Steel for gears and method for manufacturing gears using the same

Abstract

The present disclosure provides a gear steel and a method for manufacturing a gear using the steel. The steel comprises, based on the total weight of the steel, 0.10-0.30 wt% C, 0.60-0.80 wt% Si, 0.25-0.75 wt% Mn, 1.80-2.20 wt% Cr, 0.50-1.50 wt% Ni, 0.20-0.40 wt% Mo, 0.025-0.050 wt% Nb, 0.030-0.050 wt% V, and the remainder Fe and unavoidable impurities, wherein the contents of Nb and V satisfy the following <Relationship 1>: 0.055<[Nb]+[V]<0.100…………<Relationship 1>, wherein [Nb] represents the content of Nb, and [V] represents the content of V.

Description

Technical Field

[0001] The present disclosure relates to a gear steel and a method of manufacturing a gear using the same, and more particularly, to a gear steel having improved fatigue resistance and a method of manufacturing a gear using the same. Background Art

[0002] In automobiles, steels with various physical properties are used for auto parts.

[0003] Among automotive parts, gears are parts that require fatigue resistance and are generally made of carburizing steel. In particular, research on carburizing steel for gears aims to improve fatigue resistance by increasing the alloy content of Cr and Si elements, inducing carbide formation, and improving tempering resistance to increase strength.

[0004] However, there is a limitation: alloys with high Cr and Si content can only be carburized in a carburizing heat treatment apparatus that maintains a vacuum atmosphere. Therefore, carburizing heat treatment apparatuses in an air atmosphere cannot perform carburizing heat treatment on alloys with high Cr and Si content.

[0005] Additionally, carburized parts such as gears are typically manufactured only after numerous steps including rolling, rolling heat treatment, forging, ISO heat treatment, carburizing heat treatment, and shot peening.

[0006] Therefore, research is continuously being conducted into technologies for simplifying the parts manufacturing process in order to achieve cost savings and improve productivity.

[0007] The description given in the related art is only for understanding the background of the present disclosure and should not be considered as prior art already known to those skilled in the art. Summary of the Invention

[0008] The present disclosure provides a gear steel that can be carburized in an air atmosphere and has improved fatigue resistance compared to conventional steel, and a method of manufacturing a gear using the same.

[0009] According to an embodiment of the present disclosure, a gear steel is provided. The steel comprises, based on the total weight of the steel, 0.10-0.30 wt% C, 0.60-0.80 wt% Si, 0.25-0.75 wt% Mn, 1.80-2.20 wt% Cr, 0.50-1.50 wt% Ni, 0.20-0.40 wt% Mo, 0.025-0.050 wt% Nb, and 0.030-0.050 wt% V, and the remainder Fe and unavoidable impurities, wherein the contents of Nb and V satisfy the following <Relationship 1>:

[0010] 0.055<[Nb]+[V]<0.100…………<Relationship 1>,

[0011] Here, [Nb] represents the Nb content, and [V] represents the V content.

[0012] The steel may further include: P: 0.020 wt % or less; and S: 0.020 wt % or less.

[0013] Metal carbide or metal nitride (MX) precipitates may form in the steel in a fraction of 0.03-0.07%.

[0014] The MX precipitates may be at least one of Nb-based carbides, Nb-based nitrides, V-based carbides, V-based nitrides, Nb+V-based carbides, and Nb+V-based nitrides.

[0015] The size of MX precipitates can be 150 nm or less.

[0016] MX precipitates can be found in 100 μm 2 The precipitates are formed at a density of 50 or more.

[0017] A method for manufacturing a gear according to an embodiment of the present disclosure may include:

[0018] The molten metal preparation step comprises preparing a molten metal, wherein the molten metal comprises: C: 0.10-0.30 wt%; Si: 0.60-0.80 wt%; Mn: 0.25-0.75 wt%; Cr: 1.80-2.20 wt%; Ni: 0.50-1.50 wt%; Mo: 0.20-0.40 wt%; Nb: 0.025-0.050 wt%; V: 0.030-0.050 wt%; and the remainder Fe and unavoidable impurities, wherein the contents of Nb and V satisfy the following <Relationship 1>:

[0019] 0.055<[Nb]+[V]<0.100…………<Relationship 1>,

[0020] Wherein [Nb] represents the Nb content, [V] represents the V content;

[0021] a pre-rolling heat treatment step, wherein the cast molten metal is heat treated after casting;

[0022] a rolled steel molding step of rolling and molding the heat-treated cast steel into rolled steel;

[0023] a forged steel molding step of forging and molding the heat-treated rolled steel into planetary gears;

[0024] a forged steel heat treatment step of heat treating the forged steel;

[0025] a molded part processing step of processing the forged steel into a final molded part; and

[0026] Carburizing heat treatment step, carburizing the molded parts.

[0027] The heat treatment temperature in the pre-rolling heat treatment step may be maintained below the liquidus of the cast steel.

[0028] The heat treatment temperature in the pre-rolling heat treatment step may be 1180-1430°C.

[0029] The wrought steel heat treatment step may be carried out under ISO heat treatment conditions.

[0030] The carburizing heat treatment step may be performed under the following conditions: heat treatment temperature: 850-940° C.; carbon potential (CP): 0.7-1.0; and heat treatment duration: 100 minutes or longer.

[0031] The pitting area of ​​the molded part after the carburizing heat treatment step can be less than 12 mm as measured by fatigue testing (SAE J 1619) 2 .

[0032] The tooth bending fatigue test result of the molded part after the carburizing heat treatment step may be 4,000 cycles or more.

[0033] MX precipitates may form in the molded part after the carburizing heat treatment step in a fraction of 0.03-0.07%, the size of the MX precipitates being below 150 nm, and the number of MX precipitates being per 100 μm 2 There are more than 50.

[0034] The MX precipitates may be at least one of Nb-based carbides, Nb-based nitrides, V-based carbides, V-based nitrides, Nb+V-based carbides, and Nb+V-based nitrides. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The above and other aspects, features and advantages of the present disclosure will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

[0036] Figure 1 is a table listing the components used in Examples and Comparative Examples;

[0037] Figure 2 is a table showing the precipitate states and physical properties in Examples and Comparative Examples;

[0038] Figure 3 is a graph showing the fraction of non-solid-dissolved precipitates generated according to the pre-rolling heat treatment temperature in the pre-rolling heat treatment step;

[0039] Figure 4 is a graph showing the fraction of fine MX precipitates generated according to the heat treatment temperature of a molded part in a carburizing heat treatment step; and

[0040] Figure 5A and Figure 5B are views respectively showing MX precipitates and their components on the gear sample manufactured according to Example 1 analyzed by EDS (Energy Dispersive Spectrometer). DETAILED DESCRIPTION

[0041] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. However, the present disclosure is not limited to the embodiments disclosed below, but can be implemented in various forms. These embodiments are provided solely to complete the disclosure of the present disclosure and to enable those skilled in the art to fully understand the scope of the present disclosure.

[0042] The gear steel according to an embodiment of the present disclosure, which can be used to manufacture gears for automotive parts, controls the fraction, number, and size of generated fine precipitates by optimizing the contents of main alloy components.

[0043] Specifically, the gear steel according to an embodiment of the present disclosure contains: C: 0.10-0.30 wt%; Si: 0.60-0.80 wt%; Mn: 0.25-0.75 wt%; Ni: 0.50-1.50 wt%; Cr: 1.80-2.20 wt%; Mo: 0.20-0.40 wt%; Nb: 0.025-0.050 wt%; V: 0.030-0.050 wt%; and the balance Fe and unavoidable impurities. The steel may further contain P: 0.020 wt% or less; and S: 0.020 wt% or less.

[0044] In this embodiment, the contents of Nb and V affect the generation of fine precipitates, and therefore the contents of Nb and V preferably satisfy the following <Relationship 1>:

[0045] 0.055<[Nb]+[V]<0.100…………<Relationship 1>,

[0046] Here, [Nb] represents the Nb content, and [V] represents the V content.

[0047] In the present disclosure, the reasons for limiting alloy components and their contents are as follows: Unless otherwise specified, when "%" and "fraction" represent the units of the amount of a component, "%" and "fraction" refer to "wt%" and "volume fraction", respectively.

[0048] The content of carbon (C) is preferably 0.10-0.30%.

[0049] Carbon (C) is an element responsible for forming metal carbide or metal nitride (MX) precipitates and forming a solid solution in the matrix, thereby increasing the strength of steel. To fully enhance the strength of steel, a carbon (C) content of 0.10% or more is required. However, when the carbon (C) content exceeds 0.30%, toughness decreases significantly. Therefore, the carbon (C) content is preferably limited to 0.10-0.30%.

[0050] The content of silicon (Si) is preferably 0.60-0.80%.

[0051] Silicon (Si) is an element that improves the temper softening resistance of carburizing heat-treated steel. To improve the durability of steel, the steel requires a silicon (Si) content of 0.60% or more. However, when the silicon (Si) content exceeds 0.80%, oxides form on the surface of the steel subjected to carburizing heat treatment in an air atmosphere, thereby interfering with the diffusion of carbon. Therefore, to ensure the durability of the steel and prevent oxides from forming on the surface of the steel during carburizing heat treatment in an air atmosphere, the silicon (Si) content is preferably limited to 0.60-0.80%.

[0052] The content of manganese (Mn) is preferably 0.25-0.75%.

[0053] Manganese (Mn) is an element that helps deoxidize steel and forms a solid solution in the matrix to improve hardenability. To improve the bending fatigue strength of steel, the steel requires a manganese (Mn) content of 0.25% or more. However, when the manganese (Mn) content exceeds 0.75%, the hardness of the matrix increases, which leads to a significant decrease in workability. Therefore, to prevent a decrease in the bending fatigue strength and workability of the steel, the manganese (Mn) content is preferably limited to 0.25-0.75%.

[0054] The content of nickel (Ni) is preferably 0.50-1.50%.

[0055] Nickel (Ni) is an element that improves the hardenability and toughness of steel. To improve steel's fatigue resistance, steel requires a Ni content of 0.50% or more. However, Ni is an expensive element. Therefore, to reduce production costs, the Ni content is preferably limited to 0.50-1.50%.

[0056] The content of chromium (Cr) is preferably 1.80-2.20%.

[0057] Chromium (Cr) is an element that increases the strength and hardenability of steel. To improve the durability of steel, a chromium (Cr) content of 1.80% or more is required. However, when the chromium (Cr) content exceeds 2.20%, oxides and carbides form on the surface of the steel during carburizing heat treatment in an air atmosphere, thereby interfering with carbon diffusion. Therefore, to ensure the durability of the steel and prevent oxide formation on the surface of the steel during carburizing heat treatment in an air atmosphere, the chromium (Cr) content is preferably limited to 1.80-2.20%.

[0058] The content of molybdenum (Mo) is preferably 0.20-0.40%.

[0059] Molybdenum (Mo) is an element that improves hardenability. To increase the hardness of steel after carburizing heat treatment, the steel requires a Mo content of 0.20% or more. However, when the Mo content exceeds 0.40%, the effect of increasing hardness is minimal, making it ineffective from a cost perspective. Therefore, the Mo content is preferably limited to 0.20-0.40%.

[0060] The content of niobium (Nb) is preferably 0.025-0.050%.

[0061] Niobium (Nb) is an element that forms complex MX precipitates during the carburizing heat treatment. MX precipitates are a factor that prevents precipitation strengthening and grain coarsening of steel. In order to form appropriate MX precipitates, steel requires a niobium (Nb) content of 0.025% or more. However, an increase in the niobium (Nb) content leads to an increase in the solid solution temperature, and the Nb element that does not form a solid solution during the heat treatment of steel rolling forms coarse MX precipitates. The coarse MX precipitates thus formed cannot effectively interfere with the potential migration of dislocations, and therefore contribute little to improving fatigue life. Therefore, in order to allow niobium (Nb) to form the most solid solution during the heat treatment of steel rolling to increase the formation of fine MX precipitates, the niobium (Nb) content is preferably limited to 0.025-0.050%.

[0062] The content of vanadium (V) is preferably 0.030-0.050%.

[0063] Similar to niobium (Nb), vanadium (V) is an element that forms complex MX precipitates during carburizing heat treatment. To enhance precipitation strengthening in steel, a vanadium (V) content of 0.030% or greater is required. However, when the vanadium (V) content exceeds 0.050%, the effect of enhancing precipitation strengthening is minimal, making it ineffective from a cost perspective. Therefore, the vanadium (V) content is preferably limited to 0.030-0.050%.

[0064] Phosphorus (P) and sulfur (S) are unavoidable impurities in steel, and their contents are preferably kept as low as possible. Considering the process of removing phosphorus (P) and sulfur (S), the contents of each are preferably limited to 0.020 wt% or less.

[0065] The remaining components other than the above components include iron (Fe) and impurities inevitably contained.

[0066] In addition, in the Examples of the present disclosure, the total content of niobium (Vb) and vanadium (V), which are elements responsible for forming the composite MX precipitates, satisfies the following <Relationship 1>:

[0067] 0.055<[Nb]+[V]<0.100…………<Relationship 1>

[0068] Here, [Nb] represents the Nb content, and [V] represents the V content.

[0069] When the combined content of niobium (Nb) and vanadium (V), which contribute to the formation of MX precipitates, is below the lower limit of this range, the combined content of niobium (Nb) and vanadium (V) prevents the formation of the desired level of fine MX precipitates, and thus, the effect of improving physical properties such as strength and fatigue resistance cannot be expected. On the other hand, when the combined content of niobium (Nb) and vanadium (V) exceeds the upper limit of this range, the solution temperature of the steel increases, and thus, niobium (Nb) and vanadium (V) that do not form a solid solution during the heat treatment of steel rolling lead to the formation of coarse MX precipitates. The coarse MX precipitates thus formed cannot effectively interfere with the movement of dislocations, thereby reducing the effect of improving the fatigue life of the steel.

[0070] The MX precipitates are formed of niobium (Nb) and vanadium (V) and are at least one of Nb-based carbides, Nb-based nitrides, V-based carbides, V-based nitrides, Nb+V-based carbides, and Nb+V-based nitrides.

[0071] When the conditions are satisfied, MX precipitates are formed in a fraction of 0.03-0.07% in the gear steel according to the embodiment of the present disclosure.

[0072] At this time, the size of the MX precipitates is 150 nm or less, and ... 2 The precipitates are formed at a density of 50 or more.

[0073] Next, a method for manufacturing a gear using the gear steel prepared by the above-mentioned alloy design will be described.

[0074] The method for manufacturing a gear according to an embodiment of the present disclosure includes: a molten metal preparation step; a pre-rolling heat treatment step, in which the cast molten metal is heat treated after being cast; a rolled steel molding step, in which the heat-treated cast steel is rolled and molded into rolled steel; a forged steel molding step, in which the heat-treated rolled steel is forged and molded into a planetary gear; a forged steel heat treatment step, in which the forged steel is heat treated; a molded part processing step, in which the forged steel is processed into a final molded part; and a carburizing heat treatment step, in which the molded part is carburized.

[0075] In the molten metal preparation step, molten metal is prepared according to the alloy design of the gear steel described above. The molten metal contains: C: 0.10-0.30 wt%; Si: 0.60-0.80 wt%; Mn: 0.25-0.75 wt%; Cr: 1.80-2.20 wt%; Ni: 0.50-1.50 wt%; Mo: 0.20-0.40 wt%; Nb: 0.025-0.050 wt%; V: 0.030-0.050 wt%; and the balance Fe and unavoidable impurities. At this point, the total content of Nb and V satisfies <Relationship 1>.

[0076] In the pre-rolling heat treatment step, cast steel obtained by a typical continuous casting method is heat treated so that Nb and V, which are elements improving moldability and forming MX precipitates, are formed into a solid solution to the greatest extent.

[0077] In this case, the heat treatment temperature of the cast steel prior to rolling can be maintained below the liquidus of the cast steel. For example, the heat treatment temperature can be maintained at 1180-1430°C. When the heat treatment temperature is below the lower limit of this range, Nb and V do not form a solid solution in the rolled steel, resulting in the formation of coarse MX precipitates before rolling and during the carburizing heat treatment. The resulting coarse MX precipitates reduce fatigue life improvement.

[0078] In the roll-molding step, the heat-treated cast steel is rolled and molded using a typical rolling method.

[0079] In the forging steel molding step, rolled steel is forged into a gear shape using a typical forging method.

[0080] In order to improve workability and minimize deformation during subsequent heat treatment, a forging steel heat treatment step is performed.

[0081] For this purpose, a forged steel heat treatment step is carried out under ISO heat treatment conditions.

[0082] Here, the ISO heat treatment conditions are intended to improve the banded structure and suppress the precipitation of the bainite structure while maintaining the temperature below Ac1 after austenitization.

[0083] In the molded part processing step, the heat-treated forged steel is processed into a gear as a final molded part.

[0084] This machining is performed as post-machining such as typical roughing or finishing.

[0085] In the carburizing heat treatment step, the processed gear is heated in a carburizing atmosphere to diffuse and penetrate carbon (C) into the gear surface, and then quenched to improve the physical properties of the gear. In this case, Nb and V, which are precipitate-forming elements among the elements that form a solid solution in the forged steel, generate MX precipitates.

[0086] In order to obtain the desired level of MX precipitates, the carburizing heat treatment step is preferably performed under the following conditions: heat treatment temperature: 850-940°C; carbon potential (CP): 0.7-1.0; and heat treatment duration: 100 minutes or more.

[0087] Hereinafter, the present disclosure is described with reference to Examples and Comparative Examples.

[0088] Experiments were conducted to produce the final product according to the production conditions of commercially produced gears. Figure 1 Gear samples were manufactured using the molten metal produced by changing the contents of the various components as shown, according to the above-mentioned gear manufacturing method.

[0089] The rolled steel was heat treated at 1200° C. for 3 hours, while the forged steel was carburized at 920° C. for 200 minutes. At this time, the carbon potential (CP) was maintained at 0.8.

[0090] The fraction and number of MX precipitates as well as the pitting area and tooth bending fatigue life of the gears manufactured according to the examples and comparative examples were measured. The results are summarized in Figure 2 middle.

[0091] To measure the pitting corrosion resistance of gears, a gear durability tester from Powertrain was used. Sun gear parts were manufactured and tested in practice. Vibration values ​​were detected and measured as a criterion for gear pitting corrosion. The test conditions were as follows:

[0092] -RPM: 3200 (rpm)

[0093] -Torque: 180 (Nm)

[0094] -Flow rate: 1 (L / min)

[0095] -Oil temperature: 80 (℃)

[0096] -Time: 16.67 (hr)

[0097] In order to measure the tooth bending fatigue life, a spur gear specimen with a module of 4.23 was manufactured and measured using a repeated tension and compression tester, and the test conditions were as follows:

[0098] -Test standard: SAE J 1619

[0099] -Test frequency: 10Hz

[0100] -Load condition: R = 0.1 (R = minimum load / maximum load = 0.1)

[0101] from Figure 1 and Figure 2 It can be seen from the data that the embodiments according to the present disclosure meet all the requirements of the present disclosure, including the fraction and amount of MX precipitates and the pitting area and tooth bending fatigue life of the gear.

[0102] For example, in Examples 1 to 7 according to the present disclosure, MX precipitates with a size of 150 nm or less are present in a fraction of 0.03-0.07% and a 2 The density of precipitates is more than 50. Therefore, the pitting area of ​​the gear measured by fatigue test is less than 12mm. 2 , and the tooth bending fatigue test (SAE J 1619) results are more than 4,000 cycles.

[0103] In particular, Comparative Examples 1 to 4, in which at least one alloying element among Si, Cr, Ni and Mo does not satisfy the content range suggested in the present disclosure, show results in which the fraction and amount of MX precipitates and the pitting area of ​​the gear do not satisfy the standards of the present disclosure.

[0104] Comparative Examples 5 to 7, while falling within the respective content requirements for C, Si, Mn, Cr, Ni, and Mo, do not meet the recommended content range for at least one of Nb and V. In these Comparative Examples, the fraction and amount of MX precipitates do not meet the requirements of the present disclosure. In particular, Comparative Example 7 also fails to meet the criteria of the present disclosure with respect to the pitting area of ​​the gear.

[0105] Next, in order to examine the fraction of non-solid solution precipitates generated according to the pre-rolling heat treatment temperature, the steel sheets made of Figure 1 The cast steel formed from the composition of Example 1 was heat treated. The fraction of non-solid-dissolved precipitates was measured, and the fraction of non-solid-dissolved precipitates is shown in FIG. Figure 3 middle.

[0106] from Figure 3It can be seen from FIG. 1 that when heat treatment is performed at 1100° C., which is lower than the lower limit of the range suggested in the present disclosure, the generated fraction of non-solid solution precipitates is about 0.025%.

[0107] In contrast, when heat treatment was performed at 1200° C., which falls within the range suggested in the present disclosure, no non-solid-dissolved precipitates were formed.

[0108] Based on the results, the pre-rolling heat treatment temperature was changed as shown in Table 1. In the subsequent process, gears were manufactured according to the manufacturing method suggested in the present disclosure, and the generation fraction of fine MX precipitates was measured and shown in Table 1.

[0109] In the carburizing heat treatment step, the carburizing heat treatment conditions were maintained at 920° C. for 200 minutes, and the carbon potential (CP) was maintained at 0.8.

[0110] Table 1

[0111]

[0112] As shown in Table 1, in Comparative Examples 1-1 and 1-2 in which the pre-rolling heat treatment temperature was lower than the lower limit of the range suggested in the present disclosure, the fraction of fine MX precipitates generated was less than 0.030%.

[0113] In contrast, in Examples 1-1 and 1-2, in which the pre-rolling heat treatment temperature satisfies the range suggested in the present disclosure, the fraction of fine MX precipitates produced is greater than 0.030%.

[0114] In order to examine the generation fraction of fine MX precipitates according to the heat treatment temperature in the carburizing heat treatment step, the carbonized ... Figure 1 The molded article formed from the components of Example 1 was heat treated. The fraction of fine MX precipitates was measured, and the fraction of fine MX precipitates is shown in FIG. Figure 4 middle.

[0115] from Figure 4 As can be seen in Figure 3 , the fraction of MX precipitates of the molded part increases with carburizing heat treatment time. In particular, when the carburizing heat treatment is performed at 850° C., the fraction of fine MX precipitates exceeds 0.030% after 100 minutes of carburizing.

[0116] When the carburizing heat treatment was carried out at a temperature of 920°C, it was observed that the fraction of fine MX precipitates increased by more than 0.030% after about 50 minutes of heat treatment.

[0117] Therefore, in order to keep the fraction of fine MX precipitates above 0.030%, the carburizing heat treatment should be performed at 850°C or higher for 100 minutes or longer.

[0118] Based on the results, the carburizing heat treatment of the molded parts was changed as shown in the following Table 2. The fraction of fine MX precipitates thus produced was measured and is also summarized in Table 2.

[0119] In the pre-rolling heat treatment step, the heat treatment conditions were maintained at 1200° C. for 3 hours, and then in the carburizing heat treatment step, the heat treatment conditions were changed to carburizing heat treatment conditions and maintained for 200 minutes, with the carbon potential (CP) maintained at 0.8.

[0120] Table 2

[0121]

[0122] As shown in Table 2, in Comparative Examples 2-1 and 2-2 in which the carburizing heat treatment temperature was lower than the lower limit of the range suggested in the present disclosure, the fraction of fine MX precipitates generated was less than 0.030%.

[0123] In Comparative Example 2-3 in which the carburizing heat treatment temperature was higher than the upper limit of the range suggested in the present disclosure, the fraction of fine MX precipitates produced was also less than 0.030%.

[0124] In contrast, in Examples 2-1 to 2-4 in which the carburizing heat treatment temperature satisfies the range suggested in the present disclosure, the fraction of fine MX precipitates produced is greater than 0.030%.

[0125] Then, the components of the MX precipitates produced on the gear samples manufactured according to Example 1 were analyzed by EDS, and the results are shown in FIG. Figure 5A and Figure 5B The number and size of MX precipitates generated on the gear samples were measured.

[0126] like Figure 5A and Figure 5B As shown, Nb, V, C and N were detected in the MX precipitates, indicating that the MX precipitates were formed by at least one of Nb-based carbides, Nb-based nitrides, V-based carbides, V-based nitrides, Nb+V-based carbides and Nb+V-based nitrides.

[0127] At this time, the average size of the MX precipitates was measured to be 51 nm and the number of MX precipitates was 100 μm. 2 With 130.

[0128] As described above, the contents of Cr and Si in the steel according to the embodiment of the present disclosure are kept similar to those in conventional steel, and carburizing heat treatment can be performed even in an air atmosphere.

[0129] Compared with conventional steel, this steel contains an increased content of Ni and thus exhibits improved toughness and fatigue resistance.

[0130] Considering the temperature of the rolling process, the total content of Nb and V in this steel is increased compared to conventional steels to generate fine precipitates, thereby increasing the fraction of MX precipitates generated after carburizing heat treatment, thereby contributing to improvement in fatigue resistance.

[0131] Furthermore, physical properties are improved by adjusting the contents of main alloy components, and thus post-processes such as a CSP (conventional shot peening) process can be eliminated.

[0132] It will be understood by those skilled in the art that the present disclosure may be practiced in other specific forms without changing the technical ideas and basic features of the present disclosure. Therefore, it should be understood that the above embodiments are illustrative in all aspects and not restrictive. The scope of the present disclosure is defined by the scope of the appended claims rather than by the specific embodiments. It should be understood that all variations and modifications derived from the scope of the claims and their equivalents are included within the scope of the present disclosure.

Claims

1. A gear steel, comprising the following components based on the total weight of the steel: C: 0.10-0.30wt%; Si: 0.60-0.80wt%; Mn: 0.25-0.75wt%; Cr: 1.80-2.20wt%; Ni: 0.50-1.50wt%; Mo: 0.20-0.40wt%; Nb: 0.025-0.050wt%; V: 0.030-0.050 wt%; and The balance of Fe and unavoidable impurities, The contents of Nb and V satisfy the following <Relationship 1>: 0.055<[Nb]+[V]<0.100…………<Relationship 1>, [Nb] represents the Nb content, and [V] represents the V content.

2. The steel according to claim 1, further comprising, based on the total weight of the steel: P: 0.020 wt% or less; and S: 0.020 wt% or less.

3. The steel according to claim 1, wherein Metal carbide or metal nitride precipitates, namely, MX precipitates, are formed in the steel at a volume fraction of 0.03-0.07%.

4. The steel according to claim 3, wherein The MX precipitates are at least one of Nb-based carbides, Nb-based nitrides, V-based carbides, V-based nitrides, Nb+V-based carbides, and Nb+V-based nitrides.

5. The steel according to claim 3, wherein The size of the MX precipitates is 150 nm or less.

6. The steel according to claim 3, wherein The MX precipitates are present at a density of 100 μm. 2 The precipitates are formed at a density of 50 or more.

7. A method for manufacturing a gear, the method comprising: A molten metal preparation step, wherein the molten metal comprises, based on the total weight of the molten metal, 0.10-0.30 wt% C, 0.60-0.80 wt% Si, 0.25-0.75 wt% Mn, 1.80-2.20 wt% Cr, 0.50-1.50 wt% Ni, 0.20-0.40 wt% Mo, 0.025-0.050 wt% Nb, and 0.030-0.050 wt% V, and the remainder Fe and unavoidable impurities, wherein the contents of Nb and V satisfy the following <Relationship 1>: 0.055<[Nb]+[V]<0.100…………<Relationship 1>, [Nb] represents the Nb content, and [V] represents the V content; a pre-rolling heat treatment step of heat-treating the cast molten metal after casting the molten metal; a steel rolling step of rolling the heat-treated cast steel into rolled steel; a steel forging step of forging the heat-treated rolled steel into planetary gears; a forged steel heat treatment step of heat treating the forged steel; a part processing step of processing the forged steel into a final part; and Carburizing heat treatment step, performing carburizing treatment on the workpiece, The heat treatment temperature in the pre-rolling heat treatment step is 1180°C-1430°C, and The carburizing heat treatment step is carried out under the following conditions: The heat treatment temperature is 850° C. to 940° C., the carbon potential, ie, CP, is 0.7 to 1.0, and the heat treatment duration is more than 100 minutes.

8. The method according to claim 7, wherein: The forged steel heat treatment step is carried out under ISO heat treatment conditions.

9. The method according to claim 7, wherein: The pitting area of ​​the part after the carburizing heat treatment step is less than 12 mm as measured by fatigue testing, i.e., SAE J1619. 2 .

10. The method according to claim 7, wherein: The tooth bending fatigue test result of the article after the carburizing heat treatment step, namely SAE J 1619, was more than 4,000 cycles.

11. The method according to claim 7, wherein: After the carburizing heat treatment step, 0.03-0.07% of the volume fraction of metal carbide or metal nitride precipitates, namely MX precipitates, is formed in the workpiece. The size of the MX precipitates is less than 150 nm, and the number of the MX precipitates is 100 μm. 2 There are more than 50.

12. The method according to claim 11, wherein The MX precipitates are at least one of Nb-based carbides, Nb-based nitrides, V-based carbides, V-based nitrides, Nb+V-based carbides, and Nb+V-based nitrides.

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