Fatigue wear-resistant steel for constant velocity joint, and manufacturing method therefor

Abstract

Disclosed in the present invention is a fatigue wear-resistant steel for a constant velocity joint, comprising Fe and inevitable impurities, and further comprising the following chemical elements having mass percentage contents as follows: C: 0.49-0.56%, Si: 0.15-0.35%, Mn: 0.70-0.90%, S: 0.010-0.030%, Cr: 0.15-0.40%, Ni: 0.05-0.20%, V: 0.05-0.10%, Al: 0.020-0.040%, N: 0.006-0.012%, Sn: 0.005-0.015%, and Cu≤0.10%. Also disclosed in the present invention is a manufacturing method for the steel for a constant velocity joint, comprising the steps of: smelting and casting; heating and hot rolling; and cooling: performing cooling to 680-720°C at a cooling rate of 1.5-2.0°C / s after hot rolling, and then performing air cooling to room temperature.

Description

A fatigue-resistant constant velocity universal joint steel and its manufacturing method Technical Field

[0001] This invention relates to a type of steel and its manufacturing method, and more particularly to a type of steel for constant velocity universal joints and its manufacturing method. Background Technology

[0002] A constant velocity joint is a type of universal joint that can transmit power at a constant speed without changes in rotational angular velocity. Its outer wheel bears the maximum load due to its structural characteristics. If fatigue wear occurs, it is prone to noise and premature failure. Therefore, it is necessary to ensure that the contact surfaces of the constant velocity joint have fatigue wear resistance.

[0003] To improve the fatigue performance of constant velocity universal joints, existing technologies generally involve adjusting the surface induction hardening process of the parts, or combining it with nitriding, carburizing, or tempering to enhance the fatigue performance of the parts.

[0004] Chinese patent document CN106256915A, published on December 28, 2016, entitled "Alloy Steel for High-Toughness Constant Velocity Universal Joint Outer Wheel and Manufacturing Method Thereof," discloses an alloy steel for a high-toughness constant velocity universal joint outer wheel. This patent document improves torsional strength by adding Mo, B, and Ti. However, the addition of a certain amount of Ti easily produces coarse and angular liquid-precipitated TiN inclusions, which are detrimental to fatigue performance. Furthermore, quenching and tempering treatments are used before high-frequency heat treatment, increasing process costs.

[0005] Chinese patent document CN112981236A, published on June 18, 2021, entitled "A steel for the inner raceway of a constant velocity universal joint and its production method", discloses a steel for the inner raceway of a constant velocity universal joint. The patent document provides a carburized steel with cold forging properties, but does not reflect the influence of the material on the wear performance of the parts.

[0006] With increasingly stringent requirements for NVH (Noise, Vibration, and Harshness), in addition to structural design and component heat treatment processes, it is also necessary to improve the performance of steel to ensure the high fatigue performance of constant velocity universal joints and achieve noise reduction. Summary of the Invention

[0007] One of the objectives of this invention is to provide a fatigue-resistant constant velocity joint steel, which, through reasonable composition design, achieves good fatigue-resistant wear resistance and good machinability.

[0008] To achieve the above objectives, the present invention provides a steel for constant velocity universal joints, which, in addition to containing Fe and unavoidable impurities, also contains the following chemical elements in the following mass percentages:

[0009] C: 0.49-0.56%, Si: 0.15-0.35%, Mn: 0.70-0.90%, S: 0.010-0.030%, Cr: 0.15-0.40%, Ni: 0.05-0.20%, V: 0.05-0.10%, Al: 0.020-0.040%, N: 0.006-0.012%, Sn: 0.005-0.015%, Cu ≤0.10% (inclusive). The constant velocity universal joint steel of this invention exhibits good fatigue wear resistance and good machinability.

[0010] Unless otherwise specified, the content of each element in the steel of this invention refers to its mass percentage.

[0011] Preferably, in the constant velocity universal joint steel of the present invention, the mass percentage content of each chemical element is as follows:

[0012] C: 0.49-0.56%, Si: 0.15-0.35%, Mn: 0.70-0.90%, S: 0.010-0.030%, Cr: 0.15-0.40%, Ni: 0.05-0.20%, V: 0.05-0.10%, Al: 0.020-0.040%, N: 0.006-0.012%, Sn: 0.005-0.015%, Cu≤0.10% (inclusive); balance is Fe and unavoidable impurities.

[0013] The design principles of each chemical element in the constant velocity universal joint steel described in this invention are as follows:

[0014] C: In the constant velocity joint steel described in this invention, carbon (C) has a significant impact on the steel's strength, toughness, and induction hardening hardness, and is an important means of ensuring the steel's strength and quenching hardness. When the C content in the steel is too low, the steel's hardness and strength are insufficient. However, it should be noted that when the C content in the steel is too high, the steel's ductility and toughness will decrease. Therefore, in order to ensure that the constant velocity joint steel described in this invention has both high strength and hardness as well as good ductility and toughness, the mass percentage of C in the constant velocity joint steel described in this invention is controlled between 0.49% and 0.56%.

[0015] Si: In the constant velocity universal joint steel described in this invention, Si can effectively improve the quenching hardness of the steel. With other elements constant, increasing the Si content in the steel can improve the quenching hardness to a certain extent. However, it should be noted that when the Si content in the steel is too high, bainite or martensite structures are easily formed in the steel. Therefore, in the constant velocity universal joint steel described in this invention, the mass percentage of Si is controlled between 0.15% and 0.35%.

[0016] Mn: In the constant velocity joint steel described in this invention, Mn, as an alloying element, can effectively improve the strength and hardenability of the steel. Simultaneously, controlling the Mn content within a certain range can also contribute to the steel's ductility and toughness. When the Mn content in the steel is too low, hardenability is insufficient; however, when the Mn content is too high, it not only negatively impacts the steel's ductility and toughness but also weakens its contribution to strength. Therefore, in the constant velocity joint steel described in this invention, the mass percentage of Mn is controlled between 0.70% and 0.90%.

[0017] S: In the constant velocity joint steel described in this invention, sulfur (S) effectively improves the machinability of the steel. The machinability of the steel improves with increasing S content, but its hot workability deteriorates. The addition of S in this invention is primarily to form MnS inclusions with Mn in the steel, thereby achieving chip breaking and improving machinability. However, when the S content in the steel is too high, it easily clogs the gate during casting; when the S content is too low, its chip breaking ability is poor, which is detrimental to turning. Therefore, in the constant velocity joint steel described in this invention, the mass percentage of S is controlled between 0.010% and 0.030%.

[0018] Cr: In the constant velocity universal joint steel described in this invention, Cr can effectively improve the strength and hardenability of the steel. Since the universal joint steel components require surface induction hardening treatment, adding a certain amount of Cr can ensure the depth and hardness of the hardened layer. It should be noted that when the Cr content in the steel is too low, its hardenability is insufficient; while when the Cr content in the steel is too high, more alloy carbides will form. Therefore, in the constant velocity universal joint steel described in this invention, the mass percentage of Cr is controlled between 0.15% and 0.40%.

[0019] Ni: In the constant velocity joint steel described in this invention, Ni element can improve the strength of the steel, and at the same time, Ni element is also beneficial to improving the ductility and toughness of the steel. Appropriate addition of a certain amount of Ni can improve the hot plasticity of the steel. Therefore, taking into account both production costs and the beneficial effects of Ni element, the mass percentage content of Ni element in the constant velocity joint steel described in this invention is controlled between 0.05% and 0.20%.

[0020] V: In the constant velocity universal joint steel described in this invention, vanadium (V) is an important precipitation strengthening element. Adding an appropriate amount of V to the steel can form a vanadium precipitate in the ferrite, significantly improving the material's strength without affecting its ductility and toughness. This precipitate not only inhibits grain growth at grain boundaries but also acts as a hard phase, resisting matrix fatigue spalling on the wear surface. However, it should be noted that when the V content in the steel is too low, the number of precipitates is small, and their effect is not significant; while when the V content is too high, it greatly increases the cost. Therefore, considering both production costs and the beneficial effects of V, the mass percentage of V in the constant velocity universal joint steel described in this invention is controlled between 0.05% and 0.10%.

[0021] Al: In the constant velocity universal joint steel described in this invention, Al plays a deoxidizing role during the steelmaking process. The resulting composite deoxidation product acts as a nucleation site for MnS precipitation, effectively improving chip formation. Furthermore, Al can combine with N to form AlN particles, effectively refining the grain size and preventing overheating during forming heating, thus avoiding thermal defects that could affect material properties. Similarly, this precipitated phase acts as a hard phase on the contact surface of the parts, resisting wear of the rolling elements and preventing fatigue spalling of the matrix. However, when the Al content in the steel is too high, secondary oxidation and nozzle nodule formation are prone to occur during casting. Therefore, in the constant velocity universal joint steel described in this invention, the mass percentage of Al is controlled between 0.020% and 0.040%.

[0022] N: In the constant velocity universal joint steel described in this invention, nitrogen (N) can react with alloying elements Al and V to form nitrides or nitrocarbides. These nitrides or nitrocarbides can refine grains through precipitation or strengthen the steel by precipitation, thereby improving its strength and toughness. Simultaneously, these dispersed hard precipitates act as load-bearing points on the contact surfaces of parts, resisting contact fatigue and preventing fatigue wear and spalling of the matrix. It should be noted that the precipitate formation and its driving force increase with the increase of N content in the steel. When the N content in the steel is too low, its effect is not significant; while when the N content is too high, the remaining nitrogen after the precipitate formation dissolves in the steel, easily forming porosity. Therefore, in the constant velocity universal joint steel described in this invention, the mass percentage of N is controlled between 0.006-0.012%.

[0023] Sn: In the constant velocity joint steel described in this invention, Sn element can improve the wear resistance of the steel and reduce the friction coefficient of the metal surface, thereby enhancing the wear resistance of the steel. However, when the Sn content in the steel is too high, it is prone to segregation near grain boundaries or defects, affecting hot workability. Therefore, in the constant velocity joint steel described in this invention, the mass percentage content of Sn element is controlled between 0.005% and 0.015%.

[0024] Preferably, in the constant velocity universal joint steel of the present invention, the mass percentage content of each chemical element satisfies: 5Sn + Cu ≤ Ni. In this formula, the element symbol represents the mass percentage content of the corresponding element in the steel. In the steel of the present invention, Cu is an optional added element. If the Cu content in the steel is 0, then the above relationship preferably satisfies 5Sn ≤ Ni.

[0025] In this invention, Sn and Cu are both hot-brittle elements that affect the hot workability of steel. Ni, on the other hand, contributes to the ductility and toughness of hot workability. The inventors discovered that by controlling the content of Sn, Cu, and Ni in steel to satisfy 5Sn + Cu ≤ Ni, Ni can counteract the negative effects of Sn and Cu on hot workability, ensuring that the steel does not crack during continuous casting, hot rolling, and subsequent forging.

[0026] Preferably, the constant velocity universal joint steel of the present invention further contains Mo, and its mass percentage is:

[0027] 0 < Mo ≤ 0.04%.

[0028] In some implementations, to achieve better results, appropriate amounts of at least one of Mo and Cu elements can be added to the steel. Wherein:

[0029] Mo: In the constant velocity joint steel described in this invention, Mo can significantly improve the hardenability and strength of the steel. Therefore, considering both the beneficial effects and cost of Mo, the mass percentage of Mo in the constant velocity joint steel described in this invention is controlled to be 0 < Mo ≤ 0.04%.

[0030] Cu: In the constant velocity joint steel described in this invention, Cu is an element that is beneficial for improving hardenability. Depending on the hardenability and performance requirements of the material, a certain amount of Cu can be added. However, when the Cu content in the steel is too high, it can easily cause hot brittleness, affecting the quality of the steel. Therefore, when Cu is added to the constant velocity joint steel described in this invention, the mass percentage of Cu is controlled to be 0 < Cu ≤ 0.10%.

[0031] Preferably, the unavoidable impurities in the constant velocity universal joint steel of the present invention include P, wherein P ≤ 0.020%, more preferably P ≤ 0.011%.

[0032] In the constant velocity universal joint steel described in this invention, phosphorus (P) is an unavoidable impurity element in steel, and the content of impurity elements in the steel should not be too high; theoretically, the lower the better. Therefore, taking into account steelmaking conditions and cost control factors, the mass percentage content of phosphorus in the constant velocity universal joint steel described in this invention is controlled to P ≤ 0.020%.

[0033] Preferably, the microstructure of the constant velocity universal joint steel of the present invention is ferrite + pearlite, wherein the ferrite microstructure contains V (C, N) precipitates and AlN precipitates. More preferably, the volume percentage of ferrite in the microstructure of the steel is 10-20%, for example 13.1-16.8%, and the volume percentage of pearlite is 80-90%, for example 83.2-86.9%.

[0034] In this invention, by controlling the V, C, and N elements within the aforementioned ranges and employing the manufacturing method employed, a large number of V(C, N) precipitates can be obtained during the rolling and cooling process. These V(C, N) precipitates are distributed on the ferrite matrix, which can improve the strength of the material. Simultaneously, since this ferrite is a structure that precipitates first along the grain boundaries, the V(C, N) precipitates subsequently pin the grain boundaries, preventing grain growth and thus maintaining good strength and toughness in the universal joint steel for the drive shaft. If these precipitates were to precipitate in austenite, their contribution to strength would be weakened due to the higher precipitation temperature and larger size. In addition, V(C, N) and AlN precipitates refine the grains and increase the number of grain boundaries in universal joint parts. On the other hand, these dispersed high-hardness precipitates act as bearing points on the wear contact surface of the parts, contacting the rolling elements. Since the hardness of the precipitates is higher than that of tempered martensite, they can effectively resist the wear of the rolling elements, avoiding adhesive wear and spalling caused by large-area contact between the matrix and the rolling elements, thereby improving fatigue wear performance.

[0035] Preferably, in the constant velocity universal joint steel of the present invention, the equivalent diameter of the V (C, N) precipitates is 6-14 nm, and the number (distribution density) is 65-95 precipitates / μm. 2 .

[0036] Preferably, the end-quenching hardness J3 of the steel for constant velocity universal joints described in this invention is ≥59HRC.

[0037] Preferably, the steel used in the constant velocity universal joint of the present invention did not peel off after a two-month bench torsional fatigue test.

[0038] Preferably, in the constant velocity universal joint steel described in this invention, the surface hardness after surface induction hardening and tempering heat treatment is 710-740HV.

[0039] Another objective of this invention is to provide a method for manufacturing steel for constant velocity joints. This method, through reasonable process design, yields a steel for constant velocity joints with good fatigue wear resistance and good machinability.

[0040] To achieve the above objectives, the present invention provides a method for manufacturing steel for constant velocity universal joints, comprising the following steps performed sequentially:

[0041] Smelting and casting;

[0042] Heating and hot rolling;

[0043] Cooling: After hot rolling, the steel is cooled to 680-720℃ at a cooling rate of 1.5-2.0℃ / s, and then air-cooled to room temperature. Unless otherwise specified, the cooling rate of the steel in this invention refers to the average cooling rate.

[0044] In this invention, since V(C, N) precipitates in austenite with relatively large size, the precipitation strengthening effect is weak. Therefore, after rolling, a relatively fast cooling rate is applied to cool the round steel from the austenite phase region to the ferrite transformation phase region, creating conditions for the precipitation of V(C, N) at grain boundaries and in ferrite.

[0045] Preferably, in the smelting step of the manufacturing method described in this invention, after the VD vacuum treatment is completed and atmospheric pressure is restored, the wire is fed in the following order: calcium wire, aluminum wire, chromium nitride wire, and sulfur wire, for supplementary feeding and composition adjustment. This order first utilizes calcium and aluminum for deep deoxidation and inclusion modification, then adds chromium nitride to adjust the composition, and finally adds sulfur wire. This order can minimize the oxidation loss of sulfides, while using the oxides generated in the previous steps as nucleation sites for manganese sulfide, improving the sulfide morphology, and effectively solving the problem of continuous casting nozzle blockage caused by sulfur-containing and aluminum-containing steel.

[0046] Preferably, in the casting step of the manufacturing method described in this invention, the continuous casting start temperature is 1503-1520℃.

[0047] Preferably, in the heating step of the manufacturing method of the present invention, the heating temperature is 1150-1200℃.

[0048] Preferably, in the hot rolling step of the manufacturing method described in this invention, the final rolling temperature is 920-980°C.

[0049] Preferably, after the cooling step of the manufacturing method described in this invention, forging, surface induction hardening and tempering can also be performed, wherein the forging temperature is 850-950℃, the surface induction hardening temperature is 840-900℃, and the tempering temperature is 180-200℃.

[0050] The fatigue-resistant wear-resistant constant velocity universal joint steel and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0051] The constant velocity joint steel of this invention, through a rational chemical composition design, utilizes V (C, N) and AlN precipitates to refine the grains while providing a wear-resistant hard phase on the contact surface. This phase acts as a load-bearing point, effectively preventing fatigue wear of the matrix. Simultaneously, the addition of Sn reduces the steel's coefficient of friction, improving wear resistance and giving the constant velocity joint steel of this invention excellent fatigue wear resistance. Furthermore, the addition of trace amounts of S enhances machinability, resulting in constant velocity joint steel of this invention possessing not only high fatigue wear resistance but also good machinability.

[0052] In some embodiments, the end-quench hardness J3 of the constant velocity universal joint steel of the present invention is ≥59HRC, and its surface hardness after surface induction hardening and tempering is 710-740HV. It has high hardness and good fatigue wear resistance.

[0053] In some preferred embodiments, the manufacturing method of the constant velocity universal joint steel of the present invention solves the problem of castability of sulfur-containing and aluminum-containing steel by feeding calcium wire, aluminum wire, chromium nitride wire and sulfur wire in the order of feeding after the VD vacuum treatment is completed and the pressure is restored. Moreover, the process design is reasonable, the casting performance is good, the production process window is wide, and mass production can be realized. Attached Figure Description

[0054] Figure 1 is a metallographic diagram of the steel used in the constant velocity universal joint according to Embodiment 1 of the present invention. Detailed Implementation

[0055] The fatigue-resistant constant velocity universal joint steel and its manufacturing method described in this invention will be further explained and described below with reference to the accompanying drawings and specific embodiments. However, this explanation and description do not constitute an undue limitation on the technical solution of this invention.

[0056] Examples 1-7 and Comparative Examples 1-3

[0057] The constant velocity universal joint steels of Examples 1-7 and the comparative steels of Comparative Examples 1-3 of the present invention were prepared through the following steps:

[0058] (1) Smelting and casting: electric furnace smelting, LF refining, VD vacuum treatment, continuous casting. After the VD vacuum treatment is completed and atmospheric pressure is restored, the feed and composition are adjusted in the order of Ca wire, Al wire, chromium nitride wire and S wire, and the continuous casting start temperature is controlled at 1503-1520℃.

[0059] (2) Heating and hot rolling: control the heating temperature at 1150-1200℃ and the final rolling temperature at 920-980℃;

[0060] (3) Cooling: After hot rolling, the temperature is cooled to 680-720℃ at a cooling rate of 1.5-2.0℃ / s, and then air-cooled to room temperature.

[0061] In some embodiments, after the cooling step, forging, surface induction hardening and tempering can also be performed, wherein the forging temperature can be controlled at 850-950°C, the surface induction hardening temperature can be controlled at 840-900°C, and the tempering temperature can be controlled at 180-200°C.

[0062] Tables 1-1 and 1-2 list the chemical composition ratios of the constant velocity universal joint steel of Examples 1-7 and the comparative steel of Comparative Examples 1-3. In Tables 1-1 and 1-2, a content of 0 for certain elements indicates that the corresponding elements were not intentionally added to the steel as beneficial elements.

[0063] Table 1-1. (wt.%, balance is Fe and other unavoidable impurities besides P)

[0064] Table 1-2. (wt.%, balance is Fe and other unavoidable impurities other than P)

[0065] Table 2 lists the specific process parameters of the constant velocity universal joint steel of Examples 1-7 and the comparative steel of Comparative Examples 1-3 of the present invention.

[0066] Table 2.

[0067] In all embodiments, the steel exhibited good surface quality during hot rolling and subsequent forging processes, with no visible hot-working cracks observed. The steel composition of Comparative Example 1 did not meet the preferred 5Sn+Cu≤Ni requirement of this invention, and network cracks were visible on the surface of the steel after forging.

[0068] Samples were taken from the constant velocity universal joint steel of Examples 1-7 and the comparative steel of Comparative Examples 1-3, and their microstructure was observed using a metallographic microscope. The observation results are listed in Table 3. Wherein:

[0069] Size and quantity of V(C, N) precipitates: The V(C, N) precipitates were observed and photographed using a transmission electron microscope (TEM). The size (equivalent diameter) and quantity (distribution density) of the precipitates were measured and statistically calculated using the analysis software built into the TEM.

[0070] Table 3 lists the microstructure observation results of the constant velocity universal joint steel of Examples 1-7 of the present invention and the comparative steel of Comparative Examples 1-3.

[0071] Table 3.

[0072] As can be seen from Table 3 above, the microstructure of the constant velocity universal joint steel in Examples 1-7 of the present invention is ferrite + pearlite. The ferrite microstructure contains V(C, N) precipitates, with equivalent diameters of 6-14 nm and a number of 65-95 precipitates / μm. 2 In all embodiments, the volume percentage of ferrite in the steel was 13.1-16.8%, and the volume percentage of pearlite was 83.2-86.9%.

[0073] Figure 1 shows the metallographic structure of the steel for the constant velocity universal joint according to Embodiment 1 of the present invention.

[0074] As shown in Figure 1, the metallographic structure of Example 1 is ferrite + pearlite.

[0075] Samples were taken again from the constant velocity universal joint steel of Examples 1-7 and the comparative steel of Comparative Examples 1-3, and forged, induction hardened, and tempered. The forging temperature was 850-950℃, the induction hardening temperature was controlled at 840-900℃, and the tempering temperature was controlled at 180-200℃. Relevant performance tests were performed, and the parts underwent a two-month bench fatigue test. The morphology of the contact surfaces (whether there was spalling) was observed. The test results are listed in Table 4. (The table includes the information provided.)

[0076] End-hardenability test: The end-hardenability of steel is tested according to GB / T225-2006 "End-hardening test method (Jominy test) for hardenability of steel". The hardness of the induction hardened layer is tested using a Vickers hardness tester.

[0077] Torsional fatigue test of parts: fatigue test was conducted in accordance with QC / T 1020-2015 "Test Method for Constant Velocity Universal Joints and Their Assemblies of Automobiles" and the surface morphology was observed.

[0078] Table 4 lists the relevant performance test results of the constant velocity universal joint steel of Examples 1-7 of the present invention and the comparative steel of Comparative Examples 1-3.

[0079] Table 4.

[0080] As can be seen from Table 4 above, the end-quench hardness J3 of the constant velocity universal joint steel in Examples 1-7 of the present invention is ≥59HRC, and the surface hardness of the constant velocity universal joint steel in Examples 1-7 after surface induction hardening and tempering is 710-740HV. Furthermore, after two months of bench torsional fatigue testing, the surface morphology of the contact surfaces was observed. The parts processed in Examples 1-7 did not experience spalling, while Comparative Examples 1-3 showed varying degrees of wear and spalling.

[0081] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0082] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A type of steel for constant velocity universal joints, characterized in that, In addition to Fe and unavoidable impurities, the steel contains the following chemical elements in the following mass percentages: C: 0.49-0.56%, Si: 0.15-0.35%, Mn: 0.70-0.90%, S: 0.010-0.030%, Cr: 0.15-0.40%, Ni: 0.0 5-0.20%, V: 0.05-0.10%, Al: 0.020-0.040%, N: 0.006-0.012%, Sn: 0.005-0.015%, Cu≤0.10%.

2. The steel for constant velocity universal joints as described in claim 1, characterized in that, The mass percentage of each chemical element in the steel is as follows: C: 0.49-0.56%, Si: 0.15-0.35%, Mn: 0.70-0.90%, S: 0.010-0.030%, Cr: 0.15-0.40%, Ni: 0.05-0.20%, V: 0.05-0.10%, Al: 0.020-0.040%, N: 0.006-0.012%, Sn: 0.005-0.015%, Cu≤0.10%; balance Fe and unavoidable impurities.

3. The steel for constant velocity universal joints as described in claim 1 or 2, characterized in that, The mass percentage content of each chemical element in the steel satisfies: 5Sn+Cu≤Ni, where the element symbols are substituted with the corresponding mass percentage content of the element.

4. The steel for constant velocity universal joints as described in claim 1 or 2, characterized in that, The steel used in the constant velocity universal joint also contains Mo, with a mass percentage of: 0 < Mo ≤ 0.04%.

5. The steel for constant velocity universal joints as described in claim 1 or 2, characterized in that, The unavoidable impurities include P, where P ≤ 0.020%.

6. The steel for constant velocity universal joints as described in claim 1 or 2, characterized in that, The microstructure of the constant velocity universal joint steel is ferrite + pearlite, wherein the ferrite structure contains V (C, N) precipitates and AlN precipitates.

7. The steel for constant velocity universal joints as described in claim 6, characterized in that, The equivalent diameter of the V(C, N) precipitates is 6-14 nm, and the number is 65-95 per μm. 2 .

8. The steel for constant velocity universal joints as described in claim 1 or 2, characterized in that, The steel used in the constant velocity universal joint has an end-quench hardness J3≥59HRC, and preferably, the steel does not show any surface spalling after a 2-month bench torsional fatigue test.

9. The steel for constant velocity universal joints as described in claim 1 or 2, characterized in that, The surface hardness of the steel used in the constant velocity universal joint after surface induction hardening and tempering is 710-740HV.

10. A method for manufacturing steel for constant velocity universal joints as described in any one of claims 1-9, characterized in that, The method includes the following steps performed sequentially: Smelting and casting; Heating and hot rolling; Cooling: After hot rolling, the temperature is cooled to 680-720℃ at a cooling rate of 1.5-2.0℃ / s, and then air-cooled to room temperature.

11. The method as described in claim 10, characterized in that, In the smelting process, after the VD vacuum treatment is completed and atmospheric pressure is restored, the feed and composition are adjusted in the order of Ca wire, Al wire, chromium nitride wire and S wire.

12. The method as described in claim 10, characterized in that, In the casting process, the initial casting temperature is 1503-1520℃.

13. The method as described in claim 10, characterized in that, During the heating step, the heating temperature is 1150-1200℃.

14. The method as described in claim 10, characterized in that, In the hot rolling process, the final rolling temperature is 920-980℃.

15. The method as described in claim 10, characterized in that, The method further includes forging, surface induction hardening and tempering after the cooling step, wherein the forging temperature is 850-950℃, the surface induction hardening temperature is 840-900℃, and the tempering temperature is 180-200℃.

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