A carburizing steel for cold forming, a manufacturing method, a heat treatment method, and a part

Abstract

The application relates to a carburizing steel for cold forming, a manufacturing method, a heat treatment method and a part, which comprises, in percentage by mass, C 0.15-0.19%, Si 0.10-0.30%, Mn 0.45-0.70%, P <=0.030%, S 0.015-0.035%, Cr 0.90-1.20%, Nb 0.035-0.050%, Al 0.040-0.060%, B 0.0015-0.0025%, Cu <=0.20%, N 0.007-0.020%, and the rest is Fe and inevitable impurities. The application can solve the problems that the existing carburizing steel is used for producing a cold extrusion part, spheroidizing annealing takes a long time, the production cost is high, and the efficiency is low in the prior art.

Description

Technical Field

[0001] This application relates to the field of heat treatment technology for carburizing steel, and particularly to a carburizing steel for cold forming, a manufacturing method, a heat treatment method, and a part. Background Technology

[0002] Currently, the steel used in high-speed, heavy-duty gears both domestically and internationally is primarily carburized steel. Because these gears are manufactured using specialized processes and require heat treatment, and are used under heavy loads, the carburized steel used must meet specific performance requirements. First, carburized steel should have good formability, such as ease of forming and machinability. Second, it should have good hardenability and minimal dimensional and shape deformation after quenching. Finally, after carburizing, it should possess good mechanical properties, such as bending fatigue strength, good resistance to repeated impacts around the tooth root or oil pockets, and good resistance to pitting corrosion on the tooth surface.

[0003] To improve production efficiency and reduce production costs, parts such as cylindrical gears in high-speed, heavy-duty gears are mostly produced using cold extrusion processes. Typically, for cold extrusion forming, the hardness of carburized bars in the rolled state is too high, requiring post-rolling heat treatment to reduce hardness. Domestically, cold-extruded parts widely use carburized steel as specified in GB / T5216. The national standard GB / T5216 specifies the following alloy element content: C 0.17~0.23%, Si 0.17~0.37%, Mn 0.60~0.95%, P≤0.030%, S≤0.035%, Cr 0.30~0.65%; Ni 0.35%~0.75%, Mo 0.15~0.25%, Cu≤0.25%, and austenite grain size not coarser than grade 5. The billet undergoes spheroidizing annealing, with a holding time of 22–30 hours. After annealing, the billet's hardness ranges from 135 to 145 HB. This results in high resistance during cold extrusion, making it difficult to form and leading to areas of significant localized deformation. This makes it prone to internal microcracks, increasing the risk of fatigue failure. Furthermore, residual stress in cold-extruded parts can cause grain coarsening or abnormal grain formation during carburizing heat treatment. Therefore, for cold-formed parts using continuous furnace carburizing or converter carburizing, the equipment must have intermediate cooling capabilities to refine the grains, placing special requirements on the carburizing heat treatment equipment.

[0004] In summary, the production of cold-extruded parts using existing carburized steel involves time-consuming spheroidizing annealing, resulting in high production costs and low efficiency. The intercooling process requires specialized equipment and adds to the production steps, further reducing efficiency. Furthermore, the pitting corrosion resistance of existing carburized steel materials is insufficient to meet the market demand for increasingly higher torque in heavy-duty gearboxes. Summary of the Invention

[0005] This application provides a carburizing steel for cold forming, a manufacturing method, a heat treatment method, and a part, to solve the problems in the related art where cold extruded parts are produced using existing carburizing steel, resulting in long spheroidizing annealing time, high production costs, and low efficiency.

[0006] In a first aspect, a carburizing steel for cold forming is provided, comprising, by mass percentage: C 0.15–0.19%, Si 0.10–0.30%, Mn 0.45–0.70%, P ≤0.030%, S 0.015–0.035%, Cr 0.90–1.20%, Nb 0.035–0.050%, Al 0.040–0.060%, B 0.0015–0.0025%, Cu ≤0.20%, N 0.007–0.020%, with the remainder being Fe and unavoidable impurities.

[0007] Secondly, a method for manufacturing the cold-forming carburizing steel as described above is provided, comprising the following steps:

[0008] (1) Converter smelting: First, scrap steel is loaded into the converter, and then molten iron is directly loaded into the converter for top and bottom combined oxygen blowing smelting.

[0009] (2) LF refining treatment: steel is tapped with slag left at the bottom of the eccentric furnace, and pre-deoxidation is carried out after the furnace to ensure that the O content is not greater than 10PPM and the S content is not greater than 0.035%. The temperature is controlled at 1520~1620℃, and the N in the ladle is ≤60PPM. Refining takes 30~50min.

[0010] (3) RH / VD vacuum treatment: vacuum degree ≤133Pa, temperature control 1530~1650℃, vacuum time 30~50min;

[0011] (4) Continuous casting: Argon gas sealing protection is adopted for long nozzle and argon blowing protection for tundish casting. Covering agent is added to the molten steel surface in tundish for protection. The temperature of the straightening section of continuous casting is 900-980℃.

[0012] (5) Rolling: The initial rolling temperature of the continuously cast billet after austenitization heating is 1100~1160℃, and the hot rolling temperature is 980~1050℃.

[0013] In some embodiments, during converter smelting, the mass ratio of molten iron to scrap steel is 10:1 to 3.

[0014] And / or, the amount of covering agent added is 0.60 to 0.79 kg / ton of steel;

[0015] And / or, in continuous casting, the casting speed of the billet is 1.10 to 1.50 m / min.

[0016] Thirdly, a heat treatment method for the aforementioned cold-forming carburizing steel is provided, comprising the following steps:

[0017] a. Forging: Cold-forming carburized steel is heated and then forged into a forging billet;

[0018] b. Austenitization using residual heat from forging: Using residual heat from forging, the forged billet is transferred to a heating furnace for holding at a temperature of 900–950°C for 60–100 minutes to austenitize it.

[0019] c. Intermediate cooling: The austenitized forging billet is placed in the intermediate cooling zone for cooling;

[0020] d. Three-stage isothermal spheroidizing annealing:

[0021] First stage: Transfer the forged billet after intermediate cooling to an isothermal furnace, with an isothermal temperature of 550℃~620℃ and an isothermal time of 1~2h;

[0022] Second stage: Heat the forging billet after the first stage isothermal treatment to 755℃~770℃, and the isothermal time is 1~2h;

[0023] Third stage: Cool the forged billet after the second stage isothermal treatment in the furnace to 660-680℃, and the isothermal time is 4-5 hours.

[0024] e. Furnace cooling: After the forging billet undergoes three-stage isothermal spheroidizing annealing, it is cooled in the furnace to a certain temperature and then removed from the furnace and air-cooled to room temperature.

[0025] f. Cold extrusion forming: The air-cooled forging billet is cold-extruded to form a blank;

[0026] g. The blank is subjected to rough machining, fine machining, high-temperature carburizing, quenching and low-temperature tempering in sequence to obtain the part.

[0027] In some embodiments, the high-temperature carburizing process includes:

[0028] The billet is placed into a vacuum carburizing furnace and preheated at 620-680℃ for 1.5-2.5 hours, while maintaining the vacuum pressure inside the furnace at <200Pa.

[0029] Then the billet is heated to 980-1020℃ for strong carburizing treatment for 3-5 hours, the ambient carbon potential is controlled at 0.95-1.25%C, and the vacuum carburizing pressure in the furnace is controlled at 800-1700Pa;

[0030] The billet was then transferred to the diffusion zone and subjected to high-temperature diffusion at 960–990°C for 1–2 hours, with the ambient carbon potential controlled at 0.85–1.15%C and the vacuum pressure inside the furnace controlled at 600–1100Pa.

[0031] Next, the blank is subjected to low-temperature diffusion at 920-950℃ for 1-2 hours, with the ambient carbon potential controlled at 0.75-0.95%C and the vacuum pressure inside the furnace controlled at 300-500Pa.

[0032] Finally, the blank is cooled to 820-860℃ and held for 1-1.5 hours. The process layer depth of the high-temperature carburizing treatment is controlled to be 0.7-1.5 mm.

[0033] In some embodiments, when the blank is a thin-walled structure, the quenching process adopts the limiting pressure quenching, the pressure quenching oil temperature is controlled at 60-80°C, the oil spraying time is 110-130s, and the blank is demolded when it cools down to 100-120°C.

[0034] In some embodiments, when the blank is a thick-walled structure, the quenching process is direct oil quenching. The blank is placed in a quenching oil bath with an oil temperature of 110-155°C, and the quenching oil is stirred with a stirrer. When the blank is cooled to the same temperature as the oil, it is drained from the oil.

[0035] In some embodiments, the low-temperature tempering process includes heating the quenched blank to 160°C to 200°C and holding it at that temperature for 1 to 3 hours.

[0036] In some embodiments, during forging, the carburizing steel for cold forming is heated to 1130°C to 1250°C, and the final forging temperature is 980°C to 1070°C.

[0037] And / or, during intermediate cooling, cool to 550-620°C within 300-350 seconds;

[0038] And / or, during furnace cooling, the furnace is cooled to 500°C.

[0039] Fourthly, a part is provided that is manufactured using the heat treatment method for cold-forming carburized steel as described in any of the above.

[0040] The beneficial effects of the technical solution provided in this application include:

[0041] This application provides a cold-forming carburizing steel, a manufacturing method, a heat treatment method, and a part. The C content is set to 0.15–0.19%, the Mn content to 0.45–0.70%, the Nb content to 0.035–0.050%, the B content to 0.0015–0.0025%, and the N content to 0.007–0.020%. By reducing the C and Mn content, the hardness of the steel is reduced, enabling it to soften rapidly during spheroidizing annealing and significantly shortening the spheroidizing annealing time. Adding trace amounts of B compensates for the decreased hardenability caused by low C and Mn content, ensuring hardenability without affecting hardness. The appropriate addition of Nb and N ensures the formation of a large amount of niobium nitride and a small amount of niobium carbide and niobium carbonitride second-phase particles. The formed NbN not only improves the mechanical properties of the steel but also pins grain boundaries and inhibits grain growth, with significant effects. These second-phase particles can strongly pin grain boundaries during carburizing of cold-extruded parts, significantly increasing the grain coarsening temperature. This eliminates the need for the intermediate cooling process in the carburizing of ordinary carburized steel cold-extruded parts, improving production efficiency and reducing production costs. Furthermore, this application meets the production requirements of the cold extrusion process and the performance requirements of the parts without adding precious metals such as Ni and Mo, further reducing raw material production costs.

[0042] The forging billet of carburizing steel for cold forming adopts a three-stage isothermal spheroidizing annealing heat treatment process. Compared with the existing isothermal normalizing process + spheroidizing annealing, the heat treatment time is significantly shortened and the production capacity is saved. At the same time, the metallographic structure of different parts of the forging billet is more consistent, the spheroidization morphology of cementite is good, the matrix hardness is low, the dispersion difference is small, and the hardness is controlled at 112~123HB, which can well meet the requirements of subsequent production.

[0043] Because the hardness of the forged billet is lower before cold extrusion, the risk of microcracks caused by the high resistance of cold extrusion is reduced; at the same time, the intermediate cooling process in the carburizing process of cold extruded parts is eliminated, which improves production efficiency. The grains of the carburized parts are finer, which reduces the deformation of the parts during heat treatment and improves the surface precision of the parts. This makes the pitting resistance of the parts fully meet the market demand for the increasing torque of heavy-duty gearboxes. Attached Figure Description

[0044] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0045] Figure 1 A schematic diagram of the internal gear component of a heavy-duty gearbox provided in an embodiment of this application;

[0046] Figure 2 This is a schematic diagram of the sun gear component of a heavy-duty gearbox provided in an embodiment of this application. Detailed Implementation

[0047] 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.

[0048] This application provides a carburizing steel for cold forming, which, by mass percentage, comprises: C 0.15–0.19%, Si 0.10–0.30%, Mn 0.45–0.70%, P ≤0.030%, S 0.015–0.035%, Cr 0.90–1.20%, Nb 0.035–0.050%, Al 0.040–0.060%, B 0.0015–0.0025%, Cu ≤0.20%, N 0.007–0.020%, with the remainder being Fe and unavoidable impurities.

[0049] In this application, the carbon content (C:C) ensures the matrix strength and hardness of the material. When the C content is too high, the parts have high hardness and strong resistance to cold deformation, but are prone to cracking during cold extrusion; conversely, when the content is too low, the parts lack sufficient hardness and fail to meet performance requirements. Therefore, this application reduces the C content, controlling it within the range of 0.15% to 0.19%, to lower the hardness of the steel, enabling it to soften rapidly during spheroidizing annealing and significantly shortening the spheroidizing annealing time.

[0050] Regarding Si: Si and Mn act as common deoxidizers, affecting the deformation of inclusions in steel. Simultaneously, Si has a high solid solubility in iron, significantly strengthening ferrite and improving matrix toughness. However, when excessive Si is added, the hardness of the steel increases, as the silicon oxides constituting the deoxidation products are hard. Therefore, this application controls the Si content within the range of 0.10–0.30%.

[0051] For Mn: To prevent the precipitation of low-melting-point FeS, which leads to hot brittleness, at grain boundaries, Mn is added to precipitate stable MnS, making chips easier to break and thus improving the machinability of the steel. However, adding too much Mn will significantly increase the hardness of the steel, making the hardness after spheroidizing annealing insufficient for the cold extrusion process. Therefore, the Mn content is controlled within the range of 0.45% to 0.70% to reduce the hardness of the steel, enabling it to soften rapidly during spheroidizing annealing and significantly shortening the spheroidizing annealing time.

[0052] Regarding phosphorus (P): P dissolved in ferrite in steel increases strength; however, excessive P content significantly reduces plasticity and increases hardness. Therefore, this application controls the P content to be ≤0.030%.

[0053] Regarding sulfur (S): The sulfides formed by adding sulfur to steel disrupt the continuity of the metal matrix structure, acting as stress concentration sources under external forces, reducing the cutting resistance of the tool and lowering the cutting temperature. Sulfides typically have low melting points and gradually soften as the cutting temperature increases, exhibiting good plastic deformation capabilities, thus acting as a lubricant, reducing friction, and lessening the friction between chips and the tool, thereby reducing tool wear. Furthermore, sulfides also have a coating and friction-reducing effect; when low-hardness sulfides coat the surface of high-hardness oxides, they reduce tool wear and improve the surface finish of the machined material. However, excessive sulfur can form eutectic compounds (Fe-FeS, Fe-FeS-FeO) with oxygen and iron, which can easily lead to cracking during rolling. Therefore, the target control range for sulfur in this application is 0.015% to 0.035%.

[0054] Regarding Cr: Cr in steel can significantly improve strength, hardness and wear resistance, but it does not improve plasticity and toughness. Therefore, this application adds 0.90 to 1.20% Cr to steel to enhance strength without affecting the plasticity and toughness of the steel.

[0055] For Niobium (Nb): In steel, Nb precipitates as carbides or carbonitrides, pinning austenite grain boundaries, delaying recrystallization, and forming numerous deformation bands and subgrain boundaries, thus ensuring a fine microstructure after phase transformation. For cold-extruded carburizing steels requiring carburizing heat treatment, niobium carbonitrides can effectively inhibit grain growth and increase the grain coarsening temperature. However, when the Nb content is greater than 0.05%, the effect of controlling grain growth is not significant. This is because higher Nb content tends to form large precipitates in the steel, reducing the number of fine precipitates and weakening the pinning force. Therefore, the target control range for Nb is 0.035%–0.050%.

[0056] Regarding Al: Al has a strong affinity for both O and N, and is used as a deoxidizer and nitrogen stabilizer in steelmaking. Al can refine the intrinsic grain size of steel and increase the temperature at which steel grains coarsen. However, when the solid-solution Al content in steel exceeds a certain value, austenite grains tend to grow and coarsen. Therefore, the control range of Al in this application is 0.040% to 0.060%.

[0057] Regarding boron (B): When B exists in steel in the form of acid-soluble boron, it can compensate for the decrease in hardenability caused by the reduction in C and Mn content, thus ensuring the strength of the material. Simultaneously, the addition of B does not significantly affect the hardenability of the steel; therefore, the target range for B is controlled at 0.0015–0.0025%.

[0058] For nitrogen (N): Controlling the N content to 0.007–0.020% allows Nb and N to fully combine to form niobium nitride, minimizing the formation of TiC or Ti(C,N). The resulting NbN not only improves the mechanical properties of the steel but also pins grain boundaries, significantly inhibiting grain growth. Furthermore, it can form small amounts of second-phase particles such as niobium carbide and niobium carbonitride. These second-phase particles strongly pin grain boundaries during carburizing of cold-extruded parts, significantly increasing the grain coarsening temperature. This eliminates the need for the intermediate cooling process in carburizing ordinary carburized steel cold-extruded parts, improving production efficiency and reducing production costs.

[0059] Furthermore, this application meets the production requirements of the cold extrusion process and the performance requirements of the parts without adding precious metal elements such as Ni and Mo, thereby further reducing the production cost of raw materials.

[0060] This application also provides a method for manufacturing the above-mentioned carburizing steel for cold forming, which includes the following steps:

[0061] (1) Converter smelting: First, the scrap steel is loaded into the converter, and then the molten iron is directly loaded into the converter for top and bottom combined oxygen blowing smelting.

[0062] In converter smelting, the mass ratio of molten iron to scrap steel can be determined according to actual needs. For example, the mass ratio of molten iron to scrap steel is 10:1 to 3.

[0063] Before oxygen blowing, nitrogen can be supplied first, and then oxygen blowing can continue until the end. The chemical energy of the steel reaction can replace electrical energy to ensure the amount of decarburization of the steel during the smelting process, the carbon content at the smelting endpoint, and the temperature.

[0064] The duration of nitrogen supply is determined according to actual needs. For example, nitrogen supply can be carried out for 2 minutes.

[0065] (2) LF refining treatment: steel is tapped with slag left at the bottom of the eccentric furnace, and pre-deoxidation is carried out after the furnace to ensure that the O content is not greater than 10PPM and the S content is not greater than 0.035%. The temperature is controlled at 1520~1620℃, and the N in the ladle is ≤60PPM. Refining is carried out for 30~50min.

[0066] During the LF refining process, one or more alloys, such as ferromanganese, ferrosilicon, or composite refining deoxidizers, can be added to the ladle.

[0067] (3) RH / VD vacuum treatment: The refined molten steel is placed in a vacuum furnace for vacuum degassing treatment. The vacuum degree is ≤133Pa, the temperature is controlled at 1530~1650℃, and the vacuum time is 30~50min to obtain the molten steel after vacuum degassing.

[0068] (4) Continuous casting: Argon gas sealing protection is adopted for long nozzle and argon blowing protection for tundish casting. Covering agent is added to the molten steel surface in tundish for protection. The temperature of the straightening section of continuous casting is 900~980℃.

[0069] In continuous casting, an arc-shaped continuous casting machine is used. Continuous casting includes several parts: casting, primary cooling, secondary cooling, straightening, and flame cutting.

[0070] The amount of covering agent added is 0.60-0.79 kg / ton of steel; the covering agent is a low-carbon covering agent, and the insertion depth of the submerged nozzle is 100-130 mm to avoid the impact of molten steel and cause excessive fluctuations in the liquid surface.

[0071] The secondary cooling section adopts a weak cooling process, and the mold protective slag is a high-basicity protective slag with an basicity of 4. The mold uses electromagnetic stirring with a current of 600-700A and a frequency of 4-5Hz, and the continuous casting billet pulling speed is 1.10-1.50m / min.

[0072] (5) Rolling: The initial rolling temperature of the continuously cast billet after austenitization heating is 1100~1160℃, and the hot rolling temperature is 980~1050℃.

[0073] This application also provides a heat treatment method for the above-mentioned cold-forming carburized steel, which includes the following steps:

[0074] a. Forging: Cold-forming carburized steel is heated and then forged into a forging billet;

[0075] Round steel bars with a specification of Φ70mm~Φ110mm are cut into segments of 50mm~205mm using a CNC round steel cutting machine to ensure accurate cutting dimensions, high flatness of the end face of the segments, and no burrs.

[0076] During forging, the induction heating is carried out to 1130℃~1250℃, and the final forging temperature is 980℃~1070℃; the forging billet is obtained after forging.

[0077] b. Austenitization using residual heat from forging: Using residual heat from forging, the forged billet is quickly transferred to a heating furnace for holding at a temperature of 900–950°C for 60–100 minutes to fully austenitize it.

[0078] Among them, the heating furnace can be a mesh belt furnace.

[0079] c. Intermediate cooling: The austenitized forging billet is placed in the intermediate cooling zone for cooling;

[0080] Specifically, during intermediate cooling, the temperature is rapidly reduced to 550-620°C within 300-350 seconds to minimize the presence of proeutectoid ferrite in the forging billet.

[0081] d. Three-stage isothermal spheroidizing annealing:

[0082] First stage: The forging billet after intermediate cooling treatment is immediately transferred to an isothermal furnace, the isothermal temperature is 550℃~620℃, and the isothermal time is 1~2h;

[0083] Second stage: Heat the forging billet after the first stage isothermal treatment to 755℃~770℃, and the isothermal time is 1~2h;

[0084] Third stage: Cool the forged billet after the second stage isothermal treatment in the furnace to 660-680℃, and the isothermal time is 4-5 hours.

[0085] In step d, a three-stage isothermal spheroidizing annealing process is adopted, which cleverly combines the isothermal normalizing process of the forging billet before the cold extrusion process with the spheroidizing annealing process. This not only saves the energy required for heating, but also significantly shortens the spheroidizing annealing time.

[0086] The first stage of the three-stage isothermal spheroidizing annealing process aims to obtain a uniform pearlite + ferrite microstructure with banded structure ≤1.5 grade. The second stage directly heats the material from 550℃~620℃ in the first stage to 755~770℃, saving heating energy and causing partial dissolution of cementite in the pearlite. During the holding process, the lamellar pearlite, due to its different radii of curvature, has varying solubility at different locations, causing carbon diffusion and disrupting the carbon concentration balance. This results in the spheroidization of cementite, with carbides distributed as small spheres or dots on the ferrite matrix. The third stage cools the material from 755~770℃ to 660~680℃ to prolong the spheroidization process of cementite, further reducing the hardness of the forging billet and obtaining a microstructure with good cold extrusion properties, a grain size of 7~9 grade, and a hardness of 112~123HB.

[0087] In summary, the three-stage isothermal spheroidizing annealing heat treatment process for cold-forming carburizing steel billets significantly shortens the heat treatment time and saves production capacity compared to the existing isothermal normalizing process + spheroidizing annealing. At the same time, the metallographic structure of different parts of the billet is more consistent, the cementite spheroidization morphology is good, the matrix hardness is low, the dispersion difference is small, and the hardness is controlled at 112-123HB, which can well meet the requirements of subsequent production.

[0088] Because the hardness of the forged billet is lower before cold extrusion, the risk of microcracks caused by the high resistance of cold extrusion is reduced; at the same time, the intermediate cooling process in the carburizing process of cold extruded parts is eliminated, which improves production efficiency. The grains of the carburized parts are finer, which reduces the deformation of the parts during heat treatment and improves the surface precision of the parts. This makes the pitting resistance of the parts fully meet the market demand for the increasing torque of heavy-duty gearboxes.

[0089] e. Furnace cooling: After the forging billet undergoes three-stage isothermal spheroidizing annealing, it is cooled in the furnace to a certain temperature and then removed from the furnace and air-cooled to room temperature. The furnace cooling temperature can be determined according to actual needs. For example, as an example, it can be cooled in the furnace to 500°C.

[0090] f. Cold extrusion forming: The air-cooled forging billet is cold-extruded to form a blank;

[0091] After air cooling, the forged billet is shot-peened to remove the surface oxide scale, then machined and impregnated with a polymer lubricant. After air drying, it is cold-extruded on a press to obtain the billet.

[0092] g. The blank is subjected to rough machining, fine machining, high-temperature carburizing, quenching and low-temperature tempering in sequence to obtain the part.

[0093] High-temperature carburizing treatment includes:

[0094] After setting the carburizing time and diffusion time according to the technical requirements of the parts and compiling the corresponding process curves, the blanks are loaded into the vacuum carburizing furnace for high-temperature carburizing. The blanks are preheated at 620-680℃ for 1.5-2.5 hours and the vacuum pressure inside the furnace is kept <200Pa.

[0095] Then the billet is heated to 980-1020℃ for strong carburizing treatment for 3-5 hours, the ambient carbon potential is controlled at 0.95-1.25%C, and the vacuum carburizing pressure in the furnace is controlled at 800-1700Pa;

[0096] The billet was then transferred to the diffusion zone and subjected to high-temperature diffusion at 960–990°C for 1–2 hours, with the ambient carbon potential controlled at 0.85–1.15%C and the vacuum pressure inside the furnace controlled at 600–1100Pa.

[0097] Next, the blank is subjected to low-temperature diffusion at 920-950℃ for 1-2 hours, with the ambient carbon potential controlled at 0.75-0.95%C and the vacuum pressure inside the furnace controlled at 300-500Pa.

[0098] Finally, the blank is cooled to 820-860℃ and held for 1-1.5 hours. The process layer depth of the high-temperature carburizing treatment is controlled to be 0.7-1.5 mm.

[0099] The purpose of high-temperature carburizing is to improve the production efficiency of shallow-walled parts and reduce energy consumption; at the same time, the oxygen-deficient environment of the high-temperature carburizing process can reduce the depth of the intergranular oxide layer on the surface of the parts and improve the fatigue life of the parts.

[0100] The above quenching treatment has different methods for different types of parts.

[0101] For example, when the blank is a thin-walled structure, the quenching process adopts the limiting pressure quenching, the pressure quenching oil temperature is controlled at 60-80℃, the oil spraying time is 110-130s, and the blank is demolded when it cools down to 100-120℃.

[0102] Due to their structural characteristics, thin-walled parts are prone to heat treatment deformation after carburizing and quenching. If form-limiting pressure quenching is not used, the tooth shape, tooth direction, roundness and taper of the parts will exceed the precision requirements of the parts, and qualified parts cannot be produced.

[0103] Furthermore, the advantages of pressure quenching are: ensuring the pass rate of thin-walled parts after carburizing and quenching, enabling the mass production of thin-walled parts to be stable.

[0104] Among them, the rear auxiliary gear sleeve, front auxiliary gear sleeve, first and second gear sleeve, third gear sleeve, and internal gear ring are all thin-walled structural parts.

[0105] For example, when the blank is a thick-walled structure, the quenching process is direct oil quenching. The blank is placed in a quenching oil bath with an oil temperature of 110-155℃, and the quenching oil is stirred with a mixer. When the blank is cooled to the same temperature as the oil, it is drained from the oil.

[0106] The above-mentioned low-temperature tempering treatment includes: placing the quenched blank in a tempering furnace, heating it to 160℃~200℃, and holding it at that temperature for 1~3 hours.

[0107] The structural characteristics of thick-walled parts determine that they are not sensitive to deformation during carburizing and quenching heat treatment. Direct quenching can ensure the dimensional accuracy of the parts, eliminating the need for costly pressure quenching.

[0108] Furthermore, the advantages of direct quenching are: high production efficiency, no need for dedicated pressure quenching equipment, and low production cost.

[0109] Among them, gearbox shaft gears, constant mesh gears, constant mesh gear pairs, reverse gears, and ramp gears are all thick-walled structural parts.

[0110] Example 1:

[0111] Example 1 is a carburizing steel for cold forming, used in heavy-duty gearbox internal gear parts. Its chemical composition by mass percentage includes: C 0.16%, Si 0.23%, Mn 0.58%, P 0.019%, S 0.024%, Cr 1.09%, Nb 0.043%, Al 0.051%, B 0.0019%, Cu 0.17%, N 0.011%, with the remainder being Fe and unavoidable impurities.

[0112] The manufacturing method of the carburized steel for cold forming in this embodiment is as follows:

[0113] (1) Converter smelting: Hot molten iron and high-quality scrap steel are charged into the converter in a ratio of 10:3 and top and bottom composite blowing is carried out. Before oxygen blowing, nitrogen is supplied for 2 minutes, and then oxygen is blown until the end. The chemical energy of the steel reaction is used to replace electrical energy to ensure the amount of decarburization of molten steel, carbon at the end of smelting and temperature.

[0114] (2) LF refining treatment: steel is tapped with slag left at the bottom of the eccentric furnace. Pre-deoxidation is carried out after the furnace. One or more alloys of ferromanganese, ferrosilicon or composite refining deoxidizer are added to the ladle to make the O content reach 8PPM and the S content reach 0.027%. The temperature is controlled at 1580℃ and the N in the ladle is 56PPM. Refining is carried out for 40 minutes.

[0115] (3) Vacuum treatment: The refined molten steel is placed in a vacuum furnace for vacuum degassing treatment. The vacuum degree in the furnace is 100Pa, and the molten steel after vacuum degassing is obtained. The temperature is controlled at 1620℃ and the vacuum time is 35min.

[0116] (4) Continuous casting: An arc-shaped continuous casting machine is used. Continuous casting includes several parts: casting, primary cooling, secondary cooling, straightening, and flame cutting. Argon gas sealing protection is used for long nozzles and argon blowing protection in the tundish. Low-carbon covering agent is added to the tundish steel level at a rate of 0.65 kg / ton of steel. The immersion nozzle is inserted to a depth of 110 mm to avoid excessive molten steel impact and surface fluctuations. The secondary cooling section uses a weak cooling process, and the mold flux uses a high-basicity flux with a basicity of 4. The mold uses electromagnetic stirring with a current of 660 A and a frequency of 5 Hz. The billet casting speed is 1.20 m / min, and the temperature of the straightening section is 950℃.

[0117] (5) Rolling: The initial rolling temperature of the continuously cast billet after austenitization heating is 1140℃, and the hot rolling temperature is 1020℃.

[0118] The manufacturing method of the heavy-duty gearbox internal gear parts in this embodiment includes the following steps:

[0119] (1) Round steel cutting

[0120] The Φ110mm round steel bar is cut into 205mm segments using a CNC round steel cutting machine to ensure accurate cutting dimensions, high flatness of the segment end face, and no burrs.

[0121] (2) Billet heating and forging

[0122] The round steel material is induction heated to 1180℃ after cutting, and then forged into a gear-type part forging billet. The final forging temperature is controlled at 1000℃.

[0123] (3) Austenitizing and heat preservation of forging residual heat

[0124] Using the residual heat from forging, the red-hot forging billet after final forging is quickly transferred to a mesh belt furnace for heat preservation. The temperature of the mesh belt furnace is set to 930℃ and the heat preservation time is 90 minutes, so that the forging billet can be fully austenitized.

[0125] (4) Intercooler

[0126] The austenitized forging billet is placed in the intermediate cooling zone and rapidly cooled to 580°C for 320 seconds to minimize the presence of proeutectoid ferrite in the forging billet.

[0127] (5) Three-stage isothermal spheroidizing annealing

[0128] First stage: The forging billet after intermediate cooling is immediately transferred to the isothermal furnace, the isothermal temperature is controlled at 580℃, and the isothermal time is 1.5h;

[0129] Second stage: Heat the forging billet after the first stage isothermal treatment to 760℃, and the isothermal time is 1.5h;

[0130] The third stage: After the second stage isothermal treatment, the forging billet is cooled in the furnace to 670°C for 4.5 hours. After cooling in the furnace to 500°C, it is taken out of the furnace and air-cooled to room temperature.

[0131] (6) Cold extrusion forming

[0132] After three-stage isothermal spheroidizing annealing, the forging blank is shot-peened to remove the surface oxide scale, machined, impregnated with a polymer lubricant, air-dried, and then cold-extruded into the internal gear tooth shape on a press to obtain the gear blank.

[0133] (7) Rough and fine machining

[0134] After roughing and finishing processes, the gear blank becomes a pre-finished internal gear.

[0135] (8) High-temperature carburizing and quenching treatment

[0136] The semi-finished parts are loaded into a continuous furnace for high-temperature carburizing, and preheated at 650℃ for 2 hours.

[0137] The semi-finished parts were then heated to 1000℃ for strong infiltration treatment for 4 hours, while the carbon potential of the environment was controlled at 1.15%C.

[0138] The semi-finished parts were then transferred to the diffusion zone and subjected to high-temperature diffusion at 970°C for 1.5 hours, while controlling the environmental carbon potential at 1.05% C.

[0139] Next, the semi-finished parts were subjected to low-temperature diffusion at 940℃ for 1.5 hours, with the environmental carbon potential controlled at 0.85%C.

[0140] Finally, the semi-finished parts are cooled to 830℃ and kept at that temperature for 1 hour. The depth of the high-temperature carburizing process layer is controlled to be 0.7-1.2 mm.

[0141] Pressure quenching: The semi-finished parts after high-temperature carburizing are subjected to limiting pressure quenching. The pressure quenching oil temperature is controlled at 70℃, the oil spraying time is maintained at 120s, and the semi-finished parts are demolded when the temperature drops to 110℃.

[0142] (7) Low-temperature tempering

[0143] The cold-extruded gear parts, after high-temperature carburizing and quenching, are placed in a tempering furnace for low-temperature tempering at 180℃, with a holding time of 3 hours.

[0144] The internal gear, after low-temperature tempering, undergoes phosphating to obtain the finished part, the structure of which is as follows: Figure 1 As shown.

[0145] Compared to the current production process of internal gears, which involves "carburizing steel material → forging → isothermal normalizing → spheroidizing annealing → cold extrusion forming → stress-relief annealing → rough and finish machining → continuous furnace conventional carburizing → intermediate cooling treatment → pressure quenching → low-temperature tempering → phosphating treatment," this embodiment offers the following advantages: ① It employs an Nb / B composite microalloying technology to produce special carburizing steel for cold forming. The internal gear blanks forged using this carburizing steel undergo a three-stage isothermal spheroidizing annealing process, significantly shortening the spheroidizing annealing time. Furthermore, the hardness after spheroidizing annealing is lower than that of currently produced parts, ranging from 112 to 116 HB, making subsequent cold extrusion forming easier, reducing deformation resistance, increasing the lifespan of the cold extrusion die, and reducing the tendency for micro-cracks on the surface of the cold-extruded internal gears. ② It eliminates the stress-relief annealing process after cold extrusion and the intermediate cooling process during carburizing, saving production costs and improving production efficiency. ③ Cold-extruded internal gears can be carburized at high temperatures, reducing the carburizing time to 60% of current production times, significantly improving carburizing efficiency.

[0146] Example 2:

[0147] Example 2 is a carburizing steel for cold forming, used in heavy-duty gearbox sun gear parts. Its chemical composition by mass percentage includes: C 0.18%, Si 0.26%, Mn 0.63%, P 0.021%, S 0.028%, Cr 1.15%, Nb 0.046%, Al 0.055%, B 0.0021%, Cu 0.12%, N 0.015%, with the remainder being Fe and unavoidable impurities.

[0148] The manufacturing method of the carburized steel for cold forming in this embodiment is as follows:

[0149] (1) Converter smelting: Hot molten iron and high-quality scrap steel are charged into the converter in a ratio of 10:2 and top and bottom combined blowing is carried out. Before oxygen blowing, nitrogen is supplied for 2 minutes, and then oxygen is blown until the end. The chemical energy of the steel reaction is used to replace electrical energy to ensure the amount of decarburization of molten steel, carbon at the end of smelting and temperature.

[0150] (2) LF refining treatment: steel is tapped with slag left at the bottom of the eccentric furnace. Pre-deoxidation is carried out after the furnace. One or more alloys of ferromanganese, ferrosilicon or composite refining deoxidizer are added to the ladle to make the O content reach 10PPM and the S content reach 0.028%. The temperature is controlled at 1610℃ and the N in the ladle is 56PPM. Refining is carried out for 50 minutes.

[0151] (3) Vacuum treatment: The refined molten steel is placed in a vacuum furnace for vacuum degassing treatment. The vacuum degree in the furnace is 100Pa, and the molten steel after vacuum degassing is obtained. The temperature is controlled at 1620℃ and the vacuum time is 45min.

[0152] (4) Continuous casting: An arc-shaped continuous casting machine is used. Continuous casting includes several parts: casting, primary cooling, secondary cooling, straightening, and flame cutting. Argon gas sealing protection is used for long nozzles and argon blowing protection in the tundish. Low-carbon covering agent is added to the tundish steel level at a rate of 0.76 kg / ton of steel. The immersion nozzle is inserted to a depth of 127 mm to avoid excessive molten steel impact and surface fluctuations. The secondary cooling section uses a weak cooling process, and the mold flux uses a high-basicity flux with a basicity of 4. The mold uses electromagnetic stirring with a current of 690 A and a frequency of 4.5 Hz. The billet casting speed is 1.15 m / min, and the temperature of the straightening section is 970℃.

[0153] (5) Rolling: The initial rolling temperature of the continuously cast billet after austenitization heating is 1160℃, and the hot rolling temperature is 1050℃.

[0154] The manufacturing method of the heavy-duty gearbox sun gear component in this embodiment includes the following steps:

[0155] (1) Round steel cutting

[0156] The Φ75mm round steel bar is cut into 68mm segments using a CNC round steel cutting machine to ensure accurate cutting dimensions, high flatness of the segment end face, and no burrs.

[0157] (2) Billet heating and forging

[0158] The round steel material is induction heated to 1250℃ after cutting, and then forged into a gear-type part forging billet. The final forging temperature is controlled at 1050℃.

[0159] (3) Austenitizing and heat preservation of forging residual heat

[0160] Using the residual heat from forging, the red-hot forging billet after final forging is quickly transferred to a mesh belt furnace for heat preservation. The temperature of the mesh belt furnace is set to 950℃ and the heat preservation time is 100 minutes to fully austenitize the forging billet.

[0161] (4) Intercooler

[0162] The austenitized forging billet is placed in the intermediate cooling zone and rapidly cooled to 620°C for 350 seconds to minimize the presence of proeutectoid ferrite in the forging billet.

[0163] (5) Three-stage isothermal spheroidizing annealing

[0164] First stage: Immediately transfer the forged billet after intermediate cooling to the isothermal furnace, control the isothermal temperature at 620℃, and the isothermal time is 2 hours.

[0165] Second stage: Heat the forging billet after the first stage isothermal treatment to 770℃, and the isothermal time is 2 hours.

[0166] The third stage: After the second stage of isothermal treatment, the forging billet is cooled in the furnace to 680°C for 5 hours. After cooling in the furnace to 500°C, it is taken out of the furnace and air-cooled to room temperature.

[0167] (6) Cold extrusion forming

[0168] After three-stage isothermal spheroidizing annealing, the forging blank is shot-peened to remove the surface oxide scale, machined, impregnated with a polymer lubricant, air-dried, and then cold-extruded into the sun gear tooth shape on a press to obtain the tooth blank.

[0169] (7) Rough and fine machining

[0170] After roughing and finishing processes, the gear blank becomes a pre-finished sun gear.

[0171] (8) High-temperature carburizing and quenching treatment

[0172] The semi-finished parts are loaded into a continuous furnace for high-temperature carburizing, and preheated at 680℃ for 2.5 hours.

[0173] The semi-finished parts were then heated to 1020℃ for strong infiltration treatment for 3.5 hours, while controlling the environmental carbon potential at 1.25%C.

[0174] The semi-finished parts were then transferred to the diffusion zone and subjected to high-temperature diffusion at 990°C for 1 hour, while controlling the environmental carbon potential to be 1.15%C.

[0175] Next, the semi-finished parts were subjected to low-temperature diffusion at 950℃ for 1 hour, while controlling the environmental carbon potential to be 0.95%C.

[0176] Finally, the semi-finished parts are cooled to 860℃ and kept at that temperature for 1 hour. The depth of the high-temperature carburizing process layer is controlled to be 1.1-1.5 mm.

[0177] Direct quenching: The semi-finished parts after high-temperature carburizing are placed in a quenching oil bath at 140°C. The stirring speed of the agitator in the quenching oil bath is controlled. When the semi-finished parts are cooled to the same temperature as the oil, they are drained from the oil.

[0178] (7) Low-temperature tempering

[0179] The cold-extruded gear parts, after high-temperature carburizing and quenching, are placed in a tempering furnace for low-temperature tempering at 160℃, with a holding time of 2 hours.

[0180] After low-temperature tempering, the sun gear undergoes gear grinding and phosphating to obtain the finished part, the structure of which is as follows: Figure 2 As shown.

[0181] Compared to the current production process of sun gears, which involves "carburizing steel material → forging → isothermal normalizing → spheroidizing annealing → cold extrusion forming → stress-relief annealing → rough and finish machining → continuous furnace conventional carburizing → intermediate cooling treatment → oil quenching → low-temperature tempering → gear grinding → phosphating treatment," this embodiment offers the following advantages: ① It employs an Nb / B composite microalloying technology to produce carburizing steel specifically for cold forming. The sun gear blanks forged using this carburizing steel undergo segmented isothermal spheroidizing annealing, significantly shortening the spheroidizing annealing process time. Furthermore, the hardness after spheroidizing annealing is lower than that of currently produced parts, ranging from 119 to 123 HB, making subsequent cold extrusion forming easier, reducing deformation resistance, increasing the lifespan of cold extrusion dies, and reducing the tendency for micro-cracks on the surface of the cold-extruded sun gear. ② It eliminates the stress-relief annealing process after cold extrusion and the intermediate cooling process during carburizing, saving production costs and improving production efficiency. ③ Cold-extruded sun gears can be carburized at high temperatures, reducing the carburizing time to 50% of current production times, significantly improving carburizing efficiency.

[0182] In the description of this application, it should be noted that the terms "upper," "lower," etc., indicating the orientation or positional relationship are based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application. Unless otherwise expressly specified and limited, the terms "installed," "connected," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication between two elements. For those skilled in the art, the specific meaning of the above terms in this application can be understood according to the specific circumstances.

[0183] It should be noted that in this application, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0184] 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 herein.

Claims

1. A heat treatment method for carburizing steel for cold forming, characterized in that, The carburizing steel for cold forming, by mass percentage, comprises: C 0.15–0.19%, Si 0.10–0.30%, Mn 0.45–0.70%, P ≤0.030%, S 0.015–0.035%, Cr 0.90–1.20%, Nb 0.035–0.050%, Al 0.040–0.060%, B 0.0015–0.0025%, Cu ≤0.20%, N 0.007–0.020%, with the remainder being Fe and unavoidable impurities; The heat treatment method includes the following steps: a. Forging: Cold-forming carburized steel is heated and then forged into a forging billet; b. Austenitization using residual heat from forging: Using residual heat from forging, the forged billet is transferred to a heating furnace for holding at a temperature of 900–950°C for 60–100 minutes to austenitize it. c. Intermediate cooling: The austenitized forging billet is placed in the intermediate cooling zone for cooling; d. Three-stage isothermal spheroidizing annealing: First stage: Transfer the forged billet after intermediate cooling to an isothermal furnace, with an isothermal temperature of 550℃~620℃ and an isothermal time of 1~2h; Second stage: Heat the forging billet after the first stage isothermal treatment to 755℃~770℃, and the isothermal time is 1~2h; Third stage: Cool the forged billet after the second stage isothermal treatment in the furnace to 660-680℃, and the isothermal time is 4-5 hours. e. Furnace cooling: After the forging billet undergoes three-stage isothermal spheroidizing annealing, it is cooled in the furnace to a certain temperature and then removed from the furnace and air-cooled to room temperature. f. Cold extrusion forming: The air-cooled forging billet is cold-extruded to form a blank; g. The blank is subjected to rough machining, fine machining, high-temperature carburizing, quenching and low-temperature tempering in sequence to obtain the part.

2. A heat treatment method for carburizing steel for cold forming as described in claim 1, characterized in that, The method for manufacturing the carburizing steel for cold forming includes the following steps: (1) Converter smelting: First, scrap steel is loaded into the converter, and then molten iron is directly loaded into the converter for top and bottom combined oxygen blowing smelting. (2) LF refining treatment: steel is tapped with slag left at the bottom of the eccentric furnace, and pre-deoxidation is carried out after the furnace to ensure that the O content is not greater than 10ppm and the S content is not greater than 0.035%. The temperature is controlled at 1520~1620℃, and the N in the ladle is ≤60ppm. The refining time is 30~50min. (3) RH / VD vacuum treatment: vacuum degree ≤133Pa, temperature control 1530~1650℃, vacuum time 30~50min; (4) Continuous casting: Argon gas sealing protection is adopted for long nozzle and argon blowing protection for tundish casting. Covering agent is added to the molten steel surface in tundish for protection. The temperature of the straightening section of continuous casting is 900-980℃. (5) Rolling: The initial rolling temperature of the continuously cast billet after austenitization heating is 1100~1160℃, and the hot rolling temperature is 980~1050℃.

3. The heat treatment method for carburizing steel for cold forming as described in claim 2, characterized in that: The manufacturing method further includes the following steps: In converter smelting, the mass ratio of molten iron to scrap steel is 10:1 to 3. And / or, the amount of covering agent added is 0.60 to 0.79 kg / ton of steel; And / or, in continuous casting, the casting speed of the billet is 1.10 to 1.50 m / min.

4. The heat treatment method for carburizing steel for cold forming as described in claim 1, characterized in that: High-temperature carburizing treatment includes: The billet is placed into a vacuum carburizing furnace and preheated at 620-680℃ for 1.5-2.5 hours, while maintaining the vacuum pressure inside the furnace at <200Pa. Then the billet is heated to 980-1020℃ for strong carburizing treatment for 3-5 hours, the ambient carbon potential is controlled at 0.95-1.25%C, and the vacuum carburizing pressure in the furnace is controlled at 800-1700Pa; The billet was then transferred to the diffusion zone and subjected to high-temperature diffusion at 960–990°C for 1–2 hours, with the ambient carbon potential controlled at 0.85–1.15%C and the vacuum pressure inside the furnace controlled at 600–1100Pa. Next, the blank is subjected to low-temperature diffusion at 920-950℃ for 1-2 hours, with the ambient carbon potential controlled at 0.75-0.95%C and the vacuum pressure inside the furnace controlled at 300-500Pa. Finally, the blank is cooled to 820~860℃ and held for 1~1.5h. The process layer depth of the high-temperature carburizing treatment is controlled to be 0.7~1.5mm.

5. The heat treatment method for carburizing steel for cold forming as described in claim 1, characterized in that: When the blank is a thin-walled structure, the quenching process adopts the limiting pressure quenching, the pressure quenching oil temperature is controlled at 60-80℃, the oil spraying time is 110-130s, and the blank is demolded when it cools down to 100-120℃.

6. The heat treatment method for carburizing steel for cold forming as described in claim 1, characterized in that: When the blank is a thick-walled structure, the quenching treatment is direct oil quenching. The blank is placed in a quenching oil bath with an oil temperature of 110-155℃, and the quenching oil is stirred with a stirrer. When the blank is cooled to the same temperature as the oil, it is drained from the oil.

7. The heat treatment method for carburizing steel for cold forming as described in claim 1, characterized in that, Low-temperature tempering treatment includes heating the quenched blank to 160℃~200℃ and holding it at that temperature for 1~3 hours.

8. The heat treatment method for carburizing steel for cold forming as described in claim 1, characterized in that: During forging, the carburizing steel for cold forming is heated to 1130℃~1250℃, and the final forging temperature is 980℃~1070℃; And / or, during intermediate cooling, cool to 550–620°C within 300–350 s; And / or, during furnace cooling, the furnace is cooled to 500°C.

9. A part, characterized in that: It is manufactured using the heat treatment method for cold-forming carburizing steel as described in any one of claims 1-8.

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