CN113278882A

CN113278882A - Nb microalloying high contact fatigue performance carburized gear steel and development method thereof

Info

Publication number: CN113278882A
Application number: CN202110427671.6A
Authority: CN
Inventors: 金国忠; 丁毅; 汪开忠; 胡芳忠; 杨志强; 杨少朋; 胡乃悦; 陈世杰; 郝震宇
Original assignee: Maanshan Iron and Steel Co Ltd
Current assignee: Maanshan Iron and Steel Co Ltd
Priority date: 2021-04-21
Filing date: 2021-04-21
Publication date: 2021-08-20
Anticipated expiration: 2041-04-21
Also published as: CN113278882B

Abstract

The invention discloses steel for a high-contact fatigue gear, which is used for enabling a material to be carburized through microalloying and carburization optimizationTrace amount of carbide is precipitated in the process. On the premise of ensuring the hardness of the carburized layer, the carbon content in the martensite matrix is reduced through the fixing effect of the microalloy elements on carbon, so that the toughness of the material is improved; the carburization temperature is increased by adding Nb, and the contact fatigue performance of the gear steel is improved by more than 30 percent by matching the process, namely the rated fatigue life L is prolonged under the condition that the contact stress is 4.0GPa₁₀≥1.0×10⁷Median fatigue life L₅₀≥3.5×10⁷The content of the residual austenite of the carburized layer is more than 15 percent, and the matrix martensite hardness is lower than 12.0 GPa.

Description

Nb microalloying high contact fatigue performance carburized gear steel and development method thereof

Technical Field

The invention belongs to the field of alloy gear steel, and relates to Nb microalloying carburized gear steel with high contact fatigue property and a development method thereof, which are suitable for manufacturing carburized gears in the fields of manufacturing locomotives, automobiles and the like.

Background

Gear steel is a key material with large consumption and high requirement in the field of special steel, and is widely applied to the fields of machinery, traffic, energy and the like. The performance requirements of the gear steel not only influence the technical and economic indexes such as the service life of equipment, but also influence the requirements such as the use safety. The working environment of the gear is complex and severe, and the main failure modes are meshing surface abrasion, pit peeling caused by contact fatigue, crack or fracture caused by tooth root bending fatigue and the like. The material is generally required to have good toughness and wear resistance, so the performance of the material can be reflected by the contact fatigue of the material. In view of the complex working environment of gears, many scholars also make relevant studies on contact fatigue:

marly et al believe that a lower oxygen content can effectively reduce the amount of oxide inclusions and the gear, which is beneficial to improving the contact fatigue performance of the gear steel; meanwhile, the Nb content can control the grain size of a carburized layer of the gear steel and improve the hardness of the carburized layer, so that the fatigue crack initiation and propagation resistance are improved, which is one of the reasons for better contact fatigue performance after Nb microalloying. The hardness of martensite is closely related to the C content, and in general, the carbon content in martensite is comparable to the nominal carbon content of the material. Since C is the most typical stable austenite element, primarily distributed in the martensite phase from which austenite is transformed, this results in a carbon content in the martensite that is significantly higher than the nominal carbon content of the steel for testing. According to the study of Makinson et al on the AISI 4320 steel penetration layer, the C content is increased from 0.25% to 1.1%, the average nano indentation hardness of the martensite is increased from 5.5GPa to 10.5GPa, namely, the hardness is increased by about 10.6% for every 0.1% increase of the C content, the Young's modulus of the martensite is reduced from 263GPa to 245GPa, and the Young's modulus is reduced by about 0.8% for every 0.1% increase of the C content. For low carbon carburized gear steel, the surface strength of the carburized material is ensured, and simultaneously, the core part has certain toughness, which is realized by the cooperation of material components.

In order to solve the problems, through search, the invention patents of Chinese patent, publication No. CN 104775075, publication No. 2015, 04 and 02 disclose a fine grain carburized gear steel and a manufacturing method thereof, and the invention particularly relates to a fine grain carburized gear steel and a manufacturing method thereof. The steel comprises the following chemical components in percentage by weight: 0.18 to 0.23 percent of C, 0.15 to 0.35 percent of Si, 0.70 to 0.90 percent of Mn, less than or equal to 0.030 percent of P, less than or equal to 0.030 percent of S, 0.40 to 0.65 percent of Cr, 0.15 to 0.25 percent of Mo, 0.40 to 0.70 percent of Ni, 0.025 to 0.050 percent of Al, less than or equal to 0.20 percent of Cu, and [ N ]]≤0.0070％，[O]≤0.0015％，[H]Less than or equal to 0.0002 percent, and the balance of Fe and inevitable impurities. And a rolling production process with the finish rolling temperature of 850-. Compared with the prior carburized gear steel SAE 8620, the steel of the invention has the contact fatigue life (L)₁₀) The improvement is more than 30 percent. With the increasing performance requirements of high-performance gear steels in industries such as high-speed rail and automobiles, the high-performance gear steels cannot meet the current requirements of high-contact fatigue performance materials, so that carburized gear steels with better performance need to be developed.

The invention discloses a high-performance Mn-Cr series steel for wind power output gears and a production method thereof, wherein the patent publication number is CN110863158A, the publication number is 3/6/2020, the patent name is high, the addition amount of Cr and Ni is greatly reduced, the Mn-Cr series steel for gears with high hardenability and excellent low-temperature impact performance is provided by alloy component design and reasonable production process control, the end hardenability test is carried out according to GB/T225, the end hardenability J9, J15 and J25 are controlled to be equivalent to that of CiNiMo series, the impact performance test is carried out according to GB/T229, the KV-40 ℃ and KV2 are equivalent to that of CiNiMo series, the austenite grain size test is carried out according to GB/T6394 at the high temperature carburization grain size, the austenite grain size is not less than 8.5 grade, and the grain size is not more than 18.5 mu m.

Disclosure of Invention

1. Problems to be solved

The invention provides steel for a high-contact fatigue gear, which is used for separating out trace carbides in the carburizing process of materials through microalloying and carburizing optimization. On the premise of ensuring the hardness of the carburized layer, the carbon content in the martensite matrix is reduced through the fixing effect of the microalloy elements on carbon, so that the toughness of the material is improved. The contact fatigue performance of the gear steel is improved by more than 30 percent through the cooperation of the process, namely the rated fatigue life L is prolonged under the condition that the contact stress is 4.0GPa₁₀≥1.0×10⁷Median fatigue life L₅₀≥3.5×10⁷The content of the residual austenite of the carburized layer is more than 15 percent, and the matrix martensite hardness is lower than 12.0 GPa.

The invention also provides a preparation method and a carburizing process of the high contact fatigue gear steel, which adopts the process production of electric arc furnace smelting, LF refining, RH vacuum treatment, round billet continuous casting and rolling (finishing) finished products; after the parts are machined and formed, the parts are carburized at 930 +/-20 ℃, cooled to 860 +/-20 ℃ and subjected to oil quenching, and finally the samples are tempered at 200 +/-10 ℃.

2. Technical scheme

In order to solve the problems, the technical scheme adopted by the invention is as follows:

the carburized gear steel with Nb microalloying and high contact fatigue performance comprises the following chemical components in percentage by weight: 0.18% -0.22%, Si: 0.20-0.30%, Mn: 0.80-0.90%, Cr: 0.50% -0.70%, Mo: 0.15% -0.25%, Nb: 0.025 to 0.060%, Ni: 0.40-0.55%, Al: 0.020% -0.040%, P: less than or equal to 0.010 percent, S: 0.005% -0.035%, T.O: no more than 20ppm, [ H ]: 2.0ppm or less, [ N ]: 80-120ppm, and the balance of Fe and inevitable impurity elements, and performing 930 ℃ carburized grain size inspection according to GB/T6394 to obtain the gear steel carburized layer with the grain size more than or equal to 8.0 grade; the core grain size is more than or equal to 9.0 grade.

Further, the rated fatigue life L is 4.0GPa₁₀≥1.0×10⁷Median fatigue life L₅₀≥3.5×10⁷The content of the residual austenite of the carburized layer is more than 15 percent, and the matrix martensite hardness is lower than 12.0 GPa.

Further, the content of Nb and the content of solid-solution carbon satisfy: Δ Xc { [ Nb ] -log [ Nb ] }/7.735, where Δ Xc is the solid solution carbon content and [ Nb ] is the Nb content.

C: c is the most basic effective strengthening element in steel, and is the most effective element for affecting hardenability. The method mainly comprises the steps of obtaining a martensite structure in the carburizing and quenching process of the gear steel, ensuring the strength of the steel and ensuring that the martensite structure of the gear steel matrix has enough strength, wherein the content of C cannot be lower than 0.18 percent, and in order to ensure the toughness of a material core part, the content of C cannot be higher than 0.22 percent, so that the content of C is determined to be 0.18 to 0.22 percent, and in the actual operation process, the content of C can be controlled to be 0.20 to 0.22 percent.

Si: si improves the strength and hardness of steel through solid solution strengthening, and can also improve the hardenability of gear steel, and the content of Si is more than 0.20 percent. Si is a deoxidizer, so that a carburized layer is easily oxidized, the toughness of the carburized layer is reduced, and the fatigue strength of the gear is reduced. Therefore, in order to ensure the deoxidation effect and improve the toughness of the infiltrated layer, the content of Si is not higher than 0.30 percent. The Si content is controlled to be 0.20-0.30%, and in the actual operation process, the Si content can be controlled to be 0.25-0.30%.

Mn: mn is an effective deoxidizer and desulfurizer, and also an element for ensuring hardenability. Therefore, the Mn content should be more than 0.80%. However, excessive Mn lowers the toughness of the carburized layer and lowers the fatigue strength of the gear, so that the Mn content should be less than 0.90%. The Mn content is controlled to be 0.80-0.90%, and in the actual operation process, the Mn content can be controlled to be 0.87-0.90%.

Cr: cr can effectively improve the hardenability and strength of the steel. During the carburization process, Cr and C are combined to form fine Cr-rich carbide, free carbon in steel is fixed, the carbon content in martensite is reduced, and the toughness of the material is improved, so that at least more than 0.50% of Cr is ensured. However, too high Cr deteriorates cold workability of steel, so that Cr is not more than 0.60%. Therefore, the Cr content is controlled to be 0.50-0.70%, and in the actual operation process, the Cr content can be controlled to be 0.61-0.68%.

Mo: mo can obviously improve the hardenability of steel and prevent temper brittleness and overheating tendency. In addition, the reasonable matching of the Mo element and the Cr element in the invention can obviously improve the hardenability and the tempering resistance and promote the precipitation of carbide in the material. And if the Mo content is too low, the effect is limited, if the Mo content is too high, the formation of a grain boundary ferrite film is promoted, the thermoplasticity of the steel is not facilitated, the reheating crack tendency of the steel is increased, and the cost is higher. Therefore, the content of Mo is controlled to be 0.15-0.25%, and the content of Mo can be controlled to be 0.18-0.21% in the actual operation process.

Nb: nb is a strong carbide former, and typically one part of Nb can fix 7.74 parts of carbon. In the carburizing process, Nb and C form carbide, so that the C content of martensite in a carburized layer and a matrix is reduced, and the toughness of the martensite is improved. Further, when Nb is present in a solid solution state in combination with C, the pearlite transformation is strongly delayed, so that the stability of the super-cooled austenite can be increased, and the hardenability of the steel can be improved. Therefore, the Nb content should not be less than 0.025%. Too high Nb causes the above effect to be insignificant and reaches saturation. Therefore, the Nb content is controlled to be 0.025-0.060 percent, and the Nb content can be controlled to be 0.020-0.050 percent in the actual operation process.

Ni: ni is an austenite forming element, can effectively improve the core toughness of the steel, reduce the ductile-brittle transition temperature and improve the low-temperature impact property. A proper amount of Ni can stabilize austenite, so that the material can also retain a certain amount of retained austenite after quenching, the material has the effect of improving the fatigue strength of the steel material, and the Ni content is more than 0.40%. Too high Ni content reduces machinability after hot working. Therefore, the Ni content is controlled to be 0.40-0.55%, and in the actual operation process, the Ni content can be controlled to be 0.53-0.55%.

Al: al is an effective deoxidizer and can form AlN refined grains, and when the Al content is less than 0.025%, the effect is not obvious, and when the Al content is more than 0.050%, coarse inclusions are easily formed, thus deteriorating the performance of the steel. Therefore, the Al content should be controlled to be 0.025% -0.050%, and in the actual operation process, the Al content can be controlled to be 0.028% -0.040%.

[ N ]: the [ N ] can form compounds with B, Al and the like to refine grains, reasonable Al/[ N ] has obvious effect on grain refinement, and excessive [ N ] can form continuous casting defects such as bubbles and the like. Therefore, the content of [ N ] should be controlled to 80-120ppm, and in the actual operation process, the content of [ N ] can be controlled to 85-101 ppm.

P and S: s is easy to form MnS inclusion with manganese in steel, so that the steel is hot-brittle, but a small amount of S is added, the machinability of the gear steel can be obviously improved while the product performance is not influenced, and the MnS has the effect of refining grains; p is an element with strong segregation tendency, increases the cold brittleness of steel, reduces the plasticity and is harmful to the uniformity of the product structure and performance. Controlling P to be less than or equal to 0.010 percent, and S: 0.005-0.035%, in the actual operation process, the P content can be controlled to be 0.007-0.009%, and the S content can be controlled to be 0.014-0.022%.

T.O and [ H ]: forming oxide inclusions in the steel by the T.O, and controlling the T.O to be less than or equal to 20 ppm; [H] white spots are formed in steel, the product performance is seriously influenced, and the [ H ] is controlled to be less than or equal to 2.0 ppm.

Further, the content of Nb and the content of solid-solution carbon satisfy: and delta Xc { [ Nb ] -log [ Nb ] }/7.735, wherein [ Nb ] is the Nb content, and delta Xc is the content of solid solution carbon in the steel, so that a certain C content is kept in martensite for ensuring the hardness of the material, but higher C can cause the toughness of the material to be lower. Therefore, Δ X is 0.005 to 0.020. The data obtained by simulation according to the formula are consistent with the data measured in the actual production process.

A method for developing Nb microalloyed carburized gear steel with high contact fatigue performance comprises the following steps:

(1) smelting in an electric arc furnace;

(2) LF refining;

(3) RH vacuum treatment;

(4) continuous casting;

(5) rolling;

(6) and (5) slowly cooling.

Furthermore, in the step (5), the soaking temperature of the steel billet in the heating furnace is controlled to be 1230-1280 ℃, and the total heating time of preheating, heating and soaking is controlled to be 5.0-10.0 h. In actual operation, the soaking temperature is controlled to be 1268-1280 ℃, and the total heating time is controlled to be 6.5 h.

Furthermore, in the rolling process in the step (5), the initial rolling temperature is 1120-1180 ℃, and the final rolling temperature is 920-980 ℃. In actual operation, the initial rolling temperature is controlled to be 1130-1150 ℃, and the final rolling temperature is controlled to be 920-946 ℃.

Furthermore, in the step (6), after rolling, cooling to 600-660 ℃ by a cooling bed, entering a pit for slow cooling for more than or equal to 48 hours, and polishing and peeling after leaving the pit to ensure that the surface has no decarburization and zero defect. In actual operation, the pit entry temperature is controlled to be 638-659 ℃, and the finishing temperature is controlled to be 920-946 ℃.

Furthermore, after the sample is processed and cleaned, a vacuum carburizing furnace is adopted, the carburization is carried out by adopting a process of twice carburization and twice diffusion at 930 +/-20 ℃, after the carburization, the sample is cooled to the room temperature, then is heated to 860 +/-20 ℃, oil quenching is carried out, and finally the sample is tempered at 200 +/-10 ℃.

In addition, in the actual gear steel production process, the soaking temperature, the initial rolling temperature, the final rolling temperature and the pit entering temperature fluctuate within a small range and are within an error range.

3. Advantageous effects

Compared with the prior art, the invention has the beneficial effects that:

(1) the invention adopts specific components and a reasonable preparation method, the produced steel for the high-contact fatigue gear is subjected to 930 ℃ carburized grain size inspection according to GB/T6394, and the grain size of the carburized layer is more than or equal to 9.5 grade; the core grain size is more than or equal to 8.5 grade; rated fatigue life L under the condition that the compressive stress is 4.0GPa₁₀≥1.0×10⁷Median fatigue life L₅₀≥3.5×10⁷The grain size of the austenite of the infiltrated layer is more than 9.0 grade, the grain size of the austenite of the core is more than 8.0 grade, the content of the retained austenite of the infiltrated layer is more than 15 percent, and the hardness of the matrix martensite is lower than 12.0 GPa;

(2) according to the invention, Nb is added to play a role in carbon fixation, and carbide is formed by Nb and C in the carburizing process, so that the C content of martensite in a carburized layer and a matrix is reduced, and the toughness of the martensite is improved;

(3) the nitride of Nb has good thermal stability, the melting point is about 1900 ℃, the high temperature is not easy to melt, the carburizing temperature can be effectively improved, and the size increase of precipitated phases is avoided;

(4) the precipitated phase Nb (C, N) can obviously improve the difficulty of dislocation slippage, generate dislocation plugging, and effectively hinder crack propagation, thereby improving the fatigue limit of the material.

Drawings

The technical solutions of the present invention will be described in further detail below with reference to the accompanying drawings and examples, but it should be understood that these drawings are designed for illustrative purposes only and thus do not limit the scope of the present invention. Furthermore, unless otherwise indicated, the drawings are intended to be illustrative of the structural configurations described herein and are not necessarily drawn to scale.

FIG. 1 is grain size of carburized layer after carburization in example 1;

FIG. 2 is grain size of carburized layer after carburization in example 2;

FIG. 3 is grain size of carburized layer after carburization in example 3;

FIG. 4 is grain size of carburized layer after carburization for example 4;

FIG. 5 is the core grain size after carburization in example 1;

FIG. 6 is the core grain size after carburization in example 2;

FIG. 7 is the core grain size after carburization in example 3;

FIG. 8 is the core grain size after carburization in example 4;

FIG. 9 shows contact fatigue performance of the examples.

Detailed Description

The following detailed description of exemplary embodiments of the invention refers to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration exemplary embodiments in which the invention may be practiced. Although these exemplary embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, it should be understood that other embodiments may be realized and that various changes to the invention may be made without departing from the spirit and scope of the present invention. The following more detailed description of the embodiments of the invention is not intended to limit the scope of the invention, as claimed, but is presented for purposes of illustration only and not limitation to describe the features and characteristics of the invention, to set forth the best mode of carrying out the invention, and to sufficiently enable one skilled in the art to practice the invention. Accordingly, the scope of the invention is to be limited only by the following claims.

Example 1

Gear steel generally requires that its material have good toughness and wear resistance, which can reflect the material performance through the contact fatigue of the material. The defects left by metallurgy or heat treatment such as non-metallic inclusions, large-particle carbides or cavities and the like usually exist in the fatigue area, so that in order to improve the carburizing hardenability of the gear steel, reduce the non-metallic inclusions and reduce the grain size, the C content of a carburized layer and the martensite in a matrix is reduced through the carbon fixation effect of Nb by adding Nb, the toughness of the martensite is improved, and the stability of the austenite is improved; on the other hand, Nb (C, N) is used as a precipitated phase, and the pinning effect is utilized to prevent the crystal grains from growing, thereby improving the carburizing performance and the material plasticity. In addition, for low carbon carburized gear steel, the surface strength of the carburized material is ensured, and simultaneously, the core part has certain toughness, which needs to be realized by means of matching of material components.

The invention adopts gear steel with specific components to produce 4 furnaces of steel in a symbiotic way, adopts EAF electric arc furnace smelting, LF refining, RH vacuum treatment and CCM continuous casting, and performs round steel rolling after the continuous casting billet is heated at 1230-1280 ℃ and the heat preservation is more than or equal to 5 hours, and the start rolling temperature is as follows: 1120-1180 ℃, the finishing temperature is 930-980 ℃, the steel is cooled to be more than or equal to 650 ℃ through a cooling bed after rolling, the steel enters a pit for slow cooling for 48 hours, and the comparative steel 2 furnace is produced according to the process.

After rolling, the test steel is processed into 6 groups of contact fatigue test samples, carburized at 930 ℃, oil quenched at 860 ℃ and tempered at 200 ℃. And finally, checking the grain size and the fatigue performance of the material, and measuring the carbon content of the surface carburized layer and the hardness of the matrix martensite.

Wherein, table 1 shows chemical components of each example of the invention, table 2 shows solid solution carbon content of each example of the invention, table 3 shows steel rolling production process parameters of each example of the invention, table 4 shows grain size grade grain size after carburization, retained austenite content of carburized layer and matrix martensite hardness of each example of the invention, and table 5 shows contact fatigue rated fatigue life and median fatigue life of each example of the invention.

Table 1 chemical composition (%)

TABLE 2 solid solution carbon content of each example of the present invention

Examples	△Xc＝{[Nb]-log[Nb]}/7.735
		Example 1	0.0023
Example 2	0.0038
		Example 3	0.0056
Example 4	0.0073
		Comparative example 1	0.0012
Comparative example 2	0.0004

Table 2 shows the solid solution carbon content of each example of the invention, and the Nb content and the solid solution carbon content of the invention satisfy: and delta Xc { [ Nb ] -log [ Nb ] }/7.735, wherein [ Nb ] is the Nb content, and delta Xc is the solid solution carbon content in the steel type, and the solid solution carbon content is increased along with the increase of the Nb addition amount.

TABLE 3 Steel Rolling production Process parameters of the examples of the present invention

TABLE 4 grain size, retained austenite content of carburized layer, and matrix martensite hardness after carburization for examples of the invention

Table 4 shows the results of the detection of the grain size level of austenite, the content of retained austenite, and the matrix martensite hardness after two times of carburizing and diffusion carburization at 930 ℃, as shown in table 3, the grain size of the carburized layer is above 8.0 grade and the grain size of the comparative carburized layer is 7.5 grade after the gear steel in examples 1 to 4 of the present invention is carburized; the grain size of the core is above 7.0 grade. Compared with the comparative examples with less Nb content, in examples 1 to 4 of the present invention, the grain size increases with the increase of Nb content, and the retained austenite content of the carburized layer gradually increases, because Nb acts as carbon fixation, and Nb and C form carbide during carburization, so that the C content of martensite in the carburized layer and the matrix is reduced, and the toughness of martensite is improved while the stability of austenite is improved. Nb is firstly dissolved in austenite in a solid state, so that C is promoted to diffuse into the austenite, the content of C in the austenite is increased, the stability of the austenite is improved, and more retained austenite exists in a carburized layer.

TABLE 5 contact fatigue rated fatigue life and median fatigue life for various examples of the invention

Sample number	Contact stress/GPa	L₁₀/cycle	L₅₀/cycle
				Example 1	4.0	1.10×10⁷	3.92×10⁷
Example 2	4.0	1.29×10⁷	3.34×10⁷
				Examples3	4.0	1.76×10⁷	5.42×10⁷
Example 4	4.0	3.29×10⁷	6.97×10⁷
				Comparative example 1	4.0	0.57×10⁷	2.52×10⁷
Comparative example 2	4.0	0.45×10⁷	1.96×10⁷

Table 5 shows that the contact fatigue rated fatigue life of each example and each comparative example of the invention is compared with the median fatigue life, the contact fatigue of the example is improved by more than 30 percent compared with the contact fatigue performance of the comparative example gear steel through the optimization of microalloying, carburizing heat treatment process and the like, and the rated fatigue life L is under the condition that the compressive stress is 4.0GPa₁₀≥1.0×10⁷Median fatigue life L₅₀≥3.5×10⁷. The contact fatigue life of the pinion steel is related to the addition amount of Nb, when Nb is added into a certain amount, the precipitated phase Nb (C, N) can obviously improve the dislocation slippage difficulty and hinder the crack propagation, thereby improving the fatigue limit of the material, in addition, due to the pinning effect of the precipitated phase, the grains are prevented from growing, and the smaller grains can improve the plasticity and the impact resistance of the material.

The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and those skilled in the art can make various changes and modifications without departing from the scope of the present invention, and the present invention also includes equivalent embodiments.

Claims

1. The carburized gear steel with Nb microalloying and high contact fatigue performance is characterized in that the gear steel comprises the following chemical components in percentage by weight: 0.18% -0.22%, Si: 0.20-0.30%, Mn: 0.80-0.90%, Cr: 0.50% -0.70%, Mo: 0.15% -0.25%, Nb: 0.025 to 0.060%, Ni: 0.40-0.55%, Al: 0.020% -0.040%, P: less than or equal to 0.010 percent, S: 0.005% -0.035%, T.O: no more than 20ppm, [ H ]: 2.0ppm or less, [ N ]: 80-120ppm, and the balance of Fe and inevitable impurity elements, wherein after the prepared gear steel is subjected to a carburizing process at 930 ℃, the grain size of a carburized layer is more than or equal to 8.0 grade, and the grain size of a core is more than or equal to 9.0 grade.

2. The Nb microalloyed high contact fatigue property carburized gear steel according to claim 1, characterized in that the rated fatigue life L is 4.0GPa₁₀≥1.0×10⁷Median fatigue life L₅₀≥3.5×10⁷The content of the residual austenite of the carburized layer is more than 15 percent, and the matrix martensite hardness is lower than 12.0 GPa.

3. The Nb microalloyed high contact fatigue property carburized gear steel according to claim 1, characterized in that the Nb content and the solid solution carbon content satisfy: Δ Xc { [ Nb ] -log [ Nb ] }/7.735, where Δ Xc is the solid solution carbon content and [ Nb ] is the Nb content.

4. A method for developing Nb microalloyed high contact fatigue property carburized gear steel according to claim 1, characterized by comprising the following steps:

(1) smelting in an electric arc furnace;

(2) LF refining;

(3) RH vacuum treatment;

(4) continuous casting;

(5) rolling;

(6) and (5) slowly cooling.

5. The method for developing the carburized pinion steel with Nb microalloying and high contact fatigue performance according to the claim 4, characterized in that in the step (5), the soaking temperature of the billet in a heating furnace is controlled to be 1230-1280 ℃, and the total heating time of preheating, heating and soaking is controlled to be 5.0-10.0 h.

6. The method for developing the Nb microalloyed carburized gear steel with high contact fatigue performance as claimed in claim 4, wherein in the step (5), the initial rolling temperature is 1120-1180 ℃, and the final rolling temperature is 920-980 ℃.

7. The method for developing the Nb microalloyed carburized gear steel with high contact fatigue performance according to claim 4, characterized in that in the step (6), the steel is cooled to 600-660 ℃ by a cooling bed after being rolled, then is put into a pit for slow cooling, the slow cooling time is not less than 48h, and then is polished and scalped after being taken out of the pit.

8. The method for developing Nb microalloyed carburized gear steel with high contact fatigue performance as claimed in claim 4, wherein after the sample is cleaned, the sample is carburized twice at 930 +/-20 ℃ by using a vacuum carburizing furnace, cooled to room temperature, heated to 860 +/-20 ℃ for oil quenching, and finally tempered at 200 +/-10 ℃.