High-strength and high-hardness steel, internal spline for automobile and preparation method thereof

Abstract

The present invention relates to a high-strength and high-hardness steel, an internal spline for automobiles and a preparation method thereof, and belongs to the technical field of manufacturing iron-based alloys. The high-strength and high-hardness steel comprises C: 0.18-0.25%; Si: 0.17-0.37%; Mn: 1.25-2.80%; Cr: 0.10-0.40%; Mo: 0.05-0.10%; Ni: 0.38-1.10%; V: 0.25-0.35%; S≤0.035%; P≤0.035%; the remainder is Fe. The high-strength and high-hardness steel of the present invention is used to prepare an internal spline for automobiles, and the mechanical properties of the internal spline are improved through the mutual cooperation between the elements, and the cracks of the internal spline are avoided, and the hardness of the internal spline is above 488HV1, and the tensile strength is above 1350MPa.

Description

Technical Field

[0001] The invention relates to the technical field of iron-based alloy manufacturing, and in particular to a high-strength and high-hardness steel, an internal spline for automobiles and a preparation method thereof. Background Art

[0002] The automobile transmission system is one of the most critical components of automobile power. It bears the maximum load and all power during the operation of the transmission system, and is subjected to bending moments and torques with constantly changing directions and wear from long-term high-speed operation. Its performance greatly affects the reliability and service life of the automobile.

[0003] In the transmission system, the internal spline is a mechanical connector used to connect the drive shaft and the gear. It uses the pattern holes of the gear and the pattern on the drive shaft to achieve the functions of fastening and transmitting torque. The working principle of the internal spline of the car relies on the friction between the two components.

[0004] During the use of the existing internal spline, the internal spline is broken due to cracks, which eventually leads to the paralysis of the entire transmission system and makes it impossible to continue to use. Therefore, there is an urgent need for an internal spline with better performance and no cracks to ensure the long-term use of the transmission system. Summary of the invention

[0005] In view of the above analysis, the present invention aims to provide a high-strength and high-hardness steel, an internal spline for automobiles and a preparation method thereof, so as to solve the problem of cracks in existing internal splines.

[0006] On the one hand, the present invention provides a high-strength and high-hardness steel, which comprises the following chemical components, calculated by mass percentage, C: 0.18 ~ 0.25%; Si: 0.17 ~ 0.37%; Mn: 1.25~2.80%; Cr: 0.10~0.40%; Mo: 0.05~0.10%; Ni: 0.38~1.10%; V: 0.25-0.35%; S≤0.035%; P≤0.035%; the balance is Fe.

[0007] Furthermore, the following chemical components are included in terms of mass percentage: C: 0.18 ~0.20%, Si: 0.20 ~0.37%; Mn: 2.25~2.80%; Cr: 0.10~0.25%; Mo: 0.05~0.08%; Ni: 0.75~1.10%; V: 0.30-0.35%; S≤0.035%; P≤0.035%; and the balance is Fe.

[0008] In a second aspect, the present invention provides an internal spline for automobiles, which is prepared using the high-strength and high-hardness steel of the present invention.

[0009] In a third aspect, the present invention provides a method for preparing an internal spline for an automobile, comprising the following steps:

[0010] S1: Material testing and forging to obtain forging blanks;

[0011] S2: quenching and once tempering the forging blank;

[0012] S3: spline drawing to obtain a blank;

[0013] S4: performing medium frequency quenching on the obtained blank;

[0014] S5: placing the blank after medium frequency quenching in a heat treatment furnace for secondary tempering;

[0015] S6: After fine machining, the internal spline is obtained.

[0016] Furthermore, in step S2, the heating temperature of the quenching is 850-1000°C; and the holding time is 70-90 minutes.

[0017] Furthermore, in step S2, the heating temperature of the primary tempering is 470-550°C, and the holding time is 2-2.5h.

[0018] Further, in step S4, the heating temperature of the medium frequency quenching is 850-930° C., and the frequency is 800-2000 Hz.

[0019] Further, in step S4, the blank is heated by medium frequency quenching, cooled for 5-20 seconds after being heated, and then cooled at a speed not higher than 90°C.

[0020] Furthermore, in step S5, the heating temperature of the secondary tempering is 500-600°C, and the holding time is 1.3-1.7h.

[0021] Furthermore, in step S5, the heating temperature of the secondary tempering is 500-550°C.

[0022] Furthermore, the hardness of the internal spline is above 488HV1, and the tensile strength is above 1350MPa.

[0023] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0024] 1. The high-strength and high-hardness steel of the present invention is used to prepare the internal spline of the automobile. The chemical composition is strictly controlled, the content of C, Cr and Mo elements is controlled to be relatively low, the content of Mn element is increased, and Ni and V elements are added. Through the mutual cooperation between the elements, the mechanical properties of the internal spline are improved, and cracks of the internal spline are avoided;

[0025] 2. The preparation method of the internal spline of the present invention adopts forging, quenching after forging and primary tempering; medium frequency quenching and secondary tempering are performed after spline drawing. The overall strength of the obtained internal spline is relatively high, and the hardness from the edge to the center of the internal spline is excessively distributed, avoiding cracks in the internal spline, and the hardness of the internal spline is above 488HV1, and the tensile strength is above 1350MPa.

[0026] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The accompanying drawings are only used for the purpose of illustrating specific embodiments and are not to be considered as limiting the present invention. In the entire drawings, the same reference symbols represent the same components;

[0028] Figure 1 It is a schematic diagram of the forging blank structure;

[0029] Figure 2 Schematic diagram of the placement of heat treatment furnace for secondary tempering;

[0030] Figure 3 This is a schematic diagram of hardness test points;

[0031] Figure 4 The result of the flaw detection;

[0032] Figure 5 This is a physical diagram of Example 1;

[0033] Figure 6 This is a physical picture of the fracture of comparative example 1. DETAILED DESCRIPTION

[0034] The preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0035] Internal splines are a mechanical connector used to connect drive shafts and gears. They are mostly used in transmission systems such as automobiles, and their performance greatly affects the reliability and service life of the automobile. Existing internal splines have reduced the use of transmission systems such as automobiles due to cracking problems. In severe cases, the entire transmission system is paralyzed. At present, internal splines are mostly made of 42CrMo steel material, combined with forging, high-frequency quenching and low-temperature tempering and other preparation methods. However, cracks appear in the resulting internal splines during use, resulting in the paralysis of the entire transmission system.

[0036] Therefore, the present invention provides a high-strength and high-hardness steel, which comprises the following chemical components, calculated by mass percentage, C: 0.18 ~ 0.25%; Si: 0.17 ~ 0.37%; Mn: 1.25~2.80%; Cr: 0.10~0.40%; Mo: 0.05~0.10%; Ni: 0.38~1.10%; V: 0.25-0.35%; S≤0.035%; P≤0.035%; the balance is Fe.

[0037] Specifically, the high-strength and high-hardness steel includes the following chemical components in terms of mass percentage: C: 0.18 ~0.20%; Si: 0.20 ~0.37%; Mn: 2.25~2.80%; Cr: 0.10~0.25%; Mo: 0.05~0.08%; Ni: 0.75~1.10%; V: 0.30-0.35%; S≤0.035%; P≤0.035%; the balance is Fe.

[0038] Specifically, the high-strength and high-hardness steel includes the following chemical components in terms of mass percentage: C: 0.20%; Si: 0.20%; Mn: 2.25%; Cr: 0.25%; Mo: 0.08%; Ni: 0.75%; V: 0.30%; S≤0.035%; P≤0.035%; and the balance is Fe.

[0039] Specifically, the high-strength and high-hardness steel of the present invention is used to prepare automotive inner splines.

[0040] Compared with the prior art, the present invention strictly controls the chemical composition in the internal spline, controls the content of C, Cr and Mo elements to a relatively low content, increases the content of Mn element, and adds Ni and V elements. Through the mutual coordination between the elements, an internal spline with stable performance and no cracks is obtained, thereby ensuring that the transmission system of automobiles and the like has a longer service life.

[0041] The present invention also provides a method for preparing an internal spline for an automobile, comprising the following steps:

[0042] S1: Material testing and forging to obtain forging blanks;

[0043] S2: quenching and once tempering the forging blank;

[0044] S3: spline drawing to obtain a blank;

[0045] S4: performing medium frequency quenching on the obtained blank;

[0046] S5: placing the blank after medium frequency quenching in a heat treatment furnace for secondary tempering;

[0047] S6: After fine machining, the internal spline is obtained.

[0048] Compared with the prior art, the preparation method of the internal spline provided by the present invention is to perform quenching, primary tempering, spline drawing, medium frequency quenching and secondary tempering after forging. With the cooperation of the above operations, the obtained internal spline has no cracks and has relatively stable performance.

[0049] It should be noted that in step S1, the material refers to the rod used for forging. The rod used in the present invention can be purchased or smelted by itself, and its chemical composition satisfies C: 0.18 ~ 0.25%; Si: 0.17 ~ 0.37%; Mn: 1.25~2.80%; Cr: 0.10~0.40%; Mo: 0.05~0.10%; Ni: 0.38~1.10%; V: 0.25-0.35%; S≤0.035%; P≤0.035%; the remainder is Fe.

[0050] It should be noted that in step S1, the forging refers to die forging, in which the bar is placed in a die, and a forging blank is obtained after die forging. The obtained forging blank is disc-shaped, and a circular boss is arranged at the center thereof, and an annular groove is opened on the side of the forging blank away from the circular boss; and a through hole is opened at the center of the circular boss (refer to Figure 1 ). The molds used in the present invention are all designed with internal splines, and those skilled in the art can make them, so they will not be described in detail here.

[0051] It should be noted that the qualified materials are heated and placed in the mold for forging. The heating temperature is controlled at 980-1060℃ to ensure that the material has sufficient plasticity. On the one hand, it ensures that the material can fill the mold and ensure the integrity of the forging blank; on the other hand, it prevents the material from cracking during the forging process.

[0052] Specifically, in step S2, the heating temperature of the quenching is 850-1000°C; and the holding time is 70-90 minutes.

[0053] In the quenching stage, the forging billet needs to be heated to 850-1000℃ and kept warm for 70-90 minutes. Since the thickness of the boss in the forging billet is greater than the thickness of other positions, the overall temperature of the forging billet cannot be balanced in a short time. Therefore, it takes a certain amount of time to ensure that the temperature of the boss of the forging billet is balanced with other positions or inside and outside.

[0054] After the forging blank is quenched, the structure of the forging blank is martensite. When the quenching heating temperature is lower than 850℃, the austenite phase transformation of the forging blank is not achieved, and quenching cannot be achieved, and the performance of the internal spline cannot be guaranteed. When the quenching heating temperature is higher than 1000℃, the austenite lattice in the forging blank grows with the increase of the heating temperature, and defects such as microcracks are easily generated, affecting the safety factor of the internal spline.

[0055] Preferably, the heating temperature of the quenching may be 850°C, 870°C, 890°C, 900°C, 930°C, 950°C, 980°C or 1000°C.

[0056] Specifically, in step S2, the heating temperature of the primary tempering is 470-550°C, and the holding time is 2-2.5h.

[0057] Specifically, the structure after one tempering is troostite, and the hardness is HB230-300.

[0058] It should be noted that in order to obtain troostite to improve the comprehensive mechanical properties of the forging blank, the stress generated by high-temperature quenching must be eliminated to avoid defects such as cracks caused by stress. The hardness after the first tempering is controlled at HB230-300. When the heating temperature of the first tempering is lower than 470℃, or when the heating temperature of the first tempering is higher than 550℃, the required hardness of the forging blank cannot be achieved, that is, the mechanical properties of the final internal spline cannot be achieved.

[0059] It should be noted that in step S3, key teeth need to be machined in the through hole through spline drawing. When drawing the spline, the plane used by the tooling must be parallel to the effective area of ​​the tooling on the broaching machine workbench. The contact surface between the part and the broaching machine must be completely machined, and there must be no black skin, burrs, or bumps. The part and the workbench surface in contact with it must be blown clean of iron filings and cutting fluid, so as to achieve accurate and stable spline drawing action.

[0060] Specifically, in step S4, the heating temperature of the medium frequency quenching is 850-930° C., and the frequency is 800-2000 Hz.

[0061] Specifically, in step S4, the blank is heated by medium frequency quenching, cooled for 5-20 seconds after being heated, and then cooled at a speed not higher than 90°C.

[0062] It should be noted that the internal spline is a mechanical connector used to connect the drive shaft and the gear. The key teeth processed in the through hole are in direct contact with the drive shaft. The wear resistance is improved through medium-frequency quenching, and the service life is extended to maintain good use effect with related matching parts.

[0063] It should be noted that the medium frequency quenching of the present invention uses induction heating to heat the key teeth processed in the through hole, the heating temperature is controlled at 850-930°C, the frequency is controlled at 800-2000Hz, and when the heating temperature reaches the control point, it is placed in water and quickly cooled for 5-20 seconds to ensure that the 0.3-1.0mm from the inner wall of the through hole to the inner spline edge is hardened to improve wear resistance. After rapid cooling, it is then cooled at a speed not higher than 90°C to ensure that the inner spline has good strength.

[0064] Specifically, the heating temperature of the secondary tempering is 500-600° C., and the holding time is 1.5±0.2h.

[0065] In step S5, the heating temperature of the secondary tempering is 500-550°C.

[0066] It should be noted that in step S5, the intermediate frequency quenched blank is placed in a heat treatment furnace for secondary tempering. The heating temperature of the secondary tempering is 500-600°C, which belongs to high temperature tempering. When placing the intermediate frequency quenched blank in the heat treatment furnace, it can be placed in one layer or in multiple layers. However, when placing multiple layers, the through hole of the internal spline needs to be exposed to prevent better heating and heat preservation (refer to Figure 2 After secondary tempering, the internal stress in the internal spline is greatly eliminated, avoiding stress deformation and cracks.

[0067] Preferably, the heating temperature of the secondary tempering may be 500°C, 520°C, 550°C, 570°C, 580°C or 600°C.

[0068] In the present invention, on the one hand, the chemical elements of the internal spline are strictly controlled, and the performance of the internal spline is improved by the coordination between the chemical elements; on the other hand, by coordinating corresponding preparation methods, such as quenching and primary tempering, medium frequency quenching and secondary tempering, the internal spline finally obtained has high mechanical properties and no cracks are generated.

[0069] In order to describe the present invention more clearly, the following examples and comparative examples are provided to further illustrate the present invention: The internal spline for automobile of the present invention is made of high-strength and high-hardness steel.

[0070] Example 1

[0071] A method for preparing an internal spline for an automobile comprises the following steps:

[0072] S1: Material testing and forging to obtain forging blanks;

[0073] The chemical composition of the material is calculated by mass percentage, including C: 0.20%; Si: 0.20%; Mn: 2.25%; Cr: 0.25%; Mo: 0.08%; Ni: 0.75%; V: 0.30%; S: 0.025%; P: 0.015%; the balance is Fe;

[0074] S2: quenching and once tempering the forging blank;

[0075] The heating temperature of quenching is 870℃, the holding time is 80min, and then the forging blank is placed in water for 5s, then taken out and cooled to room temperature in air;

[0076] The heating temperature of the first tempering is 470℃ and the holding time is 2h;

[0077] S3: Spline drawing to obtain the blank; refer to Figure 1 The blank is disc-shaped, with a circular boss at its center, and an annular groove is provided on the side of the blank away from the circular boss; and a through hole is provided at the center of the circular boss, and a spline process is performed in the through hole, and key teeth are processed in the through hole;

[0078] S4: medium frequency quenching of the blank after spline drawing;

[0079] The blank is heated by medium frequency quenching to 900°C, then placed in water for cooling for 10 seconds, and then cooled at a speed not higher than 90°C, with a frequency of 1000Hz;

[0080] S5: Place the parts after medium frequency quenching in a heat treatment furnace for secondary tempering. Figure 2 ;

[0081] The heating temperature of the secondary tempering is 550°C and the holding time is 1.5h;

[0082] S6: After fine machining, the internal spline is obtained.

[0083] Example 2

[0084] The preparation process of Example 2 is substantially the same as that of Example 1, except that the chemical composition of the material in Example 2 includes C: 0.18%; Si: 0.17%; Mn: 1.25%; Cr: 0.10%; Mo: 0.05%; Ni: 0.38%; V: 0.25%; S: 0.015%; P: 0.025%; and the balance is Fe.

[0085] Example 3

[0086] The preparation process of Example 3 is substantially the same as that of Example 1, except that the chemical composition of the material in Example 3 includes C: 0.25%; Si: 0.37%; Mn: 2.80%; Cr: 0.40%; Mo: 0.10%; Ni: 1.10%; V: 0.35%; S: 0.022%; P: 0.028%; and the balance is Fe.

[0087] Example 4

[0088] The preparation process of Example 4 is substantially the same as that of Example 1, except that in step S2 of Example 4, the heating temperature of the quenching is 850°C and the holding time is 90 minutes. The heating temperature of the first tempering is 550°C and the holding time is 2 hours.

[0089] Example 5

[0090] The preparation process of Example 5 is substantially the same as that of Example 1, except that in step S2 of Example 5, the heating temperature of the quenching is 1000°C and the holding time is 70 minutes. The heating temperature of the first tempering is 500°C and the holding time is 2.2 hours.

[0091] Example 6

[0092] The preparation process of Example 6 is substantially the same as that of Example 1, except that, in Example 6, S4: performing medium frequency quenching on the blank after spline drawing;

[0093] The blank is heated by medium frequency quenching to 850°C, then cooled in water for 5 seconds, and then cooled at a speed not higher than 90°C, with a frequency of 800Hz;

[0094] S5: Place the parts after medium frequency quenching in a heat treatment furnace for secondary tempering;

[0095] The heating temperature of the secondary tempering is 500° C. and the holding time is 1.7 h.

[0096] Example 7

[0097] The preparation process of Example 7 is substantially the same as that of Example 1, except that, in Example 7, S4: performing medium frequency quenching on the splined blank;

[0098] The blank is heated by medium frequency quenching to 930°C, then cooled in water for 20 seconds, and then cooled at a speed not higher than 90°C, with a frequency of 2000Hz;

[0099] S5: Place the parts after medium frequency quenching in a heat treatment furnace for secondary tempering;

[0100] The heating temperature of the secondary tempering is 600° C., and the holding time is 1.3 h.

[0101] Comparative Example 1

[0102] The preparation process of Comparative Example 1 is substantially the same as that of Example 1, except that Comparative Example 1 uses 42CrMo steel, and the chemical composition includes C: 0.425%; Si: 0.227%; Mn: 0.625%; S: 0.016%; P: 0.013%; Cr: 1.036%; Mo: 0.188%; Cu: 0.025%; Ni: 0.013%; and the balance is Fe.

[0103] Preparation method:

[0104] S2: subjecting the forging blank to a quenching and tempering treatment, with a heating temperature of 840°C, followed by cooling in oil to 50-100°C, and tempering at 160°C;

[0105] S3: spline drawing to obtain a blank;

[0106] S4: performing high-frequency quenching on the obtained blank; the heating temperature is 980° C. and the frequency is 80 kHz;

[0107] S5: placing the high-frequency quenched blank in a heat treatment furnace for low-temperature tempering; the tempering temperature is 170°C;

[0108] S6: After fine machining, the internal spline is obtained.

[0109] Comparative Example 2

[0110] The preparation process of Comparative Example 2 is substantially the same as that of Example 1, except that the chemical composition of the material in Comparative Example 2 includes C: 0.20%; Si: 0.20%; Mn: 2.25%; Cr: 0.25%; Mo: 0.08%; S: 0.025%; P: 0.015%; and the balance is Fe.

[0111] Comparative Example 3

[0112] The preparation process of Comparative Example 3 is substantially the same as that of Example 1, except that in step S2 of Comparative Example 3, the heating temperature of the quenching is 1050°C, the holding time is 60 min, and the heating temperature of the primary tempering is 450°C, and the holding time is 3 h.

[0113] Comparative Example 4

[0114] The preparation process of Comparative Example 4 is substantially the same as that of Example 1, except that, in Comparative Example 4, S4: medium frequency quenching is performed on the splined blank;

[0115] The blank is heated by medium frequency quenching to 830°C, then cooled in water for 10 seconds, and then cooled at a speed not higher than 90°C, with a frequency of 500Hz;

[0116] S5: Place the parts after medium frequency quenching in a heat treatment furnace for secondary tempering;

[0117] The heating temperature of the secondary tempering is 300° C. and the holding time is 1.5 h.

[0118] Performance Testing

[0119] The above-mentioned Examples 1-7 and Comparative Examples 1-4 were subjected to performance tests, mainly including hardness test, tensile strength test and crack test, wherein the sampling points for hardness test are as follows: Figure 3 As shown, and the distance between the three points is 10-20mm, the test results are shown in Table 1.

[0120]

[0121] Combined with Examples 1-7 and Comparative Examples 1-4 and Tables 1 and Figure 4-6 It can be seen that by using the components and preparation methods provided in Examples 1-7 of the present invention, the obtained internal splines have high mechanical properties and no cracks are generated, and the hardness of the internal splines is above 488HV1 and the tensile strength is above 1350MPa.

[0122] It can be seen from Examples 1-7 and Comparative Examples 1-4 and Table 1 that by using the components and preparation methods provided by Examples 1-7 of the present invention, the hardness transition of the internal spline obtained is smooth, the difference between point 1 and point 3 is relatively small, and the tendency to crack is small.

[0123] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. An internal spline for automobile, characterized in that: Made of high-strength and high-hardness steel; including the following chemical compositions by mass percentage: C: 0.18 ~ 0.20%; Si: 0.20 ~ 0.37%; Mn: 2.25 ~ 2.80%; Cr: 0.10 ~ 0.25%; Mo: 0.05 ~ 0.08%; Ni: 0.75 ~ 1.10%; V: 0.30-0.35%; S≤0.035%; P≤0.035%; the balance is Fe; The method comprises the following preparation steps: S1: Material testing and forging to obtain forging blanks; S2: quenching and primary tempering the forging blank; the heating temperature of the primary tempering is 470-550°C, and the holding time is 2-2.5h; the microstructure after the primary tempering is sorbite, and the hardness is HB230-300; S3: spline drawing to obtain a blank; S4: performing medium frequency quenching on the obtained blank; the blank is heated by medium frequency quenching, cooled for 5-20 seconds after being heated, and then cooled at a speed not higher than 90°C; S5: placing the intermediate frequency quenched blank in a heat treatment furnace for secondary tempering; the secondary tempering has a heating temperature of 500-600° C. and a holding time of 1.3-1.7 h; S6: After fine machining, an internal spline is obtained; the hardness of the internal spline is above 488HV1, and the tensile strength is above 1350MPa.

2. A method for preparing an internal spline for an automobile as claimed in claim 1, characterized in that: The following steps are involved: S1: Material testing and forging to obtain forging blanks; S2: quenching and primary tempering the forging blank; the heating temperature of the primary tempering is 470-550°C, and the holding time is 2-2.5h; the microstructure after the primary tempering is sorbite, and the hardness is HB230-300; S3: spline drawing to obtain a blank; S4: performing medium frequency quenching on the obtained blank; the blank is heated by medium frequency quenching, cooled for 5-20 seconds after being heated, and then cooled at a speed not higher than 90°C; S5: placing the intermediate frequency quenched blank in a heat treatment furnace for secondary tempering; the secondary tempering has a heating temperature of 500-600° C. and a holding time of 1.3-1.7 h; S6: After fine machining, an internal spline is obtained; the hardness of the internal spline is above 488HV1, and the tensile strength is above 1350MPa.

3. The method for preparing an internal spline for an automobile according to claim 2, characterized in that: In step S2, the heating temperature of the quenching is 850-1000°C; and the holding time is 70-90 minutes.

4. The method for preparing an internal spline for an automobile according to claim 2, characterized in that: In step S2, the heating temperature of the primary tempering is 500°C, and the holding time is 2.2h.

5. The method for preparing an internal spline for an automobile according to claim 2, characterized in that: In step S4, the heating temperature of the medium frequency quenching is 850-930° C. and the frequency is 800-2000 Hz.

6. The method for preparing an internal spline for an automobile according to claim 2, characterized in that: In step S5, the heating temperature of the secondary tempering is 550°C and the holding time is 1.5h.

7. The internal spline for automobile according to claim 2, characterized in that: The hardness of the internal spline is above 488HV1, and the tensile strength is above 1350MPa.

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