A steel for small-size steel balls for mine grinding and a method for manufacturing small-size steel balls

Abstract

The application relates to the field of metal materials, and discloses a steel for small-size steel balls for mine grinding and a preparation method of the small-size steel balls. The steel for the steel balls comprises the following components: C: 0.4%-0.9%, Mn: 3.0%-5.0%, Si: 1.4%-1.8%, Cr: 0.8%-1.3%, Ni: 0.3%-0.6%, Mo: 0.2%-0.4%, V: 0.15%-0.25%, Nb: 0.02%-0.06%, the rest is Fe and inevitable impurities, and the contents of Mn, V and Nb need to be controlled in coordination. The small-size steel balls are forged and hot-rolled, and are subjected to non-heat-insulation grading quenching and deep cryogenic treatment. The steel ball components and the preparation process realize low-temperature martensite transformation, obtain fine and uniform martensite structure, and the steel ball has high hardness, high impact toughness and excellent impact fatigue resistance.

Description

Technical Field

[0001] This invention relates to the field of metallic materials, specifically to a steel for small-sized steel balls used in mining grinding and a method for preparing small-sized steel balls. Background Technology

[0002] Grinding steel balls are the most important grinding media consumable in ball mills used in mining. In mineral processing, ball mills typically employ a multi-compartment ball distribution process. Steel balls can be categorized into three main types based on size: large-sized steel balls (usually with a diameter greater than 60mm), medium-sized steel balls (approximately 40-60mm in diameter), and small-sized steel balls (approximately 10-40mm in diameter). Large-sized steel balls are primarily used in the crushing chamber of the ball mill. In this chamber, the material particles are relatively large, typically the feed material freshly processed by a crusher. Large-sized steel balls, with their large mass and strong impact force, mainly rely on impact crushing to break and pulverize coarse materials. Medium-sized steel balls are usually located in the transition chamber or the front section of the fine grinding chamber, playing a bridging role. When the material is crushed to a certain fineness, excessive impact force can lead to energy waste. At this point, medium-sized steel balls, through a combination of impact and grinding, further reduce the material particle size, creating conditions for the efficient grinding of small-sized steel balls. Small steel balls are mainly used in the fine grinding chamber of ball mills, responsible for further grinding the ore to a fine particle size that meets the beneficiation requirements.

[0003] Mining grinding operations involving small-sized steel balls primarily involve low-energy impact and continuous wear. Unlike large-sized steel balls, which are mainly subjected to impact, small-sized steel balls have a large total surface area, resulting in a higher frequency and area of ​​contact with the ore, leading to more significant abrasive wear on their surfaces. To slow down the wear rate of small-sized steel balls, they need to possess extremely high hardness. To withstand repeated low-energy impacts and prevent the balls from cracking, they also need to have impact toughness.

[0004] Currently, the grinding steel balls commonly used in mines are mainly chromium-based wear-resistant steel balls. For example, patent CN104831157A discloses a wear-resistant high-chromium cast ball, whose components by weight percentage include: C: 2-2.2%, Cr: 10-12%, Si: 0.4-0.6%, Mn: 0.5-0.7%, Nd: 0.5-2%, B: 0.1-0.5%, with the remainder being Fe and unavoidable impurities. Although this chromium-based wear-resistant steel ball has improved hardness, high-chromium cast iron balls are prone to carbide segregation and coarse carbides, and are prone to micro-scraping or cracking under impact loads. Moreover, its alloy cost is relatively high, which cannot meet the requirements of high efficiency and low cost for small-sized steel balls in mines.

[0005] Therefore, there is still significant room for improvement in the existing methods for preparing small-sized steel balls for mining grinding. Summary of the Invention

[0006] In view of the above-mentioned problems in the prior art, the main objective of the present invention is to provide steel for small-sized steel balls used in mining grinding and a method for preparing small-sized steel balls.

[0007] According to one aspect of the present invention, a steel for small-sized steel balls used in mining grinding is provided, the steel comprising the following components by mass percentage: C: 0.4%~0.9%, Mn: 3.0%~5.0%, Si: 1.4%~1.8%, Cr: 0.8%~1.3%, Ni: 0.3%~0.6%, Mo: 0.2%~0.4%, V: 0.15%~0.25%, Nb: 0.02%~0.06%, with the balance being Fe and unavoidable impurities, and

[0008] ,

[0009] Wherein, [Mn] is the mass percentage of Mn contained in the steel, [V] is the mass percentage of V contained in the steel, and [Nb] is the mass percentage of Nb contained in the steel.

[0010] According to one embodiment of the present invention, the component comprises the following components by mass percentage: C: 0.5%~0.7%, Mn: 4.0%~4.5%, Si: 1.5%~1.7%, Cr: 0.9%~1.1%, Ni: 0.3%~0.5%, Mo: 0.2%~0.4%, V: 0.17%~0.21%, Nb: 0.03%~0.05%, with the remainder being Fe and unavoidable impurities, the total mass percentage of impurities being less than 0.05%.

[0011] According to another aspect of the present invention, a method for preparing small-sized steel balls for mining grinding is provided, wherein the small-sized steel balls for mining grinding are prepared using steel as described in any of the above embodiments, and the method for preparing the small-sized steel balls for mining grinding includes the following steps:

[0012] The raw materials are formulated based on the target composition of steel for small-sized steel balls used in mining grinding, and the raw materials are smelted by vacuum induction melting and cast into ingots.

[0013] The ingot is heated and forged to obtain a billet.

[0014] The billet is heated, kept at a constant temperature, and hot-rolled into a bar billet with the same diameter as the target steel ball;

[0015] The bar billet is rolled into a steel ball blank with the same diameter as the target steel ball using a ball rolling mill, and the final rolling temperature is controlled above 850°C.

[0016] The steel ball blank is immediately placed in a salt bath furnace to cool to 230~250°C, and then immediately placed in constant temperature water for water cooling to the water temperature. After water cooling, the steel ball blank is placed in a liquid nitrogen environment for a predetermined time for deep cryogenic treatment.

[0017] The steel ball blank, after cryogenic treatment, is heated and tempered.

[0018] According to one embodiment of the present invention, the temperature of the constant temperature water used for water cooling is 25~40°C.

[0019] According to one embodiment of the present invention, the steel ball blank is held in a liquid nitrogen environment for 5 to 35 minutes during the cryogenic treatment.

[0020] According to one embodiment of the present invention, the vacuum degree is controlled to not exceed 5 × 10⁻⁶ during the vacuum induction melting process. -3 Pa, with a melting temperature of 1520~1560℃.

[0021] According to one embodiment of the present invention, heating the ingot and performing billet forging includes heating the ingot to 1060~1120°C and then performing billet forging, with the final forging temperature controlled above 910°C.

[0022] According to one embodiment of the present invention, heating and holding the billet and hot rolling it into a bar billet with the same diameter as the target steel ball includes heating the billet to 1160~1210℃ and holding it for 2~3 hours, hot rolling it into a bar billet in the range of 930~1110℃, and then air cooling it to room temperature.

[0023] According to one embodiment of the present invention, the tempering temperature is 200~300℃ and the holding time is 1~10h.

[0024] According to one embodiment of the present invention, the diameter of the target steel ball is 10~40mm.

[0025] Compared with the prior art, the steel for small-sized steel balls used in mining grinding and the method for preparing small-sized steel balls of the present invention have at least one of the following beneficial effects:

[0026] (1) The steel used for small-sized steel balls for mining grinding in this invention has a carbon content controlled at 0.4%~0.9%, and is combined with 3.0%~5.0% Mn, along with microalloying elements such as Cr, Mo, and Ni, to reduce the martensitic transformation temperature (martensitic transformation initiation temperature M) of the steel. s and the martensitic transformation end temperature M f ) Adjusted to M s =230~250℃, M f=100~120℃, thus achieving low-temperature martensitic transformation. Low-temperature martensitic transformation can obtain a finer and more uniform martensitic structure, significantly improving the hardness, wear resistance and toughness of the steel ball, ensuring that the steel ball has basic impact toughness and is suitable for low-energy impact conditions in mining.

[0027] (2) In this invention, the Mn content is synergistically controlled with the Nb and V content. Mn can reduce the austenitizing temperature and promote the full solid solution of Nb and V in austenite. This allows Nb, V and C to combine to form nanoscale precipitates MC (M is Nb and V) during the graded quenching, deep cooling and low temperature tempering process of steel for small-sized steel balls. This significantly improves the hardness of the steel balls, increases the proportion of high-angle grain boundaries, and hinders the propagation of microcracks caused by repeated low-energy impacts. This makes the steel balls less prone to breakage under low-energy impact conditions in mines, achieving a synergy between ultra-high hardness and basic impact toughness.

[0028] (3) When preparing small steel balls for mining grinding, the present invention has formulated a non-insulated graded quenching process for steel composition. The steel balls after final rolling are directly subjected to non-insulated salt bath furnace quenching and water quenching treatment to achieve low-temperature martensitic transformation. This not only significantly improves the hardness, wear resistance and toughness of the steel balls, but also significantly reduces the risk of cracking during the heat treatment of small steel balls. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 A flowchart illustrating a method for preparing small-sized steel balls for mining grinding according to an embodiment of the present invention is shown. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0032] Furthermore, the reference to "embodiment" herein means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0033] It should be understood that the embodiments of the invention shown in the exemplary embodiments are merely illustrative. Although only a few embodiments have been described in detail in this invention, those skilled in the art will readily recognize that various modifications are possible without substantially departing from the teachings of the invention. Accordingly, all such modifications should be included within the scope of the invention. Other substitutions, modifications, variations, and deletions can be made to the design, operating conditions, and parameters of the following exemplary embodiments without departing from the spirit of the invention.

[0034] According to one aspect of the present invention, a steel for small-sized steel balls used in mining grinding is provided. This steel for small-sized steel balls used in mining grinding comprises the following components by mass percentage: C: 0.4%~0.9%, Mn: 3.0%~5.0%, Si: 1.4%~1.8%, Cr: 0.8%~1.3%, Ni: 0.3%~0.6%, Mo: 0.2%~0.4%, V: 0.15%~0.25%, Nb: 0.02%~0.06%, with the remainder being Fe and unavoidable impurities.

[0035] ,

[0036] Wherein, [Mn] is the mass percentage of Mn contained in the steel, [V] is the mass percentage of V contained in the steel, and [Nb] is the mass percentage of Nb contained in the steel.

[0037] The steel used for small-sized steel balls in mining grinding of the present invention has a carbon content controlled at 0.4%~0.9%, and is combined with 3.0%~5.0% Mn, along with microalloying elements such as Cr, Mo, and Ni, to control the martensitic transformation temperature (martensitic transformation initiation temperature M) of the steel. s and the martensitic transformation end temperature M f ) Adjusted to M s =230~250℃, M f =100~120℃, thus achieving low-temperature martensitic transformation. Low-temperature martensitic transformation can obtain a finer and more uniform martensitic structure, significantly improving the hardness, wear resistance and toughness of steel balls, ensuring that steel balls have basic impact toughness and are suitable for low-energy impact conditions in mining.

[0038] Furthermore, this invention synergistically controls the Mn content in conjunction with the Nb and V contents. Mn lowers the austenitizing temperature and promotes the full dissolution of Nb and V in austenite, allowing Nb, V, and C to combine and form nanoscale precipitates MC (M being Nb and V) (size ≤30nm) during the graded quenching, deep cryogenic, and low-temperature tempering processes of steel for small-sized steel balls. These nanoscale MC precipitates, on the one hand, hinder dislocation movement through the Orovan mechanism, significantly increasing the hardness of the steel ball (up to HRC65 or higher); on the other hand, they effectively refine the original austenite grains (average grain size ≤5.5μm) and carbide particles, increasing the proportion of high-angle grain boundaries and hindering the propagation of microcracks caused by repeated low-energy impacts. This makes the steel ball less prone to fracture under low-energy impact conditions in mines, achieving a synergy between ultra-high hardness and basic impact toughness. Simultaneously, Cr and Fe precipitate together to form composite carbides. The competitive precipitation behavior of Nb, V, and Cr multi-component carbide-forming elements can significantly suppress carbide coarsening, achieve a substantial refinement of the precipitate size, and obtain small-sized, diffusely distributed composite carbide precipitates.

[0039] The steel composition design of the small-sized steel balls for mining grinding in this invention eliminates the need for precious metals such as Co and Ti. Using Mn as the core alloying element, combined with conventional microalloying elements, the alloy cost is reduced by more than 35% compared to high-end wear-resistant steel balls. Simultaneously, the 3.0%~5.0% Mn content significantly improves the hardenability of the steel. Combined with the synergistic effect of Cr and Mo, this ensures that the core and surface of the small-sized steel balls achieve sufficient martensite structure during the non-heat-holding graded quenching process, avoiding uneven hardenability caused by insufficient hardenability, thus meeting the needs of mass production. Testing shows that the wear rate of the steel balls of this invention is reduced by more than 65% compared to traditional mining steel balls, and the service life is extended by more than 1.3 times with the increase in hardness.

[0040] In some embodiments of the present invention, the steel for small-sized steel balls used in mining grinding comprises the following components by mass percentage: C: 0.5%~0.7%, Mn: 4.0%~4.5%, Si: 1.5%~1.7%, Cr: 0.9%~1.1%, Ni: 0.3%~0.5%, Mo: 0.2%~0.4%, V: 0.17%~0.21%, Nb: 0.03%~0.05%, with the remainder being Fe and unavoidable impurities, the total mass percentage of impurities being less than 0.05%.

[0041] According to another aspect of the present invention, a method for preparing small-sized steel balls for mining grinding is provided, wherein the small-sized steel balls to be prepared have the composition described in the above embodiments. Figure 1 As shown, the method for preparing small-sized steel balls for mining grinding includes the following steps:

[0042] Step S1: Based on the target composition of the steel for small-sized steel balls used in mining grinding, the raw materials are batched and smelted using vacuum induction melting, and then cast into ingots;

[0043] Step S2: Heat the ingot and perform forging to obtain a billet;

[0044] Step S3: Heat and hold the billet at the specified temperature and then hot roll it into a bar billet with the same diameter as the target steel ball;

[0045] Step S4: Roll the bar billet into a steel ball blank with the same diameter as the target steel ball using a ball rolling mill, and control the final rolling temperature above 850℃.

[0046] Step S5: Immediately place the steel ball blank into a salt bath furnace to cool to 230~250℃, then immediately place the steel ball blank into constant temperature water for water cooling to the water temperature. After water cooling, place the steel ball blank into a liquid nitrogen environment for a predetermined time for deep cryogenic treatment.

[0047] Step S6: Heat the cryogenically treated steel ball blank for tempering.

[0048] In preparing small-sized steel balls for mining grinding, this invention employs a non-insulated, staged quenching process based on optimized steel composition. The final rolled steel balls are directly subjected to non-insulated salt bath furnace quenching and water quenching. This is because the martensitic transformation temperature is controlled to M0 when designing the steel composition. s =230~250℃, M f =100~120℃, combined with the heat-free graded quenching process of this invention, to achieve low-temperature martensitic transformation. The low-temperature martensitic transformation temperature is lower, the supercooling is greater, and the nucleation driving force is higher, thus making it easier to obtain an ultra-fine microstructure, significantly improving the hardness, wear resistance, toughness and fatigue resistance of the steel ball, ensuring that the steel ball has basic impact toughness, and is suitable for low-energy impact conditions in mining.

[0049] Moreover, the low-temperature martensitic transformation temperature is lower, and compared with the conventional martensitic transformation, its martensitic phase transformation volume expansion is dispersed in M s To M f The transformation is completed gradually over a wide temperature range of approximately 130°C, rather than being concentrated in a narrow temperature range. At the same time, due to the lower transformation temperature, the phase transformation stress can be significantly reduced. For small-sized steel balls, this effectively avoids the risk of cracking caused by high tensile stress in the core due to the internal and external transformation.

[0050] The following provides a detailed example of each step.

[0051] In step S1, the raw materials are batched based on the target composition of the steel used for small-sized steel balls in mining grinding, and the raw materials are smelted by vacuum induction melting and cast into ingots.

[0052] In some embodiments of the present invention, the target composition of the steel for the small-sized steel balls used in mining grinding includes the following components by mass percentage: C: 0.4%~0.9%, Mn: 3.0%~5.0%, Si: 1.4%~1.8%, Cr: 0.8%~1.3%, Ni: 0.3%~0.6%, Mo: 0.2%~0.4%, V: 0.15%~0.25%, Nb: 0.02%~0.06%, with the remainder being Fe and unavoidable impurities.

[0053] ,

[0054] Wherein, [Mn] is the mass percentage of Mn contained in the steel, [V] is the mass percentage of V contained in the steel, and [Nb] is the mass percentage of Nb contained in the steel.

[0055] As mentioned above, using this composition can control the martensitic transformation temperature of the steel to M. s =230~250℃, M f =100~120℃, thus achieving low-temperature martensitic transformation. By synergistically controlling the Mn content with the Nb and V contents, Nb, V, and C combine to form nanoscale precipitates MC (M = Nb and V) (size ≤30nm) during staged quenching, deep cryogenic treatment, and low-temperature tempering of steel for small-sized steel balls. If the Mn content is not synergistically controlled with the Nb and V contents, when the relative Mn content is too low, Nb and V cannot be fully dissolved during direct quenching with residual heat from hot rolling. The undissolved Nb and V will exist in the austenite as coarse, undissolved carbides. These coarse carbides not only fail to provide effective precipitation strengthening but also become crack initiation sites, significantly reducing the precipitation strengthening effect. When the relative Mn content is too high, it leads to an increase in residual austenite and an insufficient number of precipitates MC. A lack of sufficient precipitates reduces the precipitation strengthening effect, resulting in decreased hardness and wear resistance.

[0056] In some embodiments of the present invention, the target composition of the steel for the small-sized steel balls used in mining grinding includes the following components by mass percentage: C: 0.5%~0.7%, Mn: 4.0%~4.5%, Si: 1.5%~1.7%, Cr: 0.9%~1.1%, Ni: 0.3%~0.5%, Mo: 0.2%~0.4%, V: 0.17%~0.21%, Nb: 0.03%~0.05%, with the remainder being Fe and unavoidable impurities, the total mass percentage of impurities being less than 0.05%.

[0057] In some embodiments of the present invention, the vacuum level is controlled to not exceed 5 × 10⁻⁶ during vacuum induction melting. -3The melting temperature is 1520~1560℃, and the casting temperature is 30~50℃ above the liquidus line. This vacuum environment can effectively reduce the introduction of impurities, ensure compositional uniformity, and lay the foundation for low-temperature martensitic transformation.

[0058] In step S2, the ingot is heated and forged to obtain a billet.

[0059] In some embodiments of the present invention, the ingot is heated to 1060~1120°C and then subjected to billet forging, with the final forging temperature controlled above 910°C. After forging, air cooling is performed to refine the microstructure, eliminate casting defects, and improve the density of the billet.

[0060] In step S3, the billet is heated, kept at a constant temperature, and hot-rolled into a bar billet with the same diameter as the target steel ball.

[0061] In some embodiments of the present invention, the billet is heated to 1160~1210℃ and held for 2~3 hours, then hot-rolled into a bar billet in the range of 930~1110℃, and then air-cooled to room temperature to obtain a uniform hot-rolled structure, which is suitable for subsequent rolling into balls.

[0062] In some embodiments of the present invention, the diameter of the target steel ball is 10~40mm, and the billet is hot-rolled into a Φ10~40mm bar billet.

[0063] In step S4, the bar billet is rolled into a steel ball blank with the same diameter as the target steel ball using a ball rolling equipment, and the final rolling temperature is controlled above 850°C.

[0064] In some embodiments of this invention, a dedicated ball rolling mill is used to directly roll Φ10~40mm bar billets into small-sized steel ball blanks of the corresponding diameter, ensuring that the diameter of the steel ball is consistent with the diameter of the bar billet. The final rolling temperature is ≥850℃, and the ball directly enters the quenching process after hot rolling, reducing energy consumption. If the final rolling temperature is below 850℃, insufficient Nb solid solution will form coarse undissolved carbides, which not only fail to provide precipitation strengthening but may also become crack initiation sources, leading to a decrease in hardness. At the same time, controlling the final rolling temperature above 850℃ provides the necessary cooling space for subsequent direct quenching without heat preservation. Since there is a temperature drop during the 5~8s transfer process from the mill to the salt bath furnace, when the final rolling temperature is above 850℃, even after the temperature drop, the temperature entering the salt bath furnace can still ensure that the steel ball enters the quenching process in a fully austenitic state. However, if the final rolling temperature is below 850℃, the actual temperature of the steel ball blank entering the salt bath furnace may fall into the "ferrite" or "pearlite" transformation zone, resulting in the appearance of pre-transformation products, affecting the final hardness.

[0065] In step S5, the steel ball blank is immediately placed in a salt bath furnace to cool to 230~250°C, and then the steel ball is immediately placed in constant temperature water for water cooling to the water temperature. After water cooling, the steel ball is placed in a liquid nitrogen environment for a predetermined time for cryogenic treatment.

[0066] In some embodiments of the present invention, "immediately placing the steel ball blank into the salt bath furnace" means placing the steel ball blank into the salt bath furnace within 5 to 10 seconds. After the steel ball blank is placed into the salt bath furnace, the surface of the steel ball is rapidly cooled to the martensitic transformation initiation temperature M. s The following phase transformation occurs, resulting in a high-hardness, wear-resistant, ultra-fine martensite structure on the surface. Due to the relatively mild cooling rate of the salt bath furnace, the core temperature lags behind the surface temperature. Even when the surface is fully martensitized, the core temperature remains at M... s Points or above or M s -M f During this period, martensitic transformation has not yet occurred, or a small amount of martensitic transformation may begin in some areas. At final rolling above 850℃, Nb and V are fully dissolved in austenite. During salt bath quenching, due to the rapid cooling rate, carbides do not have time to precipitate, and Nb and V atoms remain in a solid solution state within the supercooled austenite and the transformed martensitic matrix. The microstructure of the steel ball blank consists of martensite and retained austenite. At this stage, the retained austenite, as a toughening phase, effectively absorbs and buffers the volumetric expansion stress generated by the surface martensitic transformation, preventing stress concentration and accumulation in the core, and significantly reducing the risk of microcracks or even cracking due to excessive phase transformation stress during quenching.

[0067] In some embodiments of the present invention, water cooling of the steel ball blank cooled to 230-250°C by immediately immersing it in constant-temperature water refers to immersing the steel ball blank in constant-temperature water within 5-10 seconds, with the temperature of the constant-temperature water being 25-40°C. During water quenching, the surface layer before water quenching is already fully martensite. During the rapid cooling process of water quenching, the surface structure is basically stable, while the untransformed austenite in the core also transforms into martensite. The rapid cooling during the water quenching stage inhibits the precipitation of carbides, and Nb and V atoms remain mainly dissolved in the martensitic matrix. At the same time, the high-density dislocations generated by the martensitic phase transformation during water quenching further increase, providing richer nucleation sites for the formation of nano-precipitates in the subsequent deep cryogenic and tempering stages. Cooling in a salt bath furnace causes the surface to transform and expand first, applying compressive stress to the core, and then water cooling causes the core to transform later. During the transformation, the core is constrained by the outer shell, reducing the risk of steel ball cracking.

[0068] In some embodiments of the present invention, after water cooling, the steel ball is placed in a liquid nitrogen environment and kept at a predetermined temperature for a predetermined time for cryogenic treatment. The steel ball blank is kept in the liquid nitrogen environment for 5 to 35 minutes, preferably 25 to 35 minutes. Immediate deep cooling after water quenching ensures that the unstable microstructure (such as austenite) that was not transformed in the first two quenching processes continues to transform into martensite, further increasing hardness. Simultaneously, it stabilizes the final untransformed residual austenite, improving toughness, resulting in a final martensite volume percentage greater than 98%.

[0069] By controlling the cooling rate through staged quenching, thermal stress was reduced; simultaneously, the volumetric expansion stress of the martensitic transformation was balanced, solving the cracking problem commonly seen in ultra-high hardness steel balls. Deep cryogenic treatment controlled the stability of the retained austenite, making the final untransformed austenite very stable and improving toughness. At the same time, the number of dislocation defects in the martensite increased, providing nucleation sites for the formation of tempering precipitates and laying the foundation for further precipitation strengthening.

[0070] Furthermore, the heat treatment process without heat preservation in this invention eliminates the heat preservation step in the heat preservation stage, which not only shortens the heat treatment cycle but also avoids the austenite stabilization caused by heat preservation, ensuring that both the core and the surface layer are fully transformed into martensite with a martensite transformation rate of ≥98% and significantly improving the uniformity of the microstructure.

[0071] In step S6, the cryogenically treated steel ball is heated and tempered.

[0072] In some embodiments of the present invention, the tempering temperature is 200-300°C, and the holding time is 1-10 hours. During tempering at 200-300°C, Nb and V atoms dissolved in the martensitic matrix combine with carbon atoms to precipitate dispersed nanoscale MC carbides (V, Nb composite carbides) and Cr, Fe composite precipitated carbides. The competitive precipitation behavior of the multi-component carbide-forming elements Nb, V, and Cr can significantly inhibit carbide coarsening and achieve a substantial refinement of the precipitated phase size. These nanoscale precipitates can produce a significant precipitation strengthening effect, further improving the hardness of the steel ball. During tempering, a small amount of untransformed retained austenite transforms into stable nanofilm-like retained austenite, which can effectively passivate microcrack tips, improve toughness and fatigue resistance, without significantly reducing hardness, thus achieving a synergistic effect of hardness and toughness. If the tempering temperature is too low, the nano-precipitates will not precipitate sufficiently, resulting in insufficient strengthening effect; while if the temperature is too high, the softening of martensite and the coarsening of the nano-precipitates will also lead to a decrease in hardness.

[0073] Optionally, in some embodiments of the present invention, after tempering, the steel ball blanks are finished and screened. Internal defects can be detected by eddy current testing, and the roundness of the steel balls (≤0.001mm) can be detected by a roundness meter to screen out qualified products, ensuring the consistency of batch supply and adapting to the grinding needs of ball mills and rod mills.

[0074] The method of the present invention will be further described and illustrated below with reference to embodiments.

[0075] Example 1

[0076] The target composition of the small steel balls for mining grinding prepared in this embodiment is: C: 0.6%, Mn: 4.3%, Si: 1.6%, Cr: 1.0%, Ni: 0.4%, Mo: 0.3%, V: 0.2%, Nb: 0.04%, with the remainder being Fe and unavoidable impurities, and the target diameter is 25 mm.

[0077] The raw materials were prepared according to the target composition of the steel balls and smelted using vacuum induction melting. During vacuum induction melting, the vacuum degree was controlled to not exceed 5 × 10⁻⁶. -3 The steel, with a melting temperature of 1540℃, is refined and alloyed before being tapped at an adjusted temperature and cast into ingots. The ingots are then heated to 1090℃ for forging, with the final forging temperature controlled above 910℃ to obtain billets. These billets are heated to 1185℃ and held for 2.5 hours, then hot-rolled into bars between 980 and 1020℃, and subsequently air-cooled to room temperature. The bars are then heated to 980℃ and rolled into 25mm diameter steel ball blanks using a ball rolling mill, with the final rolling temperature controlled above 850℃. The steel ball blanks are immediately placed in a salt bath furnace to cool to 240℃, then immediately immersed in 30℃ constant-temperature water for water cooling, and finally placed in a liquid nitrogen environment (-196℃) for 16 minutes. Then, the steel ball blank is heated to 250℃ and held for 5 hours for tempering treatment. The steel ball blank is then finished and screened. Internal defects can be detected by eddy current testing, and the roundness of the steel ball can be detected by a roundness meter (≤0.001mm) to screen out qualified products.

[0078] Example 2

[0079] The target composition of the small steel balls for mining grinding prepared in this embodiment is: C: 0.4%, Mn: 3.0%, Si: 1.4%, Cr: 0.8%, Ni: 0.3%, Mo: 0.2%, V: 0.15%, Nb: 0.02%, with the remainder being Fe and unavoidable impurities, and the target diameter is 10 mm.

[0080] The raw materials were prepared according to the target composition of the steel balls and smelted using vacuum induction melting. During vacuum induction melting, the vacuum degree was controlled to not exceed 5 × 10⁻⁶. -3The steel, with a melting temperature of 1520℃, is refined and alloyed before being tapped at an adjusted temperature and cast into ingots. The ingots are then heated to 1060℃ for forging, with the final forging temperature controlled above 910℃ to obtain billets. These billets are heated to 1160℃ and held for 3 hours, then hot-rolled into bars between 930 and 1000℃, and then air-cooled to room temperature. The bars are heated to 950℃ and rolled into 10mm diameter steel ball blanks using a ball rolling mill, with the final rolling temperature controlled above 850℃. The steel ball blanks are immediately placed in a salt bath furnace to cool to 250℃, then immediately immersed in 40℃ constant-temperature water for water cooling, and finally placed in a liquid nitrogen environment for 5 minutes. Then, the steel ball blank is heated to 280℃ and held for 1 hour for tempering treatment. The steel ball blank is then finished and screened. Internal defects can be detected by eddy current testing, and the roundness of the steel ball can be detected by a roundness meter (≤0.001mm) to screen out qualified products.

[0081] Example 3

[0082] The target composition of the small-sized steel balls for mining grinding prepared in this embodiment is: C: 0.5%, Mn: 4.0%, Si: 1.5%, Cr: 0.9%, Ni: 0.3%, Mo: 0.2%, V: 0.17%, Nb: 0.03%, with the remainder being Fe and unavoidable impurities. The target diameter is 20 mm.

[0083] The raw materials were prepared according to the target composition of the steel balls and smelted using vacuum induction melting. During vacuum induction melting, the vacuum degree was controlled to not exceed 5 × 10⁻⁶. -3 The steel, with a melting temperature of 1530℃, is refined and alloyed before being tapped at an adjusted temperature and cast into ingots. The ingots are heated to 1080℃ for forging, with the final forging temperature controlled above 910℃ to obtain billets. The billets are heated to 1170℃ and held for 2.5 hours, then hot-rolled into bars between 950 and 1000℃, and then air-cooled to room temperature. The bars are heated to 980℃ and rolled into 20mm diameter steel ball blanks using a ball rolling mill, with the final rolling temperature controlled above 850℃. The steel ball blanks are immediately placed in a salt bath furnace to cool to 240℃, then immediately immersed in 30℃ constant-temperature water for water cooling, and finally placed in a liquid nitrogen environment for 12 minutes. Then, the steel ball blank is heated to 230℃ and held for 8 hours for tempering treatment. The steel ball blank is then finished and screened. Internal defects can be detected by eddy current testing, and the roundness of the steel ball can be detected by a roundness meter (≤0.001mm) to screen out qualified products.

[0084] Example 4

[0085] The target composition of the small-sized steel balls for mining grinding prepared in this embodiment is: C: 0.7%, Mn: 4.5%, Si: 1.7%, Cr: 1.1%, Ni: 0.5%, Mo: 0.4%, V: 0.21%, Nb: 0.05%, with the remainder being Fe and unavoidable impurities. The target diameter is 30 mm.

[0086] The raw materials were prepared according to the target composition of the steel balls and smelted using vacuum induction melting. During vacuum induction melting, the vacuum degree was controlled to not exceed 5 × 10⁻⁶. -3 The steel, with a melting temperature of 1550℃, is refined and alloyed before being tapped at an adjusted temperature and cast into ingots. The ingots are then heated to 1100℃ for forging, with the final forging temperature controlled above 910℃ to obtain billets. These billets are heated to 1200℃ and held for 2.5 hours, then hot-rolled into bars between 950 and 1110℃, and air-cooled to room temperature. The bars are then rolled into 30mm diameter steel ball blanks using a ball rolling mill, with the final rolling temperature controlled above 850℃. The steel ball blanks are immediately placed in a salt bath furnace to cool to 240℃, then immediately immersed in 35℃ constant-temperature water for water cooling, and finally placed in a liquid nitrogen environment for 25 minutes. Then, the steel ball blank is heated to 200℃ and held for 10 hours for tempering treatment. The steel ball blank is then finished and screened. Internal defects can be detected by eddy current testing, and the roundness of the steel ball can be detected by a roundness meter (≤0.001mm) to screen out qualified products.

[0087] Example 5

[0088] The target composition of the small-sized steel balls for mining grinding prepared in this embodiment is: C: 0.9%, Mn: 5.0%, Si: 1.8%, Cr: 1.3%, Ni: 0.6%, Mo: 0.4%, V: 0.25%, Nb: 0.06%, with the remainder being Fe and unavoidable impurities. The target diameter is 40 mm.

[0089] The raw materials were prepared according to the target composition of the steel balls and smelted using vacuum induction melting. During vacuum induction melting, the vacuum degree was controlled to not exceed 5 × 10⁻⁶. -3The steel, with a melting temperature of 1560℃, is refined and alloyed before being tapped at an adjusted temperature and cast into ingots. The ingots are then heated to 1120℃ for forging, with the final forging temperature controlled above 910℃ to obtain billets. These billets are heated to 1210℃ and held for 3 hours, then hot-rolled into bars between 950 and 1110℃, and then air-cooled to room temperature. The bars are then rolled into 40mm diameter steel ball blanks using a ball rolling mill, with the final rolling temperature controlled above 850℃. The steel ball blanks are immediately placed in a salt bath furnace to cool to 230℃, then immediately immersed in 25℃ constant-temperature water for water cooling, and finally immediately placed in a liquid nitrogen environment for 35 minutes. Then, the steel ball blank is heated to 300℃ and held for 3 hours for tempering treatment. The steel ball blank is then finished and screened. Internal defects can be detected by eddy current testing, and the roundness of the steel ball can be detected by a roundness meter (≤0.001mm) to screen out qualified products.

[0090] Comparative Example 1

[0091] The diameter and operation of the small steel balls for mining grinding prepared in Comparative Example 1 are basically the same as those in Example 1. The main difference between the two is the different C content. The steel balls prepared in Comparative Example 1 have a C mass percentage of 2.1%.

[0092] Comparative Example 2

[0093] The diameter and operation of the small steel balls for mining grinding prepared in Comparative Example 2 are basically the same as those in Example 1. The main difference between the two is the Mn content. The steel balls prepared in Comparative Example 2 have an Mn mass percentage of 0.8%.

[0094] Comparative Example 3

[0095] The diameter and operation of the small steel balls for mining grinding prepared in Comparative Example 3 are basically the same as those in Example 1. The main difference between the two is the Mn content. The steel balls prepared in Comparative Example 3 have an Mn mass percentage of 8%.

[0096] Comparative Example 4

[0097] The diameter and operation of the small steel balls for mining grinding prepared in Comparative Example 4 are basically the same as those in Example 1. The main difference is that the steel balls prepared in Comparative Example 4 do not contain V and Nb.

[0098] Comparative Example 5

[0099] The diameter and composition of the small steel balls for mining grinding prepared in Comparative Example 5 are basically the same as those in Example 1. The main difference is that after hot rolling into steel ball blanks, Comparative Example 5 cools the steel ball blanks to 150°C in a salt bath furnace, then closes the salt bath furnace and cools them to room temperature in the furnace.

[0100] Comparative Example 6

[0101] The diameter and composition of the small steel balls for mining grinding prepared in Comparative Example 6 are basically the same as those in Example 1. The main difference is that after hot rolling into steel ball blanks, Comparative Example 6 uses 30°C constant temperature water to cool the steel ball blanks to room temperature.

[0102] Comparative Example 7

[0103] The diameter and composition of the small steel balls for mining grinding prepared in Comparative Example 7 are basically the same as those in Example 1. The main difference between the two is that Comparative Example 7 was not subjected to cryogenic treatment.

[0104] The properties of the steel balls prepared in Examples 1-5 and Comparative Examples 1-7 were tested, and the results are shown in Table 1 below:

[0105] Table 1. Test Results of Steel Ball Performance

[0106]

[0107] As can be seen from the data in Table 1, Examples 1-5 of the present invention all achieved excellent comprehensive performance, with high hardness, impact toughness and impact fatigue life, and no cracking during heat treatment, which verifies the effectiveness of the composition design and the heat-free graded quenching process of the present invention.

[0108] The steel ball in Comparative Example 1 has a high carbon content. Although the excessive carbon content increases the hardness of the steel ball, it also causes the martensitic structure to become brittle, resulting in a sharp decrease in impact toughness. During the quenching process, the large phase transformation stress and structural stress cannot be buffered by the tough structure, leading to cracking of the steel ball and a low impact fatigue life, which fails to meet the basic requirements for impact resistance of steel balls in mining grinding.

[0109] The steel ball in Comparative Example 2 had a low manganese content, which reduced the hardenability of the steel. This resulted in insufficient martensite during the staged quenching process, leading to a significant decrease in hardness and wear resistance. Although no cracking occurred, the insufficient hardness caused severe wear under grinding conditions, resulting in a short service life.

[0110] The steel ball in Comparative Example 3 had a high manganese content. While excessive manganese stabilized the austenite, it resulted in an excessively low martensite transformation temperature. After quenching, a large amount of retained austenite remained, leading to insufficient hardness. Simultaneously, the excessive soft-phase retained austenite also reduced the overall strength of the material, with impact toughness and fatigue life inferior to Examples 1-5. This indicates that a higher manganese content is not always better; it must be controlled in conjunction with V and Nb to achieve the desired strengthening effect.

[0111] The steel ball in Comparative Example 4 did not contain the microalloying elements V and Nb. Due to the lack of V and Nb, nano-sized MC carbides could not precipitate during the subsequent tempering process, resulting in a lack of precipitation strengthening effect. Therefore, its hardness and fatigue life were lower than those of Examples 1-5. This demonstrates the crucial role of adding V and Nb in further improving the hardness and fatigue resistance of the steel ball through precipitation strengthening.

[0112] Comparative Example 5 employed a salt bath furnace cooling and quenching process. Due to the relatively mild cooling rate of the salt bath furnace, the core of the steel ball could not be fully quenched, resulting in the formation of non-martensitic structures and poor microstructure uniformity. This led to significantly lower hardness and impact toughness compared to Example 1, as well as a significantly lower fatigue life, indicating that single salt bath furnace cooling cannot fully realize the performance potential of this composition system.

[0113] Comparative Example 6 employed a single water-cooling quenching process. Although water cooling achieved a hardness comparable to the examples, the excessively rapid cooling rate generated enormous thermal and structural stresses, leading to high tensile stress in the core and causing cracking. Despite its high hardness, the cracked steel ball was completely unusable.

[0114] The steel ball in Comparative Example 7 was not subjected to deep cryogenic treatment after quenching. Due to the lack of deep cryogenic treatment, unstable residual austenite remained in the microstructure after quenching, resulting in a final hardness slightly lower than that of Example 1. More importantly, these unstable austenite are prone to stress-induced martensitic transformation during impact fatigue testing, leading to localized stress concentration and early failure, significantly reducing its fatigue life. Simultaneously, the lack of deep cryogenic treatment reduced the nucleation sites for subsequent tempering, affecting the strengthening effect of the nanoprecipitates.

[0115] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the disclosed embodiments of the present invention is limited to these examples; within the framework of the embodiments of the present invention, the technical features of the above embodiments or different embodiments can also be combined, and there are many other variations of different aspects of the embodiments of the present invention as described above, which are not provided in detail for the sake of brevity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the embodiments of the present invention should be included within the protection scope of the embodiments of the present invention.

Claims

1. A method for preparing small-sized steel balls for mining grinding, characterized in that, The method for preparing small-sized steel balls for mining grinding includes the following steps: The raw materials are formulated based on the target composition of steel for small-sized steel balls used in mining grinding, and the raw materials are smelted by vacuum induction melting and cast into ingots. The ingot is heated and forged to obtain a billet. The billet is heated, kept at a constant temperature, and hot-rolled into a bar billet with the same diameter as the target steel ball; The bar billet is rolled into a steel ball blank with the same diameter as the target steel ball using a ball rolling mill, and the final rolling temperature is controlled above 850°C. The steel ball blank is immediately placed in a salt bath furnace to cool to 230~250°C, and then immediately placed in constant temperature water for water cooling to the water temperature. After water cooling, the steel ball blank is placed in a liquid nitrogen environment for a predetermined time for deep cryogenic treatment. The steel ball blank, after cryogenic treatment, is heated and tempered. The target steel ball has a diameter of 10-40 mm, and the steel used for the small-sized steel balls for mining grinding comprises the following components by mass percentage: C: 0.4%-0.9%, Mn: 3.0%-5.0%, Si: 1.4%-1.8%, Cr: 0.8%-1.3%, Ni: 0.3%-0.6%, Mo: 0.2%-0.4%, V: 0.15%-0.25%, Nb: 0.02%-0.06%, with the remainder being Fe and unavoidable impurities. ， Wherein, [Mn] is the mass percentage of Mn contained in the steel, [V] is the mass percentage of V contained in the steel, and [Nb] is the mass percentage of Nb contained in the steel.

2. The method for preparing small-sized steel balls for mining grinding according to claim 1, characterized in that, The temperature of the constant-temperature water used for water cooling is 25~40℃.

3. The method for preparing small-sized steel balls for mining grinding according to claim 1, characterized in that, During the cryogenic treatment, the steel ball blank is kept in a liquid nitrogen environment for 5 to 35 minutes.

4. The method for preparing small-sized steel balls for mining grinding according to claim 1, characterized in that, During the vacuum induction melting process, the vacuum level is controlled to not exceed 5 × 10⁻⁶. -3 Pa, with a melting temperature of 1520~1560℃.

5. The method for preparing small-sized steel balls for mining grinding according to claim 1, characterized in that, Heating the ingot and performing forging includes heating the ingot to 1060~1120℃ and then performing forging, with the final forging temperature controlled above 910℃.

6. The method for preparing small-sized steel balls for mining grinding according to claim 1, characterized in that, The process of heating and holding the billet and hot rolling it into a bar billet with the same diameter as the target steel ball includes heating the billet to 1160~1210℃ and holding it for 2~3 hours, hot rolling it into a bar billet in the range of 930~1110℃, and then air cooling it to room temperature.

7. The method for preparing small-sized steel balls for mining grinding according to claim 1, characterized in that, The tempering treatment is performed at a temperature of 200~300℃ for 1~10 hours.

8. The method for preparing small-sized steel balls for mining grinding according to claim 1, characterized in that, The steel used for small-sized steel balls in mining grinding comprises the following components by mass percentage: C: 0.5%~0.7%, Mn: 4.0%~4.5%, Si: 1.5%~1.7%, Cr: 0.9%~1.1%, Ni: 0.3%~0.5%, Mo: 0.2%~0.4%, V: 0.17%~0.21%, Nb: 0.03%~0.05%, with the remainder being Fe and unavoidable impurities, the total mass percentage of impurities being less than 0.05%.

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