CN-122013024-A - 20MnCr5 gear steel and grain size control method thereof

Abstract

The invention belongs to the technical field of gear steel metallurgy and heat treatment, and discloses a 20MnCr5 gear steel and a grain size control method thereof, wherein TiC nano particles are uniformly introduced into 20MnCr5 molten steel in a bottom blowing mode, and are matched with high-temperature diffusion treatment to fully diffuse solute atoms, and the dispersed TiC nano particles are utilized to generate a strong-effect Zener pinning effect in a carburizing process, so that stable control of austenite grains is realized, the grain size of the 20MnCr5 gear steel can stably reach more than 7.0 grade after carburization at a high temperature and has no mixed crystal phenomenon, and the fatigue strength and impact toughness of the gear steel are remarkably improved.

Inventors

• SONG BANGMIN
• WEI ZHENDIAN
• Zou hu
• LU JIHUAN
• SHI YANG

Assignees

• 芜湖新兴铸管有限责任公司

Dates

Publication Date

20260512

Application Date

20260325

Claims (10)

1. 1. A control method for the grain size of 20MnCr5 gear steel is characterized by comprising the following steps: converter smelting-LF refining-RH vacuum-continuous casting-heating-rolling-carburizing-quenching-tempering; in the LF refining step, tiC nano particles are added into molten steel in the later stage of LF refining in a bottom blowing mode.
2. 2. The control method according to claim 1, wherein in the LF refining step, the bottom blowing is controlled using pulsed air pressure.
3. 3. The control method according to claim 2, wherein the carrier gas used for the pulse gas pressure control is argon gas, the gas pressure is 0.5-2.0MPa, and the pulse frequency is 1-5Hz.
4. 4. The control method according to claim 1, wherein in the LF refining step, tiC nanoparticles have a particle diameter of 20 to 100 nm and an addition amount of 0.05 to 0.15% by mass of molten steel.
5. 5. The control method according to claim 1, wherein in the heating step, the high-temperature diffusion treatment is performed at 1200 to 1250 ℃ for 2 to 4 hours.
6. 6. The control method according to claim 1, wherein in the rolling step, a speed of the cogging mill rolling is 1.0 to 1.5m/s.
7. 7. The control method according to claim 1, wherein in the carburizing step, carburization is performed at a temperature of 930-960 ℃, and carbon potential is controlled to be 0.8% -1.2% for 4-8 hours.
8. 8. The control method according to claim 1, wherein in the quenching step, oil quenching is used to cool to room temperature, and the cooling rate is controlled to be equal to or higher than 50 ℃ per second.
9. 9. The method according to claim 1, wherein in the tempering step, the tempering temperature is 180-220 ℃ and the temperature is kept for 2-4 hours.
10. 10. The 20MnCr5 gear steel produced by the control method of claim 1, wherein the chemical components of the 20MnCr5 gear steel are, by weight, 0.17% -0.22% of C, 1.10% -1.40% of Mn, 0.90% -1.20% of Cr, 0.15% -0.35% of Si, less than or equal to 0.035% of P, less than or equal to 0.035% of S, 0.05% -0.12% of Ti, and the balance of Fe and unavoidable impurities.

Description

20MnCr5 gear steel and grain size control method thereof Technical Field The invention belongs to the field of gear steel metallurgy, and particularly relates to 20MnCr5 gear steel and a grain size control method thereof. Background 20MnCr5 is an alloy carburizing steel, has good hardenability, machinability and comprehensive mechanical properties, and is widely applied to key core components such as automobile gearbox gears, engineering machinery transmission shafts, wind power transmission systems and the like. With the development of heavy-duty and high-speed transmission technologies, gear parts are evolving towards high precision and high fatigue life, which puts severe demands on grain size control after heat treatment. In the conventional carburizing process, long-time carburization at 930 ℃ or higher is often used in order to improve the production efficiency. Under the high-temperature environment, the original austenite grains in the steel are extremely easy to grow abnormally, and coarse austenite grains or serious mixed crystal phenomenon is generated. The coarsening of the crystal grains can not only remarkably reduce the contact fatigue strength and the bending fatigue life of the gear, but also lead to the increase of quenching deformation, and directly affect the meshing precision and the service stability of the gear. At present, micro-alloy elements such as Nb, V or Ti are added to refine grains, and the principle is that generated carbonitrides such as NbC, VC or TiN are utilized to generate pinning effect on austenite grain boundaries. However, in practical production, the conventional schemes still have significant limitations, firstly, the carbide of Nb or V is easy to generate partial solid solution or Ostwald ripening at the carburizing temperature of more than 930 ℃ to lead to rapid reduction of pinning force and can not inhibit grain boundary migration in the long-time carburization process, secondly, the conventional Ti alloying process is extremely easy to form large-size TiN inclusion with sharp corners in the molten steel solidification process, so that grains can not be refined, but become a initiation source of fatigue cracks to damage the fatigue strength, and finally, the addition of excessive microalloy elements can change the CCT curve of steel to fluctuate the hardenability zone, thereby causing uneven structure and hardness of a gear core and increasing the difficulty of heat treatment process control. Disclosure of Invention The invention also aims to provide a control method of the grain size of the 20MnCr5 gear steel, which realizes the stable refinement of austenite grains under the carburization condition at high temperature and long time by a specific heat treatment process and matching with the Zener pinning effect of nano particles, eliminates the mixed crystal phenomenon, and improves the fatigue life and the dimensional stability of the gear. The invention aims to provide 20MnCr5 gear steel produced by the control method. The technical scheme of the invention is as follows: the invention provides a control method of 20MnCr5 gear steel grain size, which comprises the following steps: converter smelting-LF refining-RH vacuum-continuous casting-heating-rolling-carburizing-quenching-tempering; in the LF refining step, tiC nano particles are added into molten steel in the later stage of LF refining in a bottom blowing mode. In the LF refining step, the bottom blowing is controlled by pulse air pressure. The carrier gas used in the pulse air pressure control is argon, the air pressure is 0.5-2.0MPa, and the pulse frequency is 1-5Hz. In the LF refining step, the grain diameter of TiC nano particles is 20-100 nm, and the addition amount is 0.05-0.15% of the mass of molten steel. In the heating step, high-temperature diffusion treatment is carried out at 1200-1250 ℃ for 2-4h. In the rolling step, the rolling speed of the cogging mill is 1.0-1.5m/s. In the carburizing step, carburizing is carried out at the temperature of 930-960 ℃, and the carbon potential is controlled to be 0.8-1.2% for 4-8 hours. In the quenching step, oil quenching is used for cooling to room temperature, and the cooling rate is controlled to be more than or equal to 50 ℃ per second. In the tempering step, the tempering temperature is 180-220 ℃, and the heat preservation is carried out for 2-4 hours. The invention provides 20MnCr5 gear steel produced by the control method, which comprises the following chemical components, by weight, 0.17% -0.22% of C, 1.10% -1.40% of Mn, 0.90% -1.20% of Cr, 0.15% -0.35% of Si, less than or equal to 0.035% of P, less than or equal to 0.035% of S, 0.05% -0.12% of Ti, and the balance of Fe and unavoidable impurities. In the method, firstly, preferably, a pulse air pressure controlled bottom blowing mode is adopted in a refining stage, the van der Waals force among nano particles is overcome by utilizing the periodical oscillation pressure generated by argon, so that