Low-sulfur free-cutting crmn gear steel and method for producing the same

Abstract

The present application relates to gear steel bar steel technical field, especially to a kind of low sulfur easy cutting CrMn gear steel and its production method.The chemical components include the following mass percentage: C:0.23%~0.28%, Si:0.11%~0.19%, Mn:0.70%~0.90%, Cr:0.90%~1.00%, Mo:0.25%~0.35%, W:0.10%~0.19%, Ni:0.10%~0.19%, Al:0.020%~0.040%, S:0.030%~0.050%, Ce:0.0010%~0.0030%, Mg:0.0035%~0.0065%, the balance is Fe and inevitable impurities.The present application improves the morphology and composition of non-metallic inclusions by Ce, Mg, S and other elements, so that the steel obtains good cutting performance, improves the surface finish of gear steel machining parts, and also increases the cutting speed of parts by 5%~8%.

Description

Technical Field

[0001] This invention relates to the field of steel technology for gear steel bars, and more particularly to a low-sulfur, free-machining CrMn-based gear steel and its production method. Background Technology

[0002] Gears are key components in automobiles and mechanical transmissions, requiring steel with both high strength and high machinability. Traditional gear steels are difficult to machine due to their high alloy content and hardness. While sulfur can improve machinability, high sulfur content reduces the purity of the steel (e.g., increasing oxide inclusions) and affects mechanical properties.

[0003] Existing technologies, such as patent CN1664150A, disclose a method for manufacturing sulfur-containing free-machining gear steel and its steel pipe, which improves machinability by adjusting the sulfur content to 0.03%~0.05%. However, the sulfur content in this patent is still too high, and it fails to effectively resolve the contradiction of sulfur-oxygen control—both maintaining sulfur to improve machinability and deoxidizing to ensure steel purity.

[0004] The challenge of controlling sulfide morphology is even more complex. Sulfur in steel combines with manganese to form MnS inclusions. These inclusions are prone to elongation and deformation during hot rolling, leading to localized stress concentration and significantly reducing the material's fatigue life. This is especially true for medium-carbon gear steel, where the low dissolved oxygen content makes it difficult to promote the formation of spherical sulfides by increasing the oxygen content, further complicating sulfide morphology control. Summary of the Invention

[0005] The purpose of this invention is to provide a low-sulfur, easily machinable CrMn-based gear steel and its production method. By adding trace amounts of elements such as Ce, Mg, and S, the surface finish of the gear steel parts can be improved, while the cutting speed of the parts can be increased.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] This invention provides a low-sulfur, free-machining CrMn-based gear steel comprising the following chemical composition by mass percentage:

[0008] C: 0.23%~0.28%, Si: 0.11%~0.19%, Mn: 0.70%~0.90%, Cr: 0.90%~1.00%, Mo: 0.25%~0.35%, W: 0.10%~0.19%, Ni: 0.10%~0.19%, Al: 0.020%~0.040%, S: 0.030%~0.050%, Ce: 0.0010%~0.0030%, Mg: 0.0035%~0.0065%, with the balance being Fe and unavoidable impurities.

[0009] The principle behind adding each element in this invention is as follows:

[0010] C: C is an essential element for ensuring the strength, hardness, and hardenability of steel. It is also crucial for guaranteeing the core strength and toughness of gear steel. Furthermore, as a core alloying element, C forms carbides with Cr, W, Mo, etc. (such as M6C, M...). 23 C6, etc., significantly improves hardness and wear resistance; therefore, the mass percentage of C is controlled at 0.23%~0.28%.

[0011] Si: Si acts as a deoxidizer in steel. In carburized gear steel, it is easily oxidized. When oxygen invades from the surface, silicon near the grain boundaries or grains diffuses to the grain boundaries preferentially compared to other elements. It combines with trace amounts of oxygen dissolved on the surface to form oxides at the grain boundaries, which are distributed in a network. The oxides of silicon reduce the surface hardness and fatigue strength of the parts, weaken the grain boundaries, and increase the possibility of cracking. Therefore, the mass percentage of Si should be controlled at 0.11%~0.19%.

[0012] Mn: Mn not only acts as a deoxidizer and desulfurizer, but also improves hardenability, promotes uniform distribution of carbon, and enhances solid solution strengthening. In addition, Mn and S can form MnS inclusions, which play a notching effect and lubricant role in gear machining, improving the machinability of steel. Therefore, the mass percentage of Mn is controlled at 0.70%~0.90%.

[0013] Cr: Cr is a medium-strong carbide-forming element, which can significantly improve the comprehensive properties of steel materials such as strength, hardenability, and wear resistance. In this invention, the mass percentage of Cr is controlled at 0.90%~1.00%.

[0014] Mo: As a strong carbide-forming element in steel, Mo can strongly inhibit carbide nucleation and growth, effectively improving hardenability. The combined effect of Mo and Mn can significantly improve the stability of austenite and enhance the hardenability of steel. Mo can delay the pearlite transformation and promote the formation of acicular ferrite. Therefore, the mass percentage of Mo is controlled at 0.25%~0.35%.

[0015] W: W can significantly hinder the precipitation of austenite grain boundary carbides, refine grains, and improve the strength, plasticity, impact toughness, and hydrogen embrittlement sensitivity of steel. In addition, as a strong carbide-forming element, W synergistically forms high-temperature stable complex carbides (such as W2C) with Cr and Mo, which significantly improves high-temperature creep resistance and wear resistance. Therefore, the mass percentage of W is controlled at 0.10%~0.19%.

[0016] Ni: Ni can enhance the cross-slip ability of the matrix, lower the brittle-to-cold transition temperature, and improve toughness. It can also give carburized steel a surface layer with a small carbon gradient and a certain degree of toughness; therefore, the mass percentage of Ni is controlled between 0.10% and 0.19%.

[0017] Nb: During the hot rolling process and the subsequent carburizing process at higher temperatures, microalloyed carbonitrides can effectively prevent grain growth, refine the grains, and allow carburizing at higher carburizing temperatures, thus improving carburizing efficiency. Therefore, the mass percentage of Nb is controlled at 0.03%~0.05%.

[0018] Al: Al is used to refine grains and deoxidize, reducing the number and size of non-metallic inclusions in steel and obtaining extremely low oxygen content. At the same time, it controls the uniformity of gear steel composition, making the austenite grains of gear steel fine and free of mixed grains. Therefore, the mass percentage of Al is controlled at 0.020%~0.040%.

[0019] S: Sulfur is one of the main free-machining elements. As the sulfur content in steel increases, the machinability index of the steel improves significantly. Therefore, the mass percentage of sulfur should be controlled between 0.030% and 0.050%.

[0020] Ce: When heated to austenitization, Ce atoms inevitably segregate towards the austenite grain boundaries, inhibiting the diffusion of carbon atoms to the grain boundaries and delaying the formation of cementite. In steel, it suppresses the nucleation of proeutectoid ferrite, improves hardenability, and also modifies non-metallic inclusions. Therefore, the mass percentage of Ce is controlled between 0.0010% and 0.0030%.

[0021] Mg: Commonly used as a deoxidizer and desulfurizer in steelmaking, it reacts with O and S in molten steel to produce magnesium oxides or sulfides, reducing the O and S levels in the molten steel to even lower levels. The resulting inclusions float to the surface or are adsorbed by the protective slag and removed from the molten steel, thus playing a role in deoxidation and desulfurization. The addition of trace amounts of Mg can improve the composition, size, and morphology of inclusions in steel and enhance the deformation capacity of inclusions in steel. Therefore, the mass percentage of Mg is controlled at 0.0035%~0.0065%.

[0022] In the above technical solution, the room temperature impact energy of the gear steel is Akv≥80J.

[0023] In the above technical solution, the hardenability of the gear steel J9 is 40~48HRC, J15≥28HRC, and the bandwidth is ≤6HRC.

[0024] In the above technical solution, the sulfide aspect ratio of the gear steel is 3~6, ​​the non-metallic inclusions are Class A ≤2.0 grade and Class B ≤2.0 grade, and the austenite grain size is 5.5~7 grade.

[0025] Another aspect of the present invention provides a method for producing the above-mentioned low-sulfur, free-machining CrMn gear steel, comprising the following steps:

[0026] (1) LF refining: LF furnace produces refining slag, the basicity of the refining slag is between 4.0 and 6.0, the electrode heating time is controlled to be 26 to 35 min, and the composition is adjusted twice, coarse adjustment and fine adjustment. The coarse adjustment is to add 0.5 to 0.8 kg / t aluminum powder for deoxidation, and the fine adjustment is to control the target content of C, Si, Mn and Cr. After the molten steel is alloyed by LF, the magnesium alloy cored wire is fed into the ladle by wire feeding process.

[0027] (2) VD vacuum degassing: the pressure holding time is controlled at 10~15min, after VD rupture, argon gas is gently blown for 10~15min, the argon gas flow rate is 40~80NL / min, and after the gentle blowing is completed, it is left to stand for 10~15min;

[0028] (3) Continuous casting: The superheat of the tundish is controlled at 20~30℃, the constant casting speed is maintained, the casting speed of the billet is 0.5~0.6m / min, the electromagnetic stirring voltage at the end of solidification is 300~500V, the current intensity is 360~500A, and the frequency is 9~12Hz;

[0029] (4) Rolling: The continuous casting billet is heated. The preheating section temperature is ≤850℃, the heating section temperature is 1100~1160℃, the soaking section temperature is 1130~1170℃, the total heating time of the continuous casting billet is ≥6h, the initial rolling temperature is 1060~1140℃, and the final rolling temperature is 890~930℃.

[0030] (5) Slow cooling: After rolling, the steel is put into the pit for slow cooling. The slow cooling time is greater than 24 hours, and the temperature of the steel coming out of the slow cooling pit is lower than 180℃.

[0031] In the above technical solution, further, in step (1), the magnesium content of the magnesium alloy cored wire is 20wt%~30wt%.

[0032] In the above technical solution, further, in step (1), the refining slag uses quicklime and fluorite.

[0033] In the above technical solution, further, in step (4), the continuous casting billet is descaled by high pressure water before rolling. High pressure descaling is carried out by single-pass multi-nozzle water spraying, with descaling pressure ≥24MPa and descaling rate ≥95%.

[0034] The beneficial effects of this invention are as follows:

[0035] 1. This invention improves the morphology and composition of non-metallic inclusions by using elements such as Ce, Mg, and S, which gives the steel good cutting performance, improves the surface finish of gear steel parts, and increases the cutting speed of the parts by 5% to 8%.

[0036] 2. This invention utilizes C to form carbides with Cr, W, Mo, etc., which significantly improves hardness and wear resistance. W, Cr, and Mo synergistically form high-temperature stable complex carbides, which significantly improve high-temperature creep resistance and wear resistance. Mn improves hardenability and makes C element uniformly distributed, enhancing the solid solution strengthening effect.

[0037] 3. The hardenability of the gear steel of this invention is 40~48HRC for J9 and ≥28HRC for J15, with a bandwidth ≤6HRC; room temperature impact energy A KV ≥80J; the aspect ratio of sulfides in gear steel is 3~6, ​​the non-metallic inclusions are Class A ≤2.0 grade and Class B ≤2.0 grade, and the austenite grain size is 5.5~7 grade.

[0038] 4. In the metallographic structure of the gear steel of the present invention, the ferrite area accounts for 50% to 60%, and the pearlite area accounts for 40% to 50%. Detailed Implementation

[0039] The technical solutions 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.

[0040] This invention provides a low-sulfur, free-machining CrMn-based gear steel, comprising the following chemical composition by mass percentage:

[0041] C: 0.23%~0.28%, Si: 0.11%~0.19%, Mn: 0.70%~0.90%, Cr: 0.90%~1.00%, Mo: 0.25%~0.35%, W: 0.10%~0.19%, Ni: 0.10%~0.19%, Al: 0.020%~0.040%, S: 0.030%~0.050%, Ce: 0.0010%~0.0030%, Mg: 0.0035%~0.0065%, with the balance being Fe and unavoidable impurities.

[0042] The above-mentioned method for producing gear steel includes the following steps:

[0043] (1) LF refining: LF furnace produces refining slag, the basicity of the refining slag is between 4.0 and 6.0, the electrode heating time is controlled at 26 to 35 min, quicklime and fluorite are used to produce refining slag, and the composition is adjusted twice, coarse adjustment and fine adjustment. The coarse adjustment is to add 0.5 to 0.8 kg / t aluminum powder for deoxidation, and the fine adjustment is to control the target content of C, Si, Mn and Cr. After the molten steel is alloyed by LF, magnesium alloy cored wire (magnesium content of magnesium alloy cored wire is 20wt% to 30wt%) is fed into the ladle by wire feeding process;

[0044] (2) VD vacuum degassing: the pressure holding time is controlled at 10~15min, after VD rupture, argon gas is gently blown for 10~15min, the argon gas flow rate is 40~80NL / min, and after the gentle blowing is completed, it is left to stand for 10~15min to ensure that the inclusions float up fully;

[0045] (3) Continuous casting: The cross-sectional size of the continuously cast billet is 320mm×410mm. The superheat of the tundish is controlled at 20~30℃. The constant casting speed is 0.5~0.6m / min. The electromagnetic stirring voltage at the end of solidification is 300~500V, the current intensity is 360~500A, and the frequency is 9~12Hz. The electromagnetic stirring process can promote the floating of non-metallic inclusions in the molten steel, reduce the particle size of inclusions in the steel, and make the inclusions evenly distributed, reduce the segregation of elements such as C, S, and Cr in the steel, and thus achieve a uniform structure.

[0046] (4) Rolling: The continuous casting billet is heated. The preheating section temperature is ≤850℃, the heating section temperature is 1100~1160℃, the soaking section temperature is 130~1170℃, and the total heating time of the continuous casting billet is ≥6h to ensure the high temperature diffusion time. Before rolling, the continuous casting billet is descaled by high pressure water. Single-pass multi-nozzle water spraying high pressure descaling is adopted. The descaling pressure is ≥24MPa, the descaling rate is ≥95%, the initial rolling temperature is 1060~1140℃, and the final rolling temperature is 890~930℃. Through low-temperature precision rolling, the amount of deformation inside the austenite grains is greatly increased, which provides a large number of nucleation cores for ferrite phase transformation and promotes ferrite phase transformation.

[0047] (5) Slow cooling: After rolling, the steel is put into the slow cooling pit. The slow cooling pit must be dry and the slow cooling time is greater than 24 hours. The temperature of the steel leaving the slow cooling pit is below 180℃.

[0048] The following are specific examples.

[0049] Example 1

[0050] A low-sulfur, easily machinable CrMn gear steel has the chemical composition shown in Table 1.

[0051] The above-mentioned method for producing gear steel includes the following steps:

[0052] (1) LF refining: LF furnace produces refining slag with a basicity of 4.35. It uses lime and fluorite for slag production and controls the electrode heating time to 28 min. It performs two composition adjustments: coarse adjustment and fine adjustment. The coarse adjustment involves adding 0.8 kg / t aluminum powder for deoxidation, and the fine adjustment involves controlling the target content of C, Si, Mn and Cr. After the molten steel is alloyed by LF, magnesium alloy cored wire with a magnesium content of 25 wt% is fed into the ladle using a wire feeding process.

[0053] (2) VD vacuum degassing: the pressure holding time is controlled at 15 min. After VD rupture, argon gas is gently blown for 12 min with an argon gas flow rate of 50 NL / min. After the gentle blowing is completed, the mixture is left to stand for 12 min to ensure that the inclusions float up fully.

[0054] (3) Continuous casting: The cross-sectional dimensions of the continuously cast billet are 320mm×410mm, the superheat of the tundish is controlled at 25℃, the constant casting speed (the casting speed of the billet is controlled at 0.56m / min), the electromagnetic stirring voltage at the end of solidification is 400V, the current intensity is 400A, and the frequency is 10Hz.

[0055] (4) Rolling: The continuous casting billet is heated. The preheating section temperature is 840℃, the heating section temperature is 1130℃, the soaking section temperature is 1165℃, the total heating time of the continuous casting billet is 7h, the continuous casting billet is descaled by high pressure water before rolling, the descaling pressure is 25MPa, the descaling rate is 97%, the rolling start temperature is 1120℃, and the final rolling temperature is 930℃.

[0056] (5) Slow cooling: The slow cooling time is 26 hours. The slow cooling pit must be dry. The temperature of the steel leaving the slow cooling pit is below 180℃.

[0057] Example 2

[0058] A low-sulfur, easily machinable CrMn gear steel has the chemical composition shown in Table 1.

[0059] The above-mentioned method for producing gear steel includes the following steps:

[0060] (1) LF refining: LF furnace produces refining slag with a basicity of 5.36. It uses lime and fluorite for slag production, controls the electrode heating time to 28 min, and performs two composition adjustments: coarse adjustment and fine adjustment. The coarse adjustment involves adding 0.7 kg / t aluminum powder for deoxidation, and the fine adjustment involves controlling the target content of C, Si, Mn and Cr. After the molten steel is alloyed by LF, magnesium alloy cored wire with a magnesium content of 25 wt% is fed into the ladle using a wire feeding process.

[0061] (2) VD vacuum degassing: the pressure holding time is controlled at 15 min. After VD rupture, argon gas is gently blown for 12 min with an argon gas flow rate of 50 NL / min. After the gentle blowing is completed, the mixture is left to stand for 12 min to ensure that the inclusions float up fully.

[0062] (3) Continuous casting: The cross-sectional dimensions of the continuously cast billet are 320mm×410mm, the superheat of the tundish is controlled at 28℃, the constant casting speed (the casting speed of the billet is controlled at 0.56m / min), the electromagnetic stirring voltage at the end of solidification is 450V, the current intensity is 450A, and the frequency is 11Hz.

[0063] (4) Rolling: The continuous casting billet is heated. The preheating section temperature is 850℃, the heating section temperature is 1125℃, the soaking section temperature is 1160℃, the total heating time of the continuous casting billet is 6.5h, the continuous casting billet is descaled by high pressure water before rolling. The descaling pressure is 25MPa, the descaling rate is 98%, the rolling start temperature is 1070℃, and the final rolling temperature is 915℃.

[0064] (5) Slow cooling: The slow cooling time is 24 hours. The slow cooling pit must be dry. The temperature of the steel leaving the slow cooling pit is below 180℃.

[0065] Example 3

[0066] A low-sulfur, easily machinable CrMn gear steel has the chemical composition shown in Table 1.

[0067] The above-mentioned method for producing gear steel includes the following steps:

[0068] (1) LF refining: LF furnace produces refining slag with a basicity of 5.58. It uses lime and fluorite for slag production, controls the electrode heating time to 30 min, and performs two composition adjustments: coarse adjustment and fine adjustment. The coarse adjustment involves adding 0.6 kg / t aluminum powder for deoxidation, and the fine adjustment involves controlling the target content of C, Si, Mn and Cr. After the molten steel is alloyed by LF, magnesium alloy cored wire with a magnesium content of 25 wt% is fed into the ladle using a wire feeding process.

[0069] (2) VD vacuum degassing: the pressure holding time is controlled at 15 min. After VD rupture, argon gas is gently blown for 10 min with an argon gas flow rate of 60 NL / min. After the gentle blowing is completed, the mixture is left to stand for 12 min to ensure that the inclusions float up fully.

[0070] (3) Continuous casting: The cross-sectional dimensions of the continuously cast billet are 320mm×410mm, the superheat of the tundish is controlled at 23℃, the constant casting speed (the casting speed of the billet is controlled at 0.55m / min), the electromagnetic stirring voltage at the end of solidification is 430V, the current intensity is 460A, and the frequency is 10Hz.

[0071] (4) Rolling: The continuous casting billet is heated. The preheating section temperature is 840℃, the heating section temperature is 1120℃, the soaking section temperature is 1150℃, the total heating time of the continuous casting billet is 7h, the continuous casting billet is descaled by high pressure water before rolling, the descaling pressure is 24MPa, the descaling rate is 98%, the rolling start temperature is 1080℃, and the final rolling temperature is 930℃.

[0072] (5) Slow cooling: The slow cooling time is 24 hours. The slow cooling pit must be dry. The temperature of the steel leaving the slow cooling pit is below 180℃.

[0073] Example 4

[0074] A low-sulfur, easily machinable CrMn gear steel has the chemical composition shown in Table 1.

[0075] The above-mentioned method for producing gear steel includes the following steps:

[0076] (1) LF refining: LF furnace produces refining slag with a basicity of 5.15. It uses lime and fluorite for slag production and controls the electrode heating time to 28 min. It performs two composition adjustments: coarse adjustment and fine adjustment. The coarse adjustment involves adding 0.7 kg / t aluminum powder for deoxidation, and the fine adjustment involves controlling the target content of C, Si, Mn and Cr. After the molten steel is alloyed by LF, magnesium alloy cored wire with a magnesium content of 25 wt% is fed into the ladle using a wire feeding process.

[0077] (2) VD vacuum degassing: the pressure holding time is controlled at 15 min. After VD rupture, argon gas is gently blown for 10 min with an argon gas flow rate of 60 NL / min. After the gentle blowing is completed, the mixture is left to stand for 12 min to ensure that the inclusions float up fully.

[0078] (3) Continuous casting: The cross-sectional dimensions of the continuously cast billet are 320mm×410mm, the superheat of the tundish is controlled at 26℃, the constant casting speed (the casting speed of the billet is controlled at 0.56m / min), the electromagnetic stirring voltage at the end of solidification is 470V, the current intensity is 400A, and the frequency is 9Hz.

[0079] (4) Rolling: The continuous casting billet is heated. The preheating section temperature is 840℃, the heating section temperature is 1120℃, the soaking section temperature is 1150℃, the total heating time of the continuous casting billet is 7h, the continuous casting billet is descaled by high pressure water before rolling, the descaling pressure is 26MPa, the descaling rate is 96%, the rolling start temperature is 1090℃, and the final rolling temperature is 920℃; (5) Slow cooling: the slow cooling time is 25h, the slow cooling pit must be dry, and the temperature of the steel leaving the slow cooling pit is below 180℃.

[0080] Comparative Example 1

[0081] A CrMn-based gear steel has the chemical composition shown in Table 1.

[0082] The above-mentioned method for producing gear steel includes the following steps:

[0083] (1) LF refining: LF furnace produces refining slag, using quicklime and fluorite for slag production, and controlling the electrode heating time to 28 min;

[0084] (2) VD vacuum degassing: the pressure holding time is controlled at 15 min, after VD rupture, argon gas is softly blown for 10 min, the argon gas flow rate is 60 NL / min, and after the soft blowing is completed, it is left to stand for 5 min;

[0085] (3) Continuous casting: The cross-sectional dimensions of the continuously cast billet are 320mm×410mm, the superheat of the tundish is controlled at 26℃, and the constant casting speed (the casting speed of the billet is controlled at 0.56m / min);

[0086] (4) Rolling: The continuous casting billet is heated at 1120℃ in the heating section and 1150℃ in the soaking section. The total heating time of the continuous casting billet is 7h. Before rolling, the continuous casting billet is descaled by high pressure water at a pressure of 26MPa and a descaling rate of 96%. The rolling start temperature is 1090℃ and the final rolling temperature is 920℃.

[0087] Table 1 Chemical composition (wt%)

[0088]

[0089] Non-metallic inclusions, austenite grain size and room temperature impact resistance of the gear steels of Examples 1-4 and Comparative Example 1 were analyzed, and the results are shown in Table 2.

[0090] The gear steels of Examples 1-4 and Comparative Example 1 were normalized by holding at 910℃ for 1 hour and then end-quenched by holding at 880℃ for 30 minutes. The quenching performance was then measured, and the results are shown in Table 2.

[0091] Table 2 Measurement Results

[0092]

[0093] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the implementation. The scope of protection of the present invention should be determined by the scope defined in the claims. Other variations or modifications can be made based on the above description. Obvious variations or modifications derived therefrom are still within the scope of protection of the present invention.

Claims

1. A low-sulfur, easily machinable CrMn-based gear steel, characterized in that, Includes the following chemical components by mass percentage: C: 0.23%~0.28%, Si: 0.11%~0.19%, Mn: 0.70%~0.90%, Cr: 0.90%~1.00%, Mo: 0.25%~0.35%, W: 0.10%~0.19%, Ni: 0.10%~0.19%, Al: 0.020%~0.040%, S: 0.030%~0.050%, Ce: 0.0010%~0.0030%, Mg: 0.0035%~0.0065%, with the balance being Fe and unavoidable impurities; The gear steel has a sulfide aspect ratio of 3 to 6, non-metallic inclusions of type A ≤ 2.0 and type B ≤ 2.0, and austenite grain size of 5.5 to 7. The method for producing the gear steel includes the following steps: (1) LF refining: LF furnace produces refining slag, the basicity of the refining slag is between 4.0 and 6.0, the electrode heating time is controlled to be 26 to 35 min, and the composition is adjusted twice, coarse adjustment and fine adjustment. The coarse adjustment is to add 0.5 to 0.8 kg / t aluminum powder for deoxidation, and the fine adjustment is to control the target content of C, Si, Mn and Cr. After the molten steel is alloyed by LF, the magnesium alloy cored wire is fed into the ladle by wire feeding process. (2) VD vacuum degassing: the pressure holding time is controlled at 10~15min, after VD rupture, argon gas is gently blown for 10~15min, the argon gas flow rate is 40~80NL / min, and after the gentle blowing is completed, it is left to stand for 10~15min; (3) Continuous casting: The superheat of the tundish is controlled at 20~30℃, the constant casting speed is maintained, the casting speed of the billet is 0.5~0.6m / min, the electromagnetic stirring voltage at the end of solidification is 300~500V, the current intensity is 360~500A, and the frequency is 9~12Hz; (4) Rolling: The continuous casting billet is heated. The preheating section temperature is ≤850℃, the heating section temperature is 1100~1160℃, the soaking section temperature is 1130~1170℃, the total heating time of the continuous casting billet is ≥6h, the rolling start temperature is 1060~1140℃, and the final rolling temperature is 890~930℃. (5) Slow cooling: After rolling, the steel is put into the pit for slow cooling. The slow cooling time is greater than 24 hours, and the temperature of the steel coming out of the slow cooling pit is lower than 180℃.

2. The low-sulfur, free-machining CrMn gear steel according to claim 1, characterized in that, The room temperature impact energy A of the gear steel KV ≥80J.

3. The low-sulfur, free-machining CrMn gear steel according to claim 1, characterized in that, The hardenability of the gear steel is 40~48HRC for J9, ≥28HRC for J15, and the bandwidth is ≤6HRC.

4. The method for producing low-sulfur, free-machining CrMn gear steel according to claim 1, characterized in that, In step (1), the magnesium content of the magnesium alloy cored wire is 20wt%~30wt%.

5. The method for producing low-sulfur, free-machining CrMn gear steel according to claim 1, characterized in that, In step (1), quicklime and fluorite are used in the refining slag.

6. The method for producing low-sulfur, free-machining CrMn gear steel according to claim 1, characterized in that, In step (4), the continuous casting billet is descaled by high-pressure water before rolling. High-pressure water spraying with multiple nozzles in a single pass is used for descaling. The descaling pressure is ≥24MPa and the descaling rate is ≥95%.

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