Steel for ultra-high-strength transmission system and method for manufacturing same

Abstract

Disclosed is a steel for an ultra-high-strength transmission system, which, in addition to Fe and inevitable impurities, further comprises the following chemical elements in percent by mass: C: 0.35-0.45%, Si: 0.05-0.35%, Mn: 1.00-1.50%, Cr: 0.05-0.30%, Ni: 0.02-0.30%, Al: 0.015-0.050%, B: 0.0010-0.0035%, Ti: 0.020-0.050%, and S: 0.010-0.035%. Also disclosed is a method for manufacturing a steel for an ultra-high-strength transmission system, comprising the steps of: smelting and casting; heating and rolling; and quenching and tempering, wherein the quenching temperature is 830-950℃, the tempering temperature is 150-250℃, the holding time is 1-3 h, and air cooling or water cooling is performed after tempering.

Description

A steel for ultra-high strength transmission systems and its manufacturing method Technical Field

[0001] This invention relates to a type of steel and a method for manufacturing the same, and more particularly to a type of steel for transmission systems and a method for manufacturing the same. Background Technology

[0002] A driveshaft typically consists of a solid shaft, a telescopic sleeve, and a universal joint. It is an important component in the automotive transmission system for transmitting power. Its function is to transmit the power from the engine or motor to the wheels, thereby generating driving force for the vehicle.

[0003] The main processing technology for existing drive shafts is forging—normalizing—machining—heat treatment (normalizing, quenching and tempering, or surface induction hardening and tempering)—finishing—assembly. Drive shaft parts that are generally subjected to light loads are often made of medium carbon steel, with 45 and 50 high-quality carbon structural steel being commonly used materials. After normalizing, quenching and tempering, and partial surface hardening and tempering, the required performance indicators are obtained.

[0004] During service, the working contact surfaces of the drive shaft need to undergo induction hardening heat treatment to achieve higher hardness and improve wear resistance. Meanwhile, other parts need to have good toughness to reduce the risk of brittle fracture from impact.

[0005] With the development of new energy vehicles and new power systems, the high horsepower and torque output of the drive motor need to be transmitted to the wheels through the drive shaft. Therefore, higher requirements are placed on the performance of automotive drive shaft components.

[0006] In the prior art, existing patent literature covers the above-mentioned fields:

[0007] Chinese patent document CN112853211A, published on May 28, 2021, entitled "A Cold Forging Steel for Universal Joint Forks of Passenger Vehicles and Its Manufacturing Method", discloses a cold forging steel for universal joint forks of passenger vehicles and its manufacturing method. Its chemical composition, by mass percentage, is: C: 0.04–0.10%, Si: 0.01%–0.12%, Mn: 0.40–0.60%, P≤0.015%, S: 0.020–0.035%, Cr≤0.10%, Ni≤0.10%, Cu≤0.10%, Mo≤0.02%, Al: 0.020%–0.050%, N: 0.007%–0.009%. By spheroidizing medium carbon steel, the spheroidization rate of pearlite is ≥85% to meet the requirements of cold forging.

[0008] Chinese patent document CN113604739A, published on November 5, 2021, entitled "A Precision Forming Steel for Automobile Drive Axle CV Cage and Its Manufacturing Method," discloses a precision forming steel for automobile drive axle CV cage and its manufacturing method. Its chemical composition, by mass percentage, includes: C: 0.54–0.57%, Si: 0.17–0.37%, Mn: 0.62–0.68%, P≤0.020%, S≤0.015%, Cr: 0.10–0.30%, Ni≤0.25%, Cu≤0.2%, Al: 0.020–0.035%, N: 60–120 ppm, 2≤Al / N≤5, H≤2.0 ppm, and O≤15 ppm. The method refines the grain size by controlling the Al / N ratio in medium carbon steel.

[0009] Chinese patent document CN109852872A, published on June 7, 2019, entitled "A Steel for CV Cage of Automobile Drive System and Its Production Method", discloses a steel for CV cage of automobile drive system and its production method. Its chemical composition, by mass percentage, includes: C: 0.53-0.57%, Si: 0.22-0.32%, Mn: 0.80-0.95%, Cr: 0.20-0.30%, S≤0.010%, P≤0.015%, [O]≤0.0015%, [H]≤0.00015%. The hardenability is improved by adjusting the Mn and Cr content in medium carbon steel. Summary of the Invention

[0010] One of the objectives of this invention is to provide an ultra-high strength transmission system steel. This ultra-high strength transmission system steel optimizes the hardenability of the steel through reasonable chemical composition design, and after induction heat treatment, it has both ultra-high strength and plasticity, and has excellent comprehensive performance.

[0011] To achieve the above objectives, the present invention provides an ultra-high strength steel for transmission systems, which, in addition to containing Fe and unavoidable impurities, also contains the following chemical elements in weight percentage:

[0012] C: 0.35~0.45%, Si: 0.05~0.35%, Mn: 1.00~1.50%, Cr: 0.05~0.30%, Ni: 0.02~0.3 0%, Al: 0.015~0.050%, B: 0.0010~0.0035%, Ti: 0.020~0.050%, S: 0.010~0.035%.

[0013] Preferably, the ultra-high strength transmission system steel of the present invention has the following chemical elements in weight percentage:

[0014] C: 0.35–0.45%, Si: 0.05–0.35%, Mn: 1.00–1.50%, Cr: 0.05–0.30%, Ni: 0.02–0.30%, Al: 0.015–0.050%, B: 0.0010–0.0035%, Ti: 0.020–0.050%, S: 0.010–0.035%; balance Fe and unavoidable impurities.

[0015] The design principles of each chemical element in the ultra-high strength transmission system steel described in this invention are as follows:

[0016] C: In the ultra-high strength transmission system steel described in this invention, element C improves the hardenability of the steel, enabling the formation of a low-temperature phase transformation structure with higher hardness during surface quenching and cooling, thus significantly increasing the surface hardness of the steel. When the mass percentage of C is too high, although it increases the proportion of hard phases such as martensite and improves the strength of the steel, it leads to a decrease in toughness. When the mass percentage of C is too low, it leads to a decrease in martensite content, making it impossible for the steel to achieve high strength. Therefore, in the ultra-high strength transmission system steel described in this invention, the mass percentage of C is controlled between 0.35% and 0.45%.

[0017] Si: In the ultra-high strength transmission system steel described in this invention, Si replaces Fe atoms in the steel through substitution, hindering dislocation movement and contributing to increased steel strength. Si can also reduce the diffusion capacity of C in ferrite, preventing the formation of large carbides that precipitate at defects. However, excessively high Si content reduces the impact toughness of the steel. Therefore, in the ultra-high strength transmission system steel described in this invention, the Si content is controlled between 0.05% and 0.35%, preferably between 0.15% and 0.35%.

[0018] Mn: In the ultra-high strength transmission system steel described in this invention, Mn mainly exists in solid solution form. During the quenching process, Mn inhibits diffusion-type phase transformation, improves the hardenability of the steel, and forms a low-temperature phase transformation structure, which has high strength. When the mass percentage of Mn is too high, it leads to the formation of more retained austenite, reducing the strength of the steel. Therefore, in the ultra-high strength transmission system steel described in this invention, the mass percentage of Mn is controlled between 1.00% and 1.50%.

[0019] Cr: In the ultra-high strength transmission system steel described in this invention, chromium has a greater affinity for carbon than iron and manganese, and can form various carbides with carbon. Adding Cr to the steel inhibits diffusion-type phase transformation, improves hardenability, forms a hardened martensitic structure, and yields steel with high strength. Simultaneously, if the Cr carbides are not completely dissolved during heating, they inhibit austenite grain growth. Therefore, in the ultra-high strength transmission system steel described in this invention, considering both cost and performance, the mass percentage of Cr is controlled between 0.05% and 0.30%, preferably between 0.11% and 0.30%.

[0020] Ni: In the ultra-high strength transmission system steel described in this invention, Ni can exist in the steel in solid solution form. Ni can be used in combination with Cr to significantly improve the hardenability of the steel. Adding an appropriate amount of Ni to the steel can lower the carbon content at the eutectoid point, which is beneficial to improving the strength of the steel. However, Ni is a valuable alloying element, and adding too much Ni will lead to an increase in production costs. Therefore, in the ultra-high strength transmission system steel described in this invention, in order to obtain excellent performance while ensuring low production costs, the mass percentage of Ni is controlled between 0.02% and 0.30%, preferably between 0.06% and 0.30%.

[0021] Al: In the ultra-high strength transmission system steel described in this invention, Al can form fine AlN precipitates during the steelmaking process. These precipitates can inhibit austenite grain growth and refine the austenite grains during subsequent cooling, achieving a fine-grain strengthening effect. It should be noted that the Al content in the steel should not be too high. Excessive Al content can lead to the formation of larger Al oxides, and coarse alumina inclusions can worsen the fatigue performance of the steel. Therefore, in the ultra-high strength transmission system steel described in this invention, the mass percentage of Al is controlled between 0.015% and 0.050%.

[0022] B: In the ultra-high strength transmission system steel described in this invention, the main function of element B is to increase the hardenability of the steel, thereby saving the relatively rare and expensive element nickel. However, element B is quite reactive, exhibiting a strong affinity for both oxygen and nitrogen, easily forming boron oxide and boron nitride. This causes element B to lose its beneficial effects, resulting in increased fluctuations in the hardenability of the steel and affecting the stability of the steel's performance. Furthermore, excessive element B can form intermetallic compounds with element Fe, which agglomerate at grain boundaries, negatively impacting the toughness of the steel and increasing the tendency for temper brittleness. Therefore, in the ultra-high strength transmission system steel described in this invention, the mass percentage of element B is controlled between 0.0010% and 0.0035%.

[0023] Ti: In the ultra-high strength transmission system steel described in this invention, Ti has a strong affinity for nitrogen, oxygen, and carbon, and its precipitation temperature is relatively high. It is an effective element for fixing nitrogen and oxygen, thus preventing boron (B) in the steel from combining with oxygen and nitrogen and losing its effect on improving hardenability. Furthermore, Ti forms fine precipitates in the steel, which can improve the steel's strength and toughness, especially its impact toughness at low temperatures. However, when the mass percentage of Ti in the steel is too high, it will form coarse, angular particles during smelting and solidification, reducing the steel's impact toughness. Therefore, in the ultra-high strength transmission system steel described in this invention, the mass percentage of Ti is controlled between 0.020% and 0.050%.

[0024] S: In the ultra-high strength transmission system steel described in this invention, sulfur (S) is a free-machining element, mainly existing in the form of MnS. During machining, it can act as a source of internal stress concentration and easily breakable chips, and also plays a lubricating role between the tool and the workpiece, reducing tool wear and improving the machinability of the steel. However, sulfur-containing inclusions usually have low melting points, and when the mass percentage of S is too high, the material will experience hot brittleness and an increased tendency for decarburization. Therefore, in the ultra-high strength transmission system steel described in this invention, the mass percentage of S is controlled between 0.010% and 0.035%.

[0025] Preferably, in the ultra-high strength transmission system steel described in this invention, the mass percentage content of each chemical element further satisfies: (500B+1.1Ni+1.5Cr+3.5Mn)≥5.00%, where the symbols of each chemical element in the formula correspond to the mass percentage content of the element. More preferably, (500B+1.1Ni+1.5Cr+3.5Mn)≥5.47%.

[0026] In this invention, the strength and plasticity of steel are improved by controlling the content of the main alloying elements B, Mn, Cr, and Ni. In the steel of this invention, C, Si, Mn, Cr, Ni, and B are important alloying elements, all of which can improve the strength of the steel after quenching and tempering. However, as the strength of the steel increases, its plasticity deteriorates rapidly. In order to achieve ultra-high strength while maintaining good plasticity after quenching and tempering, a certain amount of residual austenite with good plasticity is retained during the formation of high-strength, brittle martensite to coordinate deformation and improve the strength and plasticity of the drive shaft steel. When the content of alloying elements Mn, Cr, Ni, and B in the steel is controlled to meet the condition (500*B+1.1*Ni+1.5*Cr+3.5*Mn)≥5.00%, after quenching and tempering heat treatment, the steel exhibits a fine acicular martensite and retained austenite structure, with the retained austenite content being 5-15%. This results in the invented steel possessing high strength and good plasticity, thus giving the drive shaft excellent comprehensive performance. When the content of Mn, Cr, Ni, and B elements in the steel does not meet the above relationship, it cannot be guaranteed that the steel will obtain the microstructure of the steel of this invention after quenching and tempering heat treatment, and it cannot simultaneously possess high strength and plasticity.

[0027] Preferably, the unavoidable impurities in the ultra-high strength transmission system steel of the present invention also include residual elements Cu, Mo and V, and the mass percentage content of the residual elements satisfies at least one of the following:

[0028] Cu≤0.10%;

[0029] Mo ≤ 0.05%;

[0030] V≤0.05%.

[0031] Although Cu, Mo, and V are considered beneficial elements in various instances, in this invention, to ensure the comprehensive mechanical properties of the steel, Cu, Mo, and V are designated as residual elements, and their upper limits must be controlled. In the ultra-high strength transmission system steel described in this invention, the mass percentage content of Cu is controlled to Cu ≤ 0.1%, the mass percentage content of Mo is controlled to Mo ≤ 0.05%, and the mass percentage content of V is controlled to V ≤ 0.05%.

[0032] Preferably, the unavoidable impurities in the ultra-high strength transmission system steel of the present invention include P, O, N and H, wherein the contents of P, O, N and H, in mass percentage, satisfy at least one of the following: P≤0.0150%, O≤0.0020%, N≤0.0080%, H≤0.00020%.

[0033] In the ultra-high strength transmission system steel described in this invention, P, H, O, and N are all impurity elements in the steel. Where technical conditions permit, to obtain steel with better performance and superior quality, the content of impurity elements in the steel should be reduced as much as possible. Specifically:

[0034] P: In the ultra-high strength transmission system steel described in this invention, phosphorus (P) will segregate at grain boundaries, reducing the binding energy of the grain boundaries and deteriorating the impact performance of the steel. Simultaneously, the coexistence of P and Mn elements will exacerbate the temper brittleness of the steel. Therefore, in the ultra-high strength transmission system steel described in this invention, the mass percentage content of P can be controlled to P ≤ 0.0150%.

[0035] O: In the ultra-high strength transmission system steel described in this invention, oxygen (O) reacts with Al and Si in the steel to form compounds such as Al₂O₃ and SiO₂. Therefore, to ensure the uniformity of the steel structure and its low-temperature impact resistance, the mass percentage content of oxygen in the ultra-high strength transmission system steel described in this invention can be controlled to O ≤ 0.0020%.

[0036] N: In the ultra-high strength transmission system steel described in this invention, nitrogen (N) combines with boron (B) in the steel, thereby causing N to lose its alloying effect. Therefore, in the ultra-high strength transmission system steel described in this invention, the mass percentage content of nitrogen can be controlled to N ≤ 0.0080%.

[0037] H: In the ultra-high strength transmission system steel described in this invention, and in the surface-hardened steel described in this invention, hydrogen (H) is affected by the hydrostatic pressure field of edge-shaped dislocations in the steel, and will accumulate at the defects, forming hydrogen embrittlement, leading to delayed fracture of the steel. Therefore, in the ultra-high strength transmission system steel described in this invention, the mass percentage content of H can be controlled to H≤0.00020%.

[0038] Preferably, the microstructure of the steel used in the ultra-high strength transmission system of the present invention is acicular martensite + retained austenite.

[0039] Preferably, the area content of retained austenite in the microstructure of the ultra-high strength transmission system steel of the present invention is 5-15%.

[0040] Preferably, the steel used in the ultra-high strength transmission system of the present invention has the following properties: yield strength ≥ 1200 MPa; tensile strength ≥ 1700 MPa; elongation ≥ 8%; reduction of area ≥ 30%.

[0041] Another objective of this invention is to provide a method for manufacturing ultra-high strength steel for transmission systems. This method is simple to produce, and the resulting bars have excellent mechanical properties and low cost. Furthermore, the resulting bars have good strength and plasticity, which can enable parts to have excellent comprehensive performance and have good application prospects and value.

[0042] To achieve the above objectives, the present invention provides a method for manufacturing ultra-high strength steel for transmission systems, comprising the following steps performed sequentially:

[0043] Smelting and casting;

[0044] Heating and rolling;

[0045] Induction hardening + tempering: the quenching temperature is 830~950℃; the tempering temperature is 150~250℃, the holding time is 1~3h, and the tempering is followed by air cooling or water cooling.

[0046] In this invention, an induction hardening process is used to rapidly heat the surface of the part to the austenitic region. During rapid water cooling, the part undergoes a martensitic phase transformation, increasing its strength. Low-temperature tempering reduces the internal stress of the steel, giving it high strength and good plasticity, thus resulting in an excellent overall performance for the drive shaft.

[0047] Preferably, in the heating step of the manufacturing method of the present invention, the heating temperature is controlled at 1100-1220°C and the holding time is 2-5 hours.

[0048] Preferably, in the rolling step of the manufacturing method of the present invention, the initial rolling temperature is controlled to be ≥1050℃ and the final rolling temperature is controlled to be ≥840℃, preferably ≥850℃.

[0049] Preferably, in the casting step of the manufacturing method of the present invention, the average superheat of the continuous casting tundish is 20-40°C, and the water volume of the secondary cooling section is 0.30-0.50 L / t.

[0050] The ultra-high strength transmission system steel and its manufacturing method described in this invention have the following advantages and beneficial effects compared to the prior art:

[0051] The ultra-high strength transmission system steel described in this invention, through reasonable chemical composition design and optimized process, can form fine acicular martensite and retained austenite structure after quenching and tempering heat treatment, wherein the retained austenite content is 5-15%. It not only has high strength but also good plasticity, thus giving the transmission shaft excellent comprehensive performance.

[0052] In some implementations, the steel used in ultra-high strength transmission systems shall meet at least one of the following requirements: yield strength ≥ 1200 MPa, tensile strength ≥ 1700 MPa, elongation ≥ 8%, and reduction of area ≥ 30%.

[0053] The manufacturing method of ultra-high strength transmission system steel described in this invention is simple, and the steel obtained by this manufacturing method has excellent mechanical properties, wide applicability, and can bring huge economic benefits. Attached Figure Description

[0054] Figure 1 shows the microstructure of the ultra-high strength transmission system steel of Embodiment 1 of the present invention.

[0055] Figure 2 shows the tensile stress-strain curve of the ultra-high strength transmission system steel of Embodiment 1 of the present invention after induction hardening and low-temperature tempering heat treatment. Detailed Implementation

[0056] The following will further explain and illustrate the ultra-high strength transmission system steel and its manufacturing method described in this invention, in conjunction with the accompanying drawings and specific embodiments. However, this explanation and illustration do not constitute an undue limitation on the technical solution of this invention.

[0057] Examples 1-6 and Comparative Examples 1-3

[0058] The ultra-high strength transmission system steels of Examples 1-6 and the comparative steels of Comparative Examples 1-3 of this invention were all prepared using the following steps:

[0059] (1) Smelting and casting: Smelting can be carried out in an electric furnace or converter, and then refined by LF and treated by VD or RH vacuum. In the casting process, the molten steel after smelting is cast into steel billets by continuous casting process, and the cast steel billets are hot-sent or offline for cooling.

[0060] In some implementations, during the casting process, the average superheat of the continuous casting tundish pour can be controlled at 20–40°C, and the specific water volume in the secondary cooling section can be controlled at 0.30–0.50 L / t.

[0061] (2) Heating and rolling: The ingot is heated in a walking beam furnace and then rolled to the finished size to form round steel.

[0062] In some embodiments, the billet heating temperature can be controlled at 1100–1220°C, and the holding time can be controlled at 2–5 hours; the initial rolling temperature can be controlled at ≥1050°C, and the final rolling temperature at ≥840°C.

[0063] Quenching and tempering: After the round steel is machined into a drive shaft, it undergoes induction quenching and tempering heat treatment. The induction quenching temperature can be controlled at 830-950℃, followed by rapid water quenching; the tempering temperature can be controlled at 150-250℃, and the holding time can be controlled at 1-3 hours. After tempering, it is air-cooled or water-cooled to obtain the finished product.

[0064] In some specific implementations, the size range of the round steel bars can be Φ20~50mm.

[0065] It should be noted that the composition and process of the ultra-high strength transmission system steel in Examples 1-6 of this invention meet the requirements of this invention, while the composition of the comparative steel in Comparative Examples 1-3 does not meet the requirements of this invention.

[0066] Tables 1-1 and 1-2 list the mass percentages of each chemical element in the ultra-high strength transmission system steel of Examples 1-6 of the present invention and the comparative steel of Comparative Examples 1-3.

[0067] Table 1-1. (wt%, balance Fe and other unavoidable impurities besides P, O, N and H)

[0068] Table 1-2. (wt%, balance Fe and other unavoidable impurities besides P, O, N and H)

[0069] Tables 2-1 and 2-2 list the specific process parameters of the ultra-high strength transmission system steel of Examples 1-6 of the present invention and the comparative steel of Comparative Examples 1-3.

[0070] Table 2-1.

[0071] Table 2-2.

[0072] Samples were taken from the ultra-high strength transmission system steels of Examples 1-6 and the comparative steels of Comparative Examples 1-3, and the retained austenite content in each example and comparative steel was measured. The retained austenite content could be measured using X-ray diffraction (XRD) or calculated using a scanning electron microscope with electron backscatter diffraction (EBSD) capability for micro-area measurement. The calculated retained austenite content in each example and comparative steel is shown in Table 3 below.

[0073] Table 3 lists the microstructure observation results of the ultra-high strength transmission system steel of Examples 1-6 of the present invention and the comparative steel of Comparative Examples 1-3.

[0074] Table 3.

[0075] As can be seen from Table 3 above, the microstructure of the ultra-high strength transmission system steel in Examples 1-6 of the present invention is acicular martensite and retained austenite, and the content of retained austenite is between 5-15%.

[0076] Figure 1 shows the microstructure of the ultra-high strength transmission system steel of Embodiment 1 of the present invention.

[0077] As shown in Figure 1, the microstructure of the ultra-high strength transmission system steel of Embodiment 1 of the present invention after heat treatment is acicular martensite and retained austenite, wherein the area content of retained austenite is about 5%.

[0078] To verify the performance of the ultra-high strength transmission system steel described in this invention, the ultra-high strength transmission system steels of Examples 1-6 and the comparative steels of Comparative Examples 1-3 were sampled again, and various performance tests were performed on them. The results of each performance test are listed in Table 4. The various performance tests include:

[0079] Mechanical property testing: The tensile strength, yield strength, elongation and reduction of area of ​​the ultra-high strength transmission system steels of Examples 1-6 and the comparative steels of Comparative Examples 1-3 were tested in accordance with GB / T 228.1-2010 "Metallic materials - tensile testing - Part 1: room temperature test method".

[0080] Table 4 lists the performance test results of the ultra-high strength transmission system steel of Examples 1-6 of the present invention and the comparative steel of Comparative Examples 1-3.

[0081] Table 4.

[0082] As can be seen from Table 4 above, the ultra-high strength transmission system steels of Examples 1-6 of the present invention exhibit excellent mechanical properties after induction hardening and low-temperature tempering heat treatment. Specifically, the yield strength of the ultra-high strength transmission system steel is greater than 1200 MPa, the tensile strength is greater than 1700 MPa, the elongation is greater than 8%, and the reduction of area is greater than 30%, thus giving the manufactured constant velocity transmission shaft ultra-high strength and resulting in excellent comprehensive performance of the transmission shaft components.

[0083] Figure 2 shows the tensile stress-strain curve of the ultra-high strength transmission system steel of Embodiment 1 of the present invention after induction hardening and low-temperature tempering heat treatment.

[0084] As shown in Figure 2, after tensile deformation, the ultra-high strength transmission system steel of Embodiment 1 of the present invention has a yield strength of 1446 MPa, a tensile strength of 1891 MPa, and an elongation of 11%. It can be seen that the transmission shaft steel made from the ultra-high strength transmission system steel of the present invention has good strength and plasticity.

[0085] In summary, the ultra-high strength drive shaft steel described in this invention, through reasonable chemical composition design and optimized processes, controls the microstructure of the steel after heat treatment to be acicular martensite and 5-15% retained austenite, thereby obtaining an ultra-high strength drive shaft steel with good plasticity. Furthermore, due to its excellent strength and plasticity, drive shafts mass-produced using this steel exhibit stable heat treatment quality, which is beneficial for lightweighting and reducing the weight of drive shaft assembly components, lowering costs, and also providing a long service life. Therefore, it has good prospects for promotion and practical value.

[0086] Furthermore, 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.

[0087] It should also be noted that the above examples are merely specific embodiments of the present invention, and the present invention is obviously not limited to the above embodiments, with many similar variations. All modifications that can be directly derived or conceived by those skilled in the art from the content disclosed in this invention should fall within the protection scope of this invention.

Claims

1. Steel for a transmission system, characterized in that, In addition to Fe and unavoidable impurities, the steel contains the following chemical elements in percentage by mass: C: 0.35–0.45%, Si: 0.05–0.35%, preferably 0.15–0.35%, Mn: 1.00–1.50%, Cr: 0.05–0.30%, preferably 0.11–0.30%, Ni: 0.02–0.30%, preferably 0.06–0.30%, Al: 0.015–0.050%, B: 0.0010–0.0035%, Ti: 0.020–0.050%, S: 0.010–0.035%.

2. Steel for a transmission system according to claim 1, characterized in that, The steel used in the transmission system has the following chemical elements in percentage by mass: C: 0.35–0.45%, Si: 0.05–0.35%, preferably 0.15–0.35%, Mn: 1.00–1.50%, Cr: 0.05–0.30%, preferably 0.11–0.30%, Ni: 0.02–0.30%, preferably 0.06–0.30%, Al: 0.015–0.050%, B: 0.0010–0.0035%, Ti: 0.020–0.050%, S: 0.010–0.035%; the balance being Fe and unavoidable impurities.

3. Steel for a transmission system according to claim 1 or 2, characterized in that, The chemical element mass percentage content of the steel also satisfies: (500B+1.1Ni+1.5Cr+3.5Mn)≥5.00%, preferably≥5.47%, where each chemical element symbol in the formula is substituted with the corresponding element mass percentage content.

4. Steel for a transmission system according to claim 1 or 2, characterized in that, The unavoidable impurities also include residual elements Cu, Mo, and V, and the mass percentage of the residual elements Cu, Mo, and V satisfies at least one of the following: Cu≤0.10%; Mo ≤ 0.05%; V≤0.05％。 5. Steel for a transmission system according to claim 1 or 2, characterized in that, The unavoidable impurities include P, O, N and H, wherein the contents of P, O, N and H, in mass percentage, satisfy at least one of the following: P ≤ 0.015%, O ≤ 0.0020%, N ≤ 0.0080%, H ≤ 0.00020%.

6. Steel for a transmission system according to claim 1 or 2, characterized in that, The microstructure of the steel is acicular martensite + retained austenite.

7. Steel for a transmission system according to claim 6, characterized in that, The area content of retained austenite in the microstructure is 5-15%.

8. Steel for a transmission system according to claim 1 or 2, characterized in that, The steel shall meet at least one of the following properties: yield strength ≥ 1200 MPa; tensile strength ≥ 1700 MPa; elongation ≥ 8%; reduction of area ≥ 30%.

9. Method of manufacturing a steel for a transmission system as claimed in any one of claims 1-8, characterized in that, The method includes the following steps performed sequentially: Smelting and casting; Heating and rolling; Induction hardening + tempering: the quenching temperature is 830~950℃; the tempering temperature is 150~250℃, the holding time is 1~3h, and the tempering is followed by air cooling or water cooling.

10. The method of claim 9, wherein, During the heating process, the heating temperature is controlled at 1100–1220℃, and the holding time is 2–5 hours.

11. The method of claim 9, wherein, During the rolling process, the initial rolling temperature is controlled to be ≥1050℃ and the final rolling temperature is controlled to be ≥840℃, preferably ≥850℃.

12. The method of claim 9, wherein, In the casting process, the average superheat of the tundish pouring in the continuous casting is 20-40℃, and the water volume in the secondary cooling section is 0.30-0.50L / t.

Images