A high-strength and high-toughness seamless steel tube for motor shafts with good machinability and its manufacturing method.

Abstract

This invention discloses a high-strength and high-toughness seamless steel pipe for motor shafts with good processing performance. It contains Fe and unavoidable impurity elements, and also contains the following chemical elements in the following mass percentages: C: 0.40-0.60%, 0 < Si ≤ 0.25%, Mn: 0.5-1.2%, Ti ≤ 0.045%, B ≤ 0.0045%, N: 0.0040-0.009%, Al: 0.015-0.045%, Ca + Mg: 0.001-0.006%. Accordingly, the present invention also discloses a method for manufacturing the above-mentioned high-strength and high-toughness seamless steel pipe for motor shaft, which includes the following steps: (1) obtaining a tube blank; (2) heating, piercing, hot rolling and tension reduction; (3) intermediate heat treatment: annealing the hot-rolled tube material in the temperature range of 650 to 800°C for a holding time of 20 to 80 minutes; (4) cold drawing; (5) finished product heat treatment: normalizing in the temperature range of 750 to 820°C for a holding time of 20 to 60 minutes to obtain a ferrite + pearlite structure.

Description

Technical Field

[0001] This invention relates to a steel pipe and its manufacturing method, and more particularly to a seamless steel pipe and its manufacturing method. Background Technology

[0002] In recent years, against the backdrop of carbon reduction and pollution control, the production and sales of electric vehicles have surged, and the market and users have become increasingly demanding in their requirements for the lightweighting of electric vehicles. In order to achieve the greatest possible lightweighting, in addition to using high-strength steel and lightweight materials for the main body, components that account for a small proportion of the overall vehicle weight, such as motor shafts and other vehicle structural parts, are also gradually being included in the weight reduction efforts. The use of hollow tubing to replace solid bars and forgings has become an inevitable trend in the industry.

[0003] In existing technologies, components such as the electric motor shaft in electric vehicles are crucial safety parts, responsible for transmitting motor torque to the drive wheels. Due to the high-speed operation of the motor shaft, it not only withstands immense torsional torque during actual use but also traction and braking forces from the wheels, including longitudinal, lateral, vertical, and vibration impact forces. Therefore, to meet such stringent requirements, the industry typically demands high strength and plasticity in the materials used to manufacture motor shafts to ensure adequate resistance to torsion and fatigue.

[0004] Furthermore, due to the differences between various vehicle models, different motor shafts require processing such as integral cold extrusion and machining to achieve the required torsional and fatigue resistance under high-speed rotation conditions. Therefore, when designing the steel used for these motor shafts, in addition to considering high strength, toughness, and high fatigue performance, good cold working properties must also be considered.

[0005] For example, Chinese patent document CN104962838A, published on October 7, 2015, entitled "A High-Strength Steel, High-Strength Plastic Seamless Steel Tube for Automobile Transmission Half-Shafts and Its Manufacturing Method," discloses a high-strength steel, high-strength plastic seamless steel tube for automobile transmission half-shafts and its manufacturing method. Its chemical composition is: C: 0.07–0.15%, Si: 0.1–1.0%, Mn: 2.0–2.6%, Ni: 0.05–0.6%, Cr: 0.2–1.0%, Mo: 0.1–0.6%, B: 0.001–0.006%, Cu: 0.05–0.50%; Al: 0.015–0.060%; Nb: 0.02–0.1%; V: 0.02–0.15%. In this technical solution, the steel pipe adopts a low C design, which results in a product with lower strength, but is beneficial for welding and is more suitable for automobile half shafts produced by friction stir welding.

[0006] For example, Chinese patent document CN1388834A, published on January 1, 2003, entitled "A High-Carbon Steel Pipe with Excellent Cold Working and High-Frequency Hardening Properties and Its Manufacturing Method," discloses a high-carbon steel pipe with excellent cold working and high-frequency hardening properties and its manufacturing method. The chemical composition by mass percentage is: C 0.30~0.80%, Si≤2%, Mn≤3%. In this technical solution, a special rolling technique is used to obtain a cementite structure with a density of less than 1µm, thereby improving the cold working and high-frequency hardening properties of the steel.

[0007] Unlike the existing technical solutions mentioned above, in order to solve the problems existing in the prior art, the present invention aims to develop and obtain a new high-strength and tough seamless steel pipe and its manufacturing method for use in the preparation of motor shafts. Summary of the Invention

[0008] One of the objectives of this invention is to provide a high-strength and high-toughness seamless steel tube for motor shafts with excellent processing performance. Through reasonable composition matching and process design, this high-strength and high-toughness seamless steel tube for motor shafts can obtain excellent mechanical properties. It has excellent mechanical properties before and after tempering heat treatment, and the hardness after tempering heat treatment can reach above 58HRC. The strength-ductility product (the product of tensile strength and elongation) is greater than 15000MPa%, and it can withstand torques of more than 300KN. It is particularly suitable for manufacturing motor shaft components that bear high torsional loads and has good prospects for promotion and application value.

[0009] To achieve the above objectives, the present invention provides a high-strength and high-toughness seamless steel tube for motor shafts with good machinability, which contains Fe and unavoidable impurity elements, and also contains the following chemical elements in the following mass percentages:

[0010] C: 0.40~0.60%, 0<Si≤0.25%, Mn: 0.5~1.2%, Ti≤0.045%, B≤0.0045%, N: 0.0040~0.009%, Al: 0.015~0.045%, Ca+Mg: 0.001~0.006%.

[0011] Furthermore, in the high-strength and high-toughness seamless steel pipe for motor shafts described in this invention, the mass percentage of each chemical element is as follows:

[0012] C: 0.40–0.60%, 0 < Si ≤ 0.25%, Mn: 0.5–1.2%, Ti ≤ 0.045%, B ≤ 0.0045%, N: 0.0040–0.009%, Al: 0.015–0.045%, Ca + Mg: 0.001–0.006%; balance is Fe and unavoidable impurities.

[0013] The design principles of each chemical element in the high-strength and high-toughness seamless steel tube for motor shafts described in this invention are as follows:

[0014] C: In the high-strength and high-toughness seamless steel pipe for motor shafts described in this invention, increasing the carbon (C) content is beneficial for improving the material's strength and fatigue resistance. However, the C content in the steel pipe should not be too high. When the C content is too high, it will lead to a decrease in the material's toughness and plasticity, which is not conducive to cold working and is prone to quality problems such as processing cracks and severe decarburization. Therefore, in the high-strength and high-toughness seamless steel pipe for motor shafts described in this invention, in order to ensure the quenching hardness and hardenability of the material, the mass percentage of C is controlled between 0.40% and 0.60%, thereby ensuring both the hardenability of the material and reducing the quenching crack sensitivity, which can ensure the cold working performance of the steel.

[0015] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of element C can be further preferably controlled between 0.45% and 0.55%.

[0016] Si: In the high-strength and high-toughness seamless steel pipe for motor shafts described in this invention, Si has a significant impact on cold working performance. The lower the Si content in the steel pipe, the better its cold working performance. Generally, Si is a residual element after deoxidation during steel smelting. If a lower Si content is required in the steel, the deoxidation method during steelmaking needs to be changed. Therefore, it is necessary to comprehensively control the Al and Ca contents to ensure the deoxidation level and to ensure that the corresponding non-metallic inclusions have no adverse effect on the fatigue resistance of the steel. Based on this, in the high-strength and high-toughness seamless steel pipe for motor shafts described in this invention, the mass percentage of Si is controlled to be 0 < Si ≤ 0.25%.

[0017] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of Si element can be further preferably controlled to 0 < Si ≤ 0.20%.

[0018] Mn: In the high-strength and high-toughness seamless steel pipe for motor shafts described in this invention, increasing the Mn content can improve the strength of the material. Mn can also stabilize P and S elements, preventing the formation of low-melting-point sulfides and improving the hot working performance of the material. To achieve these effects, the Mn content in the steel should not be too low. When the Mn content is too low, it cannot effectively stabilize P and S elements, failing to achieve the desired effect. Simultaneously, the Mn content should not be too high either, as excessive Mn content leads to severe cold work hardening and exacerbates mold wear. Therefore, considering the influence of Mn content on steel properties, the mass percentage of Mn in the high-strength and high-toughness seamless steel pipe for motor shafts described in this invention is controlled between 0.5% and 1.2%.

[0019] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of Mn element can be further preferably controlled between 0.6% and 1.0%.

[0020] Ti: In the high-strength and high-toughness seamless steel pipe for motor shafts described in this invention, an appropriate amount of Ti can improve the hardenability of the steel. Ti, together with C and N, can refine the grain size. However, the Ti content in the steel should not be too high, as excessive Ti content may lead to difficulties in smelting and continuous casting. Therefore, in the high-strength and high-toughness seamless steel pipe for motor shafts described in this invention, the mass percentage of Ti is controlled to Ti ≤ 0.045%.

[0021] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of Ti element can be further preferably controlled between 0.02% and 0.04%.

[0022] B: In the high-strength and high-toughness seamless steel pipe for motor shafts described in this invention, an appropriate amount of B content can improve the hardenability and plasticity of the steel, but excessive B content may lead to high-temperature brittleness of the steel. Therefore, in the high-strength and high-toughness seamless steel pipe for motor shafts described in this invention, the mass percentage content of B element is controlled to B≤0.0045%.

[0023] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of element B can be further preferably controlled between 0.002% and 0.004%.

[0024] N: In the high-strength and high-toughness seamless steel pipe for motor shafts described in this invention, N is controlled within an appropriate range so that it can refine the grains together with Ti and B elements, thereby improving the material's performance. Therefore, in the high-strength and high-toughness seamless steel pipe for motor shafts described in this invention, the mass percentage content of N element is controlled between 0.0040% and 0.009%.

[0025] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of N element can be further preferably controlled between 0.0045% and 0.0085%.

[0026] Al: In the high-strength and high-toughness seamless steel pipe for motor shafts described in this invention, Al is a deoxidizer during the smelting process. It works with elements such as Si to ensure the deoxidation effect of the steel, guaranteeing its purity and thus its fatigue resistance. However, it should be noted that the Al content in the steel should not be too high, as excessive Al content may lead to the formation of abnormal alumina inclusions. Therefore, to maximize the beneficial effects of Al, the mass percentage of Al in the high-strength and high-toughness seamless steel pipe for motor shafts described in this invention is controlled between 0.015% and 0.045%.

[0027] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of N element can be further preferably controlled between 0.015% and 0.035%.

[0028] Ca and Mg: In the high-strength and high-toughness seamless steel pipe for motor shafts described in this invention, appropriate amounts of Ca and Mg can improve the shape and properties of non-metallic inclusions, thereby improving the fatigue performance of the material. Therefore, in order to leverage the beneficial effects of Ca and Mg elements in the high-strength and high-toughness seamless steel pipe for motor shafts described in this invention, the inventors may consider adding Ca and / or Mg elements to the steel, requiring the sum of the mass percentages of Ca and Mg elements, "Ca + Mg", to be controlled between 0.001% and 0.006%.

[0029] Of course, in some preferred embodiments, in order to obtain better implementation results, the sum of the mass percentages of Ca and Mg elements, "Ca+Mg", can be further preferably controlled between 0.001 and 0.004%.

[0030] Furthermore, in the high-strength and high-toughness seamless steel tube for motor shafts described in this invention, among unavoidable impurities, P≤0.020%, S≤0.008%, and O≤0.0030%.

[0031] Furthermore, in the high-strength and high-toughness seamless steel tube for motor shafts described in this invention, among unavoidable impurities, P≤0.015%, S≤0.005%, and O≤0.0025%.

[0032] In the high-strength and high-toughness seamless steel pipe for motor shafts described in this invention, both P and S elements are impurity elements in the steel pipe. Under the condition that technical conditions permit, in order to obtain pipes with better performance and higher quality, the content of impurity elements in the high-strength and high-toughness seamless steel pipe for motor shafts should be reduced as much as possible.

[0033] In this invention, P and S are both impurity elements introduced into the raw materials or production process of steel. P element can cause grain boundary embrittlement, which deteriorates the toughness and processing performance of the material; while S element can combine to form low-melting-point sulfides, thereby causing a decrease in the processing performance and mechanical properties of the steel.

[0034] Therefore, in this invention, the content of P, S, and O elements in the steel must be strictly controlled, and controlled to P ≤ 0.020%, S ≤ 0.008%, and O ≤ 0.0030%. Of course, in some preferred embodiments, to obtain better implementation results, the content of P, S, and O elements can be further controlled to meet the following conditions: P ≤ 0.015%, S ≤ 0.005%, and O ≤ 0.0025%.

[0035] Furthermore, in the high-strength and high-toughness seamless steel pipe for motor shafts described in this invention, the mass percentage content of each chemical element further satisfies at least one of the following:

[0036] C: 0.45–0.55%,

[0037] 0 < Si ≤ 0.20%,

[0038] Mn: 0.6–1.0%,

[0039] Ti: 0.02–0.04%,

[0040] B: 0.002~0.004%,

[0041] N: 0.0045~0.0085%

[0042] Al: 0.015–0.035%

[0043] Ca+Mg: 0.001~0.004%.

[0044] Furthermore, in the high-strength and high-toughness seamless steel pipe for motor shafts described in this invention, its microstructure is ferrite + pearlite.

[0045] Furthermore, in the high-strength and high-toughness seamless steel pipe for motor shafts described in this invention, the microstructure after quenching and tempering heat treatment is martensite plus a residual austenite in a volume ratio of 1-25%.

[0046] Furthermore, in the high-strength and high-toughness seamless steel tube for motor shafts described in this invention, its yield strength Rp 0.2 ≥400MPa, tensile strength R m ≥550MPa, elongation A 50 ≥22%.

[0047] Furthermore, in the high-strength and high-toughness seamless steel tube for motor shafts described in this invention, its properties after quenching and tempering heat treatment satisfy the following: yield strength R p0.2 ≥1000MPa, tensile strength R m ≥1400MPa, elongation A 50 ≥5%, with a hardness ≥58HRC, a strength-ductility product greater than 15000MPa%, and a torsional strength ≥300KN.

[0048] Accordingly, another objective of the present invention is to provide a method for manufacturing the above-mentioned high-strength and high-toughness seamless steel tube for motor shafts. This manufacturing method has optimized the process design, and its manufacturing process is simple and easy to implement. It can effectively prepare the above-mentioned high-strength and high-toughness seamless steel tube for motor shafts of the present invention and has very good application prospects.

[0049] To achieve the above objectives, the present invention proposes a method for manufacturing the aforementioned high-strength and high-toughness seamless steel tube for motor shafts, comprising the following steps:

[0050] (1) Obtaining a tube blank;

[0051] (2) Heating, piercing, hot rolling and tension reduction;

[0052] (3) Intermediate heat treatment: The hot-rolled tube is annealed in the temperature range of 650-800℃ and the holding time is 20-80min;

[0053] (4) Cold drawing;

[0054] (5) Finished product heat treatment: normalizing is carried out in the temperature range of 750 to 820℃, and the holding time is 20 to 60 minutes to obtain ferrite + pearlite structure.

[0055] In the above technical solution of the present invention, during the tube blank manufacturing process in step (1), the operator can specifically use an electric furnace or converter to smelt and refine the tube blank, and specifically use continuous casting to cut the tube blank to obtain the required size. In order to meet the requirements of automotive motor shaft for steel pipe dimensional accuracy and surface condition during cold working, the hot-rolled tube material obtained in step (1) must undergo further intermediate heat treatment and cold drawing to the required specifications and dimensional accuracy, and then undergo further finished product heat treatment to finally obtain finished raw material with good cold working performance.

[0056] It should be noted that in step (3) above, during the intermediate heat treatment process, the hot-rolled tube is annealed in the temperature range of 650 to 800°C and held for 20 to 80 minutes to ensure the smooth progress of the cold drawing process in the subsequent step (4).

[0057] Furthermore, in the heat treatment of the finished product in step (5) of the present invention, the cold-drawn tube material prepared in step (4) needs to be normalized at a temperature range of 750-820°C for 20-60 minutes, and the furnace atmosphere needs to be controlled to ensure no complete decarburization (i.e., no area of ​​full ferrite structure on the surface of the steel tube), and the semi-decarburization depth is ≤150μm. In this technical solution, by controlling the combination of normalizing temperature and cooling method, it can ensure that the steel tube obtains a ferrite + pearlite structure, thereby obtaining a normalized tube with strength and toughness that meet the requirements of cold working.

[0058] Furthermore, in the manufacturing method described in this invention, in step (2), the tube blank is heated and held at 1210-1280°C for 30-150 minutes.

[0059] Furthermore, in the manufacturing method described in this invention, in step (2), the perforation temperature is controlled to be 1150–1250°C.

[0060] Furthermore, in the manufacturing method described in this invention, in step (2), the hot rolling temperature is controlled to be 1000-1200°C.

[0061] Furthermore, in the manufacturing method described in this invention, in step (2), the tension reduction temperature is controlled to be 950–1000°C.

[0062] Compared with the prior art, the high-strength and high-toughness seamless steel pipe for motor shafts and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0063] In this invention, the inventors can obtain a novel high-strength and high-toughness seamless steel pipe for motor shafts through reasonable component matching and process design. The microstructure of the high-strength and high-toughness seamless steel pipe for motor shafts after heat treatment is ferrite + pearlite, and the microstructure after further quenching and tempering heat treatment is martensite + residual austenite with a volume ratio of 1-25%.

[0064] The high-strength and high-toughness seamless steel tube for motor shafts prepared in this invention exhibits Rp in its finished state. 0.2 ≥400MPa, tensile strength R m ≥550MPa, elongation A 50 ≥22%; however, the strength of this type of high-strength and high-toughness seamless steel pipe for motor shafts can be significantly improved after further quenching and tempering heat treatment, and its yield strength Rp 0.2 ≥1000MPa, tensile strength R m ≥1400MPa, elongation A 50 It has a strength of ≥5%, and the hardness after heat treatment can reach 58HRC or above. The strength-ductility product (the product of tensile strength and elongation) is greater than 15000MPa%, and it can withstand torque of more than 300KN. It is particularly suitable for manufacturing motor shaft parts that bear high torsional loads, and has good promotion prospects and application value. Detailed Implementation

[0065] The high-strength and tough seamless steel pipe for motor shafts and its manufacturing method described in this invention will be further explained and illustrated below with reference to specific embodiments. However, this explanation and illustration do not constitute an undue limitation on the technical solution of this invention.

[0066] Examples 1-10 and Comparative Examples 1-2

[0067] The high-strength and high-toughness seamless steel tubes for motor shafts in Examples 1-10 of this invention and the comparative steel tubes in Comparative Examples 1-2 were all prepared using the following steps:

[0068] (1) According to the mass percentage of chemical elements shown in Table 1, the tube blanks are smelted and refined in an electric furnace or converter and then continuously cast and cut to obtain the required size tube blanks.

[0069] (2) Heating, piercing, hot rolling and tension reduction: The tube blank is heated at 1210-1280℃ and held for 30-150 minutes, then pierced at 1150-1250℃, and then hot rolled at 1000-1200℃. After hot rolling, tension reduction is further carried out, and the tension reduction temperature is controlled at 950-1000℃ to process hot rolled tube material.

[0070] (3) Intermediate heat treatment: The hot-rolled pipe is annealed in the temperature range of 650 to 800℃ and the holding time is 20 to 80 minutes.

[0071] (4) Cold drawing.

[0072] (5) Finished product heat treatment: The cold-drawn tube material is normalized in the temperature range of 750 to 820℃ and the holding time is 20 to 60 minutes to obtain ferrite + pearlite structure.

[0073] It should be noted that the chemical element composition and related process design of the high-strength and tough seamless steel pipes for motor shafts in Examples 1-10 of this invention all meet the design specifications of this invention. However, although the comparative steel pipes in Comparative Examples 1-2 were also prepared using the above process steps, their chemical element composition and / or related process parameters do not conform to the design of this invention.

[0074] Table 1 lists the mass percentage of each chemical element in the high-strength and tough seamless steel tubes for motor shafts in Examples 1-10 and the comparative steel tubes in Comparative Examples 1-2.

[0075] Table 1. (The balance is Fe and other unavoidable impurities besides P, S, and O)

[0076]

[0077] Table 2 lists the specific process parameters used in the above manufacturing process steps for the high-strength and tough seamless steel tubes for motor shafts in Examples 1-10 and the comparative steel tubes in Comparative Examples 1-2.

[0078] Table 2.

[0079]

[0080] Samples were taken from the high-strength and tough seamless steel pipes for motor shafts of Examples 1-10 and the comparative steel pipes of Comparative Examples 1-2, and the microstructure of the steel pipes of each example and comparative example before quenching and tempering heat treatment was observed. Various properties of each example and comparative example were tested to obtain the room temperature mechanical properties before quenching and tempering. The test results of the relevant mechanical properties are listed in Table 3.

[0081] The relevant performance testing methods are as follows:

[0082] Tensile test: The yield strength, tensile strength and elongation values ​​of the steel pipes of each embodiment and comparative example before tempering were obtained by testing according to GB / T 228.1-2010 Metallic materials, tensile testing - Part 1: Tensile testing at room temperature.

[0083] Table 3 lists the performance test results of the high-strength and tough seamless steel tubes for motor shafts in Examples 1-10 (without quenching and tempering heat treatment) and the comparative steel tubes in Comparative Examples 1-2.

[0084] Table 3.

[0085]

[0086]

[0087] As can be seen from Table 3 above, before undergoing quenching and tempering heat treatment, the high-strength and high-toughness seamless steel tubes for motor shafts in Examples 1-10 already possess excellent mechanical properties, with a yield strength R... p0.2 Its tensile strength R is between 417-506 MPa. m Its elongation A is between 564-672 MPa. 50 Between 23% and 30%. Furthermore, observations revealed that, before undergoing quenching and tempering heat treatment, the microstructure of the high-strength, high-toughness seamless steel tubes for motor shafts in Examples 1-10 consisted entirely of ferrite and pearlite.

[0088] Accordingly, to further illustrate that the high-strength and high-toughness seamless steel pipe for motor shafts described in this invention still possesses excellent performance after quenching and tempering heat treatment, the inventors further sampled the high-strength and high-toughness seamless steel pipes for motor shafts of the prepared examples 1-10 and the comparative steel pipes of comparative examples 1-2, and subjected the steel pipes of each example and comparative example to quenching and tempering heat treatment. The process of quenching and tempering heat treatment was controlled as follows: the quenching temperature was 820-880℃, the holding time was 15-30min, and the water cooling was used; the tempering temperature was 150-300℃, the holding time was 20-40min, and the air cooling was used.

[0089] After completing the quenching and tempering heat treatment of the steel pipes in the examples and comparative examples, the inventors further conducted mechanical property tests on the steel pipes after quenching and tempering heat treatment to measure the performance of the steel pipes after quenching and tempering heat treatment. The relevant mechanical property test results are listed in Table 4 below.

[0090] When testing the mechanical properties of the steel pipes of Examples 1-10 and Comparative Examples 1-2 after quenching and tempering heat treatment, the tensile test process is the same as the test process in Table 3 above. The yield strength, tensile strength and elongation of the steel pipes of Examples 1-10 and Comparative Examples 1-2 after quenching and tempering heat treatment can be measured accordingly.

[0091] In addition to the properties mentioned above, other properties of the steel pipes in the embodiments and comparative examples after quenching and tempering heat treatment were further tested. The relevant testing methods are described below:

[0092] Hardness test: The hardness (HRC) of the steel pipes of Examples 1-10 and Comparative Examples 1-2 after quenching and tempering heat treatment was measured using a Rockwell hardness tester.

[0093] Torsional performance test: A static torsion test device was used to measure the torsional performance of the steel pipes of Examples 1-10 and Comparative Examples 1-2 after quenching and tempering heat treatment.

[0094] Table 4 lists the performance test results of the high-strength and tough seamless steel tubes for motor shafts in Examples 1-10 and the comparative steel tubes in Comparative Examples 1-2 after quenching and tempering heat treatment.

[0095] Table 4.

[0096]

[0097] Note: In Table 4 above, Rm×A50 is the strength-ductility product, which is the product of tensile strength and elongation.

[0098] As shown in Table 4, after quenching and tempering heat treatment, the high-strength and high-toughness seamless steel pipes for motor shafts in Examples 1-10 of this invention exhibit significantly better overall performance compared to the comparative steel pipes of Comparative Examples 1-2. Furthermore, compared to before quenching and tempering heat treatment, the yield strength and tensile strength of the high-strength and high-toughness seamless steel pipes for motor shafts in Examples 1-10 after quenching and tempering heat treatment are significantly improved, and the elongation A is also significantly increased. 50 It decreased accordingly.

[0099] As can be seen from Table 4, the high-strength and high-toughness seamless steel pipes for motor shafts obtained in Examples 1-10 of this invention all possess excellent mechanical properties, with a yield strength R... p0.2 Its tensile strength R is between 1059-1169 MPa. m Between 1400-1766 MPa, with an elongation between 9-15%, its strong plasticity R m ×A 50 It has a strength between 15499-24724 MPa%, a hardness between 59-64 HRC, and a torsional strength between 346-597 KN.

[0100] Compared to Examples 1-10, Comparative Examples 1-2 all contain chemical compositions and / or processes that do not meet the design requirements of this invention. This design results in at least one mechanical property of the comparative steel pipes prepared in Comparative Examples 1-2 failing to meet the requirements of this invention. As shown in Table 4, the torsional resistance of Comparative Examples 1 and 2 is lower than that of Examples 1-10.

[0101] In addition, after completing the above-mentioned mechanical property tests, the inventors also took samples of the steel pipes of Examples 1-10 and Comparative Examples 1-2 after quenching and tempering heat treatment, and observed and analyzed their microstructure. The relevant observation and analysis results are shown in Table 5 below.

[0102] Table 5 lists the microstructure observation and analysis results of the high-strength and tough seamless steel tubes for motor shafts in Examples 1-10 and the comparative steel tubes in Comparative Examples 1-2 after quenching and tempering heat treatment.

[0103] Table 5.

[0104]

[0105] As can be seen from Table 5 above, after quenching and tempering heat treatment, the high-strength and tough seamless steel pipes for motor shafts in Examples 1-10 all obtained a microstructure of martensite + retained austenite, and the volume ratio of retained austenite was specifically between 3-24%.

[0106] 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.

[0107] 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 high-strength and high-toughness seamless steel pipe for motor shafts, characterized in that, The mass percentages of its chemical elements are as follows: C: 0.40–0.60%, 0 < Si ≤ 0.25%, Mn: 0.5–1.2%, Ti ≤ 0.045%, B ≤ 0.0045%, N: 0.0040–0.009%, Al: 0.015–0.045%, Ca + Mg: 0.001–0.006%; the balance is Fe and unavoidable impurities. Its properties after quenching and tempering heat treatment meet the following requirements: yield strength Rp 0.2 ≥1000MPa, tensile strength R m ≥1400MPa, elongation A 50 ≥5%, with a hardness ≥58HRC, a strength-ductility product greater than 15000MPa%, and a torsional strength ≥300KN.

2. The high-strength and high-toughness seamless steel pipe for motor shafts as described in claim 1, characterized in that, In unavoidable impurities, P ≤ 0.020%, S ≤ 0.008%, and O ≤ 0.0030%.

3. The high-strength and high-toughness seamless steel pipe for motor shafts as described in claim 2, characterized in that, In unavoidable impurities, P ≤ 0.015%, S ≤ 0.005%, and O ≤ 0.0025%.

4. The high-strength and high-toughness seamless steel pipe for motor shafts as described in claim 1, characterized in that, Its mass percentage content of each chemical element further satisfies at least one of the following conditions: C：0.45～0.55％， 0 < Si ≤ 0.20%, Mn: 0.6–1.0%, Ti: 0.02–0.04%, B：0.002～0.004％， N：0.0045～0.0085％, Al：0.015～0.035％, Ca+Mg: 0.001~0.004%.

5. The high-strength and high-toughness seamless steel pipe for motor shafts as described in claim 1, characterized in that, Its microstructure consists of ferrite and pearlite.

6. The high-strength and high-toughness seamless steel pipe for motor shafts as described in claim 1, characterized in that, Its microstructure after heat treatment is martensite plus a residual austenite in a volume ratio of 1-25%.

7. The high-strength and high-toughness seamless steel pipe for motor shafts as described in claim 1, characterized in that, Its yield strength Rp 0.2 ≥400MPa, tensile strength R m ≥550MPa, elongation A 50 ≥22%.

8. A method for manufacturing a high-strength and high-toughness seamless steel tube for a motor shaft as described in any one of claims 1-7, characterized in that, Including the following steps: (1) Obtaining a tube blank; (2) Heating, piercing, hot rolling and tension reduction; (3) Intermediate heat treatment: The hot-rolled tube is annealed in the temperature range of 650-800℃ and the holding time is 20-80min; (4) Cold drawing; (5) Finished product heat treatment: normalizing is carried out in the temperature range of 750 to 820℃, and the holding time is 20 to 60 minutes to obtain ferrite + pearlite structure.

9. The manufacturing method as described in claim 8, characterized in that, In step (2), the tube blank is heated at 1210-1280℃ and held for 30-150 minutes.

10. The manufacturing method as described in claim 8, characterized in that, In step (2), the perforation temperature is controlled to be 1150-1250℃.

11. The manufacturing method as described in claim 8, characterized in that, In step (2), the hot rolling temperature is controlled to be 1000-1200℃.

12. The manufacturing method as described in claim 8, characterized in that, In step (2), the tension reduction temperature is controlled to be 950-1000℃.

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