20MnCrS5 carburizing steel and method for manufacturing carburizing steel parts

Abstract

The invention relates to 20MnCrS5 carburizing steel and a method for manufacturing carburizing steel parts. The 20MnCrS5 carburizing steel comprises 0.17%-0.20% of C, less than or equal to 0.25% of Si, 1.10%-1.40% of Mn, 1.00%-1.30% of Cr, 0.02%-0.05% of Al, 0.02%-0.04% of S, 0.1%-0.4% of Ni, 0.015%-0.03% of Nb, less than or equal to 0.08% of Mo, 0.1%-0.25% of Cu, less than or equal to 0.015% of P, less than or equal to 0.008% of N and the balance Fe and inevitable impurities. The method comprises the following steps: forging a part by using 20MnCrS5 carburizing steel; the forged part is subjected to pre-oxidation; the pre-oxidized part is subjected to vacuum carburizing treatment, and the carburizing temperature ranges from 950 DEG C to 1000 DEG C; and the carburized part is subjected to high-pressure gas quenching. The Nb element is added on the basis of traditional 20MnCrS5 steel, meanwhile, the content of Ni and the content of Cu are increased, the high-temperature carburizing technology and the follow-up cooling technology are improved, and therefore the problem that carburizing steel parts are prone to thermal deformation in the high-temperature carburizing technology is solved.

Description

Technical Field

[0001] The present invention relates to the field of gear steel production, and particularly to a 20MnCrS5 carburizing steel and a method for manufacturing carburized steel components. Background Art

[0002] With the development of the automotive field towards cost reduction and high quality, the demand for shaft and gear parts with small thermal deformation, stable production, and short production processes is increasing day by day. Shaft and gear parts usually use Cr series, Mn-Cr series, and Cr-Mo series steels. Among them, 20MnCrS5 is widely used. After surface high-temperature carburizing treatment, it has extremely high hardness, and at the same time, the core has good strength and toughness. It mainly inhibits the growth of austenite grains at high temperatures through AlN precipitation phases, achieving the purpose of refining grains and controlling abnormal grain growth. However, with the optimization of the high-temperature carburizing time process, the carburizing temperature has been continuously increased. And the AlN precipitation phase will dissolve at 950°C. When vacuum carburizing at higher temperatures, its inhibitory effect on austenite grain growth weakens, resulting in uneven grain sizes and ultimately exacerbating thermal deformation. Therefore, the conventional service temperature of traditional 20MnCrS5 is below 980°C. At a higher temperature of 1000°C, there is grain growth, and the problem of increased thermal deformation after high-temperature carburizing heat treatment exists.

[0003] The problem of grain size in high-temperature carburizing heat treatment can be improved by adding Nb elements. However, when stabilizing high-temperature and high-solid grains through Nb elements, since the Nb elements refine grains, it has an adverse effect on the hardenability of the steel, causing partial high-temperature phase transformation in the shaft and gear parts obtained by the traditional nitrogen quenching method, generating ferrite, reducing hardness, and at the same time having a greater hardness dispersion, which further exacerbates thermal deformation. Summary of the Invention

[0004] In order to solve the problem that carburized steel components made of existing 20MnCrS5 carburizing steel are prone to thermal deformation, the present invention adds Nb elements while increasing the contents of Ni and Cu in the traditional 20MnCrS5 carburizing steel, and improves the high-temperature carburizing process and subsequent cooling process, thereby improving the thermal deformation problem of 20MnCrS5 carburized steel components.

[0005] The technical solutions provided by the present invention are specifically as follows:

[0006] In the first aspect, the present invention provides a 20MnCrS5 carburizing steel. By weight percentage, the 20MnCrS5 carburizing steel includes:

[0007] C: 0.17% to 0.20%, Si: ≤0.25%, Mn: 1.10% to 1.40%, Cr: 1.00% to 1.30%, Al: 0.02% to 0.05%, S: 0.02% to 0.04%, Ni: 0.1% to 0.4%, Nb: 0.015% to 0.03%, Mo: ≤0.08%, Cu: 0.1% to 0.25%, P: ≤0.015%, N: ≤0.008%, the balance being Fe and unavoidable impurities.

[0008] In some embodiments provided by the present invention, the 20MnCrS5 carburizing steel comprises: C: 0.19% to 0.20%, Si: 0.05% to 0.15%, Mn: 1.10% to 1.30%, Cr: 1.10% to 1.25%, Al: 0.030% to 0.040%, Mo: 0.03% to 0.05%, Ni: 0.15% to 0.25%, Nb: 0.18% to 0.25%, Cu: 0.15% to 0.22%, P: 0.012% to 0.015%, S: 0.020% to 0.025%, N: 0.005% to 0.008%, the balance being Fe and unavoidable impurities.

[0009] In some embodiments provided by the present invention, the 20MnCrS5 carburizing steel comprises: C: 0.19%, Si: 0.10%, Mn: 1.26%, Cr: 1.14%, Al: 0.035%, Mo: 0.04%, Ni: 0.20%, Nb: 0.02%, Cu: 0.19%, P: 0.013%, S: 0.024%, N: 0.006%, the balance being Fe and unavoidable impurities.

[0010] In some embodiments provided by the present invention, the 20MnCrS5 carburizing steel comprises: C: 0.20%, Si: 0.10%, Mn: 1.19%, Cr: 1.20%, Al: 0.037%, Mo: 0.04%, Ni: 0.20%, Nb: 0.02%, Cu: 0.21%, P: 0.014%, S: 0.025%, N: 0.006%, the balance being Fe and unavoidable impurities.

[0011] In the present invention, Nb is added to the traditional 20MnCrS5 carburizing steel. By pinning the austenite grain boundaries with the Nb element, the growth of austenite grains at high temperatures is inhibited, thereby refining the grains, and solving the problem that in the traditional 20MnCrS5 carburizing steel, due to the dissolution of the AlN precipitation phase at a carburizing temperature of ≥950 °C, the inhibition effect of austenite grain growth is weakened, the grain size is uneven, and the thermal deformation is aggravated; the present invention increases the content of Ni to alleviate the decrease in permeability caused by Nb; by adding Cu to replace part of Ni, the strength and hardness of the components are improved, and the low-temperature impact toughness is improved, thereby reducing the thermal deformation of the components.

[0012] In a second aspect, the present invention provides a method for manufacturing carburizing steel components, including:

[0013] Forging components using the above-mentioned 20MnCrS5 carburizing steel;

[0014] Performing pre-oxidation on the forged components;

[0015] Performing vacuum carburizing treatment on the pre-oxidized components at a carburizing temperature of 950 - 1000 °C;

[0016] Performing high-pressure gas quenching on the carburized components.

[0017] In the method for manufacturing carburizing steel components provided by the present invention, first, pre-oxidation is performed on the forged components to promote the subsequent high-temperature carburizing process; then, vacuum carburizing treatment at 950 - 1000 °C is performed on the pre-oxidized components to improve the carburizing efficiency; finally, high-pressure gas quenching is performed on the carburized components, and the uniformity of heat exchange during gas quenching is used to reduce the thermal deformation of the components.

[0018] In some embodiments provided by the present invention, the vacuum carburizing treatment of the pre-oxidized components includes:

[0019] (S1) Convective heating

[0020] Sending the pre-oxidized parts into a vacuum carburizing furnace with a vacuum degree lower than 10 mbar, filling with nitrogen for convective heating, with a heating time of 40 - 60 min and a heating temperature of 950 - 1000 °C;

[0021] (S2) Vacuum heating

[0022] Evacuating to ≤10 mbar and performing vacuum heating, with a heating time of 15 - 30 min and a heating temperature of 950 - 1000 °C;

[0023] (S3) 7 - 9 times of pulse carburizing

[0024] The 1st - 3rd times: carburizing temperature is 950 - 1000 °C, pulse time is 1 - 3 min, carburizing time is 5 - 10 min, acetylene flow rate is 4000 - 10000 L / h;

[0025] The 4th - 6th times: carburizing temperature is 950 - 1000 °C, pulse time is 30 - 60 s, carburizing time is 8 - 15 min, acetylene flow rate is 2000 - 8000 L / h;

[0026] The 7th - 9th times: carburizing temperature is 950 - 1000 °C, pulse time is 30 - 60 s, carburizing time is 3 - 5 min, acetylene flow rate is 3000 - 9000 L / h;

[0027] (S4) Vacuum heat preservation

[0028] After carburizing, conduct vacuum heat preservation at 950 - 1000 °C for 1 - 2 h; then conduct vacuum heat preservation at 820 - 950 °C for 30 - 60 min.

[0029] The present invention makes carburizing uniform by means of pulse carburizing, and makes the C element fully diffuse inward by vacuum heat preservation after carburizing. Reducing the temperature before final quenching is beneficial to reducing the deformation caused by volume shrinkage during the cooling process of the parts.

[0030] In some embodiments provided by the present invention, forging parts using 20MnCrS5 carburizing steel includes: heating 20MnCrS5 carburizing steel to 1200 ± 30 °C to make its alloy elements uniformly dissolve in the steel and fully austenitize, and the final forging temperature is 1050 ± 30 °C.

[0031] In some embodiments provided by the present invention, before pre - oxidation, the method for manufacturing carburizing steel parts provided by the present invention further includes: normalizing the forged parts.

[0032] In some embodiments provided by the present invention, before pre - oxidation, the method for manufacturing carburizing steel parts provided by the present invention further includes: after normalizing, air - cooling the parts simultaneously using upper and lower fans.

[0033] In some embodiments provided by the present invention, when performing high - pressure gas quenching on the carburized parts, the high - pressure gas is helium, and the quenching pressure is 7 - 20 bar.

[0034] In some embodiments provided by the present invention, the method for manufacturing carburizing steel parts provided by the present invention further includes: tempering heat treatment, the tempering temperature is 200 - 300 °C, and the tempering time is 120 - 180 min.

[0035] In some embodiments provided by the present invention, the temperature of pre - oxidation is 380 °C ± 30 °C.

[0036] In some embodiments provided by the present invention, the carburizing steel parts are shaft gear parts.

[0037] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0038] 1. The 20MnCrS5 carburizing steel provided by the present invention maintains the good mechanical properties and wear resistance of the traditional 20MnCrS5 carburizing steel, and is more suitable for carburizing at temperatures above 1000°C.

[0039] 2. The carburizing temperature of the method for manufacturing carburizing steel parts provided by the present invention is 950 - 1000°C, with high carburizing efficiency and small thermal deformation of the obtained parts. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0041] Figure 1 It is the original austenite grain diagram of Example 1;

[0042] Figure 2 It is the core structure diagram of Example 1;

[0043] Figure 3 It is the tooth profile deviation diagram of Example 1;

[0044] Figure 4 It is the tooth direction deviation diagram of Example 1;

[0045] Figure 5 It is the original austenite grain diagram of Example 2;

[0046] Figure 6 It is the core structure diagram of Example 2;

[0047] Figure 7 It is the tooth profile deviation diagram of Example 1;

[0048] Figure 8 It is the tooth direction deviation diagram of Example 1;

[0049] Figure 9 It is the original austenite grain diagram of Comparative Example 1. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0050] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0051] Unless otherwise specified, J9 refers to the hardness at a distance of 9 mm from the end in the end-quench test. This index mainly controls the hardenability of the material, so that workpieces of a certain size can reach the required hardness range after quenching.

[0052] In order to solve the problem of increased thermal deformation of components after the carburizing temperature is increased in the vacuum carburizing heat treatment of traditional 20MnCrS5 carburizing steel, the present invention has improved the composition of traditional 20MnCrS5 carburizing steel, and at the same time proposed a manufacturing method for components suitable for the new 20MnCrS5 carburizing steel, including: improved forging process, high-temperature carburizing process and cooling process. This is of great significance for accelerating the production process of carburized components, stabilizing the production process, reducing the straightening fracture rate and saving costs.

[0053] Compared with the traditional 20MnCrS5 carburizing steel and its component manufacturing method, the present invention solves the problem of grain size growth at higher carburizing temperatures by adding Nb, and reduces the reduction of hardenability caused by Nb refining grains by increasing the content of the hardenability component Ni to cooperate with the subsequent high-pressure helium quenching process. Compared with traditional oil quenching and high-pressure nitrogen quenching, the helium quenching used in the present invention has a smaller quenching pressure and significantly improves the problem of thermal deformation during the cooling process. At the same time, the improvements in the forging process, machining, hot charging and other processes of the present invention all have beneficial effects on improving the thermal deformation of components.

[0054] The 20MnCrS5 carburizing steel provided by the present invention, by weight percentage, includes:

[0055] C: 0.17% - 0.20%, Si: ≤0.25%, Mn: 1.10% - 1.40%, Cr: 1.00% - 1.30%, Al: 0.02% - 0.05%, S: 0.02% - 0.04%, Ni: 0.1% - 0.4%, Nb: 0.015% - 0.03%, Mo: ≤0.08%, Cu: 0.1% - 0.25%, P: ≤0.015%, N: ≤0.008%, and the rest are Fe and unavoidable impurities.

[0056] In the present invention, in order to cooperate with the high-temperature vacuum carburizing heat treatment equipment and the subsequent high-pressure gas quenching cooling method, Nb is added to increase the contents of Ni and Cu alloying elements. The key of the present invention lies in that not only the Nb element pins the austenite grain boundaries, playing a role in inhibiting the growth of high-temperature austenite grains, making the grains refined, thereby improving the hot deformation. Moreover, the hardenability of the material is generally controlled and adjusted by the hardness at J9. Adding Ni can significantly improve the hardenability of the material at J9, improve the hardenability of the gear steel under high-pressure gas quenching, and improve the hot deformation through the cooling process. However, the cost of Ni element is relatively high. Therefore, the function of Cu element with the same effect is used to replace a part of Ni element.

[0057] In the 20MnCrS5 carburizing steel provided by the present invention, the specific design principles of each element are as follows:

[0058] C: C is an important element to ensure the strength and hardenability of the material. However, when the C content is too high, the hardness after controlled forging and normalizing is relatively high, the cutting performance deteriorates, and cracks are likely to occur. Moreover, the design of low-carbon steel is also beneficial to the welding performance and low-temperature toughness. At the same time, the C element is extremely easy to form carbides with alloying elements in the steel, resulting in uneven distribution of the C element, affecting the hardenability and increasing the hot deformation. Therefore, the upper limit of the C content is appropriately reduced in the present invention, and alloying elements are added to make up for the hardenability and reduce the range of hot deformation fluctuations.

[0059] Si: It mainly plays a role of solid solution strengthening in the steel. At the same time, the Si element is a deoxidizing element, which can appropriately improve the deformation of inclusions in the steel. However, too high content of it will affect the removal of P element and reduce the welding performance of the steel. Therefore, it is controlled at ≤0.25%.

[0060] Mn: It is an important strengthening and toughening element in the steel. Increasing the manganese content in the steel can expand the austenite phase region, lower the high-temperature transformation temperature, expand the rolling range, and promote grain refinement. At the same time, it can significantly improve the hardenability of the steel. However, too high Mn content will make the hardness of the steel too high after normalizing, reducing the cutting performance. At the same time, too high hardenability will make the gear steel more likely to generate cracks after carburizing and quenching. Therefore, the Mn content is controlled at 1.10% - 1.40%.

[0061] Cr: It can improve the hardenability of the material, and at the same time improve the strength, hardness and wear resistance of the steel. However, it will also lead to a decrease in plasticity and toughness. Therefore, the Cr content in the steel is controlled at 1.00% - 1.30%.

[0062] Al: The Al element, like the Si element, is a deoxidizer. At the same time, the Al element can form AlN with the N element, which can pin the grain boundaries at high temperatures and inhibit the coarsening of austenite grains during high-temperature carburizing. However, too high Al will increase the smelting difficulty and instead cause the coarsening of austenite grains. Therefore, the Al content is controlled at 0.02% - 0.05%.

[0063] S: Since sulfides generally have a low melting point, adding an appropriate amount of S element to gear steel can improve the cutting force during the grinding process, increase tool life, and improve the finish of the machined surface. However, excessive S element content will form eutectic compounds with O and Fe, increasing the tendency to crack, so the S content is controlled at 0.02% to 0.04%.

[0064] Ni: It mainly exists in the form of solid solution in steel. It can not only improve the strength and hardness of the material through solid solution strengthening, but also improve the low-temperature impact toughness. Ni also plays a role in stabilizing austenitization and improving the hardenability of the material. However, the production cost of Ni is relatively high, so it is controlled at 0.1% to 0.4%.

[0065] Nb: Nb element can form fine precipitate phase in steel, which can inhibit the growth of austenite grains during high-temperature carburizing process. However, too high Nb content will make the precipitate phase coarse and reduce the low-temperature impact toughness of the material. Therefore, the Nb content is controlled at ≤0.03%.

[0066] Mo: Mo significantly improves the hardenability of the material when it is dissolved in steel. However, considering the production cost, the Mo content is controlled at ≤0.08%.

[0067] Cu: The solid solution strengthening effect of Cu in steel is similar to that of Ni. In the present invention, Cu is used to replace the strengthening effect of the precious alloy element Ni. At the same time, Cu can also improve the corrosion resistance of carburized steel, but too high Cu will increase the cracking tendency, so the Cu element in steel is controlled to ≤0.25%.

[0068] P: The composite effect with Cu can effectively improve the weather resistance and corrosion resistance of steel, but it is not good for the toughness and plasticity of steel. Therefore, the P content is controlled below 0.015%.

[0069] N: Nitrogen can combine with aluminum in aluminum-added steel to form aluminum nitride. This second phase precipitated at high temperature is beneficial to inhibit the growth of austenite grains and refine austenite grains. However, if the nitrogen content is higher than 0.007%, it is easy to be enriched in the steel, which seriously damages the plasticity and toughness of the steel, so its content is controlled below 0.008%.

[0070] The method for manufacturing carburized steel parts provided by the present invention comprises:

[0071] Forging parts using the above-mentioned 20MnCrS5 carburizing steel;

[0072] Pre-oxidation of forged parts;

[0073] The pre-oxidized parts are vacuum carburized at a temperature of 950-1000°C;

[0074] Perform high-pressure gas quenching on the carburized components.

[0075] Preferably, the carburized steel components are gear steel parts.

[0076] The method for manufacturing gear steel parts provided by the present invention specifically includes the following steps:

[0077] (1) Forge the steel bar after rolling the above-mentioned gear steel, including: heating the gear steel bar in an intermediate frequency heating furnace to 1200 ± 30 °C to uniformly dissolve its alloying elements in the steel and fully austenitize it, and the final forging temperature is 1050 ± 30 °C; then upset it on a closed single-point press to After upsetting, forge it into a billet on a vertical die forging machine. Compared with forging relative to rolls and horizontal die forging machines using a vertical die forging machine, for shaft parts, the longitudinal grain distribution is more uniform, and the grains are not prone to abnormal growth; and the tissue segregation is reduced, the residual stress in the cross-sectional direction is small, and it is not prone to deformation during subsequent heat treatment. Then normalize the forged parts. The normalization is divided into three zones. The first zone normalization temperature is ≥ 700 °C, the second zone normalization temperature is ≥ 800 °C, and the third zone normalization temperature is 930 ± 10 °C. If the normalization time is too short, the tissue segregation cannot be eliminated and the forging stress cannot be completely released. If the time is too long, surface oxidation and decarburization will occur. Therefore, the normalization time is 110 ± 10 minutes. At the same time, multi-zone normalization can slow down the release process of forging stress before normalization, reduce thermal deformation. And weaken the thermal deformation amount caused by the phase transformation of the parts in different temperature ranges, making the residual stress caused by the phase transformation smaller and more uniformly distributed. After normalization, air-cooling is carried out simultaneously by the upper and lower fans, so that the cooling of the upper and lower regions is uniform. Among them, the working frequency of the upper fan is 40 Hz, the working frequency of the lower fan is 20 Hz, and the air-cooling duration is 150 - 200 s.

[0078] (2) Machining process: After normalizing the forgings, the parts are made into the required parts through machining processes such as shot blasting, rough turning, finish turning, drilling, hobbing, and rolling splines. To reduce the influence of heat treatment thermal deformation on the post-heat treatment processing process, reduce the tool feed rate. If the feed rate is too large, dislocation deformation will occur during the processing process. At the same time, machining allowances need to be reserved for the above-mentioned pre-heat machining processes.

[0079] (3) Hot-fitting method: The hot-fitting method used for the shaft parts of the present invention is three-point support type hanging vertical installation. Hanging vertical installation has a significant improvement in bending deformation caused by gravity compared with the traditional horizontal placement. The number of parts loaded in one furnace is 40 - 300. Reducing the loading quantity is beneficial to more uniform and stable temperature during the heating and cooling processes, and is beneficial to improving thermal deformation.

[0080] (4) Cleaning: Clean the oil stains on the surface of the parts after machining to make the carburizing process more uniform.

[0081] (5) Pre-oxidation treatment: Feed the cleaned parts into a continuous pre-oxidation furnace. The temperature of the pre-oxidation furnace is 380°C ± 30°C, and the oxidation gas is air. The purpose of pre-oxidation is: on the one hand, to clean the residual oil on the part surface through high temperature, and on the other hand, to form a dense oxide layer on the part surface, which can promote the subsequent high-temperature carburizing process.

[0082] (6) Carburizing heat treatment: Feed the pre-oxidized parts into a vacuum carburizing furnace. The vacuum degree in the furnace is lower than 10 mbar, and nitrogen is filled to 1200 mbar for convective heating. The heating time is 40 - 60 min, and the heating temperature is 950 - 1000°C. Then evacuate to 10 mbar for vacuum heating. The heating time is 15 - 30 min, and the heating temperature is 950 - 1000°C. The first-stage convective heating can make the parts heated evenly from low temperature to high temperature, thereby reducing the thermal deformation process caused by the release of residual stress during heating. The second-stage vacuum heating makes the heating process more stable. Then carry out pulse carburizing, pulse carburizing 7 - 9 times: For the 1st - 3rd pulse carburizing, the temperature is 950 - 1000°C, the pulse time is 1 - 3 min, the carburizing time is 5 - 10 min, and the acetylene flow rate is 4000 - 10000 L / h; For the 4th - 6th pulse carburizing, the temperature remains unchanged, the carburizing time is extended, and the acetylene flow rate is appropriately reduced. Among them, the carburizing temperature is 950 - 1000°C, the pulse time is 30 - 60 s, the carburizing time is 8 - 15 min, and the acetylene flow rate is 2000 - 8000 L / h; For the 7th - 9th pulse carburizing, the temperature remains unchanged, the carburizing time is reduced, and the acetylene flow rate is increased. Among them, the carburizing temperature is 950 - 1000°C, the pulse time is 30 - 60 s, the carburizing time is 3 - 5 min, and the acetylene flow rate is 3000 - 9000 L / h. Controlling reasonable pulse times and flow rates can, on the one hand, improve the utilization rate of acetylene, and on the other hand, control the carbon potential in the furnace, making the C element diffuse stably, the carburized layer more uniform, and reducing the thermal deformation during the cooling process. After carburizing, carry out vacuum holding to make the C element diffuse inward sufficiently. The holding time is 1 - 2 h. Finally, reduce the temperature for holding. The holding temperature is 820 - 950°C, and the holding time is 30 - 60 min. Reducing the temperature before final quenching is beneficial to reducing the deformation caused by volume contraction during the cooling process.

[0083] (7) Cooling method: The cooling method adopts high-pressure helium quenching. Compared with traditional oil quenching, the heat exchange of gas quenching is more uniform and the thermal deformation is smaller. Using helium quenching has a higher cooling efficiency than nitrogen quenching, and can be quenched at a lower pressure, which also has an obvious improvement on thermal deformation. The quenching pressure is 7 - 20 bar, and the quenching time is 10 min.

[0084] (8) Tempering heat treatment: After quenching, carry out sufficient tempering to eliminate the residual stress during quenching. The tempering temperature is 200 - 300°C, and the tempering time is 120 - 180 min.

[0085] The technical solution provided by the present invention will be described in detail below in conjunction with embodiments. Unless otherwise specified, the parts processed in the following embodiments and comparative examples are all shaft gear parts of the same specification.

[0086] Embodiment 1

[0087] The chemical composition of the molten steel after alloying treatment in the refining process of the steel bar used for forging is C: 0.19%, Si: 0.10%, Mn: 1.26%, Cr: 1.14%, Al: 0.035%, Mo: 0.04%, Ni: 0.20%, Nb: 0.02%, Cu: 0.19%, P: 0.013%, S: 0.024%, N: 0.006%, and the rest are Fe and inevitable impurities;

[0088] Forging process: Heat the gear steel bar in an intermediate frequency heating furnace to 1200 °C, and the final forging temperature is detected as 1035 °C; then upset it on a closed single-point press to After upsetting, forge it into a billet on a vertical die forging machine, and then normalize the forged parts. The normalization is divided into three zones. The normalization temperature in the first zone is 880 °C, the normalization temperature in the second zone is 920 °C, and the normalization temperature in the third zone is 930 °C. The normalization time is 120 minutes. After normalization, air cooling is carried out simultaneously by upper and lower fans. Among them, the working frequency of the upper fan is 40 Hz, the working frequency of the lower fan is 20 Hz, and the air cooling duration is 180 s.

[0089] Machining: After normalizing the forgings, the parts are subjected to machining processes such as shot blasting, rough turning, finish turning, drilling, hobbing, and rolling splines.

[0090] Hot fitting method: The hot fitting method used is three-point support type hanging vertical installation, and the furnace loading capacity is 180 parts per furnace.

[0091] Cleaning: Clean the oil stain on the surface of the parts after machining to make the carburizing process more uniform.

[0092] Pre-oxidation treatment: Feed the cleaned parts into a continuous pre-oxidation furnace, and the temperature of the pre-oxidation furnace is 380 °C.

[0093] Carburizing heat treatment: The pre-oxidized parts are sent into a vacuum carburizing furnace. The vacuum degree in the furnace is lower than 10 mbar, and nitrogen is filled to 1200 mbar for convective heating. The heating time is 50 min, and the heating temperature is 1000 °C. Then, it is evacuated to 10 mbar for vacuum heating. The heating time is 15 min, and the heating temperature is 970 °C. Then, pulse carburizing is carried out 7 times. The temperature of the first pulse carburizing is 970 °C, the acetylene pulse time is 3 min, the nitrogen inlet time is 3 min, and the acetylene flow rate is 6500 L / h. The temperature of the second pulse carburizing is 970 °C, the pulse time is 60 s, the nitrogen inlet time is 5 min, and the acetylene flow rate is 4300 L / h. The temperature of the third pulse carburizing is 970 °C, the pulse time is 60 s, the nitrogen inlet time is 7 min, and the acetylene flow rate is 4300 L / h. The temperature of the fourth pulse carburizing is 970 °C, the pulse time is 60 s, the nitrogen inlet time is 9 min, and the acetylene flow rate is 4300 L / h. The temperature of the fifth pulse carburizing is 970 °C, the pulse time is 60 s, the nitrogen inlet time is 11 min, and the acetylene flow rate is 4300 L / h. The temperature of the sixth pulse carburizing is 970 °C, the pulse time is 50 s, the nitrogen inlet time is 13 min, and the acetylene flow rate is 4600 L / h. The temperature of the seventh pulse carburizing is 970 °C, the pulse time is 30 s, the carburizing time is 150 s, and the acetylene flow rate is 5800 L / h. After carburizing, vacuum holding is carried out at 970 °C to make the C element diffuse inward fully and evenly. The holding time is 60 min. Finally, the temperature is lowered for holding. The holding temperature is 920 °C, and the holding time is 25 min. Lowering the temperature before final quenching is beneficial to reducing the deformation caused by volume shrinkage during the cooling process.

[0094] Cooling method: The cooling method adopts high-pressure helium gas quenching to cool to room temperature. The quenching pressure is 10 bar. Top and bottom circulating air cooling is adopted, 11 s at the top and 13 s at the bottom. The total quenching time is 8 min.

[0095] Tempering heat treatment: After quenching, sufficient tempering is carried out. The tempering temperature is 175 °C, and the tempering time is 150 min.

[0096] The original austenite grains obtained through the above process are as Figure 1 shown, and the core structure is as Figure 2 shown. It can be seen that the original austenite grains are fine, the average grain size reaches grade 8, and no obvious grain growth appears. The core structure is a typical martensite and bainite structure, with martensite structure being the main one. The detection data of various properties and structures are shown in Table 1. The detection data of heat treatment deformation are shown in Table 2. Among them, the tooth profile deviation diagram is as Figure 3 shown, and the tooth direction deviation is as Figure 4 shown. It can be seen that after heat treatment, various properties are good, the thermal deformation is small, and it meets the technical requirements.

[0097] Table 1

[0098] Grain size J9 hardness before carburizing Surface hardness after carburizing Core hardness after carburizing Band structure Grade 8 36HRC 734HV 375HV Grade 0

[0099] Table 2

[0100] Axial average deflection Straightening fracture rate Tooth profile shape deviation Tooth direction deviation Cumulative pitch deviation Radial runout 25μm 2‰ 2.5μm 4.2μm 16.2μm 13.9μm

[0101] Example 2

[0102] After alloying treatment in the refining process of the steel bar used for forging, the chemical composition of the molten steel is as follows: C: 0.20%, Si: 0.10%, Mn: 1.19%, Cr: 1.20%, Al: 0.037%, Mo: 0.04%, Ni: 0.20%, Nb: 0.02%, Cu: 0.21%, P: 0.014%, S: 0.025%, N: 0.006%, and the rest is Fe and inevitable impurities;

[0103] Forging process: Heat the gear steel bar in an intermediate frequency heating furnace to 1200°C, and the final forging temperature is detected to be 1037°C; then upset it on a closed single-point press to After upsetting, forge it into a billet on a vertical die forging machine, and then normalize the forged part. The normalization is divided into three zones. The normalization temperature in the first zone is 870°C, the second zone is 920°C, and the third zone is 930°C. The normalization time is 120 minutes. After normalization, air cooling is carried out simultaneously by upper and lower fans. Among them, the working frequency of the upper fan is 40Hz, the working frequency of the lower fan is 20Hz, and the air cooling duration is 180s.

[0104] Machining: After normalizing the forgings, the parts are processed through shot blasting, rough turning, finish turning, drilling, hobbing, rolling splines and other machining processes.

[0105] Hot installation method: The hot installation method used is three-point support type hanging vertical installation, and the furnace loading capacity is 108 parts per furnace.

[0106] Cleaning: Clean the oil stain on the surface of the parts after machining to make the carburizing process more uniform.

[0107] Pre-oxidation treatment: Send the cleaned parts into a continuous pre-oxidation furnace, and the temperature of the pre-oxidation furnace is 380°C.

[0108] Carburizing heat treatment: The pre-oxidized parts are sent into a vacuum carburizing furnace. The vacuum degree in the furnace is lower than 10 mbar, and nitrogen is filled to 1200 mbar for convective heating. The heating time is 50 min, and the heating temperature is 1000 °C. Then, it is evacuated to 10 mbar for vacuum heating. The heating time is 10 min, and the heating temperature is 1000 °C. Then, pulse carburizing is carried out 6 times. The temperature of the first pulse carburizing is 1000 °C, the pulse time is 3 min respectively, the nitrogen inlet time is 3 min, and the acetylene flow rate is 9000 L / h. The temperature of the second pulse carburizing is 1000 °C, the pulse time is 50 s, the nitrogen inlet time is 2 min, and the acetylene flow rate is 7000 L / h. The temperature of the third pulse carburizing is 1000 °C, the pulse time is 50 s, the nitrogen inlet time is 2 min, and the acetylene flow rate is 5900 L / h. The temperature of the fourth pulse carburizing is 1000 °C, the pulse time is 50 s, the nitrogen inlet time is 5 min, and the acetylene flow rate is 5300 L / h. The temperature of the fifth pulse carburizing is 1000 °C, the pulse time is 50 s, the nitrogen inlet time is 5 min, and the acetylene flow rate is 6200 L / h. The temperature of the sixth pulse carburizing is 1000 °C, the pulse time is 30 s, the nitrogen inlet time is 7 min, and the acetylene flow rate is 7700 L / h. After carburizing, vacuum heat preservation is carried out at 1000 °C to make the C element diffuse inward fully and evenly. The heat preservation time is 50 min. Finally, the temperature is lowered for heat preservation. The heat preservation temperature is 920 °C, and the heat preservation time is 25 min. Lowering the temperature before the final quenching is beneficial to reducing the deformation caused by volume shrinkage during the cooling process.

[0109] Cooling method: The cooling method adopts high-pressure helium gas quenching to cool to room temperature. The quenching pressure is 8 bar. Top and bottom circulating air cooling is adopted, 11 s at the top and 13 s at the bottom. The total quenching time is 8 min.

[0110] Tempering heat treatment: Full tempering is carried out after quenching. The tempering temperature is 180 °C, and the tempering time is 150 min.

[0111] The original austenite grains obtained through the above process are as Figure 5 shown, and the core structure is as Figure 6 shown. Compared with Example 1, in Example 2, the carburizing temperature is increased to 1000 °C. It can be seen that no obvious grain growth occurs in the original austenite grains. At the same time, in Example 2, the number of hot-loaded workpieces into the furnace, the number of carburizing acetylene pulses and the flow rate, etc. are adjusted, and the core structure is normal, mainly martensite structure. The detection data of various properties and structures are shown in Table 3. The detection data of various heat treatment deformations are shown in Table 4. Among them, the tooth profile deviation diagram is as Figure 7 shown, and the tooth direction deviation is as Figure 8As shown, it can be seen that after heat treatment, all properties are good, the thermal deformation is small, and the technical requirements are met. At the same time, due to the increase in carburizing temperature, compared with the 970°C vacuum carburizing process in Example 1, the carburizing time and holding time in Example 2 are significantly reduced, shortening the production cycle.

[0112] Table 3

[0113] Grain size J9 hardness before carburizing Surface hardness after carburizing Core hardness after carburizing Band structure Grade 8 37HRC 716HV 368HV Grade 0

[0114] Table 4

[0115] Axial average deflection Straightening fracture rate Tooth profile shape deviation Tooth direction deviation Cumulative pitch deviation Radial runout 32μm 3‰ 3.8μm 5.4μm 12.3μm 10.5μm

[0116] Comparative Example 1

[0117] The chemical composition of the molten steel after alloying treatment in the refining process of the steel bar used for forging is C: 0.19%, Si: 0.12%, Mn: 1.20%, Cr: 1.20%, Al: 0.035%, Mo: 0.04%, P: 0.018%, S: 0.025%, N: 0.007%, and the rest are Fe and inevitable impurities;

[0118] In this comparative example, the forging process, machining process, hot charging method, and subsequent cleaning, pre-oxidation, vacuum carburizing, and cooling and tempering processes used are the same as those in Example 2. The shaft gear part is the typical shaft gear steel composition used before improvement. After carburizing and quenching at 1000°C, the original austenite grains are as Figure 9 shown, and obvious non-uniformity in grain size appears, with mixed grain phenomenon. The specific tissue properties before and after heat treatment are shown in Table 5. The grain size does not meet the design requirement of ≥6 levels. Although the core hardness after carburizing meets the design requirement, according to experimental measurements, due to the existence of mixed grains, the hardness dispersion between different specimens is relatively large, close to 75HV, which is not conducive to the stability of subsequent properties. Therefore, the process optimization in Example 2 not only has practical significance for optimizing the production cycle, but also has a significant effect on the stability of high-temperature carburizing performance.

[0119] Table 5

[0120] Grain size J9 hardness before carburizing Surface hardness after carburizing Core hardness after carburizing Core hardness scatter after carburizing Grade 5 - 8 35HRC 705HV 332HV 75HV

[0121] In the description of this specification, the descriptions referring to terms such as "one embodiment / way", "some embodiments / ways", "example", "specific example" or "some examples" etc. mean that the specific features, structures, materials or characteristics described in connection with the embodiment / way or example are included in at least one embodiment / way or example of the present invention. In this specification, the schematic expressions of the above terms do not necessarily refer to the same embodiment / way or example. Moreover, the specific features, structures, materials or characteristics described can be combined in a suitable manner in any one or more embodiments / ways or examples. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments / ways or examples described in this specification and the features of different embodiments / ways or examples.

[0122] It should be noted that in the present invention, relational terms such as "first" and "second" etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or further includes elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "comprising one..." does not exclude the existence of additional identical elements in the process, method, article or device comprising the said element. In the present invention, the meaning of "a plurality" is at least two, such as two, three, etc., unless otherwise specifically defined.

[0123] The above are only specific embodiments of the present invention, enabling those skilled in the art to understand or implement the present invention. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown herein, but rather to the broadest scope consistent with the principles and novel features claimed herein.

Claims

1. A 20MnCrS5 carburizing steel, characterized in that: By weight percentage, the 20MnCrS5 carburizing steel includes: C: 0.17% - 0.20%, Si: ≤0.25%, Mn: 1.10% - 1.40%, Cr: 1.00% - 1.30%, Al: 0.02% - 0.05%, S: 0.02% - 0.04%, Ni: 0.1% - 0.4%, Nb: 0.015% - 0.03%, Mo: ≤0.08%, Cu: 0.1% - 0.25%, P: ≤0.015%, N: ≤0.008%, and the balance is Fe and inevitable impurities.

2. The 20MnCrS5 case-hardening steel according to claim 1, characterized in that: The 20MnCrS5 carburizing steel includes: C: 0.19% - 0.20%, Si: 0.05% - 0.15%, Mn: 1.10% - 1.30%, Cr: 1.10% - 1.25%, Al: 0.030% - 0.040%, Mo: 0.03% - 0.05%, Ni: 0.15% - 0.25%, Nb: 0.18% - 0.25%, Cu: 0.15% - 0.22%, P: 0.012% - 0.015%, S: 0.020% - 0.025%, N: 0.005% - 0.008%, and the balance is Fe and inevitable impurities.

3. A method for manufacturing carburized steel components, characterized in that, Including: Forging components using the 20MnCrS5 carburizing steel according to claim 1 or 2; Pre-oxidizing the forged components; Performing vacuum carburizing on the pre-oxidized components at a carburizing temperature of 950 - 1000 °C; Performing high-pressure gas quenching on the carburized components.

4. The method for manufacturing carburized steel components according to claim 3, characterized in that: The performing vacuum carburizing on the pre-oxidized components includes: (S1) Convection heating Feeding the pre-oxidized parts into a vacuum carburizing furnace with a vacuum degree lower than 10 mbar, filling with nitrogen for convection heating, with a heating time of 40 - 60 min and a heating temperature of 950 - 1000 °C; (S2) Vacuum heating Pumping to a vacuum of ≤10 mbar for vacuum heating, with a heating time of 15 - 30 min and a heating temperature of 950 - 1000 °C; (S3) 7 - 9 times of pulse carburizing For the 1st - 3rd times: carburizing temperature 950 - 1000 °C, pulse time 1 - 3 min, carburizing time 5 - 10 min, acetylene flow rate 4000 - 10000 L / h; For the 4th - 6th times: carburizing temperature 950 - 1000 °C, pulse time 30 - 60 s, carburizing time 8 - 15 min, acetylene flow rate 2000 - 8000 L / h; For the 7th - 9th times: carburizing temperature 950 - 1000 °C, pulse time 30 - 60 s, carburizing time 3 - 5 min, acetylene flow rate 3000 - 9000 L / h; (S4) Vacuum holding After carburizing, performing vacuum holding at 950 - 1000 °C for 1 - 2 h; then performing vacuum holding at 820 - 950 °C for 30 - 60 min.

5. The method for manufacturing carburized steel components according to claim 3, characterized in that: The forging components using the 20MnCrS5 carburizing steel according to claim 1 or 2 includes: Heat the 20MnCrS5 carburizing steel to 1200 ± 30 °C to uniformly dissolve its alloying elements in the steel and fully austenitize it, and the final forging temperature is 1050 ± 30 °C.

6. The method for manufacturing carburized steel components according to claim 3, characterized in that: Before pre-oxidation, it also includes: normalizing the forged parts.

7. The method for manufacturing carburized steel components according to claim 6, characterized in that: Before pre-oxidation, it also includes: after normalizing, use upper and lower fans to air-cool the parts simultaneously.

8. The method for manufacturing carburizing steel components according to claim 3, characterized in that: When performing high-pressure gas quenching on the carburized parts, the high-pressure gas is helium, and the quenching pressure is 7 - 20 bar.

9. The method for manufacturing carburized steel components according to claim 3, characterized in that: It also includes: Tempering heat treatment, the tempering temperature is 200 - 300 °C, and the tempering time is 120 - 180 min.

10. The method for manufacturing carburized steel components according to claim 3, characterized in that: The temperature of pre-oxidation is 380 °C ± 30 °C.

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