STEEL FOR HIGH-TEMPERATURE CARBURIZED GEAR SHAFT AND MANUFACTURING METHOD FOR STEEL

Abstract

Disclosed are a steel for a high-temperature carburized gear shaft and a manufacturing method for the steel. The steel for the high-temperature carburized gear shaft comprises the following chemical components in percentage by mass: 0.17-0.22% of C, 0.05-0.35% of Si, 0.80-1.40% of Mn, 0.010-0.035% of S, 0.80-1.40% of Cr, 0.020-0.046% of Al, 0.006-0.020% of N, 0.002-0.030% of Nb, V0.02%, and Ti0.01%. Also disclosed is a manufacturing method for the steel for the high-temperature carburized gear shaft, comprising the steps of: smelting and casting; heating; forging or rolling; and finishing. By reasonably controlling chemical element compositions of the steel, the steel for the gear shaft in the present invention can maintain proper austenite grain size and stability at high temperature, maintains 5-8 grades of the austenite grain size before and after the high-temperature vacuum carburizing at 940-1050 C., can be effectively applied to high-end parts such as a gearbox for a vehicle or a speed reducer and a differential of a new energy vehicle, and has good application prospects and value.

Claims

1. A steel for a high-temperature carburized gear shaft, comprising the following chemical components in percentage by mass: 0.17-0.22% C, 0.05-0.35% Si, 0.80-1.40% Mn, 0.010-0.035% S, 0.80-1.40% Cr, 0.020-0.046% Al, 0.006-0.020% N, 0.002-0.030% Nb, 0.02% or less V, and 0.01% or less Ti.

2. The steel for a high-temperature carburized gear shaft according to claim 1, wherein the balance is Fe and inevitable impurities.

3. The steel for a high-temperature carburized gear shaft according to claim 1, wherein the steel further comprises at least one of elements Ni, Mo and Cu, in percentage by mass, 0.25% or less Ni, 0.10% or less Mo, and 0.20% or less Cu.

4. The steel for a high-temperature carburized gear shaft according to claim 1, wherein the steel further comprises, in percentage by mass, 0.015% or less P, 0.0020% or less O, 0.0002% or less H, 0.0010% or less B, and 0.003% or less Ca.

5. The steel for a high-temperature carburized gear shaft according to claim 1, wherein the contents of the elements Nb, V, Al, N and C in the steel for a high-temperature carburized gear shaft satisfy the following formula: a microalloying element coefficient r.sub.M/X=(20*[Nb]/93[V]/51+[Al]/27)/([N]/14+[C]/120), and the microalloying element coefficient r.sub.M/X ranges from 0.5 to 3.0, wherein each chemical element in the formula is substituted with a numerical value before a percentage sign of the percentage content by mass of the corresponding chemical element.

6. The steel for a high-temperature carburized gear shaft according to claim 1, wherein the steel for a high-temperature carburized gear shaft has a hardenability of 30-43 HRC at a representative position J9 mm, and maintains 5-8 grades of an austenite grain size before and after high-temperature vacuum carburizing.

7. A manufacturing method for the steel for a high-temperature carburized gear shaft according to claim 1, comprising the steps of: smelting and casting; heating; forging or rolling; and finishing.

8. The manufacturing method for the steel for a high-temperature carburized gear shaft according to claim 7, wherein in the heating step, a heating temperature of a preheating section is not higher than 700 C., a temperature of a first heating section is not higher than 980 C., a temperature of a second heating section is 950-1200 C., and a temperature of a soaking section is 1050-1250 C.

9. The manufacturing method for the steel for a high-temperature carburized gear shaft according to claim 7, wherein in the forging or rolling step, a final forging temperature or a final rolling temperature is 900 C. or more.

10. The manufacturing method for the steel for a high-temperature carburized gear shaft according to claim 7, wherein the finishing step comprises at least one of scalping, annealing and tempering.

11. The steel for a high-temperature carburized gear shaft according to claim 2, wherein the steel further comprises, in percentage by mass, 0.015% or less P, 0.0020% or less 0, 0.0002% or less H, 0.0010% or less B, and 0.003% or less Ca.

12. The steel for a high-temperature carburized gear shaft according to claim 3, wherein the steel further comprises, in percentage by mass, 0.015% or less P, 0.0020% or less 0, 0.0002% or less H, 0.0010% or less B, and 0.003% or less Ca.

13. The steel for a high-temperature carburized gear shaft according to claim 2, wherein the steel for a high-temperature carburized gear shaft has a hardenability of 30-43 HRC at a representative position J9 mm, and maintains 5-8 grades of an austenite grain size before and after high-temperature vacuum carburizing.

14. The steel for a high-temperature carburized gear shaft according to claim 3, wherein the steel for a high-temperature carburized gear shaft has a hardenability of 30-43 HRC at a representative position J9 mm, and maintains 5-8 grades of an austenite grain size before and after high-temperature vacuum carburizing.

Description

DETAILED DESCRIPTION

[0060] Embodiments of the present invention will be described below with specific embodiments, and other advantages and effects of the present invention will be readily apparent to those skilled in the art from the disclosure of this specification. Although the present invention will be described in connection with preferred embodiments, it is not intended that the features of the present invention are only limited to this embodiment. On the contrary, the description of the invention in connection with the embodiments is intended to cover other alternatives or modifications that may be derived based on the claims of the present invention. The following description contains numerous specific details in order to provide a thorough understanding of the present invention. The present invention may also be practiced without these details. In addition, some specific details will be omitted from the description in order to avoid confusing or obscuring the focus of the present invention. It should be noted that the examples of the present invention and the features in the examples can be combined with each other without conflict.

Examples 1-8 and Comparative Examples 1-4

[0061] Steels for a high-temperature carburized gear shaft in Examples 1-8 are all manufactured by using the following steps: [0062] (1) smelting and casting are carried out according to the chemical composition shown in the following Table 1: wherein the smelting can be carried out by using a 50 kg vacuum induction furnace, a 150 kg vacuum induction furnace, or a 500 kg vacuum induction furnace, or the smelting also can be carried out by using electric furnace smelting+external refining+vacuum degassing, or the smelting can be carried out by using converter smelting+external refining+vacuum degassing. And the casting can be carried out by die casting or continuous casting. [0063] (2) Heating: a steel slab is first heated to be not higher than 700 C. in a preheating section, and then continues to be heated in a first heating section, wherein a set heating temperature is not higher than 980 C. At this stage, the temperature of the steel slab is 600-980 C.; after heat preservation, continue to heat to 950-1200 C. in a second heating section, and enter a soaking section after heat preservation. The temperature of the soaking section is 1050-1250 C., and the temperature of a core of the steel slab and the temperature of the surface of the steel slab are kept the same by heat preservation. [0064] (3) Forging or rolling: the final forging or final rolling temperature is controlled to be 900 C. or more. [0065] (4) Finishing: the finishing includes scalping or annealing or normalizing.

[0066] Specific processes for the steels for the high-temperature carburized gear shaft in Examples 1-8 and steels in Comparative examples 1-4 are as follows:

[0067] Example 1: Smelting is carried out on a 50 kg vacuum induction furnace according to the chemical composition shown in Table 1 below. Molten steel is cast into steel ingots, and heated and forged into billets, and the steel ingots are first heated to 700 C. in a preheating section, then continue to heat to 900 C. in a first heating section. And after heat preservation, continue to heat to 1000 C. in a second heating section. After heat preservation, enter a soaking section having a temperature of 1100 C. Then, after heat preservation, proceed with subsequent forging to finally create bars with 60 mm, wherein the final forging temperature is controlled to be 910 C., and after forging, normalize at 920 C. for 100 minutes.

[0068] Example 2: Smelting is carried out on a 150 kg vacuum induction furnace according to the chemical composition shown in Table 1 below. Molten steel is cast into steel ingots, heated and forged into billets, and the steel ingots are first heated to 650 C. in a preheating section, then continue to heat to 950 C. in a first heating section. And after heat preservation, continue to heat to 1100 C. in a second heating section. Then, after heat preservation, enter a soaking section having a temperature of 1200 C., and after heat preservation, proceed with subsequent forging to finally create bars with 75 mm, wherein the final forging temperature is controlled to be 1000 C., and after forging, perform turning scalping.

[0069] Example 3: perform electric furnace smelting according to the chemical composition shown in Table 1, and perform refining and vacuum treatment, and then cast into a continuously cast billet of 320 mm425 mm, and the continuously cast billet is first heated to 600 C. in a preheating section, then continues to heat to 980 C. in a first heating section. And after heat preservation, continue to heat to 1200 C. in a second heating section. Then, after heat preservation, enter a soaking section having a temperature of 1220 C., and after heat preservation, perform subsequent rolling. The steel slab is discharged from a heating furnace, and begins to be rolled after high-pressure water descaling and finally is rolled into bars with 120 mm, wherein a final rolling temperature is controlled to be 1000 C. After rolling, perform air cooling, annealing treatment at 650 C. for 12 hours, and inspect by ultrasonic inspection and magnetic powder inspection and the like.

[0070] Example 4: perform electric furnace smelting according to the chemical composition shown in Table 1, and perform refining and vacuum treatment, and then cast into a continuously cast billet of 280 mm280 mm, and the continuously cast billet is first heated to 620 C. in a preheating section, then continues to heat to 950 C. in a first heating section. And after heat preservation, continue to heat to 1150 C. in a second heating section. Then, after heat preservation, enter a soaking section having a temperature of 1200 C. And after heat preservation, proceed with subsequent rolling. The steel slab is discharged from a heating furnace, and begins to be rolled after high-pressure water descaling, and finally is rolled into bars with 90 mm, wherein a final rolling temperature is controlled to be 970 C. After rolling, perform air cooling, grinding wheel scalping, and inspect by ultrasonic inspection and magnetic powder inspection and the like.

[0071] Example 5: perform electric furnace smelting according to the chemical composition shown in Table 1, and perform refining and vacuum treatment, and then cast into a continuously cast billet of 320 mm425 mm, and the continuously cast billet is first heated to 600 C. in a preheating section, then continues to heat to 950 C. in a first heating section. And after heat preservation, continue to heat to 1200 C. in a second heating section. Then, after heat preservation, enter a soaking section, having a temperature of 1230 C. And after heat preservation, perform subsequent rolling. The steel slab is discharged from a heating furnace, and begins to be rolled into an intermediate slab after high-pressure water descaling, wherein the first final rolling temperature is controlled to be 1050 C. and the intermediate slab has a size of 220 mm220 mm. The intermediate slab is then preheated to 680 C., and subsequently is first heated to 1050 C., then heated to 1200 C. And after heat preservation, perform soaking, the soaking temperature being 1220 C., and the slab after soaking is discharged from the furnace, and begins to be rolled after high-pressure water descaling, thereby obtaining a finished product bar having a specification of 50 mm, wherein the second final rolling temperature is controlled to be 950 C. After rolling, perform air cooling, isothermal annealing treatment, i.e., keeping at 900 C. for 90 min, followed by air cooling to 600 C., and keeping for 120 min, then discharge from the furnace, and air cooling, and then inspect by ultrasonic inspection and magnetic powder inspection and the like.

[0072] Example 6: perform electric furnace smelting according to the chemical composition shown in Table 1, and perform refining and vacuum treatment, and then cast into a continuously cast billet of 280 mm280 mm, and the continuously cast billet is first heated to 680 C. in a preheating section, then continues to heat to 900 C. in a first heating section. And after heat preservation, continue to heat to 1180 C. in a second heating section. Then, after heat preservation, enter a soaking section having a temperature of 1200 C. And after heat preservation, perform subsequent rolling. The steel slab is discharged from a heating furnace, and begins to be rolled into an intermediate slab after high-pressure water descaling, wherein the first final rolling temperature is controlled to be 1000 C. and the intermediate slab has a size of 140 mm140 mm. The intermediate slab is then preheated to 700 C., and subsequently heated to 1100 C., then heated to 1220 C. And after heat preservation, perform soaking, the soaking temperature being 1220 C., and the slab after soaking is discharged from the furnace, and begins to be rolled into a finished product bar having a specification of 20 mm after high-pressure water descaling, wherein the second final rolling temperature is controlled to be 920 C. After rolling, perform air cooling, turning scalping, and inspect by ultrasonic inspection and magnetic powder inspection and the like.

[0073] Example 7: perform converter smelting according to the chemical composition shown in Table 1, and perform refining and vacuum treatment, and then cast into a die cast slab, and the cast slab is first heated to 620 C. in a preheating section, then continues to heat to 950 C. in a first heating section. And after heat preservation, continue to heat to 1150 C. in a second heating section. Then, after heat preservation, enter a soaking section having a temperature of 1200 C. And after heat preservation, perform subsequent rolling. The steel slab is discharged from a heating furnace, and begins to be rolled after high-pressure water descaling and finally is rolled into bars with 110 mm, wherein the final rolling temperature is controlled to be 970 C. After rolling, perform air cooling, grinding wheel scalping, and inspect by ultrasonic inspection and magnetic powder inspection and the like.

[0074] Example 8: perform converter smelting according to the chemical composition shown in Table 1, and perform refining and vacuum treatment, and then cast into a die cast slab, and the cast slab is first heated to 600 C. in a preheating section, then continues to heat to 950 C. in a first heating section. And after heat preservation, continue to heat to 1200 C. in a second heating section. Then, after heat preservation, enter a soaking section having a temperature of 1230 C. And after heat preservation, perform subsequent rolling. The steel slab is discharged from a heating furnace, and begins to be rolled into an intermediate slab after high-pressure water descaling, wherein the first final rolling temperature is controlled to be 1050 C. and the intermediate slab has a size of 260 mm260 mm. The intermediate slab is then preheated to 680 C., and subsequently is first heated to 1050 C., and then heated to 1200 C. And after heat preservation, perform soaking, the soaking temperature being 1220 C., and the slab after soaking is discharged from the furnace, and begins to be rolled into a finished product bar having a specification of 60 mm after high-pressure water descaling, wherein the second final rolling temperature is controlled to be 950 C. After rolling, perform air cooling, and then inspect by ultrasonic inspection and magnetic powder inspection and the like.

[0075] Steels in Comparative examples 1 and 2 are from commercial materials.

[0076] Comparative example 3: The implementation method thereof is the same as that in Example 1, including: perform smelting in a 50 kg vacuum induction furnace according to the chemical composition shown in Table 1, cast molten steel into steel ingots, heat and forge into billets, and the steel ingots are first heated to 700 C. in a preheating section, then continue to heat to 900 C. in a first heating section. And after heat preservation, continue to heat to 1000 C. in a second heating section. Then, after heat preservation, enter a soaking section having a temperature of 1100 C. And after heat preservation, perform subsequent forging and finally forge into bars with 60 mm, wherein the final forging temperature is controlled to be 910 C., and after forging, normalize at 920 C. for 100 minutes.

[0077] Comparative example 4: The implementation method thereof is the same as that in Example 5, including: perform electric furnace smelting according to the chemical composition shown in Table 1, and perform refining and vacuum treatment, and then cast into a continuously cast billet of 320 mm425 mm, and the continuously cast billet is heated to 600 C. in a preheating section, then continues to heat to 950 C. in a first heating section. And after heat preservation, continue to heat to 1200 C. in a second heating section. Then, after heat preservation, enter a soaking section having a temperature of 1230 C. And after heat preservation, perform subsequent rolling. The steel slab is discharged from a heating furnace, and begins to be rolled into an intermediate slab after high-pressure water descaling, wherein the first final rolling temperature is controlled to be 1050 C. and the intermediate slab has a size of 220 mm220 mm. The intermediate slab is then preheated to 680 C., and subsequently is first heated to 1050 C., and then heated to 1200 C. And after heat preservation, perform soaking, the soaking temperature being 1220 C., and the slab after soaking is discharged from the furnace, and begins to be rolled into a finished product bar having a specification of 50 mm after high-pressure water descaling, wherein the second final rolling temperature is controlled to be 950 C. After rolling, perform air cooling, isothermal annealing treatment, i.e., keeping at 900 C. for 90 min, followed by air cooling to 600 C., and keeping for 120 min, then discharge from the furnace, and perform air cooling, and then inspect by ultrasonic inspection and magnetic powder inspection and the like.

[0078] Table 1 lists the mass percentage of each chemical element and a microalloying element coefficient r.sub.M/X of the steels for the high-temperature carburized gear shaft in Examples 1-8 and comparative steels in Comparative examples 1-4.

[0079] Table 2 lists the specific process parameters of the steels for the high-temperature carburized gear shaft in Examples 1-8 and comparative steels in Comparative examples 1-4 in the above process steps.

TABLE-US-00001 TABLE 1 (%, the balance being Fe and other inevitable impurities besides P, B, V, and Ti) No. C Si Mn P S Cr Ni Mo Cu Al V Ti Nb N B r.sub.M/X Example 1 0.17 0.28 1.35 0.007 0.015 1.39 0.24 0.07 0.19 0.037 0.013 0.007 0.016 0.02 0.0002 1.60 Example 2 0.22 0.06 0.81 0.006 0.018 1.16 0.21 0.08 0.16 0.046 0.018 0 0.013 0.017 0.0003 1.36 Example 3 0.18 0.27 1.3 0.006 0.016 1.4 0.19 0.04 0.13 0.039 0 0 0.027 0.02 0.0002 2.48 Example 4 0.22 0.27 0.92 0.006 0.011 0.99 0.18 0.06 0.19 0.041 0.013 0.002 0.003 0.018 0.0004 0.61 Example 5 0.19 0.12 1.31 0.008 0.024 0.86 0.22 0.07 0.06 0.027 0.015 0.001 0.014 0.009 0.0004 1.67 Example 6 0.20 0.34 1.33 0.01 0.034 1.02 0.15 0.06 0.13 0.021 0.003 0.002 0.025 0.006 0.0003 2.91 Example 7 0.21 0.28 1.28 0.008 0.022 1.25 0.1 0.03 0.15 0.041 0.015 0 0.015 0.02 0.0002 1.40 Example 8 0.22 0.31 1.08 0.011 0.027 1.32 0 0 0 0.031 0.011 0.001 0.012 0.014 0.0003 1.24 Comparative 0.16 0.14 1.35 0.009 0.031 1.01 0.01 0.02 0.01 0.038 0.013 0.001 0.002 0.013 0.0002 0.70 example 1 Comparative 0.2 0.13 1.29 0.012 0.025 1.36 0.18 0.07 0.1 0.026 0.003 0.002 0.001 0.016 0.0002 0.40 example 2 Comparative 0.21 0.27 1.35 0.005 0.014 1.22 0.16 0.06 0.09 0.036 0.014 0.021 0.032 0.012 0.0003 3.05 example 3 Comparative 0.23 0.28 1.33 0.006 0.016 1.13 0.1 0.05 0.11 0.04 0.018 0 0 0.017 0.0002 0.36 example 4

[0080] Note: r.sub.M/x=(20*[Nb]/93[V]/51+[Al]/27)/([N]/14+[C]/120), wherein each chemical element in the formula is substituted with a numerical value before the percentage sign of the percentage content by mass of the corresponding chemical element.

TABLE-US-00002 TABLE 2 Step (2) Heating Step (3) temperature of a Temperature of Temperature of a Temperature of Final forging or Step (1) preheating a first heating second heating a soaking final rolling Intermediate Bar No. Smelting mode section ( C.) section ( C.) section ( C.) section ( C.) temperature ( C.) slab size specification Example 1 Smelting in a 700 900 1000 1100 910 60 mm 50 kg vacuum induction furnace Example 2 Smelting in a 650 950 1100 1200 1000 75 mm 150 kg vacuum induction furnace Example 3 Electric 600 980 1200 1220 1000 120 mm furnace smelting Example 4 Electric 620 950 1150 1200 970 90 mm furnace smelting Example 5 Electric 600 950 1200 1230 1050 220 mm 50 mm furnace 680 1050 1200 1220 950 220 mm smelting Example 6 Electric 680 900 1180 1200 1000 140 mm 20 mm furnace 700 1100 1220 1220 920 140 mm smelting Example 7 Converter 620 950 1150 1200 970 110 mm smelting Example 8 Converter 600 950 1200 1230 1050 260 mm 60 mm smelting 680 1050 1200 1220 950 260 mm Comparative Electric 60 mm example 1 furnace smelting Comparative Electric 90 mm example 2 furnace smelting Comparative Smelting in a 700 900 1000 1100 910 60 mm example 3 50 kg vacuum induction furnace Comparative Electric 600 950 1200 1230 1050 220 mm 50 mm example 4 furnace 680 1050 1200 1220 950 220 mm smelting

[0081] In Table 2 above, Examples 5, 6, and 8 and Comparative example 4 have two columns of parameters in Step (2) and Step (3) in the above process of the present invention because the steel slab is first rolled to a specified intermediate slab size, and then heated and rolled again to a final finished product size during rolling in the above three Examples.

[0082] The obtained steels for the high-temperature carburized gear shaft in Examples 1-8 and comparative steels in Comparative examples 1-4 are respectively sampled and subjected to a simulated carburizing quenching test, a hardenability test and a hardness test, and the test results of the obtained steels in the Examples and Comparative examples are respectively shown in Table 3.

[0083] The relevant methods for the simulated carburizing quenching test, hardenability test and hardness test are described below: [0084] simulated carburizing quenching test: hold at 940 C. for 5 hours; hold at 960 C., 980 C. and 1000 C. for 4 hours, respectively; hold at 1020 C. for 3 hours; and hold at 1050 C. for 2 hours, then perform water quenching, and take samples to observe the structures of the steels in the Examples and Comparative examples, and evaluate their austenite grain sizes according to the standard ASTM E112.

[0085] Hardenability test: for the steels in the Examples and the steels in the Comparative examples, samples are taken and prepared from hot-rolled round steel according to the national standard GB/T 225, and subjected to an end hardenability test (Jominy test) with reference to GB/T 5216, wherein the normalizing temperature is controlled to be 92010 C., and the quenching temperature is controlled to be 8705 C. And a Rockwell hardness test is conducted according to GB/T 230.2 to obtain a hardness value (HRC) at a specific location, such as hardness at 9 mm from a quenching end, i.e., J9 mm. The above process parameters may also be determined by negotiation.

[0086] Table 3 lists the test results of the steels for the high-temperature carburized gear shaft in Examples 1-8 and the comparative steels in Comparative examples 1-4.

TABLE-US-00003 TABLE 3 Grain size of Grain size of Grain size of Grain size of Grain size of Grain size of austenite under austenite under austenite under austenite under austenite under austenite under the heat the heat the heat the heat the heat the heat preservation preservation preservation preservation preservation preservation condition of condition of condition of condition of condition of condition of Hardenability 940 C. 5 h 960 C. 4 h 980 C. 4 h 1000 C. 4 h 1020 C. 3 h 1050 C. 2 h at J9 mm No. (Grade) (Grade) (Grade) (Grade) (Grade) (Grade) (HRC) Example 1 7.5 7 6.5 6 5.5 5 39 Example 2 8 7.5 6 6 5.5 5.5 (1) 31 Example 3 7 7 6 6 5 5 38 Example 4 7 7 6 6 5.5 (1) 5 (0) 32 Example 5 6.5 6.5 6.5 6 5.5 (1) 5 (0) 33 Example 6 7 7 6 6 5 5 (1) 40 Example 7 7 7 6.5 6 5 (1) 5 (0) 39 Example 8 7.5 6.5 6 5 5 (1) 5 (0) 38 Comparative 6 5.5 5.5 (1) 5 (00) 5.5 (1) 4 (0) 29 example 1 Comparative 6 6 (1) 5 (1) 5 (0) 5 (0) 4 (00) 39 example 2 Comparative 7 6.5 5.5 5 (00) 5.5 (1) 4 (0) 41 example 3 Comparative 7 6 5 (1) 5 (0) 5 (0) 4 (00) 40 example 4

[0087] As can be seen from Table 3, after the steels for the high-temperature carburized gear shaft in Examples 1-8 of the present invention are subjected to simulated carburizing at four temperatures not exceeding 1000 C. in the simulated carburizing quenching test, the austenite grain sizes are maintained within the range of 5-8 grades, and no phenomena such as mixed crystals or abnormal coarse grains are observed. And the workability of the resulting steels meets the technical requirements, wherein the steels in Example 1 and Example 3 have a grain size of 5 grade after being heated at 1040 C. for 2 h.

[0088] The mixed crystal phenomenon (1 grade) is observed after the comparative steel in Comparative example 2 is subjected to simulated carburizing and quenching at a temperature of 960 C., wherein 6 (1) represents an average grain size of 6 grade, and 1 grade abnormal coarsening occurring in a local region. After continuing to increase the simulated carburizing temperature of the comparative steels in Comparative examples 1, 3, and 4 to 980 C. or higher, the abnormal growth of the austenite grains becomes severer, wherein 5.5 (1) represents an average grain size of 5.5 grade, and 1 grade coarsening occurring in a local region. In Comparative example 3, it can be seen that TiN type inclusions are present in the steel, adversely affecting the fatigue performance. The comparative steel in Comparative example 1 has a lower hardenability, and does not meet the requirements of 20MnCrS5H high-hardenability gear steel specified in EN 10084-2008.

[0089] To sum up, it can be seen that, in the present invention, by a reasonable chemical composition design and an optimized process, the steel for the high-temperature carburized gear shaft according to the present invention can have high temperature austenite grain stability, high hardenability, narrow hardenability bandwidth and good high-temperature grain stability. It is also free-cutting and suitable for high-temperature carburizing. And it has a hardenability of 30-43 HRC at a representative position J9 mm, and maintains 5-8 grades of the austenite grain size before and after the high-temperature vacuum carburizing at up to 1000 C. A bar rolled or forged with the high-hardenability steel for the gear shaft can be effectively processed into a gear, and has suitable strength and toughness after heat treatment such as high-temperature carburizing. The steel for the gear shaft can be effectively applied to high-end parts such as a gearbox for an automobile or a speed reducer and a differential for a new energy vehicle, and has good application prospects and value.

[0090] In addition, the combinations of various technical features in the present invention are not limited to the combinations described in the claims of the present invention or the combinations described in the specific examples, and all technical features described in the present invention can be freely combined or integrated in any way unless there is a conflict between the technical features.

[0091] It should also be noted that the examples listed above are only specific examples of the present invention. Obviously, the present invention is not limited to the above examples, and similar variations or modifications made accordingly that can be directly derived or easily conceived by those skilled in the art from the contents disclosed by the present invention should fall within the protection scope of the present invention.