Isothermal annealing-free cold extrusion gear steel and production method thereof

Abstract

The invention belongs to the technical field of ferrous metallurgy, and relates to gear steel for isothermal annealing-free cold extrusion and a production method thereof. The steel comprises the following chemical components in percentage by weight: 0.18 to 0.25 percent of C, 0.15 to 0.35 percent of Si, 1.20 to 1.60 percent of Mn, 0.80 to 1.20 percent of Cr, 0.020 to 0.050 percent of Al, 0.010 to 0.030 percent of Ti, 0.015 to 0.040 percent of Nb, less than or equal to 0.015 percent of S, less than or equal to 0.015 percent of P, less than or equal to 0.008 percent of N, less than or equal to 0.0015 percent of O and the balance of Fe and inevitable impurities. The gear steel is produced through the steps of converter smelting, LF refining, RH vacuum treatment, continuous casting, heating rolling and controlled cooling. The gear steel can be directly subjected to cold extrusion forming without isothermal annealing treatment, and has excellent cold extrusion performance and mechanical performance.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy technology, and relates to a gear steel for cold extrusion without isothermal annealing and its production method. Background Technology

[0002] The drawbacks of traditional gear manufacturing processes: Traditional gear manufacturing often employs a process of hot forging followed by precision machining, resulting in low material utilization, high energy consumption during hot forging, increased processing costs, and environmental pollution. Furthermore, the gears experience shrinkage during the cooling process after hot forging, affecting dimensional accuracy. The metal and tool wear during steel machining are also significant.

[0003] Cold forging offers significant advantages such as material saving, energy saving, low cost, and high efficiency. Furthermore, cold-forged gears, due to the distribution of metal flow lines along the tooth profile and their dense microstructure, exhibit superior strength, wear resistance of the tooth surface, heat treatment performance, and meshing noise compared to machined gears. However, cold-forged gears suffer from high resistance to plastic deformation in the cold state, which can severely reduce die life. Currently, spheroidizing annealing is commonly used to reduce the hardness of the rolled material, but the annealing time often lasts for tens of hours, significantly increasing costs and reducing production efficiency.

[0004] With increasing global focus on energy conservation and environmental protection, the manufacturing industry faces pressure to reduce energy consumption and carbon emissions. The isothermal annealing process in traditional gear steel production is energy-intensive, while the isothermal annealing-free cold extrusion technology for gear steel optimizes the production process, reducing or eliminating the isothermal annealing step. This effectively reduces energy consumption and carbon emissions, aligning with the trend of green manufacturing.

[0005] Modern industrial sectors such as automotive and aerospace are placing increasingly higher demands on gear performance. Gears are required not only to possess high strength, wear resistance, and fatigue life, but also excellent dimensional accuracy and surface quality. Cold-extruded gear steel without isothermal annealing, through rational chemical composition design and advanced manufacturing processes, can meet these high-performance requirements, providing strong support for the development of modern industry.

[0006] Therefore, developing a gear steel that can be directly cold-extruded without isothermal annealing is of great significance for improving gear production efficiency and reducing production costs. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the prior art and provide a gear steel for cold extrusion without isothermal annealing and its production method. This gear steel can be directly cold extruded without isothermal annealing and has excellent cold extrusion performance and mechanical properties.

[0008] In a first aspect, the present invention provides a gear steel for cold extrusion without isothermal annealing, wherein the chemical composition of the steel by weight percentage is: C: 0.18-0.25%, Si: 0.15-0.35%, Mn: 1.20-1.60%, Cr: 0.80-1.20%, Al: 0.020-0.050%, Ti: 0.010-0.030%, Nb: 0.015-0.040%, S≤0.015%, P≤0.015%, N≤0.008%, O≤0.0015%, with the remainder being Fe and unavoidable impurities; the steel has a Brinell hardness of 130-150 HBW, a tensile strength of 600-700 MPa, and a reduction of area exceeding 60%.

[0009] Preferably, the chemical composition of the steel by weight percentage is: C: 0.20-0.23%, Si: 0.20-0.30%, Mn: 1.30-1.50%, Cr: 0.90-1.10%, Al: 0.025-0.045%, Ti: 0.015-0.025%, Nb: 0.020-0.035%, S≤0.012%, P≤0.012%, N≤0.006%, O≤0.0012%, with the remainder being Fe and unavoidable impurities.

[0010] Secondly, the present invention also provides a method for producing gear steel for cold extrusion without isothermal annealing, comprising the following steps:

[0011] (1) Converter smelting: Top and bottom blowing converter is used for smelting, and the final C content is controlled at 0.08-0.12% and the final temperature is 1620-1660℃;

[0012] (2) LF refining: The molten steel after tapping from the converter is sent to the LF refining furnace. A high-basicity slag system is used, and silicon-calcium alloy is added to carry out slag formation, deoxidation, desulfurization and composition adjustment. The refining time is controlled at 40-60 minutes, and the temperature of the molten steel at the end of the refining is 1550-1580℃.

[0013] (3) RH vacuum treatment: The molten steel after LF refining is sent into the RH vacuum treatment device, the vacuum degree is ≤5Pa, the treatment time is 15-25 minutes, and the temperature of the molten steel at the end of the treatment is controlled to be 1520-1550℃.

[0014] (4) Continuous casting: The continuous casting process is carried out using a fully protected casting process. The cross-section of the continuously cast billet is 280 or 300 mm square billet, the casting speed is 0.65-0.75 m / min, and the crystallizer stirring + electromagnetic stirring is used.

[0015] (5) Heating and rolling: Heat the continuous casting billet to 1220-1250℃, hold for 4-6 hours, and then perform multi-pass rolling with a total reduction rate ≥90% and a final rolling temperature of 850-900℃.

[0016] (6) Controlled cooling: After rolling, water cooling is adopted, with a cooling rate of 5-10℃ / s. After cooling to 600-650℃, air cooling is carried out to room temperature.

[0017] Furthermore, in the LF refining process, the slag basicity R = 3.5-4.5, and the silicon-calcium alloy wire feeding rate is 1.5-2.5 m / t steel.

[0018] Furthermore, in the continuous casting process, the crystallizer stirring current is 200-250A and the stirring frequency is 2.5-3Hz, the end electromagnetic stirring current is 150-200A and the stirring frequency is 5-8Hz; the crystallizer water flow rate is 2600-3000L / min, and the secondary cooling water flow rate is 0.3-0.4L / kg steel.

[0019] The rationale for the design of the chemical composition of the gear steel in this invention is as follows:

[0020] C: Carbon is the main element affecting the strength and hardness of steel. Too low a carbon content will result in insufficient steel strength; too high a carbon content will increase the hardness of the steel, reducing its plasticity and cold extrusion performance. This invention controls the C content at 0.18-0.25%, ensuring both sufficient strength and good cold extrusion performance of the steel.

[0021] Silicon (Si): Silicon is a beneficial element in steel, improving its strength and hardness. However, excessive silicon content increases the brittleness and reduces the plasticity of steel. This invention controls the Si content to 0.15-0.35% to ensure that the steel has good comprehensive mechanical properties.

[0022] Mn: Manganese can improve the strength and toughness of steel, while lowering its phase transformation temperature, which is beneficial for obtaining a fine grain structure. This invention controls the Mn content at 1.20-1.60% to improve the mechanical properties and cold working properties of the steel.

[0023] Cr: Chromium can improve the hardenability and wear resistance of steel, and also improve its corrosion resistance. This invention controls the Cr content at 0.80-1.20% to ensure the gears have good performance.

[0024] Al: Aluminum is a strong deoxidizer, which can effectively reduce the oxygen content in steel, while refining the grain size and improving the strength and toughness of the steel. This invention controls the Al content at 0.020-0.050% to maximize its deoxidizing and grain-refining effects.

[0025] Ti and Nb: Both titanium and niobium are strong carbonitride forming elements, capable of forming stable carbonitrides, effectively refining grains, and improving the strength and toughness of steel. Simultaneously, they can also inhibit grain growth in steel during heating. This invention controls the Ti content at 0.010-0.030% and the Nb content at 0.015-0.040% to achieve a good grain refining effect.

[0026] S and P: Sulfur and phosphorus are harmful elements in steel, which reduce the plasticity and toughness of steel and increase its brittleness. This invention strictly controls S ≤ 0.015% and P ≤ 0.015% to ensure that the steel has good plasticity and toughness.

[0027] Nitrogen and oxygen are gaseous impurities in steel that can form pores and oxide inclusions, reducing the performance of the steel. This invention controls N ≤ 0.008% and O ≤ 0.0015% to improve the purity of the steel.

[0028] The beneficial effects of this invention are:

[0029] (1) This invention rationally designs the chemical composition of gear steel, especially optimizing the content of elements such as C, Mn, and Cr, and adding appropriate amounts of microalloying elements such as Ti and Nb, so that the steel can be directly cold extruded without isothermal annealing, which greatly shortens the production cycle and reduces the production cost.

[0030] (2) This invention adopts a process flow of converter-LF-RH refining-continuous casting-controlled rolling and cooling. Through precise composition design and optimized heat treatment process, a triple match of hardness, strength, and plasticity is achieved. In particular, the design of omitting the isothermal annealing process significantly improves production efficiency and reduces manufacturing costs while ensuring forming quality. Actual tests have verified that this material exhibits excellent flowability, dimensional stability, and surface finish during gear cold extrusion molding, fully meeting the cold extrusion manufacturing requirements of high-precision gear parts.

[0031] (3) The production process of the present invention is simple and feasible, easy to industrialize, and has broad application prospects. Detailed Implementation

[0032] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0033] The performance test results of Examples 1-3 are shown in Table 1.

[0034] Example 1:

[0035] A cold extrusion gear steel without isothermal annealing and its production method are disclosed. The chemical composition of the steel by weight percentage is as follows: C: 0.20%, Si: 0.25%, Mn: 1.40%, Cr: 1.00%, Al: 0.035%, Ti: 0.020%, Nb: 0.025%, S: 0.010%, P: 0.010%, N: 0.005%, O: 0.0010%, with the remainder being Fe and unavoidable impurities.

[0036] The production method includes the following steps:

[0037] (1) Converter smelting: Top and bottom blown converter is used for smelting, and the final C content is controlled at 0.10% and the final temperature is 1640℃.

[0038] (2) LF refining: The molten steel after tapping from the converter is sent to the LF refining furnace. A high basicity slag system is adopted, with slag basicity R=4.0. Silicon-calcium alloy is added for deoxidation. The wire feed rate is 2.0m / t steel. Slag formation, deoxidation, desulfurization and composition adjustment are carried out. The refining time is controlled at 50 minutes. The temperature of molten steel at the end of refining is 1560℃.

[0039] (3) RH vacuum treatment: The molten steel after LF refining is sent into the RH vacuum treatment device with a vacuum degree of 3Pa and a treatment time of 20 minutes. The temperature of the molten steel at the end of the treatment is controlled to be 1530℃.

[0040] (4) Continuous casting: The continuous casting process is carried out using a fully protected casting process. The cross-section of the continuous casting billet is 280mm square billet, the casting speed is 0.75m / min, the crystallizer stirring current is 200A and the stirring frequency is 3Hz, the end electromagnetic stirring current is 150A and the stirring frequency is 8Hz, the crystallizer water flow rate is 2800L / min, and the secondary cooling water flow rate is 0.40L / kg steel.

[0041] (5) Heating and rolling: The continuous casting billet is heated to 1220℃ and held for 6 hours. Then, it is rolled in multiple passes with a total reduction rate of 92% and a final rolling temperature of 880℃.

[0042] (6) Controlled cooling: After rolling, water cooling is adopted, with a cooling rate of 8℃ / s. After cooling to 620℃, air cooling is carried out to room temperature.

[0043] Example 2:

[0044] A type of gear steel for cold extrusion without isothermal annealing and its production method, wherein the chemical composition by weight percentage is: C: 0.21%, Si: 0.20%, Mn: 1.30%, Cr: 0.90%, Al: 0.040%, Ti: 0.025%, Nb: 0.020%, S: 0.012%, P: 0.012%, N: 0.006%, O: 0.0012%, with the remainder being Fe and unavoidable impurities.

[0045] The production method includes the following steps:

[0046] (1) Converter smelting: Top and bottom blown converter is used for smelting, and the final C content is controlled at 0.08% and the final temperature is 1620℃.

[0047] (2) LF refining: The molten steel after tapping from the converter is sent to the LF refining furnace. A high basicity slag system is adopted, with slag basicity R=3.5. Silicon-calcium alloy is added for deoxidation. The wire feed rate is 1.5m / t steel. Slag formation, deoxidation, desulfurization and composition adjustment are carried out. The refining time is controlled at 40 minutes. The temperature of molten steel at the end of refining is 1550℃.

[0048] (3) RH vacuum treatment: The molten steel after LF refining is sent into the RH vacuum treatment device with a vacuum degree of 5 Pa and a treatment time of 15 minutes. The temperature of the molten steel at the end of the treatment is controlled to be 1520℃.

[0049] (4) Continuous casting: The continuous casting process is carried out using a fully protected casting process. The cross-section of the continuous casting billet is 280mm square billet, the casting speed is 0.70m / min, and electromagnetic stirring is used. The stirring current of the crystallizer is 200A and the stirring frequency is 3Hz. The electromagnetic stirring current at the end is 150A and the stirring frequency is 5Hz. The water flow rate of the crystallizer is 2600L / min, and the secondary cooling water flow rate is 0.35L / kg steel.

[0050] (5) Heating and rolling: The continuous casting billet is heated to 1230℃ and held for 5 hours. Then, it is rolled in multiple passes with a total reduction rate of 90% and a final rolling temperature of 850℃.

[0051] (6) Controlled cooling: After rolling, water cooling is adopted, with a cooling rate of 5℃ / s. After cooling to 600℃, air cooling is carried out to room temperature.

[0052] Example 3:

[0053] A type of gear steel for cold extrusion without isothermal annealing and its production method, wherein the chemical composition by weight percentage is: C: 0.23%, Si: 0.30%, Mn: 1.50%, Cr: 1.10%, Al: 0.040%, Ti: 0.030%, Nb: 0.035%, S: 0.005%, P: 0.008%, N: 0.004%, O: 0.0008%, with the remainder being Fe and unavoidable impurities.

[0054] The production method includes the following steps:

[0055] (1) Converter smelting: Top and bottom blown converter is used for smelting, and the final C content is controlled at 0.12% and the final temperature is 1660℃.

[0056] (2) LF refining: The molten steel after being tapped from the converter is sent to the LF refining furnace. A high basicity slag system is adopted, with slag basicity R=4.5. Silicon-calcium alloy is added for deoxidation. The wire feed rate is 2.5m / t steel. Slag formation, deoxidation, desulfurization and composition adjustment are carried out. The refining time is controlled at 60 minutes. The temperature of molten steel at the end of refining is 1580℃.

[0057] (3) RH vacuum treatment: The molten steel after LF refining is sent into the RH vacuum treatment device with a vacuum degree of 2Pa and a treatment time of 25 minutes. The temperature of the molten steel at the end of the treatment is controlled to be 1550℃.

[0058] (4) Continuous casting: The continuous casting process is carried out using a fully protected casting process. The cross-section of the continuous casting billet is 300mm square billet, the casting speed is 0.65m / min, and electromagnetic stirring is used. The stirring current of the crystallizer is 200A and the stirring frequency is 2.5Hz. The end electromagnetic stirring current is 200A and the stirring frequency is 5Hz. The crystallizer water flow rate is 3000L / min, and the secondary cooling water flow rate is 0.30L / kg steel.

[0059] (5) Heating and rolling: The continuous casting billet is heated to 1250℃ and held for 4 hours. Then, it is rolled in multiple passes with a total reduction of 95% and a final rolling temperature of 900℃.

[0060] (6) Controlled cooling: After rolling, water cooling is adopted at a rate of 10℃ / s. After cooling to 650℃, air cooling is carried out to room temperature.

[0061] Table 1. Performance test results of gear steel produced in Examples 1-3.

[0062]

[0063]

[0064] In summary, the gear steel for cold extrusion without isothermal annealing produced using the technical solution of this invention exhibits a Brinell hardness precisely controlled within the range of 130-150 HBW and a tensile strength stable at 600-700 MPa, forming a hard-strength combination suitable for cold working. Its excellent reduction of area exceeding 60% confirms the material's superior plastic deformation capability. It requires no isothermal annealing pretreatment and directly meets the requirements of the cold extrusion forming process. Actual process verification shows that this material has excellent cold extrusion forming performance and fully complies with the technical specifications for precision cold extrusion forming of gear parts.

[0065] The above embodiments are preferred implementations of the present invention. In addition, the present invention can be implemented in other ways. Any obvious substitutions without departing from the concept of the present technical solution are within the protection scope of the present invention.

Claims

1. A gear steel for cold extrusion without isothermal annealing, characterized in that, The steel's chemical composition by weight percentage is as follows: C: 0.18-0.25%, Si: 0.15-0.35%, Mn: 1.20-1.60%, Cr: 0.80-1.20%, Al: 0.020-0.050%, Ti: 0.010-0.030%, Nb: 0.015-0.040%, S≤0.015%, P≤0.015%, N≤0.008%, O≤0.0015%, with the remainder being Fe and unavoidable impurities; the steel has a Brinell hardness of 130-150 HBW, a tensile strength of 600-700 MPa, and a reduction of area exceeding 60%.

2. The gear steel for cold extrusion without isothermal annealing as described in claim 1, characterized in that, The chemical composition of the steel by weight percentage is as follows: C: 0.20-0.23%, Si: 0.20-0.30%, Mn: 1.30-1.50%, Cr: 0.90-1.10%, Al: 0.025-0.045%, Ti: 0.015-0.025%, Nb: 0.020-0.035%, S≤0.012%, P≤0.012%, N≤0.006%, O≤0.0012%, with the remainder being Fe and unavoidable impurities.

3. A method for producing gear steel for cold extrusion without isothermal annealing as described in claim 1 or 2, characterized in that, Includes the following steps: (1) Converter smelting: Top and bottom blowing converter is used for smelting, and the final C content is controlled at 0.08-0.12% and the final temperature is 1620-1660℃; (2) LF refining: The molten steel after tapping from the converter is sent to the LF refining furnace. A high-basicity slag system is used, and silicon-calcium alloy is added to carry out slag formation, deoxidation, desulfurization and composition adjustment. The refining time is controlled at 40-60 minutes, and the temperature of the molten steel at the end of the refining is 1550-1580℃. (3) RH vacuum treatment: The molten steel after LF refining is sent into the RH vacuum treatment device, the vacuum degree is ≤5Pa, the treatment time is 15-25 minutes, and the temperature of the molten steel at the end of the treatment is controlled to be 1520-1550℃. (4) Continuous casting: The continuous casting process is carried out using a fully protected casting process. The cross-section of the continuously cast billet is 280 or 300 mm square billet, the casting speed is 0.65-0.75 m / min, and the crystallizer stirring + electromagnetic stirring is used. (5) Heating and rolling: Heat the continuous casting billet to 1220-1250℃, hold for 4-6 hours, and then perform multi-pass rolling with a total reduction rate ≥90% and a final rolling temperature of 850-900℃. (6) Controlled cooling: After rolling, water cooling is adopted, with a cooling rate of 5-10℃ / s. After cooling to 600-650℃, air cooling is carried out to room temperature.

4. The method for producing gear steel for cold extrusion without isothermal annealing as described in claim 3, characterized in that, In the LF refining process, the slag basicity R = 3.5-4.5, and the silicon-calcium alloy wire feeding rate is 1.5-2.5 m / t steel.

5. The method for producing gear steel for cold extrusion without isothermal annealing as described in claim 4, characterized in that, In the continuous casting process, the crystallizer stirring current is 200-250A and the stirring frequency is 2.5-3Hz, the end electromagnetic stirring current is 150-200A and the stirring frequency is 5-8Hz; the crystallizer water flow rate is 2600-3000L / min, and the secondary cooling water flow rate is 0.3-0.4L / kg steel.

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