Low-sulfur free-cutting CrMn series gear steel and production method thereof

Abstract

The invention relates to the technical field of steel for gear steel bars, in particular to low-sulfur free-cutting CrMn series gear steel and a production method thereof. Comprising the following chemical components in percentage by mass: 0.23 percent to 0.28 percent of C, 0.11 percent to 0.19 percent of Si, 0.70 percent to 0.90 percent of Mn, 0.90 percent to 1.00 percent of Cr, 0.25 percent to 0.35 percent of Mo, 0.10 percent to 0.19 percent of W, 0.10 percent to 0.19 percent of Ni, 0.020 percent to 0.040 percent of Al, 0.030 percent to 0.050 percent of S, 0.0010 percent to 0.0030 percent of Ce, 0.0035 percent to 0.0065 percent of Mg and the balance of Fe and inevitable impurities. The form and components of non-metallic inclusions are improved through Ce, Mg, S and other elements, so that the steel obtains good cutting performance, the surface elimination smoothness of gear steel machining parts is improved, and the cutting machining speed of the parts is increased by 5%-8%.

Description

Technical Field

[0001] The present invention relates to the technical field of steel for gear steel bars, and in particular to a low-sulfur, free-cutting CrMn series gear steel and a production method thereof. Background Art

[0002] Gears, as key components in automobiles and mechanical transmissions, require steel with both high strength and excellent machinability. Traditional gear steels are difficult to machine due to their high alloy content and high hardness. While sulfur can improve machinability, high sulfur content can reduce steel purity (for example, increasing oxide inclusions), impacting mechanical properties.

[0003] Existing technologies, such as patent CN1664150A, disclose a method for manufacturing sulfur-containing free-cutting gear steel and its pipe. This method improves machinability by adjusting the sulfur content to 0.03% to 0.05%. However, the sulfur content in this patent is still relatively high and fails to effectively address the conflict between sulfur and oxygen control—retaining sulfur to improve machinability while simultaneously removing oxygen to ensure steel purity.

[0004] Further complicating matters is the challenge of controlling sulfide morphology. Sulfur in steel reacts with manganese to form MnS inclusions, which are prone to elongation and deformation during hot rolling, leading to localized stress concentrations and significantly reducing the material's fatigue life. This is particularly challenging for medium-carbon gear steel, as its low dissolved oxygen content makes it difficult to promote the formation of spherical sulfides by increasing the oxygen content. Summary of the Invention

[0005] The purpose of the present invention is to provide a low-sulfur, free-cutting CrMn gear steel and a production method thereof, by compositely adding trace amounts of elements such as Ce, Mg, and S, to improve the surface finish of gear steel machined parts and at the same time increase the cutting speed of the parts.

[0006] In order to achieve the above object, the technical solution of the present invention is as follows: In one aspect, the present invention provides a low-sulfur, free-cutting CrMn gear steel comprising the following chemical components in percentage by mass: 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.

[0007] The principles for adding each element in the present invention are as follows: C: C is an essential element to ensure the strength, hardness and hardenability of steel. In order to ensure the strength and toughness of the core of gear steel, C, as a core alloying element, 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%; Si: Si acts as a deoxidizer in steel and is easily oxidized in carburized gear steel. When oxygen invades from the surface, silicon at the grain boundary or near the grain diffuses to the grain boundary before other elements, combining with trace oxygen dissolved in the surface to form oxides at the grain boundary and distributing them in a network. Silicon oxides reduce the surface hardness and fatigue strength of parts, weaken grain boundaries, and increase the possibility of cracking. Therefore, the mass percentage of Si is controlled at 0.11%~0.19%; Mn: Mn not only acts as a deoxidizer and desulfurizer, but also improves hardenability, makes the carbon element evenly distributed, and enhances the solid solution strengthening effect. In addition, Mn and S can form MnS inclusions, which play a notch effect and lubricant role in gear processing, improving the cutting performance of steel materials. Therefore, the mass percentage of Mn is controlled at 0.70%~0.90%; Cr: Cr is a medium-strong carbide-forming element that can significantly improve the strength, hardenability, wear resistance and other comprehensive properties of steel materials. The mass percentage of Cr in the present invention is controlled to be 0.90% to 1.00%; Mo: As a strong carbide-forming element in steel, Mo can strongly hinder the nucleation and growth of carbides, effectively improving hardenability. The combined effect of Mo and Mn can significantly improve the stability of austenite and improve the hardenability of steel. Mo can delay pearlite transformation and promote the formation of acicular ferrite structure. Therefore, the mass percentage of Mo is controlled at 0.25%~0.35%; W: W can significantly hinder the precipitation of carbides at the austenite grain boundaries, refine grains, and improve the strength, plasticity, impact toughness, and hydrogen embrittlement sensitivity of steel. In addition, as a strong carbide-forming element, W can form high-temperature stable complex carbides (such as W2C) in conjunction with Cr and Mo, significantly improving high-temperature creep resistance and wear resistance. Therefore, the mass percentage of W is controlled at 0.10%~0.19%; Ni: Ni can enhance the cross-slip ability of the matrix, reduce the cold-brittle transition temperature, and improve toughness. It can also make the carburized steel obtain a surface layer with a small carbon gradient and a certain toughness. Therefore, the mass percentage of Ni is controlled at 0.10%~0.19%; Nb: During the hot rolling process and the later higher temperature carburizing process, microalloyed carbonitrides can effectively prevent grain growth, refine grains, and allow carburizing at higher carburizing temperatures, thereby improving carburizing efficiency. Therefore, the mass percentage of Nb is controlled at 0.03%~0.05%; Al: Al is used to refine grains and deoxidize, reducing the number and size of non-metallic inclusions in the steel and achieving extremely low oxygen content. At the same time, the gear steel composition is controlled to be uniform, making the austenite grains of the gear steel fine and free of mixed crystals. Therefore, the mass percentage of Al is controlled at 0.020%~0.040%; S: Sulfur is one of the main free-cutting elements. As the sulfur content in steel increases, the cutting performance index of the steel is significantly improved. Therefore, the mass percentage of sulfur is controlled at 0.030%~0.050%; Ce: When heated to austenitization, Ce atoms will inevitably segregate to the austenite grain boundaries, inhibiting the diffusion of carbon atoms to the grain boundaries and delaying the formation of cementite. In the steel, Ce inhibits the nucleation of proeutectoid ferrite, improves hardenability, and modifies non-metallic inclusions. Therefore, the mass percentage of Ce is controlled within 0.0010%~0.0030%; Mg: It is commonly used as a deoxidizer and desulfurizer in steelmaking. It reacts with O and S in the molten steel to generate magnesium oxide or sulfide, reducing the O and S in the molten steel to a lower level. The generated inclusions float up or are adsorbed by the protective slag and removed from the molten steel, playing a role in deoxidation and desulfurization. The addition of trace Mg can improve the composition, size and morphology of inclusions in steel and enhance the deformation ability of inclusions in steel. Therefore, the mass percentage of Mg is controlled at 0.0035%~0.0065%.

[0008] In the above technical solution, further, the room temperature impact energy of the gear steel is Akv≥80J.

[0009] In the above technical solution, further, the hardenability J9 of the gear steel is 40~48HRC, J15≥28HRC, and the bandwidth is ≤6HRC.

[0010] In the above technical solution, further, the aspect ratio of sulfides in the gear steel is 3 to 6, the non-metallic inclusions are Class A ≤ Grade 2.0 and Class B ≤ Grade 2.0, and the austenite grain size is 5.5 to 7.

[0011] Another aspect of the present invention provides a method for producing the above-mentioned low-sulfur, free-cutting CrMn gear steel, comprising the following steps: (1) LF refining: LF furnace-made refining slag, the refining slag basicity is between 4.0 and 6.0, the electrode heating time is controlled to be 26 to 35 minutes, 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 using the wire feeding process; (2) VD vacuum degassing: the pressure holding time is controlled at 10~15min, and argon is soft-blown for 10~15min after VD breaks the air, with an argon flow rate of 40~80NL / min. After the soft 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 pulling speed is constant, the billet pulling 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; (4) Rolling: heating the continuous casting billet, 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 starting rolling temperature is 1060~1140℃, and the finishing rolling temperature is 890~930℃; (5) Slow cooling: After rolling, the steel is put into the slow cooling pit for more than 24 hours, and the temperature of the steel out of the slow cooling pit is lower than 180℃.

[0012] In the above technical solution, further, in step (1), the magnesium content of the magnesium alloy cored wire is 20wt%~30wt%.

[0013] In the above technical solution, further, in step (1), lime and fluorite are used to make the refined slag.

[0014] In the above technical solution, further, in step (4), the continuous casting billet is descaled with high-pressure water before rolling, and a single-pass multi-nozzle water spraying high-pressure descaling is adopted, the descaling pressure is ≥24MPa, and the descaling rate is ≥95%.

[0015] The beneficial effects of the present invention are: 1. The present invention improves the morphology and composition of non-metallic inclusions by adding elements such as Ce, Mg, and S, thereby enabling the steel to obtain good cutting performance, improving the surface finish of gear steel parts, and increasing the cutting speed of parts by 5% to 8%.

[0016] 2. The present invention utilizes C to form carbides with Cr, W, Mo, etc., which significantly improves hardness and wear resistance. W cooperates with Cr and Mo to form high-temperature stable complex carbides, which significantly improves high-temperature creep resistance and wear resistance. Mn improves hardenability, makes the C element evenly distributed, and enhances the solid solution strengthening effect.

[0017] 3. The hardenability J9 of the gear steel of the present invention is 40~48HRC, J15 is ≥28HRC, and the bandwidth is ≤6HRC; the 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, Class B ≤2.0, and the austenite grain size is 5.5~7.

[0018] 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 DESCRIPTION

[0019] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0020] The present invention provides a low-sulfur, free-cutting CrMn gear steel comprising the following chemical components in percentage by mass: 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.

[0021] The production method of the gear steel comprises the following steps: (1) LF refining: LF furnace is used to produce 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 minutes, lime 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, the magnesium alloy cored wire (the magnesium content of the magnesium alloy cored wire is 20 wt% to 30 wt%) is fed into the ladle by the wire feeding process; (2) VD vacuum degassing: the pressure holding time is controlled at 10~15min, and argon is soft-blown for 10~15min after VD breaks the air, with an argon flow rate of 40~80NL / min. After the soft blowing is completed, it is left to stand for 10~15min to ensure that the inclusions are fully floated; (3) Continuous casting: The cross-sectional size of the continuous casting billet is 320 mm × 410 mm, the superheat of the tundish is controlled at 20-30 ° C, the pulling speed is constant, the billet pulling speed is 0.5-0.6 m / min, the electromagnetic stirring voltage at the end of solidification is 300-500 V, the current intensity is 360-500 A, and the frequency is 9-12 Hz. 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 uniformly distributed, reduce the segregation of elements such as C, S, and Cr in the steel, and thus uniform the structure; (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. The continuous casting billet is descaled with high-pressure water before rolling. A single-pass multi-nozzle water spray high-pressure descaling is used. The descaling pressure is ≥24MPa and the descaling rate is ≥95%. The starting rolling temperature is 1060~1140℃ and the finishing rolling temperature is 890~930℃. By means of low-temperature finishing rolling, the amount of deformation inside the austenite grains is greatly increased, which provides a large number of nucleation cores for the ferrite phase transformation and promotes the ferrite phase transformation. (5) Slow cooling: After rolling, the steel is put into the slow cooling pit. The slow cooling pit must be dry. The slow cooling time is more than 24 hours. The temperature of the steel out of the slow cooling pit is lower than 180℃.

[0022] The following are specific examples.

[0023] Example 1 A low-sulfur, free-cutting CrMn gear steel, the chemical composition of which is shown in Table 1.

[0024] The production method of the gear steel comprises the following steps: (1) LF refining: LF furnace is used to produce refining slag with a basicity of 4.35. White lime and fluorite are used for slag making. The electrode heating time is controlled at 28 min. The composition is adjusted twice, coarsely and finely. The coarse adjustment is to add 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 with a magnesium content of 25 wt% is fed into the ladle by the wire feeding process. (2) VD vacuum degassing: the pressure holding time is controlled at 15 minutes, and argon is soft-blown for 12 minutes after the VD breaks the air, with an argon flow rate of 50NL / min. After the soft blowing is completed, it is left to stand for 12 minutes to ensure that the inclusions are fully floated; (3) Continuous casting: The cross-sectional size of the continuous casting billet is 320 mm × 410 mm, the superheat of the tundish is controlled at 25 °C, the pulling speed is constant (the billet pulling speed is controlled at 0.56 m / min), the electromagnetic stirring voltage at the end of solidification is 400 V, the current intensity is 400 A, and the frequency is 10 Hz; (4) Rolling: The continuous casting slab is heated with a preheating section temperature of 840°C, a heating section temperature of 1130°C, a soaking section temperature of 1165°C, and a total heating time of 7 hours. The continuous casting slab is descaled with high-pressure water before rolling, with a descaling pressure of 25 MPa and a descaling rate of 97%. The starting rolling temperature is 1120°C and the final rolling temperature is 930°C. (5) Slow cooling: The slow cooling time is 26 hours. The slow cooling pit must be dry and the temperature of the steel leaving the slow cooling pit must be lower than 180℃.

[0025] Example 2 A low-sulfur, free-cutting CrMn gear steel, the chemical composition of which is shown in Table 1.

[0026] The production method of the gear steel comprises the following steps: (1) LF refining: LF furnace is used to produce refining slag with a basicity of 5.36. Lime and fluorite are used for slag making. The electrode heating time is controlled at 28 min. The composition is adjusted twice, coarsely and finely. The coarse adjustment is to add 0.7 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 with a magnesium content of 25 wt% is fed into the ladle by the wire feeding process. (2) VD vacuum degassing: the pressure holding time is controlled at 15 minutes, and argon is soft-blown for 12 minutes after the VD breaks the air, with an argon flow rate of 50NL / min. After the soft blowing is completed, it is left to stand for 12 minutes to ensure that the inclusions are fully floated; (3) Continuous casting: The cross-sectional size of the continuous casting billet is 320 mm × 410 mm, the superheat of the tundish is controlled at 28 °C, the pulling speed is constant (the billet pulling speed is controlled at 0.56 m / min), the electromagnetic stirring voltage at the end of solidification is 450 V, the current intensity is 450 A, and the frequency is 11 Hz; (4) Rolling: The continuous casting slab is heated with a preheating section temperature of 850°C, a heating section temperature of 1125°C, a soaking section temperature of 1160°C, and a total heating time of 6.5 h. The continuous casting slab is descaled with high-pressure water before rolling, with a descaling pressure of 25 MPa and a descaling rate of 98%. The starting rolling temperature is 1070°C and the final rolling temperature is 915°C. (5) Slow cooling: The slow cooling time is 24 hours. The slow cooling pit must be dry and the temperature of the steel leaving the slow cooling pit must be lower than 180℃.

[0027] Example 3 A low-sulfur, free-cutting CrMn gear steel, the chemical composition of which is shown in Table 1.

[0028] The production method of the gear steel comprises the following steps: (1) LF refining: LF furnace is used to produce refining slag with a basicity of 5.58. Lime and fluorite are used for slag making. The electrode heating time is controlled to be 30 min. The composition is adjusted twice, coarsely and finely. The coarse adjustment is to add 0.6 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 with a magnesium content of 25 wt% is fed into the ladle by the wire feeding process. (2) VD vacuum degassing: the pressure holding time is controlled at 15 minutes, and argon is soft-blown for 10 minutes after the VD breaks the air, with an argon flow rate of 60NL / min. After the soft blowing is completed, it is left to stand for 12 minutes to ensure that the inclusions are fully floated; (3) Continuous casting: The cross-sectional size of the continuous casting billet is 320 mm × 410 mm, the superheat of the tundish is controlled at 23 °C, the pulling speed is constant (the billet pulling speed is controlled at 0.55 m / min), the electromagnetic stirring voltage at the end of solidification is 430 V, the current intensity is 460 A, and the frequency is 10 Hz; (4) Rolling: The continuous casting slab is heated with a preheating section temperature of 840°C, a heating section temperature of 1120°C, a soaking section temperature of 1150°C, and a total heating time of 7 hours. The continuous casting slab is descaled with high-pressure water before rolling, with a descaling pressure of 24 MPa and a descaling rate of 98%. The starting rolling temperature is 1080°C and the final rolling temperature is 930°C. (5) Slow cooling: The slow cooling time is 24 hours. The slow cooling pit must be dry and the temperature of the steel leaving the slow cooling pit must be lower than 180℃.

[0029] Example 4 A low-sulfur, free-cutting CrMn gear steel, the chemical composition of which is shown in Table 1.

[0030] The production method of the gear steel comprises the following steps: (1) LF refining: LF furnace is used to produce refining slag with a basicity of 5.15. Lime and fluorite are used for slag making. The electrode heating time is controlled at 28 min. The composition is adjusted twice, coarsely and finely. The coarse adjustment is to add 0.7 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 with a magnesium content of 25 wt% is fed into the ladle by the wire feeding process. (2) VD vacuum degassing: the pressure holding time is controlled at 15 minutes, and argon is soft-blown for 10 minutes after the VD breaks the air, with an argon flow rate of 60NL / min. After the soft blowing is completed, it is left to stand for 12 minutes to ensure that the inclusions are fully floated; (3) Continuous casting: The cross-sectional size of the continuous casting billet is 320 mm × 410 mm, the superheat of the tundish is controlled at 26 °C, the pulling speed is constant (the billet pulling speed is controlled at 0.56 m / min), the electromagnetic stirring voltage at the end of solidification is 470 V, the current intensity is 400 A, and the frequency is 9 Hz; (4) Rolling: The continuous casting slab is heated with a preheating section temperature of 840°C, a heating section temperature of 1120°C, a soaking section temperature of 1150°C, and a total heating time of 7h. The continuous casting slab is descaled with high-pressure water before rolling. The descaling pressure is 26MPa, the descaling rate is 96%, the rolling start temperature is 1090°C, and the final rolling temperature is 920°C. (5) Slow cooling: The slow cooling time is 25h. The slow cooling pit must be dry, and the temperature of the steel exiting the slow cooling pit is lower than 180°C.

[0031] Comparative Example 1 A CrMn gear steel, the chemical composition of which is shown in Table 1.

[0032] The production method of the gear steel comprises the following steps: (1) LF refining: LF furnace produces refining slag, using lime and fluorite for slag making, and controlling the electrode heating time to 28 minutes; (2) VD vacuum degassing: the pressure holding time is controlled at 15 minutes, and argon is soft-blown for 10 minutes after the VD breaks the air, with an argon flow rate of 60NL / min. After the soft blowing is completed, it is left to stand for 5 minutes; (3) Continuous casting: The cross-sectional size of the continuous casting billet is 320 mm × 410 mm, the superheat of the tundish is controlled at 26 ° C, and the casting speed is constant (the casting billet speed is controlled at 0.56 m / min); (4) Rolling: The continuous casting billet is heated, the temperature of the heating section is 1120℃, the temperature of the soaking section 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℃.

[0033] Table 1 Chemical composition (wt%)

[0034] The gear steels of Examples 1-4 and Comparative Example 1 were analyzed for non-metallic inclusions, austenite grain size, and room temperature impact resistance. The results are shown in Table 2. The gear steels of Examples 1-4 and Comparative Example 1 were normalized at 910°C for 1 hour and end quenched at 880°C for 30 minutes. The quenching properties were measured. The results are shown in Table 2.

[0035] Table 2 Measurement results

[0036] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the scope of implementation. The scope of protection of the present invention shall be determined by the scope defined in the claims. Other variations or modifications may be made based on the above description. Obvious variations or modifications derived therefrom remain within the scope of protection of the present invention.

Claims

1. A low-sulfur, free-cutting CrMn gear steel, characterized in that: The chemical composition includes the following mass percentages: 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.

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

4. The low-sulfur, free-cutting CrMn gear steel according to claim 1, characterized in that: The gear steel has a sulfide aspect ratio of 3 to 6, non-metallic inclusions of type A ≤ level 2.0 and type B ≤ level 2.0, and an austenite grain size of 5.5 to 7.

5. A method for producing the low-sulfur, free-cutting CrMn gear steel according to any one of claims 1 to 4, characterized in that: The following steps are involved: (1) LF refining: LF furnace-made refining slag, the refining slag basicity is between 4.0 and 6.0, the electrode heating time is controlled to be 26 to 35 minutes, 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 using the wire feeding process; (2) VD vacuum degassing: the pressure holding time is controlled at 10~15min, and argon is soft-blown for 10~15min after VD breaks the air, with an argon flow rate of 40~80NL / min. After the soft 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 pulling speed is constant, the billet pulling 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; (4) Rolling: heating the continuous casting billet, 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 slow cooling pit for more than 24 hours, and the temperature of the steel out of the slow cooling pit is lower than 180℃.

6. The method for producing low-sulfur free-cutting CrMn gear steel according to claim 5, characterized in that: In step (1), the magnesium content of the magnesium alloy cored wire is 20 wt% to 30 wt%.

7. The method for producing low-sulfur free-cutting CrMn gear steel according to claim 5, characterized in that: In step (1), lime and fluorite are used to make the refined slag.

8. The method for producing low-sulfur free-cutting CrMn gear steel according to claim 5, characterized in that: In step (4), the continuous casting billet is descaled with high-pressure water before rolling, using a single-pass multi-nozzle water spraying high-pressure descaling method, with a descaling pressure of ≥24 MPa and a descaling rate of ≥95%.

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