A magnesium treated crmn gear steel and method of production thereof

Abstract

The present application relates to gear steel bar steel technical field, especially to a kind of magnesium treatment CrMn gear steel and its production method.The gear steel includes the following mass percentage of chemical components: C:0.20%~0.25%, Si:0.03%~0.10%, Mn:0.91%~1.00%, Cr:1.01%~1.19%, Mo:0.25%~0.35%, Nb:0.03%~0.05%, Al:0.020%~0.040%, [O]:≤0.0012%, [N]:≤0.006%, [H]:≤0.00015%, Ce:0.0010%~0.0030%, Mg:0.0070%~0.0090%, the balance is Fe and inevitable impurities.The present application utilizes the trace amount of Mg, Ce and other elements in steel and the mutual action of O, N gas and C, Si, Mn, Cr, Mo, Nb, Al in steel, improves non-metallic inclusion in gear steel, reduces the hindrance of inclusion to austenite grain boundary, thereby improves the hardenability of steel.

Description

Technical Field

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

[0002] With the steady development of the automotive industry, customers are increasingly demanding gear steel and have stricter requirements for its quality. The width of the hardenability band is an important indicator for measuring the quality of hardenable gear steel. Therefore, in the smelting technology of automotive gear steel, the performance of gear steel is improved by controlling the width of the hardenability band.

[0003] The end hardenability of gear steel reflects the core hardness. Better end hardenability results in a deeper hardened layer, less deformation after heat treatment, less grinding required, higher meshing precision, and improved machining and performance stability. Many factors influence the end hardenability of gear steel, primarily three: chemical composition, austenite grain size, and degree of austenitization. Among these, the chemical composition and its uniformity are most significant. C, Mn, Cr, and Mo all contribute to improved end hardenability. Typically, the end hardenability bandwidth is used to represent the end hardenability performance; a smaller bandwidth indicates better hardenability.

[0004] Therefore, how to achieve better hardenability in gear steel has become a pressing technical problem to be solved in this field. Summary of the Invention

[0005] The purpose of this invention is to provide a magnesium-treated CrMn gear steel and its production method. By adding trace amounts of elements such as Ce and Mg, the non-metallic inclusions in the gear steel are improved, and the obstruction of the inclusions to the austenite grain boundaries is reduced, thereby producing high-quality gear steel with strong hardenability and narrow hardenability bandwidth.

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

[0007] This invention provides a magnesium-treated CrMn gear steel, wherein the gear steel comprises the following chemical composition by mass percentage:

[0008] C: 0.20%~0.25%, Si: 0.03%~0.10%, Mn: 0.91%~1.00%, Cr: 1.01%~1.19%, Mo: 0.25%~0.35%, Nb: 0.03%~0.05%, Al: 0.020%~0.040%, [O]: ≤0.0012%, [N]: ≤0.006%, [H]: ≤0.00015%, Ce: 0.0010%~0.0030%, Mg: 0.0070%~0.0090%, balance being Fe and unavoidable impurities.

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

[0010] C: Carbon plays a crucial role in ensuring the quenching properties of steel. Furthermore, as a core alloying element, C forms carbides with Cr, Mo, and others (such as M6C, M...). 23 C6, etc., significantly improves hardness and wear resistance; therefore, the mass percentage of C is controlled at 0.20%~0.25%.

[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 0.03%~0.10%.

[0012] Mn: In addition to acting as a deoxidizer and desulfurizer, Mn also improves hardenability, promotes uniform distribution of carbon, and enhances solid solution strengthening. Furthermore, Mo forms complex carbides, improving high-temperature wear resistance, hardenability, and resistance to temper brittleness. The effect is even more significant when it is combined with Cr. Therefore, the mass percentage of Mn is controlled at 0.91%~1.00%.

[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 1.01%~1.19%.

[0014] Mo: As a strong carbide-forming element in steel, Mo can strongly inhibit the nucleation and growth of carbides, 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 transformation of pearlite and promote the formation of acicular ferrite. Therefore, the mass percentage of Mo is controlled at 0.25%~0.35%.

[0015] Nb: During forging, hot rolling, and subsequent carburizing at higher temperatures, microalloyed carbonitrides can effectively prevent grain growth and refine grains, enabling carburizing at higher temperatures and improving carburizing efficiency. Simultaneously, Nb can refine grains, suppress segregation, and improve toughness and strength, while also reducing the segregation and banding tendency of C, Mn, and Cr elements. Therefore, the mass percentage of Nb is controlled between 0.03% and 0.05%.

[0016] Al: Adding a certain amount of Al refines the grains and deoxidizes the steel, reducing the number and size of non-metallic inclusions. At the same time, it controls the uniformity of the gear steel composition, making the austenite grains of the gear steel fine and free of mixed grains. Therefore, the mass percentage of Al is controlled between 0.020% and 0.040%.

[0017] N: N combines with elements such as C and Al and precipitates to form corresponding precipitates, which are used to refine the grains. Considering the relative content control of Al and N, which play an active role, the mass percentage of N is controlled at ≤0.006%.

[0018] O: The lower the O content in steel, the fewer the number and smaller the size of non-metallic inclusions in the steel, and the better the ductility and toughness of the non-metallic inclusions. The mass percentage of O should be controlled at ≤0.0012%.

[0019] H: When H accumulates in steel, it significantly reduces the ductility and toughness of the material, leading to brittle fracture. During cooling, H accumulates due to a sharp drop in solubility, forming defects such as white spots, which seriously impair fatigue strength and impact toughness. Therefore, the mass percentage of H is controlled at ≤0.00015%.

[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, delaying the formation of cementite, suppressing the nucleation of proeutectoid ferrite in steel, improving hardenability, and Ce plays a modifying role in non-metallic inclusions. In addition, as a rare earth element, Ce improves the morphology of inclusions, enhances purity and impact toughness. Therefore, the mass percentage of Ce is controlled at 0.0010%~0.0030%.

[0021] Mg: Commonly used as a deoxidizer and desulfurizer in steelmaking, it reacts with O in molten steel to produce magnesium oxide, thus playing a deoxidizing role. At the same time, the addition of trace amounts of Mg can improve the composition, size, and morphology of inclusions in steel, and enhance the deformation ability of inclusions in steel. In addition, Mg works synergistically with elements such as Ce, O, and S to form fine composite inclusions (such as MgAl2O4-MnS-RE2O3), improving morphology (spheroidization) and reducing harmful inclusions (such as MnS or TiN alone), thereby improving fatigue life and machinability. Therefore, the mass percentage of Mg is controlled at 0.0070%~0.0090%.

[0022] In the above technical solution, the hardenability of the gear steel is J9, which is 28~36HRC, J15, which is ≥24HRC, and the bandwidth is less than 5HRC.

[0023] In the above technical solution, the non-metallic inclusions of the gear steel are of type A ≤ 2.0 grade and type B ≤ 2.0 grade, and the austenite grain size is 5.5~7 grade.

[0024] In the above technical solution, the diameter of the finished gear steel is further 90~150mm.

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

[0026] (1) LF refining: LF furnace produces refining slag, the basicity of the refining slag is between 1.5 and 3.5, the electrode heating time is controlled to be 26 to 35 min, after the molten steel is alloyed in the LF refining furnace, the magnesium alloy cored wire is fed into the ladle by the wire feeding process.

[0027] (2) VD vacuum degassing: the pressure holding time is controlled at 15~20min, after VD rupture, argon gas is gently blown for 5~10min, the argon gas flow rate is 90~140NL / min, and after the gentle blowing is completed, it is left to stand for 5~10min;

[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 controlled at 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 ≤840℃, the heating section temperature is 1100~1160℃, the soaking section temperature is 1140~1180℃, the total heating time of the continuous casting billet is ≥6h, the initial rolling temperature is 1060~1140℃, and the final rolling temperature is 850~880℃.

[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 35wt%~45wt%, and the amount of magnesium alloy cored wire added is 39~64kg per 100 tons of molten steel.

[0032] In the above technical solution, further, in step (4), the continuous casting billet is descaled by high pressure water before rolling, and high pressure descaling is carried out by single-pass multi-nozzle water spraying, with a descaling pressure ≥20MPa.

[0033] The beneficial effects of this invention are as follows:

[0034] 1. This invention utilizes magnesium treatment to modify Al2O3 inclusions in gear steel into fine MgO·Al2O3 (magnesium aluminum spinel) or MgAl2O4 inclusions. At the same time, magnesium can inhibit the formation of long strip-shaped MnS and promote its transformation into spherical or spindle-shaped (Mn,Mg)S composite sulfides.

[0035] 2. The magnesium treatment of this invention improves the strength and toughness of steel by refining inclusions and promoting the formation of acicular ferrite. Composite inclusions reduce stress concentration and significantly extend fatigue life. Spherical sulfide inclusions (such as Mg-Mn-S) can reduce tool wear and improve machinability. Unmodified long strip-shaped MnS is prone to anisotropy of mechanical properties, while magnesium treatment makes sulfides short and dispersed, improving transverse properties. The nanoscale magnesium-containing precipitates formed by magnesium treatment (such as MgO+TiN) can pin austenite grain boundaries, inhibit grain coarsening in the weld heat-affected zone (HAZ), and induce intragranular acicular ferrite (IAF) nucleation, improving the low-temperature toughness of the HAZ (significantly increasing impact energy at -40℃).

[0036] 3. The present invention uses electromagnetic stirring technology to promote the floating of non-metallic inclusions in molten steel, reduce the particle size of inclusions in steel, and make inclusions evenly distributed, thereby reducing the segregation of elements such as C and Cr in steel and thus achieving a uniform microstructure.

[0037] 4. This invention significantly increases the amount of deformation inside austenite grains through low-temperature rolling, providing a large number of nucleation sites for ferrite phase transformation and promoting ferrite phase transformation. The metallographic structure of the gear steel of this invention consists of ferrite with an area ratio of 50% to 60% and pearlite with an area ratio of 40% to 50%.

[0038] 5. The hardenability of the gear steel of the present invention is J9 28~36HRC, J15 ≥24HRC, and the bandwidth is less than 5HRC; 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. 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 magnesium-treated CrMn gear steel, comprising the following chemical composition by weight percentage:

[0041] C: 0.20%~0.25%, Si: 0.03%~0.10%, Mn: 0.91%~1.00%, Cr: 1.01%~1.19%, Mo: 0.25%~0.35%, Nb: 0.03%~0.05%, Al: 0.020%~0.040%, [O]: ≤0.0012%, [N]: ≤0.006%, [H]: ≤0.00015%, Ce: 0.0010%~0.0030%, Mg: 0.0070%~0.0090%, 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 1.5 and 3.5, the electrode heating time is controlled to be 26 to 35 min, after the molten steel is alloyed in the LF refining furnace, magnesium alloy cored wire is fed into the ladle by wire feeding process, the magnesium content of magnesium alloy cored wire is 35wt% to 45wt%, and the amount of magnesium alloy cored wire added is 39 to 64 kg per 100 tons of molten steel;

[0044] (2) VD vacuum degassing: the pressure holding time is controlled at 15~20min, after VD rupture, argon gas is gently blown for 5~10min, the argon gas flow rate is 90~140NL / min, and after the gentle blowing is completed, it is left to stand for 5~10min to ensure that the inclusions float up fully;

[0045] (3) Continuous casting: The superheat of the tundish is controlled at 20~30℃, the casting speed is constant, the casting speed of the billet is controlled at 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 and Cr in the steel, and thus make the structure uniform;

[0046] (4) Rolling: The continuous casting billet is heated, with the preheating section temperature ≤840℃, the heating section temperature 1100~1160℃, the soaking section temperature 1140~1180℃, and the total heating time of the continuous casting billet ≥6h. Before rolling, the continuous casting billet is descaled by high-pressure water spray: single-pass multi-nozzle high-pressure water descaling, water descaling pressure ≥20Mpa, initial rolling temperature 1060~1140℃, and final rolling temperature 850~880℃. Through low-temperature precision rolling, the amount of deformation inside the austenite grains is greatly increased, providing a large number of nucleation sites for ferrite phase transformation and promoting 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 magnesium-treated 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 produces refining slag with a basicity of 2.6. The electrode heating time is controlled at 28 min. After the molten steel is alloyed in a 100-ton LF refining furnace, 45 kg of magnesium alloy cored wire with a magnesium content of 40 wt% (calculated based on a yield of 40%) is fed into the ladle using a wire feeding process.

[0053] (2) VD vacuum degassing: the pressure holding time is controlled at 18 min. After VD rupture, argon gas is gently blown for 7 min with an argon gas flow rate of 100 NL / min. After the gentle blowing is completed, let it stand for 7 min to ensure that the inclusions float up fully.

[0054] (3) Continuous casting: The cross-sectional dimensions of the continuously cast billet are 280mm×320mm. The superheat of the tundish is controlled at 25℃. The constant casting speed 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 water descaling pressure is 25MPa, the initial rolling temperature is 1120℃, and the final rolling temperature is 930℃.

[0056] (5) Slow cooling: The pit must be dry before the product is put into the pit and slowly cooled for 26 hours. The product must be taken out of the pit at a temperature below 180°C.

[0057] Example 2

[0058] A magnesium-treated 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 produces refining slag with a basicity of 1.7. The electrode heating time is controlled at 32 min. After the molten steel is alloyed in a 100-ton LF refining furnace, 53 kg of magnesium alloy cored wire with a magnesium content of 38 wt% (calculated based on a yield of 40%) is fed into the ladle using a wire feeding process.

[0061] (2) VD vacuum degassing: the pressure holding time is controlled at 18 min. After VD rupture, argon gas is gently blown for 8 min with an argon gas flow rate of 90 NL / min. After the gentle blowing is completed, the mixture is left to stand for 8 min to ensure that the inclusions float up fully.

[0062] (3) Continuous casting: The cross-sectional dimensions of the continuously cast billet are 280mm×320mm. The superheat of the tundish is controlled at 28℃. The constant casting speed 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 temperature of the preheating section is 820℃, 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 6.5h, the continuous casting billet is descaled by high pressure water before rolling, the water descaling pressure is 21MPa, the initial rolling temperature is 1100℃, and the final rolling temperature is 860℃.

[0064] (5) Slow cooling: The pit must be dry before the product is put into the pit and slowly cooled for 25.5 hours. The product must be taken out of the pit at a temperature below 180°C.

[0065] Example 3

[0066] A magnesium-treated 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 produces refining slag with a basicity of 2.9. The electrode heating time is controlled at 32 min. After the molten steel is alloyed in a 100-ton LF refining furnace, 48 kg of magnesium alloy cored wire with a magnesium content of 39 wt% (calculated based on a yield of 40%) is fed into the ladle using a wire feeding process.

[0069] (2) VD vacuum degassing: the pressure holding time is controlled at 18 min. After VD rupture, argon gas is gently blown for 6 min with an argon gas flow rate of 110 NL / min. After the gentle blowing is completed, the mixture is left to stand for 7 min to ensure that the inclusions float up fully.

[0070] (3) Continuous casting: The cross-sectional dimensions of the continuously cast billet are 280mm×320mm. The superheat of the tundish is controlled at 23℃. The constant casting speed 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 820℃, the heating section temperature is 1130℃, the soaking section temperature is 1140℃, 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 water descaling pressure is 22MPa, the initial rolling temperature is 1100℃, and the final rolling temperature is 865℃.

[0072] (5) Slow cooling: The pit must be dry before the product is put into the pit and slowly cooled for 25 hours. The product must be kept below 180°C before leaving the pit.

[0073] Example 4

[0074] A magnesium-treated 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 produces refining slag with a basicity of 2.9. The electrode heating time is controlled at 33 min. After the molten steel is alloyed in a 100-ton LF refining furnace, 42 kg of magnesium alloy cored wire with a magnesium content of 45 wt% (calculated based on a yield of 40%) is fed into the ladle using a wire feeding process.

[0077] (2) VD vacuum degassing: the pressure holding time is controlled at 16 min. After VD rupture, argon gas is gently blown for 8 min with an argon gas flow rate of 110 NL / min. After the gentle blowing is completed, the mixture is left to stand for 8 min to ensure that the inclusions float to the surface.

[0078] (3) Continuous casting: The cross-sectional dimensions of the continuously cast billet are 280mm×320mm. The superheat of the tundish is controlled at 26℃. The constant casting speed is controlled at 0.56m / min. The electromagnetic stirring voltage at the end of solidification is 470V, the current intensity is 420A, and the frequency is 9Hz.

[0079] (4) Rolling: The continuous casting billet is heated. The preheating section temperature is 810℃, the heating section temperature is 1130℃, the soaking section temperature is 1170℃, the total heating time of the continuous casting billet is 7h, the continuous casting billet is descaled by high pressure water before rolling, the water descaling pressure is 22MPa, the initial rolling temperature is 1080℃, and the final rolling temperature is 850℃.

[0080] (5) Slow cooling: The pit must be dry before the product is put into the pit and slowly cooled for 25 hours. The product must be kept below 180°C before leaving the pit.

[0081] Comparative Example 1

[0082] A CrMn gear steel has the chemical composition shown in Table 1.

[0083] The above-mentioned method for producing gear steel includes the following steps:

[0084] (1) LF refining: LF produces refining slag, and molten steel is alloyed in a 100-ton LF refining furnace;

[0085] (2) VD vacuum degassing: the pressure holding time is controlled at 18 min, after VD rupture, argon gas is gently blown for 7 min, and after the gentle blowing is completed, it is left to stand for 7 min;

[0086] (3) Continuous casting: The cross-sectional dimensions of the continuously cast billet are 280mm×320mm. The superheat of the tundish is controlled at 25℃. The constant casting speed is controlled at 0.56 m / min. The electromagnetic stirring voltage at the end of solidification is 400V, the current intensity is 400A, and the frequency is 10Hz.

[0087] (4) Rolling: The continuous casting billet is heated at 1130℃ in the heating section and 1165℃ 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 with a descaling pressure of 25MPa. The initial rolling temperature is 1120℃ and the final rolling temperature is 930℃.

[0088] Table 1 Chemical composition (wt%)

[0089]

[0090] Non-metallic inclusions and austenite grain size were analyzed in the gear steels of Examples 1-4 and Comparative Example 1. The results are shown in Table 2.

[0091] 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.

[0092] Table 2 Measurement Results

[0093]

[0094] 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 magnesium-treated CrMn gear steel, characterized in that, The gear steel comprises the following chemical composition by mass percentage: C: 0.20%~0.25%, Si: 0.03%~0.10%, Mn: 0.91%~1.00%, Cr: 1.01%~1.19%, Mo: 0.25%~0.35%, Nb: 0.03%~0.05%, Al: 0.020%~0.040%, [O]: ≤0.0012%, [N]: ≤0.006%, [H]: ≤0.00015%, Ce: 0.0010%~0.0030%, Mg: 0.0070%~0.0090%, balance being Fe and unavoidable impurities; The non-metallic inclusions in the gear steel are of type A ≤ 2.0 grade and type B ≤ 2.0 grade, with austenite grain size of 5.5~7 grade; 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 1.5 and 3.5, the electrode heating time is controlled to be 26 to 35 min, after the molten steel is alloyed in the LF refining furnace, the magnesium alloy cored wire is fed into the ladle by the wire feeding process. (2) VD vacuum degassing: the pressure holding time is controlled at 15~20min, after VD rupture, argon gas is gently blown for 5~10min, the argon gas flow rate is 90~140NL / min, and after the gentle blowing is completed, it is left to stand for 5~10min; (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 controlled at 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 ≤840℃, the heating section temperature is 1100~1160℃, the soaking section temperature is 1140~1180℃, the total heating time of the continuous casting billet is ≥6h, the initial rolling temperature is 1060~1140℃, and the final rolling temperature is 850~880℃. (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 magnesium-treated CrMn gear steel according to claim 1, characterized in that, The hardenability of the gear steel is J9, which is 28~36 HRC, and J15, which is ≥24 HRC, with a bandwidth of less than 5 HRC.

3. The magnesium-treated CrMn gear steel according to claim 1, characterized in that, The diameter of the finished gear steel is 90~150mm.

4. The magnesium-treated CrMn gear steel according to claim 1, characterized in that, In step (1), the magnesium content of the magnesium alloy cored wire is 35%~45wt%, and the amount of magnesium alloy cored wire added is 39~64kg per 100 tons of molten steel.

5. The magnesium-treated 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 descaling is carried out by single-pass multi-nozzle water spraying with a descaling pressure ≥20MPa.

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