Manufacturing method of gear steel for cold forging machining

Abstract

The invention relates to a manufacturing method of gear steel for cold forging machining, and belongs to the technical field of steel preparation. The embodiment of the invention provides a manufacturing method of gear steel for cold forging machining. The method comprises the steps that a preformed blank is obtained; and the preformed blank is subjected to reduction annealing, the gear steel for cold forging machining is obtained, and the hardness of the gear steel for cold forging machining ranges from 120 HB to 160 HB. Traditional spheroidizing annealing is replaced with three-stage coupling temperature control of reduction annealing, the temperature and time parameters in the reduction annealing process are controlled, a soft-hard phase composite structure of ferrite (30%-80%) and spheroidizing pearlite (50%-95%) is obtained, the ferrite serves as a plastic phase to absorb deformation energy, spherical carbide in the spheroidizing pearlite evenly disperses stress, and therefore the spheroidizing performance of the spheroidizing pearlite is improved. The hardness is precisely controlled within the range of 120 HB-160 HB through the synergistic effect of the two, the plasticity reserve (cracking prevention) needed by cold forging forming is met, the enough initial strength is kept, and the requirement of the cold forging technology for materials can be met.

Description

Technical Field

[0001] This application relates to the field of steel preparation technology, and in particular to a method for manufacturing gear steel for cold forging. Background Technology

[0002] In modern machinery manufacturing, gears, as the core component of the transmission system, directly affect the operating efficiency and reliability of the entire mechanical system. In recent years, with technological advancements and industrial development, the requirements for gear precision, strength, and manufacturing costs have been increasing, prompting continuous innovation and optimization in gear manufacturing technology. Cold forging technology, as a volumetric plastic forming process of metals at room temperature, has been widely used in recent years in the field of precision forgings for transmission systems in robots, passenger cars, commercial vehicles, and high-end agricultural machinery. Its core principle is to force the metal blank to undergo plastic deformation by applying a strong unit extrusion force within the mold cavity, directly forming high-precision, high-surface-quality parts. Currently, the amount of cold forged parts used in ordinary passenger cars abroad has reached 40-45 kg, of which tooth-shaped parts account for more than 10 kg, with a single gear weighing over 1 kg and achieving a tooth profile precision of level 7. The introduction of the split-flow forging theory has further promoted the industrialization process. By establishing a split-flow cavity or channel, the material partially flows to a specific area when filling the cavity, effectively solving the forming problem of complex-shaped parts.

[0003] Although cold forging technology has advantages such as high forming accuracy (superior to warm forging and hot forging) and good surface quality, it still faces problems in gear steel applications: during the split forging process, stress concentration easily occurs when the material flows into the split cavity, leading to cracking of the formed part; traditional processes require spheroidizing annealing to adjust the material hardness to the ≤220HBW level required for cold forging, but this has the problems of high energy consumption and low efficiency. Furthermore, conventional gear steels such as 20CrMnTi have excessive hardness in the unannealed state, making direct cold forging difficult. Summary of the Invention

[0004] This application provides a method for manufacturing gear steel for cold forging, in order to solve the following technical problem: how to solve the problem that existing materials must undergo spheroidizing annealing treatment to achieve the hardness requirements required for cold forging.

[0005] This application provides a method for manufacturing gear steel for cold forging, the method comprising:

[0006] Preformed blanks are obtained;

[0007] The preformed billet is subjected to reduced annealing to obtain gear steel for cold forging, wherein the hardness of the gear steel for cold forging is 120HB to 160HB.

[0008] Optionally, the step of subjecting the preformed billet to reduced annealing to obtain gear steel for cold forging includes:

[0009] The preformed blank is heated to 800℃ to 1000℃ at a heating rate of 0.5℃ / s to 30℃ / s, and held for 10 min to 120 min.

[0010] The preformed blank after the first heat preservation is cooled to 600℃~800℃ at a cooling rate of 0.05℃ / s~5℃ / s, and then heat preservation is carried out for 10min~360min.

[0011] After the second heat preservation, the preformed billet is cooled to 400℃ to 600℃ at a cooling rate of 0.05℃ / s to 5℃ / s, and then further cooled to room temperature at a cooling rate of 1℃ / s to 10℃ / s to obtain gear steel for cold forging, wherein the hardness of the gear steel for cold forging is 120HB to 145HB.

[0012] Optionally, the step of subjecting the preformed billet to reduced annealing to obtain gear steel for cold forging includes:

[0013] The preformed blank is heated to 600℃~800℃ at a heating rate of 0.5℃ / s~30℃ / s, and held for 10min~120min for the first time.

[0014] The preformed blank after the first heat preservation is cooled to 400℃~700℃ at a cooling rate of 0.05℃ / s~5℃ / s, and then heat preservation is carried out for 10min~360min.

[0015] After the second heat preservation, the preformed billet is cooled to 400℃ to 600℃ at a cooling rate of 0.05℃ / s to 5℃ / s, and then further cooled to room temperature at a cooling rate of 1℃ / s to 10℃ / s to obtain gear steel for cold forging, wherein the hardness of the gear steel for cold forging is 140HB to 160HB.

[0016] Optionally, the total duration of the reduced annealing is 2 hours to 8 hours.

[0017] Optionally, the microstructure of the cold-forged gear steel, in terms of area fraction, is: ferrite: 30%–80%, spheroidized pearlite: 50%–95%.

[0018] Optionally, after obtaining the gear steel for cold forging, the method further includes:

[0019] The cold-forged gear steel is subjected to cold forging and heat treatment in sequence to obtain the finished gear steel.

[0020] Optionally, the microstructure of the finished gear steel is martensite and retained austenite, wherein the area fraction of the retained austenite is ≤10%.

[0021] Optionally, the martensite grade is from 0 to 3.

[0022] The technical solutions provided in this application have the following advantages compared with the prior art:

[0023] This application provides a method for manufacturing gear steel for cold forging. The method includes: obtaining a preformed billet; and subjecting the preformed billet to reduced-temperature annealing to obtain gear steel for cold forging, wherein the hardness of the gear steel for cold forging is 120HB to 160HB. By replacing traditional spheroidizing annealing with a three-stage coupled temperature control of reduced-temperature annealing, and by controlling the temperature and time parameters during the reduced-temperature annealing process, a soft-hard phase composite structure of ferrite (30% to 80%) and spheroidized pearlite (50% to 95%) is obtained. Ferrite acts as a plastic phase to absorb deformation energy, while the spheroidized carbides in the spheroidized pearlite uniformly disperse stress. The synergistic effect of the two precisely controls the hardness within the range of 120HB to 160HB, which satisfies the plastic reserve required for cold forging (to prevent cracking) and maintains sufficient initial strength, thus meeting the material requirements of the cold forging process. Attached Figure Description

[0024] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0025] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 A schematic flowchart illustrating a method for manufacturing cold-forged gear steel according to an embodiment of this application;

[0027] Figure 2 The metallographic structure of the cold-forging gear steel provided in Embodiment 1 of this application is magnified 3000 times.

[0028] Figure 3 The metallographic structure of the cold-forged gear steel provided in Embodiment 1 of this application is magnified 1000 times.

[0029] Figure 4 The image shown is a metallographic diagram of the cold-forged gear steel provided in Embodiment 1 of this application, magnified 100 times. Detailed Implementation

[0030] 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.

[0031] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1 to 6" cover all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the terms "including" and "comprise" as used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship; "and / or" indicates that multiple situations can exist individually or simultaneously; expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0032] Figure 1 This is a schematic flowchart illustrating a method for manufacturing cold-forged gear steel according to an embodiment of this application.

[0033] Please see Figure 1 This application provides a method for manufacturing gear steel for cold forging, the method comprising:

[0034] S1. Obtain the preformed blank;

[0035] Select suitable metal materials and determine appropriate material composition based on the gear's operating environment and requirements to ensure the quality and stability of raw materials. Cut the raw materials into preforms of the required size and shape for subsequent processing.

[0036] S2. The preformed billet is subjected to reduced annealing to obtain gear steel for cold forging, wherein the hardness of the gear steel for cold forging is 120HB to 160HB.

[0037] Traditional gear steel typically requires spheroidizing annealing before cold forging to achieve the hardness required for cold forging. However, spheroidizing annealing is a time-consuming and costly process. This application aims to eliminate the need for annealing, allowing gear steel bars to directly reach the hardness range required for cold forging, thereby improving production efficiency and reducing costs.

[0038] By precisely controlling parameters such as temperature, heating rate, holding time, and cooling rate, gear steel can achieve ideal microstructure and hardness during the reduced annealing process. This application provides two parameter combinations for reduced annealing to meet different hardness requirements.

[0039] In some embodiments, the step of subjecting the preformed billet to reduced annealing to obtain gear steel for cold forging includes:

[0040] The preformed blank is heated to 800℃ to 1000℃ at a heating rate of 0.5℃ / s to 30℃ / s, and held for 10 min to 120 min.

[0041] The preformed blank after the first heat preservation is cooled to 600℃~800℃ at a cooling rate of 0.05℃ / s~5℃ / s, and then heat preservation is carried out for 10min~360min.

[0042] After the second heat preservation, the preformed billet is cooled to 400℃ to 600℃ at a cooling rate of 0.05℃ / s to 5℃ / s, and then further cooled to room temperature at a cooling rate of 1℃ / s to 10℃ / s to obtain gear steel for cold forging, wherein the hardness of the gear steel for cold forging is 120HB to 145HB.

[0043] In some embodiments, the step of subjecting the preformed billet to reduced annealing to obtain gear steel for cold forging includes:

[0044] The preformed blank is heated to 600℃~800℃ at a heating rate of 0.5℃ / s~30℃ / s, and held for 10min~120min for the first time.

[0045] The preformed blank after the first heat preservation is cooled to 400℃~700℃ at a cooling rate of 0.05℃ / s~5℃ / s, and then heat preservation is carried out for 10min~360min.

[0046] After the second heat preservation, the preformed billet is cooled to 400℃ to 600℃ at a cooling rate of 0.05℃ / s to 5℃ / s, and then further cooled to room temperature at a cooling rate of 1℃ / s to 10℃ / s to obtain gear steel for cold forging, wherein the hardness of the gear steel for cold forging is 140HB to 160HB.

[0047] By setting a heating range of 800℃~1000℃ or 600℃~800℃ (selected according to hardness requirements) and combining it with a rapid heating rate of 0.5℃ / s~30℃ / s, the excessive grain growth is suppressed while ensuring sufficient austenite formation. This stage rapidly crosses the pearlite-austenite phase transformation critical point, forming uniform and fine austenite grains, providing a favorable nucleation substrate for subsequent phase transformation.

[0048] In the first cooling stage (0.05℃ / s~5℃ / s), by slowly cooling to the subcritical region of 600℃~800℃ or 400℃~700℃, the cementite is transformed from lamellar to spherical shape by utilizing carbon atom diffusion kinetics. Holding at this stage (10-360 minutes) provides sufficient time for carbide spheroidization, forming a 50-95% spheroidized pearlite structure. Spheroidized carbides effectively reduce dislocation movement resistance during cold forging and improve the material's plastic deformation capacity. The wide range of holding times (10-360 minutes) provides process adjustment flexibility for materials of different specifications.

[0049] By precisely controlling the ratio of ferrite (30%–80%) to spheroidized pearlite (50%–95%), a composite structure of soft and hard phases is formed. Ferrite, as a plastic phase, absorbs deformation energy, while the spheroidized carbides in the spheroidized pearlite uniformly disperse stress. The synergistic effect of the two precisely controls the hardness within the range of 120HB–160HB, satisfying the plasticity reserve required for cold forging (to prevent cracking) while maintaining sufficient initial strength.

[0050] In the second heat preservation stage, the dynamic recovery and recrystallization mechanism during the heat preservation process is utilized to eliminate the early work hardening effect. Combined with subsequent rapid cooling (1℃ / s~10℃ / s), the degree of supercooling is controlled to form acicular / fine acicular martensite (grade 0~3), ensuring that the residual austenite content of the final gear steel is ≤10%, thus ensuring dimensional stability and fatigue strength.

[0051] Microstructure refinement is achieved through a two-stage cooling process (first cooling at 0.05℃ / s to 5℃ / s to 400℃ to 600℃, then cooling at 1℃ / s to 10℃ / s to room temperature). The final result is a composite microstructure of acicular / fine acicular martensite (acicular length ≤8μm) and retained austenite (≤10%), ensuring a hardness of 120HB to 160HB while maintaining sufficient plasticity reserve. In this embodiment, room temperature refers to 15℃ to 40℃.

[0052] In some implementations, the total duration of the reduced annealing is 2 to 8 hours.

[0053] In this embodiment, by replacing traditional spheroidizing annealing with a three-stage coupled temperature control method that eliminates the need for annealing, the total time is shortened to 2-8 hours, significantly reducing material processing time and thus shortening the overall production cycle. For manufacturing industries pursuing high-efficiency production, this means faster order fulfillment and increased production capacity. The shortened production cycle directly improves production efficiency; more materials can be processed and more products produced in the same amount of time, which is crucial for enhancing a company's competitiveness and market share. The reduced time not only decreases the time spent on manpower and equipment but also lowers energy consumption. During the annealing process, precise control of temperature and time parameters enables the achievement of ideal tissue transformation in a shorter time, thereby reducing energy consumption and production costs. In fields such as robotics and new energy vehicles that require rapid response to market changes, a shorter production cycle means faster adjustments to production plans to meet diverse market demands. This advantage of annealing technology allows companies to respond more flexibly to market fluctuations.

[0054] In some embodiments, the microstructure of the cold-forged gear steel, in terms of area fraction, is: ferrite: 30%–80%, spheroidized pearlite: 50%–95%.

[0055] Ferrite, as a soft phase, effectively reduces the material's resistance to cold deformation and enhances its plastic deformation capacity. During cold forging, its good plasticity helps the material better fill the mold cavity, reducing mold wear and the risk of material cracking caused by high hardness. The core characteristic of spheroidized pearlite is that carbides are uniformly distributed in a spherical or granular form within the ferrite matrix. Compared to traditional lamellar pearlite, spheroidized pearlite exhibits superior cold working properties. The dispersed spherical distribution of carbides in spheroidized pearlite ensures material strength while providing a more uniform stress distribution for metal flow during cold forging, supporting the one-time forming of complex tooth profiles. Spheroidal carbides also effectively reduce dislocation movement resistance during cold forging, further improving the material's plastic deformation capacity. The composite microstructure of ferrite and spheroidized pearlite achieves a balance between strength and toughness: ferrite provides ductility, while spheroidized pearlite strengthens the matrix through spheroidized cementite. This combination allows the material to meet the load-bearing requirements of gears (hardness 120HB~160HB) after cold forging while maintaining sufficient impact resistance. The spheroidized carbides in the spheroidized pearlite dissolve more easily during subsequent heat treatment, facilitating uniform diffusion of carbon and forming a high-strength martensitic structure. This pre-optimized microstructure ensures that the retained austenite in the final gear steel is controlled to ≤10%, guaranteeing dimensional stability. For example, the area fraction of ferrite can be 30%, 40%, 50%, 60%, 70%, 80%, etc.; the area fraction of spheroidized pearlite can be 50%, 60%, 70%, 80%, 90%, 95%, etc.

[0056] In some embodiments, after obtaining the gear steel for cold forging, the method further includes:

[0057] The cold-forged gear steel is subjected to cold forging and heat treatment in sequence to obtain the finished gear steel.

[0058] In the manufacturing process of gear steel for cold forging, after obtaining gear steel with the required hardness (120HB~160HB) through reduced-annealing treatment, two key steps are required: cold forging and heat treatment, to finally obtain the finished gear steel. The obtained gear steel is placed in a cold forging die for extrusion and cold forging. The strong unit extrusion force applied within the die cavity forces the metal blank to undergo plastic deformation, directly forming a high-precision, high-surface-quality gear shape. The cold forging process requires careful control of parameters such as cold forging force and temperature according to the shape and requirements of the gear to ensure its accuracy and performance. Cold forging allows for the direct acquisition of gears close to their final shape, reducing subsequent machining and improving production efficiency.

[0059] After cold forging, gears require heat treatment, such as quenching and tempering, to improve their hardness and wear resistance. They are then precision-machined, including tooth profile correction and surface finish treatment, to meet the product's usage requirements.

[0060] In some embodiments, the microstructure of the finished gear steel is martensite and retained austenite, wherein the area fraction of the retained austenite is ≤10%.

[0061] In the manufacturing process of cold-forged gear steel, the cold-forged material undergoes heat treatment to form martensite and retained austenite (retained austenite area fraction ≤10%) in the microstructure of the finished gear steel. Martensite, as a hard phase, significantly improves the hardness and tensile strength of the gear steel, enabling it to withstand high-load conditions. This is crucial for transmission components such as gears, as gears need to withstand significant forces and torques during operation. Controlling the retained austenite area fraction to ≤10% effectively reduces the risk of dimensional deviations and deformation during gear service. This is because retained austenite may transform into martensite under stress or temperature changes, leading to volume expansion. By controlling the retained austenite content, the adverse effects of this transformation can be mitigated.

[0062] In some embodiments, the martensite grade is 0 to 3.

[0063] In this embodiment, the martensite grade is from 0 to 3, as detailed below:

[0064] The classification of martensite is based on the size and morphological characteristics of its acicular structure, among which:

[0065] Grade 0: Hidden acicular martensite, with no obvious visible acicular structure, and needle length ≤2μm;

[0066] Grade 1: Fine acicular martensite, needle length 2μm~4μm;

[0067] Grade 2: Medium-sized acicular martensite, needle length 4μm~6μm;

[0068] Grade 3: Slightly coarse acicular martensite, needle length 6μm~8μm.

[0069] In the manufacturing process of gear steel for cold forging, the martensite grade is controlled between 0 and 3. This design has many positive effects, mainly reflected in the following aspects:

[0070] Refining the matrix structure improves strength and hardness: When the martensite grade is 0 to 3, its acicular structure is relatively fine and uniform, which helps to refine the matrix structure of gear steel. A refined microstructure can improve the strength and hardness of the material, enabling gear steel to exhibit better load-bearing capacity and durability under high loads.

[0071] Improve the plasticity and toughness of materials:

[0072] Lower grades of martensite (such as grade 0 or 1) generally have better plastic deformation capacity, meaning that the material can more easily fill the die cavity during cold forging, reducing the risk of cracking. At the same time, the refined martensitic structure also helps to improve the toughness of the material, making the gear steel less prone to embrittlement under low temperature or impact loads.

[0073] Improving gear wear resistance: Martensite, as a hard phase, with its refined acicular structure, can increase the hardness of the material, thereby improving the wear resistance of gears. This is of great significance for extending the service life of gears and reducing maintenance costs.

[0074] Optimizing heat treatment deformation: Controlling the martensite grade within the range of 0 to 3 helps reduce the inhomogeneity of microstructure transformation during heat treatment, thereby reducing the risk of deformation of gear steel during quenching, tempering, and other processes. This helps maintain the dimensional accuracy and shape stability of gears.

[0075] Improving the fatigue strength of gear steel: A refined martensitic structure can more effectively disperse stress and reduce stress concentration, thereby improving the fatigue strength of gear steel. This is especially important for gears subjected to alternating loads, as fatigue failure is one of the main forms of gear failure.

[0076] In summary, this application presents a method for obtaining cold-forged gear steel by reducing or eliminating annealing. This method not only shortens processing time and reduces energy consumption and production costs, but also achieves an ideal microstructure and hardness range by precisely controlling the temperature and time parameters during the reduced-annealing process, thus meeting the material requirements of the cold forging process. The application of this innovative technology brings new development opportunities and challenges to the gear manufacturing industry and is expected to promote the application and promotion of cold forging technology in a wider range of fields.

[0077] The present application is further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national / industry standards; if there is no corresponding national / industry standard, they are performed according to general international standards, conventional conditions, or conditions recommended by the manufacturer.

[0078] Example 1

[0079] In this embodiment, the constituent elements and mass percentages of the gear steel are: C 0.15%, Si 0.08%, Mn 1.20%, P 0.008%, S 0.020%, Cr 1.30%, Al 0.025%, N 0.0075%, with the balance being Fe and unavoidable impurities.

[0080] The preformed billet was heated to 950℃ at a heating rate of 10℃ / s and held for 0.5h; then cooled to 750℃ at a cooling rate of 0.05℃ / s and held for 4h; then cooled to 500℃ at a cooling rate of 0.1℃ / s and then cooled to room temperature at a cooling rate of 3℃ / s. The resulting cold-forged gear steel had a hardness of 130HB.

[0081] Example 2

[0082] In this embodiment, the constituent elements and mass percentages of the gear steel are: C 0.17%, Si 0.20%, Mn 0.5%, P 0.008%, S 0.025%, Cr 1.2%, Ni 0.5%, Al 0.025%, N 0.0075%, with the balance being Fe and unavoidable impurities.

[0083] The preformed billet was heated to 800℃ at a heating rate of 10℃ / s and held for 1 hour; then cooled to 740℃ at a cooling rate of 0.05℃ / s and held for 3.5 hours; then cooled to 500℃ at a cooling rate of 0.1℃ / s and continued to cool to room temperature at a cooling rate of 5℃ / s. The resulting cold-forged gear steel had a hardness of 150HB.

[0084] Example 3

[0085] In this embodiment, the constituent elements and mass percentages of the gear steel are: C 0.17%, Si 0.20%, Mn 0.9%, P 0.008%, S 0.025%, Cr 1.0%, Ni 0.5%, Ti 0.06%, Al 0.025%, N 0.0080%, with the balance being Fe and unavoidable impurities.

[0086] The preformed billet was heated to 760℃ at a heating rate of 10℃ / s and held for 1.5h; then cooled to 700℃ at a cooling rate of 0.05℃ / s and held for 4.5h; then cooled to 500℃ at a cooling rate of 0.1℃ / s and then cooled to room temperature at a cooling rate of 2℃ / s. The resulting cold-forged gear steel had a hardness of 135HB.

[0087] Appendix Figure 2-4 Detailed explanation:

[0088] Figure 2 The metallographic diagram of the cold-forged gear steel provided in Embodiment 1 of this application is magnified 3000 times. Figure 2 It can be seen that the obtained spheroidized tissue has a spheroidization rate of over 80%.

[0089] Figure 3 The metallographic diagram of the cold-forged gear steel provided in Embodiment 1 of this application is magnified 1000 times. Figure 3 It can be seen that localized spheroidized pearlite structures and pearlite lamellar structures can meet the molding requirements for different hardness levels.

[0090] Figure 4 The metallographic structure diagram of the cold-forging gear steel provided in Embodiment 1 of this application is magnified by 100x.

[0091] Depend on Figure 4 It can be seen that there is a high ferrite ratio.

[0092] Furthermore, one or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

[0093] Compared with traditional spheroidizing annealing, the total time of the reduced annealing process in this embodiment of the invention is significantly shortened to only 2-8 hours, which significantly shortens the production cycle, improves production efficiency, and reduces production costs.

[0094] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed in this application.

Claims

1. A method for manufacturing gear steel for cold forging, the method comprising: Preformed blanks are obtained; The preformed billet is subjected to reduced annealing to obtain gear steel for cold forging, wherein the hardness of the gear steel for cold forging is 120HB to 160HB.

2. The method according to claim 1, characterized in that, The step of reducing or eliminating annealing the preformed billet to obtain gear steel for cold forging includes: The preformed blank is heated to 800℃ to 1000℃ at a heating rate of 0.5℃ / s to 30℃ / s, and held for 10 min to 120 min. The preformed blank after the first heat preservation is cooled to 600℃~800℃ at a cooling rate of 0.05℃ / s~5℃ / s, and then heat preservation is carried out for 10min~360min. After the second heat preservation, the preformed billet is cooled to 400℃ to 600℃ at a cooling rate of 0.05℃ / s to 5℃ / s, and then further cooled to room temperature at a cooling rate of 1℃ / s to 10℃ / s to obtain gear steel for cold forging.

3. The method according to claim 2, characterized in that, The hardness of the gear steel used for cold forging is 120HB to 145HB.

4. The method according to claim 1, characterized in that, The step of reducing or eliminating annealing the preformed billet to obtain gear steel for cold forging includes: The preformed blank is heated to 600℃~800℃ at a heating rate of 0.5℃ / s~30℃ / s, and held for 10min~120min for the first time. The preformed blank after the first heat preservation is cooled to 400℃~700℃ at a cooling rate of 0.05℃ / s~5℃ / s, and then heat preservation is carried out for 10min~360min. After the second heat preservation, the preformed billet is cooled to 400℃ to 600℃ at a cooling rate of 0.05℃ / s to 5℃ / s, and then further cooled to room temperature at a cooling rate of 1℃ / s to 10℃ / s to obtain gear steel for cold forging.

5. The method according to claim 4, characterized in that, The hardness of the gear steel used for cold forging is 140HB to 160HB.

6. The method according to claim 1, characterized in that, The total duration of the reduced annealing is 2 to 8 hours.

7. The method according to claim 1, characterized in that, The microstructure of the cold-forged gear steel, in terms of area fraction, is: ferrite: 30%–80%, spheroidized pearlite: 50%–95%.

8. The method according to claim 1, characterized in that, After obtaining the gear steel for cold forging, the method further includes: The cold-forged gear steel is subjected to cold forging and heat treatment in sequence to obtain the finished gear steel.

9. The method according to claim 8, characterized in that, The microstructure of the finished gear steel consists of martensite and retained austenite, wherein the area fraction of the retained austenite is ≤10%.

10. The method according to claim 9, characterized in that, The martensite is classified into grades 0 to 3.

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