An ultra-high hardenability low segregation high toughness large-size s550m grinding ball steel and a preparation method thereof

Abstract

The application relates to a super-high hardenability low-segregation high-toughness large-size S550M mill ball steel and a preparation method thereof, and belongs to the technical field of metallurgy and materials. The mill ball steel chemical composition comprises C: 0.86%-0.90%, Si: 0.23%-0.30%, Mn: 0.92%-0.98%, Cr: 1.08%-1.13%, Mo: 0.04%-0.06%, Al: 0.020%-0.030%, Ti<=0.010%, Nb: 0.010%-0.020%, Ca: 0.0005%-0.0020%, P<=0.020%, S<=0.008%, N: 0.0050%-0.0080%, O<=0.0015%, H<=0.00015%, and the balance is Fe and inevitable impurities. The mill ball steel hardenability, segregation degree and toughness are synergistically improved through component and process design.

Description

Technical Field

[0001] This invention relates to a large-size S550M grinding ball steel with ultra-high hardenability, low segregation, and high toughness, and its preparation method, belonging to the field of metallurgy and materials technology. Background Technology

[0002] Grinding balls are core consumable components in grinding operations in industries such as mining, cement, and power. Their performance directly affects grinding efficiency and equipment operating costs. With the expansion of mineral resource mining and the increasing size of equipment, the performance requirements for grinding ball steel are becoming increasingly stringent. S550M is a high-end grinding ball steel grade, requiring a hardness of up to 550 HBW after heat treatment, and possessing excellent hardenability, wear resistance, and impact fatigue resistance. It is particularly suitable for manufacturing large-diameter (Φ80 mm and above) grinding balls.

[0003] In the existing technology, the research and development of high-carbon chromium grinding ball steel has made some progress, but there are still limitations. Patent CN110846557A discloses high-carbon chromium grinding ball steel and its preparation method. It reduces carbon segregation and bendability by optimizing the content of Cr, N, Al, and Ti and controlling the process. However, its design with an upper limit of 1.10% for Cr content and a Di value requirement of ≥5.25 is difficult to meet the requirements of larger-sized grinding balls for ultimate hardenability, and it does not address the core toughness guarantee under large-sized conditions. Patent CN117107153A discloses a high hardenability, high hardness boron-titanium microalloyed grinding ball steel and its production method. It uses high carbon (C: 0.92%-1.00%) and adds boron and titanium microalloying to improve hardenability. However, excessively high carbon content will increase the risk of carbide inhomogeneity and brittleness, which is not conducive to impact toughness. Moreover, its process does not effectively solve the problem of center segregation in large-section continuous casting billets. Patent CN119592866A discloses a 390 mm × 510 mm cross-section large square billet B3 grinding ball steel and its preparation method. It primarily focuses on the cleanliness and internal quality of the large square billet B3 steel, but its composition (C content approximately 0.60%) and performance targets differ from S550M. Patent CN120272808A discloses a method for producing fine-grained high-carbon grinding ball steel, proposing to refine the grains by controlling Al, Mn, Cr, and the final rolling temperature, but it does not address the synergistic optimization design of hardenability and segregation specific to the high-carbon, high-chromium system of S550M.

[0004] In these existing technologies and practical production applications, S550M steel generally suffers from the following technical defects: First, to achieve high hardenability, large-diameter round bars must obtain high and uniform hardness from the surface to the core during subsequent heat treatment. However, traditional composition design (e.g., Cr ≤ 1.10%) may result in insufficient Di value margin, leading to a decrease in the core hardness of large spheres. Second, high-carbon high-alloy steel is extremely prone to severe central carbon segregation during continuous casting solidification, with a segregation ratio (C0 / C0) of [missing value]. 80First, it is difficult to keep the hardness at a low level (e.g., ≤1.08), which affects the uniformity of hardness and the isotropy of the material. Second, while pursuing high hardness and high hardenability, the impact toughness of the material is often sacrificed, resulting in insufficient anti-breakage ability of the grinding balls during use.

[0005] Therefore, there is an urgent need to develop a new type of S550M grinding ball steel that, while meeting the requirements of ultra-high hardness, also has higher hardenability reserve (higher Di value), lower carbon segregation level, and better impact toughness, especially suitable for the stable production of large-size products with a diameter of Φ80mm and above. Summary of the Invention

[0006] To address the aforementioned shortcomings of existing S550M grinding ball steel technology, this invention provides a large-size S550M grinding ball steel with ultra-high hardenability, low segregation, and high toughness, along with its preparation method. This invention achieves a synergistic improvement in hardenability, segregation, and toughness of the grinding ball steel through unique composition design and refined process control throughout the entire process.

[0007] A large-size S550M grinding ball steel with ultra-high hardenability, low segregation, and high toughness, comprising, by weight percentage: C: 0.86%~0.90%, Si: 0.23%~0.30%, Mn: 0.92%~0.98%, Cr: 1.08%~1.13%, Mo: 0.04%~0.06%, Al: 0.020%~0.030%, Ti≤0.010%, Nb: 0.010%~0.020%, Ca: 0.0005%~0.0020%, P≤0.020%, S≤0.008%, N: 0.0050%~0.0080%, O≤0.0015%, H≤0.00015%, with the balance being Fe and unavoidable impurities.

[0008] In the above technical solution, the critical diameter Di value of the hardenability index of the ball steel is [0.062 + 0.409 × (%C) - 0.135 (%C) × (%C)] × [1 + 3.333 × (%Mn)] × [1 + 0.7 × (%Si)] × [1 + 2.16 × (%Cr)] × [1 + 3 × (%Mo)] × [1 + 0.363 × (%Ni)] × [1 + 0.365 × (%Cu)] ≥ 5.75; the ratio of carbon content at the center of the same cross-section of the round steel to carbon content at a radius of 80% from the center is ≤ 1.08.

[0009] Furthermore, the critical diameter Di value of the hardenability index of the ball steel is 5.75~6.40.

[0010] Furthermore, the carbon content (C0) at the center of the same cross-section of the round steel and the carbon content (C0) at a radius of 80% from the center are... 80 The ratio C0 / C80 The value is 1.02 to 1.08.

[0011] In the above technical solution, the specifications of the grinding ball steel are Φ80~Φ120 mm.

[0012] In the above technical solution, the austenitic grain size of the grinding ball steel is grade 7.0 to 8.0.

[0013] In the above technical solution, the microstructure of the ball steel consists of pearlite and a small amount of network cementite, wherein pearlite is the main matrix structure, accounting for 88% to 92% of the total structure, and network cementite accounts for 8% to 12% of the total structure.

[0014] In the above technical solution, the center point hardness of the grinding ball steel is 374~382 HBW, the half radius hardness is 380~390 HBW, and the difference between the two is 6~8 HBW; the core impact energy at room temperature is 10~14 J.

[0015] Within the original composition framework of S550M, this invention optimizes the Cr content to the range of 1.08%~1.13% and adds 0.04%~0.06% Mo, which significantly improves the hardenability factor and increases the Di value to ≥5.75. This provides sufficient assurance for the core hardness of ultra-large grinding balls.

[0016] This invention innovatively introduces 0.010%~0.020% Nb, the precipitated Nb(CN) can effectively pin the austenite grain boundaries, inhibit grain growth during heating and rolling, and help refine the final austenite grains (7.0~8.0 grade), thereby improving toughness while increasing strength.

[0017] This invention uses Ti (≤0.010%) to fix free nitrogen, protecting the grain refinement effect of AlN, and transforms non-deformable inclusions such as Al2O3 and MnS into low-melting-point calcium aluminates through Ca treatment (0.0005%~0.0020%), thereby achieving inclusion morphology control and significantly improving the fatigue life and impact toughness of steel.

[0018] Another objective of this invention is to provide a method for preparing the aforementioned large-size S550M grinding ball steel with ultra-high hardenability, low segregation, and high toughness. The method includes smelting and refining, continuous casting, billet placement and holding in the furnace or hot-pressing the billet, heated rolling, and slow cooling after rolling. In the continuous casting step, the billet cross-section is ≥300 mm × 300 mm; the superheat of the molten steel is controlled at 15~25℃; a combined process of electromagnetic stirring in the crystallizer and electromagnetic stirring at the end of solidification is employed, with the crystallizer electromagnetic stirring parameters being 400~500 A / 2 Hz and the solidification end electromagnetic stirring parameters being 520~650 A / 7 Hz. The central solid fraction fs corresponding to the light reduction range at the end of solidification is 0.3~0.7, and the total reduction is 10~40 mm. In the heated rolling step, the billet is slowly heated to 1190~1230℃ and held for 2~4 h; after exiting the furnace, it is air-cooled for 1~3 hours. The first stage of rolling is at a temperature of 1000~1080℃. After rolling, the temperature is air-cooled again for 1~3 minutes. The second stage of rolling is at a temperature of 900~1050℃, and the final rolling temperature is 800~900℃.

[0019] Furthermore, in the heating and rolling step, the slow heating process is as follows: in the preheating section, the temperature is increased to 600-650℃ at a rate of 4-6℃ / min; then it enters the first heating section, where the temperature is increased to 750-850℃ at a rate of 0.5-1.5℃ / min; next, it enters the second heating section, where the temperature is increased to 1200-1260℃ at a rate of 1.5-3℃ / min; finally, it enters the soaking section and is held at 1190-1230℃ for 2-4 hours; the total heating time is controlled to be 630-760 min.

[0020] Furthermore, in the heated rolling step, the cumulative reduction rate of the two-stage rolling is 55%.

[0021] This invention utilizes a combined continuous casting process involving low superheat casting, dual-stage electromagnetic stirring, and precise reduction. In particular, by precisely controlling the light reduction interval within a "pasty zone" with a central solid fraction fs of 0.3~0.7, it effectively compensates for solidification shrinkage, breaks up dendrite bridging, and reduces the carbon segregation ratio (C0 / C50). 80 The stability is maintained at an optimal level of ≤1.08.

[0022] This invention employs a high-temperature, long-duration diffusion annealing and two-stage controlled rolling process during heated rolling. The prolonged high-temperature heating promotes the dissolution of carbides and homogenization of composition. The two-stage controlled rolling, especially the second stage which involves ≥50% large deformation in the non-recrystallized austenite region (800~900℃), accumulates distortion energy, providing ample sites for ferrite / pearlite phase deformation nuclei during subsequent cooling, further refining the transformation microstructure.

[0023] In the preparation method of the ultra-high hardenability, low segregation, high toughness, large-size S550M grinding ball steel of the present invention, in the smelting and refining steps, the raw materials are proportioned according to the target grinding ball steel composition and smelted in a converter or electric furnace; after tapping, the steel enters an LF furnace for white slag refining, and the white slag is held for 30-45 min; at the end of the refining stage, a silicon-calcium wire is fed into the molten steel for calcium treatment; then, RH vacuum circulation degassing treatment is performed, and the steel is held at a vacuum degree of 20-67 Pa for 18-30 min; after vacuum treatment, soft blowing is performed for 20-40 min.

[0024] Furthermore, the gas used in the soft blowing process is argon, and the soft blowing is performed until the slag surface moves slightly.

[0025] Furthermore, the smelting and refining steps can be performed using conventional operating procedures in the art.

[0026] This invention achieves a total oxygen content of ≤15 ppm and an extremely low level of inclusions in steel through LF white slag refining, RH deep degassing, and calcium treatment, ensuring high purity and uniformity of the material.

[0027] In the preparation method of the ultra-high hardenability, low segregation, high toughness, large-size S550M grinding ball steel of the present invention, the inlet temperature of the billet for inlet heat preservation is 300~450℃, the heat preservation time is 24~48 h, and the outlet temperature is 20~200℃; or, the hot-pressing temperature of the billet is 520~590℃.

[0028] In the preparation method of large-size S550M grinding ball steel with ultra-high hardenability, low segregation, and high toughness described in this invention, the slow cooling step after rolling adopts slow cooling in a pit. In summer, the pit temperature is 300~600℃, the time is 24~48 hours, and the pit temperature is 20~200℃. In winter, the pit temperature is 350~600℃, the time is 24~48 hours, and the pit temperature is 20~200℃.

[0029] The high-temperature slow cooling step after rolling used in this invention can avoid the formation of hard and brittle structures such as martensite or bainite, and obtain a uniform and fine pearlite structure with a small amount of cementite, laying the foundation for obtaining uniform and excellent performance.

[0030] The beneficial effects of this invention are:

[0031] 1. In terms of composition design, this invention improves hardenability by combining Cr and Mo elements, and introduces Nb elements to refine austenite grains, thereby improving toughness while increasing strength. At the same time, it uses Ti to fix free nitrogen and Ca treatment to significantly improve the fatigue life and impact toughness of steel.

[0032] 2. This invention employs a combined continuous casting process with low superheat casting, dual-stage electromagnetic stirring, and precise pressure reduction to precisely control segregation. Combined with LF white slag refining, RH deep degassing, and calcium treatment, it ensures the high purity and uniformity of the ball steel. Furthermore, a high-temperature, long-duration diffusion annealing and two-stage controlled rolling process refines the grains, and a post-rolling high-temperature slow cooling process yields a uniform and fine pearlitic matrix structure, guaranteeing the performance stability of the resulting ball steel.

[0033] 3. The S550M grinding ball steel prepared by this invention, in large sizes from Φ80 to Φ120 mm, not only meets and exceeds the high-level requirements for hardenability (Di value) and hardness (≥370 HBW), but also achieves excellent uniformity with a center point hardness and 1 / 2 radius hardness difference ≤8 HBW. Extremely low carbon segregation and cleanliness, combined with refined grains and controlled inclusions, result in significantly improved impact toughness compared to similar products while maintaining high hardness, effectively reducing the breakage rate of grinding balls during use and extending their service life. Attached Figure Description

[0034] Figure 1 This is a microstructure diagram of the grinding ball steel obtained in Example 2 of the present invention.

[0035] Figure 2 This is a microstructure diagram of the grinding ball steel obtained in Comparative Example 1 of the present invention. Detailed Implementation

[0036] The following non-limiting embodiments are intended to enable those skilled in the art to more fully understand the invention, but do not limit the invention in any way.

[0037] Unless otherwise specified, the experimental methods described in the following examples are conventional methods; the reagents and materials described are commercially available unless otherwise specified.

[0038] One of the specific implementation methods: A method for preparing large-size S550M grinding ball steel with ultra-high hardenability, low segregation, and high toughness includes the following steps: (1) Smelting and refining: The raw materials are proportioned according to the target ball steel composition and smelted in a converter or electric furnace. After tapping, the steel enters the LF furnace for white slag refining. The white slag is held for 30-45 min. At the end of the refining process, a silicon-calcium wire is fed into the molten steel for calcium treatment. Then, RH vacuum circulation degassing treatment is carried out and held at a vacuum degree of 20-67 Pa for 18-30 min. After vacuum treatment, soft blowing is carried out for 20-40 min.

[0039] (2) Continuous casting: The large billet continuous casting machine is used for casting, and the cross section of the billet is ≥300 mm×300 mm; the superheat of the molten steel is controlled at 15~25℃; the process of combining the electromagnetic stirring of the crystallizer and the electromagnetic stirring at the end of solidification is adopted. The electromagnetic stirring parameters of the crystallizer are 400~500 A / 2Hz, the electromagnetic stirring parameters at the end of solidification are 500~650 A / 7Hz, the central solid fraction fs corresponding to the light reduction range at the end of solidification is 0.3~0.7, and the total reduction is 10~40 mm.

[0040] (3) Insulation of billet in pit or hot delivery of billet: The insulation temperature of the billet in pit is 300~450℃, the insulation time is 24~48 h, and the exit temperature is 20~200℃; or, the hot delivery temperature of the billet is 520~590℃.

[0041] (4) Heating and rolling: The steel billet is slowly heated to 1190~1230℃ and held for 2~4 hours; after exiting the furnace, it is air-cooled for 1~3 minutes. The first stage rolling temperature is 1000~1080℃. After rolling, it is air-cooled again for 1~3 minutes. The second stage rolling temperature is 900~1050℃. The final rolling temperature is 800~900℃.

[0042] (5) Slow cooling after rolling: Slow cooling is adopted in the pit. In summer, the temperature in the pit is 300~600℃, the time is 24~48 hours, and the temperature at the exit of the pit is 20~200℃. In winter, the temperature in the pit is 350~600℃, the time is 24~48 hours, and the temperature at the exit of the pit is 20~200℃.

[0043] In the method of the present invention, in step (1), the chemical composition of the target grinding ball steel, by weight fraction, includes: C: 0.86%~0.90%, Si: 0.23%~0.30%, Mn: 0.92%~0.98%, Cr: 1.08%~1.13%, Mo: 0.04%~0.06%, Al: 0.020%~0.030%, Ti≤0.010%, Nb: 0.010%~0.020%, Ca: 0.0005%~0.0020%, P≤0.020%, S≤0.008%, N: 0.0050%~0.0080%, O≤0.0015%, H≤0.00015%, with the balance being Fe and unavoidable impurities.

[0044] In the method of the present invention, the slow heating process in step (4) is as follows: in the preheating section, the temperature is increased to 600-650℃ at a rate of 4-6℃ / min; then it enters the first heating section and is heated to 750-850℃ at a rate of 0.5-1.5℃ / min; then it enters the second heating section and is heated to 1200-1260℃ at a rate of 1.5-3℃ / min; finally, it enters the homogenization section and is kept at 1190-1230℃ for 2-4 hours; the total heating time is controlled to be 630-760 min.

[0045] In the method described in this invention, the critical diameter Di value of the hardenability index of the ball steel is [0.062 + 0.409 × (%C) - 0.135 (%C) × (%C)] × [1 + 3.333 × (%Mn)] × [1 + 0.7 × (%Si)] × [1 + 2.16 × (%Cr)] × [1 + 3 × (%Mo)] × [1 + 0.363 × (%Ni)] × [1 + 0.365 × (%Cu)] ≥ 5.75; the ratio of carbon content at the center of the same cross-section of the round steel to carbon content at a radius of 80% from the center is ≤ 1.08.

[0046] In the method described in this invention, the grinding ball steel has a specification of Φ80~Φ120 mm.

[0047] In the method of the present invention, the austenitic grain size of the grinding ball steel is grade 7.0 to 8.0.

[0048] In the method described in this invention, the microstructure of the ball steel consists of pearlite and a small amount of network cementite, wherein pearlite is the main matrix structure, accounting for 88% to 92% of the total structure, and network cementite accounts for 8% to 12% of the total structure.

[0049] In the method described in this invention, the center point hardness of the grinding ball steel is 374~382 HBW, the half-radius hardness is 380~390 HBW, and the difference between the two is 6~8 HBW; the core impact energy at room temperature is 10~14 J.

[0050] Example 1 A method for preparing large-size S550M grinding ball steel with ultra-high hardenability, low segregation, and high toughness includes the following steps: (1) Smelting and refining: The raw materials were weighed according to the chemical composition ratio of Example 1 in Table 1 (the ratio of each raw material was calculated according to the element content in the table and the proportion of different alloys). The electric furnace was used for smelting and heating. The tapping temperature was controlled at 1610℃ and the tapping carbon was controlled at 0.07%. After tapping, the ladle was hoisted to the LF furnace station. After entering the station, the temperature was measured at 1510℃. Then, the bottom blowing argon system was immediately turned on and medium-intensity stirring was used (the molten steel surface was slightly moved and not violently churned) to prevent the molten steel from forming a crust and to homogenize the temperature of the molten steel. Slag and deoxidizer (lime was added at 4.5 kg / t steel, silicon carbide at 1.36 kg / t steel, and aluminum granules at 0.63 kg / t steel) were added through the high-level silo. Then, the slag was heated and the temperature was increased. After the slag covered the surface of the molten steel, the electrode was inserted into the slag layer and the temperature was increased to 1570℃. During the slag formation stage, the heating rate is 5℃ / min. After 12 minutes of energization, the slag surface changes from black to gray, then gradually turns grayish-white or milky-white. At this point, FeO in the slag is ≤0.5%, and the basicity CaO / SiO2 ratio is controlled at 6. The white slag is maintained for 30 minutes. At the end of refining, silicon-calcium wire is fed into the molten steel through a wire feeder for calcium treatment to modify Al2O3 inclusions and improve castability. The wire feeding speed is controlled at 2.5 m / s, and the feeding amount is 1.8 m / t steel. After LF refining, the molten steel is hoisted to the RH station for vacuum circulation degassing treatment, maintained at a vacuum of 67 Pa for 20 minutes to remove gases (H, O, N) and further remove inclusions. After vacuum treatment, soft blowing is performed using argon gas for 30 minutes. The soft blowing intensity is based on slight movement of the slag surface and no exposure of the molten steel, promoting full floating of inclusions while avoiding slag entrapment and secondary oxidation.

[0051] (2) Continuous casting: Continuous casting is carried out using a 390 mm × 480 mm large billet continuous casting machine. Molten steel is poured into the tundish via a ladle turret, and the superheat of the molten steel in the tundish is controlled at 25℃. A combined process of electromagnetic stirring in the crystallizer and electromagnetic stirring at the end of solidification is adopted. The electromagnetic stirring parameters in the crystallizer are 450 A / 2.5 Hz, and the electromagnetic stirring parameters at the end of solidification are 600 A / 7 Hz. The central solid fraction fs corresponding to the light reduction range at the end of solidification is 0.3~0.7, and the total reduction is 25 mm.

[0052] (3) Hot charging of billet: The billet is hot-charged into the furnace at a temperature of 560℃.

[0053] (4) Heating and rolling: The billet obtained in step (3) is cold-loaded into a walking beam furnace and heated to 600°C at a rate of 5.2°C / min in the preheating section; then it enters the first heating section and is heated to 750°C at a rate of 0.92°C / min; then it enters the second heating section and is heated to 1220°C at a rate of 2.1°C / min; finally it enters the soaking section and is held at 1220°C for 2.4 h; the total heating time is controlled to be 630 min. Then it is rolled into Φ100 mm round steel in two stages: after exiting the furnace, it is air-cooled for 2.5 min, and the first stage rolling temperature is 1050°C; after rolling by 3 BD mills, it is air-cooled again for 3 min in front of the fourth mill, the second stage rolling temperature is 960°C, and the final rolling temperature is 850°C; the cumulative reduction rate of the two stages is 55%.

[0054] (5) Slow cooling after rolling: The rolled round steel obtained in step (4) is quickly bundled and sent into the heat preservation pit for slow cooling. The temperature in the pit is 380℃, the time is 48 h, and the temperature out of the pit is 120℃.

[0055] Example 2 The difference between this embodiment and embodiment 1 is that the chemical composition and Di value are different in step (1), as shown in Table 1. The remaining steps are the same as in embodiment 1.

[0056] Example 3 The difference between this embodiment and embodiment 1 is that the chemical composition and Di value are different in step (1), as shown in Table 1. The remaining steps are the same as in embodiment 1.

[0057] Comparative Example 1 The difference between this comparative example and Example 1 is that the chemical composition is different in step (1), as shown in Table 1, and no calcium treatment is performed during the refining process. The remaining steps are the same as in Example 1.

[0058] Comparative Example 2 The difference between this comparative example and Example 2 is that: in step (2), the total reduction is 10 mm; in step (4), the round steel is rolled into Φ100 mm by controlled rolling: rolling is carried out immediately after exiting the furnace, the initial rolling temperature is 1050℃, the final rolling temperature is 850℃, the reduction rate is 55%, and the remaining steps are the same as in Example 2.

[0059] The chemical composition of the grinding ball steel obtained in Examples 1-3 and Comparative Examples 1 and 2 of this invention is shown in Table 1. The hardness (hardness at 1 / 2 of the cross section and hardness at the center point) of the grinding ball steel was tested according to the test method of GB / T 231.1 Metallic Materials Brinell Hardness Test Part 1. The austenite grain size of the grinding ball steel was tested according to GB / T 6394 Metal Average Grain Size Determination Method. The microstructure of the grinding ball steel was examined according to GB / T 13298 Metal Microstructure Inspection Method, and then the carbon segregation ratio was calculated and evaluated. The results are shown in Table 2.

[0060] As shown in Table 2, the S550M grinding ball steel produced by this invention has a Di value significantly higher than 5.75, and a carbon segregation ratio C0 / C 80 The hardness remains stable at an excellent level of 1.06-1.08, with fine and uniform grains (grade 7.0-8.0). The core hardness of large-size (Φ100 mm) ball steel has a very small difference from the hardness at half the radius (≤8 HBW), demonstrating its exceptional hardenability and microstructure uniformity. Simultaneously, the core impact energy reaches over 10 J, exhibiting good toughness reserves, and the inclusion level is ≤1.0. All indicators meet and exceed high-standard requirements.

[0061] Comparative Example 1 used the S650G target composition without adding Nb and Ca. Although the process was the same as Example 1, the Di value was 7.04, the grain coarsening was to grade 6.5, the difference between the core hardness and the hardness at half the radius increased to 15 HBW, the impact energy decreased to 8 J, and the inclusion level increased to ≤1.5. For grinding ball steel, there is an optimal range for the Di value. If the Di value is too low, hardenability is insufficient, the core hardness is low, the grinding ball wears quickly, and deformation is severe. If the Di value is too high, cost increases, toughness decreases, and it is prone to brittle fracture, which may lead to premature failure. In Comparative Example 1, the Di value was 7.04. Although this significantly improved hardenability, toughness decreased, making it prone to breakage during use, and the excessive Mn content could lead to coarse grains. In embodiments 1-3 of this invention, the Di value is between 5.75 and 6.38, which is relatively moderate. While ensuring high hardenability, the grain size is refined through microalloying (adding Nb, Ca, etc.), and impurities such as P and S are strictly controlled. This ensures that the hardness of the steel ball surface and at a certain depth meets the wear resistance requirements, while also ensuring good toughness in the core by controlling the Di value to avoid excessive levels. This demonstrates the crucial role of the composition design of this invention in improving hardenability, refining grain size, and enhancing toughness.

[0062] Although Comparative Example 2 used the preferred chemical composition of this invention, the insufficient continuous casting reduction (10 mm) and the lack of a two-stage isothermal controlled rolling process resulted in severe segregation (C0 / C). 80=1.12), grain coarsening (6.5 grade), the hardness difference between the core and the half radius is as high as 25 HBW, and the impact energy is only 7 J, which fully demonstrates the necessity of the process control of the present invention to give full play to the advantages of the composition and achieve synergistic performance improvement.

[0063] Figure 1 This is a microstructure diagram of the S550M grinding ball steel prepared in Example 2 of the present invention. Figure 2 The image shows the microstructure of the grinding ball steel obtained in Comparative Example 1 of this invention. It can be seen that the grinding ball steel obtained in Example 2 has a uniform and fine microstructure with a grain size of 8.0 grade. The pearlite accounts for about 90% and the secondary cementite accounts for about 10%. In contrast, the grinding ball steel obtained in Comparative Example 1 has a pearlite ratio of ≥99%, ferrite of about 1%, and no secondary cementite.

[0064] In summary, this invention, through synergistic innovation in composition design and process control, significantly improves the hardenability, segregation control, hardness uniformity, and toughness of grinding ball steel. Particularly in large-diameter sizes (Φ80~Φ120 mm), both hardenability (Di value) and hardness (≥370 HBW) meet and exceed high-level requirements. This invention achieves a balance between high wear resistance and high breakage resistance, effectively solving the problem of early failure in large-diameter grinding balls due to insufficient core hardening or uneven microstructure during service. It significantly improves the product's service life and safety, possessing extremely high industrial application value and promising prospects for widespread adoption.

[0065] Table 1 Chemical composition (by weight) of the examples and comparative examples

[0066] Table 2. Performance results of the grinding ball steel obtained in the examples and comparative examples.

Claims

1. A large-size S550M grinding ball steel with ultra-high hardenability, low segregation, and high toughness, characterized in that: The chemical composition of the grinding ball steel, by weight percentage, includes: C: 0.86%~0.90%, Si: 0.23%~0.30%, Mn: 0.92%~0.98%, Cr: 1.08%~1.13%, Mo: 0.04%~0.06%, Al: 0.020%~0.030%, Ti≤0.010%, Nb: 0.012%~0.018%, Ca: 0.0005%~0.0020%, P≤0.020%, S≤0.008%, N: 0.0050%~0.0080%, O≤0.0015%, H≤0.00015%, with the balance being Fe and unavoidable impurities; The hardenability index critical diameter Di value of the ball steel is calculated as follows: [0.062 + 0.409 × (%C) - 0.135 (%C) × (%C)] × [1 + 3.333 × (%Mn)] × [1 + 0.7 × (%Si)] × [1 + 2.16 × (%Cr)] × [1 + 3 × (%Mo)] × [1 + 0.363 × (%Ni)] × [1 + 0.365 × (%Cu)] = 5.75~6.38; the ratio of carbon content at the center of the same cross-section of the round steel to the carbon content at a radius of 80% from the center is ≤1.

08. The grinding ball steel has a specification of Φ80~Φ120 mm; the austenite grain size of the grinding ball steel is grade 7.0~8.0; the microstructure of the grinding ball steel consists of pearlite and a small amount of network cementite, wherein pearlite is the main matrix structure, accounting for 88%~92% of the total structure, and network cementite accounts for 8%~12% of the total structure. The center point hardness of the grinding ball steel is 374~382 HBW, the half-radius hardness is 380~390 HBW, and the difference between the two is 6~8 HBW; the core impact energy at room temperature is 10~14 J; The preparation method of the ball steel includes smelting and refining, continuous casting, hot-pressing of the billet in the pit or hot-pressing of the billet, hot rolling, and slow cooling after rolling.

2. The method for preparing large-size S550M grinding ball steel with ultra-high hardenability, low segregation, and high toughness as described in claim 1, characterized in that: The preparation method includes smelting and refining, continuous casting, billet placement in the pit for heat preservation or billet hot-pressing, heated rolling, and slow cooling after rolling, wherein, In the continuous casting step, the cross-section of the billet is ≥300 mm × 300 mm; the superheat of the molten steel is controlled at 15~25℃; a combined process of electromagnetic stirring in the crystallizer and electromagnetic stirring at the end of solidification is adopted, with the electromagnetic stirring parameters in the crystallizer being 400~500 A / 2Hz, the electromagnetic stirring parameters at the end of solidification being 520~650 A / 7Hz, the central solid fraction fs corresponding to the light reduction range at the end of solidification being 0.3~0.7, and the total reduction being 10~40 mm; In the heating and rolling step, the steel billet is slowly heated to 1190~1230℃ and held for 2~4 hours; after exiting the furnace, it is air-cooled for 1~3 minutes. The first stage rolling temperature is 1000~1080℃. After rolling, it is air-cooled again for 1~3 minutes. The second stage rolling temperature is 900~1050℃. The final rolling temperature is 800~900℃. In the post-rolling slow cooling step, slow cooling in the pit is adopted. In summer, the pit temperature is 300~600℃, the time is 24~48 hours, and the pit exit temperature is 20~200℃. In winter, the pit temperature is 350~600℃, the time is 24~48 hours, and the pit exit temperature is 20~200℃.

3. The method for preparing large-size S550M grinding ball steel with ultra-high hardenability, low segregation, and high toughness according to claim 2, characterized in that: In the heating and rolling step, the slow heating process is as follows: in the preheating section, the temperature is increased to 600-650℃ at a rate of 4-6℃ / min; then it enters the first heating section and is heated to 750-850℃ at a rate of 0.5-1.5℃ / min; next, it enters the second heating section and is heated to 1200-1260℃ at a rate of 1.5-3℃ / min; finally, it enters the soaking section and is held at 1190-1230℃ for 2-4 hours; the total heating time is controlled to be 630-760 min.

4. The method for preparing large-size S550M grinding ball steel with ultra-high hardenability, low segregation, and high toughness according to claim 2, characterized in that: In the smelting and refining steps, the raw materials are proportioned according to the target ball steel composition and smelted in a converter or electric furnace. After tapping, the steel enters an LF furnace for white slag refining, with the white slag held for 30-45 minutes. At the end of the refining process, a silicon-calcium wire is fed into the molten steel for calcium treatment. Then, RH vacuum circulation degassing is performed, held at a vacuum of 20-67 Pa for 18-30 minutes. After vacuum treatment, soft blowing is performed for 20-40 minutes.

5. The method for preparing large-size S550M grinding ball steel with ultra-high hardenability, low segregation, and high toughness according to claim 2, characterized in that: The initial temperature of the cast billet in the pit for heat preservation is 300~450℃, the heat preservation time is 24~48 h, and the final temperature after exiting the pit is 20~200℃; or, the initial temperature of the cast billet for hot-pressing is 520~590℃.

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