A high-hardness high-wear-resistance high-carbon cast steel shot and a continuous fluidized liquid nitrogen cryogenic treatment system and method for producing the same

Abstract

This invention discloses a high-hardness, high-wear-resistant high-carbon cast steel shot, and a continuous fluidized bed liquid nitrogen cryogenic treatment system and method for its production, belonging to the field of metal abrasive manufacturing technology. It includes: a fluidized bed reactor with a processing chamber for containing spherical metal abrasive particles; a gas distribution plate at the bottom of the processing chamber with multiple micropores of 20-50 μm in diameter; a liquid nitrogen supply unit connected to the bottom of the fluidized bed reactor; and a gas circulation unit including a cryogenic nitrogen fan. The fan's inlet is connected to the top of the processing chamber, and its outlet is connected to the area below the gas distribution plate via a heat exchanger, forming a bottom-up cryogenic nitrogen flow. This invention solves the problems of high residual austenite, high breakage rate, uneven cryogenic treatment, and high cost in existing high-carbon cast steel shot, achieving high uniformity of microstructure, significantly reduced breakage rate, and stable hardness dispersion in high-performance cast steel shot.

Description

Technical Field

[0001] This invention relates to the field of metal abrasive manufacturing technology, and more particularly to a high-strength, high-toughness, high-carbon cast steel shot, as well as a continuous fluidized liquid nitrogen cryogenic treatment system and method for its production. This cast steel shot is suitable for high-end manufacturing scenarios with stringent requirements for surface integrity, fatigue life, and cleaning efficiency, such as shot peening and surface cleaning of components like aero-engine blades, high-speed rail brake discs, and automotive suspension springs. Background Technology

[0002] Metal abrasives are special materials made of hard metals, possessing a certain degree of hardness and toughness. They primarily remove dirt, rust, and other unevenness from the surface of parts through friction and impact, thereby improving the surface quality and performance of the parts. Metal abrasives can be classified according to their raw materials into cast iron, steel, and non-ferrous metal categories. Steel shot can be further divided into cast steel shot (sand) and cold-drawn steel wire. Cast steel includes high-carbon and low-carbon steel shot and steel grit, while cold-drawn steel wire includes steel wire cut shot and stainless steel cut shot. Among metal abrasives, steel shot (sand) is widely used due to its advantages such as good elasticity, fatigue resistance, moderate hardness, high toughness, and impact resistance. It also offers advantages such as fast cleaning speed, low consumption, and effective extension of the service life of vulnerable parts in shot blasting equipment, resulting in excellent overall performance.

[0003] Steel shot (grit) is generally classified into two types according to its shape: shot and grit, as detailed below: Shot: These are round abrasives suitable for most shot blasting and surface strengthening operations.

[0004] Sand: It has sharp edges and is suitable for processing harder or stubborn surface impurities, and has stronger cutting ability.

[0005] Cast steel shot is a spherical metal abrasive widely used in surface treatment. Its performance directly affects the surface quality, residual stress distribution, and service life of the workpiece. Traditional high-carbon cast steel shot (carbon content 0.85%-1.10%) is usually centrifuged and then subjected to ordinary quenching and tempering treatment. Although it can achieve high hardness (HRC55-60), it has the following problems: High residual austenite content: Due to the high carbon and high alloy composition, the martensite transformation completion temperature (Mf) is lower than the room temperature. After conventional quenching, the residual austenite can reach 15%-25%. During the use of high carbon cast steel shot, the residual austenite slowly transforms into martensite, causing the high carbon cast steel shot to have unstable dimensions and microcracks. High breakage rate: The presence of coarse carbides and internal stress concentration in the structure of high carbon cast steel shot makes it easy to break during impact, generating dust, reducing cleaning efficiency and polluting the environment. Retained austenite hardly transforms at room temperature; it only continues to transform into martensite under stress-induced conditions, at temperatures below Md (the upper limit temperature for "deformation-induced martensite"), or after tempering / deep cryogenic treatment. However, existing cryogenic equipment is mostly batch-type static tanks, suitable for large-sized, regularly shaped workpieces such as cutting tools, molds, and bearings. For cryogenic treatment of small spheres, there are issues with sphere agglomeration and uneven refrigerant contact. High-carbon cast steel shot typically has a particle size of Φ0.2-2.0mm and a high bulk density; in cryogenic tanks, it easily agglomerates and adheres, preventing the internal spheres from fully contacting the refrigerant, resulting in "overcooling of the outer layer and undercooling of the inner layer," leading to inconsistent microstructure transformation. Phase transformation stress easily causes microcracks. If original casting defects (porosity, inclusions) are present, cryogenic treatment may induce brittle fracture, actually increasing the breakage rate.

[0006] In the existing technology, fluidized beds are widely used in chemical drying, powder heat treatment, and surface coating. In the field of heat treatment, they are mainly used for medium and high temperature processes such as tempering, annealing, and carburizing, with temperatures exceeding >200℃. There are no public reports of combining fluidized beds with liquid nitrogen cryogenic (-196℃) for the treatment of metal abrasives. Summary of the Invention

[0007] In order to overcome the shortcomings of the prior art, the purpose of this invention is to provide a high-strength and high-toughness high-carbon cast steel shot, its preparation method and special equipment, to solve the problems of high residual austenite, high breakage rate, uneven cryogenic treatment and high cost in the prior art, and to achieve high uniformity of microstructure, significantly reduced breakage rate and stable hardness dispersion of high-performance cast steel shot.

[0008] One of the objectives of this invention is to provide a continuous fluidized liquid nitrogen cryogenic treatment system for spherical metal abrasives, which is used for uniform cryogenic treatment of spherical metal abrasives.

[0009] The second objective of this invention is to provide a deep cryogenic treatment method for high-carbon cast steel shot, which combines isothermal quenching with fluidized liquid nitrogen deep cryogenic treatment.

[0010] The third objective of this invention is to provide a high-strength, high-toughness, high-carbon cast steel shot with specific composition, structure, and performance indicators.

[0011] The fourth objective of this invention is to provide an integrated production line for high-carbon cast steel shot, which integrates granulation, heat treatment, cryogenic treatment and tempering.

[0012] The fifth objective of this invention is to provide One of the objectives of this invention is achieved through the following technical solution: A continuous fluidized liquid nitrogen cryogenic treatment system for spherical metal abrasives, comprising: The fluidized bed reactor has an internal processing chamber for containing spherical metal abrasives. At the bottom of the processing chamber is a gas distribution plate with multiple micropores of 20-50 μm in diameter. A liquid nitrogen supply unit, connected to the bottom of the fluidized bed reactor, is used to introduce liquid nitrogen into the processing chamber; The gas circulation unit includes a low-temperature nitrogen blower. The blower's inlet is connected to the top of the processing chamber, and the blower's outlet is connected to the bottom of the gas distribution plate after passing through a cold energy recovery heat exchanger. This is used to form a low-temperature nitrogen flow from bottom to top, so that the spherical metal abrasive is in a fluidized suspension state at -196°C to -180°C in the processing chamber. The temperature and differential pressure monitoring unit includes multiple temperature sensors and differential pressure sensors, which are arranged at different heights in the processing chamber to monitor the fluidization state and temperature uniformity in real time. The continuous feed and discharge unit includes a feed screw conveyor, a discharge airlock valve, and a material buffer bin, which is used to achieve continuous input and output of spherical metal abrasives while maintaining a sealed processing chamber and a low temperature environment. The heat exchanger is used to precool the newly introduced liquid nitrogen using the low-temperature exhaust gas discharged from the treatment chamber.

[0013] The second objective of this invention is achieved by the following technical solution: A method for cryogenic treatment of high-carbon cast steel shot, using a system comprising the following steps: S1. The spherical metal abrasive material, which has been isothermally quenched, is continuously fed into the fluidized bed reactor. S2. Start the liquid nitrogen supply unit and gas circulation unit to maintain the temperature inside the processing chamber at -196℃ to -180℃; control the low-temperature nitrogen flow rate to 0.3-0.6m / s; S3. Adjust the feed rate so that the residence time of the spherical metal abrasive in the processing chamber is 30-90 minutes; S4. The spherical metal abrasive after cryogenic treatment is continuously discharged through the discharge air lock valve and directly sent to the tempering process, where it is tempered at 200-250℃ for 1-2 hours.

[0014] Furthermore, the isothermal quenching process is as follows: after austenitization at 900-920℃, it is held in a salt bath at 260-300℃ for 60-90 minutes to obtain a microstructure mainly composed of bainite.

[0015] The third objective of this invention is achieved by the following technical solution: A high-strength, high-toughness, high-carbon cast steel shot has the following comprehensive characteristics: The chemical composition, by mass percentage, is: C 0.85-1.10%, Si 0.3-0.6%, Mn 0.6-1.0%, Cr 0.2-0.5%, Mo 0.1-0.3%; The microstructure is predominantly bainite, with a retained austenite content of less than 4%. The hardness is 56-60 HRC, and the hardness dispersion within the same batch is ≤ ±1.0 HRC; In the ASTM B677 standard cyclic test, the impact breakage rate is ≤3.5%.

[0016] The fourth objective of this invention is achieved by the following technical solution: A high-carbon cast steel shot is prepared by a cryogenic treatment method for high-carbon cast steel shot. The spherical metal abrasive is high-carbon cast steel shot with an initial residual austenite content of 15%-25%, which is reduced to below 4% after cryogenic treatment.

[0017] The fifth objective of this invention is achieved by the following technical solution: Applications of high-carbon cast steel shot in shot peening strengthening treatment of aero-engine blades or high-speed rail brake discs.

[0018] The sixth objective of this invention is achieved by the following technical solution: An integrated production line for high-carbon cast steel shot is connected in sequence to: a centrifugal granulation device, an isothermal quenching furnace, a continuous fluidized liquid nitrogen cryogenic treatment system, a low-temperature tempering furnace, and an automatic screening and packaging unit.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention provides a high-hardness, high-wear-resistant high-carbon cast steel shot, and a continuous fluidized liquid nitrogen cryogenic treatment system and method for its production. It integrates "isothermal quenching pretreatment + continuous fluidized deep cryogenic treatment + low-temperature tempering" into the production of high-carbon cast steel shot. The cryogenic treatment still induces the transformation of retained austenite into martensite; therefore, the amount of retained austenite in the final high-carbon cast steel shot microstructure is extremely low, typically <5%, far lower than that of ordinary quenching + low-temperature tempering. The cryogenic treatment causes martensite to shrink in an ultra-low temperature environment, forcing the lattice to shrink and driving solute atoms such as carbon atoms to precipitate from the lattice, forming nanoscale carbide particles. On the cryogenically optimized matrix, tempering further promotes the precipitation of more uniform and finer (<20nm) alloy carbides (such as M2C, MC). These precipitated phases disperse and strengthen the matrix, improving the wear resistance of the high-carbon cast steel shot without significantly sacrificing toughness. In a fluidized bed reactor, high-carbon cast steel shot is subjected to cryogenic nitrogen gas generated by liquid nitrogen spray, causing the workpiece to suspend and tumble in a fluid-like state within the furnace. This fluidized state allows for deep cryogenic treatment, ensuring full contact between the high-carbon cast steel shot and the cooling medium, eliminating dead zones and ensuring uniform heat exchange on the surface of each shot. This avoids the uneven cooling problems caused by workpiece contact in traditional static cryogenic treatments. The hardness dispersion of high-carbon cast steel shot within the same batch is ≤±1.0HRC, meeting the consistency requirements of high-end manufacturing. Ultimately, the breakage rate of high-carbon cast steel shot in surface shot peening is significantly reduced, with an impact breakage rate ≤3.5%, meeting the ASTM B677 standard and representing a reduction of over 40% compared to traditional products. When used for shot peening of aero-engine blades or high-speed rail brake discs, batch performance fluctuations can be reduced by over 30%. Detailed Implementation

[0020] The present invention will be further described below with reference to specific embodiments. It should be noted that, without conflict, the embodiments or technical features described below can be arbitrarily combined to form new embodiments.

[0021] "Quenched workpiece" refers to an intermediate state workpiece in which a metallic material (such as steel) is heated to the austenitizing temperature and then rapidly cooled (usually in water, oil, polymer solution or gas) to suppress diffusion-type phase transformation, thereby obtaining a non-equilibrium structure (mainly martensite, possibly accompanied by retained austenite) with high hardness but high internal stress and high brittleness.

[0022] Austenite (γ-Fe) is a high-temperature stable phase that typically exists in steel at temperatures above 727°C.

[0023] During conventional quenching (such as oil quenching or water quenching) to room temperature: when austenite cools to below the Ms point (the temperature at which martensite begins to transform, typically 200-300℃), some of it transforms into martensite. However, if the Mf point (the temperature at which martensite transformation ends) is below room temperature (common in high-carbon steel), a large amount of austenite remains untransformed and is retained at room temperature, becoming retained austenite. Retained austenite is unstable and may transform into martensite during subsequent use, causing dimensional changes or microcracks.

[0024] This embodiment first provides a continuous fluidized liquid nitrogen cryogenic treatment system for spherical metal abrasives, including: The fluidized bed reactor has an internal processing chamber for containing spherical metal abrasives. At the bottom of the processing chamber is a gas distribution plate with multiple micropores of 20-50 μm in diameter. A liquid nitrogen supply unit, connected to the bottom of the fluidized bed reactor, is used to introduce liquid nitrogen into the processing chamber; The gas circulation unit includes a low-temperature nitrogen blower. The blower's inlet is connected to the top of the processing chamber, and the blower's outlet is connected to the bottom of the gas distribution plate after passing through a heat exchanger. This is used to form a low-temperature nitrogen flow from bottom to top, so that the spherical metal abrasive is in a fluidized suspension state at -196°C to -180°C in the processing chamber. The temperature and differential pressure monitoring unit includes multiple temperature sensors and differential pressure sensors, which are arranged at different heights in the processing chamber to monitor the fluidization state and temperature uniformity in real time. The continuous feed and discharge unit includes a feed screw conveyor, a discharge airlock valve, and a material buffer bin, which is used to achieve continuous input and output of spherical metal abrasives while maintaining a sealed processing chamber and a low temperature environment. The heat exchanger is used to precool the newly introduced liquid nitrogen using the low-temperature exhaust gas discharged from the treatment chamber.

[0025] The working principle of the fan forming a low-temperature nitrogen flow from bottom to top is as follows: Initial startup: Liquid nitrogen is injected into the system, and some of it vaporizes to produce nitrogen gas at -196°C; The blower starts and draws air from the top of the fluidized bed (at this time, the bed contains stationary steel shot and low-temperature nitrogen).

[0026] Establish pressure differential: The blower pressurizes the gas at the outlet side (bottom of the fluidized bed reactor) (e.g., to 0.05-0.1 MPa gauge pressure) and creates a slight negative pressure at the inlet side (top of the bottom of the fluidized bed reactor). The gas naturally flows from the high-pressure zone to the low-pressure zone, but the path is forcibly guided by the pipeline as follows: blower outlet → bottom inlet chamber → through the gas distribution plate → upward through the steel shot layer → back to the blower inlet from the top of the bed. Airflow direction is locked: all pipes, valves, and distribution plates are designed to allow only unidirectional flow; gas cannot flow backward. Therefore, the blower creates a low-temperature nitrogen gas flow from bottom to top.

[0027] This embodiment further provides a method for cryogenic treatment of high-carbon cast steel shot, using a system that includes the following steps: S1. The spherical metal abrasive material, which has been isothermally quenched, is continuously fed into the fluidized bed reactor. S2. Start the liquid nitrogen supply unit and gas circulation unit to maintain the temperature inside the processing chamber at -196℃ to -180℃; control the low-temperature nitrogen flow rate to 0.3-0.6m / s; S3. Adjust the feed rate so that the residence time of the spherical metal abrasive in the processing chamber is 30-90 minutes; S4. The spherical metal abrasive after cryogenic treatment is continuously discharged through the discharge air lock valve and directly sent to the tempering process, where it is tempered at 200-250℃ for 1-2 hours.

[0028] In this embodiment, the isothermal quenching process is as follows: after austenitization at 900-920℃, the sample is held in a salt bath at 260-300℃ for 60-90 minutes to obtain a microstructure mainly composed of bainite.

[0029] The cryogenic treatment method for high-carbon cast steel shot is a key step in the preparation of high-strength and high-toughness high-carbon cast steel shot, as detailed below: This embodiment provides a method for cryogenic treatment of high-strength, high-toughness, high-carbon cast steel shot and its continuous fluidized liquid nitrogen treatment, specifically including the following steps: (1) Melting and centrifugal granulation The alloy is smelted using a medium-frequency induction furnace, and the chemical composition (mass percentage) is controlled as follows: The composition of the material is: C 0.95%, Si 0.45%, Mn 0.8%, Cr 0.35%, Mo 0.2%, and the total content of S and P is <0.02%.

[0030] After the melt temperature is raised to 1600℃, it is poured into a high-speed rotating centrifugal disc (speed 2200rpm) to obtain spherical high-carbon cast steel shot with a particle size of Φ0.8-1.2mm.

[0031] (2) Isothermal quenching treatment The high-carbon cast steel shot was austenitized by holding it at 910°C for 45 minutes, and then quickly transferred to a 280°C nitrate bath for isothermal holding for 75 minutes. After that, it was taken out, cleaned and dried.

[0032] After this treatment, the microstructure is mainly composed of bainite, with a residual austenite content of approximately 6%.

[0033] Characteristics of 280℃ isothermal quenching (isothermal transformation, TTT): Lower bainite transformation: When held at 280℃ (bainite nose temperature), diffusion-type bainite transformation occurs.

[0034] Sufficient time: Incubation for 75 minutes allows the transformation to approach equilibrium, and the residual austenite mass stabilizes in the 5-7% range.

[0035] Small volume effect: The volume change of bainite transformation is only 1-2%, with little stress accumulation, which does not hinder the transformation.

[0036] (3) The continuous fluidized bed liquid nitrogen cryogenic treatment system of the present invention for spherical metal abrasives is used for continuous fluidized bed liquid nitrogen cryogenic treatment: the isothermally quenched high carbon cast steel shot is continuously fed into the fluidized bed reactor, the liquid nitrogen supply unit and the gas circulation unit are started, and the following parameters are controlled: Processing chamber temperature: -185℃ (within the range of -196℃ to -180℃); Low-temperature nitrogen flow rate: 0.45 m / s (within the range of 0.3-0.6 m / s); Feed rate: 600 kg / h; Liquid nitrogen flow rate: 15 L / min; Stay duration: 45 minutes (within the range of 30-90 minutes); Discharge temperature: -185℃.

[0037] Under these conditions, high-carbon cast steel shot is in a stable fluidized suspension state, with uniform heat transfer and no agglomeration or localized overcooling.

[0038] (4) Low-temperature tempering After cryogenic treatment, the high-carbon cast steel shot is continuously discharged through the discharge airlock valve and directly sent into the tempering furnace. After being held at 220℃ for 2 hours, it is air-cooled.

[0039] This embodiment also provides an integrated continuous production line for high-carbon cast steel shot, with each unit connected sequentially according to the process order: Medium-frequency induction melting furnace: 2-ton capacity, equipped with an online composition spectrometer; Centrifugal granulation device: high-speed rotating disc (2200 rpm), with a water-cooled collection tank below; Drying and preheating section: 150℃ hot air drying, followed by 300℃ preheating; Isothermal quenching unit: includes an austenitizing electric furnace (910℃) and a 280℃ nitrate isothermal bath; Continuous fluidized liquid nitrogen cryogenic treatment system; Low-temperature tempering furnace: electric heating, temperature control accuracy ±5℃, 220℃×2h; Automatic screening and magnetic separation unit: multi-layer vibrating screen and permanent magnet drum; Packaging line: Automatic weighing, nitrogen filling, and bag sealing.

[0040] Optical micrograph of the high-carbon cast steel shot prepared in Example 1 after polishing and etching with 4% nitric acid alcohol (magnification: 1000×).

[0041] The main body of the structure is needle-like / lamellar lower bainite, exhibiting a typical feathery substructure; Carbides are dispersed in the ferrite matrix in the form of fine particles with a size of <0.5μm; No coarse network carbides or martensitic twins were observed; Occasionally, white residual phases were observed at the grain boundaries, which were confirmed by EBSD to be residual austenite, with an area ratio of <4%.

[0042] Comparative Example Using traditional oil quenching and tempering processes (without isothermal quenching and cryogenic treatment), steel shot with the same composition was obtained, but its residual austenite content reached 22.1%, the breakage rate was 6.8%, and the hardness dispersion reached ±2.5HRC, which was significantly inferior to that of the present embodiment.

[0043] The residual austenite content of the high-carbon cast steel shot obtained in the examples and comparative examples was characterized by X-ray diffraction (XRD). The test conditions were as follows: Cu-Kα radiation (λ=1.5406Å), tube voltage 40kV, tube current 40mA, scanning range 2θ=30°-100°, step size 0.02°, and scanning rate 2° / min.

[0044] Test results show that: No obvious diffraction peaks were observed at the characteristic diffraction peak positions of austenite (γ-Fe) in the sample of Example 1—namely, 2θ≈43.5° (corresponding to the (111) crystal plane) and 2θ≈50.8° (corresponding to the (200) crystal plane). The intensity of these peaks was close to the background noise level. Quantitative analysis using Rietveld full-spectrum fitting revealed a residual austenite content of 3.7 wt%. The comparative sample showed clear and significant diffraction peaks at the same angle, with the peak intensity at 43.5° being about 32% of the main martensite peak (2θ≈44.7°). Quantitative results showed that the content of retained austenite was as high as 22.1 wt%.

[0045] The continuous fluidized liquid nitrogen cryogenic treatment system of this embodiment includes: Fluidized bed reactor: A vertical cylindrical stainless steel container with an inner diameter of 500 mm and a height of 1200 mm, forming a processing chamber inside; Gas distribution plate: Located at the bottom of the reactor, it is made of porous sintered metal with a pore size of 30μm and a porosity of 35%; Liquid nitrogen storage tank: connected to the bottom of the reactor via pipeline; Liquid nitrogen flow control valve: precisely regulates the liquid nitrogen input rate; Low-temperature nitrogen circulating fan: The inlet is connected to the exhaust port at the top of the reactor, and the outlet is connected to the area below the gas distribution plate after passing through a heat exchanger; Cold energy recovery heat exchanger: shell and tube structure. The shell side (outer shell side) carries -180°C low-temperature nitrogen exhaust gas (from the fan outlet), which is used to pre-cool the new liquid nitrogen. The tube side (internal pipes) carries newly input room temperature or slightly cooled liquid nitrogen (from the liquid nitrogen storage tank). The pre-cooled liquid nitrogen is then sprayed into the bottom of the reactor to supplement the cooling capacity or regulate the temperature, so as to avoid the new liquid nitrogen from partially vaporizing due to absorbing ambient heat, which would require more liquid nitrogen to maintain the low temperature.

[0046] Feed screw conveyor: Inclined setting to feed high-carbon cast steel shot into the upper part of the reactor; Discharge air lock valve: Dual valves open and close alternately to prevent air from entering; Temperature sensor array: Three sets are arranged along the height of the reactor to monitor the temperature difference in real time; Differential pressure sensor: measures the pressure drop across the distribution plate to determine the fluidization state; Insulation layer: Encloses the entire reactor and uses vacuum multilayer insulation material.

[0047] Workflow arrow markings: Liquid nitrogen → Liquid nitrogen flow control valve → Below the gas distribution plate → Vaporization and ascent → Steel shot suspension (fluidization) → Top exhaust → Low-temperature nitrogen circulation fan → Feed screw and discharge gas lock valve → Return to below the distribution plate.

[0048] The above embodiments are merely preferred embodiments of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-substantial changes and substitutions made by those skilled in the art based on the present invention shall fall within the scope of protection claimed by the present invention.

Claims

1. A continuous fluidized liquid nitrogen cryogenic treatment system for spherical metal abrasives, characterized in that, include: A fluidized bed reactor has an internal processing chamber for containing spherical metal abrasives. The bottom of the processing chamber is provided with a gas distribution plate, which has multiple micropores with a pore size of 20-50 μm. A liquid nitrogen supply unit is connected to the bottom of the fluidized bed reactor for introducing liquid nitrogen into the processing chamber; The gas circulation unit includes a low-temperature nitrogen fan. The fan's inlet is connected to the top of the processing chamber, and the fan's outlet is connected to the bottom of the gas distribution plate after passing through a cold energy recovery heat exchanger. This is used to form a low-temperature nitrogen flow from bottom to top, so that the spherical metal abrasive is in a fluidized suspension state at -196°C to -180°C in the processing chamber. The temperature and differential pressure monitoring unit includes multiple temperature sensors and differential pressure sensors, which are respectively arranged at different heights of the processing chamber to monitor the fluidization state and temperature uniformity in real time. The continuous feed and discharge unit includes a feed screw conveyor, a discharge airlock valve, and a material buffer bin, which is used to achieve continuous input and output of spherical metal abrasives while maintaining a sealed processing chamber and a low temperature environment. The cold energy recovery heat exchanger is used to precool newly input liquid nitrogen using the low-temperature exhaust gas discharged from the processing chamber.

2. A method for cryogenic treatment of high-carbon cast steel shot, employing the system described in claim 1, characterized in that, Includes the following steps: S1. The spherical metal abrasive material, which has been isothermally quenched, is continuously fed into the fluidized bed reactor. S2. Start the liquid nitrogen supply unit and the gas circulation unit, and control the temperature inside the processing chamber to be maintained at -196℃ to -180℃; control the low-temperature nitrogen flow rate to be 0.3-0.6m / s; S3. Adjust the feed rate so that the residence time of the spherical metal abrasive in the processing chamber is 30-90 minutes; S4. The spherical metal abrasive after cryogenic treatment is continuously discharged through the discharge air lock valve and directly sent to the tempering process, where it is tempered at 200-250℃ for 1-2 hours.

3. The cryogenic treatment method for high-carbon cast steel shot as described in claim 2, characterized in that, The isothermal quenching process is as follows: after austenitization at 900-920℃, the sample is held in a salt bath at 260-300℃ for 60-90 minutes to obtain a microstructure mainly composed of bainite.

4. A high-strength, high-toughness, high-carbon cast steel shot, characterized in that, It has the following comprehensive characteristics: The chemical composition, by mass percentage, is: C 0.85-1.10%, Si 0.3-0.6%, Mn 0.6-1.0%, Cr 0.2-0.5%, Mo 0.1-0.3%; The microstructure is predominantly bainite, with a retained austenite content of less than 4%. The hardness is 56-60 HRC, and the hardness dispersion within the same batch is ≤ ±1.0 HRC; In the ASTM B677 standard cyclic test, the impact breakage rate is ≤3.5%.

5. A high-strength, high-toughness, high-carbon cast steel shot, characterized in that, The high-carbon cast steel shot is prepared by a cryogenic treatment method according to any one of claims 2-3. The spherical metal abrasive is high-carbon cast steel shot with an initial residual austenite content of 15%-25%, which is reduced to below 4% after cryogenic treatment.

6. An integrated production line for high-carbon cast steel shot, characterized in that, The following components are connected in sequence: a centrifugal granulation device, an isothermal quenching furnace, a continuous fluidized liquid nitrogen cryogenic treatment system as described in claim 1, a low-temperature tempering furnace, and an automatic screening and packaging unit.

7. The use of the high-strength, high-toughness, high-carbon cast steel shot as described in claim 5 in shot peening strengthening treatment of the surface of aero-engine blades or high-speed rail brake discs.

8. The use of the high-strength, high-toughness, high-carbon cast steel shot as described in claim 7 in shot peening strengthening treatment of the surface of aero-engine blades or high-speed rail brake discs, characterized in that, The high-strength, high-toughness, high-carbon cast steel shot has an impact breakage rate of ≤3.5%, a hardness of 56-60 HRC, and a hardness dispersion of ≤±1.0 HRC within the same batch.