Steel for automobile high-rotating-speed motor shaft and production method thereof

Abstract

The invention relates to steel for an automobile high-rotating-speed motor shaft and a production method thereof, and belongs to the technical field of iron-based alloys. The steel comprises the following element components in percentage by mass: 0.16 to 0.22 percent of C, 0.1 to 0.2 percent of Si, 1.2 to 1.5 percent of Mn, 1.1 to 1.4 percent of Cr, 0.01 to 0.03 percent of S, 0.005 to 0.010 percent of Nb, 0.01 to 0.02 percent of V, 0.005 to 0.010 percent of Ce, 0.02 to 0.05 percent of Al, 0.010 to 0.015 percent of N, less than or equal to 0.020 percent of P, less than or equal to 0.20 percent of Cu, less than or equal to 0.20 percent of Ni, less than or equal to 0.10 percent of Mo, less than or equal to 0.001 percent of Ti, less than or equal to 0.0009 percent of O, less than or equal to 0.04 percent of As, less than or equal to 0.03 percent of Sn, less than or equal to 0.005 percent of Sb, less than or equal to 0.002 percent of Pb and the balance The technological process comprises the steps of converter or electric furnace primary smelting, ladle refining furnace (LF furnace) refining, RH furnace or VD furnace vacuum degassing, CCM (large section) continuous casting, heating forging, stack cooling, finishing, surface and internal flaw detection and packaging. And the requirements of high homogenization, long service life, high comprehensive performance and the like of the steel for the high-speed motor shaft of the automobile are met.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of iron-based alloy in metallurgy, more particularly relates to a steel for automobile motor shaft and a production method thereof. BACKGROUND

[0002] The automobile high-speed motor shaft is one of the core components of the new energy automobile driving system, mainly used for connecting the motor rotor and the transmission system, and working for a long time in the environment of high speed, high torque and high frequency vibration, transmitting torque and supporting the rotor to rotate stably, directly affecting the power output, efficiency and reliability of the motor. Its performance directly determines the limit speed, torque output and service life of the motor. With the iteration of technology, it is upgrading from a "single bearing" to a "lightweight, integrated and high-reliability" composite functional part, becoming a key breakthrough for vehicle enterprises to improve vehicle power and endurance. With the increasing demand for "high power density" and "long endurance" of new energy vehicles, higher speed requirements are put forward for the motor shaft, and the material needs to have composite properties such as high homogenization, fatigue resistance and lightweight.

[0003] In order to obtain the automobile high-speed motor shaft with high homogenization, fatigue resistance, lightweight and other composite properties, at present, domestic mainly uses CrMo, CrNiMo alloy steel material or carbon fiber resin composite material to produce such motor shaft. Since Mo, Ni alloy and carbon fiber resin composite material all belong to valuable materials, especially carbon fiber resin composite material. Its material production process is long, and the production cost is high, thereby seriously restricting the development of China's new energy automobile industry. Therefore, our company designs the composition based on low alloy steel 20MnCr5, and adopts the "vacuum degassing + continuous casting" short process technology, improves the purity of the steel, reduces the oxygen content and the content of residual harmful elements, and improves the uniformity of the steel organization and other key quality indicators, develops and produces the high-speed motor shaft steel for automobiles with high homogenization, high reliability and long service life, so as to promote the sustainable and rapid development of the automobile industry. SUMMARY

[0004] In order to obtain the automobile high-speed motor shaft with high homogenization, fatigue resistance, lightweight and other composite properties, the present application replaces the valuable alloy steel of CrMo and CrNiMo by designing the special composition of low alloy steel 20MnCr5, and adopts the "vacuum degassing + continuous casting" process with good production continuity, stable quality and short process, improves the mechanical comprehensive performance by adding appropriate amount of special trace elements; reduces the number of non-metallic inclusions and optimizes the inclusion composition to improve the purity of the steel and realize the long-life high-reliability characteristics; the steel homogeneity is improved by designing the special continuous casting process and large reduction large deformation forging process, and a new production method of the automobile high-speed motor shaft steel meeting the market demand is developed.

[0005] The technical scheme adopted by the present application is: a kind of automobile high-speed motor shaft steel, chemical composition design: according to mass percentage C:0.16~0.22%, Si:0.1~0.2%, Mn:1.2~1.5%, Cr:1.1~1.4%, S:0.01~0.03%, Nb:0.005~0.010%, V:0.01~0.02%, Ce:0.005~0.010%, Al:0.02~0.05%, N:0.010~0.015%, P≤0.020%, Cu≤0.20%, Ni≤0.20%, Mo≤0.10%, Ti≤0.001%, O≤0.0009%, As≤0.04%, Sn≤0.03%, Sb≤0.005%, Pb≤0.002%, the balance is Fe and inevitable impurities.

[0006] 1) Determination of C content C is the most basic element in steel, and also the most economical strengthening element. Among all elements, carbon has the largest ability to improve the strength of steel, and the strengthening of carbon to quenching and tempering steel is about 9 times that of chromium and 18 times that of manganese, so in order to ensure that the gear has good strength and toughness after carburizing heat treatment, the steel needs to have a suitable carbon content. The C content range of the present application is determined to be 0.16~0.22%.

[0007] 2) Determination of Si content Si is solid-solved in ferrite phase, has a strong solid solution strengthening effect, can improve the strength, elastic limit and hardenability, but at the same time reduces the plasticity and toughness of ferrite. The Si content range of the present application steel is determined to be 0.1~0.2%.

[0008] 3) Determination of Mn content Mn is an important deoxidizing element in steelmaking, Mn reacts with oxygen (O) in steel to form manganese oxide (MnO), reducing the oxygen content in steel and reducing oxidation inclusions; manganese is easily dissolved in ferrite or austenite, significantly improving the strength of steel through solid solution strengthening effect, and manganese can inhibit the growth of austenite grains, refine the microstructure of steel, produce fine grain strengthening effect, and improve the strength and toughness of steel. The Mn content range of the present application is determined to be 1.2~1.5%.

[0009] 4) Determination of Cr content Cr is a carbide forming element, which can improve the hardenability, wear resistance and corrosion resistance of steel. In addition, Cr reduces the activity of C, which can reduce the tendency of overheating and surface decarburization rate of steel, but if the Cr content is too high, it is easy to form large bulk carbides by combining with carbon in steel. This insoluble carbide reduces the toughness of steel and reduces the service life of the material. Therefore, the Cr content range of the present application is determined to be 1.1~1.4%.

[0010] 5) Determination of S content S and Mn form manganese sulfide (MnS), and the MnS is spindle-shaped or strip-shaped, can cut off the metal continuity of the steel, reduce the friction and resistance during cutting, make the cutting chip break at the same time, protect the cutting tool and reduce the wear, and significantly improve the cutting efficiency. In addition, when the steel is cold punched or cold extruded, the appropriate amount of sulfur can reduce the deformation resistance of the material, reduce the work hardening, and improve the cold working formability. The content of S in the application is determined to be 0.01-0.03%.

[0011] 6) Determination of the content of Nb and V By adding Nb and V into the steel, the CCT curve of the steel can be moved to the right and down, the stability of austenite can be increased, and most of them exist in the form of precipitates of carbides, nitrides and carbonitrides. The precipitates usually have nanoscale size, and these dispersed small size precipitates can pin the austenite grain boundary and hinder the growth of austenite grain, thereby refining the austenite grain of the steel. Through fine-grain strengthening, the strength and toughness of the steel are improved. However, excessive addition of Nb and V will cause coarsening of carbonitride, thereby reducing the strength and toughness. The content of Nb in the application is designed to be 0.005-0.010%, and the content of V is designed to be 0.01%-0.02%.

[0012] Determination of the content of Ce Ce has strong affinity with oxygen (O) in steel, can react with them to form stable oxides, effectively remove oxidized inclusions in steel, and Ce can react with common strip or chain-shaped brittle inclusions to convert them into spherical or short rod-shaped composite inclusions. The inclusions are uniformly distributed in the steel, weaken the fragmentation between the matrix, and improve the mechanical properties of the steel. In addition, Ce and C, N elements in the steel form cerium carbide or cerium carbonitride compounds, which hinder the growth of austenite grains during steel solidification and heating, and play a role in refining the grains. The content of Ce in the application is designed to be: 0.005-0.010%.

[0013] 7) Determination of the content of Al Al is added as a deoxidizing element in steel, in addition to reducing the dissolved oxygen in the molten steel, Al and N form dispersed and fine aluminum nitride which can refine the grains. However, when the content of Al is too high, large particles of Al2O3 and other brittle inclusions are easily formed during the smelting process of the molten steel, which reduces the purity of the molten steel and affects the service life of the steel. The content of Al in the application is determined to be 0.02-0.05%.

[0014] 8) Determination of the content of N N is a strong solid solution strengthening element, when N is dissolved in the iron matrix, it forms interstitial solid solution with iron atoms, hinders dislocation movement through lattice distortion, thereby significantly improving the strength of the steel; in addition, N element is also a grain refining element, it precipitates with Nb, V, C and the like in the form of nitride, etc., which can significantly improve the austenite grain stability at high temperature, and prevent high temperature austenite grain growth. The range of the nitrogen content of the present application is determined as N: 0.010-0.015%.

[0015] 9) Determination of Ti content Ti is extremely easy to combine with N element or C element to form TiN or Ti(C, N) type non-metallic inclusions during the solidification of molten steel, which is usually a "angular" blocky inclusion in form, the hardness of the inclusion is high, which seriously affects the uniformity of the structure, and stress concentration is easily caused at the corners during operation, which seriously reduces the fatigue life of the material. The range of Ti content of the present application is determined as ≤0.001%.

[0016] 10) Determination of O content Oxygen is easy to form oxide inclusions with Al in steel, which significantly deteriorates the strength, toughness, fatigue performance and process performance of the steel due to the damage to the continuity of the matrix and the stress concentration effect. A large number of fatigue life tests have shown that the reduction of oxygen content is significantly beneficial to improving the purity of the steel, especially reducing the content and size of brittle oxide inclusions. The range of O content of the present application is determined as ≤0.0009%.

[0017] 11) Determination of P content P in steel seriously causes segregation during solidification, which weakens the grain boundary bonding force, increases the toughness transition temperature, and leads to deterioration of the toughness, processing performance and fatigue performance of the steel. The range of P content of the present application is determined as ≤0.020%.

[0018] 12) Determination of As, Sn, Sb, Pb content As, Sn, Sb, Pb and other trace elements are low-melting non-ferrous metals, which exist in steel and cause soft spots on the surface of parts, so they are regarded as harmful elements in steel. The range of the content of these elements in the present application is determined as As ≤0.04%, Sn ≤0.03%, Sb ≤0.005%, Pb ≤0.002%.

[0019] The steel material of the present application has the following main technical indexes: The macrostructure of the steel material of the present application is rated according to ASTM E381 for steel macrostructure, S ≤1, R ≤1, C ≤1, and shrinkage holes, cracks and subsurface bubbles are not allowed.

[0020] The steel material is tested for austenite grain size according to ASTM E112, and after holding at 960 DEG C for 4 hours and water quenching, the grain size is greater than or equal to 7 levels.

[0021] The end quenching property of the steel material is tested according to GB / T 225, and the heat treatment system is: normalizing at 920 DEG C ± 10 DEG C, holding for 1 hour, and air cooling; end quenching at 900 DEG C ± 5 DEG C, water cooling, and J10: 40-45HRC.

[0022] The sampling position and preparation of the mechanical property sample of the steel material are performed according to GB / T 2975, the sample is made of a diameter φ15mm heat treatment blank, the longitudinal mechanical property of the steel material is measured, the heat treatment process is: 860 DEG C ± 10 DEG C oil cooling; tempering at 170-200 DEG C air cooling. According to GB / T 228, the tensile strength (Rm) is greater than or equal to 1600MPa, and the yield strength (ReL) is greater than or equal to 1300MPa.

[0023] The non-metallic inclusions of the steel material are tested according to the K method of DIN 50602 standard, and K4(O) is less than or equal to 1.0.

[0024] The steel material is tested for contact fatigue strength according to GB / T 14229, under the conditions of room temperature (20 DEG C ± 5 DEG C) and 3000MPa contact stress, data statistics and analysis are performed, and finally the contact fatigue cycle of the steel material is required to be greater than or equal to 9*10 7 Times.

[0025] The present application is a production method of a steel for a high-speed motor shaft of an automobile, and the process flow is: converter or electric furnace primary refining→ ladle refining furnace (LF furnace) refining→ RH furnace or VD furnace vacuum degassing→ continuous casting CCM (large section)→ heating and forging→ stack cooling→ finishing→ surface and internal flaw detection→ packaging.

[0026] The main steps are as follows: (1) Molten steel smelting: Converter or electric furnace primary refining: high-quality molten iron, scrap steel and raw materials are added into the converter or electric furnace for primary refining, oxygen is blown into the top of the furnace mouth for oxidation reaction, nitrogen is blown into the bottom for stirring, 20-30 cubic meters of oxygen and 3-5 cubic meters of nitrogen are blown into per ton of steel, and active slagging agent (CaO-MgO-CaF2) is added to remove harmful elements P and Ti. The end point carbon of the primary refining furnace is 0.08%-0.12% when tapping, the tapping temperature is greater than or equal to 1630 DEG C, and the slag is blocked during tapping. Al iron (100-150Kg) is added for pre-deoxidization before tapping, and part of the alloy (primary composition) is added. After the tapping is completed, it is quickly lifted to the refining LF furnace for smelting.

[0027] Ladle refining: in the LF refining, carbon powder, ferrosilicon particles are floated into the molten steel surface for diffusion deoxidation, and Al particles are added for precipitation deoxidation. CaO-SiO2-Al2O3 composite white slag agent is added to react with harmful elements in the steel, so that they are effectively removed. During the smelting process, first, carbon powder (30-50 Kg), ferrosilicon particles (50-80 Kg) are floated into the molten steel, then Al particles (50-100 Kg) are added, and then CaO-SiO2-Al2O3 composite white slag agent (200-250 Kg) is added. Cover the ladle cover, connect the nitrogen gas at the bottom of the ladle, then insert the electrode into the slag for submerged arc power supply. Every 10-15 min, power off to measure the temperature of the molten steel and take samples for analysis. According to the target requirements, add the required main elements (Ce, V, Nb, etc.). The number of refining furnace temperature sampling is controlled at 2-3 times, until the composition meets the product requirements. The refining time is ≥45 min, and the molten steel soft blowing time is ≥10 min.

[0028] Vacuum degassing: In the RH or VD vacuum degassing, the maximum vacuum degree in the vacuum furnace is ≤100 bar, and the molten steel vacuum circulation treatment time is ≥20 min to ensure that harmful gases in the steel are effectively removed. After vacuum treatment, argon is blown into the bottom of the ladle, and the argon flow is controlled so that the molten steel is not exposed to the air. The molten steel soft blowing time is ≤10 min, while feeding 80-100 m of silicon-calcium wire to convert Al2O3 or MgO·Al2O3 in the molten steel into lower melting point calcium aluminate and composite inclusions, further removing harmful inclusions.

[0029] (2) Continuous casting: The whole process is protected by argon to prevent secondary pollution and oxidation of the molten steel; preferably, the continuous casting adopts large section φ600mm and above section, adopts light press-down control technology (press-down amount 20mm-25mm), and the pouring speed is 0.7-0.8m / min; appropriate water flow ratio to steel flow (0.5-0.7L / kg) is adopted, and intermediate ladle and pouring end double linkage electromagnetic stirring are used to break the columnar crystal zone bridging during continuous casting billet solidification; through the above various control technologies, the steel organization is more dense and uniform.

[0030] (3) Heating forging: The continuous casting billet is sawn into 2-2.5m, and is sent to a heating furnace in a neutral or weak oxidizing atmosphere (the temperature is controlled at 1100-1200℃, and the total heating time is ≥4.5h). After discharging, it is subjected to 3 upsetting (deformation amounts are 30%-40%, 20%-25%, and 25%-35% respectively), and then enters the drawing machine to draw into φ20mm-φ60mm round bar, and is stacked for cooling after discharging.

[0031] (5) Finishing: including straightening, chamfering and other finishing processes to ensure that the size, bending degree and other indicators meet the requirements.

[0032] (6) 100% non-destructive testing is carried out on the surface and inside. Only products that pass the inspection can be considered qualified products.

[0033] The advantages of this invention are: 1. This invention replaces the precious alloy steels CrMo and CrNiMo with a unique composition of low-alloy steel 20MnCr5, and combines this with the synergistic effect of trace elements to obtain high-performance steel. Secondly, this invention adopts a "vacuum degassing + continuous casting" process with good production continuity, stable quality, and short process, which greatly reduces production costs. At the same time, it optimizes and improves the smelting process, reduces the number of non-metallic inclusions and optimizes the composition of inclusions, improves the purity of steel, and achieves the characteristics of long service life and high reliability. Finally, by designing a unique continuous casting process and a large-reduction and large-deformation forging process, the homogeneity of the steel is improved, and the steel structure is made uniform and fine.

[0034] 2. Special smelting process: (1) Converter or electric furnace: High-quality molten iron, scrap steel and raw materials are added to the converter or electric furnace for primary refining. Oxygen is blown into the top of the furnace mouth for oxidation reaction, and nitrogen is blown into the bottom for stirring. 20-30 cubic meters of oxygen and 3-5 cubic meters of nitrogen are blown into each ton of steel, and active slag-forming agent (CaO-MgO-CaF2) is added to remove harmful elements P and Ti. When tapping steel, Al iron (100-150Kg) is added first for pre-deoxidation, and then some alloy is added (initial composition adjustment).

[0035] (2) Refining furnace and vacuum degassing furnace: During LF refining, carbon powder and ferrosilicon particles are floated on the surface of molten steel for diffusion deoxidation. At the same time, Al particles are added for precipitation deoxidation. CaO-SiO2-Al2O3 composite white slag agent is added to react with harmful elements in the steel to effectively remove them. During RH or VD vacuum degassing, the highest vacuum degree in the vacuum furnace is ≤100 bar. The vacuum circulation treatment time of molten steel is maintained at ≥20 min to ensure that harmful gases in the steel are effectively removed. After the vacuum treatment is completed, the soft blowing time of molten steel is ≤10 min. At the same time, 80-100 m of silicon-calcium wire is fed in to convert Al2O3 or MgO·Al2O3 in the molten steel into calcium aluminates and composite inclusions with lower melting points, further removing harmful inclusions.

[0036] (3) Argon gas is used for protection during the entire casting process to prevent secondary pollution and oxidation of molten steel. Preferably, the continuous casting adopts a large cross section of φ600mm or above, and adopts light pressure control technology (pressure reduction of 20mm to 25mm) and casting speed of 0.7 to 0.8m / min. An appropriate steel flow ratio of water (0.5 to 0.7L / kg) is adopted, and the tundish and the casting end are used for dual linkage electromagnetic stirring to break the bridging of columnar crystal zones during the solidification of the continuous casting billet. Through the above control technologies, the steel structure is made more dense and uniform.

[0037] 3. Special forging process: The continuous casting billet is sawed into 2-2.5 m, and is sent to a heating furnace in a neutral or weak oxidizing atmosphere by a lifting appliance (temperature is controlled at 1100-1200 DEG C, and total heating time is greater than or equal to 4.5 h). After being discharged, the billet is passed through 3 pads (deformation amounts are 30%-40%, 20%-25%, and 25%-35% respectively), and then is drawn into a round bar with a diameter of 20-60 mm by a drawing machine, and is stacked for cooling after being discharged. DETAILED DESCRIPTION

[0038] The application will be further described in detail below in combination with examples, which are exemplary and intended to explain the application, and cannot be understood as limiting the application.

[0039] The chemical composition (wt%) of the automobile high-speed motor shaft steel according to the embodiments of the application is shown in Table 1 and Table 2.

[0040] Table 1

[0041] Table 2

[0042] Table 3 Non-metallic inclusions of the steel materials of the examples

[0043] Table 4 Macroscopic data of the steel materials of the examples

[0044] Table 5 Grain size data of the steel materials of the examples

[0045] Table 6 Mechanical property data of the steel materials of the examples

[0046] Table 7 End quenching property data of the steel materials of the examples

[0047] Table 8 Contact fatigue life data of the steel materials of the examples

[0048] The application is a production method of an automobile high-speed motor shaft steel, and the process flow is converter or electric furnace primary refining→ ladle refining furnace (LF furnace) refining→ RH furnace or VD furnace vacuum degassing→ continuous casting CCM (large section)→ heating forging→ stacking for cooling→ finishing→ surface and internal flaw detection→ packaging.

[0049] Specifically, high-quality molten iron, scrap steel and raw materials are added into a converter or an electric furnace for primary refining, oxygen is blown into the top of the furnace mouth for oxidation reaction, nitrogen is blown into the bottom for stirring, 20-30 cubic meters of oxygen and 3-5 cubic meters of nitrogen are blown into per ton of steel, and 3-3.5 tons of active slagging agent (CaO-MgO-CaF2) is added to remove harmful elements P and Ti, the end-point carbon of the primary refining furnace is 0.08%-0.12% when tapping, the tapping temperature is greater than or equal to 1630 DEG C, and the tapping is performed by blocking slag, Al iron (100-150 Kg) is added first for pre-deoxidization, and then part of the alloy (primary composition) is added, after the tapping is completed, the molten steel is quickly lifted to the refining LF furnace; in the LF refining furnace, carbon powder (30-50 Kg), silicon iron particles (50-80 Kg) are first floated onto the surface of the molten steel for diffusion deoxidization, Al particles (50-100 Kg) are added for precipitation deoxidization, CaO-SiO2-Al2O3 composite white slag agent (200-250 Kg) is added to react with harmful elements in the steel, a ladle shield is covered, nitrogen is connected at the bottom of the ladle (flow rate 300 L / min), an electrode is inserted into the slag for submerged arc power supply, the power supply is stopped every 10-15 min to measure the temperature of the molten steel and take samples for analysis, the required main elements (Ce, V, Nb, etc.) are added according to the target requirements, the number of times of temperature measurement and sampling in the refining furnace is controlled to be 2-3 times, until the composition reaches the product requirements. The refining time is greater than or equal to 45 min, and the soft blowing time of the molten steel is greater than or equal to 10 min; When RH or VD vacuum degassing is performed, the maximum vacuum degree in the vacuum furnace is less than or equal to 100 bar, the molten steel is kept in vacuum circulation for greater than or equal to 20 min, harmful gases in the steel are effectively removed, after the vacuum treatment is completed, argon is blown into the bottom of the ladle, the argon flow rate is controlled so that the molten steel is not exposed to air, the soft blowing time of the molten steel is less than or equal to 10 min, and 80-100 m of silicon-calcium wire is fed at the same time; continuous casting is performed on a large section of greater than or equal to φ600 mm, light press-down control technology (press-down amount 20-25 mm) is used, the pouring speed is 0.7-0.8 m / min, and the ratio of the steel flow to the water amount (0.5-0.7 L / kg); the continuous casting billet is sawn into 2-2.5 m, is sent to a heating furnace in a neutral or weak oxidizing atmosphere by a lifting device (the temperature is controlled to be 1100-1200 DEG C, and the total heating time is greater than or equal to 4.5 h). After being discharged, the billet is subjected to 3 pads (deformation amounts are 30%-40%, 20%-25% and 25%-35%, respectively), and then is drawn into a round bar with a diameter of φ20 mm-φ60 mm by a drawing machine, is cooled to room temperature by piling, and then is subjected to subsequent flaw detection and finishing.

[0050] As shown in Tables 1, 2, 3, 4, 5, 6, 7 and 8, the steel for automobile high-speed motor shafts in the embodiments of the present application has stable and excellent performance in various indexes, the macroscopic quality, hardenability, microstructure density, mechanical properties and contact fatigue life of the present application all meet the requirements of the steel for automobile high-speed motor shafts.

Claims

1. A type of steel for high-speed motor shafts in automobiles, characterized in that: The chemical composition by mass percentage is as follows: C: 0.16–0.22%, Si: 0.1–0.2%, Mn: 1.2–1.5%, Cr: 1.1–1.4%, S: 0.01–0.03%, Nb: 0.005–0.010%, V: 0.01–0.02%, Ce: 0.005–0.010%, Al: 0.02–0.05%, N: 0.010–0.015%, P≤0.020%, Cu≤0.20%, Ni≤0.20%, Mo≤0.10%, Ti≤0.001%, O≤0.0009%, As≤0.04%, Sn≤0.03%, Sb≤0.005%, Pb≤0.002%, with the balance being Fe and unavoidable impurities.

2. The steel for high-speed motor shafts in automobiles according to claim 1, characterized in that: According to ASTM E381, the low-magnification microstructure of steel should meet the following requirements: S≤1, R≤1, C≤1, and there should be no shrinkage cavities, cracks, or subcutaneous bubbles. Non-metallic inclusions should be tested according to the K method of DIN 50602, with K4(O)≤1.

0.

3. The steel for high-speed motor shafts in automobiles according to claim 1, characterized in that: The austenite grain size was tested according to ASTM E112. After holding at 960℃ for 4 hours, the grain size was water quenched and the grain size was ≥7.

4. The steel for high-speed motor shafts in automobiles according to claim 1, characterized in that: The end-hardenability was tested according to GB / T225. The heat treatment regime was: normalizing at 920±10℃, holding for 1 hour, and air cooling; end-quenching at 900±5℃, and water cooling, with J10 at 40~45HRC. The sampling location and preparation of mechanical property specimens were performed according to GB / T 2975. Specimens were prepared from heat-treated blanks with a diameter of φ15mm. The longitudinal mechanical properties of the steel were determined. The heat treatment process was: oil cooling at 860℃±10℃; tempering at 170~200℃ and air cooling. The tests were conducted according to GB / T 228, with tensile strength Rm≥1600MPa and yield strength ReL≥1300MPa. The contact fatigue strength test was performed according to GB / T 14229. Under room temperature (20℃±5℃) and contact stress of 3000MPa, the contact fatigue cycle count of the steel was ≥9×10⁻⁶. 7 Second-rate.

5. A method for producing the steel for the high-speed motor shaft of an automobile as described in claim 1, characterized in that: The steps include: I. Steelmaking: including primary refining, refining and vacuum degassing. Primary refining involves adding steelmaking raw materials and auxiliary materials to a converter or electric furnace for primary refining. Oxygen is blown into the top of the furnace mouth for oxidation reaction, and nitrogen is blown into the bottom for stirring. Active slag-forming agent CaO-MgO-CaF2 is added to remove harmful elements P and Ti. Slag blocking is used when tapping. Deoxidizer is added first for pre-deoxidation when tapping, and then some alloy initial adjustment composition is added. After tapping, the steel is quickly hoisted to the refining furnace. Steel ladle refining furnace refining: First, deoxidizer is floated onto the surface of the molten steel for diffusion deoxidation, and deoxidizer is added at the same time for precipitation deoxidation. CaO-SiO2-Al2O3 composite white slag agent is added to react with harmful elements in the steel. The steel ladle is covered with a protective cover, and nitrogen gas is connected to the bottom of the steel ladle. Then, electrodes are inserted into the slag for submerged arc energization. The molten steel is measured and sampled for analysis by intermittent power interruption. The required alloying elements are added according to the target requirements. The refining time is ≥45min, and the soft blowing time of molten steel is ≥10min. Vacuum degassing: Remove harmful gases. After vacuum treatment, argon is blown into the bottom of the ladle. The argon flow rate is controlled so that the molten steel is not exposed to the air. The soft blowing time of the molten steel is ≤10min. At the same time, silicon-calcium wire is fed in to convert Al2O3 or MgO·Al2O3 in the molten steel into calcium aluminates and composite inclusions with lower melting points, further removing harmful inclusions. II. Continuous casting: The continuous casting process is used to pour molten steel into continuously cast round billets with a cross section of φ600mm or larger.

3. Heating and forging: The continuously cast billet is sawn and cut into blanks, and then sent to a heating furnace with a neutral or weakly oxidizing atmosphere for heating. After exiting the furnace, it is upset three times and then drawn into a drawing machine. After drawing, it is cooled by water in a water tank to further refine the microstructure. Finally, it is drawn into a round bar and then cooled on the production line.

6. The method for producing steel for high-speed motor shafts of automobiles according to claim 5, characterized in that: The primary refining process involves adding high-quality molten iron, scrap steel, and raw materials together to a converter or electric furnace for primary refining. 20-30 cubic meters of oxygen and 3-5 cubic meters of nitrogen are blown into each ton of steel. The final carbon content at the end of the primary refining furnace is 0.08%-0.12%, and the tapping temperature is ≥1630℃. 100-150 kg of Al iron is added before tapping for pre-deoxidation.

7. The method for producing steel for high-speed motor shafts of automobiles according to claim 5, characterized in that: The refining process involves adding 30-50 kg of carbon powder and 50-80 kg of ferrosilicon particles to the surface of the molten steel for diffusion deoxidation, while simultaneously adding 50-100 kg of Al particles for precipitation deoxidation. Then, 200-250 kg of CaO-SiO2-Al2O3 composite white slag agent is added to react with harmful elements in the steel. The ladle is then covered with a protective cover, and nitrogen gas is introduced to the bottom of the ladle at a flow rate of 300 L / min. Electrodes are then inserted into the slag for submerged arc energization. The power is stopped every 10-15 minutes to measure the temperature of the molten steel and take samples for analysis. The required alloying elements are added according to the target requirements. Temperature measurements and sampling in the refining furnace are controlled to be performed 2-3 times until the composition meets the product requirements.

8. The method for producing steel for high-speed automotive motor shafts according to claim 5, characterized in that: During RH or VD vacuum degassing, the maximum vacuum level in the vacuum furnace is ≤100 bar, and the molten steel is kept in vacuum circulation for ≥20 min. After vacuum treatment, the molten steel is soft-blown for ≤10 min, and 80-100 m of silicon-calcium wire is fed in at the same time.

9. The method for producing steel for high-speed motor shafts of automobiles according to claim 5, characterized in that: Argon gas is used for continuous casting to form round billets. Light reduction control technology is adopted: reduction amount is 20mm to 25mm, casting speed is 0.7 to 0.8m / min, steel flow ratio is 0.5 to 0.7L / kg, and dual-linkage electromagnetic stirring is used in the tundish and at the end of casting to break the bridging of columnar crystal zones during the solidification of the continuous casting billet.

10. The method for producing steel for high-speed motor shafts of automobiles according to claim 5, characterized in that: The continuously cast billet is sawn into 2-2.5m lengths and transported to a heating furnace with a neutral or weakly oxidizing atmosphere via a hoist. The temperature is controlled at 1100-1200℃, and the total heating time is ≥4.5h. After exiting the furnace, the billet undergoes three upsetting processes with deformation amounts of 30%-40%, 20%-25%, and 25%-35%, respectively. After drawing, the billet is cooled by water in a water tank and finally drawn into round bars with a diameter of φ20mm-φ60mm, which are then cooled in a stack after being removed from the production line.

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