A kind of outer hard and inner flexible steel-based composite material grinding ball and its preparation method and application

Abstract

The application discloses a steel-based composite material grinding ball with outer hardness and inner toughness as well as a preparation method and application thereof, the diameter of the steel-based composite material grinding ball is 100-150 mm, the steel-based composite material grinding ball comprises a surface hardening layer and a core region from outside to inside, the average Rockwell hardness of the surface hardening layer is 58-63 HRC, the average Rockwell hardness of the core region is 50-53 HRC, and the unnotched impact toughness of the core region is 10-14.5 J / cm 2 The method of the application realizes gradient distribution of the organizational structure and mechanical properties by component design and reasonable control of the cooling path and heat treatment process, and fundamentally improves the fracture resistance and service stability of the large-size grinding ball.

Description

Technical Field

[0001] This invention belongs to the field of wear-resistant materials technology, specifically relating to a steel-based composite grinding ball that is hard on the outside and tough on the inside, its preparation method, and its application. Background Technology

[0002] Grinding balls are the key grinding media in ball mills, widely used in industries such as mining, cement, and thermal power. During their service life, they endure complex conditions of high impact and strong abrasive wear for extended periods. This requires grinding balls to possess both high hardness to resist abrasive cutting and high toughness to absorb impact energy and prevent breakage. However, with the continuous increase in mill diameter and operating energy level, the mechanical load on grinding balls is becoming increasingly severe. In particular, large-sized grinding balls (diameter > 100 mm) are heavy and have a large mass, making them more prone to fracture, spalling, or premature failure during long-term service. Existing materials are clearly insufficient in balancing these two types of performance.

[0003] Although high-chromium cast iron grinding balls have excellent initial hardness and wear resistance and are widely used in grinding medium- and high-hardness ores, their typical microstructure is tempered martensite + eutectic carbides, resulting in high overall brittleness and an impact toughness of only 3~5 J / cm². In the high-drop operating environment of large ball mills, microcracks are prone to rapid propagation, leading to ball spalling, fragmentation, or even breakage, which seriously affects the stability and economy of the system.

[0004] Carbide-containing ductile iron (CADI) grinding balls have improved toughness, but their initial hardness is low. Especially in the early stages of service when no significant work hardening has occurred, the low surface hardness and severe abrasive cutting lead to localized spalling and excessively rapid wear, which cannot meet the service life requirements of high-hardness ores under high impact strength.

[0005] Furthermore, according to the national standard GB / T 17445-2022 "Cast Grinding Balls", the hardness difference between the surface and core of conventional grinding balls typically does not exceed 3 HRC. While this near-homogeneous performance distribution facilitates quality control, it makes it difficult for both the surface and core to simultaneously meet the requirements of high hardness and high toughness. Therefore, developing a large-size grinding ball with a gradient performance structure of "hard outer layer and tough inner layer" to achieve a high degree of synergy between high wear resistance and impact resistance is currently a core technological bottleneck facing the industry. Summary of the Invention

[0006] The purpose of this invention is to provide a steel-based composite grinding ball that is hard on the outside and tough on the inside.

[0007] The present invention also aims to provide a method for preparing the above-mentioned steel-based composite grinding ball with external hardness and internal toughness.

[0008] The final objective of this invention is to provide the application of the above-mentioned hard-outer-hard-inner-tough steel-based composite grinding balls as a key grinding medium in large ball mills of crushing or grinding equipment.

[0009] The first objective of this invention can be achieved through the following technical solution: a steel-based composite grinding ball with an outer hardness and an inner toughness, wherein the diameter of the steel-based composite grinding ball is 100-150 mm, and the steel-based composite grinding ball comprises, from the outside to the inside, a surface hardening layer and a core region, wherein the average Rockwell hardness of the surface hardening layer is 58-63 HRC, the average Rockwell hardness of the core region is 50-53 HRC, and the unnotched impact toughness of the core region is 10-14.5 J / cm. 2 .

[0010] Preferably, in some embodiments of the present invention, the average Rockwell hardness of the surface hardened layer is 58.2~62.5 HRC, the average Rockwell hardness of the core region is 50.1~52.3 HRC, and the notched impact toughness of the core region is 12.7~14.2 J / cm. 2 .

[0011] The metal matrix structure of the surface hardened layer consists of tempered martensite, bainite, and retained austenite. The surface hardened layer has a high carbide content and fine, uniform graphite. The metal matrix structure of the core region consists of bainite and retained austenite. Compared to the carbide content and graphite size in the surface hardened layer, the core region has a low carbide content and large spherical graphite.

[0012] Preferably, in some embodiments of the present invention, the volume percentage of carbides in the surface hardening layer accounts for about 10-15% of the total volume of the surface hardening layer, the volume percentage of carbides in the core region accounts for about 5-10% of the total volume of the core region, the size of graphite in the surface hardening layer is 10-40 μm, and the size of spherical graphite in the core region is 40-70 μm.

[0013] Preferably, the thickness of the surface hardening layer of the present invention is 15~45 mm, more preferably 30~40 mm.

[0014] The steel-based composite grinding ball with hard outer surface and tough inner surface provided by the present invention is a steel-based composite grinding ball containing graphite and carbides with a gradient structure of hard outer surface and tough inner surface. Through the coordinated control of composition design, solidification path control and differentiated heat treatment process, the grinding ball can meet the service conditions of impact abrasive wear of large ball mills, and has both high surface hardness and high core toughness, which significantly improves wear resistance and fracture resistance.

[0015] The second objective of this invention can be achieved through the following technical solution: the method for preparing the above-mentioned steel-based composite grinding ball with external hardness and internal toughness includes the following steps:

[0016] (1) Design of the ratio of raw materials and compound inoculant:

[0017] (1.1) Raw material ratio design: The raw materials contain the following components in mass percentage: C 2.7~4.0%, Si 1.5~3.0%, Mn 0.4~2.0%, Cr 0.2~3.0%, W 0.1~2.0%, Ni 0.5~5.0%, V 0.2~2.0%, Nb 0.01~2.0%, P < 0.05%, S < 0.03%, with the balance being Fe.

[0018] (1.2) Composite inoculant formulation design: The composite inoculant contains the following components in mass percentage: Mg 3~8%, Zr 3~10%, rare earth 3~10%, Ti 5~10%, Ca 3~10%, Al 1~3%, Si 48~52%, Fe balance;

[0019] (2) Smelting and incubation:

[0020] (2.1) Smelting:

[0021] Raw materials are added to the smelting furnace, and the furnace temperature is maintained at 1450~1500℃ during the smelting process. The resulting molten metal is then subjected to two-stage inoculation after slag removal, pre-furnace analysis and composition fine-tuning.

[0022] (2.2) Incubation treatment:

[0023] First stage of furnace inoculation: Before tapping, add 0.3-0.5% of the total mass of the composite inoculant to the molten metal and mix well to carry out the first stage of furnace inoculation;

[0024] Second stage of enhanced inoculation: When the temperature of the molten metal drops to 1380~1420℃ before casting, add 0.3~0.5% of the total mass of the composite inoculator to the molten metal again to carry out the second stage of enhanced inoculation;

[0025] (3) Casting and shaping:

[0026] Control the pouring temperature to 1320~1380℃, pour the molten metal into a metal mold or a metal mold covered with sand, and make grinding balls with a diameter of 100~150mm;

[0027] (4) Heat treatment process:

[0028] (4.1) Austenitization: Temperature 880~920℃, holding time 5~8h;

[0029] (4.2) Isothermal quenching: Cool to 280~320℃ and hold for 3~6 hours;

[0030] (4.3) Surface enhanced cooling: Water cooling or spray cooling should be performed immediately after taking it out of the isothermal furnace at 280~320℃;

[0031] (4.4) Low temperature tempering: temperature 250~320℃, hold for 1.5~4.5h, and you will get a steel-based composite grinding ball with hard exterior and tough interior.

[0032] In the above-mentioned method for preparing steel-based composite grinding balls with external hardness and internal toughness:

[0033] More preferably, the furnace charge in step (1.1) contains the following components by mass percentage: C 3.2~3.3%, Si 1.8~2.0%, Mn 0.8~1.0%, Cr 1.0~1.5%, W 0.6~0.8%, Ni 3.5~4.5%, V 0.3~0.5%, Nb 0.05~0.08%, with the balance being Fe.

[0034] In the above-mentioned components of this invention: C element provides a source for graphite and carbide precipitation; Si element promotes graphite formation; Mn element increases austenite stability; Cr element promotes the formation of M3C type carbides and improves wear resistance; W element promotes the formation of MC type carbides; Ni element stabilizes austenite and improves core toughness; V and Nb elements refine grains; phosphorus (P) content is minimized to avoid brittle phases; sulfide content is minimized to improve toughness; Fe element is the balance and is the main matrix element.

[0035] In step (3) of this invention, a metal mold or metal mold sand coating process is used to improve the surface cooling rate. This process can effectively promote the whitening of the surface structure, increase the content of surface eutectic carbides, and refine the surface spherical graphite.

[0036] Preferably, in step (4.1), austenitization is carried out at a temperature of 880~920℃ and a holding time of 5~8h to homogenize the microstructure, dissolve alloying elements, and form austenite.

[0037] Preferably, in step (4.2), isothermal quenching is carried out by cooling to 280~320℃ and holding for 3~6h, so that the matrix structure is transformed into bainite and retained austenite, thereby achieving the basic overall toughness and hardness.

[0038] Preferably, in step (4.3), the surface layer is strengthened by cooling: after being taken out of the isothermal furnace at 280~320℃, it is immediately subjected to water cooling or spray cooling to cause the surface layer of the sphere to undergo martensitic transformation and form martensite. The surface structure consists of martensite, bainite, retained austenite, eutectic carbide and spheroidal graphite. The core is cooled more slowly and the temperature is still higher, so as to maintain bainite, retained austenite, eutectic carbide and spheroidal graphite.

[0039] In step (4.3) of this invention, after being taken out of the isothermal furnace at 280~320℃, it is immediately subjected to water cooling or spray cooling to transform the residual austenite in the surface metal matrix into martensite, forming a high-hardness layer, while the core is slowly cooled to maintain high toughness.

[0040] Preferably, in step (4.4), the low-temperature tempering is carried out at a temperature of 250~320℃ for 1.5~4.5h to eliminate stress, stabilize the structure, and improve the overall performance, thereby obtaining a steel-based composite grinding ball with hard exterior and tough interior.

[0041] The grinding balls in this invention are made using a metal mold or a metal mold sand casting process, and the structural gradient is achieved through differentiated cooling paths and heat treatment processes.

[0042] The last technical problem of the present invention can be solved by the following technical solution: the application of the above-mentioned steel-based composite grinding balls with hard outer surface and tough inner surface as key grinding media in large ball mills of crushing or grinding equipment.

[0043] The structural features of the steel-based composite grinding ball with an outer hardness and an inner toughness in this invention are as follows:

[0044] Grinding ball diameter: preferably 100~150 mm;

[0045] Casting process: Prepared using metal mold casting or metal mold sand-coated casting process;

[0046] Tissue gradient source: Controlling the evolution of surface and core tissue structure through composition design, solidification path control, and heat treatment optimization;

[0047] The compositional distribution and microstructure characteristics of this steel-based composite grinding ball, which is hard on the outside and tough on the inside:

[0048] Surface hardening layer:

[0049] Rich in carbides, with fine and uniform spherical graphite;

[0050] The microstructure of the metal matrix consists of tempered martensite, bainite, and retained austenite.

[0051] It possesses excellent hardness and wear resistance.

[0052] Heart region:

[0053] It contains relatively few carbides, is rich in spherical graphite, and has a relatively large size;

[0054] The microstructure of the metal matrix consists of bainite and retained austenite;

[0055] It possesses good toughness and crack resistance.

[0056] Key parameters for microstructure control of this steel-based composite grinding ball with a hard outer surface and a tough inner surface:

[0057] Surface hardening layer thickness: 15~45 mm, preferably 30~40 mm.

[0058] Differentiated organizational control methods:

[0059] Differential distribution of spherical graphite and carbides is achieved by controlling the solidification path (i.e., regulating the cooling rate);

[0060] By combining isothermal quenching, surface strengthening cooling, and low-temperature tempering in a combined heat treatment process, the phase transformation path is controlled to form a microstructure gradient.

[0061] The grinding ball structure design of this invention fully combines the high-hardness carbide reinforcing phase and the energy absorption and buffering mechanism of spherical graphite, effectively improving the performance of the material under high impact and strong abrasive wear combined loads. It is suitable as a key grinding medium in large ball mills in industries such as mining, cement, and thermal power, and has broad engineering application prospects.

[0062] The present invention has the following advantages:

[0063] (1) The method of the present invention achieves a gradient distribution of microstructure and mechanical properties through component design and reasonable control of cooling path and heat treatment process, thereby fundamentally improving the fracture resistance and service stability of large-size grinding balls (diameter > 100 mm).

[0064] (2) The method of the present invention specifically includes the following two core implementation methods:

[0065] (2.1) Cooling path control regulates solidification structure:

[0066] By taking advantage of the characteristic that the cooling rate of large-diameter grinding balls gradually decreases from the surface to the center, the composition of the molten metal and the inoculation conditions are designed to induce a differentiated distribution of solidification paths.

[0067] By employing metal mold or metal mold sand casting process, the surface cooling rate is increased, forming a structure with high carbide content and fine spheroidal graphite, thereby enhancing wear resistance;

[0068] Slow central cooling allows for sufficient graphite precipitation and reduced carbides, creating a region with superior toughness and inhibiting fracture.

[0069] (2.2) Differentiated heat treatment process design:

[0070] A combined heat treatment process of isothermal quenching, surface strengthening cooling, and low-temperature tempering is adopted: First, the grinding ball is subjected to isothermal quenching, held at a medium temperature range (e.g., 280~320℃) to transform the metal matrix into bainite and retained austenite, thereby improving the overall toughness and basic hardness. Then, immediately after isothermal treatment, the surface area of ​​the grinding ball undergoes rapid strengthening cooling (water cooling or water mist cooling) to induce some of the retained austenite to transform into martensite, significantly improving its hardness and wear resistance. Finally, the entire ball undergoes low-temperature tempering to remove stress and stabilize the microstructure, transforming the surface matrix microstructure into a composite structure of tempered martensite, bainite, and retained austenite. Through the above heat treatment path design, the final result is: Surface microstructure: tempered martensite, bainite, retained austenite, eutectic carbides, and spheroidal graphite, exhibiting high hardness and excellent wear resistance; Core microstructure: bainite, retained austenite, eutectic carbides, and spheroidal graphite, exhibiting good toughness and impact resistance.

[0071] (3) By controlling the solidification structure and heat treatment path, the method of the present invention achieves a synergistic match between high surface hardness and high core toughness, and successfully develops wear-resistant, fracture-resistant and long-life grinding ball material suitable for the high-intensity impact and wear environment of large ball mills, filling the gap in existing materials for grinding balls with "hard outside and tough inside" structure. Attached Figure Description

[0072] The present invention will now be further described with reference to the accompanying drawings and embodiments.

[0073] Figure 1 To implement the surface and core metallographic structures of the steel-based composite grinding ball with hard exterior and tough interior in Case 1, the following are shown: (a) surface metallographic structure (graphite morphology can be observed without corrosion), (b) core metallographic structure (graphite morphology can be observed without corrosion), (c) surface metallographic structure (carbides can be observed after 4% nitric acid alcohol corrosion), (d) core metallographic structure (carbides can be observed after 4% nitric acid alcohol corrosion), (e) surface metallographic structure (tempered martensite, bainite and a small amount of retained austenite after 4% nitric acid alcohol corrosion), and (f) core metallographic structure (bainite and retained austenite after 4% nitric acid alcohol corrosion).

[0074] Figure 2 The graphite morphology diagrams are shown for Examples 1, 5, and Comparative Example 2, where (a) is the surface graphite morphology (metallic type) of Example 1, (b) is the core graphite morphology (metallic type) of Example 1, (c) is the surface graphite morphology (metallic type with sand coating) of Example 5, (d) is the core graphite morphology (metallic type with sand coating) of Example 5, (e) is the surface graphite morphology (sand type) of Comparative Example 2, and (f) is the core graphite morphology (sand type) of Comparative Example 2.

[0075] Figure 3The metallographic diagrams are of Examples 1, 5 and Comparative Example 2, where (a) is the metallographic diagram of Example 1 (metallic mold), (b) is the metallographic diagram of Example 5 (metallic mold with sand coating), and (c) is the metallographic diagram of Comparative Example 2 (sand mold). The white surface in the diagrams is eutectic carbide.

[0076] Figure 4 The images show the metallographic structures of Example 1 and Comparative Example 3, where (a) is the metallographic structure of Example 1 and (b) is the metallographic structure of Comparative Example 3. The white precipitate in the images is a eutectic carbide. Detailed Implementation

[0077] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0078] Unless otherwise specified in the following implementation plan, the test conditions are generally as per standard test conditions or the test conditions recommended by the reagent company. Unless otherwise specified, all materials and reagents used are commercially available.

[0079] Unless otherwise specified, the terms used in the following implementation methods and embodiments generally have the meanings commonly understood by those skilled in the art.

[0080] Example 1: Nickel content 4 wt.%

[0081] The method for preparing the steel-based composite grinding ball with an outer hardness and an inner toughness provided in this embodiment is based on a metal mold casting process. The structural gradient is achieved through differentiated cooling paths and heat treatment processes. The specific method includes the following steps:

[0082] (1) Design of the ratio of raw materials and compound inoculant:

[0083] (1.1) Raw material ratio design: The raw materials contain the following components in mass percentage: C 3.6%, Si 2.8%, Mn 0.9%, Cr 1.2%, W 0.7%, Ni 4%, V 0.4%, Nb 0.06%, P < 0.05%, S < 0.03%, Fe balance;

[0084] (1.2) Composite inoculant formulation design: The composite inoculant contains the following components in mass percentage: Mg 8%, Zr 5%, rare earth 5%, Ti 5%, Ca 5%, Al 2%, Si 48%, Fe balance;

[0085] (2) Smelting and incubation:

[0086] (2.1) Smelting:

[0087] Raw materials are added to the smelting furnace, and the furnace temperature is maintained at 1480℃ during the smelting process. The resulting molten metal is then subjected to two-stage inoculation after slag removal, pre-furnace analysis and composition fine-tuning.

[0088] The raw materials include iron, ferroalloys and carbon raisers. Ferroalloys include ferrosilicon, high manganese ferroalloys, high chromium ferroalloys, ferrotungsten, ferronickel, ferrovanadium and ferroniobium. The carbon raiser is graphite.

[0089] (2.2) Incubation treatment:

[0090] First stage of furnace inoculation: When the temperature of the molten metal stabilizes at 1450℃, add the composite inoculant to the molten metal 3 minutes before tapping. The amount of composite inoculant added is 0.4% of the total mass of the molten metal. Stir evenly and let stand for 2 minutes to promote the diffusion of elemental reactions.

[0091] The second stage of enhanced inoculation: Two minutes before pouring, the temperature of the molten metal is reduced to 1380℃, and a composite inoculant is added to the molten metal again. The amount of composite inoculant added is 0.4% of the total mass of the molten metal to promote graphite nucleation and improve the uniformity of carbide distribution.

[0092] (3) Casting and shaping:

[0093] The pouring temperature is controlled at 1350℃, and the molten metal is poured into a metal mold to form a grinding ball with a diameter of 150mm.

[0094] (4) Heat treatment process:

[0095] (4.1) Austenitization: Temperature 920℃, holding time 6h, to homogenize the microstructure, dissolve alloying elements, and form austenite;

[0096] (4.2) Isothermal quenching: Salt bath cooling to 320℃, holding for 6h, the microstructure transforms into bainite and retained austenite, achieving the basic overall toughness and hardness;

[0097] (4.3) Surface strengthening cooling: After being taken out of the 320℃ isothermal furnace, the surface of the grinding ball is immediately cooled with water to cause the surface of the grinding ball to undergo martensitic transformation;

[0098] (4.4) Low temperature tempering: Temperature 300℃, hold for 4h to eliminate stress, stabilize the structure and improve the comprehensive performance, thus obtaining a steel-based composite grinding ball with hard exterior and tough interior.

[0099] (5) Metallographic structure:

[0100] Metallographic structure refers to the morphology of the microstructure observed under a metallographic microscope, including graphite morphology, carbide morphology, and the morphology of the metal matrix. The morphology of the metal matrix can be used to determine whether it is martensite or bainite, etc.

[0101] The metallographic structure of the grinding ball consists of graphite, eutectic carbides and a metal matrix.

[0102] (5.1) Graphite distribution: The graphite in the surface area is spherical and small in size, and evenly distributed, such as... Figure 1 In (a), the graphite diameter in the core region is larger, such as Figure 1 (b);

[0103] (5.2) Carbide distribution: The surface area has a high carbide content, such as... Figure 1 (c) The carbide content in the heart region is lower, such as Figure 1 (d), Note: Figure 1 In (c), the white phase is a carbide. Figure 1 In (d), the white network structure is a carbide;

[0104] (5.3) Metal matrix structure:

[0105] The microstructure of the hardened metal matrix on the surface of the grinding ball consists of tempered martensite, bainite, and a small amount of retained austenite, such as... Figure 1 (e);

[0106] The microstructure of the metal matrix in the core region consists of bainite and retained austenite, such as... Figure 1 (f), note that the acicular structure is tempered martensite or bainite, and the white structure between the acicular structures is retained austenite;

[0107] (6) Mechanical properties:

[0108] (6.1) Hardness: The average hardness of the surface hardened layer is 62.5 HRC, the thickness is 40 mm, and the average hardness of the core area is 50.1 HRC;

[0109] The hardness test adopts the standard: GB / T 230.1-2018 "Metallic materials Rockwell hardness test - Part 1: Test method";

[0110] (6.2) Core impact toughness 14.2 J / cm²; The impact toughness test adopts the standard: GB / T 229-2020 "Metallic Materials Charpy Pendulum Impact Test Method";

[0111] (6.3) Surface area impact abrasive wear performance: 5J impact abrasive wear weight loss rate: 3.65mg / min;

[0112] The impact abrasive wear test adopts the standard: T / CFA 0018-2016 (i.e. T / CFA 010604-3--2016) "Impact Abrasive Wear Test Method for Steel Materials".

[0113] Example 2: Nickel content 2.5 wt.%

[0114] (1) Design of the ratio of raw materials and compound inoculant:

[0115] (1.1) Raw material ratio design: The raw materials contain the following components in mass percentage: C 3.6%, Si 2.8%, Mn 0.9%, Cr 1.2%, W 0.7%, Ni 2.5%, V 0.4%, Nb 0.06%, P < 0.05%, S < 0.03%, Fe balance;

[0116] (1.2) Compound inoculant formulation design: Same as Example 1:

[0117] (2) Smelting and incubation: Same as in Example 1;

[0118] (3) Casting and molding: Same as in Example 1;

[0119] (4) Heat treatment process:

[0120] (4.1) Austenitization: Same as in Example 1;

[0121] (4.2) Isothermal quenching: Same as in Example 1;

[0122] (4.3) Surface-enhanced cooling: Same as in Example 1;

[0123] (4.4) Low temperature tempering: temperature 270℃, hold for 4h, stress is eliminated, the structure is stabilized, and the comprehensive performance is improved, thus obtaining a steel-based composite grinding ball with hard exterior and tough interior.

[0124] (5) Metallographic structure:

[0125] The metallographic structure of the grinding ball consists of graphite, eutectic carbides, and a metallic matrix;

[0126] The hardened metal matrix on the surface of the grinding ball consists of tempered martensite, bainite, and a small amount of retained austenite.

[0127] The metal matrix in the core region consists of bainite and retained austenite;

[0128] (6) Mechanical properties:

[0129] Hardness: The average hardness of the surface hardened layer is 60.4 HRC, the thickness of the surface hardened layer is 30 mm, and the average hardness of the core is 51.8 HRC;

[0130] Cardiac impact toughness: 13.5 J / cm 2 ;

[0131] Surface area impact abrasive wear performance: 5J; Impact abrasive wear weight loss rate: 3.92mg / min.

[0132] Example 3: Ni content 1.7 wt%

[0133] (1) Design of the ratio of raw materials and compound inoculant:

[0134] (1.1) Raw material ratio design: The raw materials contain the following elements in mass percentage: C 3.6%, Si 2.8%, Mn 0.9%, Cr 1.2%, W 0.7%, Ni 1.7%, V 0.4%, Nb 0.06%, P < 0.05%, S < 0.03%, Fe balance;

[0135] (1.2) Compound inoculant formulation design: Same as Example 1:

[0136] (2) Smelting and incubation: Same as in Example 1;

[0137] (3) Casting and molding: Same as in Example 1;

[0138] (4) Heat treatment process

[0139] (4.1) Austenitization: Same as in Example 1;

[0140] (4.2) Isothermal quenching: Same as in Example 1;

[0141] (4.3) Surface-enhanced cooling: Same as in Example 1;

[0142] (4.4) Low temperature tempering: temperature 250℃, hold for 4h, stress is eliminated, the structure is stabilized, and the comprehensive performance is improved, thus obtaining a steel-based composite grinding ball with hard exterior and tough interior.

[0143] (5) Metallographic structure:

[0144] The metallographic structure of the grinding ball consists of graphite, eutectic carbides, and a metallic matrix;

[0145] The hardened metal matrix on the surface of the grinding ball consists of tempered martensite, bainite, and a small amount of retained austenite.

[0146] The metal matrix in the core region consists of bainite and retained austenite;

[0147] (6) Mechanical properties:

[0148] Hardness: The average hardness of the surface hardened layer is 58.2 HRC, the thickness of the surface hardened layer is 15mm, and the average hardness of the core is 52.3 HRC;

[0149] Cardiac impact toughness: 12.7 J / cm 2 ;

[0150] Surface area impact abrasive wear performance: 5 J impact abrasive wear weight loss rate: 4.16 mg / min.

[0151] Example 4: Air-mist cooling

[0152] (1) Design of the ratio of raw materials and compound inoculant:

[0153] (1.1) Raw material proportioning design: Same as Example 1;

[0154] (1.2) Compound inoculant formulation design: Same as Example 1:

[0155] (2) Smelting and incubation: Same as in Example 1;

[0156] (3) Casting and molding: Same as in Example 1;

[0157] (4) Heat treatment process:

[0158] (4.1) Austenitization: Same as in Example 1;

[0159] (4.2) Isothermal quenching: Same as in Example 1;

[0160] (4.3) Surface strengthening cooling: After being taken out of the 320℃ isothermal furnace, the surface of the grinding ball is immediately cooled by air mist to cause the surface of the grinding ball to undergo martensitic transformation;

[0161] (4.4) Low-temperature tempering: Same as in Example 1;

[0162] (5) Metallographic structure:

[0163] The metallographic structure of the grinding ball consists of graphite, eutectic carbides, and a metallic matrix;

[0164] The hardened metal matrix on the surface of the grinding ball consists of tempered martensite, bainite, and a small amount of retained austenite.

[0165] The metal matrix in the core region consists of bainite and retained austenite;

[0166] (6) Mechanical properties:

[0167] Hardness: The average hardness of the surface hardened layer is 61.2 HRC, the thickness of the surface hardened layer is 30 mm, and the average hardness of the core is 50.5 HRC;

[0168] Cardiac impact toughness: 14.0 J / cm 2 ;

[0169] Surface area impact abrasive wear performance: 5J; Impact abrasive wear weight loss rate: 3.73mg / min.

[0170] Example 5 Metal mold coated with sand

[0171] (1) Design of the ratio of raw materials and compound inoculant:

[0172] (1.1) Raw material proportioning design: Same as Example 1;

[0173] (1.2) Compound inoculant formulation design: Same as Example 1:

[0174] (2) Smelting and incubation: Same as in Example 1;

[0175] (3) Casting and molding: The casting mold adopts a metal mold with sand coating (a 2mm thick coated sand is applied to the inner surface of the metal mold, and the cooling rate is between that of the metal mold and the sand mold), and the rest is the same as in Example 1;

[0176] (4) Heat treatment process:

[0177] (4.1) Austenitization: Same as in Example 1;

[0178] (4.2) Isothermal quenching: Same as in Example 1;

[0179] (4.3) Surface-enhanced cooling: Same as in Example 1;

[0180] (4.4) Low-temperature tempering: Same as in Example 1;

[0181] (5) Metallographic structure:

[0182] The metallographic structure of the grinding ball consists of graphite, eutectic carbides, and a metallic matrix;

[0183] The microstructure of the hardened metal matrix on the surface of the grinding ball consists of tempered martensite, bainite, and a small amount of retained austenite.

[0184] The microstructure of the metal matrix in the core region consists of bainite and retained austenite;

[0185] (6) Mechanical properties:

[0186] Hardness: The average hardness of the surface hardened layer is 60.8 HRC, the thickness of the surface hardened layer is 35 mm, and the average hardness of the core is 51.2 HRC.

[0187] Cardiac impact toughness: 14.0 J / cm 2 ;

[0188] Surface area impact abrasive wear performance: 5J; Impact abrasive wear weight loss rate: 3.88mg / min.

[0189] Example 6: Metal mold sand coating and air mist cooling

[0190] (1) Design of the ratio of raw materials and compound inoculant:

[0191] (1.1) Raw material proportioning design: Same as Example 1;

[0192] (1.2) Compound inoculant formulation design: Same as Example 1:

[0193] (2) Smelting and incubation: Same as in Example 1;

[0194] (3) Casting and molding: The casting mold adopts a metal mold with sand coating (a 2mm thick coated sand is applied to the inner surface of the metal mold, and the cooling rate is between that of the metal mold and the sand mold), and the rest is the same as in Example 1;

[0195] (4) Heat treatment process

[0196] (4.1) Austenitization: Same as in Example 1;

[0197] (4.2) Isothermal quenching: Same as in Example 1;

[0198] (4.3) Surface strengthening cooling: After being taken out of the 320℃ isothermal furnace, the surface of the grinding ball is immediately cooled by air mist to cause the surface of the grinding ball to undergo martensitic transformation;

[0199] (4.4) Low-temperature tempering: Same as in Example 1;

[0200] (5) Metallographic structure:

[0201] The metallographic structure of the grinding ball consists of graphite, eutectic carbides, and a metallic matrix;

[0202] The microstructure of the hardened metal matrix on the surface of the grinding ball consists of tempered martensite, bainite, and a small amount of retained austenite.

[0203] The core region is composed of bainite and retained austenite;

[0204] (6) Mechanical properties:

[0205] Hardness: The average hardness of the surface hardened layer is 58.6 HRC, the thickness of the surface hardened layer is 30 mm, and the average hardness of the core is 50.7 HRC.

[0206] Cardiac impact toughness: 13.8 J / cm 2 ;

[0207] Surface area impact abrasive wear performance: 5J; Impact abrasive wear weight loss rate: 4.09mg / min.

[0208] Comparative Example 1: Heat treatment without surface enhancement cooling

[0209] (1) Design of the ratio of raw materials and compound inoculant:

[0210] (1.1) Raw material proportioning design: Same as Example 1;

[0211] (1.2) Compound inoculant formulation design: Same as Example 1:

[0212] (2) Smelting and incubation: Same as in Example 1;

[0213] (3) Casting and molding: Same as in Example 1;

[0214] (4) Heat treatment process

[0215] (4.1) Austenitization: Same as in Example 1;

[0216] (4.2) Isothermal quenching: Same as in Example 1;

[0217] (4.3) The furnace was cooled without water-cooled surface strengthening;

[0218] (4.4) Low-temperature tempering: Same as in Example 1;

[0219] (5) Metallographic structure:

[0220] The metallographic structure of the grinding ball consists of graphite, eutectic carbides, and a metallic matrix;

[0221] The microstructure of the metal matrix in the core region consists of bainite and retained austenite;

[0222] (6) Mechanical properties:

[0223] Hardness: The average surface hardness is 54.6 HRC, and the average core hardness is 52.8 HRC;

[0224] Impact toughness of the heart region: 12.3 J / cm 2 ;

[0225] Surface area impact abrasive wear performance: 5J; Impact abrasive wear weight loss rate: 4.25mg / min.

[0226] Comparative Example 2: Sand casting, cooling without surface strengthening

[0227] (1) Design of the ratio of raw materials and compound inoculant:

[0228] (1.1) Raw material proportioning design: Same as Example 1;

[0229] (1.2) Compound inoculant formulation design: Same as Example 1:

[0230] (2) Smelting and incubation: Same as in Example 1;

[0231] (3) Casting: Sand was used for the mold, and the rest was the same as in Example 1;

[0232] (4) Heat treatment process

[0233] (4.1) Austenitization: Same as in Example 1;

[0234] (4.2) Isothermal quenching: Same as in Example 1;

[0235] (4.3) The furnace was cooled without water cooling or air mist surface strengthening;

[0236] (4.4) Low temperature tempering: temperature 300℃, hold for 4h, stress is eliminated, the structure is stabilized, and the comprehensive performance is improved, thus obtaining a steel-based composite grinding ball with hard exterior and tough interior.

[0237] (5) Metallographic structure:

[0238] The metallographic structure of the grinding ball consists of graphite, eutectic carbides, and a metallic matrix;

[0239] The surface metal matrix of the grinding ball consists of bainite and retained austenite;

[0240] The core region is composed of bainite and retained austenite;

[0241] (6) Mechanical properties:

[0242] Hardness: Average surface hardness 52.9 HRC, average core hardness 52.6 HRC;

[0243] Impact toughness of the heart region: 12.0 J / cm 2 ;

[0244] Surface area impact abrasive wear performance: 5J; Impact abrasive wear weight loss rate: 4.58mg / min.

[0245] Compared with Example 1, the surface hardening degree is insufficient, the hardness distribution is uniform, and there is almost no hardness difference between the surface hardened layer and the core area.

[0246] Comparative Example 3: No V, Ti, or Nb added.

[0247] (1) Design of the ratio of raw materials and compound inoculant:

[0248] (1.1) Raw material ratio design: V, Ti, and Nb are not added to the raw materials, and the rest is the same as in Example 1;

[0249] (1.2) Compound inoculant formulation design: No Ti added, otherwise the same as in Example 1:

[0250] (2) Smelting and incubation: Same as in Example 1;

[0251] (3) Casting and molding: Same as in Example 1;

[0252] (4) Heat treatment process: Same as in Example 1;

[0253] (5) Metallographic structure:

[0254] The metallographic structure of the grinding ball consists of graphite, eutectic carbides, and a metallic matrix;

[0255] The surface metal matrix of the grinding ball consists of bainite and retained austenite;

[0256] The microstructure of the metal matrix in the core region consists of bainite and retained austenite;

[0257] (6) Mechanical properties:

[0258] Hardness: The average hardness of the surface hardened layer is 59.8 HRC, the thickness of the surface hardened layer is 35 mm, and the average hardness of the core is 49.5 HRC.

[0259] Impact toughness of the heart region: 11.8 J / cm 2 ;

[0260] Surface area impact abrasive wear performance: 5J; Impact abrasive wear weight loss rate: 4.12mg / min.

[0261] Through the above Examples 1-6 and Comparative Examples 1-3, it can be found that:

[0262] 1. The role of nickel content

[0263] Comparing Examples 1-3, we found that:

[0264] As the Ni content decreased from 4.0 wt.% and 2.5 wt.% to 1.7 wt.%, and the optimal tempering temperature gradually decreased from 300℃, 270℃, and 250℃, the surface hardness gradually decreased from 62.5 HRC and 60.4 HRC to 58.2 HRC; the impact toughness decreased from 14.2 J / cm. 2 Decreased to 12.7 J / cm 2 The wear loss rate increased from 3.65 and 3.92 to 4.16 mg / min. Therefore, by adjusting the composition (increasing the Ni content) and the heat treatment process (increasing the tempering temperature), the surface hardness can be improved, the wear loss rate can be reduced, and the toughness and wear resistance can be significantly improved.

[0265] 2. The function of cooling methods

[0266] By comparing Example 1 with Examples 4 and 6, and Comparative Examples 1-2, it was found that:

[0267] Water-cooled strengthening (Example 1): Fastest cooling rate, highest hardness (62.5 HRC), deepest hardened layer, and best wear resistance (3.65 mg / min).

[0268] Air cooling (Examples 4 and 6): The cooling rate is lower, the hardness decreases (from 62.5 HRC to 61.2 HRC and 58.6 HRC), the wear rate increases relatively, but the toughness is slightly improved;

[0269] No surface cooling (Comparative Example 1, Comparative Example 2): Hardness decreased significantly (54.6, 52.9 HRC), the hardened layer was shallow or even disappeared, and the wear resistance deteriorated significantly;

[0270] Conclusion: Surface-enhanced cooling (especially water cooling) is a key process to ensure the "external hardness" effect.

[0271] 3. The role of casting methods

[0272] By comparing Example 1 with Examples 5, 6 and Comparative Example 2, it was found that:

[0273] Metallic type (Example 1): Rapid cooling rate; fine and uniform spherical graphite in the surface hardened layer, with a graphite diameter of 10-15 μm, such as... Figure 2 In (a), the spheroidal graphite in the central region is larger, such as... Figure 2 In (b), the graphite diameter is 40-45 μm, with a high surface carbide content, accounting for 13-15% by volume, such as... Figure 3 Figure (a) shows the best wear resistance;

[0274] Metal-coated sand type (Examples 5 and 6): Compared to the metal mold, the cooling rate is slower, and the spherical graphite in the surface hardening layer is larger than that in the metal mold. The graphite morphology in the surface hardening layer of Example 5 is as follows. Figure 2 In (c), the graphite diameter is 10~40μm, and the graphite morphology in the core region is as follows: Figure 2 (d), graphite diameter 40~70μm, metallographic structure diagram of Example 5 (metal mold coated sand) as shown. Figure 3 In Figure (b), the surface carbide content is reduced compared to the metallic type, accounting for 10-13% of the volume.

[0275] Sand mold (Comparative Example 2): slowest cooling rate, coarse spheroidal graphite, significant individual variation in spheroidal graphite, graphite morphology in the surface hardened layer as follows Figure 2 (e) , graphite diameter 10~50μm, graphite morphology in the core region as follows Figure 2 (f), graphite diameter 30~80μm, comparative example 2 metallographic structure diagram (sand mold) as shown Figure 3 (c) has relatively few carbides, with a volume ratio of <10%, and this process has the worst wear resistance.

[0276] Conclusion: Metal mold casting is superior to sand casting, and sand casting is superior to sand casting. Metal mold casting is the ideal process for obtaining high surface hardness and uniform microstructure.

[0277] 4. The role of microalloying elements

[0278] like Figure 4 As shown, by comparing Example 1 and Comparative Example 3, it was found that:

[0279] Example 1 contains V, Ti, and Nb: carbides are dispersedly distributed, such as... Figure 4 (a) Figure;

[0280] In Comparative Example 3, removing V, Ti, and Nb resulted in a coarser microstructure, with coarser carbides forming a network, such as... Figure 4 (b) diagram;

[0281] Conclusion: Microalloying elements (V, Ti, Nb) are crucial for refining carbides and improving toughness and wear resistance.

[0282] In summary:

[0283] The steel-based composite grinding ball of the present invention, which is hard on the outside and tough on the inside, achieves the following through composition design (Ni content optimization, V / Ti / Nb microalloying), inoculation treatment, control of solidification cooling rate, and differentiated heat treatment (especially surface strengthening cooling):

[0284] High surface hardness (58.2~62.5 HRC), high core toughness (12.7~14.2 J / cm). 2 );

[0285] Compared with Comparative Examples 1-2:

[0286] The grinding balls of this invention have higher hardness and a deeper hardened layer, resulting in significantly improved wear resistance.

[0287] Maintaining core toughness avoids the problems of easy breakage and rapid initial wear of CADI balls found in traditional high-chromium cast iron.

[0288] The process can be implemented on existing production lines and has good prospects for industrialization.

[0289] The embodiments of the present invention have been described in detail above, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and these variations still fall within the protection scope of the present invention.

Claims

1. A steel-based composite mill ball that is hard on the outside and tough on the inside, characterised in that, The diameter of the steel-based composite material grinding ball is 100-150 mm, the steel-based composite material grinding ball comprises a surface hardening layer and a core region from outside to inside, the average Rockwell hardness of the surface hardening layer is 58-63 HRC, the average Rockwell hardness of the core region is 50-53 HRC, and the unnotched impact toughness of the core region is 10-14.5 J / cm 2 ; The preparation method of the outer-hard inner-soft steel-based composite material grinding ball, Comprise the following steps: (1) raw material and composite inoculant ratio design: (1.1) raw material ratio design: the raw material contains the following mass percentage of ingredients: C 2.7~4.0%, Si 1.5~3.0%, Mn 0.4~2.0%, Cr 0.2~3.0%, W 0.1~2.0%, Ni 0.5~5.0%, V 0.2~2.0%, Nb 0.01~2.0%, P <0.05%, S <0.03%, and the balance is Fe; (1.2) composite inoculant ratio design: the composite inoculant contains the following mass percentage of ingredients: Mg 3~8%, Zr 3~10%, rare earth 3~10%, Ti 5~10%, Ca 3~10%, Al 1~3%, Si 48~52%, and the balance is Fe; (2) smelting and inoculation: (2.1) smelting: Add raw materials in the smelting furnace, maintain the furnace temperature at 1450~1500℃ during smelting, and after the obtained metal liquid is deslagged, analyzed in front of the furnace and adjusted in composition, carry out two-stage inoculation; (2.2) inoculation treatment: First-stage front-of-furnace inoculation: add 0.3~0.5% of the total mass of the composite inoculant to the metal liquid before tapping and mix well to carry out first-stage front-of-furnace inoculation; Second-stage strengthening inoculation: when the temperature of the metal liquid is reduced to 1380~1420℃ before pouring, add 0.3~0.5% of the total mass of the composite inoculant to the metal liquid again to carry out second-stage strengthening inoculation; (3) casting forming: Control the pouring temperature to be 1320~1380℃, pour the metal liquid into a metal mold or a metal mold covered with sand mold to make a grinding ball with a diameter of 100~150mm; (4) heat treatment process: (4.1) austenitizing: temperature 880~920℃, holding time 5~8h; (4.2) isothermal quenching: reduce to 280~320℃, hold for 3~6h; (4.3) surface layer strengthening cooling: take out from the 280~320℃ isothermal furnace and immediately carry out water cooling or spray cooling; (4.4) low-temperature tempering: temperature 250~320℃, holding time 1.5~4.5h, to obtain the outer-hard inner-soft steel-based composite material grinding ball.

2. The outer hard and inner ductile steel-based composite ball according to claim 1, characterized in that, The metal matrix structure of the surface hardening layer is tempered martensite, bainite and residual austenite, the surface hardening layer has high carbide content and fine and uniform graphite, the metal matrix structure of the core region is bainite and residual austenite, and compared with the carbide content and graphite size of the surface hardening layer, the core region has low carbide content and large spherical graphite.

3. The outer hard and inner ductile steel matrix composite ball according to claim 1, characterized in that, The thickness of the surface hardening layer is 15~45 mm.

4. The method of producing a steel-based composite ball having a hard outer layer and a soft inner layer according to any one of claims 1 to 3, characterized in that, Comprise the following steps: (1) raw material and composite inoculant ratio design: (1.1) Raw material ratio design: the raw material contains the following mass percentage of ingredients: C 2.7~4.0%, Si 1.5~3.0%, Mn 0.4~2.0%, Cr 0.2~3.0%, W 0.1~2.0%, Ni 0.5~5.0%, V 0.2~2.0%, Nb 0.01~2.0%, P <0.05%, S <0.03%, and the balance is Fe; (1.2) Compound inoculant ratio design: the compound inoculant contains the following mass percentage of ingredients: Mg 3~8%, Zr 3~10%, rare earth 3~10%, Ti 5~10%, Ca 3~10%, Al 1~3%, Si 48~52%, and the balance is Fe; (2) Melting and inoculation: (2.1) Melting: Add raw materials in the melting furnace, keep the furnace temperature at 1450~1500℃ during the melting process, and after the obtained metal liquid is deslagged, analyzed in front of the furnace and adjusted in composition, carry out two-stage inoculation; (2.2) Inoculation treatment: First-stage inoculation in front of the furnace: add 0.3~0.5% of the total mass of the compound inoculant into the metal liquid before tapping and mix well to carry out the first-stage inoculation in front of the furnace; Second-stage strengthening inoculation: when the temperature of the metal liquid is reduced to 1380~1420℃ before pouring, add 0.3~0.5% of the total mass of the compound inoculant into the metal liquid again to carry out the second-stage strengthening inoculation; (3) Casting forming: Control the pouring temperature to be 1320~1380℃, pour the metal liquid into a metal mold or a metal mold covered sand mold to make a grinding ball with a diameter of 100~150mm; (4) Heat treatment process: (4.1) Austenitizing: temperature 880~920℃, holding time 5~8h; (4.2) Isothermal quenching: reduce to 280~320℃, hold for 3~6h; (4.3) Surface layer strengthening cooling: take out from the 280~320℃ isothermal furnace and immediately carry out water cooling or spray cooling; (4.4) Low-temperature tempering: temperature 250~320℃, holding time 1.5~4.5h, to obtain the steel-based composite grinding ball with hard outer layer and tough inner layer.

5. The method of manufacturing an outer hard and inner ductile steel-based composite ball according to claim 4, characterized in that, In step (1.1), the raw material contains the following mass percentage of ingredients: C 3.2~3.3%, Si 1.8~2.0%, Mn 0.8~1.0%, Cr 1.0~1.5%, W 0.6~0.8%, Ni 3.5~4.5%, V 0.3~0.5%, Nb 0.05~0.08%, and the balance is Fe.

6. The method of manufacturing an outer hard and inner ductile steel-based composite ball according to claim 4, characterized in that, In step (1.2), the rare earth is La, Ce or LaCe alloy.

7. Use of the steel-based composite grinding ball with hard outer layer and tough inner layer as claimed in any one of claims 1~3 as key grinding medium in a large ball mill of a crushing or grinding device.

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