Cr-Ni-Mo series high-temperature carburizing gear steel with structure gradient distribution and manufacturing method thereof

Abstract

The invention discloses Cr-Ni-Mo series high-temperature carburizing gear steel with structure gradient distribution, which contains Fe and inevitable impurities, and further contains the following chemical elements in percentage by mass: 0.18% to 0.25% of C, 0.15% to 0.35% of Si, 0.70% to 0.90% of Mn, 0.015% to 0.025% of S, 0.015% to 0.050% of Al, 0.40% to 0.60% of Cr, 0.15% to 0.25% of Mo, 0.40% to 0.70% of Ni, 0.005% to 0.03% of Nb and 0.008% to 0.015% of N. The invention further discloses a manufacturing method of the Cr-Ni-Mo series high-temperature carburizing gear steel with the structure gradient distribution. The manufacturing method comprises the following steps: smelting and casting; heating is performed; rolling is conducted; and high-temperature carburizing heat treatment.

Description

Technical Field

[0001] This invention relates to a type of steel and a method for manufacturing the same, and more particularly to a carburized gear steel and a method for manufacturing the same. Background Technology

[0002] Gears are the most widely used transmission components in modern machinery, playing a crucial role in transmitting torque, changing the direction of rotation, and controlling speed. The main failure modes of gears are tooth surface spalling and tooth fracture. To improve the contact fatigue and bending fatigue life of gears, effective surface modification and strengthening processes are the most effective methods. Carburizing and hardening is the most commonly used surface modification and strengthening method, which hardens the gear surface, increasing strength, while maintaining a soft phase structure in the gear core, thus improving toughness.

[0003] Currently, carburizing processes include general gas carburizing and vacuum carburizing. During the carburizing heat treatment of gears, internal oxidation occurs on the gear surface (about 0.3 mm), causing the gear surface to soften and the hardness to gradually decrease from low to high ("head-down phenomenon"). This will cause the gear surface to peel off.

[0004] Furthermore, gear steel is typically carburized at around 930℃ for approximately 10 hours. However, when the carburizing temperature is increased to 960–1000℃, the entire carburizing heat treatment time is reduced by 50% compared to the conventional 930℃ temperature. However, when the carburizing temperature reaches 960℃ or higher, the austenite grains tend to coarsen, leading to reduced gear fatigue performance and even mixed grain formation. Summary of the Invention

[0005] One of the objectives of this invention is to provide a Cr-Ni-Mo series high-temperature carburizing gear steel with a gradient microstructure distribution. This high-temperature carburizing gear steel can solve the problem of surface softening in current carburized gears, suppress the phenomenon of the hardness gradient curve of the carburized layer dropping, and meet the requirement that no austenite grains mix during vacuum carburizing at 980℃, so as to achieve a good match between strength and toughness of the gear steel and improve the fatigue life of the gear.

[0006] To achieve the above objectives, the present invention provides a Cr-Ni-Mo series high-temperature carburizing gear steel with a gradient microstructure, which contains Fe and unavoidable impurities, and also contains the following chemical elements in the following mass percentages:

[0007] C: 0.18% ~ 0.25%, Si: 0.15% ~ 0.35%, Mn: 0.70% ~ 0.90%, S: 0.015% ~ 0.025%, Al: 0.015% ~ 0.050%, Cr: 0.40% to 0.60%, Mo: 0.15% to 0.25%, Ni: 0.40% to 0.70%, Nb: 0.005% to 0.03%, N: 0.008% to 0.015%.

[0008] Furthermore, in the high-temperature carburizing gear steel described in this invention, the mass percentage content of each chemical element is as follows:

[0009] C: 0.18%–0.25%, Si: 0.15%–0.35%, Mn: 0.70%–0.90%, S: 0.015%–0.025%, Al: 0.015%–0.050%, Cr: 0.40%–0.60%, Mo: 0.15%–0.25%, Ni: 0.40%–0.70%, Nb: 0.005%–0.03%, N: 0.008%–0.015%; balance Fe and other unavoidable impurities.

[0010] This high-temperature carburizing gear steel achieves low cost, resistance to surface softening, and high strength by rationally controlling the alloy content of C, Si, Mn, Cr, Ni, and Mo. Furthermore, this invention incorporates Nb, which is micro-alloyed and utilizes the high-temperature stability of Nb precipitates NbC and Nb(C,N) to effectively suppress grain growth in the high-temperature carburizing environment.

[0011] Specifically, the design principles of each chemical element in the high-temperature carburizing gear steel described in this invention are as follows:

[0012] C: In the high-temperature carburizing gear steel described in this invention, carbon (C) is the most fundamental and effective strengthening element in steel, and it is also the most effective element for ensuring the hardenability of the steel. When the C content in the steel is too low, the tensile strength of the steel cannot be effectively guaranteed, which will lead to a decrease in the gear's resistance to deformation; however, the C content in the steel should not be too high either. When the C content in the steel increases, it will also increase the hardness of the steel, affecting the subsequent processing of the material, and may cause the hardenability of the corresponding position of the gear to exceed the design requirements. Therefore, considering the influence of C content on the performance of steel, in the high-temperature carburizing gear steel described in this invention, the mass percentage of C is controlled between 0.18% and 0.25%.

[0013] Si: In the high-temperature carburizing gear steel described in this invention, Si has a strong solid solution strengthening effect, which can significantly improve the yield strength, hardenability, and tempering resistance of the steel. However, it should be noted that the Si content in the steel should not be too high, as excessive Si will promote intergranular oxidation during the carburizing process. Therefore, to ensure the toughness of the material, the mass percentage of Si in the high-temperature carburizing gear steel described in this invention is controlled between 0.15% and 0.35%.

[0014] Mn: In the high-temperature carburized gear steel described in this invention, Mn element has the effects of solid solution strengthening and grain refinement, thereby improving the strength of the steel. It also significantly improves the hardenability of the steel. However, it should be noted that Mn element also lowers the austenitizing temperature of the steel, promotes austenite grain growth, and increases the overheating sensitivity of the steel. Therefore, in the high-temperature carburized gear steel described in this invention, the mass percentage of Mn element is controlled between 0.70% and 0.90%.

[0015] S: In the high-temperature carburized gear steel described in this invention, sulfur (S) can combine with manganese (Mn) to form MnS inclusions, thereby improving the machinability of the gear steel and enhancing the surface finish of the workpiece. Simultaneously, during the austenite transformation process, ferrite precipitates not only at the original austenite grain boundaries but also with MnS as the nucleus. This increased number of ferrite nucleation sites refines the ferrite-pearlite microstructure. Therefore, to maximize the beneficial effects of sulfur, the mass percentage of sulfur in the high-temperature carburized gear steel described in this invention is controlled between 0.015% and 0.025%.

[0016] Al: In the high-temperature carburizing gear steel described in this invention, Al can combine with N to form AlN. The formed AlN can effectively inhibit austenite grain growth and refine austenite grains during rolling heating and high-temperature carburizing. However, it should be noted that AlN will aggregate and dissolve at around 940℃. As the temperature increases, Al has a detrimental effect on inhibiting austenite grain size; at the same time, a large amount of brittle Al2O3 inclusions is detrimental to the fatigue life of gear steel. Therefore, in the high-temperature carburizing gear steel described in this invention, the mass percentage of Al is controlled between 0.015% and 0.050%.

[0017] Cr: In the high-temperature carburizing gear steel described in this invention, Cr element can effectively improve the hardenability of the steel by increasing the stability of supercooled austenite, thereby achieving a strengthening effect, and also has a good influence on the impact toughness of the steel. Therefore, in order to give full play to the excellent effects of Cr element, the mass percentage of Cr element in the high-temperature carburizing gear steel described in this invention is controlled between 0.40% and 0.60%.

[0018] Mo: In the high-temperature carburizing gear steel described in this invention, the proper combination of Mo and Cr elements can significantly improve the hardenability and tempering resistance of the steel, and molybdenum can refine the grains. However, it should be noted that the Mo content in the steel should not be too high. When the Mo content in the steel is too high, it will lead to the formation of a ferrite film at the grain boundaries, which is detrimental to the hot plasticity of the steel, increases the tendency of the steel to reheat cracking, and increases the production cost. Therefore, considering the production cost and the beneficial effects of adding Mo, the mass percentage of Mo in the high-temperature carburizing gear steel described in this invention is controlled between 0.15% and 0.25%.

[0019] Ni: In the high-temperature carburizing gear steel described in this invention, Ni has a positive effect on the impact toughness of the steel. Adding an appropriate amount of Ni to the steel can also effectively refine the microstructure and achieve a strengthening effect. Simultaneously, Ni can also increase the stability of supercooled austenite and improve the hardenability of the steel. Therefore, in the high-temperature carburizing gear steel described in this invention, the mass percentage of Ni is controlled between 0.40% and 0.70%.

[0020] Nb: In the high-temperature carburizing gear steel described in this invention, Nb is a very effective alloying element for refining grains. The Nb(C,N) formed can pin grain boundaries, hindering austenite grain growth and effectively reducing carburizing and quenching deformation. However, the Nb content in the steel should not be too high. When the Nb content is too high, the carbonitride precipitates of Nb in the billet cannot dissolve back into the gear steel matrix. With subsequent hot working or heat treatment, some precipitates will coarsen, and the coarse precipitates cannot prevent austenite grain growth during high-temperature carburizing. Therefore, in the high-temperature carburizing gear steel described in this invention, the mass percentage of Nb is controlled between 0.005% and 0.03%.

[0021] N: In the high-temperature carburized gear steel described in this invention, the main role of N is to form nitrides with Al and Nb. Under high-temperature conditions, the formed AlN and Nb (C, N) can be dispersed at the austenite grain boundaries, hindering the migration of austenite grains and inhibiting the coarsening of austenite grains. However, it should be noted that the N content in the steel should not be too high. When the N content in the steel is too high, coarse nitride particles will be formed, affecting the fatigue performance of the steel. Therefore, in order to exert the beneficial effects of N, the mass percentage of N in the high-temperature carburized gear steel described in this invention is controlled between 0.008% and 0.015%.

[0022] Furthermore, in the high-temperature carburizing gear steel described in this invention, its hardenability DI value ranges as follows: 1.50 ≤ DI ≤ 2.50, wherein:

[0023] DI = 0.54 × C × (1 + 3.333 × Mn) × (1 + 0.7 × Si) × (1 + 0.363 × Ni) × (1 + 2.16 × Cr) × (1 + 3 × Mo) × (1 + 0.365 × Cu), where each chemical element is represented by its mass percentage value before the percentage sign.

[0024] Furthermore, in the high-temperature carburized gear steel described in this invention, the carbon equivalent Ceq ranges as follows: 0.510≤Ceq≤0.560, where Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15, and each chemical element is represented by the value before the percentage sign of its mass percentage content.

[0025] Furthermore, in the unavoidable impurities of the high-temperature carburized gear steel described in this invention, the content of each impurity element satisfies at least one of the following: P≤0.018%, Ti≤0.01%, Cu≤0.05%.

[0026] In the above technical solution, P, Ti and Cu are all impurity elements in steel. When technical conditions permit, in order to obtain steel with better performance and higher quality, the content of impurity elements in steel should be reduced as much as possible.

[0027] Furthermore, in the high-temperature carburized gear steel described in this invention, the microstructure of its carburized layer is as follows: the surface layer is high-carbon acicular martensite, the subsurface layer is high-carbon acicular martensite + lath martensite, and the innermost layer of the carburized layer is lath martensite + bainite.

[0028] Furthermore, in the high-temperature carburized gear steel described in this invention, the surface hardness of the carburized layer is 810-870HV0.2, and the tensile strength is 2100-2450MPa.

[0029] Another objective of this invention is to provide a method for manufacturing Cr-Ni-Mo series high-temperature carburizing gear steel with a gradient microstructure distribution. This method can further refine the microstructure and substructure of the gear steel through two quenching processes during carburizing heat treatment, thereby improving the strength and toughness of the gear steel. Furthermore, the deep cryogenic treatment can further promote the conversion of retained austenite in the surface layer of the carburized layer into martensite, reduce the content of retained austenite, increase the surface hardness of the carburized layer, and improve the wear resistance of the gear.

[0030] To achieve the above objectives, the present invention provides a method for manufacturing high-temperature carburized gear steel, comprising the following steps:

[0031] (1) Smelting and casting;

[0032] (2) Heating;

[0033] (3) Rolling;

[0034] (4) High-temperature carburizing heat treatment: the carburizing temperature is 930~980℃ and the carburizing time is 2~4h; then quenching is performed once at a temperature of 830~860℃, and the temperature is held and then cooled to room temperature; then quenching is performed twice at a temperature of 780~830℃, and the temperature is held and then cooled to room temperature; then cryogenic treatment is performed at a temperature of -130~-230℃, and the temperature is held for 2~5h; then tempering is performed at a temperature of 150~200℃, and the temperature is held and then air-cooled to room temperature.

[0035] In this invention, different carburized layer depths can be obtained by high-temperature vacuum carburizing at 930–980℃ for 2–4 hours. Two quenching processes further refine the grains and increase the strength and plasticity of the gear steel. When the quenching temperature is too low, the amount of carbides incorporated into the matrix decreases, reducing the carbon concentration in the carburized layer and thus lowering the overall hardness of the carburized layer. When the quenching temperature is too high, carbon atoms on the surface of the carburized layer diffuse more rapidly towards the innermost layer, resulting in a decrease in the surface hardness and an increase in the hardness of the subsurface layer. Furthermore, although the effective carburized layer depth increases at higher quenching temperatures, the hardness curve of the carburized layer tends to show a gradual decrease in hardness after a period of increase (i.e., a "bowing down" phenomenon). Therefore, in the manufacturing method described in this invention, the primary quenching temperature is controlled at 830–860℃, and the secondary quenching temperature is controlled at 780–830℃.

[0036] In this invention, due to the transformation point M of the high-carbon martensite on the surface of the gear steel during the carburizing heat treatment process... f The hardness is very low (typically below zero), so quenching and cooling to room temperature leaves a large amount of austenite. Since retained austenite is unstable, it reduces surface hardness and decreases the surface wear resistance of gear steel. However, during cryogenic treatment, the retained austenite in the carburized layer of the gear steel can be fully transformed into martensite. Due to the reduced retained austenite content and refined microstructure, with smaller and more numerous carbide particles, the hardness and wear resistance of the gear steel can be improved.

[0037] In this invention, tempered martensite is obtained by controlling the tempering temperature to 150–200°C and holding it at that temperature for 60 minutes. Air cooling to room temperature balances the strength and plasticity of the gear steel.

[0038] Furthermore, in step (4) of the manufacturing method described in this invention, after carburizing treatment at a carburizing temperature of 930-980°C and a carburizing time of 2-4 hours, the austenite grain size is not less than grade 8.0.

[0039] Furthermore, in step (2) of the manufacturing method of the present invention, segmented heating is performed, wherein the temperature of the preheating section is controlled to be ≤800℃, the temperature of the first heating section is 1040~1100℃, the temperature of the second heating section is 1180~1220℃, the temperature of the soaking section is 1160~1200℃, and the total heating time is controlled to be not less than 180min.

[0040] In this invention, by controlling the temperatures of the preheating section, the first heating section, the second heating section, and the soaking section, the complete dissolution of the Nb precipitate can be ensured, and Nb can be fully dissolved back into the matrix and diffused uniformly, thereby helping to avoid the formation of coarse precipitates during rolling. Furthermore, by controlling the heating time to be no less than 180 minutes, the final quality and properties of the steel can be ensured.

[0041] Furthermore, in step (3) of the manufacturing method described in this invention, the final rolling temperature is controlled to be ≥970°C.

[0042] In this invention, to avoid rolling in the nose temperature zone of 900-950°C where Nb precipitates, the final rolling temperature can be controlled to be ≥970°C to reduce the residence time in the nose temperature zone.

[0043] The Cr-Ni-Mo high-temperature carburizing gear steel with a gradient microstructure distribution and its manufacturing method described in this invention have the following advantages and beneficial effects compared to the prior art:

[0044] The high-temperature carburized gear steel of this invention, through reasonable control of the alloy content of C, Si, Mn, Cr, Ni, and Mo, combined with optimized processes, has the advantages of low cost, resistance to surface softening, and high strength. Although the alloy content of Cr and Ni in the high-temperature carburized gear steel of this invention is low, the hardness of the carburized layer is not reduced, and there is no downward trend in the hardness gradient curve of the carburized layer. In addition, the tensile strength of the high-temperature carburized gear steel of this invention reaches 2100-2420 MPa after carburizing heat treatment.

[0045] The high-temperature carburizing gear steel of this invention incorporates Nb. By microalloying Nb and utilizing the high-temperature stability of Nb precipitates NbC and Nb(C,N), grain growth in the high-temperature carburizing environment can be effectively suppressed, achieving austenite grain size ≥8.0 grade in vacuum carburizing at ≤980℃.

[0046] The manufacturing method of high-temperature carburized gear steel described in this invention combines optimized design and manufacturing processes. The two quenching processes of carburizing heat treatment can further refine the microstructure and substructure of the gear steel, thereby improving its strength and toughness. Furthermore, the cryogenic treatment further promotes the conversion of residual austenite in the surface layer of the carburized layer into martensite, reduces the content of residual austenite, increases the surface hardness of the carburized layer, and improves the wear resistance of the gear. Attached Figure Description

[0047] Figure 1 The metallographic morphology of the surface layer of the high-temperature carburized gear steel in Example 3 is shown.

[0048] Figure 2The metallographic morphology of the subsurface layer of the carburized layer of the high-temperature carburized gear steel in Example 3 is shown.

[0049] Figure 3 The metallographic morphology of the innermost layer of the carburized layer of the high-temperature carburized gear steel in Example 3 is shown.

[0050] Figure 4 The hardness gradient curves of the carburized layer of the high-temperature carburized gear steel of Examples 1-6 and the comparative steel of Comparative Examples 1-2 are shown.

[0051] Figure 5 The image shows the austenite grain size of the high-temperature carburized gear steel of Example 3 after water quenching at 980°C for 4 hours. Detailed Implementation

[0052] The following will further explain and illustrate the Cr-Ni-Mo series high-temperature carburizing gear steel with a gradient microstructure and its manufacturing method described in this invention, in conjunction with the accompanying drawings and specific embodiments. However, this explanation and illustration do not constitute an undue limitation on the technical solution of this invention.

[0053] Examples 1-6 and Comparative Examples 1-2

[0054] The high-temperature carburized gear steels in Examples 1-6 were all prepared using the following steps:

[0055] (1) Smelting and Casting: Smelting is carried out in an electric furnace, with the tapping temperature controlled between 1650℃ and 1680℃. Then, LF refining is performed, with the final LF refining temperature controlled to ≥1660℃. Next, VD vacuum degassing is performed, with the settling time controlled to ≥15min. Low superheat casting is used, with the molten steel superheat controlled between 17-33℃ and the billet pulling speed between 0.62-0.68m / min. Electromagnetic stirring and light pressure are employed. The electromagnetic stirring frequency in the crystallizer is controlled at 2.5Hz, and the current is controlled between 150-300A. The final electromagnetic stirring frequency is controlled at 8Hz, and the current is controlled at 300A. The continuously cast billet is hot-charged into the furnace, with the charging temperature controlled to ≤450℃.

[0056] (2) Heating: Segmented heating is adopted, in which the steel billet is hot-charged into the heating furnace, which is a regenerative walking beam type heating furnace. The temperature of the preheating section is controlled at ≤800℃, the temperature of the first heating section is controlled at 1040~1100℃, the temperature of the second heating section is controlled at 1180~1220℃, the temperature of the soaking section is controlled at 1160~1200℃, and the total heating time is controlled at not less than 180min.

[0057] (3) Rolling: Control the final rolling temperature ≥970℃

[0058] (4) High-temperature carburizing heat treatment: The carburizing temperature is controlled at 930~980℃ and the carburizing time is controlled at 2~4h; then, a first quenching is performed, the first quenching temperature is controlled at 830~860℃, and after holding for 10min, it is oil-cooled to room temperature; then, a second quenching is performed, the second quenching temperature is controlled at 780~830℃, and after holding for 120min, it is oil-cooled to room temperature; then, a cryogenic treatment is performed, the cryogenic temperature is controlled at -130~-230℃, and the holding time is controlled at 2~5h; then, tempering is performed, the tempering temperature is controlled at 150~200℃, and after holding for 60min, it is air-cooled to room temperature.

[0059] In this invention, the chemical composition design and related processes of the high-temperature carburizing gear steels in Examples 1-6 all meet the design specifications of this invention. Correspondingly, Comparative Examples 1 and 2 are 20CrNiMo gear steels prepared according to standard GB / T 3077-2015, and their chemical composition designs are also listed in Table 1 below. It should be noted that Comparative Examples 1 and 2 undergo conventional carburizing heat treatment without secondary quenching or cryogenic treatment.

[0060] Table 1 lists the mass percentage of each chemical element in the high-temperature carburized gear steels of Examples 1-6 and the comparative steels of Comparative Examples 1-2.

[0061] Table 1. (wt.%, balance Fe and other unavoidable impurities besides P, Ti and Cu)

[0062] serial number C Si Mn S Al Cr Mo Ni Nb N P Ti Cu DI Cep Example 1 0.22 0.22 0.81 0.025 0.04 0.53 0.15 0.47 0.017 0.0083 0.007 0.009 0.030 1.87 0.524 Example 2 0.25 0.16 0.72 0.023 0.015 0.55 0.17 0.51 0.028 0.0115 0.0068 0.001 0.036 1.94 0.540 Example 3 0.21 0.26 0.82 0.024 0.023 0.60 0.23 0.54 0.03 0.01 0.0083 0.008 0.030 2.35 0.551 Example 4 0.20 0.25 0.81 0.018 0.035 0.45 0.24 0.57 0.02 0.0107 0.0109 0.007 0.036 1.95 0.513 Example 5 0.18 0.33 0.89 0.021 0.050 0.57 0.21 0.67 0.005 0.015 0.0058 0.005 0.032 2.17 0.531 Example 6 0.23 0.24 0.82 0.016 0.028 0.43 0.25 0.42 0.01 0.0092 0.0089 0.002 0.035 2.13 0.533 Comparative Example 1 0.19 0.21 0.95 0.003 0.031 0.66 0.30 0.75 0 0.01 0.005 0.005 0.060 2.94 0.594 Comparative Example 2 0.17 0.18 0.65 0.015 0.015 0.45 0.20 0.35 0 0.0121 0.006 0.006 0.031 1.18 0.434

[0063] Tables 2-1 and 2-2 list the specific process parameters for the high-temperature carburized gear steels of Examples 1-6 and the comparative steels of Comparative Examples 1-2 in the above process steps.

[0064] Table 2-1.

[0065]

[0066] Table 2-2.

[0067]

[0068] Note: Comparative Examples 1 and 2 were subjected to traditional carburizing heat treatment, without secondary quenching or deep cryogenic treatment.

[0069] Samples were taken from the high-temperature carburized gear steels of Examples 1-6 prepared above, and the microstructure of the carburized layer was observed using a metallographic microscope, wherein:

[0070] Figure 1 The metallographic morphology of the surface layer of the high-temperature carburized gear steel in Example 3 is shown.

[0071] Figure 2The metallographic morphology of the subsurface layer of the carburized layer of the high-temperature carburized gear steel in Example 3 is shown.

[0072] Figure 3 The metallographic morphology of the innermost layer of the carburized layer of the high-temperature carburized gear steel in Example 3 is shown.

[0073] From the above Figure 1 , 2 As can be seen from point 3, the high-temperature carburized gear steel produced by the manufacturing method described in this invention has a microstructure gradient. The surface microstructure of the carburized layer is mainly high-carbon acicular martensite, the subsurface microstructure of the carburized layer is high-carbon acicular martensite + lath martensite, and the innermost microstructure of the carburized layer is lath martensite + bainite.

[0074] In addition, various relevant performance tests were conducted on the high-temperature carburized gear steels of Examples 1-6 prepared above:

[0075] (1) Hardness test: The Vickers hardness tester was used to measure the hardness curve from the surface layer to the innermost layer of the carburized layer. The load of the hardness tester was 200g and the interval was 0.2mm. The surface hardness test results are listed in Table 3.

[0076] also, Figure 4 The hardness gradient curves of the carburized layer of the high-temperature carburized gear steel of Examples 1-6 and the comparative steel of Comparative Examples 1-2 are also shown.

[0077] (2) Mechanical property testing: The mechanical properties of the high-temperature carburized gear steel of Examples 1-6 and the comparative steel of Comparative Examples 1-2 were tested using a universal mechanical testing machine to obtain the tensile strength of each example and comparative example, and the test results are listed in Table 3.

[0078] Table 3 lists the test results of various relevant properties of the high-temperature carburized gear steels of Examples 1-6 and the comparative steels of Comparative Examples 1-2.

[0079] Table 3.

[0080]

[0081]

[0082] From Table 3 above and Figure 4 It can be seen that the hardness of the carburized layer of the high-temperature carburized gear steel in Examples 1-6 gradually decreases from the surface to the innermost layer, without any softening of the surface layer, i.e., the hardness curve gradually decreases after increasing from low to high. In contrast, Comparative Examples 1 and 2 show a downward trend in hardness curves, indicating softening of the surface layer, and the secondary surface layer in Comparative Examples 1 and 2 is harder than the surface layer. Furthermore, the surface hardness of Examples 1-6 is between 827-870 HV0.2, and the tensile strength is between 2120-2420 MPa.

[0083] In addition, according to GB / T 6394-2017, the grain size of the high-temperature carburized gear steel of Examples 1-6 and the comparative steel of Comparative Examples 1-2 was tested. Examples 1-6 were subjected to simulated carburizing heat treatment at 980℃, and Comparative Examples 1-2 were subjected to simulated carburizing heat treatment at 930℃. The test results are recorded in Table 4.

[0084] also, Figure 5 The image shows the austenite grain size of the high-temperature carburized gear steel of Example 3 after water quenching at 980°C for 4 hours.

[0085] Table 4 lists the austenite grain size test results of the high-temperature carburized gear steels of Examples 1-6 and the comparative steels of Comparative Examples 1-2.

[0086] Table 4.

[0087] serial number Austenite grain size (grade) Example 1 9.5 Example 2 8.5 Example 3 9.0 Example 4 9.5 Example 5 8.0 Example 6 8.0 Comparative Example 1 Mixed crystal 7.5(0.0) Comparative Example 2 Mixed crystal 7.0(0.0)

[0088] Combining Table 4 above and Figure 5 It can be seen that the austenite grain size of the high-temperature carburized gear steel produced by the manufacturing method described in this invention is not less than grade 8.0.

[0089] Furthermore, the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0090] It should also be noted that the above examples are merely specific embodiments of the present invention, and the present invention is obviously not limited to the above embodiments, with many similar variations. All modifications that can be directly derived or conceived by those skilled in the art from the content disclosed in this invention should fall within the protection scope of this invention.

Claims

1. A Cr-Ni-Mo high-temperature carburizing gear steel with a gradient microstructure, containing Fe and unavoidable impurities, characterized in that... It also contains the following chemical elements in the following percentages by mass: C: 0.18% ~ 0.25%, Si: 0.15% ~ 0.35%, Mn: 0.70% ~ 0.90%, S: 0.015% ~ 0.025%, Al: 0.015% ~ 0.050%, Cr: 0.40% to 0.60%, Mo: 0.15% to 0.25%, Ni: 0.40% to 0.70%, Nb: 0.005% to 0.03%, N: 0.008% to 0.015%.

2. The Cr-Ni-Mo high-temperature carburizing gear steel as described in claim 1, characterized in that, Its mass percentage content of each chemical element is as follows: C: 0.18%–0.25%, Si: 0.15%–0.35%, Mn: 0.70%–0.90%, S: 0.015%–0.025%, Al: 0.015%–0.050%, Cr: 0.40%–0.60%, Mo: 0.15%–0.25%, Ni: 0.40%–0.70%, Nb: 0.005%–0.03%, N: 0.008%–0.015%; balance Fe and other unavoidable impurities.

3. The Cr-Ni-Mo high-temperature carburizing gear steel as described in claim 1 or 2, characterized in that, Its hardenability DI value range is: 1.50≤DI≤2.50, where: DI = 0.54 × C × (1 + 3.333 × Mn) × (1 + 0.7 × Si) × (1 + 0.363 × Ni) × (1 + 2.16 × Cr) × (1 + 3 × Mo) × (1 + 0.365 × Cu), where each chemical element is represented by its mass percentage value before the percentage sign.

4. The Cr-Ni-Mo high-temperature carburizing gear steel as described in claim 1 or 2, characterized in that, Its carbon equivalent Ceq range is: 0.510≤Ceq≤0.560, where Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15, and each chemical element is substituted with the value before the percentage sign of its mass content.

5. The Cr-Ni-Mo high-temperature carburizing gear steel as described in claim 1 or 2, characterized in that, In unavoidable impurities, the content of each impurity element must satisfy at least one of the following conditions: P≤0. 0.18%, Ti≤0.01%, Cu≤0.05%.

6. The Cr-Ni-Mo high-temperature carburizing gear steel as described in claim 1 or 2, characterized in that, The microstructure of its carburized layer is as follows: the surface layer is high-carbon acicular martensite, the subsurface layer is high-carbon acicular martensite + lath martensite, and the innermost layer is lath martensite + bainite.

7. The Cr-Ni-Mo high-temperature carburizing gear steel as described in claim 1 or 2, characterized in that, The surface hardness of its carburized layer is 810-870 HV0.2, and the tensile strength is 2100-2450 MPa.

8. The method for manufacturing Cr-Ni-Mo high-temperature carburizing gear steel as described in any one of claims 1-7, characterized in that, It includes the following steps: (1) Smelting and casting; (2) Heating; (3) Rolling; (4) High-temperature carburizing heat treatment: the carburizing temperature is 930~980℃ and the carburizing time is 2~4h; then quenching is performed once at a temperature of 830~860℃, and the temperature is held and then cooled to room temperature; then quenching is performed twice at a temperature of 780~830℃, and the temperature is held and then cooled to room temperature; then cryogenic treatment is performed at a temperature of -130~-230℃, and the temperature is held for 2~5h; then tempering is performed at a temperature of 150~200℃, and the temperature is held and then air-cooled to room temperature.

9. The manufacturing method as described in claim 8, characterized in that, In step (4), after carburizing at a temperature of 930 to 980°C and a carburizing time of 2 to 4 hours, the austenite grain size is not less than grade 8.

0.

10. The manufacturing method as described in claim 8, characterized in that, In step (2), segmented heating is carried out, wherein the temperature of the preheating section is controlled to be ≤800℃, the temperature of the first heating section is 1040~1100℃, the temperature of the second heating section is 1180~1220℃, the temperature of the heat soaking section is 1160~1200℃, and the total heating time is controlled to be not less than 180min.

11. The manufacturing method as described in claim 8, characterized in that, In step (3), the final rolling temperature is controlled to be ≥970℃.

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