ball (19) according to any one of claims. 9-12, characterized in that continuous casting is carried out at a temperature of 5-40°C, preferably 10-15°C above the solidification temperature. 14. A method for manufacturing a grinding ball (19) according to any one of claims. 9-13, characterized in that the solidification of the rod (12) is initiated in the mold (9), which is at least partially metallic and cooled. 15. A method for manufacturing a grinding ball (19) according to any one of claims. 9-14, characterized in that the solidification of the rod (12) is initiated in the presence of one or more magnetic stirrers (11). 16. A method for manufacturing a grinding ball (19) according to any one of claims. 9-15, characterized in that the forming stage is carried out by rolling and/or forging. 17. A method for grinding rocks in a semi-autogenous grinding mill (1), including the use of a grinding ball (19) according to any one of claims. 1-8.