High frequency quenched steel for automotive turbine shaft and method of manufacturing the same

Abstract

This invention relates to a high-frequency quenched steel for automotive turbine shafts and its manufacturing method, belonging to the field of special steel smelting. The steel's chemical composition by weight percentage is: C: 0.43–0.47%, Si: 0.10–0.25%, Mn: 0.45–0.65%, Cr: 1.80–2.00%, P: ≤0.020%, S: 0.018–0.028%, Al: 0.015–0.035%, Cu: 0.05–0.06%, Ni: 0.05–0.06%, Mo: 0.05–0.06%, Ca ≤0.0005%, with the balance being Fe and unavoidable impurities. Manufacturing process: converter primary refining → refining → billet casting → billet heating in heating furnace → rough rolling → intermediate rolling → finish rolling into round steel → water cooling after rolling → high-pressure nitrogen air cooling and conveying → surface peeling of round steel → surface high-frequency induction hardening → low-temperature tempering in nitrogen atmosphere holding furnace, to obtain a high-frequency quenched steel for automotive turbine shafts. The center of the round steel has a pearlite and ferrite structure with an actual grain size of 7.0-8.0 grade; the hardness within 2mm of the round steel surface is 57-62 HRC, the round steel surface has no soft spots, and the hardness at the center of the round steel is 23-26 HRC.

Description

Technical Field

[0001] This invention belongs to the technical field of special steel smelting and rolling, specifically relating to a high-frequency quenched steel for automotive turbine shafts and its manufacturing method. Background Technology

[0002] The turbine shaft, located inside a car engine, plays a crucial role in transmitting engine power. During operation, the turbine shaft can reach speeds exceeding 160,000 rpm. During this high-speed rotation, its surface experiences significant friction, while the center of the turbine shaft bears substantial pressure. Traditional automotive turbine shafts typically use steel with uneven surface hardness and some soft spots. Furthermore, the steel's microstructure at the center is not dense, exhibiting poor grain uniformity and coarse grains. Therefore, after prolonged operation, the durability of both the surface and center of the traditional turbine shaft is poor. Fatigue failure first occurs at the surface and center, leading to a precipitous drop in operational stability. This results in vibration during transmission, ultimately causing fatigue fracture of the turbine shaft. Summary of the Invention

[0003] This invention develops a steel for automotive turbine shafts from the perspective of chemical composition and manufacturing method, and obtains a high-frequency quenched steel for automotive turbine shafts. The steel has good surface hardness uniformity and no soft spots on the surface. The steel has a dense structure at the center, and the actual grains at the center are small and uniform, which can significantly improve the durability, stability and fatigue life of automotive turbine shafts.

[0004] The round steel obtained in this application has a center porosity of ≤0.5 grade; the actual grain size at the center of the round steel is grade 7.0-8.0; there is no decarburization on the surface of the round steel; the hardness within 2mm of the surface of the round steel is 57-62HRC; there are no soft spots on the surface of the round steel; and the hardness at the center of the round steel is 23-26HRC.

[0005] The technical solution adopted by the present invention to solve the above problems is as follows: a high-frequency quenched steel for automotive turbine shafts, wherein the chemical composition of the steel by weight percentage is C: 0.43-0.47%, Si: 0.10-0.25%, Mn: 0.45-0.65%, Cr: 1.80-2.00%, P: ≤0.020%, S: 0.018-0.028%, Al: 0.015-0.035%, Cu: 0.05-0.06%, Ni: 0.05-0.06%, Mo: 0.05-0.06%, Ca ≤0.0005%, and the balance is Fe and unavoidable impurities.

[0006] The main functions and design basis of each chemical element in the steel of this invention are as follows:

[0007] C: Carbon is the most important element affecting the surface hardness of steel after high-frequency quenching. As the carbon content increases, the surface hardness of the steel increases, effectively improving its wear resistance. Simultaneously, the carbon content of the steel in this invention also affects the center hardness of the round steel. With increasing carbon content, the hardness at the center of the round steel increases after rolling and cooling. However, the center of the automotive turbine shaft needs sufficient toughness, and excessively high carbon content is detrimental to the toughness of the center of the round steel. The fluctuation range of carbon content has a significant impact on the surface hardness range of the steel after high-frequency quenching. Therefore, the fluctuation of carbon content should be controlled within a narrow range. The carbon content range of the steel in this invention is C: 0.45~0.47%.

[0008] Adding a small amount of ferrosilicon in the early stages of refining can serve as a pre-deoxidation agent. Therefore, the silicon content in this invention is selected within the range of 0.10–0.25%.

[0009] Mn: Adding appropriate amounts of manganese and sulfur during the refining process can significantly improve the machinability of steel by precipitating manganese sulfide during continuous casting solidification. Furthermore, adding an appropriate amount of manganese can also increase the wear resistance of the steel surface. Therefore, the manganese content selected in this invention is in the range of 0.45% to 0.65%.

[0010] Cr: Sufficient chromium alloying element is added to the steel of this invention. The addition of chromium alloying element can effectively improve the hardenability of the steel, thereby increasing the hardness of the center position of the round steel after nitrogen air cooling. Therefore, the chromium content of this invention is selected in the range of Cr: 1.80~2.00%.

[0011] P: Phosphorus is a harmful element in the steel of this invention. The presence of phosphorus increases the cold brittleness of the steel. Therefore, the phosphorus content in the steel of this invention is controlled to be P:≤0.020%.

[0012] S: Adding appropriate amounts of manganese and sulfur during the refining process can significantly improve the machinability of steel by releasing manganese sulfide during continuous casting solidification. Therefore, the sulfur content should be selected within the range of 0.018–0.028%.

[0013] In the refining process, sulfur wire is first fed into the molten steel. After the sulfur wire is fed in, the calcium in the molten steel combines with sulfur to form high-melting-point non-metallic inclusions, CaS. Then, aluminum wire is fed into the molten steel for precipitation and deoxidation. During the deoxidation process, a large amount of non-metallic inclusions, Al2O3, are formed in the molten steel. The argon flow rate is increased to a stirring effect (argon flow rate parameter: 180-200 L / min). The numerous fine, dispersed Al2O3 inclusions in the molten steel collide with the CaS in the steel and are adsorbed by the Al2O3 inclusions. The CaS adheres to the surface of the Al2O3 inclusions and floats to the slag layer along with the Al2O3 inclusions, effectively removing the CaS inclusions from the steel and thus reducing the calcium content in the molten steel. In this invention, adding aluminum to the steel can effectively remove CaS inclusions from the steel, reduce the calcium content in the molten steel, and simultaneously deoxidize the steel. Therefore, the aluminum content range selected in this invention is Al: 0.015-0.035%.

[0014] Cu, Ni, Mo: Residual copper, nickel, and molybdenum in scrap steel have a certain impact on the stability of the hardness at the center of the round steel after nitrogen air cooling. To reduce the fluctuation of the hardness at the center of the round steel after nitrogen air cooling and to improve the hardness at the center, this invention selects to add a small amount of copper plate, nickel plate, and ferromolybdenum to the steel at the end of the refining process to fine-tune the chemical composition of the molten steel. This ensures that the center of the round steel has sufficiently high hardness after nitrogen air cooling, and that the hardness at the center can be stably controlled within a narrow range, reaching 23-26 HRC. Considering that copper, nickel, and molybdenum are all precious metal elements, to reduce steel production costs, a small amount of copper, nickel, and molybdenum are added to the steel of this invention, ensuring that the copper, nickel, and molybdenum content in the steel can be stably controlled within a narrow range. Therefore, the selection range of copper, nickel, and molybdenum elements in this invention is: Cu: 0.05-0.06%, Ni: 0.05-0.06%, Mo: 0.05-0.06%.

[0015] Ca: The steel of this invention is a sulfur-containing steel. Residual calcium in the steel will combine with sulfur to form non-metallic inclusions such as calcium sulfide, reducing the purity of the molten steel and affecting the fatigue life of the high-frequency quenched automotive turbine shaft steel. Therefore, to improve the fatigue life of the high-frequency quenched automotive turbine shaft, the Ca content of the steel of this invention must be controlled to ≤0.0005%.

[0016] The manufacturing method of the above-mentioned high-frequency quenched steel for automotive turbine shafts includes the following process steps:

[0017] Step 1: Steel Smelting and Casting: A converter is used for primary and secondary refining. The primary refining process uses high-quality molten iron and high-quality scrap steel, with the Cu, Ni, and Mo content in the scrap steel all ≤0.05%. During refining, ferrosilicon, ferrochrome, and ferromanganese alloys are added sequentially to adjust the chemical composition of the steel. Then, sulfur wire is fed into the molten steel. After the sulfur wire is added, the calcium in the molten steel combines with the sulfur to form CaS. Simultaneously, aluminum wire is fed into the molten steel for precipitation and deoxidation. During the deoxidation process, a large amount of non-metallic inclusions, Al2O3, are formed in the molten steel. Argon gas is used to stir the molten steel. The argon flow rate is 180-200 L / min. The numerous fine, dispersed Al2O3 inclusions in the molten steel are stirred by the argon gas, causing them to collide with and be adsorbed by the CaS in the steel. These inclusions then float to the slag layer along with the Al2O3 inclusions, effectively removing the CaS inclusions and thus reducing the calcium content in the molten steel to ≤0.0005%. At the end of the refining process, small amounts of copper, nickel, and ferromolybdenum are added to finely adjust the chemical composition of the molten steel. Finally, the molten steel is cast into billets.

[0018] Step 2, Rolling: The billet is heated in the furnace. When the temperature at the center of the billet reaches 1150-1200℃, it is held at this temperature for 2-3 hours. The continuous casting billet is rolled by roughing, intermediate rolling and finishing rolling. The roughing rolling adopts large reduction rolling. The cumulative reduction rate of roughing rolling is controlled at 60-65%. The roughing rolling is carried out in the plastic temperature range of 1110-1140℃. After roughing rolling, it is rolled into round steel with φ20-30mm by intermediate rolling and finishing rolling. The final rolling temperature is not lower than 900℃.

[0019] The roughing temperature of this application should be selected from the optimal plasticity temperature range of the steel of this invention. The optimal roughing plasticity temperature range is determined by using the same rolling pressure and the same heat number to roll billets in different temperature ranges. The temperature ranges of the billets are 900-930℃, 930-960℃, 960-990℃, 990-1020℃, 1050-1080℃, 1080-1110℃, 1110-1140℃, 1140-1170℃, and 1170-1200℃. After roughing, the elongation of the billets in different temperature ranges is measured. The temperature range with the highest elongation is the optimal roughing plasticity temperature range of the steel of this invention, which is 1110-1140℃. The roughing is carried out with a large reduction and in the high plasticity temperature range, resulting in a uniform and fine austenite structure at the center of the material after roughing.

[0020] Step 3: Post-rolling cooling: The high-temperature round steel after rolling is rapidly cooled to 720-740℃ within 5 seconds by water cooling. Rapid water cooling preserves the fine austenite structure formed in the center after rough rolling, providing suitable metallographic structure and temperature conditions for subsequent phase transformation. At the same time, it avoids the steel from staying in a high-temperature environment for a long time and inhibits the formation of iron oxide scale on the steel.

[0021] After water cooling, the round steel bars are transported via a conveyor line at a speed of 0.2 m / s or higher. Nitrogen gas is blown upwards from the bottom of the conveyor line to cool the round steel bars, with a pressure of 1.0-1.2 kPa and an air volume of 5-6 m³ / s. 3 / s, preferably control the steel cooling rate: 10-12℃ / s, high pressure nitrogen cools the center of the round steel to 200-230℃, during the nitrogen cooling process, the fine austenite structure in the center of the round steel is transformed into a uniform fine pearlite and ferrite structure, with an actual grain size of 7.0-8.0 grade; after cooling, the surface of the round steel is peeled off to remove the decarburized layer;

[0022] Step 4: After surface peeling, the round steel is surface quenched using high-frequency induction heating. The temperature within 2mm of the surface of the round steel is heated to 880-890℃, while the temperature at the center of the round steel is ≤230℃. Then, a high-pressure water ring is immediately used to spray water quench and cool the round steel. A circular baffle is set between the high-pressure water ring and the high-frequency induction quenching device to prevent water from splashing onto the surface of the steel being induction heated during the high-pressure water ring spraying process, thereby preventing the formation of soft spots on the steel surface. The round steel after high-frequency induction quenching is immediately placed in a holding furnace for low-temperature tempering. An inert atmosphere is used in the holding furnace.

[0023] In one implementation method, in step two, the roughing process is performed in three passes, and the cumulative compression ratio of the three passes is 60-65%.

[0024] In one implementation method, in step three, the decarburized layer depth on the surface of the round steel during the rolling and post-rolling cooling process is ≤0.12mm, and the peeling depth on the surface of the round steel is 0.12mm.

[0025] In one implementation method, in step four, when the peeled round steel is subjected to high-frequency induction heating, the current frequency of the high-frequency induction heating is set to 60KHz, and the high-frequency induction heating time is 8-10s.

[0026] In one implementation method, in step four, the round steel after high-frequency induction hardening is immediately placed in a holding furnace at a temperature of 230°C with a nitrogen protective atmosphere for low-temperature tempering for more than 2 hours.

[0027] Compared with the prior art, the advantages of the present invention are as follows:

[0028] This invention targets high-frequency quenched automotive turbine shaft steel. The steel composition is medium carbon, low calcium, with the addition of small amounts of copper, nickel, and molybdenum to stabilize the residual elements copper, nickel, and molybdenum. A rational steelmaking process effectively reduces the harmful element calcium in the molten steel, controls the total surface area reduction in the three passes of the rough rolling process, and ensures that the entire rough rolling process is carried out within the optimal rough rolling plasticity temperature range. Rough rolling refines the austenite structure at the center of the material, resulting in a uniform and fine austenite structure. Rapid water cooling of high-temperature round steel after rolling is to preserve the fine austenite structure in the center after rough rolling, and at the same time inhibit the formation of iron oxide scale on the steel surface. After water cooling, the center of the steel is cooled to 200-230℃ by high-pressure nitrogen air cooling, so that the fine austenite structure in the center is transformed into fine and uniform ferrite and pearlite structure during the nitrogen air cooling process. By controlling the decarburization of the round steel surface through the rolling process, the depth of the decarburized layer on the round steel surface can be stably controlled to ≤0.12mm. In terms of peeling, the decarburized layer of this application is shallower. Then, a peeling machine is used to peel the surface of the round steel to completely remove the decarburization on the surface of the round steel.

[0029] After surface peeling, the round steel bars undergo surface quenching using high-frequency induction heating. The cooling process is controlled, and a circular baffle is placed between the high-frequency induction quenching device and the high-pressure water ring to prevent water splashes onto the surface of the round steel being heated by the molten steel, thus preventing the formation of soft spots on the steel surface. After high-frequency induction quenching, the round steel bars are placed in a holding furnace with a nitrogen protective atmosphere for low-temperature tempering to prevent the re-formation of a decarburized layer on the surface.

[0030] Based on the above technical means, this application finally obtained a high-frequency quenched steel for automotive turbine shafts, characterized by: a center porosity of ≤0.5 grade; an actual grain size of 7.0-8.0 grade at the center of the round steel; no decarburization on the surface of the round steel; a hardness of 57-62 HRC within 2 mm of the surface of the round steel; no soft spots on the surface of the round steel; and a hardness of 23-26 HRC at the center of the round steel. The turbine shaft possesses the characteristics of high surface hardness, low center hardness, and high toughness, making it an ideal, durable shaft steel that is not easily broken, and a high-quality steel for turbine shafts. Attached Figure Description

[0031] Figure 1 This is a grain size diagram of the center position of the round steel in Embodiment 1 of the present invention, magnified by 100 times.

[0032] Figure 2 This is a grain size diagram of the center position of the round steel in Embodiment 2 of the present invention, magnified by 100 times. Detailed Implementation

[0033] The present invention will be further described in detail below with reference to the embodiments. The embodiments are exemplary and intended to explain the present invention, but should not be construed as limiting the present invention.

[0034] Example 1 and Example 2:

[0035] The three embodiments involve a manufacturing method for high-frequency quenched automotive turbine shaft steel: converter primary refining → refining → continuous casting of square billets (240mm*240mm) → heating of the continuously cast square billets in a heating furnace → rough rolling → intermediate rolling → finish rolling into round bars → water cooling after rolling → high-pressure nitrogen air cooling → surface peeling of round bars → high-frequency induction quenching → low-temperature tempering in a nitrogen furnace. Two batches of high-frequency quenched automotive turbine shaft steel were manufactured.

[0036] The process employs a converter for smelting, using high-quality molten iron and high-quality scrap steel for primary refining. The high-quality scrap steel has Cu, Ni, and Mo contents of 0.04%, 0.04%, and 0.05% (Example 1), and 0.05%, 0.05%, and 0.04% (Example 2), respectively. During refining, ferrosilicon, ferrochrome, and ferromanganese alloys are added sequentially to adjust the chemical composition of the steel. Then, sulfur wire is fed into the molten steel. After the sulfur wire is added, calcium in the molten steel combines with sulfur to form CaS. At this point, aluminum wire is fed into the molten steel for precipitation and deoxidation. During the deoxidation process, a large number of non-metallic inclusions are formed in the molten steel. Al2O3 was used, and the argon flow rate was increased to 181 L / min and 198 L / min. The numerous fine, dispersed Al2O3 inclusions in the molten steel collided with and were adsorbed by the CaS in the steel during argon agitation. These inclusions floated to the slag layer along with the Al2O3 inclusions, effectively removing CaS inclusions and thus reducing the calcium content in the molten steel. The calcium content was stabilized at 0.0004% (Example 1) and 0.0005% (Example 2). At the end of refining, small amounts of copper, nickel, and ferromolybdenum were added to fine-tune the chemical composition of the molten steel. After smelting, the molten steel was cast onto a continuous casting platform to produce continuously cast billets.

[0037] The continuously cast billet is heated in a heating furnace. The temperature at the center of the billet is heated to 1158℃ (Example 1) and 1195℃ (Example 2). The center of the billet is held at this temperature for 2 hours (Example 1) and 3 hours (Example 2). The billet is rolled using roughing, intermediate rolling, and finishing rolling. The roughing rolling uses a large reduction rolling process. The total reduction rate of the roughing rolling three passes is controlled at 61% (Example 1) and 65% (Example 2). The roughing rolling temperature should be selected within the optimal plasticity temperature range of the steel of this invention. The optimal roughing rolling plasticity temperature range is determined by using the same temperature range during the roughing rolling process. The same heat number billets were rolled at different temperature ranges: 900-930℃, 930-960℃, 960-990℃, 990-1020℃, 1050-1080℃, 1080-1110℃, 1110-1140℃, 1140-1170℃, and 1170-1200℃. After rough rolling, the elongation of the billets in different temperature ranges was measured. The temperature range with the highest elongation was identified as the optimal rough rolling plasticity temperature range for the steel of this invention, which was 1110-1140℃. Rough rolling employed a large reduction and was performed in the high plasticity temperature range, resulting in a uniform and fine austenite structure at the center of the material after rough rolling. The rough-rolled material was then subjected to intermediate rolling and finish rolling to finally produce round bars with diameters of φ21mm (Example 1) and φ29mm (Example 2).

[0038] High-temperature round steel bars (≥900℃) after rolling were rapidly cooled to 728℃ (Example 1) and 737℃ (Example 2) within 5 seconds using water cooling. This rapid water cooling preserved the fine austenite structure formed in the center after rough rolling, providing suitable metallographic structure and temperature conditions for subsequent phase transformation. Simultaneously, it avoided prolonged exposure of the steel to high temperatures, inhibiting the formation of iron oxide scale. The water-cooled round steel bars were transported via a conveyor line at a speed of 0.2 m / s. Fans at the bottom of the conveyor line blew nitrogen gas upwards onto the round steel bars at pressures of 1.0 kPa (Example 1) and 1.2 kPa (Example 2), with an air volume of 5.1 m³ / s. 3 / s (Example 1), 6m 3 / s

[0039] (Example 2) The steel cooling rate was 10℃ / s (Example 1) and 12℃ / s (Example 2). After being cooled to 229℃ (Example 1) and 203℃ (Example 2) at the center of the round steel by high-pressure nitrogen air cooling, the fine austenite structure at the center of the round steel was transformed into a uniform and fine pearlite and ferrite structure, with an actual grain size of grade 7.0 (Example 1) and 8.0 (Example 2). The decarburized layer depth on the surface of the round steel was controlled to be 0.09mm (Example 1) and 0.12mm (Example 2) during the rolling and cooling process. Then, a peeling machine was used to peel off the surface of the round steel, with a peeling depth of [missing information].

[0040] After surface peeling, the round steel bars were quenched using high-frequency induction heating. The current frequency of the high-frequency induction heating was 60 kHz, and the heating time was 8 seconds (Example 1) and 10 seconds (Example 2). After high-frequency induction heating, the temperature within 2 mm of the surface of the round steel bars was 881℃ (Example 1) and 889℃ (Example 2), respectively, and the temperature at the center of the round steel bars was ≤230℃. Immediately after quenching, the steel bars were cooled using a high-pressure water ring spray. A circular baffle was installed between the high-pressure water ring and the high-frequency induction quenching device to prevent water droplets from splashing onto the surface of the steel bars during high-frequency induction heating, thus preventing the formation of soft spots on the steel surface. The high-frequency induction quenched round steel bars were immediately placed in a holding furnace at 230℃ with a nitrogen protective atmosphere for low-temperature tempering for 2 hours.

[0041] The smelting composition of the steel obtained in Example 1 and Example 2 is shown in Table 1.

[0042] Table 1 Smelting composition (wt%)

[0043]

[0044] Table 2 shows the porosity of the center of the round steel obtained in Examples 1 and 2, the actual grain size at the center position, and the depth of the decarburized layer on the surface.

[0045] Table 2

[0046]

[0047] The hardness of the round steel bars obtained in Examples 1 and 2 within 2 mm of the surface, and the hardness of the center of the round steel bars are shown in the figure.

[0048] Table 3.

[0049]

[0050] This invention relates to a high-frequency quenched steel for automotive turbine shafts. The steel composition is medium carbon, low calcium, with small amounts of copper, nickel, and molybdenum added to stabilize the residual elements copper, nickel, and molybdenum. A rational steelmaking process effectively reduces the harmful element calcium in the molten steel, controls the total surface area reduction in the three passes of the rough rolling process, and ensures that the entire rough rolling process is carried out within the optimal rough rolling plasticity temperature range. Rough rolling refines the austenite structure at the center of the material, resulting in a uniform and fine austenite structure. Rapid water cooling of high-temperature round steel after rolling helps retain the fine austenite structure in the center after rough rolling, while also inhibiting the formation of iron oxide scale. After water cooling, the center of the steel is cooled to 200-230℃ by high-pressure nitrogen air cooling, and the fine austenite structure in the center is transformed into fine and uniform ferrite and pearlite structure. The decarburization of the round steel surface is controlled by the rolling process, and the depth of the decarburized layer on the surface of the round steel can be stably controlled to ≤0.12mm. Then, a peeling machine is used to peel off the surface of the round steel to completely remove the decarburization.

[0051] After surface peeling, the round steel is quenched using high-frequency induction heating. The high-frequency quenching cooling process is controlled, and a circular baffle is placed between the high-frequency induction quenching device and the high-pressure water ring to prevent water splashing onto the surface of the round steel being heated by the high-frequency molten steel, thus preventing the formation of soft spots on the steel surface. After high-frequency induction quenching, the round steel is subjected to low-temperature tempering in a furnace with a nitrogen protective atmosphere to prevent the re-formation of a decarburized layer on the surface. Ultimately, a high-frequency quenched steel for automotive turbine shafts was invented, characterized by: a center porosity ≤0.5 grade; an actual grain size of 7.0-8.0 grade at the center of the round steel; no decarburization on the surface of the round steel; a hardness of 57-62 HRC within 2 mm of the surface of the round steel; no soft spots on the surface of the round steel; and a hardness of 23-26 HRC at the center of the round steel.

[0052] This invention manufactures a high-frequency quenched steel for automotive turbine shafts, filling the gap in high-quality turbine steel in China.

Claims

1. A method for manufacturing high-frequency quenched steel for automotive turbine shafts, characterized in that: The chemical composition of the steel, by weight percentage, is: C: 0.43–0.47%, Si: 0.10–0.25%, Mn: 0.45–0.65%, Cr: 1.80–2.00%, P: ≤0.020%, S: 0.018–0.028%, Al: 0.015–0.035%, Cu: 0.05–0.06%, Ni: 0.05–0.06%, Mo: 0.05–0.06%, Ca≤0.0005%, balance being Fe and unavoidable impurities; manufacturing method includes: Step 1: Steel Smelting and Casting: A converter is used for primary and secondary refining. The primary refining process uses high-quality molten iron and high-quality scrap steel, with the Cu, Ni, and Mo content in the scrap steel all ≤0.05%. During refining, ferrosilicon, ferrochrome, and ferromanganese alloys are added sequentially to adjust the chemical composition of the steel. Then, sulfur wire is fed into the molten steel. After the sulfur wire is added, the calcium in the molten steel combines with the sulfur to form CaS. Simultaneously, aluminum wire is fed into the molten steel for precipitation and deoxidation. During the deoxidation process, a large amount of non-metallic inclusions, Al2O3, are formed in the molten steel. Argon gas is used to stir the molten steel. The argon flow rate is 180-200 L / min. The numerous fine, dispersed Al2O3 inclusions in the molten steel are stirred by the argon gas, causing them to collide with and be adsorbed by the CaS in the steel. These inclusions then float to the slag layer along with the Al2O3 inclusions, effectively removing the CaS inclusions and thus reducing the calcium content in the molten steel to ≤0.0005%. At the end of the refining process, small amounts of copper, nickel, and ferromolybdenum are added to finely adjust the chemical composition of the molten steel. After smelting, the molten steel is cast into billets. Step 2, Rolling: The billet is heated in the furnace. When the temperature at the center of the billet reaches 1150-1200℃, it is held at this temperature for 2-3 hours. The continuous casting billet is rolled by roughing, intermediate rolling and finishing rolling. The roughing rolling adopts large reduction rolling. The cumulative reduction rate of roughing rolling is controlled at 60-65%. The roughing rolling is carried out in the plastic temperature range of 1110-1140℃. After roughing rolling, it is rolled into round steel with φ20-30mm by intermediate rolling and finishing rolling. The final rolling temperature is not lower than 900℃. Step 3: Post-rolling cooling: The high-temperature round steel after rolling is rapidly cooled to 720-740℃ within 5 seconds using water cooling. After water cooling, the round steel is transported via a conveyor line at a speed of 0.2m / s or higher. Nitrogen gas is blown upwards from the bottom of the conveyor line to cool the round steel. The nitrogen blowing pressure is set at 1.0-1.2kPa, the air volume is 5-6m³ / s, and the cooling rate is controlled at 10-12℃ / s. The high-pressure nitrogen gas cools the center of the round steel to 200-230℃, transforming the fine austenite structure in the center of the round steel into a uniform and fine pearlite and ferrite structure, with an actual grain size of 7.0-8.0 grade. After cooling, the surface of the round steel is peeled to remove the decarburized layer. Step 4: After surface peeling, the round steel is surface quenched using high-frequency induction heating. The temperature within 2mm of the surface of the round steel is heated to 880-890℃, while the temperature at the center of the round steel is ≤230℃. Then, a high-pressure water ring is immediately used to spray water quench and cool the round steel. A circular baffle is set between the high-pressure water ring and the high-frequency induction quenching device to prevent water from splashing onto the surface of the steel being induction heated during the high-pressure water ring spraying process, thereby preventing the formation of soft spots on the steel surface. The round steel after high-frequency induction quenching is immediately placed in a holding furnace for low-temperature tempering. An inert atmosphere is used in the holding furnace.

2. The manufacturing method of high-frequency quenched steel for automotive turbine shafts according to claim 1, characterized in that: The porosity of the round steel center is ≤0.5 grade; the center of the round steel has a pearlite and ferrite structure with an actual grain size of 7.0-8.0 grade; there is no decarburization on the surface of the round steel; the hardness within 2mm of the surface of the round steel is 57-62HRC; there are no soft spots on the surface of the round steel; the hardness at the center of the round steel is 23-26HRC.

3. The manufacturing method of high-frequency quenched steel for automotive turbine shafts according to claim 1, characterized in that: In step two, the roughing process is set to three passes, and the cumulative compression ratio of the three passes is 60-65%.

4. The manufacturing method of high-frequency quenched steel for automotive turbine shafts according to claim 1, characterized in that: In step three, the decarburized layer depth on the surface of the round steel during the rolling and post-rolling cooling process is ≤0.12mm, and the peeling depth on the surface of the round steel is 0.12mm.

5. The method for manufacturing high-frequency quenched steel for automotive turbine shafts according to claim 1, characterized in that: In step four, when performing high-frequency induction heating on the peeled round steel, the current frequency of the high-frequency induction heating is set to 60KHz, and the high-frequency induction heating time is 8-10s.

6. The method for manufacturing high-frequency quenched steel for automotive turbine shafts according to claim 1, characterized in that: In step four, the round steel bars after high-frequency induction hardening are immediately placed in a holding furnace at 230°C with a nitrogen protective atmosphere for low-temperature tempering for more than 2 hours.

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