A composite carburizing heat treatment method of high niobium carburizing steel and high niobium carburizing steel part

Abstract

This invention provides a composite carburizing heat treatment method for high-niobium carburizing steel and high-niobium carburized steel parts. Addressing the problem of shallow effective carburizing depth and slow carburizing rate in 20CrNi2MoNb high-niobium carburizing steel with a niobium content ≥0.06% where niobium forms highly dispersed, extremely fine NbC particles that hinder carbon atom diffusion during conventional carburizing, resulting in a carbon concentration ≥0.80%, this invention innovatively designs a composite process involving two carburizing stages and intermediate segmented high-temperature tempering. Through intermediate high-temperature tempering, the highly dispersed, extremely fine NbC particles formed in the first carburizing stage mature and grow, releasing carbon atom diffusion channels, eliminating lattice distortion and internal stress, and significantly reducing the content of niobium in solid solution in the matrix, precipitating it as NbC particles. This clears the way for rapid carbon atom diffusion in the second carburizing stage. This significantly improves the carburizing efficiency of high-niobium carburizing steel, increasing the effective carburizing depth (≥0.8% carbon concentration) by over 280%, while simultaneously forming a gentle and ideal carbon concentration gradient, fully leveraging the fine-grain strengthening and dispersion strengthening properties of high-niobium steel.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology for metallic materials, and in particular to a composite carburizing heat treatment method for high-niobium carburizing steel and high-niobium carburizing steel parts. Background Technology

[0002] Carburized steel is a key material for core components such as bearings and gears in mechanical equipment transmission systems. Through carburizing heat treatment, a high-carbon hardened layer is formed on the surface of the workpiece, achieving a balance between high surface hardness and high wear resistance and high core toughness and high strength, thus meeting the requirements for long fatigue life and high load-bearing capacity of the components. To adapt to the continuous upgrading requirements of high-end equipment for component performance, a high-niobium alloyed 20CrNi2MoNb carburizing steel has been developed domestically. This steel, with the addition of 0.06~0.07% niobium, achieves a significant grain refinement effect and possesses the performance potential of high strength, high toughness, long fatigue life, and high wear resistance, making it a preferred upgrade material for high-end bearings and gears. However, in actual industrial production, a serious technical bottleneck has emerged when using conventional one-time carburizing processes to treat this high-niobium carburizing steel: under conventional carburizing processes, it is difficult for the workpiece to achieve the expected surface carbon concentration distribution; the effective carburized layer depth with a carbon concentration ≥0.8% is severely shallow, and the overall carburizing rate is extremely slow (see...). Figure 1 This cannot meet the efficiency and performance requirements of industrialized production.

[0003] Systematic mechanistic studies have revealed that the core cause of the aforementioned technical bottleneck lies in the fact that niobium, a strong carbide-forming element, has a high niobium content in 20CrNi2MoNb steel. During conventional carburizing, this niobium strongly binds with carbon atoms diffusing from the workpiece surface inward, forming a large number of highly dispersed, extremely fine NbC particles at grain boundaries and dislocations. These NbC particles strongly pin grain boundaries and dislocations, directly blocking the diffusion channels of carbon atoms and severely hindering further diffusion of carbon atoms into the steel matrix. This ultimately leads to a significant decrease in carburizing efficiency, insufficient effective carburized layer depth, and poor carbon concentration gradient distribution. A review of existing technologies reveals that related technical solutions for niobium-containing carburizing steel generally impose strict limits on the niobium content in the steel. For example, US patent application US 20210292880A1 discloses a carburized steel component that strictly limits the niobium content in the steel matrix to between 0% and 0.06%. This method achieves grain refinement through conventional carburizing followed by reheating and quenching. However, it does not address the core technical problem of niobium hindering carbon atom diffusion in high-niobium carburized steel with a niobium content exceeding 0.06%, nor does it propose corresponding process improvement solutions. In summary, in the existing technology, for 20CrNi2MoNb high-niobium carburized steel with a niobium content ≥0.06%, there is still no heat treatment process that can effectively overcome the carburizing obstacle caused by high niobium while fully utilizing the excellent performance potential of high-niobium steel. This has become the core problem restricting the industrial-scale application of this high-niobium carburized steel. Summary of the Invention

[0004] The purpose of this invention is to address the problems of hindered carbon diffusion, poor carbon concentration distribution in the carburizing layer (severely insufficient depth of carbon concentration above 0.8% on the working surface after removing machining allowance), and slow carburizing rate in the conventional carburizing process of high-niobium carburizing steel. This invention provides a composite carburizing heat treatment method for high-niobium carburizing steel and high-niobium carburizing steel parts. It effectively eliminates the hindering effect of high niobium on the carburizing process, significantly improves carburizing efficiency, forms an ideal carbon concentration gradient in the carburizing layer, and fully utilizes the fine-grain strengthening and dispersion strengthening properties of high-niobium steel, achieving a breakthrough in the industrial application of high-niobium carburizing steel.

[0005] According to one objective of the present invention, a composite carburizing heat treatment method for high-niobium carburizing steel is provided, wherein the high-niobium carburizing steel is 20CrNi2MoNb carburizing steel with a niobium element mass percentage ≥0.06%, and the method comprises the following steps performed sequentially: S1. First carburizing treatment: After pre-oxidation, the workpiece is placed in a carburizing atmosphere to complete austenitizing heating and first carburizing. After carburizing, the workpiece is cooled and homogenized, and then quenched. S2. High-temperature tempering treatment: The workpiece treated in S1 is subjected to segmented high-temperature tempering, and after tempering, it is air-cooled to room temperature. S3, Second Carburizing Treatment: The workpiece treated in S2 is placed in a carburizing atmosphere again to complete austenitizing heating and second carburizing. After achieving the target carburized layer depth and carbon concentration distribution, it is cooled down, homogenized, quenched, and then tempered.

[0006] Furthermore, in step S1, the temperature of the pre-oxidation treatment is 510~530℃, and the pre-oxidation treatment is carried out in an air-heated furnace.

[0007] Furthermore, in step S1, the austenitizing heating temperature for the first carburizing is 930~950℃, and the carbon potential of the carburizing atmosphere is 1.15~1.40%; after carburizing, the temperature is lowered to 860~880℃, and oil quenching is performed after homogenization.

[0008] Furthermore, in step S2, the segmented high-temperature tempering specifically involves: first heating the workpiece to 680~690℃ for the first stage of heat preservation tempering, then cooling it in the furnace to 620~630℃ for the second stage of heat preservation tempering, and finally air-cooling it to room temperature after both stages of tempering are completed.

[0009] Furthermore, the holding time for the first stage of heat preservation and tempering is 14~18h, and the holding time for the second stage of heat preservation and tempering is 14~18h.

[0010] Furthermore, in step S3, the austenitizing heating temperature for the second carburizing is 930~950℃, and the carbon potential of the carburizing atmosphere is 1.15~1.40%; after carburizing, the temperature is lowered to 860~880℃, and after homogenization, oil cooling or molten salt bath quenching is used.

[0011] Furthermore, in step S3, the tempering temperature after quenching and cooling is 200~400℃.

[0012] Furthermore, after step S3 is completed, the workpiece is subjected to secondary quenching and low-temperature tempering treatment, specifically: the workpiece is heated to 815~820℃ for oil quenching, and then subjected to low-temperature tempering at 180~200℃.

[0013] Furthermore, the high-temperature tempering in step S2 causes the dispersed fine NbC particles formed during the first carburizing process to mature and grow, releasing carbon atom diffusion channels, eliminating lattice distortion and internal stress generated during the carburizing process, and providing an ordered matrix environment for carbon atom diffusion in step S3.

[0014] According to another objective of the present invention, the present invention provides a high-niobium carburized steel part, which is prepared by the above-mentioned composite carburizing heat treatment method for high-niobium carburized steel. In the carburized layer of the part, the effective carburized layer depth is ≥1.0 mm with a carbon concentration ≥0.8%, the carbon concentration gradient of the carburized layer is gentle from the surface to the inside, and NbC reinforcing particles are dispersed in the carburized layer.

[0015] The technical solution of this invention employs a composite process of two carburizing stages with intermediate high-temperature tempering. This process allows the fine, dispersed NbC particles generated during the initial carburizing to mature and grow, releasing carbon diffusion channels, eliminating lattice distortion and internal stress. It also significantly reduces the content of niobium in solid solution in the matrix, causing it to precipitate as NbC particles. This significantly removes the hindrance of high niobium to carbon diffusion, effectively increasing the carburizing rate and effective carburized layer depth. It can form a gentle and reasonable surface carbon concentration gradient, avoiding carbide network or abnormal aggregation, while retaining the dispersion strengthening effect of NbC. This fully leverages the comprehensive properties of high-niobium carburized steel, including high strength, high toughness, high wear resistance, and high fatigue life. Attached Figure Description

[0016] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0017] Figure 1The carbon concentration distribution curve of the carburized layer after conventional one-time carburizing process of 20CrNi2MoNb steel is shown in the present invention. Figure 2 This is the carbon concentration distribution curve of the carburized layer after the composite carburizing heat treatment process of the present invention on 20CrNi2MoNb steel. Figure 3 The image shows the metallographic structure of the 20CrNi2MoNb steel of this invention after the first carburizing and high-temperature tempering treatment. Figure 4 Microstructure of 20CrNi2MoNb steel after being treated by the composite process of this invention and then quenched and tempered twice; Figure 5 The carbon concentration distribution curve of the carburized layer after two conventional carburizing processes on 20CrNi2MoNb steel without intermediate high-temperature tempering is shown in the prior art of this invention. Detailed Implementation

[0018] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," and "counterclockwise," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention.

[0020] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the stated features. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified. Furthermore, the terms "installed," "connected," and "linked" should be interpreted broadly; for example, they may refer to a fixed connection, a detachable connection, or an integral connection; they may refer to a mechanical connection or an electrical connection; they may refer to a direct connection or an indirect connection through an intermediate medium; and they may refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0021] The 20CrNi2MoNb high-niobium carburizing steel used in the following examples and comparative examples has the following chemical composition by mass percentage: C 0.19-0.20%, Si 0.32-0.33%, Mn 0.63%, Cr 0.55-0.56%, Ni 1.8%, Mo 0.25-0.26%, Nb 0.06-0.07%, V 0.094-0.097%, with the balance being Fe and impurities.

[0022] Example 1 This embodiment provides a composite carburizing heat treatment method for high-niobium carburizing steel, the specific steps of which are as follows: 1. First carburizing treatment: The workpiece is placed in the air heating furnace of the single-push plate carburizing furnace production line and heated to 520℃ for pre-oxidation treatment. Then the workpiece is moved into the main carburizing furnace, which is divided into a heating zone, a strong carburizing zone 1, a strong carburizing zone 2, and a diffusion zone. The carbon potential of the carburizing atmosphere in the furnace is controlled at 1.20~1.40%, the heating temperature is controlled at 935~945℃, and the cycle time is 74 minutes to complete the first carburizing. After carburizing, the workpiece enters the cooling zone to cool to 870℃. After homogenization, it is quenched in oil for cooling and then removed for online cleaning.

[0023] 2. Segmented high-temperature tempering treatment: After the first carburizing, the workpiece is transferred into a multi-purpose furnace and heated to 690℃ and held for 16 hours to complete the first stage of high-temperature tempering; then the workpiece is cooled to 630℃ and held for 16 hours to complete the second stage of high-temperature tempering; after tempering, the workpiece is taken out of the furnace and air-cooled to room temperature.

[0024] 3. Second carburizing treatment: Place the workpiece after high-temperature tempering in a multi-purpose furnace and reheat it to 940℃. Hold it at this temperature for 9 hours in a carburizing atmosphere with a carbon potential of 1.3~1.4% to complete the second carburizing. After carburizing, cool it down to 870℃ for uniform temperature, then quench it in oil to cool it. Take it out and clean it. Then heat the workpiece to 350℃ for tempering and air cool it after taking it out of the furnace.

[0025] 4. Secondary quenching and low-temperature tempering: The workpiece is placed in a rotary hearth furnace, heated to 818℃ and held for oil quenching, then heated to 190℃ and held for low-temperature tempering, and finally air-cooled after being removed from the furnace to complete the entire heat treatment process.

[0026] In this embodiment, the first carburizing process lasted 23.23 hours, the second carburizing process lasted 9 hours, and the total carburizing time was 32.23 hours. Metallographic analysis, microhardness testing, and carbon concentration distribution detection were performed on the treated workpiece. The results showed that the effective carburized layer depth with a carbon concentration ≥0.8% reached 1.35 mm, with a smooth transition in carbon concentration from the surface to the interior, and no network carbide precipitation or aggregation. A large number of fine NbC reinforcing particles were dispersed in the carburized layer, and the matrix structure was uniform and fine. The surface hardness, core toughness, wear resistance, and contact fatigue life of the workpiece all met the requirements for high-end bearings.

[0027] Example 2 This embodiment provides a composite carburizing heat treatment method for high-niobium carburizing steel, the specific steps of which are as follows: 1. First carburizing treatment: Place the workpiece in an air heating furnace and heat it to 510℃ for pre-oxidation treatment; then place the workpiece in a carburizing atmosphere, control the carbon potential at 1.15~1.30%, heat it to 930℃ for austenitization and hold it at that temperature for 79 minutes to complete the first carburizing; after carburizing, cool it to 860℃ for homogenization, quench it in oil for cooling, clean it, and set it aside for use.

[0028] 2. Segmented high-temperature tempering treatment: The workpiece after the first carburizing is heated to 680℃ and held for 18 hours to complete the first stage of high-temperature tempering; then it is cooled to 620℃ and held for 18 hours to complete the second stage of high-temperature tempering; after tempering, it is air-cooled to room temperature.

[0029] 3. Second carburizing treatment: After high-temperature tempering, the workpiece is placed in a carburizing atmosphere again, with the carbon potential controlled at 1.20~1.35%, heated to 930℃ for austenitization and held for 11 hours to complete the second carburizing. After carburizing, the temperature is lowered to 860℃ for homogenization, then oil quenched and cooled. After cleaning, it is heated to 400℃ for tempering and then air-cooled after being taken out of the furnace.

[0030] 4. Secondary quenching and low-temperature tempering: The workpiece is heated to 815℃ and held for heat treatment, then oil-quenched, followed by heating to 180℃ and holding for low-temperature tempering. After being removed from the furnace, it is air-cooled to complete the entire heat treatment process.

[0031] The performance of the workpiece after processing in this embodiment was tested. The results showed that the effective carburized layer depth of the workpiece with a carbon concentration ≥0.8% reached 1.29mm, the carbon concentration gradient was gentle, there was no abnormal tissue precipitation, the workpiece had excellent strength and toughness matching, and the wear resistance and fatigue performance met the requirements of high-end gears.

[0032] Example 3 This embodiment provides a composite carburizing heat treatment method for high-niobium carburizing steel, the specific steps of which are as follows: 1. First carburizing treatment: Place the workpiece in an air heating furnace and heat it to 530℃ for pre-oxidation treatment; then place the workpiece in a carburizing atmosphere, control the carbon potential at 1.25~1.40%, heat it to 950℃ for austenitization and hold it at that temperature for 72 minutes to complete the first carburizing; after carburizing, cool it to 880℃ for homogenization, quench it in oil for cooling, clean it, and set it aside for use.

[0033] 2. Segmented high-temperature tempering treatment: The workpiece after the first carburizing is heated to 685℃ and held for 14 hours to complete the first stage of high-temperature tempering; then it is cooled to 625℃ and held for 14 hours to complete the second stage of high-temperature tempering; after tempering, it is air-cooled to room temperature.

[0034] 3. Second carburizing treatment: After high-temperature tempering, the workpiece is placed in a carburizing atmosphere again, with the carbon potential controlled at 1.25~1.40%, heated to 950℃ for austenitization and held for 7 hours to complete the second carburizing. After carburizing, the temperature is lowered to 880℃ for homogenization, quenched in a molten salt bath, cleaned, heated to 200℃ for tempering, and then air-cooled after removal from the furnace.

[0035] 4. Secondary quenching and low-temperature tempering: The workpiece is heated to 820℃ and held for heat treatment, then oil-quenched, followed by heating to 200℃ and holding for low-temperature tempering. After being removed from the furnace, it is air-cooled to complete the entire heat treatment process.

[0036] The performance of the workpiece after processing in this embodiment was tested. The results showed that the effective carburized layer depth of the workpiece with a carbon concentration ≥0.8% reached 1.37mm, the carburized layer structure was uniform, NbC particles were dispersed, and the surface hardness and impact toughness of the workpiece met the design requirements.

[0037] Comparative Example 1 This comparative example uses the conventional one-time carburizing process of existing technology to process the same batch of workpieces. The specific steps are as follows: the workpieces are heated to 940~945℃ and held in a carburizing atmosphere with a carbon potential of 1.2~1.45% for 38 hours to complete the one-time carburizing; after carburizing, the temperature is lowered to 880℃ for uniform temperature, and then oil quenched for cooling. Subsequently, the same tempering, secondary quenching and low-temperature tempering treatment as in Example 1 is performed.

[0038] The workpieces treated in this comparative example were tested, and the results showed that the effective carburized layer depth of the workpiece with a carbon concentration ≥0.8% was only 0.33mm, which was far lower than the test results of Example 1. This could not meet the effective carburized layer depth requirements for high-end bearings and gears, and seriously affected the performance and fatigue life of the workpieces.

[0039] Comparative Example 2 The only difference between this comparative example and Example 1 is that the S2 segmented high-temperature tempering treatment step is omitted, and the second carburizing treatment is performed directly after the first carburizing is completed. The remaining process parameters are completely consistent with those of Example 1, and the total carburizing time is the same as that of Example 1.

[0040] The workpiece treated in this comparative example was tested, and the results showed that the effective carburized layer depth in the workpiece with a carbon concentration ≥0.8% was only 0.35mm. Figure 5 Compared to Example 1, a significant decrease was observed, consistent with the shortcomings of conventional one-time carburizing processes; this fully demonstrates that the segmented high-temperature tempering step in this invention is a core and essential technical feature for solving the problem of hindered carbon diffusion in high-niobium carburizing steel and improving carburizing efficiency.

[0041] The principle of this invention is based on an in-depth analysis of the carburizing inhibition mechanism of high-niobium steel. Through analysis and research, it was found that during conventional carburizing, the high niobium content forms a large number of highly dispersed, extremely fine NbC particles, which are pinned to grain boundaries and dislocations, severely hindering the further diffusion of carbon atoms. The composite process of this invention, through intermediate high-temperature tempering, allows the large number of highly dispersed, extremely fine NbC particles formed during the first carburizing process to mature and grow (see...). Figure 3 This process releases diffusion channels, eliminates internal stress, and results in a more ordered atomic arrangement. It also significantly reduces the amount of niobium in solid solution in the matrix, causing it to precipitate as NbC particles, thus creating a stable environment for subsequent diffusion. This process has the following beneficial effects: 1. Eliminating diffusion barriers: (1) High-temperature tempering ripens and grows highly dispersed and extremely fine NbC particles. Result: The number of NbC particles decreases, the average size increases slightly, and the spacing increases. (2) Releasing diffusion channels: When NbC particles grow and the spacing increases, the grain boundaries and diffusion channels that were originally "blocked" by them are reopened. (3) Eliminating internal stress and defect reorganization: The lattice distortion and internal stress caused by the first carburizing are released in the tempering process, and the atomic arrangement becomes more ordered, creating a stable environment for subsequent diffusion. (4) Sufficiently reducing the solid solution niobium in the matrix, which precipitates in the form of NbC particles, reduces the hindering effect of niobium on the second carburizing.

[0042] 2. Formation of an ideal carbon concentration gradient: The niobium carbide particles formed during the high-temperature tempering process remain stable during the subsequent second carburizing heating process and will not redissolve (the carburizing temperature is lower than the dissolution temperature of niobium carbide; the NbC phase is stable below 1050℃). These undissolved niobium carbide particles are themselves excellent reinforcing phases. The final carburized layer is a composite carburized layer composed of the carbon concentration gradient in the matrix and the dispersed niobium carbide particles, resulting in a smoother and more ideal carbon concentration distribution. See the example below. Figure 2 and Figure 4 .

[0043] 3. Improved carburizing efficiency: By eliminating the hindering effect of highly dispersed, extremely fine NbC particles on carburizing, the rate of the second carburizing is significantly increased, enabling the acquisition of a deeper effective carburized layer with a carbon concentration ≥0.80% in a shorter time. A comparison of these is shown in [link to comparison]. Figure 1 , Figure 2 .

[0044] This composite process, which involves two carburizing stages with an intermediate high-temperature tempering, is specifically designed for carburizing steel with high niobium content (especially niobium content exceeding conventional limits, such as above 0.06%). It is the first of its kind proposed in this application and effectively resolves the contradiction between the performance of high niobium steel in terms of "high strength, toughness, fatigue life, and wear resistance" and the difficulty of carburizing.

[0045] In summary, compared with the prior art, the present invention has the following significant and outstanding advantages: This invention completely eliminates the obstruction of high niobium content to the carburizing process, significantly improving carburizing efficiency. Through a core design incorporating segmented high-temperature tempering between two carburizing processes, the fine, dispersed NbC particles formed in the first carburizing stage mature and grow, releasing blocked carbon atom diffusion channels. Simultaneously, it eliminates lattice distortion and internal stress, clearing obstacles for carbon atom diffusion. Furthermore, it significantly reduces the amount of niobium in the matrix in solid solution, causing it to precipitate as NbC particles. Compared to conventional one-step carburizing processes, this invention achieves a qualitative leap in carburizing efficiency with a shorter total carburizing time. The effective carburized layer depth for carbon concentrations ≥0.80% is increased by over 280%, completely solving the industry pain points of slow carburizing rate and shallow effective carburized layer in high-niobium carburizing steel.

[0046] This invention achieves an ideal carbon concentration distribution in the carburized layer, fully guaranteeing the performance of the workpiece. Through a composite process design, the final carburized layer exhibits a smooth transition in carbon concentration from the surface to the interior, without significant carbide network precipitation or aggregation. This avoids the problems of low surface carbon concentration, easy fatigue spalling, and insufficient lifespan that are common in conventional carburizing processes. Simultaneously, the dispersed NbC particles in the carburized layer remain stable and insoluble at the carburizing temperature, serving as an excellent reinforcing phase and achieving dispersion strengthening of the carburized layer, significantly improving the wear resistance and contact fatigue life of the workpiece.

[0047] This invention achieves a perfect match between the performance potential of high-niobium steel and the carburizing process, breaking through the bottleneck of industrial application. It does not require limiting the niobium content in the steel and is specifically designed for 20CrNi2MoNb high-niobium carburizing steel with a niobium content ≥0.06%. While solving the carburizing problem, it fully retains the advantages of fine-grained strengthening and dispersion strengthening brought by high-niobium alloying. This allows the workpiece to simultaneously possess excellent performance matching of high surface hardness and high wear resistance with high core strength and high toughness, fully leveraging the performance potential of high-niobium carburizing steel and clearing the core obstacles for the industrial promotion and application of this steel in high-end bearings, gears, and other core components.

[0048] The process is highly compatible and adaptable to the needs of industrial mass production. All process steps of this invention can be completed on existing conventional carburizing heat treatment equipment without the need for additional specialized equipment. The process parameters are highly controllable and stable, and can be directly adapted to existing industrial heat treatment production lines for bearings and gear parts, making it extremely valuable for widespread application.

[0049] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A composite carburizing heat treatment method for high-niobium carburizing steel, characterized in that, The high-niobium carburizing steel is a 20CrNi2MoNb carburizing steel with a niobium content ≥0.06% by mass, and includes the following steps performed sequentially: S1. First carburizing treatment: After pre-oxidation, the workpiece is placed in a carburizing atmosphere to complete austenitizing heating and first carburizing. After carburizing, the workpiece is cooled down to homogenize and then quenched. S2. High-temperature tempering treatment: The workpiece treated in S1 is subjected to segmented high-temperature tempering, and after tempering, it is air-cooled to room temperature. S3, Second Carburizing Treatment: The workpiece treated in S2 is placed in a carburizing atmosphere again to complete austenitizing heating and second carburizing. After achieving the target carburized layer depth and carbon concentration distribution, it is cooled down to homogenize and then quenched. Finally, tempering treatment is completed.

2. The composite carburizing heat treatment method for high-niobium carburizing steel according to claim 1, characterized in that, In step S1, the temperature of the pre-oxidation treatment is 510~530℃, and the pre-oxidation treatment is carried out in an air-heated furnace.

3. The composite carburizing heat treatment method for high-niobium carburizing steel according to claim 1, characterized in that, In step S1, the austenitizing heating temperature for the first carburizing is 930~950℃, and the carbon potential of the carburizing atmosphere is 1.15~1.40%. After carburizing, the temperature is lowered to 860~880℃, and oil quenching is performed after homogenization.

4. The composite carburizing heat treatment method for high-niobium carburizing steel according to claim 1, characterized in that, In step S2, the segmented high-temperature tempering is specifically as follows: first, the workpiece is heated to 680~690℃ for the first stage of heat preservation tempering, and then cooled in the furnace to 620~630℃ for the second stage of heat preservation tempering. After the two stages of tempering are completed, the workpiece is air-cooled to room temperature.

5. The composite carburizing heat treatment method for high-niobium carburizing steel according to claim 4, characterized in that, The holding time for the first stage of heat preservation and tempering is 14~18h, and the holding time for the second stage of heat preservation and tempering is 14~18h.

6. The composite carburizing heat treatment method for high-niobium carburizing steel according to claim 1, characterized in that, In step S3, the austenitizing heating temperature for the second carburizing is 930~950℃, and the carbon potential of the carburizing atmosphere is 1.15~1.40%. After carburizing, the temperature is lowered to 860~880℃, and after homogenization, oil cooling or molten salt bath quenching is used.

7. The composite carburizing heat treatment method for high-niobium carburizing steel according to claim 1, characterized in that, In step S3, the tempering temperature after quenching and cooling is 200~400℃.

8. The composite carburizing heat treatment method for high-niobium carburizing steel according to claim 1, characterized in that, After step S3 is completed, the workpiece is subjected to secondary quenching and low-temperature tempering treatment. Specifically, the workpiece is heated to 815~820℃ for oil quenching, and then tempered at 180~200℃.

9. The composite carburizing heat treatment method for high-niobium carburizing steel according to claim 1, characterized in that, The high-temperature tempering in step S2 causes the dispersed fine NbC particles formed during the first carburizing process to mature and grow, releasing carbon atom diffusion channels, eliminating lattice distortion and internal stress generated during the carburizing process, and providing an ordered matrix environment for carbon atom diffusion in step S3.

10. A high-niobium carburized steel component, characterized in that, The component is prepared by the composite carburizing heat treatment method of high niobium carburizing steel according to any one of claims 1-9. In the carburized layer of the component, the effective carburized layer depth is ≥1.0 mm with a carbon concentration ≥0.8%, the carbon concentration gradient of the carburized layer is gentle from the surface to the inside, and NbC reinforcing particles are dispersed in the carburized layer.

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