Easily processable seamless steel tube with high strength and toughness for an electric motor shaft and method for manufacturing the same.

Abstract

The present invention discloses a readily machinable seamless steel tube with high strength and toughness for an electric motor shaft and a method for its production. The seamless steel tube contains iron and unavoidable impurities, as well as the following chemical elements in mass percentages: C: 0.40–0.60%, O < 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%. It possesses excellent mechanical properties before and after thermal treatment by quenching and tempering, such as a hardness of not less than 58 HRC, a strength-ductility product of greater than 15000 MPa% and a torsional strength of not less than 300 KN after thermal treatment by quenching and tempering.

Description

Technical area

[0001] The invention relates to a steel pipe and a method for producing the same, in particular a seamless steel pipe and a method for producing the same. State of the art

[0002] In recent years, the number of electric vehicles produced and sold has increased significantly, driven by efforts to reduce carbon emissions and environmental impacts. The market is placing ever-increasing demands on the lightweight construction of electric vehicles themselves. To achieve maximum lightweight construction, in addition to using lightweight materials such as high-strength steel in the body, the weight of smaller components, such as the electric motor shaft and other structural parts of the vehicle, which account for only a relatively small proportion of the total vehicle weight, must also be reduced. A common practice in the industry is to replace a solid rod or forging with a hollow tube.

[0003] It is well known in the art that components such as the electric motor shaft of an electric vehicle serve to transmit the torque of the electric motor to the drive wheels and are considered very important safety components. Since the electric motor shaft rotates at a high speed, during practical operation, the electric motor shaft is subjected not only to enormous torque but also to traction and braking forces from the vehicle wheels, such as vertical forces, lateral forces, perpendicular forces, and impact forces due to vibrations. Due to the stringent application conditions, the material used in the practical manufacture of the electric motor shaft must have high strength and ductility to ensure sufficient torsional strength and fatigue resistance.

[0004] Depending on the different vehicle types, the necessary torsional strength and fatigue resistance at high speeds can only be achieved by cold pressing, cutting, etc., on the various electric motor shafts. Therefore, when designing such a steel for an electric motor shaft, high toughness, high fatigue resistance, and good cold workability must be considered simultaneously.

[0005] For example, a high-strength steel, a seamless steel pipe with a high product of strength and ductility for a vehicle wheel axle, and a method for manufacturing the same are disclosed in Chinese Patent Publication No. CN 104962838 A, titled "High-strength steel, seamless steel pipe with a high product of strength and ductility for a vehicle wheel axle, and a method for manufacturing the same," published on October 7, 2015. The steel contains the following chemical components: 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, a design of the steel pipe with a low C content is used, so that the strength of the manufactured finished product is relatively low, which facilitates welding and is suitable for the wheel axle of the vehicle manufactured by friction stir welding.

[0006] For example, a high-carbon steel pipe with excellent cold workability and high-frequency quenching hardenability and a manufacturing method thereof are disclosed in Chinese Patent Publication No. CN 1388834 A, titled "High-carbon steel pipe with excellent cold workability and high-frequency quenching hardenability and a manufacturing method thereof," published on January 1, 2003. The steel pipe disclosed in this patent application contains the following chemical components in mass percentage: C 0.30-0.80%, Si ≤ 2%, Mn ≤ 3%. This technical solution uses a special rolling technique to obtain a cementite microstructure of less than 1 µm, thereby improving the cold workability and high-frequency quenching hardenability of the steel.

[0007] Compared with the above conventional technical solutions, in order to eliminate the problem in the prior art, the present invention is intended to develop and provide a novel seamless steel pipe having high strength and toughness used for manufacturing an electric motor shaft and a method for manufacturing the same. Disclosure of the invention

[0008] One of the objectives of the present invention is to provide a readily processable seamless steel tube with high strength and toughness for an electric motor shaft. Due to appropriate component selection and process design, it exhibits excellent mechanical properties, such as excellent mechanical properties before and after thermal treatment by quenching and tempering. It has a hardness of not less than 58 HRC, a strength-ductility product (i.e., the product of tensile strength and elongation) of greater than 15,000 MPa%, and a torsional strength of not less than 300 KN after thermal treatment by quenching and tempering. This steel tube is particularly suitable for use in components operating under high torsional loads, such as an electric motor shaft, and can be well applied and widely used.

[0009] To achieve this objective, the present invention provides a readily processable seamless steel tube with high strength and toughness for an electric motor shaft. The steel tube contains Fe and unavoidable impurity elements, as well as the following chemical elements in mass percentage: 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%.

[0010] Furthermore, the seamless steel pipe with high strength and toughness for an electric motor shaft according to the invention comprises the following chemical elements in mass percentage: 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%; and the remaining amount of Fe and unavoidable impurities.

[0011] Preferably, the mass percentages of the individual chemical elements are: C: 0.4 - 0.57%, Si: 0.01 - 0.23%, Mn: 0.5 - 1.2%, Ti: 0 - 0.043%, B: 0 - 0.0042%, N: 0.004 - 0.009%, Al: 0.016 - 0.045%, Ca+Mg: 0.001 - 0.006%.

[0012] In the seamless steel pipe with high strength and toughness for an electric motor shaft according to the invention, the adjustment of the chemical elements is based on the following principles: C: In the seamless steel pipe with high strength and toughness for an electric motor shaft according to the present invention, increasing the content of element C improves the strength and fatigue resistance of the material. However, the content of element C in the steel pipe should not be too high, otherwise the ductility of the material will decrease and quality problems such as easy cracking during processing and excessive decarburization will occur, which is detrimental to cold processing. Therefore, in the seamless steel pipe with high strength and toughness for an electric motor shaft according to the present invention, the mass percentage of element C is kept between 0.40% and 0.60% to ensure quenching hardness and hardenability, thereby improving the through-hardenability of the material and reducing susceptibility to cracking during quenching.This ensures that the steel can be cold worked.

[0013] Of course, in some preferred embodiments, the mass percentage of element C may be more preferably kept between 0.45% - 0.55% in order to achieve a better effect in execution.

[0014] Si: In the seamless steel pipe with high strength and toughness for an electric motor shaft according to the present invention, the Si element has a significant influence on cold workability. The lower the Si element content in the steel pipe, the better the cold workability of the steel pipe. Si is usually an element that remains after the steel is deoxidized during forging. If a low Si content in the steel is required, the deoxidation method during forging of the liquid steel should be modified. Therefore, the level of deoxidation should be ensured by comprehensively controlling the Al and Ca contents so that the corresponding non-metallic inclusions do not adversely affect the fatigue resistance of the steel.On this basis, in the seamless steel pipe with high strength and toughness for an electric motor shaft according to the invention, the mass percentage of the element Si is kept such that 0 < Si ≤ 0.25%.

[0015] Of course, in some preferred embodiments, the mass percentage of the Si element may be more preferably maintained such that 0 < Si ≤ 0.20% to achieve a better performance effect. In some preferred embodiments, the mass percentage of the Si element is maintained such that 0.01 ≤ Si ≤ 0.20%. In some preferred embodiments, the mass percentage of the Si element is maintained such that 0.01 ≤ Si ≤ 0.23%.

[0016] Mn: In the seamless steel pipe with high strength and toughness for an electric motor shaft according to the present invention, increasing the content of the Mn element can improve the strength of the material. Furthermore, Mn can stabilize the P and S elements to prevent the formation of low-melting point sulfides and improve the hot workability of the material. To achieve the above-mentioned effect, the Mn element content in the steel should not be too low, otherwise the P and S elements cannot be stabilized and the desired effect cannot be achieved. Furthermore, the Mn element content should not be too high, otherwise serious deformation and hardening will be caused during cold processing, which will exacerbate mold wear.Therefore, taking into account the influence of the content of the element Mn on the properties of the steel, in the seamless steel pipe with high strength and toughness for an electric motor shaft according to the invention, the mass percentage of the element Mn is kept between 0.5% - 1.2%.

[0017] Of course, in some preferred embodiments, the mass percentage of the element Mn may be more preferably kept between 0.6% - 1.0% in order to achieve a better effect in execution.

[0018] Ti: In the seamless steel pipe with high strength and toughness for an electric motor shaft according to the present invention, an appropriate amount of Ti element can improve the hardenability of the steel. The Ti element, in combination with the elements C and N, can refine the grains. However, the Ti element content in the steel should not be too high. Otherwise, continuous casting during forging will be difficult. Therefore, in the seamless steel pipe with high strength and toughness for an electric motor shaft according to the present invention, the mass percentage of Ti element, if added, is maintained at Ti ≤ 0.045%.

[0019] Of course, in some preferred embodiments, the mass percentage of the element Ti may be more preferably kept between 0.02% - 0.04% in order to achieve a better effect in execution.

[0020] B: In the seamless steel pipe with high strength and toughness for an electric motor shaft according to the present invention, an appropriate amount of B can improve the through-hardenability and plasticity of the steel. However, an excessive content of element B can lead to brittleness of the steel at high temperatures. Therefore, in the seamless steel pipe with high strength and toughness for an electric motor shaft according to the present invention, the mass percentage of element B, if added, is maintained at B ≤ 0.0045%.

[0021] Of course, in some preferred embodiments, the mass percentage of element B may be more preferably kept between 0.002% - 0.004% in order to achieve a better effect in execution.

[0022] N: In the seamless steel tube with high strength and toughness for an electric motor shaft according to the invention, the amount of N should be controlled within an appropriate range to refine the grains together with the elements Ti and B, thereby improving the material properties. Therefore, in the seamless steel tube with high strength and toughness for an electric motor shaft according to the invention, the mass percentage of the element N is kept between 0.0040% and 0.009%.

[0023] Of course, in some preferred embodiments, the mass percentage of the element N may be more preferably kept between 0.0045% - 0.0085% in order to achieve a better effect in execution.

[0024] Al: In the seamless steel tube with high strength and toughness for an electric motor shaft according to the invention, Al serves as a deoxidizer during forging. In conjunction with elements such as Si, it can ensure the deoxidation effect in the steel, thereby ensuring the purity of the steel and thus the fatigue resistance of the material. However, care must be taken to ensure that the content of the element Al in the steel is not too high. Otherwise, abnormal aluminum oxide inclusions may form. To utilize the beneficial effects of the element Al, the mass percentage of the element Al in the seamless steel tube with high strength and toughness for an electric motor shaft according to the invention is kept between 0.015% and 0.045%.

[0025] Of course, in some preferred embodiments, the mass percentage of the element N may be more preferably kept between 0.015% - 0.035% in order to achieve a better effect in execution.

[0026] Ca and Mg: In the seamless steel tube with high strength and toughness for an electric motor shaft according to the present invention, appropriate amounts of Ca and Mg can improve the shape and properties of the non-metallic inclusions, thereby improving the fatigue resistance of the material and the like. Therefore, to utilize the advantageous effects of the elements Ca and Mg, the inventor proposes that the element Ca and / or Mg be added to the seamless steel tube with high strength, toughness, and cuttability for an electric motor shaft according to the present invention, so that the sum of the mass percentages of the elements Ca and Mg, Ca+Mg, should be kept between 0.001% and 0.006%.

[0027] Of course, in some preferred embodiments, the sum of the mass percentages of the elements Ca and Mg, Ca+Mg, may be kept more preferably between 0.001% - 0.004% in order to achieve a better effect in execution.

[0028] Furthermore, in the seamless steel pipe with high strength and toughness for an electric motor shaft according to the invention, the unavoidable impurities P ≤ 0.020%, S ≤ 0.008% and O ≤ 0.0030% apply.

[0029] Furthermore, in the seamless steel pipe with high strength and toughness for an electric motor shaft according to the invention, the unavoidable impurities P ≤ 0.015%, S ≤ 0.005% and O ≤ 0.0025% apply.

[0030] In the seamless steel tube with high strength and toughness for an electric motor shaft according to the invention, the elements P and S are impurities in the steel tube. The contents of such impurity elements in the seamless steel tube with high strength and toughness for an electric motor shaft should be kept as low as technically achievable in order to obtain a tube with better properties and high quality.

[0031] In the invention, P and S are impurities present in the raw material or auxiliary materials for the steel, or introduced during the production process. The element P can lead to embrittlement at crystal boundaries, thus impairing the toughness and workability of the material. The element S can react to form low-melting-point sulfides, which reduce the workability and mechanical properties of the steel.

[0032] Therefore, in the invention, the contents of the elements P, S, and O in the steel should be kept narrow, so that P ≤ 0.020%, S ≤ 0.008%, and O ≤ 0.0030%. Of course, in some preferred embodiments, the contents of the elements P, S, and O can be kept such that P ≤ 0.015%, S ≤ 0.005%, and O ≤ 0.0025% to achieve a better performance.

[0033] In the seamless steel pipe with high strength and toughness for an electric motor shaft according to the invention, the mass percentages of each chemical element further satisfy 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%.

[0034] Furthermore, in the seamless steel tube according to the invention with high strength and toughness for an electric motor shaft, the microstructure of the seamless steel tube is formed from ferrite and pearlite. Preferably, the seamless steel tube is normalized.

[0035] Furthermore, in the seamless steel tube according to the invention with high strength and toughness for an electric motor shaft, the microstructure of the seamless steel tube after thermal treatment by quenching and tempering is formed from martensite and residual austenite, which has a volumetric phase fraction of 1%-25%. Preferably, the volumetric phase fraction of residual austenite is 3%-24%.

[0036] Furthermore, in the seamless steel pipe with high strength and toughness for an electric motor shaft according to the invention, it is provided that the seamless steel pipe has a yield strength Rp 0,2of ≥ 400 MPa, a tensile strength R m of ≥ 550 MPa, and an expansion A 50 of ≥ 22%. Preferably, the seamless steel pipe is normalized.

[0037] Furthermore, in the seamless steel pipe with high strength and toughness for an electric motor shaft according to the invention, it is provided that the seamless steel pipe has a yield strength Rp 0,2 of ≥ 420 MPa, a tensile strength R m of ≥ 570 MPa, and an expansion A 50 of ≥ 25%. Preferably, the seamless steel pipe is normalized.

[0038] Furthermore, in the seamless steel pipe with high strength and toughness for an electric motor shaft according to the invention, it is provided that the seamless steel pipe has a yield strength Rp 0,2 of ≥ 430 MPa, a tensile strength R m of ≥ 590 MPa, and an expansion A 50 of ≥ 25%. Preferably, the seamless steel pipe is normalized.

[0039] Furthermore, in the seamless steel pipe with high strength and toughness for an electric motor shaft according to the invention, it is provided that the seamless steel pipe after the thermal treatment by quenching and tempering has the following properties: a yield strength Rp 0,2 of ≥ 1000 MPa, a tensile strength R m of ≥ 1400 MPa, an elongation A 50 of ≥ 5%, a hardness of ≥ 58 HRC, a product of strength and ductility of greater than 15000 MPa%, and a torsional strength of ≥ 300 KN.

[0040] Furthermore, in the seamless steel pipe with high strength and toughness for an electric motor shaft according to the invention, it is provided that the seamless steel pipe after the thermal treatment by quenching and tempering has the following properties: a yield strength Rp 0,2 of ≥ 1050 MPa, a tensile strength R m of ≥ 1500 MPa, an elongation A 50of ≥ 7%, a hardness of ≥ 58 HRC, a product of strength and ductility of greater than 16000 MPa%, and a torsional strength of ≥ 350 KN.

[0041] Furthermore, in the seamless steel pipe with high strength and toughness for an electric motor shaft according to the invention, it is provided that the seamless steel pipe after the thermal treatment by quenching and tempering has the following properties: a yield strength Rp 0,2 of ≥ 1100 MPa, a tensile strength R m of ≥ 1600 MPa, an elongation A 50 of ≥ 7%, a hardness of ≥ 60 HRC, a product of strength and ductility of greater than 18000 MPa%, and a torsional strength of ≥ 400 KN.

[0042] Accordingly, another object of the invention is to provide a method for producing the above-described seamless steel pipe with high strength and toughness for an electric motor shaft, in which the processes are optimized so that the production of the seamless steel pipe with high strength and toughness for an electric motor shaft according to the invention is simple, effective, easy to carry out and well applicable.

[0043] To achieve this object, the present invention also provides a method for producing the above-described seamless steel tube with high strength and toughness for an electric motor shaft. The method comprises the following steps: (1) Producing a blank; (2) heating, drilling, hot rolling and stretch reducing; (3) intermediate thermal treatment: annealing of the hot-rolled tube material in a temperature range of 650 - 800°C for a temperature holding period of 20 - 80 min; (4) cold drawing; (5) thermal treatment of the cold-drawn tube: normalizing the tube in a temperature range of 750-820°C for a temperature holding period of 20-60 minutes to obtain a microstructure of ferrite and pearlite. Preferably, the method further comprises the following step: (6) Quenching and tempering: Quenching at a quenching temperature of 820 - 880°C for a temperature holding period of 15 - 30 min, followed by water cooling, and tempering at a tempering temperature of 150 - 300°C for a temperature holding period of 20 - 40 min, followed by air cooling.

[0044] In the above technical solution according to the invention, the operator can cast the blank, in particular by forging and refining using an electric furnace or converter during the blank production process in step (1), and obtain blanks of the required size, in particular by continuous casting and cutting. To meet the requirements for the dimensional accuracy of the steel pipe and the surface condition during cold processing in the field of electric motor shafts of vehicles, the hot-rolled pipe material obtained in step (1) should be further subjected to intermediate thermal treatment, cold drawing to the required specification and dimensional accuracy, and then thermal treatment of the finished product (the cold-drawn pipe) to finally obtain a finished raw material with good cold workability.

[0045] It should be noted that annealing in a temperature range of 650 - 800°C and maintaining the temperature for 20 - 80 min during the intermediate thermal treatment in step (3) ensures the smooth running of the cold drawing in the subsequent step (4).

[0046] Furthermore, the tube produced by cold drawing in step (4) is to be normalized in a temperature range of 750-820°C during the thermal treatment of the finished product in step (5) of the invention and maintained at this temperature for 20-60 minutes. The atmosphere in the furnace is controlled to ensure no complete decarburization (i.e., the absence of areas of solid ferrite structure on the surface of the steel tube) and a partially decarburized depth of ≤ 150 µm. In this technical solution, by combining the adjustment of the normalization temperature with the control of the cooling, the production of a steel tube with a ferrite-pearlite structure can be ensured, thus obtaining a normalized tube whose strength and toughness can meet the requirements of cold working.

[0047] Furthermore, the manufacturing method according to the invention provides that in step (2) the blank is heated to 1210 - 1280°C and kept at that temperature for 30 - 150 minutes.

[0048] Furthermore, the manufacturing method according to the invention provides that in step (2) the temperature for drilling is controlled in a range of 1150 - 1250°C.

[0049] Furthermore, the manufacturing method according to the invention provides that in step (2) a tandem hot rolling takes place, in which the temperature for tandem hot rolling is controlled in a range of 1000 - 1200°C.

[0050] Furthermore, the manufacturing method according to the invention provides that in step (2) the temperature for stretch reduction is controlled in a range of 950 - 1000°C.

[0051] Furthermore, the method for production according to the invention provides that the method comprises at least one of the following steps: in step (1) the blank is produced by forging by means of an electric furnace or converter, refining and casting, and after continuous casting is cut into blanks of the required size, and that the step preferably further comprises an intermediate thermal treatment and cold drawing to the required specification with the required dimensional accuracy; in step (5) the normalized tube is not completely decarburized and the partially decarburized depth is ≤ 150 µm.

[0052] Compared with the prior art, the seamless steel pipe with high strength and toughness for an electric motor shaft according to the present invention and the method for producing the same have the following advantages and favorable effects.

[0053] The present invention achieves a novel seamless steel tube with high strength and toughness for an electric motor shaft by appropriately adjusting the components and designing the processes. The microstructure of the seamless steel tube with high strength and toughness for an electric motor shaft after thermal treatment is composed of ferrite and pearlite, and the microstructure after further thermal treatment by quenching and tempering is composed of martensite and residual austenite, which has a volumetric phase fraction of 1%-25%.

[0054] The seamless steel tube with high strength and toughness for an electric motor shaft produced according to the invention in the finished state has a yield strength Rp 0,2 of ≥ 400 MPa, a tensile strength R m of ≥ 550 MPa, and an expansion A 50of ≥ 22%. After thermal treatment by quenching and tempering, the strength of the seamless steel tube with high strength and toughness for an electric motor shaft can be further increased, e.g., it then has a yield strength Rp 0,2 of ≥ 1000 MPa, a tensile strength R m of ≥ 1400 MPa and an elongation A 50 of ≥ 5%. It has a hardness of no less than 58 HRC, a strength-ductility ratio (i.e., the product of tensile strength and elongation) of greater than 15,000 MPa%, and a torsional strength of no less than 300 kN after thermal treatment by quenching and tempering. This steel tube is particularly suitable for use in components operating under high torsional loads, such as electric motor shafts, and can be well-applied and widely used. Detailed embodiments

[0055] The seamless steel tube with high strength and toughness for an electric motor shaft according to the invention and the method for producing the same are explained and described in more detail below in connection with the specific embodiments. However, the explanation and description do not represent a limitation on the technical solutions according to the invention. Examples 1-10 and comparative example 1-2

[0056] The seamless steel pipes with high strength and toughness for an electric motor shaft according to Examples 1-10 of the invention and the comparative steel pipes according to Comparative Examples 1 and 2 were manufactured by the following steps: (1) An ingot was cast according to the mass percentages of the chemical elements shown in Table 1 by forging and refining by means of an electric furnace or converter, continuously cast, and cut into ingots of the required size. (2) Heating, drilling, hot rolling and stretch reducing: The blank was heated to 1210 - 1280°C and held for 30 - 150 min, then drilled at a high temperature between 1150 - 1250°C, and subjected to tandem hot rolling at a high temperature between 1000 - 1200°C and then stretch reducing at a controlled temperature for stretch reduction in the range of 950 - 1000°C to obtain a hot-rolled pipe material through the processing. (3) Intermediate thermal treatment: The hot-rolled tube material was annealed in a temperature range of 650 - 800°C for a temperature holding time of 20 - 80 min. (4) Cold drawing. (5) thermal treatment of the cold-drawn tube: normalizing the cold-drawn tube in a temperature range of 750 - 820°C for a temperature holding time of 20 - 60 min to obtain a microstructure of ferrite and pearlite.

[0057] It should be noted that the chemical compositions and the respective process configurations of the seamless steel pipes with high strength and toughness for an electric motor shaft according to Examples 1-10 of the invention meet the requirements of the invention. Although the comparative steel pipes according to Comparative Examples 1 and 2 were also produced using the steps described above, their chemical compositions and / or their respective process parameters do not meet the requirements of the invention.

[0058] Table 1 summarizes the mass percentages of each chemical element in the high-strength, high-toughness seamless steel tubes for electric motor shafts according to Examples 1-10 and the comparative steel tubes according to Comparative Examples 1 and 2. Table 1. (The residual amount of Fe and unavoidable impurities other than P, S, and O) number Chemical element C (wt%) Si (wt%) Mn (wt%) P (wt%) S (wt%) Ti (wt%) B (wt%) N (wt%) O (wt%) Al (wt%) Ca+Mg (wt%) Example 1 0,492 0,14 0,77 0,016 0,0045 0,0347 0,0031 0,0069 0,0022 0,0360 0,0040 Example 2 0,487 0,19 0,77 0,020 0,0050 0,0238 0,0031 0,0086 0,0017 0,0355 0,0026 Example 3 0,562 0,17 0,64 0,019 0,0014 0,0308 0,0 0,0044 0,0021 0,0165 0,0010 Example 4 0,547 0,04 1,05 0,004 0,0041 0,0408 0,0041 0,0087 0,0014 0,0356 0,0058 Example 5 0,448 0,08 0,95 0,015 0,0059 0,0321 0,0035 0,0079 0,0011 0,0445 0,0041 Example 6 0,510 0,10 0,62 0,003 0,0046 0,0221 0,0018 0,0089 0,0020 0,0202 0,0042 Example 7 0,560 0,20 0,53 0,012 0,0001 0,0 0,0 0,0051 0,0012 0,0431 0,0052 Example 8 0,449 0,01 0,85 0,009 0,0055 0,0269 0,0035 0,0056 0,0029 0,0422 0,0019 Example 9 0,465 0,15 1,18 0,003 0,0006 0,0401 0,0028 0,0069 0,0015 0,0302 0,0020 Example 10 0,404 0,23 0,78 0,009 0,0037 0,0426 0,0031 0,0079 0,001 0,0348 0,0033 Comparison example 1 0,450 0,42 0,62 0,015 0,0060 0,02 0,0042 0,0120 0,0029 0,0041 0,0021 Comparison example 2 0,682 0,28 0,83 0,012 0,0021 0,04 0,0021 0,0055 0,0015 0,0042 0,0055

[0059] Table 2 summarizes the specific process parameters used in the manufacturing steps described above for the seamless steel tubes with high strength and toughness for an electric motor shaft according to Examples 1-10 and for the comparative steel tubes according to Comparative Examples 1 and 2. Table 2. number Step (2) Step (3) Step (5) Heating temperature for blank (°C) Time for maintaining the heating temperature (min) Temperature for drilling (°C) Temperature for tandem warm water (°C) Temperature for stretch reduction (°C) Glowing temperature (°C) Time for holding the annealing temperature (min) Normalization temperature (°C) Time for maintaining normalization temperature (min) Example 1 1245 146 1152 1080 977 745 76 815 41 Example 2 1235 43 1161 1058 958 669 55 751 22 Example 3 1278 147 1200 1190 993 664 30 819 56 Example 4 1245 147 1198 1038 987 722 31 813 41 Example 5 1221 132 1249 1144 999 752 63 766 50 Example 6 1250 91 1213 1005 976 695 44 772 26 Example 7 1246 100 1217 1198 951 718 31 810 45 Example 8 1269 79 1204 1148 995 682 71 785 48 Example 9 1271 68 1162 1166 972 687 30 752 59 Example 10 1248 61 1180 1070 963 685 20 796 43 Comparison example 1 1200 149 1189 1168 972 740 29 797 28 Comparison example 2 1268 114 1221 1180 977 675 38 650 37

[0060] Samples were taken from the finished seamless high-strength, high-toughness steel tubes for an electric motor shaft according to Examples 1-10 and from the comparative steel tubes according to Comparative Examples 1 and 2, respectively. The microstructures of the steel tubes from the Examples and Comparative Examples before thermal treatment by quenching and tempering were observed, and their properties were measured to obtain the mechanical properties at normal temperature before quenching and tempering, the measurement results of which are listed in Table 3.

[0061] The means of measuring the properties in question is described below.

[0062] Tensile test: The tensile test was conducted according to GB / T 228.1-2010, Metal Material, Tensile Test, Part 1: Method of Room Temperature Tensile Test, to determine the yield strength, tensile strength, and elongation values ​​of the steel pipes from the examples and comparative examples before thermal treatment by quenching and tempering at room temperature.

[0063] Table 3 summarizes the property test results of the seamless high-strength and toughness steel tubes for an electric motor shaft according to Examples 1-10 and the comparative steel tubes according to Comparative Examples 1 and 2 before thermal treatment by quenching and tempering. Table 3. number Yield strength Rp 0,2 (MPa) Tensile strength R m (MPa) Stretch A 50 (%) Example 1 428 666 30 Example 2 485 616 28 Example 3 479 590 33 Example 4 428 573 33 Example 5 419 564 31 Example 6 417 623 33 Example 7 450 579 23 Example 8 457 632 27 Example 9 506 638 27 Example 10 431 672 30 Comparison example 1 430 603 22 Comparison example 2 515 634 29

[0064] From Table 3 above, it can be seen that the seamless steel tubes with high strength and toughness for an electric motor shaft according to Examples 1 - 10 already have excellent mechanical properties before the thermal treatment by quenching and tempering, the yield strength of which Rp 0,2 between 417 - 506 MPa, tensile strength R m between 564 - 672 MPa and elongation A 50 between 23% and 30%. It is also observed that the microstructure of the seamless steel tubes with high strength and toughness for an electric motor shaft according to Examples 1 to 10 is composed of ferrite and pearlite before thermal treatment by quenching and tempering.

[0065] To further demonstrate that the high-strength, high-toughness seamless steel tubes for an electric motor shaft according to the present invention still retain excellent properties after the quenching and tempering thermal treatment, samples were taken from each of the finished, high-strength, high-toughness seamless steel tubes for an electric motor shaft produced in Examples 1-10 and the comparative steel tubes produced in Comparative Examples 1 and 2, and subjected to quenching and tempering thermal treatment. The quenching and tempering thermal treatment was controlled such that quenching was performed at a quenching temperature of 860°C for a temperature holding period of 20 minutes, followed by water cooling; and tempering was performed at a tempering temperature of 250°C for a temperature holding period of 30 minutes, followed by air cooling.

[0066] After the thermal treatment by quenching and tempering of the steel pipe samples according to the examples and the comparative examples, the mechanical properties of the steel pipes after the thermal treatment by quenching and tempering were further measured to investigate their properties after the thermal treatment by quenching and tempering, which are summarized in the following Table 4.

[0067] When measuring the mechanical properties of the steel pipes according to Examples 1-10 and Comparative Examples 1 and 2 after the thermal treatment by quenching and tempering, the tensile test was the same as the test in Table 3 above, so that the yield strength, tensile strength, and elongation of the steel pipes according to Examples 1-10 and Comparative Examples 1 and 2 after the thermal treatment by quenching and tempering could also be determined.

[0068] In addition to the properties mentioned above, other properties of the steel pipes according to the examples and comparative examples were measured after thermal treatment by quenching and tempering. The measurement method is described below.

[0069] Hardness test: The hardness (HRC) of the steel pipes according to Examples 1 - 10 and Comparative Examples 1 and 2 after thermal treatment by quenching and tempering were measured using a Rockwell hardness tester.

[0070] Torsional strength test: The torsional strengths of the steel pipes according to Examples 1-10 and Comparative Examples 1 and 2 after thermal treatment by quenching and tempering were measured using a static torsional testing machine.

[0071] Table 4 summarizes the property test results of the seamless high-strength and toughness steel tubes for an electric motor shaft according to Examples 1-10 and the comparative steel tubes according to Comparative Examples 1 and 2 after thermal treatment by quenching and tempering. Table 4. number Yield strength Rp 0,2 (MPa) Tensile strength R m (MPa) Stretch A 50 (%) Rm × A 50 (MPa%) HRC Torsional strength (KN) Example 1 1066 1400 15 21000 64 416 Example 2 1087 1692 9 15228 62 430 Example 3 1108 1646 10 16460 60 459 Example 4 1154 1604 10 16040 61 391 Example 5 1060 1698 11 18678 61 404 Example 6 1059 1766 14 24724 63 597 Example 7 1134 1724 9 15516 59 426 Example 8 1163 1630 12 19560 60 346 Example 9 1154 1702 10 17020 64 466 Example 10 1169 1409 11 15499 60 404 Comparison example 1 1157 1404 9 12636 61 259 Comparison example 2 1024 1802 4 7208 66 287

[0072] Note: In Table 4 above, Rm × A50 represents the product of strength and ductility, specifically the product of tensile strength and elongation.

[0073] From Table 4, it can be seen that the overall properties of the seamless steel tubes with high strength and toughness for an electric motor shaft according to Examples 1-10 of the invention are significantly better than the comparative steel tubes according to Comparative Examples 1 and 2 after the thermal treatment by quenching and tempering. Compared with the properties before the thermal treatment by quenching and tempering, the yield strength and the tensile strength of the seamless steel tubes with high strength and toughness for an electric motor shaft according to Examples 1-10 are significantly increased after the thermal treatment by quenching and tempering, while the elongation A 50 decreases accordingly.

[0074] It can be seen from Table 4 that the seamless steel pipes with high strength and toughness for an electric motor shaft according to Examples 1-10 of the invention have excellent mechanical properties, the yield strength Rp of which 0,2 between 1059 - 1169 MPa, tensile strength R m between 1400 - 1766 MPa, elongation between 9% - 15%, product of strength and ductility R m × A 50 between 15499 - 24724 MPa%, hardness between 59 - 64 HRC and torsional strength between 346 - 597 KN.

[0075] In contrast to Examples 1-10, Comparative Examples 1 and 2 contain a chemical component and / or process that cannot meet the requirements of the invention. At least one of the mechanical properties of the comparative steel pipes produced according to Comparative Examples 1 and 2 may not meet the requirements of the invention. As shown in Table 4, the torsional strengths of the steel pipes according to Comparative Examples 1 and 2 are lower than those of Examples 1-10.

[0076] In addition, following the mechanical property measurements described above, samples were taken again from the steel pipes according to Examples 1-10 and Comparative Examples 1 and 2 after thermal treatment by quenching and tempering to observe and evaluate their microstructures. The results are shown in Table 5 below. According to the invention, a ZEISS optical microscope was used to observe the microstructures, and X-ray diffraction (XRD) was used to evaluate the proportion of residual austenite.

[0077] Table 5 summarizes the results of observation of the microstructures of the seamless steel tubes with high strength and toughness for an electric motor shaft according to Examples 1-10 and the comparative steel tubes according to Comparative Examples 1 and 2 after thermal treatment by quenching and tempering. Table 5. number Microstructure volumetric phase fraction of remaining austenite (%) Example 1 Martensite+remaining austenite 22 Example 2 Martensite+remaining austenite 16 Example 3 Martensite+remaining austenite 5 Example 4 Martensite+remaining austenite 18 Example 5 Martensite+remaining austenite 9 Example 6 Martensite+remaining austenite 20 Example 7 Martensite+remaining austenite 15 Example 8 Martensite+remaining austenite 3 Example 9 Martensite+remaining austenite 24 Example 10 Martensite+remaining austenite 11 Comparison example 1 Martensite+remaining austenite 7 Comparison example 2 Martensite 0

[0078] In Table 5 above, it can be seen that the microstructures of the seamless steel tubes with high strength and toughness for an electric motor shaft according to Examples 1 - 10 after the thermal treatment by quenching and tempering are formed from martensite and residual austenite, which has a volumetric phase fraction particularly between 3% - 24%.

[0079] It should be noted that the combination of the technical features herein should not be limited to the combinations specified in the claims or to the combinations specified in the specific embodiments, and that all the technical features specified herein can be freely combined or composed with each other as long as they do not conflict with each other.

[0080] It should also be noted that the above-mentioned embodiments represent only specific embodiments of the invention. Of course, the present invention is not limited to the above-mentioned embodiments. Changes or modifications that a person skilled in the art can readily derive from the disclosure of the invention or that are readily conceivable are intended to be included within the scope of the invention. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] CN 104962838 A

[0005] CN 1388834 A

[0006] Cited non-patent literature

[0000] High-strength steel, seamless steel tube with a high product of strength and ductility for a wheel axle of a vehicle and method for producing the same”, which was published on 7 October 2015

[0005]

Claims

[1] Easily processable seamless steel tube with high strength and toughness for an electric motor shaft, containing Fe and unavoidable impurity elements, characterized by that the seamless steel pipe further contains the following chemical elements in mass percentage: 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%. [2] Seamless steel tube with high strength and toughness for an electric motor shaft according to claim 1, characterized by that the mass percentages of the individual chemical elements are: 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%; and the remaining amount of Fe and unavoidable impurities, and preferably the mass percentages of the individual chemical elements are: C: 0.4 - 0.57%, Si: 0.01 - 0.23%, Mn: 0.5 - 1.2%, Ti: 0 - 0.043%, B: 0 - 0.0042%, N: 0.004 - 0.009%, Al: 0.016 - 0.045%, Ca+Mg: 0.001 - 0.006%. [3] Seamless steel tube with high strength and toughness for an electric motor shaft according to claim 1 or 2, characterized by that the unavoidable impurities P ≤ 0.020%, S ≤ 0.008% and O ≤ 0.0030% apply. [4] Seamless steel tube with high strength and toughness for an electric motor shaft according to claim 3, characterized by that the unavoidable impurities P ≤ 0.015%, S ≤ 0.005% and O ≤ 0.0025% apply. [5] Seamless steel tube with high strength and toughness for an electric motor shaft according to claim 1 or 2, characterized by that the mass percentages of the individual chemical elements further satisfy 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%. [6] Seamless steel tube with high strength and toughness for an electric motor shaft according to claim 1 or 2, characterized by that the microstructure of the seamless steel pipe is formed by ferrite and pearlite. [7] Seamless steel tube with high strength and toughness for an electric motor shaft according to claim 1 or 2, characterized by that the microstructure of the seamless steel pipe after the thermal treatment by quenching and tempering is formed from martensite and residual austenite, which has a volumetric phase fraction of 1% - 25%, preferably 3% - 24%. [8] Seamless steel tube with high strength and toughness for an electric motor shaft according to claim 1 or 2, characterized bythat the seamless steel pipe has a yield strength Rp 0,2 of ≥ 400 MPa, a tensile strength R m of ≥ 550 MPa, and an expansion A 50 of ≥ 22%. [9] Seamless steel tube with high strength and toughness for an electric motor shaft according to claim 1 or 2, characterized by that the seamless steel pipe after thermal treatment by quenching and tempering has the following properties: a yield strength Rp 0,2 of ≥1000 MPa, a tensile strength R m of ≥ 1400 MPa, an elongation A 50 of ≥ 5%, a hardness of ≥ 58 HRC, a product of strength and ductility of greater than 15000 MPa%, and a torsional strength of ≥ 300 KN. [10] A method for producing a seamless steel tube with high strength and toughness for an electric motor shaft according to any one of claims 1-9, characterized by that the procedure includes the following steps: (1) Producing a blank; (2) heating, drilling, hot rolling and stretch reducing; (3) intermediate thermal treatment: annealing of the hot-rolled tube material in a temperature range of 650 - 800°C for a temperature holding period of 20 - 80 min; (4) cold drawing; (5) thermal treatment of the cold-drawn tube: normalising the tube in a temperature range of 750 - 820°C for a temperature holding period of 20 - 60 min to obtain a microstructure of ferrite and pearlite, and preferably the method further comprises the following step: (6) Quenching and tempering: Quenching at a quenching temperature of 820 - 880°C for a temperature holding period of 15 - 30 min, followed by water cooling, and tempering at a tempering temperature of 150 - 300°C for a temperature holding period of 20 - 40 min, followed by air cooling. [11] A method of manufacturing according to claim 10, characterized bythat in step (2) the blank is heated to 1210 - 1280°C and kept at that temperature for 30 - 150 min. [12] A method of manufacturing according to claim 10, characterized by that in step (2) the temperature for drilling is controlled in a range of 1150 - 1250°C. [13] A method of manufacturing according to claim 10, characterized by that in step (2) a tandem hot rolling takes place in which the temperature for tandem hot rolling is controlled in a range of 1000 - 1200°C. [14] A method of manufacturing according to claim 10, characterized by that in step (2) the temperature for stretch reduction is controlled in a range of 950 - 1000°C. [15] A method of manufacturing according to claim 10, characterized byin step (1) the blank is produced by forging by means of an electric furnace or converter, refining and casting, and after continuous casting is cut into blanks of the required size, and that the step preferably further comprises an intermediate thermal treatment and cold drawing to the required specification with the required dimensional accuracy; and / or in step (5) the normalized tube does not have complete decarburization and the partially decarburized depth is ≤ 150 µm.