Steel for Low-Temperature Forged Gears and Method for Producing the Same

Abstract

A low-temperature forged gear steel is disclosed, which, in addition to Fe and unavoidable impurities, further contains the following chemical elements in mass%: 0.15-0.17% C; 0.10-0.20% Si; 1.0-1.10% Mn; 0.80-0.90% Cr and 0.02-0.04% Al. Correspondingly, a manufacturing method for the low-temperature forged gear steel is further disclosed. The manufacturing method includes the following steps: (1) smelting and casting; (2) heating; (3) forging or rolling; and (4) spheroidizing annealing: heating to 750-770°C and holding, then cooling to 700-720°C at a cooling rate of 5-15°C / h and holding, then cooling to 660-680°C at a cooling rate of 3-12°C / h and holding, then cooling to 500°C or less at a cooling rate of 5-20°C / h, and then cooling by tapping.

Description

Technical Field

[0001] The present invention relates to steel and a method for manufacturing the same, and particularly to steel for low-temperature forged gears and a method for manufacturing the same.

Background Art

[0002] Gears are the main components of automotive gearboxes. In recent years, facing the extensive automotive market accompanied by the rapid development of the automotive industry, the demand for automotive gearboxes has been maintained at a high level for a long time.

[0003] Due to the unique shape of gears and high dimensional accuracy requirements, many enterprises manufacture gears by using finishing machining after hot forging. However, the use of this gear manufacturing method not only has a low material utilization rate, but also requires a large amount of energy consumption in hot forging, greatly increasing the processing cost and causing environmental pollution.

[0004] Therefore, some component processing enterprises adopt low-temperature forging technology to manufacture gears. And the low-temperature forging of components generally has the following advantages: (1) The dimensional accuracy and positional accuracy of near-net shape bodies are high, and an ideal blank can be provided for subsequent high-efficiency and high-precision machining; (2) For near-net shape components, many components do not need to be machined subsequently, and the waste of raw materials can be greatly reduced; (3) High productivity, low energy consumption, can effectively reduce the manufacturing cost, shorten the manufacturing cycle, improve the competitiveness of products, and can greatly improve the performance and quality of precision formed products compared with conventional cut products; (4) Compared with the conventional forming process, the low-temperature forging technology of components improves the manufacturing conditions and greatly reduces environmental pollution; and (5) For low-temperature precision forming, due to the reduction of the heat treatment process and the progress of the flash-free process, the energy consumption and environmental pollution are greatly reduced by the low-temperature forging technology of components. Therefore, it can be seen that the low-temperature forging technology is more in line with the future trends of clean manufacturing and green environmental protection, and can create favorable conditions for sustainable development.

[0005] However, the low-temperature forging technology has high requirements for the plasticity of materials. The shape of the gear is complex, and excellent plasticity of the material is required for low-temperature forging processing. Therefore, when manufacturing gears by low-temperature forging technology, due to the lack of plasticity of the steel, problems such as cracking and micro-cracks often occur during extrusion processing. This problem leads to an increase in the scrap rate of processed parts and an increase in subsequent inspection costs.

[0006] Also, hardenability is always one of the important requirement indicators for gear steel. The size of the hardenability band width has a great influence on heat treatment deformation. The narrower the hardenability band width, the smaller the separation range, which is beneficial to gear machining, and the meshing accuracy is improved. Currently, the Chinese national standard GB / T 5216-2014 "Structural Steel with Specific Hardenability Band Width" requires a hardenability band width of 10HRC or more. In the automotive industry, the requirements for the hardenability band width are more stringent than the national standard, but only limit it to a band width of 7HRC or less. With the increasing requirements for the precision machining and assembly dimensional tolerances of automotive gears in the automotive industry, reducing the hardenability band width of gear steel has become a hot research topic for gear steel at home and abroad in recent years. In particular, for some low-temperature forging gear steels that require high precision, the requirements for hardenability are even more stringent.

[0007] Based on this, the object of the present invention is to provide a low-temperature forging gear steel that not only has excellent plastic properties and low-temperature processing properties, but also has a narrow hardenability band width. The low-temperature forging gear steel can be effectively applied to low-temperature forging gears, has a wide application range, and has good prospects for popularization and application value.

Summary of the Invention

[0008] One of the objectives of the present invention is to provide a steel for low-temperature forged gears. The steel for low-temperature forged gears adopts a reasonable chemical composition design, has not only appropriate strength, but also excellent plastic properties and low-temperature forging properties. At the same time, the steel for low-temperature forged gears has a narrow hardening band width of quenching and tempering, and the hardening property of each position (i.e., J1.5, J3, J5, J7, and J9) has a band width within 4 HRC. And the steel for low-temperature forged gears can effectively meet the requirements, has wide applicability, and has good prospects for popularization and application value.

[0009] It should be noted that J1.5 represents the hardness at 1.5 mm from the end of the gear steel, J3 represents the hardness at 3 mm from the end of the gear steel, J5 represents the hardness at 5 mm from the end of the gear steel, J7 represents the hardness at 7 mm from the end of the gear steel, and J9 represents the hardness at 9 mm from the end of the gear steel.

[0010] In order to achieve the above object, the present invention provides a steel for low-temperature forged gears, which further contains the following chemical elements in mass% in addition to Fe and inevitable impurities: 0.15 - 0.17% of C, 0.10 - 0.20% of Si, 1.0 - 1.10% of Mn, 0.80 - 0.90% of Cr, and 0.02 - 0.04% of Al.

[0011] In a preferred embodiment, the steel for low-temperature forged gears according to the present invention consists of the following chemical elements in mass%: 0.15 - 0.17% of C, 0.10 - 0.20% of Si, 1.0 - 1.10% of Mn, 0.80 - 0.90% of Cr, and 0.02 - 0.04% of Al; the balance is Fe and inevitable impurities.

[0012] In the steel for low-temperature forged gears according to the present invention, the design principle of each chemical element is specifically as follows:

[0013] C: In the steel for low-temperature forging gears according to the present invention, the addition of an appropriate amount of C element can ensure that the steel has good hardenability and appropriate strength, which is beneficial for improving the wear resistance of the final parts processed from the steel. However, it should be noted that an increase in the content of C element in the steel increases the hardness of the steel, which may cause the material strength to be too high during subsequent processing, an increase in die defects during low-temperature forging, and an increase in downstream processing costs. Therefore, the content of C element in the steel should not be too high. If the content of C element in the steel is too low, the steel cannot be guaranteed to achieve a high tensile strength. As a result, the structural strength of the center part of the gear decreases, the deformation resistance performance of the gear deteriorates, and the service life of the gear decreases. That is, C is an important element that affects hardenability. In order to narrow the hardenability of the gear steel, in the steel for low-temperature forging gears according to the present invention, the mass content of C element is controlled to be 0.15 - 0.17%.

[0014] Si: In the steel for low-temperature forging gears according to the present invention, the Si element is a ferrite-forming element with a strong solid solution strengthening effect, which can effectively increase the strength of the steel. In addition, since the Si element can reduce the diffusion ability of C in ferrite, by adding an appropriate amount of Si element to the steel, the formation of coarse grains of carbides during spheroidizing annealing and precipitation at defect sites can be avoided. However, it should be noted that the content of Si element in the steel should not be too high. When the content of Si element in the steel is too high, the plasticity of the steel decreases. Therefore, in the steel for low-temperature forging gears according to the present invention, the mass content of Si element is controlled to be 0.10 - 0.20%.

[0015] Mn: In the steel for low-temperature forging gears according to the present invention, in the steel, the Mn element easily forms plastic MnS with the S element, thereby enhancing the cutting-off effect and improving the cutting performance in the subsequent gear finishing process. However, it should be noted that if the content of the Mn element in the steel is too high, it will lead to an increase in the strength and hardness of the steel, and deteriorate the die defects during subsequent low-temperature forging. Therefore, in order to avoid the steel having too high strength and improve the machinability of the steel, and to reduce the variation in hardenability by quenching, in the steel for low-temperature forging gears according to the present invention, the mass content of the Mn element, which is a major element affecting the hardenability by quenching of the gear steel, is controlled to be 1.0 to 1.10%.

[0016] Cr: In the steel for low-temperature forging gears according to the present invention, the addition of the Cr element to the steel inhibits the diffusion-type phase transformation of the steel and is not helpful for the formation of diffusion nuclei during spheroidization. If the content of the Cr element in the steel is too high, coarse carbide grains will be formed, and the low-temperature deformation characteristics will deteriorate. Also, Cr may greatly affect the hardenability by quenching of the gear steel. And in order to reduce the variation range of the hardenability by quenching of the gear steel, in the steel for low-temperature forging gears according to the present invention, the mass content of the Cr element is controlled to be 0.80 to 0.90%.

[0017] Al: In the steel for low-temperature forging gears according to the present invention, the Al element can form fine AlN precipitates during the steelmaking process, and can suppress the growth of austenite grains during the subsequent cooling process, thereby effectively refining the austenite grains and achieving the purpose of improving the toughness of the steel at low temperatures. However, it should be noted that the content of the Al element in the steel should not be too high. If the content of the Al element in the steel is too high, large Al oxides will be formed, and as a result, large-sized type B inclusions will be formed, and the hard inclusions of coarse aluminum oxide will reduce the fatigue resistance of the steel and cause chipping phenomena during machining. Based on this, in order to exert the beneficial effects of the Al element, in the steel for low-temperature forging gears according to the present invention, the mass content of the Al element is controlled to be 0.02 to 0.04%.

[0018] Preferably, in the steel for cryogenic forging according to the present invention, among inevitable impurities, the content of impurity elements satisfies at least one of the following: P ≦ 0.015%, S ≦ 0.003%, N ≦ 0.012% and O ≦ 0.003%, preferably O ≦ 0.002% and B ≦ 0.0002%.

[0019] In the above technical solution, P, S, N, O and B are all impurity elements in the steel. In order to obtain steel with better performance and quality, if technical conditions permit, it is necessary to reduce the content of impurity elements in the steel as much as possible.

[0020] The impurity element P may combine with Fe to form a hard and brittle Fe 3 P phase. As a result, the steel has low-temperature brittleness during cold working and the plasticity of the steel decreases; when the steel is subjected to an impact load, intergranular fracture occurs and a large cleavage surface is formed; and the P element in the steel segregates at the grain boundary, reducing the grain boundary binding energy and the plasticity of the steel. Therefore, in order to avoid increasing the brittleness of the steel, in the present invention, the mass content of the P element is controlled to P ≦ 0.015%.

[0021] The impurity element S, which is an impurity element, easily combines with Fe to form an FeS phase with a melting point of 989 °C, causing high-temperature brittleness of the steel during hot working. Therefore, in order to avoid the high-temperature brittleness of the steel, in the present invention, the mass content of S is controlled to S ≦ 0.003%.

[0022] The impurity element N can form AlN or TiN in the steel and can play a role in refining austenite grains. However, when the content of the N element in the steel increases, the enrichment at the defect part increases, and at the same time, coarse nitride precipitation particles are formed, affecting the service life of the steel. Therefore, in the present invention, it is necessary to control the mass content of the N element to N ≦ 0.012%.

[0023] The impurity element O, together with the Al element and Ti element in the steel, Al 2 O 3Since there is a possibility of forming compounds such as TiO, in order to ensure the uniformity of the steel structure, in the low-temperature forging steel for gears according to the present invention, the mass content of O is controlled to O≤0.003%, preferably the mass content of O is controlled to O≤0.002%.

[0024] The impurity element B has a great influence on the hardenability of the material. The B element segregates at the austenite grain boundary, and when austenite decomposes, it becomes difficult for new phases to nucleate at the austenite grain boundary. Therefore, the incubation period of austenite decomposition becomes longer, so the rate of diffusion phase transformation decreases and martensite transformation is promoted, thus improving the hardenability of the steel. However, since the position of B segregation is not constant, the variation in the hardenability of the material becomes large. Therefore, in the present invention, in order to ensure the hardenability of the gear steel, the mass content of B is controlled to B≤0.0002%.

[0025] Preferably, the low-temperature forging steel for gears according to the present invention further contains at least one of the following chemical elements: 0 < Ca ≤ 0.005% and 0 < Ti ≤ 0.008%.

[0026] In the above technical solution, the above Ca element and Ti element can further improve the properties of the low-temperature forging steel for gears according to the present invention. The design principles of the Ca element and Ti element are as follows:

[0027] Ca: In the low-temperature forging steel for gears according to the present invention, by adding an appropriate amount of Ca element into the steel, CaS can be formed, thereby improving the size and morphology of inclusions and improving the impact toughness of the steel. However, it should be noted that the content of Ca element in the steel should not be too high. Therefore, in the present invention, the mass content of Ca element can be controlled to 0 < Ca ≤ 0.005%.

[0028] Ti: In the steel for low-temperature forging gears according to the present invention, the Ti element can form corresponding compounds with the C element and N element in the steel. Here, the formation temperature of TiN is 1400 °C or higher, and TiN usually precipitates in the liquid phase or δ ferrite. Thereby, the purpose of refining austenite grains is achieved. However, it should be noted that if the content of the Ti element in the steel is too high, coarse TiN precipitates will be formed, resulting in a decrease in the fatigue resistance of the steel. Based on this, in the present invention, the mass content of the Ti element can be controlled to 0 < Ti ≤ 0.008%.

[0029] It should be noted that the addition of the above elements increases the cost of the material. Considering the combination of performance and cost reduction, in the technical solution of the present invention, at least one of the above elements can be preferably added.

[0030] Preferably, the steel for low-temperature forging gears according to the present invention has a fine structure of ferrite + spherical carbide.

[0031] Preferably, the steel for low-temperature forging gears according to the present invention has mechanical properties satisfying a yield strength of 180 - 220 MPa, a tensile strength of 380 - 430 MPa, an elongation rate of 37% or more, and a reduction of area of 68% or more, and / or the steel for low-temperature forging gears has hardenability satisfying J1.5: 38 - 42 HRC, J3: 35 - 39 HRC, J5: 30 - 34 HRC, J7: 26 - 30 HRC, and J9: 21 - 25 HRC, and the above hardenability has a band width of 4 HRC or less respectively.

[0032] Correspondingly, another object of the present invention is to provide a manufacturing method of the above steel for low-temperature forging gears. This manufacturing method is simple to manufacture. The steel for low-temperature forging gears manufactured by this manufacturing method not only has excellent plastic properties and low-temperature processing properties, but also satisfies the requirements of gear steel with narrow hardenability, and has good prospects for popularization and application value.

[0033] To achieve the above object, the present invention provides a manufacturing method of the above steel for low-temperature forging gears, including the following steps: (1) Smelting and casting; (2) Heating; (3) Forging or rolling; and (4) Spheroidizing annealing: After heating and holding at 750 - 770°C, cooling and holding at 700 - 720°C at a cooling rate of 5 - 15°C / h, then cooling and holding at 660 - 680°C at a cooling rate of 3 - 12°C / h, and then cooling to 500°C or below at a cooling rate of 5 - 20°C / h, followed by tapping (i.e., discharging from the furnace) and cooling.

[0034] In the manufacturing method of the low - temperature forging gear steel according to the present invention, by controlling the process conditions, especially the parameters of the heat treatment process, the forging or rolling rod is controlled, and then the spheroidizing annealing process is adopted. As a result, the low - temperature forging gear steel manufactured by the manufacturing method according to the present invention can obtain a ferrite + spheroidal carbide matrix structure in which a large amount of ferrite exists in the matrix, thereby effectively ensuring that the low - temperature forging gear steel of the present invention has good plasticity, removing the internal stress of the steel, and obtaining good structural uniformity.

[0035] In step (1) of the manufacturing method of the present invention, it should be noted that in the smelting process, electric furnace smelting or converter smelting can be adopted, and in the casting process, die - casting or continuous casting can be adopted.

[0036] Therefore, in step (3), a forging process or a rolling process can be adopted. When using the forging process, it can be directly forged to the final size of the round steel. When using the rolling process, the steel slab can also be directly rolled to the final specification. In some embodiments, during the rolling process, the steel slab can be rolled to a predetermined intermediate slab size, and then the intermediate slab can be heated and rolled to obtain the final finished - product size.

[0037] Preferably, in the manufacturing method of the present invention, in step (2), the heating temperature is controlled at 1080 - 1200°C.

[0038] In the above technical solution, in step (2), the steel can be austenitized by controlling the heating temperature at 1080 - 1200°C. Since the elements in the steel are uniformly diffused and the segregation of the material is reduced, the uniformity of the structure of the round steel becomes good, and the fluctuations in hardenability during subsequent forging or rolling and cooling are reduced.

[0039] Preferably, in the manufacturing method of the present invention, in step (3), the final rolling temperature or the final forging temperature is controlled at 860 - 980°C.

[0040] Preferably, in the manufacturing method of the present invention, in step (4), it is heated to 750 - 770°C and held for 4 hours or more, then cooled to 700 - 720°C at a cooling rate of 5 - 15°C / h and held for 3.5 hours or more, cooled to 660 - 680°C at a cooling rate of 3 - 12°C / h and held for 3.5 hours or more, cooled to 500°C or less at a cooling rate of 5 - 20°C / h, and then tapped (i.e., discharged from the furnace) and cooled.

[0041] The steel for low-temperature forging gears and its manufacturing method according to the present invention have the following advantages and beneficial effects compared with the prior art: The steel for low-temperature forging gears according to the present invention, in combination with a specific heat treatment process, by fully utilizing the influence of various alloying elements on phase transformation and microstructure, can form a uniform matrix structure of ferrite + spherical carbide using a reasonable chemical composition design. In addition, the present invention effectively controls the contents of impurities such as P, N, and O in the steel, ensuring that the obtained steel for low-temperature forging gears has a narrow hardenability band width, etc., while having appropriate strength, excellent plasticity, and elongation rate.

[0042] In the present invention, the steel for low-temperature forging gears of the present invention not only has suitable strength, but also excellent plastic properties and low-temperature forging properties. The steel for low-temperature forging gears has mechanical properties satisfying a yield strength of 180 to 220 MPa, a tensile strength of 380 to 430 MPa, an elongation rate of 37% or more, and a reduction of area of 68% or more. On the other hand, the steel for low-temperature forging gears also has a narrow hardenability band width, and the steel for low-temperature forging gears has a hardenability satisfying J1.5: 38 to 42 HRC, J3: 35 to 39 HRC, J5: 30 to 34 HRC, J7: 26 to 30 HRC, and J9: 21 to 25 HRC. And the above-mentioned hardenability has a band width of 4 HRC or less respectively.

[0043] In addition, the steel for low-temperature forging gears according to the present invention has a reasonable chemical composition and process design, a wide process window, can realize mass commercial production in a rod manufacturing line, has a very wide application range, and further has good prospects for popularization and application value. It should be noted.

Brief Description of Drawings

[0044]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0045] Hereinafter, the steel for low-temperature forging gears according to the present invention and its manufacturing method will be further described and described with reference to the drawings and specific examples, but this description and description do not unduly limit the technical solution of the present invention.

Examples

[0046] Examples 1 to 9 The steel for low-temperature forging gears of Examples 1 to 9 were all manufactured by the following steps: (1) Smelting and casting according to the chemical composition shown in Table 1: Smelting was carried out using an electric furnace or a converter, and the casting process adopted die casting or continuous casting. (2) Heating: The heating temperature was controlled at 1080 - 1200 °C. (3) Forging or rolling: The final rolling temperature or the final forging temperature was controlled at 860 - 980 °C. (4) Spheroidizing annealing: After heating and holding at 750 - 770 °C, it was cooled and held at 700 - 720 °C at a cooling rate of 5 - 15 °C / h, then cooled and held at 660 - 680 °C at a cooling rate of 3 - 12 °C / h, and then cooled to 500 °C or below at a cooling rate of 5 - 20 °C / h, and then tapped and cooled.

[0047] The low-temperature forging gear steel of Examples 1 - 9 of the present invention was manufactured using the above processes, and its chemical composition and related process parameters satisfied the control conditions of the design specifications of the present invention.

[0048] It should be noted that in the forging process or rolling process of the above step (3), a forging process or a rolling process can be adopted. When using the forging process, it was possible to directly forge to the final size of the round steel. When using the rolling process, it was also possible to directly roll the steel slab to the final specifications. In some embodiments, during the rolling process, the steel slab was rolled to a predetermined intermediate slab size, and then the intermediate slab was reheated and rolled to obtain the final finished product size after being discharged from the heating furnace.

[0049] In Examples 1 - 9, in all examples except Example 6, a rolling process of rolling the steel slab in step (3) was adopted. And the final rolling temperature during the rolling process was controlled at 860 - 980 °C; after rolling the steel slab into an intermediate slab with a dimension specification of 215×215 mm, the intermediate slab was reheated, and after discharging the intermediate slab from the heating furnace, the intermediate slab was re-rolled into a final finished round steel with a specification of φ25 - 45 mm.

[0050] Accordingly, in Example 6 of the present invention, a forging process is adopted for the operation in the above step (3), and in Example 6, the final forging temperature is controlled to be 860 to 980 °C, so as to directly forge finished round steel having a specification of φ25 to 45 mm.

[0051] Table 1 lists the chemical element compositions (mass %) of the low-temperature forging steel for gears in Examples 1 to 9.

[0052]

Table 1

[0053] Tables 2-1 and 2-2 list the specific process parameters of the low-temperature forging steel for gears in Examples 1-9 in the above process steps.

[0054]

Table 2

[0055]

Table 3

[0056] The low-temperature forging steel for gears obtained in Examples 1 to 9 was sampled and subjected to various related performance tests. The obtained performance test results are listed in Table 3 respectively.

[0057] The low-temperature forging steel for gears in Examples 1 to 9 was tested in accordance with GB / T 228.1-2010 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature", and the yield strength, tensile strength, elongation, and reduction of area of the steel in the examples were measured.

[0058] Table 3 lists the mechanical property test results of the low-temperature forging steel for gears in Examples 1 to 9.

[0059]

Table 4

[0060] Correspondingly, after the mechanical property tests were completed, the low-temperature forging gear steels of Examples 1 to 9 were sampled and subjected to a quenching hardenability test. The obtained performance test results are listed in Table 4.

[0061] The low-temperature forging gear steels of Examples 1 to 9 were tested according to GB / T 225-2006 "Steel - Quenching Hardenability Test by End Quenching", and the quenching hardenability of the steels of the examples was measured.

[0062] Table 4 lists the quenching hardenability test results of the low-temperature forging gear steels of Examples 1 to 9.

[0063]

Table 5

[0064] As can be seen from Table 3, in the present invention, the low-temperature forging gear steels of Examples 1 to 9 exhibit very excellent mechanical properties. Also, the low-temperature forging gear steels of Examples 1 to 9 all had a yield strength of 182 to 218 MPa, a tensile strength of 382 to 426 MPa, an elongation rate of 37% or more, and a reduction of area of 68% or more. The mechanical properties of the low-temperature forging gear steels of the examples were excellent, and the steel showed good plasticity and reduction of area at low temperatures, indicating excellent low-temperature processing characteristics.

[0065] Therefore, as can be seen from Table 4, the quenching hardenability of the low-temperature forging gear steels of Examples 1 to 9 satisfied J1.5: 39.4 to 41.6 HRC, J3: 36.8 to 38.6 HRC, J5: 30.4 to 32.5 HRC, J7: 27.4 to 29.5 HRC, and J9: 22.3 to 24.7 HRC, and the quenching hardenability band width at each position was 4 HRC or less.

[0066] Figure 1 is a photograph of the microstructure of the low-temperature forging gear steel of Example 4 under an optical microscope.

[0067] Figure 2 is a photograph of an image of the low-temperature forging gear steel of Example 4 by a scanning electron microscope (SEM).

[0068] As can be seen in conjunction with FIGS. 1 and 2, the steel for low-temperature forging gears of Example 4 of the present invention had a microstructure of ferrite + spherical carbide.

[0069] In summary, the present invention developed a steel for low-temperature forging gears with a narrow hardening band width through a reasonable chemical composition design combined with an optimized process. And this steel for low-temperature forging gears not only has better strength, but also excellent plasticity and elongation rate, can be effectively applied to low-temperature forging processed gears, has a very wide application range, and can be seen to have good prospects for popularization and application value.

[0070] Furthermore, the steel for low-temperature forging gears according to the present invention has a reasonable chemical composition and process design, a wide process window, and can realize mass commercial production on a rod manufacturing line.

[0071] It should be noted that the steel for low-temperature forging gears according to the present invention has a reasonable chemical composition and process design, a wide process window, can realize mass commercial production on a rod manufacturing line, and has good prospects for popularization and application value.

[0072] Also, the combination of technical features in the present invention is not limited to the combinations described in the claims of the present invention or the combinations described in specific embodiments. All technical features described in the present invention can be freely combined or integrated in any way as long as there is no contradiction between the technical features.

[0073] Only specific embodiments of the present invention are exemplified above. Obviously, the present invention is not limited to the above embodiments, and it should be noted that there are many similar variations. All variations directly derived or conceivable by those skilled in the art from the content disclosed in the present invention are intended to be included within the protection scope of the present invention.

Claims

1. A steel material for low-temperature forging gears, consisting of the following chemical elements by mass%, 0.15 to 0.17% C; 0.10 to 0.20% Si; 1.0 to 1.10% Mn; 0.80 to 0.90% Cr and 0.02 to 0.04% Al; the balance being Fe and inevitable impurities; Among the inevitable impurities, the content of impurity elements is: P ≤ 0.015%, S ≤ 0.003%, N ≤ 0.012%, O ≤ 0.003% and B ≤ 0.0002%, A steel material for low-temperature forging gears having a microstructure of ferrite + spherical carbide.

2. The steel material for low-temperature forging gears according to Claim 1, satisfying O ≤ 0.002%.

3. The steel material for low-temperature forging gears according to Claim 1, further containing at least one of the following chemical elements: 0 < Ca ≤ 0.005% and 0 < Ti ≤ 0.008%.

4. The steel material for low-temperature forging gears has mechanical properties satisfying a yield strength of 180 to 220 MPa, a tensile strength of 380 to 430 MPa, an elongation of 37% or more, and a reduction of area of 68% or more, and / or The steel material for low-temperature forging gears has hardenability satisfying J1.5: 38 - 42 HRC, J3: 35 - 39 HRC, J5: 30 - 34 HRC, J7: 26 - 30 HRC, and J9: 21 - 25 HRC according to Chinese national standard GB / T 225 - 2006, and the hardenability has a bandwidth of 4 HRC or less respectively, The steel material for low-temperature forging gears according to Claim 1.

5. A manufacturing method of the steel material for low-temperature forging gears according to any one of Claims 1 to 4, the manufacturing method including the following steps: (1) Smelting and casting; (2) Heating; (3) Forging or rolling; and (4) Spheroidizing annealing: After heating and holding at 750 - 770 °C, cooling and holding at 700 - 720 °C at a cooling rate of 5 - 15 °C / h, cooling and holding at 660 - 680 °C at a cooling rate of 3 - 12 °C / h, then cooling to 500 °C or less at a cooling rate of 5 - 20 °C / h, and then discharging from the furnace and cooling.

6. The manufacturing method according to Claim 5, controlling the heating temperature to 1080 - 1200 °C in step (2).

7. The manufacturing method according to Claim 5, controlling the final rolling temperature or the final forging temperature to 860 - 980 °C in step (3).

8. In step (4), after heating to 750 to 770 °C and holding for 4 h or more, cooling to 700 to 720 °C at a cooling rate of 5 to 15 °C / h and holding for 3.5 h or more, cooling to 660 to 680 °C at a cooling rate of 3 to 12 °C / h and holding for 3.5 h or more, cooling to 500 °C or less at a cooling rate of 5 to 20 °C / h, and then discharging from the furnace and cooling, the manufacturing method according to claim 5.

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