A process method for regulating the core hardness of low-carbon alloy steel 20MnCr5 after carburizing and quenching

Abstract

The present application belongs to the technical field of improving the performance of metal parts, and particularly relates to a process for regulating the core hardness of low-carbon alloy steel 20MnCr5 after carburizing and quenching. For the problem of core hardness exceeding the upper limit or lower limit, the low-carbon alloy steel 20MnCr5 parts to be regulated are put into a heating furnace for complete austenitization; after the austenitization is completed, the temperature is lowered to a proper quenching temperature and kept for a certain time, and the core of the parts precipitates proeutectoid ferrite structure with the holding time, so as to solve the problem of core hardness exceeding the upper limit or lower limit. The present application regulates the proeutectoid ferrite of the core of the parts and the quenching cooling speed without affecting the mechanical properties of the parts, so as to meet the design requirements of the core hardness of the parts, and thus solve the problem of core hardness exceeding the specification requirements after quenching due to the fluctuation of the composition of the raw materials.

Description

Technical Field

[0001] This invention belongs to the technical field of improving the matrix properties of metal parts, and specifically relates to a process improvement method for controlling the core hardness of low-carbon alloy steel 20MnCr5 after quenching by adjusting the content of proeutectoid ferrite in the core. Background Technology

[0002] Low-carbon alloy steel 20MnCr5 is widely used in transmission components for automobiles, ships, aerospace, and other industries. The principle of carburizing and quenching process is to heat the part to a certain temperature and hold it for a certain time to make it fully austenitic, and then perform carburizing and diffusion; then cool it to the quenching temperature and homogenize it, and finally quickly transfer it into the quenching medium for cooling. The surface of the part obtains acicular martensite to obtain high hardness, and the core obtains low-carbon martensite structure to obtain high comprehensive mechanical properties.

[0003] The core hardness of low-carbon alloy steel 20MnCr5 after quenching is generally 30-45 HRC. With the development of the manufacturing industry, some customers are demanding a narrower range of core hardness (35-41 HRC). Existing technologies show the influence of the quenching process on core hardness. For example, the study on "the influence of the hardenability bandwidth of 20CrMoH steel on core hardness" points out that excessively high core hardness in gears significantly impacts their operating conditions. The hardenability of steel often determines the core hardness of a part. Lowering the quenching temperature reduces core hardness, while increasing the quenching temperature increases it. However, this study does not investigate the influence of the proeutectoid ferrite content on core hardness.

[0004] For 20MnCr5, the technical problem to be solved by this invention is how to precisely control the core hardness of 20MnCr5 within a narrow range of 35 to 41 HRC under a specific proeutectoid ferrite content in the core through the quenching process. Summary of the Invention

[0005] Based on the problems pointed out in the background section, this invention provides a process improvement method for adjusting the core hardness of 20MnCr5 low-carbon alloy steel after quenching. This invention adjusts the "core proeutectoid ferrite content + quenching oil cooling capacity" after carburizing and carburizing to solve the problem that the core hardness of 20MnCr5 low-carbon alloy steel after carburizing and quenching exceeds the requirements of the drawing specifications. It accurately and effectively controls the core hardness within the range of 35 to 41 HRC, and further controls it within a narrower range of 38 to 41 HRC.

[0006] The technical solution adopted by this invention based on the above objectives is: a process improvement method for adjusting the core hardness of low-carbon alloy steel 20MnCr5 after quenching, comprising the following steps:

[0007] (1) After cleaning the 20MnCr5 low carbon alloy steel parts, put them into the UBT364836G pre-oxidation furnace and close the furnace door.

[0008] (2) Select the pre-oxidation program, with a pre-oxidation temperature of 350-450℃ and a pre-oxidation time of 45-60 minutes. Start the circulating fan and start the program.

[0009] (3) After pre-oxidation, the parts are transferred into the front chamber of the UBQ364836G multi-purpose furnace. The air exchange time in the front chamber is 15–25 minutes, and the rear chamber is kept at a temperature of 780–800℃. After air exchange, the parts are automatically transferred to the rear chamber for heating, with a nitrogen flow rate of 3.2 ± 0.2 m³ / h. 3 The methanol flow rate is 3.0 ± 0.2 L / h, and the heating rate is set to 4℃ / min. The homogenization time is 20–30 minutes to ensure complete austenitization of the parts. The strong carburizing and diffusion temperatures are 880℃–920℃, the strong carburizing potential is 1.0C%–1.2C%, and the diffusion carburizing potential is 0.7C%–0.8C.

[0010] (4) After carburizing, the billet is cooled to 20MnCr5 by conventional quenching process at a rate of 1℃ / min. If the core hardness of the billet exceeds 35-41HRC after conventional quenching, the subsequent quenching process of the billet is adjusted.

[0011] If the core hardness is below 35 HRC, the part will be quenched at 845℃~865℃ after carburizing, with a quenching holding time of 30min~60min and a carbon potential of 0.7C%~0.8C% during quenching holding. After quenching holding, the part will be quickly transferred into quenching oil (model KR228) for quenching. The maximum cooling rate of this quenching oil is 95~103℃ / s. The quenching oil temperature is 60℃~120℃, the residence time in the quenching oil is 20~30 minutes, and the stirring speed of the oil bath is 200~600RPM.

[0012] If the core hardness is higher than 41 HRC, after carburizing, quench at 760℃~730℃ for 0~25 min, with a carbon potential of 0.6C%~0.75C%. After quenching, quickly transfer the parts into quenching oil (model KR488) for further quenching. The maximum cooling rate of this quenching oil is 75~85℃ / s; the quenching oil temperature is 60℃~120℃; the residence time in the quenching oil is 20~30 minutes; and the stirring speed of the oil bath is 200~600 RPM.

[0013] (5) After quenching, the parts are drained of oil for 15-25 minutes before being removed from the multi-purpose furnace. After cleaning, they are tempered at 170℃-190℃ for 120-180 minutes, with a nitrogen flow rate of 5-8 m³ / h during tempering. 3 / h.

[0014] (6) After tempering, the parts are removed from the tempering furnace and air-cooled to room temperature.

[0015] The composition requirements for 20MnCr5 are as follows (by weight percentage): C: 0.17-0.22, Si≤0.4, Mn: 1.10-1.40, P≤0.025; S≤0.035, Cr: 1.0~1.3, Mo≤0.25, Ni≤0.250; the remainder is iron and unavoidable impurities.

[0016] The beneficial effects of this invention are: by adjusting the "core proeutectoid ferrite content + quenching oil cooling capacity," that is, after austenitization and carburizing of low-carbon alloy steel 20MnCr5, the quenching holding temperature and quenching cooling rate are adjusted to achieve controllable core hardness after quenching. This method not only effectively controls the range of core hardness values ​​after carburizing and quenching of low-carbon alloy steel 20MnCr5, but also ensures that the mechanical properties are not significantly affected, thus ensuring good service performance of the parts. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the experimental sample.

[0018] Figure 2 These are core microstructure diagrams of low-carbon alloy steel 20MnCr5 with different quenching and holding temperatures and times, and different thicknesses.

[0019] Figure 3 This is a graph showing the core hardness of low-carbon alloy steel 20MnCr5 under different quenching and holding temperatures and quenching and holding times.

[0020] Figure 4 This is a schematic diagram of the fabrication of a micro-stretched sample.

[0021] Figure 5 This is a graph showing the relationship between strain and stress in a tensile specimen of 20MnCr5 low-carbon alloy steel.

[0022] Figure 6 The images show the metallographic structure of 20MnCr5 low-carbon alloy steel after quenching and tempering in quenching oils with different cooling capacities. Detailed Implementation

[0023] The present invention will now be further described with reference to specific embodiments. The following embodiments are intended to illustrate the present invention and not to further limit the present invention.

[0024] Conventional quenching process for low-carbon alloy steel 20MnCr5:

[0025] (1) After cleaning, the low-carbon alloy steel 20MnCr5 is placed into the pre-oxidation furnace of model UBT364836G and the furnace door is closed.

[0026] (2) Select the pre-oxidation program, pre-oxidation temperature 420℃, and pre-oxidation time 60 minutes when the temperature reaches the set point. Start the circulating fan and start the program.

[0027] (3) After pre-oxidation, the parts (thickness 3-5mm) are transferred into the front chamber of the UBQ364836G multi-purpose furnace. The air exchange time in the front chamber is 20 minutes, and the rear chamber is kept at 800℃. After air exchange, the parts are automatically transferred to the rear chamber for heating, with a nitrogen flow rate of 3.2m³ / h. 3 The methanol flow rate was 3.0 L / h, and the heating rate was set at 4℃ / min. The homogenization time was 20 minutes. The strong percolation and diffusion temperatures were 910℃, the strong percolation carbon potential was 1.0 C%, and the diffusion carbon potential was 0.8 C%.

[0028] (4) After carburizing, a quenching process is carried out. The quenching temperature is 780℃~840℃, the quenching time is 15~60min, and the carbon potential is 0.7C%~0.8C during quenching.

[0029] (5) After quenching and heat preservation, the parts are automatically transferred into KR488 quenching oil (maximum quenching cooling rate 80-85℃ / s) in less than 30 seconds; the quenching oil temperature is 110℃, the residence time in the quenching oil is 20 minutes, the stirring speed of the oil tank is 400RPM, and the quenching oil flows from the bottom of the oil tank upwards.

[0030] (6) After quenching, the parts are drained of oil for 15 minutes before being removed from the multi-purpose furnace. After cleaning, they are tempered at 180℃ for 120 minutes with a nitrogen flow rate of 5 m³ / h. 3 / h.

[0031] (7) After tempering, the parts are removed from the tempering furnace and air-cooled to room temperature.

[0032] Example 1

[0033] (1) After cleaning Trial 01 low carbon alloy steel 20MnCr5, put it into the UBT364836G pre-oxidation furnace and close the furnace door.

[0034] (2) Select the pre-oxidation program, pre-oxidation temperature 420℃, and pre-oxidation time 60 minutes when the temperature reaches the set point. Start the circulating fan and start the program.

[0035] (3) After pre-oxidation, the parts are transferred into the front chamber of the UBQ364836G multi-purpose furnace. The air exchange time in the front chamber is 20 minutes, and the rear chamber is kept at a temperature of 800℃. After air exchange, the parts are automatically transferred to the rear chamber for heating, with a nitrogen flow rate of 3.2 m³ / s. 3The methanol flow rate was 3.0 L / h, and the heating rate was set at 4℃ / min. The homogenization time was 20 minutes. The strong percolation and diffusion temperatures were 910℃, with a strong percolation carbon potential of 1.0C% and a diffusion carbon potential of 0.8C%.

[0036] (4) After carburizing, the above-mentioned conventional quenching process was adopted (quenching temperature 840℃, 15min, quenching oil KR488). The core hardness of the steel billet after quenching was 42.5HRC. The subsequent quenching process of the steel billet was adjusted to a quenching temperature of 750℃ and a quenching holding time of 0 minutes (0min means that quenching is performed immediately after the set quenching temperature is reached, without holding). The carbon potential during quenching holding was 0.7C.

[0037] (5) After the quenching and heat preservation is completed, the parts are automatically transferred into the KR488 quenching oil. The transfer time is <30 seconds. The quenching oil temperature is 110℃, the residence time in the quenching oil is 20 minutes, the stirring speed of the oil tank is 400RPM, and the quenching oil flows from the bottom of the oil tank upwards.

[0038] (6) After quenching, the parts are drained of oil for 15 minutes before being removed from the multi-purpose furnace. After cleaning, they are tempered at 180℃ for 120 minutes with a nitrogen flow rate of 5 m³ / h. 3 / h.

[0039] (7) After tempering, the parts are removed from the tempering furnace and air-cooled to room temperature.

[0040] Example 1 yielded a core hardness of 40.8 HRC for 20MnCr5.

[0041] Example 2

[0042] (1) After cleaning Trial 02 low carbon alloy steel 20MnCr5, put it into the UBT364836G pre-oxidation furnace and close the furnace door.

[0043] (2) Select the pre-oxidation program, pre-oxidation temperature 420℃, and pre-oxidation time 60 minutes when the temperature reaches the set point. Start the circulating fan and start the program.

[0044] (3) After pre-oxidation, the parts are transferred into the front chamber of the UBQ364836G multi-purpose furnace. The air exchange time in the front chamber is 20 minutes, and the rear chamber is kept at a temperature of 800℃. After air exchange, the parts are automatically transferred to the rear chamber for heating, with a nitrogen flow rate of 3.2 m³ / s. 3 The methanol flow rate was 3.0 L / h, and the heating rate was set at 4℃ / min. The homogenization time was 20 minutes. The strong percolation and diffusion temperatures were 910℃, the strong percolation carbon potential was 1.0 C%, and the diffusion carbon potential was 0.8 C%.

[0045] (4) After carburizing, conventional quenching process is adopted (quenching temperature 840℃, 15min, quenching oil KR488). The core hardness of 20MnCr5 billet after quenching is 42.5HRC. The subsequent quenching process of billet is controlled, specifically: quenching temperature is 750℃, quenching holding time is 5 minutes, and carbon potential is 0.7C during quenching holding time.

[0046] (5) After the quenching and heat preservation is completed, the parts are automatically transferred into the KR488 quenching oil. The transfer time is <30 seconds. The quenching oil temperature is 110℃, the residence time in the quenching oil is 20 minutes, the stirring speed of the oil tank is 400RPM, and the quenching oil flows from the bottom of the oil tank upwards.

[0047] (6) After quenching, the parts are drained of oil for 15 minutes before being removed from the multi-purpose furnace. After cleaning, they are tempered at 180℃ for 120 minutes with a nitrogen flow rate of 5 m³ / h. 3 / h.

[0048] (7) After tempering, the parts are removed from the tempering furnace and air-cooled to room temperature.

[0049] Example 2 yielded a core hardness of 39.9 HRC for 20MnCr5.

[0050] Example 3

[0051] (1) After cleaning Trial 03 low carbon alloy steel 20MnCr5, put it into the UBT364836G pre-oxidation furnace and close the furnace door.

[0052] (2) Select the pre-oxidation program, pre-oxidation temperature 420℃, and pre-oxidation time 60 minutes when the temperature reaches the set point. Start the circulating fan and start the program.

[0053] (3) After pre-oxidation, the parts are transferred into the front chamber of the UBQ364836G multi-purpose furnace. The air exchange time in the front chamber is 20 minutes, and the rear chamber is kept at a temperature of 800℃. After air exchange, the parts are automatically transferred to the rear chamber for heating, with a nitrogen flow rate of 3.2 m³ / s. 3 The methanol flow rate was 3.0 L / h, and the heating rate was set at 4℃ / min. The homogenization time was 20 minutes. The strong percolation and diffusion temperatures were 910℃, the strong percolation carbon potential was 1.0 C%, and the diffusion carbon potential was 0.8 C%.

[0054] (4) After carburizing, conventional quenching process is adopted (quenching temperature 840℃, 15min, quenching oil KR488). The core hardness of 20MnCr5 billet after quenching is 42.5HRC. The subsequent quenching process of billet is controlled, specifically the quenching temperature is 750℃, the quenching holding time is 15 minutes, and the carbon potential is 0.7C during the quenching holding time.

[0055] (5) After the quenching and heat preservation is completed, the parts are automatically transferred into the KR488 quenching oil. The transfer time is <30 seconds. The quenching oil temperature is 110℃, the residence time in the quenching oil is 20 minutes, the stirring speed of the oil tank is 400RPM, and the quenching oil flows from the bottom of the oil tank upwards.

[0056] (6) After quenching, the parts are drained of oil for 15 minutes before being removed from the multi-purpose furnace. After cleaning, they are tempered at 180℃ for 120 minutes with a nitrogen flow rate of 5 m³ / h. 3 / h.

[0057] (7) After tempering, the parts are removed from the tempering furnace and air-cooled to room temperature.

[0058] Example 3 yielded a core hardness of 39.2 HRC for 20MnCr5.

[0059] Example 4

[0060] Compared with Example 3, the difference is that the quenching process of the billet after step (4) is adjusted, specifically the quenching temperature is 750°C, the quenching holding time is 25 minutes, the carbon potential is 0.7C during the quenching holding time, and other operations are the same as in Example 3.

[0061] Example 4 yielded a core hardness of 38.6 HRC for 20MnCr5.

[0062] Example 5

[0063] Compared with Example 3, the difference is that the quenching process of the billet after step (4) is adjusted, specifically the quenching temperature is 720°C, the quenching holding time is 15 minutes, the carbon potential is 0.7C during the quenching holding time, and the other operations are the same as in Example 3.

[0064] Example 5 yielded a core hardness of 33.5 HRC for 20MnCr5.

[0065] Example 6

[0066] Compared with Example 3, the difference is that the quenching process of the billet after step (4) is adjusted, specifically the quenching temperature is 800℃, the quenching holding time is 15 minutes, the carbon potential is 0.7C during the quenching holding time, and other operations are the same as in Example 3.

[0067] Example 6 yielded a core hardness of 42.2 HRC for 20MnCr5.

[0068] Example 7

[0069] (1) After cleaning the low-carbon alloy steel 20MnCr5, put it into the UBT364836G pre-oxidation furnace and close the furnace door.

[0070] (2) Select the pre-oxidation program, pre-oxidation temperature 420℃, and pre-oxidation time 60 minutes when the temperature reaches the set point. Start the circulating fan and start the program.

[0071] (3) After pre-oxidation, the parts (with an effective thickness of 10-16mm) are transferred into the front chamber of the UBQ364836G multi-purpose furnace. The air exchange time in the front chamber is 20 minutes, and the rear chamber is kept at 800℃. After air exchange, the parts are automatically transferred to the rear chamber for heating, with a nitrogen flow rate of 3.2m³ / h. 3 The methanol flow rate was 3.0 L / h, and the heating rate was set at 4℃ / min. The homogenization time was 20 minutes. The strong percolation and diffusion temperatures were 910℃, the strong percolation carbon potential was 1.0 C%, and the diffusion carbon potential was 0.8 C%.

[0072] (4) After carburizing, conventional quenching process is adopted (quenching temperature 840℃, quenching oil KR488). The core hardness of 20MnCr5 billet after quenching is 26.2HRC. The subsequent quenching process of billet is controlled, specifically: quenching temperature is 850℃, quenching holding time is 30 minutes, and carbon potential is 0.8C during quenching holding time.

[0073] (5) After the quenching and heat preservation is completed, the parts are automatically transferred into the KR228 quenching oil (maximum cooling rate is 95~103℃ / s) with a transfer time of <30 seconds; the quenching oil temperature is 80℃, the residence time in the quenching oil is 20 minutes, the stirring speed of the oil tank is 400RPM, and the quenching oil flows from the bottom of the oil tank upwards.

[0074] (6) After quenching, the parts are drained of oil for 15 minutes before being removed from the multi-purpose furnace. After cleaning, they are tempered at 180℃ for 120 minutes with a nitrogen flow rate of 5 m³ / h. 3 / h.

[0075] (7) After tempering, the parts are removed from the tempering furnace and air-cooled to room temperature.

[0076] Example 7 yielded a core hardness of 38.8 HRC for 20MnCr5.

[0077] Example 8

[0078] Compared with Example 7, the difference is that the quenching conditions in step (4) are adjusted to: temperature of 850°C, quenching and holding for 60 minutes, and other operations are the same as in Example 7.

[0079] Example 8 yielded a core hardness of 38.6 HRC for 20MnCr5.

[0080] Example 9

[0081] Compared with Example 7, the difference is that KR228 in step (5) is replaced with KR488, and the other operations are the same as in Example 7.

[0082] Example 9 yielded a core hardness of 26.5 HRC for 20MnCr5.

[0083] Figure 1 This is a schematic diagram of the experimental sample. The sample has a "C"-shaped opening with a thickness of 3.3 mm at the open end, and the thickness gradually increases towards the center of the torso, reaching a maximum of 15.89 mm at the center of the torso.

[0084] from Figure 2 It can be seen that under normal quenching temperature and holding time (840℃ for 15 minutes) for low-carbon alloy steel 20MnCr5, the core microstructure of both the thinnest part (head) and the thickest part (body) consists of low-carbon martensite and bainite. Due to the greater thickness of the body, the bainite content in the core is higher than that in the head. When the temperature is reduced to 750℃, significant ferrite precipitation occurs in the core regions of both the head and body, and the amount of precipitation increases with the extension of holding time (0 minutes, 5 minutes, 15 minutes).

[0085] from Figure 3 (a) It can be seen that for the low-carbon alloy steel 20MnCr5, with the same holding time (15 minutes), as the quenching holding temperature decreased (from 840℃ to 750℃), the core hardness of the sample head decreased from 42.5 HRC to 39.2 HRC. At the same quenching holding temperature (750℃), as the quenching holding time increased (from 0 minutes to 15 minutes), the core hardness of the sample head decreased from 40.8 HRC to 39.2 HRC. The core hardness of the sample body also showed a decreasing trend with decreasing quenching holding temperature and increasing quenching holding time.

[0086] Figure 4 This diagram illustrates the fabrication of miniature tensile specimens. The specimens are taken from the torso of a "C"-shaped test piece. Three miniature tensile specimens can be fabricated from each "C"-shaped test piece.

[0087] from Figure 5 It can be seen that the yield strength and tensile strength of the 20MnCr5 low-carbon alloy steel sample, which was fully austenitized and quenched at 840℃, were not significantly different from those of the quenched sample with a quenching temperature of 750℃ and a core containing an appropriate amount of ferrite; the strain hardening rate was also not significantly different.

[0088] Combination Figure 2 , Figure 3It is known that the ferrite content in the core is the fundamental reason for reducing the core hardness of low-carbon alloy steel. Although the increase in bainite content in the core also reduces core hardness with increasing sample thickness, for specific functional areas of the part, a feasible method to reduce core hardness is to adjust the temperature (760℃~730℃) and time (10min~15min) to allow an appropriate amount of ferrite (grade 3~5) to precipitate in the core. Figure 5 Mechanical experiments show that an appropriate amount of non-networked core ferrite has no significant effect on its mechanical properties.

[0089] Figure 6 These are metallographic images of 20MnCr5 low-carbon alloy steel after quenching and tempering in quenching oils with different cooling capacities. After changing the furnace number of the steel mill used for the raw materials, under the same heat treatment process, the core microstructure of the part is martensite and a large amount of bainite, as shown below. Figure 6 As shown in (a), under normal quenching process, the core hardness is 26.2 HRC, lower than the drawing requirement (30-45 HRC). After changing the quenching oil from isothermal quenching oil KR488 to rapid quenching oil KR228, the quenching temperature increased from 840℃ to 850℃, and the core microstructure became martensitic. Figure 6 As shown in (b), the core hardness is 38.8 HRC. Therefore, when the core structure contains non-martensite such as bainite and troostite after quenching, the core hardness can be increased by improving the cooling capacity to obtain a fully martensitic structure.

[0090] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A process for controlling the core hardness of low-carbon alloy steel 20MnCr5 after carburizing and quenching, characterized in that, After pre-oxidation and carburizing, the low-carbon alloy steel 20MnCr5 parts are quenched; the core hardness is controlled at 35~41HRC through the quenching process. The specific preparation steps are as follows: (1) After cleaning, the low-carbon alloy steel 20MnCr5 parts are placed in a pre-oxidation furnace to remove the oil film and oxide film on the surface of the parts; the pre-oxidation temperature in step (1) is 350~450℃ and the pre-oxidation time is 45~60 minutes; (2) After the pre-oxidation is completed, the low carbon alloy steel 20MnCr5 parts are transferred into a multi-purpose furnace for heating and carburizing; the carburizing temperature is 880℃~920℃, the strong carburizing potential is 1.0C%~1.2C, the diffusion carburizing potential is 0.7C%~0.8C, and the parts are completely austenitized. (3) When the core hardness of the part is lower than 35HRC after conventional quenching, the temperature will be lowered to 845℃~865℃ for quenching after carburizing. The quenching holding time is 30min~60min, and the carbon potential is 0.7C%~0.8C% during quenching holding. After quenching and heat preservation, the parts are quickly transferred into quenching oil for quenching. The transfer time into the quenching oil is ≤30s. The maximum cooling rate of the quenching oil is 95~103℃ / s. The temperature of the quenching oil is 60℃~120℃. The residence time in the quenching oil is 20~30 minutes. The stirring speed of the oil tank is 200~600RPM. When the core hardness is higher than 41 HRC after conventional quenching, the part is quenched at 760℃~730℃ after carburizing, with a quenching holding time of 0~25 min and a carbon potential of 0.6C%~0.75C during quenching holding. After quenching holding, the part is quickly transferred into quenching oil for quenching. The maximum cooling rate of the quenching oil is 75~85℃ / s; the quenching oil temperature is 60℃~120℃; the residence time in the quenching oil is 20~30 minutes; and the stirring speed of the oil bath is 200~600 RPM. (4) After quenching, the parts are drained of oil and removed from the multi-purpose furnace. After cleaning, they are tempered at a temperature of 170℃~190℃ and a holding time of 120~180 minutes. Finally, they are air-cooled to room temperature after being removed from the tempering furnace to obtain low-carbon alloy steel with a core hardness of 35~41HRC.

2. The process method for controlling the core hardness of low-carbon alloy steel 20MnCr5 after carburizing and quenching according to claim 1, characterized in that: The carburizing conditions in step (2) are: nitrogen flow rate 3.2 ± 0.2 m³ / h. 3 / h, methanol flow rate 3.0±0.2L / h, heating rate 4℃ / min.

3. The process method for controlling the core hardness of low-carbon alloy steel 20MnCr5 after carburizing and quenching according to claim 1, characterized in that: The conventional quenching conditions for 20MnCr5 are: 780℃~840℃; quenching holding time 15~60min; and carbon potential of 0.7C%~0.8C during quenching holding.

4. The process method for controlling the core hardness of low-carbon alloy steel 20MnCr5 after carburizing and quenching according to claim 1, characterized in that: Step (4) Oil draining time: 15-25 min; nitrogen flow rate during tempering: 5-8 m³ / min. 3 / h.

5. The process method for controlling the core hardness of low-carbon alloy steel 20MnCr5 after carburizing and quenching according to claim 1, characterized in that: The core hardness of low-carbon alloy steel 20MnCr5 is 38~41HRC.

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