A gear steel for automotive transmissions and its manufacturing method

Abstract

This invention discloses a gear steel for automotive transmissions and its manufacturing method, comprising: C: 0.19%~0.21%, Si: 0.23%~0.28%, Mn: 0.95%~0.98%, Cr: 1.08%~1.12%, Ti: 0.05%~0.07%, S: 0.028%~0.035%, P≤0.015%, Als: 0.008%~0.025%, Ni≤0.05%, Cu≤0.05%, Mo≤0.01%, residual elements Sn≤0.01%, Pb≤0.005%, As≤0.02%, Bi≤0.008%, Sb≤0.008%, total oxygen content: 0.001%~0.002%, total nitrogen content ≤0.004%, with the balance being Fe and unavoidable impurity elements, and the Mn / S ratio ranging from 27 to 35. This application controls the carbon-oxygen product at the end of the converter smelting process, which helps reduce residual oxygen in the steel; the aluminum-containing modifier modifies the ladle top slag, which helps improve aluminum yield; the high-basicity slag operation in the early stage of refining effectively removes and controls oxygen in the molten steel, while the lower-basicity slag operation in the later stage of refining stabilizes the sulfur content and yield in the subsequent sulfur feeding line; the sulfides are mainly composed of single manganese sulfide sulfides, with a core of calcium aluminate and an outer layer of manganese sulfide composite sulfides, and are in a diffuse distribution state, which effectively ensures the machinability of gear steel.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, and in particular relates to a gear steel for automotive transmissions and its manufacturing method. Background Technology

[0002] The automotive transmission is a core component of the automotive engine. Because automotive transmission gears operate under conditions of high speed, high load, and constantly changing speed and load, the surface of the transmission gears must have high strength, high hardness, and good wear resistance. The gear core must also have good strength and toughness. At the same time, the transmission gears must have high meshing precision and low operating noise. High precision and low noise are closely related to the machinability of gear steel. Therefore, all gear steel manufacturers have invested a lot of effort in improving the machinability of their products.

[0003] Against the backdrop of energy conservation and environmental protection, the direct cold forging process followed by machining of hot-rolled gear round steel without hot forging is gaining increasing attention, thus placing higher demands on the machinability of gear steel. Currently, the main method to improve the machinability of gear steel is by adding elements such as lead, tellurium, and sulfur. Due to environmental protection requirements, my country generally uses sulfur as the free-machining element.

[0004] A Chinese patent with publication number CN114892070A discloses a sulfur-containing gear steel and a method for producing it. The chemical composition of the sulfur-containing gear steel, by mass percentage, includes: C: 0.18-0.19%, S: 0.020-0.030%, Si: ≤0.10%, Mn: 1.33-1.37%, Cr: 1.23-1.27%, Al: 0.033-0.041%, N: 100-120ppm, with the remainder being Fe and unavoidable impurities. The mass ratio of Te to S satisfies: Te / S = 0.05-0.10. The sulfides in the sulfur-containing gear steel are MnTe-encapsulated MnS double-layer composite inclusions with an average aspect ratio of 1.5-2.0. The above method uses Te to adjust the sulfides in the gear steel, thereby improving the sulfide morphology, increasing its hardness, and reducing the deformation during forging. However, the sulfur recovery rate in molten steel is unstable, and under the condition of ensuring a suitable manganese-sulfur ratio, the segregation of sulfur in the billet is not effectively controlled. Summary of the Invention

[0005] The purpose of this application is to provide a gear steel for automotive transmissions and a method for manufacturing the same, which effectively stabilizes the sulfur yield and controls sulfide precipitation to improve the machinability of the gear steel.

[0006] To solve the above-mentioned technical problems, this application mainly adopts the following technical solutions: In the first aspect, this application provides a gear steel for automotive transmissions, the chemical composition of which, by mass percentage, comprises: C: 0.19%~0.21%, Si: 0.23%~0.28%, Mn: 0.95%~0.98%, Cr: 1.08%~1.12%, Ti: 0.05%~0.07%, S: 0.028%~0.035%, P≤0.015%, Als: 0.008%~0.025%, Ni≤0.05%, Cu≤0.05%, Mo≤0.01%, residual elements Sn≤0.01%, Pb≤0.005%, As≤0.02%, Bi≤0.008%, Sb≤0.008%, total oxygen content: 0.001%~0.002%, total nitrogen content ≤0.004%, and the balance being Fe and unavoidable impurity elements, wherein the Mn:S ratio ranges from 27 to 35.

[0007] In a second aspect, this application provides a method for manufacturing gear steel for automotive transmissions as described in the first aspect embodiment above, specifically including the following steps: Step 1: Converter smelting: Control the carbon-oxygen product at the end of the blowing process to ≤0.0024%, and add an aluminum-containing modifier to modify the ladle top slag; Step 2: Refining LF furnace. High-alkalinity slag is used in the early stage of refining. During the refining process, aluminum wire precipitation deoxidation is combined with calcium carbide diffusion deoxidation. In the later stage of refining, lower-alkalinity slag is used. After pre-calcium treatment, the soft blowing time is guaranteed to be ≥10min before leaving the station. Step 3: Refining RH furnace, using high vacuum large circulation operation, high vacuum time ≥5min; after the final calcium treatment interval is ≥8min, feed in the sulfur line, and control the soft blowing time to ≥10min before leaving the station; Step 4: Continuous casting, with full-process protective casting. Electric stirring is used throughout the process, including both the mold and the end-point stirring. The mold is 105±2.5m in diameter. 3 The system operates in a weak cooling mode with a flow rate of / h, while the secondary cooling system uses a medium cooling mode. The specific water flow rate is controlled between 0.55 and 0.60, and the standard pull speed is 0.9 ± 0.05 m / min. Step 5: Heat the steel billet, and control the temperature of the soaking zone at 1220±15℃; Step Six: High-Pressure Water Descaling; Step 7: Rolling.

[0008] In some embodiments of this application, in step one, the binary basicity of the aluminum-containing modifier is 5 to 6.

[0009] In some embodiments of this application, in step two, quartz is added in the later stage of refining to control the binary basicity of the final slag at 1.5~2.5, and after pre-calcification treatment, the Al content in the molten steel is controlled at 0.020%~0.035%, and Ca≥0.0010%.

[0010] In some embodiments of this application, in step three, the ultimate vacuum degree of the high vacuum large circulation is ≤20Pa, the Al content after final calcium treatment is controlled at 0.010%~0.025%, the calcium-aluminum ratio is controlled at 0.08~0.15, and the S content is controlled at 0.030%~0.035% after the sulfur wire is fed in.

[0011] In some embodiments of this application, in step four, the intermediate package uses a high-SiO2 covering agent with an alkalinity of 1.0 to 1.5.

[0012] In some embodiments of this application, in step four, the crystallizer electric stirring parameters are selected as 350±10A / 3±0.5Hz, and the end electric stirring parameters are selected as 250±10A / 8±0.5Hz.

[0013] In some embodiments of this application, in step four, the cooling water distribution ratio for zones 1 to 4 is set to a 35:21:38:6 pattern.

[0014] In some embodiments of this application, in step seven, the temperature is controlled at 810~840°C after the final rolling is completed.

[0015] Compared with the prior art, the advantages and positive effects of this invention are as follows: The manufacturing method of gear steel for automotive transmissions in this application includes processes such as converter smelting, refining LF furnace, refining RH furnace, continuous casting, billet heating, high-pressure water descaling, and rolling. Controlling the carbon-oxygen product at the end of the converter smelting process helps reduce residual oxygen in the steel, decreases inclusions, and makes the molten steel purer. Modifying the ladle top slag with an aluminum-containing modifier helps improve aluminum yield, enhance desulfurization and dephosphorization, and reduce alumina inclusions. High-basicity slag operation in the early refining stage ensures effective removal and control of oxygen in the molten steel, while lower-basicity slag operation in the later refining stage stabilizes subsequent processes. The sulfur content and yield in the molten steel during sulfur feeding effectively kept the manganese-sulfur ratio within the design range. An interval of at least 8 minutes between final calcium treatment and sulfur feeding prevented the combination of sulfur and calcium to form calcium sulfide, thus stabilizing the sulfur content in the molten steel and effectively serving as a nucleation site for sulfide precipitation during solidification. The sulfides were primarily composed of single manganese sulfides, with a core of calcium aluminate and an outer layer of manganese sulfides, exhibiting a dispersed distribution with an aspect ratio ≤5, effectively ensuring the machinability of the gear steel. The continuous casting process employed full-process protective casting, utilizing both electric stirring in the crystallizer and at the end of the process. The crystallizer used a 105±2.5m... 3 The / h weak cooling mode effectively avoids the problem of billet cracks easily occurring when titanium-containing and sulfur-containing gear steel solidifies at high temperatures. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 Metallographic images of the gear steel for automotive transmissions manufactured according to Embodiment 1 of the present invention; Figure 2 Metallographic images of the gear steel for automotive transmissions manufactured according to Embodiment 2 of the present invention; Figure 3 Metallographic images of the gear steel for automotive transmissions manufactured according to Example 3 of this invention; Figure 4 Metallographic images of the gear steel for automotive transmissions manufactured according to Example 4 of this invention; Figure 5 Metallographic images of the gear steel for automotive transmissions manufactured according to Embodiment 5 of the present invention; Figure 6 Metallographic image of the gear steel for automotive transmissions manufactured according to Embodiment 6 of the present invention. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] The following disclosure provides many different embodiments or examples for implementing various structures of the invention. To simplify the disclosure, specific examples of components and arrangements are described below. These are merely examples and are not intended to limit the invention. Furthermore, reference numerals and / or letters may be repeated in different examples; such repetition is for simplification and clarity and does not in itself indicate a relationship between the various embodiments and / or arrangements discussed. In addition, examples of various specific processes and materials are provided in this invention, but those skilled in the art will recognize the application of other processes and / or the use of other materials.

[0020] In the first aspect, this application provides a gear steel for automotive transmissions, the chemical composition of which, by mass percentage, comprises: C: 0.19%~0.21%, Si: 0.23%~0.28%, Mn: 0.95%~0.98%, Cr: 1.08%~1.12%, Ti: 0.05%~0.07%, S: 0.028%~0.035%, P≤0.015%, Als: 0.008%~0.025%, Ni≤0.05%, Cu≤0.05%, Mo≤0.01%, residual elements Sn≤0.01%, Pb≤0.005%, As≤0.02%, Bi≤0.008%, Sb≤0.008%, total oxygen content: 0.001%~0.002%, total nitrogen content ≤0.004%, and the balance being Fe and unavoidable impurity elements, wherein the Mn:S ratio ranges from 27 to 35.

[0021] Specifically, the sulfur content and the corresponding manganese-sulfur ratio in this application are within a suitable range. Precise control of sulfur is mainly achieved through low-basicity refining slag, matching of calcium treatment and sulfur addition timing, sulfur addition method, and the use of low-basicity high-SiO2 covering agent in the ladle. Manganese control is mainly achieved through stabilizing the converter charging system, using self-produced scrap steel to reduce steel output fluctuations, improving the metering accuracy of the LF refining process, and reducing slag oxygen in the early stage of refining. This achieves narrow compositional control of manganese content and meets the design requirements for the manganese-sulfur ratio. The prepared gear steel achieves machinability by controlling the precipitation distribution and morphology of sulfides.

[0022] In a second aspect, this application provides a method for manufacturing gear steel for automotive transmissions as described in the first aspect embodiment above, specifically including the following steps: Step 1: Converter smelting: Control the carbon-oxygen product at the end of the blowing process to ≤0.0024%, and add an aluminum-containing modifier to modify the ladle top slag.

[0023] Specifically, by controlling the carbon-oxygen product at the end of the blowing process to ≤0.0024%, the less residual oxygen and inclusions in the steel at the target carbon content, the purer the molten steel.

[0024] In some embodiments of this application, slag discharge from the ladle is strictly controlled during the tapping process.

[0025] Specifically, controlling slag discharge from the ladle and preventing slag from entering the ladle helps improve the purity of the molten steel. Steel slag contains high levels of iron oxide and other impurities. If these impurities enter the ladle, they will react chemically with elements in the molten steel, leading to an increase in inclusions and thus reducing the cleanliness of the steel. The high FeO content in steel slag can oxidize alloying elements (such as silicon and manganese), and in severe cases, can cause the molten steel composition to deviate from the expected levels. Therefore, controlling slag discharge can improve the recovery rate of alloying elements.

[0026] Specifically, measures to control slag discharge from the ladle include using a sliding plate to block slag, which allows the slag layer thickness in the ladle to be stably controlled below 50mm.

[0027] In some embodiments of this application, in step one, the binary basicity of the aluminum-containing modifier is 5 to 6.

[0028] Step 2: Refining the LF furnace. In the early stage of refining, high-alkalinity slag is used. During the refining process, aluminum wire precipitation deoxidation is combined with calcium carbide diffusion deoxidation. In the later stage of refining, lower-alkalinity slag is used. After pre-calcium treatment, the soft blowing time is guaranteed to be ≥10min before leaving the station.

[0029] Specifically, using high-basicity slag in the early stages of refining helps to increase the activity of calcium oxide and achieve deep desulfurization. At the same time, it can inhibit the formation of brittle alumina inclusions and promote their transformation into low-melting-point, easily floating calcium aluminates, thus ensuring the effective removal and control of oxygen in the molten steel and improving the cleanliness of the molten steel.

[0030] Specifically, aluminum wire feeding precipitation deoxidation during the refining process involves feeding aluminum wire into the molten steel in the ladle. The aluminum reacts chemically with the oxygen in the molten steel to form insoluble alumina precipitate, thus removing the oxygen from the steel. This method offers high deoxidation efficiency, enabling deep final deoxidation, controlling the aluminum content in the molten steel, and further improving its quality.

[0031] Specifically, calcium carbide diffusion deoxidation works by increasing the temperature of molten steel, allowing oxygen atoms to gain sufficient energy to overcome the binding force of surrounding atoms and diffuse towards the surface of the molten steel. At the surface, the oxygen atoms react with the added calcium carbide or other substances, generating gases (such as carbon monoxide) that escape, thus achieving deoxidation. This method can more thoroughly remove oxygen from molten steel, improving its purity.

[0032] The refining process employs a combination of aluminum wire feeding precipitation deoxidation and calcium carbide diffusion deoxidation, integrating the advantages of both methods. Aluminum wire feeding precipitation deoxidation quickly and effectively removes most of the oxygen from the molten steel, while calcium carbide diffusion deoxidation further removes any remaining oxygen, improving the purity of the molten steel. This combined approach allows for more comprehensive control of the oxygen content in the molten steel, reduces other impurities, and improves the quality of the steel and the performance of the final product.

[0033] In some embodiments of this application, in step two, quartz is added in the later stage of refining to control the binary basicity of the final slag at 1.5~2.5. Under the condition of good deoxidation in the early stage, the use of lower basicity slag in the later stage stabilizes the sulfur content and yield in the molten steel during the subsequent sulfur feeding line, effectively achieving the manganese-sulfur ratio within the design range; pre-calcium treatment is performed before soft blowing at the station, and after the pre-calcium treatment, the Al content in the molten steel is controlled at 0.020%~0.035%, and Ca≥0.0010%.

[0034] Step 3: Refining RH furnace, using high vacuum large circulation operation, high vacuum time ≥5min; after the final calcium treatment interval is ≥8min, feed in the sulfur line, and control the soft blowing time to ≥10min before leaving the station; In some embodiments of this application, in step three, the ultimate vacuum degree of the high vacuum large circulation is ≤20Pa. After the circulation ends and the vacuum is broken, final calcium treatment is performed. After the final calcium treatment, the Al content is controlled at 0.010%~0.025%, and the calcium-aluminum ratio is controlled at 0.08~0.15, which is conducive to the precipitation of calcium aluminate inclusions. After the sulfur wire is fed in, the S content is controlled at 0.030%~0.035%.

[0035] Specifically, in the early stage of refining, high-basicity refining slag ensured the effective removal and control of oxygen in the molten steel. In the later stage, the basicity of the refining slag was reduced to ensure the stability of the sulfur content in the molten steel by feeding the sulfur line with RH. The final calcium treatment and the sulfur line feeding were spaced 8 minutes apart to avoid the combination of sulfur and calcium to form CaS, thereby stabilizing the sulfur content in the molten steel. When refining was completed, the oxygen content was controlled at 10~20ppm and the calcium-aluminum ratio was controlled at 0.08~0.15. This allowed the calcium aluminates in the molten steel to preferentially precipitate mainly as low-melting-point 3CaO*Al2O3, 12CaO*7Al2O3, CaO*Al2O3 and their composite inclusions, which could effectively serve as heterogeneous nucleation sites for sulfides during the solidification process.

[0036] Step 4: Continuous casting, with full-process protective casting. Electric stirring is used throughout the process, including both the mold and the end-point stirring. The mold is 105±2.5m in diameter. 3 The weak cooling mode of / h effectively avoids the problem of billet cracks that are prone to occur when titanium and sulfur-containing gear steel solidifies in the high-temperature section; the secondary cooling adopts the medium cooling mode, the specific water volume is controlled at 0.55~0.60, and the standard drawing speed is 0.9±0.05m / min.

[0037] Specifically, sulfide precipitation mainly occurs during the solidification process of molten steel. As the temperature decreases, sulfides spontaneously form when the equilibrium concentration product of manganese and sulfur reaches the specified requirements. The amount and size of the precipitates are directly related to the cooling rate. Therefore, a higher water volume is set in the solidified pasty zone (secondary cooling zone 3). Considering the crack sensitivity of titanium- and sulfur-containing gear steel, the precipitation distribution is directly related to the sulfur segregation between secondary dendrites. Therefore, a dual electric stirring mode with matching crystallizer electric stirring and end electric stirring is adopted to improve the situation.

[0038] In some embodiments of this application, in step four, a high-SiO2 covering agent with an alkalinity of 1.0~1.5 is used in the tundish. The high-SiO2 covering agent, once added to the molten steel surface, can quickly and evenly cover the surface, effectively isolates air, has a low thermal conductivity, and effectively prevents crust formation on the surface and freezing of the nozzle.

[0039] In some embodiments of this application, in step four, the crystallizer electric stirring parameters are selected as 350±10A / 3±0.5Hz, and the end electric stirring parameters are selected as 250±10A / 8±0.5Hz. This can effectively improve sulfur segregation during continuous casting.

[0040] In some embodiments of this application, in step four, the secondary cooling water distribution ratio for zones 1 to 4 is set at 35:21:38:6. This effectively promotes the fine dispersion and precipitation of sulfides in the solid-liquid two-phase region at the solidification front by using a high water volume in zone 3.

[0041] Step 5: Heating the billet. Place the 200mm square billet into the heating furnace and heat it. The temperature of the soaking zone is controlled at 1220±15℃. Step 6: High-pressure water descaling to remove surface iron oxide scale.

[0042] Step 7: Rolling. In addition to single sulfides with a large number and small size of homogeneous nuclei, composite sulfides using specific types of calcium aluminates as heterogeneous nucleation cores are more beneficial for the distribution of low aspect ratio and fine dispersion of sulfides after rolling, taking advantage of the fact that calcium aluminates are not easily deformed.

[0043] In some embodiments of this application, in step seven, the temperature is controlled at 810~840°C after the final rolling is completed.

[0044] Specifically, the gear steel produced using the method of this disclosure has a microstructure of ferrite + pearlite. The product has advantages such as fine and dispersed sulfide distribution and good machinability. The gears used in automotive gearboxes manufactured by users have the characteristics of high machining accuracy and good matching.

[0045] Example 1: The gear steel bar for automotive gearboxes has a specification of φ60mm. Its manufacturing method includes: converter smelting, refining LF furnace, refining RH furnace, continuous casting, billet heating, high-pressure water descaling and rolling process. The top-and-bottom combined blowing converter adopts the bottom blowing large annular gap process. The carbon-oxygen product at the end of the blowing process is controlled at 0.0024%. During the tapping process, the slag discharge from the ladle is strictly controlled. At the same time, an aluminum-containing modifier is added to modify the top slag of the ladle. The binary basicity of the high-basicity aluminum-containing modifier is 5.5. In the early stage of the LF process refining, high-basicity slag is used, and the binary basicity is controlled at 5.3. The refining process adopts aluminum wire feeding precipitation deoxidation combined with calcium carbide diffusion deoxidation process. In the later stage of refining, lower-basicity slag is used, with a final slag basicity of 2.2. After pre-calcium treatment, the Al content in the molten steel is controlled at 0.020% and the Ca content at 0.0010%. The steel is then blown out of the station after 10 minutes. The RH refining process adopts high vacuum large circulation operation with an ultimate vacuum of 16 Pa and a high vacuum time of more than 5 minutes. After final calcium treatment, the Al content is controlled at 0.025% and the calcium-aluminum ratio is controlled at 0.09. After feeding the sulfur line, the S content is controlled at 0.032%. The interval between final calcium treatment and feeding the sulfur line is ≥8 minutes. Before leaving the station, the soft blowing time is controlled at 10 minutes. The continuous casting process utilizes full protective casting. A covering agent with a basicity of 1.2 and a SiO2 content of 36% is used in the tundish. Electric stirring is employed throughout the process, including both the crystallizer and end-point stirring. The crystallizer is a 105m... 3 The system operates in a weak cooling mode with a flow rate of 0.60. The standard pulling speed is 0.9 m / min. The crystallizer electric stirring parameters are 350±10A / 3±0.5Hz, and the terminal electric stirring parameters are 250±10A / 8±0.5Hz. The secondary cooling system is divided into four zones, with the water distribution ratio for zones 1 to 4 set at 35:21:38:6. By using a high cooling rate in zone 3, the system effectively promotes the fine dispersion precipitation of sulfides in the two-phase region at the solidification front.

[0046] The end face size of the continuously cast billet is 200mm*200mm. The chemical composition and mass percentage of the billet are as follows: C: 0.20%, Si: 0.25%, Mn: 0.96%, Cr: 1.10%, Ti: 0.06%, S: 0.031%, P: 0.012%, Als: 0.015%, Ni: 0.01%, Cu: 0.01%, Mo: 0.002%, residual elements Sn: 0.002%, Pb: 0.001%, As: 0.005%, Bi: 0.002%, Sb: 0.002%, total oxygen content 15ppm, total nitrogen content 35ppm, balance is Fe and unavoidable impurities, of which the manganese-sulfur ratio is 31, the remainder is iron and unavoidable impurity elements; The heating process involves placing a 200mm square billet into a heating furnace for heating. The temperature in the soaking zone is controlled at 1220±15℃. After heating, the surface of the billet is descaled with high-pressure water. Then, it is rolled into finished bar products. After the final rolling, the temperature is controlled at 820~830℃.

[0047] Combination Figure 1 As shown, after slow cooling, samples were taken for testing of sulfides. The main types were single manganese sulfides and composite sulfides with a calcium aluminate core and a manganese sulfide outer layer. The aspect ratios varied from 3 to 6, and they were dispersed, which effectively ensured the machinability of gear steel for automotive gearboxes.

[0048] In this embodiment, the gear steel for automotive transmissions successfully achieved a uniform and well-machinable distribution of sulfides in the finished bar stock. The microstructure, as tested, is ferrite + pearlite with no supercooled microstructure and a total oxygen content of 15 ppm. After processing by downstream users, the sulfur-containing gear steel for automotive transmissions exhibits high machining precision and good matching performance.

[0049] Example 2: The hot-rolled bar stock for automotive gearboxes has a specification of φ50mm. Its manufacturing method includes: converter smelting, refining LF furnace, refining RH furnace, continuous casting, billet heating, high-pressure water descaling and rolling process. The top-and-bottom combined blowing converter adopts the bottom blowing large annular gap process. The carbon-oxygen product at the end of the blowing process is controlled at 0.0024%. During the tapping process, the slag discharge from the ladle is strictly controlled. At the same time, an aluminum-containing modifier is added to modify the top slag of the ladle. The binary basicity of the high-basicity aluminum-containing modifier is 5.0. In the early stage of the LF process refining, high basicity slag is used, and the binary basicity is controlled at 5.5. The refining process adopts aluminum wire feeding precipitation deoxidation combined with calcium carbide diffusion deoxidation process. In the later stage of refining, lower basicity slag is used, with a final slag basicity of 2.1. After pre-calcium treatment, the Al content in the molten steel is controlled at 0.023% and the Ca content at 0.0015%. The steel is then blown out of the station in 13 minutes. The RH refining process adopts high vacuum large circulation operation, with an ultimate vacuum of 20Pa and a high vacuum time of more than 5 minutes. After final calcium treatment, the Al content is controlled at 0.018% and the calcium-aluminum ratio is controlled at 0.15. After feeding the sulfur line, the S content is controlled at 0.035%. The interval between final calcium treatment and feeding the sulfur line is ≥8 minutes. Before leaving the station, the soft blowing time is controlled at 13 minutes. The continuous casting process utilizes full-protection casting. The tundish employs a covering agent with a basicity of 1.5 and a SiO2 content of 36%. Electric stirring is used throughout the process, including both the crystallizer and end-point stirring. The crystallizer is a 106m³. 3 The system operates in a weak cooling mode with a flow rate of 0.60. The standard pulling speed is 0.9 m / min. The crystallizer electric stirring parameters are 350±10A / 3±0.5Hz, and the terminal electric stirring parameters are 250±10A / 8±0.5Hz. The secondary cooling system is divided into four zones, with the water distribution ratio for zones 1 to 4 set at 35:21:38:6. By using a high cooling rate in zone 3, the system effectively promotes the fine dispersion precipitation of sulfides in the two-phase region at the solidification front.

[0050] The end face size of the continuously cast billet is 200mm*200mm. The chemical composition and mass percentage of the billet are as follows: C: 0.21%, Si: 0.26%, Mn: 0.945%, Cr: 1.11%, Ti: 0.06%, S: 0.035%, P: 0.013%, Als: 0.016%, Ni: 0.01%, Cu: 0.01%, Mo: 0.001%, residual elements Sn: 0.001%, Pb: 0.001%, As: 0.008%, Bi: 0.002%, Sb: 0.001%, total oxygen content 16ppm, total nitrogen content 32ppm, balance is Fe and unavoidable impurities, of which the manganese-sulfur ratio is 27, the remainder is iron and unavoidable impurity elements; The heating process involves placing a 200mm square billet into a heating furnace for heating. The temperature in the soaking zone is controlled at 1220±15℃. After heating, the surface of the billet is descaled with high-pressure water. Then, it is rolled into finished bar products. After the final rolling, the temperature is controlled at 810~830℃.

[0051] Combination Figure 2 As shown, after slow cooling, samples were taken for testing of sulfides. The main types were single manganese sulfides and composite sulfides with a calcium aluminate core and a manganese sulfide outer layer. The aspect ratios varied from 3 to 6, and they were dispersed, which effectively ensured the machinability of gear steel for automotive gearboxes.

[0052] In this embodiment, the gear steel for automotive transmissions successfully achieved a uniform and dispersed distribution of sulfides in the finished bar stock, resulting in good machinability. The microstructure, as tested, is ferrite + pearlite with no supercooled structure and a total oxygen content of 16 ppm. After processing by downstream users, the sulfur-containing gear steel for automotive transmissions exhibits high machining precision and good matching performance.

[0053] Example 3: The hot-rolled bar stock for automotive gearboxes has a specification of φ55mm. Its manufacturing method includes: converter smelting, refining LF furnace, refining RH furnace, continuous casting, billet heating, high-pressure water descaling and rolling process. The top-and-bottom combined blowing converter adopts the bottom blowing large annular gap process. The carbon-oxygen product at the end of the blowing process is controlled at 0.0022%. During the tapping process, the slag discharge from the ladle is strictly controlled. At the same time, an aluminum-containing modifier is added to modify the top slag of the ladle. The binary basicity of the high-basicity aluminum-containing modifier is 5.5. In the early stage of the LF process refining, high basicity slag operation is adopted, and the binary basicity is controlled at 5.1. The refining process adopts aluminum wire feeding precipitation deoxidation combined with calcium carbide diffusion deoxidation process. In the later stage of refining, lower basicity slag operation is adopted, with a final slag basicity of 1.9. After pre-calcium treatment, the Al content in the molten steel is controlled at 0.021% and the Ca content is 0.0013%. The steel is then blown out of the station in 12 minutes. The RH refining process adopts high vacuum large circulation operation, with an ultimate vacuum degree of 20Pa and a high vacuum time of more than 5 minutes. After final calcium treatment, the Al content is controlled at 0.020% and the calcium-aluminum ratio is controlled at 0.11. After feeding the sulfur line, the S content is controlled at 0.031%. The interval between final calcium treatment and feeding the sulfur line is ≥8 minutes. Before leaving the station, the soft blowing time is controlled at 15 minutes. The continuous casting process utilizes full protective casting. A covering agent with a basicity of 1.2 and a SiO2 content of 38% is used in the tundish. Electric stirring is employed throughout the process, including both the crystallizer and end-point stirring. The crystallizer is a 105m... 3The system operates in a weak cooling mode with a flow rate of 0.60. The standard pulling speed is 0.9 m / min. The crystallizer electric stirring parameters are 350±10A / 3±0.5Hz, and the terminal electric stirring parameters are 250±10A / 8±0.5Hz. The secondary cooling system is divided into four zones, with the water distribution ratio for zones 1 to 4 set at 35:21:38:6. By using a high cooling rate in zone 3, the system effectively promotes the fine dispersion precipitation of sulfides in the two-phase region at the solidification front.

[0054] The end face size of the continuously cast billet is 200mm*200mm. The chemical composition and mass percentage of the billet are as follows: C: 0.19%, Si: 0.23%, Mn: 0.98%, Cr: 1.12%, Ti: 0.05%, S: 0.028%, P: 0.015%, Als: 0.025%, Ni: 0.01%, Cu: 0.01%, Mo: 0.001%, residual elements Sn: 0.002%, Pb: 0.001%, As: 0.005%, Bi: 0.001%, Sb: 0.001%, total oxygen content 20ppm, total nitrogen content 33ppm, balance is Fe and unavoidable impurities, of which the manganese-sulfur ratio is 35, the remainder is iron and unavoidable impurity elements; The heating process involves placing a 200mm square billet into a heating furnace for heating. The temperature in the soaking zone is controlled at 1220±15℃. After heating, the surface of the billet is descaled with high-pressure water. Then, it is rolled into finished bar products. After the final rolling, the temperature is controlled at 820~840℃.

[0055] Combination Figure 3 As shown, after slow cooling, samples were taken for testing of sulfides. The main types were single manganese sulfides and composite sulfides with a calcium aluminate core and a manganese sulfide outer layer. The aspect ratios varied from 3 to 6, and they were dispersed, which effectively ensured the machinability of gear steel for automotive gearboxes.

[0056] In this embodiment, the gear steel for automotive transmissions successfully achieved a uniform and well-machinable distribution of sulfides in the finished bar stock. The microstructure, as tested, is ferrite + pearlite with no supercooled microstructure and a total oxygen content of 17 ppm. After processing by downstream users, the sulfur-containing gear steel for automotive transmissions exhibits high machining precision and good matching performance.

[0057] Example 4: The hot-rolled bar stock for automotive gearboxes has a specification of φ45mm. Its manufacturing method includes: converter smelting, refining LF furnace, refining RH furnace, continuous casting, billet heating, high-pressure water descaling and rolling process. The top-and-bottom combined blowing converter adopts the bottom blowing large annular gap process. The carbon-oxygen product at the end of the blowing process is controlled at 0.0023%. During the tapping process, the slag discharge from the ladle is strictly controlled. At the same time, an aluminum-containing modifier is added to modify the top slag of the ladle. The binary basicity of the high-basicity aluminum-containing modifier is 5.3. In the early stage of the LF process refining, high-basicity slag is used, and the binary basicity is controlled at 5.2. The refining process adopts aluminum wire feeding precipitation deoxidation combined with calcium carbide diffusion deoxidation process. In the later stage of refining, lower-basicity slag is used, with a final slag basicity of 2.0. After pre-calcium treatment, the Al content in the molten steel is controlled at 0.022% and the Ca content at 0.0014%. The steel is then blown out of the station in 13 minutes. The RH refining process adopts high vacuum large circulation operation with an ultimate vacuum of 19 Pa and a high vacuum time of more than 5 minutes. After final calcium treatment, the Al content is controlled at 0.022% and the calcium-aluminum ratio is controlled at 0.10. After feeding the sulfur line, the S content is controlled at 0.030%. The interval between final calcium treatment and feeding the sulfur line is ≥8 minutes. Before leaving the station, the soft blowing time is controlled at 14 minutes. The continuous casting process utilizes full protective casting. The tundish employs a covering agent with a basicity of 1.2 and a SiO2 content of 38%. Electric stirring is used throughout the process, including both the crystallizer and end-point stirring. The crystallizer is a 103m³ / h. 3 The system operates in a weak cooling mode with a flow rate of 0.60. The standard pulling speed is 0.9 m / min. The crystallizer electric stirring parameters are 350±10A / 3±0.5Hz, and the terminal electric stirring parameters are 250±10A / 8±0.5Hz. The secondary cooling system is divided into four zones, with the water distribution ratio for zones 1 to 4 set at 35:21:38:6. By using a high cooling rate in zone 3, the system effectively promotes the fine dispersion precipitation of sulfides in the two-phase region at the solidification front.

[0058] The end face size of the continuously cast billet is 200mm*200mm. The chemical composition and mass percentage of the billet are as follows: C: 0.21%, Si: 0.28%, Mn: 0.95%, Cr: 1.08%, Ti: 0.07%, S: 0.035%, P: 0.015%, Als: 0.008%, Ni: 0.05%, Cu: 0.05%, Mo: 0.01%, residual elements Sn: 0.01%, Pb: 0.005%, As: 0.02%, Bi: 0.008%, Sb: 0.008%, total oxygen content 10ppm, total nitrogen content 40ppm, balance is Fe and unavoidable impurities, of which the manganese-sulfur ratio is 27.1, the remainder is iron and unavoidable impurity elements; The heating process involves placing a 200mm square billet into a heating furnace for heating. The temperature in the soaking zone is controlled at 1220±15℃. After heating, the surface of the billet is descaled with high-pressure water. Then, it is rolled into finished bar products. After the final rolling, the temperature is controlled at 820~830℃.

[0059] Combination Figure 4As shown, after slow cooling, samples were taken for testing of sulfides. The main types were single manganese sulfides and composite sulfides with a calcium aluminate core and a manganese sulfide outer layer. The aspect ratios varied from 3 to 6, and they were dispersed, which effectively ensured the machinability of gear steel for automotive gearboxes.

[0060] In this embodiment, the gear steel for automotive transmissions successfully achieved a uniform and dispersed distribution of sulfides in the finished bar stock, resulting in good machinability. The microstructure, as tested, is ferrite + pearlite with no supercooled structure and a total oxygen content of 16 ppm. After processing by downstream users, the sulfur-containing gear steel for automotive transmissions exhibits high machining precision and good matching performance.

[0061] Example 5: The hot-rolled bar stock for automotive gearboxes has a specification of φ65mm. Its manufacturing method includes: converter smelting, refining LF furnace, refining RH furnace, continuous casting, billet heating, high-pressure water descaling and rolling process. The top-and-bottom combined blowing converter adopts the bottom blowing large annular gap process. The carbon-oxygen product at the end of the blowing process is controlled at 0.0022%. During the tapping process, the slag discharge from the ladle is strictly controlled. At the same time, an aluminum-containing modifier is added to modify the top slag of the ladle. The binary basicity of the high-basicity aluminum-containing modifier is 5.2. In the early stage of the LF process refining, high-basicity slag operation is adopted, and the binary basicity is controlled at 5.1. The refining process adopts aluminum wire feeding precipitation deoxidation combined with calcium carbide diffusion deoxidation process. In the later stage of refining, lower-basicity slag operation is adopted, with a final slag basicity of 2.3. After pre-calcium treatment, the Al content in the molten steel is controlled at 0.022% and the Ca content at 0.0015%. The steel is then blown out of the station in 11 minutes. The RH refining process adopts high vacuum large circulation operation with an ultimate vacuum of 18 Pa and a high vacuum time of more than 5 minutes. After final calcium treatment, the Al content is controlled at 0.018% and the calcium-aluminum ratio is controlled at 0.11. After feeding the sulfur line, the S content is controlled at 0.030%. The interval between final calcium treatment and feeding the sulfur line is ≥8 minutes. Before leaving the station, the soft blowing time is controlled at 15 minutes. The continuous casting process utilizes full protective casting. The tundish employs a covering agent with a basicity of 1.0 and a SiO2 content of 37%. Electric stirring is used throughout the process, including both the crystallizer and end-point stirring. The crystallizer is a 107.5m... 3 The system operates in a weak cooling mode with a flow rate of 0.55. The standard pulling speed is 0.95 m / min. The crystallizer electric stirring parameters are 350±10A / 3±0.5Hz, and the terminal electric stirring parameters are 250±10A / 8±0.5Hz. The secondary cooling system is divided into four zones, with the water distribution ratio for zones 1 to 4 set at 35:21:38:6. By using a high cooling rate in zone 3, the system effectively promotes the fine dispersion precipitation of sulfides in the two-phase region at the solidification front.

[0062] The end face size of the continuously cast billet is 200mm*200mm. The chemical composition and mass percentage of the billet are as follows: C: 0.19%, Si: 0.24%, Mn: 0.97%, Cr: 1.11%, Ti: 0.07%, S: 0.031%, P: 0.010%, Als: 0.017%, Ni: 0.01%, Cu: 0.01%, Mo: 0.002%, residual elements Sn: 0.001%, Pb: 0.001%, As: 0.002%, Bi: 0.002%, Sb: 0.002%, total oxygen content 17ppm, total nitrogen content 32ppm, balance is Fe and unavoidable impurities, of which the manganese-sulfur ratio is 31, the remainder is iron and unavoidable impurity elements; The heating process involves placing a 200mm square billet into a heating furnace for heating. The temperature in the soaking zone is controlled at 1220±15℃. After heating, the surface of the billet is descaled with high-pressure water. Then, it is rolled into finished bar products. After the final rolling, the temperature is controlled at 830~840℃.

[0063] Combination Figure 5 As shown, after slow cooling, samples were taken for testing of sulfides. The main types were single manganese sulfides and composite sulfides with a calcium aluminate core and a manganese sulfide outer layer. The aspect ratios varied from 3 to 6, and they were dispersed, which effectively ensured the machinability of gear steel for automotive gearboxes.

[0064] In this embodiment, the gear steel for automotive transmissions successfully achieved a uniform and well-machinable distribution of sulfides in the finished bar stock. The microstructure, as tested, is ferrite + pearlite with no supercooled microstructure and a total oxygen content of 17 ppm. After processing by downstream users, the sulfur-containing gear steel for automotive transmissions exhibits high machining precision and good matching performance.

[0065] Example 6: The hot-rolled bar stock for automotive gearboxes has a specification of φ70mm. Its manufacturing method includes: converter smelting, refining LF furnace, refining RH furnace, continuous casting, billet heating, high-pressure water descaling and rolling process. The top-and-bottom combined blowing converter adopts the bottom blowing large annular gap process. The carbon-oxygen product at the end of the blowing process is controlled at 0.0022%. During the tapping process, the slag discharge from the ladle is strictly controlled. At the same time, an aluminum-containing modifier is added to modify the top slag of the ladle. The binary basicity of the high-basicity aluminum-containing modifier is 5.1. In the early stage of the LF process refining, high-basicity slag operation is adopted, and the binary basicity is controlled at 5.2. The refining process adopts aluminum wire feeding precipitation deoxidation combined with calcium carbide diffusion deoxidation process. In the later stage of refining, lower-basicity slag operation is adopted, with a final slag basicity of 2.1. After pre-calcium treatment, the Al content in the molten steel is controlled at 0.020% and the Ca content at 0.0015%. The steel is then blown out of the station in 12 minutes. The RH refining process adopts high vacuum large circulation operation with an ultimate vacuum of 19 Pa and a high vacuum time of more than 5 minutes. After final calcium treatment, the Al content is controlled at 0.010% and the calcium-aluminum ratio is controlled at 0.08. After feeding the sulfur line, the S content is controlled at 0.032%. The interval between final calcium treatment and feeding the sulfur line is ≥8 minutes. Before leaving the station, the soft blowing time is controlled at 16 minutes. The continuous casting process utilizes full protective casting. The tundish employs a covering agent with a basicity of 1.2 and a SiO2 content of 36%. Electric stirring is used throughout the process, including both the crystallizer and end-point stirring. The crystallizer is a 102.5m... 3 The system operates in a weak cooling mode with a flow rate of 0.60. The secondary cooling system uses a medium cooling mode with a specific water flow rate of 0.60 and a standard pulling speed of 0.85 m / min. The crystallizer electric stirring parameters are 350±10A / 3±0.5Hz, and the terminal electric stirring parameters are 250±10A / 8±0.5Hz. The secondary cooling system is divided into four zones, with the water distribution ratio for zones 1 to 4 set at 35:21:38:6. By using a high cooling rate in zone 3, the system effectively promotes the fine dispersion precipitation of sulfides in the two-phase region at the solidification front.

[0066] The end face size of the continuously cast billet is 200mm*200mm. The chemical composition and mass percentage of the billet are as follows: C: 0.21%, Si: 0.24%, Mn: 0.97%, Cr: 1.10%, Ti: 0.05%, S: 0.031%, P: 0.013%, Als: 0.015%, Ni: 0.01%, Cu: 0.01%, Mo: 0.002%, residual elements Sn: 0.001%, Pb: 0.001%, As: 0.005%, Bi: 0.002%, Sb: 0.002%, total oxygen content 15ppm, total nitrogen content 31ppm, balance is Fe and unavoidable impurities, of which the manganese-sulfur ratio is 31, the remainder is iron and unavoidable impurity elements; The heating process involves placing a 200mm square billet into a heating furnace for heating. The temperature in the soaking zone is controlled at 1220±15℃. After heating, the surface of the billet is descaled with high-pressure water. Then, it is rolled into finished bar products. After the final rolling, the temperature is controlled at 810~830℃.

[0067] Combination Figure 6 As shown, after slow cooling, samples were taken for testing of sulfides. The main types were single manganese sulfides and composite sulfides with a calcium aluminate core and a manganese sulfide outer layer. The aspect ratios varied from 3 to 6, and they were dispersed, which effectively ensured the machinability of gear steel for automotive gearboxes.

[0068] In this embodiment, the gear steel for automotive transmissions successfully achieved a uniform and well-machinable distribution of sulfides in the finished bar stock. The microstructure, as tested, is ferrite + pearlite with no supercooled microstructure and a total oxygen content of 15 ppm. After processing by downstream users, the sulfur-containing gear steel for automotive transmissions exhibits high machining precision and good matching performance.

[0069] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A gear steel for automotive transmissions, characterized in that, Its chemical composition, by mass percentage, includes: C: 0.19%~0.21%, Si: 0.23%~0.28%, Mn: 0.95%~0.98%, Cr: 1.08%~1.12%, Ti: 0.05%~0.07%, S: 0.028%~0.035%, P≤0.015%, Als: 0.008%~0.025%, Ni≤0.05%, Cu≤0.05%, Mo≤0.01%, residual elements Sn≤0.01%, Pb≤0.005%, As≤0.02%, Bi≤0.008%, Sb≤0.008%, total oxygen content: 0.001%~0.002%, total nitrogen content ≤0.004%, with the balance being Fe and unavoidable impurity elements, wherein the Mn / S ratio ranges from 27 to 35.

2. A method for manufacturing gear steel for automotive transmissions as described in claim 1, characterized in that, Specifically, the following steps are included: Step 1: Converter smelting: Control the carbon-oxygen product at the end of the blowing process to ≤0.0024%, and add an aluminum-containing modifier to modify the ladle top slag; Step 2: Refining LF furnace. High-alkalinity slag is used in the early stage of refining. During the refining process, aluminum wire precipitation deoxidation is combined with calcium carbide diffusion deoxidation. In the later stage of refining, lower-alkalinity slag is used. After pre-calcium treatment, the soft blowing time is guaranteed to be ≥10min before leaving the station. Step 3: Refining RH furnace, using high vacuum large circulation operation, high vacuum time ≥5min; after the final calcium treatment interval is ≥8min, feed in the sulfur line, and control the soft blowing time to ≥10min before leaving the station; Step 4: Continuous casting, with full-process protective casting. Electric stirring is used throughout the process, including both the mold and the end-point stirring. The mold is 105±2.5m in diameter. 3 The system operates in a weak cooling mode with a flow rate of / h, while the secondary cooling system uses a medium cooling mode. The specific water flow rate is controlled between 0.55 and 0.60, and the standard pull speed is 0.9 ± 0.05 m / min. Step 5: Heat the steel billet, and control the temperature of the soaking zone at 1220±15℃; Step Six: High-Pressure Water Descaling; Step 7: Rolling.

3. The method for manufacturing gear steel for automotive transmissions according to claim 2, characterized in that, In step one, the binary basicity of the aluminum-containing modifier is 5-6.

4. The method for manufacturing gear steel for automotive transmissions according to claim 2, characterized in that, In step two, quartz is added in the later stage of refining to control the binary basicity of the final slag at 1.5~2.

5. After pre-calcification treatment, the Al content in the molten steel is controlled at 0.020%~0.035%, and Ca≥0.0010%.

5. The method for manufacturing gear steel for automotive transmissions according to claim 2, characterized in that, In step three, the ultimate vacuum degree of the high vacuum large circulation is ≤20Pa, the Al content after final calcium treatment is controlled at 0.010%~0.025%, the calcium-aluminum ratio is controlled at 0.08~0.15, and the S content is controlled at 0.030%~0.035% after the sulfur wire is fed in.

6. The method for manufacturing gear steel for automotive transmissions according to claim 2, characterized in that, In step four, the intermediate package uses a high-SiO2 covering agent with an alkalinity of 1.0~1.

5.

7. The method for manufacturing gear steel for automotive transmissions according to claim 2, characterized in that, In step four, the crystallizer electric stirring parameters are selected as 350±10A / 3±0.5Hz, and the end electric stirring parameters are selected as 250±10A / 8±0.5Hz.

8. The method for manufacturing gear steel for automotive transmissions according to claim 2, characterized in that, In step four, the cooling water distribution ratio for zones 1 to 4 is set at 35:21:38:

6.

9. The method for manufacturing gear steel for automotive transmissions according to claim 2, characterized in that, In step seven, the temperature is controlled at 810~840℃ after the final rolling.

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