A magnesium treated crmn ti gear steel and method of production thereof

Abstract

The present application relates to gear steel bar steel technical field, specifically, especially relates to a kind of magnesium treatment CrMnTi gear steel and its production method.The gear steel includes the following mass percentage of chemical composition: C:0.21%~0.25%, Si:0.11%~0.19%, Mn:0.90%~1.00%, Cr:0.81%~0.99%, Mo:0.20%~0.30%, Ti:0.040%~0.065%, Nb:0.03%~0.05%, Al:0.020%~0.040%, B:0.0020%~0.0040%, [N]:0.008%~0.012%, Ce:0.001%~0.003%, Mg:0.0045%~0.0065%, the balance is Fe and inevitable impurities.The present application utilizes the trace amount of Mg, Ce, B and other elements in steel and C, Si, Mn, Cr, Mo, Ti, Nb interaction, the organization state of steel, non-metallic inclusion is regulated, simultaneously improves the mechanical properties of steel, hardenability.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of gear steel bar, in particular, especially relates to a magnesium treated CrMnTi gear steel and a production method thereof. BACKGROUND

[0002] Due to the low price of main chemical elements of 20CrMnTi steel, small grain growth tendency, and the fact that a suitable substitute steel has not been established, the 20CrMnTiH still dominates the gear industry in China in recent years, so the physical quality of the main steel grade 20CrMnTiH should be continuously improved and stabilized.

[0003] With the increase of titanium content in 20CrMnTi steel, titanium inclusions are easily formed. Because the viscosity of the steel is large, it is difficult for TiN to float during solidification of the molten steel, so it often exists in the form of inclusions in the steel. Coarse TiN particles can cause longitudinal cracks in the ingot during forging and rolling, and can also cause quenching cracks during subsequent processing and use, reducing the fatigue life of the steel. At the same time, Ti is easy to form TiC with C in the steel, which prevents the grain of austenite from coarsening. On the other hand, the more TiN and TiC, the less carbon in the solid solution, which reduces the strength of the steel, and because these carbon and nitrogen particles can act as new phase nuclei during the decomposition of austenite, they accelerate the transformation of austenite, increase the critical cooling rate of the steel, and partially offset the effect of elements Mn and Cr on the hardenability, resulting in a decrease in the hardenability of 20CrMnTi steel at a certain austenitizing temperature. SUMMARY

[0004] The purpose of the present application is to provide a magnesium treated CrMnTi gear steel and a production method thereof. By adding trace amounts of Ce, Mg, B and other elements, the non-metallic inclusions in the gear steel are improved, and the inclusions hinder the austenite grain boundary, thereby improving the hardenability of the gear steel.

[0005] In order to achieve the above-mentioned purpose, the technical scheme of the present application is as follows:

[0006] In one aspect, the present application provides a magnesium treated CrMnTi gear steel, which comprises the following chemical components by mass percentage:

[0007] C: 0.21%~0.25%, Si: 0.11%~0.19%, Mn: 0.90%~1.00%, Cr: 0.81%~0.99%, Mo: 0.20%~0.30%, Ti: 0.040%~0.065%, Nb: 0.03%~0.05%, Al: 0.020%~0.040%, B: 0.002%~0.004%, [N]: 0.008%~0.012%, Ce: 0.001%~0.003%, Mg: 0.0045%~0.0065%, the balance being Fe and inevitable impurities.

[0008] The principle of adding each element in the application is as follows:

[0009] C: C element is used to ensure the strength of steel material and improve the hardenability of steel material, in order to ensure the strength and toughness of gear core, in addition, as a core alloying element, it forms carbides (such as M6C, M 23 C6, etc.) with Cr, Mo, etc., which significantly improves hardness and wear resistance, and the mass percentage of C is controlled at 0.21%~0.25%;

[0010] Si: Si element in steel plays the role of deoxidizer, and in carburized gear steel, it is easy to oxidize, when oxygen invades from the surface, silicon near the grain boundary or grain boundary preferentially diffuses to the grain boundary, combines with trace oxygen dissolved on the surface into oxides, and is distributed in a network, the silicon oxide reduces the surface hardness and fatigue strength of the part, weakens the grain boundary and increases the possibility of cracking of the part, therefore, the mass percentage of Si is controlled at 0.11%~0.19%;

[0011] Mn: Mn element not only plays the role of deoxidizer and desulfurizer, but also has the effect of improving hardenability, making C element uniformly distributed, and enhancing solid solution strengthening effect, therefore, the mass percentage of Mn is controlled at 0.90%~1.00%;

[0012] Cr: Cr is a medium-strong carbide forming element, which can significantly improve the strength, hardenability, wear resistance and other comprehensive properties of steel material, and the mass percentage of Cr in the application is controlled at 0.81%~0.99%;

[0013] Mo: Mo, as a strong carbide forming element in steel, can strongly hinder the nucleation and growth of carbides, and can effectively improve the hardenability, the combined effect of Mo and Mn can also significantly improve the stability of austenite and the hardenability of steel, Mo element can delay pearlite transformation and promote the formation of acicular ferrite structure, in addition, Mo forms complex carbides, which improves high temperature wear resistance, hardenability and temper brittleness resistance, and the effect is more significant when combined with Cr, therefore, the mass percentage of Mo is controlled at 0.20%~0.30%;

[0014] Ti: Ti element is mainly added to refine the austenite grain size, the effect of Ti element in the range of 0.040%~0.065% on the hardenability of the steel is small, and the size of TiN inclusions and the content of TiN are avoided to be too large, at the same time, Ti in the steel combines with N to form TiN, so that B element in the steel can exist in the form of "effective B", thereby affecting the stability of the hardenability of the steel;

[0015] Nb: In the process of heating, rolling and later higher temperature carburizing, micro-alloy carbonitride can effectively prevent grain growth, refine the grain, carburize at a higher carburizing temperature, improve the carburizing efficiency, in addition, Nb can refine the grain, suppress segregation, improve toughness and strength, and reduce C, Mn and Cr element segregation and banded structure tendency, therefore, the mass percentage of Nb is controlled in the range of 0.03%~0.05%;

[0016] Al: Al is used to refine the grain and deoxidize, reduce the number and size of non-metallic inclusions in the steel, and obtain very low oxygen content, at the same time, the composition of the gear steel is controlled to be uniform, so that the austenite grain of the gear steel is small and there is no mixed crystal phenomenon, therefore, the mass percentage of Al is controlled in the range of 0.020%~0.040%;

[0017] B: B element exists in the form of acid-soluble boron in the steel, replaces some valuable alloying elements, and improves the hardenability of the steel, acid-insoluble boron in the form of carbon and nitride can refine the grain and improve the strength and toughness of the steel, in addition, B element can reduce the generation of large size TiN in cooperation with Ti, Ti has stronger nitrogen fixation ability than B, and TiN is formed preferentially, B can inhibit the coarsening of TiN by competing with nitrogen fixation, thereby reducing the harm of TiN to the plasticity of the steel, therefore, the mass percentage of B is controlled in the range of 0.002%~0.004%;

[0018] N: N combines with C, Al, B, Ti and other elements and precipitates to form corresponding precipitates for grain refinement, considering the relative content control of Al and B as positive effect, the mass percentage of N is controlled in the range of 0.008%~0.012%;

[0019] Ce: When heated to austenitizing, Ce atoms must be segregated to the austenite grain boundary, which inhibits the diffusion of carbon atoms to the grain boundary, delays the formation of cementite, and inhibits the nucleation of proeutectoid ferrite in the steel, thereby improving the hardenability, in addition, Ce element has a modifying effect on non-metallic inclusions, and as a rare earth element, Ce can improve the inclusion morphology, improve the purity and impact toughness, therefore, the content of Ce is controlled in the range of 0.001%~0.003%;

[0020] Mg: commonly used as a steelmaking deoxidizer, reacts with O in the molten steel to form magnesium oxide, reducing the O in the molten steel to a lower level, the generated inclusions float or are absorbed by the protective slag to remove from the molten steel, play a deoxidizing role, at the same time, the addition of trace Mg can improve the composition, size and morphology of inclusions in steel, enhance the deformation of inclusions in steel, in addition, Mg cooperates with Ce, O, S and other elements to form fine composite inclusions (such as MgAl2O4-MnS-RE2O3), improve the morphology (spheroidization), reduce harmful inclusions (such as single MnS or TiN), thereby improve the fatigue life and machinability, therefore, the mass percentage of Mg is controlled at 0.0045%~0.0065%.

[0021] In the technical solution, further, the hardenability J9 of the gear steel is 36~42HRC, and J15≥24HRC.

[0022] In the technical solution, further, the non-metallic inclusions of the gear steel are A class ≤2.0 level, B class ≤2.0 level, and the austenite grain size is 6~7 level.

[0023] Another aspect of the present application provides a production method of the above-mentioned magnesium-treated CrMnTi gear steel, comprising the following steps:

[0024] (1) LF refining: the electrode heating time is controlled at 26~35min, after the molten steel is treated by alloying in the LF refining furnace, the magnesium-containing alloy cored wire is fed into the ladle by wire feeding process;

[0025] (2) VD vacuum degassing: the holding time is controlled at 10~15min, after the VD breaks the vacuum, soft blowing is carried out for 10~15min with argon gas, the argon gas flow is 50~70NL / min, and after the soft blowing is finished, the static state is maintained for 10~15min;

[0026] (3) continuous casting: the superheat degree of the tundish is controlled at 20~30℃, the constant speed is maintained, the casting blank drawing speed is 0.65~0.75m / min, the crystallizer electromagnetic stirring voltage is 250~500V, the current intensity is 310~490A, and the frequency is 4~8Hz, the solidification end electromagnetic stirring voltage is 300~500V, the current intensity is 360~500A, and the frequency is 9~12Hz;

[0027] (4) rolling: the continuous casting blank is heated, the preheating segment temperature is ≤840℃, the heating segment temperature is 1080~1140℃, the soaking segment temperature is 1120~1160℃, the total heating time of the continuous casting blank is ≥6h, the opening rolling temperature is 1060~1100℃, and the final rolling temperature is 870~900℃;

[0028] (5) slow cooling: after rolling, the steel enters the pit for slow cooling, the slow cooling time is greater than 24h, and the temperature of the steel out of the slow cooling pit is lower than 180℃.

[0029] In the technical solution, further, in step (1), the magnesium content of the magnesium-containing alloy cored wire is 20wt%-30wt%.

[0030] In the technical solution, further, in step (3), the continuous casting whole-process protection pouring is adopted, wherein the molten steel in the ladle and the tundish is added with low-oxidizing and alkaline covering agent, the protection sleeve is used from the ladle to the tundish and from the tundish to the crystallizer, and argon is blown for protection, and the protection slag is added in the crystallizer.

[0031] In the technical solution, further, in step (4), the high-pressure water descaling is performed on the continuous casting billet before rolling, the single-pass multi-nozzle water high-pressure descaling is adopted, the descaling pressure is greater than or equal to 24MPa, and the descaling rate is greater than or equal to 95%.

[0032] The beneficial effects of the present application are as follows:

[0033] 1. The present application utilizes magnesium treatment to modify the Al2O3 inclusions in gear steel into fine MgO·Al2O3 or MgAl2O4 inclusions, and magnesium can inhibit the formation of long strip-shaped MnS and promote its transformation into spherical or spindle-shaped (Mn, Mg) S complex sulfide. At the same time, Mg treatment provides effective nucleation sites for TiN, generating nanoscale TiN complex inclusions, enhancing intragranular nucleation ability, significantly refining the size of TiN inclusions in steel, and promoting their dispersion distribution, thereby improving the strength and toughness of the steel.

[0034] 2. The magnesium treatment in the present application refines inclusions and promotes the formation of acicular ferrite, thereby improving the strength and toughness of the steel, the complex inclusions reduce stress concentration and significantly prolong the fatigue life, the spherical sulfide inclusions (such as Mg-Mn-S) can reduce tool wear and improve cutting machinability, the unmodified long strip-shaped MnS easily leads to mechanical property anisotropy, while magnesium treatment makes the sulfide short and dispersed, thereby improving the transverse performance, the nanoscale magnesium-containing precipitates (such as MgO+TiN) formed by magnesium treatment can pin the austenite grain boundaries, inhibit the grain coarsening in the heat-affected zone (HAZ), and induce intragranular acicular ferrite (IAF) nucleation, thereby improving the low-temperature toughness of the HAZ (the impact energy at -40℃ is significantly increased).

[0035] 3. The present application adjusts the crystallizer and terminal electromagnetic stirring parameters, thereby improving the segregation of the continuous casting billet and promoting the floating of non-metallic inclusions in the steel, the non-metallic inclusions in the steel are A class ≤2.0 level and B class ≤2.0 level, and the austenite grain size is 6-7 level.

[0036] 4. The gear steel of the present application has a hardenability J9 of 36-42HRC and a hardenability J15 of ≥24HRC.

[0037] 5. The gear steel of the present application has a metallographic structure of 50%-60% of ferrite and 40%-50% of pearlite. DETAILED DESCRIPTION

[0038] The technical solutions in the embodiments of the present application will be clearly and completely described in combination with the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all the other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0039] The present application provides a magnesium-treated CrMnTi gear steel, comprising the following chemical components by mass percentage:

[0040] C: 0.21%~0.25%, Si: 0.11%~0.19%, Mn: 0.90%~1.00%, Cr: 0.81%~0.99%, Mo: 0.20%~0.30%, Ti: 0.040%~0.065%, Nb: 0.03%~0.05%, Al: 0.020%~0.040%, B: 0.0020%~0.0040%, [N]: 0.008%~0.012%, Ce: 0.001%~0.003%, Mg: 0.0045%~0.0065%, and the balance of Fe and inevitable impurities.

[0041] The production method of the above gear steel comprises the following steps:

[0042] (1) LF refining: the electrode heating time is controlled to be 26~35 min, after the molten steel is treated by alloying in the LF refining furnace, the magnesium-containing cored wire is fed into the ladle by using the wire feeding process, the magnesium content of the magnesium-containing cored wire is 20wt%~30wt%, and the adding amount of the magnesium-containing cored wire is 37~81 kg per 100 tons of molten steel;

[0043] (2) VD vacuum degassing: the pressure maintaining time is controlled to be 10~15 min, after the VD is broken, soft blowing of 10~15 min of argon gas is performed, the argon gas flow is 50~70 NL / min, after the soft blowing is finished, the static state is maintained for 10~15 min to ensure that the inclusions are fully floated up;

[0044] (3) Continuous casting: the overheat degree of the tundish is controlled at 20-30°C, the constant speed is maintained, the casting blank drawing speed is controlled at 0.65-0.75 m / min, the whole process adopts protective pouring (the low oxidizing and alkaline covering agent is added into the molten steel in the ladle and tundish, the protective sleeve is used and argon is blown from the ladle to the tundish, and the tundish is added with protective slag), the electromagnetic stirring voltage of the tundish is 250-500 V, the current intensity is 310-490 A, and the frequency is 4-8 Hz, the electromagnetic stirring voltage of the solidification end 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 the non-metallic inclusions in the molten steel, reduce the size of the inclusions in the steel, and make the inclusions uniformly distributed, and at the same time, the element segregation of the steel is reduced, so that the uniform structure is obtained, thereby reducing the number and depth of the surface defects such as the transverse crack, longitudinal crack and corner crack of the steel blank;

[0045] (4) Rolling: the continuous casting blank is heated, the temperature of the preheating section is ≤840°C, the temperature of the heating section is 1080-1140°C, the temperature of the soaking section is 1120-1160°C, the total heating time of the continuous casting blank is ≥6 h, the high-temperature diffusion time is ensured, the high-pressure water descaling control is performed on the continuous casting blank before rolling, the single-pass multi-nozzle water high-pressure descaling is adopted, the water descaling pressure is ≥24 MPa, the water descaling rate is ≥95%, the rolling-in temperature is 1060-1100°C, and the finish rolling temperature is 870-900°C; through the low-temperature finish rolling mode, the number of the internal deformation of the austenite grain is greatly increased, a large number of nucleation cores are provided for the ferrite phase change, and the ferrite phase change is promoted.

[0046] (5) Slow cooling: after rolling, the steel is put into the pit for slow cooling, the slow cooling pit must be dry, the slow cooling time is greater than 24 h, and the temperature of the steel out of the slow cooling pit is lower than 180°C.

[0047] The following is a specific embodiment.

[0048] Example 1

[0049] A magnesium-treated CrMnTi gear steel, the chemical composition of which is shown in Table 1.

[0050] The production method of the above gear steel comprises the following steps:

[0051] (1) LF refining: the electrode heating time is 28 min, after the molten steel is treated by alloying in a 100-ton LF refining furnace, 48 kg of magnesium-containing alloy cored wire with a magnesium content of 26 wt% is fed into the ladle by using the wire feeding process;

[0052] (2) VD vacuum degassing: the VD pressure holding time is controlled at 15 min, after the VD is broken, soft blowing of argon is performed for 10 min, the argon flow is 50 NL / min, after the soft blowing is completed, the molten steel is statically placed for 12 min to ensure that the inclusions are fully floated;

[0053] (3) Continuous casting: the cross-sectional size of the continuous casting billet is 320 mm x 410 mm, the superheat in the tundish is controlled to be 25°C, the constant casting speed is adopted, the casting billet casting speed is controlled to be 0.70 m / min, the whole process is protected casting (low oxidizing and alkaline covering agent is added into the molten steel in the ladle and tundish, the protective tube is used and argon is blown from the ladle to the tundish and from the tundish to the mold, and the protective slag is added into the mold), the electromagnetic stirring voltage of the mold is 300 V, the current intensity is 350 A, and the frequency is 4 Hz, the electromagnetic stirring voltage of the solidification end is 300 V, the current intensity is 400 A, and the frequency is 11 Hz;

[0054] (4) Rolling: the continuous casting billet is heated, the temperature in the preheating section is 830°C, the temperature in the heating section is 1110°C, the temperature in the soaking section is 1130°C, the total heating time of the continuous casting billet is 6 h, the high-pressure water descaling control before rolling is adopted, the single-pass multi-nozzle water high-pressure descaling is used, the water descaling pressure is 25 MPa, the water descaling rate is 96%, the rolling-in temperature is 1080°C, and the finish rolling temperature is 885°C;

[0055] (5) Slow cooling: the steel is slow cooled in the pit, the slow cooling pit must be dry, the slow cooling time is 25 h, and the temperature of the steel out of the slow cooling pit is lower than 180°C.

[0056] Example 2

[0057] A magnesium-treated CrMnTi gear steel, the chemical composition of which is shown in Table 1.

[0058] The production method of the above gear steel comprises the following steps:

[0059] (1) LF refining: the electrode heating time is 30 min, after the molten steel is treated by alloying in a 100-ton LF refining furnace, 46 kg of magnesium-containing alloy cored wire with the magnesium content of 25 wt% is fed into the ladle by using the wire feeding process;

[0060] (2) VD vacuum degassing: the VD pressure holding time is controlled to be 13 min, after the VD is broken, soft blowing of argon is performed for 13 min, the argon flow rate is 60 NL / min, after the soft blowing is completed, the static state is maintained for 13 min to ensure that the inclusions are fully floated;

[0061] (3) Continuous casting: the cross-sectional size of the continuous casting billet is 320 mm x 410 mm, the superheat in the tundish is controlled to be 25°C, the constant casting speed is adopted, the casting billet casting speed is controlled to be 0.69 m / min, the whole process is protected casting (low oxidizing and alkaline covering agent is added into the molten steel in the ladle and tundish, the protective tube is used and argon is blown from the ladle to the tundish and from the tundish to the mold, and the protective slag is added into the mold), the electromagnetic stirring voltage of the mold is 400 V, the current intensity is 420 A, and the frequency is 6 Hz, the electromagnetic stirring voltage of the solidification end is 330 V, the current intensity is 450 A, and the frequency is 12 Hz;

[0062] (4) Rolling: the continuous casting billet is heated, the preheating temperature is 820°C, the heating temperature is 1090°C, the soaking temperature is 1140°C, the total heating time of the continuous casting billet is 6.5h, the high-pressure water descaling control before rolling is adopted, the single-pass multi-jet water high-pressure descaling is used, the water descaling pressure is 24MPa, the water descaling rate is 95%, the rolling-in temperature is 1090°C, and the finishing temperature is 875°C;

[0063] (5) Slow cooling: the steel enters the pit for slow cooling, the slow cooling pit must be dry, the slow cooling time is 24h, and the steel temperature out of the slow cooling pit is lower than 180°C.

[0064] Example 3

[0065] A magnesium-treated CrMnTi gear steel has the chemical composition shown in Table 1.

[0066] The production method of the above gear steel comprises the following steps:

[0067] (1) LF refining: the electrode heating time is 30min, after the molten steel is treated by alloying in a 100-ton LF refining furnace, 50kg of magnesium-containing alloy cored wire with a magnesium content of 28wt% (calculated according to the yield rate of 40%) is fed into the ladle by wire feeding process;

[0068] (2) VD vacuum degassing: the VD pressure holding time is controlled to be 12min, after the VD breaks the vacuum, soft blowing of 12min of argon gas is performed with an argon gas flow of 55NL / min, and after the soft blowing is completed, 12min of standing is performed to ensure that the inclusions float up fully;

[0069] (3) Continuous casting: the continuous casting billet has a cross-sectional size of 320mmx410mm, the tundish superheat is controlled to be 25°C, the constant speed is controlled, the casting billet speed is controlled to be 0.73m / min, the whole process is protected pouring (the molten steel in the ladle and the tundish is added with low oxidizing and alkaline covering agent, the large ladle to the tundish and the tundish to the crystallizer is protected by a protective sleeve and argon blowing protection, and the crystallizer is added with protective slag), the crystallizer electromagnetic stirring voltage is 400V, the current intensity is 420A, and the frequency is 7Hz, the solidification end electromagnetic stirring voltage is 350V, the current intensity is 380A, and the frequency is 9Hz;

[0070] (4) Rolling: the continuous casting billet is heated, the preheating temperature is 815°C, the heating temperature is 1020°C, the soaking temperature is 1150°C, the total heating time of the continuous casting billet is 6.5h, the high-pressure water descaling control before rolling is adopted, the single-pass multi-jet water high-pressure descaling is used, the water descaling pressure is 24.5MPa, the water descaling rate is 95.6%, the rolling-in temperature is 1080°C, and the finishing temperature is 870°C;

[0071] (5) Slow cooling: the steel enters the pit for slow cooling, the slow cooling pit must be dry, the slow cooling time is 26h, and the steel temperature out of the slow cooling pit is lower than 180°C.

[0072] Example 4

[0073] A magnesium treated CrMnTi gear steel having a chemical composition as shown in Table 1.

[0074] A production method of the above gear steel, comprising the following steps:

[0075] (1) LF refining: electrode heating time is 32 min, after the molten steel is alloyed by a 100-ton LF refining furnace, 52 kg of magnesium-containing alloy cored wire with a magnesium content of 25 wt% (calculated according to the recovery rate of 40%) is fed into the ladle by wire feeding process;

[0076] (2) VD vacuum degassing: the VD pressure holding time is controlled at 15 min, after VD breaking, soft blowing of argon is carried out for 15 min, the argon flow rate is 70 NL / min, after the soft blowing is completed, it is statically placed for 15 min to ensure that the inclusions are fully floated;

[0077] (3) Continuous casting: the cross-sectional size of the continuous casting billet is 320 mm x 410 mm, the tundish superheat is controlled at 25℃, the constant speed is controlled at 0.71 m / min, the whole process is protected casting (low oxidizing and alkaline covering agent is added to the molten steel in the ladle and tundish, the protective sleeve is used and argon is blown from the ladle to the tundish and the tundish to the crystallizer, and the protective slag is added to the crystallizer), the electromagnetic stirring voltage of the crystallizer is 450 V, the current intensity is 330 A, and the frequency is 7 Hz, the electromagnetic stirring voltage of the solidification end is 350 V, the current intensity is 400 A, and the frequency is 10 Hz;

[0078] (4) Rolling: the continuous casting billet is heated, the preheating temperature is 810℃, the heating temperature is 1090℃, the soaking temperature is 1130℃, the total heating time of the continuous casting billet is 6.5 h, the high-pressure water descaling control is used before rolling, the single-pass multi-nozzle water high-pressure descaling is used, the water descaling pressure is 25.5 MPa, the water descaling rate is 96.2%, the rolling-in temperature is 1070℃, and the finish rolling temperature is 890℃;

[0079] (5) Slow cooling: the steel is slow-cooled in the pit, the slow-cooling pit must be dry, the slow-cooling time is 24.5 h, and the temperature of the steel out of the slow-cooling pit is lower than 180℃.

[0080] Comparative Example 1

[0081] A CrMnTi gear steel having a chemical composition as shown in Table 1.

[0082] A production method of the above gear steel, comprising the following steps:

[0083] (1) LF refining: electrode heating time is 28 min, the molten steel is alloyed by a 100-ton LF refining furnace;

[0084] (2) VD vacuum degassing: VD pressure holding time is controlled at 15 min, soft blowing 10 min of argon after VD breaking vacuum, and standing for 5 min after soft blowing;

[0085] (3) Continuous casting: the cross-sectional size of the continuous casting billet is 320 mm x 410 mm, constant speed, the casting billet drawing speed is controlled at 0.70 m / min, the solidification end electromagnetic stirring voltage is 300 V, the current intensity is 400 A, and the frequency is 11 Hz;

[0086] (4) Rolling: the continuous casting billet is heated, the heating section temperature is 1110℃, the soaking section temperature is 1130℃, the total heating time of the continuous casting billet is 6 h, the opening rolling temperature is 1080℃, and the final rolling temperature is 920℃.

[0087] Table 1 Chemical composition (wt%)

[0088]

[0089] The gear steels of Examples 1-4 and Comparative Example 1 are subjected to non-metallic inclusion and austenite grain size analysis, and the results are shown in Table 2.

[0090] The gear steels of Examples 1-4 and Comparative Example 1 are subjected to non-metallic inclusion and austenite grain size analysis, and the results are shown in Table 2.

[0091] Table 2 Determination results

[0092]

[0093] The above examples are merely preferred embodiments of the present application, and are not intended to limit the embodiments. The protection scope of the present application should be defined by the scope defined in the claims. Other different forms of changes or variations can be made on the basis of the above description. The changes or variations thus derived are still within the protection scope of the present application.

Claims

1. A magnesium treated CrMnTi gear steel, characterized in that, The gear steel comprises the following chemical components in mass percentage: C: 0.21%~0.25%, Si: 0.11%~0.19%, Mn: 0.90%~1.00%, Cr: 0.81%~0.99%, Mo: 0.20%~0.30%, Ti: 0.040%~0.065%, Nb: 0.03%~0.05%, Al: 0.020%~0.040%, B: 0.002%~0.004%, [N]: 0.008%~0.012%, Ce: 0.001%~0.003%, Mg: 0.0045%~0.0065%, the balance being Fe and inevitable impurities; The non-metallic inclusions of the gear steel are A ≤ 2.0 level, B ≤ 2.0 level, and the austenite grain size is 6~7 level; The production method of the gear steel comprises the following steps: (1) LF refining: the electrode heating time is controlled to be 26~35 min, after the molten steel is treated by alloying in the LF refining furnace, the magnesium-containing cored wire is fed into the ladle by using the wire feeding process; (2) VD vacuum degassing: the pressure maintaining time is controlled to be 10~15 min, after the VD is broken, soft blowing of argon is carried out for 10~15 min, the argon flow is 50~70 NL / min, and after the soft blowing is finished, the steel is placed for 10~15 min; (3) Continuous casting: the tundish superheat is controlled to be 20~30 ℃, the constant speed is maintained, the casting blank drawing speed is 0.65~0.75 m / min, the crystallizer electromagnetic stirring voltage is 250~500 V, the current intensity is 310~490 A, and the frequency is 4~8 Hz, the solidification end electromagnetic stirring voltage is 300~500 V, the current intensity is 360~500 A, and the frequency is 9~12 Hz; (4) Rolling: the continuous casting blank is heated, the preheating temperature is ≤840 ℃, the heating temperature is 1080~1140 ℃, the soaking temperature is 1120~1160 ℃, the total heating time of the continuous casting blank is ≥6 h, the opening rolling temperature is 1060~1100 ℃, and the final rolling temperature is 870~900 ℃; (5) Slow cooling: after rolling, the steel is placed in the pit for slow cooling, the slow cooling time is greater than 24 h, and the temperature of the steel taken out of the slow cooling pit is lower than 180 ℃.

2. The magnesium treated CrMnTi gear steel of claim 1, wherein, The hardenability J9 of the gear steel is 36~42 HRC, and J15 is ≥24 HRC.

3. The magnesium treated CrMnTi gear steel of claim 1, wherein, In step (1), the magnesium content of the magnesium-containing cored wire is 20wt%~30wt%, and the adding amount of the magnesium-containing cored wire is 37~81 kg per 100 tons of molten steel.

4. The magnesium treated CrMnTi gear steel of claim 1, wherein, In step (3), the whole continuous casting is protected, wherein the low oxidizing and alkaline covering agent is added into the molten steel in the ladle and the tundish, the protection sleeve is used and argon blowing protection is carried out from the ladle to the tundish and from the tundish to the crystallizer, and the protection slag is added into the crystallizer.

5. The magnesium treated CrMnTi gear steel of claim 1, wherein, In step (4), the high-pressure water descaling is carried out on the continuous casting blank before rolling, the single-pass multi-nozzle water high-pressure descaling is used, the descaling pressure is ≥24 MPa, and the descaling rate is ≥95%.

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