Nb / Ti / RE composite microalloyed 20CrMoH steel and preparation method thereof

Abstract

The invention relates to Nb / Ti / RE composite microalloyed 20CrMoH steel and a preparation method thereof, and relates to the technical field of steel for high-end robot gears. The Nb / Ti / RE composite microalloyed 20CrMoH steel comprises the following chemical components in percentage by mass: 0.15 to 0.25 percent of C, 0.2 to 0.25 percent of Si, 0.80 to 1.0 percent of Mn, 0.8 to 1.0 percent of Cr, 0.15 to 0.25 percent of Mo, 0.015 to 0.025 percent of Nb, 0.015 to 0.025 percent of Ti, 0.020 to 0.040 percent of Al, 0.0030 to 0.0050 percent of RE, less than or equal to 0.008 percent of P, less than or equal to 0.008 percent of S and the balance of Fe. And the balance of Fe and other inevitable impurities. On the basis of rare earth Y element and Nb / Ti / RE composite microalloying cooperation, a multi-stage rolling process design is combined, a banded structure appearing in traditional CrMo series gear steel hot rolling is eliminated, the structure distribution uniformity is improved, and the mechanical property stability is improved. The gear steel produced through the method is uniform in structure and stable in performance, the performance requirement of high-end gear steel can be met, and remarkable economic benefits are generated.

Description

Technical Field

[0001] The present invention relates to the technical field of steel for high-end robot gears, and in particular to a Nb / Ti / RE composite microalloyed 20CrMoH steel and a preparation method thereof. Background Art

[0002] As my country's manufacturing industry transforms and upgrades toward highly intelligent and automated production, precision equipment such as industrial robots and high-end machine tools are placing higher demands on the performance of core transmission components. As core components of power transmission systems, gears must simultaneously meet multiple performance requirements, including bending strength, contact fatigue strength, and wear resistance, under complex alternating stress conditions. In cutting-edge applications such as high-speed, heavy-load, and precision transmission, traditional gear steels are no longer able to meet stringent standards for fatigue life, dimensional stability, and noise control.

[0003] The metallurgical quality of gear steel directly determines its service performance. The current production process encompasses a complete system of steelmaking, continuous casting, hot rolling, controlled cooling, and heat treatment. Solute element segregation during the continuous casting process leads to the formation of dendritic microstructure, which, after subsequent heat treatment, easily forms a banded structure composed of alternating proeutectoid ferrite and pearlite. This microstructural defect not only causes anisotropy in the material's mechanical properties but also leads to premature cracking in gears under cyclic loading, reducing their service life. Therefore, overcoming the bottleneck in controlling banded microstructure and achieving homogenized microstructure control in gear steel has become a key issue in improving the reliability of transmission systems in high-end equipment.

[0004] In recent years, numerous researchers have conducted extensive research to eliminate banded structure. Microalloying and adjusting rolling process parameters have been shown to effectively improve the microstructure uniformity of gear steel. For example, Chinese patent CN118480734A discloses a method for producing rare earth, low-sulfur, free-cutting 20CrMnTi gear steel. The steel's composition, by weight, is as follows: C 0.19-0.21%, Si 0.30-0.35%, Mn 0.90-1.00%, P ≤ 0.020%, S 0.020-0.030%, Cr 1.20-1.30%, Ti 0.05-0.07%, Al 0.030-0.040%, RE 0.0030-0.0040%, with the remainder being Fe and impurities. The production method includes controlling the steelmaking and rolling processes. By adding RE and controlling the S content, the present invention, in conjunction with specific process steps, can effectively improve the cutting performance and internal quality of the gear steel. Narrow composition control effectively improves the microstructure uniformity of gear steel and keeps the hardenability bandwidth within a narrow range (35≤J9≤41, 26≤J15≤32). However, excessive addition of Ti to gear steel can easily form TiN with residual N in the steel, reducing the fatigue life of the material.

[0005] Chinese patent CN118792580A discloses a method for producing carburized gear steel for automotive applications. During the converter smelting stage, the process utilizes a bottom-blowing nitrogen-argon switching technique to precisely control carbon content and strictly limit impurities such as phosphorus and sulfur to reduce the oxidizing properties of the molten steel. White slag treatment and calcium wire treatment are implemented in the refining process to effectively improve inclusion morphology and prevent nozzle blockage. A soft-blowing process also promotes inclusion flotation. The continuous casting process emphasizes tundish baking quality and molten steel protection, employing electromagnetic stirring and secondary cooling to enhance surface uniformity. Furthermore, the principle of chromium alloying is utilized to significantly improve the mechanical properties of the hot-rolled round steel, thereby significantly enhancing the overall lifespan and safety of the pump rod. However, the production process for this gear steel is complex and requires manufacturers to have converter steelmaking equipment, hindering the widespread adoption of this technology.

[0006] As one of the key materials for high-end gear manufacturing, 20CrMoH, a low-carbon, low-alloy carburized gear steel, has become the core material for gear components in the automotive, machinery manufacturing and other fields due to its good hardenability, heat treatment stability and weldability.

[0007] However, since CrMo gear steel is prone to produce banded structure during hot rolling, in order to improve the product competitiveness of CrMo gear steel, it is particularly necessary to conduct in-depth research on the formation mechanism and regulation method of its banded structure. Summary of the Invention

[0008] To address the existing technical problems in the gear steel production process, such as the formation of titanium nitride from titanium, which reduces the fatigue life of the material; the complicated smelting process, the need to control impurity content, the difficulty in operation; the banded structure during hot rolling that reduces mechanical properties; the uneven hardness distribution; and the poor welding quality, the present invention proposes a Nb / Ti / RE microalloyed 20CrMoH steel and a preparation method that can solve the above-mentioned technical problems. The technical solution is as follows:

[0009] A Nb / Ti / RE composite microalloyed 20CrMoH steel has the following chemical compositions by mass: C 0.15-0.25%, Si 0.2-0.25%, Mn 0.80-1.0%, Cr 0.8-1.0%, Mo 0.15-0.25%, Nb 0.015-0.025%, Ti 0.015-0.025%, Al 0.020-0.040%, P ≤0.008%, S ≤0.008%, RE 0.0030-0.0040%, with the remainder being Fe and unavoidable impurities.

[0010] Optionally, the Nb / Ti / RE composite microalloyed 20CrMoH steel is a square billet with a cross-sectional size of 24×24-58 mm×58 mm.

[0011] Optionally, the microstructure of the Nb / Ti / RE composite microalloyed 20CrMoH steel is uniformly distributed without significant banded structure. High-magnification microstructure observation reveals that the gear steel is composed of massive ferrite and bainite, with a volume fraction of bainite of 41-52%, a volume fraction of ferrite of 48-59%, and an average length of ferrite grains of 17-28 μm and an average width of 8-18 μm.

[0012] Optionally, the impact toughness of the Nb / Ti / RE composite microalloyed 20CrMoH steel is 52-79 J, and the hardness fluctuation is ±25 HV.

[0013] A preparation method based on the Nb / Ti / RE composite microalloyed 20CrMoH steel, the preparation method of the Nb / Ti / RE composite microalloyed 20CrMoH steel specifically comprises the following steps:

[0014] S1. Billet preparation: Raw materials are weighed according to the chemical composition of Nb / Ti / RE composite microalloyed 20CrMoH steel, and then subjected to electric furnace smelting, LF refining, RH vacuum degassing and continuous casting to obtain 20CrMoH steel billets:

[0015] S2. Billet heating: heating and heat-insulating the S120CrMoH steel billet to obtain a heated billet;

[0016] S3, three-stage controlled rolling: the steel billet after heating in S2 is subjected to three-stage controlled rolling of rough rolling stage, intermediate rolling stage and finishing rolling stage to obtain three-stage controlled rolled steel;

[0017] S4, air cooling: The S3 three-stage controlled-rolled steel is air-cooled and annealed to room temperature to obtain gear steel.

[0018] Optionally, the cross-sectional size of the 20CrMoH steel billet of S1 is 138×138-140 mm×140 mm.

[0019] Optionally, the heating temperature of the S2 heating and insulation treatment is 1200±30°C, and the insulation time is 120±5min.

[0020] Optionally, the starting rolling temperature in the S3 rough rolling stage is 1150±30℃, the finishing rolling temperature is 1050±30℃, the reduction is 13-17%, and the thickness after rolling is 116-121mm; the starting rolling temperature in the intermediate rolling stage is 1050±30℃, the finishing rolling temperature is 950±30℃, the reduction is 45-75%, and the thickness after rolling is 29-67mm; the starting rolling temperature in the finishing rolling stage is 950±30℃, the finishing rolling temperature is 900±30℃, the reduction is 13-17%, and the thickness after rolling is 24-58mm.

[0021] Optionally, the S4 air cooling annealing rate is 2-3°C / s.

[0022] The innovative technical principle of the present invention is as follows: through Nb / Ti / RE composite microalloying, (Nb / Ti)C second phase particles are precipitated during the high-temperature solidification process, and the three-stage controlled rolling process is coupled to promote the precipitation of (Nb / Ti)C second phase particles during the high-temperature hot rolling process, effectively refine the austenite grain structure, and become the nucleation points of ferrite and bainite in the subsequent cooling process, promote the uniform phase transformation, and effectively improve the banded structure caused by component segregation in gear steel; at the same time, Nb / Ti / RE composite microalloying can appropriately reduce the amount of Ti element added in the gear steel, reducing the risk of reducing the life of the gear steel due to the generation of TiN; in addition, the addition of rare earth element Y can improve the morphology of inclusions and precipitates, and effectively improve the comprehensive performance of the gear steel.

[0023] Compared with the prior art, the above technical solution has at least the following beneficial effects:

[0024] The above scheme, the present invention proposes a Nb / Ti / RE composite microalloyed 20CrMoH steel and a preparation method, which can solve the technical problems existing in the prior art in the production process of gear steel, such as the titanium element forming TiN to reduce the fatigue life of the material, the complicated smelting process, the need to control the impurity content, the difficulty of operation, the banded structure in the hot rolling process causing the mechanical properties to be reduced, the hardness distribution is uneven, and the welding quality is damaged.

[0025] This invention utilizes a Nb / Ti / RE composite microalloy to reduce the amount of Ti added to the steel, effectively mitigating the risk of TiN formation affecting the fatigue life of gear steel. The appropriate addition of the rare earth element Y, due to its strong affinity for oxygen and sulfur, preferentially forms rare earth oxides and sulfides, effectively reducing the content of harmful impurities and brittle inclusions in the steel. Rare earth elements also promote the dispersion of carbides, improving hardenability and resulting in more uniform hardness and microstructure in gears.

[0026] The present invention uses electric furnace smelting, LF refining, RH vacuum degassing and continuous casting to ensure that the alloy composition of the 20CrMoH steel billet is precise, reduce the content of dissolved O, N and H elements in the steel, significantly reduce the content of inclusions such as TiN in the steel, reduce the risk of hydrogen-induced delayed cracking, help improve steel quality, and increase the service life of gears.

[0027] The present invention can improve the component segregation of the steel billet produced during the continuous casting process by performing a heating and heat-insulating treatment on the continuous casting steel billet, promote the uniform distribution of elements, and effectively inhibit the formation of banded structure in the subsequent rolling process.

[0028] This invention addresses the problem of banded structure formation during hot rolling of conventional CrMo gear steels. By optimizing rolling process parameters, the steel achieves improved microstructure homogenization. A three-step rolling process combined with temperature control suppresses banded structure formation and improves microstructure uniformity.

[0029] The present invention suppresses the formation of a large amount of proeutectoid ferrite in the gear steel by air cooling annealing to room temperature, and prevents the formation of banded structure during the cooling process; at the same time, the blocky ferrite and bainite generated during the air cooling process are evenly distributed, ensuring that the gear steel obtains excellent performance.

[0030] The Nb / Ti / RE composite microalloyed 20CrMoH steel prepared by the present invention has a uniform microstructure distribution and no significant banded structure. High-magnification microstructure observation reveals that the gear steel is composed of massive ferrite and bainite, with the volume fraction of bainite being 41-52% and the volume fraction of ferrite being 48-59%. The average length of the ferrite grains is 17-28 μm, and the average width is 8-18 μm.

[0031] In summary, compared with conventional technologies, the present invention produces Nb / Ti / RE composite microalloyed 20CrMoH steel with an impact toughness of 52-79J and a hardness fluctuation of ±25HV through furnace smelting, LF refining, RH vacuum degassing and continuous casting, heating and heat-holding treatment, three-stage controlled rolling, and air-cooling annealing to room temperature. This method, based on conventional CrMo-based gear steel, introduces the rare earth element yttrium (Y) and an innovative multi-stage rolling process. This Nb / Ti / RE composite microalloyed 20CrMoH gear steel and its preparation process achieve uniform microhardness and microstructure distribution, without noticeable banding, significantly improving performance consistency. The method is simple, easy to operate, environmentally friendly, low-cost, short-process, and highly efficient, making it suitable for large-scale industrial production and promotion. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0033] Figure 1 1 is a low-magnification and high-magnification metallographic structure diagram of a Nb / Ti / RE composite microalloyed 20CrMoH steel according to Example 1 of the present invention, wherein (a) is a low-magnification metallographic structure diagram, and (b) is a high-magnification metallographic structure diagram;

[0034] Figure 2 1 is a low-magnification and high-magnification metallographic structure diagram of a Nb / Ti / RE composite microalloyed 20CrMoH steel according to Example 2 of the present invention, wherein (a) is a low-magnification metallographic structure diagram, and (b) is a high-magnification metallographic structure diagram;

[0035] Figure 3 1 is a low-magnification and high-magnification metallographic structure diagram of a Nb / Ti / RE composite microalloyed 20CrMoH steel according to Example 3 of the present invention, wherein (a) is a low-magnification metallographic structure diagram, and (b) is a high-magnification metallographic structure diagram;

[0036] Figure 4 1 is a low-magnification and high-magnification metallographic structure diagram of a Nb / Ti / RE composite microalloyed 20CrMoH steel according to Example 4 of the present invention, wherein (a) is a low-magnification metallographic structure diagram, and (b) is a high-magnification metallographic structure diagram;

[0037] Figure 5 These are low-magnification and high-magnification metallographic microstructure diagrams of a Nb / Ti / RE composite microalloyed 20CrMoH steel according to Comparative Example 1 of the present invention, wherein (a) is a low-magnification metallographic microstructure diagram, and (b) is a high-magnification metallographic microstructure diagram;

[0038] Figure 6 These are low-magnification and high-magnification metallographic microstructure diagrams of a Nb / Ti / RE composite microalloyed 20CrMoH steel according to Comparative Example 2 of the present invention, wherein (a) is a low-magnification metallographic microstructure diagram, and (b) is a high-magnification metallographic microstructure diagram;

[0039] Figure 7 is the hardness fluctuation curve of the gear steel after rolling of Examples 1-4 of the present invention;

[0040] Figure 8 This is the hardness fluctuation curve of the gear steel after rolling in Comparative Example 1-2 of the present invention. DETAILED DESCRIPTION

[0041] The technical solution of the present invention is described below in conjunction with the accompanying drawings.

[0042] In the embodiments of the present invention, words such as "exemplarily" and "for example" are used to indicate examples, illustrations, or explanations. Any embodiment or design described as an "exemplary" in the present invention should not be interpreted as being preferred or advantageous over other embodiments or designs. Rather, the use of the word "exemplary" is intended to present concepts in a concrete manner. Furthermore, in the embodiments of the present invention, "and / or" can mean both or either of the two.

[0043] In the embodiments of the present invention, “image” and “picture” may sometimes be used interchangeably. It should be noted that when the distinction between them is not emphasized, the meanings they intend to express are the same.

[0044] In the embodiments of the present invention, sometimes a subscript such as W1 may be written as a non-subscript such as W1. When the difference is not emphasized, the meanings to be expressed are the same.

[0045] In order to make the technical problems, technical solutions and advantages to be solved by the present invention clearer, a detailed description will be given below with reference to the accompanying drawings and specific embodiments.

[0046] A Nb / Ti / RE composite microalloyed 20CrMoH steel has the following chemical compositions by mass: C 0.15-0.25%, Si 0.2-0.25%, Mn 0.80-1.0%, Cr 0.8-1.0%, Mo 0.15-0.25%, Nb 0.015-0.025%, Ti 0.015-0.025%, Al 0.020-0.040%, P ≤0.008%, S ≤0.008%, RE 0.0030-0.0040%, with the remainder being Fe and unavoidable impurities.

[0047] Particularly, the Nb / Ti / RE composite microalloyed 20CrMoH steel is a square billet with a cross-sectional size of 24×24-58 mm×58 mm.

[0048] In particular, the microstructure of the Nb / Ti / RE composite microalloyed 20CrMoH steel is uniformly distributed, without significant banded structure. High-magnification microstructure observation revealed that the gear steel is composed of massive ferrite and bainite, with the volume fraction of bainite being 41-52% and the volume fraction of ferrite being 48-59%. The average length of the ferrite grains is 17-28 μm, and the average width is 8-18 μm.

[0049] In particular, the impact toughness of the Nb / Ti / RE composite microalloyed 20CrMoH steel is 52-79 J, and the hardness fluctuates within ±25 HV.

[0050] A preparation method based on the Nb / Ti / RE composite microalloyed 20CrMoH steel, the preparation method of the Nb / Ti / RE composite microalloyed 20CrMoH steel specifically comprises the following steps:

[0051] S1. Billet preparation: Raw materials are weighed according to the chemical composition of Nb / Ti / RE composite microalloyed 20CrMoH steel, and then subjected to electric furnace smelting, LF refining, RH vacuum degassing and continuous casting to obtain 20CrMoH steel billets:

[0052] S2. Billet heating: heating and heat-insulating the S120CrMoH steel billet to obtain a heated billet;

[0053] S3, three-stage controlled rolling: the steel billet after heating in S2 is subjected to three-stage controlled rolling of rough rolling stage, intermediate rolling stage and finishing rolling stage to obtain three-stage controlled rolled steel;

[0054] S4, air cooling: The S3 three-stage controlled-rolled steel is air-cooled and annealed to room temperature to obtain gear steel.

[0055] In particular, the cross-sectional dimensions of the 20CrMoH steel billet of S1 are 138×138-140 mm×140 mm.

[0056] In particular, the heating temperature of the S2 heating and holding treatment is 1200±30°C, and the holding time is 120±5min.

[0057] In particular, the starting rolling temperature in the S3 rough rolling stage is 1150±30℃, the finishing rolling temperature is 1050±30℃, the reduction is 13-17%, and the thickness after rolling is 116-121mm; the starting rolling temperature in the intermediate rolling stage is 1050±30℃, the finishing rolling temperature is 950±30℃, the reduction is 45-75%, and the thickness after rolling is 29-67mm; the starting rolling temperature in the finishing rolling stage is 950±30℃, the finishing rolling temperature is 900±30℃, the reduction is 13-17%, and the thickness after rolling is 24-58mm.

[0058] In particular, the cooling rate of S4 air cooling annealing is 2-3°C / s.

[0059] Example 1

[0060] The Nb / Ti / RE composite microalloyed 20CrMoH steel of this embodiment has the following chemical composition by mass percentage: C 0.25%, Si 0.23%, Mn 0.88%, Cr 0.85%, Mo 0.23%, Nb 0.018%, Ti 0.023%, Al 0.040%, RE 0.0030%, P ≤ 0.008%, S ≤ 0.008%; the remainder is Fe and unavoidable impurities.

[0061] A preparation method based on the Nb / Ti / RE composite microalloyed 20CrMoH steel, the preparation method of the Nb / Ti / RE composite microalloyed 20CrMoH steel specifically comprises the following steps:

[0062] S1. Billet preparation: Raw materials were weighed according to the chemical composition of Nb / Ti / RE composite microalloyed 20CrMoH steel, and then subjected to electric furnace smelting, LF refining, RH vacuum degassing, and continuous casting to obtain a 20CrMoH steel billet with a cross-sectional size of 140×140 mm.

[0063] S2. Billet heating: The 20CrMoH steel billet of S1 is heated and held at 1200° C. for 120 min to obtain a heated billet;

[0064] S3, three-stage controlled rolling: the steel billet after heating in S2 is subjected to three-stage controlled rolling, namely, rough rolling stage, intermediate rolling stage, and finishing rolling stage. The starting rolling temperature in the rough rolling stage is 1150±30°C, the finishing rolling temperature is 1050±30°C, the reduction is 15%, and the thickness after rolling is 119mm; the starting rolling temperature in the intermediate rolling stage is 1050±30°C, the finishing rolling temperature is 950±30°C, the reduction is 45%, and the thickness after rolling is 65mm; the starting rolling temperature in the finishing rolling stage is 950±30°C, the finishing rolling temperature is 900±30°C, the reduction is 16%, and the thickness after rolling is 55mm, thereby obtaining a three-stage controlled rolled steel;

[0065] S4. Air cooling: The S3 three-stage controlled-rolled steel is air-cooled and annealed to room temperature at a rate of 2°C / s to obtain gear steel.

[0066] like Figure 1 As shown in the figure, the microstructure of the Nb / Ti / RE composite microalloyed 20CrMoH steel prepared in this embodiment is uniformly distributed, and no significant banded structure is produced. High-magnification microstructure observation reveals that the gear steel is composed of massive ferrite and bainite, with the volume fraction of bainite being 51.4% and the volume fraction of ferrite being 48.6%. The average length of the ferrite grains is 23±5μm, and the average width is 14±4μm.

[0067] The impact toughness of the Nb / Ti / RE composite microalloyed 20CrMoH steel prepared in this embodiment is 56±4J, indicating that the microstructure and properties are evenly distributed. Figure 7 The hardness curve of Example 1 shows that the hardness fluctuation range of the gear steel is relatively small between 279-330 HV, indicating that the performance consistency of different parts is good.

[0068] Example 2

[0069] The Nb / Ti / RE composite microalloyed 20CrMoH steel of this embodiment has the following chemical compositions by mass: C 0.15%, Si 0.21%, Mn 0.85%, Cr 1.0%, Mo 0.21%, Nb 0.017%, Ti 0.015%, Al 0.028%, RE 0.0035%, P ≤ 0.008%, S ≤ 0.008%; the remainder is Fe and unavoidable impurities.

[0070] A preparation method based on the Nb / Ti / RE composite microalloyed 20CrMoH steel, the preparation method of the Nb / Ti / RE composite microalloyed 20CrMoH steel specifically comprises the following steps:

[0071] S1. Billet preparation: Raw materials were weighed according to the chemical composition of Nb / Ti / RE composite microalloyed 20CrMoH steel, and then subjected to electric furnace smelting, LF refining, RH vacuum degassing, and continuous casting to obtain a 20CrMoH steel billet with a cross-sectional size of 140×140 mm.

[0072] S2. Billet heating: The 20CrMoH steel billet of S1 is heated and held at 1200° C. for 120 min to obtain a heated billet;

[0073] S3, three-stage controlled rolling: the steel billet after heating in S2 is subjected to three-stage controlled rolling, namely, rough rolling stage, intermediate rolling stage, and finishing rolling stage. The starting rolling temperature in the rough rolling stage is 1150±30°C, the finishing rolling temperature is 1050±30°C, the reduction is 13%, and the thickness after rolling is 122mm; the starting rolling temperature in the intermediate rolling stage is 1050±30°C, the finishing rolling temperature is 950±30°C, the reduction is 55%, and the thickness after rolling is 55mm; the starting rolling temperature in the finishing rolling stage is 950±30°C, the finishing rolling temperature is 900±30°C, the reduction is 17%, and the thickness after rolling is 45.5mm, thereby obtaining a three-stage controlled rolled steel;

[0074] S4. Air cooling: The S3 three-stage controlled-rolled steel is air-cooled and annealed to room temperature at a rate of 2°C / s to obtain gear steel.

[0075] like Figure 2 As shown in the figure, the microstructure of the Nb / Ti / RE composite microalloyed 20CrMoH steel prepared in this embodiment is uniformly distributed, and no significant banded structure is produced. High-magnification microstructure observation reveals that the gear steel is composed of massive ferrite and bainite, with the volume fraction of bainite being 54.6% and the volume fraction of ferrite being 45.4%. The average length of the ferrite grains is 20±3μm, and the average width is 12±4μm.

[0076] The impact toughness of the Nb / Ti / RE composite microalloyed 20CrMoH steel prepared in this embodiment is 62±7 J, indicating that the microstructure and properties are evenly distributed. Figure 7 The hardness curve of Example 2 shows that the hardness fluctuation range of the gear steel is relatively small between 310-350 HV, indicating that the performance consistency of different parts is good.

[0077] Example 3

[0078] The Nb / Ti / RE composite microalloyed 20CrMoH steel of this embodiment has the following chemical composition by mass percentage: C 0.22%, Si 0.25%, Mn 0.80%, Cr 0.9%, Mo 0.25%, Nb 0.025%, Ti 0.019%, Al 0.020%, RE 0.0040%, P ≤ 0.008%, S ≤ 0.008%; the remainder is Fe and unavoidable impurities.

[0079] A preparation method based on the Nb / Ti / RE composite microalloyed 20CrMoH steel, the preparation method of the Nb / Ti / RE composite microalloyed 20CrMoH steel specifically comprises the following steps:

[0080] S1. Billet preparation: Raw materials were weighed according to the chemical composition of Nb / Ti / RE composite microalloyed 20CrMoH steel, and then subjected to electric furnace smelting, LF refining, RH vacuum degassing, and continuous casting to obtain a 20CrMoH steel billet with a cross-sectional size of 140×140 mm.

[0081] S2. Billet heating: The S120CrMoH steel billet is heated and held at 1200°C for 120 minutes to obtain a heated billet.

[0082] S3, three-stage controlled rolling: the steel billet after heating in S2 is subjected to three-stage controlled rolling, namely, rough rolling stage, intermediate rolling stage, and finishing rolling stage. The starting rolling temperature in the rough rolling stage is 1150±30°C, the finishing rolling temperature is 1050±30°C, the reduction is 17%, and the thickness after rolling is 116mm; the starting rolling temperature in the intermediate rolling stage is 1050±30°C, the finishing rolling temperature is 950±30°C, the reduction is 65%, and the thickness after rolling is 40.7mm; the starting rolling temperature in the finishing rolling stage is 950±30°C, the finishing rolling temperature is 900±30°C, the reduction is 13%, and the thickness after rolling is 34.4mm, thereby obtaining a three-stage controlled rolled steel;

[0083] S4. Air cooling: The S3 three-stage controlled-rolled steel is air-cooled and annealed to room temperature at a rate of 3°C / s to obtain gear steel.

[0084] like Figure 3 As shown in the figure, the microstructure of the Nb / Ti / RE composite microalloyed 20CrMoH steel prepared in this embodiment is uniformly distributed, and no significant banded structure is produced; high-magnification microstructure observation reveals that the gear steel is composed of massive ferrite and bainite, with the volume fraction of bainite being 55.5% and the volume fraction of ferrite being 44.5%. The average length of the ferrite grains is 25±3μm, and the average width is 13±5μm.

[0085] The impact toughness of the Nb / Ti / RE composite microalloyed 20CrMoH steel prepared in this embodiment is 70±5J, indicating that the microstructure and properties are evenly distributed. Figure 7 The hardness curve of Example 3 shows that the hardness fluctuation range of the gear steel is relatively small between 300-350 HV, indicating that the performance consistency of different parts is good.

[0086] Example 4

[0087] The Nb / Ti / RE composite microalloyed 20CrMoH steel of this embodiment has the following chemical composition by mass percentage: C 0.18%, Si 0.2%, Mn 1.0%, Cr 0.8%, Mo 0.15%, Nb 0.015%, Ti 0.025%, Al 0.033%, RE 0.0050%, P ≤ 0.008%, S ≤ 0.008%; the remainder is Fe and unavoidable impurities.

[0088] A preparation method based on the Nb / Ti / RE composite microalloyed 20CrMoH steel, the preparation method of the Nb / Ti / RE composite microalloyed 20CrMoH steel specifically comprises the following steps:

[0089] S1. Billet preparation: Raw materials were weighed according to the chemical composition of Nb / Ti / RE composite microalloyed 20CrMoH steel, and then subjected to electric furnace smelting, LF refining, RH vacuum degassing, and continuous casting to obtain a 20CrMoH steel billet with a cross-sectional size of 140×140 mm.

[0090] S2. Billet heating: The S120CrMoH steel billet is heated and held at 1200°C for 120 minutes to obtain a heated billet.

[0091] S3, three-stage controlled rolling: the steel billet after heating in S2 is subjected to three-stage controlled rolling, namely, rough rolling stage, intermediate rolling stage, and finishing rolling stage. The starting rolling temperature in the rough rolling stage is 1150±30°C, the finishing rolling temperature is 1050±30°C, the reduction is 15%, and the thickness after rolling is 119mm; the starting rolling temperature in the intermediate rolling stage is 1050±30°C, the finishing rolling temperature is 950±30°C, the reduction is 75%, and the thickness after rolling is 30mm; the starting rolling temperature in the finishing rolling stage is 950±30°C, the finishing rolling temperature is 900±30°C, the reduction is 15%, and the thickness after rolling is 25.3mm, thereby obtaining a three-stage controlled rolled steel;

[0092] S4. Air cooling: The S3 three-stage controlled-rolled steel is air-cooled and annealed to room temperature at a rate of 3°C / s to obtain gear steel.

[0093] like Figure 4 As shown in the figure, the microstructure of the Nb / Ti / RE composite microalloyed 20CrMoH steel prepared in this embodiment is uniformly distributed, and no significant banded structure is produced. High-magnification microstructure observation reveals that the gear steel is composed of massive ferrite and bainite, with the volume fraction of bainite being 58.5% and the volume fraction of ferrite being 41.5%. The average length of the ferrite grains is 24±4μm, and the average width is 14±3μm.

[0094] The impact toughness of the Nb / Ti / RE composite microalloyed 20CrMoH steel prepared in this embodiment is 73±6 J, indicating that the microstructure and properties are evenly distributed. Figure 7 The hardness curve of Example 4 shows that the hardness fluctuation range of the gear steel is relatively small between 351-401 HV, indicating that the performance consistency of different parts is good.

[0095] Comparative Example 1

[0096] The chemical composition of the Nb / Ti / RE composite microalloyed 20CrMoH steel in this comparative example is as follows, by mass percentage: C 0.25%, Si 0.23%, Mn 0.88%, Cr 0.85%, Mo 0.23%, Nb 0.018%, Ti 0.023%, Al 0.040%, RE 0.0030%, P ≤ 0.008%, S ≤ 0.008%; the remainder is Fe and unavoidable impurities.

[0097] A preparation method based on the Nb / Ti / RE composite microalloyed 20CrMoH steel, the preparation method of the Nb / Ti / RE composite microalloyed 20CrMoH steel specifically comprises the following steps:

[0098] S1. Billet preparation: Raw materials were weighed according to the chemical composition of Nb / Ti / RE composite microalloyed 20CrMoH steel, and then subjected to electric furnace smelting, LF refining, RH vacuum degassing, and continuous casting to obtain a 20CrMoH steel billet with a cross-sectional size of 140×140 mm.

[0099] S2. Billet heating: The S120CrMoH steel billet is heated and held at 1200°C for 120 minutes to obtain a heated billet.

[0100] S3, one-stage controlled rolling: the steel billet heated in S2 is subjected to a one-stage rolling process, with the starting rolling temperature being 1150±30°C, the finishing rolling temperature being 950±30°C, the reduction being 30%, and the thickness after rolling being 98mm to obtain a one-stage controlled rolled steel;

[0101] S4. Air cooling: The S3 one-stage controlled-rolled steel is air-cooled and annealed to room temperature at a rate of 3°C / s to obtain gear steel.

[0102] like Figure 5 As shown in the figure, the microstructure of the Nb / Ti / RE composite microalloyed 20CrMoH steel prepared in this comparative example is unevenly distributed, resulting in a significant banded structure. High-magnification microstructure observation revealed that the gear steel is composed of massive ferrite and bainite, with a volume fraction of bainite of 58.5% and a volume fraction of ferrite of 41.5%. The average length of the ferrite grains is 25±5μm, and the average width is 16±5μm.

[0103] The impact toughness of the Nb / Ti / RE composite microalloyed 20CrMoH steel prepared in this comparative example is 46±12 J, indicating that the distribution of microstructure and properties is uneven. Figure 8The hardness curve of Comparative Example 1 shows that the hardness fluctuation range of the gear steel is large between 305-420HV, indicating that the performance consistency of different parts is poor.

[0104] Comparative Example 2

[0105] The chemical composition of the Nb / Ti composite microalloyed 20CrMoH steel in this comparative example is as follows, by mass percentage: C 0.22%, Si 0.23%, Mn 0.93%, Cr 0.85%, Mo 0.17%, Nb 0.018%, Ti 0.023%, Al 0.029%, P ≤ 0.008%, S ≤ 0.008%; the remainder is Fe and unavoidable impurities.

[0106] A preparation method based on the Nb / Ti composite microalloyed 20CrMoH steel, the preparation method of the Nb / Ti composite microalloyed 20CrMoH steel specifically comprises the following steps:

[0107] S1. Billet preparation: Raw materials were weighed according to the chemical composition of Nb / Ti composite microalloyed 20CrMoH steel, and then subjected to electric furnace smelting, LF refining, RH vacuum degassing, and continuous casting to obtain a 20CrMoH steel billet with a cross-sectional size of 140×140 mm.

[0108] S2. Billet heating: The S120CrMoH steel billet is heated and held at 1200°C for 120 minutes to obtain a heated billet.

[0109] S3, three-stage controlled rolling: the steel billet after heating in S2 is subjected to three-stage controlled rolling, namely, rough rolling stage, intermediate rolling stage, and finishing rolling stage. The starting rolling temperature in the rough rolling stage is 1150±30°C, the finishing rolling temperature is 1050±30°C, the reduction is 15%, and the thickness after rolling is 119mm; the starting rolling temperature in the intermediate rolling stage is 1050±30°C, the finishing rolling temperature is 950±30°C, the reduction is 55%, and the thickness after rolling is 53.5mm; the starting rolling temperature in the finishing rolling stage is 950±30°C, the finishing rolling temperature is 900±30°C, the reduction is 15%, and the thickness after rolling is 45.5mm, thereby obtaining a three-stage controlled rolled steel;

[0110] S4. Air cooling: The S3 three-stage controlled-rolled steel is air-cooled and annealed to room temperature at a rate of 2°C / s to obtain gear steel.

[0111] like Figure 6As shown in the figure, the microstructure of the Nb / Ti composite microalloyed 20CrMoH steel prepared in this comparative example is unevenly distributed, resulting in a significant banded structure. High-magnification microstructure observation revealed that the gear steel is composed of massive ferrite and bainite, with a volume fraction of bainite of 58.5% and a volume fraction of ferrite of 41.5%. The average length of the ferrite grains is 25±6μm, and the average width is 16±6μm.

[0112] The impact toughness of the Nb / Ti composite microalloyed 20CrMoH steel prepared in this comparative example is 59±13 J, indicating that the distribution of microstructure and properties is uneven. Figure 8 The hardness curve of Comparative Example 2 shows that the hardness fluctuation range of the gear steel is large between 315-384HV, indicating that the performance consistency of different parts is poor.

[0113] A comparison of Examples 1-4 and Comparative Examples 1-2 shows that in Examples 1-4, a three-stage controlled rolling process is adopted for the gear steel, and appropriate rare earth alloying is performed. The obtained gear steel has uniform structure, impact toughness of 52-79J, hardness fluctuation of ±25HV, and good performance stability; in Comparative Example 1, a one-stage rolling process is adopted, and the rolling reduction is small (30%), the obtained gear steel has poor structure uniformity, obvious banded structure can be observed in the low-magnification metallographic structure, the steel performance fluctuates greatly, the impact toughness is 46±12J, and the hardness fluctuation range is 305-420HV; in Comparative Example 2, no rare earth elements are added, and although a three-stage controlled rolling process is adopted, banded structure can still be observed in the metallographic structure, the steel performance fluctuates greatly, the impact toughness is 59±13J, and the hardness fluctuation range is 315-384HV.

[0114] The above scheme, the present invention proposes a Nb / Ti / RE composite microalloyed 20CrMoH steel and a preparation method, which can solve the technical problems existing in the prior art in the production process of gear steel, such as the titanium element forming TiN to reduce the fatigue life of the material, the complicated smelting process, the need to control the impurity content, the difficulty of operation, the banded structure in the hot rolling process causing the mechanical properties to be reduced, the hardness distribution is uneven, and the welding quality is damaged.

[0115] This invention utilizes a Nb / Ti / RE composite microalloy to reduce the amount of Ti added to the steel, effectively mitigating the risk of TiN formation affecting the fatigue life of gear steel. The appropriate addition of the rare earth element Y, due to its strong affinity for oxygen and sulfur, preferentially forms rare earth oxides and sulfides, effectively reducing the content of harmful impurities and brittle inclusions in the steel. Rare earth elements also promote the dispersion of carbides, improving hardenability and resulting in more uniform hardness and microstructure in gears.

[0116] The present invention uses electric furnace smelting, LF refining, RH vacuum degassing and continuous casting to ensure that the alloy composition of the 20CrMoH steel billet is precise, reduce the content of dissolved O, N and H elements in the steel, significantly reduce the content of inclusions such as TiN in the steel, reduce the risk of hydrogen-induced delayed cracking, help improve steel quality, and increase the service life of gears.

[0117] The present invention can improve the component segregation of the steel billet produced during the continuous casting process by performing a heating and heat-insulating treatment on the continuous casting steel billet, promote the uniform distribution of elements, and effectively inhibit the formation of banded structure in the subsequent rolling process.

[0118] This invention addresses the problem of banded structure formation during hot rolling of conventional CrMo gear steels. By optimizing rolling process parameters, the steel achieves improved microstructure homogenization. A three-step rolling process combined with temperature control suppresses banded structure formation and improves microstructure uniformity.

[0119] The present invention suppresses the formation of a large amount of proeutectoid ferrite in the gear steel by air cooling annealing to room temperature, and prevents the formation of banded structure during the cooling process; at the same time, the blocky ferrite and bainite generated during the air cooling process are evenly distributed, ensuring that the gear steel obtains excellent performance.

[0120] The Nb / Ti / RE composite microalloyed 20CrMoH steel prepared by the present invention has a uniform microstructure distribution and no significant banded structure. High-magnification microstructure observation reveals that the gear steel is composed of massive ferrite and bainite, with the volume fraction of bainite being 41-52% and the volume fraction of ferrite being 48-59%. The average length of the ferrite grains is 17-28 μm, and the average width is 8-18 μm.

[0121] In summary, compared with conventional technologies, the present invention produces Nb / Ti / RE composite microalloyed 20CrMoH steel with an impact toughness of 52-79J and a hardness fluctuation of ±25HV through furnace smelting, LF refining, RH vacuum degassing and continuous casting, heating and heat-holding treatment, three-stage controlled rolling, and air-cooling annealing to room temperature. This method, based on conventional CrMo-based gear steel, introduces the rare earth element yttrium (Y) and an innovative multi-stage rolling process. This Nb / Ti / RE composite microalloyed 20CrMoH gear steel and its preparation process achieve uniform microhardness and microstructure distribution, without noticeable banding, significantly improving performance consistency. The method is simple, easy to operate, environmentally friendly, low-cost, short-process, and highly efficient, making it suitable for large-scale industrial production and promotion.

[0122] It should be understood that the term "and / or" as used herein simply describes a relationship between associated objects, indicating that three possible relationships exist. For example, "A and / or B" can represent: A alone, A and B together, or B alone. A and B can be singular or plural. Furthermore, the character " / " as used herein generally indicates an "or" relationship between the associated objects, but it may also indicate an "and / or" relationship. For specific understanding, please refer to the context.

[0123] In this disclosure, "at least one" means one or more, and "plurality" means two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c" can mean: a, b, c, ab, ac, bc, or abc, where a, b, and c can be single or plural.

[0124] It should be understood that in various embodiments of the present invention, the size of the serial numbers of the above-mentioned processes does not mean the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0125] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present invention should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A Nb / Ti / RE composite microalloyed 20CrMoH steel, characterized in that: The chemical composition of the Nb / Ti / RE composite microalloyed 20CrMoH steel is as follows, by mass percentage: C 0.15-0.25%, Si 0.2-0.25%, Mn 0.80-1.0%, Cr0.8-1.0%, Mo 0.15-0.25%, Nb 0.015-0.025%, Ti 0.015-0.025%, Al 0.020-0.040%, P≤0.008%, S≤0.008%, RE 0.0030-0.0050%; the remainder is Fe and unavoidable impurities.

2. The Nb / Ti / RE composite microalloyed 20CrMoH steel according to claim 1, characterized in that: The Nb / Ti / RE composite microalloyed 20CrMoH steel is a square billet with a cross-sectional size of 24×24-58 mm×58 mm.

3. The Nb / Ti / RE composite microalloyed 20CrMoH steel according to claim 1, characterized in that: The microstructure of the Nb / Ti / RE composite microalloyed 20CrMoH steel is uniformly distributed, without significant banded structure. High-magnification microstructure observation revealed that the gear steel is composed of massive ferrite and bainite, with the volume fraction of bainite being 41-52% and the volume fraction of ferrite being 48-59%. The average length of the ferrite grains is 17-28 μm, and the average width is 8-18 μm.

4. The Nb / Ti / RE composite microalloyed 20CrMoH steel according to claim 1, characterized in that: The impact toughness of the Nb / Ti / RE composite microalloyed 20CrMoH steel is 52-79 J, and the hardness fluctuates within ±25 HV.

5. A method for preparing the Nb / Ti / RE composite microalloyed 20CrMoH steel according to any one of claims 1 to 4, characterized in that: The preparation method of the Nb / Ti / RE composite microalloyed 20CrMoH steel comprises the following steps: S1. Billet preparation: Raw materials are weighed according to the chemical composition of Nb / Ti / RE composite microalloyed 20CrMoH steel, and then subjected to electric furnace smelting, LF refining, RH vacuum degassing and continuous casting to obtain 20CrMoH steel billets: S2. Billet heating: heating and heat-insulating the S120CrMoH steel billet to obtain a heated billet; S3, three-stage controlled rolling: the steel billet after heating in S2 is subjected to three-stage controlled rolling of rough rolling stage, intermediate rolling stage and finishing rolling stage to obtain three-stage controlled rolled steel; S4, air cooling: The S3 three-stage controlled-rolled steel is air-cooled and annealed to room temperature to obtain gear steel.

6. The method for preparing Nb / Ti / RE composite microalloyed 20CrMoH steel according to claim 1, characterized in that: The cross-sectional dimensions of the S1 20CrMoH steel billet are 138×138-140mm×140mm.

7. The method for preparing Nb / Ti / RE composite microalloyed 20CrMoH steel according to claim 1, characterized in that: The heating temperature of S2 heating and insulation treatment is 1200±30℃, and the insulation time is 120±5min.

8. The method for preparing Nb / Ti / RE composite microalloyed 20CrMoH steel according to claim 1, characterized in that: The starting rolling temperature of the S3 rough rolling stage is 1150±30℃, the final rolling temperature is 1050±30℃, the reduction is 13-17%, and the thickness after rolling is 116-121mm; the starting rolling temperature of the intermediate rolling stage is 1050±30℃, the final rolling temperature is 950±30℃, the reduction is 45-75%, and the thickness after rolling is 29-67mm; the starting rolling temperature of the finishing rolling stage is 950±30℃, the final rolling temperature is 900±30℃, the reduction is 13-17%, and the thickness after rolling is 24-58mm.

9. The method for preparing Nb / Ti / RE composite microalloyed 20CrMoH steel according to claim 1, characterized in that: The cooling rate of S4 air cooling annealing is 2-3°C / s.

Images