High-bearing-capacity and high-precision surface strengthening steel and micro-deformation heat treatment process thereof

Abstract

The invention relates to the technical field of alloy steel processing, in particular to high-bearing-capacity high-precision surface strengthened steel and a micro-deformation heat treatment process thereof, and the high-bearing-capacity high-precision surface strengthened steel comprises carbon, chromium, silicon, manganese, nitrogen, boron, oxygen, sulfur, phosphorus, nickel, molybdenum and other metal elements. According to the method, a semi-finished product is subjected to surface severe deformation microalloying treatment, surface carbonitriding treatment, isothermal quenching (including waste heat austenitizing, water isothermal quenching and deep cooling), acid pickling and blackening treatment, and the final product is obtained. By adopting the design of reducing the carbon content component, the core part of the bearing has higher toughness, and the surface has high strength and high hardness after surface carburization, so that a part product with good core part toughness and high surface strength is finally obtained. According to the technology, the deformation of a final product is extremely low, the precision is improved, and the subsequent machining procedure can be removed. The amount of retained austenite in the surface structure of the final product is less than 3%, the surface hardness is not less than 63 HRC, and the lowest hardness of the core part is not greater than 45 HRC.

Description

Technical Field

[0001] This invention relates to the field of alloy steel processing technology, specifically to a high-load-bearing, high-precision surface-strengthened steel and its micro-deformation heat treatment process. Background Technology

[0002] With the rapid development of high-end equipment fields such as new energy vehicles, rail transit, and aerospace, the service environments of some surface-strengthened components, such as bearings and gears, are becoming increasingly demanding, thus placing higher requirements on their overall load-bearing capacity, surface wear resistance, and surface contact fatigue resistance. Currently, these high-performance surface-strengthened components mostly use high-carbon steel (carbon content 0.9~1.1wt%) for surface chemical heat treatment. Among these, carbonitriding is widely used due to its advantages such as low penetration temperature, fast penetration rate, high hardenability, and good wear resistance and contact fatigue resistance. Existing research shows that carbonitriding can increase surface hardness while forming residual compressive stress, which is beneficial to improving contact fatigue resistance. In addition, carbonitriding also helps to increase the content of retained austenite in the surface microstructure. A higher content of retained austenite can improve the plastic deformation capacity of bearing surfaces during compressive stress contact, delay crack initiation, increase crack initiation energy, and thus improve contact fatigue resistance.

[0003] In existing technologies, carbonitriding is typically followed by quenching and tempering to obtain tempered martensite and retained austenite structures. This structure has sufficient hardness but insufficient toughness. Particularly when used in high-load-bearing rolling bearings, under high contact stress, the bearing surface temperature rises sharply. At this point, the high content of retained austenite easily transforms into martensite, causing bearing deformation and severely affecting dimensional accuracy. Therefore, the tempered martensite and retained austenite structures obtained by carbonitriding of high-carbon steel limit its use in extremely high-load-bearing components such as rolling bearings, for example, angular contact ball bearings used in wind turbine generators and heavy machinery. Furthermore, contact fatigue failure in high-load, high-precision components often originates from the subsurface layer. This is related not only to the properties of the subsurface but also to the transition layer between the surface, subsurface, and matrix.

[0004] To address this, a high-load-bearing, high-precision surface-strengthened steel and its micro-deformation heat treatment process are proposed. Summary of the Invention

[0005] The purpose of this invention is to provide a high-load-bearing, high-precision surface-strengthened steel and its micro-deformation heat treatment process, in order to solve the problems in the prior art where surface-strengthened parts such as bearings and gears are prone to deformation under extreme heavy loads, resulting in low dimensional accuracy and contact fatigue originating from the subsurface layer.

[0006] To achieve the above objectives, the present invention provides the following technical solution: In a first aspect, the present invention provides a high-load-bearing and high-precision surface-strengthened steel, comprising, by weight percentage: carbon 0.60~0.80, chromium 1.0~1.3, silicon 0.6~0.8, manganese 0.5~0.8, nitrogen 0.01~0.03, boron 0.02~0.04, oxygen ≤0.0008, sulfur ≤0.01, phosphorus ≤0.015, nickel 0.3~0.5, molybdenum 0.05~0.10, with the remainder being Fe and impurities.

[0007] Preferably, the microstructure of the carburized layer of the high-load-bearing and high-precision surface-strengthened steel is, by volume percentage: 43%–47% bainite, 40%–45% carbides, 10%–15% tempered martensite, and 2.0%–3.0% retained austenite.

[0008] Preferably, the matrix microstructure of the high load-bearing and high-precision surface-strengthened steel is, by volume percentage: 75%–80% bainite, 7%–10% tempered martensite, 5%–8% carbides, and 5.0%–8.0% retained austenite.

[0009] Preferably, the carburized layer of the high-load-bearing and high-precision surface-strengthened steel has a depth of 250~400μm, a surface hardness of 63~68HRC, a matrix hardness of 32~38HRC, and a gradient decrease in hardness from the surface to the matrix, with a gradient layer thickness of 250~400μm.

[0010] Secondly, the present invention also provides a micro-deformation heat treatment process for high-load-bearing and high-precision surface-strengthened steel, comprising the following steps: S1: After performing a severe surface deformation micro-alloying treatment on the semi-finished product, the surface of the semi-finished product is coated with alloy powder, and then the coated surface of the semi-finished product is subjected to severe deformation treatment. S2: The semi-finished product after severe deformation is subjected to surface carbonitriding treatment to obtain a semi-finished product with a single component of chromium or nickel metal, or a uniform mixture of chromium and nickel metal. S3: The semi-finished product after surface carbonitriding treatment is subjected to isothermal quenching to obtain the quenched semi-finished product. S4: The semi-finished product after isothermal quenching is pickled and blackened to obtain the final product.

[0011] Preferably, the alloy powder in step S1 is a single component of chromium or nickel metal, or a powder of chromium or nickel uniformly mixed.

[0012] Preferably, the severe deformation treatment in step S1 is to continuously treat the surface coated with metal powder using methods such as rolling, impact, and shot peening.

[0013] Preferably, the surface carbonitriding treatment in step S2 includes a preliminary low-temperature diffusion treatment, a carbonitriding treatment, and a final low-temperature diffusion treatment, which correspond to the following three steps in sequence: S21: Preliminary low-temperature diffusion treatment: Place the semi-finished product after S1 treatment into a controlled atmosphere vacuum multi-purpose furnace, heat to 300~350℃, and keep warm for 3~6 hours; S22: Carbonitriding treatment: Introduce a carbonitriding atmosphere into the semi-finished product after step S21, heat to 650~750℃, hold for 6~8 hours, and control the carbon potential at 0.5%~0.6%; S23: Final low-temperature diffusion treatment: Cool the semi-finished product after S22 treatment to 600~640℃ and maintain it for 2~3 hours, with the carbon potential controlled at 0.7%~0.8%.

[0014] Preferably, the pickling process in step S4 is as follows: pickling with a pickling solution with a sulfuric acid concentration of 10% to 15%, the pickling temperature is 80 to 90°C, and the pickling time is 10 to 15 minutes.

[0015] Preferably, the blackening treatment in step S4 is as follows: the pickled semi-finished product is treated with an oxidation solution. The temperature of the oxidation solution when entering the tank is 120~125℃, and the temperature when exiting the tank is 125~130℃. The treatment time is 10~20min. The oxidation solution composition is CuSO4 4~8g / L, Na3PO4 7~15g / L, NaNO2 2~4g / L, and SeO2 4~8g / L.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention employs a reduced carbon content composition to achieve higher toughness in the bearing core. After surface carburizing, the surface exhibits high strength and hardness, ultimately resulting in a component product with good core toughness and high surface strength. The use of a low Cr content composition helps reduce carbide segregation and residual austenite content during heat treatment. Intense surface deformation induces Cr alloying elements to diffuse into the surface, increasing the surface Cr content. Ultimately, the surface has a high Cr content, while the matrix has a low Cr content, leading to the formation of a large amount of chromium carbides on the surface, thus improving surface hardness. By performing intense surface deformation alloying treatment before carbonitriding, numerous crystal structure defects are formed on the surface, which facilitates the diffusion of carbon and nitrogen elements during carbonitriding, reducing the co-diffusion temperature and shortening the time. Furthermore, the higher Cr or Ni content on the surface enhances surface hardness, with the hardness decreasing gradually from the surface to the core.

[0017] 2. This invention divides the surface carbonitriding treatment into three stages. The first stage allows elements such as Cr or Ni to further diffuse to the surface, preparing for the subsequent second and third stages. During the isothermal quenching process, the first stage utilizes the residual heat temperature of carbonitriding, which helps save energy and shorten the process flow. The final product has a residual austenite content of less than 3% in its surface structure, a surface hardness of not less than 63 HRC, and a minimum core hardness of not more than 45 HRC. The technology of this invention results in extremely low deformation of the final product, improving its precision. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the microstructure of the surface carburized layer of the final product of this invention, the carburized bearing. Figure 2 This is a schematic diagram of the matrix microstructure of the carburized bearing, the final product of this invention. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0020] Please refer to Figures 1 to 2 A high-load-bearing and high-precision surface-strengthened steel is provided, comprising, by weight percentage: carbon 0.60~0.80, chromium 1.0~1.3, silicon 0.6~0.8, manganese 0.5~0.8, nitrogen 0.01~0.03, boron 0.02~0.04, oxygen ≤0.0008, sulfur ≤0.01, phosphorus ≤0.015, nickel 0.3~0.5, molybdenum 0.05~0.10, with the remainder being Fe and impurities.

[0021] Specifically, the microstructure of the carburized layer of high-load-bearing and high-precision surface-strengthened steel, by volume percentage, is: 43%–47% bainite, 40%–45% carbides, 10%–15% tempered martensite, and 2.0%–3.0% retained austenite.

[0022] The microstructure of the matrix of high load-bearing and high-precision surface-strengthened steel, by volume percentage, is: 75%–80% bainite, 7%–10% tempered martensite, 5%–8% carbides, and 5.0%–8.0% retained austenite.

[0023] The carburized layer depth of high load-bearing and high-precision surface-strengthened steel is 250~400μm, the surface hardness is 63~68HRC, the matrix hardness is 32~38HRC, the hardness decreases from the surface to the matrix, and the thickness of the gradient layer is 250~400μm.

[0024] The elements of this application are: C: Carbon is the main element for achieving high hardness, existing primarily in solid solution and carbide forms. High-load-bearing, high-precision surface-strengthened steels generally have a hardness greater than 60 HRC, requiring a carbon content greater than 0.6 wt%. However, when the carbon content exceeds 0.8 wt%, brittleness increases dramatically. Furthermore, because this invention employs surface micro / nano-structure refinement technology, it facilitates carbon diffusion, meaning a higher carbon content can be obtained during carbonitriding. Therefore, the carbon content in the matrix of this invention is lower than that of common high-carbon surface-strengthened steels. Specifically, the carbon content in this invention is controlled at 0.6%–0.80%.

[0025] Cr: Chromium improves hardenability and is also the main carbide (Cr 23 C6 and Cr7C3 are chromium-forming elements that improve hardness and wear resistance and reduce contact fatigue failure. However, when the Cr content is high, it is easy to form banded structures (carbide segregation), which can also lead to an increase in the content of retained austenite. Taking all factors into consideration, the chromium content in this invention is controlled at 1.0~1.3wt%.

[0026] Si: Silicon improves hardenability, especially when added together with Mn, further enhancing hardenability. Si also exhibits good tempering stability, but a silicon content exceeding 0.8 wt% increases brittleness. Therefore, in this invention, the silicon content is controlled at 0.6~0.8 wt%.

[0027] Mn: Manganese improves hardenability and is usually added simultaneously with Si. Manganese helps to eliminate the harmful effects of sulfur. However, when the Mn content is greater than 0.8 wt%, banded segregation is easily formed, and high Mn content also increases retained austenite. Taking all factors into consideration, the manganese content in this invention is controlled at 0.5~0.8 wt%.

[0028] Nitrogen (N) can inhibit austenite grain growth, refine grains, and improve strength and toughness. It also enhances hardenability, especially when added simultaneously with boron (B). However, nitrogen has limited solubility in Fe, and excessive nitrogen content can easily lead to defects such as bubbles and inclusions. Therefore, the nitrogen content in this invention is controlled at 0.01~0.03 wt%.

[0029] B: Boron can significantly improve hardenability and is often added in combination with nitrogen. The B / N ratio is controlled at 1.0~1.5. Boron can also replace elements such as Cr, Mn, Mo, and Ni. However, excessive addition of boron can easily lead to poor hot working performance. Taking all factors into consideration, the amount of boron in this invention is controlled at 0.02~0.04 wt%.

[0030] Ni: While nickel increases strength, it also significantly improves toughness and hardenability. However, high nickel content easily leads to the formation of retained austenite, resulting in a high content of retained austenite in the matrix. Under heavy loads, this retained austenite transforms into martensite, causing bearing deformation and reducing dimensional accuracy. Therefore, to reduce the presence of retained austenite, N and B are used instead of Ni. Considering all factors, Ni is controlled at 0.3~0.5wt% in this invention.

[0031] Mo: Molybdenum can form recalcitrant carbides, which strengthen the matrix and improve hardenability. However, excessive content can easily lead to banded structures (carbide segregation), and Mo is also relatively expensive. Considering all factors, the Mo content in this invention is controlled at 0.05~0.10 wt%.

[0032] S and P: Oxygen, sulfur, and phosphorus are harmful impurity elements in bearing steel. In this invention, the contents of oxygen, sulfur, and phosphorus are controlled at ≤0.008wt%, ≤0.01wt%, and ≤0.015wt%, respectively.

[0033] This application also provides a micro-deformation heat treatment process for the high-load-bearing and high-precision surface-strengthened steel described above, comprising the following steps: S1: After performing a severe surface deformation micro-alloying treatment on the semi-finished product, the surface of the semi-finished product is coated with alloy powder, and then the coated surface of the semi-finished product is subjected to severe deformation treatment. S2: The semi-finished product after severe deformation is subjected to surface carbonitriding treatment to obtain a semi-finished product with a single component of chromium or nickel metal, or a uniform mixture of chromium and nickel metal. S3: The semi-finished product after surface carbonitriding treatment is subjected to isothermal quenching to obtain the quenched semi-finished product. S4: The semi-finished product after isothermal quenching is pickled and blackened to obtain the final product.

[0034] The alloy powder in step S1 is a single component of chromium or nickel metal, or a powder of chromium or nickel uniformly mixed.

[0035] The severe deformation treatment in step S1 involves continuously treating the surface coated with metal powder using methods such as rolling, impact, and shot peening.

[0036] The surface carbonitriding treatment in step S2 includes a preliminary low-temperature diffusion treatment, a carbonitriding treatment, and a final low-temperature diffusion treatment, which correspond to the following three steps in sequence: S21: Place the semi-finished product after S1 treatment into a controlled atmosphere vacuum multi-purpose furnace, heat to 300~350℃ and keep warm for 3~6 hours to allow chromium or nickel elements to diffuse more evenly to the surface of the semi-finished product. S22: Introduce a co-diffusion atmosphere into the semi-finished product after step S21, heat to 650~750℃, hold for 6~8 hours, and control the carbon potential at 0.5%~0.6%; S23: Cool the semi-finished product after S22 treatment to 600~640℃ and maintain it for 2~3 hours, with the carbon potential controlled at 0.7%~0.8%.

[0037] The pickling process in step S4 is as follows: pickling is performed using a pickling solution with a sulfuric acid concentration of 10% to 15%, the temperature is set to 80 to 90°C, and the pickling time is 10 to 15 minutes.

[0038] The blackening treatment in step S4 is as follows: the pickled semi-finished product is treated with an oxidation solution. The temperature of the oxidation solution when entering the tank is 120~125℃, and the temperature when exiting the tank is 125~130℃. The treatment time is 10~20min. The composition of the oxidation solution is CuSO4 4~8g / L, Na3PO4 7~15g / L, NaNO2 2~4g / L, and SeO2 4~8g / L.

[0039] The final products include components such as bearings, gears, drive wheels, and pistons that have undergone surface chemical and heat treatment. Example

[0040] A carburized bearing for automobiles was prepared, with the following chemical composition (wt%): C 0.60, Cr 1.3, Si 0.6, Mn 0.8, N 0.01, B 0.02, O ≤0.0008, S ≤0.01, P ≤0.015, Ni 0.5, Mo 0.05, with the remainder being Fe and impurities. The preparation process is as follows: bearing semi-finished product → surface micro-alloying treatment with severe deformation → surface carbonitriding treatment (including preliminary low-temperature diffusion treatment, carbonitriding treatment, and final low-temperature diffusion treatment) → isothermal quenching (including residual heat austenitization, water isothermal quenching, and deep cryogenic treatment) → pickling and blackening treatment → final product. Specific process parameters are as follows: S1: The bearing semi-finished product undergoes a surface cold working and severe deformation micro-alloying treatment. The surface of the bearing semi-finished product is coated with a single-component Cr metal, and the surface coated with metal powder is continuously subjected to severe plastic deformation treatment using a rolling method.

[0041] S2: Surface carbonitriding treatment is performed, consisting of three processes: preliminary low-temperature diffusion treatment, carbonitriding treatment, and final low-temperature diffusion treatment. In the preliminary low-temperature diffusion treatment, the sample after the first step is placed in a controlled atmosphere vacuum multi-purpose furnace and heated to 300℃ for 6 hours. Subsequently, propane, methanol, nitrogen, and ammonia are introduced. During the carbonitriding treatment, the temperature is raised to 650℃ and held for 8 hours, with the carbon potential controlled at 0.6%. Then, in the final low-temperature diffusion treatment, the temperature is maintained at 600℃ for 3 hours, with the carbon potential controlled at 0.7%.

[0042] S3: Isothermal quenching is performed in three stages: residual heat austenitization, isothermal quenching, and cryogenic treatment. Residual heat austenitization involves stopping the co-diffusion atmosphere from the second step and evacuating the furnace until the vacuum level is less than 1×10⁻³ Pa. The sample is heated to 800℃ and held for 60 minutes. During isothermal quenching, the sample is rapidly immersed in 70℃ water. After cooling to 240℃, it is removed and placed in another furnace with the same vacuum temperature for 2 hours. During cryogenic treatment, the sample is finally cooled to room temperature in cold water at -50℃.

[0043] S4: Pre-blackening treatment is performed. The pickling solution has a sulfuric acid concentration of 10%, the temperature is 80℃, and the pickling time is 15 min. After pickling, a blackening treatment is performed. The blackening treatment involves an oxidation solution temperature of 120℃ upon entering the tank and 125℃ upon exiting the tank, with a treatment time of 20 min. The oxidation solution composition is CuSO4 4 g / L, Na3PO4 15 g / L, NaNO2 4 g / L, and SeO2 4 g / L. The final microstructure of the bearing carburized layer (volume fraction) is: 43% bainite, 45% carbides, 10% tempered martensite, and 2.0% retained austenite; the matrix microstructure (volume fraction) is: 80% bainite, 7% tempered martensite, 5% carbides, and 8.0% retained austenite. The carburized layer depth is 400 μm, the surface hardness is 63 HRC, the matrix hardness is 32 HRC, and the hardness gradient layer thickness is 400 μm. Example

[0044] A carburized gear for new energy vehicles was prepared, with the following chemical composition (wt%): C 0.80, Cr 1.0, Si 0.8, Mn 0.5, N 0.03, B 0.04, O ≤0.0008, S ≤0.01, P ≤0.015, Ni 0.3, Mo 0.10, with the remainder being Fe and impurities. The preparation process involved: semi-finished gear → microalloying treatment with severe surface deformation → surface carbonitriding treatment (including preliminary low-temperature diffusion treatment, carbonitriding treatment, and final low-temperature diffusion treatment) → isothermal quenching (including residual heat austenitization, water isothermal quenching, and deep cryogenic treatment) → pickling and blackening treatment → final product. Specific process parameters are as follows: S1: The bearing semi-finished product undergoes a severe surface deformation micro-alloying treatment. The surface of the bearing semi-finished product is coated with a mixture of Cr and Ni metals, and the surface coated with metal powder is subjected to severe plastic deformation treatment continuously using an impact method.

[0045] S2: Surface carbonitriding treatment is performed, consisting of three processes: preliminary low-temperature diffusion treatment, carbonitriding treatment, and final low-temperature diffusion treatment. In the preliminary low-temperature diffusion treatment, the sample after the first step is placed in a controlled atmosphere vacuum multi-purpose furnace and heated to 350℃ for 3 hours. Subsequently, propane, methanol, nitrogen, and ammonia are introduced. During the carbonitriding treatment, the temperature is raised to 750℃ and held for 6 hours, with the carbon potential controlled at 0.5%. Then, in the final low-temperature diffusion treatment, the temperature is maintained at 640℃ for 2 hours, with the carbon potential controlled at 0.8%.

[0046] S3: Isothermal quenching is performed in three stages: residual heat austenitization, isothermal quenching, and cryogenic treatment. Residual heat austenitization involves stopping the co-diffusion atmosphere from step two and evacuating the furnace until the vacuum level is less than 1×10⁻³ Pa. The sample is heated to 850℃ and held for 30 minutes. During isothermal quenching, the sample is rapidly immersed in 90℃ water. After cooling to 210℃, it is removed and placed in another furnace with the same vacuum temperature for 4 hours. During cryogenic treatment, the sample is finally cooled to room temperature in cold water at -75℃.

[0047] S4: Pickling and blackening treatments are performed. The pickling solution has a sulfuric acid concentration of 15%, a temperature of 90℃, and a pickling time of 10 minutes. After pickling, blackening treatment is performed. The oxidation solution temperature is 125℃ upon entering the tank and 130℃ upon exiting the tank, with a treatment time of 10 minutes. The composition of the oxidation solution is CuSO4 8g / L, Na3PO4 7g / L, NaNO2 2g / L, and SeO2 8g / L. The final microstructure of the carburized layer of the gear is (volume fraction): 47% bainite, 40% carbides, 10% tempered martensite, and 3.0% retained austenite; the matrix microstructure (volume fraction) is: 75% bainite, 10% tempered martensite, 8% carbides, and 7.0% retained austenite. The carburized layer depth is 250μm, the surface hardness is 68HRC, the matrix hardness is 38HRC, and the hardness gradient layer thickness is 250μm. Example

[0048] A carburized transmission wheel for new energy vehicles was prepared, with the following chemical composition (wt%): C 0.90, Cr 1.1, Si 0.7, Mn 0.7, N 0.02, B 0.03, O ≤0.0008, S ≤0.01, P ≤0.015, Ni 0.4, Mo 0.06, with the remainder being Fe and impurities. The preparation process involved: semi-finished transmission wheel → surface micro-alloying treatment with severe deformation → surface carbonitriding treatment (including preliminary low-temperature diffusion treatment, carbonitriding treatment, and final low-temperature diffusion treatment) → isothermal quenching (including residual heat austenitization, water isothermal quenching, and deep cryogenic treatment) → pickling and blackening treatment → final product. Specific process parameters are as follows: S1: The semi-finished product undergoes severe surface deformation and micro-alloying treatment by cold working. The surface of the semi-finished product is coated with metallic Cr, and the surface coated with metallic powder is continuously subjected to severe deformation treatment by shot peening.

[0049] S2: Surface carbonitriding treatment is performed, consisting of three processes: preliminary low-temperature diffusion treatment, carbonitriding treatment, and final low-temperature diffusion treatment. In the preliminary low-temperature diffusion treatment, the sample after the first step is placed in a controlled atmosphere vacuum multi-purpose furnace and heated to 310℃ for 5 hours. Subsequently, propane, methanol, nitrogen, and ammonia are introduced. During the carbonitriding treatment, the temperature is raised to 710℃ and held for 6.5 hours, with the carbon potential controlled at 0.54%. Then, in the final low-temperature diffusion treatment, the temperature is maintained at 630℃ for 2.5 hours, with the carbon potential controlled at 0.77%.

[0050] S3: Isothermal quenching is performed in three stages: residual heat austenitization, isothermal quenching, and cryogenic treatment. Residual heat austenitization involves stopping the co-diffusion atmosphere from the second step and evacuating the furnace until the vacuum level is less than 1×10⁻³ Pa. The sample is heated to 820℃ and held for 40 minutes. During isothermal quenching, the sample is rapidly immersed in 80℃ water. After cooling to 220℃, it is removed and placed in another furnace with the same vacuum temperature for 3 hours. During cryogenic treatment, the sample is finally cooled to room temperature in cold water at -60℃.

[0051] S4: Pickling and blackening treatments were performed. The pickling solution had a sulfuric acid concentration of 12%, a temperature of 85℃, and a pickling time of 12 minutes. After pickling, blackening treatment was performed. The oxidation solution temperature upon entering the tank was 122℃, and the exit temperature was 128℃, with a treatment time of 14 minutes. The oxidation solution composition was CuSO4 6g / L, Na3PO4 11g / L, NaNO2 3g / L, and SeO2 5g / L. The final microstructure of the carburized layer of the transmission wheel (volume fraction) was: 44% bainite, 43% carbides, 11% tempered martensite, and 2.0% retained austenite; the matrix microstructure (volume fraction) was: 78% bainite, 8% tempered martensite, 7% carbides, and 7.0% retained austenite. The carburized layer depth was 320μm, the surface hardness was 65HRC, the matrix hardness was 40HRC, and the hardness gradient layer thickness was 320μm.

[0052] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A high-load-bearing, high-precision surface-strengthened steel, characterized in that, It includes, by weight percentage: carbon 0.60~0.80, chromium 1.0~1.3, silicon 0.6~0.8, manganese 0.5~0.8, nitrogen 0.01~0.03, boron 0.02~0.04, oxygen ≤0.0008, sulfur ≤0.01, phosphorus ≤0.015, nickel 0.3~0.5, molybdenum 0.05~0.10, with the remainder being Fe and impurities.

2. The high-load-bearing, high-precision surface-strengthened steel according to claim 1, characterized in that, The microstructure of the carburized layer of the high-load-bearing and high-precision surface-strengthened steel, by volume percentage, is: 43%–47% bainite, 40%–45% carbides, 10%–15% tempered martensite, and 2.0%–3.0% retained austenite.

3. The high-load-bearing, high-precision surface-strengthened steel according to claim 1, characterized in that, The microstructure of the high-load-bearing and high-precision surface-strengthened steel, by volume percentage, is: 75%–80% bainite, 7%–10% tempered martensite, 5%–8% carbides, and 5.0%–8.0% retained austenite.

4. The high-load-bearing, high-precision surface-strengthened steel according to claim 1, characterized in that, The high-load-bearing and high-precision surface-strengthened steel has a carburized layer depth of 250~400μm, a surface hardness of 58~62HRC, and a matrix hardness of 32~38HRC.

5. A micro-deformation heat treatment process for high-load-bearing and high-precision surface-strengthened steel according to any one of claims 1 to 4, characterized in that, Includes the following steps: S1: After the semi-finished product undergoes severe surface deformation micro-alloying treatment, alloy powder is coated on the surface of the semi-finished product, and then the coated surface of the semi-finished product undergoes severe deformation treatment. S2: The semi-finished product after severe deformation is subjected to surface carbonitriding treatment to obtain a semi-finished product with a single component of chromium or nickel metal, or a uniform mixture of chromium and nickel metal. S3: The semi-finished product after surface carbonitriding is subjected to isothermal quenching to obtain the quenched semi-finished product. S4: The semi-finished product after isothermal quenching is pickled and blackened to obtain the final product.

6. The micro-deformation heat treatment process for high-load-bearing and high-precision surface-strengthened steel according to claim 5, characterized in that, The alloy powder in step S1 is a single component of chromium or nickel metal, or a powder that is a uniform mixture of chromium and nickel.

7. The micro-deformation heat treatment process for high-load-bearing and high-precision surface-strengthened steel according to claim 5, characterized in that, The severe deformation treatment in step S1 involves continuously treating the surface coated with metal powder using methods such as rolling, impact, and shot peening.

8. The micro-deformation heat treatment process for high-load-bearing and high-precision surface-strengthened steel according to claim 5, characterized in that, The surface carbonitriding treatment in step S2 includes a preliminary low-temperature diffusion treatment, a carbonitriding treatment, and a final low-temperature diffusion treatment, which correspond to the following three steps in sequence: S21: Preliminary low-temperature diffusion treatment: Place the semi-finished product after S1 treatment into a controlled atmosphere vacuum multi-purpose furnace, heat to 300~350℃, and keep warm for 3~6 hours to allow chromium or nickel elements to diffuse more evenly to the surface of the semi-finished product. S22: Carbonitriding treatment: Introduce a carbonitriding atmosphere into the semi-finished product after step S21, heat to 650~750℃, hold for 6~8 hours, and control the carbon potential at 0.5%~0.6%; S23: Final low-temperature diffusion treatment: Cool the semi-finished product after S22 treatment to 600~640℃ and keep it at that temperature for 2~3 hours, while controlling the carbon potential at 0.7%~0.8%.

9. The micro-deformation heat treatment process for high-load-bearing and high-precision surface-strengthened steel according to claim 5, characterized in that, The pickling process in step S4 is as follows: pickling is performed using a pickling solution with a sulfuric acid concentration of 10% to 15%, the pickling temperature is 80 to 90°C, and the pickling time is 10 to 15 minutes.

10. The micro-deformation heat treatment process for high-load-bearing and high-precision surface-strengthened steel according to claim 8, characterized in that, The blackening treatment in step S4 is as follows: the pickled semi-finished product is treated with an oxidation solution. The temperature of the oxidation solution when entering the tank is 120~125℃, and the temperature when exiting the tank is 125~130℃. The treatment time is 10~20min. The composition of the oxidation solution is CuSO4 4~8g / L, Na3PO4 7~15g / L, NaNO2 2~4g / L, and SeO2 4~8g / L.

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